HTTP参考手册

RFC 2616: HTTP/1.1

本备忘录的状态本文件规定了互联网社区的互联网标准跟踪协议，并要求对改进进行讨论和提出建议。这份备忘录的分发是无限的。摘要超文本传输协议（HTTP）是分布式协作超媒体信息系统的应用级协议。它是一种通用的，无状态的协议，它可以用于超出超文本使用的许多任务，如名称服务器和分布式对象管理系统，通过扩展其请求方法，错误代码和头文件[47]。 HTTP的一个特性是数据表示的输入和协商，允许独立于正在传输的数据构建系统。 HTTP自1990年以来一直由全球网络全球信息倡议使用。该规范定义了被称为“HTTP / 1.1”的协议，并且是对RFC 2068 [33]的更新。

1 介绍

1.1 目的

The Hypertext Transfer Protocol (HTTP) is an application-level    protocol for distributed, collaborative, hypermedia information    systems. HTTP has been in use by the World-Wide Web global    information initiative since 1990. The first version of HTTP,    referred to as HTTP/0.9, was a simple protocol for raw data transfer    across the Internet. HTTP/1.0, as defined by [RFC 1945](https://tools.ietf.org/html/rfc1945) [[6](about:blank#ref-6)], improved    the protocol by allowing messages to be in the format of MIME-like    messages, containing metainformation about the data transferred and    modifiers on the request/response semantics. However, HTTP/1.0 does    not sufficiently take into consideration the effects of hierarchical    proxies, caching, the need for persistent connections, or virtual    hosts. In addition, the proliferation of incompletely-implemented    applications calling themselves "HTTP/1.0" has necessitated a    protocol version change in order for two communicating applications    to determine each other's true capabilities.     This specification defines the protocol referred to as "HTTP/1.1".    This protocol includes more stringent requirements than HTTP/1.0 in    order to ensure reliable implementation of its features.     Practical information systems require more functionality than simple    retrieval, including search, front-end update, and annotation. HTTP    allows an open-ended set of methods and headers that indicate the    purpose of a request [[47](about:blank#ref-47)]. It builds on the discipline of reference    provided by the Uniform Resource Identifier (URI) [[3](about:blank#ref-3)], as a location    (URL) [[4](about:blank#ref-4)] or name (URN) [[20](about:blank#ref-20)], for indicating the resource to which a      method is to be applied. Messages are passed in a format similar to    that used by Internet mail [[9](about:blank#ref-9)] as defined by the Multipurpose    Internet Mail Extensions (MIME) [[7](about:blank#ref-7)].     HTTP is also used as a generic protocol for communication between    user agents and proxies/gateways to other Internet systems, including    those supported by the SMTP [[16](about:blank#ref-16)], NNTP [[13](about:blank#ref-13)], FTP [[18](about:blank#ref-18)], Gopher [[2](about:blank#ref-2)],    and WAIS [[10](about:blank#ref-10)] protocols. In this way, HTTP allows basic hypermedia    access to resources available from diverse applications.

1.2 要求

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",    "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this    document are to be interpreted as described in [RFC 2119](https://tools.ietf.org/html/rfc2119) [[34](about:blank#ref-34)].     An implementation is not compliant if it fails to satisfy one or more    of the MUST or REQUIRED level requirements for the protocols it    implements. An implementation that satisfies all the MUST or REQUIRED    level and all the SHOULD level requirements for its protocols is said    to be "unconditionally compliant"; one that satisfies all the MUST    level requirements but not all the SHOULD level requirements for its    protocols is said to be "conditionally compliant."

1.3 术语

This specification uses a number of terms to refer to the roles    played by participants in, and objects of, the HTTP communication.     connection       A transport layer virtual circuit established between two programs       for the purpose of communication.     message       The basic unit of HTTP communication, consisting of a structured       sequence of octets matching the syntax defined in [section 4](about:blank#section-4) and       transmitted via the connection.     request       An HTTP request message, as defined in [section 5](about:blank#section-5).     response       An HTTP response message, as defined in [section 6](about:blank#section-6).      resource       A network data object or service that can be identified by a URI,       as defined in [section 3.2](about:blank#section-3.2). Resources may be available in multiple       representations (e.g. multiple languages, data formats, size, and       resolutions) or vary in other ways.     entity       The information transferred as the payload of a request or       response. An entity consists of metainformation in the form of       entity-header fields and content in the form of an entity-body, as       described in [section 7](about:blank#section-7).     representation       An entity included with a response that is subject to content       negotiation, as described in [section 12](about:blank#section-12). There may exist multiple       representations associated with a particular response status.     content negotiation       The mechanism for selecting the appropriate representation when       servicing a request, as described in [section 12](about:blank#section-12). The       representation of entities in any response can be negotiated       (including error responses).     variant       A resource may have one, or more than one, representation(s)       associated with it at any given instant. Each of these       representations is termed a `varriant'.  Use of the term `variant'       does not necessarily imply that the resource is subject to content       negotiation.     client       A program that establishes connections for the purpose of sending       requests.     user agent       The client which initiates a request. These are often browsers,       editors, spiders (web-traversing robots), or other end user tools.     server       An application program that accepts connections in order to       service requests by sending back responses. Any given program may       be capable of being both a client and a server; our use of these       terms refers only to the role being performed by the program for a       particular connection, rather than to the program's capabilities       in general. Likewise, any server may act as an origin server,       proxy, gateway, or tunnel, switching behavior based on the nature       of each request.      origin server       The server on which a given resource resides or is to be created.     proxy       An intermediary program which acts as both a server and a client       for the purpose of making requests on behalf of other clients.       Requests are serviced internally or by passing them on, with       possible translation, to other servers. A proxy MUST implement       both the client and server requirements of this specification. A       "transparent proxy" is a proxy that does not modify the request or       response beyond what is required for proxy authentication and       identification. A "non-transparent proxy" is a proxy that modifies       the request or response in order to provide some added service to       the user agent, such as group annotation services, media type       transformation, protocol reduction, or anonymity filtering. Except       where either transparent or non-transparent behavior is explicitly       stated, the HTTP proxy requirements apply to both types of       proxies.     gateway       A server which acts as an intermediary for some other server.       Unlike a proxy, a gateway receives requests as if it were the       origin server for the requested resource; the requesting client       may not be aware that it is communicating with a gateway.     tunnel       An intermediary program which is acting as a blind relay between       two connections. Once active, a tunnel is not considered a party       to the HTTP communication, though the tunnel may have been       initiated by an HTTP request. The tunnel ceases to exist when both       ends of the relayed connections are closed.     cache       A program's local store of response messages and the subsystem       that controls its message storage, retrieval, and deletion. A       cache stores cacheable responses in order to reduce the response       time and network bandwidth consumption on future, equivalent       requests. Any client or server may include a cache, though a cache       cannot be used by a server that is acting as a tunnel.     cacheable       A response is cacheable if a cache is allowed to store a copy of       the response message for use in answering subsequent requests. The       rules for determining the cacheability of HTTP responses are       defined in [section 13](about:blank#section-13). Even if a resource is cacheable, there may       be additional constraints on whether a cache can use the cached       copy for a particular request.      first-hand       A response is first-hand if it comes directly and without       unnecessary delay from the origin server, perhaps via one or more       proxies. A response is also first-hand if its validity has just       been checked directly with the origin server.     explicit expiration time       The time at which the origin server intends that an entity should       no longer be returned by a cache without further validation.     heuristic expiration time       An expiration time assigned by a cache when no explicit expiration       time is available.     age       The age of a response is the time since it was sent by, or       successfully validated with, the origin server.     freshness lifetime       The length of time between the generation of a response and its       expiration time.     fresh       A response is fresh if its age has not yet exceeded its freshness       lifetime.     stale       A response is stale if its age has passed its freshness lifetime.     semantically transparent       A cache behaves in a "semantically transparent" manner, with       respect to a particular response, when its use affects neither the       requesting client nor the origin server, except to improve       performance. When a cache is semantically transparent, the client       receives exactly the same response (except for hop-by-hop headers)       that it would have received had its request been handled directly       by the origin server.     validator       A protocol element (e.g., an entity tag or a Last-Modified time)       that is used to find out whether a cache entry is an equivalent       copy of an entity.     upstream/downstream       Upstream and downstream describe the flow of a message: all       messages flow from upstream to downstream.      inbound/outbound       Inbound and outbound refer to the request and response paths for       messages: "inbound" means "traveling toward the origin server",       and "outbound" means "traveling toward the user agent"

1.4 整体操作

The HTTP protocol is a request/response protocol. A client sends a    request to the server in the form of a request method, URI, and    protocol version, followed by a MIME-like message containing request    modifiers, client information, and possible body content over a    connection with a server. The server responds with a status line,    including the message's protocol version and a success or error code,    followed by a MIME-like message containing server information, entity    metainformation, and possible entity-body content. The relationship    between HTTP and MIME is described in appendix 19.4.     Most HTTP communication is initiated by a user agent and consists of    a request to be applied to a resource on some origin server. In the    simplest case, this may be accomplished via a single connection (v)    between the user agent (UA) and the origin server (O).            request chain ------------------------>        UA -------------------v------------------- O           <----------------------- response chain     A more complicated situation occurs when one or more intermediaries    are present in the request/response chain. There are three common    forms of intermediary: proxy, gateway, and tunnel. A proxy is a    forwarding agent, receiving requests for a URI in its absolute form,    rewriting all or part of the message, and forwarding the reformatted    request toward the server identified by the URI. A gateway is a    receiving agent, acting as a layer above some other server(s) and, if    necessary, translating the requests to the underlying server's    protocol. A tunnel acts as a relay point between two connections    without changing the messages; tunnels are used when the    communication needs to pass through an intermediary (such as a    firewall) even when the intermediary cannot understand the contents    of the messages.            request chain -------------------------------------->        UA -----v----- A -----v----- B -----v----- C -----v----- O           <------------------------------------- response chain     The figure above shows three intermediaries (A, B, and C) between the    user agent and origin server. A request or response message that    travels the whole chain will pass through four separate connections.    This distinction is important because some HTTP communication options      may apply only to the connection with the nearest, non-tunnel    neighbor, only to the end-points of the chain, or to all connections    along the chain. Although the diagram is linear, each participant may    be engaged in multiple, simultaneous communications. For example, B    may be receiving requests from many clients other than A, and/or    forwarding requests to servers other than C, at the same time that it    is handling A's request.     Any party to the communication which is not acting as a tunnel may    employ an internal cache for handling requests. The effect of a cache    is that the request/response chain is shortened if one of the    participants along the chain has a cached response applicable to that    request. The following illustrates the resulting chain if B has a    cached copy of an earlier response from O (via C) for a request which    has not been cached by UA or A.            request chain ---------->        UA -----v----- A -----v----- B - - - - - - C - - - - - - O           <--------- response chain     Not all responses are usefully cacheable, and some requests may    contain modifiers which place special requirements on cache behavior.    HTTP requirements for cache behavior and cacheable responses are    defined in [section 13](about:blank#section-13).     In fact, there are a wide variety of architectures and configurations    of caches and proxies currently being experimented with or deployed    across the World Wide Web. These systems include national hierarchies    of proxy caches to save transoceanic bandwidth, systems that    broadcast or multicast cache entries, organizations that distribute    subsets of cached data via CD-ROM, and so on. HTTP systems are used    in corporate intranets over high-bandwidth links, and for access via    PDAs with low-power radio links and intermittent connectivity. The    goal of HTTP/1.1 is to support the wide diversity of configurations    already deployed while introducing protocol constructs that meet the    needs of those who build web applications that require high    reliability and, failing that, at least reliable indications of    failure.     HTTP communication usually takes place over TCP/IP connections. The    default port is TCP 80 [[19](about:blank#ref-19)], but other ports can be used. This does    not preclude HTTP from being implemented on top of any other protocol    on the Internet, or on other networks. HTTP only presumes a reliable    transport; any protocol that provides such guarantees can be used;    the mapping of the HTTP/1.1 request and response structures onto the    transport data units of the protocol in question is outside the scope    of this specification.      In HTTP/1.0, most implementations used a new connection for each    request/response exchange. In HTTP/1.1, a connection may be used for    one or more request/response exchanges, although connections may be    closed for a variety of reasons (see [section 8.1](about:blank#section-8.1)).

2 符号约定和通用语法

2.1 增强BNF

All of the mechanisms specified in this document are described in    both prose and an augmented Backus-Naur Form (BNF) similar to that    used by [RFC 822](https://tools.ietf.org/html/rfc822) [[9](about:blank#ref-9)]. Implementors will need to be familiar with the    notation in order to understand this specification. The augmented BNF    includes the following constructs:     name = definition       The name of a rule is simply the name itself (without any       enclosing "<" and ">") and is separated from its definition by the       equal "=" character. White space is only significant in that       indentation of continuation lines is used to indicate a rule       definition that spans more than one line. Certain basic rules are       in uppercase, such as SP, LWS, HT, CRLF, DIGIT, ALPHA, etc. Angle       brackets are used within definitions whenever their presence will       facilitate discerning the use of rule names.     "literal"       Quotation marks surround literal text. Unless stated otherwise,       the text is case-insensitive.     rule1 | rule2       Elements separated by a bar ("|") are alternatives, e.g., "yes |       no" will accept yes or no.     (rule1 rule2)       Elements enclosed in parentheses are treated as a single element.       Thus, "(elem (foo | bar) elem)" allows the token sequences "elem       foo elem" and "elem bar elem".     \*rule       The character "\*" preceding an element indicates repetition. The       full form is "<n>\*<m>element" indicating at least <n> and at most       <m> occurrences of element. Default values are 0 and infinity so       that "\*(element)" allows any number, including zero; "1\*element"       requires at least one; and "1\*2element" allows one or two.     [[rule]()]       Square brackets enclose optional elements; "[foo bar]" is       equivalent to "\*1(foo bar)".      N rule       Specific repetition: "<n>(element)" is equivalent to       "<n>\*<n>(element)"; that is, exactly <n> occurrences of (element).       Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three       alphabetic characters.     #rule       A construct "#" is defined, similar to "\*", for defining lists of       elements. The full form is "<n>#<m>element" indicating at least       <n> and at most <m> elements, each separated by one or more commas       (",") and OPTIONAL linear white space (LWS). This makes the usual       form of lists very easy; a rule such as          ( \*LWS element \*( \*LWS "," \*LWS element ))       can be shown as          1#element       Wherever this construct is used, null elements are allowed, but do       not contribute to the count of elements present. That is,       "(element), , (element) " is permitted, but counts as only two       elements. Therefore, where at least one element is required, at       least one non-null element MUST be present. Default values are 0       and infinity so that "#element" allows any number, including zero;       "1#element" requires at least one; and "1#2element" allows one or       two.     ; comment       A semi-colon, set off some distance to the right of rule text,       starts a comment that continues to the end of line. This is a       simple way of including useful notes in parallel with the       specifications.     implied \*LWS       The grammar described by this specification is word-based. Except       where noted otherwise, linear white space (LWS) can be included       between any two adjacent words (token or quoted-string), and       between adjacent words and separators, without changing the       interpretation of a field. At least one delimiter (LWS and/or        separators) MUST exist between any two tokens (for the definition       of "token" below), since they would otherwise be interpreted as a       single token.

2.2 基本规则

The following rules are used throughout this specification to    describe basic parsing constructs. The US-ASCII coded character set    is defined by ANSI X3.4-1986 [[21](about:blank#ref-21)].          OCTET          = <any 8-bit sequence of data>        CHAR           = <any US-ASCII character (octets 0 - 127)>        UPALPHA        = <any US-ASCII uppercase letter "A".."Z">        LOALPHA        = <any US-ASCII lowercase letter "a".."z">        ALPHA          = UPALPHA | LOALPHA        DIGIT          = <any US-ASCII digit "0".."9">        CTL            = <any US-ASCII control character                         (octets 0 - 31) and DEL (127)>        CR             = <US-ASCII CR, carriage return (13)>        LF             = <US-ASCII LF, linefeed (10)>        SP             = <US-ASCII SP, space (32)>        HT             = <US-ASCII HT, horizontal-tab (9)>        <">            = <US-ASCII double-quote mark (34)>     HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all    protocol elements except the entity-body (see appendix 19.3 for    tolerant applications). The end-of-line marker within an entity-body    is defined by its associated media type, as described in [section 3.7](about:blank#section-3.7).         CRLF           = CR LF     HTTP/1.1 header field values can be folded onto multiple lines if the    continuation line begins with a space or horizontal tab. All linear    white space, including folding, has the same semantics as SP. A    recipient MAY replace any linear white space with a single SP before    interpreting the field value or forwarding the message downstream.         LWS            = [CRLF] 1\*( SP | HT )     The TEXT rule is only used for descriptive field contents and values    that are not intended to be interpreted by the message parser. Words    of \*TEXT MAY contain characters from character sets other than ISO-    8859-1 [[22](about:blank#ref-22)] only when encoded according to the rules of [RFC 2047](https://tools.ietf.org/html/rfc2047)    [[14](about:blank#ref-14)].         TEXT           = <any OCTET except CTLs,                         but including LWS>     A CRLF is allowed in the definition of TEXT only as part of a header    field continuation. It is expected that the folding LWS will be    replaced with a single SP before interpretation of the TEXT value.     Hexadecimal numeric characters are used in several protocol elements.         HEX            = "A" | "B" | "C" | "D" | "E" | "F"                       | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT      Many HTTP/1.1 header field values consist of words separated by LWS    or special characters. These special characters MUST be in a quoted    string to be used within a parameter value (as defined in [section](about:blank#section-3.6) [3.6](about:blank#section-3.6)).         token          = 1\*<any CHAR except CTLs or separators>        separators     = "(" | ")" | "<" | ">" | "@"                       | "," | ";" | ":" | "\" | <">                       | "/" | "[" | "]" | "?" | "="                       | "{" | "}" | SP | HT     Comments can be included in some HTTP header fields by surrounding    the comment text with parentheses. Comments are only allowed in    fields containing "comment" as part of their field value definition.    In all other fields, parentheses are considered part of the field    value.         comment        = "(" \*( ctext | quoted-pair | comment ) ")"        ctext          = <any TEXT excluding "(" and ")">     A string of text is parsed as a single word if it is quoted using    double-quote marks.         quoted-string  = ( <"> \*(qdtext | quoted-pair ) <"> )        qdtext         = <any TEXT except <">>     The backslash character ("\") MAY be used as a single-character    quoting mechanism only within quoted-string and comment constructs.         quoted-pair    = "\" CHAR

3 协议参数

3.1 HTTP版本

HTTP uses a "<major>.<minor>" numbering scheme to indicate versions    of the protocol. The protocol versioning policy is intended to allow    the sender to indicate the format of a message and its capacity for    understanding further HTTP communication, rather than the features    obtained via that communication. No change is made to the version    number for the addition of message components which do not affect    communication behavior or which only add to extensible field values.    The <minor> number is incremented when the changes made to the    protocol add features which do not change the general message parsing    algorithm, but which may add to the message semantics and imply    additional capabilities of the sender. The <major> number is    incremented when the format of a message within the protocol is    changed. See [RFC 2145](https://tools.ietf.org/html/rfc2145) [[36](about:blank#ref-36)] for a fuller explanation.      The version of an HTTP message is indicated by an HTTP-Version field    in the first line of the message.         HTTP-Version   = "HTTP" "/" 1\*DIGIT "." 1\*DIGIT     Note that the major and minor numbers MUST be treated as separate    integers and that each MAY be incremented higher than a single digit.    Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is    lower than HTTP/12.3. Leading zeros MUST be ignored by recipients and    MUST NOT be sent.     An application that sends a request or response message that includes    HTTP-Version of "HTTP/1.1" MUST be at least conditionally compliant    with this specification. Applications that are at least conditionally    compliant with this specification SHOULD use an HTTP-Version of    "HTTP/1.1" in their messages, and MUST do so for any message that is    not compatible with HTTP/1.0. For more details on when to send    specific HTTP-Version values, see [RFC 2145](https://tools.ietf.org/html/rfc2145) [[36](about:blank#ref-36)].     The HTTP version of an application is the highest HTTP version for    which the application is at least conditionally compliant.     Proxy and gateway applications need to be careful when forwarding    messages in protocol versions different from that of the application.    Since the protocol version indicates the protocol capability of the    sender, a proxy/gateway MUST NOT send a message with a version    indicator which is greater than its actual version. If a higher    version request is received, the proxy/gateway MUST either downgrade    the request version, or respond with an error, or switch to tunnel    behavior.     Due to interoperability problems with HTTP/1.0 proxies discovered    since the publication of [RFC 2068](https://tools.ietf.org/html/rfc2068)[[33](about:blank#ref-33)], caching proxies MUST, gateways    MAY, and tunnels MUST NOT upgrade the request to the highest version    they support. The proxy/gateway's response to that request MUST be in    the same major version as the request.        Note: Converting between versions of HTTP may involve modification       of header fields required or forbidden by the versions involved.

3.2 统一资源标识符

URIs have been known by many names: WWW addresses, Universal Document    Identifiers, Universal Resource Identifiers [[3](about:blank#ref-3)], and finally the    combination of Uniform Resource Locators (URL) [[4](about:blank#ref-4)] and Names (URN)    [[20](about:blank#ref-20)]. As far as HTTP is concerned, Uniform Resource Identifiers are    simply formatted strings which identify--via name, location, or any    other characteristic--a resource.

3.2.1 一般语法

URIs in HTTP can be represented in absolute form or relative to some    known base URI [[11](about:blank#ref-11)], depending upon the context of their use. The two    forms are differentiated by the fact that absolute URIs always begin    with a scheme name followed by a colon. For definitive information on    URL syntax and semantics, see "Uniform Resource Identifiers (URI):    Generic Syntax and Semantics," [RFC 2396](https://tools.ietf.org/html/rfc2396) [[42](about:blank#ref-42)] (which replaces RFCs    1738 [[4](about:blank#ref-4)] and [RFC 1808](https://tools.ietf.org/html/rfc1808) [[11](about:blank#ref-11)]). This specification adopts the    definitions of "URI-reference", "absoluteURI", "relativeURI", "port",    "host","abs\_path", "rel\_path", and "authority" from that    specification.     The HTTP protocol does not place any a priori limit on the length of    a URI. Servers MUST be able to handle the URI of any resource they    serve, and SHOULD be able to handle URIs of unbounded length if they    provide GET-based forms that could generate such URIs. A server    SHOULD return 414 (Request-URI Too Long) status if a URI is longer    than the server can handle (see [section 10.4.15](about:blank#section-10.4.15)).        Note: Servers ought to be cautious about depending on URI lengths       above 255 bytes, because some older client or proxy       implementations might not properly support these lengths.

3.2.2 http URL

The "http" scheme is used to locate network resources via the HTTP    protocol. This section defines the scheme-specific syntax and    semantics for http URLs.     http\_URL = "http:" "//" host [ ":" port ] [ abs\_path [ "?" query ]]     If the port is empty or not given, port 80 is assumed. The semantics    are that the identified resource is located at the server listening    for TCP connections on that port of that host, and the Request-URI    for the resource is abs\_path ([section 5.1.2](about:blank#section-5.1.2)). The use of IP addresses    in URLs SHOULD be avoided whenever possible (see [RFC 1900](https://tools.ietf.org/html/rfc1900) [[24](about:blank#ref-24)]). If    the abs\_path is not present in the URL, it MUST be given as "/" when    used as a Request-URI for a resource ([section 5.1.2](about:blank#section-5.1.2)). If a proxy    receives a host name which is not a fully qualified domain name, it    MAY add its domain to the host name it received. If a proxy receives    a fully qualified domain name, the proxy MUST NOT change the host    name.

3.2.3 URI比较

When comparing two URIs to decide if they match or not, a client    SHOULD use a case-sensitive octet-by-octet comparison of the entire    URIs, with these exceptions:        - A port that is empty or not given is equivalent to the default         port for that URI-reference;          - Comparisons of host names MUST be case-insensitive;          - Comparisons of scheme names MUST be case-insensitive;          - An empty abs\_path is equivalent to an abs\_path of "/".     Characters other than those in the "reserved" and "unsafe" sets (see    [RFC 2396](https://tools.ietf.org/html/rfc2396) [[42](about:blank#ref-42)]) are equivalent to their ""%" HEX HEX" encoding.     For example, the following three URIs are equivalent:        [http://abc.com:80/~smith/home.html](http://abc.com/~smith/home.html) [http://ABC.com/%7Esmith/home.html](http://abc.com/%7Esmith/home.html) [http://ABC.com:/%7esmith/home.html](http://abc.com/%7esmith/home.html)

3.3 日期/时间格式

3.3.1 完整日期

HTTP applications have historically allowed three different formats    for the representation of date/time stamps:        Sun, 06 Nov 1994 08:49:37 GMT  ; [RFC 822](https://tools.ietf.org/html/rfc822), updated by [RFC 1123](https://tools.ietf.org/html/rfc1123)       Sunday, 06-Nov-94 08:49:37 GMT ; [RFC 850](https://tools.ietf.org/html/rfc850), obsoleted by [RFC 1036](https://tools.ietf.org/html/rfc1036)       Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format     The first format is preferred as an Internet standard and represents    a fixed-length subset of that defined by [RFC 1123](https://tools.ietf.org/html/rfc1123) [[8](about:blank#ref-8)] (an update to    [RFC 822](https://tools.ietf.org/html/rfc822) [[9](about:blank#ref-9)]). The second format is in common use, but is based on the    obsolete [RFC 850](https://tools.ietf.org/html/rfc850) [[12](about:blank#ref-12)] date format and lacks a four-digit year.    HTTP/1.1 clients and servers that parse the date value MUST accept    all three formats (for compatibility with HTTP/1.0), though they MUST    only generate the [RFC 1123](https://tools.ietf.org/html/rfc1123) format for representing HTTP-date values    in header fields. See [section 19.3](about:blank#section-19.3) for further information.        Note: Recipients of date values are encouraged to be robust in       accepting date values that may have been sent by non-HTTP       applications, as is sometimes the case when retrieving or posting       messages via proxies/gateways to SMTP or NNTP.      All HTTP date/time stamps MUST be represented in Greenwich Mean Time    (GMT), without exception. For the purposes of HTTP, GMT is exactly    equal to UTC (Coordinated Universal Time). This is indicated in the    first two formats by the inclusion of "GMT" as the three-letter    abbreviation for time zone, and MUST be assumed when reading the    asctime format. HTTP-date is case sensitive and MUST NOT include    additional LWS beyond that specifically included as SP in the    grammar.         HTTP-date    = [rfc1123](https://tools.ietf.org/html/rfc1123)-date | [rfc850](https://tools.ietf.org/html/rfc850)-date | asctime-date        [rfc1123](https://tools.ietf.org/html/rfc1123)-date = wkday "," SP date1 SP time SP "GMT"        [rfc850](https://tools.ietf.org/html/rfc850)-date  = weekday "," SP date2 SP time SP "GMT"        asctime-date = wkday SP date3 SP time SP 4DIGIT        date1        = 2DIGIT SP month SP 4DIGIT                       ; day month year (e.g., 02 Jun 1982)        date2        = 2DIGIT "-" month "-" 2DIGIT                       ; day-month-year (e.g., 02-Jun-82)        date3        = month SP ( 2DIGIT | ( SP 1DIGIT ))                       ; month day (e.g., Jun  2)        time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT                       ; 00:00:00 - 23:59:59        wkday        = "Mon" | "Tue" | "Wed"                     | "Thu" | "Fri" | "Sat" | "Sun"        weekday      = "Monday" | "Tuesday" | "Wednesday"                     | "Thursday" | "Friday" | "Saturday" | "Sunday"        month        = "Jan" | "Feb" | "Mar" | "Apr"                     | "May" | "Jun" | "Jul" | "Aug"                     | "Sep" | "Oct" | "Nov" | "Dec"        Note: HTTP requirements for the date/time stamp format apply only       to their usage within the protocol stream. Clients and servers are       not required to use these formats for user presentation, request       logging, etc.

3.3.2 Delta秒

Some HTTP header fields allow a time value to be specified as an    integer number of seconds, represented in decimal, after the time    that the message was received.         delta-seconds  = 1\*DIGIT

3.4 字符集

HTTP uses the same definition of the term "character set" as that    described for MIME:      The term "character set" is used in this document to refer to a    method used with one or more tables to convert a sequence of octets    into a sequence of characters. Note that unconditional conversion in    the other direction is not required, in that not all characters may    be available in a given character set and a character set may provide    more than one sequence of octets to represent a particular character.    This definition is intended to allow various kinds of character    encoding, from simple single-table mappings such as US-ASCII to    complex table switching methods such as those that use ISO-2022's    techniques. However, the definition associated with a MIME character    set name MUST fully specify the mapping to be performed from octets    to characters. In particular, use of external profiling information    to determine the exact mapping is not permitted.        Note: This use of the term "character set" is more commonly       referred to as a "character encoding." However, since HTTP and       MIME share the same registry, it is important that the terminology       also be shared.     HTTP character sets are identified by case-insensitive tokens. The    complete set of tokens is defined by the IANA Character Set registry    [[19](about:blank#ref-19)].         charset = token     Although HTTP allows an arbitrary token to be used as a charset    value, any token that has a predefined value within the IANA    Character Set registry [[19](about:blank#ref-19)] MUST represent the character set defined    by that registry. Applications SHOULD limit their use of character    sets to those defined by the IANA registry.     Implementors should be aware of IETF character set requirements [[38](about:blank#ref-38)]    [[41](about:blank#ref-41)].

3.4.1 缺少字符集

Some HTTP/1.0 software has interpreted a Content-Type header without    charset parameter incorrectly to mean "recipient should guess."    Senders wishing to defeat this behavior MAY include a charset    parameter even when the charset is ISO-8859-1 and SHOULD do so when    it is known that it will not confuse the recipient.     Unfortunately, some older HTTP/1.0 clients did not deal properly with    an explicit charset parameter. HTTP/1.1 recipients MUST respect the    charset label provided by the sender; and those user agents that have    a provision to "guess" a charset MUST use the charset from the      content-type field if they support that charset, rather than the    recipient's preference, when initially displaying a document. See    [section 3.7.1](about:blank#section-3.7.1).

3.5 内容编码

Content coding values indicate an encoding transformation that has    been or can be applied to an entity. Content codings are primarily    used to allow a document to be compressed or otherwise usefully    transformed without losing the identity of its underlying media type    and without loss of information. Frequently, the entity is stored in    coded form, transmitted directly, and only decoded by the recipient.         content-coding   = token     All content-coding values are case-insensitive. HTTP/1.1 uses    content-coding values in the Accept-Encoding ([section 14.3](about:blank#section-14.3)) and    Content-Encoding ([section 14.11](about:blank#section-14.11)) header fields. Although the value    describes the content-coding, what is more important is that it    indicates what decoding mechanism will be required to remove the    encoding.     The Internet Assigned Numbers Authority (IANA) acts as a registry for    content-coding value tokens. Initially, the registry contains the    following tokens:     gzip An encoding format produced by the file compression program         "gzip" (GNU zip) as described in [RFC 1952](https://tools.ietf.org/html/rfc1952) [[25](about:blank#ref-25)]. This format is a         Lempel-Ziv coding (LZ77) with a 32 bit CRC.     compress         The encoding format produced by the common UNIX file compression         program "compress". This format is an adaptive Lempel-Ziv-Welch         coding (LZW).          Use of program names for the identification of encoding formats         is not desirable and is discouraged for future encodings. Their         use here is representative of historical practice, not good         design. For compatibility with previous implementations of HTTP,         applications SHOULD consider "x-gzip" and "x-compress" to be         equivalent to "gzip" and "compress" respectively.     deflate         The "zlib" format defined in [RFC 1950](https://tools.ietf.org/html/rfc1950) [[31](about:blank#ref-31)] in combination with         the "deflate" compression mechanism described in [RFC 1951](https://tools.ietf.org/html/rfc1951) [[29](about:blank#ref-29)].      identity         The default (identity) encoding; the use of no transformation         whatsoever. This content-coding is used only in the Accept-         Encoding header, and SHOULD NOT be used in the Content-Encoding         header.     New content-coding value tokens SHOULD be registered; to allow    interoperability between clients and servers, specifications of the    content coding algorithms needed to implement a new value SHOULD be    publicly available and adequate for independent implementation, and    conform to the purpose of content coding defined in this section.

3.6 转移编码

Transfer-coding values are used to indicate an encoding    transformation that has been, can be, or may need to be applied to an    entity-body in order to ensure "safe transport" through the network.    This differs from a content coding in that the transfer-coding is a    property of the message, not of the original entity.         transfer-coding         = "chunked" | transfer-extension        transfer-extension      = token \*( ";" parameter )     Parameters are in  the form of attribute/value pairs.         parameter               = attribute "=" value        attribute               = token        value                   = token | quoted-string     All transfer-coding values are case-insensitive. HTTP/1.1 uses    transfer-coding values in the TE header field ([section 14.39](about:blank#section-14.39)) and in    the Transfer-Encoding header field ([section 14.41](about:blank#section-14.41)).     Whenever a transfer-coding is applied to a message-body, the set of    transfer-codings MUST include "chunked", unless the message is    terminated by closing the connection. When the "chunked" transfer-    coding is used, it MUST be the last transfer-coding applied to the    message-body. The "chunked" transfer-coding MUST NOT be applied more    than once to a message-body. These rules allow the recipient to    determine the transfer-length of the message ([section 4.4](about:blank#section-4.4)).     Transfer-codings are analogous to the Content-Transfer-Encoding    values of MIME [[7](about:blank#ref-7)], which were designed to enable safe transport of    binary data over a 7-bit transport service. However, safe transport    has a different focus for an 8bit-clean transfer protocol. In HTTP,    the only unsafe characteristic of message-bodies is the difficulty in    determining the exact body length ([section 7.2.2](about:blank#section-7.2.2)), or the desire to    encrypt data over a shared transport.      The Internet Assigned Numbers Authority (IANA) acts as a registry for    transfer-coding value tokens. Initially, the registry contains the    following tokens: "chunked" ([section 3.6.1](about:blank#section-3.6.1)), "identity" ([section](about:blank#section-3.6.2) [3.6.2](about:blank#section-3.6.2)), "gzip" ([section 3.5](about:blank#section-3.5)), "compress" ([section 3.5](about:blank#section-3.5)), and "deflate"    ([section 3.5](about:blank#section-3.5)).     New transfer-coding value tokens SHOULD be registered in the same way    as new content-coding value tokens ([section 3.5](about:blank#section-3.5)).     A server which receives an entity-body with a transfer-coding it does    not understand SHOULD return 501 (Unimplemented), and close the    connection. A server MUST NOT send transfer-codings to an HTTP/1.0    client.

3.6.1 分块传输编码

The chunked encoding modifies the body of a message in order to    transfer it as a series of chunks, each with its own size indicator,    followed by an OPTIONAL trailer containing entity-header fields. This    allows dynamically produced content to be transferred along with the    information necessary for the recipient to verify that it has    received the full message.         Chunked-Body   = \*chunk                         last-chunk                         trailer                         CRLF         chunk          = chunk-size [ chunk-extension ] CRLF                         chunk-data CRLF        chunk-size     = 1\*HEX        last-chunk     = 1\*("0") [ chunk-extension ] CRLF         chunk-extension= \*( ";" chunk-ext-name [ "=" chunk-ext-val ] )        chunk-ext-name = token        chunk-ext-val  = token | quoted-string        chunk-data     = chunk-size(OCTET)        trailer        = \*(entity-header CRLF)     The chunk-size field is a string of hex digits indicating the size of    the chunk. The chunked encoding is ended by any chunk whose size is    zero, followed by the trailer, which is terminated by an empty line.     The trailer allows the sender to include additional HTTP header    fields at the end of the message. The Trailer header field can be    used to indicate which header fields are included in a trailer (see    [section 14.40](about:blank#section-14.40)).      A server using chunked transfer-coding in a response MUST NOT use the    trailer for any header fields unless at least one of the following is    true:     a)the request included a TE header field that indicates "trailers" is      acceptable in the transfer-coding of the  response, as described in      [section 14.39](about:blank#section-14.39); or,     b)the server is the origin server for the response, the trailer      fields consist entirely of optional metadata, and the recipient      could use the message (in a manner acceptable to the origin server)      without receiving this metadata.  In other words, the origin server      is willing to accept the possibility that the trailer fields might      be silently discarded along the path to the client.     This requirement prevents an interoperability failure when the    message is being received by an HTTP/1.1 (or later) proxy and    forwarded to an HTTP/1.0 recipient. It avoids a situation where    compliance with the protocol would have necessitated a possibly    infinite buffer on the proxy.     An example process for decoding a Chunked-Body is presented in    appendix 19.4.6.     All HTTP/1.1 applications MUST be able to receive and decode the    "chunked" transfer-coding, and MUST ignore chunk-extension extensions    they do not understand.

3.7 媒体类型

HTTP uses Internet Media Types [[17](about:blank#ref-17)] in the Content-Type ([section](about:blank#section-14.17) [14.17](about:blank#section-14.17)) and Accept ([section 14.1](about:blank#section-14.1)) header fields in order to provide    open and extensible data typing and type negotiation.         media-type     = type "/" subtype \*( ";" parameter )        type           = token        subtype        = token     Parameters MAY follow the type/subtype in the form of attribute/value    pairs (as defined in [section 3.6](about:blank#section-3.6)).     The type, subtype, and parameter attribute names are case-    insensitive. Parameter values might or might not be case-sensitive,    depending on the semantics of the parameter name. Linear white space    (LWS) MUST NOT be used between the type and subtype, nor between an    attribute and its value. The presence or absence of a parameter might    be significant to the processing of a media-type, depending on its    definition within the media type registry.      Note that some older HTTP applications do not recognize media type    parameters. When sending data to older HTTP applications,    implementations SHOULD only use media type parameters when they are    required by that type/subtype definition.     Media-type values are registered with the Internet Assigned Number    Authority (IANA [[19](about:blank#ref-19)]). The media type registration process is    outlined in [RFC 1590](https://tools.ietf.org/html/rfc1590) [[17](about:blank#ref-17)]. Use of non-registered media types is    discouraged.

