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RFC2068 - Hypertext Transfer Protocol -- HTTP/1.1

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Network Working Group R. Fielding Request for Comments: 2068 UC Irvine Category: Standards Track J. Gettys J. Mogul DEC H. Frystyk T. Berners-Lee MIT/LCS January 1997 Hypertext Transfer Protocol -- HTTP/1.1 Status of this Memo This document specifies
  Network Working Group R. Fielding
Request for Comments: 2068 UC Irvine
Category: Standards Track J. Gettys
J. Mogul
DEC
H. Frystyk
T. Berners-Lee
MIT/LCS
January 1997

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.

Abstract

The Hypertext Transfer Protocol (HTTP) is an application-level
protocol for distributed, collaborative, hypermedia information
systems. It is a generic, stateless, object-oriented protocol which
can be used for many tasks, such as name servers and distributed
object management systems, through extension of its request methods.
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".

Table of Contents

1 Introduction.............................................7
1.1 Purpose ..............................................7
1.2 Requirements .........................................7
1.3 Terminology ..........................................8
1.4 Overall Operation ...................................11
2 Notational Conventions and Generic Grammar..............13
2.1 Augmented BNF .......................................13
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 ...................................18
3.2.2 http URL .........................................19
3.2.3 URI Comparison ...................................20
3.3 Date/Time Formats ...................................21
3.3.1 Full Date ........................................21
3.3.2 Delta Seconds ....................................22
3.4 Character Sets ......................................22
3.5 Content Codings .....................................23
3.6 Transfer Codings ....................................24
3.7 Media Types .........................................25
3.7.1 Canonicalization and Text Defaults ...............26
3.7.2 Multipart Types ..................................27
3.8 Product Tokens ......................................28
3.9 Quality Values ......................................28
3.10 Language Tags ......................................28
3.11 Entity Tags ........................................29
3.12 Range Units ........................................30
4 HTTP Message............................................30
4.1 Message Types .......................................30
4.2 Message Headers .....................................31
4.3 Message Body ........................................32
4.4 Message Length ......................................32
4.5 General Header Fields ...............................34
5 Request.................................................34
5.1 Request-Line ........................................34
5.1.1 Method ...........................................35
5.1.2 Request-URI ......................................35
5.2 The Resource Identified by a Request ................37
5.3 Request Header Fields ...............................37
6 Response................................................38
6.1 Status-Line .........................................38
6.1.1 Status Code and Reason Phrase ....................39
6.2 Response Header Fields ..............................41
7 Entity..................................................41
7.1 Entity Header Fields ................................41
7.2 Entity Body .........................................42
7.2.1 Type .............................................42
7.2.2 Length ...........................................43
8 Connections.............................................43
8.1 Persistent Connections ..............................43
8.1.1 Purpose ..........................................43
8.1.2 Overall Operation ................................44
8.1.3 Proxy Servers ....................................45
8.1.4 Practical Considerations .........................45
8.2 Message Transmission Requirements ...................46
9 Method Definitions......................................48
9.1 Safe and Idempotent Methods .........................48

9.1.1 Safe Methods .....................................48
9.1.2 Idempotent Methods ...............................49
9.2 OPTIONS .............................................49
9.3 GET .................................................50
9.4 HEAD ................................................50
9.5 POST ................................................51
9.6 PUT .................................................52
9.7 DELETE ..............................................53
9.8 TRACE ...............................................53
10 Status Code Definitions................................53
10.1 Informational 1xx ..................................54
10.1.1 100 Continue ....................................54
10.1.2 101 Switching Protocols .........................54
10.2 Successful 2xx .....................................54
10.2.1 200 OK ..........................................54
10.2.2 201 Created .....................................55
10.2.3 202 Accepted ....................................55
10.2.4 203 Non-Authoritative Information ...............55
10.2.5 204 No Content ..................................55
10.2.6 205 Reset Content ...............................56
10.2.7 206 Partial Content .............................56
10.3 Redirection 3xx ....................................56
10.3.1 300 Multiple Choices ............................57
10.3.2 301 Moved Permanently ...........................57
10.3.3 302 Moved Temporarily ...........................58
10.3.4 303 See Other ...................................58
10.3.5 304 Not Modified ................................58
10.3.6 305 Use Proxy ...................................59
10.4 Client Error 4xx ...................................59
10.4.1 400 Bad Request .................................60
10.4.2 401 Unauthorized ................................60
10.4.3 402 Payment Required ............................60
10.4.4 403 Forbidden ...................................60
10.4.5 404 Not Found ...................................60
10.4.6 405 Method Not Allowed ..........................61
10.4.7 406 Not Acceptable ..............................61
10.4.8 407 Proxy Authentication Required ...............61
10.4.9 408 Request Timeout .............................62
10.4.10 409 Conflict ...................................62
10.4.11 410 Gone .......................................62
10.4.12 411 Length Required ............................63
10.4.13 412 Precondition Failed ........................63
10.4.14 413 Request Entity Too Large ...................63
10.4.15 414 Request-URI Too Long .......................63
10.4.16 415 Unsupported Media Type .....................63
10.5 Server Error 5xx ...................................64
10.5.1 500 Internal Server Error .......................64
10.5.2 501 Not Implemented .............................64

