RFC 2068 HTTP/1.1 January 1997
Any HTTP/1.1 message containing an entity-body SHOULD include a
Content-Type header field defining the media type of that body. If
and only if the media type is not given by a Content-Type field, the
recipient MAY attempt to guess the media type via inspection of its
content and/or the name extension(s) of the URL used to identify the
resource. If the media type remains unknown, the recipient SHOULD
treat it as type "application/octet-stream".
7.2.2 Length
The length of an entity-body is the length of the message-body after
any transfer codings have been removed. Section 4.4 defines how the
length of a message-body is determined.
8 Connections
8.1 Persistent Connections
8.1.1 Purpose
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 requires a client to make multiple
requests of the same server in a short amount of time. Analyses of
these performance problems are available [30][27]; analysis and
results from a prototype implementation are in [26].
Persistent HTTP connections have a number of advantages:
o By opening and closing fewer TCP connections, CPU time is saved,
and memory used for TCP protocol control blocks is also saved.
o 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.
o 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.
o 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.
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8.1.2 Overall Operation
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 may
assume that the server will maintain a persistent connection.
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.
8.1.2.1 Negotiation
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 section 19.7.1 for more information on backwards
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 section 4.4.
8.1.2.2 Pipelining
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.
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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.
8.1.3 Proxy Servers
It is especially important that proxies correctly implement the
properties of the Connection header field as specified in 14.2.1.
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 persistent connection with an
HTTP/1.0 client.
8.1.4 Practical Considerations
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 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 MAY 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 request without user
interaction so long as the request method is idempotent (see section
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9.1.2); other methods MUST NOT be automatically retried, although
user agents MAY offer a human operator the choice of retrying the
request.
However, this automatic retry SHOULD NOT be repeated if the second
request 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 maintain AT MOST 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 of the Internet or other networks.
8.2 Message Transmission Requirements
General requirements:
o 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.
o 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 (section 3.6), a zero length chunk and
empty footer 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.
o An HTTP/1.1 (or later) client MUST be prepared to accept a 100
(Continue) status followed by a regular response.
o An HTTP/1.1 (or later) server that receives a request from a
HTTP/1.0 (or earlier) client MUST NOT transmit the 100 (continue)
response; it SHOULD either wait for the request to be completed
normally (thus avoiding an interrupted request) or close the
connection prematurely.
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Upon receiving a method subject to these requirements from an
HTTP/1.1 (or later) client, an HTTP/1.1 (or later) server MUST either
respond with 100 (Continue) status and continue to read from the
input stream, or respond with an error status. If it responds with an
error status, it MAY close the transport (TCP) connection or it MAY
continue to read and discard the rest of the request. It MUST NOT
perform the requested method if it returns an error status.
Clients SHOULD remember the version number of at least the most
recently used server; if an HTTP/1.1 client has seen an HTTP/1.1 or
later response from the server, and it sees the connection close
before receiving any status from the server, the client SHOULD retry
the request without user interaction so long as the request method is
idempotent (see section 9.1.2); other methods MUST NOT be
automatically retried, although user agents MAY offer a human
operator the choice of retrying the request.. If the client does
retry the request, the client
o MUST first send the request header fields, and then
o MUST wait for the server to respond with either a 100 (Continue)
response, in which case the client should continue, or with an
error status.
If an HTTP/1.1 client has not seen an HTTP/1.1 or later response from
the server, it should assume that the server implements HTTP/1.0 or
older and will not use the 100 (Continue) response. If in this case
the client sees the connection close before receiving any status from
the server, the client SHOULD retry the request. If the client does
retry the request to this HTTP/1.0 server, it should use the
following "binary exponential backoff" algorithm to be assured of
obtaining a reliable response:
1. Initiate a new connection to the server
2. Transmit the request-headers
3. 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.
4. Compute T = R * (2**N), where N is the number of previous retries
of this request.
5. Wait either for an error response from the server, or for T seconds
(whichever comes first)
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6. If no error response is received, after T seconds transmit the body
of the request.
7. 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.
No matter what the server version, if an error status is received,
the client
o MUST NOT continue and
o MUST close the connection if it has not completed sending the
message.
An HTTP/1.1 (or later) client that sees the connection close after
receiving a 100 (Continue) but before receiving any other status
SHOULD retry the request, and need not wait for 100 (Continue)
response (but MAY do so if this simplifies the implementation).
9 Method Definitions
The set of common methods for HTTP/1.1 is defined below. Although
this set can be expanded, additional methods cannot be assumed to
share the same semantics for separately extended clients and servers.
The Host request-header field (section 14.23) MUST accompany all
HTTP/1.1 requests.
