the Internet Backbone

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