180 M15 180 M14 180 M13 200 M18 200 M17 200 M16 ACK M19 ACK M20 RTP media packets BYE M22 BYE M21 200 M23 200 M24 Figure 10.1 SIP-to-SIP call with authentication, proxies, and record-route. domain. That proxy then looks up the Request-URI and locates a registration for the called party. The INVITE is forwarded to the destination UAS, a Record-Route header having been inserted to ensure that the proxy is pres- ent in all future requests by either party. This is because a direct routed SIP mes- sage to Ada would be blocked by the firewall. The called party receives the INVITE request and sends 180 Ringing and 200 OK responses, which are routed back to the caller using the Via header chain from the initial INVITE. The ACK sent by the caller includes a Route header built from the Record-Route header field in the 200 OK response. This routing skips the first proxy but includes the firewall proxy. The media session begins with the user agents exchanging RTP and RTCP packets. The call terminates when the called party, Ada, sends a BYE, which includes a Route header generated from the Record-Route header field in the INVITE. Note that the CSeq for the called user agent is initialized to Call Flow Examples 1000. The acknowledgment of the BYE with a 200 OK response causes both sides to stop sending media packets. INVITE sip:ada@language.org SIP/2.0 <Request-URI Via: SIP/2.0/UDP 12.26.17.91:5060;branch=z9hG4bK454 Max-Forwards: 70 From: Charles Babbage <sip:babbage@analyticalsoc.org>;tag=9382 To: sip:ada@language.org Call-ID: E6-32-9a-34-91-e7@analyticalsoc.org CSeq: 1 INVITE <CSeq initialized to 1 Contact: sip:babbage@client.analyticalsoc.org> Subject: RE: Software User-Agent: Difference Engine 1 Content-Type: application/sdp Content-Length: 137 o=Babbage 2890844534 2890844534 IN IP4 12.26.17.91 t=0 0 C=IN IP4 12.26.17.91 <Babbage's IP address m=audio 49170 RTP/AVP 0 <Port number a=rtpmap 0 PCMU/8000 <Codecinfo SIP/2.0 407 Proxy Authentication Required Via: SIP/2.0/UDP 12.26.17.91:5060;branch=z9hG4bK454 From: Charles Babbag