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

Hi Anura:
You can add means, whether the relevant RVs are dependent or not, so  
average delays add.
Higher order statistics, as you point out, do not add simply.   
Convolution ASSUMES that the RVs are independent.

Regards,
Phil Chimento

On Mar 21, 2006, at 12:31, Anura Jayasumana wrote:

>
>
> 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
>> RIPE Network Coordination Centre          http:// 
>> www.amsterdamned.org/~henk
>> P.O.Box 10096          Singel 258         Phone: +31.20.5354414
>> 1001 EB Amsterdam      1016 AB Amsterdam  Fax: +31.20.5354445
>> The Netherlands        The Netherlands    Mobile: +31.6.55861746
>> --------------------------------------------------------------------- 
>> ---------
>>
>> 1160438400. Watch this space...
>>
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>
> Anura P. Jayasumana
> 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>

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