," RFC 3255, 2003 Mankin, A., et al., "Change Process for the Session Initiation Protocol (SIP)," RFC 3427, 2002. Call Flow Examples In this chapter, many of the concepts and details presented in the preceding chapters will be illustrated with examples. Each example includes a call flow dia- gram, a discussion of the example, followed by the message details. Each mes- sage is labeled in the figure with a message number for easy reference. For more examples of the protocol, refer to the SIP specification [1] and the SIP call flows [2, 3] documents. The purpose of the examples in this chapter is to illustrate aspects of the SIP protocol. The interoperation scenarios with the PSTN and with an H.323 network are not intended to fully define the interworking or show a complete parameter mapping between the protocols. Likewise, simplifications such as minimal authentication and direct client-to-gateway messaging are used to make the examples more clear. 10.1 SIP Call with Authentication, Proxies, and Record-Route Figure 10.1 shows a basic SIP call between two SIP user agents involving two proxy servers. Rather than perform a DNS query on the SIP URI of the called party, the calling SIP phone sends the INVITE request to a proxy server for address resolution. The proxy server requires authentication to perform this service and replies with a 407 Proxy Authorization Required response. Using the nonce from the challenge, the caller resends the INVITE with the caller's username and password credentials encrypted. The proxy checks the credentials, and finding them to be correct, performs the DNS lookup on the Request-URI. The INVITE is then forwarded to the proxy server listed in the DNS SRV record that handles the language.org SIP: Understanding the Session Initiation Protocol Proxy Firewall Babbage Server Proxy Location 12.26.17.91 15.16.17.18 Server 10.14.92.1 Server 1.2.3.4 INVITE sip:ada@language.org M1 407 M2 ACK M3 INVITE sip:ada@language.org M4 100 M5 language.org? M6 10.14.92.1 M7 INVITE M8 1.2.3.4 100 M9 INVITE M12 