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

SIP supports the following while establishing and terminating sessions between user
agents:
• Determines the end system to be used for communication.
• Determines the willingness of the called user to participate in a call.
• Determines the media and media parameters to be used for a particular session.
• Establishes session parameters at both the ends of a session.
• Manages a session by including transfer and termination of sessions, modifying
session parameters, and invoking services.
Following are the salient features of SIP:
• Session-agnostic
• Transport independent
• Provides user mobility
• Provides stack configuration capabilities
SIP Components
This section discusses the components of the SIP infrastructure.
The SIP infrastructure contains the following components:
• User Agent
• Proxy Server
• Redirect Server
• Registrar Server
• Back 2 Back User Agent
This section addresses the following topics:
• “User Agent” (page 18)
• “Proxy Server” (page 19)
• “Redirect Server” (page 19)
• “Registrar” (page 20)
• “Back 2 Back User Agent” (page 20)
User Agent
A user agent (UA) is an application that can initiate, receive, and terminate a call in a
SIP session. The user agent initiating a call acts as a client while sending the initial
INVITE request and acts as a server while receiving a BYE request from the client. In
a SIP session, a user agent can be any end-user device, such as a cell phone, multimedia
handset, personal computer (PC), personal digital assistant (PDA), or a softphone.
18 Introduction