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Versions: 00 01 02 03 RFC 4068

Mobile IP Working Group                            Rajeev Koodli, Editor
INTERNET DRAFT                                     Nokia Research Center
30 January 2004


                     Fast Handovers for Mobile IPv6
                  draft-ietf-mipshop-fast-mipv6-01.txt


   Status of This Memo

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

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

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

   The list of current Internet-Drafts can be accessed at:
        http://www.ietf.org/ietf/1id-abstracts.txt
   The list of Internet-Draft Shadow Directories can be accessed at:
        http://www.ietf.org/shadow.html.

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.


   Abstract

   Mobile IPv6 enables a Mobile Node to maintain its connectivity to
   the Internet when moving from an Access Router to another, a process
   referred to as handover.  During handover, there is a period when
   the Mobile Node is unable to send or receive packets due to both
   link switching delay and IP protocol operations.  This ``handover
   latency'' resulting from standard Mobile IPv6 procedures, namely
   movement detection, new Care of Address configuration and Binding
   Update, is often unacceptable to real-time traffic such as Voice
   over IP. Reducing the handover latency could be beneficial to non
   real-time, throughput-sensitive applications as well.  This document
   specifies a protocol to improve handover latency due to Mobile IPv6
   procedures.  This document does not address improving the link
   switching latency.




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                                 Contents


Status of This Memo                                                    i

Abstract                                                               i

 1. Introduction                                                       2

 2. Terminology                                                        2

 3. Protocol Overview                                                  4
     3.1. Addressing the Handover Latency . . . . . . . . . . . . .    4
     3.2. Protocol Operation  . . . . . . . . . . . . . . . . . . .    6
     3.3. Protocol Operation of Network-initiated Handover  . . . .    9

 4. Protocol Details                                                  10

 5. Miscellaneous                                                     13
     5.1. Handover Capability Exchange  . . . . . . . . . . . . . .   13
     5.2. Determining New Care of Address . . . . . . . . . . . . .   13
     5.3. Packet Loss . . . . . . . . . . . . . . . . . . . . . . .   14
     5.4. DAD Handling  . . . . . . . . . . . . . . . . . . . . . .   14
     5.5. Fast or Erroneous Movement  . . . . . . . . . . . . . . .   15

 6. Message Formats                                                   16
     6.1. New Neighbor Discovery Messages . . . . . . . . . . . . .   16
           6.1.1. Router Solicitation for Proxy Advertisement
                 (RtSolPr)  . . . . . . . . . . . . . . . . . . . .   16
           6.1.2. Proxy Router Advertisement (PrRtAdv)  . . . . . .   18
     6.2. Inter-Access Router Messages  . . . . . . . . . . . . . .   21
           6.2.1. Handover Initiate (HI)  . . . . . . . . . . . . .   21
           6.2.2. Handover Acknowledge (HAck) . . . . . . . . . . .   23
     6.3. New Mobility Header Messages  . . . . . . . . . . . . . .   25
           6.3.1. Fast Binding Update (FBU) . . . . . . . . . . . .   25
           6.3.2. Fast Binding Acknowledgment (FBack) . . . . . . .   26
           6.3.3. Fast Neighbor Advertisement (FNA) . . . . . . . .   28
     6.4. New Options . . . . . . . . . . . . . . . . . . . . . . .   29
           6.4.1. IP Address Option . . . . . . . . . . . . . . . .   29
           6.4.2. New Router Prefix Information Option  . . . . . .   30
           6.4.3. Link-layer Address (LLA) Option . . . . . . . . .   31
           6.4.4. Neighbor Advertisement Acknowledgment (NAACK) . .   32

 7. Configurable Parameters                                           33

 8. Security Considerations                                           33




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 9. IANA Considerations                                               34

10. Contributors                                                      36

11. Acknowledgments                                                   36

12. Normative References                                              36

13. Informative References                                            37

 A. Change Log                                                        37

 B. Contact Information                                               37







































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   1. Introduction

   Mobile IPv6 [3] describes the protocol operations for a mobile node
   to maintain connectivity to the Internet during its handover from
   one access router to another.  These operations involve movement
   detection, IP address configuration, and location update.  The
   combined handover latency is often sufficient to affect real-time
   applications.  Throughput-sensitive applications can also benefit
   from reducing this latency.  This document describes a protocol to
   reduce the handover latency.

   This specification addresses the following problem:  how to allow a
   mobile node to send packets as soon as it detects a new subnet link,
   and how to deliver packets to a mobile node as soon as its attachment
   is detected by the new access router.  The protocol defines IP
   protocol messages necessary for its operation regardless of link
   technology.  It does this without depending on specific link-layer
   features while allowing link-specific customizations.  By definition,
   this specification considers handovers that inter-work with Mobile
   IP: once attached to its new access router, a MN engages in Mobile IP
   operations including Return Routability [3].  There are no special
   requirements for a mobile node to behave differently with respect to
   its standard Mobile IP operations.


   2. Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
   NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", "OPTIONAL", and
   "silently ignore" in this document are to be interpreted as described
   in RFC 2119 [1].

   The following terminology and abbreviations are used in this
   document.  The reference handover scenario is illustrated in
   Figure 1.

      Mobile Node (MN)
               A Mobile IPv6 host

      Access Point (AP)
               A Layer 2 device connected to a subnet that offers
               wireless connectivity to a MN. An Access Point Identifier
               (AP-ID) refers the AP's L2 address.  Sometimes, AP-ID is
               also referred to as Base Station Subsystem ID (BSSID).

      Access Router (AR)
               The MN's default router





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      Previous Access Router (PAR)
               The MN's default router prior to its handover

      New Access Router (NAR)
               The MN's anticipated default router subsequent to its
               handover

      Previous CoA (PCoA)
               The MN's Care of Address valid on PAR.

      New CoA (NCoA)
               The MN's Care of Address valid on NAR

      Handover
               A process of terminating existing connectivity and
               obtaining new IP connectivity.

      Router Solicitation for Proxy Advertisement (RtSolPr)
               A message from the MN to the PAR requesting information
               for a potential handover

      Proxy Router Advertisement (PrRtAdv)
               A message from the PAR to the MN that aids in movement
               detection.  The message also acts as a trigger for
               network-initiated handover.

      (AP-ID, AR-Info) tuple
               Contains an access router's L2 and IP addresses, and
               prefix valid on the interface to which the Access Point
               (identified by AP-ID) is attached.

      Assigned Addressing
               A particular type of NCoA configuration in which the NAR
               assigns an IPv6 address for the MN. The method by which
               NAR manages its address pool is not specified in this
               document.

      Fast Binding Update (FBU)
               A message from the MN instructing its PAR to redirect its
               traffic (towards NAR)

      Fast Binding Acknowledgment (FBack)
               A message from the PAR in response to FBU

      Fast Neighbor Advertisement (FNA)
               A message from the MN to the NAR to announce attachment
               and to confirm use of NCoA when the MN has not received
               FBACK




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      Handover Initiate (HI)
               A message from the PAR to the NAR to initiate handover

      Handover Acknowledge (HAck)
               A message from the NAR to the PAR as a response to HI




             v            +------------+
           +-+            |  Previous  |        <
           | | ---------- |   Access   | ------ > ----\
           +-+            |   Router   |        <      \
               MN         |   (PAR)    |                \
            |             +------------+            +---------------+
            |                   ^            IP     | Correspondent |
            |                   |         Network   |  Node         |
            V                   |                   +---------------+
                                v                        /
             v            +------------+                /
           +-+            |    New     |        <      /
           | | ---------- |   Access   | ------ > ----/
           +-+            |   Router   |        <
              MN          |   (NAR)    |
                          +------------+



               Figure 1: Reference Scenario for Handover



   3. Protocol Overview

   3.1. Addressing the Handover Latency

   The ability to immediately send packets from a new subnet link
   depends on the ``IP connectivity'' latency, which in turn depends
   on the movement detection latency and the new CoA configuration
   latency.  Once a MN is IP-capable on the new subnet link, it can send
   a Binding Update to its Home Agent and one or more correspondents.
   Once its correspondents successfully process the Binding Update,
   which typically involves the Return Routability procedure, the MN can
   receive packets at the new CoA. So, the ability to receive packets
   from correspondents directly at its new CoA depends on the Binding
   Update latency as well as the IP connectivity latency.

