t been stored in the database and includes a To tag: SIP/2.0 200 OK Via: SIP/2.0/UDP 200.201.202.203:5060;branch=z9hG4bKus19 To: Werner Heisenberg <sip:werner.heisenberg@munich.de>;tag=8771 From: Werner Heisenberg <sip:werner.heisenberg@munich.de> tag=3431 Call-ID: 23@200.201.202.203 CSeq: 1 REGISTER Contact: k<sip:werner.heisenberg@munich.de>;expires=360 Content-Length: 0 Introduction to SIP The Contact URI is returned along with an expires parameter, which indicates how long the registration is valid, which in this case is 1 hour (3,600 sec- onds). If Heisenberg wants the registration to be valid beyond that interval, he must send another REGISTER request within the expiration interval. Registration is typically automatically performed on initialization of a SIP device and at regular intervals determined by the expiration interval chosen by the registrar server. Registration is an additive process-more than one device can be registered against a SIP URI. If more than one device is registered, a proxy may forward the request to either or both devices, either in a sequential or parallel search. Additional register operations can be used to clear registrations or retrieve a list of currently registered devices. 2.4 SIP Presence and Instant Message Example This example shows how SIP is used in a presence and instant messaging appli- cation. Presence information can be thought of as the state of a user or device at a particular instant. It can be as simple as whether a particular user is signed in or not, whether they are active at their station, or idle or away. For a mobile device, presence information can include the actual location in terms of coordinates, or in general terms such as "in the office," "on travel," or "in the lab." Presence information can even include information about the status or mood of the user, whether they are working, relaxing, or socializing. For all these examples, a pres- ence protocol is mainly concerned about establishing subscriptions or long-term relationships between devices about