SIP, H.323 and MGCP/Megaco

A number of signalling protocols have been developed in different camps to address the need for real-time session signalling over packet-based networks. Each of these protocols has different origins and different supporters with differing priorities.

H.323 was developed in the Enterprise LAN community as a video-conferencing technique and has much in common with ISDN signalling protocols such as Q.931.

MGCP/Megaco comes from the telco engineering world and is closely associated with intra-domain control of softswitches and media gateways etc.

The IETF developed SIP, reusing many familiar Internet elements: SMTP, HTTP, URLs, MIME, DNS.

Despite all being signalling protocols, they are not equals and peers - they can and will coexist; however, there is some debate as to what extent.