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Versions: 00 01 02 03 04 05 06 07 08 RFC 4066

IETF Seamoby Working Group
Internet Draft                                            Marco Liebsch
                                                             Ajoy Singh
                                                              (Editors)
                                                         Hemant Chaskar
                                                          Daichi Funato
                                                            Eunsoo Shim
draft-ietf-seamoby-card-protocol-02.txt
Expires: December 2003                                        June 2003


                     Candidate Access Router Discovery


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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Abstract

   To enable seamless IP-layer handover of a mobile node (MN) from one
   access router (AR) to another, the MN is required to discover the
   identities of candidate ARs (CARs) for handover, along with their
   capabilities, prior to the initiation of the IP-layer handover. The
   act of discovery of CARs has two aspects to it: Identifying the IP
   addresses of the CARs and finding the capabilities of those CARs.
   This process is called "candidate access router discovery" (CARD).
   At the time of IP-layer handover, that CAR, whose capabilities is a
   good match to the preferences of the MN, may be chosen as the target
   AR for handover. The protocol described in this document allows a
   mobile node to perform CARD.



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TABLE OF CONTENTS


   1.  INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . .  4

   2.  TERMINOLOGY. . . . . . . . . . . . . . . . . . . . . . . . .  5

   3.  CARD PROTOCOL FUNCTIONS. . . . . . . . . . . . . . . . . . .  6
     3.1 Reverse Address Translation. . . . . . . . . . . . . . . .  6
     3.2 Discovery of CAR Capabilities. . . . . . . . . . . . . . .  6

   4.  CARD PROTOCOL OPERATION. . . . . . . . . . . . . . . . . . .  7
     4.1 Conceptual Data Structures . . . . . . . . . . . . . . . . 10
     4.2 Mobile Node - Access Router Operation. . . . . . . . . . . 10
       4.2.1 Mobile Node Operation. . . . . . . . . . . . . . . . . 10
       4.2.2 Current Access Router Operation. . . . . . . . . . . . 11
     4.3 Current Access Router - Candidate Access Router Operation. 12
       4.3.1 Current Access Router Operation. . . . . . . . . . . . 12
       4.3.2 Candidate Access Router Operation. . . . . . . . . . . 12
     4.4 CARD Signaling Failure Recovery. . . . . . . . . . . . . . 13
       4.4.1 MN-AR Signaling Failure. . . . . . . . . . . . . . . . 13
       4.4.2 AR-AR Signaling Failure. . . . . . . . . . . . . . . . 13
     4.5 CARD Protocol Message Piggybacking on the MN-AR Interface. 14
     4.6 CARD Protocol Security . . . . . . . . . . . . . . . . . . 14

   5.  PROTOCOL MESSAGES. . . . . . . . . . . . . . . . . . . . . . 15
     5.1 CARD Messages for the Mobile Node-Access Router interface. 15
       5.1.1 CARD Main Header Format. . . . . . . . . . . . . . . . 15
       5.1.2 CARD Options Format. . . . . . . . . . . . . . . . . . 17
         5.1.2.1 CARD Request Option. . . . . . . . . . . . . . . . 18
         5.1.2.2 CARD Reply Option. . . . . . . . . . . . . . . . . 18
       5.1.3 Sub-Options Format . . . . . . . . . . . . . . . . . . 19
         5.1.3.1 L2 ID Sub-Option . . . . . . . . . . . . . . . . . 20
         5.1.3.2 Preferences Sub-Option . . . . . . . . . . . . . . 21
         5.1.3.3 Requirements Sub-Option. . . . . . . . . . . . . . 21
         5.1.3.4 Capability Container Sub-Option. . . . . . . . . . 22
         5.1.3.5 Address Sub-Option . . . . . . . . . . . . . . . . 23
       5.1.4 Capability AVP Encoding Rule . . . . . . . . . . . . . 23
     5.2 CARD Messages for the inter-Access Router Protocol
              Operation . . . . . . . . . . . . . . . . . . . . . . 24
       5.2.1 Protocol Transport . . . . . . . . . . . . . . . . . . 24
       5.2.2 Protocol Main Header . . . . . . . . . . . . . . . . . 25
       5.2.3 Protocol Payload Types . . . . . . . . . . . . . . . . 25
     5.3 Overview on sub-options'/payload types' usage. . . . . . . 26

   6.  SECURITY CONSIDERATIONS. . . . . . . . . . . . . . . . . . . 27
     6.1 Assumptions  . . . . . . . . . . . . . . . . . . . . . . . 27
     6.2 Security Association between AR and AR . . . . . . . . . . 27


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     6.3 Security Association between AR and MN . . . . . . . . . . 28
     6.4 DoS Attack . . . . . . . . . . . . . . . . . . . . . . . . 28

   7.  PROTOCOL CONSTANTS . . . . . . . . . . . . . . . . . . . . . 29

   8.  IANA CONSIDERATIONS  . . . . . . . . . . . . . . . . . . . . 29

   9.  NORMATIVE REFERENCES . . . . . . . . . . . . . . . . . . . . 30

   10. INFORMATIVE REFERENCES . . . . . . . . . . . . . . . . . . . 30

   11. AUTHORS' ADDRESSES . . . . . . . . . . . . . . . . . . . . . 31

   12. IPR STATEMENTS . . . . . . . . . . . . . . . . . . . . . . . 32

   13. COPYRIGHT NOTICE . . . . . . . . . . . . . . . . . . . . . . 32

   14. ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . . 32

   Appendix A  MAINTENANCE OF ADDRESS MAPPING TABLES IN
               ACCESS ROUTERS . . . . . . . . . . . . . . . . . . . 33
   Appendix A.1 Centralized Approach using a Server Functional
                 Entity . . . . . . . . . . . . . . . . . . . . . . 33
   Appendix   A.1.1 Approach  . . . . . . . . . . . . . . . . . . . 33
   Appendix   A.1.2 Associated Protocol Operation . . . . . . . . . 34
   Appendix   A.1.3 Associated Protocol Messages. . . . . . . . . . 36
   Appendix     A.1.3.1 CARD Message Transport for the Interface
                between an AR and the CARD Server . . . . . . . . . 36
   Appendix     A.1.3.2 Protocol Main Header. . . . . . . . . . . . 36
   Appendix     A.1.3.3 Protocol Payload Types. . . . . . . . . . . 37
   Appendix   A.1.4 Associated Security Considerations. . . . . . . 37
   Appendix     A.1.4.1 Security Associations . . . . . . . . . . . 37
   Appendix     A.1.4.2 DoS Attack. . . . . . . . . . . . . . . . . 38
   Appendix     A.1.4.3 CAR Table Contamination . . . . . . . . . . 38
   Appendix   A.1.5 Associated IPR statements . . . . . . . . . . . 39

   Appendix A.2  Decentralized Approach using Mobile Terminals'
                 Handover . . . . . . . . . . . . . . . . . . . . . 40
   Appendix   A.2.1 Approach. . . . . . . . . . . . . . . . . . . . 40
   Appendix   A.2.2 Associated Protocol Operation . . . . . . . . . 40
   Appendix   A.2.3 Associated Protocol Messages. . . . . . . . . . 42
   Appendix   A.2.4 Associated Security Considerations. . . . . . . 43
   Appendix   A.2.5 Associated IPR Statements . . . . . . . . . . . 43

   Appendix B  APPLICATION SCENARIOS. . . . . . . . . . . . . . . . 44
   Appendix B.1 CARD Operation in a Mobile-IPv6 Enabled Wireless
                LAN Network . . . . . . . . . . . . . . . . . . . . 44
   Appendix B.2 CARD operation in a Fast Mobile-IPv6 enabled
                network . . . . . . . . . . . . . . . . . . . . . . 47



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Conventions used in this document

   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 [1].


1. INTRODUCTION

   IP mobility protocols, such as Mobile IP, enable mobile nodes to
   execute IP-level handover among access routers. Additionally, work
   is underway [7][15] to extend the mobility protocols to allow
   seamless IP handover. The pre-requisite for the seamless IP mobility
   protocols is the knowledge of candidate access routers (CARs) to
   which a mobile node can be handed over to. The CAR discovery
   protocol enables to acquire information about the access routers
   that are candidates for the mobile node's next handover.

   The CAR discovery involves identifying a CAR's IP address as well as
   its capabilities that the mobile node might use for its handover
   decision. There are cases when a mobile node has a choice of
   candidates to perform handover to different CARs. The mobile node
   would choose one based on a match between the mobile node's
   requirements on a handover candidate and the CAR's capabilities.
   However, the decision algorithm itself is out of scope of this
   document.

   The problem statement of the CAR discovery is discussed in [2]. In
   this document, a protocol is described to perform CAR discovery.
   Section 3 describes two main functions of the CAR discovery
   protocol. Then, section 4 describes the core part of the CARD
   protocol operation. Finally, the protocol messages' format is
   described in section 5.

   In Appendix A, two optional approaches are described to build a
   local table (CAR table), holding CARs' IP addresses and associated
   access points' layer-2 addresses, dynamically in access routers.
   This mapping is required in access routers to identify an individual
   CAR's IP address and to perform reverse address translation.
   However, the core protocol, as described in this document up to
   section 5, assumes this local CAR table (section 4.1) in access
   routers to be available and filled with the IP addresses of the CARs
   (and their associated APs' L2 addresses) throughout the core part of
   the draft.







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2. TERMINOLOGY

   This document uses terminology defined in TERMS [6].

   In addition, the following terms are used:

   Mobile Node (MN)

   A Mobile Node is an IP host capable of moving its point of
   attachment to the Internet.

   Access Point (AP)

   A radio transceiver by which a MN obtains Layer 2 connectivity with
   the wired network.

   Access Router (AR)

   An IP router residing in an access network and connected to one or
   more APs. An AR offers IP connectivity to MNs.

   Candidate AR (CAR)

   An AR to which a MN has a choice of performing IP-level handover.

