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Low power devices such as common wireless router platforms are not capable of performing reliable full packet capture due to resource constraints. In order for such devices to be used to perform link-level measurement on IEEE 802.11 networks, a packet sampling technique is required in order to reliably capture a representative sample of frames. The traditional Berkeley packet filter mechanism found in UNIX-like operating systems does not directly support packet sampling as it provides no way of generating pseudo-random numbers and does not allow a filter program to keep state between invocations. This paper explores the use of the IEEE 802.11 frame check sequence as a source of pseudo-random numbers for use when deciding whether to sample a packet. This theory is tested by analysing the distribution of frame check sequences from a large, real world capture. Finally, a BPF program fragment is presented which can be used to efficiently select packets for sampling.
Date of Conference: 23-27 June 2009