Re: Uniform Probabilistic Sampling in PSAMP-PROTO

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

Hi Benoit,

Sorry for the late reply.
Basically, I agree with your proposal.
Please find comments in line.

--On 07.12.2005 18:29 Uhr +0100 Benoit Claise wrote:

> Dear all,
>
> We're trying to model "Uniform Probabilistic Sampling" in [PSAMP-PROTO].
>
> [PSAMP-MIB] specifies:
>
> 5.2.5. Uniform Probabilistic Sampling
>
> Capability objects are not specified for the uniform probabilistic
> sampling method. It has only one parameter in the
> psampSampUniProbParamSetTable, the psampSampUniProbProbability. This
> object gives the probability that a packet is sampled. The
> probability is equal for every packet. The given value must be
> divided by 4294967295 (=2^32-1), so a value of 0 means no packet is
> sampled (probability is 0) and a value of 4294967295 means every
> packet is sampled (probability is 1).
>
> psampSampUniProbProbability OBJECT-TYPE
> SYNTAX Unsigned32 (0..4294967295)
> MAX-ACCESS read-create
> STATUS current
> DESCRIPTION
> "This object gives the probability that a packet is sampled.
> The probability is equal for every packet. The given value
> must be divided by 4294967295 (=2^32-1), so a value of 0
> means no packet is sampled (probability is 0) and a value of
> 4294967295 means every packet is sampled (probability is
> 1)."
>
>
> First of all, I've got an issue with the psampSampUniProbProbability MIB. How do we represent a probability of 1/2 as this number not even?
> Should we use a different number (1000000000), specifying a range in the SYNTAX. I think so!

I support this proposal.  If we follow it, setting and reading
the probability would become less error-prone.  From reading the
value you can much easier identify a probalility of 0.5: 500000000
or 0.01: 10000000.  Still the problem of counting the zeros
remains as problem.  The disadvantage is less precision.  But I
think the remaining precision of 1 of a billion is still sufficient.

> 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.
>
> What to do now?
>
> Solution 1:
> We export the probability with a float, and we approximate this value with the MIB variable.

Approximation is not an issue, because the precision of the Integer
in the MIB would be greater than the precision of a float32 by several
orders of magnitude.  So, we do not loose information by converting a
float32 to a 32 bit integer (in this case).
Approximation would be an issue if we chose a float64.

> 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?

I am fine with Solution 1 as well as with Solution 2.
Are there any other opinions?

> 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].

The question is how much precision we need here.
In general, I think that single precision is sufficient.

Thanks,

   Juergen

> Regards, Benoit.



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