RE: Draft: PI addressing derived from AS numbers

[Pekka Savola <pekkas@netcore.fi>]

On Sat, 1 Feb 2003, Randy Bush wrote:
> > In the long term, i'm rather convinced that an architectural change will 
> > be required.  I can't see other scalable alternatives.  
> 
> this is also what i see, though i am anxiously awaiting enlightenment.

Well, speaking of enlightenment, I've had a few ideas lately.  I'm pretty 
sure someone has thought along the same lines at some point, but here goes 
anyway.

The current multihoming practises seem to have an impact on the global
routing table on two different axels:

 1) the absolute number of routes
 2) the number of changes in the routing tables due to the large number of 
routes, required processing and update propagation etc.

For example, consider (in future) 10,000 routes coming behind a
next-to-the-origin-AS ISP.   If something bad happens to the ISP, all 
10,000 routes will be withdrawn, and a secondary path will be selected 
(perhaps 3,000 will go to ISP X, 3,000 will go to ISP Y, and 4,000 go 
nowhere, simplifying).

The absolute numbers _might_ be manageable if changes were manageable: I'm
fairly certain of that at least to the degree of O(10^6) -- memory is
cheap.

A way to make changes manageable could be to change how BGP works with
regard to nested routes from multiple sources.  Or really, devise another
protocol.  Here's a raw thought:

When BGP is used for multihoming, all the paths are advertised throughout,
not only the current best paths.  There has to be a marking on which ones
aren't the best paths of course.  When a transit AS becomes unreachable,
instead of withdrawing the routes and waiting for updates, the only thing
that is signalled is "ASX went down, switch to alternative path(s) if
any".  So, changes would not be processed per prefix, but mainly per
(transit) AS, in an "aggregated" fashion. In that way, convergence could
be near-instantaneuous and the amount of processing for BGP updates (at
the cost of about N extra routes in RIB) needed when multihoming minimal.

-- 
Pekka Savola                 "You each name yourselves king, yet the
Netcore Oy                    kingdom bleeds."
Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings