ol.com> Content-Type: application/sdp Content-Length: 159 o=schroed5244 2890844526 2890844526 IN IP4 100.101.102.103 s=Phone Call C=IN IP4 100.101.102.103 m=audio 49170 RTP/AVP 0 a=rtpmap: 0 PCMU/8000 The proxy looks up the SIP URI in the Request-URI (sip : wer- ner.heisenberg@munich.de) in its database and locates Heisenberg. This completes the two-step process: 1. DNS lookup by user agent to locate the IP address of the proxy; data- base lookup is performed by the proxy to locate the IP address; SIP: Understanding the Session Initiation Protocol 2. The INVITE is then forwarded to Heisenberg's IP address with the addition of a second Via header field stamped with the address of the proxy: INVITE sip:werner.heisenberg@200.201.202.203 SIP/2.0 Via: SIP/2.0/UDP proxy.munich.de:5060;branch=z9hG4bK83842.1 Via: SIP/2.0/UDI 100.101.102.103:5060;branch=z9hG4bkmp17a Max-Forwards: 69 To: Heisenberg <sip:werner.heisenberg@munich.de> From: E. Schroedinger <sip:schroed5244@aol.com>;tag=4 Call-ID: 10@100.101.102.103 CSeq: 1 INVITE Contact: <sip:schroed5244@pc33.aol.com> Content-Type: application/sdp Content-Length: 159 o=schroed5244 2890844526 2890844526 IN IP4 100.101.102.103 s=Phone Call C=IN IP4 100.101.102.103 m=audio 49172 RTP/AVP 0 a=rtpmap: PCMU/8000 From the presence of two Via header fields, Heisenberg knows that the INVITE has been routed through a proxy server. Having received the INVITE, a 180 Ringing response is sent by Heisenberg to the proxy: SIP/2.0 180 Ringing Via: SIP/2.0/UDP proxy.munich.de:5060;branch=z9hG4bK83842.1 :received=100.101.10 Via: SIP/2.0/UDP 100.101.102.103:5060;branch=z9hG4bKmp17a To: Heisenberg p:werner.heisenberg@munich.de>;tag=314159 From: E. Schroedinger <sip:schroed5244@aol.com>;tag=4 Call-ID: 10@100.101.102.103 CSeq: 1 INVITE Contact: <sip:werner.heisenberg@200.201.202.203> Content-Length: 0 Again, this response contains the Via header fields, and the To, From, Call-ID, and CSeq header fields from the INVITE request. The response is then sent to the address in the first Via header field, 