Network Working Group                                           A. Patel
Internet-Draft                                                  K. Leung
Expires: August 11, 2005 February 23, 2006                                 Cisco Systems
                                                               M. Khalil
                                                               H. Akhtar
                                                         Nortel Networks
                                                            K. Chowdhury
                                                        Starent Networks
                                                       February 10,
                                                         August 22, 2005

                Authentication Protocol for Mobile IPv6
                  draft-ietf-mip6-auth-protocol-04.txt
                  draft-ietf-mip6-auth-protocol-05.txt

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Copyright Notice

   Copyright (C) The Internet Society (2005).  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
   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
     3.1
     3.1.  General Terms  . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Operational flow . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Mobility message authentication option . . . . . . . . . . . .  7
     5.1
     5.1.  MN-HA authentication mobility option . . . . . . . . . . .  8
       5.1.1
       5.1.1.  Processing Considerations  . . . . . . . . . . . . . .  9
     5.2
     5.2.  MN-AAA authentication mobility option  . . . . . . . . . .  9
       5.2.1 10
       5.2.1.  Processing Considerations  . . . . . . . . . . . . . . 10
     5.3
     5.3.  Authentication Failure Detection at the Mobile Node  . . . 10 11
   6.  Mobility message replay protection option  . . . . . . . . . . 11 12
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14 15
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15 16
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   10. Normative References . . . . . . . . . . . . . . . . . . . . . 17
   Appendix A.  Rationale for mobility message replay protection
                option  . . . . . . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 17
   A.  Rationale for mobility message replay protection option  . . . 19
   Intellectual Property and Copyright Statements . . . . . . . . 20 . . 21

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 an Authentication, Authorization and
       Accounting (AAA) infrastructure for authenticating

   The draft draft-patil-mip6-whyauthdataoption-01.txt describes the subscriber
   2.  require dynamic assignment
   architecture of Home Agent cdma2000 networks and home addresses
   3.  have constraints on the number motivation of messages involved in setting up
       IPsec SAs
   4.  include Mobile Nodes that do not support IKEv1

   This indicates using the need for a solution that does not necessarily
   require an IPsec SA
   authentication option for securing the signaling messages that deal
   with the Registration process of a Mobile Node with a home agent. IP in that architecture.

   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 a host 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 does not imply that the availability of
   such a solution deprecates the use of IPsec for securing Mobile IPv6
   signaling between Mobile Nodes and Home Agents.  Home agents still
   have to implement and support registrations from Mobile Nodes that
   are secured via IPsec as well as with the authentication option.

   The authentication mechanism proposed here is similar to the
   authentication mechanism used in Mobile IPv4 [RFC3344].

2.  Overview

   This document presents a lightweight mechanism to authenticate the
   Mobile Node at the Home Agent or at the Authentication, Authorization
   and Accounting (AAA) server in Home network (AAAH) based on a shared
   key based mobility security association between the Mobile Node and
   the respective authenticating entity.  This shared key based mobility
   security association (shared-key based SA) may be statically
   provisioned or dynamically created.  The term "security "mobility security
   association" referred to in this document is understood to be a
   "shared-key based Mobile IPv6 authentication" security association.  The Mobile Node MUST use only
   one means of authentication, based on either the shared key based
   authentication or IPsec security association with a selected Home
   Agent at any given time.  However a MN that implements both schemes
   of authentication may choose any appropriate scheme with the chosen
   HA.  The determination of what authentication scheme to use is beyond
   the scope of this document.  Home agents still have to implement and
   support registrations from Mobile Nodes that are secured via IPsec as
   well as with the authentication option.

   This document introduces new mobility options to aid in
   authentication of the Mobile Node to the Home Agent 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.  The Home Agent SHOULD prevent
   these operations for a Mobile Node when such protection cannot be
   provided by the network.

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.

3.1

3.1.  General Terms

   First (size, input)

      Some formulas in this specification use a functional form "First
      (size, input)" to indicate truncation of the "input" data so that
      only the first "size" bits remain to be used.

4.  Operational flow

   The figure below describes the sequence of messages sent and received
   between the MN and HA in the registration process.  Binding Update
   (BU) and Binding Acknowledgement (BA) messages are used in the
   registration process.

