3 Protocol Parameters
3.1 HTTP Version
HTTP uses a "<major>.<minor>" numbering scheme to indicate versions
of the protocol. The protocol versioning policy is intended to allow
the sender to indicate the format of a message and its capacity for
understanding further HTTP communication, rather than the features
obtained via that communication. No change is made to the version
number for the addition of message components which do not affect
communication behavior or which only add to extensible field values.
The <minor> number is incremented when the changes made to the
protocol add features which do not change the general message parsing
algorithm, but which may add to the message semantics and imply
additional capabilities of the sender. The <major> number is
incremented when the format of a message within the protocol is
changed. See RFC 2145  for a fuller explanation.
The version of an HTTP message is indicated by an HTTP-Version field
in the first line of the message.
HTTP-Version = "HTTP" "/" 1*DIGIT "." 1*DIGIT
Note that the major and minor numbers MUST be treated as separate
integers and that each MAY be incremented higher than a single digit.
Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is
lower than HTTP/12.3. Leading zeros MUST be ignored by recipients and
MUST NOT be sent.
An application that sends a request or response message that includes
HTTP-Version of "HTTP/1.1" MUST be at least conditionally compliant
with this specification. Applications that are at least conditionally
compliant with this specification SHOULD use an HTTP-Version of
"HTTP/1.1" in their messages, and MUST do so for any message that is
not compatible with HTTP/1.0. For more details on when to send
specific HTTP-Version values, see RFC 2145 .
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, 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 , 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. For definitive information on
URL syntax and semantics, see "Uniform Resource Identifiers (URI):
Generic Syntax and Semantics," RFC 2396  (which replaces RFCs
1738  and RFC 1808 ). This specification adopts the
definitions of "URI-reference", "absoluteURI", "relativeURI", "port",
"host","abs_path", "rel_path", and "authority" from that
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 ). 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
3.2.3 URI Comparison
When comparing two URIs to decide if they match or not, a client
SHOULD use a case-sensitive octet-by-octet comparison of the entire
URIs, with these exceptions:
- A port that is empty or not given is equivalent to the default
port for that URI-reference;
- Comparisons of host names MUST be case-insensitive;
- Comparisons of scheme names MUST be case-insensitive;
- An empty abs_path is equivalent to an abs_path of "/".
Characters other than those in the "reserved" and "unsafe" sets (see
RFC 2396 ) are equivalent to their ""%" HEX HEX" encoding.
For example, the following three URIs are equivalent:
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 ; RFC 822, updated by RFC 1123
Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036
Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format
The first format is preferred as an Internet standard and represents
a fixed-length subset of that defined by RFC 1123  (an update to
RFC 822 ). The second format is in common use, but is based on the
obsolete RFC 850  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.
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
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
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 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  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 
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 provided by the sender; and those user agents that have
a provision to "guess" a charset MUST use the charset from the
content-type field if they support that charset, rather than the
recipient's preference, when initially displaying a document. See
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 = token
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
gzip An encoding format produced by the file compression program
"gzip" (GNU zip) as described in RFC 1952 . 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
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  in combination with
the "deflate" compression mechanism described in RFC 1951 .
The default (identity) encoding; the use of no transformation
whatsoever. This content-coding is used only in the Accept-
Encoding header, and SHOULD NOT be used in the Content-Encoding
New content-coding value tokens SHOULD be registered; to allow
interoperability between clients and servers, specifications of the
content coding algorithms needed to implement a new value SHOULD be
publicly available and adequate for independent implementation, and
conform to the purpose of content coding defined in this section.
3.6 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.
parameter = attribute "=" value
attribute = token
value = token | quoted-string
All transfer-coding values are case-insensitive. HTTP/1.1 uses
transfer-coding values in the TE header field (section 14.39) 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 , 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.
