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negotiation
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... An entity included with a response that is subject to content
negotiation, as described in section 12. There may exist multiple
representations associated with a particular response status. ...
... content negotiation ...
... representations is termed a `varriant'. Use of the term `variant'
does not necessarily imply that the resource is subject to content
negotiation. ...
... 14.17) and Accept (section 14.1) header fields in order to provide
open and extensible data typing and type negotiation.
...
...
HTTP content negotiation (section 12) uses short "floating point"
numbers to indicate the relative importance ("weight") of various
...
... Negotiation ...
... body is not defined by this specification, but might be defined by
future extensions to HTTP. Content negotiation MAY be used to select
the appropriate response format. If no response body is included, the
...
... representations, each with its own specific location, and agent-
driven negotiation information (section 12) is being provided so that
the user (or user agent) can select a preferred representation and
...
... Content Negotiation ...
... entity types. For that reason, HTTP
has provisions for several mechanisms for "content negotiation" --
the process of selecting the best representation for a given response
when there are multiple representations available.
...
...
Note: This is not called "format negotiation" because the
alternate representations may be of the same media type, but use
...
... Any response containing an entity-body MAY be subject to negotiation,
including error responses.
...
...
There are two kinds of content negotiation which are possible in
HTTP: server-driven and agent ...
... HTTP: server-driven and agent-driven negotiation. These two kinds of
negotiation are orthogonal and thus may be used separately or in
...
... agent-driven negotiation. These two kinds of
negotiation are orthogonal and thus may be used separately or in
combination. One method of combination, referred to as transparent
...
... combination. One method of combination, referred to as transparent
negotiation, occurs when a cache uses the agent-driven negotiation ...
... negotiation, occurs when a cache uses the agent-driven negotiation
information provided by the origin server in order to provide
server-driven negotiation ...
... negotiation
information provided by the origin server in order to provide
server-driven negotiation for subsequent requests.
...
... Server-driven Negotiation ...
... an algorithm located at the server, it is called server-driven
negotiation. Selection is based on the available representations of
the response (the dimensions over which it can vary; e.g. language,
...
...
Server-driven negotiation is advantageous when the algorithm for
selecting from among the available representations is difficult to
...
...
Server-driven negotiation has disadvantages:
...
... HTTP/1.1 includes the following request-header fields for enabling
server-driven negotiation through description of user agent
capabilities and user preferences: Accept (section 14.1), Accept-
...
... server uses to select a representation that is subject to server-
driven negotiation. See section 13.6 for use of the Vary header field
by caches ...
... Agent-driven Negotiation ...
...
With agent-driven negotiation, selection of the best representation
for a response is performed by the user agent ...
...
Agent-driven negotiation is advantageous when the response would vary
over commonly-used dimensions (such as type, language, or encoding ...
...
Agent-driven negotiation suffers from the disadvantage of needing a
second request to obtain the best alternate representation. This
second request is only efficient when caching is used. In addition,
...
... status codes for enabling agent-driven negotiation when the server is
unwilling or unable to provide a varying response using server-driven
negotiation ...
... negotiation when the server is
unwilling or unable to provide a varying response using server-driven
negotiation.
...
... Transparent Negotiation ...
... negotiation is a combination of both server-driven and
agent-driven negotiation. When a cache is supplied with a form of the
list of available representations of the response (as in agent ...
... list of available representations of the response (as in agent-driven
negotiation) and the dimensions of variance are completely understood
by the cache, then the cache ...
... cache, then the cache becomes capable of performing server-
driven negotiation on behalf of the origin server for subsequent
requests on that resource.
...
...
Transparent negotiation has the advantage of distributing the
negotiation work that would otherwise be required of the origin
...
... Transparent negotiation has the advantage of distributing the
negotiation work that would otherwise be required of the origin
server and also removing the second request delay of agent ...
... removing the second request delay of agent-driven
negotiation when the cache is able to correctly guess the right
response.
...
...
This specification does not define any mechanism for transparent
negotiation, though it also does not prevent any such mechanism from
being developed as an extension that could be used within HTTP/1.1.
...
...
Use of server-driven content negotiation (section 12.1), as indicated
by the presence of a Vary header field in a response, alters the
...
... representations of a cacheable response subject to server-driven
negotiation. The set of header fields named by the Vary field value
is known as the "selecting" request-headers ...
... header field with any
cacheable response that is subject to server-driven negotiation.
Doing so allows a cache to properly interpret future requests on that
...
... cache to properly interpret future requests on that
resource and informs the user agent about the presence of negotiation
on that resource. A server MAY include a Vary header field with a
...
... header field with a
non-cacheable response that is subject to server-driven negotiation,
since this might provide the user agent with useful information about
...