





Internet Research Task Force (IRTF)                               Y. Qiu
Request for Comments: 5726               Institute for Infocomm Research
Category: Experimental                                      F. Zhao, Ed.
ISSN: 2070-1721                                                   Google
                                                               R. Koodli
                                                           Cisco Systems
                                                           February 2010


                 Mobile IPv6 Location Privacy Solutions

Abstract

   Mobile IPv6 (RFC 3775) enables a mobile node to remain reachable
   while it roams on the Internet.  However, the location and movement
   of the mobile node can be revealed by the IP addresses used in
   signaling or data packets.  In this document, we consider the Mobile
   IPv6 location privacy problem described in RFC 4882, and propose
   efficient and secure techniques to protect location privacy of the
   mobile node.  This document is a product of the IP Mobility
   Optimizations (MobOpts) Research Group.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Research Task
   Force (IRTF).  The IRTF publishes the results of Internet-related
   research and development activities.  These results might not be
   suitable for deployment.  This RFC represents the consensus of the IP
   Mobility Optimizations Research Group of the Internet Research Task
   Force (IRTF).  Documents approved for publication by the IRSG are not
   a candidate for any level of Internet Standard; see Section 2 of RFC
   5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5726.










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Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.








































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Table of Contents

   1. Introduction ....................................................5
   2. Conventions and Terminology .....................................6
      2.1. Conventions ................................................6
      2.2. Terminology ................................................6
   3. Requirements ....................................................8
   4. Solution Overview ...............................................9
   5. Reverse-Tunneled Correspondent Binding Update ..................11
      5.1. The Procedure .............................................12
      5.2. Route-Optimized Payload Packets ...........................14
      5.3. Mobile Node Operation .....................................15
           5.3.1. Conceptual Data Structures .........................15
           5.3.2. Reverse-Tunneled Correspondent Binding
                  Update to the Correspondent Node ...................15
           5.3.3. Reverse-Tunneled Correspondent Binding
                  Acknowledgement from the Correspondent Node ........16
           5.3.4. Route-Optimized Payload Packets ....................16
           5.3.5. Receiving ICMP Error Message .......................17
           5.3.6. Binding Error from the Correspondent Node ..........17
           5.3.7. Binding Refresh Request from the
                  Correspondent Node .................................17
      5.4. Home Agent Operation ......................................17
      5.5. Correspondent Node Operation ..............................18
           5.5.1. Conceptual Data Structures .........................18
           5.5.2. Reverse-Tunneled Correspondent Binding
                  Update from the Mobile Node ........................18
           5.5.3. Reverse-tunneled Correspondent Binding
                  Acknowledgement to the Mobile Node .................18
           5.5.4. Route-Optimized Payload Packets ....................18
           5.5.5. ICMP Error Message to the Mobile Node ..............19
           5.5.6. Binding Error to the Mobile Node ...................19
           5.5.7. Binding Refresh Request to the Mobile Node .........19
      5.6. Summary ...................................................20
   6. IP Address Location Privacy Solution Using the Pseudo
      Home Address ...................................................20
      6.1. Home Binding Update .......................................20
           6.1.1. Pseudo Home Address Registration ...................20
           6.1.2. Home De-Registration ...............................21
      6.2. Correspondent Binding Update Using the Pseudo Home
           Address ...................................................22
           6.2.1. Return Routability Procedure .......................22
           6.2.2. Route-Optimized Correspondent Binding Update .......24
           6.2.3. Reverse-tunneled Correspondent Binding Update ......25
           6.2.4. Using Different Pseudo Home Addresses with
                  Different Correspondent Nodes ......................25
      6.3. Payload Packets ...........................................25
           6.3.1. Reverse Tunneling Mode .............................25



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           6.3.2. Route Optimization Mode ............................26
      6.4. Prefix Discovery ..........................................26
      6.5. Mobile Node Operation .....................................26
           6.5.1. Conceptual Data Structures .........................26
           6.5.2. Binding Update to the Home Agent ...................27
           6.5.3. Binding Acknowledgement from the Home Agent ........27
           6.5.4. Home Test Init to the Home Agent ...................28
           6.5.5. Home Test from the Home Agent ......................28
           6.5.6. Route-Optimized Payload Packets ....................29
           6.5.7. Receiving Binding Refresh Request ..................29
      6.6. Home Agent Operation ......................................29
           6.6.1. Conceptual Data Structures .........................30
           6.6.2. Binding Update from the Mobile Node ................30
           6.6.3. Binding Acknowledgement to the Mobile Node .........31
           6.6.4. Home Test Init from the Mobile Node ................31
           6.6.5. Home Test to the Mobile Node .......................32
      6.7. Correspondent Node Operation ..............................32
   7. Extensions to Mobile IPv6 ......................................32
      7.1. Encrypted Home Address Destination Option .................32
      7.2. Encrypted Home Address Routing Header .....................33
      7.3. Pseudo Home Address Mobility Option .......................34
      7.4. Pseudo Home Address Acknowledgement Mobility Option .......35
   8. Security Considerations ........................................37
      8.1. Home Binding Update .......................................37
      8.2. Correspondent Binding Update ..............................38
      8.3. Route-Optimized Payload Packets ...........................38
   9. Related Work ...................................................39
   10. IANA Considerations ...........................................40
   11. Conclusion ....................................................40
   12. Acknowledgements ..............................................41
   13. References ....................................................41
      13.1. Normative References .....................................41
      13.2. Informative References ...................................42
   Appendix A. Profiling Attack: Discussion ..........................44
     A.1. The Care-of Address ........................................44
     A.2. Profiling on the Encrypted Home Address ....................44
     A.3. The IPsec SPI ..............................................45
     A.4. The IPsec Sequence Number ..................................45
     A.5. The Regular Interval of Signaling Messages..................46
     A.6. The Sequence Number in the Binding Update Message ..........46
     A.7. Multiple Concurrent Sessions ...............................46
     A.8. Summary ....................................................47









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1.  Introduction

   The IP address location privacy problem is concerned with unwittingly
   revealing the current location of a mobile node to eavesdroppers and
   to communicating parties.  In the presence of mobility as specified
   in Mobile IPv6 [6], there are two related problems: disclosing the
   care-of address to a correspondent node, and revealing the home
   address to an eavesdropper (please see the terminology below).  A
   detailed description of the location privacy problem can be found in
   RFC 4882 [11].  This document assumes that the reader is familiar
   with the basic operation of Mobile IPv6 specified in RFC 3775, as
   well as the location privacy problem described in RFC 4882.

   In order to protect location privacy, a mobile node must not disclose
   the binding between its care-of address and its home address.  In
   this document, we propose a set of extensions to the Mobile IPv6
   specification to address the IP address location privacy problem.
   Related to the IP address location privacy is "profiling", where the
   activities of a mobile node are linked and then analyzed.  Profiled
   activities may contribute to compromising a mobile node's location
   privacy, especially when combined with additional information.
   Furthermore, once location privacy is compromised, it may lead to
   more targeted profiling.  Solutions to thwart profiling are
   important; however, they are not central to this document.  We
   discuss profiling in the appendix.

   We propose two IP address location privacy solutions in this
   document.  With the first solution (as described in Section 5), the
   mobile node can communicate with the correspondent node by using the
   real home address without location privacy being breached by
   eavesdroppers.  This is done by using parameters generated during the
   return routability procedure to mask the real home address, which
   provides an evolution towards location privacy protection based on
   return routability messages already specified in RFC 3775.  With the
   second solution (as described in Section 6), an IPsec tunnel mode
   security association with a non-null encryption algorithm is
   negotiated to encrypt signaling messages (including the real home
   address therein) exchanged between the mobile node and the home
   agent, for example, during the home binding update procedure.
   Furthermore, during the return routability procedure and the
   correspondent binding update procedure, a "pseudo home address" (the
   definition of this new term and many other commonly used mobility
   related terms is provided in Section 2) is used to replace the real
   home address in various messages, which allows the mobile node to
   hide its real home address from both the correspondent node and
   eavesdroppers without the need for additional extensions to the
   correspondent node.  Moreover, the mobile node may mask the pseudo




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   home address by using the mechanism specified in Section 5 to further
   enhance location privacy protection.  Each of these two solutions can
   be implemented on its own without relying on the other.

   The solutions presented in this document are designed based on the
   following assumptions.  First, we focus on location privacy issues
   arising when the mobile node attaches to a foreign link; location
   privacy issues when the mobile node attaches to its home link, if
   any, are outside the scope of this document.  Second, we assume that
   IPsec [2] is used to secure mobility signaling messages exchanged
   between the mobile node and the home agent; therefore, location
   privacy solutions when other security mechanisms are used are beyond
   the scope of this document.  Third, we assume that eavesdroppers are
   passive attackers, e.g., an eavesdropper along the path traversed by
   traffic flows from or to the mobile node.  We make this assumption
   because messages generated by active attackers can either be
   discarded based on local policy at a mobile node or the mobile node
   could choose to treat such messages like those of any other
   correspondent nodes.  Thus, specific threats to location privacy
   posed by active attackers are also beyond the scope of this document.
   Fourth, in order to simplify analysis, we assume that both the
   correspondent node and the home agent are fixed nodes; if either is
   mobile, the same analysis and solutions for the mobile node may also
   apply.  Finally, the same solution applies to each of the care-of
   addresses if a mobile node maintains more than one care-of address.

   This document represents the consensus of the MobOpts Research Group.
   It has been reviewed by the Research Group members active in the
   specific area of work.  At the request of their chairs, this document
   has been comprehensively reviewed by multiple active contributors to
   the IETF Mobile IP related working groups.

2.  Conventions and Terminology

2.1.  Conventions

   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 [1].

2.2.  Terminology

   In this document, we introduce two new terms, "pseudo home address"
   and "encrypted home address".  The definition of these two terms is
   provided in the following.






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   o  Pseudo Home Address (pHoA): A unicast IPv6 address formed to
      replace the real home address used in certain Mobile IPv6
      signaling or data packets.  Without explicit indication, the
      pseudo home address looks like a regular IPv6 address [5].

   o  Encrypted Home Address (eHoA): The output when applying an
      encryption algorithm to the real home address or the pseudo home
      address with additional inputs, e.g., a key.  The real home
      address can be recovered from the encrypted home address by using
      a decryption algorithm.

