[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits] [IPR]

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

IETF Seamoby Working Group
Internet Draft                                            Marco Liebsch
Category: Experimental                                       Ajoy Singh
                                                              (Editors)
                                                         Hemant Chaskar
                                                          Daichi Funato
                                                            Eunsoo Shim
draft-ietf-seamoby-card-protocol-08.txt
Expires: March 2005                                      September 2004


                     Candidate Access Router Discovery


Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance
   with RFC 3668.

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

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

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

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

Abstract

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



Liebsch and Singh (editors)  Expires March 2005               [Page 1]

Internet-Draft     Candidate Access Router Discovery    September 2004



TABLE OF CONTENTS

   1. INTRODUCTION...................................................3
   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........................................5
      4.1 Conceptual Data Structures.................................8
      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
   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.......................................30
      6.1 Veracity of CARD Information..............................30
      6.2 Security Association between AR and AR....................31
      6.3 Security Association between AR and MN....................31
      6.4 Router Certificate Exchange...............................32
      6.5 DoS Attack................................................33
      6.6 Replay Attacks............................................33
   7. PROTOCOL CONSTANTS............................................34
   8. IANA CONSIDERATIONS...........................................34
   9. NORMATIVE REFERENCES..........................................34
   10. INFORMATIVE REFERENCES.......................................35
   11. AUTHORS' ADDRESSES...........................................36
   12. IPR STATEMENTS...............................................36
   13. DISCLAIMER OF VALIDITY.......................................37
   14. COPYRIGHT NOTICE.............................................37
   15. ACKNOWLEDGEMENTS.............................................37

   Appendix A   MAINTENANCE OF ADDRESS MAPPING TABLES IN
                ACCESS ROUTERS. . . . . . . . . . . . . . . . . . . 39
   Appendix A.1 Centralized Approach using a Server Functional
                Entity. . . . . . . . . . . . . . . . . . . . . . . 39
   Appendix A.2 Decentralized Approach using Mobile Terminals'
                Handover. . . . . . . . . . . . . . . . . . . . . . 40

   Appendix B   APPLICATION SCENARIOS . . . . . . . . . . . . . . . 43
   Appendix B.1 CARD Operation in a Mobile IPv6 Enabled Wireless
                LAN Network . . . . . . . . . . . . . . . . . . . . 43
   Appendix B.2 CARD Operation in a Fast Mobile IPv6 enabled
                network . . . . . . . . . . . . . . . . . . . . . . 46






Liebsch and Singh (editors)    Expires March 2005             [Page 2]

Internet-Draft     Candidate Access Router Discovery    September 2004


Conventions used in this document

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

1.   INTRODUCTION

   IP mobility protocols, such as Mobile IP, enable mobile nodes to
   execute IP-level handover among access routers. Additionally, work
   is underway [Kood03][Malk03] to extend the mobility protocols to
   allow seamless IP handover. The pre-requisite for the seamless IP
   mobility protocols is the knowledge of candidate access routers
   (CARs) to which a mobile node can be 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 as well as the
   capabilities that the mobile node might use for a handover decision.
   There are cases when a mobile node has a choice of CARs. The mobile
   node chooses one based on 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 scope of this
   document.

   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,
   necessary for reverse address translation. The default method is
   static configuration. Appendix B contains two sample scenarios for
   using CARD.

2.   TERMINOLOGY

   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.



Liebsch and Singh (editors)    Expires March 2005             [Page 3]

Internet-Draft     Candidate Access Router Discovery    September 2004

   Candidate AR (CAR)

   An AR to which a 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 a MN when the AR is being considered as a handover
   candidate.

   L2 ID

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

   CARD Initiating Trigger

   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 a 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 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, while others can change with time.

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


Liebsch and Singh (editors)    Expires March 2005             [Page 4]

Internet-Draft     Candidate Access Router Discovery    September 2004


4.   CARD PROTOCOL OPERATION

   The CARD protocol allows MNs to resolve the L2 ID of one or more APs
   to the IP address of the associated CARs. The L2 IDs are typically
   discovered during 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
   between a MN and its current AR (Section 5.1.2), and between a MN's
   current AR and individual CARs respectively (Section 5.2.2).

