HTTP/1.1, part 1: URIs, Connections, and Message ParsingDay Software23 Corporate Plaza DR, Suite 280Newport BeachCA92660USA+1-949-706-5300+1-949-706-5305fielding@gbiv.comhttp://roy.gbiv.com/One Laptop per Child21 Oak Knoll RoadCarlisleMA01741USAjg@laptop.orghttp://www.laptop.org/Hewlett-Packard CompanyHP Labs, Large Scale Systems Group1501 Page Mill Road, MS 1177Palo AltoCA94304USAJeffMogul@acm.orgMicrosoft Corporation1 Microsoft WayRedmondWA98052USAhenrikn@microsoft.comAdobe Systems, Incorporated345 Park AveSan JoseCA95110USALMM@acm.orghttp://larry.masinter.net/Microsoft Corporation1 Microsoft WayRedmondWA98052paulle@microsoft.comWorld Wide Web ConsortiumMIT Computer Science and Artificial Intelligence LaboratoryThe Stata Center, Building 3232 Vassar StreetCambridgeMA02139USAtimbl@w3.orghttp://www.w3.org/People/Berners-Lee/World Wide Web ConsortiumW3C / ERCIM2004, rte des LuciolesSophia-AntipolisAM06902Franceylafon@w3.orghttp://www.raubacapeu.net/people/yves/greenbytes GmbHHafenweg 16MuensterNW48155Germany+49 251 2807760+49 251 2807761julian.reschke@greenbytes.dehttp://greenbytes.de/tech/webdav/HTTPbis Working Group
The Hypertext Transfer Protocol (HTTP) is an application-level
protocol for distributed, collaborative, hypertext information
systems. HTTP has been in use by the World Wide Web global information
initiative since 1990. This document is Part 1 of the seven-part specification
that defines the protocol referred to as "HTTP/1.1" and, taken together,
obsoletes RFC 2616. Part 1 provides an overview of HTTP and
its associated terminology, defines the "http" and "https" Uniform
Resource Identifier (URI) schemes, defines the generic message syntax
and parsing requirements for HTTP message frames, and describes
general security concerns for implementations.
Discussion of this draft should take place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org). The current issues list is
at
and related documents (including fancy diffs) can be found at
.
The changes in this draft are summarized in .
The Hypertext Transfer Protocol (HTTP) is an application-level
request/response protocol that uses extensible semantics and MIME-like
message payloads for flexible interaction with network-based hypertext
information systems. HTTP relies upon the Uniform Resource Identifier (URI)
standard to indicate request targets and
relationships between resources.
Messages are passed in a format similar to that used by Internet mail
and the Multipurpose Internet Mail Extensions
(MIME) (see Appendix A of for the differences
between HTTP and MIME messages).
HTTP is a generic interface protocol for information systems. It is
designed to hide the details of how a service is implemented by presenting
a uniform interface to clients that is independent of the types of
resources provided. Likewise, servers do not need to be aware of each
client's purpose: an HTTP request can be considered in isolation rather
than being associated with a specific type of client or a predetermined
sequence of application steps. The result is a protocol that can be used
effectively in many different contexts and for which implementations can
evolve independently over time.
HTTP is also designed for use as a generic protocol for translating
communication to and from other Internet information systems.
HTTP proxies and gateways provide access to alternative information
services by translating their diverse protocols into a hypertext
format that can be viewed and manipulated by clients in the same way
as HTTP services.
One consequence of HTTP flexibility is that the protocol cannot be
defined in terms of what occurs behind the interface. Instead, we
are limited to defining the syntax of communication, the intent
of received communication, and the expected behavior of recipients.
If the communication is considered in isolation, then successful
actions should be reflected in corresponding changes to the
observable interface provided by servers. However, since multiple
clients may act in parallel and perhaps at cross-purposes, we
cannot require that such changes be observable beyond the scope
of a single response.
This document is Part 1 of the seven-part specification of HTTP,
defining the protocol referred to as "HTTP/1.1" and obsoleting
.
Part 1 describes the architectural elements that are used or
referred to in HTTP and defines the URI schemes specific to
HTTP-based resources, overall network operation, connection
management, and HTTP message framing and forwarding requirements.
Our goal is to define all of the mechanisms necessary for HTTP message
handling that are independent of message semantics, thereby defining the
complete set of requirements for message parsers and
message-forwarding intermediaries.
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 .
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."
This specification uses the Augmented Backus-Naur Form (ABNF) notation
of .
The following core rules are included by
reference, as defined in , Appendix B.1:
ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
DIGIT (decimal 0-9), DQUOTE (double quote),
HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
OCTET (any 8-bit sequence of data), SP (space),
VCHAR (any visible character),
and WSP (whitespace).
One extension to the ABNF rules of is used to
improve readability.
A construct "#" is defined, similar to "*", for defining lists of
elements. The full form is "<n>#<m>element" indicating at least
<n> and at most <m> elements, each separated by a single comma
(",") and optional whitespace (OWS).
For compatibility with legacy list rules, recipients SHOULD accept empty
list elements. In other words, consumers would follow the list productions:
shows the collected ABNF, with the list rules
expanded as explained above.
HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all
protocol elements except the entity-body (see for
tolerant applications). The end-of-line marker within an entity-body
is defined by its associated media type, as described in Section 2.3 of .
This specification uses three rules to denote the use of linear
whitespace: OWS (optional whitespace), RWS (required whitespace), and
BWS ("bad" whitespace).
The OWS rule is used where zero or more linear whitespace characters may
appear. OWS SHOULD either not be produced or be produced as a single SP
character. Multiple OWS characters that occur within field-content SHOULD
be replaced with a single SP before interpreting the field value or
forwarding the message downstream.
RWS is used when at least one linear whitespace character is required to
separate field tokens. RWS SHOULD be produced as a single SP character.
Multiple RWS characters that occur within field-content SHOULD be
replaced with a single SP before interpreting the field value or
forwarding the message downstream.
BWS is used where the grammar allows optional whitespace for historical
reasons but senders SHOULD NOT produce it in messages. HTTP/1.1
recipients MUST accept such bad optional whitespace and remove it before
interpreting the field value or forwarding the message downstream.
Many HTTP/1.1 header field values consist of words separated by whitespace
or special characters. These special characters MUST be in a quoted
string to be used within a parameter value (as defined in
).
A string of text is parsed as a single word if it is quoted using
double-quote marks.
The backslash character ("\") MAY be used as a single-character
quoting mechanism only within quoted-string and comment constructs.
The ABNF rules below are defined in other parts:
HTTP was created with a specific architecture in mind, the World Wide Web,
and has evolved over time to support the scalability needs of a worldwide
hypertext system. Much of that architecture is reflected in the terminology
and syntax productions used to define HTTP.
Uniform Resource Identifiers (URIs) are used
throughout HTTP as the means for identifying resources. URI references
are used to target requests, redirect responses, and define relationships.
HTTP does not limit what a resource may be; it merely defines an interface
that can be used to interact with a resource via HTTP. More information on
the scope of URIs and resources can be found in .
This specification adopts the definitions of "URI-reference",
"absolute-URI", "relative-part", "fragment", "port", "host",
"path-abempty", "path-absolute", "query", and "authority" from
. In addition, we define a partial-URI rule for
protocol elements that allow a relative URI without a fragment.
Each protocol element in HTTP that allows a URI reference will indicate in
its ABNF production whether the element allows only a URI in absolute form
(absolute-URI), any relative reference (relative-ref), or some other subset
of the URI-reference grammar. Unless otherwise indicated, URI references
are parsed relative to the request target (the default base URI for both
the request and its corresponding response).
The "http" scheme is used to locate network resources via the HTTP
protocol.
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-target
for the resource is path-absolute (). The use of IP addresses
in URLs SHOULD be avoided whenever possible (see ). If
the path-absolute is not present in the URL, it MUST be given as "/" when
used as a request-target for a resource (). If a proxy
receives a host name which is not a fully qualified domain name, it
MAY add its domain to the host name it received. If a proxy receives
a fully qualified domain name, the proxy MUST NOT change the host
name.
TBD: Define and explain purpose of https scheme.
Note: the "https" scheme is defined in .
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 path-absolute is equivalent to a path-absolute of "/".Characters other than those in the "reserved" set are equivalent to their
percent-encoded octets (see , Section 2.1).
For example, the following three URIs are equivalent:
HTTP 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.
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).
A more complicated situation occurs when one or more intermediaries
are present in the request/response chain. There are three common
forms of intermediary: proxy, gateway, and tunnel. A proxy is a
forwarding agent, receiving requests for a URI in its absolute form,
rewriting all or part of the message, and forwarding the reformatted
request toward the server identified by the URI. A gateway is a
receiving agent, acting as a layer above some other server(s) and, if
necessary, translating the requests to the underlying server's
protocol. A tunnel acts as a relay point between two connections
without changing the messages; tunnels are used when the
communication needs to pass through an intermediary (such as a
firewall) even when the intermediary cannot understand the contents
of the messages.
The figure above shows three intermediaries (A, B, and C) between the
user agent and origin server. A request or response message that
travels the whole chain will pass through four separate connections.