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"
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 = *chunk
chunk = chunk-size [ chunk-extension ] CRLF
chunk-size = 1*HEX
last-chunk = 1*("0") [ chunk-extension ] CRLF
chunk-extension= *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
chunk-ext-name = token
chunk-ext-val = token | quoted-string
chunk-data = chunk-size(OCTET)
trailer = *(entity-header CRLF)
The chunk-size field is a string of hex digits indicating the size of
the chunk. The chunked encoding is ended by any chunk whose size is
zero, followed by the trailer, which is terminated by an empty line.
The trailer allows the sender to include additional HTTP header
fields at the end of the message. The Trailer header field can be
used to indicate which header fields are included in a trailer (see
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
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  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 )
type = token
subtype = token
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.
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 ). The media type registration process is
outlined in RFC 1590 . Use of non-registered media types is
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-body; a bare CR
or LF MUST NOT be substituted for CRLF within any of the HTTP control
structures (such as header fields and multipart boundaries).
If an entity-body is encoded with a content-coding, the underlying
data MUST be in a form defined above prior to being encoded.
The "charset" parameter is used with some media types to define the
character set (section 3.4) 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
, 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
empty; HTTP applications MUST NOT transmit the epilogue (even if the
original multipart contains an epilogue). These restrictions exist in
order to preserve the self-delimiting nature of a multipart message-
body, wherein the "end" of the message-body is indicated by the
ending multipart boundary.
In general, HTTP treats a multipart message-body no differently than
any other media type: strictly as payload. The one exception is the
"multipart/byteranges" type (appendix 19.2) when it appears in a 206
(Partial Content) response, which will be interpreted by some HTTP
caching mechanisms as described in sections 13.5.4 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 .
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.
product = token ["/" product-version]
product-version = token
User-Agent: CERN-LineMode/2.15 libwww/2.17b3
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.
qvalue = ( "0" [ "." 0*3DIGIT ] )
| ( "1" [ "." 0*3("0") ] )
"Quality values" is a misnomer, since these values merely represent
relative degradation in desired quality.
3.10 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-
The syntax and registry of HTTP language tags is the same as that
defined by RFC 1766 . 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
subtag = 1*8ALPHA
White space is not allowed within the tag and all tags are case-
insensitive. The name space of language tags is administered by the
IANA. Example tags include:
en, en-US, en-cockney, i-cherokee, x-pig-latin
where any two-letter primary-tag is an ISO-639 language abbreviation
and any two-letter initial subtag is an ISO-3166 country code. (The
last three tags above are not registered tags; all but the last are
examples of tags which could be registered in future.)
3.11 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
weak = "W/"
opaque-tag = quoted-string
A "strong entity tag" MAY be shared by two entities of a resource
only if they are equivalent by octet equality.
A "weak entity tag," indicated by the "W/" prefix, MAY be shared by
two entities of a resource only if the entities are equivalent and
could be substituted for each other with no significant change in
semantics. A weak entity tag can only be used for weak comparison.
An entity tag MUST be unique across all versions of all entities
associated with a particular resource. A given entity tag value MAY
be used for entities obtained by requests on different URIs. The use
of the same entity tag value in conjunction with entities obtained by
requests on different URIs does not imply the equivalence of those
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 = "bytes"
other-range-unit = token
The only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1
implementations MAY ignore ranges specified using other units.
HTTP/1.1 has been designed to allow implementations of applications
that do not depend on knowledge of ranges.
4 HTTP Message
4.1 Message Types
HTTP messages consist of requests from client to server and responses
from server to client.
HTTP-message = Request | Response ; HTTP/1.1 messages
Request (section 5) and Response (section 6) messages use the generic
message format of RFC 822  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-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
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 . Each header field consists
of a name followed by a colon (":") and the field value. Field names
are case-insensitive. The field value MAY be preceded by any amount
of LWS, though a single SP is preferred. Header fields can be
extended over multiple lines by preceding each extra line with at
least one SP or HT. Applications ought to follow "common form", where
one is known or indicated, when generating HTTP constructs, since
there might exist some implementations that fail to accept anything
beyond the common forms.
