Spatially coordinated packet sampling can be implemented by using a deterministic function of packet content to determine the selection decision for a given packet. In this way, a given packet may be selected at either all points that it passes, or none. Selection amongst the set of packets should appear as random as possible. In this paper we calculate the empirical entropy of selection of bits from various fields from the packet header. Based on this study they propose using the IP identification field (IPID) to seed the selection decision. In order to sample at a given desired sampling rate, a mask is applied to the IPID, with selection occurring if the field matches a given value after masking. By applying a number of non-overlapping masks in succession, any rate within a given range can be specified. If the field contents were actually random, the long term sampling rate would be the ratio of ID value that survive the mask, to the total number of possible values. After researchin...