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Versions: 00 01 02 03 04 05 06 07 RFC 4285

Network Working Group                                           A. Patel
Internet-Draft                                                  K. Leung
Expires: June 23, 2005                                     Cisco Systems
                                                               M. Khalil
                                                               H. Akhtar
                                                         Nortel Networks
                                                            K. Chowdhury
                                                        Starent Networks
                                                       December 23, 2004


                Authentication Protocol for Mobile IPv6
                  draft-ietf-mip6-auth-protocol-02.txt

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   material or to cite them other than as "work in progress."

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   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on June 23, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   IPsec is specified as the sole means of securing all signaling
   messages between the Mobile Node and Home agent for Mobile IPv6.  A
   flexible model for security between the mobile node and home agent is
   required from the perspective of deployment of the Mobile IPv6



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   protocol.  One instance of such deployment need comes from networks
   that are built on 3GPP2 standards.  This document proposes an
   alternate method for securing the signaling messages that are
   responsible for performing Registration of a mobile node at a home
   agent.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Operational flow . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Mobility message authentication option . . . . . . . . . . . .  7
     5.1   MN-HA authentication mobility option . . . . . . . . . . .  8
       5.1.1   Processing Considerations  . . . . . . . . . . . . . .  9
     5.2   MN-AAA authentication mobility option  . . . . . . . . . .  9
       5.2.1   Processing Considerations  . . . . . . . . . . . . . . 10
     5.3   Authentication Failure Detection at the Mobile Node  . . . 10
   6.  Mobility message replay protection option  . . . . . . . . . . 11
     6.1   Timestamp option . . . . . . . . . . . . . . . . . . . . . 12
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   10.   Normative References . . . . . . . . . . . . . . . . . . . . 16
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 16
   A.  Rationale for message identification option  . . . . . . . . . 18
       Intellectual Property and Copyright Statements . . . . . . . . 19
























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1.  Introduction

   The base Mobile IPv6 specification [RFC3775] specifies the signaling
   messages, Binding Update (BU) and Binding Acknowledgement (BA),
   between the Mobile node and Home agent to be secured by the IPsec
   Security Associations (IPsec SAs) that are established between these
   two entities.  This security model for Mobile IPv6 does not fit in
   very well for deployment scenarios which:

   1.  rely on the use of a Authentication, Authorization and Accounting
       (AAA) infrastructure for authenticating the subscriber
   2.  require dynamic assignment of home agent and home addresses
   3.  have constraints on the number of messages involved in setting up
       a IPsec SAs using protocols like IKEv1
   4.  include mobile nodes that do not support IKEv1

   The conclusion drawn thereby is the need for a solution that does not
   necessarily require an IPsec SA for securing the signaling messages
   that deal with the Registration process of a mobile node with a home
   agent.

   This document proposes a solution for securing the Binding update and
   Binding acknowledgment messages between the Mobile node and Home
   agent using an authentication option which is included in these
   messages.  Such a mechanism enables IPv6 mobility in hosts without
   having to establish an IPsec SA with its home agent.  A mobile node
   can implement Mobile IPv6 without having to integrate it with the
   IPsec module, in which case the Binding update and Binding
   Acknowldegement messages (between MN-HA) are secured with the
   authentication option.  It should be noted that it does not imply
   that the availability of such a solution deprecates the use of IPsec
   for securing Mobile IPv6 signaling between MNs and HAs.  Home agents
   however have to implement and support registrations from mobile nodes
   that are secured via IPsec as well as with the authentication option.

















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2.  Overview

   This document presents a lightweight mechanism to authenticate the
   mobile node at the HA or at the Authentication, Authorization and
   Accounting (AAA) server in Home network (AAAH) based on a shared key
   based security association between the mobile node and the respective
   authenticating entity.  This shared key based security association
   (shared-key based SA) may be statically provisioned or dynamically
   created.  The term "security association" referred to in this
   document is understood to be a "shared-key based Mobile IPv6
   authentication" security association.

   This document introduces new mobility options to aid in
   authentication of the mobile node to the HA or AAAH server.  The
   confidentiality protection of Return Routability messages and
   authentication/integrity protection of Mobile Prefix Discovery (MPD)
   is outside the scope of this document.


































