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RFC 5268 - Mobile IPv6 Fast Handovers


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Network Working Group                                     R. Koodli, Ed.
Request for Comments: 5268                              Starent Networks
Obsoletes: 4068                                                June 2008
Category: Standards Track

                       Mobile IPv6 Fast Handovers

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   Mobile IPv6 enables a Mobile Node (MN) to maintain its connectivity
   to the Internet when moving from one Access Router to another, a
   process referred to as handover.  During handover, there is a period
   during which the Mobile Node is unable to send or receive packets
   because of 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 (VoIP).  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.

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................3
   3. Protocol Overview ...............................................6
      3.1. Addressing the Handover Latency ............................6
      3.2. Protocol Operation .........................................8
      3.3. Protocol Operation during Network-Initiated Handover ......11
   4. Protocol Details ...............................................11
   5. Other Considerations ...........................................15
      5.1. Handover Capability Exchange ..............................15
      5.2. Determining New Care-of Address ...........................16
      5.3. Prefix Management .........................................16
      5.4. Packet Loss ...............................................17
      5.5. DAD Handling ..............................................18
      5.6. Fast or Erroneous Movement ................................19
   6. Message Formats ................................................20
      6.1. New Neighborhood Discovery Messages .......................20
           6.1.1. Router Solicitation for Proxy Advertisement
                  (RtSolPr) ..........................................20
           6.1.2. Proxy Router Advertisement (PrRtAdv) ...............22
      6.2. Inter - Access Router Messages ............................25
           6.2.1. Handover Initiate (HI) .............................25
           6.2.2. Handover Acknowledge (HAck) ........................27
      6.3. New Mobility Header Messages ..............................28
           6.3.1. Fast Binding Update (FBU) ..........................28
           6.3.2. Fast Binding Acknowledgment (FBack) ................30
      6.4. Unsolicited Neighbor Advertisement (UNA) ..................31
      6.5. New Options ...............................................32
           6.5.1. IP Address/Prefix Option ...........................33
           6.5.2. Link-Layer Address (LLA) Option ....................34
           6.5.3. Mobility Header Link-Layer Address (MH-LLA)
                  Option .............................................35
           6.5.4. Binding Authorization Data for FMIPv6 (BADF) .......35
           6.5.5. Neighbor Advertisement Acknowledgment (NAACK) ......36
   7. Related Protocol and Device Considerations .....................37
   8. Evolution from and Compatibility with RFC 4068 .................38
   9. Configurable Parameters ........................................39
   10. Security Considerations .......................................39
      10.1. Peer Authorization Database Entries when Using IKEv2 .....41
      10.2. Security Policy Database Entries .........................42
   11. IANA Considerations ...........................................42
   12. Acknowledgments ...............................................43
   13. References ....................................................44
      13.1. Normative References .....................................44
      13.2. Informative References ...................................45
   Appendix A. Contributors ..........................................46
   Appendix B. Changes since RFC 4068 ................................46

1.  Introduction

   Mobile IPv6 [RFC3775] 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
   link-layer procedures, 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 problems: 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 interwork with Mobile IP.
   Once attached to its new access router, an MN engages in Mobile IP
   operations including Return Routability [RFC3775].  There are no
   special requirements for a mobile node to behave differently with
   respect to its standard Mobile IP operations.

   This specification is applicable when a mobile node has to perform IP
   layer operations as a result of handovers.  This specification does
   not address improving the link switching latency.  It does not modify
   or optimize procedures related to signaling with the home agent of a
   mobile node.  Indeed, while targeted for Mobile IPv6, it could be
   used with any mechanism that allows communication to continue despite
   movements.  Finally, this specification does not address bulk
   movement of nodes using aggregate prefixes.

2.  Terminology

   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 RFC 2119 [RFC2119].
   The use of the term, "silently ignore" is not defined in RFC 2119.
   However, the term is used in this document and can be similarly
   construed.

   The following terminology and abbreviations are used in this document
   in addition to those defined in [RFC3775].  The reference handover
   scenario is illustrated in Figure 1.

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

          Figure 1: Reference Scenario for Handover

   Mobile Node (MN): A Mobile IPv6 host.

   Access Point (AP): A Layer 2 device connected to an IP subnet that
   offers wireless connectivity to an MN.  An Access Point Identifier
   (AP-ID) refers the AP's L2 address.  Sometimes, AP-ID is also
   referred to as a Basic Service Set IDentifier (BSSID).

   Access Router (AR): The MN's default router.

   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's subnet.

   New CoA (NCoA): The MN's Care-of Address valid on NAR's subnet.

   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 provides information about neighboring links facilitating
   expedited movement detection.  The message can also act 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.  The triplet [Router's L2
   address, Router's IP address, and Prefix] is called "AR-Info".  See
   Section 5.3.

   Neighborhood Discovery: The process of resolving neighborhood AP-IDs
   to AR-Info.

   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 (toward NAR).

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

   Predictive Fast Handover: The fast handover in which an MN is able to
   send an FBU when it is attached to the PAR, which then establishes
   forwarding for its traffic (even before the MN attaches to the NAR).

   Reactive Fast Handover: The fast handover in which an MN is able to
   send the FBU only after attaching to the NAR.

   Unsolicited Neighbor Advertisement (UNA): The message in [RFC4861]
   with 'O' bit cleared.

   Fast Neighbor Advertisement (FNA): This message from RFC 4068
   [RFC4068] is deprecated.  The UNA message above is the preferred
   message in this specification.

   Handover Initiate (HI): A message from the PAR to the NAR regarding
   an MN's handover.

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

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 an 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 process the Binding Update successfully,
   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 an MN to quickly detect that it has moved to a
   new subnet by providing the new access point and the associated
   subnet prefix information when the MN is still connected to its
   current subnet (i.e., PAR in Figure 1).  For instance, an MN may
   discover available access points using link-layer specific mechanisms
   (e.g., a "scan" in Wireless Local Area Network (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 or at any time while connected to its current router.  The
   result of resolving an identifier associated with an access point is
   a [AP-ID, AR-Info] tuple, which an 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 coordinates including its
   prefix, IP address, and L2 address.  The "Router Solicitation for
   Proxy Advertisement (RtSolPr)" and "Proxy Router Advertisement
   (PrRtAdv)" messages in Section 6.1 are used for aiding movement
   detection.

   Through the RtSolPr and PrRtAdv messages, the MN also formulates a
   prospective new CoA (NCoA) when it is 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)" (see Section 6.3.2) 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 an unsolicited
   Neighbor Advertisement message (with the 'O' bit set to zero)
   [RFC4861]; NAR responds to this UNA message in case it wishes to
   provide a different IP address to use.  In this way, NCoA
   configuration latency is reduced.

   The information provided in the PrRtAdv message can be used even when
   DHCP [RFC3315] is used to configure an NCoA on the NAR's link.  In
   this case, the protocol supports forwarding using PCoA, and the MN
   performs DHCP once it attaches to the NAR's link.  The MN still
   formulates an NCoA for FBU processing; however, it MUST NOT send data
   packets using the NCoA in the FBU.

