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Re: [tcpm] Comments on draft-ietf-tcpm-accurate-ecn

Mirja Kühlewind <mirja.kuehlewind@tik.ee.ethz.ch> Fri, 13 July 2018 20:59 UTC

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From: Mirja Kühlewind <mirja.kuehlewind@tik.ee.ethz.ch>
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Date: Fri, 13 Jul 2018 16:59:11 -0400
Cc: Bob Briscoe <ietf@bobbriscoe.net>, "Scharf, Michael (Nokia - DE/Stuttgart)" <michael.scharf@nokia.com>, "draft-ietf-tcpm-accurate-ecn@ietf.org" <draft-ietf-tcpm-accurate-ecn@ietf.org>, "tcpm@ietf.org" <tcpm@ietf.org>
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To: Yuchung Cheng <ycheng@google.com>
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Subject: Re: [tcpm] Comments on draft-ietf-tcpm-accurate-ecn
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> Am 13.07.2018 um 16:45 schrieb Yuchung Cheng <ycheng@google.com>:
> 
>>>>> 3. Leave ACE-count and ACE option optional (i.e. MAY)
>>>> 
>>>> I don’t understand this. If both is optional, you don’t have any feedback. Or what do you mean by „leave ACE-count optional“?
>>> use-case: We can negotiate DCTCP-style ECN for the internet.
>>> 
>>> Then interested parties can progressively experiment on more accurate
>>> "options" (!= TCP-option)
>> 
>> As Appendix A of RFC7560 says I don’t think it is a safe option for the Internet where packet loss more likely then in a full ECN-enabled data center.
> 
> Again I disagree RFC7560 Appendix A is a big problem based on my
> experience with at times loss-heavy ECN-enabled data-center (Google
> data-center runs very hot and uses a DCTCP-variant).
> 
> We can quabble forever w/o data. That's why I asked for some
> (non-simulation) data.

I’m not sure if that is a question about data. With the feedback scheme that is used by DCTCP a single ACK loss can already cause feedback to get lost and I don’t think that a safe option to be deployed on the Internet. Actually in high loss situation the chance to at least get some feedback is pretty high, however, if you only have a single CE mark, chances to misses that are much higher.

The other problem with the feedback scheme used by DCTCP is that is does not provide feedback for control packets (as it relies on the ACK/SEQ number with does not increase for control packets) and therefore cannot be used with ECN++.

Mirja

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