Re: How to describe SSM sessions in SDP?

[Toerless Eckert <eckert@cisco.com>]

> I've forgotten - does "draft-ietf-mmusic-sdp-srcfilter-00.txt" describe our 
> current consensus for how SSM sessions are described in SDP - e.g.,
>          c=IN IP4 232.3.4.5/127
>          a=incl:IN IP4 232.3.4.5 192.168.9.10
> ?

It seems to describe that, yes. It is a bit sad though, that it, like IGMPv3
tries to provide just an adequate solution for ISM (normal multicast) in
the first place, even though most likely most users of both IGMPv3 and
such an SDDP extension will want to use it for SSM.

Problem is: There is nothing in this extened session description telling you
if the filtering is just voluntarily - which is what filtering is for ISM,
or if this is rather a necessary provisioning of the source address for
the benefit of SSM.

So an upgraded application that can recognize this extension can not 
differentiate if this is a SSM session, so it can not warn the user that
he won't see traffic in case that there's just insufficient stack below -
like only IGMPv2 or the like. Sure, the IGMPv3 API doesn't provide 
sufficient information either, but that just makes it worse.

So an application that would like to learn if that's an SSM session, or
if it can optionally express the filtering needs more attributes than
just the list of sources. It would need at least another binary bit
"Must include" or the like. Could rather call it "SSM" anyhow.

My 2cents.

Cheers
	Toerless