9.1 Safe and Idempotent Methods
9.1.1 Safe Methods
Implementers should be aware that the software represents the user in
their interactions over the Internet, and should be careful to allow
the user to be aware of any actions they may take which may have an
unexpected significance to themselves or others.
In particular, the convention has been established that the GET and
HEAD methods should never have the significance of taking an action
other than retrieval. These methods should be considered "safe." This
allows user agents to represent other methods, such as POST, PUT and
DELETE, in a special way, so that the user is made aware of the fact
that a possibly unsafe action is being requested.
Naturally, it is not possible to ensure that the server does not
generate side-effects as a result of performing a GET request; in
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fact, some dynamic resources consider that a feature. The important
distinction here is that the user did not request the side-effects,
so therefore cannot be held accountable for them.
9.1.2 Idempotent Methods
Methods may also have the property of "idempotence" in that (aside
from error or expiration issues) the side-effects of N > 0 identical
requests is the same as for a single request. The methods GET, HEAD,
PUT and DELETE share this property.
9.2 OPTIONS
The OPTIONS method represents a request for information about the
communication options available on the request/response chain
identified by the Request-URI. This method allows the client to
determine the options and/or requirements associated with a resource,
or the capabilities of a server, without implying a resource action
or initiating a resource retrieval.
Unless the server's response is an error, the response MUST NOT
include entity information other than what can be considered as
communication options (e.g., Allow is appropriate, but Content-Type
is not). Responses to this method are not cachable.
If the Request-URI is an asterisk ("*"), the OPTIONS request is
intended to apply to the server as a whole. A 200 response SHOULD
include any header fields which indicate optional features
implemented by the server (e.g., Public), including any extensions
not defined by this specification, in addition to any applicable
general or response-header fields. As described in section 5.1.2, an
"OPTIONS *" request can be applied through a proxy by specifying the
destination server in the Request-URI without any path information.
If the Request-URI is not an asterisk, the OPTIONS request applies
only to the options that are available when communicating with that
resource. A 200 response SHOULD include any header fields which
indicate optional features implemented by the server and applicable
to that resource (e.g., Allow), including any extensions not defined
by this specification, in addition to any applicable general or
response-header fields. If the OPTIONS request passes through a
proxy, the proxy MUST edit the response to exclude those options
which apply to a proxy's capabilities and which are known to be
unavailable through that proxy.
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9.3 GET
The GET method means retrieve whatever information (in the form of an
entity) is identified by the Request-URI. If the Request-URI refers
to a data-producing process, it is the produced data which shall be
returned as the entity in the response and not the source text of the
process, unless that text happens to be the output of the process.
The semantics of the GET method change to a "conditional GET" if the
request message includes an If-Modified-Since, If-Unmodified-Since,
If-Match, If-None-Match, or If-Range header field. A conditional GET
method requests that the entity be transferred only under the
circumstances described by the conditional header field(s). The
conditional GET method is intended to reduce unnecessary network
usage by allowing cached entities to be refreshed without requiring
multiple requests or transferring data already held by the client.
The semantics of the GET method change to a "partial GET" if the
request message includes a Range header field. A partial GET requests
that only part of the entity be transferred, as described in section
14.36. The partial GET method is intended to reduce unnecessary
network usage by allowing partially-retrieved entities to be
completed without transferring data already held by the client.
The response to a GET request is cachable if and only if it meets the
requirements for HTTP caching described in section 13.
9.4 HEAD
The HEAD method is identical to GET except that the server MUST NOT
return a message-body in the response. The metainformation contained
in the HTTP headers in response to a HEAD request SHOULD be identical
to the information sent in response to a GET request. This method can
be used for obtaining metainformation about the entity implied by the
request without transferring the entity-body itself. This method is
often used for testing hypertext links for validity, accessibility,
and recent modification.
The response to a HEAD request may be cachable in the sense that the
information contained in the response may be used to update a
previously cached entity from that resource. If the new field values
indicate that the cached entity differs from the current entity (as
would be indicated by a change in Content-Length, Content-MD5, ETag
or Last-Modified), then the cache MUST treat the cache entry as
stale.
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9.5 POST
The POST method is used to request that the destination server accept
the entity enclosed in the request as a new subordinate of the
resource identified by the Request-URI in the Request-Line. POST is
designed to allow a uniform method to cover the following functions:
o Annotation of existing resources;
o Posting a message to a bulletin board, newsgroup, mailing list,
or similar group of articles;
o Providing a block of data, such as the result of submitting a
form, to a data-handling process;
o Extending a database through an append operation.