   In order to reduce the Binding Update latency, the protocol specifies
   a binding between the Previous CoA (PCoA) and NCoA.  An MN sends a
   "Fast Binding Update" (see Section 6.3.1) message to its Previous
   Access Router to establish this tunnel.  When feasible, the MN SHOULD
   send an FBU from the PAR's link.  Otherwise, the MN should send the
   FBU immediately after detecting attachment to the NAR.  An FBU
   message MUST contain the Binding Authorization Data for FMIPv6 (BADF)
   option (see Section 6.5.4) in order to ensure that only a legitimate
   MN that owns the PCoA is able to establish a binding.  Subsequent
   sections describe the protocol mechanics.  In any case, the result is
   that the 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 the PAR, again until it completes
   Binding Update.  And, PAR MUST 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 a PCoA as a source IP address
   are not dropped due to ingress filtering.  Even though the MN is
   IP-capable on the new link, it cannot use the NCoA directly with its
   correspondents without the correspondents first establishing a
   binding cache entry (for the NCoA).  Forwarding support for the 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 it is attached to a new subnet link, unless the
   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 the NAR to send Neighbor
   Solicitation before the MN attaches, subsequent retransmissions of
   address resolution are separated by a default period of one second
   each.  In order to circumvent this delay, an MN announces its
   attachment immediately with an UNA message that allows the NAR to
   forward packets to the MN right away.  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 an MN sends an FBU from the

   PAR's link, FBack can be delivered after the NAR considers the NCoA
   acceptable for use.  This is especially useful when addresses are
   assigned by the access router.  The NAR can also rely on its trust
   relationship with the PAR before providing forwarding support for the
   MN.  That is, it may create a forwarding entry for the NCoA, subject
   to "approval" from the PAR, which it trusts.  In addition, buffering
   for handover traffic at the NAR may be desirable.  Even though the
   Neighbor Discovery protocol provides a small buffer (typically one or
   two packets) for packets awaiting address resolution, this buffer may
   be inadequate for traffic, such as VoIP, already in progress.  The
   routers may also wish to maintain a separate buffer for servicing the
   handover traffic.  Finally, the access routers could transfer
   network-resident contexts, such as access control, Quality of Service
   (QoS), and header compression, in conjunction with handover (although
   the context transfer process itself is not specified in this
   document).  For all these operations, the protocol provides "Handover
   Initiate (HI)" and "Handover Acknowledge (HAck)" messages (see
   Section 6.2).  Both of these messages SHOULD be used.  The access
   routers MUST have the necessary security association established by
   means outside the scope of this document.

3.2.  Protocol Operation

   The protocol begins when an MN sends an RtSolPr message 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 containing one or more
   [AP-ID, AR-Info] tuples.  The MN may send an 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 an RtSolPr, the MN
   will have discovered the available APs by link-specific methods.  The
   RtSolPr and PrRtAdv messages do not establish any state at the access
   router; 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 to the PAR.
   The purpose of the FBU is to authorize the PAR to bind the PCoA to
   the NCoA, so that arriving packets can be tunneled to the new
   location of the MN.  The FBU should be sent from the 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, the FBU is sent from the new link.

   The format and semantics of FBU processing are specified in Section
   6.3.1.  The FBU message MUST contain the BADF option (see Section
   6.5.4) to secure the message.

   Depending on whether an FBack is received on the previous link (which
   clearly depends on whether the 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 is already in progress by the time the MN
          handovers to the NAR.  The MN SHOULD send the UNA immediately
          after attaching to the NAR, so that arriving as well as
          buffered packets can be forwarded to the MN right away.
          Before sending FBack to the MN, the PAR can determine whether
          the NCoA is acceptable to the 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 the
          FBU is carried in an HI message (from PAR to NAR), and NAR MAY
          assign the proposed NCoA.  Such an assigned NCoA MUST be
          returned in HAck (from NAR to PAR), 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 the FBack on the previous link because
          the MN has not sent the FBU or 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 the PAR has processed
          the FBU successfully.  Hence, the MN (re)sends the FBU message
          to the PAR immediately after sending the UNA message.  If the
          NAR chooses to supply a different IP address to use than the
          NCoA, it MAY send a Router Advertisement with "Neighbor
          Advertisement Acknowledge (NAACK)" option in which it includes
          an alternate IP address for the MN to use.  Detailed UNA
          processing rules are specified in Section 6.4.

   The scenario in which an MN sends an FBU and receives an FBack on
   PAR's link is illustrated in Figure 2.  For convenience, this
   scenario is characterized as the "predictive" mode of operation.  The
   scenario in which the MN sends an FBU from the NAR's link is
   illustrated in Figure 3.  For convenience, this scenario is
   characterized as the "reactive" mode of operation.  Note that the
   reactive mode also includes the case in which an FBU has been sent
   from the PAR's link but an FBack has not yet been received.  The
   figure is intended to illustrate that the FBU is forwarded through
   the NAR, but it is processed only by the PAR.

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

            Figure 2: Predictive Fast Handover

     MN                    PAR                    NAR
      |                     |                      |
      |------RtSolPr------->|                      |
      |<-----PrRtAdv--------|                      |
      |                     |                      |
   disconnect               |                      |
      |                     |                      |
      |                     |                      |
   connect                  |                      |
      |-------UNA-----------|--------------------->|
      |-------FBU-----------|---------------------)|
      |                     |<-------FBU----------)|
      |                     |----------HI--------->|
      |                     |<-------HAck----------|
      |                     |(HI/HAck if necessary)|
      |                   forward                  |
      |              packets(including FBAck)=====>|
      |                     |                      |
      |<=================================== deliver packets
      |                                            |

            Figure 3: Reactive Fast Handover

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

3.3.  Protocol Operation during Network-Initiated Handover

   In some wireless technologies, the handover control may reside in the
   network even though the decision to undergo handover may be mutually
   arrived at between the MN and the network.  In such networks, the PAR
   can send an unsolicited PrRtAdv containing the link-layer address, IP
   address, and subnet prefix 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 the
   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
   topology map of links and subnets.  Such usage of PrRtAdv is
   decoupled from the actual handover; see Section 6.1.2.

4.  Protocol Details

   All descriptions refer to Figure 1.

   After discovering one or more nearby access points, the MN sends
   RtSolPr to the PAR 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 can send RtSolPr at any
   time, e.g., when one or more new access points are discovered.  The
   MN can also send RtSolPr more than once during its attachment to PAR.
   The trigger for sending RtSolPr can originate from a link-specific
   event, such as the promise of a 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., UNA) can be transmitted immediately.  Implementations SHOULD
   make use of such triggers whenever available.

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

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

      1.  If the PAR does not have an entry corresponding to the new
          access point, it MUST respond indicating that the new access
          point 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 access point is connected to the PAR's current
          interface (to which MN is attached), the PAR MUST respond with
          a Code value indicating that the new access point is connected
          to the current interface, but not send any prefix information.
          This scenario could arise, for example, when several wireless
          access points are bridged into a wired network.  No further
          protocol action is necessary.

      3.  If the new access point is known and the PAR has information
          about it, then the PAR MUST respond indicating that the new
          access point is known and supply the [AP-ID, AR-Info] tuple.
          If the new access point 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 access
          point, the PAR SHOULD supply neighborhood [AP-ID, AR-Info]
          tuples that are subject to path MTU restrictions (i.e.,
          provide any 'n' tuples without exceeding the link MTU).

   When further protocol action is necessary, some implementations MAY
   choose to begin buffering copies of incoming packets at the PAR.  If
   such First in First Out (FIFO) buffering is used, the PAR MUST
   continue forwarding the packets to the PCoA (i.e., buffer and
   forward).  While the protocol does not forbid such an implementation
   support, care must be taken to ensure that the PAR continues
   forwarding packets to the PCoA (i.e., uses a buffer and forward
   approach).  The PAR SHOULD stop buffering once it begins forwarding
   packets to the NCoA.