   To allow a MN to retrieve a CARs' address and capability
   information, the CARD Request and CARD Reply messages used between a
   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 CARs' 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 the CAR(s) and associated capabilities MAY be used
   by the MN to perform target access router selection during its IP
   handover. The current AR returns replies based on 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 a 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
   returns only these capabilities of interest to the requesting MN.
   The format of this optional Preferences message parameter is
   described in Section 5.1.3.2.

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




Liebsch and Singh (editors)    Expires March 2005             [Page 5]

Internet-Draft     Candidate Access Router Discovery    September 2004

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

   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 for 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 router's certified key, and thereby validate
   signatures on CARD messages, and other messages between the MN and
   AR. The MN only needs to send the trusted anchor option if it does
   not have the certification path for the router already cached. If the
   mobile node has the certification path cached, either through
   preconfiguration, previous receipt of the chain from this router, or
   through having received the chain through a previous router, then the
   trusted anchor does not need to be sent. More information about
   certificate exchange and its use in CARD security can be found in
   Section 6.


Liebsch and Singh (editors)    Expires March 2005             [Page 6]

Internet-Draft     Candidate Access Router Discovery    September 2004


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






          +--------------+  (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 reception 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 ID(s) to its current AR
   using the MN-AR CARD Request message (2m). In case 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, in case the MN has not attached
   any L2 ID message parameters, it just reads out all CARs' IP address
   information using the reverse address translation information (L2 ID
   to IP address mapping) from its local CAR table. The current AR then
   returns to the MN using the MN-AR CARD Reply message (3m) the IP
   address of the CAR(s), each CAR's set of L2 IDs with CANDIDATE
   indicated in the L2 ID sub-option status field, and, in case
   capability information has been requested, associated capabilities.



Liebsch and Singh (editors)    Expires March 2005             [Page 7]

Internet-Draft     Candidate Access Router Discovery    September 2004

   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

   AR(s) SHALL maintain a 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 respectively.

   ARs SHOULD also keep and maintain individual CARs' capabilities in
   the local CAR table, taking the associated capability lifetime into
   account. If the lifetime of an individual capability entry has
   expired, the respective capability 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 ARs' CAR table may be implemented
   differently, hence 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 and so that a 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, a MN sends a CARD Request to its current AR,
   requesting it to resolve the L2 ID of nearby access points to the IP
   address of associated CARs, and also to obtain capability parameters
   associated with these CARs. In case the requesting MN 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 CARD Request MUST have set the C-flag. 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.



Liebsch and Singh (editors)    Expires March 2005             [Page 8]

Internet-Draft     Candidate Access Router Discovery    September 2004

   In case the MN appends multiple L2 ID sub-options to a CARD Request,
   the AR MUST assume 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 matching parameters that
   belong together. Hence, 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 a 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.

   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 CARs' 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.

   A MN uses exponential backoff to retransmit the CARD Request in the
   event 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 not be
   possible, 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 a L2 ID.

   In some cases a 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 being sent towards the MN.

   RESOLVER ERROR Status-Code indication:



Liebsch and Singh (editors)    Expires March 2005             [Page 9]

Internet-Draft     Candidate Access Router Discovery    September 2004

   In case 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 a L2 ID is encountered that shares a CAR with a previously
   resolved L2 ID, the AR returns MATCH to the MN. This status code is
   an indicator 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 CAR. A 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:
   In case 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. Since the
   received parameters' Context-IDs cannot be correlated with a 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, a MN can use CARD when it boots up
   initially to determine whether piggyback operation is possible. A 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 a 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 a MN's MN-AR CARD Request, the connected AR SHALL
   resolve the requested APs' L2 ID to the IP address of the associated
   CAR(s). In case no L2 ID parameter has been sent with the MN-AR CARD
   Request message, the 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 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


Liebsch and Singh (editors)    Expires March 2005            [Page 10]

Internet-Draft     Candidate Access Router Discovery    September 2004

   Capability Container sub-option. In case an AR cannot resolve a
   particular L2 ID, a L2 ID sub-option is sent back to the MN,
   indicating 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. Here, the AR marks the Status-Code of
   individual L2 IDs as CANDIDATE, indicating to the MN, that the
   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 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 along with associated capabilities
   are then delivered to the MN using the MN-AR CARD Reply message. The
   CARs' IP address as well as the capabilities SHALL be encoded
   according to the format for CARD protocol message parameters as
   defined in Section 5.1.3 of this document. The capabilities are
   encoded as attribute-value pairs, which are 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 a MN-AR CARD Request or when it detects that
   one or multiple of its local CAR table's capability entries'
   lifetime is about to expire. An AR SHOULD preferentially utilize its
   CAR table to fulfill requests rather than signaling 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 CAR(s)
   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


