path: root/doc/rfc/rfc4881.txt

Network Working Group                                   K. El Malki, Ed.
Request for Comments: 4881                                       Athonet
Category: Experimental                                         June 2007


                  Low-Latency Handoffs in Mobile IPv4

Status of This Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

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
   minimizing 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.

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 .................................................11
      3.2. Network-Initiated Handoff .................................13
      3.3. Mobile-Initiated Handoff ..................................15
      3.4. Obtaining and Proxying nFA Advertisements .................17
           3.4.1. Inter-FA Solicitation ..............................17
           3.4.2. Tunneled nFA Advertisements ........................18
      3.5. Caching Router Advertisements .............................19



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      3.6. Movement Detection, MN, and FA Considerations .............19
      3.7. L2 Address Considerations .................................21
      3.8. Applicability of PRE-REGISTRATION Handoff .................21
   4. The POST-REGISTRATION Handoff Method ...........................23
      4.1. Two-Party Handoff .........................................24
      4.2. Three-Party Handoff .......................................28
      4.3. Renewal or Termination of Tunneling Service ...............34
      4.4. When Will the MN Perform a Mobile IPv4 Registration? ......34
      4.5. Handoff Request (HRqst) Message Format ....................36
      4.6. Handoff Reply (HRply) Message Format ......................38
      4.7. Handoff to Third (HTT) Message Format .....................40
      4.8. Applicability of POST-REGISTRATION Handoff Method .........40
   5. Combined Handoff Method ........................................41
   6. Layer 2 and Layer 3 Handoff Timing Considerations ..............42
   7. Reverse Tunneling Support ......................................42
   8. Handoff Signaling Failure Recovery .............................43
      8.1. PRE-REGISTRATION Signaling Failure Recovery ...............43
           8.1.1. Failure of PrRtSol and PrRtAdv .....................43
           8.1.2. Failure of Inter-FA Solicitation and
                  Advertisement ......................................44
      8.2. POST-REGISTRATION Signaling Failure Recovery ..............44
           8.2.1. HRqst Message Dropped ..............................44
           8.2.2. HRply Message Dropped ..............................45
   9. Generalized Link Layer and IPv4 Address (LLA) Extension ........46
      9.1. 3GPP2 IMSI Link Layer Address and Connection ID
           Extension .................................................47
      9.2. 3GPP IMSI Link Layer Address Extension ....................48
      9.3. Ethernet Link Layer Address Extension .....................49
      9.4. IEEE 64-Bit Global Identifier (EUI-64) Address Extension ..50
      9.5. Solicited IPv4 Address Extension ..........................51
      9.6. Access Point Identifier Extension .........................52
      9.7. FA IPv4 Address Extension .................................53
   10. IANA Considerations ...........................................53
      10.1. New Extension Values .....................................53
      10.2. Generalized Link Layer and IP Address Identifier (LLA) ...54
      10.3. New Message Type and Code ................................54
   11. Security Considerations .......................................55
   12. Acknowledgements ..............................................57
   13. References ....................................................57
      13.1. Normative References .....................................57
      13.2. Informative References ...................................58
   Appendix A - Gateway Foreign Agents................................59
   Appendix B - Low Latency Handoffs for Multiple-Interface MNs.......60
   Appendix C - PRE_REGISTRATION Message Summary......................61







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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 minimizing the period of
   time during which an 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 are 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, and 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, and Section 9 describes protocol extensions required by 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 an 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.






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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 (CoA) of a Mobile Node (MN) prior to a layer 3
         (L3) handoff.

      nFA - new Foreign Agent, the FA anticipated to be handling an 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 an MN's point of layer 2 (L2) connection
         from one wireless access point to another.

      L3 handoff - Movement of an MN between FAs that involves changing
         the care-of address 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 an 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.

      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.




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

      bidirectional edge tunnel (BET) -  A bidirectional tunnel
         established between two FAs for purposes of temporarily routing
         an 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 an MN or FA, or an L2
         identifier that can be resolved to the IPv4 address of an 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 an MN may only begin the registration process after
        an L2 handoff to nFA (new FA) has completed.

      - 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:



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      - 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 allows for service to continue
   uninterrupted while the handoff is being processed by the network
   without requiring the MN's involvement.

   Three methods are presented in this document 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, and

      - 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 with 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 bidirectional 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
   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 set up and move bidirectional
   tunnels as required to give the MN continued connectivity.





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   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 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.



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   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

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, and

      - 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,
        and

      - 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 Home Agent (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 Authentication,
   Authorization, and Accounting (AAA) transaction must occur to
   authenticate the MN with a new domain.






















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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

   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 Generalized 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
         Generalized Link Layer and IP Address (LLA) Extension (see
         Section 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 neighboring 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.





