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Network Working Group                                         C. PerkinsRequest for Comments: 4721                         Nokia Research CenterObsoletes:3012                                               P. CalhounUpdates:3344                                        Cisco Systems, Inc.Category: Standards Track                                    J. Bharatia                                                         Nortel Networks                                                            January 2007Mobile 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 updatesRFC 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 byRFC 3344.  This   document obsoletesRFC 3012.Perkins, et al.             Standards Track                     [Page 1]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 2007Table of Contents1. Introduction ....................................................21.1. Terminology ................................................32. Mobile IP Agent Advertisement Challenge Extension ...............42.1. Handling of Solicited Agent Advertisements .................43. Operation .......................................................53.1. Mobile Node Processing of Registration Requests ............53.2. Foreign Agent Processing of Registration Requests ..........6            3.2.1. Foreign Agent Algorithm for Tracking Used                   Challenges .........................................83.3. Foreign Agent Processing of Registration Replies ...........93.4. Home Agent Processing of Challenge Extensions .............103.5. Mobile Node Processing of Registration Replies ............114. Mobile-Foreign Challenge Extension .............................115. Generalized Mobile IP Authentication Extension .................126. Mobile-AAA Authentication Subtype ..............................137. Reserved SPIs for Mobile IP ....................................148. SPIs for RADIUS AAA Servers ....................................149. Configurable Parameters ........................................1510. Error Values ..................................................1611. IANA Considerations ...........................................1612. Security Considerations .......................................1713. Acknowledgements ..............................................1814. Normative References ..........................................18Appendix A. Changes sinceRFC 3012 ................................20Appendix B. Verification Infrastructure ...........................21Appendix C. Message Flow for FA Challenge Messaging with               Mobile-AAA Extension ..................................22Appendix D. Message Flow for FA Challenge Messaging with               MN-FA Authentication ..................................23Appendix E. Example Pseudo-code for Tracking Used Challenges ......241.  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 additionalPerkins, et al.             Standards Track                     [Page 2]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 2007   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 inRFC 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 seeSection 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.Perkins, et al.             Standards Track                     [Page 3]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 20072.  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.Perkins, et al.             Standards Track                     [Page 4]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 2007   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 inSection 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, seeSection 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.  SeeAppendix 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 inSection 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.Perkins, et al.             Standards Track                     [Page 5]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 2007   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 inSection 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 (seeSection 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.Perkins, et al.             Standards Track                     [Page 6]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 2007   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 (Section3.7.1 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 (seeSection 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 (seeSection 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, seeSection 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 (seeSection 10).   If the Mobile-AAA Authentication extension (seeSection 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 authenticationPerkins, et al.             Standards Track                     [Page 7]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 2007   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 inAppendix 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.Perkins, et al.             Standards Track                     [Page 8]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 2007   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 inSection 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 ReplyPerkins, et al.             Standards Track                     [Page 9]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 2007   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 (seeSection 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.Perkins, et al.             Standards Track                    [Page 10]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 20073.5.  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 inSection 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 inSection 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 byPerkins, et al.             Standards Track                    [Page 11]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 2007   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 IndexPerkins, et al.             Standards Track                    [Page 12]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 2007   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))          (seeSection 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 inSection 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.Perkins, et al.             Standards Track                    [Page 13]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 20077.  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-numbers8.  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 (seeSection 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 toSection 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 usedPerkins, et al.             Standards Track                    [Page 14]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 2007   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.Perkins, et al.             Standards Track                    [Page 15]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 200710.  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 Values11.  IANA Considerations   The following are currently assigned by IANA forRFC 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 inSection 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 (seeSection 5).  New      subtypes of the Generalized Authentication extension, other than      the number (1) for the MN-AAA authentication extension specified      inSection 6, must be specified and approved by a designated      expert.Perkins, et al.             Standards Track                    [Page 16]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 2007      The Mobile Node - Foreign Agent (MN-FA) Challenge extension,      defined inSection 4, is a router advertisement extension as      defined inRFC 1256 [RFC1256] and extended inRFC 3344 [RFC3344].      IANA has assigned a value of 132 for this purpose.      The Code values defined inSection 10 are error codes as defined      inRFC 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 inSection 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 inSection 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 (seeSection 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 (seeSection 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.Perkins, et al.             Standards Track                    [Page 17]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 2007Section 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 inSection 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.Perkins, et al.             Standards Track                    [Page 18]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 2007   [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)",RFC2865, 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.Perkins, et al.             Standards Track                    [Page 19]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 2007Appendix A.  Changes sinceRFC 3012   The following is the list of changes fromRFC 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 insection 3.2 andsection 11.Perkins, et al.             Standards Track                    [Page 20]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 2007Appendix 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 valuePerkins, et al.             Standards Track                    [Page 21]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 2007   BAD_AUTHENTICATION (seeSection 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.Perkins, et al.             Standards Track                    [Page 22]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 2007   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 (seeAppendix 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.Perkins, et al.             Standards Track                    [Page 23]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 2007   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 (seeSection 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);   }Perkins, et al.             Standards Track                    [Page 24]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 2007Authors' 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.comPerkins, et al.             Standards Track                    [Page 25]

RFC 4721       Mobile IPv4 Challenge/Response Extensions    January 2007Full Copyright Statement   Copyright (C) The IETF Trust (2007).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   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, THE IETF TRUST,   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.Intellectual Property   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   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 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 athttp://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.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Perkins, et al.             Standards Track                    [Page 26]