Subject: RE: [ippm] Composition of Metrics

From: "STEPHAN Emile RD-CORE-LAN" <[email-masked]>
Date: Thu, 2 Jun 2005 14:08:53 +0200

Dear Maurizio

2 years ago I tried to introduce this point in the IPPM WG, see http://www.watersprings.org/pub/id/draft-stephan-ippm-spatial-metrics-01.txt. This ID defines metrics for spatial composition and spatial decomposition. This point was presented and discussed in the IPPM WG. 
My understanding of the IPPM WG conclusion is that it is quite tricky to define composition metrics respectful of the RFC2330 because it is possible to prove that such composition is not deterministic.

Consequently Lei and I are focusing on the definition of spatial metrics to decompose end-to-end measures and one-to-group metrics in http://www.ietf.org/internet-drafts/draft-stephan-ippm-multimetrics-00.txt. 

Regards
Emile

> -----Message d'origine-----
> De : [email-masked] [mailto:[email-masked]] De la part de
> Maurizio Molina
> Envoyé : mercredi 1 juin 2005 20:47
> À : [email-masked]
> Objet : [ippm] Composition of Metrics
> 
> Dear all,
> In RFC 2330 (Framework for IP Performance Metrics), the Spatial and
> temporal composition of metrics is defined (see extract below).
> However, in none of the RFCs defining the metrics the problem appears to
> be tackled. Could somebody clarify me if there is a reason why this
> piece of work has not been undertaken?
> Thanks,
> Maurizio
> 
> 
> 9.1. Spatial Composition of Metrics
> .....
>    When the definition of a metric includes a conjecture that the metric
>    across the path is related to the metric across the subpaths of the
>    path, that conjecture constitutes a claim that the metric exhibits
>    spatial composition.  The definition should then include:
>  +    the specific conjecture applied to the metric,
>  +    a justification of the practical utility of the composition in
>       terms of making accurate measurements of the metric on the path,
>  +    a justification of the usefulness of the composition in terms of
>       making analysis of the path using A-frame concepts more effective,
>       and
>  +    an analysis of how the conjecture could be incorrect.
> 
> 9.2. Temporal Composition of Formal Models and Empirical Metrics
> 
> ......
> 
>    When the definition of a metric includes a conjecture that the metric
>    across the path at a given time T is related to the metric across the
>    path for a set of other times, that conjecture constitutes a claim
>    that the metric exhibits temporal composition.  The definition should
>    then include:
> 
>  +    the specific conjecture applied to the metric,
>  +    a justification of the practical utility of the composition in
>       terms of making accurate measurements of the metric on the path,
>       and
>  +    a justification of the usefulness of the composition in terms of
>       making analysis of the path using A-frame concepts more effective.
> 
> 
> 
> 
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---

From: Maurizio Molina <[email-masked]>
Date: Thu, 09 Jun 2005 17:21:14 +0100

Hi Emile,

I compared your two drafts, and summarizing I would say (correct me if
I’m wrong) that:

1) Type-P-METRIC-XX-trajectory in the old draft corresponds to the
Type-P-METRIC-stream in the new one.

2) The new draft contains the one-to-group metrics

3) The asynchronous metrics were ruled out.

Regarding the last point, I understand that if you have a set of hosts
<H0, H1, …Hn> along a path and a set of delays <dT1, dT2, …dTn> obtained
by *different* probes (thus asynchronous...) is too naïve to say that
the delay’s sum is a good representation of the singleton one-way-delay
on the path. The path delay and its partitioning in the different
portions could of course be obtained if we had a trajectory metric,
which is a good thing but is not given by any currently widely deployed
measurement protocol.

So, it looks like the IPPM WG preferred just to prepare the container
for something that it is still to come and not to say anything about how
to compose existing metric. What I think would be useful is giving
guidelines about the composition of metric *statistics* (e.g. the mean
-> trivial - the variance or the percentiles -> not trivial because the
independence on the several path portions comes into play). I know that
testing such an independence hypothesis is difficult or impossible, is
it a reason for not saying anything about how to compose metric
statistics (which is something that operators would like to do)?.
Furthermore, in the fourth bullet point of RFC 2330 there was exactly
the statement that the specification of the composition would come along
with some reasoning about its correctness (see below).
Apologies if I'm raising a point that has already been discussed and
buried in te WG. In this case, I'd appreciate pointers to mail archives,
etc.
Regards,
Maurizio