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rfc:rfc2461

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

                                                          W. Simpson
                                                          Daydreamer
                                                       December 1998
             Neighbor Discovery for IP Version 6 (IPv6)

Status of this Memo

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

Copyright Notice

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

Abstract

 This document specifies the Neighbor Discovery protocol for IP
 Version 6.  IPv6 nodes on the same link use Neighbor Discovery to
 discover each other's presence, to determine each other's link-layer
 addresses, to find routers and to maintain reachability information
 about the paths to active neighbors.
 Table of Contents
 1.  INTRODUCTION.............................................    3
 2.  TERMINOLOGY..............................................    4
    2.1.  General.............................................    4
    2.2.  Link Types..........................................    7
    2.3.  Addresses...........................................    8
    2.4.  Requirements........................................    9
 3.  PROTOCOL OVERVIEW........................................    9
    3.1.  Comparison with IPv4................................   13
    3.2.  Supported Link Types................................   15
 4.  MESSAGE FORMATS..........................................   17
    4.1.  Router Solicitation Message Format..................   17
    4.2.  Router Advertisement Message Format.................   18
    4.3.  Neighbor Solicitation Message Format................   21
    4.4.  Neighbor Advertisement Message Format...............   23
    4.5.  Redirect Message Format.............................   26

Narten, et. al. Standards Track [Page 1] RFC 2461 Neighbor Discovery for IPv6 December 1998

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

Narten, et. al. Standards Track [Page 2] RFC 2461 Neighbor Discovery for IPv6 December 1998

    8.1.  Validation of Redirect Messages.....................   71
    8.2.  Router Specification................................   72
    8.3.  Host Specification..................................   73
 9.  EXTENSIBILITY - OPTION PROCESSING........................   74
 10.  PROTOCOL CONSTANTS......................................   75
 11.  SECURITY CONSIDERATIONS.................................   76
 12.  RENUMBERING CONSIDERATIONS..............................   78
 References...................................................   80
 Authors' Addresses...........................................   81
 Appendix A: Multihomed Hosts.................................   82
 Appendix B: Future Extensions................................   84
 Appendix C: State Machine for the Reachability State.........   85
 Appendix D: Summary of ISROUTER Rules........................   88
 Appendix E: Implementation Issues............................   89
     Appendix E.1: Reachability confirmations.................   89
 Appendix F: Changes since RFC 1970...........................   91
 Full Copyright Statement.....................................   93

1. INTRODUCTION

 This specification defines the Neighbor Discovery (ND) protocol for
 Internet Protocol Version 6 (IPv6).  Nodes (hosts and routers) use
 Neighbor Discovery to determine the link-layer addresses for
 neighbors known to reside on attached links and to quickly purge
 cached values that become invalid.  Hosts also use Neighbor Discovery
 to find neighboring routers that are willing to forward packets on
 their behalf.  Finally, nodes use the protocol to actively keep track
 of which neighbors are reachable and which are not, and to detect
 changed link-layer addresses.  When a router or the path to a router
 fails, a host actively searches for functioning alternates.
 Unless specified otherwise (in a document that covers operating IP
 over a particular link type) this document applies to all link types.
 However, because ND uses link-layer multicast for some of its
 services, it is possible that on some link types (e.g., NBMA links)
 alternative protocols or mechanisms to implement those services will
 be specified (in the appropriate document covering the operation of
 IP over a particular link type).  The services described in this
 document that are not directly dependent on multicast, such as
 Redirects, Next-hop determination, Neighbor Unreachability Detection,
 etc., are expected to be provided as specified in this document.  The
 details of how one uses ND on NBMA links is an area for further
 study.
 The authors would like to acknowledge the contributions of the IPNGWG
 working group and, in particular, (in alphabetical order) Ran
 Atkinson, Jim Bound, Scott Bradner, Alex Conta, Stephen Deering,

Narten, et. al. Standards Track [Page 3] RFC 2461 Neighbor Discovery for IPv6 December 1998

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

2. TERMINOLOGY

2.1. General

 IP          - Internet Protocol Version 6.  The terms IPv4 and
               IPv6 are used only in contexts where necessary to avoid
               ambiguity.
 ICMP        - Internet Message Control Protocol for the Internet
               Protocol Version 6.  The terms ICMPv4 and ICMPv6 are
               used only in contexts where necessary to avoid
               ambiguity.
 node        - a device that implements IP.
 router      - a node that forwards IP packets not explicitly
               addressed to itself.
 host        - any node that is not a router.
 upper layer - a protocol layer immediately above IP.  Examples are
               transport protocols such as TCP and UDP, control
               protocols such as ICMP, routing protocols such as OSPF,
               and internet or lower-layer protocols being "tunneled"
               over (i.e., encapsulated in) IP such as IPX, AppleTalk,
               or IP itself.
 link        - a communication facility or medium over which nodes can
               communicate at the link layer, i.e., the layer
               immediately below IP.  Examples are Ethernets (simple
               or bridged), PPP links, X.25, Frame Relay, or ATM
               networks as well as internet (or higher) layer
               "tunnels", such as tunnels over IPv4 or IPv6 itself.
 interface   - a node's attachment to a link.
 neighbors   - nodes attached to the same link.
 address     - an IP-layer identifier for an interface or a set of
               interfaces.
 anycast address
             - an identifier for a set of interfaces (typically
               belonging to different nodes).  A packet sent to an