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3.  Terminology

   The keywords "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.














































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4.  Operational flow


         MN                                                  HA/AAAH
         |                   BU to HA                           |
   (a)   |----------------------------------------------------->|
         | (including MN-ID option [optional],                  |
         |  Message ID option [optional], authentication option)|
         |                                                      |
         |                                                      |
         |                                   HA/AAAH authenticates MN
         |                                                      |
         |                                                      |
         |                  BA to MN                            |
   (b)   |<-----------------------------------------------------|
         | (including MN-ID option [optional],                  |
         |  Message ID option [optional], authentication option)|
         |                                                      |


   Mobile Node MAY use Mobile Node Identifier Option as defined in
   [MN_Ident] to identify itself while authenticating with the HA.  The
   mobile node MUST use the Mobile Node Identifier option as defined in
   [MN_Ident] to identify itself while authenticating with the AAA
   infrastructure.

   MN MAY use Message Identifier option as defined in Section 6 for
   replay protection.























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5.  Mobility message authentication option

   This section defines the message authentication mobility option that
   may be used to secure Binding Update and Binding Acknowledgement
   messages.  This option can be used along with IPsec or preferably as
   an alternate mechanism to authenticate Binding Update and Binding
   Acknowledgement messages in absence of IPsec.

   This document also defines subtype numbers, which identify the mode
   of authentication and the peer entity to authenticate the message.
   Two subtype numbers are specified in this document.  It is expected
   that other subtypes will be defined by other documents in the future.

   Only one instance of an authentication option of a particular subtype
   can be present in the message.  One message may contain multiple
   instances of authentication options with different subtype values.

   When a Binding Update or Binding Acknowledgement is received without
   an authentication option and the entity receiving it is configured to
   use authentication option or has the shared-key based security
   association for authentication option, the entity should silently
   discard the received message.

       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
                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                       |  Option Type  | Option Length |  Subtype      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          SPI                                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Authentication Data . . .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      Option Type:

         AUTH-OPTION-TYPE to be defined by IANA.  An 8-bit identifier of
         the type mobility option.

      Option Length:

         8-bit unsigned integer, representing the length in octets of
         the Sub-type, Security Parameter Index (SPI) and Authentication
         Data fields.

      Subtype:





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         A number assigned to identify the entity and/or mechanism to be
         used to authenticate the message.

      SPI:

         Security Parameter Index

      Authentication Data:

         This field has the information to authenticate the relevant
         mobility entity.  This protects the message beginning at the
         Mobility Header upto and including the SPI field.

      Alignment requirements :

         The alignment requirement for this option is 4n + 1 octets.

5.1  MN-HA authentication mobility option

   The format of the MN-HA authentication mobility option is as defined
   in Section 5.  This option uses the subtype value of 1.  The MN-HA
   authentication mobility option is used to authenticate the Binding
   Update and Binding Acknowledgement messages based on the shared-key
   based security association between the mobile node and the HA.

   The shared-key based security association between MN and HA as per
   this specification consists of a SPI, a key, an authentication
   algorithm and the replay protection mechanism in use.  The SPI is a
   number in range [0-4294967296], where the range [0-255] is reserved.
   The key consists of an arbitrary value and is 16 octets in length.
   The authentication algorithm is HMAC_SHA1.  The replay protection
   mechanism may use the Sequence number as specified in [RFC3775] or
   the option as defined in Section 6.

   This must be the last option in a message with mobility header if it
   is the only authentication option in the message.  It must occur
   before the MN-AAA authentication option if both options are present
   in the message.

   The authentication data is calculated on the message starting from
   the mobility header till (including) the SPI value of this option.

   Authentication Data = First (96, HMAC_SHA1(MN-HA Shared key, Mobility
   Data))

   Mobility Data = care-of address | home address | Mobility Header(MH)
   Data




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   MH Data is the content of the Mobility Header till (including) the
   SPI field of this option.

   The first 96 bits from the MAC result are used as the Authentication
   Data field.

5.1.1  Processing Considerations

   MN MUST include this option in a BU if it has a shared-key based
   security association with the HA.  The HA MUST include this option in
   the BA if IPsec is not used and it has a shared-key based security
   association with the mobile node.

   MN or HA receiving this option MUST verify the authentication data in
   the option.  If authentication fails, HA MUST discard the BU and send
   BA with Status Code MIPV6-AUTH-FAIL, if the HA has a SA with the
   mobile node.

5.2  MN-AAA authentication mobility option

   The format of the MN-AAA authentication mobility option is as defined
   in Section 5.  This option uses the subtype value of 2.  The MN-AAA
   authentication mobility option is used to authenticate the Binding
   Update message based on the shared security association between MN
   and AAA server in Home network (AAAH).  It is not used in Binding
   Acknowledgement message.  The corresponding Binding Acknowledgement
   messages must be authenticated using the MN-HA authentication option.

