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User agents are both the computer
that is being used to make a call, and the target computer that is being
called. These make the two endpoints of the communication
session. There are two components
to a user agent: a client and a server. When a user agent makes a request (such
as initiating a session),it is the User Agent Client
(UAC), and the user agent
responding to the request is the User Agent Server (UAS). Because the user
agent will send a message, and then respond to another, it
will switch back and forth
between these roles throughout a session. Even though other devices that we’ll
discuss are optional to various degrees, User Agents must exist for a SIP
session to be established. Without them, it would be like trying to make a
phone call without having another person to call. One UA will invite the other
into a session, and SIP can then be used to manage and tear down the session
when it is complete. During this time, the UAC will use SIP to send requests to
the UAS, which will acknowledge the request and respond to it. Just as a
conversation between two people on the phone consists of conveying a message or
asking a question and then waiting for a response, the UAC and UAS will
exchange messages and swap roles in a similar manner throughout the session.
Without this interaction, communication couldn’t exist.
Although a user agent is often a
software application installed on a computer, it can also be a PDA, USB phone
that connects to a computer, or a gateway that connects the network to the
Public Switched Telephone Network. In any of these situations however, the user
agent will continue to act as both a client and a server, as it sends and
responds to messages.
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