Re: [AVTCORE] Mirja Kühlewind's Discuss on draft-ietf-avtcore-rtp-circuit-breakers-15: (with DISCUSS and COMMENT)

Colin Perkins <csp@csperkins.org> Wed, 04 May 2016 11:02 UTC

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Hi,

> On 3 May 2016, at 13:03, Mirja Kuehlewind <ietf@kuehlewind.net>; wrote:
> 
> Mirja Kühlewind has entered the following ballot position for
> draft-ietf-avtcore-rtp-circuit-breakers-15: 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-rtp-circuit-breakers/
> 
> 
> 
> ----------------------------------------------------------------------
> DISCUSS:
> ----------------------------------------------------------------------
> 
> This is about one point in section 7 (ECN) that I think is wrong but I
> would like to get some feedback from the authors:
> "then ECN-CE marked packets SHOULD
>   be treated as if they were lost when calculating if the congestion-
>   based RTP circuit breaker"
> (also section 5: "The count of ECN-CE marked packets
>   contained in those ECN feedback reports is counted towards the number
>   of lost packets reported")
> 
> We are currently discussion mechanisms where the AQM in the congested
> network node sends  much more CE markings than one would see loss (when
> using TCP) for the same level of congestion. When treating ECN-CE similar
> to loss, such a different behavior could trigger the circuit breaker
> unnecessarily. Potentially ECN-CE might not need to be considered here at
> all, because as long as there are (only) ECN-CE marks (and no loss) all
> data is transmitted correctly to the receiver and therefore there is no
> need to trigger a circuit breaker.

This is echoing the language in RFCs 3168 and 6679. My understanding is that both require CE marks to be treated the same as lost packets for congestion control, and the circuit breaker is essentially a form of congestion control.

> Further also in section 7: "ECN-CE marked packets SHOULD be treated as if
> they were lost for the
>   purposes of congestion control"
> 
> This document should not impose any SHOULDs for congestion control as
> this doc is only about circuit breaker sand therefore the sentence above
> should be removed.

Not sure I agree. The circuit breaker needs to treat lost and CE-marked packets the same as a congestion control algorithm, and the ECN specifications say that the response to CE marks needs to be the same as the response to loss. 

> ----------------------------------------------------------------------
> COMMENT:
> ----------------------------------------------------------------------
> 
> Few more minor comments:
> 
> 1) reference [I-D.ietf-tsvwg-circuit-breaker] should be normative

This was discussed in relation to Ben’s AD review. I think you can implement the RTP circuit breaker without reading the TSV draft, so informative seems correct. However, I don’t much care either way.

> 2) How is the loss rate in 4.3 calculated if some (but no all) RR are
> lost?

There’s no obvious way for the receiver of the RRs to know that some were lost, so the calculation will proceed as if the reporting interval was longer. 

> 3) I don't think that most of the text on congestion control in section 2
> (as well as the abstract) is necessary for this doc (but it also don't
> really hurt)

I agree it’s not necessary, but I tend to think motivation text is helpful in specifications.

Cheers,
Colin




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