Subject: [PCN] Pre-congestion and ECN

From: Michael Menth <[email-masked]>
Date: Tue, 10 Jul 2007 08:31:06 +0200

Hi Phil and others,,

 > > I think it makes sense to define the term "pre-congestion": a link with
 > > capacity C is pre-congested with regard to a rate threshold R<C if its
 > > current traffic rate is above R. This applies both to the
 > > configurable-admission-rate and configurable-termination-rate
 > > thresholds, i.e., there are different types of pre-congestion. We
 > > introduced this notion in Section 2 of  the "Three State PCN Marking"
 > > draft:
 > > http://www.ietf.org/internet-drafts/draft-babiarz-pcn-3sm-00.txt

 > would this definition also catch ECN (which 3168 considers 
congestion, rather than pre-congestion)?

Congestion observed in real queues by ECN can be seen as a special form 
of "pre-congestion", namely with regard to the link rate R=C. It is 
possible to view it as such, but I find it a bit pathologic because the 
"pre-" is not really met since R=C and not R<C.

This makes the difference between PCN and ECN explicit: ECN starts 
marking packets when real queueing (=congestion) occurs while PCN 
already marks packets when pre-congestion occurs, i.e., when a rate R<C 
is exceeded and no physical queues arise.

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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