Subject: [AVT] RTP Retransmissions (discussion restart)

From: Carsten Burmeister <[email-masked]>
Date: Mon, 07 Jan 2002 16:31:57 +0100

Dear All,

Happy New Year!

I could not attend the last IETF meeting, but from the pre-minutes I saw
that the retransmissions were discussed. As there were also some discussions
on the mailing list before the meeting and in parallel, I would like to try
to summarize those and re-start the discussions.

There are two drafts dealing with retransmissions in RTP: The AVT work item
draft draft-ietf-avt-rtp-selret-03.txt and the proposal of David
draft-leon-rtp-retransmission-01.txt. There have been a couple of changes in
both drafts during the last iteration(s), however I would like to discuss
the drafts only as they are now.

The main difference between the drafts is the following: The AVT-draft
allows sending the retransmissions in the same RTP/RTCP session *and*
sending them in a different RTP/RTCP session. Both possibilities are
separated by the means of some signalling, e.g. SDP. The Leon-draft allows
sending the retransmissions in a different RTP/RTCP session, but forbids
sending the retransmissions in the same RTP/RTCP session.

We propose to allow both options in the AVT-draft, because we think that
there are different applications with different requirements on the
retransmission scheme. The applications we have in mind (e.g. MPEG-4
streaming over unreliable networks) can benefit significantly from sending
the retransmissions in the same session as the original data, because of the
much faster recovery scheme. Norihito Takatori (from Mitsubishi) also
supported this option. David Leon wants to send the retransmissions in a
different RTP session. Especially for the conversational text payload
format, where the indication of lost packets to the receiving application is
recommended, this might be beneficial (however using this indication
together with retransmissions is a little bit more tricky anyway).

To us these discussions already reveal that there is a need for more than
one very restrictive retransmission scheme. As our payload formats can build
these schemes and they can be separated, e.g. by SDP, we think that this
would be a good solution for all.

David has a different opinion and wants to restrict the retransmission
scheme to send the retransmissions always on a different RTP/RTCP session.
Our application design would have problems with that and the performance
would be decreased, e.g. because of the need for additional timers, RTP
sessions and the longer response times.

Are there more opinions or comments?

BR,
Carsten

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
http://www1.ietf.org/mailman/listinfo/avt

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From: Bill Foster <[email-masked]>
Date: Mon, 07 Jan 2002 08:14:47 -0800

At 04:31 PM 1/7/2002 +0100, Carsten Burmeister wrote:
>supported this option.

I also support this option. The ability to detect the loss of RTX packets 
more quickly by using the same sequence number space as the RTP packets is 
important for some applications. There may be some advantages in some 
applications for having a separate RTP session, but in other applications 
it is a distinct disadvantage. I think we should allow both.

>David Leon wants to send the retransmissions in a

-----------------------------
Bill Foster
Phone: (250) 758-9418
Fax:     (781) 998-6492