Subject: [MMUSIC] I-D ACTION:draft-camarillo-mmusic-alt-00.txt

From: [email-masked]
Date: Tue, 04 Feb 2003 06:44:47 -0500

A New Internet-Draft is available from the on-line Internet-Drafts directories.

	Title		: The Alternative Semantics for the Session
                          Description Protocol Grouping Framework
	Author(s)	: G. Camarillo, J. Rosenberg
	Filename	: draft-camarillo-mmusic-alt-00.txt
	Pages		: 8
	Date		: 2003-2-3
	
This document defines the alternative (ALT) semantics for the SDP
grouping framework. The ALT semantics allow offering alternative
media configurations to establish a particular media stream.

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From: Paul Kyzivat <[email-masked]>
Date: Tue, 11 Feb 2003 11:20:21 -0500

Gonzalo,

I appreciate the value of this kind of mechanism. But I have a few 
questions about the details.

My main concern is whether it is possible to be certain of ordering of 
media content as usage shifts from one alternative stream to another. 
There are several issues in this regard:

- multiple streams may be established in parallel, but only one should 
be used. In some cases, e.g. UDP, the only way to attempt establishment 
is to send media over the stream. So by definition more than one is 
being used. And if this is done in parallel there is no way to know the 
order in which the media will arrive.

- "During the session, a sender MAY choose to use any of the grouped m 
lines at a particular point in time to send data. This makes it possible 
to use the ALT semantics as a fail-over mechanism for ongoing sessions."

This is described as being useful for failover, but not restricted to 
that. If a sender simply stops using one stream and begins using 
another, there is no guarantee of the order in which the resultant media 
will arrive at the recipient. This may again result in ambiguity about 
the order of media content.

This gets worse if it is permissible to subsequently resume using a 
stream that had previously been used and then abandoned. As you bounce 
between streams it will rapidly become impossible to know the ordering 
of media content.

I think there need to be more constraints over how an alternative is 
selected and how transitions between them are subsequently selected.

	Paul

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