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Versions: 00 01 02 draft-ietf-mipshop-3gfh

Network Working Group                                          H. Yokota
Internet-Draft                               KDDI R&D Laboratories, Inc.
Expires: August 5, 2006                                       G. Dommety
                                                     Cisco Systems, Inc.
                                                           February 2006


            Mobile IPv6 Fast Handovers for 3G CDMA Networks
                    draft-yokota-mipshop-3gfh-02.txt

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   This Internet-Draft will expire on August 5, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   Mobile IPv6 is designed to maintain its connectivity while moving
   from one network to another.  It is adopted in 3G CDMA networks as a
   way to maintain connectivity when the mobile node moves between
   access provider networks.  However, this handover procedure requires
   not only movement detection, but also the acquisition of a new
   care-of address and the sending of a binding update message to the
   home agent before the traffic begins to direct to the new location.



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   During this period, packets destined for the mobile node will be
   lost, which may not be acceptable for real-time application such as
   Voice over IP (VoIP) or video telephony.  This document specifies
   fast handover methods and selective bi-casting methods in the 3G
   context in order to reduce latency and packet loss during handover.

Table of Contents

   1.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Network reference model for Mobile IPv6 over 3G networks . . .  6
   5.  Fast handover procedures . . . . . . . . . . . . . . . . . . . 10
     5.1   Predictive fast handover . . . . . . . . . . . . . . . . . 10
     5.2   Reactive fast handover . . . . . . . . . . . . . . . . . . 14
   6.  Selective bi-casting . . . . . . . . . . . . . . . . . . . . . 18
     6.1   Bi-casting by the PAR to the MN with multiple
           interfaces (A-1) . . . . . . . . . . . . . . . . . . . . . 20
     6.2   Bi-casting by the HA to the MN with multiple
           interfaces (A-2) . . . . . . . . . . . . . . . . . . . . . 21
     6.3   Bi-casting by the PAR to the MN with a single
           interface (B-1)  . . . . . . . . . . . . . . . . . . . . . 22
     6.4   Bi-casting by the HA to the MN with a single
           interfaces (B-2) . . . . . . . . . . . . . . . . . . . . . 24
     6.5   Message Format . . . . . . . . . . . . . . . . . . . . . . 26
       6.5.1   Simultaneous bindings flag and the HI message
               indication flag extensions to (F)BU message  . . . . . 26
       6.5.2   New mobility option for bi-casting lifetime  . . . . . 27
       6.5.3   New status code for simultaneous bindings  . . . . . . 27
       6.5.4   New Option for vendor-specific information . . . . . . 28
     6.6   MN and AR/HA operations  . . . . . . . . . . . . . . . . . 29
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 30
   8.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 31
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 32
       Intellectual Property and Copyright Statements . . . . . . . . 33















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1.  Requirements notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [1].














































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2.  Introduction

   Mobile IPv6 allows mobile nodes (MNs) to maintain persistent IPv6
   addresses while roaming around in IPv6 networks and it is adopted in
   3G CDMA networks for handing off between different access provider
   networks [2].  During handover, however, the mobile node (MN) needs
   to switch the radio networks, to obtain a new Care-of Address (CoA)
   and to re-register with the home agent (HA), which causes a
   communication disruption.  This is not desirable for real-time
   applications such as VoIP and video telephony.  To reduce this
   disruption time or latency, a fast handover protocol for Mobile IPv6
   [3] is proposed.  In this proposal, there are two modes called
   "predictive" fast handover and "reactive" fast handover.  This
   document first specifies how these fast handover modes can be applied
   in the 3G context and shows that several Mobile IPv6 bootstrapping
   procedures can be omitted.  If the MN has more than one network
   interface, even smoother handover can be realized by transmitting
   packets destined for the MN to both networks, where the mobile node
   resides and will move to.  This document defines this mechanism as
   selective bi-casting and shows several use cases with their handover
   procedures.






























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3.  Terminology

   This document refers to [2] for Mobile IPv6 fast handover
   terminology.  Terms that first appear in this document are defined
   below:

   Forward Pilot Channel:
        A portion of the Forward Channel that carries the pilot.  The
        Forward Channel is a portion of the physical layer channels
        transmitted from the access network to the MN.

   Sector:
        Part of the access network that provides one CDMA channel.  The
        area covered by one base station may be split into several
        sectors.

   Home Link Prefix (HLP):
        The prefix address assigned to the home link where the MN should
        send the binding update message.  This is one of the bootstrap
        parameters for the MN.

   Packet Data Serving Node (PDSN):
        An entity that routes MN originated or MN terminated packet data
        traffic.  A PDSN establishes, maintains and terminates link
        layer sessions to MNs [2].  A PDSN can be the access router in
        the visited access provider network.

























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4.  Network reference model for Mobile IPv6 over 3G networks

   Figure 1 shows a simplified reference model of the Mobile IP enabled
   3G networks.  The home agent (HA) of the mobile node (MN) resides in
   the home IP network and the MN roams around the access provider
   networks.  The home network and the access provider networks are
   managed by either the same or different operator(s).  Prior to the
   Mobile IPv6 registration, the MN establishes a link-layer connection
   with the access router (AR), which is also called PDSN (Packet Data
   Serving Node) in 3G networks, of the access provider network to which
   the MN is attached.  When the MN moves from one access provider
   network to another, a Mobile IPv6 handover is performed.  The figure
   shows the situation, where the MN moves from the PAR (previous access
   router) to the NAR (new access router) and in the case of 3G
   networks, the PAR and the NAR are equivalent to the oPDSN (old PDSN)
   and the nPDSN (new PDSN).






