Network Working Group                                           J. Arkko
Internet-Draft                                                  Ericsson
Expires: April 11, 2006                                  October 8, 2005

     Failure Detection and Locator Pair Exploration Design for IPv6

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   This draft discusses the issues of detecting failures in a currently
   used address pair between two hosts and picking a new address pair to
   be used when a failure occurs.  The draft also discusses the roles of
   a multihoming protocol versus network attachment functions at IP and
   link layers.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements language  . . . . . . . . . . . . . . . . . . . .  4
   3.  Related Work . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  8
         4.1.  Available Addresses  . . . . . . . . . . . . . . . . .  8
         4.2.  Locally Operational Addresses  . . . . . . . . . . . .  9
         4.3.  Operational Address Pairs  . . . . . . . . . . . . . .  9
         4.4.  Primary Address Pair . . . . . . . . . . . . . . . . . 11
         4.5.  Miscellaneous  . . . . . . . . . . . . . . . . . . . . 11
   5.  Architectural Considerations . . . . . . . . . . . . . . . . . 12
   6.  Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
         6.1.  State Machines . . . . . . . . . . . . . . . . . . . . 14
         6.2.  Failure Detection  . . . . . . . . . . . . . . . . . . 19
         6.3.  Alternative Locator Pair Exploration . . . . . . . . . 19
               6.3.1.  Exploration Order  . . . . . . . . . . . . . . 19
               6.3.2.  Exploration Protocol . . . . . . . . . . . . . 21
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 23
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
         8.1.  Normative References . . . . . . . . . . . . . . . . . 24
         8.2.  Informative References . . . . . . . . . . . . . . . . 24
   Appendix A.  Contributors  . . . . . . . . . . . . . . . . . . . . 27
   Appendix B.  Acknowledgements  . . . . . . . . . . . . . . . . . . 28
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 29
   Intellectual Property and Copyright Statements . . . . . . . . . . 30

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1.  Introduction

   The SHIM6 working group is extending IPv6 to support multihoming.
   The focus of the group is to look at an IP layer (or layer 3.5)
   mechanism that hides multihoming from applications [23].  This
   mechanism needs to detect when a switch to another address or
   addresses becomes necessary.  We call this failure detection.

   This draft discusses what requirements such a component of the SHIM6
   protocol has, and how these requirements can be achieved.  The draft
   is structured as follows: Section 3 discusses what kind of solutions
   have been used in other similar protocols.  Section 4 defines a set
   of useful terms and discusses them, and Section 5 discusses the
   architectural implications of failure detection designs.  Finally,
   Section 6 describes one possible solution involving a mechanism to
   detect failures and an exploration protocol for working address

   For the purposes of this draft, we consider an address to be
   synonymous with a locator.  There may be other, higher level
   identifiers such as security associations, FQDNs, CGA public keys,
   HBA bindings, or HITs that tie the different locators used by a node

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2.  Requirements language

   In this document, the key words "MAY", "MUST, "MUST NOT", "OPTIONAL",
   "RECOMMENDED", "SHOULD", and "SHOULD NOT", are to be interpreted as
   described in [1].

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3.  Related Work

   Another SHIM6 document [10] discusses what kind of mechanisms can be
   used to detect whether the peer is still reachable at the currently
   used address.  Two proposed mechanisms, Correspondent Unreachability
   Detection (CUD) and Forced Bidirectional Communication (FBD) are
   presented.  CUD is based on getting upper layer positive feedback,
   and IPv6 NUD-like probing if there is no feedback.  FBD is based on
   forcing bidirectional communication by adding keepalive messages when
   there is no other, payload traffic.

   In SCTP [11], the addresses of the endpoints are learned in the
   connection setup phase either through listing them explicitly or via
   giving a DNS name that points to them.  In order to provide a
   failover mechanism between multihomed hosts, SCTP has the following

   o  One of the peer's addresses is selected as the primary address by
      the application running on top of SCTP.  All data packets are sent
      to this address until there is a reason to choose another address,
      such as the failure of the primary address.

   o  Testing the reachability of the peer endpoint's addresses.  This
      is done both via observing the data packets sent to the peer or
      via a periodic heartbeat when there is no data packets to send.