   In addition, we use commonly adopted mobility-related terms as
   defined in [6] and [11] throughout this document.  Some of these
   terms are provided below for easier reference.  Nevertheless, we
   assume that readers are familiar with the basic operation of the
   Mobile IPv6 protocol as defined in RFC 3775 [6], RFC 3776 [7], and
   RFC 4877 [8].

   o  Mobile Node (MN): A Mobile IPv6 compliant mobile node that can
      roam on the Internet

   o  Correspondent Node (CN): An IPv6 node that communicates with the
      mobile node

   o  Home Network: The network where the mobile node is normally
      present when it is not roaming

   o  Visited Network: The network that the mobile node uses to access
      the Internet when it is roaming

   o  Home Agent (HA): A router on the mobile node's home network that
      provides forwarding support when the mobile node is roaming

   o  Home Address (HoA): The mobile node's unicast IP address valid on
      its home network

   o  Care-of Address (CoA): The mobile node's unicast IP address valid
      on the visited network

   o  Return Routability (RR): A procedure which enables secure binding
      between the care-of address and the home address when no pre-
      existing security association exists between the mobile node and
      the correspondent node

   o  Home Test Init (HoTI) / Home Test (HoT) / Care-of Test Init (CoTI)
      / Care-of Test (CoT): Messages used during the return routability
      procedure




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   o  Binding Update (BU): A message used by the mobile node to securely
      bind its care-of address to its home address at the correspondent
      node or the home agent

   o  Binding Acknowledgement (BA): A response to the Binding Update

   o  Message Authentication Code (MAC): The value, which is computed
      using HMAC_SHA1 in this document, that protects both a message's
      integrity and its authenticity

   o  Route Optimization: A mechanism that allows direct routing of
      packets between a roaming mobile node and its correspondent node,
      without having to traverse the home network

   o  Reverse Tunneling or Bidirectional Tunneling: A mechanism used for
      packet forwarding between a roaming mobile node and its
      correspondent node via its home agent

3.  Requirements

   In this section, we describe the requirements that should be met by
   the Mobile IPv6 location privacy solutions, hereafter referred to as
   "the solution".  These are some of the basic requirements set forth
   in order to make the solution readily implementable by those familiar
   with Mobile IPv6 and the related security protocols used with it
   (such as IKEv2 [4] and IPsec).

   R01: The solution must follow the framework and architecture of IPv6
        and Mobile IPv6 (as specified in RFC 3775, RFC 3776, and RFC
        4877).

   R02: The solution must not interfere with the operation of IPsec.
        This means that the principles and the operation specified in
        RFC 3776 and RFC 4877 need to be followed.  For example, the
        IPsec security association and policy must be identified by the
        real home address.

   R03: The solution should provide back-compatibility in order for
        different Mobile IPv6 entities to work together even though they
        may have different capabilities.  This requires the mobile node
        to be able to detect whether the home agent or the correspondent
        node supports the use of the location privacy solutions.

   R04: The overhead resulting from the solution, in terms of payloads
        or messages transmitted and memory, should be kept minimal.






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4.  Solution Overview

   The IP address location privacy solutions proposed in this document
   intend to conceal the binding between the mobile node's real home
   address and its care-of address from eavesdroppers and the
   correspondent node.  In this section, we present an overview of the
   proposed solutions.

   With the Mobile IPv6 specification, during the home binding update
   procedure, both the real home address and the care-of address are in
   the cleartext when either the IPsec tunnel mode or the IPsec
   transport mode is used with no encryption.  As described in
   Section 6.1, the solution to prevent the real home address being
   leaked to eavesdroppers on the MN-HA path during the home binding
   update procedure is to set up an IPsec tunnel mode security
   association with a non-null encryption algorithm to encrypt home
   binding signaling messages and the real home address therein.  This
   method is also used to enable location privacy protection during
   other mobility signaling message exchanges between the home agent and
   the mobile node, such as the prefix discovery procedure (see
   Section 6.4).

   When communicating with the correspondent node with the reverse
   tunneling mode, the mobile node can hide its current location from
   the correspondent node and eavesdroppers along the HA-CN path, since
   the care-of address is not included in payload packets transmitted on
   that path.  Also, an IPsec security association with a non-null
   encryption algorithm established between the mobile node and the home
   agent can conceal the real home address carried in payload packets
   from eavesdroppers along the MN-HA path.

   In order to communicate with a correspondent node in the route
   optimization mode, the mobile node needs to perform the return
   routability procedure followed by the correspondent binding update
   procedure.  With the current Mobile IPv6 specification, the real home
   address and the care-of address in the correspondent Binding Update
   message and payload packets are visible to eavesdroppers.  Therefore,
   in order to send and receive packets through the optimized route and
   protect location privacy at the same time, the mobile node needs to
   disclose its care-of address and conceal its real home address.
   There are two different scenarios and we propose a different solution
   for each scenario.

   One scenario is that the correspondent node is able to obtain the
   mobile node's real home address and initiates communication with the
   mobile node by using the real home address.  In this case, the mobile
   node needs to continue to use the real home address with the
   correspondent node in order to maintain session continuity, and to



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   conceal the real home address from eavesdroppers.  The solution for
   this scenario (hereinafter referred to as "reverse-tunneled
   correspondent binding update") is described in Section 5.  With this
   solution, the mobile node exchanges the same return routability
   signaling messages as defined in RFC 3775 with the correspondent node
   and then derives a privacy management key from keygen tokens and uses
   this key to encrypt the real home address.  Finally, it reverse-
   tunnels an extended correspondent Binding Update message via the home
   agent to register the encrypted home address and the real home
   address at the correspondent node.  After the correspondent
   registration, the mobile node and the correspondent node use the
   registered encrypted home address, instead of the real home address
   in payload packets exchanged via the optimized route.  The encrypted
   home address is different for different correspondent nodes since the
   privacy management key would be different.

   The other scenario is that the mobile node prefers to conceal its
   real home address from both the correspondent node and the
   eavesdroppers (typically the mobile node initiates communication in
   this case, since the correspondent node does not know the real home
   address).  The solution for this scenario is described in
   Section 6.2.  With this solution, the mobile node first obtains a
   home keygen token generated based on the pseudo home address during
   the home address test procedure.  Subsequently, the mobile node sends
   the correspondent Binding Update message to register the binding
   between the pseudo home address and the care-of address at the
   correspondent node via the optimized route.  After the correspondent
   registration, the mobile node and the correspondent node use the
   registered pseudo home address, instead of the real home address, in
   payload packets exchanged via the optimized route.  Note that the use
   of the pseudo home address is completely transparent to the
   correspondent node.

   Furthermore, it is feasible to throttle "profiling" on the pseudo
   home address by using a combination of these two solutions.  That is,
   the mobile node uses the pseudo home address in the extended home
   address test procedure to obtain a home keygen token; then, it uses
   the pseudo home address instead of the real home address in the
   reverse-tunneled correspondent binding update procedure.  With this
   solution, the encrypted pseudo home address used in route optimized
   payload packets looks different to eavesdroppers each time, after a
   new round of the return routability procedure is completed.

   Before a pseudo home address is used with a correspondent node, it
   MUST be registered with the home agent during the home registration
   procedure.  The mobile node indicates the requested pseudo home
   address in a new mobility option, called the Pseudo Home Address
   option (see Section 7.3), carried in the home Binding Update message,



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   and the home agent indicates the status of pseudo home address
   registration in another new mobility option, called Pseudo Home
   Address Acknowledgement option (see Section 7.4), carried in the home
   Binding Acknowledgement message.  The pseudo home address MUST be
   routable in order for the home agent to intercept packets destined at
   this pseudo home address.  It is statistically difficult for other
   nodes to derive the real home address from the pseudo home address.
   A detailed description of pseudo home address generation is provided
   in Section 6.1.1.1.

   With extensions introduced in this document, a mobile node is able to
   discover whether the home agent and the correspondent node support
   the location privacy solutions or not.  When present in the home
   Binding Update message, the Pseudo Home Address mobility option
   indicates that the mobile node requests the use of the location
   privacy solutions.  If such a Binding Update message is valid and the
   home agent supports the location privacy solutions for this
   particular mobile node, it responds with the Pseudo Home Address
   Acknowledgement mobility option in the Binding Acknowledgement
   message.  Otherwise, if the home agent does not support the location
   privacy solutions, it does not include the Pseudo Home Address
   Acknowledgement mobility option in the Binding Acknowledgement
   message.  Similarly, the presence of the Encrypted Home Address
   destination option in the correspondent Binding Update message
   indicates to the correspondent node that the mobile node requests the
   use of the location privacy solutions.  If such a Binding Update
   message is valid and the correspondent node supports the location
   privacy solutions for this particular mobile node, it responds with
   the Encrypted Home Address routing header in the correspondent
   Binding Acknowledgement message to the mobile node.  If the
   correspondent node does not support the location privacy solutions,
   it rejects the mobile node's request by returning an ICMP Parameter
   Problem message with Code value set to 2.  Furthermore, a home agent
   that recognizes such extensions but does not wish to provide location
   privacy protection MAY redirect the mobile node to another home
   agent.  If the request for using the location privacy solutions is
   rejected, the mobile node may either proceed without location privacy
   protection, or try with a different home agent or a correspondent
   node, or abort the operation.

5.  Reverse-Tunneled Correspondent Binding Update

   In this section, we describe a solution that protects location
   privacy against eavesdroppers when the mobile node uses the real home
   address during communication with the correspondent node via the
   optimized route.  Note that this solution does not require any change
   to return routability signaling messages.  The detailed description
   is as follows.



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5.1.  The Procedure

   After the return routability procedure is completed, if the mobile
   node needs to protect location privacy, and at the same time still
   uses the real home address with the correspondent node, the mobile
   node derives a privacy management key, Kpm, from the Kbm, where Kpm =
   HMAC_SHA1 (Kbm, 0).  The mobile node uses Kpm to generate the
   encrypted home address as follows.

      encrypted home address = Enc(Kpm, the home address)

      Where Enc() is a symmetric key encryption algorithm.  AES is the
      default encryption algorithm.

   Kpm changes upon every change of Kbm, which itself changes when
   return routability is run (e.g., upon change of care-of address,
   expiry of keygen token, etc.).  So, Kpm is not re-used when a care-of
   address changes.

