Keith,
I am not sure there is that much difference. In the current /48 provider based global addresses, each subnet is identified by a /64 prefix. Both approaches will generate /64 routes. I suspect that only in the largest sites, will the subnet field also be used for aggregation inside of the site.operationally I think it would be a mess to have site-locals routed differently within a site than globals. it's not that you can't do it, it's that it makes life more difficult, and GUPIs seem to be a better way to solve the same problem.
This is all about tradeoffs. For example in the site-local prefix FEC0::/10, there are 54-bits available for global identification and subnet numbering. If 16-bits are for subnets, then there are 38-bits left for global site identification. Is this big enough for a global token? Probably not if it a random number or based on some other existing global identifier. Might be OK if it was centrally allocated, but who is going to do that? Also, what about sites that need more than 16-bits of subnets?
We could use a shorter prefix, but how much of the total IPv6 address space do we want to use for this? For example, a /2 prefix would allow 16-bits of subnets and a 46-bit token. But this would use 1/4 of the total IPv6 address space. That doesn't seem wise.
So I am not sure there is any solution that has the three properties:
1) globally unique
2) 16-bit subnet field
3) Uses a limited amount of the IPv6 address space
We may have to pick the two we think are the most important. The current site-local definition has 2) and 3). My global site local proposal has 1) and 3). Andrew White's draft has 1) and 3).
Bob
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