[ns] Random Exponential Marking

[Lloyd Wood <l.wood@eim.surrey.ac.uk>]

forwarding since it mentions ns scripts...

http://netlab.caltech.edu/pub.html

has the paper, as well as stuff on the similarly-named Random Early
Marking.

L.

<L.Wood@surrey.ac.uk>PGP<http://www.ee.surrey.ac.uk/Personal/L.Wood/>

---------- Forwarded message ----------
Date: Fri, 26 Jan 2001 18:12:15 -0800
From: Steven Low <slow@caltech.edu>
To: Vishal Misra <misra@newworld.cs.umass.edu>
Cc: end2end-interest@ISI.EDU, sanjeewa@caltech.edu
Subject: Re: [e2e] [Fwd: RED-->ECN]


>         AQM in the absence of ECN can be counter productive. If you try to
> control the buffer to low levels, then you reduce delays. However that
> means that loss must go up almost quadratically (taking the
> simplified throughput equation for TCP, sqrt(p)*RTT = Constant since link
> capacity is not changing). That increases the probability of redundant
> retransmissions as also the probability of flows going into
> timeouts. Goodput suffers. Hence, decoupling congestion measure and
> performance measure may not help if your feedback mechanism is drops
> instead of marks, and will in all likelihood hurt performance. Thus, ECN
> is *critical* for the success of AQM schemes.
> 
> -Vishal

Hi Vishal,

I completely agree that ECN would be great for AQM.  I agree also with
your observation that larger delay in fact reduces loss rate.  
However I'm not sure if I agree that AQM without ECN *necessarily*
leads to worse performance.  I think it probably depends on 1) various
parameters of the scenario, and 2) the specific AQM scheme.  Our
preliminary ns-2 simulations with REM actually shows improvement in
goodput and delay over DropTail (using NewReno) both with or without
ECN (see preprint "REM: Active Queue Management" on our website at the
end of this email).  I can imagine one can probably design scenarios
where what you describe is the case.  An interesting question is to
characterize the conditions under which AQM without ECN helps or
hurts.

Another question is whether we should rely on large (queueing) delay to 
keep loss rate down?   This doesn't seem a good idea, nor necessary.

Steven
ps. We hope to post our ns-2 scripts so that anyone interested can
play with REM.
__________________________________________________________________
Steven Low, Assoc Prof of CS & EE 
slow@caltech.edu			netlab.caltech.edu
Tel: (626) 395-6767			Caltech MC256-80
Fax: (626) 792-4257			Pasadena CA 91125