   This must be the last option in a message with mobility header.  If
   both Mobile-Home and Mobile-AAA authentication mobility options are
   present, the Mobile-Home Authentication Extension MUST appear prior
   to the Mobile-AAA Authentication option.  The corresponding response
   MUST include the Mobile-Home Authentication Extension, and MUST NOT
   include the Mobile-AAA Authentication Extension.

   The mobile node MAY use Mobile Node Identifier option [MN_Ident] to
   enable the Home Agent to make use of available AAA infrastructure.

   The authentication data is calculated on the message starting from
   the mobility header till (including) the SPI value of this option.

   The authentication data shall be calculated as follows:

   Authentication data = hash_fn(MN-AAA Shared key, MAC_Mobility Data)

   hash_fn() is decided by the value of SPI field in the authentication
   option.  The SPI field in the MN-AAA authentication option also
   defines how the mobility options in BU are mapped to AAA attributes



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   for authentication.

   SPI = HMAC_SHA1_SPI:

   hash_fn() is HMAC_SHA1.  When HMAC_SHA1_SPI is used, the BU is
   authenticated by AAA using HMAC_SHA1 authentication.

   MAC_Mobility Data = SHA1(care-of address | home address | MH Data)

   MH Data is the content of the Mobility Header till (including) the
   SPI field of this option.

5.2.1  Processing Considerations

   The MN-AAA authentication mobility option MUST be verified by the AAA
   infrastructure that has the shared secret with the mobile node.  The
   HA relays the authenticating information to the home AAA.  The HA
   relies on the home AAA to admit or reject the Binding Update from the
   mobile node.

5.2.1.1  Home Agent Considerations

   Upon receiving a BU from the mobile node, the HA MUST extract the
   MN-AAA authentication data and the SPI from the MN-AAA authentication
   mobility option and extract the Mobile Node Identifier from the
   Mobile Node Identifier mobility option [MN_Ident] (if present).

   The HA MUST include the extracted MN-AAA authentication data, SPI and
   the Mobile Node Identifier in AAA specific Attribute-Value Pairs
   (AVPs) while initiating the authentication procedure via AAA
   infrastructure.

5.3  Authentication Failure Detection at the Mobile Node

   In case of authentication failure, the HA MUST send a Binding
   Acknowledgement with status code MIPV6-AUTH-FAIL to the mobile node,
   if an SA to be used between MN and HA for authentication exists.
   This MAY need administrative intervention to resolve the cause of the
   authentication failure.

   Upon receiving a Binding Acknowledgement with status code
   MIPV6-AUTH-FAIL, the mobile node SHOULD stop sending new Binding
   Updates to the responding HA.








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6.  Mobility message replay protection option

   The Mobility message replay protection option MAY be used in a
   Binding Update/Binding Acknowledgement messages when authenticated
   using the mobility authentication option as described in Section 5.

   The Replay Protection option is used to let the home agent verify
   that a Binding Update has been freshly generated by the mobile node,
   not replayed by an attacker from some previous Binding Update.  The
   Replay Protection option when included is used by the mobile node for
   matching BA with BU.

   The subtype field in the identification option specifies the style of
   replay protection used.  This document specifies timestamps as one
   style of replay protection, as described in Section 6.1.  The
   Identification field in the Replay Protection option in a new Binding
   Update MUST not be the same as in an immediately preceding Binding
   Update.

   The style of replay protection in effect between a mobile node and
   the HA is part of the shared-key based mobility security association.
   A mobile node and the HA MUST agree on which method of replay
   protection will be used (even though the security association may be
   dynamically derived.  This can be done as part of policy at the HA).

   If the policy at HA mandates replay protection using this option (as
   opposed to the sequence number in Mobility Header in Binding Update)
   and the Binding Update from MN does not include this option, HA
   discards the BU and sets the Status Code in BA to MIPV6-MESG-ID-REQD.

   When mobility message identification option is used along with
   authentication option, the mobile node SHOULD set the Sequence Number
   in the mobility header in Binding Update to 0 and SHOULD ignore the
   Sequence Number in Mobility Header in BA.  Appendix A provides
   details regarding why message identification option MAY be used when
   using the authentication option.


       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                       |  Option Type  | Option Length |   Subtype     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                  Identification ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






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      Option Type:

         MESG-ID-OPTION-TYPE to be defined by IANA.  An 8-bit identifier
         of the type mobility option.

      Option Length:

         8-bit unsigned integer, representing the length in octets of
         the Subtype and Identification field.

