





Network Working Group                                           J. Arkko
Request for Comments: 3776                                      Ericsson
Category: Standards Track                                 V. Devarapalli
                                                   Nokia Research Center
                                                               F. Dupont
                                                       GET/ENST Bretagne
                                                               June 2004


         Using IPsec to Protect Mobile IPv6 Signaling Between
                      Mobile Nodes and Home Agents

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   Mobile IPv6 uses IPsec to protect signaling between the home agent
   and the mobile node.  Mobile IPv6 base document defines the main
   requirements these nodes must follow.  This document discusses these
   requirements in more depth, illustrates the used packet formats,
   describes suitable configuration procedures, and shows how
   implementations can process the packets in the right order.

Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.    Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.    Packet Formats . . . . . . . . . . . . . . . . . . . . . . .  5
         3.1   Binding Updates and Acknowledgements . . . . . . . . .  5
         3.2   Return Routability Signaling . . . . . . . . . . . . .  7
         3.3   Prefix Discovery . . . . . . . . . . . . . . . . . . .  8
         3.4   Payload Packets  . . . . . . . . . . . . . . . . . . .  9
   4.    Requirements . . . . . . . . . . . . . . . . . . . . . . . .  9
         4.1   Mandatory Support  . . . . . . . . . . . . . . . . . . 10
         4.2   Policy Requirements  . . . . . . . . . . . . . . . . . 10
         4.3   IPsec Protocol Processing  . . . . . . . . . . . . . . 13
         4.4   Dynamic Keying . . . . . . . . . . . . . . . . . . . . 15
   5.    Example Configurations . . . . . . . . . . . . . . . . . . . 16



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         5.1   Format . . . . . . . . . . . . . . . . . . . . . . . . 17
         5.2   Manual Configuration . . . . . . . . . . . . . . . . . 18
               5.2.1 Binding Updates and Acknowledgements . . . . . . 18
               5.2.2 Return Routability Signaling . . . . . . . . . . 19
               5.2.3 Prefix Discovery . . . . . . . . . . . . . . . . 20
               5.2.4 Payload Packets  . . . . . . . . . . . . . . . . 21
         5.3   Dynamic Keying . . . . . . . . . . . . . . . . . . . . 22
               5.3.1 Binding Updates and Acknowledgements . . . . . . 22
               5.3.2 Return Routability Signaling . . . . . . . . . . 23
               5.3.3 Prefix Discovery . . . . . . . . . . . . . . . . 24
               5.3.4 Payload Packets  . . . . . . . . . . . . . . . . 25
   6.    Processing Steps within a Node . . . . . . . . . . . . . . . 25
         6.1   Binding Update to the Home Agent . . . . . . . . . . . 25
         6.2   Binding Update from the Mobile Node  . . . . . . . . . 26
         6.3   Binding Acknowledgement to the Mobile Node . . . . . . 27
         6.4   Binding Acknowledgement from the Home Agent  . . . . . 28
         6.5   Home Test Init to the Home Agent . . . . . . . . . . . 29
         6.6   Home Test Init from the Mobile Node  . . . . . . . . . 30
         6.7   Home Test to the Mobile Node . . . . . . . . . . . . . 30
         6.8   Home Test from the Home Agent  . . . . . . . . . . . . 31
         6.9   Prefix Solicitation Message to the Home Agent  . . . . 31
         6.10  Prefix Solicitation Message from the Mobile Node . . . 31
         6.11  Prefix Advertisement Message to the Mobile Node  . . . 32
         6.12  Prefix Advertisement Message from the Home Agent . . . 32
         6.13  Payload Packet to the Home Agent . . . . . . . . . . . 32
         6.14  Payload Packet from the Mobile Node  . . . . . . . . . 32
         6.15  Payload Packet to the Mobile Node  . . . . . . . . . . 32
         6.16  Payload Packet from the Home Agent . . . . . . . . . . 32
         6.17  Establishing New Security Associations . . . . . . . . 32
         6.18  Rekeying Security Associations . . . . . . . . . . . . 33
         6.19  Movements and Dynamic Keying . . . . . . . . . . . . . 34
   7.    Implementation Considerations  . . . . . . . . . . . . . . . 35
         7.1   IPsec  . . . . . . . . . . . . . . . . . . . . . . . . 35
         7.2   IKE  . . . . . . . . . . . . . . . . . . . . . . . . . 36
         7.3   Bump-in-the-Stack  . . . . . . . . . . . . . . . . . . 37
   8.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 37
   9.    Security Considerations  . . . . . . . . . . . . . . . . . . 37
   10    References . . . . . . . . . . . . . . . . . . . . . . . . . 38
         10.1  Normative References . . . . . . . . . . . . . . . . . 38
         10.2  Informative References . . . . . . . . . . . . . . . . 38
   11.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 39
   12.   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 39
   13.   Full Copyright Statement . . . . . . . . . . . . . . . . . . 40








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1.  Introduction

   This document illustrates the use of IPsec in securing Mobile IPv6
   [7] traffic between mobile nodes and home agents.  In Mobile IPv6, a
   mobile node is always expected to be addressable at its home address,
   whether it is currently attached to its home link or is away from
   home.  The "home address" is an IP address assigned to the mobile
   node within its home subnet prefix on its home link.  While a mobile
   node is at home, packets addressed to its home address are routed to
   the mobile node's home link.

   While a mobile node is attached to some foreign link away from home,
   it is also addressable at a care-of address.  A care-of address is an
   IP address associated with a mobile node that has a subnet prefix
   from a particular foreign link.  The association between a mobile
   node's home address and care-of address is known as a "binding" for
   the mobile node.  While away from home, a mobile node registers its
   primary care-of address with a router on its home link, requesting
   this router to function as the "home agent" for the mobile node.  The
   mobile node performs this binding registration by sending a "Binding
   Update" message to the home agent.  The home agent replies to the
   mobile node by returning a "Binding Acknowledgement" message.

   Any other nodes communicating with a mobile node are referred to as
   "correspondent nodes".  Mobile nodes can provide information about
   their current location to correspondent nodes, again using Binding
   Updates and Acknowledgements.  Additionally, return routability test
   is performed between the mobile node, home agent, and the
   correspondent node in order to authorize the establishment of the
   binding.  Packets between the mobile node and the correspondent node
   are either tunneled via the home agent, or sent directly if a binding
   exists in the correspondent node for the current location of the
   mobile node.

   Mobile IPv6 tunnels payload packets between the mobile node and the
   home agent in both directions.  This tunneling uses IPv6
   encapsulation [6].  Where these tunnels need to be secured, they are
   replaced by IPsec tunnels [2].

   Mobile IPv6 also provides support for the reconfiguration of the home
   network.  Here, the home subnet prefixes may change over time.
   Mobile nodes can learn new information about home subnet prefixes
   through the "prefix discovery" mechanism.

   This document discusses security mechanisms for the control traffic
   between the mobile node and the home agent.  If this traffic is not
   protected, mobile nodes and correspondent nodes are vulnerable to
   man-in-the-middle, hijacking, passive wiretapping, impersonation, and



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   denial-of-service attacks.  Any third parties are also vulnerable to
   denial-of-service attacks, for instance if an attacker could direct
   the traffic flowing through the home agent to a innocent third party.
   These attacks are discussed in more detail in Section 15.1 of the
   Mobile IPv6 base specification [7].

   In order to avoid these attacks, the base specification uses IPsec
   Encapsulating Security Payload (ESP) [3] to protect control traffic
   between the home agent and the mobile node.  This control traffic
   consists of various messages carried by the Mobility Header protocol
   in IPv6 [5].  The traffic takes the following forms:

   o  Binding Update and Acknowledgement messages exchanged between the
      mobile node and the home agent, as described in Sections 10.3.1,
      10.3.2, 11.7.1, and 11.7.3 of the base specification [7].

   o  Return routability messages Home Test Init and Home Test that pass
      through the home agent on their way to a correspondent node, as
      described in Section 10.4.6 of the base specification [7].

   o  ICMPv6 messages exchanged between the mobile node and the home
      agent for the purposes of prefix discovery, as described in
      Sections 10.6 and 11.4 of the base specification [7].

