





Network Working Group                                            H. Jang
Request for Comments: 5270                                       SAMSUNG
Category: Informational                                           J. Jee
                                                                    ETRI
                                                                  Y. Han
                                                                     KUT
                                                                 S. Park
                                                     SAMSUNG Electronics
                                                                  J. Cha
                                                                    ETRI
                                                               June 2008


         Mobile IPv6 Fast Handovers over IEEE 802.16e Networks

Status of This Memo

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

Abstract

   This document describes how a Mobile IPv6 Fast Handover can be
   implemented on link layers conforming to the IEEE 802.16e suite of
   specifications.  The proposed scheme tries to achieve seamless
   handover by exploiting the link-layer handover indicators and thereby
   synchronizing the IEEE 802.16e handover procedures with the Mobile
   IPv6 fast handover procedures efficiently.






















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  IEEE 802.16e Handover Overview . . . . . . . . . . . . . . . .  4
   4.  Network Topology Acquisition and Network Selection . . . . . .  5
   5.  Interaction between FMIPv6 and IEEE 802.16e  . . . . . . . . .  6
     5.1.  Access Router Discovery  . . . . . . . . . . . . . . . . .  6
     5.2.  Handover Preparation . . . . . . . . . . . . . . . . . . .  7
     5.3.  Handover Execution . . . . . . . . . . . . . . . . . . . .  8
     5.4.  Handover Completion  . . . . . . . . . . . . . . . . . . .  9
   6.  The Examples of Handover Scenario  . . . . . . . . . . . . . . 10
     6.1.  Predictive Mode  . . . . . . . . . . . . . . . . . . . . . 10
     6.2.  Reactive Mode  . . . . . . . . . . . . . . . . . . . . . . 12
   7.  IEEE 802.21 Considerations . . . . . . . . . . . . . . . . . . 14
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 15
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 15
     10.2. Informative References . . . . . . . . . . . . . . . . . . 16

1.  Introduction

   Mobile IPv6 Fast Handover protocol (FMIPv6) [RFC5268] was proposed to
   complement the Mobile IPv6 (MIPv6) [RFC3775] by reducing the handover
   latency for the real-time traffic.  FMIPv6 assumes the support from
   the link-layer technology; however, the specific link-layer
   information available and its available timing differs according to
   the particular link-layer technology in use, as pointed out in
   [RFC4260], which provides an FMIPv6 solution for the IEEE 802.11
   networks.  So, this document is proposed to provide an informational
   guide to the developers about how to optimize the FMIPv6 handover
   procedures, specifically in the IEEE 802.16e networks
   [IEEE802.16][IEEE802.16e].

   The proposed scheme achieves seamless handover by exploiting the
   link-layer handover indicators and designing an efficient
   interleaving scheme of the 802.16e and the FMIPv6 handover
   procedures.  The scheme targets a hard handover, which is the default
   handover type of IEEE 802.16e.  For the other handover types, i.e.,
   the Macro Diversity Handover (MDHO) and Fast Base Station Switching
   (FBSS), the base stations in the same diversity set are likely to
   belong to the same subnet for diversity, and FMIPv6 might not be
   needed.  Regarding the MDHO and the FBSS deployment with FMIPv6,
   further discussion will be needed and is not in the scope of this
   document.





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   We begin with a summary of handover procedures of [IEEE802.16e] and
   then present the optimized complete FMIPv6 handover procedures by
   using the link-layer handover indicators.  The examples of handover
   scenarios are described for both the predictive mode and reactive
   mode.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document is to be interpreted as described in [RFC2119].

   Most of terms used in this document are defined in MIPv6 [RFC3775]
   and FMIPv6 [RFC5268].

   The following terms come from the IEEE 802.16e specification
   [IEEE802.16e].

      MOB_NBR-ADV

         An IEEE 802.16e neighbor advertisement message sent
         periodically by a base station.

      MOB_MSHO-REQ

         An IEEE 802.16e handover request message sent by a mobile node.

      MOB_BSHO-RSP

         An IEEE 802.16e handover response message sent by a base
         station.