10.5.3 502 Bad Gateway .................................64
10.5.4 503 Service Unavailable .........................64
10.5.5 504 Gateway Timeout .............................64
10.5.6 505 HTTP Version Not Supported ..................65
11 Access Authentication..................................65
11.1 Basic Authentication Scheme ........................66
11.2 Digest Authentication Scheme .......................67
12 Content Negotiation....................................67
12.1 Server-driven Negotiation ..........................68
12.2 Agent-driven Negotiation ...........................69
12.3 Transparent Negotiation ............................70
13 Caching in HTTP........................................70
13.1.1 Cache Correctness ...............................72
13.1.2 Warnings ........................................73
13.1.3 Cache-control Mechanisms ........................74
13.1.4 Explicit User Agent Warnings ....................74
13.1.5 Exceptions to the Rules and Warnings ............75
13.1.6 Client-controlled Behavior ......................75
13.2 Expiration Model ...................................75
13.2.1 Server-Specified Expiration .....................75
13.2.2 Heuristic Expiration ............................76
13.2.3 Age Calculations ................................77
13.2.4 Expiration Calculations .........................79
13.2.5 Disambiguating Expiration Values ................80
13.2.6 Disambiguating Multiple Responses ...............80
13.3 Validation Model ...................................81
13.3.1 Last-modified Dates .............................82
13.3.2 Entity Tag Cache Validators .....................82
13.3.3 Weak and Strong Validators ......................82
13.3.4 Rules for When to Use Entity Tags and Last-
modified Dates..........................................85
13.3.5 Non-validating Conditionals .....................86
13.4 Response Cachability ...............................86
13.5 Constructing Responses From Caches .................87
13.5.1 End-to-end and Hop-by-hop Headers ...............88
13.5.2 Non-modifiable Headers ..........................88
13.5.3 Combining Headers ...............................89
13.5.4 Combining Byte Ranges ...........................90
13.6 Caching Negotiated Responses .......................90
13.7 Shared and Non-Shared Caches .......................91
13.8 Errors or Incomplete Response Cache Behavior .......91
13.9 Side Effects of GET and HEAD .......................92
13.10 Invalidation After Updates or Deletions ...........92
13.11 Write-Through Mandatory ...........................93
13.12 Cache Replacement .................................93
13.13 History Lists .....................................93
14 Header Field Definitions...............................94
14.1 Accept .............................................95

