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RFC 2326 Real Time Streaming Protocol April 199812.40 UnsupportedThe Unsupported response header lists the features not supported bythe server. In the case where the feature was specified via theProxy-Require field (Section 12.32), if there is a proxy on the pathbetween the client and the server, the proxy MUST insert a messagereply with an error message "551 Option Not Supported".See Section 12.32 for a usage example.12.41 User-AgentSee [H14.42]12.42 VarySee [H14.43]12.43 ViaSee [H14.44].12.44 WWW-AuthenticaSee [H14.46].13 CachingIn HTTP, response-request pairs are cached. RTSP differssignificantly in that respect. Responses are not cacheable, with theexception of the presentation description returned by DESCRIBE orincluded with ANNOUNCE. (Since the responses for anything butDESCRIBE and GET_PARAMETER do not return any data, caching is notreally an issue for these requests.) However, it is desirable for thecontinuous media data, typically delivered out-of-band with respectto RTSP, to be cached, as well as the session description.On receiving a SETUP or PLAY request, a proxy ascertains whether ithas an up-to-date copy of the continuous media content and itsdescription. It can determine whether the copy is up-to-date byissuing a SETUP or DESCRIBE request, respectively, and comparing theLast-Modified header with that of the cached copy. If the copy is notup-to-date, it modifies the SETUP transport parameters as appropriateand forwards the request to the origin server. Subsequent controlcommands such as PLAY or PAUSE then pass the proxy unmodified. Theproxy delivers the continuous media data to the client, whilepossibly making a local copy for later reuse. The exact behaviorallowed to the cache is given by the cache-response directivesSchulzrinne, et. al. Standards Track [Page 62]
RFC 2326 Real Time Streaming Protocol April 1998described in Section 12.8. A cache MUST answer any DESCRIBE requestsif it is currently serving the stream to the requestor, as it ispossible that low-level details of the stream description may havechanged on the origin-server.Note that an RTSP cache, unlike the HTTP cache, is of the "cutthrough"variety. Rather than retrieving the whole resource from theorigin server, the cache simply copies the streaming data as itpasses by on its way to the client. Thus, it does not introduceadditional latency.To the client, an RTSP proxy cache appears like a regular mediaserver, to the media origin server like a client. Just as an HTTPcache has to store the content type, content language, and so on forthe objects it caches, a media cache has to store the presentationdescription. Typically, a cache eliminates all transport-references(that is, multicast information) from the presentation description,since these are independent of the data delivery from the cache tothe client. Information on the encodings remains the same. If thecache is able to translate the cached media data, it would create anew presentation description with all the encoding possibilities itcan offer.14 ExamplesThe following examples refer to stream description formats that arenot standards, such as RTSL. The following examples are not to beused as a reference for those formats.14.1 Media on Demand (Unicast)Client C requests a movie from media servers A ( audio.example.com)and V (video.example.com). The media description is stored on a webserver W . The media description contains descriptions of thepresentation and all its streams, including the codecs that areavailable, dynamic RTP payload types, the protocol stack, and contentinformation such as language or copyright restrictions. It may alsogive an indication about the timeline of the movie.In this example, the client is only interested in the last part ofthe movie.C->W: GET /twister.sdp HTTP/1.1Host: www.example.comAccept: application/sdpW->C: HTTP/1.0 200 OKContent-Type: application/sdpSchulzrinne, et. al. Standards Track [Page 63]
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