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

[Eric Dumazet]

On Sun, 2012-08-05 at 18:54 +0200, Richard Scheffenegger wrote:
> Hi Eric,
> 
> ----- Original Message ----- 
> From: "Eric Dumazet" <eric.dumazet at gmail.com>
> To: "Andrew McGregor" <andrewmcgr at gmail.com>
> Cc: <codel at lists.bufferbloat.net>
> Sent: Sunday, August 05, 2012 7:30 AM
> Subject: Re: [Codel] [RFC PATCH] codel: ecn mark at target
> 
> 
> > 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].
> 
> That's true for the first mark; any subsequent mark (during the same window) 
> should have no effect - thus a high marking rate (marking fraction per 
> window) should not be that much worse... Of course, the queue can never know 
> the effective window of the tcp stream it is marking...
> 

Once your cwnd is 1 packet, and RTT is 100ms, what can you get from
this, if all your packets have ECN mark ?

> As a test, when the marking is done really instead of drop, do you see 
> fairness betwenn the ecn and legacy tcp flows? (if not, the ecn 
> implementation may be faulty).

Yes its fine as mentioned in my test : codel , and I get 50/50 split
between my two flows.

It could be a flaw in linux implementation, I admit we had so many bugs
that it could very well be still buggy.