      Each time data packet retransmission is initiated (or when a
      heartbeat is not answered within the estimated round-trip time) an
      error counter is incremented.  When a configured error limit is
      reached, the particular destination address is marked as inactive.
      The reception of an acknowledgement or heartbeat response clears
      the counter.

   o  Retransmission: When retransmitting the endpoint attempts pick the
      most "divergent" source-destination pair from the original source-
      destination pair to which the packet was transmitted.  Rules for
      such selection are, however, left as implementation decisions in

   SCTP does not define how local knowledge (such as information learned
   from the link layer) should be used.  SCTP also has no mechanism to
   deal with dynamic changes to the set of available addresses, although
   mechanisms for that are being developed [18].

   The MOBIKE protocol is currently being specified [16] [15].  This

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   protocol operates in a mixed IPv4/IPv6 environment, and typically has
   to work through NATs.  The current design is assumed to need to work
   only in symmetric connectivity scenarios.

   Some of the issues that have been discussed in the MOBIKE design
   phase include the following:

   o  Single address vs. multiple peer addresses.  A simple approach is
      to have the peers be aware of just the current address of the
      other side instead of all possible ones.  Assuming that one of the
      peers will request the other to start sending to a new address
      this works well.  However, this approach is unable to deal with
      problems that affect both nodes.  For instance, two nodes
      connected by two separate point-to-point links will be unable to
      switch to the other link if a failure occurs on the first one.

   o  Addresses vs. address pairs.  Are tests and current paths
      individual peer addresses, or pairs of peer and own addresses
      (paths)?  It seems that some failure scenarios require the use of
      a path rather than a single address.  A network failure may make
      it impossible to communicate between a particular pair of
      addresses, even if those addresses have some other connectivity.

   o  Where the connectivity information comes from.  Does it come from
      local stack (such as interface up/down, router advertisement),
      from reception of ESP packets, from IKEv2 keepalives, or through
      some MOBIKE-defined mechanism?

   The mobility and multihoming specification for the HIP protocol [14]
   leaves the determination of when address updates are sent to a local
   policy, but suggests the use of local information and ICMP error

   Network attachment procedures are also relevant for multihoming.  The
   IPv6 and MIP6 working groups have standardized mechanisms to learn
   about networks that a node has attached to.  Basic IPv6 Neighbor
   Discovery was, however, designed primarily for static situations.
   The fully dynamic detection procedure has turned out to be a
   relatively complex procedure for mobile hosts, and it was not fully
   anticipated at the time IPv6 Neighbor Discovery or DHCP were being
   designed.  As a result, enhanced or optimized mechanisms are being
   designed in the DHC and DNA working groups [6] [7].

   ICE [17], STUN [12], and TURN [24] are also related mechanisms.  They
   are primarily used for NAT detection and communication through NATs

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   in IPv4 environment, for application such as as voice over IP.  STUN
   uses a server in the Internet to discover the presence and type of
   NATs and the client's public IP addresses and ports.  TURN makes it
   possible to receive incoming connections in hosts behind NATs.  ICE
   makes use of these protocols in peer-to-peer cooperative fashion,
   allowing participants to discover, create and verify mutual
   connectivity, and then use this connectivity for multimedia streams.
   While these mechanisms are not designed for dynamic and failure
   situations, they have many of the same requirements for the
   exploration of connectivity, as well as the requirement to deal with

   Related work in the IPv6 area includes RFC 3484 [5] which defines
   source and destination address selection rules for IPv6 in situations
   where multiple candidate address pairs exist.  RFC 3484 considers
   only a static situation, however, and does not take into account the
   effect of failures.  In the MULTI6 working group [22] considers how
   applications can re-initiate connections after failures in the best
   way.  This work differs from the shim-layer approach selected for
   further development in the working group with respect to the timing
   of the address selection.  In the shim-layer approach failure
   detection and the selection of new addresses happens at any time,
   while [22] considers only the case when an application re-establishes

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4.  Definitions

   This section defines terms useful in discussing the failure detection
   problem space.

