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Sometimes it is possible that a network interface graph shows a speed, higher than physically possible. This article explains how that can happen. |
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As the calculation of interface speed needs data at two points in time, timing is of the essence. Consider an SNMP device being under heavy load and failing to respond in a timely manner. This can lead to both decreased in or increased throughput, depending on the exact scenario.
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The OIDs that are being queried for interface throughput calculation can be 32-bit or 64-bit. While it is more likely for a 32-bit counter to overflow, 64-bit counters can also overflow. The result of such an overflow is with utmost certainty a off throughput calculation.
Original text:
As there is no direct possibility to poll bandwidth data from, e.g., a switch, the calculation of the bandwidth is basically like this:
(B(t_2) - B(t_1)) / (t_2 - t_1) |
Where t_2 > t_1 are points in time, and B(t) is the total amount of bytes (octets) passed by an interface, starting from 0 when the device boots.
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The counter B may have some delay in getting the current value, as there might be high traffic on the interface and counters not being updated instantaneously. This systematic error tends to compensate by difference calculation roughly, thus, B_error is taken to be minor for the moment.
The counter for the amount of bytes can be 32-bit or 64-bit sized. Having the smaller counters, you risk encountering a counter overflow. The bigger the polling interval, the higher this risk; thus, you should remain at 1 min whenever possible, also for other reasons. With the increasing development of Checkmk, measures have been implemented to detect and compensate counter overflow, which nevertheless will not be perfect.
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an off throughput calculation.
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