message-header = field-name ":" [ field-value ]
field-name = token
field-value = *( field-content | LWS )
field-content = <the OCTETs making up the field-value
and consisting of either *TEXT or combinations
of token, separators, and quoted-string>
The field-content does not include any leading or trailing LWS:
linear white space occurring before the first non-whitespace
character of the field-value or after the last non-whitespace
character of the field-value. Such leading or trailing LWS MAY be
removed without changing the semantics of the field value. Any LWS
that occurs between field-content MAY be replaced with a single SP
before interpreting the field value or forwarding the message
The order in which header fields with differing field names are
received is not significant. However, it is "good practice" to send
general-header fields first, followed by request-header or response-
header fields, and ending with the entity-header fields.
Multiple message-header fields with the same field-name MAY be
present in a message if and only if the entire field-value for that
header field is defined as a comma-separated list [i.e., #(values)].
It MUST be possible to combine the multiple header fields into one
"field-name: field-value" pair, without changing the semantics of the
message, by appending each subsequent field-value to the first, each
separated by a comma. The order in which header fields with the same
field-name are received is therefore significant to the
interpretation of the combined field value, and thus a proxy MUST NOT
change the order of these field values when a message is forwarded.
4.3 Message Body
The message-body (if any) of an HTTP message is used to carry the
entity-body associated with the request or response. The message-body
differs from the entity-body only when a transfer-coding has been
applied, as indicated by the Transfer-Encoding header field (section
message-body = entity-body
| <entity-body encoded as per Transfer-Encoding>
Transfer-Encoding MUST be used to indicate any transfer-codings
applied by an application to ensure safe and proper transfer of the
message. Transfer-Encoding is a property of the message, not of the
entity, and thus MAY be added or removed by any application along the
request/response chain. (However, section 3.6 places restrictions on
when certain transfer-codings may be used.)
The rules for when a message-body is allowed in a message differ for
requests and responses.
The presence of a message-body in a request is signaled by the
inclusion of a Content-Length or Transfer-Encoding header field in
the request's message-headers. A message-body MUST NOT be included in
a request if the specification of the request method (section 5.1.1)
does not allow sending an entity-body in requests. A server SHOULD
read and forward a message-body on any request; if the request method
does not include defined semantics for an entity-body, then the
message-body SHOULD be ignored when handling the request.
For response messages, whether or not a message-body is included with
a message is dependent on both the request method and the response
status code (section 6.1.1). All responses to the HEAD request method
MUST NOT include a message-body, even though the presence of entity-
header fields might lead one to believe they do. All 1xx
(informational), 204 (no content), and 304 (not modified) responses
MUST NOT include a message-body. All other responses do include a
message-body, although it MAY be of zero length.
4.4 Message Length
The transfer-length of a message is the length of the message-body as
it appears in the message; that is, after any transfer-codings have
been applied. When a message-body is included with a message, the
transfer-length of that body is determined by one of the following
(in order of precedence):
1.Any response message which "MUST NOT" include a message-body (such
as the 1xx, 204, and 304 responses and any response to a HEAD
request) is always terminated by the first empty line after the
header fields, regardless of the entity-header fields present in
2.If a Transfer-Encoding header field (section 14.41) is present and
has any value other than "identity", then the transfer-length is
defined by use of the "chunked" transfer-coding (section 3.6),
unless the message is terminated by closing the connection.
3.If a Content-Length header field (section 14.13) is present, its
decimal value in OCTETs represents both the entity-length and the
transfer-length. The Content-Length header field MUST NOT be sent
if these two lengths are different (i.e., if a Transfer-Encoding
header field is present). If a message is received with both a
Transfer-Encoding header field and a Content-Length header field,
the latter MUST be ignored.
4.If the message uses the media type "multipart/byteranges", and the
ransfer-length is not otherwise specified, then this self-
elimiting media type defines the transfer-length. This media type
UST NOT be used unless the sender knows that the recipient can arse
it; the presence in a request of a Range header with ultiple byte-
range specifiers from a 1.1 client implies that the lient can parse
A range header might be forwarded by a 1.0 proxy that does not
understand multipart/byteranges; in this case the server MUST
delimit the message using methods defined in items 1,3 or 5 of
5.By the server closing the connection. (Closing the connection
cannot be used to indicate the end of a request body, since that
would leave no possibility for the server to send back a response.)
