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RFC2616 - Hypertext Transfer Protocol -- HTTP/1.1

时间:2005-02-16 来源: 作者: 点击:
Network Working Group R. Fielding Request for Comments: 2616 UC Irvine Obsoletes: 2068 J. Gettys Category: Standards Track Compaq/W3C J. Mogul Compaq H. Frystyk W3C/MIT L. Masinter Xerox P. Leach Microsoft T. Berners-Lee W3C/MIT June 1999 Hypertext T
  Network Working Group R. Fielding
Request for Comments: 2616 UC Irvine
Obsoletes: 2068 J. Gettys
Category: Standards Track Compaq/W3C
J. Mogul
Compaq
H. Frystyk
W3C/MIT
L. Masinter
Xerox
P. Leach
Microsoft
T. Berners-Lee
W3C/MIT
June 1999

Hypertext Transfer Protocol -- HTTP/1.1

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1999). All Rights Reserved.

Abstract

The Hypertext Transfer Protocol (HTTP) is an application-level
protocol for distributed, collaborative, hypermedia information
systems. It is a generic, stateless, protocol which can be used for
many tasks beyond its use for hypertext, such as name servers and
distributed object management systems, through extension of its
request methods, error codes and headers [47]. A feature of HTTP is
the typing and negotiation of data representation, allowing systems
to be built independently of the data being transferred.

HTTP has been in use by the World-Wide Web global information
initiative since 1990. This specification defines the protocol
referred to as "HTTP/1.1", and is an update to RFC2068 [33].

Table of Contents

1 Introduction ...................................................7
1.1 Purpose......................................................7
1.2 Requirements .................................................8
1.3 Terminology ..................................................8
1.4 Overall Operation ...........................................12
2 Notational Conventions and Generic Grammar ....................14
2.1 Augmented BNF ...............................................14
2.2 Basic Rules .................................................15
3 Protocol Parameters ...........................................17
3.1 HTTP Version ................................................17
3.2 Uniform Resource Identifiers ................................18
3.2.1 General Syntax ...........................................19
3.2.2 http URL .................................................19
3.2.3 URI Comparison ...........................................20
3.3 Date/Time Formats ...........................................20
3.3.1 Full Date ................................................20
3.3.2 Delta Seconds ............................................21
3.4 Character Sets ..............................................21
3.4.1 Missing Charset ..........................................22
3.5 Content Codings .............................................23
3.6 Transfer Codings ............................................24
3.6.1 Chunked Transfer Coding ..................................25
3.7 Media Types .................................................26
3.7.1 Canonicalization and Text Defaults .......................27
3.7.2 Multipart Types ..........................................27
3.8 Product Tokens ..............................................28
3.9 Quality Values ..............................................29
3.10 Language Tags ...............................................29
3.11 Entity Tags .................................................30
3.12 Range Units .................................................30
4 HTTP Message ..................................................31
4.1 Message Types ...............................................31
4.2 Message Headers .............................................31
4.3 Message Body ................................................32
4.4 Message Length ..............................................33
4.5 General Header Fields .......................................34
5 Request .......................................................35
5.1 Request-Line ................................................35
5.1.1 Method ...................................................36
5.1.2 Request-URI ..............................................36
5.2 The Resource Identified by a Request ........................38
5.3 Request Header Fields .......................................38
6 Response ......................................................39
6.1 Status-Line .................................................39
6.1.1 Status Code and Reason Phrase ............................39
6.2 Response Header Fields ......................................41

7 Entity ........................................................42
7.1 Entity Header Fields ........................................42
7.2 Entity Body .................................................43
7.2.1 Type .....................................................43
7.2.2 Entity Length ............................................43
8 Connections ...................................................44
8.1 Persistent Connections ......................................44
8.1.1 Purpose ..................................................44
8.1.2 Overall Operation ........................................45
8.1.3 Proxy Servers ............................................46
8.1.4 Practical Considerations .................................46
8.2 Message Transmission Requirements ...........................47
8.2.1 Persistent Connections and Flow Control ..................47
8.2.2 Monitoring Connections for Error Status Messages .........48
8.2.3 Use of the 100 (Continue) Status .........................48
8.2.4 Client Behavior if Server Prematurely Closes Connection ..50
9 Method Definitions ............................................51
9.1 Safe and Idempotent Methods .................................51
9.1.1 Safe Methods .............................................51
9.1.2 Idempotent Methods .......................................51
9.2 OPTIONS .....................................................52
9.3 GET .........................................................53
9.4 HEAD ........................................................54
9.5 POST ........................................................54
9.6 PUT .........................................................55
9.7 DELETE ......................................................56
9.8 TRACE .......................................................56
9.9 CONNECT .....................................................57
10 Status Code Definitions ......................................57
10.1 Informational 1xx ...........................................57
10.1.1 100 Continue .............................................58
10.1.2 101 Switching Protocols ..................................58
10.2 Successful 2xx ..............................................58
10.2.1 200 OK ...................................................58
10.2.2 201 Created ..............................................59
10.2.3 202 Accepted .............................................59
10.2.4 203 Non-Authoritative Information ........................59
10.2.5 204 No Content ...........................................60
10.2.6 205 Reset Content ........................................60
10.2.7 206 Partial Content ......................................60
10.3 Redirection 3xx .............................................61
10.3.1 300 Multiple Choices .....................................61
10.3.2 301 Moved Permanently ....................................62
10.3.3 302 Found ................................................62
10.3.4 303 See Other ............................................63
10.3.5 304 Not Modified .........................................63
10.3.6 305 Use Proxy ............................................64
10.3.7 306 (Unused) .............................................64

