IKEv2 Mobility and Multihoming                                T. Kivinen
(mobike)                                                   Safenet, Inc.
Internet-Draft                                             H. Tschofenig
Expires: January 19, 2006                                        Siemens
                                                           July 18, 2005

                     Design of the MOBIKE Protocol

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on January 19, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).


   The MOBIKE (IKEv2 Mobility and Multihoming) working group is
   developing extensions for the Internet Key Exchange Protocol version
   2 (IKEv2).  These extensions should enable an efficient management of
   IKE and IPsec Security Associations when a host possesses multiple IP
   addresses and/or where IP addresses of an IPsec host change over time
   (for example, due to mobility).

Kivinen & Tschofenig    Expires January 19, 2006                [Page 1]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

   This document discusses the involved network entities, and the
   relationship between IKEv2 signaling and information provided by
   other protocols.  Design decisions for the MOBIKE protocol,
   background information and discussions within the working group are

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Scenarios  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.1   Mobility Scenario  . . . . . . . . . . . . . . . . . . . .  7
     3.2   Multihoming Scenario . . . . . . . . . . . . . . . . . . .  8
     3.3   Multihomed Laptop Scenario . . . . . . . . . . . . . . . .  9
   4.  Framework  . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   5.  Design Considerations  . . . . . . . . . . . . . . . . . . . . 13
     5.1   Indicating Support for MOBIKE  . . . . . . . . . . . . . . 13
     5.2   Changing a Preferred Address and Multi-homing Support  . . 13
       5.2.1   Storing a single or multiple addresses . . . . . . . . 14
       5.2.2   Indirect or Direct Indication  . . . . . . . . . . . . 15
       5.2.3   Connectivity Tests using IKEv2 Dead-Peer Detection . . 16
     5.3   Simultaneous Movements . . . . . . . . . . . . . . . . . . 17
     5.4   NAT Traversal  . . . . . . . . . . . . . . . . . . . . . . 18
     5.5   Changing addresses or changing the paths . . . . . . . . . 20
     5.6   Return Routability Tests . . . . . . . . . . . . . . . . . 20
     5.7   Employing MOBIKE results in other protocols  . . . . . . . 23
     5.8   Scope of SA changes  . . . . . . . . . . . . . . . . . . . 24
     5.9   Zero Address Set . . . . . . . . . . . . . . . . . . . . . 25
     5.10  IPsec Tunnel or Transport Mode . . . . . . . . . . . . . . 25
     5.11  Message Representation . . . . . . . . . . . . . . . . . . 26
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 28
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 29
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 30
   9.  Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 31
   10.   References . . . . . . . . . . . . . . . . . . . . . . . . . 32
     10.1  Normative references . . . . . . . . . . . . . . . . . . . 32
     10.2  Informative References . . . . . . . . . . . . . . . . . . 32
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 34
       Intellectual Property and Copyright Statements . . . . . . . . 35

Kivinen & Tschofenig    Expires January 19, 2006                [Page 2]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

1.  Introduction

   The purpose of IKEv2 is to mutually authenticate two hosts, establish
   one or more IPsec Security Associations (SAs) between them, and
   subsequently manage these SAs (for example, by rekeying or deleting).
   IKEv2 enables the hosts to share information that is relevant to both
   the usage of the cryptographic algorithms that should be employed
   (e.g., parameters required by cryptographic algorithms and session
   keys) and to the usage of local security policies, such as
   information about the traffic that should experience protection.

   IKEv2 assumes that an IKE SA is created implicitly between the IP
   address pair that is used during the protocol execution when
   establishing the IKEv2 SA.  This means that, in each host, only one
   IP address pair is stored for the IKEv2 SA as part of a single IKEv2
   protocol session, and, for tunnel mode SAs, the hosts places this
   single pair in the outer IP headers.  Existing documents make no
   provision to change this pair after an IKE SA is created.

   There are scenarios where one or both of the IP addresses of this
   pair may change during an IPsec session.  In principle, the IKE SA
   and all corresponding IPsec SAs could be re-established after the IP
   address has changed.  However, this can be problematic, as the device
   might be too slow for this task.  Moreover, manual user interaction
   (for example when using SecurID cards) might be required as part of
   the IKEv2 authentication procedure.  Therefore, an automatic
   mechanism is neeed that updates the IP addresses associated with the
   IKE SA and the IPsec SAs.  MOBIKE provides such a mechanism.

   The work of the MOBIKE working group and therefore this document is
   based on the assumption that the mobility and multi-homing extensions
   are developed for IKEv2 [I-D.ietf-ipsec-ikev2].  As IKEv2 is built on
   the architecture described in RFC2401bis [I-D.ietf-ipsec-rfc2401bis],
   all protocols developed within the MOBIKE working group must be
   compatible with both IKEv2 and the architecture described in
   RFC2401bis.  The document does not aim to neither provide support
   IKEv1 [RFC2409] nor the architecture described in RFC2401 [RFC2401].

   This document is structured as follows.  After introducing some
   important terms in Section 2 a number of relevant usage scenarios are
   discussed in Section 3.  Section 4 discusses the interoperation of
   MOBIKE with other protocols and processes that may run in the local
   machine.  Finally, Section 5 discusses design considerations
   affecting the MOBIKE protocol.  The document concludes in Section 6
   with security considerations.

Kivinen & Tschofenig    Expires January 19, 2006                [Page 3]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

2.  Terminology

   This section introduces the terminology that is used in this


      A peer is an IKEv2 endpoint.  In addition, a peer implements the
      MOBIKE extensions, as defined in this and related documents.

   Available address:

      An address is said to be available if the following conditions are

      *  The address has been assigned to an interface.

      *  If the address is an IPv6 address, we additionally require (a)
         that the address is valid as defined in RFC 2461 [RFC2461], and
         (b) that the address is not tentative as defined in RFC 2462
         [RFC2462].  In other words, we require the address assignment
         to be complete.

         Note that this explicitly allows an address to be optimistic as
         defined in [I-D.ietf-ipv6-optimistic-dad].

      *  If the address is an IPv6 address, it is a global unicast or
         unique site-local address, as defined in [I-D.ietf-ipv6-unique-
         local-addr].  That is, it is not an IPv6 link-local.  Where
         IPv4 is considered, it is not an RFC 1918 [RFC1918] address.

      *  The address and interface is acceptable for sending and
         receiving traffic according to a local policy.

      This definition is taken from [I-D.arkko-multi6dt-failure-


   Locally Operational Address:

      An address is said to be locally operational if it is available
      and its use is locally known to be possible and permitted.  This
      definition is taken from [I-D.arkko-multi6dt-failure-detection].

