In my vision the /48s being given out as "PI" today can be used for the
ID portion, while every transit will give a "location" /48 to the site
that needs it. Over the DFZ the src/dst will be in DFZ/location style,
but when it arrives at the endsite it will be in PI mode again. NAT
(that evil thing) is useful and when you do it twice you actually still
have the same packet and have achieved a tunnel without the overhead of
it. The signaling of what to use is the tricky part though.
I think that dual NAT can be used in a somewhat benign way. If it's
done by bilateral agreement between networks with globally unique
prefixes, and the mappings at each end are symmetrical, it seems like
it's basically equivalent to a tunnel with a kind of header compression
and without the PMTU reduction issues. And if the addresses used at the
host are unique, it gets rid of many of the problems caused by
overlapping use of RFC 1918 addresses in IPv4. There's still some
issues related to traceability of traffic over the network, but maybe
those are manageable.
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