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RFC 4861

 
 
 

Neighbor Discovery for IP version 6 (IPv6)

Part 3 of 5, p. 38 to 58
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6.  Router and Prefix Discovery

   This section describes router and host behavior related to the Router
   Discovery portion of Neighbor Discovery.  Router Discovery is used to
   locate neighboring routers as well as learn prefixes and
   configuration parameters related to stateless address
   autoconfiguration.

   Prefix Discovery is the process through which hosts learn the ranges
   of IP addresses that reside on-link and can be reached directly
   without going through a router.  Routers send Router Advertisements
   that indicate whether the sender is willing to be a default router.
   Router Advertisements also contain Prefix Information options that
   list the set of prefixes that identify on-link IP addresses.

   Stateless Address Autoconfiguration must also obtain subnet prefixes
   as part of configuring addresses.  Although the prefixes used for
   address autoconfiguration are logically distinct from those used for
   on-link determination, autoconfiguration information is piggybacked
   on Router Discovery messages to reduce network traffic.  Indeed, the
   same prefixes can be advertised for on-link determination and address
   autoconfiguration by specifying the appropriate flags in the Prefix
   Information options.  See [ADDRCONF] for details on how
   autoconfiguration information is processed.

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6.1.  Message Validation

6.1.1.  Validation of Router Solicitation Messages

   Hosts MUST silently discard any received Router Solicitation
   Messages.

   A router MUST silently discard any received Router Solicitation
   messages that do not satisfy all of the following validity checks:

      - The IP Hop Limit field has a value of 255, i.e., the packet
        could not possibly have been forwarded by a router.

      - ICMP Checksum is valid.

      - ICMP Code is 0.

      - ICMP length (derived from the IP length) is 8 or more octets.

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

      - If the IP source address is the unspecified address, there is no
        source link-layer address option in the message.

   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.

   The contents of any defined options that are not specified to be used
   with Router Solicitation messages MUST be ignored and the packet
   processed as normal.  The only defined option that may appear is the
   Source Link-Layer Address option.

   A solicitation that passes the validity checks is called a "valid
   solicitation".

6.1.2.  Validation of Router Advertisement Messages

   A node MUST silently discard any received Router Advertisement
   messages that do not satisfy all of the following validity checks:

      - IP Source Address is a link-local address.  Routers must use
        their link-local address as the source for Router Advertisement
        and Redirect messages so that hosts can uniquely identify
        routers.

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      - The IP Hop Limit field has a value of 255, i.e., the packet
        could not possibly have been forwarded by a router.

      - ICMP Checksum is valid.

      - ICMP Code is 0.

      - ICMP length (derived from the IP length) is 16 or more octets.

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

   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.

   The contents of any defined options that are not specified to be used
   with Router Advertisement messages MUST be ignored and the packet
   processed as normal.  The only defined options that may appear are
   the Source Link-Layer Address, Prefix Information and MTU options.

   An advertisement that passes the validity checks is called a "valid
   advertisement".

6.2.  Router Specification

6.2.1.  Router Configuration Variables

   A router MUST allow for the following conceptual variables to be
   configured by system management.  The specific variable names are
   used for demonstration purposes only, and an implementation is not
   required to have them, so long as its external behavior is consistent
   with that described in this document.  Default values are specified
   to simplify configuration in common cases.

   The default values for some of the variables listed below may be
   overridden by specific documents that describe how IPv6 operates over
   different link layers.  This rule simplifies the configuration of
   Neighbor Discovery over link types with widely differing performance
   characteristics.

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   For each interface:

      IsRouter       A flag indicating whether routing is enabled on
                     this interface.  Enabling routing on the interface
                     would imply that a router can forward packets to or
                     from the interface.

                     Default: FALSE

      AdvSendAdvertisements
                     A flag indicating whether or not the router sends
                     periodic Router Advertisements and responds to
                     Router Solicitations.

                     Default: FALSE

                     Note that AdvSendAdvertisements MUST be FALSE by
                     default so that a node will not accidentally start
                     acting as a router unless it is explicitly
                     configured by system management to send Router
                     Advertisements.

      MaxRtrAdvInterval
                     The maximum time allowed between sending
                     unsolicited multicast Router Advertisements from
                     the interface, in seconds.  MUST be no less than 4
                     seconds and no greater than 1800 seconds.

                     Default: 600 seconds

      MinRtrAdvInterval
                     The minimum time allowed between sending
                     unsolicited multicast Router Advertisements from
                     the interface, in seconds.  MUST be no less than 3
                     seconds and no greater than .75 *
                     MaxRtrAdvInterval.

