User Datagram Protocol (UDP) and Transmission Control Protocol (TCP) are the two basic transport protocols in an IP network, where TCP is used if a reliable connection required while UDP is used when reliability is not critical but simplicity is important.
Due to the time-sensitive nature of audio and video traffic, UDP is the logical choice as the transport technology. However, there are many short-comings for UDP, for example, more information was needed on a packet-by-packet basis than UDP offered. Therefore, other technologies, such as RTP, are adopted to compensate the shortages of UDP, as the transport technology of time sensitive multimedia traffic. As a result, VoIP traffic rides on top of RTP, which rides on top of UDP.
Though there are various signaling system for various VOIP and multimedia communication architecture, namely, H.323, MGCP, SIP etc., the transport technologies and protocols could be used to transport media with any signaling systems. In other words, the transport technologies and protocols are fairly independent of the signaling protocols.
In the following table, we listed the main transport technologies and protocols for time sensitive audio-video traffic, and their corresponding key functions:
| Protocol Name | Functions | Standard# |
| Real-Time Transport Protocol (RTP) | For delivering audio and video over the Internet. | RFC 3550 |
| Real Time Control Protocol (RTCP) | Provides out-of-band control information for an RTP flow. | RFC 3550 |
| Real Time Streaming Protocol (RTSP) | For use in streaming media systems which allows a client to remotely control a streaming media server and allowing time-based access to files on a server. | RFC 2326 |
| Reliable User Datagram Protocol (RUDP) | Used as the transport protocol for MGCP based network. | RFC 1151 |
| Secure Real-time Transport Protocol (SRTP) | Provides encryption, message authentication and integrity, and replay protection to the RTP data in both unicast and multicast applications. | RFC 3711 |
| Stream Control Transmission Protocol (SCTP) | Provides transport services to ensure reliable, in-sequence transport of messages with congestion control. | RFC 2960 |
| ZRTP | An extension to RTP which describes a method of Diffie-Hellman key agreement for SRTP. | Draft |
Related Terms: VOIP Transport Protocol, VOIP, RTP, RTCP, RTSP, RUDP, SCPT, ZRTP
Reference Links: http://www.javvin.com/protocolVOIP.html: VOIP Protocols