The IMS: IP Multimedia Concepts And Services, Second Edition

12.3. SIP architecture

Elements in SIP can be classified as User Agents (UAs) and intermediaries (servers). In an ideal world, communications between two end points (or UAs) happen without the need for intermediaries. But, this is not always the case as network administrators and service providers would like to keep track of traffic in their network.

Figure 12.2 depicts a typical network setup, which is referred to as the "SIP trapezoid".

A SIP UA or terminal is the end point of dialogs: it sends and receives SIP requests and responses, it is the end point of multimedia streams and it is, usually, the User Equipment (UE), which comprises an application in a terminal or a dedicated hardware appliance. The UA consists of two parts:

• User Agent Client (UAC) – the caller application that initiates requests;

• User Agent Server (UAS) – accepts, redirects, rejects requests and sends responses to incoming requests on behalf of the user.

Gateways are special cases of UAs.

Figure 12.2. SIP trapezoids.

SIP intermediaries are logical entities through which SIP messages pass on their way to their final destination. These intermediaries are used to route and redirect requests. These servers include:

• Proxy server – receives and forwards SIP requests. It can interpret or re-write certain parts of SIP messages that do not disturb the state of a request or dialog at the end points, including the ...