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RFC 4861

 Errata 
Draft STD
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Neighbor Discovery for IP version 6 (IPv6)

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Network Working Group                                          T. Narten
Request for Comments: 4861                                           IBM
Obsoletes: 2461                                              E. Nordmark
Category: Standards Track                               Sun Microsystems
                                                              W. Simpson
                                                              Daydreamer
                                                              H. Soliman
                                                    Elevate Technologies
                                                          September 2007


               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.

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Table of Contents

   1. Introduction ....................................................4
   2. Terminology .....................................................4
      2.1. General ....................................................4
      2.2. Link Types .................................................8
      2.3. Addresses ..................................................9
      2.4. Requirements ..............................................10
   3. Protocol Overview ..............................................10
      3.1. Comparison with IPv4 ......................................14
      3.2. Supported Link Types ......................................16
      3.3. Securing Neighbor Discovery Messages ......................18
   4. Message Formats ................................................18
      4.1. Router Solicitation Message Format ........................18
      4.2. Router Advertisement Message Format .......................19
      4.3. Neighbor Solicitation Message Format ......................22
      4.4. Neighbor Advertisement Message Format .....................23
      4.5. Redirect Message Format ...................................26
      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 ..............................36
      5.3. Garbage Collection and Timeout Requirements ...............37
   6. Router and Prefix Discovery ....................................38
      6.1. Message Validation ........................................39
           6.1.1. Validation of Router Solicitation Messages .........39
           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 ..................45
           6.2.3. Router Advertisement Message Content ...............45
           6.2.4. Sending Unsolicited Router Advertisements ..........47
           6.2.5. Ceasing To Be an Advertising Interface .............47
           6.2.6. Processing Router Solicitations ....................48
           6.2.7. Router Advertisement Consistency ...................50
           6.2.8. Link-local Address Change ..........................50
      6.3. Host Specification ........................................51
           6.3.1. Host Configuration Variables .......................51
           6.3.2. Host Variables .....................................51
           6.3.3. Interface Initialization ...........................52
           6.3.4. Processing Received Router Advertisements ..........53
           6.3.5. Timing out Prefixes and Default Routers ............56
           6.3.6. Default Router Selection ...........................56
           6.3.7. Sending Router Solicitations .......................57

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   7. Address Resolution and Neighbor Unreachability Detection .......59
      7.1. Message Validation ........................................59
           7.1.1. Validation of Neighbor Solicitations ...............59
           7.1.2. Validation of Neighbor Advertisements ..............60
      7.2. Address Resolution ........................................60
           7.2.1. Interface Initialization ...........................61
           7.2.2. Sending Neighbor Solicitations .....................61
           7.2.3. Receipt of Neighbor Solicitations ..................62
           7.2.4. Sending Solicited Neighbor Advertisements ..........63
           7.2.5. Receipt of Neighbor Advertisements .................64
           7.2.6. Sending Unsolicited Neighbor Advertisements ........66
           7.2.7. Anycast Neighbor Advertisements ....................67
           7.2.8. Proxy Neighbor Advertisements ......................68
      7.3. Neighbor Unreachability Detection .........................68
           7.3.1. Reachability Confirmation ..........................69
           7.3.2. Neighbor Cache Entry States ........................70
           7.3.3. Node Behavior ......................................71
   8. Redirect Function ..............................................73
      8.1. Validation of Redirect Messages ...........................74
      8.2. Router Specification ......................................75
      8.3. Host Specification ........................................76
   9. Extensibility - Option Processing ..............................76
   10. Protocol Constants ............................................78
   11. Security Considerations .......................................79
      11.1. Threat Analysis ..........................................79
      11.2. Securing Neighbor Discovery Messages .....................81
   12. Renumbering Considerations ....................................81
   13. IANA Considerations ...........................................83
   14. References ....................................................84
      14.1. Normative References .....................................84
      14.2. Informative References ...................................84
   Appendix A: Multihomed Hosts ......................................87
   Appendix B: Future Extensions .....................................88
   Appendix C: State Machine for the Reachability State ..............89
   Appendix D: Summary of IsRouter Rules .............................91
   Appendix E: Implementation Issues .................................92
   Appendix F: Changes from RFC 2461 .................................94
   Acknowledgments ...................................................95

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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., Non-Broadcast
   Multi-Access (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 are addressed in [IPv6-NBMA].  In addition, [IPv6-3GPP]
   and[IPv6-CELL] discuss the use of this protocol over some cellular
   links, which are examples of NBMA links.

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 Control Message 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.

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   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-layer (or lower-layer) protocols being
                 "tunneled" over (i.e., encapsulated in) IP such as
                 Internetwork Packet Exchange (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-layer (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].

                 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.