   The nodes may also optionally protect payload traffic passing through
   the home agent, as described in Section 5.5 of the base specification
   [7].  If multicast group membership control protocols or stateful
   address autoconfiguration protocols are supported, payload data
   protection support is required.

   The control traffic between the mobile node and the home agent
   requires message authentication, integrity, correct ordering and
   anti-replay protection.  The mobile node and the home agent must have
   an IPsec security association to protect this traffic.  IPsec does
   not proving correct ordering of messages.  Correct ordering of the
   control traffic is ensured by a sequence number in the Binding Update
   and Binding Acknowledgement messages.  The sequence number in the
   Binding Updates also provides protection to a certain extent.  It
   fails in some scenarios, for example, if the Home Agent loses the
   Binding Cache state.  Full protection against replay attacks is
   possible only when IKE is used.

   Great care is needed when using IKE [4] to establish security
   associations to Mobile IPv6 home agents.  The right kind of addresses
   must be used for transporting IKE.  This is necessary to avoid
   circular dependencies in which the use of a Binding Update triggers
   the need for an IKE exchange that cannot complete prior to the
   Binding Update having been completed.



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   The mobile IPv6 base document defines the main requirements the
   mobile nodes and home agents must follow when securing the above
   traffic.  This document discusses these requirements in more depth,
   illustrates the used packet formats, describes suitable configuration
   procedures, and shows how implementations can process the packets in
   the right order.

   We begin our description by showing the required wire formats for the
   protected packets in Section 3.  Section 4 describes rules which
   associated Mobile IPv6, IPsec, and IKE implementations must observe.
   Section 5 discusses how to configure either manually keyed IPsec
   security associations or how to configure IKE to establish them
   automatically.  Section 6 shows examples of how packets are processed
   within the nodes.

   All implementations of Mobile IPv6 mobile node and home agent MUST
   support at least the formats described in Section 3 and obey the
   rules in Section 4.

   The configuration and processing sections are informative, and should
   only be considered as one possible way of providing the required
   functionality.

   Note that where this document indicates a feature MUST be supported
   and SHOULD be used, this implies that all implementations must be
   capable of using the specified feature, but there may be cases where,
   for instance, a configuration option disables to use of the feature
   in a particular situation.

2.  Terminology

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [1].

3.  Packet Formats

3.1.  Binding Updates and Acknowledgements

   When the mobile node is away from its home, the BUs sent by it to the
   home agent MUST support at least the following headers in the
   following order:

      IPv6 header (source = care-of address,
                   destination = home agent)
      Destination Options header
         Home Address option (home address)
      ESP header in transport mode



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      Mobility header
         Binding Update
            Alternate Care-of Address option (care-of address)

   Note that the Alternate Care-of Address option is used to ensure that
   the care-of address is protected by ESP.  The home agent considers
   the address within this option as the current care-of address for the
   mobile node.  The home address is not protected by ESP directly, but
   the use of a specific home address with a specific security
   association is required by policy.

   The Binding Acknowledgements sent back to the mobile node when it is
   away from home MUST support at least the following headers in the
   following order:

      IPv6 header (source = home agent,
                   destination = care-of address)
      Routing header (type 2)
         home address
      ESP header in transport mode
      Mobility header
         Binding Acknowledgement

   When the mobile node is at home, the above rules are different as the
   mobile node can use its home address as a source address.  This
   typically happens for the de-registration Binding Update when the
   mobile is returning home.  In this situation, the Binding Updates
   MUST support at least the following headers in the following order:

      IPv6 header (source = home address,
                   destination = home agent)
      ESP header in transport mode
      Mobility header
         Binding Update

   The Binding Acknowledgement messages sent to the home address MUST
   support at least the following headers in the following order:

      IPv6 header (source = home agent,
                   destination = home address)
      ESP header in transport mode
      Mobility header
         Binding Acknowledgement








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3.2.  Return Routability Signaling

   When the Home Test Init messages tunneled to the home agent are
   protected by IPsec, they MUST support at least the following headers
   in the following order:

      IPv6 header (source = care-of address,
                   destination = home agent)
      ESP header in tunnel mode
      IPv6 header (source = home address,
                   destination = correspondent node)
      Mobility Header
         Home Test Init

   This format assumes that the mobile node's current care-of address is
   used as the outer header destination address in the security
   association.  As discussed in Section 4.3, this requires the home
   agent to update the destination address when the mobile node moves.
   Policy entries and security association selectors stay the same,
   however, as the inner packets do not change upon movements.

   Note that there are trade-offs in using care-of addresses as the
   destination addresses versus using the home address and attaching an
   additional Home Address destination option and/or Routing header to
   the packets.  The basis for requiring support for at least the
   care-of address case has been discussed in Section 7.

   Similarly, when the Home Test messages tunneled from the home agent
   are protected by IPsec, they MUST support at least the following
   headers in the following order:

      IPv6 header (source = home agent,
                   destination = care-of address)
      ESP header in tunnel mode
      IPv6 header (source = correspondent node,
                   destination = home address)
      Mobility Header
         Home Test

   The format used to protect return routability packets relies on the
   destination of the tunnel packets to change for the mobile node as it
   moves.  The home agent's address stays the same, but the mobile
   node's address changes upon movements, as if the security
   association's outer header destination address had changed.  When the
   mobile node adopts a new care-of address, it adopts also a new source
   address for outgoing tunnel packets.  The home agent accepts packets
   sent like this, as the outer source address in tunnel packets is not
   checked according to the rules in RFC 2401.  (We note, however, that



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   some implementations are known to make source address checks.) For a
   discussion of the role of source addresses in outer tunnel headers,
   see Section 5.1.2.1 of RFC 2401 [2].  Note also that the home agent
   requires the packets to be authenticated regardless of the source
   address change, hence the "new" sender must possess the same keys for
   the security association as it had in the previous location.  This
   proves that the sender is the same entity, regardless of the changes
   in the addresses.

   The process is more complicated in the home agent side, as the home
   agent has stored the previous care-of address in its Security
   Association Database as the outer header destination address.  When
   IKE is being used, the mobile node runs it on top of its current
   care-of address, and the resulting tunnel-mode security associations
   will use the same addresses as IKE run over.  In order for the home
   agent to be able to tunnel a Home Test message to the mobile node, it
   uses the current care-of address as the destination of the tunnel
   packets, as if the home agent had modified the outer header
   destination address in the security association used for this
   protection.  This implies that the same security association can be
   used in multiple locations, and no new configuration or
   re-establishment of IKE phases is needed per movement.  Section 5.2.2
   discusses the security policy and security association database
   entries that are needed to accomplish this.

3.3.  Prefix Discovery

   If IPsec is used to protect prefix discovery, requests for prefixes
   from the mobile node to the home agent MUST support at least the
   following headers in the following order.

      IPv6 header (source = care-of address,
                   destination = home agent)
      Destination Options header
         Home Address option (home address)
      ESP header in transport mode
      ICMPv6
         Mobile Prefix Solicitation

   Again if IPsec is used, solicited and unsolicited prefix information
   advertisements from the home agent to the mobile node MUST support at
   least the following headers in the following order.

      IPv6 header (source = home agent,
                   destination = care-of address)
      Routing header (type 2)
         home address
      ESP header in transport mode



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      ICMPv6
         Mobile Prefix Advertisement

3.4.  Payload Packets

   If IPsec is used to protect payload packets tunneled to the home
   agent from the mobile node, we use a format similar to the one in
   Section 3.2.  However, instead of the MobilityHeader, these packets
   may contain any legal IPv6 protocol(s):

      IPv6 header (source = care-of address,
                   destination = home agent)
      ESP header in tunnel mode
      IPv6 header (source = home address,
                   destination = correspondent node)
      Any protocol

   Similarly, when the payload packets are tunneled from the home agent
   to the mobile node with ESP encapsulation, they MUST support at least
   the following headers in the following order:

      IPv6 header (source = home agent,
                   destination = care-of address)
      ESP header in tunnel mode
      IPv6 header (source = correspondent node,
                   destination = home address)
      Any protocol

4.  Requirements

   This section describes mandatory rules for all Mobile IPv6 mobile
   nodes and home agents.  These rules are necessary in order for it to
   be possible to enable IPsec communications despite movements,
   guarantee sufficient security, and to ensure correct processing order
   of packets.