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

   Figure 1: Home Registration with Authentication Protocol

   Mobile Node MAY MUST use the Mobile Node Identifier Option Option, specifically
   the MN-NAI mobility option as defined in [MN_Ident] or Home Address to identify
   itself while authenticating with the Home Agent.  The mobile node
   uses the Mobile Node Identifier option as defined in [MN_Ident] to
   identify itself as may be required for use with some existing AAA
   infrastructure designs.

   Mobile Node MAY use Message Identifier option as defined in Section 6
   for additional replay protection.

   The authentication option described in Section 5 may be used by the
   mobile node to transfer authentication data when the mobile node and
   the home agent are utilizing an SPI to index between multiple
   mobility security associations.  For the case when there is only one
   such mobility security association, and no SPI is needed, the Mobile
   Node and Home Agent can use the Binding Authorization Data option as
   defined in the base Mobile IPv6 specification [RFC3775] for this same
   purpose.  Since that option does not have any SPI, the Mobile Node
   and the Home Agent implicitly agree that the mobility security
   association to be used is the only mobility security association that
   is defined for their mutual authentication needs.

5.  Mobility message authentication option

   This section defines a 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 the 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.  Other subtypes
   may be defined for use 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 mobility
   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 ....
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 2

      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:

         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.

5.1

5.1.  MN-HA authentication mobility option

   The format of the MN-HA authentication mobility option is as defined
   in Figure 2.  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 Home
   Agent.

   The shared-key based mobility security association between Mobile
   Node and Home Agent used within 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.  If the Timestamp option is used for replay protection as
   defined in Section 6, the mobility security association includes a
   "close enough" field to account for clock drift.  A default value of
   7 seconds MAY be used.  This value SHOULD be greater than 3 seconds.

   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 Section 5.2 authentication option if both options
   are present in the message.

   The authentication data is calculated on the message starting from
   the mobility header upto and 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

   MH Data is the content of the Mobility Header upto and including the
   SPI field of this option.  The Checksum field in Mobility Header MUST
   be set to 0 to calculate the Mobility Data.

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

5.1.1

5.1.1.  Processing Considerations

   The assumption is that Mobile Node has a shared-key based security
   association with the Home Agent.  The Mobile Node MUST include this
   option in a BU if it has a shared-key based mobility security
   association with the Home Agent.  The Home Agent MUST include this
   option in the BA if it received this option in the corresponding BU
   and Home Agent has a shared-key based mobility security association
   with the Mobile Node.

   The Mobile Node or Home Agent receiving this option MUST verify the
   authentication data in the option.  If authentication fails, the Home
   Agent MUST send BA with Status Code MIPV6-AUTH-FAIL.  If the Home
   Agent does not have shared-key based SA, Home Agent MUST discard the
   BU.  The Home Agent MAY log such events.

5.2

5.2.  MN-AAA authentication mobility option

   The format of the MN-AAA authentication mobility option is as defined
   in Figure 2.  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 mobility security association
   between Mobile Node and AAA server in Home network (AAAH).  It is not
   used in Binding Acknowledgement messages.  The corresponding Binding
   Acknowledgement messages must be authenticated using the MN-HA
   authentication option Section 5.1.

   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 option MUST appear prior to
   the Mobile-AAA Authentication option.  The
   corresponding response MUST include the Mobile-Home Authentication
   option, and MUST NOT include the Mobile-AAA Authentication option.

   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 upto and 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.

   SPI = HMAC_SHA1_SPI:

   If SPI has the well-known value HMAC_SHA1_SPI, then hash_fn() is
   HMAC_SHA1.  When HMAC_SHA1_SPI is used, the BU is authenticated by
   AAA using HMAC_SHA1 authentication.  In that case, MAC_Mobility Data
   is calculated as follows:

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

   MH Data is the content of the Mobility Header upto and including the
   SPI field of this option.

5.2.1

5.2.1.  Processing Considerations

   Interaction between the HA and the AAA server is beyond the scope of
   this document.

   When the Home Agent receives a Binding Update with the Mobile-AAA
   authentication option, the Binding Update is authenticated by an
   entity external to the Home Agent, typically a AAA server.

