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RE: And Now for Something Completely Different (was Re: IPv6 news)

  • From: Church, Chuck
  • Date: Tue Oct 18 13:41:47 2005


	I've been thinking a bunch about this IPv6 multihoming issue.
It seems that the method of hierarchical summarization will keep the
global tables small for all single-homed end user blocks.  But the
multihomed ones will be the problem.  The possible solution I've been
thinking about is 'adjacency blocks', for lack of a better term.  In
theory, the first customer to home to two different ISPs causes a new
large address block to be advertised upstream by these two ISPs.
Further customers homing to these two ISPs get an allocation out of this
same block.  The two ISPs will only announce the large block.  Of course
there are issues involving failure and scalability...
	Failure would involve an ISP losing contact with end customer,
but still announcing the aggregate upstream.  This can be solved by
requiring that two ISPs must have a direct peering agreement, before
they can accept dual-homed customers.  Or a possible method (maybe using
communities?) where ISP B will announce the customer's actual block (the
small one) to it's upstreams, if notified by ISP A that it's not
reachable by them.  When ISP A resumes contact with end customer, ISP B
retracts the smaller prefix.
	Scalability is an obvious issue, as the possible number of these
'adjacency blocks' would be N * (N-1), where N is the number of ISPs in
the world.  Obviously pretty large.  But I feel the number of ISPs that
people would actually dual home to (due to reputation, regional
existence, etc) is a few orders of magnitude smaller.  ~100,000 prefixes
(each can be an ASN, I suppose) should cover all needs, doing some
simple math.
	The downside is that end customers are going to lose the ability
to prefer traffic from one ISP versus another for inbound traffic.  That
alone might be a show-stopper, not sure how important it is.  Since IPv6
is a whole new ballgame, maybe it's ok to change the rules...
	Looking for any thoughts about it.  I'm sure there's things I
haven't considered, but the people I've bounced it off of haven't seen
any obvious problems.  Flame-retardant clothes on, just in case though.


>Every multi-homer will be needing their own ASN, so that's what
>up your routing tables. It's economy there. Btw, a lot of ASNs
>one network only. People surely think multihoming is important to them
>(and I cannot blame them for that).

>Hierarchical routing is one possible solution, with a lot of drawbacks
>and problems. Forget about geographic hierarchies; there's always
>who do not peer. Visibility radius limitation is another (I cannot
>the idea is new, I only don't know what it's called).