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

1 Introduction
With the increase in the mobility of people and the number of mobile devices that they
use, it is important to stay connected irrespective of the geographical location of a
person and the mobile device used. The Session Initiation Protocol (SIP) provides
converged communication between various mobile devices and enables seamless data
and voice transfer. This protocol establishes a session between the two end points in
the network and the actual data is transferred using protocols, such as Real-Time
Transport Protocol (RTP) , Real-Time Streaming Protocol (RTSP) and the Media
Gateway Control Protocol (MEGACO), and the Session Description Protocol (SDP).
SIP also enables personal mobility so that users can maintain a single externally visible
identifier irrespective of their geographical location.
This chapter discusses the SIP architecture and the various components in the SIP
infrastructure that enable it to facilitate users to remain connected in different network
locations.
This chapter addresses the following topics:
• “SIP Overview”
• “SIP Components” (page 18)
• “SIP Architecture” (page 20)
• “SIP Messages” (page 22)
• “Call Flow Scenarios” (page 29)
• “Implementations of SIP on HP-UX” (page 33)
SIP Overview
SIP is an application layer signaling protocol, developed by the Internet Engineering
Task Force (IETF), that enables the creation, modification, and termination of sessions
that are independent of the underlying transport protocols and session type being
established.
Like hypertext transfer protocol (HTTP), SIP is a client-server protocol, in which requests
are issued by the clients and responses are managed by the servers. Clients and servers
exchange messages that contain the information required for establishing a session.
The SIP infrastructure includes network hosts called the proxy server to which user
agents can send registrations, invitations to sessions, and other requests to locate session
participants and for other functions. The infrastructure also contains entities such as
redirect server, registrar, and location service to redirect requests, alter the contact
address, and locate an address. These entities along with the name mapping service in
SIP support personal mobility and enable users to maintain a single externally visible
identifier, regardless of the network location of the user. SIP also contains a back 2 back
user agent (B2BUA) that is similar to the proxy server. However, B2BUA serves as both
a user agent client and a user agent server.
SIP Overview 17