   The protocol enables a MN to quickly detect that it has moved to
   a new subnet by providing the new access point and the associated



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   subnet prefix information when the MN is still connected to its
   current subnet (i.e., PAR in Figure 1).  For instance, a MN may
   discover available access points using link-layer specific mechanisms
   (e.g., a ``scan'' in WLAN) and then request subnet information
   corresponding to one or more of those discovered access points.  The
   MN may do this after performing router discovery.  The MN may also do
   this at any time while connected to its current point of attachment.
   The result of resolving an identifier associated with an access point
   is a [AP-ID, AR-Info] tuple, which a MN can use in readily detecting
   movement:  when attachment to an access point with AP-ID takes place,
   the MN knows the corresponding new router's co-ordinates including
   its prefix, IP address and MAC (or L2) address.  The ``Router
   Solicitation for Proxy Advertisement (RtSolPr)'' and ``Proxy Router
   Advertisement (PrRtAdv)'' messages 6.1 are used for aiding movement
   detection.

   Through the RtSolPr and PrRtAdv messages, the MN also formulates
   a prospective new CoA (NCoA), while still present on the PAR's
   link.  Hence, the latency due to new prefix discovery subsequent to
   handover is eliminated.  Furthermore, this prospective address can
   be used immediately after attaching to the new subnet link (i.e.,
   NAR's link) when the MN has received a ``Fast Binding Acknowledgment
   (FBack)'' message prior to its movement.  In the event it moves
   without receiving an FBack, the MN can still start using NCoA after
   announcing its attachment through a ``Fast Neighbor Advertisement
   (FNA)'' message; NAR responds to FNA in case the tentative address
   is already in use.  In this way, NCoA configuration latency is
   reduced.  Under some limited conditions where the probability of
   address collision is considered insignificant, it may be possible to
   use NCoA immediately after attaching to the new link.  Even so, all
   implementations MUST support the mechanism specified in this document
   to avoid potential address conflicts and SHOULD use them.

   In order to reduce the Binding Update latency, the protocol specifies
   a tunnel between the Previous CoA (PCoA) and NCoA. A MN sends a
   ``Fast Binding Update'' message to its Previous Access Router to
   establish this tunnel.  When feasible, the MN SHOULD send FBU from
   PAR's link.  Otherwise, it should send it immediately after detecting
   attachment to NAR. Subsequent sections describe the protocol
   mechanics.  In any case, the result is that PAR begins tunneling
   packets arriving for PCoA to NCoA. Such a tunnel remains active
   until the MN completes the Binding Update with its correspondents.
   In the opposite direction, the MN SHOULD reverse tunnel packets
   to PAR, again until it completes Binding Update.  And, PAR SHOULD
   forward the inner packet in the tunnel to its destination (i.e., to
   the MN's correspondent).  Such a reverse tunnel ensures that packets
   containing PCoA as source IP addres are not dropped due to ingress
   filtering.  Readers may observe that even though the MN is IP-capable
   on the new link, it cannot use NCoA directly with its correspondents



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   without the correspondents first establishing a binding cache entry
   (for NCoA). Forwarding support for PCoA is provided through a reverse
   tunnel between the MN and the PAR.

   Setting up a tunnel alone does not ensure that the MN receives
   packets as soon as attaching to a new subnet link, unless NAR can
   detect the MN's presence.  A neighbor discovery operation involving
   a neighbor's address resolution (i.e., Neighbor Solicitation and
   Neighbor Advertisement) typically results in considerable delay,
   sometimes lasting multiple seconds.  For instance, when arriving
   packets trigger NAR to send Neighbor Solicitation before the MN
   attaches, subsequent re-transmissions of address resolution are
   separated by a default period of one second each.  In order to
   circumvent this delay, a MN announces its attachment through the FNA
   message that allows NAR to consider MN to be reachable.  If there
   is no existing entry, FNA allows NAR to create one.  If NAR already
   has an entry, FNA updates the entry while taking potential address
   conflicts into consideration.  Through tunnel establishment for PCoA
   and fast advertisement, the protocol provides expedited forwarding of
   packets to the MN.

   The protocol also provides the following important functionalities.
   The access routers can exchange messages to confirm that a proposed
   NCoA is acceptable.  For instance, when a MN sends FBU from PAR's
   link, FBack can be delivered after NAR considers NCoA acceptable
   to use.  This is especially useful when addresses are assigned by
   the access router.  The NAR can also rely on its trust relationship
   with PAR before providing forwarding support for the MN. That is,
   it may create a forwarding entry for NCoA subject to ``approval''
   from PAR which it trusts.  Finally, the access routers could transfer
   network-resident contexts, such as access control, QoS, header
   compression, in conjunction with handover.  For all these operations,
   the protocol provides ``Handover Initiate (HI)'' and ``Handover
   Acknowledge (HAck)'' messages.  Both of these messages MUST be
   supported and SHOULD be used.  The access routers MUST have necessary
   security association established by means outside the scope of this
   document.


   3.2. Protocol Operation

   The protocol begins when a MN sends RtSolPr to its access router
   to resolve one or more Access Point Identifiers to subnet-specific
   information.  In response, the access router (e.g., PAR in Figure 1)
   sends a PrRtAdv message which contains one or more [AP-ID, AR-Info]
   tuples.  The MN may send RtSolPr at any convenient time, for instance
   as a response to some link-specific event (a ``trigger'') or simply
   after performing router discovery.  However, the expectation is that
   prior to sending RtSolPr, the MN has discovered the available APs



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   by link-specific methods.  The RtSolPr and PrRtAdv messages do not
   establish any state at the access router, and their packet formats
   are defined in Section 6.1.

   With the information provided in the PrRtAdv message, the MN
   formulates a prospective NCoA and sends an FBU message.  The purpose
   of FBU is to authorize PAR to bind PCoA to NCoA, so that arriving
   packets can be tunneled to the new location.  The FBU SHOULD be
   sent from PAR's link whenever feasible.  For instance, an internal
   link-specific trigger could enable FBU transmission from the previous
   link.  When it is not feasible, FBU is sent from the new link.  Care
   must be taken to ensure that NCoA used in FBU does not conflict with
   an address already in use by some other node on link.  For this, FBU
   encapsulation within FNA MUST be implemented and SHOULD be used (See
   below) when FBU is sent from NAR's link.

   The format and semantics of FBU processing are specified in
   Section 6.3.1.

   Depending on whether an FBack is received or not on the previous
   link, which clearly depends on whether FBU was sent in the first
   place, there are two modes of operation.

    1. The MN receives FBack on the previous link.  This means that
       packet tunneling would already be in progress by the time the
       MN handovers to NAR. The MN SHOULD send FNA immediately after
       attaching to NAR, so that arriving as well as buffered packets
       can be forwarded to the MN right away.

       Before sending FBack to MN, PAR can determine whether NCoA is
       acceptable to NAR through the exchange of HI and HAck messages.
       When assigned addressing (i.e., addresses are assigned by the
       router) is used, the proposed NCoA in FBU is carried in HI, and
       NAR MAY assign the proposed NCoA. Such an assigned NCoA MUST be
       returned in HAck, and PAR MUST in turn provide the assigned NCoA
       in FBack.  If there is an assigned NCoA returned in FBack, the MN
       MUST use the assigned address (and not the proposed address in
       FBU) upon attaching to NAR.