                                  Home IP Network
                             +........................+
                             . +--------+  +--------+ .
                             . |   HA   |--|  AAA   | .
                             . +--------+  +--------+ .
                             +../......\..............+
                               /        \
             Access Provider Network   Access Provider Network
                  +.............+      +.............+
                  . +---------+ .      . +---------+ .
                  . |   PAR   | .      . |   NAR   | .
                  . | (oPDSN) | .      . | (nPDSN) | .
                  . +---------+ .      . +---------+ .
                  .   |    :|   .      .   :|    |   .
                  .   |    :|L2link  L2link:|    |   .
                  .   |    :|   .      .   :|    |   .
                  . +--+  +:--+ .      .  +:--+ +--+ .
                  . |BS|  |:BS| .      .  |:BS| |BS| .
                  . +--+  +:--+ .      .  +:--+ +--+ .
                  .        :|   .      .   :|        .
                  .      +----+ .      .  +----+     .
                  .      | MN |---------> | MN |     .
                  .      +----+ .      .  +----+     .
                  +.............+      +.............+

                  Figure 1: Reference Model for Mobile IP



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   In 3G CDMA networks, pilot channels transmitted by base stations
   allow the MN to obtain a rapid and accurate C/I (carrier to
   interference) estimate.  This estimate is based on measuring the
   strength of the Forward Pilot Channel or the pilot, which is
   associated with a sector of a base station (BS).  The MN searches for
   the pilots and maintains those with sufficient signal strength in the
   pilot sets.  The MN sends measurement results, which include the
   offsets of the PN (pseudorandom) code in use and the C/Is in the
   pilot sets, to provide the access network (AN) with the estimate of
   sectors in its neighborhood.  There are several triggers for the MN
   to send those estimates, e.g. when the strength of a pilot in the
   pilot sets is higher enough than that of the current pilot, the MN
   sends the estimates to the access network.  If the serving access
   network finds that the sector associated with the highest pilot
   strength belongs to a different AR, it attempts to close the
   connection with the MN.  The MN then attempts to get a new traffic
   channel assigned in the new access network, which is followed by
   establishing a new connection with the new AR.  The MN can
   continually search for pilots without disrupting the data
   communication and a timely handover is assisted by the network.  If
   the air interface information can be used as a trigger for the
   handover between access routers, fast and smooth handover of Mobile
   IPv6 can be realized in 3G CDMA networks.

   To assist the handover of the MN to the new AR, various types of
   information can be considered: the pilot sets, which include the
   candidates of the target sectors or BSs, the cell information where
   the MN resides, the serving nodes in the radio access network and the
   location of the MN if available.  In this document, a collection of
   such information is called "handover assist information".  In 3G
   networks, the link-layer address of the new access point defined in
   [3] may not be available.  If this is the case, the handover assist
   information SHOULD be used instead.

   Figure 2 shows the protocol sequence from the attachment to the
   network to the Mobile IPv6 registration.  After the traffic channel
   is assigned, the MN first establishes a link-layer connection between
   itself and the access router.  As the link-layer protocol, PPP can be
   considered and in this figure, a PPP handshake is depicted as an
   example.  Then the MN registers with the HA by sending a Binding
   Update message.  There are several parameters for using Mobile IPv6
   such as the home address (HoA), the care-of address (CoA), the home
   agent address (HA) and the home link prefix (HLP).  These addresses
   are required prior to sending a Binding Update and obtaining these
   values is called bootstrapping.  One such method is proposed in [5],
   where the bootstrapping information is obtained during the link-layer
   establishment phase.




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                 MN                     AR             HA            AAA
        +--      |                    (PDSN)           |              |
        |        |         LCP          |              |              |
        |(1)     |<-------------------->|              |              |
        |        |     CHAP or PAP      | Access-Request/Accept       |
        |(2)     |<-------------------->|<---------|----------------->|
        |        |                +------------+   |   |              |
        |(3)     |                | HA,HLP,HoA |<--+   |              |
        |        |                +------------+       |              |
        |        |    IPv6CP(IF-ID)     |              |              |
        |(4)     |<--------|----------->|              |              |
    (a)<     +---------+   |            |              |              |
        |(5) | LL-addr |<--+            |              |              |
        |    +---------+                |              |              |
        |        |     RA(prefix)       |              |              |
        |(6)     |<-----|---------------|              |              |
        |     +-----+   |               |              |              |
        |(7)  | CoA |<--+               |              |              |
        |     +-----+                   |              |              |
        |        |   DHCPv6(HA,HLP,HoA) |              |              |
        |(8)     |<-----------|-------->|              |              |
        |    +------------+   |         |              |              |
        |(9) | HA,HLP,HoA |<--+         |              |              |
        |    +------------+             |              |              |
        *--      |                      | BU           |              |
    (b)          |------------------------------------>|Authentication|
                 |                      |              |  query/reply |
    (c)          |                      |              |<------------>|
                 |                      | BA           |              |
    (d)          |<------------------------------------|              |
                 |                      |              |              |

                  Figure 2: MIPv6 operation in 3G network

   The procedure for the initial registration is as follows:

      (a) The link-layer connection establishment and the bootstrapping
      phase

      (a-1) The LCP configure-request/response messages are exchanged.

      (a-2) CHAP or PAP authentication is conducted.

      (a-3) The bootstrapping parameters are conveyed from the HA and
      stored in the AR (PDSN).

      (a-4) Unique interface IDs are negotiated in IPv6CP.




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      (a-5) The MN configures its link-local address based on the
      obtained interface ID.

      (a-6) A router advertisement containing the prefix is received by
      the MN.

      (a-7) The MN configures its CoA based on the obtained prefix.

      (a-8) DHCPv6 is used to obtain the bootstrap parameters such as
      the home agent address, the home link prefix and the home address.

      (a-9) The MN configures its HoA based on the obtained parameters.

      (b) A binding update is sent to the HA.

      (c) The HA asks the AAA to authenticate the MN for the initial
      registration.

      (d) The binding acknowledgment is sent back to the MN.

   As is shown in Figure 2, it takes a considerable amount of time to
   establish a link-layer connection and all of the above sequences run
   every time the MN attaches to a new access network.  It is therefore
   beneficial if packets on the fly to the MN are saved not only during
   the time period where the MN switches to the new radio channel but
   also during the time period where the MN establishes the link-layer
   connection.
