      MOB_BSHO-REQ

         An IEEE 802.16e handover request message sent by a base
         station.

      MOB_HO-IND

         An IEEE 802.16e handover indication message sent by a mobile
         node.

      BSID

         An IEEE 802.16e base station identifier.






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3.  IEEE 802.16e Handover Overview

   Compared with the handover in the WLAN (Wireless Local Area Network),
   the IEEE 802.16e handover mechanism consists of more steps since the
   802.16e embraces the functionality for elaborate parameter adjustment
   and procedural flexibility.

   When a mobile node (MN) stays in a link, it listens to the Layer 2
   neighbor advertisement messages, named MOB_NBR-ADV, from its serving
   base station (BS).  A BS broadcasts them periodically to identify the
   network and announce the characteristics of neighbor BSs.  Receiving
   this, the MN decodes this message to find out information about the
   parameters of neighbor BSs for its future handover.  With the
   provided information in a MOB_NBR-ADV, the MN may minimize the
   handover latency by obtaining the channel number of neighbors and
   reducing the scanning time, or may select the better target BS based
   on the signal strength, Quality-of-Service (QoS) level, service
   price, etc.

   The handover procedure is conceptually divided into two steps:
   "handover preparation" and "handover execution" [SH802.16e].  The
   handover preparation can be initiated by either an MN or a BS.

   During this period, neighbors are compared by the metrics such as
   signal strength or QoS parameters, and a target BS is selected among
   them.  If necessary, the MN may try to associate (initial ranging)
   with candidate BSs to expedite a future handover.  Once the MN
   decides to handover, it notifies its intent by sending a MOB_MSHO-REQ
   message to the serving BS (s-BS).  The BS then replies with a
   MOB_BSHO-RSP containing the recommended BSs to the MN after
   negotiating with candidates.  Optionally, it may confirm handover to
   the target BS (t-BS) over backbone when the target is decided.
   Alternatively, the BS may trigger handover with a MOB_BSHO-REQ
   message.

   After handover preparation, handover execution starts.  The MN sends
   a MOB_HO-IND message to the serving BS as a final indication of its
   handover.  Once it makes a new attachment, it conducts 802.16e
   ranging through which it can acquire physical parameters from the
   target BS, tuning its parameters to the target BS.  After ranging
   with the target BS successfully, the MN negotiates basic capabilities
   such as maximum transmit power and modulator/demodulator type.  It
   then performs authentication and key exchange procedures, and finally
   registers with the target BS.  If the target BS has already learned
   some contexts such as authentication or capability parameters through
   backbone, it may omit the corresponding procedures.  For the details
   of the 802.16 handover procedures, refer to Section 6.3.22 of
   [IEEE802.16e].  After completing registration, the target BS starts



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   to serve the MN and communication via target BS is available.
   However, in case the MN moves to a different subnet, it should
   reconfigure a new IP address and reestablish an IP connection.  To
   resume the active session of the previous link, the MN should also
   perform IP layer handover.

4.  Network Topology Acquisition and Network Selection

   This section describes how discovery of adjacent networks and
   selection of target network work in the IEEE 802.16e for background
   information.

   An MN can learn the network topology and acquire the link information
   in several ways.  First of all, it can do that via L2 neighbor
   advertisements.  A BS supporting mobile functionality shall broadcast
   a MOB_NBR-ADV message periodically that includes the network topology
   information (its maximum interval is 1 second).  This message
   includes BSIDs and channel information of neighbor BSs, and it is
   used to facilitate the MN's synchronization with neighbor BSs.  An MN
   can collect the necessary information of the neighbor BSs through
   this message for its future handover.

