Subject: [ippm] Comments on draft-ietf-ippm-spatial-composition-02.txt

From: "Fardid, Reza" <[email-masked]>
Date: Fri, 17 Nov 2006 15:38:59 -0800

Al,

I have one small comment about the latest draft:

5.1.5 Composition Function

If Ep_i = 1 for any i, then CompEp = 1, e.g., when active measurements
over subpath i cannot be made or reported for a duration of time.

This degenerate, yet possible, case renders the resulting CompEp
useless.

This case can either be included in 5.1.8 as a Source of Error, or
excluded from CompEp by adding a statement after its composition
function is defined:  "CompEp is only defined over Ep_i != 1 for all i"
.

Regards,

Reza

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From: Al Morton <[email-masked]>
Date: Tue, 21 Nov 2006 15:39:13 -0500

At 06:38 PM 11/17/2006, Fardid, Reza wrote:

...

If Ep_i = 1 for any i, then CompEp =
1, e.g., when active measurements over subpath i cannot be made or
reported for a duration of time.

This degenerate, yet possible, case
renders the resulting CompEp useless.

This case can either be included in
5.1.8 as a Source of Error, or excluded from CompEp by adding a statement
after its composition function is defined:  “CompEp is only defined
over Ep_i != 1 for all i” .

Hi Reza,

Thanks for your comment on the 02 version of the draft.

I think that you've actually raised several points:

* In theory, Ep_i could equal 1 on subpath i.  In that case, I don't

think the equation for CompEp is rendered useless.  A result of
CompEp = 1

means that the Complete Path does not support packet transfer,

because one or more of the subpaths does not have connectivity.

* In practice, when measurements cannot be initiated on a subpath

(and perhaps the measurement system gives up during the test
interval),

there won't be a Ep_i for subpath i reported, it should be
designated

as "undefined".  I think we should distinguish this from
the case

where the measurement system continued to send packets throughout

the test interval, but all were declared lost (as in the first
bullet).

We definitely need to mention the possibility of failed
measurements.

Something like:

"If any Ep_i is undefined, possibly because a measurement system
failed

to report a value, then CompEp is undefined for that measurement
interval.

* Should we include the Connectivity Metric within the scope of

this draft, and examine whether it makes sense to use a

Composed Connectivity Metric?  I don't know if this would see a

lot of use, but perhaps we should have it for completeness.

Al