[e2e] congestion collapse definition

Jon Crowcroft Jon.Crowcroft at cl.cam.ac.uk
Tue Sep 8 21:34:04 PDT 2009



i recall being taught about this pre-internet
and when (just pre 1988 tcp fixen)
load increase leads to fall in throughput
(as in aloha, csma, or any contended media)
due to more time being spent contending,
and colliding,
than in resolving conention
and getting a "slot" - whether the slot is
on media, or in a buffer

the original religion was that under such loads,
one should switch to reservation schemes...
but then the control theory approach that kk/raj jain, van et al
introduced, led to graceful asymptote to some operating point
with maximum goodput,
rather than collapse, was a shift in religion - some
design rules emerged for parameters (like buffer sizing, packet sizes,
media loss due to non congestive reasons) that would then allow these
control feedback schemes to work - otherwise you'd still get collapse
the "buffer madness" turned out to be a new religion...plus
there are assumptions about the balance of traffic matrix 
and the rate of change of number of flows (i.e. what
had been 2nd order effects) that are
violated in real world scenarios so that you
still get congestive collapse - i.e. correlated flows (flash crowds)
and accidental topological ddos  attacks (which the underprvisioned
3g access might be yet another example of?) and all compounded by mis-construing the 
desgn rules...

i.e. we've moved on to the next set of problems...:)

In missive <20090909025805.D6ACB6BE609 at mercury.lcs.mit.edu>, Noel Chiappa typed:

 >>    > From: "David P. Reed" <dpreed at reed.com>
 >>    > If I look at Wikipedia, for example, at the definition of congestion
 >>    > collapse there, it says that CC is characterized by large buffering
 >>    > delays AND lost packets. 
 >>I was responsible for the original version of that page, so any faults in it
 >>can probably be laid at my door... :-)
 >>When writing it, I was probably thinking of the examples of congestive
 >>collapse I had seen (e.g. the ARPANet TCP email meltdown, TFTP Sorcerer's
 >>Apprentice Syndrome, etc) in which high packet drop rates were seen. Of
 >>course, that was 'back in the day', when routers had a _lot_ less buffering
 >>than they do now, so the symptoms naturally differed somewhat.
 >>    > should this phenomenon be included in "congestion collapse" (I believe
 >>    > so), or should we invent a new more specific name (Buffer Madness?).
 >>Both, I think.
 >>In general terms, they are both examples of the same basic concept; people
 >>give the network a lot more traffic than it can handle, and as a result things
 >>turn to total alimentary byproduct. To me, the term 'congestive collapse' is a
 >>perfect term for that general situation, because the network is, well,
 >>congested; so congested it effectively ceases to function. (Ceases to function
 >>effectively?) Congestion + failure -> congestive collapse. (Somehow
 >>'congestive failure' doesn't have the same ring - probably the alliteration.)
 >>At the same time, they are interestingly different (in terms of the
 >>symptoms), so this subvariant (which we will probably be seeing more of,
 >>until the lessons gets into the global conciousness) could probably use a
 >>specific term. 'Buffer Madness' sounds good to me; another possibility is
 >>some term related to blockage of the alimentary canal, where stuff just keeps
 >>building up - but my brain won't cough up, right at this moment, the term I'm
 >>thinking of.
 >>	Noel



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