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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 5534

Network Working Group                                           J. Arkko
Internet-Draft                                                  Ericsson
Expires: April 27, 2006                                       I. Beijnum
                                                                   Muada
                                                        October 24, 2005


    Failure Detection and Locator Pair Exploration Protocol for IPv6
                              Multihoming
                 draft-ietf-shim6-failure-detection-02

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   This Internet-Draft will expire on April 27, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document defines a mechanism for the detection of communication
   failures between two communicating hosts at IP layer, and an
   exploration protocol for switching to another pair of interfaces
   and/or addresses between the same hosts if a working pair can be
   found.  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  . . . . . . . . . . . . . . . . . . . . . . . . .  7
       4.1.  Available Addresses  . . . . . . . . . . . . . . . . . .  7
       4.2.  Locally Operational Addresses  . . . . . . . . . . . . .  8
       4.3.  Operational Address Pairs  . . . . . . . . . . . . . . .  8
       4.4.  Current Address Pair . . . . . . . . . . . . . . . . . . 10
       4.5.  Miscellaneous  . . . . . . . . . . . . . . . . . . . . . 10
   5.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . . 11
       5.1.  Failure Detection  . . . . . . . . . . . . . . . . . . . 11
       5.2.  Alternative Address Pair Exploration . . . . . . . . . . 11
       5.3.  Exploration Order  . . . . . . . . . . . . . . . . . . . 12
       5.4.  Protocol Design  . . . . . . . . . . . . . . . . . . . . 13
       5.5.  Example Protocol Runs  . . . . . . . . . . . . . . . . . 14
       5.6.  Limitations  . . . . . . . . . . . . . . . . . . . . . . 17
   6.  Protocol Definition  . . . . . . . . . . . . . . . . . . . . . 18
       6.1.  SHIM6 Probe Message  . . . . . . . . . . . . . . . . . . 18
       6.2.  SHIM6 Event Option . . . . . . . . . . . . . . . . . . . 19
       6.3.  REAP Event Message . . . . . . . . . . . . . . . . . . . 19
       6.4.  REAP Options . . . . . . . . . . . . . . . . . . . . . . 21
             6.4.1.  Payload Reception Report . . . . . . . . . . . . 21
             6.4.2.  Event Reception Report . . . . . . . . . . . . . 22
       6.5.  State Machine  . . . . . . . . . . . . . . . . . . . . . 23
       6.6.  Protocol Constants . . . . . . . . . . . . . . . . . . . 23
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 27
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
       9.1.  Normative References . . . . . . . . . . . . . . . . . . 28
       9.2.  Informative References . . . . . . . . . . . . . . . . . 28
   Appendix A.  Contributors  . . . . . . . . . . . . . . . . . . . . 31
   Appendix B.  Acknowledgements  . . . . . . . . . . . . . . . . . . 32
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33
   Intellectual Property and Copyright Statements . . . . . . . . . . 34















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1.  Introduction

   The SHIM6 protocol extends IPv6 to support multihoming.  This
   protocol is an IP layer mechanism that hides multihoming from
   applications [18].  A part of the SHIM6 solution involves detecting
   when a currently used pair of addresses (or interfaces) between two
   communication hosts has failed, and picking another pair when this
   occurs.  We call the former failure detection, and the latter locator
   pair exploration.

   This draft defines the mechanism and protocol to achieve both failure
   detection and locator pair exploration.  This protocol is called
   REAchability Protocol (REAP).  It designed to be carried within the
   SHIM6 protocol, but may also be used in other contexts.

   The draft is structured as follows: Section 3 discusses prior work in
   this space, Section 4 defines a set of useful terms, Section 5 gives
   an overview of REAP, and Section 6 specifies the message formats and
   behaviour in detail.  Section 7 discusses the security considerations
   of REAP.

   For the purposes of this draft, we consider an address to be
   synonymous with a locator.  We assume that there are other, higher
   level identifiers such as CGA public keys or HBA bindings that tie
   the different locators used by a node together [17].


























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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 [2].














































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3.  Related Work

   In SCTP [10], 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 selects one of the
   peer's addresses 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.

   SCTP also tests 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.
   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.

   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 [20].

   The MOBIKE protocol [15] provides multihoming and mobility for VPN
   connections.  Its failure detection and locator pair exploration is
   designed to work across mixed IPv4/IPv6 environments and NATs, as
   long as a path that allows bidirectional communication can be found.

