Subject: [PCN] RE: Pre-congestion and ECN

From: "Charzinski, Joachim" <[email-masked]>
Date: Wed, 11 Jul 2007 08:13:02 +0200

Hi Michael,

> 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.

Isn't this also a question of time scales? ECN would diagnose 
"congestion" (which from the way ECN works should mostly be 
considered pre-congestion) when a queue is found significantly 
filled. This can happen also when R<C in terms of medium term 
mean values but R>C during the very recent (mostly less than 1s) 
past. 
As I understood it, PCN is supposed to cover the longer time scales. 
Thus, in PCN notation, ECN would actually signal pre-congestion in 
most cases. 

Best regards

	Joachim.

----------------------------------------------------------
Nokia Siemens Networks
 
Joachim Charzinski
Principal Innovator

Machtlfinger Str. 1
D-81379 Muenchen
Germany
Tel: +49 89 722 46803

[email-masked] 
http://www.nokiasiemensnetworks.com/global/

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---

From: Michael Menth <[email-masked]>
Date: Thu, 12 Jul 2007 00:51:07 +0200

Hi Joachim,

thanks for this subtle question! Please see inline!

Charzinski, Joachim wrote:
> Isn't this also a question of time scales? 
Yes, that is absolutely true. The sensitivity to time scales (in other 
words: reaction time of the markers when a certain rate is exceeded) can 
be controlled by the token bucket sizes of the PCN meters and markers 
which can be configured. In a similar way, the sensitivity to time 
scales of ECN can be controlled by the parameters of its underlying RED 
queue and the averaging parameter.

> ECN would diagnose 
Basically, if ECN signals congestion, PCN will also signal 
pre-congestion, but the opposite is not necessarily true. Thus, PCN 
reacts earlier than ECN where "earlier" relates to the observed average 
rate.

However, this statement is not always true (this is probably what you 
suspected) when we consider the time it takes until packets are marked. 
Take an ECN marker that starts marking packets already at a very low 
average fill state of the shortly-averaged physical size of its RED 
queue. It is compared to a PCN meter and marker with R<C  and a large 
token bucket. If a large burst arrives, it is possible that  ECN marks 
packets but PCN does not. However, we get this behavior only because we 
provoked it by the very different parameters controlling the reaction 
time of the marking algorithms.

> As I understood it, PCN is supposed to cover the longer time scales. 
That's a good question! How fast should PCN react with admission-stop or 
flow termination when the admissible or supportable rate is exceeded? 
Within 100ms, 1s or 10s? The answer has a direct impact on the 
configuration of the meters.

What parameters are used for ECN markers in practice?

> Thus, in PCN notation, ECN would actually signal pre-congestion in 
I would express that differently. ECN signals congestion, but possibly 
earlier (with respect to the first marked packet) than PCN signals 
pre-congestion. And sometimes the duration of the congestion is too 
short for PCN to react. However, this all depends on the configured 
parameters ...

Best 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

---

From: <[email-masked]>
Date: Thu, 12 Jul 2007 11:08:58 +0100

Michael - re your original point will try & add "pre-congestion" to terminology on lines you suggest

phil

> -----Original Message-----
> From: Charzinski, Joachim [mailto:[email-masked]]
> Sent: 11 July 2007 07:13
> To: [email-masked]; [email-masked]
> Subject: [PCN] RE: Pre-congestion and ECN
> 
> Hi Michael,
> 
> Nokia Siemens Networks GmbH & Co. KG - Sitz der Gesellschaft: München /
> Registered office: Munich - Registergericht: München / Commercial
> registry: Munich, HRA 88537 - WEEE-Reg.-Nr.: DE 52984304 - Persönlich
> haftende Gesellschafterin / General Partner: Nokia Siemens Networks
> Management GmbH - Geschäftsleitung / Board of Directors: Joachim Malterer,
> Lydia Sommer - Sitz der Gesellschaft: München / Registered office: Munich
> - Registergericht: München / Commercial registry: Munich, HRB 163416
> 
> 
> 
>

---

From: Michael Menth <[email-masked]>
Date: Thu, 12 Jul 2007 14:50:08 +0200

Hi Phil,

here is a compact summary regarding your question about the relation 
between PCN and ECN taking Joachim's comments into account. As you had 
that question, therefore this might be useful for the draft.

"ECN signals congestion using ECN-marking when the line rate C is
exceeded by the current traffic and physical queuing occurs. PCN
signals pre-congestion using PCN-marking when a configured rate
R<C is exceeded whereby physical queueing does not necessarily
occur. Thus, PCN marks packets already at lower rate than ECN
does. However, the reaction speed of ECN and PCN marking depends
on configured parameters. If ECN is configured to react fast and
PCN is configured to react slowly, packets may be ECN-marked but
not PCN-marked in the presence of short-time bursts."

Regards,

    Michael