Subject: PROTO-11: Accuracy Report Interpretation

From: Benoit Claise <[email-masked]>
Date: Thu, 02 Mar 2006 14:56:15 +0100

Dear all,

We need a way of specifying the accuracy of our reported information, as 
PSAMP frameworks specifies: "the Report Stream must include information 
that enables the accuracy of measurements to be determined.". Here is 
some proposed text.

_Section 7.4.4 Accuracy Report Interpretation_

In order for the Collecting Process to determine the inherent accuracy 
of the reported quantities (for example timestamps), the PSAMP Device 
SHOULD send an Accuracy Report Interpretation. 

The Accuracy Report Interpretation MUST be exported by an Option 
Template Record with a double scope that contains the TemplateId 
[PSAMP-INFO] and the Information Element for which the accuracy is 
required.  The accuracy SHOULD be reported either with the fixedAccuracy 
Information Element [PSAMP-INFO], or with the proportionalAccuracy 
Information Element [PSAMP-INFO].

Accuracy Report Interpretation using the fixedAccuracy Information Element
 Scope:            TemplateId
                        InformationElementId
 Non-scope:     fixedAccuracy

Accuracy Report Interpretation using the proportionalAccuracy 
Information Element
 Scope:            TemplateId
                        InformationElementId
 Non-scope:     proportionalAccuracy

For example, the accuracy of an Information Element whose Abstract Data 
Type is dateTimeMilliSeconds [IPFIX-INFO], for which the unit is 
specified as milliseconds, can be specified with the fixedAccuracy 
Information Element with the milliseconds units. In this case, the 
accuracy is the Information Element value +/- the value reported in the 
fixedAccuracy.

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 proportionalAccuracy Information 
Element with the octet units. In this case, the accuracy is the 
Information Element value +/- the value reported in the 
proportionalAccuracy time Information Element value.

Alternatively to reporting either the fixedAccuracy Information Element 
or the proportionalAccuracy 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.

If the accuracy of a reported quantity changes on the Metering Process, 
a new Accuracy Report Interpretation MUST be generated. The Collecting 
Process MUST keep the accuracy of the latest Accuracy Report Interpretation.

Example 1 (to be developed)
Records (using template 12):
1. Scope: TemplateId = 5, InformationElementId = 152
Non-scope: fixedAccuracy = 2
=> we have a accuracy of +/- 2ms)

Example 2 (to be developed):
1. Scope: TemplateId = 5, InformationElementId = 152
Non-scope: proportionalAccuracy = 0.005
=> we had an accuracy of  +/- 0.5%

---------------------
For the PSAMP information model:

fixedAccuracy

       Description:
          Describes the accuracy of an Information Element, as a negative
          and positive relative error in absolute value.
       Abstract Data Type: float64
       ElementId: 320
       Status: current
       Units: The units of the Information Element for which is accuracy is
          specified.

proportionalAccuracy

    Description:
          Describes the accuracy of an Information Element as a negative
          and positive relative error, expressed as a value between 0 and 1.
       Abstract Data Type: float64
       ElementId: 321
       Status: current
       Units: None

 informationElementID

     Description:
          Contains the ID of another Information Element.
       Abstract Data Type: unsigned16
       Data Type Semantics: identifier
       ElementId: 303
       Status: current
       Units: None

Regards, Benoit.

---

From: Andrew Johnson <[email-masked]>
Date: Thu, 02 Mar 2006 16:07:22 +0000

One comment inline:

Benoit Claise wrote:
> required.

If we make the templateId optional then you can have a mechanism for
setting the accuracy for all uses of an element.  Is this useful /
desirable?  It seems to me it is, since the measuring accuracy on
a router (or measurement device) will generally be fixed.

Andrew

>  The accuracy SHOULD be reported either with the fixedAccuracy 

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---

From: Benoit Claise <[email-masked]>
Date: Thu, 02 Mar 2006 23:22:44 +0100

Andrew Johnson wrote:
> One comment inline:
>
> Benoit Claise wrote:
>> Dear all,
>>
>> We need a way of specifying the accuracy of our reported information, 
>> as PSAMP frameworks specifies: "the Report Stream must include 
>> information that enables the accuracy of measurements to be 
>> determined.". Here is some proposed text.
>>
>> In order for the Collecting Process to determine the inherent 
>> accuracy of the reported quantities (for example timestamps), the 
>> PSAMP Device SHOULD send an Accuracy Report Interpretation.
Good point.

"The Accuracy Report Interpretation MUST be exported by an Option 
Template Record with a double scope that contains the TemplateId 
[PSAMP-INFO] and the Information Element for which the accuracy is 
required. "
becomes
"The Accuracy Report Interpretation MUST be exported by an Option 
Template Record with a scope that contains the Information Element for 
which the accuracy is required. In case the accuracy is specific to a 
template, a second scope containing the templateId value MUST be added 
to the Option Template Record."

Regards, Benoit.
>> For example, the accuracy of an Information Element whose Abstract 
>> Data Type is dateTimeMilliSeconds [IPFIX-INFO], for which the unit is 
>> accuracy is the Information Element value +/- the value reported in 
>> the fixedAccuracy.
>>
>> Alternatively to reporting either the fixedAccuracy Information 
>> Element or the proportionalAccuracy 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.
>>
>> If the accuracy of a reported quantity changes on the Metering 
>> Process, a new Accuracy Report Interpretation MUST be generated. The 
>> Collecting Process MUST keep the accuracy of the latest Accuracy 
>> Report Interpretation.
>>

---

From: Juergen Quittek <[email-masked]>
Date: Fri, 03 Mar 2006 01:50:45 +0100

Benoit and Andrew,

--On 3/2/06 11:22 PM +0100 Benoit Claise wrote:
> "The Accuracy Report Interpretation MUST be exported by an Option Template Record with a double scope that contains the TemplateId [PSAMP-INFO] and the Information Element for which the accuracy is required. "
> becomes
> "The Accuracy Report Interpretation MUST be exported by an Option Template Record with a scope that contains the Information Element for which the accuracy is required. In case the accuracy is specific to a template, a second scope containing the
> templateId value MUST be added to the Option Template Record."
>
> Regards, Benoit.
>>
>> Andrew
>>
>>

The example does not (yet) show how the unit (ms) is determined.

>>> => we had an accuracy of  +/- 0.5%

What about

Description
  Specifies the maximum possible difference (positive or negative) between
  the reported value and the actual value for a given Information Element. ?

>>>           and positive relative error, expressed as a value between 0 and 1.

What about

Description
  Specifies the maximum possible ratio (positive or negative) between the
  error (difference between reported value and the actual value) and the
  actual reported value for a given Information Element. ?

Thanks,

    Juergen