[Codel] [RFC PATCH] codel: ecn mark at target

[Eric Dumazet]

On Mon, 2012-08-06 at 12:09 -0700, Andrew McGregor wrote:
> On 6/08/2012, at 10:50 AM, Dave Taht <dave.taht at gmail.com> wrote:
> 
> > This discussion is getting mildly off-track. My intent in posting this patch
> > was to prove how wrong the "ecn mark at target" idea was by example,
> > and in doing so, shed light on those new to codel, on how the algorithm
> > actually works, and to encourage those that didn't grok it, to read and
> > run the code in whatever scenarios would help more people to
> > grokking in fullness.
> > 
> > I hadn't expected to twiddle a bug!
> 
> Well, so drop at target is wrong wrt deployed TCPs.  Ok, fine.
> 
> So, instead, how about this: mark instead of dropping, but only for
> the first few iterations around the while loop in dequeue (so that
> huge backlogs can be drained).  The question then is, how many is a
> few?  I suppose that can be answered empirically.
> 
> Andrew

Lets take the analogy with RED.

Once a MAX_THRESH was reached, it did a hard_mark.

So we could choose a threshold to drop instead of marking.

Possible easy choices :

1) A threshold on sojourn time. For example force_drop = 2 * target

2) A threshold on 'count'. For example force_drop if count > 1000