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RFC 6775

 
 
 

Neighbor Discovery Optimization for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)

Part 2 of 3, p. 15 to 40
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4.  New Neighbor Discovery Options and Messages

   This section defines new Neighbor Discovery message options used by
   this specification.  The Address Registration Option is used by
   hosts, whereas the Authoritative Border Router Option and 6LoWPAN
   Context Option are used in the substitutable router-to-router
   interaction.  This section also defines the new router-to-router
   Duplicate Address Request and Duplicate Address Confirmation
   messages.

4.1.  Address Registration Option

   The routers need to know the set of host IP addresses that are
   directly reachable and their corresponding link-layer addresses.
   This needs to be maintained as the radio reachability changes.  For
   this purpose, an Address Registration Option (ARO) is introduced,
   which can be included in unicast NS messages sent by hosts.  Thus, it
   can be included in the unicast NS messages that a host sends as part
   of NUD to determine that it can still reach a default router.  The
   ARO is used by the receiving router to reliably maintain its Neighbor
   Cache.  The same option is included in corresponding NA messages with
   a Status field indicating the success or failure of the registration.
   This option is always host initiated.

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   The information contained in the ARO is also included in the multihop
   DAR and DAC messages used between 6LRs and 6LBRs, but the option
   itself is not used in those messages.

   The ARO is required for reliability and power saving.  The lifetime
   field provides flexibility to the host to register an address that
   should be usable (continue to be advertised by the 6LR in the routing
   protocol, etc.) during its intended sleep schedule.

   The sender of the NS also includes the EUI-64 [EUI64] of the
   interface from which it is registering an address.  This is used as a
   unique ID for the detection of duplicate addresses.  It is used to
   tell the difference between the same node re-registering its address
   and a different node (with a different EUI-64) registering an address
   that is already in use by someone else.  The EUI-64 is also used to
   deliver an NA carrying an error Status code to the EUI-64-based
   link-local IPv6 address of the host (see Section 6.5.2).

   When the ARO is used by hosts, an SLLAO (Source Link-Layer Address
   Option) [RFC4861] MUST be included, and the address that is to be
   registered MUST be the IPv6 source address of the NS message.

    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      |   Length = 2  |    Status     |   Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Reserved            |     Registration Lifetime     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                            EUI-64                             +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Type:                   33

   Length:                 8-bit unsigned integer.  The length of the
                           option in units of 8 bytes.  Always 2.

   Status:                 8-bit unsigned integer.  Indicates the status
                           of a registration in the NA response.  MUST
                           be set to 0 in NS messages.  See below.

   Reserved:               This field is unused.  It MUST be initialized
                           to zero by the sender and MUST be ignored by
                           the receiver.

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   Registration Lifetime:  16-bit unsigned integer.  The amount of time
                           in units of 60 seconds that the router should
                           retain the NCE for the sender of the NS that
                           includes this option.

   EUI-64:                 64 bits.  This field is used to uniquely
                           identify the interface of the Registered
                           Address by including the EUI-64 identifier
                           [EUI64] assigned to it unmodified.

   The Status values used in NAs are:

          +--------+--------------------------------------------+
          | Status |                 Description                |
          +--------+--------------------------------------------+
          |    0   |                   Success                  |
          |    1   |              Duplicate Address             |
          |    2   |             Neighbor Cache Full            |
          |  3-255 | Allocated using Standards Action [RFC5226] |
          +--------+--------------------------------------------+

                                  Table 1

4.2.  6LoWPAN Context Option

   The 6LoWPAN Context Option (6CO) carries prefix information for
   LoWPAN header compression and is similar to the PIO of [RFC4861].
   However, the prefixes can be remote as well as local to the LoWPAN,
   since header compression potentially applies to all IPv6 addresses.
   This option allows for the dissemination of multiple contexts
   identified by a CID for use as specified in [RFC6282].  A context may
   be a prefix of any length or an address (/128), and up to 16 6COs may
   be carried in an RA message.

    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      |     Length    |Context Length | Res |C|  CID  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Reserved           |         Valid Lifetime        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                       Context Prefix                          .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 1: 6LoWPAN Context Option Format

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   Type:            34

   Length:          8-bit unsigned integer.  The length of the option
                    (including the Type and Length fields) in units of
                    8 bytes.  May be 2 or 3, depending on the length of
                    the Context Prefix field.

   Context Length:  8-bit unsigned integer.  The number of leading bits
                    in the Context Prefix field that are valid.  The
                    value ranges from 0 to 128.  If it is more than 64,
                    then the Length MUST be 3.

   C:               1-bit context Compression flag.  This flag indicates
                    if the context is valid for use in compression.  A
                    context that is not valid MUST NOT be used for
                    compression but SHOULD be used in decompression in
                    case another compressor has not yet received the
                    updated context information.  This flag is used to
                    manage the context life cycle based on the
                    recommendations in Section 7.2.

   CID:             4-bit Context Identifier for this prefix
                    information.  The CID is used by context-based
                    header compression as specified in [RFC6282].  The
                    list of CIDs for a LoWPAN is configured on the 6LBR
                    that originates the context information for the
                    6LoWPAN.

   Res, Reserved:   This field is unused.  It MUST be initialized to
                    zero by the sender and MUST be ignored by the
                    receiver.

   Valid Lifetime:  16-bit unsigned integer.  The length of time in
                    units of 60 seconds (relative to the time the packet
                    is received) that the context is valid for the
                    purpose of header compression or decompression.  A
                    value of all zero bits (0x0) indicates that this
                    context entry MUST be removed immediately.

   Context Prefix:  The IPv6 prefix or address corresponding to the CID
                    field.  The valid length of this field is included
                    in the Context Length field.  This field is padded
                    with zeros in order to make the option a multiple of
                    8 bytes.

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4.3.  Authoritative Border Router Option

   The Authoritative Border Router Option (ABRO) is needed when RA
   messages are used to disseminate prefixes and context information
   across a route-over topology.  In this case, 6LRs receive PIOs from
   other 6LRs.  This implies that a 6LR can't just let the most recently
   received RA win.  In order to be able to reliably add and remove
   prefixes from the 6LoWPAN, we need to carry information from the
   authoritative 6LBR.  This is done by introducing a version number
   that the 6LBR sets and that 6LRs propagate as they propagate the
   prefix and context information with this ABRO.  When there are
   multiple 6LBRs, they would have separate version number spaces.
   Thus, this option needs to carry the IP address of the 6LBR that
   originated that set of information.