    2. The MN does not receive FBack on the previous link.  One cause
       for this is that the MN has not sent the FBU. The other is that
       the MN has left the link after sending the FBU, which itself
       may be lost, but before receiving an FBack.  Without receiving
       an FBack in the latter case, the MN cannot ascertain whether
       PAR has successfully processed the FBU. Hence, it (re)sends an
       FBU as soon as it attaches to NAR. In order to enable NAR to
       forward packets immediately (when FBU has been processed) and
       to allow NAR to verify if NCoA is acceptable, the MN SHOULD
       encapsulate FBU in FNA. If NAR detects that NCoA is in use when



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       processing FNA, for instance while creating a neighbor entry, it
       MUST discard the inner FBU packet and send a Router Advertisement
       with ``Neighbor Advertisement Acknowledge (NAACK)'' option in
       which NAR MAY include an alternate IP address for the MN to
       use.  This discarding avoids rare but the undesirable outcome of
       address collision.  Detailed FNA processing rules are specified
       in Section 6.3.3.

   The scenario in which a MN sends FBU and receives FBack on PAR's
   link is illustrated in Figure 2.  For convenience, this scenario is
   characterized as ``predictive'' mode of operation.  The scenario in
   which the MN sends FBU from NAR's link is illustrated in Figure 3.
   For convenience, this scenario is characterized as ``reactive''
   mode of operation.  Note that the reactive mode also includes the
   case when FBU has been sent from PAR's link but FBack has not been
   received yet.

   Finally, the PrRtAdv message may be sent unsolicited, i.e., without
   the MN first sending RtSolPr.  This mode is described in Section 3.3.




              MN                    PAR                  NAR
               |                     |                    |
               |------RtSolPr------->|                    |
               |<-----PrRtAdv--------|                    |
               |                     |                    |
               |------FBU----------->|--------HI--------->|
               |                     |<------HAck---------|
               |          <--FBack---|--FBack--->         |
               |                     |                    |
            disconnect             forward                |
               |                   packets===============>|
               |                     |                    |
               |                     |                    |
           connect                   |                    |
               |                     |                    |
               |--------- FNA --------------------------->|
               |<=================================== deliver packets
               |                                          |




                 Figure 2: ``Predictive'' Fast Handover






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              MN                    PAR                  NAR
               |                     |                    |
               |------RtSolPr------->|                    |
               |<-----PrRtAdv--------|                    |
               |                     |                    |
            disconnect               |                    |
               |                     |                    |
               |                     |                    |
            connect                  |                    |
               |------FNA[FBU]-------|------------------->|
               |                     |<-----FBU-----------|
               |                     |------FBack-------->|
               |                   forward                |
               |                   packets===============>|
               |                     |                    |
               |<=================================== deliver packets
               |                                          |




                  Figure 3: ``Reactive'' Fast Handover



   3.3. Protocol Operation of Network-initiated Handover

   In some wireless technologies, the handover control may reside in
   the network even though the decision to undergo handover may be
   co-operatingly arrived at between the MN and the network.  On such
   networks, the PAR can send an unsolicited PrRtAdv containing the link
   layer address, IP address and subnet prefixes of the NAR when the
   network decides that a handover is imminent.  The MN MUST process
   this PrRtAdv to configure a new care of address on the new subnet,
   and MUST send an FBU to PAR prior to switching to the new link.
   After transmitting PrRtAdv, the PAR MUST continue to forward packets
   to the MN on its current link until the FBU is received.  The rest of
   the operation is the same as that described in Section 3.2.

   The unsolicited PrRtAdv also allows the network to inform the MN
   about geographically adjacent subnets without the MN having to
   explicitly request that information.  This can reduce the amount of
   wireless traffic required for the MN to obtain a neighborhood links
   and subnet topology map.  Such usage of PrRtAdv is decoupled from the
   actual handover.  See Section 6.1.2.





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   4. Protocol Details

   All description makes use of Figure 1 as the reference.

   After discovering one or more nearby access points, the MN sends
   RtSolPr in order to resolve access point identifiers to subnet router
   information.  A convenient time to do this is after performing router
   discovery.  However, the MN MAY send RtSolPr at any time during its
   attachment to PAR. The trigger for sending RtSolPr can emanate from
   a link-specific event, such as the promise of better signal strength
   from another access point coupled with fading signal quality with the
   current access point.  Such events, often broadly referred to as ``L2
   triggers'', are outside the scope of this document.  Nevertheless,
   they serve as events that invoke this protocol.  For instance, when a
   ``link up'' indication is obtained on the new link, protocol messages
   (e.g., FNA) can be immediately transmitted.  Implementations SHOULD
   make use of such triggers whenever available.

   The RtSolPr message may contain one or more AP-IDs.  A wildcard
   requests all available tuples.

   As a response to RtSolPr, PAR sends a PrRtAdv message which indicates
   one of the following possible conditions.

    1. If the PAR does not have an entry corresponding to the new
       attachment point, it MUST respond indicating that the new
       point of attachment is unknown.  The MN MUST stop fast handover
       protocol operations on the current link.  The MN MAY send an FBU
       from its new link.

    2. If the new point of attachment is connected to the PAR's current
       interface (to which MN is attached) itself, PAR MUST respond
       indicating that the point of attachment is known but is connected
       to current interface.  This could happen, for example, when
       several wireless access points are bridged into a wired network.
       No further protocol action is necessary.

    3. If the new point of attachment is known and the PAR has
       information about it, then PAR MUST respond indicating that the
       point of attachment is known and supply the [AP-ID, AR-Info]
       tuple.  If the new point of attachment is known, but does not
       support fast handover, the PAR MUST indicate this with Code 3
       (See Section 6.1.2).

    4. If a wildcard is supplied as an identifier for the new point of
       attachment, the PAR SHOULD supply neighborhood [AP-ID, AR-Info]
       tuples subject to path MTU restrictions (i.e., provide any `n'
       tuples without exceeding the link MTU).




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   When further protocol action is necessary, PAR MAY begin buffering
   copies of incoming packets in FIFO order but MUST continue forwarding
   the packets to PCoA on PAR's link (i.e., buffer and forward).
   Such buffering is useful when the MN leaves without sending the
   FBU message from the PAR's link.  The PAR SHOULD stop buffering
   after processing the FBU message.  The size of the buffer is an
   implementation-specific consideration.

   The method by which Access Routers exchange information about their
   neighbors and thereby allow construction of PrRtAdvs with information
   about the new subnet is outside the scope of this document.

   The RtSolPr and PrRtAdv messages MUST be implemented by a MN and
   an access router that supports fast handovers.  However, when
   the parameters necessary for the MN to send packets immediately
   upon attaching to the NAR are supplied by the link layer handover
   mechanism itself, use of above messages is optional on such links.

   After PrRtAdv message is processed, the MN sends FBU and includes
   the proposed NCoA. The MN SHOULD send FBU from PAR's link whenever
   ``anticipation'' of handover is feasible.  When anticipation is
   not feasible or if it has not received an FBack, the MN sends FBU
   immediately after attaching to NAR's link.  This FBU SHOULD be
   encapsulated in a FNA message.  The encapsulation allows NAR to
   discard the (inner) FBU packet if address conflict is detected as a
   result of (outer) FNA packet processing (see FNA processing below).
   In response to FBU, PAR establishes a binding between PCoA (``Home
   Address'') and NCoA, and sends FBack to MN. Prior to establishing
   this binding, PAR SHOULD send a HI message to NAR, and receive HAck
   in response.  In order to determine the NAR's address for the HI
   message, the PAR can perform longest prefix match of NCoA (in FBU)
   with the prefix list of neighboring access routers.  When the source
   IP address of FBU is PCoA, i.e., the FBU is sent from the PAR's link,
   the HI message MUST have a Code value set to 0.  See Section 6.2.1.
   When the source IP address of FBU is not PCoA, i.e., the FBU is sent
   from the NAR's link, the HI message MUST have a Code value of 1.  See
   Section 6.2.1.