   Capability of an AR

   A characteristic of the service offered by an AR that may be of
   interest to a MN when the AR is being considered as a handover
   candidate.

   L2 ID

   Identifier of an AP that uniquely identifies that AP. For example,
   in 802.11 PCF, this could be a MAC address of an AP.
















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3. CARD PROTOCOL FUNCTIONS

   A CARD protocol accomplishes the following functions.


   3.1 Reverse Address Translation

   If a MN can listen to L2 IDs of new APs prior to making decision
   about IP-level handover to CARs, a mechanism is needed for reverse
   address translation. This function of the CARD protocol enables the
   MN to map the received L2 ID of an AP to the IP address of the
   associated CAR that connects to the AP. To get the CAR's IP address,
   the MN sends the L2 ID of the AP to the current AR and the current
   AR provides the associated CAR's IP address to the MN.

   In cases where the MN can acquire IP connectivity with CARs prior to
   making handover decisions, this functionality is trivially realized,
   since the MN can request CARs individually for reverse address
   translation.


   3.2 Discovery of CAR Capabilities

   Information about capabilities of CARs can assist the MN in making
   optimized handover decisions. This capability information serves as
   input to the target AR selection algorithm. Some of the capability
   parameters of CARs can be static, while some others can change with
   time.

   Definition of capabilities is out of scope of the CARD protocol
   design. Encoding rules for capabilities and the format of a
   capability container for capability transport are specified in
   section 5.

   There are two approaches for MNs to acquire address and capability
   information of CARs. One is that the MN sends an explicit request to
   its current AR and the current AR provides address and capability
   information to the MN. The other is that the current AR either
   periodically transmits address and capability information of CARs to
   the MNs over download channels, or link-layer mechanisms trigger
   unsolicited transmission of CARs' address and capability
   information.









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4. CARD PROTOCOL OPERATION

   The CARD protocol is used to allow MNs resolving the L2 ID of one or
   more APs, which are candidates the MN may initiate a handover to, to
   the IP address of the associated CARs, as well as to discover these
   CARs' capabilities.  Furthermore, the protocol allows populating
   ARs' CAR tables (section 4.1) with the capabilities of CARs.

   For this, the CARD protocol makes use of a CARD Request and CARD
   Reply protocol message handshake between a MN and its current AR,
   and between a MN's current AR and individual CARs respectively. CARD
   Request and CARD Reply messages are used on the interface between a
   MN and its current AR to allow MNs retrieving CARs' address and
   capability parameter specific information from the network. To allow
   ARs populating and maintaining their local CAR table with capability
   parameter information of CARs, a CARD Request and CARD Reply
   protocol message handshake is also used on the interface between a
   MN's current AR and CARs to allow updating ARs' CAR table entries
   with CARs' capability information.

   An access point's L2 ID, a CAR's IP address and associated
   capability information is carried as CARD protocol message parameter
   with a CARD Request or a CARD Reply message respectively. A CAR's
   capabilities are specified as a list of attribute-value pairs, which
   is conveyed in a Capability Container message parameter.

   The CARD protocol enables the MN's current AR to exchange
   capabilities with CARs and to subsequently convey appropriate
   capabilities to the connected MNs. Information about the CAR(s) and
   associated capabilities MAY be used by the MN to perform target
   access router selection during its IP handover. The current AR
   initiates capability exchange with a CAR either when it receives a
   CARD Request message from a MN, containing possibly parameters
   carrying identifier(s) (L2 ID) of newly discovered AP(s), or when it
   detects that some of its CAR table's capability entries are about to
   expire. Upon completion of the MN-solicited capability exchange
   between a MN's current AR and CARs, the current AR MUST notify the
   desired capabilities to the MN by sending a CARD Reply message
   having the desired message parameters appended. The current AR MAY
   also send periodically unsolicited CARD Reply messages to all
   connected MNs. This behavior of the AR SHALL depend upon the local
   policies of the network service providers and need to be configured
   administratively.

   The unsolicited CARD Reply SHALL be broadcast from ARs to all the
   connected MNs. For unsolicited CARD Reply messages sent to connected
   MNs, the AR MUST set the U-flag of the CARD Reply to indicate to MNs
   that this particular CARD Reply message has been sent unsolicited.



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   The CARD protocol also enables a MN to optionally indicate its
   preferences on capabilities of interest to its current AR, which
   allows the MN's current AR performing optional capability pre-
   filtering for optimization purposes. Appending the optional
   Preferences message parameter for a CARD Request message, which is
   sent to the MN's current AR, the MN can indicate a list of
   capability attributes, which are of interest to the MN, to its
   current AR. The AR now returns only these capabilities of interest
   to the requesting MN. The format of this optional Preferences
   message parameter is described in section 5.1.3.2.

   Optionally, the MN can provide its current AR with a list of
   capability attribute-value pairs, indicating not only the capability
   parameters (attributes) as required for capability pre-filtering,
   but also a specific value for a particular capability. This allows
   the MN's current AR performing CAR pre-filtering and to send only
   address and capability information of CARs, whose capability values
   meet the requirements of the MN, back to the requesting MN. The
   format of this optional Requirements message parameter is described
   in section 5.1.3.3.

   As an example, using the optional Preferences message parameter, a
   MN may indicate to its current AR that it is interested only in
   IEEE802.11 interface specific capability parameters, since this is
   the only interface the MN has implemented. Hence, the MN's current
   AR sends back only CARs' IEEE802.11 specific capabilities.
   Similarly, using the optional Requirements message parameter, a MN
   MAY indicate to its current AR that it is only interested in CARs
   that can satisfy a given QoS constraint. Here, a MN sends the
   respective QoS attribute with the QoS constraint value to its
   current AR using the optional Requirements message parameter. The
   QoS constraint is denoted as an attribute-value pair and
   encapsulated with the Requirements message parameter, which is
   appended to the MN-originated CARD Request message. Based on the
   received optional list of attributes in the Preferences parameter or
   a list of attribute-value pairs in the Requirements message
   parameter, the MN's current AR MAY use these parameters for deciding
   the content of the solicited CARD Reply message, which is to be sent
   back to the MN. Alternatively, in case no optimization with regard
   to capability or CAR pre-filtering is performed by the MN's current
   AR, the current AR MAY choose to silently ignore the optional
   Requirements and Preferences message parameter as received in the
   CARD Request message.








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   The CARD protocol operation, as described in this section,
   distinguishes signaling messages exchanged between a MN and its
   connecting AR and signaling messages exchanged between ARs. Hence,
   description of signaling messages described in the following
   sections have a preceding identifier, referring to the associated
   interface. Messages that are exchanged between a MN and AR are
   precluded with "MN-AR", messages between ARs with "AR-AR"
   respectively.



          +--------------+  (3)AR-AR CARD Request   +----------+
          |  Current     |------------------------->|    CAR   |
          |     AR       |<-------------------------|          |
          +--------------+  (4)AR-AR CARD Reply     +----------+
              ^      |
              |      |    MN-AR
      MN-AR   |      | CARD Reply(5)
   CARD Request(2)   V
           +--------------+
           |    Mobile    |
           |     Node     |<-- CARD Init Trigger
           +--------------+       (1)


                Figure 1: MN initiated CARD Protocol Overview


   Figure 1 describes the operation of the MN initiated CARD
   Request/Reply-based protocol operation. On reception of access
   points' L2 IDs or the appearance of a CARD initiation trigger (1),
   the MN passes on one or more L2 ID(s) to its current AR using the
   MN-AR CARD Request message (2). The MN's current AR resolves the L2
   ID to the IP address of the associated CAR or, in case the MN has
   not attached one or more L2 ID message parameters, it just reads out
   all CARs' IP address information using the reverse address
   translation information (L2 ID to IP address mapping) from its local
   CAR table. In case one or more capability entries have expired in
   the current AR's CAR table, the current AR then directly contacts
   the CAR and performs capability discovery with it by performing an
   AR-AR CARD Request (3) and AR-AR CARD Reply (4) protocol message
   handshake to retrieve individual CARs' capability information. The
   current AR then updates capability entries in its local CAR table
   and passes on the IP address of the CAR(s) and associated
   capabilities to the MN using the MN-AR CARD Reply message (5).

   Since the MN-AR CARD Request is sent when a MN discovers new AP(s)
   during link layer scanning, sometimes a MN might send frequent MN-AR
   CARD Requests, thereby overwhelming its current AR with CARD Request
   signaling messages. To counteract this problem, the AR SHOULD set


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   the R-flag (rate limiting) of a subsequent CARD Reply message for
   flow-control purposes (section 5.1.2.2), thereby requesting the MN
   to reduce the generation rate of MN-AR CARD Requests. Upon receipt
   of the MN-AR CARD Reply with the R-flag set, the requesting MN MUST
   reduce the rate of generation of MN-AR CARD Requests. The exact
   implementation of a rate-limiting algorithm should be decided by the
   implementers.


   4.1 Conceptual Data Structures

   AR(s) SHALL maintain a L2-L3 address mapping table (CAR table) that
   will be used to resolve L2 IDs of candidate APs to the IP address of
   associated CARs. This address-mapping table can be configured
   statically for the CARD protocol operation. Optionally, the CAR
   table MAY be populated dynamically, using either a server-based or a
   handover-based approach, as described in appendices A.1 and A.2
   respectively.

   ARs SHALL also keep and maintain individual CARs' capabilities in
   the local CAR table, taking the associated capability lifetime into
   account. If the lifetime of an individual capability entry has
   expired, the respective capability is to be discovered and to be
   updated when requested from a connected MN. The ARs' CAR table may
   be implemented differently by the different implementations, hence
   additional details are not provided here.