This distinction is important because some HTTP communication options
may apply only to the connection with the nearest, non-tunnel
neighbor, only to the end-points of the chain, or to all connections
along the chain. Although the diagram is linear, each participant may
be engaged in multiple, simultaneous communications. For example, B
may be receiving requests from many clients other than A, and/or
forwarding requests to servers other than C, at the same time that it
is handling A's request.
Any party to the communication which is not acting as a tunnel may
employ an internal cache for handling requests. The effect of a cache
is that the request/response chain is shortened if one of the
participants along the chain has a cached response applicable to that
request. The following illustrates the resulting chain if B has a
cached copy of an earlier response from O (via C) for a request which
has not been cached by UA or A.
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 1 of .
In fact, there are a wide variety of architectures and configurations
of caches and proxies currently being experimented with or deployed
across the World Wide Web. These systems include national hierarchies
of proxy caches to save transoceanic bandwidth, systems that
broadcast or multicast cache entries, organizations that distribute
subsets of cached data via CD-ROM, and so on. HTTP systems are used
in corporate intranets over high-bandwidth links, and for access via
PDAs with low-power radio links and intermittent connectivity. The
goal of HTTP/1.1 is to support the wide diversity of configurations
already deployed while introducing protocol constructs that meet the
needs of those who build web applications that require high
reliability and, failing that, at least reliable indications of
failure.
HTTP communication usually takes place over TCP/IP connections. The
default port is TCP 80 (), but other ports can be used. This does
not preclude HTTP from being implemented on top of any other protocol
on the Internet, or on other networks. HTTP only presumes a reliable
transport; any protocol that provides such guarantees can be used;
the mapping of the HTTP/1.1 request and response structures onto the
transport data units of the protocol in question is outside the scope
of this specification.
In HTTP/1.0, most implementations used a new connection for each
request/response exchange. In HTTP/1.1, a connection may be used for
one or more request/response exchanges, although connections may be
closed for a variety of reasons (see ).
TBD: Configured to use HTTP to proxy HTTP or other protocols.TBD: Interception of HTTP traffic for initiating access control.TBD: Profiles of HTTP defined by other protocol.
Extensions of HTTP like WebDAV.TBD: Instructions on composing HTTP requests via hypertext formats.
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 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 is case-sensitive.
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 .
The HTTP version of an application is the highest HTTP version for
which the application is at least conditionally compliant.
Proxy and gateway applications need to be careful when forwarding
messages in protocol versions different from that of the application.
Since the protocol version indicates the protocol capability of the
sender, a proxy/gateway MUST NOT send a message with a version
indicator which is greater than its actual version. If a higher
version request is received, the proxy/gateway MUST either downgrade
the request version, or respond with an error, or switch to tunnel
behavior.
Due to interoperability problems with HTTP/1.0 proxies discovered
since the publication of , 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.
HTTP applications have historically allowed three different formats
for the representation of date/time stamps:
The first format is preferred as an Internet standard and represents
a fixed-length subset of that defined by . The
other formats are described here only for
compatibility with obsolete implementations.
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 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 whitespace beyond that specifically included as SP in the
grammar.
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.
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.
Parameters are in the form of attribute/value pairs.
All transfer-coding values are case-insensitive. HTTP/1.1 uses
transfer-coding values in the TE header field () and in
the Transfer-Encoding header field ().
Whenever a transfer-coding is applied to a message-body, the set of
transfer-codings MUST include "chunked", unless the message indicates it
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 ().
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 (), 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" (),
"gzip", "compress", and "deflate" (Section 2.2 of ).
New transfer-coding value tokens SHOULD be registered in the same way
as new content-coding value tokens (Section 2.2 of ).
A server which receives an entity-body with a transfer-coding it does
not understand SHOULD return 501 (Not Implemented), and close the
connection. A server MUST NOT send transfer-codings to an HTTP/1.0
client.
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.
The chunk-size field is a string of hex digits indicating the size of
the chunk-data in octets. 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
true:
the request included a TE header field that indicates "trailers" is
acceptable in the transfer-coding of the response, as described in
; or,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.
A process for decoding the "chunked" transfer-coding
can be represented in pseudo-code as:
All HTTP/1.1 applications MUST be able to receive and decode the
"chunked" transfer-coding, and MUST ignore chunk-ext extensions
they do not understand.
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 whitespace. By
convention, the products are listed in order of their significance
for identifying the application.
Examples:
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).
Both transfer codings (TE request header, )
and content negotiation (Section 4 of ) use 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.
Note: "Quality values" is a misnomer, since these values merely represent
relative degradation in desired quality.
HTTP messages consist of requests from client to server and responses
from server to client.
Request () and Response () messages use the generic
message format of 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.
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.
Whitespace (WSP) MUST NOT be sent between the start-line and the first
header field. The presence of whitespace might be an attempt to trick a
noncompliant implementation of HTTP into ignoring that field or processing
the next line as a new request, either of which may result in security
issues when implementations within the request chain interpret the
same message differently. HTTP/1.1 servers MUST reject such a message
with a 400 (Bad Request) response.
HTTP header fields follow the same general format as Internet messages in
Section 2.1 of . Each header field consists
of a name followed by a colon (":"), optional whitespace, and the field
value. Field names are case-insensitive.
Historically, HTTP has allowed field-content with text in the ISO-8859-1
character encoding (allowing other character sets
through use of encoding). In practice, most HTTP
header field-values use only a subset of the US-ASCII charset
. Newly defined header fields SHOULD constrain
their field-values to US-ASCII characters. Recipients SHOULD treat other
(obs-text) octets in field-content as opaque data.
No whitespace is allowed between the header field-name and colon. For
security reasons, any request message received containing such whitespace
MUST be rejected with a response code of 400 (Bad Request) and any such
whitespace in a response message MUST be removed.
The field value MAY be preceded by optional whitespace; a single SP is
preferred. The field-value does not include any leading or trailing white
space: OWS occurring before the first non-whitespace character of the
field-value or after the last non-whitespace character of the field-value
is ignored and MAY be removed without changing the meaning of the header
field.
Historically, HTTP header field values could be extended over multiple
lines by preceding each extra line with at least one space or horizontal
tab character (line folding). This specification deprecates such line
folding except within the message/http media type
().
HTTP/1.1 senders MUST NOT produce messages that include line folding
(i.e., that contain any field-content that matches the obs-fold rule) unless
the message is intended for packaging within the message/http media type.
HTTP/1.1 recipients SHOULD accept line folding and replace any embedded
obs-fold whitespace with a single SP prior to interpreting the field value
or forwarding the message downstream.
Comments can be included in some HTTP header fields by surrounding
the comment text with parentheses. Comments are only allowed in
fields containing "comment" as part of their field value definition.
In all other fields, parentheses are considered part of the field
value.
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.
Note: the "Set-Cookie" header as implemented in
practice (as opposed to how it is specified in )
can occur multiple times, but does not use the list syntax, and thus cannot
be combined into a single line. (See Appendix A.2.3 of
for details.) Also note that the Set-Cookie2 header specified in
does not share this problem.
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 ().
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, 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 2 of )
explicitly disallows an entity-body in requests.
When a request message contains both a message-body of non-zero
length and a method that does not define any semantics for that
request message-body, then an origin server SHOULD either ignore
the message-body or respond with an appropriate error message
(e.g., 413). A proxy or gateway, when presented the same request,
SHOULD either forward the request inbound with the message-body or
ignore the message-body when determining a response.
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 (). 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.
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):
Any response message which "MUST NOT" include a message-body (such
as the 1xx, 204, and 304 responses and any response to a HEAD
request) is always terminated by the first empty line after the
header fields, regardless of the entity-header fields present in
the message.
If a Transfer-Encoding header field ()
is present and the "chunked" transfer-coding ()
is used, the transfer-length is defined by the use of this transfer-coding.
If a Transfer-Encoding header field is present and the "chunked" transfer-coding
is not present, the transfer-length is defined by the sender closing the connection.
If a Content-Length header field () 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.
If the message uses the media type "multipart/byteranges", and the
transfer-length is not otherwise specified, then this self-delimiting
media type defines the transfer-length. This media type
MUST NOT be used unless the sender knows that the recipient can parse
it; the presence in a request of a Range header with multiple byte-range
specifiers from a 1.1 client implies that the client can parse
multipart/byteranges responses.
A range header might be forwarded by a 1.0 proxy that does not
understand multipart/byteranges; in this case the server MUST
delimit the message using methods defined in items 1, 3 or 5 of
this section.
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 (), thus allowing this mechanism
to be used for messages when the message length cannot be determined
in advance.
Messages MUST NOT include both a Content-Length header field and a
transfer-coding. If the message does include a
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.
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 field names can be extended reliably only in
combination with a change in the protocol version. However, new or
experimental header fields may be given the semantics of general
header fields if all parties in the communication recognize them to
be general-header fields. Unrecognized header fields are treated as
entity-header fields.
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.
The Request-Line begins with a method token, followed by the
request-target 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.
The Method token indicates the method to be performed on the
resource identified by the request-target. The method is case-sensitive.
The request-target
identifies the resource upon which to apply the request.
