MIP6 J. Kempf
Internet-Draft DoCoMo Communications Labs USA
Expires: August 13, 2004 J. Arkko
Ericsson
February 13, 2004
The Mobile IPv6 Bootstrapping Problem
draft-kempf-mip6-bootstrap-00.txt
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Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document discusses the creation of a security association
between a mobile node and a home agent that is previously unknown to
it. This problem is called the bootstrapping problem. The document
discusses several different usage scenarios, as well as related
issues involving informing the mobile node of changes in the home
network topology and mobility management service. Limitations of the
base Mobile IPv6 protocol in dealing with the scenarios are outlined.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Requirements language . . . . . . . . . . . . . . . . 4
2. Mobile IPv6 Configuration . . . . . . . . . . . . . . . . . 5
3. Issues in Bootstrapping . . . . . . . . . . . . . . . . . . 6
3.1 Addressing . . . . . . . . . . . . . . . . . . . . . . 6
3.1.1 Dynamic Home Address Assignment . . . . . . . . 6
3.1.2 Dynamic Home Agent Assignment . . . . . . . . . 7
3.1.3 Management requirements . . . . . . . . . . . . 7
3.2 Security Infrastructure . . . . . . . . . . . . . . . 8
3.2.1 Integration with AAA Infrastructure . . . . . . 8
3.2.2 "Opportunistic" or "Local" Discovery . . . . . . 8
3.3 Topology Change . . . . . . . . . . . . . . . . . . . 8
3.3.1 Dormant Mode Mobile Nodes . . . . . . . . . . . 8
3.3.2 Use of ICMP . . . . . . . . . . . . . . . . . . 9
4. Bootstrapping Scenarios . . . . . . . . . . . . . . . . . . 10
5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . 12
Normative References . . . . . . . . . . . . . . . . . . . . 13
Informative References . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 14
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . 16
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1. Introduction
The bootstrapping problem for Mobile IPv6 [4, 5] is one of the issues
the MIP6 WG is chartered to solve. The text in the charter says:
"A bootstrap mechanism for setting up security associations
between the mobile node and home agent that would enable easier
deployment of Mobile IPv6. This bootstrap mechanism is intended
to be used when the device is turned on the very first time and
activates MIPv6. The WG should investigate and define the scope
before solving the problem."
In addition to easier deployment, reasons for bootstrapping include
the following:
o Resilience to network renumbering, provisioning of new home
agents, and other network management operations.
o Increasing the efficiency of communications through the selection
of an appropriate home agent.
o Load balancing.
o Hiding the topological location of the mobile node.
The current Mobile IPv6 procedure for establishing an IPsec security
association between the Mobile Node and home agent requires either
manual keying or IKEv1 [2]. With manual keying, the security
associations have to be bound to specific home addresses and home
agent addresses. With IKEv1, the mobile node needs to have a
statically defined home address in order that the home agent can make
an authorization decision and identify the credentials during the IKE
Phase 1 ISAKMP exchange. In this document, we discuss why these
constraints may be problematic in some deployment scenarios.
In addition, Mobile IPv6 defines mechanisms for dynamic home agent
and home prefix discovery. While these mechanisms are not
specifically related to security, changes in the home agent address
and mobile node home address also imply changes in authorization
information related to security policies. As a result, the discovery
mechanisms might not be as easily deployed as would be desirable.
This document defines the bootstrap problem. In Section 2 we review
the Mobile IPv6 mechanisms for configuration and security association
establishment; bootstrapping issues related to addressing,
infrastructure, and topology changes are discussed in Section 3.
Some scenarios for possible use of a bootstrapping mechanism are
outlined in Section 4. Finally, some conclusions are presented in
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Section 5.
1.1 Requirements language
In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
"recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
described in [1].
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2. Mobile IPv6 Configuration
The Mobile IPv6 protocol needs no configuration or security set-up
mechanisms for its route optimization functionality. However, it
requires that mobile nodes and home agents have been assigned to each
other via configuration.
All mobile node - home agent communications are protected by IPsec,
as described in Section 5.1 of [4] and [5]. In order to protect
messages exchanged between the mobile node and the home agent with
IPsec, appropriate security policy database entries are needed. A
mobile node must be prevented from using its security association to
send a message on the behalf of another mobile node using the same
home agent. This is achieved through the use of the security policy
database, and via an authorization check in IKEv1 when dynamic keying
is used.
