[tcpm] Possible error in accurate-ecn

Bob Briscoe <research@bobbriscoe.net> Tue, 10 November 2020 00:31 UTC

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To: Richard Scheffenegger <rscheff@gmx.at>, Mirja Kuehlewind <ietf@kuehlewind.net>, Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>, tcpm IETF list <tcpm@ietf.org>
From: Bob Briscoe <research@bobbriscoe.net>
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Date: Tue, 10 Nov 2020 00:31:13 +0000
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Subject: [tcpm] Possible error in accurate-ecn
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Mirja, Richard, Ilpo, tcpm list,

I've just been reading through the accurate-ecn draft to double-check. I 
think there's a problem with the following text...  Data Receiver Safety Procedures

    An AccECN Data Receiver:

    o  SHOULD immediately send an ACK whenever a data packet marked CE
       arrives after the previous [data] 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 no greater
       than 6.

    For the avoidance of doubt, the 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.

In the first bullet, I think it doesn't matter whether the previous 
packet marked CE was a data packet or a pure ACK (i.e we should remove 
the second occurrence of 'data' that I have put in [square brackets].

The second bullet doesn't consider the possibility that the 'n'th CE 
mark might arrive on a pure ACK. Then, the wording as it stands says the 
Data Receiver MUST immediately ACK a pure ACK. I know TCP never ACKs a 
pure ACK, but I'm not actually sure it does any harm to do so in this 
case (it cannot cause an infinite loop of ACKs). However, given it would 
be unorthodox, we maybe ought to rule it out by rewording anyway?

Thoughts anyone?


Bob Briscoe                               http://bobbriscoe.net/