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

From: "Fardid, Reza" <[email-masked]>
Date: Wed, 22 Nov 2006 16:42:17 -0800

Hi All,

My comments are inline.

Regards,

-Reza

-----Original Message-----
From: Al Morton [mailto:[email-masked]] 
Sent: Tuesday, November 21, 2006 12:39 PM
To: Fardid, Reza
Cc: [email-masked]
Subject: Re: [ippm] Comments on
draft-ietf-ippm-spatial-composition-02.txt

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

...
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"
.

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.

>> Yes, I agree. Pardon the choice of words to describe the result of
CompEd.

* 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.

>> In either case, whether Ep_i is "undefined" or Ep_i = 1 for subpath
i, CompEp over the 

>> other (remaining) subpaths is defined, measurable, and of interest. 

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.

>> I suggest:

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

>> to report a value, then CompEp is only defined over the other
subpaths 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.

>> If anything, only for completeness. The Connectivity Metric over

>> individual subpaths is relevant in the context of the above CompEp

>> exception by making it a precondition to Ep_i measurement.

>> However, it can be argued that using it in this context is out 

>> of the scope of this draft and Composed metrics.

Al

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From: Al Morton <[email-masked]>
Date: Mon, 27 Nov 2006 18:26:55 -0500

At 07:42 PM 11/22/2006, Fardid, Reza wrote:

Al wrote:

* 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

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

We definitely need to mention the possibility of failed
measurements.

>> In either
case, whether Ep_i is “undefined” or Ep_i = 1 for subpath i, CompEp over
the

>> other
(remaining) subpaths is defined, measurable, and of interest.

But the CompEp that results from the composition of

sub-paths A, B, and C will be undefined if the Ep

for any of the subpaths is undefined.  Right?

Sub-Paths

++   A
++ ++   B   ++ ++   C   ++

Src ||.......|| ||.......|| ||.......|| Dst

++       ++
++       ++
++       ++

++
++

Src ||...............................|| Dst

++        Composed
Metric        ++

I think we want to concentrate on the composed metric for

one specific path.  If other composed metrics don't use

the sub-path whose results are undefined, they don't suffer from

this issue.

Something like:

>> I
suggest:

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

>> to report a
value, then CompEp is only defined over the other subpaths for
that

>> measurement
interval.

This might be more clear:

If any Ep_i is undefined in a particular measurement interval,

possibly because a measurement system failed to report a value,

then any CompEp that uses subpath i for that measurement interval

is undefined.

Al