   Another method for acquisition of network topology is scanning, which
   is the process to seek and monitor available BSs in order to find
   suitable handover targets.  While a MOB_NBR-ADV message includes
   static information about neighbor BSs, scanning provides rather
   dynamic parameters such as link quality parameters.  Since the
   MOB_NBR-ADV message delivers a list of neighbor BSIDs periodically
   and scanning provides a way to sort out some adequate BSs, it is
   recommended that when new BSs are found in the advertisement, the MN
   identifies them via scanning and resolves their BSIDs to the
   information of the subnet where the BS is connected.  The
   association, an optional initial ranging procedure occurring during
   scanning, is one of the helpful methods to facilitate the impending
   handover.  The MN is able to get ranging parameters and service
   availability information for the purpose of proper selection of the
   target BS and expediting a potential future handover to it.  The
   detailed explanation of association is described in Section 6.3.22 of
   [IEEE802.16e].

   Besides the methods provided by 802.16e, the MN may rely on other
   schemes.  For instance, the topology information may be provided
   through the MIIS (Media Independent Information Service)
   [IEEE802.21], which has been developed by the IEEE 802.21 working
   group.  The MIIS is a framework by which the MN or network can obtain
   network information to facilitate network selection and handovers.





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   After learning about neighbors, the MN may compare them to find a BS,
   which can serve better than the serving BS.  The target BS may be
   determined by considering various criteria such as required QoS,
   cost, user preference, and policy.  How to select the target BS is
   not in the scope of this document.

5.  Interaction between FMIPv6 and IEEE 802.16e

   In this section, a set of primitives is introduced for an efficient
   interleaving of the IEEE 802.16e and the FMIPv6 procedures as below.
   The following sections present the handover procedures in detail by
   using them.

      o NEW_LINK_DETECTED (NLD)

         A trigger from the link layer to the IP layer in the MN to
         report that a new link has been detected.

      o LINK_HANDOVER_IMPEND (LHI)

         A trigger from the link layer to the IP layer in the MN to
         report that a link-layer handover decision has been made and
         its execution is imminent.

      o LINK_SWITCH (LSW)

         A control command from the IP layer to the link layer in the MN
         in order to force the MN to switch from an old BS to a new BS.

      o LINK_UP (LUP)

         A trigger from the link layer to the IP layer in the MN to
         report that the MN completes the link-layer connection
         establishment with a new BS.

5.1.  Access Router Discovery

   Once a new BS is detected through reception of a MOB_NBR-ADV and
   scanning, an MN may try to learn the associated access router (AR)
   information as soon as possible.  In order to enable its quick
   discovery in the IP layer, the link layer (802.16) triggers a
   NEW_LINK_DETECTED primitive to the IP layer (FMIPv6) on detecting a
   new BS.

   Receiving the NEW_LINK_DETECTED from the link layer, the IP layer
   tries to learn the associated AR information by exchanging an RtSolPr
   (Router Solicitation for Proxy Advertisement) and a PrRtAdv (Proxy
   Router Advertisement) with the PAR (Previous Access Router).



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   According to [RFC5268], the MN may send an RtSolPr at any convenient
   time.  However, this proposal recommends that, if feasible, the MN
   send it as soon as possible after receiving the NEW_LINK_DETECTED for
   quick router discovery because detection of a new BS usually implies
   MN's movement, which may result in handover.

   Transmission of RtSolPr messages may cause the signaling overhead
   problem that is mentioned in Section 2 of [RFC4907].  To rate-limit
   the retransmitted RtSolPr messages, FMIPv6 provides a back-off
   mechanism.  It is also possible that attackers may forge a MOB_NBR-
   ADV message so that it can contain a bunch of bogus BSIDs or may send
   a flood of MOB_NBR-ADV messages each of which contains different
   BSIDs.  This problem is mentioned in Section 8.

5.2.  Handover Preparation

   When the MN decides to change links based on its policy such as the
   degrading signal strength or increasing packet loss rate, it
   initiates handover by sending a MOB_MSHO-REQ to the BS and will
   receive a MOB_BSHO-RSP from the BS as a response.  Alternatively, the
   BS may initiate handover by sending a MOB_BSHO-REQ to the MN.