Narten, et. al. Standards Track [Page 4] RFC 2461 Neighbor Discovery for IPv6 December 1998

               anycast address is delivered to one of the interfaces
               identified by that address (the "nearest" one,
               according to the routing protocol's measure of
               distance).  See [ADDR-ARCH].
               Note that an anycast address is syntactically
               indistinguishable from a unicast address.  Thus, nodes
               sending packets to anycast addresses don't generally
               know that an anycast address is being used.  Throughout
               the rest of this document, references to unicast
               addresses also apply to anycast addresses in those
               cases where the node is unaware that a unicast address
               is actually an anycast address.
 prefix      - a bit string that consists of some number of initial
               bits of an address.
 link-layer address
             - a link-layer identifier for an interface.  Examples
               include IEEE 802 addresses for Ethernet links and E.164
               addresses for ISDN links.
 on-link     - an address that is assigned to an interface on a
               specified link.  A node considers an address to be on-
               link if:
  1. it is covered by one of the link's prefixes, or
  1. a neighboring router specifies the address as

the target of a Redirect message, or

  1. a Neighbor Advertisement message is received for

the (target) address, or

  1. 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.

Narten, et. al. Standards Track [Page 5] RFC 2461 Neighbor Discovery for IPv6 December 1998

 reachability
             - whether or not the one-way "forward" path to a
               neighbor is functioning properly.  In particular,
               whether packets sent to a neighbor are reaching the
               IP layer on the neighboring machine and are being
               processed properly by the receiving IP layer.  For
               neighboring routers, reachability means that packets
               sent by a node's IP layer are delivered to the
               router's IP layer, and the router is indeed
               forwarding packets (i.e., it is configured as a
               router, not a host).  For hosts, reachability means
               that packets sent by a node's IP layer are delivered
               to the neighbor host's IP layer.
 packet      - an IP header plus payload.
 link MTU    - the maximum transmission unit, i.e., maximum packet
               size in octets, that can be conveyed in one piece
               over a link.
 target      - an address about which address resolution
               information is sought, or an address which is the
               new first-hop when being redirected.
 proxy       - a router that responds to Neighbor Discovery query
               messages on behalf of another node.  A router acting
               on behalf of a mobile node that has moved off-link
               could potentially act as a proxy for the mobile
               node.
 ICMP destination unreachable indication
             - an error indication returned to the original sender
               of a packet that cannot be delivered for the reasons
               outlined in [ICMPv6].  If the error occurs on a node
               other than the node originating the packet, an ICMP
               error message is generated.  If the error occurs on
               the originating node, an implementation is not
               required to actually create and send an ICMP error
               packet to the source, as long as the upper-layer
               sender is notified through an appropriate mechanism
               (e.g., return value from a procedure call).  Note,
               however, that an implementation may find it
               convenient in some cases to return errors to the
               sender by taking the offending packet, generating an
               ICMP error message, and then delivering it (locally)
               through the generic error handling routines.

Narten, et. al. Standards Track [Page 6] RFC 2461 Neighbor Discovery for IPv6 December 1998

 random delay
             - when sending out messages, it is sometimes necessary to
               delay a transmission for a random amount of time in
               order to prevent multiple nodes from transmitting at
               exactly the same time, or to prevent long-range
               periodic transmissions from synchronizing with each
               other [SYNC].  When a random component is required, a
               node calculates the actual delay in such a way that the
               computed delay forms a uniformly-distributed random
               value that falls between the specified minimum and
               maximum delay times.  The implementor must take care to
               insure that the granularity of the calculated random
               component and the resolution of the timer used are both
               high enough to insure that the probability of multiple
               nodes delaying the same amount of time is small.
 random delay seed
             - If a pseudo-random number generator is used in
               calculating a random delay component, the generator
               should be initialized with a unique seed prior to being
               used.  Note that it is not sufficient to use the
               interface token alone as the seed, since interface
               tokens will not always be unique.  To reduce the
               probability that duplicate interface tokens cause the
               same seed to be used, the seed should be calculated
               from a variety of input sources (e.g., machine
               components) that are likely to be different even on
               identical "boxes".  For example, the seed could be
               formed by combining the CPU's serial number with an
               interface token.

2.2. Link Types

 Different link layers have different properties.  The ones of concern
 to Neighbor Discovery are:
 multicast      - a link that supports a native mechanism at the
                  link layer for sending packets to all (i.e.,
                  broadcast) or a subset of all neighbors.
 point-to-point - a link that connects exactly two interfaces.  A
                  point-to-point link is assumed to have multicast
                  capability and have a link-local address.
 non-broadcast multi-access (NBMA)
                - a link to which more than two interfaces can attach,
                  but that does not support a native form of multicast
                  or broadcast (e.g., X.25, ATM, frame relay, etc.).

