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Network Working Group                                          R. KoodliRequest for Comments: 4882                        Nokia Siemens NetworksCategory: Informational                                         May 2007IP Address Location Privacy and Mobile IPv6: Problem StatementStatus of This Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The IETF Trust (2007).Abstract   In this document, we discuss location privacy as applicable to Mobile   IPv6.  We document the concerns arising from revealing a Home Address   to an onlooker and from disclosing a Care-of Address to a   correspondent.Table of Contents1. Introduction ....................................................22. Definitions .....................................................33. Problem Definition ..............................................43.1. Disclosing the Care-of Address to the Correspondent Node ...43.2. Revealing the Home Address to Onlookers ....................43.3. Problem Scope ..............................................44. Problem Illustration ............................................55. Conclusion ......................................................76. Security Considerations .........................................77. Acknowledgments .................................................88. References ......................................................88.1. Normative References .......................................88.2. Informative References .....................................8Appendix A. Background ............................................10Koodli                       Informational                      [Page 1]

RFC 4882                 MIP6 Location Privacy                  May 20071.  Introduction   The problems of location privacy, and privacy when using IP for   communication, have become important.  IP privacy is broadly   concerned with protecting user communication from unwittingly   revealing information that could be used to analyze and gather   sensitive user data.  Examples include gathering data at certain   vantage points, collecting information related to specific traffic,   and monitoring (perhaps) certain populations of users for activity   during specific times of the day, etc.  In this document, we refer to   this as the "profiling" problem.   Location privacy is concerned with the problem of revealing roaming,   which we define here as the process of a Mobile Node (MN) moving from   one network to another with or without ongoing sessions.  A constant   identifier with global scope can reveal roaming.  Examples are a   device identifier such as an IP address, and a user identifier such   as a SIP [RFC3261] URI [RFC3986].  Often, a binding between these two   identifiers is available, e.g., through DNS [RFC1035].  Traffic   analysis of such IP and Upper Layer Protocol identifiers on a single   network can indicate device and user roaming.  Roaming could also be   inferred by means of profiling constant fields in IP communication   across multiple network movements.  For example, an Interface   Identifier (IID) [RFC2462] in the IPv6 address that remains unchanged   across networks could suggest roaming.  The Security Parameter Index   (SPI) in the IPsec [RFC4301] header is another field that may be   subject to such profiling and inference.  Inferring roaming in this   way typically requires traffic analysis across multiple networks, or   colluding attackers, or both.  When location privacy is compromised,   it could lead to more targeted profiling of user communication.   As can be seen, the location privacy problem spans multiple protocol   layers.  Nevertheless, we can examine problems encountered by nodes   using a particular protocol layer.  Roaming is particularly important   to Mobile IP, which defines a global identifier (Home Address) that   can reveal device roaming, and in conjunction with a corresponding   user identifier (such as a SIP URI), can also reveal user roaming.   Furthermore, a user may not wish to reveal roaming to   correspondent(s), which translates to the use of a Care-of Address.   As with a Home Address, the Care-of Address can also reveal the   topological location of the Mobile Node.   This document scopes the problem of location privacy for the Mobile   IP protocol.  The primary goal is to prevent attackers on the path   between the Mobile Node (MN) and the Correspondent Node (CN) from   detecting roaming due to the disclosure of the Home Address.  The   attackers are assumed to be able to observe, modify, and inject   traffic at one point between the MN and the CN.  The attackers areKoodli                       Informational                      [Page 2]

RFC 4882                 MIP6 Location Privacy                  May 2007   assumed not to be able to observe at multiple points and correlate   observations to detect roaming.  For this reason, MAC addresses,   IIDs, and other fields that can be profiled to detect roaming are   only in scope to the extent that they can be used by an attacker at   one point.  Upper layer protocol identifiers that expose roaming are   out of scope except that a solution to the problem described here   needs to be usable as a building block in solutions to those   problems.   This document also considers the problem from the exposure of a   Care-of Address to the CN.   This document is only concerned with IP address location privacy in   the context of Mobile IPv6.  It does not address the overall privacy   problem.  For instance, it does not address privacy issues related to   MAC addresses or the relationship of IP and MAC addresses [HADDAD],   or the Upper Layer Protocol addresses.  Solutions to the problem   specified here should provide protection against roaming disclosure   due to using Mobile IPv6 addresses from a visited network.   This document assumes that the reader is familiar with the basic   operation of Mobile IPv6 [RFC3775] and the associated terminology   defined therein.  For convenience, we provide some definitions below.2.  Definitions   o  Mobile Node (MN): A Mobile IPv6 Mobile Node that freely roams      around networks   o  Correspondent Node (CN): A Mobile IPv6 that node corresponds with      an MN   o  Home Network: The network where the MN is normally present when it      is not roaming   o  Visited Network: A network that an MN uses to access the Internet      when it is roaming   o  Home Agent: A router on the MN's home network that provides      forwarding support when the MN is roaming   o  Home Address (HoA): The MN's unicast IP address valid on its home      network   o  Care-of Address (CoA): The MN's unicast IP address valid on the      visited networkKoodli                       Informational                      [Page 3]