   The method by which access routers exchange information about their
   neighbors and thereby allow construction of Proxy Router
   Advertisements with information about neighboring subnets is outside
   the scope of this document.

   The RtSolPr and PrRtAdv messages MUST be implemented by an 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 the above messages is optional on such
   links.

   After a PrRtAdv message is processed, the MN sends an FBU at a time
   determined by link-specific events, and includes the proposed NCoA.
   The MN SHOULD send the FBU from the PAR's link whenever
   "anticipation" of handover is feasible.  When anticipation is not
   feasible or when it has not received an FBack, the MN sends an FBU
   immediately after attaching to NAR's link.  In response to the FBU,
   the PAR establishes a binding between the PCoA ("Home Address") and
   the NCoA, and sends the FBack to the MN.  Prior to establishing this
   binding, the PAR SHOULD send an HI message to the NAR, and receive
   HAck in response.  In order to determine the NAR's address for the HI
   message, the PAR can perform the longest prefix match of NCoA (in
   FBU) with the prefix list of neighboring access routers.  When the
   source IP address of the FBU is the 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 the 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 an HI message with Code 0, the
   NAR:

      1.  determines whether the NCoA supplied in the HI message is
          unique before beginning to defend it.  It sends a Duplicate
          Address Detection (DAD) probe [RFC4862] for NCoA to verify
          uniqueness.  However, in deployments where the probability of
          address collisions is considered extremely low (and hence not
          an issue), the parameter DupAddrDetectTransmits (see
          [RFC4862]) is set to zero on the NAR, allowing it to avoid
          performing DAD on the NCoA.  The NAR similarly sets
          DupAddrDetectTransmits to zero in other deployments where DAD
          is not a concern.  Once the NCoA is determined to be unique,
          the NAR starts proxying [RFC4861] the address for
          PROXY_ND_LIFETIME during which the MN is expected to connect
          to the NAR.  In case there is already an NCoA present in its
          data structure (for instance, it has already processed an HI
          message earlier), the NAR MAY verify if the LLA is the same as
          its own or that of the MN itself.  If so, the NAR MAY allow
          the use of the NCoA.

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

      3.  MAY create a host route entry for the PCoA (on the interface
          to which the MN is attaching to) in case the 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 SHOULD also set up a reverse tunnel to the PAR in this
          case.

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

   When the Code value in HI is 1, the NAR MUST skip the above
   operations.  Sending an HI message with Code 1 allows the NAR to
   validate the neighbor cache entry it creates for the MN during UNA
   processing.  That is, the NAR can make use of the knowledge that its
   trusted peer (i.e., the PAR) has a trust relationship with the MN.

   If HAck contains an assigned NCoA, the FBack MUST include it, and the
   MN MUST use the address provided in the FBack.  The PAR MAY send the
   FBack to the previous link as well to facilitate faster reception in
   the event that the MN is still present.  The result of the FBU and
   FBack processing is that PAR begins tunneling the MN's packets to the
   NCoA.  If the MN does not receive an FBack message even after
   retransmitting the 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:

      1.  sends an UNA message (see Section 6.4).  If the MN has not
          received an FBack by the time UNA is being sent, it SHOULD
          send an FBU message following the UNA message.

      2.  joins the all-nodes multicast group and the solicited-node
          multicast group corresponding to the NCoA.

      3.  starts a DAD probe for NCoA, see [RFC4862].

   When a NAR receives an UNA message, it:

      1.  deletes its proxy neighbor cache entry, if it exists, updates
          the state to STALE [RFC4861], and forwards arriving and
          buffered packets.

      2.  updates an entry in INCOMPLETE state [RFC4861], if it exists,
          to STALE and forwards arriving and buffered packets.  This
          would be the case if NAR had previously sent a Neighbor
          Solicitation that went unanswered perhaps because the MN had
          not yet attached to the link.

   The buffer for handover traffic should be linked to this UNA
   processing.  The exact mechanism is implementation dependent.

   The NAR may choose to provide a different IP address other than the
   NCoA.  This is possible if it is proxying the NCoA.  In such a case,
   it:

      1.  MAY send a Router Advertisement with the NAACK option in which
          it includes an alternate IP address for use.  This message
          MUST be sent to the source IP address present in UNA using the
          same Layer 2 address present in UNA.

   If the MN receives an IP address in the NAACK option, it MUST use it
   and send an FBU using the new CoA.  As a special case, the address
   supplied in NAACK could be the PCoA itself, in which case the MN MUST
   NOT send any more FBUs.  The Status codes for the NAACK option are
   specified in Section 6.5.5.

   Once the MN has confirmed its NCoA (either through DAD or when
   provided for by the NAR), it SHOULD send a Neighbor Advertisement
   message with the 'O' bit set, to the all-nodes multicast address.
   This message allows MN's neighbors to update their neighbor cache
   entries.

   For data forwarding, the PAR tunnels packets 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 the
   PAR receives a reverse tunneled packet, it must verify if a secure
   binding exists for the MN identified by the PCoA in the tunneled
   packet, before forwarding the packet.

5.  Other Considerations

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 the PAR does not support the fast handover protocol
   and stop sending any more RtSolPr messages.

   Even if an MN's current access router is capable of providing fast
   handover support, 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 the FBU.  The PAR
   MUST use the NCoA present in the FBU in its HI message.  The NAR MUST
   verify if the NCoA present in HI is already in use.  In any case, the
   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, the PAR MUST include it in the
   FBack message.  However, the MN itself does not have to wait on PAR's
   link for this exchange to take place.  It can handover any time after
   sending the FBU message; sometimes it may be forced to handover
   without sending the FBU.  In any case, it can still confirm using
   NCoA from NAR's link by sending the UNA message.

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

   When DHCP is used, the protocol supports forwarding for PCoA only.
   In this case, the MN MUST perform DHCP operations once it attaches to
   the NAR even though it formulates an NCoA for transmitting the FBU.
   This is indicated in the PrRtAdv message with Code = 5.

5.3.  Prefix Management

   As defined in Section 2, the Prefix part of "AR-Info" is the prefix
   valid on the interface to which the AP is attached.  This document
   does not specify how this Prefix is managed, it's length and
   assignment policies.  The protocol operation specified in this
   document works regardless of these considerations.  Often, but not
   necessarily always, this Prefix may be the aggregate prefix (such as
   /48) valid on the interface.  In some deployments, each MN may have
   its own per-mobile prefix (such as a /64) used for generating the
   NCoA.  Some point-to-point links may use such a deployment.

   When per-mobile prefix assignment is used, the "AR-Info" advertised
   in PrRtAdv still includes the (aggregate) prefix valid on the
   interface to which the target AP is attached, unless the access
   routers communicate with each other (using HI and HAck messages) to

   manage the per-mobile prefix.  The MN still formulates an NCoA using
   the aggregate prefix.  However, an alternate NCoA based on the
   per-mobile prefix is returned by NAR in the HAck message.  This
   alternate NCoA is provided to the MN in either the FBack message or
   in the NAACK option.

5.4.  Packet Loss

   Handover involves link switching, which may not be exactly
   coordinated 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 the 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 the NAR and then sends an
   FBU message, packets arriving at the PAR until the FBU is processed
   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.

   Whereas buffering can enable a smooth handover, the buffer size and
   the rate at which buffered packets are eventually forwarded are
   important considerations when providing buffering support.  There are
   a number of aspects to consider:

   o  Some applications transmit less data over a given period of data
      than others, and this implies different buffering requirements.
      For instance, Voice over IP typically needs smaller buffers
      compared to high-resolution streaming video, as the latter has
      larger packet sizes and higher arrival rates.

   o  When the mobile node appears on the new link, having the buffering
      router send a large number of packets in quick succession may
      overtax the resources of the router, the mobile node itself, or
      the path between these two.

