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RFC 4721 - Mobile IPv4 Challenge/Response Extensions (Revised)


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Network Working Group                                         C. Perkins
Request for Comments: 4721                         Nokia Research Center
Obsoletes: 3012                                               P. Calhoun
Updates: 3344                                        Cisco Systems, Inc.
Category: Standards Track                                    J. Bharatia
                                                         Nortel Networks
                                                            January 2007

          Mobile IPv4 Challenge/Response Extensions (Revised)

Status of This Memo

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

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   Mobile IP, as originally specified, defines an authentication
   extension (the Mobile-Foreign Authentication extension) by which a
   mobile node can authenticate itself to a foreign agent.
   Unfortunately, that extension does not provide the foreign agent any
   direct guarantee that the protocol is protected from replays and does
   not allow for the use of existing techniques (such as Challenge
   Handshake Authentication Protocol (CHAP)) for authenticating portable
   computer devices.

   In this specification, we define extensions for the Mobile IP Agent
   Advertisements and the Registration Request that allow a foreign
   agent to use a challenge/response mechanism to authenticate the
   mobile node.

   Furthermore, this document updates RFC 3344 by including a new
   authentication extension called the Mobile-Authentication,
   Authorization, and Accounting (AAA) Authentication extension.  This
   new extension is provided so that a mobile node can supply
   credentials for authorization, using commonly available AAA
   infrastructure elements.  This authorization-enabling extension MAY
   co-exist in the same Registration Request with authentication
   extensions defined for Mobile IP Registration by RFC 3344.  This
   document obsoletes RFC 3012.

Table of Contents

   1. Introduction ....................................................2
      1.1. Terminology ................................................3
   2. Mobile IP Agent Advertisement Challenge Extension ...............4
      2.1. Handling of Solicited Agent Advertisements .................4
   3. Operation .......................................................5
      3.1. Mobile Node Processing of Registration Requests ............5
      3.2. Foreign Agent Processing of Registration Requests ..........6
            3.2.1. Foreign Agent Algorithm for Tracking Used
                   Challenges .........................................8
      3.3. Foreign Agent Processing of Registration Replies ...........9
      3.4. Home Agent Processing of Challenge Extensions .............10
      3.5. Mobile Node Processing of Registration Replies ............11
   4. Mobile-Foreign Challenge Extension .............................11
   5. Generalized Mobile IP Authentication Extension .................12
   6. Mobile-AAA Authentication Subtype ..............................13
   7. Reserved SPIs for Mobile IP ....................................14
   8. SPIs for RADIUS AAA Servers ....................................14
   9. Configurable Parameters ........................................15
   10. Error Values ..................................................16
   11. IANA Considerations ...........................................16
   12. Security Considerations .......................................17
   13. Acknowledgements ..............................................18
   14. Normative References ..........................................18
   Appendix A. Changes since RFC 3012 ................................20
   Appendix B. Verification Infrastructure ...........................21
   Appendix C. Message Flow for FA Challenge Messaging with
               Mobile-AAA Extension ..................................22
   Appendix D. Message Flow for FA Challenge Messaging with
               MN-FA Authentication ..................................23
   Appendix E. Example Pseudo-code for Tracking Used Challenges ......24

1.  Introduction

   Mobile IP defines the Mobile-Foreign Authentication extension to
   allow a mobile node to authenticate itself to a foreign agent.  Such
   authentication mechanisms are mostly external to the principal
   operation of Mobile IP, since the foreign agent can easily route
   packets to and from a mobile node whether or not the mobile node is
   reporting a legitimately owned home address to the foreign agent.
   Unfortunately, that extension does not provide the foreign agent any
   direct guarantee that the protocol is protected from replays and does
   not allow for the use of CHAP [RFC1994] for authenticating portable
   computer devices.  In this specification, we define extensions for
   the Mobile IP Agent Advertisements and the Registration Request that
   allow a foreign agent to use a challenge/ response mechanism to
   authenticate the mobile node.  Furthermore, an additional

   authentication extension, the Mobile-AAA authentication extension, is
   provided so that a mobile node can supply credentials for
   authorization using commonly available AAA infrastructure elements.
   The foreign agent may be able to interact with an AAA infrastructure
   (using protocols outside the scope of this document) to obtain a
   secure indication that the mobile node is authorized to use the local
   network resources.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   This document uses the term Security Parameters Index (SPI) as
   defined in the base Mobile IP protocol specification [RFC3344].  All
   SPI values defined in this document refer to values for the SPI as
   defined in that specification.

   The following additional terminology is used in addition to that
   defined in [RFC3344]:

   previously used challenge:

      The challenge is a previously used challenge if the mobile node
      sent the same challenge to the foreign agent in a previous
      Registration Request, and if that previous Registration Request
      passed all validity checks performed by the foreign agent.  The
      foreign agent may not be able to keep records for all previously
      used challenges, but see Section 3.2 for minimal requirements.

   security association:

      A "mobility security association", as defined in [RFC3344].

   unknown challenge:

      Any challenge from a particular mobile node that the foreign agent
      has no record of having put either into one of its recent Agent
      Advertisements or into a registration reply message to that mobile
      node.

   unused challenge:

      A challenge that has not already been accepted by the foreign
      agent from the mobile node in the Registration Request, i.e., a
      challenge that is neither unknown nor previously used.