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      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 Generalized 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 Generalized Link Layer
         and IP Address (LLA) Extension (see Section 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-triggered case (see 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 (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 Generalized Link Layer and IP Address
         Extension (see Sections 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 Section 3.7.
         Figures 2 and 3 illustrate this procedure.




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      5. If the registration is successful, packets for the MN are
         tunneled 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-REGISTRATION 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 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 that 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



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   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
   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)

   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 Section 9.  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.
















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   MN                    oFA                 nFA                 HA/GFA
    |                     | L2-Target~~~~~~~~>|                    |
    |                     |    Trigger        |                    |
    |                     |...................|                    |
    |<--------------------------------------- |                    |
    |     (PrRtAdv)       |...................|                    |
    |                     | Tunneled 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 Section 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 that can be
   resolved to the nFA's IPv4 address).  As an example, a Wireless LAN
   MN may perform a scan to obtain the Base Station Identifier (BSSID)
   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.






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   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 Section 3.4.1.  The extension containing the identifier is a sub-
   type of the Generalized Link Layer Address Extension described in
   Section 9.

   Two extension sub-types 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



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   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 Section
   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 Figure 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 neighboring FAs,
   involving shared keys and the default algorithms of [1] (see the
   Security Considerations of this document).

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 BSSID.  In the
   latter case, the oFA must implement a mechanism to resolve the Base



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   Station Identifier to an IPv4 address.  The default mechanism is to
   use a configured table of neighboring Base Station Identifiers (e.g.,
   BSSID) to FA IPv4 address mappings in each FA.  Other automated
   discovery mechanisms may also be used.

   If oFA does not cache advertisements (see Section 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 Encapsulating Security Payload (ESP) [10] to
   prevent attacks (see the Security Considerations section of this
   document).  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 identifiers 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.









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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
   exchange SHOULD be performed in advance of handoff.  A process SHOULD
   be in place at the oFA to solicit its neighboring 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 second.  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
   neighboring 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 neighboring 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



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   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].

   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 an
   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., the Address Resolution Protocol (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
   Section 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.



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   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
   wireless system on which this handoff method is being implemented.
   Consider an Ethernet-like system such as 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
   Generalized 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
   Generalized 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-REGISTRATION 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



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   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 is able to obtain an L2 trigger
   informing it 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
   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 Section
   3.6.













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4.  The POST-REGISTRATION Handoff Method

   The POST-REGISTRATION handoff method uses bidirectional 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
   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
   neighboring 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.









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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
   using the suffixes (s), (t), and (r), respectively.

      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:

         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:




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         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 that 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).

         b) If nFA sends the HRply, the H bit is set and the N and R
            bits are unset, indicating that 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 is required 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).



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      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 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.




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   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 set up 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 handoffs, 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



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   send an Agent Advertisement to the MN after L2 handoff has completed
   (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:






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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 mapped 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 oFA and nFA exchange an 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 an HRply(s).  Section 4.7
            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 an
         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 under way 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.






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         b) The oFA exchanges an 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:

         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.


























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   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 signaling 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 that 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
   sometime 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



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   be influenced by network engineering and implementation
   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 canceled".

   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].




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

      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.

















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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.

    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              16 (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 Generic
                        Routing Encapsulation (GRE) [4] as defined in
                        [1,5] for the tunnel.  Extensions of HRqst
                        containing GRE type and key Fields are outside
                        the scope of this document.





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      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 a 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
                        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 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.











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4.6.  Handoff Reply (HRply) 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.

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

      Type              17 (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
                        bidirectional 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 an 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



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                        require service.  A value of 0 on an HRply(r)
                        indicates that the tunnel lifetime will be
                        terminated.  A value of 0xffff indicates an
                        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.

      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 a 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 defined in [1].

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



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4.7.  Handoff to Third (HTT) Message Format

   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 an 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 an 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
   time at which a handoff must occur, such as when an 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



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   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 the 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.

   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
   second).

   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 Section 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 Sections 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: 1 second).



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   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.

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.








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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 neighboring
         FA's Advertisement or loss of the neighboring 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, 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 is dropped.  To reduce the
   probability that PrRtAdv and PrRtSol are lost, the MN and FA MAY
   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 signaling.  In this case, the MN solicits an FA
   Advertisement when it connects to nFA at L2 (L2-LU), as described in
   Section 3.6, and performs a standard Mobile IPv4 Registration with
   the nFA as specified in [1].










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8.1.2.  Failure of Inter-FA Solicitation and Advertisement

   The solicitation from an FA to another neighboring FA may fail or the
   corresponding advertisement from the neighboring 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 Section 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 handoffs.  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 that 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 an 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 that 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 an
   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 canceled.  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.











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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
   document defines six sub-types.  Future RFCs should allocate their
   own sub-type and define their own address formats.

       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

        138 (skippable) [1]  - when used in Registration Requests
        140 (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 sub-types 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.





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

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

       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

            1 (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 Identity, 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.


















