HP-UX C SIP Stack Programmer's Guide (Novembery 2007)

2 HP-UX C SIP Stack Programmer’s Guide
Messages
USER AGENT In SIP, a User Agent (UA) is the endpoint entity. User Agents initiate and
terminate sessions by exchanging requests and responses. RFC 3261 defines the
User Agent as an application, which contains both a User Agent client and User
Agent server, as follows:
 User Agent Client (UAC)—a client application that initiates
SIP requests.
 User Agent Server (UAS)—a server application that contacts
the user when a SIP request is received and that returns a
response on behalf of the user.
Some of the devices that can have a UA function in a SIP network are
workstations, IP-phones, telephony gateways, call agents, and automated
answering services.
PROXY SERVER A Proxy server is an intermediary entity that acts as both a server and a client,
for the purpose of making requests on behalf of other clients. Requests are
serviced either internally or by passing them on, possibly after translation, to
other servers. A Proxy interprets, and, if necessary, rewrites a request message
before forwarding it.
REDIRECT SERVER A Redirect server is a server that accepts a SIP request, maps the SIP address of
the called party into zero (if there is no known address) or more new addresses
and returns them to the client. Unlike Proxy servers, Redirect Servers do not
pass the request onto other servers.
REGISTRAR A Registrar is a server that accepts REGISTER requests for the purpose of
updating a location database with the contact information of the user specified in
the request.
B2BUA A B2BUA is a logical entity that receives a request, processes it as a User Agent
Server (UAS) and, in order to determine how the request should be answered,
acts as a User Agent Client (UAC) and generates requests. A B2BUA must
maintain call state and actively participate in sending requests and responses for
dialogs in which it is involved. The B2BUA has tighter control of the call than a
Proxy—for example, a Proxy cannot disconnect a call or alter the messages.
MESSAGES The following sections deal with SIP messages.
MESSAGE TYPES There are two types of SIP messages:
 Requests—sent from the client to the server.