      In particular, transmitting a large amount of buffered packets in
      succession can congest the path between the buffering router and
      the mobile node.  Furthermore, nodes (such as a base station) on
      the path between the buffering router and the mobile node may drop
      such packets.  If a base station buffers too many such packets,
      they may contribute to additional jitter for packets arriving
      behind them, which is undesirable for real-time communication.

   o  Since routers are not involved in end-to-end communication, they
      have no knowledge of transport conditions.

   o  The wireless connectivity of the mobile node may vary over time.
      It may achieve a smaller or higher bandwidth on the new link,
      signal strength may be weak at the time it just enters the area of
      this access point, and so on.

   As a result, it is difficult to design an algorithm that would
   transmit buffered packets at appropriate spacing under all scenarios.
   The purpose of fast handovers is to avoid packet loss.  Yet, draining
   buffered packets too fast can, by itself, cause loss of the packets,
   as well as blocking or loss of following packets meant for the mobile
   node.

   This specification does not restrict implementations from providing
   specialized buffering support for any specific situation.  However,
   attention must be paid to the rate at which buffered packets are
   forwarded to the MN once attachment is complete.  Routers
   implementing this specification MUST implement at least the default
   algorithm, which is based on the original arrival rates of the
   buffered packets.  A maximum of 5 packets MAY be sent one after
   another, but all subsequent packets SHOULD use a sending rate that is
   determined by metering the rate at which packets have entered the
   buffer, potentially using smoothing techniques such as recent
   activity over a sliding time window and weighted averages [RFC3290].

   It should be noted, however, that this default algorithm is crude and
   may not be suitable for all situations.  Future revisions of this
   specification may provide additional algorithms, once enough
   experience of the various conditions in deployed networks is
   attained.

5.5.  DAD Handling

   Duplicate Address Detection (DAD) was defined in [RFC4862] to avoid
   address duplication on links when stateless address
   auto-configuration is used.  The use of DAD to verify the uniqueness
   of an IPv6 address configured through stateless auto-configuration
   adds delays to a handover.  The probability of an interface
   identifier duplication on the same subnet is very low; however, it
   cannot be ignored.  Hence, the protocol specified in this document
   SHOULD only be used in deployments where the probability of such
   address collisions is extremely low or it is not a concern (because
   of the address management procedure deployed).  The protocol requires
   the NAR to send a DAD probe before it starts defending the NCoA.
   However, this DAD delay can be turned off by setting
   DupAddrDetectTransmits to zero on the NAR [RFC4862].

   This document specifies messages that can be used to provide
   duplicate-free addresses, but the document does not specify how to
   create or manage such duplicate-free addresses.  In some cases, the
   NAR may already have the knowledge required to assess whether or not
   the MN's address is a duplicate before the MN moves to the new
   subnet.  For example, in some deployments, the NAR may maintain a
   pool of duplicate-free addresses in a list for handover purposes.  In
   such cases, the NAR can provide this disposition in the HAck message
   (see Section 6.2.2) or in the NAACK option (see Section 6.5.5).

5.6.  Fast or Erroneous Movement

   Although this specification is for fast handover, the protocol is
   limited in terms of how fast an MN can move.  A special case of fast
   movement is ping-pong, where an 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 but it either moves 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
   resolved 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 Home Agent and preferably that of every CN
   with which it is in communication.  An 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 cost over
   the air interface 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.

   An MN returning to the PAR before updating the necessary bindings
   when present on the NAR MUST send a Fast Binding Update with the 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 to the NAR.  The PAR, upon receiving this
   Fast Binding Update, will check its set of outgoing (temporary fast
   handover) tunnels.  If it finds a match, it SHOULD terminate that
   tunnel; i.e., start delivering packets directly to the node instead.
   In order for the PAR to process such an FBU, the lifetime of the
   security association has to be at least that of the tunnel itself.

   Temporary tunnels for the purposes of fast handovers should use short
   lifetimes (of the order of at most a few tens of seconds or less).
   The lifetime of such tunnels should be enough to allow an 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 the loss of
   packets since routing can change and the PAR may forward packets
   toward another router before the MN actually connects to that router.
   If the MN discovers itself on an unanticipated access router, it
   SHOULD send a new Fast Binding Update to the PAR.  This FBU
   supersedes the existing binding at the PAR, and the packets will be
   redirected to the newly confirmed location of the MN.

6.  Message Formats

   All the ICMPv6 messages have a common Type specified in [RFC4443].
   The messages are distinguished based on the Subtype field (see
   below).  For all the ICMPv6 messages, the checksum is defined in
   [RFC4443].

6.1.  New Neighborhood 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.  All the
   Link-Layer Address options have the format defined in Section 6.5.2.

    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          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Subtype    |    Reserved   |            Identifier         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Options ...
   +-+-+-+-+-+-+-+-+-+-+-+-

   Figure 4: Router Solicitation for Proxy Advertisement (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.

   ICMP Fields:

      Type: 154

      Code: 0

      Checksum: The ICMPv6 checksum.

      Subtype: 2

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

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

   Valid Options:

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

      New Access Point Link-Layer Address: The link-layer address or
      identification of the access 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.  See the LLA
      option below.

   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 can be avoided when
   the receiver needs to send packets back to the sender (of the RtSolPr
   message).

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

   A Proxy Router Advertisement (PrRtAdv) message should be received by
   the MN in response to an RtSolPr.  If such a message is not received
   in a timely manner (no less than twice the typical round trip time
   (RTT) over the access link or 100 milliseconds if RTT is not known),
   it SHOULD resend the RtSolPr message.  Subsequent retransmissions can
   be up to RTSOLPR_RETRIES, but MUST use an exponential backoff in
   which the timeout period (i.e., 2xRTT or 100 milliseconds) is doubled
   prior to each instance of retransmission.  If Proxy Router
   Advertisement is not received by the time the MN disconnects from the
   PAR, the MN SHOULD send an FBU immediately after configuring a new
   CoA.

   When RtSolPr messages are sent more than once, they MUST be rate
   limited with MAX_RTSOLPR_RATE per second.  During each use of an
   RtSolPr, exponential backoff is used for retransmissions.

6.1.2.  Proxy Router Advertisement (PrRtAdv)

   Access routers send Proxy Router Advertisement messages gratuitously
   if the handover is network-initiated or as a response to an RtSolPr
   message from an 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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Subtype    |    Reserved   |           Identifier          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    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.

   ICMP Fields:

      Type: 154

      Code: 0, 1, 2, 3, 4, or 5.  See below.

      Checksum: The ICMPv6 checksum.

      Subtype: 3

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

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

   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 the LLA option format below.

      New Access Point Link-Layer Address: The link-layer address or
      identification of the access 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 the Code is 0 or 1.

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

      New Router Prefix Information Option: Specifies the prefix of the
      access router the message is proxied for and is used for address
      auto-configuration.  This option MUST be included when the Code is
      0 or 1.  However, when this prefix is the same as what is used in
      the New Router's IP Address option (above), the Prefix Information
      option need not be present.

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

   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, 4, and 5 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 Option-Code field in
   the New Access Point LLA option in this case is 1 reflecting the LLA
   of the access 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 has
   been sent unsolicited.  If a New CoA option is present following the
   New Router Prefix Information option, the MN MUST use the supplied
   NCoA and send an FBU immediately or else stand to lose service.  This
   message acts as a network-initiated handover trigger; see Section
   3.3.  The Option-Code field in the New Access Point LLA option (see
   below) in this case is 1 reflecting the LLA of the access 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.  Each New Access Point LLA option contains an
   Option-Code value (described below) that indicates a specific
   outcome.