3.7.1 标准化和文本默认值

Internet media types are registered with a canonical form. An    entity-body transferred via HTTP messages MUST be represented in the    appropriate canonical form prior to its transmission except for    "text" types, as defined in the next paragraph.     When in canonical form, media subtypes of the "text" type use CRLF as    the text line break. HTTP relaxes this requirement and allows the    transport of text media with plain CR or LF alone representing a line    break when it is done consistently for an entire entity-body. HTTP    applications MUST accept CRLF, bare CR, and bare LF as being    representative of a line break in text media received via HTTP. In    addition, if the text is represented in a character set that does not    use octets 13 and 10 for CR and LF respectively, as is the case for    some multi-byte character sets, HTTP allows the use of whatever octet    sequences are defined by that character set to represent the    equivalent of CR and LF for line breaks. This flexibility regarding    line breaks applies only to text media in the entity-body; a bare CR    or LF MUST NOT be substituted for CRLF within any of the HTTP control    structures (such as header fields and multipart boundaries).     If an entity-body is encoded with a content-coding, the underlying    data MUST be in a form defined above prior to being encoded.     The "charset" parameter is used with some media types to define the    character set ([section 3.4](about:blank#section-3.4)) of the data. When no explicit charset    parameter is provided by the sender, media subtypes of the "text"    type are defined to have a default charset value of "ISO-8859-1" when    received via HTTP. Data in character sets other than "ISO-8859-1" or    its subsets MUST be labeled with an appropriate charset value. See    [section 3.4.1](about:blank#section-3.4.1) for compatibility problems.

3.7.2 多部分类型

MIME provides for a number of "multipart" types -- encapsulations of    one or more entities within a single message-body. All multipart    types share a common syntax, as defined in [section 5.1.1 of RFC 2046](https://tools.ietf.org/html/rfc2046#section-5.1.1)      [[40](about:blank#ref-40)], and MUST include a boundary parameter as part of the media type    value. The message body is itself a protocol element and MUST    therefore use only CRLF to represent line breaks between body-parts.    Unlike in [RFC 2046](https://tools.ietf.org/html/rfc2046), the epilogue of any multipart message MUST be    empty; HTTP applications MUST NOT transmit the epilogue (even if the    original multipart contains an epilogue). These restrictions exist in    order to preserve the self-delimiting nature of a multipart message-    body, wherein the "end" of the message-body is indicated by the    ending multipart boundary.     In general, HTTP treats a multipart message-body no differently than    any other media type: strictly as payload. The one exception is the    "multipart/byteranges" type (appendix 19.2) when it appears in a 206    (Partial Content) response, which will be interpreted by some HTTP    caching mechanisms as described in sections [13.5.4](about:blank#section-13.5.4) and [14.16](about:blank#section-14.16). In all    other cases, an HTTP user agent SHOULD follow the same or similar    behavior as a MIME user agent would upon receipt of a multipart type.    The MIME header fields within each body-part of a multipart message-    body do not have any significance to HTTP beyond that defined by    their MIME semantics.     In general, an HTTP user agent SHOULD follow the same or similar    behavior as a MIME user agent would upon receipt of a multipart type.    If an application receives an unrecognized multipart subtype, the    application MUST treat it as being equivalent to "multipart/mixed".        Note: The "multipart/form-data" type has been specifically defined       for carrying form data suitable for processing via the POST       request method, as described in [RFC 1867](https://tools.ietf.org/html/rfc1867) [[15](about:blank#ref-15)].

3.8 产品代币

Product tokens are used to allow communicating applications to    identify themselves by software name and version. Most fields using    product tokens also allow sub-products which form a significant part    of the application to be listed, separated by white space. By    convention, the products are listed in order of their significance    for identifying the application.         product         = token ["/" product-version]        product-version = token     Examples:         User-Agent: CERN-LineMode/2.15 libwww/2.17b3        Server: Apache/0.8.4      Product tokens SHOULD be short and to the point. They MUST NOT be    used for advertising or other non-essential information. Although any    token character MAY appear in a product-version, this token SHOULD    only be used for a version identifier (i.e., successive versions of    the same product SHOULD only differ in the product-version portion of    the product value).

3.9 质量值

HTTP content negotiation ([section 12](about:blank#section-12)) uses short "floating point"    numbers to indicate the relative importance ("weight") of various    negotiable parameters.  A weight is normalized to a real number in    the range 0 through 1, where 0 is the minimum and 1 the maximum    value. If a parameter has a quality value of 0, then content with    this parameter is `not acceptable' for the client. HTTP/1.1    applications MUST NOT generate more than three digits after the    decimal point. User configuration of these values SHOULD also be    limited in this fashion.         qvalue         = ( "0" [ "." 0\*3DIGIT ] )                       | ( "1" [ "." 0\*3("0") ] )     "Quality values" is a misnomer, since these values merely represent    relative degradation in desired quality.

3.10 语言标签

A language tag identifies a natural language spoken, written, or    otherwise conveyed by human beings for communication of information    to other human beings. Computer languages are explicitly excluded.    HTTP uses language tags within the Accept-Language and Content-    Language fields.     The syntax and registry of HTTP language tags is the same as that    defined by [RFC 1766](https://tools.ietf.org/html/rfc1766) [[1](about:blank#ref-1)]. In summary, a language tag is composed of 1    or more parts: A primary language tag and a possibly empty series of    subtags:          language-tag  = primary-tag \*( "-" subtag )         primary-tag   = 1\*8ALPHA         subtag        = 1\*8ALPHA     White space is not allowed within the tag and all tags are case-    insensitive. The name space of language tags is administered by the    IANA. Example tags include:         en, en-US, en-cockney, i-cherokee, x-pig-latin      where any two-letter primary-tag is an ISO-639 language abbreviation    and any two-letter initial subtag is an ISO-3166 country code. (The    last three tags above are not registered tags; all but the last are    examples of tags which could be registered in future.)

3.11 实体标签

Entity tags are used for comparing two or more entities from the same    requested resource. HTTP/1.1 uses entity tags in the ETag ([section](about:blank#section-14.19) [14.19](about:blank#section-14.19)), If-Match ([section 14.24](about:blank#section-14.24)), If-None-Match ([section 14.26](about:blank#section-14.26)), and    If-Range ([section 14.27](about:blank#section-14.27)) header fields. The definition of how they    are used and compared as cache validators is in [section 13.3.3](about:blank#section-13.3.3). An    entity tag consists of an opaque quoted string, possibly prefixed by    a weakness indicator.        entity-tag = [ weak ] opaque-tag       weak       = "W/"       opaque-tag = quoted-string     A "strong entity tag" MAY be shared by two entities of a resource    only if they are equivalent by octet equality.     A "weak entity tag," indicated by the "W/" prefix, MAY be shared by    two entities of a resource only if the entities are equivalent and    could be substituted for each other with no significant change in    semantics. A weak entity tag can only be used for weak comparison.     An entity tag MUST be unique across all versions of all entities    associated with a particular resource. A given entity tag value MAY    be used for entities obtained by requests on different URIs. The use    of the same entity tag value in conjunction with entities obtained by    requests on different URIs does not imply the equivalence of those    entities.

3.12 范围单位

HTTP/1.1 allows a client to request that only part (a range of) the    response entity be included within the response. HTTP/1.1 uses range    units in the Range ([section 14.35](about:blank#section-14.35)) and Content-Range ([section 14.16](about:blank#section-14.16))    header fields. An entity can be broken down into subranges according    to various structural units.        range-unit       = bytes-unit | other-range-unit       bytes-unit       = "bytes"       other-range-unit = token     The only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1    implementations MAY ignore ranges specified using other units.      HTTP/1.1 has been designed to allow implementations of applications    that do not depend on knowledge of ranges.

4 HTTP消息

4.1 消息类型

HTTP messages consist of requests from client to server and responses    from server to client.         HTTP-message   = Request | Response     ; HTTP/1.1 messages     Request ([section 5](about:blank#section-5)) and Response ([section 6](about:blank#section-6)) messages use the generic    message format of [RFC 822](https://tools.ietf.org/html/rfc822) [[9](about:blank#ref-9)] for transferring entities (the payload    of the message). Both types of message consist of a start-line, zero    or more header fields (also known as "headers"), an empty line (i.e.,    a line with nothing preceding the CRLF) indicating the end of the    header fields, and possibly a message-body.          generic-message = start-line                           \*(message-header CRLF)                           CRLF                           [ message-body ]         start-line      = Request-Line | Status-Line     In the interest of robustness, servers SHOULD ignore any empty    line(s) received where a Request-Line is expected. In other words, if    the server is reading the protocol stream at the beginning of a    message and receives a CRLF first, it should ignore the CRLF.     Certain buggy HTTP/1.0 client implementations generate extra CRLF's    after a POST request. To restate what is explicitly forbidden by the    BNF, an HTTP/1.1 client MUST NOT preface or follow a request with an    extra CRLF.

4.2 消息头

HTTP header fields, which include general-header ([section 4.5](about:blank#section-4.5)),    request-header ([section 5.3](about:blank#section-5.3)), response-header ([section 6.2](about:blank#section-6.2)), and    entity-header ([section 7.1](about:blank#section-7.1)) fields, follow the same generic format as    that given in [Section 3.1 of RFC 822](https://tools.ietf.org/html/rfc822#section-3.1) [[9](about:blank#ref-9)]. Each header field consists    of a name followed by a colon (":") and the field value. Field names    are case-insensitive. The field value MAY be preceded by any amount    of LWS, though a single SP is preferred. Header fields can be    extended over multiple lines by preceding each extra line with at    least one SP or HT. Applications ought to follow "common form", where    one is known or indicated, when generating HTTP constructs, since    there might exist some implementations that fail to accept anything      beyond the common forms.         message-header = field-name ":" [ field-value ]        field-name     = token        field-value    = \*( field-content | LWS )        field-content  = <the OCTETs making up the field-value                         and consisting of either \*TEXT or combinations                         of token, separators, and quoted-string>     The field-content does not include any leading or trailing LWS:    linear white space occurring before the first non-whitespace    character of the field-value or after the last non-whitespace    character of the field-value. Such leading or trailing LWS MAY be    removed without changing the semantics of the field value. Any LWS    that occurs between field-content MAY be replaced with a single SP    before interpreting the field value or forwarding the message    downstream.     The order in which header fields with differing field names are    received is not significant. However, it is "good practice" to send    general-header fields first, followed by request-header or response-    header fields, and ending with the entity-header fields.     Multiple message-header fields with the same field-name MAY be    present in a message if and only if the entire field-value for that    header field is defined as a comma-separated list [i.e., #(values)].    It MUST be possible to combine the multiple header fields into one    "field-name: field-value" pair, without changing the semantics of the    message, by appending each subsequent field-value to the first, each    separated by a comma. The order in which header fields with the same    field-name are received is therefore significant to the    interpretation of the combined field value, and thus a proxy MUST NOT    change the order of these field values when a message is forwarded.

4.3 消息正文

The message-body (if any) of an HTTP message is used to carry the    entity-body associated with the request or response. The message-body    differs from the entity-body only when a transfer-coding has been    applied, as indicated by the Transfer-Encoding header field ([section](about:blank#section-14.41) [14.41](about:blank#section-14.41)).         message-body = entity-body                     | <entity-body encoded as per Transfer-Encoding>     Transfer-Encoding MUST be used to indicate any transfer-codings    applied by an application to ensure safe and proper transfer of the    message. Transfer-Encoding is a property of the message, not of the      entity, and thus MAY be added or removed by any application along the    request/response chain. (However, [section 3.6](about:blank#section-3.6) places restrictions on    when certain transfer-codings may be used.)     The rules for when a message-body is allowed in a message differ for    requests and responses.     The presence of a message-body in a request is signaled by the    inclusion of a Content-Length or Transfer-Encoding header field in    the request's message-headers. A message-body MUST NOT be included in    a request if the specification of the request method ([section 5.1.1](about:blank#section-5.1.1))    does not allow sending an entity-body in requests. A server SHOULD    read and forward a message-body on any request; if the request method    does not include defined semantics for an entity-body, then the    message-body SHOULD be ignored when handling the request.     For response messages, whether or not a message-body is included with    a message is dependent on both the request method and the response    status code ([section 6.1.1](about:blank#section-6.1.1)). All responses to the HEAD request method    MUST NOT include a message-body, even though the presence of entity-    header fields might lead one to believe they do. All 1xx    (informational), 204 (no content), and 304 (not modified) responses    MUST NOT include a message-body. All other responses do include a    message-body, although it MAY be of zero length.

4.4 消息长度

The transfer-length of a message is the length of the message-body as    it appears in the message; that is, after any transfer-codings have    been applied. When a message-body is included with a message, the    transfer-length of that body is determined by one of the following    (in order of precedence):     1.Any response message which "MUST NOT" include a message-body (such      as the 1xx, 204, and 304 responses and any response to a HEAD      request) is always terminated by the first empty line after the      header fields, regardless of the entity-header fields present in      the message.     2.If a Transfer-Encoding header field ([section 14.41](about:blank#section-14.41)) is present and      has any value other than "identity", then the transfer-length is      defined by use of the "chunked" transfer-coding ([section 3.6](about:blank#section-3.6)),      unless the message is terminated by closing the connection.     3.If a Content-Length header field ([section 14.13](about:blank#section-14.13)) is present, its      decimal value in OCTETs represents both the entity-length and the      transfer-length. The Content-Length header field MUST NOT be sent      if these two lengths are different (i.e., if a Transfer-Encoding        header field is present). If a message is received with both a      Transfer-Encoding header field and a Content-Length header field,      the latter MUST be ignored.     4.If the message uses the media type "multipart/byteranges", and the      ransfer-length is not otherwise specified, then this self-      elimiting media type defines the transfer-length. This media type      UST NOT be used unless the sender knows that the recipient can arse      it; the presence in a request of a Range header with ultiple byte-      range specifiers from a 1.1 client implies that the lient can parse      multipart/byteranges responses.         A range header might be forwarded by a 1.0 proxy that does not        understand multipart/byteranges; in this case the server MUST        delimit the message using methods defined in items 1,3 or 5 of        this section.     5.By the server closing the connection. (Closing the connection      cannot be used to indicate the end of a request body, since that      would leave no possibility for the server to send back a response.)     For compatibility with HTTP/1.0 applications, HTTP/1.1 requests    containing a message-body MUST include a valid Content-Length header    field unless the server is known to be HTTP/1.1 compliant. If a    request contains a message-body and a Content-Length is not given,    the server SHOULD respond with 400 (bad request) if it cannot    determine the length of the message, or with 411 (length required) if    it wishes to insist on receiving a valid Content-Length.     All HTTP/1.1 applications that receive entities MUST accept the    "chunked" transfer-coding ([section 3.6](about:blank#section-3.6)), thus allowing this mechanism    to be used for messages when the message length cannot be determined    in advance.     Messages MUST NOT include both a Content-Length header field and a    non-identity transfer-coding. If the message does include a non-    identity transfer-coding, the Content-Length MUST be ignored.     When a Content-Length is given in a message where a message-body is    allowed, its field value MUST exactly match the number of OCTETs in    the message-body. HTTP/1.1 user agents MUST notify the user when an    invalid length is received and detected.

4.5 一般标题字段

There are a few header fields which have general applicability for    both request and response messages, but which do not apply to the    entity being transferred. These header fields apply only to the      message being transmitted.         general-header = Cache-Control            ; [Section 14.9](about:blank#section-14.9)                       | Connection               ; [Section 14.10](about:blank#section-14.10)                       | Date                     ; [Section 14.18](about:blank#section-14.18)                       | Pragma                   ; [Section 14.32](about:blank#section-14.32)                       | Trailer                  ; [Section 14.40](about:blank#section-14.40)                       | Transfer-Encoding        ; [Section 14.41](about:blank#section-14.41)                       | Upgrade                  ; [Section 14.42](about:blank#section-14.42)                       | Via                      ; [Section 14.45](about:blank#section-14.45)                       | Warning                  ; [Section 14.46](about:blank#section-14.46)     General-header field names can be extended reliably only in    combination with a change in the protocol version. However, new or    experimental header fields may be given the semantics of general    header fields if all parties in the communication recognize them to    be general-header fields. Unrecognized header fields are treated as    entity-header fields.

5 请求

A request message from a client to a server includes, within the    first line of that message, the method to be applied to the resource,    the identifier of the resource, and the protocol version in use.          Request       = Request-Line              ; [Section 5.1](about:blank#section-5.1)                         \*(( general-header        ; [Section 4.5](about:blank#section-4.5)                          | request-header         ; [Section 5.3](about:blank#section-5.3)                          | entity-header ) CRLF)  ; [Section 7.1](about:blank#section-7.1)                         CRLF                         [ message-body ]          ; [Section 4.3](about:blank#section-4.3)

5.1 请求线

The Request-Line begins with a method token, followed by the    Request-URI and the protocol version, and ending with CRLF. The    elements are separated by SP characters. No CR or LF is allowed    except in the final CRLF sequence.          Request-Line   = Method SP Request-URI SP HTTP-Version CRLF

5.1.1 方法

The Method  token indicates the method to be performed on the    resource identified by the Request-URI. The method is case-sensitive.         Method         = "OPTIONS"                ; [Section 9.2](about:blank#section-9.2)                       | "GET"                    ; [Section 9.3](about:blank#section-9.3)                       | "HEAD"                   ; [Section 9.4](about:blank#section-9.4)                       | "POST"                   ; [Section 9.5](about:blank#section-9.5)                       | "PUT"                    ; [Section 9.6](about:blank#section-9.6)                       | "DELETE"                 ; [Section 9.7](about:blank#section-9.7)                       | "TRACE"                  ; [Section 9.8](about:blank#section-9.8)                       | "CONNECT"                ; [Section 9.9](about:blank#section-9.9)                       | extension-method        extension-method = token     The list of methods allowed by a resource can be specified in an    Allow header field ([section 14.7](about:blank#section-14.7)). The return code of the response    always notifies the client whether a method is currently allowed on a    resource, since the set of allowed methods can change dynamically. An    origin server SHOULD return the status code 405 (Method Not Allowed)    if the method is known by the origin server but not allowed for the    requested resource, and 501 (Not Implemented) if the method is    unrecognized or not implemented by the origin server. The methods GET    and HEAD MUST be supported by all general-purpose servers. All other    methods are OPTIONAL; however, if the above methods are implemented,    they MUST be implemented with the same semantics as those specified    in [section 9](about:blank#section-9).

5.1.2 请求URI

The Request-URI is a Uniform Resource Identifier ([section 3.2](about:blank#section-3.2)) and    identifies the resource upon which to apply the request.         Request-URI    = "\*" | absoluteURI | abs\_path | authority     The four options for Request-URI are dependent on the nature of the    request. The asterisk "\*" means that the request does not apply to a    particular resource, but to the server itself, and is only allowed    when the method used does not necessarily apply to a resource. One    example would be         OPTIONS \* HTTP/1.1     The absoluteURI form is REQUIRED when the request is being made to a    proxy. The proxy is requested to forward the request or service it    from a valid cache, and return the response. Note that the proxy MAY    forward the request on to another proxy or directly to the server      specified by the absoluteURI. In order to avoid request loops, a    proxy MUST be able to recognize all of its server names, including    any aliases, local variations, and the numeric IP address. An example    Request-Line would be:         GET [http://www.w3.org/pub/WWW/TheProject.html](http://www.w3.org/pub/WWW/TheProject.html) HTTP/1.1     To allow for transition to absoluteURIs in all requests in future    versions of HTTP, all HTTP/1.1 servers MUST accept the absoluteURI    form in requests, even though HTTP/1.1 clients will only generate    them in requests to proxies.     The authority form is only used by the CONNECT method ([section 9.9](about:blank#section-9.9)).     The most common form of Request-URI is that used to identify a    resource on an origin server or gateway. In this case the absolute    path of the URI MUST be transmitted (see [section 3.2.1](about:blank#section-3.2.1), abs\_path) as    the Request-URI, and the network location of the URI (authority) MUST    be transmitted in a Host header field. For example, a client wishing    to retrieve the resource above directly from the origin server would    create a TCP connection to port 80 of the host "www.w3.org" and send    the lines:         GET /pub/WWW/TheProject.html HTTP/1.1        Host: www.w3.org     followed by the remainder of the Request. Note that the absolute path    cannot be empty; if none is present in the original URI, it MUST be    given as "/" (the server root).     The Request-URI is transmitted in the format specified in [section](about:blank#section-3.2.1) [3.2.1](about:blank#section-3.2.1). If the Request-URI is encoded using the "% HEX HEX" encoding    [[42](about:blank#ref-42)], the origin server MUST decode the Request-URI in order to    properly interpret the request. Servers SHOULD respond to invalid    Request-URIs with an appropriate status code.     A transparent proxy MUST NOT rewrite the "abs\_path" part of the    received Request-URI when forwarding it to the next inbound server,    except as noted above to replace a null abs\_path with "/".        Note: The "no rewrite" rule prevents the proxy from changing the       meaning of the request when the origin server is improperly using       a non-reserved URI character for a reserved purpose.  Implementors       should be aware that some pre-HTTP/1.1 proxies have been known to       rewrite the Request-URI.

5.2 由请求标识的资源

The exact resource identified by an Internet request is determined by    examining both the Request-URI and the Host header field.     An origin server that does not allow resources to differ by the    requested host MAY ignore the Host header field value when    determining the resource identified by an HTTP/1.1 request. (But see    [section 19.6.1.1](about:blank#section-19.6.1.1) for other requirements on Host support in HTTP/1.1.)     An origin server that does differentiate resources based on the host    requested (sometimes referred to as virtual hosts or vanity host    names) MUST use the following rules for determining the requested    resource on an HTTP/1.1 request:     1. If Request-URI is an absoluteURI, the host is part of the      Request-URI. Any Host header field value in the request MUST be      ignored.     2. If the Request-URI is not an absoluteURI, and the request includes      a Host header field, the host is determined by the Host header      field value.     3. If the host as determined by rule 1 or 2 is not a valid host on      the server, the response MUST be a 400 (Bad Request) error message.     Recipients of an HTTP/1.0 request that lacks a Host header field MAY    attempt to use heuristics (e.g., examination of the URI path for    something unique to a particular host) in order to determine what    exact resource is being requested.

5.3 请求标题字段

The request-header fields allow the client to pass additional    information about the request, and about the client itself, to the    server. These fields act as request modifiers, with semantics    equivalent to the parameters on a programming language method    invocation.         request-header = Accept                   ; [Section 14.1](about:blank#section-14.1)                       | Accept-Charset           ; [Section 14.2](about:blank#section-14.2)                       | Accept-Encoding          ; [Section 14.3](about:blank#section-14.3)                       | Accept-Language          ; [Section 14.4](about:blank#section-14.4)                       | Authorization            ; [Section 14.8](about:blank#section-14.8)                       | Expect                   ; [Section 14.20](about:blank#section-14.20)                       | From                     ; [Section 14.22](about:blank#section-14.22)                       | Host                     ; [Section 14.23](about:blank#section-14.23)                       | If-Match                 ; [Section 14.24](about:blank#section-14.24)                         | If-Modified-Since        ; [Section 14.25](about:blank#section-14.25)                       | If-None-Match            ; [Section 14.26](about:blank#section-14.26)                       | If-Range                 ; [Section 14.27](about:blank#section-14.27)                       | If-Unmodified-Since      ; [Section 14.28](about:blank#section-14.28)                       | Max-Forwards             ; [Section 14.31](about:blank#section-14.31)                       | Proxy-Authorization      ; [Section 14.34](about:blank#section-14.34)                       | Range                    ; [Section 14.35](about:blank#section-14.35)                       | Referer                  ; [Section 14.36](about:blank#section-14.36)                       | TE                       ; [Section 14.39](about:blank#section-14.39)                       | User-Agent               ; [Section 14.43](about:blank#section-14.43)     Request-header field names can be extended reliably only in    combination with a change in the protocol version. However, new or    experimental header fields MAY be given the semantics of request-    header fields if all parties in the communication recognize them to    be request-header fields. Unrecognized header fields are treated as    entity-header fields.

6 消息响应

After receiving and interpreting a request message, a server responds    with an HTTP response message.         Response      = Status-Line               ; [Section 6.1](about:blank#section-6.1)                        \*(( general-header        ; [Section 4.5](about:blank#section-4.5)                         | response-header        ; [Section 6.2](about:blank#section-6.2)                         | entity-header ) CRLF)  ; [Section 7.1](about:blank#section-7.1)                        CRLF                        [ message-body ]          ; [Section 7.2](about:blank#section-7.2)

6.1 状态行

The first line of a Response message is the Status-Line, consisting    of the protocol version followed by a numeric status code and its    associated textual phrase, with each element separated by SP    characters. No CR or LF is allowed except in the final CRLF sequence.         Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF

6.1.1 状态码和原因短语

The Status-Code element is a 3-digit integer result code of the    attempt to understand and satisfy the request. These codes are fully    defined in [section 10](about:blank#section-10). The Reason-Phrase is intended to give a short    textual description of the Status-Code. The Status-Code is intended    for use by automata and the Reason-Phrase is intended for the human    user. The client is not required to examine or display the Reason-    Phrase.      The first digit of the Status-Code defines the class of response. The    last two digits do not have any categorization role. There are 5    values for the first digit:        - 1xx: Informational - Request received, continuing process        - 2xx: Success - The action was successfully received,         understood, and accepted        - 3xx: Redirection - Further action must be taken in order to         complete the request        - 4xx: Client Error - The request contains bad syntax or cannot         be fulfilled        - 5xx: Server Error - The server failed to fulfill an apparently         valid request     The individual values of the numeric status codes defined for    HTTP/1.1, and an example set of corresponding Reason-Phrase's, are    presented below. The reason phrases listed here are only    recommendations -- they MAY be replaced by local equivalents without    affecting the protocol.        Status-Code    =             "100"  ; [Section 10.1.1](about:blank#section-10.1.1): Continue           | "101"  ; [Section 10.1.2](about:blank#section-10.1.2): Switching Protocols           | "200"  ; [Section 10.2.1](about:blank#section-10.2.1): OK           | "201"  ; [Section 10.2.2](about:blank#section-10.2.2): Created           | "202"  ; [Section 10.2.3](about:blank#section-10.2.3): Accepted           | "203"  ; [Section 10.2.4](about:blank#section-10.2.4): Non-Authoritative Information           | "204"  ; [Section 10.2.5](about:blank#section-10.2.5): No Content           | "205"  ; [Section 10.2.6](about:blank#section-10.2.6): Reset Content           | "206"  ; [Section 10.2.7](about:blank#section-10.2.7): Partial Content           | "300"  ; [Section 10.3.1](about:blank#section-10.3.1): Multiple Choices           | "301"  ; [Section 10.3.2](about:blank#section-10.3.2): Moved Permanently           | "302"  ; [Section 10.3.3](about:blank#section-10.3.3): Found           | "303"  ; [Section 10.3.4](about:blank#section-10.3.4): See Other           | "304"  ; [Section 10.3.5](about:blank#section-10.3.5): Not Modified           | "305"  ; [Section 10.3.6](about:blank#section-10.3.6): Use Proxy           | "307"  ; [Section 10.3.8](about:blank#section-10.3.8): Temporary Redirect           | "400"  ; [Section 10.4.1](about:blank#section-10.4.1): Bad Request           | "401"  ; [Section 10.4.2](about:blank#section-10.4.2): Unauthorized           | "402"  ; [Section 10.4.3](about:blank#section-10.4.3): Payment Required           | "403"  ; [Section 10.4.4](about:blank#section-10.4.4): Forbidden           | "404"  ; [Section 10.4.5](about:blank#section-10.4.5): Not Found           | "405"  ; [Section 10.4.6](about:blank#section-10.4.6): Method Not Allowed           | "406"  ; [Section 10.4.7](about:blank#section-10.4.7): Not Acceptable             | "407"  ; [Section 10.4.8](about:blank#section-10.4.8): Proxy Authentication Required           | "408"  ; [Section 10.4.9](about:blank#section-10.4.9): Request Time-out           | "409"  ; [Section 10.4.10](about:blank#section-10.4.10): Conflict           | "410"  ; [Section 10.4.11](about:blank#section-10.4.11): Gone           | "411"  ; [Section 10.4.12](about:blank#section-10.4.12): Length Required           | "412"  ; [Section 10.4.13](about:blank#section-10.4.13): Precondition Failed           | "413"  ; [Section 10.4.14](about:blank#section-10.4.14): Request Entity Too Large           | "414"  ; [Section 10.4.15](about:blank#section-10.4.15): Request-URI Too Large           | "415"  ; [Section 10.4.16](about:blank#section-10.4.16): Unsupported Media Type           | "416"  ; [Section 10.4.17](about:blank#section-10.4.17): Requested range not satisfiable           | "417"  ; [Section 10.4.18](about:blank#section-10.4.18): Expectation Failed           | "500"  ; [Section 10.5.1](about:blank#section-10.5.1): Internal Server Error           | "501"  ; [Section 10.5.2](about:blank#section-10.5.2): Not Implemented           | "502"  ; [Section 10.5.3](about:blank#section-10.5.3): Bad Gateway           | "503"  ; [Section 10.5.4](about:blank#section-10.5.4): Service Unavailable           | "504"  ; [Section 10.5.5](about:blank#section-10.5.5): Gateway Time-out           | "505"  ; [Section 10.5.6](about:blank#section-10.5.6): HTTP Version not supported           | extension-code        extension-code = 3DIGIT       Reason-Phrase  = \*<TEXT, excluding CR, LF>     HTTP status codes are extensible. HTTP applications are not required    to understand the meaning of all registered status codes, though such    understanding is obviously desirable. However, applications MUST    understand the class of any status code, as indicated by the first    digit, and treat any unrecognized response as being equivalent to the    x00 status code of that class, with the exception that an    unrecognized response MUST NOT be cached. For example, if an    unrecognized status code of 431 is received by the client, it can    safely assume that there was something wrong with its request and    treat the response as if it had received a 400 status code. In such    cases, user agents SHOULD present to the user the entity returned    with the response, since that entity is likely to include human-    readable information which will explain the unusual status.

6.2 响应标题字段

The response-header fields allow the server to pass additional    information about the response which cannot be placed in the Status-    Line. These header fields give information about the server and about    further access to the resource identified by the Request-URI.         response-header = Accept-Ranges           ; [Section 14.5](about:blank#section-14.5)                        | Age                     ; [Section 14.6](about:blank#section-14.6)                        | ETag                    ; [Section 14.19](about:blank#section-14.19)                        | Location                ; [Section 14.30](about:blank#section-14.30)                        | Proxy-Authenticate      ; [Section 14.33](about:blank#section-14.33)                          | Retry-After             ; [Section 14.37](about:blank#section-14.37)                        | Server                  ; [Section 14.38](about:blank#section-14.38)                        | Vary                    ; [Section 14.44](about:blank#section-14.44)                        | WWW-Authenticate        ; [Section 14.47](about:blank#section-14.47)     Response-header field names can be extended reliably only in    combination with a change in the protocol version. However, new or    experimental header fields MAY be given the semantics of response-    header fields if all parties in the communication recognize them to    be response-header fields. Unrecognized header fields are treated as    entity-header fields.

7 实体

Request and Response messages MAY transfer an entity if not otherwise    restricted by the request method or response status code. An entity    consists of entity-header fields and an entity-body, although some    responses will only include the entity-headers.     In this section, both sender and recipient refer to either the client    or the server, depending on who sends and who receives the entity.

7.1 实体标题字段

Entity-header fields define metainformation about the entity-body or,    if no body is present, about the resource identified by the request.    Some of this metainformation is OPTIONAL; some might be REQUIRED by    portions of this specification.         entity-header  = Allow                    ; [Section 14.7](about:blank#section-14.7)                       | Content-Encoding         ; [Section 14.11](about:blank#section-14.11)                       | Content-Language         ; [Section 14.12](about:blank#section-14.12)                       | Content-Length           ; [Section 14.13](about:blank#section-14.13)                       | Content-Location         ; [Section 14.14](about:blank#section-14.14)                       | Content-MD5              ; [Section 14.15](about:blank#section-14.15)                       | Content-Range            ; [Section 14.16](about:blank#section-14.16)                       | Content-Type             ; [Section 14.17](about:blank#section-14.17)                       | Expires                  ; [Section 14.21](about:blank#section-14.21)                       | Last-Modified            ; [Section 14.29](about:blank#section-14.29)                       | extension-header         extension-header = message-header     The extension-header mechanism allows additional entity-header fields    to be defined without changing the protocol, but these fields cannot    be assumed to be recognizable by the recipient. Unrecognized header    fields SHOULD be ignored by the recipient and MUST be forwarded by    transparent proxies.

7.2 实体主体

The entity-body (if any) sent with an HTTP request or response is in    a format and encoding defined by the entity-header fields.         entity-body    = \*OCTET     An entity-body is only present in a message when a message-body is    present, as described in [section 4.3](about:blank#section-4.3). The entity-body is obtained    from the message-body by decoding any Transfer-Encoding that might    have been applied to ensure safe and proper transfer of the message.

7.2.1 类型

When an entity-body is included with a message, the data type of that    body is determined via the header fields Content-Type and Content-    Encoding. These define a two-layer, ordered encoding model:         entity-body := Content-Encoding( Content-Type( data ) )     Content-Type specifies the media type of the underlying data.    Content-Encoding may be used to indicate any additional content    codings applied to the data, usually for the purpose of data    compression, that are a property of the requested resource. There is    no default encoding.     Any HTTP/1.1 message containing an entity-body SHOULD include a    Content-Type header field defining the media type of that body. If    and only if the media type is not given by a Content-Type field, the    recipient MAY attempt to guess the media type via inspection of its    content and/or the name extension(s) of the URI used to identify the    resource. If the media type remains unknown, the recipient SHOULD    treat it as type "application/octet-stream".

7.2.2 实体长度

The entity-length of a message is the length of the message-body    before any transfer-codings have been applied. [Section 4.4](about:blank#section-4.4) defines    how the transfer-length of a message-body is determined.

8 连接

8.1 永久性连接

8.1.1 目的

Prior to persistent connections, a separate TCP connection was    established to fetch each URL, increasing the load on HTTP servers    and causing congestion on the Internet. The use of inline images and    other associated data often require a client to make multiple    requests of the same server in a short amount of time. Analysis of    these performance problems and results from a prototype    implementation are available [[26](about:blank#ref-26)] [[30](about:blank#ref-30)]. Implementation experience and    measurements of actual HTTP/1.1 ([RFC 2068](https://tools.ietf.org/html/rfc2068)) implementations show good    results [[39](about:blank#ref-39)]. Alternatives have also been explored, for example,    T/TCP [[27](about:blank#ref-27)].     Persistent HTTP connections have a number of advantages:        - By opening and closing fewer TCP connections, CPU time is saved         in routers and hosts (clients, servers, proxies, gateways,         tunnels, or caches), and memory used for TCP protocol control         blocks can be saved in hosts.        - HTTP requests and responses can be pipelined on a connection.         Pipelining allows a client to make multiple requests without         waiting for each response, allowing a single TCP connection to         be used much more efficiently, with much lower elapsed time.        - Network congestion is reduced by reducing the number of packets         caused by TCP opens, and by allowing TCP sufficient time to         determine the congestion state of the network.        - Latency on subsequent requests is reduced since there is no time         spent in TCP's connection opening handshake.        - HTTP can evolve more gracefully, since errors can be reported         without the penalty of closing the TCP connection. Clients using         future versions of HTTP might optimistically try a new feature,         but if communicating with an older server, retry with old         semantics after an error is reported.     HTTP implementations SHOULD implement persistent connections.

8.1.2 整体操作

A significant difference between HTTP/1.1 and earlier versions of    HTTP is that persistent connections are the default behavior of any    HTTP connection. That is, unless otherwise indicated, the client    SHOULD assume that the server will maintain a persistent connection,    even after error responses from the server.     Persistent connections provide a mechanism by which a client and a    server can signal the close of a TCP connection. This signaling takes    place using the Connection header field ([section 14.10](about:blank#section-14.10)). Once a close    has been signaled, the client MUST NOT send any more requests on that    connection.

8.1.2.1 交涉

An HTTP/1.1 server MAY assume that a HTTP/1.1 client intends to    maintain a persistent connection unless a Connection header including    the connection-token "close" was sent in the request. If the server    chooses to close the connection immediately after sending the    response, it SHOULD send a Connection header including the    connection-token close.     An HTTP/1.1 client MAY expect a connection to remain open, but would    decide to keep it open based on whether the response from a server    contains a Connection header with the connection-token close. In case    the client does not want to maintain a connection for more than that    request, it SHOULD send a Connection header including the    connection-token close.     If either the client or the server sends the close token in the    Connection header, that request becomes the last one for the    connection.     Clients and servers SHOULD NOT assume that a persistent connection is    maintained for HTTP versions less than 1.1 unless it is explicitly    signaled. See [section 19.6.2](about:blank#section-19.6.2) for more information on backward    compatibility with HTTP/1.0 clients.     In order to remain persistent, all messages on the connection MUST    have a self-defined message length (i.e., one not defined by closure    of the connection), as described in [section 4.4](about:blank#section-4.4).

