    Network Working Group                                        T. Narten
    Request for Comments: 2461                                         IBM
    Obsoletes: 1970                                            E. Nordmark
    Category: Standards Track                             Sun Microsystems
                                                                W. Simpson
                                                                Daydreamer
                                                             December 1998

                   Neighbor Discovery for IP Version 6 (IPv6)

    Status of this Memo

       This document specifies an Internet standards track protocol for the
       Internet community, and requests discussion and suggestions for
       improvements.  Please refer to the current edition of the "Internet
       Official Protocol Standards" (STD 1) for the standardization state
       and status of this protocol.  Distribution of this memo is unlimited.

    Copyright Notice

       Copyright (C) The Internet Society (1998).  All Rights Reserved.

    Abstract

       This document specifies the Neighbor Discovery protocol for IP
       Version 6.  IPv6 nodes on the same link use Neighbor Discovery to
       discover each other's presence, to determine each other's link-layer
       addresses, to find routers and to maintain reachability information
       about the paths to active neighbors.

       Table of Contents

       1.  INTRODUCTION.............................................    3
       2.  TERMINOLOGY..............................................    4
          2.1.  General.............................................    4
          2.2.  Link Types..........................................    7
          2.3.  Addresses...........................................    8
          2.4.  Requirements........................................    9
       3.  PROTOCOL OVERVIEW........................................    9
          3.1.  Comparison with IPv4................................   13
          3.2.  Supported Link Types................................   15
       4.  MESSAGE FORMATS..........................................   17
          4.1.  Router Solicitation Message Format..................   17
          4.2.  Router Advertisement Message Format.................   18
          4.3.  Neighbor Solicitation Message Format................   21
          4.4.  Neighbor Advertisement Message Format...............   23
          4.5.  Redirect Message Format.............................   26

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          4.6.  Option Formats......................................   28
             4.6.1.  Source/Target Link-layer Address...............   28
             4.6.2.  Prefix Information.............................   29
             4.6.3.  Redirected Header..............................   31
             4.6.4.  MTU............................................   32
       5.  CONCEPTUAL MODEL OF A HOST...............................   33
          5.1.  Conceptual Data Structures..........................   33
          5.2.  Conceptual Sending Algorithm........................   35
          5.3.  Garbage Collection and Timeout Requirements.........   37
       6.  ROUTER AND PREFIX DISCOVERY..............................   37
          6.1.  Message Validation..................................   38
             6.1.1.  Validation of Router Solicitation Messages.....   38
             6.1.2.  Validation of Router Advertisement Messages....   39
          6.2.  Router Specification................................   40
             6.2.1.  Router Configuration Variables.................   40
             6.2.2.  Becoming An Advertising Interface..............   44
             6.2.3.  Router Advertisement Message Content...........   44
             6.2.4.  Sending Unsolicited Router Advertisements......   46
             6.2.5.  Ceasing To Be An Advertising Interface.........   46
             6.2.6.  Processing Router Solicitations................   47
             6.2.7.  Router Advertisement Consistency...............   48
             6.2.8.  Link-local Address Change......................   49
          6.3.  Host Specification..................................   50
             6.3.1.  Host Configuration Variables...................   50
             6.3.2.  Host Variables.................................   50
             6.3.3.  Interface Initialization.......................   51
             6.3.4.  Processing Received Router Advertisements......   51
             6.3.5.  Timing out Prefixes and Default Routers........   54
             6.3.6.  Default Router Selection.......................   54
             6.3.7.  Sending Router Solicitations...................   55
       7.  ADDRESS RESOLUTION AND NEIGHBOR UNREACHABILITY DETECTION.   56
          7.1.  Message Validation..................................   57
             7.1.1.  Validation of Neighbor Solicitations...........   57
             7.1.2.  Validation of Neighbor Advertisements..........   58
          7.2.  Address Resolution..................................   58
             7.2.1.  Interface Initialization.......................   59
             7.2.2.  Sending Neighbor Solicitations.................   59
             7.2.3.  Receipt of Neighbor Solicitations..............   60
             7.2.4.  Sending Solicited Neighbor Advertisements......   61
             7.2.5.  Receipt of Neighbor Advertisements.............   62
             7.2.6.  Sending Unsolicited Neighbor Advertisements....   64
             7.2.7.  Anycast Neighbor Advertisements................   65
             7.2.8.  Proxy Neighbor Advertisements..................   65
          7.3.  Neighbor Unreachability Detection...................   66
             7.3.1.  Reachability Confirmation......................   66
             7.3.2.  Neighbor Cache Entry States....................   67
             7.3.3.  Node Behavior..................................   68
       8.  REDIRECT FUNCTION........................................   70

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          8.1.  Validation of Redirect Messages.....................   71
          8.2.  Router Specification................................   72
          8.3.  Host Specification..................................   73
       9.  EXTENSIBILITY - OPTION PROCESSING........................   74
       10.  PROTOCOL CONSTANTS......................................   75
       11.  SECURITY CONSIDERATIONS.................................   76
       12.  RENUMBERING CONSIDERATIONS..............................   78

       References...................................................   80
       Authors' Addresses...........................................   81
       Appendix A: Multihomed Hosts.................................   82
       Appendix B: Future Extensions................................   84
       Appendix C: State Machine for the Reachability State.........   85
       Appendix D: Summary of ISROUTER Rules........................   88
       Appendix E: Implementation Issues............................   89
           Appendix E.1: Reachability confirmations.................   89
       Appendix F: Changes since RFC 1970...........................   91
       Full Copyright Statement.....................................   93

    1.  INTRODUCTION

       This specification defines the Neighbor Discovery (ND) protocol for
       Internet Protocol Version 6 (IPv6).  Nodes (hosts and routers) use
       Neighbor Discovery to determine the link-layer addresses for
       neighbors known to reside on attached links and to quickly purge
       cached values that become invalid.  Hosts also use Neighbor Discovery
       to find neighboring routers that are willing to forward packets on
       their behalf.  Finally, nodes use the protocol to actively keep track
       of which neighbors are reachable and which are not, and to detect
       changed link-layer addresses.  When a router or the path to a router
       fails, a host actively searches for functioning alternates.

       Unless specified otherwise (in a document that covers operating IP
       over a particular link type) this document applies to all link types.
       However, because ND uses link-layer multicast for some of its
       services, it is possible that on some link types (e.g., NBMA links)
       alternative protocols or mechanisms to implement those services will
       be specified (in the appropriate document covering the operation of
       IP over a particular link type).  The services described in this
       document that are not directly dependent on multicast, such as
       Redirects, Next-hop determination, Neighbor Unreachability Detection,
       etc., are expected to be provided as specified in this document.  The
       details of how one uses ND on NBMA links is an area for further
       study.

       The authors would like to acknowledge the contributions of the IPNGWG
       working group and, in particular, (in alphabetical order) Ran
       Atkinson, Jim Bound, Scott Bradner, Alex Conta, Stephen Deering,

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    RFC 2461              Neighbor Discovery for IPv6          December 1998

       Richard Draves, Francis Dupont, Robert Elz, Robert Gilligan, Robert
       Hinden, Allison Mankin, Dan McDonald, Charles Perkins, Matt Thomas,
       and Susan Thomson.

    2.  TERMINOLOGY

    2.1.  General

       IP          - Internet Protocol Version 6.  The terms IPv4 and
                     IPv6 are used only in contexts where necessary to avoid
                     ambiguity.

       ICMP        - Internet Message Control Protocol for the Internet
                     Protocol Version 6.  The terms ICMPv4 and ICMPv6 are
                     used only in contexts where necessary to avoid
                     ambiguity.

       node        - a device that implements IP.

       router      - a node that forwards IP packets not explicitly
                     addressed to itself.

       host        - any node that is not a router.

       upper layer - a protocol layer immediately above IP.  Examples are
                     transport protocols such as TCP and UDP, control
                     protocols such as ICMP, routing protocols such as OSPF,
                     and internet or lower-layer protocols being "tunneled"
                     over (i.e., encapsulated in) IP such as IPX, AppleTalk,
                     or IP itself.

       link        - a communication facility or medium over which nodes can
                     communicate at the link layer, i.e., the layer
                     immediately below IP.  Examples are Ethernets (simple
                     or bridged), PPP links, X.25, Frame Relay, or ATM
                     networks as well as internet (or higher) layer
                     "tunnels", such as tunnels over IPv4 or IPv6 itself.

       interface   - a node's attachment to a link.

       neighbors   - nodes attached to the same link.

       address     - an IP-layer identifier for an interface or a set of
                     interfaces.

       anycast address
                   - an identifier for a set of interfaces (typically
                     belonging to different nodes).  A packet sent to an

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                     anycast address is delivered to one of the interfaces
                     identified by that address (the "nearest" one,
                     according to the routing protocol's measure of
                     distance).  See [ADDR-ARCH].

                     Note that an anycast address is syntactically
                     indistinguishable from a unicast address.  Thus, nodes
                     sending packets to anycast addresses don't generally
                     know that an anycast address is being used.  Throughout
                     the rest of this document, references to unicast
                     addresses also apply to anycast addresses in those
                     cases where the node is unaware that a unicast address
                     is actually an anycast address.

       prefix      - a bit string that consists of some number of initial
                     bits of an address.

       link-layer address
                   - a link-layer identifier for an interface.  Examples
                     include IEEE 802 addresses for Ethernet links and E.164
                     addresses for ISDN links.

       on-link     - an address that is assigned to an interface on a
                     specified link.  A node considers an address to be on-
                     link if:

                        - it is covered by one of the link's prefixes, or

                        - a neighboring router specifies the address as
                          the target of a Redirect message, or

                        - a Neighbor Advertisement message is received for
                          the (target) address, or

                        - any Neighbor Discovery message is received from
                          the address.

       off-link    - the opposite of "on-link"; an address that is not
                     assigned to any interfaces on the specified link.

       longest prefix match
                   - The process of determining which prefix (if any) in
                     a set of prefixes covers a target address.  A target
                     address is covered by a prefix if all of the bits in
                     the prefix match the left-most bits of the target
                     address.  When multiple prefixes cover an address,
                     the longest prefix is the one that matches.

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       reachability
                   - whether or not the one-way "forward" path to a
                     neighbor is functioning properly.  In particular,
                     whether packets sent to a neighbor are reaching the
                     IP layer on the neighboring machine and are being
                     processed properly by the receiving IP layer.  For
                     neighboring routers, reachability means that packets
                     sent by a node's IP layer are delivered to the
                     router's IP layer, and the router is indeed
                     forwarding packets (i.e., it is configured as a
                     router, not a host).  For hosts, reachability means
                     that packets sent by a node's IP layer are delivered
                     to the neighbor host's IP layer.

       packet      - an IP header plus payload.

       link MTU    - the maximum transmission unit, i.e., maximum packet
                     size in octets, that can be conveyed in one piece
                     over a link.

       target      - an address about which address resolution
                     information is sought, or an address which is the
                     new first-hop when being redirected.

       proxy       - a router that responds to Neighbor Discovery query
                     messages on behalf of another node.  A router acting
                     on behalf of a mobile node that has moved off-link
                     could potentially act as a proxy for the mobile
                     node.

       ICMP destination unreachable indication
                   - an error indication returned to the original sender
                     of a packet that cannot be delivered for the reasons
                     outlined in [ICMPv6].  If the error occurs on a node
                     other than the node originating the packet, an ICMP
                     error message is generated.  If the error occurs on
                     the originating node, an implementation is not
                     required to actually create and send an ICMP error
                     packet to the source, as long as the upper-layer
                     sender is notified through an appropriate mechanism
                     (e.g., return value from a procedure call).  Note,
                     however, that an implementation may find it
                     convenient in some cases to return errors to the
                     sender by taking the offending packet, generating an
                     ICMP error message, and then delivering it (locally)
                     through the generic error handling routines.

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       random delay
                   - when sending out messages, it is sometimes necessary to
                     delay a transmission for a random amount of time in
                     order to prevent multiple nodes from transmitting at
                     exactly the same time, or to prevent long-range
                     periodic transmissions from synchronizing with each
                     other [SYNC].  When a random component is required, a
                     node calculates the actual delay in such a way that the
                     computed delay forms a uniformly-distributed random
                     value that falls between the specified minimum and
                     maximum delay times.  The implementor must take care to
                     insure that the granularity of the calculated random
                     component and the resolution of the timer used are both
                     high enough to insure that the probability of multiple
                     nodes delaying the same amount of time is small.

       random delay seed
                   - If a pseudo-random number generator is used in
                     calculating a random delay component, the generator
                     should be initialized with a unique seed prior to being
                     used.  Note that it is not sufficient to use the
                     interface token alone as the seed, since interface
                     tokens will not always be unique.  To reduce the
                     probability that duplicate interface tokens cause the
                     same seed to be used, the seed should be calculated
                     from a variety of input sources (e.g., machine
                     components) that are likely to be different even on
                     identical "boxes".  For example, the seed could be
                     formed by combining the CPU's serial number with an
                     interface token.

    2.2.  Link Types

       Different link layers have different properties.  The ones of concern
       to Neighbor Discovery are:

       multicast      - a link that supports a native mechanism at the
                        link layer for sending packets to all (i.e.,
                        broadcast) or a subset of all neighbors.

       point-to-point - a link that connects exactly two interfaces.  A
                        point-to-point link is assumed to have multicast
                        capability and have a link-local address.

       non-broadcast multi-access (NBMA)
                      - a link to which more than two interfaces can attach,
                        but that does not support a native form of multicast
                        or broadcast (e.g., X.25, ATM, frame relay, etc.).

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                        Note that all link types (including NBMA) are
                        expected to provide multicast service for IP (e.g.,
                        using multicast servers), but it is an issue for
                        further study whether ND should use such facilities
                        or an alternate mechanism that provides the
                        equivalent ND services.

       shared media   - a link that allows direct communication among a
                        number of nodes, but attached nodes are configured
                        in such a way that they do not have complete prefix
                        information for all on-link destinations.  That is,
                        at the IP level, nodes on the same link may not know
                        that they are neighbors; by default, they
                        communicate through a router.  Examples are large
                        (switched) public data networks such as SMDS and B-
                        ISDN.  Also known as "large clouds".  See [SH-
                        MEDIA].

       variable MTU   - a link that does not have a well-defined MTU (e.g.,
                        IEEE 802.5 token rings).  Many links (e.g.,
                        Ethernet) have a standard MTU defined by the link-
                        layer protocol or by the specific document
                        describing how to run IP over the link layer.

       asymmetric reachability
                      - a link where non-reflexive and/or non-transitive
                        reachability is part of normal operation.  (Non-
                        reflexive reachability means packets from A reach B
                        but packets from B don't reach A.  Non-transitive
                        reachability means packets from A reach B, and
                        packets from B reach C, but packets from A don't
                        reach C.)  Many radio links exhibit these
                        properties.

