Re: [RFC PATCH 28/28] tcp: AccECN sysctl documentation

[Dave Taht <dave.taht@gmail.com>]

On Wed, Mar 18, 2020 at 2:44 AM Ilpo Järvinen <ilpo.jarvinen@helsinki.fi> wrote:
>
> From: Ilpo Järvinen <ilpo.jarvinen@cs.helsinki.fi>
>
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@cs.helsinki.fi>
> ---
>  Documentation/networking/ip-sysctl.txt | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
> index 5f53faff4e25..ecca6e1d6bea 100644
> --- a/Documentation/networking/ip-sysctl.txt
> +++ b/Documentation/networking/ip-sysctl.txt
> @@ -301,15 +301,21 @@ tcp_ecn - INTEGER
>                 0 Disable ECN.  Neither initiate nor accept ECN.
>                 1 Enable ECN when requested by incoming connections and
>                   also request ECN on outgoing connection attempts.
> -               2 Enable ECN when requested by incoming connections
> +               2 Enable ECN or AccECN when requested by incoming connections
>                   but do not request ECN on outgoing connections.

Changing existing user-behavior for this default seems to be overly
optimistic. Useful for testing, but...

> +               3 Enable AccECN when requested by incoming connections and
> +                 also request AccECN on outgoing connection attempts.
> +           0x102 Enable AccECN in optionless mode for incoming connections.
> +           0x103 Enable AccECN in optionless mode for incoming and outgoing
> +                 connections.

In terms of the logic bits here, it might make more sense

0: disable ecn
1: enable std ecn on in or out
2: enable std ecn when requested on in (the default)
3: essentially unused
4: enable accecn when requested on in
5: enable std ecn and accecn on in or out
6: enable accecn and ecn on in but not out

Do we have any data on how often the tcp ns bit is a source of
firewalling problems yet?

0x102 strikes me as a bit more magical than required and I don't know
what optionless means in this context.

>         Default: 2
>
>  tcp_ecn_fallback - BOOLEAN
>         If the kernel detects that ECN connection misbehaves, enable fall
>         back to non-ECN. Currently, this knob implements the fallback
> -       from RFC3168, section 6.1.1.1., but we reserve that in future,
> -       additional detection mechanisms could be implemented under this
> +       from RFC3168, section 6.1.1.1., as well as the ECT codepoint mangling
> +       detection during the Accurate ECN handshake, but we reserve that in
> +       future, additional detection mechanisms could be implemented under this
>         knob. The value is not used, if tcp_ecn or per route (or congestion
>         control) ECN settings are disabled.
>         Default: 1 (fallback enabled)
> --
> 2.20.1
>


-- 
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Dave Täht
CTO, TekLibre, LLC
http://www.teklibre.com
Tel: 1-831-435-0729