Sections 10.6 and 11.4 of [4] describe the operation of a protocol by
which an mobile node that is away from home can discover it's home
agent and by which the home agent can push prefix changes to the
mobile node. The protocol is an extension of ICMPv6.
Prefix information propagation is envisioned as a way for network
renumbering events in the home network to be propagated to the mobile
node so that the mobile node can form a new home address. Home agent
discovery is envisioned as a way for the mobile node to find a home
agent given the prefix of its home network. While not strictly
involved in the mobile node - home agent IPsec security relation,
this protocol is required for bootstrapping the mobile node - home
agent connection.
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3. Issues in Bootstrapping
3.1 Addressing
In this section, we discuss the problems caused by the currently
tight binding to home addresses and home agent addresses.
3.1.1 Dynamic Home Address Assignment
Currently, the home agent uses the mobile node's home address for
authorization. When manual keying is used, this happens through the
security policy database, which specifies that a certain security
association may only use a specific home address. When dynamic
keying is used, the home agent ensures that the IKE Phase 1 identity
is authorized to request security associations for the given home
address. Mobile IPv6 uses IKEv1, which is unable to update the
security policy database based on a dynamically assigned home
address. As a result, static home address assignment is really the
only home address configuration technique compatible with the current
specification.
However, support for dynamic home address assignment would be
desirable for the following reasons:
Prefix changes in the home network
The Mobile IPv6 specification contains support for a mobile node
to autoconfigure a home address based on its discovery of prefix
information on the home subnet [4].
DHCP-based address assignment
Some ISPs may want to use DHCPv6 from the home network to
configure home addresses [7].
Addressing privacy
It may be desirable to establish randomly generated addresses as
in RFC 3041 [3] and use them for a short period of time.
Unfortunately, current protocols make it possible to use such
addresses only from the visited network. As a result, these
addresses can not be used for communications lasting longer than
the attachment to a particular visited network.
Ease of deployment
In order to make deployment of Mobile IPv6 easy, it would be
desirable to free users and administrators from the task of
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allocating home addresses and specifying them in the security
policy database.
3.1.2 Dynamic Home Agent Assignment
Currently, the address of the home agent is specified in the security
policy database. Support for multiple home agents requires the
configuration of multiple security policy database entries.
However, support for dynamic home agent assignment would be desirable
for the following reasons:
Home agent discovery
The Mobile IPv6 specification contains support for a mobile node
to autoconfigure a home agent address based on a discovery
protocol [4].
Independent network management
An ISP may also want to dynamically assign home agents in
different subnets, that is, not require that a roamed mobile node
have a fixed home subnet.
Local home agents
The mobile node's home ISP may want to allow a local roaming
partner ISP to assign a local home agent for the mobile node.
This is useful both from the point of view of communications
efficiency, and has also been mentioned as one approach to support
location privacy.
Ease of deployment
ISP may want to allow "opportunistic" discovery and utilization of
its mobility services without any prearranged contact. These
scenarios will require dynamic home address assignment.
3.1.3 Management requirements
As described earlier, new addresses invalidate configured security
policy databases and authorization tables. Regardless of the
specific protocols used, there is a need for either an automatic
system for updating the security policy entries, or manual
configuration. These requirements apply to both home agents and
mobile nodes, but it can not be expected that mobile node users are
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capable of performing the required tasks.
3.2 Security Infrastructure
3.2.1 Integration with AAA Infrastructure
The current IKEv1-based dynamic key exchange protocol described in
[5] has no integration with backend authentication, authorization and
accounting techniques unless the authentication credentials and trust
relationships use certificates.
Using certificates may require the ISP to deploy a PKI, which may not
be possible or desirable in certain circumstances. Where a
traditional AAA infrastructure is used, the home agent is not able to
leverage authentication and authorization information established
between the mobile node, the foreign AAA server, and the home AAA
server when the mobile node gains access to the foreign network, in
order to authenticate the mobile node's identity and determine if the
mobile node is authorized for mobility service.
