20:16 05 August 2017
Session Initiation Protocol (SIP) is a signaling, standardised protocol widely used for setting up, modifying and terminating two-party or multiparty voice or video communications over the Internet.
Working with other services in carrying voice for a call, SIP provides support by supplying the following information: user location, user availability, user capabilities, session setup and session management.
Typically, SIP sends messages in User Datagram Protocol (UDP) on port 5060 and uses port 5061 for a second line in a two-party communication. Many Voice over Internet Protocol (VoIP) providers can change these ports allowing users to use 5063, 5064, or 5068 to keep multiple ATA devices in the same Natted LAN network.
Included in the invitation when setting up a call are parameters describing exactly what form the audio or video will use. When both endpoints are ready to start exchanging data, Real Transport Protocol (RTP) is used to exchange voice and data packets.
SIP is also used to ensure that ATA devices remain registered with the provider by communication with their server. Information that is passed includes the IP address where the ATA can be located and other information that keeps the server updated with any information that may have changed since the last registration.