      When the Option-Code field in the New Access Point LLA option is
      5, handover to that access point does not require a change of CoA.
      This would be the case, for instance, when a number of access
      points are connected to the same router interface, or when network
      based mobility management mechanisms ensure that the specific
      mobile node always observes the same prefix regardless of whether
      there is a separate router attached to the target access point.
      No other options are required in this case.

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

      When the Option-Code field in the New Access 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 only present for a subset of access points requested.
   The Option-Code field values (defined above including a value of 1)
   distinguish different outcomes for individual access points.

   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.

   A Proxy Router Advertisement with Code 5 means that the MN may use
   the new router information present for detecting movement to a new
   subnet, but the MN must perform DHCP [RFC3315] upon attaching to the
   NAR's link.  The PAR and NAR will forward packets to the PCoA of the
   MN.  The MN must still formulate an NCoA for transmitting FBU (using
   the information sent in this message), but that NCoA will not be used
   for forwarding packets.

   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 an 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              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Subtype    |S|U| Reserved  |           Identifier          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    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

   ICMP Fields:

      Type: 154

      Code: 0 or 1.  See below

      Checksum: The ICMPv6 checksum.

      Subtype: 4

      'S' flag: 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' flag: Buffer flag.  When set, the destination SHOULD buffer
      any packets toward 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.

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

   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 a host route can be established if necessary.

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

   The PAR uses a Code value of 0 when it processes an FBU with PCoA as
   source IP address.  The PAR uses a Code value of 1 when it processes
   an FBU whose source IP address is not PCoA.

   If a Handover Acknowledge (HAck) message is not received as a
   response in a short time period (no less than twice the typical round
   trip time (RTT) between source and destination, or 100 milliseconds
   if RTT is not known), the Handover Initiate SHOULD be resent.
   Subsequent retransmissions can be up to HI_RETRIES, but MUST use
   exponential backoff in which the timeout period (i.e., 2xRTT or 100
   milliseconds) is doubled during each instance of retransmission.

6.2.2.  Handover Acknowledge (HAck)

   The Handover Acknowledgment message is a new ICMPv6 message that MUST
   be sent (typically by the NAR to the 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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Subtype    |     Reserved  |           Identifier          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    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.

   ICMP Fields:

      Type: 154

      Code:

         0: Handover Accepted, NCoA valid
         1: Handover Accepted, NCoA not valid or in use
         2: Handover Accepted, NCoA assigned (used in Assigned
         addressing)
         3: Handover Accepted, use PCoA
         4: Message sent unsolicited, usually to trigger an HI message
         128: Handover Not Accepted, reason unspecified
         129: Administratively prohibited
         130: Insufficient resources

      Checksum: The ICMPv6 checksum.

      Subtype: 5

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

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

   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 the 'S' bit is not set, e.g., Code 2 above.

      Upon receiving an 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 the PCoA (instead of accepting or
      assigning an NCoA), establish a host route entry for the PCoA, and
      set up a tunnel to the PAR to forward the MN's packets sent with
      the PCoA as a 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.  The NAR indicates forwarding support
      for PCoA using Code value 3 in the HAck message.  Subsequently,
      the PAR establishes a tunnel to the NAR in order to forward
      packets arriving for the PCoA.

      When responding to an 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
   [RFC3775].  The Fast Binding Update, Fast Binding Acknowledgment, and
   the (deprecated) Fast Neighbor Advertisement messages use the
   Mobility Header.

6.3.1.  Fast Binding Update (FBU)

   The Fast Binding Update message has a Mobility Header Type value of
   8.  The FBU is identical to the Mobile IPv6 Binding Update (BU)
   message.  However, the processing rules are slightly different.

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

      'A' flag: MUST be set to one to request that PAR send a Fast
      Binding Acknowledgment message.

      'H' flag: MUST be set to one.  See [RFC3775].

      'L' flag: See [RFC3775].

      'K' flag: See [RFC3775].

      Reserved: This field is unused.  MUST be set to zero.

      Sequence Number: See [RFC3775].

      Lifetime: The requested time in seconds for which the sender
      wishes to have a binding.

      Mobility Options: MUST contain an alternate CoA option set to the
      NCoA when an FBU is sent from the PAR's link.  MUST contain the
      Binding Authorization Data for the FMIP (BADF) option.  See
      Section 6.5.4.  MAY contain the Mobility Header LLA option (see
      Section 6.5.3).

   The MN sends an FBU message any time after receiving a PrRtAdv
   message.  If the MN moves prior to receiving a PrRtAdv message, it
   SHOULD send an FBU to the PAR after configuring the NCoA on the NAR
   according to Neighbor Discovery and IPv6 Address Configuration
   protocols.  When the MN moves without having received a PrRtAdv
   message, it cannot transmit an UNA message upon attaching to the
   NAR's link.

   The source IP address is the PCoA when the FBU is sent from the PAR's
   link, and the source IP address is the NCoA when the FBU sent from
   the NAR's link.  When the source IP address is the PCoA, the MN MUST
   include the alternate CoA option set to NCoA.  The PAR MUST process
   the FBU even though the address in the alternate CoA option is
   different from that in the source IP address, and ensure that the
   address in the alternate CoA option is used in the New CoA option in
   the HI message to the NAR.

   The FBU MUST also include the Home Address Option set to PCoA.  An
   FBU message MUST be protected so that the PAR is able to determine
   that the FBU message is sent by an MN that legitimately owns the
   PCoA.

6.3.2.  Fast Binding Acknowledgment (FBack)

   The Fast Binding Acknowledgment message has a Mobility Header Type
   value of 9.  The FBack message is sent by the PAR to acknowledge
   receipt of a Fast Binding Update message in which the 'A' bit is set.
   If PAR sends an HI message to the NAR after processing an FBU, the
   FBack message SHOULD NOT be sent to the MN before the PAR receives a
   HAck message from the NAR.  The PAR MAY send the FBack immediately in
   the reactive mode however.  The Fast Binding Acknowledgment MAY also
   be sent to the MN on the old link.

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

         Figure 9: Fast Binding Acknowledgment (FBack) Message

   IP Fields:

         Source address: The IP address of the Previous Access Router

         Destination Address: The NCoA, and optionally the PCoA

      Status: 8-bit unsigned integer indicating the disposition of the
      Fast Binding Update.  Values of the Status field that are 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 [RFC3775].

      Reserved: An unused field.  MUST be set to zero.

      Sequence Number: Copied from the 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 an "alternate" CoA if Status is 1.
      MUST contain the Binding Authorization Data for FMIP (BADF)
      option.  See 6.4.5.

6.4.  Unsolicited Neighbor Advertisement (UNA)

   This is the same message as in [RFC4861] with the requirement that
   the 'O' bit is always set to zero.  Since this is an unsolicited
   message, the 'S' bit is zero, and since this is sent by an MN, the
   'R' bit is also zero.

   If the NAR is proxying the NCoA (as a result of HI and HAck
   exchange), then UNA processing has additional steps (see below).  If
   the NAR is not proxying the NCoA (for instance, HI and HAck exchange
   has not taken place), then UNA processing follows the same procedure
   as specified in [RFC4861].  Implementations MAY retransmit UNA
   subject to the specification in Section 7.2.6 of [RFC4861] while
   noting that the default RetransTimer value is large for handover
   purposes.