The lack of connection to the AAA infrastructure also means the home
agent does not know where to issue accounting records at appropriate
times during the mobile node's session, as determined by the business
relationship between the home ISP and the mobile node's owner.
Presumably, some backend AAA protocol between the home agent and home
AAA could be utilized but IKEv1 does not contain support for
exchanging the right kind of information, primarily the NAI [6], with
the mobile node.
3.2.2 "Opportunistic" or "Local" Discovery
The home agent discovery protocol does not support "opportunistic" or
local discovery mechanisms in a roaming partner's local access
network. It is expected that the mobile node must know the prefix of
the home subnet in order to be able to discover a home agent, it must
either obtain that information through prefix update or have it
statically configured. A more typical pattern for interdomain
service discovery in the Internet is that the client (mobile node in
this case) knows the domain name of the service, and uses DNS in some
manner to find the server in the other domain. For local service
discovery, DHCP is typically used.
3.3 Topology Change
3.3.1 Dormant Mode Mobile Nodes
The description of the protocol to push prefix information to mobile
nodes in Section 10.6 has an implicit assumption that the mobile node
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is active and taking IP traffic. In fact, many, if not most, mobile
devices will be in a low power "dormant mode" to save battery power,
or even switched off, so they will miss any propagation of prefix
information. As a practical matter, if this protocol is used, an ISP
will need to keep the old prefix around and handle any queries to the
old home agent anycast address on the old subnet, whereby the mobile
node asks for a new home agent as described in Section 11.4, until
all mobile nodes are accounted for. Even then, since some mobile
nodes are likely to be turned off for long periods, some owners would
need to be contacted by other means, reducing the utility of the
protocol.
3.3.2 Use of ICMP
Many ISPs now routinely block ICMP at firewalls as a blanket security
measure, to remove the possibility of ping attacks, etc. Requiring
them to pass the Mobile IPv6 prefix update and home agent discovery
messages is likely to meet with a skeptical response.
While the ICMP messages associated with the prefix update are
required to be sent within the mobile node - home agent IPsec
security association, the home agent discovery message is sent to an
anycast address. Securing anycast messages is, however, difficult
with IPsec. As a result, at least some of the ICMP messages have to
be processed in the clear. While the specific threats relating to
the discovery of home agent addresses are not that significant, it is
at least necessary for the ICMP messages to pass firewalls.
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4. Bootstrapping Scenarios
In this section, we discuss four different scenarios involving
bootstrapping.
The simplest bootstrapping scenario involves the creation of the
security association "from thin air", i.e., without any pre-existing
relationship. This could be achieved using, for instance, SSH-style
leap-of-faith or other weak authentication mechanisms [9].
Unfortunately, as discussed in Section 6, some of the assumptions of
the base Mobile IPv6 protocol rely on there being at least some
administrative relationship between the mobile node and its home
agent. As a result, either this approach should be ruled out, or the
assumptions of the base protocol removed through extensions.
Another scenario involves turning an existing security association in
the user's home network for a different purpose into a new security
association suitable for protecting Mobile IPv6. For instance, an
existing security association for a VPN service could be used to
generate suitable Mobile IPv6 security associations, on a
first-come-first-served home address basis.
The third scenario is similar to the second one, but utilizes a
security association from one of the access networks to which the
node is connected, if available, rather than from the home network.
The network access security relationship is used in order to create a
security association suitable for Mobile IPv6. For instance, when
the mobile node boots and connects to the network for the first time,
it could create a security association with the access operator's
home agent. This home agent could then be used as the mobile node
moves into a different position or even into a different access
network.
The fourth scenario involves turning an existing security association
with a home agent into a new one. For instance, the existing
security association for one home address can be used to communicate
changed addresses and home agents. Based on this the parties can
modify their security policy entries and authorization tables.
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5. Conclusions
The ability to bootstrap security associations for Mobile IPv6 is
necessary for many purposes, and can be expected to have a
significant impact on the speed with which the protocol can be
deployed. New protocol mechanisms are required for this bootstrap to
become possible, however, as the base Mobile IPv6 protocol does not
accommodate for it.
We expect the bootstrapping mechanisms to focus on scenarios 2, 3,
and 4 described in Section 4, i.e., bootstrapping based on an
existing home network security association, network access security
association, or the modification of an existing security association
for Mobile IPv6.