14.2 Accept-Charset .....................................97
14.3 Accept-Encoding ....................................97
14.4 Accept-Language ....................................98
14.5 Accept-Ranges ......................................99
14.6 Age ................................................99
14.7 Allow .............................................100
14.8 Authorization .....................................100
14.9 Cache-Control .....................................101
14.9.1 What is Cachable ...............................103
14.9.2 What May be Stored by Caches ...................103
14.9.3 Modifications of the Basic Expiration Mechanism 104
14.9.4 Cache Revalidation and Reload Controls .........105
14.9.5 No-Transform Directive .........................107
14.9.6 Cache Control Extensions .......................108
14.10 Connection .......................................109
14.11 Content-Base .....................................109
14.12 Content-Encoding .................................110
14.13 Content-Language .................................110
14.14 Content-Length ...................................111
14.15 Content-Location .................................112
14.16 Content-MD5 ......................................113
14.17 Content-Range ....................................114
14.18 Content-Type .....................................116
14.19 Date .............................................116
14.20 ETag .............................................117
14.21 Expires ..........................................117
14.22 From .............................................118
14.23 Host .............................................119
14.24 If-Modified-Since ................................119
14.25 If-Match .........................................121
14.26 If-None-Match ....................................122
14.27 If-Range .........................................123
14.28 If-Unmodified-Since ..............................124
14.29 Last-Modified ....................................124
14.30 Location .........................................125
14.31 Max-Forwards .....................................125
14.32 Pragma ...........................................126
14.33 Proxy-Authenticate ...............................127
14.34 Proxy-Authorization ..............................127
14.35 Public ...........................................127
14.36 Range ............................................128
14.36.1 Byte Ranges ...................................128
14.36.2 Range Retrieval Requests ......................130
14.37 Referer ..........................................131
14.38 Retry-After ......................................131
14.39 Server ...........................................132
14.40 Transfer-Encoding ................................132
14.41 Upgrade ..........................................132

14.42 User-Agent .......................................134
14.43 Vary .............................................134
14.44 Via ..............................................135
14.45 Warning ..........................................137
14.46 WWW-Authenticate .................................139
15 Security Considerations...............................139
15.1 Authentication of Clients .........................139
15.2 Offering a Choice of Authentication Schemes .......140
15.3 Abuse of Server Log Information ...................141
15.4 Transfer of Sensitive Information .................141
15.5 Attacks Based On File and Path Names ..............142
15.6 Personal Information ..............................143
15.7 Privacy Issues Connected to Accept Headers ........143
15.8 DNS Spoofing ......................................144
15.9 Location Headers and Spoofing .....................144
16 Acknowledgments.......................................144
17 References............................................146
18 Authors' Addresses....................................149
19 Appendices............................................150
19.1 Internet Media Type message/http ..................150
19.2 Internet Media Type multipart/byteranges ..........150
19.3 Tolerant Applications .............................151
19.4 Differences Between HTTP Entities and
MIME Entities...........................................152
19.4.1 Conversion to Canonical Form ...................152
19.4.2 Conversion of Date Formats .....................153
19.4.3 Introduction of Content-Encoding ...............153
19.4.4 No Content-Transfer-Encoding ...................153
19.4.5 HTTP Header Fields in Multipart Body-Parts .....153
19.4.6 Introduction of Transfer-Encoding ..............154
19.4.7 MIME-Version ...................................154
19.5 Changes from HTTP/1.0 .............................154
19.5.1 Changes to Simplify Multi-homed Web Servers and
Conserve IP Addresses .................................155
19.6 Additional Features ...............................156
19.6.1 Additional Request Methods .....................156
19.6.2 Additional Header Field Definitions ............156
19.7 Compatibility with Previous Versions ..............160
19.7.1 Compatibility with HTTP/1.0 Persistent
Connections............................................161

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, and 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 that indicate the purpose of a
request. It builds on the discipline of reference provided by the
Uniform Resource Identifier (URI) [3][20], as a location (URL) [4] or
name (URN) , 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 as defined by the Multipurpose Internet Mail Extensions
(MIME).

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

This specification uses the same words as RFC1123 [8] for defining
the significance of each particular requirement. These words are:

MUST
This word or the adjective "required" means that the item is an
absolute requirement of the specification.

SHOULD
This word or the adjective "recommended" means that there may
exist valid reasons in particular circumstances to ignore this
item, but the full implications should be understood and the case
carefully weighed before choosing a different course.

MAY
This word or the adjective "optional" means that this item is
truly optional. One vendor may choose to include the item because
a particular marketplace requires it or because it enhances the
product, for example; another vendor may omit the same item.

An implementation is not compliant if it fails to satisfy one or more
of the MUST requirements for the protocols it implements. An
implementation that satisfies all the MUST and all the SHOULD
requirements for its protocols is said to be "unconditionally
compliant"; one that satisfies all the MUST requirements but not all
the SHOULD 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,
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 `variant.' 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.

