Figure 2.13 Overview of Registrar
Each registration has a limited life span.The expires header field or the expires parameter of the contact header field determines for how long the registration is valid.The user agent must refresh the registration within the life span. Otherwise it will expire and the user will become unavailable.
2.2.2.2.4 Redirect server
The entity that receives a request and sends back a reply containing a list of the current location of a particular user is called redirect server. A redirect server receives requests and looks up the intended recipient of the request in the location database, created by a registrar. It then creates a list of current locations of the user and sends it to the request originator in a response within SIP
3xx redirection responses class.
The originator of the request then extracts the list of destinations and sends another request directly to them. Figure 2.14 shows a typical redirection.
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Redirect Server
INVITE #1
302 Moved Temporarily
INVITE #2
User Agent A