Kivinen & Tschofenig    Expires January 19, 2006                [Page 4]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

   Operational address pair:

      A pair of operational addresses are said to be an operational
      address pair, if and only if bidirectional connectivity can be
      shown between the two addresses.  Note that sometimes it is
      necessary to consider connectivity on a per-flow level between two
      endpoints needs to be tested.  This differentiation might be
      necessary to address certain Network Address Translation types or
      specific firewalls.  This definition is taken from [I-D.arkko-
      multi6dt-failure-detection] and adapted for the MOBIKE context.
      Although it is possible to further differentiate unidirectional
      and bidirectional operational address pairs, only bidirectional
      connectivity is relevant to this document and unidirectional
      connectivity is out of scope.


      The sequence of routers traversed by the MOBIKE and IPsec packets
      exchanged between the two peers.  Note that this path may be
      affected not only by the involved source and destination IP
      addresses, but also by the transport protocol.  Since MOBIKE and
      IPsec packets have a different appearance on the wire they might
      be routed along a different path, for example by load balancers.
      This definition is taken from [RFC2960] and adapted to the MOBIKE

   Primary Path:

      The sequence of routers traversed by an IP packet that carries the
      default source and destination addresses is said to be the Primary
      Path.  This definition is taken from [RFC2960] and adapted to the
      MOBIKE context.

   Preferred Address:

      The IP address of a peer to which MOBIKE and IPsec traffic should
      be sent by default.  A given peer has only one active preferred
      address at a given point in time, except for the small time period
      where it switches from an old to a new preferred address.  This
      definition is taken from [I-D.ietf-hip-mm] and adapted to the
      MOBIKE context.

Kivinen & Tschofenig    Expires January 19, 2006                [Page 5]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

   Peer Address Set:

      We denote the two peers of a MOBIKE session by peer A and peer B.
      A peer address set is the subset of locally operational addresses
      of peer A that is sent to peer B. A policy available at peer A
      indicates which addresses are included in the peer address set.
      Such a policy might be created either manually or automatically
      through interaction with other mechanisms that indicate new
      available addresses.

   Terminology regarding NAT types (e.g.  Full Cone, Restricted Cone,
   Port Restricted Cone and Symmetric), can be found in Section 5 of
   [RFC3489].  For mobility related terminology (e.g.  Make-before-break
   or Break-before-make) see [RFC3753].

Kivinen & Tschofenig    Expires January 19, 2006                [Page 6]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

3.  Scenarios

   In this section we discuss three typical usage scenarios for the
   MOBIKE protocol.

3.1  Mobility Scenario

   Figure 1 shows a break-before-make mobility scenario where a mobile
   node changes its point of network attachment.  Prior to the change,
   the mobile node had established an IPsec connection with a security
   gateway which offered, for example, access to a corporate network.
   The IKEv2 exchange that facilitated the set up of the IPsec SA(s)
   took place over the path labeled as 'old path'.  The involved packets
   carried the MN's "old" IP address and were forwarded by the "old"
   access router (OAR) to the security gateway (GW).

   When the MN changes its point of network attachment, it obtains a new
   IP address using statefu address configuration techniques or via the
   stateless address autoconfiguration mechanism.  The goal of MOBIKE,
   in this scenario, is to enable the MN and the GW to continue using
   the existing SAs and to avoid setting up a new IKE SA.  A protocol
   exchange, denoted by 'MOBIKE Address Update', enables the peers to
   update their state as necessary.

   Note that in a break-before-make scenario the MN obtains the new IP
   address after it can no longer be reached at the old IP address.  In
   a make-before-break scenario, the MN is, for a given period of time,
   reachable at both the old and the new IP address.  MOBIKE should work
   in both the above scenarios.

Kivinen & Tschofenig    Expires January 19, 2006                [Page 7]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

                          (Initial IKEv2 Exchange)
       Old IP   +--+        +---+                    v
       address  |MN|------> |OAR| -------------V     v
                +--+        +---+ Old path     V     v
                 .                          +----+   v>>>>> +--+
                 .move                      | R  | -------> |GW|
                 .                          |    |    >>>>> |  |
                 v                          +----+   ^      +--+
                +--+        +---+ New path     ^     ^
       New IP   |MN|------> |NAR|--------------^     ^
       address  +--+        +---+                    ^
                          (MOBIKE Address Update)

              ---> = Path taken by data packets
              >>>> = Signaling traffic (IKEv2 and MOBIKE)
              ...> = End host movement

                        Figure 1: Mobility Scenario

3.2  Multihoming Scenario

   Another MOBIKE usage scenario is depicted in Figure 2.  In this
   scenario, the MOBIKE peers are equipped with multiple interfaces (and
   multiple IP addresses).  Peer A has two interface cards with two IP
   addresses, IP_A1 and IP_A2, and peer B has two IP addresses, IP_B1
   and IP_B2.  Each peer selects one of its IP addresses as the
   preferred address which is used for subsequent communication.
   Various reasons, (e.g hardware or network link failures), may require
   a peer to switch from one interface to another.

     +------------+                                  +------------+
     | Peer A     |           *~~~~~~~~~*            | Peer B     |
     |            |>>>>>>>>>> * Network   *>>>>>>>>>>|            |
     |      IP_A1 +-------->+             +--------->+ IP_B1      |
     |            |         |             |          |            |
     |      IP_A2 +********>+             +*********>+ IP_B2      |
     |            |          *           *           |            |
     +------------+           *~~~~~~~~~*            +------------+

              ---> = Path taken by data packets
              >>>> = Signaling traffic (IKEv2 and MOBIKE)
              ***> = Potential future path through the network
                     (if Peer A and Peer B change their preferred

Kivinen & Tschofenig    Expires January 19, 2006                [Page 8]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

                      Figure 2: Multihoming Scenario

   Note that MOBIKE does not aim to support load balancing between
   multiple IP addresses.  That is, each peer uses only one of the
   available IP addresses at a given point in time.

3.3  Multihomed Laptop Scenario

   The third scenario we consider is about a laptop, which has multiple
   interface cards and therefore several ways to connect to the network.
   It may for example have a fixed Ethernet card, a WLAN interface, a
   GPRS adaptor, a Bluetooth interface or USB hardware.  Not all
   interfaces are connected to the network at all times for a number of
   reasons (e.g., cost, availability of certain link layer technologies,
   user convenience).  The mechanism that determines which interfaces
   are connected to the network at any given point in time is outside
   the scope of the MOBIKE protocol and, as such, this document.
   However, as the laptop changes its point of attachment to the
   network, the set of IP addresses under which the laptop is reachable,
   changes too.

   Even if IP addresses change due to interface switching or mobility,
   the IP address obtained via the configuration payloads within IKEv2
   remain unaffected.  The IP address obtained via the IKEv2
   configuration payloads allow the configuration of the inner IP
   address of the IPsec tunnel.  As such, applications might not detect
   any change at all.

