Session Initiation Protocol (SIP)

The Session Initiation Protocol (SIP) is a communications protocol primarily used in VoIP (Voice over IP) and other real-time communication services such as video calls. It ensures that a session between two or more participants is established, managed, and terminated. SIP regulates the establishment, management, and termination of real-time communications such as VoIP or video chats. It is therefore not responsible for the audio transmission itself, but for managing the connection—like a moderator bringing two conversation partners together. You can think of SIP as the "phone book" that determines how the connection is established and terminated, but not how the actual communication proceeds—that is handled by another protocol such as RTP.

Today’s basis for VoIP: SIP is standard for Internet telephony (e.g. in softphones or IP telephone systems), but can be partially replaced by WebRTC in browsers.

Classification in the OSI model:

• Application layer (Layer 7)
  SIP is an application layer protocol because it provides the logic to initiate, manage, and terminate communication sessions in real time.