2.  Mobile IP Agent Advertisement Challenge Extension

   This section defines a new extension to the Router Discovery Protocol
   [RFC1256] for use by foreign agents that need to issue a challenge
   for authenticating mobile nodes.

      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     |          Challenge ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 1.  The Challenge Extension

   Type:

      24

   Length:

      The length of the Challenge value in octets; SHOULD be at least 4.

   Challenge:

      A random value that SHOULD be at least 32 bits.

   The Challenge extension, illustrated in Figure 1, is inserted in the
   Agent Advertisements by the foreign agent in order to communicate a
   previously unused challenge value that can be used by the mobile node
   to compute an authentication for its next registration request
   message.  The challenge is selected by the foreign agent to provide
   local assurance that the mobile node is not replaying any earlier
   registration request.  Eastlake et al. [RFC4086] provides more
   information on generating pseudo-random numbers suitable for use as
   values for the challenge.

   Note that the storage of different Challenges received in Agent
   Advertisements from multiple foreign agents is implementation
   specific and hence out of scope for this specification.

2.1.  Handling of Solicited Agent Advertisements

   When a foreign agent generates an Agent Advertisement in response to
   a Router Solicitation [RFC1256], some additional considerations come
   into play.  According to the Mobile IP base specification [RFC3344],
   the resulting Agent Advertisement may be either multicast or unicast.

   If the solicited Agent Advertisement is multicast, the foreign agent
   MUST NOT generate a new Challenge value and update its window of
   remembered advertised Challenges.  It must instead re-use the most
   recent of the CHALLENGE_WINDOW Advertisement Challenge values
   (Section 9).

   If the agent advertisement is unicast back to the soliciting mobile
   node, it MUST be handled as follows: If the challenge most recently
   unicast to the soliciting mobile node has not been previously used
   (as defined in Section 1.1), it SHOULD be repeated in the newly
   issued unicast agent advertisement.  Otherwise, a new challenge MUST
   be generated and remembered as the most recent challenge issued to
   the mobile node.  For further discussion of this, see Section 12.

3.  Operation

   This section describes modifications to the Mobile IP registration
   process [RFC3344] that may occur after the foreign agent issues a
   Mobile IP Agent Advertisement containing the Challenge on its local
   link.  See Appendix C for a diagram showing the canonical message
   flow for messages related to the processing of the foreign agent
   challenge values.

3.1.  Mobile Node Processing of Registration Requests

   Retransmission behavior for Registration Requests is identical to
   that specified in Mobile IP specification [RFC3344].  A retransmitted
   Registration Request MAY use the same Challenge value as given in the
   original Registration Request.

   Whenever the Agent Advertisement contains the Challenge extension, if
   the mobile node does not have a security association with the foreign
   agent, then it MUST include the Challenge value in a Mobile-Foreign
   Challenge extension to the Registration Request message.  If, on the
   other hand, the mobile node does have a security association with the
   foreign agent, it SHOULD include the Challenge value in its
   Registration Request message.

   If the mobile node has a security association with the Foreign Agent,
   it MUST include a Mobile-Foreign Authentication extension in its
   Registration Request message, according to the base Mobile IP
   specification [RFC3344].  When the Registration Request contains the
   Mobile-Foreign Challenge extension specified in Section 4, the
   Mobile-Foreign Authentication MUST follow the Challenge extension in
   the Registration Request.  The mobile node MAY also include the
   Mobile-AAA Authentication extension.

   If both the Mobile-Foreign Authentication and the Mobile-AAA
   Authentication extensions are present, the Mobile-Foreign Challenge
   extension MUST precede the Mobile-AAA Authentication extension, and
   the Mobile-AAA Authentication extension MUST precede the Mobile-
   Foreign Authentication extension.

   If the mobile node does not have a security association with the
   foreign agent, the mobile node MUST include the Mobile-AAA
   Authentication extension as, defined in Section 6, when it includes
   the Mobile-Foreign Challenge extension.  In addition, the mobile node
   SHOULD include the NAI extension [RFC2794] to enable the foreign
   agent to make use of available verification infrastructure that
   requires this.  The SPI field of the Mobile-AAA Authentication
   extension specifies the particular secret and algorithm (shared
   between the mobile node and the verification infrastructure) that
   must be used to perform the authentication.  If the SPI value is
   chosen as CHAP_SPI (see Section 9), then the mobile node specifies
   CHAP-style authentication [RFC1994] using MD5 [RFC1321].

   In either case, the Mobile-Foreign Challenge extension followed by
   one of the above specified authentication extensions MUST follow the
   Mobile-Home Authentication extension, if present.

   A mobile node MAY include the Mobile-AAA Authentication extension in
   the Registration Request when the mobile node registers directly with
   its home agent (using a co-located care-of address).  In this case,
   the mobile node uses an SPI value of CHAP_SPI (Section 8) in the
   Mobile Node-Authentication, Authorization, and Accounting (MN-AAA)
   Authentication extension and MUST NOT include the Mobile-Foreign
   Challenge extension.  Also, replay protection for the Registration
   Request in this case is provided by the Identification field defined
   by [RFC3344].

3.2.  Foreign Agent Processing of Registration Requests

   Upon receipt of the Registration Request, if the foreign agent has
   issued a Challenge as part of its Agent Advertisements, and if it
   does not have a security association with the mobile node, then the
   foreign agent SHOULD check that the Mobile-Foreign Challenge
   extension exists, and that it contains a challenge value previously
   unused by the mobile node.  This ensures that the mobile node is not
   attempting to replay a previous advertisement and authentication.  In
   this case, if the Registration Request does not include a Challenge
   extension, the foreign agent MUST send a Registration Reply with the
   Code field set to missing_challenge.