   The Source Address in UNA MUST be the NCoA.  The destination address
   is typically the all-nodes multicast address; however, some
   deployments may not prefer transmission to a multicast address.  In
   such cases, the destination address SHOULD be the NAR's IP address.

   The Target Address MUST include the NCoA, and the Target link-layer
   address MUST include the MN's LLA.

   The MN sends an UNA message to the NAR, as soon as it regains
   connectivity on the new link.  Arriving or buffered packets can be
   immediately forwarded.  If the NAR is proxying the NCoA, it creates a
   neighbor cache entry in STALE state but forwards packets as it
   determines bidirectional reachability according to the standard
   Neighbor Discovery procedure.  If there is an entry in INCOMPLETE
   state without a link-layer address, it sets it to STALE, again
   according to the procedure in [RFC4861].

   The NAR MAY wish to provide a different IP address to the MN than the
   one in the UNA message.  In such a case, the NAR MUST delete the
   proxy entry for the NCoA and send a Router Advertisement with the
   NAACK option containing the new IP address.

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

6.5.  New Options

   All the options, with the exception of Binding Data Authorization for
   FMIPv6 (BADF) discussed in Section 6.5.4, use Type, Length, and
   Option-Code format shown in Figure 10.

   The Type values are defined from the Neighbor Discovery options
   space.  The Length field is in units of 8 octets, except for the
   Mobility Header Link-Layer Address option, whose Length field is in
   units of octets in accordance with Section 6.2 in [RFC3775].  And,
   Option-Code provides additional information for each of the options
   (see individual options below).

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Type     |     Length    |  Option-Code  |               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                                  ...                          ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 10: Option Format

6.5.1.  IP Address/Prefix Option

   This option is sent in the Proxy Router Advertisement, the Handover
   Initiate, and Handover Acknowledge messages.

    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   | Prefix Length |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Reserved                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                             IPv6 Address                      +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 11: IPv6 Address/Prefix Option

   Type: 17

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

   Option-Code:

         1: Old Care-of Address
         2: New Care-of Address
         3: NAR's IP address
         4: NAR's Prefix, sent in PrRtAdv.  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.

   Prefix Length: 8-bit unsigned integer that indicates the length of
   the IPv6 Address Prefix.  The value ranges from 0 to 128.

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

   IPv6 address: The IP address defined by the Option-Code field.

6.5.2.  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     |  Option-Code  |       LLA...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 12: Link-Layer Address Option

   Type: 19

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

   Option-Code:

         0: wildcard requesting resolution for all nearby access points
         1: Link-Layer Address of the New Access 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 access point identified by the LLA belongs to the
         current interface of the router
         6: No prefix information available for the access point
         identified by the LLA
         7: No fast handovers support available for the access point
         identified by the LLA

      LLA: The variable length link-layer address.

   The LLA option does not have a length field for the LLA itself.  The
   implementations must consult the specific link layer over which the
   protocol is run in order to determine the content and length of the
   LLA.

   Depending on the size of individual LLA option, appropriate padding
   MUST be used to ensure that the entire option size is a multiple of 8
   octets.

   The New Access Point Link-Layer Address contains the link-layer
   address of the access 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
   an 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 to which the Proxy Router
   Solicitation message refers.

6.5.3.  Mobility Header Link-Layer Address (MH-LLA) Option

   This option is identical to the LLA option, but is carried in the
   Mobility Header messages, e.g., FBU.  In the future, other Mobility
   Header messages may also make use of this option.  The format of the
   option is shown in Figure 13.  There are no alignment requirements
   for this 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    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Option-Code   |                  LLA                     ....
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 13: Mobility Header Link-Layer Address Option

   Type: 7

   Length: The size of this option in octets not including the Type and
   Length fields.

   Option-Code: 2 Link-Layer Address of the MN.

   LLA: The variable length link-layer address.

6.5.4.  Binding Authorization Data for FMIPv6 (BADF)

   This option MUST be present in FBU and FBack messages.  The security
   association between the MN and the PAR is established by companion
   protocols [RFC5269].  This option specifies how to compute and verify
   a Message Authentication Code (MAC) using the established security
   association.

   The format of this option is shown in Figure 14.

    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      | Option Length |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            SPI                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                         Authenticator                         |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 14: Binding Authorization Data for FMIPv6 (BADF) Option

   Type: 21

   Option Length: The length of the Authenticator in bytes

   SPI: Security Parameter Index.  SPI = 0 is reserved for the
   Authenticator computed using SEND-based handover keys.

   Authenticator: Same as in RFC 3775, with "correspondent" replaced by
   the PAR's IP address, and Kbm replaced by the shared key between the
   MN and the PAR.

   The default MAC calculation is done using HMAC_SHA1 with the first 96
   bits used for the MAC.  Since there is an Option Length field,
   implementations can use other algorithms such as HMAC_SHA256.

   This option MUST be the last Mobility Option present.

6.5.5.  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    | Option-Code   |    Status     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Reserved                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 15: Neighbor Advertisement Acknowledgment Option

   Type: 20

   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)

   Option-Code: 0

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

         1: NCoA is invalid, perform address configuration
         2: NCoA is invalid, use the supplied NCoA.  The supplied NCoA
         (in the form of an IP Address Option) MUST be present following
         the Reserved field.
         3: NCoA is invalid, use NAR's IP address as NCoA in FBU
         4: PCoA supplied, do not send FBU
         128: Link-Layer Address unrecognized

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

   The NAR responds to UNA with the NAACK option to notify the MN to use
   a different NCoA than the one that the MN has used.  If the NAR
   proposes a different NCoA, the Router Advertisement MUST use the
   source IP address in the UNA message as the destination address, and
   use the L2 address present in UNA.  The MN MUST use the NCoA if it is
   supplied with the NAACK option.  If the NAACK indicates that the
   Link-Layer Address is unrecognized, for instance, if the MN uses an
   LLA valid on PAR's link but the same LLA is not valid on NAR's link
   due to a different access technology, the MN MUST NOT use the NCoA or
   the PCoA and SHOULD start immediately the process of acquiring a
   different NCoA at the NAR.

   In the future, new option types may be defined.

7.  Related Protocol and Device Considerations

   The protocol specified here, as a design principle, introduces no or
   minimal changes to related protocols.  For example, no changes to the
   base Mobile IPv6 protocol are needed in order to implement this
   protocol.  Similarly, no changes to the IPv6 stateless address auto-
   configuration protocol [RFC4862] and DHCP [RFC3315] are introduced.
   The protocol specifies an optional extension to Neighbor Discovery
   [RFC4861] in which an access router may send a router advertisement
   as a response to the UNA message (see Section 6.4).  Other than this
   extension, the specification does not modify Neighbor Discovery
   behavior (including the procedures performed when attached to the PAR
   and when attaching to the NAR).

   The protocol does not require changes to any intermediate Layer 2
   device between an MN and its access router that supports this
   specification.  This includes the wireless access points, switches,
   snooping devices, and so on.

8.  Evolution from and Compatibility with RFC 4068

   This document has evolved from [RFC4068].  Specifically, a new
   handover key establishment protocol (see [RFC5269]) has been defined
   to enable a security association between a mobile node and its access
   router.  This allows the secure update of the routing of packets
   during a handover.  In the future, new specifications may be defined
   to establish such security associations depending on the particular
   deployment scenario.