      Subtype:

         8-bit unsigned integer indicating the style of replay
         protection in use.

      Identification:

         The Identification field carries Subtype specific data for
         replay protection.

      Alignment requirements :

         This option does not have any specific alignment requirements.

6.1  Timestamp option

   The format of the timestamp mobility option is as defined in Section
   6.  This option uses the subtype value of 1.  The Identification
   field carries timestamp for replay protection.

   The basic principle of timestamp replay protection is that the node
   generating a message inserts the current time of day, and the node
   receiving the message checks that this timestamp is sufficiently
   close to its own time of day.  Unless specified differently in the
   shared-key based security association between the nodes, a default
   value of 7 seconds MAY be used to limit the time difference.  This
   value SHOULD be greater than 3 seconds.  Obviously the two nodes must
   have adequately synchronized time-of-day clocks.

   The mobile node MUST set the Identification field to a 64-bit value
   formatted as specified by the Network Time Protocol [RFC1305].  The
   low-order 32 bits of the NTP format represent fractional seconds, and
   those bits which are not available from a time source SHOULD be
   generated from a good source of randomness.  Note, however, that when
   using timestamps, the 64-bit Identification used in a Binding Update
   from the mobile node MUST be greater than that used in any previous
   Binding Update.




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   After successful authentication of Binding Update (either locally at
   the HA or when a success indication is received from the AAA server),
   the home agent MUST check the Identification field for validity.  In
   order to be valid, the timestamp contained in the Identification
   field MUST be close enough to the home agent's time of day clock and
   the timestamp MUST be greater than all previously accepted timestamps
   for the requesting mobile node.

   If the timestamp is valid, the home agent copies the entire
   Identification field into the Identification field in the BA it
   returns to the mobile node.  If the timestamp is not valid, the home
   agent copies only the low-order 32 bits into the BA, and supplies the
   high-order 32 bits from its own time of day.  If the timestamp field
   is not valid but the authentication of the BU succeeds, HA MUST send
   a Binding Acknowledgement with status code MIPV6-ID-MISMATCH.  HA
   does not create a binding cache entry if the timestamp check fails.

   If the mobile node receives a Binding Acknowledgement with the code
   MIPV6-ID-MISMATCH, the mobile node MUST authenticate the BA by
   processing the MN-HA authentication mobility option.  If
   authentication succeeds, the mobile node MUST adjust its timestamp
   and send subsequent Binding Update using the updated value.  Upon
   receiving a BA that does not contain the MIPV6-ID-MISMATCH status
   code, the mobile node MUST compare the Identification value in the BA
   to the Identification value it sent in the corresponding BU.  If the
   values match, the mobile node proceeds to process the MN-HA
   authentication data in the BA.  If the values do not match, the MN
   silently discards the BA.























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7.  Security Considerations

   This document proposes new authentication options to authenticate the
   control message between MN, HA and/or home AAA (as an alternative to
   IPsec).  The new options provide for authentication of Binding Update
   and Binding Acknowledgement messages.  The MN-AAA authentication
   options provides for authentication with AAA infrastructure.  It can
   be used to generate a per session key between MN and HA for
   subsequent authentication of BU/BA between MN and HA via the MN-HA
   authentication option.

   This memo also introduces an optional replay protection mechanism
   Section 6, to prevent replay attacks.  The sequence number field in
   the Binding Update is not used if this mechanism is used.  This memo
   defines the timestamp option to be used for message identification.




































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8.  IANA Considerations

   IANA services are required for this document.  The values for new
   mobility options and status codes must be assigned from the Mobile
   IPv6 [RFC3775] numbering space.

   The values for Mobility Option types AUTH-OPTION-TYPE and
   MESG-ID-OPTION-TYPE, as defined in Section 5 and Section 6 need to be
   assigned.  The suggested values are 8 for the AUTH-OPTION-TYPE and 9
   for the MESG-ID-OPTION-TYPE Mobility Option.

   The values for status codes MIPV6-ID-MISMATCH, MIPv6-AUTH-FAIL and
   MIPV6-MESG-ID-REQD as defined in Section 6.1, Section 6 and Section
   5.3 also need to be assigned.  The suggested values are 144 for
   MIPV6-ID-MISMATCH 145 for MIPV6-MESG-ID-REQD and 146 for
   MIPV6-AUTH-FAIL.

   IANA should record values for these new Mobility Options and the new
   Status Codes.

   A new section for enumerating algorithms identified by specific SPIs
   within the range 0-255 is to be added to

   http://www.isi.edu/in-notes/iana/assignments/mobility-parameters

   The currently defined values are as follows:

   The value 0 should not be assigned.