   If a mobile node retransmits a Registration Request with the same
   Challenge extension, and if the foreign agent still has a pending
   Registration Request record in effect for the mobile node, then the
   foreign agent forwards the Registration Request to the Home Agent
   again.  The foreign agent SHOULD check that the mobile node is
   actually performing a retransmission, by verifying that the relevant
   fields of the retransmitted request (including, if present, the
   mobile node NAI extension [RFC2794]) are the same as represented in
   the visitor list entry for the pending Registration Request (Section
   3.7.1 of [RFC3344]).  This verification MUST NOT include the
   "remaining Lifetime of the pending registration" or the
   Identification field, since those values are likely to change even
   for requests that are merely retransmissions and not new Registration
   Requests.  In all other circumstances, if the foreign agent receives
   a Registration Request with a Challenge extension containing a
   Challenge value previously used by that mobile node, the foreign
   agent SHOULD send a Registration Reply to the mobile node, containing
   the Code value stale_challenge.

   The foreign agent MUST NOT accept any Challenge in the Registration
   Request unless it was offered in the last Registration Reply or
   unicast Agent Advertisement sent to the mobile node or advertised as
   one of the last CHALLENGE_WINDOW (see Section 9) Challenge values
   inserted into the immediately preceding Agent Advertisements.  If the
   Challenge is not one of the recently advertised values, the foreign
   Agent SHOULD send a Registration Reply with Code value
   unknown_challenge (see Section 10).  The foreign agent MUST maintain
   the last challenge used by each mobile node that has registered using
   any one of the last CHALLENGE_WINDOW challenge values.  This last
   challenge value can be stored as part of the mobile node's
   registration records.  Also, see Section 3.2.1 for a possible
   algorithm that can be used to satisfy this requirement.

   Furthermore, the foreign agent MUST check that there is either a
   Mobile-Foreign or a Mobile-AAA Authentication extension after the
   Challenge extension.  Any registration message containing the
   Challenge extension without either of these authentication extensions
   MUST be silently discarded.  If the registration message contains a
   Mobile-Foreign Authentication extension with an incorrect
   authenticator that fails verification, the foreign agent MAY send a
   Registration Reply to the mobile node with Code value mobile node
   failed authentication (see Section 10).

   If the Mobile-AAA Authentication extension (see Section 6) is present
   in the message, or if a Network Access Identifier (NAI) extension is
   included indicating that the mobile node belongs to a different
   administrative domain, the foreign agent may take actions outside the
   scope of this protocol specification to carry out the authentication

   of the mobile node.  If the registration message contains a Mobile-
   AAA Authentication extension with an incorrect authenticator that
   fails verification, the foreign agent MAY send a Registration Reply
   to the mobile node with Code value fa_bad_aaa_auth.  If the Mobile-
   AAA Authentication extension is present in the Registration Request,
   the foreign agent MUST NOT remove the Mobile-AAA Authentication
   extension and the Mobile-Foreign Challenge extension from the
   Registration Request before forwarding to the home agent.  Appendix C
   provides an example of an action that could be taken by a foreign
   agent.

   In the event that the Challenge extension is authenticated through
   the Mobile-Foreign Authentication extension and the Mobile-AAA
   Authentication extension is not present, the foreign agent MAY remove
   the Challenge extension from the Registration Request without
   disturbing the authentication value used for the computation.  If the
   Mobile-AAA Authentication extension is present and a security
   association exists between the foreign agent and the home agent, the
   Mobile-Foreign Challenge extension and the Mobile-AAA Authentication
   extension MUST precede the Foreign-Home Authentication extension.

   If the foreign agent does remove the Challenge extension and
   applicable authentication from the Registration Request message, then
   it SHOULD store the Identification field from the Registration
   Request message as part of its record-keeping information about the
   particular mobile node in order to protect against replays.

3.2.1.  Foreign Agent Algorithm for Tracking Used Challenges

   If the foreign agent maintains a large CHALLENGE_WINDOW, it becomes
   more important for scalability purposes to compare incoming
   challenges efficiently against the set of Challenge values that have
   been advertised recently.  This can be done by keeping the Challenge
   values in order of advertisement, and by making use of the mandated
   behavior that mobile nodes MUST NOT use Challenge values that were
   advertised before the last advertised Challenge value that the mobile
   node attempted to use.  The pseudo-code in Appendix E accomplishes
   this objective.  The maximum amount of total storage required by this
   algorithm is equal to Size*(CHALLENGE_WINDOW + (2*N)), where N is the
   current number of mobile nodes for which the foreign agent is storing
   challenge values.  Note that whenever the stored challenge value is
   no longer in the CHALLENGE_WINDOW, it can be deleted from the foreign
   agent's records, perhaps along with all other registration
   information for the mobile node if it is no longer registered.

   It is presumed that the foreign agent keeps an array of advertised
   Challenges, a record of the last advertised challenge used by a
   mobile node, and also a record of the last challenge provided to a
   mobile node in a Registration Reply or unicast Agent Advertisement.

