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

[Eric Dumazet <eric.dumazet@gmail.com>]

As far as CoDel is concerned, we could have two stages of control :

The current one, to mark/drop packet as specified by Kathleen & Van

Then, for marked (ecn) packets, pass a second codel stage, but dropping
packets this time, to make sure we dont allow queue to become too large.


On Sun, 2012-08-05 at 09:53 -0700, Andrew McGregor wrote:
> Well, there's a lot of people at the IETF who really want to do other things with ECN, but it seems like the simple version is far too aggressive.
> 
> So, I think the desirable properties are something like:
> 1) Allow ECN flows to achieve the same or slightly higher throughput to maintain an incentive to deploy it.
> 2) Still drop ECN flows eventually to avoid too much queue buildup.
> 3) Account somehow for the fact that marking takes longer to control the queue (but we don't know how much longer).
> 
> Maybe mark ECN instead of dropping, but if we end up trying to mark/drop twice in one round, drop the later packets?
> 
> Oh, and ECN nonce deployment is negligible, to the extent that there are proposals in the IETF to reuse the bits for other things, and there is no pushback on that.
> 
> Andrew
> 
> On 4/08/2012, at 10:30 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> 
> > On Sat, 2012-08-04 at 20:06 -0700, Andrew McGregor wrote:
> >> Well, thanks Eric for trying it.
> >> 
> >> Hmm.  How was I that wrong?  Because I was supporting that idea.
> >> 
> >> Time to think.
> > 
> > No problem Andrew ;)
> > 
> > Its seems ECN is not well enough understood.
> > 
> > ECN marking a packet has the same effect for the sender : reducing cwnd
> > exactly like a packet drop. Only difference is avoiding the
> > retransmit[s].
> > 
> > It cannot be used only to send a 'small' warning, while other competing
> > non ECN flows have no signal.
> > 
> > As far as packet schedulers are concerned, there should be no difference
> > in ECN marking and dropping a packet. I believe linux packet schedulers
> > are fine in this area.
> > 
> > Now, there are fundamental issues with ECN itself, out of Codel scope,
> > thats for sure.
> > 
> > How widely has been RFC 3540 deployed, anybody knows ?
> > 
> > 
> > 
>