1. Overview of SIP
Session Initiation Protocol (Session Initiation Protocol, SIP). SIP is an application layer signaling control protocol, the main purpose of which is to establish, modify and release the application layer protocol of multimedia sessions in the IP network. Its main applications include but not limited to voice, messaging, video, call control, etc. Participants in a session can communicate via multicast, mesh unicast, or a mixture of the two.
The location of the SIP protocol
SIP, like HTTP, belongs to the application layer protocol, so it can be transmitted through UDP (more commonly used), or it can be transmitted with TCP.
2. Common terms
Transaction: It is an event on the client side and the server side, including the request sent from the first client to the server side, and a transaction is marked by the branch and cseq parameters.
Session (session): It is a collection of multimedia sender and receiver combinations. It can be understood as a series of signal actions such as off-hook, dial-up, call, and hang-up for a call.
The session is established after the media exchange (exchange SDP)
Dialogue (dialog): A dialogue is an end-to-end SIP relationship between two UAs that lasts for a period of time. Use call-ID, local tag, remote tag (the other party's tag) to mark a conversation.