Narten, et. al. Standards Track [Page 7] RFC 2461 Neighbor Discovery for IPv6 December 1998

                  Note that all link types (including NBMA) are
                  expected to provide multicast service for IP (e.g.,
                  using multicast servers), but it is an issue for
                  further study whether ND should use such facilities
                  or an alternate mechanism that provides the
                  equivalent ND services.
 shared media   - a link that allows direct communication among a
                  number of nodes, but attached nodes are configured
                  in such a way that they do not have complete prefix
                  information for all on-link destinations.  That is,
                  at the IP level, nodes on the same link may not know
                  that they are neighbors; by default, they
                  communicate through a router.  Examples are large
                  (switched) public data networks such as SMDS and B-
                  ISDN.  Also known as "large clouds".  See [SH-
                  MEDIA].
 variable MTU   - a link that does not have a well-defined MTU (e.g.,
                  IEEE 802.5 token rings).  Many links (e.g.,
                  Ethernet) have a standard MTU defined by the link-
                  layer protocol or by the specific document
                  describing how to run IP over the link layer.
 asymmetric reachability
                - a link where non-reflexive and/or non-transitive
                  reachability is part of normal operation.  (Non-
                  reflexive reachability means packets from A reach B
                  but packets from B don't reach A.  Non-transitive
                  reachability means packets from A reach B, and
                  packets from B reach C, but packets from A don't
                  reach C.)  Many radio links exhibit these
                  properties.

2.3. Addresses

 Neighbor Discovery makes use of a number of different addresses
 defined in [ADDR-ARCH], including:
 all-nodes multicast address
             - the link-local scope address to reach all nodes.
               FF02::1
 all-routers multicast address
             - the link-local scope address to reach all routers.
               FF02::2

Narten, et. al. Standards Track [Page 8] RFC 2461 Neighbor Discovery for IPv6 December 1998

 solicited-node multicast address
             - a link-local scope multicast address that is computed
               as a function of the solicited target's address.  The
               function is described in [ADDR-ARCH]. The function is
               chosen so that IP addresses which differ only in the
               high-order bits, e.g., due to multiple high-order
               prefixes associated with different providers, will map
               to the same solicited-node address thereby reducing the
               number of multicast addresses a node must join.
 link-local address
             - a unicast address having link-only scope that can be
               used to reach neighbors.  All interfaces on routers
               MUST have a link-local address.  Also, [ADDRCONF]
               requires that interfaces on hosts have a link-local
               address.
 unspecified address
             - a reserved address value that indicates the lack of an
               address (e.g., the address is unknown).  It is never
               used as a destination address, but may be used as a
               source address if the sender does not (yet) know its
               own address (e.g., while verifying an address is unused
               during address autoconfiguration [ADDRCONF]).  The
               unspecified address has a value of 0:0:0:0:0:0:0:0.

2.4. Requirements

 The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
 SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
 document, are to be interpreted as described in [KEYWORDS].
 This document also makes use of internal conceptual variables to
 describe protocol behavior and external variables that an
 implementation must allow system administrators to change.  The
 specific variable names, how their values change, and how their
 settings influence protocol behavior are provided to demonstrate
 protocol behavior.  An implementation is not required to have them in
 the exact form described here, so long as its external behavior is
 consistent with that described in this document.

3. PROTOCOL OVERVIEW

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

Narten, et. al. Standards Track [Page 9] RFC 2461 Neighbor Discovery for IPv6 December 1998

   Router Discovery: How hosts locate routers that reside on an
              attached link.
   Prefix Discovery: How hosts discover the set of address prefixes
              that define which destinations are on-link for an
              attached link.  (Nodes use prefixes to distinguish
              destinations that reside on-link from those only
              reachable through a router.)
   Parameter Discovery: How a node learns such link parameters as the
              link MTU or such Internet parameters as the hop limit
              value to place in outgoing packets.
   Address Autoconfiguration: How nodes automatically configure an
              address for an interface.
   Address resolution: How nodes determine the link-layer address of
              an on-link destination (e.g., a neighbor) given only the
              destination's IP address.
   Next-hop determination: The algorithm for mapping an IP destination
              address into the IP address of the neighbor to which
              traffic for the destination should be sent.  The next-
              hop can be a router or the destination itself.
   Neighbor Unreachability Detection: How nodes determine that a
              neighbor is no longer reachable.  For neighbors used as
              routers, alternate default routers can be tried.  For
              both routers and hosts, address resolution can be
              performed again.
   Duplicate Address Detection: How a node determines that an address
              it wishes to use is not already in use by another node.
   Redirect:  How a router informs a host of a better first-hop node
              to reach a particular destination.
 Neighbor Discovery defines five different ICMP packet types: A pair
 of Router Solicitation and Router Advertisement messages, a pair of
 Neighbor Solicitation and Neighbor Advertisements messages, and a
 Redirect message.  The messages serve the following purpose:
   Router Solicitation: When an interface becomes enabled, hosts may
              send out Router Solicitations that request routers to
              generate Router Advertisements immediately rather than
              at their next scheduled time.