   To meet the security obligations outlined in Section 12, the foreign
   agent SHOULD use one of the already stored, previously unused
   challenges when responding to an unauthenticated Registration Request
   or Agent Solicitation.  If none of the already stored challenges are
   previously unused, the foreign agent SHOULD generate a new challenge,
   include it in the response, and store it in the per-Mobile data
   structure.

3.3.  Foreign Agent Processing of Registration Replies

   The foreign agent SHOULD include a new Mobile-Foreign Challenge
   extension in any Registration Reply, successful or not.  If the
   foreign agent includes this extension in a successful Registration
   Reply, the extension SHOULD precede a Mobile-Foreign authentication
   extension if present.  Suppose that the Registration Reply includes a
   Challenge extension from the home agent, and that the foreign agent
   wishes to include another Challenge extension with the Registration
   Reply for use by the mobile node.  In that case, the foreign agent
   MUST delete the Challenge extension from the home agent from the
   Registration Reply, along with any Foreign-Home authentication
   extension, before appending the new Challenge extension to the
   Registration Reply.

   One example of a situation where the foreign agent MAY omit the
   inclusion of a Mobile-Foreign Challenge extension in the Registration
   Reply would be when a new challenge has been multicast recently.

   If a foreign agent has conditions in which it omits the inclusion of
   a Mobile-Foreign Challenge extension in the Registration Reply, it
   still MUST respond with an agent advertisement containing a
   previously unused challenge in response to a subsequent agent
   solicitation from the same mobile node.  Otherwise (when the said
   conditions are not met), the foreign agent MUST include a previously
   unused challenge in any Registration Reply, successful or not.

   If the foreign agent does not remove the Challenge extension from the
   Registration Request received from the mobile node, then the foreign
   agent SHOULD store the Challenge value as part of the pending
   registration request list [RFC3344].  Also, if the Registration Reply
   coming from the home agent does not include the Challenge extension,
   the foreign agent SHOULD NOT reject the registration request.  If the
   Challenge extension is present in the Registration Reply, it MUST be
   the same Challenge value that was included in the Registration Reply

   received from the home agent, the foreign agent MUST insert a Foreign
   Agent (FA) Error extension with Status value ha_wrong_challenge in
   the Registration Reply sent to the mobile node (see Section 10).

   A mobile node MUST be prepared to use a challenge from a unicast or
   multicast Agent Advertisement in lieu of one returned in a
   Registration Reply, and it MUST solicit for one if it has not already
   received one either in a Registration Reply or a recent
   advertisement.

   If the foreign agent receives a Registration Reply with the Code
   value ha_bad_aaa_auth, the Registration Reply with this Code value
   MUST be relayed to the mobile node.  In this document, whenever the
   foreign agent is required to reject a Registration Request, it MUST
   put the given code in the usual Code field of the Registration Reply,
   unless the Registration Reply has already been received from the home
   agent.  In this case, the foreign agent MUST preserve the value of
   the Code field set by the home agent and MUST put its own rejection
   code in the Status field of the FA Error extension (defined in
   [RFC4636]).

3.4.  Home Agent Processing of Challenge Extensions

   If the home agent receives a Registration Request with the Mobile-
   Foreign Challenge extension and recognizes the extension, the home
   agent MUST include the Challenge extension in the Registration Reply.
   The Challenge extension MUST be placed after the Mobile-Home
   authentication extension, and the extension SHOULD be authenticated
   by a Foreign-Home Authentication extension.

   The home agent may receive a Registration Request with the Mobile-AAA
   Authentication extension.  If the Mobile-AAA Authentication extension
   is used by the home agent as an authorization-enabling extension and
   the verification fails due to an incorrect authenticator, the home
   agent MAY reject the Registration Reply with the error code
   ha_bad_aaa_auth.

   Since the extension type for the Challenge extension is within the
   range 128-255, the home agent MUST process such a Registration
   Request even if it does not recognize the Challenge extension
   [RFC3344].  In this case, the home agent will send a Registration
   Reply to the foreign agent that does not include the Challenge
   extension.

3.5.  Mobile Node Processing of Registration Replies

   A mobile node might receive the error code in the Registration Reply
   from the foreign agent as a response to the Registration Request.
   The error codes are defined in Section 10.

   In any case, if the mobile node attempts to register again after such
   an error, it MUST use a new Challenge value in such a registration,
   obtained either from an Agent Advertisement, or from a Challenge
   extension to the Registration Reply containing the error.

   In the co-located care-of address mode, the mobile node receives a
   Registration Reply without the Challenge extension and processes the
   Registration Reply as specified in [RFC3344].  In this case, when the
   mobile node includes the MN-AAA Authentication Extension, the
   Challenge value 0 is recommended for the authenticator computation
   mentioned in Section 8.

4.  Mobile-Foreign Challenge Extension

   This section specifies a new Mobile IP Registration extension that is
   used to satisfy a Challenge in an Agent Advertisement.  The Challenge
   extension to the Registration Request message is used to indicate the
   challenge that the mobile node is attempting to satisfy.

      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     |          Challenge ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 2.  The Mobile-Foreign Challenge Extension

   Type:

      132 (skippable).  (See [RFC3344]).

   Length:

      Length of the Challenge value.

   Challenge:

      The Challenge field is copied from the Challenge field found in
      the received Challenge extension.

   Suppose that the mobile node has successfully registered using one of
   the Challenge Values within the CHALLENGE_WINDOW values advertised by

   the foreign agent.  In that case, in any new Registration Request the
   mobile node MUST NOT use any Challenge Value that was advertised by
   the foreign agent before the Challenge Value in the mobile node's
   last Registration Request.