8.1.2.2 流水线

A client that supports persistent connections MAY "pipeline" its    requests (i.e., send multiple requests without waiting for each    response). A server MUST send its responses to those requests in the    same order that the requests were received.     Clients which assume persistent connections and pipeline immediately    after connection establishment SHOULD be prepared to retry their    connection if the first pipelined attempt fails. If a client does    such a retry, it MUST NOT pipeline before it knows the connection is    persistent. Clients MUST also be prepared to resend their requests if    the server closes the connection before sending all of the    corresponding responses.     Clients SHOULD NOT pipeline requests using non-idempotent methods or    non-idempotent sequences of methods (see [section 9.1.2](about:blank#section-9.1.2)). Otherwise, a    premature termination of the transport connection could lead to    indeterminate results. A client wishing to send a non-idempotent    request SHOULD wait to send that request until it has received the    response status for the previous request.

8.1.3 代理服务器

It is especially important that proxies correctly implement the    properties of the Connection header field as specified in [section](about:blank#section-14.10) [14.10](about:blank#section-14.10).     The proxy server MUST signal persistent connections separately with    its clients and the origin servers (or other proxy servers) that it    connects to. Each persistent connection applies to only one transport    link.     A proxy server MUST NOT establish a HTTP/1.1 persistent connection    with an HTTP/1.0 client (but see [RFC 2068](https://tools.ietf.org/html/rfc2068) [[33](about:blank#ref-33)] for information and    discussion of the problems with the Keep-Alive header implemented by    many HTTP/1.0 clients).

8.1.4 实际考虑

Servers will usually have some time-out value beyond which they will    no longer maintain an inactive connection. Proxy servers might make    this a higher value since it is likely that the client will be making    more connections through the same server. The use of persistent    connections places no requirements on the length (or existence) of    this time-out for either the client or the server.      When a client or server wishes to time-out it SHOULD issue a graceful    close on the transport connection. Clients and servers SHOULD both    constantly watch for the other side of the transport close, and    respond to it as appropriate. If a client or server does not detect    the other side's close promptly it could cause unnecessary resource    drain on the network.     A client, server, or proxy MAY close the transport connection at any    time. For example, a client might have started to send a new request    at the same time that the server has decided to close the "idle"    connection. From the server's point of view, the connection is being    closed while it was idle, but from the client's point of view, a    request is in progress.     This means that clients, servers, and proxies MUST be able to recover    from asynchronous close events. Client software SHOULD reopen the    transport connection and retransmit the aborted sequence of requests    without user interaction so long as the request sequence is    idempotent (see [section 9.1.2](about:blank#section-9.1.2)). Non-idempotent methods or sequences    MUST NOT be automatically retried, although user agents MAY offer a    human operator the choice of retrying the request(s). Confirmation by    user-agent software with semantic understanding of the application    MAY substitute for user confirmation. The automatic retry SHOULD NOT    be repeated if the second sequence of requests fails.     Servers SHOULD always respond to at least one request per connection,    if at all possible. Servers SHOULD NOT close a connection in the    middle of transmitting a response, unless a network or client failure    is suspected.     Clients that use persistent connections SHOULD limit the number of    simultaneous connections that they maintain to a given server. A    single-user client SHOULD NOT maintain more than 2 connections with    any server or proxy. A proxy SHOULD use up to 2\*N connections to    another server or proxy, where N is the number of simultaneously    active users. These guidelines are intended to improve HTTP response    times and avoid congestion.

8.2 消息传输要求

8.2.1 持续性连接和流量控制

HTTP/1.1 servers SHOULD maintain persistent connections and use TCP's    flow control mechanisms to resolve temporary overloads, rather than    terminating connections with the expectation that clients will retry.    The latter technique can exacerbate network congestion.

8.2.2 监视错误状态消息的连接

An HTTP/1.1 (or later) client sending a message-body SHOULD monitor    the network connection for an error status while it is transmitting    the request. If the client sees an error status, it SHOULD    immediately cease transmitting the body. If the body is being sent    using a "chunked" encoding ([section 3.6](about:blank#section-3.6)), a zero length chunk and    empty trailer MAY be used to prematurely mark the end of the message.    If the body was preceded by a Content-Length header, the client MUST    close the connection.

8.2.3 使用100（继续）状态

The purpose of the 100 (Continue) status (see [section 10.1.1](about:blank#section-10.1.1)) is to    allow a client that is sending a request message with a request body    to determine if the origin server is willing to accept the request    (based on the request headers) before the client sends the request    body. In some cases, it might either be inappropriate or highly    inefficient for the client to send the body if the server will reject    the message without looking at the body.     Requirements for HTTP/1.1 clients:        - If a client will wait for a 100 (Continue) response before         sending the request body, it MUST send an Expect request-header         field ([section 14.20](about:blank#section-14.20)) with the "100-continue" expectation.        - A client MUST NOT send an Expect request-header field ([section](about:blank#section-14.20) [14.20](about:blank#section-14.20)) with the "100-continue" expectation if it does not intend         to send a request body.     Because of the presence of older implementations, the protocol allows    ambiguous situations in which a client may send "Expect: 100-    continue" without receiving either a 417 (Expectation Failed) status    or a 100 (Continue) status. Therefore, when a client sends this    header field to an origin server (possibly via a proxy) from which it    has never seen a 100 (Continue) status, the client SHOULD NOT wait    for an indefinite period before sending the request body.     Requirements for HTTP/1.1 origin servers:        - Upon receiving a request which includes an Expect request-header         field with the "100-continue" expectation, an origin server MUST         either respond with 100 (Continue) status and continue to read         from the input stream, or respond with a final status code. The         origin server MUST NOT wait for the request body before sending         the 100 (Continue) response. If it responds with a final status         code, it MAY close the transport connection or it MAY continue           to read and discard the rest of the request.  It MUST NOT         perform the requested method if it returns a final status code.        - An origin server SHOULD NOT send a 100 (Continue) response if         the request message does not include an Expect request-header         field with the "100-continue" expectation, and MUST NOT send a         100 (Continue) response if such a request comes from an HTTP/1.0         (or earlier) client. There is an exception to this rule: for         compatibility with [RFC 2068](https://tools.ietf.org/html/rfc2068), a server MAY send a 100 (Continue)         status in response to an HTTP/1.1 PUT or POST request that does         not include an Expect request-header field with the "100-         continue" expectation. This exception, the purpose of which is         to minimize any client processing delays associated with an         undeclared wait for 100 (Continue) status, applies only to         HTTP/1.1 requests, and not to requests with any other HTTP-         version value.        - An origin server MAY omit a 100 (Continue) response if it has         already received some or all of the request body for the         corresponding request.        - An origin server that sends a 100 (Continue) response MUST         ultimately send a final status code, once the request body is         received and processed, unless it terminates the transport         connection prematurely.        - If an origin server receives a request that does not include an         Expect request-header field with the "100-continue" expectation,         the request includes a request body, and the server responds         with a final status code before reading the entire request body         from the transport connection, then the server SHOULD NOT close         the transport connection until it has read the entire request,         or until the client closes the connection. Otherwise, the client         might not reliably receive the response message. However, this         requirement is not be construed as preventing a server from         defending itself against denial-of-service attacks, or from         badly broken client implementations.     Requirements for HTTP/1.1 proxies:        - If a proxy receives a request that includes an Expect request-         header field with the "100-continue" expectation, and the proxy         either knows that the next-hop server complies with HTTP/1.1 or         higher, or does not know the HTTP version of the next-hop         server, it MUST forward the request, including the Expect header         field.         - If the proxy knows that the version of the next-hop server is         HTTP/1.0 or lower, it MUST NOT forward the request, and it MUST         respond with a 417 (Expectation Failed) status.        - Proxies SHOULD maintain a cache recording the HTTP version         numbers received from recently-referenced next-hop servers.        - A proxy MUST NOT forward a 100 (Continue) response if the         request message was received from an HTTP/1.0 (or earlier)         client and did not include an Expect request-header field with         the "100-continue" expectation. This requirement overrides the         general rule for forwarding of 1xx responses (see [section 10.1](about:blank#section-10.1)).

8.2.4 服务器过早关闭连接时的客户端行为

If an HTTP/1.1 client sends a request which includes a request body,    but which does not include an Expect request-header field with the    "100-continue" expectation, and if the client is not directly    connected to an HTTP/1.1 origin server, and if the client sees the    connection close before receiving any status from the server, the    client SHOULD retry the request.  If the client does retry this    request, it MAY use the following "binary exponential backoff"    algorithm to be assured of obtaining a reliable response:        1. Initiate a new connection to the server        2. Transmit the request-headers        3. Initialize a variable R to the estimated round-trip time to the          server (e.g., based on the time it took to establish the          connection), or to a constant value of 5 seconds if the round-          trip time is not available.        4. Compute T = R \* (2\*\*N), where N is the number of previous          retries of this request.        5. Wait either for an error response from the server, or for T          seconds (whichever comes first)        6. If no error response is received, after T seconds transmit the          body of the request.        7. If client sees that the connection is closed prematurely,          repeat from step 1 until the request is accepted, an error          response is received, or the user becomes impatient and          terminates the retry process.      If at any point an error status is received, the client        - SHOULD NOT continue and        - SHOULD close the connection if it has not completed sending the         request message.

9 方法定义

The set of common methods for HTTP/1.1 is defined below. Although    this set can be expanded, additional methods cannot be assumed to    share the same semantics for separately extended clients and servers.     The Host request-header field ([section 14.23](about:blank#section-14.23)) MUST accompany all    HTTP/1.1 requests.

9.1 安全、幂等方法

9.1.1 安全方法

Implementors should be aware that the software represents the user in    their interactions over the Internet, and should be careful to allow    the user to be aware of any actions they might take which may have an    unexpected significance to themselves or others.     In particular, the convention has been established that the GET and    HEAD methods SHOULD NOT have the significance of taking an action    other than retrieval. These methods ought to be considered "safe".    This allows user agents to represent other methods, such as POST, PUT    and DELETE, in a special way, so that the user is made aware of the    fact that a possibly unsafe action is being requested.     Naturally, it is not possible to ensure that the server does not    generate side-effects as a result of performing a GET request; in    fact, some dynamic resources consider that a feature. The important    distinction here is that the user did not request the side-effects,    so therefore cannot be held accountable for them.

9.1.2 幂等方法

Methods can also have the property of "idempotence" in that (aside    from error or expiration issues) the side-effects of N > 0 identical    requests is the same as for a single request. The methods GET, HEAD,    PUT and DELETE share this property. Also, the methods OPTIONS and    TRACE SHOULD NOT have side effects, and so are inherently idempotent.      However, it is possible that a sequence of several requests is non-    idempotent, even if all of the methods executed in that sequence are    idempotent. (A sequence is idempotent if a single execution of the    entire sequence always yields a result that is not changed by a    reexecution of all, or part, of that sequence.) For example, a    sequence is non-idempotent if its result depends on a value that is    later modified in the same sequence.     A sequence that never has side effects is idempotent, by definition    (provided that no concurrent operations are being executed on the    same set of resources).

9.2 OPTIONS

The OPTIONS method represents a request for information about the    communication options available on the request/response chain    identified by the Request-URI. This method allows the client to    determine the options and/or requirements associated with a resource,    or the capabilities of a server, without implying a resource action    or initiating a resource retrieval.     Responses to this method are not cacheable.     If the OPTIONS request includes an entity-body (as indicated by the    presence of Content-Length or Transfer-Encoding), then the media type    MUST be indicated by a Content-Type field. Although this    specification does not define any use for such a body, future    extensions to HTTP might use the OPTIONS body to make more detailed    queries on the server. A server that does not support such an    extension MAY discard the request body.     If the Request-URI is an asterisk ("\*"), the OPTIONS request is    intended to apply to the server in general rather than to a specific    resource. Since a server's communication options typically depend on    the resource, the "\*" request is only useful as a "ping" or "no-op"    type of method; it does nothing beyond allowing the client to test    the capabilities of the server. For example, this can be used to test    a proxy for HTTP/1.1 compliance (or lack thereof).     If the Request-URI is not an asterisk, the OPTIONS request applies    only to the options that are available when communicating with that    resource.     A 200 response SHOULD include any header fields that indicate    optional features implemented by the server and applicable to that    resource (e.g., Allow), possibly including extensions not defined by    this specification. The response body, if any, SHOULD also include    information about the communication options. The format for such a      body is not defined by this specification, but might be defined by    future extensions to HTTP. Content negotiation MAY be used to select    the appropriate response format. If no response body is included, the    response MUST include a Content-Length field with a field-value of    "0".     The Max-Forwards request-header field MAY be used to target a    specific proxy in the request chain. When a proxy receives an OPTIONS    request on an absoluteURI for which request forwarding is permitted,    the proxy MUST check for a Max-Forwards field. If the Max-Forwards    field-value is zero ("0"), the proxy MUST NOT forward the message;    instead, the proxy SHOULD respond with its own communication options.    If the Max-Forwards field-value is an integer greater than zero, the    proxy MUST decrement the field-value when it forwards the request. If    no Max-Forwards field is present in the request, then the forwarded    request MUST NOT include a Max-Forwards field.

9.3 GET

The GET method means retrieve whatever information (in the form of an    entity) is identified by the Request-URI. If the Request-URI refers    to a data-producing process, it is the produced data which shall be    returned as the entity in the response and not the source text of the    process, unless that text happens to be the output of the process.     The semantics of the GET method change to a "conditional GET" if the    request message includes an If-Modified-Since, If-Unmodified-Since,    If-Match, If-None-Match, or If-Range header field. A conditional GET    method requests that the entity be transferred only under the    circumstances described by the conditional header field(s). The    conditional GET method is intended to reduce unnecessary network    usage by allowing cached entities to be refreshed without requiring    multiple requests or transferring data already held by the client.     The semantics of the GET method change to a "partial GET" if the    request message includes a Range header field. A partial GET requests    that only part of the entity be transferred, as described in [section](about:blank#section-14.35) [14.35](about:blank#section-14.35). The partial GET method is intended to reduce unnecessary    network usage by allowing partially-retrieved entities to be    completed without transferring data already held by the client.     The response to a GET request is cacheable if and only if it meets    the requirements for HTTP caching described in [section 13](about:blank#section-13).     See [section 15.1.3](about:blank#section-15.1.3) for security considerations when used for forms.

9.4 HEAD

The HEAD method is identical to GET except that the server MUST NOT    return a message-body in the response. The metainformation contained    in the HTTP headers in response to a HEAD request SHOULD be identical    to the information sent in response to a GET request. This method can    be used for obtaining metainformation about the entity implied by the    request without transferring the entity-body itself. This method is    often used for testing hypertext links for validity, accessibility,    and recent modification.     The response to a HEAD request MAY be cacheable in the sense that the    information contained in the response MAY be used to update a    previously cached entity from that resource. If the new field values    indicate that the cached entity differs from the current entity (as    would be indicated by a change in Content-Length, Content-MD5, ETag    or Last-Modified), then the cache MUST treat the cache entry as    stale.

9.5 POST

The POST method is used to request that the origin server accept the    entity enclosed in the request as a new subordinate of the resource    identified by the Request-URI in the Request-Line. POST is designed    to allow a uniform method to cover the following functions:        - Annotation of existing resources;        - Posting a message to a bulletin board, newsgroup, mailing list,         or similar group of articles;        - Providing a block of data, such as the result of submitting a         form, to a data-handling process;        - Extending a database through an append operation.     The actual function performed by the POST method is determined by the    server and is usually dependent on the Request-URI. The posted entity    is subordinate to that URI in the same way that a file is subordinate    to a directory containing it, a news article is subordinate to a    newsgroup to which it is posted, or a record is subordinate to a    database.     The action performed by the POST method might not result in a    resource that can be identified by a URI. In this case, either 200    (OK) or 204 (No Content) is the appropriate response status,    depending on whether or not the response includes an entity that    describes the result.      If a resource has been created on the origin server, the response    SHOULD be 201 (Created) and contain an entity which describes the    status of the request and refers to the new resource, and a Location    header (see [section 14.30](about:blank#section-14.30)).     Responses to this method are not cacheable, unless the response    includes appropriate Cache-Control or Expires header fields. However,    the 303 (See Other) response can be used to direct the user agent to    retrieve a cacheable resource.     POST requests MUST obey the message transmission requirements set out    in [section 8.2](about:blank#section-8.2).     See [section 15.1.3](about:blank#section-15.1.3) for security considerations.

9.6 PUT

The PUT method requests that the enclosed entity be stored under the    supplied Request-URI. If the Request-URI refers to an already    existing resource, the enclosed entity SHOULD be considered as a    modified version of the one residing on the origin server. If the    Request-URI does not point to an existing resource, and that URI is    capable of being defined as a new resource by the requesting user    agent, the origin server can create the resource with that URI. If a    new resource is created, the origin server MUST inform the user agent    via the 201 (Created) response. If an existing resource is modified,    either the 200 (OK) or 204 (No Content) response codes SHOULD be sent    to indicate successful completion of the request. If the resource    could not be created or modified with the Request-URI, an appropriate    error response SHOULD be given that reflects the nature of the    problem. The recipient of the entity MUST NOT ignore any Content-\*    (e.g. Content-Range) headers that it does not understand or implement    and MUST return a 501 (Not Implemented) response in such cases.     If the request passes through a cache and the Request-URI identifies    one or more currently cached entities, those entries SHOULD be    treated as stale. Responses to this method are not cacheable.     The fundamental difference between the POST and PUT requests is    reflected in the different meaning of the Request-URI. The URI in a    POST request identifies the resource that will handle the enclosed    entity. That resource might be a data-accepting process, a gateway to    some other protocol, or a separate entity that accepts annotations.    In contrast, the URI in a PUT request identifies the entity enclosed    with the request -- the user agent knows what URI is intended and the    server MUST NOT attempt to apply the request to some other resource.    If the server desires that the request be applied to a different URI,      it MUST send a 301 (Moved Permanently) response; the user agent MAY    then make its own decision regarding whether or not to redirect the    request.     A single resource MAY be identified by many different URIs. For    example, an article might have a URI for identifying "the current    version" which is separate from the URI identifying each particular    version. In this case, a PUT request on a general URI might result in    several other URIs being defined by the origin server.     HTTP/1.1 does not define how a PUT method affects the state of an    origin server.     PUT requests MUST obey the message transmission requirements set out    in [section 8.2](about:blank#section-8.2).     Unless otherwise specified for a particular entity-header, the    entity-headers in the PUT request SHOULD be applied to the resource    created or modified by the PUT.

9.7 DELETE

The DELETE method requests that the origin server delete the resource    identified by the Request-URI. This method MAY be overridden by human    intervention (or other means) on the origin server. The client cannot    be guaranteed that the operation has been carried out, even if the    status code returned from the origin server indicates that the action    has been completed successfully. However, the server SHOULD NOT    indicate success unless, at the time the response is given, it    intends to delete the resource or move it to an inaccessible    location.     A successful response SHOULD be 200 (OK) if the response includes an    entity describing the status, 202 (Accepted) if the action has not    yet been enacted, or 204 (No Content) if the action has been enacted    but the response does not include an entity.     If the request passes through a cache and the Request-URI identifies    one or more currently cached entities, those entries SHOULD be    treated as stale. Responses to this method are not cacheable.

9.8 TRACE

The TRACE method is used to invoke a remote, application-layer loop-    back of the request message. The final recipient of the request    SHOULD reflect the message received back to the client as the    entity-body of a 200 (OK) response. The final recipient is either the      origin server or the first proxy or gateway to receive a Max-Forwards    value of zero (0) in the request (see [section 14.31](about:blank#section-14.31)). A TRACE request    MUST NOT include an entity.     TRACE allows the client to see what is being received at the other    end of the request chain and use that data for testing or diagnostic    information. The value of the Via header field ([section 14.45](about:blank#section-14.45)) is of    particular interest, since it acts as a trace of the request chain.    Use of the Max-Forwards header field allows the client to limit the    length of the request chain, which is useful for testing a chain of    proxies forwarding messages in an infinite loop.     If the request is valid, the response SHOULD contain the entire    request message in the entity-body, with a Content-Type of    "message/http". Responses to this method MUST NOT be cached.

9.9 CONNECT

This specification reserves the method name CONNECT for use with a    proxy that can dynamically switch to being a tunnel (e.g. SSL    tunneling [[44](about:blank#ref-44)]).

10 状态码定义

Each Status-Code is described below, including a description of which    method(s) it can follow and any metainformation required in the    response.

10.1 信息1xx

This class of status code indicates a provisional response,    consisting only of the Status-Line and optional headers, and is    terminated by an empty line. There are no required headers for this    class of status code. Since HTTP/1.0 did not define any 1xx status    codes, servers MUST NOT send a 1xx response to an HTTP/1.0 client    except under experimental conditions.     A client MUST be prepared to accept one or more 1xx status responses    prior to a regular response, even if the client does not expect a 100    (Continue) status message. Unexpected 1xx status responses MAY be    ignored by a user agent.     Proxies MUST forward 1xx responses, unless the connection between the    proxy and its client has been closed, or unless the proxy itself    requested the generation of the 1xx response. (For example, if a      proxy adds a "Expect: 100-continue" field when it forwards a request,    then it need not forward the corresponding 100 (Continue)    response(s).)

10.1.1 100延续

The client SHOULD continue with its request. This interim response is    used to inform the client that the initial part of the request has    been received and has not yet been rejected by the server. The client    SHOULD continue by sending the remainder of the request or, if the    request has already been completed, ignore this response. The server    MUST send a final response after the request has been completed. See    [section 8.2.3](about:blank#section-8.2.3) for detailed discussion of the use and handling of this    status code.

10.1.2 101 切换协议

The server understands and is willing to comply with the client's    request, via the Upgrade message header field ([section 14.42](about:blank#section-14.42)), for a    change in the application protocol being used on this connection. The    server will switch protocols to those defined by the response's    Upgrade header field immediately after the empty line which    terminates the 101 response.     The protocol SHOULD be switched only when it is advantageous to do    so. For example, switching to a newer version of HTTP is advantageous    over older versions, and switching to a real-time, synchronous    protocol might be advantageous when delivering resources that use    such features.

10.2 成功2xx

This class of status code indicates that the client's request was    successfully received, understood, and accepted.

10.2.1 200 OK

The request has succeeded. The information returned with the response    is dependent on the method used in the request, for example:     GET    an entity corresponding to the requested resource is sent in           the response;     HEAD   the entity-header fields corresponding to the requested           resource are sent in the response without any message-body;     POST   an entity describing or containing the result of the action;      TRACE  an entity containing the request message as received by the           end server.

10.2.2 201创建

The request has been fulfilled and resulted in a new resource being    created. The newly created resource can be referenced by the URI(s)    returned in the entity of the response, with the most specific URI    for the resource given by a Location header field. The response    SHOULD include an entity containing a list of resource    characteristics and location(s) from which the user or user agent can    choose the one most appropriate. The entity format is specified by    the media type given in the Content-Type header field. The origin    server MUST create the resource before returning the 201 status code.    If the action cannot be carried out immediately, the server SHOULD    respond with 202 (Accepted) response instead.     A 201 response MAY contain an ETag response header field indicating    the current value of the entity tag for the requested variant just    created, see [section 14.19](about:blank#section-14.19).

10.2.3 202接受

The request has been accepted for processing, but the processing has    not been completed.  The request might or might not eventually be    acted upon, as it might be disallowed when processing actually takes    place. There is no facility for re-sending a status code from an    asynchronous operation such as this.     The 202 response is intentionally non-committal. Its purpose is to    allow a server to accept a request for some other process (perhaps a    batch-oriented process that is only run once per day) without    requiring that the user agent's connection to the server persist    until the process is completed. The entity returned with this    response SHOULD include an indication of the request's current status    and either a pointer to a status monitor or some estimate of when the    user can expect the request to be fulfilled.

10.2.4 203非权威信息

The returned metainformation in the entity-header is not the    definitive set as available from the origin server, but is gathered    from a local or a third-party copy. The set presented MAY be a subset    or superset of the original version. For example, including local    annotation information about the resource might result in a superset    of the metainformation known by the origin server. Use of this    response code is not required and is only appropriate when the    response would otherwise be 200 (OK).

10.2.5 204无内容

The server has fulfilled the request but does not need to return an    entity-body, and might want to return updated metainformation. The    response MAY include new or updated metainformation in the form of    entity-headers, which if present SHOULD be associated with the    requested variant.     If the client is a user agent, it SHOULD NOT change its document view    from that which caused the request to be sent. This response is    primarily intended to allow input for actions to take place without    causing a change to the user agent's active document view, although    any new or updated metainformation SHOULD be applied to the document    currently in the user agent's active view.     The 204 response MUST NOT include a message-body, and thus is always    terminated by the first empty line after the header fields.

10.2.6 205重置内容

The server has fulfilled the request and the user agent SHOULD reset    the document view which caused the request to be sent. This response    is primarily intended to allow input for actions to take place via    user input, followed by a clearing of the form in which the input is    given so that the user can easily initiate another input action. The    response MUST NOT include an entity.

10.2.7 206部分内容

The server has fulfilled the partial GET request for the resource.    The request MUST have included a Range header field ([section 14.35](about:blank#section-14.35))    indicating the desired range, and MAY have included an If-Range    header field ([section 14.27](about:blank#section-14.27)) to make the request conditional.     The response MUST include the following header fields:        - Either a Content-Range header field ([section 14.16](about:blank#section-14.16)) indicating         the range included with this response, or a multipart/byteranges         Content-Type including Content-Range fields for each part. If a         Content-Length header field is present in the response, its         value MUST match the actual number of OCTETs transmitted in the         message-body.        - Date        - ETag and/or Content-Location, if the header would have been sent         in a 200 response to the same request         - Expires, Cache-Control, and/or Vary, if the field-value might         differ from that sent in any previous response for the same         variant     If the 206 response is the result of an If-Range request that used a    strong cache validator (see [section 13.3.3](about:blank#section-13.3.3)), the response SHOULD NOT    include other entity-headers. If the response is the result of an    If-Range request that used a weak validator, the response MUST NOT    include other entity-headers; this prevents inconsistencies between    cached entity-bodies and updated headers. Otherwise, the response    MUST include all of the entity-headers that would have been returned    with a 200 (OK) response to the same request.     A cache MUST NOT combine a 206 response with other previously cached    content if the ETag or Last-Modified headers do not match exactly,    see 13.5.4.     A cache that does not support the Range and Content-Range headers    MUST NOT cache 206 (Partial) responses.

10.3 重定向3xx

This class of status code indicates that further action needs to be    taken by the user agent in order to fulfill the request.  The action    required MAY be carried out by the user agent without interaction    with the user if and only if the method used in the second request is    GET or HEAD. A client SHOULD detect infinite redirection loops, since    such loops generate network traffic for each redirection.        Note: previous versions of this specification recommended a       maximum of five redirections. Content developers should be aware       that there might be clients that implement such a fixed       limitation.

10.3.1 300 多项选择

The requested resource corresponds to any one of a set of    representations, each with its own specific location, and agent-    driven negotiation information ([section 12](about:blank#section-12)) is being provided so that    the user (or user agent) can select a preferred representation and    redirect its request to that location.     Unless it was a HEAD request, the response SHOULD include an entity    containing a list of resource characteristics and location(s) from    which the user or user agent can choose the one most appropriate. The    entity format is specified by the media type given in the Content-    Type header field. Depending upon the format and the capabilities of      the user agent, selection of the most appropriate choice MAY be    performed automatically. However, this specification does not define    any standard for such automatic selection.     If the server has a preferred choice of representation, it SHOULD    include the specific URI for that representation in the Location    field; user agents MAY use the Location field value for automatic    redirection. This response is cacheable unless indicated otherwise.

10.3.2 301 永久性移动

The requested resource has been assigned a new permanent URI and any    future references to this resource SHOULD use one of the returned    URIs.  Clients with link editing capabilities ought to automatically    re-link references to the Request-URI to one or more of the new    references returned by the server, where possible. This response is    cacheable unless indicated otherwise.     The new permanent URI SHOULD be given by the Location field in the    response. Unless the request method was HEAD, the entity of the    response SHOULD contain a short hypertext note with a hyperlink to    the new URI(s).     If the 301 status code is received in response to a request other    than GET or HEAD, the user agent MUST NOT automatically redirect the    request unless it can be confirmed by the user, since this might    change the conditions under which the request was issued.        Note: When automatically redirecting a POST request after       receiving a 301 status code, some existing HTTP/1.0 user agents       will erroneously change it into a GET request.

10.3.3 302 找到

The requested resource resides temporarily under a different URI.    Since the redirection might be altered on occasion, the client SHOULD    continue to use the Request-URI for future requests.  This response    is only cacheable if indicated by a Cache-Control or Expires header    field.     The temporary URI SHOULD be given by the Location field in the    response. Unless the request method was HEAD, the entity of the    response SHOULD contain a short hypertext note with a hyperlink to    the new URI(s).      If the 302 status code is received in response to a request other    than GET or HEAD, the user agent MUST NOT automatically redirect the    request unless it can be confirmed by the user, since this might    change the conditions under which the request was issued.        Note: [RFC 1945](https://tools.ietf.org/html/rfc1945) and [RFC 2068](https://tools.ietf.org/html/rfc2068) specify that the client is not allowed       to change the method on the redirected request.  However, most       existing user agent implementations treat 302 as if it were a 303       response, performing a GET on the Location field-value regardless       of the original request method. The status codes 303 and 307 have       been added for servers that wish to make unambiguously clear which       kind of reaction is expected of the client.

10.3.4 303 参见其他

The response to the request can be found under a different URI and    SHOULD be retrieved using a GET method on that resource. This method    exists primarily to allow the output of a POST-activated script to    redirect the user agent to a selected resource. The new URI is not a    substitute reference for the originally requested resource. The 303    response MUST NOT be cached, but the response to the second    (redirected) request might be cacheable.     The different URI SHOULD be given by the Location field in the    response. Unless the request method was HEAD, the entity of the    response SHOULD contain a short hypertext note with a hyperlink to    the new URI(s).        Note: Many pre-HTTP/1.1 user agents do not understand the 303       status. When interoperability with such clients is a concern, the       302 status code may be used instead, since most user agents react       to a 302 response as described here for 303.

10.3.5 304 未修改

If the client has performed a conditional GET request and access is    allowed, but the document has not been modified, the server SHOULD    respond with this status code. The 304 response MUST NOT contain a    message-body, and thus is always terminated by the first empty line    after the header fields.     The response MUST include the following header fields:        - Date, unless its omission is required by [section 14.18.1](about:blank#section-14.18.1)      If a clockless origin server obeys these rules, and proxies and    clients add their own Date to any response received without one (as    already specified by [[RFC 2068], section 14.19](https://tools.ietf.org/html/rfc2068#section-14.19)), caches will operate    correctly.        - ETag and/or Content-Location, if the header would have been sent         in a 200 response to the same request        - Expires, Cache-Control, and/or Vary, if the field-value might         differ from that sent in any previous response for the same         variant     If the conditional GET used a strong cache validator (see [section](about:blank#section-13.3.3) [13.3.3](about:blank#section-13.3.3)), the response SHOULD NOT include other entity-headers.    Otherwise (i.e., the conditional GET used a weak validator), the    response MUST NOT include other entity-headers; this prevents    inconsistencies between cached entity-bodies and updated headers.     If a 304 response indicates an entity not currently cached, then the    cache MUST disregard the response and repeat the request without the    conditional.     If a cache uses a received 304 response to update a cache entry, the    cache MUST update the entry to reflect any new field values given in    the response.

10.3.6 使用代理

The requested resource MUST be accessed through the proxy given by    the Location field. The Location field gives the URI of the proxy.    The recipient is expected to repeat this single request via the    proxy. 305 responses MUST only be generated by origin servers.        Note: [RFC 2068](https://tools.ietf.org/html/rfc2068) was not clear that 305 was intended to redirect a       single request, and to be generated by origin servers only.  Not       observing these limitations has significant security consequences.

10.3.7 306（未使用）

The 306 status code was used in a previous version of the    specification, is no longer used, and the code is reserved.

10.3.8 307 临时重定向

The requested resource resides temporarily under a different URI.    Since the redirection MAY be altered on occasion, the client SHOULD    continue to use the Request-URI for future requests.  This response    is only cacheable if indicated by a Cache-Control or Expires header    field.     The temporary URI SHOULD be given by the Location field in the    response. Unless the request method was HEAD, the entity of the    response SHOULD contain a short hypertext note with a hyperlink to    the new URI(s) , since many pre-HTTP/1.1 user agents do not    understand the 307 status. Therefore, the note SHOULD contain the    information necessary for a user to repeat the original request on    the new URI.     If the 307 status code is received in response to a request other    than GET or HEAD, the user agent MUST NOT automatically redirect the    request unless it can be confirmed by the user, since this might    change the conditions under which the request was issued.

10.4 客户端错误4xx

The 4xx class of status code is intended for cases in which the    client seems to have erred. Except when responding to a HEAD request,    the server SHOULD include an entity containing an explanation of the    error situation, and whether it is a temporary or permanent    condition. These status codes are applicable to any request method.    User agents SHOULD display any included entity to the user.     If the client is sending data, a server implementation using TCP    SHOULD be careful to ensure that the client acknowledges receipt of    the packet(s) containing the response, before the server closes the    input connection. If the client continues sending data to the server    after the close, the server's TCP stack will send a reset packet to    the client, which may erase the client's unacknowledged input buffers    before they can be read and interpreted by the HTTP application.

10.4.1 400 错误请求

The request could not be understood by the server due to malformed    syntax. The client SHOULD NOT repeat the request without    modifications.

10.4.2 401 未经授权

The request requires user authentication. The response MUST include a    WWW-Authenticate header field ([section 14.47](about:blank#section-14.47)) containing a challenge    applicable to the requested resource. The client MAY repeat the    request with a suitable Authorization header field ([section 14.8](about:blank#section-14.8)). If    the request already included Authorization credentials, then the 401    response indicates that authorization has been refused for those    credentials. If the 401 response contains the same challenge as the    prior response, and the user agent has already attempted    authentication at least once, then the user SHOULD be presented the    entity that was given in the response, since that entity might    include relevant diagnostic information. HTTP access authentication    is explained in "HTTP Authentication: Basic and Digest Access    Authentication" [[43](about:blank#ref-43)].

10.4.3 402 付款需要

This code is reserved for future use.

10.4.4 403 禁止

The server understood the request, but is refusing to fulfill it.    Authorization will not help and the request SHOULD NOT be repeated.    If the request method was not HEAD and the server wishes to make    public why the request has not been fulfilled, it SHOULD describe the    reason for the refusal in the entity.  If the server does not wish to    make this information available to the client, the status code 404    (Not Found) can be used instead.

10.4.5 404 未找到

The server has not found anything matching the Request-URI. No    indication is given of whether the condition is temporary or    permanent. The 410 (Gone) status code SHOULD be used if the server    knows, through some internally configurable mechanism, that an old    resource is permanently unavailable and has no forwarding address.    This status code is commonly used when the server does not wish to    reveal exactly why the request has been refused, or when no other    response is applicable.

10.4.6 405 方法不允许

The method specified in the Request-Line is not allowed for the    resource identified by the Request-URI. The response MUST include an    Allow header containing a list of valid methods for the requested    resource.

10.4.7 406 非接受范围内

The resource identified by the request is only capable of generating    response entities which have content characteristics not acceptable    according to the accept headers sent in the request.     Unless it was a HEAD request, the response SHOULD include an entity    containing a list of available entity characteristics and location(s)    from which the user or user agent can choose the one most    appropriate. The entity format is specified by the media type given    in the Content-Type header field. Depending upon the format and the    capabilities of the user agent, selection of the most appropriate    choice MAY be performed automatically. However, this specification    does not define any standard for such automatic selection.        Note: HTTP/1.1 servers are allowed to return responses which are       not acceptable according to the accept headers sent in the       request. In some cases, this may even be preferable to sending a       406 response. User agents are encouraged to inspect the headers of       an incoming response to determine if it is acceptable.     If the response could be unacceptable, a user agent SHOULD    temporarily stop receipt of more data and query the user for a    decision on further actions.

10.4.8 407 需要代理验证

This code is similar to 401 (Unauthorized), but indicates that the    client must first authenticate itself with the proxy. The proxy MUST    return a Proxy-Authenticate header field ([section 14.33](about:blank#section-14.33)) containing a    challenge applicable to the proxy for the requested resource. The    client MAY repeat the request with a suitable Proxy-Authorization    header field ([section 14.34](about:blank#section-14.34)). HTTP access authentication is explained    in "HTTP Authentication: Basic and Digest Access Authentication"    [[43](about:blank#ref-43)].

10.4.9 408 请求超时

The client did not produce a request within the time that the server    was prepared to wait. The client MAY repeat the request without    modifications at any later time.

10.4.10 409 冲突

The request could not be completed due to a conflict with the current    state of the resource. This code is only allowed in situations where    it is expected that the user might be able to resolve the conflict    and resubmit the request. The response body SHOULD include enough      information for the user to recognize the source of the conflict.    Ideally, the response entity would include enough information for the    user or user agent to fix the problem; however, that might not be    possible and is not required.     Conflicts are most likely to occur in response to a PUT request. For    example, if versioning were being used and the entity being PUT    included changes to a resource which conflict with those made by an    earlier (third-party) request, the server might use the 409 response    to indicate that it can't complete the request. In this case, the    response entity would likely contain a list of the differences    between the two versions in a format defined by the response    Content-Type.

