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Mobile IP Working Group                                 David B. Johnson
INTERNET DRAFT                                Carnegie Mellon University
22 November 1995                                         Charles Perkins
                                                         IBM Corporation



                    Route Optimization in Mobile IP

                    draft-ietf-mobileip-optim-03.txt


Status of This Memo

   This document is a product of the Mobile IP Working Group of the
   Internet Engineering Task Force (IETF).  Comments should be submitted
   to working group mailing list at mobile-ip@tadpole.com.

   Distribution of this document is unlimited.

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at
   any time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress".

   To learn the current status of any Internet-Draft, please check
   the "1id-abstracts.txt" listing contained in the Internet- Drafts
   Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
   munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).



















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Abstract

   This document defines extensions to the operation of the base
   Mobile IP protocol to allow for optimization of datagram routing from
   a correspondent node to a mobile node.  Without Route Optimization,
   all datagrams destined to a mobile node are routed through that
   mobile node's home agent, which then tunnels each datagram to the
   mobile node's current location.  The protocol extensions described
   here provide a means for correspondent nodes that implement them
   to cache the binding of a mobile node and to then tunnel their own
   datagrams for the mobile node directly to that location, bypassing
   the possibly lengthy route for each datagram to and from the mobile
   node's home agent.  Extensions are also provided to allow datagrams
   in flight when a mobile node moves, and datagrams sent based on an
   out-of-date cached binding, to be forwarded directly to the mobile
   node's new binding.



































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                                Contents



Status of This Memo                                                    i

Abstract                                                              ii

 1. Introduction                                                       1

 2. Route Optimization Overview                                        3
     2.1. Binding Caching . . . . . . . . . . . . . . . . . . . . .    3
     2.2. Foreign Agent Handoff . . . . . . . . . . . . . . . . . .    3
     2.3. Binding Cache Updates . . . . . . . . . . . . . . . . . .    5
     2.4. Registration Key Establishment Using the Home Agent . . .    7
     2.5. Registration Key Establishment Using Diffie-Hellman . . .    8

 3. Route Optimization Message Formats                                10
     3.1. Binding Warning Message . . . . . . . . . . . . . . . . .   11
     3.2. Binding Request Message . . . . . . . . . . . . . . . . .   12
     3.3. Binding Update Message  . . . . . . . . . . . . . . . . .   13
     3.4. Binding Acknowledge Message . . . . . . . . . . . . . . .   15
     3.5. Registration Request Message  . . . . . . . . . . . . . .   17

 4. Route Optimization Extension Formats                              18
     4.1. Previous Foreign Agent Notification Extension . . . . . .   19
     4.2. Route Optimization Authentication Extension . . . . . . .   21
     4.3. Home Agent Registration Key Request Extension . . . . . .   22
     4.4. Mobile Node Registration Key Reply Extension  . . . . . .   23
     4.5. Foreign Agent Registration Key Reply Extension  . . . . .   24
     4.6. Diffie-Hellman Registration Key Request Extension . . . .   25
     4.7. Diffie-Hellman Registration Key Reply Extension . . . . .   27
     4.8. Mobile Service Extension  . . . . . . . . . . . . . . . .   28

 5. Mobility Security Association Management                          29

 6. Binding Cache Considerations                                      31
     6.1. Cache Management  . . . . . . . . . . . . . . . . . . . .   31
     6.2. Receiving Binding Warning Messages  . . . . . . . . . . .   31
     6.3. Receiving Binding Update Messages . . . . . . . . . . . .   32
     6.4. Using Special Tunnels . . . . . . . . . . . . . . . . . .   32

 7. Home Agent Considerations                                         34
     7.1. Rate Limiting . . . . . . . . . . . . . . . . . . . . . .   34
     7.2. Receiving Binding Request Messages  . . . . . . . . . . .   34
     7.3. Receiving Registration Key Requests . . . . . . . . . . .   34



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     7.4. Receiving Special Tunnels . . . . . . . . . . . . . . . .   35

 8. Foreign Agent Considerations                                      36
     8.1. Establishing a Registration Key . . . . . . . . . . . . .   36
     8.2. Previous Foreign Agent Notification . . . . . . . . . . .   37
     8.3. Receiving Tunneled Datagrams  . . . . . . . . . . . . . .   38
     8.4. Rate Limiting . . . . . . . . . . . . . . . . . . . . . .   39
     8.5. Sending Special Tunnels . . . . . . . . . . . . . . . . .   39

 9. Mobile Node Considerations                                        40
     9.1. Establishing a Registration Key . . . . . . . . . . . . .   40
     9.2. Notifying Previous Foreign Agents . . . . . . . . . . . .   40

References                                                            42

Appendix A. Using a Master Key at the Home Agent                      43

Chairs' Addresses                                                     44

Authors' Addresses                                                    45































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1. Introduction

   The base Mobile IP protocol [6] allows any mobile node to move about,
   changing its point of attachment to the Internet, while continuing
   to be addressed by its home IP address.  Correspondent nodes sending
   IP datagrams to a mobile node address them to the mobile node's home
   address in the same way as to any destination.

   While the mobile node is connected to the Internet away from its
   home network, it is served by a "home agent" on its home network
   and is associated with a "care-of address" indicating its current
   location.  The association between a mobile node's home address and
   its care-of address is known as a "mobility binding".  The care-of
   address is generally the address of a "foreign agent" on the network
   being visited by the mobile node, which forwards arriving datagrams
   locally to the mobile node.  Alternatively, the care-of address may
   be temporarily assigned to the mobile node using DHCP [3] or other
   means.  All IP datagrams addressed to the mobile node are routed by
   the normal IP routing mechanisms to the mobile node's home network,
   where they are intercepted by the mobile node's home agent, which
   then tunnels each datagram to the mobile node's current care-of
   address.  Datagrams sent by a mobile node use the foreign agent as a
   default router but require no other special handling or routing.

   This scheme allows transparent interoperation with mobile nodes,
   but forces all datagrams for a mobile node to be routed through its
   home agent; datagrams to the mobile node are often routed along
   paths that are significantly longer than optimal.  For example, if a
   mobile node, say MN1, is visiting some subnet, even datagrams from
   a correspondent node on this same subnet must be routed through the
   Internet to MN1's home agent on MN1's home network, only to then
   be tunneled back to the original subnet to MN1's foreign agent for
   delivery to MN1.  This indirect routing can significantly delay the
   delivery of the datagram to MN1 and places an unnecessary burden on
   the networks and routers along this path through the Internet.  If
   the correspondent node in this example is actually another mobile
   node, say MN2, then datagrams from MN1 to MN2 must likewise be routed
   through MN2's home agent on MN2's home network and back to the
   original subnet for delivery to MN1.

   This document defines extensions to the base Mobile IP protocol to
   allow for the optimization of datagram routing from a correspondent
   node to a mobile node.  These extensions provide a means for nodes
   that implement them to cache the binding of a mobile node and to then
   tunnel their own datagrams directly to the care-of address indicated
   in that binding, bypassing the possibly lengthy route to and from
   that mobile node's home agent.  Extensions are also provided to allow
   datagrams in flight when a mobile node moves, and datagrams sent



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   based on an out-of-date cached binding, to be forwarded directly
   to the mobile node's new care-of address.  These extensions are
   collectively referred to as Route Optimization in this document.

   All operation of Route Optimization that changes the routing of IP
   datagrams to the mobile node is authenticated using the same type of
   authentication mechanism used in the base Mobile IP protocol.  This
   authentication generally relies on a "mobility security association"
   established in advance between the node sending a message and the
   node receiving the message that must authenticate it.  When the
   required mobility security association has not been established, a
   Mobile IP implementation using Route Optimization operates in the
   same way as the base Mobile IP protocol.

   Section 2 of this document provides an overview of the operation of
   Route Optimization.  Section 3 defines the message types used by
   Route Optimization, and Section 4 defines the message extensions
   used.  Section 5 discusses the problem of managing the mobility
   security associations needed to provide authentication of all
   messages that affect the routing of datagrams to a mobile node.
   The final four sections of this document define in detail the
   operation of Route Optimization from the point of view of each of the
   entities involved:  considerations for nodes maintaining a binding
   cache are presented in Section 6, mobile node considerations in
   Section 9, foreign agent considerations in Section 8, and home agent
   considerations in Section 7.

























