netdev - Re: [PATCH v15 net-next 14/14] tcp: accecn: try to fit AccECN option with SACK

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Message-ID: <CANn89iLFEM+mbU2d0AEH3O+3zg5Cwm7pdmaadUxXifUqLuaQLg@mail.gmail.com>
Date: Thu, 21 Aug 2025 06:04:20 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: chia-yu.chang@...ia-bell-labs.com
Cc: pabeni@...hat.com, linux-doc@...r.kernel.org, corbet@....net, 
	horms@...nel.org, dsahern@...nel.org, kuniyu@...zon.com, bpf@...r.kernel.org, 
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	mirja.kuehlewind@...csson.com, cheshire@...le.com, rs.ietf@....at, 
	Jason_Livingood@...cast.com, vidhi_goel@...le.com
Subject: Re: [PATCH v15 net-next 14/14] tcp: accecn: try to fit AccECN option
 with SACK

On Fri, Aug 15, 2025 at 1:40 AM <chia-yu.chang@...ia-bell-labs.com> wrote:
>
> From: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
>
> As SACK blocks tend to eat all option space when there are
> many holes, it is useful to compromise on sending many SACK
> blocks in every ACK and attempt to fit the AccECN option
> there by reducing the number of SACK blocks. However, it will
> never go below two SACK blocks because of the AccECN option.
>
> As the AccECN option is often not put to every ACK, the space
> hijack is usually only temporary. Depending on the reuqired
> AccECN fields (can be either 3, 2, 1, or 0, cf. Table 5 in
> AccECN spec) and the NOPs used for alignment of other
> TCP options, up to two SACK blocks will be reduced. Please
> find below tables for more details:
>
> +====================+=========================================+
> | Number of | Required | Remaining |  Number of  |    Final    |
> |   SACK    |  AccECN  |  option   |  reduced    |  number of  |
> |  blocks   |  fields  |  spaces   | SACK blocks | SACK blocks |
> +===========+==========+===========+=============+=============+
> |  x (<=2)  |  0 to 3  |    any    |      0      |      x      |
> +-----------+----------+-----------+-------------+-------------+
> |     3     |    0     |    any    |      0      |      3      |
> |     3     |    1     |    <4     |      1      |      2      |
> |     3     |    1     |    >=4    |      0      |      3      |
> |     3     |    2     |    <8     |      1      |      2      |
> |     3     |    2     |    >=8    |      0      |      3      |
> |     3     |    3     |    <12    |      1      |      2      |
> |     3     |    3     |    >=12   |      0      |      3      |
> +-----------+----------+-----------+-------------+-------------+
> |  y (>=4)  |    0     |    any    |      0      |      y      |
> |  y (>=4)  |    1     |    <4     |      1      |     y-1     |
> |  y (>=4)  |    1     |    >=4    |      0      |      y      |
> |  y (>=4)  |    2     |    <8     |      1      |     y-1     |
> |  y (>=4)  |    2     |    >=8    |      0      |      y      |
> |  y (>=4)  |    3     |    <4     |      2      |     y-2     |
> |  y (>=4)  |    3     |    <12    |      1      |     y-1     |
> |  y (>=4)  |    3     |    >=12   |      0      |      y      |
> +===========+==========+===========+=============+=============+
>

It is unclear if this chart takes into account the TCP TS option ?

Can you clarify this point ?