10.4.11 410 退出

The requested resource is no longer available at the server and no    forwarding address is known. This condition is expected to be    considered permanent. Clients with link editing capabilities SHOULD    delete references to the Request-URI after user approval. If the    server does not know, or has no facility to determine, whether or not    the condition is permanent, the status code 404 (Not Found) SHOULD be    used instead. This response is cacheable unless indicated otherwise.     The 410 response is primarily intended to assist the task of web    maintenance by notifying the recipient that the resource is    intentionally unavailable and that the server owners desire that    remote links to that resource be removed. Such an event is common for    limited-time, promotional services and for resources belonging to    individuals no longer working at the server's site. It is not    necessary to mark all permanently unavailable resources as "gone" or    to keep the mark for any length of time -- that is left to the    discretion of the server owner.

10.4.12 411 所需长度

The server refuses to accept the request without a defined Content-    Length. The client MAY repeat the request if it adds a valid    Content-Length header field containing the length of the message-body    in the request message.

10.4.13 412 先决条件失败

The precondition given in one or more of the request-header fields    evaluated to false when it was tested on the server. This response    code allows the client to place preconditions on the current resource    metainformation (header field data) and thus prevent the requested    method from being applied to a resource other than the one intended.

10.4.14 413 请求实体过大

The server is refusing to process a request because the request    entity is larger than the server is willing or able to process. The    server MAY close the connection to prevent the client from continuing    the request.     If the condition is temporary, the server SHOULD include a Retry-    After header field to indicate that it is temporary and after what    time the client MAY try again.

10.4.15 414 请求URI过长

The server is refusing to service the request because the Request-URI    is longer than the server is willing to interpret. This rare    condition is only likely to occur when a client has improperly    converted a POST request to a GET request with long query    information, when the client has descended into a URI "black hole" of    redirection (e.g., a redirected URI prefix that points to a suffix of    itself), or when the server is under attack by a client attempting to    exploit security holes present in some servers using fixed-length    buffers for reading or manipulating the Request-URI.

10.4.16 415 非所支持的媒体类型

The server is refusing to service the request because the entity of    the request is in a format not supported by the requested resource    for the requested method.

10.4.17 416 请求范围不满足条件

A server SHOULD return a response with this status code if a request    included a Range request-header field ([section 14.35](about:blank#section-14.35)), and none of    the range-specifier values in this field overlap the current extent    of the selected resource, and the request did not include an If-Range    request-header field. (For byte-ranges, this means that the first-    byte-pos of all of the byte-range-spec values were greater than the    current length of the selected resource.)     When this status code is returned for a byte-range request, the    response SHOULD include a Content-Range entity-header field    specifying the current length of the selected resource (see [section](about:blank#section-14.16) [14.16](about:blank#section-14.16)). This response MUST NOT use the multipart/byteranges content-    type.

10.4.18 417 期望失败

The expectation given in an Expect request-header field (see [section](about:blank#section-14.20) [14.20](about:blank#section-14.20)) could not be met by this server, or, if the server is a proxy,    the server has unambiguous evidence that the request could not be met    by the next-hop server.

10.5 服务器错误 5xx

Response status codes beginning with the digit "5" indicate cases in    which the server is aware that it has erred or is incapable of    performing the request. Except when responding to a HEAD request, the    server SHOULD include an entity containing an explanation of the    error situation, and whether it is a temporary or permanent    condition. User agents SHOULD display any included entity to the    user. These response codes are applicable to any request method.

10.5.1 500 内部服务器错误

The server encountered an unexpected condition which prevented it    from fulfilling the request.

10.5.2 501 未实施

The server does not support the functionality required to fulfill the    request. This is the appropriate response when the server does not    recognize the request method and is not capable of supporting it for    any resource.

10.5.3 502 错误网关

The server, while acting as a gateway or proxy, received an invalid    response from the upstream server it accessed in attempting to    fulfill the request.

10.5.4 503 服务不可用

The server is currently unable to handle the request due to a    temporary overloading or maintenance of the server. The implication    is that this is a temporary condition which will be alleviated after    some delay. If known, the length of the delay MAY be indicated in a    Retry-After header. If no Retry-After is given, the client SHOULD    handle the response as it would for a 500 response.        Note: The existence of the 503 status code does not imply that a       server must use it when becoming overloaded. Some servers may wish       to simply refuse the connection.

10.5.5 504 网关超时

The server, while acting as a gateway or proxy, did not receive a    timely response from the upstream server specified by the URI (e.g.    HTTP, FTP, LDAP) or some other auxiliary server (e.g. DNS) it needed    to access in attempting to complete the request.        Note: Note to implementors: some deployed proxies are known to       return 400 or 500 when DNS lookups time out.

10.5.6 505 HTTP版本不支持

The server does not support, or refuses to support, the HTTP protocol    version that was used in the request message. The server is    indicating that it is unable or unwilling to complete the request    using the same major version as the client, as described in [section](about:blank#section-3.1) [3.1](about:blank#section-3.1), other than with this error message. The response SHOULD contain    an entity describing why that version is not supported and what other    protocols are supported by that server.

11 访问验证

HTTP provides several OPTIONAL challenge-response authentication    mechanisms which can be used by a server to challenge a client    request and by a client to provide authentication information. The    general framework for access authentication, and the specification of    "basic" and "digest" authentication, are specified in "HTTP    Authentication: Basic and Digest Access Authentication" [[43](about:blank#ref-43)]. This    specification adopts the definitions of "challenge" and "credentials"    from that specification.

12 内容交涉

Most HTTP responses include an entity which contains information for    interpretation by a human user. Naturally, it is desirable to supply    the user with the "best available" entity corresponding to the    request. Unfortunately for servers and caches, not all users have the    same preferences for what is "best," and not all user agents are    equally capable of rendering all entity types. For that reason, HTTP    has provisions for several mechanisms for "content negotiation" --    the process of selecting the best representation for a given response    when there are multiple representations available.        Note: This is not called "format negotiation" because the       alternate representations may be of the same media type, but use       different capabilities of that type, be in different languages,       etc.      Any response containing an entity-body MAY be subject to negotiation,    including error responses.     There are two kinds of content negotiation which are possible in    HTTP: server-driven and agent-driven negotiation. These two kinds of    negotiation are orthogonal and thus may be used separately or in    combination. One method of combination, referred to as transparent    negotiation, occurs when a cache uses the agent-driven negotiation    information provided by the origin server in order to provide    server-driven negotiation for subsequent requests.

12.1 服务器驱动交涉

If the selection of the best representation for a response is made by    an algorithm located at the server, it is called server-driven    negotiation. Selection is based on the available representations of    the response (the dimensions over which it can vary; e.g. language,    content-coding, etc.) and the contents of particular header fields in    the request message or on other information pertaining to the request    (such as the network address of the client).     Server-driven negotiation is advantageous when the algorithm for    selecting from among the available representations is difficult to    describe to the user agent, or when the server desires to send its    "best guess" to the client along with the first response (hoping to    avoid the round-trip delay of a subsequent request if the "best    guess" is good enough for the user). In order to improve the server's    guess, the user agent MAY include request header fields (Accept,    Accept-Language, Accept-Encoding, etc.) which describe its    preferences for such a response.     Server-driven negotiation has disadvantages:        1. It is impossible for the server to accurately determine what          might be "best" for any given user, since that would require          complete knowledge of both the capabilities of the user agent          and the intended use for the response (e.g., does the user want          to view it on screen or print it on paper?).        2. Having the user agent describe its capabilities in every          request can be both very inefficient (given that only a small          percentage of responses have multiple representations) and a          potential violation of the user's privacy.        3. It complicates the implementation of an origin server and the          algorithms for generating responses to a request.         4. It may limit a public cache's ability to use the same response          for multiple user's requests.     HTTP/1.1 includes the following request-header fields for enabling    server-driven negotiation through description of user agent    capabilities and user preferences: Accept ([section 14.1](about:blank#section-14.1)), Accept-    Charset ([section 14.2](about:blank#section-14.2)), Accept-Encoding ([section 14.3](about:blank#section-14.3)), Accept-    Language ([section 14.4](about:blank#section-14.4)), and User-Agent ([section 14.43](about:blank#section-14.43)). However, an    origin server is not limited to these dimensions and MAY vary the    response based on any aspect of the request, including information    outside the request-header fields or within extension header fields    not defined by this specification.     The Vary  header field can be used to express the parameters the    server uses to select a representation that is subject to server-    driven negotiation. See [section 13.6](about:blank#section-13.6) for use of the Vary header field    by caches and [section 14.44](about:blank#section-14.44) for use of the Vary header field by    servers.

12.2 代理驱动交涉

With agent-driven negotiation, selection of the best representation    for a response is performed by the user agent after receiving an    initial response from the origin server. Selection is based on a list    of the available representations of the response included within the    header fields or entity-body of the initial response, with each    representation identified by its own URI. Selection from among the    representations may be performed automatically (if the user agent is    capable of doing so) or manually by the user selecting from a    generated (possibly hypertext) menu.     Agent-driven negotiation is advantageous when the response would vary    over commonly-used dimensions (such as type, language, or encoding),    when the origin server is unable to determine a user agent's    capabilities from examining the request, and generally when public    caches are used to distribute server load and reduce network usage.     Agent-driven negotiation suffers from the disadvantage of needing a    second request to obtain the best alternate representation. This    second request is only efficient when caching is used. In addition,    this specification does not define any mechanism for supporting    automatic selection, though it also does not prevent any such    mechanism from being developed as an extension and used within    HTTP/1.1.      HTTP/1.1 defines the 300 (Multiple Choices) and 406 (Not Acceptable)    status codes for enabling agent-driven negotiation when the server is    unwilling or unable to provide a varying response using server-driven    negotiation.

12.3 公正交涉

Transparent negotiation is a combination of both server-driven and    agent-driven negotiation. When a cache is supplied with a form of the    list of available representations of the response (as in agent-driven    negotiation) and the dimensions of variance are completely understood    by the cache, then the cache becomes capable of performing server-    driven negotiation on behalf of the origin server for subsequent    requests on that resource.     Transparent negotiation has the advantage of distributing the    negotiation work that would otherwise be required of the origin    server and also removing the second request delay of agent-driven    negotiation when the cache is able to correctly guess the right    response.     This specification does not define any mechanism for transparent    negotiation, though it also does not prevent any such mechanism from    being developed as an extension that could be used within HTTP/1.1.

13 HTTP缓存

HTTP is typically used for distributed information systems, where    performance can be improved by the use of response caches. The    HTTP/1.1 protocol includes a number of elements intended to make    caching work as well as possible. Because these elements are    inextricable from other aspects of the protocol, and because they    interact with each other, it is useful to describe the basic caching    design of HTTP separately from the detailed descriptions of methods,    headers, response codes, etc.     Caching would be useless if it did not significantly improve    performance. The goal of caching in HTTP/1.1 is to eliminate the need    to send requests in many cases, and to eliminate the need to send    full responses in many other cases. The former reduces the number of    network round-trips required for many operations; we use an    "expiration" mechanism for this purpose (see [section 13.2](about:blank#section-13.2)). The    latter reduces network bandwidth requirements; we use a "validation"    mechanism for this purpose (see [section 13.3](about:blank#section-13.3)).     Requirements for performance, availability, and disconnected    operation require us to be able to relax the goal of semantic    transparency. The HTTP/1.1 protocol allows origin servers, caches,      and clients to explicitly reduce transparency when necessary.    However, because non-transparent operation may confuse non-expert    users, and might be incompatible with certain server applications    (such as those for ordering merchandise), the protocol requires that    transparency be relaxed        - only by an explicit protocol-level request when relaxed by         client or origin server        - only with an explicit warning to the end user when relaxed by         cache or client     Therefore, the HTTP/1.1 protocol provides these important elements:        1. Protocol features that provide full semantic transparency when          this is required by all parties.        2. Protocol features that allow an origin server or user agent to          explicitly request and control non-transparent operation.        3. Protocol features that allow a cache to attach warnings to          responses that do not preserve the requested approximation of          semantic transparency.     A basic principle is that it must be possible for the clients to    detect any potential relaxation of semantic transparency.        Note: The server, cache, or client implementor might be faced with       design decisions not explicitly discussed in this specification.       If a decision might affect semantic transparency, the implementor       ought to err on the side of maintaining transparency unless a       careful and complete analysis shows significant benefits in       breaking transparency.

13.1.1 高速缓存的正确性

A correct cache MUST respond to a request with the most up-to-date    response held by the cache that is appropriate to the request (see    sections [13.2.5](about:blank#section-13.2.5), [13.2.6](about:blank#section-13.2.6), and [13.12](about:blank#section-13.12)) which meets one of the following    conditions:        1. It has been checked for equivalence with what the origin server          would have returned by revalidating the response with the          origin server ([section 13.3](about:blank#section-13.3));         2. It is "fresh enough" (see [section 13.2](about:blank#section-13.2)). In the default case,          this means it meets the least restrictive freshness requirement          of the client, origin server, and cache (see [section 14.9](about:blank#section-14.9)); if          the origin server so specifies, it is the freshness requirement          of the origin server alone.           If a stored response is not "fresh enough" by the most          restrictive freshness requirement of both the client and the          origin server, in carefully considered circumstances the cache          MAY still return the response with the appropriate Warning          header (see [section 13.1.5](about:blank#section-13.1.5) and 14.46), unless such a response          is prohibited (e.g., by a "no-store" cache-directive, or by a          "no-cache" cache-request-directive; see [section 14.9](about:blank#section-14.9)).        3. It is an appropriate 304 (Not Modified), 305 (Proxy Redirect),          or error (4xx or 5xx) response message.     If the cache can not communicate with the origin server, then a    correct cache SHOULD respond as above if the response can be    correctly served from the cache; if not it MUST return an error or    warning indicating that there was a communication failure.     If a cache receives a response (either an entire response, or a 304    (Not Modified) response) that it would normally forward to the    requesting client, and the received response is no longer fresh, the    cache SHOULD forward it to the requesting client without adding a new    Warning (but without removing any existing Warning headers). A cache    SHOULD NOT attempt to revalidate a response simply because that    response became stale in transit; this might lead to an infinite    loop. A user agent that receives a stale response without a Warning    MAY display a warning indication to the user.

13.1.2 警告

Whenever a cache returns a response that is neither first-hand nor    "fresh enough" (in the sense of condition 2 in [section 13.1.1](about:blank#section-13.1.1)), it    MUST attach a warning to that effect, using a Warning general-header.    The Warning header and the currently defined warnings are described    in [section 14.46](about:blank#section-14.46). The warning allows clients to take appropriate    action.     Warnings MAY be used for other purposes, both cache-related and    otherwise. The use of a warning, rather than an error status code,    distinguish these responses from true failures.     Warnings are assigned three digit warn-codes. The first digit    indicates whether the Warning MUST or MUST NOT be deleted from a    stored cache entry after a successful revalidation:      1xx  Warnings that describe the freshness or revalidation status of      the response, and so MUST be deleted after a successful      revalidation. 1XX warn-codes MAY be generated by a cache only when      validating a cached entry. It MUST NOT be generated by clients.     2xx  Warnings that describe some aspect of the entity body or entity      headers that is not rectified by a revalidation (for example, a      lossy compression of the entity bodies) and which MUST NOT be      deleted after a successful revalidation.     See [section 14.46](about:blank#section-14.46) for the definitions of the codes themselves.     HTTP/1.0 caches will cache all Warnings in responses, without    deleting the ones in the first category. Warnings in responses that    are passed to HTTP/1.0 caches carry an extra warning-date field,    which prevents a future HTTP/1.1 recipient from believing an    erroneously cached Warning.     Warnings also carry a warning text. The text MAY be in any    appropriate natural language (perhaps based on the client's Accept    headers), and include an OPTIONAL indication of what character set is    used.     Multiple warnings MAY be attached to a response (either by the origin    server or by a cache), including multiple warnings with the same code    number. For example, a server might provide the same warning with    texts in both English and Basque.     When multiple warnings are attached to a response, it might not be    practical or reasonable to display all of them to the user. This    version of HTTP does not specify strict priority rules for deciding    which warnings to display and in what order, but does suggest some    heuristics.

13.1.3 缓存控制机制

The basic cache mechanisms in HTTP/1.1 (server-specified expiration    times and validators) are implicit directives to caches. In some    cases, a server or client might need to provide explicit directives    to the HTTP caches. We use the Cache-Control header for this purpose.     The Cache-Control header allows a client or server to transmit a    variety of directives in either requests or responses. These    directives typically override the default caching algorithms. As a    general rule, if there is any apparent conflict between header    values, the most restrictive interpretation is applied (that is, the    one that is most likely to preserve semantic transparency). However,      in some cases, cache-control directives are explicitly specified as    weakening the approximation of semantic transparency (for example,    "max-stale" or "public").     The cache-control directives are described in detail in [section 14.9](about:blank#section-14.9).

13.1.4 显式用户代理警告

Many user agents make it possible for users to override the basic    caching mechanisms. For example, the user agent might allow the user    to specify that cached entities (even explicitly stale ones) are    never validated. Or the user agent might habitually add "Cache-    Control: max-stale=3600" to every request. The user agent SHOULD NOT    default to either non-transparent behavior, or behavior that results    in abnormally ineffective caching, but MAY be explicitly configured    to do so by an explicit action of the user.     If the user has overridden the basic caching mechanisms, the user    agent SHOULD explicitly indicate to the user whenever this results in    the display of information that might not meet the server's    transparency requirements (in particular, if the displayed entity is    known to be stale). Since the protocol normally allows the user agent    to determine if responses are stale or not, this indication need only    be displayed when this actually happens. The indication need not be a    dialog box; it could be an icon (for example, a picture of a rotting    fish) or some other indicator.     If the user has overridden the caching mechanisms in a way that would    abnormally reduce the effectiveness of caches, the user agent SHOULD    continually indicate this state to the user (for example, by a    display of a picture of currency in flames) so that the user does not    inadvertently consume excess resources or suffer from excessive    latency.

13.1.5 规则和警告的例外情况

In some cases, the operator of a cache MAY choose to configure it to    return stale responses even when not requested by clients. This    decision ought not be made lightly, but may be necessary for reasons    of availability or performance, especially when the cache is poorly    connected to the origin server. Whenever a cache returns a stale    response, it MUST mark it as such (using a Warning header) enabling    the client software to alert the user that there might be a potential    problem.      It also allows the user agent to take steps to obtain a first-hand or    fresh response. For this reason, a cache SHOULD NOT return a stale    response if the client explicitly requests a first-hand or fresh one,    unless it is impossible to comply for technical or policy reasons.

13.1.6 客户端控制的行为

While the origin server (and to a lesser extent, intermediate caches,    by their contribution to the age of a response) are the primary    source of expiration information, in some cases the client might need    to control a cache's decision about whether to return a cached    response without validating it. Clients do this using several    directives of the Cache-Control header.     A client's request MAY specify the maximum age it is willing to    accept of an unvalidated response; specifying a value of zero forces    the cache(s) to revalidate all responses. A client MAY also specify    the minimum time remaining before a response expires. Both of these    options increase constraints on the behavior of caches, and so cannot    further relax the cache's approximation of semantic transparency.     A client MAY also specify that it will accept stale responses, up to    some maximum amount of staleness. This loosens the constraints on the    caches, and so might violate the origin server's specified    constraints on semantic transparency, but might be necessary to    support disconnected operation, or high availability in the face of    poor connectivity.

13.2 到期模型

13.2.1 服务器指定的到期

HTTP caching works best when caches can entirely avoid making    requests to the origin server. The primary mechanism for avoiding    requests is for an origin server to provide an explicit expiration    time in the future, indicating that a response MAY be used to satisfy    subsequent requests. In other words, a cache can return a fresh    response without first contacting the server.     Our expectation is that servers will assign future explicit    expiration times to responses in the belief that the entity is not    likely to change, in a semantically significant way, before the    expiration time is reached. This normally preserves semantic    transparency, as long as the server's expiration times are carefully    chosen.      The expiration mechanism applies only to responses taken from a cache    and not to first-hand responses forwarded immediately to the    requesting client.     If an origin server wishes to force a semantically transparent cache    to validate every request, it MAY assign an explicit expiration time    in the past. This means that the response is always stale, and so the    cache SHOULD validate it before using it for subsequent requests. See    [section 14.9.4](about:blank#section-14.9.4) for a more restrictive way to force revalidation.     If an origin server wishes to force any HTTP/1.1 cache, no matter how    it is configured, to validate every request, it SHOULD use the "must-    revalidate" cache-control directive (see [section 14.9](about:blank#section-14.9)).     Servers specify explicit expiration times using either the Expires    header, or the max-age directive of the Cache-Control header.     An expiration time cannot be used to force a user agent to refresh    its display or reload a resource; its semantics apply only to caching    mechanisms, and such mechanisms need only check a resource's    expiration status when a new request for that resource is initiated.    See [section 13.13](about:blank#section-13.13) for an explanation of the difference between caches    and history mechanisms.

13.2.2 启发式过期

Since origin servers do not always provide explicit expiration times,    HTTP caches typically assign heuristic expiration times, employing    algorithms that use other header values (such as the Last-Modified    time) to estimate a plausible expiration time. The HTTP/1.1    specification does not provide specific algorithms, but does impose    worst-case constraints on their results. Since heuristic expiration    times might compromise semantic transparency, they ought to used    cautiously, and we encourage origin servers to provide explicit    expiration times as much as possible.

13.2.3 年龄计算

In order to know if a cached entry is fresh, a cache needs to know if    its age exceeds its freshness lifetime. We discuss how to calculate    the latter in [section 13.2.4](about:blank#section-13.2.4); this section describes how to calculate    the age of a response or cache entry.     In this discussion, we use the term "now" to mean "the current value    of the clock at the host performing the calculation." Hosts that use    HTTP, but especially hosts running origin servers and caches, SHOULD    use NTP [[28](about:blank#ref-28)] or some similar protocol to synchronize their clocks to    a globally accurate time standard.      HTTP/1.1 requires origin servers to send a Date header, if possible,    with every response, giving the time at which the response was    generated (see [section 14.18](about:blank#section-14.18)). We use the term "date\_value" to denote    the value of the Date header, in a form appropriate for arithmetic    operations.     HTTP/1.1 uses the Age response-header to convey the estimated age of    the response message when obtained from a cache. The Age field value    is the cache's estimate of the amount of time since the response was    generated or revalidated by the origin server.     In essence, the Age value is the sum of the time that the response    has been resident in each of the caches along the path from the    origin server, plus the amount of time it has been in transit along    network paths.     We use the term "age\_value" to denote the value of the Age header, in    a form appropriate for arithmetic operations.     A response's age can be calculated in two entirely independent ways:        1. now minus date\_value, if the local clock is reasonably well          synchronized to the origin server's clock. If the result is          negative, the result is replaced by zero.        2. age\_value, if all of the caches along the response path          implement HTTP/1.1.     Given that we have two independent ways to compute the age of a    response when it is received, we can combine these as         corrected\_received\_age = max(now - date\_value, age\_value)     and as long as we have either nearly synchronized clocks or all-    HTTP/1.1 paths, one gets a reliable (conservative) result.     Because of network-imposed delays, some significant interval might    pass between the time that a server generates a response and the time    it is received at the next outbound cache or client. If uncorrected,    this delay could result in improperly low ages.     Because the request that resulted in the returned Age value must have    been initiated prior to that Age value's generation, we can correct    for delays imposed by the network by recording the time at which the    request was initiated. Then, when an Age value is received, it MUST    be interpreted relative to the time the request was initiated, not      the time that the response was received. This algorithm results in    conservative behavior no matter how much delay is experienced. So, we    compute:        corrected\_initial\_age = corrected\_received\_age                             + (now - request\_time)     where "request\_time" is the time (according to the local clock) when    the request that elicited this response was sent.     Summary of age calculation algorithm, when a cache receives a    response:        /\*        \* age\_value        \*      is the value of Age: header received by the cache with        \*              this response.        \* date\_value        \*      is the value of the origin server's Date: header        \* request\_time        \*      is the (local) time when the cache made the request        \*              that resulted in this cached response        \* response\_time        \*      is the (local) time when the cache received the        \*              response        \* now        \*      is the current (local) time        \*/        apparent\_age = max(0, response\_time - date\_value);       corrected\_received\_age = max(apparent\_age, age\_value);       response\_delay = response\_time - request\_time;       corrected\_initial\_age = corrected\_received\_age + response\_delay;       resident\_time = now - response\_time;       current\_age   = corrected\_initial\_age + resident\_time;     The current\_age of a cache entry is calculated by adding the amount    of time (in seconds) since the cache entry was last validated by the    origin server to the corrected\_initial\_age. When a response is    generated from a cache entry, the cache MUST include a single Age    header field in the response with a value equal to the cache entry's    current\_age.     The presence of an Age header field in a response implies that a    response is not first-hand. However, the converse is not true, since    the lack of an Age header field in a response does not imply that the      response is first-hand unless all caches along the request path are    compliant with HTTP/1.1 (i.e., older HTTP caches did not implement    the Age header field).

13.2.4 到期计算

In order to decide whether a response is fresh or stale, we need to    compare its freshness lifetime to its age. The age is calculated as    described in [section 13.2.3](about:blank#section-13.2.3); this section describes how to calculate    the freshness lifetime, and to determine if a response has expired.    In the discussion below, the values can be represented in any form    appropriate for arithmetic operations.     We use the term "expires\_value" to denote the value of the Expires    header. We use the term "max\_age\_value" to denote an appropriate    value of the number of seconds carried by the "max-age" directive of    the Cache-Control header in a response (see [section 14.9.3](about:blank#section-14.9.3)).     The max-age directive takes priority over Expires, so if max-age is    present in a response, the calculation is simply:        freshness\_lifetime = max\_age\_value     Otherwise, if Expires is present in the response, the calculation is:        freshness\_lifetime = expires\_value - date\_value     Note that neither of these calculations is vulnerable to clock skew,    since all of the information comes from the origin server.     If none of Expires, Cache-Control: max-age, or Cache-Control: s-    maxage (see [section 14.9.3](about:blank#section-14.9.3)) appears in the response, and the response    does not include other restrictions on caching, the cache MAY compute    a freshness lifetime using a heuristic. The cache MUST attach Warning    113 to any response whose age is more than 24 hours if such warning    has not already been added.     Also, if the response does have a Last-Modified time, the heuristic    expiration value SHOULD be no more than some fraction of the interval    since that time. A typical setting of this fraction might be 10%.     The calculation to determine if a response has expired is quite    simple:        response\_is\_fresh = (freshness\_lifetime > current\_age)

13.2.5 消除期满值的歧义

Because expiration values are assigned optimistically, it is possible    for two caches to contain fresh values for the same resource that are    different.     If a client performing a retrieval receives a non-first-hand response    for a request that was already fresh in its own cache, and the Date    header in its existing cache entry is newer than the Date on the new    response, then the client MAY ignore the response. If so, it MAY    retry the request with a "Cache-Control: max-age=0" directive (see    [section 14.9](about:blank#section-14.9)), to force a check with the origin server.     If a cache has two fresh responses for the same representation with    different validators, it MUST use the one with the more recent Date    header. This situation might arise because the cache is pooling    responses from other caches, or because a client has asked for a    reload or a revalidation of an apparently fresh cache entry.

13.2.6 消除多重反应的歧义

Because a client might be receiving responses via multiple paths, so    that some responses flow through one set of caches and other    responses flow through a different set of caches, a client might    receive responses in an order different from that in which the origin    server sent them. We would like the client to use the most recently    generated response, even if older responses are still apparently    fresh.     Neither the entity tag nor the expiration value can impose an    ordering on responses, since it is possible that a later response    intentionally carries an earlier expiration time. The Date values are    ordered to a granularity of one second.     When a client tries to revalidate a cache entry, and the response it    receives contains a Date header that appears to be older than the one    for the existing entry, then the client SHOULD repeat the request    unconditionally, and include         Cache-Control: max-age=0     to force any intermediate caches to validate their copies directly    with the origin server, or         Cache-Control: no-cache     to force any intermediate caches to obtain a new copy from the origin    server.      If the Date values are equal, then the client MAY use either response    (or MAY, if it is being extremely prudent, request a new response).    Servers MUST NOT depend on clients being able to choose    deterministically between responses generated during the same second,    if their expiration times overlap.

13.3 验证模型

When a cache has a stale entry that it would like to use as a    response to a client's request, it first has to check with the origin    server (or possibly an intermediate cache with a fresh response) to    see if its cached entry is still usable. We call this "validating"    the cache entry. Since we do not want to have to pay the overhead of    retransmitting the full response if the cached entry is good, and we    do not want to pay the overhead of an extra round trip if the cached    entry is invalid, the HTTP/1.1 protocol supports the use of    conditional methods.     The key protocol features for supporting conditional methods are    those concerned with "cache validators." When an origin server    generates a full response, it attaches some sort of validator to it,    which is kept with the cache entry. When a client (user agent or    proxy cache) makes a conditional request for a resource for which it    has a cache entry, it includes the associated validator in the    request.     The server then checks that validator against the current validator    for the entity, and, if they match (see [section 13.3.3](about:blank#section-13.3.3)), it responds    with a special status code (usually, 304 (Not Modified)) and no    entity-body. Otherwise, it returns a full response (including    entity-body). Thus, we avoid transmitting the full response if the    validator matches, and we avoid an extra round trip if it does not    match.     In HTTP/1.1, a conditional request looks exactly the same as a normal    request for the same resource, except that it carries a special    header (which includes the validator) that implicitly turns the    method (usually, GET) into a conditional.     The protocol includes both positive and negative senses of cache-    validating conditions. That is, it is possible to request either that    a method be performed if and only if a validator matches or if and    only if no validators match.         Note: a response that lacks a validator may still be cached, and       served from cache until it expires, unless this is explicitly       prohibited by a cache-control directive. However, a cache cannot       do a conditional retrieval if it does not have a validator for the       entity, which means it will not be refreshable after it expires.

13.3.1 最后修改日期

The Last-Modified entity-header field value is often used as a cache    validator. In simple terms, a cache entry is considered to be valid    if the entity has not been modified since the Last-Modified value.

13.3.2 实体标记缓存验证器

The ETag response-header field value, an entity tag, provides for an    "opaque" cache validator. This might allow more reliable validation    in situations where it is inconvenient to store modification dates,    where the one-second resolution of HTTP date values is not    sufficient, or where the origin server wishes to avoid certain    paradoxes that might arise from the use of modification dates.     Entity Tags are described in [section 3.11](about:blank#section-3.11). The headers used with    entity tags are described in sections [14.19](about:blank#section-14.19), [14.24](about:blank#section-14.24), [14.26](about:blank#section-14.26) and [14.44](about:blank#section-14.44).

13.3.3 强弱验证者

Since both origin servers and caches will compare two validators to    decide if they represent the same or different entities, one normally    would expect that if the entity (the entity-body or any entity-    headers) changes in any way, then the associated validator would    change as well. If this is true, then we call this validator a    "strong validator."     However, there might be cases when a server prefers to change the    validator only on semantically significant changes, and not when    insignificant aspects of the entity change. A validator that does not    always change when the resource changes is a "weak validator."     Entity tags are normally "strong validators," but the protocol    provides a mechanism to tag an entity tag as "weak." One can think of    a strong validator as one that changes whenever the bits of an entity    changes, while a weak value changes whenever the meaning of an entity    changes. Alternatively, one can think of a strong validator as part    of an identifier for a specific entity, while a weak validator is    part of an identifier for a set of semantically equivalent entities.        Note: One example of a strong validator is an integer that is       incremented in stable storage every time an entity is changed.         An entity's modification time, if represented with one-second       resolution, could be a weak validator, since it is possible that       the resource might be modified twice during a single second.        Support for weak validators is optional. However, weak validators       allow for more efficient caching of equivalent objects; for       example, a hit counter on a site is probably good enough if it is       updated every few days or weeks, and any value during that period       is likely "good enough" to be equivalent.     A "use" of a validator is either when a client generates a request    and includes the validator in a validating header field, or when a    server compares two validators.     Strong validators are usable in any context. Weak validators are only    usable in contexts that do not depend on exact equality of an entity.    For example, either kind is usable for a conditional GET of a full    entity. However, only a strong validator is usable for a sub-range    retrieval, since otherwise the client might end up with an internally    inconsistent entity.     Clients MAY issue simple (non-subrange) GET requests with either weak    validators or strong validators. Clients MUST NOT use weak validators    in other forms of request.     The only function that the HTTP/1.1 protocol defines on validators is    comparison. There are two validator comparison functions, depending    on whether the comparison context allows the use of weak validators    or not:        - The strong comparison function: in order to be considered equal,         both validators MUST be identical in every way, and both MUST         NOT be weak.        - The weak comparison function: in order to be considered equal,         both validators MUST be identical in every way, but either or         both of them MAY be tagged as "weak" without affecting the         result.     An entity tag is strong unless it is explicitly tagged as weak.    [Section 3.11](about:blank#section-3.11) gives the syntax for entity tags.     A Last-Modified time, when used as a validator in a request, is    implicitly weak unless it is possible to deduce that it is strong,    using the following rules:        - The validator is being compared by an origin server to the         actual current validator for the entity and,         - That origin server reliably knows that the associated entity did         not change twice during the second covered by the presented         validator.     or        - The validator is about to be used by a client in an If-         Modified-Since or If-Unmodified-Since header, because the client         has a cache entry for the associated entity, and        - That cache entry includes a Date value, which gives the time         when the origin server sent the original response, and        - The presented Last-Modified time is at least 60 seconds before         the Date value.     or        - The validator is being compared by an intermediate cache to the         validator stored in its cache entry for the entity, and        - That cache entry includes a Date value, which gives the time         when the origin server sent the original response, and        - The presented Last-Modified time is at least 60 seconds before         the Date value.     This method relies on the fact that if two different responses were    sent by the origin server during the same second, but both had the    same Last-Modified time, then at least one of those responses would    have a Date value equal to its Last-Modified time. The arbitrary 60-    second limit guards against the possibility that the Date and Last-    Modified values are generated from different clocks, or at somewhat    different times during the preparation of the response. An    implementation MAY use a value larger than 60 seconds, if it is    believed that 60 seconds is too short.     If a client wishes to perform a sub-range retrieval on a value for    which it has only a Last-Modified time and no opaque validator, it    MAY do this only if the Last-Modified time is strong in the sense    described here.     A cache or origin server receiving a conditional request, other than    a full-body GET request, MUST use the strong comparison function to    evaluate the condition.     These rules allow HTTP/1.1 caches and clients to safely perform sub-    range retrievals on values that have been obtained from HTTP/1.0      servers.

13.3.4 何时使用实体标签和上次修改日期的规则

We adopt a set of rules and recommendations for origin servers,    clients, and caches regarding when various validator types ought to    be used, and for what purposes.     HTTP/1.1 origin servers:        - SHOULD send an entity tag validator unless it is not feasible to         generate one.        - MAY send a weak entity tag instead of a strong entity tag, if         performance considerations support the use of weak entity tags,         or if it is unfeasible to send a strong entity tag.        - SHOULD send a Last-Modified value if it is feasible to send one,         unless the risk of a breakdown in semantic transparency that         could result from using this date in an If-Modified-Since header         would lead to serious problems.     In other words, the preferred behavior for an HTTP/1.1 origin server    is to send both a strong entity tag and a Last-Modified value.     In order to be legal, a strong entity tag MUST change whenever the    associated entity value changes in any way. A weak entity tag SHOULD    change whenever the associated entity changes in a semantically    significant way.        Note: in order to provide semantically transparent caching, an       origin server must avoid reusing a specific strong entity tag       value for two different entities, or reusing a specific weak       entity tag value for two semantically different entities. Cache       entries might persist for arbitrarily long periods, regardless of       expiration times, so it might be inappropriate to expect that a       cache will never again attempt to validate an entry using a       validator that it obtained at some point in the past.     HTTP/1.1 clients:        - If an entity tag has been provided by the origin server, MUST         use that entity tag in any cache-conditional request (using If-         Match or If-None-Match).        - If only a Last-Modified value has been provided by the origin         server, SHOULD use that value in non-subrange cache-conditional         requests (using If-Modified-Since).         - If only a Last-Modified value has been provided by an HTTP/1.0         origin server, MAY use that value in subrange cache-conditional         requests (using If-Unmodified-Since:). The user agent SHOULD         provide a way to disable this, in case of difficulty.        - If both an entity tag and a Last-Modified value have been         provided by the origin server, SHOULD use both validators in         cache-conditional requests. This allows both HTTP/1.0 and         HTTP/1.1 caches to respond appropriately.     An HTTP/1.1 origin server, upon receiving a conditional request that    includes both a Last-Modified date (e.g., in an If-Modified-Since or    If-Unmodified-Since header field) and one or more entity tags (e.g.,    in an If-Match, If-None-Match, or If-Range header field) as cache    validators, MUST NOT return a response status of 304 (Not Modified)    unless doing so is consistent with all of the conditional header    fields in the request.     An HTTP/1.1 caching proxy, upon receiving a conditional request that    includes both a Last-Modified date and one or more entity tags as    cache validators, MUST NOT return a locally cached response to the    client unless that cached response is consistent with all of the    conditional header fields in the request.        Note: The general principle behind these rules is that HTTP/1.1       servers and clients should transmit as much non-redundant       information as is available in their responses and requests.       HTTP/1.1 systems receiving this information will make the most       conservative assumptions about the validators they receive.        HTTP/1.0 clients and caches will ignore entity tags. Generally,       last-modified values received or used by these systems will       support transparent and efficient caching, and so HTTP/1.1 origin       servers should provide Last-Modified values. In those rare cases       where the use of a Last-Modified value as a validator by an       HTTP/1.0 system could result in a serious problem, then HTTP/1.1       origin servers should not provide one.