4.1.  Available Addresses

   SHIM6 nodes need to be aware of what addresses they themselves have.
   If a node loses the address it is currently using for communications,
   another address must replace this address.  And if a node loses an
   address that the node's peer knows about, the peer must be informed.
   Similarly, when a node acquires a new address it may generally wish
   the peer to know about it.

   Definition.  Available address.  An address is said to be available
   if the following conditions are fulfilled:

   o  The address has been assigned to an interface of the node.

   o  If the address is an IPv6 address, we additionally require that
      (a) the address is valid in the sense of RFC 2461 [2], and that
      (b) the address is not tentative in the sense of RFC 2462 [3].  In
      other words, the address assignment is complete so that
      communications can be started.

      Note this explicitly allows an address to be optimistic in the
      sense of [8] even though implementations are probably better off
      using other addresses as long as there is an alternative.

   o  The address is a global unicast, unique local address [9], or an
      unambiguous IPv6 link-local or IPv4 RFC 1918 address.  That is, it
      is not an IPv6 site-local address.  Where IPv6 link-local or RFC
      1918 addresses are used, their use needs to be unambiguous.  The
      precise meaning of ambiguous has not been defined yet, but one
      approach is requiring that at most one link-local address be used
      per node within the same connection between two peers.

         Note: Given RFC 3484 [5] rules for preferring smallest scope,
         it is likely that many IPv6 flows at least start with even
         link-local addresses.

   o  The address and interface is acceptable for use according to a
      local policy.

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   Available addresses are discovered and monitored through mechanisms
   outside the scope of SHIM6 (and HIP or MOBIKE).  These mechanisms
   include IPv6 Neighbor Discovery and Address Autoconfiguration [2]
   [3], DHCP [4], enhanced network detection mechanisms detected by the
   DNA working group, and corresponding IPv4 mechanisms, such as [6].

4.2.  Locally Operational Addresses

   Two different granularity levels are needed for failure detection.
   The coarser granularity is for individual addresses:

   Definition.  Locally Operational Address.  An available address is
   said to be locally operational when its use is known to be possible
   locally: the interface is up, a relevant default router (if
   applicable) is known to be reachable, and no other local information
   points to the address being unusable.

   Locally operational addresses are discovered and monitored through
   mechanisms outside SHIM6 (and HIP or MOBIKE).  These mechanisms
   include IPv6 Neighbor Discovery [2], corresponding IPv4 mechanisms,
   and link layer specific mechanisms.

   It is also possible for hosts to learn about routing failures for a
   particular selected source prefix.  Protocols for distributing this
   information are being designed [19] [22].  The development of such
   protocols would be possible, however.  Potential approaches include
   overloading information in current IPv6 Router Advertisement or
   adding some new information in them.  Similarly, hosts could learn
   information from servers that query the BGP routing tables.

4.3.  Operational Address Pairs

   The existence of locally operational addresses are 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.  For this reason we need the
   definition of a second level of granularity, for pairs of addresses:

   Definition.  Bidirectionally operational address pair.  A pair of
   locally operational addresses are said to be an operational address
   pair, iff bidirectional connectivity can be shown between the
   addresses.  That is, a packet sent with one of the addresses in the
   source field and the other in the destination field reaches the
   destination, and vice versa.

   Unfortunately, there are scenarios where bidirectionally operational
   address pairs do not exist.  For instance, ingress filtering or
   network failures may result in one address pair being operational in

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   one direction while another one is operational from the other
   direction.  The following definition captures this general situation:

   Definition.  Undirectionally operational address pair.  A pair of
   locally operational addresses are said to be an unidirectionally
   operational address pair, iff packets sent with the first address as
   the source and the second address as the destination can be shown to
   reach the destination.

