Calculating bandwidth

[Jack C_1]

Hello all,

I've set up SNMP on linux, which is all fine and dandy. I am trying to write a small app that checks how much bandwidth is going through the interface at any given time. I did it in perl, and it works... but the numbers are wrong!

Basically, I pull ifInOctets from snmp. This is the bytes of data that have come in the interface. But when I take the change in that, over the change in time since the last check, the numbers are not right! Low values seem to be ok, but if I generate A LOT of traffic, then it shoots up to 45 MegaBYTES!!! Obviously impossibly over 100-MBit network.

I also tested it against MRTG. MRTG seems to get the right values. What have I done wrong?

Thanks,

-Jack C
http://www.CREPinc.com

[Steven E. Protter]

Does snmp always return valid values?

I think to help an snmp guru(not me) might need to see your application source code. Could be a simple division error or something.

SEP

[Jack C_1]

That's true. I check pretty closly though...

One thing I do know is that SNMP counters reset after a certain number. I acounted for that in the program.

Perhaps my timeer is not accurate enough?

Thanks,

-Jack C

[Petr Simik_1 · Accepted solution]

try this scripts - it's checking Lan interface traffic.

[Ragu_3]

Another shot, test it with mrtg-rrdtool.
Take random samples across a fixed time period. Can you show me this perl script?
The sampling is not done right.

[Jack C_1]

Yes I will post the script on monday, sorry I don't have it with me right now. Also, I will test the script monday, because unfourtunatly I'm on windows right now.

Thanks guys,

-Jack C

[Jack C_1]

OK. The shell script does not run because I do not have the binary therein. (lanadmin).

My perl script is attatched. Sorry it is not very good, as I am not a real perl person.

Thanks again,
-Jack C
http://www.crepinc.com

[Al Licause]

A while back I wrote a similar script for Tru64unix. I pulled both input bytes and
output bytes. I didn't use snmp but the figures are the same.

I have a feeling that if you are only capturing and calculating bandwidth based on outpackets, you are shortchanging the calculation.

Bandwidth of a particular interface is made up of both input and output traffic.

Try including the ifoutOctets as well as see if this brings you any closer the expected result.

[Al Licause]

Re-reading your original note, I'm wondering if you are forgetting a conversion factor some place ?

I am from the old school of bits/second for throughput numbers so I use the following:

(((ibytes_previous + obytes_previous)*8) - ((ibytes + obytes)*8))/seconds

[Jack C_1]

Well, my data is not total throughput. If you look at the script, you see that I have a separate area for in/out on each interface (except lo). So only IN bandwidth and OUT bandidth are shown. And yes, it is in Bytes Per Second.

Here is a thought. After comparing it to a known level of bandwidth, it seems that I get one low value, then one high. Perhaps, if my timing was not acurate enough, that could happen. Ideas?

Thanks

-Jack C
http://www.crepinc.com