Simple definition
SIP (Session Initiation Protocol) is a signaling protocol used to establish, modify, and terminate communication sessions such as VoIP calls.
Technical definition
SIP uses requests and responses between endpoints, PBXs, proxies, and carriers. It uses SIP URIs and commonly relies on SDP to describe media parameters; the audio or video stream is normally carried separately by RTP.
How it works / role
A phone can first register with a PBX using REGISTER. During a call, an INVITE, provisional responses, 200 OK, and ACK establish the session, while BYE terminates it. Intermediate devices must preserve consistent signaling and addressing.
What is it used for?
Register endpoints, route calls, negotiate session parameters, and control IP telephony features.
Practical example
A SIP phone calls an external number: the PBX receives the INVITE, selects the carrier trunk, negotiates the session, and RTP then carries the voice media.
Common issues
- NAT or firewall changes or blocks signaling
- SIP ALG alters messages or advertised addresses
- Incorrect credentials, registrar, or proxy settings
- The call is established but media fails because of a separate RTP issue
Key takeaway: SIP mainly controls signaling; a SIP session can succeed while the RTP media path still fails.