   The ABRO MUST be included in all RA messages in the case when RAs are
   used to propagate information between routers (as described in
   Section 8.2).

    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      |  Length = 3   |          Version Low          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Version High         |        Valid Lifetime         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                          6LBR Address                         +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Type:                       35

   Length:                     8-bit unsigned integer.  The length of
                               the option in units of 8 bytes.
                               Always 3.

   Version Low, Version High:  Together, Version Low and Version High
                               constitute the Version Number field, a
                               32-bit unsigned integer where Version Low
                               is the least significant 16 bits and
                               Version High is the most significant

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                               16 bits.  The version number
                               corresponding to this set of information
                               contained in the RA message.  The
                               authoritative 6LBR originating the prefix
                               increases this version number each time
                               its set of prefix or context information
                               changes.

   Valid Lifetime:             16-bit unsigned integer.  The length of
                               time in units of 60 seconds (relative to
                               the time the packet is received) that
                               this set of border router information is
                               valid.  A value of all zero bits (0x0)
                               assumes a default value of 10,000
                               (~one week).

   Reserved:                   This field is unused.  It MUST be
                               initialized to zero by the sender and
                               MUST be ignored by the receiver.

   6LBR Address:               IPv6 address of the 6LBR that is the
                               origin of the included version number.

4.4.  Duplicate Address Messages

   For the multihop DAD exchanges between a 6LR and 6LBR as specified in
   Section 8.2, there are two new ICMPv6 message types called the
   Duplicate Address Request (DAR) and the Duplicate Address
   Confirmation (DAC).  We avoid reusing the NS and NA messages for this
   purpose, since these messages are not subject to the hop limit=255
   check as they are forwarded by intermediate 6LRs.  The information
   contained in the messages is otherwise the same as would be in an NS
   carrying an ARO, with the message format inlining the fields that are
   in the ARO.

   The DAR and DAC use the same message format with different ICMPv6
   type values, and the Status field is only meaningful in the DAC
   message.

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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Status     |   Reserved    |     Registration Lifetime     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                            EUI-64                             +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                       Registered Address                      +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IP fields:

   IPv6 Source:            A non-link-local address of the sending
                           router.

   IPv6 Destination:       In a DAR, a non-link-local address of a 6LBR.
                           In a DAC, this is just the source from the
                           DAR.

   Hop Limit:              Set to MULTIHOP_HOPLIMIT on transmit.  MUST
                           be ignored on receipt.

   ICMP Fields:

   Type:                   157 for the DAR and 158 for the DAC.

   Code:                   Set to zero on transmit.  MUST be ignored on
                           receipt.

   Checksum:               The ICMP checksum.  See [RFC4443].

   Status:                 8-bit unsigned integer.  Indicates the status
                           of a registration in the DAC.  MUST be set to
                           0 in the DAR.  See Table 1.

   Reserved:               This field is unused.  It MUST be initialized
                           to zero by the sender and MUST be ignored by
                           the receiver.

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   Registration Lifetime:  16-bit unsigned integer.  The amount of time
                           in units of 60 seconds that the 6LBR should
                           retain the DAD table entry (Section 8.2.2)
                           for the Registered Address.  A value of 0
                           indicates in a DAR that the DAD table entry
                           should be removed.

   EUI-64:                 64 bits.  This field is used to uniquely
                           identify the interface of the Registered
                           Address by including the EUI-64 identifier
                           [EUI64] assigned to it unmodified.

   Registered Address:     128-bit field.  Carries the host address that
                           was contained in the IPv6 Source field in the
                           NS that contained the ARO sent by the host.

5.  Host Behavior

   Hosts in a LoWPAN use the ARO in the NS messages they send as a way
   to maintain the Neighbor Cache in the routers, thereby removing the
   need for multicast NSs to do address resolution.  Unlike in
   [RFC4861], the hosts initiate updating the information they receive
   in RAs by sending RSs before the information expires.  Finally, when
   NUD indicates that one or all default routers have become
   unreachable, then the host uses RSs to find a new set of default
   routers.

5.1.  Forbidden Actions

   A host MUST NOT multicast an NS message.

5.2.  Interface Initialization

   When the interface on a host is initialized, it follows the
   specification in [RFC4861].  A link-local address is formed based on
   the EUI-64 identifier [EUI64] assigned to the interface as per
   [RFC4944] or the appropriate IP-over-foo document for the link, and
   then the host sends RS messages as described in [RFC4861]
   Section 6.3.7.

   There is no need to join the solicited-node multicast address, since
   nobody multicasts NSs in this type of network.  A host MUST join the
   all-nodes multicast address.

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5.3.  Sending a Router Solicitation

   The RS is formatted as specified in [RFC4861] and sent to the IPv6
   all-routers multicast address (see [RFC4861] Section 6.3.7 for
   details).  An SLLAO MUST be included to enable unicast RAs in
   response.  An unspecified source address MUST NOT be used in RS
   messages.

   If the link layer supports a way to send packets to some kind of
   all-routers anycast link-layer address, then that MAY be used to
   convey these packets to a router.

   Since hosts do not depend on multicast RAs to discover routers, the
   hosts need to intelligently retransmit RSs whenever the default
   router list is empty, one of its default routers becomes unreachable,
   or the lifetime of the prefixes and contexts in the previous RA is
   about to expire.  The RECOMMENDED rate of retransmissions is to
   initially send up to 3 (MAX_RTR_SOLICITATIONS) RS messages separated
   by at least 10 seconds (RTR_SOLICITATION_INTERVAL) as specified in
   [RFC4861], and then switch to slower retransmissions.  After the
   initial retransmissions, the host SHOULD do truncated binary
   exponential backoff [ETHERNET] of the retransmission timer for each
   subsequent retransmission, truncating the increase of the
   retransmission timer at 60 seconds (MAX_RTR_SOLICITATION_INTERVAL).
   In all cases, the RS retransmissions are terminated when an RA is
   received.  See Section 9 for protocol constants.