For compatibility with HTTP/1.0 applications, HTTP/1.1 requests
containing a message-body MUST include a valid Content-Length header
field unless the server is known to be HTTP/1.1 compliant. If a
request contains a message-body and a Content-Length is not given,
the server SHOULD respond with 400 (bad request) if it cannot
determine the length of the message, or with 411 (length required) if
it wishes to insist on receiving a valid Content-Length.
All HTTP/1.1 applications that receive entities MUST accept the
"chunked" transfer-coding (section 3.6), thus allowing this mechanism
to be used for messages when the message length cannot be determined
Messages MUST NOT include both a Content-Length header field and a
non-identity transfer-coding. If the message does include a non-
identity transfer-coding, the Content-Length MUST be ignored.
When a Content-Length is given in a message where a message-body is
allowed, its field value MUST exactly match the number of OCTETs in
the message-body. HTTP/1.1 user agents MUST notify the user when an
invalid length is received and detected.
4.5 General Header Fields
There are a few header fields which have general applicability for
both request and response messages, but which do not apply to the
entity being transferred. These header fields apply only to the
message being transmitted.
general-header = Cache-Control ; Section 14.9
| Connection ; Section 14.10
| Date ; Section 14.18
| Pragma ; Section 14.32
| Trailer ; Section 14.40
| Transfer-Encoding ; Section 14.41
| Upgrade ; Section 14.42
| Via ; Section 14.45
| Warning ; Section 14.46
General-header field names can be extended reliably only in
combination with a change in the protocol version. However, new or
experimental header fields may be given the semantics of general
header fields if all parties in the communication recognize them to
be general-header fields. Unrecognized header fields are treated as
A request message from a client to a server includes, within the
first line of that message, the method to be applied to the resource,
the identifier of the resource, and the protocol version in use.
Request = Request-Line ; Section 5.1
*(( general-header ; Section 4.5
| request-header ; Section 5.3
| entity-header ) CRLF) ; Section 7.1
[ message-body ] ; Section 4.35.1 Request-Line
The Request-Line begins with a method token, followed by the
Request-URI and the protocol version, and ending with CRLF. The
elements are separated by SP characters. No CR or LF is allowed
except in the final CRLF sequence.
Request-Line = Method SP Request-URI SP HTTP-Version CRLF
The Method token indicates the method to be performed on the
resource identified by the Request-URI. The method is case-sensitive.
Method = "OPTIONS" ; Section 9.2
| "GET" ; Section 9.3
| "HEAD" ; Section 9.4
| "POST" ; Section 9.5
| "PUT" ; Section 9.6
| "DELETE" ; Section 9.7
| "TRACE" ; Section 9.8
| "CONNECT" ; Section 9.9
extension-method = token
The list of methods allowed by a resource can be specified in an
Allow header field (section 14.7). The return code of the response
always notifies the client whether a method is currently allowed on a
resource, since the set of allowed methods can change dynamically. An
origin server SHOULD return the status code 405 (Method Not Allowed)
if the method is known by the origin server but not allowed for the
requested resource, and 501 (Not Implemented) if the method is
unrecognized or not implemented by the origin server. The methods GET
and HEAD MUST be supported by all general-purpose servers. All other
methods are OPTIONAL; however, if the above methods are implemented,
they MUST be implemented with the same semantics as those specified
in section 9.
The Request-URI is a Uniform Resource Identifier (section 3.2) and
identifies the resource upon which to apply the request.
