Re: Geographic v. topological address allocation

["Sean M. Doran" <smd () clock org>]

Vadim Antonov <avg () pluris com> writes:

> Sean Doran wrote:
>
> > As tli pointed out the top of the hierarchy is not
> > arbitrary, it must be default free.
>
> Sorry for the nitpicking, but this definition has at least two
> flaws:
>
> a) there's a bi-partite backbone configuration, where each half
>    has default pointing to the other half.  Both do not have to
>    carry full routes.  (Of course, this scheme has problems with
>    packets destined to the blue, or can be extended to more than
>    two partitions).

In effect, this entails the synthesis by equivalent areas
of a superior level into which each party can default.
I touched on this briefly in my previous message.

With variable length addressing this kind of joint
level-n-plus-one synthesis is easy; the new area simply
encompasses sufficient bits to distinguish each
level-n/level-n-plus-one IS participating in it.

> b) multihomed non-transit networks may want to be default-free
>    and carry full routing to improve load sharing of outgoing
>    traffic.  Since they are non-transit, they cannot be considered
>    "top of hierarchy".

Yes, I believe I also mentioned Yakov's "pull" (did you see his
slides at the IETF (and NANOG?) with which he presented
his push/pull definitions?) can be used to optimize
routing when strict hierarchical routing is inefficient.

> Bingo. 

We are in sync, Vadim.  Surprise surprise.

        Sean.

P.S.:

> Note that IXPs are not "top" of the hierarchy, but just 
> aggregators for essentially point-to-point links between
> tier-1 backbones.

This is a good way of putting it.  I will steal it and use
it myself from time to time.