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5.  Fast handover procedures

   There are two modes defined in [3] according to the timing of sending
   FBU (Fast Binding Update); one is called "predictive mode," where the
   MN sends FBU and receives FBAck (Fast Binding Ack) on PAR (Previous
   Access Router)'s link and the other is called "reactive mode," where
   the MN sends FBU from NAR (New Access Router)'s link.  In the
   predictive mode, the time and place the MN hands off is indicated
   sufficiently before the time it actually happens.  In cellular
   systems, handovers are controlled by the network and the predictive
   mode is well applied although the reactive mode is possible as well.
   On the other hand, in wireless LAN networks, for example, the MN
   controls its handover and it is not easy to know where it moves to
   until it starts to scan the channels.  In this case, the reactive
   mode is well applied.

5.1  Predictive fast handover

   Figure 3 shows the predictive mode of MIPv6 fast handover operation.
   When the MN finds a sector or a BS whose pilot signal is sufficiently
   strong, it initiates handover according to the following sequence:

      (a) A router solicitation for proxy router advertisement is sent
      to the PAR.

      (b) A proxy router advertisement containing the prefix in the NAR
      is sent back to the MN.

      (c) The MN creates an NCoA (new CoA) and sends the Fast Binding
      Update (FBU) storing the NCoA to the PAR, which in turn sends the
      Handover Initiate (HI) to the NAR.

      (d) The NAR sends the Handover Acknowledge (HAck) back to the PAR,
      which in turn sends the FBU acknowledgment (FBAck) to the MN.

      (e) The PAR starts forwarding packets toward the NCoA and the NAR
      captures and buffers them.

      (f) The connection associated with the PAR is closed and the
      traffic channel is assigned in the new access network.

      (g) The MN establishes the link layer connection with or without
      authentication.

      (h) The MN sends the Fast Neighbor Advertisement (FNA).

      (i) The NAR starts delivering packets to the MN.




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      (j) The MN sends the BU to the HA.

      (k) The HA sends the BA back to the MN.

      (l) The HA starts delivering packets to the MN via the NAR.


         MN            PAR             NAR            HA             AAA
         |    RtSolPr   |               |              |              |
    (a)  |------------->|               |              |              |
         |    PrRtAdv   |               |              |              |
    (b)  |<-------------|               |              |              |
         |      FBU     |      Hl       |              |              |
    (c)  |------------->|-------------->|              |              |
         |     FBack    |     HAck      |              |              |
    (d)  |<-------------|<--------------|              |              |
         |              |forward packets|              |              |
    (e)  |              |==============>|              |              |
         |              |         +-----------+        |              |
         |              |         | buffering |        |              |
         |              |         +-----------+        |              |
    (f) handover        |               |              |              |
         |              | link layer connection        |              |
    (g)  |/----------------------------\|/...........................\|
         |\----------------------------/|\.........................../|
         |             FNA              |              |              |
    (h)  |----------------------------->|              |              |
         |       deliver packets        |              |              |
    (i)  |<=============================|              |              |
         |              |    BU         |              |              |
    (j)  |-------------------------------------------->|              |
         |              |    BA         |              |              |
    (k)  |<--------------------------------------------|              |
         |       deliver packets        |              |              |
    (l)  |<=============================|<=============|              |


         Figure 3: MIPv6 Fast handover operation (predictive mode)

   It is assumed that the NAR can be identified by the PAR leveraging
   the handover assist information from the MN.  To perform the
   predictive mode, the FBAck MUST be received by the MN before the
   connection with the current access network is closed.  If the MN
   fails to send the FBU before handover, it SHOULD fall back to the
   reactive mode.  Even if the MN successfully sends the FBU, its
   reception by the PAR may be delayed for various reasons such as
   congestion.  If the NAR receives the HI triggered by the delayed FBU
   after the reception of the FNA ((c) comes after (h)), then the NAR



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   SHOULD send the HAck with handover not accepted.

   In (a), RtSolPr MUST include the MN and the New Access Point Link-
   Layer Address (LLA) options according to [3].  As for the MN-LLA
   option, the only available identifier is the interface ID, so it
   SHOULD be used for the MN-LLA.  As for the New AP-LLA, the handover
   assist information may be applied.  Since the LLA is assumed to be an
   IEEE identifier, even if the length field of the LLA option is in
   units of 8 octets, the actual length can be obtained by knowing that
   the length of an IEEE identifier is 6 octets.  If the interface ID of
   the MN is generated in the EUI-64-based format, the MN-LLA can be
   constructed from it.  However, if the LLA is not well-known, the
   length of the LLA becomes ambiguous.  If this is the case, it is
   necessary to use a new option defined in Section 6.5.4 and the
   corresponding length in it.

   In (b), PrRtAdv MUST include options for the LLA, IP address and
   prefix of the NAR.  The PAR SHOULD be able to identify the NAR from
   the handover assist information provided by the MN.

   There are several ways to configure a unique IP address for the MN.
   If a globally unique prefix is assigned per each link as introduced
   in [5], the MN can use any interface ID except that of the other peer
   to create a unique IP address.  If this is the case, however, the PAR
   cannot provide the MN with a correct prefix for the new network since
   such a prefix is selected by the NAR and provided in the router
   advertisement ((a-6) in Figure 2).  Still, the NCoA MUST be included
   in the FBU for the PAR to resolve the IP address of the NAR, so that
   the MN configures a temporary NCoA with the prefix of the NAR and the
   correct NCoA MUST be assigned by the NAR.  Therefore, in (c), the PAR
   MUST send the HI with the S flag set when it receives the FBU from
   the MN.  On the other hand, if more than one MN connected to an AR
   share the same prefix, each MN MUST have a unique interface ID.
   Unless it is guaranteed that each MN connected to the network
   including a roaming case is preconfigured with a unique interface ID,
   it MUST be agreed or provided by the NAR via the HI/Hack exchange.

