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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: June 16, 2007                                    I. van Beijnum
                                                       December 13, 2006


    Failure Detection and Locator Pair Exploration Protocol for IPv6
                              Multihoming
                 draft-ietf-shim6-failure-detection-07

Status of this Memo

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   This Internet-Draft will expire on June 16, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2006).













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Abstract

   This document specifies how the level 3 multihoming shim protocol
   (SHIM6) detects failures between two communicating hosts.  It also
   specifies an exploration protocol for switching to another pair of
   interfaces and/or addresses between the same hosts if a failure
   occurs and an operational pair can be found.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Requirements language  . . . . . . . . . . . . . . . . . . . .  5
   3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  6
       3.1.  Available Addresses  . . . . . . . . . . . . . . . . . .  6
       3.2.  Locally Operational Addresses  . . . . . . . . . . . . .  7
       3.3.  Operational Address Pairs  . . . . . . . . . . . . . . .  7
       3.4.  Primary Address Pair . . . . . . . . . . . . . . . . . .  9
       3.5.  Current Address Pair . . . . . . . . . . . . . . . . . .  9
   4.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . . 10
       4.1.  Failure Detection  . . . . . . . . . . . . . . . . . . . 10
       4.2.  Full Reachability Exploration  . . . . . . . . . . . . . 12
       4.3.  Exploration Order  . . . . . . . . . . . . . . . . . . . 13
   5.  Protocol Definition  . . . . . . . . . . . . . . . . . . . . . 15
       5.1.  Keepalive Message  . . . . . . . . . . . . . . . . . . . 15
       5.2.  Probe Message  . . . . . . . . . . . . . . . . . . . . . 16
       5.3.  Keepalive Timeout Option Format  . . . . . . . . . . . . 21
   6.  Behaviour  . . . . . . . . . . . . . . . . . . . . . . . . . . 22
       6.1.  Incoming payload packet  . . . . . . . . . . . . . . . . 22
       6.2.  Outgoing payload packet  . . . . . . . . . . . . . . . . 23
       6.3.  Keepalive timeout  . . . . . . . . . . . . . . . . . . . 23
       6.4.  Send timeout . . . . . . . . . . . . . . . . . . . . . . 24
       6.5.  Retransmission . . . . . . . . . . . . . . . . . . . . . 24
       6.6.  Reception of the Keepalive message . . . . . . . . . . . 24
       6.7.  Reception of the Probe message State=Exploring . . . . . 25
       6.8.  Reception of the Probe message State=InboundOk . . . . . 25
       6.9.  Reception of the Probe message State=Operational . . . . 25
       6.10. Graphical Representation of the State Machine  . . . . . 26
   7.  Protocol Constants . . . . . . . . . . . . . . . . . . . . . . 27
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 28
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 30
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
       10.1. Normative References . . . . . . . . . . . . . . . . . . 31
       10.2. Informative References . . . . . . . . . . . . . . . . . 31
   Appendix A.  Example Protocol Runs . . . . . . . . . . . . . . . . 33
   Appendix B.  Contributors  . . . . . . . . . . . . . . . . . . . . 38
   Appendix C.  Acknowledgements  . . . . . . . . . . . . . . . . . . 39
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 40



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   Intellectual Property and Copyright Statements . . . . . . . . . . 41


















































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1.  Introduction

   The SHIM6 protocol [I-D.ietf-shim6-proto] extends IPv6 to support
   multihoming.  It is an IP layer mechanism that hides multihoming from
   applications.  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 document specifies the mechanisms and protocol messages to
   achieve both failure detection and locator pair exploration.  This
   part of the SHIM6 protocol is called the REAchability Protocol
   (REAP).

   The document is structured as follows: Section 3 defines a set of
   useful terms, Section 4 gives an overview of REAP, and Section 5
   specifies the message formats and behaviour in detail.  Section 8
   discusses the security considerations of REAP.

   In this specification, we consider an address to be synonymous with a
   locator.  Other parts of the SHIM6 protocol ensure that the different
   locators used by a node actually belong together.  That is, REAP is
   not responsible for ensuring that it ends up with a legitimate
   locator.


























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2.  Requirements language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].














































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3.  Definitions

   This section defines terms useful for discussing failure detection
   and locator pair exploration.

3.1.  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  The address is valid in the sense of RFC 2461 [RFC2461].

   o  The address is not tentative in the sense of RFC 2462 [RFC2462].
      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 Optimistic DAD [RFC4429] even though implementations
      may prefer using other addresses as long as there is an
      alternative.

   o  The address is a global unicast, unique local address [RFC4193],
      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.

   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 SHIM6.  SHIM6 implementations MUST be able to
   employ information provided by IPv6 Neighbor Discovery [RFC2461],
   Address Autoconfiguration [RFC2462], and DHCP [RFC3315] (when DHCP is
   implemented).  This information includes the availability of a new
   address and status changes of existing addresses (such as when an
   address becomes invalid).



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3.2.  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 default router (if needed) suitable
   for this 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 SHIM6 protocol.  SHIM6 implementations MUST be
   able to employ information provided from Neighbor Unreachability
   Detection [RFC2461].  Implementations MAY also employ additional,
   link layer specific mechanisms.