Narten, et. al. Standards Track [Page 10] RFC 2461 Neighbor Discovery for IPv6 December 1998

   Router Advertisement: Routers advertise their presence together
              with various link and Internet parameters either
              periodically, or in response to a Router Solicitation
              message.  Router Advertisements contain prefixes that
              are used for on-link determination and/or address
              configuration, a suggested hop limit value, etc.
   Neighbor Solicitation: Sent by a node to determine the link-layer
              address of a neighbor, or to verify that a neighbor is
              still reachable via a cached link-layer address.
              Neighbor Solicitations are also used for Duplicate
              Address Detection.
   Neighbor Advertisement: A response to a Neighbor Solicitation
              message.  A node may also send unsolicited Neighbor
              Advertisements to announce a link-layer address change.
   Redirect:  Used by routers to inform hosts of a better first hop
              for a destination.
 On multicast-capable links, each router periodically multicasts a
 Router Advertisement packet announcing its availability.  A host
 receives Router Advertisements from all routers, building a list of
 default routers.  Routers generate Router Advertisements frequently
 enough that hosts will learn of their presence within a few minutes,
 but not frequently enough to rely on an absence of advertisements to
 detect router failure; a separate Neighbor Unreachability Detection
 algorithm provides failure detection.
 Router Advertisements contain a list of prefixes used for on-link
 determination and/or autonomous address configuration; flags
 associated with the prefixes specify the intended uses of a
 particular prefix.  Hosts use the advertised on-link prefixes to
 build and maintain a list that is used in deciding when a packet's
 destination is on-link or beyond a router.  Note that a destination
 can be on-link even though it is not covered by any advertised on-
 link prefix.  In such cases a router can send a Redirect informing
 the sender that the destination is a neighbor.
 Router Advertisements (and per-prefix flags) allow routers to inform
 hosts how to perform Address Autoconfiguration.  For example, routers
 can specify whether hosts should use stateful (DHCPv6) and/or
 autonomous (stateless) address configuration.  The exact semantics
 and usage of the address configuration-related information is
 specified in [ADDRCONF].
 Router Advertisement messages also contain Internet parameters such
 as the hop limit that hosts should use in outgoing packets and,

Narten, et. al. Standards Track [Page 11] RFC 2461 Neighbor Discovery for IPv6 December 1998

 optionally, link parameters such as the link MTU.  This facilitates
 centralized administration of critical parameters that can be set on
 routers and automatically propagated to all attached hosts.
 Nodes accomplish address resolution by multicasting a Neighbor
 Solicitation that asks the target node to return its link-layer
 address.  Neighbor Solicitation messages are multicast to the
 solicited-node multicast address of the target address.  The target
 returns its link-layer address in a unicast Neighbor Advertisement
 message.  A single request-response pair of packets is sufficient for
 both the initiator and the target to resolve each other's link-layer
 addresses; the initiator includes its link-layer address in the
 Neighbor Solicitation.
 Neighbor Solicitation messages can also be used to determine if more
 than one node has been assigned the same unicast address.  The use of
 Neighbor Solicitation messages for Duplicate Address Detection is
 specified in [ADDRCONF].
 Neighbor Unreachability Detection detects the failure of a neighbor
 or the failure of the forward path to the neighbor.  Doing so
 requires positive confirmation that packets sent to a neighbor are
 actually reaching that neighbor and being processed properly by its
 IP layer.  Neighbor Unreachability Detection uses confirmation from
 two sources.  When possible, upper-layer protocols provide a positive
 confirmation that a connection is making "forward progress", that is,
 previously sent data is known to have been delivered correctly (e.g.,
 new acknowledgments were received recently).  When positive
 confirmation is not forthcoming through such "hints", a node sends
 unicast Neighbor Solicitation messages that solicit Neighbor
 Advertisements as reachability confirmation from the next hop.  To
 reduce unnecessary network traffic, probe messages are only sent to
 neighbors to which the node is actively sending packets.
 In addition to addressing the above general problems, Neighbor
 Discovery also handles the following situations:
   Link-layer address change - A node that knows its link-layer
         address has changed can multicast a few (unsolicited)
         Neighbor Advertisement packets to all nodes to quickly update
         cached link-layer addresses that have become invalid.  Note
         that the sending of unsolicited advertisements is a
         performance enhancement only (e.g., unreliable).  The
         Neighbor Unreachability Detection algorithm ensures that all
         nodes will reliably discover the new address, though the
         delay may be somewhat longer.

