SIP (Session Initiation Protocol) describes a technology for the signaling of data streams, that can be used in the internet over TCP/IP networks to model classical fixed-line telephony.

The most common usage for SIP is VoIP - Voice over IP. In this case, telephone calls are not placed using the conventional telephone network (PSTN), but instead are routed over the familiar internet protocol TCP/IP. With a broadband connection to the internet, telephone calls between VoIP users can be made without any additional costs!

Additional possibilities include the use of instant messaging services and video conferencing.