Allgemeine HeaderWarning
Beispiel:
Warning: 199 warnagent "Misc. warning" Tue, 15 Nov 1994 12:45:26 GMT
Original Beschreibung (engl.):
The Warning general-header field is used to carry additional
information about the status or transformation of a message which
might not be reflected in the message. This information is typically
used to warn about a possible lack of semantic transparency from
caching operations or transformations applied to the entity body of
the message.
Warning headers are sent with responses using:
Warning = "Warning" ":" 1#warning-value
warning-value = warn-code SP warn-agent SP warn-text
[SP warn-date]
warn-code = 3DIGIT
warn-agent = ( host [ ":" port ] ) | pseudonym
; the name or pseudonym of the server adding
; the Warning header, for use in debugging
warn-text = quoted-string
warn-date = <"> HTTP-date <">
A response MAY carry more than one Warning header.
The warn-text SHOULD be in a natural language and character set that
is most likely to be intelligible to the human user receiving the
response. This decision MAY be based on any available knowledge, such
as the location of the cache or user, the Accept-Language field in a
request, the Content-Language field in a response, etc. The default
language is English and the default character set is ISO-8859-1.
If a character set other than ISO-8859-1 is used, it MUST be encoded
in the warn-text using the method described in RFC 2047 [14].
Warning headers can in general be applied to any message, however
some specific warn-codes are specific to caches and can only be
applied to response messages. New Warning headers SHOULD be added
after any existing Warning headers. A cache MUST NOT delete any
Warning header that it received with a message. However, if a cache
successfully validates a cache entry, it SHOULD remove any Warning
headers previously attached to that entry except as specified for
specific Warning codes. It MUST then add any Warning headers received
in the validating response. In other words, Warning headers are those
that would be attached to the most recent relevant response.
When multiple Warning headers are attached to a response, the user
agent ought to inform the user of as many of them as possible, in the
order that they appear in the response. If it is not possible to
inform the user of all of the warnings, the user agent SHOULD follow
these heuristics:
- Warnings that appear early in the response take priority over
those appearing later in the response.
- Warnings in the user's preferred character set take priority
over warnings in other character sets but with identical warn-
codes and warn-agents.
Systems that generate multiple Warning headers SHOULD order them with
this user agent behavior in mind.
Requirements for the behavior of caches with respect to Warnings are
stated in section 13.1.2.
This is a list of the currently-defined warn-codes, each with a
recommended warn-text in English, and a description of its meaning.
110 Response is stale
MUST be included whenever the returned response is stale.
111 Revalidation failed
MUST be included if a cache returns a stale response because an
attempt to revalidate the response failed, due to an inability to
reach the server.
112 Disconnected operation
SHOULD be included if the cache is intentionally disconnected from
the rest of the network for a period of time.
113 Heuristic expiration
MUST be included if the cache heuristically chose a freshness
lifetime greater than 24 hours and the response's age is greater
than 24 hours.
199 Miscellaneous warning
The warning text MAY include arbitrary information to be presented
to a human user, or logged. A system receiving this warning MUST
NOT take any automated action, besides presenting the warning to
the user.
214 Transformation applied
MUST be added by an intermediate cache or proxy if it applies any
transformation changing the content-coding (as specified in the
Content-Encoding header) or media-type (as specified in the
Content-Type header) of the response, or the entity-body of the
response, unless this Warning code already appears in the response.
299 Miscellaneous persistent warning
The warning text MAY include arbitrary information to be presented
to a human user, or logged. A system receiving this warning MUST
NOT take any automated action.
If an implementation sends a message with one or more Warning headers
whose version is HTTP/1.0 or lower, then the sender MUST include in
each warning-value a warn-date that matches the date in the response.
If an implementation receives a message with a warning-value that
includes a warn-date, and that warn-date is different from the Date
value in the response, then that warning-value MUST be deleted from
the message before storing, forwarding, or using it. (This prevents
bad consequences of naive caching of Warning header fields.) If all
of the warning-values are deleted for this reason, the Warning header
MUST be deleted as well.
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