    2.3.  Addresses

       Neighbor Discovery makes use of a number of different addresses
       defined in [ADDR-ARCH], including:

       all-nodes multicast address
                   - the link-local scope address to reach all nodes.
                     FF02::1

       all-routers multicast address
                   - the link-local scope address to reach all routers.
                     FF02::2

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       solicited-node multicast address
                   - a link-local scope multicast address that is computed
                     as a function of the solicited target's address.  The
                     function is described in [ADDR-ARCH]. The function is
                     chosen so that IP addresses which differ only in the
                     high-order bits, e.g., due to multiple high-order
                     prefixes associated with different providers, will map
                     to the same solicited-node address thereby reducing the
                     number of multicast addresses a node must join.

       link-local address
                   - a unicast address having link-only scope that can be
                     used to reach neighbors.  All interfaces on routers
                     MUST have a link-local address.  Also, [ADDRCONF]
                     requires that interfaces on hosts have a link-local
                     address.

       unspecified address
                   - a reserved address value that indicates the lack of an
                     address (e.g., the address is unknown).  It is never
                     used as a destination address, but may be used as a
                     source address if the sender does not (yet) know its
                     own address (e.g., while verifying an address is unused
                     during address autoconfiguration [ADDRCONF]).  The
                     unspecified address has a value of 0:0:0:0:0:0:0:0.

    2.4.  Requirements

       The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
       SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
       document, are to be interpreted as described in [KEYWORDS].

       This document also makes use of internal conceptual variables to
       describe protocol behavior and external variables that an
       implementation must allow system administrators to change.  The
       specific variable names, how their values change, and how their
       settings influence protocol behavior are provided to demonstrate
       protocol behavior.  An implementation is not required to have them in
       the exact form described here, so long as its external behavior is
       consistent with that described in this document.

    3.  PROTOCOL OVERVIEW

       This protocol solves a set of problems related to the interaction
       between nodes attached to the same link.  It defines mechanisms for
       solving each of the following problems:

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         Router Discovery: How hosts locate routers that reside on an
                    attached link.

         Prefix Discovery: How hosts discover the set of address prefixes
                    that define which destinations are on-link for an
                    attached link.  (Nodes use prefixes to distinguish
                    destinations that reside on-link from those only
                    reachable through a router.)

         Parameter Discovery: How a node learns such link parameters as the
                    link MTU or such Internet parameters as the hop limit
                    value to place in outgoing packets.

         Address Autoconfiguration: How nodes automatically configure an
                    address for an interface.

         Address resolution: How nodes determine the link-layer address of
                    an on-link destination (e.g., a neighbor) given only the
                    destination's IP address.

         Next-hop determination: The algorithm for mapping an IP destination
                    address into the IP address of the neighbor to which
                    traffic for the destination should be sent.  The next-
                    hop can be a router or the destination itself.

         Neighbor Unreachability Detection: How nodes determine that a
                    neighbor is no longer reachable.  For neighbors used as
                    routers, alternate default routers can be tried.  For
                    both routers and hosts, address resolution can be
                    performed again.

         Duplicate Address Detection: How a node determines that an address
                    it wishes to use is not already in use by another node.

         Redirect:  How a router informs a host of a better first-hop node
                    to reach a particular destination.

       Neighbor Discovery defines five different ICMP packet types: A pair
       of Router Solicitation and Router Advertisement messages, a pair of
       Neighbor Solicitation and Neighbor Advertisements messages, and a
       Redirect message.  The messages serve the following purpose:

         Router Solicitation: When an interface becomes enabled, hosts may
                    send out Router Solicitations that request routers to
                    generate Router Advertisements immediately rather than
                    at their next scheduled time.

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         Router Advertisement: Routers advertise their presence together
                    with various link and Internet parameters either
                    periodically, or in response to a Router Solicitation
                    message.  Router Advertisements contain prefixes that
                    are used for on-link determination and/or address
                    configuration, a suggested hop limit value, etc.

         Neighbor Solicitation: Sent by a node to determine the link-layer
                    address of a neighbor, or to verify that a neighbor is
                    still reachable via a cached link-layer address.
                    Neighbor Solicitations are also used for Duplicate
                    Address Detection.

         Neighbor Advertisement: A response to a Neighbor Solicitation
                    message.  A node may also send unsolicited Neighbor
                    Advertisements to announce a link-layer address change.

         Redirect:  Used by routers to inform hosts of a better first hop
                    for a destination.

       On multicast-capable links, each router periodically multicasts a
       Router Advertisement packet announcing its availability.  A host
       receives Router Advertisements from all routers, building a list of
       default routers.  Routers generate Router Advertisements frequently
       enough that hosts will learn of their presence within a few minutes,
       but not frequently enough to rely on an absence of advertisements to
       detect router failure; a separate Neighbor Unreachability Detection
       algorithm provides failure detection.

       Router Advertisements contain a list of prefixes used for on-link
       determination and/or autonomous address configuration; flags
       associated with the prefixes specify the intended uses of a
       particular prefix.  Hosts use the advertised on-link prefixes to
       build and maintain a list that is used in deciding when a packet's
       destination is on-link or beyond a router.  Note that a destination
       can be on-link even though it is not covered by any advertised on-
       link prefix.  In such cases a router can send a Redirect informing
       the sender that the destination is a neighbor.

       Router Advertisements (and per-prefix flags) allow routers to inform
       hosts how to perform Address Autoconfiguration.  For example, routers
       can specify whether hosts should use stateful (DHCPv6) and/or
       autonomous (stateless) address configuration.  The exact semantics
       and usage of the address configuration-related information is
       specified in [ADDRCONF].

       Router Advertisement messages also contain Internet parameters such
       as the hop limit that hosts should use in outgoing packets and,

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       optionally, link parameters such as the link MTU.  This facilitates
       centralized administration of critical parameters that can be set on
       routers and automatically propagated to all attached hosts.

       Nodes accomplish address resolution by multicasting a Neighbor
       Solicitation that asks the target node to return its link-layer
       address.  Neighbor Solicitation messages are multicast to the
       solicited-node multicast address of the target address.  The target
       returns its link-layer address in a unicast Neighbor Advertisement
       message.  A single request-response pair of packets is sufficient for
       both the initiator and the target to resolve each other's link-layer
       addresses; the initiator includes its link-layer address in the
       Neighbor Solicitation.

       Neighbor Solicitation messages can also be used to determine if more
       than one node has been assigned the same unicast address.  The use of
       Neighbor Solicitation messages for Duplicate Address Detection is
       specified in [ADDRCONF].

       Neighbor Unreachability Detection detects the failure of a neighbor
       or the failure of the forward path to the neighbor.  Doing so
       requires positive confirmation that packets sent to a neighbor are
       actually reaching that neighbor and being processed properly by its
       IP layer.  Neighbor Unreachability Detection uses confirmation from
       two sources.  When possible, upper-layer protocols provide a positive
       confirmation that a connection is making "forward progress", that is,
       previously sent data is known to have been delivered correctly (e.g.,
       new acknowledgments were received recently).  When positive
       confirmation is not forthcoming through such "hints", a node sends
       unicast Neighbor Solicitation messages that solicit Neighbor
       Advertisements as reachability confirmation from the next hop.  To
       reduce unnecessary network traffic, probe messages are only sent to
       neighbors to which the node is actively sending packets.

       In addition to addressing the above general problems, Neighbor
       Discovery also handles the following situations:

         Link-layer address change - A node that knows its link-layer
               address has changed can multicast a few (unsolicited)
               Neighbor Advertisement packets to all nodes to quickly update
               cached link-layer addresses that have become invalid.  Note
               that the sending of unsolicited advertisements is a
               performance enhancement only (e.g., unreliable).  The
               Neighbor Unreachability Detection algorithm ensures that all
               nodes will reliably discover the new address, though the
               delay may be somewhat longer.

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         Inbound load balancing - Nodes with replicated interfaces may want
               to load balance the reception of incoming packets across
               multiple network interfaces on the same link.  Such nodes
               have multiple link-layer addresses assigned to the same
               interface.  For example, a single network driver could
               represent multiple network interface cards as a single
               logical interface having multiple link-layer addresses.

               Load balancing is handled by allowing routers to omit the
               source link-layer address from Router Advertisement packets,
               thereby forcing neighbors to use Neighbor Solicitation
               messages to learn link-layer addresses of routers.  Returned
               Neighbor Advertisement messages can then contain link-layer
               addresses that differ depending on who issued the
               solicitation.

         Anycast addresses - Anycast addresses identify one of a set of
               nodes providing an equivalent service, and multiple nodes on
               the same link may be configured to recognize the same Anycast
               address.  Neighbor Discovery handles anycasts by having nodes
               expect to receive multiple Neighbor Advertisements for the
               same target.  All advertisements for anycast addresses are
               tagged as being non-Override advertisements.  This invokes
               specific rules to determine which of potentially multiple
               advertisements should be used.

         Proxy advertisements - A router willing to accept packets on behalf
               of a target address that is unable to respond to Neighbor
               Solicitations can issue non-Override Neighbor Advertisements.
               There is currently no specified use of proxy, but proxy
               advertising could potentially be used to handle cases like
               mobile nodes that have moved off-link.  However, it is not
               intended as a general mechanism to handle nodes that, e.g.,
               do not implement this protocol.

    3.1.  Comparison with IPv4

       The IPv6 Neighbor Discovery protocol corresponds to a combination of
       the IPv4 protocols ARP [ARP], ICMP Router Discovery [RDISC], and ICMP
       Redirect [ICMPv4].  In IPv4 there is no generally agreed upon
       protocol or mechanism for Neighbor Unreachability Detection, although
       Hosts Requirements [HR-CL] does specify some possible algorithms for
       Dead Gateway Detection (a subset of the problems Neighbor
       Unreachability Detection tackles).

       The Neighbor Discovery protocol provides a multitude of improvements
       over the IPv4 set of protocols:

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          Router Discovery is part of the base protocol set; there is no
          need for hosts to "snoop" the routing protocols.

          Router advertisements carry link-layer addresses; no additional
          packet exchange is needed to resolve the router's link-layer
          address.

          Router advertisements carry prefixes for a link; there is no need
          to have a separate mechanism to configure the "netmask".

          Router advertisements enable Address Autoconfiguration.

          Routers can advertise an MTU for hosts to use on the link,
          ensuring that all nodes use the same MTU value on links lacking a
          well-defined MTU.

          Address resolution multicasts are "spread" over 4 billion (2^32)
          multicast addresses greatly reducing address resolution related
          interrupts on nodes other than the target.  Moreover, non-IPv6
          machines should not be interrupted at all.

          Redirects contain the link-layer address of the new first hop;
          separate address resolution is not needed upon receiving a
          redirect.

          Multiple prefixes can be associated with the same link.  By
          default, hosts learn all on-link prefixes from Router
          Advertisements.  However, routers may be configured to omit some
          or all prefixes from Router Advertisements.  In such cases hosts
          assume that destinations are off-link and send traffic to routers.
          A router can then issue redirects as appropriate.

          Unlike IPv4, the recipient of an IPv6 redirect assumes that the
          new next-hop is on-link.  In IPv4, a host ignores redirects
          specifying a next-hop that is not on-link according to the link's
          network mask.  The IPv6 redirect mechanism is analogous to the
          XRedirect facility specified in [SH-MEDIA].  It is expected to be
          useful on non-broadcast and shared media links in which it is
          undesirable or not possible for nodes to know all prefixes for
          on-link destinations.

          Neighbor Unreachability Detection is part of the base
          significantly improving the robustness of packet delivery in the
          presence of failing routers, partially failing or partitioned
          links and nodes that change their link-layer addresses.  For
          instance, mobile nodes can move off-link without losing any
          connectivity due to stale ARP caches.

    Narten, et. al.             Standards Track                    [Page 14]

    RFC 2461              Neighbor Discovery for IPv6          December 1998

          Unlike ARP, Neighbor Discovery detects half-link failures (using
          Neighbor Unreachability Detection) and avoids sending traffic to
          neighbors with which two-way connectivity is absent.

          Unlike in IPv4 Router Discovery the Router Advertisement messages
          do not contain a preference field.  The preference field is not
          needed to handle routers of different "stability"; the Neighbor
          Unreachability Detection will detect dead routers and switch to a
          working one.

          The use of link-local addresses to uniquely identify routers (for
          Router Advertisement and Redirect messages) makes it possible for
          hosts to maintain the router associations in the event of the site
          renumbering to use new global prefixes.

          Using the Hop Limit equal to 255 trick Neighbor Discovery is
          immune to off-link senders that accidentally or intentionally send
          ND messages.  In IPv4 off-link senders can send both ICMP
          Redirects and Router Advertisement messages.

          Placing address resolution at the ICMP layer makes the protocol
          more media-independent than ARP and makes it possible to use
          standard IP authentication and security mechanisms as appropriate
          [IPv6-AUTH, IPv6-ESP].

    3.2.  Supported Link Types

       Neighbor Discovery supports links with different properties.  In the
       presence of certain properties only a subset of the ND protocol
       mechanisms are fully specified in this document:

         point-to-point - Neighbor Discovery handles such links just like
                          multicast links.  (Multicast can be trivially
                          provided on point to point links, and interfaces
                          can be assigned link-local addresses.)  Neighbor
                          Discovery should be implemented as described in
                          this document.

         multicast      - Neighbor Discovery should be implemented as
                          described in this document.

         non-broadcast multiple access (NBMA)
                        - Redirect, Neighbor Unreachability Detection and
                          next-hop determination should be implemented as
                          described in this document.  Address resolution,
                          and the mechanism for delivering Router
                          Solicitations and Advertisements on NBMA links is
                          not specified in this document.  Note that if

    Narten, et. al.             Standards Track                    [Page 15]

    RFC 2461              Neighbor Discovery for IPv6          December 1998

                          hosts support manual configuration of a list of
                          default routers, hosts can dynamically acquire the
                          link-layer addresses for their neighbors from
                          Redirect messages.

         shared media   - The Redirect message is modeled after the
                          XRedirect message in [SH-MEDIA] in order to
                          simplify use of the protocol on shared media
                          links.

                          This specification does not address shared media
                          issues that only relate to routers, such as:

                           - How routers exchange reachability information
                             on a shared media link.

                           - How a router determines the link-layer address
                             of a host, which it needs to send redirect
                             messages to the host.

                           - How a router determines that it is the first-
                             hop router for a received packet.

                          The protocol is extensible (through the definition
                          of new options) so that other solutions might be
                          possible in the future.

         variable MTU   - Neighbor Discovery allows routers to specify a MTU
                          for the link, which all nodes then use.  All nodes
                          on a link must use the same MTU (or Maximum
                          Receive Unit) in order for multicast to work
                          properly.  Otherwise when multicasting a sender,
                          which can not know which nodes will receive the
                          packet, could not determine a minimum packet size
                          all receivers can process.

         asymmetric reachability
                        - Neighbor Discovery detects the absence of
                          symmetric reachability; a node avoids paths to a
                          neighbor with which it does not have symmetric
                          connectivity.

                          The Neighbor Unreachability Detection will
                          typically identify such half-links and the node
                          will refrain from using them.

                          The protocol can presumably be extended in the
                          future to find viable paths in environments that

    Narten, et. al.             Standards Track                    [Page 16]

    RFC 2461              Neighbor Discovery for IPv6          December 1998

                          lack reflexive and transitive connectivity.

    4.  MESSAGE FORMATS

    4.1.  Router Solicitation Message Format

       Hosts send Router Solicitations in order to prompt routers to
       generate Router Advertisements quickly.

          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             |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                            Reserved                           |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |   Options ...
         +-+-+-+-+-+-+-+-+-+-+-+-

       IP Fields:

          Source Address
                         An IP address assigned to the sending interface, or
                         the unspecified address if no address is assigned
                         to the sending interface.

          Destination Address
                         Typically the all-routers multicast address.

