On the Code Length of TCAM Coding Schemes

All high-speed Internet devices need to implement classification, i.e. they must determine whether incoming packet headers belong to a given subset of a search space. To do it, they encode the subset using ternary arrays in special high-speed devices called TCAMs (ternary content-addressable memories). However, the optimal coding for arbitrary subsets is unknown. In particular, to encode an arbitrary range subset of the space of all W-bit values, previous works have successively reduced the upper-bound on the code length from 2W-2 to 2W-4, then 2W-5, and finally W TCAM entries. In this paper, we prove that this final result is optimal for typical prefix coding and cannot be further improved, i.e. the bound of W is tight. To do so, we introduce new analytical tools based on independent sets and alternating paths.