What is SIP?

SIP, which stands for Session Initiation Protocol, is the underlying communication protocol that is the industry standard for managing real-time multimedia sessions over the internet. In simpler terms, it is the "language" that powers most modern Voice over IP (VoIP) phone systems, video conferences, and instant messaging.

SIP's primary role is signaling. It is responsible for all the "behind the scenes" work required to set up and manage a communication session between two or more participants (endpoints). This includes tasks like:

• Locating the user you are trying to call.
• Making their phone ring.
• Negotiating the parameters of the call.
• Transferring or terminating the call when it's finished.

A complete communication session managed by SIP can be broken down into three key parts:

1. Session Initiation (The Signaling): This is the core function performed by SIP itself. It initiates the request to communicate (e.g., "I want to call this number") and manages all the control signals throughout the call's lifecycle.
2. Session Description (The Negotiation): Once the two endpoints agree to communicate, they need to establish the "rules" for the session. This is handled by another protocol called SDP (Session Description Protocol). SDP negotiates the technical details, such as which audio or video codecs will be used and which network ports the media should be sent to.
3. The Media Stream (The Payload): It is important to note that SIP only sets up and manages the call; it does not carry the actual voice or video data itself. The actual media stream is transported by a separate protocol, typically RTP (Real-time Transport Protocol) or its secure version, sRTP. This is the "payload" that contains the content of your conversation.