   Both types of operational pairs are discovered and monitored through
   the following mechanisms:

   o  Positive feedback from upper layer protocols.  For instance, TCP
      can indicate to the IP layer that it is making progress.  This is
      similar to how IPv6 Neighbor Unreachability Detection can in some
      cases be avoided when upper layers provide information about
      bidirectional connectivity [2].  In the case of unidirectional
      connectivity, the upper layer protocol responses come back using
      another address pair, but show that the messages sent using the
      first address pair have been received.

   o  Negative feedback from upper layer protocols.  It is conceivable
      that upper layer protocols give an indication of a problem to the
      SHIM6 layer.  For instance, TCP could indicate that there's either
      congestion or lack of connectivity in the path because it is not
      getting ACKs.

   o  Explicit reachability tests, such as keepalives or probes added
      when there's only unidirectional payload traffic [10].

   o  ICMP error messages.  Given the ease of spoofing ICMP messages,
      one should be careful to not trust these blindly, however.  Our
      suggestion is to use ICMP error messages only as a hint to perform
      an explicit reachability test, but not as a reason to disrupt
      ongoing communications without other indications of problems.  The
      situation may be different when certain verifications of the ICMP
      messages are being performed [21].  These verifications can ensure
      that (practically) only on-path attackers can spoof the messages.
      Such verifications are not possible for all transport protocols,

   Note that some protocols, such as HIP [14] and MOBIKE [16], perform a
   return routability test of an address before it is taken into use.
   The purpose of this test is to ensure that fraudulent peers do not

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   trick others into redirecting traffic streams onto innocent victims
   [26].  Such tests can at the same time work as a means to ensure that
   an address pair is operational.  Note, however, that some advanced
   optimizations attempt to postpone the reachability tests so that they
   do not increase movement-related latency [25].

4.4.  Primary Address Pair

   Contrary to SCTP which has a specific congestion avoidance design
   suitable for multi-homing, IP-layer solutions need to avoid sending
   packets concurrently over multiple paths; TCP behaves rather poorly
   in such circumstances.  For this reason it is necessary to choose a
   particular pair of addresses as the primary address pair which is
   used until problems occur, at least for the same session.

   A primary address pair need not be operational at all times.  If
   there is no traffic to send, we may not know if the primary address
   pair is operational.  Nevertheless, it makes sense to assume that the
   address pair that worked in some time ago continues to work for new
   communications as well.

4.5.  Miscellaneous

   Addresses can become deprecated [2].  When other operational
   addresses exist, nodes generally wish to move their communications
   away from the deprecated addresses.

   Similarly, IPv6 source address selection [5] may guide the selection
   of a particular source address - destination address pair.

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5.  Architectural Considerations

   Architecturally, a number of questions arises.  One simple question
   is whether there needs to be communications between a multihoming
   solution residing at the IP layer and upper layer protocols?  Upon
   changing to a new address pair, transport layer protocol SHOULD be
   notified so that it can perform a slow start, or some other form of
   adaptation to the possibly changed conditions.  This is necessary,
   for instance, when switching from a high-bandwidth LAN interface to a
   low bandwidth cellular interface.  (Note that this notification can
   not be done in protocol designs where the end points are not the
   final hosts, such as where a gateway is used.)

   A more fundamental question is which protocols should be responsible
   for which parts of the problem.  It seems clear that no multihoming
   solution should take on the task of lower layers and other IP
   functions for discovering its own addresses or testing local
   connectivity.  Protocols such as DHCP or Neighbor and Router
   Discovery do this already.

   But it is less clear which protocol(s) should discover end-to-end
   connectivity problems or recover from them.  One answer is that this
   is clearly within the domain of multihoming protocol.  By performing
   testing and failure detection of the used path and switching to a new
   path if necessary, the transport and application protocols can work

   On the other hand, one could argue that transport and application
   protocols would have more knowledge about the situation, and have a
   better ability to decide when a move is required.  For instance, they
   know what the required throughput and congestion status is.  Also, it
   would be unfortunate if both the IP layer and transport/application
   layer took action for the same problem, for instance by switching to
   a new address at the IP layer and throttling back due to "congestion"
   at the transport layer.

   One can also envision that applications would be able to tell the IP
   or transport layer that the current connection in unsatisfactory and
   an exploration for a better one would be desirable.  This would
   require an API to be developed, however.

   Generally speaking, we can divide information that a host has into
   three categories: local information from "lower layers" such as IPv6
   Neighbor Discovery, transit and congestion condition information from
   either from the multihoming protocol itself or from transport layer
   protocols and (where available) ECN, and application layer policies
   that dictate what the requirements are for acceptable connections.

