It's a tree
No, it's not a tree
I thought we were sort of an autonomous collective!
Watery marketers lobbing Powerpoints is no basis for a form of architecture
Network engineers spend a lot of time making sure that their networks, and
the Internet, are not trees. Multiple peering and transit relationships
make the network robust - and cyclic.
The core of the current Internet routing architecture in the US
is a couple of dozen Tier 1 providers who almost all interconnect with
each other,
with each pair almost always connected in more than two places
(usually an East Coast and a West Coast location plus others.)
- Most of the Tier 2 providers are connected to at least two upstreams,
either both Tier 1 or a Tier 1 and a Tier 2.
- There's no well-defined boundary between Tier 2 and Tier 3,
but the Tier 3 types of folks may not be as diverse.
- Some big hosting companies are owned by Tier 1 carriers,
and may just get connectivity from their parent company,
but it usually still has physically diverse connections to diverse switches.
- Many other hosting companies are independent of the carriers,
and tend to have feeds from multiple carriers (usually multiple Tier 1
for the big players).
- Many big end-user companies have multiple large internet feeds
from multiple carriers; even small companies with a couple of T1s
often try to get some diversity (in which case the ISP run by the
local telco is often one of their providers.)
- If you want physically diverse access to your building,
you usually need to buy at least a couple of T3s -
some local telcos will still do diverse T1 access, but most don't,
or else they have it in their tariff rate but *your* street doesn't have it.
As Jim and others have said, it's extremely not tree-like -
we want to maximize the number of careless drunken backhoe drivers
it takes to take down our circuits, as well as maximizing the
number of equipment failures and operator mistakes it takes,
and trying to minimize the damage any problem causes.
DNS's namespace is tree-like, but the actual implementation of the
DNS name server networks is very forested and meshy.
The biggest problems are all at layer 9.
