Source address selection insufficient?

[Erik Nordmark <Erik.Nordmark@sun.com>]

When reading draft-huitema-multi6-hosts-03.txt (which is good because
it works out enough details to raise questions) I wonder
if address selection can solve ingress filtering.

Taking the canonical picture from the draft
             /-- ( A ) ---(      ) --- ( C ) --\
   X (site X)             ( IPv6 )              (Site Y) Y
             \-- ( B ) ---(      ) --- ( D ) --/

This has 4 locator pairs: 
	A:X-C:Y
	A:X-D:Y
	B:X-C:Y
	B:X-D:Y

The set of locator pairs that work when sending out from site X
might be A:X-C:Y and B:X-D:Y
but the set of locator pairs that work when sending from site Y might
be the other two: A:X-D:Y and B:X-C:Y.

Thus the intersection of the two ingress filtering constraints is the empty
set.

This can happen due to normal routing as far as I can tell.
The constraints for X appear if X routes packet to C out through A
and packets to D out through B.
The constraints for Y appear if Y routes packets to A out through D
and packets to B out through C.

Am I missing something?

If the above is true it seems like we need something other than
source address selection (relaxed filtering, source-based routing,
or locator rewriting).

  Erik