      Note 1: A part of the problem in ensuring that an address is
      operational is making sure that after a change in link layer
      connectivity we are still connected to the same IP subnet.
      Mechanisms such as DNA CPL [I-D.ietf-dna-cpl] or DNAv6
      [I-D.ietf-dna-protocol] can be used to ensure this.

      Note 2: In theory, it would also be possible for hosts to learn
      about routing failures for a particular selected source prefix, if
      only suitable protocols for this purpose existed.  Some proposals
      in this space have been made, see, for instance
      [I-D.bagnulo-shim6-addr-selection] and
      [I-D.huitema-multi6-addr-selection], but none have been
      standardized to date.

3.3.  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 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 when 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.  Unidirectionally operational address pair.  A pair of
   locally operational addresses are said to be an unidirectionally
   operational address pair when packets sent with the first address as
   the source and the second address as the destination can be shown to
   reach the destination.

   SHIM6 implementations MUST support the discovery of operational
   address pairs through the use of explicit rechability tests and
   Forced Bidirectional Communication (FBD), described later in this
   specification.  In addition, implementations MAY employ the following
   additional 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 [RFC2461].

      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  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 or move an address pair to a lower
      place in the list of address pairs to be probed, 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, as
      explained by Gont in [I-D.ietf-tcpm-icmp-attacks].  These
      verifications can ensure that (practically) only on-path attackers
      can spoof the messages.







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3.4.  Primary Address Pair

   The primary address pair consists of the addresses that upper layer
   protocols use in their interaction with the SHIM6 layer.  Use of the
   primary address pair means that the communication is compatible with
   regular non-SHIM6 communication and no context ID needs to be
   present.

3.5.  Current Address Pair

   SHIM6 needs to avoid sending packets which belong to the same
   transport connection concurrently over multiple paths.  This is
   because congestion control in commonly used transport protocols is
   based upon a notion of a single path.  While routing can introduce
   path changes as well and transport protocols have means to deal with
   this, frequent changes will cause problems.  Efficient congestion
   control over multiple paths is a considered research at the time this
   specification is written.

   For these reasons 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.

      It is theoretically possible to support multiple current address
      pairs for different transport sessions or SHIM6 contexts.
      However, this is not supported in this version of the SHIM6
      protocol.

   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 be operational
   for new communications as well.


















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4.  Protocol Overview

   This section discusses the design of the reachability detection and
   full reachability exploration mechanisms, and gives on overview of
   the REAP protocol.

   Exploring the full set of communication options between two hosts
   that both have two or more addresses is an expensive operation as the
   number of combinations to be explored increases very quickly with the
   number of addresses.  For instance, with two addresses on both sides,
   there are four possible address pairs.  Since we can't assume that
   reachability in one direction automatically means reachability for
   the complement pair in the other direction, the total number of two-
   way combinations is eight.  (Combinations = nA * nB * 2.)

   An important observation in multihoming is that failures are
   relatively infrequent, so that an operational pair that worked a few
   seconds ago is very likely to be still operational.  So it makes
   sense to have a light-weight protocol that confirms existing
   reachability, and only invoke heavier exploration when a there is a
   suspected failure.

4.1.  Failure Detection

   Failure detection consists of three parts: tracking local
   information, tracking remote peer status, and finally verifying
   reachability.  Tracking local information consists of using, for
   instance, reachability information about the local router as an
   input.  Nodes SHOULD employ techniques listed in Section 3.1 and
   Section 3.2 to be track the local situation.  It is also necessary to
   track remote address information from the peer.  For instance, if the
   peer's currently used address is no longer in use, mechanism to relay
   that information is needed.  The Update Request message in the SHIM6
   protocol is used for this purpose [I-D.ietf-shim6-proto].  Finally,
   when the local and remote information indicates that communication
   should be possible and there are upper layer packets to be sent,
   reachability verification is necessary to ensure that the peers
   actually have an operational pair.

   A technique called Forced Bidirectional Detection (FBD, originally
   defined in an earlier SHIM6 document [I-D.ietf-shim6-reach-detect])
   is employed for the reachability verification.  Reachability for the
   currently used address pair in a SHIM6 context is determined by
   making sure that whenever there is data traffic in one direction,
   there is also traffic in the other direction.  This can be data
   traffic as well, but also transport layer acknowledgments or a REAP
   reachability keepalive if there is no other traffic.  This way, it is
   no longer possible to have traffic in only one direction, so whenever



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   there is data traffic going out, but there are no return packets,
   there must be a failure, so the full exploration mechanism is
   started.

   A more detailed description of the current pair reachability
   evaluation mechanism:

   1.  To avoid the other side from concluding there is a reachability
       failure, it's necessary for a host implementing the failure
       detection mechanism to generate periodic keepalives when there is
       no other traffic.

       FBD works by generating REAP keepalives if the node is receiving
       packets from its peer but not sending any of its own.  The
       keepalives are sent at certain intervals so that the other side
       knows there is a reachability problem when it doesn't receive any
       incoming packets for its Send Timeout period.  The host
       communicates its Send Timeout value to the peer as an Keepalive
       Timeout Option (section 5.3) in the I2, I2bis, R2, or UPDATE
       messages.  The peer then maps this value to its Keepalive Timeout
       value.

