Subject: [AVT] Comments on draft-ietf-avt-rtp-retransmission-04.txt

From: Colin Perkins <[email-masked]>
Date: Thu, 02 Jan 2003 19:34:55 -0500

I've just read the -04 version of the retransmission draft. It seems to be
in reasonably good shape, but I do have some questions and comments:

 - At the end of section 3.1, after discussing the choice between SSRC and
   session multiplexing, the draft states "both multiplexing approaches
   SHOULD be supported". Is this intended to be a requirement for both the
   sender and receiver? 

 - In section 4, the draft mentions the possibility that the retransmission
   stream may include packets with multiple clock rates. How is this to be
   signalled in the SDP?

 - In section 4, the draft states that and profile-specific payload header
   must be included in the retransmitted packet. Is this sent before the
   OSN payload header or after it?

 - Section 4 is unclear how RTCP for the retransmission stream is handled.
   Is the sender expected to send periodic SDES and SR reports? BYEs? 

 - In section 5.3, the description of how to associate streams when SSRC
   multiplexing is used seems error-prone. Would it not be possible to
   signal the mapping explicitly?

 - In section 6.1, when the original and retransmission streams are sent
   in the same RTP session, how are the RTCP timing rules modified? Does
   the sender get twice the RTCP bandwidth since it appears as two SSRCs?

 - In section 6.2, a pointer to the RTCP Reporting Extensions draft may be
   appropriate, since that defines a receiver RTT report.

 - In section 8, note that the correct syntax is "a=fmtp:<number> ...".

 - In section 12, how does this draft affect SRTP? Is it possible to use 
   the two formats together?

I also think it would be appropriate if this draft describes the level of
reliability that can be expected from a session using retransmission, and
perhaps describes when this is acceptable, and when TCP should be used?

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

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From: [email-masked]
Date: Thu, 16 Jan 2003 10:17:24 -0600

Colin, 

I had missed one of your comments. My answer below:

> > >Here again, we thought that the RTP retransmission stream had to be
> > >treated like any other RTP stream as far as the RTCP 
> timing rules are
> > >concerned. This means that we can't specify the fraction 
> of the RTCP
> > >bandwidth available for the retransmission stream separatly.
> > The original
> > >stream and the retransmission stream are seen as two
> > independent senders
> > >that are sharing the RTCP bandwidths allocated to senders in
> > the session as
> > >would any other sender do.
> >
> > Okay, but the draft says "the receiver sends report blocks
> > for the original
> > and the retransmission streams in the same RTCP RR packet".
> > It is not clear
> > that two separate RTP sessions are used.

This is the SSRC-multiplexing case so there's a single RTP session. The receiver thus receives two SSRC sources, one for the original stream and one for the retransmission stream in this session. As usual, it should send one report block for each source in its RR packets. Is something creating confusion?

Thanks, David.