5.  Generalized Mobile IP Authentication Extension

   Several new authentication extensions have been designed for various
   control messages proposed for extensions to Mobile IP.  A new
   authentication extension is required for a mobile node to present its
   credentials to any other entity other than the ones already defined;
   the only entities defined in the base Mobile IP specification
   [RFC3344] are the home agent and the foreign agent.  The purpose of
   the generalized authentication extension defined here is to collect
   together data for all such new authentication applications into a
   single extension type with subtypes.

      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      |    Subtype    |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              SPI                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Authenticator ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 3.  The Generalized Mobile IP Authentication Extension

   Type:

      36 (not skippable).  (See [RFC3344]).

   Subtype:

      A number assigned to identify the kind of endpoints or other
      characteristics of the particular authentication strategy.

   Length:

      4 plus the number of octets in the Authenticator; MUST be at least
      20.

   SPI:

      Security Parameters Index

   Authenticator:

      The variable length Authenticator field

   In this document, only one subtype is defined:

   1     Mobile-AAA Authentication subtype
          (Hashed Message Authentication Code-MD5 (HMAC-MD5))
          (see Section 6).

6.  Mobile-AAA Authentication Subtype

   The Generalized Authentication extension with subtype 1 will be
   referred to as a Mobile-AAA Authentication extension.  The mobile
   node MAY include a Mobile-AAA Authentication extension in any
   Registration Request.  This extension MAY co-exist in the same
   Registration Request with Authentication extensions defined for
   Mobile IP Registration ([RFC3344]).  If the mobile node does not
   include a Mobile-Foreign Authentication extension, then it MUST
   include the Mobile-AAA Authentication extension whenever the
   Challenge extension is present.  If both are present, the Mobile-AAA
   Authentication extension MUST precede the Mobile-Foreign
   Authentication extension.

   If the Mobile-AAA Authentication extension is present, the Mobile-
   Home Authentication extension MUST appear prior to the Mobile-AAA
   Authentication extension.  The corresponding response MUST include
   the Mobile-Home Authentication extension and MUST NOT include the
   Mobile-AAA Authentication extension.

   The default algorithm for computation of the authenticator is HMAC-
   MD5 [RFC2104] computed on the following data, in the order shown:

      Preceding Mobile IP data || Type, Subtype, Length, SPI

   where the Type, Length, Subtype, and SPI are as shown in Section 5.
   The Preceding Mobile IP data refers to the UDP payload (the
   Registration Request or Registration Reply data) and all prior
   extensions in their entirety.  The resulting function call, as
   described in [RFC2104], would be:

      hmac_md5(data, datalen, Key, KeyLength, authenticator);

   Each mobile node MUST support the ability to produce the
   authenticator by using HMAC-MD5 as shown.  Just as with Mobile IP, it
   must be possible to configure the use of any arbitrary 32-bit SPI
   outside of the SPIs in the reserved range 0-255 for selection of this
   default algorithm.

7.  Reserved SPIs for Mobile IP

   Mobile IP defines several authentication extensions for use in
   Registration Requests and Replies.  Each authentication extension
   carries a Security Parameters Index (SPI) that should be used to
   index a table of security associations.  Values in the range 0-255
   are reserved for special use.  A list of reserved SPI numbers is to
   be maintained by IANA at the following URL:

      http://www.iana.org/assignments/mobileip-numbers

8.  SPIs for RADIUS AAA Servers

   Some AAA servers only admit a single security association and thus do
   not use the SPI numbers for Mobile IP authentication extensions for
   use when determining the security association that would be necessary
   for verifying the authentication information included with the
   Authentication extension.

   SPI number CHAP_SPI (see Section 9) is reserved for indicating the
   following procedure for computing authentication data (called the
   "authenticator"), which is used by many RADIUS servers [RFC2865]
   today.

   To compute the authenticator, apply MD5 [RFC1321] computed on the
   following data, in the order shown:

      High-order octet from Challenge || Key ||

      MD5(Preceding Mobile IP data ||

      Type, Subtype (if present), Length, SPI) ||

      Least-order 237 octets from Challenge

   where Type, Length, SPI, and possibly Subtype are the fields of the
   authentication extension in use.  For instance, all four of these
   fields would be in use when SPI == CHAP_SPI is used with the
   Generalized Authentication extension.  In case of co-located care-of
   address, the Challenge value 0 is used (refer to Section 3.5).  Since
   the RADIUS protocol cannot carry attributes of length greater than
   253, the preceding Mobile IP data, type, subtype (if present),
   length, and SPI are hashed using MD5.  Finally, the least significant
   237 octets of the challenge are concatenated.  If the challenge has
   fewer than 238 octets, this algorithm includes the high-order octet
   in the computation twice but ensures that the challenge is used

   exactly as is.  Additional padding is never used to increase the
   length of the challenge; the input data is allowed to be shorter than
   237 octets long.

9.  Configurable Parameters

   Every Mobile IP agent supporting the extensions defined in this
   document SHOULD be able to configure each parameter in the following
   table.  Each table entry contains the name of the parameter, the
   default value, and the section of the document in which the parameter
   first appears.

