The decision the algorithm is trying to make is when to drop packets. RED drops packets when the maximum queue level rises. CoDel drops packets when the minimum queue level rises. If the queue is never (nearly) empty, the interface is oversubscribed and packet drops are in order. But because it looks at minimum, not maximum, CoDel will not drop packets in bursts so long as those bursts pass within the (5 ms) interval used to measure minimum queue level.
1/sqrt bit controls how many packets get dropped as a function of time and I gather it is control loop tuning used to strike a good balance between responsiveness, stability and do no harm.
Kevin Gross
On Tue, May 8, 2012 at 7:04 PM, Dave Taht
<dave.taht@gmail.com> wrote:
Both the acm queue article and jim's blog entry this morning were way
above mensa's standards.
Nobody has attempted to explain the elegant simplicity of the
algorithm itself in the inverse sqrt however! I have a good grip on
it, and am trying, but can barely explain it to myself. Anyone else
care to dig through the codel code and try to put it into english?
Nice bit in ReadWrite News:
http://www.readwriteweb.com/enterprise/2012/05/good-news-for-solving-bufferbloat-codel-provides-no-knobs-solution.php
Bob Cringley lays down the challenge for us here on the bloat list,
and details the opportunity.
http://www.cringely.com/2012/05/beginning-of-the-end-for-bufferbloat/
He closes with:
"My advice to Cisco, Netgear, D-Link and others is that this could be
an important moment in their businesses if they choose to approach it
correctly. It’s a chance to get all of us to buy new routers, perhaps
new everything. Think of the music industry bonanza when we all
shifted our record libraries from vinyl to CDs. It could be the same
for networking equipment. But for that to happen the vendors have to
finally acknowledge bufferbloat and use their marketing dollars to
teach us all why we should upgrade ASAP. Everybody would win.
Take our money, please."
With the cerowrt project, at least, I've hoped to make that shift
possible, and to some extent... happen.
We have *working code*, and *proof of concept*. What's next? Where do
we go from here?
--
Dave Täht
SKYPE: davetaht
US Tel: 1-239-829-5608
http://www.bufferbloat.net
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