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RFC 1970


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Neighbor Discovery for IP Version 6 (IPv6)

Part 1 of 3, p. 1 to 27
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Network Working Group                                          T. Narten
Request for Comments: 1970                                           IBM
Category: Standards Track                                    E. Nordmark
                                                        Sun Microsystems
                                                              W. Simpson
                                                              Daydreamer
                                                             August 1996


               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.

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........................................   10
      3.1.  Comparison with IPv4................................   14
      3.2.  Supported Link Types................................   16
   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.............................   25
      4.6.  Option Formats......................................   27
         4.6.1.  Source/Target Link-layer Address...............   28
         4.6.2.  Prefix Information.............................   29
         4.6.3.  Redirected Header..............................   31

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         4.6.4.  MTU............................................   31
   5.  CONCEPTUAL MODEL OF A HOST...............................   32
      5.1.  Conceptual Data Structures..........................   33
      5.2.  Conceptual Sending Algorithm........................   35
      5.3.  Garbage Collection and Timeout Requirements.........   36
   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....   38
      6.2.  Router Specification................................   39
         6.2.1.  Router Configuration Variables.................   39
         6.2.2.  Becoming An Advertising Interface..............   43
         6.2.3.  Router Advertisement Message Content...........   43
         6.2.4.  Sending Unsolicited Router Advertisements......   45
         6.2.5.  Ceasing To Be An Advertising Interface.........   45
         6.2.6.  Processing Router Solicitations................   46
         6.2.7.  Router Advertisement Consistency...............   47
         6.2.8.  Link-local Address Change......................   48
      6.3.  Host Specification..................................   48
         6.3.1.  Host Configuration Variables...................   48
         6.3.2.  Host Variables.................................   48
         6.3.3.  Interface Initialization.......................   50
         6.3.4.  Processing Received Router Advertisements......   50
         6.3.5.  Timing out Prefixes and Default Routers........   52
         6.3.6.  Default Router Selection.......................   53
         6.3.7.  Sending Router Solicitations...................   54
   7.  ADDRESS RESOLUTION AND NEIGHBOR UNREACHABILITY DETECTION.   55
      7.1.  Message Validation..................................   55
         7.1.1.  Validation of Neighbor Solicitations...........   55
         7.1.2.  Validation of Neighbor Advertisements..........   56
      7.2.  Address Resolution..................................   57
         7.2.1.  Interface Initialization.......................   57
         7.2.2.  Sending Neighbor Solicitations.................   57
         7.2.3.  Receipt of Neighbor Solicitations..............   58
         7.2.4.  Sending Solicited Neighbor Advertisements......   59
         7.2.5.  Receipt of Neighbor Advertisements.............   59
         7.2.6.  Sending Unsolicited Neighbor Advertisements....   61
         7.2.7.  Anycast Neighbor Advertisements................   62
         7.2.8.  Proxy Neighbor Advertisements..................   62
      7.3.  Neighbor Unreachability Detection...................   63
         7.3.1.  Reachability Confirmation......................   63
         7.3.2.  Neighbor Cache Entry States....................   64
         7.3.3.  Node Behavior..................................   66
   8.  REDIRECT FUNCTION........................................   68
      8.1.  Validation of Redirect Messages.....................   68
      8.2.  Router Specification................................   69
      8.3.  Host Specification..................................   70
   9.  EXTENSIBILITY - OPTION PROCESSING........................   71

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   10.  PROTOCOL CONSTANTS......................................   72
   11.  SECURITY CONSIDERATIONS.................................   73
   REFERENCES...................................................   75
   AUTHORS' ADDRESSES...........................................   76
   APPENDIX A: MULTIHOMED HOSTS.................................   77
   APPENDIX B: FUTURE EXTENSIONS................................   78
   APPENDIX C: STATE MACHINE FOR THE REACHABILITY STATE.........   78
   APPENDIX D: IMPLEMENTATION ISSUES............................   80
      Appendix D.1: Reachability confirmations..................   80

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 the IPNGWG
   working group and, in particular, (in alphabetical order) Ran
   Atkinson, Jim Bound, Scott Bradner, Alex Conta, Stephen Deering,
   Francis Dupont, Robert Elz, Robert Gilligan, Robert Hinden, Allison
   Mankin, Dan McDonald, Charles Perkins, Matt Thomas, and Susan
   Thomson.

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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
                 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].

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                 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.

   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

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                 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.

   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

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                 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.).
                    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

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                    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

   solicited-node multicast address
               - a link-local scope multicast address that is computed
                 as a function of the solicited target's address.  The
                 solicited-node multicast address is formed by taking
                 the low-order 32 bits of the target IP address and
                 appending those bits to the 96-bit prefix
                 FF02:0:0:0:0:1 to produce a multicast address within
                 the range FF02::1:0:0 to FF02::1:FFFF:FFFF.  For
                 example, the solicited node multicast address

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                 corresponding to the IP address 4037::01:800:200E:8C6C
                 is FF02::1:200E:8C6C.  IP addresses that 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

   Throughout this document, the words that are used to define the
   significance of the particular requirements are capitalized.  These
   words are:

   MUST
        This word or the adjective "REQUIRED" means that the item is an
        absolute requirement of this specification.

   MUST NOT
        This phrase means the item is an absolute prohibition of this
        specification.

   SHOULD
        This word or the adjective "RECOMMENDED" means that there may
        exist valid reasons in particular circumstances to ignore this
        item, but the full implications should be understood and the
        case carefully weighed before choosing a different course.

   SHOULD NOT
        This phrase means that there may exist valid reasons in
        particular circumstances when the listed behavior is acceptable
        or even useful, but the full implications should be understood
        and the case carefully weighted before implementing any behavior

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        described with this label.

   MAY  This word or the adjective "OPTIONAL" means that this item is
        truly optional.  One vendor may choose to include the item
        because a particular marketplace requires it or because it
        enhances the product, for example, another vendor may omit the
        same item.

   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:

    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.

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    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.

    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

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   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,
   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,

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   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.

     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.

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          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:

     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

Top      ToC       Page 15 
     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.

     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].

Top      ToC       Page 16 
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 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.

Top      ToC       Page 17 
   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
                    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.

Top      ToC       Page 18 
   Hop Limit      255

   Priority       15

   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.

Valid Options:

   Source link-layer address
                  The link-layer address of the sender, if known.

   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 ...
     +-+-+-+-+-+-+-+-+-+-+-+-

Top      ToC       Page 19 
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

   Priority       15

   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           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.

Top      ToC       Page 20 
   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 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

Top      ToC       Page 21 
                  able to chose 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.

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

Top      ToC       Page 22 
   Priority       15

   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           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.  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.

Top      ToC       Page 23 
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

   Priority       15

   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.

Top      ToC       Page 24 
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.

Top      ToC       Page 25 
Possible options:

   Target link-layer address
                  The link-layer address for the target, i.e., the
                  sender of the advertisement.  MUST 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.

4.5.  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 ...
     +-+-+-+-+-+-+-+-+-+-+-+-

Top      ToC       Page 26 
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

   Priority       15

   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.

Top      ToC       Page 27 
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 576 octets.



(page 27 continued on part 2)

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