          Hop Limit      255

          Authentication Header
                         If a Security Association for the IP Authentication
                         Header exists between the sender and the
                         destination address, then the sender SHOULD include
                         this header.

       ICMP Fields:

          Type           133

          Code           0

          Checksum       The ICMP checksum.  See [ICMPv6].

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

    Narten, et. al.             Standards Track                    [Page 17]

    RFC 2461              Neighbor Discovery for IPv6          December 1998

       Valid Options:

          Source link-layer address
                         The link-layer address of the sender, if known.
                         MUST NOT be included if the Source Address is the
                         unspecified address.  Otherwise it SHOULD be
                         included on link layers that have addresses.

          Future versions of this protocol may define new option types.
          Receivers MUST silently ignore any options they do not recognize
          and continue processing the message.

    4.2.  Router Advertisement Message Format

       Routers send out Router Advertisement message periodically, or in
       response to a Router Solicitation.

          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             |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         | Cur Hop Limit |M|O|  Reserved |       Router Lifetime         |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                         Reachable Time                        |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                          Retrans Timer                        |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |   Options ...
         +-+-+-+-+-+-+-+-+-+-+-+-

       IP Fields:

          Source Address
                         MUST be the link-local address assigned to the
                         interface from which this message is sent.

          Destination Address
                         Typically the Source Address of an invoking Router
                         Solicitation or the all-nodes multicast address.

          Hop Limit      255

          Authentication Header
                         If a Security Association for the IP Authentication
                         Header exists between the sender and the
                         destination address, then the sender SHOULD include
                         this header.

    Narten, et. al.             Standards Track                    [Page 18]

    RFC 2461              Neighbor Discovery for IPv6          December 1998

       ICMP Fields:

          Type           134

          Code           0

          Checksum       The ICMP checksum.  See [ICMPv6].

          Cur Hop Limit  8-bit unsigned integer.  The default value that
                         should be placed in the Hop Count field of the IP
                         header for outgoing IP packets.  A value of zero
                         means unspecified (by this router).

          M              1-bit "Managed address configuration" flag.  When
                         set, hosts use the administered (stateful) protocol
                         for address autoconfiguration in addition to any
                         addresses autoconfigured using stateless address
                         autoconfiguration.  The use of this flag is
                         described in [ADDRCONF].

          O              1-bit "Other stateful configuration" flag.  When
                         set, hosts use the administered (stateful) protocol
                         for autoconfiguration of other (non-address)
                         information.  The use of this flag is described in
                         [ADDRCONF].

          Reserved       A 6-bit unused field.  It MUST be initialized to
                         zero by the sender and MUST be ignored by the
                         receiver.

          Router Lifetime
                         16-bit unsigned integer.  The lifetime associated
                         with the default router in units of seconds.  The
                         maximum value corresponds to 18.2 hours.  A
                         Lifetime of 0 indicates that the router is not a
                         default router and SHOULD NOT appear on the default
                         router list.  The Router Lifetime applies only to
                         the router's usefulness as a default router; it
                         does not apply to information contained in other
                         message fields or options.  Options that need time
                         limits for their information include their own
                         lifetime fields.

          Reachable Time 32-bit unsigned integer.  The time, in
                         milliseconds, that a node assumes a neighbor is
                         reachable after having received a reachability
                         confirmation.  Used by the Neighbor Unreachability
                         Detection algorithm (see Section 7.3).  A value of

    Narten, et. al.             Standards Track                    [Page 19]

    RFC 2461              Neighbor Discovery for IPv6          December 1998

                         zero means unspecified (by this router).

          Retrans Timer  32-bit unsigned integer.  The time, in
                         milliseconds, between retransmitted Neighbor
                         Solicitation messages.  Used by address resolution
                         and the Neighbor Unreachability Detection algorithm
                         (see Sections 7.2 and 7.3).  A value of zero means
                         unspecified (by this router).

       Possible options:

          Source link-layer address
                         The link-layer address of the interface from which
                         the Router Advertisement is sent.  Only used on
                         link layers that have addresses.  A router MAY omit
                         this option in order to enable inbound load sharing
                         across multiple link-layer addresses.

          MTU            SHOULD be sent on links that have a variable MTU
                         (as specified in the document that describes how to
                         run IP over the particular link type).  MAY be sent
                         on other links.

          Prefix Information
                         These options specify the prefixes that are on-link
                         and/or are used for address autoconfiguration.  A
                         router SHOULD include all its on-link prefixes
                         (except the link-local prefix) so that multihomed
                         hosts have complete prefix information about on-
                         link destinations for the links to which they
                         attach.  If complete information is lacking, a
                         multihomed host may not be able to choose the
                         correct outgoing interface when sending traffic to
                         its neighbors.

          Future versions of this protocol may define new option types.
          Receivers MUST silently ignore any options they do not recognize
          and continue processing the message.

    Narten, et. al.             Standards Track                    [Page 20]

    RFC 2461              Neighbor Discovery for IPv6          December 1998

    4.3.  Neighbor Solicitation Message Format

       Nodes send Neighbor Solicitations to request the link-layer address
       of a target node while also providing their own link-layer address to
       the target.  Neighbor Solicitations are multicast when the node needs
       to resolve an address and unicast when the node seeks to verify the
       reachability of a neighbor.

             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             |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |                           Reserved                            |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |                                                               |
            +                                                               +
            |                                                               |
            +                       Target Address                          +
            |                                                               |
            +                                                               +
            |                                                               |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |   Options ...
            +-+-+-+-+-+-+-+-+-+-+-+-

       IP Fields:

          Source Address
                         Either an address assigned to the interface from
                         which this message is sent or (if Duplicate Address
                         Detection is in progress [ADDRCONF]) the
                         unspecified address.

          Destination Address
                         Either the solicited-node multicast address
                         corresponding to the target address, or the target
                         address.

          Hop Limit      255

          Authentication Header
                         If a Security Association for the IP Authentication
                         Header exists between the sender and the
                         destination address, then the sender SHOULD include
                         this header.

    Narten, et. al.             Standards Track                    [Page 21]

    RFC 2461              Neighbor Discovery for IPv6          December 1998

       ICMP Fields:

          Type           135

          Code           0

          Checksum       The ICMP checksum.  See [ICMPv6].

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

          Target Address
                         The IP address of the target of the solicitation.
                         It MUST NOT be a multicast address.

       Possible options:

          Source link-layer address
                         The link-layer address for the sender.  MUST NOT be
                         included when the source IP address is the
                         unspecified address.  Otherwise, on link layers
                         that have addresses this option MUST be included in
                         multicast solicitations and SHOULD be included in
                         unicast solicitations.

          Future versions of this protocol may define new option types.
          Receivers MUST silently ignore any options they do not recognize
          and continue processing the message.

    Narten, et. al.             Standards Track                    [Page 22]

    RFC 2461              Neighbor Discovery for IPv6          December 1998

    4.4.  Neighbor Advertisement Message Format

       A node sends Neighbor Advertisements in response to Neighbor
       Solicitations and sends unsolicited Neighbor Advertisements in order
       to (unreliably) propagate new information quickly.

           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             |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |R|S|O|                     Reserved                            |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                                                               |
          +                                                               +
          |                                                               |
          +                       Target Address                          +
          |                                                               |
          +                                                               +
          |                                                               |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |   Options ...
          +-+-+-+-+-+-+-+-+-+-+-+-

       IP Fields:

          Source Address
                         An address assigned to the interface from which the
                         advertisement is sent.

          Destination Address
                         For solicited advertisements, the Source Address of
                         an invoking Neighbor Solicitation or, if the
                         solicitation's Source Address is the unspecified
                         address, the all-nodes multicast address.

                         For unsolicited advertisements typically the all-
                         nodes multicast address.

          Hop Limit      255

       Authentication Header
                         If a Security Association for the IP Authentication
                         Header exists between the sender and the
                         destination address, then the sender SHOULD include
                         this header.

    Narten, et. al.             Standards Track                    [Page 23]

    RFC 2461              Neighbor Discovery for IPv6          December 1998

       ICMP Fields:

          Type           136

          Code           0

          Checksum       The ICMP checksum.  See [ICMPv6].

          R              Router flag.  When set, the R-bit indicates that
                         the sender is a router.  The R-bit is used by
                         Neighbor Unreachability Detection to detect a
                         router that changes to a host.

          S              Solicited flag.  When set, the S-bit indicates that
                         the advertisement was sent in response to a
                         Neighbor Solicitation from the Destination address.
                         The S-bit is used as a reachability confirmation
                         for Neighbor Unreachability Detection.  It MUST NOT
                         be set in multicast advertisements or in
                         unsolicited unicast advertisements.

          O              Override flag.  When set, the O-bit indicates that
                         the advertisement should override an existing cache
                         entry and update the cached link-layer address.
                         When it is not set the advertisement will not
                         update a cached link-layer address though it will
                         update an existing Neighbor Cache entry for which
                         no link-layer address is known.  It SHOULD NOT be
                         set in solicited advertisements for anycast
                         addresses and in solicited proxy advertisements.
                         It SHOULD be set in other solicited advertisements
                         and in unsolicited advertisements.

          Reserved       29-bit unused field.  It MUST be initialized to
                         zero by the sender and MUST be ignored by the
                         receiver.

          Target Address
                         For solicited advertisements, the Target Address
                         field in the Neighbor Solicitation message that
                         prompted this advertisement.  For an unsolicited
                         advertisement, the address whose link-layer address
                         has changed.  The Target Address MUST NOT be a
                         multicast address.

    Narten, et. al.             Standards Track                    [Page 24]

    RFC 2461              Neighbor Discovery for IPv6          December 1998

       Possible options:

          Target link-layer address
                         The link-layer address for the target, i.e., the
                         sender of the advertisement.  This option MUST be
                         included on link layers that have addresses when
                         responding to multicast solicitations.  When
                         responding to a unicast Neighbor Solicitation this
                         option SHOULD be included.

                         The option MUST be included for multicast
                         solicitations in order to avoid infinite Neighbor
                         Solicitation "recursion" when the peer node does
                         not have a cache entry to return a Neighbor
                         Advertisements message.  When responding to unicast
                         solicitations, the option can be omitted since the
                         sender of the solicitation has the correct link-
                         layer address; otherwise it would not have be able
                         to send the unicast solicitation in the first
                         place. However, including the link-layer address in
                         this case adds little overhead and eliminates a
                         potential race condition where the sender deletes
                         the cached link-layer address prior to receiving a
                         response to a previous solicitation.

          Future versions of this protocol may define new option types.
          Receivers MUST silently ignore any options they do not recognize
          and continue processing the message.

    Narten, et. al.             Standards Track                    [Page 25]

    RFC 2461              Neighbor Discovery for IPv6          December 1998

    4.5.  Redirect Message Format

       Routers send Redirect packets to inform a host of a better first-hop
       node on the path to a destination.  Hosts can be redirected to a
       better first-hop router but can also be informed by a redirect that
       the destination is in fact a neighbor.  The latter is accomplished by
       setting the ICMP Target Address equal to the ICMP Destination
       Address.

           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             |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                           Reserved                            |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                                                               |
          +                                                               +
          |                                                               |
          +                       Target Address                          +
          |                                                               |
          +                                                               +
          |                                                               |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                                                               |
          +                                                               +
          |                                                               |
          +                     Destination Address                       +
          |                                                               |
          +                                                               +
          |                                                               |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |   Options ...
          +-+-+-+-+-+-+-+-+-+-+-+-

       IP Fields:

          Source Address
                         MUST be the link-local address assigned to the
                         interface from which this message is sent.

          Destination Address
                         The Source Address of the packet that triggered the
                         redirect.

          Hop Limit      255

    Narten, et. al.             Standards Track                    [Page 26]

    RFC 2461              Neighbor Discovery for IPv6          December 1998

          Authentication Header
                         If a Security Association for the IP Authentication
                         Header exists between the sender and the
                         destination address, then the sender SHOULD include
                         this header.

       ICMP Fields:

          Type           137

          Code           0

          Checksum       The ICMP checksum.  See [ICMPv6].

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

          Target Address An IP address that is a better first hop to use for
                         the ICMP Destination Address.  When the target is
                         the actual endpoint of communication, i.e., the
                         destination is a neighbor, the Target Address field
                         MUST contain the same value as the ICMP Destination
                         Address field.  Otherwise the target is a better
                         first-hop router and the Target Address MUST be the
                         router's link-local address so that hosts can
                         uniquely identify routers.

          Destination Address
                         The IP address of the destination which is
                         redirected to the target.

       Possible options:

          Target link-layer address
                         The link-layer address for the target.  It SHOULD
                         be included (if known).  Note that on NBMA links,
                         hosts may rely on the presence of the Target Link-
                         Layer Address option in Redirect messages as the
                         means for determining the link-layer addresses of
                         neighbors.  In such cases, the option MUST be
                         included in Redirect messages.

          Redirected Header
                         As much as possible of the IP packet that triggered
                         the sending of the Redirect without making the
                         redirect packet exceed 1280 octets.

    Narten, et. al.             Standards Track                    [Page 27]

    RFC 2461              Neighbor Discovery for IPv6          December 1998

    4.6.  Option Formats

       Neighbor Discovery messages include zero or more options, some of
       which may appear multiple times in the same message.  All options are
       of the form:

            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     |              ...              |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           ~                              ...                              ~
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Fields:

          Type           8-bit identifier of the type of option.  The
                         options defined in this document are:

                               Option Name                             Type

                            Source Link-Layer Address                    1
                            Target Link-Layer Address                    2
                            Prefix Information                           3
                            Redirected Header                            4
                            MTU                                          5

          Length         8-bit unsigned integer.  The length of the option
                         (including the type and length fields) in units of
                         8 octets.  The value 0 is invalid.  Nodes MUST
                         silently discard an ND packet that contains an
                         option with length zero.

    4.6.1.  Source/Target Link-layer Address

          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     |    Link-Layer Address ...
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Fields:

          Type
                         1 for Source Link-layer Address
                         2 for Target Link-layer Address

    Narten, et. al.             Standards Track                    [Page 28]

    RFC 2461              Neighbor Discovery for IPv6          December 1998

          Length         The length of the option (including the type and
                         length fields) in units of 8 octets.  For example,
                         the length for IEEE 802 addresses is 1 [IPv6-
                         ETHER].

          Link-Layer Address
                         The variable length link-layer address.

                         The content and format of this field (including
                         byte and bit ordering) is expected to be specified
                         in specific documents that describe how IPv6
                         operates over different link layers.  For instance,
                         [IPv6-ETHER].

       Description
                         The Source Link-Layer Address option contains the
                         link-layer address of the sender of the packet.  It
                         is used in the Neighbor Solicitation, Router
                         Solicitation, and Router Advertisement packets.

                         The Target Link-Layer Address option contains the
                         link-layer address of the target.  It is used in
                         Neighbor Advertisement and Redirect packets.

                         These options MUST be silently ignored for other
                         Neighbor Discovery messages.

    4.6.2.  Prefix Information

           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     | Prefix Length |L|A| Reserved1 |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                         Valid Lifetime                        |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                       Preferred Lifetime                      |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                           Reserved2                           |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                                                               |
          +                                                               +
          |                                                               |
          +                            Prefix                             +
          |                                                               |
          +                                                               +
          |                                                               |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Narten, et. al.             Standards Track                    [Page 29]

    RFC 2461              Neighbor Discovery for IPv6          December 1998

       Fields:

          Type           3

          Length         4

          Prefix Length  8-bit unsigned integer.  The number of leading bits
                         in the Prefix that are valid.  The value ranges
                         from 0 to 128.