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   The division of work is largely left as an open issue as far as this
   document is concerned, but our description works from a point of view
   of a multihoming protocol at the IP layer.  We also note that in the
   CELP proposal [20], both IP, transport, and application layer
   entities could share their connectivity status in a common
   information pool.  This may also be a useful approach.

   Finally, the last architectural question is about the difference
   between mobility and multihoming.  Given our definitions above,
   there's no fundamental difference with respect to how the
   multihoming/mobility protocol learns the addresses it has available.
   However, a practical difference is that in a multihoming scenario
   there are alternative addresses, whereas in mobility changes to a new
   address are forced due to the old address no longer being available.
   Interestingly, with the exception of MOBIKE, existing mobility
   protocols do not employ any failure detection mechanisms of their
   own, and rely solely on link layer and neighbor discovery mechanisms.

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6.  Solution

   We need to keep track of the host's own available addresses,
   operational addresses, and operational address pairs, and to explore
   for other operational pairs when a failure occurs.  We will first
   describe two general state machines that illustrate the overall
   process, and then discuss the details of the reachability tests
   needed for ensuring operational status, and the exploration protocol.

6.1.  State Machines

   Addresses can be in the AVAILABLE and OPERATIONAL states.  The state
   transitions relating to this are shown in Figure 1.

     Address becomes |              |
     available       |              |
   ----------------->|              |
                     |  AVAILABLE   |
   <-----------------|              |
     Address is no   |              |
    longer available |              |
                        |       / \
                Address |        | Address
                becomes |        | is no longer
            operational |        | operational
                        |        |
                       \ /       |
                     |              |
     Address is no   |              |
    longer available |              |
   <-----------------| OPERATIONAL  |
                     |              |
                     |              |
                     |              |

          Figure 1. Address state machine.

   When an address becomes operational, it SHOULD be reported as a new
   address to the peer.  Similarly, when an address is no longer
   operational or available, the peer SHOULD be informed.

   In addition, a particular address can be either preferred or
   deprecated.  This is not shown in the state machine.

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   Another state machine describes address pair selection.  A node runs
   the address pair selection state machine to choose the currently used
   primary address pair, the one which is used for sending outgoing
   packets.  A node runs one of these state machines towards each
   different peer, tracking the known address pairs and their status.
   Each peer also has its own state machine for talking back to the
   node; there is no guarantee that the same address pairs (in reverse
   order) have the same state; lack of bidirectionally operational pair
   would result in a different state on both sides, for instance.

   The state machine can be in the NO PRIMARY, TESTING PRIMARY, and
   PRIMARY OPERATIONAL states.  The chosen address pair is known to be
   operational in the PRIMARY OPERATIONAL state, and is either
   unverified or non-operational in the other states.

   Figure 2 shows the state machine:

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                         |                |
                         |                |
                         |                |
                         |                |
                         |       NO       |
                         |     PRIMARY    |
                         |                |
                   +-----|                |<---------------+
                   |     |                |                |
                   |     +----------------+                |
                   |         / \    / \                    |
               Add |          |      |                     |
             pair: |   Delete |      | Test         Delete |
              Send |   pair & |      | fail &       pair & |
              test |     Last |      | Last           Last |
                   |          |      |                     |
                   |     +----------------+                |
                   |     |                |                |
                   +---->|                |<----+          |
                         |                |     | Test     |
    Connect: Send test   |                |     | fail &   |
   --------------------->|     TESTING    |     | !Last    |
                         |     PRIMARY    |+----+          |
          +------------->|                |                |
          |              |                |<----+          |
          |        +---->|                |     |          |
          |        |     +----------------+     |          |
   Policy | ICMP | |          |      |          |          |
   change | Timer: |      ULP |      | Test     | Delete   |
          |   Send | feedback:|      | OK:      | pair &   |
          |   test |    Reset |      | Reset    | !Last    |
          |        |    timer |      | timer    |          |
          |        |         \ /    \ /         |          |
          |        |     +----------------+     |          |
          |        +-----|                |     |          |
          |              |                |-----+          |
          +--------------|                |                |
                         |                |                |
                   +-----|   OPERATIONAL  |                |
     ULP feedback: |     |     PRIMARY    |                |
       Reset timer |     |                |----------------+
                   +---->|                |
                         |                |

          Figure 2. Pair selection state machine.

