Subject: [PCN] Feedback on performance draft

From: Michael Menth <[email-masked]>
Date: Fri, 08 Dec 2006 12:12:46 +0100

Dear Anna et al.,

I've read your draft
http://www.ietf.org/internet-drafts/draft-zhang-pcn-performance-evaluation-00.txt
My overall impression: nice document and interesting and informative 
results! I've got some comments.

ad 3.7/5.1)
PCN works well also with small ingress-egress aggregation, i.e., 
over-admission is not observed with the simulated traffic.

This can be explained as follows. If the admissible rate of an interface 
is exceeded, ALL packets flowing over the respective link are 
admission-stop labelled and their egress routers signal this information 
back to the respective ingress routers. As a consequence, all ingress 
routers having at least a single flow in progress are informed about the 
bottleneck. Therefore, this kind of implicit signalling is independent 
of the aggregation level.

ad 4.2/5.2)
If the preemption threshold is exceeded, only those packets are labelled 
that exceed the {tolerable interface, excess traffic, preemption} rate. 
The observation was that preemption marked packets were not distributed 
equally over all flows, but some
flows had all off their packets labelled and some others had no packet 
labelled at all. I think this is due to the periodic nature of the 
aggregated process. The phase (= transmission instant within a period) 
of a flow within the aggregated process determines the "queue" 
encountered by its packets in any period. Randomizing the transmission 
time breaks this phenomenon if the randomization is strong enough. 
Applying a jitter offset to every transmission instant which is equally 
distributed in [0, period length] should lead to strong over-preemption 
for all simulated scenarios.
Without randomization, this effect could be provoked by choosing 
different period length for the traffic and adjusting the packet sizes 
accordingly. Try prime numbers if you want to avoid short cycles: period 
lengths of 13, 17, 19, 23, 29, 31 ms!

Kind regards,

    Michael

-- 
Dr. Michael Menth, Assistant Professor
University of Wuerzburg, Institute of Computer Science
Am Hubland, D-97074 Wuerzburg, Germany, room B206
phone: (+49)-931/888-6644, fax: (+49)-931/888-6632
mailto:[email-masked]
http://www3.informatik.uni-wuerzburg.de/research/ngn

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