Response Codes in SIP

The response codes are consistent with, and extend, HTTP/1.1 response
   codes.  Not all HTTP/1.1 response codes are appropriate, and only
   those that are appropriate are given here.  Other HTTP/1.1 response
   codes SHOULD NOT be used.  Also, SIP defines a new class, 6xx.

21.1 Provisional 1xx

   Provisional responses, also known as informational responses,
   indicate that the server contacted is performing some further action
   and does not yet have a definitive response.  A server sends a 1xx
   response if it expects to take more than 200 ms to obtain a final
   response.  Note that 1xx responses are not transmitted reliably.
   They never cause the client to send an ACK.  Provisional (1xx)
   responses MAY contain message bodies, including session descriptions.

21.1.1 100 Trying

   This response indicates that the request has been received by the
   next-hop server and that some unspecified action is being taken on
   behalf of this call (for example, a database is being consulted).
   This response, like all other provisional responses, stops
   retransmissions of an INVITE by a UAC.  The 100 (Trying) response is
   different from other provisional responses, in that it is never
   forwarded upstream by a stateful proxy.

21.1.2 180 Ringing

   The UA receiving the INVITE is trying to alert the user.  This
   response MAY be used to initiate local ringback.

21.1.3 181 Call Is Being Forwarded

   A server MAY use this status code to indicate that the call is being
   forwarded to a different set of destinations.

21.1.4 182 Queued

   The called party is temporarily unavailable, but the server has
   decided to queue the call rather than reject it.  When the callee
   becomes available, it will return the appropriate final status
   response.  The reason phrase MAY give further details about the
   status of the call, for example, "5 calls queued; expected waiting
   time is 15 minutes".  The server MAY issue several 182 (Queued)
   responses to update the caller about the status of the queued call.

21.1.5 183 Session Progress

   The 183 (Session Progress) response is used to convey information
   about the progress of the call that is not otherwise classified.  The
   Reason-Phrase, header fields, or message body MAY be used to convey
   more details about the call progress.

Prev - Next >>