   In [3], the FNA MUST include the LLA of the MN, but the point-to-
   point link layer connection makes it unnecessary.  The only required
   information is the NCoA to check if there is a corresponding buffer,
   thus in (h), the function of the FNA can be realized in several ways.

   o  Since the establishment of the link layer connection in (g)
      indicates readiness of data communication on the MN side, the NAR
      immediately checks if there is a buffer that has packets destined
      for the NCoA and starts delivering if any. (elimination of FNA)





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   o  The FNA equivalent information can be conveyed in the phase of the
      link layer connection, e.g. by conveying the NCoA in a PPP IPCP
      with vendor specific extension as defined in [6].  Only when this
      message is received by the NAR, it checks if there is a buffer for
      the NCoA.  (L2 implementation of FNA)

   o  The MN sends the FNA as defined in [3] with the LLA of the MN,
      which may be derived from the EUI-64 based interface ID. (standard
      implementation of FNA)

   When PPP IPCP option is used as the means for the L2 implementation
   of FNA, it SHOULD be confirmed that the NAR supports this option,
   otherwise it may cause a longer delay by the Configure-Reject
   message.

   The primary benefit of this mode is that the packets destined for the
   MN can be buffered at the NAR, and packet loss due to handover will
   be much lower than that of the normal MIPv6 opration.  Regarding the
   bootstrapping, the following benefits can be obtained, too:

   o  Since the HA, HLP and HoA are not changed during the fast
      handover, bootstrapping information is not required.

   o  Since the NCoA including the interface ID can be obtained or
      configured via the fast handover procedures, a router
      advertisement is not required.

   Therefore, as shown in Figure 4, bootstrapping procedures (a-3) to
   (a-9) can be omitted from the standard MIPv6 operation in Figure 2.
   Also, if the security policy permits, the NAR can know the MN by the
   NAI in the PPP link setup and the authentication in (2) may be
   omitted.  Note that another authentication is conducted in the MIPv6
   registration phase and presumably the same AAA is referred to.


















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                  MN          oPDSN       nPDSN        HA          AAA
          +--     |           (PAR)       (NAR)         |           |
          |       |        LCP  |           |           |           |
          | (1)   |<----------------------->|           |           |
          |       |        CHAP |           | Access-Request/Accept |
          | (2)   |<----------------------->|<-------|------------->|
          |+...................................+     |  |           |
          |.      |       +------------+    |  .     |  |           |
          |.(3)*  |       | HA,HLP,HoA |<------------+  |           |
          |.      |       +------------+    |  .        |           |
          |.      |                         |  .        |           |
          |.      |    IPv6CP(IF-ID)        |  .        |           |
          |.(4)*  |<---------|------------->|  .        |           |
      (a)< .    +---------+  |  |           |  .        |           |
          |.(5)*| LL-addr |<-+  |           |  .        |           |
          |.    +---------+     |           |  .        |           |
          |.      |                         |  .        |           |
          |.      |       RA(prefix)        |  .        |           |
          |.(6)*  |<---------|--------------|  .        |           |
          |.    +-----+      |  |           |  .        |           |
          |.(7)*| CoA |<-----+  |           |  .        |           |
          |.    +-----+         |           |  .        |           |
          |.      |                         |  .        |           |
          |.      |    HCPv6(HA,HL,HoA)     |  .        |           |
          |.(8)*  |<---------|------------->|  .        |           |
          |.    +-----+      |  |           |  .        |           |
          |.(9)*| HoA |<-----+  |           |  .        |           |
          |.    +-----+         |           |  .        |           |
          |+...................................+        |           |
          *--     |             |           |           |           |

   Figure 4: Procedures that can be omitted in the link-layer connection


5.2  Reactive fast handover

   When the MN cannot receive the FBAck on the PAR's link or the network
   does not support the predictive fast handover, the reactive fast
   handover can be applied.  To support the predictive fast handover,
   the PAR must accurately resolve the address of the NAR from the lower
   layer information such as the link-layer address of the new access
   point or the base station, which is not always feasible in some
   cases.  To minimize packet loss in this situation, the PAR instead of
   the NAR can buffer packets for the MN until the MN regains
   connectivity with the NAR.  The NAR obtains the information of the
   PAR from the MN on the NAR's link and receives packets buffered at
   the PAR.  In this case, the PAR does not need to know the IP address
   of the NAR or the NCoA and just waits for the NAR to contact the PAR.



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   However, since the PAR needs to know when to buffer packets for the
   MN, the PAR obtains the timing of buffering from the MN via the FBU
   or the lower layer signaling, e.g. an indication of the release of
   the connection with the MN.  Details of the procedure are as follows:

      (a) A router solicitation for proxy router advertisement MAY be
      sent to the PAR.

      (b) The proxy router advertisement MAY be sent to the MN, but the
      prefix of the NAR MAY not be included.

      (c) The MN sends the FBU or the access network indicates the close
      of the connection with the MN by the lower layer signaling.  The
      PAR MAY start buffering packets destined for the PCoA.

      (d) The connection associated with the PAR is closed and the
      traffic channel is assigned in the new access network.

      (e) The MN establishes the link layer connection.  Since the IP
      address of the MN is guaranteed to be unique, the MN SHOULD not
      perform DAD

      (f) The MN sends the Fast Binding Update (FBU) to the NAR either
      or not being encapsulated by the Fast Neighbor Advertisement
      (FNA).

      (g) The NAR decapsulates the FBU if encapsulated and sends it to
      the PAR.

      (h) The PAR sends the Handover Initiate (HI) to the NAR with the
      Code set to 1.

      (i) The NAR sends the Handover Acknowledge (HAck) back to the PAR.

      (j) The PAR sends the FBAck to the NAR.

      (k) If the PAR buffers packets destined for the PCoA, it starts
      forwarding them as well as newly arriving ones to the NAR.

      (l) The NAR delivers the packets to the MN.

      (m) The MN sends the BU to the HA.

      (n) The HA sends the BA back to the MN.

      (o) The HA starts delivering packets to the MN via the NAR.