   Existing mechanisms at lower layers or in IKEv2 are used to detect
   failures, and upon failure MOBIKE attempts to explore all
   combinations of addresses to find a working pair.  Such exploration
   is necessary when a problem affects 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.
   While both communicating hosts are aware of each others' addresses,
   only one end of the communication is in charge of deciding what
   address pair to use, however.

   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
   messages.



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   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 [13] [7].

   ICE [16], STUN [11], and TURN [25] are also related mechanisms.  They
   are primarily used for NAT detection and communication through NATs
   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
   middleboxes.

   Related work in the IPv6 area includes RFC 3484 [6] 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 [24] 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 [24] considers only the case when an application re-establishes
   connections.

   An earlier SHIM6 document [19] discussed 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) were 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.  FBD is the chosen
   mechanism in this document.





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4.  Definitions

   This section defines terms useful in discussing the problem space.

4.1.  Available Addresses

   Multihoming 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 [3], and that
      (b) the address is not tentative in the sense of RFC 2462 [4].  In
      other words, the address assignment is complete so that
      communications can be started.

      Note that 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 address.  That is, it is not an IPv6
      site-local address.  Where IPv6 link-local addresses are used,
      their use needs to be unambiguous as follows.  At most one link-
      local address may be used per node within the same connection
      between two peers.

         IPv4 compatibility note: If this protocol were defined to
         handle IPv4, then RFC 1918 addresses would also need to be
         allowed.


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

   Available addresses are discovered and monitored through mechanisms
   outside the scope of the protocol described here.  These mechanisms
   include IPv6 Neighbor Discovery and Address Autoconfiguration [3]
   [4], DHCP [5], and DNA mechanisms [7].




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      IPv4 compatibility note: If IPv4 was supported in this protocol,
      then also mechanisms defined in [13] would need to be supported.

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, at least one default router (if
   applicable) that could be used to send a packet with this address as
   a source address 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 the protocol described here.  These mechanisms
   include IPv6 Neighbor Discovery [3] and link layer specific
   mechanisms.

      IPv4 compatibility note: In IPv4, mechanisms such as those defined
      in [13] could be used.

   It is also possible for hosts to learn about routing failures for a
   particular selected source prefix, if suitable protocols for this
   purpose exist.  Some proposals in this space have been made, see, for
   instance [21] and [24].  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



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   network failures may result in one address pair being operational in
   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 could be 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 [3].  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
      multihoming 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.

   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 [23].  These verifications can ensure
      that (practically) only on-path attackers can spoof the messages.

   Note a multihoming protocol needs to 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 trick others into
   redirecting traffic streams onto innocent victims [26].  This test
   can at the same time work as a means to ensure that an address pair
   is operational, as discussed in Section 5.2.





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4.4.  Current Address Pair

   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 current address pair which is used until problems occur, at
   least for the same session.

   A current 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 [3].  When other operational
   addresses exist, nodes generally wish to move their communications
   away from the deprecated addresses.

   Similarly, IPv6 source address selection [6] may guide the selection
   of a particular source address - destination address pair.





























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5.  Protocol Overview

   This section discusses the design of the failure detection and
   address pair exploration mechanisms, and gives on overview of the
   REAP protocol.

5.1.  Failure Detection

   This process consists of three tasks.  First, it is necessary to
   track 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.  Nodes SHOULD employ techniques listed in Section 4.1
   and Section 4.2 to be aware of the local situation.

   Similarly, it is necessary to track remote address information from
   the peer.  For instance, the peer may inform that its currently used
   address is no longer in use.  Techniques outside the scope of this
   document are used for this, for further information see [18].

   The third task is to ensure verify reachability with the peer when
   the local and remote information indicates that communication should
   be possible.  This needs to be performed only if there's upper layer
   packets to be sent, however.

   This document defines the protocol mechanisms only for the third
   task.  We employ a technique called Forced Bidirectional Detection
   (FBD) to ensure reachability.  This technique tests reachability only
   when there's payload traffic.  When there is no payload traffic, no
   tests will be performed, and no failure are assumed to exist.

   Similarly, when there is bidirectional payload traffic, there is no
   need for FBD to do anything, as packets are already flowing as
   expected.  However, if one of the peers is only receiving but not
   sending any other traffic, then FBD sends occasional keepalives to
   the other peer in order to let the peer know that its payload traffic
   is getting through.  As a result, a node that is sending something to
   the peer but receives nothing in return can assume that there's a
   failure.

