





Network Working Group                                    M. Liebsch, Ed.
Request for Comments: 4066                                 A. Singh, Ed.
Category: Experimental                                        H. Chaskar
                                                               D. Funato
                                                                 E. Shim
                                                               July 2005


                Candidate Access Router Discovery (CARD)

Status of This Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

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 and capabilities of candidate ARs (CARs) for handover
   prior to the initiation of the handover.  The act of discovery of
   CARs has two aspects: identifying the IP addresses of the CARs and
   finding their capabilities.  This process is called "candidate access
   router discovery" (CARD).  At the time of IP-layer handover, the CAR,
   whose capabilities are a good match to the preferences of the MN, is
   chosen as the target AR for handover.  The protocol described in this
   document allows a mobile node to perform CARD.

Table of Contents

   1.  Introduction..................................................  2
   2.  Terminology...................................................  3
   3.  CARD Protocol Functions.......................................  4
       3.1.  Reverse Address Translation.............................  4
       3.2.  Discovery of CAR Capabilities...........................  4
   4.  CARD Protocol Operation.......................................  4
       4.1.  Conceptual Data Structures..............................  7
       4.2.  Mobile Node - Access Router Operation...................  8
       4.3.  Current Access Router - Candidate Access Router
             Operation............................................... 11
       4.4.  CARD Protocol Message Piggybacking on the MN-AR
             Interface............................................... 13



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   5.  Protocol Messages............................................. 14
       5.1.  CARD Messages for the Mobile Node-Access Router
             Interface............................................... 14
       5.2.  CARD Inter-Access Router Messages....................... 28
   6.  Security Considerations....................................... 31
       6.1.  Veracity of CARD Information............................ 31
       6.2.  Security Association between AR and AR.................. 31
       6.3.  Security Association between AR and MN.................. 32
       6.4.  Router Certificate Exchange............................. 32
       6.5.  DoS Attack.............................................. 34
       6.6.  Replay Attacks.......................................... 34
   7.  Protocol Constants............................................ 34
   8.  IANA Considerations........................................... 35
   9.  Normative References.......................................... 35
   10. Informative References........................................ 35
   11. Contributors.................................................. 36
   12. Acknowledgements.............................................. 36
   Appendix A.  Maintenance of Address Mapping Tables in
                Access Routers....................................... 37
       Appendix A.1. Centralized Approach Using a Server Functional
                     Entity.......................................... 37
       Appendix A.2. Decentralized Approach Using Mobile Terminals'
                     Handover........................................ 38
   Appendix B.  Application Scenarios................................ 40
       Appendix B.1. CARD Operation in a Mobile IPv6-Enabled Wireless
                     LAN Network..................................... 40
       Appendix B.2. CARD Operation in a Fast Mobile IPv6-Enabled
                     Network......................................... 43

1.  Introduction

   IP mobility protocols, such as Mobile IP, enable mobile nodes to
   execute IP-level handover among access routers.  Work is underway
   [Kood03][Malk03] to extend the mobility protocols to allow seamless
   IP handover.  Seamless IP mobility protocols will require knowledge
   of candidate access routers (CARs) to which a mobile node can be
   transferred.  The CAR discovery protocol enables the acquisition of
   information about the access routers that are candidates for the
   mobile node's next handover.

   CAR discovery involves identifying a CAR's IP address and the
   capabilities that the mobile node might use for a handover decision.
   There are cases in which a mobile node has a choice of CARs.  The
   mobile node chooses one according to a match between the mobile
   node's requirements for a handover candidate and the CAR's
   capabilities.  However, the decision algorithm itself is out of the
   scope of this document.




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   The problem statement for CAR discovery is documented in [TKCK02].
   In this document, a protocol is described to perform CAR discovery.
   Section 3 describes two main functions of the CAR discovery protocol.
   Section 4 describes the core part of the CARD protocol operation.
   The protocol message format is described in Section 5.  Section 6
   discusses security considerations, and Section 7 contains a table of
   protocol parameters.  Appendix A contains two alternative techniques
   for dynamically constructing the CAR table mapping between the access
   point L2 ID and Access Router IP address, which is necessary for
   reverse address translation.  The default method is static
   configuration.  Appendix B contains two sample scenarios for using
   CARD.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [Brad97].

   This document uses terminology defined in [MaKo03].

   In addition, the following terms are used:

   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 an MN has a choice when performing IP-level
      handover.

   Capability of an AR

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

   L2 ID

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








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   CARD Initiating Trigger

      An L2 trigger used to initiate the CARD process.  For example, a
      MN can initiate CARD as soon as it detects the L2 ID of a new AP
      during link layer scan.

   Access Point (AP)

      A wireless access point, identified by a MAC address, providing
      service to the wired network for wireless nodes.

3.  CARD Protocol Functions

   The CARD protocol accomplishes the following functions.

3.1.  Reverse Address Translation

   If an MN can listen to the L2 IDs of new APs prior to making a
   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.

3.2.  Discovery of CAR Capabilities

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

   A definition of capabilities is out of the scope of this document.
   Encoding rules for capabilities and the format of a capability
   container for capability transport are specified in Section 5.

4.  CARD Protocol Operation

   The CARD protocol allows MNs to resolve the L2 ID of one or more APs
   to the IP addresses of the associated CARs.  The L2 IDs are typically
   discovered during an operation by the MN and are potential handover
   candidates.  Additionally, CARD allows MNs to discover particular
   capabilities associated with the CARs, such as available bandwidth,
   that might influence the handover decision of the MN.  Furthermore,
   the protocol allows ARs to populate and maintain their local CAR
   table (Section 4.1) with the capabilities of CARs.  For this, the
   CARD protocol makes use of CARD Request and CARD Reply messages



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   between an MN and its current AR (Section 5.1.2), and between an MN's
   current AR and individual CARs, respectively (Section 5.2.2).

   To allow an MN to retrieve a CAR's address and capability
   information, the CARD Request and CARD Reply messages used between an
   MN and its current AR may contain one or more access points' L2 IDs
   and the IP addresses of associated CARs, respectively.  Optionally,
   the CARD Reply messages can also contain a CAR's capability
   information.  A CAR's capabilities are specified as a list of
   attribute-value pairs, which are conveyed in a Capability Container
   message parameter.

   Information about CARs and associated capabilities MAY be used by the
   MN to perform target access router selection during its IP handover.
   The current AR returns replies according to its CAR table (see
   Section 4.1) and returns a RESOLVER ERROR (see Section 5.1.3.1) if
   the request cannot be resolved.

   The CARD protocol also enables an MN to optionally indicate its
   preferences on capabilities of interest to its current AR by
   including the Preferences message parameter in the CARD Request
   message.  The MN's current AR MAY use this information to perform
   optional capability pre-filtering for optimization purposes, and it
   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) required for capability pre-filtering, but
   also a specific value for a particular capability.  This allows the
   MN's current AR to perform 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.

   For example, using the optional Preferences message parameter, an MN
   may indicate to its current AR that it is interested only in
   IEEE802.11a interface-specific capability parameters, as this is the
   only interface the MN has implemented.  The MN's current AR sends
   back only CARs with IEEE802.11a-specific capabilities.  Similarly,
   using the optional Requirements message parameter, an MN may indicate
   to its current AR that it is only interested in CARs that can satisfy
   a given QoS constraint.  Here, an 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



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   Requirements message parameter, which is appended to the MN-
   originated CARD Request message.  The Requirements message parameter
   may be used to indicate the cutoff values of the capabilities for any
   desired CARs.  According to 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, if the
   MN's current AR does not perform optimization with regard to
   capability or CAR pre-filtering, the current AR MAY choose to
   silently ignore the optional Requirements and Preferences message
   parameter as received in the CARD Request message.