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   The notation used in Figure 2 is explained below:


      An event representing the desire of the application to send a
      packet to a new peer, or an indication from a peer wishing to
      connect to us.

   Test OK

      An event representing a successful completion of the reachability

   Test fail

      An event representing failure to complete the reachability test.

   ULP feedback

      An event representing positive indication from an upper layer
      protocol that the packets we have sent to the peer are getting


      An event representing the reception of an ICMP error message.


      An event representing timer elapsing.

   Add pair

      An event representing the addition of a new possible address pair,
      either through learning a new local address or being told of a new
      remote address.  Note that this does not usually result in any
      immediate action, unless we are currently lacking an operational
      primary pair.

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   Delete pair

      An event representing the deletion of the currently chosen primary
      address pair, or learning that one of the addresses is in the pair
      is no longer operational.

   Policy change

      An event representing the desire of the local or remote end to
      change to a different address pair, despite the current one being
      operational.  This can be due to the availability of the higher-
      bandwidth connection, cost, or other issues.


      A condition that tells whether or not the currently chosen primary
      pair is the only known address pair.

   Send test

      An action to initiate the reachability test for a particular pair.
      This test is typically embedded in the SHIM6 connection setup
      exchange when run initially, and a separate exchange later.

      Note that due to potentially asymmetric connectivity, both sides
      have to perform their own tests, and make their own primary pair

   Reset timer

      An action to reset a timer so that it will send an event after a
      specified time.

   The state machines also assumes an underlying multihoming signaling
   capability, consisting of the following abstract message exchanges:


      Establishes a connection between the peers.  May also exchange
      locator sets and test reachability at the same time.

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      Verifies reachability using a specific address pair.


      Informs the peer about new locators.


      Informs the peer about losing some locators.

   Note that the above state machine leaves open how specific address
   pairs are chosen or how the tests are actually performed.  These
   issues will be discussed in the next sections.  We have also, on
   purpose, decided to avoid attaching functional labels such as
   "backup" to other address pairs beyond the primary pair.  It is our
   belief that a general design does not need these labels.

6.2.  Failure Detection

   This process consists of three tasks:

   o  Tracking local information from lower and upper layers.  For
      instance, when link layer informs that we have no connection then
      we know there is a failure.

   o  Performing a reachability process as described in in [10] for
      ensuring that there is reachability when the local information
      says there should be.

   o  Following commands from the peer regarding the availability of

6.3.  Alternative Locator Pair Exploration

6.3.1.  Exploration Order

   The pair selection state machine assumes an ability to pick primary
   and alternative address pairs.

   This process results in a combinatorial explosion when there are many
   addresses on both sides.  Do both sides track all possible
   combinations of addresses?  If a failure occurs, shall all
   combinations be tested before giving up?  Are such tests performed in
   parallel or in sequence, and what kind of backoff procedures should

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   be applied?

   Our suggestion is that nodes MUST first consult RFC 3484 [5] Section
   4 rules to determine what combinations of addresses are legal from a
   local point of view, as this reduces the search space.  RFC 3484 also
   provides a priority ordering among different address pairs, making
   the search possibly faster.  Nodes SHOULD also use local information,
   such as known quality of service parameters or interface types to
   determine what addresses are preferred over others, and try pairs
   containing such addresses first.  In some cases we can also learn the
   peer's preferences through the multihoming protocol.

      Discussion note 1: It may also be possible to simulate preferences
      by choosing to not tell the peer about some (non-preferred)

      Discussion note 2: The preferences may either be learned
      dynamically or be configured.  It is believed, however, that
      dynamic learning based purely on the SHIM6 protocol is too hard
      and not the task this layer should do.  Solutions where multiple
      protocols share their information in a common pool of locators
      could provide this information from transport protocols, however

   The reception of packets from the peer with a given address pair is a
   good hint that the address pair works, particularly when these
   packets are authenticated multihoming protocol packets.  However, the
   reception of these packets alone is an insufficient reason to switch
   to a new address, as in an unidirectional connectivity case the
   return path may not work.