          L              1-bit on-link flag.  When set, indicates that this
                         prefix can be used for on-link determination.  When
                         not set the advertisement makes no statement about
                         on-link or off-link properties of the prefix.  For
                         instance, the prefix might be used for address
                         configuration with some of the addresses belonging
                         to the prefix being on-link and others being off-
                         link.

          A              1-bit autonomous address-configuration flag.  When
                         set indicates that this prefix can be used for
                         autonomous address configuration as specified in
                         [ADDRCONF].

          Reserved1      6-bit unused field.  It MUST be initialized to zero
                         by the sender and MUST be ignored by the receiver.

          Valid Lifetime
                         32-bit unsigned integer.  The length of time in
                         seconds (relative to the time the packet is sent)
                         that the prefix is valid for the purpose of on-link
                         determination.  A value of all one bits
                         (0xffffffff) represents infinity.  The Valid
                         Lifetime is also used by [ADDRCONF].

          Preferred Lifetime
                         32-bit unsigned integer.  The length of time in
                         seconds (relative to the time the packet is sent)
                         that addresses generated from the prefix via
                         stateless address autoconfiguration remain
                         preferred [ADDRCONF].  A value of all one bits
                         (0xffffffff) represents infinity.  See [ADDRCONF].

          Reserved2      This field is unused.  It MUST be initialized to
                         zero by the sender and MUST be ignored by the
                         receiver.

    Narten, et. al.             Standards Track                    [Page 30]

    RFC 2461              Neighbor Discovery for IPv6          December 1998

          Prefix         An IP address or a prefix of an IP address.  The
                         Prefix Length field contains the number of valid
                         leading bits in the prefix.  The bits in the prefix
                         after the prefix length are reserved and MUST be
                         initialized to zero by the sender and ignored by
                         the receiver.  A router SHOULD NOT send a prefix
                         option for the link-local prefix and a host SHOULD
                         ignore such a prefix option.

       Description
                         The Prefix Information option provide hosts with
                         on-link prefixes and prefixes for Address
                         Autoconfiguration.

                         The Prefix Information option appears in Router
                         Advertisement packets and MUST be silently ignored
                         for other messages.

    4.6.3.  Redirected Header

           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     |            Reserved           |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                           Reserved                            |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                                                               |
          ~                       IP header + data                        ~
          |                                                               |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Fields:

          Type           4

          Length         The length of the option in units of 8 octets.

          Reserved       These fields are unused.  They MUST be initialized
                         to zero by the sender and MUST be ignored by the
                         receiver.

          IP header + data
                         The original packet truncated to ensure that the
                         size of the redirect message does not exceed 1280
                         octets.

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       Description
                         The Redirected Header option is used in Redirect
                         messages and contains all or part of the packet
                         that is being redirected.

                         This option MUST be silently ignored for other
                         Neighbor Discovery messages.

    4.6.4.  MTU

           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     |           Reserved            |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                              MTU                              |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Fields:

          Type           5

          Length         1

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

          MTU            32-bit unsigned integer.  The recommended MTU for
                         the link.

       Description
                         The MTU option is used in  Router Advertisement
                         messages to insure that all nodes on a link use the
                         same MTU value in those cases where the link MTU is
                         not well known.

                         This option MUST be silently ignored for other
                         Neighbor Discovery messages.

                         In configurations in which heterogeneous
                         technologies are bridged together, the maximum
                         supported MTU may differ from one segment to
                         another.  If the bridges do not generate ICMP
                         Packet Too Big messages, communicating nodes will
                         be unable to use Path MTU to dynamically determine
                         the appropriate MTU on a per-neighbor basis.  In
                         such cases, routers use the MTU option to specify

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                         the maximum MTU value that is supported by all
                         segments.

    5.  CONCEPTUAL MODEL OF A HOST

       This section describes a conceptual model of one possible data
       structure organization that hosts (and to some extent routers) will
       maintain in interacting with neighboring nodes.  The described
       organization is provided to facilitate the explanation of how the
       Neighbor Discovery protocol should behave.  This document does not
       mandate that implementations adhere to this model as long as their
       external behavior is consistent with that described in this document.

       This model is only concerned with the aspects of host behavior
       directly related to Neighbor Discovery.  In particular, it does not
       concern itself with such issues as source address selection or the
       selecting of an outgoing interface on a multihomed host.

    5.1.  Conceptual Data Structures

       Hosts will need to maintain the following pieces of information for
       each interface:

          Neighbor Cache
                       - A set of entries about individual neighbors to
                         which traffic has been sent recently.  Entries are
                         keyed on the neighbor's on-link unicast IP address
                         and contain such information as its link-layer
                         address, a flag indicating whether the neighbor is
                         a router or a host (called IsRouter in this
                         document), a pointer to any queued packets waiting
                         for address resolution to complete, etc.

                         A Neighbor Cache entry also contains information
                         used by the Neighbor Unreachability Detection
                         algorithm, including the reachability state, the
                         number of unanswered probes, and the time the next
                         Neighbor Unreachability Detection event is
                         scheduled to take place.

          Destination Cache
                       - A set of entries about destinations to which
                         traffic has been sent recently.  The Destination
                         Cache includes both on-link and off-link
                         destinations and provides a level of indirection
                         into the Neighbor Cache; the Destination Cache maps
                         a destination IP address to the IP address of the
                         next-hop neighbor.  This cache is updated with

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                         information learned from Redirect messages.
                         Implementations may find it convenient to store
                         additional information not directly related to
                         Neighbor Discovery in Destination Cache entries,
                         such as the Path MTU (PMTU) and round trip timers
                         maintained by transport protocols.

          Prefix List  - A list of the prefixes that define a set of
                         addresses that are on-link.  Prefix List entries
                         are created from information received in Router
                         Advertisements.  Each entry has an associated
                         invalidation timer value (extracted from the
                         advertisement) used to expire prefixes when they
                         become invalid.  A special "infinity" timer value
                         specifies that a prefix remains valid forever,
                         unless a new (finite) value is received in a
                         subsequent advertisement.

                         The link-local prefix is considered to be on the
                         prefix list with an infinite invalidation timer
                         regardless of whether routers are advertising a
                         prefix for it.  Received Router Advertisements
                         SHOULD NOT modify the invalidation timer for the
                         link-local prefix.

          Default Router List
                       - A list of routers to which packets may be sent.
                         Router list entries point to entries in the
                         Neighbor Cache; the algorithm for selecting a
                         default router favors routers known to be reachable
                         over those whose reachability is suspect.  Each
                         entry also has an associated invalidation timer
                         value (extracted from Router Advertisements) used
                         to delete entries that are no longer advertised.

       Note that the above conceptual data structures can be implemented
       using a variety of techniques.  One possible implementation is to use
       a single longest-match routing table for all of the above data
       structures.  Regardless of the specific implementation, it is
       critical that the Neighbor Cache entry for a router is shared by all
       Destination Cache entries using that router in order to prevent
       redundant Neighbor Unreachability Detection probes.

       Note also that other protocols (e.g., IPv6 Mobility) might add
       additional conceptual data structures.  An implementation is at
       liberty to implement such data structures in any way it pleases.  For
       example, an implementation could merge all conceptual data structures
       into a single routing table.

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       The Neighbor Cache contains information maintained by the Neighbor
       Unreachability Detection algorithm.  A key piece of information is a
       neighbor's reachability state, which is one of five possible values.
       The following definitions are informal; precise definitions can be
       found in Section 7.3.2.

          INCOMPLETE  Address resolution is in progress and the link-layer
                      address of the neighbor has not yet been determined.

          REACHABLE   Roughly speaking, the neighbor is known to have been
                      reachable recently (within tens of seconds ago).

          STALE       The neighbor is no longer known to be reachable but
                      until traffic is sent to the neighbor, no attempt
                      should be made to verify its reachability.

          DELAY       The neighbor is no longer known to be reachable, and
                      traffic has recently been sent to the neighbor.
                      Rather than probe the neighbor immediately, however,
                      delay sending probes for a short while in order to
                      give upper layer protocols a chance to provide
                      reachability confirmation.

          PROBE       The neighbor is no longer known to be reachable, and
                      unicast Neighbor Solicitation probes are being sent to
                      verify reachability.

    5.2.  Conceptual Sending Algorithm

       When sending a packet to a destination, a node uses a combination of
       the Destination Cache, the Prefix List, and the Default Router List
       to determine the IP address of the appropriate next hop, an operation
       known as "next-hop determination".  Once the IP address of the next
       hop is known, the Neighbor Cache is consulted for link-layer
       information about that neighbor.

       Next-hop determination for a given unicast destination operates as
       follows.  The sender performs a longest prefix match against the
       Prefix List to determine whether the packet's destination is on- or
       off-link.  If the destination is on-link, the next-hop address is the
       same as the packet's destination address.  Otherwise, the sender
       selects a router from the Default Router List (following the rules
       described in Section 6.3.6).  If the Default Router List is empty,
       the sender assumes that the destination is on-link.

       For efficiency reasons, next-hop determination is not performed on
       every packet that is sent.  Instead, the results of next-hop
       determination computations are saved in the Destination Cache (which

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       also contains updates learned from Redirect messages).  When the
       sending node has a packet to send, it first examines the Destination
       Cache.  If no entry exists for the destination, next-hop
       determination is invoked to create a Destination Cache entry.

       Once the IP address of the next-hop node is known, the sender
       examines the Neighbor Cache for link-layer information about that
       neighbor.  If no entry exists, the sender creates one, sets its state
       to INCOMPLETE, initiates Address Resolution, and then queues the data
       packet pending completion of address resolution.  For multicast-
       capable interfaces Address Resolution consists of sending a Neighbor
       Solicitation message and waiting for a Neighbor Advertisement.  When
       a Neighbor Advertisement response is received, the link-layer
       addresses is entered in the Neighbor Cache entry and the queued
       packet is transmitted.  The address resolution mechanism is described
       in detail in Section 7.2.

       For multicast packets the next-hop is always the (multicast)
       destination address and is considered to be on-link.  The procedure
       for determining the link-layer address corresponding to a given IP
       multicast address can be found in a separate document that covers
       operating IP over a particular link type (e.g., [IPv6-ETHER]).

       Each time a Neighbor Cache entry is accessed while transmitting a
       unicast packet, the sender checks Neighbor Unreachability Detection
       related information according to the Neighbor Unreachability
       Detection algorithm (Section 7.3).  This unreachability check might
       result in the sender transmitting a unicast Neighbor Solicitation to
       verify that the neighbor is still reachable.

       Next-hop determination is done the first time traffic is sent to a
       destination.  As long as subsequent communication to that destination
       proceeds successfully, the Destination Cache entry continues to be
       used.  If at some point communication ceases to proceed, as
       determined by the Neighbor Unreachability Detection algorithm, next-
       hop determination may need to be performed again.  For example,
       traffic through a failed router should be switched to a working
       router.  Likewise, it may be possible to reroute traffic destined for
       a mobile node to a "mobility agent".

       Note that when a node redoes next-hop determination there is no need
       to discard the complete Destination Cache entry.  In fact, it is
       generally beneficial to retain such cached information as the PMTU
       and round trip timer values that may also be kept in the Destination
       Cache entry.

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       Routers and multihomed hosts have multiple interfaces.  The remainder
       of this document assumes that all sent and received Neighbor
       Discovery messages refer to the interface of appropriate context.
       For example, when responding to a Router Solicitation, the
       corresponding Router Advertisement is sent out the interface on which
       the solicitation was received.

    5.3.  Garbage Collection and Timeout Requirements

       The conceptual data structures described above use different
       mechanisms for discarding potentially stale or unused information.

       From the perspective of correctness there is no need to periodically
       purge Destination and Neighbor Cache entries.  Although stale
       information can potentially remain in the cache indefinitely, the
       Neighbor Unreachability Detection algorithm ensures that stale
       information is purged quickly if it is actually being used.

       To limit the storage needed for the Destination and Neighbor Caches,
       a node may need to garbage-collect old entries.  However, care must
       be taken to insure that sufficient space is always present to hold
       the working set of active entries.  A small cache may result in an
       excessive number of Neighbor Discovery messages if entries are
       discarded and rebuilt in quick succession.  Any LRU-based policy that
       only reclaims entries that have not been used in some time (e.g., ten
       minutes or more) should be adequate for garbage-collecting unused
       entries.

       A node should retain entries in the Default Router List and the
       Prefix List until their lifetimes expire.  However, a node may
       garbage collect entries prematurely if it is low on memory.  If not
       all routers are kept on the Default Router list, a node should retain
       at least two entries in the Default Router List (and preferably more)
       in order to maintain robust connectivity for off-link destinations.

       When removing an entry from the Prefix List there is no need to purge
       any entries from the Destination or Neighbor Caches.  Neighbor
       Unreachability Detection will efficiently purge any entries in these
       caches that have become invalid.  When removing an entry from the
       Default Router List, however, any entries in the Destination Cache
       that go through that router must perform next-hop determination again
       to select a new default router.

    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

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       configuration parameters related to 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.

    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.

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

          - 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.

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       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.

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

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

          - 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.

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       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.

       For each multicast interface:

          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 *

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                         MaxRtrAdvInterval.

                         Default: 0.33 * MaxRtrAdvInterval

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

                         Default: FALSE

          AdvOtherConfigFlag
                         The TRUE/FALSE value to be placed in the "Other
                         stateful 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 that
                         current diameter of the Internet.  The value zero
                         means unspecified (by this router).

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

          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.

                         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 MUST 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

                       Automatic address configuration [ADDRCONF]
                       defines additional information associated with
                       each 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
                                 MUST 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).

                            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
       variables described above.  However, external router behavior MUST be

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       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 multicast 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.  See [ADDRCONF].

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

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

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       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

          - shutting down the system.

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       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 insure 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.

       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.

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        - 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, 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.  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.

    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).

        - Values of the M or O flags.

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

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        - 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 6.2.7 then the comparison of the
          lifetimes can not 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 change rarely, 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
       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.

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    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.

       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
                           (unicast) IP packets.

                           Default: The value specified in the "Assigned
                           Numbers" RFC [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.

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                           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.

    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, such as stateful
       autoconfiguration.  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.

       Some Router Advertisement fields (e.g., Cur Hop Limit, Reachable Time
       and Retrans Timer) may contain a value denoting 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:

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          - 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,
       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 recompute 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 Neighbor Unreachability
       Detection messages 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 insure that
       a new random value gets recomputed 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

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       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 default 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
       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

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            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 increase the 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.

          Note: Implementations can choose to process the on-link aspects of
          the prefixes separately from the 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,

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       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).
            An implementation may choose to always return the same router or
            cycle through the router list in a round-robin fashion as long
            as it always returns a reachable or a probably reachable router
            when one is available.

         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.

         3) If the Default Router List is empty, assume that all
            destinations are on-link as specified in Section 5.2.

    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.

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          - 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
       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.

       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.
       Unsolicited Router Advertisements may be incomplete (see Section
       6.2.3); solicited advertisements are expected to contain complete
       information.

       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.

    7.  ADDRESS RESOLUTION AND NEIGHBOR UNREACHABILITY DETECTION

       This section describes the functions related to Neighbor Solicitation
       and Neighbor Advertisement messages and includes descriptions of
       address resolution and the Neighbor Unreachability Detection
       algorithm.

