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

Neal Cardwell <ncardwell@google.com> Sat, 07 August 2021 14:57 UTC

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From: Neal Cardwell <ncardwell@google.com>
Date: Sat, 7 Aug 2021 10:57:10 -0400
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To: Vidhi Goel <vidhi_goel=40apple.com@dmarc.ietf.org>
Cc: Yuchung Cheng <ycheng@google.com>, Praveen Balasubramanian <pravb@microsoft.com>, "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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I also agree with Yuchung’s suggestion, for all of the reasons he provided.

best,
neal


On Fri, Aug 6, 2021 at 3:59 PM Vidhi Goel <vidhi_goel=
40apple.com@dmarc.ietf.org> wrote:

> I agree with Yuchung’s suggestion for all the reasons he provided. And its
> better to have it at one place.
>
> Vidhi
>
> On Aug 6, 2021, at 12:53 PM, Yuchung Cheng <
> ycheng=40google.com@dmarc.ietf.org> wrote:
>
> Hi WG
>
> I have been wondering if we (= IETF) should just update RFC5681 directly,
> instead of another RFC3465-bis with experimental status.
>
> Appropriate byte counting is essential but the RFC5681 of L=1 is
> detrimental. There are far more people who read RFC5681 to implement the
> new stack instead of RFC3465. So we should fold the experimental RFC3465
> updates into RFC5681 directly, and obsolete RFC3465.
>
> This is orthogonal to the final value of L :-)
>
>
>
>
> On Tue, Aug 3, 2021 at 9:42 AM Yuchung Cheng <ycheng@google.com> wrote:
>
>>
>>
>> On Mon, Aug 2, 2021 at 6:12 PM Yuchung Cheng <ycheng@google.com> wrote:
>>
>>>
>>>
>>> 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.
>>>
>> Sorry please ignore my previous message about the cwnd cap. It is
>> completely unnecessary -- since with ack-clocking and appropriate counting,
>> a correct sender would never release more than a cwnd-worth of data. I was
>> imagining the multiple application-limited burst could let the receiver
>> keep holding up ACKs, but that can never exceed a cwnd worth of data.
>>
>>
>>>
>>>>
>>>
>>>> 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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>>>> tcpm@ietf.org
>>>> https://www.ietf.org/mailman/listinfo/tcpm
>>>>
>>> _______________________________________________
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