SIP (Session Initiation Protocol)

What is SIP?

The Session Initiation Protocol (SIP) is a signaling protocol used for initiating, maintaining, modifying, and terminating real-time sessions that involve video, voice, messaging, and other communications applications and services between two or more endpoints.

How it Works

SIP works like the HTTP of communications. It is a text-based protocol that sends messages between endpoints to set up a call. It handles:

• User Location: Determining where the end system to be used for communication is.
• User Availability: Determining the willingness of the called party to engage in communications.
• User Capabilities: Determining the media and media parameters to be used.
• Session Setup: "Ringing" establishment of session parameters at both caller and called party.
• Session Management: Including transfer and termination of sessions, modifying session parameters, and invoking services.

SIP vs. WebRTC

WebRTC does not mandate a specific signaling protocol, but SIP is often used as the signaling layer over WebSockets to set up WebRTC calls, especially when bridging into legacy VoIP networks.