Subject: [PCN] Why is CL useful if SM works?

From: Michael Menth <[email-masked]>
Date: Tue, 10 Feb 2009 21:05:52 +0100

Hi all,

SM and CL were agreed to be both interesting to have as PCN mechanism at 
the meeting in Philadelphia.

I am not so sure about the benefit of CL. SM is good enough for networks 
with large aggregates and single-path routing. CL does not a particular 
better job in these cases although it requires two DSCPs for signalling 
which is almost a show stopper. Therefore, I fear that CL in the form as 
discussed so far is not useful.

When we have SM working, only another mechanism can be of interest that 
copes with multipath routing and small ingress-egress-aggregates (not 
necessarily small link aggregation). And if possible, waste of 
codepoints should be avoided.

Comments? Please give feedback, have I overlooked something?

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: Tom Taylor <[email-masked]>
Date: Wed, 11 Feb 2009 06:50:35 -0500

You may be right, depending on the interpretation of the admission marking 
threshold. If one chooses the admission stop point for the CLE on the basis that 
you want the statistical probability that you reach the maximum supportable rate 
to be less than x%, then there is a mathematical dependency between the 
admission marking threshold and the excess marking threshold that involves the 
statistical properties of the traffic flows. This dependency means that you can 
operate your network based observed flows most of the time, and use PCN marking 
only to cope with situations requiring flow termination.

I can spell out the details. The catch is that in practice, statistical 
variances may be too high so you end up enforcing lower link occupancies than 
threshold marking would permit.

Tom

---

From: Michael Menth <[email-masked]>
Date: Wed, 11 Feb 2009 13:37:35 +0100

Hi Tom,

you say the benefit of CL vs. SM is possibly increased resource 
efficiency since the admissible rate can be set to a larger value in CL 
than in SM because CL can cope better with variable traffic, right? So 
the argument for CL (as discussed so far) is possibly improved efficiency.

I doubt that improved efficiency alone warrants the coexistence of two 
standards and the use of another DSCP. Are there other benefits and 
opinions?

Regards,

    Michael

Tom Taylor schrieb:
> You may be right, depending on the interpretation of the admission 
> marking threshold. If one chooses the admission stop point for the CLE 
> on the basis that you want the statistical probability that you reach 
> the maximum supportable rate to be less than x%, then there is a 
> mathematical dependency between the admission marking threshold and 
> the excess marking threshold that involves the statistical properties 
> of the traffic flows. This dependency means that you can operate your 
> network based observed flows most of the time, and use PCN marking 
> I can spell out the details. The catch is that in practice, 
> statistical variances may be too high so you end up enforcing lower 
> link occupancies than threshold marking would permit.
>
> Tom
>
> Michael Menth wrote:
>> Hi all,
>>
>> SM and CL were agreed to be both interesting to have as PCN mechanism 
>> at the meeting in Philadelphia.
>>
>> I am not so sure about the benefit of CL. SM is good enough for 
>> networks with large aggregates and single-path routing. CL does not a 
>> particular better job in these cases although it requires two DSCPs 
>> for signalling which is almost a show stopper. Therefore, I fear that 
>> CL in the form as discussed so far is not useful.
>>
>> When we have SM working, only another mechanism can be of interest 
>> that copes with multipath routing and small ingress-egress-aggregates 
>> (not necessarily small link aggregation). And if possible, waste of

---

From: ken carlberg <[email-masked]>
Date: Wed, 11 Feb 2009 07:43:48 -0500

On Feb 11, 2009, at 7:37 AM, Michael Menth wrote:

> I doubt that improved efficiency alone warrants the coexistence of  
> two standards and the use of another DSCP. Are there other benefits  
> and opinions?

what other criteria do you feel needs to be in place?

Also, out of curiosity (and just a general question), what are the  
chances that these efforts are initially migrated to experimental  
status by the IESG and the group is asked to use values from the  
experimental pool?

-ken

---

From: Michael Menth <[email-masked]>
Date: Wed, 11 Feb 2009 14:12:03 +0100

Hi Ken,

> what other criteria do you feel needs to be in place?
Coexistence of two similar PCN standards - a simple and a complex one - 
might make sense if this extends the applicability of PCN. Examples are 
networks with multipath routing or networks with low ingress-egress 
aggregation (with possibly large link aggregation).

Regards,

    Michael

---

From: "Anna Charny (acharny)" <[email-masked]>
Date: Wed, 11 Feb 2009 08:56:10 -0500

Hi Michael,

I think the area when CL is useful is exactly where SM performance
suffers - which is in the presence of a very large number of small
ingress-egress aggregates which together take a large portion of
bottleneck capacity. Should this be the case in a network, CL clearly
has superior performance over SM.

Best,
Anna 

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf Of
Michael Menth
Sent: Tuesday, February 10, 2009 3:06 PM
To: [email-masked]
Subject: [PCN] Why is CL useful if SM works?

Hi all,

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

_______________________________________________
PCN mailing list
[email-masked]
https://www.ietf.org/mailman/listinfo/pcn

---

From: Michael Menth <[email-masked]>
Date: Wed, 11 Feb 2009 15:43:47 +0100

Hi Anna,

> has superior performance over SM.
The reason why CL and SM do not work well for small 
ingress-egress-aggregates (IEAs) is that many of these IEAs empty most 
of the time and cannot block. Figs. 5c and 5d in
http://www3.informatik.uni-wuerzburg.de/~menth/Publications/papers/Menth08-Sub-8.pdf
show that CL (exhaustive marking) reduces overadmission compared to SM 
(excess marking) if the average number of flows per IEA is about 1. 
Figs. 5a and 5b show that CL (exhaustive marking) cannot fight overload 
either the average number of flows per IEA it is smaller like 0.5.  So 
CL just mitigates this situation, but I would not say that it can handle 
it. 100% overadmission is a lot.

It's fine that SM cannot cope with
a) small IEAs and
b) multipath routing
since it is just not designed for that. MPLS networks with fat pipes are 
happy to have this simple mechanism. But an improved PCN mechanism 
should be able to handle a) and b) especially if it costs another DSCP.

Regards,

    Michael