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Versions: 00 01 02 03 04 05 07 08 09 10 11 RFC 4881

Network Working Group                                K. El Malki, Editor
INTERNET-DRAFT                                                   Athonet
Expires: April 2006                                       3 October 2005



                     Low Latency Handoffs in Mobile IPv4
             <draft-ietf-mobileip-lowlatency-handoffs-v4-11.txt>


Status of this memo

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Copyright Notice

   Copyright (C) The Internet Society (2005).  All Rights Reserved.

Abstract

   Mobile IPv4 describes how a Mobile Node can perform IPv4-layer
   handoffs between subnets served by different Foreign Agents.  In
   certain cases, the latency involved in these handoffs can be above
   the threshold required for the support of delay-sensitive or real-
   time services.  The aim of this document is to present two methods to
   achieve low-latency Mobile IPv4 handoffs.  In addition, a combination
   of these two methods is described.  The described techniques allow
   greater support for real-time services on a Mobile IPv4 network by
   minimising the period of time when a Mobile Node is unable to send or
   receive IPv4 packets due to the delay in the Mobile IPv4 Registration
   process.



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TABLE OF CONTENTS

   1. Introduction.....................................................3
      1.1. Terminology.................................................4
      1.2. The Techniques..............................................5
      1.3. L2 triggers.................................................7
      1.4. Requirements language.......................................9
   2. Requirements.....................................................9
   3. The PRE-REGISTRATION Handoff Method.............................10
      3.1. Operation..................................................10
      3.2. Network-Initiated Handoff..................................13
      3.3. Mobile-Initiated Handoff...................................14
      3.4. Obtaining and Proxying nFA Advertisements..................16
         3.4.1. Inter-FA Solicitation.................................16
         3.4.2. Tunneled nFA Advertisements...........................17
      3.5. Caching Router Advertisements..............................17
      3.6. Movement Detection, MN and FA Considerations...............18
      3.7. L2 Address Considerations..................................19
      3.8. Applicability of PRE-REGISTRATION Handoff..................20
   4. The POST-REGISTRATION Handoff Method............................21
      4.1. Two Party Handoff..........................................22
      4.2. Three Party Handoff........................................26
      4.3. Renewal or Termination of Tunneling Service................31
      4.4. When will the MN perform a Mobile IPv4 Registration?.......31
      4.5. Handoff Request (HRqst) Message format.....................32
      4.6. Handoff Reply (HRply) Message..............................34
      4.7. Handoff To Third (HTT) Message.............................36
      4.8. Applicability of POST-REGISTRATION Handoff Method..........36
   5. Combined Handoff Method.........................................37
   6. Layer 2 and Layer 3 Handoff timing Considerations...............38
   7. Reverse Tunneling Support.......................................39
   8. Handoff Signaling Failure Recovery..............................39
      8.1. PRE-REGISTRATION Signaling Failure Recovery................39
         8.1.1. Failure of PrRtSol and PrRtAdv........................39
         8.1.2. Failure of Inter-FA solicitation and advertisement....40
      8.2. POST-REGISTRATION Signaling Failure Recovery...............40
         8.2.1. HRqst Message Dropped.................................40
         8.2.2. HRply Message Dropped.................................41
   9. Generalized Link Layer and IPv4 Address (LLA) Extension.........41
      9.1. 3GPP2 IMSI Link Layer Address and Connection ID Extension..42
      9.2. 3GPP IMSI Link Layer Address Extension.....................43
      9.3. Ethernet Link Layer Address Extension......................44
      9.4. IEEE 64-Bit Global Identifier (EUI-64) Address Extension...44
      9.5. Solicited IPv4 Address Extension...........................45
      9.6. Access Point Identifier Extension..........................45
      9.7. FA IPv4 Address Extension..................................46
   10. IANA Considerations............................................46
      10.1. New Extension values......................................47
      10.2. Generalized Link Layer and IP Address Identifier (LLA) Sub-
      type values.....................................................47



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      10.3. New Message Type and Code.................................47
   11. Security Considerations........................................48
   12. Acknowledgements...............................................50
   13. Editor's Address...............................................50
   14. Contributing Authors...........................................50
   15. References.....................................................51
      15.1. Normative References......................................51
      15.2. Informative References....................................51
   Appendix A - Gateway Foreign Agents................................52
   Appendix B - Low Latency Handoffs for Multiple-Interface MNs.......53
   Appendix C - PRE_REGISTRATION Message Summary......................54


1.
   Introduction

   Mobile IPv4 [1] describes how a Mobile Node (MN) can perform IPv4-
   layer handoff between subnets served by different Foreign Agents
   (FAs).  In certain cases, the latency involved in handoff can be
   above the threshold required for the support of delay-sensitive or
   real-time services.  The aim of this document is to present two
   techniques to achieve low-latency Mobile IPv4 handoff during movement
   between FAs.  A further combination of these two techniques is also
   described.  The presented techniques allow greater support for real-
   time services on a Mobile IPv4 network by minimising the period of
   time during which a MN is unable to send or receive IPv4 packets due
   to the delay in the Mobile IPv4 Registration process.  One or more of
   these techniques may be required to achieve fast Mobile IPv4 handoffs
   over different wireless technologies (e.g. WLAN, Cellular, WiMAX,
   Flash-OFDM etc.).  Each wireless technology has different layer-2
   handoff procedures and the best low latency technique for each
   scenario should be used to optimize the handoff performance.  Further
   deployment and experimentation is required to determine which
   technique is best suited to the wireless technologies in terms of
   implementation and performance.  Therefore the authors encourage
   further performance measurements and work on Low-Latency-over-foo
   specifications in collaboration with the appropriate wireless
   technology fora to describe the applicability to different wireless
   layer-2s.

   In the rest of this section, terminology used throughout the document
   is presented, the handoff techniques are briefly described, and the
   use of link layer information is outlined.  In Section 2, a brief
   description of requirements is presented.  Section 3 describes the
   details of the PRE-REGISTRATION handoff technique, while Section 4
   describes the details of the POST-REGISTRATION handoff technique.  In
   Section 5, a combined method using the two handoff techniques
   together is presented.  Section 6 discusses Layer 2 and Layer 3
   handoff timing considerations.  Section 7 discusses reverse tunneling
   support, Section 8 describes mechanisms to recover from message
   failures while Section 9 describes protocol extensions required by



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   the handoff techniques.  Sections 10 and 11 discuss IANA and security
   considerations.  Finally the three appendices discuss additional
   material related to the handoff techniques.  Appendix A gives a short
   introduction to Regional Registrations [11] which can be used
   together with low latency handoffs.  Appendix B discusses low latency
   handoff when a MN has multiple wireless L2 interfaces, in which case
   the techniques in this document may not be necessary.  Appendix C
   provides a summary of the messages used in PRE-REGISTRATION.


1.1.
     Terminology

   This section presents a few terms used throughout the document.

      oFA - old Foreign Agent (FA), the FA involved in handling the
         care of address of a Mobile Node (MN) prior to a Layer 3
         (L3) handoff.

      nFA - new Foreign Agent, the FA anticipated to be handling a
         MN's care of address after completion of an L3 handoff.

      aFA - anchor Foreign Agent, the FA that is currently handling
         the network end of the tunnel in POST-REGISTRATION.

      L2 handoff - Movement of a MN's point of Layer 2 (L2)
         connection from one wireless access point to another.

      L3 handoff - Movement of a MN between FAs which involves
         changing the care-of address (CoA) at Layer 3 (L3).

      L2 trigger - Information from L2 that informs L3 of particular
         events before and after L2 handoff.  The descriptions of L2
         triggers in this document are not specific to any particular
         L2, but rather represent generalizations of L2 information
         available from a wide variety of L2 protocols.

      L2-MT - An L2 trigger that occurs at the MN informing of
         movement to a certain nFA (Mobile Trigger).

      L2-ST or source trigger - An L2 trigger that occurs at oFA,
         informing the oFA that L2 handoff is about to occur.

      L2-TT or target trigger - An L2 trigger that occurs at nFA,
         informing the nFA that a MN is about to be handed off to
         nFA.

      L2-LU - An L2 trigger that occurs at the MN or nFA, informing
         that the L2 link between MN and nFA is established.





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      L2-LD - An L2 trigger that occurs at the oFA, informing the oFA
         that the L2 link between MN and oFA is lost.

      low latency handoff - L3 handoff in which the period of time
         during which the MN is unable to receive packets is
         minimized.

      low loss handoff - L3 handoff in which the number of packets
         dropped or delayed is minimized.  Low loss handoff is often
         called smooth handoff.

      seamless handoff - L3 handoff that is both low latency and low
         loss.

      bi-directional edge tunnel (BET) -  A bidirectional tunnel
         established between two FAs for purposes of temporarily
         routing a MN's traffic to/from it on a new subnet without
         requiring the MN to change CoA.

      ping-pong - Rapid back and forth movement between two wireless
         access points often due to failure of L2 handoff.  Ping-pong
         can occur if radio conditions for both the old and new
         access points are about equivalent and less than optimal for
         establishing a good, low-error L2 connection.

      network-initiated handoff - L3 handoff in which oFA or nFA
         initiates the handoff.

      mobile-initiated handoff - L3 handoff in which the MN initiates
         the handoff.

      MN or FA identifier - An IPv4 address of a MN or FA, or an L2
         identifier that can be resolved to the IPv4 address of a MN
         or FA.  If the identifier is an L2 identifier, it may be
         specific to the L2 technology.

1.2.
     The Techniques

   Mobile IPv4 was originally designed without any assumptions about the
   underlying link layers over which it would operate so that it could
   have the widest possible applicability.  This approach has the
   advantage of facilitating a clean separation between L2 and L3 of the
   protocol stack, but it has negative consequences for handoff latency.
   The strict separation between L2 and L3 results in the following
   built-in sources of delay:

     - The MN may only communicate with a directly connected FA.  This
       implies that a MN may only begin the registration process after
       an L2 handoff to nFA (new FA) has completed.




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     - The registration process takes some non-zero time to complete as
       the Registration Requests propagate through the network.  During
       this period of time the MN is not able to send or receive
       IPv4 packets.

   This document presents techniques for reducing these built-in delay
   components of Mobile IPv4.  The techniques can be divided into two
   general categories, depending on which of the above problems they
   are attempting to address:

     - Allow the MN to communicate with the nFA while still connected
       to the oFA.

     - Provide for data delivery to the MN at the nFA even before the
       formal registration process has completed.

   The first category of techniques allows the MN to "pre-build" its
   registration state on the nFA prior to an underlying L2 handoff.
   The second category of techniques allow for service to continue
   uninterrupted while the handoff is being processed by the network.

   Three methods are presented in this draft to achieve low-latency L3
   handoff, one for each category described above and one as a
   combination of the two:

   - PRE-REGISTRATION handoff method,

   - POST-REGISTRATION handoff method,

   - combined handoff method.

   The PRE-REGISTRATION handoff method allows the MN to be involved in
   an anticipated IPv4-layer handoff.  The MN is assisted by the network
   in performing an L3 handoff before it completes the L2 handoff.  The
   L3 handoff can be either network-initiated or mobile-initiated.
   Accordingly, L2 triggers are used both in the MN and in the FA to
   trigger particular L3 handoff events.  The PRE-REGISTRATION method
   coupled to L2 mobility helps to achieve seamless handoffs between
   FAs.  The basic Mobile IPv4 concept involving advertisement followed
   by registration is supported and the PRE-REGISTRATION handoff method
   relies on Mobile IPv4 security.  No new messages are proposed, except
   for an extension to the Agent Solicitation message in the
   mobile-initiated case.

   The POST-REGISTRATION handoff method proposes extensions to the
   Mobile IPv4 protocol to allow the oFA (old FA) and nFA (new FA) to
   utilize L2 triggers to set up a bi-directional tunnel between oFA and
   nFA that allows the MN to continue using its oFA while on nFA's
   subnet.  This enables a rapid establishment of service at the new
   point of attachment which minimizes the impact on real-time



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   applications.  The MN must eventually perform a formal Mobile IPv4
   registration after L2 communication with the new FA is established,
   but this can be delayed as required by the MN or FA.  Until the MN
   performs registration, the FAs will setup and move bidirectional
   tunnels as required to give the MN continued connectivity.

   The combined method involves running a PRE-REGISTRATION and a
   POST-REGISTRATION handoff in parallel.  If the PRE-REGISTRATION
   handoff can be performed before the L2 handoff completes, the
   combined method resolves to a PRE-REGISTRATION handoff.  However, if
   the PRE-REGISTRATION handoff does not complete within an access
   technology dependent time period, the oFA starts forwarding traffic
   for the MN to the nFA as specified in the POST-REGISTRATION handoff
   method.  This provides for a useful backup mechanism when completion
   of a PRE-REGISTRATION handoff cannot always be guaranteed before the
   L2 handoff completion.

   It should be noted that the methods described in this document may be
   applied to MNs having a single interface (e.g. Wireless LAN
   interface) or multiple interfaces (e.g. one WLAN and one cellular
   interface).  However, the case of multiply-interfaced MNs needs
   special consideration, since the handoff methods described in this
   document may not be required in all cases (see Appendix B).

1.3.
     L2 triggers

   An L2 trigger is a signal of an L2 event.  In this document, the L2
   events relate to the L2 handoff process.  One possible event is early
   notice of an upcoming change in the L2 point of attachment of the
   mobile node to the access network.  Another possible event is the
   completion of relocation of the mobile node's L2 point of attachment
   to a new L2 access point.  This information may come explicitly from
   L2 in a solicited or unsolicited manner, or it may be derived from L2
   messages.  Although the protocols outlined in this document make use
   of specific L2 information, Mobile IPv4 should be kept independent of
   any specific L2.  L2 triggers are an abstraction mechanism for a
   technology specific trigger.  Therefore, an L2 trigger that is made
   available to the Mobile IPv4 stack is assumed to be generic and
   technology independent.  The precise format of these triggers is not
   covered in this document, but the information required to be
   contained in the L2 triggers for low latency handoffs is specified.

