Книга: Practical Common Lisp

The Shoutcast Protocol

The Shoutcast Protocol

The Shoutcast protocol was invented by the folks at Nullsoft, the makers of the Winamp MP3 software. It was designed to support Internet audio broadcasting—Shoutcast DJs send audio data from their personal computers to a central Shoutcast server that then turns around and streams it out to any connected listeners.

The server you'll build is actually only half a true Shoutcast server—you'll use the protocol that Shoutcast servers use to stream MP3s to listeners, but your server will be able to serve only songs already stored on the file system of the computer where the server is running.

You need to worry about only two parts of the Shoutcast protocol: the request that a client makes in order to start receiving a stream and the format of the response, including the mechanism by which metadata about what song is currently playing is embedded in the stream.

The initial request from the MP3 client to the Shoutcast server is formatted as a normal HTTP request. In response, the Shoutcast server sends an ICY response that looks like an HTTP response except with the string "ICY"[298] in place of the normal HTTP version string and with different headers. After sending the headers and a blank line, the server streams a potentially endless amount of MP3 data.

The only tricky thing about the Shoutcast protocol is the way metadata about the songs being streamed is embedded in the data sent to the client. The problem facing the Shoutcast designers was to provide a way for the Shoutcast server to communicate new title information to the client each time it started playing a new song so the client could display it in its UI. (Recall from Chapter 25 that the MP3 format doesn't make any provision for encoding metadata.) While one of the design goals of ID3v2 had been to make it better suited for use when streaming MP3s, the Nullsoft folks decided to go their own route and invent a new scheme that's fairly easy to implement on both the client side and the server side. That, of course, was ideal for them since they were also the authors of their own MP3 client.

Their scheme was to simply ignore the structure of MP3 data and embed a chunk of self-delimiting metadata every n bytes. The client would then be responsible for stripping out this metadata so it wasn't treated as MP3 data. Since metadata sent to a client that isn't ready for it will cause glitches in the sound, the server is supposed to send metadata only if the client's original request contains a special Icy-Metadata header. And in order for the client to know how often to expect metadata, the server must send back a header Icy-Metaint whose value is the number of bytes of MP3 data that will be sent between each chunk of metadata.

The basic content of the metadata is a string of the form "StreamTitle='title';" where title is the title of the current song and can't contain single quote marks. This payload is encoded as a length-delimited array of bytes: a single byte is sent indicating how many 16-byte blocks follow, and then that many blocks are sent. They contain the string payload as an ASCII string, with the final block padded out with null bytes as necessary.

Thus, the smallest legal metadata chunk is a single byte, zero, indicating zero subsequent blocks. If the server doesn't need to update the metadata, it can send such an empty chunk, but it must send at least the one byte so the client doesn't throw away actual MP3 data.

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