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       Neighbor Solicitation and Advertisement messages are also used for
       Duplicate Address Detection as specified by [ADDRCONF].  In
       particular, Duplicate Address Detection sends Neighbor Solicitation
       messages with an unspecified source address targeting its own
       "tentative" address.  Such messages trigger nodes already using the
       address to respond with a multicast Neighbor Advertisement indicating
       that the address is in use.

    7.1.  Message Validation

    7.1.1.  Validation of Neighbor Solicitations

       A node MUST silently discard any received Neighbor 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.

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

          - ICMP Checksum is valid.

          - ICMP Code is 0.

          - ICMP length (derived from the IP length) is 24 or more octets.

          - Target Address is not a multicast address.

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

          - If the IP source address is the unspecified address, the IP
            destination address is a solicited-node multicast address.

          - 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 Neighbor 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.

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

    7.1.2.  Validation of Neighbor Advertisements

       A node MUST silently discard any received Neighbor Advertisement
       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.

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

          - ICMP Checksum is valid.

          - ICMP Code is 0.

          - ICMP length (derived from the IP length) is 24 or more octets.

          - Target Address is not a multicast address.

          - If the IP Destination Address is a multicast address the
            Solicited flag is zero.

          - 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 Neighbor Advertisement messages MUST be ignored and the packet
       processed as normal.  The only defined option that may appear is the
       Target Link-Layer Address option.

       A Neighbor Advertisements that passes the validity checks is called a
       "valid advertisement".

    7.2.  Address Resolution

       Address resolution is the process through which a node determines the
       link-layer address of a neighbor given only its IP address.  Address
       resolution is performed only on addresses that are determined to be
       on-link and for which the sender does not know the corresponding
       link-layer address.  Address resolution is never performed on
       multicast addresses.

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    7.2.1.  Interface Initialization

       When a multicast-capable interface becomes enabled the node MUST join
       the all-nodes multicast address on that interface, as well as the
       solicited-node multicast address corresponding to each of the IP
       addresses assigned to the interface.

       The set of addresses assigned to an interface may change over time.
       New addresses might be added and old addresses might be removed
       [ADDRCONF].  In such cases the node MUST join and leave the
       solicited-node multicast address corresponding to the new and old
       addresses, respectively.  Note that multiple unicast addresses may
       map into the same solicited-node multicast address; a node MUST NOT
       leave the solicited-node multicast group until all assigned addresses
       corresponding to that multicast address have been removed.

    7.2.2.  Sending Neighbor Solicitations

       When a node has a unicast packet to send to a neighbor, but does not
       know the neighbor's link-layer address, it performs address
       resolution.  For multicast-capable interfaces this entails creating a
       Neighbor Cache entry in the INCOMPLETE state and transmitting a
       Neighbor Solicitation message targeted at the neighbor.  The
       solicitation is sent to the solicited-node multicast address
       corresponding to the target address.

       If the source address of the packet prompting the solicitation is the
       same as one of the addresses assigned to the outgoing interface, that
       address SHOULD be placed in the IP Source Address of the outgoing
       solicitation.  Otherwise, any one of the addresses assigned to the
       interface should be used.  Using the prompting packet's source
       address when possible insures that the recipient of the Neighbor
       Solicitation installs in its Neighbor Cache the IP address that is
       highly likely to be used in subsequent return traffic belonging to
       the prompting packet's "connection".

       If the solicitation is being sent to a solicited-node multicast
       address, the sender MUST include its link-layer address (if it has
       one) as a Source Link-Layer Address option.  Otherwise, the sender
       SHOULD include its link-layer address (if it has one) as a Source
       Link-Layer Address option.  Including the source link-layer address
       in a multicast solicitation is required to give the target an address
       to which it can send the Neighbor Advertisement.  On unicast
       solicitations, an implementation MAY omit the Source Link-Layer
       Address option. The assumption here is that if the sender has a
       peer's link-layer address in its cache, there is a high probability
       that the peer will also have an entry in its cache for the sender.
       Consequently, it need not be sent.

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       While waiting for address resolution to complete, the sender MUST,
       for each neighbor, retain a small queue of packets waiting for
       address resolution to complete.  The queue MUST hold at least one
       packet, and MAY contain more.  However, the number of queued packets
       per neighbor SHOULD be limited to some small value.  When a queue
       overflows, the new arrival SHOULD replace the oldest entry.  Once
       address resolution completes, the node transmits any queued packets.

       While awaiting a response, the sender SHOULD retransmit Neighbor
       Solicitation messages approximately every RetransTimer milliseconds,
       even in the absence of additional traffic to the neighbor.
       Retransmissions MUST be rate-limited to at most one solicitation per
       neighbor every RetransTimer milliseconds.

       If no Neighbor Advertisement is received after MAX_MULTICAST_SOLICIT
       solicitations, address resolution has failed.  The sender MUST return
       ICMP destination unreachable indications with code 3 (Address
       Unreachable) for each packet queued awaiting address resolution.

    7.2.3.  Receipt of Neighbor Solicitations

       A valid Neighbor Solicitation that does not meet any the following
       requirements MUST be silently discarded:

        - The Target Address is a "valid" unicast or anycast address
          assigned to the receiving interface [ADDRCONF],

        - The Target Address is a unicast address for which the node is
          offering proxy service, or

        - The Target Address is a "tentative" address on which Duplicate
          Address Detection is being performed [ADDRCONF].

       If the Target Address is tentative, the Neighbor Solicitation should
       be processed as described in [ADDRCONF].  Otherwise, the following
       description applies.  If the Source Address is not the unspecified
       address and, on link layers that have addresses, the solicitation
       includes a Source Link-Layer Address option, then the recipient
       SHOULD create or update the Neighbor Cache entry for the IP Source
       Address of the solicitation.  If an entry does not already exist, the
       node SHOULD create a new one and set its reachability state to STALE
       as specified in Section 7.3.3.  If an entry already exists, and the
       cached link-layer address differs from the one in the received Source
       Link-Layer option, the cached address should be replaced by the
       received address and the entry's reachability state MUST be set to
       STALE.

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       If a Neighbor Cache entry is created the IsRouter flag SHOULD be set
       to FALSE.  This will be the case even if the Neighbor Solicitation is
       sent by a router since the Neighbor Solicitation messages do not
       contain an indication of whether or not the sender is a router.  In
       the event that the sender is a router, subsequent Neighbor
       Advertisement or Router Advertisement messages will set the correct
       IsRouter value.  If a Neighbor Cache entry already exists its
       IsRouter flag MUST NOT be modified.

       If the Source Address is the unspecified address the node MUST NOT
       create or update the Neighbor Cache entry.

       After any updates to the Neighbor Cache, the node sends a Neighbor
       Advertisement response as described in the next section.

    7.2.4.  Sending Solicited Neighbor Advertisements

       A node sends a Neighbor Advertisement in response to a valid Neighbor
       Solicitation targeting one of the node's assigned addresses.  The
       Target Address of the advertisement is copied from the Target Address
       of the solicitation.  If the solicitation's IP Destination Address is
       not a multicast address, the Target Link-Layer Address option MAY be
       omitted; the neighboring node's cached value must already be current
       in order for the solicitation to have been received.  If the
       solicitation's IP Destination Address is a multicast address, the
       Target Link-Layer option MUST be included in the advertisement.
       Furthermore, if the node is a router, it MUST set the Router flag to
       one; otherwise it MUST set the flag to zero.

       If the Target Address is either an anycast address or a unicast
       address for which the node is providing proxy service, or the Target
       Link-Layer Address option is not included, the Override flag SHOULD
       be set to zero.  Otherwise, the Override flag SHOULD be set to one.
       Proper setting of the Override flag ensures that nodes give
       preference to non-proxy advertisements, even when received after
       proxy advertisements, and also ensures that the first advertisement
       for an anycast address "wins".

       If the source of the solicitation is the unspecified address, the
       node MUST set the Solicited flag to zero and multicast the
       advertisement to the all-nodes address.  Otherwise, the node MUST set
       the Solicited flag to one and unicast the advertisement to the Source
       Address of the solicitation.

       If the Target Address is an anycast address the sender SHOULD delay
       sending a response for a random time between 0 and
       MAX_ANYCAST_DELAY_TIME seconds.

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       Because unicast Neighbor Solicitations are not required to include a
       Source Link-Layer Address, it is possible that a node sending a
       solicited Neighbor Advertisement does not have a corresponding link-
       layer address for its neighbor in its Neighbor Cache.  In such
       situations, a node will first have to use Neighbor Discovery to
       determine the link-layer address of its neighbor (i.e, send out a
       multicast Neighbor Solicitation).

    7.2.5.  Receipt of Neighbor Advertisements

       When a valid Neighbor Advertisement is received (either solicited or
       unsolicited), the Neighbor Cache is searched for the target's entry.
       If no entry exists, the advertisement SHOULD be silently discarded.
       There is no need to create an entry if none exists, since the
       recipient has apparently not initiated any communication with the
       target.

       Once the appropriate Neighbor Cache entry has been located, the
       specific actions taken depend on the state of the Neighbor Cache
       entry, the flags in the advertisement and the actual link-layer
       address supplied.

       If the target's Neighbor Cache entry is in the INCOMPLETE state when
       the advertisement is received, one of two things happens.  If the
       link layer has addresses and no Target Link-Layer address option is
       included, the receiving node SHOULD silently discard the received
       advertisement.  Otherwise, the receiving node performs the following
       steps:

        - It records the link-layer address in the Neighbor Cache entry.

        - If the advertisement's Solicited flag is set, the state of the
          entry is set to REACHABLE, otherwise it is set to STALE.

        - It sets the IsRouter flag in the cache entry based on the Router
          flag in the received advertisement.

        - It sends any packets queued for the neighbor awaiting address
          resolution.

       Note that the Override flag is ignored if the entry is in the
       INCOMPLETE state.

       If the target's Neighbor Cache entry is in any state other than
       INCOMPLETE when the advertisement is received, processing becomes
       quite a bit more complex.  If the Override flag is clear and the
       supplied link-layer address differs from that in the cache, then one
       of two actions takes place: if the state of the entry is REACHABLE,

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       set it to STALE, but do not update the entry in any other way;
       otherwise, the received advertisement should be ignored and MUST NOT
       update the cache.  If the Override flag is set, both the Override
       flag is clear and the supplied link-layer address is the same as that
       in the cache, or no Target Link-layer address option was supplied,
       the received advertisement MUST update the Neighbor Cache entry as
       follows:

        - The link-layer address in the Target Link-Layer Address option
          MUST be inserted in the cache (if one is supplied and is different
          than the already recorded address).

        - If the Solicited flag is set, the state of the entry MUST be set
          to REACHABLE.  If the Solicited flag is zero and the link-layer
          address was updated with a different address the state MUST be set
          to STALE.  Otherwise, the entry's state remains unchanged.

          An advertisement's Solicited flag should only be set if the
          advertisement is a response to a Neighbor Solicitation.  Because
          Neighbor Unreachability Detection Solicitations are sent to the
          cached link-layer address, receipt of a solicited advertisement
          indicates that the forward path is working.  Receipt of an
          unsolicited advertisement, however, suggests that a neighbor has
          urgent information to announce (e.g., a changed link-layer
          address).  If the urgent information indicates a change from what
          a node is currently using, the node should verify the reachability
          of the (new) path when it sends the next packet.  There is no need
          to update the state for unsolicited advertisements that do not
          change the contents of the cache.

        - The IsRouter flag in the cache entry MUST be set based on the
          Router flag in the received advertisement.  In those cases where
          the IsRouter flag changes from TRUE to FALSE as a result of this
          update, the node MUST remove that router from the Default Router
          List and update the Destination Cache entries for all destinations
          using that neighbor as a router as specified in Section 7.3.3.
          This is needed to detect when a node that is used as a router
          stops forwarding packets due to being configured as a host.

       The above rules ensure that the cache is updated either when the
       Neighbor Advertisement takes precedence (i.e., the Override flag is
       set) or when the Neighbor Advertisement refers to the same link-layer
       address that is currently recorded in the cache.  If none of the
       above apply, the advertisement prompts future Neighbor Unreachability
       Detection (if it is not already in progress) by changing the state in
       the cache entry.

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    7.2.6.  Sending Unsolicited Neighbor Advertisements

       In some cases a node may be able to determine that its link-layer
       address has changed (e.g., hot-swap of an interface card) and may
       wish to inform its neighbors of the new link-layer address quickly.
       In such cases a node MAY send up to MAX_NEIGHBOR_ADVERTISEMENT
       unsolicited Neighbor Advertisement messages to the all-nodes
       multicast address.  These advertisements MUST be separated by at
       least RetransTimer seconds.

       The Target Address field in the unsolicited advertisement is set to
       an IP address of the interface, and the Target Link-Layer Address
       option is filled with the new link-layer address.  The Solicited flag
       MUST be set to zero, in order to avoid confusing the Neighbor
       Unreachability Detection algorithm.  If the node is a router, it MUST
       set the Router flag to one; otherwise it MUST set it to zero.  The
       Override flag MAY be set to either zero or one.  In either case,
       neighboring nodes will immediately change the state of their Neighbor
       Cache entries for the Target Address to STALE, prompting them to
       verify the path for reachability.  If the Override flag is set to
       one, neighboring nodes will install the new link-layer address in
       their caches.  Otherwise, they will ignore the new link-layer
       address, choosing instead to probe the cached address.

       A node that has multiple IP addresses assigned to an interface MAY
       multicast a separate Neighbor Advertisement for each address.  In
       such a case the node SHOULD introduce a small delay between the
       sending of each advertisement to reduce the probability of the
       advertisements being lost due to congestion.

       A proxy MAY multicast Neighbor Advertisements when its link-layer
       address changes or when it is configured (by system management or
       other mechanisms) to proxy for an address.  If there are multiple
       nodes that are providing proxy services for the same set of addresses
       the proxies SHOULD provide a mechanism that prevents multiple proxies
       from multicasting advertisements for any one address, in order to
       reduce the risk of excessive multicast traffic.

       Also, a node belonging to an anycast address MAY multicast
       unsolicited Neighbor Advertisements for the anycast address when the
       node's link-layer address changes.

       Note that because unsolicited Neighbor Advertisements do not reliably
       update caches in all nodes (the advertisements might not be received
       by all nodes), they should only be viewed as a performance
       optimization to quickly update the caches in most neighbors.  The
       Neighbor Unreachability Detection algorithm ensures that all nodes
       obtain a reachable link-layer address, though the delay may be

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       slightly longer.

    7.2.7.  Anycast Neighbor Advertisements

       From the perspective of Neighbor Discovery, anycast addresses are
       treated just like unicast addresses in most cases.  Because an
       anycast address is syntactically the same as a unicast address, nodes
       performing address resolution or Neighbor Unreachability Detection on
       an anycast address treat it as if it were a unicast address.  No
       special processing takes place.

       Nodes that have an anycast address assigned to an interface treat
       them exactly the same as if they were unicast addresses with two
       exceptions.  First, Neighbor Advertisements sent in response to a
       Neighbor Solicitation SHOULD be delayed by a random time between 0
       and MAX_ANYCAST_DELAY_TIME to reduce the probability of network
       congestion.  Second, the Override flag in Neighbor Advertisements
       SHOULD be set to 0, so that when multiple advertisements are
       received, the first received advertisement is used rather than the
       most recently received advertisement.

