Polling strategy and Script files.

The goal of this How to is to provide to LoriotPro users more information on the polling process and the polling strategies. 

Before stating a polling campaign, you should know what you want to measure and have an idea of how the SNMP object changes over the time. 

Finding  the SNMP object you want to measure. 

Here we use an example of a network interface. We want to measure at regular interval how much octets go in and out of this interface. 

Get the interface instance from the simple query tool by clicking on one of the interface, you then get the list of interfaces and the ifindex of the interface.

Creating the SCRIPT file 

A script file is a simple text file that contains the list of objects and their index, polled according to the polling interval. The extension of this file are .sci. 

Use the Mib Scripter tool to create your sci file and/or a simple editor. 

In our example we decide to collect the sysname, sysuptime of the equipment and the ifinoctet, ifoutoctet values of a specific network interface. The sysuptime value is required if calculations have to be made over a time period.

 The sysuptime value is available in the mib2.

OID LEN (8) : 1.3.6.1.2.1.1.3.

iso(1).org(3).dod(6).internet(1).mgmt(2).mib-2(1).system(1).sysuptime(3).sysuptimeinstance(0) 

Definition: "The time (in hundredths of a second) since the network management portion of the system las last re-initialized." 

Example of sci file:

Using the Script file

The script file can be used in the the Host properties windows.

In the example below you see the sci file that is loaded

Use the Load polling script to select your script file and check the box Out CSV.
The result will be save in a csv file in the www/log under the the poll_name_date format.

You will have in our example a file generated for each day containing the values of interface in octets and out octets taken each minute. 

Exemple : poll_128.1.1.1__Oct_03_2003.csv

 The data are separated by comma (csv format)

1. The date
2. The timestamp added by LoriotPro
3. The sysname of the host
4. The time since last boot
5. The ifinoctets of the interface
6. The ifoutoctets of the interface 

The csv file looks like this: 

Using the results

To calculate the number of octets received between each polling, it is necessary to check that the counter has not been reset between two pollings or that it has not wrapped. 

A counter wraps when it reaches its maximum value (counter max value), this could occur frequently on high speed links. You can use the following table to get an idea of time between polling to set if the line is used at a theoretically full speed. 

We assume that the counter contains octets and not bits (like the ifinoctet SNMP object) 

The max value of a 32 bit counter is :   4 294 967 296 (octets)
The max value of a 64 bit counter is :   1.8 10e19 (octets) 

If your link is loaded you should imperatively define a polling interval to avoid this case. The polling interval should be inferior to the worst case, in our example below 5,7 minutes. If possible, increase the polling period to a ratio of 4 of the counter wrap  time. 

We could imagine the simple case below: 

We voluntary set for the example a theoretically fully loaded Ethernet after the second counter restart

 The raw data of the counter would be: 

To calculate the number of real data exchanged, some calculations are necessary. 

The following formula gives you the number of octets transmitted for a polling interval. 

raw value (t) – raw value (t-1)  

The sum of these results on the needed interval period (hour), day, month etc.. would give you the number of octets transmitted over that period. 

But if the raw value (t) – raw value (t-1) is negative the counter has wrapped. Notice that it is one way to be sure that the counter has wrapped. It is indeed important to notice that the result of the calculation raw value (t) – raw value (t-1) can be positive tough the counter has wrapped.

In case the calculation is negative, you should then calculate the amount of data with the formula: 

raw value (t) + (counter max value – raw value (t-1))  

In our example, the real values are

And if Sysuptime(t)-sysuptime(t-1) is negative the equipment has restarted. The formula is not valid anymore. Use the raw value (t) value as value, you have lost anyway the data value between the last polling and the shutdown. 

The throughput in octets/s can be calculated for each polling interval by the formula: 

raw value (t) – raw value (t-1)
Sysuptime(t) - Sysuptime(t-1)

Or when the counter wraps

 raw value(t) + (counter max – (raw value( t-1)))
Sysuptime(t) - Sysuptime(t-1) 

We have to read the Sysuptime value (the time since the equipment has been started) to avoid an eventual time skew introduced by the network delay between the SNMP request and the response. This is necessary if you want to calculate a precise throughput.  

end of document