                     Default: 0.33 * MaxRtrAdvInterval If
                     MaxRtrAdvInterval >= 9 seconds; otherwise, the
                     Default is MaxRtrAdvInterval.

      AdvManagedFlag
                     The TRUE/FALSE value to be placed in the "Managed
                     address configuration" flag field in the Router
                     Advertisement.  See [ADDRCONF].

                     Default: FALSE

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      AdvOtherConfigFlag
                     The TRUE/FALSE value to be placed in the "Other
                     configuration" flag field in the Router
                     Advertisement.  See [ADDRCONF].

                     Default: FALSE

      AdvLinkMTU     The value to be placed in MTU options sent by the
                     router.  A value of zero indicates that no MTU
                     options are sent.

                     Default: 0

      AdvReachableTime
                     The value to be placed in the Reachable Time field
                     in the Router Advertisement messages sent by the
                     router.  The value zero means unspecified (by this
                     router).  MUST be no greater than 3,600,000
                     milliseconds (1 hour).

                     Default: 0

      AdvRetransTimer The value to be placed in the Retrans Timer field
                     in the Router Advertisement messages sent by the
                     router.  The value zero means unspecified (by this
                     router).

                     Default: 0

      AdvCurHopLimit
                     The default value to be placed in the Cur Hop Limit
                     field in the Router Advertisement messages sent by
                     the router.  The value should be set to the current
                     diameter of the Internet.  The value zero means
                     unspecified (by this router).

                     Default:  The value specified in the "Assigned
                     Numbers" [ASSIGNED] that was in effect at the time
                     of implementation.

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      AdvDefaultLifetime
                     The value to be placed in the Router Lifetime field
                     of Router Advertisements sent from the interface,
                     in seconds.  MUST be either zero or between
                     MaxRtrAdvInterval and 9000 seconds.  A value of
                     zero indicates that the router is not to be used as
                     a default router.  These limits may be overridden
                     by specific documents that describe how IPv6
                     operates over different link layers.  For instance,
                     in a point-to-point link the peers may have enough
                     information about the number and status of devices
                     at the other end so that advertisements are needed
                     less frequently.

                     Default: 3 * MaxRtrAdvInterval

      AdvPrefixList
                     A list of prefixes to be placed in Prefix
                     Information options in Router Advertisement
                     messages sent from the interface.

                     Default: all prefixes that the router advertises
                     via routing protocols as being on-link for the
                     interface from which the advertisement is sent.
                     The link-local prefix SHOULD NOT be included in the
                     list of advertised prefixes.

                     Each prefix has an associated:

                        AdvValidLifetime
                             The value to be placed in the Valid
                             Lifetime in the Prefix Information option,
                             in seconds.  The designated value of all
                             1's (0xffffffff) represents infinity.
                             Implementations MAY allow AdvValidLifetime
                             to be specified in two ways:

                               - a time that decrements in real time,
                                 that is, one that will result in a
                                 Lifetime of zero at the specified time
                                 in the future, or

                               - a fixed time that stays the same in
                                 consecutive advertisements.

                             Default: 2592000 seconds (30 days), fixed
                             (i.e., stays the same in consecutive
                             advertisements).

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                        AdvOnLinkFlag
                             The value to be placed in the on-link flag
                             ("L-bit") field in the Prefix Information
                             option.

                             Default: TRUE

                   Stateless address configuration [ADDRCONF] defines
                   additional information associated with each of the
                   prefixes:

                        AdvPreferredLifetime
                             The value to be placed in the Preferred
                             Lifetime in the Prefix Information option,
                             in seconds.  The designated value of all
                             1's (0xffffffff) represents infinity.  See
                             [ADDRCONF] for details on how this value is
                             used.  Implementations MAY allow
                             AdvPreferredLifetime to be specified in two
                             ways:

                               - a time that decrements in real time,
                                 that is, one that will result in a
                                 Lifetime of zero at a specified time in
                                 the future, or

                               - a fixed time that stays the same in
                                 consecutive advertisements.

                             Default: 604800 seconds (7 days), fixed
                             (i.e., stays the same in consecutive
                             advertisements).  This value MUST NOT be
                             larger than AdvValidLifetime.

                        AdvAutonomousFlag
                             The value to be placed in the Autonomous
                             Flag field in the Prefix Information
                             option.  See [ADDRCONF].