       The interval after which keepalives are sent is named Keepalive
       Interval.  This document doesn't specify a value for Keepalive
       Interval, but recognizes that an often used approach is sending
       keepalives at one-half to one-third of the Keepalive Timeout
       interval, so that multiple keepalives are generated and have time
       to reach the correspondent before it times out.  An upper bound
       on this interval would be (Keepalive Timeout - 2) seconds, so
       that one keepalive has time to reach the other side, assuming a
       maximum one-way delay of 2 seconds.

   2.  Whenever outgoing data packets are generated, a timer is started
       to reflect the requirement that the peer should generate return
       traffic from data packets.  The timeout value is set to the value
       of Send Timeout.

       For the purposes of this specification, "data packet" refers to
       any packet that is part of a SHIM6 context, including both upper
       layer protocol packets and SHIM6 protocol messages except those
       defined in this specification.

   3.  Whenever incoming data packets are received, the timer associated
       with the return traffic from the peer is stopped, and another
       timer is started to reflect the requirement for this node to
       generate return traffic.  This timeout value is set to the value
       of Keepalive Timeout.




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       These two timers are mutually exclusive.  In other words, either
       the node is expecting to see traffic from the peer based the
       traffic that the node sent earlier or the node is expecting to
       respond to the peer based on the traffic that the peer sent
       earlier (or the node is in an idle state).

   4.  The reception of a REAP keepalive packet leads to stopping the
       timer associated with the return traffic from the peer.

   5.  Keepalive Interval seconds after the last data packet has been
       received for a context, and if no other packet has been sent
       within this context since the data packet has been received, a
       REAP keepalive packet is generated for the context in question
       and transmitted to the correspondent.  A host may send the
       keepalive sooner than Keepalive Interval seconds if
       implementation considerations warrant this, but should take care
       to to avoid sending keepalives at an excessive rate.  REAP
       keepalive packets SHOULD continue to be sent at the Keepalive
       Interval until either a data packet in the SHIM6 context has been
       received from the peer or the Keepalive Timeout expires.
       Keepalives are not sent at all when a data packet was sent since
       the last received data packet.

   6.  Send Timeout seconds after the transmission of a data packet with
       no return traffic on this context, a full reachability
       exploration is started.

   Section 7 provides some suggested defaults for these timeout values.
   Experience from the deployment of the SHIM6 protocol is needed in
   order to determine what values are most suitable.  The setting of
   these values is also related to various parameters in transport
   protocols, such as TCP keepalive interval.

4.2.  Full Reachability 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
   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



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   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 parallel 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 Probe
   messages, in sequence, until it gets an Probe message from B
   indicating that (a) B has received one of A's messages and,
   obviously, (b) that B's Probe message gets back to A. B uses the same
   algorithm, but starts the process from the reception of the first
   Probe message from A.

   Upon changing to a new address pair, the network path traversed most
   likely has changed, so that the ULP SHOULD be informed.  This can be
   a signal for the ULP to adapt due to the change in path so that, for
   example, TCP could initiate a slow start procedure, although it's
   likely that the circumstances that led to the selection of a new path
   already caused enough packet loss to trigger slow start.

   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 inter-layer communication mechanism
   to be developed, however.  In any case, this is another issue that we
   treat as being outside the scope of pure address exploration.

   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 8, 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.

4.3.  Exploration Order

   The exploration process assumes an ability to choose address pairs
   for testing, in some sequence.  This process may result in a
   combinatorial explosion when there are many addresses on both sides,
   but a back-off procedure is employed to avoid a "signaling storm".

   Nodes first consult the RFC 3484 default address selection rules
   [RFC3484] Section 4 rules to determine what combinations of addresses
   are allowed 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.  (Additional



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   mechanisms may be defined in the future for arriving at an initial
   ordering of address pairs before testing starts
   [I-D.ietf-shim6-locator-pair-selection].)  Nodes may 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.  The SHIM6 protocol also
   carries preference information in its messages.


      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.

   Out of the set of possible candidate address pairs, nodes SHOULD
   attempt to test through all of them until an operational pair is
   found, and retrying the process as is necessary.  However, all nodes
   MUST perform this process sequentially and with exponential back-off.
   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.

   Section 7 suggests default values for the timers associated with the
   exploration process.  The value Initial Probe Timeout (0.5 seconds)
   specifies the interval between initial attempts to send probes;
   Number of Initial Probes (4) specifies how many initial probes can be
   sent before the exponential backoff procedure needs to be employed.
   This process increases the time between every probe if there is no
   response.  Typically, each increase doubles the time but this
   specification does not mandate a particular increase.

   Finally, Max Probe Timeout (60 seconds) specifies a limit beyond
   which the probe interval may not grow.  If the exploration process
   reaches this interval, it will continue sending at this rate until a
   suitable response is triggered or the SHIM6 context is garbage
   collected, because upper layer protocols using the SHIM6 context in
   question are no longer attempting to send packets.  Reaching the Max
   Probe Timeout may also serve as a hint to the garbage collection
   process that the context is no longer usable.







