MIP6 Working Group Rajeev Koodli
Internet-Draft Nokia Research Center
Intended status: Informational February 19, 2007
Expires: August 23, 2007
IP Address Location Privacy and Mobile IPv6: Problem Statement
draft-ietf-mip6-location-privacy-06.txt
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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 Home Address
to an on-looker and from disclosing Care of Address to a
correspondent.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Problem Definition . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Disclosing the Care-of Address to the Correspondent
Node . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Revealing the Home Address to On-lookers . . . . . . . . . 5
3.3. Problem Scope . . . . . . . . . . . . . . . . . . . . . . 5
4. Problem Illustration . . . . . . . . . . . . . . . . . . . . . 6
5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . . 9
Appendix A. Background . . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . . . 11
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1. 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 on-going 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[rfc2396]. 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 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 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 targetted 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 Care-of Address. As
with 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 are
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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 which 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 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
[draft-haddad], or the Upper Layer Protocol addresses. Solution 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
a MN
o Home Network: The network where the MN is normally present when it
is not roaming
o Visited Network: A network which a MN uses to access Internet when
it is roaming
o Home Agent: A router on the MN's home network which 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 network
o Reverse Tunneling or Bidirectional Tunneling: A mechanism used for
packet forwarding between the MN and its Home Agent
o Route Optimization: A mechanism which allows direct routing of
packets between a roaming MN and its CN, without having to
traverse the home network.
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3. Problem Definition
3.1. 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 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 On-lookers
When a Mobile IP MN roams from its home network to a visited network
or from one visited network to another, use of Home Address in
communication reveals to an on-looker that the MN has roamed. When a
binding of 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
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 on-looker is present on the MN - CN path (when route
optimization is used). It may also be compromised when the on-looker
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 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 a 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.
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.
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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
- peer communication are likely to see a user-friendly identifier
such as a SIP URI, and the communication end-points 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. And, an IP
address cannot also 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 on-lookers.
With its correspondents, the MN can either communicate directly or
reverse tunnel its packets through the Home Agent. Using reverse
tunneling does not reveal 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.
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 on-looker on the MN - HA path can determine that the
MN has roamed and could possibly also determine that the user has
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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 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 which registers ownership of a
prefix chunk. Since an ISP or an organization is not, rightly,
required to provide a blue-print 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 targetted
profiling of user data. An eavesdropper may specifically track the
traffic containing the Home Address, and monitor the movement of the
Mobile Node with 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 Care-of Address is revealed, and modulate actions based
on such a knowledge. Such an 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. Solutions should be careful in considering
the cost of change of either Care-of Address or Home Address on
signaling.
When roaming, a 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, a MN can avoid revealing its
Care-of Address to its home network correspondents simply by using
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.
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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 Care-of Address to a correspondent and revealing Home
Address to an on-looker can compromise the location privacy of a
Mobile Node, and hence that of a user. We have seen that
bidirectional tunneling allows a MN to protect its Care-of Address to
the CN. And, ESP encryption of inner IP packet allows the MN to
protect its Home Address from the on-lookers 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 on-lookers in the data packets as well as
in Mobile IPv6 signaling messages. The solutions to this problem are
expected to be protocol specifications assuming the existing Mobile
IPv6 functional entities, namely, the Mobile Node, its Home Agent and
the Correspondent Node.
6. IANA Considerations
There are no IANA considerations introduced by this draft.
7. 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 on-lookers 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 a MN wishes to use
route optimization. Regardless of whether Care-of Address is an on-
link address of the MN on the visited link or that of a co-operating
proxy, mere presence of Binding Cache entry is sufficient for a CN to
ascertain roaming. Hence, a 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 in Problem
Illustration. Use of IP addresses with privacy extensions [rfc3041]
could be especially useful for Care-of Addresses if appropriate
tradeoffs with Return Routability signaling are taken into account.
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8. Acknowledgment
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.
9. References
9.1. Normative References
[rfc3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
in IPv6", RFC 3775, June 2004,
<ftp://ftp.isi.edu/in-notes/rfc3775>.
9.2. Informative References
[draft-haddad]
Haddad, W. et al., "Privacy for Mobile and Multi-homed
Nodes: MoMiPriv Problem Statement (work in progress)",
October 2004.
[rfc1035] Mockapetris, P., "Domain names - implementation and
specification", RFC 1035, November 1987,
<ftp://ftp.isi.edu/in-notes/rfc1035>.
[rfc2396] Berners-Lee, T., Fielding, R., and L. Manister, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998, <ftp://ftp.isi.edu/in-notes/rfc2396>.
[rfc2461] Narten, T., Nordmark, E., and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461,
December 1998, <ftp://ftp.isi.edu/in-notes/rfc2461>.
[rfc2462] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998,
<ftp://ftp.isi.edu/in-notes/rfc2462>.
[rfc3041] Narten, T. and R. Draves, "Privacy Extensions for
Stateless Address Autoconfiguration in IPv6", RFC 3041,
January 2001, <ftp://ftp.isi.edu/in-notes/rfc3041>.
[rfc3261] Rosenberg, J. et al., "SIP: Session Initiation
Protocol", RFC 3261, July 2004,
<ftp://ftp.isi.edu/in-notes/rfc3261>.
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[rfc3825] Polk, J. and J. Schnizlein, "DHCP Option for Coordinate-
based Location Configuration Information", RFC 3825,
July 2004, <ftp://ftp.isi.edu/in-notes/rfc3825>.
[rfc4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005,
<ftp://ftp.isi.edu/in-notes/rfc4301>.
Appendix 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 end-point it is used to,
the ``time of the 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'' timezone (for location
privacy or otherwise) which can reveal that the user is in a
different timezone when messages are sent during ``normal'' time of
the day. Furthermore, tools exist on the Internet which can map an
IP address to the physical address of an ISP or the organization
which owns the prefix chunk. Taking this to another step, with in-
built GPS receivers on IP hosts, applications can be devised to map
geo-locations to IP network information. Even without GPS receivers,
geo-location 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 Research Center
975 Page Mill Road, 200
Palo Alto, CA 94304
USA
Email: rajeev.koodli@nokia.com
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