[e2e] Latency Variation and Contention.
Sireen Habib Malik
s.malik at tuhh.de
Tue Aug 16 03:22:55 PDT 2005
Have not read the paper, however, I think that if,
RTT = Round Trip Time, and
dRTT = variations in RTT,
then "dRTT" is a weak/poor indicator of congestion.
A congestion signal based upon "dRTT/RTT" would give a much better idea,
Detlef Bosau wrote:
> Hi to all.
> Recently, I found the following paper by Sherif M. ElRakabawy,
> Alexander Klemm and Christoph Lindemann:
> The paper proposes a congestion control algorithm for ad hoc networks.
> Perhaps, this paper is interesting within the context of our latency
> However, I´m not yet convinced of this work.
> If I leave out some sheets of paper, some simulations and many words,
> the paper basically assumes that in ad hoc networks a TCP sender can
> measurethe degree of network contention using the variance of
> (recently seen) round trip times:
> -If the variance is close to zero, the network is hardly loaded.
> -If the variance is "high" (of course "high" is to be defined) there
> is a high degree of contention on this network.
> Afterwards the authors propose a sender pacing scheme, where a TCP
> flow´s rate is decreased with respect to the so measured "degree of
> What I do not yet understand is basic assumption: variance 0 <=> no
> load; variance high <=> heavy load.
> Perhaps the main difficulty is that I believed this myself for years
> and it was an admittedly difficult task to convince me that I was
> wrong %-)
> journal = " IEEE/ACM TRANSACTIONS ON NETWORKING",
> volume ="11",
> number = "3",
> month = "June",
> year = "2003",
> title = "Delay--Based Congestion Avoidance for TCP",
> author = "Jim Martin and Arne Nilsson and Injong Rhee",
> eventually did the job.
> More precisely, I looked at the latencies themselves, not the variances.
> Let´s consider a simple example.
> A network B
> "network" is some shared media packet switching network.
> Let´s place a TCP sender on A and the according sink on B.
> The simple question is (and I thought about this years ago without
> really coming to an end - I´m afraid I didn´t want to):
> Is a variance close to zero really equivalent for a low load situation?
> And does increasing variance indicate increasing load?
> Isn´t it possible that a variance close to zero is a consequence of a
> fully loaded network? And _decreasing_ load in that situation would
> cause the latencies to vary?
> If we could reliably identify a low load situation from a varaince
> close to zero, we could use the latencies themselves as a load
> indicator because we could reliably identify a "no load latency" and
> thus could identify imminent congestion by latency observation.
> One could even think of a "latency-congestion scale" which is
> calibrated first by variance observation in order to get the
> "unloaded" mark and second by drop observation and some loss
> differentation technique to get the "imminent congestion" mark.
> To my knowledge, this is extensively discussed in literature - until
> Martin, Nilsson and Rhee found the mentioned results.
> Now, back to my example and the basic question: Does the assumption,
> latency variations indicate the degree of contention in an ad hoch
> network, really hold?
> I admit, I personally do not yet see an evidence for this.
M.Sc.-Ing. Sireen Malik
Hamburg University of Technology
FSP 4-06 (room 5.012)
Schwarzenbergstrasse 95 (IVD)
Tel: +49 (40) 42-878-3443
Fax: +49 (40) 42-878-2941
E-Mail: s.malik at tuhh.de
--Everything should be as simple as possible, but no simpler (Albert Einstein)
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