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Versions: (draft-kivinen-mobike-protocol) 00 01 02 03 04 05 06 07 08 RFC 4555

Network Working Group                                     P. Eronen, Ed.
Internet-Draft                                                     Nokia
Expires: December 30, 2005                                 June 28, 2005


            IKEv2 Mobility and Multihoming Protocol (MOBIKE)
                   draft-ietf-mobike-protocol-00.txt

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Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document describes the MOBIKE protocol, a mobility and
   multihoming extension to IKEv2.  The purpose of MOBIKE is to update
   the (outer) IP addresses associated with IKE and IPsec Security
   Associations (SAs).  The main scenario for MOBIKE is making it
   possible for a remote access VPN user to move from one address to
   another without re-establishing all security associations with the
   VPN gateway.





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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1   Motivation . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2   MOBIKE protocol overview . . . . . . . . . . . . . . . . .  4
     1.3   Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  MOBIKE protocol exchanges  . . . . . . . . . . . . . . . . . .  6
     2.1   Signaling support for MOBIKE . . . . . . . . . . . . . . .  6
     2.2   Additional addresses . . . . . . . . . . . . . . . . . . .  6
     2.3   Changing path of IPsec SAs . . . . . . . . . . . . . . . .  7
     2.4   Updating additional addresses  . . . . . . . . . . . . . .  8
     2.5   Path testing . . . . . . . . . . . . . . . . . . . . . . .  9
     2.6   Return routability check . . . . . . . . . . . . . . . . . 10
     2.7   NAT prevention . . . . . . . . . . . . . . . . . . . . . . 11
   3.  Payload formats  . . . . . . . . . . . . . . . . . . . . . . . 13
     3.1   MOBIKE_SUPPORTED notification payload  . . . . . . . . . . 13
     3.2   ADDITIONAL_ADDRESS notification payload  . . . . . . . . . 13
     3.3   CHANGE_PATH notification payload . . . . . . . . . . . . . 13
     3.4   UNACCEPTABLE_PATH notification payload . . . . . . . . . . 13
     3.5   COOKIE2 notification payload . . . . . . . . . . . . . . . 14
     3.6   NAT_PREVENTION notification payload  . . . . . . . . . . . 14
     3.7   NAT_PREVENTED notification payload . . . . . . . . . . . . 14
   4.  Security considerations  . . . . . . . . . . . . . . . . . . . 15
   5.  IANA considerations  . . . . . . . . . . . . . . . . . . . . . 18
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     7.1   Normative references . . . . . . . . . . . . . . . . . . . 18
     7.2   Informative references . . . . . . . . . . . . . . . . . . 19
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 20
       Intellectual Property and Copyright Statements . . . . . . . . 21





















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1.  Introduction

1.1  Motivation

   IKEv2 is used for performing mutual authentication and establishing
   and maintaining IPsec security associations (SAs).  In the current
   specifications, the IPsec and IKE SAs are created implicitly between
   the IP addresses that are used when the IKE_SA is established.  These
   IP addresses are then used as the outer (tunnel header) addresses for
   tunnel mode IPsec packets.  Currently, it is not possible to change
   these addresses after the IKE_SA has been created.

   There are scenarios where these IP addresses might change.  One
   example is mobility: a host changes its point of network attachment,
   and receives a new IP address.  Another example is a multihoming host
   that would like to change to a different interface if, for instance,
   the currently used address stops working for some reason.

   In some cases, the the problem can be solved by simply creating new
   IKE and IPsec SAs after the IP address has changed.  In static
   multihoming scenarios, it may even be possible to have several IKE
   and IPsec SAs simultaneously, and perform some kind of dynamic
   routing over them [RFC3884].  However, this can be problematic for
   several reasons.  Creating a new IKE_SA may require user interaction
   for authentication (entering a code from a token card, for instance).
   Creating new SAs often also involves expensive calculations and
   possibly a large number of roundtrips.  Due to these reasons, a
   mechanism for updating the IP addresses of existing IKE and IPsec SAs
   is needed.  The MOBIKE protocol described in this document provides
   such a mechanism.

