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

A user agent can function in the following roles:
• User Agent Client
A user agent client (UAC) is a client application that initiates a new SIP request.
The application acts as a UAC only for the duration of the transaction when it sends
a SIP request. If the application receives a request, UAC assumes the role of a user
agent server for processing that transaction. The requests issued by a UAC are
INIVITE, ACK, OPTIONS, BYE, CANCEL, and REGISTER.
• User Agent Server
A user agent server (UAS) is a server application that receives a request, processes
the request, and generates a response. An application acts as a UAS only for the
duration of the transaction when it receives a SIP response. If the application sends
a request, UAS assumes the role of a user agent server for processing that
transaction.
A SIP end point can perform the role of both UAC and UAS. However, during a
transaction, it can perform only one role. Depending on the UA that initiated the
request, an end point functions as a UAC or a UAS.
Proxy Server
A proxy server is an intermediate entity that receives SIP requests and forwards them
on behalf of the requestor. Proxy servers route requests to the user’s current location,
authenticate and authorize users for services, and implement provider call-routing
policies. It also enforces polices for a user and provides name mapping.
For each new request, a proxy can function either in stateful or stateless mode. When
it functions as a stateless proxy server, it acts as a simple forwarding element. A stateless
proxy server discards information about a message after the message is forwarded. A
stateful proxy server remembers information (specifically, transaction state) about each
incoming request and also about requests it sends as a result of processing the incoming
request. It uses this information to influence the processing of future messages associated
with that request.
SIP proxy servers use presence services to track users and to check the willingness and
ability of users to communicate with other users on the network. A presence service is
an application of the SIP-specific event notification mechanism (RFC 3265 (Session
Initiation Protocol (SIP) - Specific Event Notification)). It enables a user to be located
regardless of the physical location. It also accepts presence information from users,
stores, and distributes the presence information to other interested users.
Redirect Server
A redirect server accepts a SIP request, maps the address into zero or more new
addresses, and returns these addresses to the client. The redirect server uses the location
service to obtain information about a user's possible locations. Unlike a proxy server,
SIP Components 19