13.3.5 非验证条件

The principle behind entity tags is that only the service author    knows the semantics of a resource well enough to select an    appropriate cache validation mechanism, and the specification of any    validator comparison function more complex than byte-equality would    open up a can of worms. Thus, comparisons of any other headers    (except Last-Modified, for compatibility with HTTP/1.0) are never    used for purposes of validating a cache entry.

13.4 响应可缓存性

Unless specifically constrained by a cache-control ([section 14.9](about:blank#section-14.9))    directive, a caching system MAY always store a successful response    (see [section 13.8](about:blank#section-13.8)) as a cache entry, MAY return it without validation    if it is fresh, and MAY return it after successful validation. If    there is neither a cache validator nor an explicit expiration time    associated with a response, we do not expect it to be cached, but    certain caches MAY violate this expectation (for example, when little    or no network connectivity is available). A client can usually detect    that such a response was taken from a cache by comparing the Date    header to the current time.        Note: some HTTP/1.0 caches are known to violate this expectation       without providing any Warning.     However, in some cases it might be inappropriate for a cache to    retain an entity, or to return it in response to a subsequent    request. This might be because absolute semantic transparency is    deemed necessary by the service author, or because of security or    privacy considerations. Certain cache-control directives are    therefore provided so that the server can indicate that certain    resource entities, or portions thereof, are not to be cached    regardless of other considerations.     Note that [section 14.8](about:blank#section-14.8) normally prevents a shared cache from saving    and returning a response to a previous request if that request    included an Authorization header.     A response received with a status code of 200, 203, 206, 300, 301 or    410 MAY be stored by a cache and used in reply to a subsequent    request, subject to the expiration mechanism, unless a cache-control    directive prohibits caching. However, a cache that does not support    the Range and Content-Range headers MUST NOT cache 206 (Partial    Content) responses.     A response received with any other status code (e.g. status codes 302    and 307) MUST NOT be returned in a reply to a subsequent request    unless there are cache-control directives or another header(s) that    explicitly allow it. For example, these include the following: an    Expires header ([section 14.21](about:blank#section-14.21)); a "max-age", "s-maxage",  "must-    revalidate", "proxy-revalidate", "public" or "private" cache-control    directive ([section 14.9](about:blank#section-14.9)).

13.5 构建来自缓存的响应

The purpose of an HTTP cache is to store information received in    response to requests for use in responding to future requests. In    many cases, a cache simply returns the appropriate parts of a    response to the requester. However, if the cache holds a cache entry    based on a previous response, it might have to combine parts of a new    response with what is held in the cache entry.

13.5.1 端到端和逐跳报头

For the purpose of defining the behavior of caches and non-caching    proxies, we divide HTTP headers into two categories:        - End-to-end headers, which are  transmitted to the ultimate         recipient of a request or response. End-to-end headers in         responses MUST be stored as part of a cache entry and MUST be         transmitted in any response formed from a cache entry.        - Hop-by-hop headers, which are meaningful only for a single         transport-level connection, and are not stored by caches or         forwarded by proxies.     The following HTTP/1.1 headers are hop-by-hop headers:        - Connection       - Keep-Alive       - Proxy-Authenticate       - Proxy-Authorization       - TE       - Trailers       - Transfer-Encoding       - Upgrade     All other headers defined by HTTP/1.1 are end-to-end headers.     Other hop-by-hop headers MUST be listed in a Connection header,    ([section 14.10](about:blank#section-14.10)) to be introduced into HTTP/1.1 (or later).

13.5.2 不可修改的标题

Some features of the HTTP/1.1 protocol, such as Digest    Authentication, depend on the value of certain end-to-end headers. A    transparent proxy SHOULD NOT modify an end-to-end header unless the    definition of that header requires or specifically allows that.      A transparent proxy MUST NOT modify any of the following fields in a    request or response, and it MUST NOT add any of these fields if not    already present:        - Content-Location        - Content-MD5        - ETag        - Last-Modified     A transparent proxy MUST NOT modify any of the following fields in a    response:        - Expires     but it MAY add any of these fields if not already present. If an    Expires header is added, it MUST be given a field-value identical to    that of the Date header in that response.     A  proxy MUST NOT modify or add any of the following fields in a    message that contains the no-transform cache-control directive, or in    any request:        - Content-Encoding        - Content-Range        - Content-Type     A non-transparent proxy MAY modify or add these fields to a message    that does not include no-transform, but if it does so, it MUST add a    Warning 214 (Transformation applied) if one does not already appear    in the message (see [section 14.46](about:blank#section-14.46)).        Warning: unnecessary modification of end-to-end headers might       cause authentication failures if stronger authentication       mechanisms are introduced in later versions of HTTP. Such       authentication mechanisms MAY rely on the values of header fields       not listed here.     The Content-Length field of a request or response is added or deleted    according to the rules in [section 4.4](about:blank#section-4.4). A transparent proxy MUST    preserve the entity-length ([section 7.2.2](about:blank#section-7.2.2)) of the entity-body,    although it MAY change the transfer-length ([section 4.4](about:blank#section-4.4)).

13.5.3 组合标题

When a cache makes a validating request to a server, and the server    provides a 304 (Not Modified) response or a 206 (Partial Content)    response, the cache then constructs a response to send to the    requesting client.     If the status code is 304 (Not Modified), the cache uses the entity-    body stored in the cache entry as the entity-body of this outgoing    response. If the status code is 206 (Partial Content) and the ETag or    Last-Modified headers match exactly, the cache MAY combine the    contents stored in the cache entry with the new contents received in    the response and use the result as the entity-body of this outgoing    response, (see 13.5.4).     The end-to-end headers stored in the cache entry are used for the    constructed response, except that        - any stored Warning headers with warn-code 1xx (see [section](about:blank#section-14.46) [14.46](about:blank#section-14.46)) MUST be deleted from the cache entry and the forwarded         response.        - any stored Warning headers with warn-code 2xx MUST be retained         in the cache entry and the forwarded response.        - any end-to-end headers provided in the 304 or 206 response MUST         replace the corresponding headers from the cache entry.     Unless the cache decides to remove the cache entry, it MUST also    replace the end-to-end headers stored with the cache entry with    corresponding headers received in the incoming response, except for    Warning headers as described immediately above. If a header field-    name in the incoming response matches more than one header in the    cache entry, all such old headers MUST be replaced.     In other words, the set of end-to-end headers received in the    incoming response overrides all corresponding end-to-end headers    stored with the cache entry (except for stored Warning headers with    warn-code 1xx, which are deleted even if not overridden).        Note: this rule allows an origin server to use a 304 (Not       Modified) or a 206 (Partial Content) response to update any header       associated with a previous response for the same entity or sub-       ranges thereof, although it might not always be meaningful or       correct to do so. This rule does not allow an origin server to use       a 304 (Not Modified) or a 206 (Partial Content) response to       entirely delete a header that it had provided with a previous       response.

13.5.4 组合字节范围

A response might transfer only a subrange of the bytes of an entity-    body, either because the request included one or more Range    specifications, or because a connection was broken prematurely. After    several such transfers, a cache might have received several ranges of    the same entity-body.     If a cache has a stored non-empty set of subranges for an entity, and    an incoming response transfers another subrange, the cache MAY    combine the new subrange with the existing set if both the following    conditions are met:        - Both the incoming response and the cache entry have a cache         validator.        - The two cache validators match using the strong comparison         function (see [section 13.3.3](about:blank#section-13.3.3)).     If either requirement is not met, the cache MUST use only the most    recent partial response (based on the Date values transmitted with    every response, and using the incoming response if these values are    equal or missing), and MUST discard the other partial information.

13.6 缓存协商响应

Use of server-driven content negotiation ([section 12.1](about:blank#section-12.1)), as indicated    by the presence of a Vary header field in a response, alters the    conditions and procedure by which a cache can use the response for    subsequent requests. See [section 14.44](about:blank#section-14.44) for use of the Vary header    field by servers.     A server SHOULD use the Vary header field to inform a cache of what    request-header fields were used to select among multiple    representations of a cacheable response subject to server-driven    negotiation. The set of header fields named by the Vary field value    is known as the "selecting" request-headers.     When the cache receives a subsequent request whose Request-URI    specifies one or more cache entries including a Vary header field,    the cache MUST NOT use such a cache entry to construct a response to    the new request unless all of the selecting request-headers present    in the new request match the corresponding stored request-headers in    the original request.     The selecting request-headers from two requests are defined to match    if and only if the selecting request-headers in the first request can    be transformed to the selecting request-headers in the second request      by adding or removing linear white space (LWS) at places where this    is allowed by the corresponding BNF, and/or combining multiple    message-header fields with the same field name following the rules    about message headers in [section 4.2](about:blank#section-4.2).     A Vary header field-value of "\*" always fails to match and subsequent    requests on that resource can only be properly interpreted by the    origin server.     If the selecting request header fields for the cached entry do not    match the selecting request header fields of the new request, then    the cache MUST NOT use a cached entry to satisfy the request unless    it first relays the new request to the origin server in a conditional    request and the server responds with 304 (Not Modified), including an    entity tag or Content-Location that indicates the entity to be used.     If an entity tag was assigned to a cached representation, the    forwarded request SHOULD be conditional and include the entity tags    in an If-None-Match header field from all its cache entries for the    resource. This conveys to the server the set of entities currently    held by the cache, so that if any one of these entities matches the    requested entity, the server can use the ETag header field in its 304    (Not Modified) response to tell the cache which entry is appropriate.    If the entity-tag of the new response matches that of an existing    entry, the new response SHOULD be used to update the header fields of    the existing entry, and the result MUST be returned to the client.     If any of the existing cache entries contains only partial content    for the associated entity, its entity-tag SHOULD NOT be included in    the If-None-Match header field unless the request is for a range that    would be fully satisfied by that entry.     If a cache receives a successful response whose Content-Location    field matches that of an existing cache entry for the same Request-    ]URI, whose entity-tag differs from that of the existing entry, and    whose Date is more recent than that of the existing entry, the    existing entry SHOULD NOT be returned in response to future requests    and SHOULD be deleted from the cache.

13.7 共享和非共享高速缓存

For reasons of security and privacy, it is necessary to make a    distinction between "shared" and "non-shared" caches. A non-shared    cache is one that is accessible only to a single user. Accessibility    in this case SHOULD be enforced by appropriate security mechanisms.    All other caches are considered to be "shared." Other sections of      this specification place certain constraints on the operation of    shared caches in order to prevent loss of privacy or failure of    access controls.

13.8 错误或不完整的响应缓存行为

A cache that receives an incomplete response (for example, with fewer    bytes of data than specified in a Content-Length header) MAY store    the response. However, the cache MUST treat this as a partial    response. Partial responses MAY be combined as described in [section](about:blank#section-13.5.4) [13.5.4](about:blank#section-13.5.4); the result might be a full response or might still be    partial. A cache MUST NOT return a partial response to a client    without explicitly marking it as such, using the 206 (Partial    Content) status code. A cache MUST NOT return a partial response    using a status code of 200 (OK).     If a cache receives a 5xx response while attempting to revalidate an    entry, it MAY either forward this response to the requesting client,    or act as if the server failed to respond. In the latter case, it MAY    return a previously received response unless the cached entry    includes the "must-revalidate" cache-control directive (see [section](about:blank#section-14.9) [14.9](about:blank#section-14.9)).

13.9 GET和HEAD的副作用

Unless the origin server explicitly prohibits the caching of their    responses, the application of GET and HEAD methods to any resources    SHOULD NOT have side effects that would lead to erroneous behavior if    these responses are taken from a cache. They MAY still have side    effects, but a cache is not required to consider such side effects in    its caching decisions. Caches are always expected to observe an    origin server's explicit restrictions on caching.     We note one exception to this rule: since some applications have    traditionally used GETs and HEADs with query URLs (those containing a    "?" in the rel\_path part) to perform operations with significant side    effects, caches MUST NOT treat responses to such URIs as fresh unless    the server provides an explicit expiration time. This specifically    means that responses from HTTP/1.0 servers for such URIs SHOULD NOT    be taken from a cache. See [section 9.1.1](about:blank#section-9.1.1) for related information.

13.10 更新或删除后无效

The effect of certain methods performed on a resource at the origin    server might cause one or more existing cache entries to become non-    transparently invalid. That is, although they might continue to be    "fresh," they do not accurately reflect what the origin server would    return for a new request on that resource.      There is no way for the HTTP protocol to guarantee that all such    cache entries are marked invalid. For example, the request that    caused the change at the origin server might not have gone through    the proxy where a cache entry is stored. However, several rules help    reduce the likelihood of erroneous behavior.     In this section, the phrase "invalidate an entity" means that the    cache will either remove all instances of that entity from its    storage, or will mark these as "invalid" and in need of a mandatory    revalidation before they can be returned in response to a subsequent    request.     Some HTTP methods MUST cause a cache to invalidate an entity. This is    either the entity referred to by the Request-URI, or by the Location    or Content-Location headers (if present). These methods are:        - PUT        - DELETE        - POST     In order to prevent denial of service attacks, an invalidation based    on the URI in a Location or Content-Location header MUST only be    performed if the host part is the same as in the Request-URI.     A cache that passes through requests for methods it does not    understand SHOULD invalidate any entities referred to by the    Request-URI.

13.11 强制性写入

All methods that might be expected to cause modifications to the    origin server's resources MUST be written through to the origin    server. This currently includes all methods except for GET and HEAD.    A cache MUST NOT reply to such a request from a client before having    transmitted the request to the inbound server, and having received a    corresponding response from the inbound server. This does not prevent    a proxy cache from sending a 100 (Continue) response before the    inbound server has sent its final reply.     The alternative (known as "write-back" or "copy-back" caching) is not    allowed in HTTP/1.1, due to the difficulty of providing consistent    updates and the problems arising from server, cache, or network    failure prior to write-back.

13.12 缓存替换

If a new cacheable (see sections [14.9.2](about:blank#section-14.9.2), [13.2.5](about:blank#section-13.2.5), [13.2.6](about:blank#section-13.2.6) and [13.8](about:blank#section-13.8))    response is received from a resource while any existing responses for    the same resource are cached, the cache SHOULD use the new response    to reply to the current request. It MAY insert it into cache storage    and MAY, if it meets all other requirements, use it to respond to any    future requests that would previously have caused the old response to    be returned. If it inserts the new response into cache storage  the    rules in [section 13.5.3](about:blank#section-13.5.3) apply.        Note: a new response that has an older Date header value than       existing cached responses is not cacheable.

13.13 历史列表

User agents often have history mechanisms, such as "Back" buttons and    history lists, which can be used to redisplay an entity retrieved    earlier in a session.     History mechanisms and caches are different. In particular history    mechanisms SHOULD NOT try to show a semantically transparent view of    the current state of a resource. Rather, a history mechanism is meant    to show exactly what the user saw at the time when the resource was    retrieved.     By default, an expiration time does not apply to history mechanisms.    If the entity is still in storage, a history mechanism SHOULD display    it even if the entity has expired, unless the user has specifically    configured the agent to refresh expired history documents.     This is not to be construed to prohibit the history mechanism from    telling the user that a view might be stale.        Note: if history list mechanisms unnecessarily prevent users from       viewing stale resources, this will tend to force service authors       to avoid using HTTP expiration controls and cache controls when       they would otherwise like to. Service authors may consider it       important that users not be presented with error messages or       warning messages when they use navigation controls (such as BACK)       to view previously fetched resources. Even though sometimes such       resources ought not to cached, or ought to expire quickly, user       interface considerations may force service authors to resort to       other means of preventing caching (e.g. "once-only" URLs) in order       not to suffer the effects of improperly functioning history       mechanisms.

14 标题字段定义

This section defines the syntax and semantics of all standard    HTTP/1.1 header fields. For entity-header fields, both sender and    recipient refer to either the client or the server, depending on who    sends and who receives the entity.

14.1 接收

The Accept request-header field can be used to specify certain media    types which are acceptable for the response. Accept headers can be    used to indicate that the request is specifically limited to a small    set of desired types, as in the case of a request for an in-line    image.         Accept         = "Accept" ":"                         #( media-range [ accept-params ] )         media-range    = ( "\*/\*"                         | ( type "/" "\*" )                         | ( type "/" subtype )                         ) \*( ";" parameter )        accept-params  = ";" "q" "=" qvalue \*( accept-extension )        accept-extension = ";" token [ "=" ( token | quoted-string ) ]     The asterisk "\*" character is used to group media types into ranges,    with "\*/\*" indicating all media types and "type/\*" indicating all    subtypes of that type. The media-range MAY include media type    parameters that are applicable to that range.     Each media-range MAY be followed by one or more accept-params,    beginning with the "q" parameter for indicating a relative quality    factor. The first "q" parameter (if any) separates the media-range    parameter(s) from the accept-params. Quality factors allow the user    or user agent to indicate the relative degree of preference for that    media-range, using the qvalue scale from 0 to 1 ([section 3.9](about:blank#section-3.9)). The    default value is q=1.        Note: Use of the "q" parameter name to separate media type       parameters from Accept extension parameters is due to historical       practice. Although this prevents any media type parameter named       "q" from being used with a media range, such an event is believed       to be unlikely given the lack of any "q" parameters in the IANA       media type registry and the rare usage of any media type       parameters in Accept. Future media types are discouraged from       registering any parameter named "q".      The example         Accept: audio/\*; q=0.2, audio/basic     SHOULD be interpreted as "I prefer audio/basic, but send me any audio    type if it is the best available after an 80% mark-down in quality."     If no Accept header field is present, then it is assumed that the    client accepts all media types. If an Accept header field is present,    and if the server cannot send a response which is acceptable    according to the combined Accept field value, then the server SHOULD    send a 406 (not acceptable) response.     A more elaborate example is         Accept: text/plain; q=0.5, text/html,                text/x-dvi; q=0.8, text/x-c     Verbally, this would be interpreted as "text/html and text/x-c are    the preferred media types, but if they do not exist, then send the    text/x-dvi entity, and if that does not exist, send the text/plain    entity."     Media ranges can be overridden by more specific media ranges or    specific media types. If more than one media range applies to a given    type, the most specific reference has precedence. For example,         Accept: text/\*, text/html, text/html;level=1, \*/\*     have the following precedence:         1) text/html;level=1        2) text/html        3) text/\*        4) \*/\*     The media type quality factor associated with a given type is    determined by finding the media range with the highest precedence    which matches that type. For example,         Accept: text/\*;q=0.3, text/html;q=0.7, text/html;level=1,                text/html;level=2;q=0.4, \*/\*;q=0.5     would cause the following values to be associated:         text/html;level=1         = 1        text/html                 = 0.7        text/plain                = 0.3          image/jpeg                = 0.5        text/html;level=2         = 0.4        text/html;level=3         = 0.7        Note: A user agent might be provided with a default set of quality       values for certain media ranges. However, unless the user agent is       a closed system which cannot interact with other rendering agents,       this default set ought to be configurable by the user.

14.2 字符集接收

The Accept-Charset request-header field can be used to indicate what    character sets are acceptable for the response. This field allows    clients capable of understanding more comprehensive or special-    purpose character sets to signal that capability to a server which is    capable of representing documents in those character sets.        Accept-Charset = "Accept-Charset" ":"               1#( ( charset | "\*" )[ ";" "q" "=" qvalue ] )      Character set values are described in [section 3.4](about:blank#section-3.4). Each charset MAY    be given an associated quality value which represents the user's    preference for that charset. The default value is q=1. An example is        Accept-Charset: iso-8859-5, unicode-1-1;q=0.8     The special value "\*", if present in the Accept-Charset field,    matches every character set (including ISO-8859-1) which is not    mentioned elsewhere in the Accept-Charset field. If no "\*" is present    in an Accept-Charset field, then all character sets not explicitly    mentioned get a quality value of 0, except for ISO-8859-1, which gets    a quality value of 1 if not explicitly mentioned.     If no Accept-Charset header is present, the default is that any    character set is acceptable. If an Accept-Charset header is present,    and if the server cannot send a response which is acceptable    according to the Accept-Charset header, then the server SHOULD send    an error response with the 406 (not acceptable) status code, though    the sending of an unacceptable response is also allowed.

14.3 编码接收

The Accept-Encoding request-header field is similar to Accept, but    restricts the content-codings ([section 3.5](about:blank#section-3.5)) that are acceptable in    the response.         Accept-Encoding  = "Accept-Encoding" ":"                             1#( codings [ ";" "q" "=" qvalue ] )        codings          = ( content-coding | "\*" )     Examples of its use are:         Accept-Encoding: compress, gzip        Accept-Encoding:        Accept-Encoding: \*        Accept-Encoding: compress;q=0.5, gzip;q=1.0        Accept-Encoding: gzip;q=1.0, identity; q=0.5, \*;q=0     A server tests whether a content-coding is acceptable, according to    an Accept-Encoding field, using these rules:        1. If the content-coding is one of the content-codings listed in          the Accept-Encoding field, then it is acceptable, unless it is          accompanied by a qvalue of 0. (As defined in [section 3.9](about:blank#section-3.9), a          qvalue of 0 means "not acceptable.")        2. The special "\*" symbol in an Accept-Encoding field matches any          available content-coding not explicitly listed in the header          field.        3. If multiple content-codings are acceptable, then the acceptable          content-coding with the highest non-zero qvalue is preferred.        4. The "identity" content-coding is always acceptable, unless          specifically refused because the Accept-Encoding field includes          "identity;q=0", or because the field includes "\*;q=0" and does          not explicitly include the "identity" content-coding. If the          Accept-Encoding field-value is empty, then only the "identity"          encoding is acceptable.     If an Accept-Encoding field is present in a request, and if the    server cannot send a response which is acceptable according to the    Accept-Encoding header, then the server SHOULD send an error response    with the 406 (Not Acceptable) status code.     If no Accept-Encoding field is present in a request, the server MAY    assume that the client will accept any content coding. In this case,    if "identity" is one of the available content-codings, then the    server SHOULD use the "identity" content-coding, unless it has    additional information that a different content-coding is meaningful    to the client.        Note: If the request does not include an Accept-Encoding field,       and if the "identity" content-coding is unavailable, then       content-codings commonly understood by HTTP/1.0 clients (i.e.,         "gzip" and "compress") are preferred; some older clients       improperly display messages sent with other content-codings.  The       server might also make this decision based on information about       the particular user-agent or client.        Note: Most HTTP/1.0 applications do not recognize or obey qvalues       associated with content-codings. This means that qvalues will not       work and are not permitted with x-gzip or x-compress.

14.4 语言接收

The Accept-Language request-header field is similar to Accept, but    restricts the set of natural languages that are preferred as a    response to the request. Language tags are defined in [section 3.10](about:blank#section-3.10).         Accept-Language = "Accept-Language" ":"                          1#( language-range [ ";" "q" "=" qvalue ] )        language-range  = ( ( 1\*8ALPHA \*( "-" 1\*8ALPHA ) ) | "\*" )     Each language-range MAY be given an associated quality value which    represents an estimate of the user's preference for the languages    specified by that range. The quality value defaults to "q=1". For    example,         Accept-Language: da, en-gb;q=0.8, en;q=0.7     would mean: "I prefer Danish, but will accept British English and    other types of English." A language-range matches a language-tag if    it exactly equals the tag, or if it exactly equals a prefix of the    tag such that the first tag character following the prefix is "-".    The special range "\*", if present in the Accept-Language field,    matches every tag not matched by any other range present in the    Accept-Language field.        Note: This use of a prefix matching rule does not imply that       language tags are assigned to languages in such a way that it is       always true that if a user understands a language with a certain       tag, then this user will also understand all languages with tags       for which this tag is a prefix. The prefix rule simply allows the       use of prefix tags if this is the case.     The language quality factor assigned to a language-tag by the    Accept-Language field is the quality value of the longest language-    range in the field that matches the language-tag. If no language-    range in the field matches the tag, the language quality factor    assigned is 0. If no Accept-Language header is present in the    request, the server      SHOULD assume that all languages are equally acceptable. If an    Accept-Language header is present, then all languages which are    assigned a quality factor greater than 0 are acceptable.     It might be contrary to the privacy expectations of the user to send    an Accept-Language header with the complete linguistic preferences of    the user in every request. For a discussion of this issue, see    [section 15.1.4](about:blank#section-15.1.4).     As intelligibility is highly dependent on the individual user, it is    recommended that client applications make the choice of linguistic    preference available to the user. If the choice is not made    available, then the Accept-Language header field MUST NOT be given in    the request.        Note: When making the choice of linguistic preference available to       the user, we remind implementors of  the fact that users are not       familiar with the details of language matching as described above,       and should provide appropriate guidance. As an example, users       might assume that on selecting "en-gb", they will be served any       kind of English document if British English is not available. A       user agent might suggest in such a case to add "en" to get the       best matching behavior.

14.5 范围接收

   The Accept-Ranges response-header field allows the server to       indicate its acceptance of range requests for a resource:            Accept-Ranges     = "Accept-Ranges" ":" acceptable-ranges           acceptable-ranges = 1#range-unit | "none"        Origin servers that accept byte-range requests MAY send            Accept-Ranges: bytes        but are not required to do so. Clients MAY generate byte-range       requests without having received this header for the resource       involved. Range units are defined in [section 3.12](about:blank#section-3.12).        Servers that do not accept any kind of range request for a       resource MAY send            Accept-Ranges: none        to advise the client not to attempt a range request.

14.6 年龄

   The Age response-header field conveys the sender's estimate of the       amount of time since the response (or its revalidation) was       generated at the origin server. A cached response is "fresh" if       its age does not exceed its freshness lifetime. Age values are       calculated as specified in [section 13.2.3](about:blank#section-13.2.3).             Age = "Age" ":" age-value            age-value = delta-seconds        Age values are non-negative decimal integers, representing time in       seconds.        If a cache receives a value larger than the largest positive       integer it can represent, or if any of its age calculations       overflows, it MUST transmit an Age header with a value of       2147483648 (2^31). An HTTP/1.1 server that includes a cache MUST       include an Age header field in every response generated from its       own cache. Caches SHOULD use an arithmetic type of at least 31       bits of range.

14.7 允许度

   The Allow entity-header field lists the set of methods supported       by the resource identified by the Request-URI. The purpose of this       field is strictly to inform the recipient of valid methods       associated with the resource. An Allow header field MUST be       present in a 405 (Method Not Allowed) response.            Allow   = "Allow" ":" #Method        Example of use:            Allow: GET, HEAD, PUT        This field cannot prevent a client from trying other methods.       However, the indications given by the Allow header field value       SHOULD be followed. The actual set of allowed methods is defined       by the origin server at the time of each request.        The Allow header field MAY be provided with a PUT request to       recommend the methods to be supported by the new or modified       resource. The server is not required to support these methods and       SHOULD include an Allow header in the response giving the actual       supported methods.         A proxy MUST NOT modify the Allow header field even if it does not       understand all the methods specified, since the user agent might       have other means of communicating with the origin server.

14.8 授权

   A user agent that wishes to authenticate itself with a server--       usually, but not necessarily, after receiving a 401 response--does       so by including an Authorization request-header field with the       request.  The Authorization field value consists of credentials       containing the authentication information of the user agent for       the realm of the resource being requested.            Authorization  = "Authorization" ":" credentials        HTTP access authentication is described in "HTTP Authentication:       Basic and Digest Access Authentication" [[43](about:blank#ref-43)]. If a request is       authenticated and a realm specified, the same credentials SHOULD       be valid for all other requests within this realm (assuming that       the authentication scheme itself does not require otherwise, such       as credentials that vary according to a challenge value or using       synchronized clocks).        When a shared cache (see [section 13.7](about:blank#section-13.7)) receives a request       containing an Authorization field, it MUST NOT return the       corresponding response as a reply to any other request, unless one       of the following specific exceptions holds:        1. If the response includes the "s-maxage" cache-control          directive, the cache MAY use that response in replying to a          subsequent request. But (if the specified maximum age has          passed) a proxy cache MUST first revalidate it with the origin          server, using the request-headers from the new request to allow          the origin server to authenticate the new request. (This is the          defined behavior for s-maxage.) If the response includes "s-          maxage=0", the proxy MUST always revalidate it before re-using          it.        2. If the response includes the "must-revalidate" cache-control          directive, the cache MAY use that response in replying to a          subsequent request. But if the response is stale, all caches          MUST first revalidate it with the origin server, using the          request-headers from the new request to allow the origin server          to authenticate the new request.        3. If the response includes the "public" cache-control directive,          it MAY be returned in reply to any subsequent request.

14.9 缓存控制

The Cache-Control general-header field is used to specify directives    that MUST be obeyed by all caching mechanisms along the    request/response chain. The directives specify behavior intended to    prevent caches from adversely interfering with the request or    response. These directives typically override the default caching    algorithms. Cache directives are unidirectional in that the presence    of a directive in a request does not imply that the same directive is    to be given in the response.        Note that HTTP/1.0 caches might not implement Cache-Control and       might only implement Pragma: no-cache (see [section 14.32](about:blank#section-14.32)).     Cache directives MUST be passed through by a proxy or gateway    application, regardless of their significance to that application,    since the directives might be applicable to all recipients along the    request/response chain. It is not possible to specify a cache-    directive for a specific cache.      Cache-Control   = "Cache-Control" ":" 1#cache-directive      cache-directive = cache-request-directive          | cache-response-directive      cache-request-directive =            "no-cache"                          ; [Section 14.9.1](about:blank#section-14.9.1)          | "no-store"                          ; [Section 14.9.2](about:blank#section-14.9.2)          | "max-age" "=" delta-seconds         ; [Section 14.9.3](about:blank#section-14.9.3), 14.9.4          | "max-stale" [ "=" delta-seconds ]   ; [Section 14.9.3](about:blank#section-14.9.3)          | "min-fresh" "=" delta-seconds       ; [Section 14.9.3](about:blank#section-14.9.3)          | "no-transform"                      ; [Section 14.9.5](about:blank#section-14.9.5)          | "only-if-cached"                    ; [Section 14.9.4](about:blank#section-14.9.4)          | cache-extension                     ; [Section 14.9.6](about:blank#section-14.9.6)       cache-response-directive =            "public"                               ; [Section 14.9.1](about:blank#section-14.9.1)          | "private" [ "=" <"> 1#field-name <"> ] ; [Section 14.9.1](about:blank#section-14.9.1)          | "no-cache" [ "=" <"> 1#field-name <"> ]; [Section 14.9.1](about:blank#section-14.9.1)          | "no-store"                             ; [Section 14.9.2](about:blank#section-14.9.2)          | "no-transform"                         ; [Section 14.9.5](about:blank#section-14.9.5)          | "must-revalidate"                      ; [Section 14.9.4](about:blank#section-14.9.4)          | "proxy-revalidate"                     ; [Section 14.9.4](about:blank#section-14.9.4)          | "max-age" "=" delta-seconds            ; [Section 14.9.3](about:blank#section-14.9.3)          | "s-maxage" "=" delta-seconds           ; [Section 14.9.3](about:blank#section-14.9.3)          | cache-extension                        ; [Section 14.9.6](about:blank#section-14.9.6)      cache-extension = token [ "=" ( token | quoted-string ) ]      When a directive appears without any 1#field-name parameter, the    directive applies to the entire request or response. When such a    directive appears with a 1#field-name parameter, it applies only to    the named field or fields, and not to the rest of the request or    response. This mechanism supports extensibility; implementations of    future versions of the HTTP protocol might apply these directives to    header fields not defined in HTTP/1.1.     The cache-control directives can be broken down into these general    categories:        - Restrictions on what are cacheable; these may only be imposed by         the origin server.        - Restrictions on what may be stored by a cache; these may be         imposed by either the origin server or the user agent.        - Modifications of the basic expiration mechanism; these may be         imposed by either the origin server or the user agent.        - Controls over cache revalidation and reload; these may only be         imposed by a user agent.        - Control over transformation of entities.        - Extensions to the caching system.

14.9.1 什么是缓存

By default, a response is cacheable if the requirements of the    request method, request header fields, and the response status    indicate that it is cacheable. [Section 13.4](about:blank#section-13.4) summarizes these defaults    for cacheability. The following Cache-Control response directives    allow an origin server to override the default cacheability of a    response:     public       Indicates that the response MAY be cached by any cache, even if it       would normally be non-cacheable or cacheable only within a non-       shared cache. (See also Authorization, [section 14.8](about:blank#section-14.8), for       additional details.)     private       Indicates that all or part of the response message is intended for       a single user and MUST NOT be cached by a shared cache. This       allows an origin server to state that the specified parts of the         response are intended for only one user and are not a valid       response for requests by other users. A private (non-shared) cache       MAY cache the response.         Note: This usage of the word private only controls where the        response may be cached, and cannot ensure the privacy of the        message content.     no-cache        If the no-cache directive does not specify a field-name, then a       cache MUST NOT use the response to satisfy a subsequent request       without successful revalidation with the origin server. This       allows an origin server to prevent caching even by caches that       have been configured to return stale responses to client requests.        If the no-cache directive does specify one or more field-names,       then a cache MAY use the response to satisfy a subsequent request,       subject to any other restrictions on caching. However, the       specified field-name(s) MUST NOT be sent in the response to a       subsequent request without successful revalidation with the origin       server. This allows an origin server to prevent the re-use of       certain header fields in a response, while still allowing caching       of the rest of the response.         Note: Most HTTP/1.0 caches will not recognize or obey this        directive.

14.9.2 缓存可能存储什么内容

no-store       The purpose of the no-store directive is to prevent the       inadvertent release or retention of sensitive information (for       example, on backup tapes). The no-store directive applies to the       entire message, and MAY be sent either in a response or in a       request. If sent in a request, a cache MUST NOT store any part of       either this request or any response to it. If sent in a response,       a cache MUST NOT store any part of either this response or the       request that elicited it. This directive applies to both non-       shared and shared caches. "MUST NOT store" in this context means       that the cache MUST NOT intentionally store the information in       non-volatile storage, and MUST make a best-effort attempt to       remove the information from volatile storage as promptly as       possible after forwarding it.        Even when this directive is associated with a response, users       might explicitly store such a response outside of the caching       system (e.g., with a "Save As" dialog). History buffers MAY store       such responses as part of their normal operation.         The purpose of this directive is to meet the stated requirements       of certain users and service authors who are concerned about       accidental releases of information via unanticipated accesses to       cache data structures. While the use of this directive might       improve privacy in some cases, we caution that it is NOT in any       way a reliable or sufficient mechanism for ensuring privacy. In       particular, malicious or compromised caches might not recognize or       obey this directive, and communications networks might be       vulnerable to eavesdropping.

14.9.3 基本到期机制的修改

The expiration time of an entity MAY be specified by the origin    server using the Expires header (see [section 14.21](about:blank#section-14.21)). Alternatively,    it MAY be specified using the max-age directive in a response. When    the max-age cache-control directive is present in a cached response,    the response is stale if its current age is greater than the age    value given (in seconds) at the time of a new request for that    resource. The max-age directive on a response implies that the    response is cacheable (i.e., "public") unless some other, more    restrictive cache directive is also present.     If a response includes both an Expires header and a max-age    directive, the max-age directive overrides the Expires header, even    if the Expires header is more restrictive. This rule allows an origin    server to provide, for a given response, a longer expiration time to    an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be    useful if certain HTTP/1.0 caches improperly calculate ages or    expiration times, perhaps due to desynchronized clocks.     Many HTTP/1.0 cache implementations will treat an Expires value that    is less than or equal to the response Date value as being equivalent    to the Cache-Control response directive "no-cache". If an HTTP/1.1    cache receives such a response, and the response does not include a    Cache-Control header field, it SHOULD consider the response to be    non-cacheable in order to retain compatibility with HTTP/1.0 servers.         Note: An origin server might wish to use a relatively new HTTP        cache control feature, such as the "private" directive, on a        network including older caches that do not understand that        feature. The origin server will need to combine the new feature        with an Expires field whose value is less than or equal to the        Date value. This will prevent older caches from improperly        caching the response.      s-maxage        If a response includes an s-maxage directive, then for a shared        cache (but not for a private cache), the maximum age specified by        this directive overrides the maximum age specified by either the        max-age directive or the Expires header. The s-maxage directive        also implies the semantics of the proxy-revalidate directive (see        [section 14.9.4](about:blank#section-14.9.4)), i.e., that the shared cache must not use the        entry after it becomes stale to respond to a subsequent request        without first revalidating it with the origin server. The s-        maxage directive is always ignored by a private cache.     Note that most older caches, not compliant with this specification,    do not implement any cache-control directives. An origin server    wishing to use a cache-control directive that restricts, but does not    prevent, caching by an HTTP/1.1-compliant cache MAY exploit the    requirement that the max-age directive overrides the Expires header,    and the fact that pre-HTTP/1.1-compliant caches do not observe the    max-age directive.     Other directives allow a user agent to modify the basic expiration    mechanism. These directives MAY be specified on a request:     max-age       Indicates that the client is willing to accept a response whose       age is no greater than the specified time in seconds. Unless max-       stale directive is also included, the client is not willing to       accept a stale response.     min-fresh       Indicates that the client is willing to accept a response whose       freshness lifetime is no less than its current age plus the       specified time in seconds. That is, the client wants a response       that will still be fresh for at least the specified number of       seconds.     max-stale       Indicates that the client is willing to accept a response that has       exceeded its expiration time. If max-stale is assigned a value,       then the client is willing to accept a response that has exceeded       its expiration time by no more than the specified number of       seconds. If no value is assigned to max-stale, then the client is       willing to accept a stale response of any age.     If a cache returns a stale response, either because of a max-stale    directive on a request, or because the cache is configured to    override the expiration time of a response, the cache MUST attach a    Warning header to the stale response, using Warning 110 (Response is    stale).      A cache MAY be configured to return stale responses without    validation, but only if this does not conflict with any "MUST"-level    requirements concerning cache validation (e.g., a "must-revalidate"    cache-control directive).     If both the new request and the cached entry include "max-age"    directives, then the lesser of the two values is used for determining    the freshness of the cached entry for that request.

