Perhaps the most famous VoIP protocol is the H.323 standardized by International Telecommunication Union (ITU). H.323 is very sophisticated partially due to the fact that it is not just a strict VoIP protocol but also one for interactive data sharing, videoconferencing and audio/video processing. It is actually a conglomeration of several smaller protocols, namely:
Another smaller but very robust protocol is more geared to just VoIP. It is called Session Initiation Protocol and usually referred to as simply SIP. It consists of MIME, SDP and SIP itself.
MGCP (Media Gateway Control Protocol supported by Level 3, Bellcore, Cisco and Nortel) has also gained a substantial share of the VoIP market. Other main VoIP related protocols include Megaco H.248 (Media Gateway Control boasting great interoperability and backed by GTE, ICG, Netcom, Level 3,Sprint and founded by joint cooperation of IETF and the ITU-T Study Group 16), RVP over IP, SAPv2, SGCP and Skinny.
All these protocols are not always 100% compatible with each other therefore some possible standardization might by showing up there in the future.
And important factor in protocols application is bandwidth optimization. Signaling proxy operations are normally faster then the full proxy modes but perhaps the fastest are approaches like those of TDMoIP which deal with bundling together conversations going in the same direction.