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         MN           oPDSN           nPDSN            HA            AAA
         |            (PAR)           (NAR)            |              |
         |   RtSolPr    |               |              |              |
    (a)  |------------->|               |              |              |
         |   PrRtAdv    |               |              |              |
    (b)  |<-------------|               |              |              |
         | FBU/lower-layer sig.         |              |              |
    (c)  | . . . . . . >|               |              |              |
         |        +-----------+         |              |              |
         |        | buffering |         |              |              |
         |        +-----------+         |              |              |
    (d) handover        |               |              |              |
         |              | link layer connection        |              |
    (e)  |/----------------------------\|/...........................\|
         |\----------------------------/|\.........................../|
         |        FNA[FBU]/FBU          |              |              |
    (f)  |----------------------------->|              |              |
         |              |     FBU       |              |              |
    (g)  |              |<--------------|              |              |
         |              |      HI       |              |              |
    (h)  |              |-------------->|              |              |
         |              |     HAck      |              |              |
    (i)  |              |<--------------|              |              |
         |              |     FBack     |              |              |
    (j)  |              |-------------->|              |              |
         |              |forward packets|              |              |
    (k)  |              |==============>|              |              |
         |        deliver packets       |              |              |
    (l)  |<=============================|              |              |
         |              |     BU        |              |              |
    (m)  |-------------------------------------------->|              |
         |              |     BA        |              |              |
    (n)  |<--------------------------------------------|              |
         |        deliver packets       |              |              |
    (o)  |<=============================|<=============|              |

          Figure 5: MIPv6 Fast handover operation (reactive mode)

   To indicate the PAR to buffer packets destined for the PCoA, in (c),
   the MN SHOULD not include information on the NCoA in the FBU and the
   PAR SHOULD accept it.  Or, when the PAR is indicated that the session
   with the MN has been closed by the lower layer signaling when the PAR
   attempts to send the FBAck, the PAR MAY start buffering.

   An L2-based fast handover is possible as defined in [7] by extending
   the L2 link from the previous access network to the new access
   network via the PAR and the NAR.  The timing of the fast handover
   trigger is the same as the reactive fast handover method (without



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   buffering) in this section.  In the case of the L2-based fast
   handover, however, once the L2 link is extended to the new location,
   it is maintained until the MN becomes inactive (dormant) and the link
   is released.  As long as the L2 link is extended, the path, on which
   packets are conveyed, is not optimal in length.  In the case of
   Mobile IPv6 fast handover, when the new location is registered with
   the HA, the packets are directed to the NAR.












































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6.  Selective bi-casting

   If the MN has the capability to receive more than one radio signal,
   even smoother handover can be realized.  This situation happens when,
   for instance, the MN can receive multiple channels of the same radio
   system at the same time, or the MN has multiple interfaces of
   different radio systems such as 3G and WiFi.  Especially, when the MN
   is running a real-time and interactive application, long-time
   buffering at the PAR or NAR is not always beneficial for the MN.  If
   this is the case, it will be helpful to deliver packets destined for
   the MN via both the old and new points of attachment at the same time
   during handover.  Even if the MN has only a single interface, the
   above function has some benefits when the timing of handover cannot
   be acquired precisely in advance.  This type of use case was
   originally proposed in [9].  Mobile IPv4 allows simultaneous bindings
   [8] and bi-casting is realized by retaining the old care-of address
   in the binding cache and sending packets destined for the MN towards
   both the old and new care-of addresses.  Since bi-casting consumes
   double the network resources, it must be limited to smooth handover.
   In this document, bi-casting used for a short period of time for
   smooth handover is called "selective bi-casting."  Figure 6 shows
   that the simultaneous bindings and bi-casting are performed at the
   PAR, which copies packets destined for the MN and transmits them not
   only to the old point of attachment but also to the new point of
   attachment via the NAR.  The above operation is more effective when
   the predictive fast handover is applied because in the case of the
   reactive fast handover, all the actions are taken after the MN has
   moved to the new location.  By that time, it is not necessary to
   deliver packets to the old point of attachment.  As another scenario,
   Figure 7 shows that simultaneous bindings are performed at the HA.
   This scenario is likely to happen when the MN is connected to
   multiple different networks such as 3G and WiFi at the same time.
   Also, if the access networks in Figure 1 are operated by different
   providers, it may be difficult for the ARs in these networks to
   cooperate with each other.  In this case as well, the HA must handle
   simultaneous bindings and bi-casting.















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                                +----------+
                                |    HA    |
                                +----------+
                                  /
                                 /
                         +------/-+      +--------+
                         |  PAR --|------|-- NAR  |
                         +------|-+      +-|------+
                                |          |
                                |          |
                           +----|+        +|----+
                           | BS ||        || BS |
                           +----|+        +|----+
                                \         /
                                 \     | /
                                  >    |<
                                  +------+
                                  |  MN  |-->
                                  +------+


                 Figure 6: Selective bi-casting scenario 1



                                +----------+
                                |    HA    |
                                +----------+
                                  /      \
                                 /        \
                         +------/-+      +-\------+
                         |  PAR | |      | | NAR  |
                         +------|-+      +-|------+
                                |          |
                                |          |
                           +----|+        +|----+
                           | BS ||        || BS |
                           +----|+        +|----+
                                \         /
                                 \     | /
                                  >    |<
                                  +------+
                                  |  MN  |-->
                                  +------+


                 Figure 7: Selective bi-casting scenario 2




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   From the above observations, selective bi-casting can be categorized
   from the following viewpoints:

   [No. of interfaces on the MN]
      (A) multiple interfaces
      (B) single interface

   [the node where bi-casting is performed]
      (1) at the PAR
      (2) at the HA

   The procedures for each combination are described in the following
   section.

6.1  Bi-casting by the PAR to the MN with multiple interfaces (A-1)

   As shown in Figure 8, the MN has two interfaces with the link layer
   addresses: LLA1 and LLA2.  This case is typical of handover between
   different access systems such as 3G and WiFi.  Details of the
   sequence are as follows:

      (a) The interface with LLA1 acquires the global IP address PCoA.