Narten, et. al. Standards Track [Page 12] RFC 2461 Neighbor Discovery for IPv6 December 1998

   Inbound load balancing - Nodes with replicated interfaces may want
         to load balance the reception of incoming packets across
         multiple network interfaces on the same link.  Such nodes
         have multiple link-layer addresses assigned to the same
         interface.  For example, a single network driver could
         represent multiple network interface cards as a single
         logical interface having multiple link-layer addresses.
         Load balancing is handled by allowing routers to omit the
         source link-layer address from Router Advertisement packets,
         thereby forcing neighbors to use Neighbor Solicitation
         messages to learn link-layer addresses of routers.  Returned
         Neighbor Advertisement messages can then contain link-layer
         addresses that differ depending on who issued the
         solicitation.
   Anycast addresses - Anycast addresses identify one of a set of
         nodes providing an equivalent service, and multiple nodes on
         the same link may be configured to recognize the same Anycast
         address.  Neighbor Discovery handles anycasts by having nodes
         expect to receive multiple Neighbor Advertisements for the
         same target.  All advertisements for anycast addresses are
         tagged as being non-Override advertisements.  This invokes
         specific rules to determine which of potentially multiple
         advertisements should be used.
   Proxy advertisements - A router willing to accept packets on behalf
         of a target address that is unable to respond to Neighbor
         Solicitations can issue non-Override Neighbor Advertisements.
         There is currently no specified use of proxy, but proxy
         advertising could potentially be used to handle cases like
         mobile nodes that have moved off-link.  However, it is not
         intended as a general mechanism to handle nodes that, e.g.,
         do not implement this protocol.

3.1. Comparison with IPv4

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

Narten, et. al. Standards Track [Page 13] RFC 2461 Neighbor Discovery for IPv6 December 1998

    Router Discovery is part of the base protocol set; there is no
    need for hosts to "snoop" the routing protocols.
    Router advertisements carry link-layer addresses; no additional
    packet exchange is needed to resolve the router's link-layer
    address.
    Router advertisements carry prefixes for a link; there is no need
    to have a separate mechanism to configure the "netmask".
    Router advertisements enable Address Autoconfiguration.
    Routers can advertise an MTU for hosts to use on the link,
    ensuring that all nodes use the same MTU value on links lacking a
    well-defined MTU.
    Address resolution multicasts are "spread" over 4 billion (2^32)
    multicast addresses greatly reducing address resolution related
    interrupts on nodes other than the target.  Moreover, non-IPv6
    machines should not be interrupted at all.
    Redirects contain the link-layer address of the new first hop;
    separate address resolution is not needed upon receiving a
    redirect.
    Multiple prefixes can be associated with the same link.  By
    default, hosts learn all on-link prefixes from Router
    Advertisements.  However, routers may be configured to omit some
    or all prefixes from Router Advertisements.  In such cases hosts
    assume that destinations are off-link and send traffic to routers.
    A router can then issue redirects as appropriate.
    Unlike IPv4, the recipient of an IPv6 redirect assumes that the
    new next-hop is on-link.  In IPv4, a host ignores redirects
    specifying a next-hop that is not on-link according to the link's
    network mask.  The IPv6 redirect mechanism is analogous to the
    XRedirect facility specified in [SH-MEDIA].  It is expected to be
    useful on non-broadcast and shared media links in which it is
    undesirable or not possible for nodes to know all prefixes for
    on-link destinations.
    Neighbor Unreachability Detection is part of the base
    significantly improving the robustness of packet delivery in the
    presence of failing routers, partially failing or partitioned
    links and nodes that change their link-layer addresses.  For
    instance, mobile nodes can move off-link without losing any
    connectivity due to stale ARP caches.

Narten, et. al. Standards Track [Page 14] RFC 2461 Neighbor Discovery for IPv6 December 1998

    Unlike ARP, Neighbor Discovery detects half-link failures (using
    Neighbor Unreachability Detection) and avoids sending traffic to
    neighbors with which two-way connectivity is absent.
    Unlike in IPv4 Router Discovery the Router Advertisement messages
    do not contain a preference field.  The preference field is not
    needed to handle routers of different "stability"; the Neighbor
    Unreachability Detection will detect dead routers and switch to a
    working one.
    The use of link-local addresses to uniquely identify routers (for
    Router Advertisement and Redirect messages) makes it possible for
    hosts to maintain the router associations in the event of the site
    renumbering to use new global prefixes.
    Using the Hop Limit equal to 255 trick Neighbor Discovery is
    immune to off-link senders that accidentally or intentionally send
    ND messages.  In IPv4 off-link senders can send both ICMP
    Redirects and Router Advertisement messages.
    Placing address resolution at the ICMP layer makes the protocol
    more media-independent than ARP and makes it possible to use
    standard IP authentication and security mechanisms as appropriate
    [IPv6-AUTH, IPv6-ESP].

3.2. Supported Link Types

 Neighbor Discovery supports links with different properties.  In the
 presence of certain properties only a subset of the ND protocol
 mechanisms are fully specified in this document:
   point-to-point - Neighbor Discovery handles such links just like
                    multicast links.  (Multicast can be trivially
                    provided on point to point links, and interfaces
                    can be assigned link-local addresses.)  Neighbor
                    Discovery should be implemented as described in
                    this document.
   multicast      - Neighbor Discovery should be implemented as
                    described in this document.
   non-broadcast multiple access (NBMA)
                  - Redirect, Neighbor Unreachability Detection and
                    next-hop determination should be implemented as
                    described in this document.  Address resolution,
                    and the mechanism for delivering Router
                    Solicitations and Advertisements on NBMA links is
                    not specified in this document.  Note that if

Narten, et. al. Standards Track [Page 15] RFC 2461 Neighbor Discovery for IPv6 December 1998

                    hosts support manual configuration of a list of
                    default routers, hosts can dynamically acquire the
                    link-layer addresses for their neighbors from
                    Redirect messages.
   shared media   - The Redirect message is modeled after the
                    XRedirect message in [SH-MEDIA] in order to
                    simplify use of the protocol on shared media
                    links.
                    This specification does not address shared media
                    issues that only relate to routers, such as:
  1. How routers exchange reachability information

on a shared media link.