   The MN can additionally request from the AR a certification path that
   is anchored at a certificate from a shared, trusted anchor.  The MN
   includes in the CARD Request message a list of trusted anchors for
   which the MN has a certificate, and the AR replies with the
   certification path.  If no match is found, the AR returns the trusted
   anchor names from the CARD Request.  The MN can ask for a chain for
   either the current AR or a CAR.  If the trusted anchor list is
   accompanied by an AP L2 ID for the MN's current AP, the returned
   chain is for the current AR.  If the L2 ID is for an AP that the MN
   has heard during scanning and is not connected to the current AR, the
   returned chain is for a CAR.  The chain is returned as a sequence of
   CARD Reply messages, each message containing a single certificate,
   the L2 identifier for the AP sent in the CARD Request, and a router
   address for the CAR (or for the AR itself if a request was made for
   the AR).  When the chain is complete, the MN can use it to obtain the
   AR's certified key and thereby validate signatures on CARD messages
   and other messages between the MN and the current AR.  The MN only
   has to send the trusted anchor option if it does not have the
   certification path for the AR already cached.  If the MN has the
   certification path cached, through preconfiguration, through previous
   receipt of the chain from this router, or by having received the
   chain through a previous router, then the trusted anchor does not
   have to be sent.  More information about certificate exchange and its
   use in CARD security can be found in Section 6.

   The CARD protocol operation, as described in this section,
   distinguishes signaling messages exchanged between an MN and its
   current AR from those exchanged between ARs.  Hence, descriptions of
   signaling messages in the following sections have preceding
   identifiers referring to the associated interface.  Messages that are
   exchanged between an MN and AR are designated as "MN-AR", and
   messages between ARs are designated as "AR-AR".






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

                Figure 1: MN-initiated CARD Protocol Overview

   Figure 1 describes the operation of the MN-AR CARD Request/Reply
   protocol and AR-AR CARD Request/Reply protocol.  On receipt of the
   access points' L2 IDs or the appearance of a CARD initiation trigger
   (1m), the MN may pass on one or more AP L2 IDs to its current AR
   using the MN-AR CARD Request message (2m).  If the MN wants its AR to
   perform capability discovery in addition to reverse address
   translation, this must be indicated in the MN-AR CARD Request message
   by setting the C-flag.  If the C-flag is not set, the AR receiving
   the CARD Request message will perform only reverse address
   translation.  The MN's current AR resolves the L2 ID to the IP
   address of the associated CAR or, if the MN has not attached any L2
   ID message parameters, 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.  The current AR then
   returns to the MN using the MN-AR CARD Reply message (3m), the IP
   addresses of any CARs, each CAR's set of L2 IDs with CANDIDATE
   indicated in the L2 ID sub-option status field, and, if capability
   information has been requested, associated capabilities.

   For the AR-AR CARD Request/Reply protocol, the requesting AR sends a
   CARD Request message to its peer when the CAR table entries time out
   (1a).  The peer returns a CARD Reply message with the requested
   information (2a).

4.1.  Conceptual Data Structures

   ARs SHALL maintain an L2-L3 address mapping table (CAR table) that is
   used to resolve L2 IDs of candidate APs to the IP address of the
   associated CAR.  By default, this address-mapping table is configured
   statically for the CARD protocol operation.  Optionally, the CAR
   table MAY be populated dynamically.  Two possible approaches are
   described in Appendices A.1 and A.2.




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   ARs SHOULD also keep and maintain individual CARs' capabilities in
   the local CAR table, with the associated capability lifetime taken
   into account.  If the lifetime of an individual capability entry has
   expired, the respective capability information is updated.  An AR may
   also initiate capability exchange prior to expiration of the
   capabilities associated with a CAR in the CAR table, thereby
   populating its CAR table.  The AR's CAR table may be implemented
   differently; therefore additional details are not provided here.  ARs
   MUST maintain their own AP-to-AR mappings and capability information
   in their CAR tables, in order to provide newly booted MNs with this
   information so that an MN can obtain the AR's certification path.

   MNs SHOULD maintain discovered address and capability information of
   CARs in a local cache to avoid requesting the same information
   repeatedly and to select an appropriate target AR from the list of
   CARs as quickly as possible when a handover is imminent.

4.2.  Mobile Node - Access Router Operation

4.2.1.  Mobile Node Operation

   To initiate CARD, an 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 obtain capability parameters
   associated with these CARs.  If the requesting MN wants its current
   AR to resolve specific L2 IDs, the MN-AR CARD Request MUST contain
   the CARD protocol-specific L2 ID message parameters.  If the MN wants
   its AR to perform only reverse address translation without appending
   the CARs' capabilities, the MN refrains from setting the C-flag in
   the CARD Request message.  If the MN wants to perform capability
   discovery, the MN MUST set the C-flag in the CARD Request message.
   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.

   If the MN appends multiple L2 ID sub-options to a CARD Request, the
   AR MUST assume that each L2 ID is associated with an AP that connects
   to a different CAR.  Since L2 IDs, address information, and
   capability information are transmitted with separate sub-options,
   each sub-option carries a Context-ID, to allow parameters that belong
   together to be matched.  Therefore, the MN MUST assign different
   Context-ID values to the L2 ID sub-options it appends to the CARD
   Request message.  The Status-Code field of the L2 ID sub-option MUST
   always be set to NONE (0x00) by the MN.  The MN MUST set the sequence
   number to a randomly generated value, and the AR MUST include the
   sequence number in all messages of the reply.  If the reply spans
   multiple messages, each message contains the same sequence number.



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   Upon receipt of the corresponding MN-AR CARD Reply message, the MN
   correlates the CARD Reply with the appropriate CARD Request message
   and then processes all MN-AR CARD Reply message parameters to
   retrieve its CAR's address and capability information.  If the MN is
   unable to correlate the CARD Reply with any previously sent CARD
   Request messages, the MN SHOULD silently discard the reply.  This may
   happen when the MN reboots after sending a CARD Request message to
   the connected AR.

   An MN uses exponential backoff to retransmit the CARD Request in the
   event that a CARD Reply is not received within CARD_REQUEST_RETRY
   seconds.  The retransmitted CARD Request MUST have the same sequence
   number as the original.  With the exception of certification paths,
   which are large by nature, an AR SHOULD attempt to limit the
   information in a CARD Reply to a single message.  Should that be
   impossible, the AR MAY send the reply in multiple messages.  The last
   message of a reply MUST always have the L-flag set in the CARD Reply
   option to indicate that the message is the last for the associated
   sequence number.  An AR retransmitting replies to a CARD Request MUST
   always send the full CARD Reply sequence.  The Trusted Anchor sub-
   option and the Router Certificate sub-option provide a means whereby
   the MN can request specific certificates in a certification path, in
   the event that the CARD Reply carrying a certification path spans
   multiple messages and one of them is lost.  However, a request for
   specific certificates that were not received in the initial CARD
   Reply MUST be treated as a new request by the MN and MUST use a
   different sequence number.

   Processing the Context-ID of Address sub-options allows the MN to
   assign the resolved IP address of a specific CAR to an L2 ID.

   In some cases, an L2 ID parameter is present in a CARD Reply message.
   The Status-Code field in the L2 ID parameter indicates one of the
   following reasons for its being sent toward the MN.

   RESOLVER ERROR Status-Code indication:
      If the MN's current AR could not resolve a particular L2 ID, this
      status code is returned to the MN.

   MATCH Status-Code indication:
      If an L2 ID is encountered that shares a CAR with a previously
      resolved L2 ID, the AR returns MATCH to the MN.  This status code
      indicates that the Context-ID of this particular L2 ID sub-option
      has been set to the Context-ID of the associated CAR's Address and
      Capability Container sub-option, which is sent with this CARD
      Reply message.  This approach avoids sending the same CAR's
      address and capability information multiple times with the same
      CARD Reply message in case two or more L2 IDs resolve to the same



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      CAR.  An MN uses the Context-ID received in the L2 ID sub-option
      as the key to find the serving CAR of the given AP from the
      content of the received CARD Reply message.

   CANDIDATE Status-Code indication:
      If the MN does not append any L2 ID to the CARD Request, the AR
      sends back the L2 ID and address information of all CARs.  Because
      the received parameters' Context-IDs cannot be correlated with an
      L2 ID's Context-ID of a previously sent request, the AR chooses
      values for the Context-ID and marks these candidate L2 IDs with
      CANDIDATE in the status code of the distributed L2 IDs.  However,
      individual values of L2 IDs' Context-ID allow the MN to assign a
      particular L2 ID to the associated Address and the possibly
      received Capability Container sub-option.

      As described in Section 4.5, an MN can use CARD when it initially
      boots up to determine whether piggyback operation is possible.  An
      MN can also use CARD initially to determine the capabilities and
      certificates for an AR on which it boots up or if it cannot obtain
      the certificates beforehand.  To do this, the MN includes an L2
      Identifier option with its current AP L2 ID and the requested
      information.  The AR replies with its own information.

