Network Working Group
R. Fielding
Request for Comments: 2616
Obsoletes: 2068
Category: Standards Track
Compaq/W3C
H. Frystyk
L. Masinter
T. Berners-Lee
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.
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 RFC 2068 [33].
Fielding, et al.
Standards Track
Table of Contents
Introduction ...................................................7
Purpose......................................................7
Requirements .................................................8
Terminology ..................................................8
Overall Operation ...........................................12
Notational Conventions and Generic Grammar ....................14
Augmented BNF ...............................................14
Basic Rules .................................................15
Protocol Parameters ...........................................17
HTTP Version ................................................17
Uniform Resource Identifiers ................................18
General Syntax ...........................................19
http URL .................................................19
URI Comparison ...........................................20
Date/Time Formats ...........................................20
Full Date ................................................20
Delta Seconds ............................................21
Character Sets ..............................................21
Missing Charset ..........................................22
Content Codings .............................................23
Transfer Codings ............................................24
Chunked Transfer Coding ..................................25
Media Types .................................................26
Canonicalization and Text Defaults .......................27
Multipart Types ..........................................27
Product Tokens ..............................................28
Quality Values ..............................................29
Language Tags ...............................................29
Entity Tags .................................................30
Range Units .................................................30
HTTP Message ..................................................31
Message Types ...............................................31
Message Headers .............................................31
Message Body ................................................32
Message Length ..............................................33
General Header Fields .......................................34
Request .......................................................35
Request-Line ................................................35
Method ...................................................36
Request-URI ..............................................36
The Resource Identified by a Request ........................38
Request Header Fields .......................................38
Response ......................................................39
Status-Line .................................................39
Status Code and Reason Phrase ............................39
Response Header Fields ......................................41
Fielding, et al.
Standards Track
Entity ........................................................42
Entity Header Fields ........................................42
Entity Body .................................................43
Type .....................................................43
Entity Length ............................................43
Connections ...................................................44
Persistent Connections ......................................44
Purpose ..................................................44
Overall Operation ........................................45
Proxy Servers ............................................46
Practical Considerations .................................46
Message Transmission Requirements ...........................47
Persistent Connections and Flow Control ..................47
Monitoring Connections for Error Status Messages .........48
Use of the 100 (Continue) Status .........................48
Client Behavior if Server Prematurely Closes Connection ..50
Method Definitions ............................................51
Safe and Idempotent Methods .................................51
Safe Methods .............................................51
Idempotent Methods .......................................51
OPTIONS .....................................................52
GET .........................................................53
HEAD ........................................................54
POST ........................................................54
PUT .........................................................55
DELETE ......................................................56
TRACE .......................................................56
CONNECT .....................................................57
Status Code Definitions ......................................57
Informational 1xx ...........................................57
100 Continue .............................................58
101 Switching Protocols ..................................58
Successful 2xx ..............................................58
200 OK ...................................................58
201 Created ..............................................59
202 Accepted .............................................59
203 Non-Authoritative Information ........................59
204 No Content ...........................................60
205 Reset Content ........................................60
206 Partial Content ......................................60
Redirection 3xx .............................................61
300 Multiple Choices .....................................61
301 Moved Permanently ....................................62
302 Found ................................................62
303 See Other ............................................63
304 Not Modified .........................................63
305 Use Proxy ............................................64
306 (Unused) .............................................64
Fielding, et al.
Standards Track
307 Temporary Redirect ...................................65
Client Error 4xx ............................................65
400 Bad Request .........................................65
401 Unauthorized ........................................66
402 Payment Required ....................................66
403 Forbidden ...........................................66
404 Not Found ...........................................66
405 Method Not Allowed ..................................66
406 Not Acceptable ......................................67
407 Proxy Authentication Required .......................67
408 Request Timeout .....................................67
409 Conflict ............................................67
410 Gone ................................................68
411 Length Required .....................................68
412 Precondition Failed .................................68
413 Request Entity Too Large ............................69
414 Request-URI Too Long ................................69
415 Unsupported Media Type ..............................69
416 Requested Range Not Satisfiable .....................69
417 Expectation Failed ..................................70
Server Error 5xx ............................................70
500 Internal Server Error ................................70
501 Not Implemented ......................................70
502 Bad Gateway ..........................................70
503 Service Unavailable ..................................70
504 Gateway Timeout ......................................71
505 HTTP Version Not Supported ...........................71
Access Authentication ........................................71
Content Negotiation ..........................................71
Server-driven Negotiation ...................................72
Agent-driven Negotiation ....................................73
Transparent Negotiation .....................................74
Caching in HTTP ..............................................74
Cache Correctness ........................................75
Warnings .................................................76
Cache-control Mechanisms .................................77
Explicit User Agent Warnings .............................78
Exceptions to the Rules and Warnings .....................78
Client-controlled Behavior ...............................79
Expiration Model ............................................79
Server-Specified Expiration ..............................79
Heuristic Expiration .....................................80
Age Calculations .........................................80
Expiration Calculations ..................................83
Disambiguating Expiration Values .........................84
Disambiguating Multiple Responses ........................84
Validation Model ............................................85
Last-Modified Dates ......................................86
Fielding, et al.