   The protocol has improved from the experiences in implementing
   [RFC4068], and from experimental usage.  The input has improved the
   specification of parameter fields (such as lifetime, codepoints,
   etc.)  as well as inclusion of new parameter fields in the existing
   messages.  As of this writing, there are two publicly available
   implementations, [fmipv6] and [tarzan], and multiple proprietary
   implementations.  Some experience suggests that the protocol meets
   the delay and packet loss requirements when used appropriately with
   particular radio access protocols.  For instance, see [RFC5184] and
   [mip6-book].  Nevertheless, it is important to recognize that
   handover performance is a function of both IP layer operations, which
   this protocol specifies, and the particular radio access technology
   itself, which this protocol relies upon but does not modify.

   An existing implementation of [RFC4068] needs to be updated in order
   to support this specification.  The primary addition is the
   establishment of a security association between an MN and its access
   router (i.e., MN and PAR).  One way to establish such a security
   association is specified in [RFC5269].  An implementation that
   complies with the specification in this document is likely to also
   work with [RFC4068], except for the Binding Authorization Data for
   FMIPv6 option (see Section 6.5.4) that can only be processed when
   security association is in place between a mobile node and its access
   router.  This specification deprecates the Fast Neighbor
   Advertisement (FNA) message.  However, it is acceptable for a NAR to
   process this message from a mobile node as specified in [RFC4068].

9.  Configurable Parameters

   Mobile nodes rely on configuration parameters shown in the table
   below.  Each mobile node MUST have a configuration mechanism to
   adjust the parameters.  Such a configuration mechanism may be either
   local (such as a command line interface) or based on central
   management of a number of mobile nodes.

   +-------------------+---------------+---------------+
   |   Parameter Name  | Default Value |   Definition  |
   +-------------------+---------------+---------------+
   |  RTSOLPR_RETRIES  |       3       | Section 6.1.1 |
   |  MAX_RTSOLPR_RATE |       3       | Section 6.1.1 |
   |    FBU_RETRIES    |       3       | Section 6.3.1 |
   | PROXY_ND_LIFETIME |  1.5 seconds  | Section 6.2.2 |
   |     HI_RETRIES    |       3       | Section 6.2.1 |
   +-------------------+---------------+---------------+

10.  Security Considerations

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

      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 an 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 association
      that MUST be used to secure FBU.  The current version of this
      protocol relies on a companion protocol [RFC5269] to establish
      such a security association.  Using the shared handover key from
      [RFC5269], the Authenticator in BADF option (see Section 6.5.4)
      MUST be computed, and the BADF option included in FBU and FBack
      messages.

      Secure FBU, malicious or inadvertent 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 an MN with a
      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 UNA.  Therefore, any inadvertent or
      malicious redirection to a host is avoided.  It is still possible
      to jam a NAR's buffer with redirected traffic.  However, since a
      NAR's handover state corresponding to an NCoA has a finite (and
      short) lifetime corresponding to a small multiple of anticipated
      handover latency, the extent of this vulnerability is arguably
      small.

      Sending an FBU from a NAR's link: A malicious node may send an FBU
      from a NAR's link providing an unsuspecting node's address as an
      NCoA.  This is similar to base Mobile IP where the MN can provide
      some other node's IP address as its CoA to its Home Agent; here
      the PAR acts like a "temporary Home Agent" having a security
      association with the Mobile Node and providing forwarding support
      for the handover traffic.  As in base Mobile IP, this misdelivery
      is traceable to the MN that has a security association with the
      router.  So, it is possible to isolate such an MN if it continues
      to misbehave.  Similarly, an MN that has a security association
      with the PAR may provide the LLA of some other node on NAR's link,
      which can cause misdelivery of packets (meant for the NCoA) to an
      unsuspecting node.  It is possible to trace the MN in this case as
      well.

   Apart from the above, the RtSolPr (Section 6.1.1) and PrRtAdv
   (Section 6.1.2) messages inherit the weaknesses of Neighbor Discovery
   protocol [RFC4861].  Specifically, when its access router is
   compromised, the MN's RtSolPr message may be answered by an attacker
   that provides a rogue router as the resolution.  Should the MN attach
   to such a rogue router, its communication can be compromised.
   Similarly, a network-initiated PrRtAdv message (see Section 3.3) from
   an attacker could cause an MN to handover to a rogue router.  Where
   these weaknesses are a concern, a solution such as Secure Neighbor
   Discovery (SEND) [RFC3971] SHOULD be considered.

   The protocol provides support for buffering packets during an MN's
   handover.  This is done by securely exchanging the Handover Initiate
   (HI) and Handover Acknowledgment (HAck) messages in response to the
   FBU message from an MN.  It is possible that an MN may fail, either
   inadvertently or purposely, to undergo handover to the NAR, which
   typically provides buffering support.  This can cause the NAR to
   waste its memory containing the buffered packets, and in the worst
   case, could create resource exhaustion concerns.  Hence,
   implementations must limit the size of the buffer as a local policy
   configuration, which may consider parameters such as the average
   handover delay, expected size of packets, and so on.

   The Handover Initiate (HI) and Handover Acknowledgement (HAck)
   messages exchanged between the PAR and NAR MUST be protected using
   end-to-end security association(s) offering integrity and data origin
   authentication.

   The PAR and the NAR MUST implement IPsec [RFC4301] for protecting the
   HI and HAck messages.  IPsec Encapsulating Security Payload (ESP)
   [RFC4303] in transport mode with mandatory integrity protection
   SHOULD be used for protecting the signaling messages.
   Confidentiality protection of these messages is not required.

   The security associations can be created by using either manual IPsec
   configuration or a dynamic key negotiation protocol such as Internet
   Key Exchange Protocol version 2 (IKEv2) [RFC4306].  If IKEv2 is used,
   the PAR and the NAR can use any of the authentication mechanisms, as
   specified in RFC 4306, for mutual authentication.  However, to ensure
   a baseline interoperability, the implementations MUST support shared
   secrets for mutual authentication.  The following sections describe
   the Peer Authorization Database (PAD) and Security Policy Database
   (SPD) entries specified in [RFC4301] when IKEv2 is used for setting
   up the required IPsec security associations.

10.1.  Peer Authorization Database Entries when Using IKEv2

   This section describes PAD entries on the PAR and the NAR.  The PAD
   entries are only example configurations.  Note that the PAD is a
   logical concept and a particular PAR or NAR implementation can
   implement the PAD in any implementation specific manner.  The PAD
   state may also be distributed across various databases in a specific
   implementation.

   PAR PAD:

      - IF remote_identity = nar_identity_1
      THEN authenticate (shared secret/certificate/EAP) and authorize
      CHILD_SA for remote address nar_address_1

   NAR PAD:

      - IF remote_identity = par_identity_1
      THEN authenticate (shared secret/certificate/EAP) and authorize
      CHILD_SAs for remote address par_address_1

   The list of authentication mechanisms in the above examples is not
   exhaustive.  There could be other credentials used for authentication
   stored in the PAD.

10.2.  Security Policy Database Entries

   This section describes the security policy entries on the PAR and the
   NAR required to protect the HI and HAck messages.  The SPD entries
   are only example configurations.  A particular PAR or NAR
   implementation could configure different SPD entries as long as they
   provide the required security.

   In the examples shown below, the identity of the PAR is assumed to be
   par_1, the address of the PAR is assumed to be par_address_1, and the
   address of the NAR is assumed to be nar_address_1.

