Subject: [ippm] comments on draft-morton-ippm-composition-00.txt

From: Maurizio Molina <[email-masked]>
Date: Tue, 09 Aug 2005 15:52:56 +0100

Hi Al,
some comments on your draft:

- sec 3.1:
...This means that the complete path metric may be composed from:

    +  the same metric for each sub-path

    +  multiple metrics for each sub-path (possibly one that is the same

      as the complete path metric)

    +  a single sub-path metrics that is different from the complete

      path metric

=>Comment: I understand the first example, not the other two. Shouldn't
we stick, for simplicity, to the first case?

- sec. 3.2:
For each metric, the applicable circumstances are defined, in terms of
whether the composition:
Requires the same test packets to traverse all sub-paths, or may use
similar packets sent and collected separately in each sub-path.

=> Comment: I think the sentence brings some confusion, mixing
de-composition and composition. The draft should focus on composition
only, and should say it explicitly. Also, by mentioning test packets you
seem to limit to active measurements, whereas in other parts of the
draft you leave the door open to passive measurement as well.

- Sec. 4:
One-way Delay Composition Metrics and Statistics

=> Comment: Shouldn’t it read “Composition of One-way Delay Metrics and
Statistics”?

- Sec. 4.1.8
"...Therefore, the sub-path measurements may differ   from the performance experienced by packets on the complete path. Measurements employing sufficient sub-path address pairs mightproduce bounds on the extent of this error."

=> Comment: Not sure what you mean by “sufficient sub-path address
pairs". I think this is however a delicate point and Lei's concerns, if
I understood well, were also about this. Could you clarify?

Thanks,
Maurizio

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---

From: Al Morton <[email-masked]>
Date: Tue, 16 Aug 2005 15:52:56 -0400

Hi Maurizio,  Thanks for your comments, see responses below.
(sorry for the delay, I was on vacation last week...)

>we stick, for simplicity, to the first case?

I don't see a strong reason to restrict to only the first case.
I know of several methods for composition of Delay Variation
that fall into the 2nd and 3rd categories.

>draft you leave the door open to passive measurement as well.

Perhaps the text could be clarified, because I didn't mean to include
the idea of de-composition in this requirement. I only wanted to
include the possibility that some metrics may have more the stringent
single-source-packet requirement (like the multi-metrics draft describes).
I think that passive measurement is attractive to many,
so I'll revise the wording (remove "test").

>=> Comment: Shouldn't it read "Composition of One-way Delay Metrics and
>Statistics"?

OK, let's try that.

>- Sec. 4.1.8
>"...Therefore, the sub-path measurements may differ   from the performance 
>experienced by packets on the complete path. Measurements employing 
>sufficient sub-path address pairs mightproduce bounds on the extent of 
>this error."
>
>=> Comment: Not sure what you mean by "sufficient sub-path address

This is really a general concern for all spatial compositions.
Network and equipment designs sometimes employ parallel links,
processors, etc., and an address hashing function may determine what exact
path a particular packet will take. See the draft below for more detail:
http://www.ietf.cnri.reston.va.us/internet-drafts/draft-ietf-bmwg-hash-stuffing-03.txt

So, measurements that terminate within a sub-path will necessarily
use different source and destination addresses from those used
by packets that traverse the complete path, and possibly traverse the sub-path
in different ways. However, multiple tests (using different address pairs)
tend to cover more possible alternatives within a given sub-path.

hope this helps,
Al