The four options for request-target 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
The absolute-URI 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 absolute-URI. 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:
To allow for transition to absolute-URIs in all requests in future
versions of HTTP, all HTTP/1.1 servers MUST accept the absolute-URI
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 7.9 of ).
The most common form of request-target 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 , path-absolute) as
the request-target, 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.example.org" and send
the lines:
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).
If a proxy receives a request without any path in the request-target and
the method specified is capable of supporting the asterisk form of
request-target, then the last proxy on the request chain MUST forward the
request with "*" as the final request-target.
The request-target is transmitted in the format specified in
. If the request-target is percent-encoded
(, Section 2.1), the origin server
MUST decode the request-target in order to
properly interpret the request. Servers SHOULD respond to invalid
request-targets with an appropriate status code.
A transparent proxy MUST NOT rewrite the "path-absolute" part of the
received request-target when forwarding it to the next inbound server,
except as noted above to replace a null path-absolute 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-target.
HTTP does not place a pre-defined limit on the length of a request-target.
A server MUST be prepared to receive URIs of unbounded length and
respond with the 414 (URI Too Long) status if the received
request-target would be longer than the server wishes to handle
(see Section 8.4.15 of ).
Various ad-hoc limitations on request-target length are found in practice.
It is RECOMMENDED that all HTTP senders and recipients support
request-target lengths of 8000 or more OCTETs.
The exact resource identified by an Internet request is determined by
examining both the request-target 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
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:
If request-target is an absolute-URI, the host is part of the
request-target. Any Host header field value in the request MUST be
ignored.If the request-target is not an absolute-URI, and the request includes
a Host header field, the host is determined by the Host header
field value.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.
After receiving and interpreting a request message, a server responds
with an HTTP response message.
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.
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 8 of . The Reason Phrase exists for the sole
purpose of providing a textual description associated with the numeric
status code, out of deference to earlier Internet application protocols
that were more frequently used with interactive text clients.
A client SHOULD ignore the content of the Reason Phrase.
The first digit of the Status-Code defines the class of response. The
last two digits do not have any categorization role. There are 5
values for the first digit:
1xx: Informational - Request received, continuing process
2xx: Success - The action was successfully received,
understood, and accepted
3xx: Redirection - Further action must be taken in order to
complete the request
4xx: Client Error - The request contains bad syntax or cannot
be fulfilled
5xx: Server Error - The server failed to fulfill an apparently
valid request
Prior to persistent connections, a separate TCP connection was
established to fetch each URL, increasing the load on HTTP servers
and causing congestion on the Internet. The use of inline images and
other associated data often require a client to make multiple
requests of the same server in a short amount of time. Analysis of
these performance problems and results from a prototype
implementation are available . Implementation experience and
measurements of actual HTTP/1.1 (RFC 2068) implementations show good
results . Alternatives have also been explored, for example,
T/TCP .
Persistent HTTP connections have a number of advantages:
By opening and closing fewer TCP connections, CPU time is saved
in routers and hosts (clients, servers, proxies, gateways,
tunnels, or caches), and memory used for TCP protocol control
blocks can be saved in hosts.
HTTP requests and responses can be pipelined on a connection.
Pipelining allows a client to make multiple requests without
waiting for each response, allowing a single TCP connection to
be used much more efficiently, with much lower elapsed time.
Network congestion is reduced by reducing the number of packets
caused by TCP opens, and by allowing TCP sufficient time to
determine the congestion state of the network.
Latency on subsequent requests is reduced since there is no time
spent in TCP's connection opening handshake.
HTTP can evolve more gracefully, since errors can be reported
without the penalty of closing the TCP connection. Clients using
future versions of HTTP might optimistically try a new feature,
but if communicating with an older server, retry with old
semantics after an error is reported.
HTTP implementations SHOULD implement persistent connections.
A significant difference between HTTP/1.1 and earlier versions of
HTTP is that persistent connections are the default behavior of any
HTTP connection. That is, unless otherwise indicated, the client
SHOULD assume that the server will maintain a persistent connection,
even after error responses from the server.
Persistent connections provide a mechanism by which a client and a
server can signal the close of a TCP connection. This signaling takes
place using the Connection header field (). Once a close
has been signaled, the client MUST NOT send any more requests on that
connection.
An HTTP/1.1 server MAY assume that a HTTP/1.1 client intends to
maintain a persistent connection unless a Connection header including
the connection-token "close" was sent in the request. If the server
chooses to close the connection immediately after sending the
response, it SHOULD send a Connection header including the
connection-token close.
An HTTP/1.1 client MAY expect a connection to remain open, but would
decide to keep it open based on whether the response from a server
contains a Connection header with the connection-token close. In case
the client does not want to maintain a connection for more than that
request, it SHOULD send a Connection header including the
connection-token close.
If either the client or the server sends the close token in the
Connection header, that request becomes the last one for the
connection.
Clients and servers SHOULD NOT assume that a persistent connection is
maintained for HTTP versions less than 1.1 unless it is explicitly
signaled. See for more information on backward
compatibility with HTTP/1.0 clients.
In order to remain persistent, all messages on the connection MUST
have a self-defined message length (i.e., one not defined by closure
of the connection), as described in .
A client that supports persistent connections MAY "pipeline" its
requests (i.e., send multiple requests without waiting for each
response). A server MUST send its responses to those requests in the
same order that the requests were received.
Clients which assume persistent connections and pipeline immediately
after connection establishment SHOULD be prepared to retry their
connection if the first pipelined attempt fails. If a client does
such a retry, it MUST NOT pipeline before it knows the connection is
persistent. Clients MUST also be prepared to resend their requests if
the server closes the connection before sending all of the
corresponding responses.
Clients SHOULD NOT pipeline requests using non-idempotent methods or
non-idempotent sequences of methods (see Section 7.1.2 of ). Otherwise, a
premature termination of the transport connection could lead to
indeterminate results. A client wishing to send a non-idempotent
request SHOULD wait to send that request until it has received the
response status for the previous request.
It is especially important that proxies correctly implement the
properties of the Connection header field as specified in .
The proxy server MUST signal persistent connections separately with
its clients and the origin servers (or other proxy servers) that it
connects to. Each persistent connection applies to only one transport
link.
A proxy server MUST NOT establish a HTTP/1.1 persistent connection
with an HTTP/1.0 client (but see for information and
discussion of the problems with the Keep-Alive header implemented by
many HTTP/1.0 clients).
Servers will usually have some time-out value beyond which they will
no longer maintain an inactive connection. Proxy servers might make
this a higher value since it is likely that the client will be making
more connections through the same server. The use of persistent
connections places no requirements on the length (or existence) of
this time-out for either the client or the server.
When a client or server wishes to time-out it SHOULD issue a graceful
close on the transport connection. Clients and servers SHOULD both
constantly watch for the other side of the transport close, and
respond to it as appropriate. If a client or server does not detect
the other side's close promptly it could cause unnecessary resource
drain on the network.
A client, server, or proxy MAY close the transport connection at any
time. For example, a client might have started to send a new request
at the same time that the server has decided to close the "idle"
connection. From the server's point of view, the connection is being
closed while it was idle, but from the client's point of view, a
request is in progress.
This means that clients, servers, and proxies MUST be able to recover
from asynchronous close events. Client software SHOULD reopen the
transport connection and retransmit the aborted sequence of requests
without user interaction so long as the request sequence is
idempotent (see Section 7.1.2 of ). Non-idempotent methods or sequences
MUST NOT be automatically retried, although user agents MAY offer a
human operator the choice of retrying the request(s). Confirmation by
user-agent software with semantic understanding of the application
MAY substitute for user confirmation. The automatic retry SHOULD NOT
be repeated if the second sequence of requests fails.
Servers SHOULD always respond to at least one request per connection,
if at all possible. Servers SHOULD NOT close a connection in the
middle of transmitting a response, unless a network or client failure
is suspected.
Clients that use persistent connections SHOULD limit the number of
simultaneous connections that they maintain to a given server. A
single-user client SHOULD NOT maintain more than 2 connections with
any server or proxy. A proxy SHOULD use up to 2*N connections to
another server or proxy, where N is the number of simultaneously
active users. These guidelines are intended to improve HTTP response
times and avoid congestion.
HTTP/1.1 servers SHOULD maintain persistent connections and use TCP's
flow control mechanisms to resolve temporary overloads, rather than
terminating connections with the expectation that clients will retry.
The latter technique can exacerbate network congestion.
An HTTP/1.1 (or later) client sending a message-body SHOULD monitor
the network connection for an error status while it is transmitting
the request. If the client sees an error status, it SHOULD
immediately cease transmitting the body. If the body is being sent
using a "chunked" encoding (), a zero length chunk and
empty trailer MAY be used to prematurely mark the end of the message.
If the body was preceded by a Content-Length header, the client MUST
close the connection.
The purpose of the 100 (Continue) status (see Section 8.1.1 of ) is to
allow a client that is sending a request message with a request body
to determine if the origin server is willing to accept the request
(based on the request headers) before the client sends the request
body. In some cases, it might either be inappropriate or highly
inefficient for the client to send the body if the server will reject
the message without looking at the body.
Requirements for HTTP/1.1 clients:
If a client will wait for a 100 (Continue) response before
sending the request body, it MUST send an Expect request-header
field (Section 9.2 of ) with the "100-continue" expectation.
