Subject: (mobile-ip) Route Optimization in Mobile IP : Previous Foreign Agent Notification

From: zhong chen <[email-masked]>
Date: Wed, 5 Nov 1997 15:30:15 -0800 (PST)

I have a problem understanding the meaning of the "Authenticator"
field appeared at the end of "Previous Foreign Agent Notification
extension". The MN sends the extension to its new Foreign Agent to
indicate it has a previous Foreign Agent and asks the new Foreign Agent to
send a "Binding Update" message to its previous Foreign Agent so that the
previous Foreign Agent can update its visitor list or create a new binding
cache entry for the Mobile Node. 

Assumptions:

1. the Mobile Node shares a secret key with its previous Foreign Agent
2. the Mobile Node has eatablished a new secret key with its new Foreign
   Agent
3. Mobile Node's previous Foreign Agent wants to authenticate the "Binding
   Update" message from the new Foreign Agent by means of "Route
   Optimization Authentication extension" but lacks a shared secret key
   with the new Foreign Agent

According to the draft (Page 21), the "Authenticator" is:

               The authenticator value to be used in the Route
               Optimization Authentication extension in the Binding
               Update message sent by the new foreign agent to the
               mobile node's previous foreign agent.  This authenticator
               is calculated only over the Binding Update message body.

I wonder how the Mobile Node can calculate the "authenticator" given the
fact that the expected "Binding Update" message contains a couple of
fields which can not be determined by the Mobile Node. Here is a list of
all fields to appear in the "Binding Update" message:

A		"acknowledge" bit, not known
I		"identification present" bit, not known
M		"minimal encapsulation" bit, not known
G		"GREncapsulation" bit, not known
Lifetime	included in "Previous Foreign Agent Notification
		extension", known
Home Address	known to the Mobile Node
Care-of-addr	included in "Previous Foreign Agent Notification
		extension", known
Identification  not known

I am not sure if I am thinking all right, anyway, I want to know the
correct answer, your reply and time will be highly appreciated.

Thanks!

***********************************************
Zhong CHEN

Dept. of Computer Science
Portland State University
P.O.Box 751
Portland, OR 97207

Tel: (503)725,4138(O)
http://www.cs.pdx.edu/~zchen/
***********************************************

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