14.9.4 缓存重新验证和重新加载控件

Sometimes a user agent might want or need to insist that a cache    revalidate its cache entry with the origin server (and not just with    the next cache along the path to the origin server), or to reload its    cache entry from the origin server. End-to-end revalidation might be    necessary if either the cache or the origin server has overestimated    the expiration time of the cached response. End-to-end reload may be    necessary if the cache entry has become corrupted for some reason.     End-to-end revalidation may be requested either when the client does    not have its own local cached copy, in which case we call it    "unspecified end-to-end revalidation", or when the client does have a    local cached copy, in which case we call it "specific end-to-end    revalidation."     The client can specify these three kinds of action using Cache-    Control request directives:     End-to-end reload       The request includes a "no-cache" cache-control directive or, for       compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field       names MUST NOT be included with the no-cache directive in a       request. The server MUST NOT use a cached copy when responding to       such a request.     Specific end-to-end revalidation       The request includes a "max-age=0" cache-control directive, which       forces each cache along the path to the origin server to       revalidate its own entry, if any, with the next cache or server.       The initial request includes a cache-validating conditional with       the client's current validator.     Unspecified end-to-end revalidation       The request includes "max-age=0" cache-control directive, which       forces each cache along the path to the origin server to       revalidate its own entry, if any, with the next cache or server.       The initial request does not include a cache-validating         conditional; the first cache along the path (if any) that holds a       cache entry for this resource includes a cache-validating       conditional with its current validator.     max-age       When an intermediate cache is forced, by means of a max-age=0       directive, to revalidate its own cache entry, and the client has       supplied its own validator in the request, the supplied validator       might differ from the validator currently stored with the cache       entry. In this case, the cache MAY use either validator in making       its own request without affecting semantic transparency.        However, the choice of validator might affect performance. The       best approach is for the intermediate cache to use its own       validator when making its request. If the server replies with 304       (Not Modified), then the cache can return its now validated copy       to the client with a 200 (OK) response. If the server replies with       a new entity and cache validator, however, the intermediate cache       can compare the returned validator with the one provided in the       client's request, using the strong comparison function. If the       client's validator is equal to the origin server's, then the       intermediate cache simply returns 304 (Not Modified). Otherwise,       it returns the new entity with a 200 (OK) response.        If a request includes the no-cache directive, it SHOULD NOT       include min-fresh, max-stale, or max-age.     only-if-cached       In some cases, such as times of extremely poor network       connectivity, a client may want a cache to return only those       responses that it currently has stored, and not to reload or       revalidate with the origin server. To do this, the client may       include the only-if-cached directive in a request. If it receives       this directive, a cache SHOULD either respond using a cached entry       that is consistent with the other constraints of the request, or       respond with a 504 (Gateway Timeout) status. However, if a group       of caches is being operated as a unified system with good internal       connectivity, such a request MAY be forwarded within that group of       caches.     must-revalidate       Because a cache MAY be configured to ignore a server's specified       expiration time, and because a client request MAY include a max-       stale directive (which has a similar effect), the protocol also       includes a mechanism for the origin server to require revalidation       of a cache entry on any subsequent use. When the must-revalidate       directive is present in a response received by a cache, that cache       MUST NOT use the entry after it becomes stale to respond to a         subsequent request without first revalidating it with the origin       server. (I.e., the cache MUST do an end-to-end revalidation every       time, if, based solely on the origin server's Expires or max-age       value, the cached response is stale.)        The must-revalidate directive is necessary to support reliable       operation for certain protocol features. In all circumstances an       HTTP/1.1 cache MUST obey the must-revalidate directive; in       particular, if the cache cannot reach the origin server for any       reason, it MUST generate a 504 (Gateway Timeout) response.        Servers SHOULD send the must-revalidate directive if and only if       failure to revalidate a request on the entity could result in       incorrect operation, such as a silently unexecuted financial       transaction. Recipients MUST NOT take any automated action that       violates this directive, and MUST NOT automatically provide an       unvalidated copy of the entity if revalidation fails.        Although this is not recommended, user agents operating under       severe connectivity constraints MAY violate this directive but, if       so, MUST explicitly warn the user that an unvalidated response has       been provided. The warning MUST be provided on each unvalidated       access, and SHOULD require explicit user confirmation.     proxy-revalidate       The proxy-revalidate directive has the same meaning as the must-       revalidate directive, except that it does not apply to non-shared       user agent caches. It can be used on a response to an       authenticated request to permit the user's cache to store and       later return the response without needing to revalidate it (since       it has already been authenticated once by that user), while still       requiring proxies that service many users to revalidate each time       (in order to make sure that each user has been authenticated).       Note that such authenticated responses also need the public cache       control directive in order to allow them to be cached at all.

14.9.5 无变换指令

no-transform       Implementors of intermediate caches (proxies) have found it useful       to convert the media type of certain entity bodies. A non-       transparent proxy might, for example, convert between image       formats in order to save cache space or to reduce the amount of       traffic on a slow link.        Serious operational problems occur, however, when these       transformations are applied to entity bodies intended for certain       kinds of applications. For example, applications for medical         imaging, scientific data analysis and those using end-to-end       authentication, all depend on receiving an entity body that is bit       for bit identical to the original entity-body.        Therefore, if a message includes the no-transform directive, an       intermediate cache or proxy MUST NOT change those headers that are       listed in [section 13.5.2](about:blank#section-13.5.2) as being subject to the no-transform       directive. This implies that the cache or proxy MUST NOT change       any aspect of the entity-body that is specified by these headers,       including the value of the entity-body itself.

14.9.6 高速缓存控制扩展

The Cache-Control header field can be extended through the use of one    or more cache-extension tokens, each with an optional assigned value.    Informational extensions (those which do not require a change in    cache behavior) MAY be added without changing the semantics of other    directives. Behavioral extensions are designed to work by acting as    modifiers to the existing base of cache directives. Both the new    directive and the standard directive are supplied, such that    applications which do not understand the new directive will default    to the behavior specified by the standard directive, and those that    understand the new directive will recognize it as modifying the    requirements associated with the standard directive. In this way,    extensions to the cache-control directives can be made without    requiring changes to the base protocol.     This extension mechanism depends on an HTTP cache obeying all of the    cache-control directives defined for its native HTTP-version, obeying    certain extensions, and ignoring all directives that it does not    understand.     For example, consider a hypothetical new response directive called    community which acts as a modifier to the private directive. We    define this new directive to mean that, in addition to any non-shared    cache, any cache which is shared only by members of the community    named within its value may cache the response. An origin server    wishing to allow the UCI community to use an otherwise private    response in their shared cache(s) could do so by including         Cache-Control: private, community="UCI"     A cache seeing this header field will act correctly even if the cache    does not understand the community cache-extension, since it will also    see and understand the private directive and thus default to the safe    behavior.      Unrecognized cache-directives MUST be ignored; it is assumed that any    cache-directive likely to be unrecognized by an HTTP/1.1 cache will    be combined with standard directives (or the response's default    cacheability) such that the cache behavior will remain minimally    correct even if the cache does not understand the extension(s).

14.10 连接

The Connection general-header field allows the sender to specify    options that are desired for that particular connection and MUST NOT    be communicated by proxies over further connections.     The Connection header has the following grammar:         Connection = "Connection" ":" 1#(connection-token)        connection-token  = token     HTTP/1.1 proxies MUST parse the Connection header field before a    message is forwarded and, for each connection-token in this field,    remove any header field(s) from the message with the same name as the    connection-token. Connection options are signaled by the presence of    a connection-token in the Connection header field, not by any    corresponding additional header field(s), since the additional header    field may not be sent if there are no parameters associated with that    connection option.     Message headers listed in the Connection header MUST NOT include    end-to-end headers, such as Cache-Control.     HTTP/1.1 defines the "close" connection option for the sender to    signal that the connection will be closed after completion of the    response. For example,         Connection: close     in either the request or the response header fields indicates that    the connection SHOULD NOT be considered `persistent' ([section 8.1](about:blank#section-8.1))    after the current request/response is complete.     HTTP/1.1 applications that do not support persistent connections MUST    include the "close" connection option in every message.     A system receiving an HTTP/1.0 (or lower-version) message that    includes a Connection header MUST, for each connection-token in this    field, remove and ignore any header field(s) from the message with    the same name as the connection-token. This protects against mistaken    forwarding of such header fields by pre-HTTP/1.1 proxies. See [section](about:blank#section-19.6.2) [19.6.2](about:blank#section-19.6.2).

14.11 内容编码

The Content-Encoding entity-header field is used as a modifier to the    media-type. When present, its value indicates what additional content    codings have been applied to the entity-body, and thus what decoding    mechanisms must be applied in order to obtain the media-type    referenced by the Content-Type header field. Content-Encoding is    primarily used to allow a document to be compressed without losing    the identity of its underlying media type.         Content-Encoding  = "Content-Encoding" ":" 1#content-coding     Content codings are defined in [section 3.5](about:blank#section-3.5). An example of its use is         Content-Encoding: gzip     The content-coding is a characteristic of the entity identified by    the Request-URI. Typically, the entity-body is stored with this    encoding and is only decoded before rendering or analogous usage.    However, a non-transparent proxy MAY modify the content-coding if the    new coding is known to be acceptable to the recipient, unless the    "no-transform" cache-control directive is present in the message.     If the content-coding of an entity is not "identity", then the    response MUST include a Content-Encoding entity-header ([section](about:blank#section-14.11) [14.11](about:blank#section-14.11)) that lists the non-identity content-coding(s) used.     If the content-coding of an entity in a request message is not    acceptable to the origin server, the server SHOULD respond with a    status code of 415 (Unsupported Media Type).     If multiple encodings have been applied to an entity, the content    codings MUST be listed in the order in which they were applied.    Additional information about the encoding parameters MAY be provided    by other entity-header fields not defined by this specification.

14.12 内容语言

The Content-Language entity-header field describes the natural    language(s) of the intended audience for the enclosed entity. Note    that this might not be equivalent to all the languages used within    the entity-body.         Content-Language  = "Content-Language" ":" 1#language-tag      Language tags are defined in [section 3.10](about:blank#section-3.10). The primary purpose of    Content-Language is to allow a user to identify and differentiate    entities according to the user's own preferred language. Thus, if the    body content is intended only for a Danish-literate audience, the    appropriate field is         Content-Language: da     If no Content-Language is specified, the default is that the content    is intended for all language audiences. This might mean that the    sender does not consider it to be specific to any natural language,    or that the sender does not know for which language it is intended.     Multiple languages MAY be listed for content that is intended for    multiple audiences. For example, a rendition of the "Treaty of    Waitangi," presented simultaneously in the original Maori and English    versions, would call for         Content-Language: mi, en     However, just because multiple languages are present within an entity    does not mean that it is intended for multiple linguistic audiences.    An example would be a beginner's language primer, such as "A First    Lesson in Latin," which is clearly intended to be used by an    English-literate audience. In this case, the Content-Language would    properly only include "en".     Content-Language MAY be applied to any media type -- it is not    limited to textual documents.

14.13 内容长度

The Content-Length entity-header field indicates the size of the    entity-body, in decimal number of OCTETs, sent to the recipient or,    in the case of the HEAD method, the size of the entity-body that    would have been sent had the request been a GET.         Content-Length    = "Content-Length" ":" 1\*DIGIT     An example is         Content-Length: 3495     Applications SHOULD use this field to indicate the transfer-length of    the message-body, unless this is prohibited by the rules in [section](about:blank#section-4.4) [4.4](about:blank#section-4.4).      Any Content-Length greater than or equal to zero is a valid value.    [Section 4.4](about:blank#section-4.4) describes how to determine the length of a message-body    if a Content-Length is not given.     Note that the meaning of this field is significantly different from    the corresponding definition in MIME, where it is an optional field    used within the "message/external-body" content-type. In HTTP, it    SHOULD be sent whenever the message's length can be determined prior    to being transferred, unless this is prohibited by the rules in    [section 4.4](about:blank#section-4.4).

14.14 内容位置

The Content-Location entity-header field MAY be used to supply the    resource location for the entity enclosed in the message when that    entity is accessible from a location separate from the requested    resource's URI. A server SHOULD provide a Content-Location for the    variant corresponding to the response entity; especially in the case    where a resource has multiple entities associated with it, and those    entities actually have separate locations by which they might be    individually accessed, the server SHOULD provide a Content-Location    for the particular variant which is returned.         Content-Location = "Content-Location" ":"                          ( absoluteURI | relativeURI )     The value of Content-Location also defines the base URI for the    entity.     The Content-Location value is not a replacement for the original    requested URI; it is only a statement of the location of the resource    corresponding to this particular entity at the time of the request.    Future requests MAY specify the Content-Location URI as the request-    URI if the desire is to identify the source of that particular    entity.     A cache cannot assume that an entity with a Content-Location    different from the URI used to retrieve it can be used to respond to    later requests on that Content-Location URI. However, the Content-    Location can be used to differentiate between multiple entities    retrieved from a single requested resource, as described in [section](about:blank#section-13.6) [13.6](about:blank#section-13.6).     If the Content-Location is a relative URI, the relative URI is    interpreted relative to the Request-URI.     The meaning of the Content-Location header in PUT or POST requests is    undefined; servers are free to ignore it in those cases.

14.15 内容MD5

The Content-MD5 entity-header field, as defined in [RFC 1864](https://tools.ietf.org/html/rfc1864) [[23](about:blank#ref-23)], is    an MD5 digest of the entity-body for the purpose of providing an    end-to-end message integrity check (MIC) of the entity-body. (Note: a    MIC is good for detecting accidental modification of the entity-body    in transit, but is not proof against malicious attacks.)          Content-MD5   = "Content-MD5" ":" md5-digest         md5-digest   = <base64 of 128 bit MD5 digest as per [RFC 1864](https://tools.ietf.org/html/rfc1864)>     The Content-MD5 header field MAY be generated by an origin server or    client to function as an integrity check of the entity-body. Only    origin servers or clients MAY generate the Content-MD5 header field;    proxies and gateways MUST NOT generate it, as this would defeat its    value as an end-to-end integrity check. Any recipient of the entity-    body, including gateways and proxies, MAY check that the digest value    in this header field matches that of the entity-body as received.     The MD5 digest is computed based on the content of the entity-body,    including any content-coding that has been applied, but not including    any transfer-encoding applied to the message-body. If the message is    received with a transfer-encoding, that encoding MUST be removed    prior to checking the Content-MD5 value against the received entity.     This has the result that the digest is computed on the octets of the    entity-body exactly as, and in the order that, they would be sent if    no transfer-encoding were being applied.     HTTP extends [RFC 1864](https://tools.ietf.org/html/rfc1864) to permit the digest to be computed for MIME    composite media-types (e.g., multipart/\* and message/rfc822), but    this does not change how the digest is computed as defined in the    preceding paragraph.     There are several consequences of this. The entity-body for composite    types MAY contain many body-parts, each with its own MIME and HTTP    headers (including Content-MD5, Content-Transfer-Encoding, and    Content-Encoding headers). If a body-part has a Content-Transfer-    Encoding or Content-Encoding header, it is assumed that the content    of the body-part has had the encoding applied, and the body-part is    included in the Content-MD5 digest as is -- i.e., after the    application. The Transfer-Encoding header field is not allowed within    body-parts.     Conversion of all line breaks to CRLF MUST NOT be done before    computing or checking the digest: the line break convention used in    the text actually transmitted MUST be left unaltered when computing    the digest.         Note: while the definition of Content-MD5 is exactly the same for       HTTP as in [RFC 1864](https://tools.ietf.org/html/rfc1864) for MIME entity-bodies, there are several ways       in which the application of Content-MD5 to HTTP entity-bodies       differs from its application to MIME entity-bodies. One is that       HTTP, unlike MIME, does not use Content-Transfer-Encoding, and       does use Transfer-Encoding and Content-Encoding. Another is that       HTTP more frequently uses binary content types than MIME, so it is       worth noting that, in such cases, the byte order used to compute       the digest is the transmission byte order defined for the type.       Lastly, HTTP allows transmission of text types with any of several       line break conventions and not just the canonical form using CRLF.

14.16 内容范围

The Content-Range entity-header is sent with a partial entity-body to    specify where in the full entity-body the partial body should be    applied. Range units are defined in [section 3.12](about:blank#section-3.12).         Content-Range = "Content-Range" ":" content-range-spec         content-range-spec      = byte-content-range-spec        byte-content-range-spec = bytes-unit SP                                  byte-range-resp-spec "/"                                  ( instance-length | "\*" )         byte-range-resp-spec = (first-byte-pos "-" last-byte-pos)                                       | "\*"        instance-length           = 1\*DIGIT     The header SHOULD indicate the total length of the full entity-body,    unless this length is unknown or difficult to determine. The asterisk    "\*" character means that the instance-length is unknown at the time    when the response was generated.     Unlike byte-ranges-specifier values (see [section 14.35.1](about:blank#section-14.35.1)), a byte-    range-resp-spec MUST only specify one range, and MUST contain    absolute byte positions for both the first and last byte of the    range.     A byte-content-range-spec with a byte-range-resp-spec whose last-    byte-pos value is less than its first-byte-pos value, or whose    instance-length value is less than or equal to its last-byte-pos    value, is invalid. The recipient of an invalid byte-content-range-    spec MUST ignore it and any content transferred along with it.     A server sending a response with status code 416 (Requested range not    satisfiable) SHOULD include a Content-Range field with a byte-range-    resp-spec of "\*". The instance-length specifies the current length of      the selected resource. A response with status code 206 (Partial    Content) MUST NOT include a Content-Range field with a byte-range-    resp-spec of "\*".     Examples of byte-content-range-spec values, assuming that the entity    contains a total of 1234 bytes:        . The first 500 bytes:        bytes 0-499/1234        . The second 500 bytes:        bytes 500-999/1234        . All except for the first 500 bytes:        bytes 500-1233/1234        . The last 500 bytes:        bytes 734-1233/1234     When an HTTP message includes the content of a single range (for    example, a response to a request for a single range, or to a request    for a set of ranges that overlap without any holes), this content is    transmitted with a Content-Range header, and a Content-Length header    showing the number of bytes actually transferred. For example,         HTTP/1.1 206 Partial content        Date: Wed, 15 Nov 1995 06:25:24 GMT        Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT        Content-Range: bytes 21010-47021/47022        Content-Length: 26012        Content-Type: image/gif     When an HTTP message includes the content of multiple ranges (for    example, a response to a request for multiple non-overlapping    ranges), these are transmitted as a multipart message. The multipart    media type used for this purpose is "multipart/byteranges" as defined    in appendix 19.2. See appendix 19.6.3 for a compatibility issue.     A response to a request for a single range MUST NOT be sent using the    multipart/byteranges media type.  A response to a request for    multiple ranges, whose result is a single range, MAY be sent as a    multipart/byteranges media type with one part. A client that cannot    decode a multipart/byteranges message MUST NOT ask for multiple    byte-ranges in a single request.     When a client requests multiple byte-ranges in one request, the    server SHOULD return them in the order that they appeared in the    request.      If the server ignores a byte-range-spec because it is syntactically    invalid, the server SHOULD treat the request as if the invalid Range    header field did not exist. (Normally, this means return a 200    response containing the full entity).     If the server receives a request (other than one including an If-    Range request-header field) with an unsatisfiable Range request-    header field (that is, all of whose byte-range-spec values have a    first-byte-pos value greater than the current length of the selected    resource), it SHOULD return a response code of 416 (Requested range    not satisfiable) ([section 10.4.17](about:blank#section-10.4.17)).        Note: clients cannot depend on servers to send a 416 (Requested       range not satisfiable) response instead of a 200 (OK) response for       an unsatisfiable Range request-header, since not all servers       implement this request-header.

14.17 内容类型

The Content-Type entity-header field indicates the media type of the    entity-body sent to the recipient or, in the case of the HEAD method,    the media type that would have been sent had the request been a GET.         Content-Type   = "Content-Type" ":" media-type     Media types are defined in [section 3.7](about:blank#section-3.7). An example of the field is         Content-Type: text/html; charset=ISO-8859-4     Further discussion of methods for identifying the media type of an    entity is provided in [section 7.2.1](about:blank#section-7.2.1).

14.18 日期

The Date general-header field represents the date and time at which    the message was originated, having the same semantics as orig-date in    [RFC 822](https://tools.ietf.org/html/rfc822). The field value is an HTTP-date, as described in [section](about:blank#section-3.3.1) [3.3.1](about:blank#section-3.3.1); it MUST be sent in [RFC 1123](https://tools.ietf.org/html/rfc1123) [[8](about:blank#ref-8)]-date format.         Date  = "Date" ":" HTTP-date     An example is         Date: Tue, 15 Nov 1994 08:12:31 GMT     Origin servers MUST include a Date header field in all responses,    except in these cases:         1. If the response status code is 100 (Continue) or 101 (Switching          Protocols), the response MAY include a Date header field, at          the server's option.        2. If the response status code conveys a server error, e.g. 500          (Internal Server Error) or 503 (Service Unavailable), and it is          inconvenient or impossible to generate a valid Date.        3. If the server does not have a clock that can provide a          reasonable approximation of the current time, its responses          MUST NOT include a Date header field. In this case, the rules          in [section 14.18.1](about:blank#section-14.18.1) MUST be followed.     A received message that does not have a Date header field MUST be    assigned one by the recipient if the message will be cached by that    recipient or gatewayed via a protocol which requires a Date. An HTTP    implementation without a clock MUST NOT cache responses without    revalidating them on every use. An HTTP cache, especially a shared    cache, SHOULD use a mechanism, such as NTP [[28](about:blank#ref-28)], to synchronize its    clock with a reliable external standard.     Clients SHOULD only send a Date header field in messages that include    an entity-body, as in the case of the PUT and POST requests, and even    then it is optional. A client without a clock MUST NOT send a Date    header field in a request.     The HTTP-date sent in a Date header SHOULD NOT represent a date and    time subsequent to the generation of the message. It SHOULD represent    the best available approximation of the date and time of message    generation, unless the implementation has no means of generating a    reasonably accurate date and time. In theory, the date ought to    represent the moment just before the entity is generated. In    practice, the date can be generated at any time during the message    origination without affecting its semantic value.

14.18.1 无时钟原点服务器操作

Some origin server implementations might not have a clock available.    An origin server without a clock MUST NOT assign Expires or Last-    Modified values to a response, unless these values were associated    with the resource by a system or user with a reliable clock. It MAY    assign an Expires value that is known, at or before server    configuration time, to be in the past (this allows "pre-expiration"    of responses without storing separate Expires values for each    resource).

14.19 ETag

The ETag response-header field provides the current value of the    entity tag for the requested variant. The headers used with entity    tags are described in sections [14.24](about:blank#section-14.24), [14.26](about:blank#section-14.26) and [14.44](about:blank#section-14.44). The entity tag    MAY be used for comparison with other entities from the same resource    (see [section 13.3.3](about:blank#section-13.3.3)).        ETag = "ETag" ":" entity-tag     Examples:        ETag: "xyzzy"       ETag: W/"xyzzy"       ETag: ""

14.20 期望

The Expect request-header field is used to indicate that particular    server behaviors are required by the client.        Expect       =  "Expect" ":" 1#expectation        expectation  =  "100-continue" | expectation-extension       expectation-extension =  token [ "=" ( token | quoted-string )                                \*expect-params ]       expect-params =  ";" token [ "=" ( token | quoted-string ) ]      A server that does not understand or is unable to comply with any of    the expectation values in the Expect field of a request MUST respond    with appropriate error status. The server MUST respond with a 417    (Expectation Failed) status if any of the expectations cannot be met    or, if there are other problems with the request, some other 4xx    status.     This header field is defined with extensible syntax to allow for    future extensions. If a server receives a request containing an    Expect field that includes an expectation-extension that it does not    support, it MUST respond with a 417 (Expectation Failed) status.     Comparison of expectation values is case-insensitive for unquoted    tokens (including the 100-continue token), and is case-sensitive for    quoted-string expectation-extensions.      The Expect mechanism is hop-by-hop: that is, an HTTP/1.1 proxy MUST    return a 417 (Expectation Failed) status if it receives a request    with an expectation that it cannot meet. However, the Expect    request-header itself is end-to-end; it MUST be forwarded if the    request is forwarded.     Many older HTTP/1.0 and HTTP/1.1 applications do not understand the    Expect header.     See [section 8.2.3](about:blank#section-8.2.3) for the use of the 100 (continue) status.

14.21 到期

The Expires entity-header field gives the date/time after which the    response is considered stale. A stale cache entry may not normally be    returned by a cache (either a proxy cache or a user agent cache)    unless it is first validated with the origin server (or with an    intermediate cache that has a fresh copy of the entity). See [section](about:blank#section-13.2) [13.2](about:blank#section-13.2) for further discussion of the expiration model.     The presence of an Expires field does not imply that the original    resource will change or cease to exist at, before, or after that    time.     The format is an absolute date and time as defined by HTTP-date in    [section 3.3.1](about:blank#section-3.3.1); it MUST be in [RFC 1123](https://tools.ietf.org/html/rfc1123) date format:        Expires = "Expires" ":" HTTP-date     An example of its use is        Expires: Thu, 01 Dec 1994 16:00:00 GMT        Note: if a response includes a Cache-Control field with the max-       age directive (see [section 14.9.3](about:blank#section-14.9.3)), that directive overrides the       Expires field.     HTTP/1.1 clients and caches MUST treat other invalid date formats,    especially including the value "0", as in the past (i.e., "already    expired").     To mark a response as "already expired," an origin server sends an    Expires date that is equal to the Date header value. (See the rules    for expiration calculations in [section 13.2.4](about:blank#section-13.2.4).)      To mark a response as "never expires," an origin server sends an    Expires date approximately one year from the time the response is    sent. HTTP/1.1 servers SHOULD NOT send Expires dates more than one    year in the future.     The presence of an Expires header field with a date value of some    time in the future on a response that otherwise would by default be    non-cacheable indicates that the response is cacheable, unless    indicated otherwise by a Cache-Control header field ([section 14.9](about:blank#section-14.9)).

14.22 From

The From request-header field, if given, SHOULD contain an Internet    e-mail address for the human user who controls the requesting user    agent. The address SHOULD be machine-usable, as defined by "mailbox"    in [RFC 822](https://tools.ietf.org/html/rfc822) [[9](about:blank#ref-9)] as updated by [RFC 1123](https://tools.ietf.org/html/rfc1123) [[8](about:blank#ref-8)]:         From   = "From" ":" mailbox     An example is:         From: webmaster@w3.org     This header field MAY be used for logging purposes and as a means for    identifying the source of invalid or unwanted requests. It SHOULD NOT    be used as an insecure form of access protection. The interpretation    of this field is that the request is being performed on behalf of the    person given, who accepts responsibility for the method performed. In    particular, robot agents SHOULD include this header so that the    person responsible for running the robot can be contacted if problems    occur on the receiving end.     The Internet e-mail address in this field MAY be separate from the    Internet host which issued the request. For example, when a request    is passed through a proxy the original issuer's address SHOULD be    used.     The client SHOULD NOT send the From header field without the user's    approval, as it might conflict with the user's privacy interests or    their site's security policy. It is strongly recommended that the    user be able to disable, enable, and modify the value of this field    at any time prior to a request.

14.23 Host

The Host request-header field specifies the Internet host and port    number of the resource being requested, as obtained from the original    URI given by the user or referring resource (generally an HTTP URL,      as described in [section 3.2.2](about:blank#section-3.2.2)). The Host field value MUST represent    the naming authority of the origin server or gateway given by the    original URL. This allows the origin server or gateway to    differentiate between internally-ambiguous URLs, such as the root "/"    URL of a server for multiple host names on a single IP address.         Host = "Host" ":" host [ ":" port ] ; [Section 3.2.2](about:blank#section-3.2.2)     A "host" without any trailing port information implies the default    port for the service requested (e.g., "80" for an HTTP URL). For    example, a request on the origin server for    <[http://www.w3.org/pub/WWW/](http://www.w3.org/pub/WWW/)> would properly include:         GET /pub/WWW/ HTTP/1.1        Host: www.w3.org     A client MUST include a Host header field in all HTTP/1.1 request    messages . If the requested URI does not include an Internet host    name for the service being requested, then the Host header field MUST    be given with an empty value. An HTTP/1.1 proxy MUST ensure that any    request message it forwards does contain an appropriate Host header    field that identifies the service being requested by the proxy. All    Internet-based HTTP/1.1 servers MUST respond with a 400 (Bad Request)    status code to any HTTP/1.1 request message which lacks a Host header    field.     See sections [5.2](about:blank#section-5.2) and [19.6.1.1](about:blank#section-19.6.1.1) for other requirements relating to    Host.

14.24 If-Match

The If-Match request-header field is used with a method to make it    conditional. A client that has one or more entities previously    obtained from the resource can verify that one of those entities is    current by including a list of their associated entity tags in the    If-Match header field. Entity tags are defined in [section 3.11](about:blank#section-3.11). The    purpose of this feature is to allow efficient updates of cached    information with a minimum amount of transaction overhead. It is also    used, on updating requests, to prevent inadvertent modification of    the wrong version of a resource. As a special case, the value "\*"    matches any current entity of the resource.         If-Match = "If-Match" ":" ( "\*" | 1#entity-tag )     If any of the entity tags match the entity tag of the entity that    would have been returned in the response to a similar GET request    (without the If-Match header) on that resource, or if "\*" is given      and any current entity exists for that resource, then the server MAY    perform the requested method as if the If-Match header field did not    exist.     A server MUST use the strong comparison function (see [section 13.3.3](about:blank#section-13.3.3))    to compare the entity tags in If-Match.     If none of the entity tags match, or if "\*" is given and no current    entity exists, the server MUST NOT perform the requested method, and    MUST return a 412 (Precondition Failed) response. This behavior is    most useful when the client wants to prevent an updating method, such    as PUT, from modifying a resource that has changed since the client    last retrieved it.     If the request would, without the If-Match header field, result in    anything other than a 2xx or 412 status, then the If-Match header    MUST be ignored.     The meaning of "If-Match: \*" is that the method SHOULD be performed    if the representation selected by the origin server (or by a cache,    possibly using the Vary mechanism, see [section 14.44](about:blank#section-14.44)) exists, and    MUST NOT be performed if the representation does not exist.     A request intended to update a resource (e.g., a PUT) MAY include an    If-Match header field to signal that the request method MUST NOT be    applied if the entity corresponding to the If-Match value (a single    entity tag) is no longer a representation of that resource. This    allows the user to indicate that they do not wish the request to be    successful if the resource has been changed without their knowledge.    Examples:         If-Match: "xyzzy"        If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"        If-Match: \*     The result of a request having both an If-Match header field and    either an If-None-Match or an If-Modified-Since header fields is    undefined by this specification.

14.25 If-Modified-Since

The If-Modified-Since request-header field is used with a method to    make it conditional: if the requested variant has not been modified    since the time specified in this field, an entity will not be    returned from the server; instead, a 304 (not modified) response will    be returned without any message-body.         If-Modified-Since = "If-Modified-Since" ":" HTTP-date      An example of the field is:         If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT     A GET method with an If-Modified-Since header and no Range header    requests that the identified entity be transferred only if it has    been modified since the date given by the If-Modified-Since header.    The algorithm for determining this includes the following cases:        a) If the request would normally result in anything other than a          200 (OK) status, or if the passed If-Modified-Since date is          invalid, the response is exactly the same as for a normal GET.          A date which is later than the server's current time is          invalid.        b) If the variant has been modified since the If-Modified-Since          date, the response is exactly the same as for a normal GET.        c) If the variant has not been modified since a valid If-          Modified-Since date, the server SHOULD return a 304 (Not          Modified) response.     The purpose of this feature is to allow efficient updates of cached    information with a minimum amount of transaction overhead.        Note: The Range request-header field modifies the meaning of If-       Modified-Since; see [section 14.35](about:blank#section-14.35) for full details.        Note: If-Modified-Since times are interpreted by the server, whose       clock might not be synchronized with the client.        Note: When handling an If-Modified-Since header field, some       servers will use an exact date comparison function, rather than a       less-than function, for deciding whether to send a 304 (Not       Modified) response. To get best results when sending an If-       Modified-Since header field for cache validation, clients are       advised to use the exact date string received in a previous Last-       Modified header field whenever possible.        Note: If a client uses an arbitrary date in the If-Modified-Since       header instead of a date taken from the Last-Modified header for       the same request, the client should be aware of the fact that this       date is interpreted in the server's understanding of time. The       client should consider unsynchronized clocks and rounding problems       due to the different encodings of time between the client and       server. This includes the possibility of race conditions if the       document has changed between the time it was first requested and       the If-Modified-Since date of a subsequent request, and the         possibility of clock-skew-related problems if the If-Modified-       Since date is derived from the client's clock without correction       to the server's clock. Corrections for different time bases       between client and server are at best approximate due to network       latency.     The result of a request having both an If-Modified-Since header field    and either an If-Match or an If-Unmodified-Since header fields is    undefined by this specification.

14.26 If-None-Match

The If-None-Match request-header field is used with a method to make    it conditional. A client that has one or more entities previously    obtained from the resource can verify that none of those entities is    current by including a list of their associated entity tags in the    If-None-Match header field. The purpose of this feature is to allow    efficient updates of cached information with a minimum amount of    transaction overhead. It is also used to prevent a method (e.g. PUT)    from inadvertently modifying an existing resource when the client    believes that the resource does not exist.     As a special case, the value "\*" matches any current entity of the    resource.         If-None-Match = "If-None-Match" ":" ( "\*" | 1#entity-tag )     If any of the entity tags match the entity tag of the entity that    would have been returned in the response to a similar GET request    (without the If-None-Match header) on that resource, or if "\*" is    given and any current entity exists for that resource, then the    server MUST NOT perform the requested method, unless required to do    so because the resource's modification date fails to match that    supplied in an If-Modified-Since header field in the request.    Instead, if the request method was GET or HEAD, the server SHOULD    respond with a 304 (Not Modified) response, including the cache-    related header fields (particularly ETag) of one of the entities that    matched. For all other request methods, the server MUST respond with    a status of 412 (Precondition Failed).     See [section 13.3.3](about:blank#section-13.3.3) for rules on how to determine if two entities tags    match. The weak comparison function can only be used with GET or HEAD    requests.      If none of the entity tags match, then the server MAY perform the    requested method as if the If-None-Match header field did not exist,    but MUST also ignore any If-Modified-Since header field(s) in the    request. That is, if no entity tags match, then the server MUST NOT    return a 304 (Not Modified) response.     If the request would, without the If-None-Match header field, result    in anything other than a 2xx or 304 status, then the If-None-Match    header MUST be ignored. (See [section 13.3.4](about:blank#section-13.3.4) for a discussion of    server behavior when both If-Modified-Since and If-None-Match appear    in the same request.)     The meaning of "If-None-Match: \*" is that the method MUST NOT be    performed if the representation selected by the origin server (or by    a cache, possibly using the Vary mechanism, see [section 14.44](about:blank#section-14.44))    exists, and SHOULD be performed if the representation does not exist.    This feature is intended to be useful in preventing races between PUT    operations.     Examples:         If-None-Match: "xyzzy"        If-None-Match: W/"xyzzy"        If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"        If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"        If-None-Match: \*     The result of a request having both an If-None-Match header field and    either an If-Match or an If-Unmodified-Since header fields is    undefined by this specification.

14.27 If-Range

If a client has a partial copy of an entity in its cache, and wishes    to have an up-to-date copy of the entire entity in its cache, it    could use the Range request-header with a conditional GET (using    either or both of If-Unmodified-Since and If-Match.) However, if the    condition fails because the entity has been modified, the client    would then have to make a second request to obtain the entire current    entity-body.     The If-Range header allows a client to "short-circuit" the second    request. Informally, its meaning is `if the entity is unchanged, send    me the part(s) that I am missing; otherwise, send me the entire new    entity'.          If-Range = "If-Range" ":" ( entity-tag | HTTP-date )      If the client has no entity tag for an entity, but does have a Last-    Modified date, it MAY use that date in an If-Range header. (The    server can distinguish between a valid HTTP-date and any form of    entity-tag by examining no more than two characters.) The If-Range    header SHOULD only be used together with a Range header, and MUST be    ignored if the request does not include a Range header, or if the    server does not support the sub-range operation.     If the entity tag given in the If-Range header matches the current    entity tag for the entity, then the server SHOULD provide the    specified sub-range of the entity using a 206 (Partial content)    response. If the entity tag does not match, then the server SHOULD    return the entire entity using a 200 (OK) response.

14.28 If-Unmodified-Since

The If-Unmodified-Since request-header field is used with a method to    make it conditional. If the requested resource has not been modified    since the time specified in this field, the server SHOULD perform the    requested operation as if the If-Unmodified-Since header were not    present.     If the requested variant has been modified since the specified time,    the server MUST NOT perform the requested operation, and MUST return    a 412 (Precondition Failed).        If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date     An example of the field is:         If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT     If the request normally (i.e., without the If-Unmodified-Since    header) would result in anything other than a 2xx or 412 status, the    If-Unmodified-Since header SHOULD be ignored.     If the specified date is invalid, the header is ignored.     The result of a request having both an If-Unmodified-Since header    field and either an If-None-Match or an If-Modified-Since header    fields is undefined by this specification.