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   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 (e.g.,
                      as indicated by the on-link flag in the Prefix
                      Information option), 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
                 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
                 transmission unit over a link.

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

   proxy       - a node 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
                 computed delay forms a uniformly distributed random
                 value that falls between the specified minimum and
                 maximum delay times.  The implementor must take care to
                 ensure that the granularity of the calculated random
                 component and the resolution of the timer used are both
                 high enough to ensure 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 identifier alone as the seed, since interface

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                 identifiers will not always be unique.  To reduce the
                 probability that duplicate interface identifiers 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 identifier.  Additional information on
                 randomness and random number generation can be found in
                 [RAND].

2.2.  Link Types

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

   multicast capable
                  - 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 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
                    applications that need it (e.g., using multicast
                    servers).  However, it is an issue for further study
                    whether ND should use such facilities or an
                    alternate mechanism that provides the equivalent
                    multicast capability for ND.

   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 Switched
                    Multimegabit Data Service (SMDS) and Broadband
                    Integrated Services Digital Network (B-ISDN).  Also
                    known as "large clouds".  See [SH-MEDIA].

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   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
                 function is described in [ADDR-ARCH].  The function is
                 chosen so that IP addresses that differ only in the
                 most significant bits, e.g., due to multiple prefixes
                 associated with different providers, will map to the
                 same solicited-node address thereby reducing the number
                 of multicast addresses a node must join at the link
                 layer.

   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.

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   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 stateless address autoconfiguration [ADDRCONF]).
                 The unspecified address has a value of 0:0:0:0:0:0:0:0.

   Note that this specification does not strictly comply with the
   consistency requirements in [ADDR-SEL] for the scopes of source and
   destination addresses.  It is possible in some cases for hosts to use
   a source address of a larger scope than the destination address in
   the IPv6 header.

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:

     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 link parameters (such as the
                link MTU) or Internet parameters (such as the hop limit
                value) to place in outgoing packets.

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     Address Autoconfiguration: Introduces the mechanisms needed in
                order to allow nodes to configure an address for an
                interface in a stateless manner.  Stateless address
                autoconfiguration is specified in [ADDRCONF].

     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 whether or not
                an address it wishes to use is 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 determining whether another address shares
                the same link (on-link determination) and/or address
                configuration, a suggested hop limit value, etc.

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     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 DHCPv6 and/or autonomous
   (stateless) address configuration.

   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

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   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.

     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.

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           Neighbor Discovery allows a router to perform load balancing
           for traffic addressed to itself 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, e.g.,
           who issued the solicitation.  This specification does not
           define a mechanism that allows hosts to Load-balance incoming
           packets.  See [LD-SHRE].

     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.  A non-Override
           advertisement is one that does not update or replace the
           information sent by another advertisement.  These
           advertisements are discussed later in the context of Neighbor
           advertisement messages.  This invokes specific rules to
           determine which of potentially multiple advertisements should
           be used.

     Proxy advertisements - A node willing to accept packets on behalf
           of a target address that is unable to respond to Neighbor
           Solicitations can issue non-Override Neighbor Advertisements.
           Proxy advertisements are used by Mobile IPv6 Home Agents to
           defend mobile nodes' addresses when they move 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 Address Resolution Protocol [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 the Hosts Requirements document
   [HR-CL] does specify some possible algorithms for Dead Gateway
   Detection (a subset of the problems Neighbor Unreachability Detection
   tackles).

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   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 16 million (2^24)
      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, which
      significantly improves the robustness of packet delivery in the
      presence of failing routers, partially failing or partitioned
      links, or nodes that change their link-layer addresses.  For

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      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.

      By setting the Hop Limit to 255, 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
      generic IP-layer authentication and security mechanisms as
      appropriate.

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.)

     multicast      - Neighbor Discovery operates over multicast capable
                      links 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 are

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                      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.

     variable MTU   - Neighbor Discovery allows routers to specify an
                      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
                      that all receivers can process (or Maximum Receive
                      Unit).

     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.

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                      The protocol can presumably be extended in the
                      future to find viable paths in environments that
                      lack reflexive and transitive connectivity.

3.3.  Securing Neighbor Discovery Messages

   Neighbor Discovery messages are needed for various functions.
   Several functions are designed to allow hosts to ascertain the
   ownership of an address or the mapping between link-layer and IP-
   layer addresses.  Vulnerabilities related to Neighbor Discovery are
   discussed in Section 11.1.  A general solution for securing Neighbor
   Discovery is outside the scope of this specification and is discussed
   in [SEND].  However, Section 11.2 explains how and under which
   constraints IPsec Authentication Header (AH) or Encapsulating
   Security Payload (ESP) can be used to secure Neighbor Discovery.



(page 18 continued on part 2)

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