Here are my first thoughts on a tiered model, with attributes of each tier. Top level (Tier 1): -- provide transit as their principal business -- have at least some default-free routers -- have connectivity at >1 geographically separated major exchange -- need special measures to deal with BGP scaling issues inside their AS (or multiple AS) such as confederations, clusters, etc. 2nd Level -- provide transit within a geographic area -- may have default-free routers -- limit operations to a geographic area; may connect to multiple exchanges within that area 3rd Level -- do not provide commercial transit services, although they may incidentally provide transit among their customers -- do not do BGP peering with any "downstream" organizations. Their user base is part of their AS. -- May peer with multiple upstream providers 4th Level -- do not run BGP -- Internet access through provider only