   The rules in the following sections apply only to the communications
   between home agents and mobile nodes.  They should not be taken as
   requirements on how IPsec in general is used by mobile nodes.












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4.1.  Mandatory Support

   The following requirements apply to both home agents and mobile
   nodes:

   o  Manual configuration of IPsec security associations MUST be
      supported.  The configuration of the keys is expected to take
      place out-of-band, for instance at the time the mobile node is
      configured to use its home agent.

   o  Automatic key management with IKE [4] MAY be supported.  Only
      IKEv1 is discussed in this document.  Other automatic key
      management mechanisms exist and will appear beyond IKEv1, but this
      document does not address the issues related to them.

   o  ESP encapsulation of Binding Updates and Acknowledgements between
      the mobile node and home agent MUST be supported and MUST be used.

   o  ESP encapsulation of the Home Test Init and Home Test messages
      tunneled between the mobile node and home agent MUST be supported
      and SHOULD be used.

   o  ESP encapsulation of the ICMPv6 messages related to prefix
      discovery MUST be supported and SHOULD be used.

   o  ESP encapsulation of the payload packets tunneled between the
      mobile node and home agent MAY be supported and used.

   o  If multicast group membership control protocols or stateful
      address autoconfiguration protocols are supported, payload data
      protection MUST be supported for those protocols.

4.2.  Policy Requirements

   The following requirements apply to both home agents and mobile
   nodes:

   o  As required in the base specification [7], when a packet destined
      to the receiving node is matched against IPsec security policy or
      selectors of a security association, an address appearing in a
      Home Address destination option is considered as the source
      address of the packet.

      Note that the home address option appears before IPsec headers.
      Section 11.3.2 of the base specification describes one possible
      implementation approach for this: The IPsec policy operations can
      be performed at the time when the packet has not yet been modified
      per Mobile IPv6 rules, or has been brought back to its normal form



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      after Mobile IPv6 processing.  That is, the processing of the Home
      Address option is seen as a fixed transformation of the packets
      that does not affect IPsec processing.

   o  Similarly, a home address within a Type 2 Routing header destined
      to the receiving node is considered as the destination address of
      the packet, when a packet is matched against IPsec security policy
      or selectors of a security association.

      Similar implementation considers apply to the Routing header
      processing as was described above for the Home Address destination
      option.

   o  When IPsec is used to protect return routability signaling or
      payload packets, this protection MUST only be applied to the
      return routability packets entering the IPv6 encapsulated tunnel
      interface between the mobile node and the home agent.  This can be
      achieved, for instance, by defining the security policy database
      entries specifically for the tunnel interface.  That is, the
      policy entries are not generally applied on all traffic on the
      physical interface(s) of the nodes, but rather only on traffic
      that enters this tunnel.

   o  The authentication of mobile nodes MAY be based either on machine
      or user credentials.  Note that multi-user operating systems
      typically allow all users of a node to use any of the IP addresses
      assigned to the node.  This limits the capability of the home
      agent to restrict the use of a home address to a particular user
      in such environment.  Where user credentials are applied in a
      multi-user environment, the configuration should authorize all
      users of the node to control all home addresses assigned to the
      node.

   o  When the mobile node returns home and de-registers with the Home
      Agent, the tunnel between the home agent and the mobile node's
      care-of address is torn down.  The security policy entries, which
      were used for protecting tunneled traffic between the mobile node
      and the home agent MUST be made inactive (for instance, by
      removing them and installing them back later through an API).  The
      corresponding security associations could be kept as they are or
      deleted depending on how they were created.  If the security
      associations were created dynamically using IKE, they are
      automatically deleted when they expire.  If the security
      associations were created through manual configuration, they MUST
      be retained and used later when the mobile node moves away from
      home again.  The security associations protecting Binding Updates
      and Acknowledgements, and prefix discovery SHOULD NOT be deleted
      as they do not depend on care-of addresses and can be used again.



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   The following rules apply to mobile nodes:

   o  The mobile node MUST use the Home Address destination option in
      Binding Updates and Mobile Prefix Solicitations, sent to the home
      agent from a care-of address.

   o  When the mobile node receives a changed set of prefixes from the
      home agent during prefix discovery, there is a need to configure
      new security policy entries, and there may be a need to configure
      new security associations.  It is outside the scope of this
      specification to discuss automatic methods for this.

   The following rules apply to home agents:

   o  The home agent MUST use the Type 2 Routing header in Binding
      Acknowledgements and Mobile Prefix Advertisements sent to the
      mobile node, again due to the need to have the home address
      visible when the policy checks are made.

   o  It is necessary to avoid the possibility that a mobile node could
      use its security association to send a Binding Update on behalf of
      another mobile node using the same home agent.  In order to do
      this, the security policy database entries MUST unequivocally
      identify a single security association for protecting Binding
      Updates between any given home address and home agent when
      manually keyed IPsec security associations are used.  When dynamic
      keying is used, the security policy database entries MUST
      unequivocally identify the IKE phase 1 credentials which can be
      used to authorize the creation of security associations for
      protecting Binding Updates for a particular home address.  How
      these mappings are maintained is outside the scope of this
      specification, but they may be maintained, for instance, as a
      locally administered table in the home agent.  If the phase 1
      identity is a Fully Qualified Domain Name (FQDN), secure forms of
      DNS may also be used.

   o  When the set of prefixes advertised by the home agent changes,
      there is a need to configure new security policy entries, and
      there may be a need to configure new security associations.  It is
      outside the scope of this specification to discuss automatic
      methods for this, if new home addresses are required.










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4.3.  IPsec Protocol Processing

   The following requirements apply to both home agents and mobile
   nodes:

   o  When securing Binding Updates, Binding Acknowledgements, and
      prefix discovery, both the mobile nodes and the home agents MUST
      support and SHOULD use the Encapsulating Security Payload (ESP)
      [3] header in transport mode and MUST use a non-null payload
      authentication algorithm to provide data origin authentication,
      connectionless integrity and optional anti-replay protection.

      Mandatory support for encryption and integrity protection
      algorithms is as defined in RFC 2401 [2], RFC 2402 [8], and RFC
      2406 [3].  Care is needed when selecting suitable encryption
      algorithms for ESP, however.  Currently available integrity
      protection algorithms are in general considered to be secure.  The
      encryption algorithm, DES, mandated by the current IPsec standards
      is not, however.  This is particularly problematic when IPsec
      security associations are configured manually, as the same key is
      used for a long time.

   o  Tunnel mode IPsec ESP MUST be supported and SHOULD be used for the
      protection of packets belonging to the return routability
      procedure.  A non-null encryption transform and a non-null
      authentication algorithm MUST be applied.

      Note that the return routability procedure involves two message
      exchanges from the mobile node to the correspondent node.  The
      purpose of these exchanges is to assure that the mobile node is
      live at the claimed home and care-of addresses.  One of the
      exchanges is sent directly to and from the correspondent node,
      while another one is tunneled through the home agent.  If an
      attacker is on the mobile node's link and the mobile node's
      current link is an unprotected wireless link, the attacker would
      able to see both sets of messages, and launch attacks based on it
      (these attacks are discussed further in Section 15.4 of the base
      specification [7].)  One can prevent the attack by making sure
      that the packets tunneled through the home agent are encrypted.

      Note that this specification concerns itself only with on-the-wire
      formats, and does not dictate specific implementations mechanisms.
      In the case of IPsec tunnel mode, the use of IP-in-IP
      encapsulation followed by IPsec transport mode encapsulation may
      also be possible.