4.2.2.  Current Access Router Operation

   Upon receipt of an MN's MN-AR CARD Request, the connected AR SHALL
   resolve the requested APs' L2 ID to the IP address of any associated
   CARs.  If no L2 ID parameter has been sent with the MN-AR CARD
   Request message, the receiving AR retrieves all CARs' IP addresses
   and, if the C-flag was set in the request, the capability
   information.

   In the first case, where the AR resolves only requested L2 IDs, the
   AR does not send back the L2 ID to the requesting MN.  If, however,
   two or more L2 IDs match the same CAR information, the L2 ID sub-
   option is sent back to the MN, indicating a MATCH in the Status-Code
   field of the L2 ID.  Furthermore, the AR sets the Context-ID of the
   returned L2 ID to the value of the resolved CAR's L2 ID, Address, and
   Capability Container sub-option.  If an AR cannot resolve a
   particular L2 ID, an L2 ID sub-option is sent back to the MN,
   indicating a RESOLVER ERROR in the L2 ID sub-option's Status-Code
   field.

   In the second case, where the AR did not receive any L2 ID with a
   CARD Request, all candidate APs' L2 IDs are sent to a requesting MN
   with the CARD Reply message.  The AR marks the Status-Code of
   individual L2 IDs as CANDIDATE, indicating to the MN that the




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   associated Context-ID cannot be matched with the ID of a previously
   sent request.

   In any case, the AR MUST set the Context-ID of the Address and the
   Capability Container sub-option to the same value as that of the
   associated L2 ID sub-option.

   Optionally, when allowed by local policies and supported by
   respective ARs for capability discovery, 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 and associated capabilities are
   then delivered to the MN using the MN-AR CARD Reply message.  The
   CARs' IP address and 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 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.

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 an MN-AR CARD Request or when it detects that one or
   more of its local CAR table's capability entries' lifetimes are about
   to expire.  An AR SHOULD preferentially utilize its CAR table to
   fulfill requests rather than signal the CAR directly, and it SHOULD
   keep the CAR table up to date for this purpose, in order to avoid
   injecting unnecessary delays into the MN response.

   The AR SHOULD issue an AR-AR CARD Request to the respective CARs if
   complete capability information of a CAR is not available in the
   current AR's CAR table, or if such information is expired or about to
   expire.  The AR-AR CARD Request message format is defined in Section
   5.2.2.  The sequence number on the AR-AR interface starts with zero
   when the AR reboots.  The sending AR MUST increment the sequence
   number in the CARD Request by one each time it sends a CARD Request
   message.

   The AR MAY append its own capabilities, which are encoded as
   attribute-value pairs and encapsulated with the Capability Container
   message parameter, to the released AR-AR CARD Request.  If the AR-AR
   CARD Request conveys the current AR's capabilities to the CAR, the
   associated Capability Container can have any value set for the
   Context-ID, as there is no need for the receiving CAR to process this



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   field due to the absence of an L2 ID and an Address sub-option.
   Furthermore, the current AR MAY set the P-flag in the Capability
   Container sub-option to inform the CAR about its own capability to
   perform CARD protocol message piggybacking.

   Optionally, a current AR MAY append the Preferences sub-option to the
   AR-AR CARD Request to obtain only capability parameters of interest
   from a CAR.

   Upon receipt of the AR-AR CARD Reply, 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
   store 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 values of the table
   entries' timeouts shall depend upon the nature of individual
   capabilities.

   Optionally, CARs can send unsolicited CARD Reply messages to globally
   adjacent ARs if the configuration of their APs or capabilities
   changes dynamically.  If the current AR receives an unsolicited CARD
   Reply message from a CAR for which there is an entry in its local CAR
   table, the current AR checks that the sequence number of the received
   CARD Reply has increased compared to that of the previously received
   unsolicited CARD Reply message, which has been sent from the same
   CAR.  Then, the current AR can update its local CAR table according
   to the received capabilities.  If a new CAR is added, an AR may
   receive a CARD Reply from a CAR that is not in its CAR table, or from
   a CAR that has rebooted.  In this case, the sequence number is 0.
   The requirement that ARs share an IPsec security association,
   detailed in Section 6, ensures that an AR never accepts CARD
   information from an unauthenticated source.

4.3.2.  Candidate Access Router Operation

   Upon receipt of an AR-AR CARD Request, a CAR shall extract the
   sending AR's capabilities, if the sending AR has included its
   capabilities.  The CAR SHALL store the received capabilities in its
   CAR table and set the timer for individual capabilities
   appropriately.  The values of the table entries' timeouts depend on
   the nature of capabilities in the AR-AR CARD Reply message.  The CAR
   must include the same sequence number in the AR-AR CARD Reply Message
   as that received in the AR-AR CARD Request Message.  The AR-AR CARD
   Reply shall include the CAR's capabilities as list of attribute-value
   pairs in the Capability Container message parameter.  If the sending
   AR has appended an optional Preferences sub-option, the CAR MAY
   perform capability filtering and send back only those capabilities of
   interest to the requesting AR, identified according to the



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   Preferences sub-option.  Because the AR-AR CARD Reply is based on a
   previously received AR-AR CARD Request, the CAR MUST set the U-flag
   of the AR-AR CARD Reply to 0.

   Optionally, the CAR MAY send an unsolicited CARD Reply message to
   globally adjacent ARs if one or more of its capability parameters
   change.  Each unsolicited CARD Reply message should have as
   destination address the adjacent AR's unicast address and must have
   the U-flag set.  Consecutive unsolicited CARD Reply messages MUST
   have the sequence number incremented accordingly, starting with 0
   when the AR boots.

4.4.  CARD Protocol Message Piggybacking on the MN-AR Interface

   CARD supports another mode of CAR information distribution, in which
   the capabilities are piggybacked on fast handover protocol messages.
   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 [Kood03]
   signaling messages, the 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.  The format of
   these messages and parameters is described in Section 5.1.

   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
   4.2.1.  If the MN supports CARD-protocol message piggybacking, the
   P-flag in this very first CARD Request message is set.  On receipt of
   the CARD Request message, the current AR learns about the MN's
   piggybacking capability.  To indicate its piggybacking capability,
   the AR sets the P-flag in the CARD Reply message.  If the AR does not
   support piggybacking, all subsequent CARD-protocol messages between
   the MN and the AR are sent stand-alone, using the CARD main header.
   If 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 Router Solicitation
   for Proxy (RtSolPr) and Proxy Router Advertisement (PrRtAdv)
   messages.  During the CARD process, an MN learns about CARs'
   piggybacking capability at the discovery phase, as the Capability
   Container (described in Section 5.1.3.4) also carries a P-flag.  This
   allows the MN to perform CARD protocol message piggybacking
   immediately after a handover to a selected CAR, assuming that this
   CAR supports CARD protocol piggybacking.

   If a MN prefers the reverse address translation function of the Fast
   Mobile IPv6 protocol, it can use CARD protocol message piggybacking
   to retrieve only the CARs' capability information.  To indicate that



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   reverse address translation is not required, the piggybacked CARD
   Request message MUST have the A-flag set.  This causes the current AR
   to append only Capability Container sub-options.  To associate a
   Capability Container sent as a parameter of the CARD Reply message to
   the IP address for the appropriate CAR, the Context-ID of an
   individual Capability Container MUST be used as an index, pointing to
   the associated IP address in the PrRtAdv message options.  The
   Context-ID of individual Capability Containers is set appropriately
   by the MN's current AR.  Details about how individual Context-ID
   values can be associated with a particular IP address option of the
   PrRtAdv message is out of the scope of this document.

5.  Protocol Messages

5.1.  CARD Messages for the Mobile Node-Access Router Interface

5.1.1.  MN-AR Transport

   The MN-AR interface uses ICMP for transport.  Because ICMP messages
   are limited to a single packet, and because ICMP contains no
   provisions for retransmitting packets if signaling is lost, the CARD
   protocol incorporates provisions for improving transport performance
   on the MN-AR interface.  MNs SHOULD limit the amount of information
   requested in a single ICMP packet, as ICMP has no provision for
   fragmentation above the IP level.

   MNs and ARs use the Experimental ICMP-type main header [Ke04] when
   CARD protocol messages cannot be conveyed via ICMP-type Fast Mobile
   IPv6 [Kood03].  The MN-AR interface MUST implement and SHOULD use the
   CARD ICMP-type header for transport.  If available, the MN-AR
   interface MAY use the ICMP-type Fast Mobile IPv6 [Kood03] for
   transport (Section 4.4).