                             Default: TRUE

   The above variables contain information that is placed in outgoing
   Router Advertisement messages.  Hosts use the received information to
   initialize a set of analogous variables that control their external
   behavior (see Section 6.3.2).  Some of these host variables (e.g.,
   CurHopLimit, RetransTimer, and ReachableTime) apply to all nodes
   including routers.  In practice, these variables may not actually be
   present on routers, since their contents can be derived from the

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   variables described above.  However, external router behavior MUST be
   the same as host behavior with respect to these variables.  In
   particular, this includes the occasional randomization of the
   ReachableTime value as described in Section 6.3.2.

   Protocol constants are defined in Section 10.

6.2.2.  Becoming an Advertising Interface

   The term "advertising interface" refers to any functioning and
   enabled interface that has at least one unicast IP address assigned
   to it and whose corresponding AdvSendAdvertisements flag is TRUE.  A
   router MUST NOT send Router Advertisements out any interface that is
   not an advertising interface.

   An interface may become an advertising interface at times other than
   system startup.  For example:

      - changing the AdvSendAdvertisements flag on an enabled interface
        from FALSE to TRUE, or

      - administratively enabling the interface, if it had been
        administratively disabled, and its AdvSendAdvertisements flag is
        TRUE, or

      - enabling IP forwarding capability (i.e., changing the system
        from being a host to being a router), when the interface's
        AdvSendAdvertisements flag is TRUE.

   A router MUST join the all-routers multicast address on an
   advertising interface.  Routers respond to Router Solicitations sent
   to the all-routers address and verify the consistency of Router
   Advertisements sent by neighboring routers.

6.2.3.  Router Advertisement Message Content

   A router sends periodic as well as solicited Router Advertisements
   out its advertising interfaces.  Outgoing Router Advertisements are
   filled with the following values consistent with the message format
   given in Section 4.2:

      - In the Router Lifetime field: the interface's configured
        AdvDefaultLifetime.

      - In the M and O flags: the interface's configured AdvManagedFlag
        and AdvOtherConfigFlag, respectively.

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      - In the Cur Hop Limit field: the interface's configured
        CurHopLimit.

      - In the Reachable Time field: the interface's configured
        AdvReachableTime.

      - In the Retrans Timer field: the interface's configured
        AdvRetransTimer.

      - In the options:

           o Source Link-Layer Address option: link-layer address of the
             sending interface.  This option MAY be omitted to
             facilitate in-bound load balancing over replicated
             interfaces.

           o MTU option: the interface's configured AdvLinkMTU value if
             the value is non-zero.  If AdvLinkMTU is zero, the MTU
             option is not sent.

           o Prefix Information options: one Prefix Information option
             for each prefix listed in AdvPrefixList with the option
             fields set from the information in the AdvPrefixList entry
             as follows:

                - In the "on-link" flag: the entry's AdvOnLinkFlag.

                - In the Valid Lifetime field: the entry's
                  AdvValidLifetime.

                - In the "Autonomous address configuration" flag: the
                  entry's AdvAutonomousFlag.

                - In the Preferred Lifetime field: the entry's
                  AdvPreferredLifetime.

   A router might want to send Router Advertisements without advertising
   itself as a default router.  For instance, a router might advertise
   prefixes for stateless address autoconfiguration while not wishing to
   forward packets.  Such a router sets the Router Lifetime field in
   outgoing advertisements to zero.

   A router MAY choose not to include some or all options when sending
   unsolicited Router Advertisements.  For example, if prefix lifetimes
   are much longer than AdvDefaultLifetime, including them every few
   advertisements may be sufficient.  However, when responding to a
   Router Solicitation or while sending the first few initial

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   unsolicited advertisements, a router SHOULD include all options so
   that all information (e.g., prefixes) is propagated quickly during
   system initialization.

   If including all options causes the size of an advertisement to
   exceed the link MTU, multiple advertisements can be sent, each
   containing a subset of the options.

6.2.4.  Sending Unsolicited Router Advertisements

   A host MUST NOT send Router Advertisement messages at any time.

   Unsolicited Router Advertisements are not strictly periodic: the
   interval between subsequent transmissions is randomized to reduce the
   probability of synchronization with the advertisements from other
   routers on the same link [SYNC].  Each advertising interface has its
   own timer.  Whenever a multicast advertisement is sent from an
   interface, the timer is reset to a uniformly distributed random value
   between the interface's configured MinRtrAdvInterval and
   MaxRtrAdvInterval; expiration of the timer causes the next
   advertisement to be sent and a new random value to be chosen.