   The main scenario for MOBIKE is making it possible for a remote
   access VPN user to move from one address to another without re-
   establishing all security associations with the VPN gateway.  For
   instance, a user could start from fixed Ethernet in the office, and
   then disconnect the laptop and move to office wireless LAN.  When
   leaving the office the laptop could start using GPRS, and switch to a
   different wireless LAN when the user arrives home.  MOBIKE updates
   only the outer (tunnel header) addresses of IPsec SAs, and the
   addresses and others traffic selectors used inside the tunnel stay
   unchanged.  Thus, mobility can be (mostly) invisible to applications
   and their connections using the VPN.

   More complex scenarios arise when the VPN gateway also has several
   network interfaces: these interfaces could be connected to different
   networks or ISPs, they may have may be a mix of IPv4 and IPv6
   addresses, and the addresses may change over time.  Furthermore, both
   parties could be VPN gateways relaying traffic for other parties.



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1.2  MOBIKE protocol overview

   Since MOBIKE allows both parties to have several addresses, this
   leads us to an important question: there are up to N*M pairs of IP
   addresses that could potentially be used.  How to decide which of
   these pairs should be used?  The decision has to take into account
   several factors.  First, the parties have may preferences about which
   interface should be used, due to performance and cost reasons, for
   instance.  Second, the decision is constrained by the fact that some
   of the pairs may not work at all due to incompatible IP versions,
   outages somewhere in the network, problems at the local link at
   either end, and so on.

   MOBIKE solves this problem by taking a simple approach: the party
   that initiated the IKE_SA (the "client" in remote access VPN
   scenario) is responsible for deciding which address pair is used for
   the IPsec SAs, and collecting the information it needs to make this
   decision (such as determining which address pairs work or do not
   work).  The other party (the "gateway" in remote access VPN scenario)
   simply tells the initiator what addresses it has, but does not update
   the IPsec SAs until it receives a message from the initiator to do
   so.

   Making the decision at the initiator is consistent with how normal
   IKEv2 works: the initiator decides which addresses it uses when
   contacting the responder.  It also makes sense especially when the
   initiator is the mobile node: it is in better position to decide
   which of its network interfaces should be used for both upstream and
   downstream traffic.

   The details of exactly how the initiator makes the decision, what
   information is used in making it, how the information is collected,
   how preferences affect the decision, and when a decision needs to be
   changed, are largely beyond the scope of MOBIKE.  This does not mean
   that these details are unimportant: on the contrary, they are likely
   to be crucial in any real system.  However, MOBIKE is concerned with
   these details only to the extent that they are visible in IKEv2/IPsec
   messages exchanged between the peers (and thus need to be
   standardized to ensure interoperability).  Issues such as mobility
   detection and local policies are also not specific to MOBIKE, but
   apply to existing mobility protocols such as Mobile IPv4 [MIP4] as
   well.

   One important aspect of this information gathering that has to be
   visible in the messages is determining whether a certain pair of
   addresses can be used.  IKEv2 Dead Peer Detection (DPD) feature can
   provide information that the currently used pair does or does not
   work.  There are, however, some complications in using it for other



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   addresses, and thus MOBIKE adds a new IKEv2 message that can be used
   to "test" whether some particular pair of addresses works or not,
   without yet committing to changing the addresses currently in use.

   MOBIKE also has to deal with situations where the network contains
   NATs or stateful packet filters (for brevity, the rest of this
   document talks simply about NATs).  When the addresses used for IPsec
   SAs are changed, MOBIKE can enable or disable IKEv2 NAT Traversal as
   needed.  However, if the party "outside" the NAT changes its IP
   address, it may no longer be able to send packets to the party
   "behind" the NAT, since the packets may not (depending on the exact
   type of NAT) match the NAT mapping state.  Here MOBIKE assumes that
   the initiator is the party "behind" the NAT, and does not fully
   support the case where the responder's addresses change when NATs are
   present.

   Updating the addresses of IPsec SAs naturally has to take into
   account several security considerations.  MOBIKE includes two
   features design to address these considerations.  First, a "return
   routability" check can be used to verify the addresses provided by
   the peer.  This makes it more difficult flood third parties with
   large amounts of traffic.  Second, a "NAT prevention" feature ensures
   that IP addresses have not been modified by NATs, IPv4/IPv6
   translation agents, or other similar devices.  This feature is mainly
   intended for site-to-site VPNs where the administrators may know
   beforehand that NATs are not present, and thus any modification to
   the packet can be considered to be an attack.