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 cachable 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.

cachable
A response is cachable if a cache is allowed to store a copy of
the response message for use in answering subsequent requests. The
rules for determining the cachability of HTTP responses are
defined in section 13. Even if a resource is cachable, 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.

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 cachable, and some requests may
contain modifiers which place special requirements on cache behavior.
HTTP requirements for cache behavior and cachable 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, 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]. Implementers 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. Whitespace 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 whitespace (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 whitespace (LWS) can be included
between any two adjacent words (token or quoted-string), and
between adjacent tokens and delimiters (tspecials), without
changing the interpretation of a field. At least one delimiter
(tspecials) must exist between any two tokens, 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 headers 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.

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 RFC1522
[14].

TEXT = <any OCTET except CTLs,
but including LWS>

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.

token = 1*<any CHAR except CTLs or tspecials>

tspecials = "(" | ")" | "<" | ">" | "@"
| "," | ";" | ":" | "\" | <">
| "/" | "[" | "]" | "?" | "="
| "{" | "}" | 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 | 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) <"> )

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.

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.

Applications sending Request or Response messages, as defined by this
specification, MUST include an HTTP-Version of "HTTP/1.1". Use of
this version number indicates that the sending application is at
least conditionally compliant with this specification.

The HTTP version of an application is the highest HTTP version for
which the application is at least conditionally compliant.

Proxy and gateway applications must 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 never 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, respond with an error, or switch to tunnel
behavior. Requests with a version lower than that of the
proxy/gateway's version MAY be upgraded before being forwarded; 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 , and finally the
combination of Uniform Resource Locators (URL) and Names (URN). 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, 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.

URI = ( absoluteURI | relativeURI ) [ "#" fragment ]

absoluteURI = scheme ":" *( uchar | reserved )

relativeURI = net_path | abs_path | rel_path

net_path = "//" net_loc [ abs_path ]
abs_path = "/" rel_path
rel_path = [ path ] [ ";" params ] [ "?" query ]

path = fsegment *( "/" segment )
fsegment = 1*pchar
segment = *pchar

params = param *( ";" param )
param = *( pchar | "/" )

scheme = 1*( ALPHA | DIGIT | "+" | "-" | "." )
net_loc = *( pchar | ";" | "?" )

query = *( uchar | reserved )
fragment = *( uchar | reserved )

pchar = uchar | ":" | "@" | "&" | "=" | "+"
uchar = unreserved | escape
unreserved = ALPHA | DIGIT | safe | extra | national

escape = "%" HEX HEX
reserved = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+"
extra = "!" | "*" | "'" | "(" | ")" | ","
safe = "$" | "-" | "_" | "."
unsafe = CTL | SP | <"> | "#" | "%" | "<" | ">"
national = <any OCTET excluding ALPHA, DIGIT,
reserved, extra, safe, and unsafe>

For definitive information on URL syntax and semantics, see RFC1738
[4] and RFC1808 [11]. The BNF above includes national characters not
allowed in valid URLs as specified by RFC1738, since HTTP servers
are not restricted in the set of unreserved characters allowed to
represent the rel_path part of addresses, and HTTP proxies may
receive requests for URIs not defined by RFC1738.

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 should be cautious about depending on URI lengths
above 255 bytes, because some older client or proxy implementations
may 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 ]

host = <A legal Internet host domain name
or IP address (in dotted-decimal form),
as defined by Section 2.1 of RFC1123>

port = *DIGIT

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. The use of IP addresses in URL's 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).

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:

o A port that is empty or not given is equivalent to the default
port for that URI;

o Comparisons of host names MUST be case-insensitive;

o Comparisons of scheme names MUST be case-insensitive;

o An empty abs_path is equivalent to an abs_path of "/".

Characters other than those in the "reserved" and "unsafe" sets (see
section 3.2) are equivalent to their ""%" HEX HEX" encodings.

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 RFC1123
Sunday, 06-Nov-94 08:49:37 GMT ; RFC850, obsoleted by 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 RFC1123 (an update to RFC
822). The second format is in common use, but is based on the
obsolete RFC850 [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 RFC1123 format for representing HTTP-date values
in header fields.

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. 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 = rfc1123-date | rfc850-date | asctime-date

rfc1123-date = wkday "," SP date1 SP time SP "GMT"
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 Seconds

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 Character Sets

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
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