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5.  Protocol Definition

5.1.  Keepalive Message

   The format of the keepalive message 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  |  Hdr Ext Len  |0|  Type = 66  |  Reserved1  |0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Checksum           |R|                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                             |
   |                    Receiver Context Tag                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Reserved2                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                            Options                            +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Next Header, Hdr Ext Len, 0, 0, Checksum

      These are as specified in Section 5.3 of the SHIM6 protocol
      description [I-D.ietf-shim6-proto].

   Type

      This field identifies the Probe message and MUST be set to 66
      (Keepalive).

   Reserved1

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

   R

      This is a 1-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 47-bit field for the Context Tag the receiver has
      allocated for the context.





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   Reserved2

      This is a 32-bit field reserved for future use.  It is set to zero
      on transmit, and MUST be ignored on receipt.

   Options

      This MAY contain one or more SHIM6 options.The inclusion of the
      latter options is not necessary, however, as there are currently
      no defined options that are useful in a Keepalive message.  These
      options are provided only for future extensibility reasons.

   A valid message conforms to the format above, has a Receiver Context
   Tag that matches to context known by the receiver, is valid shim
   control message as defined in Section 12.2 of the SHIM6 protocol
   description [I-D.ietf-shim6-proto], and its shim context state is
   ESTABLISHED.  The receiver processes a valid message by inspecting
   its options, and executing any actions specified for such options.


      Discussion: It may appear prudent to include additional fields
      that would provide at least a basic level of security, but since
      data packets also indicate ongoing reachability, just as
      keepalives, and those packets don't have such fields, there is
      little or no reason to include them in a keepalive.

   The processing rules for this message are the given in more detail in
   Section 6.

5.2.  Probe Message

   This message performs REAP exploration.  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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  |  Hdr Ext Len  |0|  Type = 67  |   Reserved  |0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Checksum           |R|                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                             |
   |                    Receiver Context Tag                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Precvd| Psent |Sta|                 Reserved2                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                      First probe sent                         +
   |                                                               |
   +                      Source address                           +



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   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                      First probe sent                         +
   |                                                               |
   +                      Destination address                      +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      First probe nonce                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      First probe data                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               /
   /                      Nth probe sent                           /
   |                                                               |
   +                      Source address                           +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                      Nth probe sent                           +
   |                                                               |
   +                      Destination address                      +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Nth probe nonce                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Nth probe data                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                      First probe received                     +
   |                                                               |
   +                      Source address                           +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                      First probe received                     +
   |                                                               |
   +                      Destination address                      +



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   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      First probe nonce                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      First probe data                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                      Nth probe received                       +
   |                                                               |
   +                      Source address                           +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                      Nth probe received                       +
   |                                                               |
   +                      Destination address                      +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Nth probe nonce                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Nth probe data                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                          Options                              +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                          Options                              +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Next Header, Hdr Ext Len, 0, 0, Checksum

      These are as specified in Section 5.3 of the SHIM6 protocol
      description [I-D.ietf-shim6-proto].

   Type

      This field identifies the Probe message and MUST be set to 67
      (Probe).





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   Reserved

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

   R

      This is a 1-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 47-bit field for the Context Tag the receiver has
      allocated for the context.

   Psent

      This is a 4-bit field that indicates the number of sent probes
      included in this probe message.  The first set of probe fields
      pertains to the current message and MUST be present, so the
      minimum value for this field is 1.  Additional sent probe fields
      are copies of the same fields sent in (recent) earlier probes and
      may be included or omitted as per any logic employed by the
      implementation.

   Precvd

      This is a 4-bit field that indicates the number of received probes
      included in this probe messsage.  Received probe fields are copies
      of the same fields received in (recent) earlier probes and may be
      included or omitted as per any logic employed by the
      implementation.

      The fields probe source, probe destination, probe nonce and probe
      data may be repeated, depending on the value of Psent and
      Preceived.

   Sta (State)

      This 2-bit State field is used to inform the peer about the state
      of the sender.  It has three legal values:

      0 (Operational) implies that the sender both (a) believes it has
      no problem communicating and (b) believes that the recipient also
      has no problem communicating.

      1 (Exploring) implies that the sender has a problem communicating
      with the recipient, e.g., it has not seen any traffic from the



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      recipient even when it expected some.

      2 (InboundOk) implies that the sender believes it has no problem
      communicating, i.e., it at least sees packets from the recipient,
      but that the recipient either has a problem or has not yet
      confirmed to the sender that the problem has been solved.

   Reserved2

      MUST be set to 0 upon transmission and MUST be ignored upon
      reception.

   Probe source

      This 128-bit field contains the source IPv6 address used to send
      the probe.

   Probe destination

      This 128-bit field contains the destination IPv6 address used to
      send the probe.

   Probe nonce

      This is a 32-bit field that is initialized by the sender with a
      value that allows it to determine which sent probes a received
      probe correlates with.  It is highly recommeded that the nonce
      field is at least moderately hard to guess so that even on-path
      attackers can't deduce the next nonce value that will be used.
      This value SHOULD be generated using a random number generator
      that is known to have good randomness properties as outlined in
      RFC 1750 [RFC1750].

   Probe data

      This is a 32-bit field with no fixed meaning.  The probe data
      field is copied back with no changes.  Future flags may define a
      use for this field.


