Session Initiation Protocol (SIP), shown in Figure
is a protocol
developed by the Internet Engineering Task Force (IETF) as a simpler
alternative to H.323. Like H.323, SIP defines a distributed architecture.
Unlike H.323, SIP only defines how sessions are to be set up and torn down. It
utilizes other IETF protocols to define other aspects of VoIP and multimedia
sessions. Examples include Session Description Protocol (SDP) for capabilities
exchange, Universal Resource Locaters (URLs) for addressing, Domain Name
Systems (DNSs) for service location, and Telephony Routing over IP (TRIP) for
call routing.
SIP was designed as a multimedia protocol that could take
advantage of the architecture and messages already found in popular Internet
applications. By using a distributed architecture that uses URLs for naming and
text-based messaging, SIP attempts to take advantage of the Internet model for
building VoIP networks and applications. In addition to VoIP, SIP is used for
videoconferencing and instant messaging.
Although the IETF has made
great progress in defining extensions that allow SIP to work with legacy voice
networks, the primary motivation behind the protocol is to support
next-generation communication models. These next-generation communication
models utilize the Internet and Internet applications.
As a protocol
used in a distributed architecture, SIP allows companies to build large-scale
networks that are scalable, resilient, and redundant. It provides mechanisms
for interconnecting with other VoIP networks. Intelligence and new features can
be added to the endpoints or the SIP proxy, or can be used to redirect servers.