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Subtype    |             Reserved                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Options ...
   +-+-+-+-+-+-+-+-+-+-+-+- - - -

   IP Fields:

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




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

      Hop Limit:     255

   ICMP Fields:

      Type:          Experimental Mobility type (assigned by IANA for
                     IPv4 and IPv6, see [Ke04]).

      Code:          0

      Checksum:      The ICMP checksum.

      Subtype:       Experimental Mobility subtype for CARD; see [Ke04].

      Reserved:      This field is currently unused.  It MUST be
                     initialized to 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 support
                     processing of the CARD Request message on the
                     receiver side, further sub-options may 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.

   Valid Sub-Options:

   Support level is indicated in parentheses.

      Layer-2 ID (mandatory):
                     The Layer-2 ID sub-option [5.1.3.1] 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.








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      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.

      Trusted Anchor (mandatory):
                     The Trusted Anchor sub-option carries the name of a
                     trusted anchor for which the MN has a certificate.
                     The format of the Trusted Anchor sub-option is
                     described in Section 5.1.3.6.

      Router Certificate (mandatory):
                     The Router Certificate sub-option carries one
                     certificate in the path for the current AR or for a
                     CAR.  The chain includes certificates starting at a
                     trusted anchor, which the AR shares in common with
                     the MN, to the router itself.  The format of the
                     Router Certificate sub-option is described in
                     Section 5.1.3.7.

      Preferences (optional):
                     The Preferences sub-option carries information
                     about attributes of interest to the requesting
                     entity.  Attributes are encoded according to the
                     AVP encoding rule, which is described in Section
                     5.1.4.  For proper settings of AVP Code and Data
                     field, see Section 5.1.3.2.  This sub-option is
                     used only if optional capability pre-filtering is
                     performed on ARs, and it provides 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 the MN's current AR.  This
                     parameter conveys MN specific attribute-value pairs
                     to allow the MN's current AR to send only
                     information about CARs of interest back to the
                     requesting MN.  CARs are filtered on ARs according
                     to the CARs' capability parameters and given policy
                     or threshold, as encoded in the Requirements sub-



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                     option.  Attribute-value pairs are encoded
                     according to the AVP encoding rule, which is
                     described in Section 5.1.4.  Rules for proper
                     setting of the AVP Code and Data field for the
                     Requirements sub-option are described in Section
                     5.1.3.3.

   CARD Requests that fail to elicit a response are retransmitted.  The
   initial retransmission occurs after a CARD_REQUEST_RETRY wait period.
   Retransmissions MUST be made with exponentially increasing wait
   intervals (doubling the wait each time).  CARD Requests should be
   retransmitted until either a response (which might be an error) has
   been obtained or CARD_RETRY_MAX seconds have occurred.  ARs MUST
   discard any CARD Requests having the same sequence number after
   CARD_RETRY_MAX seconds.  If a CARD Reply spans multiple ICMP
   messages, the same sequence number MUST be used in each message.

   MNs that retransmit a CARD Request use the same CARD sequence number.
   This allows the AR to cache its reply to the original request and
   then to send it again, should a duplicate request arrive.  This
   cached information should only be held for a maximum of
   CARD_RETRY_MAX seconds after receipt of the request.  Sequence
   numbers SHOULD be chosen randomly.  Random sequence numbers avoid
   duplicates if MNs restart frequently and simplify sequence-number
   maintenance on both the MN and AR when MNs frequently appear and
   disappear due to movement between CARs.

5.1.2.  CARD Options Format

   All options are of the following 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     |Vers.|        ...              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                              ...                              ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

      Type:          8-bit identifier of the type of option, assigned by
                     IANA.  See [Ke04] for CARD Request and CARD Reply
                     values.

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



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      Vers.:         3-bit version code.  For this specification,
                     Vers.=1.

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     |Vers.|P|C|A|T|     Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sequence Number                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Sub-Options
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -  -  -

   Fields:

      Type:    Assigned by IANA for IPv4 and IPv6; see [Ke04].

      Length:  The length of the option in units of 8 octets, including
               the type and length fields as well as sub-options.

      Vers.:   3-bit version code.  For this specification, Vers.=1.

               Flags:   P-flag:  Indicates the CARD-protocol message
                                 piggybacking capability of the CARD
                                 Request message sender.  A description
                                 for proper use of this flag can be
                                 found in Section 4.4 of this document.

                        C-flag:  Indicates that the requesting entity is
                                 also interested in associated CARs'
                                 capabilities.  If the MN wants the AR
                                 to append CARs' capability parameters
                                 to the CARD Reply in addition to
                                 address information, the MN must set
                                 this flag.

                        A-flag:  Indicates that the requesting entity
                                 does NOT want the receiver of this
                                 message to perform reverse address
                                 translation.  This flag is set if CARD
                                 protocol messages are piggybacked with
                                 a protocol that performs reverse
                                 address translation.  For details,
                                 refer to Section 4.4 of this document.





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                        T-flag:  Indicates that the requesting entity is
                                 interested in obtaining all
                                 certificates from the responder.  This
                                 flag is only valid on the AR-AR
                                 interface.

               The flag combination A=1 and C=0 is invalid, and the flag
               T=1 is invalid on the MN-AR interface.  The AR MUST
               discard an invalid message and log an appropriate error
               message.

      Reserved:
               Initialized to zero, ignored on receipt.

      Sequence Number:
               Allows requests to be correlated with replies.

   Valid Sub-Options:

      - L2 ID sub-option
      - Preferences sub-option
      - Requirements sub-option
      - Trusted Anchor sub-option

   To ensure that requirements on boundary alignment are met, individual
   sub-options MUST meet the 64-bit boundary alignment requirements
   respectively.  This will ensure that the entire CARD Request option
   meets the 8n alignment constraint.

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     |Vers.|P|U|L|     Reserved      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Sequence Number                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Sub-Options
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -

   Fields:

      Type:    Assigned by IANA for IPv4 and IPv6 [Ke04].

      Length:  The length of the option in units of 8 octets, including
               the type and length fields as well as sub-options.




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      Vers.:   3-bit version code.  For this specification, Vers.=1.

               Flags:   P-flag:  Indicates the CARD-protocol message
                                 piggybacking capability of the CARD
                                 Reply message sender.  A description
                                 for proper use of this flag can be
                                 found in Section 4.4 of this document.

                        U-flag:  Indicates an unsolicited CARD Reply.
                                 This flag is only valid on the AR-AR
                                 interface.

                        L-flag:  Set if this message is the last message
                                 in a multiple ICMP message reply.  This
                                 flag is only valid on the MN-AR
                                 interface.

               The flag U=1 on an AR-MN message is invalid.  An invalid
               message should be discarded and an appropriate error
               message logged.

      Reserved:
               Initialized to zero, ignored on receipt.

      Sequence Number:
               Allows requests to be correlated with replies.

   Valid Sub-Options:

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

   To ensure requirements on boundary alignment are met, individual
   sub-options MUST meet 64-bit boundary alignment requirements
   respectively.  This will ensure that the entire CARD Request option
   meets the 8n alignment constraint.

5.1.3.  Sub-Options Format

   All sub-options are of the following 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 . . .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   Sub-Option Type:  8-bit identifier of the type of option.  The
                     sub-options defined in this document are listed
                     in the table below.  The table also indicates
                     on which interfaces the sub-option is valid.

          Description                Type              Interface
              |                       |               /         \
              |                       |            MN-AR       AR-AR
      ---------------------------------------------------------------
            L2 ID                    0x01            x
            Address                  0x02            x
            Capability Container     0x03            x           x
            Preferences              0x04            x           x
            Requirements             0x05            x
            Trusted Anchor           0x06            x
            Router Certificate       0x07            x           x

   Sub-Option-Length: 8-bit unsigned integer indicating the length of
                      the sub-option, including the sub-option type and
                      sub-option length fields.  Sub-option lengths are
                      in units of 8 octets, aligned on a 64-bit
                      boundary.  Sub-options that are shorter are padded
                      with null octets; the extent of the padding is
                      determined by the sub-option contents.

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  |  Status Code  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    L2-Type                    |     L2 ID . . .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -

   Sub-Option Type:
                  0x01

   Sub-Option Length:
                  Length of the sub-option.