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   The following rules apply to mobile nodes:

   o  When ESP is used to protect Binding Updates, there is no
      protection for the care-of address which appears in the IPv6
      header outside the area protected by ESP.  It is important for the
      home agent to verify that the care-of address has not been
      tampered with.  As a result, the attacker would have redirected
      the mobile node's traffic to another address.  In order to prevent
      this, Mobile IPv6 implementations MUST use the Alternate Care-of
      Address mobility option in Binding Updates sent by mobile nodes
      while away from home.  The exception to this is when the mobile
      node returns home and sends a Binding Update to the home agent in
      order to de-register.  In this case no Alternate Care-of Address
      option is needed, as described in Section 3.1.

      When IPsec is used to protect return routability signaling or
      payload packets, the mobile node MUST set the source address it
      uses for the outgoing tunnel packets to the current primary care-
      of address.  The mobile node starts to use a new primary care-of
      address immediately after sending a Binding Update to the home
      agent to register this new address.  Similarly, it starts to use
      the new address as the required destination address of tunneled
      packets received from the home agent.

   The following rules apply to home agents:

   o  When IPsec is used to protect return routability signaling or
      payload packets, IPsec security associations are needed to provide
      this protection.  When the care-of address for the mobile node
      changes as a result of an accepted Binding Update, special
      treatment is needed for the next packets sent using these security
      associations.  The home agent MUST set the new care-of address as
      the destination address of these packets, as if the outer header
      destination address in the security association had changed.
      Similarly, the home agent starts to expect the new source address
      in the tunnel packets received from the mobile node.

      Such address changes can be implemented, for instance, through an
      API from the Mobile IPv6 implementation to the IPsec
      implementation.  It should be noted that the use of such an API
      and the address changes MUST only be done based on the Binding
      Updates received by the home agent and protected by the use of
      IPsec.  Address modifications based on other sources, such as
      Binding Updates to the correspondent nodes protected by return
      routability, or open access to an API from any application may
      result in security vulnerabilities.





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4.4.  Dynamic Keying

   The following requirements apply to both home agents and mobile
   nodes:

   o  If anti-replay protection is required, dynamic keying MUST be
      used.  IPsec can provide anti-replay protection only if dynamic
      keying is used (which may not always be the case).  IPsec also
      does not guarantee correct ordering of packets, only that they
      have not been replayed.  Because of this, sequence numbers within
      the Mobile IPv6 messages are used to ensure correct ordering.
      However, if the 16 bit Mobile IPv6 sequence number space is cycled
      through, or the home agent reboots and loses its state regarding
      the sequence numbers, replay and reordering attacks become
      possible.  The use of dynamic keying, IPsec anti-replay
      protection, and the Mobile IPv6 sequence numbers can together
      prevent such attacks.

   o  If IKE version 1 is used with preshared secrets in main mode, it
      determines the shared secret to use from the IP address of the
      peer.  With Mobile IPv6, however, this may be a care-of address
      and does not indicate which mobile node attempts to contact the
      home agent.  Therefore, if preshared secret authentication is used
      in IKEv1 between the mobile node and the home agent then
      aggressive mode MUST be used.  Note also that care needs to be
      taken with phase 1 identity selection.  Where the ID_IPV6_ADDR
      Identity Payloads is used, unambiguous mapping of identities to
      keys is not possible.  (The next version of IKE may not have these
      limitations.)

   Note that the difficulties with main mode and preshared secrets in
   IKE version 1 are well known for dynamic addresses.  With static
   addresses, there has not been a problem.  With Mobile IPv6, however,
   the use of the care-of addresses to run IKE to the home agent
   presents a problem even when the home address stays stable.  Further
   discussion about the use of care-of addresses in this way appears in
   Section 7.

   The following rules apply to mobile nodes:

   o  In addition to the rules above, if dynamic keying is used, the key
      management protocol MUST use the care-of address as the source
      address in the protocol exchanges with the mobile node's home
      agent.







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   o  However, the IPsec security associations with the mobile node's
      home agent use home addresses.  That is, the IPsec security
      associations MUST be requested from the key management protocol
      using the home address of the mobile node as the client identity.

      The security associations for protecting Binding Updates and
      Acknowledgements are requested for the Mobility header protocol in
      transport mode and for specific IP addresses as endpoints.  No
      other selectors are used.  Similarly, the security associations
      for protecting prefix discovery are requested for the ICMPv6
      protocol and the specific IP addresses, again without other
      selectors.  Security associations for payload and return
      routability protection are requested for a specific tunnel
      interface and either the payload protocol or the Mobility header
      protocol, in tunnel mode.  In this case one requested endpoint is
      an IP address and the other one is a wildcard, and there are no
      other selectors.

   o  If the mobile node has used IKE version 1 to establish security
      associations with its home agent, it should follow the procedures
      discussed in Section 11.7.1 and 11.7.3 of the base specification
      [7] to determine whether the IKE endpoints can be moved or if IKE
      phase 1 has to be re-established.

   The following rules apply to home agents:

   o  If the home agent has used IKE version 1 to establish security
      associations with the mobile node, it should follow the procedures
      discussed in Section 10.3.1 and 10.3.2 of the base specification
      [7] to determine whether the IKE endpoints can be moved or if IKE
      phase 1 has to be re-established.

5.  Example Configurations

   In the following we describe the Security Policy Database (SPD) and
   Security Association Database (SAD) entries necessary to protect
   Binding Updates and Binding Acknowledgements exchanged between the
   mobile node and the home agent.

   Section 5.1 introduces the format we use in the description of the
   SPD and the SAD.  Section 5.2 describes how to configure manually
   keyed IPsec security associations without dynamic keying, and Section
   5.3 describes how to use dynamic keying.








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5.1.  Format

   The format used in the examples is as follows.  The SPD description
   has the format

     <node> "SPD OUT:"
       "-" <spdentry>
       "-" <spdentry>
       ...
       "-" <spdentry>

     <node> "SPD IN:"
       "-" <spdentry>
       "-" <spdentry>
       ...
       "-" <spdentry>

   Where <node> represents the name of the node, and <spdentry> has the
   following format:

     "IF" <condition> "THEN USE SA " <sa> |
     "IF" <condition> "THEN USE SA " <pattern> |

   Where <condition> is a boolean expression about the fields of the
   IPv6 packet, <sa> is the name of a specific security association, and
   <pattern> is a specification for a security association to be
   negotiated via IKE [4].  The SAD description has the format

     <node> "SAD:"
       "-" <sadentry>
       "-" <sadentry>
       ...
       "-" <sadentry>

   Where <node> represents the name of the node, and <sadentry> has the
   following format:

     <sa> "(" <dir> ","
              <spi> ","
              <destination> ","
              <ipsec-proto> ","
              <mode> ")" ":"
          <rule>








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   Where <dir> is "IN" or "OUT", <spi> is the SPI of the security
   association, <destination> is its destination, <ipsec-proto> is in
   our case "ESP", <mode> is either "TUNNEL" or "TRANSPORT", and <rule>
   is an expression which describes the IPsec selectors, i.e., which
   fields of the IPv6 packet must have which values.

   We will be using an example mobile node in this section with the home
   address "home_address_1".  The user's identity in this mobile node is
   "user_1".  The home agent's address is "home_agent_1".

5.2.  Manual Configuration

5.2.1.  Binding Updates and Acknowledgements

   Here are the contents of the SPD and SAD for protecting Binding
   Updates and Acknowledgements:

     mobile node SPD OUT:
       - IF source = home_address_1 & destination = home_agent_1 &
            proto = MH
         THEN USE SA SA1

     mobile node SPD IN:
       - IF source = home_agent_1 & destination = home_address_1 &
            proto = MH
         THEN USE SA SA2

     mobile node SAD:
       - SA1(OUT, spi_a, home_agent_1, ESP, TRANSPORT):
         source = home_address_1 & destination = home_agent_1 &
         proto = MH
       - SA2(IN, spi_b, home_address_1, ESP, TRANSPORT):
         source = home_agent_1 & destination = home_address_1 &
         proto = MH

     home agent SPD OUT:
       - IF source = home_agent_1 & destination = home_address_1 &
            proto = MH
         THEN USE SA SA2

     home agent SPD IN:
       - IF source = home_address_1 & destination = home_agent_1 &
            proto = MH
         THEN USE SA SA1

     home agent SAD:
       - SA2(OUT, spi_b, home_address_1, ESP, TRANSPORT):
         source = home_agent_1 & destination = home_address_1 &



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         proto = MH
       - SA1(IN, spi_a, home_agent_1, ESP, TRANSPORT):
         source = home_address_1 & destination = home_agent_1 &
         proto = MH

   In the above, "MH" refers to the protocol number for the Mobility
   Header [7].