Kivinen & Tschofenig    Expires January 19, 2006                [Page 9]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

4.  Framework

   The working group will develop a MOBIKE protocol which needs to
   perform the following operations:

   o  inform the other peer about the peer address set

   o  inform the other peer about the preferred address

   o  test connectivity along a path and thereby to detect an outage

   o  change the preferred address

   o  change the peer address set

   o  Ability to deal with Network Address Translation devices

   The technical details of these functions are discussed below.
   Although MOBIKE will have to interact with other mechanisms, the
   working group is chartered to leave this aspect outside the scope.

   When a MOBIKE peer initiates a protocol exchange it needs to define a
   peer address set based on the IP addresses available to it.  The peer
   may want to make this set available to the other peer.  The IKEv2
   Initiator does not need to indicate which of the addresses in the
   peer address set is its preferred address.  This is because the
   Initiator has to place its preferred address into the source IP
   address field of the IP header with the initial message exchange.
   Additionally, the Initiator expects incoming signaling messages to
   arrive at this address.  The peer address set and the preferred
   address are defined based on interaction with other components within
   a host.  In some cases, the peer address set may be available before
   the initial protocol exchange and does not change during the lifetime
   of the IKE-SA.  The preferred address might change due to policy
   reasons.  Section 3 describes three scenarios in which the peer
   address set is modified (by adding or deleting addresses).  In these
   scenarios the preferred address may change as well.

   A modification of the peer address set or a change of the preferred
   address typically is the result of the MOBIKE peer's local policy and
   by the interaction with other protocols (such as DHCP or IPv6
   Neighbor Discovery).

   Figure 3 shows an example protocol interaction between a pair of
   MOBIKE peers.  MOBIKE interacts with the IPsec engine using the
   PF_KEY API [RFC2367].  Using this API, the MOBIKE daemon can create
   entries in the Security Association (SAD) and Security Policy

Kivinen & Tschofenig    Expires January 19, 2006               [Page 10]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

   Databases (SPD).  The IPsec engine may also interact with IKEv2 and
   MOBIKE daemon using this API.  The content of the Security Policy and
   Security Association Databases determines what traffic is protected
   with IPsec in which fashion.  MOBIKE, on the other hand, receives
   information from a number of sources that may run both in kernel-mode
   and in user-mode.  Information relevant for MOBIKE might be stored in
   a database.  The contents of such a database, along with the
   occurrence of events of which the MOBIKE process is notified, form
   the basis on which MOBIKE decides regarding the set of available
   addresses, the peer address set, and the preferred address.  Policies
   may also affect the selection process.

   The a peer address set and the preferred address needs to be
   available to the other peer.  In order to address certain failure
   cases, MOBIKE should perform connectivity tests between the peers
   (potentially over a number of different paths).  Although a number of
   address pairs may be available for such tests, the most important is
   the pair (source address, destination address) of the primary path.
   This is because this pair is selected for sending and receiving
   MOBIKE signaling and IPsec traffic.  If a problem along this primary
   path is detected (e.g., due to a router failure) it is necessary to
   switch to a new primary path.  In order to be able to do so quickly,
   it may be helpful to perform connectivity tests of other paths
   periodically.  Such a technique would also help in identifying
   previously disconnected paths that become operational.

Kivinen & Tschofenig    Expires January 19, 2006               [Page 11]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

                           +-------------+       +---------+
                           |User-space   |       | MOBIKE  |
                           |Protocols    |  +-->>| Daemon  |
                           |relevant for |  |    |         |
                           |MOBIKE       |  |    +---------+
                           +-------------+  |         ^
   User Space                    ^          |         ^
   ++++++++++++++++++++++++++++ API ++++++ API ++++ PF_KEY ++++++++
   Kernel Space                  v          |         v
                               _______      |         v
       +-------------+        /       \     |    +--------------+
       |Routing      |       / Trigger \    |    | IPsec        |
       |Protocols    |<<-->>|  Database |<<-+  +>+ Engine       |
       |             |       \         /       | | (+Databases) |
       +-----+---+---+        \_______/        | +------+-------+
             ^   ^               ^             |        ^
             |   +---------------+-------------+--------+-----+
             |                   v             |        |     |
             |             +-------------+     |        |     |
      I      |             |Kernel-space |     |        |     |   I
      n      |   +-------->+Protocols    +<----+-----+  |     |   n
      t      v   v         |relevant for |     |     v  v     v   t
      e +----+---+-+       |MOBIKE       |     |   +-+--+-----+-+ e
      r |  Input   |       +-------------+     |   | Outgoing   | r
      f |  Packet  +<--------------------------+   | Interface  | f
    ==a>|Processing|===============================| Processing |=a>
      c |          |                               |            | c
      e +----------+                               +------------+ e
      s                                                           s
              ===> = IP packets arriving/leaving a MOBIKE node
              <->  = control and configuration operations

                            Figure 3: Framework

   Please note that Figure 3 illustrates an example of how a MOBIKE
   implementation could work.  Hence, it serves illustrative purposes

   Extensions of the PF_KEY interface required by MOBIKE are also within
   the scope of the working group.  Finally, certain optimizations for
   wireless environments are also covered.

Kivinen & Tschofenig    Expires January 19, 2006               [Page 12]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

5.  Design Considerations

   This section discusses aspects affecting the design of the MOBIKE

5.1  Indicating Support for MOBIKE

   In order for MOBIKE to function, both peers must implement the MOBIKE
   extension of IKEv2.  If one or none of the peers supports MOBIKE,
   then, whenever an IP address changes, IKEv2 will have to be re-run in
   order to create a new IKE SA and the respective IPsec SAs.  In
   MOBIKE, a peer needs to be confident that its address change messages
   are understood by the other peer.  If these messages are not
   understood, it is possible that connectivity between the peers is

   One way to ensure that a peer receives feedback on whether or not its
   messages are understood by the other peer, is by using IKEv2
   messaging for MOBIKE and to mark some messages as "critical".
   According to the IKEv2 specification, such messages either have to be
   understood by the receiver, or an error message has to be returned to
   the sender.

   A second way to ensure receipt of the above-mentioned feedback is by
   using Vendor ID payloads that are exchanged during the initial IKEv2
   exchange.  These payloads would then indicate whether or not a given
   peer supports the MOBIKE protocol.

   A third approach would use the Notify payload which is also used for
   NAT detection (via NAT_DETECTION_SOURCE_IP and

   Both a Vendor ID and a Notify payload may be used to indicate the
   support of certain extensions.

   Note that a MOBIKE peer could also attempt to execute MOBIKE
   opportunistically with the critical bit set when an address change
   has occurred.  The drawback of this approach is, however, that an
   unnecessary MOBIKE message exchange is introduced.

   Although Vendor ID payloads and Notifications are technically
   equivalent, Notifications are already used in IKEv2 as a capability
   negotiation mechanism.  Hence, Notifications and Vendor ID payloads
   are the preferred mechanisms.