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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

            2 (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

            3 (skippable) [1]

         Length

            7 (includes the Sub-Type field)

         Sub-Type

            3

         MAC

            Contains the 48-bit Ethernet MAC Address.

























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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

            4 (skippable) [1]

         Length

            9 (includes the Sub-Type field)

         Sub-Type

            4

         MAC

            Contains the 64-bit Global Identifier Address.

























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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

            5 (skippable) [1]

         Length

            5 (includes the Sub-Type field)

         Sub-Type

            5

         IPv4 Address

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
























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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

            6 (skippable) [1]

         Length

            5 (includes the Sub-Type field)

         Sub-Type

            6

         AP ID

            Contains the 32-bit Access Point Identifier.






















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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

            7 (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
   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



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   Mobile IPv4 number space for Router Discovery messages maintained by
   IANA.  The value assigned by IANA is 140.  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 138 by IANA.  This new extension uses 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 4.5 and 4.6 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
   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].







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   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 secrets or 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 an 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 drop 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 secrets or public keys may also be used.  The Handoff Request
   and Reply messages MUST be authenticated using the FA-FA
   authentication extension [11] that uses the default algorithm
   specified in [7].  The absence of this security would allow
   impersonation attacks and denial-of-service attacks.



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   The minimal requirement is that all FAs involved in low latency
   handoffs MUST support manual pre-configuration of peer-to-peer
   security associations with neighboring FAs, involving shared secrets
   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 an 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 typically
   be 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
   security between MN and FA, so the same security considerations in
   [1] apply.  Should malicious nodes be able to generate or modify L2



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   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 attack.  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, Gianluca Verin, and Jonathan Wood for valuable comments
   and suggestions on the whole document.  The authors also thank the
   Mobile IPv4 WG chairs, Phil Roberts and Basavaraj Patil, for their
   input.

13.  References

13.1.  Normative References

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

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

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

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

   [5]  Plummer, D., "Ethernet Address Resolution Protocol: Or
        Converting Network Protocol Addresses to 48.bit Ethernet Address
        for Transmission on Ethernet Hardware", STD 37, 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]  Perkins, C., Calhoun, P., and J. Bharatia, "Mobile IPv4
        Challenge/Response Extensions (Revised)", RFC 4721, January
        2007.





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

   [9]  Postel, J., "Internet Control Message Protocol", STD 5, RFC 792,
        September 1981.

   [10] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303,
        December 2005.

   [11] Fogelstroem, E., Jonsson, A., and C. Perkins, "Mobile IPv4
        Regional Registration", RFC 4857, June 2007.

   [12] Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within ESP
        and AH", RFC 2404, November 1998.

13.2.  Informative References

   [13] TIA/EIA/IS-2000.

   [14] Narten, T. and 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] Kaufman, C., Ed., "Internet Key Exchange (IKEv2) Protocol", RFC
        4306, December 2005.

























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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 an MN have in common.  Figure A.1 provides an
   example of an MN's initial registration through the GFA.  If this is
   the first registration message, the message MUST be forwarded to the
   HA.  All packets sent to the MN will be delivered to the GFA, which
   in turn will forward the packets to the FA servicing the MN.

                RegReq    +-----+   RegReq
             +----------->| oFA |--------------+
             |            +-----+              |
             |                                 v
          +----+                            +-----+ RegReq  +----+
          | 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.

                           +-----+
                           | oFA |
                           +-----+
          +----+                            +-----+         +----+
          | MN |                            | GFA |         | HA |
          +----+                            +-----+         +----+
             |                                 ^
             |             +-----+             |
             +------------>| nFA |-------------+
               RegRegReq   +-----+  RegRegReq


           Figure A.2 - Regional Registration through GFA

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





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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
   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



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   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 with 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)
      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)



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      Destination Layer 2 Address: Default Router (i.e., oFA Address)
      LLA Extension (defined in this spec): containing the MN's L2
      address that must be used by nFA.  This will typically be an
      Ethernet MAC address but other types can be used as specified in
      Section 9 of this document.

   Although this is not mandated, an MN implementation may set the S bit
   (see Section 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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Contributing Authors

   Pat Calhoun
   Cisco Systems
   EMail: pcalhoun@cisco.com

   Tom Hiller
   Lucent Technologies
   EMail: tom.hiller@lucent.com

   James Kempf
   NTT DoCoMo USA Labs
   EMail: kempf@docomolabs-usa.com

   Peter J. McCann
   Motorola Labs
   EMail: pete.mccann@motorola.com

   Ajoy Singh
   Motorola
   EMail: asingh1@email.mot.com

   Hesham Soliman
   Elevate Technologies
   EMail: Hesham@elevatemobile.com

   Sebastian Thalanany
   US Cellular
   EMail: Sebastian.thalanany@uscellular.com

Editor's Address

   Karim El Malki
   Athonet
   EMail: karim@athonet.com
















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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

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

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.







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