   The HI message contains the PCoA, link-layer address and the NCoA of
   the MN. In response to processing a HI message with Code 0, the NAR

    1. determines whether NCoA supplied in the HI message is a valid
       address for use, and if it is, starts proxying [4] the address
       for PROXY_ND_LIFETIME during which the MN is expected to connect
       to NAR. The NAR MAY use the link-layer address to verify if a
       corresponding IP address exists in its forwarding tables.






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    2. allocates NCoA for the MN when assigned addressing is used,
       creates a proxy neighbor cache entry and begins defending it.
       The NAR MAY allocate NCoA proposed in HI.

    3. MAY create a host route entry for PCoA in case NCoA cannot
       be accepted or assigned.  This host route entry SHOULD be
       implemented such that until the MN's presence is detected, either
       through explicit announcement by the MN or by other means,
       arriving packets do not invoke neighbor discovery.  The NAR MAY
       also set up a reverse tunnel to PAR in this case.

    4. provides the status of handover request in Handover Acknowledge
       (HAck) message.

   When the Code value in HI is 1, NAR MUST skip the above operations
   since it would have performed those operations during FNA processing.
   However, it SHOULD be prepared to process any other options which
   may be defined in the future.  Sending a HI message with Code 1
   allows NAR to, loosely speaking, validate the neighbor cache entry
   it creates for the MN during FNA processing.  That is, NAR can make
   use of the knowledge that its trusted peer (i.e., PAR) has a trust
   relationship with the MN.

   If HAck contains an assigned NCoA, FBack MUST include it, and the MN
   MUST use the address provided in FBack.  The PAR SHOULD send FBack to
   previous link as well to facilitate faster reception should the MN be
   still present there.  The result of FBU and FBack processing is that
   PAR begins tunneling MN's packets to NCoA. If the MN does not receive
   an FBack message even after re-transmitting FBU for FBU_RETRIES, it
   must assume that fast handover support is not available and stop the
   protocol operation.

   As soon as the MN establishes link connectivity with the NAR, it
   SHOULD send a Fast Neighbor Advertisement (FNA) message (see 6.3.3).
   If the MN has not received an FBack by the time FNA is being sent, it
   SHOULD encapsulate the FBU in FNA and send them together.

   When the NCoA corresponding to the FNA message is acceptable, the NAR
   MUST,

    1. delete its proxy neighbor cache entry, if any is present.

    2. create a neighbor cache entry and set its state to REACHABLE
       without over-writing an existing entry for a different layer 2
       address.

    3. forward any buffered packets

    4. enable the host route entry, if any is present, for PCoA.



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   When the NCoA corresponding to the FNA message is not acceptable, the
   NAR MUST

    1. discard the inner (FBU) packet.

    2. send a Router Advertisement with the NAACK option, and MAY
       include an alternate NCoA for use.  This message MUST be sent
       to the source IP address present in FNA using the same Layer 2
       address present in FNA.

   If the MN receives a Router Advertisement with a NAACK option,
   it MUST use the address, if any, provided in the NAACK option.
   Otherwise, the MN should configure another NCoA. Subsequently, the MN
   SHOULD send an FBU using the new CoA. As a special case, the address
   supplied in NAACK could be PCoA itself, in which case the MN MUST NOT
   send any more FBUs.

   Once the MN has confirmed its NCoA, it SHOULD send a Neighbor
   Advertisement message.  This message allows MN's neighbors to update
   their neighbor cache entries with the MN's addresses.

   For data forwarding, the PAR tunnels packets to the MN using
   its global IP address valid on the interface to which the MN was
   attached.  The MN reverse tunnels its packets to the same global
   address of PAR. The tunnel end-point addresses must be configured
   accordingly.  When PAR receives a reverse tunneled packet, it must
   verify if a secure binding exists for the MN identified by PCoA in
   the tunneled packet, before forwarding the packet.


   5. Miscellaneous

   5.1. Handover Capability Exchange

   The MN expects a PrRtAdv in response to its RtSolPr message.  If the
   MN does not receive a PrRtAdv message even after RTSOLPR_RETRIES, it
   must assume that PAR does not support the fast handover protocol and
   stop sending any more RtSolPr messages.

   Even if a MN's current access router is capable of fast handover, the
   new access router to which the MN attaches may be incapable of fast
   handover.  This is indicated to the MN during ``runtime'', through
   the PrRtAdv message with a Code value of 3 (see Section 6.1.2).


   5.2. Determining New Care of Address

   Typically, the MN formulates its prospective NCoA using the
   information provided in a PrRtAdv message, and sends FBU. The PAR



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   MUST use the NCoA present in FBU in its HI message.  The NAR MUST
   verify if NCoA present in HI is already in use.  In any case, NAR
   MUST respond to HI using a HAck, in which it may include another NCoA
   to use, especially when assigned address configuration is used.  If
   there is a CoA present in HAck, PAR MUST include it in the FBack
   message.

   If PrRtAdv message carries a NCoA, the MN MUST use it as its
   prospective NCoA.


   5.3. Packet Loss

   Handover involves link switching, which may not be exactly
   co-ordinated with fast handover signaling.  Furthermore, the
   arrival pattern of packets is dependent on many factors, including
   application characteristics, network queuing behaviors etc.  Hence,
   packets may arrive at NAR before the MN is able to establish its
   link there.  These packets will be lost unless they are buffered
   by the NAR. Similarly, if the MN attaches to NAR and then sends an
   FBU message, packets arriving at PAR will be lost unless they are
   buffered.  This protocol provides an option to indicate request for
   buffering at the NAR in the HI message.  When the PAR requests this
   feature (for the MN), it SHOULD also provide its own support for
   buffering.


   5.4. DAD Handling

   Duplicate Address Detection (DAD) was defined in [5] to avoid address
   duplication on links when stateless address autoconfiguration is
   used.  The use of DAD to verify the uniqueness of an IPv6 address
   configured through stateless autoconfiguration adds delays to a
   handover.

   The probability of an interface identifier duplication on the same
   subnet is very low, however it cannot be ignored.  In this draft
   certain precautions are proposed to minimize the effects of a
   duplicate address occurrence.

   In some cases the NAR may already have the knowledge required to
   assess whether the MN's address is a duplicate or not before the MN
   moves to the new subnet.  For example, the NAR can have a list of all
   nodes on its subnet, perhaps for access control, and by searching
   this list, it can confirm whether the MN's address is a duplicate
   or not.  The result of this search is sent back to the PAR in the
   HAck message.  If such knowledge is not available at the NAR, it may
   indicate this by not confirming NCoA in the HAck message.  The NAR
   may also indicate this in the NAACK option as a response to the FNA



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   message.  In such cases, the MN would have to follow the address
   configuration procedure according to [4] after attaching to the NAR.


   5.5. Fast or Erroneous Movement

   Although this specification is for fast handover, the protocol has
   its limits in terms of how fast a MN can move.  A special case
   of fast movement is ping-pong, where a MN moves between the same
   two access points rapidly.  Another instance of the same problem
   is erroneous movement i.e., the MN receives information prior to
   a handover that it is moving to a new access point and but it is
   either moved to a different one or it aborts movement altogether.
   All of the above behaviors are usually the result of link layer
   idiosyncrasies and thus are often tackled at the link layer itself.

   IP layer mobility, however, introduces its own limits.  IP layer
   handovers should occur at a rate suitable for the MN to update the
   binding of, at least, its HA and preferably that of every CN with
   which it is in communication.  A MN that moves faster than necessary
   for this signaling to complete, which may be of the order of few
   seconds, may start losing packets.  The signaling overhead over the
   air and in the network may increase significantly, especially in the
   case of rapid movement between several access routers.  To avoid the
   signaling overhead, the following measures are suggested.

