486 Busy Here client error response. If there is a possibility that the call to the specified Request-URI could be answered in other locations, this response should not be sent. 5.6.2 603 Decline This response has the same effect as the 600 Busy Everywhere but does not give away any information about the call state of the server. This response could indicate the called party is busy, or simply does not want to accept the call. 5.6.3 604 Does Not Exist Anywhere This response is similar to the 404 Not Found response but indicates that the user in the Request-URI cannot be found anywhere. This response should only be sent by a server that has access to all information about the user. 5.6.4 606 Not Acceptable This response can be used to implement some session negotiation capability in SIP. This response indicates that some aspect of the desired session is not accept- able to the UAS, and as a result, the session cannot be established. The response may contain a Warning header field with a numerical code describing exactly what was not acceptable. The request can be retried with different media session information. An example of simple negotiation with SIP is shown in Figure 5.6. If more complicated negotiation capability is required, another protocol should be used. SIP: Understanding the Session Initiation Protocol INVITE sdp UAC1 606 Not acceptable INVITE sdp UAC2 100 Trying Figure 5.6 Session negotiation with SIP. References Rosenberg, J., et al., "SIP: Session Initiation Protocol," RFC 3261, 2002. Camarillo, G., et al., "Integrated Services Digital Network (ISDN) User Part (ISUP) to Session Initiation Protocol (SIP) Mapping," RFC 3398, 2002. Roach, A., "SIP Specific Events," RFC 3265, 2002. Donovan, S., and J. Rosenberg, "Session Timers in the Session Initiation Protocol (SIP)," IETF Internet-Draft, Work in Progress, November 2002. Peterson, J., "Enhancements for Authenticated Identity Management in the Session Initia- tion Protocol (SIP),' IETF Internet-Draft, Work in Progress, February 2003. Peterson, J., "SIP