Standards Track
Entity Tag Cache Validators ..............................86
Weak and Strong Validators ...............................86
Rules for When to Use Entity Tags and Last-Modified Dates.89
Non-validating Conditionals ..............................90
Response Cacheability .......................................91
Constructing Responses From Caches ..........................92
End-to-end and Hop-by-hop Headers ........................92
Non-modifiable Headers ...................................92
Combining Headers ........................................94
Combining Byte Ranges ....................................95
Caching Negotiated Responses ................................95
Shared and Non-Shared Caches ................................96
Errors or Incomplete Response Cache Behavior ................97
Side Effects of GET and HEAD ................................97
Invalidation After Updates or Deletions ...................97
Write-Through Mandatory ...................................98
Cache Replacement .........................................99
History Lists .............................................99
Header Field Definitions ....................................100
Accept .....................................................100
Accept-Charset .............................................102
Accept-Encoding ............................................102
Accept-Language ............................................104
Accept-Ranges ..............................................105
Age ........................................................106
Allow ......................................................106
Authorization ..............................................107
Cache-Control ..............................................108
What is Cacheable .......................................109
What May be Stored by Caches ............................110
Modifications of the Basic Expiration Mechanism .........111
Cache Revalidation and Reload Controls ..................113
No-Transform Directive ..................................115
Cache Control Extensions ................................116
Connection ...............................................117
Content-Encoding .........................................118
Content-Language .........................................118
Content-Length ...........................................119
Content-Location .........................................120
Content-MD5 ..............................................121
Content-Range ............................................122
Content-Type .............................................124
Date .....................................................124
Clockless Origin Server Operation ......................125
ETag .....................................................126
Expect ...................................................126
Expires ..................................................127
From .....................................................128
Fielding, et al.
Standards Track
Host .....................................................128
If-Match .................................................129
If-Modified-Since ........................................130
If-None-Match ............................................132
If-Range .................................................133
If-Unmodified-Since ......................................134
Last-Modified ............................................134
Location .................................................135
Max-Forwards .............................................136
Pragma ...................................................136
Proxy-Authenticate .......................................137
Proxy-Authorization ......................................137
Range ....................................................138
Byte Ranges ...........................................138
Range Retrieval Requests ..............................139
Referer ..................................................140
Retry-After ..............................................141
Server ...................................................141
TE .......................................................142
Trailer ..................................................143
Transfer-Encoding..........................................143
Upgrade ..................................................144
User-Agent ...............................................145
Vary .....................................................145
Via ......................................................146
Warning ..................................................148
WWW-Authenticate .........................................150
15 Security Considerations .......................................150
Personal Information....................................151
Abuse of Server Log Information .........................151
Transfer of Sensitive Information .......................151
Encoding Sensitive Information in URI's .................152
Privacy Issues Connected to Accept Headers ..............152
Attacks Based On File and Path Names .......................153
DNS Spoofing ...............................................154
Location Headers and Spoofing ..............................154
Content-Disposition Issues .................................154
Authentication Credentials and Idle Clients ................155
Proxies and Caching ........................................155
Denial of Service Attacks on Proxies....................156
Acknowledgments .............................................156
References ..................................................158
Authors' Addresses ..........................................162
Appendices ..................................................164
Internet Media Type message/http and application/http ......164
Internet Media Type multipart/byteranges ...................165
Tolerant Applications ......................................166
Differences Between HTTP Entities and RFC 2045 Entities ....167
Fielding, et al.