5.3

5.3.  Authentication Failure Detection at the Mobile Node

   In case of authentication failure, the Home Agent MUST send a Binding
   Acknowledgement with status code MIPV6-AUTH-FAIL to the Mobile Node,
   if an SA to be used between Mobile Node and Home Agent for
   authentication exists.

   Upon receiving a Binding Acknowledgement with status code
   MIPV6-AUTH-FAIL, MIPV6-AUTH-
   FAIL, the Mobile Node SHOULD stop sending new Binding Updates to the
   Home Agent.

6.  Mobility message replay protection option

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

   The mobility message replay protection option is used to let the Home
   Agent verify that a Binding Update has been freshly generated by the
   Mobile Node and not replayed by an attacker from some previous
   Binding Update.  This is especially useful for cases where the Home
   Agent does not maintain stateful information about the Mobile Node
   after the binding entry has been removed.  The Home Agent does the
   replay protection check after the Binding Update has been
   authenticated.  The mobility message replay protection option when
   included is used by the Mobile Node for matching BA with BU.

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

   When the Home Agent receives the mobility message replay protection
   option in Binding Update, it SHOULD include the mobility message
   replay protection option in Binding Acknowledgement.  Appendix A
   provides details regarding why the mobility message replay protection
   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 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                  Timestamp ...                                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                  Timestamp                                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 3

      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 Timestamp field.

      Timestamp:

         This field carries the 64 bit timestamp.

      Alignment requirements :

         The alignment requirement for this option is 8n + 2.

   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 mobility 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.  The two nodes must have
   adequately synchronized time-of-day clocks.

   The Mobile Node MUST set the Timestamp 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 Timestamp used in a Binding Update from
   the Mobile Node MUST be greater than that used in any previous
   successful Binding Update.

   After successful authentication of Binding Update (either locally at
   the Home Agent or when a success indication is received from the AAA
   server), the Home Agent MUST check the Timestamp field for validity.
   In order to be valid, the timestamp contained in the Timestamp 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 Timestamp
   field into the Timestamp 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, Home Agent MUST send a Binding Acknowledgement with status
   code MIPV6-ID-MISMATCH.  The Home Agent 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 Timestamp value in the
   BA to the Timestamp 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
   Mobile Node silently discards the BA.

7.  Security Considerations

   This document proposes new authentication options to authenticate the
   control message between Mobile Node, Home Agent 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
   Mobile Node and Home Agent for subsequent authentication of BU/BA
   between Mobile Node and Home Agent via the MN-HA authentication
   option.

   This specification also introduces an optional replay protection
   mechanism in 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 mobility
   message replay protection.

8.  IANA Considerations

   IANA services are required for this specification.  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, 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, Section 6 and Section 5.3
   also need to be assigned.  The suggested values are 144 for
   MIPV6-ID-MISMATCH 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 5.2.

   The value 5 is reserved for use by 3GPP2.

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

   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:

   1 MN-HA authentication mobility option Section 5.1

   2 MN-AAA authentication mobility option Section 5.2

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

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

10.  Normative References

   [MN_Ident]
              Patel et. al., A., "Mobile Node Identifier Option for
              Mobile IPv6", draft-ietf-mip6-mn-ident-option-01.txt draft-ietf-mip6-mn-ident-option-03.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. 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. 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
   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

Appendix A.  Rationale for mobility message replay protection 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 Home Agent should accept a BU
   with a Sequence Number greater than the Sequence Number from previous
   Binding Update.  This implicitly assumes that the Home Agent 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 Home Agent 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
   Mobile Node to register with a different Home Agent during each
   mobility session.  Thus, it is unreasonable to expect each Home Agent
   in the network to maintain state about the Mobile Node.  Also, if the
   Home Agent 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 Home Agent does not save state
   information for every Mobile Node) would be for the Home Agent to
   reject the first BU and assign a (randomly generated) starting
   sequence number for the session and force the Mobile Node 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 mobility message replay protection
   option to prevent replay attacks.  Specifically, timestamps are used
   to prevent replay attacks as described in Section 6.

   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 Mobile Node and CN, the liveness proof is provided
   by the use of nonces which the CN generates.

Authors' Addresses

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

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

   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@nortel.com

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

   Phone: +1 972-684-4732
   Email: haseebak@nortel.com
   Kuntal Chowdhury
   Starent Networks
   30 International Place
   Tewksbury, MA  01876
   US

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

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