Re: [tcpm] Hystart survey of large server operators

Neal Cardwell <ncardwell@google.com> Thu, 29 July 2021 16:59 UTC

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From: Neal Cardwell <ncardwell@google.com>
Date: Thu, 29 Jul 2021 12:58:51 -0400
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To: Yuchung Cheng <ycheng@google.com>
Cc: Bob Briscoe <ietf@bobbriscoe.net>, huitema@huitema.net, "tcpm@ietf.org" <tcpm@ietf.org>, "draft-ietf-tcpm-hystartplusplus@ietf.org" <draft-ietf-tcpm-hystartplusplus@ietf.org>
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Subject: Re: [tcpm] Hystart survey of large server operators
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On Thu, Jul 29, 2021 at 12:40 PM Yuchung Cheng <ycheng@google.com> wrote:

>
>
> On Thu, Jul 29, 2021 at 9:28 AM Bob Briscoe <ietf@bobbriscoe.net> wrote:
>
>> Yuchung,
>>
>> On 29/07/2021 00:35, Yuchung Cheng wrote:
>>
>> same experience with Chrisitian
>>
>> For more than half a decade Google traffic used Linux CUBIC Hystart. This
>> includes google.com and YouTube public Internet traffic, as well as
>> internal Google TCP traffic. When we deployed pacing around 2013 we
>> disabled the Hystart ACK-train mechanism (that Hystart++ prohibits now) as
>> it causes false SS exit. But we continued to use the stock Hystart Delay
>> mechanism. ~2014-15 we switched to BBR which uses a different startup more
>> robust to delay jitters.
>>
>>
>> [BB] You or Neal (sry can't remember which) were one of those I surveyed
>> who said they had disabled Hystart. So perhaps this was a misunderstanding
>> over the ACK-train part.
>>
> Yes this must be mis-communication. Since that was in 2018, it's either
> "we don't use hystart b/c BBR doesn't use hystart" or "we don't use hystart
> ack-train when hystart was used in Cubic before BBR".
>

Yes, my sense of our experience with CUBIC Hystart matches Yuchung's,
including the likely sources of mis-communication.

thanks,
neal