Re: [AVTCORE] Magnus Westerlund's Discuss on draft-ietf-avtcore-cc-feedback-message-08: (with DISCUSS and COMMENT)

Colin Perkins <csp@csperkins.org> Tue, 22 September 2020 16:19 UTC

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From: Colin Perkins <csp@csperkins.org>
In-Reply-To: <160078165368.7234.14368794515227742740@ietfa.amsl.com>
Date: Tue, 22 Sep 2020 17:19:21 +0100
Cc: The IESG <iesg@ietf.org>, draft-ietf-avtcore-cc-feedback-message@ietf.org, avtcore-chairs@ietf.org, avt@ietf.org, Bernard Aboba <bernard.aboba@gmail.com>
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To: Magnus Westerlund <magnus.westerlund@ericsson.com>
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Subject: Re: [AVTCORE] Magnus Westerlund's Discuss on draft-ietf-avtcore-cc-feedback-message-08: (with DISCUSS and COMMENT)
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Good point – it’s probably worth adding some words to this draft to say that the guidance in Section 7 of RFC 6679 should be followed when using this feedback format to carry ECN marking information.

Colin



> On 22 Sep 2020, at 14:34, Magnus Westerlund via Datatracker <noreply@ietf.org> wrote:
> 
> Magnus Westerlund has entered the following ballot position for
> draft-ietf-avtcore-cc-feedback-message-08: Discuss
> 
> When responding, please keep the subject line intact and reply to all
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> 
> Please refer to https://www.ietf.org/iesg/statement/discuss-criteria.html
> for more information about IESG DISCUSS and COMMENT positions.
> 
> 
> The document, along with other ballot positions, can be found here:
> https://datatracker.ietf.org/doc/draft-ietf-avtcore-cc-feedback-message/
> 
> 
> 
> ----------------------------------------------------------------------
> DISCUSS:
> ----------------------------------------------------------------------
> 
> In Section 7.
> 
> there is discussion that CCFB will replace the ECN FB format. I find that
> appropriate however there is one function in ECN FB format that is not
> discussed here. Section 7.2.1 in RFC 6679 states
> 
> An immediate or
>      early (depending on the RTP/AVPF mode) ECN feedback packet SHOULD
>      be generated on receipt of the first ECT- or ECN-CE-marked packet
>      from a sender that has not previously sent any ECT traffic.  Each
>      regular RTCP report MUST also contain an ECN Summary Report
>      (Section 5.2).  Reception of subsequent ECN-CE-marked packets MUST
>      result in additional early or immediate ECN feedback packets being
>      sent unless no timely feedback is required.
> 
> There are no specification in this document that says that on reception of
> ECN-CE marks the feedback packet should be sent using early or immediate. That
> might not be required given a correctly configured session, where reporting
> occur on the time scale. However, I think some discussion of the usage of early
> reporting for ECN-CE mark is needed if longer reporting intervals are used.
> 
> Where there any discussion in the WG of this subject?
> 
> 
> ----------------------------------------------------------------------
> COMMENT:
> ----------------------------------------------------------------------
> 
> A. Section 3.1:
> The document in one paragraph states the below:
> 
> The value of num_reports MAY be zero, indicating that there are no
>   packet metric blocks included for that SSRC.
> 
> When reading this I did wonder what value should be set. This is given almost
> at the end of the section in the below sentence.
> 
> If no packets are received from an SSRC in a reporting interval, a
>   report block MAY be sent with begin_seq set to the highest sequence
>   number previously received from that SSRC and num_reports set to zero
>   (or, the report can simply to omitted).
> 
> I think it would be clearer if the recommendation for begin_seq when
> num_reports = 0 to be stated in the context of the first one. Then a more
> focused sentence for the second part can explain the context in that paragraph.
> 
> B. Section 11:
> 
> Since RTP is an unreliable transport, a
>   sender can intentionally leave a gap in the RTP sequence number space
>   without causing harm, to check that the receiver is correctly
>   reporting losses.
> 
> I think "without causing harm" is overstating it. I think without serious harm
> is more correct. Depending on jitter buffer and usage of NACK to report packet
> loss a gap in the sequence number can have cause retransmission requests. Also
> if the gap is in the wrong place in a packet sequence, for example in the
> middle of sequence of packets for a single video frame it could trigger a
> repair action despite all data have been received.
> 
> I would state it is possible to insert gap with some consideration for the
> media so that minimal impact is had.
> 
> 
> 



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