   A MN returning to the PAR before updating the necessary bindings when
   present on NAR MUST send a Fast Binding Update with Home Address
   equal to the MN's PCoA and a lifetime of zero, to the PAR. The MN
   should have a security association with the PAR since it performed
   a fast handover from it to the NAR. The PAR, on receiving this
   Fast Binding Update, will check its set of outgoing (temporary
   fast handover) tunnels.  If it finds a match it SHOULD tear down
   that tunnel; i.e., stop forwarding packets for this MN and start
   delivering packets directly to the node instead.  The MN SHOULD NOT
   make any attempt to use any of the fast handover mechanisms described
   in this specification and SHOULD revert back to standard Mobile IPv6.

   Temporary tunnels for the purposes of fast handovers should use short
   lifetimes (in the order of a small number of seconds or less).  The
   lifetime of such tunnels should be enough to allow a MN to update all
   its active bindings.  The default lifetime of the tunnel should be
   the same as the lifetime value in the FBU message.

   The effect of erroneous movement is typically limited to loss of
   packets since routing can change and the PAR may forward packets
   towards another router before the MN actually connects to that
   router.  If the MN discovers itself on an unanticipated access
   router, a Fast Binding Update to the PAR SHOULD be sent.  Since



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   Fast Binding Updates are authenticated, they supersede the existing
   binding and packets MUST be redirected to the new confirmed location
   of the MN.


   6. Message Formats

   6.1. New Neighbor Discovery Messages

   6.1.1. Router Solicitation for Proxy Advertisement (RtSolPr)

   Mobile Nodes send Router Solicitation for Proxy Advertisement in
   order to prompt routers for Proxy Router Advertisements.  For all the
   ICMP messages, the checksum is defined in [2].  All the link-layer
   address options have the format defined in 6.4.3.



   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      |     Code      |          Checksum             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |          Identifier           |           Reserved            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |   Options ...
  +-+-+-+-+-+-+-+-+-+-+-+-




       Figure 4: Router Solicitation for Proxy (RtSolPr) Message



 IP Fields:

   Source Address
                  An IP address assigned to the sending interface

   Destination Address
                  The address of the Access Router or the all routers
                  multicast address.

   Hop Limit      255. See RFC 2461.

 Authentication Header
                  If a Security Association for the IP Authentication
                  Header exists between the sender and the



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                  destination address, then the sender SHOULD include
                  this header.

 ICMP Fields:

   Type           To be assigned by IANA.

   Code           0

   Checksum       The ICMPv6 checksum.

   Identifier     MUST be set by the sender so that replies can be
                  matched to this Solicitation.

   Reserved       MUST be set to zero by the sender and ignored by
                  the receiver.

 Valid Options:

   Source Link-layer Address
                  When known, the link-layer address of the sender
                  SHOULD be included using the Link-Layer Address
                  option. See LLA option format below.

   New Attachment Point Link-layer Address
                  The link-layer address or identification of the
                  attachment point for which the MN requests routing
                  advertisement information. It MUST be included
                  in all RtSolPr messages. More than one such address
                  or identifier can be present. This field can also
                  be a wildcard address with all bits set to zero.

   Future versions of this protocol may define new option types.
   Receivers MUST silently ignore any options that they do not recognize
   and continue processing the rest of the message.

   Including the source LLA option allows the receiver to record the
   sender's L2 address so that neighbor discovery, when the receiver
   needs to send packets back to the sender (of RtSolPr message), can be
   avoided.

   When a wildcard is used form New Attachment Point LLA, no other New
   Attachment Point LLA options must be present.

   A Proxy Router Advertisement (PrRtAdv) message should be received by
   the MN as a response to RtSolPr.  If such a message is not received
   in a short time period but no less than twice the typical round
   trip time (RTT) over the access link or 100 ms if RTT is not known,
   it SHOULD resend RtSolPr message at most RTSOLPR_RETRIES, waiting



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   for the same time during each instance of retransmission.  If Proxy
   Router Advertisement is not received by the time the MN disconnects
   from the PAR, the MN SHOULD send FBU immediately after configuring a
   new CoA.


   6.1.2. Proxy Router Advertisement (PrRtAdv)

   Access routers send out Proxy Router Advertisement message
   gratuitously if the handover is network-initiated or as a response
   to RtSolPr message from a MN, providing the link-layer address,
   IP address and subnet prefixes of neighboring routers.  All the
   link-layer address options have the format defined in 6.4.3.



   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      |     Code      |          Checksum             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |          Identifier           |           Reserved            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |   Options ...
  +-+-+-+-+-+-+-+-+-+-+-+-



         Figure 5: Proxy Router Advertisement (PrRtAdv) Message



 IP Fields:

   Source Address
                  MUST be the link-local address assigned to the
                  interface from which this message is sent.

   Destination Address
                  The Source Address of an invoking Router
                  Solicitation for Proxy Advertisement or the address
                  of the node the Access Router is instructing to
                  handover.

   Hop Limit      255. See RFC 2461.

 Authentication Header
                  If a Security Association for the IP Authentication
                  Header exists between the sender and the



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                  destination address, then the sender SHOULD include
                  this header.

 ICMP Fields:

   Type           To be assigned by IANA.

   Code           0, 1, 2, 3 or 4. See below.

   Checksum       The ICMPv6 checksum.

   Identifier     Copied from Router Solicitation for Proxy
                  Advertisement or set to Zero if unsolicited.

   Reserved       MUST be set to zero by the sender and ignored by
                  the receiver.

 Valid Options in the following order:

   Source Link-layer Address
                  When known, the link-layer address of the sender
                  SHOULD be included using the Link-Layer Address
                  option. See LLA option format below.

   New Attachment Point Link-layer Address
                  The link-layer address or identification of the
                  attachment point is copied from RtSolPr
                  message. This option MUST be present.

   New Router's Link-layer Address
                  The link-layer address of the Access Router for
                  which this message is proxied for. This option MUST be
                  included when Code is 0 or 1.

   New Router's IP Address
                  The IP address of NAR. This option MUST be
                  included when Code is 0 or 1.

   New Router Prefix Information Option.
                  Specifies the prefixes of the Access
                  Router the message is proxied for and is used
                  for address autoconfiguration. This option MUST be
                  included when Code is 0 or 1.

   New CoA Option
                  MAY be present when PrRtAdv is sent
                  unsolicited. PAR MAY compute new CoA using NAR's
                  prefix information and the MN's L2 address, or by
                  any other means.



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   Future versions of this protocol may define new option types.
   Receivers MUST silently ignore any options they do not recognize and
   continue processing the message.

   Currently, Code values 0, 1, 2, 3 and 4 are defined.

   A Proxy Router Advertisement with Code 0 means that the MN should
   use the [AP-ID, AR-Info] tuple (present in the options above) for
   movement detection and NCoA formulation.  The Sub-type field in the
   New Attachment Point LLA option in this case is 1 reflecting the
   LLA of the attachment point for which the rest of the options are
   related.  Multiple tuples may be present.

   A Proxy Router Advertisement with Code 1 means that the message is
   sent unsolicited.  If a New CoA option is present following the New
   Router Prefix Information option, the MN SHOULD use the supplied NCoA
   and send FBU immediately or else stand to lose service.  This message
   acts as a network-initiated handover trigger.  See Section 3.3.  The
   Sub-type field in the New Attachment Point LLA option (see below) in
   this case is 1 reflecting the LLA of the attachment point for which
   the rest of the options are related.

   A Proxy Router Advertisement with Code 2 means that no new
   router information is present.  Multiple New Attachment Point LLA
   options may be present.  The Sub-type field values (defined below)
   distinguish different outcomes.

   When the Sub-type field in the New Attachment Point LLA option is 5,
   handover to that attachment point does not require change of CoA. No
   other options are required in this case.

   When the Sub-type field in the New Attachment Point LLA option is 6,
   PAR is not aware of the Prefix Information requested.  The MN SHOULD
   attempt to send FBU as soon as it regains connectivity with the NAR.
   No other options are required in this case.

   When the Sub-type field in the New Attachment Point LLA option is
   7, it means that the NAR does not support fast handover.  The MN
   MUST stop fast handover protocol operations.  No other options are
   required in this case.