5.2  Changing a Preferred Address and Multi-homing Support

   From MOBIKE's point of view, support for multi-homing is inherently

Kivinen & Tschofenig    Expires January 19, 2006               [Page 13]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

   provided by the fact that it manages a set of peer addresses, rather
   than a single address.  Further, MOBIKE provides mechanisms to change
   a peer's preferred IP address.  Each peer needs to learn the
   preferred address and the peer address set.

5.2.1  Storing a single or multiple addresses

   One design decision is whether an IKE-SA should be associated with a
   single IP address or multiple IP addresses.  One option is that a
   peer can provide a list of addresses to its counterpart which can
   then be used as destination addresses.

   Note that MOBIKE does not support load balancing, i.e., only one IP
   address is set to a preferred address at a time and changing the
   preferred address typically requires some MOBIKE signaling.

   Another option is to only communicate one address to the other peer
   and both peers only use that address when communicating.  If this
   address cannot be used anymore then an address update is sent to the
   other peer that changes the preferred address.

   Alternatively, if peer A, for example,provides a peer address set
   with multiple IP addresses then peer B can recover from a failure of
   the preferred address without further communication with peer A. That
   is, if it detects that the primary path does not work anymore it can
   either switch to a new preferred address locally (i.e., changing the
   source IP address of outgoing MOBIKE messages) or to try an IP
   address from A's peer address set (i.e., changing the destination
   address).  If peer B only received a single IP address from peer A
   for A then peer B can only change its own preferred address.  Peer B
   would have to wait for an address update from peer A with a new IP
   address in order to fix the problem.

   The main advantage of storing only a single IP address for the remote
   peer is that it makes retransmission handling easier.  Moreover, it
   simplifies the recovery procedure.  The peer whose IP address changed
   must start the recovery process and send the new IP address to the
   other peer.  However, connectivity failures along the path are not
   well addressed with advertising a single IP address.

   The single IP address approach does not work if both peers change
   their IP addresses at the same time, for example if both hosts move
   simultaneously, even though multiple addresses are available to the
   two peers.  The IKEv2 implementation might also require window size
   to be larger than 1 because the MOBIKE peer needs to be able to send
   the IP address change notifications before it switches to another
   address.  Depending on the occurrence of return routability checks,
   retransmissions policies and similar message exchanges a window size

Kivinen & Tschofenig    Expires January 19, 2006               [Page 14]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

   larger than 1 might be required to deal with more than one pending
   response at the same time.  Furthermore, the single IP address
   approach does not really benefit much from indirect indications as
   the peer receiving these indications might not be able to fix the
   situation by itself (e.g., even if a peer receives an ICMP host
   unreachable message for the old IP address, it cannot try another IP
   address, since it does not know any).

   The problems with IP address lists lie mostly in their complexity.
   Notification and recovery processes are more complicated.  Both ends
   can recover from the IP address changes.  However, both peers should
   not attempt to recover at the same time or nearly the same time as
   this could cause them to lose connectivity.  The MOBIKE protocol is
   required to prevent this.

   The previous discussion is independent of the question of how many
   addresses to send in a single MOBIKE message.  A MOBIKE message might
   be able to carry more than one IP address (with the IP address list
   approach) or a single address only.  A NAT does not change addresses
   carried inside the MOBIKE message but it can change IP address (and
   transport layer addresses) in the IP header and Transport Protocol
   header (if NAT traversal is enabled as part of configuration or
   dynamically through the NAT discovery mechanism.  Furthermore, a
   MOBIKE message carrying the peer address set could be idempotent
   (i.e., always resending the full address list) or the protocol may
   allow add/delete operations to be specified.  [I-D.dupont-ikev2-
   addrmgmt], for example, offers an approach which defines add/delete
   operations.  The same is true for the dynamic address reconfiguration
   extension for SCTP [I-D.ietf-tsvwg-addip-sctp].

5.2.2  Indirect or Direct Indication

   An indication to change the preferred IP address might be either
   direct or indirect.

   Direct indication:

      A direct indication means that the other peer will send an message
      with the address change.  This can, for example, be accomplished
      by having MOBIKE sending an authenticated address update
      notification with a different preferred address.

   Indirect indication:

      An indirect indication to change the preferred address can be
      obtained by observing the environment.  An indirect indication

Kivinen & Tschofenig    Expires January 19, 2006               [Page 15]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

      might, for example, be the receipt of an ICMP message or
      information about a link failure.  This information should be seen
      as a hint and should not cause a change of the remote peer's
      preferred address.  Depending on the local policy, MOBIKE may
      decide to perform a dead-peer detection exchange for the preferred
      address pair (or another address pair from the peer address set).
      When a peer detects that the other end started to use a different
      source IP address than before, it might want to authorize the new
      preferred address (if not already authorized).  Authorization aims
      to ensure that a particular peer is allowed to use the indicated
      address.  Claiming to be at an arbitrary address without
      performing a return-routability test or without verifying that the
      IP address is listed within a certificate opens the door for
      various denial of service attacks.  Hence a peer may also start a
      connectivity test of this particular address.

   If more information is available to the MOBIKE daemon then a more
   intelligent decision regarding the selection of a new primary path
   can be made.

5.2.3  Connectivity Tests using IKEv2 Dead-Peer Detection

   This section discusses the suitability of the IKEv2 dead-peer
   detection (DPD) mechanism for connectivity tests between address
   pairs.  The basic IKEv2 DPD mechanism is not modified by MOBIKE but
   it needs to be investigated whether it can be used for MOBIKE
   purposes as well.

   The IKEv2 DPD mechanism involves sending an empty informational
   exchange packet to a given address of the remote peer.  On receipt,
   the remote peer responds with an acknowledgement.  If no
   acknowledgement is received after a certain timeout period (and after
   couple of retransmissions), the remote peer is considered to be not
   reachable at the address in question.  On the other hand, receipt of
   IPsec protected acknowledgement is a guarantee that the other peer is
   reachable at the address in question.

   When reusing dead-peer detection in MOBIKE for connectivity tests it
   seems to be reasonable to try other IP addresses (if they are
   available) in case of an unsuccessful connectivity test for a given
   address pair.  Dead-peer detection messages using a different address
   pair should use the same message-id as the original dead-peer
   detection message (i.e. they are simply retransmissions of the dead-
   peer detection packet using different destination IP address).  If
   different message-id is used then it violates constraints placed by
   the IKEv2 specification on the DPD message with regard to the
   mandatory ACK for each message-id, causing the IKEv2 SA to be

Kivinen & Tschofenig    Expires January 19, 2006               [Page 16]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

   If MOBIKE strictly follows the guidelines of the dead-peer detection
   mechanism in IKEv2 then an IKE-SA should be marked as dead and
   deleted if the connectivity test for all available address pairs
   fails.  Note that this is not in-line with the approach used, for
   example, in SCTP where a failed connectivity test for an address does
   not lead to (a) the IP address or IP address pair to be marked as
   dead and (b) delete state.  Connectivity tests will be continued for
   the address pairs in hope that the problem will recover soon.  This
   comparison with SCTP aims to point at another IETF protocol that aims
   to address the multi-homing problem (although with a different scope
   and a different layer).