   For the first few advertisements (up to
   MAX_INITIAL_RTR_ADVERTISEMENTS) sent from an interface when it
   becomes an advertising interface, if the randomly chosen interval is
   greater than MAX_INITIAL_RTR_ADVERT_INTERVAL, the timer SHOULD be set
   to MAX_INITIAL_RTR_ADVERT_INTERVAL instead.  Using a smaller interval
   for the initial advertisements increases the likelihood of a router
   being discovered quickly when it first becomes available, in the
   presence of possible packet loss.

   The information contained in Router Advertisements may change through
   actions of system management.  For instance, the lifetime of
   advertised prefixes may change, new prefixes could be added, a router
   could cease to be a router (i.e., switch from being a router to being
   a host), etc.  In such cases, the router MAY transmit up to
   MAX_INITIAL_RTR_ADVERTISEMENTS unsolicited advertisements, using the
   same rules as when an interface becomes an advertising interface.

6.2.5.  Ceasing To Be an Advertising Interface

   An interface may cease to be an advertising interface, through
   actions of system management such as:

      - changing the AdvSendAdvertisements flag of an enabled interface
        from TRUE to FALSE, or

      - administratively disabling the interface, or

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      - shutting down the system.

   In such cases, the router SHOULD transmit one or more (but not more
   than MAX_FINAL_RTR_ADVERTISEMENTS) final multicast Router
   Advertisements on the interface with a Router Lifetime field of zero.
   In the case of a router becoming a host, the system SHOULD also
   depart from the all-routers IP multicast group on all interfaces on
   which the router supports IP multicast (whether or not they had been
   advertising interfaces).  In addition, the host MUST ensure that
   subsequent Neighbor Advertisement messages sent from the interface
   have the Router flag set to zero.

   Note that system management may disable a router's IP forwarding
   capability (i.e., changing the system from being a router to being a
   host), a step that does not necessarily imply that the router's
   interfaces stop being advertising interfaces.  In such cases,
   subsequent Router Advertisements MUST set the Router Lifetime field
   to zero.

6.2.6.  Processing Router Solicitations

   A host MUST silently discard any received Router Solicitation
   messages.

   In addition to sending periodic, unsolicited advertisements, a router
   sends advertisements in response to valid solicitations received on
   an advertising interface.  A router MAY choose to unicast the
   response directly to the soliciting host's address (if the
   solicitation's source address is not the unspecified address), but
   the usual case is to multicast the response to the all-nodes group.
   In the latter case, the interface's interval timer is reset to a new
   random value, as if an unsolicited advertisement had just been sent
   (see Section 6.2.4).

   In all cases, Router Advertisements sent in response to a Router
   Solicitation MUST be delayed by a random time between 0 and
   MAX_RA_DELAY_TIME seconds. (If a single advertisement is sent in
   response to multiple solicitations, the delay is relative to the
   first solicitation.)  In addition, consecutive Router Advertisements
   sent to the all-nodes multicast address MUST be rate limited to no
   more than one advertisement every MIN_DELAY_BETWEEN_RAS seconds.

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   A router might process Router Solicitations as follows:

    - Upon receipt of a Router Solicitation, compute a random delay
      within the range 0 through MAX_RA_DELAY_TIME.  If the computed
      value corresponds to a time later than the time the next multicast
      Router Advertisement is scheduled to be sent, ignore the random
      delay and send the advertisement at the already-scheduled time.

    - If the router sent a multicast Router Advertisement (solicited or
      unsolicited) within the last MIN_DELAY_BETWEEN_RAS seconds,
      schedule the advertisement to be sent at a time corresponding to
      MIN_DELAY_BETWEEN_RAS plus the random value after the previous
      advertisement was sent.  This ensures that the multicast Router
      Advertisements are rate limited.

    - Otherwise, schedule the sending of a Router Advertisement at the
      time given by the random value.

   Note that a router is permitted to send multicast Router
   Advertisements more frequently than indicated by the
   MinRtrAdvInterval configuration variable so long as the more frequent
   advertisements are responses to Router Solicitations.  In all cases,
   however, unsolicited multicast advertisements MUST NOT be sent more
   frequently than indicated by MinRtrAdvInterval.