5.4.  Processing a Router Advertisement

   The processing of RAs is as in [RFC4861], with the addition of
   handling the 6CO and triggering address registration when a new
   address has been configured.  Furthermore, the SLLAO MUST be included
   in the RA.  Unlike in [RFC4861], the maximum value of the RA Router
   Lifetime field MAY be up to 0xFFFF (approximately 18 hours).

   Should the host erroneously receive a PIO with the L (on-link) flag
   set, then that PIO MUST be ignored.

5.4.1.  Address Configuration

   Address configuration follows [RFC4862].  For an address not derived
   from an EUI-64, the M flag of the RA determines how the address can
   be configured.  If the M flag is set in the RA, then DHCPv6 MUST be
   used to assign the address.  If the M flag is not set, then the
   address can be configured by any other means (and duplicate detection
   is performed as part of the registration process).

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   Once an address has been configured, it will be registered by
   unicasting an NS with an ARO to one or more routers.

5.4.2.  Storing Contexts

   The host maintains a conceptual data structure for the context
   information it receives from the routers.  This structure is called
   the context table.  It includes the CID, the prefix (from the Context
   Prefix field in the 6CO), the Compression bit, and the Valid
   Lifetime.  A context table entry that has the Compression bit clear
   is used for decompression when receiving packets but MUST NOT be used
   for compression when sending packets.

   When a 6CO is received in an RA, it is used to add or update the
   information in the context table.  If the CID field in the 6CO
   matches an existing context table entry, then that entry is updated
   with the information in the 6CO.  If the Valid Lifetime field in the
   6CO is zero, then the entry is immediately deleted.

   If there is no matching entry in the context table, and the Valid
   Lifetime field is non-zero, then a new context is added to the
   context table.  The 6CO is used to update the created entry.

   When the 6LBR changes the context information, a host might not
   immediately notice.  And in the worst case, a host might have stale
   context information.  For this reason, 6LBRs use the recommendations
   in Section 7.2 for carefully managing the context life cycle.  Nodes
   should be careful about using header compression in RA messages that
   include 6COs.

5.4.3.  Maintaining Prefix and Context Information

   The prefix information is timed out as specified in [RFC4861].  When
   the Valid Lifetime for a context table entry expires, the entry is
   placed in a receive-only mode, which is the equivalent of receiving a
   6CO for that context with C=0.  The entry is held in receive-only
   mode for a period of twice the default Router Lifetime, after which
   the entry is removed.

   A host should inspect the various lifetimes to determine when it
   should next initiate sending an RS to ask for any updates to the
   information.  The lifetimes that matter are the default Router
   Lifetime, the Valid Lifetime in the PIOs, and the Valid Lifetime in
   the 6CO.  The host SHOULD unicast one or more RSs to the router well
   before the shortest of those lifetimes (across all the prefixes and
   all the contexts) expires and then switch to multicast RS messages if
   there is no response to the unicasts.  The retransmission behavior
   for the RSs is specified in Section 5.3.

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5.5.  Registration and Neighbor Unreachability Detection

   Hosts send unicast NS messages to register their IPv6 addresses, and
   also to do NUD to verify that their default routers are still
   reachable.  The registration is performed by the host including an
   ARO in the NS it sends.  Even if the host doesn't have data to send,
   but is expecting others to try to send packets to the host, the host
   needs to maintain its NCEs in the routers.  This is done by sending
   NS messages with an ARO to the router well in advance of the
   Registration Lifetime expiring.  NS messages are retransmitted up to
   MAX_UNICAST_SOLICIT times using a minimum timeout of RETRANS_TIMER
   until the host receives an NA message with an ARO.

   Hosts that receive RA messages from multiple default routers SHOULD
   attempt to register with more than one of them in order to increase
   the robustness of the network.

   Note that NUD probes can be suppressed by reachability confirmations
   from transport protocols or applications as specified in [RFC4861].

   When a host knows it will no longer use a router it is registered to,
   it SHOULD de-register with the router by sending an NS with an ARO
   containing a lifetime of 0.  To handle the case when a host loses
   connectivity with the default router involuntarily, the host SHOULD
   use a suitably low Registration Lifetime.

5.5.1.  Sending a Neighbor Solicitation

   The host triggers sending NS messages containing an ARO when a new
   address is configured, when it discovers a new default router, or
   well before the Registration Lifetime expires.  Such an NS MUST
   include an SLLAO, since the router needs to record the link-layer
   address of the host.  An unspecified source address MUST NOT be used
   in NS messages.

5.5.2.  Processing a Neighbor Advertisement

   A host handles NA messages as specified in [RFC4861], with added
   logic described in this section for handling the ARO.

   In addition to the normal validation of an NA and its options, the
   ARO (if present) is verified as follows.  If the Length field is not
   two, the option is silently ignored.  If the EUI-64 field does not
   match the EUI-64 of the interface, the option is silently ignored.

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   If the Status field is zero, then the address registration was
   successful.  The host saves the Registration Lifetime from the ARO
   for use to trigger a new NS well before the lifetime expires.  If the
   Status field is not equal to zero, the address registration has
   failed.

5.5.3.  Recovering from Failures

   The procedure for maintaining reachability information about a
   neighbor is the same as in [RFC4861] Section 7.3, with the exception
   that address resolution is not performed.

   The address registration procedure may fail for two reasons: no
   response to NSs is received (NUD failure), or an ARO with a failure
   Status (Status > 0) is received.  In the case of NUD failure, the
   entry for that router will be removed; thus, address registration is
   no longer of importance.  When an ARO with a non-zero Status field is
   received, this indicates that registration for that address has
   failed.  A failure Status of one indicates that a duplicate address
   was detected, and the procedure described in [RFC4862] Section 5.4.5
   is followed.  The host MUST NOT use the address it tried to register.
   If the host has valid registrations with other routers, these MUST be
   removed by registering with each using a zero ARO lifetime.

   A Status code of two indicates that the Neighbor Cache of that router
   is full.  In this case, the host SHOULD remove this router from its
   default router list and attempt to register with another router.  If
   the host's default router list is empty, it needs to revert to
   sending RSs as specified in Section 5.3.