   In order to properly abstract from the L2, it is assumed that one of
   the three entities - the MN, oFA, or nFA - is made aware of the need
   for an L2 handoff, and that the nFA or MN can optionally also be made
   aware that an L2 handoff has completed.  A specific L2 will often
   dictate when a trigger is received and which entity will receive it.
   Certain L2s provide advance triggers on the network-side, while
   others provide advance triggers on the MN.  Also, the particular
   timing of the trigger with respect to the actual L2 handoff may



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   differ from technology to technology.   For example, some wireless
   links may provide such a trigger well in advance of the actual
   handoff.   In contrast, other L2s may provide little or no
   information in anticipation of the L2 handoff.

   An L2 trigger may be categorized according to whether it is received
   by the MN, oFA, or nFA.  Table 1 gives such a categorization along
   with information contained in the trigger.  The methods presented in
   this document operate based on different types of L2 triggers as
   shown in Table 1.  Once the L2 trigger is received, the handoff
   processes described hereafter are initiated.  The three triggers: L2-
   ST, L2-TT and L2-MT are independent of each other and are not
   expected to occur together since each will trigger a different type
   of handoff behaviour.


   +-------------+----------------------+------------------------------+
   | L2 trigger  |       Mobile         |           Source             |
   |             |       Trigger        |           Trigger            |
   |             |       (L2-MT)        |           (L2-ST)            |
   +-------------+----------------------+------------------------------+
   | Recipient   |          MN          |             oFA              |
   +-------------+----------------------+--------------+---------------+
   | Method      | PRE                  | PRE          | POST          |
   |             | mobile-initiated     | network-     | source        |
   |             |                      | initiated    | trigger       |
   +-------------+----------------------+--------------+---------------+
   | When?       | sufficiently before  | sufficiently | sufficiently  |
   |             | the L2 handoff       | before L2    | before L2     |
   |             | so that MN can       | handoff for  | handoff for   |
   |             | solicit PrRtAdv      | FA to send   | oFA & nFA to  |
   |             | from oFA.            | PrRtAdv      | exchange      |
   |             |                      | to MN.       | HRqst/HRply.  |
   +-------------+----------------------+--------------+---------------+
   | Parameters  | nFA identifier       | nFA identifier, MN identifier|
   +-------------+----------------------+------------------------------+

                            Table 1 - L2 Trigger
                          (continued on next page)














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   +------------+----------------------+---------------+---------------+
   | L2 trigger |       Target         |  Link-Up      |  Link-Down    |
   |            |       Trigger        |  (L2-LU)      |   (L2-LD)     |
   |            |       (L2-TT)        |               |               |
   |------------+----------------------+---------------+---------------+
   | Recipient  |          nFA         |  MN or nFA    |     oFA       |
   |------------+-----------+----------+---------------+---------------+
   | Method     | PRE       |  POST    |  PRE & POST   |    POST       |
   |            | network   |  target  |               |               |
   |            | initiated |  trigger |               |               |
   |------------+----------------------+---------------+---------------+
   | When?      |                      | when radio    |  when radio   |
   |            |      same as         | link between  |  link between |
   |            |   source trigger     | MN & nFA  is  |  MN and oFA   |
   |            |                      | established   |  is lost      |
   |------------+----------------------+---------------+---------------+
   | Parameters | oFA identifier       | @MN: nFA IPv4 | MN identifier |
   |            | MN identifier        | or L2 addr.   |               |
   |            |                      | @nFA: MN IPv4 |               |
   |            |                      | or L2 addr.   |               |
   +------------+----------------------+---------------+---------------+

                           Table 1 - L2 Trigger
                     (continued from previous page)


1.4.
     Requirements language

   In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
   "recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
   described in [2].


2.
   Requirements

   The following requirements are applicable to low-latency handoff
   techniques and are supported by the methods in this document:

   - to provide low-latency and low loss handoff for real time services,

   - to have no dependence on a wireless L2 technology,

   - to support inter- and intra-access technology handoffs,

   - to limit wireless bandwidth usage.








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3.
   The PRE-REGISTRATION Handoff Method

   The PRE-REGISTRATION handoff method is based on the normal Mobile
   IPv4 handoff procedure specified in [1], according to which:

   - an advertisement for an FA is received by an MN,
   - the advertisement allows the MN to perform movement detection,
   - the MN registers with the FA.

   The basic messages specified in [1] are extended to carry information
   required to achieve fast handoffs.  The PRE-REGISTRATION method
   allows both the MN and FA to initiate the layer-3 handoff and it can
   make use of L2 triggers on either the FA or MN side, depending on
   whether network-initiated or mobile-initiated handoff occurs.

   PRE-REGISTRATION supports the normal Mobile IPv4 model [1] and
   optionally also the Regional Registration model [11].  There can be
   advantages in implementing [11] together with Low Latency Handoff
   mechanisms, in particular in cases where the HA is at a distance (in
   terms of delay) from the nFA.  The time required for the handoff
   procedure to complete can be reduced by using a closer local HA,
   called Gateway Foreign Agent (GFA) in [11].  However implementation
   of [11] is not required by PRE-REGISTRATION.  PRE-REGISTRATION also
   supports movement where a new AAA transaction must occur to
   authenticate the MN with a new domain.

3.1.
     Operation

   The PRE-REGISTRATION Handoff mechanism is summarized in Figure 1.

                              +---------+
                              | HA (GFA)|<---------+
                              +---------+          | 4.  (Reg)RegReq
                                                   | 5.  (Reg)RegReply
                                                   v
                     +-----+    1a.  PrRtSol    +-----+
                     |     | -----------------> | nFA |
                     | oFA |    1b.  PrRtAdv    |     |
                     +-----+ <----------------- +-----+
                      ^   |                       ^
        (2a.  PrRtSol)|   | 2b                    |
                      |   | PrRtAdv               | 3.  (Reg)RegReq
                      |   |                       |
                      |   v   --------------------+
                     +-----+ /
                     | MN  |
                     +-----+    - - - - - ->
                                  Movement

              Figure 1 - PRE-REGISTRATION Handoff Protocol



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   The following steps provide more detail on the protocol:

        1. Message 1a is a Proxy Router (Agent) Solicitation (PrRtSol)
           from oFA to nFA.  It is a Mobile IP agent solicitation
           containing an identifier for the nFA (i.e. IP address or L2
           address) in a Generalised Link Layer and IP Address Extension
           (see section 9.).  When message 1a is received by the nFA
           containing nFA's correct identifier in the LLA extension, the
           nFA MUST return the Proxy Router Advertisement (Agent
           Advertisement) in message 1b.  Message 1b is simply nFA's
           agent advertisement containing the nFA layer 2 address in a
           Generalised Link Layer and IP Address (LLA) Extension (see
           9.3). Messages 1a and 1b SHOULD occur in advance of the
           PRE-REGISTRATION Handoff in order not to delay the handoff.
           For this to occur, oFA SHOULD solicit and cache
           advertisements from neighbouring nFAs using messages 1a and
           1b, thus decoupling the timing of this exchange from the rest
           of the PRE-REGISTRATION Handoff.  When the L3 handoff is
           initiated by a target L2 trigger at nFA (L2-TT), message 1b
           equals message 2b and is sent unsolicited directly to MN
           (tunneled by nFA to MN through oFA) instead of being relayed
           by oFA.

        2. Message 2a is a Proxy Router Solicitation (PrRtSol) from MN
           to oFA.  It is different from a normal Router (Agent)
           Solicitation since it is soliciting an advertisement from a
           router different from the one receiving this message.  It is
           a Mobile IP agent solicitation containing an identifier for
           the nFA (i.e. IP address or L2 address) in a Generalised Link
           Layer and IP Address Extension (see section 9.).  The
           presence of message 2a indicates that the handoff is mobile-
           initiated and its absence means that the handoff is network-
           initiated.  In mobile-initiated handoff, message 2a occurs if
           there is an L2 trigger in the MN to solicit for a Proxy
           Router Advertisement (PrRtAdv).  When message 2a is received
           by the oFA it MUST return the Proxy Router Advertisement
           (Agent Advertisement) in message 2b.  This is simply nFA's
           agent advertisement containing the nFA layer 2 address in a
           Generalised Link Layer and IP Address (LLA) Extension (see
           9.3).  In network-initiated source-triggered handoff, the L2
           trigger occurs at oFA and oFA MUST relay the Agent
           Advertisement in message 2b without the need for the MN to
           solicit.  Note that it is also possible for nFA to advertise
           directly to the MN in the network-initiated target-trigger
           case (section 3.2).

        3. The MN performs movement detection upon receipt of a
           solicited or unsolicited Agent Advertisement and, if
           required, it sends a Registration Request (RegReq) message
           [1] in message 3 to nFA.  When a local Gateway Foreign Agent



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           (GFA) is present this message can optionally be a Regional
           Registration Request (RegRegReq) [11].  Message 3 is routed
           through oFA since the MN is not directly connected to nFA
           prior to the L2 handoff.

        4. Messages 4 and 5 complete the standard Mobile IPv4
           Registration [1] or optionally Regional Registration [11]
           initiated with message 3.  The Registration Request MUST
           contain the MN's layer 2 address in a Generalised Link Layer
           and IP Address Extension (see 3.7 and 9). This identifier may
           be a plain Ethernet address or an identifier specific to the
           wireless technology.  If the MN is not already connected to
           nFA, the Registration Reply in message 5 MUST be buffered by
           the nFA and unicast to the MN on-link as soon as the MN
           connects to nFA (i.e. L2-LU trigger at nFA which can be
           implemented by the MN sending an Agent Solicitation or
           optionally using special layer 2 techniques which are outside
           the scope of this document).  This is necessary since the MN
           may have to detach from oFA, due to the wireless L2
           connection, before it receives the Reply.  The MN's L2
           address is obtained using the extensions in Section 9, as
           described in 3.7.  Figures 2 and 3 illustrate this procedure.

        5. If the Registration is successful packets for the MN are
           tunnelled from the HA (or GFA) to the nFA and then to the MN.

   PRE-REGISTRATION is not dependent on [11].  However if the HA is at a
   distance (in terms of delay) from the nFA, the use of a local GFA may
   reduce the time required for the handoff procedure to complete.

   The time at which the L2 trigger is received by the oFA or MN,
   thereby triggering the PRE-REGISTRATION handoff, compared to the time
   at which the actual L2 handoff occurs is important for the optimal
   performance of the low latency handoff.  That is, in the optimal case
   the L2 trigger will be received and the four messaging steps of PRE-
   REG described above will be completed (i.e. up to when the
   Registration Request is processed by HA or GFA) before the MN moves.
   Optimally the Registration Reply and the first packet redirected by
   the HA (or GFA) to nFA will reach the MN at the moment in which the
   MN's L2 link to nFA is fully established.  The MN would therefore not
   suffer any disruption due to the L3 handoff.  This cannot always be
   guaranteed unless particular implementation techniques are used. To
   alleviate a part of this timing problem the MN MAY set the S bit [1]
   in Low Latency Registration Requests sent by the MN. This allows the
   MN to receive packets at both oFA and nFA during the short layer 2
   handoff time. Other techniques may be required such as L2 techniques
   or buffering but these are outside the scope of this document.  In
   addition further handoff smoothing considerations may be required to
   prevent the loss of packets in-flight between HA (or GFA) and oFA
   while the MN performs a PRE-REGISTRATION handoff.  These are also



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   outside the scope of this document.

   Figures 2, 3, and 4 contain message timing diagrams for the network-
   initiated and mobile-initiated PRE-REGISTRATION handoff procedures.

3.2.
     Network-Initiated Handoff

   As described in Table 1, a PRE-REGISTRATION handoff can be initiated
   at oFA by a source trigger or at nFA by a target trigger.  Figures 2
   and 3 contain message timing diagrams for PRE-REGISTRATION network-
   initiated handoff for source and target triggers.

   A source-triggered network-initiated handoff occurs when an L2
   trigger is received at the oFA informing it of a certain MN's
   upcoming movement from oFA to nFA.  The L2 trigger contains
   information including the MN's identifier (i.e. the IPv4 address
   itself or an identifier which can be resolved to the IPv4 address)
   and the nFA's identifier.  An identifier may be an IPv4 address or
   something specific to the wireless technology (e.g. Base Station or
   Access Point Identifier).  A target-triggered network-initiated
   handoff occurs when an L2 trigger is received at the nFA informing it
   of a certain MN's upcoming movement from oFA.  This type of trigger
   is also shown in Table 1 and contains information including the MN's
   identifier and the oFA's identifier.


   MN                    oFA                 nFA                 HA/GFA
    |                     |<~~~~~~ L2-Source  |                    |
    |                     |           Trigger |                    |
    |<--------------------|                   |                    |
    |     PrRtAdv         |                   |                    |
    |                     |                   |                    |
    |---------------------------------------->|                    |
    |   RegReq or         |                   |                    |
    |   RegRegReq (routed via oFA)            |------------------->|
    |                                         | RegReq or RegRegReq|
    |                                         |                    |
    |                          Buffered ~~~~~>|<-------------------|
    |---------------------------------------->|    (Reg)RegReply   |
    | Agent Solicitation                      |                    |
    | (sent when MN connects to nFA)          |                    |
    |                                         |                    |
    |<----------------------------------------|                    |
    |              (Reg)RegReply              |                    |
    |              (sent when nFA receives Solicitation or L2-LU)  |

         Figure 2 - PRE-REGISTRATION Handoff Message Timing Diagram
                     (Network-Initiated, Source Trigger)





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   In a source-triggered handoff, when oFA receives the trigger (L2-ST)
   it MUST send message 2b, the Proxy Router Advertisement (PrRtAdv), to
   the MN.  The PrRtAdv is nFA's agent advertisement [1] with one of the
   link-layer extensions described in sections 9 (i.e. 9.3).  The use of
   the contents of this extension is described in section 3.7.  Messages
   1a and 1b SHOULD be exchanged by oFA and nFA before the L2 trigger is
   received (see section 3.4.1).  Message 2a is not used.