         Discussion: One potential use of this field relates to
         communicating delays between reception of a probe and
         transmission of a reply to it.








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   Options

      For future extensions.

5.3.  Keepalive Timeout Option Format

   Either side of a SHIM6 context can notify the peer of the value that
   it would prefer the peer to use as its Keepalive Timeout value.  If
   the host is using a non-default Send Timeout value, it SHOULD
   communicate this value as a Keepalive Timeout value to the peer in
   the below option.  This option MAY be sent in the I2, I2bis, R2, or
   UPDATE messages.  The option SHOULD only need to be sent once in a
   given shim6 association.  If a host receives this option it SHOULD
   update its Keepalive Timeout value for the correspondent.

    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 = 10         |0|        Length  = 4            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +           Reserved            |      Keepalive Timeout        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Type

      This field identifies the option and MUST be set to 10 (Keepalive
      Timeout).

   Length

      This field MUST be set as specified in Section 5.14 of the SHIM6
      protocol description [I-D.ietf-shim6-proto].  That is, it is set
      to 4.

   Reserved

      16-bit field reserved for future use.  Set to zero upon transmit
      and MUST be ignored upon receipt.

   Keepalive Timeout

      Value in seconds corresponding to suggested Keepalive Timeout
      value for the peer.






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6.  Behaviour

   The required behaviour of REAP nodes is specified below in the form
   of a state machine.  The externally observable behaviour of an
   implementation MUST conform to this state machine, but there is no
   requirement that the implementation actually employs a state machine.
   Intermixed with the following description we also provide a state
   machine description in a tabular form.  That form is only
   informational, however.

   On a given context with a given peer, the node can be in one of three
   states: Operational, Exploring, or InboundOK.  In the Operational
   state the underlying address pairs are assumed to be operational.  In
   the Exploring state this node has observed a problem and has
   currently not seen any traffic from the peer.  Finally, in the
   InboundOK state this node sees traffic from the peer, but peer may
   not yet see any traffic from this node so that the exploration
   process needs to continue.

   The node maintains also the Send timer (Send Timeout seconds) and
   Keepalive timer (Keepalive Timeout seconds).  The Send timer reflects
   the requirement that when this node sends a payload packet there
   should be some return traffic (either payload packets or Keepalive
   messages) within Send Timeout seconds.  The Keepalive timer reflects
   the requirement that when this node receives a payload packet there
   should a similar response towards the peer.  The Keepalive timer is
   only used within the Operational state, and the Send timer in the
   Operational and InboundOK states.  No timer is running in the
   Exploring state.  As explained in Section 4.1, the two timers are
   mutually exclusive.  That is, either the Keepalive timer is running
   or the Send timer is running (or no timer is running).

   Note that Appendix A gives some examples of typical protocol runs to
   illustrate the behaviour.

6.1.  Incoming payload packet

   Upon the reception of a payload packet in the Operational state, the
   node starts the Keepalive timer if it is not yet running, and stops
   the Send timer if it was running.

   If the node is in the Exploring state it transitions to the InboundOK
   state, sends a Probe message, and starts the Send timer.  It fills
   the Psent and corresponding Probe source address, Probe destination
   address, Probe nonce, and Probe data fields with information about
   recent Probe messages that have not yet been reported as seen by the
   peer.  It also fills the Precvd and corresponding Probe source
   address, Probe destination address, Probe nonce, and Probe data



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   fields with information about recent Probe messages it has seen from
   the peer.  When sending a Probe message, the State field MUST be set
   to a value that matches the conceptual state of the sender after
   sending the Probe.  In this case the node therefore sets the Sta
   field to 2 (InboundOk).  The IP source and and destination addresses
   for sending the Probe message are selected as discussed in
   Section 4.3.

   In the InboundOK state the node stops the Send timer if it was
   running, but does not do anything else.

   The reception of SHIM6 control messages other than the Keepalive and
   Probe messages are treated similarly with payload packets.

   While the Keepalive timer is running, the node SHOULD send Keepalive
   messages to the peer with an interval of Keepalive Interval seconds.
   Conceptually, a separate timer is used to distinguish between the
   interval between Keepalive messages and the overall Keepalive Timeout
   interval.  However, this separate timer is not modelled in the
   tabular or graphical state machines.  When sent, the Keepalive
   message is constructed as described in Section 5.1.  It is sent using
   the current address pair.

     Operational            Exploring                InboundOk
     -------------------------------------------------------------
     STOP Send;             SEND Probe InboundOk;    STOP Send
     START Keepalive        START Send;
                            GOTO InboundOk

6.2.  Outgoing payload packet

   Upon sending a payload packet in the Operational state, the node
   stops the Keepalive timer if it was running and starts the Send timer
   if it was not running.  In the Exploring state there is no effect,
   and in the InboundOK state the node simply starts the Send timer if
   it was not yet running.  (The sending of SHIM6 control messages is
   again treated similarly here.)

     Operational              Exploring              InboundOk
     -----------------------------------------------------------
     START Send;              -                      START Send
     STOP Keepalive

6.3.  Keepalive timeout

   Upon a timeout on the Keepalive timer, the node sends one last
   Keepalive message and cancels the timer governing the sending of the
   next Keepalive message.  This can only happen in the Operational



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   state.

