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Mobile IP Working Group                                  Charles Perkins
INTERNET DRAFT                                          Sun Microsystems
25 Feburary 1999                                        David B. Johnson
                                              Carnegie Mellon University



                    Route Optimization in Mobile IP

                    draft-ietf-mobileip-optim-08.txt


Status of This Memo

   This document is a submission by the Mobile IP Working Group of the
   Internet Engineering Task Force (IETF).  Comments should be submitted
   to the mobile-ip@SmallWorks.COM mailing list.

   Distribution of this memo is unlimited.

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  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."

   The list of current Internet-Drafts can be accessed at:

      http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at:

      http://www.ietf.org/shadow.html.


Abstract

   This document defines extensions to 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 to cache the binding of a mobile node and to






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   then tunnel their own datagrams for the mobile node directly to that
   location, bypassing the route for each datagram through 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   out-of-date cached binding, to be forwarded directly to the mobile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                                                               i

 1. Introduction                                                       1

 2. Terminology                                                        2

 3. Route Optimization Overview                                        2
     3.1. Binding Caches  . . . . . . . . . . . . . . . . . . . . .    3
     3.2. Foreign Agent Smooth Handoff  . . . . . . . . . . . . . .    4

 4. Route Optimization Message Formats                                 6
     4.1. Binding Warning Message . . . . . . . . . . . . . . . . .    6
     4.2. Binding Request Message . . . . . . . . . . . . . . . . .    7
     4.3. Binding Update Message  . . . . . . . . . . . . . . . . .    8
     4.4. Binding Acknowledge Message . . . . . . . . . . . . . . .   11
     4.5. Route Optimization Authentication Extension . . . . . . .   12
     4.6. Modified Registration Request Message . . . . . . . . . .   12

 5. Format of Smooth Handoff Extensions                               13
     5.1. Previous Foreign Agent Notification Extension . . . . . .   13
     5.2. Modified Mobility Agent Advertisement Extension . . . . .   15

 6. Binding Warning Extension                                         16

 7. Miscellaneous Home Agent Operations                               17
     7.1. Home Agent Rate Limiting  . . . . . . . . . . . . . . . .   17
     7.2. Mobility Security Association Management  . . . . . . . .   17
     7.3. Managing Binding Updates for Correspondent Nodes  . . . .   18

 8. Miscellaneous Foreign Agent Operations                            18
     8.1. Previous Foreign Agent Notification . . . . . . . . . . .   18
     8.2. Maintaining Binding Caches  . . . . . . . . . . . . . . .   20
     8.3. Rate Limiting . . . . . . . . . . . . . . . . . . . . . .   20

 9. Security Considerations                                           20

10. Summary                                                           20

11. Acknowledgement                                                   21

 A. Using a Master Key at the Home Agent                              21




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References                                                            23

Chairs' Addresses                                                     24

















































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1. Introduction

   The base Mobile IP protocol [12], allows any mobile node to move
   about, changing its point of attachment to the Internet, while
   continuing to be identified by its home IP address.  Correspondent
   nodes sending IP datagrams to a mobile node at its home address
   in the same way as with any other destination.  This scheme
   allows transparent interoperation between mobile nodes and their
   correspondent nodes, but forces all datagrams for a mobile node to
   be routed through its home agent.  Thus, datagrams to the mobile
   node are often routed along paths that are significantly longer than
   optimal.  For example, if a mobile node is visiting some subnet,
   even datagrams from a correspondent node on the same subnet must be
   routed through the Internet to the mobile node's home agent (on its
   home network), only then to be tunneled back to the original subnet
   for final delivery.  This indirect routing delays the delivery of the
   datagrams to mobile nodes, and places an unnecessary burden on the
   networks and routers along their paths through the Internet.

   In this document, we will define extensions to the operation of
   the base Mobile IP protocol to allow for better routing, so that
   datagrams can be routed from a correspondent node to a mobile node
   without going to the home agent first.  We refer collectively to
   these extensions as Route Optimization.