   The mobile node generates a correspondent Binding Update message and
   reverse-tunnels this message to the correspondent node via the home
   agent.  The format of this message after encapsulation is:

       IPv6 header (source = care-of address,
                    destination = home agent)
       ESP header in tunnel mode
       IPv6 header (source = home address,
                    destination = correspondent node)
       Destination option header
           Encrypted Home Address option (encrypted home address)
       Parameters:
           Alternative Care-of Address option (care-of address)
           sequence number (within the Binding Update message header)
           home nonce index (within the Nonce Indices option)
           care-of nonce index (within the Nonce Indices option)
           First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent
               | BU)))

   This packet is protected by the IPsec security association with a
   non-null encryption algorithm.  If the home agent can process this
   packet successfully, it forwards the following packet to the
   correspondent node.

       IPv6 header (source = home address,
                    destination = correspondent node)
       Destination option header
           Encrypted Home Address option (encrypted home address)




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       Parameters:
           Alternative Care-of Address option (care-of address)
           sequence number (within the Binding Update message header)
           home nonce index (within the Nonce Indices option)
           care-of nonce index (within the Nonce Indices option)
           First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent
               | BU)))

   After receiving a reverse-tunneled correspondent Binding Update
   message, the correspondent node performs the operation as described
   in Section 5.5.  If the correspondent Binding Update message is
   processed successfully and an acknowledgement is requested, the
   correspondent node constructs a Binding Acknowledgement message shown
   below.

       IPv6 header (source = correspondent node,
                    destination = home address)
       Encrypted Home Address routing header
           encrypted home address
       Parameters:
           sequence number (within the Binding Update message header)
           First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent
               | BA)))

   Upon receiving this Binding Acknowledgement message, the home agent
   applies the IPsec security association with a non-null encryption
   algorithm to this message and forwards the following packet to the
   mobile node.

       IPv6 header (source = home agent,
                    destination = care-of address)
       ESP header in tunnel mode
       IPv6 header (source = correspondent node,
                    destination = home address)
       Encrypted Home Address routing header
           encrypted home address
       Parameters:
           sequence number (within the Binding Update message header)
           First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent
               | BA)))

   The reverse-tunneled correspondent binding update procedure is
   completed after the mobile node processes the received Binding
   Acknowledgement message.  Note that when the mobile node communicates
   with a different correspondent node, the encrypted home address looks
   different.





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   To delete an established Binding Cache entry at the correspondent
   node, the mobile node reverse-tunnels the following Binding Update
   message via the home agent.  Note that the Encrypted Home Address
   option is optional during the correspondent binding de-registration
   and only the home keygen token is used to generate Kbm and Kpm, if
   needed, in this case.

       IPv6 header (source = care-of address,
                    destination = home agent)
       ESP header in tunnel mode
       IPv6 header (source = home address,
                    destination = correspondent node)
       Destination option header (optional)
           Encrypted Home Address option (encrypted home address)
       Parameters:
           Alternative Care-of Address option (care-of address)
           sequence number (within the Binding Update message header)
           home nonce index (within the Nonce Indices option)
           care-of nonce index (within the Nonce Indices option)
           First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent
               | BU)))

   If an acknowledgement is requested, the correspondent node returns
   the following Binding Acknowledgement message to the mobile node.

       IPv6 header (source = correspondent node,
                    destination = home address)
       Encrypted Home Address routing header (optional)
           encrypted home address
       Parameters:
           sequence number (within the Binding Update message header)
           First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent
               | BA)))

   Since the destination IP address in this message is the home address,
   the home agent will receive this message and forward it to the mobile
   node via the reverse tunnel.

5.2.  Route-Optimized Payload Packets

   After the correspondent registration is completed successfully,
   subsequent payload packets are exchanged via the optimized route
   between the mobile node and the correspondent node.  In such packets,
   only the encrypted home address carried in the Encrypted Home Address
   destination option and the Encrypted Home Address routing header are
   visible to eavesdroppers.





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   The format of payload packets sent from the mobile node to the
   correspondent node is:

       IPv6 header (source = care-of address,
                    destination = correspondent node)
       Destination option header
           Encrypted Home Address option (encrypted home address)
       Payload

   The format of payload packets sent from the correspondent node to the
   mobile node is:

       IPv6 header (source = correspondent node,
                    destination = care-of address)
       Encrypted Home Address routing header
           encrypted home address
       Payload

5.3.  Mobile Node Operation

5.3.1.  Conceptual Data Structures

   The Binding Update List entry for the correspondent registration is
   extended with a new field to store the current encrypted home address
   used with a particular correspondent node.  The encrypted home
   address is stored when the mobile node sends a reverse-tunneled
   correspondent Binding Update message, and the state of the
   corresponding Binding Update List entry is updated when the mobile
   node successfully processes the correspondent Binding Acknowledgement
   message.  Note that the encrypted home address field is not valid in
   the Binding Update List entry for the home registration.

   Given that the encrypted home address is 128 bits long, it is
   expected that each encrypted home address or the combination of the
   encrypted home address and the correspondent node's IP address stored
   in the Binding Update List is unique.  Therefore, the mobile node can
   use the encrypted home address (or use it together with the
   correspondent node's IP address) as a primary key to look up the
   Binding Update List.

5.3.2.  Reverse-Tunneled Correspondent Binding Update to the
        Correspondent Node

   After the return routability procedure, if the mobile node chooses to
   use the location privacy solution with the correspondent node, e.g.,
   based on the mobile node's configuration, it generates the encrypted
   home address, updates or creates a new correspondent Binding Update
   List entry to store the encrypted home address, then forwards the



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   correspondent Binding Update message through the reverse tunnel
   established with the home agent.  Note that the MAC is generated in
   the same way as specified in RFC 3775, and it does not cover the
   encrypted home address.

5.3.3.  Reverse-Tunneled Correspondent Binding Acknowledgement from the
        Correspondent Node

   When the mobile node receives a Binding Acknowledgement message from
   the correspondent node in response to a previously sent reverse-
   tunneled correspondent Binding Update message, if this Binding
   Acknowledgement message contains an Encrypted Home Address routing
   header, the mobile node considers that the correspondent node
   supports the location privacy solution.  The mobile node
   authenticates this message based on RFC 3775.  If authentication is
   successful, the mobile node decrypts the encrypted home address and
   compares the result with the real home address, or compares the
   encrypted home address with the one stored in the Binding Update List
   entry.  If they match, the mobile node considers that the
   correspondent registration is successful and updates the state of the
   corresponding Binding Update List entry.  If they do not match, the
   mobile node MAY start the correspondent binding update procedure
   again.

5.3.4.  Route-Optimized Payload Packets

   In order to maintain session continuity, upper layers of the IP stack
   in the mobile node still use the real home address, even after the
   reverse-tunneled correspondent registration.

   A possible way of implementation is as follows.  When the Mobile IP
   sublayer at the mobile node receives a packet from the upper layer,
   the normal processing as specified in RFC 3775 is performed.
   Subsequently, the Home Address option is replaced with the Encrypted
   Home Address option carrying the encrypted home address stored in the
   corresponding Binding Update List entry, and then the mobile node
   forwards the packet to the correspondent node via the optimized
   route.

   On the other hand, when the mobile node receives a payload packet
   carrying the Encrypted Home Address routing header, the mobile node
   uses the encrypted home address (optionally together with the IP
   address of the correspondent node) to look up the Binding Update
   List.  If an entry is found, the mobile node accepts this packet,
   replaces the Encrypted Home Address option with the Home Address
   option carrying the real home address, and continues with processing
   based on RFC 3775.  If no entry is found, the mobile node silently
   drops the received packet.



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5.3.5.  Receiving ICMP Error Message

   The mobile node may receive an ICMP Parameter Problem, Code 2,
   message forwarded by the home agent via the bidirectional tunnel, for
   example, when the correspondent node does not support the use of the
   Encrypted Home Address option.  If such a message is received, the
   mobile node SHOULD not attempt to use the location privacy solution
   with the correspondent node.  The mobile node may choose either not
   to communicate with the correspondent node, or to communicate without
   location privacy protection.

5.3.6.  Binding Error from the Correspondent Node

   When the mobile node communicates with a correspondent node by using
   the encrypted home address, a Binding Error message with the Status
   field set as 1 (unknown binding for Home Address destination option)
   may be received by the mobile node if there is no valid Binding Cache
   entry established at the correspondent node.  Note that we do not
   specify a new Status value to be used in this case because the
   implementation of the Binding Update List entry can contain an
   indication of whether an encrypted home address is currently used
   with the correspondent node.  Upon receiving the Binding Error
   message, the mobile node can find out which encrypted home address is
   invalid by looking at the Home Address field of the Binding Error
   message.  The mobile node may then perform the correspondent binding
   update procedure to establish a valid binding for the encrypted home
   address.

5.3.7.  Binding Refresh Request from the Correspondent Node

   When the mobile node receives a Binding Refresh Request message sent
   from the correspondent node and forwarded by the home agent via the
   bidirectional tunnel, the mobile node needs to perform the
   correspondent binding update procedure to refresh the binding for the
   encrypted home address at the correspondent node.

5.4.  Home Agent Operation

   With the solution described in this section (i.e., Section 5), there
   is no new home agent operation to be specified.  That is, the home
   agent behaves based on RFC 3775 when processing signaling or data
   packets.









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5.5.  Correspondent Node Operation

5.5.1.  Conceptual Data Structures

   The Binding Cache entry is extended with a new field to store the
   current encrypted home address used with a particular mobile node.
   The encrypted home address is stored when the correspondent node
   successfully processes a reverse-tunneled correspondent Binding
   Update message.

   Given that the encrypted home address is 128 bits long, it is
   expected that each encrypted home address or the combination of the
   care-of address and the encrypted home address stored in the Binding
   Cache entry is unique.  Therefore, the correspondent node can use the
   encrypted home address (or use it together with the care-of address)
   as a primary key to look up the Binding Cache.

5.5.2.  Reverse-Tunneled Correspondent Binding Update from the Mobile
        Node

   When receiving a reverse-tunneled Binding Update message with the
   Encrypted Home Address option, if the correspondent node supports the
   location privacy solution, it verifies the message by using the same
   method as defined in RFC 3775.  If this verification succeeds, the
   correspondent node generates Kpm and uses it to decrypt the encrypted
   home address, and compares the result with the source IP address.  If
   they match, the correspondent node stores the encrypted home address
   in the corresponding Binding Cache entry.

