navigating complexity of BGP AS graph

measured by downstream peering richness
entropy of outbound links distribution (for ~2K transit AS)
sum p log p where p ~ #prefixes routed via an AS link
takes into account how often each link is taken
nov 2000: 4.23 bits 
may 2000: 4.24 bits
lots of individual ASes changed though
opposite changes w.net effect 0
another invariant?