   Route Optimization extensions provide a means for nodes 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 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 care-of address.

   All operation of Route Optimization that changes the routing of IP
   datagrams to the mobile node is authenticated using the same type of
   mechanisms used in the base Mobile IP protocol.  This authentication
   generally relies on a mobility security association established
   in advance between the sender and receiver of such messages.  The
   association can be created using ISAKMP [7], SKIP [1], or any of the
   registration key establishment methods specified in [10].

   After Section 2 gives some extra terminology, Section 3 provides
   an overview of the basic protocol operations associated with
   Route Optimization.  Section 4 defines the message types used
   to update binding caches.  Subsequent sections show the formats
   for the messages, explaining the function of fields within each
   message.  Home agent considerations in Section 7, and foreign agent
   considerations in Section 8.




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2. Terminology

   This document introduces the following terminology, in addition to
   that used to describe the base Mobile IP protocol:

      Binding cache

         A cache of mobility bindings of mobile nodes, maintained by a
         node for use in tunneling datagrams to those mobile nodes.

      Binding update

         A message indicating a mobile node's current mobility binding,
         and in particular its care-of address.

      Registration Lifetime

         The registration lifetime is the time duration for which a
         binding is valid.  The term remaining registration lifetime
         means the amount of time remaining for which a registration
         lifetime is still valid, at some time after the registration
         was approved by the home agent.

      Security Parameters Index (SPI)

         An index identifying a security context between a pair of
         nodes among the contexts available in the Mobility Security
         Association.  SPI values 0 through 255 are reserved and MUST
         NOT be used in any Mobility Security Association.

      Triangle Routing

         A situation in which a Correspondent Host's packets to a Mobile
         Host follow a path which is longer than the optimal path
         because the packets must be forwarded to the Mobile Host via a
         Home Agent.

   This protocol specification uses conventional meanings [2] for
   capitalized words such as MUST, SHOULD, etc., to indicate requirement
   levels for various protocol features.


3. Route Optimization Overview

   This section provides an overview of the protocols and operations of
   Route Optimization.  These can be divided into two main parts:

    1. Updating binding caches




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    2. Managing smooth handoffs between foreign agents

   The first part of the document goes into detail about binding cache
   maintenance, and then smooth handoff is considered.


3.1. Binding Caches

   Route Optimization provides a means for any node to maintain a
   binding cache containing the care-of address 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 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.

   Any node may maintain a binding cache to optimize its own
   communication with mobile nodes.  A node may create or update a
   binding cache entry for a mobile node only when it has received
   and authenticated the mobile node's mobility binding.  As before,
   each binding in the binding cache also has an associated lifetime,
   specified in the Binding Update message in which the node obtained
   the binding.  After the expiration of this time period, the binding
   is deleted from the cache.  In addition, a node cache may use any
   reasonable strategy for managing the space within the binding 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.

   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.  The home agent should then send
   a Binding Update message to the original source node, informing it
   of the mobile node's current mobility binding.  No acknowledgment
   for such a 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.  For
   a Binding Update to be authenticated by the original source node,



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   the source node and the home agent must have established a mobility
   security association.

   Similarly, when any node (e.g., a foreign agent) receives a tunneled
   datagram, if it has a binding cache entry for the destination mobile
   node (and thus 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 this mobile node.
   In this case, the receiving node should send a Binding Warning
   message to the mobile node's home agent, advising it to send a
   Binding Update message to the node that tunneled this datagram.  The
   mobile node's home agent can be determined from the binding cache
   entry, because the home agent address is learned from the Binding
   Update that established this cache entry.  The address of the node
   that tunneled this datagram can be determined from the datagram's
   header, since the address of the node tunneling this datagram is
   the outer source address of the encapsulated datagram.  As in the
   case of a Binding Update sent by the mobile node's home agent, no
   acknowledgment 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.

   When sending an IP datagram, if the sending node has a binding cache
   entry for the destination node, it should tunnel the datagram to the
   mobile node's care-of address using the encapsulation techniques used
   by home agents, and described in [8, 9, 3, 4].