Standards Track
MIME-Version ............................................167
Conversion to Canonical Form ............................167
Conversion of Date Formats ..............................168
Introduction of Content-Encoding ........................168
No Content-Transfer-Encoding ............................168
Introduction of Transfer-Encoding .......................169
MHTML and Line Length Limitations .......................169
Additional Features ........................................169
Content-Disposition .....................................170
Compatibility with Previous Versions .......................170
Changes from HTTP/1.0 ...................................171
Compatibility with HTTP/1.0 Persistent Connections ......172
Changes from RFC 2068 ...................................172
Index .......................................................175
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 RFC 1945 [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
Fielding, et al.
Standards Track
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 RFC 2119 [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.
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.
An HTTP request message, as defined in section 5.
An HTTP response message, as defined in section 6.
Fielding, et al.
Standards Track
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.
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).
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.
A program that establishes connections for the purpose of sending
user agent
The client which initiates a request. These are often browsers,
editors, spiders (web-traversing robots), or other end user tools.
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 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.
Fielding, et al.
Standards Track
origin server
The server on which a given resource resides or is to be created.
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
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 th the requesting client
may not be aware that it is communicating with a gateway.
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.
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.
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.
Fielding, et al.
Standards Track
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.
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.
A response is fresh if its age has not yet exceeded its freshness
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.
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.
Fielding, et al.
Standards Track
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
UA -----v----- A -----v----- B -----v----- C -----v----- O
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
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
any protocol that provides such gu
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.
Fielding, et al.
Standards Track
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 RFC 822 [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 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.
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".
The character "*" preceding an element indicates repetition. The
full form is "*element" indicating at least
and at most
occurrences of element. Default values are 0 and infinity so
that "*(element)" allows any number, "1*element"
r and "1*2element" allows one or two.
Square brackets enclo "[foo bar]" is
equivalent to "*1(foo bar)".
Fielding, et al.
Standards Track
Specific repetition: "(element)" is equivalent to
"*(element)"; that is, exactly
occurrences of (element).
Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three
alphabetic characters.
A construct "#" is defined, similar to "*", for defining lists of
elements. The full form is "#element" indicating at least
and at most
elements, each separated by one or more commas
(",") and OPTIONAL linear white space (LWS). This makes the usual
for a rule such as
( *LWS element *( *LWS "," *LWS element ))
can be shown as
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,
"1#element" r and "1#2element" allows one or
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].
Fielding, et al.
Standards Track
= UPALPHA | LOALPHA
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.
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.
= [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-
] only when encoded according to the rules of RFC 2047
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.
= "A" | "B" | "C" | "D" | "E" | "F"
| "a" | "b" | "c" | "d" | "e" | "f" | DIGIT
Fielding, et al.
Standards Track
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
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
= "(" *( ctext | quoted-pair | comment ) ")"
A string of text is parsed as a single word if it is quoted using
double-quote marks.
quoted-string
*(qdtext | quoted-pair )
= <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 "." 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.
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
incremented when the format of a message within the protocol is
changed. See RFC 2145 [36] for a fuller explanation.
Fielding, et al.
Standards Track
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 [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
Due to interoperability problems with HTTP/1.0 proxies discovered
since the publication of RFC 2068[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.
Fielding, et al.
Standards Track
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," RFC 2396 [42] (which replaces RFCs
1738 [4] and RFC 1808 [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 RFC 1900 [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
Fielding, et al.
Standards Track
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-
- Comparisons of host names MUST be case-
- Comparisons of scheme names MUST be case-
- An empty abs_path is equivalent to an abs_path of "/".
Characters other than those in the "reserved" and "unsafe" sets (see
RFC 2396 [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
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 :37 GMT RFC 822, updated by RFC 1123
Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036
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 [8] (an update to
RFC 822 [9]). The second format is in common use, but is based on the
obsolete RFC 850 [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 format for representing HTTP-date values
in header fields. See 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.
Fielding, et al.
Standards Track
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
= 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
= 2DIGIT SP month SP 4DIGIT
day month year (e.g., 02 Jun 1982)
= 2DIGIT "-" month "-" 2DIGIT
day-month-year (e.g., 02-Jun-82)
= month SP ( 2DIGIT | ( SP 1DIGIT ))
month day (e.g., Jun
= 2DIGIT ":" 2DIGIT ":" 2DIGIT
00:00:00 - 23:59:59
= "Mon" | "Tue" | "Wed"
| "Thu" | "Fri" | "Sat" | "Sun"
= "Monday" | "Tuesday" | "Wednesday"
| "Thursday" | "Friday" | "Saturday" | "Sunday"
= "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
3.4 Character Sets
HTTP uses the same definition of the term "character set" as that
described for MIME:
Fielding, et al.
