ed in the To header and it will not vary during the session; that is, all of the requests from Laura to Bob will have the same To field. Laura Figure 5-19 P2 does not need to (1) INVITE (2) INVITE be in the signalling Record-Route:P1 (3) INVITE path. Record-Route:P1 (4) INVITE Record-Route:P3 Record-Route:P1 (5) 200 OK (6) 200 OK Contact: Bob (7) 200 OK Contact: Bob Record-Route:P3 (8) 200 OK Contact: Bob Record-Route:P3 Record-Route:P1 Contact: Bob Record-Route:P3 Record-Route:P1 Record-Route:P3 Record-Route:P1 Record-Route:P1 (9) ACK Route:P3 Route:Bob (10) ACK Route: Bob (11) ACK Conversation (12) BYE Route: P3 (13) ACK Route: Bob Route: Bob (14) ACK (15) 200 OK (16) 200 OK (17) 200 OK Chapter 5 university Figure 5-20 Laura 131160.1.112 The To header does not change through (1) INVITE SIP: Bob.Johnson@company.com SIP/2.0 a session. To: SIP:Bob.Johnson@company.com (2) INVITE SIP: Bob@131.160.1.112 SIP/2.0 To: SIP:Bob.Johnson@company.com (3) INVITE SIP: Bob.Johnson@university.com SIP/2.0 To: SIP:Bob.Johnson@company.com Laura places the same SIP URL in the Request-URI SO it will send the request to the SIP at company.com. This proxy performs a parallel search by trying two different SIP URLs: SIP:Bob@131.160.1.112 and SIP:Bob@ university.com. Both INVITEs sent by the proxy at company.com have the same To header, but they have different Request-URIs. Via Via headers store all the proxies that handle the request. Hence, they contain the path taken by the request (Figure 5-21). This information is used for detecting routing loops. If a request is forwarded in a loop, any proxy can notice it simply by inspecting the Via headers. If it finds its address there, the proxy knows that it has already handled this request. A typical Via header looks like the following: Via: SIP/2.0/UDP workstation1234 company.com Via headers are also used to route responses towards the client who gen- erated the request. This way, a SIP response traverses the same set of prox- ies as the request, but in the opposite direction. SIP Bodies Bo