A client MUST NOT send an Expect request-header field (Section 9.2 of )
with the "100-continue" expectation if it does not intend
to send a request body.
Because of the presence of older implementations, the protocol allows
ambiguous situations in which a client may send "Expect: 100-continue"
without receiving either a 417 (Expectation Failed) status
or a 100 (Continue) status. Therefore, when a client sends this
header field to an origin server (possibly via a proxy) from which it
has never seen a 100 (Continue) status, the client SHOULD NOT wait
for an indefinite period before sending the request body.
Requirements for HTTP/1.1 origin servers:
Upon receiving a request which includes an Expect request-header
field with the "100-continue" expectation, an origin server MUST
either respond with 100 (Continue) status and continue to read
from the input stream, or respond with a final status code. The
origin server MUST NOT wait for the request body before sending
the 100 (Continue) response. If it responds with a final status
code, it MAY close the transport connection or it MAY continue
to read and discard the rest of the request. It MUST NOT
perform the requested method if it returns a final status code.
An origin server SHOULD NOT send a 100 (Continue) response if
the request message does not include an Expect request-header
field with the "100-continue" expectation, and MUST NOT send a
100 (Continue) response if such a request comes from an HTTP/1.0
(or earlier) client. There is an exception to this rule: for
compatibility with , a server MAY send a 100 (Continue)
status in response to an HTTP/1.1 PUT or POST request that does
not include an Expect request-header field with the "100-continue"
expectation. This exception, the purpose of which is
to minimize any client processing delays associated with an
undeclared wait for 100 (Continue) status, applies only to
HTTP/1.1 requests, and not to requests with any other HTTP-version
value.
An origin server MAY omit a 100 (Continue) response if it has
already received some or all of the request body for the
corresponding request.
An origin server that sends a 100 (Continue) response MUST
ultimately send a final status code, once the request body is
received and processed, unless it terminates the transport
connection prematurely.
If an origin server receives a request that does not include an
Expect request-header field with the "100-continue" expectation,
the request includes a request body, and the server responds
with a final status code before reading the entire request body
from the transport connection, then the server SHOULD NOT close
the transport connection until it has read the entire request,
or until the client closes the connection. Otherwise, the client
might not reliably receive the response message. However, this
requirement is not be construed as preventing a server from
defending itself against denial-of-service attacks, or from
badly broken client implementations.
Requirements for HTTP/1.1 proxies:
If a proxy receives a request that includes an Expect request-header
field with the "100-continue" expectation, and the proxy
either knows that the next-hop server complies with HTTP/1.1 or
higher, or does not know the HTTP version of the next-hop
server, it MUST forward the request, including the Expect header
field.
If the proxy knows that the version of the next-hop server is
HTTP/1.0 or lower, it MUST NOT forward the request, and it MUST
respond with a 417 (Expectation Failed) status.
Proxies SHOULD maintain a cache recording the HTTP version
numbers received from recently-referenced next-hop servers.
A proxy MUST NOT forward a 100 (Continue) response if the
request message was received from an HTTP/1.0 (or earlier)
client and did not include an Expect request-header field with
the "100-continue" expectation. This requirement overrides the
general rule for forwarding of 1xx responses (see Section 8.1 of ).
If an HTTP/1.1 client sends a request which includes a request body,
but which does not include an Expect request-header field with the
"100-continue" expectation, and if the client is not directly
connected to an HTTP/1.1 origin server, and if the client sees the
connection close before receiving any status from the server, the
client SHOULD retry the request. If the client does retry this
request, it MAY use the following "binary exponential backoff"
algorithm to be assured of obtaining a reliable response:
Initiate a new connection to the server
Transmit the request-headers
Initialize a variable R to the estimated round-trip time to the
server (e.g., based on the time it took to establish the
connection), or to a constant value of 5 seconds if the round-trip
time is not available.
Compute T = R * (2**N), where N is the number of previous
retries of this request.
Wait either for an error response from the server, or for T
seconds (whichever comes first)
If no error response is received, after T seconds transmit the
body of the request.
If client sees that the connection is closed prematurely,
repeat from step 1 until the request is accepted, an error
response is received, or the user becomes impatient and
terminates the retry process.
If at any point an error status is received, the client
SHOULD NOT continue andSHOULD close the connection if it has not completed sending the
request message.
This section defines the syntax and semantics of HTTP/1.1 header fields
related to message framing and transport protocols.
For entity-header fields, both sender and recipient refer to either the
client or the server, depending on who sends and who receives the entity.
The general-header field "Connection" allows the sender to specify
options that are desired for that particular connection and MUST NOT
be communicated by proxies over further connections.
The Connection header's value has the following grammar:
HTTP/1.1 proxies MUST parse the Connection header field before a
message is forwarded and, for each connection-token in this field,
remove any header field(s) from the message with the same name as the
connection-token. Connection options are signaled by the presence of
a connection-token in the Connection header field, not by any
corresponding additional header field(s), since the additional header
field may not be sent if there are no parameters associated with that
connection option.
Message headers listed in the Connection header MUST NOT include
end-to-end headers, such as Cache-Control.
HTTP/1.1 defines the "close" connection option for the sender to
signal that the connection will be closed after completion of the
response. For example,
in either the request or the response header fields indicates that
the connection SHOULD NOT be considered `persistent' ()
after the current request/response is complete.
An HTTP/1.1 client that does not support persistent connections MUST
include the "close" connection option in every request message.
An HTTP/1.1 server that does not support persistent connections MUST
include the "close" connection option in every response message that
does not have a 1xx (informational) status code.
A system receiving an HTTP/1.0 (or lower-version) message that
includes a Connection header MUST, for each connection-token in this
field, remove and ignore any header field(s) from the message with
the same name as the connection-token. This protects against mistaken
forwarding of such header fields by pre-HTTP/1.1 proxies. See .
The entity-header field "Content-Length" indicates the size of the
entity-body, in decimal number of OCTETs, sent to the recipient or,
in the case of the HEAD method, the size of the entity-body that
would have been sent had the request been a GET.
An example is
Applications SHOULD use this field to indicate the transfer-length of
the message-body, unless this is prohibited by the rules in .
Any Content-Length greater than or equal to zero is a valid value.
describes how to determine the length of a message-body
if a Content-Length is not given.
Note that the meaning of this field is significantly different from
the corresponding definition in MIME, where it is an optional field
used within the "message/external-body" content-type. In HTTP, it
SHOULD be sent whenever the message's length can be determined prior
to being transferred, unless this is prohibited by the rules in
.
The general-header field "Date" represents the date and time at which
the message was originated, having the same semantics as orig-date in
Section 3.6.1 of . The field value is an
HTTP-date, as described in ;
it MUST be sent in rfc1123-date format.
An example is
Origin servers MUST include a Date header field in all responses,
except in these cases:
If the response status code is 100 (Continue) or 101 (Switching
Protocols), the response MAY include a Date header field, at
the server's option.If the response status code conveys a server error, e.g. 500
(Internal Server Error) or 503 (Service Unavailable), and it is
inconvenient or impossible to generate a valid Date.If the server does not have a clock that can provide a
reasonable approximation of the current time, its responses
MUST NOT include a Date header field. In this case, the rules
in MUST be followed.
A received message that does not have a Date header field MUST be
assigned one by the recipient if the message will be cached by that
recipient or gatewayed via a protocol which requires a Date. An HTTP
implementation without a clock MUST NOT cache responses without
revalidating them on every use. An HTTP cache, especially a shared
cache, SHOULD use a mechanism, such as NTP , to synchronize its
clock with a reliable external standard.
Clients SHOULD only send a Date header field in messages that include
an entity-body, as in the case of the PUT and POST requests, and even
then it is optional. A client without a clock MUST NOT send a Date
header field in a request.
The HTTP-date sent in a Date header SHOULD NOT represent a date and
time subsequent to the generation of the message. It SHOULD represent
the best available approximation of the date and time of message
generation, unless the implementation has no means of generating a
reasonably accurate date and time. In theory, the date ought to
represent the moment just before the entity is generated. In
practice, the date can be generated at any time during the message
origination without affecting its semantic value.
Some origin server implementations might not have a clock available.
An origin server without a clock MUST NOT assign Expires or Last-Modified
values to a response, unless these values were associated
with the resource by a system or user with a reliable clock. It MAY
assign an Expires value that is known, at or before server
configuration time, to be in the past (this allows "pre-expiration"
of responses without storing separate Expires values for each
resource).
The request-header field "Host" specifies the Internet host and port
number of the resource being requested, as obtained from the original
URI given by the user or referring resource (generally an http URI,
as described in ). The Host field value MUST represent
the naming authority of the origin server or gateway given by the
original URL. This allows the origin server or gateway to
differentiate between internally-ambiguous URLs, such as the root "/"
URL of a server for multiple host names on a single IP address.