5.2.  Alternative Address Pair Exploration

   As explained in previous section, the currently used address pair may
   become invalid either through one of the addresses being becoming
   unavailable or inoperational, or the pair itself being declared
   inoperational.  An exploration process attempts to find another
   operational pair so that communications can resume.

   What makes this process hard is the requirement to support



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   unidirectionally operational address pairs.  It is insufficient to
   probe address pairs by a simple request - response protocol.
   Instead, the party that first detects the problem starts a process
   where it tries each of the different address pairs in turn by sending
   a message to its peer.  These messages carry information about the
   state of connectivity between the peers, such as whether the sender
   has seen any traffic from the peer recently.  When the peer receives
   a message that indicates a problem, it assists the process by
   starting its own exploration to the other direction, again sending
   information about the recently received payload traffic or signaling
   messages.

   Specifically, when A decides that it needs to explore for an
   alternative address pair to B, it will initiate a set of Event
   messages, in sequence, until it gets an Event message from B
   indicating that (a) B has received one of A's messages and,
   obviously, (b) that B's Event message gets back to A. B uses the same
   algorithm, but starts the process from the reception of the first
   Event message from A.

   Upon changing to a new address pair, transport layer protocol needs
   to be informed so that it can perform a slow start, or some other
   form of adaptation to the possibly changed conditions.  However, this
   functionality is outside the scope of REAP and is rather seen as a
   general multihoming issue.

   Similarly, 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.  In
   any case, this is another issue that we treat as being outside the
   scope of pure address exploration.

5.3.  Exploration Order

   The exploration process assumes an ability to pick current and
   alternative address pairs.  This process results in a combinatorial
   explosion when there are many addresses on both sides.  However, not
   all combinations are legal.  In order to avoid congestion, we also
   can not explore all pairs without performing some kind of a back-off
   procedure.

   Nodes MUST first consult RFC 3484 [6] 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 MAY also use local information, such as known quality
   of service parameters or interface types to determine what addresses



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   are preferred over others, and try pairs containing such addresses
   first.  The multihoming protocol also carries preference information
   in its messages [18].


      Discussion note: The preferences may either be learned dynamically
      or be configured.  It is believed, however, that dynamic learning
      based purely on the multihoming 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 [22].

   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.

      IPv4 compatibility note: 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.

   Out of the set of possible candidate address pairs, nodes SHOULD
   attempt a test through all of them until a working pair is found.
   However, all nodes 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.

5.4.  Protocol Design

   REAP is designed as a modular part of SHIM6 in the hopes that it may
   also be useful in other contexts.  This document defines how it is
   carried within SHIM6, but the actual protocol messages are self-
   contained so that it could be carried by other protocols as well.

   Similarly, while this document defines a SHIM6 message that carries
   REAP, this message is only used when no other SHIM6 message is about
   to be sent that could be used to carry the REAP information.  For
   instance, a Locator List Update message can be used to carry a REAP
   message that conveys reachability information.

   The REAP design allows performing both failure detection and address



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   pair exploration in the same sequence of messages, without a need to
   designate a specific point when the current address pair is declared
   inoperational and the search for a new pair begins.  This is useful,
   as the loss of a small number of packets is not a proof that a
   problem exists.  Integrated failure detection and exploration allows
   us to test multiple address pairs simultaneously, including the
   current pair in case it starts working again.

   REAP also integrates a return routability function, making it
   unnecessary to perform another roundtrip before a newly discovered
   address can be taken into use.

   This document defines a minimal set of parameters that are carried by
   the messages of the protocol.  Specifically, we have limited the
   parameters to those that are necessary to find a working path.  We
   note there may be extensions that are needed in the future for
   various reasons, such as the desire to support load balancing or
   finding best paths.  An option format has been specified to allow
   this.

5.5.  Example Protocol Runs

   This section has examples of REAP protocol runs in typical scenarios.
   We start with the simplest scenario of two hosts, A and B, that have
   a SHIM6 connection with each other but are not currently sending any
   data.  As neither side sends anything, they also do not expect
   anything back, so there are no messages at all:

    Peer A                                        Peer B
      |                                             |
      |                                             |
      |                                             |
      |                                             |
      |                                             |
      |                                             |
      |                                             |
      |                                             |

   Our second example involves an active connection with bidirectional
   payload packet flows.  Here the reception of data from the peer is
   taken as an indication of reachability, so again there are no extra
   packes:









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    Peer A                                        Peer B
      |                                             |
      |              payload packet                 |
      |-------------------------------------------->|
      |                                             |
      |              payload packet                 |
      |<--------------------------------------------|
      |                                             |
      |              payload packet                 |
      |-------------------------------------------->|
      |                                             |
      |                                             |