  1. How a router determines the link-layer address

of a host, which it needs to send redirect

                       messages to the host.
  1. How a router determines that it is the first-

hop router for a received packet.

                    The protocol is extensible (through the definition
                    of new options) so that other solutions might be
                    possible in the future.
   variable MTU   - Neighbor Discovery allows routers to specify a MTU
                    for the link, which all nodes then use.  All nodes
                    on a link must use the same MTU (or Maximum
                    Receive Unit) in order for multicast to work
                    properly.  Otherwise when multicasting a sender,
                    which can not know which nodes will receive the
                    packet, could not determine a minimum packet size
                    all receivers can process.
   asymmetric reachability
                  - Neighbor Discovery detects the absence of
                    symmetric reachability; a node avoids paths to a
                    neighbor with which it does not have symmetric
                    connectivity.
                    The Neighbor Unreachability Detection will
                    typically identify such half-links and the node
                    will refrain from using them.
                    The protocol can presumably be extended in the
                    future to find viable paths in environments that

Narten, et. al. Standards Track [Page 16] RFC 2461 Neighbor Discovery for IPv6 December 1998

                    lack reflexive and transitive connectivity.

4. MESSAGE FORMATS

4.1. Router Solicitation Message Format

 Hosts send Router Solicitations in order to prompt routers to
 generate Router Advertisements quickly.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Reserved                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Options ...
   +-+-+-+-+-+-+-+-+-+-+-+-
 IP Fields:
    Source Address
                   An IP address assigned to the sending interface, or
                   the unspecified address if no address is assigned
                   to the sending interface.
    Destination Address
                   Typically the all-routers multicast address.
    Hop Limit      255
    Authentication Header
                   If a Security Association for the IP Authentication
                   Header exists between the sender and the
                   destination address, then the sender SHOULD include
                   this header.
 ICMP Fields:
    Type           133
    Code           0
    Checksum       The ICMP checksum.  See [ICMPv6].
    Reserved       This field is unused.  It MUST be initialized to
                   zero by the sender and MUST be ignored by the
                   receiver.

Narten, et. al. Standards Track [Page 17] RFC 2461 Neighbor Discovery for IPv6 December 1998

 Valid Options:
    Source link-layer address
                   The link-layer address of the sender, if known.
                   MUST NOT be included if the Source Address is the
                   unspecified address.  Otherwise it SHOULD be
                   included on link layers that have addresses.
    Future versions of this protocol may define new option types.
    Receivers MUST silently ignore any options they do not recognize
    and continue processing the message.

4.2. Router Advertisement Message Format

 Routers send out Router Advertisement message periodically, or in
 response to a Router Solicitation.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Cur Hop Limit |M|O|  Reserved |       Router Lifetime         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Reachable Time                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Retrans Timer                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Options ...
   +-+-+-+-+-+-+-+-+-+-+-+-
 IP Fields:
    Source Address
                   MUST be the link-local address assigned to the
                   interface from which this message is sent.
    Destination Address
                   Typically the Source Address of an invoking Router
                   Solicitation or the all-nodes multicast address.
    Hop Limit      255
    Authentication Header
                   If a Security Association for the IP Authentication
                   Header exists between the sender and the
                   destination address, then the sender SHOULD include
                   this header.

Narten, et. al. Standards Track [Page 18] RFC 2461 Neighbor Discovery for IPv6 December 1998

 ICMP Fields:
    Type           134
    Code           0
    Checksum       The ICMP checksum.  See [ICMPv6].
    Cur Hop Limit  8-bit unsigned integer.  The default value that
                   should be placed in the Hop Count field of the IP
                   header for outgoing IP packets.  A value of zero
                   means unspecified (by this router).
    M              1-bit "Managed address configuration" flag.  When
                   set, hosts use the administered (stateful) protocol
                   for address autoconfiguration in addition to any
                   addresses autoconfigured using stateless address
                   autoconfiguration.  The use of this flag is
                   described in [ADDRCONF].
    O              1-bit "Other stateful configuration" flag.  When
                   set, hosts use the administered (stateful) protocol
                   for autoconfiguration of other (non-address)
                   information.  The use of this flag is described in
                   [ADDRCONF].
    Reserved       A 6-bit unused field.  It MUST be initialized to
                   zero by the sender and MUST be ignored by the
                   receiver.
    Router Lifetime
                   16-bit unsigned integer.  The lifetime associated
                   with the default router in units of seconds.  The
                   maximum value corresponds to 18.2 hours.  A
                   Lifetime of 0 indicates that the router is not a
                   default router and SHOULD NOT appear on the default
                   router list.  The Router Lifetime applies only to
                   the router's usefulness as a default router; it
                   does not apply to information contained in other
                   message fields or options.  Options that need time
                   limits for their information include their own
                   lifetime fields.
    Reachable Time 32-bit unsigned integer.  The time, in
                   milliseconds, that a node assumes a neighbor is
                   reachable after having received a reachability
                   confirmation.  Used by the Neighbor Unreachability
                   Detection algorithm (see Section 7.3).  A value of