A "host" without any trailing port information implies the default
port for the service requested (e.g., "80" for an HTTP URL). For
example, a request on the origin server for
<http://www.example.org/pub/WWW/> would properly include:
A client MUST include a Host header field in all HTTP/1.1 request
messages. If the requested URI does not include an Internet host
name for the service being requested, then the Host header field MUST
be given with an empty value. An HTTP/1.1 proxy MUST ensure that any
request message it forwards does contain an appropriate Host header
field that identifies the service being requested by the proxy. All
Internet-based HTTP/1.1 servers MUST respond with a 400 (Bad Request)
status code to any HTTP/1.1 request message which lacks a Host header
field.
See Sections
and
for other requirements relating to Host.
The request-header field "TE" indicates what extension transfer-codings
it is willing to accept in the response and whether or not it is
willing to accept trailer fields in a chunked transfer-coding. Its
value may consist of the keyword "trailers" and/or a comma-separated
list of extension transfer-coding names with optional accept
parameters (as described in ).
The presence of the keyword "trailers" indicates that the client is
willing to accept trailer fields in a chunked transfer-coding, as
defined in . This keyword is reserved for use with
transfer-coding values even though it does not itself represent a
transfer-coding.
Examples of its use are:
The TE header field only applies to the immediate connection.
Therefore, the keyword MUST be supplied within a Connection header
field () whenever TE is present in an HTTP/1.1 message.
A server tests whether a transfer-coding is acceptable, according to
a TE field, using these rules:
The "chunked" transfer-coding is always acceptable. If the
keyword "trailers" is listed, the client indicates that it is
willing to accept trailer fields in the chunked response on
behalf of itself and any downstream clients. The implication is
that, if given, the client is stating that either all
downstream clients are willing to accept trailer fields in the
forwarded response, or that it will attempt to buffer the
response on behalf of downstream recipients.
Note: HTTP/1.1 does not define any means to limit the size of a
chunked response such that a client can be assured of buffering
the entire response.If the transfer-coding being tested is one of the transfer-codings
listed in the TE field, then it is acceptable unless it
is accompanied by a qvalue of 0. (As defined in , a
qvalue of 0 means "not acceptable.")If multiple transfer-codings are acceptable, then the
acceptable transfer-coding with the highest non-zero qvalue is
preferred. The "chunked" transfer-coding always has a qvalue
of 1.
If the TE field-value is empty or if no TE field is present, the only
transfer-coding is "chunked". A message with no transfer-coding is
always acceptable.
The general field "Trailer" indicates that the given set of
header fields is present in the trailer of a message encoded with
chunked transfer-coding.
An HTTP/1.1 message SHOULD include a Trailer header field in a
message using chunked transfer-coding with a non-empty trailer. Doing
so allows the recipient to know which header fields to expect in the
trailer.
If no Trailer header field is present, the trailer SHOULD NOT include
any header fields. See for restrictions on the use of
trailer fields in a "chunked" transfer-coding.
Message header fields listed in the Trailer header field MUST NOT
include the following header fields:
Transfer-EncodingContent-LengthTrailer
The general-header "Transfer-Encoding" field indicates what (if any)
type of transformation has been applied to the message body in order
to safely transfer it between the sender and the recipient. This
differs from the content-coding in that the transfer-coding is a
property of the message, not of the entity.
Transfer-codings are defined in . An example is:
If multiple encodings have been applied to an entity, the transfer-codings
MUST be listed in the order in which they were applied.
Additional information about the encoding parameters MAY be provided
by other entity-header fields not defined by this specification.
Many older HTTP/1.0 applications do not understand the Transfer-Encoding
header.
The general-header "Upgrade" allows the client to specify what
additional communication protocols it supports and would like to use
if the server finds it appropriate to switch protocols. The server
MUST use the Upgrade header field within a 101 (Switching Protocols)
response to indicate which protocol(s) are being switched.
For example,
The Upgrade header field is intended to provide a simple mechanism
for transition from HTTP/1.1 to some other, incompatible protocol. It
does so by allowing the client to advertise its desire to use another
protocol, such as a later version of HTTP with a higher major version
number, even though the current request has been made using HTTP/1.1.
This eases the difficult transition between incompatible protocols by
allowing the client to initiate a request in the more commonly
supported protocol while indicating to the server that it would like
to use a "better" protocol if available (where "better" is determined
by the server, possibly according to the nature of the method and/or
resource being requested).
The Upgrade header field only applies to switching application-layer
protocols upon the existing transport-layer connection. Upgrade
cannot be used to insist on a protocol change; its acceptance and use
by the server is optional. The capabilities and nature of the
application-layer communication after the protocol change is entirely
dependent upon the new protocol chosen, although the first action
after changing the protocol MUST be a response to the initial HTTP
request containing the Upgrade header field.
The Upgrade header field only applies to the immediate connection.
Therefore, the upgrade keyword MUST be supplied within a Connection
header field () whenever Upgrade is present in an
HTTP/1.1 message.
The Upgrade header field cannot be used to indicate a switch to a
protocol on a different connection. For that purpose, it is more
appropriate to use a 301, 302, 303, or 305 redirection response.
This specification only defines the protocol name "HTTP" for use by
the family of Hypertext Transfer Protocols, as defined by the HTTP
version rules of and future updates to this
specification. Any token can be used as a protocol name; however, it
will only be useful if both the client and server associate the name
with the same protocol.
The general-header field "Via" MUST be used by gateways and proxies to
indicate the intermediate protocols and recipients between the user
agent and the server on requests, and between the origin server and
the client on responses. It is analogous to the "Received" field defined in
Section 3.6.7 of and is intended to be used for tracking message forwards,
avoiding request loops, and identifying the protocol capabilities of
all senders along the request/response chain.
The received-protocol indicates the protocol version of the message
received by the server or client along each segment of the
request/response chain. The received-protocol version is appended to
the Via field value when the message is forwarded so that information
about the protocol capabilities of upstream applications remains
visible to all recipients.
The protocol-name is optional if and only if it would be "HTTP". The
received-by field is normally the host and optional port number of a
recipient server or client that subsequently forwarded the message.
However, if the real host is considered to be sensitive information,
it MAY be replaced by a pseudonym. If the port is not given, it MAY
be assumed to be the default port of the received-protocol.
Multiple Via field values represents each proxy or gateway that has
forwarded the message. Each recipient MUST append its information
such that the end result is ordered according to the sequence of
forwarding applications.
Comments MAY be used in the Via header field to identify the software
of the recipient proxy or gateway, analogous to the User-Agent and
Server header fields. However, all comments in the Via field are
optional and MAY be removed by any recipient prior to forwarding the
message.
For example, a request message could be sent from an HTTP/1.0 user
agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
forward the request to a public proxy at p.example.net, which completes
the request by forwarding it to the origin server at www.example.com.
The request received by www.example.com would then have the following
Via header field:
Proxies and gateways used as a portal through a network firewall
SHOULD NOT, by default, forward the names and ports of hosts within
the firewall region. This information SHOULD only be propagated if
explicitly enabled. If not enabled, the received-by host of any host
behind the firewall SHOULD be replaced by an appropriate pseudonym
for that host.
For organizations that have strong privacy requirements for hiding
internal structures, a proxy MAY combine an ordered subsequence of
Via header field entries with identical received-protocol values into
a single such entry. For example,
could be collapsed to
Applications SHOULD NOT combine multiple entries unless they are all
under the same organizational control and the hosts have already been
replaced by pseudonyms. Applications MUST NOT combine entries which
have different received-protocol values.
The Message Header Registry located at should be updated
with the permanent registrations below (see ):
Header Field NameProtocolStatusReferenceConnectionhttpstandardContent-LengthhttpstandardDatehttpstandardHosthttpstandardTEhttpstandardTrailerhttpstandardTransfer-EncodinghttpstandardUpgradehttpstandardViahttpstandard
The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
The entry for the "http" URI Scheme in the registry located at
should be updated to point to of this document
(see ).
This document serves as the specification for the Internet media types
"message/http" and "application/http". The following is to be registered with
IANA (see ).
The message/http type can be used to enclose a single HTTP request or
response message, provided that it obeys the MIME restrictions for all
"message" types regarding line length and encodings.
message
http
none
version, msgtype
The HTTP-Version number of the enclosed message
(e.g., "1.1"). If not present, the version can be
determined from the first line of the body.
The message type -- "request" or "response". If not
present, the type can be determined from the first
line of the body.
only "7bit", "8bit", or "binary" are permitted
none
none
This specification (see ).
nonenonenone
See Authors Section.
COMMON
none
IESG
The application/http type can be used to enclose a pipeline of one or more
HTTP request or response messages (not intermixed).
application
http
none
version, msgtype
The HTTP-Version number of the enclosed messages
(e.g., "1.1"). If not present, the version can be
determined from the first line of the body.
The message type -- "request" or "response". If not
present, the type can be determined from the first
line of the body.
HTTP messages enclosed by this type
are in "binary" format; use of an appropriate
Content-Transfer-Encoding is required when
transmitted via E-mail.
none
none
This specification (see ).
nonenonenone
See Authors Section.
COMMON
none
IESG
This section is meant to inform application developers, information
providers, and users of the security limitations in HTTP/1.1 as
described by this document. The discussion does not include
definitive solutions to the problems revealed, though it does make
some suggestions for reducing security risks.
HTTP clients are often privy to large amounts of personal information
(e.g. the user's name, location, mail address, passwords, encryption
keys, etc.), and SHOULD be very careful to prevent unintentional
leakage of this information.