   MN                    oFA                 nFA                 HA/GFA
    |                     | L2-Target~~~~~~~~>|                    |
    |                     |    Trigger        |                    |
    |                     |...................|                    |
    |<--------------------------------------- |                    |
    |     (PrRtAdv)       |...................|                    |
    |                     | Tunnelled Agent Advertisement          |
    |                     |                   |                    |
    |---------------------------------------->|                    |
    |   RegReq.  or       |                   |                    |
    |   RegRegReq (routed via oFA)            |------------------->|
    |                                         | RegReq or RegRegReq|
    |                                         |                    |
    |                          Buffered ~~~~~>|<-------------------|
    |---------------------------------------->|    (Reg)RegReply   |
    | Agent Solicitation                      |                    |
    | (sent when MN connects to nFA)          |                    |
    |                                         |                    |
    |<----------------------------------------|                    |
    |              (Reg)RegReply              |                    |
    |              (sent when nFA receives Solicitation or L2-LU)  |

         Figure 3 - PRE-REGISTRATION Handoff Message Timing Diagram
                     (Network-Initiated, Target Trigger)

   In a target-triggered handoff, when nFA receives the trigger (L2-TT)
   it MUST tunnel an Agent Advertisement to the MN through oFA to
   initiate the L3 handoff.  The inner Advertisement is unicast by nFA
   to MN, thus nFA treats the target-trigger as a Router (Agent)
   Solicitation.  This Advertisement is tunneled to oFA, which functions
   as a normal router, decapsulating the Advertisement and forwarding it
   to the MN.  This message MUST be authenticated to prevent attacks
   (see 3.4.2).

3.3.
     Mobile-Initiated Handoff

   As shown in Table 1, a mobile-initiated handoff occurs when an L2
   trigger is received at the MN informing that it will shortly move to
   nFA.  The L2 trigger contains information such as the nFA's
   identifier (i.e. nFA's IPv4 address or an identifier which can be
   resolved to the nFA's IPv4 address).  As an example, a Wireless LAN



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   MN may perform a scan to obtain the BSSID identifier of the Access
   Point that is a potential handoff target (i.e. its signal is becoming
   stronger). The message timing diagram is shown in Figure 4.


   MN                    oFA                 nFA               HA/GFA
    |<~~~~~ L2-Trigger    |                   |                    |
    |                     |                   |                    |
    |-------------------->|                   |                    |
    |      PrRtSol        |                   |                    |
    |                     |                   |                    |
    |<--------------------|                   |                    |
    |      PrRtAdv        |                   |                    |
    |                     |                   |                    |
    |---------------------------------------->|                    |
    |   RegReq or         |                   |                    |
    |   RegRegReq (routed via oFA)            |------------------->|
    |                                         | RegReq or RegRegReq|
    |                                         |                    |
    |                          Buffered ~~~~~>|<-------------------|
    |---------------------------------------->|    (Reg)RegReply   |
    | Agent Solicitation                      |                    |
    | (sent when MN connects to nFA)          |                    |
    |                                         |                    |
    |<----------------------------------------|                    |
    |              (Reg)RegReply              |                    |
    |              (sent when nFA receives Solicitation or L2-LU)  |


         Figure 4 - PRE-REGISTRATION Handoff Message Timing Diagram
                             (Mobile-Initiated)


   As a consequence of the L2 trigger (L2-MT) the MN MUST send message
   1a, the Proxy Router Solicitation (PrRtSol).  This message is a
   unicast agent solicitation to oFA for a Proxy Router Advertisement
   (PrRtAdv).  This solicitation MUST have a TTL=1 as in [1].  The Proxy
   Router Advertisement Solicitation unicast to oFA is an agent
   solicitation with a special extension.  The solicitation MUST have an
   extension containing an FA identifier (i.e. IPv4 address or L2
   address contained in an LLA extension, see section 9) because the MN
   is soliciting another specific FA's advertisement from the oFA.  This
   specific FA will be the MN's nFA.  The identifier is the IPv4 address
   of the nFA or another identifier that can be used by the oFA to
   resolve to nFA's IPv4 address.  If the identifier is not an IPv4
   address, it MAY be specific to the underlying wireless technology,
   for example, an Access Point or Base Station Identifier (e.g. WLAN
   BSSID) that can be mapped by oFA to the nFA IPv4 address as described
   in 3.4.1.  The extension containing the identifier is a subtype of
   the Generalised Link-Layer Address extension described in Section 9.



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   Two extension subtypes have been defined to contain the nFA's IPv4
   address and an access point identifier.  They are called the
   Solicited Agent IPv4 Address Extension and the Access Point
   Identifier Extension, and are described in Sections 9.5 and 9.6.
   These two extensions SHOULD NOT be present in the same PrRtSol
   message.

   When oFA receives the PrRtSol message it MUST reply to the MN with
   the Proxy Router Advertisement (PrRtAdv, message 2b).  The PrRtAdv is
   simply the agent advertisement for the requested nFA, proxied by oFA.
   In order to expedite the handoff, the actual nFA advertisement SHOULD
   be cached by the oFA following a previous exchange with nFA, shown in
   messages 1a and 1b, as specified in Section 3.5.  The PrRtAdv message
   MUST contain the nFA's L2 address (using the LLA extension in 9.3).
   This is further described in section 3.7.

3.4.
     Obtaining and Proxying nFA Advertisements

   Since L2 triggers are involved in initiating PRE-REGISTRATION
   handoff, the trigger timing SHOULD be arranged such that a full L3
   PRE-REGISTRATION handoff can complete before the L2 handoff process
   completes.  That is, the L2 handoff should be completed after the
   MN's Registration with the nFA is performed (message 3 in Fig.1).
   The Registration MAY be transmitted in more than one copy (default
   recommendation: 2) to reduce the probability that it is lost due to
   errors on the wireless link. This would not apply to reliable
   wireless links where retransmissions are performed at layer 2 in case
   of error to guarantee packet delivery.

   A PRE-REGISTRATION handoff in this case requires the MN to receive an
   agent advertisement from the nFA through the old wireless access
   point.  How to achieve this is discussed in the following
   subsections.  Messages exchanged between FAs MUST be authenticated to
   prevent impersonation attacks.  The minimal requirement is that all
   FAs involved in low latency handoffs MUST support manual pre-
   configuration of security associations with other neighbouring FAs,
   involving shared keys and the default algorithms of [1] (see Security
   Considerations).

3.4.1.
       Inter-FA Solicitation

   This applies to the network-initiated source-triggered (L2-ST) and
   mobile-initiated (L2-MT) cases only.  Inter-FA solicitation assumes
   that oFA has access to the IPv4 address of the nFA.  The IPv4 address
   of nFA is obtained by means of an L2 trigger at oFA in the
   network-initiated case (see Section 3.2) or by means of the extension
   to the Proxy Router Solicitation (PrRtSol) from the MN in the
   mobile-initiated case (see Section 3.3). This extension to the
   PrRtSol may contain an IPv4 address or another identifier, for
   example an identifier of a Wireless Base Station such as the WLAN



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   BSSID. In the latter case the receiving oFA must implement a
   mechanism to resolve the Base Station Identifier to an IPv4 address.
   The default mechanism is to use a configured table of neighbouring
   BSSID to FA IPv4 address mappings in each FA. Other automated
   discovery mechanisms may also be used.

   If oFA does not cache advertisements (see 3.5) once it receives an L2
   trigger and obtains the address of the nFA for a specific MN it MUST
   send a unicast agent solicitation (PrRtSol) to nFA.  The nFA replies
   to the oFA by unicasting an Agent Advertisement with appropriate
   extensions (PrRtAdv).  This method removes the TTL limitation of [1]
   for Mobile IPv4 messages (i.e. TTL=1 is not applicable here).  The
   TTL limitation cannot be applied since oFA and nFA may be more than
   one hop away and since it is unnecessary for a secured unicast
   message.  The ICMP solicitations and advertisements MUST be
   authenticated and integrity protected.  These messages MUST be
   protected using ESP [10] to prevent attacks (see Security
   Considerations section).  An FA MUST NOT accept ICMP solicitations or
   advertisements from sources that are not authenticated.

   As a practical matter, oFA SHOULD pre-solicit and cache
   advertisements from known neighboring FAs (see section 3.5) to avoid
   performing the solicitation during an actual handoff procedure.

3.4.2.
       Tunneled nFA Advertisements

   This applies to the network-initiated target-triggered (L2-TT) case
   only.  Following a target trigger (L2-TT) the nFA MUST send a
   tunneled agent advertisement to the MN through oFA.  Tunneling nFA
   advertisements assumes that the nFA is aware of the IPv4 address for
   oFA and the MN.  These IPv4 addresses are obtained by means of the FA
   and MN contained in an L2 trigger received at nFA in the network-
   initiated case (see Section 3.2).  However in [1] the TTL must be 1
   on Agent Advertisements from the nFA.  Therefore tunneling
   advertisements is applicable if the TTL limitation of [1] is relaxed.
   For this purpose, a pre-established security association between oFA
   and nFA MUST be in place to authenticate this message and relax the
   TTL limitation.  If the implementation requires this, a tunnel SHOULD
   be configured when the inter-FA security association is established.
   The tunneled ICMP advertisement MUST be secured using tunnel mode ESP
   [10] between nFA and oFA.  An FA MUST NOT accept tunneled ICMP
   packets destined to it from sources that are not authenticated.

3.5.
     Caching Router Advertisements

   In the mobile-initiated (L2-MT) case and the network-initiated
   source-triggered (L2-ST) case, the message exchange 1 in Figure 1
   could impose an additional latency on the L3 handoff process if done
   as part of the handoff procedure.  In order to remove this source of
   latency, the inter-FA Router (Agent) Solicitation and Advertisement



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   exchange SHOULD be performed in advance of handoff.  A process SHOULD
   be in place at the oFA to solicit its neighbouring nFAs at a
   predefined time interval (MIN_SOLICITATION_INTERVAL).  This interval
   SHOULD NOT be set too small to avoid unnecessary consumption of
   network bandwidth and nFA processing resources.  The minimum value of
   MIN_SOLICITATION_INTERVAL is 1 sec.  If the FA Challenge/Response
   mechanism in [7] is used then the MIN_SOLICITATION_INTERVAL MUST be
   set to a value smaller then the window of time in which a challenge
   remains valid so that the nFA challenge does not expire before the MN
   issues the Registration Request.  Therefore the recommended default
   value for the MIN_SOLICITATION_INTERVAL in oFA is (0.5 * nFA's
   CHALLENGE_WINDOW * nFA's Agent advertisement interval).  The
   CHALLENGE_WINDOW and Agent advertisement interval are defined in [7]
   and [1] respectively.  The minimum requirement is that the
   MIN_SOLICITATION_INTERVAL MUST be manually configurable, while
   possible autoconfiguration mechanisms are outside the scope of this
   document.  To allow advertisement caching in certain implementations
   and in cases where the nFA advertisement interval is very small, it
   MAY be necessary for the implementation in nFA to allow different
   CHALLENGE_WINDOW and agent advertisement interval settings for its
   nFA-oFA interface.

   The oFA SHOULD cache the most recent advertisement from its
   neighbouring nFAs.  This advertisement MUST be sent to the MN in
   message 2b with a TTL=1.  The oFA SHOULD also have a mechanism in
   place to create a list of neighbouring nFAs.  The minimum requirement
   for each FA is that it SHOULD allow manual configuration of a list of
   nFA addresses that an MN could possibly perform an L3 handoff to.
   The FA addresses in this list will depend on deployment and radio
   coverage.  It is also possible to specify another protocol to achieve
   nFA discovery, but this is outside the scope of this document.

3.6.
     Movement Detection, MN and FA Considerations

   When the MN receives an Agent Advertisement with a Mobility Agent
   extension, it performs actions according to the following movement
   detection mechanism: the MN SHOULD be "Eager" to perform new
   bindings.  This means that the MN SHOULD perform Registrations with
   any new FA from which it receives an advertisement (i.e. MN is
   Eager), as long as there are no locally-defined policies in the MN
   that discourage the use of the discovered FA.  For example, the MN
   could have a policy based on the cost of service.  The method by
   which the MN determines whether the FA is a new FA is described in
   [1] and MAY use an FA-NAI extension [11]. By being "Eager" to perform
   Registrations the MN reduces latency times.

   The MN also needs to change its default router from oFA to nFA.  The
   MN MUST change its default router to nFA as soon as the
   PRE-REGISTRATION procedure has completed (i.e. Registration Reply is
   received by MN) as described in [1].



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   Overall the MN behaves as described in [1] with the following
   changes: the specified movement detection mechanism mentioned above
   and the ability to use the L2-MT to initiate an agent solicitation
   with a special extension (PrRtSol).  Also, when the MN receives a L2-
   LU trigger (i.e. new interface or link is up) it MUST immediately
   send an Agent Solicitation [1] on that interface.  An nFA that
   receives an Agent Solicitation [1] will use it as an L2-LU trigger
   event and according to [1] it will record the MN's IPv4/Layer 2
   addresses (i.e. ARP entry).  At that point the nFA starts delivering
   data to the MN including the previously buffered Registration Reply.
   The nFA MAY also use other L2 mechanisms to detect earlier that the
   MN has attached to the new link and to start forwarding data to it.
   The MN SHOULD NOT attempt to retransmit a low latency Registration
   Request (i.e. Registration Request containing an LLA extension
   described in 9.) when it does not receive the Registration Reply.

   When moving from a PRE-REGISTRATION network to a normal Mobile IPv4
   [1] network the MN will no longer receive PrRtAdv messages (i.e.
   agent advertisements with the LLA extension).  If the MN still
   receives L2-MTs it will attempt to send PrRtSol messages.  The normal
   FA will reply with a normal agent advertisement [1].  If the MN does
   not receive a PrRtAdv in reply to its PrRtSol, it MAY retransmit the
   PrRtSol message once after PRE_SOL_INTERVAL seconds and then for
   another PRE_SOL_ATTEMPTS times with exponential backoff of the
   transmission interval.  If a PrRtAdv is not received within
   PRE_SOL_INTERVAL seconds after the last PrRtSol attempt, the MN MUST
   stop sending PrRtSol messages until after a Registration with a new
   FA is performed.  The default value for PRE_SOL_ATTEMPTS is 2 and for
   PRE_SOL_INTERVAL it is 1 second.  It should be noted that the
   performance of the movement detection mechanism mandated in
   PRE-REGISTRATION (i.e. eager to register) may have sub-optimal
   behaviour in a standard Mobile IPv4 [1] network.  Therefore standard
   movement detection mechanisms [1] should be used in plain Mobile IPv4
   networks. Instead when the MN moves from a normal Mobile IPv4 [1]
   network to a PRE-REGISTRATION network, the MN starts receiving L2-MT
   triggers or PrRtAdv messages.  When the MN receives L2-MT triggers or
   PrRtAdv messages it SHOULD follow the PRE-REGISTRATION procedure.