   Other failure codes may be defined in future documents.

5.6.  Next-Hop Determination

   The IP address of the next hop for a destination is determined as
   follows.  Destinations to the link-local prefix (fe80::) are always
   sent on the link to that destination.  It is assumed that link-local
   addresses are formed as specified in Section 5.2 from the EUI-64, and
   address resolution is not performed.  Packets are sent to link-local
   destinations by reversing the procedure in Appendix A of [RFC4291].

   Multicast addresses are considered to be on-link and are resolved as
   specified in [RFC4944] or the appropriate IP-over-foo document.  Note
   that [RFC4944] only defines how to represent a multicast destination
   address in the LoWPAN header.  Support for multicast scopes larger
   than link-local needs an appropriate multicast routing algorithm.

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   All other prefixes are assumed to be off-link [RFC5889].  Anycast
   addresses are always considered to be off-link.  They are therefore
   sent to one of the routers in the default router list.

   A LoWPAN node is not required to maintain a minimum of one buffer per
   neighbor as specified in [RFC4861], since packets are never queued
   while waiting for address resolution.

5.7.  Address Resolution

   The address registration mechanism and the SLLAO in RA messages
   provide sufficient a priori state in routers and hosts to resolve an
   IPv6 address to its associated link-layer address.  As all prefixes
   except the link-local prefix and multicast addresses are always
   assumed to be off-link, multicast-based address resolution between
   neighbors is not needed.

   Link-layer addresses for neighbors are stored in NCEs [RFC4861].  In
   order to achieve LoWPAN compression, most global addresses are formed
   using a link-layer address.  Thus, a host can reduce memory usage by
   optimizing for this case and only storing link-layer address
   information if it differs from the link-layer address corresponding
   to the Interface ID of the IPv6 address (i.e., differs in more than
   the on-link/global bit being inverted).

5.8.  Sleeping

   It is often advantageous for battery-powered hosts in LoWPANs to keep
   a low duty cycle.  The optimizations described in this document
   enable hosts to sleep, as further described in this section.  Routers
   may want to cache traffic destined to a host that is sleeping, but
   such functionality is out of the scope of this document.

5.8.1.  Picking an Appropriate Registration Lifetime

   As all ND messages are initiated by the hosts, this allows a host to
   sleep or otherwise be unreachable between NS/NA message exchanges.
   The ARO attached to NS messages indicates to a router to keep the NCE
   for that address valid for the period in the Registration Lifetime
   field.  A host should choose a sleep time appropriate for its energy
   characteristics and set a Registration Lifetime larger than the sleep
   time to ensure that the registration is renewed successfully
   (considering, for example, clock drift and additional time for
   potential retransmissions of the re-registration).  External
   configuration of a host should also consider the stability of the
   network (how quickly the topology changes) when choosing its sleep

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   time (and thus Registration Lifetime).  A dynamic network requires a
   shorter sleep time so that routers don't keep invalid NCEs for nodes
   longer than necessary.

5.8.2.  Behavior on Wakeup

   When a host wakes up from a sleep period, it SHOULD refresh its
   current address registrations that will time out before the next
   wakeup.  This is done by sending NS messages with an ARO as described
   in Section 5.5.1.  The host may also need to refresh its prefix and
   context information by sending a new unicast RS (the maximum Router
   Lifetime is about 18 hours, whereas the maximum Registration Lifetime
   is about 45.5 days).  If after wakeup the host (using NUD) determines
   that some or all previous default routers have become unreachable,
   then the host will send multicast RSs to discover new default
   router(s) and restart the address registration process.

6.  Router Behavior for 6LRs and 6LBRs

   Both 6LRs and 6LBRs maintain the Neighbor Cache [RFC4861] based on
   the AROs they receive in NA messages from hosts, ND packets from
   other nodes, and, potentially, a routing protocol used in the 6LoWPAN
   as outlined in Section 3.5.

   The routers SHOULD NOT garbage-collect Registered NCEs (see
   Section 3.4), since they need to retain them until the Registration
   Lifetime expires.  Similarly, if NUD on the router determines that
   the host is UNREACHABLE (based on the logic in [RFC4861]), the NCE
   SHOULD NOT be deleted but rather retained until the Registration
   Lifetime expires.  A renewed ARO should mark the cache entry as
   STALE.  Thus, for 6LoWPAN routers, the Neighbor Cache doesn't behave
   like a cache.  Instead, it behaves as a registry of all the host
   addresses that are attached to the router.

   Routers MAY implement the Default Router Preference (Prf) extension
   [RFC4191] and use that to indicate to the host whether the router is
   a 6LBR or a 6LR.  If this is implemented, then 6LRs with no route to
   a border router MUST set Prf to (11) for low preference, other 6LRs
   MUST set Prf to (00) for normal preference, and 6LBRs MUST set Prf to
   (01) for high preference.

6.1.  Forbidden Actions

   Even if a router in a route-over topology can reach both a host and
   another target, because of radio propagation it generally cannot know
   whether the host can directly reach the other target.  Therefore, it
   cannot assume that Redirect will actually work from one host to
   another.  Therefore, it SHOULD NOT send Redirect messages.  The only

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   potential exception to this "SHOULD NOT" is when the deployment/
   implementation has a way to know how the host can reach the intended
   target.  Hence, it is RECOMMENDED that the implementation by default
   does not send Redirect messages but can be configurable when the
   deployment calls for this.  In contrast, for mesh-under topologies,
   the same considerations about Redirects apply as in [RFC4861].

   A router MUST NOT set the L (on-link) flag in the PIOs, since that
   might trigger hosts to send multicast NSs.

6.2.  Interface Initialization

   The 6LBR router interface initialization behavior is the same as in
   [RFC4861].  However, in a dynamic configuration scenario (see
   Section 8.1), a 6LR comes up as a non-router and waits to receive the
   advertisement for configuring its own interface address first, before
   setting its interfaces to be advertising interfaces and turning into
   a router.

