Re: [aqm] adoption call: draft-welzl-ecn-benefits

[Gorry Fairhurst <[e-mail]>]

On 29/08/2014 14:52, David Collier-Brown wrote:
> On 08/29/2014 09:16 AM, Scheffenegger, Richard wrote:
>> Hi Gorry,
>>
>>>> Given QUIC includes FEC to hide losses, I guess it is a good example to
>>>> consider whether ECN still offers sufficient benefits over and above
>>>> just removing losses.
>>>>
>>> GF: And then, isn't the implication of AQM to significantly increase the
>>> number of "losses" unless we use ECN?
>>>
>>> Indeed, I have the impression we are confusing many on these points -
>>> ECN could change the reaction to congestion signal, and FEC (even
>>> opportunistic CC-friendly FEC) can also change the way things react to
>>> congestion signals.
>> I don't think that an AQM's implication is automatically to increase the number of losses; that may happen to specific flows (in particular, unresponsive ones), but for responsive (non-ECN) ones, the expectation would be to de-correlate the losses, and for TCP, to only have around 1 loss per window when necessary - instead of a burst loss of one window and the expensive recovery...
>>
>> Perhaps it's that perception that also poses an obstacle to AQM deployment, because of the believe that a dynamic but lower mark/drop threshold will cause more losses?
>
> Goodness gracious, from the point of  view of a queuing network, AQM
> reduces losses overall, in the process of minimizing delay and keeping
> bandwidth use just below the theoretical maximum.
>
> Oversimplifying, we try to keep the buffers empty, so that if we get a
> burst we can handle it without losses and without affecting other
> communications. We signal via a loss or other indicator if the
> non-bursty flow is enough to cause congestion, which keeps the buffers
> near-empty and the system uncongested.
>
Yep that's what I think is the goal.

> Failing to do so fills the buffers without signalling there is
> congestion, and induces delay on everyone who's dependant on the buffer.
> Not to mention allowing the congestion to go unreported!
>
> It's not a tradeoff discussion: it's arguably one about correctness.
>
> --dave
> [Somebody like Neil Gunther could explain the math of this better, but
> the behaviour is well-known in the trade, and cordially hated.
> Congestion control is superior to admission control, which is what I
> often use to prevent the server equivalent of congestive collapse (:-))]
>

I agree that AQM often reduces losses (what I said above didn't say 
this) but it sometimes it can increase loss (e.g.  a large RTT or less 
responsive CC), but I don't recall saying something on this in the 
draft. If you see something in the draft that isn't correct then let us 
know.

Gorry