Two questions about selection between HMIP and FMIP in Mobile IPv6

Hello, everyone,

          I am confused about a question in selection
between HMIP and
FMIP in Mobile IPv6. 
         which protocol should be used by a MN roaming from
one AP to
the other?  What does the selection depend on? The two-layer
information? As if it does depend on the two-layer
information,what are
they namely?
         
         Another question: 

          In the draft-jang-dhc-haopt-02.txt,in DHCPv6, Home
Network
Identifier Option(HNI option) is defined to carry the
identifier of the
target home   network. The target  may be local home agent
(eg. MAP in
HMIP) or real home agent in given home domain.  In this
Option, if the
id-type is 0, it means the Home Network Identifier  will be
NULL; But In
HMIP,after the MN received the response from the MAP which
acted as one
local home agent, the MN should send the BU to the real home
agent .  
          The question comes out. During bootstrapping, as
if the MN
make a decision on performing HMIP,  the MN will send a DHCP
request
including the HNI option (id-type is 0), then it will not
receive the
information about real home agent.   

          Anyone can give me the detailed explaination or a
scenario?
Thanks.



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