   4.2 Mobile Node - Access Router Operation

   4.2.1 Mobile Node Operation

   To initiate CARD, a MN sends a CARD Request to its current AR,
   requesting it to resolve the L2 ID of nearby access points to the IP
   address of associated CARs, and also to obtain capability parameters
   associated with these CARs. In case the requesting MN want its
   current AR to resolve specific L2 IDs, the MN-AR CARD Request SHOULD
   contain the CARD protocol specific L2 ID message parameters,
   carrying the L2 ID of respective access points, for which reverse
   address translation to associated CARs' IP address as well as CARs'
   capability information is being requested. The CARD Request MAY also
   contain the Preferences or Requirements message parameter,
   indicating the MN's preferences on capability attributes of interest
   or its requirements on CARs' capability attribute-value pairs to its
   current AR. For example, using the Preferences message parameter, a
   MN may indicate that it is only interested in these CAR(s)
   supporting a specific air interface technology. Similarly, using the
   Requirements message parameter, a MN can indicate the list of
   capability attributes and associated capabilities' values to its
   current AR. The Requirements message parameter may be used to


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   indicate the cut off values of the capabilities for the desired
   CAR(s). The MN's current AR MAY use the Preferences and Requirement
   message parameter to decide about a sub-set of the CAR(s) that can
   satisfy the MN's need.

   Upon receipt of the corresponding MN-AR CARD Reply message, which
   has been sent by the MN's current AR in response to the MN's
   previously sent request, the MN processes all MN-AR CARD Reply
   message parameters to retrieve its CARs' address and capability
   information.


   4.2.2 Current Access Router Operation

   Upon receipt of the requesting MN's MN-AR CARD-Request, containing
   one or multiple L2 ID message parameters, the connected AR SHALL
   resolve the requested APs' L2 ID to the IP address of the associated
   CAR(s). In case no L2 ID parameter has been sent with the MN-AR CARD
   Request message, the MN's current AR retrieves all CARs' IP address
   and capability information from its local CAR table. Optionally,
   when allowed by local policies and supported by respective ARs, the
   AR MAY retrieve a subset of capabilities or CARs, satisfying the
   optionally appended Preferences and Requirement message parameter,
   from its local CAR table. CARs' address information along with
   associated capabilities are then delivered to the MN using the MN-AR
   CARD Reply message, having the Address message parameters and
   appropriate Capability Container parameters appended. The CARs' IP
   address as well as the capabilities SHALL be encoded according to
   the format for CARD protocol message parameters as defined in
   section 5.1.3 of this document. The capabilities are encoded as
   attribute-value pairs, which are to be encapsulated in a Capability
   Container message parameter according to the format defined in
   section 5.1.3.4. The responding current AR shall copy the sequence
   number received in the MN-AR CARD Request to the MN-AR CARD Reply.

   The CARD protocol optionally allows service providers to configure
   AR to send periodic unsolicited CARD Reply Messages to all connected
   mobile nodes. The unsolicited CARD Reply is delivered as broadcast
   message to MN(s). The current AR sets the U-flag of the unsolicited
   CARD Reply to indicate that the message is being sent unsolicited.
   The interval between consecutive periodic broadcast is a
   configurable parameter and SHALL be configured by the network
   administrators.









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   4.3 Current Access Router - Candidate Access Router Operation

   4.3.1 Current Access Router Operation

   The MN's current AR MAY initiate capability exchange with CARs
   either when it receives a MN-AR CARD Request or when it detects that
   one or multiple of its local CAR table's capability entries'
   lifetime is about to expire.

   Upon receipt of a MN-AR CARD Request, the MN's current AR retrieves
   the IP address of the associated CAR(s) from its local CARD table.
   Then the AR SHOULD issue an AR-AR CARD Request to the respective
   CAR(s) if complete capability information of a CAR is not available
   in the current AR's CAR table. The AR MAY also issue the AR-AR CARD
   Request when it detects that one or multiple of its local CAR
   table's entries are about to expire. The AR-AR CARD Request message
   format is defined in section 5.2.2. The AR MUST set the sequence
   number of the CARD Request to one more than the previously used
   sequence number value. The AR MAY append its own capabilities,
   encoded as attribute-value pairs and encapsulated with the
   Capability Container message parameter, to the released AR-AR CARD
   Request. The MN's current AR SHALL use the IPsec ESP for
   authenticating the AR-AR CARD Request. The IPsec ESP MAY be also
   used for encrypting the capability information.

   Upon receipt of the AR-AR CARD Reply, which has been sent by the CAR
   in response to the previously sent request, the MN's current AR
   SHALL extract the capability information from the payload of the
   received message and buffer the received capabilities in its local
   CAR table. The lifetime of individual capabilities is to be set
   according to the lifetime indicated for each capability received.
   The value of the table entries' timeout shall depend upon the nature
   of individual capabilities. Then the AR MUST send the MN-AR CARD
   Reply to the Mobile Node.


   4.3.2 Candidate Access Router Operation

   Upon receipt of a AR-AR CARD Request, a CAR shall extract the
   capabilities of the MN's current AR from the payload of the received
   message. The CAR SHALL buffer the received capabilities in its CAR
   table and set the timer for individual capabilities appropriately.
   The value of the table entries' timeout depends upon the nature of
   capabilities received. The CAR then MUST respond with the AR-AR CARD
   Reply message. The CAR MUST include the same sequence number
   received in AR-AR CARD Request message to the AR-AR CARD Reply
   message. The AR-AR CARD Reply shall include the CAR's capabilities
   as list of attribute-value pairs in the Capability Container message



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   parameter. The CAR SHALL use IPsec ESP for authentication or
   optionally encryption of the AR-AR CARD Reply message.


   4.4 CARD Signaling Failure Recovery

   For a variety of reasons, the packets carrying CARD protocol
   signaling may be dropped. In this section we consider mechanisms for
   recovery from the CARD signaling failures. Broadly the CARD
   signaling failures can be categorized in MN-AR signaling failures
   and AR-AR signaling failures.


   4.4.1 MN-AR Signaling Failure

   It is likely that either a CARD Request or CARD Reply may be dropped
   due to poor radio link conditions. A MN SHALL detect the loss of a
   MN-AR CARD Request or MN-AR CARD Reply Message using a timeout
   mechanism (MN_AR_CARD_TIMEOUT). The AR SHALL start a timer
   (MN_AR_CARD_TIMER) after sending a MN-AR CARD Request message with
   the given sequence number. The MN SHALL stop the timer as soon as
   the reply to the MN-AR CARD Request is received by it. Upon
   expiration of the MN_AR_CARD_TIMER, the MN SHALL declare the
   outstanding message as lost, resends the same message and restart
   the MN_AR_CARD_TIMER. The MN shall retry the MN-AR CARD Request for
   a pre-configured number of times (MN_AR_CARD_RETRIES) before
   declaring the protocol message exchange aborted. The MN SHALL
   silently discard any duplicate MN-AR CARD Reply messages received
   from its current AR.


   4.4.2 AR-AR Signaling Failure

   It is likely that a AR-AR CARD Request or AR-AR CARD Reply may be
   dropped due to congestion at the intermediate routers or poor link
   conditions. The MN's current AR SHALL detect the loss of an AR-AR
   CARD Request or an AR-AR CARD Reply message using a timeout
   mechanism (AR_AR_CARD_TIMEOUT). The current AR SHALL start a timer
   (AR_AR_CARD_TIMER) after sending the AR-AR CARD Request with the
   given sequence number. The current AR SHALL stop the timer as soon
   as the reply to the AR-AR CARD Request is received by it. Upon
   expiration of the AR_AR_CARD_TIMER, the MN's current AR SHALL
   declare the outstanding AR-AR CARD Request as lost and then resends
   the same message to the CAR. The current AR SHALL retry the AR-AR
   CARD Request message for a pre-configured number of times
   (AR_AR_CARD_RETRIES) before declaring the protocol message exchange
   as aborted. The current AR SHALL silently discard any duplicate AR-
   AR CARD Reply received from the CAR.




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   4.5 CARD Protocol Message Piggybacking on the MN-AR Interface

   To allow MNs and ARs appending the ICMP-option type CARD Request and
   CARD Reply (section 5.1.2) to the ICMP-type Fast Mobile IPv6
   signaling messages (CARD protocol message piggybacking), MN and AR
   should know about the signaling peer's capability for CARD protocol
   message piggybacking. This requires dynamic discovery of
   piggybacking capability using the P-flag in the MN-AR CARD Request
   and the MN-AR CARD Reply message, as well as in the Capability
   Container message parameter, as described in detail in section 5.1.
   When not receiving an unsolicited CARD Reply message from the MN's
   current AR, the MN sends the very first CARD Request to its current
   AR using the ICMP-type CARD main header for transport, as described
   in section 5.1.1. In case the MN supports CARD protocol message
   piggybacking, the P-flag in this very first CARD Request message is
   to be set. On reception of the CARD Request message, the MN's
   current AR learns about the MN's piggybacking capability. To
   indicate its own capability to convey CARD protocol messages with
   Fast Mobile IPv6 protocol messages, the AR sets the P-flag in the
   CARD Reply message. In case the AR does not support CARD protocol
   message piggybacking, all subsequent CARD protocol messages between
   the MN and this particular AR are to be sent stand-alone, using the
   CARD main header. In case both nodes, the MN and its current AR,
   support CARD protocol message piggybacking, subsequent CARD protocol
   messages can be conveyed as an option via the Fast Mobile IPv6
   RtSolPr and PrRtAdv message. During the CARD process, a MN learns
   about its CARs' piggybacking capability already during the discovery
   phase, since the Capability Container, as described in section
   5.1.3.4, carries also a P-flag, which is to be set appropriately
   from respective CARs whose capabilities are encapsulated. This
   allows the MN to immediately perform CARD protocol message
   piggybacking after a handover to a selected CAR, assumed this CAR
   supports CARD protocol piggybacking.

   An application scenario of the CARD-function enabled Fast Mobile-
   IPv6 protocol, which carries CARD protocol messages between a MN and
   its current AR by means of CARD protocol message piggybacking, is
   described in Appendix B.2.