5.2.2.  Return Routability Signaling

   In the following we describe the necessary SPD and SAD entries to
   protect return routability signaling between the mobile node and the
   home agent.  Note that the rules in the SPD are ordered, and the ones
   in the previous section must take precedence over these ones.  In
   other words, the higher precedence entries must occur first in the
   RFC 2401 [2] ordered list of SPD entries.

     mobile node SPD OUT:
       - IF interface = IPv6 IPv6 tunnel to home_agent_1 &
            source = home_address_1 & destination = any &
            proto = MH
         THEN USE SA SA3

     mobile node SPD IN:
       - IF interface = IPv6 tunnel from home_agent_1 &
            source = any & destination = home_address_1 &
            proto = MH
         THEN USE SA SA4

     mobile node SAD:
       - SA3(OUT, spi_c, home_agent_1, ESP, TUNNEL):
         source = home_address_1 & destination = any & proto = MH
       - SA4(IN, spi_d, care_of_address_1, ESP, TUNNEL):
         source = any & destination = home_address_1 & proto = MH

     home agent SPD OUT:
       - IF interface = IPv6 tunnel to home_address_1 &
            source = any & destination = home_address_1 &
            proto = MH
         THEN USE SA SA4

     home agent SPD IN:
       - IF interface = IPv6 tunnel from home_address_1 &
            source = home_address_1 & destination = any &
            proto = MH
         THEN USE SA SA3





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     home agent SAD:
       - SA4(OUT, spi_d, care_of_address_1, ESP, TUNNEL):
         source = any & destination = home_address_1 & proto = MH
       - SA3(IN, spi_c, home_agent_1, ESP, TUNNEL):
         source = home_address_1 & destination = any & proto = MH

   The security association from the home agent to the mobile node uses
   the current care-of address as the destination.  As discussed
   earlier, this address is updated in the SAD as the mobile node moves.
   It can be initialized to the home address before the mobile node has
   registered.

5.2.3.  Prefix Discovery

   In the following we describe some additional SPD and SAD entries to
   protect prefix discovery.  Note that the SPDs described above protect
   all ICMPv6 traffic between the mobile node and the home agent, as
   IPsec may not have the ability to distinguish between different
   ICMPv6 types.

     mobile node SPD OUT:
       - IF source = home_address_1 & destination = home_agent_1 &
            proto = ICMPv6
         THEN USE SA SA5.

     mobile node SPD IN:
       - IF source = home_agent_1 & destination = home_address_1 &
            proto = ICMPv6
         THEN USE SA SA6

     mobile node SAD:
       - SA5(OUT, spi_e, home_agent_1, ESP, TRANSPORT):
         source = home_address_1 & destination = home_agent_1 &
         proto = ICMPv6
       - SA6(IN, spi_f, home_address_1, ESP, TRANSPORT):
         source = home_agent_1 & destination = home_address_1 &
         proto = ICMPv6

     home agent SPD OUT:
       - IF source = home_agent_1 & destination = home_address_1 &
            proto = ICMPv6
         THEN USE SA SA6

     home agent SPD IN:
       - IF source = home_address_1 & destination = home_agent_1 &
            proto = ICMPv6
         THEN USE SA SA5




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     home agent SAD:
       - SA6(OUT, spi_f, home_address_1, ESP, TRANSPORT):
         source = home_agent_1 & destination = home_address_1 &
         proto = ICMPv6
       - SA5(IN, spi_e, home_agent_1, ESP, TRANSPORT):
         source = home_address_1 & destination = home_agent_1 &
         proto = ICMPv6

5.2.4.  Payload Packets

   It is also possible to perform some additional, optional, protection
   of tunneled payload packets.  This protection takes place in a
   similar manner to the return routability protection above, but
   requires a different value for the protocol field.  The necessary SPD
   and SAD entries are shown below.  It is assumed that the entries for
   protecting Binding Updates and Acknowledgements, and the entries to
   protect Home Test Init and Home Test messages take precedence over
   these entries.

     mobile node SPD OUT:
       - IF interface = IPv6 tunnel to home_agent_1 &
            source = home_address_1 & destination = any &
            proto = X
         THEN USE SA SA7

     mobile node SPD IN:
       - IF interface = IPv6 tunnel from home_agent_1 &
            source = any & destination = home_address_1 &
            proto = X
         THEN USE SA SA8

     mobile node SAD:
       - SA7(OUT, spi_g, home_agent_1, ESP, TUNNEL):
         source = home_address_1 & destination = any & proto = X
       - SA8(IN, spi_h, care_of_address_1, ESP, TUNNEL):
         source = any & destination = home_address_1 & proto = X

     home agent SPD OUT:
       - IF interface = IPv6 tunnel to home_address_1 &
            source = any & destination = home_address_1 &
            proto = X
         THEN USE SA SA8

     home agent SPD IN:
       - IF interface = IPv6 tunnel from home_address_1 &
            source = home_address_1 & destination = any &
            proto = X
         THEN USE SA SA7



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     home agent SAD:
       - SA8(OUT, spi_h, care_of_address_1, ESP, TUNNEL):
         source = any & destination = home_address_1 & proto = X
       - SA7(IN, spi_g, home_agent_1, ESP, TUNNEL):
         source = home_address_1 & destination = any & proto = X

   If multicast group membership control protocols such as MLDv1 [9] or
   MLDv2 [11] need to be protected, these packets may use a link-local
   address rather than the home address of the mobile node.  In this
   case the source and destination can be left as a wildcard and the SPD
   entries will work solely based on the used interface and the
   protocol, which is ICMPv6 for both MLDv1 and MLDv2.

   Similar problems are encountered when stateful address
   autoconfiguration protocols such as DHCPv6 [10] are used.  The same
   approach is applicable for DHCPv6 as well.  DHCPv6 uses the UDP
   protocol.

   Support for multiple layers of encapsulation (such as ESP
   encapsulated in ESP) is not required by RFC 2401 [2] and is also
   otherwise often problematic.  It is therefore useful to avoid setting
   the protocol X in the above entries to either AH or ESP.

5.3.  Dynamic Keying

   In this section we show an example configuration that uses IKE to
   negotiate security associations.

5.3.1.  Binding Updates and Acknowledgements

   Here are the contents of the SPD for protecting Binding Updates and
   Acknowledgements:

     mobile node SPD OUT:
       - IF source = home_address_1 & destination = home_agent_1 &
            proto = MH
         THEN USE SA ESP TRANSPORT: local phase 1 identity = user_1

     mobile node SPD IN:
       - IF source = home_agent_1 & destination = home_address_1 &
            proto = MH
         THEN USE SA ESP TRANSPORT: local phase 1 identity = user_1

     home agent SPD OUT:
       - IF source = home_agent_1 & destination = home_address_1 &
            proto = MH
         THEN USE SA ESP TRANSPORT: peer phase 1 identity = user_1




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     home agent SPD IN:
       - IF source = home_address_1 & destination = home_agent_1 &
            proto = MH
         THEN USE SA ESP TRANSPORT: peer phase 1 identity = user_1

   We have omitted details of the proposed transforms in the above, and
   all details related to the particular authentication method such as
   certificates beyond listing a specific identity that must be used.

   We require IKE version 1 to be run using the care-of addresses but
   still negotiate IPsec SAs that use home addresses.  The extra
   conditions set by the home agent SPD for the peer phase 1 identity to
   be "user_1" must be verified by the home agent.  The purpose of the
   condition is to ensure that the IKE phase 2 negotiation for a given
   user's home address can not be requested by another user.  In the
   mobile node, we simply set our local identity to be "user_1".

