[e2e] "congestion" avoidance...

Jon Crowcroft J.Crowcroft at cs.ucl.ac.uk
Tue Apr 17 03:20:44 PDT 2001

so the problem i have with the current "ECN pricing" thinking is that
it ignores users preferences for stability and predictability over
cheapness (and we have a LOT of evidence gathered from mobile phone
contracts, web and traditional telewphony as well as airline procing 
and so on i can cite)

the shadow price for a packet (smart market) is one model, but leads
to potential rapid fluctation in price aroudn flash crowd periods,
which are all to common in IP networks

the simple alternative  is a shadow price for a virtual circuit - this
gives a stable price for the throughput over the "lifetime" of a

if you Mix this, you can get into nasty arbritrage situations....and i
don't buy into the story that yo ucan offer users a choce via risk
brokers - for the very reason that the traffic is highly dynamic...

also, risk brokers form markets  themselves.....

what i was thinking ewas to "democratise" (disintermediate) the risk
broker and let users form their +own+ cartels dynamically...

i.e. we napsterise congestion pricing for packets and flows...

In message < at mail.reed.com>, "David P. Reed" t

 >>Interesting thoughts.  However, money or something like it needs to enter 
 >>into the thinking.  I.e. some notion of sharing responsibility for costs 
 >>imposed on others.
 >>IE: At a point of congestion, the "indirect channels" among competing flows 
 >>provide a way of signalling (at some bitrate) for a bargaining scheme.
 >>What range of bargaining schemes can be piggybacked on this signalling channel?
 >>For example, what if a single (urgency) bit per packet (like the ECN flag, 
 >>but provided by the source to the congested queue) could be modulated at 
 >>the source, tracked in a state variable at a router queue, and coupled into 
 >>a bit in each outgoing packet that controls rate like ECN.
 >>At 10:06 AM 4/17/01 +0100, Jon Crowcroft wrote:
 >>>there have been some steps recently to look at a range of rate and
 >>>window based mechanisms for sharing the net amongst a set of sources (or
 >>>sinks if we include receiver based multicast schemes) - i was looking
 >>>at these and wondering if it isnt time to revisit some of the
 >>>congestion control and avoidance thinking
 >>>some schemes have been proposed that smooth the adjustment so that
 >>>over an RTT we creep up to the operating rate, and creep down, on a
 >>>packet by packet (inter-packet delay adjustment) basis
 >>>(RAP from Handley et al)
 >>>other schemes have proposed different powers for the increase decrease
 >>>function (and assert that so long as we decrease x^n, and increase
 >>>x^(n-1), we ought to be "ok" for some definition of ok)
 >>>(binomial adjustment etc from
 >>>the TCP AIMD with fast retransmit scheme has several motivating factors
 >>>some intended, some lucky happenstance (serendipitous)...
 >>>1/ sampling network conditions and eliminating noise:
 >>>currently, this operates over the RTT timescale, but is memoryless
 >>>after that....estimates for loss effectively im,plicit in the AIMD
 >>>operation, but the noise filter (number of dupacks) is somewhat
 >>>2/ safe/stable operation:
 >>>given feedback controller, its reasonable to operate this over
 >>>packet conservation/self clocking makes it more smooth
 >>>3/ relating end system rate adjustment timescale to buffering provisioning
 >>>the AIMD scheme has the bandwidth/delay product wrth of network
 >>>buffering as a necessary side effect - other adjustment schemes might
 >>>need less (some might need more but that almost certainly means they
 >>>are trouble:-)
 >>>4/ social coupling - we have a target operating point which will be
 >>>some fraction of a bottleneck link
 >>>if we take a flow f, and a flux (sum of flows into a bottleneck) F,
 >>>then the idea is that we get a share proportional to the _resource_ we
 >>>use, which (approximately) includes 1/RTT as a factor (kelly et al, le
 >>>boudec et al)  the idea is that a set of fs in an F are coupled by the
 >>>loss or ECN feedback function, and by some reaction period being at
 >>>least in the same ballpark....
 >>>in fact, though we don't have to have smooth functions at all, nor do
 >>>we have to sample only the average loss rate, nor choose the sample
 >>>rate to be an RTT - the RTT is a way of _loosely _ coupling things,
 >>>but is perhaps too strong
 >>>what if someone wanted a _rate_ that persited for all (or a larger
 >>>part) of a connection? how could we work out some sort of congestion
 >>>model that accommodated both packet and connection timescales?
 >>>at least one factor seems missing, and that is some estimate of the
 >>>number (and rate of change of number) of flows....if we alter the
 >>>sample period, and sample bot hte hcongestion feedback Mean, AND its
 >>>variance, we might be able to (assuming the social coupling function
 >>>was still "social") estimate this
 >>>obviosuly if people want to they can behave anti-socially (but that is
 >>>and wil lalways be true unti lwe do pricing or othewr forms of
 >>>admission control) - letsassume they behave "nicely"....
 >>>could someone choose to operate a "very slow" congestion control
 >>>scheme? why not? lets say i run a connection that takes 1/10 of the
 >>>capacity, but there are 5 other connections, then why should i react
 >>>to loss unless my longer term loss (or ecn) rate  tells me that
 >>>there's now 9+ other flows? currently,  if i run any adjustment
 >>>scheme based just on average, i have a chance of adjusting wrongly...
 >>>more importantly, maybe
 >>>secondly, how about re-examining the social coupling function? why
 >>>shouldn't ten people _agree_ a different congestion partition function
 >>>(e.g. they have an application that requires n sources)
 >>>i guess this could be implemented via the Congestion Manager type API,
 >>>but i am interested in the general family of functions that fit this
 >>>more general model - for example, it seems to me that you can have
 >>>radically different increase/decrease if you have
 >>>a) a different sample period and a more accurate deascripotion of the
 >>>evolutuon of the loss/load process over time (e.g. some sort of fancy
 >>>bayesian predictor)
 >>>b) a different share/social function - e.g. if we have 10 sources
 >>>agreeing on a different load, then how do they distribute this
 >>>information and how do  we make sure they aren't penalized by any
 >>>extra fancy stuff people might later add!



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