   The Keepalive message is constructed as described in Section 5.1.  It
   is sent using the current address pair.

     Operational              Exploring              InboundOk
     -----------------------------------------------------------
     SEND Keepalive;          -                      -
     STOP Keepalive

6.4.  Send timeout

   Upon a timeout on the Send timer, the node enters the Exploring
   state, sends a Probe message, and stops the Keepalive timer if it was
   running.  The Probe message is constructed as explained in
   Section 6.1, except that the Sta field is set to 1 (Exploring).

   Operational              Exploring              InboundOk
   -----------------------------------------------------------
   SEND Probe Exploring;    -                      SEND Probe Exploring;
   STOP Keepalive;                                 GOTO Exploring
   GOTO Exploring

6.5.  Retransmission

   While in the Exploring state the node keeps retransmitting its Probe
   messages to different (or same) addresses as defined in Section 4.3.
   A similar process is employed in the InboundOk state, except that
   upon such retransmission the Send timer is started if it was not
   running already.

   The Probe messages are constructed as explained in Section 6.1,
   except that the Sta field is set to 1 (Exploring) or 2 (InboundOk),
   depending on which state the sender is in.

     Operational             Exploring              InboundOk
     ----------------------------------------------------------
     -                       SEND Probe Exploring   SEND Probe InboundOk
                                                    START Send

6.6.  Reception of the Keepalive message

   Upon the reception of a Keepalive message in the Operational state,
   the node stops the Send timer, if it was running.  If the node is in
   the Exploring state it transitions to the InboundOK state, sends a
   Probe message, and starts the Send timer.  The Probe message is
   constructed as explained in Section 6.1.




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   In the InboundOK state the Send timer is stopped, if it was running.

     Operational            Exploring                InboundOk
     -----------------------------------------------------------
     STOP Send              SEND Probe InboundOk;    STOP Send
                            START Send;
                            GOTO InboundOk

6.7.  Reception of the Probe message State=Exploring

   Upon receiving a Probe with State set to Exploring, the node enters
   the InboundOK state, sends a Probe as described in Section 6.1, stops
   the Keepalive timer if it was running, and restarts the Send timer.

     Operational             Exploring               InboundOk
     -----------------------------------------------------------
     SEND Probe InboundOk;   SEND Probe InboundOk;   SEND Probe
     STOP Keepalive;         START Send;                  InboundOk;
     RESTART Send;           GOTO InboundOk          RESTART Send
     GOTO InboundOk

6.8.  Reception of the Probe message State=InboundOk

   Upon the reception of a Probe message with State set to InboundOk,
   the node sends a Probe message, restarts the Send timer, stops the
   Keepalive timer if it was running, and transitions to the Operational
   state.

   The Probe message is constructed as explained in Section 6.1, except
   that the Sta field is set to 0 (Operational).

     Operational             Exploring               InboundOk
     -------------------------------------------------------------
     SEND Probe Operational; SEND Probe Operational; SEND Probe
     RESTART Send;           RESTART Send;                Operational;
     STOP Keepalive          GOTO Operational        RESTART Send;
                                                     GOTO Operational

6.9.  Reception of the Probe message State=Operational

   Upon the reception of a Probe message with State set to Operational,
   the node stops the Send timer if it was running, starts the Keepalive
   timer if it was not yet running, and transitions to the Operational
   state.  The Probe message is constructed as explained in Section 6.1,
   except that the Sta field is set to 0 (Operational).

      Note: This terminates the exploration process when both parties
      are happy and know that their peer is happy as well.



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     Operational              Exploring              InboundOk
     -----------------------------------------------------------
     STOP Send                STOP Send;             STOP Send;
     START Keepalive          START Keepalive        START Keepalive
                              GOTO Operational       GOTO Operational

   The reachability detection and exploration process has no effect on
   payload communications until a new operational address pairs have
   actually been confirmed.  Prior to that the payload packets continue
   to be sent to the previously used addresses.

6.10.  Graphical Representation of the State Machine

   In the PDF version of this specification, an informational drawing
   illustrates the state machine.  Where the text and the drawing
   differ, the text takes precedence.



































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7.  Protocol Constants

   The following protocol constants are defined:

   Send Timeout                              10 seconds
   Keepalive Interval                           Not specified here
   Initial Probe Timeout                    0.5 seconds
   Number of Initial Probes                   4 probes
   Max Probe Timeout                         60 seconds

   Alternate values of the Send Timeout may be selected by a host and
   communicated to the peer in the Keepalive Timeout Option.







































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8.  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 operational.  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.  Unprotected
   indications SHOULD NOT be taken as a proof of connectivity problems.
   However, REAP has weak resistance against incorrect information even
   from unprotected indications 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 [I-D.ietf-tcpm-icmp-attacks], link
   layer security, or the use of SEND [RFC3971] to protect IPv6 Router
   and Neighbor Discovery.

   Other parts of the SHIM6 protocol ensure that the set of addresses we
   are switching between actually belong together.  REAP itself provides
   no such assurances.  Similarly, REAP provides some protection against
   third party flooding attacks [AURA02]; when REAP is run its Probe
   nonces 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.  SHIM6 does this by performing
   binding of all operations to context tags.