   If there is uncertainty as to which mode to choose (e.g. MN receives
   messages from both PRE-REGISTRATION and normal FAs) the MN decides
   based on its Registration status with the current FA. If the MN
   already has a valid normal Mobile IPv4 Registration [1] with the
   advertising FA, it SHOULD give priority to the PRE-REGISTRATION
   procedure. Otherwise it SHOULD give priority to normal Mobile IPv4
   [1] Registration procedure. The MN SHOULD NOT attempt to perform PRE-
   REGISTRATION and standard Mobile IPv4 [1] Registrations in parallel.

3.7.
     L2 Address Considerations

   Some special considerations should be taken with respect to the



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   wireless system on which this handoff method is being implemented.
   Consider an Ethernet-like system (e.g. IEEE 802.11) for example.  In
   PRE-REGISTRATION the MN is registering with an FA (nFA) that is not
   its current first-hop router, therefore the L2 address of the
   Ethernet frame containing the MN's Registration Request reaching the
   nFA is not the MN's address.  Therefore the FA MUST NOT use the
   Ethernet address of the incoming Registration Request as the MN's L2
   address as specified in [1].  This applies to the cases where the
   wireless access points are bridges or routers and independently of
   whether the FA is implemented in the wireless access points
   themselves.  In this case the MN's Registration Request (or Regional
   Registration Request) MUST use an L2 address extension to the
   Registration message.  Such an L2 address is either the same L2
   address that remains constant as the MN moves, or it is the MN's L2
   address at nFA.  To communicate its L2 address, the MN includes a
   Generalised Link Layer and IP Address Extension (see Section 9) with
   its Registration Request (or Regional Registration Request) message.
   If this extension is present the FA MUST use the L2 address contained
   in the extension to communicate with the MN.  If a particular
   wireless L2 technology has defined a special interface to the
   wireless network that allows the FA to resolve the mapping between an
   MN's IPv4 address and its L2 address without the need to use the
   extension, the L2 address extension contents may be discarded.  For
   the same reasons above, the MN MUST NOT use the source L2 address of
   the Agent Advertisement message (PrRtAdv) as its default router's L2
   address.  Therefore the nFA MUST include a Generalised Link Layer and
   IP Address Extension (see Section 9.3) with its Agent Advertisement
   (PrRtAdv) messages.

3.8.
     Applicability of PRE-REGISTRATION Handoff

   The PRE-REGISTRATON Handoff method is applicable to scenarios where a
   period of service disruption due to layer 3 is not acceptable, for
   example when performing real-time communications, and therefore where
   an anticipation of the layer 3 handoff is required.  Security for the
   PRE-REGISTRATION handoff method is based on the same security model
   as [1] including the use of AAA.  A prerequisite for PRE-REGISTRATION
   is that the FA or MN are able to obtain an L2 trigger informing them
   of a pending L2 handoff procedure.  The target of the L2 handoff is
   another access point or radio network that is in the coverage area of
   a new FA.  The L2 trigger information may be in the form of
   Identifiers that need to be resolved to IPv4 addresses using methods
   that may be specific to the wireless network and are not considered
   here.  If, for example, the oFA or MN determines that the IPv4
   address of the new FA matches oFA's address then the PRE-REGISTRATION
   handoff SHOULD NOT be initiated.

   The L2 trigger must allow enough time for the PRE-REGISTRATION
   handoff procedure to be performed.  In many wireless L2 technologies,
   the L2 handoff procedure involves a number of message exchanges



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   before the effective L2 handoff is performed.  For such technologies,
   PRE-REGISTRATION handoff can be initiated at the beginning of the L2
   handoff procedure and completed before the L2 handoff is completed.
   It is efficient to engineer the network such that this succession of
   events is ensured.

   The PRE-REGISTRATION Handoff method is applicable in the following
   cases:

   - when the MN has locally defined policies that determine a
     preference for one access over another, for example due to service
     cost within the same or different technology, and therefore where
     it is necessary to allow the MN to select the appropriate FA with
     which to connect,

   - when L2 security between the MN and the FA is either not present
     or cannot be relied upon to provide adequate security,

   - when the trigger to initiate the handoff is received at the MN.

   In the first case it is necessary to involve eventual local MN
   policies in the movement detection procedure as described in 3.6.


4.
   The POST-REGISTRATION Handoff Method

   The POST-REGISTRATION handoff method uses bi-directional edge tunnels
   (BETs) or unidirectional tunnels to perform low latency change in the
   L2 point of attachment for the MN without requiring any involvement
   by the MN.  Figure 5 illustrates the basic POST-REGISTRATION handoff.

                            +------+
                            |  CN  |
                            +------+
                               |
                              ...
                               |
                            +------+   BET    +------+
                            | aFA  |==========| nFA  |
                            +------+          +------+
                                                  | wireless link
                                                  |
                                  movement    +------+
                                 --------->   |  MN  |
                                              +------+

                      Figure 5 - POST-REGISTRATION Concept

   Following a successful Mobile IPv4 Registration between MN and oFA,
   the oFA becomes the mobility anchor point for the MN, called the



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   anchor FA (aFA).  When the MN moves from oFA to nFA, rather than
   performing signaling over the wireless link to register with the nFA,
   the MN can defer the L3 handoff and continue to use its aFA (i.e. oFA
   in this case).  If the MN moves to a third FA before registering with
   the nFA, in certain cases described later, the third FA signals aFA
   to move the wireless link end of the BET from nFA to it.  The network
   end of the BET remains anchored at aFA until the MN performs the
   Mobile IPv4 Registration.

   Messages between oFA/aFA and nFA MUST be authenticated.  The minimal
   requirement is that all FAs involved in low latency handoffs MUST
   support manual pre-configuration of security associations with other
   neighbouring FAs, involving shared keys and the default algorithms of
   [1].  POST-REGISTRATION FAs MUST implement the inter-FA
   authentication extension (FA-FA authentication extension) specified
   in [11] and MAY additionally use other security mechanisms.

4.1.
     Two Party Handoff

   Two party handoff occurs when the MN moves from oFA to nFA.  Normally
   this movement would result in a new Mobile IPv4 Registration at nFA.
   However in POST-REGISTRATION, the MN and nFA MAY delay this but
   maintain connectivity using the BET (or alternatively unidirectional
   tunnel) between oFA and nFA.  The protocol is shown in Figure 6.


            1a) L2-ST ~~~~> +------+ 2) HRqst +------+ <~~~ 1b) L2-TT
                            | oFA  |<-------->| nFA  |
                4a) L2-LD~> +------+ 3) HRply +------+ <~~~ 4b) L2-LU
                               ^                  ^
                     old L2    |                  |     new L2
                               +-------+    +-----+
                                       |    |
                                       |    |
                                       V    V
                                      +------+  movement
                       4c) L2-LU ---> |  MN  | --------->
                                      +------+

               Figure 6 - Two Party Handoff (POST-REGISTRATION)

   The following describes the progress of a two party handoff.  The
   numbered items refer to steps in Figure 6.  The source triggered
   HRqst/HRply message for tunnel creation, the target triggered
   HRqst/HRply message for tunnel creation and the HRqst/HRply to extend
   or terminate a BET (or unidirectional tunnel) are identified
   respectively using the suffixes (s), (t) and (r).

     1) Either the oFA or nFA receives an L2 trigger informing it that a
        certain MN is about to move from oFA to nFA.  The two cases are:



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           a) The L2 trigger is a source trigger (L2-ST) at oFA.  The
              trigger contains the MN's L2 address and an identifier for
              the nFA (the IPv4 address itself or an L2 address that can
              be resolved to the IPv4 address of the nFA).

           b) The L2 trigger is a target trigger (L2-TT) at nFA.  The
              trigger contains the MN's L2 address and an identifier for
              the oFA (the IPv4 address itself or an L2 address that can
              be resolved to the IPv4 address of the oFA).

     2) The FA receiving the trigger sends a Handoff Request (HRqst) to
        the other FA.  There are two cases:

           a) If oFA is sending the HRqst, the H bit is set and the N
              bit is unset, indicating it is an HRqst(s).  The HRqst(s)
              contains the lifetime of the tunnel the oFA is willing to
              support, the MN's IPv4 home address, the MN's HA address
              and an LLA option with the MN's L2 address.  If the
              lifetime is zero and the T bit is not set, the oFA is not
              willing to tunnel any packets for MN.  A positive lifetime
              and a set T bit indicate that the oFA is willing to tunnel
              for the indicated time.  Section 4.5 describes the
              HRqst(s) and Section 9 describes the LLA option.

           b) If nFA is sending the HRqst, the N bit is set and the H
              bit is unset, indicating it is an HRqst(t).  If the T bit
              is set, nFA has requested a reverse tunnel and the
              HRqst(t) contains the lifetime of the tunnel the nFA is
              requesting.  The HRqst(t) also contains an LLA option with
              the MN's L2 address.  The MN's IPv4 home address and HA
              address are not sent, unless they are discovered by some
              means outside the scope of this document (for example, as
              part of the L2-TT).  Section 4.5 describes the HRqst(t).

     3) The FA receiving the HRqst sends a Handoff Reply (HRply) to the
        other FA.  There are two cases:

           a) If oFA is sending the HRply, the N bit is set and the H
              and R bits are unset, indicating that the reply is in
              response to a HRqst(t), i.e. it is an HRply(t).  If the T
              bit is set, the HRply(t) contains the tunnel lifetime the
              oFA is willing to provide, otherwise the tunnel lifetime
              is set to zero indicating that the oFA is not willing to
              provide tunnel service.  If both HRply(t) and HRqst(t)
              have the T bit set and non-zero lifetime a BET is
              established.  The HRply(t) also contains the MN's home
              subnet IPv4 address, the MN's HA address, and an LLA
              option containing the MN's L2 address.  Section 4.6
              describes the HRply(t).




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           b) If nFA sends the HRply, the H bit is set and the N and R
              bits are unset, indicating this is a response to HRqst(s),
              i.e. it is an HRply(s).  If the T bit is set, the nFA
              indicates that it requests a reverse tunnel, and the
              lifetime field is set with the requested tunnel lifetime.
              The T Bit can be set in HRply only if the oFA had set the
              T bit in the corresponding HRqst or if the nFA requires to
              reverse tunnel incoming packets to oFA because ingress
              filtering is enabled on its network.  This establishes a
              BET.  The tunnel lifetime requested by the nFA must be
              less than or equal to the tunnel lifetime offered by oFA
              in the HRqst(s).  Section 4.6 describes the HRply(s).

     4) The point during the L2 handoff in which the MN is no longer
        connected on a given link is signaled by an L2-LD trigger at oFA
        and MN.  Completion of L2 handoff is signaled by an L2-LU
        trigger at nFA and MN.  The trigger is handled as follows:

           a) When oFA receives the L2-LD trigger, it begins forwarding
             MN-bound packets through the forward tunnel to nFA.

           b) When the nFA receives the L2-LU trigger, it begins
              delivering packets tunneled from oFA to MN and forwards
              outbound packets from MN using normal routing mechanisms
              or through a reverse tunnel to oFA or HA.  The nFA at this
              point may not yet be the default router of the MN (see
              section 4.4), therefore to receive all outbound packets
              from the MN the nFA must send a unicast proxy ARP message
              (used in [1]) to the MN upon receiving an L2-LU trigger.
              This proxy ARP message is an ARP Reply [5] sent by the nFA
              on behalf of oFA, therefore supplying the nFA link-layer
              address in the Sender Hardware Address field and the oFA
              IPv4 address in the Target Protocol Address field.

           c) When the MN receives the L2-LU, it MAY initiate the Mobile
              IPv4 Registration process by soliciting an Agent
              Advertisement as described in [1].  If the Registration is
              successful the nFA takes over the role of anchor FA (aFA)
              from the oFA.  Alternatively the MN MAY defer the Mobile
              IPv4 Registration (see section 4.4).

     5) The oFA becomes an aFA if the MN moves to a third FA before
        having performed a Mobile IPv4 Registration with nFA.

     6) Should L2 handoff fail in Step 4 (due to L2 reasons) and a
        ping-pong situation arise, the oFA may be able to determine this
        case through the trigger mechanism (i.e. FA sees successive
        L2-ST/L2-TT followed by L2-LD and then L2-LU).  The FA that
        originated the HRqst can in this case cancel the tunnel by
        sending an HRqst(r) to the other FA with lifetime zero.  It will



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        then simply continue delivering packets to MN exactly as if no
        handoff had been pending.  Section 4.5 describes the HRqst(r).

   If the oFA sets the B bit in HRqst/HRply and the nFA has not
   requested a reverse tunnel by setting the T bit, the nFA SHOULD
   tunnel outgoing packets from the MN to the HA because the MN has
   requested this service from the oFA.  The nFA SHOULD offer this
   service only if no security between the nFA and the MN's HA is
   required, or if there is an existing nFA-HA security association.

   The actual timing of BET or unidirectional tunnel placement depends
   on the available L2 triggers.  The forward tunnel from oFA to nFA is
   constructed using one of the tunneling procedures described in [1]
   for the HA to FA tunnel with the difference that the ends of the
   tunnel are at the oFA and nFA, respectively.  The reverse tunnel from
   nFA to oFA is constructed as described in [3] with the difference
   that the network end of the tunnel is at the oFA instead of the HA.
   If both forward and reverse tunnels are established then a BET has
   been established.  With optimal L2 trigger information, as described
   above, the FAs can setup the BET immediately when the L2 handoff is
   initiated, start tunneling MN-bound data when the link to the MN goes
   down and the nFA can use the link up trigger to start delivering
   packets.  In the absence of optimal L2 trigger information, the HRply
   can act as the trigger to start tunneling MN-bound data, but in this
   case, the period of packet delivery disruption to the MN could still
   be present and additional measures may be required to provide
   uninterrupted service.  Particular implementation and deployment
   scenarios could require techniques to smooth the handoff by providing
   a means to convey packets arriving during the L2 handoff.  The exact
   techniques are outside the scope of this document.