   On receiving either a MOB_BSHO-RSP or a MOB_BSHO-REQ, the link layer
   triggers a LINK_HANDOVER_IMPEND in order to signal the IP layer of
   arrival of MOB_BSHO-REQ/MOB_BSHO-RSP quickly.  At this time, the
   target BS decided in the link layer is delivered to the IP layer as a
   parameter of the primitive.  The primitive is used to report that a
   link-layer handover decision has been made and its execution is
   imminent.  It can be helpfully used for FMIPv6 as an indication to
   start the handover preparation procedure, that is to send an FBU
   (Fast Binding Update) message to the PAR.

   To avoid erroneous results due to unreliable and inconsistent
   characteristics of link, for instance, to move to the unpredicted
   network or to stay in the current network after sending an FBU,
   Section 2 of [RFC4907] advises the use of a combination of signal
   strength data with other techniques rather than relying only on
   signal strength for handover decision.  For example, the
   LINK_HANDOVER_IMPEND may be sent after validating filtered signal
   strength measurements with other indications of link loss such as
   lack of beacon reception.

   Once the IP layer receives the LINK_HANDOVER_IMPEND, it checks
   whether or not the specified target network belongs to a different
   subnet based on the information collected during the Access Router
   Discovery step.  If the target proves to be in the same subnet, the
   MN can continue to use the current IP address after handover, and
   there is no need to perform FMIPv6.  Otherwise, the IP layer



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   formulates a prospective NCoA (New Care-of Address) with the
   information provided in the PrRtAdv message and sends an FBU message
   to the PAR.

   When the FBU message arrives in the PAR successfully, the PAR and the
   NAR (New Access Router) process it according to [RFC5268].  The PAR
   sets up a tunnel between the PCoA (Previous Care-of Address) and NCoA
   by exchanging HI (Handover Initiate) and HAck (Handover Acknowledge)
   messages with the NAR, forwarding the packets destined for the MN to
   the NCoA.  The NCoA is confirmed or re-assigned by the NAR in the
   HAck and, finally delivered to the MN through the FBack (Fast Binding
   Acknowledgment) in case of predictive mode.

   After the MN sends a MOB_HO-IND to the serving BS, data packet
   transfer between the MN and the BS is no longer allowed.  Note that
   when a MOB_HO-IND is sent out before an FBack arrives in the MN, it
   is highly probable that the MN will operate in reactive mode because
   the serving BS releases all the MN's connections and resources after
   receiving a MOB_HO-IND.  Therefore, if possible, the MN should
   exchange FBU and FBack messages with the PAR before sending a MOB_HO-
   IND to the BS so as to operate in predictive mode.

5.3.  Handover Execution

   If the MN receives an FBack message on the previous link, it runs in
   predictive mode after handover.  Otherwise, it should run in reactive
   mode.  In order for the MN to operate in predictive mode as far as
   possible after handover, implementations may allow use of a
   LINK_SWITCH primitive.  The LINK_SWITCH is a command in order to
   force the MN to switch from an old BS to a new BS and the similar
   concept has introduced for the wireless LAN in [RFC5184].  When it is
   applied, the MN's IP layer issues a LINK_SWITCH primitive to the link
   layer on receiving the FBack message in the previous link.  Until it
   occurs, the link layer keeps the current (previous) link if feasible
   and postpones sending a MOB_HO-IND message while waiting for the
   FBack message.

   After switching links, the MN synchronizes with the target BS and
   performs the 802.16e network entry procedure.  The MN exchanges the
   RNG-REQ/RSP, SBC-REQ/RSP, PKM-REQ/RSP, and REG-REQ/RSP messages with
   the target BS.  Some of these messages may be omitted if the
   (previously) serving BS transferred the context to the target BS over
   the backbone beforehand.  When the network entry procedure is
   completed and the link layer is ready for data transmission, it
   informs the IP layer of the fact with a LINK_UP primitive.






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   Section 2 of [RFC4907] recommends that link indications should be
   designed with built-in damping.  The LINK_UP primitive defined in
   this document is generated by the link layer state machine based on
   the 802.16e link layer message exchanges, that is, the IEEE 802.16e
   network entry and the service flow creation procedures.  Therefore,
   the LINK_UP is typically less sensitive to changes in transient link
   conditions.  The link may experience an intermittent loss.  Even in
   such a case, the following FMIPv6 operation is performed only when
   the MN handovers to the link with a different subnet and there is no
   signaling overhead as a result of a intermittent loss.

