Re: [tcpm] Seeking WG opinions on ACKing ACKs with good cause (was: Possible error in accurate-ecn)

Martin Duke <> Tue, 16 March 2021 19:11 UTC

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From: Martin Duke <>
Date: Tue, 16 Mar 2021 12:11:10 -0700
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To: Bob Briscoe <>
Cc: "Scheffenegger, Richard" <>, tcpm IETF list <>, Mirja Kuehlewind <>, Yoshifumi Nishada <>
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Subject: Re: [tcpm] Seeking WG opinions on ACKing ACKs with good cause (was: Possible error in accurate-ecn)
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Hi Bob,

On Mon, Mar 15, 2021 at 3:57 PM Bob Briscoe <> wrote:

> [BB] I think Martin is not disagreeing with your assessment. He was just
> pointing out that the knock-on effects of ACKing pure ACKs originally
> stems from allowing pure ACKs to be ECN capable in the first place. He's
> not saying that's bad. Just it's the root cause of the new phenomenon
> we're seeing here.


> [BB] I was going to say roughly the same as Richard in response to
> Martin here.
> Just because some apparent DupACKs with an increased CE count are not
> DupACKs, does not imply that a true DupACK cannot have increase CE count.
> That's why we need other tests, like lack of SACK when negotiated, or
> timestamp evidence.
> Bob

Absolutely: by the same token, a window update could also be a genuine
duplicate ACK, but TCP uses a heuristic.

If people are uncomfortable with the heuristic, we could eliminate it by
- pure counter updates are not duplicate acks, as an extension to the 5681
definition, and
- A receiver that gets both an OOO packet (which might be CE marked) and
one or more CE-marked ACKs must send *two* acks; one that updates the
counter and one that does not. The one that does not is unambiguously a
duplicate ack.

I don't think this is necessary, but it would resolve the problem.