   Context-ID:    Associates the L2 ID, IP address and other parameters
                  that belong to the same AR IP address but are encoded
                  in separate sub-options.






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   Status Code:   This field allows ARs to inform a requesting entity
                  about processing results for a particular L2 ID.  The
                  L2 ID sub-option MUST be sent back to the requesting
                  entity with a CARD Reply message.

                  The following status codes are specified:

               0x00:    NONE - This value MUST be set when the L2 ID is
                        included in a CARD Request.

               0x01:    CANDIDATE - MUST be set in a CARD Reply when a
                        L2 ID sub-option is included with information
                        about candidate APs' L2 IDs.  Candidate L2 IDs
                        are sent if the CARD Request did not include a
                        specific L2 ID for resolution.  If CANDIDATE is
                        set, the AR MUST set the Context-ID field of
                        individual parameters to a value that allows
                        associated L2 ID, address, and capability
                        information to be matched on the receiver side.

               0x02:    MATCH - MUST be set in the CARD Reply to
                        identify that this L2 ID matches previously
                        resolved CAR information for a different L2 ID.
                        If MATCH is set, the AR sets the Context-ID in
                        the L2-ID sub-option to identify the matching
                        previously resolved L2 ID.

               0x03:    RESOLVER ERROR - MUST be set in the CARD Reply
                        if the L2 ID cannot be resolved.  The AR sets
                        this value for the Status Code in the returned
                        L2 ID sub-option.

   L2 type:       Indicates the interface type.  Allocated by IANA
                  [Ke04].

   L2 ID:         The variable length Layer-2 identifier of an
                  individual CAR's access point.  The length without
                  padding is determined by the L2 type.

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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





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   Sub-Option Type:
                  0x04

   Sub-Option Length:
                  Length of the sub-option.

   Preferences:   List of capability attribute values (see Section
                  5.1.4).

   Only ATTRIBUTE (AVP Code; see Section 5.1.4) fields MUST be present
   and set for individual capabilities, which are of interest to the
   requesting entity.  The LIFETIME and VALUE (Data) indicator will not
   be processed and can be omitted.  The AVP LENGTH indicator is also
   not present, as the preferences are indicated only with a list of
   16-bit encoded ATTRIBUTE fields.  If 64-bit boundary alignment
   requirements cannot be met with the list of ATTRIBUTE values, padding
   the missing 16-bit MUST be done with an ATTRIBUTE value of 0x0000.
   An ATTRIBUTE code of 0x0 is reserved so that the end of the ATTRIBUTE
   code list can be determined when an ATTRIBUTE value of 0x0 is read.

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

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:
                  0x05

   Sub-Option Length:
                  Length of the sub-option.

   Requirements:  AVP-encoded requirements (see Section 5.1.4)

   AVPs MUST be encoded according to the rule described in Section
   5.1.4.  Both the ATTRIBUTE (AVP Code) and VALUE (Data) fields MUST be
   present and set appropriately.  The end of the Requirements list can
   be determined when an ATTRIBUTE value of 0x0 is read.

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





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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:
                  0x03

   Sub-Option Length:
                  Length of the sub-option.

   Context-ID:    Associates the L2 ID, IP address, and other parameters
                  that belong to the same AR IP address but are encoded
                  in separate sub-options.

   Flags:         P-flag:  Indicates piggybacking capability of the CAR
                           whose capabilities are conveyed in this
                           Capability Container.  This flag allows an MN
                           to know after a CARD process whether a
                           selected new AR can perform piggybacking.

   Reserved:      Initialized to zero, ignored on receipt.

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

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:
                  0x02






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   Sub-Option Length:
                  Length of the sub-option.  For IPv4, the length is 1
                  (8 octets); for IPv6 the length is 3 (24 octets).

   Context-ID:    Associates the L2 ID, IP address, and other parameters
                  that belong to the same AR IP address but are encoded
                  in separate sub-options.

   Address Type:  Indicates the type of the address.

                                       0x01  IPv4
                                       0x02  IPv6

   Address:       The Candidate Access Router's IP address.

5.1.3.6.  Trusted Anchor 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 |      Component                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Trusted Anchor Name
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - -

   Sub-Option Type:
                  0x06

   Sub-Option Length:
                  Length of the sub-option.

   Reserved:      Initialized to zero, ignored on receipt.

   Component:     A 2 octet unsigned integer field set to 65,535 if the
                  sender desires to retrieve all the certificates in the
                  certification path.  Otherwise, it is set to the
                  component identifier corresponding to the certificate
                  that the receiver wants to retrieve.

   Trusted Anchor Name:
                  DER encoding for the X.501 name of certification path
                  component(see [Arkko04] for more detail on
                  certification path component name encoding).

   A CARD Request message containing Trusted Anchor sub-options MUST NOT
   contain any other sub-options, except for a single L2 ID sub-option
   identifying the AP of interest.




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   Trusted anchor sub-options SHOULD be retransmitted for individual
   components not received within CARD_REQUEST_RETRY seconds, rather
   than retransmitting a request for the whole list.  Subsequent
   retransmissions SHOULD take into account any received options and
   only request those that have not been received.

5.1.3.7.  Router Certificate 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  | Reserved      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          All Components       |        Component              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                          Certificate...                       |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Padding...                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Sub-Option Type:
                  0x07

   Sub-Option Length:
                  Length of the sub-option.

   Context-ID:    Associates the L2 ID, IP address and other parameters
                  that belong to the same AR IP address but are encoded
                  in separate sub-options.

   Reserved:      Initialized to zero, ignored on receipt.

   All Components:
                  2 octet unsigned integer giving the total number of
                  certificates in the certification path.

   Component:     2 octet unsigned integer giving the location of this
                  certificate in the certification path.

   Certificate:   Variable-length field containing the X.509v3 router
                  certificate encoded in ASN.1 (see [Arkko04] for more
                  detail on a certificate profile that includes
                  encoding).




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   Padding:       Variable-length field making the option length a
                  multiple of 8, beginning after the ASN.1 encoding of
                  the certificate and continuing to the end of the
                  option, as specified by the Length field.

   A CARD Reply containing a Router Certificate sub-option MUST NOT
   include more than one such sub-option, and the CARD Reply MUST
   contain the matching L2 ID sub-option and router Address sub-option
   for the router possessing the chain with the Context-ID field set to
   a nonzero value, and with no other sub-options.  Any other sub-
   options included in a CARD Reply SHOULD be ignored.  If the reply
   spans multiple ICMP messages, the L2 ID sub-option and router Address
   sub-option MUST be included in the first message sent, and the
   Context-ID field in the Router Certificate sub-options in all the
   messages MUST be set to the same value as that in the L2 ID and
   Address sub-options.  The replying AR SHOULD order the returned
   certification path so that the certificate immediately after the
   trust anchor in the path is the first certificate sent, in order to
   allow immediate verification.  The trust anchor certificate itself
   SHOULD NOT be sent.

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            |  AVP Length   |   Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Attribute Lifetime       |           Data . . .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -

   AVP Code:      Identifies the attribute uniquely.  The AVP Code
                  0x0000 is reserved and MUST NOT be assigned to a
                  capability.

   AVP Length:    The 2 octet AVP length field indicates the number of
                  octets in this AVP, including the AVP Code, AVP
                  Length, Reserved, Lifetime, and Data fields.

   Reserved:      Initialized to zero, ignored on receipt.

   Lifetime:      Specifies the lifetime of the encoded capability in
                  seconds.  In the case of a static capability, the
                  Lifetime field MUST be set to the maximum value
                  (0xffff), which indicates that the lifetime of this
                  capability parameter never expires.  A lifetime value
                  of 0x0000 deletes a capability entry.




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   Data:          This variable-length field has the Value of the
                  capability attribute encoded.

   Because an AVP Code of 0x0 is reserved, it can be used by the sub-
   option list parsing to determine when the end of a list of
   Capabilities has been reached and where the sub-option padding
   starts.  AVPs themselves are not zero padded.

   Note: This document provides no detailed information on how to encode
   the individual capability attribute values, which is to be encoded in
   the Data field.  Details on the interpretation of individual
   capability parameters are out of the scope of this document.

