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

Martin Duke <> Fri, 12 March 2021 21:37 UTC

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From: Martin Duke <>
Date: Fri, 12 Mar 2021 13:37:10 -0800
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To: Bob Briscoe <>
Cc: Yoshifumi Nishida <>, 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,

Although I don't think the difference between these alternatives is all
that large, I believe I would go with (B) -- allow acks of acks at a rate
less than 100%. I suppose that in some corner cases it will prevent drops
out of a shallow queue, which isn't a big deal, but why not do it? To me,
the real complexity here is marking ACKs in the first place, which forces
you to do a bit of accounting about the number of acks you've sent.

It would be good to also add language to extend the RFC5681 definition of
"Duplicate ACK"


 DUPLICATE ACKNOWLEDGMENT: An acknowledgment is considered a
      "duplicate" in the following algorithms when (a) the receiver of
      the ACK has outstanding data, (b) the incoming acknowledgment
      carries no data, (c) the SYN and FIN bits are both off, (d) the
      acknowledgment number is equal to the greatest acknowledgment
      received on the given connection (TCP.UNA from [RFC793
<>]) and (e)
      the advertised window in the incoming acknowledgment equals the
      advertised window in the last incoming acknowledgment.

      Alternatively, a TCP that utilizes selective acknowledgments
      (SACKs) [RFC2018, RFC2883
<>] can leverage the SACK
information to
      determine when an incoming ACK is a "duplicate" (e.g., if the ACK
      contains previously unknown SACK information).


to include "does not increase the ACE counter" as another criterion. Thus
TCP treats this as it would a pure window update, which is functionally
similar. This should solve any spurious retransmission issues.

On Fri, Mar 12, 2021 at 2:54 AM Bob Briscoe <> wrote:

