Re: [tcpm] Possible error in accurate-ecn

Bob Briscoe <research@bobbriscoe.net> Tue, 10 November 2020 16:39 UTC

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To: Mirja Kuehlewind <ietf@kuehlewind.net>
Cc: tcpm IETF list <tcpm@ietf.org>, Richard Scheffenegger <rscheff@gmx.at>
References: <47df9b8b-515e-d40d-3473-599b0a3e3876@bobbriscoe.net> <6031BE2B-4D33-426F-BA17-DDF15CF821DE@kuehlewind.net>
From: Bob Briscoe <research@bobbriscoe.net>
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Date: Tue, 10 Nov 2020 16:38:57 +0000
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Subject: Re: [tcpm] Possible error in accurate-ecn
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Mirja,

On 10/11/2020 10:08, Mirja Kuehlewind 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]. 
> [MK] 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. 

[BB] Neither the current text, nor my proposed edit, nor the text you 
propose below, says this. And I don't think it needs to. Surely this 
falls into the case of the second bullet ('n' CE marks in a row before 
needing to feed back anything).

> [MK] 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. 

[BB] An AQM will typically mark packets irrespective of their type or 
size (that's a feature not a bug). So there's no reason for the receiver 
to treat some marks differently to others, whatever type or size of 
packets they're on.

Let's revisit your last sentence above with a more general scenario - 
with more than one pure ACK in between the data - actually quite a 
common pattern. I.e. a server receives a small request, sends its larger 
response, then when that response has completed, the client sends 
another small request. So the server's TCP will see one data packet, a 
load of pure ACKs then one data packet again.

Now consider the case where both arriving data packets (the requests) 
are CE-marked. There are three relevant cases for the marking of the 
pure ACKs in between:
1) no CE-marked pure ACKs
2) some non-CE-marked pure ACKs.
3) all CE-marked pure ACKs

In cases 1 & 2, there has been a transition from none to some CE marks, 
but there hasn't been an opportunity to report it yet, so use the first 
opportunity to ACK a data packet.
In case 3, surely the server doesn't need to send feedback unless there 
are 'n' CE marks to report.

So how about this text to replace the first bullet:

    An AccECN Data Receiver:

    o  SHOULD send an ACK at the first opportunity (in response to the
       arrival of a data packet) whenever a mix of non-CE marked
       and CE marked packets have arrived since any previous ACK.

Moving on to the second bullet...
>> 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? 
> [MK] 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.

[BB] By the end of your last para you start to realize that, if the n'th 
CE mark arrives on pure ACK, it doesn't say anything about whether the 
previous (n-1) CE marks were on data or pure ACKs.

Consider another common scenario; a server delivering chunks of video 
over a high BDP path (e.g. 500 packets per RTT), and going idle between 
times (perhaps in response to an "exceeded high watermark" data packet 
from the server). Now consider the possibility that the reverse path 
bottleneck builds a queue during each chunk, and starts CE marking 
towards the end of each chunk. Then, after the server stops sending a 
chunk, it continues to see a couple of hundred CE-marked pure ACKs 
arriving.

However, by your proposed rule, it doesn't send any feedback, 'cos it 
has no data to acknowledge. So the client cannot regulate its ACK 
stream. That's just tough, 'cos the client will stop sending more pure 
ACKs after it has received the last data, which is before any later 
feedback from the server can reach it.

However, if the client later sends a data packet that the server can 
acknowledge (e.g. "a low watermark" message), the server's ACE counter 
will have wrapped dozens of times, and the client won't be able to work 
out how many. That doesn't matter if a few round trips have elapsed - 
the congestion info will have become stale. But it does matter if it's 
fairly soon.

Instead, if we keep the text of the second bullet... if the server's 'n' 
was 2, it would send a pure ACK for every other pure ACK it received. 
But, what if these pure ACKs from the server were also CE-marked? Then 
the client would feed back pure ACKs using its 'n'. This regression 
would eventually die out, but I don't like it.

This conversation about the second bullet presumes AccECN is being used 
for ACK congestion control. I'm not comfortable with writing anything 
specific about ACK congestion control into a draft on the standards 
track. But I don't want to preclude AccECN being used for ACK CC.


I believe the second bullet needs something changed to limit the ACKing 
of pure ACKs. However, before we put effort into wordsmithing, I'd like 
to hear whether you agree with the general idea of not defining what to 
do for ACK CC, but not precluding it.

Cheers




Bob

>
> Mirja
>
>
>
>> Thoughts anyone?
>>
>>
>> Bob
>>
>> --
>> ________________________________________________________________
>> Bob Briscoe
>> http://bobbriscoe.net/
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> tcpm mailing list
> tcpm@ietf.org
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________________________________________________________________
Bob Briscoe                               http://bobbriscoe.net/