Request-URI = "*" | absoluteURI | abs_path | authority
The four options for Request-URI are dependent on the nature of the
request. The asterisk "*" means that the request does not apply to a
particular resource, but to the server itself, and is only allowed
when the method used does not necessarily apply to a resource. One
example would be
OPTIONS * HTTP/1.1
The absoluteURI form is REQUIRED when the request is being made to a
proxy. The proxy is requested to forward the request or service it
from a valid cache, and return the response. Note that the proxy MAY
forward the request on to another proxy or directly to the server
specified by the absoluteURI. In order to avoid request loops, a
proxy MUST be able to recognize all of its server names, including
any aliases, local variations, and the numeric IP address. An example
Request-Line would be:
GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.1
To allow for transition to absoluteURIs in all requests in future
versions of HTTP, all HTTP/1.1 servers MUST accept the absoluteURI
form in requests, even though HTTP/1.1 clients will only generate
them in requests to proxies.
The authority form is only used by the CONNECT method (section 9.9).
The most common form of Request-URI is that used to identify a
resource on an origin server or gateway. In this case the absolute
path of the URI MUST be transmitted (see section 3.2.1, abs_path) as
the Request-URI, and the network location of the URI (authority) MUST
be transmitted in a Host header field. For example, a client wishing
to retrieve the resource above directly from the origin server would
create a TCP connection to port 80 of the host "www.w3.org" and send
GET /pub/WWW/TheProject.html HTTP/1.1
followed by the remainder of the Request. Note that the absolute path
cannot be empty; if none is present in the original URI, it MUST be
given as "/" (the server root).
The Request-URI is transmitted in the format specified in section
3.2.1. If the Request-URI is encoded using the "% HEX HEX" encoding
, the origin server MUST decode the Request-URI in order to
properly interpret the request. Servers SHOULD respond to invalid
Request-URIs with an appropriate status code.
A transparent proxy MUST NOT rewrite the "abs_path" part of the
received Request-URI when forwarding it to the next inbound server,
except as noted above to replace a null abs_path with "/".
Note: The "no rewrite" rule prevents the proxy from changing the
meaning of the request when the origin server is improperly using
a non-reserved URI character for a reserved purpose. Implementors
should be aware that some pre-HTTP/1.1 proxies have been known to
rewrite the Request-URI.
5.2 The Resource Identified by a Request
The exact resource identified by an Internet request is determined by
examining both the Request-URI and the Host header field.
An origin server that does not allow resources to differ by the
requested host MAY ignore the Host header field value when
determining the resource identified by an HTTP/1.1 request. (But see
section 18.104.22.168 for other requirements on Host support in HTTP/1.1.)
An origin server that does differentiate resources based on the host
requested (sometimes referred to as virtual hosts or vanity host
names) MUST use the following rules for determining the requested
resource on an HTTP/1.1 request:
1. If Request-URI is an absoluteURI, the host is part of the
Request-URI. Any Host header field value in the request MUST be
2. If the Request-URI is not an absoluteURI, and the request includes
a Host header field, the host is determined by the Host header
3. If the host as determined by rule 1 or 2 is not a valid host on
the server, the response MUST be a 400 (Bad Request) error message.
Recipients of an HTTP/1.0 request that lacks a Host header field MAY
attempt to use heuristics (e.g., examination of the URI path for
something unique to a particular host) in order to determine what
exact resource is being requested.
5.3 Request Header Fields
The request-header fields allow the client to pass additional
information about the request, and about the client itself, to the
server. These fields act as request modifiers, with semantics
equivalent to the parameters on a programming language method
request-header = Accept ; Section 14.1
| Accept-Charset ; Section 14.2
| Accept-Encoding ; Section 14.3
| Accept-Language ; Section 14.4
| Authorization ; Section 14.8
| Expect ; Section 14.20
| From ; Section 14.22
| Host ; Section 14.23
| If-Match ; Section 14.24
| If-Modified-Since ; Section 14.25
| If-None-Match ; Section 14.26
| If-Range ; Section 14.27
| If-Unmodified-Since ; Section 14.28
| Max-Forwards ; Section 14.31
| Proxy-Authorization ; Section 14.34
| Range ; Section 14.35
| Referer ; Section 14.36
| TE ; Section 14.39
| User-Agent ; Section 14.43
Request-header field names can be extended reliably only in
combination with a change in the protocol version. However, new or
experimental header fields MAY be given the semantics of request-
header fields if all parties in the communication recognize them to
be request-header fields. Unrecognized header fields are treated as
After receiving and interpreting a request message, a server responds
with an HTTP response message.