Liebsch and Singh (editors)    Expires March 2005            [Page 11]

Internet-Draft     Candidate Access Router Discovery    September 2004

   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. In case 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, since there is no need for the receiving CAR to process
   this field due to the absence of a 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, which has been sent by the CAR
   in response to the previously sent request, the MN's current AR
   SHALL extract the capability information from the payload of the
   received message and 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 value of the table entries' timeout 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. In case 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
   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 value of the table entries' timeout depends upon
   the nature of capabilities in the AR-AR CARD Reply message. The CAR


Liebsch and Singh (editors)    Expires March 2005            [Page 12]

Internet-Draft     Candidate Access Router Discovery    September 2004

   must include the same sequence number in the AR-AR CARD Reply
   Message as 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.
   In case the sending AR has appended an optional Preferences sub-
   option, the CAR MAY perform capability filtering and send back only
   these capabilities, which are of interest to the requesting AR,
   identified according to the Preferences sub-option. Since 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 in case one or more of its capability
   parameters change. The unsolicited CARD Reply messages should have
   as destination address the adjacent ARs' unicast address and must
   have the U-flag set. Consecutive unsolicited CARD Reply messages
   MUST have the sequence number incremented respectively, 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 distributed 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. In case the MN supports CARD protocol message piggybacking,
   the P-flag in this very first CARD Request message is set. On
   reception of the CARD Request message, 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. In
   case 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. In case both nodes, the MN and its
   current AR, support CARD protocol message piggybacking, subsequent
   CARD protocol messages can be conveyed as an option via the Fast
   Mobile IPv6 Router Solicitation for Proxy (RtSolPr) and Proxy Router
   Advertisement (PrRtAdv) messages. During the CARD process, a MN
   learns about CAR's piggybacking capability during the discovery
   phase, since the Capability Container, as described in Section
   5.1.3.4, also carries a P-flag. This allows the MN to immediately


Liebsch and Singh (editors)    Expires March 2005            [Page 13]

Internet-Draft     Candidate Access Router Discovery    September 2004

   perform CARD protocol message piggybacking after a handover to a
   selected CAR, assumed 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
   reverse address translation is not required, the piggybacked CARD
   Request message MUST have the A-flag set. These 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, since ICMP has no provision for
   fragmentation above the IP level.

   Hosts and Access Routers 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:


Liebsch and Singh (editors)    Expires March 2005            [Page 14]

Internet-Draft     Candidate Access Router Discovery    September 2004


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

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

         Hop Limit:     255

         ICMP Fields:

         Type           Experimental Mobility type (To be 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 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 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



Liebsch and Singh (editors)    Expires March 2005            [Page 15]

Internet-Draft     Candidate Access Router Discovery    September 2004

                        point associated with the CAR, whose IP address
                        and capability information is to be resolved.

         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 AR or for a
                        CAR. The chain includes certificates starting
                        at a trusted anchor, which the router shares in
                        common with the mobile node, to the router
                        itself. The format of the Router Certification
                        sub-option is described in Section 5.1.3.7.

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


Liebsch and Singh (editors)    Expires March 2005            [Page 16]

Internet-Draft     Candidate Access Router Discovery    September 2004

                        according to CARs' capability parameters and
                        given policy or threshold, as encoded in the
                        Requirements sub-option. Attribute-value pairs
                        are encoded according to the AVP encoding rule
                        as described in Section 5.1.4. 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 which 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 for CARD_RETRY_MAX seconds occurs. 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 which retransmit a CARD Request use the same CARD sequence
   number. This allows an AR to cache its reply to the original request
   and then 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 randomly chosen. 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 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
                        option including the type and length fields in
                        units of 8 octets. The value 0 is invalid.


Liebsch and Singh (editors)    Expires March 2005            [Page 17]

Internet-Draft     Candidate Access Router Discovery    September 2004


         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:    To be 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 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
                        interested also 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 in case CARD protocol messages are
                        piggybacked with a protocol that performs
                        reverse address translation. For details refer
                        to Section 4.4 of this document.