   The keepalive mechanism in this specification is vulnerable to
   spoofing.  On path-attackers that can see a SHIM6 context tag can
   send spoofed Keepalive messages once per Send Timeout interval, to
   prevent two SHIM6 nodes from sending Keepalives themselves.  This
   vulnerability is only relevant to nodes involved in a one-way
   communication.  The result of the attack is that the nodes enter the
   exploration phase needlessly, but they should be able to confirm
   connectivity unless, of course, the attacker is able to prevent the
   exploration phase from completing.  Off-path attackers may not be



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   able to generate spoofed results, given that the context tags are 47-
   bit random numbers.

   The exploration phase is vulnerable to attackers that are on the
   path.  Off-path attackers would find it hard to guess either the
   context tag or the correct probe identifiers.  Given that IPsec
   operates above the shim layer, it is not possible to protect the
   exploration phase against on-path attackers.  This is similar to the
   ability to protect other Shim6 control exchanges.  There are
   mechanisms in place to prevent the redirection of communications to
   wrong addresses, but on-path attackers can cause denial-of-service,
   move communications to less-preferred address pairs, and so on.

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


























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9.  IANA Considerations

   No IANA actions are required.
















































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10.  References

10.1.  Normative References

   [RFC1750]  Eastlake, D., Crocker, S., and J. Schiller, "Randomness
              Recommendations for Security", RFC 1750, December 1994.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2461]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor
              Discovery for IP Version 6 (IPv6)", RFC 2461,
              December 1998.

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

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484, February 2003.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, October 2005.

   [RFC4429]  Moore, N., "Optimistic Duplicate Address Detection (DAD)
              for IPv6", RFC 4429, April 2006.

10.2.  Informative References

   [AURA02]   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.

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

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

   [I-D.ietf-dna-cpl]



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              Nordmark, E. and J. Choi, "DNA with unmodified routers:
              Prefix list based approach", draft-ietf-dna-cpl-02 (work
              in progress), January 2006.

   [I-D.ietf-dna-protocol]
              Kempf, J., "Detecting Network Attachment in IPv6 Networks
              (DNAv6)", draft-ietf-dna-protocol-01 (work in progress),
              June 2006.

   [I-D.ietf-hip-mm]
              Nikander, P., "End-Host Mobility and Multihoming with the
              Host Identity Protocol", draft-ietf-hip-mm-04 (work in
              progress), June 2006.

   [I-D.ietf-shim6-locator-pair-selection]
              Bagnulo, M., "Default Locator-pair selection algorithm for
              the SHIM6 protocol",
              draft-ietf-shim6-locator-pair-selection-01 (work in
              progress), October 2006.

   [I-D.ietf-shim6-proto]
              Bagnulo, M. and E. Nordmark, "Level 3 multihoming shim
              protocol", draft-ietf-shim6-proto-07 (work in progress),
              December 2006.

   [I-D.ietf-shim6-reach-detect]
              Beijnum, I., "Shim6 Reachability Detection",
              draft-ietf-shim6-reach-detect-01 (work in progress),
              October 2005.

   [I-D.ietf-tcpm-icmp-attacks]
              Gont, F., "ICMP attacks against TCP",
              draft-ietf-tcpm-icmp-attacks-00 (work in progress),
              February 2006.

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

   [RFC3971]  Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
              Neighbor Discovery (SEND)", RFC 3971, March 2005.









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Appendix A.  Example Protocol Runs

   This appendix 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:

               EXAMPLE 1: No communications

    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:

          EXAMPLE 2: Bidirectional communications

    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:







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         EXAMPLE 3: Unidirectional communications

    Peer A                                        Peer B
      |                                             |
      |              payload packet                 |
      |-------------------------------------------->|
      |                                             |
      |              payload packet                 |
      |-------------------------------------------->|
      |                                             |
      |              payload packet                 |
      |-------------------------------------------->|
      |                                             |
      |              Keepalive id=p                 |
      |<--------------------------------------------|
      |                                             |
      |              payload packet                 |
      |-------------------------------------------->|
      |                                             |
      |                                             |

   The next example involves a failure scenario.  Here A has addresses A
   and B has addresses B1 and B2.  The currently used address pairs are
   (A, B1) and (B1, A).  All connections via B1 become broken, which
   leads to an exploration process:

              EXAMPLE 4: Failure scenario

    Peer A                                        Peer B
      |                                             |
   State:                                           | State:
   Operational                                      | Operational
      |            (A,B1) payload packet            |
      |-------------------------------------------->|
      |                                             |
      |            (B1,A) payload packet            |
      |<--------------------------------------------| At time T1
      |                                             | path A<->B1
      |            (A,B1) payload packet            | becomes
      |----------------------------------------/    | broken
      |                                             |
      |           ( B1,A) payload packet            |
      |   /-----------------------------------------|
      |                                             |
      |            (A,B1) payload packet            |
      |----------------------------------------/    |
      |                                             |
      |            (B1,A) payload packet            |