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2. Route Optimization Overview

2.1. Binding Caching

   Route Optimization provides a means for any node that wishes to
   optimize its own communication with mobile nodes to maintain a
   "binding cache" containing the mobility binding of one or more mobile
   nodes.  When sending an IP datagram to a mobile node, if the sender
   has a binding cache entry for the destination mobile node, it may
   tunnel the datagram directly to the care-of address indicated in the
   cached mobility binding.

   In the absence of any binding cache entry, datagrams destined for
   a mobile node will be routed to the mobile node's home network in
   the same way as any other IP datagram, and are then tunneled to the
   mobile node's current care-of address by the mobile node's home
   agent.  This is the only routing mechanism supported by the base
   Mobile IP protocol.  With Route Optimization, as a side effect of
   this indirect routing of a datagram to a mobile node, the original
   sender of the datagram may be informed of the mobile node's current
   mobility binding, giving the sender an opportunity to cache the
   binding.

   A node may create a binding cache entry for a mobile node only when
   it has received and authenticated the mobile node's mobility binding.
   Likewise, a node may update an existing binding cache entry for a
   mobile node, such as after the mobile node has moved to a new foreign
   agent, only when it has received and authenticated the mobile node's
   new mobility binding.

   A binding cache will, by necessity, have a finite size.  Any node
   implementing a binding cache may manage the space in its cache using
   any local cache replacement policy.  If a datagram is sent by a
   node to a destination for which it has dropped the cache entry from
   its binding cache, the datagram will be routed normally, leading to
   the mobile node's home network, where it will be intercepted by the
   mobile node's home agent and tunneled to the mobile node's care-of
   address.  As when a binding cache entry is initially created, this
   indirect routing to the mobile node will result in the original
   sender of the datagram being informed of the mobile node's current
   mobility binding, allowing it to add this entry again to its binding
   cache.


2.2. Foreign Agent Handoff

   When a mobile node moves and registers with a new foreign agent, the
   base Mobile IP protocol does not notify the mobile node's previous



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   foreign agent.  IP datagrams intercepted by the home agent after
   the new registration are tunneled to the mobile node's new care-of
   address, but datagrams in flight that had already been intercepted
   by the home agent and tunneled to the old care-of address when the
   mobile node moved are lost and are assumed to be retransmitted by
   higher-level protocols if neeeded.  The old foreign agent eventually
   deletes the mobile node's registration after the expiration of the
   lifetime period established when the mobile node registered there.

   Route Optimization provides a means for the mobile node's previous
   foreign agent to be reliably notified of the mobile node's new
   mobility binding, allowing datagrams in flight to the mobile
   node's previous foreign agent to be forwarded to its new care-of
   address.  This notification also allows any datagrams tunneled to the
   mobile node's previous foreign agent from correspondent nodes with
   out-of-date binding cache entries for the mobile node (that have not
   yet learned that the mobile node has moved), to be forwarded to its
   new care-of address.  Finally, this notification allows any resources
   consumed by the mobile node's binding at the previous foreign agent
   (such as radio channel reservations) to be released immediately,
   rather than waiting for the mobile node's registration to expire.

   Optionally, during registration with a new foreign agent, the mobile
   node and the foreign agent may establish a new shared secret key
   as a "registration key".  When the mobile node later registers
   with a new foreign agent, if it established a registration key
   during registration with its previous foreign agent, it may use
   this key to notify the previous foreign agent that it has moved.
   This notification may also optionally include the mobile node's new
   care-of address, allowing the previous foreign agent to create a
   binding cache entry for the mobile node to serve as a "forwarding
   pointer" to its new location.  Any tunneled datagrams for the mobile
   node that arrive at this previous foreign agent after this binding
   cache entry has been created will then be re-tunneled by this
   foreign agent to the mobile node's new location using the mobility
   binding in this binding cache entry.  The registration key is used
   to authenticate the notification sent to the previous foreign agent.
   Other uses of the registration key are possible, such as for use as
   an encryption key for providing privacy over a wireless link between
   the mobile node and its foreign agent, but such uses are beyond the
   scope of this document.  Once established, the registration key for a
   mobile node can be stored by the foreign agent with the mobile node's
   visitor list entry.

   As part of the registration procedure, the mobile node may
   request that its new foreign agent attempt to notify its previous
   foreign agent on its behalf, by including a Previous Foreign Agent
   Notification extension in its Registration Request message sent to



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   the new foreign agent.  The new foreign agent then builds a Binding
   Update message and transmits it to the mobile node's previous foreign
   agent as part of registration, requesting an acknowledgement from
   the previous foreign agent.  The Previous Foreign Agent Notification
   extension includes only those values needed to construct the Binding
   Update message that are not already contained in the Registration
   Request message.  The authenticator for the Binding Update message is
   computed by the mobile node based on its registration key shared with
   its previous foreign agent.

   The mobile node is responsible for occasionally retransmitting a
   Binding Update message to its previous foreign agent until the
   matching Binding Acknowledge message is received, or until the mobile
   node can be sure of the expiration of its registration with that
   foreign agent.

   The binding cache entry created at the mobile node's previous foreign
   agent is treated in the same way as any other binding cache entry.
   In particular, it is possible that this binding cache entry will be
   deleted from the cache at any time.  In this case, the foreign agent
   will be unable to re-tunnel subsequently arriving tunneled datagrams
   for the mobile node, directly to the mobile node's new location.

   Suppose a node (for instance, such a previous foreign agent) receives
   a datagram that has been tunneled to this node, but this node is
   unable to deliver the datagram locally to the destination mobile node
   (the node is not the mobile node itself, and it is not a foreign
   agent with a visitor list entry for the mobile node).  To attempt
   delivery of the datagram in this case, the node must encapsulate the
   datagram as a "special tunnel" datagram (Section 8.5), destined to
   the mobile node.  This use of a "special tunnel" avoids a possible
   routing loop in the case in which the case-of address registered
   at the home agent is the address of this node, but this node has
   forgotten that it is serving as the mobile node's foreign agent,
   perhaps because the node has crashed and lost its visitor list state.
   The "special tunnel" allows the home agent to see the address of the
   node that tunneled the datagram, and to avoid tunneling the datagram
   back to the same node.


2.3. Binding Cache Updates

   When a mobile node's home agent intercepts a datagram from the
   home network and tunnels it to the mobile node, the home agent may
   deduce that the original source of the datagram has no binding
   cache entry for the destination mobile node.  In this case, the
   home agent MAY send a Binding Update message to the original source
   node, informing it of the mobile node's current mobility binding.



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   No acknowledgement for this Binding Update message is needed, since
   additional future datagrams from this source node intercepted by the
   home agent for the mobile node will cause transmission of another
   Binding Update.  In order for this Binding Update to be authenticated
   by the original source node, the home agent and this source node must
   have established a mobility security association.

   Similarly, when any node receives a tunneled datagram that was
   tunneled to this node, if it is not the current foreign agent
   serving the destination (it has no visitor list entry for this mobile
   node), the node receiving this tunneled datagram may deduce that
   the tunneling node has an out-of-date binding cache entry for the
   destination mobile node.  In this case, the receiving node MAY send a
   Binding Warning message to the tunneling node, advising it that its
   binding cache entry for the mobile node is out-of-date.  As in the
   case of a Binding Update sent by the mobile node's home agent, no
   acknowledgement of this Binding Warning is needed, since additional
   future datagrams for the mobile node tunneled by the same node will
   cause the transmission of another Binding Warning.

   However, unlike the Binding Update message, no authentication of the
   Binding Warning message is necessary, since it does not directly
   affect the routing of IP datagrams to the mobile node.  Instead, when
   a node receives a Binding Warning message, that node sends a Binding
   Request message to the indicated mobile node's home agent, requesting
   the mobile node's current mobility binding.  The home agent then
   answers this Binding Request message with a Binding Update message;
   when the Binding Update message is received, the node may then create
   a binding cache entry for the mobile node.  In order for this node
   and the home agent to exchange these Binding Request and Binding
   Update messages, they must have established a mobility security
   association.

