FW: Comments on sFlow RFC 3176

From: Sonia Panchen (sonia_panchen@inmon.com)
Date: 04/25/02

  • Next message: Sonia Panchen: "RE: Comments on sFlow RFC 3176"

    Tanja Zseby from the Fraunhofer Institute FOKUS/Global Networking made some
    good comments about sFlow as described in RFC 3176 which may be of broader

    -----Original Message-----
    From: Sonia Panchen [mailto:sonia_panchen@inmon.com]
    Sent: Tuesday, April 23, 2002 1:17 PM
    To: Tanja Zseby
    Cc: quittek@ccrle.nec.de; zander@fokus.gmd.de; carle@fokus.gmd.de
    Subject: RE: Survey of traffic flow measurements at saint2002

    Thank you very much for taking the time to reply. Your comments are
    valuable, and we will include the feedback in the next documents.
    I have made some comments in-line below.
    Would it be OK to post this to the discussion group on www.sflow.org? I
    think some of this discussion may be useful to others too.


    > -----Original Message-----
    > From: Tanja Zseby [mailto:zseby@fokus.gmd.de]
    > Sent: Tuesday, April 23, 2002 8:56 AM
    > To: Sonia Panchen
    > Cc: quittek@ccrle.nec.de; zander@fokus.gmd.de; carle@fokus.gmd.de
    > Subject: Re: Survey of traffic flow measurements at saint2002
    > Hi Sonia,
    > I have some comments on RFC3176:
    > - formula (1): I always associated with the term "sampling rate" the
    > sampling probability p (e.g. 1/10) that means sampled_packets to
    > total_packets. I think in the sFlowBilling paper you use it in th same
    > way. But in RFC 3176 it is total_packets/sampled_packets. This I found
    > confusing.
    This is a good point. We at least should be consistent!

    > - section 2.1.2: it is mentioned that a variation of +/- 10% of the mean
    > value is sufficient. But it is not mentioned for what it is sufficient.
    > I assume it is meant that you get a sufficiently accuracte prediction of
    > the error ? Or do you refer to certain apllications ?
    We found through simulation, experiments, and statistical analysis that the
    results we obtained from processing the samples (ie scaling as described in
    the sFlowBilling paper) did not vary significantly provided that the random
    number generation resulted in values within +/-10% of the mean.

    > - section 2.2.: was hard to understand for me. It was not clear to me
    > what exactly you sample here and what parameter you want to estimate. I
    > understand that you are polling the IF counters regularily. Does the
    > sampling process select which counters you read or at which time
    > intervals ?
    I see your point. The "sampling" of network interface statistics is really
    local polling or time-based sampling of the interface counters.
    This is probably more understandable when read in conjunction with the
    specifics of the MIB and datagram. For example the MIB
    (sFlowCounterSamplingInterval) allows the polling interval to be set and the
    datagram (if_counters) defines the counters which are read.

    > - 3.2 (sflow MIB) sampling rate definition: see comment above.
    > Furthermore the sentence "a sampling rate of 0 disables sampling" could
    > be confusing. Disabling sampling sounds to me like capturing all packets
    > but I guess it is meant that no packet is selected for the
    > sample, correct ?
    This is also a good point, and I could see how it might be confusing.
    But you are correct. Disabling sampling was meant to mean no packets are

    > Hope this was some constructive input.
    This was very constructive - thank you. It is always good to get a different

    The RFC was written in the form of an implementation guide or ERS/interface
    specification. I think you point out the value of a more narrative overview

    > Regards
    > Tanja
    > --
    > Dipl.-Ing. Tanja Zseby
    > FhI FOKUS/Global Networking Email: zseby@fokus.fhg.de
    > Kaiserin-Augusta-Allee 31 Phone:
    > +49-30-3463-7153
    > D-10589 Berlin, Germany Fax:
    > +49-30-3463-8153
    > ------------------------------------------------------------------
    > --------------------
    > "Living on earth is expensive but it includes a free trip around
    > the sun." (Anonymous)
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