      (b) The MN sends the BU to the HA from the link with PCoA with S=0
      and N=0 (the default behavior), which are defined in
      Section 6.5.1, and receives the BA from the HA.

      (c) The MN receives packets destined for PCoA from the link with
      LLA1 via the PAR.

      (d) When the MN detects that handover is imminent, it opens the
      interface with LLA2 and acquires the global IP address NCoA.

      (e) The MN inserts NCoA into the FBU and sends it with S=1 and N=0
      from the link with PCoA to the PAR.

      (f) The MN receives the FBack from the PAR.

      (g) The PAR sends packets destined for PCoA directly to the link
      with LLA1 and also forwards them to the NAR.  The forwarded
      packets are received by the MN on the link with NCoA.

      (h) When the MN is ready to use the link with LLA2 as the primary
      one, it sends the BU to the HA with S=0 and N=0.

      (i) The MN starts to receive packets via the NAR.

   As shown in this example, since the NCoA is assigned on the link with



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   LLA2 and the NAR has the neighbor cache entry (NCE) for the NCoA,
   there is no need to send the HI from the PAR.  To suppress sending
   the HI, a new flag 'N' is defined in the FBU.  Also, to indicate the
   PAR to bi-cast packets, a new flag 'S' is defined in the FBU.  If the
   MN requests selective bi-casting and the valid NCoA has been
   assigned, the MN SHOULD send the FBU to the PAR with the S flag set
   and the N flag unset requesting bi-casting but not sending the HI.
   If the PAR receives this FBU, it SHOULD not send the HI to the NAR.

        +..MN..+
       LLA1   LLA2            PAR                NAR                 HA
        |      |               |                  |                  |
        |link-layer connection |                  |                  |
   (a)  |/--------------------\|                  |                  |
        |\--------------------/|                  |                  |
        |      |               BU(S=0,N=0)/BA                        |
   (b)  |<---------------------------------------------------------->|
        |      |               |                  |                  |
   (c)  |<=====================|<====================================|
        |      |       link-layer connection      |                  |
   (d)  |      |/--------------------------------\|                  |
        |      |\--------------------------------/|                  |
        |     FBU(S=1,N=0)     |                  |                  |
   (e)  |--------------------->|                  |                  |
        |        FBack         |                  |                  |
   (f)  |<---------------------|                  |                  |
        |      |               |                  |                  |
   (g)  |<====================+|<====================================|
        |      |              +=================>+|                  |
        |      |<================================+|                  |
        |      |               |                  |                  |
        |      |               BU(S=0,N=0)/BA                        |
   (h)  |      |<--------------------------------------------------->|
        :      |               |                  |                  |
   (i)  :      |<=================================|<=================|

                        Figure 8: Combination (A-1)


6.2  Bi-casting by the HA to the MN with multiple interfaces (A-2)

   This case happens when the access network where the PAR belongs and
   the one where the NAR belongs are administrated by different
   providers or are different access systems.  The cellular network is
   typically a closed network and the ARs can access external nodes only
   via the HA.  If this is the case, the PAR and the NAR cannot directly
   communicate with each other.  Details of the sequence of this case
   are shown in Figure 9 and below:



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      (a) through (d) are the same as those in the case (A-1).

      (e) The MN sends the BU to the HA from the link with NCoA with S=1
      and N=0, which means that the MN requests the HA to bi-cast but
      not to send the HI.

      (f) The HA forwards packets destined for the MN to both the PAR
      and the NAR.  Those packets can be received by the MN on either
      one or both of the links.

      (g) When the bi-casting lifetime, which is defined in
      Section 6.5.2, is expired, packets are forwarded only to the NAR.


        +..MN..+
       LLA1   LLA2            PAR                NAR                 HA
        |      |               |                  |                  |
        |link-layer connection |                  |                  |
   (a)  |/--------------------\|                  |                  |
        |\--------------------/|                  |                  |
        |      |               BU(S=0,N=0)/BA     |                  |
   (b)  |<---------------------------------------------------------->|
        |      |               |                  |                  |
   (c)  |<=====================|<====================================|
        |      |       link-layer connection      |                  |
   (d)  |      |/--------------------------------\|                  |
        |      |\--------------------------------/|                  |
        |      |               |                  |                  |
        |      |                   BU(S=1,N=0)/BA                    |
   (e)  |      |<--------------------------------------------------->|
        |      |               |                  |                  |
   (f)  |<=====================|<====================================|
        |      |<=================================|<=================|
        :      |               |                  |                  |
   (g)  :      |<=================================|<=================|

                        Figure 9: Combination (A-2)


6.3  Bi-casting by the PAR to the MN with a single interface (B-1)

   This case is typical of handover with the same access system within
   the same provider network.  Details of the sequence of this case are
   shown in Figure 10 and below:

      (a) through (c) are the same as those in the case (A-1).





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      (d) The MN sends the RtSolPr to the PAR.

      (e) The MN receives the PrRtAdv with a valid NCoA from the PAR.

      (f) The MN sends the FBU with S=1 and N=1 to the PAR.

      (g) The PAR sends the HI to the NAR.  The 'U' bit MUST not be set
      if the 'S' bit of the FBU is set.

      (h) The PAR receives the HAck with a valid NCoA from the NAR.

      (i) The PAR sends the FBack both to the MN and the NAR.

      (j) The PAR sends packets destined for PCoA directly to the MN and
      also forwards them to the NAR.

      (k) The MN moves to the new access network and configures NCoA by
      establishing the link-layer connection.  At this point, the MN can
      receive the forwarded packets from the NAR.

      (l) The MN sends the BU with S=0 and N=0 to the HA.

      (m) The MN starts to receive packets only via the NAR.

   In (g), it is necessary that the PAR sends the HI to the NAR to
   create the neighbor cache entry (NCE) for the NCoA on the NAR.

