5.2.  CARD Inter-Access Router Messages

5.2.1.  AR-AR Transport

   Because the types of access networks in which CARD might be useful
   are not currently deployed or, if they have been deployed, have not
   been extensively measured, it is difficult to know whether congestion
   will be a problem for inter-router CARD.  Part of the research task
   in preparing CARD for consideration as a candidate for possible
   standardization is to quantify this issue.  However, in order to
   avoid potential interference with production applications (should a
   prototype CARD deployment involve running over the public Internet),
   it seems prudent to recommend a default transport protocol that
   accommodates congestion.

   This suggests that implementations of CARD MUST support and that
   prototype deployments of CARD SHOULD use the Stream Control Transport
   Protocol (SCTP) [Stew00] as the transport protocol between routers,
   especially if deployment over the public Internet is contemplated.
   SCTP supports congestion control, fragmentation, and partial
   retransmission based on a programmable retransmission timer.  SCTP
   also supports many advanced and complex features, such as multiple
   streams and multiple IP addresses for failover, that are not
   necessary for experimental implementation and prototype deployment of
   CARD.  The use of these SCTP features for CARD is not recommended at
   this time.

   The SCTP Payload Data Chunk carries the CARD messages.  CARD messages
   on the inter-router interface consist of just the CARD Request or
   CARD Reply options.  The User Data part of each SCTP message contains
   the CARD option for the message type.  For instance, a CARD Reply
   message is constructed by including the CARD Reply option and all the
   appropriate sub-options within the User Data part of an SCTP message.





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   A single stream is used for CARD with in-sequence delivery of SCTP
   messages.  Each message, unless fragmented, corresponds to a single
   CARD query or response.  Unsolicited CARD Reply messages can also be
   sent to peers to notify them of changes in network configuration or
   capabilities.  A single stream provides simplicity.  Use of multiple
   streams to prevent head-of-line blocking is for future study.  Since
   timeliness is not an issue with inter-router CARD, and since there
   being more than one CARD transaction between two routers active at
   any one time is unlikely, having ordered delivery simplifies the
   implementation.  The Payload Protocol Identifier in the SCTP header
   is 'CARD'.  CARD uses the Seamoby SCTP port number [Ke04].

   The format of Payload Data Chunk taken from [Stew00] is shown in the
   following diagram.

    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 = 0    | Reserved|U|B|E|    Length                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              TSN                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Stream Identifier S      |   Stream Sequence Number n    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Payload Protocol Identifier                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                 User Data (seq n of Stream S)                 /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         'U' bit      The Unordered bit.  MUST be set to 0 (zero).
         'B' bit      The Beginning fragment bit.  See [Stew00].

         'E' bit      The Ending fragment bit.  See [Stew00].

         TSN          Transmission Sequence Number.  See [Stew00].

         Stream Identifier S
                      Identifies the CARD stream.

         Stream Sequence Number n
                      Sequence number.  See [Stew00].

         Payload Protocol Identifier
                      Set to 'CARD'.

         User Data    Contains the CARD message.



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   In order to avoid generating congestion on startup, ARs MUST wait a
   random amount of time between 0 and CARD_STARTUP_WAIT seconds upon
   reboot before sending an AR-AR CARD Request to one of its CARs.  An
   AR that receives a CARD Request from another AR that is not in its
   CAR table MUST NOT solicit the AR but rather MUST wait until the AR
   sends an unsolicited CARD Reply advertising the AR's information.  An
   AR that is starting up MUST send unsolicited CARD Replies to all its
   CARs to make sure that their CAR tables are properly populated.

   The frequency of unsolicited CARD Reply messages MUST be strictly
   limited to CARD_MIN_UPDATE_INTERVAL, in order to avoid overwhelming
   CARs with traffic.  ARs are free to discard messages that arrive more
   frequently.

   If a CARD deployment will never run over the public Internet, and if
   it is known that congestion is not a problem in the access network,
   alternative transport protocols MAY be appropriate vehicles for
   experimentation.  Implementations of CARD MAY support UDP for such
   purposes.  In that case, the researcher MUST be careful to
   accommodate good Internet transport protocol engineering practices,
   such as using retransmits with exponential backoff.  In addition,
   whether SCTP is an appropriate transport protocol for all inter-
   router CARD operations is an open research question.  Investigation
   of this issue (for example, to determine whether a lighter-weight
   protocol might be more appropriate than SCTP) may be of interest to
   some researchers.

5.2.2.  Protocol Payload Types

   The AR-AR interface MUST insert the CARD Request option and CARD
   Reply option directly into the body of the SCTP User Data field.  The
   sequence number for the CARD Request on the AR-AR interface MUST be
   initialized to zero when the AR reboots, and MUST be incremented
   every time a CARD Request message is sent.  The replying AR MUST
   include a sequence number from the CARD Request in the CARD Reply.
   If an unsolicited CARD Reply is sent, the sending AR MUST increment
   the sequence number.  Sequentially increasing sequence numbers allows
   the receiving AR to determine whether the information has already
   been received.

   On the AR-AR interface, the Capability Container parameter is used to
   convey capabilities between ARs.  Optionally, the Preferences
   parameter can be used for capability pre-filtering during the inter-
   AR capability discovery procedure.  Payload types and encoding rules
   are the same as those described for the respective 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.
   Additionally, an AR can set the T flag in the CARD Request header in



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   order to obtain the certificates for the CAR.  The description of
   sub-options in Section 5.1.3 includes information on what flag
   settings are prohibited on the AR-AR interface.

6.  Security Considerations

6.1.  Veracity of CARD Information

   The veracity of the CARD protocol depends on the ability of an AR to
   obtain accurate information about geographically neighboring ARs, and
   to provide accurate information about its own APs and capabilities to
   other ARs.  The CARD protocol described in the body of this document
   does not contain any support for determining the AR-to-AP mapping or
   capabilities, either for a specific AR or for a CAR.  Therefore,
   methods for determining the accuracy of the information exchanged
   between ARs are out of scope for the base CARD protocol.  The
   appendices of this document describe procedures for discovering the
   identities of the geographically adjacent ARs and APs (including
   capabilities) and discuss relevant security considerations.
   Alternatively, this information could be statically configured into
   the AR.

6.2.  Security Association between AR and AR

   CARD contains support allowing ARs to exchange capability
   information.  If this protocol is not protected from modification, a
   malicious attacker can modify 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 messages
   between ARs MUST be authenticated.  The messages also SHOULD be
   encrypted for privacy of the information, if required.
   Confidentiality might be required if the traffic between two ARs in
   an operator's network traversed the public Internet, for example.

   Two ARs engaging in the CARD protocol MUST use IKE [HarCar98] to
   negotiate an IPsec ESP security association for message
   authentication.  If confidentiality is desired, the two ARs MUST
   additionally negotiate an ESP security association for encryption.
   Replay protection SHOULD also be enabled with IKE.  To protect CARD
   protocol messages between ARs, IPsec ESP [AtKe98] MUST be used with a
   non-null integrity protection and origin authentication algorithm and
   SHOULD be used with a non-null encryption algorithm for protecting
   the confidentiality of the CARD information.

   An AR can provide the certificates for its CARs if the certificates
   are available.  The AR requests certificates from its CARs by setting
   the T flag in the CARD Request message.  All certificates are sent.



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   If CARD is used to exchange information between different
   administrative domains, additional security policy issues may apply.
   Such issues are out of the scope of this document.  Use of CARD
   between administrative domains is not recommended at this time, until
   the policy issues involved are more thoroughly understood.

6.3.  Security Association between AR and MN

   A malicious node can send bogus CARD Reply messages to MNs by
   masquerading as the AR.  The MN MUST authenticate the CARD Reply
   messages from the AR.  Since establishing an IPSec security
   association between the MN and AR is likely to be a performance
   issue, IKE is not an appropriate mechanism for setting up the
   security association.  Instead, the SEND security association is used
   [Arkko04].  ARs MUST include a SEND Signature Option on CARD Reply
   messages.  The format of the signature option is the same for both
   IPv4 and IPv6 CARD, though SEND itself is only defined for IPv6.  A
   Mobile IPv4 ICMP Foreign Agent Advertisement option type code for the
   SEND signature option [Ke04] has been allocated.

   No authentication is required for CARD Requests since CARD
   information is provided by the AR to optimize link access.  In
   contrast, CARD Reply authentication is required because a bogus AR
   could provide the MN with CARD information that would lead the MN to
   handover to a bogus router, which could steal traffic or propagate a
   denial of service attack on the MN.  The asymmetry of the
   authentication requirement is the same as that involving Router
   Advertisements in IPv6 router discovery [Arkko04].

