Elements of a SIP Network
There are three main elements in a SIP Network: user agents, servers, and loca-
tion services.User Agents
User agentsare the end devices in a SIP network. They originate SIP requests to
establish media sessions, and send and receive media. A user agent can be a
SIP phone or SIP client software running on a PC or palmtop. Alternatively, a
user agent can be a gateway to another network, such as a PSTN gateway,
which allows a SIP phone to receive and make calls to the PSTN.
Auser agent client(UAC) is the part of the user agent that initiates requests,
while the user agent server(UAS) is the part of the user agent that generates
responses to received requests. Every SIP user agent contains both a UAC and
a UAS. During the course of a session, both parts are typically used. This is dif-
ferent from most other client-server architectures, such as web browsing. Dur-
ing a web browsing session, a PC is always the HTTP client (web browser
software), and the web server is always the HTTP server.
SIP user agents are usually assumed to be intelligent, in the sense of being
part of a fully qualified Internet host as defined in RFC 1121 and RFC 1122 [8],
[7], and support many other basic Internet protocols including DHCP, DNS,
IMCP, and so on.Servers
Servers are intermediary devices that are located within the SIP-enabled net-
work and assist user agents in session establishment and other functions.
There are three types of SIP servers defined in RFC 3261:
■■ A SIPproxyreceives SIP requests from a user agent or another proxy
and forwards or proxies the request to another location.
■■ Aredirect serverreceives a request from a user agent or proxy and
returns a redirection response (3xx), indicating where the request
should be retried.
■■ Aregistrar serverreceives SIP registration requests and updates the user
agent’s information into a location service or other database.
SIP proxy, redirect, and registrar servers are purely signaling relay elements.
They have no media capabilities and do not initiate requests except on behalf
of a user agent.
SIP servers are optional for SIP-based communications, as will be discussed
in Chapter 20, “Peer-to-Peer SIP.”106 Chapter 6