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Last modified 07:10, 24 Apr 2017
Table of contents
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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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