Response = Status-Line ; Section 6.1
*(( general-header ; Section 4.5
| response-header ; Section 6.2
| entity-header ) CRLF) ; Section 7.1
[ message-body ] ; Section 7.26.1 Status-Line
The first line of a Response message is the Status-Line, consisting
of the protocol version followed by a numeric status code and its
associated textual phrase, with each element separated by SP
characters. No CR or LF is allowed except in the final CRLF sequence.
Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF
6.1.1 Status Code and Reason Phrase
The Status-Code element is a 3-digit integer result code of the
attempt to understand and satisfy the request. These codes are fully
defined in section 10. The Reason-Phrase is intended to give a short
textual description of the Status-Code. The Status-Code is intended
for use by automata and the Reason-Phrase is intended for the human
user. The client is not required to examine or display the Reason-
The first digit of the Status-Code defines the class of response. The
last two digits do not have any categorization role. There are 5
values for the first digit:
- 1xx: Informational - Request received, continuing process
- 2xx: Success - The action was successfully received,
understood, and accepted
- 3xx: Redirection - Further action must be taken in order to
complete the request
- 4xx: Client Error - The request contains bad syntax or cannot
- 5xx: Server Error - The server failed to fulfill an apparently
The individual values of the numeric status codes defined for
HTTP/1.1, and an example set of corresponding Reason-Phrase's, are
presented below. The reason phrases listed here are only
recommendations -- they MAY be replaced by local equivalents without
affecting the protocol.
"100" ; Section 10.1.1: Continue
| "101" ; Section 10.1.2: Switching Protocols
| "200" ; Section 10.2.1: OK
| "201" ; Section 10.2.2: Created
| "202" ; Section 10.2.3: Accepted
| "203" ; Section 10.2.4: Non-Authoritative Information
| "204" ; Section 10.2.5: No Content
| "205" ; Section 10.2.6: Reset Content
| "206" ; Section 10.2.7: Partial Content
| "300" ; Section 10.3.1: Multiple Choices
| "301" ; Section 10.3.2: Moved Permanently
| "302" ; Section 10.3.3: Found
| "303" ; Section 10.3.4: See Other
| "304" ; Section 10.3.5: Not Modified
| "305" ; Section 10.3.6: Use Proxy
| "307" ; Section 10.3.8: Temporary Redirect
| "400" ; Section 10.4.1: Bad Request
| "401" ; Section 10.4.2: Unauthorized
| "402" ; Section 10.4.3: Payment Required
| "403" ; Section 10.4.4: Forbidden
| "404" ; Section 10.4.5: Not Found
| "405" ; Section 10.4.6: Method Not Allowed
| "406" ; Section 10.4.7: Not Acceptable
| "407" ; Section 10.4.8: Proxy Authentication Required
| "408" ; Section 10.4.9: Request Time-out
| "409" ; Section 10.4.10: Conflict
| "410" ; Section 10.4.11: Gone
| "411" ; Section 10.4.12: Length Required
| "412" ; Section 10.4.13: Precondition Failed
| "413" ; Section 10.4.14: Request Entity Too Large
| "414" ; Section 10.4.15: Request-URI Too Large
| "415" ; Section 10.4.16: Unsupported Media Type
| "416" ; Section 10.4.17: Requested range not satisfiable
| "417" ; Section 10.4.18: Expectation Failed
| "500" ; Section 10.5.1: Internal Server Error
| "501" ; Section 10.5.2: Not Implemented
| "502" ; Section 10.5.3: Bad Gateway
| "503" ; Section 10.5.4: Service Unavailable
| "504" ; Section 10.5.5: Gateway Time-out
| "505" ; Section 10.5.6: HTTP Version not supported
extension-code = 3DIGIT
Reason-Phrase = *<TEXT, excluding CR, LF>
HTTP status codes are extensible. HTTP applications are not required
to understand the meaning of all registered status codes, though such
understanding is obviously desirable. However, applications MUST
understand the class of any status code, as indicated by the first
digit, and treat any unrecognized response as being equivalent to the
x00 status code of that class, with the exception that an
unrecognized response MUST NOT be cached. For example, if an
unrecognized status code of 431 is received by the client, it can
safely assume that there was something wrong with its request and
treat the response as if it had received a 400 status code. In such
cases, user agents SHOULD present to the user the entity returned
with the response, since that entity is likely to include human-
readable information which will explain the unusual status.
