Space-Stretch Tradeoff in Routing Revisited

We present several new proofs of lower bounds for the space-stretch tradeoff in labeled network routing. First, we give a new proof of an important result of Cyril Gavoille and Marc Gengler that any routing scheme with stretch < 3 must use Ω(n) bits of space at some node on some network with n vertices, even if port numbers can be changed. Compared to the original proof, our proof is significantly shorter and, we believe, conceptually and technically simpler. A small extension of the proof can show that, in fact, any constant fraction of the n nodes must use Ω(n) bits of space on some graph. Our main contribution is a new result that if port numbers are chosen adversarially, then stretch < 2k+1 implies some node must use Ω(n^(1/k) log n) bits of space on some graph, assuming a girth conjecture by Erdős. We conclude by showing that all known methods of proving a space lower bound in the labeled setting, in fact, require the girth conjecture.