3.2. Foreign Agent Smooth 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
   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 likely to be lost and are assumed to be
   retransmitted by higher-level protocols if needed.  The old foreign
   agent eventually deletes its visitor list entry for the mobile node
   after the expiration of the registration lifetime.

   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



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   the mobile node's previous foreign agent, from correspondent nodes
   with out-of-date binding cache entries for the mobile node, to be
   forwarded to its new care-of address.  Finally, this notification
   allows any resources consumed by the mobile node at the previous
   foreign agent (such as radio channel reservations) to be released
   immediately, rather than waiting for its registration lifetime to
   expire.

   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
   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 acknowledgment from the
   previous foreign agent.  The 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 using the
   security association shared with its previous foreign agent.  This
   notification will typically 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 [5]
   to its new location.  Any tunneled datagrams for the mobile node that
   arrive at its previous foreign agent after the forwarding pointer has
   been created can then be re-tunneled to the mobile node's new care-of
   address.

   For this smooth handoff to be secure, during registration with a
   new foreign agent, the mobile node and the foreign agent have to
   have a security association.  The security association is used to
   authenticate the notification sent to the previous foreign agent.

   The Mobility Agent Advertisement extension of the agent advertisement
   message is revised under Route Optimization to include a bit
   indicating that the foreign agent sending the advertisement supports
   smooth handoffs.

   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 that foreign agent has expired its binding.
   The mobile node is likely to select a small timeout value for the
   lifetime available to such bindings sent to previous foreign agents.








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4. Route Optimization Message Formats

   Route Optimization defines four message types used for management
   of binding cache entries.  These message types fit in the numbering
   space defined in the base Mobile IP specification for messages sent
   to UDP port 434.  Each of these messages begins with a one-octet
   field indicating the type of the message.  The binding cache
   management messages in this section are carried by way of UDP, sent
   to port 434.

   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.


4.1. Binding Warning Message

   A Binding Warning message is used to transmit advice that one or
   more correspondent nodes or foreign agents are likely to have either
   no binding cache entry or an out-of-date binding cache entry for
   some mobile node.  When any node detunnels a datagram destined for
   the mobile node, if it is not the current foreign agent for the
   destination mobile node, it SHOULD send a Binding Warning message to
   the mobile node's home 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      |                   Reserved                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Mobile Node Home Address                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Target Node Addresses ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The format of the Binding Warning message is illustrated above, and
   contains the following fields:




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      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.

      Target Node Addresses
               One or more addresses of nodes that need to receive
               Binding Update messages.  Each node should be the target
               of a Binding Update message sent by the home agent.

   A home agent will receive a Binding Warning message if a node
   maintaining a binding cache entry for one of the home agent's mobile
   nodes uses an out-of-date entry.  When a home agent receives a
   Binding Warning message, it should send a Binding Update message to
   each target node address identified in the Binding Warning, giving it
   the current binding for the mobile node identified in the mobile node
   home address field of the Binding Warning.

   When a mobile node receives a new Care-of Address, it MAY send a
   Binding Warning message to its Home Agent, requesting that the home
   agent send Binding Update messages to one or more correspondent
   nodes.  This feature MAY be used by the mobile node when it returns
   to its home network, so that the Home Agent will send out Binding
   Updates with zero lifetimes to all the mobile node's correspondent
   nodes.  It is important for the correspondent nodes to delete their
   binding cache entries for the mobile node when the mobile node no
   longer has a Care-of Address.


4.2. 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.

    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                        +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   The format of the Binding Request message is illustrated above, and
   contains the following fields:

      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.

   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 is required to 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 make no further attempt to satisfy Binding Requests
   on behalf of that mobile node.  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.


4.3. Binding Update Message

   The Binding Update message is used for notification of a mobile
   node's current mobility binding.  It should be sent by the mobile
   node's home agent in response to a Binding Request message, a Binding
   Warning message, or the reception of a Binding Warning extension to
   a Registration Request.  It should 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.