Narten, et. al. Standards Track [Page 19] RFC 2461 Neighbor Discovery for IPv6 December 1998

                   zero means unspecified (by this router).
    Retrans Timer  32-bit unsigned integer.  The time, in
                   milliseconds, between retransmitted Neighbor
                   Solicitation messages.  Used by address resolution
                   and the Neighbor Unreachability Detection algorithm
                   (see Sections 7.2 and 7.3).  A value of zero means
                   unspecified (by this router).
 Possible options:
    Source link-layer address
                   The link-layer address of the interface from which
                   the Router Advertisement is sent.  Only used on
                   link layers that have addresses.  A router MAY omit
                   this option in order to enable inbound load sharing
                   across multiple link-layer addresses.
    MTU            SHOULD be sent on links that have a variable MTU
                   (as specified in the document that describes how to
                   run IP over the particular link type).  MAY be sent
                   on other links.
    Prefix Information
                   These options specify the prefixes that are on-link
                   and/or are used for address autoconfiguration.  A
                   router SHOULD include all its on-link prefixes
                   (except the link-local prefix) so that multihomed
                   hosts have complete prefix information about on-
                   link destinations for the links to which they
                   attach.  If complete information is lacking, a
                   multihomed host may not be able to choose the
                   correct outgoing interface when sending traffic to
                   its neighbors.
    Future versions of this protocol may define new option types.
    Receivers MUST silently ignore any options they do not recognize
    and continue processing the message.

Narten, et. al. Standards Track [Page 20] RFC 2461 Neighbor Discovery for IPv6 December 1998

4.3. Neighbor Solicitation Message Format

 Nodes send Neighbor Solicitations to request the link-layer address
 of a target node while also providing their own link-layer address to
 the target.  Neighbor Solicitations are multicast when the node needs
 to resolve an address and unicast when the node seeks to verify the
 reachability of a neighbor.
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |     Code      |          Checksum             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Reserved                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                                                               |
      +                       Target Address                          +
      |                                                               |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Options ...
      +-+-+-+-+-+-+-+-+-+-+-+-
 IP Fields:
    Source Address
                   Either an address assigned to the interface from
                   which this message is sent or (if Duplicate Address
                   Detection is in progress [ADDRCONF]) the
                   unspecified address.
    Destination Address
                   Either the solicited-node multicast address
                   corresponding to the target address, or the target
                   address.
    Hop Limit      255
    Authentication Header
                   If a Security Association for the IP Authentication
                   Header exists between the sender and the
                   destination address, then the sender SHOULD include
                   this header.

Narten, et. al. Standards Track [Page 21] RFC 2461 Neighbor Discovery for IPv6 December 1998

 ICMP Fields:
    Type           135
    Code           0
    Checksum       The ICMP checksum.  See [ICMPv6].
    Reserved       This field is unused.  It MUST be initialized to
                   zero by the sender and MUST be ignored by the
                   receiver.
    Target Address
                   The IP address of the target of the solicitation.
                   It MUST NOT be a multicast address.
 Possible options:
    Source link-layer address
                   The link-layer address for the sender.  MUST NOT be
                   included when the source IP address is the
                   unspecified address.  Otherwise, on link layers
                   that have addresses this option MUST be included in
                   multicast solicitations and SHOULD be included in
                   unicast solicitations.
    Future versions of this protocol may define new option types.
    Receivers MUST silently ignore any options they do not recognize
    and continue processing the message.

Narten, et. al. Standards Track [Page 22] RFC 2461 Neighbor Discovery for IPv6 December 1998

4.4. Neighbor Advertisement Message Format

 A node sends Neighbor Advertisements in response to Neighbor
 Solicitations and sends unsolicited Neighbor Advertisements in order
 to (unreliably) propagate new information quickly.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type      |     Code      |          Checksum             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |R|S|O|                     Reserved                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    |                                                               |
    +                       Target Address                          +
    |                                                               |
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Options ...
    +-+-+-+-+-+-+-+-+-+-+-+-
 IP Fields:
    Source Address
                   An address assigned to the interface from which the
                   advertisement is sent.
    Destination Address
                   For solicited advertisements, the Source Address of
                   an invoking Neighbor Solicitation or, if the
                   solicitation's Source Address is the unspecified
                   address, the all-nodes multicast address.
                   For unsolicited advertisements typically the all-
                   nodes multicast address.
    Hop Limit      255
 Authentication Header
                   If a Security Association for the IP Authentication
                   Header exists between the sender and the
                   destination address, then the sender SHOULD include
                   this header.