   4.6 CARD Protocol Security

   The MN-AR and AR-AR messages SHALL be protected using IPsec ESP
   [10]. It is safe to assume that there will be an appropriate SA
   between a MN and its connected AR, which MAY be used to secure MN-AR
   CARD Message. It is also assumed that neighboring ARs SHALL
   establish an appropriate SA to secure the AR-AR CARD messages.




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5. PROTOCOL MESSAGES

   5.1 CARD Messages for the Mobile Node-Access Router Interface

   5.1.1 CARD Main Header Format

   Hosts and Access Routers use the CARD ICMP-type main header when
   CARD protocol messages, which are to be exchanged between a MN and
   an AR, cannot be conveyed via another outgoing ICMP-type message, as
   for example the Fast Mobile-IPv6 'Router Solicitation for Proxy' or
   'Proxy Router Advertisement' [7] 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      |     Code      |          Checksum             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Reserved                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Options ...
     +-+-+-+-+-+-+-+-+-+-+-+- - - -

   IP Fields:

         Source Address:
                        An IP address assigned to the sending
                        interface.

         Destination Address:
                        An IP address assigned to the receiving
                        interface.

         Hop Limit:     255

         Encapsulating Security Payload (ESP) Header:
                        The sender SHOULD include the Encapsulating
                        Security Payload (ESP) Header, based on the
                        previously established Security Association
                        between the sender and the receiver.

      ICMP Fields:

         Type           T.B.A (To be assigned)

         Code           0

         Checksum       The ICMP checksum.



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         Reserved       This field is currently unused. It MUST be
                        initialized with zero by the sender and MUST be
                        ignored by the receiver.

   Valid Options:

         CARD Request: The CARD Request allows entities to request CARD
                       specific information from ARs. To process the
                       CARD Request message on the receiver side,
                       further sub-options must be carried, serving as
                       input to the reverse address translation
                       function and/or capability discovery function.

         CARD Reply:   The CARD Reply carries parameters, previously
                       requested with a CARD Request, back to the
                       sender of the CARD Request. In case of
                       unsolicited address information and capabilities
                       are to be sent to a node, the sender uses the
                       CARD Reply without getting an explicit CARD
                       Request before. Further sub-options will be
                       associated with the CARD Reply message.

   Valid Sub-Options:

         Layer-2 ID (mandatory):
                        The Layer-2 ID sub-option carries information
                        about the type of an access point as well as
                        the Layer-2 address of the access point
                        associated with the CAR, whose IP address and
                        capability information is to be resolved.

         Preferences sub-option (optional):
                        The Preferences sub-option carries information
                        about attributes of interest to the requesting
                        entity. Attributes are encoded according to the
                        AVP encoding rule as described in section
                        5.1.4. For proper settings of AVP Code and Data
                        field, please see section 5.1.3.2. This sub-
                        option is used only in case of performing
                        optional capability pre-filtering on ARs and
                        allows for providing only capabilities of
                        interest to a requesting MN.

         Requirements (optional):
                        The Requirements sub-option carries information
                        about attribute-value pairs required for pre-
                        filtering of CARs on a MN's current AR. This
                        parameter conveys MN specific attribute-value
                        pairs to allow a MN's current AR to send only
                        CARs of interest, meaning CARs matching the


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                        MN's requirements according to the attributes'
                        values, back to the requesting MN. CARs are
                        filtered on ARs according to CARs' capability
                        parameters and given policy or threshold, as
                        encoded in the Requirements sub-option.
                        Attribute-value pairs are encoded according to
                        the AVP encoding rule as described in section
                        5.1.4. Setting rules of AVP Code and Data field
                        for the Requirements sub-option are described
                        in section 5.1.3.3.

         Capability container (mandatory):
                        The Capability container sub-option carries
                        information about a single CAR's capabilities.
                        The format of this sub-option is described in
                        section 5.1.3.4.

         Address (mandatory):
                        The Address sub-option carries information on
                        an individual CAR's resolved IP address. The
                        format of the Address sub-option is described
                        in section 5.1.3.5.


   5.1.2 CARD Options Format

   All options are of the form:

      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     |              ...              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                              ...                              ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Fields:

         Type:          8-bit identifier of the type of option. The
                        options defined in this document are:

               Option Name                             Type
            --------------------------------------------------
            MN-AR CARD Request                         T.B.A
            MN-AR CARD Reply                           T.B.A


         Length:        8-bit unsigned integer. The length of the
                        option including the type and length fields in
                        units of octets.  The value 0 is invalid.


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   5.1.2.1 CARD Request 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     |P|         Reserved            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Sequence Number                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Sub-Options
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -  -  -

   Fields:

      Type:    T.B.A

      Length:  The length of the option in units of octets, including
               the type and length fields as well as sub-options.

      Flags:   P-flag:  Indicates CARD protocol message piggybacking
                        capability of the CARD Request message sender.
                        A description for proper use of this flag can
                        be found in section 4.5 of this document.

               Reserved bits MUST be initialized with 0.

      Sequence Number:
               Allows correlating requests with replies.


   Valid Sub-Options:

      - L2 ID sub-option
      - Preferences sub-option
      - Requirements sub-option


    5.1.2.2 CARD Reply 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     |P|U|R|       Reserved          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Sequence Number                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Sub-Options
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -


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   Fields:

      Type:    T.B.A

      Length:  The length of the option in units of octets, including
               the type and length fields as well as sub-options.

      Flags:   P-flag:  Indicates CARD protocol message piggybacking
                        capability of the CARD Request message sender.
                        A description for proper use of this flag can
                        be found in section 4.5 of this document.

               U-flag:  Indicates an unsolicited CARD Reply.
                        A description for proper use of this flag can
                        be found in section 4 of this document.

               R-flag:  Indicates exceeding CARD Request rate
                        limitation. A description for proper use of
                        this flag can be found in section 4 of this
                        document.

               Reserved bits MUST be initialized with 0.

      Sequence Number:
               Allows correlating requests with replies.


   Valid Sub-Options:

      - L2 ID sub-option
      - Capability Container sub-option
      - Address sub-option


   5.1.3 Sub-Options Format

   All Sub-Options are of the form:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Sub-Option Type|Sub-Option Len |       Sub-Option Data . . .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Sub-Option Type:  8-bit identifier of the type of option. The
                     Sub-Options defined in this document are:



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            Sub-Option Name                         Type
            --------------------------------------------
            L2 ID                                   T.B.A
            Preferences                             T.B.A
            Requirements                            T.B.A
            Capability Container                    T.B.A
            Address                                 T.B.A


   Option-Length: 8-bit unsigned integer. The length of the
                  option including the type and length fields in
                  units of octets. The value 0 is invalid.


   5.1.3.1 L2 ID Sub-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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Sub-Option Type|Sub-Option Len |   Context-ID  |M|  L2-Type    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      L2 ID . . .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -


   Sub-Option Type:
                  T.B.A

   Sub-Option Length:
                  Length of the Sub-Option (including type and length
                  fields as well as L2 type indicator) in units of 8
                  octets.

   Context-ID:    Identifies associated L2 ID, IP address and
                  capability information, when coming with separated
                  sub-options.

   M-flag:        This flag indicates that the Context-ID of this
                  particular L2 ID sub-option has been modified by the
                  MN's current AR and set to the same value as a
                  preceding L2 ID received in the same CARD Request
                  message. This adjustment appears in case this L2 ID's
                  associated access point is served by the same CAR as
                  a preceding access point's L2 ID, hence, the same
                  Capability Container and Address sub-option,
                  describing a CAR's IP address and associated
                  capabilities, is valid for this particular L2 ID.




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   L2 type:       Indicates the interface type (optional)
                  (Ethernet, IEEE802.11b, ...).

                  If the L2 type indicator is not used, this field MUST
                  be set to 0.

   L2 ID:         The variable length layer-2 identifier of an
                  individual CAR's access point.


   5.1.3.2 Preferences Sub-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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Sub-Option Type|Sub-Option Len |         Preferences
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Sub-Option Type:
                  T.B.A

   Sub-Option Length:
                  Length of the Sub-Option (including type and length
                  fields) in units of 8 octets.

   Preferences:   AVP encoded preferences (see section 5.1.4).

   AVPs MUST be encoded according to the AVP encoding rule described in
   section 5.1.4. Only ATTRIBUTES (AVP Code) need to be set. The VALUE
   indicator (Data) will not be processed and can be omitted. The 'AVP
   Length' field is to be set appropriately.

   The use of the Preferences sub-option is optional and for
   optimization purpose.

   5.1.3.3 Requirements Sub-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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Sub-Option Type|Sub-Option Len |         Requirements
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Sub-Option Type:
                  T.B.A

   Sub-Option Length:
                  Length of the Sub-Option (including type and length
                  fields) in units of octets.


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   Requirements:  AVP encoded requirements (see section 5.1.4)

   AVPs MUST be encoded according to the rule described in section
   5.1.4. Both, ATTRIBUTES (AVP Code) and VALUES (Data) MUST be set.

   The use of the Requirements sub-option is optional and for
   optimization purpose.


   5.1.3.4 Capability Container Sub-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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Sub-Option Type|Sub-Option Len |   Context-ID  |P|  Reserved   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           AVPs
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -


   Sub-Option Type:
                  T.B.A

   Sub-Option Length:
                  Length of the Sub-Option (including type and length
                  fields as well as AVPs) in units of 8 octets.

   Context-ID:    Identifies L2 ID, IP address and capability triples,
                  coming with separate sub-options.

   Flags:         P-flag: Indicates piggybacking capability of a CAR.
                  This flag allows a MN already after a CARD process to
                  know about a selected new AR's piggybacking
                  capability.

                  Reserved bits MUST be initialized with 0.

   AVPs:          AVPs are a method of encapsulating capability
                  information relevant for the CARD protocol. See
                  section 5.1.4 for the AVP encoding rule.










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   5.1.3.5 Address Sub-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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Sub-Option Type|Sub-Option Len |  Context-ID   | Address Type  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            Address . . .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - -


   Sub-Option Type:
                  T.B.A

   Sub-Option Length:
                  Length of the Sub-Option (including type and length
                  fields) in units of octets.

