Re: [tcpm] Possible error in accurate-ecn

Mirja Kuehlewind <ietf@kuehlewind.net> Tue, 10 November 2020 10:08 UTC

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From: Mirja Kuehlewind <ietf@kuehlewind.net>
In-Reply-To: <47df9b8b-515e-d40d-3473-599b0a3e3876@bobbriscoe.net>
Date: Tue, 10 Nov 2020 11:08:03 +0100
Cc: Richard Scheffenegger <rscheff@gmx.at>, =?utf-8?Q?Ilpo_J=C3=A4rvinen?= <ilpo.jarvinen@helsinki.fi>, tcpm IETF list <tcpm@ietf.org>
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To: Bob Briscoe <research@bobbriscoe.net>
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Subject: Re: [tcpm] Possible error in accurate-ecn
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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?

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.

Mirja



> 
> Thoughts anyone?
> 
> 
> Bob
> 
> --
> ________________________________________________________________
> Bob Briscoe                               
> http://bobbriscoe.net/