Remote Audio Output Protocol
Remote Audio Output Protocol (RAOP), is an RTSP/RTP based protocol made Apple Inc. It powers the AirTunes technology built into AirPort Express wireless (802.11b/g) access point. The major difference from the RTSP protocol is an initial Asymmetric key verification made by iTunes to verify it is talking to a AirPort Express, and vice versa. The data channel is also encrypted by AES, with a random key protected by the assymetric key mentioned above.
The key stored in iTunes has been extracted by Jon Lech Johansen, enabling 3rd party RAOP clients, but the key stored in the AirPort Express is not yet know, and 3rd party server are thus not yet possible without knowing the random AES key iTunes generated.