5.4.  Handover Completion

   When the MN's IP layer receives a LINK_UP primitive from the link
   layer, it should check whether it has moved into the target network
   predicted by FMIPv6.  In case the target BS is within the same
   subnet, the MN does not perform the FMIPv6 operation.

      *  If the MN discovers itself in the predicted target network and
         receives an FBack message in the previous link, the MN's IP
         layer sends an UNA (Unsolicited Neighbor Advertisement) to the
         NAR (predictive mode).

      *  If the MN has moved to the target network without receiving an
         FBack message in the previous link, the IP layer sends an UNA
         and also an FBU message immediately after sending the UNA
         message (reactive mode).  The NAR may provide a different IP
         address by using an RA (Router Advertisement) with a NAACK
         (Neighbor Advertisement Acknowledge) option other than the
         formulated NCoA by the MN.

      *  The MN may discover itself in the unpredicted network
         (erroneous movement).  If this is the case, the MN moves to the
         network that is not the target specified in the
         LINK_HANDOVER_IMPEND primitive.  For the recovery from such an
         invalid indication, which is mentioned in Section 2 of
         [RFC4907], the MN should send a new FBU to the PAR according to
         Section 5.6 of [RFC5268] so that the PAR can update the
         existing binding entry and redirect the packets to the new
         confirmed location.

   In both cases of predictive and reactive modes, once the MN has moved
   into the new link, it uses the NCoA formulated by the MN as a source
   address of the UNA, irrespective of NCoA availability.  It then
   starts a Duplicate Address Detection (DAD) probe for NCoA according
   to [RFC4862].  In case the NAR provides the MN with a new NCoA, the
   MN MUST use the provided NCoA instead of the NCoA formulated by the
   MN.



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   When the NAR receives an UNA message, it deletes its proxy neighbor
   cache entry if it exists, and forwards buffered packets to the MN
   after updating the neighbor cache properly.  Detailed UNA processing
   rules are specified in Section 6.4 of [RFC5268].

6.  The Examples of Handover Scenario

   In this section, the recommended handover procedures over 802.16e
   network are shown for both predictive and reactive modes.  It is
   assumed that the MN handovers to the network that belongs to a
   different subnet.

   In the following figures, the messages between the MN's Layer 2 (MN
   L2) and the BS are the IEEE 802.16 messages, while messages between
   the MN's Layer 3 (MN L3) and the PAR and messages between PAR and NAR
   are the FMIPv6 messages.  The messages between the MN L2 and the MN
   L3 are primitives introduced in this document.

6.1.  Predictive Mode

   The handover procedures in the predictive mode are briefly described
   as follows.  Figure 3 illustrates these procedures.

      1.   A BS broadcasts a MOB_NBR-ADV periodically.

      2.   If the MN discovers a new neighbor BS in this message, it may
           perform scanning for the BS.

      3.   When a new BS is found through the MOB_NBR-ADV and scanning,
           the MN's link layer notifies it to the IP layer by a
           NEW_LINK_DETECTED primitive.

      4.   The MN tries to resolve the new BS's BSID to the associated
           AR by exchange of RtSolPr and PrRtAdv messages with the PAR.

      5.   The MN initiates handover by sending a MOB_MSHO-REQ message
           to the BS and receives a MOB_BSHO-RSP from the BS.
           Alternatively, the BS may initiate handover by sending a
           MOB_BSHO-REQ to the MN.

      6.   When the MN receives either a MOB_BSHO-RSP or a MOB_BSHO-REQ
           from the BS, its link layer triggers a LINK_HANDOVER_IMPEND
           primitive to the IP layer.








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      7.   On reception of the LINK_HANDOVER_IMPEND, the MN's IP layer
           identifies that the target delivered along with the
           LINK_HANDOVER_IMPEND belongs to a different subnet and sends
           an FBU message to the PAR.  On receiving this message, the
           PAR establishes tunnel between the PCoA and the NCoA by
           exchange of HI and HAck messages with the NAR, and it
           forwards packets destined for the MN to the NCoA.  During
           this time, the NAR may confirm NCoA availability in the new
           link via HAck.