   Router Solicitations in which the Source Address is the unspecified
   address MUST NOT update the router's Neighbor Cache; solicitations
   with a proper source address update the Neighbor Cache as follows.
   If the router already has a Neighbor Cache entry for the
   solicitation's sender, the solicitation contains a Source Link-Layer
   Address option, and the received link-layer address differs from that
   already in the cache, then the link-layer address SHOULD be updated
   in the appropriate Neighbor Cache entry, and its reachability state
   MUST also be set to STALE.  If there is no existing Neighbor Cache
   entry for the solicitation's sender, the router creates one, installs
   the link- layer address and sets its reachability state to STALE as
   specified in Section 7.3.3.  If there is no existing Neighbor Cache
   entry and no Source Link-Layer Address option was present in the
   solicitation, the router may respond with either a multicast or a
   unicast router advertisement.  Whether or not a Source Link-Layer
   Address option is provided, if a Neighbor Cache entry for the
   solicitation's sender exists (or is created) the entry's IsRouter
   flag MUST be set to FALSE.

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6.2.7.  Router Advertisement Consistency

   Routers SHOULD inspect valid Router Advertisements sent by other
   routers and verify that the routers are advertising consistent
   information on a link.  Detected inconsistencies indicate that one or
   more routers might be misconfigured and SHOULD be logged to system or
   network management.  The minimum set of information to check
   includes:

    - Cur Hop Limit values (except for the unspecified value of zero
      other inconsistencies SHOULD be logged to system network
      management).

    - Values of the M or O flags.

    - Reachable Time values (except for the unspecified value of zero).

    - Retrans Timer values (except for the unspecified value of zero).

    - Values in the MTU options.

    - Preferred and Valid Lifetimes for the same prefix.  If
      AdvPreferredLifetime and/or AdvValidLifetime decrement in real
      time as specified in Section 6.2.1 then the comparison of the
      lifetimes cannot compare the content of the fields in the Router
      Advertisement, but must instead compare the time at which the
      prefix will become deprecated and invalidated, respectively.  Due
      to link propagation delays and potentially poorly synchronized
      clocks between the routers such comparison SHOULD allow some time
      skew.

   Note that it is not an error for different routers to advertise
   different sets of prefixes.  Also, some routers might leave some
   fields as unspecified, i.e., with the value zero, while other routers
   specify values.  The logging of errors SHOULD be restricted to
   conflicting information that causes hosts to switch from one value to
   another with each received advertisement.

   Any other action on reception of Router Advertisement messages by a
   router is beyond the scope of this document.

6.2.8.  Link-local Address Change

   The link-local address on a router should rarely change, if ever.
   Nodes receiving Neighbor Discovery messages use the source address to
   identify the sender.  If multiple packets from the same router
   contain different source addresses, nodes will assume they come from
   different routers, leading to undesirable behavior.  For example, a

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   node will ignore Redirect messages that are believed to have been
   sent by a router other than the current first-hop router.  Thus, the
   source address used in Router Advertisements sent by a particular
   router must be identical to the target address in a Redirect message
   when redirecting to that router.

   Using the link-local address to uniquely identify routers on the link
   has the benefit that the address a router is known by should not
   change when a site renumbers.

   If a router changes the link-local address for one of its interfaces,
   it SHOULD inform hosts of this change.  The router SHOULD multicast a
   few Router Advertisements from the old link-local address with the
   Router Lifetime field set to zero and also multicast a few Router
   Advertisements from the new link-local address.  The overall effect
   should be the same as if one interface ceases being an advertising
   interface, and a different one starts being an advertising interface.

6.3.  Host Specification

6.3.1.  Host Configuration Variables

   None.

6.3.2.  Host Variables

   A host maintains certain Neighbor-Discovery-related variables in
   addition to the data structures defined in Section 5.1.  The specific
   variable names are used for demonstration purposes only, and an
   implementation is not required to have them, so long as its external
   behavior is consistent with that described in this document.

   These variables have default values that are overridden by
   information received in Router Advertisement messages.  The default
   values are used when there is no router on the link or when all
   received Router Advertisements have left a particular value
   unspecified.

   The default values in this specification may be overridden by
   specific documents that describe how IP operates over different link
   layers.  This rule allows Neighbor Discovery to operate over links
   with widely varying performance characteristics.

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   For each interface:

        LinkMTU        The MTU of the link.
                       Default: The valued defined in the specific
                       document that describes how IPv6 operates over
                       the particular link layer (e.g., [IPv6-ETHER]).

        CurHopLimit    The default hop limit to be used when sending IP
                       packets.

                       Default: The value specified in the "Assigned
                       Numbers" [ASSIGNED] that was in effect at the
                       time of implementation.

