1xx: Provisional/Informational Responses



Informational responses are used to indicate call progress. Normally the responses are end to end (except 100 Trying). The main objective of informational responses is to stop retransmission of INVITE requests.

Informational responses include the following responses −

100 Trying

  • This special case response is only a hop-by-hop request.

  • It is never forwarded and may not contain a message body.

  • It is used to avoid the retransmission of INVITE requests.

180 Ringing

  • This response is used to indicate that an INVITE has been received by the user agent and alerting is taking place.

181 Call is Being Forwarded

  • This response is used to indicate that the call has been forwarded to another endpoint.

  • It gives the status of the caller, as a forwarding operation may result in the call taking longer to be answered.

182 Call Queued

  • This response is used to indicate that the INVITE has been received and will be processed in a queue.

183 Session Progress

  • It indicates that information about the progress of a session may be present in a message body or media stream.

  • Unlike a 100 Trying response, a 183 is an end-to-end response.

  • It establishes early dialog.

  • A typical use of this response is to allow a UAC to hear a ringtone recorded announcement in calls through a gateway into the PSTN.

session_initiation_protocol_response_codes.htm
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