      8.   The MN receives the FBack message before its handover and
           sends a MOB_HO-IND message as a final indication of handover.
           Issue of a MOB_HO-IND may be promoted optionally by using a
           LINK_SWITCH command from the IP layer.  Afterwards it
           operates in predictive mode in the new link.

      9.   The MN conducts handover to the target BS and performs the
           IEEE 802.16e network entry procedure.

      10.  As soon as the network entry procedure is completed, the MN's
           link layer signals the IP layer with a LINK_UP.  On receiving
           this, the IP layer identifies that it has moved to a
           predicted target network and received the FBack message in
           the previous link.  It issues an UNA to the NAR by using the
           NCoA as a source IP address.  At the same time, it starts to
           perform DAD for the NCoA.

      11.  When the NAR receives the UNA from the MN, it delivers the
           buffered packets to the MN.






















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        (MN L3  MN L2)                   s-BS   PAR          t-BS   NAR
          |      |                        |      |            |      |
    1-2.  |      |<---MOB_NBR-ADV --------|      |            |      |
          |      |<-------Scanning------->|      |            |      |
    3.    |<-NLD-|                        |      |            |      |
    4.    |--------------(RtSolPr)-------------->|            |      |
          |<--------------PrRtAdv----------------|            |      |
          |      |                        |      |            |      |
    5.    |      |------MOB_MSHO-REQ----->|      |            |      |
          |      |<-----MOB_BSHO-RSP------|      |            |      |
          |      |  or                    |      |            |      |
          |      |<-----MOB_BSHO-REQ------|      |            |      |
    6.    |<-LHI-|                        |      |            |      |
    7.    |------------------FBU---------------->|            |      |
          |      |                        |      |--------HI-------->|
          |      |                        |      |<------HACK--------|
          |<-----------------FBack---------------|-->         |      |
          |      |                        |    Packets==============>|
    8.    |(LSW)>|-------MOB_HO-IND------>|      |            |      |
       disconnect|                        |      |            |      |
       connect   |                        |      |            |      |
    9.    |      |<---------IEEE 802.16 network entry-------->|      |
    10.   |<-LUP-|                        |      |            |      |
          |----------------------------UNA-------------------------->|
    11.   |<==================================================== Packets
          |      |                        |      |                   |

               Figure 3. Predictive Fast Handover in 802.16e

6.2.  Reactive Mode

   The handover procedures in the reactive mode are described as
   follows.  Figure 4 is illustrating these procedures.

      1. ~ 7.  The same as procedures of predictive mode.

      8.   The MN does not receive the FBack message before handover and
           sends a MOB_HO-IND message as a final indication of handover.
           Afterwards, it operates in reactive mode in the new link.

      9.   The MN conducts handover to the target network and performs
           the 802.16e network entry procedure.









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      10.  As soon as the network entry procedure is completed, the MN's
           link layer signals the IP layer with a LINK_UP.  On receiving
           this, the IP layer identifies that it has moved to the
           predicted target network without receiving the FBack in the
           previous link.  The MN issues an UNA to the NAR by using NCoA
           as a source IP address and starts to perform DAD for the
           NCoA.  Additionally, it sends an FBU to the PAR in the
           reactive mode.

      11.  When the NAR receives the UNA and the FBU from the MN, it
           forwards the FBack to the PAR.  The FBack and Packets are
           forwarded from the PAR and delivered to the MN (NCoA) through
           the NAR.  The NAR may supply a different IP address than the
           NCoA by sending an RA with a NAACK option to the MN.

