proxy shim with hash in upper address bits?

[Iljitsch van Beijnum <iljitsch@muada.com>]

[catching up]

After reading Marcelo's draft about a proxy implementation of shim6 a  
while ago I was somewhat disheartened: this is extremely hard to do,  
mostly because an unsuspecting host would need to receive an HBA- 
compatible address.

I know we had discussions in the past about putting crypto bits in  
the _upper_ rather than the _lower_ bits of an address, which would  
solve this problem very nicely. That would (of course) be  
incompatible with the idea that shim6 identifiers are also PA  
routable locators, but that's actually an advantage: by doing this  
we'd be creating an identifier space for shim6 that's distinct from  
regular routable address space and also from all forms of ULA.

The only problem that I see is that under the normal regime (/48 per  
site), the number of hash bits would be very limited. The only way to  
solve that would be to annex most of the subnet bits, so we can have  
52 - 56 hash bits.

Thoughts?