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tcpudpnetwork-protocolsicecastinternet-radio

Icecast transport layer protocol - TCP or UDP?

I don't seem to find a answer, so I'm asking you. Does a stock Icecast2 server use TCP or UDP to broadcast the streaming data? I know that it uses a custom HTTP based Application Layer protocol, so one might think its TCP, but on the other hand it is a broadcast application, so UDP would be more logical to me. If it uses TCP nonetheless, why does it do that?

Solution

  • Icecast and SHOUTcast both use TCP for both the source streams and streaming to end clients. There are many reasons this is beneficial:

    • The codecs used by most internet radio stations do not lend themselves well to having lost chunks of data. If the stream were corrupt, either by lost or out-of-order packets, the decoder will sometimes be able to re-sync and continue, but many will simply fail.

    • Most internet radio stations have no real latency requirement. Nobody knows or cares if they get the audio delayed by a few seconds. It is actually typical to crank up the buffer size to allow clients to start playback quickly, causing delays of 10-30 seconds.

    • It is important to be compatible with HTTP. I suspect that when Nullsoft originally built SHOUTcast, their goal was to get up and running with it as simply as possible, so it makes sense that they mimicked HTTP. I suspect that the reason Icecast and SHOUTcast are so popular is that it is easy to write a client for them because it is essentially HTTP. Now that web-based players are a reality (with Flash and even HTML5), it is critical that the protocol be compatible with HTTP as many browsers do not support other streaming protocols. (Flash has its own protocol, but it is not nearly as simple as HTTP to implement.) If a client can play a file streamed from an HTTP server, it can stream from Icecast (and SHOUTcast if it is lenient in its HTTP implementation).

    • You mentioned broadcast... I don't know if you meant in the sense of UDP broadcast packets, but those do not work well in practice over the internet. Therefore, the only benefit to using UDP would be to reduce overhead, but I think you will see that for the reasons above, the few bytes of overhead don't outweigh the benefits of TCP for this type of application.

    In short, this is not a telephony application where latency matters and custom clients can be used.