   Note that IKEv2 implementations may have a window size of 1, and
   therefore be unable to initiate a dead-peer detection exchange while
   another exchange is pending.  As a result, all other exchanges are
   subject to an identical retransmission policy as used for the dead-
   peer detection.  To use a different policy for different message
   types seems to be reasonable.

   The dead-peer detection mechanism for the other IP address pairs can
   also be executed simultaneously if the window size larger than 1,
   meaning that after the initial timeout period of the preferred
   address expires, DPD packets are sent simultaneously to all other
   address pairs.  It is necessary to differentiate acknowledgement
   messages in order to determine which address pair is operational.
   The source IP address of the acknowledgement can be used for this

   The protocol should also be nice to the network, meaning, that when
   some core link goes down, and a large number of MOBIKE clients notice
   this, they should not start sending a large number of messages while
   trying to recover from the problem.  This may be particularly
   unfortunate because packets may be dropped because of congestion in
   the first place.  If MOBIKE clients simultaneously try to test all
   possible address pairs by executing connectivity tests then the
   congestion problem only gets worse.

   Also note that the IKEv2 dead-peer detection is not sufficient for
   the return routability check.  See Section 5.6 for more information.

5.3  Simultaneous Movements

   MOBIKE does not aim to provide a mobility solution that allows
   simultaneous movements.  Instead, the MOBIKE working group focuses on
   selected scenarios as described in Section 3.  Some of the scenarios
   assume that one peer has a fixed set of addresses (from which some
   subset might be in use).  Thus it cannot move to an address that is
   unknown to the other peer.  Situations in which both peers move

Kivinen & Tschofenig    Expires January 19, 2006               [Page 17]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

   simultaneously are outside the scope of the MOBIKE WG.  MOBIKE has
   not been chartered to deal with the rendezvous problem, or with the
   introduction of new entities in the network.

   Note that if only a single address is stored in the peer address set
   (instead of an address list) we might end up in the case where it
   seems that both peers changed their addresses at the same time (e.g.,
   if both nodes change their addresses at the same time).  This is
   something that the MOBIKE protocol must deal with.

   Three cases can be differentiated:

   o  Two mobile nodes obtain a new address at the same time, and then
      being unable to tell each other where they are (in a break-before-
      make scenario).  This problem is called the rendezvous problem,
      and is traditionally solved by introducing another third entity,
      for example, the home agents (in Mobile IPv4/IPv6) or forwarding
      agents (in the Host Identity Protocol).  Essentially, solving this
      problem requires the existence of additional infrastructure nodes.

   o  Simultaneous changes to addresses such that at least one of the
      new addresses is known to the other peer before the change

   o  No simultaneous changes at all.

5.4  NAT Traversal

   IKEv2 supports legacy NAT traversal.  This feature is known as NAT-T
   which allows IKEv2 to work even if a NAT along the path between the
   Initiator and the Responder modifies the source and possibly the
   destination IP address.  With NAPT even the transport protocol
   identifiers are modified (which then requires UDP encapsulation for
   exchanged IPsec protected data traffic).  Therefore, the MOBIKE
   daemon needs to obtain to required IP address informationfrom the IP
   header (if a NAT was detected that modified the IP header) rather
   than from the protected payload.  This problem is not new and is an
   issues of every mobility protocol where the most important
   information exchanged is the IP address .

   One of the goals in the MOBIKE protocol is to securely exchange one
   or more addresses of the peer address set and to securely set the
   primary address.  If no other protocol is used to securely retrieve
   the IP address and port information allocated by the NAT then it is

Kivinen & Tschofenig    Expires January 19, 2006               [Page 18]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

   not possible to tackle all attacks against MOBIKE.  Section 6
   discusses this aspect in more detail.  Various approaches to solve
   the problem have been presented:

   o  Securely retrieving IP address and port information allocated by
      the NAT using a protocol different from MOBIKE.  This approach is
      outside the scope of the MOBIKE working group since other working
      groups, such as MIDCOM and NSIS, already deal with this problem.
      The MOBIKE protocol can benefit from the interaction with these
      protocols but the interaction with these protocols it outside the
      scope of this document.

   o  Design a protocol in such a way that NAT boxes are able to inspect
      (or even participate) in the protocol exchange.  This approach was
      taken with the Host Identity Protocol but is not an option for
      IKEv2 and MOBIKE since most IKEv2 messages are encrypted with the
      established IKE SA.  This prevents the NAT from learning required
      information from the protocol exchange in a similar fashion as in

   o  Disable NAT-T by indicating the desire never to use information
      from the (unauthenticated) header.  While this approach prevents
      some security problems it effectively disallows the protocol to
      work in environments with NATs.

   There is no way to distinguish the whether there is a NAT device
   along the path that modifies the header information in packets or an
   adversary mounting an attack.  If a NAT is detected during the
   creation of an IKE SA, that should automatically disable the MOBIKE
   extensions and use NAT-T.

   A design question is whether NAT detection capabilities should be
   enabled only during the initial IKEv2 exchange or also during
   subsequent message exchanges.  If MOBIKE is executed with no NAT
   along the path when the IKE SA was created, then a NAT which appears
   after moving to a new network cannot be dealt with if NAT detection
   is enabled only during the initial exchange.  Hence, it is desirable
   to also support a scenario where a MOBIKE peer moves from a subnet
   that is not behind a NAT to a network that is.

   A NAT prevention mechanism can be used to make sure that no adversary
   can interact with the protocol if no NAT is expected between the
   Initiator and the Responder. (reference?  Explanation?)

   Whether or not MOBIKE should support NAT traversal is one of the most

Kivinen & Tschofenig    Expires January 19, 2006               [Page 19]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

   important design decisions.

5.5  Changing addresses or changing the paths

   A design decision is whether it is enough for the MOBIKE protocol to
   detect dead addresses, or it also needs to detect dead paths.  Dead
   address detection refers to the ability to establish whether or not a
   given IP address pair is operational.  Dead path detection refers to
   the ability to establish whether or not all possible (local/remote)
   address pairs are operational (or at least find one such pair).

   While performing just one address change is simpler, the existence of
   locally operational addresses is not, however, a guarantee that
   communications can be established with the peer.  A failure in the
   routing infrastructure can prevent the sent packets from reaching
   their destination.