      +------------------+---------------+---------------------+
      | Parameter Name   | Default Value | Section of Document |
      +------------------+---------------+---------------------+
      | CHALLENGE_WINDOW | 2             | 3.2                 |
      |                  |               |                     |
      | CHAP_SPI         | 2             | 8                   |
      +------------------+---------------+---------------------+

                      Table 1.  Configurable Parameters

   Note that CHALLENGE_WINDOW SHOULD be at least 2.  This makes it far
   less likely that mobile nodes will register using a Challenge value
   that is outside the set of values allowable by the foreign agent.

10.  Error Values

   Each entry in the following table contains the name of the Code
   [RFC3344] to be returned in a Registration Reply, the value for the
   Code, and the section in which the error is mentioned in this
   specification.

      +--------------------+-------+--------------------------+
      | Error Name         | Value | Section of Document      |
      +--------------------+-------+--------------------------+
      | unknown_challenge  | 104   | 3.2                      |
      |                    |       |                          |
      | mobile node failed | 67    | 3.2; also see [RFC3344]  |
      | authentication     |       |                          |
      |                    |       |                          |
      | missing_challenge  | 105   | 3.1, 3.2                 |
      |                    |       |                          |
      | stale_challenge    | 106   | 3.2                      |
      |                    |       |                          |
      | fa_bad_aaa_auth    | 108   | 3.2                      |
      |                    |       |                          |
      | ha_bad_aaa_auth    | 144   | 3.4                      |
      |                    |       |                          |
      | ha_wrong_challenge | 109   | 3.2                      |
      +--------------------+-------+--------------------------+

                            Table 2.  Error Values

11.  IANA Considerations

   The following are currently assigned by IANA for RFC 3012 [RFC3012]
   and are applicable to this document.  IANA has recorded these values
   as part of this document.

      The Generalized Mobile IP Authentication extension defined in
      Section 5 is a Mobile IP registration extension.  IANA has
      assigned a value of 36 for this extension.

      A new number space is to be created for enumerating subtypes of
      the Generalized Authentication extension (see Section 5).  New
      subtypes of the Generalized Authentication extension, other than
      the number (1) for the MN-AAA authentication extension specified
      in Section 6, must be specified and approved by a designated
      expert.

      The Mobile Node - Foreign Agent (MN-FA) Challenge extension,
      defined in Section 4, is a router advertisement extension as
      defined in RFC 1256 [RFC1256] and extended in RFC 3344 [RFC3344].
      IANA has assigned a value of 132 for this purpose.

      The Code values defined in Section 10 are error codes as defined
      in RFC 3344 ([RFC3344]).  They correspond to error values
      conventionally associated with rejection by the foreign agent
      (i.e., values from the range 64-127).  The Code value 67 is a
      pre-existing value that is to be used in some cases with the
      extension defined in this specification.  IANA has recorded the
      values as defined in Section 10.

      A new section for enumerating algorithms identified by specific
      SPIs within the range 0-255 has been added by IANA.  The CHAP_SPI
      number (2) discussed in Section 8 is assigned from this range of
      reserved SPI numbers.  New assignments from this reserved range
      must be specified and approved by the Mobile IP working group.
      SPI number 1 should not be assigned unless in the future the
      Mobile IP working group decides that SKIP is not important for
      enumeration in the list of reserved numbers.  SPI number 0 should
      not be assigned.

   Additionally, the new error codes fa_bad_aaa_auth, ha_bad_aaa_auth,
   and ha_wrong_challenge are defined by this document.  Among these,
   ha_wrong_challenge may appear in the Status code of the FA Error
   extension, defined in [RFC4636].

12.  Security Considerations

   In the event that a malicious mobile node attempts to replay the
   authenticator for an old Mobile-Foreign Challenge, the foreign agent
   would detect it, since the agent always checks whether it has
   recently advertised the Challenge (see Section 3.2).  Allowing mobile
   nodes with different IP addresses or NAIs to use the same Challenge
   value does not represent a security vulnerability, as the
   authentication data provided by the mobile node will be computed over
   data that is different (at least the mobile node's IP address will
   vary).

   If the foreign agent chooses a Challenge value (see Section 2) with
   fewer than 4 octets, the foreign agent SHOULD include the value of
   the Identification field in the records it maintains for the mobile
   node.  The foreign agent can then determine whether the Registration
   messages using the short Challenge value are in fact unique and thus
   assuredly not replayed from any earlier registration.

   Section 8 (SPI For RADIUS AAA Servers) defines a method of computing
   the Generalized Mobile IP Authentication extension's authenticator
   field, using MD5 in a manner that is consistent with RADIUS
   [RFC2865].  The use of MD5 in the method described in Section 8 is
   less secure than HMAC-MD5 [RFC2104] and MUST be avoided whenever
   possible.

   Note that an active attacker may try to prevent successful
   registrations by sending a large number of Agent Solicitations or
   bogus Registration Requests, each of which could cause the foreign
   agent to respond with a fresh challenge, invalidating the challenge
   that the MN is currently trying to use.  To prevent such attacks, the
   foreign agent MUST NOT invalidate previously unused challenges when
   responding to unauthenticated Registration Requests or Agent
   Solicitations.  In addition, the foreign agent MUST NOT allocate new
   storage when responding to such messages, as this would also create
   the possibility of denial of service.

   The Challenge extension specified in this document need not be used
   for co-located care-of address mode.  In this case, replay protection
   is provided by the Identification field in the Registration Request
   message [RFC3344].

