<html><head><style type="text/css"><!-- DIV {margin:0px;} --></style></head><body><div style="font-family:times new roman, new york, times, serif;font-size:12pt"><DIV style="FONT-SIZE: 12pt; FONT-FAMILY: times new roman, new york, times, serif">Creeping up into other people's topic, if I am allowed...<BR></DIV>
<DIV style="FONT-SIZE: 12pt; FONT-FAMILY: times new roman, new york, times, serif">----- Original Message ----<BR>From: Lachlan Andrew <lachlan.andrew@gmail.com><BR>To: dpapadimitriou@psg.com; dimitri.papadimitriou@alcatel-lucent.be<BR>Cc: iccrg IRTF list <iccrg@cs.ucl.ac.uk><BR>Sent: Tuesday, August 7, 2007 6:30:31 PM<BR>Subject: Re: [Iccrg] draft-irtf-iccrg-welzl-congestion-control-open-research-00.txt<BR></DIV>
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<DIV style="FONT-SIZE: 12pt; FONT-FAMILY: times new roman, new york, times, serif">[lots of stuff deleted, so as to focus on a specific point...]<BR></DIV></DIV>
<DIV style="FONT-SIZE: 12pt; FONT-FAMILY: times new roman, new york, times, serif"><BR>>From a control theory point of view, there's recently been a lot of<BR>work on the minimum information-theoretic signal rate required to<BR>"stabilise" a system (e.g,<BR><<A href="http://www.ee.unimelb.edu.au/staff/gnair/NairPIEEE.pdf" target=_blank>http://www.ee.unimelb.edu.au/staff/gnair/NairPIEEE.pdf</A>>). I'm not<BR>sure how much of that relates to congestion control, where<BR>undershooting the rate is much preferable to overshooting.<BR><BR>To decide how much feed back is "needed", we need to decide what<BR>performance measures to use, and what performance level is "needed" --<BR>e.g. do we need unused bandwidth to be taken up again within x<BR>RTTs/seconds/minutes of being released by another flow? Do we need<BR>overload to abate within x RTTs?<BR></DIV>
<DIV style="FONT-SIZE: 12pt; FONT-FAMILY: times new roman, new york, times, serif"><DC> I am assuming that "how much feedback is needed" to mean how often feedback info</DIV>
<DIV style="FONT-SIZE: 12pt; FONT-FAMILY: times new roman, new york, times, serif">should be received from the context above. From a control theoretical point of view, a CC</DIV>
<DIV style="FONT-SIZE: 12pt; FONT-FAMILY: times new roman, new york, times, serif">scheme tries to "follow" the dynamics of the disturbance of the system, which in this case can</DIV>
<DIV style="FONT-SIZE: 12pt; FONT-FAMILY: times new roman, new york, times, serif">be taken as available bandwidth (could be something else, such as queue levels). Suppose that as the result of a disturbance at time To, the response curve of the monitored variable has a maximum frequency component of Fmax. Then, we need to "sample" this variable at least as much as 2 x Fmax, the Nyquist sampling frequency. Obviously that this fact plays a role only on schemes that the "monitored variable" is multibit. For a single bit variable, such as loss, one needs to sample the system as often as the "bit flips". In practical terms, it means that for loss based AIMD CC the controller needs to be informed at every loss event.</DIV>
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<DIV style="FONT-SIZE: 12pt; FONT-FAMILY: times new roman, new york, times, serif"><BR>Cheers,<BR>Lachlan<BR><BR>-- <BR>Lachlan Andrew Dept of Computer Science, Caltech<BR>1200 E California Blvd, Mail Code 256-80, Pasadena CA 91125, USA<BR>Phone: +1 (626) 395-8820 Fax: +1 (626) 568-3603<BR><BR>_______________________________________________<BR>Iccrg mailing list<BR>Iccrg@cs.ucl.ac.uk<BR><A href="http://oakham.cs.ucl.ac.uk/mailman/listinfo/iccrg" target=_blank>http://oakham.cs.ucl.ac.uk/mailman/listinfo/iccrg</A></DIV>
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