5.6  Return Routability Tests

   Changing the preferred address and subsequently using it for
   communication is associated with an authorization decision: Is a peer
   allowed to use this address?  Does this peer own this address?  Two
   mechanisms have been proposed in the past to allow a peer to
   determine the answer to this question:

   o  The addresses a peer is using are part of a certificate.
      [RFC3554] which is introduced by this approach.  If the other peer
      is, for example, a security gateway with a limited set of fixed IP
      addresses, then the security gateway may have a certificate with
      all the IP addresses appear in the certificate.

   o  A return routability check is performed by the remote peer before
      the address is updated in that peer's Security Association
      Database.  This is done in order provide a certain degree of
      confidence to the remote peer that local peer is reachable at the
      indicated address.

   Without taking an authorization decision a malicious peer can
   redirect traffic towards a third party or a blackhole.

   A MOBIKE peer should not use an IP addressed provided by another
   MOBIKE peer as a primary address without computing the authorization
   decision.  If the addresses are part of the certificate then it is
   not necessary to execute the weaker return-routability test.  The
   return-routability test is a form of authorization check, although it
   provides weaker guarantees then the inclusion of the IP address as

Kivinen & Tschofenig    Expires January 19, 2006               [Page 20]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

   part of a certificate.  If multiple addresses are communicated to the
   remote peer then some of these addresses may be already verified even
   if the primary address is still operational.

   Another option is to use the [I-D.dupont-mipv6-3bombing] approach
   which suggests to perform a return routability test only when an
   address update needs to be sent from some address other than the
   indicated preferred address.

   Finally it would be possible not to execute return routability checks
   at all.  In case of indirect change notifications we only move to the
   new preferred address after successful dead-peer detection (i.e., a
   response to a DPD test) on the new address, which is already a return
   routability check.  With a direct notification the authenticated peer
   may have provided an authenticated IP address.  Thus it is would be
   possible to simply trust the MOBIKE peer to provide a proper IP
   address.  There is no way an adversary can successfully launch an
   attack by injecting faked addresses since it does not know the IKE SA
   and the corresponding keying material.A protection against an
   internal attacker, i.e. the authenticated peer forwarding its traffic
   to the new address, is not provided.  This might be an issue when
   extensions are added to IKEv2 that do not require authentication of
   end points (e.g., opportunistic security using anonymous Diffie-
   Hellman).  On the other hand we know the identity of the peer in that
   case.  The identity of the IKEv2 Initiator and the IKEv2 Responder
   can take various forms: IP address, FQDN, DN, email address alike
   identifiers, etc.

   It seems that there it is also a policy issue when to schedule a
   return routability test.

   The basic format of the return routability check could be similar to
   dead-peer detection, but the problem is that if that fails then the
   IKEv2 specification requires the IKE SA to be deleted.  Because of
   this a different type of exchange is required and thereby avoiding
   modifications to the IKEv2 specification.

   There are potential attacks if a return routability check does not
   include some kind of nonce.  The valid end point could send an
   address update notification for a third party, trying to get all the
   traffic to be sent there, causing a denial of service attack.  If the
   return routability checks does not contain any cookies or other
   random information not known to the other end, then that valid node
   could reply to the return routability checks even when it cannot see
   the request.  This might cause a peer to move the traffic to a
   location where the original recipient cannot be reached.

   The IKEv2 NAT-T mechanism does not perform return routability checks.

Kivinen & Tschofenig    Expires January 19, 2006               [Page 21]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

   It simply uses the last seen source IP address used by the other peer
   as the destination address to send response packets.  An adversary
   can change those IP addresses, and can cause the response packets to
   be sent to wrong IP address.  The situation is self-fixing when the
   adversary is no longer able to modify packets and the first packet
   with an unmodified IP address reaches the other peer.  Mobility
   environments make this attack more difficult for an adversary since
   it requires the adversary to be located somewhere on the individual
   paths ({CoA1, ..., CoAn} towards the destination IP address) have a
   shared path or if the adversary is located near the MOBIKE client
   then it needs to follow the user mobility patterns.  With IKEv2
   NAT-T, the genuine client can cause third party bombing by
   redirecting all the traffic pointed to him to third party.  As the
   MOBIKE protocol tries to provide equal or better security than IKEv2
   NAT-T mechanism it should protect against these attacks.

   There may be return routability information available from the other
   parts of the system too (as shown in Figure 3), but the checks done
   may have a different quality.  There are multiple levels for return
   routability checks:

   o  None, no tests

   o  A party willing to answer the return routability check is located
      along the path to the claimed address ().  This is the basic form
      of return routability test.

   o  There is an answer from the tested address, and that answer was
      authenticated, integrity and replay protected.

   o  There was an authenticated, integrity and replay protected answer
      from the peer, but it is not guaranteed to originate at the tested
      address or path to it (because the peer can construct a response
      without seeing the request).

   The return routability checks do not protect against 3rd party
   bombing if the attacker is along the path, as the attacker can
   forward the return routability checks to the real peer (even if those
   packets are cryptographically authenticated).

   If the address to be tested is carried inside the MOBIKE payload,
   then the adversary cannot forward packets.  Thus 3rd party bombings
   are prevented.

Kivinen & Tschofenig    Expires January 19, 2006               [Page 22]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

   If the reply packet can be constructed without seeing the request
   packet (for example, if there is no nonce, challenge or similar
   mechanism to show liveness), then the genuine peer can cause 3rd
   party bombing, by replying to those requests without seeing them at

   Other levels might only provide a guarantee that there is a node at
   the IP address which replied to the request.  There is no indication
   as to whether or not the reply is fresh, and whether or not the
   request may have been transmitted from a different source address.

5.7  Employing MOBIKE results in other protocols

   If MOBIKE has learned about new locations or verified the validity of
   a remote address through a return routability check, can this
   information be useful for other protocols?

   When considering the basic MOBIKE VPN scenario, the answer is no.
   Transport and application layer protocols running inside the VPN
   tunnel are unaware of the outer addresses or their status.

   Similarly, IP layer tunnel termination at a gateway rather than a
   host endpoint limits the benefits for "other protocols" that could be
   informed -- all application protocols at the other side are unaware
   of IPsec, IKE, or MOBIKE.

   However, it is conceivable that future uses or extensions of the
   MOBIKE protocol make such information distribution useful.  For
   instance, if transport mode MOBIKE and SCTP were made to work
   together, it would potentially be useful for SCTP to learn about the
   new addresses at the same time as MOBIKE.  Similarly, various IP
   layer mechanisms may make use of the fact that a return routability
   test of a specific type has been performed.  However, care should be
   exercised in all these situations .

   [I-D.crocker-celp] discusses the use of common locator information
   pools in a IPv6 multi-homing context; it assumed that both transport
   and IP layer solutions are be used in order to support multi-homing,
   and that it would be beneficial for different protocols to coordinate
   their results in some way, for instance by sharing throughput
   information of address pairs.  This may apply to MOBIKE as well,
   assuming it co-exists with non-IPsec protocols that are faced with
   the same or similar multi-homing choices.

   Nevertheless, all of this is outside the scope of current MOBIKE base
   protocol design and may be addressed in future work. (so why do you
   elaborate so much on this stuff?)