       (MN L3  MN L2)                   s-BS   PAR          t-BS   NAR
          |      |                        |      |            |      |
    1-2.  |      |<---MOB_NBR-ADV & Scan--|      |            |      |
          |      |<-------Scanning------->|      |            |      |
    3.    |<-NLD-|                        |      |            |      |
    4.    |--------------(RtSolPr)-------------->|            |      |
          |<--------------PrRtAdv----------------|            |      |
          |      |                        |      |            |      |
    5.    |      |------MOB_MSHO-REQ----->|      |            |      |
          |      |<-----MOB_BSHO-RSP------|      |            |      |
          |      |  or                    |      |            |      |
          |      |<-----MOB_BSHO-REQ------|      |            |      |
    6.    |<-LHI-|                        |      |            |      |
    7.    |--------FBU----X--->           |      |            |      |
    8.    |      |-------MOB_HO-IND------>|      |            |      |
       disconnect|                        |      |            |      |
       connect   |                        |      |            |      |
    9.    |      |<---------IEEE 802.16 network entry-------->|      |
    10.   |<-LUP-|                        |      |            |      |
          |----------------------------UNA-------------------------->|
          |----------------------------FBU--------------------------)|
    11.   |      |                        |      |<-------FBU-------)|
          |      |                        |      |<-----HI/HAck----->|
          |      |                        |      |  (if necessary)   |
          |      |                        | Packets & FBack=========>|
          |<=========================================================|
          |      |                        |      |            |      |

                Figure 4. Reactive Fast Handover in 802.16e







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7.  IEEE 802.21 Considerations

   It is worth noting that great research has been conducted on defining
   generic services in the IEEE 802.21 working group that facilitate
   handovers between heterogeneous access links.  The standard works are
   named as a Media Independent Handover (MIH) Service [IEEE802.21], and
   propose three kinds of services: Media Independent Event Service
   (MIES), Media Independent Command Service (MICS), and Media
   Independent Information Service (MIIS).

   An MIES defines the events triggered from lower layers (physical and
   link) to higher layers (network and above) in order to report changes
   of physical and link-layer conditions.  On the other hand, an MICS
   supports the commands sent from higher layers to lower layers, and it
   provides users with a way of managing the link behavior relevant to
   handovers and mobility.  An MIIS provides a framework by which the MN
   or network can obtain network information to facilitate network
   selection and handovers.

   Although the purpose of IEEE 802.21 has been developed to enhance the
   user experience of MNs roaming between heterogeneous networks, the
   results may be utilized to optimize the handover performance in a
   homogeneous network.  When the MIH primitives are available for
   handover in the 802.16e network, the MN can use them instead of the
   primitives proposed in this document.  Table 1 shows examples of the
   mapping between the proposed primitives and the MIH primitives.

           +-------------------------+-------------------------+
           |   Proposed primitives   |      MIH primitives     |
           +===================================================+
           |  NEW_LINK_DETECTED      |  LINK_DETECTED          |
           +---------------------------------------------------+
           |  LINK_HANDOVER_IMPEND   |  LINK_HANDOVER_IMMINENT |
           +---------------------------------------------------+
           |  LINK_SWITCH            |  HANDOVER_COMMIT        |
           +---------------------------------------------------+
           |  LINK_UP                |  LINK_UP                |
           +---------------------------------------------------+

            Table 1. The Proposed Primitives and MIH Primitives

8.  Security Considerations

   The primitives defined in this document are used only for local
   indication inside of the MN, so no security mechanism is required to
   protect those primitives.  However, FMIPv6 messages and IEEE 802.16e
   messages, which may trigger the primitives, need to be protected.




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   Security considerations of the FMIPv6 specification [RFC5268] are
   applicable to this document.  It is also worthwhile to note that the
   IEEE802.16e has a security sub-layer that provides subscribers with
   privacy and authentication over the broadband wireless network.  This
   layer has two main component protocols: a privacy key management
   protocol (PKM) for key management and authentication and an
   encapsulation protocol for encrypting data.  From the perspective of
   the 802.16e, FMIPv6 messages are considered as data and are delivered
   securely by using those protocols.