Standards Track
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
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] 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]
3.4.1 Missing Charset
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 pr and those user agents that have
a provision to "guess" a charset MUST use the charset from the
Fielding, et al.
Standards Track
content-type field if they support that charset, rather than the
recipient's preference, when initially displaying a document. See
section 3.7.1.
3.5 Content Codings
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
All content-coding values are case-insensitive. HTTP/1.1 uses
content-coding values in the Accept-Encoding (section 14.3) and
Content-Encoding (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
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 [25]. This format is a
Lempel-Ziv coding (LZ77) with a 32 bit CRC.
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.
The "zlib" format defined in RFC 1950 [31] in combination with
the "deflate" compression mechanism described in RFC 1951 [29].
Fielding, et al.
Standards Track
The default (identity) 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
New content-coding value tokens SHOULD 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 Transfer Codings
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.
= attribute "=" 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) and in
the Transfer-Encoding header field (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).
Transfer-codings are analogous to the Content-Transfer-Encoding
values of MIME [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), or the desire to
encrypt data over a shared transport.
Fielding, et al.
Standards Track
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), "identity" (section
3.6.2), "gzip" (section 3.5), "compress" (section 3.5), and "deflate"
(section 3.5).
New transfer-coding value tokens SHOULD be registered in the same way
as new content-coding value tokens (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
3.6.1 Chunked Transfer Coding
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
last-chunk
= chunk-size [ chunk-extension ] CRLF
chunk-data CRLF
chunk-size
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)
= *(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).
Fielding, et al.
Standards Track
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
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; 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 Media Types
HTTP uses Internet Media Types [17] in the Content-Type (section
14.17) and Accept (section 14.1) header fields in order to provide
open and extensible data typing and type negotiation.
media-type
= type "/" subtype *( ";" parameter )
Parameters MAY follow the type/subtype in the form of attribute/value
pairs (as defined in 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.
Fielding, et al.
Standards Track
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]). The media type registration process is
outlined in RFC 1590 [17]. Use of non-registered media types is
discouraged.
3.7.1 Canonicalization and Text Defaults
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- 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) 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 for compatibility problems.
3.7.2 Multipart Types
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
Fielding, et al.
Standards Track
[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, the epilogue of any multipart message MUST be
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 and 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 [15].
3.8 Product Tokens
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.
= token ["/" product-version]
product-version = token
User-Agent: CERN-LineMode/2.15 libwww/2.17b3
Server: Apache/0.8.4
Fielding, et al.
Standards Track
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 Quality Values
HTTP content negotiation (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.
= ( "0" [ "." 0*3DIGIT ] )
| ( "1" [ "." 0*3("0") ] )
"Quality values" is a misnomer, since these values merely represent
relative degradation in desired quality.
3.10 Language Tags
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 [1]. In summary, a language tag is composed of 1
or more parts: A primary language tag and a possibly empty series of
language-tag
= primary-tag *( "-" subtag )
primary-tag
= 1*8ALPHA
= 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
Fielding, et al.
Standards Track
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 all but the last are
examples of tags which could be registered in future.)
3.11 Entity Tags
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
14.19), If-Match (section 14.24), If-None-Match (section 14.26), and
If-Range (section 14.27) header fields. The definition of how they
are used and compared as cache validators is in section 13.3.3. An
entity tag consists of an opaque quoted string, possibly prefixed by
a weakness indicator.
entity-tag = [ weak ] opaque-tag
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
3.12 Range Units
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) and Content-Range (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
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.
Fielding, et al.
Standards Track
HTTP/1.1 has been designed to allow implementations of applications
that do not depend on knowledge of ranges.
4 HTTP Message
4.1 Message Types
HTTP messages consist of requests from client to server and responses
from server to client.
HTTP-message
= Request | R HTTP/1.1 messages
Request (section 5) and Response (section 6) messages use the generic
message format of RFC 822 [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)
[ 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 Message Headers
HTTP header fields, which include general-header (section 4.5),
request-header (section 5.3), response-header (section 6.2), and
entity-header (section 7.1) fields, follow the same generic format as
that given in Section 3.1 of RFC 822 [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
Fielding, et al.
Standards Track
beyond the common forms.
message-header = field-name ":" [ field-value ]
field-name
field-value
= *( field-content | LWS )
field-content
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 me}