   The third example is the first one that involves an actual REAP
   message.  Here the hosts communicate in just one direction, so REAP
   messages are needed to indicate to the peer that sends payload
   packets that its packets are getting through:

    Peer A                                        Peer B
      |                                             |
      |              payload packet                 |
      |-------------------------------------------->|
      |                                             |
      |              payload packet                 |
      |-------------------------------------------->|
      |                                             |
      |              payload packet                 |
      |-------------------------------------------->|
      |                                             |
      |      REAP Event id=p,                       |
      |                 iseeyou=yes,                |
      |                 payload reception report    |
      |<--------------------------------------------|
      |                                             |
      |              payload packet                 |
      |-------------------------------------------->|
      |                                             |
      |                                             |

   Finally, our last example involves a failure scenario.  Here A has
   addresses A1 and A2 and B has addresses B1 and B2.  The currently
   used address pairs are (A1, B1) and (B1, A1).  The first of these
   becomes broken, which leads to an exploration process:

    Peer A                                        Peer B
      |                                             |
      |           (A1,B1) payload packet            |
      |-------------------------------------------->|



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      |                                             |
      |           (B1,A1) payload packet            |
      |<--------------------------------------------|
      |                                             | Path A1->B1
      |           (A1,B1) payload packet            | is now
      |----------------------------------------/    | broken
      |                                             |
      |                                             | Eventually, B
      |                                             | sends a com-
      |       (B1,A1) REAP Event id=p,              | plaint that
      |                          iseeyou=no         | it is not rec-
      |<--------------------------------------------| eiving anything
      |                                             |
   A realizes                                       |
   that it needs                                    |
   to start the                                     |
   exploration                                      |
      |                                             |
      |   (A1, B1) REAP Event id=q,                 |
      |                       iseeyou=yes           |
      |                       payload reception rep |
      |                       event reception rep(p)| But it gets lost
      |-------------------------------------/       | due to broken path
      |                                             |
   Retransmission                                   |
   to a different                                   |
   address                                          |
      |                                             |
      |   (A1, B2) REAP Event id=r,                 |
      |                       iseeyou=yes           |
      |                       payload reception rep |
      |                       event reception rep(p)| This one gets
      |-------------------------------------------->| through
      |                                             |
      |                                             |
      |                                             | B now knows
      |                                             | that A has no
      |    (B1,A1) REAP Event id=p,                 | problem to receive
      |                       iseeyou=yes,          | its packets and
      |                       event reception rep(r)| This one gets
      |<--------------------------------------------| that A has found
      |                                             | a new path to B
      |                                             |
      |           (A1,B2) payload packet            |
      |-------------------------------------------->| Payload packets
      |                                             | flow again
      |           (B1,A1) payload packet            |
      |<--------------------------------------------|



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      |                                             |

5.6.  Limitations

   REAP is designed to support failure recovery even in the case of
   having only unidirectionally operational address pairs.  However, due
   to security concerns discussed in Section 7, the exploration process
   can typically be run only for a session that has already been
   established.  Specifically, while REAP would in theory be capable of
   exploration even during connection establishment, its use within the
   SHIM6 protocol does not allow this.

   REAP does not support IPv4, but could be extended to do so.  We have
   noted IPv4 compatibility issues where they exist.





































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6.  Protocol Definition

6.1.  SHIM6 Probe Message

   The SHIM6 Probe message carries REAP messages when no other SHIM6
   message needs to be sent.  Its format is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       59      |  Hdr Ext Len  |0|  Type = TBD |   Reserved1 |0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Checksum           |           Reserved2           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Receiver Context Tag                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Options                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Next Header

      This value MUST be set to NO_NXT_HDR (59).

   Type

      This field identifies the Probe message and MUST be set to < TBD
      by IANA > (Probe).

   Reserved1

      This is a 7-bit field reserved for future use.  It is set to zero
      on transmit, and MUST be ignored on receipt.

   Reserved2

      This is a 16-bit field reserved for future use.  It is set to zero
      on transmit, and MUST be ignored on receipt.

   Receiver Context Tag

      This is a 32-bit field for the Context Tag the receiver has
      allocated for the context.







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   Options

      This MUST contain at least the SHIM6 Event option and MAY contain
      other options.

