ng Via: SIP/2.0/UDR 45.2.32.1:5060;branch=z9hG4bK67865 To: <sip:support@chaos.info>;tag=a5ff34d9ee201 From: A. N. Sarkovskii<sip:sarkovskii@45.2.32.1>;tag=7643545 Call-ID: 0140092501 CSeq: 1 INVITE Contact: ksip:agent7@67.42.2.32> Content-Length: 0 The two responses are identical except for having different To tags and Contact URIs. Finally, one of the two UAs answers and sends a 200 OK response: SIP/2.0 200 OK Via: SIP/2.0/UDP 45.2.32.1:5060;branch=z9hG4bK67865 SIP: Understanding the Session Initiation Protocol To: <sip:support@chaos.info>;tag=343214112 From: A. N. Sarkovskii sarkovskii@45. 2.32.1> ;tag=7643545 Call-ID: 0140092501 CSeq: 1 INVITE Contact: <sip:agent42@67.42.2.1> Content-Type: application/sdp Content-Length: (SDP not shown) The forking proxy server sends a CANCEL to the second UA to stop that phone alerting. If both UAs had answered, the forking proxy would have forwarded both 200 OK responses back to the caller who then would have had to choose which one, probably accepting one and sending a BYE to the other one. A stateful proxy usually sends a 100 Trying response when it receives an INVITE. A stateless proxy never sends a 100 Trying response. A 100 Trying response received by a proxy is never forwarded-it is a single hop only response. A proxy handling a TCP request must be stateful, since a user agent will assume reliable transport and rely on the proxy for retransmissions on any UDP hops in the signaling path. The only limit to the number of proxies that can forward a message is con- trolled by the Max-Forwards header field, which is decremented by each proxy that touches the request. If the Max-Forwards count goes to zero, the proxy discards the message and sends a 483 Too Many Hops response back to the originator. A SIP session timer [8] has been proposed to limit the time period over which a stateful proxy must maintain state information. In the initial INVITE request, a Session-Expires header field indicates a timer interval after which stateful proxies may discard state information about