





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.




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      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



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                      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



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                      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.



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   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.



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   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



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                     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.













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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.





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   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.






















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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.





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   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.







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   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.























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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





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      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.




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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




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      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                             +
      |                                                               |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   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.





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      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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   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.



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   [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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   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.






























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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.






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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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   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.
































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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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    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.




































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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.
























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