6.3.  Processing a Router Solicitation

   A router processes RS messages as specified in [RFC4861].  The
   differences relate to the inclusion of ABROs in the RA messages and
   the exclusive use of unicast RAs.  If a 6LR has received an ABRO from
   a 6LBR, then it will include that option unmodified in the RA
   messages it sends.  And, if the 6LR has received RAs -- whether with
   the same prefixes and context information or different -- from a
   different 6LBR, then it will need to keep those prefixes and that
   context information separately so that the RAs the 6LR sends will
   maintain the association between the ABRO and the prefixes and
   context information.  The router can tell which 6LBR originated the
   prefixes and context information from the 6LBR Address field in the
   ABRO.  When a router has information tied to multiple ABROs, a single
   RS will result in multiple RAs each containing a different ABRO.

   When the ABRO Valid Lifetime associated with a 6LBR times out, all
   information related to that 6LBR MUST be removed.  As an
   implementation note, it is recommended that RAs are sent sufficiently
   more frequently than the ABRO Valid Lifetime so that missing an RA
   does not result in removing all information related to a 6LBR.

   An RS might be received from a host that has not yet registered its
   address with the router.  Thus, the router MUST NOT modify an
   existing NCE based on the SLLAO from the RS.  However, a router MAY
   create a Tentative NCE based on the SLLAO.  Such a Tentative NCE
   SHOULD be timed out in TENTATIVE_NCE_LIFETIME seconds, unless a
   registration converts it into a Registered NCE.

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   A 6LR or 6LBR MUST include an SLLAO in the RAs it sends; this is
   required so that the hosts will know the link-layer address of the
   router.  Unlike in [RFC4861], the maximum value of the RA Router
   Lifetime field MAY be up to 0xFFFF (approximately 18 hours).

   Unlike [RFC4861], which suggests multicast RAs, this specification
   improves the exchange by always unicasting RAs in response to RSs.
   This is possible, since the RS always includes an SLLAO, which is
   used by the router to unicast the RA.

6.4.  Periodic Router Advertisements

   A router does not need to send any periodic RA messages, since the
   hosts will solicit updated information by sending RSs before the
   lifetimes expire.

   However, if the routers use RAs to distribute prefix and/or context
   information across a route-over topology, that might require periodic
   RA messages.  Such RAs are sent using the configurable
   MinRtrAdvInterval and MaxRtrAdvInterval as per [RFC4861].

6.5.  Processing a Neighbor Solicitation

   A router handles NS messages as specified in [RFC4861], with added
   logic described in this section for handling the ARO.

   In addition to the normal validation of an NS and its options, the
   ARO is verified as follows (if present).  If the Length field is not
   two, or if the Status field is not zero, then the NS is silently
   ignored.

   If the source address of the NS is the unspecified address, or if no
   SLLAO is included, then any included ARO is ignored, that is, the NS
   is processed as if it did not contain an ARO.

6.5.1.  Checking for Duplicates

   If the NS contains a valid ARO, then the router inspects its Neighbor
   Cache on the arriving interface to see if it is a duplicate.  It
   isn't a duplicate if (1) there is no NCE for the IPv6 source address
   of the NS or (2) there is such an NCE and the EUI-64 is the same.
   Otherwise, it is a duplicate address.  Note that if multihop DAD
   (Section 8.2) is used, then the checks are slightly different, to
   take into account Tentative NCEs.  In the case where it is a
   duplicate address, then the router responds with a unicast NA message
   with the ARO Status field set to one (to indicate that the address is
   a duplicate) as described in Section 6.5.2.  In this case, there is
   no modification to the Neighbor Cache.

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6.5.2.  Returning Address Registration Errors

   Address registration errors are not sent back to the source address
   of the NS due to a possible risk of L2 address collision.  Instead,
   the NA is sent to the link-local IPv6 address with the Interface ID
   part derived from the EUI-64 field of the ARO as per [RFC4944].  In
   particular, this means that the universal/local bit needs to be
   inverted.  The NA is formatted with a copy of the ARO from the NS,
   but with the Status field set to indicate the appropriate error.

   The error is sent to the link-local address with the Interface ID
   derived from the EUI-64.  Thus, if the ARO was from and for a short
   address, the L2 destination address for the NA with the ARO error
   will be the 64-bit unique address.

6.5.3.  Updating the Neighbor Cache

   If the ARO did not result in a duplicate address being detected as
   above, then if the Registration Lifetime is non-zero the router
   creates (if it didn't exist) or updates (otherwise) an NCE for the
   IPv6 source address of the NS.  If the Neighbor Cache is full and a
   new entry needs to be created, then the router responds with a
   unicast NA with the ARO Status field set to two (to indicate that the
   router's Neighbor Cache is full) as described in Section 6.5.2.

   The Registration Lifetime and the EUI-64 are recorded in the NCE.  A
   unicast NA is then sent in response to the NS.  This NA SHOULD
   include a copy of the ARO, with the Status field set to zero.  A
   TLLAO (Target Link-Layer Address Option) [RFC4861] is not required in
   the NA, since the host already knows the router's link-layer address
   from RAs.

   If the ARO contains a zero Registration Lifetime, then any existing
   NCE for the IPv6 source address of the NS MUST be deleted and an NA
   sent as above.

   Should the Registration Lifetime in an NCE expire, then the router
   MUST delete the cache entry.

   The addition and removal of Registered NCEs would result in notifying
   the routing protocol.

   Note: If the substitutable multihop DAD (Section 8.2) is used, then
   the updating of the Neighbor Cache is slightly different due to
   Tentative NCEs.

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6.5.4.  Next-Hop Determination

   In order to deliver a packet destined for a 6LN registered with a
   router, next-hop determination is slightly different for routers than
   for hosts (see Section 5.6).  The routing table is checked to
   determine the next-hop IP address.  A Registered NCE determines if
   the next-hop IP address is on-link.  It is the responsibility of the
   routing protocol of the router to maintain on-link information about
   its registered neighbors.  Tentative NCEs MUST NOT be used to
   determine on-link status of the registered nodes.

