12.3 Transparent Negotiation
Transparent 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-driven
negotiation) and the dimensions of variance are completely understood
by the 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
server and also 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 and used within HTTP/1.1. An HTTP/1.1
cache performing transparent negotiation MUST include a Vary header
field in the response (defining the dimensions of its variance) if it
is cachable to ensure correct interoperation with all HTTP/1.1
clients. The agent-driven negotiation information supplied by the
origin server SHOULD be included with the transparently negotiated
response.
13 Caching in HTTP
HTTP is typically used for distributed information systems, where
performance can be improved by the use of response caches. The
HTTP/1.1 protocol includes a number of elements intended to make
caching work as well as possible. Because these elements are
inextricable from other aspects of the protocol, and because they
interact with each other, it is useful to describe the basic caching
design of HTTP separately from the detailed descriptions of methods,
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headers, response codes, etc.
Caching would be useless if it did not significantly improve
performance. The goal of caching in HTTP/1.1 is to eliminate the need
to send requests in many cases, and to eliminate the need to send
full responses in many other cases. The former reduces the number of
network round-trips required for many operations; we use an
"expiration" mechanism for this purpose (see section 13.2). The
latter reduces network bandwidth requirements; we use a "validation"
mechanism for this purpose (see section 13.3).
Requirements for performance, availability, and disconnected
operation require us to be able to relax the goal of semantic
transparency. The HTTP/1.1 protocol allows origin servers, caches,
and clients to explicitly reduce transparency when necessary.
However, because non-transparent operation may confuse non-expert
users, and may be incompatible with certain server applications (such
as those for ordering merchandise), the protocol requires that
transparency be relaxed
o only by an explicit protocol-level request when relaxed by client
or origin server
o only with an explicit warning to the end user when relaxed by cache
or client
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RFC 2068 HTTP/1.1 January 1997
Therefore, the HTTP/1.1 protocol provides these important elements:
1. Protocol features that provide full semantic transparency when this
is required by all parties.
2. Protocol features that allow an origin server or user agent to
explicitly request and control non-transparent operation.
3. Protocol features that allow a cache to attach warnings to
responses that do not preserve the requested approximation of
semantic transparency.
A basic principle is that it must be possible for the clients to
detect any potential relaxation of semantic transparency.
Note: The server, cache, or client implementer may be faced with
design decisions not explicitly discussed in this specification. If
a decision may affect semantic transparency, the implementer ought
to err on the side of maintaining transparency unless a careful and
complete analysis shows significant benefits in breaking
transparency.