   The Generalized Mobile IP Authentication extension includes a subtype
   field that is used to identify characteristics of the particular
   authentication strategy.  This document only defines one subtype, the
   Mobile-AAA Authentication subtype that uses HMAC-MD5.  If it is
   necessary to move to a new message authentication algorithm in the
   future, this could be accomplished by defining a new subtype that
   uses a different one.

13.  Acknowledgements

   The authors would like to thank Pete McCann, Ahmad Muhanna, Henrik
   Levkowetz, Kent Leung, Alpesh Patel, Madjid Nakhjiri, Gabriel
   Montenegro, Jari Arkko, and other MIP4 WG participants for their
   useful discussions.

14.  Normative References

   [RFC1256]  Deering, S., "ICMP Router Discovery Messages", RFC 1256,
              September 1991.

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              April 1992.

   [RFC1994]  Simpson, W., "PPP Challenge Handshake Authentication
              Protocol (CHAP)", RFC 1994, August 1996.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:  Keyed-
              Hashing for Message Authentication", RFC 2104, February
              1997.

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

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)", RFC
              2865, June 2000.

   [RFC2794]  Calhoun, P. and C. Perkins, "Mobile IP Network Access
              Identifier Extension for IPv4", RFC 2794, March 2000.

   [RFC3012]  Perkins, C. and P. Calhoun, "Mobile IPv4
              Challenge/Response Extensions", RFC 3012, November 2000.

   [RFC3344]  Perkins, C., "IP Mobility Support for IPv4", RFC 3344,
              August 2002.

   [RFC4086]  Eastlake, D., 3rd, Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              June 2005.

   [RFC4636]  Perkins, C., "Foreign Agent Error Extension for Mobile
              IPv4", RFC 4636, October 2006.

Appendix A.  Changes since RFC 3012

   The following is the list of changes from RFC 3012 ([RFC3012]):

   o  Foreign agent recommended to include a Challenge in every
      Registration Reply, so that mobile node can re-register without
      waiting for an Advertisement.

   o  Foreign agent MUST record applicable challenge values used by each
      mobile node.

   o  Mobile node forbidden to use Challenge values which were
      advertised previous to the last Challenge value which it had used
      for a registration.

   o  Challenge definitions are cleaned up.

   o  Programming suggestion added as an appendix.

   o  HMAC_CHAP_SPI option is added for Generalized Mobile IP
      Authentication extension.  Upon receipt of HMAC_CHAP_SPI, HMAC-MD5
      is used instead of MD5 for computing the authenticator.

   o  Added fa_bad_aaa_auth and ha_bad_aaa_auth error codes to report
      authentication errors caused while processing Mobile-AAA
      Authentication extension.  Also, added the error code
      ha_wrong_challenge to indicate that Challenge value differs in the
      Registration Reply received from the home agent compare to the one
      sent to the home agent in the Registration Request.

   o  Processing of the Mobile-AAA Authentication extension is clarified
      for the foreign agent and the home agent.

   o  Co-existence of the Mobile-AAA Authentication extension in the
      same Registration Request is made explicit.

   o  The situation in which the foreign agent sets missing_challenge is
      clarified further.

   o  The use of Mobile-AAA Authentication extension is allowed by the
      mobile node with co-located care-of address.

   o  Added protection against bogus Registration Reply and Agent
      Advertisement.  Also, the processing of the Challenge is clarified
      if it is received in the multicast/unicast Agent Advertisement.

   o  Added reference of FA Error extension in the References section
      and also updated relevant text in section 3.2 and section 11.

Appendix B.  Verification Infrastructure

   The Challenge extensions in this protocol specification are expected
   to be useful to help the foreign agent manage connectivity for
   visiting mobile nodes, even in situations where the foreign agent
   does not have any security association with the mobile node or the
   mobile node's home agent.  In order to carry out the necessary
   authentication, it is expected that the foreign agent will need the
   assistance of external administrative systems, which have come to be
   called AAA systems.  For the purposes of this document, we call the
   external administrative support the "verification infrastructure".
   The verification infrastructure is described to motivate the design
   of the protocol elements defined in this document and is not strictly
   needed for the protocol to work.  The foreign agent is free to use
   any means at its disposal to verify the credentials of the mobile
   node.  It could, for instance, rely on a separate protocol between
   the foreign agent and the Mobile IP home agent and still not require
   any modification to the mobile node.

   In order to verify the credentials of the mobile node, we assume that
   the foreign agent has access to a verification infrastructure that
   can return a secure notification to the foreign agent that the
   authentication has been performed, along with the results of that
   authentication.  This infrastructure may be visualized as shown in
   Figure 4.

      +----------------------------------------------------+
      |                                                    |
      |  Verification and Key Management Infrastructure    |
      |                                                    |
      +----------------------------------------------------+
               ^ |                                  ^ |
               | |                                  | |
               | v                                  | v
        +---------------+                    +---------------+
        |               |                    |               |
        | foreign agent |                    |   home agent  |
        |               |                    |               |
        +---------------+                    +---------------+

                  Figure 4.  The Verification Infrastructure

   After the foreign agent gets the Challenge authentication, it MAY
   pass the authentication to the (here unspecified) infrastructure and
   await a Registration Reply.  If the Reply has a positive status
   (indicating that the registration was accepted), the foreign agent
   accepts the registration.  If the Reply contains the Code value

   BAD_AUTHENTICATION (see Section 10), the foreign agent takes actions
   indicated for rejected registrations.

