Subject: Re: [PCN] Admission control for SM/CL

From: SATOH Daisuke <[email-masked]>
Date: Fri, 04 Mar 2011 16:02:02 +0900

Dear Michael and All,

How about the following?
An ingress admits only one flow if it has not sent another
packet to an egress for the duration of a measurement interval. Then the
igress does not admit new flows until it receives PCN feedback.

The above does not need a new mechanism of probe.
It is true that overadmission may occur but I believe quantity of
overadmission is small. If the quantity is large, it must be caused by
flash crowd. PCN does not work in the case of flash crowd.

Daisuke

> 
> Hi all,
> 
> if IEAs do not carry any traffic, there is no appropriate PCN feedback. In
> such a case admission requests must be admitted. This makes SM/CL
> prone to overadmission when the average number of flows per IEA is low.
> I feel that we should do something against that. There is a rather simple
> solution.
> 
> Let DMI be the duration of a measurement interval. An ingress node should
> send a probe/hello packet towards an egress node if it has not sent another
> packet to that node for DMI/n time (n>=1).
> 
> This modification avoids that measured PCN rates are zero and so the 
> decision
> point always gets a meaningful indication about the pre-congestion state of
> an IEA-specific path. For CL, n=1 is sufficient while SM requires about 
> n=10 or
> n=20 to work well. Let DMI=200 ms, then the additional traffic rate per 
> IEA is
> at most 1.3 kbit/s for CL and 26 kbit/s for SM. This seems to be a 
> lightweight
> improvement for CL, while the overhead is larger for SM.
> 
> SM does not only work well in case of empty IEAs but also for IEAs with only
> one or two flows. There is a probability that SM admits new flows even 
> in the
> presence of pre-congestion as only a small fraction of PCN traffic is
> re-marked. The presented mechanism also helps SM to work better also in 
> these
> cases at the expense of some more overhead.
> 
> I'd like to hear opinions about this feature and whether we should 
> include it
> in the current drafts.
> 
> Best wishes,
> 
>      Michael
> 
> -- 
> Prof. Dr. habil. Michael Menth
> University of Tuebingen
> Faculty of Science
> Department of Computer Science
> Chair of Communication Networks
> Sand 13, 72076 Tuebingen, Germany
> phone: (+49)-7071/29-70505
> fax: (+49)-7071/29-5220
> mailto:[email-masked]
> http://www-kn.informatik.uni-tuebingen.de
-- 
NTTアドバンステクノロジ（株）　ネットワークテクノロジセンタ
佐藤　大輔　(Daisuke SATOH, Ph.D.)
mailto: [email-masked]

---

From: Michael Menth <[email-masked]>
Date: Fri, 04 Mar 2011 08:31:46 +0100

Hi Daisuke,

> The above does not need a new mechanism of probe.

That's also a step forward. It certainly helps a bit although it's not
so effective.

> flash crowd. PCN does not work in the case of flash crowd.

what's a flash crowd? A situation with extremely high load and for
exactly that AC is needed. The fact that PCN-based AC sometimes
overadmits a bit does not mean that it does not work in general with
flash crowds. That would be bad. If AC works only in the presence of
moderate load, then it's cheaper to do without it.

The problem with the current SM and excess traffic marking is that If
the AR of a bottleneck link is limited to 100 voice flows and its load
stems from 100 different IEAs, we can expect a *permanent* overadmission
of about 20-25% if the currently offered load is 5 times larger than for
what the system was designed for.

Let's take a similar scenario. AR=500 flows that come from 1000 IEAs. In
that case your solution described above cannot avoid overadmission.

But that's possibly not too bad if we really limit the applicability of
PCN to IEAs with at least 5 flows on average. My proposal was thought to
extend the applicability of PCN towards lower aggregation levels.

Best wishes,

Michael

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From: SATOH Daisuke <[email-masked]>
Date: Fri, 04 Mar 2011 20:12:32 +0900

Hi Michael,

On Fri, 04 Mar 2011 08:31:46 +0100

> moderate load, then it's cheaper to do without it.

I use flash crowd extreamly high load as congestion just after disaster.

> what the system was designed for.

This problem is not directly related to admisson control problem in the
case of no flow in IEA. The problem with the current SM is a problem of low
flow aggregation in IEA.

> that case your solution described above cannot avoid overadmission.

That's true but I believe the probability is very low. The probability
that 1000 IEA generate one or more flows simultaneously within the next
measurement interval although they generate no flow in the current
measurement interval, is very low. However, it may happen just after
earthquake happens.

I agree your method is more effective than mine. I believe probing is
 effective for measuring traffic etc. However, I hesitate to agree using
 probing only for the case of no flow in IEA. "MOTTAINAI" in Japanese.

Best regards,

Daisuke

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From: Michael Menth <[email-masked]>
Date: Wed, 09 Mar 2011 09:41:31 +0100

Hi Daisuke,

>  probing only for the case of no flow in IEA. "MOTTAINAI" in Japanese.

I don't say do probing if there is no other flow, I say send a probe if
no other packet has been sent for X time. This is more general and also
helps with on/off traffic.

Best wishes,

Michael

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From: SATOH Daisuke <[email-masked]>
Date: Thu, 10 Mar 2011 09:17:49 +0900

Hi Michael,

My description is not appropriate. I hesitate that equipment has a
mechanism of sending probes only for the case of no flow in IEA. Sending
probes can be used for admission control etc. In PCN, we do not use
sending probes for admission control.

Best regards,
Daisuke

On Wed, 09 Mar 2011 09:41:31 +0100