1.3  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [KEYWORDS].

   IPsec Security Association (SA)

      An ESP or AH Security Association.

   Path

      A particular combination of source IP address and destination IP
      address (note: this definition does not consider the route taken
      by the packets in the network).








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2.  MOBIKE protocol exchanges

2.1  Signaling support for MOBIKE

   Implementations that wish to use MOBIKE for a particular IKE_SA MUST
   include a MOBIKE_SUPPORTED notification in the IKE_SA_INIT request
   and response messages.

      Initiator                   Responder
     -----------                 -----------
      HDR, SAi1, KEi, Ni,
           N(MOBIKE_SUPPORTED),
           [N(NAT_DETECTION_*)]  -->

                             <--  HDR, SAr1, KEr, Nr,
                                       [N(NAT_DETECTION_*)],
                                       [CERTREQ],
                                       N(MOBIKE_SUPPORTED)

   The MOBIKE_SUPPORTED notification payload is described in Section 3.

2.2  Additional addresses

   Both the initiator and responder MAY include one or more
   ADDITIONAL_ADDRESS notification payloads in the IKE_AUTH exchange (in
   case of multiple IKE_AUTH exchanges, in the message containing the SA
   payload).

      Initiator                   Responder
     -----------                 -----------
      HDR, SK { IDi, [CERT], [IDr], AUTH,
                [CP(CFG_REQUEST)]
                SAi2, TSi, TSr,
                [N(ADDITIONAL_ADDRESS)*] }  -->

                             <--  HDR, SK { IDr, [CERT], AUTH,
                                            [CP(CFG_REPLY)],
                                            SAr2, TSi, TSr,
                                            [N(ADDITIONAL_ADDRESS)*] }

   The recipient stores this information, but no other action is taken
   at this time.









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2.3  Changing path of IPsec SAs

   In MOBIKE, the initiator of the IKE_SA decides what addresses are
   used in the IPsec SAs.  That is, the responder never updates any
   IPsec SAs without receiving an explicit CHANGE_PATH request from the
   initiator.  (As described below, the responder can, however, update
   the IKE_SA in some circumstances.)

   The description in this section assumes that the initiator has
   already decided what the new addresses should be.  How this decision
   is made is beyond the scope of this specification.  When this
   decision has been made, the initiator

   o  Updates the IKE_SA and IPsec SAs with the new addresses, and sets
      the "pending_update" flag in the IKE_SA.

   o  If NAT Traversal is not enabled, and the responder supports NAT
      Traversal (as indicated by NAT detection payloads in the
      IKE_SA_INIT exchange), and the initiator either suspects or knows
      that a NAT is likely to be present, enables NAT Traversal.

   o  When the window size allows, sends an INFORMATIONAL request
      containing the CHANGE_PATH notification payload (which does not
      contain any data), and clears the "pending_update" flag.

      Initiator                   Responder
     -----------                 -----------
      HDR, SK { N(CHANGE_PATH),
                N(COOKIE2),
                [N(NAT_DETECTION_*),]
                [N(NAT_PREVENTION)] } -->

   o  If a new address change occurs while waiting for the response,
      starts again from the first step (and ignores responses to this
      CHANGE_PATH request).

   Note that if the responder has NAT Traversal enabled, it can update
   the addresses in both the IKE_SA and IPsec SAs as usual (if it
   implements the "SHOULD" from [IKEv2] Section 2.23.

   When processing an INFORMATIONAL request containing the CHANGE_PATH
   notification, the responder

   o  Compares the Message ID with the latest_update_received counter in
      the IKE_SA.  If latest_update_received is greater than the
      received Message ID, the reply is sent as usual, but no other
      action is taken; otherwise, updates the latest_update_received
      counter.