   A Proxy Router Advertisement with Code 3 means that new router
   information is present only for a subset of attachment points
   requested.  The Sub-type field values (defined above including a
   value of 1) distinguish different outcomes for individual access
   points.




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   A Proxy Router Advertisement with Code 4 means that the subnet
   information regarding neighboring access points is sent unsolicited,
   but the message is not a handover trigger, unlike when the message is
   sent with Code 1.  Multiple tuples may be present.

   When a wildcard AP identifier is supplied in the RtSolPr message,
   the PrRtAdv message should include any 'n' [Access Point Identifier,
   Link-layer address option, Prefix Information Option] tuples
   corresponding to the PAR's neighborhood.


   6.2. Inter-Access Router Messages

   6.2.1. Handover Initiate (HI)

   The Handover Initiate (HI) is an ICMPv6 message sent by an Access
   Router (typically PAR) to another Access Router (typically NAR) to
   initiate the process of a MN's handover.




    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      |    Code       |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Identifier             |S|U|       Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Options...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-



                Figure 6: Handover Initiate (HI) Message



 IP Fields:

   Source Address
                  The IP address of the PAR

   Destination Address
                  The IP address of the NAR

   Hop Limit      255. See RFC 2461.

 Authentication Header



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                  The authentication header MUST be used when this
                  message is sent.

 ICMP Fields:

   Type           To be assigned by IANA.

   Code           0 or 1. See below

   Checksum       The ICMPv6 checksum.

   Identifier     MUST be set by the sender so replies can be matched
                  to this message.

   S              Assigned address configuration flag. When set, this
                  message requests a new CoA to be returned by the
                  destination. May be set when Code = 0. MUST be 0
                  when Code = 1.

   U              Buffer flag. When set, the destination SHOULD buffer
                  any packets towards the node indicated in the options
                  of this message. Used when Code = 0, SHOULD be set
                  to 0 when Code = 1.

  Reserved       MUST be set to zero by the sender and ignored by
                 the receiver.


 Valid Options:

   Link-layer address of MN
                  The link-layer address of the MN that is
                  undergoing handover to the destination (i.e., NAR).
                  This option MUST be included so that the destination
                  can recognize the MN.

   Previous Care of Address
                  The IP address used by the MN while
                  attached to the originating router. This option
                  SHOULD be included so that host route can be
                  established in case necessary.

   New Care of Address
                  The IP address the MN wishes to use when
                  connected to the destination. When the `S' bit is
                  set, NAR MAY assign this address.






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   A Code value of 0 is used when PAR receives FBU with PCoA as source
   IP address.  A Code value of 1 is used PAR receives FBU whose source
   IP address is not PCoA.

   If Handover Acknowledge (HAck) message is not received as a response,
   the Handover Initiate SHOULD be re-sent up to HI_RETRIES times using
   a short retransmission timer with a value no less than twice the
   round trip time between source and destination or 100 ms if RTT is
   not known.


   6.2.2. Handover Acknowledge (HAck)

   The Handover Acknowledgment message is a new ICMPv6 message that MUST
   be sent (typically by NAR to PAR) as a reply to the Handover Initiate
   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      |    Code       |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Identifier             |H|T|R|      Reserved           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Options...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-



             Figure 7: Handover Acknowledge (HAck) Message




 IP Fields:

   Source Address
                Copied from the destination address of the Handover
                Initiate Message to which this message is a
                response.

   Destination Address
                Copied from the source address of the Handover
                Initiate Message to which this message is a
                response.

   Hop Limit      255. See RFC 2461.



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 Authentication Header
                  The authentication header MUST be used when this
                  message is sent.

 ICMP Fields:

   Type         To be assigned by IANA.

   Code
                0: Handover Accepted, NCoA valid
                1: Handover Accepted, NCoA not valid
                2: Handover Accepted, NCoA in use
                3: Handover Accepted, NCoA assigned
                   (used in Assigned addressing)
                4: Handover Accepted, NCoA not assigned
                   (used in Assigned addressing)
              128: Handover Not Accepted, reason unspecified
              129: Administratively prohibited
              130: Insufficient resources

   Checksum       The ICMPv6 checksum.

   Identifier     Copied from the corresponding field in the Handover
                  Initiate message this message is in response to.

   Reserved       MUST be set to zero by the sender and ignored by
                  the receiver.

 Valid Options:

   New Care of Address
        If the S flag in the Handover Initiate message is set,
        this option MUST be used to provide NCoA the MN should
        use when connected to this router. This option MAY be
        included even when `S' bit is not set, e.g., Code 2
        above.

   Upon receiving a HI message, the NAR MUST respond with a Handover
   Acknowledge message.  If the `S' flag is set in the HI message, the
   NAR SHOULD include the New Care of Address option and a Code 3.

   The NAR MAY provide support for PCoA (instead of accepting or
   assigning NCoA) and establish a host route entry for PCoA, and set up
   a tunnel to the PAR to forward MN's packets sent with PCoA as source
   IP address.  This host route entry SHOULD be used to forward packets
   once the NAR detects that the particular MN is attached to its link.





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   When responding to a HI message containing a Code value 1, the Code
   values 1, 2, and 4 in the HAck message are not relevant.

   Finally, the new access router can always refuse handover, in which
   case it should indicate the reason in one of the available Code
   values.


   6.3. New Mobility Header Messages

   Mobile IPv6 uses a new IPv6 header type called Mobility Header [3].
   The Fast Binding Update, Fast Binding Acknowledgment and Fast
   Neighbor Advertisement messages use the Mobility Header.


   6.3.1. Fast Binding Update (FBU)

   The Fast Binding Update message is identical to the Mobile IPv6
   Binding Update (BU) message.  However, the processing rules are
   slightly different.  A tentative value of 8 for the Mobility Header
   Type for FBU is to be assigned by IANA.



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





              Figure 8: Fast Binding Update (FBU) Message

      IP fields:

         Source address      The PCoA or NCoA

         Destination Address
                             The IP address of the Previous Access
                             Router



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      `A' flag        MUST be set to one to request PAR to send a Fast
                      Binding Acknowledgement message.

      `H' flag        MUST be set to one.  See [3].

      `L' flag        See [3].

      `K' flag        See [3].

      Reserved        This field is unused.  MUST be set zero.

      Sequence Number See [3].

      Lifetime        See [3].

      Mobility Options
                      MUST contain alternate CoA option set to NCoA IP
                      address when FBU is sent from PAR's link.

   The MN sends FBU message any time after receiving a PrRtAdv message.
   If the MN moves prior to receiving a PrRtAdv message, it SHOULD send
   a FBU to the PAR after configuring NCoA on the NAR according to
   Neighbor Discovery and IPv6 Address Configuration protocols.

   The source IP address is PCoA when FBU is sent from PAR's link, and
   the source IP address is NCoA when sent from NAR's link.  When FBU is
   sent from NAR's link, it SHOULD be encapsulated within FNA.

   The FBU MUST also include the Home Address Option and the Home
   Address is PCoA. A FBU message MUST be protected so that PAR is able
   to determine that the FBU message is sent by a genuine MN.


   6.3.2. Fast Binding Acknowledgment (FBack)

   The Fast Binding Acknowledgment message is sent by the PAR to
   acknowledge receipt of a Fast Binding Update message in which the `A'
   bit is set.  The Fast Binding Acknowledgment message SHOULD NOT be
   sent to the MN before the PAR receives a HAck message from the NAR.
   The Fast Binding Acknowledgment MAY also be sent to the MN on the old
   link.  A tentative value of 8 for the Mobility Header Type for FBACK
   is to be assigned by IANA.

