[Mip4] home registrations and regional registrations question

["Niko A. Fikouras" <niko@comnets.uni-bremen.de>]

It seems that I am having a problem telling the difference between home 
and regional registrations and whose IP address goes in the COA field of 
the registration request. I have taken a couple of paragraphs from the 
"draft-ietf-mobileip-reg-tunnel-04.txt" document to explain the reason 
for my confusion. I would be very grateful if one would clear this one 
out for me.

In section "5. Regional Registration", page 11 we find:
... "Then, the mobile node can
perform a regional registration with this foreign agent and GFA. The
mobile node issues a Regional Registration Request message to the GFA
via the new foreign agent." ...

... and below that:
... "The care-of address should be set
to the address of the local foreign agent, or else zero if the local
foreign agent is not advertising its own care-of address." ...

I am under the impression that the aforementioned segment refers to 
regional registrations, if so then should perhaps the last paragraph 
read as stated in section "4.1. Mobile Node Considerations", page 8, that:

" If the mobile node determines that it is in a visited domain, it
SHOULD either use the advertised GFA address in the care-of address
field in the Registration Request message, or set this field to zero
to request to be assigned a GFA"

Otherwise, how is the FA supposed to figure out what GFA to relay the 
registration request to? From what I gather, if the mobile node wants to 
do a home registration, then it places the FA's address in the COA field 
of the RegReq. On receipt by the FA, the RegReq is forwarded to the HA 
avoiding the GFA.
If the mobile node wishes to make a regional registration it places the 
selected GFA's address in the COA field of the RegReq. This forces the 
FA to relay the RegReq to the GFA, who in turn terminates it and issues 
a RegReply. If that is correct, and assuming that all RegReqs with the 
purpose of renewing the lifetime of a binding are home registrations 
then a GFA would never come across such a registration and expire 
bindings. If that model of operations is not correct, then all RegReqs 
(home registrations or not) must go through the GFA. But then two 
questions arise, how does the FA know what GFA to forward the RegReq to 
and how does the GFA know which RegReqs to forward to the HA and which not?

The first question is based on information given in a paragraph of 
section "5. Regional registration", page 12 that states:
"If the advertised GFA is not the same as the one the mobile node has
registered as its care-of address, and if the mobile node is still
within the same domain as it was when it registered that care-of
address, the mobile node MAY try to perform a regional registration
with its registered GFA."

From this it is derived that a hierarchy may host multiple GFAs. As 
such, on receipt of RegReq a FA would not explicitly know what GFA to 
forward it to.

Best Regards
niko


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