   These checks also imply that the configuration of the home agent is
   user-specific: every user or home address requires a specific
   configuration entry.  It would be possible to alleviate the
   configuration tasks by using certificates that have home addresses in
   the Subject AltName field.  However, it is not clear if all IKE
   implementations allow one address to be used for carrying the IKE
   negotiations when another address is mentioned in the used
   certificates.  In any case, even this approach would have required
   user-specific tasks in the certification authority.

5.3.2.  Return Routability Signaling

   Protection for the return routability signaling can be configured in
   a similar manner as above.

     mobile node SPD OUT:
       - IF interface = IPv6 tunnel to home_agent_1 &
            source = home_address_1 & destination = any &
            proto = MH
         THEN USE SA ESP TUNNEL: outer destination = home_agent_1 &
                                 local phase 1 identity = user_1

     mobile node SPD IN:
       - IF interface = IPv6 tunnel from home_agent_1 &
            source = any & destination = home_address_1 &
            proto = MH
         THEN USE SA ESP TUNNEL: outer destination = home_agent_1 &
                                 local phase 1 identity = user_1






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     home agent SPD OUT:
       - IF interface = IPv6 tunnel to home_address_1 &
            source = any & destination = home_address_1 &
            proto = MH
         THEN USE SA ESP TUNNEL: outer destination = home_address_1 &
                                 peer phase 1 identity = user_1

     home agent SPD IN:
       - IF interface = IPv6 tunnel from home_address_1 &
            source = home_address_1 & destination = any &
            proto = MH
         THEN USE SA ESP TUNNEL: outer destination = home_address_1 &
                                 peer phase 1 identity = user_1

   The security association from the home agent to the mobile node uses
   the current care-of address as the destination.  As discussed
   earlier, this address is updated in the SAD as the mobile node moves.
   The SPD entries can be written using the home address (as above), if
   the care-of address update in the SAD is also done upon the creation
   of security associations.

5.3.3.  Prefix Discovery

   In the following we describe some additional SPD entries to protect
   prefix discovery with IKE.  (Note that when actual new prefixes are
   discovered, there may be a need to enter new manually configured SPD
   entries to specify the authorization policy for the resulting new
   home addresses.)

     mobile node SPD OUT:
       - IF source = home_address_1 & destination = home_agent_1 &
            proto = ICMPv6
         THEN USE SA ESP TRANSPORT: local phase 1 identity = user_1

     mobile node SPD IN:
       - IF source = home_agent_1 & destination = home_address_1 &
            proto = ICMPv6
         THEN USE SA ESP TRANSPORT: local phase 1 identity = user_1

     home agent SPD OUT:
       - IF source = home_agent_1 & destination = home_address_1 &
            proto = ICMPv6
         THEN USE SA ESP TRANSPORT: peer phase 1 identity = user_1

     home agent SPD IN:
       - IF source = home_address_1 & destination = home_agent_1 &
            proto = ICMPv6
         THEN USE SA ESP TRANSPORT: peer phase 1 identity = user_1



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5.3.4.  Payload Packets

   Protection for the payload packets happens similarly to the
   protection of return routability signaling.  As in the manually keyed
   case, these SPD entries have lower priority than the above ones.

      mobile node SPD OUT:
        - IF interface = IPv6 tunnel to home_agent_1 &
             source = home_address_1 & destination = any &
             proto = X
          THEN USE SA ESP TUNNEL: outer destination = home_agent_1 &
                                  local phase 1 identity = user_1

      mobile node SPD IN:
        - IF interface = IPv6 tunnel from home_agent_1 &
             source = any & destination = home_address_1 &
             proto = X
          THEN USE SA ESP TUNNEL: outer destination = home_agent_1 &
                                  local phase 1 identity = user_1

      home agent SPD OUT:
        - IF interface = IPv6 tunnel to home_address_1 &
             source = any & destination = home_address_1 &
             proto = X
          THEN USE SA ESP TUNNEL: outer destination = home_address_1 &
                                  peer phase 1 identity = user_1

      home agent SPD IN:
        - IF interface = IPv6 tunnel from home_address_1 &
             source = home_address_1 & destination = any &
             proto = X
          THEN USE SA ESP TUNNEL: outer destination = home_address_1 &
                                  peer phase 1 identity = user_1

6.  Processing Steps within a Node

6.1.  Binding Update to the Home Agent

   Step 1.  At the mobile node, Mobile IPv6 module first produces the
   following packet:

      IPv6 header (source = home address,
                   destination = home agent)
      Mobility header
         Binding Update

   Step 2.  This packet is matched against the IPsec SPD on the mobile
   node and we make a note that IPsec must be applied.



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   Step 3.  Then, we add the necessary Mobile IPv6 options but do not
   change the addresses yet, as described in Section 11.3.2 of the base
   specification [7].  This results in:

      IPv6 header (source = home address,
                   destination = home agent)
      Destination Options header
         Home Address option (care-of address)
      Mobility header
         Binding Update

   Step 4.  Finally, IPsec headers are added and the necessary
   authenticator values are calculated:

      IPv6 header (source = home address,
                   destination = home agent)
      Destination Options header
         Home Address option (care-of address)
      ESP header (SPI = spi_a)
      Mobility header
         Binding Update

   Here spi_a is the SPI value that was either configured manually, or
   agreed upon in an earlier IKE negotiation.

   Step 5.  Before sending the packet, the addresses in the IPv6 header
   and the Destination Options header are changed:

      IPv6 header (source = care-of address,
                   destination = home agent)
      Destination Options header
         Home Address option (home address)
      ESP header (SPI = spi_a)
      Mobility header
         Binding Update

6.2.  Binding Update from the Mobile Node

   Step 1.  The following packet is received at the home agent:

      IPv6 header (source = care-of address,
                   destination = home agent)
      Destination Options header
         Home Address option (home address)
      ESP header (SPI = spi_a)
      Mobility header
         Binding Update




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   Step 2.  The home address option is processed first, which results in

      IPv6 header (source = home address,
                   destination = home agent)
      Destination Options header
         Home Address option (care-of address)
      ESP header (SPI = spi_a)
      Mobility header
         Binding Update

   Step 3.  ESP header is processed next, resulting in

       IPv6 header (source = home address,
                    destination = home agent)
       Destination Options header
          Home Address option (care-of address)
       Mobility header
          Binding Update

   Step 4.  This packet matches the policy required for this security
   association (source = home address, destination = home agent, proto =
   MH).

   Step 5.  Mobile IPv6 processes the Binding Update.  The Binding
   Update is delivered to the Mobile IPv6 module.

   Step 6.  If there are any security associations in the security
   association database for the protection of return routability or
   payload packets for this mobile node, those security associations are
   updated with the new care-of address.

6.3.  Binding Acknowledgement to the Mobile Node

   Step 1.  Mobile IPv6 produces the following packet:

      IPv6 header (source = home agent,
                   destination = home address)
      Mobility header
         Binding Acknowledgement

   Step 2.  This packet matches the IPsec policy entries, and we
   remember that IPsec has to be applied.









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   Step 3.  Then, we add the necessary Route Headers but do not change
   the addresses yet, as described in Section 9.5.4 of the base
   specification [7].  This results in:

      IPv6 header (source = home agent,
                   destination = home address)
      Routing header (type 2)
         care-of address
      Mobility header
         Binding Acknowledgement

   Step 4.  We apply IPsec:

      IPv6 header (source = home agent,
                   destination = home address)
      Routing header (type 2)
         care-of address
      ESP header (SPI = spi_b)
      Mobility header
         Binding Acknowledgement

   Step 5.  Finally, before sending the packet out we change the
   addresses in the IPv6 header and the Route header:

      IPv6 header (source = home agent,
                   destination = care-of address)
      Routing header (type 2)
         home address
      ESP header (SPI = spi_b)
      Mobility header
         Binding Acknowledgement

6.4.  Binding Acknowledgement from the Home Agent

   Step 1.  The following packet is received at the mobile node

      IPv6 header (source = home agent,
                   destination = care-of address)
      Routing header (type 2)
         home address
      ESP header (SPI = spi_b)
      Mobility header
         Binding Acknowledgement








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   Step 2.  After the routing header is processed the packet becomes

      IPv6 header (source = home agent,
                   destination = home address)
      Routing header (type 2)
         care-of address
      ESP header (SPI = spi_b)
      Mobility header
         Binding Acknowledgement

   Step 3.  ESP header is processed next, resulting in:

      IPv6 header (source = home agent,
                   destination = home address)
      Routing header (type 2)
         care-of address
      Mobility header
         Binding Acknowledgement

   Step 4.  This packet matches the policy required for this security
   association (source = home agent, destination = home address, proto =
   MH).