RFC 4882                 MIP6 Location Privacy                  May 2007   o  Reverse Tunneling or Bidirectional Tunneling: A mechanism used for      packet forwarding between the MN and its Home Agent   o  Route Optimization: A mechanism that allows direct routing of      packets between a roaming MN and its CN, without having to      traverse the home network3.  Problem Definition3.1.  Disclosing the Care-of Address to the Correspondent Node   When a Mobile IP MN roams from its home network to a visited network   or from one visited network to another, use of Care-of Address in   communication with a correspondent reveals that the MN has roamed.   This assumes that the correspondent is able to associate the Care-of   Address to the Home Address, for instance, by inspecting the Binding   Cache Entry.  The Home Address itself is assumed to have been   obtained by whatever means (e.g., through DNS lookup).3.2.  Revealing the Home Address to Onlookers   When a Mobile IP MN roams from its home network to a visited network   or from one visited network to another, use of a Home Address in   communication reveals to an onlooker that the MN has roamed.  When a   binding of a Home Address to a user identifier (such as a SIP URI) is   available, the Home Address can be used to also determine that the   user has roamed.  This problem is independent of whether the MN uses   a Care-of Address to communicate directly with the correspondent   (i.e., uses route optimization), or the MN communicates via the Home   Agent (i.e., uses reverse tunneling).  Location privacy can be   compromised when an onlooker is present on the MN - CN path (when   route optimization is used).  It may also be compromised when the   onlooker is present on the MN - HA path (when bidirectional tunneling   without encryption is used; see below).3.3.  Problem Scope   With existing Mobile IPv6 solutions, there is some protection against   location privacy.  If a Mobile Node uses reverse tunneling with   Encapsulating Security Payload (ESP) encryption, then the Home   Address is not revealed on the MN - HA path.  So, eavesdroppers on   the MN - HA path cannot determine roaming.  They could, however,   still profile fields in the ESP header; however, this problem is not   specific to Mobile IPv6 location privacy.   When an MN uses reverse tunneling (regardless of ESP encryption), the   correspondent does not have access to the Care-of Address.  Hence, it   cannot determine that the MN has roamed.Koodli                       Informational                      [Page 4]

RFC 4882                 MIP6 Location Privacy                  May 2007   Hence, the location privacy problem is particularly applicable when   Mobile IPv6 route optimization is used or when reverse tunneling is   used without protecting the inner IP packet containing the Home   Address.4.  Problem Illustration   This section is intended to provide an illustration of the problem   defined in the previous section.   Consider a Mobile Node at its home network.  Whenever it is involved   in IP communication, its correspondents can see an IP address valid   on the home network.  Elaborating further, the users involved in   peer-to-peer communication are likely to see a user-friendly   identifier such as a SIP URI, and the communication endpoints in the   IP stack will see IP addresses.  Users uninterested in or unaware of   IP communication details will not see any difference when the MN   acquires a new IP address.  Of course, any user can "tcpdump" or   "ethereal" a session, capture IP packets, and map the MN's IP address   to an approximate geo-location.  This mapping may reveal the home   location of a user, but a correspondent cannot ascertain whether the   user has actually roamed or not.  Assessing the physical location   based on IP addresses has some similarities to assessing the   geographical location based on the area code of a telephone number.   The granularity of the physical area corresponding to an IP address   can vary depending on how sophisticated the available tools are, how   often an ISP conducts its network re-numbering, etc.  Also, an IP   address cannot guarantee that a peer is at a certain geographic area   due to technologies such as VPN and tunneling.   When the MN roams to another network, the location privacy problem   consists of two parts: revealing information to its correspondents   and to onlookers.   With its correspondents, the MN can either communicate directly or   reverse-tunnel its packets through the Home Agent.  Using reverse   tunneling does not reveal the Care-of Address of the MN, although   end-to-end delay may vary depending on the particular scenario.  With   those correspondents with which it can disclose its Care-of Address   "on the wire", the MN has the option of using route-optimized   communication.  The transport protocol still sees the Home Address   with route optimization.  Unless the correspondent runs some packet   capturing utility, the user cannot see which mode (reverse tunneling   or route optimization) is being used, but knows that it is   communicating with the same peer whose URI it knows.  This is similar   to conversing with a roaming cellphone user whose phone number, like   the URI, remains unchanged.Koodli                       Informational                      [Page 5]