      IP fields:

         Source address      The IP address of the Previous Access
                             Router

         Destination Address The NCoA



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                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |    Status     |K|  Reserved   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           Sequence #          |           Lifetime            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                        Mobility options                       .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





         Figure 9: Fast Binding Acknowledgment (FBack) Message



      Status
                        8-bit unsigned integer indicating the
                        disposition of the Fast Binding Update.  Values
                        of the Status field less than 128 indicate that
                        the Binding Update was accepted by the receiving
                        node.  The following such Status values are
                        currently defined:

                        0 Fast Binding Update accepted
                        1 Fast Binding Update accepted but NCoA is
                        invalid.  Use NCoA supplied in ``alternate'' CoA


                        Values of the Status field greater than or equal
                        to 128 indicate that the Binding Update was
                        rejected by the receiving node.  The following
                        such Status values are currently defined:

                        128 Reason unspecified
                        129 Administratively prohibited
                        130 Insufficient resources
                        131 Incorrect interface identifier length


      `K' flag          See [3].

      Reserved          An unused field.  MUST be set to zero.




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      Sequence Number   Copied from FBU message for use by the MN in
                        matching this acknowledgment with an outstanding
                        FBU.

      Lifetime
                        The granted lifetime in seconds for which the
                        sender of this message will retain a binding for
                        traffic redirection.

      Mobility Options  MUST contain ``alternate'' CoA if Status is 1.


   6.3.3. Fast Neighbor Advertisement (FNA)

   A MN sends a Fast Neighbor Advertisement to announce itself to the
   NAR. A tentative value of 8 for the Mobility Header Type for FBACK
   is to be assigned by IANA. When the Mobility Header Type is FNA,
   the Payload Proto field may be set to IPv6 in order to assist FBU
   encapsulation.



                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |            Reserved           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .                                                               .
    .                        Mobility Options                       .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



          Figure 10: Fast Neighbor Advertisement (FNA) Message

      IP fields:

         Source address      NCoA

         Destination Address NAR's IP address

      Mobility Options  MUST contain the Link-Layer Address of the MN in
                        LLA option format.  See below.

   The MN sends Fast Neighbor Advertisement to the NAR, as soon as it
   regains connectivity on the new link.  Arriving or buffered packets
   can be immediately forwarded.  If NAR is proxying NCoA, it creates
   a neighbor cache entry in REACHABLE state.  If there is no entry at
   all, it creates one and sets it to REACHABLE. If there is an entry



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   in INCOMPLETE state without a link-layer address, it sets it to
   REACHABLE. During the process of creating a neighbor cache entry, NAR
   can also detect if NCoA is in use, thus avoiding address collisions.
   Since FBU is encapsulated within FNA when sent from NAR's link, NAR
   drops FBU in case it detects any collision.

   The combination of NCoA (present in source IP address) and the
   Link-Layer Address (present as a Mobility Option) SHOULD be used to
   distinguish the MN from other nodes.


   6.4. New Options

   6.4.1. IP Address Option

   This option is sent in the Proxy Router Advertisement, the Handover
   Initiate, and Handover Acknowledge messages.  The Proxy Router
   Advertisement and Handover Acknowledgment messages only contain the
   NCoA while the Handover Initiate message may include both NCoA and
   PCoA. The format used here was first documented 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    | Prefix Length |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +                                                               +
  |                                                               |
  +                          IPv6 Address                         +
  |                                                               |
  +                                                               +
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                     Figure 11: IPv6 Address Option




   Type
        To be assigned by IANA.

   Length
        The size of this option in octets excluding the Type,
        Sub-type and Length fields.



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   Sub-Type
        1   Old Care-of Address
        2   New Care-of Address

   Prefix Length
        The Length of the IPv6 Address Prefix

   IPv6 address
        The IP address for the unit defined by the Type field.



   6.4.2. New Router Prefix Information Option

   This option is sent in the PrRtAdv message in order to provide the
   prefix information valid on the NAR.



  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   | Prefix Length |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +                                                               +
  |                                                               |
  +                            Prefix                             +
  |                                                               |
  +                                                               +
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



            Figure 12: New Router Prefix Information Option



   Type
        To be assigned by IANA.

   Length
        The size of this option in octets excluding the Type,
        Sub-type and Length fields.

   Sub-Type
        0



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   Prefix Length
        8-bit unsigned integer.  The number of leading bits in the
        Prefix that are valid.  The value ranges from 0 to 128.

   Prefix
        An IP address or a prefix of an IP address.  The Prefix Length
        field contains the number of valid leading bits in the prefix.
        The bits in the prefix after the prefix length are reserved
        and MUST be initialized to zero by the sender and ignored by
        the receiver.


   6.4.3. Link-layer Address (LLA) Option



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



                  Figure 13: Link-Layer Address Option



   Type
        To be assigned by IANA.

   Length
        The size of this option in octets excluding the Type,
        Sub-type and Length fields.

   Sub-Type
        0 wildcard requesting resolution for all nearby access points
        1 Link-layer Address of the New Attachment Point
        2 Link-layer Address of the MN
        3 Link-layer Address of the NAR (i.e., Proxied Originator)
        4 Link-layer Address of the source of RtSolPr or PrRtAdv
          message
        5 The attachment point identified by the LLA belongs to the
          current interface of the router
        6 No prefix information available for the attachment point
          identified by the LLA
        7 No fast handovers support available for the attachment point
          identified by the LLA



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   LLA
        The variable length link-layer address.

   The New Attachment Point Link Layer address contains the link-layer
   address of the attachment point for which handover is about to
   be attempted.  This is used in the Router Solicitation for Proxy
   Advertisement message.

   The MN Link-Layer address option contains the link-layer address of a
   MN. It is used in the Handover Initiate message.

   The NAR (i.e., Proxied Originator) Link-Layer address option contains
   the Link Layer address of the Access Router for which the Proxy
   Router Solicitation message refers to.

   The option must use appropriate padding for alignment on byte
   boundaries.


   6.4.4. Neighbor Advertisement Acknowledgment (NAACK)



   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    |     Status    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                          Reserved                             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



        Figure 14: Neighbor Advertisement Acknowledgment Option


   Type
        To be assigned by IANA.

   Length
        8-bit unsigned integer.  Length of the option, in 8
        octets. The length is 1 when a new CoA is not supplied. The
        length is 3 when a new CoA is present (immediately following
        the Reserved field)

   Sub-Type
        0




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   Status
        8-bit unsigned integer indicating the disposition of the Fast
        Neighnor Advertisement message. The following Status
        values are currently defined:

           1   The New CoA is invalid
           2   The New CoA is invalid, use the supplied CoA. The New
               CoA MUST be present following the Reserved field.
         128   Link Layer Address unrecognized

   Since the NAACK option is carried in a Router Advertisement, the
   Length field is units of 8 octets unlike in other options.

   The NAR responds to FNA with the NAACK option to notify the MN
   to use a different NCoA if there is address collision.  If the
   NCoA is invalid, the Router Advertisement MUST use the NCoA as the
   destination address but use the L2 address present in FNA. The MN
   SHOULD use the NCoA if it is supplied with the NAACK option.  If the
   NAACK indicates that the Link Layer Address is unrecognized the MN
   MUST NOT use the NCoA or the PCoA and SHOULD start immediately the
   process of acquiring a NCoA at the NAR.

   In the future, new option types may be defined.


   7. Configurable Parameters

      Parameter Name       Default Value            Definition
      -------------------  ----------------------   -------
      RTSOLPR_RETRIES      2                        Section 6.1.1
      FBU_RETRIES          3                        Section 4
      PROXY_ND_LIFETIME    1.5 seconds              Section 6.2.2
      HI_RETRIES           4                        Section 6.2.1




   8. Security Considerations

   The following security vulnerabilities are identified, and suggested
   solutions mentioned.

    1. Insecure FBU: in this case, packets meant for one address could
       be stolen, or redirected to some unsuspecting node.  This concern
       is the same as that in a MN and Home Agent relationship.