   A valid SHIM6 Probe message conforms to the format above and has a
   Receiver Context Tag that matches to context known by the receiver.
   The receiver processes a valid message by inspecting its options, and
   executing any actions specified for the SHIM6 Event option found
   within the options.

6.2.  SHIM6 Event Option

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Type = TBD        |0|            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                          REAP Event                           ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      This value MUST be set to < TBD by IANA > (Event Option).

   0

      This value MUST be set to 0, as in other SHIM6 options.

   Length

      This is the length of the option and MUST be calculated as
      specified in Section 5.14 of [18].

   The processing rules for this option are the ones defined for the
   REAP Event field in Section 6.3.

6.3.  REAP Event Message

   REAP Event messages are the only messages in the REAP protocol.
   Their format is as follows:










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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Message Type = 1          |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Y| Res |                   Identifier                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                          REAP Options                         ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Message Type

      This value identifies the REAP message, and MUST be set to 1
      (Event).

   Length

      This is the length of the message excluding the Message Type and
      Length fields, expressed in bytes.

   Y (The "I See You" flag)

      This flag is set to 1 if the sender receives either payload
      packets or REAP messages from the peer, and 0 otherwise.  The
      determination of when the sender receives something is made during
      the last Exploration Init Timeout seconds (see Section 6.6) when
      traffic was expected, i.e., when there was either payload traffic
      or REAP messages.

      Upon reception, a value of 1 indicates that the receiver does not
      need to change its behaviour as the sender is already seeing its
      packets.  A value of 0 indicates that the receiver MUST explore
      different outgoing address pairs.

   Res

      This 3-bit reserved field MUST be set to zero when sending, and
      ignored on receipt.

   Identifier

      This identifies this particular instance of an Event message.
      This value SHOULD be generated using a random number generator
      that is known to have good randomness properties [1].  Upon
      reception, Identifier values are copied onto Event Reception
      Report options.  This allows them to be used for both identifying
      which Events were received as well as for performing a return
      routability test.



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   REAP Options

      This field contains zero or REAP options.  Unrecognized options
      MUST be ignored upon receipt.  All implementations MUST support
      the options defined in Section 6.4, however.

6.4.  REAP Options

   The general option format is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Option Type          |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                         Option Data                           ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Option Type

      This value identifies the option.

   Length

      This is the length of the option excluding the Option Type and
      Length fields, expressed in bytes.

   Option Data

      Option-specific content.

6.4.1.  Payload Reception Report

   This option SHOULD be included in any Event message when the sender
   has recently (within the last Exploration Init Timeout seconds)
   received payload packets from the peer.  Its format is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Option Type = 1         |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                          Suboptions                           ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+







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   Option Type

      This value identifies the option and MUST be set to 1 (Payload
      Reception Report).

   Length

      This is the length of the option excluding the Option Type and
      Length fields, expressed in bytes.

   Suboptions

      This field is reserved for possible future REAP Options that are
      carried (recursively) within this option.  Unrecognized options
      MUST be ignored upon receipt.  Currently there are no defined
      options that can be carried here.

         IPv4 compatibility note: If IPv4 and NATs would need to be
         supported, then it might be necessary to indicate what
         addresses and port numbers were used in the received payload
         packets.

6.4.2.  Event Reception Report

   This option MUST be included in any Event message when the sender has
   recently (within the last Exploration Init Timeout seconds) received
   Event messages from the peer.  Depending on MTU and timing
   considerations, the sender MAY, however, include options for only
   some of the received Event messages.  All implementations MUST
   support sending of at least five such options, however.

   The format of this option is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Option Type = 2         |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |R|                         Identifier                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                          Suboptions                           ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Option Type

      This value identifies the option and MUST be set to 1 (Payload
      Reception Report).




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   Length

      This is the length of the option excluding the Option Type and
      Length fields, expressed in bytes.

   R

      This is a 1 bit reserved field, MUST be set to zero and ignored on
      receipt.

   Identifier

      This 31 bit field carries the identifier of the Event message that
      was recently received.

   Suboptions

      This field is reserved for possible future REAP Options that are
      carried (recursively) within this option.  Unrecognized options
      MUST be ignored upon receipt.  Currently there are no defined
      options that can be carried here.

         IPv4 compatibility note: If IPv4 and NATs would need to be
         supported, then it might be necessary to indicate what
         addresses and port numbers were used in the received payload
         packets.

6.5.  State Machine

   A suggested state machine to implement REAP is shown below.