      DISCUSSION: An alternative design would be to send a Binding
      Warning-like message directly to the home agent, which would
      then send a Binding Update to the correspondent node.  This
      accomplishes the same thing in 2 messages instead of 3
      (Binding Warning, Binding Request, Binding Update), but the
      authentication of the binding may be more difficult without
      a request-update exchange between the correspondent node and
      the home agent.  In particular, the two-way exchange between
      the correspondent node and the home agent provides a means
      to return the new value to be used as a nonce for the next
      authentication, in the case in which nonces are being used
      for replay protection.

   Each Binding Update message indicates the maximum lifetime of any
   binding cache entry created from the Binding Update.  When sending



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   the Binding Update message, the home agent SHOULD set this lifetime
   to the remaining service lifetime associated with the mobile node's
   current registration.  Any binding cache entry established or updated
   in response to this Binding Update message must be marked to be
   deleted after the expiration of this period.  A node wanting to
   provide continued service with a particular binding cache entry may
   attempt to reconfirm that mobility binding before the expiration of
   this lifetime period.  Such reconfirmation of a binding cache entry
   may be appropriate when the node has indications (such as an open
   transport-level connection to the mobile node) that the binding
   cache entry is still needed.  This reconfirmation is performed by
   the node sending a Binding Request message to the mobile node's home
   agent, requesting it to reply with the mobile node's current mobility
   binding in a new Binding Update message.


2.4. Registration Key Establishment Using the Home Agent

   One method of establishing a registration key is for the mobile node
   to request its home agent to generate one during registration, and
   for the home agent to send a copy of this key to both the mobile
   node and its new foreign agent.  The home agent in this case acts
   as a "key distribution center" (KDC) for the mobile node and the
   foreign agent.  The mobile node requests this by including a Home
   Agent Registration Key Request extension in its Registration Request
   message.  The home agent sends the generated key to the mobile node
   and to its foreign agent by including a Mobile Node Registration Key
   Reply extension and a Foreign Agent Registration Key Reply extension
   its Registration Reply message.  The Mobile Node Registration Key
   Reply extension contains a copy of the chosen key encrypted under a
   key and algorithm shared between the home agent and the mobile node,
   and a Foreign Agent Registration Key Reply extension contains a copy
   of the same key encrypted under a key and algorithm shared between
   the home agent and the foreign agent.

   In order for the registration key to be established using this
   method, the foreign agent must have a mobility security association
   with the home agent.  For example, if mobile nodes from some home
   network often visit this foreign agent, it may in general be worth
   the effort of creating such a mobility security association between
   this foreign agent and the home agent serving that home network.  If
   no mobility security association exists, a mobile node may instead be
   able to establish a registration key with its home agent using the
   alternative method described in the next section.







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2.5. Registration Key Establishment Using Diffie-Hellman

   An alternate method defined in this document for establishing a
   registration key is for the mobile node and its foreign agent to
   execute the Diffie-Hellman key exchange algorithm [2] as part of the
   mobile node's registration.  The Diffie-Hellman algorithm is a public
   key cryptosystem that allows two parties to establish a shared secret
   key, such that the shared secret key cannot be determined by other
   parties overhearing the messages exchanged during the algorithm.  It
   is used, for example, in other protocols that require a key exchange,
   such as in the Cellular Digital Packet Data (CDPD) system [1].
   Briefly, the Diffie-Hellman algorithm involves the use of two large
   public numbers, a Prime number (p) and a Generator (g).  The Prime
   number and the Generator must be known by both parties involved in
   the algorithm, but need not be kept secret; these values may be
   different for each execution of the algorithm and are not used once
   the algorithm completes.  Each party chooses a private random number,
   produces a Computed Value based on this random number and the Prime
   and the Generator, and sends the Computed Value in a message to the
   other party.  Each party then computes the (same) shared secret key
   using its own private random number, the Computed Value received from
   the other party, and the Prime and Generator values.

   To use this algorithm during registration with a foreign agent,
   the mobile node includes a Diffie-Hellman Registration Key Request
   extension in its Registration Request message, containing its values
   for the Prime and Generator, along with the Computed Value from its
   own private random number.  The foreign agent then choses its own
   private random number and includes a Diffie-Hellman Registration Key
   Reply extension in its Registration Reply message to the mobile node;
   the extension includes the foreign agent's own Computed Value based
   on its chosen random number and the supplied Prime and Generator
   values from the mobile node.  The mobile node and foreign agent each
   independently form the (same) shared secret key from their own chosen
   random number, the Computed Value supplied by the other party, and
   the Prime and Generator values.

   The Diffie-Hellman algorithm allows the mobile node and its foreign
   agent to establish a registration key without any pre-existing
   mobility security associations, but the Diffie-Hellman algorithm
   itself is covered by a patent in the United States.  The method of
   establishing a registration key using Diffie-Hellman thus may not be
   usable by those who have not licensed the patent.  An implementation
   of the Diffie-Hellman key exchange algorithm is available, though, in
   the free RSAREF toolkit from RSA Laboratories [7].

   Also, establishing a registration key using Diffie-Hellman is
   computationally more expensive than the method described in



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   Section 2.4.  The use of Diffie-Hellman described here, though, is
   designed to allow the Diffie-Hellman computations to be overlapped
   with other activities.  The mobile node may choose (or be manually
   configured with) the Prime and Generator values at any time, or
   may use the same two values for a number of registrations.  The
   mobile node may also choose its private random number and compute
   its Computed Value at any time.  For example, after completing
   one registration, the mobile node may choose the private random
   number for its next registration and begin the computation of
   its new Computed Value based on this random number, such that it
   has completed this computation before it is needed in its next
   registration; in its simplest form, the mobile node may use the
   same private random number and Computed Value for any number of
   registrations.  The foreign agent may choose its private random
   number and begin computation of its Computed Value based on this
   number as soon as it receives the mobile node's Registration Request
   message, and need only complete this computation before it sends
   the matching Registration Reply message for the mobile node's
   registration.

   The Agent Advertisement message (Mobile Service extension) is revised
   under Route Optimization to include a bit to indicate that the
   foreign agent sending the advertisement supports this method of
   registration key establishment using the Diffie-Hellman algorithm.
   If this bit is not set in the Agent Advertisement from the foreign
   agent, the mobile node MUST NOT send a Diffie-Hellman Registration
   Key Request extension to the foreign agent in its Registration
   Request message.

      DISCUSSION: This could be extended to support other similar
      key exchange algorithms, either by adding a new Request
      and Reply extension for each, or by adding a field in the
      extensions to indicate which algorithm is to be used.
      Diffie-Hellman seems the only obvious choice, though,
      currently.
















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3. Route Optimization Message Formats

   Route Optimization defines four message types used for management
   of binding cache entries.  Each of these messages begins with a
   one-octet field indicating the type of the message.

   The following Type codes are defined in this document:

      16 = Binding Warning message
      17 = Binding Request message
      18 = Binding Update message
      19 = Binding Acknowledge message

   Route Optimization also requires one minor change to existing
   Mobile IP messages:  a new flag bit must be added to the Registration
   Request message, replacing a previously unused, reserved bit in the
   message.

   This section describes each of the new Route Optimization messages
   and the change to Registration Request message.































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3.1. Binding Warning Message

   A Binding Warning message is used to advise a node that it appears
   to have either no binding cache entry or an out-of-date binding
   cache entry for some mobile node.  When any node receives a datagram
   tunneled to itself, if it is not the current foreign agent for the
   destination mobile node, it MAY send a Binding Warning message to the
   node that originated the tunneled datagram.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |                   Reserved                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Mobile Node Home Address                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type

         16

      Reserved

         Sent as 0; ignored on reception.

      Mobile Node Home Address

         The home address of the mobile node to which the Binding
         Warning message refers.






















