Superfluous advertisement (was: Friday's Random Comment)

Sun May 1 12:49:12 UTC 2016

> >>    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