The SIP Network Architecture

SIP defines two basic classes of network entities—clients and servers.
Strictly speaking, a client, also known as a user agent client, is an application
program that sends SIP requests.A server is an entity that responds to
those requests. Thus, SIP is a client-server protocol. VoIP calls using SIP
are originated by a client and terminated at a server.A client may be found
within a user’s device, which could be, for example, a SIP phone. Clients
may also be found within the same platform as a server. For example, SIP
enables the use of proxies, which act as both clients and servers.
Four different types of servers are available—proxy servers, redirect
servers, user agent servers, and registrars.A proxy server acts similarly to a
proxy server used for Web access from a corporate local area network
(LAN). Clients send requests to the proxy, which either handles those
requests itself or forwards them on to other servers, perhaps after performing
some translation. To those other servers, it appears as though the message
is coming from the proxy rather than some entity hidden behind it.
Given that a proxy both receives requests and sends requests, it incorporates
both server and client functionality. Figure 8-7 shows an example of
the operation of a proxy server. It does not take much imagination to realize
how this type of functionality can be used for call forwarding/follow-me
services.
A redirect server is a server that accepts SIP requests, maps the destination
address to zero or more new addresses, and returns the translated
address to the originator of the request. Thereafter, the originator of the
request may send requests to the address(es) returned by the redirect
server. A redirect server does not initiate any SIP requests of its own.
Figure 8-8 shows an example of the operation of a redirect server. This
can be another means of providing the call forwarding/follow-me service
that can be provided by a proxy server. This difference is that, in the case of a redirect sever, the originating client does the actual forwarding of the call.
The redirect server simply provides the information necessary to enable the
originating client to do so, after which the redirect server is no longer
involved.

A user-agent server accepts SIP requests and contacts the user. A
response from the user to the user-agent server results in a SIP response on
behalf of the user. In reality, a SIP device, such as a SIP-enabled phone, will
function as both a user-agent client and a user-agent server. Acting as a
user-agent client, it is able to initiate SIP requests. Acting as a user-agent
server, it can receive and respond to SIP requests. In practical terms, this
means that it is able to initiate calls and receive calls. This enables SIP, a
client-server protocol, to be used for peer-to-peer communication.
A registrar is a server that accepts SIP REGISTER requests. SIP
includes the concept of user registration, whereby a user signals to the network
that it is available at a particular address. Such registration is performed
by the issuance of a REGISTER request from the user to the
registrar. Typically, a registrar will be combined with a proxy or redirect
server. Registration in SIP serves a similar purpose to location updating in
a GSM network; it is a means by which a user can signal to the network
that he or she is available at a particular location.
Given that practical implementations involve the combination of a useragent
client and a user-agent server and the combining of registrars with
either proxy servers or redirection servers, a real network may well involve
only user agents and the redirection or proxy servers.