Kivinen & Tschofenig    Expires January 19, 2006               [Page 23]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

5.8  Scope of SA changes

   Most sections of this document discuss design considerations for
   updating and maintaining addresses in the database entries that
   relate to an IKE-SA.  However, changing the preferred address also
   affects the entries of the IPsec SA database.  The outer tunnel
   header addresses (source and destination IP addresses) need to be
   modified according to the primary path to allow the IPsec protected
   data traffic to travel along the same path as the MOBIKE packets (if
   we only consider the IP header information).  If the MOBIKE messages
   and the IPsec protected data traffic travel along a different path
   then NAT handling is severely complicated.

   The basic question is then how the IPsec SAs are changed to use the
   new address pair (the same address pair as the MOBIKE signaling
   traffic -- the primary path).  One option is that when the IKE SA
   address is changed then automatically all IPsec SAs associated with
   it are moved along with it to new address pair.  Another option is to
   have a separate exchange to move the IPsec SAs separately.

   If IPsec SAs should be updated separately then a more efficient
   format than the notification payload is needed to preserve bandwidth.
   A notification payload can only store one SPI per payload.  A
   separate payload which would have list of IPsec SA SPIs and new
   preferred address.  If there is a large number of IPsec SAs, those
   payloads can be quite large unless ranges of SPI values are
   supported.  If we automatically move all IPsec SAs when the IKE SA
   moves, then we only need to keep track which IKE SA was used to
   create the IPsec SA, and fetch the IP addresses.  Note that IKEv2
   [I-D.ietf-ipsec-ikev2] already requires implementations to keep track
   which IPsec SAs are created using which IKE SA.

   If we do allow each IPsec SA address set to be updated separately,
   then we can support scenarios, where the machine has fast and/or
   cheap connections and slow and/or expensive connections, and it wants
   to allow moving some of the SAs to the slower and/or more expensive
   connection, and prevent the move, for example, of the news video
   stream from the WLAN to the GPRS link.

   On the other hand, even if we tie the IKE SA update to the IPsec SA
   update, then we can create separate IKE SAs for this scenario, e.g.,
   we create one IKE SA which have both links as endpoints, and it is
   used for important traffic, and then we create another IKE SA which
   have only the fast and/or cheap connection, which is then used for
   that kind of bulk traffic.

Kivinen & Tschofenig    Expires January 19, 2006               [Page 24]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

5.9  Zero Address Set

   One of the features which is potentially useful is for the peer to
   announce that it will now disconnect for some time, i.e. it will not
   be reachable at all.  For instance, a laptop might go to suspend
   mode.  In this case the it could send address notification with zero
   new addresses, which means that it will not have any valid addresses
   anymore.  The responder of that kind of notification would then
   acknowledge that, and could then temporarily disable all SAs and
   therefore stop sending traffic.  If any of the SAs gets any packets
   they are simply dropped.  This could also include some kind of ACK
   spoofing to keep the TCP/IP sessions alive (or simply set the TCP/IP
   keepalives and timeouts large enough not to cause problems), or it
   could simply be left to the applications, e.g. allow TCP/IP sessions
   to notice the link is broken.

   The local policy could then indicate how long the peer should allow
   remote peers to remain disconnected.

   From a technical point of view this feature addresses two aspects:

   o  There is no need to transmit IPsec data traffic.  IPsec protected
      data needs to be dropped which saves bandwidth.  This does not
      provide a functional benefit, i.e., nothing breaks if this feature
      is not provided.

   o  MOBIKE signaling messages are also ignored.  The IKE-SA must not
      be deleted and the suspend functionality (realized with the zero
      address set) may require the IKE-SA to be tagged with a lifetime
      value since the IKE-SA should not be kept in alive for an
      undefined period of time.  Note that IKEv2 does not require that
      the IKE-SA has a lifetime associated with it.  In order to prevent
      the IKE-SA from being deleted the dead-peer detection mechanism
      needs to be suspended as well.

   Due to the fact that this extension would be complicated, a first
   version of the MOBIKE protocol will not provide this feature.

5.10  IPsec Tunnel or Transport Mode

   Current MOBIKE design is focused only on the VPN type usage and
   tunnel mode.  Transport mode behaviour would also be useful, but will
   be discussed in future documents.

Kivinen & Tschofenig    Expires January 19, 2006               [Page 25]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

5.11  Message Representation

   The IP address change notifications can be sent either via an
   informational exchange already specified in the IKEv2, or via a
   MOBIKE specific message exchange.  Using informational exchange has
   the main advantage that it is already specified in the IKEv2 and
   implementations incorporate the functionality already.

   Another question is the format of the address update notifications.
   The address update notifications can include multiple addresses, of
   which some may be IPv4 and some IPv6 addresses.  The number of
   addresses is most likely going to be limited in typical environments
   (with less than 10 addresses).  The format may need to indicate a
   preference value for each address.  The format could either contain a
   preference number that determines the relative order of the
   addresses, or it could simply be ordered, according to preference,
   list of IP addresses.  While two addresses can have the same
   preference value an ordered list avoids this situation.

   Even if load balancing is currently outside the scope of MOBIKE,
   future work might include.  The selected format needs to be flexible
   enough to include additional information (e.g. to enable load
   balancing).  This may be realized with an reserved field, which can
   later be used to store additional information.  As there may arise
   other information which may have to be tied to an address in the
   future, a reserved field seems like a prudent design in any case.

   There are two formats that place IP address lists into a message.
   One includes each IP address as separate payload (where the payload
   order indicates the preference value, or the payload itself might
   include the preference value), or we can put the IP address list as
   one payload to the exchange, and that one payload will then have
   internal format which includes the list of IP addresses.

   Having multiple payloads with each one having carrying one IP address
   makes the protocol probably easier to parse, as we can already use
   the normal IKEv2 payload parsing procedures..  It also offers an easy
   way for the extensions, as the payload probably contains only the
   type of the IP address (or the type is encoded to the payload type),
   and the IP address itself, and as each payload already has length
   associated to it, we can detect if there is any extra data after the
   IP address.  Some implementations might have problems parsing IKEv2
   payloads that are longer than a certain threshold, but if the sender
   sends them in the most preferred first, the receiver can only use the
   first addresses.

   Having all IP addresses in one big MOBIKE specified internal format
   provides more compact encoding, and keeps the MOBIKE implementation

Kivinen & Tschofenig    Expires January 19, 2006               [Page 26]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

   more concentrated to one module.  It also avoids problems of packets
   arriving in an order different from what they were sent.

   Another choice is which type of payloads to use.  IKEv2 already
   specifies a notify payload.  It includes some extra fields (SPI size,
   SPI, protocol etc), which gives 4 bytes of the extra overhead, and
   there is the notify data field, which could include the MOBIKE
   specific data.

