Re: [Bloat] RED against bufferbloat

[Bob Briscoe <bob.briscoe@bt.com>]

Sahil,

At 06:46 25/02/2015, Mikael Abrahamsson wrote:
>On Tue, 24 Feb 2015, sahil grover wrote:
>
>>(i) First of all,i want to know whether RED was implemented or not?
>>if not then what were the reasons(major) ?
>
>RED has been available on most platforms, but it was generally not 
>turned on. It also needs configuration from an operator, and it's 
>hard to know how to configure.

About a decade ago my company (BT) widely deployed RED in the 
upstream 'head-end' of our global MPLS network, i.e. the likely 
bottleneck in the customer edge router where the customer's LAN 
traffic enters their access link. We deployed it as WRED, i.e. 
different configurations of RED across the various diffserv classes, 
in order to minimise queuing latency in all the classes, including 
the lowest priority class. A configuration calculator was developed 
to help the engineers during set up. We still use this setup 
successfuly today, including for all our particularly latency 
sensitive customers in the finance sector.

We did not deploy RED on our broadband platform (ie public Internet), 
altho in retrospect we should have done, because any AQM is much 
better than none. We're fixing that now.

>>(ii)Second, as we all know RED controls the  average queue size from
>>growing.
>>So it also controls delay in a way or  we can say  is a solution to
>>bufferbloat problem. Then why it was not considered.
>
>It was designed to fix "bufferbloat" long before the bufferbloat 
>word was even invented. It's just that in practice, it doesn't work 
>very well. RED is configured with a drop probability slope at 
>certain buffer depths, and that's it. It doesn't react or change 
>depending on conditions. You have to guess at configure-time.
>
>What we need are mechanisms that work better in real life and that 
>are adaptive.

If you were prepared to read a paper, I would have suggested:
"The New AQM Kids on the Block: An Experimental Evaluation of CoDel and PIE"
<http://infocom2014.ieee-infocom.org/GI14-slides/GI14-s2-3.pdf>

This compares CoDel and PIE against Adaptive RED, which was a variant 
of RED proposed by Sally Floyd & co-authors in 2001 and available 
since Linux kernel version 3.3. ARED addressed the configuration 
sensitivity problem of RED by adapting the parameters to link rate 
and load conditions.

The paper convinced me that ARED is good enough (in the paper's 
simulations it was often better than PIE or CoDel), at least for 
links with fixed rate (or only occasionally varying rate like DSL).* 
This is important for us because it means we can consider deploying 
AQM by adding soft controls on top of the RED implementations we 
already have in existing equipment. This could reduce deployment 
completion time from decades to a few months.

* I'm not sure ARED would be able to cope with the rapidly changing 
rate of a wireless link tho.

HTH


Bob



________________________________________________________________
Bob Briscoe,                                                  BT