5.3 Protocols

Transmission Control Protocol (TCP) ^[7], ^[8] and User Datagram Protocol (UDP) ^[9] are two Internet Protocol transport protocols that can be used for transmitting compressed video over the Internet. TCP, a reliable protocol, guarantees delivery of all packets and in order, while UDP does not guarantee delivery of packets or the ordering of received packets. TCP uses retransmissions to guarantee that all packets arrive.

Most streaming video applications do not require guaranteed in-order arrival of all packets, and cannot tolerate the unbounded delay of using TCP to send compressed video data. So UDP is the transport protocol generally used for video streaming over IP networks.

The Real-Time Transport Protocol (RTP) is frequently used with UDP for streaming of video over IP networks. RTP provides functionality suited for carrying real-time content and for synchronizing different streams with timing properties. RTP specifies a header at the beginning of each packet that includes fields for payload type, time stamp, and sequence number. The RTP specification was published as RFC 1889 ^[10] by the Audio/Video Transport Working Group of the Internet Engineering Task Force (IETF).

RFC 1889 defines also the Real-Time Transport Control Protocol, RTCP, which works in conjunction with RTP. RTCP defines a syntax for providing feedback of quality-of-service (QoS) parameters to the participants of an RTP session.

RTP can be used with many different audio or video compression standards. The Audio/Video Transport Working Group also has published several RFCs that specify carriage of specific video compression standards over RTP, in general by adding standard-specific RTP header extensions. For example, RFC 2032 ^[11] "RTP Payload Format for H.261 Video Streams," describes a recommended syntax for an H.261 specific header to be included in an RTP packet, after the basic RTP header. In order to be error resilient, higher layer syntax elements from the H.261 bit stream are redundantly repeated in each packet header, in a fixed length format.

RFC 2038, ^[12] "RTP Payload Format for MPEG1/MPEG2 Video," similarly describes a recommended syntax for MPEG video data to be streamed using RTP. RFC 2038 applies only to MPEG elementary streams. RFC 2038 requires that coded pictures be fragmented into separate packets. New pictures must be at the start of a packet. Certain picture layer parameters are repeated in the MPEG specific RTP header extension.

RFC 2429, ^[13] "RTP Payload Format for the 1998 Version of ITU-T Rec. H.263 Video (H.263+)," describes a syntax for streaming H.263 over RTP. In addition to providing syntax for an H.263+ payload header, it provides an optional Video Redundancy Coding Header that works with H.263+'s reference picture selection to improve error resilience.

RFB 3016, "RTP Payload Format for MPEG-4 Audio/Visual Streams," does not provide an MPEG-4-specific RTP header extension. It does provide rules for fragmenting the MPEG-4 Visual Bitstream into RTP packets. An IETF Internet Draft, draft-ietf-avt-mpeg4-multisl-04.txt, "RTP Payload Format for MPEG-4 Streams," provides an MPEG-4-specific RTP header extension.

The Real-Time Streaming Protocol (RTSP) is a session control protocol for initiating and direction streaming of multimedia over IP. RTSP provides VCR-like control functions, such as PLAY, PAUSE, RESUME, FAST-FORWARD, and FAST-REWIND. RTSP is not used to deliver compressed video data itself, but is used in conjunction with other protocols such as RTP.

^[7]Postel, J., Transmission Control Protocol, RFC 793, 1981.

^[8]Allman, M., Paxson, V., and Stevens, W., TCP congestion control, RFC 2581, 1999.

^[9]Postel, J., User Datagram Protocol, Request for comments RFC 768, ISI, August 1980.

^[10]Schulzrinne, H. et al., RTP: A transport protocol for real-time applications, IETF RFC 1889, January 1996.

^[11]Turletti, T. and Huitema, C., RTP payload format for H.261 video streams, IETF RFC 2032, October 1996.

^[12]Hoffman, D. and Fernando, G., RTP payload format for MPEG1/MPEG2 video, IETF RFC 2038, October 1996.

^[13]Zhu, C., RTP payload format for H.263 video streams, IETF RFC 2190, September 1997.