   Figures 7 and 8 show timing diagrams for source trigger (L2-ST) and
   target trigger (L2-TT) two party handoff, respectively.


                 MN                    nFA                 oFA
                  |                     |                   |
                  |                     |     HRqst(s)      |<~~~ L2-ST
                  |                     |<------------------|
                  |                     |     HRply(s)      |
                  |                     |------------------>|
                  |                     |                   |
                 --------------------------------------------<~~~ L2-LD
                                   L2 Handoff
                 --------------------------------------------<~~~ L2-LU
                  |                     |                   |
                  |<------------------->|                   |
                  |    MN's traffic     |                   |

               Figure 7 - Two Party Source Trigger Handoff Timing



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                 MN                    nFA                 oFA
                  |                     |                   |
                  |           L2-TT ~~~>|     HRqst(t)      |
                  |                     |------------------>|
                  |                     |     HRply(t)      |
                  |                     |<------------------|
                  |                     |                   |
                 --------------------------------------------<~~~ L2-LD
                                   L2 Handoff
                 --------------------------------------------<~~~ L2-LU
                  |                     |                   |
                  |<------------------->|                   |
                  |    MN's traffic     |                   |

               Figure 8 - Two Party Target Trigger Handoff Timing

   Once the tunnel between aFA and the current FA is in place, it is
   torn down by one of the following events:

     1) The aFA decides to stop tunneling because the lifetime it sent
        expires and was not renewed, or the aFA or current FA decide to
        terminate tunnel service prematurely for some other reason
        (refer to section 4.3).

     2) The MN completes the process by performing a Mobile IPv4
        Registration with the current FA.  This may be initiated by the
        FA that sends an Agent Advertisement or by the MN that solicits
        for an Agent Advertisement as in [1].

     3) The MN moves to a third FA (see section 4.2)

4.2.
     Three Party Handoff

   Three party handoff is applicable when an MN, which has already
   established an aFA and is receiving tunneled packets through its
   current FA, moves to a new FA without performing a Mobile IPv4
   Registration.

   The need for the Three Party Handoff function depends on the wireless
   system in which POST-REGISTRATION is being implemented.  For radio L2
   protocols in which it is possible for the MN to move so rapidly from
   one FA to another such that a probability exists that the Mobile IPv4
   Registration with nFA will not complete before the MN moves on, HTT
   (Handoff To Third) SHOULD be implemented.  Certain wireless systems
   and implementations do not allow such fast movement between FAs and
   may force the Mobile IPv4 Registration to occur soon after L2
   handoff, in which case three party handoff is not applicable.  If
   this three party handoff feature is not implemented, the FA SHOULD
   send an Agent Advertisement to the MN after L2 handoff has completed




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   (L2-LU at nFA) and/or the MN SHOULD solicit an Agent Advertisement
   after L2 handoff (L2-LU at MN).

                                    +------+
                               +--->| aFA  |<---+
                               |    +------+    |
                  4b) HRqst(r) |                | 3) HRqst(t)
                      HRply(r) |                |    HRply(t)
                               |                |
                               v    2a) HRqst   v
            1a) L2-ST ~~~> +------+     HTT  +------+ <~~~ 1b) L2-TT
                           | oFA  |<-------->| nFA  |
           4a) L2-LD ~~~>  +------+ 2b) HTT  +------+ <~~~ 5a) L2-LU
                              ^         HRply    ^
                      old L2  |                  |  new L2
                              +-------+    +-----+
                                      |    |
                                      |    |
                                      V    V
                                     +------+  movement
                      5b) L2-LU ~~~> |  MN  | --------->
                                     +------+

                         Figure 9 - Three Party Handoff

   The L3 handoff can be deferred either because of a decision by the
   MN/FA (i.e. MN does not send Agent Solicitations and FA does not send
   Agent Advertisements) or it may result from rapid movement between
   oFA and nFA that does not allow enough time for the registration to
   complete.  This scenario is shown in Figure 9.  In this case, oFA
   must inform nFA (i.e. the third FA) to contact aFA about moving the
   radio end of the tunnel.  This is performed with the HTT message.
   The general idea behind the three party handoff procedure is that the
   oFA supplies nFA with the same information it would have obtained via
   an L2-TT if handoff had occurred from aFA to nFA, then the nFA
   performs an HRqst(t)/HRply(t) sequence with aFA in order to move the
   BET to nFA.  When the L2 handoff is complete, oFA sends an HRqst(r)
   to aFA to terminate the previous BET.

   The following describes the progress of a three party handoff.  The
   numbered items refer to steps in Figure 9.

     1) Either the oFA or nFA receives an L2 trigger informing it that a
        certain MN is about to be moved.  The two cases are:

           a) The L2 trigger is a source trigger (L2-ST) at oFA.  The
              trigger contains the MN's L2 address and an identifier for
              the nFA (the IPv4 address itself or an L2 address that can
              be mapped to the IPv4 address of the nFA).




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           b) The L2 trigger is a target trigger (L2-TT) at nFA.  The
              trigger contains the MN's L2 address and an identifier for
              the oFA (the IPv4 address itself or an L2 address that can
              be resolved to the IPv4 address of the oFA).

     2) The oFA and nFA exchange a HTT/HRply or HRqst/HTT pair.  HTT is
        indicated by setting both the H and N bits in the HRqst or
        HRply.  The HTT message MUST NOT have any tunnel flag bits set,
        because the tunnel is negotiated between the aFA and nFA, not
        oFA and nFA.  There are two cases:

           a) The L2 trigger is an L2-ST.  The oFA sends HTT to nFA
              containing the MN's home IPv4 address, the MN's HA
              address, an LLA containing the aFA's IPv4 address, and an
              LLA containing the L2 address of the MN.  This is enough
              information for nFA to perform a target triggered handoff
              with aFA.  The nFA responds with a HRply(s).  Section 4.8
              describes the HTT.

           b) The L2 trigger is an L2-TT.  The nFA sends HRqst(t) to oFA,
             exactly as if a two party handoff were occurring.  The
             oFA responds with HTT containing the same information as
             in a) above.  This is enough information for nFA to
             perform a target triggered handoff with aFA.

     3) Upon receipt of the HTT, the nFA first checks its Visitor Cache
        to see whether it is already tunneling for MN.  If so, Step 6 is
        performed.  If not, nFA performs a target triggered handoff with
        aFA, exactly as in Section 4.1, exchanging a HRqst(t)/HRply(t)
        pair.  Because aFA receives no L2 trigger indicating when L2
        handoff starts, it may start tunneling to nFA upon transmission
        of the HRply(t).

     4) Once the L2 handoff is underway and the MN gets disconnected at
        L2, aFA and oFA exchange messages canceling tunnel service
        between aFA and oFA and allowing aFA to start the tunnel with
        nFA.

           a) The point in the L2 handoff process where the MN gets
              disconnected from oFA is signaled at oFA by L2-LD.

           b) The oFA exchanges a HRqst(r)/HRply(r) pair having lifetime
              zero with aFA.  This cancels tunnel service between oFA
              and aFA.  If aFA has not already established a tunnel to
              nFA, it must do so immediately upon receipt of the
              HRqst(r).  The aFA provides tunneling service exactly as
              described in Section 4.1 Step 4a.

     5) Completion of L2 handoff is signaled by an L2-LU trigger at nFA
        and/or MN.  The nFA and MN handle the trigger as follows:



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           a) The nFA provides packet delivery service to the MN exactly
              as described in Section 4.1, Step 4b.

           b) The MN either defers or initiates Mobile IPv4 Registration
              when it receives the L2-LU, as in Section 4.1

     6) In the special case where nFA and aFA are the same (i.e. the MN
        is moving back to the original anchor FA), aFA recognizes that
        it is tunneling to oFA when it checks its Visitor Cache in Step
        3.  In this case, there is no need for aFA to send the
        HRqst(t)/HRply(t) in Step 3.  Upon receipt of the L2-LU trigger
        on handoff completion, the aFA begins routing packets to MN and
        the tunnel to nFA is torn down.  The oFA still exchanges the
        HRqst(r)/HRply(r) with aFA in Step 4b because oFA cannot know a
        priori that aFA and nFA are the same, but they are redundant.

   Figures 10 and 11 show timing diagrams for source trigger (L2-ST) and
   target trigger (L2-TT) three party handoff, respectively.


          MN               nFA            oFA              aFA
           |                |   L2-ST ~~~> |                |
           |                |              |                |
           |                |<-------------|                |
           |                |       HTT    |                |
           |                |------------->|                |
           |                |    HRply(s)  |                |
           |                |------------------------------>|
           |                |   HRqst(t)   |                |
           |                |<------------------------------|
           |                |    HRply(t)  |                |
           |                |              |                |
          ----------------------------------<~~~ L2-LD      |
                                           |--------------->|
                        L2 Handoff         |     HRqst(r)   |
                                           |                |
                                           |<---------------|
                                           |     HRply(r)   |
                                           |                |
          ----------------------------------<~~~ L2-LU      |
           | MN's traffic   |              |                |
           |<-------------->|              |                |

              Figure 10 - Three Party Source Trigger Handoff Timing









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          MN               nFA            oFA              aFA
           |                |              |                |
           |                |<~~~ L2-TT    |                |
           |                |------------->|                |
           |                |    HRqst(t)  |                |
           |                |<-------------|                |
           |                |    HTT       |                |
           |                |------------------------------>|
           |                |   HRqst(t)   |                |
           |                |<------------------------------|
           |                |    HRply(t)  |                |
           |                |              |                |
          ----------------------------------<~~~ L2-LD      |
                                           |--------------->|
                        L2 Handoff         |     HRqst(r)   |
                                           |                |
                                           |<---------------|
                                           |     HRply(r)   |
                                           |                |
          ----------------------------------<~~~ L2-LU      |
           | MN's traffic   |              |                |
           |<-------------->|              |                |

              Figure 11 - Three Party Target Trigger Handoff Timing

   Unlike two party handoff, the timing of BET establishment between aFA
   and nFA cannot fully depend on the availability of L2 trigger
   information because aFA does not receive an L2 trigger signalling L2
   handoff.  The two timing extremes at which aFA can place the BET with
   nFA are:

     1) At the earliest, aFA MAY start tunneling packets using the BET
        to nFA after sending the HRply(t) to nFA in response to the
        request for target-triggered handoff

     2) At the latest, aFA MAY start tunneling packets using the BET to
        nFA and tear down the BET with oFA when receiving the HRqst(r)
        from oFA indicating the MN has disconnected.

   In addition, aFA MAY continue tunneling to oFA if 1) is selected,
   until the HRqst(r) is received.  In this case, the result may be
   duplicated packets at the MN because the MN will receive packets
   through oFA on the old L2 until it disconnects (L2-LD).  If 2) is
   selected, the additional latency will add to the MN's L3 service
   disruption period.  Of course, aFA can choose to place the BET some
   time between 1) and 2) if reliable bounds are available on the
   duration of time between L2-ST/L2-TT and the MN's disconnection
   (L2-LD).  The exact selection of when to establish the BET is likely
   to be influenced by network engineering and implementation




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   considerations, including whether a handoff smoothing solution is
   used, and is beyond the scope of this specification.

4.3.
     Renewal or Termination of Tunneling Service

   To prevent a BET from expiring when its lifetime runs out, the MN's
   current FA signals the aFA to either renew or terminate the BET.
   This may be the case when the MN defers Mobile IPv4 Registration.  If
   no such signal is received, the aFA will terminate the BET when the
   lifetime expires.  In addition, the current FA or aFA may need to
   terminate the BET prior to the lifetime expiring.  In order to avoid
   error conditions in which tunnels do not expire even though the MN to
   which they apply is no longer reachable, FAs SHOULD set the tunnel
   lifetime field to some value other that 0xffff, which indicates "good
   until cancelled".

   Figure 12 illustrates the message exchange that occurs between the FA
   needing to terminate or extend the tunnel (designated FA(1) in the
   figure) and the other FA (designated FA(2) in the figure).  The
   HRqst(r)/HRply(r) is indicated by setting the R bit in the
   HRqst/HRply messages.  If the HRqst(r) is renewing a BET then it
   contains a non-zero lifetime, otherwise if the lifetime is set to
   zero it indicates tunnel termination.  The aFA has complete control
   over whether a tunnel is extended or terminated, and it MAY reply to
   a request for extension with a shorter lifetime than was requested.

                                     HRqst(r)
                            +------+ <--------  +------+
                            | FA(2)| ---------> | FA(1)|
                            +------+ HRply(r)   +------+

                      Figure 12 - BET Renewal or Termination

4.4.
     When will the MN perform a Mobile IPv4 Registration?

   The MN/FA have control over when to perform the Mobile IPv4
   Registration.  Although the MN/FA may decide to defer Mobile IPv4
   Registration for a certain period, three possible events can lead to
   the need to terminate tunneling service.  If this occurs the MN MUST
   perform the Mobile IPv4 Registration.  These events are:

     1) The end of life for the BET is pending and a request by the
        current FA to aFA for renewal has been denied, or alternatively
        the current FA or aFA needs to terminate the BET prematurely.
        The FA in this case MUST initiate the Mobile IPv4 Registration
        by sending an Agent Advertisement to the MN as in [1].

     2) The MN itself decides to perform a Mobile IPv4 Registration and
        initiates it by sending an Agent solicitation as in [1].




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     3) During a source triggered handoff, the oFA attempts to perform
        BET handoff but nFA is not capable of performing it.  The FA in
        this case MUST initiate the Mobile IPv4 Registration by sending
        the MN an Agent Advertisement as in [1].  Note that this
        situation will never arise during target triggered handoff
        because an HRqst(t) will not be sent to oFA by an nFA that
        doesn't support POST-REGISTRATION.

