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Re: Uniform Probabilistic Sampling in PSAMP-PROTO

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.



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

regards

Andrew

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