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

Vidhi Goel <vidhi_goel@apple.com> Mon, 09 August 2021 20:33 UTC

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From: Vidhi Goel <vidhi_goel@apple.com>
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Date: Mon, 09 Aug 2021 13:32:52 -0700
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Cc: Yuchung Cheng <ycheng=40google.com@dmarc.ietf.org>, "tcpm@ietf.org" <tcpm@ietf.org>, "mallman@icir.org" <mallman@icir.org>
To: Yoshifumi Nishida <nsd.ietf@gmail.com>, Neal Cardwell <ncardwell@google.com>
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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 like Yoshi’s suggestion to fold the initial window into RFC5681-bis.

> That sounds reasonable to me as well. Although the initial window is a somewhat independent issue that might evolve independently from the congestion algorithm itself, so I can imagine advantages to keeping it separate.

I think Initial Window is not independent from congestion control module, because lets say in future with higher BDPs we decide to set IW=1000, then the congestion control algorithm should be able to immediately detect congestion, if any, even at the start of the connection and for such a high initial value, probably requires other ways to detect congestion besides packet loss.
Yes, it is separate from how the algorithm works itself, but still deciding the initial value depends a lot on the congestion control algorithm.

Thanks,
Vidhi

> On Aug 9, 2021, at 10:07 AM, Yoshifumi Nishida <nsd.ietf@gmail.com> wrote:
> 
> Yes, that was my intention. Sorry for being unclear.
> I thought updating RFC5681 could be impactful. So, if it would happen, I would like to make sure we won't have another update for a long time.
> --
> Yoshi
> 
> On Mon, Aug 9, 2021 at 9:53 AM Neal Cardwell <ncardwell@google.com <mailto:ncardwell@google.com>> wrote:
> I think Yoshifumi is suggesting that if the WG re-spins  RFC5681 then in addition to folding in discussion of ABC/RFC3465 the RFC5681bis could also include the IW10 content in RFC6928. That could help save time in avoiding promoting RFC6928 from experimental to proposed standard.
> 
> That sounds reasonable to me as well. Although the initial window is a somewhat independent issue that might evolve independently from the congestion algorithm itself, so I can imagine advantages to keeping it separate.
> 
> neal
> 
> 
> On Mon, Aug 9, 2021 at 12:39 PM Yuchung Cheng <ycheng=40google.com@dmarc.ietf.org <mailto:40google.com@dmarc.ietf.org>> wrote:
> Sorry I don't understand your suggestion. Is that related to ABC? could you explain more
> 
> On Mon, Aug 9, 2021 at 1:41 AM Yoshifumi Nishida <nsd.ietf@gmail.com <mailto:nsd.ietf@gmail.com>> wrote:
> I don't have a strong opinion on this yet, But, if we *could* move in this direction, it might be good to think about the IW explanation in RFC5681 as well?
> if we do this, we might not need to discuss promoting RFC6928.
> --
> Yoshi
> 
> On Sat, Aug 7, 2021 at 7:57 AM Neal Cardwell <ncardwell=40google.com@dmarc.ietf.org <mailto:40google.com@dmarc.ietf.org>> wrote:
> 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 <mailto: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 <mailto: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 <mailto:ycheng@google.com>> wrote:
>> 
>> 
>> On Mon, Aug 2, 2021 at 6:12 PM Yuchung Cheng <ycheng@google.com <mailto:ycheng@google.com>> wrote:
>> 
>> 
>> On Mon, Aug 2, 2021 at 5:53 PM Neal Cardwell <ncardwell=40google.com@dmarc.ietf.org <mailto:40google.com@dmarc.ietf.org>> wrote:
>> 
>> 
>> On Mon, Aug 2, 2021 at 8:46 PM Praveen Balasubramanian <pravb=40microsoft.com@dmarc.ietf.org <mailto: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=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=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=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=814d488c61260521b1b3cc97063700a5a6667c8f>
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e73ebb0881ea5534ce606c1d71b4ac44db5c6930 <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 <mailto:tcpm-bounces@ietf.org>> On Behalf Of Neal Cardwell
>> Sent: Monday, August 2, 2021 4:18 PM
>> To: Vidhi Goel <vidhi_goel@apple.com <mailto:vidhi_goel@apple.com>>
>> Cc: Extensions <tcpm@ietf.org <mailto:tcpm@ietf.org>>; Mark Allman <mallman@icir.org <mailto: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 <mailto:vidhi_goel@apple.com>> wrote:
>> 
>>  
>> 
>> On Mon, Aug 2, 2021 at 3:37 PM Mark Allman <mallman@icir.org <mailto: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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