Re: [MMUSIC] RTSP multicast question.

["Yedidia Amit" <amit.yedidia at elbitsystems.com>]

Thank you all for your comments.
Now lets consider again the following scenario:
One media server which manage multiple live streams only - all the streams are sent by multicast to the local network.
Several clients (e.g. VLC) who send DESCRIBE receive SDP with the multicast details.
What is stopping them to receive the stream properly.( Even if they send SETUP and PLAY they will be answered with 200 ok)
All other controls are not relevant for this case (like PAUSE will not be supported -since this is live multicast)
What is wrong with that? Don't forget that I control the server to do whatever I want.

-----Original Message-----
From: Magnus Westerlund [mailto:magnus.westerlund at ericsson.com] 
Sent: Monday, August 04, 2008 6:37 PM
To: HAAS Christian
Cc: Yedidia Amit; mmusic at ietf.org
Subject: Re: [MMUSIC] RTSP multicast question.

HAAS Christian skrev:
> Hello!
> 
> I have to point out one paragraph since it puzzles me a bit - if not even
> throws over my view of RTSP so far :)
> 
> From: Magnus Westerlund [mailto:magnus.westerlund at ericsson.com] 
>> I would also like to point out that using the same session ID for 
>> multiple clients are not allowed according to this text in section 12.37:
>>
>>    Note that a session identifier identifies a RTSP session across
>>    transport sessions or connections. Control messages for more than one
>>    RTSP URL may be sent within a single RTSP session. Hence, it is
>>    possible that clients use the same session for controlling many
>>    streams constituting a presentation, as long as all the streams come
>>    from the same server. (See example in Section 14). However, multiple
>>    "user" sessions for the same URL from the same client MUST use
>>    different session identifiers.
> 
> I'd have read this paragraph (and others) that controlling one stream from
> several different clients is indeed possible - especially with the first
> sentence.;
> For example, in RFC 2326, 1.1 'Invitation of a media server to a conference':
> "... Several parties in the conference may take turns 'pushing the remote
> control buttons' ".
> So far I have interpreted RTSP to be a control mechanism that allows sharing
> of the controls [for shared resources, such as multicast streams, where this
> makes sense].
> How the session identifier is distributed was out of the scope, but that I'd
> have SDP considered for.
> 
> I interpreted the last sentence of your quoted paragraph with respect to
> multiple dedicated streams. When starting a RealPlayer twice for example on
> the same host, requesting the same file will generate two streams (two
> sessions).
> 
> This was the base of my second idea for our use case to have all clients keep
> alive the one session the server controls; and the session identifier I'd
> have taken from SDP (for example the 's' field).
> 

Well, then this is one more example of the inconsistencies of RFC 2326.

Having multiple client instances control the same session may be of 
interest, but it is not a thought through solution as one needs to 
ensure consistent handling of the RTSP state and provide a working 
security model for this. Something that I would claim is not in place in 
either RFC 2326 or in the RTSP 2.0 draft.

If you go read RTSP 2.0 it says in section 3.4 (version 18):

    If one has an established conference or group session, it is possible
    to have an RTSP server distribute media to the whole group.
    Transmission to the group is simplest when controlled by a single
    participant or leader of the conference.  Shared control might be
    possible, but would require further investigation and possibly
    extensions.

Cheers

Magnus Westerlund

IETF Transport Area Director & TSVWG Chair
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