   Context-ID:    Identifies L2 ID, IP address and capability triples,
                  coming with separate sub-options.

   Address Type:  Indicates the type of the address.

                              0x01  IPv4
                              0x02  IPv6

   Address:       The Candidate Access Router's IP address.


   5.1.4 Capability AVP encoding rule


      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        AVP Code       |S| Res |          AVP Length           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Attribute Lifetime  (present if S = 0)            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Data . . .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -








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   AVP Code:      Identifies the attribute uniquely.

   Flags:         S-flag (Static)   1: identifies a static attribute,
                                       lifetime field is not present.
                                       Data will follow immediately
                                       after the AVP Length field.

                                    0: identifies a dynamic attribute,
                                       lifetime field indicates the
                                       attribute's lifetime.

                  Reserved (Res) flags MUST be set to 0.

   Lifetime:      Specifies the lifetime of the encoded capability
                  in seconds. This field is only present if the encoded
                  capability has a lifetime associated and the S-bit
                  has not been set.

   AVP Length:    The two octet AVP length field indicates the
                  number of octets in this AVP, including the AVP Code,
                  AVP Flags, AVP Length, Lifetime (if present) and
                  Data.


   In case the encoded capability is static and does not change with
   the time, the S-flag MUST be set and the 32-bit Lifetime field is
   not present in the encoded capability. In this case the Data field
   follows immediately the AVP length field. If there is a timeout
   associated with the encoded capability, the S-flag MUST NOT be set
   and the Lifetime field MUST be present.

   Note: This document provides no detailed information on how to
   encode the capability attribute's value, which is to be encoded in
   the Data field of the generic message format described above. Also
   details on the interpretation of individual capability parameters is
   out of scope of this document.


   5.2 CARD Messages for the inter-Access Router Protocol Operation

   5.2.1 Protocol Transport

   For the CARD protocol operation on the network side between a MN's
   current AR and CARs, UDP [9] is used as transport for CARD protocol
   messages. The associated UDP port for the CARD protocol operation is
   T.B.A.

   To authenticate protocol messages between ARs, the IPsec ESP SHOULD
   be used [10].


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   5.2.2 Protocol Main Header

   Protocol main header comprises the first 8 octets:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Version|  Res. |     Type      |            Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Sequence Number                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            Payload ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -


   Version:       Indicates the version of the protocol.
                  The version described in this document is version 1.

   Res.:          This field is currently reserved and MUST be
                  set to 0.

   Type:          Message type.


         Message types specified for this interface:

              Message                     Type
          --------------------------------------
          AR-AR CARD Request              0x01
          AR-AR CARD Reply                0x02


   Length:        Length of the subsequent payload in octets.

   Sequence number:
                  Allows correlating requests with responses.


   5.2.3 Protocol Payload Types

   Payload types and encoding rules are the same as described for the
   various sub-option types in section 5.1 for the MN-AR interface. The
   same TLV-encoded format is used to attach the options as payload to
   the protocol main header.







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   5.3 Overview on sub-options'/payload types' usage

   The following table indicates, which sub-options or payload types
   are relevant for the various interfaces in CARD protocol functions.


         Description                Type              Interface
             |                       |               /         \
             |                       |            MN-AR       AR-AR
     ---------------------------------------------------------------
         L2 ID                     T.B.A            x
         Preferences               T.B.A            x           x
         Requirements              T.B.A            x
         Capability Container      T.B.A            x           x
         Address                   T.B.A            x




































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6. SECURITY CONSIDERATIONS

   6.1 Assumptions

   It is important to note that it is assumed in the protocol that each
   AR has the correct information in the CAR table about the identities
   of the geographically neighboring APs and their associated ARs and
   the association relationship between the APs and the ARs. It is
   assumed that the ARs registered in the CAR table at each AR are
   authorized to participate in the CARD protocol.

   So any security concern regarding the procedure to discover the
   identities is not considered here. Verifying the authorization
   status of particular ARs with respect to participating in the CARD
   protocol is a part of the discovery procedures and thus is not
   considered here either. The appendices of this draft describe
   procedures for discovering the identities of the geographically ARs
   and APs and relevant security considerations.

   It is assumed also that each AR has the correct information about
   APs associated with the AR or capability to get it. It could be done
   as static configuration at the AR or a protocol could be used
   between the AR and the APs for dynamic discovery and exchange of
   information such as MAC addresses and operating channels of the APs.
   It is out of scope of this draft.


   6.2 Security Association between AR and AR

   Each AR receives capability information from its neighboring ARs. If
   the message is not protected from modification, a malicious attacker
   can modify the information, which can cause undesirable impacts on
   the applications using the information. Also if the information is
   delivered in plain text, a third party can read it.

   To prevent the information from being compromised, the CARD REPLY
   messages between ARs SHOULD be authenticated. The messages also MAY
   be encrypted for privacy of the information.

   How to establish a security association is out of scope of this
   memo. But it is assumed that the two CARs can establish a security
   association. IPsec ESP is the default mechanism for message
   authentication between ARs. Also, IPsec ESP is the default method
   for message encryption.

   Which capability information is collected in the CAR table and
   allowed to be disclosed depends on the administration policy. In
   particular, if the CARD protocol runs between ARs in different
   domains as well as within the same domain, different policies could


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   be established regarding capability information disclosure. The
   policy can be implemented locally at each AR and thus it is not
   dealt with here.


   6.3 Security Association between AR and MN

   A malicious node can send bogus CARD REPLY messages to MNs by
   masquerading the AR. So the MN SHOULD authenticate the CARD REPLY
   messages from the AR.


   6.4 DoS Attack

   An AR can be overwhelmed with CARD REQUEST messages or even CARD
   REPLY messages. A MN can also be overwhelmed with CARD REPLY
   messages. The AR or MN SHOULD implement a rate limiting policy about
   sending or responding to the messages so that it does not send or
   process more than a certain number of messages per period. The AR
   should also implement a rate limiting policy in accepting CARD
   REQUEST messages from any particular AR or MN.

   An attacker can send a huge list of capability information by
   masquerading ARs. It can cause overflow in the buffer for the CAR
   table at ARs or MNs. So the AR or the MN should put a limit on the
   size of the capability information for an AR.


























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7. PROTOCOL CONSTANTS

   MN_AR_CARD_TIMEOUT: This timer value indicates the timeout of an
                       expected CARD Reply message on a MN after a
                       previously released CARD Request message has
                       been sent to the MN's current AR.

   MN_AR_CARD_RETRIES: This value indicates the number of retries when
                       sending a MN-AR CARD Request from a MN before
                       declaring the message exchange aborted.

   AR_AR_CARD_TIMEOUT: This timer value indicates the timeout of an
                       expected CARD Reply message on an AR after a
                       previously released CARD Request message has
                       been sent to a CAR.

   AR_AR_CARD_RETRIES: This value indicates the number of retries when
                       sending an AR-AR CARD Request from a MN's
                       current AR to a CAR before declaring the message
                       exchange aborted.


8. IANA CONSIDERATIONS

   This section is to provide the Internet Assigned Numbers Authority
   (IANA) with guidelines to allow assignment and registration of
   values related to the Candidate Access Router Discovery protocol, in
   accordance with [11].

   The protocol described in this document requires a new ICMP type to
   be assigned by the IANA for the CARD protocol main header (section
   5.1.1). Furthermore, two new ICMP-option types (section 5.1.2) are
   to be assigned through IETF consensus [11] for the protocol
   operation between a Mobile Node and its current Access Router. The
   new ICMP options to be assigned by the IANA shall be used for the
   CARD Request (section 5.1.2.1) and the CARD Reply (section 5.1.2.2)
   options. The protocol also requires a UDP port number to be assigned
   through IETF consensus for the inter-Access Router CARD protocol
   operation (section 5.2.1).












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9. NORMATIVE REFERENCES

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

   [4] Kempf, J.,"Problem Description: Reasons For Performing Context
       Transfers Between Nodes in an IP Access Network", RFC 3374,
       September 2002.

   [8] Narten, T., et al., "Neighbor Discovery for IP Version 6
       (IPv6)", RFC 2461, December 1998.

   [9] Postel, J., "User Datagram Protocol", RFC 768, August 1980.

   [10]Atkinson, R., Kent, S.,"IP Encapsulating Security Payload
       (ESP)", RFC 2406, November 1998.

   [11]Narten, T., Alvestrand, H., "Guidelines for Writing an IANA
       Considerations Section in RFCs", RFC 2434, October 1998.


10. INFORMATIVE REFERENCES

   [2] Trossen, D., Krishanmurthi, G. Chaskar, H., Kempf, J. "Issues in
       candidate access router discovery for seamless IP-level
       handoffs", draft-ietf-seamoby-cardiscovery-issues-04.txt, work
       in progress, October 2002.

   [3] Krishanmurti, G., "Requirements for CAR Discovery Protocolsö,
       draft-ietf-seamoby-card-requirements-02.txt, work in progress,
       October 2002.

   [5] Kenward, B.,"General Requirements for Context Transfer", draft-
       ietf-seamoby-ct-reqs-05.txt, work in progress, October 2002.

   [6] Manner, J., Kojo, M. (Ed), "Mobility Related Terminology",
       draft-ietf-seamoby-mobility-terminology-04.txt, work in
       progress, April 2003.

   [7] Koodli, R, et al., "Fast handoffs for Mobile IPv6", draft-ietf-
       mobileip-fast-mipv6-06.txt, work in progress, March 2003.

   [12]Funato, D. et al.,"Geographically Adjacent Access Router
       Discovery Protocolö, draft-funato-seamoby-gaard-01.txt, work in
       progress, June 2002.

   [13]Trossen, D. et al.,"A Dynamic Protocol for Candidate Access-
       Router Discovery", draft-trossen-seamoby-dycard-01.txt, work in
       progress, March 2003.