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    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 ...
   +-+-+-+-+-+-+-+-

   The format of the Binding Update message is illustrated above, and
   contains the following fields:

      Type     18

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

      I        The 'I' (identification present) bit is set by the node
               sending the Binding Update message if the identification
               field is present in the message.

      M        If the 'M' (minimal encapsulation) bit is set, datagrams
               may be tunneled to the mobile node using the minimal
               encapsulation protocol [9].

      G        If the 'G' (Generic Record Encapsulation, or GRE) bit is
               set, datagrams may be tunneled to the mobile node using
               GRE [3].

      Rsvd     Reserved.  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.



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      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.

   Each Binding Update message indicates the binding's maximum lifetime.
   When sending the Binding Update message, the home agent should set
   this lifetime to the remaining registration lifetime.  A node wanting
   to provide continued service with a particular binding cache entry
   may attempt to reconfirm that mobility binding before the expiration
   of the registration lifetime.  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.  Note that the node
   maintaining the binding should also keep track of the home agent's
   address, to be able to fill in the destination IP address of future
   Binding Requests.

   When a node receives a Binding Update message, it is required to
   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 4.5), which is required.  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.

   Under all circumstances, the sending of Binding Update messages is
   subject to the rate limiting restriction described in Section 7.1.

   When using nonces for replay protection, the identification field in
   the Binding Update message is used differently in this case, to still



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   allow replay protection even though the Binding Update is not being
   sent in reply to a request directly from the target node.  In this
   case, the home agent is required to set the high-order 32 bits of the
   identification field to the value of the nonce that will be used by
   the home agent in the next Binding Update message sent to this node.
   The low-order 32 bits of the identification field are required to be
   set to the value of the nonce being used for this message.

   Thus, on each Binding Update message, the home agent communicates to
   the target node, the value of the nonce that will be used next time,
   and if no Binding Updates are lost in the network, the home agent and
   the target node can remain synchronized with respect to the nonces
   being used.  If, however, the target node receives a Binding Update
   with what it believes to be an incorrect nonce, it may resynchronize
   with the home agent by using a Binding Request message.


4.4. 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 a
   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      |            Reserved           |    Status     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Mobile Node Home Address                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Identification                        +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The format of the Binding Acknowledge message is illustrated above,
   and contains the following fields:

      Type     19

      Status   If the Status is nonzero, this acknowledgment is
               negative.  For instance, if the Binding Update was
               not accepted, but the incoming datagram has the
               Acknowledge flag set, then the status code should be set
               appropriately in the Binding Acknowledge message.

      Reserved Sent as 0; ignored on reception.




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      Mobile Node Home Address
               Copied from the Binding Update message being
               acknowledged.

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

   Allowable values for the Status include:

      128 reason unspecified
      129 administratively prohibited
      130 insufficient resources
      131 sending node failed authentication
      133 identification mismatch
      134 poorly formed Binding Update

   Up-to-date values of the Code field are specified in the most recent
   "Assigned Numbers" [13].


4.5. Route Optimization Authentication Extension

   The Route Optimization Authentication extension is used to
   authenticate certain Route Optimization management messages.  It has
   the same format and default algorithm support requirements as the
   three authentication extensions defined for base Mobile IP [12], but
   is distinguished by its type, which is 35.  The authenticator value
   is computed, as before, from the 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.  This extension is required to be used in
   any Binding Update message.


4.6. Modified 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 any Binding Update message.
   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.




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

    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|V|P|r|           Lifetime            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     (Unchanged ...)
   +-+-+-+-+-+-+-+-+-+-+-+-+

      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.

      r        reserved


5. Format of Smooth Handoff Extensions

   This section specifies the format for messages which are used
   to enable smooth handoff from a mobile node's previous foreign
   agent to its new foreign agent when a mobile node initiates a new
   registration.


5.1. 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 may
   then delete the mobile node's visitor list entry and, if a new
   care-of address is included in the Binding Update message, create a
   binding cache entry for the mobile node with 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.