   One suggested good implementation strategy is to record the
   reachability test result (an on/off value) and multiply this by the
   age of the information.  This allows recently tested address pairs to
   be chosen before old ones.

   Out of the set of possible candidate address pairs, nodes SHOULD
   attempt a test through all of them, but MUST do this sequentially and
   using an exponential back-off procedure.

   This sequential process is necessary in order to avoid a "signaling
   storm" when an outage occurs (particularly for a complete site).
   However, it also limits the number of addresses that can in practice
   be used for multihoming, considering that transport and application
   layer protocols will fail if the switch to a new address pair takes
   too long.  For instance, we can assume that an initial timeout value

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   is 0.1 seconds and there are four addresses on both sides.  Going
   through all sixteen address pairs and doubling the timeout value at
   every trial would take 3200 seconds!

   Finally, as has been noted in the context of MOBIKE, the existence of
   NATs can require that peers continuously monitor the operational
   status of address pairs, as otherwise NAT state related to a
   particular communication is lost, and the peer on the outer side of
   the NAT can no longer reach the peer inside the NAT.

6.3.2.  Exploration Protocol

   The exploration for a working address pair is not easy, as
   unidirectional reachability needs to be considered.  This is because
   the test of a single pair may not result in a working paths to send
   both the request and response packets.  The following protocol could
   be used to avoid this problem:

    Peer A                                        Peer B
      |                                             |
      |  Poll 1 (src=A1, dst=B1)                    |
      |                                             |
      |               Poll 2 (src=B1, dst=A1) OK: 1 |
      |        X------------------------------------|
      |                                             |
      |  Poll 3 (src=A2, dst=B1)                    |
      |------------------------------X              |
      |                                             |
      |          Poll 4 (src=B2, dst=A1) OK: 1      |
      |                                             |
      |  Poll 5 (src=A1, dst=B1) OK: 4              |
      |                                             |

   When B receives the first Poll message, it memorizes that it has
   gotten it.  The Poll message from B, however, is lost so A tries
   again with another pair.  This is lost too, but B continues its own
   testing process by sending its second Poll message, which is received
   by A. The messages carry identifiers, and a list of identifiers that
   were found messages the sender had itself successfully received

   In the end of the example case, A and B know that they have a working
   path from A to B using (A1, B1) and from B to A using (B2, A1).

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   More generally, when A decides that it needs to test for
   connectivity, it will initiate a set of Poll messages, in sequence,
   until it gets a Poll message from B indicating that (a) B has
   received one of A's Poll messages and, obviously, (b) that B's Poll
   message is getting through.  B uses the same algorithm, but starts
   the process from the reception of the first Poll message from A.

   Note that this protocol can be implemented in different ways.  One
   approach is to rely on data packets, such as TCP payload packets and
   acknowledgements.  This method has the benefit that it likely passes
   easily through firewalls and other middleboxes.  One exception to
   this are stateful firewalls that wish to know what happened "earlier"
   in the connection, but it seems that such firewalls are fundamentally
   incompatible with multi-homing anyway.  One drawback of this method
   is, however, that the the number of available payload packets may not
   match the need in a situation where a lot of address pairs need to be

   Another approach is to have a completely separate protocol for the
   exploration.  This would need to be explicitly allowed in firewalls
   before it could be used.  On the other hand, then it would be very
   clear for the firewall administrators what they are letting through.

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7.  Security Considerations

   Attackers may 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-

   SHIM6 does not provide any protection of its own for indications 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 [21], link layer security, or the use of [13] to
   protect IPv6 Router and Neighbor Discovery.

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8.  References

8.1.  Normative References

   [1]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

   [2]   Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
         for IP Version 6 (IPv6)", RFC 2461, December 1998.

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

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

   [5]   Draves, R., "Default Address Selection for Internet Protocol
         version 6 (IPv6)", RFC 3484, February 2003.