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   [14]Shim, E., Gitlin, R.,"Fast Handoff Using Neighbor Information",
       draft-shim-mobileip-neighbor-00.txt, work in progress,
       November 2000.

   [15]El Malki, K. et. al, "Low Latency Handoffs in Mobile IPv4",
       draft-ietf-mobileip-lowlatency-handoffs-v4-05.txt, work in
       progress, June 2003.



11. AUTHORS' ADDRESSES

   Hemant Chaskar
   Nokia Research Center
   5 Wayside Road
   Burlington, MA 01803, USA
   Phone: +1 781-993-3785
   Email: Hemant.Chaskar@nokia.com

   Daichi Funato
   NTT DoCoMo USA Labs
   181 Metro Drive, Suite 300
   San Jose, CA 95110, USA
   Phone: +1 408-451-4736
   Email: funato@docomolabs-usa.com

   Marco Liebsch
   NEC Network Laboratories
   Kurfuersten-Anlage 36 , 69115 Heidelberg
   Germany
   Phone: +49 6221-90511-46
   Email: marco.liebsch@ccrle.nec.de

   Eunsoo Shim
   NEC Laboratories America, Inc.
   4 Independence Way
   Princeton, NJ 08540, USA
   Phone: +1 609-951-2909
   Email: eunsoo@nec-labs.com

   Ajoy Singh
   Motorola Inc
   1501 West Shure Dr, USA
   Phone: +1 847-632-6941
   Email: asingh1@email.mot.com






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12. IPR STATEMENTS

   The IETF has been notified of intellectual property rights claimed
   in regard to some or all of the specification contained in this
   document. For more information consult the online list of claimed
   rights.

   Please refer to http://www.ietf.org/ietf/IPR for more information.


13. COPYRIGHT NOTICE

   "Copyright (C) The Internet Society (date). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph
   are included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."


14. ACKNOWLEDGEMENTS

   The CARD design team would like to thank Erik Nordmark for providing
   valuable insight about the piggybacking of CARD options upon Fast
   Mobile-IPv6 messages. In addition, the design team would like to
   thank (in alphabetical order) Dirk Trossen, Govind Krishnamurthi,
   James Kempf, Madjid Nakhjiri, Pete McCann, Rajeev Koodli, Robert C.
   Chalmers and other members of the Seamoby WG for their valuable
   comments on the previous versions of the draft as well as for the
   general CARD related discussion and feedback.



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APPENDIX A: MAINTENANCE OF ADDRESS MAPPING TABLES IN ACCESS ROUTERS

   This appendix gives information on two optional CAR table
   maintenance schemes for reverse address mapping in access routers.


   Appendix A.1 Centralized Approach using a Server Functional Entity

   Appendix A.1.1 Approach

   The centralized approach works when the MN has IP-layer connectivity
   with only the current AR. However, the MN can scan L2 beacons from
   neighboring AP(s) and thereby deduce their L2 ID(s). For example,
   802.11 families of MN or cellular handsets with the mobile-assisted
   handover capability can do this.

   Figure A.1.1 illustrates the centralized CARD operation. In this
   operation, ARs have registered their address information with a CARD
   server in advance. When a MN discovers the L2 ID of APs during L2
   scanning, the MN passes one or more L2 ID(s) to its current AR and
   the AR resolves it to the IP address of the AR. For this, the AR
   first checks whether the mapping information is locally available in
   its CAR table. If not, the MN's current AR queries a CARD server
   with the L2 ID. In response, the CARD server returns the IP address
   of the CAR to the current AR. Then, the current AR directly contacts
   the respective CAR and performs capability discovery with it. The
   current AR then passes the IP address of the CAR and associated
   capabilities to the MN. The current AR stores the resolved IP
   address within its local CAR table.

   The centralized CARD protocol operation introduces additional
   signaling messages, which are exchanged between the MN's current AR
   and the CARD server. The signaling messages are shown with the
   preceding identifier "AR-Server", referring to the associated
   interface.

   An initial idea of performing reverse address translation using a
   centralized server has been described in [12].












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                                +----------+
                  +------------>|   CARD   |<-------------+
                  |+------------|  Server  |-------------+|
                  ||            +----------+             ||
                  ||                                     ||
                  ||             ~~~~~~~~~~~             ||
      (3)AR-Server||(4)AR-Server{           }            || (0) CARD
           CARD   ||    CARD   {             }           ||Registration
          Request ||   Reply  {    IP Cloud   }          |Request/Reply
                  ||           {             }           ||
                  ||            {           }            ||
                  |V             ~~~~~~~~~~~             V|
              +---------+  (5)AR-AR CARD Request   +-----+-----+
              | Current |------------------------->| CAR | CAR |
              |   AR    |<-------------------------|  1  |  2  |
              +---------+  (6)AR-AR CARD Reply     +-----+-----+
                 ^ |                                  |     |
        (2)MN-AR | |(7)MN-AR                          |     |
           CARD  | |   CARD                           |     |
          Request| V   REPLY                        +---+ +---+
           +--------------+    (1) AP1 L2 ID     +--|AP1| |AP2|
           |    Mobile    |<---------------------+  +---+ +---+
           |     Node     |<--------------------------------+
           +--------------+    (1) AP2 L2 ID

           Figure A.1.1: Centralized Approach for L2-L3 mapping


   Appendix A.1.2 Associated Protocol Operation

   Figure A.1.2 shows the timing diagram of the centralized CARD
   protocol operation. In this figure, the CAR registration process is
   done before the CARD discovery process (0). CARs register with the
   CARD server when the CAR is initialized or when the status of the
   APs' L2 ID, which are associated with the CAR, changes. The CAR MAY
   also periodically register with the CARD server to update the list
   of current AP(s) that it supports.

   The CAR discovery process in the centralized approach is initiated
   as soon as a MN discovers the L2 ID of a nearby AP during the
   periodic L2 scanning. The MN sends the L2 ID to its current AR with
   a MN-AR CARD Request message. If the identity and capability
   information of the requested CAR is not available in the AR's local
   CAR table, the current AR subsequently sends an AR-Server CARD
   Request message to the CARD server to resolve the IP address of the
   serving AR of the newly discovered AP. The CARD server then resolves
   the received L2 ID(s) to the IP address of the associated CAR(s) and



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   returns the identity of the CAR(s) to the requesting AR using the
   AR-Server CARD Reply message.


     MN                current AR         CARD Server               CAR
     |                     |                   |             (Candidate
     |                     |                   |                 Access
     |                     |                   |                Router)
     |                     |                   |                     |
     |                     |                   |  Registration Req   |
     |                     |                   |<--------------------|
     |  <~ ~ ~ L2-SCAN (1) |                   |  Registration Reply |
     |                     |                   |-------------------->|
     |                     |                   |                     |
     |                     |                   |                     |
     |MN-AR CARD Request(2)|                   |                     |
     |-------------------->|                   |                     |
     |                     |  AR-Server        |                     |
     |                     |  CARD Request(3)  |                     |
     |                     |------------------>|                     |
     |                     |    AR-Server      |                     |
     |                     |   CARD Reply(4)   |                     |
     |                     |<----------------- |                     |
     |                     |                   |                     |
     |                     |          AR-AR CARD Request(5)          |
     |                     |---------------------------------------->|
     |                     |                   |                     |
     |                     |          AR-AR CARD Reply (6)           |
     |                     |<----------------------------------------|
     |                     |                   |                     |
     | MN-AR CARD Reply(7) |                   |                     |
     |<--------------------|                   |                     |
     |                     |                   |                     |
     |                     |                   |                     |
     |                     |                   |                     |


   Figure A.1.2 CARD procedure using a server function for maintenance
                of reverse address translation information in ARs' CAR
                tables.


   Upon receipt of the AR-Server CARD Reply message, the MN's current
   AR extracts the IP address of the CAR and subsequently requests
   remaining capabilities by sending an AR-AR CARD Request message to


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   the CAR. The CAR conveys its capabilities to the requesting AR in an
   AR-AR CARD Reply message. Upon receipt of the AR-AR CARD Reply
   message, the current AR caches the CAR's capabilities as well as the
   associated L2-L3 mapping information in its local CAR table and
   conveys the requested capabilities and address information to the MN
   using the MN-AR CARD Reply message.


   Appendix A.1.3 Associated Protocol Messages

   A.1.3.1 CARD Message Transport for the interface between an AR
           and the CARD Server

   For the centralized CARD operation between an AR and the CARD
   server, UDP is used as transport protocol for CARD protocol
   messages. A UDP port is T.B.A.

   To authenticate protocol messages between ARs, IPsec ESP is to be
   used.


   A.1.3.2 Protocol Main Header

   The protocol main header for this interface is the same as used for
   the interface between ARs on network side, and is described in
   section 5.2.2.

   Because ARs need to register with the CARD server function, two
   additional message types have been specified, which is a CARD
   Registration Request and a CARD Registration Reply message.

   The following table lists message types specified for CARD as used
   between an AR and the CARD server function:


            Message types specified for this interface:

                 Message                     Type
               ------------------------------------
               AR-Server CARD Request        0x03
               AR-Server CARD Reply          0x04
               CARD Registration Request     0x05
               CARD Registration Reply       0x06


   For the registration related message types, an additional payload
   type is required and described in section A.1.3.3.





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   A.1.3.3 Protocol Payload Types

   Payload types and encoding rules are the same as described for the
   various sub-option types in section 5.1 for the MN-AR interface. The
   same TLV-encoded format is used to append the options to the
   protocol main header.

   For the registration of an AR with a CARD server function, an
   additional payload type is required to indicate the lifetime of the
   associated AR's registration. The lifetime option is encoded as
   follows:


       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     |           Reserved            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Lifetime                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      Type:    T.B.A

      Length:  0x08  -  Length of the lifetime payload option in octets
                        (including type and length fields).

      Reserved:
               To be initialized with 0.

      Lifetime:
               Indicates the lifetime of a registration in seconds.
               If the lifetime is set to '0', this indicates a de-
               registration with a CARD server function.