6.5.5.  Address Resolution between Routers

   There needs to be a mechanism somewhere for the routers to discover
   each other's link-layer addresses.  If the routing protocol used
   between the routers provides this, then there is no need for the
   routers to use the ARO between each other.  Otherwise, the routers
   SHOULD use the ARO.  When routers use the ARO to register with each
   other and multihop DAD (Section 8.2) is in use, then care must be
   taken to ensure that there isn't a flood of ARO-carrying messages
   sent to the 6LBR as each router hears an ARO from their neighboring
   routers.  The details for this scenario are out of scope of this
   document.

   Routers MAY also use multicast NSs as in [RFC4861] to resolve each
   others link-layer addresses.  Thus, routers MAY multicast NSs for
   other routers, for example, as a result of receiving some routing
   protocol update.  Routers MUST respond to multicast NSs.  This
   implies that routers MUST join the solicited-node multicast addresses
   as specified in [RFC4861].

7.  Border Router Behavior

   A 6LBR handles the sending of RAs and processing of NSs from hosts as
   specified above in Section 6.  A 6LBR SHOULD always include an ABRO
   in the RAs it sends, listing itself as the 6LBR address.  This
   requires that the 6LBR maintain the version number in stable storage
   and increase the version number when some information in its RAs
   changes.  The information whose change affects the version is in the
   PIOs (the prefixes or their lifetimes) and in the 6CO (the prefixes,
   CIDs, or lifetimes).

   In addition, a 6LBR is somehow configured with the prefix or prefixes
   that are assigned to the LoWPAN and advertises those in RAs as in
   [RFC4861].  In the case of route-over, those prefixes can be
   disseminated to all the 6LRs using the technique discussed in

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   Section 8.1.  However, there might be mechanisms outside of the scope
   of this document that can be used as a substitute for prefix
   dissemination in the route-over topology (see Section 1.4).

   If the 6LoWPAN uses header compression [RFC6282] with context, then
   the 6LBR needs to manage the CIDs and advertise those in RAs by
   including 6COs in its RAs so that directly attached hosts are
   informed about the CIDs.  Below, we specify things to consider when
   the 6LBR needs to add, remove, or change the context information.  In
   the case of route-over, the context information is disseminated to
   all the 6LRs using the technique discussed in Section 8, unless a
   different specification provides a substitute for this multihop
   distribution.

7.1.  Prefix Determination

   The prefix or prefixes used in a LoWPAN can be manually configured or
   can be acquired using DHCPv6 Prefix Delegation [RFC3633].  For a
   LoWPAN that is isolated from the network either permanently or
   occasionally, the 6LBR can assign a ULA prefix using [RFC4193].  The
   ULA prefix should be stored in stable storage so that the same prefix
   is used after a failure of the 6LBR.  If the LoWPAN has multiple
   6LBRs, then they should be configured with the same set of prefixes.
   The set of prefixes is included in the RA messages as specified in
   [RFC4861].

7.2.  Context Configuration and Management

   If the LoWPAN uses header compression [RFC6282] with context, then
   the 6LBR must be configured with context information and related
   CIDs.  If the LoWPAN has multiple 6LBRs, then they MUST be configured
   with the same context information and CIDs.  As noted in [RFC6282],
   maintaining consistency of context information is crucial for
   ensuring that packets will be decompressed correctly.

   The context information carried in RA messages originates at 6LBRs
   and must be disseminated to all the routers and hosts within the
   LoWPAN.  RAs include one 6CO for each context.

   For the dissemination of context information using the 6CO, a strict
   life cycle SHOULD be used in order to ensure that the context
   information stays synchronized throughout the LoWPAN.  New context
   information SHOULD be introduced into the LoWPAN with C=0, to ensure
   that it is known by all nodes that may have to perform header
   decompression based on this context information.  Only when it is
   reasonable to assume that this information was successfully
   disseminated SHOULD an option with C=1 be sent, enabling the actual
   use of the context information for compression.

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   Conversely, to avoid the situation where nodes send packets that make
   use of previous values of contexts -- which would result in ambiguity
   when receiving a packet that uses a recently changed context -- old
   values of a context SHOULD be taken out of use for a while before new
   values are assigned to this specific context.  That is, in
   preparation for a change of context information, its dissemination
   SHOULD continue for at least MIN_CONTEXT_CHANGE_DELAY with C=0.  Only
   when it is reasonable to assume that the fact that the context is now
   invalid was successfully disseminated should the CID be taken out of
   dissemination or reused with a different Context Prefix field.  In
   the latter case, dissemination of the new value again SHOULD start
   with C=0, as above.

8.  Substitutable Feature Behavior

   Normally, in a 6LoWPAN multihop network, the RA messages are used to
   disseminate prefixes and context information to all the 6LRs in a
   route-over topology.  If all routers are configured to use a
   substitute mechanism for such information distribution, any remaining
   use of the 6LoWPAN-ND mechanisms is governed by the substitute
   specification.

   There is also the option for a 6LR to perform multihop DAD (for IPv6
   addresses not derived from an EUI-64) against a 6LBR in a route-over
   topology by using the DAR and DAC messages.  This is substitutable
   because there might be other ways to either allocate a unique
   address, such as DHCPv6 [RFC3315], or use other future mechanisms for
   multihop DAD.  Again, in this case, any remaining use of the
   6LoWPAN-ND mechanisms is governed by the substitute specification.

   To be clear: Implementations MUST support the features described in
   Sections 8.1 and 8.2, unless the implementation supports some
   alternative ("substitute") from some other specification.

8.1.  Multihop Prefix and Context Distribution

   The multihop distribution relies on RS messages and RA messages sent
   between routers, and using the ABRO version number to control the
   propagation of the information (prefixes and context information)
   that is being sent in the RAs.

   This multihop distribution mechanism can handle arbitrary information
   from an arbitrary number of 6LBRs.  However, the semantics of the
   context information requires that all the 6LNs use the same
   information whether they send, forward, or receive compressed
   packets.  Thus, the manager of the 6LBRs needs to somehow ensure that
   the context information is in synchrony across the 6LBRs.  This can
   be handled in different ways.  One possible way to ensure it is to

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   treat the context and prefix information as originating from some
   logical or virtual source, which in essence means that it looks like
   the information is distributed from a single source.