We very strongly recommend that a convenient interface be provided
for the user to control dissemination of such information, and that
designers and implementors be particularly careful in this area.
History shows that errors in this area often create serious security
and/or privacy problems and generate highly adverse publicity for the
implementor's company.
A server is in the position to save personal data about a user's
requests which might identify their reading patterns or subjects of
interest. This information is clearly confidential in nature and its
handling can be constrained by law in certain countries. People using
HTTP to provide data are responsible for ensuring that
such material is not distributed without the permission of any
individuals that are identifiable by the published results.
Implementations of HTTP origin servers SHOULD be careful to restrict
the documents returned by HTTP requests to be only those that were
intended by the server administrators. If an HTTP server translates
HTTP URIs directly into file system calls, the server MUST take
special care not to serve files that were not intended to be
delivered to HTTP clients. For example, UNIX, Microsoft Windows, and
other operating systems use ".." as a path component to indicate a
directory level above the current one. On such a system, an HTTP
server MUST disallow any such construct in the request-target if it
would otherwise allow access to a resource outside those intended to
be accessible via the HTTP server. Similarly, files intended for
reference only internally to the server (such as access control
files, configuration files, and script code) MUST be protected from
inappropriate retrieval, since they might contain sensitive
information. Experience has shown that minor bugs in such HTTP server
implementations have turned into security risks.
Clients using HTTP rely heavily on the Domain Name Service, and are
thus generally prone to security attacks based on the deliberate
mis-association of IP addresses and DNS names. Clients need to be
cautious in assuming the continuing validity of an IP number/DNS name
association.
In particular, HTTP clients SHOULD rely on their name resolver for
confirmation of an IP number/DNS name association, rather than
caching the result of previous host name lookups. Many platforms
already can cache host name lookups locally when appropriate, and
they SHOULD be configured to do so. It is proper for these lookups to
be cached, however, only when the TTL (Time To Live) information
reported by the name server makes it likely that the cached
information will remain useful.
If HTTP clients cache the results of host name lookups in order to
achieve a performance improvement, they MUST observe the TTL
information reported by DNS.
If HTTP clients do not observe this rule, they could be spoofed when
a previously-accessed server's IP address changes. As network
renumbering is expected to become increasingly common , the
possibility of this form of attack will grow. Observing this
requirement thus reduces this potential security vulnerability.
This requirement also improves the load-balancing behavior of clients
for replicated servers using the same DNS name and reduces the
likelihood of a user's experiencing failure in accessing sites which
use that strategy.
By their very nature, HTTP proxies are men-in-the-middle, and
represent an opportunity for man-in-the-middle attacks. Compromise of
the systems on which the proxies run can result in serious security
and privacy problems. Proxies have access to security-related
information, personal information about individual users and
organizations, and proprietary information belonging to users and
content providers. A compromised proxy, or a proxy implemented or
configured without regard to security and privacy considerations,
might be used in the commission of a wide range of potential attacks.
Proxy operators should protect the systems on which proxies run as
they would protect any system that contains or transports sensitive
information. In particular, log information gathered at proxies often
contains highly sensitive personal information, and/or information
about organizations. Log information should be carefully guarded, and
appropriate guidelines for use developed and followed. ().
Proxy implementors should consider the privacy and security
implications of their design and coding decisions, and of the
configuration options they provide to proxy operators (especially the
default configuration).
Users of a proxy need to be aware that they are no trustworthier than
the people who run the proxy; HTTP itself cannot solve this problem.
The judicious use of cryptography, when appropriate, may suffice to
protect against a broad range of security and privacy attacks. Such
cryptography is beyond the scope of the HTTP/1.1 specification.
They exist. They are hard to defend against. Research continues.
Beware.
HTTP has evolved considerably over the years. It has
benefited from a large and active developer community--the many
people who have participated on the www-talk mailing list--and it is
that community which has been most responsible for the success of
HTTP and of the World-Wide Web in general. Marc Andreessen, Robert
Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois
Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob
McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc
VanHeyningen deserve special recognition for their efforts in
defining early aspects of the protocol.
This document has benefited greatly from the comments of all those
participating in the HTTP-WG. In addition to those already mentioned,
the following individuals have contributed to this specification:
Gary Adams, Harald Tveit Alvestrand, Keith Ball, Brian Behlendorf,
Paul Burchard, Maurizio Codogno, Mike Cowlishaw, Roman Czyborra,
Michael A. Dolan, Daniel DuBois, David J. Fiander, Alan Freier, Marc Hedlund, Greg Herlihy,
Koen Holtman, Alex Hopmann, Bob Jernigan, Shel Kaphan, Rohit Khare,
John Klensin, Martijn Koster, Alexei Kosut, David M. Kristol,
Daniel LaLiberte, Ben Laurie, Paul J. Leach, Albert Lunde,
John C. Mallery, Jean-Philippe Martin-Flatin, Mitra, David Morris,
Gavin Nicol, Ross Patterson, Bill Perry, Jeffrey Perry, Scott Powers, Owen Rees,
Luigi Rizzo, David Robinson, Marc Salomon, Rich Salz,
Allan M. Schiffman, Jim Seidman, Chuck Shotton, Eric W. Sink,
Simon E. Spero, Richard N. Taylor, Robert S. Thau,
Bill (BearHeart) Weinman, Francois Yergeau, Mary Ellen Zurko,
Josh Cohen.
Thanks to the "cave men" of Palo Alto. You know who you are.
Jim Gettys (the editor of ) wishes particularly
to thank Roy Fielding, the editor of , along
with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen
Holtman, John Franks, Josh Cohen, Alex Hopmann, Scott Lawrence, and
Larry Masinter for their help. And thanks go particularly to Jeff
Mogul and Scott Lawrence for performing the "MUST/MAY/SHOULD" audit.
The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik
Frystyk implemented RFC 2068 early, and we wish to thank them for the
discovery of many of the problems that this document attempts to
rectify.
This specification makes heavy use of the augmented BNF and generic
constructs defined by David H. Crocker for . Similarly, it
reuses many of the definitions provided by Nathaniel Borenstein and
Ned Freed for MIME . We hope that their inclusion in this
specification will help reduce past confusion over the relationship
between HTTP and Internet mail message formats.
Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
International Organization for StandardizationHTTP/1.1, part 2: Message SemanticsDay Softwarefielding@gbiv.comOne Laptop per Childjg@laptop.orgHewlett-Packard CompanyJeffMogul@acm.orgMicrosoft Corporationhenrikn@microsoft.comAdobe Systems, IncorporatedLMM@acm.orgMicrosoft Corporationpaulle@microsoft.comWorld Wide Web Consortiumtimbl@w3.orgWorld Wide Web Consortiumylafon@w3.orggreenbytes GmbHjulian.reschke@greenbytes.deHTTP/1.1, part 3: Message Payload and Content NegotiationDay Softwarefielding@gbiv.comOne Laptop per Childjg@laptop.orgHewlett-Packard CompanyJeffMogul@acm.orgMicrosoft Corporationhenrikn@microsoft.comAdobe Systems, IncorporatedLMM@acm.orgMicrosoft Corporationpaulle@microsoft.comWorld Wide Web Consortiumtimbl@w3.orgWorld Wide Web Consortiumylafon@w3.orggreenbytes GmbHjulian.reschke@greenbytes.deHTTP/1.1, part 5: Range Requests and Partial ResponsesDay Softwarefielding@gbiv.comOne Laptop per Childjg@laptop.orgHewlett-Packard CompanyJeffMogul@acm.orgMicrosoft Corporationhenrikn@microsoft.comAdobe Systems, IncorporatedLMM@acm.orgMicrosoft Corporationpaulle@microsoft.comWorld Wide Web Consortiumtimbl@w3.orgWorld Wide Web Consortiumylafon@w3.orggreenbytes GmbHjulian.reschke@greenbytes.deHTTP/1.1, part 6: CachingDay Softwarefielding@gbiv.comOne Laptop per Childjg@laptop.orgHewlett-Packard CompanyJeffMogul@acm.orgMicrosoft Corporationhenrikn@microsoft.comAdobe Systems, IncorporatedLMM@acm.orgMicrosoft Corporationpaulle@microsoft.comWorld Wide Web Consortiumtimbl@w3.orgWorld Wide Web Consortiumylafon@w3.orggreenbytes GmbHjulian.reschke@greenbytes.deAugmented BNF for Syntax Specifications: ABNFBrandenburg InternetWorking675 Spruce Dr.SunnyvaleCA94086US+1.408.246.8253dcrocker@bbiw.netTHUS plc.1/2 Berkeley Square99 Berkely StreetGlasgowG3 7HRUKpaul.overell@thus.netKey words for use in RFCs to Indicate Requirement LevelsHarvard Universitysob@harvard.eduUniform Resource Identifier (URI): Generic SyntaxWorld Wide Web Consortiumtimbl@w3.orghttp://www.w3.org/People/Berners-Lee/Day Softwarefielding@gbiv.comhttp://roy.gbiv.com/Adobe Systems IncorporatedLMM@acm.orghttp://larry.masinter.net/Coded Character Set -- 7-bit American Standard Code for Information InterchangeAmerican National Standards InstituteNetwork Performance Effects