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

Praveen Balasubramanian <pravb@microsoft.com> Tue, 03 August 2021 00:46 UTC

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From: Praveen Balasubramanian <pravb@microsoft.com>
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Subject: Re: [tcpm] =?utf-8?q?=5BEXTERNAL=5D_Re=3A__Linux_doesn=E2=80=99t_imp?= =?utf-8?q?lement_RFC3465?=
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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?

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<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