   If a set of 6LBRs behave as a single one (using mechanisms out of
   scope of this document) so that the prefixes and contexts and the
   ABRO version number will be the same from all the 6LBRs, then those
   6LBRs can pick a single IP address to use in the ABRO.

8.1.1.  6LBRs Sending Router Advertisements

   6LBRs supporting multihop prefix and context distribution MUST
   include an ABRO in each of their RAs.  The ABRO Version Number field
   is used to keep prefix and context information consistent throughout
   the LoWPAN, along with the guidelines in Section 7.2.  Each time any
   information in the set of PIOs or 6COs changes, the ABRO version is
   increased by one.

   This requires that the 6LBR maintain the PIO, 6CO, and ABRO Version
   Number in stable storage, since an old version number will be
   silently ignored by the 6LRs.

8.1.2.  Routers Sending Router Solicitations

   In a 6LoWPAN, unless substituted, multihop distribution is done using
   RA messages.  Thus, on interface initialization, a router (6LR) MUST
   send RS messages following the rules specified for hosts in
   [RFC4861].  This in turn will cause the routers to respond with RA
   messages that can then be used to initially seed the prefix and
   context information.

8.1.3.  Routers Processing Router Advertisements

   If multihop distribution is not done using RA messages, then the
   routers follow [RFC4861], which states that they merely do some
   consistency checks; in this case, nothing in Section 8.1 applies.
   Otherwise, the routers will check and record the prefix and context
   information from the received RAs, and use that information as
   follows.

   If a received RA does not contain an ABRO, then the RA MUST be
   silently ignored.

   The router uses the 6LBR Address field in the ABRO to check if it has
   previously received information from the 6LBR.  If it finds no such
   information, then it just records the 6LBR address, Version, Valid
   Lifetime, and the associated prefixes and context information.  If
   the 6LBR is previously known, then the Version Number field MUST be

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   compared against the recorded version number for that 6LBR.  If the
   version number received in the packet is less than the stored version
   number, then the information in the RA is silently ignored.
   Otherwise, the recorded information and version number are updated.

8.1.4.  Storing the Information

   The router keeps state for each 6LBR that it sees with an ABRO.  This
   includes the version number, the Valid Lifetime, and the complete set
   of PIOs and 6COs.  The prefixes are timed out based on the Valid
   Lifetime in the PIO.  The Context Prefix is timed out based on the
   Valid Lifetime in the 6CO.

   While the prefixes and context information are stored in the router,
   their valid and preferred lifetimes are decremented as time passes.
   This ensures that when the router is in turn later advertising that
   information in the RAs it sends, the 'expiry time' doesn't
   accidentally move further into the future.  For example, if a 6CO
   with a Valid Lifetime of 10 minutes is received at time T, and the
   router includes this in an RA it sends at time T+5 minutes, the Valid
   Lifetime in the 6CO it sends will be only 5 minutes.

8.1.5.  Sending Router Advertisements

   When multihop distribution is performed using RA messages, the
   routers MUST ensure that the ABRO always stays together with the
   prefixes and context information received with that ABRO.  Thus, if
   the router has received prefix P1 with an ABRO saying it is from one
   6LBR, and prefix P2 from another 6LBR, then the router MUST NOT
   include the two prefixes in the same RA message.  Prefix P1 MUST be
   in an RA that includes an ABRO from the first 6LBR, etc.  Note that
   multiple 6LBRs might advertise the same prefix and context
   information, but they still need to be associated with the 6LBRs that
   advertised them.

   The routers periodically send RAs as in [RFC4861].  This is for the
   benefit of the other routers receiving the prefixes and context
   information.  The routers also respond to RSs by unicasting RA
   messages.  In both cases, the above constraint of keeping the ABRO
   together with 'its' prefixes and context information applies.

   When a router receives new information from a 6LBR, that is, either
   it hears from a new 6LBR (a new 6LBR address in the ABRO) or the ABRO
   version number of an existing 6LBR has increased, then it is useful
   to send out a few triggered updates.  The recommendation is to behave
   the same as when an interface has become an advertising interface as
   described in [RFC4861], that is, send up to three RA messages.  This
   ensures rapid propagation of new information to all the 6LRs.

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8.2.  Multihop Duplicate Address Detection

   The ARO can be used, in addition to registering an address in a 6LR,
   to have the 6LR verify that the address isn't used by some other host
   known to the 6LR.  However, that isn't sufficient in a route-over
   topology (or in a LoWPAN with multiple 6LBRs), since some host
   attached to another 6LR could be using the same address.  There might
   be different ways for the 6LRs to coordinate such duplicate address
   detection in the future, or addresses could be assigned using a
   DHCPv6 server that verifies uniqueness as part of the assignment.

   This specification offers a substitutable simple technique for 6LRs
   and 6LBRs to perform DAD that reuses the information from the ARO in
   the DAR and DAC messages.  This technique is not needed when the
   Interface ID in the address is based on an EUI-64, since those are
   assumed to be globally unique.  The technique assumes that either the
   6LRs register with all the 6LBRs or the network uses some out-of-
   scope mechanism to keep the DAD tables in the 6LBRs synchronized.

   The multihop DAD mechanism is used synchronously the first time an
   address is registered with a particular 6LR.  That is, the ARO is not
   returned to the host until multihop DAD has been completed against
   the 6LBRs.  For existing registrations in the 6LR, multihop DAD needs
   to be repeated against the 6LBRs to ensure that the entry for the
   address in the 6LBRs does not time out, but that can be done
   asynchronously with the response to the hosts.  One method to achieve
   this is to track how much is left of the lifetime the 6LR registered
   with the 6LBRs and to re-register with the 6LBR when this lifetime is
   about to run out.

   For synchronous multihop DAD, the 6LR performs some additional checks
   to ensure that it has an NCE it can use to respond to the host when
   it receives a response from a 6LBR.  This consists of checking for an
   already existing (Tentative or Registered) NCE for the Registered
   Address with a different EUI-64.  If such a Registered NCE exists,
   then the 6LR SHOULD respond that the address is a duplicate.  If such
   a Tentative NCE exists, then the 6LR SHOULD silently ignore the ARO,
   thereby relying on the host retransmitting the ARO.  This is needed
   to handle the case when multiple hosts try to register the same IPv6
   address at the same time.  If no NCE exists, then the 6LR MUST create
   a Tentative NCE with the EUI-64 and the SLLAO.  This entry will be
   used to send the response to the host when the 6LBR responds
   positively.