   Step 5.  The Binding Acknowledgement is delivered to the Mobile IPv6
   module.

6.5.  Home Test Init to the Home Agent

   Step 1.  The mobile node constructs a Home Test Init message:

      IPv6 header (source = home address,
                   destination = correspondent node)
      Mobility header
         Home Test Init

   Step 2.  Mobile IPv6 determines that this packet should go to the
   tunnel to the home agent.

   Step 3.  The packet is matched against IPsec policy entries for the
   interface, and we find that IPsec needs to be applied.

   Step 4.  IPsec tunnel mode headers are added.  Note that we use a
   care-of address as a source address for the tunnel packet.

      IPv6 header (source = care-of address,
                   destination = home agent)
      ESP header (SPI = spi_c)
      IPv6 header (source = home address,



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                   destination = correspondent node)
      Mobility header
         Home Test Init

   Step 5.  The packet is sent directly to the home agent using IPsec
   encapsulation.

6.6.  Home Test Init from the Mobile Node

   Step 1.  The home agent receives the following packet:

      IPv6 header (source = care-of address,
                   destination = home agent)
      ESP header (SPI = spi_c)
      IPv6 header (source = home address,
                   destination = correspondent node)
      Mobility Header
         Home Test Init

   Step 2.  IPsec processing is performed, resulting in:

      IPv6 header (source = home address,
                   destination = correspondent node)
      Mobility Header
         Home Test Init

   Step 3.  The resulting packet matches the policy required for this
   security association and the packet can be processed further.

   Step 4.  The packet is then forwarded to the correspondent node.

6.7.  Home Test to the Mobile Node

   Step 1.  The home agent receives a Home Test packet from the
   correspondent node:

      IPv6 header (source = correspondent node,
                   destination = home address)
      Mobility Header
         Home Test Init

   Step 2.  The home agent determines that this packet is destined to a
   mobile node that is away from home, and decides to tunnel it.

   Step 3.  The packet matches the IPsec policy entries for the tunnel
   interface, and we note that IPsec needs to be applied.





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   Step 4.  IPsec is applied, resulting in a new packet.  Note that the
   home agent must keep track of the location of the mobile node, and
   update the tunnel endpoint address in the security association(s)
   accordingly.

      IPv6 header (source = home agent,
                   destination = care-of address)
      ESP header (SPI = spi_d)
      IPv6 header (source = correspondent node,
                   destination = home address)
      Mobility Header
         Home Test Init

   Step 5.  The packet is sent directly to the care-of address using
   IPsec encapsulation.

6.8.  Home Test from the Home Agent

   Step 1.  The mobile node receives the following packet:

      IPv6 header (source = home agent,
                   destination = care-of address)
      ESP header (SPI = spi_d)
      IPv6 header (source = correspondent node,
                   destination = home address)
      Mobility Header
         Home Test Init

   Step 2.  IPsec is processed, resulting in:

      IPv6 header (source = correspondent node,
                   destination = home address)
      Mobility Header
         Home Test Init

   Step 3.  This matches the policy required for this security
   association (source = any, destination = home address).

   Step 4.  The packet is given to Mobile IPv6 processing.

6.9.  Prefix Solicitation Message to the Home Agent

   This procedure is similar to the one presented in Section 6.1.

6.10.  Prefix Solicitation Message from the Mobile Node

   This procedure is similar to the one presented in Section 6.2.




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6.11.  Prefix Advertisement Message to the Mobile Node

   This procedure is similar to the one presented in Section 6.3.

6.12.  Prefix Advertisement Message from the Home Agent

   This procedure is similar to the one presented in Section 6.4.

6.13.  Payload Packet to the Home Agent

   This procedure is similar to the one presented in Section 6.5.

6.14.  Payload Packet from the Mobile Node

   This procedure is similar to the one presented in Section 6.6.

6.15.  Payload Packet to the Mobile Node

   This procedure is similar to the one presented in Section 6.7.

6.16.  Payload Packet from the Home Agent

   This procedure is similar to the one presented in Section 6.8.

6.17.  Establishing New Security Associations

   Step 1.  The mobile node wishes to send a Binding Update to the home
   agent.

     IPv6 header (source = home address,
                  destination = home agent)
     Mobility header
        Binding Update

   Step 2.  There is no existing security association to protect the
   Binding Update, so the mobile node initiates IKE.  The IKE packets
   are sent as shown in the following examples.  The first packet is an
   example of an IKE packet sent from the mobile node, and the second
   one is from the home agent.  The examples shows also that the phase 1
   identity used for the mobile node is a FQDN.

     IPv6 header (source = care-of address,
                  destination = home agent)
        UDP
        IKE
           ... IDii = ID_FQDN mn123.ha.net ...





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     IPv6 header (source = home agent
                  destination = care-of address)
        UDP
        IKE
           ... IDir = ID_FQDN ha.net ...

   Step 3.  IKE phase 1 completes, and phase 2 is initiated to request
   security associations for protecting traffic between the mobile
   node's home address and the home agent.  These addresses will be used
   as selectors.  This involves sending and receiving additional IKE
   packets.  The below example shows again one packet sent by the mobile
   node and another sent by the home agent.  The example shows also that
   the phase 2 identity used for the mobile node is the mobile node's
   home address.

     IPv6 header (source = care-of address,
                  destination = home agent)
        UDP
        IKE
           ... IDci = ID_IPV6_ADDR home address ...

     IPv6 header (source = home agent,
                  destination = care-of address)
        UDP
        IKE
           ... IDcr = ID_IPV6_ADDR home agent ...

   Step 4.  The remaining steps are as shown in Section 6.1.

6.18.  Rekeying Security Associations

   Step 1.  The mobile node and the home agent have existing security
   associations.  Either side may decide at any time that the security
   associations need to be rekeyed, for instance, because the specified
   lifetime is approaching.

   Step 2.  Mobility header packets sent during rekey may be protected
   by the existing security associations.

   Step 3.  When the rekeying is finished, new security associations are
   established.  In practice there is a time interval during which an
   old, about-to-expire security association and newly established
   security association will both exist.  The new ones should be used as
   soon as they become available.

   Step 4.  A notification of the deletion of the old security
   associations is received.  After this, only the new security
   associations can be used.



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   Note that there is no requirement that the existence of the IPsec and
   IKE security associations is tied to the existence of bindings.  It
   is not necessary to delete a security association if a binding is
   removed, as a new binding may soon be established after this.

   Since cryptographic acceleration hardware may only be able to handle
   a limited number of active security associations, security
   associations may be deleted via IKE in order to keep the number of
   active cryptographic contexts to a minimum.  Such deletions should
   not be interpreted as a sign of losing a contact to the peer or as a
   reason to remove a binding.  Rather, if additional traffic needs to
   be sent, it is preferable to bring up another security association to
   protect it.

6.19.  Movements and Dynamic Keying

   In this section we describe the sequence of events that relate to
   movement with IKE-based security associations.  In the initial state,
   the mobile node is not registered in any location and has no security
   associations with the home agent.  Depending on whether the peers
   will be able to move IKE endpoints to new care-of addresses, the
   actions taken in Step 9 and 10 are different.

   Step 1.  Mobile node with the home address A moves to care-of address
   B.

   Step 2.  Mobile node runs IKE from care-of address B to the home
   agent, establishing a phase 1.  The home agent can only act as the
   responder before it knows the current location of the mobile node.

   Step 3.  Protected by this phase 1, mobile node establishes a pair of
   security associations for protecting Mobility Header traffic to and
   from the home address A.

