Re: [tcpm] Linux doesn’t implement RFC3465

Yuchung Cheng <> Wed, 28 July 2021 23:20 UTC

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From: Yuchung Cheng <>
Date: Wed, 28 Jul 2021 16:20:14 -0700
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To: Vidhi Goel <>
Cc: Mark Allman <>, " Extensions" <>, Neal Cardwell <>
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Subject: Re: [tcpm] =?utf-8?q?Linux_doesn=E2=80=99t_implement_RFC3465?=
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Thank you Vidhi and Mark for supporting an ABC update.

To give this a start, my recommended update is:

1) if the sender uses some form of pacing to send data packets, L is
RECOMMENDED to be the effective window worth of packets. Pacing here refers
to spread packet transmission following a rate based on the congestion
window and round trip. Additionally if SACK is supported, the same L
applies in slow start after RTO

2) otherwise L is RECOMMENDED to be 8


On Wed, Jul 28, 2021 at 2:13 PM Vidhi Goel <vidhi_goel=> wrote:

> Resurrecting the 3465 thread.
> In the TCPM meeting at IETF 111, we discussed about this issue of L=2
> which is a MUST in RFC 3465. This is a very strict requirement and stacks
> like Linux already doesn’t follow it.
> The basic principle in the Linux code is that the sender should use the
> ACKs to learn about the capacity of the path (both in volumetric and rate
> dimensions), and should not ignore that information. This allows the sender
> to quickly grow and achieve high throughput, even in the presence of
> stretch ACKs, which are pervasive, due to TSO/GSO, GRO/LRO, etc.
> Considering bursts is important, but that can be tackled as an orthogonal
> issue. Bursts are avoided in the Linux TCP ecosystem by the combination of
> TSO autosizing, pacing, TSQ, and fair queueing.
> As Neal described, the congestion controller should use the information in
> stretch ACKs to increase its congestion window so that it correctly adjusts
> the *cwnd* based on available link capacity.
> Burstiness is an orthogonal issue which can be solved by pacing.
> QUIC loss recovery (RFC 9002) also follows this approach.
> *Mark*,
> As a lot of transport and congestion control drafts reference RFC 3465, do
> you think we should update this RFC to reflect the current deployment? This
> would also be useful for someone who is just starting with a new
> implementation.
> Thanks,
> Vidhi
> On Nov 27, 2019, at 5:14 AM, Mark Allman <> wrote:
> +Mark Allman
> Just to clear it up, I *was* at BBN long ago when the ABC document
> was written.  It's a cool place to work.  I recommend it.  But, I
> now hang out at ICSI.
> I believe that ABC was written to solve the problem with ACK
> counting by counting the number of bytes acknowledged for
> misbehaving receivers. Limiting the increase to 2*MSS was a good
> solution to avoid bursts at the time.
> The main motivation behind ABC was to counteract delayed ACKs.  The
> common approach at the time was to just bump cwnd by one MSS every
> time an ACK rolled in.  So, if an ACK covered two segments because
> the receiver was delaying the ACKs then the growth rate during slow
> start was 1.5x per RTT instead of the 2x that was really
> envisioned.  During congestion avoidance the growth was 1 MSS every
> 2 RTTs instead of the envisioned every RTT.
> A secondary motivation was to counteract these ACK division attacks
> that Savage taught us about.  I.e., we could ACK an MSS-sized packet
> one byte at a time and the sender would then increase the cwnd by
> MSS*MSS bytes in the prevalent ACK counting scheme (i.e., cwnd would
> get bumped by MSS bytes for every ACK).
> The limit has two roots ...
> (1) The limit is important in slow starts that follow an RTO.  As
>    the RFC discusses, in this case we might retransmit a single
>    packet and this will cause the receiver's window to slide a
>    great deal.  Therefore, an ACK may indicate that a ton of data
>    has left the network, but that isn't really the case.  So, we
>    don't want to increase the cwnd based on all the new bytes
>    ACKed.
>    I have since mostly decided that this use of L is crude.
>    Probably there is a more elegant way to do it by using the
>    scoreboard and the SACK information to get a better
>    understanding of what left the network and when.  That said, in
>    this case L is simple and probably about right most of the
>    time.
> (2) I think there was some general conservativeness to bursts and
>    using L everywhere quelled some of the worry.  Here L=2 was used
>    to exactly offset delayed ACKs.
> I agree that increasing the congestion window and controlling the
> burst rate are orthogonal issues.
> Yes.  In fact, we did subsequent research on mitigating bursts
> because we never really thought of ABC as somehow the way to control
> bursts (research papers available, but never went into RFCs).
> And, in a world that leverages stretch ACKs as a routine I think
> Linux's approach of not using an L may well be correct.  Documenting
> that and the reasoning behind it in modern networks seems useful to
> me.
> allman
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