5.5.3.  Reverse-tunneled Correspondent Binding Acknowledgement to the
        Mobile Node

   If an acknowledgement to the reverse-tunneled correspondent Binding
   Update message is requested by the mobile node, the correspondent
   node returns a Binding Acknowledgement message with the Encrypted
   Home Address routing header, if it supports the location privacy
   solution.  The MAC in the Binding Acknowledgement message is
   generated in the same way as specified in RFC 3775 and does not cover
   the encrypted home address carried in the Encrypted Home Address
   routing header.

5.5.4.  Route-Optimized Payload Packets

   In order to maintain session continuity, upper layers of the IP stack
   in the correspondent node still use the real home address, even after
   the reverse-tunneled correspondent registration.





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   A possible way of implementation is as follows.  When the IP layer at
   the correspondent node finishes processing the packet received from
   the upper layer based on RFC 3775, the Type 2 routing header together
   with the real home address therein is replaced with the Encrypted
   Home Address routing header with the encrypted home address found in
   the corresponding Binding Cache entry.  Then, this packet is
   forwarded to the mobile node via the optimized route.

   On the other hand, when the correspondent node receives a payload
   packet with the Encrypted Home Address option, it uses the encrypted
   home address (optionally together with the care-of address of the
   mobile node) to look up the Binding Cache.  If there is an entry, the
   correspondent node replaces the Encrypted Home Address option with
   the Home Address option carrying the real home address before
   forwarding the packet to the upper layer.  If no matching entry is
   found, the correspondent node sends a Binding Error message to the
   source IP address, i.e., the care-of address of the mobile node.

5.5.5.  ICMP Error Message to the Mobile Node

   When receiving a reverse-tunneled correspondent Binding Update
   message with the Encrypted Home Address option, if the correspondent
   node does not support location privacy extensions, it sends an ICMP
   Parameter Problem, Code 2, message to the source IP address (i.e.,
   the home address of the mobile node) and the home agent then forwards
   this ICMP message to the mobile node via the bidirectional tunnel.

5.5.6.  Binding Error to the Mobile Node

   When the correspondent node receives a payload packet with the
   Encrypted Home Address option for which there is no valid Binding
   Cache entry, it returns a Binding Error message with the Status code
   set as 1 back to the source IP address of the packet.  Furthermore,
   the Home Address field in the Binding Error message MUST be copied
   from the Encrypted Home Address field in the Encrypted Home Address
   destination option of the offending packet, or set to the unspecified
   address if no such option appears in the packet.

5.5.7.  Binding Refresh Request to the Mobile Node

   When the correspondent node realizes that a Binding Cache entry is
   about to expire, it sends a Binding Refresh Request message to the
   real home address of the mobile node stored in the Binding Cache
   entry.







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5.6.  Summary

   With the solution in Section 5, the real home address is visible in
   the Binding Update and Binding Acknowledgement messages along the
   HA-CN path.  Like Mobile IPv6 itself, it has not been designed to
   change the communications between the home network and the
   correspondent node; the same issues would affect non-mobile hosts as
   well.  This solution meets all the requirements set forth for the
   location privacy solutions and provides a simple way to provide
   location privacy protection while allowing the use of the real home
   address with the correspondent node.

6.  IP Address Location Privacy Solution Using the Pseudo Home Address

6.1.  Home Binding Update

   When the mobile node attaches to a foreign link, it first performs
   the home binding update procedure for the real home address with the
   home agent, as specified in RFC 3775.  For hiding the real home
   address, we require the use of IPsec Encapsulating Security Payload
   (ESP) [3] in tunnel mode.  In order to provide location privacy, a
   non-null encryption transform must be used so that the real home
   address is encrypted and encapsulated, and made invisible to
   eavesdroppers on the MN-HA path.  The packet formats and processing
   rules are the same as specified in RFC 3775 and RFC 4877.

6.1.1.  Pseudo Home Address Registration

6.1.1.1.  Generation

   To protect location privacy in the route optimization mode, the
   mobile node replaces the real home address used in certain signaling
   and payload packets with the pseudo home address.  Different from the
   encrypted home address, the pseudo home address needs to be routable
   so that the home agent can intercept packets with the pseudo home
   address used as the destination address.  The pseudo home address is
   generated by concatenating one of the home network prefixes with a
   random bit string.  There are many ways to generate such a random bit
   string, for example, by using a random number generator or a secure
   encryption or hash algorithm.

   Using the pseudo home address instead of the real home address even
   in return routability and binding update to the correspondent has the
   following advantages.  First, the pseudo home address does not reveal
   the identity of a mobile node since it is not (or should not be)
   publicly known.  Hence, the signaling on the HA-CN is path is more
   secure since attackers will not be able to determine the identity of
   the mobile node based on the pseudo home address.  Second, the mobile



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   node can communicate with a correspondent without disclosing its real
   home address.  Finally, the chosen pseudo home address can be
   different with different correspondents for both signaling and data
   traffic purposes.

   The prefix used to form the pseudo home address MUST be managed by
   the same home agent so that it can forward the return routability
   messages.  Even though it does not have to be the same as that used
   in the real home address, the prefix is highly recommended to be
   different.  For instance, a home agent may use a different prefix
   pool for location privacy purposes for a set of mobile nodes.  This
   ensures that the real home address and the pseudo home address are
   not co-related (assuming the mobile node chooses different interface
   identifiers (IIDs)).

6.1.1.2.  Registration

   The mobile node MUST register the pseudo home address to be used with
   the home agent before actually using it with a correspondent node.
   To do so, the mobile node indicates a pseudo home address in the
   Pseudo Home Address mobility option in the Binding Update message
   sent to the home agent.  If the home agent supports the location
   privacy solution, it performs the Duplicate Address Detection to
   detect whether this pseudo home address conflicts with other pseudo
   home addresses submitted from different mobile nodes.  Based on the
   result, the home agent indicates whether to accept the pseudo home
   address by setting the appropriate status code in the Pseudo Home
   Address Acknowledgement option in the Binding Acknowledgement
   message.  If the home agent prefers the use of a different home
   network prefix from that of the requested pseudo home address, the
   home agent returns the new pseudo home address in the Pseudo Home
   Address Acknowledgement mobility option to the mobile node.

   The mobile node MAY register the pseudo home address when it is about
   to communicate with a correspondent node with location privacy
   protection.  The default lifetime of registered pseudo home addresses
   is the same as the Home Binding Cache entry; however, a mobile node
   may choose any value and a home agent may grant any value.  The
   mobile node can add or delete any pseudo home address by using the
   Pseudo Home Address mobility option in the home Binding Update
   message.  The home agent does not have to recover the real home
   address from the pseudo home address.

6.1.2.  Home De-Registration

   When the mobile node returns to its home link, the home de-
   registration procedure is the same as specified in RFC 3775, i.e.,
   the real home address is used as the source IP address in the Binding



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   Update message and the destination IP address in the Binding
   Acknowledgement message.  The de-registration of the real home
   address results in automatic de-registration of all pseudo home
   addresses.  When the mobile node decides to disconnect from the home
   agent while at its foreign link, the format of the Binding Update and
   Acknowledgement is the same as that defined for the home
   registration, except that the Lifetime field is set to zero.  The
   home agent deletes the corresponding Binding Cache entry including
   the registered pseudo home address, if any.

6.2.  Correspondent Binding Update Using the Pseudo Home Address

6.2.1.  Return Routability Procedure

   The location privacy solution specified in this section does not
   introduce any change to the care-of address test procedure as
   specified in RFC 3775.  In the following, we highlight the extensions
   to the home address test procedure, during which the mobile node
   obtains a home keygen token generated based on the pseudo home
   address.

   The mobile node generates and sends a Home Test Init message to the
   home agent.  The format of this message is:

       IPv6 header (source = care-of address, destination = home agent)
       ESP header in tunnel mode
       IPv6 header (source = home address, destination = correspondent)
       Mobility Header (HoTI)
           Home Init Cookie
           Pseudo Home Address mobility option (pseudo home address)

   The difference from what is specified in RFC 3775 is that the mobile
   node includes a Pseudo Home Address mobility option (see Section 7.3)
   in the Home Test Init message.  A new option for carrying the pseudo
   home address is necessary because the security association between
   the mobile node and the home agent is based on the real home address.
   The pseudo home address contained in the Pseudo Home Address option
   is selected by the mobile node from a set of pseudo home addresses
   that have been registered with the home agent during the home
   registration procedure.  Note that the Home Test Init message is
   protected by an IPsec security association in the ESP tunnel mode
   with a non-null encryption algorithm and a non-null authentication
   algorithm, as specified in RFC 3776.

   When receiving a Home Test Init message, the home agent performs the
   operation as specified in Section 6.6.4.  If this operation succeeds
   when the Pseudo Home Address mobility option is present in the Home
   Test Init message, the home agent generates a Home Test Init message



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   and forwards it to the correspondent node.  As shown in the
   following, the pseudo home address carried in the Pseudo Home Address
   mobility option is used as the source IP address in the forwarded
   Home Test Init message.

       IPv6 header (source = pseudo home address,
                    destination = correspondent)
       Mobility Header (HoTI)
           Home Init Cookie

   The forwarded Home Test Init message looks the same to the
   correspondent node as what is specified in RFC 3775 and the
   correspondent node does not realize that the pseudo home address is
   used, and just generates a home keygen token using the same algorithm
   as specified in RFC 3775.

      home keygen token =
          First (64, HMAC_SHA1 (Kcn, (pseudo home address | nonce | 0)))

   The correspondent node then replies with a Home Test message.  As
   shown in the following, the format of this message is the same as
   that specified in RFC 3776, and the pseudo home address is used as
   the destination IP address.

       IPv6 header (source = correspondent,
                    destination = pseudo home address)
       Mobility Header (HoT)
           Home Init Cookie
           Home Keygen Token
           Home Nonce Index

   When the home agent intercepts the Home Test message using proxy
   Neighbor Discovery, it performs the operation as specified in
   Section 6.6.5.  If this operation succeeds, the home agent generates
   the following Home Test message and forwards to the mobile node.

