Basically, SIP is a text based application layer protocol similar to HTTP. It is used to control the setup and teardown of multimedia sessions in an IP network. VoIP is just one of the applications that can be handled using SIP. SIP also has a means of setting up video calls and fax calls. For example, SIP is used for presence and instant messaging communications. SIP is also extensible to support media and applications that are yet to be defined.
This was first published in September 2005