14.29 Last-Modified

The Last-Modified entity-header field indicates the date and time at    which the origin server believes the variant was last modified.         Last-Modified  = "Last-Modified" ":" HTTP-date      An example of its use is         Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT     The exact meaning of this header field depends on the implementation    of the origin server and the nature of the original resource. For    files, it may be just the file system last-modified time. For    entities with dynamically included parts, it may be the most recent    of the set of last-modify times for its component parts. For database    gateways, it may be the last-update time stamp of the record. For    virtual objects, it may be the last time the internal state changed.     An origin server MUST NOT send a Last-Modified date which is later    than the server's time of message origination. In such cases, where    the resource's last modification would indicate some time in the    future, the server MUST replace that date with the message    origination date.     An origin server SHOULD obtain the Last-Modified value of the entity    as close as possible to the time that it generates the Date value of    its response. This allows a recipient to make an accurate assessment    of the entity's modification time, especially if the entity changes    near the time that the response is generated.     HTTP/1.1 servers SHOULD send Last-Modified whenever feasible.

14.30 Location

The Location response-header field is used to redirect the recipient    to a location other than the Request-URI for completion of the    request or identification of a new resource. For 201 (Created)    responses, the Location is that of the new resource which was created    by the request. For 3xx responses, the location SHOULD indicate the    server's preferred URI for automatic redirection to the resource. The    field value consists of a single absolute URI.         Location       = "Location" ":" absoluteURI     An example is:         Location: [http://www.w3.org/pub/WWW/People.html](http://www.w3.org/pub/WWW/People.html)        Note: The Content-Location header field ([section 14.14](about:blank#section-14.14)) differs       from Location in that the Content-Location identifies the original       location of the entity enclosed in the request. It is therefore       possible for a response to contain header fields for both Location       and Content-Location. Also see [section 13.10](about:blank#section-13.10) for cache       requirements of some methods.

14.31 Max-Forwards

The Max-Forwards request-header field provides a mechanism with the    TRACE ([section 9.8](about:blank#section-9.8)) and OPTIONS ([section 9.2](about:blank#section-9.2)) methods to limit the    number of proxies or gateways that can forward the request to the    next inbound server. This can be useful when the client is attempting    to trace a request chain which appears to be failing or looping in    mid-chain.         Max-Forwards   = "Max-Forwards" ":" 1\*DIGIT     The Max-Forwards value is a decimal integer indicating the remaining    number of times this request message may be forwarded.     Each proxy or gateway recipient of a TRACE or OPTIONS request    containing a Max-Forwards header field MUST check and update its    value prior to forwarding the request. If the received value is zero    (0), the recipient MUST NOT forward the request; instead, it MUST    respond as the final recipient. If the received Max-Forwards value is    greater than zero, then the forwarded message MUST contain an updated    Max-Forwards field with a value decremented by one (1).     The Max-Forwards header field MAY be ignored for all other methods    defined by this specification and for any extension methods for which    it is not explicitly referred to as part of that method definition.

14.32 Pragma

The Pragma general-header field is used to include implementation-    specific directives that might apply to any recipient along the    request/response chain. All pragma directives specify optional    behavior from the viewpoint of the protocol; however, some systems    MAY require that behavior be consistent with the directives.         Pragma            = "Pragma" ":" 1#pragma-directive        pragma-directive  = "no-cache" | extension-pragma        extension-pragma  = token [ "=" ( token | quoted-string ) ]     When the no-cache directive is present in a request message, an    application SHOULD forward the request toward the origin server even    if it has a cached copy of what is being requested. This pragma    directive has the same semantics as the no-cache cache-directive (see    [section 14.9](about:blank#section-14.9)) and is defined here for backward compatibility with    HTTP/1.0. Clients SHOULD include both header fields when a no-cache    request is sent to a server not known to be HTTP/1.1 compliant.      Pragma directives MUST be passed through by a proxy or gateway    application, regardless of their significance to that application,    since the directives might be applicable to all recipients along the    request/response chain. It is not possible to specify a pragma for a    specific recipient; however, any pragma directive not relevant to a    recipient SHOULD be ignored by that recipient.     HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had    sent "Cache-Control: no-cache". No new Pragma directives will be    defined in HTTP.        Note: because the meaning of "Pragma: no-cache as a response       header field is not actually specified, it does not provide a       reliable replacement for "Cache-Control: no-cache" in a response

14.33 代理验证

The Proxy-Authenticate response-header field MUST be included as part    of a 407 (Proxy Authentication Required) response. The field value    consists of a challenge that indicates the authentication scheme and    parameters applicable to the proxy for this Request-URI.         Proxy-Authenticate  = "Proxy-Authenticate" ":" 1#challenge     The HTTP access authentication process is described in "HTTP    Authentication: Basic and Digest Access Authentication" [[43](about:blank#ref-43)]. Unlike    WWW-Authenticate, the Proxy-Authenticate header field applies only to    the current connection and SHOULD NOT be passed on to downstream    clients. However, an intermediate proxy might need to obtain its own    credentials by requesting them from the downstream client, which in    some circumstances will appear as if the proxy is forwarding the    Proxy-Authenticate header field.

14.34 代理授权

The Proxy-Authorization request-header field allows the client to    identify itself (or its user) to a proxy which requires    authentication. The Proxy-Authorization field value consists of    credentials containing the authentication information of the user    agent for the proxy and/or realm of the resource being requested.         Proxy-Authorization     = "Proxy-Authorization" ":" credentials     The HTTP access authentication process is described in "HTTP    Authentication: Basic and Digest Access Authentication" [[43](about:blank#ref-43)] . Unlike    Authorization, the Proxy-Authorization header field applies only to    the next outbound proxy that demanded authentication using the Proxy-    Authenticate field. When multiple proxies are used in a chain, the      Proxy-Authorization header field is consumed by the first outbound    proxy that was expecting to receive credentials. A proxy MAY relay    the credentials from the client request to the next proxy if that is    the mechanism by which the proxies cooperatively authenticate a given    request.

14.35 范围

14.35.1 字节范围

Since all HTTP entities are represented in HTTP messages as sequences    of bytes, the concept of a byte range is meaningful for any HTTP    entity. (However, not all clients and servers need to support byte-    range operations.)     Byte range specifications in HTTP apply to the sequence of bytes in    the entity-body (not necessarily the same as the message-body).     A byte range operation MAY specify a single range of bytes, or a set    of ranges within a single entity.         ranges-specifier = byte-ranges-specifier        byte-ranges-specifier = bytes-unit "=" byte-range-set        byte-range-set  = 1#( byte-range-spec | suffix-byte-range-spec )        byte-range-spec = first-byte-pos "-" [last-byte-pos]        first-byte-pos  = 1\*DIGIT        last-byte-pos   = 1\*DIGIT     The first-byte-pos value in a byte-range-spec gives the byte-offset    of the first byte in a range. The last-byte-pos value gives the    byte-offset of the last byte in the range; that is, the byte    positions specified are inclusive. Byte offsets start at zero.     If the last-byte-pos value is present, it MUST be greater than or    equal to the first-byte-pos in that byte-range-spec, or the byte-    range-spec is syntactically invalid. The recipient of a byte-range-    set that includes one or more syntactically invalid byte-range-spec    values MUST ignore the header field that includes that byte-range-    set.     If the last-byte-pos value is absent, or if the value is greater than    or equal to the current length of the entity-body, last-byte-pos is    taken to be equal to one less than the current length of the entity-    body in bytes.     By its choice of last-byte-pos, a client can limit the number of    bytes retrieved without knowing the size of the entity.          suffix-byte-range-spec = "-" suffix-length        suffix-length = 1\*DIGIT     A suffix-byte-range-spec is used to specify the suffix of the    entity-body, of a length given by the suffix-length value. (That is,    this form specifies the last N bytes of an entity-body.) If the    entity is shorter than the specified suffix-length, the entire    entity-body is used.     If a syntactically valid byte-range-set includes at least one byte-    range-spec whose first-byte-pos is less than the current length of    the entity-body, or at least one suffix-byte-range-spec with a non-    zero suffix-length, then the byte-range-set is satisfiable.    Otherwise, the byte-range-set is unsatisfiable. If the byte-range-set    is unsatisfiable, the server SHOULD return a response with a status    of 416 (Requested range not satisfiable). Otherwise, the server    SHOULD return a response with a status of 206 (Partial Content)    containing the satisfiable ranges of the entity-body.     Examples of byte-ranges-specifier values (assuming an entity-body of    length 10000):        - The first 500 bytes (byte offsets 0-499, inclusive):  bytes=0-         499        - The second 500 bytes (byte offsets 500-999, inclusive):         bytes=500-999        - The final 500 bytes (byte offsets 9500-9999, inclusive):         bytes=-500        - Or bytes=9500-        - The first and last bytes only (bytes 0 and 9999):  bytes=0-0,-1        - Several legal but not canonical specifications of the second 500         bytes (byte offsets 500-999, inclusive):          bytes=500-600,601-999          bytes=500-700,601-999

14.35.2 范围检索请求

HTTP retrieval requests using conditional or unconditional GET    methods MAY request one or more sub-ranges of the entity, instead of    the entire entity, using the Range request header, which applies to    the entity returned as the result of the request:        Range = "Range" ":" ranges-specifier      A server MAY ignore the Range header. However, HTTP/1.1 origin    servers and intermediate caches ought to support byte ranges when    possible, since Range supports efficient recovery from partially    failed transfers, and supports efficient partial retrieval of large    entities.     If the server supports the Range header and the specified range or    ranges are appropriate for the entity:        - The presence of a Range header in an unconditional GET modifies         what is returned if the GET is otherwise successful. In other         words, the response carries a status code of 206 (Partial         Content) instead of 200 (OK).        - The presence of a Range header in a conditional GET (a request         using one or both of If-Modified-Since and If-None-Match, or         one or both of If-Unmodified-Since and If-Match) modifies what         is returned if the GET is otherwise successful and the         condition is true. It does not affect the 304 (Not Modified)         response returned if the conditional is false.     In some cases, it might be more appropriate to use the If-Range    header (see [section 14.27](about:blank#section-14.27)) in addition to the Range header.     If a proxy that supports ranges receives a Range request, forwards    the request to an inbound server, and receives an entire entity in    reply, it SHOULD only return the requested range to its client. It    SHOULD store the entire received response in its cache if that is    consistent with its cache allocation policies.

14.36 Referer

The Referer[sic] request-header field allows the client to specify,    for the server's benefit, the address (URI) of the resource from    which the Request-URI was obtained (the "referrer", although the    header field is misspelled.) The Referer request-header allows a    server to generate lists of back-links to resources for interest,    logging, optimized caching, etc. It also allows obsolete or mistyped    links to be traced for maintenance. The Referer field MUST NOT be    sent if the Request-URI was obtained from a source that does not have    its own URI, such as input from the user keyboard.         Referer        = "Referer" ":" ( absoluteURI | relativeURI )     Example:         Referer: [http://www.w3.org/hypertext/DataSources/Overview.html](http://www.w3.org/hypertext/DataSources/Overview.html)      If the field value is a relative URI, it SHOULD be interpreted    relative to the Request-URI. The URI MUST NOT include a fragment. See    [section 15.1.3](about:blank#section-15.1.3) for security considerations.

14.37 重试后

The Retry-After response-header field can be used with a 503 (Service    Unavailable) response to indicate how long the service is expected to    be unavailable to the requesting client. This field MAY also be used    with any 3xx (Redirection) response to indicate the minimum time the    user-agent is asked wait before issuing the redirected request. The    value of this field can be either an HTTP-date or an integer number    of seconds (in decimal) after the time of the response.         Retry-After  = "Retry-After" ":" ( HTTP-date | delta-seconds )     Two examples of its use are         Retry-After: Fri, 31 Dec 1999 23:59:59 GMT        Retry-After: 120     In the latter example, the delay is 2 minutes.

14.38 服务器

The Server response-header field contains information about the    software used by the origin server to handle the request. The field    can contain multiple product tokens ([section 3.8](about:blank#section-3.8)) and comments    identifying the server and any significant subproducts. The product    tokens are listed in order of their significance for identifying the    application.         Server         = "Server" ":" 1\*( product | comment )     Example:         Server: CERN/3.0 libwww/2.17     If the response is being forwarded through a proxy, the proxy    application MUST NOT modify the Server response-header. Instead, it    SHOULD include a Via field (as described in [section 14.45](about:blank#section-14.45)).        Note: Revealing the specific software version of the server might       allow the server machine to become more vulnerable to attacks       against software that is known to contain security holes. Server       implementors are encouraged to make this field a configurable       option.

14.39 TE

The TE request-header field indicates what extension transfer-codings    it is willing to accept in the response and whether or not it is    willing to accept trailer fields in a chunked transfer-coding. Its    value may consist of the keyword "trailers" and/or a comma-separated    list of extension transfer-coding names with optional accept    parameters (as described in [section 3.6](about:blank#section-3.6)).         TE        = "TE" ":" #( t-codings )        t-codings = "trailers" | ( transfer-extension [ accept-params ] )     The presence of the keyword "trailers" indicates that the client is    willing to accept trailer fields in a chunked transfer-coding, as    defined in [section 3.6.1](about:blank#section-3.6.1). This keyword is reserved for use with    transfer-coding values even though it does not itself represent a    transfer-coding.     Examples of its use are:         TE: deflate        TE:        TE: trailers, deflate;q=0.5     The TE header field only applies to the immediate connection.    Therefore, the keyword MUST be supplied within a Connection header    field ([section 14.10](about:blank#section-14.10)) whenever TE is present in an HTTP/1.1 message.     A server tests whether a transfer-coding is acceptable, according to    a TE field, using these rules:        1. The "chunked" transfer-coding is always acceptable. If the          keyword "trailers" is listed, the client indicates that it is          willing to accept trailer fields in the chunked response on          behalf of itself and any downstream clients. The implication is          that, if given, the client is stating that either all          downstream clients are willing to accept trailer fields in the          forwarded response, or that it will attempt to buffer the          response on behalf of downstream recipients.           Note: HTTP/1.1 does not define any means to limit the size of a          chunked response such that a client can be assured of buffering          the entire response.        2. If the transfer-coding being tested is one of the transfer-          codings listed in the TE field, then it is acceptable unless it          is accompanied by a qvalue of 0. (As defined in [section 3.9](about:blank#section-3.9), a          qvalue of 0 means "not acceptable.")         3. If multiple transfer-codings are acceptable, then the          acceptable transfer-coding with the highest non-zero qvalue is          preferred.  The "chunked" transfer-coding always has a qvalue          of 1.     If the TE field-value is empty or if no TE field is present, the only    transfer-coding  is "chunked". A message with no transfer-coding is    always acceptable.

14.40 预告

The Trailer general field value indicates that the given set of    header fields is present in the trailer of a message encoded with    chunked transfer-coding.         Trailer  = "Trailer" ":" 1#field-name     An HTTP/1.1 message SHOULD include a Trailer header field in a    message using chunked transfer-coding with a non-empty trailer. Doing    so allows the recipient to know which header fields to expect in the    trailer.     If no Trailer header field is present, the trailer SHOULD NOT include    any header fields. See [section 3.6.1](about:blank#section-3.6.1) for restrictions on the use of    trailer fields in a "chunked" transfer-coding.     Message header fields listed in the Trailer header field MUST NOT    include the following header fields:        . Transfer-Encoding        . Content-Length        . Trailer

14.41 传输编码

The Transfer-Encoding general-header field indicates what (if any)    type of transformation has been applied to the message body in order    to safely transfer it between the sender and the recipient. This    differs from the content-coding in that the transfer-coding is a    property of the message, not of the entity.       Transfer-Encoding       = "Transfer-Encoding" ":" 1#transfer-coding     Transfer-codings are defined in [section 3.6](about:blank#section-3.6). An example is:       Transfer-Encoding: chunked      If multiple encodings have been applied to an entity, the transfer-    codings MUST be listed in the order in which they were applied.    Additional information about the encoding parameters MAY be provided    by other entity-header fields not defined by this specification.     Many older HTTP/1.0 applications do not understand the Transfer-    Encoding header.

14.42 升级

The Upgrade general-header allows the client to specify what    additional communication protocols it supports and would like to use    if the server finds it appropriate to switch protocols. The server    MUST use the Upgrade header field within a 101 (Switching Protocols)    response to indicate which protocol(s) are being switched.         Upgrade        = "Upgrade" ":" 1#product     For example,         Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11     The Upgrade header field is intended to provide a simple mechanism    for transition from HTTP/1.1 to some other, incompatible protocol. It    does so by allowing the client to advertise its desire to use another    protocol, such as a later version of HTTP with a higher major version    number, even though the current request has been made using HTTP/1.1.    This eases the difficult transition between incompatible protocols by    allowing the client to initiate a request in the more commonly    supported protocol while indicating to the server that it would like    to use a "better" protocol if available (where "better" is determined    by the server, possibly according to the nature of the method and/or    resource being requested).     The Upgrade header field only applies to switching application-layer    protocols upon the existing transport-layer connection. Upgrade    cannot be used to insist on a protocol change; its acceptance and use    by the server is optional. The capabilities and nature of the    application-layer communication after the protocol change is entirely    dependent upon the new protocol chosen, although the first action    after changing the protocol MUST be a response to the initial HTTP    request containing the Upgrade header field.     The Upgrade header field only applies to the immediate connection.    Therefore, the upgrade keyword MUST be supplied within a Connection    header field ([section 14.10](about:blank#section-14.10)) whenever Upgrade is present in an    HTTP/1.1 message.      The Upgrade header field cannot be used to indicate a switch to a    protocol on a different connection. For that purpose, it is more    appropriate to use a 301, 302, 303, or 305 redirection response.     This specification only defines the protocol name "HTTP" for use by    the family of Hypertext Transfer Protocols, as defined by the HTTP    version rules of [section 3.1](about:blank#section-3.1) and future updates to this    specification. Any token can be used as a protocol name; however, it    will only be useful if both the client and server associate the name    with the same protocol.

14.43 用户代理

The User-Agent request-header field contains information about the    user agent originating the request. This is for statistical purposes,    the tracing of protocol violations, and automated recognition of user    agents for the sake of tailoring responses to avoid particular user    agent limitations. User agents SHOULD include this field with    requests. The field can contain multiple product tokens ([section 3.8](about:blank#section-3.8))    and comments identifying the agent and any subproducts which form a    significant part of the user agent. By convention, the product tokens    are listed in order of their significance for identifying the    application.         User-Agent     = "User-Agent" ":" 1\*( product | comment )     Example:         User-Agent: CERN-LineMode/2.15 libwww/2.17b3

14.44 变化

The Vary field value indicates the set of request-header fields that    fully determines, while the response is fresh, whether a cache is    permitted to use the response to reply to a subsequent request    without revalidation. For uncacheable or stale responses, the Vary    field value advises the user agent about the criteria that were used    to select the representation. A Vary field value of "\*" implies that    a cache cannot determine from the request headers of a subsequent    request whether this response is the appropriate representation. See    [section 13.6](about:blank#section-13.6) for use of the Vary header field by caches.         Vary  = "Vary" ":" ( "\*" | 1#field-name )     An HTTP/1.1 server SHOULD include a Vary header field with any    cacheable response that is subject to server-driven negotiation.    Doing so allows a cache to properly interpret future requests on that    resource and informs the user agent about the presence of negotiation      on that resource. A server MAY include a Vary header field with a    non-cacheable response that is subject to server-driven negotiation,    since this might provide the user agent with useful information about    the dimensions over which the response varies at the time of the    response.     A Vary field value consisting of a list of field-names signals that    the representation selected for the response is based on a selection    algorithm which considers ONLY the listed request-header field values    in selecting the most appropriate representation. A cache MAY assume    that the same selection will be made for future requests with the    same values for the listed field names, for the duration of time for    which the response is fresh.     The field-names given are not limited to the set of standard    request-header fields defined by this specification. Field names are    case-insensitive.     A Vary field value of "\*" signals that unspecified parameters not    limited to the request-headers (e.g., the network address of the    client), play a role in the selection of the response representation.    The "\*" value MUST NOT be generated by a proxy server; it may only be    generated by an origin server.

14.45 途径

The Via general-header field MUST be used by gateways and proxies to    indicate the intermediate protocols and recipients between the user    agent and the server on requests, and between the origin server and    the client on responses. It is analogous to the "Received" field of    [RFC 822](https://tools.ietf.org/html/rfc822) [[9](about:blank#ref-9)] and is intended to be used for tracking message forwards,    avoiding request loops, and identifying the protocol capabilities of    all senders along the request/response chain.        Via =  "Via" ":" 1#( received-protocol received-by [ comment ] )       received-protocol = [ protocol-name "/" ] protocol-version       protocol-name     = token       protocol-version  = token       received-by       = ( host [ ":" port ] ) | pseudonym       pseudonym         = token     The received-protocol indicates the protocol version of the message    received by the server or client along each segment of the    request/response chain. The received-protocol version is appended to    the Via field value when the message is forwarded so that information    about the protocol capabilities of upstream applications remains    visible to all recipients.      The protocol-name is optional if and only if it would be "HTTP". The    received-by field is normally the host and optional port number of a    recipient server or client that subsequently forwarded the message.    However, if the real host is considered to be sensitive information,    it MAY be replaced by a pseudonym. If the port is not given, it MAY    be assumed to be the default port of the received-protocol.     Multiple Via field values represents each proxy or gateway that has    forwarded the message. Each recipient MUST append its information    such that the end result is ordered according to the sequence of    forwarding applications.     Comments MAY be used in the Via header field to identify the software    of the recipient proxy or gateway, analogous to the User-Agent and    Server header fields. However, all comments in the Via field are    optional and MAY be removed by any recipient prior to forwarding the    message.     For example, a request message could be sent from an HTTP/1.0 user    agent to an internal proxy code-named "fred", which uses HTTP/1.1 to    forward the request to a public proxy at nowhere.com, which completes    the request by forwarding it to the origin server at www.ics.uci.edu.    The request received by www.ics.uci.edu would then have the following    Via header field:         Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1)     Proxies and gateways used as a portal through a network firewall    SHOULD NOT, by default, forward the names and ports of hosts within    the firewall region. This information SHOULD only be propagated if    explicitly enabled. If not enabled, the received-by host of any host    behind the firewall SHOULD be replaced by an appropriate pseudonym    for that host.     For organizations that have strong privacy requirements for hiding    internal structures, a proxy MAY combine an ordered subsequence of    Via header field entries with identical received-protocol values into    a single such entry. For example,         Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy          could be collapsed to         Via: 1.0 ricky, 1.1 mertz, 1.0 lucy      Applications SHOULD NOT combine multiple entries unless they are all    under the same organizational control and the hosts have already been    replaced by pseudonyms. Applications MUST NOT combine entries which    have different received-protocol values.

14.46 警告

The Warning general-header field is used to carry additional    information about the status or transformation of a message which    might not be reflected in the message. This information is typically    used to warn about a possible lack of semantic transparency from    caching operations or transformations applied to the entity body of    the message.     Warning headers are sent with responses using:         Warning    = "Warning" ":" 1#warning-value         warning-value = warn-code SP warn-agent SP warn-text                                              [SP warn-date]         warn-code  = 3DIGIT        warn-agent = ( host [ ":" port ] ) | pseudonym                        ; the name or pseudonym of the server adding                        ; the Warning header, for use in debugging        warn-text  = quoted-string        warn-date  = <"> HTTP-date <">     A response MAY carry more than one Warning header.     The warn-text SHOULD be in a natural language and character set that    is most likely to be intelligible to the human user receiving the    response. This decision MAY be based on any available knowledge, such    as the location of the cache or user, the Accept-Language field in a    request, the Content-Language field in a response, etc. The default    language is English and the default character set is ISO-8859-1.     If a character set other than ISO-8859-1 is used, it MUST be encoded    in the warn-text using the method described in [RFC 2047](https://tools.ietf.org/html/rfc2047) [[14](about:blank#ref-14)].     Warning headers can in general be applied to any message, however    some specific warn-codes are specific to caches and can only be    applied to response messages. New Warning headers SHOULD be added    after any existing Warning headers. A cache MUST NOT delete any    Warning header that it received with a message. However, if a cache    successfully validates a cache entry, it SHOULD remove any Warning    headers previously attached to that entry except as specified for      specific Warning codes. It MUST then add any Warning headers received    in the validating response. In other words, Warning headers are those    that would be attached to the most recent relevant response.     When multiple Warning headers are attached to a response, the user    agent ought to inform the user of as many of them as possible, in the    order that they appear in the response. If it is not possible to    inform the user of all of the warnings, the user agent SHOULD follow    these heuristics:        - Warnings that appear early in the response take priority over         those appearing later in the response.        - Warnings in the user's preferred character set take priority         over warnings in other character sets but with identical warn-         codes and warn-agents.     Systems that generate multiple Warning headers SHOULD order them with    this user agent behavior in mind.     Requirements for the behavior of caches with respect to Warnings are    stated in [section 13.1.2](about:blank#section-13.1.2).     This is a list of the currently-defined warn-codes, each with a    recommended warn-text in English, and a description of its meaning.     110 Response is stale      MUST be included whenever the returned response is stale.     111 Revalidation failed      MUST be included if a cache returns a stale response because an      attempt to revalidate the response failed, due to an inability to      reach the server.     112 Disconnected operation      SHOULD be included if the cache is intentionally disconnected from      the rest of the network for a period of time.     113 Heuristic expiration      MUST be included if the cache heuristically chose a freshness      lifetime greater than 24 hours and the response's age is greater      than 24 hours.     199 Miscellaneous warning      The warning text MAY include arbitrary information to be presented      to a human user, or logged. A system receiving this warning MUST      NOT take any automated action, besides presenting the warning to      the user.      214 Transformation applied      MUST be added by an intermediate cache or proxy if it applies any      transformation changing the content-coding (as specified in the      Content-Encoding header) or media-type (as specified in the      Content-Type header) of the response, or the entity-body of the      response, unless this Warning code already appears in the response.     299 Miscellaneous persistent warning      The warning text MAY include arbitrary information to be presented      to a human user, or logged. A system receiving this warning MUST      NOT take any automated action.     If an implementation sends a message with one or more Warning headers    whose version is HTTP/1.0 or lower, then the sender MUST include in    each warning-value a warn-date that matches the date in the response.     If an implementation receives a message with a warning-value that    includes a warn-date, and that warn-date is different from the Date    value in the response, then that warning-value MUST be deleted from    the message before storing, forwarding, or using it. (This prevents    bad consequences of naive caching of Warning header fields.) If all    of the warning-values are deleted for this reason, the Warning header    MUST be deleted as well.

14.47 WWW身份验证

The WWW-Authenticate response-header field MUST be included in 401    (Unauthorized) response messages. The field value consists of at    least one challenge that indicates the authentication scheme(s) and    parameters applicable to the Request-URI.         WWW-Authenticate  = "WWW-Authenticate" ":" 1#challenge     The HTTP access authentication process is described in "HTTP    Authentication: Basic and Digest Access Authentication" [[43](about:blank#ref-43)]. User    agents are advised to take special care in parsing the WWW-    Authenticate field value as it might contain more than one challenge,    or if more than one WWW-Authenticate header field is provided, the    contents of a challenge itself can contain a comma-separated list of    authentication parameters.

15 安全考虑

This section is meant to inform application developers, information    providers, and users of the security limitations in HTTP/1.1 as    described by this document. The discussion does not include    definitive solutions to the problems revealed, though it does make    some suggestions for reducing security risks.

15.1 个人信息

HTTP clients are often privy to large amounts of personal information    (e.g. the user's name, location, mail address, passwords, encryption    keys, etc.), and SHOULD be very careful to prevent unintentional    leakage of this information via the HTTP protocol to other sources.    We very strongly recommend that a convenient interface be provided    for the user to control dissemination of such information, and that    designers and implementors be particularly careful in this area.    History shows that errors in this area often create serious security    and/or privacy problems and generate highly adverse publicity for the    implementor's company.

15.1.1 滥用服务器日志信息

A server is in the position to save personal data about a user's    requests which might identify their reading patterns or subjects of    interest. This information is clearly confidential in nature and its    handling can be constrained by law in certain countries. People using    the HTTP protocol to provide data are responsible for ensuring that    such material is not distributed without the permission of any    individuals that are identifiable by the published results.

15.1.2 敏感信息的传递

Like any generic data transfer protocol, HTTP cannot regulate the    content of the data that is transferred, nor is there any a priori    method of determining the sensitivity of any particular piece of    information within the context of any given request. Therefore,    applications SHOULD supply as much control over this information as    possible to the provider of that information. Four header fields are    worth special mention in this context: Server, Via, Referer and From.     Revealing the specific software version of the server might allow the    server machine to become more vulnerable to attacks against software    that is known to contain security holes. Implementors SHOULD make the    Server header field a configurable option.     Proxies which serve as a portal through a network firewall SHOULD    take special precautions regarding the transfer of header information    that identifies the hosts behind the firewall. In particular, they    SHOULD remove, or replace with sanitized versions, any Via fields    generated behind the firewall.     The Referer header allows reading patterns to be studied and reverse    links drawn. Although it can be very useful, its power can be abused    if user details are not separated from the information contained in      the Referer. Even when the personal information has been removed, the    Referer header might indicate a private document's URI whose    publication would be inappropriate.     The information sent in the From field might conflict with the user's    privacy interests or their site's security policy, and hence it    SHOULD NOT be transmitted without the user being able to disable,    enable, and modify the contents of the field. The user MUST be able    to set the contents of this field within a user preference or    application defaults configuration.     We suggest, though do not require, that a convenient toggle interface    be provided for the user to enable or disable the sending of From and    Referer information.     The User-Agent ([section 14.43](about:blank#section-14.43)) or Server ([section 14.38](about:blank#section-14.38)) header    fields can sometimes be used to determine that a specific client or    server have a particular security hole which might be exploited.    Unfortunately, this same information is often used for other valuable    purposes for which HTTP currently has no better mechanism.

15.1.3 编码URI中的敏感信息

Because the source of a link might be private information or might    reveal an otherwise private information source, it is strongly    recommended that the user be able to select whether or not the    Referer field is sent. For example, a browser client could have a    toggle switch for browsing openly/anonymously, which would    respectively enable/disable the sending of Referer and From    information.     Clients SHOULD NOT include a Referer header field in a (non-secure)    HTTP request if the referring page was transferred with a secure    protocol.     Authors of services which use the HTTP protocol SHOULD NOT use GET    based forms for the submission of sensitive data, because this will    cause this data to be encoded in the Request-URI. Many existing    servers, proxies, and user agents will log the request URI in some    place where it might be visible to third parties. Servers can use    POST-based form submission instead

15.1.4 连接到接收头的隐私问题

Accept request-headers can reveal information about the user to all    servers which are accessed. The Accept-Language header in particular    can reveal information the user would consider to be of a private    nature, because the understanding of particular languages is often      strongly correlated to the membership of a particular ethnic group.    User agents which offer the option to configure the contents of an    Accept-Language header to be sent in every request are strongly    encouraged to let the configuration process include a message which    makes the user aware of the loss of privacy involved.     An approach that limits the loss of privacy would be for a user agent    to omit the sending of Accept-Language headers by default, and to ask    the user whether or not to start sending Accept-Language headers to a    server if it detects, by looking for any Vary response-header fields    generated by the server, that such sending could improve the quality    of service.     Elaborate user-customized accept header fields sent in every request,    in particular if these include quality values, can be used by servers    as relatively reliable and long-lived user identifiers. Such user    identifiers would allow content providers to do click-trail tracking,    and would allow collaborating content providers to match cross-server    click-trails or form submissions of individual users. Note that for    many users not behind a proxy, the network address of the host    running the user agent will also serve as a long-lived user    identifier. In environments where proxies are used to enhance    privacy, user agents ought to be conservative in offering accept    header configuration options to end users. As an extreme privacy    measure, proxies could filter the accept headers in relayed requests.    General purpose user agents which provide a high degree of header    configurability SHOULD warn users about the loss of privacy which can    be involved.

15.2 基于文件和路径名称的攻击

Implementations of HTTP origin servers SHOULD be careful to restrict    the documents returned by HTTP requests to be only those that were    intended by the server administrators. If an HTTP server translates    HTTP URIs directly into file system calls, the server MUST take    special care not to serve files that were not intended to be    delivered to HTTP clients. For example, UNIX, Microsoft Windows, and    other operating systems use ".." as a path component to indicate a    directory level above the current one. On such a system, an HTTP    server MUST disallow any such construct in the Request-URI if it    would otherwise allow access to a resource outside those intended to    be accessible via the HTTP server. Similarly, files intended for    reference only internally to the server (such as access control    files, configuration files, and script code) MUST be protected from    inappropriate retrieval, since they might contain sensitive    information. Experience has shown that minor bugs in such HTTP server    implementations have turned into security risks.

15.3 DNS欺骗

Clients using HTTP rely heavily on the Domain Name Service, and are    thus generally prone to security attacks based on the deliberate    mis-association of IP addresses and DNS names. Clients need to be    cautious in assuming the continuing validity of an IP number/DNS name    association.     In particular, HTTP clients SHOULD rely on their name resolver for    confirmation of an IP number/DNS name association, rather than    caching the result of previous host name lookups. Many platforms    already can cache host name lookups locally when appropriate, and    they SHOULD be configured to do so. It is proper for these lookups to    be cached, however, only when the TTL (Time To Live) information    reported by the name server makes it likely that the cached    information will remain useful.     If HTTP clients cache the results of host name lookups in order to    achieve a performance improvement, they MUST observe the TTL    information reported by DNS.     If HTTP clients do not observe this rule, they could be spoofed when    a previously-accessed server's IP address changes. As network    renumbering is expected to become increasingly common [[24](about:blank#ref-24)], the    possibility of this form of attack will grow. Observing this    requirement thus reduces this potential security vulnerability.     This requirement also improves the load-balancing behavior of clients    for replicated servers using the same DNS name and reduces the    likelihood of a user's experiencing failure in accessing sites which    use that strategy.

15.4 位置标题和欺骗

If a single server supports multiple organizations that do not trust    one another, then it MUST check the values of Location and Content-    Location headers in responses that are generated under control of    said organizations to make sure that they do not attempt to    invalidate resources over which they have no authority.

15.5 内容处置问题

RFC 1806 [35]中提到了HTTP中经常实现的Content-Disposition（请参阅第19.5.1节）标头，它有许多非常严重的安全考虑因素。内容处置不是HTTP标准的一部分，但是由于它被广泛实施，我们正在记录它对使用者的使用和风险。有关详细信息，请参阅RFC 2183 [49]（about：blank＃ref-49）。

15.6 身份验证凭证和空闲客户端

Existing HTTP clients and user agents typically retain authentication    information indefinitely. HTTP/1.1. does not provide a method for a    server to direct clients to discard these cached credentials. This is    a significant defect that requires further extensions to HTTP.    Circumstances under which credential caching can interfere with the    application's security model include but are not limited to:        - Clients which have been idle for an extended period following         which the server might wish to cause the client to reprompt the         user for credentials.        - Applications which include a session termination indication         (such as a `logout' or `commit' button on a page) after which         the server side of the application `knows' that there is no         further reason for the client to retain the credentials.     This is currently under separate study. There are a number of work-    arounds to parts of this problem, and we encourage the use of    password protection in screen savers, idle time-outs, and other    methods which mitigate the security problems inherent in this    problem. In particular, user agents which cache credentials are    encouraged to provide a readily accessible mechanism for discarding    cached credentials under user control.

15.7 代理和缓存

By their very nature, HTTP proxies are men-in-the-middle, and    represent an opportunity for man-in-the-middle attacks. Compromise of    the systems on which the proxies run can result in serious security    and privacy problems. Proxies have access to security-related    information, personal information about individual users and    organizations, and proprietary information belonging to users and    content providers. A compromised proxy, or a proxy implemented or    configured without regard to security and privacy considerations,    might be used in the commission of a wide range of potential attacks.     Proxy operators should protect the systems on which proxies run as    they would protect any system that contains or transports sensitive    information. In particular, log information gathered at proxies often    contains highly sensitive personal information, and/or information    about organizations. Log information should be carefully guarded, and    appropriate guidelines for use developed and followed. ([Section](about:blank#section-15.1.1) [15.1.1](about:blank#section-15.1.1)).      Caching proxies provide additional potential vulnerabilities, since    the contents of the cache represent an attractive target for    malicious exploitation. Because cache contents persist after an HTTP    request is complete, an attack on the cache can reveal information    long after a user believes that the information has been removed from    the network. Therefore, cache contents should be protected as    sensitive information.     Proxy implementors should consider the privacy and security    implications of their design and coding decisions, and of the    configuration options they provide to proxy operators (especially the    default configuration).     Users of a proxy need to be aware that they are no trustworthier than    the people who run the proxy; HTTP itself cannot solve this problem.     The judicious use of cryptography, when appropriate, may suffice to    protect against a broad range of security and privacy attacks. Such    cryptography is beyond the scope of the HTTP/1.1 specification.

15.7.1 对代理的拒绝服务攻击

They exist. They are hard to defend against. Research continues.    Beware.