of HTTP/1.1, CSS1, and PNGImproving HTTP LatencyRequirements for Internet Hosts - Application and SupportUniversity of Southern California (USC), Information Sciences InstituteBraden@ISI.EDUNetwork Time Protocol (Version 3) Specification, ImplementationUniversity of Delaware, Electrical Engineering Departmentmills@udel.eduRenumbering Needs WorkCERN, Computing and Networks Divisionbrian@dxcoms.cern.chcisco Systemsyakov@cisco.comHypertext Transfer Protocol -- HTTP/1.0MIT, Laboratory for Computer Sciencetimbl@w3.orgUniversity of California, Irvine, Department of Information and Computer Sciencefielding@ics.uci.eduW3 Consortium, MIT Laboratory for Computer Sciencefrystyk@w3.orgMultipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message BodiesInnosoft International, Inc.ned@innosoft.comFirst Virtual Holdingsnsb@nsb.fv.comMIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII TextUniversity of Tennesseemoore@cs.utk.eduHypertext Transfer Protocol -- HTTP/1.1University of California, Irvine, Department of Information and Computer Sciencefielding@ics.uci.eduMIT Laboratory for Computer Sciencejg@w3.orgDigital Equipment Corporation, Western Research Laboratorymogul@wrl.dec.comMIT Laboratory for Computer Sciencefrystyk@w3.orgMIT Laboratory for Computer Sciencetimbl@w3.orgHTTP State Management MechanismBell Laboratories, Lucent Technologiesdmk@bell-labs.comNetscape Communications Corp.montulli@netscape.comUse and Interpretation of HTTP Version NumbersWestern Research Laboratorymogul@wrl.dec.comDepartment of Information and Computer Sciencefielding@ics.uci.eduMIT Laboratory for Computer Sciencejg@w3.orgW3 Consortiumfrystyk@w3.orgHypertext Transfer Protocol -- HTTP/1.1University of California, Irvinefielding@ics.uci.eduW3Cjg@w3.orgCompaq Computer Corporationmogul@wrl.dec.comMIT Laboratory for Computer Sciencefrystyk@w3.orgXerox Corporationmasinter@parc.xerox.comMicrosoft Corporationpaulle@microsoft.comW3Ctimbl@w3.orgHTTP Over TLSRTFM, Inc.ekr@rtfm.comHTTP State Management MechanismBell Laboratories, Lucent Technologiesdmk@bell-labs.comEpinions.com, Inc.lou@montulli.orgRegistration Procedures for Message Header FieldsNine by NineGK-IETF@ninebynine.orgBEA Systemsmnot@pobox.comHP LabsJeffMogul@acm.orgMedia Type Specifications and Registration ProceduresSun Microsystemsned.freed@mrochek.comklensin+ietf@jck.comGuidelines and Registration Procedures for New URI SchemesAT&T Laboratoriestony+urireg@maillennium.att.comQualcomm, Inc.hardie@qualcomm.comAdobe SystemsLMM@acm.orgInternet Message FormatQualcomm IncorporatedHTTP Cookies: Standards, Privacy, and PoliticsAnalysis of HTTP Performance ProblemsAnalysis of HTTP PerformanceUSC/Information Sciences Institutetouch@isi.eduUSC/Information Sciences Institutejohnh@isi.eduUSC/Information Sciences Institutekatia@isi.edu(original report dated Aug. 1996)
Although this document specifies the requirements for the generation
of HTTP/1.1 messages, not all applications will be correct in their
implementation. We therefore recommend that operational applications
be tolerant of deviations whenever those deviations can be
interpreted unambiguously.
Clients SHOULD be tolerant in parsing the Status-Line and servers
tolerant when parsing the Request-Line. In particular, they SHOULD
accept any amount of WSP characters between fields, even though
only a single SP is required.
The line terminator for message-header fields is the sequence CRLF.
However, we recommend that applications, when parsing such headers,
recognize a single LF as a line terminator and ignore the leading CR.
The character set of an entity-body SHOULD be labeled as the lowest
common denominator of the character codes used within that body, with
the exception that not labeling the entity is preferred over labeling
the entity with the labels US-ASCII or ISO-8859-1. See .
Additional rules for requirements on parsing and encoding of dates
and other potential problems with date encodings include:
HTTP/1.1 clients and caches SHOULD assume that an RFC-850 date
which appears to be more than 50 years in the future is in fact
in the past (this helps solve the "year 2000" problem).An HTTP/1.1 implementation MAY internally represent a parsed
Expires date as earlier than the proper value, but MUST NOT
internally represent a parsed Expires date as later than the
proper value.All expiration-related calculations MUST be done in GMT. The
local time zone MUST NOT influence the calculation or comparison
of an age or expiration time.If an HTTP header incorrectly carries a date value with a time
zone other than GMT, it MUST be converted into GMT using the
most conservative possible conversion.
HTTP has been in use by the World-Wide Web global information initiative
since 1990. The first version of HTTP, later referred to as HTTP/0.9,
was a simple protocol for hypertext data transfer across the Internet
with only a single method and no metadata.
HTTP/1.0, as defined by , added a range of request
methods and MIME-like messaging that could include metadata about the data
transferred and modifiers on the request/response semantics. However,
HTTP/1.0 did not sufficiently take into consideration the effects of
hierarchical proxies, caching, the need for persistent connections, or
name-based virtual hosts. The proliferation of incompletely-implemented
applications calling themselves "HTTP/1.0" further necessitated a
protocol version change in order for two communicating applications
to determine each other's true capabilities.
HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
requirements that enable reliable implementations, adding only
those new features that will either be safely ignored by an HTTP/1.0
recipient or only sent when communicating with a party advertising
compliance with HTTP/1.1.
It is beyond the scope of a protocol specification to mandate
compliance with previous versions. HTTP/1.1 was deliberately
designed, however, to make supporting previous versions easy. It is
worth noting that, at the time of composing this specification
(1996), we would expect commercial HTTP/1.1 servers to:
recognize the format of the Request-Line for HTTP/0.9, 1.0, and
1.1 requests;understand any valid request in the format of HTTP/0.9, 1.0, or
1.1;respond appropriately with a message in the same major version
used by the client.
And we would expect HTTP/1.1 clients to:
recognize the format of the Status-Line for HTTP/1.0 and 1.1
responses;understand any valid response in the format of HTTP/0.9, 1.0, or
1.1.
For most implementations of HTTP/1.0, each connection is established
by the client prior to the request and closed by the server after
sending the response. Some implementations implement the Keep-Alive
version of persistent connections described in Section 19.7.1 of .
This section summarizes major differences between versions HTTP/1.0
and HTTP/1.1.
The requirements that clients and servers support the Host request-header,
report an error if the Host request-header () is
missing from an HTTP/1.1 request, and accept absolute URIs ()
are among the most important changes defined by this
specification.
Older HTTP/1.0 clients assumed a one-to-one relationship of IP
addresses and servers; there was no other established mechanism for
distinguishing the intended server of a request than the IP address
to which that request was directed. The changes outlined above will
allow the Internet, once older HTTP clients are no longer common, to
support multiple Web sites from a single IP address, greatly
simplifying large operational Web servers, where allocation of many
IP addresses to a single host has created serious problems. The
Internet will also be able to recover the IP addresses that have been
allocated for the sole purpose of allowing special-purpose domain
names to be used in root-level HTTP URLs. Given the rate of growth of
the Web, and the number of servers already deployed, it is extremely
important that all implementations of HTTP (including updates to
existing HTTP/1.0 applications) correctly implement these
requirements:
Both clients and servers MUST support the Host request-header.A client that sends an HTTP/1.1 request MUST send a Host header.Servers MUST report a 400 (Bad Request) error if an HTTP/1.1
request does not include a Host request-header.Servers MUST accept absolute URIs.
Some clients and servers might wish to be compatible with some
previous implementations of persistent connections in HTTP/1.0
clients and servers. Persistent connections in HTTP/1.0 are
explicitly negotiated as they are not the default behavior. HTTP/1.0
experimental implementations of persistent connections are faulty,
and the new facilities in HTTP/1.1 are designed to rectify these
problems. The problem was that some existing 1.0 clients may be
sending Keep-Alive to a proxy server that doesn't understand
Connection, which would then erroneously forward it to the next
inbound server, which would establish the Keep-Alive connection and
result in a hung HTTP/1.0 proxy waiting for the close on the
response. The result is that HTTP/1.0 clients must be prevented from
using Keep-Alive when talking to proxies.
However, talking to proxies is the most important use of persistent
connections, so that prohibition is clearly unacceptable. Therefore,
we need some other mechanism for indicating a persistent connection
is desired, which is safe to use even when talking to an old proxy
that ignores Connection. Persistent connections are the default for
HTTP/1.1 messages; we introduce a new keyword (Connection: close) for
declaring non-persistence. See .
The original HTTP/1.0 form of persistent connections (the Connection:
Keep-Alive and Keep-Alive header) is documented in .
This specification has been carefully audited to correct and
disambiguate key word usage; RFC 2068 had many problems in respect to
the conventions laid out in .
