The Session Initiation Protocol (SIP) is the most common signaling protocol used in VOIP today to initiate, maintain, and terminate communication sessions, which include voice, video, and messaging. A session is a communication connection between two endpoints. These endpoints can be hardphones, computers, smartphones, or any other device capable of making call via the network.

The SIP protocol is an application layer protocol that can use the TCP/IP suite of protocols. This means that SIP sessions can use either TCP or UDP as it’s transport layer protocol. Typically, SIP runs on top of the UDP protocol for performance reasons. It also can run on either IPv4 or IPv6 at the network layer.

SIP is an open standard protocol and this has sparked tremendous growth in the telephony market. As a result many of the phone manufactures ship SIP-based phones for use with VOIP. With SIP, phones can determine each endpoints location, as well as the agree upon the same audio/video codecs necessary to transfer multimedia data.