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validity
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... SHOULD
This word or the adjective "recommended" means that there may
exist valid reasons in particular circumstances to ignore this
item, but the full implications should be understood and the case
carefully weighed before choosing a different course.
...
... unnecessary delay from the origin server, perhaps via one or more
proxies. A response is also first-hand if its validity has just
been checked directly with the origin server.
...
... age
The age of a response is the time since it was sent by, or
successfully validated with, the origin server.
freshness lifetime ...
... 11]. The BNF above includes national characters not
allowed in valid URLs as specified by RFC 1738(-> 4266prop | 4248prop), since HTTP ...
... 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 ...
... determine the length of the message, or with 411 (length required) if
it wishes to insist on receiving a valid Content-Length.
...
...
3. If the host as determined by rule 1 or 2 is not a valid host on
the server, the response MUST be a 400 (Bad Request) error
message ...
...
o 5xx: Server Error - The server failed to fulfill an apparently
valid request
The individual values of the numeric status codes ...
... method is
often used for testing hypertext links for validity, accessibility,
and recent modification.
...
... Request-URI. The response MUST include an
Allow header containing a list of valid methods for the requested
resource.
...
... The server refuses to accept the request without a defined Content-
Length. The client MAY repeat the request if it adds a valid
Content-Length header field ...
... can only be compared for equality with other realms on that server.
The server will service the request only if it can validate the
user-ID and password for the protection space of the Request-URI ...
... user agent may allow the user to
specify that cached entities (even explicitly stale ones) are never
validated. Or the user agent might habitually add "Cache-Control:
...
... If an origin server wishes to force a semantically transparent cache
to validate every request, it may assign an explicit expiration time
in the past. This means that the response is always stale, and so the
cache ...
... in the past. This means that the response is always stale, and so the
cache SHOULD validate it before using it for subsequent requests. See
section 14.9.4 for a more restrictive way to force revalidation.
...
... HTTP/1.1 cache, no matter how
it is configured, to validate every request, it should use the
"must-revalidate" Cache-Control directive (see section 14.9).
...
...
to force any intermediate caches to validate their copies directly
with the origin server, or
...
... cache
validator. In simple terms, a cache entry is considered to be valid
if the entity has not been modified since the Last-Modified value.
...
... times, so it may be inappropriate to expect that a cache will never
again attempt to validate an entry using a validator that it
obtained at some point in the past.
...
... the resource identified by the Request-URI. The purpose of this field
is strictly to inform the recipient of valid methods associated with
the resource. An Allow header field ...
... authenticated and a realm specified, the same credentials SHOULD
be valid for all other requests within this realm.
When a shared cache ...
... origin server to state that the specified parts of the response are
intended for only one user and are not a valid response for requests
by other users. A private (non-shared) cache may cache ...
... server replies with 304 (Not Modified),
then the cache should return its now validated copy to the client
with a 200 (OK) response. If the server replies ...
... HTTP/1.1 requests containing an entity-body, e.g., because
the request has a valid Content-Length field, uses Transfer-Encoding:
...
...
Any Content-Length greater than or equal to zero is a valid value.
Section 4.4 describes how to determine the length of a message-body
...
... cache or an user
agent cache) unless it is first validated with the origin server (or
with an intermediate cache that has a fresh copy of the entity ...
... the response is exactly the same as for a normal GET.
c)If the variant has not been modified since a valid If-Modified-Since
date, the server MUST return a 304 (Not Modified) response.
...
... Range header. (The server
can distinguish between a valid HTTP-date and any form of entity-tag ...
... header that it received with a
response. However, if a cache successfully validates a cache entry,
it SHOULD remove ...
... DNS names. Clients need to be
cautious in assuming the continuing validity of an IP number/DNS name
...