Last time this term came up, I opined that there was no "backbone" any more
and that 1996's Internet had a "hairball topology." Vadim, among others,
disagreed with me but we didn't pursue the topic. Perhaps we should have.
Back in the dimming prehistory of the universe, there was a "backbone" in
the sense that AS690 really mattered and if you didn't have someone sending
your routes into ANS (or NSFnet or ARPAnet, depending on the month and year)
then you could not really say that you were "on the Internet" since most
folks would not be able to reach you.
There is no such AS in 1996.
And in that sense, there is no backbone in 1996.
What we have these days is a _whole_bunch_ of AS's such that if any one of
them does not receive your routes somehow, then you aren't "on the Internet"
since a whole lot of other end hosts will not be able to reach your end hosts.
Sprint, MCI, ANS, Alternet, PSI, and AGIS come to mind. (There are others.)
Terminologically speaking, there's no discrete set of wires or routers or
companies you can point to and say, "there, that right there, that is the
Internet Backbone."
We tend to reserve the term "NSP" for folks who peer at enough NAPs that they
have no default route and aren't buying transit from anybody. We tend to use
the term "ISP" when we mean someone in the packet or even the session business
who _does_ have to buy transit from somebody. Once in a while I hear the term
"backbone provider" used synonomously with "NSP" (as defined above).
I am not even going to get started (here and now, at least) on the subject of
peering politics/economics. I just thought I'd chime in on the definitions of
the words we're all using.
