tion to a session for sip: Bob.Johnson@company.com 131.160.1.112 SIP server at Figure 4-8 company.com Redirect SIP server. (1) Invittion to a session for (2) You better try to reach him at sip: Bob.Johnson@company.com Bob@131.160.1.112 131.160.1.112 (3) Invittion to a session for sip: Bob@131/160.1.112 Chapter 4 User Agent. The user interacts with the UA through the aforementioned interface-often a window with a selection of buttons. When Bob clicks the Call Laura button, the UA triggers the appropriate SIP messages to estab- lish the call. Laura also has a SIP UA in her computer. When its UA receives the invi- tation from Bob's, it alerts Laura by showing a pop-up window with two buttons: Accept call from Bob and Reject call from Bob. Depending on which button Laura clicks, her UA sends SIP messages back to Bob's UA. All inter- actions between users and the SIP protocol are mediated by UAs. However, keep in mind that some systems using SIP are not directly con- nected to users. For example, Bob can redirect all session invitations received from midnight to 7 A.M. to his SIP answering machine. The machine will automatically establish sessions in order to record messages. It also contains a UA-one that does not necessarily maintain interaction with the user, but can still respond to invitations or forward invitations on Bob's behalf. The lowly wake-up call is a good example of a session created automati- cally. The UA at the hotel reception is programmed to call the guest's UA at time t. Media Tools All in all, SIP delivers a session description to a SIP UA. If the session described is a voice session, the UA will have to deliver it to the voice tool that will handle the audio. For other types of sessions, the UA will deliver the session to the proper media tool. SIP UAs are sometimes incorporated into the same user interface with media tools for the session. An audio/video session can't be established without a SIP UA, an audio tool, and a video tool. If these three are com- bined under the same user interface, t