Packet Reordering Metrics: Some Methodological Considerations

Characterizing what makes a packet reordering metric meaningful is a problem that has attracted significant interest, but it still lacks a universally accepted solution. We contribute to this discussion by investigating some theoretical concepts that make the following simple intuitions precise: (1) a metric that is inconsistent, i.e., gives different values on two similar TCP traces, should not be regarded as useful; (2) we formalize the notion of two traces being "identical modulo unimportant details" using similarity relations; (3) if "real-life" traces differ from random sequences by always satisfying certain reorder invariants, then we should only use traces satisfying these invariants when investigating the consistency of a reordering metric. We illustrate these concepts in the context of RESTORED, an approach to semantic compression of TCP traces. In particular, we discuss the consistency of two metrics defined by Jayasumana et al. (2002, http://cnrl.colostate.edu/Reorder/draft-jayasumana-reorder-density-06.txt ) with respect to the similarity notions defined in A. Hansson et al., (2006, 2005)