Monday, May 18, 2009

Scalability and accuracy of packet sampling

This chart from Packet Sampling Basics is useful for explaining why sFlow's packet sampling mechanism provides the accuracy and scalability needed for network-wide visibility. The chart shows that the accuracy of a traffic measurement (e.g. How much bandwidth is being consumed by backup traffic?) increases rapidly as the number of samples contributing to the measurement increases.

The chart shows that the percentage accuracy is independent of the number of packets on the network. This independence is the key to sFlow's scalability.  For example, a measurement will have a 5% accuracy as long as it is based on at least 1,500 samples.  Only 1,500 samples are required whether the network contains one switch or 1,000 switches,  10Mbps links or 100Gbps links.

The accuracy of sampled data is also independent of the type of traffic: traffic can consist of a small number of large connections, many small connections, traffic can arrive in bursts or spread out over time. In all cases the accuracy is determined only by the number of samples.

The packet sampling mechanism in sFlow is implemented in hardware, providing wire-speed performance.  When a switch samples a packet, the sampled packet header and packet path information is immediately sent to the central traffic analyzer. Promptly sending the sFlow data reduces the amount of memory on the switch and provides the sFlow collector with a real-time view of network activity.

Using sFlow to monitor all the switches in the network provides a robust and accurate means of monitoring traffic suitable for exacting applications such as network billing and charge-back.  The redundancy that end-to-end monitoring provides ensures that very little data is lost, even when switches fail or are taken down for maintenance.

No comments:

Post a Comment