6.2 Response Header Fields
The response-header fields allow the server to pass additional
information about the response which cannot be placed in the Status-
Line. These header fields give information about the server and about
further access to the resource identified by the Request-URI.
response-header = Accept-Ranges ; Section 14.5
| Age ; Section 14.6
| ETag ; Section 14.19
| Location ; Section 14.30
| Proxy-Authenticate ; Section 14.33
| Retry-After ; Section 14.37
| Server ; Section 14.38
| Vary ; Section 14.44
| WWW-Authenticate ; Section 14.47
Response-header field names can be extended reliably only in
combination with a change in the protocol version. However, new or
experimental header fields MAY be given the semantics of response-
header fields if all parties in the communication recognize them to
be response-header fields. Unrecognized header fields are treated as
Request and Response messages MAY transfer an entity if not otherwise
restricted by the request method or response status code. An entity
consists of entity-header fields and an entity-body, although some
responses will only include the entity-headers.
In this section, both sender and recipient refer to either the client
or the server, depending on who sends and who receives the entity.
7.1 Entity Header Fields
Entity-header fields define metainformation about the entity-body or,
if no body is present, about the resource identified by the request.
Some of this metainformation is OPTIONAL; some might be REQUIRED by
portions of this specification.
entity-header = Allow ; Section 14.7
| Content-Encoding ; Section 14.11
| Content-Language ; Section 14.12
| Content-Length ; Section 14.13
| Content-Location ; Section 14.14
| Content-MD5 ; Section 14.15
| Content-Range ; Section 14.16
| Content-Type ; Section 14.17
| Expires ; Section 14.21
| Last-Modified ; Section 14.29
extension-header = message-header
The extension-header mechanism allows additional entity-header fields
to be defined without changing the protocol, but these fields cannot
be assumed to be recognizable by the recipient. Unrecognized header
fields SHOULD be ignored by the recipient and MUST be forwarded by
7.2 Entity Body
The entity-body (if any) sent with an HTTP request or response is in
a format and encoding defined by the entity-header fields.
entity-body = *OCTET
An entity-body is only present in a message when a message-body is
present, as described in section 4.3. The entity-body is obtained
from the message-body by decoding any Transfer-Encoding that might
have been applied to ensure safe and proper transfer of the message.
When an entity-body is included with a message, the data type of that
body is determined via the header fields Content-Type and Content-
Encoding. These define a two-layer, ordered encoding model:
entity-body := Content-Encoding( Content-Type( data ) )
Content-Type specifies the media type of the underlying data.
Content-Encoding may be used to indicate any additional content
codings applied to the data, usually for the purpose of data
compression, that are a property of the requested resource. There is
no default encoding.
Any HTTP/1.1 message containing an entity-body SHOULD include a
Content-Type header field defining the media type of that body. If
and only if the media type is not given by a Content-Type field, the
recipient MAY attempt to guess the media type via inspection of its
content and/or the name extension(s) of the URI used to identify the
resource. If the media type remains unknown, the recipient SHOULD
treat it as type "application/octet-stream".
7.2.2 Entity Length
The entity-length of a message is the length of the message-body
before any transfer-codings have been applied. Section 4.4 defines
how the transfer-length of a message-body is determined.