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    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                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              SPI                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Authenticator ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type     96

      Length   14 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.

      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
               delete its visitor list entry only for the mobile node,
               but not forward datagrams for it).

      SPI      Security Parameters Index (4 bytes).  An opaque
               identifier.  The SPI is copied over into the Route
               Optimization Authentication extension by the new foreign
               agent.



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      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.

   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 [12].


5.2. Modified Mobility Agent Advertisement Extension

   Performing smooth handoffs requires one minor change to the existing
   Mobile IP Mobility Agent Advertisement extension [12].  A new flag
   bit, the 'S' bit, replaces a previously unused reserved bit in
   the extension, to indicate that the foreign agent supports smooth
   handoffs.  By default, every foreign agent that supports smooth
   handoffs SHOULD support at least the establishment of a registration
   key [10] by using Diffie-Hellman 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     |        Sequence Number        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Lifetime            |R|B|H|F|M|G|V|T|S|  reserved   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       ...      zero or more Care-of Addresses
   +-+-+-

   Thus, the proposed modification to the Mobility Agent Advertisement
   extension, illustrated above, keeps the advertisement almost the same
   as in the base Mobile IP specification, except for adding following
   bit:

      S        The 'S' smooth handoff bit is set by the foreign agent
               sending the agent advertisement message to indicate
               that it supports the smooth handoffs, and thus the
               Registration Key Request and Diffie-Hellman Registration
               Key Reply extensions [10].

   More detailed information about the handling of this extension by
   foreign agents is deferred until Section 8.1.







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6. Binding Warning Extension

   A mobile node may append a Binding Warning Extension to a
   Registration request.  The Binding Warning extension is used to
   advise a mobile node's home agent that one or more correspondent
   nodes are likely to have either no binding cache entry or an
   out-of-date binding cache entry for the mobile node sending the
   Registration Request.

    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    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Mobile Node Home Address                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Target Node Addresses ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The format of the Binding Warning extension is illustrated above, and
   contains the following fields:

      Type     TBD

      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.

      Target Node Addresses
               One or more addresses of the correspondent nodes that
               need to receive Binding Update messages.  Each node
               should be the target of a Binding Update message sent by
               the home agent.

   When a home agent receives a Binding Warning extension as part of a
   valid Registration Request, it should send a Binding Update message
   to each target node address identified in the Binding Warning, giving
   it the current binding for the mobile node identified in the mobile
   node home address field of the Binding Warning.

   When a mobile node returns to its home network, it MAY append a
   Binding Warning extension to the Registration Request message sent
   to its Home Agent, requesting that the home agent send Binding
   Update messages (naturally, with zero lifetimes) to one or more
   correspondent nodes.  It is important for the correspondent nodes
   to delete their binding cache entries for the mobile node when the
   mobile node no longer has a Care-of Address.



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7. Miscellaneous Home Agent Operations

7.1. Home Agent Rate Limiting

   A home agent is required to 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, most
   Internet nodes will not support maintenance of a binding cache.  In
   this case, continual transmissions of Binding Update messages will
   only waste processing resources at the home agent and correspondent
   node, and along the Internet path between these nodes.


7.2. 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, only the home agent is aware of
   the mobile node's mobility binding and only the home agent tunnels
   datagrams to the mobile node.  Thus, all routing of datagrams to the
   mobile node while away from its home network is controlled by the
   home agent.  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
   almost any node in the Internet.  Since no authentication or key
   distribution protocol is generally available in the Internet today,
   the Route Optimization procedures defined in this document MAY make
   use of the same type of manual key distribution discussed 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 include the same parameters as required by base Mobile IP [12].

   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 are required to have established a mobility security
   association.  This mobility security association, though, could
   conceivably be used in creating and updating binding cache entries
   at this correspondent node for all mobile nodes served by this home
   agent.  Doing so places the correspondent node in a fairly natural
   relationship with respect to the mobile nodes served by this home
   agent.  For example, the mobile nodes may represent different people



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   affiliated with the same organization owning the home agent, with
   which the user of the correspondent node often collaborates.  The
   effort of establishing such a mobility security association with
   the relevant home agent may be more easily justified (appendix A)
   than the effort of doing so with each mobile node.  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.

