21.4 Request Failure 4xx

   4xx responses are definite failure responses from a particular
   server.  The client SHOULD NOT retry the same request without
   modification (for example, adding appropriate authorization).
   However, the same request to a different server might be successful.

21.4.1 400 Bad Request

   The request could not be understood due to malformed syntax.  The
   Reason-Phrase SHOULD identify the syntax problem in more detail, for
   example, "Missing Call-ID header field".

21.4.2 401 Unauthorized

   The request requires user authentication.  This response is issued by
   UASs and registrars, while 407 (Proxy Authentication Required) is
   used by proxy servers.

21.4.3 402 Payment Required

   Reserved for future use.

21.4.4 403 Forbidden

   The server understood the request, but is refusing to fulfill it.
   Authorization will not help, and the request SHOULD NOT be repeated.

21.4.5 404 Not Found

   The server has definitive information that the user does not exist at
   the domain specified in the Request-URI.  This status is also
   returned if the domain in the Request-URI does not match any of the
   domains handled by the recipient of the request.

21.4.6 405 Method Not Allowed

   The method specified in the Request-Line is understood, but not
   allowed for the address identified by the Request-URI.

   The response MUST include an Allow header field containing a list of
   valid methods for the indicated address.

21.4.7 406 Not Acceptable

   The resource identified by the request is only capable of generating
   response entities that have content characteristics not acceptable
   according to the Accept header field sent in the request.

21.4.8 407 Proxy Authentication Required

   This code is similar to 401 (Unauthorized), but indicates that the
   client MUST first authenticate itself with the proxy.  SIP access
   authentication is explained in Sections 26 and 22.3.

   This status code can be used for applications where access to the
   communication channel (for example, a telephony gateway) rather than
   the callee requires authentication.

21.4.9 408 Request Timeout

   The server could not produce a response within a suitable amount of
   time, for example, if it could not determine the location of the user
   in time.  The client MAY repeat the request without modifications at
   any later time.

21.4.10 410 Gone

   The requested resource is no longer available at the server and no
   forwarding address is known.  This condition is expected to be
   considered permanent.  If the server does not know, or has no
   facility to determine, whether or not the condition is permanent, the
   status code 404 (Not Found) SHOULD be used instead.

21.4.11 413 Request Entity Too Large

   The server is refusing to process a request because the request
   entity-body is larger than the server is willing or able to process.
   The server MAY close the connection to prevent the client from
   continuing the request.

   If the condition is temporary, the server SHOULD include a Retry-
   After header field to indicate that it is temporary and after what
   time the client MAY try again.

21.4.12 414 Request-URI Too Long

   The server is refusing to service the request because the Request-URI
   is longer than the server is willing to interpret.

21.4.13 415 Unsupported Media Type

   The server is refusing to service the request because the message
   body of the request is in a format not supported by the server for
   the requested method.  The server MUST return a list of acceptable
   formats using the Accept, Accept-Encoding, or Accept-Language header
   field, depending on the specific problem with the content.  UAC
   processing of this response is described in Section 8.1.3.5.

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