The actual function performed by the POST method is determined by the
server and is usually dependent on the Request-URI. The posted entity
is subordinate to that URI in the same way that a file is subordinate
to a directory containing it, a news article is subordinate to a
newsgroup to which it is posted, or a record is subordinate to a
database.
The action performed by the POST method might not result in a
resource that can be identified by a URI. In this case, either 200
(OK) or 204 (No Content) is the appropriate response status,
depending on whether or not the response includes an entity that
describes the result.
If a resource has been created on the origin server, the response
SHOULD be 201 (Created) and contain an entity which describes the
status of the request and refers to the new resource, and a Location
header (see section 14.30).
Responses to this method are not cachable, unless the response
includes appropriate Cache-Control or Expires header fields. However,
the 303 (See Other) response can be used to direct the user agent to
retrieve a cachable resource.
POST requests must obey the message transmission requirements set out
in section 8.2.
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9.6 PUT
The PUT method requests that the enclosed entity be stored under the
supplied Request-URI. If the Request-URI refers to an already
existing resource, the enclosed entity SHOULD be considered as a
modified version of the one residing on the origin server. If the
Request-URI does not point to an existing resource, and that URI is
capable of being defined as a new resource by the requesting user
agent, the origin server can create the resource with that URI. If a
new resource is created, the origin server MUST inform the user agent
via the 201 (Created) response. If an existing resource is modified,
either the 200 (OK) or 204 (No Content) response codes SHOULD be sent
to indicate successful completion of the request. If the resource
could not be created or modified with the Request-URI, an appropriate
error response SHOULD be given that reflects the nature of the
problem. The recipient of the entity MUST NOT ignore any Content-*
(e.g. Content-Range) headers that it does not understand or implement
and MUST return a 501 (Not Implemented) response in such cases.
If the request passes through a cache and the Request-URI identifies
one or more currently cached entities, those entries should be
treated as stale. Responses to this method are not cachable.
The fundamental difference between the POST and PUT requests is
reflected in the different meaning of the Request-URI. The URI in a
POST request identifies the resource that will handle the enclosed
entity. That resource may be a data-accepting process, a gateway to
some other protocol, or a separate entity that accepts annotations.
In contrast, the URI in a PUT request identifies the entity enclosed
with the request -- the user agent knows what URI is intended and the
server MUST NOT attempt to apply the request to some other resource.
If the server desires that the request be applied to a different URI,
it MUST send a 301 (Moved Permanently) response; the user agent MAY
then make its own decision regarding whether or not to redirect the
request.
A single resource MAY be identified by many different URIs. For
example, an article may have a URI for identifying "the current
version" which is separate from the URI identifying each particular
version. In this case, a PUT request on a general URI may result in
several other URIs being defined by the origin server.
HTTP/1.1 does not define how a PUT method affects the state of an
origin server.
PUT requests must obey the message transmission requirements set out
in section 8.2.
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9.7 DELETE
The DELETE method requests that the origin server delete the resource
identified by the Request-URI. This method MAY be overridden by human
intervention (or other means) on the origin server. The client cannot
be guaranteed that the operation has been carried out, even if the
status code returned from the origin server indicates that the action
has been completed successfully. However, the server SHOULD not
indicate success unless, at the time the response is given, it
intends to delete the resource or move it to an inaccessible
location.
A successful response SHOULD be 200 (OK) if the response includes an
entity describing the status, 202 (Accepted) if the action has not
yet been enacted, or 204 (No Content) if the response is OK but does
not include an entity.
If the request passes through a cache and the Request-URI identifies
one or more currently cached entities, those entries should be
treated as stale. Responses to this method are not cachable.
9.8 TRACE
The TRACE method is used to invoke a remote, application-layer loop-
back of the request message. The final recipient of the request
SHOULD reflect the message received back to the client as the
entity-body of a 200 (OK) response. The final recipient is either the
origin server or the first proxy or gateway to receive a Max-Forwards
value of zero (0) in the request (see section 14.31). A TRACE request
MUST NOT include an entity.
TRACE allows the client to see what is being received at the other
end of the request chain and use that data for testing or diagnostic
information. The value of the Via header field (section 14.44) is of
particular interest, since it acts as a trace of the request chain.
Use of the Max-Forwards header field allows the client to limit the
length of the request chain, which is useful for testing a chain of
proxies forwarding messages in an infinite loop.
If successful, the response SHOULD contain the entire request message
in the entity-body, with a Content-Type of "message/http". Responses
to this method MUST NOT be cached.
10 Status Code Definitions
Each Status-Code is described below, including a description of which
method(s) it can follow and any metainformation required in the
response.
