Useful TCL script?

Thomas Magill tmagill at providecommerce.com
Sun May 23 19:26:31 CDT 2010


It was more of a quick and dirty project and a reason for me to learn
some TCL.  I wasn't aware of any way to get the value from snmp or any
cli command so I pulled the info I did have (show int) and worked with
that.  As I try to improve my TCL skills, I will probably work on this.
In fact, I have already updated it somewhat but do not have the final
r2.  This was basically a random fun project for me and thought some
people may find value in the (semi) finished product.  Thanks to
everyone for the input though.  It gives me things to try to figure out.
And I am definitely no match major so the margin of error was really the
last thing on my mind...  I was just automating what one of my team was
doing manually.

As far as the comments of using netflow, I usually agree but we have the
solarwinds netflow product which I am not a fan of at all and find it
hard to get the useful data I want.  Other products I have used allow
such better ability to drill in to data but solarwinds has let me down
in the netflow arena.

-----Original Message-----
From: James Hess [mailto:mysidia at gmail.com] 
Sent: Sunday, May 23, 2010 4:14 PM
To: Christopher Gatlin
Cc: Thomas Magill; nanog at nanog.org
Subject: Re: Useful TCL script?

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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