Re: [tcpm] Why draft-gomez-tcpm-ack-rate-request?

Jonathan Morton <chromatix99@gmail.com> Fri, 25 March 2022 15:43 UTC

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From: Jonathan Morton <chromatix99@gmail.com>
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Date: Fri, 25 Mar 2022 17:42:58 +0200
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Subject: Re: [tcpm] Why draft-gomez-tcpm-ack-rate-request?
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>>> I would also note that ARR offers an alternative signalling method (to AccECN) for effectuating Generalised ECN, as applied to pure acks in particular.  I think ARR is conceptually and mechanically simpler for implementers than AccECN is.
> 
> [BB] If I read between the lines, I believe you are categorizing the combination of AccECN and generalized-ecn (ECN++) as just one very narrow application of them: ACK congestion control. Although the combination of AccECN and ECN++ makes ACK-CC possible, that wasn't one of the original motivations for either.*

You're reading something that I didn't write.  I don't need to comment on the goals or applicability of AccECN here, only those of ECN++ and TARR.

> Whatever, if we run with this narrow focus, TARR wouldn't be enough to deploy ACK CC. It solely gives the control signal, not the measurement in the other direction that would be needed to drive that control.

Ack CC would be triggered by the detection of ack-losses or ECN signals on the acks.  These acks travel in the opposite direction to data segments, ie. from the "receiver" to the "sender".  TARR allows the sender, which is in a position to observe these losses or ECN markings *and* knows what the data-oriented CC algorithm needs, to inform the receiver what density of acks it is desirable for the receiver to send.

In other words, TARR could be tried *today* - even without ECN++, and certainly without AccECN.  As a signalling mechanism, TARR is, in fact, sufficient on its own to implement Ack CC.

As you note, ECN++ intends to allow the use of ECT on TCP ack and control packets, not just data segments, as well as on retransmitted data segments which are presently sent Not-ECT.  ECT implies, to the network, that the endpoints will use explicit congestion signals (ie. CE marks) to reduce traffic volume on the flow and in the direction that the marks were applied in the network.  Presently, TCP only does that for data segments (as per RFC-3168).

TARR adds a mechanism for an appropriate congestion response for CE marks applied to TCP acks.  This is completely in keeping with ECN++ goals.  Hence, the successful negotiation of TARR could be taken as sufficient to permit setting ECT on pure acks, if not the other cases as well.  This assumes, of course, that an appropriate Ack CC response is either written into TARR, or is explicitly implied by its presence.

 - Jonathan Morton