   Some detailed scenarios relating to case 2) will be described
   hereafter.  According to [1], when using an FA care-of address the MN
   MAY use the FA as its default router.  Otherwise it MUST choose its
   default router from those advertised in the ICMP Router Advertisement
   portion of the Agent Advertisement.  Here we assume that the FA
   router is also the MN's default router.  In POST-REGISTRATION, when a
   tunnel is established between oFA and nFA and the MN has moved to
   nFA, the oFA MUST NOT send Agent Advertisements to the MN.  In this
   case it is possible that the MN will not receive Agent Advertisements
   for extended periods of time.  According to [8] hosts will remove
   default router entries if the lifetime of the Router Advertisement
   expires and no further advertisements are received.  Note that the
   ICMP Router Advertisement lifetime is not related to the Registration
   Lifetime in the Mobility Agent Advertisement extension [1].  To avoid
   this disruption the MN MUST solicit the default router (i.e. FA)
   before the lifetime of its active default router entry runs out, or
   alternatively the FA MUST advertise as soon as the MN-nFA link is up
   (L2-LU).  This effectively means that the MN will at most be able to
   defer Mobile IPv4 Registration for as long as the remaining lifetime
   of the active default router, as configured in the ICMP Router
   Advertisements.  The MN MUST perform a Mobile IPv4 registration [1]
   when it receives an Agent Advertisement following a POST-REGISTRATION
   handoff.

4.5.
     Handoff Request (HRqst) Message format

   This is a new Mobile IPv4 message carried on UDP (destination port
   434) [1].  The UDP header is followed by the fields below.

















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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      |H|N|R|M|G|T|B|            Reserved             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            Lifetime           |          Reserved             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        MN Home Address                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          HA Address                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                         Identification                        +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Extensions ...
    +-+-+-+-+-+-+-+-

     Type              TBD (Handoff Request)

     H                 Source triggered handoff request.  When set and
                       the N bit is unset, indicates that the request
                       was the result of an L2-ST at oFA.

     N                 Target triggered handoff request.  When set and
                       the H bit is unset, indicates that the request
                       was the result of an L2-TT at nFA.

     R                 Set if the request is an HRqst(r) (i.e. a request
                       to renew the tunnel, H and N bits must be unset).

     M                 The FA issuing the HRqst will use Minimal
                       Encapsulation as defined in [1,5] for the tunnel.

     G                 The FA issuing the HRqst will use GRE [4]
                       Encapsulation as defined in [1,5] for the tunnel.
                       Extensions of HRqst containing GRE type and key
                       Fields are outside the scope of this document.

     T                 For an HRqst(s), indicates that the oFA is
                       willing to support both forward and reverse
                       tunnel service.  For an HRqst(t), indicates that
                       the nFA is requesting reverse tunnel service.

     B                 When sent in an HRqst(s), indicates that the MN
                       has requested a reverse tunnel to the HA and that
                       the nFA SHOULD use reverse tunnel to the HA if it
                       will not be reverse tunneling to the oFA.

     Lifetime          The lifetime of the tunnel in seconds.  If this



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                       is an HRqst(t), then the lifetime represents a
                       request by nFA for a reverse tunnel.  If this is
                       an HRqst(s), then the lifetime represents the
                       maximum amount of time that oFA is willing to
                       maintain both forward and reverse tunnels. If
                       this is an HRqst(r), then the lifetime represents
                       a request for the amount of time to renew the
                       tunnel's lifetime.  A value of 0 on an HRqst(s)
                       indicates that the oFA is unwilling to grant
                       tunnel service.  A value of 0 on an HRqst(t)
                       indicates that the nFA does not require reverse
                       tunnel service.  A value of 0 on an HRqst(r)
                       indicates that the tunnel should be terminated.
                       A value of 0xffff indicates infinity.

     MN Home Address   For HRqst(s), the home address of the MN.

     HA Addr           For HRqst(s), the HA address of the mobile node.

     Identification    As in defined in [1].

     Extensions        The Message MUST include an LLA (see Section 9)
                       containing the MN's L2 address and an L2 address
                       that can be mapped to an IPv4 address for the FA.
                       This Message MUST contain the FA-FA
                       Authentication Extension [11] that is used to
                       secure the HRqst message.

4.6.
     Handoff Reply (HRply) Message

   This is a new Mobile IPv4 message carried on UDP (destination port
   434) [1].  The UDP header is followed by the fields 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      |H|N|R|M|G|T|B|    Reserved     |    Code       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          Lifetime             |         Reserved              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        MN Home Address                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          HA Address                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                         Identification                        +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Extensions ...
    +-+-+-+-+-+-+-+-



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     Type              TBD (Handoff Reply)

     Code              A value indicating the result of the Handoff
                       Request.  Only two codes are currently supported,
                       0, indicating success, and 1, indicating that the
                       handoff cannot be performed. The remaining values
                       are for future use.

     Lifetime          The lifetime, in seconds, for which the
                       bi-directional tunnel for the MN will be
                       maintained.  If this is an HRply(s), then the
                       lifetime represents a request by nFA, and it can
                       be any value up to the maximum value sent in the
                       HRqst(s).  Larger values are assumed to default
                       to OFA's maximum.  If this is an HRply(t), then
                       the lifetime represents the maximum amount of
                       time that the oFA will grant to the nFA.  If this
                       is a HRply(r), then the lifetime represents the
                       amount of time by which the tunnel life will be
                       extended.  If the Code field indicates that
                       handoff failed, the Lifetime field will be
                       ignored and SHOULD be set to zero.  A value of
                       0 on an HRply(t) indicates that the oFA is
                       unwilling to grant service.  A value of 0 on an
                       HRply(s) indicates that the nFA does not require
                       service.  A value of 0 on HRply(r) indicates that
                       the tunnel lifetime will be terminated.  A value
                       of 0xffff indicates infinite lifetime.

     H                 Source triggered handoff reply.  When set and
                       the N bit is unset, indicates that the
                       reply is in response to an HRqst(s).

     N                 Target triggered handoff reply.  When set and
                       the H bit is unset, indicates that the
                       reply is in response to an HRqst(t).

     R                 Set if the reply is an HRply(r).  Neither
                       the H nor the N bit are set.

     M                 The FA issuing the HRqst will use Minimal
                       Encapsulation as defined in [1,5] for
                       the tunnel.

     G                 The FA issuing the HRqst will use GRE [4]
                       Encapsulation as defined in [1,5] for the tunnel.
                       When this flag bit is set the HRply may require
                       extensions containing the GRE type and key
                       fields, but they are outside the scope of this
                       document.



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     T                 For an HRply(s), indicates that the nFA is
                       Requesting to reverse tunnel service.  For an
                       HRply(t), indicates that the oFA is willing to
                       provide both forward and reverse tunnel service.

     B                 When sent in an HRply(t), indicates that the MN
                       has requested a reverse tunnel to the HA and that
                       the nFA SHOULD use reverse tunnel to the HA if
                       it will not be reverse tunneling to the oFA.  It
                       can be set in HRply(t) only if the T bit was
                       unset in the corresponding HRqst(t).

     MN Home Address   For HRply(t), the home IPv4 address of the MN.

     HA Addr           For HRply(t), the HA IPv4 address of the MN.

     Identification    As in defined in [1].

     Extensions        This Message MUST contain the FA-FA
                       Authentication Extension [11] that is used to
                       secure the HRply message.

4.7.
     Handoff To Third (HTT) Message

   The Handoff to Third message has the same format as the Handoff
   Request and Handoff Reply Messages, except both the H and N bits are
   set.  If the HTT message is in response to a L2-ST and is sent to
   initiate a handoff, then, with the exception of the H and N bits, the
   message has the same fields set and includes the same extensions as
   an HRqst(s).  If the HTT message is sent in response to an HRqst(t),
   then, with the exception of the H and N bits, the message has the
   same fields set and includes the same extensions as an HRply(t).  The
   tunnel bits MUST NOT be set in the HTT message because BET
   construction is not negotiated between oFA and nFA, it is negotiated
   between nFA and aFA in the ensuing HRqst(t)/HRply(t).

   In addition, the HTT MUST contain the following extensions in the
   specified order:

        Solicited IPv4 Address Option: containing aFA's Address

        LLA Option: containing the L2 address of the MN.

4.8.
     Applicability of POST-REGISTRATION Handoff Method

   The POST-REGISTRATION handoff approach allows FAs to communicate
   directly about a pending handoff, and does not require any IPv4 layer
   messages to be sent to or from a MN prior to the L2 handoff event.
   Therefore, it eliminates a possible source of handoff latency.  This
   may be required when the link layer imposes hard deadlines on the



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   time at which a handoff must occur, such as when a MN is rapidly
   moving out of a radio coverage area.  Consequently, POST-REGISTRATION
   is primarily of interest in handoff between FAs that support the same
   radio access technology.  Handoff between heterogeneous radio
   technologies will, of necessity, require interaction between the MN
   and the network, and so is not a domain of applicability for
   POST-REGISTRATION.

   Because a POST-REGISTRATION handoff is triggered by an unspecified
   mechanism that informs the oFA or nFA that an L2 handoff is pending,
   the POST-REGISTRATION approach is only applicable to networks where
   such a mechanism is available.  For example, an L2 may provide
   indications of radio signal quality that cause the oFA or nFA to send
   the POST-REGISTRATION handoff messages.  Any such indications must
   also provide each FA involved in the handoff with the identity of the
   other, so that messages can be sent to the right place.  This may
   involve mapping L2 information onto FA IPv4 addresses.  Also, the FAs
   involved in a handoff must have pre-provisioned security arrangements
   so that the POST-REGISTRATION messages can be authenticated.  If a
   handoff is to be completed as a result of the POST-REGISTRATION
   messaging, any L2 handoff indications must also be securely
   authenticated so that traffic to the old point of attachment is not
   improperly halted.

   POST-REGISTRATION handoff is appropriate in the following cases:

   - L2 triggers are available on the network to indicate that L2
     handoff is pending.

   - Pre-provisioned security mechanisms are in place to allow fast
     and secure messaging between the FAs and between the MN and an FA.

   - Access point choice by the MN is not a concern or choice requires
     user intervention and therefore is not on the critical path for
     handoff.


5.
   Combined Handoff Method

   The combined method uses both PRE-REGISTRATION and POST-REGISTRATION
   handoff.  If PRE-REGISTRATION does not complete prior to the
   expiration of a timer on the nFA, the POST-REGISTRATION mechanism is
   used to create the tunnel between oFA and nFA.  This protects the MN
   from delays caused by errors such as loss of the Mobile IPv4
   Registration Reply message involved in PRE-REGISTRATION for the
   mobile-initiated and network-initiated source-triggered cases.  It
   also protects the MN from delays caused by errors or the loss of any
   of the Mobile IPv4 messages involved in PRE-REGISTRATION for the
   network-initiated target-triggered case.




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   When the nFA receives a target-trigger it will follow the PRE-
   REGISTRATION procedure.  When the combined method is used, the nFA
   MUST also start a timer when it receives a target-trigger.  The timer
   should be set to a small value (default for target-trigger case: 1
   sec).

   According to PRE-REGISTRATION, the nFA will receive the Registration
   Request from the MN.  When the combined method is used, this
   Registration Request sent by the MN MUST contain the IPv4 address of
   the oFA in an FA IPv4 address LLA extension (see 9.7).  This same
   Registration Request message will contain multiple LLA extensions,
   since it will also contain the MN's layer 2 address in an LLA
   extension as described for PRE-REGISTRATION (see 3.7 and 9).  When
   the nFA has not started the handoff procedure using a target-trigger
   (i.e. mobile-initiated or network-initiated target-triggered cases)
   the nFA MUST start a timer as soon as it receives the low latency
   Registration Request from the MN.  This timer should be set to a
   small value (default for : 1 sec).

   In all cases the timer MUST be reset when the Registration Reply
   message is received by nFA.  If the timer expires before the
   Registration Reply is received, the nFA MUST initiate the POST-
   REGISTRATION procedure.  The nFA utilizes the oFA IPv4 address
   (previously received in the extension to the Registration Request
   message) as the destination of the POST-REGISTRATION HRqst message to
   create the tunnel between nFA and oFA.  The nFA MAY tear down this
   tunnel when it receives and forwards a successful Registration Reply
   for that MN.


6.
   Layer 2 and Layer 3 Handoff timing Considerations

   In the optimal cases considered in the PRE-REGISTRATION and
   POST-REGISTRATION handoffs it was assumed that a timely L2 trigger
   would be received in such a way that packets could be delivered to
   the MN via its nFA immediately upon connection.  In this way the MN
   does not suffer disruption due to the L3 handoff.  However such
   precise timing of the L2 trigger and handoff mechanism with respect
   to the actual L2 handoff event will not be possible in all wireless
   systems and may depend on particular implementation techniques.
   Therefore some uncertainty may exist at L3 as to exactly when the L2
   connection between the MN and the nFA becomes fully established and
   can be used for L3 traffic.  It is possible that in certain
   implementations traffic will be re-routed too early or too late with
   respect to the moment when the connection between the MN and the nFA
   becomes fully established.  The techniques that may solve this
   problem and allow the MN to receive traffic independently of the
   timing of the L2 handoff event include buffering and simultaneous
   bindings (i.e. bicasting: setting the S bit [1] in Registration
   Requests). However these are optional and are not mandated.



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7.
   Reverse Tunneling Support

   The handoff methods all support reverse tunneling.  The MN may
   request reverse tunneling [3] by setting the T bit in its
   Registration Request.  In the case of POST-REGISTRATION, if the MN
   had requested Reverse Tunneling previously at oFA, the Handoff
   message from oFA (see Section 4) includes the T bit enabled to inform
   nFA to establish a BET for the visitor entry.  Typically, the T bit
   will always be set to ensure that any delays in the MN receiving its
   new care of address do not result in any delay in uplink packet
   transmission from the MN, but local policies and particular L2
   technologies may allow the reverse tunnel to be turned off.


8.
   Handoff Signaling Failure Recovery

   In general and to a greater extent in wireless networks, packets
   carrying handoff signaling may be dropped or lost due to errors on
   the link.  In this section, we consider mechanisms for recovery from
   handoff signaling failures.