   Appendix A.1.4: Associated Security Considerations


   A.1.4.1 Security Associations

   The AR-CARD Server communication must be protected using IPsec ESP
   and a previously established security association.








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   A.1.4.2 DoS Attack

   The MN-AR communication presents opportunities to an attacker. A
   rogue MN can use CARD as a denial-of-service (DoS) attack against an
   AR. It could also flood the backend AR-Server and AR-AR
   communications. If the MN undertakes a DoS attack by flooding its
   current AR with real or bogus L2 IDs, the CARD protocol should
   prevent it.

   When the AR has an authentication scheme for MNs, it is difficult
   for a rogue MN to change its identity. Hence, one possible solution
   is to limit the number of requests from an identical MN within a
   unit of time.


   A.1.4.3 CAR Table Contamination

   When an AR allows caching, CAR table contamination could occur. A MN
   provides the current AR with unauthenticated observations of AP
   identifiers that it can hear. Then the AR asks for the authenticated
   AR information using the CARD server. The CARD server can tell only
   that there is a registered AR with the given L2 ID, but it cannot
   tell whether the AR is a CAR of the current AR. (Note that CAR needs
   to have an access point geographically adjacent to current AR's
   APs). The current AR relies on the fact that a MN provided the L2 ID
   that matches a registered AR. A malicious MN may provide a L2 ID,
   which is the L2 ID of a registered AR but not a CAR of the current
   AR, that is, has no overlapping coverage with the current AR. Then
   the current AR would build a CAR table with the IP addresses of ARs
   that are not CARs. This has implications on the size of CAR table
   that can be allowed on ARs. A more serious implication is that, if a
   large number of non-CAR entries appear in the AR'S CAR table, the AR
   spends processing resources in exchanging capabilities with them.

   There is a possible solution for this. The ARs can handle this by
   making the CARD information soft state, so that it times out the AP
   addresses if it does not receive a confirmation from another MN
   within a certain period of time. Thus, any bogus information has
   only a limited lifetime, and even within that lifetime, it cannot do
   more than to occupy a table slot in the AR's memory. In fact, the AR
   can use the number of MNs reporting a particular address to weight
   the relevance of a reported AP. So, if 20 MNs report it, the AP
   address is more likely to stick around than if only one MN reports
   it. This is an issue for implementation rather than a protocol
   issue.

   However, this issue could not be a problem in actual handover cases.
   At the time of handover, the MN or AR receives the L2 ID of the AP
   to which the MN is moving. Or the MN matches the AP L2 ID in the CAR
   table with the address of the APs it can hear. Thus, an AP's L2 ID


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   that is provided by a malicious MN, but has no wireless connectivity
   to the CAR, is filtered out when a MN or AR uses the information for
   handover, so this can do no harm at the time of handover.


   Appendix A.1.5 Associated IPR statements

   The IETF has been notified of intellectual property rights claimed
   in regard to some or all of the specification contained in this
   Appendix A.1. For more information consult the online list of
   claimed rights.

   Please refer to http://www.ietf.org/ietf/IPR for more information.







































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   Appendix A.2 Decentralized Approach using Mobile Terminals'
                Handover

   Appendix A.2.1 Approach

   This approach performs CARD over the MN-AR interface as described in
   Section 4. However, it employs one additional message, called the
   Router Identity message, over the MN-AR interface to enable ARs to
   learn about the reverse address translation tables of their
   neighboring ARs, without being dependent on any centralized server.

   In this approach, CAR identities in the CAR table of an AR are
   maintained as soft states. In other words, the entries for CARs are
   removed from CAR table if not refreshed before the timeout period.
   The entries for CAR identities in CAR table are created/refreshed
   according to following mechanism.

   The key idea behind the decentralized approach is to bootstrap and
   maintain the association between two ARs as neighbors of each other,
   using the actual handover of MNs occurring between them as input.
   The first handover between any two neighboring ARs serves as the
   bootstrap handover, which invoke the discovery procedure and the
   subsequent handover serve to refresh the association between the
   neighboring ARs. After the bootstrap handover, the MNs can perform
   CARD and thus seamless handover using the CAR information. This idea
   was presented in [13] and [14].


   Appendix A.2.2 Associated Protocol Operation

   CAR table maintenance using the Router Identity message:

   Upon the completion of an inter-AR handover, the MN SHOULD send a
   Router Identity message to its current AR. This message contains the
   identity (IP address) of the previous AR (pAR), which is already
   known to the MN. This message is sent as a specific sub-option in
   the MN-AR CARD Request (see below). It SHOULD be acknowledged with
   the MN-AR CARD Reply. The Router Identity message enables the MN's
   current AR to learn that the pAR (still) has an AP whose coverage
   overlaps with one of the APs of the current AR and vice versa. With
   this information, the MN's current AR can create or refresh an entry
   for the pAR as its neighbor. If handover cease between two
   particular ARs, the associated entries will eventually timeout and
   removed from each AR's CAR table.

   Prior to trusting the MN's report, however, the current AR may
   perform a number of checks to ensure the validity of the received
   information. As one simple method to verify the accuracy of the
   Router Identity message, the current AR sends an AR-AR CARD Request


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   message to the pAR. The AR-AR CARD Request includes the identity of
   the MN. Upon receiving this message, the pAR has to verify that the
   said MN was indeed attached to it during a reasonable past and
   respond to the current AR. In this way, each handover of a MN
   results in a bi-directional discovery process between the two
   participating ARs.

   Upon receiving positive verification response, the current AR
   creates or refreshes as applicable, the entry for the pAR in its
   local CAR table. In the former case, the current AR and the pAR
   exchange capabilities using the AR-AR CARD Request and AR-AR CARD
   Reply protocol messages. When a new entry is created, the ARs MUST
   exchange their reverse address translation tables. They may exchange
   other capabilities at this time or may defer it to later time when
   some MN undergoing handover between them performs CARD as described
   in Section 4. In the later (refresh) case, ARs may exchange
   capabilities or defer it until later time when some MN undergoing
   handover between them performs CARD as described in Section 4.


             +--------------+  (4)AR-AR CARD Request   +----------+
             |  Current     |------------------------->|    pAR   |
             |     AR       |<-------------------------|          |
             +--------------+  (5)AR-AR CARD Reply     +----------+
                 ^       |                                  .
                 |       |(3) ACK                        .
   Router        |       |                                  .
   Identity              V                                  .
   Message (2)+--------------+                      +---------------+
              |    Mobile    |                      |     Mobile    |
              |     Node     |<-- Inter AR Handoff  |     Node      |
              +--------------+       (1)            +---------------+

        Figure A.2.1 Use of Router Identity Message for CAR Table
                     Maintenance


   Finally, note that, in a handover-based protocol, a first handover
   between a pAR and a MN's current AR (without regard to direction, MN
   identity and which APs are involved) cannot use CARD, as this
   handover would bootstrap the CAR table. However, it is hoped that in
   long term such handover will only amount to a small fraction of
   total successful handover between pAR and the MN's current AR. Also,
   if the user of the MN engaging in such first handover is running a
   non-delay sensitive application at the time of handover, the user
   may not even realize its impact.






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   Appendix A.2.3 Associated Protocol Messages

   Router Identity Message Format:

   The dynamic handover based approach requires a MN to convey the
   pAR's IP address to its current AR. The pAR's IP address is conveyed
   from the MN to its current AR as using the following CARD protocol
   message sub-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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Sub-Option Type|Sub-Option Len |         pAR ID ....
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -


      Sub-Option Type:
                     T.B.A

      Sub-Option Length:
                     Length of the Sub-Option in units of octets.

      pAR ID:        IP address of the pAR.


   The MN's current AR SHOULD acknowledge the receipt of the Router
   Identity message using the ACK sub-option described below.

      0                   1
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Sub-Option Type|Sub-Option Len |     ACK       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      Sub-Option Type:
                     T.B.A

      Sub-Option Length:
                     Length of the Sub-Option in units of octets.

      ACK:           All 1's.






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   Appendix A.2.4 Associated Security Considerations

   In the design of this protocol, a few assumptions have been made
   about the security model in place between the MN and the AR, and
   between ARs. In particular, it has been assumed that prior to any
   protocol messaging, the AR has authenticated and authorized the MN
   to participate in CARD. Moreover, in order for two ARs to cooperate
   without introducing serious security concerns, they must be able to
   establish a security association. For intra-domain routers, this
   could be as simple as a shared secret key.  For the inter-domain
   scenario, the two domains must have a previously established
   relationship that can be leveraged to derive an adequate session
   key. All messages listed herein should be protected by means of
   IPsec ESP to provide authentication and to ensure message integrity.


   Appendix A.2.5 Associated IPR Statements

   The IETF has been notified of intellectual property rights claimed
   in regard to some or all of the specification contained in this
   Appendix A.2. For more information consult the online list of
   claimed rights.

   Please refer to http://www.ietf.org/ietf/IPR for more information.



























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APPENDIX B: APPLICATION SCENARIOS

   This section provides two examples of an application scenario for
   the CARD protocol operation. One scenario describes CARD protocol
   operation in a Mobile IPv6 (MIPv6) enabled network, providing access
   to the infrastructure via wireless LAN Access Points and associated
   Access Routers. A second scenario describes CARD protocol operation
   in a Mobile IPv6 enabled network, which has enhanced support for
   fast handover integrated (Fast Mobile-IPv6), also providing wireless
   LAN access to the infrastructure.


   Appendix B.1 CARD Operation in a Mobile-IPv6 Enabled Wireless LAN
                Network

   This application scenario assumes a moving MN having access to the
   infrastructure through wireless LAN (IEEE802.11) APs. Location
   tracking is performed using the Mobile IPv6 protocol.
   The following figure illustrates the assumed network sector for
   description of CARD protocol operation.