The ability of the mobile node to dynamically locate a home agent
impacts whether the mobile node can set up the IPsec security
association, and the constraints on the ability of the mobile node to
dynamically configure the IPsec security association also constrain
how dynamic home agent location can be. The current mobile node -
home agent IPsec SA bootstrapping procedure is constrained by the
requirements of IKEv1 dynamic key exchange. These constraints, in
turn, make dynamic home address assignment, dynamic home agent
assignment, and proper integration with AAA infrastructure difficult.
The IKEv2 design [8] is much less constrained in many of these areas,
and may be a good candidate for a more flexible bootstrapping
procedure.
The design intent of the prefix updating and home agent discovery
protocols described in Sections 10.6 and 11.4 of [4] is to extend the
same kind of subnet configuration service enjoyed by hosts and
routers on a local subnet (address autoconfiguration, router
discovery) to a remote mobile node, using a similar mechanism (home
address autoconfiguration, home agent discovery). This basic subnet
configuration mechanism is not well suited to a loose collection of
perhaps millions of roaming mobile nodes. Some utilization of
existing interdomain mechanisms for bootstrapping home network
mobility service from a foreign domain and standard service
configuration mechanisms for performing the same function within a
roaming partner's network is more likely to be viable. On the other
hand, IP layer mechanisms for bootstrapping Mobile IPv6 should not be
bound to mechanisms specific to a particular type of access network
technology, wireless technology, or ISP, in order to ease the use of
the mechanism across many different kinds of access networks and
ISPs.
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6. Security Considerations
This document does not propose any new protocols, and therefore does
not involve any security considerations in that sense. However,
throughout this document there are discussions of problems with
Mobile IPv6 involving the mobile node - home agent IPsec security
association.
When considering different bootstrapping solutions, it is important
to keep the security assumptions of the Mobile IPv6 protocol design
in mind. In particular, the protocol relies on the home agent's
operator to have an administrative relationship with the mobile
node's user. Through this relationship, rogue mobile nodes can be
tracked down. Completely automatic bootstrapping without any
pre-existing relationship is thus out of the question, unless
additional defenses (such as new care-of address verification) are
built into the Mobile IPv6 protocol.
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Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
[3] Narten, T. and R. Draves, "Privacy Extensions for Stateless
Address Autoconfiguration in IPv6", RFC 3041, January 2001.
[4] Johnson, D., Perkins, C. and J. Arkko, "Mobility Support in
IPv6", draft-ietf-mobileip-ipv6-24 (work in progress), July
2003.
[5] Arkko, J., Devarapalli, V. and F. Dupont, "Using IPsec to
Protect Mobile IPv6 Signaling between Mobile Nodes and Home
Agents", draft-ietf-mobileip-mipv6-ha-ipsec-06 (work in
progress), July 2003.
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Informative References
[6] Aboba, B. and M. Beadles, "The Network Access Identifier", RFC
2486, January 1999.
[7] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and M.
Carney, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
[8] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
draft-ietf-ipsec-ikev2-11 (work in progress), October 2003.
[9] Arkko, J. and P. Nikander, "How to Authenticate Unknown
Principals without Trusted Parties", To appear in Proceedings
of Security Protocols Workshop 2002, Cambridge, UK, April 2002.
[10] Montenegro, G., Patil, B., Arkko, J. and J. Kempf, "Thoughts on
Bootstrapping Mobility Securely", Presentation in MIP6 WG in
IETF-57, July 2003.
[11] Arkko, J. and C. Perkins, "Alternative (Future) Proposals for
MIPv6 Security", Presentation in MIP6 WG in IETF-57, July 2003.
Authors' Addresses
James Kempf
DoCoMo Communications Labs USA
181 Metro Drive
San Jose, CA 94043
USA
EMail: kempf@docomolabs-usa.com
Jari Arkko
Ericsson
Jorvas 02420
Finland
EMail: jari.arkko@ericsson.com
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Appendix A. Acknowledgements
The authors would like to thank Tom Hiller and Gabriel Montenegro for
interesting discussions in this problem space. Part of this draft is
based on ideas presented in [10] and [11].
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