        BaseReachableTime
                       A base value used for computing the random
                       ReachableTime value.

                       Default: REACHABLE_TIME milliseconds.

        ReachableTime  The time a neighbor is considered reachable after
                       receiving a reachability confirmation.

                       This value should be a uniformly distributed
                       random value between MIN_RANDOM_FACTOR and
                       MAX_RANDOM_FACTOR times BaseReachableTime
                       milliseconds.  A new random value should be
                       calculated when BaseReachableTime changes (due to
                       Router Advertisements) or at least every few
                       hours even if no Router Advertisements are
                       received.

        RetransTimer   The time between retransmissions of Neighbor
                       Solicitation messages to a neighbor when
                       resolving the address or when probing the
                       reachability of a neighbor.

                       Default: RETRANS_TIMER milliseconds

6.3.3.  Interface Initialization

   The host joins the all-nodes multicast address on all multicast-
   capable interfaces.

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6.3.4.  Processing Received Router Advertisements

   When multiple routers are present, the information advertised
   collectively by all routers may be a superset of the information
   contained in a single Router Advertisement.  Moreover, information
   may also be obtained through other dynamic means like DHCPv6.  Hosts
   accept the union of all received information; the receipt of a Router
   Advertisement MUST NOT invalidate all information received in a
   previous advertisement or from another source.  However, when
   received information for a specific parameter (e.g., Link MTU) or
   option (e.g., Lifetime on a specific Prefix) differs from information
   received earlier, and the parameter/option can only have one value,
   the most recently received information is considered authoritative.

   A Router Advertisement field (e.g., Cur Hop Limit, Reachable Time,
   and Retrans Timer) may contain a value denoting that it is
   unspecified.  In such cases, the parameter should be ignored and the
   host should continue using whatever value it is already using.  In
   particular, a host MUST NOT interpret the unspecified value as
   meaning change back to the default value that was in use before the
   first Router Advertisement was received.  This rule prevents hosts
   from continually changing an internal variable when one router
   advertises a specific value, but other routers advertise the
   unspecified value.

   On receipt of a valid Router Advertisement, a host extracts the
   source address of the packet and does the following:

      - If the address is not already present in the host's Default
        Router List, and the advertisement's Router Lifetime is non-
        zero, create a new entry in the list, and initialize its
        invalidation timer value from the advertisement's Router
        Lifetime field.

      - If the address is already present in the host's Default Router
        List as a result of a previously received advertisement, reset
        its invalidation timer to the Router Lifetime value in the newly
        received advertisement.

      - If the address is already present in the host's Default Router
        List and the received Router Lifetime value is zero, immediately
        time-out the entry as specified in Section 6.3.5.

   To limit the storage needed for the Default Router List, a host MAY
   choose not to store all of the router addresses discovered via
   advertisements.  However, a host MUST retain at least two router
   addresses and SHOULD retain more.  Default router selections are made
   whenever communication to a destination appears to be failing.  Thus,

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   the more routers on the list, the more likely an alternative working
   router can be found quickly (e.g., without having to wait for the
   next advertisement to arrive).

   If the received Cur Hop Limit value is non-zero, the host SHOULD set
   its CurHopLimit variable to the received value.

   If the received Reachable Time value is non-zero, the host SHOULD set
   its BaseReachableTime variable to the received value.  If the new
   value differs from the previous value, the host SHOULD re-compute a
   new random ReachableTime value.  ReachableTime is computed as a
   uniformly distributed random value between MIN_RANDOM_FACTOR and
   MAX_RANDOM_FACTOR times the BaseReachableTime.  Using a random
   component eliminates the possibility that Neighbor Unreachability
   Detection messages will synchronize with each other.

   In most cases, the advertised Reachable Time value will be the same
   in consecutive Router Advertisements, and a host's BaseReachableTime
   rarely changes.  In such cases, an implementation SHOULD ensure that
   a new random value gets re-computed at least once every few hours.

   The RetransTimer variable SHOULD be copied from the Retrans Timer
   field, if the received value is non-zero.