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   o  If the NAT_PREVENTION payload is present, processes it as
      described in Section 2.7.

   o  Checks that the (source IP address, destination IP address) pair
      in the IP header is acceptable according to local policy.  If it
      is not, replies with "HDR, SK {N(COOKIE2), N(UNACCEPTABLE_PATH)}".

   o  Updates the IP addresses in the IKE_SA and IPsec SAs with the
      values from the IP header.

   o  If NAT Traversal is supported and NAT detection payloads were
      included, enables or disables NAT Traversal.

   o  Replies with an INFORMATIONAL response:

      Initiator                   Responder
     -----------                 -----------
                             <--  HDR, SK { N(COOKIE2),
                                            [N(NAT_DETECTION_*)] }

   When the initiator receives the reply, it

   o  If the response contains the NAT_PREVENTED payload, processes it
      as described in Section 2.7.

   o  If the response contains an UNACCEPTABLE_PATH notification
      payload, the initiator MAY select another path and retry the
      exchange, keep on using the current path, or disconnect.

   o  If NAT Traversal is supported and NAT detection payloads were
      included, enables or disables NAT Traversal.


2.4  Updating additional addresses

   As described in Section 2.2, both the initiator and responder can
   send a list of additional addresses (in addition to the one used for
   IKE_SA_INIT/IKE_AUTH exchange) to the initiator in the IKE_AUTH
   exchange.  If this list of addresses changes, a new list can be sent
   in any INFORMATIONAL exchange request message.

   When the responder (of the original IKE_SA) receives an INFORMATIONAL
   request containing ADDITIONAL_ADDRESS payloads, it simply stores the
   information, but no other action is taken.







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      Initiator                   Responder
     -----------                 -----------
      HDR, SK { N(ADDITIONAL_ADDRESS)+,
                N(COOKIE2) }  -->

                             <--  HDR, SK { N(COOKIE2) }

   When the initiator receives an INFORMATIONAL request containing
   ADDITIONAL_ADDRESS, it stores the information and also determines
   whether the currently used path needs to be changed (for instance, if
   the currently used address is no longer included in the list); if it
   does, the initiator proceeds as described in the previous section.

      Initiator                   Responder
     -----------                 -----------
                             <--  HDR, SK { N(ADDITIONAL_ADDRESS)+,
                                            N(COOKIE2) }

      HDR, SK { N(COOKIE2) }  -->

   If the implementation supports window sizes greater than one, it also
   has to keep track of the Message ID of the latest update it has
   received, to avoid the situation where new information is overwritten
   by older.

   There is one additional complication: when the responder wants to
   send a new additional address list, the currently used path may no
   longer work.  In this case, the responder uses the additional address
   list received from the initiator, the list of its own addresses, and,
   if necessary, the path testing feature (see Section 2.5) to determine
   a path that works, updates the addresses in the IKE_SA (but not IPsec
   SAs), and then sends the INFORMATIONAL request.  This is the only
   time the responder uses the additional address list received from the
   initiator.

   Note that both peers can have their own policies about what addresses
   or paths are acceptable to use.  A minimal "mobile client" could have
   a policy that says that only the responder's address specified in
   local configuration is acceptable.  This kind of client does not have
   to send or process ADDITIONAL_ADDRESS notification payloads.
   Similarly, a simple "VPN gateway" that has only a single address, and
   is not going to change it, does not need to send or understand
   ADDITIONAL_ADDRESS notification payloads.

2.5  Path testing

   IKEv2 Dead Peer Detection allows the peers to detect if the currently
   used path has stopped working.  However, if either of the peers has



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   several addresses, DPD alone does not indicate which of the other
   paths might work.  The path testing feature allows the parties to
   determine whether a particular path (pair of addresses) works,
   without yet committing to changing over to these addresses.

   MOBIKE introduces a new IKEv2 exchange type, PATH_TEST, for testing
   connectivity.  This exchange is not part of any IKE_SA, so it is not
   cryptographically protected.  It also does not result in the
   responder keeping any state.

      Initiator                   Responder
     -----------                 -----------
      HDR(0,0), N(COOKIE2),
                [N(NAT_DETECTION_*)]  -->

                             <--  HDR(0,0), N(COOKIE2),
                                            [N(NAT_DETECTION_*)]

   The reason for introducing a new exchange type, instead of using
   INFORMATIONAL exchanges, is to simplify implementations by allowing
   MOBIKE to work with window size 1.

   Performing path testing over several different paths is not required
   if the node has other information that enables it to select which
   path should be used.  Also, responders do not perform path testing
   unless they update their list of additional addresses (as described
   in the previous section).  Implementations MAY do path testing even
   if the currently used path is working to e.g. detect when a better
   but previously unavailable path becomes available, or to speed up
   recovery in fault situations.

