SIP (Session Initiation Protocol)
SIP is the signaling protocol that sets up, changes and ends real-time sessions such as voice calls. Learn what SIP does and what it leaves to other protocols.
Published 8 November 2026 · 1 min read · By SpaGreen Creative
Short answer
SIP is the text-based signaling protocol, defined in RFC 3261, that locates endpoints and sets up, modifies and ends multimedia sessions such as voice calls.
SIP, the Session Initiation Protocol, is the signaling layer behind most VoIP calling. It is defined in RFC 3261 and uses plain-text requests and responses that look similar to HTTP. A client sends a request such as REGISTER to announce where it can be reached, or INVITE to start a call, and the server answers with a numeric status code such as 200 OK or 401 Unauthorized.
SIP's job is limited to finding the other party and agreeing on how the session will run. It does not carry the audio itself. The details of the media, such as codecs, IP addresses and ports, travel inside SIP messages in a separate format called SDP, and the audio then flows over RTP directly between the endpoints or through a media server.
SIP can run over UDP, TCP or TLS. The transport affects reliability, NAT behaviour and whether the signaling is encrypted. Every SIP softphone, PBX and trunk provider speaks this protocol, which is why a standards-based client can connect to servers from many different vendors.
Sources and further reading
- RFC 3261: SIP: Session Initiation Protocolwww.rfc-editor.org
This article is published by SpaGreen Creative, the maker of SIPdia. Product names mentioned belong to their owners. Where it describes other software, treat the steps as general guidance and check the official documentation linked above.
