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20.43 Warning

   The Warning header field is used to carry additional information
   about the status of a response.  Warning header field values are sent
   with responses and contain a three-digit warning code, host name, and
   warning text.

   The "warn-text" should be in a natural language that is most likely
   to be intelligible to the human user receiving the response.  This
   decision can be based on any available knowledge, such as the
   location of the user, the Accept-Language field in a request, or the
   Content-Language field in a response.  The default language is i-
   default [21].

   The currently-defined "warn-code"s are listed below, with a
   recommended warn-text in English and a description of their meaning.
   These warnings describe failures induced by the session description.
   The first digit of warning codes beginning with "3" indicates
   warnings specific to SIP.  Warnings 300 through 329 are reserved for
   indicating problems with keywords in the session description, 330
   through 339 are warnings related to basic network services requested
   in the session description, 370 through 379 are warnings related to
   quantitative QoS parameters requested in the session description, and
   390 through 399 are miscellaneous warnings that do not fall into one
   of the above categories.

      300 Incompatible network protocol: One or more network protocols
          contained in the session description are not available.

      301 Incompatible network address formats: One or more network
          address formats contained in the session description are not
          available.

      302 Incompatible transport protocol: One or more transport
          protocols described in the session description are not
          available.

      303 Incompatible bandwidth units: One or more bandwidth
          measurement units contained in the session description were
          not understood.

      304 Media type not available: One or more media types contained in
          the session description are not available.

      305 Incompatible media format: One or more media formats contained
          in the session description are not available.

      306 Attribute not understood: One or more of the media attributes
          in the session description are not supported.

      307 Session description parameter not understood: A parameter
          other than those listed above was not understood.

      330 Multicast not available: The site where the user is located
          does not support multicast.

      331 Unicast not available: The site where the user is located does
          not support unicast communication (usually due to the presence
          of a firewall).

      370 Insufficient bandwidth: The bandwidth specified in the session
          description or defined by the media exceeds that known to be
          available.

      399 Miscellaneous warning: The warning text can include arbitrary
          information to be presented to a human user or logged.  A
          system receiving this warning MUST NOT take any automated
          action.

             1xx and 2xx have been taken by HTTP/1.1.

   Additional "warn-code"s can be defined through IANA, as defined in
   Section 27.2.

   Examples:

      Warning: 307 isi.edu "Session parameter 'foo' not understood"
      Warning: 301 isi.edu "Incompatible network address type 'E.164'"

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