   After extracting information from the fixed part of the Router
   Advertisement message, the advertisement is scanned for valid
   options.  If the advertisement contains a Source Link-Layer Address
   option, the link-layer address SHOULD be recorded in the Neighbor
   Cache entry for the router (creating an entry if necessary) and the
   IsRouter flag in the Neighbor Cache entry MUST be set to TRUE.  If no
   Source Link-Layer Address is included, but a corresponding Neighbor
   Cache entry exists, its IsRouter flag MUST be set to TRUE.  The
   IsRouter flag is used by Neighbor Unreachability Detection to
   determine when a router changes to being a host (i.e., no longer
   capable of forwarding packets).  If a Neighbor Cache entry is created
   for the router, its reachability state MUST be set to STALE as
   specified in Section 7.3.3.  If a cache entry already exists and is
   updated with a different link-layer address, the reachability state
   MUST also be set to STALE.

   If the MTU option is present, hosts SHOULD copy the option's value
   into LinkMTU so long as the value is greater than or equal to the
   minimum link MTU [IPv6] and does not exceed the maximum LinkMTU value
   specified in the link-type-specific document (e.g., [IPv6-ETHER]).

   Prefix Information options that have the "on-link" (L) flag set
   indicate a prefix identifying a range of addresses that should be
   considered on-link.  Note, however, that a Prefix Information option

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   with the on-link flag set to zero conveys no information concerning
   on-link determination and MUST NOT be interpreted to mean that
   addresses covered by the prefix are off-link.  The only way to cancel
   a previous on-link indication is to advertise that prefix with the
   L-bit set and the Lifetime set to zero.  The default behavior (see
   Section 5.2) when sending a packet to an address for which no
   information is known about the on-link status of the address is to
   forward the packet to a default router; the reception of a Prefix
   Information option with the "on-link" (L) flag set to zero does not
   change this behavior.  The reasons for an address being treated as
   on-link is specified in the definition of "on-link" in Section 2.1.
   Prefixes with the on-link flag set to zero would normally have the
   autonomous flag set and be used by [ADDRCONF].

   For each Prefix Information option with the on-link flag set, a host
   does the following:

      - If the prefix is the link-local prefix, silently ignore the
        Prefix Information option.

      - If the prefix is not already present in the Prefix List, and the
        Prefix Information option's Valid Lifetime field is non-zero,
        create a new entry for the prefix and initialize its
        invalidation timer to the Valid Lifetime value in the Prefix
        Information option.

      - If the prefix is already present in the host's Prefix List as
        the result of a previously received advertisement, reset its
        invalidation timer to the Valid Lifetime value in the Prefix
        Information option.  If the new Lifetime value is zero, time-out
        the prefix immediately (see Section 6.3.5).

      - If the Prefix Information option's Valid Lifetime field is zero,
        and the prefix is not present in the host's Prefix List,
        silently ignore the option.

   Stateless address autoconfiguration [ADDRCONF] may in some
   circumstances use a larger Valid Lifetime of a prefix or ignore it
   completely in order to prevent a particular denial-of-service attack.
   However, since the effect of the same denial of service targeted at
   the on-link prefix list is not catastrophic (hosts would send packets
   to a default router and receive a redirect rather than sending
   packets directly to a neighbor), the Neighbor Discovery protocol does
   not impose such a check on the prefix lifetime values.  Similarly,
   [ADDRCONF] may impose certain restrictions on the prefix length for
   address configuration purposes.  Therefore, the prefix might be
   rejected by [ADDRCONF] implementation in the host.  However, the

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   prefix length is still valid for on-link determination when combined
   with other flags in the prefix option.

      Note: Implementations can choose to process the on-link aspects of
      the prefixes separately from the stateless address
      autoconfiguration aspects of the prefixes by, e.g., passing a copy
      of each valid Router Advertisement message to both an "on-link"
      and an "addrconf" function.  Each function can then operate
      independently on the prefixes that have the appropriate flag set.

6.3.5.  Timing out Prefixes and Default Routers

   Whenever the invalidation timer expires for a Prefix List entry, that
   entry is discarded.  No existing Destination Cache entries need be
   updated, however.  Should a reachability problem arise with an
   existing Neighbor Cache entry, Neighbor Unreachability Detection will
   perform any needed recovery.

   Whenever the Lifetime of an entry in the Default Router List expires,
   that entry is discarded.  When removing a router from the Default
   Router list, the node MUST update the Destination Cache in such a way
   that all entries using the router perform next-hop determination
   again rather than continue sending traffic to the (deleted) router.

