[e2e] TCP Loss Differentiation

[Jon Crowcroft]

hmmm - do you want the CS answer or the EE answer

i think the CS answer is that you can't differentiate 
congestion from failure causes for loss on a per packet basis 

thought experiment - what if a packet that is ECN marked gets lost?

i think the EE answer is that you could infer the likely cause
from the statistics of the channel over the recent past. of course, it
may be easier to do this at a receiver than a sender.

I am not sure if this is relevant, but it looks like we ought to
understand it anyhow:
A Survey of Results for Deletion Channels and Related Synchronization
Channels, by   	Michael Mitzenmacher 	 
http://portal.acm.org/citation.cfm?id=1427856

I suppose if the channel is delay bounded, and doesn't re-route
without telling you, then there might be some expensive way to put in
zillions of bits in the packet (or have each hop look at each packet
and see if it "should have seen" a packet before but hadn't, and set a
excplicit loss notification bit + seqno  - maybe we could send an
ICMP unreachable-from-previous hop with a copy of the next packet
(better still, with a timestamp of prev to one, and curent packet)

in multihop radio networks (especially if you have multipath routing), 
there is even a problem statistically
dis-ambiguating packet loss due to congestion and interference, since
some intererence is due to other packets on parallel flows that go
over "links" that are near enough (even on "link disjoint" routes)
to cause interference...(think "rubbernecking")

In missive <49A1323C.3040705 at web.de>, Detlef Bosau typed:

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 >>Injong Rhee wrote:
 >>> Perhaps I might add on this thread. Yes. I agree that it is not so 
 >>> clear that we have a model for non-congestion related losses. The 
 >>> motivation for this differentiation is, I guess, to disregard 
 >>> non-congestion related losses for TCP window control. So the 
 >>> motivation is valid. But maybe we should look at the problem from a 
 >>> different perspective. Instead of trying to detect non-congestion 
 >>> losses, why not try to detect congestion losses? 
 >>
 >>I did not follow the entire thread yet, but I think you miss the problem.
 >>
 >>The problem is not how to detect congestion losses. In the thread, ECN 
 >>is mentioned, perhaps other mechanisms as well.
 >>
 >>The problem is how to react properly upon packet loss?
 >>
 >>Particularly: how do we react properly upon an _individual_ packet loss? 
 >>When should a packet be disregarded for congeston control?
 >>On a statistical basis, there is an approach: the CETEN work by Alman 
 >>and Eddy. But can a reliable loss differentiation on an e-2-e basis be 
 >>achieved for individual packet?
 >>
 >>-- 
 >>Detlef Bosau                          Mail:  detlef.bosau at web.de
 >>Galileistrasse 30                     Web:   http://www.detlef-bosau.de
 >>70565 Stuttgart                       Skype: detlef.bosau
 >>Mobile: +49 172 681 9937
 >>
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 cheers

   jon