Re: [PSAMP] PSAMP-INFO IE realtiveError

[Paul Aitken <[e-mail]>]

Tanja,

> Hi Benoit and Paul,
> 
>  
> 
> here my suggestions for clarification of the error IEs in PSAMP-INFO.
> 
> -    I suggest to rename the fixedError to absoluteError

Agreed.


> -    I suggest to introduce CI limits and level to also report 
> estimation errors
> 
> -    If it is still possible I would suggest to make a few small changes 
> in PSAMP-PROTO for consistency.
> 
> -    Upper and lower CI limits can be also specified as provided 
> absolute or relative limits. So we could also add 2 more IEs (for the 
> relative CI limits)
> 
>  
> 
> New description of IEs:
> 
>  
> 
> absoluteError
> 
> This Information Element specifies the maximum possible measurement 
> error of the reported value for a given Information Element. The 

We should indicate how to connect the *Error values to specific fields, 
eg by using an option with the specific field as scope. Else, someone 
may put the *Error elements adjacent to the relative fields in the data 
record - which could work, but is open to misinterpretation.


> absoluteError has the same unit as the information element it is 
> associated to. The real value of the metric can differ by absoluteError 

"with" ------^^

> (positive or negative) from the measured value. This information element 
> provides only the error for measured values. If an information element 
> contains an estimated values (from sampling) the confidence boundaries 
> and confidence level have to be provided instead.

I would name the IEs: "the confidence boundaries and confidence level 
have to be provided instead (with the upperCILimit, lowerCILimit and 
confidenceLevel).


> relativeError
> 
> This Information Element specifies the maximum possible measurement 
> error of the reported value for a given Information Element as 
> percentage of the measured value. The real value of the metric can 
> differ by relativeError percent (positive or negative) from the measured 
> value. This information element provides only the error for measured 
> values. If an information element contains an estimated values (from 
> sampling) the confidence boundaries and confidence level have to be 
> provided instead.

Again, as above, I would specifically name the IEs for this.


> upperCILimit
> 
> This Information Element specifies the upper limit of a confidence 
> interval. It is used to provide an accuracy statement for an estimated 
> value. The confidence limits define the range in which the real value is 
> assumed to be with a certain probability p. Confidence limits always 
> need to be associated with a confidence level that defines this 
> probability p. Please note that a confidence interval only provides a 
> probability that the real values lies within the limits. That means the 
> real value can lie outside the confidence limits.
>
>
>
> lowerCILimit
> 
> This Information Element specifies the lower limit of a confidence 
> interval. For further information see the description of upperCILimit.
> 
>  
> 
> confidenceLevel
> 
> This Information Element specifies the confidence level. It is used to 
> provide an accuracy statement for estimated values. The confidence level 
> provides the probability p with which the real value lies within a given 
> range. A confidence level always needs to be associated with confidence 
> limits that define the range in which the real value is assumed to be.

We should specify that upperCILimit, lowerCILimit and confidenceLevel 
are all required, and what to do if too few of them are provided.


> Changes to PSAMP-PROTO if still possible:
> 
>  
> 
> -    Rename fixedError to absoluteError
> 
> -    Slightly modify paragraph 2
> 
> OLD:
> 
> ...  The accuracy SHOULD be reported either with the fixedError 
> Information Element [PSAMP-INFO 
> <http://tools.ietf.org/html/draft-ietf-psamp-protocol-09#ref-PSAMP-INFO>], 
> or with the relativeError Information Element [PSAMP-INFO 
> <http://tools.ietf.org/html/draft-ietf-psamp-protocol-09#ref-PSAMP-INFO>].
> 
> NEW:
> 
> ... The accuracy for a measured information elelment SHOULD be reported 
> either with the fixedError Information Element [PSAMP-INFO 
> <http://tools.ietf.org/html/draft-ietf-psamp-protocol-09#ref-PSAMP-INFO>], 
> or with the relativeError Information Element [PSAMP-INFO 
> <http://tools.ietf.org/html/draft-ietf-psamp-protocol-09#ref-PSAMP-INFO>]. 
> The accuracy for an estimated information element (from sampling) SHOULD 
> be reported with confidence limits and confidence level.[PSAMP-INFO]

Agreed. I'd also like to add something indicating how this can be done, 
eg by using an option with the correct scope.


> 
>  
> 
> -    Remove the following paragraph (very important! Otherwise it would 
> lead to confusion):
> 
> For example, the accuracy of an Information Element to estimate the 
> accuracy of a sampled flow, for which the unit would be specified in 
> octets, can be specified with the relativeError Information Element with 
> the octet units.  In this case, the error interval is the Information 
> Element value +/- the value reported in the relativeError times the 
> reported Information Element value.
> 
>  
> 
> -    Avoid the term error interval
> 
> OLD:
> 
> In this case, the error interval is the Information Element value +/- 
> the value reported in the fixedError.
> 
> NEW:
> 
> In this case, the real values lies within the range of the Information 
> Element value +/- the value reported in the absoluteError.
> 
>  
> 
>  
> 
> -    Remove the following paragraph (since absolute or relative error 
> are just different representations I would not gain something if I 
> report both)
> 
> Alternatively to reporting either the fixedError Information Element or 
> the relativeError Information Element in the Accuracy Report 
> Interpretation, both Information Elements MAY be present.  This scenario 
> could help in more complex situations where the system clock drifts, on 
> the top of having its own accuracy, during the duration of a measurement.

The intention was to say that the clock is 5 minutes slow, +/- 10 
seconds - so there's both an absolute error and a relative error.


> Sorry for the late comments, I was quite busy with PSAMP-TECH before...

Thanks for your input!


-- 
Paul Aitken
Cisco Systems Ltd, Edinburgh, Scotland.
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