   The value 3 is suggested for HMAC_SHA1_SPI as defined in section
   Section 5.2.

   The value 5 is reserved for use by 3GPP2.

   In addition, IANA needs to create a new namespace for the subtype
   field of the MN-HA and MN-AAA authentication mobility options under

   http://www.isi.edu/in-notes/iana/assignments/mobility-parameters

   The currently allocated values are as follows:

   MN-HA authentication mobility option Section 5.1     [1]

   MN-AAA authentication mobility option Section 5.2    [2]

   New values for this namespace can be allocated using Standards Action
   [RFC2434].




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9.  Acknowledgements

   The authors would like to thank Basavaraj Patil, Charlie Perkins
   Vijay Devarapalli and Jari Arkko for their thorough review and
   suggestions on the document.  The authors would like to acknowledge
   the fact that a similar authentication method was considered in base
   protocol [RFC3775] at one time.

10  Normative References

   [MN_Ident]
              Patel et. al., A., "MN Identifier Option for Mobile IPv6",
              draft-ietf-mip6-mn-ident-option-01.txt (work in progress),
              December 2004.

   [RFC1305]  Mills, D., "Network Time Protocol (Version 3)
              Specification, Implementation", RFC 1305, March 1992.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

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

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

   [RFC3775]  Johnson, D., Perkins, C. and J. Arkko, "Mobility Support
              in IPv6", RFC 3775, June 2004.


Authors' Addresses

   Alpesh Patel
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   US

   Phone: +1 408-853-9580
   EMail: alpesh@cisco.com








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   Kent Leung
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   US

   Phone: +1 408-526-5030
   EMail: kleung@cisco.com


   Mohamed Khalil
   Nortel Networks
   2221 Lakeside Blvd.
   Richardson, TX  75082
   US

   Phone: +1 972-685-0574
   EMail: mkhalil@nortelnetworks.com


   Haseeb Akhtar
   Nortel Networks
   2221 Lakeside Blvd.
   Richardson, TX  75082
   US

   Phone: +1 972-684-4732
   EMail: haseebak@nortelnetworks.com


   Kuntal Chowdhury
   Starent Networks
   2540 Coolwater Dr.
   Plano, TX  75025
   US

   Phone: +1 214 550 1416
   EMail: kchowdury@starentnetworks.com













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Appendix A.  Rationale for message identification option

   Mobile IPv6 [RFC3775] defines a Sequence Number in the mobility
   header to prevent replay attacks.  There are two aspects that stand
   out in regards to using the Sequence Number to prevent replay
   attacks.

   Firstly, the specification states that HA should accept a BU with a
   Sequence Number greater than the Sequence Number from previous
   Binding Update.  This implicitly assumes that the HA has some
   information regarding the Sequence Number from previous BU (even when
   the binding cache entry is not present).  Secondly, the specification
   states that if the HA has no binding cache entry for the indicated
   home address, it MUST accept any Sequence Number value in a received
   Binding Update from this mobile node.

   With the mechanism defined in this draft, it is possible for the MN
   to register with a different home agent during each mobility session.
   Thus, it is unreasonable to expect each HA in the network to maintain
   state about the mobile node.  Also, if the HA does not cache
   information regarding sequence number, as per the second point above,
   a replayed BU can cause a Home Agent to create a binding cache entry
   for the mobile node.  Thus, when authentication option is used,
   Sequence Number does not provide protection against replay attack.

   One solution to this problem (when HA does not save state information
   for every MN) would be for the HA to reject the first BU and assign a
   (randomly generated) starting sequence number for the session and
   force the MN to send a fresh BU with the suggested sequence number.
   While this would work in most cases, it would require an additional
   round trip and this extra signalling and latency is not acceptable in
   certain deployments (3GPP2).  Also, this rejection and using sequence
   number as a nonce in rejection is a new behavior that is not
   specified in [RFC3775].

   Thus, this specification uses the message identification option to
   prevent replay attacks.  Specifically, timestamps are used for
   message identification to prevent replay attacks as described in
   Section 6.1.

   It is important to note that as per Mobile IPv6 [RFC3775] this
   problem with sequence number exists.  Since the base specification
   mandates the use of IPsec (and naturally that goes with IKE in most
   cases), the real replay protection is provided by IPsec/IKE.  In case
   of BU/BA between MN and CN, the liveness proof is provided by the use
   of nonces which the CN generates.





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Internet-Draft    Authentication Protocol for Mobile IPv6  December 2004


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