A method comprising: generating a session description message that describes properties of a multimedia presentation to be streamed to a client device, the multimedia presentation having a plurality of media streams, and the session description message having: a mutual exclusivity extension that is separate from an Advanced Streaming Format, ASF, file header and that specifies which of the plurality of media streams are mutually exclusive; a marker extension that is separate from the ASF file header and that specifies an index into the multimedia presentation; a script command extension that is separate from the ASF file header and that specifies data that is to be presented during playback of the multimedia presentation; a leaky bucket extension that is separate from the ASF file header and that specifies how much data of the multimedia presentation to buffer when playing the multimedia presentation at a particular bit rate; a caching information extension that is separate from the ASF file header and that specifies information about caching of the multimedia presentation; an encapsulation extension that is separate from the ASF file header and that specifies codec parameters for one or more of a plurality of different codecs; a bandwidth sharing extension that is separate from the ASF file header and that specifies media streams of the plurality of media streams that share bandwidth; a stream prioritization extension that is separate from the ASF file header and that specifies which of the plurality of media streams should have priority over which other of the plurality of media streams; and include, as part of data streamed to the client device, the session description message. generating a session description message that describes properties of a multimedia presentation to be streamed to a client device, the multimedia presentation having a plurality of media streams, and the session description message having: a mutual exclusivity extension that is separate from an Advanced Streaming Format, ASF, file header and that specifies which of the plurality of media streams are mutually exclusive; a marker extension that is separate from the ASF file header and that specifies an index into the multimedia presentation; a script command extension that is separate from the ASF file header and that specifies data that is to be presented during playback of the multimedia presentation; a leaky bucket extension that is separate from the ASF file header and that specifies how much data of the multimedia presentation to buffer when playing the multimedia presentation at a particular bit rate; a caching information extension that is separate from the ASF file header and that specifies information about caching of the multimedia presentation; an encapsulation extension that is separate from the ASF file header and that specifies codec parameters for one or more of a plurality of different codecs; a bandwidth sharing extension that is separate from the ASF file header and that specifies media streams of the plurality of media streams that share bandwidth; a stream prioritization extension that is separate from the ASF file header and that specifies which of the plurality of media streams should have priority over which other of the plurality of media streams; and a mutual exclusivity extension that is separate from an Advanced Streaming Format, ASF, file header and that specifies which of the plurality of media streams are mutually exclusive; a marker extension that is separate from the ASF file header and that specifies an index into the multimedia presentation; a script command extension that is separate from the ASF file header and that specifies data that is to be presented during playback of the multimedia presentation; a leaky bucket extension that is separate from the ASF file header and that specifies how much data of the multimedia presentation to buffer when playing the multimedia presentation at a particular bit rate; a caching information extension that is separate from the ASF file header and that specifies information about caching of the multimedia presentation; an encapsulation extension that is separate from the ASF file header and that specifies codec parameters for one or more of a plurality of different codecs; a bandwidth sharing extension that is separate from the ASF file header and that specifies media streams of the plurality of media streams that share bandwidth; a stream prioritization extension that is separate from the ASF file header and that specifies which of the plurality of media streams should have priority over which other of the plurality of media streams; and include, as part of data streamed to the client device, the session description message.