Re: [AVTCORE] [tsvwg] [rtcweb] WG Last Call on changes: draft-ietf-avtcore-rtp-circuit-breakers-16

Michael Welzl <michawe@ifi.uio.no> Thu, 30 June 2016 16:28 UTC

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From: Michael Welzl <michawe@ifi.uio.no>
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Date: Thu, 30 Jun 2016 18:28:15 +0200
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To: "De Schepper, Koen (Nokia - BE)" <koen.de_schepper@nokia-bell-labs.com>
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Cc: "Black, David" <david.black@emc.com>, "rtcweb@ietf.org" <rtcweb@ietf.org>, tsvwg <tsvwg@ietf.org>, IETF AVTCore WG <avt@ietf.org>
Subject: Re: [AVTCORE] [tsvwg] [rtcweb] WG Last Call on changes: draft-ietf-avtcore-rtp-circuit-breakers-16
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Hi,


> On 30. jun. 2016, at 17.58, De Schepper, Koen (Nokia - BE) <koen.de_schepper@nokia-bell-labs.com>; wrote:
> 
> 
>> -----Original Message-----
>> From: Black, David [mailto:david.black@emc.com]
>> Sent: maandag 27 juni 2016 22:10
>> To: De Schepper, Koen (Nokia - BE)
>> Cc: tsvwg; IETF AVTCore WG; rtcweb@ietf.org; Black, David
>> Subject: RE: [tsvwg] [rtcweb] [AVTCORE] WG Last Call on changes: draft-
>> ietf-avtcore-rtp-circuit-breakers-16
>> 
>>> As long as an AQM is marking at the same rate as dropping
>> 
>> That's an interesting assumption - it should be true for AQMs vetted
>> here in the past,
> 
> And I hope for ect(0) the network will keep doing this in the future, at 
> least not to deviate too far from this.
> L4S proposes to use ect(1) just because it deviates too much, so the network 
> can detect the different end-system behavior and can couple it correctly to 
> what it is doing for drop and ect(0).
> 
> 
>> but there are easy ways for it not to hold (e.g., if
>> dropping
>> or marking is based on queue occupancy, it is possible that dropping
>> reduces queue occupancy in a fashion that marking does not).
> 
> Do you mean that this results in different mark and drop probabilities 
> for competing ECN and non-ECN flows?
> 
> The impact on the dropping/marking probability *value* applied by an AQM can change 
> due to presence of ECN flows, but I don't think this results in a different 
> drop compared to mark probability. Up to now all classic AQMs smooth their p 
> over sufficient long time to remove short fluctuations in queue sizes. 
> 
> From an end system I think for classic ECN it is safe to assume that if you 
> detect high levels of drop, it means there is a high level of 
> congestion AND the network takes care of it. If you detect a high level
> of marking, it means lots of congestion and nobody is currently taking care 
> of it. Everybody is looking at you as a sender to take care of it. That's why
> I think you should not use ECN if you don't use congestion control.
> 
> It also means that the competing drop flows are having a high level of matching
> drop. That's why I think a classic circuit breaker should also treat drop and 
> classic marking similar.
> 
> I understood ABE tries to modify the end system behavior slightly for ECN. If 
> this can be done safely with benefits that outweighs the disadvantages, 
> than that's no problem, but I think nobody is in favor to modify the AQM 
> behavior to ect(0).

I pretty much agree with everything here…


> Based on DualQ compatibility experiments with classic ECN, I also believe 
> that, if we want to further promote classic ECN too, we need some extra 
> safety measures for classic AQMs using ECN to avoid high CE marking levels, 
> where ect(0) flows start to push away too much the non-ect flows. For 
> instance PIE has (for good reasons) a max of 10% marking, and then switches 
> to drop. As far as I know such measures are not mandatory in other ECN/AQM 
> related drafts, is it?

… but here I’d like to put a note of warning: see the appendix here:
http://heim.ifi.uio.no/michawe/research/publications/CAIA-TR-150710A.pdf

here, we found that this behavior is detrimental (the 10% threshold was too small).
=> I agree with the general notion of such a threshold, but the threshold shouldn’t be too low (it can eliminate ECN’s benefits).

Cheers,
Michael