Re: [tcpm] Possible error in accurate-ecn

Mirja Kuehlewind <ietf@kuehlewind.net> Mon, 23 November 2020 11:15 UTC

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From: Mirja Kuehlewind <ietf@kuehlewind.net>
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Date: Mon, 23 Nov 2020 12:15:06 +0100
Cc: Bob Briscoe <research@bobbriscoe.net>, tcpm IETF list <tcpm@ietf.org>, Richard Scheffenegger <rscheff@gmx.at>
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Subject: Re: [tcpm] Possible error in accurate-ecn
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Hi Bob, hi all,

Sorry for my late reply. See inline.

> On 12. Nov 2020, at 23:41, Yoshifumi Nishida <nsd.ietf@gmail.com> wrote:
> 
> Hi Bob,
> 
> On Wed, Nov 11, 2020 at 3:18 PM Bob Briscoe <research@bobbriscoe.net> wrote:
> Yoshi,
> 
> On 11/11/2020 09:37, Yoshifumi Nishida wrote:
>> 
>> 
>> On Tue, Nov 10, 2020 at 2:08 AM Mirja Kuehlewind <ietf@kuehlewind.net> wrote:
>> Hi Bob,
>> 
>> Please see below.
>> 
>> > On 10. Nov 2020, at 01:31, Bob Briscoe <research@bobbriscoe.net> wrote:
>> > 
>> > 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...
>> > 
>> > 3.2.2.5.1.  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].
>> 
>> I believe it does matter. If the previous packet was a pure ACK and was CE marked, you didn’t send an ACK and so you should immediately send one now. If the previous packet was a data packet and CE marked, you don’t need to send an immediate ACK because you already did this with the previous packet. However, it text might be a but ambiguous because what is meant is “if the previous packet was a data packet and CE marked”. So this does not apply if we e.g. have a CE-marked data packets, then a pure ACK that is not CE marked, and then another CE-marked data packet.  
>> 
>> > 
>> > 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?
>> 
>> even though it won't cause an infinite loop, can they be dup acks? If so, it doesn't look good to me especially when early retransmit is activated.
> 
> [BB] I'm not expert in TCP loss recovery but, surely if TCP A has nothing outstanding, it cannot consider receipt of dup acknowledgements from TCP B as a sign of loss? Nonetheless, there would be a chance that such a dup acknowledgement might be in flight when TCP A happened to have just sent some more data, and they crossed in flight. 
> 
> To avoid this case, it would be possible for TCP B to send pure ACK(s) with SEG.SEQ = SND.NXT-1 (using the same trick as keepalive probes). Being out of window, each of these pure ACKs would elicit a further pure ACK from TCP A. However, that would acknowledge SND-NXT, so the loop would stop there. Nonetheless, if TCP B started sending data again, and if TCP B's data crossed the pure ACKs that it had recently elicited from TCP A, once these pure ACKs arrived at TCP B, I think it would treat them as DupACKs.
> 
> It's seems possible for TCP B to recognize that it elicited these DupACKs from TCP A, and to therefore work out that it could ignore them. 
> 
> I've kept on digging into this particular obscure corner of ACK congestion control, only 'cos I wanted to see if there was a useful end to this hole (so it would be worth the AccECN RFC allowing others to dig there later, rather than ruling it out).
> 
> Nonethless, the AccECN draft itself doesn't have to go anywhere near this subject - it can leave all that for a potential future draft about using AccECN for ACK congestion control. 
> 
> I personally prefer not to specify very detailed logic for ACK CC here unless it is crucial for the doc. 
> Since the doc is aiming to be a PS, I think it would be better to avoid risks for nasty corner cases caused by some tricky techniques. 

I would leave anything on ACK CC also for a separate draft.

>> I think we also can leave this as it is because if that ’n’ packet is a pure ACK that still means that you have unacked data packets with CE marks and you should trigger an ACK for those packet now (rather than waiting for the delayed ACK timer to expire). However this case is less important. Also             we should probably make sure that this doesn’t apply if there are only pure ACKs with CE marks, maybe by add “if unacknowledged data are outstanding” or something.
>>  
>> "can send a pure ack on a pure ack when the ack value advances"?  
> 
> [BB] Yes. That's a fairly safe thing to require.
> I think the best strategy here is to require an immediate ACK in response to a data packet if there are at least 'n' CE marks to be reported. And to extend that requirement to immediately ACKing a pure ACK if the ACK value has advanced since the last ACK. But just to say nothing about ACKing pure ACKs in other cases, so we don't preclude a case that can be made safe in future (e.g. with something like the keepalive approach above).
> 
> If you (Yoshi), Mirja and others agree, let's try to work out the wording of this second bullet. 
> 
> Works for me. If you could have proposed texts, I will take a look. 
> --
> Yoshi

Works for me too.

On Richard’s point that this could lead to synchronicity lost of the AccECN
counters: Only if you don’t use the option. And as long as ACK CC is not implemented, it’s probably also we can live with.

Mirja