112> Provisional Response from Proxy to Laura Upon receipt of this response, the proxy removes the Via header with its own address and sends the response to the address contained in the next Via header. This proxy takes no further action. SIP/2.0 180 Ringing Via: SIP/2.0/UDP workstation1000.university.com:5060 From: Laura Brown <sip:Laura.Brown@university.com> To: Bob Johnson <sip:Bob.Johnson@company.com>;tag=314159 Call-ID: 12345678@workstation1000.university.com CSeq: 1 INVITE Contact: Bob Johnson <sip:Bob@131.160.1.112> Final Response from Bob to Proxy When Bob accepts the call, his UA returns its SDP session description. It will receive RTP packets on UDP port 41000. SIP/2.0 200 OK Via: SIP/2.0/UDP 131.160.1.110 Via: SIP/2.0/UDP workstation1000.university.com:5060 From: Laura Brown <sip:Laura.Brown@university.com> To: Bob Johnson ip:Bob.Johnson@company.com>;tag=314159 Call-ID: 12345678@workstation1000.university.com CSeq: 1 INVITE Contact: Bob Johnson <sip:Bob@131.160.1.112> Content-Type: application/sdp Content-Length: 154 Chapter 5 o=Bob 2891234321 2891234321 IN IP4 131.160.1.112 s=Let us talk for a while c=IN IP4 131.160.1.112 t=0 0 m=audio 41000 RTP/AVP 0 Final Response from Proxy to Laura The proxy server routes the final response in the same way it routed the previous provisional response. In other words, it removes the first Via header and sends the response to the address contained in the next Via. SIP/2.0 200 OK Via: SIP/2.0/UDP workstation1000.university.com:5060 From: Laura Brown <sip:Laura.Brown@university.com> To: Bob Johnson sip:Bob.Johnson@company.com>;tag=314159 Call-ID: 12345678@workstation1000.university.com CSeq: 1 INVITE Contact: Bob Johnson <sip:Bob@131.160.1.112> Content-Type: application/sdp Content-Length: 154 o=Bob 2891234321 2891234321 IN IP4 131.160.1.112 s=Let us talk for a while c=IN IP4 131.160.1.112 m=audio 41000 RTP/AVP 0 ACK from Laura to Bob When Laura's UA receives the 200 OK final response, it sends an ACK request. This ACK is sent directly to Bob's UA, whose address is con