5

SIP Response Messages

This chapter covers the types of SIP response messages. A SIP response is a message generated by a UAS or a SIP server to reply to a request generated by a UAC. A response may contain additional header fields of information needed by the UAC, or it may be a simple acknowledgment to prevent retransmissions of the request by the UAC. Many responses direct the UAC to take specific additional steps. The responses are discussed in terms of structure and classes. Then each request type is discussed and examined in detail.

There are six classes of SIP responses. The first five classes were borrowed from HTTP; the sixth was created for SIP. The classes are shown in Table 5.1.

If a particular SIP response code is not understood by a UAC, it must be interpreted by the class of the response. For example, an unknown 599 Server Unplugged response must be interpreted by a user agent as a 500 Server Failure response.

The reason phrase is for human consumption only—the SIP protocol uses only the response code in determining behavior. Thus, a 200 Call Failed is interpreted the same as 200 OK. The reason phrases listed here are the suggested ones from the RFC document. They can be used to convey more information, especially in failure class responses—the phrase is likely to be displayed to the user. Some response codes were borrowed from HTTP, sometimes with a slightly different reason phrase. However, not all HTTP response codes are valid in SIP, and some even have a different meaning.

Unless otherwise referenced, the responses described here are defined in RFC 3261 [1].

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