       As with unicast addresses, Neighbor Unreachability Detection ensures
       that a node quickly detects when the current binding for an anycast
       address becomes invalid.

    7.2.8.  Proxy Neighbor Advertisements

       Under limited circumstances, a router MAY proxy for one or more other
       nodes, that is, through Neighbor Advertisements indicate that it is
       willing to accept packets not explicitly addressed to itself.  For
       example, a router might accept packets on behalf of a mobile node
       that has moved off-link.  The mechanisms used by proxy are identical
       to the mechanisms used with anycast addresses.

       A proxy MUST join the solicited-node multicast address(es) that
       correspond to the IP address(es) assigned to the node for which it is
       proxying.

       All solicited proxy Neighbor Advertisement messages MUST have the
       Override flag set to zero.  This ensures that if the node itself is
       present on the link its Neighbor Advertisement (with the Override
       flag set to one) will take precedence of any advertisement received
       from a proxy.  A proxy MAY send unsolicited advertisements with the
       Override flag set to one as specified in Section 7.2.6, but doing so
       may cause the proxy advertisement to override a valid entry created
       by the node itself.

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       Finally, when sending a proxy advertisement in response to a Neighbor
       Solicitation, the sender should delay its response by a random time
       between 0 and MAX_ANYCAST_DELAY_TIME seconds.

    7.3.  Neighbor Unreachability Detection

       Communication to or through a neighbor may fail for numerous reasons
       at any time, including hardware failure, hot-swap of an interface
       card, etc.  If the destination has failed, no recovery is possible
       and communication fails.  On the other hand, if it is the path that
       has failed, recovery may be possible.  Thus, a node actively tracks
       the reachability "state" for the neighbors to which it is sending
       packets.

       Neighbor Unreachability Detection is used for all paths between hosts
       and neighboring nodes, including host-to-host, host-to-router, and
       router-to-host communication.  Neighbor Unreachability Detection may
       also be used between routers, but is not required if an equivalent
       mechanism is available, for example, as part of the routing
       protocols.

       When a path to a neighbor appears to be failing, the specific
       recovery procedure depends on how the neighbor is being used.  If the
       neighbor is the ultimate destination, for example, address resolution
       should be performed again.  If the neighbor is a router, however,
       attempting to switch to another router would be appropriate.  The
       specific recovery that takes place is covered under next-hop
       determination; Neighbor Unreachability Detection signals the need for
       next-hop determination by deleting a Neighbor Cache entry.

       Neighbor Unreachability Detection is performed only for neighbors to
       which unicast packets are sent; it is not used when sending to
       multicast addresses.

    7.3.1.  Reachability Confirmation

       A neighbor is considered reachable if the node has recently received
       a confirmation that packets sent recently to the neighbor were
       received by its IP layer.  Positive confirmation can be gathered in
       two ways: hints from upper layer protocols that indicate a connection
       is making "forward progress", or receipt of a Neighbor Advertisement
       message that is a response to a Neighbor Solicitation message.

       A connection makes "forward progress" if the packets received from a
       remote peer can only be arriving if recent packets sent to that peer
       are actually reaching it.  In TCP, for example, receipt of a (new)
       acknowledgement indicates that previously sent data reached the peer.
       Likewise, the arrival of new (non-duplicate) data indicates that

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       earlier acknowledgements are being delivered to the remote peer.  If
       packets are reaching the peer, they must also be reaching the
       sender's next-hop neighbor; thus "forward progress" is a confirmation
       that the next-hop neighbor is reachable.  For off-link destinations,
       forward progress implies that the first-hop router is reachable.
       When available, this upper-layer information SHOULD be used.

       In some cases (e.g., UDP-based protocols and routers forwarding
       packets to hosts) such reachability information may not be readily
       available from upper-layer protocols.  When no hints are available
       and a node is sending packets to a neighbor, the node actively probes
       the neighbor using unicast Neighbor Solicitation messages to verify
       that the forward path is still working.

       The receipt of a solicited Neighbor Advertisement serves as
       reachability confirmation, since advertisements with the Solicited
       flag set to one are sent only in response to a Neighbor Solicitation.
       Receipt of other Neighbor Discovery messages such as Router
       Advertisements and Neighbor Advertisement with the Solicited flag set
       to zero MUST NOT be treated as a reachability confirmation.  Receipt
       of unsolicited messages only confirm the one-way path from the sender
       to the recipient node.  In contrast, Neighbor Unreachability
       Detection requires that a node keep track of the reachability of the
       forward path to a neighbor from the its perspective, not the
       neighbor's perspective.  Note that receipt of a solicited
       advertisement indicates that a path is working in both directions.
       The solicitation must have reached the neighbor, prompting it to
       generate an advertisement.  Likewise, receipt of an advertisement
       indicates that the path from the sender to the recipient is working.
       However, the latter fact is known only to the recipient; the
       advertisement's sender has no direct way of knowing that the
       advertisement it sent actually reached a neighbor.  From the
       perspective of Neighbor Unreachability Detection, only the
       reachability of the forward path is of interest.

    7.3.2.  Neighbor Cache Entry States

       A Neighbor Cache entry can be in one of five states:

          INCOMPLETE  Address resolution is being performed on the entry.
                      Specifically, a Neighbor Solicitation has been sent to
                      the solicited-node multicast address of the target,
                      but the corresponding Neighbor Advertisement has not
                      yet been received.

          REACHABLE   Positive confirmation was received within the last
                      ReachableTime milliseconds that the forward path to
                      the neighbor was functioning properly.  While

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                      REACHABLE, no special action takes place as packets
                      are sent.

          STALE       More than ReachableTime milliseconds have elapsed
                      since the last positive confirmation was received that
                      the forward path was functioning properly.  While
                      stale, no action takes place until a packet is sent.

                      The STALE state is entered upon receiving an
                      unsolicited Neighbor Discovery message that updates
                      the cached link-layer address.  Receipt of such a
                      message does not confirm reachability, and entering
                      the STALE state insures reachability is verified
                      quickly if the entry is actually being used.  However,
                      reachability is not actually verified until the entry
                      is actually used.

          DELAY       More than ReachableTime milliseconds have elapsed
                      since the last positive confirmation was received that
                      the forward path was functioning properly, and a
                      packet was sent within the last DELAY_FIRST_PROBE_TIME
                      seconds.  If no reachability confirmation is received
                      within DELAY_FIRST_PROBE_TIME seconds of entering the
                      DELAY state, send a Neighbor Solicitation and change
                      the state to PROBE.

                      The DELAY state is an optimization that gives upper-
                      layer protocols additional time to provide
                      reachability confirmation in those cases where
                      ReachableTime milliseconds have passed since the last
                      confirmation due to lack of recent traffic.  Without
                      this optimization the opening of a TCP connection
                      after a traffic lull would initiate probes even though
                      the subsequent three-way handshake would provide a
                      reachability confirmation almost immediately.

          PROBE       A reachability confirmation is actively sought by
                      retransmitting Neighbor Solicitations every
                      RetransTimer milliseconds until a reachability
                      confirmation is received.

    7.3.3.  Node Behavior

       Neighbor Unreachability Detection operates in parallel with the
       sending of packets to a neighbor.  While reasserting a neighbor's
       reachability, a node continues sending packets to that neighbor using
       the cached link-layer address.  If no traffic is sent to a neighbor,
       no probes are sent.

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       When a node needs to perform address resolution on a neighboring
       address, it creates an entry in the INCOMPLETE state and initiates
       address resolution as specified in Section 7.2.  If address
       resolution fails, the entry SHOULD be deleted, so that subsequent
       traffic to that neighbor invokes the next-hop determination procedure
       again.  Invoking next-hop determination at this point insures that
       alternate default routers are tried.

       When a reachability confirmation is received (either through upper-
       layer advice or a solicited Neighbor Advertisement) an entry's state
       changes to REACHABLE.  The one exception is that upper-layer advice
       has no effect on entries in the INCOMPLETE state (e.g., for which no
       link-layer address is cached).

       When ReachableTime milliseconds have passed since receipt of the last
       reachability confirmation for a neighbor, the Neighbor Cache entry's
       state changes from REACHABLE to STALE.

          Note: An implementation may actually defer changing the state from
          REACHABLE to STALE until a packet is sent to the neighbor, i.e.,
          there need not be an explicit timeout event associated with the
          expiration of ReachableTime.

       The first time a node sends a packet to a neighbor whose entry is
       STALE, the sender changes the state to DELAY and a sets a timer to
       expire in DELAY_FIRST_PROBE_TIME seconds.  If the entry is still in
       the DELAY state when the timer expires, the entry's state changes to
       PROBE.  If reachability confirmation is received, the entry's state
       changes to REACHABLE.

       Upon entering the PROBE state, a node sends a unicast Neighbor
       Solicitation message to the neighbor using the cached link-layer
       address.  While in the PROBE state, a node retransmits Neighbor
       Solicitation messages every RetransTimer milliseconds until
       reachability confirmation is obtained.  Probes are retransmitted even
       if no additional packets are sent to the neighbor.  If no response is
       received after waiting RetransTimer milliseconds after sending the
       MAX_UNICAST_SOLICIT solicitations, retransmissions cease and the
       entry SHOULD be deleted.  Subsequent traffic to that neighbor will
       recreate the entry and performs address resolution again.

       Note that all Neighbor Solicitations are rate-limited on a per-
       neighbor basis.  A node MUST NOT send Neighbor Solicitations to the
       same neighbor more frequently than once every RetransTimer
       milliseconds.

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       A Neighbor Cache entry enters the STALE state when created as a
       result of receiving packets other than solicited Neighbor
       Advertisements (i.e., Router Solicitations, Router Advertisements,
       Redirects, and Neighbor Solicitations).  These packets contain the
       link-layer address of either the sender or, in the case of Redirect,
       the redirection target.  However, receipt of these link-layer
       addresses does not confirm reachability of the forward-direction path
       to that node.  Placing a newly created Neighbor Cache entry for which
       the link-layer address is known in the STALE state provides assurance
       that path failures are detected quickly.  In addition, should a
       cached link-layer address be modified due to receiving one of the
       above messages the state SHOULD also be set to STALE to provide
       prompt verification that the path to the new link-layer address is
       working.

       To properly detect the case where a router switches from being a
       router to being a host (e.g., if its IP forwarding capability is
       turned off by system management), a node MUST compare the Router flag
       field in all received Neighbor Advertisement messages with the
       IsRouter flag recorded in the Neighbor Cache entry.  When a node
       detects that a neighbor has changed from being a router to being a
       host, the node MUST remove that router from the Default Router List
       and update the Destination Cache as described in Section 6.3.5.  Note
       that a router may not be listed in the Default Router List, even
       though a Destination Cache entry is using it (e.g., a host was
       redirected to it).  In such cases, all Destination Cache entries that
       reference the (former) router must perform next-hop determination
       again before using the entry.

       In some cases, link-specific information may indicate that a path to
       a neighbor has failed (e.g., the resetting of a virtual circuit).  In
       such cases, link-specific information may be used to purge Neighbor
       Cache entries before the Neighbor Unreachability Detection would do
       so.  However, link-specific information MUST NOT be used to confirm
       the reachability of a neighbor; such information does not provide
       end-to-end confirmation between neighboring IP layers.

    8.  REDIRECT FUNCTION

       This section describes the functions related to the sending and
       processing of Redirect messages.

       Redirect messages are sent by routers to redirect a host to a better
       first-hop router for a specific destination or to inform hosts that a
       destination is in fact a neighbor (i.e., on-link).  The latter is
       accomplished by having the ICMP Target Address be equal to the ICMP
       Destination Address.

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       A router MUST be able to determine the link-local address for each of
       its neighboring routers in order to ensure that the target address in
       a Redirect message identifies the neighbor router by its link-local
       address.  For static routing this requirement implies that the next-
       hop router's address should be specified using the link-local address
       of the router.  For dynamic routing this requirement implies that all
       IPv6 routing protocols must somehow exchange the link-local addresses
       of neighboring routers.

    8.1.  Validation of Redirect Messages

       A host MUST silently discard any received Redirect message that does
       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.

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

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

          - ICMP Checksum is valid.

          - ICMP Code is 0.

          - ICMP length (derived from the IP length) is 40 or more octets.

          - The IP source address of the Redirect is the same as the current
            first-hop router for the specified ICMP Destination Address.

          - The ICMP Destination Address field in the redirect message does
            not contain a multicast address.

          - The ICMP Target Address is either a link-local address (when
            redirected to a router) or the same as the ICMP Destination
            Address (when redirected to the on-link destination).

          - 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.

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

       A host MUST NOT consider a redirect invalid just because the Target
       Address of the redirect is not covered under one of the link's
       prefixes.  Part of the semantics of the Redirect message is that the
       Target Address is on-link.

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

    8.2.  Router Specification

       A router SHOULD send a redirect message, subject to rate limiting,
       whenever it forwards a packet that is not explicitly addressed to
       itself (i.e. a packet that is not source routed through the router)
       in which:

          - the Source Address field of the packet identifies a neighbor,
            and

          - the router determines that a better first-hop node resides on
            the same link as the sending node for the Destination Address of
            the packet being forwarded, and

          - the Destination Address of the packet is not a multicast
            address, and

       The transmitted redirect packet contains, consistent with the message
       format given in Section 4.5:

          - In the Target Address field: the address to which subsequent
            packets for the destination SHOULD be sent.  If the target is a
            router, that router's link-local address MUST be used.  If the
            target is a host the target address field MUST be set to the
            same value as the Destination Address field.

          - In the Destination Address field: the destination address of the
            invoking IP packet.

          - In the options:

               o Target Link-Layer Address option: link-layer address of the
                 target, if known.

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               o Redirected Header: as much of the forwarded packet as can
                 fit without the redirect packet exceeding 1280 octets in
                 size.

       A router MUST limit the rate at which Redirect messages are sent, in
       order to limit the bandwidth and processing costs incurred by the
       Redirect messages when the source does not correctly respond to the
       Redirects, or the source chooses to ignore unauthenticated Redirect
       messages.  More details on the rate-limiting of ICMP error messages
       can be found in [ICMPv6].

       A router MUST NOT update its routing tables upon receipt of a
       Redirect.

    8.3.  Host Specification

       A host receiving a valid redirect SHOULD update its Destination Cache
       accordingly so that subsequent traffic goes to the specified target.
       If no Destination Cache entry exists for the destination, an
       implementation SHOULD create such an entry.

       If the redirect contains a Target Link-Layer Address option the host
       either creates or updates the Neighbor Cache entry for the target.
       In both cases the cached link-layer address is copied from the Target
       Link-Layer Address option.  If a Neighbor Cache entry is created for
       the target its reachability state MUST be set to STALE as specified
       in Section 7.3.3.  If a cache entry already existed and it is updated
       with a different link-layer address, its reachability state MUST also
       be set to STALE.  If the link-layer address is the same as that
       already in the cache, the cache entry's state remains unchanged.

       If the Target and Destination Addresses are the same, the host MUST
       treat the Target as on-link.  If the Target Address is not the same
       as the Destination Address, the host MUST set IsRouter to TRUE for
       the target.  If the Target and Destination Addresses are the same,
       however, one cannot reliably determine whether the Target Address is
       a router.  Consequently, newly created Neighbor Cache entries should
       set the IsRouter flag to FALSE, while existing cache entries should
       leave the flag unchanged.  If the Target is a router, subsequent
       Neighbor Advertisement or Router Advertisement messages will update
       IsRouter accordingly.

