RTSP provides "VCR-style" control functionality such as pause, fast forward, reverse, and absolute positioning, which is beyond the scope of H.323 and RTP.
Both H.323 and RTSP use RTP as their standard means of actually delivering the multimedia data. This data-level compatibility makes efficient gateways between the protocols possible, since only control messages need to be translated.
|Task||Inviting users to real-time conferences||Initiating and controlling media streams to unicast and multicast addresses|
|Data transport||Bi-directional between SIP caller and callee||One-directional; media server may either play or record data, with direction indicated at stream setup time|
|third-party delivery||not yet, but planned||The Transport header may contain any address, including an address differing from the one issuing the RTSP requests.|
|Caching||No notion of content caching, as conferences are real-time||Caching similar to HTTP, where end systems contact cache to obtain content. Like some HTTP caches such as "sqid", caches may be "cut-through", in that data is delived to the client before it has been completely received by the cache.|
|Redirection||Location header; used for personal mobility and for bypassing proxies||Location header; used for load sharing between media servers|
Invites a user to one or more media sessions. Transport information is indicated in the session description included as the message body.
Invites a server to send data for a single media stream to the destination specified in the Transport header field. If left open by the client, the server may also select transport parameters and convey them to the client using the Transport response header.
Terminates the whole call/session.
Depending on URL, may terminate whole session or individual media stream.
SIP may support third-party invitations in the future, but RTSP likely will not due to its different role. If a conference participant (Alice) wants to get a media server into a conference using an MCU, it could invite it using SIP, indicating the MCU as the third party to be contacted. The media server then invites itself to the MCU using SIP and obtains the necessary transport information. Alice then sends normal RTSP commands to the media server, indicating the Conference
Session establishment is also faster, although this is not likely to be a major issue since RTSP requires its own setup.
Last updated by Henning Schulzrinne