16 本文致谢

This specification makes heavy use of the augmented BNF and generic    constructs defined by David H. Crocker for [RFC 822](https://tools.ietf.org/html/rfc822) [[9](about:blank#ref-9)]. Similarly, it    reuses many of the definitions provided by Nathaniel Borenstein and    Ned Freed for MIME [[7](about:blank#ref-7)]. We hope that their inclusion in this    specification will help reduce past confusion over the relationship    between HTTP and Internet mail message formats.     The HTTP protocol has evolved considerably over the years. It has    benefited from a large and active developer community--the many    people who have participated on the www-talk mailing list--and it is    that community which has been most responsible for the success of    HTTP and of the World-Wide Web in general. Marc Andreessen, Robert    Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois    Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob    McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc    VanHeyningen deserve special recognition for their efforts in    defining early aspects of the protocol.     This document has benefited greatly from the comments of all those    participating in the HTTP-WG. In addition to those already mentioned,    the following individuals have contributed to this specification:          Gary Adams                  Ross Patterson        Harald Tveit Alvestrand     Albert Lunde        Keith Ball                  John C. Mallery        Brian Behlendorf            Jean-Philippe Martin-Flatin        Paul Burchard               Mitra        Maurizio Codogno            David Morris        Mike Cowlishaw              Gavin Nicol        Roman Czyborra              Bill Perry        Michael A. Dolan            Jeffrey Perry        David J. Fiander            Scott Powers        Alan Freier                 Owen Rees        Marc Hedlund                Luigi Rizzo        Greg Herlihy                David Robinson        Koen Holtman                Marc Salomon        Alex Hopmann                Rich Salz        Bob Jernigan                Allan M. Schiffman        Shel Kaphan                 Jim Seidman        Rohit Khare                 Chuck Shotton        John Klensin                Eric W. Sink        Martijn Koster              Simon E. Spero        Alexei Kosut                Richard N. Taylor        David M. Kristol            Robert S. Thau        Daniel LaLiberte            Bill (BearHeart) Weinman        Ben Laurie                  Francois Yergeau        Paul J. Leach               Mary Ellen Zurko        Daniel DuBois               Josh Cohen      Much of the content and presentation of the caching design is due to    suggestions and comments from individuals including: Shel Kaphan,    Paul Leach, Koen Holtman, David Morris, and Larry Masinter.     Most of the specification of ranges is based on work originally done    by Ari Luotonen and John Franks, with additional input from Steve    Zilles.     Thanks to the "cave men" of Palo Alto. You know who you are.     Jim Gettys (the current editor of this document) wishes particularly    to thank Roy Fielding, the previous editor of this document, along    with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen    Holtman, John Franks, Josh Cohen, Alex Hopmann, Scott Lawrence, and    Larry Masinter for their help. And thanks go particularly to Jeff    Mogul and Scott Lawrence for performing the "MUST/MAY/SHOULD" audit.      The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik    Frystyk implemented [RFC 2068](https://tools.ietf.org/html/rfc2068) early, and we wish to thank them for the    discovery of many of the problems that this document attempts to    rectify.

17 参考文献

[[1]()] Alvestrand, H., "Tags for the Identification of Languages", [RFC](https://tools.ietf.org/html/rfc1766) [1766](https://tools.ietf.org/html/rfc1766), March 1995.     [[2]()] Anklesaria, F., McCahill, M., Lindner, P., Johnson, D., Torrey,        D. and B. Alberti, "The Internet Gopher Protocol (a distributed        document search and retrieval protocol)", [RFC 1436](https://tools.ietf.org/html/rfc1436), March 1993.     [[3]()] Berners-Lee, T., "Universal Resource Identifiers in WWW", [RFC](https://tools.ietf.org/html/rfc1630) [1630](https://tools.ietf.org/html/rfc1630), June 1994.     [[4]()] Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform Resource        Locators (URL)", [RFC 1738](https://tools.ietf.org/html/rfc1738), December 1994.     [[5]()] Berners-Lee, T. and D. Connolly, "Hypertext Markup Language -        2.0", [RFC 1866](https://tools.ietf.org/html/rfc1866), November 1995.     [[6]()] Berners-Lee, T., Fielding, R. and H. Frystyk, "Hypertext Transfer        Protocol -- HTTP/1.0", [RFC 1945](https://tools.ietf.org/html/rfc1945), May 1996.     [[7]()] Freed, N. and N. Borenstein, "Multipurpose Internet Mail        Extensions (MIME) Part One: Format of Internet Message Bodies",        [RFC 2045](https://tools.ietf.org/html/rfc2045), November 1996.     [[8]()] Braden, R., "Requirements for Internet Hosts -- Communication        Layers", STD 3, [RFC 1123](https://tools.ietf.org/html/rfc1123), October 1989.     [[9]()] Crocker, D., "Standard for The Format of ARPA Internet Text        Messages", STD 11, [RFC 822](https://tools.ietf.org/html/rfc822), August 1982.     [[10]()] Davis, F., Kahle, B., Morris, H., Salem, J., Shen, T., Wang, R.,         Sui, J., and M. Grinbaum, "WAIS Interface Protocol Prototype         Functional Specification," (v1.5), Thinking Machines         Corporation, April 1990.     [[11]()] Fielding, R., "Relative Uniform Resource Locators", [RFC 1808](https://tools.ietf.org/html/rfc1808),         June 1995.     [[12]()] Horton, M. and R. Adams, "Standard for Interchange of USENET         Messages", [RFC 1036](https://tools.ietf.org/html/rfc1036), December 1987.      [[13]()] Kantor, B. and P. Lapsley, "Network News Transfer Protocol", [RFC](https://tools.ietf.org/html/rfc977) [977](https://tools.ietf.org/html/rfc977), February 1986.     [[14]()] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part         Three: Message Header Extensions for Non-ASCII Text", [RFC 2047](https://tools.ietf.org/html/rfc2047),         November 1996.     [[15]()] Nebel, E. and L. Masinter, "Form-based File Upload in HTML", [RFC](https://tools.ietf.org/html/rfc1867) [1867](https://tools.ietf.org/html/rfc1867), November 1995.     [[16]()] Postel, J., "Simple Mail Transfer Protocol", STD 10, [RFC 821](https://tools.ietf.org/html/rfc821),         August 1982.     [[17]()] Postel, J., "Media Type Registration Procedure", [RFC 1590](https://tools.ietf.org/html/rfc1590),         November 1996.     [[18]()] Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9, [RFC](https://tools.ietf.org/html/rfc959) [959](https://tools.ietf.org/html/rfc959), October 1985.     [[19]()] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, [RFC 1700](https://tools.ietf.org/html/rfc1700),         October 1994.     [[20]()] Sollins, K. and L. Masinter, "Functional Requirements for         Uniform Resource Names", [RFC 1737](https://tools.ietf.org/html/rfc1737), December 1994.     [[21]()] US-ASCII. Coded Character Set - 7-Bit American Standard Code for         Information Interchange. Standard ANSI X3.4-1986, ANSI, 1986.     [[22]()] ISO-8859. International Standard -- Information Processing --         8-bit Single-Byte Coded Graphic Character Sets --         Part 1: Latin alphabet No. 1, ISO-8859-1:1987.         Part 2: Latin alphabet No. 2, ISO-8859-2, 1987.         Part 3: Latin alphabet No. 3, ISO-8859-3, 1988.         Part 4: Latin alphabet No. 4, ISO-8859-4, 1988.         Part 5: Latin/Cyrillic alphabet, ISO-8859-5, 1988.         Part 6: Latin/Arabic alphabet, ISO-8859-6, 1987.         Part 7: Latin/Greek alphabet, ISO-8859-7, 1987.         Part 8: Latin/Hebrew alphabet, ISO-8859-8, 1988.         Part 9: Latin alphabet No. 5, ISO-8859-9, 1990.     [[23]()] Meyers, J. and M. Rose, "The Content-MD5 Header Field", [RFC](https://tools.ietf.org/html/rfc1864) [1864](https://tools.ietf.org/html/rfc1864), October 1995.     [[24]()] Carpenter, B. and Y. Rekhter, "Renumbering Needs Work", [RFC](https://tools.ietf.org/html/rfc1900) [1900](https://tools.ietf.org/html/rfc1900), February 1996.     [[25]()] Deutsch, P., "GZIP file format specification version 4.3", [RFC](https://tools.ietf.org/html/rfc1952) [1952](https://tools.ietf.org/html/rfc1952), May 1996.      [[26]()] Venkata N. Padmanabhan, and Jeffrey C. Mogul. "Improving HTTP         Latency", Computer Networks and ISDN Systems, v. 28, pp. 25-35,         Dec. 1995. Slightly revised version of paper in Proc. 2nd         International WWW Conference '94: Mosaic and the Web, Oct. 1994,         which is available at         [http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/HTTPLat](http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/HTTPLatency.html) [ency.html](http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/HTTPLatency.html).     [[27]()] Joe Touch, John Heidemann, and Katia Obraczka. "Analysis of HTTP         Performance", <URL: [http://www.isi.edu/touch/pubs/http-perf96/](http://www.isi.edu/touch/pubs/http-perf96/)>,         ISI Research Report ISI/RR-98-463, (original report dated Aug.         1996), USC/Information Sciences Institute, August 1998.     [[28]()] Mills, D., "Network Time Protocol (Version 3) Specification,         Implementation and Analysis", [RFC 1305](https://tools.ietf.org/html/rfc1305), March 1992.     [[29]()] Deutsch, P., "DEFLATE Compressed Data Format Specification         version 1.3", [RFC 1951](https://tools.ietf.org/html/rfc1951), May 1996.     [[30]()] S. Spero, "Analysis of HTTP Performance Problems,"         [http://sunsite.unc.edu/mdma-release/http-prob.html](http://sunsite.unc.edu/mdma-release/http-prob.html).     [[31]()] Deutsch, P. and J. Gailly, "ZLIB Compressed Data Format         Specification version 3.3", [RFC 1950](https://tools.ietf.org/html/rfc1950), May 1996.     [[32]()] Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P.,         Luotonen, A., Sink, E. and L. Stewart, "An Extension to HTTP:         Digest Access Authentication", [RFC 2069](https://tools.ietf.org/html/rfc2069), January 1997.     [[33]()] Fielding, R., Gettys, J., Mogul, J., Frystyk, H. and T.         Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", [RFC](https://tools.ietf.org/html/rfc2068) [2068](https://tools.ietf.org/html/rfc2068), January 1997.     [[34]()] Bradner, S., "Key words for use in RFCs to Indicate Requirement         Levels", [BCP 14](https://tools.ietf.org/html/bcp14), [RFC 2119](https://tools.ietf.org/html/rfc2119), March 1997.     [[35]()] Troost, R. and Dorner, S., "Communicating Presentation         Information in Internet Messages: The Content-Disposition         Header", [RFC 1806](https://tools.ietf.org/html/rfc1806), June 1995.     [[36]()] Mogul, J., Fielding, R., Gettys, J. and H. Frystyk, "Use and         Interpretation of HTTP Version Numbers", [RFC 2145](https://tools.ietf.org/html/rfc2145), May 1997.         [jg639]     [[37]()] Palme, J., "Common Internet Message Headers", [RFC 2076](https://tools.ietf.org/html/rfc2076), February         1997. [jg640]      [[38]()] Yergeau, F., "UTF-8, a transformation format of Unicode and         ISO-10646", [RFC 2279](https://tools.ietf.org/html/rfc2279), January 1998. [jg641]     [[39]()] Nielsen, H.F., Gettys, J., Baird-Smith, A., Prud'hommeaux, E.,         Lie, H., and C. Lilley. "Network Performance Effects of         HTTP/1.1, CSS1, and PNG," Proceedings of ACM SIGCOMM '97, Cannes         France, September 1997.[jg642]     [[40]()] Freed, N. and N. Borenstein, "Multipurpose Internet Mail         Extensions (MIME) Part Two: Media Types", [RFC 2046](https://tools.ietf.org/html/rfc2046), November         1996. [jg643]     [[41]()] Alvestrand, H., "IETF Policy on Character Sets and Languages",         [BCP 18](https://tools.ietf.org/html/bcp18), [RFC 2277](https://tools.ietf.org/html/rfc2277), January 1998. [jg644]     [[42]()] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource         Identifiers (URI): Generic Syntax and Semantics", [RFC 2396](https://tools.ietf.org/html/rfc2396),         August 1998. [jg645]     [[43]()] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,         Leach, P., Luotonen, A., Sink, E. and L. Stewart, "HTTP         Authentication: Basic and Digest Access Authentication", [RFC](https://tools.ietf.org/html/rfc2617) [2617](https://tools.ietf.org/html/rfc2617), June 1999. [jg646]     [[44]()] Luotonen, A., "Tunneling TCP based protocols through Web proxy         servers," Work in Progress. [jg647]     [[45]()] Palme, J. and A. Hopmann, "MIME E-mail Encapsulation of         Aggregate Documents, such as HTML (MHTML)", [RFC 2110](https://tools.ietf.org/html/rfc2110), March         1997.     [[46]()] Bradner, S., "The Internet Standards Process -- Revision 3", [BCP](https://tools.ietf.org/html/bcp9) [9](https://tools.ietf.org/html/bcp9), [RFC 2026](https://tools.ietf.org/html/rfc2026), October 1996.     [[47]()] Masinter, L., "Hyper Text Coffee Pot Control Protocol         (HTCPCP/1.0)", [RFC 2324](https://tools.ietf.org/html/rfc2324), 1 April 1998.     [[48]()] Freed, N. and N. Borenstein, "Multipurpose Internet Mail         Extensions (MIME) Part Five: Conformance Criteria and Examples",         [RFC 2049](https://tools.ietf.org/html/rfc2049), November 1996.     [[49]()] Troost, R., Dorner, S. and K. Moore, "Communicating Presentation         Information in Internet Messages: The Content-Disposition Header         Field", [RFC 2183](https://tools.ietf.org/html/rfc2183), August 1997.

18 作者地址

Roy T. Fielding    Information and Computer Science    University of California, Irvine    Irvine, CA 92697-3425, USA     Fax: +1 (949) 824-1715    EMail: fielding@ics.uci.edu      James Gettys    World Wide Web Consortium    MIT Laboratory for Computer Science    545 Technology Square    Cambridge, MA 02139, USA     Fax: +1 (617) 258 8682    EMail: jg@w3.org      Jeffrey C. Mogul    Western Research Laboratory    Compaq Computer Corporation    250 University Avenue    Palo Alto, California, 94305, USA     EMail: mogul@wrl.dec.com      Henrik Frystyk Nielsen    World Wide Web Consortium    MIT Laboratory for Computer Science    545 Technology Square    Cambridge, MA 02139, USA     Fax: +1 (617) 258 8682    EMail: frystyk@w3.org      Larry Masinter    Xerox Corporation    3333 Coyote Hill Road    Palo Alto, CA 94034, USA     EMail: masinter@parc.xerox.com      Paul J. Leach    Microsoft Corporation    1 Microsoft Way    Redmond, WA 98052, USA     EMail: paulle@microsoft.com      Tim Berners-Lee    Director, World Wide Web Consortium    MIT Laboratory for Computer Science    545 Technology Square    Cambridge, MA 02139, USA     Fax: +1 (617) 258 8682    EMail: timbl@w3.org

19 附录

19.1 互联网媒体类型消息/ http和应用程序/ http

In addition to defining the HTTP/1.1 protocol, this document serves    as the specification for the Internet media type "message/http" and    "application/http". The message/http type can be used to enclose a    single HTTP request or response message, provided that it obeys the    MIME restrictions for all "message" types regarding line length and    encodings. The application/http type can be used to enclose a    pipeline of one or more HTTP request or response messages (not    intermixed). The following is to be registered with IANA [[17](about:blank#ref-17)].         Media Type name:         message        Media subtype name:      http        Required parameters:     none        Optional parameters:     version, msgtype         version: The HTTP-Version number of the enclosed message                  (e.g., "1.1"). If not present, the version can be                  determined from the first line of the body.         msgtype: The message type -- "request" or "response". If not                  present, the type can be determined from the first                  line of the body.        Encoding considerations: only "7bit", "8bit", or "binary" are                                 permitted        Security considerations: none         Media Type name:         application        Media subtype name:      http        Required parameters:     none        Optional parameters:     version, msgtype         version: The HTTP-Version number of the enclosed messages                  (e.g., "1.1"). If not present, the version can be                  determined from the first line of the body.         msgtype: The message type -- "request" or "response". If not                  present, the type can be determined from the first                  line of the body.        Encoding considerations: HTTP messages enclosed by this type                  are in "binary" format; use of an appropriate                  Content-Transfer-Encoding is required when                  transmitted via E-mail.        Security considerations: none

19.2 互联网媒体类型multipart / byteranges

When an HTTP 206 (Partial Content) response message includes the    content of multiple ranges (a response to a request for multiple    non-overlapping ranges), these are transmitted as a multipart    message-body. The media type for this purpose is called    "multipart/byteranges".     The multipart/byteranges media type includes two or more parts, each    with its own Content-Type and Content-Range fields. The required    boundary parameter specifies the boundary string used to separate    each body-part.         Media Type name:         multipart        Media subtype name:      byteranges        Required parameters:     boundary        Optional parameters:     none        Encoding considerations: only "7bit", "8bit", or "binary" are                                 permitted        Security considerations: none      For example:     HTTP/1.1 206 Partial Content    Date: Wed, 15 Nov 1995 06:25:24 GMT    Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT    Content-type: multipart/byteranges; boundary=THIS\_STRING\_SEPARATES     --THIS\_STRING\_SEPARATES    Content-type: application/pdf    Content-range: bytes 500-999/8000     ...the first range    --THIS\_STRING\_SEPARATES    Content-type: application/pdf    Content-range: bytes 7000-7999/8000     ...the second range    --THIS\_STRING\_SEPARATES--        Notes:        1) Additional CRLFs may precede the first boundary string in the          entity.         2) Although [RFC 2046](https://tools.ietf.org/html/rfc2046) [[40](about:blank#ref-40)] permits the boundary string to be          quoted, some existing implementations handle a quoted boundary          string incorrectly.        3) A number of browsers and servers were coded to an early draft          of the byteranges specification to use a media type of          multipart/x-byteranges, which is almost, but not quite          compatible with the version documented in HTTP/1.1.

19.3 可兼收的应用程序

Although this document specifies the requirements for the generation    of HTTP/1.1 messages, not all applications will be correct in their    implementation. We therefore recommend that operational applications    be tolerant of deviations whenever those deviations can be    interpreted unambiguously.     Clients SHOULD be tolerant in parsing the Status-Line and servers    tolerant when parsing the Request-Line. In particular, they SHOULD    accept any amount of SP or HT characters between fields, even though    only a single SP is required.     The line terminator for message-header fields is the sequence CRLF.    However, we recommend that applications, when parsing such headers,    recognize a single LF as a line terminator and ignore the leading CR.     The character set of an entity-body SHOULD be labeled as the lowest    common denominator of the character codes used within that body, with    the exception that not labeling the entity is preferred over labeling    the entity with the labels US-ASCII or ISO-8859-1. See [section 3.7.1](about:blank#section-3.7.1)    and 3.4.1.     Additional rules for requirements on parsing and encoding of dates    and other potential problems with date encodings include:        - HTTP/1.1 clients and caches SHOULD assume that an [RFC-850](https://tools.ietf.org/html/rfc850) date         which appears to be more than 50 years in the future is in fact         in the past (this helps solve the "year 2000" problem).        - An HTTP/1.1 implementation MAY internally represent a parsed         Expires date as earlier than the proper value, but MUST NOT         internally represent a parsed Expires date as later than the         proper value.        - All expiration-related calculations MUST be done in GMT. The         local time zone MUST NOT influence the calculation or comparison         of an age or expiration time.         - If an HTTP header incorrectly carries a date value with a time         zone other than GMT, it MUST be converted into GMT using the         most conservative possible conversion.

19.4 HTTP实体和RFC 2045实体之间的区别

HTTP/1.1 uses many of the constructs defined for Internet Mail ([RFC](https://tools.ietf.org/html/rfc822) [822](https://tools.ietf.org/html/rfc822) [[9](about:blank#ref-9)]) and the Multipurpose Internet Mail Extensions (MIME [[7](about:blank#ref-7)]) to    allow entities to be transmitted in an open variety of    representations and with extensible mechanisms. However, [RFC 2045](https://tools.ietf.org/html/rfc2045)    discusses mail, and HTTP has a few features that are different from    those described in [RFC 2045](https://tools.ietf.org/html/rfc2045). These differences were carefully chosen    to optimize performance over binary connections, to allow greater    freedom in the use of new media types, to make date comparisons    easier, and to acknowledge the practice of some early HTTP servers    and clients.     This appendix describes specific areas where HTTP differs from [RFC](https://tools.ietf.org/html/rfc2045) [2045](https://tools.ietf.org/html/rfc2045). Proxies and gateways to strict MIME environments SHOULD be    aware of these differences and provide the appropriate conversions    where necessary. Proxies and gateways from MIME environments to HTTP    also need to be aware of the differences because some conversions    might be required.

19.4.1 MIME版本

HTTP is not a MIME-compliant protocol. However, HTTP/1.1 messages MAY    include a single MIME-Version general-header field to indicate what    version of the MIME protocol was used to construct the message. Use    of the MIME-Version header field indicates that the message is in    full compliance with the MIME protocol (as defined in [RFC 2045](https://tools.ietf.org/html/rfc2045)[[7](about:blank#ref-7)]).    Proxies/gateways are responsible for ensuring full compliance (where    possible) when exporting HTTP messages to strict MIME environments.         MIME-Version   = "MIME-Version" ":" 1\*DIGIT "." 1\*DIGIT     MIME version "1.0" is the default for use in HTTP/1.1. However,    HTTP/1.1 message parsing and semantics are defined by this document    and not the MIME specification.

19.4.2 转换为规范形式

RFC 2045 [7]要求Internet邮件实体在被传输之前转换为规范形式，如RFC 2049 [48]的第4部分所述。本文档的3.7.1节描述了通过HTTP传输时允许“文本”媒体类型的子类型的形式。RFC 2046要求具有“文本”类型的内容将换行符表示为CRLF，并禁止在换行符序列之外使用CR或LF。当通过HTTP传输消息时，HTTP允许CRLF，裸露CR和裸LF来指示文本内容中的换行符。在可能的情况下，从HTTP到严格的MIME环境的代理或网关应该翻译3.7节中描述的文本媒体类型中的所有换行符。RFC 2049 标准形式的CRLF。但是，请注意，这可能因内容编码的存在而变得复杂，并且HTTP允许使用某些不使用八位组13和10来表示CR和LF的字符集，这与一些情况相同多字节字符集。实现者应该注意，除非原始内容已经是规范形式，否则转换将破坏应用于原始内容的任何加密校验和。因此，对于在HTTP中使用此类校验和的任何内容，建议使用规范形式。

19.4.3 日期格式的转换

HTTP/1.1 uses a restricted set of date formats ([section 3.3.1](about:blank#section-3.3.1)) to    simplify the process of date comparison. Proxies and gateways from    other protocols SHOULD ensure that any Date header field present in a    message conforms to one of the HTTP/1.1 formats and rewrite the date    if necessary.

19.4.4 内容编码的介绍

RFC 2045不包含任何等同于HTTP / 1.1的Content-Encoding标头字段的概念。由于这充当媒体类型的修饰符，因此HTTP或MIME兼容协议中的代理和网关必须在转发消息之前更改Content-Type标头字段的值或解码实体主体。（Internet邮件的Content-Type的一些实验性应用程序使用媒体类型参数“; conversions = <content-coding>”来执行等同于Content-Encoding的功能，但该参数不是RFC 2045的一部分。）

19.4.5 无内容式传输编码

HTTP does not use the Content-Transfer-Encoding (CTE) field of [RFC](https://tools.ietf.org/html/rfc2045) [2045](https://tools.ietf.org/html/rfc2045). Proxies and gateways from MIME-compliant protocols to HTTP MUST    remove any non-identity CTE ("quoted-printable" or "base64") encoding    prior to delivering the response message to an HTTP client.     Proxies and gateways from HTTP to MIME-compliant protocols are    responsible for ensuring that the message is in the correct format    and encoding for safe transport on that protocol, where "safe      transport" is defined by the limitations of the protocol being used.    Such a proxy or gateway SHOULD label the data with an appropriate    Content-Transfer-Encoding if doing so will improve the likelihood of    safe transport over the destination protocol.

19.4.6 传输编码的介绍

HTTP/1.1 introduces the Transfer-Encoding header field ([section](about:blank#section-14.41) [14.41](about:blank#section-14.41)). Proxies/gateways MUST remove any transfer-coding prior to    forwarding a message via a MIME-compliant protocol.     A process for decoding the "chunked" transfer-coding ([section 3.6](about:blank#section-3.6))    can be represented in pseudo-code as:         length := 0        read chunk-size, chunk-extension (if any) and CRLF        while (chunk-size > 0) {           read chunk-data and CRLF           append chunk-data to entity-body           length := length + chunk-size           read chunk-size and CRLF        }        read entity-header        while (entity-header not empty) {           append entity-header to existing header fields           read entity-header        }        Content-Length := length        Remove "chunked" from Transfer-Encoding

19.4.7 MHTML和线路长度限制

HTTP implementations which share code with MHTML [[45](about:blank#ref-45)] implementations    need to be aware of MIME line length limitations. Since HTTP does not    have this limitation, HTTP does not fold long lines. MHTML messages    being transported by HTTP follow all conventions of MHTML, including    line length limitations and folding, canonicalization, etc., since    HTTP transports all message-bodies as payload (see [section 3.7.2](about:blank#section-3.7.2)) and    does not interpret the content or any MIME header lines that might be    contained therein.

19.5 附加功能

一些现有HTTP实现使用的RFC 1945和RFC 2068文档协议元素，或许在大多数HTTP / 1.1应用程序中并不一致和正确。建议实现者了解这些功能，但不能依赖于他们在其他HTTP / 1.1应用程序中的存在或与其互操作。其中一些描述了所提出的实验性功能，一些描述了实验性部署发现缺乏的功能，现在在基本的HTTP / 1.1规范中进行了讨论。许多其他标题，如来自SMTP和MIME的Content-Disposition和Title，也经常被实现（参见RFC 2076 [37]）。

19.5.1 内容处理

The Content-Disposition response-header field has been proposed as a    means for the origin server to suggest a default filename if the user    requests that the content is saved to a file. This usage is derived    from the definition of Content-Disposition in [RFC 1806](https://tools.ietf.org/html/rfc1806) [[35](about:blank#ref-35)].          content-disposition = "Content-Disposition" ":"                               disposition-type \*( ";" disposition-parm )         disposition-type = "attachment" | disp-extension-token         disposition-parm = filename-parm | disp-extension-parm         filename-parm = "filename" "=" quoted-string         disp-extension-token = token         disp-extension-parm = token "=" ( token | quoted-string )     An example is          Content-Disposition: attachment; filename="fname.ext"     The receiving user agent SHOULD NOT respect any directory path    information present in the filename-parm parameter, which is the only    parameter believed to apply to HTTP implementations at this time. The    filename SHOULD be treated as a terminal component only.     If this header is used in a response with the application/octet-    stream content-type, the implied suggestion is that the user agent    should not display the response, but directly enter a `save response    as...' dialog.     See [section 15.5](about:blank#section-15.5) for Content-Disposition security issues.

19.6 与以前版本的兼容性

It is beyond the scope of a protocol specification to mandate    compliance with previous versions. HTTP/1.1 was deliberately    designed, however, to make supporting previous versions easy. It is    worth noting that, at the time of composing this specification    (1996), we would expect commercial HTTP/1.1 servers to:        - recognize the format of the Request-Line for HTTP/0.9, 1.0, and         1.1 requests;         - understand any valid request in the format of HTTP/0.9, 1.0, or         1.1;        - respond appropriately with a message in the same major version         used by the client.     And we would expect HTTP/1.1 clients to:        - recognize the format of the Status-Line for HTTP/1.0 and 1.1         responses;        - understand any valid response in the format of HTTP/0.9, 1.0, or         1.1.     For most implementations of HTTP/1.0, each connection is established    by the client prior to the request and closed by the server after    sending the response. Some implementations implement the Keep-Alive    version of persistent connections described in section 19.7.1 of [RFC](https://tools.ietf.org/html/rfc2068) [2068](https://tools.ietf.org/html/rfc2068) [[33](about:blank#ref-33)].

19.6.1 来自HTTP / 1.0的更改

This section summarizes major differences between versions HTTP/1.0    and HTTP/1.1.

19.6.1.1 简化多宿主Web服务器和节约IP的更改

地址

The requirements that clients and servers support the Host request-    header, report an error if the Host request-header ([section 14.23](about:blank#section-14.23)) is    missing from an HTTP/1.1 request, and accept absolute URIs ([section](about:blank#section-5.1.2) [5.1.2](about:blank#section-5.1.2)) are among the most important changes defined by this    specification.     Older HTTP/1.0 clients assumed a one-to-one relationship of IP    addresses and servers; there was no other established mechanism for    distinguishing the intended server of a request than the IP address    to which that request was directed. The changes outlined above will    allow the Internet, once older HTTP clients are no longer common, to    support multiple Web sites from a single IP address, greatly    simplifying large operational Web servers, where allocation of many    IP addresses to a single host has created serious problems. The    Internet will also be able to recover the IP addresses that have been    allocated for the sole purpose of allowing special-purpose domain    names to be used in root-level HTTP URLs. Given the rate of growth of    the Web, and the number of servers already deployed, it is extremely      important that all implementations of HTTP (including updates to    existing HTTP/1.0 applications) correctly implement these    requirements:        - Both clients and servers MUST support the Host request-header.        - A client that sends an HTTP/1.1 request MUST send a Host header.        - Servers MUST report a 400 (Bad Request) error if an HTTP/1.1         request does not include a Host request-header.        - Servers MUST accept absolute URIs.

19.6.2 与HTTP / 1.0持久连接的兼容性

Some clients and servers might wish to be compatible with some    previous implementations of persistent connections in HTTP/1.0    clients and servers. Persistent connections in HTTP/1.0 are    explicitly negotiated as they are not the default behavior. HTTP/1.0    experimental implementations of persistent connections are faulty,    and the new facilities in HTTP/1.1 are designed to rectify these    problems. The problem was that some existing 1.0 clients may be    sending Keep-Alive to a proxy server that doesn't understand    Connection, which would then erroneously forward it to the next    inbound server, which would establish the Keep-Alive connection and    result in a hung HTTP/1.0 proxy waiting for the close on the    response. The result is that HTTP/1.0 clients must be prevented from    using Keep-Alive when talking to proxies.     However, talking to proxies is the most important use of persistent    connections, so that prohibition is clearly unacceptable. Therefore,    we need some other mechanism for indicating a persistent connection    is desired, which is safe to use even when talking to an old proxy    that ignores Connection. Persistent connections are the default for    HTTP/1.1 messages; we introduce a new keyword (Connection: close) for    declaring non-persistence. See [section 14.10](about:blank#section-14.10).     The original HTTP/1.0 form of persistent connections (the Connection:    Keep-Alive and Keep-Alive header) is documented in [RFC 2068](https://tools.ietf.org/html/rfc2068). [[33](about:blank#ref-33)]

19.6.3 RFC 2068的修改

This specification has been carefully audited to correct and    disambiguate key word usage; [RFC 2068](https://tools.ietf.org/html/rfc2068) had many problems in respect to    the conventions laid out in [RFC 2119](https://tools.ietf.org/html/rfc2119) [[34](about:blank#ref-34)].     Clarified which error code should be used for inbound server failures    (e.g. DNS failures). ([Section 10.5.5](about:blank#section-10.5.5)).      CREATE had a race that required an Etag be sent when a resource is    first created. ([Section 10.2.2](about:blank#section-10.2.2)).     Content-Base was deleted from the specification: it was not    implemented widely, and there is no simple, safe way to introduce it    without a robust extension mechanism. In addition, it is used in a    similar, but not identical fashion in MHTML [[45](about:blank#ref-45)].     Transfer-coding and message lengths all interact in ways that    required fixing exactly when chunked encoding is used (to allow for    transfer encoding that may not be self delimiting); it was important    to straighten out exactly how message lengths are computed. (Sections    3.6, 4.4, 7.2.2, 13.5.2, 14.13, 14.16)     A content-coding of "identity" was introduced, to solve problems    discovered in caching. ([section 3.5](about:blank#section-3.5))     Quality Values of zero should indicate that "I don't want something"    to allow clients to refuse a representation. ([Section 3.9](about:blank#section-3.9))     The use and interpretation of HTTP version numbers has been clarified    by [RFC 2145](https://tools.ietf.org/html/rfc2145). Require proxies to upgrade requests to highest protocol    version they support to deal with problems discovered in HTTP/1.0    implementations ([Section 3.1](about:blank#section-3.1))     Charset wildcarding is introduced to avoid explosion of character set    names in accept headers. ([Section 14.2](about:blank#section-14.2))     A case was missed in the Cache-Control model of HTTP/1.1; s-maxage    was introduced to add this missing case. (Sections [13.4](about:blank#section-13.4), [14.8](about:blank#section-14.8), [14.9](about:blank#section-14.9),    14.9.3)     The Cache-Control: max-age directive was not properly defined for    responses. ([Section 14.9.3](about:blank#section-14.9.3))     There are situations where a server (especially a proxy) does not    know the full length of a response but is capable of serving a    byterange request. We therefore need a mechanism to allow byteranges    with a content-range not indicating the full length of the message.    ([Section 14.16](about:blank#section-14.16))     Range request responses would become very verbose if all meta-data    were always returned; by allowing the server to only send needed    headers in a 206 response, this problem can be avoided. ([Section](about:blank#section-10.2.7) [10.2.7](about:blank#section-10.2.7), 13.5.3, and 14.27)      Fix problem with unsatisfiable range requests; there are two cases:    syntactic problems, and range doesn't exist in the document. The 416    status code was needed to resolve this ambiguity needed to indicate    an error for a byte range request that falls outside of the actual    contents of a document. ([Section 10.4.17](about:blank#section-10.4.17), 14.16)     Rewrite of message transmission requirements to make it much harder    for implementors to get it wrong, as the consequences of errors here    can have significant impact on the Internet, and to deal with the    following problems:        1. Changing "HTTP/1.1 or later" to "HTTP/1.1", in contexts where          this was incorrectly placing a requirement on the behavior of          an implementation of a future version of HTTP/1.x        2. Made it clear that user-agents should retry requests, not          "clients" in general.        3. Converted requirements for clients to ignore unexpected 100          (Continue) responses, and for proxies to forward 100 responses,          into a general requirement for 1xx responses.        4. Modified some TCP-specific language, to make it clearer that          non-TCP transports are possible for HTTP.        5. Require that the origin server MUST NOT wait for the request          body before it sends a required 100 (Continue) response.        6. Allow, rather than require, a server to omit 100 (Continue) if          it has already seen some of the request body.        7. Allow servers to defend against denial-of-service attacks and          broken clients.     This change adds the Expect header and 417 status code. The message    transmission requirements fixes are in sections [8.2](about:blank#section-8.2), [10.4.18](about:blank#section-10.4.18),    8.1.2.2, 13.11, and 14.20.     Proxies should be able to add Content-Length when appropriate.    ([Section 13.5.2](about:blank#section-13.5.2))     Clean up confusion between 403 and 404 responses. ([Section 10.4.4](about:blank#section-10.4.4),    10.4.5, and 10.4.11)     Warnings could be cached incorrectly, or not updated appropriately.    ([Section 13.1.2](about:blank#section-13.1.2), 13.2.4, 13.5.2, 13.5.3, 14.9.3, and 14.46) Warning    also needed to be a general header, as PUT or other methods may have    need for it in requests.      Transfer-coding had significant problems, particularly with    interactions with chunked encoding. The solution is that transfer-    codings become as full fledged as content-codings. This involves    adding an IANA registry for transfer-codings (separate from content    codings), a new header field (TE) and enabling trailer headers in the    future. Transfer encoding is a major performance benefit, so it was    worth fixing [[39](about:blank#ref-39)]. TE also solves another, obscure, downward    interoperability problem that could have occurred due to interactions    between authentication trailers, chunked encoding and HTTP/1.0    clients.([Section 3.6](about:blank#section-3.6), 3.6.1, and 14.39)     The PATCH, LINK, UNLINK methods were defined but not commonly    implemented in previous versions of this specification. See [RFC 2068](https://tools.ietf.org/html/rfc2068)    [[33](about:blank#ref-33)].     The Alternates, Content-Version, Derived-From, Link, URI, Public and    Content-Base header fields were defined in previous versions of this    specification, but not commonly implemented. See [RFC 2068](https://tools.ietf.org/html/rfc2068) [[33](about:blank#ref-33)].

20 索引

Please see the PostScript version of this RFC for the INDEX.

	RFC 2616: HTTP/1.1
	RFC 2616: HTTP/1.1	详细

HTTP

超文本传输协议（ HTTP，HyperText Transfer Protocol ) 是互联网上应用最为广泛的一种网络协议。所有的 WWW 文件都必须遵守这个标准。

HTTP目录

1.指南 \| Guides
2.指南：基础 \| Guides: Basics
3.CSP
4.标题 \| Headers
5.方法 \| Methods
6.RFC 2616: HTTP/1.1
7.RFC 4918: WebDAV
8.RFC 5023: The Atom Publishing Protocol
9.RFC 7230: Message Syntax and Routing
10.RFC 7231: Semantics and Content
11.RFC 7232: Conditional Requests
12.RFC 7233: Range Requests
13.RFC 7234: Caching
14.RFC 7235: Authentication
15.状态 \| Status
16.HTTP 请求方法
17.HTTP状态码
18.HTTP 响应头信息
19.HTTP 教程