[e2e] Latency Variation and Contention.

[Detlef Bosau]

Hi to all.

Recently, I found the following paper by Sherif M. ElRakabawy, Alexander 
Klemm and Christoph Lindemann:

http://mobicom.cs.uni-dortmund.de/publications/TCP-AP_MobiHoc05.pdf

The paper proposes a congestion control algorithm for ad hoc networks.
Perhaps, this paper is interesting within the context of our latency 
discussion.

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 
contention".

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 %-)
However,

	@article{martin,
	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.

Detlef
-- 
Detlef Bosau
Galileistrasse 30
70565 Stuttgart
Mail: detlef.bosau at web.de
Web: http://www.detlef-bosau.de
Mobile: +49 172 681 9937