   Another option would be to have a custom payload type, which then
   includes the information needed for the MOBIKE protocol.

   MOBIKE might send the full peer address list once one of the IP
   addresses changes (either addresses are added, removed, the order
   changes or the preferred address is updated) or an incremental
   update.  Sending incremental updates provides more compact packets
   (meaning we can support more IP addresses), but on the other hand
   have more problems in the synchronization and packet reordering
   cases, i.e., the incremental updates must be processed in order, but
   for full updates we can simply use the most recent one, and ignore
   old ones, even if they arrive after the most recent one (IKEv2
   packets have message id which is incremented for each packet, thus we
   know the sending order easily).

   Note that each peer needs to communicate its peer address set to the
   other peer.

Kivinen & Tschofenig    Expires January 19, 2006               [Page 27]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

6.  Security Considerations

   As all the messages are already authenticated by the IKEv2 there is
   no problem that any attackers would modify the contents of the
   packets.  The IP addresses in the IP header of the packets are not
   authenticated, thus the protocol defined must take care that they are
   only used as an indication that something might be different, and
   that do not cause any direct actions.

   An attacker can also spoof ICMP error messages in an effort to
   confuse the peers about which addresses are not working.  At worst
   this causes denial of service and/or the use of non-preferred

   One type of attack that needs to be taken care of in the MOBIKE
   protocol is the "flooding attack" type.  See [I-D.ietf-mip6-ro-sec]
   and [Aur02] for more information about flooding attacks.

Kivinen & Tschofenig    Expires January 19, 2006               [Page 28]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

7.  IANA Considerations

   This document does not introduce any IANA considerations.

Kivinen & Tschofenig    Expires January 19, 2006               [Page 29]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

8.  Acknowledgments

   This document is the result of discussions in the MOBIKE working
   group.  The authors would like to thank Jari Arkko, Pasi Eronen,
   Francis Dupont, Mohan Parthasarathy, Paul Hoffman, Bill Sommerfeld,
   James Kempf, Vijay Devarapalli, Atul Sharma, Bora Akyol, Joe Touch,
   Udo Schilcher, Tom Henderson, Andreas Pashalidis and Maureen Stillman
   for their input.

   We would like to particularly thank Pasi Eronen for tracking open
   issues on the MOBIKE mailing list.  He helped us to make good
   progress on the document.

Kivinen & Tschofenig    Expires January 19, 2006               [Page 30]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

9.  Open Issues

   This document is not yet complete, the following open issues need to
   be addressed in a future version:

   o  Section 4 needs an example to better illustrate the functionality
      of MOBIKE

   o  Section 6 requires a more detailed discussion about the potential
      security vulnerabilities and corresponding countermeasures.

   o  Some text is needed to address the implications of unidirectional
      connectivity aspect for MOBIKE (see also issue #19).

   o  A discussion about the scalability aspects of connectivity tests
      would be benefical.

   o  More details are necessary to describe the implications of NAT
      traversal for MOBIKE.

   A complete list of issues is available at

Kivinen & Tschofenig    Expires January 19, 2006               [Page 31]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

10.  References

10.1  Normative references

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

              Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", draft-ietf-ipsec-rfc2401bis-06 (work
              in progress), April 2005.

10.2  Informative References

              Arkko, J., "Failure Detection and Locator Selection in
              Multi6", draft-arkko-multi6dt-failure-detection-00 (work
              in progress), October 2004.

   [RFC2409]  Harkins, D. and D. Carrel, "The Internet Key Exchange
              (IKE)", RFC 2409, November 1998.

   [RFC2401]  Kent, S. and R. Atkinson, "Security Architecture for the
              Internet Protocol", RFC 2401, November 1998.

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

              Nikander, P., "Mobile IP version 6 Route Optimization
              Security Design Background", draft-ietf-mip6-ro-sec-03
              (work in progress), May 2005.

              Nikander, P., "End-Host Mobility and Multi-Homing with
              Host Identity Protocol", draft-ietf-hip-mm-01 (work in
              progress), February 2005.

              Crocker, D., "Framework for Common Endpoint Locator
              Pools", draft-crocker-celp-00 (work in progress),
              February 2004.

   [RFC3489]  Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,

Kivinen & Tschofenig    Expires January 19, 2006               [Page 32]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

              "STUN - Simple Traversal of User Datagram Protocol (UDP)
              Through Network Address Translators (NATs)", RFC 3489,
              March 2003.

   [RFC2960]  Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
              Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
              Zhang, L., and V. Paxson, "Stream Control Transmission
              Protocol", RFC 2960, October 2000.

   [RFC3753]  Manner, J. and M. Kojo, "Mobility Related Terminology",
              RFC 3753, June 2004.

              Stewart, R., "Stream Control Transmission Protocol (SCTP)
              Dynamic Address  Reconfiguration",
              draft-ietf-tsvwg-addip-sctp-12 (work in progress),
              June 2005.

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

   [RFC3554]  Bellovin, S., Ioannidis, J., Keromytis, A., and R.
              Stewart, "On the Use of Stream Control Transmission
              Protocol (SCTP) with IPsec", RFC 3554, July 2003.

              Moore, N., "Optimistic Duplicate Address Detection for
              IPv6", draft-ietf-ipv6-optimistic-dad-05 (work in
              progress), February 2005.

              Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", draft-ietf-ipv6-unique-local-addr-09 (work in
              progress), January 2005.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC2367]  McDonald, D., Metz, C., and B. Phan, "PF_KEY Key
              Management API, Version 2", RFC 2367, July 1998.

   [RFC2462]  Thomson, S. and T. Narten, "IPv6 Stateless Address
              Autoconfiguration", RFC 2462, December 1998.

   [RFC2461]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor

Kivinen & Tschofenig    Expires January 19, 2006               [Page 33]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

              Discovery for IP Version 6 (IPv6)", RFC 2461,
              December 1998.

   [Aur02]    Aura, T., Roe, M., and J. Arkko, "Security of Internet
              Location Management", In Proc. 18th Annual Computer
              Security Applications Conference, pages 78-87, Las Vegas,
              NV USA, December 2002.

Authors' Addresses

   Tero Kivinen
   Safenet, Inc.
   Fredrikinkatu 47
   HELSINKI  FIN-00100

   Email: kivinen@safenet-inc.com

   Hannes Tschofenig
   Otto-Hahn-Ring 6
   Munich, Bavaria  81739

   Email: Hannes.Tschofenig@siemens.com
   URI:   http://www.tschofenig.com

Kivinen & Tschofenig    Expires January 19, 2006               [Page 34]

Internet-Draft        Design of the MOBIKE Protocol            July 2005

Intellectual Property Statement

   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

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at

Disclaimer of Validity

   This document and the information contained herein are provided on an

Copyright Statement

   Copyright (C) The Internet Society (2005).  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.


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

Kivinen & Tschofenig    Expires January 19, 2006               [Page 35]