ap: 0 PCMU/8000 The proxy forwards the 200 OK message to Schroedinger after removing the first Via header field: SIP/2.0 200 OK Via: SIP/2.0/UDP 100.101.102.103:5060;branch=z9hG4bkmp17a To: Heisenberg <sip:werner.heisenberg@munich.de>;tag=314159 From: E. Schroedinger<sip:schroed5244@aol.com>;tag=42 Call-ID: 10@100.101.102.103 CSeq: 1 INVITE Contact: <sip:werner.heisenberg@200.201.202.203> Content-Type: application/sdp SIP: Understanding the Session Initiation Protocol Content-Length: 159 o=heisenberg 2890844526 2890844526 IN IP4 200.201.202.203 C=IN IP4 2200.201.202.203 m=audio 49170 RTP/AVP 0 a=rtpmap PCMU/8000 The presence of the Contact header field with the SIP URI address of Heisenberg in the 200 OK allows Schroedinger to send the ACK directly to Heisenberg, bypassing the proxy. (Note that the Request-URI is set to Heisen- berg's Contact URI and not the URI in to To header field.) This request, and all future requests continue to use the tag in the To header field: ACK sip: werner.heisenberg@200.201.202.20 SIP/2.0 Via: SIP/2.0/UDP 100.101.102.103:5060;branch=z9hG4bkka42 Max-Forwards: 70 To: Heisenberg sip:werner.heisenberg@munich.de>;tag=314159 From: E. Schroedinger <sip:schroed5244@aol.com>;tag=42 Call-ID: 10@100.101.102.103 CSeq: 1 ACK Content-Length: 0 This shows that the proxy server is not really "in the call." It facilitates the two end points locating and contacting each other, but it can drop out of the signaling path as soon as it no longer adds any value to the exchange. A proxy server can force further messaging to route through it by inserting a Record- Route header field, which is described in Section 6.2.12. In addition, it is pos- sible to have a proxy server that does not retain any knowledge of the fact that there is a session established between Schroedinger and Heisenberg (referred to as call state information). This is discussed in Section 3.3.1. Note that the media is always end-to-end and not through the proxy. In SIP the path of the signaling messages is totally independent of the path