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

From: "Philip F. Chimento Jr." <[email-masked]>
Date: Mon, 20 Mar 2006 19:03:53 -0500

Hi Al & Steven:
I have some comments on the Framework draft that I thought I would  
post to the mailing list. I hope they are useful.

Section 1.1. Motivation: I think that this section needs a lot of  
work. Possibly I am missing the point of the draft, but it seems to  
me that the two key issues here are:
1) It is hard to combine metrics from separate measurements because  
of statistical dependencies. Independence becomes an issue when you  
go beyond first moments.
2) (The following was alluded to when discussing scalability) There  
is the issue of sampling. It is true that you cannot measure every  
end-to-end flow in a large network, but what do specific measurements  
tell you about the whole? In other words, how do you devise a  
sampling plan that (correctly) tells you something about how your  
network is behaving without collecting terabytes of data?

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.

Section 2: What is meant by metrics having a "deterministic  
relationship" ?

Section 3.1: Why would you specifically rule out forecasting ?

Section 3.2: I think this section needs some work.
    - What exactly is the weighted metric that you give as an example  
telling me?
    - There is no figure (one is referred to in the text)
    - To my knowledge, BGP doesn't advertise measurement information  
as suggested here.
    - In the last few sentences, I got worried: It doesn't seem right  
to me to say "X" is the max OWD when composed of spatial measurements  
taken at different times. If you define this spatial composition, you  
have to define it on exactly the same time intervals.

  Section 3.3: What if the end-to-end paths (to take your example)  
share one or more links? I think that non-overlapping is too much to  
expect (or doesn't yield metrics that you really want).

  Section 3.5: Do you have any examples of higher order composition?  
When is it useful?

  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.

Feel free to tell me that I have missed the point....

Regards,
Phil Chimento

_______________________________________________
ippm mailing list
[email-masked] 
https://www1.ietf.org/mailman/listinfo/ippm

---

From: Henk Uijterwaal <[email-masked]>
Date: Tue, 21 Mar 2006 16:31:49 +0100

Phil,

>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.

>Putting these together in a straightforward way doesn't give you the

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...

---

From: "Philip F. Chimento Jr." <[email-masked]>
Date: Tue, 21 Mar 2006 10:54:56 -0500

Hi Henk:

> me information about the entire network.
Sure. But the document seems to be trying to say more than that.  
Maybe I am reading too much into it...

> I agree but can't we just say so?

Of course. I think that my point is that you need to say it.

Regards,
Phil Chimento

---

From: Anura Jayasumana <[email-masked]>
Date: Tue, 21 Mar 2006 10:31:44 -0700

>> Putting these together in a straightforward way doesn't give you the

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

> [email-masked] https://www1.ietf.org/mailman/listinfo/ippm

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: <[email-masked]>
_______________________________________________
ippm mailing list
[email-masked] 
https://www1.ietf.org/mailman/listinfo/ippm

---

From: "Philip F. Chimento Jr." <[email-masked]>
Date: Tue, 21 Mar 2006 12:40:46 -0500

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:

> 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.
> Anura P. Jayasumana

---

From: Al Morton <[email-masked]>
Date: Tue, 21 Mar 2006 18:28:57 -0500

Hi Phil,

thanks for your careful review and comments.

catching-up one at a time...

>Section 2: What is meant by metrics having a "deterministic

There is a function defined that produces the "composed" metric.
I suppose I could have called this a composition function,
instead. Would that change help?

good to see you here in Dallas,
Al

---

From: "Philip F. Chimento Jr." <[email-masked]>
Date: Wed, 22 Mar 2006 09:51:09 -0500

Hi Al:

Yes, I think calling it a function would help clarify.

and thanks, same here.

Regards,
Phil Chimento