   Implementations that perform path testing MUST take steps to avoid
   causing unnecessary congestion.  TBD: add some more details here.

2.6  Return routability check

   Both the initiator and the responder can optionally verify that the
   other party can actually receive packets at the claimed address.
   This "return routability check" can be done before updating the IPsec
   SAs, immediately after updating them, or continuously during the
   connection.

   By default, return routability check SHOULD be done before updating
   the IPsec SAs.  In environments where the peer is expected to be
   well-behaving (many corporate VPNs, for instance), or the address can
   be verified by some other means (e.g., the address is included in the
   peer's certificate), the return routability check MAY be skipped or
   postponed until after the IPsec SAs have been updated.



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   Any INFORMATIONAL exchange can be used for return routability
   purposes (with one exception, described below): when a valid response
   is received, we know the other party can receive packets at the
   claimed address.

   To ensure that the peer cannot generate the correct INFORMATIONAL
   response without seeing the request, a new payload is added to all
   INFORMATIONAL messages.  The sender of an INFORMATIONAL request MUST
   include a COOKIE2 notification payload, and the recipient of an
   INFORMATIONAL request MUST copy the payload as-is to the response.
   When processing the response, the original sender MUST verify that
   the value is the same one as sent.  If the values do not match, the
   IKE_SA MUST be closed.

   There is one additional issue that must be taken into account.  If
   the destination address in the IKE_SA has been updated after the
   INFORMATIONAL request was sent, then it is possible that the request
   has been sent to several different addresses.  In this case,
   receiving the INFORMATIONAL response does not tell which address is
   the working one; thus, a new INFORMATIONAL request needs to be sent.

2.7  NAT prevention

   IKEv2/IPsec implementations that do not support NAT Traversal can, in
   fact, work across some types of one-to-one "basic" NATs and IPv4/IPv6
   translation agents in tunnel mode.  This may be considered a problem
   in some circumstances, since in some sense any modification of the IP
   addresses can be considered to be an attack.

   The "NAT prevention" feature allows both the initiator and responder
   to have a policy that prevents the use of paths that contain NATs,
   IPv4/IPv6 translation agents, or other nodes that modify the
   addresses in the IP header.  This feature is mainly intended for
   site-to-site VPN cases, where the administrators may know beforehand
   that NATs are not present, and thus any modification to the packet
   can be considered to be an attack.

   This specification addresses the issue as follows.  When an IPsec SA
   is created, the tunnel header IP addresses (and port if doing UDP
   encapsulation) are taken from the IKE_SA, not the message IP header.
   The NAT_PREVENTION payload is used to guarantee that NATs have not
   modified the address used in IKE_SA.  However, all response messages
   are still sent to the address and port the corresponding request came
   from.

   The initiator MAY include a NAT_PREVENTION payload in an IKE_SA_INIT
   request.  The responder MUST compare the NAT_PREVENTION payload with
   the values from the IP header.  If they do not match, the responder



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   replies with "HDR(A,0), N(NAT_PREVENTED)" and does not create any
   state.

   If the values do match, the responder initializes (local_address,
   local_port, peer_address, peer_port) in the to-be-created IKE_SA with
   values from the IP header.  The same applies if neither
   NAT_PREVENTION nor NAT_DETECTION_*_IP payloads were included, or if
   the responder does not support NAT Traversal.

   If the IKE_SA_INIT request included NAT_DETECTION_*_IP payloads but
   no NAT_PREVENTION payload, the situation is different since the
   initiator may at this point change from port 500 to 4500.  In this
   case, the responder initializes (local_address, local_port,
   peer_address, peer_port) from the first IKE_AUTH request.  It may
   also decide to perform a return routability check soon after the
   IKE_AUTH exchanges have been completed.

   IKEv2 requires that if an IPsec endpoint discovers a NAT between it
   and its correspondent, it MUST send all subsequent traffic to and
   from port 4500.  To simplify things, implementations that support
   both this specification and NAT Traversal MUST change to port 4500 if
   the correspondent also supports both, even if no NAT was detected
   between them.

   NAT_PREVENTION payloads can also be included when changing the path
   of IPsec SAs (see Section 2.3).  TBD: add better description.

























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3.  Payload formats

3.1  MOBIKE_SUPPORTED notification payload

   The MOBIKE_SUPPORTED notification payload is included in the
   IKE_SA_INIT messages to indicate that the implementation supports
   this specification.