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      |   /-----------------------------------------|
      |                                             |
      |            (A,B1) payload packet            |
      |----------------------------------------/    |
      |                                             |
      |                                             | Send Timeout
      |                                             | seconds after
      |                                             | T1, B happens to
      |                                             | see the problem
      |             (B1,A) Probe id=p,              | first and sends a
      |                          state=exploring    | complaint that
      |   /-----------------------------------------| it is not rec-
      |                                             | eiving anything
      |                                             | State:
      |                                             | Exploring
      |                                             |
      |             (B2,A) Probe id=q,              |
      |                          state=exploring    | But its lost,
      |<--------------------------------------------| retransmission
      |                                             | uses another pair
   A realizes                                       |
   that it needs                                    |
   to start the                                     |
   exploration. It                                  |
   picks B2 as the                                  |
   most likely candidate,                           |
   as it appeared in the                            |
   Probe                                            |
   State: InboundOk                                 |
      |                                             |
      |       (A, B2) Probe id=r,                   |
      |                     state=inboundok,        |
      |                     received probe q        | This one gets
      |-------------------------------------------->| through.
      |                                             | State:
      |                                             | Operational
      |                                             |
      |                                             |
      |       (B2,A) Probe id=s,                    |
      |                    state=operational,       | B now knows
      |                    received probe r         | that A has no
      |<--------------------------------------------| problem to receive
      |                                             | its packets
   State: Operational                               |
      |                                             |
      |            (A,B2) payload packet            |
      |-------------------------------------------->| Payload packets
      |                                             | flow again



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      |            (B2,A) payload packet            |
      |<--------------------------------------------|

   The next example shows when the failure for the current locator pair
   is in the other direction only.  A has addresses A1 and A2, and B has
   addresses B1 and B2.  The current communication is between A1 and B1,
   but A's packets no longer reach B using this pair.

              EXAMPLE 5: One-way failure

    Peer A                                        Peer B
      |                                             |
   State:                                           | State:
   Operational                                      | Operational
      |                                             |
      |           (A1,B1) payload packet            |
      |-------------------------------------------->|
      |                                             |
      |           (B1,A1) payload packet            |
      |<--------------------------------------------|
      |                                             |
      |           (A1,B1) payload packet            | At time T1
      |----------------------------------------/    | path A1->B1
      |                                             | becomes
      |                                             | broken
      |           (B1,A1) payload packet            |
      |<--------------------------------------------|
      |                                             |
      |           (A1,B1) payload packet            |
      |----------------------------------------/    |
      |                                             |
      |           (B1,A1) payload packet            |
      |<--------------------------------------------|
      |                                             |
      |           (A1,B1) payload packet            |
      |----------------------------------------/    |
      |                                             |
      |                                             | Send Timeout
      |                                             | seconds after
      |                                             | T1, B notices
      |                                             | the problem and
      |          (B1,A1) Probe id=p,                | sends a com-
      |                        state=exploring      | plaint that
      |<--------------------------------------------| it is not rec-
      |                                             | eiving anything
   A responds                                       | State: Exploring
   State: InboundOk                                 |
      |                                             |



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      |      (A1, B1) Probe id=q,                   |
      |                     state=inboundok,        |
      |                     received payload,       |
      |                     received probe q        |
      |----------------------------------------/    | But A's response
      |                                             | is lost
      |         (B2,A2) Probe id=r,                 |
      |                       state=exploring       | Next try different
      |<--------------------------------------------| locator pair
      |                                             |
      |     (A2, B2) Probe id=s,                    |
      |                    state=inboundok,         |
      |                    received payload,        |
      |                    received probes p, r     | This one gets
      |-------------------------------------------->| through
      |                                             | State: Operational
      |                                             |
      |                                             | B now knows
      |                                             | that A has no
      |      (B2,A2) Probe id=t,                    | problem to receive
      |                    state=operational,       | its packets, and
      |                    received probe s         | that A's probe
      |<--------------------------------------------| gets to B. It
      |                                             | sends a
   State: Operational                               | confirmation to A
      |                                             |
      |           (A2,B2) payload packet            |
      |-------------------------------------------->| Payload packets
      |                                             | flow again
      |           (B1,A1) payload packet            |
      |<--------------------------------------------|




















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Appendix B.  Contributors

   This draft attempts to summarize the thoughts and unpublished
   contributions of many people, including the MULTI6 WG design team
   members Marcelo Bagnulo Braun, Erik Nordmark, Geoff Huston, Kurtis
   Lindqvist, Margaret Wasserman, and Jukka Ylitalo, the MOBIKE WG
   contributors Pasi Eronen, Tero Kivinen, Francis Dupont, Spencer
   Dawkins, and James Kempf, and HIP WG contributors such as Pekka
   Nikander.  This draft is also in debt to work done in the context of
   SCTP [RFC2960] and HIP multihoming and mobility extension
   [I-D.ietf-hip-mm].








































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Appendix C.  Acknowledgements

   The authors would also like to thank Christian Huitema, Pekka Savola,
   John Loughney, Sam Xia, Hannes Tschofenig, Sebastian Barre, Thomas
   Henderson, and Deguang Le for interesting discussions in this problem
   space, and for review of this specification.













































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Authors' Addresses

   Jari Arkko
   Ericsson
   Jorvas  02420
   Finland

   Email: jari.arkko@ericsson.com


   Iljitsch van Beijnum

   Email: iljitsch@muada.com






































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Full Copyright Statement

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