Transfer-coding and message lengths all interact in ways that
required fixing exactly when chunked encoding is used (to allow for
transfer encoding that may not be self delimiting); it was important
to straighten out exactly how message lengths are computed. (Sections
, ,
,
see also , and )
The use and interpretation of HTTP version numbers has been clarified
by . Require proxies to upgrade requests to highest protocol
version they support to deal with problems discovered in HTTP/1.0
implementations ()
Quality Values of zero should indicate that "I don't want something"
to allow clients to refuse a representation. ()
Transfer-coding had significant problems, particularly with
interactions with chunked encoding. The solution is that transfer-codings
become as full fledged as content-codings. This involves
adding an IANA registry for transfer-codings (separate from content
codings), a new header field (TE) and enabling trailer headers in the
future. Transfer encoding is a major performance benefit, so it was
worth fixing . TE also solves another, obscure, downward
interoperability problem that could have occurred due to interactions
between authentication trailers, chunked encoding and HTTP/1.0
clients.(Section , ,
and )
Empty list elements in list productions have been deprecated.
()
Rules about implicit linear whitespace between certain grammar productions
have been removed; now it's only allowed when specifically pointed out
in the ABNF. The NUL character is no longer allowed in comment and quoted-string
text. The quoted-pair rule no longer allows escaping NUL, CR or LF.
Non-ASCII content in header fields and reason phrase has been obsoleted and
made opaque (the TEXT rule was removed)
()
Clarify that HTTP-Version is case sensitive.
()
Remove reference to non-existant identity transfer-coding value tokens.
(Sections and
)
Clarification that the chunk length does not include
the count of the octets in the chunk header and trailer.
()
Require that invalid whitespace around field-names be rejected.
()
Update use of abs_path production from RFC1808 to the path-absolute + query
components of RFC3986.
()
Clarify exactly when close connection options must be sent.
()
This specification uses a number of terms to refer to the roles
played by participants in, and objects of, the HTTP communication.
cache
A program's local store of response messages and the subsystem
that controls its message storage, retrieval, and deletion. A
cache stores cacheable responses in order to reduce the response
time and network bandwidth consumption on future, equivalent
requests. Any client or server may include a cache, though a cache
cannot be used by a server that is acting as a tunnel.
cacheable
A response is cacheable if a cache is allowed to store a copy of
the response message for use in answering subsequent requests. The
rules for determining the cacheability of HTTP responses are
defined in Section 1 of . Even if a resource is cacheable, there may
be additional constraints on whether a cache can use the cached
copy for a particular request.
client
A program that establishes connections for the purpose of sending
requests.
connection
A transport layer virtual circuit established between two programs
for the purpose of communication.
content negotiation
The mechanism for selecting the appropriate representation when
servicing a request, as described in Section 4 of . The
representation of entities in any response can be negotiated
(including error responses).
entity
The information transferred as the payload of a request or
response. An entity consists of metainformation in the form of
entity-header fields and content in the form of an entity-body, as
described in Section 3 of .
gateway
A server which acts as an intermediary for some other server.
Unlike a proxy, a gateway receives requests as if it were the
origin server for the requested resource; the requesting client
may not be aware that it is communicating with a gateway.
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"
message
The basic unit of HTTP communication, consisting of a structured
sequence of octets matching the syntax defined in and
transmitted via the connection.
origin server
The server on which a given resource resides or is to be created.
proxy
An intermediary program which acts as both a server and a client
for the purpose of making requests on behalf of other clients.
Requests are serviced internally or by passing them on, with
possible translation, to other servers. A proxy MUST implement
both the client and server requirements of this specification. A
"transparent proxy" is a proxy that does not modify the request or
response beyond what is required for proxy authentication and
identification. A "non-transparent proxy" is a proxy that modifies
the request or response in order to provide some added service to
the user agent, such as group annotation services, media type
transformation, protocol reduction, or anonymity filtering. Except
where either transparent or non-transparent behavior is explicitly
stated, the HTTP proxy requirements apply to both types of
proxies.
request
An HTTP request message, as defined in .
resource
A network data object or service that can be identified by a URI,
as defined in . Resources may be available in multiple
representations (e.g. multiple languages, data formats, size, and
resolutions) or vary in other ways.
response
An HTTP response message, as defined in .
representation
An entity included with a response that is subject to content
negotiation, as described in Section 4 of . There may exist multiple
representations associated with a particular response status.
server
An application program that accepts connections in order to
service requests by sending back responses. Any given program may
be capable of being both a client and a server; our use of these
terms refers only to the role being performed by the program for a
particular connection, rather than to the program's capabilities
in general. Likewise, any server may act as an origin server,
proxy, gateway, or tunnel, switching behavior based on the nature
of each request.
tunnel
An intermediary program which is acting as a blind relay between
two connections. Once active, a tunnel is not considered a party
to the HTTP communication, though the tunnel may have been
initiated by an HTTP request. The tunnel ceases to exist when both
ends of the relayed connections are closed.
upstream/downstream
Upstream and downstream describe the flow of a message: all
messages flow from upstream to downstream.
user agent
The client which initiates a request. These are often browsers,
editors, spiders (web-traversing robots), or other end user tools.
variant
A resource may have one, or more than one, representation(s)
associated with it at any given instant. Each of these
representations is termed a `variant'. Use of the term `variant'
does not necessarily imply that the resource is subject to content
negotiation.
Extracted relevant partitions from .
Closed issues:
:
"HTTP Version should be case sensitive"
()
:
"'unsafe' characters"
()
:
"Chunk Size Definition"
()
:
"Message Length"
()
:
"Media Type Registrations"
()
:
"URI includes query"
()
:
"No close on 1xx responses"
()
:
"Remove 'identity' token references"
()
:
"Import query BNF"
:
"qdtext BNF"
:
"Normative and Informative references"
:
"RFC2606 Compliance"
:
"RFC977 reference"
:
"RFC1700 references"
:
"inconsistency in date format explanation"
:
"Date reference typo"
:
"Informative references"
:
"ISO-8859-1 Reference"
:
"Normative up-to-date references"
Other changes:
Update media type registrations to use RFC4288 template.
Use names of RFC4234 core rules DQUOTE and WSP,
fix broken ABNF for chunk-data
(work in progress on )
Closed issues:
:
"Bodies on GET (and other) requests"
:
"Updating to RFC4288"
:
"Status Code and Reason Phrase"
:
"rel_path not used"
Ongoing work on ABNF conversion ():
Get rid of duplicate BNF rule names ("host" -> "uri-host", "trailer" ->
"trailer-part").
Avoid underscore character in rule names ("http_URL" ->
"http-URL", "abs_path" -> "path-absolute").
Add rules for terms imported from URI spec ("absoluteURI", "authority",
"path-absolute", "port", "query", "relativeURI", "host) -- these will
have to be updated when switching over to RFC3986.
Synchronize core rules with RFC5234.
Get rid of prose rules that span multiple lines.
Get rid of unused rules LOALPHA and UPALPHA.
Move "Product Tokens" section (back) into Part 1, as "token" is used
in the definition of the Upgrade header.
Add explicit references to BNF syntax and rules imported from other parts of the specification.
Rewrite prose rule "token" in terms of "tchar", rewrite prose rule "TEXT".
Closed issues:
:
"HTTP-date vs. rfc1123-date"
:
"WS in quoted-pair"
Ongoing work on IANA Message Header Registration ():
Reference RFC 3984, and update header registrations for headers defined
in this document.
Ongoing work on ABNF conversion ():
Replace string literals when the string really is case-sensitive (HTTP-Version).
Closed issues:
:
"Connection closing"
:
"Move registrations and registry information to IANA Considerations"
:
"need new URL for PAD1995 reference"
:
"IANA Considerations: update HTTP URI scheme registration"
:
"Cite HTTPS URI scheme definition"
:
"List-type headers vs Set-Cookie"
Ongoing work on ABNF conversion ():
Replace string literals when the string really is case-sensitive (HTTP-Date).
Replace HEX by HEXDIG for future consistence with RFC 5234's core rules.
Closed issues:
:
"Out-of-date reference for URIs"
:
"RFC 2822 is updated by RFC 5322"
Ongoing work on ABNF conversion ():
Use "/" instead of "|" for alternatives.
Get rid of RFC822 dependency; use RFC5234 plus extensions instead.
Only reference RFC 5234's core rules.
Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
whitespace ("OWS") and required whitespace ("RWS").
Rewrite ABNFs to spell out whitespace rules, factor out
header value format definitions.
Closed issues:
:
"Header LWS"
:
"Sort 1.3 Terminology"
:
"RFC2047 encoded words"
:
"Character Encodings in TEXT"
:
"Line Folding"
:
"OPTIONS * and proxies"
:
"Reason-Phrase BNF"
:
"Use of TEXT"
:
"Join "Differences Between HTTP Entities and RFC 2045 Entities"?"
:
"RFC822 reference left in discussion of date formats"
Final work on ABNF conversion ():
Rewrite definition of list rules, deprecate empty list elements.
Add appendix containing collected and expanded ABNF.
Other changes:
Rewrite introduction; add mostly new Architecture Section.
Move definition of quality values from Part 3 into Part 1;
make TE request header grammar independent of accept-params (defined in Part 3).