       IPv6 header (source = home agent, destination = care-of address)
       ESP header in tunnel mode
       IPv6 header (source = correspondent, destination = home address)
       Mobility Header (HoT)
           Home Init Cookie
           Home Keygen Token
           Home Nonce Index
           Pseudo Home Address Acknowledgement mobility option
              (pseudo home address)






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   When the mobile node receives the Home Test message, it performs
   operation as specified in Section 6.5.5.  If such operation succeeds,
   the mobile node obtains a home keygen token computed using the pseudo
   home address.  After the care-of address test is completed, the
   mobile node hashes the care-of keygen token and the home keygen token
   together to generate Kbm using the same method as specified in RFC
   3775.

6.2.2.  Route-Optimized Correspondent Binding Update

   In this procedure, the mobile node MUST use the same pseudo home
   address used during the home address test procedure.  The pseudo home
   address is carried in the Home Address option in the correspondent
   Binding Update message.

       IPv6 header (source = care-of address,
                    destination = correspondent)
       Destination option header
           Home Address destination option (pseudo home address)
       Parameters:
           sequence number (within the Binding Update message header)
           home nonce index (within the Nonce Indices option)
           care-of nonce index (within the Nonce Indices option)
           First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent
           | BU)))

   When the correspondent node receives the Binding Update message, it
   performs the same operation as specified in RFC 3775.  If such
   operation succeeds and an acknowledgement is requested by the mobile
   node, the correspondent node replies with the following Binding
   Acknowledgement message.

       IPv6 header (source = correspondent,
                    destination = care-of address)
       Parameters:
           sequence number (within the Binding Update message header)
           First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent
           | BA)))

   After the mobile node receives the Binding Acknowledgement message
   indicating that the correspondent registration succeeds, the mobile
   node can now use the pseudo home address for communicating with the
   correspondent node.








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   Such a Binding Update message may also be used by the mobile node to
   delete a previously established binding at the correspondent node.
   In this case, similar to what is specified in RFC 3775, Kbm is
   generated exclusively from the home keygen token that is based on the
   pseudo home address.

6.2.3.  Reverse-tunneled Correspondent Binding Update

   The mobile node may choose to use reverse tunneling for sending the
   Binding Update.  The format of messages during such a procedure is
   similar to what is described in Sections 5 and 6.2.1, except that a
   pseudo home address is used in place of the real home address.  The
   Encrypted Home Address destination and the Encrypted Home Address
   routing header SHOULD be used to carry the encrypted pseudo home
   address.

6.2.4.  Using Different Pseudo Home Addresses with Different
        Correspondent Nodes

   Based on its configuration and policy, the mobile node can choose to
   use the same or different pseudo home addresses when communicating
   with different correspondent nodes.  Using a different pseudo home
   address with each correspondent node may help prevent the mobile
   node's activities from being linked and correlated.  To do so, the
   mobile node selects a different but already registered pseudo home
   address and repeats the return routability procedure and the
   correspondent binding update procedure with each correspondent node.

   In addition, if the mobile node prefers, it MAY use different pseudo
   home addresses for different sessions with the same correspondent
   node.  This typically requires additional configuration at the mobile
   node that associates a specific session (for example, identified by
   the port number and the protocol number, among others) with a
   specific pseudo home address.  This document does not address details
   of this solution.

6.3.  Payload Packets

6.3.1.  Reverse Tunneling Mode

   The format of payload packets reverse-tunneled via the home agent is
   the same as that specified for the home address test procedure in
   Section 6.2.1.








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6.3.2.  Route Optimization Mode

   When the route-optimized correspondent binding update procedure is
   performed, the format of payload packets exchanged between the mobile
   node and the correspondent node is the same as specified in RFC 3775.
   The operation of the mobile node when communicating with the
   correspondent node via the route optimization mode is described in
   Section 6.5.6.

   When the reverse tunneled correspondent binding update procedure is
   performed, the format of payload packets exchanged between the mobile
   node and the correspondent node is the same as specified in Section
   5, except that the encrypted pseudo home address SHOULD be included
   in the Encrypted Home Address destination option and the Encrypted
   Home Address routing header.

6.4.  Prefix Discovery

   The solution to protect location privacy during the prefix discovery
   procedure is similar to that used during the home binding update
   procedure.

6.5.  Mobile Node Operation

   In this section, we describe the mobile node's operation when the
   location privacy solution is used.

6.5.1.  Conceptual Data Structures

6.5.1.1.  Pseudo Home Address Table

   We introduce a new data structure, called Pseudo Home Address table,
   to record the information of pseudo home addresses.  The mobile node
   may maintain a Pseudo Home Address table for each home agent it
   registers with.  Each entry in the table contains a pseudo home
   address and its associated state, i.e., "unconfirmed" or "confirmed".
   The mobile node creates or updates entries in the Pseudo Home Address
   table when sending the home Binding Update message or receiving the
   home Binding Acknowledgement message.  The pseudo home address can be
   used as a key to search the table.  There MUST NOT be any duplicated
   pseudo home addresses stored in the Pseudo Home Address table.

6.5.1.2.  Binding Update List

   The Binding Update List entry is extended with a field, called Pseudo
   Home Address.  This field MAY be implemented as a pointer that points
   to a corresponding entry in the Pseudo Home Address table.  This
   pointer is initialized as NULL when the Binding Update List entry is



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   created (for example, when the mobile node sends a Binding Update
   message or a Home Test Init message to the home agent).  For the
   binding sent to a specific home agent, the Pseudo Home Address field
   points to the first entry in the Pseudo Home Address table (or NULL
   if the table is empty), so that the mobile node can access all the
   pseudo home addresses registered at this home agent; on the other
   hand, for the binding sent to a specific correspondent node, the
   Pseudo Home Address field points to the Pseudo Home Address table
   entry that contains the actual pseudo home address used with this
   correspondent node (or NULL if no pseudo home address is used with
   this correspondent node).

6.5.2.  Binding Update to the Home Agent

   The mobile node may decide to perform the home registration with
   location privacy protection, for example, when it attaches to a
   foreign link or when it needs to extend the lifetime of a registered
   home binding.

   Since IPsec tunnel mode is used, the mobile node MUST negotiate a
   non-null encryption algorithm (for example, during the bootstrapping)
   and use it to protect the home Binding Update message as specified in
   RFC 3775 and RFC 4877.  In addition, the mobile node can register a
   pseudo home address as described above.  If the mobile node does not
   wish to register the pseudo home address at this point, but wishes to
   discover whether the home agent supports the location privacy
   solution, the mobile node includes a Pseudo Home Address mobility
   option without the Pseudo Home Address field in the Binding Update
   message sent to the home agent.

   After sending the home de-registration binding update message, in
   addition to the operation specified in RFC 3775, the mobile node MUST
   stop using any data structure specific to the location privacy
   solution and MAY delete them after the Binding Acknowledgement
   message is processed successfully.

6.5.3.  Binding Acknowledgement from the Home Agent

   With IPsec tunnel mode, the mobile node follows the rules specified
   in RFC 3775 and RFC 4877 to process the Binding Acknowledgement
   message.

   In addition, if one or more Pseudo Home Address Acknowledgement
   mobility options are present in the Binding Acknowledgement message,
   the mobile node checks the Status field in each option.  If the
   Status field in one option is 0 (Success), the pseudo home address,
   if not already present, is added into the Pseudo Home Address table,
   and the state of the corresponding entry is set to "confirmed".



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   Otherwise, the mobile node deletes any existing pseudo home address
   with the "unconfirmed" state (i.e., either an error code or no
   acknowledgement for such a pseudo home address is received) from the
   Pseudo Home Address table.

   The mobile node considers that the home agent supports the location
   privacy solution, if a valid Pseudo Home Address Acknowledgement
   mobility option with or without a Pseudo Home Address field is
   received.

   Note that the mobile node MUST determine whether the home
   registration succeeds or not based on what is specified RFC 3775.

6.5.4.  Home Test Init to the Home Agent

   To enable location privacy protection during communication with the
   correspondent node in the route optimization mode, the mobile node
   generates a Home Test Init message based on what is specified in RFC
   3775 and RFC 3776.  In addition, if the return routability procedure
   is for a new session with the correspondent node, the mobile node
   selects any pseudo home address from those already registered with
   the home agent and stored in the Pseudo Home Address table;
   otherwise, the mobile node must use the same pseudo home address as
   used with the same correspondent node before.  The selected pseudo
   home address is carried in the Pseudo Home Address mobility option of
   the generated Home Test Init message.  This Home Test Init message is
   protected by an IPsec security association with a non-null encryption
   algorithm.

   After sending the Home Test Init message to the home agent, if there
   is no Binding Update List entry existing for the correspondent node,
   the mobile node creates one entry that points to the pseudo home
   address used; otherwise, the mobile node updates the existing entry.

6.5.5.  Home Test from the Home Agent

   When the mobile node receives a Home Test message from the home
   agent, it processes the packet based on processing rules specified in
   RFC 3775 and RFC 3776.  If this is a valid packet and there is a
   Pseudo Home Address Acknowledgement option included, the mobile node
   examines the Status field inside this mobility option as follows:

   o  If the Status field indicates that the home address test procedure
      using the pseudo home address succeeds (the Status field is 0), in
      addition to what is specified in RFC 3775, the mobile node
      prepares to use the pseudo home address carried in the Pseudo Home
      Address Acknowledgement option for the correspondent registration.




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   o  If the Status field indicates that the home address test procedure
      using the pseudo home address fails (the Status field is larger
      than 127), the mobile node can take steps to correct the cause of
      the error and retransmit the Home Test Init message, subject to
      the retransmission limit specified in RFC 3775.  If this is not
      done or it fails, then the mobile node SHOULD record in its
      Binding Update List that the future home address test procedure
      SHOULD NOT use the pseudo home address with this correspondent
      node.

6.5.6.  Route-Optimized Payload Packets

   After the mobile node completes the route-optimized correspondent
   registration procedure using the pseudo home address, payload packets
   are sent to the correspondent node with the pseudo home address in
   the Home Address destination option.

   The packet processing rules when sending and receiving route-
   optimized packets are the same as in RFC 3775 except that pseudo home
   addresses are used.  In addition, if encrypted pseudo home addresses
   are used, both the mobile node and the correspondent node need to
   replace the encrypted address with the pseudo home address before
   passing them to the upper layers.

