Saturday, May 30, 2009

Measurement overhead

How much extra traffic will network monitoring generate? The goal of network-wide visibility is to improve performance on the network, so the extra traffic generated by monitoring needs to be small and must not degrade performance.

The chart looks at the overhead in terms of measurement records reported per packet on the network. Ideally the overhead associated with monitoring should be small and constant (less than 0.1% of the traffic). Since flow-oriented monitoring (e.g. NetFlow) involves the creation and export of flow records, the overhead is determined by the average number of packets in a flow. If there are a large number of packets in a flow, the overhead will be low. However, if the number of packets per flow is small then the overhead will be high and in the worst case may result in a flow record being generated and exported for every packet on the network.

In practice, the number packets per flow can vary enormously depending on the type of traffic being monitored. DNS traffic is one packet per flow, web traffic will typically have 5-10 packets per flow and video streams may have thousands of packets per flow.

The overhead generated by flow monitoring can become acute during a worm outbreak or when the network is subjected to a denial of service attack (DoS attack). In both cases large numbers of single packet flows are created and the additional overhead created by flow monitoring is likely to exacerbate the problem. The impact of this increased measurement traffic on the network is made worse by the traffic bursts that flow monitoring creates. It is precisely during these times that network visibility is most needed so that the threat can be identified and controlled.

Since sFlow is not a flow-based protocol, the overhead is completely unaffected by the number of packets per flow. sFlow's use of packet sampling limits the overhead of traffic monitoring and ensures accurate, timely, network-wide visibility without impacting network performance - even during extreme traffic situations like a denial of service attack.

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