8.1.
     PRE-REGISTRATION Signaling Failure Recovery

   Failure of PRE-REGISTRATION signaling breaks down into three cases:

     1) Loss of messages PrRtSol and PrRtAdv on the air link.

     2) Loss of the solicitation by an FA to obtain another neighbouring
        FA's Advertisement or loss of the neighbouring FA's
        advertisement.

     3) Failure of the standard Mobile IPv4 Registration.

   Of these, case 3) is handled by standard Mobile IPv4 mechanisms
   described in [1].  Case 2) is expected to be a rare event because
   spontaneous packet drop rates on the fixed network are caused by
   congestion or router failure.  Since bit error rates on wireless
   links are higher than on fixed links, case 1) is more likely to
   occur.  In the following subsections, the cases 1) and 2) are
   considered.

8.1.1.
       Failure of PrRtSol and PrRtAdv

   PRE-REGISTRATION handoff can fail in network-initiated handoff when
   the PrRtAdv sent by oFA in response to the source trigger (L2-ST) or
   the advertisement sent by nFA in response to the target trigger (L2-
   TT) fails to reach the MN.  PRE-REGISTRATION handoff can also fail in
   mobile-initiated handoff when either the PrRtSol sent from the MN or
   return PrRtAdv sent from the oFA are dropped.  To reduce the
   probability that PrRtAdv and PrRtSol are lost the MN and FA MAY



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   transmit multiple copies of these messages.  Should these messages
   fail anyway, in both cases the MN connects to the nFA without having
   received any prior signalling.  In this case the MN solicits an FA
   Advertisement when it connects to nFA at L2 (L2-LU), as described in
   3.6, and performs a standard Mobile IPv4 registration with the nFA as
   specified in [1].

8.1.2.
       Failure of Inter-FA solicitation and advertisement

   The solicitation from an FA to another neighbouring FA may fail or
   the corresponding advertisement from the neighbouring FA may be lost.
   To reduce the probability that these messages are lost, the FAs MAY
   transmit multiple copies of these messages.  If a failure occurs
   anyway, the FA soliciting the Agent Advertisement is unable to send a
   PrRtAdv in response to a source trigger or to a mobile-initiated
   PrRtSol.  In these cases, when the MN does not receive a notification
   or confirmation of a PRE-REGISTRATION handoff, the MN MUST perform a
   standard Mobile IPv4 registration as soon as it connects to the nFA
   (L2-LU) as described in 8.1.1.

8.2.
     POST-REGISTRATION Signaling Failure Recovery

   Failure occurs in POST-REGISTRATION when either the HRqst or HRply
   message is dropped.  The effects of the failure and the recovery
   procedure depend on which message is dropped, and whether the handoff
   is source or target triggered.  Since all of the POST-REGISTRATION
   signaling is going over the fixed network, it can be expected that
   spontaneous dropping of packets in the absence of congestion and
   router failure should be a relatively rare event.  Nevertheless,
   failure recovery mechanisms SHOULD be implemented.

8.2.1.
       HRqst Message Dropped

   If the HRqst message is dropped, the effect is the same for both
   source and target-triggered handoff.  In either case, the FA to which
   the HRqst was destined will never respond with an HRply message.  If
   the handoff is source-triggered, then the nFA never learns of the
   handoff, and the oFA never receives confirmation.  If the handoff is
   target-triggered, then the oFA never learns of the handoff, and the
   nFA never receives confirmation.

   The recovery procedure in this case is as follows: the oFA MUST NOT
   construct a forward tunnel when the MN moves off-link (L2-LD) if the
   handoff is source-triggered, and the nFA MUST NOT construct a reverse
   tunnel if the handoff is target-triggered.  If the nFA was not
   informed of the handoff by an HRqst message (corresponding to failure
   of source-triggered handoff) or if the handoff was not confirmed by
   an HRply message (corresponding to failure of target-triggered
   handoff) the nFA MUST unicast an Agent Advertisement to the MN as
   soon as its L2 connection is established (L2-LU at nFA).



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8.2.2.
       HRply Message Dropped

   If the HRply message is dropped, the FA sending the HRply will assume
   that the handoff has been confirmed, but the FA that is expecting to
   receive the HRply does not receive confirmation.  In this case, the
   failure recovery procedure is different for source-triggered and
   target-triggered handoffs.

   In a target-triggered handoff, the oFA assumes the handoff is
   confirmed because it has sent the HRply, but the nFA has not received
   it so it does not have confirmation.  The oFA starts tunneling
   packets to the nFA when the MN moves from its link (L2-LD).  The nFA
   MUST send a FA Advertisement to the MN as soon as its L2 link is up
   (L2-LU at nFA) and MAY drop the tunneled packets.  The nFA SHOULD
   send an ICMP Destination Unreachable [9] message to the oFA.  When
   the oFA receives this message it will terminate the tunnel and stop
   forwarding packets.  If reverse tunneling was requested the nFA MUST
   NOT reverse tunnel because it has not received handoff confirmation.

   In source-triggered handoff, the nFA assumes the handoff is confirmed
   because it has sent the HRply, but the oFA has not received it so it
   does not have confirmation.  Without failure recovery, the MN could
   move to the nFA without the oFA being able to start the forward
   tunnel for the MN's packets, and the MN would not be able to initiate
   a Mobile IPv4 registration because it does not know that the handoff
   has failed.  In this situation, the oFA MUST send out a HRqst message
   to the nFA with lifetime zero as soon as the MN leaves its link
   (L2-LD).  The oFA SHOULD continue to retransmit the HRqst message,
   with exponential backoff for CONFIG-HFAIL seconds or until it
   receives an HRply acknowledging the request to cancel the tunnel.
   The default value for CONFIG-HFAIL is 10 seconds.  When the nFA
   receives the HRqst, it MUST immediately send an Agent Advertisement
   to the MN, as is the case whenever a tunnel is cancelled.  In
   addition, the oFA MUST also drop any packets received through the
   reverse tunnel from the nFA.  The oFA SHOULD NOT send the ICMP
   Destination Unreachable message to the nFA because the nFA has been
   informed by the HRqst message to cancel the tunnel.  However, if the
   nFA receives an ICMP Destination unreachable message for the tunnel
   prior to receiving the HRqst canceling the tunnel, it MUST send an FA
   Advertisement to the MN and cancel the tunnel.


9.
   Generalized Link Layer and IPv4 Address (LLA) Extension

   This section defines the Generalized Link Layer and IPv4 Address
   (LLA) Extension, used by any node that needs to communicate Link
   Layer and IPv4 Addresses.  The format of the extension relies on sub-
   types, where each sub-type defines its own sub-structure.  This draft
   defines six sub-types.  Future RFCs should allocate their own sub-
   type and define their own address formats.



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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      |   Sub-Type    |    LLA ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type

        TBD (skippable) [1]  - when used in Registration Requests
        TBD (skippable) [1]  - when used in Agent Advertisements

      Length

        The length of the Link Layer Address + the one octet Sub-Type
        field

      Sub-Type

        This field contains the Link Layer sub-type identifier

      LLA

        Contains the Link Layer Address


      In this document, seven subtypes are defined:

            1        3GPP2 International Mobile Station Identity and
                     Connection ID [13]
            2        3GPP International Mobile Subscriber Identity [15]
            3        Ethernet 48 bit MAC address [5]
            4        64 bit Global ID, EUI-64 [6]
            5        Solicited IPv4 Address
            6        Access Point Identifier
            7        FA IPv4 Address

   The following subsections describe the extensions.


9.1.
     3GPP2 IMSI Link Layer Address and Connection ID Extension

   The IMSI Link Layer Address Extension contains the International
   Mobile Station Identity.

       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      |   Sub-Type    |    IMSI ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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         Type

            TBD (skippable) [1]

         Length

            The length of the IMSI field + the one octet Sub-Type field

         Sub-Type

            1

         IMSI

            Contains the IMSI, in the form:
                       <IMSI>:<Connection Id>
            Where the <IMSI> is an ASCII-based representation of the
            International Mobile Station Identifier, most significant
            digit first, ":" is ASCII 0x3a, and the Connection ID is the
            ASCII representation of a small, decimal number used for
            distinguishing different link-layer connections from the
            same mobile device.

9.2.
     3GPP IMSI Link Layer Address Extension

   The IMSI Link Layer Address Extension contains the International
   Mobile Station Identity.

       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      |   Sub-Type    |    IMSI ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Type

            TBD (skippable) [1]

         Length

            The length of the IMSI field + the one octet Sub-Type field

         Sub-Type

            2

         IMSI

            Contains the IMSI, a number composed of 15-digits or less,
            coded as described in [15].



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9.3.
     Ethernet Link Layer Address Extension

   The Ethernet Link Layer Address Extension contains the 48 bit
   Ethernet MAC Address, as defined in [5].

       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      |   Sub-Type    |    MAC ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Type

            TBD (skippable) [1]

         Length

            7 (includes the Sub-Type field)

         Sub-Type

            3

         MAC

            Contains the 48 bit Ethernet MAC Address.

9.4.
     IEEE 64-Bit Global Identifier (EUI-64) Address Extension

   The 64-Bit Global Identifier (EUI-64) Address Extension contains the
   64 bit address, as defined in [6].

       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      |   Sub-Type    |    MAC ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Type

            TBD (skippable) [1]

         Length

            9 (includes the Sub-Type field)

         Sub-Type

            4




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         MAC

            Contains the 64-Bit Global Identifier Address.

9.5.
     Solicited IPv4 Address Extension

   The 32-bit Solicited IPv4 Address Extension contains the IPv4 address
   of the agent (FA) being solicited.  This extension MAY be present in
   an ICMP Agent Solicitation as explained in Section 3.3.

       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      |   Sub-Type    |    IPv4 addr ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Type

            TBD (skippable) [1]

         Length

            5 (includes the Sub-Type field)

         Sub-Type

            5

         IPv4 Address

            Contains the 32-Bit IPv4 Address of the solicited node.

9.6.
     Access Point Identifier Extension

   The 32-bit Access Point Identifier Extension contains an Identifier
   of the Access Point to which the MN will move.  This may be a
   wireless L2 identifier.  The MN is able to solicit an advertisement
   from the FA servicing a certain Access Point by using this extension
   with Agent Solicitations as explained in Section 3.3.

       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      |   Sub-Type    |    AP ID...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Type

            TBD (skippable) [1]




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         Length

            5 (includes the Sub-Type field)

         Sub-Type

            6

         AP ID

            Contains the 32-Bit Access Point Identifier.

9.7.
     FA IPv4 Address Extension

   The 32-bit FA IPv4 Address Extension contains the IPv4 address of the
   agent (FA).  This extension MAY be present in a Registration Request
   message to identify the oFA as explained in section 5.

       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      |   Sub-Type    |    IPv4 addr ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Type

            TBD (skippable) [1]

         Length

            5 (includes the Sub-Type field)

         Sub-Type

            7

         IPv4 Address

            Contains the 32-Bit IPv4 Address of the FA node.


10.
    IANA Considerations

   This document defines one new extension to Mobile IPv4 Control
   messages and one new extension to Mobile IPv4 Router Discovery
   messages already maintained by IANA.  This document also defines a
   new Mobile IPv4 Control message type to be used between FAs.  To
   ensure correct interoperation based on this specification, IANA must
   reserve values in the Mobile IPv4 number space for two new extensions
   and one new message type. IANA must also manage the numbering spaces



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   created by the two new extensions, the message type and its
   associated Code field.

10.1.
      New Extension values

   Section 9 introduces two extensions.
   Generalized Link Layer and IPv4 Address (LLA) Extension for Router
   Discovery messages: A new Mobile IPv4 extension that follows after
   Mobile IPv4 ICMP Router Discovery messages (e.g. Mobile IP Agent
   Advertisements). The type value of this extension belongs to the
   Mobile IPv4 number space for Router Discovery messages maintained by
   IANA.  The value assigned by IANA is TBD.  This new extension uses
   the Link Layer and IPv4 Address Identifier (LLA) Sub-Type numbering
   space that requires IANA management (see section 10.2).

   Generalized Link Layer and IPv4 Address (LLA) Extension for Mobile IP
   Control messages: A new Mobile IPv4 extension appended to Mobile IP
   Control messages (e.g. Registration Request).  The type value of this
   extension belongs to the Mobile IPv4 number space for extensions to
   Mobile IPv4 Control messages maintained by IANA.  It MUST be in the
   skippable (128-255) range as defined in [1].  The value assigned is
   TBD by IANA.  This new extension uses a the Link Layer and IP Address
   Identifier (LLA) Sub-Type numbering space that requires IANA
   management (see section 10.2).

10.2.
      Generalized Link Layer and IP Address Identifier (LLA) Sub-type
   values

   This section describes the sub-type values that are applicable to
   both the Generalized LLA Extensions for Mobile IP Control and Router
   Discovery messages.  This specification makes use of the Sub-type
   values 1-7, and all other values other than zero (reserved) are
   available for assignment via IETF consensus [14].  The seven Sub-type
   values defined in this specification are:

            1        3GPP2 International Mobile Station Identity and
                     Connection ID [13]
            2        3GPP International Mobile Subscriber Identity [15]
            3        Ethernet 48 bit MAC address [5]
            4        64 bit Global ID, EUI-64 [6]
            5        Solicited IPv4 Address
            6        Access Point Identifier
            7        FA IPv4 Address

10.3.
      New Message Type and Code

   Sections 4.4 and 4.5 define two new Mobile IPv4 message types:
   Handoff Request and Handoff Reply. These require two type numbers to
   be assigned by IANA from the Mobile IPv4 control message type address
   space.  The Handoff Reply message also introduces its own Code field



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   that requires IANA to manage a new Code address space.  This
   specification makes use of the Code values 0-1, where 0 identifies a
   successful Handoff and 1 defines a generic handoff failure. All other
   values are available for assignment via IETF consensus [14].

