Re: [tcpm] [EXTERNAL] Re: Linux doesn’t implement RFC3465

Yuchung Cheng <ycheng@google.com> Tue, 03 August 2021 01:13 UTC

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From: Yuchung Cheng <ycheng@google.com>
Date: Mon, 2 Aug 2021 18:12:16 -0700
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To: Neal Cardwell <ncardwell=40google.com@dmarc.ietf.org>
Cc: Praveen Balasubramanian <pravb=40microsoft.com@dmarc.ietf.org>, "tcpm@ietf.org" <tcpm@ietf.org>, "mallman@icir.org" <mallman@icir.org>
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Subject: Re: [tcpm] =?utf-8?q?=5BEXTERNAL=5D_Re=3A_Linux_doesn=E2=80=99t_impl?= =?utf-8?q?ement_RFC3465?=
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On Mon, Aug 2, 2021 at 5:53 PM Neal Cardwell <ncardwell=
40google.com@dmarc.ietf.org> wrote:

>
>
> On Mon, Aug 2, 2021 at 8:46 PM Praveen Balasubramanian <pravb=
> 40microsoft.com@dmarc.ietf.org> wrote:
>
>> In experiments a few years ago on DC networks, values over L=8 resulted
>> in a noticeable increase in packet drops and retransmissions (without
>> pacing). Windows TCP has been using L=8 for many years now. If we do want
>> to specify a fallback L value for implementations that cannot pace, my
>> suggestion would be to use the value 8.
>>
>>
>>
>> Neal, are there cases where Linux is or can be deployed with infinite L
>> and no pacing?
>>
>
> Yes, "infinite L and no pacing" is the default behavior for Linux TCP,
> starting in 2013 for slow-start and then starting in 2015 for congestion
> avoidance.
>
To be more clear: both fq_pacing and TCP pacing have been disabled by
default in Linux upstream. We do not know how much Linux senders enable
them today besides the Google servers.

Regarding L = 8, to avoid another round of why or why not. We could say
inf-L causes line-rate burst up to the stretched ACK degree so put a
comfortable L if you prefer, then mention implementation practice like
yours. At the end of the day it's ad-hoc (or "art") and subject to change.
It might be sensible to cap at cwnd to disincentivize receivers /
middle-boxes bunching up 10 rounds of ACKs.

>
>

> Yuchung pasted the URLs for the exact Linux commits above, which are:
>
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9f9843a751d0a2057f9f3d313886e7e5e6ebaac9
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9cd981dcf174d26805a032aefa791436da709bee
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c22bdca94782f05b9337d8548bde51b2f38ef17f
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=814d488c61260521b1b3cc97063700a5a6667c8f
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e73ebb0881ea5534ce606c1d71b4ac44db5c6930
>
> But I understand that not everyone is in a position to read GPL-licensed
> code. :-)
>
> best regards,
> neal
>
>
>
>>
>>
>> *From:* tcpm <tcpm-bounces@ietf.org> *On Behalf Of * Neal Cardwell
>> *Sent:* Monday, August 2, 2021 4:18 PM
>> *To:* Vidhi Goel <vidhi_goel@apple.com>
>> *Cc:* Extensions <tcpm@ietf.org>rg>; Mark Allman <mallman@icir.org>
>> *Subject:* [EXTERNAL] Re: [tcpm] Linux doesn’t implement RFC3465
>>
>>
>>
>>
>>
>>
>>
>> On Mon, Aug 2, 2021 at 7:02 PM Vidhi Goel <vidhi_goel@apple.com> wrote:
>>
>>
>>
>> On Mon, Aug 2, 2021 at 3:37 PM Mark Allman <mallman@icir.org> wrote:
>>
>>
>> > The fact is that Linux CC has long moved to infinite L since 2031,
>>
>> So, if our experience is with L=\infinity and it is demonstrably OK
>> why don't we say *THAT* instead of "make L=5 or L=10"?  I would
>> submit that it makes more sense to leverage experience than it does
>>
>> to make things up.
>>
>> +1
>>
>>
>>
>> Yes, I agree that would be a great approach to take.
>>
>>
>>
>> So, we are saying it is fine to ignore L completely and simply increase
>> cwnd by bytes_acked during slow start? And if this causes large bursts to
>> be sent out (when an implementation doesn’t do pacing), that is fine?
>>
>>
>>
>> Yes, I think that is the proposal on the table, and it sounds good to me.
>>
>>
>>
>> A rationale would be:
>>
>>
>>
>> (1) Implementations SHOULD pace (RFC 7661).
>>
>>
>>
>> (2) Implementations that don't pace will generally be causing large
>> bursts for many different reasons anyway (data and/or ACK aggregation in
>> the network or end hosts), restart from idle,...) so having a constant L
>> does not provide enough protection from bursts to justify the cost in
>> reduced performance (in the form of slower slow-start). In support of this,
>> experience with this as the default behavior in Linux TCP over the
>> 2013-2021 period suggests this works well enough in practice.
>>
>>
>>
>> neal
>>
>>
>>
>>
>>
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