Session Initiation Protocol (SIP)

Distributed architecture for creating, modifying and terminating sessions with one or more participants.
Controls communications sessions.
Standardised under IETF.
Covers video conferencing, audio conferencing and one-to-one telephone calls.
Stack independent, usually run over TCP or UDP.
Uses request/response model similar to HTTP – client sends a request and gets a response from server.
Requests/responses in plain text with SIP borrowing HTTP status codes and encoding rules.
Data (voice/video) carried over Real-time Transport Protocol (RTP) providing streaming functions after SIP establishes link.
Compared to H.323, SIP is less complex, has lower overheads for VoIP.
H.323 is less extensible, less scalable and suitable only for single LAN.