   [6]   Aboba, B., "Detection of Network Attachment (DNA) in IPv4",
         draft-ietf-dhc-dna-ipv4-08 (work in progress), July 2004.

   [7]   Choi, J., "Detecting Network Attachment in IPv6 Goals",
         draft-ietf-dna-goals-00 (work in progress), June 2004.

   [8]   Moore, N., "Optimistic Duplicate Address Detection for IPv6",
         draft-ietf-ipv6-optimistic-dad-01 (work in progress),
         June 2004.

   [9]   Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
         Addresses", draft-ietf-ipv6-unique-local-addr-05 (work in
         progress), June 2004.

   [10]  Beijnum, I., "Shim6 Reachability Detection",
         draft-ietf-shim6-reach-detect-00 (work in progress), July 2005.

8.2.  Informative References

   [11]  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.

   [12]  Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, "STUN
         - Simple Traversal of User Datagram Protocol (UDP) Through
         Network Address Translators (NATs)", RFC 3489, March 2003.

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

   [14]  Nikander, P., "End-Host Mobility and Multi-Homing with Host
         Identity Protocol", draft-ietf-hip-mm-00 (work in progress),
         October 2004.

   [15]  Kivinen, T., "Design of the MOBIKE protocol",
         draft-ietf-mobike-design-00 (work in progress), June 2004.

   [16]  Eronen, P., "IKEv2 Mobility and Multihoming Protocol (MOBIKE)",
         draft-ietf-mobike-protocol-03 (work in progress),
         September 2005.

   [17]  Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
         Methodology for Network  Address Translator (NAT) Traversal for
         Multimedia Session Establishment Protocols",
         draft-ietf-mmusic-ice-02 (work in progress), July 2004.

   [18]  Stewart, R., "Stream Control Transmission Protocol (SCTP)
         Dynamic Address  Reconfiguration",
         draft-ietf-tsvwg-addip-sctp-10 (work in progress),
         January 2005.

   [19]  Bagnulo, M., "Address selection in multihomed environments",
         draft-bagnulo-shim6-addr-selection-00 (work in progress),
         October 2005.

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

   [21]  Gont, F., "ICMP attacks against TCP",
         draft-gont-tcpm-icmp-attacks-00 (work in progress),
         August 2004.

   [22]  Huitema, C., "Address selection in multihomed environments",
         draft-huitema-multi6-addr-selection-00 (work in progress),
         October 2004.

   [23]  Nordmark, E., "Level 3 multihoming shim protocol",
         draft-ietf-shim6-proto-00 (work in progress), October 2005.

   [24]  Rosenberg, J., "Traversal Using Relay NAT (TURN)",
         draft-rosenberg-midcom-turn-05 (work in progress), July 2004.

   [25]  Vogt, C., Arkko, J., Bless, R., Doll, M., and T. Kuefner,
         "Credit-Based Authorization for Mobile IPv6 Early Binding
         Updates", draft-vogt-mipv6-credit-based-authorization-00 (work

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         in progress), May 2004.

   [26]  Aura, T., Roe, M., and J. Arkko, "Security of Internet Location
         Management", In Proceedings of the 18th Annual Computer
         Security Applications Conference, Las Vegas, Nevada, USA.,
         December 2002.

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Appendix A.  Contributors

   This draft attempts to summarize the thoughts and unpublished
   contributions of many people, including the MULTI6 WG design team
   members Marcelo Bagnulo Braun, Iljitsch van Beijnum, Erik Nordmark,
   Geoff Huston, Margaret Wasserman, and Jukka Ylitalo, the MOBIKE WG
   contributors Pasi Eronen, Tero Kivinen, Francis Dupont, Spencer
   Dawkins, and James Kempf, and my colleague Pekka Nikander at
   Ericsson.  This draft is also in debt to work done in the context of
   SCTP [11].

   The protocol design in Section 6.3.2 is due to Erik, Marcelo, and

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Appendix B.  Acknowledgements

   The author would also like to thank Christian Huitema, Pekka Savola,
   and Hannes Tschofenig for interesting discussions in this problem
   space, and for their comments on earlier versions of this draft.

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Author's Address

   Jari Arkko
   Jorvas  02420


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