Re: [TERNLI] Notes from tonight's ad hoc
Bob Briscoe <rbriscoe@jungle.bt.co.uk> Thu, 03 August 2006 17:28 UTC
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Date: Thu, 03 Aug 2006 11:29:01 +0100
To: Lars Eggert <lars.eggert@netlab.nec.de>
From: Bob Briscoe <rbriscoe@jungle.bt.co.uk>
Subject: Re: [TERNLI] Notes from tonight's ad hoc
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Lars, At 08:57 03/08/2006, Lars Eggert wrote: >It's not only congestion, it's also delay, plus maybe additional >information. One work item from the ad hoc meeting was to figure out >which kinds of information transports do or could operate on. Yes, I said the TSV needs congestion and delay, but I only focused on the NET signalling congestion upwards, because...: i) If you meant specifically RTT delay (= propagation + congestion delay), the TSV can measure that as often as it needs to without explicit signalling. ii) However, if you meant specifically congestion delay I disagree - this is less useful as a congestion metric (implying I don't believe FAST TCP or Vegas are the long term direction - fine as tactical hacks). Arguments against congestion delay as a metric: a) Explicit congestion signalling is a richer way for the NET to signal how much the TSV should change its rate because, if the NET signals (or the TSV measures) congestion delay, in order to signal an equivalent amount of info to the TSV for it to know how to change its rate, the NET would also have to say what the output line rate of each interface was (meaning numerous metrics across a path - yuk). Just signalling congestion does that all in one metric, even when accumulated across a path. b) Measuring congestion delay is fine as a tactical hack to get more precise info from the network than 1-bit drop or 1-bit ECN, but it won't be useful long term as link speeds increase. Then congestion delay becomes so small relative to propagation delay that it becomes a very imprecise metric to do congestion control on (IOW, the equiv no. of bits of info per packet is currently >1, but will become <1 as the net scales). (We're in the early stages of writing up and proving the above argument as a full paper.) BTW, I do agree with FAST & Vegas (and Siris's weighted window-based cc) on another count: Unlike Reno, delay should only determine the second order behaviour, not the first order. Ie the equilibrium rate shouldn't depend on RTT, but how fast it gets there should. >>Basically, I'm saying >>a) Instead of signalling "something's changed", I believe it is >>more practical to continuously signal the "something". > >That's an interesting observation. By constantly providing the raw >information, it is up to the transports to decide what difference >constitutes a "change" vs. having the network layer decide that a >"change" has occurred. It also allows transports to process the raw >information into events any way they see fit, instead of assuming one >specific way at the layer below. If the NET says "something's changed", * 1RTT later the TSV probes. * The TSV has to probe for everything, because it doesn't know what "something" is (unless the NET allows the question "what's changed?" which is silly - it would have been better for the NET to say what changed in the first place otherwise this would take yet another RTT). * 1 more RTT later the TSV finds out the info. That's 2RTTs already. If we can get down to exactly what the "something's" are, we can know how many bits would be needed to continuously repeat them, rather than implement the more complicated probe interfaces necessary above. Additionally, when talking about changes in load/congestion, the NET doesn't know what threshold all the different TSVs would like it to use to decide when a change has been big enough to say "something's changed". Congestion changes all the time. Some changes are bigger than others. So the approach of continuously reporting the current level avoids the NET second guessing what changes TSVs are interested in. Bob ____________________________________________________________________________ Bob Briscoe, <bob.briscoe@bt.com> Networks Research Centre, BT Research B54/77 Adastral Park,Martlesham Heath,Ipswich,IP5 3RE,UK. +44 1473 645196
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- Re: [TERNLI] Notes from tonight's ad hoc Mark Allman
- RE: [TERNLI] Notes from tonight's ad hoc Kevin Fall
- Re: [TERNLI] Notes from tonight's ad hoc Wesley Eddy
- Re: [TERNLI] Notes from tonight's ad hoc Mark Allman
- Re: [TERNLI] Notes from tonight's ad hoc Michael Welzl
- RE: [TERNLI] Notes from tonight's ad hoc Kevin Fall
- RE: [TERNLI] Notes from tonight's ad hoc Michael Welzl
- Re: [TERNLI] Notes from tonight's ad hoc Mark Allman
- Re: [TERNLI] Notes from tonight's ad hoc Wesley Eddy
- Re: [TERNLI] Notes from tonight's ad hoc Michael Welzl
- Re: [TERNLI] Notes from tonight's ad hoc Joe Touch
- Re: [TERNLI] Notes from tonight's ad hoc Mark Allman
- Re: [TERNLI] Notes from tonight's ad hoc Joe Touch
- Re: [TERNLI] Notes from tonight's ad hoc Lars Eggert
- Re: [TERNLI] Notes from tonight's ad hoc Gorry Fairhurst
- Re: [TERNLI] Notes from tonight's ad hoc Spencer Dawkins
- Re: [TERNLI] Notes from tonight's ad hoc Aaron Falk
- Re: [TERNLI] Notes from tonight's ad hoc Aaron Falk
- Re: [TERNLI] Notes from tonight's ad hoc Joe Touch
- Re: [TERNLI] Notes from tonight's ad hoc Joe Touch
- Re: [TERNLI] Notes from tonight's ad hoc Gorry Fairhurst
- Re: [TERNLI] Notes from tonight's ad hoc Aaron Falk
- Re: [TERNLI] Notes from tonight's ad hoc Bob Briscoe
- Re: [TERNLI] Notes from tonight's ad hoc Lars Eggert
- Re: [TERNLI] Notes from tonight's ad hoc Bob Briscoe
- Re: [TERNLI] Notes from tonight's ad hoc Joe Touch