Re: Uniform Probabilistic Sampling in PSAMP-PROTO

[Juergen Quittek <quittek@netlab.nec.de>]

Hi Andrew,

--On 08.12.2005 16:53 Uhr +0000 Andrew Johnson wrote:

> Benoit Claise wrote:
> [SNIP]
> > Initially we wanted to model the probability in [PSAMP-PROTO] with a
> > float, which is allowed by [IPFIX-PROTO].
> > However, we've got the issue that SMIv2 doesn't support floats.
>
> Although the floating point type is not a base type in SMI there are
> a couple of proposals about how to send them.  This extract is from
> the comp.protocols.snmp SNMP FAQ:
>
>   http://www.faqs.org/faqs/snmp-faq/part2/
>   ===============================================================
>   2.40.04
>   SUBJECT: Floating Point Numbers in SMI?
>
>   You cannot add new base types to the SMI.
>
>   For now, the easiest way to have floating point numbers
>   in SNMP MIBs is to use the base type OCTET STRING and
>   encode the value in ASCII.
>
>   This is not the most elegant approach. However, it will work
>   between your agent and your management application and it will
>   be compliant to the SNMP SMI and protocol specifications.
>
>   David Perkins
>   ===============================================================
>
> Apparently this method is implemented in the NET-SNMP agent and
> manager.
>   (http://ops.ietf.org/lists/mreview/mreview.2004/msg00083.html)
>
> Whether we use the above suggested method or define a new way of
> using unsigned32 the application will have to have some way of
> converting that number before it can be used.

This would be Solution 4.
I still prefer Solution 1 or Solution 2.

> > What to do now?
> >
> > _Solution 1: _
> > We export the probability with a float, and we approximate this value
> > with the MIB variable.
> >
> > _Solution 2: _
> > We export the probability with an unsigned32, exactly the same content
> > as the MIB variable psampSampUniProbProbability
> >
> > _Solution 3: _
> > We export the probability with two values N, M.
> >     This means 2 inter-dependent I.E.s and 2 MIB variables instead of one.
> >     I don't like it too much
> >
> > I'm clearly in favor of solution 1. It's not right that we would limit
> > IPFIX because of the limitations of SMIv2
> > Feedback?
> >
> > Side question: if we go for the float solution, should we have a
> > float64? This would give us more precision
> > Note: not yet defined in [IPFIX-PROTO].
>
> FYI:
> The 32-bit float has 24 bits of precision, i.e. roughly +/-0.000006%.

It is 23 bits we are using. Precision would be    roughly +/-0.00001%
The Unsigned as Benoit suggests it would have     roughly +/-0.0000001%

> The 64-bit float has 53 bits of precision, i.e. roughly +/-0.00000000000001%
> regards

Thanks,

   Juergen

> Andrew
>
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