amine all the parameters of each message to under- stand a particular scenario. In this scenario, we do SO deliberately to display SIP messages with all their headers and parameters. SIP Call Through a Proxy In Figure 5-32, Laura calls Bob at his public address, but he isn't there. A proxy server at company.com routes the call to his current location: SIP:Bob@131.160.1.112. This example comprises three different transactions: INVITE, ACK, and Chapter 5 Proxy Proxy Figure 5-31 Provisional responses do not stop request (1) BYE retransmissions. (2) BYE (3) 100 Trying (4) BYE (5) 100 Trying (6) 200 OK (7) BYE (8) 200 OK (9) BYE (10) 200 OK (11) BYE (12) BYE (13) 200 OK INVITE from Laura's UA to SIP Proxy Laura's UA places Bob's pub- lic address in the To field and in the Request-URI. It adds a Via header with its address and creates a message body with an SDP session description. Laura wants to receive RTP packets containing PCM voice on UDP port 20002. The request is sent to the proxy at company.com because the domain part of the Request-URI is company.com. INVITE sip: Johnson@company.com SIP/2.0 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 SIP: Protocol Operation Laura SIP proxy Figure 5-32 SIP call through (1) INVITE a proxy (2) INVITE (3) 180 Ringing (4) 180 Ringing (5) 200 OK (6) 200 OK (7) ACK Conversation (8) BYE (9) 200 OK workstation1000.university.com 131.160.1.110 131.160.1.112 Call-ID: 12345678@workstation1000.university.com CSeq: 1 INVITE Contact: Laura Brown<sip:Laura@workstation1000.university.com> Content-Type: application/sdp Content-Length: 154 o=Laura 2891234526 2891234526 IN IP4 workstation1000.university.com s=Let us talk for a while C=IN IP4 138.85.27.10 t=0 0 m=audio 20002 RTP/AVP 0 INVITE from SIP Proxy to Bob The SIP proxy at company.com receives the INVITE request. The host part of the Request-URI reads Bob.Johnson. The proxy knows that Bob.Johnson may be reachable at SIP:Bob@131.160.1.11