RFC 4882                 MIP6 Location Privacy                  May 2007   Regardless of whether the MN uses route optimization or reverse   tunneling (without ESP encryption), its Home Address is revealed in   data packets.  When equipped with an ability to inspect packets "on   the wire", an onlooker on the MN - HA path can determine that the MN   has roamed and could possibly also determine that the user has   roamed.  This could compromise the location privacy even if the MN   took steps to hide its roaming information from a correspondent.   The above description is valid regardless of whether a Home Address   is statically allocated or is dynamically allocated.  In either case,   the mapping of IP address to a geo-location will most likely yield   results with the same level of granularity.  With the freely   available tools on the Internet, this granularity is the physical   address of the ISP or the organization that registers ownership of a   prefix chunk.  Since an ISP or an organization is not, rightly,   required to provide a blueprint of its subnets, the granularity   remains fairly coarse for a mobile wireless network.  However,   sophisticated attackers might be able to conduct site mapping and   obtain more fine-grained subnet information.   A compromise in location privacy could lead to more targeted   profiling of user data.  An eavesdropper may specifically track the   traffic containing the Home Address, and monitor the movement of the   Mobile Node with a changing Care-of Address.  The profiling problem   is not specific to Mobile IPv6, but could be triggered by a   compromise in location privacy due to revealing the Home Address.  A   correspondent may take advantage of the knowledge that a user has   roamed when the Care-of Address is revealed, and modulate actions   based on such knowledge.  Such information could cause concern to a   mobile user, especially when the correspondent turns out be   untrustworthy.  For these reasons, appropriate security measures on   the management interfaces on the MN to guard against the disclosure   or misuse of location information should be considered.   Applying existing techniques to thwart profiling may have   implications to Mobile IPv6 signaling performance.  For instance,   changing the Care-of Address often would cause additional Return   Routability [RFC3775] and binding management signaling.  And,   changing the Home Address often has implications on IPsec security   association management.  Careful consideration should be given to the   signaling cost introduced by changing either the Care-of Address or   the Home Address.   When roaming, an MN may treat its home network nodes as any other   correspondents.  Reverse tunneling is perhaps sufficient for home   network communication, since route-optimized communication will   traverse the identical path.  Hence, an MN can avoid revealing its   Care-of Address to its home network correspondents simply by usingKoodli                       Informational                      [Page 6]

RFC 4882                 MIP6 Location Privacy                  May 2007   reverse tunneling.  The Proxy Neighbor Advertisements [RFC2461] from   the Home Agent could serve as hints to the home network nodes that   the Mobile Node is away.  However, they will not be able to know the   Mobile Node's current point of attachment unless the MN uses route   optimization with them.5.  Conclusion   In this document, we have discussed the location privacy problem as   applicable to Mobile IPv6.  The problem can be summarized as follows:   disclosing the Care-of Address to a correspondent and revealing the   Home Address to an onlooker can compromise the location privacy of a   Mobile Node, and hence that of a user.  We have seen that   bidirectional tunneling allows an MN to protect its Care-of Address   to the CN.  And, ESP encryption of an inner IP packet allows the MN   to protect its Home Address from the onlookers on the MN - HA path.   However, with route optimization, the MN will reveal its Care-of   Address to the CN.  Moreover, route optimization causes the Home   Address to be revealed to onlookers in the data packets as well as in   Mobile IPv6 signaling messages.  The solutions to this problem are   expected to be protocol specifications that use the existing Mobile   IPv6 functional entities, namely, the Mobile Node, its Home Agent,   and the Correspondent Node.6.  Security Considerations   This document discusses the location privacy problem specific to   Mobile IPv6.  Any solution must be able to protect (e.g., using   encryption) the Home Address from disclosure to onlookers in data   packets when using route optimization or reverse tunneling.  In   addition, solutions must consider protecting the Mobile IPv6   signaling messages from disclosing the Home Address along the MN - HA   and MN - CN paths.   Disclosing the Care-of Address is inevitable if an MN wishes to use   route optimization.  Regardless of whether the Care-of Address is an   on-link address of the MN on the visited link or that of a   cooperating proxy, mere presence of a Binding Cache Entry is   sufficient for a CN to ascertain roaming.  Hence, an MN concerned   with location privacy should exercise prudence in determining whether   to use route optimization with, especially previously unacquainted,   correspondents.   The solutions should also consider the use of temporary addresses and   their implications on Mobile IPv6 signaling as discussed inSection4, "Problem Illustration".  Use of IP addresses with privacy   extensions [RFC3041] could be especially useful for Care-of AddressesKoodli                       Informational                      [Page 7]