   Implicit in this picture is the important observation that the
   foreign agent and the home agent have to be equipped to make use of
   whatever protocol is required by the challenge verification and key
   management infrastructure shown in the figure.

   The protocol messages for handling the authentication within the
   verification infrastructure and the identity of the agent performing
   the verification of the foreign agent challenge are not specified in
   this document, as those operations do not have to be performed by any
   Mobile IP entity.

Appendix C.  Message Flow for FA Challenge Messaging with Mobile-AAA
             Extension

   MN                  FA                   Verification     home agent
    |<-- Adv+Challenge--|                  Infrastructure          |
    |    (if needed)    |                         |                |
    |                   |                         |                |
    |-- RReq+Challenge->|                         |                |
    |    + Auth.Ext.    |                         |                |
    |                   |   Auth. Request, incl.  |                |
    |                   |--- RReq + Challenge --->|                |
    |                   |      + Auth.Ext         |   RReq +       |
    |                   |                         |-- Challenge -->|
    |                   |                         |                |
    |                   |                         |                |
    |                   |                         |<--- RRep ----- |
    |                   |   Authorization, incl.  |                |
    |                   |<-- RRep + Auth.Ext.-----|                |
    |                   |                         |                |
    |<-- RRep+Auth.Ext--|                         |                |
    |  + New Challenge  |                         |                |

            Figure 5.  Message Flows for FA Challenge Messaging

   In Figure 5, the following informational message flow is illustrated:

   1.  The foreign agent includes a Challenge Value in a unicast Agent
       Advertisement, if needed.  This advertisement MAY have been
       produced after receiving an Agent Solicitation from the mobile
       node (not shown in the diagram).

   2.  The mobile node creates a Registration Request including the
       advertised Challenge Value in the Challenge extension, along with
       a Mobile-AAA authentication extension.

   3.  The foreign agent relays the Registration Request either to the
       home agent specified by the mobile node or to its locally
       configured Verification Infrastructure (see Appendix B),
       according to local policy.

   4.  The foreign agent receives a Registration Reply with the
       appropriate indications for authorizing connectivity for the
       mobile node.

   5.  The foreign agent relays the Registration Reply to the mobile
       node, often along with a new Challenge Value to be used by the
       mobile node in its next Registration Request message.

Appendix D.  Message Flow for FA Challenge Messaging with MN-FA
             Authentication

         MN                  FA                     home agent
          |<-- Adv+Challenge--|                         |
          |    (if needed)    |                         |
          |                   |                         |
          |-- RReq+Challenge->|                         |
          |    + Auth.Ext.    |                         |
          |                   |--- RReq + Challenge --->|
          |                   |   + HA-FA Auth.Ext      |
          |                   |                         |
          |                   |<-- RRep + Challenge ----|
          |                   |   + HA-FA Auth.Ext      |
          |                   |                         |
          |<-- RRep+Auth.Ext--|                         |
          |  + New Challenge  |                         |

      Figure 6.  Message Flows for FA Challenge Messaging with MN-FA
                              Authentication

   In Figure 6, the following informational message flow is illustrated:

   1.  The foreign agent disseminates a Challenge Value in an Agent
       Advertisement, if needed.  This advertisement MAY have been
       produced after receiving an Agent Solicitation from the mobile
       node (not shown in the diagram).

   2.  The mobile node creates a Registration Request including the
       advertised Challenge Value in the Challenge extension, along with
       a Mobile-Foreign Authentication extension.

   3.  The foreign agent relays the Registration Request to the home
       agent specified by the mobile node.

   4.  The foreign agent receives a Registration Reply with the
       appropriate indications for authorizing connectivity for the
       mobile node.

   5.  The foreign agent relays the Registration Reply to the mobile
       node, possibly along with a new Challenge Value to be used by the
       mobile node in its next Registration Request message.  If the
       Reply contains the Code value ha_bad_aaa_auth (see Section 10),
       the foreign agent takes actions indicated for rejected
       registrations.

Appendix E.  Example Pseudo-Code for Tracking Used Challenges

   current_chal := RegistrationRequest.challenge_extension_value
   last_chal := mobile_node_record.last_used_adv_chal

   if (current_chal == mobile_node_record.RegReply_challenge) {
       update (mobile_node_record, current_chal)
       return (OK)
   }
   else if (current_chal "among" VALID_ADV_CHALLENGES[]{
      if (last_chal "among" VALID_ADV_CHALLENGES[]) {
         if (current_chal is "before" last_chal) {
             send_error(STALE_CHALLENGE)
             return (FAILURE)
         }
         else {
             update (mobile_node_record, current_chal)
             return (OK)
         }
      }
      else {
         update (mobile_node_record, current_chal)
         return (OK)
      }
   }
   else {
      send_error(UNKNOWN_CHALLENGE);
   }

Authors' Addresses

   Charles E. Perkins
   Nokia Research Center
   Communications Systems Lab
   313 Fairchild Drive
   Mountain View, California  94043

   Phone: +1 650 625-2986
   EMail: charles.perkins@nokia.com

   Pat R. Calhoun
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134

   Phone: +1 408-853-5269
   EMail: pcalhoun@cisco.com

   Jayshree Bharatia
   Nortel Networks
   2221, Lakeside Blvd
   Richardson, TX  75082

   Phone: +1 972-684-5767
   EMail: jayshree@nortel.com

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