   PAR SPD-S:

      - IF local_address = par_address_1 & remote_address =
      nar_address_1 & proto = ICMPv6 & local_icmpv6_type = HI &
      remote_icmpv6_type = HAck
      THEN use SA ESP transport mode Initiate using IDi = par_1 to
      address nar_address_1

   NAR SPD-S:

      - IF local_address = nar_address_1 & remote_address =
      par_address_1 & proto = ICMPv6 & local_icmpv6_type = HAck &
      remote_icmpv6_type = HI
      THEN use SA ESP transport mode

11.  IANA Considerations

   This document defines a new ICMPv6 message, which has been allocated
   from the ICMPv6 Type registry.

      154     FMIPv6 Messages

   This document creates a new registry for the 'Subtype' field in the
   above ICMPv6 message, called the "FMIPv6 Message Types".  IANA has
   assigned the following values.

   +---------+-------------+---------------+
   | Subtype | Description |   Reference   |
   +---------+-------------+---------------+
   |   2     |   RtSolPr   | Section 6.1.1 |
   |   3     |   PrRtAdv   | Section 6.1.2 |
   |   4     |      HI     | Section 6.2.1 |
   |   5     |     HAck    | Section 6.2.2 |
   +---------+-------------+---------------+

   The values '0' and '1' are reserved.  The upper limit is 255.  An RFC
   is required for new message assignment.

   The document defines a new Mobility Option that has received Type
   assignment from the Mobility Options Type registry.

      1.  Binding Authorization Data for FMIPv6 (BADF) option, described
          in Section 6.5.4

   The document has received Type assignments for the following (see
   [RFC4068]):

   The document defines the following Neighbor Discovery [RFC4861]
   options that have received Type assignment from IANA.

   +---------+-----------------------------------------+---------------+
   |   Type  |               Description               |   Reference   |
   +---------+-----------------------------------------+---------------+
   |    17   |         IP Address/Prefix Option        | Section 6.5.1 |
   |    19   |        Link-layer Address Option        | Section 6.5.2 |
   |    20   |  Neighbor Advertisement Acknowledgment  | Section 6.5.5 |
   |         |                  Option                 |               |
   +---------+-----------------------------------------+---------------+

   The document defines the following Mobility Header messages that have
   received Type allocation from the Mobility Header Types registry.

      1.  Fast Binding Update, described in Section 6.3.1

      2.  Fast Binding Acknowledgment, described in Section 6.3.2

   The document defines the following Mobility Option that has received
   Type assignment from the Mobility Options Type registry.

      1.  Mobility Header Link-Layer Address option, described in
          Section 6.5.3

12.  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, Emil Ivov, Syam Madanapalli, Suvidh
   Mathur, Andre Martin, Javier Martin, Koshiro Mitsuya, Gabriel
   Montenegro, Takeshi Ogawa, Sun Peng, YC Peng, Alex Petrescu, Domagoj
   Premec, Subba Reddy, K. Raghav, Ranjit Wable, and Jonathan Wood.
   Behcet Sarikaya and Frank Xia are acknowledged for the feedback on
   operation over point-to-point links.  The editor would like to
   acknowledge a contribution from James Kempf to improve this

   specification.  Vijay Devarapalli provided text for the security
   configuration between access routers in Section 10.  Thanks to Jari
   Arkko for the detailed AD Review, which has improved this document.
   The editor would also like to thank the [mipshop] working group chair
   Gabriel Montenegro and the erstwhile [mobile ip] working group chairs
   Basavaraj Patil and Phil Roberts for providing much support for this
   work.

13.  References

13.1.  Normative References

   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5269]     Kempf, J. and R. Koodli, "Distributing a Symmetric Fast
                 Mobile IPv6 (FMIPv6) Handover Key Using SEcure Neighbor
                 Discovery (SEND)", RFC 5269, June 2008.

   [RFC4443]     Conta, A., Deering, S., and M. Gupta, Ed., "Internet
                 Control Message Protocol (ICMPv6) for the Internet
                 Protocol Version 6 (IPv6) Specification", RFC 4443,
                 March 2006.

   [RFC3315]     Droms, R., Ed., Bound, J., Volz, B., Lemon, T.,
                 Perkins, C., and M. Carney, "Dynamic Host Configuration
                 Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3775]     Johnson, D., Perkins, C., and J. Arkko, "Mobility
                 Support in IPv6", RFC 3775, June 2004.

   [RFC4301]     Kent, S. and K. Seo, "Security Architecture for the
                 Internet Protocol", RFC 4301, December 2005.

   [RFC4303]     Kent, S., "IP Encapsulating Security Payload (ESP)",
                 RFC 4303, December 2005.

   [RFC4306]     Kaufman, C., Ed., "Internet Key Exchange (IKEv2)
                 Protocol", RFC 4306, December 2005.

   [RFC4861]     Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
                 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
                 September 2007.

   [RFC4862]     Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
                 Address Autoconfiguration", RFC 4862, September 2007.

13.2.  Informative References

   [fmipv6]      "fmipv6.org : Home Page", <http://fmipv6.org>.

   [mip6-book]   Koodli, R. and C. Perkins, "Mobile Internetworking with
                 IPv6, Chapter 22, John Wiley & Sons.", July 2007.

   [RFC3290]     Bernet, Y., Blake, S., Grossman, D., and A. Smith, "An
                 Informal Management Model for Diffserv Routers", RFC
                 3290, May 2002.

   [RFC3971]     Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
                 "SEcure Neighbor Discovery (SEND)", RFC 3971, March
                 2005.

   [RFC4068]     Koodli, R., Ed., "Fast Handovers for Mobile IPv6", RFC
                 4068, July 2005.

   [RFC5184]     Teraoka, F., Gogo, K., Mitsuya, K., Shibui, R., and K.
                 Mitani, "Unified Layer 2 (L2) Abstractions for Layer 3
                 (L3)-Driven Fast Handover", RFC 5184, May 2008.

   [tarzan]      "Nautilus6 - Tarzan",
                 <http://software.nautilus6.org/TARZAN/>.

Appendix A.  Contributors

   This document has its origins in the fast handover design team in the
   erstwhile [mobile ip] working group.  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.

Appendix B.  Changes since RFC 4068

   Following are the major changes and clarifications:

   o  Specified security association between the MN and its Access
      Router in the companion document [RFC5269].

   o  Specified Binding Authorization Data for Fast Handovers (BADF)
      option to carry the security parameters used for verifying the
      authenticity of FBU and FBack messages.  The handover key used for
      computing the Authenticator is specified in companion documents.

   o  Specified the security configuration for inter - access router
      signaling (HI, HAck).

   o  Added a section on prefix management between access routers and
      illustrated protocol operation over point-to-point links.

   o  Deprecated FNA, which is a Mobility Header message.  In its place,
      the Unsolicited Neighbor Advertisement (UNA) message from RFC 4861
      is used.

   o  Combined the IPv6 Address Option and IPv6 Prefix Option.

   o  Added description of DAD requirement on NAR when determining NCoA
      uniqueness in Section 4, "Protocol Details".

   o  Added a new code value for gratuitous HAck message to trigger a HI
      message.

   o  Added Option-Code 5 in PrRtAdv message to indicate NETLMM usage.

   o  Clarified protocol usage when DHCP is used for NCoA formulation
      (Sections 6.1.2, 3.1, and 5.2).  Added a new Code value (5) in
      PrRtAdv (Section 6.1.2).

   o  Clarified that IPv6 Neighbor Discovery operations are a must in
      Section 7, "Related Protocol and Device Considerations".

   o  Clarified "PAR = temporary HA" for FBUs sent by a genuine MN to an
      unsuspecting CoA.

Editor's Address

   Rajeev Koodli
   Starent Networks
   USA

   EMail: rkoodli@starentnetworks.com

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