RFC 4882                 MIP6 Location Privacy                  May 2007   if appropriate trade-offs with Return Routability signaling are taken   into account.7.  Acknowledgments   James Kempf, Qiu Ying, Sam Xia, and Lakshminath Dondeti are   acknowledged for their review and feedback.  Thanks to Jari Arkko and   Kilian Weniger for the last-call review and for suggesting   improvements and text.  Thanks to Sam Hartman for providing text to   improve the problem scope.8.  References8.1.  Normative References   [RFC3775]  Johnson, D., Perkins, C., and J. Arkko, "Mobility Support              in IPv6",RFC 3775, June 2004.8.2.  Informative References   [HADDAD]   Haddad, W., et al., "Privacy for Mobile and Multi-homed              Nodes: Problem Statement" Work in Progress, June 2006.   [RFC1035]  Mockapetris, P., "Domain names - implementation and              specification", STD 13,RFC 1035, November 1987.   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform              Resource Identifier (URI): Generic Syntax", STD 66,RFC3986, January 2005.   [RFC2461]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor              Discovery for IP Version 6 (IPv6)",RFC 2461, December              1998.   [RFC2462]  Thomson, S. and T. Narten, "IPv6 Stateless Address              Autoconfiguration",RFC 2462, December 1998.   [RFC3041]  Narten, T. and R. Draves, "Privacy Extensions for              Stateless Address Autoconfiguration in IPv6",RFC 3041,              January 2001.   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,              A., Peterson, J., Sparks, R., Handley, M., and E.              Schooler, "SIP: Session Initiation Protocol",RFC 3261,              June 2002.Koodli                       Informational                      [Page 8]

RFC 4882                 MIP6 Location Privacy                  May 2007   [RFC3825]  Polk, J., Schnizlein, J., and M. Linsner, "Dynamic Host              Configuration Protocol Option for Coordinate-based              Location Configuration Information",RFC 3825, July 2004.   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the              Internet Protocol",RFC 4301, December 2005.Koodli                       Informational                      [Page 9]

RFC 4882                 MIP6 Location Privacy                  May 2007Appendix A.  Background   The location privacy topic is broad and often has different   connotations.  It also spans multiple layers in the OSI reference   model.  Besides, there are attributes beyond an IP address alone that   can reveal hints about location.  For instance, even if a   correspondent is communicating with the same endpoint it is used to,   the "time of day" attribute can reveal a hint to the user.  Some   roaming cellphone users may have noticed that their SMS messages   carry a timestamp of their "home network" time zone (for location   privacy or otherwise), which can reveal that the user is in a   different time zone when messages are sent during a "normal" time of   the day.  Furthermore, tools exist on the Internet that can map an IP   address to the physical address of an ISP or the organization that   owns the prefix chunk.  Taking this to another step, with built-in   GPS receivers on IP hosts, applications can be devised to map geo-   locations to IP network information.  Even without GPS receivers,   geo-locations can also be obtained in environments where "Geopriv" is   supported, for instance, as a DHCP option [RFC3825].  In summary, a   user's physical location can be determined or guessed with some   certainty and with varying levels of granularity by different means,   even though IP addresses themselves do not inherently provide any   geo-location information.  It is perhaps useful to bear this broad   scope in mind as the problem of IP address location privacy in the   presence of IP Mobility is addressed.Author's Address   Rajeev Koodli   Nokia Siemens Networks   313 Fairchild Drive   Mountain View, CA 94043   EMail: rajeev.koodli@nokia.comKoodli                       Informational                     [Page 10]

RFC 4882                 MIP6 Location Privacy                  May 2007Full Copyright Statement   Copyright (C) The IETF Trust (2007).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Koodli                       Informational                     [Page 11]