Re: [aqm] Questioning each PIE heuristic

Fred Baker <fredbaker.ietf@gmail.com> Tue, 28 March 2017 11:39 UTC

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From: Fred Baker <fredbaker.ietf@gmail.com>
In-Reply-To: <77D4FC66-C99F-49D0-BB73-27A0CEF70F31@gmail.com>
Date: Tue, 28 Mar 2017 06:39:05 -0500
Cc: Rong Pan <ropan@cisco.com>, Bob Briscoe <ietf@bobbriscoe.net>, Greg White <g.white@CableLabs.com>, Preethi Natarajan <prenatar@cisco.com>, AQM IETF list <aqm@ietf.org>, "De Schepper, Koen (Koen)" <koen.de_schepper@nokia.com>, tsvwg IETF list <tsvwg@ietf.org>
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References: <9ddba389-e368-9050-3b14-aa235c99fcb8@bobbriscoe.net> <D4FDD717.2636D%ropan@cisco.com> <77D4FC66-C99F-49D0-BB73-27A0CEF70F31@gmail.com>
To: Jonathan Morton <chromatix99@gmail.com>
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Subject: Re: [aqm] Questioning each PIE heuristic
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> On Mar 28, 2017, at 1:25 AM, Jonathan Morton <chromatix99@gmail.com> wrote:
> 
> 
>> On 28 Mar, 2017, at 04:04, Rong Pan (ropan) <ropan@cisco.com> wrote:
>> 
>>> Q1. What were the reasons for introducing such a frequent suppression of
>>> the PI algorithm (the RFC just says what this code does, not why)?
>> 
>> To be work conserving and avoid any unnecessary drops are the main reasons
>> behind it. 
>> Cisco had a not so successful algorithm before that is not work
>> conserving. So we are extra cautious about being work conserving...
> 
> AQM algorithms are by definition *not* work-conserving, because they can drop packets.  By *definition*.  I therefore think you’re chasing a non-goal here, and you’re going to have to justify it much more clearly if it’s going to make it into an RFC.
> 
> 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.

I'm not convinced I understand the definitions of "work conserving" and "non work conserving" in this context. A "work conserving" scheduling algorithm keeps an interface transmitting as long as there is data in the queue, while a non-work-conserving algorithm reduces the effective rate of the interface by spacing packets out.