   The Notify Message Type for MOBIKE_SUPPORTED is TBD-BY-IANA
   (16396..40959).  The Protocol ID field is set to one (1), and SPI
   Size is set to zero.  There is no data associated with this Notify
   type.

3.2  ADDITIONAL_ADDRESS notification payload

   Both initiator and responder can include ADDITIONAL_ADDRESS payloads
   in the IKE_AUTH exchange and INFORMATIONAL exchange request messages;
   see Section 2.2 and Section 2.4 for more detailed description.

   The Notify Message Type for ADDITIONAL_ADDRESS is TBD-BY-IANA
   (16396..40959).  The Protocol ID field is set to one (1), and SPI
   Size is set to zero.  The data associated with this Notify type is
   either an IPv4 address or an IPv6 address; the type is determined by
   the payload length.

3.3  CHANGE_PATH notification payload

   This payload is included in INFORMATIONAL exchange requests sent by
   the initiator of the IKE_SA to update addresses of the IKE_SA and
   IPsec SAs (see Section 2.3).

   The Notify Message Type for CHANGE_PATH is TBD-BY-IANA
   (16396..40959).  The Protocol ID field is set to one (1), and SPI
   Size is set to zero.  There is no data associated with this Notify
   type.

3.4  UNACCEPTABLE_PATH notification payload

   The responder can include this notification payload in an
   INFORMATIONAL exchange response to indicate that the address change
   in the corresponding request message (which contained a CHANGE_PATH
   notification payload) was not carried out.

   The Notify Message Type for UNACCEPTABLE_PATH is TBD-BY-IANA
   (40..8191).  The Protocol ID field is set to one (1), and SPI Size is
   set to zero.  There is no data associated with this Notify type.





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3.5  COOKIE2 notification payload

   This payload is included in all INFORMATIONAL exchange messages for
   return routability check purposes (see Section 2.6).  It is also used
   in PATH_TEST messages to match requests and responses (see
   Section 2.5).

   The data associated with this notification MUST be between 8 and 64
   octets in length (inclusive), and MUST be chosen in a way that is
   unpredictable to the recipient.  The Notify Message Type for this
   message is TBD-BY-IANA (16396..40959).  The Protocol ID field is set
   to one (1), and SPI Size is set to zero.

3.6  NAT_PREVENTION notification payload

   See Section 2.7 for a description of this payload.

   The data associated with this notification is the SHA-1 hash
   [FIPS180-2] of the following data: IKE SPIs (in the order they appear
   in the header), the IP address and port from which the packet was
   sent, and the IP address and port to which the packet was sent.  The
   Notify Message Type for this message is TBD-BY-IANA (16396..40959).
   The Protocol ID field is set to one (1), and SPI Size is set to zero.

3.7  NAT_PREVENTED notification payload

   See Section 2.7 for a description of this payload.

   The Notify Message Type for NAT_PREVENTED is TBD-BY-IANA (40..8191).
   The Protocol ID field is set to one (1), and SPI Size is set to zero.
   There is no data associated with this Notify type.




















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4.  Security considerations

   The main goals of this specification are to not reduce the security
   offered by usual IKEv2 procedures and to counter mobility related
   threats in an appropriate manner.  In some specific cases MOBIKE is
   also capable of protecting address changes better than existing NAT
   Traversal procedures.

   The threats arising in scenarios targeted by MOBIKE are:

   Traffic redirection and hijacking

      Insecure mobility management mechanisms may allow inappropriate
      redirection of traffic.  This may allow inspection of the traffic
      as well as man-in-the-middle and session hijacking attacks.

      The scope of these attacks in the MOBIKE case is limited, as all
      traffic is protected using IPsec.  However, it should be observed
      that security associations originally created for the protection
      of a specific flow between specific addresses may be moved through
      MOBIKE.  The level of required protection may be different in a
      new location of a VPN client, for instance.

   Third-party denial-of-service through flooding

      Traffic redirection may be performed not just to gain access to
      the traffic, but also to cause a denial-of-service attack for a
      third party.  For instance, a high-speed TCP session or a
      multimedia stream may be redirected towards a victim host, causing
      its communications capabilities to suffer.

