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

[David Collier-Brown <[e-mail]>]

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.

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 (:-))]



-- 
David Collier-Brown,         | Always do right. This will gratify
System Programmer and Author | some people and astonish the rest
[e-mail]           |                      -- Mark Twain