Media Data Streaming - Scalable Continuous Media Streaming Systems - page 114

Media streaming solution. MMS employs TCP for the exchange of control messages and can
send the media data over either UDP or TCP. RealNetworks also developed their own Real-
Networks Data Transport (RDT) for use in their streaming solution. Because of the proprietary
nature of these protocols we will not cover them further in this chapter.
On the other hand, the Internet community has also developed open standards for media
streaming. This includes the Real Time Streaming Protocol (RTSP) defined in RFC 2326 [7],
the Real-time Transport Protocol (RTP) and the RTP Control Protocol (RTCP), first introduced
in RFC 1889, later revised in RFC 3550 [8], which became an official standard in May 2004.

6.2.1 Real-Time Streaming Protocol (RTSP)
The Real-Time Streaming Protocol (RTSP) is an application-layer protocol designed to control
the delivery of media data (e.g., play, pause, and seek) with embedded timing information, such
as audio and video. The protocol is independent of the lower-layer protocol. Thus, RTSP can
be carried over TCP, UDP, or other transport protocols. The syntax of RTSP shares many sim-
ilarities with HTTP/1.1, thus simplifying implementation and deployment. However, besides
the syntax similarities, RTSP differs from HTTP in many important ways.
First, unlike HTTP, RTSP is a stateful protocol, thus requiring the host to maintain state
information of a streaming session across multiple RTSP requests. Second, both the RTSP
server and client can issues RTSP requests. Finally, the media data are to be delivered out-of-
band, i.e., using a separate protocol such as, but not limited to, the Real-time Transport Protocol.
In a typical streaming application (see Figure 6.3), the client will first obtain a presentation
description file using out-of-band methods (e.g., through the web using HTTP). The presen-
tation description file describes one or more presentations , each composed of one or more
synchronized media streams. The presentation description file also contains properties of the

HTTP GET

Web Server Web Browser

Presentation Description File

RTSP SETUP

RTSP PLAY

RTP Media Data

Media Server Media Client

.

RTP Media Data

RTSP CLOSE

Figure 6.3 Protocol exchanges in a media streaming session