representativeness of flow data based on samples

Wed Jan 30 19:42:49 UTC 2002

### On Wed, 30 Jan 2002 14:02:30 -0500, Joe Abley <jabley at automagic.org>
### casually decided to expound upon nanog at merit.edu the following thoughts
### about "representativeness of flow data based on samples":

JA> For example, if I am trying to rank the top traffic sinks for my
JA> network beyond an attached peer (i.e. an ordinal rather than cardinal
JA> measurement), will I get different answers if I use a sampling rate
JA> of 1:1000 compared to 1:50, given a statistically "long enough"
JA> measurement period?

I suspect that it will just determine the smoothness of your statistics over
the long run which I assume is what you're interested in.  I guess it will
depend on the ballpark expected packet flow.  One might ask the question of
"how close do things seem/need to be?" One has to assume the sampling run
time is bigger than the sampling rate by a certain order of magnitude
because the amount of sampling error can be predicted as the square root of
the number of samples.  So what does a per-sample loss mean to you?  And how
much error can you tolerate?  Figure that out and you can narrow in on an
appropriate sampling period.

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