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3.2. Binding Request Message

   A Binding Request message is used by a node to request a mobile
   node's current mobility binding from the mobile node's home agent.
   It is sent by a node upon receiving a Binding Warning message, or by
   a node desiring to update the mobility binding in a binding cache
   entry that it holds for the mobile node.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |                  Reserved                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Mobile Node Home Address                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Identification                        +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type

         17

      Reserved

         Sent as 0; ignored on reception.

      Mobile Node Home Address

         The home address of the mobile node to which the Binding
         Request refers.

      Identification

         A 64-bit sequence number, assigned by the node sending the
         Binding Request message, used to assist in matching requests
         with replies, and in protecting against replay attacks.













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3.3. Binding Update Message

   The Binding Update message is used to notify another node of a mobile
   node's current mobility binding.  It MAY be sent by the mobile node's
   home agent in response to a Binding Request message.  It MAY also
   be sent by a mobile node, or by the foreign agent with which the
   mobile node is registering, when notifying the mobile node's previous
   foreign agent that the mobile node has moved.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |A|I|M|G|  Rsvd |            Lifetime           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Mobile Node Home Address                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Care-of Address                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Identification                        +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Extensions ...
   +-+-+-+-+-+-+-+-

      Type

         18

      Acknowledge (A)

         The Acknowledge (A) bit is set by the node sending the Binding
         Update message to request a Binding Acknowledge message be
         returned acknowledging its receipt.

      Identification Present (I)

         The Identification Present (I) bit is set by the node sending
         the Binding Update message to indicate that the Identification
         field is present in the message.

      Minimal Encapsulation (M)

         If the Minimal Encapsulation (M) bit is set, datagrams may be
         tunneled to the mobile node using the minimal encapsulation
         protocol used in the base Mobile IP protocol.





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      GRE Encapsulation (G)

         If the GRE Encapsulation (G) bit is set, datagrams may be
         tunneled to the mobile node using the GRE encapsulation
         protocol [5].

      Reserved (Rsvd)

         Sent as 0; ignored on reception.

      Lifetime

         The number of seconds remaining before the binding cache entry
         must be considered expired.  A value of all ones indicates
         infinity.  A value of zero indicates that no binding cache
         entry for the mobile node should be created and that any
         existing binding cache entry (and visitor list entry, in the
         case of a mobile node's previous foreign agent) for the mobile
         node should be deleted.  The lifetime is typically equal to the
         remaining lifetime of the mobile node's registration.

      Mobile Node Home Address

         The home address of the mobile node to which the Binding Update
         message refers.

      Care-of Address

         The current care-of address of the mobile node.  When set equal
         to the home address of the mobile node, the Binding Update
         message instead indicates that no binding cache entry for the
         mobile node should be created, and any existing binding cache
         entry (and visitor list entry, in the case of a mobile node's
         previous foreign agent) for the mobile node should be deleted.

      Identification

         If present, a 64-bit number, assigned by the node sending the
         Binding Request message, used to assist in matching requests
         with replies, and in protecting against replay attacks.

   The Route Optimization Authentication extension (Section 4.2) is
   required.








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3.4. Binding Acknowledge Message

   A Binding Acknowledge message is used to acknowledge receipt of a
   Binding Update message.  It SHOULD be sent by a node receiving the
   Binding Update message if the Acknowledge (A) bit is set in the
   Binding Update message.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |N|               Reserved                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Mobile Node Home Address                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Identification                        +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type

         19

      Negative Acknowledge (N)

         If the Negative Acknowledge (N) bit is set, this
         acknowledgement is negative.  For instance, if the
         binding update was not accepted, but the incoming datagram has
         the Acknowledge flag set, then the Negative Acknowledge (N) bit
         SHOULD be set in this Binding Acknowledge message.

            DISCUSSION: Alternatively, we could replace this bit with
            a status code, as in the Registration Reply message.  The
            mobile node could log the error, but currently has no
            real way to recover from it, so perhaps the exact reason
            for the error isn't that useful.

      Reserved

         Sent as 0; ignored on reception.

      Mobile Node Home Address

         Copied from the Binding Update message being acknowledged.







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      Identification

         Copied from the Binding Update message being acknowledged, if
         present there.















































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3.5. Registration Request Message

   One bit is added to the flag bits in the Registration Request message
   to indicate that the mobile node would like its home agent to keep
   its mobility binding "private".  Normally, the home agent sends
   Binding Update messages to correspondent nodes as needed to allow
   them to cache the mobile node's binding.  If the mobile node sets the
   Private (P) bit in the Registration Request message, the home agent
   MUST not send the mobile node's binding in Binding Update messages.
   Instead, each Binding Update message should give the mobile node's
   care-of address equal to its home address, and should give a Lifetime
   value of 0.

   Thus, the Registration Request message under Route Optimization
   begins as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |S|B|D|M|G|P|rvd|          Lifetime             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | (unchanged...)
   +-+-+-

      Private (P)

         The Private (P) bit is set by the node sending the Binding
         Update message to indicate that the home agent should keep its
         mobility binding private.  In any Binding Update message sent
         by the mobile node's home agent, the care-of address should be
         set equal to the mobile node's home address, and the Lifetime
         should be set equal to 0.



















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4. Route Optimization Extension Formats

   Route Optimization defines the following Mobile IP message
   extensions:

       96 = Previous Foreign Agent Notification Extension
       97 = Route Optimization Authentication Extension
       98 = Home Agent Registration Key Request Extension
       99 = Mobile Node Registration Key Reply Extension
      100 = Foreign Agent Registration Key Reply Extension
      101 = Diffie-Hellman Registration Key Request Extension
      102 = Diffie-Hellman Registration Key Reply Extension

   Route Optimization also requires one minor change to existing
   Mobile IP message extensions:  a new flag bit must be added to the
   Mobile Service extension, replacing a previously unused, reserved bit
   in the extension.

   This section describes each of the new Route Optimization message
   extensions and the change to Mobile Service extension.































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4.1. Previous Foreign Agent Notification Extension

   The Previous Foreign Agent Notification extension may be included
   in a Registration Request message sent to a foreign agent.  It
   requests the new foreign agent to send a Binding Update message to
   the mobile node's previous foreign agent on behalf of the mobile
   node, to notify it that the mobile node has moved.  The previous
   foreign agent deletes the mobile node's visitor list entry and, if a
   new care-of address is included in the Binding Update message, also
   creates a binding cache entry for the mobile node pointing to its new
   care-of address.  The Previous Foreign Agent Notification extension
   contains only those values not otherwise already contained in the
   Registration Request message that are needed for the new foreign
   agent to construct the Binding Update message.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Length    |         Cache Lifetime        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Previous Foreign Agent Address                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   New Care-of Address Address                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Authenticator ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type

         96

      Length

         10 plus the length of the Authenticator

      Cache Lifetime

         The number of seconds remaining before the binding cache entry
         created by the previous foreign agent must be considered
         expired.  A value of all ones indicates infinity.  A value
         of zero indicates that the previous foreign agent should not
         create a binding cache entry for the mobile node once it has
         deleted the mobile node's visitor list entry.  The Cache
         Lifetime value is copied into the Lifetime field of the Binding
         Update message.






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      Previous Foreign Agent Address

         The IP address of the mobile node's previous foreign agent
         to which the new foreign agent should send a Binding Update
         message on behalf of the mobile node.

      New Care-of Address

         The new care-of address for the new foreign agent to send in
         the Binding Update message to the previous foreign agent.  This
         SHOULD be either the care-of address being registered in this
         new registration (i.e., to cause IP datagrams from the previous
         foreign agent to be tunneled to the new foreign agent) or the
         mobile node's home address (i.e., to cause the previous foreign
         agent to only delete its visitor list entry for the mobile
         node).

      Authenticator

         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.



























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4.2. Route Optimization Authentication Extension

   The Route Optimization Authentication extension is used to
   authenticate certain Route Optimization management messages.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Length    |        Authenticator ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type

         97

      Length

         The length of the Authenticator

      Authenticator

         (variable length) A value computed from a stream of bytes
         including the shared secret, the UDP payload (that is, the
         Route Optimization management message), all prior extensions
         in their entirety, and the type and length of this extension,
         but not including the Authenticator field itself nor the UDP
         header.
