       Hence, the PAR MUST ensure that the FBU packet arrived from a
       node that legitimately owns the PCoA. The access router and its
       hosts may use any available mechanism to establish a security



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       association which MUST be used to secure FBU. The current version
       of this protocol does not specify how this security association
       is established.  However, the future work, either as part of this
       document or in a separate document, may specify this security
       establishment.

       If an access router can ensure that the source IP address in
       an arriving packet could only have originated from the node
       whose link-layer address is in the router's neighbor cache, then
       a bogus node cannot use a victim's IP address for malicious
       redirection of traffic.  Such an operation is recommended at
       least on neighbor discovery messages including the RtSolPr
       message.

    2. Secure FBU, malicious or inadvertant redirection:  in this case,
       the FBU is secured, but the target of binding happens to be an
       unsuspecting node either due to inadvertent operation or due
       to malicious intent.  This vulnerability can lead to a MN with
       genuine security association with its access router redirecting
       traffic to an incorrect address.

       However, the target of malicious traffic redirection is limited
       to an interface on an access router with which the PAR has a
       security association.  The PAR MUST verify that the NCoA to
       which PCoA is being bound actually belongs to NAR's prefix.  In
       order to do this, HI and HAck message exchanges are to be used.
       When NAR accepts NCoA in HI (with Code = 0), it proxies NCoA so
       that any arriving packets are not sent on the link until the MN
       attaches and announces itself through FNA. So, any inadvertant or
       malicious redirection to a host is avoided.  It is still possible
       to jam NAR's buffer with redirected traffic.  However, since
       NAR's handover state corresponding to NCoA has a finite (and
       short) lifetime corresponding to a small multiple of anticipated
       handover latency, the extent of this vulnerability is arguably
       small.

    3. Sending FBU from NAR's link:  a malicious node may send FBU from
       NAR's link providing an unsuspecting node's address as NCoA.
       Since FBU is encapsulated in FNA, NAR should detect the collision
       with an address in use when processing FNA, and it then drops
       FBU. When NAR is unable to detect address collisions, there is a
       vulnerability that redirection can affect an unsuspecting node.


   9. IANA Considerations

   This document defines four new ICMPv6 message types.  These messages
   must be assigned ICMPv6 type numbers from the informational message
   range in http://www.iana.org/assignments/icmpv6-parameters:



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    1. Router Solicitation for Proxy Advertisement, described in
       Section 6.1.1

    2. Proxy Router Advertisement, described in Section 6.1.2

    3. Handover Initiate, described in Section 6.2.1, and

    4. Handover Acknowledge, described in Section 6.2.2

   The messages above constitute the FMIPv6 ICMPv6 messages.  This
   document also defines three new options for the above FMIPv6
   ICMPv6 messages.  These options must be assigned Option Format Type
   numbers from a new FMIPv6 ICMPv6 Option Format section of the ICMPv6
   Parameters registry.  This new section is as follows:

   FMIPv6 ICMPv6 OPTION FORMATS

   The FMIPv6 ICMPv6 messages have options that are identified by an
   Option Format Type field in [draft-ietf-mipshop-fast-mipv6-01.txt],
   which is referred to as FMIPv6 in the following table.

      Type  Description              Reference
      ----  -----------              ---------
      TBA   IP Address Option        FMIPv6
      TBA   New Router Prefix
            Information Option       FMIPv6
      TBA   Link-layer Address
            Option                   FMIPv6



   The document defines a new Neighbor Discovery option which must be
   assigned Option Type value within the option numbering space for
   Neighbor Discovery messages:

    1. Neighbor Advertisement Acknowledgment Option

   The document defines three new Mobility Header messages which
   need type allocation from the Mobility Header Types registry at
   http://www.iana.org/assignments/mobility-header-types:

    1. Fast Binding Update, described in Section 6.3.1

    2. Fast Binding Acknowledgment, described in Section 6.3.2, and

    3. Fast Neighbor Advertisement, described in Section 6.3.3






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   10. Contributors

   This document has its origins in the fast handover design team
   effort.  The members of this design team in alphabetical order were;
   Gopal Dommety, Karim El-Malki, Mohammed Khalil, Charles Perkins,
   Hesham Soliman, George Tsirtsis and Alper Yegin.


   11. Acknowledgments

   The editor would like to thank all those who have provided feedback
   on this specification, but can only mention a few here:  Vijay
   Devarapalli, Youn-Hee Han, Suvidh Mathur, Gabriel Montenegro,
   Takeshi Ogawa, Sun Peng, and Jonathan Wood.  The editor would
   like to acknowledge contribution from James Kempf to improve this
   specification.  The editor would also like to thank the erstwhile
   [mobile ip] working group chairs Basavaraj Patil and Phil Roberts for
   providing much support for this work.


   12. Normative References



   [1]  S. Bradner, ``Key words for use in RFCs to Indicate Requirement
        Levels,'' Request for Comments (Best Current Practice) 2119,
        Internet Engineering Task Force, March 1997.

   [2]  A. Conta and S. Deering, ``Internet Control Message
        Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6)
        Specification'', "Request for Comments (Draft Standard) 2463,
        Internet Engineering Task Force, December 1998.

   [3]  D. Johnson, C. E. Perkins, and J. Arkko, ``Mobility Support in
        IPv6 (work in progress)'', Internet Engineering Task Force, June
        2003.

   [4]  T. Narten, E. Nordmark, and W. Simpson, ``Neighbor Discovery for
        IP Version 6 (IPv6)'', Request for Comments (Draft Standard)
        2461, Internet Engineering Task Force, December 1998.

   [5]  S. Thomson and T. Narten, ``IPv6 Stateless Address
        Autoconfiguration'', Request for Comments (Draft Standard) 2462,
        Internet Engineering Task Force, December 1998.








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   13. Informative References



   [6]  M. Khalil, R. Narayanan, H. Akhtar, and E. Qaddoura, ``Mobile IP
        Extensions Rationalization (MIER) (work in progress)'', Internet
        Engineering Task Force, February 2000.


   A. Change Log

   The following changes have taken place since the previous version.
   The Section numbers refer to version 06.

    -  Last Call comments incorporated

    -  Revised the security considerations section in v07

    -  Refined and added a section on network-initiated handover v07

    -  Section 3 format change

    -  Section 4 format change (i.e., no subsections).

    -  Description in Section 4.4 merged with ``Fast or Erroneous
       Movement''

    -  Section 4.5 deprecated

    -  Section 4.6 deprecated

    -  Revision of some message formats in Section 6


   B. Contact Information

   The design team member's contact information:


     Gopal Dommety
     Cisco Systems, Inc.
     170 West Tasman Drive
     San Jose, CA 95134
     Phone:+1 408 525 1404
     E-Mail: gdommety@cisco.com

     Karim El Malki
     Ericsson Radio Systems AB
     LM Ericssons Vag. 8



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     126 25 Stockholm
     SWEDEN
     Phone:  +46 8 7195803
     Fax:    +46 8 7190170
     E-mail: Karim.El-Malki@era.ericsson.se

     Mohamed Khalil
     Nortel Networks
     E-Mail: mkhalil@nortelnetworks.com

     Charles E. Perkins
     Communications Systems Lab
     Nokia Research Center
     313 Fairchild Drive
     Mountain View, California 94043
     USA
     Phone:  +1-650 625-2986
     E-Mail:  charliep@iprg.nokia.com
     Fax:  +1 650 625-2502

     Hesham Soliman
     Flarion Technologies
     E-mail: H.Soliman@flarion.com

     George Tsirtsis
     Flarion Technologies
     E-Mail: G.Tsirtsis@flarion.com

     Alper E. Yegin
     NTT DoCoMo Labs USA
     E-mail: alper@docomolabs-usa.com


   The editor's contact information:

     Rajeev Koodli
     Nokia Research Center
     313 Fairchild Drive
     Mountain View, CA 94043 USA
     Phone: +1 650 625 2359
     Fax: +1 650 625 2502
     E-Mail: Rajeev.Koodli@nokia.com










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