   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 authenticate the
   values transmitted in Route Optimization extensions.


7.3. Managing Binding Updates for Correspondent Nodes

   The home agent MAY keep a list of correspondent nodes from which it
   has received Binding Acknowledgements for Binding Updates for active
   registrations (i.e., registrations which have not yet timed out).  In
   this case, when the home agent receives a valid Registration Request,
   it MAY transmit new Binding Updates to each correspondent node that
   is on its list for the particular mobile node.  In order to know
   which correspondent nodes correctly received the Binding Updates, the
   home agent MUST set the `A' bit in the Binding Update, requesting an
   acknowledgement.

   Rate-limiting MUST be employed by a Home Agent offering this service,
   as specified in section 7.1.


8. Miscellaneous Foreign Agent Operations

   This section details various operational considerations important
   for foreign agents wishing to support smooth handoff.  This includes
   processing Previous Foreign Agent Notification messages, and the
   maintenance of up-to-date binding cache entries.


8.1. Previous Foreign Agent Notification

   When a foreign agent receives a Previous Foreign Agent Notification
   message, it creates a Binding Update for the previous foreign agent,
   using the specified SPI and precomputed authenticator sent to it by



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   the mobile node.  The Binding Update message is also required to set
   the 'A' bit, so that the previous foreign agent will know to send a
   Binding Acknowledge message back to the mobile node.

   When the previous foreign agent receives the Binding Update, it will
   authenticate the message using the mobility security association and
   SPI specified in the Binding Update.  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 message, the previous
   foreign agent will create a binding cache entry for the mobile node;
   the previous foreign agent can then tunnel datagrams to the mobile
   node's new care-of address using that binding cache, just as any node
   maintaining a binding cache.  The previous foreign agent is also
   expected to return a Binding Acknowledge message to the mobile node.

   Note that this Binding Acknowledge is addressed to the mobile node,
   and SHOULD be tunneled using the new binding cache entry.  The
   tunneled acknowledgment then SHOULD be delivered directly to the
   new foreign agent, without having to go to the home network.  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 acknowledgment from the previous foreign agent.  This
   latter approach would have the benefit that fewer datagrams would
   be transmitted over bandwidth-constrained wireless media during
   registration.

   When the Binding Acknowledge 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
   message.  The mobile node has to be certain that its previous foreign
   agent has been notified about its new care-of address, because
   otherwise the previous foreign agent could 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, and does not retransmit the message even if
   no acknowledgment is received.





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   If the acknowledgment 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 Acknowledge.


8.2. Maintaining Binding Caches

   It is possible that the binding cache entry taken by the previous
   foreign agent from the information in the Previous Foreign Agent
   Notification extension 5.1 will be deleted from its cache at any
   time.  In this case, the previous foreign agent will be unable to
   re-tunnel subsequently arriving tunneled datagrams for the mobile
   node, and would resort to using a special tunnel [11].  Mobile nodes
   SHOULD assign small lifetimes to such bindings so that they will not
   take up space in the foreign agent's binding cache for very long.


8.3. Rate Limiting

   A foreign agent MUST provide some mechanism to limit the rate at
   which it sends Binding Warning messages 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 will only waste
   processing resources at the foreign agent and correspondent node, and
   along the Internet path between these nodes.


9. Security Considerations

   The calculation of the authentication data supplied with the Route
   Optimization Authentication extension in section 4.5 is specified
   to be the same as in the base Mobile IP document for ease of
   implementation.  There is a better method available (HMAC), specified
   in RFC 2104 [6].  If the base Mobile IP specification is updated to
   use HMAC, then this route optimization specification should also be
   updated similarly.