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4.3. Home Agent Registration Key Request Extension

   The Home Agent Registration Key Request extension may be used in
   Registration Request messages to request a registration key from
   the mobile node's home agent.  The extension is included in the
   Registration Request message by the mobile node.  It is authenticated
   along with the rest of the Registration Request message, and
   thus no additional authenticator is included in the extension.
   In response to a Home Agent Registration Key Request extension,
   the home agent MAY include a Mobile Node Registration Key Reply
   extension and a Foreign Agent Registration Key Reply extension in its
   Registration Reply message, containing encrypted copies of the (same)
   new registration key for the mobile node and the foreign agent,
   respectively.

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Length    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type

         98

      Length

         0

   The mobility security association assumed to exist between the home
   agent and the mobile node will be used to encrypt the key sent in
   the Mobile Node Registration Key Reply extension, unless a Key
   Identification extension is also included with the Registration
   Request message.

   The home agent must also share a mobility security association with
   the foreign agent in order to return the requested registration key,
   and the home agent will use this security association to encrypt the
   key sent in the Foreign Agent Registration Key Reply extension.












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4.4. Mobile Node Registration Key Reply Extension

   The Mobile Node Registration Key Reply extension may be used in
   Registration Reply messages to send a registration key from the
   mobile node's home agent to the mobile node.  When used, the home
   agent MUST also include a Foreign Agent Registration Key Reply
   extension in the Registration Reply message, giving a copy of the
   same key to the mobile node's new foreign agent.  The Mobile Node
   Registration Key Reply extension is authenticated along with the
   rest of the Registration Reply message, and thus no additional
   authenticator is included in the extension.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Length    |  Mobile Node Encrypted Key ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type

         99

      Length

         The length of the Mobile Node Encrypted Key

      Mobile Node Encrypted Key

         (variable length) The registration key, chosen by the home
         agent, encrypted under the mobility security association
         between the home agent and the mobile node.  The same key must
         be sent, encrypted for the foreign agent in a Foreign Agent
         Registration Key extension in this Registration Reply message.


















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4.5. Foreign Agent Registration Key Reply Extension

   The Foreign Agent Registration Key Reply extension may be used
   in Registration Reply messages to send a registration key from
   the mobile node's home agent to the mobile node's new foreign
   agent.  When used, the home agent MUST also include a Mobile Node
   Registration Key Reply extension in the Registration Reply message,
   giving a copy of the same key to the mobile node.  The Foreign Agent
   Registration Key Reply extension is authenticated by including an
   authenticator in the extension, computed based on the mobility
   security association shared between the home agent and the foreign
   agent.  In order for this extionsion to be used, the home agent MUST
   share a mobility security association with the foreign agent.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Length    |  Foreign Agent Encrypted Key ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Authenticator ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type

         100

      Length

         The length of the Foreign Agent Encrypted Key plus the length
         of the Authenticator

      Foreign Agent Encrypted Key

         (variable length) The registration key, chosen by the home
         agent, encrypted under the mobility security association
         between the home agent and the foreign agent.  The same key
         must be sent, encrypted for the mobile node in a Mobile Node
         Registration Key extension in this Registration Reply message.

      Authenticator

         (variable length) A value computed from a stream of bytes
         including the shared secret and the fields in this extension
         other than the Authenticator field itself.







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4.6. Diffie-Hellman Registration Key Request Extension

   The Diffie-Hellman Registration Key Request extension may be included
   in a Registration Request message sent to a foreign agent.  It begins
   the Diffie-Hellman key exchange algorithm [2] between the mobile node
   and its new foreign agent, as described in Section 2.5.  The foreign
   agent SHOULD then include a Diffie-Hellman Registration Key Reply
   extension in its Registration Reply message sent to the mobile node
   in order to complete the key exchange.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Length    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Prime ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Generator ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Computed Value ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type

         101

      Length

         3 times the length of each of Prime, Generator, and
         Computed Value.  The values Prime, Generator, and
         Computed Value must all be the same length, which must be a
         multiple of 8 bits.

      Prime

         One of the two public numbers involved in the Diffie-Hellman
         key exchange algorithm.  The value Prime should be a large
         prime number.

      Generator

         One of the two public numbers involved in the Diffie-Hellman
         key exchange algorithm.  If p is the value of the Prime used
         for this Diffie-Hellman exchange, the value Generator should be
         less than p, and should be a primitive root of p.






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      Computed Value

         The public computed value from the mobile node for this
         Diffie-Hellman exchange.  The mobile node chooses a large
         random number, x.  If g is the value of the Generator and p is
         the value of the Prime, the Computed Value in the extension is
         (g**x) mod p.












































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4.7. Diffie-Hellman Registration Key Reply Extension

   The Diffie-Hellman Registration Key Reply extension SHOULD be
   included in a Registration Reply message sent by a foreign agent to a
   mobile node that included a Diffie-Hellman Registration Key Request
   extension in its Registration Request message to the foreign agent.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Length    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Computed Value ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type

         101

      Length

         The length of the Computed Value.  The length of the Computed
         Value must be a multiple of 8 bits.

      Computed Value

         The foreign agent chooses a large random number, y.  If g is
         the value of the Generator and p is the value of the Prime, the
         Computed Value in the extension is (g**y) mod p.  The values
         of the Generator and Prime are taken from the Diffie-Hellman
         Registration Key Request extension from the mobile node's
         Registration Request message.



















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4.8. Mobile Service Extension

   One bit is added to the flag bits in the Mobile Service extension
   (forming an Agent Advertisement message), when sent by a foreign
   agent, to indicate that the foreign agent supports registration
   key establishment using the Diffie-Hellman key exchange algorithm.
   When this bit is set in the Agent Advertisement, the mobile node
   MAY request a registration key be established by including a
   Diffie-Hellman Registration Key Request extension in its Registration
   Request message.  The foreign agent replies with the Diffie-Hellman
   Registration Key Reply extension in its Registration Reply to the
   mobile node.

   Thus, the Mobile Service extension under Route Optimization begins as
   follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |        Sequence Number        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Lifetime            |R|B|H|F|M|G|D|    reserved     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | (unchanged...)
   +-+-+-

      Diffie-Hellman (D)

         The Diffie-Hellman (D) bit is set by the foreign agent
         sending the Agent Advertisement message to indicate that it
         supports the registration key establishment protocol using the
         Diffie-Hellman Registration Key Request and Diffie-Hellman
         Registration Key Reply extensions.


















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5. Mobility Security Association Management

   One of the most difficult aspects of Route Optimization for Mobile IP
   in the Internet today is that of providing authentication for all
   messages that affect the routing of datagrams to a mobile node.
   In the base Mobile IP protocol, all routing of datagrams to the
   mobile node while away from its home network is controlled by the
   home agent, since only the home agent is aware of the mobile node's
   mobility binding and only the home agent tunnels datagrams to
   the mobile node.  Authentication is currently achieved based on a
   manually established mobility security association between the home
   agent and the mobile node.  Since the home agent and the mobile
   node are both owned by the same organization (both are assigned IP
   addresses within the same IP subnet), this manual configuration is
   manageable, and (for example) can be performed while the mobile node
   is at home.

   However, with Route Optimization, authentication is more difficult
   to manage, since a Binding Update may in general need to be sent to
   any node in the Internet.  Since no general authentication or key
   distribution protocol is available in the Internet today, the Route
   Optimization procedures defined in this document make use of the
   same type of manually configured mobility security associations used
   in the base Mobile IP protocol.  For use with Route Optimization,
   a mobility security association held by a correspondent node or a
   foreign agent must in general include the same parameters as required
   by the base Mobile IP protocol specification [6].