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       MN                     PAR                NAR                 HA
        |                      |                  |                  |
        |link-layer connection |                  |                  |
   (a)  |/--------------------\|                  |                  |
        |\--------------------/|                  |                  |
        |                      BU(S=0,N=0)/BA                        |
   (b)  |<---------------------------------------------------------->|
        |                      |                  |                  |
   (c)  |<=====================|<====================================|
        |       RtSoIPr        |                  |                  |
   (d)  |--------------------->|                  |                  |
        |       PrRtAdv        |                  |                  |
   (e)  |<---------------------|                  |                  |
        |     FBU(S=1,N=1)     |                  |                  |
   (f)  |--------------------->|                  |                  |
        |                      |        HI        |                  |
   (g)  |                      |----------------->|                  |
        |  (new link)          |       HAck       |                  |
   (h)  |      |               |<-----------------|                  |
        |      |  FBAck        |       FBack      |                  |
   (i)  |<---------------------|----------------->|                  |
        |      |               |                  |                  |
   (j)  |<====================+|<====================================|
        |.....>|              +=================>+|                  |
        :      |<================================+|                  |
        :      |               |                  |                  |
        :      |       link-layer connection      |                  |
   (k)  :      |/--------------------------------\|                  |
        :      |\--------------------------------/|                  |
        :      |                  BU(S=0,N=0)/BA                     |
   (l)  :      |<--------------------------------------------------->|
        :      |               |                  |                  |
   (m)  :      |<=================================|<=================|

                       Figure 10: Combination (B-1)


6.4  Bi-casting by the HA to the MN with a single interfaces (B-2)

   This case is typical of handover with the same access system between
   different provider networks.  Details of the sequence of this case
   are shown in Figure 11 and below:

      (a) through (d) are the same as those in the case (A-3).

      (e) The MN receives the PrRtAdv without a valid NCoA from the PAR.





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      (f) The MN sends the FBU with S=1 and N=1 to the HA.

      (g) The HA sends the HI to the NAR.  The 'U' bit MUST not be set
      if the 'S' bit of the FBU is set.

      (h) The HA receives the HAck from the NAR.

      (i) The HA sends the FBack to the MN and the NAR.

      (j) The HA sends packets destined for the MN to both the PAR and
      the NAR.  They can be received by the MN on either one or both of
      the links.

      (k) The MN moves to the new access network and configures NCoA by
      establishing the link-layer connection.  At this point, the MN can
      receive the forwarded packets from the NAR.

      (l) The MN sends the BU with S=0 and N=0 to the HA.

      (m) The MN starts to receive packets only via the NAR.

      (k) The MN moves to the new access network and configures NCoA by
      establishing the link-layer connection.  At this point, the MN can
      receive the forwarded packets from the NAR.

      (l) The MN sends the BU with S=0 and N=0 to the HA.

      (m) The MN starts to receive packets only via the NAR.

   In (e), by receiving the PrRtAdv without a valid NCoA (the Code is
   typically 2), the MN judges that the PAR does not have information on
   the NAR or can not send the HI directly to the NAR.  Then the MN
   sends the FBU to the HA.

   In (g), it is necessary that the HA sends the HI to the NAR to make
   the NCE for the NCoA on the NAR.















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       MN                     PAR                NAR                 HA
        |                      |                  |                  |
        |link-layer connection |                  |                  |
   (a)  |/--------------------\|                  |                  |
        |\--------------------/|                  |                  |
        |                      BU(S=0,N=0)/BA                        |
   (b)  |<---------------------------------------------------------->|
        |                      |                  |                  |
   (c)  |<=====================|<====================================|
        |       RtSoIPr        |                  |                  |
   (d)  |--------------------->|                  |                  |
        |       PrRtAdv        |                  |                  |
   (e)  |<---------------------|                  |                  |
        |                        FBU(S=1,N=1)                        |
   (f)  |----------------------------------------------------------->|
        |                      |                  |        HI        |
   (g)  |                      |                  |<-----------------|
        |                      |                  |       HAck       |
   (h)  |  (new link)          |                  |----------------->|
        |      |               |                  |      FBack       |
   (i)  |      |               |    FBack         |<-----------------|
        |<-----------------------------------------------------------|
        |      |               |                  |                  |
   (j)  |<=====================|<====================================|
        |      |<=================================|<=================|
        |.....>|       link-layer connection      |                  |
   (k)  :      |/--------------------------------\|                  |
        :      |\--------------------------------/|                  |
        :      |                  BU(S=0,N=0)/BA                     |
   (l)  :      |<--------------------------------------------------->|
        :      |               |                  |                  |
   (m)  :      |<=================================|<=================|

                       Figure 11: Combination (B-2)


6.5  Message Format

6.5.1  Simultaneous bindings flag and the HI message indication flag
       extensions to (F)BU message

   When the MN requests simultaneous bindings and bi-casting to the HA
   or the PAR, the MN sets the newly defined simultaneous bindings flag
   in the Binding Update (BU) [10] or FBU, respectively.  To suppress
   sending the HI when it is unnecessary, the HI message indication flag
   is defined in the (F)BU as well.





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                                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                       |          Sequence #           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |A|H|L|K|S|N|    Reserved       |            Lifetime           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       .                                                               .
       .                        Mobility options                       .
       .                                                               .
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   S:        Simultaneous bindings.  If the 'S' bit is set, the mobile
             node is requesting that the HA or the PAR retain its prior
             mobility bindings and bi-cast packets destined for the MN.

   N:        Indication to send the HI.  On the condition that the 'S'
             bit is set, if the 'N' bit is set, the receiver (the PAR or
             the HA) SHOULD send the HI to the NAR, otherwise the
             receiver SHOULD not send the HI.  If the 'S' bit is not
             set, the 'N' bit MUST be ignored.

   If the 'S' bit is supported, the 'N' MUST also be supported.  When
   S=0 and N=0, the AR and the HA perform according to [3] and [10],
   respectively (the default behavior).

