Re: [aqm] Questioning each PIE heuristic

Dave Dolson <> Thu, 30 March 2017 12:06 UTC

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From: Dave Dolson <>
To: Bob Briscoe <>, Jonathan Morton <>, "Rong Pan (ropan)" <>
CC: Greg White <>, tsvwg IETF list <>, "AQM IETF list" <>, "" <>
Thread-Topic: [aqm] Questioning each PIE heuristic
Date: Thu, 30 Mar 2017 12:06:48 +0000
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Subject: Re: [aqm] Questioning each PIE heuristic
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While reviewing PIE, it might be worth revisiting my comment here:
The second part was never answered to my satisfaction.

I believed I had found a bug:  I said, "I think there are a variety of traffic patterns that can allow 'p' to meander around in non-zero space even though the queue is nearly empty"

Maybe you've addressed this, because I think it was due to a heuristic.

I'm much in favor of removing the heuristics on control systems; thanks.


-----Original Message-----
From: aqm [] On Behalf Of Bob Briscoe
Sent: Thursday, March 30, 2017 2:47 AM
To: Jonathan Morton; Rong Pan (ropan)
Cc: Greg White; tsvwg IETF list; AQM IETF list;
Subject: Re: [aqm] Questioning each PIE heuristic


Picking up on an earlier point you made about avoiding heuristics by ensuring the underlying algo is sound,... that's precisely why I'm going through all the (9) PIE heuristics...

For PI2 we removed all but 2 and it worked the same or better than PIE in all our tests. I have been assessing each of the other 7 one by one for reinstatement. So far I've rejected 6. I think I can reject this last one by making the sampling time of the base PI algo dependent on the max link rate. Then when the queue goes idle, the base PI algo will decay drop down to zero no slower than the queue drains, without needing this extra heuristic. But I need to check that's realistic.

We will be writing all this up (probably in an update to the PI2 paper - I don't think the IETF PI2 spec is the right place for a critique of heuristics that it doesn't use).

Our aim is a completely sound AQM in a few lines of code and a few operations so it can be implemented everywhere with minimal resistance from developers due to performance concerns (e.g. cheap ethernet switches, cheap home gateways, carrier-grade equipment for thousands of users, etc).


On 28/03/17 07:25, Jonathan Morton wrote:
> By all means, avoid dropping packets when the queue is actually empty - that is, when you’re delivering the last packet in the queue.  In that case, there is no congestion to signal for.  But there really is no need to have any complex state-switching logic for that.  If your underlying algorithm is sound, it will naturally decay to zero packet drops if the empty-queue condition persists.
>   - Jonathan Morton

Bob Briscoe                     

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