   For a correspondent node to be able to create a binding cache entry
   for a mobile node, the correspondent node and the mobile node's
   home agent MUST have established a mobility security association.
   This mobility security association, though, may be used in creating
   and updating binding cache entries at this correspondent node
   for all mobile nodes served by this home agent.  This places the
   correspondent node in a fairly natural relationship with respect
   to the mobile nodes served by this home agent.  For example, these
   mobile nodes may represent different people affiliated with the
   organization owning the home agent and these mobile nodes, with which
   the user of this correspondent node often collaborates.  In this
   case, the effort of establishing the necessary mobility security
   association with this home agent may be justified.  It is similarly
   possible for a home agent to have a manually established mobility
   security association with the foreign agents often used by its mobile
   nodes, or for a particular mobile node to have a manually established
   mobility security association with the foreign agents serving the
   foreign networks that it often visits.





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   In general, if the movement and communication patterns of a mobile
   node or the group of mobile nodes served by the same home agent are
   sufficient to justify establishing a mobility security association
   with the mobile node's home agent, users or network administrators
   are likely to do so.  Without establishing a mobility security
   association, nodes will not currently be able to use the Route
   Optimization extensions but can use the base Mobile IP protocol.












































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6. Binding Cache Considerations

   Any node may maintain a binding cache in order to optimize its own
   communication with mobile nodes.  When sending an IP datagram, if the
   sending node has a binding cache entry for the destination node, it
   MAY tunnel the datagram to the mobile node's care-of address using
   the encapsulation techniques described for home agents in the base
   Mobile IP protocol specification [6].  Any optional encapsulation
   methods supported are indicated by flag bits in the Binding Update
   message.

   When a mobile node's home agent intercepts a datagram on the home
   network and tunnels it to the mobile node, the originating node
   generally has no binding cache entry for the destination mobile node.
   In such cases, the home agent MAY send a Binding Update message to
   the originator.

   Similarly, when a node other than the foreign agent currently
   serving a mobile node receives a datagram for a mobile node, and
   this datagram has been tunneled to that node, the node tunneling
   the datagram generally has an out-of-date binding for the mobile
   node in its binding cache.  In such cases, the node receiving the
   tunneled datagram MAY send a Binding Warning message to the tunneling
   node, which SHOULD then request a new binding for the mobile node by
   sending a Binding Request message to the mobile node's home agent.


6.1. Cache Management

   A node maintaining a binding cache may use any reasonable strategy
   for managing the space within the cache.  When a new entry needs to
   be added to the binding cache, the node MAY choose to drop any entry
   already in the cache, if needed, to make space for the new entry.
   For example, a "least-recently used" (LRU) strategy for cache entry
   replacement is likely to work well.

   Each binding in the binding cache also has an associated lifetime,
   specified by in the Binding Update message in which the node obtained
   the binding.  After the expiration of this time period, the binding
   MUST be deleted from the cache.


6.2. Receiving Binding Warning Messages

   A node maintaining and using a binding cache will receive a Binding
   Warning message if its binding cache entry for a datagram it has
   tunneled is out-of-date, as determined by the node which receives
   the tunneled datagram.  When a node receives a Binding Warning



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   message, it SHOULD request an updated binding for this mobile node
   from the mobile node's home agent, using a Binding Request message.
   Included in the Binding Request message is a 64-bit identification
   field, in the same format described in the base Mobile IP protocol
   specification, for matching the request with the returned Binding
   Update message.  The node SHOULD silently discard any Binding Update
   messages that do not correspond with its latest Binding Request
   message for this mobile node.


6.3. Receiving Binding Update Messages

   When a node receives a Binding Update message, it MUST verify
   the authentication in the message, using the mobility security
   association it shares with the mobile node's home agent.  The
   authentication data is found in the Route Optimization Authentication
   extension (Section 4.2).  If the authentication succeeds, then a
   binding cache entry SHOULD be updated for use in future transmissions
   of data to the mobile node.  Otherwise, an authentication exception
   SHOULD be raised.


6.4. Using Special Tunnels

   Whenever any node receives a tunneled datagram for for which it has
   no visitor list entry for the datagram's destination, the node may
   deduce that the tunneling node has an out-of-date binding cache entry
   for the destination mobile node.  If the node receiving the tunneled
   datagram has a binding cache entry for the destination, it SHOULD
   re-tunnel the datagram to the care-of address indicated in this
   binding cache entry.

   However, if the node receiving the tunneled datagram has no binding
   cache entry for the destination, it cannot re-tunnel the node to
   its destination.  Instead, the node SHOULD forward the datagram
   to the destination mobile node's home agent, using a special form
   of tunneling called a "special tunnel".  To tunnel the datagram
   using a special tunnel, the tunneled datagram's destination address
   is set equal to the destination address in the tunneled datagram.
   Thus, both the "inner" and "outer" destination addresses are set to
   the home address of the mobile node.  The tunneled datagram will
   thus be routed to the mobile node's home network, where it will be
   intercepted by the mobile node's home agent in the same way as other
   datagrams addressed to the mobile node.

   The home agent SHOULD then tunnel the datagram to the current care-of
   address for the mobile node.  However, the home agent MUST NOT tunnel
   the datagram to the current care-of address if the special tunnel of



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   the datagram originated at that care-of address, as indicated by the
   "outer" source address of the special tunnel datagram.  The use of
   the special tunnel format allows the home agent to identify the node
   that tunneled the datagram to it (as well as the original sender of
   the datagram).  If the home agent believes that the current care-of
   address for the mobile node is the same as the source of the special
   tunnel, then the home agent SHOULD discard the datagram; in this
   case, the foreign agent serving the mobile node appears to have lost
   its entry for the mobile node in its visitor list, for example, if
   the foreign agent has crashed and rebooted.

   After tunneling the datagram to the current care-of address for the
   mobile node, the home agent MAY notify the source of the special
   tunnel of the mobile node's current binding, by sending it a Binding
   Update message.




































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7. Home Agent Considerations

   The home agent will be the frequent source of Binding Update messages
   sent to correspondent nodes that are communicating with its mobile
   nodes.  Any correspondent node that has no binding cache entry for a
   mobile node, will send normal, untunneled datagrams through the home
   agent by the normal routing in the Internet.  When the home agent
   first receives a datagram for a mobile node, it SHOULD also send a
   Binding Update message to the originator of the datagram in hopes
   that the originator will be able to create a binding cache entry for
   that mobile node.  Then, future datagrams sent by this node to the
   mobile node should not need the involvement of the home agent.


7.1. Rate Limiting

   A home agent MUST provide some mechanism to limit the rate at which
   it sends Binding Update messages to to the same node about any given
   mobility binding.  This rate limiting is especially important because
   it is expected that, within the short term, many Internet nodes will
   not support maintenance of a binding cache.  In this case, continual
   transmissions of Binding Update messages to such a correspondent
   node will only add unnecessary overhead to at the home agent and
   correspondent node, and along the Internet path between these nodes.


7.2. Receiving Binding Request Messages

   When the home agent receives a Binding Request message, it consults
   its home list and determines the correct binding information to be
   sent to the requesting node.  Before satisfying the request, the
   home agent MUST check whether or not the mobile node has allowed
   the information to be disseminated.  If the mobile node specified
   the Private (P) bit in its Registration Request message, then the
   home agent MUST not satisfy any Binding Requests.  In this case,
   the home agent SHOULD return a Binding Update in which both the
   Care-of Address is set equal to the mobile node's home address and
   the Lifetime is set to zero.  Such a Binding Update message indicates
   that the binding cache entry for the specified mobile node should be
   deleted.


7.3. Receiving Registration Key Requests

   When the home agent receives a Registration Request message, a Home
   Agent Registration Key Request extension (Section 4.3) may be present
   in the message, requesting the home agent to provide a registration
   key to the mobile node and its foreign agent, as described in



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   Section 2.2.  In that event, the home agent employs a good algorithm
   for producing random keys [4] and encrypts the result separately for
   use by the foreign agent and by the mobile node.  The chosen key is
   encrypted under the mobility security association shared between the
   home agent and the mobile node, and the encrypted key is placed in
   a Mobile Node Registration Key Reply extension (Section 4.4) in the
   Registration Reply message.  The same key also is encrypted under the
   mobility security association shared between the home agent and the
   foreign agent, and the encrypted key is placed in a Foreign Agent
   Registration Key Reply extension (Section 4.5) in the Registration
   Reply message.

