Keisuke Sakitani writes:
I'd like to know how a HA should  behave
when it receives MPS form an MN on home link by any chance.

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Vijay Devarapalli responds to Keisuke Sakitani: Just drop it. I would
consider it a bug in the MN implementation, if the MN sends a MPS from
home link.

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Brian Haley writes: Section 6.7 says a Home Address Option MUST be
included (from the MN) and 10.6.3 says the HA MUST use a Type 2
routing header.  These two imply that a HA will only respond to an MPS
for a MN that it is the HA for, since in other cases it can't meet
these requirements.

So yes, I think the HA should drop the request as well.