Subject: [AVT][MMUSIC] draft-lennox-mmusic-sdp-source-attributes-00 and RTCP SDES

From: Colin Perkins <[email-masked]>
Date: Sun, 18 Mar 2007 11:11:24 +0100

On 27 Feb 2007, at 20:03, Jonathan Lennox wrote:
> Joerg Ott, Thomas Schierl, and I have written an Internet-Draft on  
> source-specific SDP attributes, draft-lennox-mmusic-sdp-source- 
> attributes-00, now available from the I-D repository.
>
> The draft allows individual RTP sources to be described in SDP  
> session descriptions, generalizing the mechanism used by the draft  
> on RTCP extensions for single-source multicast (draft-ietf-avt- 
> rtcpssm).  It was also originally motivated by the desire to  
> support ssrc-multiplexing of H.264 SVC, as originally used by draft- 
> wenger-avt-rtp-svc-02 and draft-schierl-mmusic-layered-codec-01,  
> though the latest revisions of those drafts have dropped support  
> for ssrc-multiplexing for other reasons.
>
> Comments are welcome.
>
> I'm sending this announcement to the AVT group as well as MMUSIC,  
> as the draft's mechanisms have non-trivial interactions with RTP  
> collision detection.   Please direct followups to only one of the  
> working group mailing lists, depending on the topic being discussed.

[Sent to both lists, since this is a cross-group issue]

There is potential overlap between the media source attributes  
defined in this draft and the information conveyed in RTCP SDES  
packets. We should be clear on how these two mechanisms relate, what  
types of information should be signalled in SDP, and what types  
should be conveyed via RTCP SDES packets.

In the current draft, for example, the media source attributes for  
conveying a textual description of the source, and the source  
language, would seem to fit the sort of information usually sent in  
RTCP SDES items.

-- 
Colin Perkins
http://csperkins.org/

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---

From: Colin Perkins <[email-masked]>
Date: Sun, 18 Mar 2007 11:21:05 +0100

> for ssrc-multiplexing for other reasons.

Can you comment on the interactions between parameters specified in  
"a=fmtp:" lines and those specified in "a=ssrc: fmtp" lines? There  
would seem to be the potential for significant confusion here, yet  
the draft goes into relatively little detail.

Also, what are the interactions between congestion control and the  
media source bandwidth attribute? The are already problems with  
reconciling this with media level "b=" attributes: will these get  
worse with media source bandwidth attributes?

-- 
Colin Perkins
http://csperkins.org/

---

From: "Jonathan Lennox" <[email-masked]>
Date: Mon, 19 Mar 2007 11:38:04 +0100

> the draft goes into relatively little detail.   

Source-specific fmtp parameters are going to need to be defined per-format;
I don't think there's much you can meaningfully say about them on a
format-independent basis.

The particular use case I'm thinking about here is when you have several
H.264 streams with different (and incompatible) parameter sets in an RTP
session, and you want to send parameter sets out of band (as recommended by
RFC 3984).  There's no way to support this with SDP at present.

> worse with media source bandwidth attributes?   

It doesn't solve the problem, but I don't think it makes it worse. Basically
this just allocates the (sending) media bandwidth among the sources.

It probably interacts with the send-bandwidth attribute, though.

-- 
Jonathan Lennox -- Layered Media
[email-masked]

---

From: Colin Perkins <[email-masked]>
Date: Wed, 28 Mar 2007 08:52:28 +0100

> RFC 3984).  There's no way to support this with SDP at present.

Sure, but there are potential problems if conflicting format  
parameters are specified at media and source level.

> this just allocates the (sending) media bandwidth among the sources.

We do need to be clear that these are not a substitute for congestion  
control, though.

-- 
Colin Perkins
http://csperkins.org/