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10.1 Informational 1xx
This class of status code indicates a provisional response,
consisting only of the Status-Line and optional headers, and is
terminated by an empty line. Since HTTP/1.0 did not define any 1xx
status codes, servers MUST NOT send a 1xx response to an HTTP/1.0
client except under experimental conditions.
10.1.1 100 Continue
The client may continue with its request. This interim response is
used to inform the client that the initial part of the request has
been received and has not yet been rejected by the server. The client
SHOULD continue by sending the remainder of the request or, if the
request has already been completed, ignore this response. The server
MUST send a final response after the request has been completed.
10.1.2 101 Switching Protocols
The server understands and is willing to comply with the client's
request, via the Upgrade message header field (section 14.41), for a
change in the application protocol being used on this connection. The
server will switch protocols to those defined by the response's
Upgrade header field immediately after the empty line which
terminates the 101 response.
The protocol should only be switched when it is advantageous to do
so. For example, switching to a newer version of HTTP is
advantageous over older versions, and switching to a real-time,
synchronous protocol may be advantageous when delivering resources
that use such features.
10.2 Successful 2xx
This class of status code indicates that the client's request was
successfully received, understood, and accepted.
10.2.1 200 OK
The request has succeeded. The information returned with the response
is dependent on the method used in the request, for example:
GET an entity corresponding to the requested resource is sent in the
response;
HEAD the entity-header fields corresponding to the requested resource
are sent in the response without any message-body;
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POST an entity describing or containing the result of the action;
TRACE an entity containing the request message as received by the end
server.
10.2.2 201 Created
The request has been fulfilled and resulted in a new resource being
created. The newly created resource can be referenced by the URI(s)
returned in the entity of the response, with the most specific URL
for the resource given by a Location header field. The origin server
MUST create the resource before returning the 201 status code. If the
action cannot be carried out immediately, the server should respond
with 202 (Accepted) response instead.
10.2.3 202 Accepted
The request has been accepted for processing, but the processing has
not been completed. The request MAY or MAY NOT eventually be acted
upon, as it MAY be disallowed when processing actually takes place.
There is no facility for re-sending a status code from an
asynchronous operation such as this.
The 202 response is intentionally non-committal. Its purpose is to
allow a server to accept a request for some other process (perhaps a
batch-oriented process that is only run once per day) without
requiring that the user agent's connection to the server persist
until the process is completed. The entity returned with this
response SHOULD include an indication of the request's current status
and either a pointer to a status monitor or some estimate of when the
user can expect the request to be fulfilled.
10.2.4 203 Non-Authoritative Information
The returned metainformation in the entity-header is not the
definitive set as available from the origin server, but is gathered
from a local or a third-party copy. The set presented MAY be a subset
or superset of the original version. For example, including local
annotation information about the resource MAY result in a superset of
the metainformation known by the origin server. Use of this response
code is not required and is only appropriate when the response would
otherwise be 200 (OK).
10.2.5 204 No Content
The server has fulfilled the request but there is no new information
to send back. If the client is a user agent, it SHOULD NOT change its
document view from that which caused the request to be sent. This
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response is primarily intended to allow input for actions to take
place without causing a change to the user agent's active document
view. The response MAY include new metainformation in the form of
entity-headers, which SHOULD apply to the document currently in the
user agent's active view.
The 204 response MUST NOT include a message-body, and thus is always
terminated by the first empty line after the header fields.
10.2.6 205 Reset Content
The server has fulfilled the request and the user agent SHOULD reset
the document view which caused the request to be sent. This response
is primarily intended to allow input for actions to take place via
user input, followed by a clearing of the form in which the input is
given so that the user can easily initiate another input action. The
response MUST NOT include an entity.
10.2.7 206 Partial Content
The server has fulfilled the partial GET request for the resource.
The request must have included a Range header field (section 14.36)
indicating the desired range. The response MUST include either a
Content-Range header field (section 14.17) indicating the range
included with this response, or a multipart/byteranges Content-Type
including Content-Range fields for each part. If multipart/byteranges
is not used, the Content-Length header field in the response MUST
match the actual number of OCTETs transmitted in the message-body.
A cache that does not support the Range and Content-Range headers
MUST NOT cache 206 (Partial) responses.
10.3 Redirection 3xx
This class of status code indicates that further action needs to be
taken by the user agent in order to fulfill the request. The action
required MAY be carried out by the user agent without interaction
with the user if and only if the method used in the second request is
GET or HEAD. A user agent SHOULD NOT automatically redirect a request
more than 5 times, since such redirections usually indicate an
infinite loop.
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