       Redirect messages apply to all flows that are being sent to a given
       destination.  That is, upon receipt of a Redirect for a Destination
       Address, all Destination Cache entries to that address should be
       updated to use the specified next-hop, regardless of the contents of
       the Flow Label field that appears in the Redirected Header option.

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       A host MAY have a configuration switch that can be set to make it
       ignore a Redirect message that does not have an IP Authentication
       header.

       A host MUST NOT send Redirect messages.

    9.  EXTENSIBILITY - OPTION PROCESSING

       Options provide a mechanism for encoding variable length fields,
       fields that may appear multiple times in the same packet, or
       information that may not appear in all packets.  Options can also be
       used to add additional functionality to future versions of ND.

       In order to ensure that future extensions properly coexist with
       current implementations, all nodes MUST silently ignore any options
       they do not recognize in received ND packets and continue processing
       the packet.  All options specified in this document MUST be
       recognized.  A node MUST NOT ignore valid options just because the ND
       message contains unrecognized ones.

       The current set of options is defined in such a way that receivers
       can process multiple options in the same packet independently of each
       other.  In order to maintain these properties future options SHOULD
       follow the simple rule:

            The option MUST NOT depend on the presence or absence of any
            other options.  The semantics of an option should depend only on
            the information in the fixed part of the ND packet and on the
            information contained in the option itself.

       Adhering to the above rule has the following benefits:

         1) Receivers can process options independently of one another.  For
            example, an implementation can choose to process the Prefix
            Information option contained in a Router Advertisement message
            in a user-space process while the link-layer address option in
            the same message is processed by routines in the kernel.

         2) Should the number of options cause a packet to exceed a link's
            MTU, multiple packets can carry subsets of the options without
            any change in semantics.

         3) Senders MAY send a subset of options in different packets.  For
            instance, if a prefix's Valid and Preferred Lifetime are high
            enough, it might not be necessary to include the Prefix
            Information option in every Router Advertisement.  In addition,
            different routers might send different sets of options.  Thus, a
            receiver MUST NOT associate any action with the absence of an

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            option in a particular packet.  This protocol specifies that
            receivers should only act on the expiration of timers and on the
            information that is received in the packets.

       Options in Neighbor Discovery packets can appear in any order;
       receivers MUST be prepared to process them independently of their
       order.  There can also be multiple instances of the same option in a
       message (e.g., Prefix Information options).

       If the number of included options in a Router Advertisement causes
       the advertisement's size to exceed the link MTU, the router can send
       multiple separate advertisements each containing a subset of the
       options.

       The amount of data to include in the Redirected Header option MUST be
       limited so that the entire redirect packet does not exceed 1280
       octets.

       All options are a multiple of 8 octets of length, ensuring
       appropriate alignment without any "pad" options.  The fields in the
       options (as well as the fields in ND packets) are defined to align on
       their natural boundaries (e.g., a 16-bit field is aligned on a 16-bit
       boundary) with the exception of the 128-bit IP addresses/prefixes,
       which are aligned on a 64-bit boundary.  The link-layer address field
       contains an uninterpreted octet string; it is aligned on an 8-bit
       boundary.

       The size of an ND packet including the IP header is limited to the
       link MTU (which is at least 1280 octets).  When adding options to an
       ND packet a node MUST NOT exceed the link MTU.

       Future versions of this protocol may define new option types.
       Receivers MUST silently ignore any options they do not recognize and
       continue processing the message.

    10.  PROTOCOL CONSTANTS

       Router constants:

                MAX_INITIAL_RTR_ADVERT_INTERVAL  16 seconds

                MAX_INITIAL_RTR_ADVERTISEMENTS    3 transmissions

                MAX_FINAL_RTR_ADVERTISEMENTS      3 transmissions

                MIN_DELAY_BETWEEN_RAS             3 seconds

                MAX_RA_DELAY_TIME                 .5 seconds

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       Host constants:

                MAX_RTR_SOLICITATION_DELAY        1 second

                RTR_SOLICITATION_INTERVAL         4 seconds

                MAX_RTR_SOLICITATIONS             3 transmissions

       Node constants:

                MAX_MULTICAST_SOLICIT             3 transmissions

                MAX_UNICAST_SOLICIT               3 transmissions

                MAX_ANYCAST_DELAY_TIME            1 second

                MAX_NEIGHBOR_ADVERTISEMENT        3 transmissions

                REACHABLE_TIME               30,000 milliseconds

                RETRANS_TIMER                 1,000 milliseconds

                DELAY_FIRST_PROBE_TIME            5 seconds

                MIN_RANDOM_FACTOR                 .5

                MAX_RANDOM_FACTOR                 1.5

       Additional protocol constants are defined with the message formats in
       Section 4.

       All protocol constants are subject to change in future revisions of
       the protocol.

       The constants in this specification may be overridden by specific
       documents that describe how IPv6 operates over different link layers.
       This rule allows Neighbor Discovery to operate over links with widely
       varying performance characteristics.

    11.  SECURITY CONSIDERATIONS

       Neighbor Discovery is subject to attacks that cause IP packets to
       flow to unexpected places.  Such attacks can be used to cause denial
       of service but also allow nodes to intercept and optionally modify
       packets destined for other nodes.

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       The protocol reduces the exposure to such threats in the absence of
       authentication by ignoring ND packets received from off-link senders.
       The Hop Limit field of all received packets is verified to contain
       255, the maximum legal value.  Because routers decrement the Hop
       Limit on all packets they forward, received packets containing a Hop
       Limit of 255 must have originated from a neighbor.

       An example of denial of service attacks is that a node on the link
       that can send packets with an arbitrary IP source address can both
       advertise itself as a default router and also send "forged" Router
       Advertisement messages that immediately time out all other default
       routers as well as all on-link prefixes.  An intruder can achieve
       this by sending out multiple Router Advertisements, one for each
       legitimate router, with the source address set to the address of
       another router, the Router Lifetime field set to zero, and the
       Preferred and Valid lifetimes set to zero for all the prefixes.  Such
       an attack would cause all packets, for both on-link and off-link
       destinations, to go to the rogue router.  That router can then
       selectively examine, modify or drop all packets sent on the link. The
       Neighbor Unreachability Detection will not detect such a black hole
       as long as the rogue router politely answers the NUD probes with a
       Neighbor Advertisement with the R-bit set.

       Many link layers are also subject to different denial of service
       attacks such as continuously occupying the link in CSMA/CD networks
       (e.g., by sending packets closely back-to-back or asserting the
       collision signal on the link), or originating packets with somebody
       else's source MAC address to confuse, e.g., Ethernet switches.

       The trust model for redirects is the same as in IPv4.  A redirect is
       accepted only if received from the same router that is currently
       being used for that destination.  It is natural to trust the routers
       on the link.  If a host has been redirected to another node (i.e.,
       the destination is on-link) there is no way to prevent the target
       from issuing another redirect to some other destination.  However,
       this exposure is no worse than it was; the target host, once
       subverted, could always act as a hidden router to forward traffic
       elsewhere.

       The protocol contains no mechanism to determine which neighbors are
       authorized to send a particular type of message (e.g., Router
       Advertisements); any neighbor, presumably even in the presence of
       authentication, can send Router Advertisement messages thereby being
       able to cause denial of service.  Furthermore, any neighbor can send
       proxy Neighbor Advertisements as well as unsolicited Neighbor
       Advertisements as a potential denial of service attack.

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    RFC 2461              Neighbor Discovery for IPv6          December 1998

       Neighbor Discovery protocol packet exchanges can be authenticated
       using the IP Authentication Header [IPv6-AUTH].  A node SHOULD
       include an Authentication Header when sending Neighbor Discovery
       packets if a security association for use with the IP Authentication
       Header exists for the destination address.  The security associations
       may have been created through manual configuration or through the
       operation of some key management protocol.

       Received Authentication Headers in Neighbor Discovery packets MUST be
       verified for correctness and packets with incorrect authentication
       MUST be ignored.

       It SHOULD be possible for the system administrator to configure a
       node to ignore any Neighbor Discovery messages that are not
       authenticated using either the Authentication Header or Encapsulating
       Security Payload.  The configuration technique for this MUST be
       documented.  Such a switch SHOULD default to allowing unauthenticated
       messages.

       Confidentiality issues are addressed by the IP Security Architecture
       and the IP Encapsulating Security Payload documents [IPv6-SA, IPv6-
       ESP].

    12.  RENUMBERING CONSIDERATIONS

       The Neighbor Discovery protocol together with IPv6 Address
       Autoconfiguration [ADDRCONF] provides mechanisms to aid in
       renumbering - new prefixes and addresses can be introduced and old
       ones can be deprecated and removed.

       The robustness of these mechanisms is based on all the nodes on the
       link receiving the Router Advertisement messages in a timely manner.
       However, a host might be turned off or be unreachable for an extended
       period of time (i.e., a machine is powered down for months after a
       project terminates).  It is possible to preserve robust renumbering
       in such cases but it does place some constraints on how long prefixes
       must be advertised.

       Consider the following example in which a prefix is initially
       advertised with a lifetime of 2 months, but on August 1st it is
       determined that the prefix needs to be deprecated and removed due to
       renumbering by September 1st.  This can be done by reducing the
       advertised lifetime to 1 week starting on August 1st and as the
       cutoff gets closer the lifetimes can be made shorter until by
       September 1st the prefix is advertised with a zero lifetime.  The
       point is that, if one or more nodes were unplugged from the link
       prior to September 1st they might still think that the prefix is
       valid since the last lifetime they received was 2 months.  Thus if a

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       node was unplugged on July 31st it thinks the prefix is valid until
       September 30th.  If that node is plugged back in prior to September
       30th it may continue to use the old prefix.  The only way to force a
       node to stop using a prefix that was previously advertised with a
       long Lifetime is to have that node receive an advertisement for that
       prefix that changes the lifetime downward.  The solution in this
       example is simple: continue advertising the prefix with a lifetime of
       0 from September 1st until October 1st.

       In general, in order to be robust against nodes that might be
       unplugged from the link it is important to track the furthest into
       the future a particular prefix can be viewed valid by any node on the
       link.  The prefix must then be advertised with a 0 Lifetime until
       that point in future. This "furthest into the future" time is simply
       the maximum, over all Router Advertisements, of the time the
       advertisement was sent plus the prefix's Lifetime contained in the
       advertisement.

       The above has an important implication on using infinite lifetimes.
       If a prefix is advertised with an infinite lifetime, and that prefix
       later needs to be renumbered, it is undesirable to continue
       advertising that prefix with a zero lifetime forever.  Thus either
       infinite lifetimes should be avoided or there must be a limit on how
       long time a node can be unplugged from the link before it is plugged
       back in again.  However, it is unclear how the network administrator
       can enforce a limit on how long time hosts such as laptops can be
       unplugged from the link.

       Network administrators should give serious consideration to using
       relatively short lifetimes (i.e., no more than a few weeks).  While
       it might appear that using long lifetimes would help insure
       robustness, in reality a host will be unable to communicate in the
       absence of properly functioning routers.  Such routers will be
       sending Router Advertisements that contain appropriate (and current)
       prefixes.  A host connected to a network that has no functioning
       routers is likely to have more serious problems than just a lack of a
       valid prefix and address.

       The above discussion does not distinguish between the preferred and
       valid lifetimes.  For all practical purposes it is probably
       sufficient to track the valid lifetime since the preferred lifetime
       will not exceed the valid lifetime.

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    REFERENCES

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

       [ADDR-ARCH]  Hinden, R. and S. Deering, "IP Version 6 Addressing
                    Architecture", RFC 2373, July 1998.

       [ANYCST]     Partridge, C., Mendez, T. and W. Milliken, "Host
                    Anycasting Service", RFC 1546, November 1993.

       [ARP]        Plummer, D., "An Ethernet Address Resolution Protocol",
                    STD 37, RFC 826, November 1982.

       [HR-CL]      Braden, R., Editor, "Requirements for Internet Hosts --
                    Communication Layers", STD 3, RFC 1122, October 1989.

       [ICMPv4]     Postel, J., "Internet Control Message Protocol", STD 5,
                    RFC 792, September 1981.

       [ICMPv6]     Conta, A. and S. Deering, "Internet Control Message
                    Protocol (ICMPv6) for the Internet Protocol Version 6
                    (IPv6) Specification", RFC 2463, December 1998.

       [IPv6]       Deering, S. and R. Hinden, "Internet Protocol, Version 6
                    (IPv6) Specification", RFC 2460, December 1998.

       [IPv6-ETHER] Crawford, M., "Transmission of IPv6 Packets over
                    Ethernet Networks", RFC 2464, December 1998.

       [IPv6-SA]    Kent, S. and R. Atkinson, "Security Architecture for the
                    Internet Protocol", RFC 2401, November 1998.

       [IPv6-AUTH]  Kent, S. and R. Atkinson, "IP Authentication Header",
                    RFC 2402, November 1998.

       [IPv6-ESP]   Kent, S. and R. Atkinson, "IP Encapsulating Security
                    Payload (ESP)", RFC 2406, November 1998.

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

       [RDISC]      Deering, S., "ICMP Router Discovery Messages", RFC 1256,
                    September 1991.

       [SH-MEDIA]   Braden, R., Postel, J. and Y. Rekhter, "Internet
                    Architecture Extensions for Shared Media", RFC 1620, May
                    1994.

    Narten, et. al.             Standards Track                    [Page 80]

    RFC 2461              Neighbor Discovery for IPv6          December 1998

       [ASSIGNED]   Reynolds, J. and J. Postel, "ASSIGNED NUMBERS", STD 2,
                    RFC 1700, October 1994. See also:
                    http://www.iana.org/numbers.html

       [SYNC]       S. Floyd, V. Jacobson, "The Synchronization of Periodic
                    Routing Messages", IEEE/ACM Transactions on Networking,
                    April 1994.  ftp://ftp.ee.lbl.gov/papers/sync_94.ps.Z

    Authors' Addresses

       Thomas Narten
       IBM Corporation
       P.O. Box 12195
       Research Triangle Park, NC 27709-2195
       USA

       Phone: +1 919 254 7798
       EMail: narten@raleigh.ibm.com

       Erik Nordmark
       Sun Microsystems, Inc.
       901 San Antonio Road
       Palo Alto, CA 94303
       USA

       Phone: +1 650 786 5166
       Fax:   +1 650 786 5896
       EMail: nordmark@sun.com

       William Allen Simpson
       Daydreamer
       Computer Systems Consulting Services
       1384 Fontaine
       Madison Heights, Michigan  48071
       USA

       EMail: Bill.Simpson@um.cc.umich.edu
              bsimpson@MorningStar.com

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    APPENDIX A: MULTIHOMED HOSTS

       There are a number of complicating issues that arise when Neighbor
       Discovery is used by hosts that have multiple interfaces.  This
       section does not attempt to define the proper operation of multihomed
       hosts with regard to Neighbor Discovery.  Rather, it identifies
       issues that require further study.  Implementors are encouraged to
       experiment with various approaches to making Neighbor Discovery work
       on multihomed hosts and to report their experiences.