   In the case that the mobile node masks the pseudo home address and
   uses the reverse-tunneled correspondent binding update procedure, the
   mobile node performs the operation specified in Section 5.3.4, except
   that the pseudo home address rather than the real home address is
   expected.

6.5.7.  Receiving Binding Refresh Request

   When the Mobile Node receives a Binding Refresh Request message from
   a correspondent node, the destination IP address may be the pseudo
   home address.  In this case, the mobile node needs to check the
   corresponding Binding Update List entry for the correspondent node.
   If the pseudo home address is invalid, the mobile node silently
   discards this message.  Otherwise, the mobile node refreshes the
   binding with the correspondent node by using the same pseudo home
   address.

6.6.  Home Agent Operation

   In this section, we describe the home agent's operation when the
   location privacy solution is used.






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6.6.1.  Conceptual Data Structures

   The Binding Cache entry is extended with a field that points to a
   list of currently accepted pseudo home addresses.  Note that each
   registered pseudo home address MUST be unique and all the registered
   pseudo home addresses SHOULD be organized in such a way that the
   associated Binding Cache entry can be quickly located when a pseudo
   home address is used as the key to look up the Binding Cache.

6.6.2.  Binding Update from the Mobile Node

   If the received Binding Update message contains one or more Pseudo
   Home Address mobility options, the home agent MUST ignore such an
   option if it does not recognize it.  If the home agent recognizes
   such an option, a Pseudo Home Address Acknowledgement mobility option
   is generated and some fields therein are set as follows:

   o  If the Pseudo Home Address field received is empty, the Status
      field is set to 0 (Success), and the Pseudo Home Address field is
      empty.

   o  If the Pseudo Home Address field received is set to all zero, the
      Status field is set is 0 (Success), and a pseudo home address
      SHOULD be included in the Pseudo Home Address field, if the home
      agent supports the dynamic pseudo home address assignment;
      otherwise, the Status field is set to 132 (Dynamic pseudo home
      address assignment not available) and the Pseudo Home Address
      field is empty.

   o  The Pseudo Home Address field received may contain an IPv6
      address.  If the format of such an IP address is incorrect, the
      Status field is set to 130 (Incorrect pseudo home address).  If
      such an IP address is invalid, for example, the prefix is not a
      valid home network prefix or this is detected as a duplicated IP
      address, the Status field is set to 131 (Invalid pseudo home
      address).  In both cases, the Pseudo Home Address field is empty.
      If the home agent suggests a different pseudo home address, the
      Status field is set to 0 (Success), and the new pseudo home
      address is included in the Pseudo Home Address field.  Otherwise,
      if the home agent accepts the requested pseudo home address, the
      Status field is set as 0 (Success), and the same IP address is
      included in the Pseudo Home Address field.

   o  If the home agent does not allow the mobile node to use the pseudo
      home address with the correspondent node, the Status field SHOULD
      be set as 129 (Administratively prohibited) and the Pseudo Home
      Address field is empty.




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   o  In case that the home agent does not accept the Pseudo Home
      Address mobility option for all other reasons, the Status field
      SHOULD be set as 128 (Failure, reason unspecified) and the Pseudo
      Home Address is empty.

   When receiving a Binding Update message protected with the IPsec
   tunnel mode, the home agent performs the operation specified in RFC
   4877.

   When receiving and successfully processing a Binding Update message
   for de-registration from the mobile node, in addition to what is
   specified in RFC 3775, the home agent MUST delete data structures
   related to the location privacy extension.

6.6.3.  Binding Acknowledgement to the Mobile Node

   When sending a Binding Acknowledgement message protected with the
   IPsec tunnel mode, the home agent performs the operation specified in
   RFC 4877.

   The processing rules related to the Pseudo Home Address
   Acknowledgement mobility option are described in Section 6.6.2.

6.6.4.  Home Test Init from the Mobile Node

   When receiving a Home Test Init message from the mobile node, the
   home agent first verifies this message based on the IPsec processing
   rules as specified in RFC 3776.  If the verification fails, the home
   agent acts based on such IPsec processing rules.  Otherwise, if the
   Pseudo Home Address option does not exist in the Home Test Init
   message, the home agent performs the operation as specified in RFC
   3775.  Otherwise, the following operation is performed.

   1.  The home agent looks up its Binding Cache by using the real home
       address as a key.  If the pseudo home address carried in the
       Pseudo Home Address option does not match any pseudo home address
       associated with the corresponding Binding Cache entry (including
       when the Pseudo Home Address field is set as zero), it MUST
       reject the Home Test Init message by sending back a Home Test
       message including the Pseudo Home Address Acknowledgement option
       with the Status field set as 131 (Invalid pseudo home address).

   2.  Otherwise, the home agent constructs a Home Test Init message
       with the pseudo home address as the source IP address, and
       forwards the Home Test Init message to the correspondent node.






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6.6.5.  Home Test to the Mobile Node

   When the home agent intercepts a Home Test message using proxy
   Neighbor Discovery, if the destination IP address matches with one of
   the real home addresses, the home agent performs the operation as
   specified in RFC 3775.  Otherwise, the home agent uses the
   destination IP address to look up the Binding Cache to find if there
   is a matched pseudo home addresses.  If not, the home agent discards
   this message silently.  When a matching pseudo home address is found,
   the home agent generates a Home Test message with a Pseudo Home
   Address Acknowledgement option and sends it to the mobile node.
   Inside the Pseudo Home Address Acknowledgement option, the Status
   field is set to zero (Success) and the Pseudo Home Address field is
   filled with the found pseudo home address.

6.7.  Correspondent Node Operation

   With the solution described in this section, when the correspondent
   node is involved in the route-optimized correspondent binding update
   procedure, there is no new operation if only pseudo home addresses
   are used without encryption.  This specification recommends using
   encrypted pseudo home addresses to thwart revealing any prefix
   information about a mobile node.  The additional operations are the
   same as specified in Section 5.5, except that the pseudo home
   address, instead of the real home address, is used.

7.  Extensions to Mobile IPv6

   This section describes the experimental extensions to Mobile IPv6
   used in this document.  For experimentation purposes, the
   experimental IPv6 Option Type, the experimental IPv6 Routing Header
   Type, and the experimental Mobility Option Type as defined in RFC
   4727 [12] and RFC 5096 [13] can be used in the Encrypted Home Address
   destination option, the Encrypted Home Address routing header, the
   Pseudo Home Address mobility option, and the Pseudo Home Address
   Acknowledgement mobility option.  In the following, we describe the
   format of each extension for illustration purpose.

7.1.  Encrypted Home Address Destination Option

   This option is used in the Destination Option extension header (Next
   Header value = 60).









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       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 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                                                               |
      +                    Encrypted Home Address                     +
      |                                                               |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Option Type

         A type for identifying the use of the encrypted home address in
         this option.  Implementations of this RFC can use the value
         0xFE.  This value is reserved in RFC 4727 for all experiments
         involving IPv6 destination options.

      Encrypted Home Address

         The encrypted home address generated from a either real or
         pseudo home address.

   The processing of other fields in the Encrypted Home Address option
   is the same as that of those fields in the Home Address option
   described in RFC 3775.  Note that if the Encrypted Home Address
   option is present in a packet, the encrypted home address therein
   MUST NOT be treated as the real source IP address by the receiver.

7.2.  Encrypted Home Address Routing Header

   The encrypted home address is carried in this routing header.

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Next Header  | Hdr Ext Len=2 | Routing Type  |Segments Left=1|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                                                               |
      +                   Encrypted Home Address                      +
      |                                                               |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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      Routing Type

         A type for identifying the use of the encrypted home address in
         this option.  Implementations of this RFC can use the value
         0xFE.  This value is reserved in RFC 4727 for all experiments
         involving IPv6 routing header.

      Encrypted Home Address

         The encrypted home address generated from a either real or
         pseudo home address.

   The processing of other fields in the Encrypted Home Address routing
   header is the same as described in RFC 3775.  Note that if this
   routing header is present in a packet, the encrypted home address
   therein MUST NOT be treated as the real destination IP address by the
   receiver.

7.3.  Pseudo Home Address Mobility Option

   This mobility option is included in the mobility header, including
   the Binding Update message and the Home Test Init message, and
   carries zero or one pseudo home address.  The alignment requirement
   for this option is 4n.

       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      |A|   Reserved  | Prefix length |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                                                               |
      +                     Pseudo Home Address                       +
      |                                                               |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type

         A unique type (together with the 'A' bit in the Reserved field)
         for identifying the Pseudo Home Address Acknowledgement
         mobility option.  For experimental purpose, the value of this
         type is 18 as reserved in RFC 5096.






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      Length

         The length of the Pseudo Home Address mobility option excluding
         the Type field and the Length field.  It MUST be 2 when the
         Pseudo Home Address field is not present; otherwise, it MUST be
         18.

      Reserved Field

         The 'A' bit, which MUST be set to zero to indicate that this is
         a Pseudo Home Address mobility option.  The rest of bits MUST
         be set as zero by the sender and ignored by the receiver.

      Prefix Length

         The length of the home network prefix of the included pseudo
         home address.  When the Pseudo Home Address field is not
         present, the Prefix Length field MUST be set as zero.

      Pseudo Home Address

         If present, the field contains a pseudo home address that the
         mobile node wants to use for location privacy protection or
         zero if the mobile node requests a pseudo home address from the
         home agent.  This field is not present if the mobile node only
         intends to discover whether the home agent supports the
         location privacy solutions.  The Length field is used to detect
         whether the Pseudo Home Address field is present in the Pseudo
         Home Address mobility option.

7.4.  Pseudo Home Address Acknowledgement Mobility Option

   This mobility option is included in the mobility header, including
   the Binding Acknowledgement message and the Home Test message sent to
   the mobile node, and carries zero or one pseudo home address.  This
   mobility option is used to indicate the status of the pseudo home
   address registration and/or whether the home agent supports the
   location privacy solutions.  The alignment requirement for this
   option is 2n.












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       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     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |A|   Reserved  | Prefix length |    Status     |   Reserved    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                                                               |
      +                     Pseudo Home Address                       +
      |                                                               |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type

         A unique type (together with the 'A' bit in the Reserved field)
         for identifying the Pseudo Home Address Acknowledgement
         mobility option.  For experimental purpose, the value of this
         type is 18 as reserved in RFC 5096.