                       -----------------------------
                      /                             \   +----+
                      |           NETWORK           |---| HA |
                      \                             /   +----+
                       -----------------------------
                        |                         |
                     +-----+                   +-----+
                     | AR1 |---------+         | AR2 |
                     +-----+         |         +-----+
                        |  subnet 1  |            |subnet 2
                     +-----+      +-----+      +-----+
                     | AP1 |      | AP2 |      | AP3 |
                     +-----+      +-----+      +-----+
                        ^            ^            ^
                         \
                          \
                           \
                            v
                         +-----+
                         | MN  | - - ->>>- - - ->>>
                         +-----+

                   Figure B.1: Assumed network topology


   A Mobile IPv6 Home Agent (HA), which is connected to the network,
   maintains location information of the MN in its binding cache.
   According to Figure B.1, the MN holds currently a care-of address
   for the subnet 1, supported by AR1, which is registered with its HA.


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   According to the MN's movement, the MN's current environment offers
   two further wireless LAN APs with increasing link-quality as
   candidate APs for a handover. To allow the MN taking a decision, on
   which AP might be the better choice, not only access link quality
   parameters, but also parameters associated with ARs should be taken
   into account for the decision process. These AR-related parameters
   can be, for example, available QoS resources or the type of access
   technologies supported from an AR. To learn about these candidate
   ARs' capabilities and associated IP address information, the MN
   performs CARD. This requires retrieving information about candidate
   APs' L2 ID, which is broadcast via beacon information from
   respective APs. Furthermore, associated link-quality parameters are
   to be retrieved to ascertain, whether or not approaching APs are
   eligible candidates for a handover. Assume AP2 and AP3 to be
   suitable candidate APs. The MN encapsulates both L2 IDs (AP2 and
   AP3) into a CARD Request message, using the L2 ID sub-option, and
   sends it to its current AR (AR1).

   AR1 resolves each L2 ID, listed as L2 ID options in the received
   CARD Request, to the associated IP address of the respective AR,
   making use of its local CAR table. According to the environment
   illustrated in Figure B.1, the associated AR IP address of the
   candidate AP2 will be the same as the MN is currently attached to,
   which is AR1. Respective IP address of the candidate AR, to which
   AP3 is connected to, is the address of AR2. Since IP addresses of
   the MN's CARs are now known to AR1, AR1 retrieves CARs' capabilities
   from the CAR table, assumed it has valid entries for respective
   capability parameters in the local CAR table. To ascertain dynamic
   capabilities, of which lifetime in AR1's CAR table has been expired,
   AR1 performs inter-AR CARD for capability discovery. Since
   capability information of AR1 is known to AR1, a respective inter-AR
   CARD Request is to be sent only to AR2. AR2 in response sends a CARD
   Reply message back to AR1, having the requested capability
   parameters encapsulated with the signaling message, all assembled in
   a capability container sub-option.
   Now, AR1 sends its own capabilities and the dynamically discovered
   ones of AR2 back to the MN via a CARD Reply message. Furthermore,
   AR1 stores the capability parameters of AR2 with the associated
   lifetimes in its local CAR table.

   On reception of the CARD Reply message, the MN performs target AR
   selection, taking AR1's and AR2's capability parameters as well as
   associated APs' link-quality parameters into account. In case the
   selected AP is AP2, no IP handover needs to be performed. In case
   AP3 and the associated AR2 are selected, the MN needs to perform an
   IP handover according to the Mobily-IPv6 protocol scenario.

   Figure B.2 illustrates the signaling flow of the previously
   described application scenario of CARD within a Mobile-IPv6 enabled
   network.


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     MN           AP1     AR1     AP2         AP3                   AR2
     |             |       |       |           |                     |
     |  connected  |       |       |           |                     |
     0-------------0-------0       |           |                     |
     |             |       |       |           |                     |
     |             |       |       |           |                     |
     |                             |           |                     |
     | <~~~~~~~~~L2-SCAN (AP2)~~~~~|           |                     |
     | <~~~~~~~~~L2-SCAN (AP3)~~~~~~~~~~~~~~~~~|                     |
     |                             |           |                     |
     | (MN-AR) CARD Req    |       |           |                     |
     |-------------------->|          (AR-AR) CARD Req               |
     |             |       |---------------------------------------->|
     |             |       |          (AR-AR) CARD Repl              |
     | (MN-AR) CARD Repl   |<----------------------------------------|
     |<--------------------|       |           |                     |
     |             |       |       |           |                     |
   [target AR      |       |       |           |                     |
   selection]      |       |       |           |                     |
     |             |       |       |           |                     |
     //           //       //      //         //                     //
   [either...]     |       |       |           |                     |
     |             |       |       |           |                     |
     |-------- L2 attach --------->|           |                     |
     |             |       |       |           |                     |
     |      connected      |       |           |                     |
     0---------------------0-------0           |                     |
     |             |       |       |           |                     |
     //            //      //      //         //                     //
   [... or]        |       |       |           |                     |
     |             |       |       |           |                     |
     |--------------- L2 attach -------------->|                     |
     |             |       |       |           |                     |
     |      connected      |       |           |                     |
     0-----------------------------------------0---------------------0
     |             |       |       |           |                     |
     |                                         |                     |
     |     MIPv6 Binding Update to the HA      |                     |
     |------------------------------------------------ - - - >       |
     |             |       |       |           |                     |

     Figure B.2: CARD protocol operation within a Mobile-IPv6 enabled
                           wireless LAN network.



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   Appendix B.2 CARD Operation in a Fast Mobile-IPv6 Enabled Network

   This application scenario assumes ARs to be able to perform the fast
   handover protocol sequence for Mobile IPv6 [7]. The MN scans for new
   APs to handover to, similar to what Figure B.1 illustrates. To
   discover candidate APs' associated ARs (CARs), the MN attaches a MN-
   AR CARD Request option to the ICMP-type Fast Mobile-IPv6 RtSolPr
   message, which is sent to the MN's current AR (pAR, previous AR).
   Candidate APs' L2 IDs are encapsulated using the CARD protocol's L2
   ID sub-options, which allows the MN to send multiple L2 IDs of
   candidate APs to its current AR (potentially replaces the "New
   Attachment Point Link-Layer Address" option of the Fast Mobile-IPv6
   protocol).

   The pAR resolves the received list of candidate APs' L2 IDs to the
   IP address of associated CARs. Furthermore, the pAR checks its local
   CAR table to retrieve information about the CARs' capabilities. In
   case one or multiple associated capability entries of a CAR have
   expired, the pAR acquires this CAR's capabilities by means of
   sending an AR-AR CARD Request to the respective CAR. The CAR replies
   to the requesting pAR with an AR-AR CARD Reply message, having all
   capabilities encapsulated in a capability container sub-option and
   attached to the CARD Reply option. On reception of the CARs'
   capability information, the pAR updates its local CAR table and
   forwards the address and capability information of the MN's CAR(s)
   to the MN by means of attaching a MN-AR CARD Reply option, carrying
   appropriate address and capability container sub-options, to the
   Fast Mobile-IPv6 PrRtAdv message. When the MN's handover is
   imminent, the MN selects its new AR and the associated new AP from
   the discovered list of CARs. According to the Fast Mobile-IPv6
   protocol, the MN notifies the pAR of the selected new AR with the
   Fast Binding Update (F-BU) message, which allows the pAR to perform
   further protocol sequences for a fast handover according to the Fast
   Mobile-IPv6 protocol.

   Optionally, the pAR could perform selection of an appropriate new AR
   on behalf of the MN after the pAR has the MN's CARs' addresses and
   associated capabilities available. To allow for selection of an
   appropriate new AR out of the list of CARs, the MN must send its
   requirements for the selection process to its pAR together with the
   MN-MN CARD Request message, appended as a list of attribute-value
   pairs carried with the CARD protocol's Requirements sub-option.
   After the pAR has selected the MN's new AR, the address and
   associated capabilities of the chosen new AR are notified to the MN
   with the CARD Reply option, which is conveyed to the MN with the
   Fast Mobile-IPv6 PrRtAdv message.



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   Note: Since the CARD protocol functions and the CARD Request and
   CARD Reply message provide all information of the RtSolPr and
   PrRtAdv in a flexible way with regard to discovery and selection of
   a new AR during a handover process, the CARD Request and CARD Reply
   options could potentially replace the Fast Mobile-IPv6 RtSolPr and
   PrRtAdv protocol messages respectively.

   Figure B.3 illustrates how CARD protocol messages and functions
   could perform together with the Fast Mobile-IPv6 protocol.


         MN                    pAR                  NAR       CAR2
          |                     |                 as CAR1       |
          |                     |                    |          |
          |-------RtSolPr------>|                    |          |
          |  [MN-AR CARD Req]   |-- AR-AR CARD Req*->|          |
          |                     |-- AR-AR CARD Req*------------>|
          |                     |<--AR-AR CARD Repl*------------|
          |                     |<--AR-AR CARD Repl*-|          |
          |<------PrRtAdv-------|                    |          |
          |  [MN-AR CARD Repl]  |                    |          |
          |                     |                    |          |
     NAR selection              |                    |          |
          |------F-BU---------->|--------HI--------->|          |
          |                     |<------HACK---------|          |
          |          <--F-BACK--|--F-BACK-->         |          |
          |                     |                    |          |
      Disconnect                |                    |          |
          |                   forward                |          |
          |                   packets===============>|          |
          |                     |                    |          |
          |                     |                    |          |
       Connect                  |                    |          |
          |                     |                    |          |
          RS (with FNA option)======================>|          |
          |<-----------RA (with NAACK option)--------|          |
          |<=================================== deliver packets |
          |                                          |          |
             Figure B.3: Fast Handover protocol sequence with
                           CARD protocol options


   *): The CARD protocol interaction between the pAR and CARs is only
   required in case the lifetime of one or multiple capability entries
   of the pAR's local CARD table have been expired. Otherwise, the pAR
   can respond to the requesting MN immediately after having the CARs'
   address and capability information retrieved from its local CAR
   table.


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