6.5.2  New mobility option for bi-casting lifetime

   The MN may request how long the HA or the PAR should retain the
   simultaneous bindings (and therefore bi-casting) by attaching the
   following mobility option in the binding update message:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     |  Type = T.B.D |   Length = 2  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       Bi-casting Lifetime     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Bi-casting Lifetime:
             The time period when the PAR or the HA retains the previous
             CoA (PCoA).


6.5.3  New status code for simultaneous bindings

   If the AR or the HA receives more (fast) binding update messages with



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   different CoAs for the same HoA than it can support, it should send a
   binding acknowledgement message with the following status code:

   Status   T.B.D.
        too many simultaneous mobility bindings


6.5.4  New Option for vendor-specific information

   If the lower layer information of the new point of attachment is not
   represented as the Link-Layer Address, the following option SHOULD be
   used.  The primary purpose of this option is to convey the handover
   assist information described in Section 4.

     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     |  Option-Code  |   VS-Length   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           VS-Value...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type           T.B.D.

   Length         The size of this option in 8 octets including the
                  Type, Length, Option-Code and VS-Length fields.

   Option-Code    Indicates the particular type of vendor-specific
                  information.  This value is administrated by the
                  vendor or organization that uses this option.

   VS-Length      The size of the VS-Value field in octets.

   VS-Value       Zero or more octets of vendor-specific information
                  data.

   This option MUST be understood by the sender (typically the MN) and
   the receiver (typically the AR or the HA).  If nodes in between do
   not support this option, they SHOULD treat this option as opaque and
   MUST not drop it.

   Depending on the size of the VS-Value field, appropriate padding MUST
   be used to ensure that the entire option size is a multiple of 8
   octets.  The VS-Length is used to disambiguate the size of the VS-
   Value.





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6.6  MN and AR/HA operations

   In order to enable bi-casting, the MN sends a BU or FBU by setting
   the 'S' flag to the HA or the PAR, respectively.  When the PAR/HA
   allows bi-casting, a successful (F)Back is returned and bi-casting is
   started.  The MN can request a desirable bi-casting lifetime to the
   PAR/HA with the bi-casting lifetime option in the (F)BU.  If the
   requested lifetime is acceptable, the PAR/HA sends an (F)Back with
   the accepted bi-casting lifetime, which is determined by the policies
   of the PAR/HA.  When bi-casting is performed at the HA, the MN is
   likely to receive duplicate packets from multiple interfaces.  In the
   case of audio or video applications, it may be necessary to
   synchronize the bi-cast flows coming from different access networks
   so that the user does not have to experience a communication
   disruption.  This may take longer than just the time for a handover.
   If this is the case, the MN may request a longer bi-cast lifetime.
   After the flows are synchronized and successfully switched on the
   application level, the MN may explicitly de-register the PCoA by
   sending an (F)BU with the lifetime field being zero.  On the side of
   the PAR/HA, the maximum value of the bi-casting lifetime must be
   configured and even if the MN does not request a bi-casting lifetime
   or does not successfully de-register the PCoA, it is deleted after
   the maximum value of the bi-casting lifetime elapses.




























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7.  Security Considerations

   The security considerations for Mobile IPv6 fast handover are
   described in [3].  When a 3G network is considered, the PAR and the
   NAR have a trusting relationship and the links between them and those
   between the ARs and the MN are usually secured.  When the MN is
   authenticated at the phase of the link-layer connection, the AR can
   distinguish the authenticated users from the others.  This may be not
   the case, however, if the access networks are operated by different
   providers.









































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8.  Conclusions

   The handover performance of the standard Mobile IPv6 is not
   sufficient for real-time communications that are not resilient to
   packet loss.  The Mobile IPv6 fast handover methods are effective for
   these applications.  This document specifies how these methods can be
   applied to 3G networks.  By introducing fast handover, not only are
   more packets saved which otherwise would be dropped, but also some of
   the bootstrapping parameters can be omitted at the link establishment
   phase, which can expedite the handover process.  For interactive
   real-time applications, in which excessive buffering is
   inappropriate, selective bi-casting is also proposed.  By retaining
   the PCoA and the NCoA in the binding cache, packets destined for the
   MN are transmitted to both the old and new points of attachment at
   the same time, whereby the applications on the MN can choose which
   flow to adopt considering the media continuity.

9.  References

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

   [2]   3GPP2 TSG-A, "Interoperability Specification (IOS) for cdma2000
         Access Network Interfaces Part 1 Overview", A.S0011-C v.1.0,
         February 2005.

   [3]   Koodli, R., Ed., "Fast Handover for Mobile IPv6", RFC 4068,
         July 2005.

   [4]   3GPP2 TSG-X, "cdma2000 Wireless IP Network Standard:
         Introduction", X.P0011-001-D v.0.5, November 2004.

   [5]   3GPP2 TSG-X, "cdma2000 Wireless IP Network Standard: Simple IP
         and Mobile IP services", X.P0011-002-D v.0.5, November 2004.

   [6]   Simpson, W., "PPP Vendor Extensions", RFC 2153, May 1997.

   [7]   3GPP2 TSG-X, "cdma2000 Wireless IP Network Standard: Packet
         Data Mobility and Resource Management", X.P0011-003-D v.0.5,
         November 2004.

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

   [9]   Malki, K., "Simultaneous Bindings for Mobile IPv6 Fast
         Handovers",  draft-elmalki-mobileip-bicasting-v6-05.txt,
         October 2003.




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   [10]  Johnson, D., "Mobility Support in IPv6", RFC 3775, June 2004.


Authors' Addresses

   Hidetoshi Yokota
   KDDI R&D Laboratories, Inc.
   2-1-15 Ohara, Fujimino
   Saitama,  356-8502
   JP

   Phone: +81 49 278 7894
   Fax:   +81 49 278 7510
   Email: yokota@kddilabs.jp


   Gopal Dommety
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134
   US

   Phone: +1 408 525 1404
   Email: gdommety@cisco.com



























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