/ˌɛs-ˌaɪ-ˈpiː/
n. “The protocol that makes voice and video calls over the internet possible.”
SIP, short for Session Initiation Protocol, is a signaling protocol used to initiate, manage, and terminate real-time communication sessions over IP networks. These sessions can include voice calls, video calls, instant messaging, and multimedia conferences. SIP is widely used in VoIP (Voice over IP) systems, unified communications, and video conferencing platforms.
Key characteristics of SIP include:
- Session Establishment: Initiates communication between endpoints, negotiating media capabilities and parameters.
- Session Management: Allows modifications during a session, such as adding video to an existing voice call.
- Session Termination: Gracefully ends communication sessions.
- Protocol Agnostic: Works independently of the underlying transport protocol, typically using UDP, TCP, or TLS.
A basic example of a SIP request (REGISTER) looks like this in textual form:
REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/UDP 192.168.1.100:5060
From: <sip:alice@example.com>
To: <sip:alice@example.com>
Call-ID: 123456@example.com
CSeq: 1 REGISTER
Contact: <sip:alice@192.168.1.100>In this snippet, a client registers its address with a SIP server, enabling incoming calls to reach the user. SIP messages include headers, methods (like INVITE, REGISTER, BYE), and URIs to coordinate communication.
In essence, SIP is the backbone of internet-based communication, managing the signaling and setup for voice, video, and messaging sessions in a standardized, interoperable way.