> Yoshi, tcpm list,
> As promised we set up a small design team on the single issue of
> occasionally ACKing pure ACKs if they carry new info (ECN marking at the IP
> layer). The design team has two solutions and everyone would be prepared to
> accept either, but preferences differ. So we're seeking wider opinions
> (more opinions obviously won't narrow the choices, but at least the WG can
> then make a decision informed by those who care).
> To understand the question, you can either read the email below. Or the 3
> slides for the tcpm meeting are briefer, and include pictures:
> Here's the current draft text in question (see here for context
> ):
> Data Receiver Safety Procedures   An AccECN Data Receiver:
>    o  SHOULD immediately send an ACK whenever a data packet marked CE
>       arrives after the previous packet was not CE.
>    o  MUST immediately send an ACK once 'n' CE marks have arrived since
>       the previous ACK, where 'n' SHOULD be 2 and MUST be in the range 2
>       to 6 inclusive.
> Only the second bullet is in question. Here is the proposed diff for each
> alternative, then we explain:
> Alternative A
>    o  MUST immediately send an ACK once 'n' CE marks have arrived since
> -     the previous ACK,
> +     the previous ACK and there is outstanding data to acknowledge,
>       where 'n' SHOULD be 2 and MUST be in the range 2 to 6 inclusive.
> Alternative B
>    o  MUST immediately send an ACK once 'n' CE marks have arrived since
> -     the previous ACK, where 'n' SHOULD be 2 and MUST be in the range 2
> +     the previous ACK, where 'n' SHOULD be 3 and MUST be in the range 3
>       to 6 inclusive.
> Extra guidance text would be required in each case too (see the end).
> Background:
> AccECN is a change to the TCP wire protocol that requires the packet count
> of congestion feedback to include any congestion experienced (CE) arriving
> on Pure ACKs (amongst other things). AccECN doesn't require Pure ACKs to be
> ECN-capable, but allows for them to be. Similarly, AccECN doesn't require
> any congestion response to CE on pure ACKs, but having the feedback
> information there allows a response to be added with a one-ended update, if
> desired/necessary. Basically the data receiver is a 'dumb reflector'.
> The above two bullets were designed to ensure that an ACK is triggered a)
> on the first sign of congestion, and b) frequently enough for the count of
> CE markings to be fed back using the 3-bit ACE field before it wraps, even
> if an occasional pure ACK is lost.
> We then realized that the wording could require an ACK to be triggered in
> response to a CE-marked pure ACK. The circumstance when this could occur
> would be when peer X sends a volley of data to Y then stops, and the path
> back from Y to X is congested (probably by other flow(s) so that many of
> the ACKs are CE-marked. The second bullet above under alternative (B) would
> require X to ACK every 'n-th' CE-marked pure ACK. However, if Y immediately
> started sending a volley of data to X, Y could misinterpret those ACKs (of
> ACKs) from X as DupACKs.
> There are two ways to deal with this:
> A) Some of us prefer to completely prevent ACKs on pure ACKs, on the basis
> that they do not want to risk sometimes generating more ACKs today
> B) Others want to ensure that these rules will cause pure ACKs to be ACKed
> when the amount of CE on the ACKs merits it. But sparingly and strongly
> damping any ACK ping-pong.
> There are complexity arguments on both sides.
> B more complex:
>     extra (non-mandatory) 'if' condition for lack of SACK options on a
> pure ACK (to decide it's not a DupACK).
> B less complex:
>     consistent handling of CE marking whether on pure ACKs or data (which
> would probably remove an 'if' condition).
> A more complex:
>     CE markings on a string of pure ACKs can build up without feeding them
> back, until released by a data packet (if ever).
>     More code at the other end to deal with the resulting risk of many
> wraps of the ACE field (or ignore?).
> A less complex:
>     less different from current TCP.
> Extra guidance text would be necessary in either case.
> * Alt A) would need text on handling the risk of many ACE wraps
>     (to be written).
> * Alt B) would need something like the following changes:
>     For the avoidance of doubt, the above change-triggered ACK mechanism
>     is deliberately worded to solely apply to data packets, and to ignore
>     the arrival of a control packet with no payload, because it is
> -  important that TCP does not acknowledge pure ACKs.  The change-
> +  important that TCP does not acknowledge pure ACKs which convey no new
> +  state information to the sender. The change-
>     triggered ACK approach can lead to some additional ACKs but it feeds
>     back the timing and the order in which ECN marks are received with
>     minimal additional complexity.  If only CE marks are infrequent, or
>     there are multiple marks in a row, the additional load will be low.
>     Other marking patterns could increase the load significantly.
> +
> +  Providing feedback on the congestion state of the return channel
> +  after a sender has ceased transmitting more data helps inform the
> +  clients TCP congestion controller about the state of the return path.
> +  Should the role of data sender and receiver subsequently change, the
> +  new sender has more up to date knowledge of the network state,
> +  preventing transmissions of inappropriate size at that moment.
>     Even though the first bullet is stated as a "SHOULD", it is important
>     for a transition to immediately trigger an ACK if at all possible, so
>     that the Data Sender can rely on change-triggered ACKs to detect
>     queue growth as soon as possible, e.g. at the start of a flow.  This
>     requirement can only be relaxed if certain offload hardware needed
>     for high performance cannot support change-triggered ACKs (although
>     high performance protocols such as DCTCP already successfully use
>     change-triggered ACKs).  One possible compromise would be for the
>     receiver to heuristically detect whether the sender is in slow-start,
>     then to implement change-triggered ACKs while the sender is in slow-
>     start, and offload otherwise.
> +   The second bullet creates a possible case where an AccECN
> implementation
> +   could sometimes ACK pure ACKs, which in turn might be mistaken for
> +   duplicate ACKs (in scenarios where TCP peers take turns to send
> +   sets of data packets). To prevent spurious transmissions in such
> +   circumstances, if SACK has been negotiated, an implementation could
> +   optionally assume that an ACK is not a Duplicate ACK if it has no SACK
> option,
> +   which would indicate it was an ACK of an  ACK. Alternatively it could
> use
> +   timestamp options to rule out DupACKs.
> Bob
> --
> ________________________________________________________________
> Bob Briscoe                     
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> tcpm mailing list