F / \ D E | | B C \ / A
Suppose A is a customer of B and C.
This is possible, but only remotely probable. In the real world, D and E are likely peered, as are B and C.
"likely?" with what probability? any measurement cite please. nothing exact; something rough would be fine.
Well, the average AS Path length is something like 4, and according to the charts Geoff has presented here and there, the graph is becoming more dense, as most people interconnect. The odds of finding an end-to-end path (4 hops) on the global 'net where no-one is peered in the middle seems pretty unlikely to me. It's not impossible, but it does seem unlikely, just given the average AS Path length and the density of the graph. For example, I suppose you could make A/B/C part of the same network which is intentionally not peered, or B/C two regional providers who are not peered with one another. You could then make D/E IXPs who have no transit connectivity between them, and then make F a tier 1 provider... But this really seems unlikely to me. How would you string together 4 AS' in a row that have no connectivity to any transit AS, even regional, like this? Two hops I can see, four I have a hard time seeing.
it is a common TE use case. but folk watching the water rise are starting to ask why the whole world should pay for A's TE.
Precisely. Tragedy of the commons. To put it in other terms, removing information reduces optimization -- but if I can get optimization by making someone else pay for the information, then, well, why not? :-) Russ