      The attackers in this threat can be either outsiders or even one
      of the participants.  In usual VPN usage scenarios attacks by
      participants can be easily dealt with.  However, this requires
      that strong authentication was performed in the initial IKEv2
      negotiation.  This may not be the case in all scenarios,
      particularly with opportunistic approaches to security.

      Normally such attacks would expire in a short time frame due to
      the lack of responses (such as transport layer acknowledgements)
      from the victim.  However, as described in [Aura02], malicious
      participants would typically be able to spoof such
      acknowledgements and maintain the traffic flow for an extended
      period of time.  For instance, if the attacker opened the TCP
      stream itself before redirecting it to the victim, the attacker
      becomes aware of the sequence number space used in this particular
      session.




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      It should also be noted, as shown in [Bombing], that without
      ingress filtering in the attacker's network such attacks are
      already possible simply by sending spoofed packets from the
      attacker to the victim directly.  Consequently, it makes little
      sense to protect against attacks of similar nature in MOBIKE.
      However, it still makes sense to limit the amplification
      capabilities provided to attackers, so that they cannot use stream
      redirection to send 1000 packets to the victim by sending just a
      few packets themselves.

      Note that a variant of the flooding attack exists in IKEv2 NAT
      Traversal functionality [PseudoNAT].  In this variant, the
      attacker has to be on the path between the participants, changing
      the addresses in the packets that pass by.  This attack is
      possible because the addresses in the outer headers are not
      protected.  When the attacker leaves the path, the correct
      situation is restored after the exchange of the next packets
      between the participants.

   Spoofing indications related to network connectivity

      Attackers may also spoof various indications from lower layers and
      the network in an effort to confuse the peers about which
      addresses are or are not working.  For example, attackers may
      spoof ICMP error messages in an effort to cause the parties to
      move their traffic elsewhere or even to disconnect.  Attackers may
      also spoof information related to network attachments, router
      discovery, and address assignments in an effort to make the
      parties believe they have Internet connectivity when in reality
      they do not.

      This may cause use of non-preferred addresses or even denial-of-
      service.

   Denial-of-service of the participants through  MOBIKE

      Inappropriate MOBIKE protocol mechanisms might make it possible
      for attackers to disconnect the participants, or to move them to
      non-operational addresses.

   MOBIKE addresses these threats using the following countermeasures:

   Payload traffic protection

      The use of IPsec protection on payload traffic protects the
      participants against disclosure of the contents of the traffic,
      should the traffic end up in an incorrect destination.  It is
      recommended that security policies be configured in a manner that



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      takes into account that a single security association can be used
      through different paths at different times.

   Protection of MOBIKE payloads

      The payloads used in MOBIKE are encrypted, integrity protected,
      and replay protected.  This assures that no one except the
      participants can, for instance, give a control message to change
      the addresses.

      Note, however, that the actual IP address communicated in these
      messages is in the outer IP header and not protected, just as in
      IKEv2 NAT Traversal.  MOBIKE adds the NAT_PREVENTION payload,
      however, which can be used to prevent modifications by outsiders.
      Where this payload is used, communication through NATs and other
      address translators is impossible, however.  This feature is
      mainly intended for site-to-site VPN cases, where the
      administrators may know beforehand that NATs are not present, and
      thus any modification to the packet can be considered to be an
      attack.

   Explicit address change

      MOBIKE allows only address changes that are explicitly requested.
      This provides additional security beyond to what IKEv2 NAT
      Traversal has, but it should be noted that the benefits of this
      can only be realized when MOBIKE is used without intervening NATs
      and NAT Traversal.

      When NAT Traversal is supported, the peer's address may be updated
      automatically to allow changes in NAT mappings.  The "continued
      return routability" feature, implemented by the COOKIE2 payload,
      allows verification of the new address after the change.  This
      limits the duration of any "third party bombing" attack by off-
      path (relative to the victim) attackers.

   Return routability tests

      This specification requires the use of return routability tests
      (under certain conditions) to ensure that third party flooding
      attacks are prevented.  The tests are authenticated messages that
      the peer has to respond to in order for the address change to be
      committed to.  The tests contain unpredictable data, and can only
      be properly responded to by someone who has the keys associated
      with the IKEv2 security association and who has seen the request
      packet for the test.