               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.


Liebsch and Singh (editors)    Expires March 2005            [Page 18]

Internet-Draft     Candidate Access Router Discovery    September 2004


               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 reception.

      Sequence Number:
               Allows correlating requests with replies.

   Valid Sub-Options:

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

   To ensure meeting requirements on boundary alignment, 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:    To be 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.

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

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


Liebsch and Singh (editors)    Expires March 2005            [Page 19]

Internet-Draft     Candidate Access Router Discovery    September 2004


               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
                       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 reception.

      Sequence Number:
               Allows correlating requests with replies.

   Valid Sub-Options:

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

   To ensure meeting requirements on boundary alignment, 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 form:

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


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








Liebsch and Singh (editors)    Expires March 2005            [Page 20]

Internet-Draft     Candidate Access Router Discovery    September 2004




         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 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.

   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.



Liebsch and Singh (editors)    Expires March 2005            [Page 21]

Internet-Draft     Candidate Access Router Discovery    September 2004

                  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 matching
                          associated L2 ID, address and capability
                          information 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Sub-Option Type:
                  0x04

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

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


Liebsch and Singh (editors)    Expires March 2005            [Page 22]

Internet-Draft     Candidate Access Router Discovery    September 2004


   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, since the preferences are indicated only with a list of
   16-bit encoded ATTRIBUTE fields. In case 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 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 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) field 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 for
   optimization purpose.

   5.1.3.4 Capability Container Sub-Option

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Sub-Option Type|Sub-Option Len |   Context-ID  |P|  Reserved   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           AVPs
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -



Liebsch and Singh (editors)    Expires March 2005            [Page 23]

Internet-Draft     Candidate Access Router Discovery    September 2004


   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 a MN
                           to know after a CARD process whether a
                           selected new AR can perform piggybacking.

   Reserved:      Initialized to zero, ignored on reception.

   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

   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


Liebsch and Singh (editors)    Expires March 2005            [Page 24]

Internet-Draft     Candidate Access Router Discovery    September 2004


   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 reception.

   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.

   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.










Liebsch and Singh (editors)    Expires March 2005            [Page 25]

Internet-Draft     Candidate Access Router Discovery    September 2004



   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 reception.

   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 certificate profile including encoding)

   Padding:       A variable length field making the option length a
                  multiple of 8, beginning after the ASN.1 encoding of
                  the certificate 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


Liebsch and Singh (editors)    Expires March 2005            [Page 26]

Internet-Draft     Candidate Access Router Discovery    September 2004

   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 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 in the L2 ID and Address sub-
   options. The replying AR SHOULD order the returned certification
   path such 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 two octet AVP length field indicates the
                  number of octets in this AVP, including the AVP Code,
                  AVP Length, Reserved, Lifetime and Data field.

   Reserved:      Initialized to zero, ignored on reception.

   Lifetime:      Specifies the lifetime of the encoded capability
                  in seconds. In 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.

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

   Since 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.


Liebsch and Singh (editors)    Expires March 2005            [Page 27]

Internet-Draft     Candidate Access Router Discovery    September 2004


   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 scope of this document.

   5.2 CARD Inter-Access Router Messages

   5.2.1 AR-AR Transport

   Since the types of access networks in which CARD might be useful are
   not today 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
   prototype deployments of CARD SHOULD use Stream Control Transport
   Protocol (SCTP) [Stew00] for 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 such 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.

   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 is
   unlikely to be more than one CARD transaction between two routers
   active at any one time, having ordered delivery simplifies the


Liebsch and Singh (editors)    Expires March 2005            [Page 28]

Internet-Draft     Candidate Access Router Discovery    September 2004

   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.

   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 an 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 sent unsolicited CARD Replies to all its
   CARs to make sure their CAR tables are properly populated.



Liebsch and Singh (editors)    Expires March 2005            [Page 29]

Internet-Draft     Candidate Access Router Discovery    September 2004

   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 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,
   including using retransmits with exponential backoff, etc. In
   addition, it is an open research question whether SCTP is an
   appropriate transport protocol for all inter-router CARD operations.
   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 in 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 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
   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 such accurate information about its own APs and
   capabilities to other ARs.  The CARD protocol described in the body


Liebsch and Singh (editors)    Expires March 2005            [Page 30]

Internet-Draft     Candidate Access Router Discovery    September 2004

   of this document does not contain any support for determining the AR
   to AP mapping or capabilities, either for a specific AR itself 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 draft 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 does contain 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.