   However, some of IEEE 802.16e management messages are sent without
   authentication.  For example, there is no protection to secure
   802.16e broadcast messages.  It may be possible for the attacker to
   maliciously forge a MOB_NBR-ADV message so that it contains the bogus
   BSIDs, or send a flood of MOB_NBR-ADV messages having different bogus
   BSIDs toward the MN.  As a result, the MN may trigger a bunch of
   NEW_LINK_DETECTED primitives and send useless consecutive RtSolPr
   messages to the PAR, finally resulting in wasting the air resources.
   Therefore, the MN SHOULD perform scanning when detecting new BSs in
   the received MOB_NBR-ADV messages in order to assure the included
   neighbor information.

   It is also possible that attackers try a DoS (Denial-of-Service)
   attack by sending a flood of MOB_BSHO-REQ messages and triggering
   LINK_HANDOVER_IMPEND primitives in the MN.  But the IEEE 802.16e
   provides a message authentication scheme for management messages
   involved in handover as well as network entry procedures by using a
   message authentication code (MAC) such as HMAC/CMAC (hashed/cipher
   MAC).  Thus, those management messages are protected from the
   malicious use by attackers who intend to trigger LINK_HANDOVER_IMPEND
   or LINK_UP primitives in the MN.

9.  Acknowledgments

   Many thanks to the IETF Mobility Working Group members of KWISF
   (Korea Wireless Internet Standardization Forum) for their efforts on
   this work.  In addition, we would like to thank Alper E. Yegin,
   Jinhyeock Choi, Rajeev Koodli, Jonne Soininen, Gabriel Montenegro,
   Singh Ajoy, Yoshihiro Ohba, Behcet Sarikaya, Vijay Devarapalli, and
   Ved Kafle who have provided technical advice.

10.  References

10.1.  Normative References

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




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

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

   [RFC5268]      Koodli, R., Ed., "Mobile IPv6 Fast Handovers",
                  RFC 5268, June 2008.

   [IEEE802.16]   "IEEE Standard for Local and Metropolitan Area
                  Networks, Part 16: Air Interface for Fixed Broadband
                  Wireless Access Systems", IEEE Std 802.16-2004,
                  October 2004.

   [IEEE802.16e]  "IEEE Standard for Local and Metropolitan Area
                  Networks, Amendment 2: Physical and Medium Access
                  Control Layers for Combined Fixed and Mobile Operation
                  in Licensed Bands and Corrigendum 1", IEEE
                  Std 802.16e-2005 and IEEE Std 802.16-2004/Cor 1-2005,
                  February 2006.

10.2.  Informative References

   [RFC4260]      McCann, P., "Mobile IPv6 Fast Handovers for 802.11
                  Networks", RFC 4260, November 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.

   [RFC4907]      Aboba, B., "Architectural Implications of Link
                  Indications", RFC 4907, June 2007.

   [IEEE802.21]   "Draft IEEE Standard for Local and Metropolitan Area
                  Networks: Media Independent Handover Services", IEEE
                  Std P802.21 D9.0, February 2008.

   [SH802.16e]    Kim, K., Kim, C., and T. Kim, "A Seamless Handover
                  Mechanism for IEEE 802.16e Broadband Wireless Access",
                  International Conference on Computational Science vol.
                  2, pp.527-534, 2005.









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Authors' Addresses

   Heejin Jang
   SAMSUNG Advanced Institute of Technology
   P.O. Box 111
   Suwon 440-600
   Korea

   EMail: heejin.jang@gmail.com


   Junghoon Jee
   Electronics and Telecommunications Research Institute
   161 Gajeong-dong, Yuseong-gu
   Daejon 305-350
   Korea

   EMail: jhjee@etri.re.kr


   Youn-Hee Han
   Korea University of Technology and Education
   Gajeon-ri, Byeongcheon-myeon
   Cheonan 330-708
   Korea

   EMail: yhhan@kut.ac.kr


   Soohong Daniel Park
   SAMSUNG Electronics
   416 Maetan-3dong, Yeongtong-gu
   Suwon 442-742
   Korea

   EMail: soohong.park@samsung.com


   Jaesun Cha
   Electronics and Telecommunications Research Institute
   161 Gajeong-dong, Yuseong-gu
   Daejon 305-350
   Korea

   EMail: jscha@etri.re.kr






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