Re: [ippm] Comments on draft-ietf-ippm-framework-compagg-00

Henk Uijterwaal wrote:

> Phil,
>
>> The problem that I have with this section is that it doesn't give me
>> very strong or concrete reasons for why I want to combine metrics.
>
>
> Suppose I have a path A-B-C.  I'm interested in the delays between them.
> Now, I can measure all 3 (A-B, B-C and A-B-C) separately.  However, if
> there is a way to add up the delays between A-B and B-C, and get the
> overall delay, then i can see myself some work and diskspace.  In 
> general,
> I'd be interested in the minimum set of measurements that still gives
> me information about the entire network.
>
>>  Section 5.1: The problem here is that piece-wise measurements do
>> not necessarily combine. IPDV and re-ordering are examples where one
>> subsection of a network can "undo" what a previous one has done.
>> Putting these together in a straightforward way doesn't give you the
>> right answer.
>

With delays,  if the packet delays in the two networks are not 
correlated, it is still possible to measure statistics of  delays of 
segments and combine them to provide end-to-end delay statistics.  When 
delays are correlated, it becomes necessary to measure correlation as 
well.... which would be more complex.
Reordering is somewhat of a harder problem. But, when  captured using RD 
(reorder density), you can combine the reordering in different segments 
and still come up with end-to-end reordering.  Combination is done using
convolution, which would be the case for delay also.  One thing going 
for RD is the fact that reordering in two network segments are less 
likely to be correlated compared to delay.  We have carried out many 
mesurements on Internet, planet-lab, simulations, etc., and are yet to 
see a case where it did not work - for reordering.
(See http://www.cnrl.colostate.edu/packet_reorder.html )
Cheers!
AJ

>
> I agree but can't we just say so?
>
> Henk
>
>
> ------------------------------------------------------------------------------ 
>
> Henk Uijterwaal                           Email: 
> henk.uijterwaal(at)ripe.net
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>
>
> 1160438400. Watch this space...
>
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Anura P. Jayasumana <http://www.engr.colostate.edu/%7Eanura>
Professor, Electrical & Computer Engineering
                           and Computer Science
Department of Electrical and Computer Engineering
Colorado State University
Fort Collins, CO 80523
Phone: (970) 491-7855
Fax: (970) 491-2249
Email: <Anura.Jayasumana@Colostate.edu>