It is important to distinguish between RTP and Real-Time Streaming Protocols. RTSP is used when viewers communicate with a unicast server. RTSP allows two-way communication, that is, viewers can communicate with the streaming server and do things like rewind the movie, go to a chapter and so on. QuickTime automatically translates the viewer interaction with the one-screen movie controller into the proper RTSP requests. By contrast, RTP is a one-way protocol used to send live or stored streams from the server to the client.

How Live Streams are Handled

When you tune into a live broadcast, the QuickTime streaming client (for example, QuickTime Player or the QuickTime Plug-in) sends a request to the QuickTime Streaming Server. The server looks for a Session Description Protocol (SDP) file and if it is found, it begins to stream the media onto the computer via RTP. An SDP file is a text file that contains information about what will be streamed and how to tune in. SDP files are created by broadcast software (such as Sorenson Broadcaster) on the computer that captures the live media but the SDP file must be copied to the streaming server before the media can be broadcast. QuickTime Player and the QuickTime Plug-in can open SDP files as if they were movies.

Transport Issues

Real-time streaming involves some data transport issues:

• Data loss: RTP uses low-level User Datagram Protocol (UDP) transport. UDP is faster and more efficient than TCP/IP, but it lacks a mechanism for reporting lost packets, so streaming over the Internet almost always involves some data loss.

• Firewalls: Because most firewalls block UDP, viewers behind a firewall may not be able to receive live streams, even though they can request them via RTSP. A proxy server software is needed to get around this. Apple provides proxy servers for most popular firewalls.

• Network address translation: Small networks using routers to connect to the Internet may also have problems receiving streaming movies. These routers typically use Network Address Translation (NAT) but streaming over RTP involves port addresses that confuse some older NAT software.

• HTTP tunnelling: When everything fails, use HTTP tunnelling to "wrap" the RTP packets inside ordinary HTTP packets. This usually allows the stream to pass through any firewall or NAT router. To enable HTTP tunnelling, each viewer must configure their QuickTime Settings control panel by checking ‘Use HTTP’ and setting the port to 80 (or the port used for HTTP transfer by your streaming server) and the server must be set up to support HTTP tunnelling.

FastStart vs. Streaming.

When deciding on the best way to deliver QuickTime movies over the web, what should be chosen—Fast Start (HTTP/FTP delivery) or streaming (RTP/RTSP delivery)? For live feeds that must be transmitted in real time, streaming is the only answer, but for other types of movies, each delivery method has its own pros and cons.

MM Vijay Sargunar and M Syed Masood, Department of Computer Science, Engineering and Applications, Crescent Engineering College, Chennai