10.3.8 307 Temporary Redirect ...................................65
10.4 Client Error 4xx ............................................65
10.4.1 400 Bad Request .........................................65
10.4.2 401 Unauthorized ........................................66
10.4.3 402 Payment Required ....................................66
10.4.4 403 Forbidden ...........................................66
10.4.5 404 Not Found ...........................................66
10.4.6 405 Method Not Allowed ..................................66
10.4.7 406 Not Acceptable ......................................67
10.4.8 407 Proxy Authentication Required .......................67
10.4.9 408 Request Timeout .....................................67
10.4.10 409 Conflict ............................................67
10.4.11 410 Gone ................................................68
10.4.12 411 Length Required .....................................68
10.4.13 412 Precondition Failed .................................68
10.4.14 413 Request Entity Too Large ............................69
10.4.15 414 Request-URI Too Long ................................69
10.4.16 415 Unsupported Media Type ..............................69
10.4.17 416 Requested Range Not Satisfiable .....................69
10.4.18 417 Expectation Failed ..................................70
10.5 Server Error 5xx ............................................70
10.5.1 500 Internal Server Error ................................70
10.5.2 501 Not Implemented ......................................70
10.5.3 502 Bad Gateway ..........................................70
10.5.4 503 Service Unavailable ..................................70
10.5.5 504 Gateway Timeout ......................................71
10.5.6 505 HTTP Version Not Supported ...........................71
11 Access Authentication ........................................71
12 Content Negotiation ..........................................71
12.1 Server-driven Negotiation ...................................72
12.2 Agent-driven Negotiation ....................................73
12.3 Transparent Negotiation .....................................74
13 Caching in HTTP ..............................................74
13.1.1 Cache Correctness ........................................75
13.1.2 Warnings .................................................76
13.1.3 Cache-control Mechanisms .................................77
13.1.4 Explicit User Agent Warnings .............................78
13.1.5 Exceptions to the Rules and Warnings .....................78
13.1.6 Client-controlled Behavior ...............................79
13.2 Expiration Model ............................................79
13.2.1 Server-Specified Expiration ..............................79
13.2.2 Heuristic Expiration .....................................80
13.2.3 Age Calculations .........................................80
13.2.4 Expiration Calculations ..................................83
13.2.5 Disambiguating Expiration Values .........................84
13.2.6 Disambiguating Multiple Responses ........................84
13.3 Validation Model ............................................85
13.3.1 Last-Modified Dates ......................................86

13.3.2 Entity Tag Cache Validators ..............................86
13.3.3 Weak and Strong Validators ...............................86
13.3.4 Rules for When to Use Entity Tags and Last-Modified Dates.89
13.3.5 Non-validating Conditionals ..............................90
13.4 Response Cacheability .......................................91
13.5 Constructing Responses From Caches ..........................92
13.5.1 End-to-end and Hop-by-hop Headers ........................92
13.5.2 Non-modifiable Headers ...................................92
13.5.3 Combining Headers ........................................94
13.5.4 Combining Byte Ranges ....................................95
13.6 Caching Negotiated Responses ................................95
13.7 Shared and Non-Shared Caches ................................96
13.8 Errors or Incomplete Response Cache Behavior ................97
13.9 Side Effects of GET and HEAD ................................97
13.10 Invalidation After Updates or Deletions ...................97
13.11 Write-Through Mandatory ...................................98
13.12 Cache Replacement .........................................99
13.13 History Lists .............................................99
14 Header Field Definitions ....................................100
14.1 Accept .....................................................100
14.2 Accept-Charset .............................................102
14.3 Accept-Encoding ............................................102
14.4 Accept-Language ............................................104
14.5 Accept-Ranges ..............................................105
14.6 Age ........................................................106
14.7 Allow ......................................................106
14.8 Authorization ..............................................107
14.9 Cache-Control ..............................................108
14.9.1 What is Cacheable .......................................109
14.9.2 What May be Stored by Caches ............................110
14.9.3 Modifications of the Basic Expiration Mechanism .........111
14.9.4 Cache Revalidation and Reload Controls ..................113
14.9.5 No-Transform Directive ..................................115
14.9.6 Cache Control Extensions ................................116
14.10 Connection ...............................................117
14.11 Content-Encoding .........................................118
14.12 Content-Language .........................................118
14.13 Content-Length ...........................................119
14.14 Content-Location .........................................120
14.15 Content-MD5 ..............................................121
14.16 Content-Range ............................................122
14.17 Content-Type .............................................124
14.18 Date .....................................................124
14.18.1 Clockless Origin Server Operation ......................125
14.19 ETag .....................................................126
14.20 Expect ...................................................126
14.21 Expires ..................................................127
14.22 From .....................................................128

