Maxwell strikes the heart (ECN: Clearing the air)

Maxwell strikes the heart (ECN: Clearing the air)

[Pete Zaitcev]

> From: Gregory Maxwell (greg@linuxpower.cx)
> Date: Sun Jan 28 2001 - 14:42:04 EST 
> 
> On Sun, Jan 28, 2001 at 01:29:52PM +0000, James Sutherland wrote: 
> > > There is nothing silly with the decision, davem is simply a modern day 
> > > internet hero. 
> > 
> > No. If it were something essential, perhaps, but it's just a minor 
> > performance tweak to cut packet loss over congested links. It's not 
> > IPv6. It's not PMTU. It's not even very useful right now! 
> 
> No. ECN is essential to the continued stability of the Internet. Without 
> probabilistic queuing (i.e. RED) and ECN the Internet will continue to have 
> retransmit synchronization and once congested stay congested until people get 
> frustrated and give it up for a little bit. 
> 
> It's a real issue, and it's actually important to have it implemented. It's 
> not just a performance hack. 

I always "knew" that the stability of the Internet is secured by the
exponential backoff in TCP. A small packet loss on uncongested links
is a part of this technique, and it existed long before ATM studies
produced RED (which infiltrated backwards). It also requires sending
stacks to "give up for a little bit" (actually to give up a lot, and
together with the slow start it produced the well known "saw" of the
window size).

So far I fail to see how a repainted NAK, kludged into a NAKless protocol,
would improve stability of the Internet. If anything, it is going to
exaggerate traffic oscillations. I would appreciate couple of links
to reputable studies or discussions on the subject.

-- Pete
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Re: Maxwell strikes the heart (ECN: Clearing the air)

[J.D. Bakker]

At 14:55 -0800 29-01-2001, Pete Zaitcev wrote:
>So far I fail to see how a repainted NAK, kludged into a NAKless protocol,
>would improve stability of the Internet. If anything, it is going to
>exaggerate traffic oscillations.

If anything RED will *reduce* oscillations by alleviating retransmit 
synchronization.

>  I would appreciate couple of links
>to reputable studies or discussions on the subject.

See

http://www.aciri.org/floyd/ecn.html
http://www.aciri.org/floyd/red.html

(I particularly like ftp://ftp.ee.lbl.gov/talks/vj-nanog-red.pdf)

RED and ECN can be considered independently (although they do work 
together quite well). You can see ECN as a way of the network telling 
the endpoints "Normally I would have dropped this datagram, but I'll 
let you get away with it this time". TCP behavior on reception of an 
ECN should be the same as when a datagram is dropped, but without the 
latency (and jitter) that is the result of a lost datagram.

JDB.
[using RED/ECN-like protocols for latency sensitive video 
communications over a wireless link]
-- 
LART. 250 MIPS under one Watt. Free hardware design files.
http://www.lart.tudelft.nl/
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