r that handles the domain company.com, we will find a user whose username is Bob.Johnson. Bob's URL might be SIP:Bob@131.160.1.112 instead, indicating that the host whose IP address is 131.160.1.112 has a user whose username is Bob. Registrations We've noted that users register their current location to a server if they wish to be found. In this example, Bob is working on his laptop, whose IP address is 131.160.1.112. His login name is Bob. He reg- isters his current position with the company server (Figure 4-6). Now Laura wants to call Bob. She has his public SIP address (SIP:Bob.Johnson@company.com) because it's printed on his business card. Chapter 4 SIP server at Figure 4-6 company.com Bob registers his current position to the server. I am "Bob.Johnson." Today I will be reachable at: sip: Bob@123.160.1.112 131.160.1.112 So when the server at company.com is contacted and asked for Bob.John- son, it knows where Bob.Johnson can be reached and a connection is made. In this situation, SIP provides two modes of operation: redirect and proxy. In the proxy mode of operation, the server contacts Bob at 131.160.1.112 and delivers Laura's session description to him (Figure 4-7). In the redirect mode, the server tells Laura to try SIP:Bob@131.160. 1.112 instead (Figure 4-8). A user may very well register several locations at one server. Or the user might register his or her locations with several servers. It's not unusual for various servers and locations to be contacted before a user is finally reached. SIP Entities The SIP protocol defines several entities, and it's vital to understand their role inside any architecture that uses SIP. User Agents User Agent (UA) is the SIP entity that interacts with the user. It usually has an interface towards the user. Say Bob wants to make a call over the Inter- net with his computer. He launches the proper program that contains a SIP The Session Initiation Protocol: SIP SIP server at Figure 4-7 company.com Proxy SIP server. (2) Invittion to a session for Bob@131.160.1.112 (1) Invit