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Re: [tcpm] A possible simplification for AccECN servers

Neal Cardwell <ncardwell@google.com> Sat, 23 November 2019 20:16 UTC

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From: Neal Cardwell <ncardwell@google.com>
Date: Sat, 23 Nov 2019 15:16:03 -0500
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To: Bob Briscoe <ietf@bobbriscoe.net>
Cc: tcpm IETF list <tcpm@ietf.org>, Mirja Kuehlewind <ietf@kuehlewind.net>, Richard Scheffenegger <rscheff@gmx.at>
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Subject: Re: [tcpm] A possible simplification for AccECN servers
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On Sat, Nov 23, 2019 at 6:28 AM Bob Briscoe <ietf@bobbriscoe.net> wrote:
>
> tcpm-ers, Mirja, Richard,
>
> draft-ietf-tcpm-accurate-ecn-09 currently says that an AccECN-enabled
> server MUST support both classic ECN and non ECN. However, that's not
> actually necessary for interoperability. Certainly, when an AccECN
> server receives a non-ECN-setup SYN, it MUST fall back to a
> non-ECN-setup SYN-ACK. But, if it receives an ECN-setup SYN, does anyone
> object if we change it from MUST to SHOULD respond with an ECN-setup
> SYN-ACK? And it MAY respond with a non-ECN-setup SYN-ACK?

I think removing that MUST and replacing with SHOULD or MAY is an
important change. Many servers are situated in datacenter networks
where the datacenter switches are CE-marking using shallow-threshold
DCTCP-style ECN. I don't see a way for such servers to feasibly
participate in classic/RFC-3168 ECN, because to do so would risk
interpreting shallow-threshold DCTCP-style CE marks as deep-threshold
classic/RFC-3168 CE marks, causing overreaction to the CE marks and
thus poor performance.

For servers in such networks it seems important to preserve the option
to participate only in non-ECN, DCTCP-ECN, and L4S-ECN conversations,
and decline classic/RFC-3168 ECN conversations.

best,
neal

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