HP-UX Java SIP Stack Programmer's Guide (February 2008)

the redirect server does not initiate its own SIP request. Instead, it redirects the request
back to the client indicating that the client must attempt a different route to reach the
recipient. This method of identifying a client irrespective of its location enables a client
to move geographically and still remain contactable with the same SIP identity. Unlike
a user agent server, the redirect server does not accept or terminate calls.
Registrar
The registrar server enables clients to alter the address at which they can be contacted.
A SIP client can send a REGISTER request to inform the registrar about the change in
the address. The registrar accepts the change and records the user's new address. Thus,
the information about the client's location is kept updated, even though the client
changes locations in the network. The registrar server also supports authentication.
A client must register with the registrar server each time a user turns on the SIP user
client, using a SIP IP phone, PC, laptop, or other SIP devices. Typically, the proxy or
registration server forwards the registration information to the location or the redirect
server. These registrations can occur at any time during a session and are also
periodically refreshed so that most recent information is stored in the location or the
redirect server. Therefore, a registrar server is co-located with a proxy or redirect server
to offer the location services.
Back 2 Back User Agent
A back 2 back user agent (B2BUA) serves both as a user agent server and a user agent
client. It is a logical entity that receives a request and processes it as a user agent server.
It acts as a user agent client to determine how the request must be handled.
Unlike a proxy server, it maintains the dialog state and participates in all requests sent
on the dialogs that it has established. The responsibility of a proxy server is limited to
routing messages. However, a B2BUA performs additional tasks, such as topology
hiding and network termination of dialogs. The B2BUA can also alter the messages to
suit different applications, such as prepaid charging, and customer care applications.
SIP Architecture
This section discusses the SIP architecture and the different layers in the SIP protocol.
It also illustrates the SIP protocol structure.
Figure 1-1 illustrates the building blocks in the SIP protocol architecture and the position
of SIP in the IP stack.
20 Introduction