   Since CARD is a discovery protocol, confidentiality is not generally
   necessary on the MN-AR interface.  In specific cases where different
   network operators share the same access network infrastructure,
   network operators may want to hide information about operator-
   specific capabilities for business reasons.  The base CARD protocol
   contains no support for such cases.  However, should such a case
   arise in the future, an AVP for an encrypted capability can be
   defined at that time.

6.4.  Router Certificate Exchange

   Because SEND is only available in IPv6, the procedures for obtaining
   certificates differ depending on whether CARD is used with IPv4 or
   IPv6.  In IPv6, when the MN receives a CARD reply with signature from
   an AR for which it does not have a certificate, it SHOULD use SEND
   DCS/DCA to obtain the AR's certificate chain.  ARs MUST be configured
   with a certification path for this purpose, and MNs MUST be
   configured with a set of certificates for shared trusted anchors to
   allow verification of the AR certificates.  An MN may not necessarily



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   need to use Cryptographically Generated Addresses (CGAs) with CARD,
   so CGA support is OPTIONAL for CARD.  A certificate profile for ARs
   is described in the SEND specification [Arkko04].

   In IPv4, there is no DCS/DCA message for obtaining the certificate.
   If the MN does not have a certificate for the AR, the MN sends a CARD
   Request message containing the L2 ID of its current AP and one
   Trusted Anchor sub-option (Section 5.1.3.6) for each shared trusted
   anchor for which the MN has a certificate, to obtain the
   certification path for the current AR.  The Component field of the
   Trusted Anchor sub-option is set to 65535 to indicate that the entire
   certification path is needed.  No other options should be included in
   the request.  The AR replies by sending a CARD Reply containing the
   L2 ID sub-option sent in the request, an Address sub-option for
   itself, and a Router Certificate sub-option (Section 5.1.3.7)
   containing one certificate in its certification path that matches one
   of the requested trust anchors, and no other sub-options, setting the
   Context-ID of all sub-options to match.  The All Components field is
   set to the path length, and the Component field is set to the number
   of this component in the path.  If the path is longer than one
   certificate, the AR sends the L2 ID sub-option and the Address sub-
   option in the first certificate and the other certificates in
   separate ICMP messages, due to the limitation on ICMP message length,
   with the same Context-ID set on each Route Certificate sub-option,
   and with the Component field properly set.  The router SHOULD NOT
   send the trusted anchor's certificate and SHOULD send certificates in
   order from the certificate after the trusted anchor.  If the trusted
   anchor option does not match any certificate, the AR returns the
   Trusted Anchor sub-options in the reply.  The MN SHOULD immediately
   conduct a Certificate Revocation List (CRL) check on any certificates
   obtained through CARD certificate exchange, to make sure that the
   certificates are still valid.

   Certification paths for CARs may be fetched in advance of handover by
   requesting them as part of the CARD protocol.  In that case, the MN
   includes Trusted Anchor sub-options in the CARD request along with
   the L2 ID sub-option for the AP for which the CAR certificate is
   desired, and the AR replies as above, except that the L2 ID, address,
   and certificates are for the CAR instead of for the AR itself.  This
   allows the MN to skip the DCS/DCA or CARD certificate exchange when
   it moves to a new router.

   Because the amount of space in an ICMP message is limited, the router
   certification paths SHOULD be kept short.







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6.5.  DoS Attack

   An AR can be overwhelmed with CARD Request messages.  The AR SHOULD
   implement a rate-limiting policy so that it does not send or process
   more than a certain number of messages per period.  The following is
   a suggested rate limiting policy.  If the number of CARD messages
   exceeds CARD_REQUEST_RATE, the AR SHOULD begin to drop messages
   randomly until the rate is reduced.  MNs SHOULD avoid sending
   messages more frequently than CARD_REQUEST_RATE.  ARs SHOULD also
   avoid sending unsolicited CARD Replies or CARD Requests more
   frequently than CARD_MIN_UPDATE_INTERVAL, but, in this case, the
   existence of an IPsec security association ensures that messages from
   unknown entities will be discarded immediately during IPsec
   processing.

   MNs MUST discard CARD Replies for which there is no outstanding CARD
   Request, as indicated by the sequence number.

6.6.  Replay Attacks

   To protect against replay attacks on the AR-AR interface, ARs SHOULD
   enable replay protection when negotiating the IPsec security
   association using IKE.

   On the MN-AR interface, the MN MUST discard any CARD Replies for
   which there is no outstanding request, as determined by the sequence
   number.  For ARs, an attacker can replay a previous request from an
   MN, but the attack is without serious consequence because the MN
   ignores the reply in any case.

7.  Protocol Constants

      Constant           Section    Default Value     Meaning
   --------------------------------------------------------------------
   CARD_REQUEST_RETRY      5.1.1    2 seconds    Wait interval before
                                                 initial retransmit
                                                 on MN-AR interface.

   CARD_RETRY_MAX          5.1.1    15 seconds   Give up on retry
                                                 on MN-AR interface.

   CARD_STARTUP_WAIT       5.2.1    1-3 seconds  Maximum startup wait
                                                 for an AR before
                                                 performing AR-AR
                                                 CARD.

   CARD_MIN_UPDATE_INTERVAL 5.2.1   60 seconds   Minimum AR-AR update
                                                 interval.



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   CARD_REQUEST_RATE        6.5     2 requests/  Maximum number of
                                      sec.       messages before
                                                 AR institutes rate
                                                 limiting.

8.  IANA Considerations

   See [Ke04] for instructions on IANA allocation.

9.  Normative References

   [Brad97]   Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [Stew00]   Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
              Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
              Zhang, L., and V. Paxson, "Stream Control Transmission
              Protocol", RFC 2960, October 2000.

   [AtKe98]   Kent, S. and R. Atkinson, "IP Encapsulating Security
              Payload (ESP)", RFC 2406, November 1998.

   [HarCar98] Harkins, D. and D. Carrel, "The Internet Key Exchange
              (IKE)", RFC 2409, November 1998.

   [Arkko04]  Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
              Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [Ke04]     Kempf, J., "Instructions for Seamoby and Experimental
              Mobility Protocol IANA Allocations", RFC 4065, July 2005.

10.  Informative References

   [TKCK02]   Trossen, D., Krishanmurthi, G. Chaskar, H., Kempf, J.,
              "Issues in candidate access router discovery for seamless
              IP-level handoffs", Work in Progress.

   [MaKo03]   Manner, J. and M. Kojo, "Mobility Related Terminology",
              RFC 3753, June 2004.

   [Kood03]   Koodli, R., Ed., "Fast Handovers for Mobile IPv6", RFC
              4068, July 2005.

   [Funa02]   Funato, D., et al., "Geographically Adjacent Access Router
              Discovery Protocol", Work in Progress.





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   [Tros03]   Trossen, D., et al., "A Dynamic Protocol for Candidate
              Access-Router Discovery", Work in Progress.

   [ShGi00]   Shim, E. and R. Gitlin, "Fast Handoff Using Neighbor
              Information", Work in Progress.

   [Malk03]   El Malki, K., et al., "Low Latency Handoffs in Mobile
              IPv4", Work in Progress.

11.  Contributors

   The authors would like to thank Vijay Devarapalli (Nokia) and Henrik
   Petander (Helsinki University of Technology) for formally reviewing
   the protocol specification document and providing valuable comments
   and input for technical discussions.  The authors would also like to
   thank James Kempf for reviewing and for providing a lot of valuable
   comments and editing help.

12.  Acknowledgements

   The authors 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
   document, as well as for the general CARD-related discussion and
   feedback.  In addition, the authors would like to thank Erik Nordmark
   for providing valuable insight about the piggybacking of CARD options
   upon Fast Mobile IPv6 messages.























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Appendix A.  Maintenance of Address Mapping Tables in Access Routers

   This appendix provides information on two optional CAR table
   maintenance schemes for reverse address mapping in access routers.
   These schemes replace static configuration of the AP L2 ID-to-CAR IP
   address mapping in the CAR table.  Details on these mechanisms are
   out of the scope of this document.  The intention of this appendix is
   to provide only a basic idea on flexible extensions to the CARD
   protocol, as described in this document.

