Subject: [AVT] Using the same CNAME for multiple hosts

From: "Ali C. Begen (abegen)" <[email-masked]>
Date: Mon, 2 Mar 2009 13:51:16 -0800

(Copying fecframe as well)

Magnus brought up a concern in the last fecframe session regarding the
use of the same CNAME by multiple hosts.

Here is a scenario:

 RTP           FEC
SOURCE  ---> ENCODER ---> FEC Stream --->   RTP
         |                                Receiver
         +--------------Source Stream -->   (RR)

This is a simple scenario, in more complex systems, there could be
different FEC encoders producing different FEC streams from the same
source stream. In other scenarios, multiple RTP source streams could be
combined and fed together to an FEC encoder. RFC 4756 has the
source/repair grouping semantics for SDP: a=group:fec s1 r1

On the RTP side, RFC 2733 used the same SSRC value for both source and
repair streams for easier association. Later, RFC 5109 did the same
thing.

Now, following the rules of RTP and based on the suggestions from
various individuals, we would like to use distinct (random) SSRC values
for the source and repair streams. This allows the sender to multiplex
source and repair streams on a single port (good for easier NAT
traversal, etc). We propose to use the CNAME field for association
between the repair and source streams. This provides us greater
flexibility.

This approach requires the use of the same CNAME by different hosts (a
common CNAME could be produced based on an algorithm). However, a naive
RTP device MAY assume that data from different flows with the same CNAME
come from the same host. Since this is not the case in our scenario,
such devices will break in our approach!

Does anybody see a potential problem here? Any objections to our
proposal? If so, please speak up. We have multiple drafts using this
approach and we need to hear objections (if any) before we complete
those drafts.

Thanks.
-acbegen

---

From: "Roni Even" <[email-masked]>
Date: Thu, 5 Mar 2009 11:15:42 +0200

Hi Ali,
I had some offline discussion with Colin and Steve Casner.
All of us think that this approach is OK. RFC 3550 does not require that the
CNAME will be attached to a specific endpoint.
The relevant text is
"
If each application creates its CNAME independently, the resulting
   CNAMEs may not be identical as would be required to provide a binding
   across multiple media tools belonging to one participant in a set of
   related RTP sessions.  If cross-media binding is required, it may be
   necessary for the CNAME of each tool to be externally configured with
   the same value by a coordination tool. "

This says that you will need a way to coordinate the CNAME and to keep it
for the relevant streams.
Example is a video with repair stream and the associated audio stream should
all have the same CNAME.

Note that you may have a case when one host (FEC encoder) may use multiple
CNAMEs.

The PROBLEM I see is with RFC 5109 mostly section 14.1 that explains why you
shall not multiplex both streams based on SSRC.
This was a change from RFC 2733. Maybe it will be good to review this text
in RFC 5109 and update the draft.

Regards
Roni Even

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf Of Ali C.
Begen (abegen)
Sent: Monday, March 02, 2009 11:51 PM
To: IETF AVT WG
Cc: [email-masked]
Subject: [AVT] Using the same CNAME for multiple hosts

Here is a scenario:

 RTP           FEC

Thanks.
-acbegen

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
https://www.ietf.org/mailman/listinfo/avt

---

From: "Ali C. Begen (abegen)" <[email-masked]>
Date: Sat, 7 Mar 2009 20:18:43 -0800

Hi Roni,

Thanks for your email. We will take this into consideration.

Regarding 5109, I agree with your comments. I am not sure why SSRC
multiplexing was not allowed in 5109. If anybody knows the history, that
might be useful.

If desired by the WG, we can make a quick update on 5109.

BR,
-acbegen

> From: Roni Even [mailto:[email-masked]] 
> Sent: Thursday, March 05, 2009 1:16 AM
> To: Ali C. Begen (abegen); 'IETF AVT WG'
> Cc: [email-masked]
> Subject: RE: [AVT] Using the same CNAME for multiple hosts
> 
> Hi Ali,