      (The text version does not have the state machine.
      Please consult either the PDF version of this draft,
      or http://www.arkko.com/publications/shim6/reap-proto.jpg)

   This state machine still has some known open issues.  One issue is
   that it does not represent other events than those present in a
   static situation.  For instance, the loss of an address, or peer
   telling us of its new addresses should also affect the state machine.
   A more serious issue is that the machine treats all flows of under 3
   seconds as something that do not need to be acknowledged.  This can
   be easily corrected, but we are struggling with the support of this
   while simultaneously not having to perform extra exploration when the
   traffic flow legitimately ends.

6.6.  Protocol Constants

   The following protocol constants are defined:



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   Exploration Init Timeout                        10 seconds
   Incoming Timeout                                 3 seconds
   Outgoing Timeout                                 3 seconds
   Give Up Timeout                                 60 seconds
   Keepalive Timeout                                3 seconds














































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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-
   service.

   This protocol does not provide any protection of its own for
   indications from other parts of the protocol stack.  However, this
   protocol has weak resistance against incorrect information from these
   sources in the sense that it performs its own tests prior to picking
   a new address pair.  Denial-of- service vulnerabilities remain,
   however, as do vulnerabilities against on path attackers.

   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 [23], link layer security, or the
   use of [12] to protect IPv6 Router and Neighbor Discovery.

   This protocol is designed to be used in situations where other parts
   of the stack have ensured that a set of addresses belong together,
   such as via SHIM6 HBAs [17].  That is, REAP itself provides no
   assurance that a set of addresses belongs to the same host.
   Similarly, REAP provides only minimal protection against third party
   flooding attacks; when REAP is run its Event identifiers can be used
   as a return routability check that the claimed address is indeed
   willing to receive traffic.  However, this needs to be complemented
   with another mechanism to ensure that the claimed address is also the
   correct host.  In SHIM6 this is performed by binding all operations
   to context tags.

   Finally, the exploration itself can cause a number of packets to be
   sent.  As a result it may be used as a tool for packet amplification
   in flooding attacks.  In order to prevent this it is required that
   the protocol employing REAP has built-in mechanisms to prevent this.
   For instance, in SHIM6 contexts are created only after a relatively
   large number of packets has been exchanged, a cost which reduces the
   attractiveness of using SHIM6 and REAP for amplification attacks.
   However, such protections are typically not present at connection
   establishment time.  When exploration would be needed for connection
   establishment to succeed, its usage would result in an amplification



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   vulnerability.  As a result, SHIM6 does not support the use of REAP
   in connection establishment stage.

















































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8.  IANA Considerations

   This document requires the allocation of a SHIM6 message Type code
   for the SHIM6 Probe message (Section 6.1).

   This document requires also the allocation of a SHIM6 option Type
   code for the SHIM6 Event option (Section 6.2).

   This document creates two new name spaces under the new SHIM6 REAP
   repository.  The first name space is for REAP Message Type
   (Section 6.3) and it has one reserved value (0) and one defined
   value, 1 (Event).  Further allocations within this 16-bit field can
   be made through Standards Action or IESG Approval.  The range from
   65000 to 65535 is reserved for experimental use.

   The second name space is for REAP Option Type (Section 6.4) and it
   has one reserved value (0) and two defined values, 1 (Payload
   Reception Report defined in Section 6.4.1) and 2 (Event Reception
   Report defined in Section 6.4.2).  Further allocations within this
   16-bit field can be made through Specification Required.  The range
   from 65535 to 65535 is reserved for experimental use.






























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9.  References

9.1.  Normative References

   [1]  Eastlake, D., Crocker, S., and J. Schiller, "Randomness
        Recommendations for Security", RFC 1750, December 1994.

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

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

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

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

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

   [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.

9.2.  Informative References

   [10]  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.

   [11]  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.

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

   [13]  Aboba, B., "Detection of Network Attachment (DNA) in IPv4",



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         draft-ietf-dhc-dna-ipv4-08 (work in progress), July 2004.

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

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

   [16]  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.

   [17]  Bagnulo, M., "Hash Based Addresses (HBA)",
         draft-ietf-shim6-hba-00 (work in progress), July 2005.

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

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

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

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

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

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

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

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

   [26]  Aura, T., Roe, M., and J. Arkko, "Security of Internet Location



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         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 [10] and HIP [14].









































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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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Authors' Addresses

   Jari Arkko
   Ericsson
   Jorvas  02420
   Finland

   Email: jari.arkko@ericsson.com


   Iljitsch van Beijnum
   Muada
   The Netherlands

   Email: iljitsch@muada.com




































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