       If a multihomed host receives Router Advertisements on all of its
       interfaces, it will (probably) have learned on-link prefixes for the
       addresses residing on each link.  When a packet must be sent through
       a router, however, selecting the "wrong" router can result in a
       suboptimal or non-functioning path.  There are number of issues to
       consider:

         1) In order for a router to send a redirect, it must determine that
            the packet it is forwarding originates from a neighbor.  The
            standard test for this case is to compare the source address of
            the packet to the list of on-link prefixes associated with the
            interface on which the packet was received.  If the originating
            host is multihomed, however, the source address it uses may
            belong to an interface other than the interface from which it
            was sent.  In such cases, a router will not send redirects, and
            suboptimal routing is likely.  In order to be redirected, the
            sending host must always send packets out the interface
            corresponding to the outgoing packet's source address.  Note
            that this issue never arises with non-multihomed hosts; they
            only have one interface.

         2) If the selected first-hop router does not have a route at all
            for the destination, it will be unable to deliver the packet.
            However, the destination may be reachable through a router on
            one of the other interfaces.  Neighbor Discovery does not
            address this scenario; it does not arise in the non-multihomed
            case.

         3) Even if the first-hop router does have a route for a
            destination, there may be a better route via another interface.
            No mechanism exists for the multihomed host to detect this
            situation.

       If a multihomed host fails to receive Router Advertisements on one or
       more of its interfaces, it will not know (in the absence of
       configured information) which destinations are on-link on the
       affected interface(s).  This leads to a number of problems:

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         1) If no Router Advertisement is received on any interfaces, a
            multihomed host will have no way of knowing which interface to
            send packets out on, even for on-link destinations.  Under
            similar conditions in the non-multihomed host case, a node
            treats all destinations as residing on-link, and communication
            proceeds.  In the multihomed case, however, additional
            information is needed to select the proper outgoing interface.
            Alternatively, a node could attempt to perform address
            resolution on all interfaces, a step involving significant
            complexity that is not present in the non-multihomed host case.

         2) If Router Advertisements are received on some, but not all
            interfaces, a multihomed host could choose to only send packets
            out on the interfaces on which it has received Router
            Advertisements.  A key assumption made here, however, is that
            routers on those other interfaces will be able to route packets
            to the ultimate destination, even when those destinations reside
            on the subnet to which the sender connects, but has no on-link
            prefix information.  Should the assumption be FALSE,
            communication would fail.  Even if the assumption holds, packets
            will traverse a sub-optimal path.

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    APPENDIX B: FUTURE EXTENSIONS

       Possible extensions for future study are:

        o Using dynamic timers to be able to adapt to links with widely
          varying delay.  Measuring round trip times, however, requires
          acknowledgments and sequence numbers in order to match received
          Neighbor Advertisements with the actual Neighbor Solicitation that
          triggered the advertisement.  Implementors wishing to experiment
          with such a facility could do so in a backwards-compatible way by
          defining a new option carrying the necessary information.  Nodes
          not understanding the option would simply ignore it.

        o Adding capabilities to facilitate the operation over links that
          currently require hosts to register with an address resolution
          server.  This could for instance enable routers to ask hosts to
          send them periodic unsolicited advertisements.  Once again this
          can be added using a new option sent in the Router Advertisements.

        o Adding additional procedures for links where asymmetric and non-
          transitive reachability is part of normal operations.  Such
          procedures might allow hosts and routers to find usable paths on,
          e.g., radio links.

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    APPENDIX C: STATE MACHINE FOR THE REACHABILITY STATE

       This appendix contains a summary of the rules specified in Sections
       7.2 and 7.3.  This document does not mandate that implementations
       adhere to this model as long as their external behavior is consistent
       with that described in this document.

       When performing address resolution and Neighbor Unreachability
       Detection the following state transitions apply using the conceptual
       model:

    State           Event                   Action                New state

    -               Packet to send.         Create entry.         INCOMPLETE
                                            Send multicast NS.
                                            Start retransmit timer

    INCOMPLETE      Retransmit timeout,     Retransmit NS         INCOMPLETE
                    less than N             Start retransmit timer
                    retransmissions.

    INCOMPLETE      Retransmit timeout,     Discard entry         -
                    N or more               Send ICMP error
                    retransmissions.

    INCOMPLETE      NA, Solicited=0,        Record link-layer     STALE
                    Override=any            address.  Send queued
                                            packets.

    INCOMPLETE      NA, Solicited=1,        Record link-layer     REACHABLE
                    Override=any            address.  Send queued
                                            packets.

    !INCOMPLETE     NA, Solicited=1,        -                     REACHABLE
                    Override=0
                    Same link-layer
                    address as cached.

    REACHABLE       NA, Solicited=1,        -                     STALE
                    Override=0
                    Different link-layer
                    address than cached.

    STALE or PROBE  NA, Solicited=1,        -                     unchanged
                    Override=0
                    Different link-layer
                    address than cached.

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    !INCOMPLETE     NA, Solicited=1,        Record link-layer     REACHABLE
                    Override=1              address (if
                                            different).

    !INCOMPLETE     NA, Solicited=0,        -                     unchanged
                    Override=0

    !INCOMPLETE     NA, Solicited=0,        -                     unchanged
                    Override=1
                    Same link-layer
                    address as cached.

    !INCOMPLETE     NA, Solicited=0,        Record link-layer     STALE
                    Override=1              address.
                    Different link-layer
                    address than cached.

    !INCOMPLETE     upper-layer reachability  -                   REACHABLE
                    confirmation

    REACHABLE       timeout, more than      -                     STALE
                    N seconds since
                    reachability confirm.

    STALE           Sending packet          Start delay timer     DELAY

    DELAY           Delay timeout           Send unicast NS probe PROBE
                                            Start retransmit timer

    PROBE           Retransmit timeout,     Retransmit NS         PROBE
                    less than N
                    retransmissions.

    PROBE           Retransmit timeout,     Discard entry         -
                    N or more
                    retransmissions.

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    RFC 2461              Neighbor Discovery for IPv6          December 1998

       The state transitions for receiving unsolicited information other
       than Neighbor Advertisement messages apply to either the source of
       the packet (for Neighbor Solicitation, Router Solicitation, and
       Router Advertisement messages) or the target address (for Redirect
       messages) as follows:

    State           Event                   Action                New state

    -               NS, RS, RA, Redirect    Create entry.         STALE

    INCOMPLETE      NS, RS, RA, Redirect    Record link-layer     STALE
                                            address.  Send queued
                                            packets.

    !INCOMPLETE     NS, RS, RA, Redirect    Update link-layer     STALE
                    Different link-layer    address
                    address than cached.

    !INCOMPLETE     NS, RS, RA, Redirect    -                     unchanged
                    Same link-layer
                    address as cached.

    Narten, et. al.             Standards Track                    [Page 87]

    RFC 2461              Neighbor Discovery for IPv6          December 1998

    APPENDIX D: SUMMARY OF ISROUTER RULES

       This appendix presents a summary of the rules for maintaining the
       IsRouter flag as specified in this document.

       The background for these rules is that the ND messages contain,
       either implicitly or explicitly, information that indicates whether
       or not the sender (or Target Address) is a host or a router.  The
       following assumptions are used:

        - The sender of a Router Solicitation is implicitly assumed to be a
          host since there is no need for routers to send such messages.

        - The sender of a Router Advertisement is implicitly assumed to be a
          router.

        - Neighbor Solicitation messages do not contain either an implicit
          or explicit indication about the sender.  Both hosts and routers
          send such messages.

          - Neighbor Advertisement messages contain an explicit "IsRouter
          flag", the R-bit.

        - The target of the redirect, when the target differs from the
          destination address in the packet being redirected, is implicitly
          assumed to be a router.  This is a natural assumption since that
          node is expected to be able to forward the packets towards the
          destination.

        - The target of the redirect, when the target is the same as the
          destination, does not carry any host vs. router information.  All
          that is known is that the destination (i.e. target) is on-link but
          it could be either a host or a router.

       The rules for setting the IsRouter flag are based on the information
       content above.  If an ND message contains explicit or implicit
       information the receipt of the message will cause the IsRouter flag
       to be updated.  But when there is no host vs. router information in
       the ND message the receipt of the message MUST NOT cause a change to
       the IsRouter state.  When the receipt of such a message causes a
       Neighbor Cache entry to be created this document specifies that the
       IsRouter flag be set to FALSE.  There is greater potential for
       mischief when a node incorrectly thinks a host is a router, than the
       other way around.  In these cases a subsequent Neighbor Advertisement
       or Router Advertisement message will set the correct IsRouter value.

    Narten, et. al.             Standards Track                    [Page 88]

    RFC 2461              Neighbor Discovery for IPv6          December 1998

    APPENDIX E: IMPLEMENTATION ISSUES

    Appendix E.1: Reachability confirmations

       Neighbor Unreachability Detection requires explicit confirmation that
       a forward-path is functioning properly.  To avoid the need for
       Neighbor Solicitation probe messages, upper layer protocols should
       provide such an indication when the cost of doing so is small.
       Reliable connection-oriented protocols such as TCP are generally
       aware when the forward-path is working.  When TCP sends (or receives)
       data, for instance, it updates its window sequence numbers, sets and
       cancels retransmit timers, etc.  Specific scenarios that usually
       indicate a properly functioning forward-path include:

        - Receipt of an acknowledgement that covers a sequence number (e.g.,
          data) not previously acknowledged indicates that the forward path
          was working at the time the data was sent.

        - Completion of the initial three-way handshake is a special case of
          the previous rule; although no data is sent during the handshake,
          the SYN flags are counted as data from the sequence number
          perspective.  This applies to both the SYN+ACK for the active open
          the ACK of that packet on the passively opening peer.

        - Receipt of new data (i.e., data not previously received) indicates
          that the forward-path was working at the time an acknowledgement
          was sent that advanced the peer's send window that allowed the new
          data to be sent.

       To minimize the cost of communicating reachability information
       between the TCP and IP layers, an implementation may wish to rate-
       limit the reachability confirmations its sends IP.  One possibility
       is to process reachability only every few packets.  For example, one
       might update reachability information once per round trip time, if an
       implementation only has one round trip timer per connection.  For
       those implementations that cache Destination Cache entries within
       control blocks, it may be possible to update the Neighbor Cache entry
       directly (i.e., without an expensive lookup) once the TCP packet has
       been demultiplexed to its corresponding control block.  For other
       implementation it may be possible to piggyback the reachability
       confirmation on the next packet submitted to IP assuming that the
       implementation guards against the piggybacked confirmation becoming
       stale when no packets are sent to IP for an extended period of time.

       TCP must also guard against thinking "stale" information indicates
       current reachability.  For example, new data received 30 minutes
       after a window has opened up does not constitute a confirmation that
       the path is currently working.  In merely indicates that 30 minutes

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    RFC 2461              Neighbor Discovery for IPv6          December 1998

       ago the window update reached the peer i.e. the path was working at
       that point in time.  An implementation must also take into account
       TCP zero-window probes that are sent even if the path is broken and
       the window update did not reach the peer.

       For UDP based applications (RPC, DNS) it is relatively simple to make
       the client send reachability confirmations when the response packet
       is received.  It is more difficult and in some cases impossible for
       the server to generate such confirmations since there is no flow
       control, i.e., the server can not determine whether a received
       request indicates that a previous response reached the client.

       Note that an implementation can not use negative upper-layer advise
       as a replacement for the Neighbor Unreachability Detection algorithm.
       Negative advise (e.g. from TCP when there are excessive
       retransmissions) could serve as a hint that the forward path from the
       sender of the data might not be working.  But it would fail to detect
       when the path from the receiver of the data is not functioning
       causing, none of the acknowledgement packets to reach the sender.

    Narten, et. al.             Standards Track                    [Page 90]

    RFC 2461              Neighbor Discovery for IPv6          December 1998

    APPENDIX F: CHANGES SINCE RFC 1970

        o Removed all references to the IPv6 priority field.

        o Replaced definition of solicited node multicast address with a
          reference to the [ADDR-ARCH] specification.  That specification
          says that "the solicited-node multicast address is formed by
          taking the low-order 24 bits of the address (unicast or anycast)
          and appending those bits to the prefix FF02:0:0:0:0:1:FF00::/104".

        o Updated the references section to list (new) RFC numbers.

        o Updated the text in section 7.2.5 and the tables in appendix C to
          have the receipt of an NS message update the state of an existing
          neighbor cache entry only if the link-layer address is different
          than the recorded link-layer address.

        o Added an explicit check in section 7.1.1 so that received NS
          messages from an unsolicited address must be sent the solicited-
          node multicast address; if sent to unicast destination, silently
          discard.

        o Added a requirement in section 6.2.1 that Lifetimes be
          configurable in either of two ways: as a fixed value that doesn't
          change over time, or one that decrements in real time.

        o Added text in section 6.2.7 to relax the consistency checks on
          prefix lifetimes when the lifetimes are configured to decrement in
          real time.  This is needed to avoid false alarms due to link
          propagation delay and lack of synchronized clocks.

        o Added text to section 6.3.4 to point out that [ADDRCONF] might
          ignore short lifetimes but that Neighbor Discovery does not ignore
          short prefix lifetimes.

        o Clarified the rules for RS and NS packets with an unspecified
          source address. Such packets MUST NOT include source link-layer
          address option; verified by receivers.

        o Clarified in section 7.2.3 that addresses for which the node
          proxies are acceptable in NS messages.  Previously the text only
          mentioned unicast and anycast addresses assigned to the interface
          (i.e., wasn't clear that proxy addresses were allowed).

        o Tightened up ambiguities an inconsistencies regarding when to set
          the IsRouter flag in Neighbor Cache entries.  Added an appendix to
          summarize these rules.

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    RFC 2461              Neighbor Discovery for IPv6          December 1998

        o Added a section on renumbering considerations to clarify how long
          prefixes have to be advertised when the lifetime(s) are reduced.

        o Added additional text to the rules in section 7 for the NS/NA
          packets used for NUD probes so that the Link-Layer Address options
          can be omitted from these packets in certain cases without causing
          an infinite NS "recursion". Specifically, added text that permits
          the Link-Layer address to be omitted in unicast solicitations
          (i.e., MAY language).

        o Changed the default AdvValidLifetime from infinity to 30 days.

        o Changed the constant "576" to "1280" in places where its context
          was that of the minimum sized IP packet that all links must be
          able to carry.

    Narten, et. al.             Standards Track                    [Page 92]

    RFC 2461              Neighbor Discovery for IPv6          December 1998

    Full Copyright Statement

       Copyright (C) The Internet Society (1998).  All Rights Reserved.

       This document and translations of it may be copied and furnished to
       others, and derivative works that comment on or otherwise explain it
       or assist in its implementation may be prepared, copied, published
       and distributed, in whole or in part, without restriction of any
       kind, provided that the above copyright notice and this paragraph are
       included on all such copies and derivative works.  However, this
       document itself may not be modified in any way, such as by removing
       the copyright notice or references to the Internet Society or other
       Internet organizations, except as needed for the purpose of
       developing Internet standards in which case the procedures for
       copyrights defined in the Internet Standards process must be
       followed, or as required to translate it into languages other than
       English.

       The limited permissions granted above are perpetual and will not be
       revoked by the Internet Society or its successors or assigns.

       This document and the information contained herein is provided on an
       "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
       TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
       BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
       HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
       MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

    Narten, et. al.             Standards Track                    [Page 93]