Protocols

Voice over IP has been implemented in various ways using both proprietary and open protocols and standards. Examples of technologies used to implement Voice over IP include:

• H.323
• IP Multimedia Subsystem (IMS)
• Media Gateway Control Protocol (MGCP)
• Session Initiation Protocol (SIP)
• Real-time Transport Protocol (RTP)
• Session Description Protocol (SDP)

The H.323 protocol was one of the first VoIP protocols that found widespread implementation for long-distance traffic, as well as local area network services. However, since the development of newer, less complex protocols, such as MGCP and SIP, H.323 deployments are increasingly limited to carrying existing long-haul network traffic. In particular, the Session Initiation Protocol (SIP) has gained widespread VoIP market penetration.
A notable proprietary implementation is the Skype protocol, which is in part based on the principles of Peer-to-Peer (P2P) networking.