   If CARD is used to exchange information between different
   administrative domains, additional security policy issues may apply.
   Such issues are out of 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


Liebsch and Singh (editors)    Expires March 2005            [Page 31]

Internet-Draft     Candidate Access Router Discovery    September 2004

   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 [SEND].

   Since CARD is a discovery protocol, confidentiality is not necessary
   in general on the MN-AR interface. In specific cases where different
   network operators are sharing 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 depend on whether CARD is used with IPv4 or
   IPv6. In IPv6, when the MN receives a CARD reply with signature from
   a router 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. A MN may not
   necessarily need to use Cryptographically Generated Addresses (CGAs)
   with CARD, 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.
   In that case, if the MN does not have a certificate for the router,
   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


Liebsch and Singh (editors)    Expires March 2005            [Page 32]

Internet-Draft     Candidate Access Router Discovery    September 2004

   (Section 5.1.3.7) containing one certificate in its certification
   path, matching 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 router sends the LD 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 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 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 option for the AP for which the CAR certificate is
   desired, and the AR replies as above, except 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.

   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. A suggested rate
   limiting policy is the following. If the number of CARD messages
   exceeds CARD_REQUEST_RATE, the AR SHOULD begin to randomly drop
   messages 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 assures 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, indicated by the sequence number.

   6.6       Replay Attacks



Liebsch and Singh (editors)    Expires March 2005            [Page 33]

Internet-Draft     Candidate Access Router Discovery    September 2004

   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 a
   MN, but the attack is without serious consequence since the MN in
   any case ignores the reply.

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.

   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.

   [Kemp02] Kempf, J., "Problem Description: Reasons For Performing
            Context Transfers Between Nodes in an IP Access Network",
            RFC 3374, September 2002.

   [NaNS98] Narten, T., et al., "Neighbor Discovery for IP Version 6
            (IPv6)", RFC 2461, December 1998.



Liebsch and Singh (editors)    Expires March 2005            [Page 34]

Internet-Draft     Candidate Access Router Discovery    September 2004

   [Post80] Postel, J., "User Datagram Protocol", RFC 768, August 1980.

   [Stew00] Stewert, R., et. al., "Stream Control Transmission
            Protocol",        RFC 2960, October, 2000.

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

   [NaAl98] Narten, T., Alvestrand, H., "Guidelines for Writing an IANA
            Considerations Section in RFCs", RFC 2434, October 1998.

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

   [Arkko04] Arkko, J., editor, Kempf, J., Sommerfelt, B., Zill, B., and
             Nikander, P., "SEcure Neighbor Discovery(SEND)", Internet
             draft, Work in progress.

   [Ke04] Kempf, J., "Instructions for Seamoby and Experimental Mobility
          Protocol IANA Allocations", Internet Draft, Work in progress.

10. INFORMATIVE REFERENCES

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

   [Kris02] Krishanmurti, G., "Requirements for CAR Discovery
            Protocols", Internet Draft, Work in progress.

   [Kenw02] Kenward, B., "General Requirements for Context
            Transfer", Internet Draft, Work in progress.

   [MaKo03] Manner, J., Kojo, M. (Ed), "Mobility Related Terminology",
            Internet Draft, Work in progress.

   [Kood03] Koodli, R, et al., "Fast handoffs for Mobile IPv6",
           Internet Draft, Work in progress.

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

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

   [ShGi00] Shim, E., Gitlin, R., "Fast Handoff Using Neighbor
            Information", Internet Draft, Work in progress.

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


Liebsch and Singh (editors)    Expires March 2005            [Page 35]

Internet-Draft     Candidate Access Router Discovery    September 2004


11. AUTHORS' ADDRESSES

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

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

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

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

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


12. IPR STATEMENTS

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

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has


Liebsch and Singh (editors)    Expires March 2005            [Page 36]

Internet-Draft     Candidate Access Router Discovery    September 2004

   made any independent effort to identify any such rights. Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-
   ipr@ietf.org.

