RFC 3261 / START HERE
SIP, before the details
SIP: Session Initiation Protocol
SIP sets up, changes, and ends communication sessions such as Internet calls. It handles signaling; the actual audio or video usually uses other protocols.
The idea in one picture
A caller sends an invitation
Participants negotiate the session
Separate protocols carry the media
A simplified picture of the idea; the chapters explain the actual mechanisms.
Why it exists
Participants need a way to locate each other, invite someone to a session, and agree whether to proceed.
A familiar example
An Internet phone sends an invitation, receives an answer, and later sends a message to end the call.
Helpful background
You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.
What you’ll learn next
Begin with chapter one, or choose a topic.
- Register a reachable contactA registrar records a binding from an address-of-record to a contact URI.→
- Invite through a proxySIP proxies route signaling toward a user; the media path can be separate.→
- A dialog outlives a transactionA dialog maintains peer relationships across individual request/response transactions.→
- Cancel a request, end a sessionCANCEL and BYE operate at different points in the call lifecycle.→
- Signaling is not the media streamA SIP message can carry an SDP offer or answer while media follows the negotiated transport.→
This introduction explains RFC 3261 (2002). Read the original RFC ↗