   MOBIKE does not provide any protection of its own for indications



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   from other parts of the protocol stack.  However, MOBIKE is resistant
   to incorrect information from these sources in the sense that it
   provides its own security for both the signaling of addressing
   information as well as actual payload data transmission.  Denial-of-
   service vulnerabilities remain, however.  Some aspects of these
   vulnerabilities can be mitigated through the use of techniques
   specific to the other parts of the stack, such as properly dealing
   with ICMP errors [ICMPAttacks], link layer security, or the use of
   [SEND] to protect IPv6 Router and Neighbor Discovery.

5.  IANA considerations

   This document does not create any new namespaces to be maintained by
   IANA, but it requires new values in namespaces that have been defined
   in the IKEv2 base specification [IKEv2].

   This document defines one new IKEv2 exchange, "PATH_TEST", whose
   value is to be allocated from the "IKEv2 Exchange Types" namespace.
   This exchange is described in Section 2.5.

   This document also defines several new IKEv2 notification payloads
   whose values are to be allocated from the "IKEv2 Notification Payload
   Types" namespace.  These notification payloads are described in
   Section 3.

6.  Acknowledgements

   This document is a collaborative effort of the entire MOBIKE WG.  We
   would particularly like to thank Jari Arkko, Francis Dupont, Paul
   Hoffman, Tero Kivinen, and Hannes Tschofenig.  This document also
   incorporates ideas and text from earlier MOBIKE protocol proposals,
   including [AddrMgmt], [Kivinen], [MOPO], and [SMOBIKE], and the
   MOBIKE design document [Design].

7.  References

7.1  Normative references

   [FIPS180-2]
              National Institute of Standards and Technology,
              "Specifications for the Secure Hash Standard",  Federal
              Information Processing Standard (FIPS) Publication 180-2,
              August 2002.

   [IKEv2]    Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
              draft-ietf-ipsec-ikev2-17 (work in progress),
              October 2004.




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

   [UDPEncap]
              Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
              Stenberg, "UDP Encapsulation of IPsec ESP Packets",
              RFC 3948, January 2005.

7.2  Informative references

   [AddrMgmt]
              Dupont, F., "Address Management for IKE version 2",
              draft-dupont-ikev2-addrmgmt-07 (work in progress),
              May 2005.

   [Aura02]   Aura, T., Roe, M., and J. Arkko, "Security of Internet
              Location Management",  Proc. 18th Annual Computer Security
              Applications Conference (ACSAC), December 2002.

   [Bombing]  Dupont, F., "A note about 3rd party bombing in Mobile
              IPv6", draft-dupont-mipv6-3bombing-02 (work in progress),
              June 2005.

   [Design]   Kivinen, T. and H. Tschofenig, "Design of the MOBIKE
              protocol", draft-ietf-mobike-design-02 (work in progress),
              February 2005.

   [ICMPAttacks]
              Gont, F., "ICMP attacks against TCP",
              draft-gont-tcpm-icmp-attacks-03 (work in progress),
              December 2004.

   [Kivinen]  Kivinen, T., "MOBIKE protocol",
              draft-kivinen-mobike-protocol-00 (work in progress),
              February 2004.

   [MIP4]     Perkins, C., "IP Mobility Support for IPv4", RFC 3344,
              August 2002.

   [MOPO]     Eronen, P., "Mobility Protocol Options for IKEv2 (MOPO-
              IKE)", draft-eronen-mobike-mopo-02 (work in progress),
              February 2005.

   [PseudoNAT]
              Dupont, F. and J-J. Bernard, "Transient pseudo-NAT attacks
              or how NATs are even more evil than you believed",
              draft-dupont-transient-pseudonat-04 (expired) (work in



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              progress), June 2004.

   [RFC3884]  Touch, J., Eggert, L., and Y. Wang, "Use of IPsec
              Transport Mode for Dynamic Routing", RFC 3884,
              September 2004.

   [SEND]     Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
              Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [SMOBIKE]  Eronen, P. and H. Tschofenig, "Simple Mobility and
              Multihoming Extensions for IKEv2 (SMOBIKE)",
              draft-eronen-mobike-simple-00 (work in progress),
              March 2004.


Author's Address

   Pasi Eronen (editor)
   Nokia Research Center
   P.O. Box 407
   FIN-00045 Nokia Group
   Finland

   Email: pasi.eronen@nokia.com



























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