14.23 Host .....................................................128
14.24 If-Match .................................................129
14.25 If-Modified-Since ........................................130
14.26 If-None-Match ............................................132
14.27 If-Range .................................................133
14.28 If-Unmodified-Since ......................................134
14.29 Last-Modified ............................................134
14.30 Location .................................................135
14.31 Max-Forwards .............................................136
14.32 Pragma ...................................................136
14.33 Proxy-Authenticate .......................................137
14.34 Proxy-Authorization ......................................137
14.35 Range ....................................................138
14.35.1 Byte Ranges ...........................................138
14.35.2 Range Retrieval Requests ..............................139
14.36 Referer ..................................................140
14.37 Retry-After ..............................................141
14.38 Server ...................................................141
14.39 TE .......................................................142
14.40 Trailer ..................................................143
14.41 Transfer-Encoding..........................................143
14.42 Upgrade ..................................................144
14.43 User-Agent ...............................................145
14.44 Vary .....................................................145
14.45 Via ......................................................146
14.46 Warning ..................................................148
14.47 WWW-Authenticate .........................................150
15 Security Considerations .......................................150
15.1 Personal Information....................................151
15.1.1 Abuse of Server Log Information .........................151
15.1.2 Transfer of Sensitive Information .......................151
15.1.3 Encoding Sensitive Information in URI's .................152
15.1.4 Privacy Issues Connected to Accept Headers ..............152
15.2 Attacks Based On File and Path Names .......................153
15.3 DNS Spoofing ...............................................154
15.4 Location Headers and Spoofing ..............................154
15.5 Content-Disposition Issues .................................154
15.6 Authentication Credentials and Idle Clients ................155
15.7 Proxies and Caching ........................................155
15.7.1 Denial of Service Attacks on Proxies....................156
16 Acknowledgments .............................................156
17 References ..................................................158
18 Authors' Addresses ..........................................162
19 Appendices ..................................................164
19.1 Internet Media Type message/http and application/http ......164
19.2 Internet Media Type multipart/byteranges ...................165
19.3 Tolerant Applications ......................................166
19.4 Differences Between HTTP Entities and RFC2045 Entities ....167

19.4.1 MIME-Version ............................................167
19.4.2 Conversion to Canonical Form ............................167
19.4.3 Conversion of Date Formats ..............................168
19.4.4 Introduction of Content-Encoding ........................168
19.4.5 No Content-Transfer-Encoding ............................168
19.4.6 Introduction of Transfer-Encoding .......................169
19.4.7 MHTML and Line Length Limitations .......................169
19.5 Additional Features ........................................169
19.5.1 Content-Disposition .....................................170
19.6 Compatibility with Previous Versions .......................170
19.6.1 Changes from HTTP/1.0 ...................................171
19.6.2 Compatibility with HTTP/1.0 Persistent Connections ......172
19.6.3 Changes from RFC2068 ...................................172
20 Index .......................................................175
21 Full Copyright Statement ....................................176

1 Introduction

1.1 Purpose

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 RFC1945 [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]. It builds on the discipline of reference
provided by the Uniform Resource Identifier (URI) [3], as a location
(URL) [4] or name (URN) [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] as defined by the Multipurpose
Internet Mail Extensions (MIME) [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], NNTP [13], FTP [18], Gopher [2],
and WAIS [10] protocols. In this way, HTTP allows basic hypermedia
access to resources available from diverse applications.

1.2 Requirements

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 RFC2119 [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 Terminology

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 and
transmitted via the connection.

request
An HTTP request message, as defined in section 5.

response
An HTTP response message, as defined in section 6.

resource
A network data object or service that can be identified by a URI,
as defined in 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.

representation
An entity included with a response that is subject to content
negotiation, as described in 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. 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. 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 Overall Operation

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.

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], 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).

2 Notational Conventions and Generic Grammar

2.1 Augmented 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 RFC822 [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 Basic Rules

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].

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.

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] only when encoded according to the rules of RFC2047
[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
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 Protocol Parameters

3.1 HTTP Version

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 RFC2145 [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 RFC2145 [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 RFC2068[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 Uniform Resource Identifiers

URIs have been known by many names: WWW addresses, Universal Document
Identifiers, Universal Resource Identifiers [3], and finally the
combination of Uniform Resource Locators (URL) [4] and Names (URN)
[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 General Syntax

URIs in HTTP can be represented in absolute form or relative to some
known base URI [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," RFC2396 [42] (which replaces RFCs
1738 [4] and RFC1808 [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).

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). The use of IP addresses
in URLs SHOULD be avoided whenever possible (see RFC1900 [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). 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 Comparison

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
RFC2396 [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/%7Esmith/home.html
/ABC.com:/%7esmith/home.html">http://ABC.com:/%7esmith/home.html

3.3 Date/Time Formats

3.3.1 Full Date

HTTP applications have historically allowed three different formats
for the representation of date/time stamps:

Sun, 06 Nov 1994 08:49:37 GMT ; RFC822, updated by
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