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*To*: Benoit Claise <bclaise@cisco.com>, psamp <psamp@ops.ietf.org>*Subject*: Re: Uniform Probabilistic Sampling in PSAMP-PROTO*From*: Juergen Quittek <quittek@netlab.nec.de>*Date*: Wed, 21 Dec 2005 09:34:00 +0100*In-reply-to*: <43971C0C.7080101@cisco.com>*References*: <43971C0C.7080101@cisco.com>

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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**References**:**Uniform Probabilistic Sampling in PSAMP-PROTO***From:*Benoit Claise <bclaise@cisco.com>

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