Appendix A.1.  Centralized Approach Using a Server Functional Entity

   The centralized approach performs CARD over the MN-AR interface as
   described in Section 4 of this document.  Additionally, the
   centralized approach introduces a new entity, the CARD server, to
   assist the current AR in performing reverse address translation.  The
   centralized approach requires that neighboring ARs register with the
   CARD server to populate the reverse address translation table.  The
   registration of AR addresses with the CARD server is performed prior
   to initiation of any reverse address translation request.

   Figure A.1 illustrates a typical scenario of the centralized CARD
   operation.  In this example, ARs have registered their address
   information with a CARD server in advance.  When an MN discovers the
   L2 ID of APs during L2 scanning, it passes one or more L2 IDs to its
   current AR, and the AR resolves them 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 it is 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 then 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 between an AR and the
   CARD server function 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 is described in [Funa02].









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                                   +----------+
                     +------------>|   CARD   |<-------------+
                     |+------------|  Server  |-------------+|
                     ||            +----------+             ||
                     ||                                     ||
                     ||             ~~~~~~~~~~~             ||
         (3)AR-Server||(4)AR-Server{           }            ||(0) CARD
             CARD    ||    CARD   {             }           ||Reg Req/
           Request   ||   Reply  {    IP Cloud   }          |  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: Centralized Approach for L2-L3 Mapping

Appendix A.2.  Decentralized Approach Using Mobile Terminals'
               Handover

   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 state.  The entries for CARs are removed from the
   CAR table if they are not refreshed before the timeout period expires
   and are created or refreshed according to the 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 to invoke the discovery procedure, and the
   subsequent handover serves to refresh the association between the
   neighboring ARs.  After the bootstrap handover, the MNs can perform



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   CARD and thus seamless handover using the CAR information.  This idea
   was presented in [ShGi00] and [Tros03].

   Maintenance of the CAR table is done by using an additional option
   for the CARD protocol operation performed between an MN and its
   current AR.  This message serves as 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), and can be sent as a
   specific sub-option in the MN-AR CARD Request message.  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 is no
   longer possible between two ARs, the associated entries eventually
   timeout and are 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.  One simple method is to verify the accuracy of the
   Router Identity message by sending an AR-AR CARD Request message to
   the pAR.  The AR-AR CARD Request includes the identity of the MN.
   Upon receiving this message, the pAR verifies that the MN was indeed
   attached to it during a reasonable past interval and responds 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 a 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 exchange to a later time
   when some MN undergoing handover between them performs CARD as
   described in Section 4.  In the latter (refresh) case, ARs may
   exchange capabilities or defer exchanges until a later time when
   another MN undergoes handover.

   Finally, note that in a handover-based protocol, a first handover
   between a pAR and an MN's current AR cannot use CARD, as this
   handover bootstraps the CAR table.  However, in the long term, such a
   handover will only amount to a small fraction of total successful
   handover between the two ARs.  Also, if the MN engaging in such a
   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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RFC 4066        Candidate Access Router Discovery (CARD)       July 2005


Appendix B.  Application Scenarios

   This section provides two examples of application scenarios for CARD
   protocol operation.  One scenario describes a CARD protocol operation
   in a Mobile IPv6 (MIPv6) 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.

   This application scenario assumes a moving MN having access to the
   infrastructure through wireless LAN (IEEE802.11) APs.  Mobility
   management is performed using the Mobile IPv6 protocol.  The
   following figure illustrates the assumed access network design.

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

                       -----------------------------
                      /                             \   +----+
                      |           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) maintains location information for the
   MN in its binding cache.  In Figure B.1, the MN holds a care-of
   address for the subnet 1, supported by AR1.  As the MN moves, the
   MN's current environment offers two further wireless LAN APs with
   increasing link-quality as candidate APs for a handover.  To



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RFC 4066        Candidate Access Router Discovery (CARD)       July 2005


   facilitate decision making, parameters associated with ARs are taken
   into account during the decision process.  The 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 IDs.  Furthermore, associated link-quality parameters are
   retrieved to ascertain whether approaching APs are eligible
   candidates for a handover.  If AP2 and AP3 are 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 the message to
   its current AR (AR1).

   AR1 resolves each L2 ID listed in L2 ID options to the associated IP
   address of the respective CAR, 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.  The corresponding IP
   address of the candidate AR, to which AP3 is connected, is the
   address of AR2.  IP addresses of the MN's CARs are now known to AR1,
   which retrieves the CARs' capabilities from the CAR table.  Assuming
   that it has valid entries for respective capability parameters to
   refresh dynamic capabilities, whose associated lifetimes in AR1's CAR
   table have expired, AR1 performs Inter-AR CARD for capability
   discovery.  Since capability information for AR1 is known to AR1, a
   respective Inter-AR CARD Request is sent only to AR2.  In response,
   AR2 sends a CARD Reply message back to AR1, encapsulating the
   requested capability parameters with the signaling message in a
   Capability Container sub-option.

   Next, 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.

   Upon receipt of the CARD Reply message, the MN performs target AR
   selection, taking AR1's and AR2's capability parameters and
   associated APs' link-quality parameters into account.  If the
   selected AP is AP2, no IP handover needs to be performed.  If AP3 and
   the associated AR2 are selected, the MN needs to perform an IP
   handover according to the Mobile IPv6 protocol operation.










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RFC 4066        Candidate Access Router Discovery (CARD)       July 2005


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

     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 Network

   This application scenario assumes that ARs can perform the fast
   handover protocol sequence for Mobile IPv6 [Kood03].  The MN scans
   for new APs for handover, similar to Figure B.1.  To discover the 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 allow the MN to send multiple L2 IDs of
   candidate APs to its current AR.  (This 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
   addresses of associated CARs.  The pAR checks its local CAR table to
   retrieve information about the CARs' capabilities.  If any table
   entries have expired, the pAR acquires this CAR's capabilities by
   sending an AR-AR CARD Request to the respective CAR.  The CAR replies
   with an AR-AR CARD Reply message, encapsulating all capabilities in a
   Capability Container sub-option and attaching them to the CARD Reply
   option.  On receipt of the CARs' capability information, the pAR
   updates its local CAR table and forwards the address and capability
   information to the MN by attaching a MN-AR CARD Reply option 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, allowing the pAR to perform 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.  The MN must send its requirements
   for the selection process to its pAR together with the MN-AR CARD
   Request message After the pAR has selected the MN's new AR, the
   address and associated capabilities of the chosen new AR are sent to
   the MN with the CARD Reply option in the Fast Mobile IPv6 PrRtAdv
   message.











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RFC 4066        Candidate Access Router Discovery (CARD)       July 2005


   Figure B.3 illustrates how CARD protocol messages and functions work
   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

   * In Figure B.3, the CARD protocol interaction between the pAR and
     CARs is only required if the lifetime of any capability entries in
     the pAR's CAR table have expired.  Otherwise, the pAR can respond
     to the requesting MN immediately after retrieving the CARs'
     addresses and capability information from its CAR table.












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RFC 4066        Candidate Access Router Discovery (CARD)       July 2005


Authors' Addresses

   Hemant Chaskar
   AirTight Networks
   339 N. Bernardo Avenue
   Mountain View, CA 94043, USA

   EMail: hemant.chaskar@airtightnetworks.net


   Daichi Funato
   NTT DoCoMo, Inc.
   Communication Systems Laboratory
   Wireless Laboratories
   3-5, Hikarinooka, Yokosuka,
   Kanagawa 239-8536, Japan

   Phone: +81-46-840-3921
   EMail: funato@mlab.yrp.nttdocomo.co.jp


   Marco Liebsch
   NEC Network Laboratories
   Kurfuersten-Anlage 36,
   69115 Heidelberg, Germany

   Phone: +49 6221-90511-46
   EMail: marco.liebsch@netlab.nec.de


   Eunsoo Shim
   Panasonic Digital Networking Laboratory
   Panasonic Corporation
   Two Research Way
   Princeton, NJ 08540

   Phone: +1-609-734-7354
   EMail: eunsoo@research.panasonic.com


   Ajoy Singh
   Motorola Inc
   2G11, 1501 West Shure Dr.
   Arlington Heights, IL 60004, USA

   Phone: +1 847-632-6941
   EMail: asingh1@email.mot.com




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RFC 4066        Candidate Access Router Discovery (CARD)       July 2005


Full Copyright Statement

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