6.3.6.  Default Router Selection

   The algorithm for selecting a router depends in part on whether or
   not a router is known to be reachable.  The exact details of how a
   node keeps track of a neighbor's reachability state are covered in
   Section 7.3.  The algorithm for selecting a default router is invoked
   during next-hop determination when no Destination Cache entry exists
   for an off-link destination or when communication through an existing
   router appears to be failing.  Under normal conditions, a router
   would be selected the first time traffic is sent to a destination,
   with subsequent traffic for that destination using the same router as
   indicated in the Destination Cache modulo any changes to the
   Destination Cache caused by Redirect messages.

   The policy for selecting routers from the Default Router List is as
   follows:

     1) Routers that are reachable or probably reachable (i.e., in any
        state other than INCOMPLETE) SHOULD be preferred over routers
        whose reachability is unknown or suspect (i.e., in the
        INCOMPLETE state, or for which no Neighbor Cache entry exists).
        Further implementation hints on default router selection when
        multiple equivalent routers are available are discussed in
        [LD-SHRE].

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     2) When no routers on the list are known to be reachable or
        probably reachable, routers SHOULD be selected in a round-robin
        fashion, so that subsequent requests for a default router do not
        return the same router until all other routers have been
        selected.

        Cycling through the router list in this case ensures that all
        available routers are actively probed by the Neighbor
        Unreachability Detection algorithm.  A request for a default
        router is made in conjunction with the sending of a packet to a
        router, and the selected router will be probed for reachability
        as a side effect.

6.3.7.  Sending Router Solicitations

   When an interface becomes enabled, a host may be unwilling to wait
   for the next unsolicited Router Advertisement to locate default
   routers or learn prefixes.  To obtain Router Advertisements quickly,
   a host SHOULD transmit up to MAX_RTR_SOLICITATIONS Router
   Solicitation messages, each separated by at least
   RTR_SOLICITATION_INTERVAL seconds.  Router Solicitations may be sent
   after any of the following events:

      - The interface is initialized at system startup time.

      - The interface is reinitialized after a temporary interface
        failure or after being temporarily disabled by system
        management.

      - The system changes from being a router to being a host, by
        having its IP forwarding capability turned off by system
        management.

      - The host attaches to a link for the first time.

      - The host re-attaches to a link after being detached for some
        time.

   A host sends Router Solicitations to the all-routers multicast
   address.  The IP source address is set to either one of the
   interface's unicast addresses or the unspecified address.  The Source
   Link-Layer Address option SHOULD be set to the host's link-layer
   address, if the IP source address is not the unspecified address.

   Before a host sends an initial solicitation, it SHOULD delay the
   transmission for a random amount of time between 0 and
   MAX_RTR_SOLICITATION_DELAY.  This serves to alleviate congestion when
   many hosts start up on a link at the same time, such as might happen

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   after recovery from a power failure.  If a host has already performed
   a random delay since the interface became (re)enabled (e.g., as part
   of Duplicate Address Detection [ADDRCONF]), there is no need to delay
   again before sending the first Router Solicitation message.

   In some cases, the random delay MAY be omitted if necessary.  For
   instance, a mobile node, using [MIPv6], moving to a new link would
   need to discover such movement as soon as possible to minimize the
   amount of packet losses resulting from the change in its topological
   movement.  Router Solicitations provide a useful tool for movement
   detection in Mobile IPv6 as they allow mobile nodes to determine
   movement to new links.  Hence, if a mobile node received link-layer
   information indicating that movement might have taken place, it MAY
   send a Router Solicitation immediately, without random delays.  The
   strength of such indications should be assessed by the mobile node's
   implementation depending on the level of certainty of the link-layer
   hints, and it is outside the scope of this specification.  Note that
   using this mechanism inappropriately (e.g., based on weak or
   transient indications) may result in Router Solicitation storms.
   Furthermore, simultaneous mobility of a large number of mobile nodes
   that use this mechanism can result in a large number of solicitations
   sent simultaneously.

   Once the host sends a Router Solicitation, and receives a valid
   Router Advertisement with a non-zero Router Lifetime, the host MUST
   desist from sending additional solicitations on that interface, until
   the next time one of the above events occurs.  Moreover, a host
   SHOULD send at least one solicitation in the case where an
   advertisement is received prior to having sent a solicitation.
   Responses to solicited advertisements may contain more information
   than unsolicited advertisements.

   If a host sends MAX_RTR_SOLICITATIONS solicitations, and receives no
   Router Advertisements after having waited MAX_RTR_SOLICITATION_DELAY
   seconds after sending the last solicitation, the host concludes that
   there are no routers on the link for the purpose of [ADDRCONF].
   However, the host continues to receive and process Router
   Advertisements messages in the event that routers appear on the link.


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