   Code Values for Mobile IP Handoff Reply Messages
   0          Successful Handoff
   1          Generic Handoff Failure
   2-255      Unallocated


11.
    Security Considerations

   For the PRE-REGISTRATION method, as discussed in Section 3.8, the oFA
   and nFA MUST share a security association to authenticate and
   integrity protect messages transported between them. In addition oFA
   must be authorized to solicit nFA based on the security association.
   The minimal requirement to establish a security association between
   FAs is that both FAs support manual pre-configuration of security
   associations involving shared keys. Other mechanisms to establish
   security associations using IKE [16] based on shared and public keys
   may also be used. The inter-FA ICMP messages (solicitations and
   advertisements) MUST be authenticated and integrity protected using
   ESP [10].  The default ESP authentication algorithm for use in this
   specification is HMAC-SHA1-96 [12].  The absence of this security
   would allow denial of service attacks from malicious nodes at any
   distance from the FA.  To secure Registration Request and Reply
   messages, PRE-REGISTRATION uses the security mechanisms already
   described in [1] and optionally [11].

   POST-REGISTRATION introduces a new change to Mobile IPv4, which is
   the possibility that a MN may receive packets from an FA with which
   it has not yet performed a Mobile IPv4 Registration.  It is not
   recommended that the MN drops packets from unknown FAs since it would
   effectively eliminate the advantages of POST-REGISTRATION.  From a
   security viewpoint, dropping packets from unknown FAs would not
   provide significant protection for an MN from any attack.  This is
   because any malicious host may use the MN's home address to send
   packets to the MN through its current known FA, therefore processing
   packets received from unknown FAs would not provide worse security
   than with normal Mobile IPv4.

   In a similar way to PRE-REGISTRATION, in POST-REGISTRATION oFA and
   nFA MUST share a security association required to protect the Handoff
   Request and Reply messages.  The minimal requirement to establish a
   security association between FAs is that the FAs support manual pre-
   configuration of security associations involving shared keys.  Other
   mechanisms to establish security associations using IKE [16] based on
   shared and public keys may also be used.  The Handoff Request and
   Reply messages MUST be authenticated using the FA-FA authentication



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   extension [11] that uses the default algorithm specified in [7].  The
   absence of this security would allow impersonation attacks and denial
   of service attacks.

   The minimal requirement is that all FAs involved in low latency
   handoffs MUST support manual pre-configuration of peer-to-peer
   security associations with neighbouring FAs, involving static keys
   and are already required to support the default algorithms of [1].
   Other mechanisms to establish security associations using IKE [16]
   based on shared or public keys may also be used.

   Since the techniques outlined in this document depend on particular
   L2 information (triggers) to optimize performance, some level of L2
   security is assumed.  Both PRE and POST-REGISTRATION techniques
   depend on L2 triggers, but the L2 security implications are different
   for the two techniques.

   In particular, in POST-REGISTRATION the L2 triggers initiate the
   establishment of tunnels that route IPv4 packets for the MN to its
   new location.  Therefore the L2 triggers MUST be secured against any
   tampering by malicious nodes, either mobile or within the wired
   network.  The L2 addresses or IPv4 addresses for the MN and the FAs
   that appear in the L2 triggers MUST correspond to the actual nodes
   that are participating in the handoff.  If there is any possibility
   that tampering may occur, the recipient of an L2 trigger MUST have
   some way of authenticating the L2 information.  Wireless networks
   that do not provide such features will be subject to impersonation
   attacks, where malicious nodes could cause FAs to believe that a MN
   has moved to other service areas or to allow a bogus MN to obtain
   unauthorized service from an FA prior to performing a Mobile IPv4
   registration.  In POST-REGISTRATION the L2 triggers would be
   typically sent between a wireless base station and the FA.  No
   standard protocol exists at this time to communicate the L2 trigger
   information but it is important that any future protocol used for
   this purpose provides adequate security. If the wireless base station
   and FA were integrated then this security threat would not apply.
   Also the layer-2 control messages on the wireless link must be
   secured appropriately to prevent a malicious node from running
   impersonation attacks and causing unwanted L2 triggers to be
   generated.  Integrity and replay protection would be required to
   avoid impersonation threats and resource consumption threats where a
   malicious node replays old messages to cause resource consumption.
   This depends on the type of L2 security of the wireless link.  For
   example in cellular technologies the control messages are secured,
   although the type of security varies depending on the cellular
   standard, but this is not typically the case in WLAN IEEE 802.11
   networks.

   In PRE-REGISTRATION the security of L2 triggers has different
   implications.  The PRE-REGISTRATION technique depends on Mobile IPv4



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   security between MN and FA, so the same security considerations in
   [1] apply.  Should malicious nodes be able to generate or modify L2
   trigger information (i.e. L2-ST or L2-TT), this would cause
   advertisements to be sent to the MN.  They would consume wireless
   resources and processing in the MN, but would not allow an
   impersonation attacks.  In order to prevent such denial of service
   attacks, there should be a limit on the number of advertisements that
   an FA (oFA) will relay to the MN as a result of the reception of L2
   triggers.  This number will depend on the L2 technology and the
   default limit is 10 per second.


12.
    Acknowledgements

   The authors want to thank Lennart Bang, Bryan Hartwell, Joel
   Hortelius and Jonathan Wood for valuable comments and suggestions on
   the whole draft.  The authors also thank the Mobile IPv4 WG chairs,
   Phil Roberts and Basavaraj Patil, for their input.


13.
    Editor's Address

   Karim El Malki
   Athonet
   E-mail: karim@athonet.com


14.
    Contributing Authors

   Pat Calhoun, Black Storm Networks
   <pcalhoun@bstormnetworks.com>

   Tom Hiller, Lucent Technologies
   <tom.hiller@lucent.com>

   James Kempf, NTT DoCoMo USA Labs
   <kempf@docomolabs-usa.com>

   Peter J.  McCann, Lucent Technologies
   <mccap@lucent.com>

   Ajoy Singh, Motorola
   <asingh1@email.mot.com>

   Hesham Soliman, Flarion
   <H.Soliman@flarion.com>

   Sebastian Thalanany, Motorola
   <sthalan1@email.mot.com>




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15.
    References

15.1.
      Normative References

     [1]  C. Perkins, Editor, "IPv4 Mobility Support for IPv4", RFC
          3344, August 2002.

     [2]  S. Bradner.  "Key words for use in RFCs to Indicate
          Requirement Levels".  BCP 14, RFC 2119, March 1997.

     [3]  G. Montenegro, "Reverse Tunneling for Mobile IPv4, revised",
          RFC 3024, January 2001.

     [4]  D. Farinacci, T.  Li, S.  Hanks, and P.  Traina,  "Generic
          Routing Encapsulation (GRE)",  RFC 2784, Internet Engineering
          Task Force, March 2000.

     [5]  D. Plummer, "An Ethernet Address Resolution Protocol - or
          Converting Network Protocol Addresses to 48.bit Ethernet
          Address for Transmission on Ethernet Hardware", RFC 826,
          November 1982.

     [6]  IEEE, "Guidelines for 64-bit Global Identifier (EUI-64)
          Registration Authority",
          http://standards.ieee.org/regauth/oui/tutorials/EUI64.html,
          March 1997.

     [7]  C. Perkins,  P.  Calhoun, "Mobile IPv4 Challenge/Response
          Extensions",  RFC 3012, November 2000.

     [8]  S. Deering, "ICMP Router Discovery", RFC 1256, September 1991

     [9]  J. Postel, "Internet Control Message Protocol," RFC 792,
          September 1981.

     [10] S. Kent, R.  Atkinson, "IPv4 Encapsulating Security Payload
          (ESP)", RFC 2406, November 1998.

     [11] E. Gustafsson, A. Jonsson and C. Perkins, "Mobile IPv4
          Regional Tunnel Management", draft-ietf-mobileip-reg-tunnel-08
          (work in progress), November 2003.

     [12] C. Madson and R. Glenn, "The Use of HMAC-SHA1-96 within ESP
          and AH", RFC 2404, November 1998.

15.2.
      Informative References

     [13] TIA/EIA/IS-2000.





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     [14] T. Narten, H, Alvestrand, "Guidelines for Writing an IANA
          Considerations Section in RFCs", BCP 26, RFC 2434, October
          1998.

     [15] 3GPP TS 23.003 (www.3gpp.org).

     [16] D. Harkins and D. Carrel, "The Internet Key Exchange (IKE)",
          RFC 2409, November 1998.


Appendix A - Gateway Foreign Agents

   The Mobile IPv4 Regional Registration specification [11] introduces
   the Gateway Foreign Agent (GFA), as a mobility agent that two FAs
   providing service to a MN have in common.  Figure A.1 provides an
   example of a MN's initial registration through the GFA.  If this is
   the first registration message, the message MUST be forwarded to the
   HA.  All packets destined for the mobile will be delivered to the
   GFA, which in turn will forward the packets to the FA servicing the
   MN.


                   Reg Req   +-----+   Reg Req
                +----------->| oFA |--------------+
                |            +-----+              |
                |                                 v
             +----+                            +-----+ Reg Req +----+
             | MN |                            | GFA |<------->| HA |
             +----+                            +-----+         +----+

                              +-----+
                              | nFA |
                              +-----+

               Figure A.1 - Initial Registrations through GFA

   If the MN moves to an nFA that is serviced by a GFA common with oFA,
   the MN MAY issue a Regional Registration Request (see Figure A.2).
   The Regional Registration message does not need to be forwarded to
   the HA, since the MN's traffic can still be delivered to the same
   GFA.  This optimized approach effectively reduces the latency
   involved in the registration process.











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                              +-----+
                              | oFA |
                              +-----+
             +----+                            +-----+         +----+
             | MN |                            | GFA |         | HA |
             +----+                            +-----+         +----+
                |                                 ^
                |             +-----+             |
                +------------>| nFA |-------------+
                 Regional Reg +-----+ Regional Reg


              Figure A.2 - Regional Registration through GFA

   Note that the GFA may also be the MN's first-hop router.


Appendix B - Low Latency Handoffs for Multiple-Interface MNs

   For MNs that have two wireless network interfaces, either on the same
   wireless network or on wireless networks having different wireless L2
   technologies, the techniques discussed in this document may be
   unnecessary if the Mobile IPv4 stack on the MN allows switching an
   IPv4 address binding between interfaces.  This Appendix discusses how
   multiple wireless interfaces can aid low latency handoff.

         +------+        +---------+
         |  HA  |--------|  (GFA)  |
         +------+        +---------+
                           /     \
                        ...       ...
                         /         \
                        /           \
                    +------+      +------+
                    | oFA  |      | nFA  |
                    +------+      +------+
                       |             |
                    +------+      +------+
                    | RN1  |      | RN2  |
                    +------+      +------+
                    +------+
                    |  MN  | --------->
                    +------+
                             Movement

     Figure B.1 - Network Model for Mobile IPv4 With Multi-Access

   Figure B.1 illustrates the normal and hierarchical MIPv4 models.  As
   shown in the figure, assume that the MN is connected to Radio Network
   1 (RN1) and is registered with oFA through which it is receiving



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   traffic.  Suppose MN enters the coverage area of RN2 and nFA and that
   it prefers connectivity to this network for reasons beyond the scope
   of this document (e.g. user preferences, cost, QoS available etc.).
   The MN activates the interface to RN2 but continues communicating
   through RN1.  The MN may solicit advertisements from nFA through the
   interface connected to RN1 to speed up the handoff process, provided
   there is no TTL restriction, or it can solicit advertisements through
   the interface connected to RN2 if it has been configured for IPv4
   traffic.

   Once the MN is registered with nFA and is successfully receiving and
   transmitting through the new network, it tears down the interface to
   RN1.  If the MN has enough time to complete this procedure without
   incurring degraded service or disconnection, the MN would experience
   a seamless multi-access handoff but it may not be possible in all
   cases, due to network coverage or for other reasons.  Should multiple
   interface handoff be possible then the low latency methods described
   in this document are not necessary.

   In order to support the possible failure of the connectivity with the
   new network (RN2/nFA) in the short period following handoff, the MN
   may use the S bit in its Mobile IPv4 Registration Request to maintain
   simultaneous bindings both its existing (HA or GFA) binding with oFA
   and a new binding with nFA.


Appendix C - PRE_REGISTRATION Message Summary

   This appendix contains a quick reference for IPv4 and Layer 2
   addresses to be used in PRE-REGISTRATION messages.

   Proxy Router Advertisement (PrRtAdv)
   This is a standard Router/Agent Advertisement [1] with the following
   characteristics:
     Source IPv4 Address: nFA IPv4 Address
     Source Layer 2 Address: oFA L2 Address
     Destination IPv4 Address: MN IPv4 Address (from PrRtSol)
     Destination Layer 2 Address: MN L2 Address (from PrRtSol)
     LLA Extension (defined in this spec): containing nFA Layer 2
     Address.

   Proxy Router Solicitation (PrRtSol)
   This is a standard Router/Agent Solicitation [1] with the following
   characteristics:
     Source IPv4 Address: MN Address
     Source Layer 2 Address: MN Address
     Destination IPv4 Address: oFA Address (from source address of
     previous Router Advertisement or PrRtAdv)
     Destination Layer 2 Address: oFA Address (from source address of
     previous Router Advertisement or PrRtAdv LLA)



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     LLA Extension (defined in this spec): depends on the layer 2
     Technology (e.g. typically for WLAN this would be the BSSID of the
     new WLAN Access Point)

   Registration Request (as seen on MN-oFA link)
   This is a Mobile IPv4 Registration Request message [1] with the
   following characteristics:
     Source IPv4 Address: MN Address
     Source Layer 2 Address: MN Address
     Destination IPv4 Address: nFA Address (from source addr of PrRtAdv)
     Destination Layer 2 Address: Default Router (i.e. oFA Address)
   Although this is not mandated, an MN implementation may set the S bit
   (see chapter 6.) in Registration Request messages to improve the
   handoff and avoid problems due to failed layer 2 handoffs and layer 2
   ping-pong effects between two base stations.

   Registration Reply (as seen on oFA-MN link)
   This is a Mobile IPv4 Registration Reply message [1] with the
   following characteristics:
     Source IPv4 Address: nFA Address
     Source Layer 2 Address: oFA Address
     Destination IPv4 Address: MN Address (from source of Registration
     Request)
     Destination Layer 2 Address: MN Address (from source of
     Registration Request)




























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Full Copyright Statement

   Copyright (C) The Internet Society (2005).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
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