   When a 6LR receives an NS containing an ARO with a non-zero
   Registration Lifetime and it has no existing Registered NCE, then
   with this mechanism the 6LR will invoke synchronous multihop DAD.

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   The 6LR will unicast a DAR message to one or more 6LBRs, where the
   DAR contains the host's address in the Registered Address field.  The
   DAR will be forwarded by 6LRs until it reaches the 6LBR; hence, its
   IPv6 Hop Limit field will not be 255 when received by the 6LBR.  The
   6LBR will respond with a DAC message, which will have a hop limit
   less than 255 when it reaches the 6LR.

   When the 6LR receives the DAC from the 6LBR, it will look for a
   matching (same IP address and EUI-64) (Tentative or Registered) NCE.
   If no such entry is found, then the DAC is silently ignored.  If an
   entry is found and the DAC had Status=0, then the 6LR will mark the
   Tentative NCE as Registered.  In all cases, when an entry is found,
   then the 6LR will respond to the host with an NA, copying the Status
   and EUI-64 fields from the DAC to an ARO in the NA.  In case the
   status is an error, then the destination IP address of the NA is
   derived from the EUI-64 field of the DAC.

   A Tentative NCE SHOULD be timed out TENTATIVE_NCE_LIFETIME seconds
   after it was created in order to allow for another host to attempt to
   register the IPv6 address.

8.2.1.  Message Validation for DAR and DAC

   A node MUST silently discard any received DAR and DAC messages for
   which at least one of the following validity checks is not satisfied:

   o  If the message includes an IP Authentication Header, the message
      authenticates correctly.

   o  ICMP Checksum is valid.

   o  ICMP Code is 0.

   o  ICMP Length (derived from the IP length) is 32 or more bytes.

   o  The Registered Address is not a multicast address.

   o  All included options have a length that is greater than zero.

   o  The IP source address is not the unspecified address, nor is it a
      multicast address.

   The contents of the Reserved field and of any unrecognized options
   MUST be ignored.  Future backward-compatible changes to the protocol
   may specify the contents of the Reserved field or add new options;
   backward-incompatible changes may use different Code values.

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   Note that due to the forwarding of the DAR and DAC messages between
   the 6LR and 6LBR, there is no hop-limit check on receipt for these
   ICMPv6 message types.

8.2.2.  Conceptual Data Structures

   A 6LBR implementing multihop DAD needs to maintain some state
   separate from the Neighbor Cache.  We call this conceptual data
   structure the DAD table.  It is indexed by the IPv6 address -- the
   Registered Address in the DAR -- and contains the EUI-64 and the
   Registration Lifetime of the host that is using that address.

8.2.3.  6LR Sending a Duplicate Address Request

   When a 6LR that implements multihop DAD receives an NS from a host,
   and subject to the above checks, the 6LR forms and sends a DAR to at
   least one 6LBR.  The DAR contains the following information:

   o  In the IPv6 source address, a global address of the 6LR.

   o  In the IPv6 destination address, the address of the 6LBR.

   o  In the IPv6 hop limit, MULTIHOP_HOPLIMIT.

   o  The Status field MUST be set to zero.

   o  The EUI-64 and Registration Lifetime are copied from the ARO
      received from the host.

   o  The Registered Address set to the IPv6 address of the host, that
      is, the sender of the triggering NS.

   When a 6LR receives an NS from a host with a zero Registration
   Lifetime, then, in addition to removing the NCE for the host as
   specified in Section 6, a DAR is sent to the 6LBRs as above.

   A router MUST NOT modify the Neighbor Cache as a result of receiving
   a DAR.

8.2.4.  6LBR Receiving a Duplicate Address Request

   When a 6LBR that implements the substitutable multihop DAD receives a
   DAR from a 6LR, it performs the message validation specified in
   Section 8.2.1.  If the DAR is valid, the 6LBR proceeds to look for
   the Registration Address in the DAD table.  If an entry is found and
   the recorded EUI-64 is different than the EUI-64 in the DAR, then it

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   returns a DAC NA with the Status set to 1 ('Duplicate Address').
   Otherwise, it returns a DAC with Status set to zero and updates the
   lifetime.

   If no entry is found in the DAD table and the Registration Lifetime
   is non-zero, then an entry is created and the EUI-64 and Registered
   Address from the DAR are stored in that entry.

   If an entry is found in the DAD table, the EUI-64 matches, and the
   Registration Lifetime is zero, then the entry is deleted from the
   table.

   In both of the above cases, the 6LBR forms a DAC with the information
   copied from the DAR and the Status field is set to zero.  The DAC is
   sent back to the 6LR, i.e., back to the source of the DAR.  The IPv6
   hop limit is set to MULTIHOP_HOPLIMIT.

8.2.5.  Processing a Duplicate Address Confirmation

   When a 6LR implementing multihop DAD receives a DAC message, then it
   first validates the message per Section 8.2.1.  For a valid DAC, if
   there is no Tentative NCE matching the Registered Address and EUI-64,
   then the DAC is silently ignored.  Otherwise, the information in the
   DAC and in the Tentative NCE is used to form an NA to send to the
   host.  The Status code is copied from the DAC to the ARO that is sent
   to the host.  In the case where the DAC indicates an error (the
   Status is non-zero), the NA is returned to the host as described in
   Section 6.5.2, and the Tentative NCE for the Registered Address is
   removed.  Otherwise, it is made into a Registered NCE.

   A router MUST NOT modify the Neighbor Cache as a result of receiving
   a DAC, unless there is a Tentative NCE matching the IPv6 address and
   EUI-64.

8.2.6.  Recovering from Failures

   If there is no response from a 6LBR after RETRANS_TIMER [RFC4861],
   then the 6LR would retransmit the DAR to the 6LBR up to
   MAX_UNICAST_SOLICIT [RFC4861] times.  After this, the 6LR SHOULD
   respond to the host with an ARO Status of zero.


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