SIP - Response Codes

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A SIP response is a message generated by a user agent server (UAS) or SIP server to reply a request generated by a client. It could be a formal acknowledgement to prevent retransmission of requests by a UAC.

  • A response may contain some additional header fields of info needed by a UAC.

  • SIP has six responses.

  • 1xx to 5xx has been borrowed from HTTP and 6xx is introduced in SIP.

  • 1xx is considered as a provisional response and the rest are final responses.

S.No. Function & Description
1 1xx: Provisional/Informational Responses

Informational responses are used to indicate call progress. Normally the responses are end to end (except 100 Trying).
2 2xx: Success Responses

This class of responses is meant for indicating that a request has been accepted.
3 3xx: Redirect Responses

Generally these class responses are sent by redirect servers in response to INVITE. They are also known as redirect class responses.
4 4xx: Client Failure Responses

Client error responses indicate that the request cannot be fulfilled as some errors are identified from the UAC side.
5 5xx: Server Failure Response

This class response is used to indicate that the request cannot be processed because of an error with the server.
6 6xx: Global Failure Response

This response class indicates that the server knows that the request will fail wherever it is tried. As a result, the request should not be sent to other locations.
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