      Length

         The length of the Pseudo Home Address Acknowledgement mobility
         option excluding the Type field and the Length field.  It MUST
         be 4 when the Pseudo Home Address field is not present;
         otherwise, it MUST be 20.

      Reserved

         The 'A' bit, which MUST be set to one to indicate that this is
         a Pseudo Home Address Acknowledgement mobility option.  The
         rest of bits MUST be set as zero by the sender and ignored by
         the receiver.

      Prefix Length

         The length of the home network prefix of the included pseudo
         home address.  When the Pseudo Home Address field is not
         present, the Prefix Length MUST be set as zero.

      Status

         It indicates the status of the pseudo home address
         registration.  Values from 0 to 127 indicate success.  Higher
         values indicate failure.  The following values are reserved:



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                0   Success
                128 Failure, reason unspecified
                129 Administratively prohibited
                130 Incorrect pseudo home address
                131 Invalid pseudo home address
                132 Dynamic pseudo home address assignment not available

      Reserved

         This field is reserved for future use.  It MUST be set to zero
         by the sender and ignored by the receiver.

      Pseudo Home Address

         If present, the field contains a pseudo home address that the
         home agent registers for the mobile node to use for location
         privacy protection.  This field is not present when the home
         agent only needs to indicate that it supports the location
         privacy solutions as a response to the query from the mobile
         node.  The Length field is used to detect whether the Pseudo
         Home Address field is present in the Pseudo Home Address
         Acknowledgement mobility option.

8.  Security Considerations

   The solutions proposed in this document address one of the security
   issues in the mobile environment, i.e., location privacy.  Throughout
   the document, we provide a detailed analysis of how the proposed
   solutions address the location privacy problem.  We carefully design
   such solutions to make sure that they fit well into the Mobile IPv6
   framework; therefore, the same threat analysis, security mechanisms
   (such as IPsec, the sequence number in binding signaling messages,
   the return routability procedure), and considerations as described in
   RFC 3775 still apply.  Nevertheless, in the following we provide an
   in-depth analysis on security threats involving the use of the
   location privacy solutions and demonstrate that the proposed
   solutions do not introduce any new vulnerability or weaken the
   strength of security protection of the original Mobile IPv6 protocol.

8.1.  Home Binding Update

   Given the strong security of the cryptography algorithm used to
   generate the encrypted home address, eavesdroppers are unable to
   derive the real home address from the encrypted home address and thus
   to correlate the care-of address with the real home address.
   Moreover, the encrypted home address can be updated to prevent
   eavesdroppers from linking the mobile node's ongoing activities.




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   During the pseudo home address registration, the home agent verifies
   that the requested pseudo home address is not in use by other mobile
   nodes; therefore, the other mobile node cannot, inadvertently or
   maliciously, intercept ongoing sessions of a victim mobile node by
   registering the same pseudo home address.

   A mobile node may attempt to register a large number of pseudo home
   addresses that may exhaust the pool of available pseudo home
   addresses and prevent other mobile nodes using location privacy
   protection.  The home agent MUST limit the number of pseudo home
   addresses that can be requested by a mobile node.  Also, with the
   IPsec security association between the home agent and the mobile
   node, if any misuse of the pseudo home address registration is
   detected, the home agent can identify the malicious mobile node and
   take further actions.

8.2.  Correspondent Binding Update

   The return routability procedure using the pseudo home address
   follows the same principle of the original return routability
   procedure, i.e., the message exchange verifies that the mobile node
   is reachable at both the pseudo home address and the care-of address
   (this is because the pseudo home address is required to be routable).
   Furthermore, the extended return routability procedure also utilizes
   the same security mechanisms as defined in RFC 3775, such as the
   nonce, the node key, and the sequence number, to protect against
   attacks.  Overall, it provides the same security strength as the
   original return routability procedure.

   The reverse-tunneled correspondent binding update procedure does not
   weaken security either.  Although the real home address is
   transferred in cleartext on the HA-CN path, eavesdroppers on this
   path can already perform more serious attacks against the mobile node
   with the Mobile IPv6 protocol.

8.3.  Route-Optimized Payload Packets

   Using the Encrypted Home Address option in route-optimized packets
   results in the same security implications when the Home Address
   option is used in such packets.  For example, the Encrypted Home
   Address option may be used by attackers to launch reflection attacks,
   e.g., by indicating the IP address of a victim node in the Encrypted
   Home Address option.  Similar to the processing rule for the Home
   Address option specified in RFC 3775, this document restricts the use
   of the Encrypted Home Address option: it can be used only if there is
   an established Binding Cache entry containing the encrypted (pseudo)
   home address.




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   With the proposed location privacy solutions, the Encrypted Home
   Address routing header is used to carry the encrypted (pseudo) home
   address.  The same threats specified in RFC 3775 for the Type 2
   routing header are also possible when the routing header carries the
   encrypted (pseudo) home address.  Similar processing rules are also
   used in this document to address such a threat: if the encrypted
   (pseudo) home address in the Encrypted Home Address routing header
   does not match with that stored in the Binding Update List entry, the
   packet will be dropped.

9.  Related Work

   Our work benefits from previous work and discussion on this topic.
   Similar to the concept of the pseudo home address, many documents
   have proposed using a temporary identity to replace the mobile node's
   home address in the IPsec security association, Mobile IPv6 signaling
   messages, and data packets.  However, the details of how to generate
   and update this identity are absent.  In the following, we provide a
   survey of related work.

   RFC 4941 [10] specifies a mechanism to generate randomized interface
   identifiers, which can be used to update the care-of address and the
   home address.  However, with our solution, the prefix of a pseudo
   home address can be different from that of the real home address and
   other pseudo home addresses, which prevents eavesdroppers from
   correlating and analyzing IP traffic based on a common prefix.
   Furthermore, we also discuss the interval of IP address update in the
   mobility scenario in order to resist the profiling attack both
   effectively and efficiently.

   In [16], the authors propose using a temporary identity, called the
   Temporary Mobile Identifier (TMI), to replace the home address, and
   discussed the feasibility of utilizing the Crypto-Based Identifier
   (CBID), Cryptographically Generated Addresses (CGA), or Mobility
   Anchor Point (MAP) to further protect location privacy.  However, as
   a 128-bit random number, the TMI is not routable; therefore, it is
   not suitable to be the source IP address in the Home Test Init
   message forwarded by the home agent to the correspondent node.
   Otherwise, the home agent cannot receive the Home Test message from
   the correspondent node.  Furthermore, the document does not specify
   how to update the TMI to address the profiling attack.

   In [14], the authors propose a mechanism that uses an identity as the
   home address and periodically updates such an identity by using a key
   and a previous identity as inputs to a cryptography algorithm.






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   In [15], the authors propose to update the mobile node's home address
   periodically to hide its movement.  The new home address is generated
   from the current local network prefix, the Binding Update session
   key, and the previous home address, and updated every time when the
   return routability procedure is performed.  The generated home
   address is random, routable, recognizable, and recoverable.

   In [18], the authors propose a mechanism to achieve both route
   optimization and location privacy at the same time.  This is done by
   discovering a tunneling agent near the correspondent node and
   bidirectionally tunneling data traffic between the mobile node and
   the tunneling agent.

10.  IANA Considerations

   This document creates a new registry "Pseudo Home Address
   Acknowledgement Status Codes" for the Status field in the Pseudo Home
   Address Acknowledgement mobility option.  The current values are
   described in Section 7.4 and are the following:

      0   Success

      128 Failure, reason unspecified

      129 Administratively prohibited

      130 Incorrect pseudo home address

      131 Invalid pseudo home address

      132 Dynamic pseudo home address assignment not available

11.  Conclusion

   In this document, we have proposed solutions to address location
   privacy issues in the context of mobility.  The main idea is to hide
   the binding between the home address and the care-of address from
   eavesdroppers and the correspondent node.  We have described two
   methods.  The first method extends the return routability to hide the
   real home address in Binding Update and data packets.  This method
   uses the real home address in return routability signaling, and does
   not require any changes to the home agent.  The second method uses
   pseudo home addresses starting from return routability signaling, and
   requires some extensions to the home agent operation.  This method
   protects revealing the real home address on the HA-CN path.  The two
   methods provide a means to hide the real home address from
   eavesdroppers, and the second method can also hide it from the
   correspondents.



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   The solutions we have proposed are for the basic Mobile IPv6 protocol
   as specified in RFC 3775.  Recently, many extensions to Mobile IPv6
   have been proposed, such as the NEMO Basic Support protocol [19],
   Dual Stack Mobile IPv6 Support [20], Multiple Care-of Addresses
   Registration [21], Binding Revocation [22], Generic Signaling Message
   [23].  It is expected that the proposed location privacy solutions
   can be applied with some modifications, if needed, to address
   location privacy issues when these extensions are used.  One of our
   future works is to clarify related issues, if any, when the location
   privacy solutions are used with new Mobile IPv6 extensions.

12.  Acknowledgements

   The authors would like to thank the co-authors of previous documents
   from which this document is derived: Vijay Devarapalli, Hannu Flinck,
   Charlie Perkins, Feng Bao, Robert Deng, James Kempf, and Jianying
   Zhou.  In addition, sincere appreciation is also extended to Claude
   Castelluccia, Francis Dupont, Gabriel Montenegro, Greg Daley, Kilian
   Weniger, Takashi Aramaki, Wassim Haddad, Heejin Jang, and Michael
   Welzl for their valuable contributions, review, and discussion.  Work
   by Fan Zhao was done while he was at University of California, Davis
   and Marvell Semiconductor, Inc.

13.  References

13.1.  Normative References

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

   [2]   Kent, S. and K. Seo, "Security Architecture for the Internet
         Protocol", RFC 4301, December 2005.

   [3]   Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303,
         December 2005.

   [4]   Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
         RFC 4306, December 2005.

   [5]   Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
         Specification", RFC 2460, December 1998.

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

   [7]   Arkko, J., Devarapalli, V., and F. Dupont, "Using IPsec to
         Protect Mobile IPv6 Signaling Between Mobile Nodes and Home
         Agents", RFC 3776, June 2004.



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   [8]   Devarapalli, V. and F. Dupont, "Mobile IPv6 Operation with
         IKEv2 and the revised IPsec Architecture", RFC 4877,
         April 2007.

   [9]   Hinden, R. and S. Deering, "IP Version 6 Addressing
         Architecture", RFC 4291, February 2006.

