ertext Transfer Protocol - HTTP/1.1," RFC 2616, 1999. [22] Freed, N., and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies," RFC 2045, 1996. SIP Response Messages This chapter covers the types of SIP response messages. A SIP response is a mes- sage generated by a UAS or a SIP server to reply to a request generated by a UAC. A response may contain additional header fields containing information needed by the UAC. Or, it may be a simple acknowledgment to prevent retrans- missions 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 sug- gested ones from the RFC document. They can be used to convey more infor- mation, especially in failure class responses-the phrase is likely to be displayed to the user. Some response codes were borrowed from HTTP, perhaps with a slightly different reason phrase. 1 However, not all HTTP response codes are valid in SIP, and some even have a different meaning. New response codes 1. Not all HTTP response codes are supported in SIP. Only the response codes described in RFC 3261 and supporting RFCs are supported in SIP. SIP: Understanding the Session Initiation Protocol Table 5.1 SIP Response Classes Class Description Action Informational Indicates status of call prior to co