   If the home agent receives a Home Agent Registration Key Request
   extension in a Registration Request message, but it does not have
   an established mobility security association with the foreign agent
   with which the mobile node is registering, the home agent MUST reject
   the registration attempt.  In this case, the home agent returns a
   Registration Reply message indicating that the foreign agent failed
   authentication.


7.4. Receiving Special Tunnels

   The home agent may also receive datagrams tunneled to the mobile
   node using a special tunnel (Section 6.4), which it has intercepted
   for the mobile node in the same way as any datagram destined to the
   mobile node while the mobile node is away from home.  The home agent
   MUST examine the tunnel source address (the "outer" address of the
   tunneled datagram).  If the tunnel source address differs from the
   care-of address shown in the home list entry for the mobile node,
   the home agent SHOULD decapsulate the inner datagram from the tunnel
   and re-tunnel it to the mobile node's care-of address.  However, if
   the tunnel source address is the same as the mobile node's care-of
   address, the home agent MUST NOT tunnel the datagram back to that
   same node; the home agent SHOULD instead discard the datagram.

   In either case, the home agent SHOULD also send a Binding Update
   message to the sender of the original datagram (the "inner" source
   address of the tunneled datagram), if it shares a mobility security
   association with this node.  The sending of this Binding Update
   message, though, is subject to the rate limiting restriction
   described in Section 7.1.









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8. Foreign Agent Considerations

   In addition to managing the resources needed to handle registrations
   in the base Mobile IP protocol, a foreign agent participating in
   Route Optimization assumes two additional responsibilities.  It
   SHOULD maintain a binding cache for forwarding datagrams to mobile
   nodes once they have moved to new care-of addresses, and it SHOULD
   support the use of a Binding Update message for norifying a mobile
   node's previous foreign agent that it has moved.


8.1. Establishing a Registration Key

   As part of the registration procedure, a mobile node and its
   new foreign agent can establish a registration key (Sections 2.4
   and 2.5).  Within this document, the registration key is used only
   for authentication of a single Binding Update message.  Other uses
   for the registration key are possible, such as the use of this key to
   provide privacy along the wireless link connecting between the mobile
   node and its foreign agent; such additional uses of the registration
   key are outside the scope of this discussion.

   This document specifies two alternative methods for establishing
   a registration key as part of a mobile node's registration with a
   foreign agent.

   The foreign agent may receive a Home Agent Registration Key
   Request extension in the mobile node's Registration Request message
   (Section 2.4).  The foreign agent need not process the Home Agent
   Registration Key Request extension.  When the Registration Reply is
   received by the foreign agent from the home agent, it MAY contain
   a Foreign Agent Registration Key Reply extension and a Mobile Node
   Registration Key Reply extension.  In this case, the foreign agent
   removes the Foreign Agent Registration Key Reply extension, and
   forwards the rest of the Registration Reply message to the mobile
   node.  The Foreign Agent Registration Key Reply extension is covered
   by its own authentication, not by the authentication covering the
   Registration Reply message itself, and thus removing the Foreign
   Agent Registration Key Reply extension does not interfere with
   the mobile node's ability to verify the authentication on the
   Registration Reply message.  The foreign agent decrypts the supplied
   registration key using the mobility security association it shares
   with the home agent, and stores the key as part fo the visitor list
   entry created for the mobile node for this registration.

   Alternatively, the foreign agent may receive a Diffie-Hellman
   Registration Key Request extension in the mobile node's Registration
   Request message (Section 2.5).  The extension contains the Prime



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   and Generator values, chosen by the mobile node, to be used for
   this execution of the Diffie-Hellman algorithm.  The foreign agent
   chooses its own private random number and returns the Diffie-Hellman
   Computed Value when it sends the Registration Reply message sent to
   the mobile node.  The foreign agent uses Prime and Generator values
   and the Computed Value from the mobile node that it received in the
   Diffie-Hellman Registration Key Request extension, together with its
   own chosen private random number, to form the registration key.  The
   foreign agent stores this registration key as part of the visitor
   list entry created for the mobile node for this registration.

   Establishing the registration key in effect establishes a mobility
   security association between the mobile node and the foreign agent.
   After the mobile node moves to a new care-of address, this mobility
   security association is used for authenticating the Binding Update
   message from the mobile node notifying this foreign agent of its
   movement.


8.2. Previous Foreign Agent Notification

   When a mobile node registers with a new foreign agent, the mobile
   node MAY request its new foreign agent to notify its previous foreign
   agent that it has moved.  The mobile node includes a Previous Foreign
   Agent Notification extension in its Registration Request message to
   the foreign agent, requesting that the foreign agent send a Binding
   Update message to its previous foreign agent.  As with all Binding
   Update messages, this Binding Update must be authenticated using the
   Route Optimization Authentication extension (Section 4.2).

   The foreign agent assembles the Binding Update message using
   the fields in the Registration Request message (including the
   Authenticator supplied in the Previous Foreign Agent Notification
   extension) and sends it to the previous foreign agent identified in
   the extension.  The Acknowledge (A) bit MUST be set in this Binding
   Update message.

   When the previous foreign agent receives the Binding Update, it will
   authenticate the message using the mobility security association
   set up with the registration key established with the mobile node
   during its registration with this mobile node.  If the message
   authentication is correct, the visitor list entry for this mobile
   node at the previous foreign agent will be deleted and a Binding
   Acknowledge message returned to the sender.  In addition, if a new
   care-of address was included in the Binding Update mesasge, the
   previous foreign agent will create a binding cache entry for the
   mobile node; the previous foreign agent can then tunnel datagrams to




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   the mobile node's new care-of address using that binding cache, just
   as any node maintaining a binding cache.

   In particular, the previous foreign agent can tunnel the Binding
   Acknowledge message back to the new care-of address specified in
   the received binding.  This creates an interesting problem for the
   new foreign agent when it receives the acknowledgment before the
   registration reply from the home agent.  It is suggested that the new
   foreign agent deliver the acknowledgment to the mobile node anyway,
   even though the mobile node is technically unregistered.  If there
   is concern that this provides a loophole for unauthorized traffic
   to the mobile node, the new foreign agent could limit the number of
   datagrams delivered to the unregistered mobile node to this single
   instance.  Alternatively, a new extension to the Registration Reply
   message can be defined to carry along the acknowledgement from the
   previous foreign agent.  This latter approach would have the benefit
   that fewer datagrams would be transmitted over bandwidth-constrained
   wireless media during registrations.

   The registration key established with this previous foreign agent is
   destroyed as a part of the processing of this Binding Update message.
   Since the previous foreign agent deletes the visitor list entry for
   the mobile node, it also deletes its record of the registration key.
   A registration key is thus useful only for the notification to the
   previous foreign agent after moving to a new care-of address.  No
   subsequent use of this registration key is possible, and thus no
   reply protection is necessary for the Binding Update message used for
   this notification.


8.3. Receiving Tunneled Datagrams

   When a foreign agent receives a datagram tunneled to itself, it
   examines its visitor list for an entry for the destination mobile
   node, as in the base Mobile IP protocol.  If a visitor list entry is
   found, the foreign agent delivers the datagram to the mobile node.

   However, if no visitor list entry is found, the foreign agent
   examines its binding cache for a cache entry for the destination
   mobile node.  If one is found, the foreign agent re-tunnels the new
   care-of address indicated in the binding cache entry.  In this case,
   the foreign agent also may deduce that the sender of the datagram has
   an out-of-date binding cache entry for this mobile node, since it
   otherwise would have tunneled the datagram directly to the correct
   new care-of address itself.  The foreign agent SHOULD then send a
   Binding Warning message to the original sender of the datagram (the
   "inner" source address of the tunnel).




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8.4. Rate Limiting

   A foreign agent MUST provide some mechanism to limit the rate at
   which it sends Binding Warning messages to to the same node about any
   given mobility binding.  This rate limiting is especially important
   because it is expected that, within the short term, many Internet
   nodes will not support maintenance of a binding cache.  In this
   case, continual transmissions of Binding Warning messages to such
   a correspondent node will only add unnecessary overhead to at the
   foreign agent and correspondent node, and along the Internet path
   between these nodes.