   Step 4.  Mobile node sends a Binding Update and receives a Binding
   Acknowledgement using the security associations created in Step 3.

   Step 5.  Mobile node establishes a pair of security associations for
   protecting return routability packets.  These security associations
   are in tunnel mode and their endpoint in the mobile node side is
   care-of address B.  For the purposes of our example, this step uses
   the phase 1 connection established in Step 2.  Multiple phase 1
   connections are also possible.

   Step 6.  The mobile node uses the security associations created in
   Step 5 to run return routability.





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   Step 7.  The mobile node moves to a new location and adopts a new
   care-of address C.

   Step 8.  Mobile node sends a Binding Update and receives a Binding
   Acknowledgement using the security associations created in Step 3.
   The home agent ensures that the next packets sent using the security
   associations created in Step 5 will have the new care-of address as
   their destination address, as if the outer header destination address
   in the security association had changed.

   Step 9.  If the mobile node and the HA have the capability to change
   the IKE endpoints, they change the address to C.  If they do not have
   the capability, both nodes remove their phase 1 connections created
   on top of the care-of address B and will establish a new IKE phase 1
   on top of the care-of address C.  This capability to change the IKE
   phase 1 end points is indicated through setting the Key Management
   Mobility Capability (K) flag [7] in the Binding Update and Binding
   Acknowledgement messages.

   Step 10.  If a new IKE phase 1 connection was setup after movement,
   the MN will not be able to receive any notifications delivered on top
   of the old IKE phase 1 security association.  Notifications delivered
   on top of the new security association are received and processed
   normally.  If the mobile node and HA were able to update the IKE
   endpoints, they can continue using the same IKE phase 1 connection.

7.  Implementation Considerations

7.1.  IPsec

   Note that packet formats and header ordering discussed in Section 3
   must be supported, but implementations may also support other
   formats.  In general, the use of formats not required here may lead
   to incorrect processing of the packets by the peer (such as silently
   discarding them), unless support for these formats has been verified
   off-line.  Such verification can take place at the same time the
   parameters of the security associations are agreed upon.  In some
   cases, however, basic IPv6 specifications call for support of options
   not discussed here.  In these cases, such a verification step might
   be unnecessary as long as the peer fully supports the relevant IPv6
   specifications.  However, no claims are made in this document about
   the validity of these other formats in the context of Mobile IPv6.
   It is also likely that systems that support Mobile IPv6 have been
   tested more extensively with the required formats.

   We have chosen to require an encapsulation format for return
   routability and payload packet protection which can only be realized
   if the destination of the IPsec packets sent from the home agent can



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   be changed as the mobile node moves.  One of the main reasons for
   choosing such a format is that it removes the overhead of twenty four
   bytes when a home address option or routing header is added to the
   tunneled packet.  Such an overhead would not be significant for the
   protection of the return routability packets, but would create an
   additional overhead if IPsec is used to protect the tunneling of
   payload packets to the home agent.  This overhead may be significant
   for real-time traffic.  Given that the use of the shorter packet
   formats for any traffic requires the existence of suitable APIs, we
   have chosen to require support for the shorter packet formats both
   for payload and return routability packets.

   In order to support the care-of address as the destination address on
   the mobile node side, the home agent must act as if the outer header
   destination address in the security association to the mobile node
   would have changed upon movements.  Implementations are free to
   choose any particular method to make this change, such as using an
   API to the IPsec implementation to change the parameters of the
   security association, removing the security association and
   installing a new one, or modification of the packet after it has gone
   through IPsec processing.  The only requirement is that after
   registering a new binding at the home agent, the next IPsec packets
   sent on this security association will be addressed to the new
   care-of address.

   We have chosen to require policy entries that are specific to a
   tunnel interface.  This means that implementations have to regard the
   Home Agent - Mobile Node tunnel as a separate interface on which
   IPsec SPDs can be based.  A further complication of the IPsec
   processing on a tunnel interface is that this requires access to the
   BITS implementation before the packet actually goes out.

7.2.  IKE

   We have chosen to require that a dynamic key management protocol must
   be able to make an authorization decision for IPsec security
   association creation with different addresses than with what the key
   management protocol is run.  We expect this to be done typically by
   configuring the allowed combinations of phase 1 user identities and
   home addresses.

   When certificate authentication is used, IKE fragmentation can be
   encountered.  This can occur when certificate chains are used, or
   even with single certificates if they are large.  Many firewalls do
   not handle fragments properly, and may drop them.  Routers in the
   path may also discard fragments after the initial one, since they





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   typically will not contain full IP headers that can be compared
   against an access list.  Where fragmentation occurs, the endpoints
   will not always be able to establish a security association.

   Fortunately, typical Mobile IPv6 deployment uses short certificate
   chains, as the mobile node is communicating directly with its home
   network.  Where the problem appears, it may be difficult (at least
   away from home) to replace the firewalls or routers with equipment
   that can properly support fragments.  It may help to store the peer
   certificates locally, or to obtain them through other means.

7.3.  Bump-in-the-Stack

   Mobile IPv6 sets high requirements for a so-called Bump-In-The-Stack
   (BITS) implementation model of IPsec.  As Mobile IPv6 specific
   modifications of the packets are required before or after IPsec
   processing, the BITS implementation has to perform also some tasks
   related to mobility.  This may increase the complexity of the
   implementation, even if it already performs some tasks of the IP
   layer (such as fragmentation).

   Specifically, Bump-in-the-Stack implementations may have to deal with
   the following issues:

   o  Processing the Home Address destination option and Routing header
      type 2 to a form suitable for IPsec processing to take place.
      This is needed, among other things, for the security association
      and policy lookups.  While relatively straightforward, the
      required processing may have a hardware effect in BITS
      implementations, if they use hardware support beyond the
      cryptographic operations.

   o  Detecting packets sent between the mobile node and its home agent
      using IPv6 encapsulation.

   o  Offering the necessary APIs for updating the IPsec and IKE
      security association endpoints.

8.  IANA Considerations

   No IANA actions are necessary based on this document.  IANA actions
   for the Mobile IPv6 protocol itself have been covered in [7].

9.  Security Considerations

   The Mobile IPv6 base specification [7] requires strong security
   between the mobile node and the home agent.  This memo discusses how
   that security can be arranged in practice, using IPsec.  The security



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   considerations related to this are documented in the base
   specification, including a discussion of the implications of using
   either manual or dynamic keying.

10.  References

10.1.  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 R. Atkinson, "Security Architecture for the
        Internet Protocol", RFC 2401, November 1998.

   [3]  Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
        (ESP)", RFC 2406, November 1998.

   [4]  Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
        RFC 2409, November 1998.

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

   [6]  Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6
        Specification", RFC 2473, December 1998.

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

10.2.  Informative References

   [8]  Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
        November 1998.

   [9]  Deering, S., Fenner, W. and B. Haberman, "Multicast Listener
        Discovery (MLD) for IPv6", RFC 2710, October 1999.

   [10] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, C. and
        M. Carney, "Dynamic Host Configuration Protocol for IPv6
        (DHCPv6)", RFC 3315, July 2003.

   [11] Vida, R. and L. Costa, Eds., "Multicast Listener Discovery
        Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.








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11.  Acknowledgements

   The authors would like to thank Greg O'Shea, Michael Thomas, Kevin
   Miles, Cheryl Madson, Bernard Aboba, Erik Nordmark, Gabriel
   Montenegro, Steven Kent, and Santeri Paavolainen for interesting
   discussions in this problem space.

12.  Authors' Addresses

   Jari Arkko
   Ericsson
   02420  Jorvas
   Finland

   EMail: jari.arkko@ericsson.com


   Vijay Devarapalli
   Nokia Research Center
   313 Fairchild Drive
   Mountain View  CA 94043
   USA

   EMail: vijayd@iprg.nokia.com


   Francis Dupont
   ENST Bretagne
   Campus de Rennes
   2, rue de la Chataigneraie
   CS 17607
   35576 Cesson-Sevigne Cedex
   France

   EMail: Francis.Dupont@enst-bretagne.fr
















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13.  Full Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 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 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 in BCP 78 and BCP 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 at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
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   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.









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