13. DISCLAIMER OF VALIDITY

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

14. COPYRIGHT NOTICE

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

15. CONTRIBUTORS

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


15. 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


Liebsch and Singh (editors)    Expires March 2005            [Page 37]

Internet-Draft     Candidate Access Router Discovery    September 2004

   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.

















































Liebsch and Singh (editors)    Expires March 2005            [Page 38]

Internet-Draft     Candidate Access Router Discovery    September 2004


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 and 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 neighboring AR(s) to 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 a MN discovers the
   L2 ID of APs during L2 scanning, the MN passes one or more L2 ID(s)
   to its current AR and the AR resolves it to the IP address of the
   AR. For this, the AR first checks whether the mapping information is
   locally available in its CAR table. If not, the MN's current AR
   queries a CARD server with the L2 ID. In response, the CARD server
   returns the IP address of the CAR to the current AR. Then, the
   current AR directly contacts the respective CAR and performs
   capability discovery with it. The current AR then passes the IP
   address of the CAR and associated capabilities to the MN. The
   current AR 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 has been described in [Funa02].








Liebsch and Singh (editors)    Expires March 2005            [Page 39]

Internet-Draft     Candidate Access Router Discovery    September 2004





                                   +----------+
                     +------------>|   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 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


Liebsch and Singh (editors)    Expires March 2005            [Page 40]

Internet-Draft     Candidate Access Router Discovery    September 2004

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

   Maintenance of the CAR table is done using an additional option for
   the CARD protocol operation performed between a 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 it to a later time when
   some MN undergoing handover between them performs CARD as described
   in Section 4. In the later (refresh) case, ARs may exchange
   capabilities or defer it until a later time when another MN
   undergoes handover.

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



Liebsch and Singh (editors)    Expires March 2005            [Page 41]

Internet-Draft     Candidate Access Router Discovery    September 2004

   first handover is running a non-delay sensitive application at the
   time of handover, the user may not even realize its impact.


















































Liebsch and Singh (editors)    Expires March 2005            [Page 42]

Internet-Draft     Candidate Access Router Discovery    September 2004

APPENDIX B: APPLICATION SCENARIOS

   This section provides two examples of an application scenario 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.


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

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

                       -----------------------------
                      /                             \   +----+
                      |           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. From 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


Liebsch and Singh (editors)    Expires March 2005            [Page 43]

Internet-Draft     Candidate Access Router Discovery    September 2004

   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 or not approaching APs are eligible
   candidates for a handover. Assume 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 it 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. Respective IP address
   of the candidate AR, to which AP3 is connected to, is the address of
   AR2. Since IP addresses of the MN's CARs are now known to AR1, AR1
   retrieves the CARs' capabilities from the CAR table, assumed it has
   valid entries for respective capability parameters To refresh
   dynamic capabilities, whose associated lifetime in AR1's CAR table
   has 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. AR2 in response sends a
   CARD Reply message back to AR1, encapsulating the requested
   capability parameters with the signaling message, in a Capability
   Container sub-option.
   Now, AR1 sends its own capabilities and the dynamically discovered
   ones of AR2 back to the MN via a CARD Reply message. Furthermore,
   AR1 stores the capability parameters of AR2 with the associated
   lifetimes in its local CAR table.

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

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








Liebsch and Singh (editors)    Expires March 2005            [Page 44]

Internet-Draft     Candidate Access Router Discovery    September 2004


     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.



Liebsch and Singh (editors)    Expires March 2005            [Page 45]

Internet-Draft     Candidate Access Router Discovery    September 2004


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

   This application scenario assumes 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 (potentially replaces the "New
   Attachment Point Link-Layer Address" option of the Fast Mobile IPv6
   protocol).

   The pAR resolves the received list of candidate APs' L2 IDs to the
   IP address of associated CARs. 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 reception of the CARs' capability information, the
   pAR updates its local CAR table and forwards the address and
   capability information to the MN of 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.



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







Liebsch and Singh (editors)    Expires March 2005            [Page 46]

Internet-Draft     Candidate Access Router Discovery    September 2004


         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 in case 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' address
   and capability information from its CAR table.














Liebsch and Singh (editors)    Expires March 2005            [Page 47]


Html markup produced by rfcmarkup 1.108, available from http://tools.ietf.org/tools/rfcmarkup/