8.5. Sending Special Tunnels

   If a foreign agent receives a tunneled datagram for a mobile node
   for which it has no visitor list entry or binding cache entry, the
   foreign agent MAY forward the datagram to the mobile node's home
   agent by sending it as a special tunnel (Section 2.2).  The home
   agent will intercept the special tunnel datagram addressed to the
   mobile node in the same way as any datagram for the mobile node while
   it is away from home.  The foreign agent sending the special tunnel
   MUST NOT send a Binding Warning message in this case, since the
   home agent will send a Binding Update message when it received and
   re-tunnels the special tunnel datagram.


























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9. Mobile Node Considerations

9.1. Establishing a Registration Key

   When a mobile node sends a Registration Request message, it can also
   request a registration key to be established between itself and the
   new foreign agent.  The key may be used later to authenticate the
   Binding Update sent to this foreign agent to notify it that the
   mobile node has moved to a new care-of address.

   The mobile node may include a Home Agent Registration Key Request
   extension in its Registration Request message to request its home
   agent to act as a "key distribution center" (KDC) for establishing a
   registration key with its new foreign agent (Section 2.4).  The home
   agent chooses a new key and returns encrypted copies of it for the
   foreign agent and the mobile node.  The mobile node receives its copy
   of the new key in the Mobile Node Registration Key Reply extension
   in the Registration Reply extension.  The mobile node decrypts the
   key using the mobility security association it shares with its home
   agent, and stores the key for later use in notifying this foreign
   agent that it has moved to a new care-of address.

   Alternatively, the mobile node may include a Diffie-Hellman
   Registration Key Request extension in its Registration Request
   message to request its new foreign agent to participate in the
   Diffie-Hellman key exchange algorithm with it.  The mobile node
   chooses a private random number and includes the Prime and Generator
   values to be used in this execution of the Diffie-Hellman algorithm,
   together with the Computed Value based on its chosen private random
   number, in the Diffie-Hellman Registration Request extension.  The
   foreign agent chooses its own private random number, and returns the
   Computed Value based on this number in a Diffie-Hellman Registration
   Key Reply extension in the Registration Reply that it returns to the
   mobile node.  The mobile node forms the registration key according to
   the Diffie-Hellman algorithm and stores it for later use in notifying
   this foreign agent that it has moved to a new care-of address.


9.2. Notifying Previous Foreign Agents

   During registration with a new foreign agent, a mobile node may
   request its new foreign agent to notify its previous foreign agent
   that it has moved.  The new foreign agent sends a Binding Update
   message to the previous foreign agent on behalf of the mobile node.

   To nofity its previous foreign agent as part of the new registration,
   the mobile node includes a Previous Foreign Agent Notification
   extension (Section 4.1) in its Registration Request message.  The



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   extension contains only those values needed by the new foreign
   agent to construct the Binding Update message that are not already
   included in the Registration Request message.  In particular, the
   new foreign agent only sends the Binding Update message on behalf of
   the mobile node.  The mobile node computes the correct Authenticator
   value for the Binding Update message, based on the registration key
   it established with its previous foreign agent, and includes this
   Authenticator value in the extension for inclusion in the Binding
   Update message by the new foreign agent.  The new foreign agent does
   not learn the previous foreign agent registration key during the new
   registration or notification.

   When the Binding Acknowledgment message from the previous foreign
   agent is received by the new foreign agent, it detunnels it and sends
   it to the mobile node.  In this way, the mobile node can discover
   that its previous foreign agent has received the Binding Update
   mesage.  This is important, because otherwise the previous foreign
   agent would often become a "black hole" for datagrams destined for
   the mobile node based on out-of-date binding cache entries at other
   nodes.  The new foreign agent has no further responsibility for
   helping to update the binding cache at the previous foreign agent.

   The mobile node expects to eventually receive a Binding
   Acknowledgement message from its previous foreign agent, signifying
   that the Binding Update message was received.  If the acknowledgement
   has not been received after sufficient time, the mobile node is
   responsible for retransmitting another Binding Update message to its
   previous foreign agent.  Although the previous foreign agent may
   have already received and processed the Binding Update message (the
   Binding Acknowledge message may have been lost in transit to the new
   foreign agent), the mobile node should continue to retransmit its
   Binding Update message until the previous foreign agent responds with
   a Binding Acknowledgement.


















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References

   [1] CDPD Forum.  Cellular Digital Packet Data system specification.
       Release 1.0, July 1993.

   [2] W. Diffie and M. E. Hellman.  New directions in cryptography.
       IEEE Transactions on Information Theory, IT-22(6):644--654,
       November 1976.

   [3] Ralph Droms.  Dynamic Host Configuration Protocol.  RFC 1541,
       October 1993.

   [4] Donald E. Eastlake 3rd, Stephen D. Crocker, and Jeffrey I.
       Schiller.  Randomness recommendations for security.  RFC 1750,
       December 1994.

   [5] Stan Hanks, Tony Li, Dino Farinacci, and Paul Traina.  Generic
       Routing Encapsulation (GRE).  RFC 1701, October 1994.

   [6] Charles Perkins, editor.  IP mobility support.  Internet Draft,
       draft-ietf-mobileip-protocol-12.txt, August 1995.  Work in
       progress.

   [7] RSA Laboratories.  RSAREF(TM) 2.0:  A free cryptographic toolkit,
       April 1994.


























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Appendix A. Using a Master Key at the Home Agent

   Rather than storing each mobility security association that it has
   established with many different correspondent nodes and foreign
   agents, a home agent may manage its mobility security associations so
   that each of them can be generated from a single "master" key.  With
   the master key, the home agent could build a key for any given other
   node by computing the node-specific key as

      MD5(node-address || master-key || node-address)

   where node-address is the IP address of the particular node for which
   the home agent is building a key, and master-key is the single master
   key held by the home agent for all mobility security associations it
   has established with correspondent nodes.  The node-specific key is
   built by computing an MD5 hash over a string consisting of the master
   key with the node-address concatenated as a prefix and as a suffix.

   Using this scheme, when establishing each mobility security
   association, the network administrator managing the home agent
   computes the node-specific key and communicates this key to the
   network administrator of the other node through some "secure"
   channel, such as over the telephone.  The mobility security
   association is configured at this other node in the same way as any
   mobility security association.  At the home agent, though, no record
   need be kept that this key has been given out.  The home agent need
   only be configured to know that this scheme is in use for all of its
   mobility security associations (perhaps only for specific set of its
   mobile nodes).

   When the home agent then needs a mobility security association as
   part of Route Optimization, it builds the node-specific key based on
   the master key and the IP address of the other node with which it
   is attempting to authenticate.  If the other node knows the correct
   node-specific key, the authentication will succeed; otherwise, it
   will fail as it should.















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Chairs' Addresses

   The working group can be contacted via the current chairs:

      Tony Li
      cisco Systems
      170 W. Tasman Drive
      San Jose, CA  95134

      Phone:  +1-408-526-8186
      E-mail: tli@cisco.com



      Jim Solomon
      Motorola, Inc.
      1301 E. Algonquin Road
      Schaumburg, IL  60196

      Phone:  +1-708-576-2753
      E-mail: solomon@comm.mot.com






























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Authors' Addresses

   Questions about this document can also be directed to the authors:

      David B. Johnson
      Computer Science Department
      Carnegie Mellon University
      5000 Forbes Avenue
      Pittsburgh, PA  15213-3891

      Phone:  +1-412-268-7399
      Fax:    +1-412-268-5576
      E-mail: dbj@cs.cmu.edu



      Charles Perkins
      Room J1-A25
      T. J. Watson Research Center
      IBM Corporation
      30 Saw Mill River Rd.
      Hawthorne, NY  10532

      Phone:  +1-914-784-7350
      Fax:    +1-914-784-7007
      E-mail: perk@watson.ibm.com

























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