   [10]  Narten, T., Draves, R., and S. Krishnan, "Privacy Extensions
         for Stateless Address Autoconfiguration in IPv6", RFC 4941,
         September 2007.

   [11]  Koodli, R., "IP Address Location Privacy and Mobile IPv6:
         Problem Statement", RFC 4882, March 2007.

   [12]  Fenner, B., "Experimental Values in IPv4, IPv6, ICMPv4, ICMPv6,
         UDP, and TCP Headers", RFC 4727, November 2006.

   [13]  Devarapalli, V., "Mobile IPv6 Experimental Messages", RFC 5096,
         December 2007.

13.2.  Informative References

   [14]  Bao, F., Deng, R., Kempf, J., Qiu, Y., and J. Zhou, "Protocol
         for Protecting Movement of Mobile Nodes in Mobile IPv6", Work
         in Progress, March 2005.

   [15]  Bao, F., Deng, R., Kempf, J., Qiu, Y., and J. Zhou, "Protocol
         for Hiding Movement of Mobile Nodes in Mobile IPv6", Work
         in Progress, March 2005.

   [16]  Castelluccia, C., Dupont, F., and G. Montenegro, "A Simple
         Privacy Extension for Mobile IPv6", Work in Progress,
         July 2006.

   [17]  Daley, G., "Location Privacy and Mobile IPv6", Work
         in Progress, January 2004.

   [18]  Weniger, K. and T. Aramaki, "Route Optimization and Location
         Privacy using Tunneling Agents (ROTA)", Work in Progress,
         October 2005.

   [19]  Devarapalli, V., Wakikawa, R., Petrescu, A., and P. Thubert,
         "Network Mobility (NEMO) Basic Support Protocol", RFC 3963,
         January 2005.

   [20]  Soliman, H., "Mobile IPv6 Support for Dual Stack Hosts and
         Routers", RFC 5555, June 2009.




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RFC 5726             MIP6 Location Privacy Solutions       February 2010


   [21]  Wakikawa, R., Devarapalli, V., Tsirtsis, G., Ernst, T., and K.
         Nagami, "Multiple Care-of Addresses Registration", RFC 5648,
         October 2009.

   [22]  Muhanna, A., Khalil, M., Gundavelli, S., Chowdhury, K., and P.
         Yegani, "Binding Revocation for IPv6 Mobility", Work
         in Progress, October 2009.

   [23]  Haley, B. and S. Gundavelli, "Mobile IPv6 Generic Signaling
         Message", Work in Progress, August 2008.









































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Appendix A.  Profiling Attack: Discussion

   Profiling attacks pose a significant threat to user privacy.  By
   collecting and analyzing (either online or offline) IP traffic,
   attackers can obtain sensitive user information.  In the context of
   mobility, although the profiling attack does not directly lead to
   compromise of location privacy in the way the disclosure of the
   binding between the home address and the care-of address does,
   attackers can infer the mobile node's roaming and track its movement
   (i.e., handover) by profiling the mobile node's communication based
   on certain fields in IP packets, such as a constant IPsec SPI used
   during the home registration.  The more information collected, the
   higher probability location privacy is compromised, which in return
   results in more targeted profiling.

   We have taken the profiling problem into consideration when designing
   the solution to IP address location privacy; however, not all aspects
   of profiling attacks are addressed since the profiling problem spans
   multiple protocol layers.  In the following, we provide a broad
   discussion on the profiling attack and protection mechanisms.  Our
   discussion is organized based on how profiling attacks can be
   performed.  Note that the following sections are not sorted based on
   any criteria or may not exhaustively list all the possible attack
   means (for example, profiling attacks based on upper-layer payloads
   in data packets are not discussed).

A.1.  The Care-of Address

   Eavesdroppers on the MN-HA path and/or the MN-CN path can profile the
   mobile node's communication by collecting packets with the same
   care-of address.  It is recommended that the mobile node periodically
   updates its care-of address by using DHCPv6 or IPv6 address privacy
   extension, even if it does not change its current attachment point.
   Furthermore, it is even better to change the network prefix of the
   care-of address periodically, since eavesdroppers may profile IP
   packets based on the common network prefix.

   Since the binding update procedure needs to be performed once the
   care-of address is changed, in order to reduce signaling overheads,
   the mobile node may choose to change its care-of address when the
   Binding Cache entry at the home agent or the correspondent node is
   about to expire.

A.2.  Profiling on the Encrypted Home Address

   Generated from either a real or pseudo home address, the encrypted
   home address can be dynamically updated, because a new key is
   generated when a new round of the return routability procedure is



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   performed, which makes the encrypted home address look different in
   subsequent Binding Update and Acknowledgement messages.
   Nevertheless, the same encrypted home address is used in payload
   packets forwarded via the optimized route before the next round of
   the return routability procedure.  Given the cost and overhead of
   updating the encrypted home address, the proposed location privacy
   solutions still provide a reasonable level of protection against such
   profiling attacks.

A.3.  The IPsec SPI

   Eavesdroppers on the MN-HA path can profile the mobile node's
   communication based on the SPI of an IPsec security association that
   is for protecting the home Binding Update and Acknowledgement message
   or for protecting bidirectional-tunneled payload packets.

   To resist this kind of profiling attack, the IPsec SPI needs to be
   periodically updated.  One way is that the mobile node and the home
   agent rekey the IPsec security association or perform re-
   authentication periodically.  This may result in more signaling
   overhead.  Another way is that the mobile node or the home agent
   generates a new SPI and then notifies each other by exchanging the
   Binding Update and Acknowledgement messages protected by an existing
   IPsec security association with a non-null encryption algorithm.  In
   this way, the information of the new SPI is hidden from
   eavesdroppers.  The new SPI MUST not conflict with other existing
   SPIs; and if the conflict is detected on one end point, another SPI
   MUST be generated and be synchronized with the other end point.  The
   new SPI is applied to the next packet that needs to be protected by
   this IPsec security association.  This solution requires close
   interaction between Mobile IP and IPsec.  For example, when the home
   agent receives a new SPI suggested by the mobile node, it needs to
   change the corresponding Security Association Database (SAD) entry.

A.4.  The IPsec Sequence Number

   The IPsec sequence number is required to be larger than that in the
   previous valid IPsec packet if the anti-replay service is enabled.
   However, if the increment of the IPsec sequence number is fixed (for
   example, the IPsec sequence number is sequentially increased), it is
   possible for eavesdroppers to identify a sequence of IPsec packets
   that are from/to the same mobile node and to track the mobile node's
   activities.  One possible solution is to randomize the increment of
   the IPsec sequence number on both end points (i.e., the mobile node
   and the home agent) of the IPsec security association.  The algorithm
   to generate randomness is implementation specific.  It can be, for
   example, any random number generator, and independently chosen by
   each end point.



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A.5.  The Regular Interval of Signaling Messages

   As described in RFC 3775, certain signaling messages may be exchanged
   on a regular basis.  For example, the correspondent registration
   needs to be performed every MAX_RR_BINDING_LIFETIME seconds and the
   home binding update procedure needs to be performed regularly, if the
   lifetime of the home Binding Cache entry is fixed.  Such timing
   allows eavesdroppers to perform traffic analyses and correlate
   different messages.  Due to background traffic and routing dynamics,
   the timing of messages observed by an eavesdropper at a certain
   vantage point may be irregular.  Nevertheless, a better solution is
   to randomize the lifetime of the Binding Cache entry in the home
   agent and the correspondent node.

A.6.  The Sequence Number in the Binding Update Message

   RFC 3775 requires that the sequence number in the Binding Update
   message be larger than that in the previous valid Binding Update
   message for a particular mobile node.  However, if the increment of
   the sequence number in the home or correspondent Binding Update
   message is fixed (for example, the sequence number is sequentially
   increased), it is possible for eavesdroppers on the MN-HA or MN-CN
   path to identify a sequence of Binding Update messages that are from
   the same mobile node and to track the mobile node's movement.  One
   possible solution is that the mobile node randomizes the increment of
   the sequence number used in subsequent Binding Update messages.  The
   algorithm to generate randomness is implementation specific.  It can
   be, for example, any random number generator.  Note that such an
   algorithm is not needed when the sequence number is encrypted, for
   example, when the home Binding Update message is protected by an
   IPsec tunnel mode security association.

A.7.  Multiple Concurrent Sessions

   It is possible for (colluded) eavesdroppers to correlate the mobile
   node's different sessions with the same or different correspondent
   nodes, for example, based on the same pseudo home address and/or the
   same care-of address.  A possible solution is to use different pseudo
   home addresses and different care-of addresses in different sessions.
   Note that the mobile node may also use the same pseudo home address
   with different correspondent nodes, if the pseudo home address is
   masked by different privacy management keys generated during the
   return routability procedure with different correspondent nodes.  In
   this way, the encrypted pseudo home addresses used with different
   correspondent nodes look different to eavesdroppers.






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A.8.  Summary

   As discussed above, there exist multiple means for eavesdroppers to
   correlate observed activities.  For example, some IP fields, which
   contain certain constant values and remain unchanged for a long time,
   allow eavesdroppers to identify and link the mobile node's activities
   deterministically.  Other means may be less reliable when used for
   traffic analysis and correlation; nevertheless, they provide
   additional hints to malicious attackers.

   The solution to the profiling attack is to update certain IP fields
   periodically.  Generally, the more frequently, the higher the
   probability that the profiling attack is resisted and also the higher
   the cost in terms of communication and processing overheads and
   complexity.  As eavesdroppers can profile activities based on
   multiple fields, it may not be cost-effective to update some fields
   more frequently than others.  Furthermore, it may reduce some
   overheads, if all the related IP fields are updated together with the
   same frequency.

   The profiling attack is a complicated issue.  A complete solution
   would have to consider tradeoffs of many different factors, such as
   complexity, effectiveness, and efficiency.




























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Authors' Addresses

   Ying Qiu
   Institute for Infocomm Research, Singapore
   1 Fusionopolis Way
   #21-01 Connexis (South Tower)
   Singapore  138632

   Phone: +65-6408 2053
   EMail: qiuying@i2r.a-star.edu.sg


   Fan Zhao (editor)
   Google Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   US

   EMail: fanzhao@google.com


   Rajeev Koodli
   Cisco Systems

   EMail: rkoodli@cisco.com


























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