10. Summary

   In this document, we have presented the current protocol definition
   for Route Optimization, by which is meant the elimination of triangle



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   routing whenever the correspondent node is able to perform the
   necessary protocol operations.  The Route Optimization protocol
   definition is largely concerned with supplying a Binding Update to
   any correspondent node that needs one (and has some realistic chance
   to process it correctly).  The Binding Update message is also used in
   conjunction with the Previous Foreign Agent Notification message to
   allow for smooth handoffs between foreign agents.


11. Acknowledgement

   Expanding the Binding Warning to allow a mobile node to send a list
   of correspondent nodes to the Home Agent was suggested by Mohamad
   Khalil, Emad Qaddoura, Haseeb Akhtar, and Liem Le of Nortel Networks.


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, for example 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 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




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   node-specific key, the authentication will succeed; otherwise, it
   will fail as it should.


















































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References

    [1] A. Aziz, T. Markson, and H. Prafullchandra.  Simple
        Key-Management For Internet Protocols (SKIP).
        draft-ietf-ipsec-skip-07.txt, August 1996.  (work in progress).

    [2] S. Bradner.  Key Words for Use in RFCs to Indicate Requirement
        Levels.  RFC 2119, March 1997.

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

    [4] Stan Hanks, Tony Li, Dino Farinacci, and Paul Traina.  Generic
        Routing Encapsulation over IPv4 networks.  RFC 1702, October
        1994.

    [5] David B. Johnson.  Scalable and Robust Internetwork Routing
        for Mobile Hosts.  In Proceedings of the 14th International
        Conference on Distributed Computing Systems, pages 2--11, June
        1994.

    [6] H. Krawczyk, M. Bellare, and R. Cannetti.  HMAC: Keyed-Hashing
        for Message Authentication.  RFC 2104, February 1997.

    [7] D. Maughan, M. Schertler, M. Schneider, and J. Turner.  Internet
        Security Association and Key Management Protocol (ISAKMP).
        draft-ietf-ipsec-isakmp-08.txt, July 1997.  (work in progress).

    [8] Charles Perkins.  IP Encapsulation within IP.  RFC 2003, May
        1996.

    [9] Charles Perkins.  Minimal Encapsulation within IP.  RFC 2004,
        May 1996.

   [10] Charles E. Perkins and David B. Johnson.  Registration Keys for
        Route Optimization.  draft-ietf-mobileip-regkey-00.txt, November
        1997.  (work in progress).

   [11] Charles E. Perkins and David B. Johnson.  Special Tunnels for
        Mobile IP.  draft-ietf-mobileip-spectun-00.txt, November 1997.
        (work in progress).

   [12] C. Perkins, Editor.  IP Mobility Support.  RFC 2002, October
        1996.

   [13] Joyce K. Reynolds and Jon Postel.  Assigned Numbers.  STD 2,
        October 1994.





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Chairs' Addresses

   The working group can be contacted via the current chairs:

       Jim Solomon                             Erik Nordmark
       Redback Networks, Inc.                  Sun Microsystems, Inc.
       1301 E. Algonquin Road                  17 Network Circle
       Schaumburg, IL 60196                    Menlo Park, California 94025
       USA                                     USA

       Phone:  +1-847-576-2753                 Phone:  +1 650 786-5166
       Fax:                                    Fax:  +1 650 786-5896
       E-mail:  solomon@redbacknetworks.com    E-mail:  nordmark@sun.com


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

       Charles E. Perkins                  David B. Johnson
       Sun Microsystems Laboratories       Computer Science Department
       Mail Stop MPK15-214
       Room 2682
       Sun Microsystems, Inc.              Carnegie Mellon University
       15 Network Circly                   5000 Forbes Avenue
       Menlo Park, California  94025       Pittsburgh, PA 15213-3891
       USA                                 USA
       Phone:  +1-650-786-6464             Phone:  +1-412-268-7399
       Fax:  +1-650-786-6445               Fax:  +1-412-268-5576
       E-mail:  charles.perkins@Sun.COM    E-mail:  dbj@cs.cmu.edu
























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