Useful TCL script?

James Hess mysidia at gmail.com
Sun May 23 18:13:49 CDT 2010


On Sun, May 23, 2010 at 5:16 PM, Christopher Gatlin
<chris at travelingtech.net> wrote:
> That is a stellar TCL script!
> I generally use netflow to glean information regarding average packet size.

Seems like a good script to me.  My only criticism would be pretty
hard to do anything about...   you're averaging an average over a
longer period of time than the underlying data that computes the
average.  dividing 5 minute average databytes approximation  by  5
minute  average  packets  approximation   is probably not a very
reliable estimate for   5 minute average packet size.

Even ignoring roundoff errors, an amount of error is introduced by
averaging  less precise samples, than the device should have access to
  (if it could compute that rolling average itself using 1 minute or
higher resolution data)
The less stable the packet size, the more error your approximation should have.

Example of 5 minute average  over    5    60-second   bps divided by
pps samples...
Versus  taking the two  5 minute average rates and dividing them.

                 60sec average after
           1min      2min    3min    4min     5min     |  Actual 5
minute average rate
                                                       |
bps        100000    200000  300000  400000   850000   |   370000
pps        600       300     900     1000     3620     |   1284
bps/pps    166.67    666.67  333.33  400      234.80   |   360.294




BPS / PPS   that would be computed by     370000  / 1284   =     288.16 bits
When 360.294  bits should be the answer, is an error of     72.134
bits  (or  9 bytes  out of 45 bytes),

Representing an introduced error of   ~20%


--
-J




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