Narten, et. al. Standards Track [Page 23] RFC 2461 Neighbor Discovery for IPv6 December 1998

 ICMP Fields:
    Type           136
    Code           0
    Checksum       The ICMP checksum.  See [ICMPv6].
    R              Router flag.  When set, the R-bit indicates that
                   the sender is a router.  The R-bit is used by
                   Neighbor Unreachability Detection to detect a
                   router that changes to a host.
    S              Solicited flag.  When set, the S-bit indicates that
                   the advertisement was sent in response to a
                   Neighbor Solicitation from the Destination address.
                   The S-bit is used as a reachability confirmation
                   for Neighbor Unreachability Detection.  It MUST NOT
                   be set in multicast advertisements or in
                   unsolicited unicast advertisements.
    O              Override flag.  When set, the O-bit indicates that
                   the advertisement should override an existing cache
                   entry and update the cached link-layer address.
                   When it is not set the advertisement will not
                   update a cached link-layer address though it will
                   update an existing Neighbor Cache entry for which
                   no link-layer address is known.  It SHOULD NOT be
                   set in solicited advertisements for anycast
                   addresses and in solicited proxy advertisements.
                   It SHOULD be set in other solicited advertisements
                   and in unsolicited advertisements.
    Reserved       29-bit unused field.  It MUST be initialized to
                   zero by the sender and MUST be ignored by the
                   receiver.
    Target Address
                   For solicited advertisements, the Target Address
                   field in the Neighbor Solicitation message that
                   prompted this advertisement.  For an unsolicited
                   advertisement, the address whose link-layer address
                   has changed.  The Target Address MUST NOT be a
                   multicast address.

Narten, et. al. Standards Track [Page 24] RFC 2461 Neighbor Discovery for IPv6 December 1998

 Possible options:
    Target link-layer address
                   The link-layer address for the target, i.e., the
                   sender of the advertisement.  This option MUST be
                   included on link layers that have addresses when
                   responding to multicast solicitations.  When
                   responding to a unicast Neighbor Solicitation this
                   option SHOULD be included.
                   The option MUST be included for multicast
                   solicitations in order to avoid infinite Neighbor
                   Solicitation "recursion" when the peer node does
                   not have a cache entry to return a Neighbor
                   Advertisements message.  When responding to unicast
                   solicitations, the option can be omitted since the
                   sender of the solicitation has the correct link-
                   layer address; otherwise it would not have be able
                   to send the unicast solicitation in the first
                   place. However, including the link-layer address in
                   this case adds little overhead and eliminates a
                   potential race condition where the sender deletes
                   the cached link-layer address prior to receiving a
                   response to a previous solicitation.
    Future versions of this protocol may define new option types.
    Receivers MUST silently ignore any options they do not recognize
    and continue processing the message.

Narten, et. al. Standards Track [Page 25] RFC 2461 Neighbor Discovery for IPv6 December 1998

4.5. Redirect Message Format

 Routers send Redirect packets to inform a host of a better first-hop
 node on the path to a destination.  Hosts can be redirected to a
 better first-hop router but can also be informed by a redirect that
 the destination is in fact a neighbor.  The latter is accomplished by
 setting the ICMP Target Address equal to the ICMP Destination
 Address.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type      |     Code      |          Checksum             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           Reserved                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    |                                                               |
    +                       Target Address                          +
    |                                                               |
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    |                                                               |
    +                     Destination Address                       +
    |                                                               |
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Options ...
    +-+-+-+-+-+-+-+-+-+-+-+-
 IP Fields:
    Source Address
                   MUST be the link-local address assigned to the
                   interface from which this message is sent.
    Destination Address
                   The Source Address of the packet that triggered the
                   redirect.
    Hop Limit      255

Narten, et. al. Standards Track [Page 26] RFC 2461 Neighbor Discovery for IPv6 December 1998

    Authentication Header
                   If a Security Association for the IP Authentication
                   Header exists between the sender and the
                   destination address, then the sender SHOULD include
                   this header.
 ICMP Fields:
    Type           137
    Code           0
    Checksum       The ICMP checksum.  See [ICMPv6].
    Reserved       This field is unused.  It MUST be initialized to
                   zero by the sender and MUST be ignored by the
                   receiver.
    Target Address An IP address that is a better first hop to use for
                   the ICMP Destination Address.  When the target is
                   the actual endpoint of communication, i.e., the
                   destination is a neighbor, the Target Address field
                   MUST contain the same value as the ICMP Destination
                   Address field.  Otherwise the target is a better
                   first-hop router and the Target Address MUST be the
                   router's link-local address so that hosts can
                   uniquely identify routers.
    Destination Address
                   The IP address of the destination which is
                   redirected to the target.
 Possible options:
    Target link-layer address
                   The link-layer address for the target.  It SHOULD
                   be included (if known).  Note that on NBMA links,
                   hosts may rely on the presence of the Target Link-
                   Layer Address option in Redirect messages as the
                   means for determining the link-layer addresses of
                   neighbors.  In such cases, the option MUST be
                   included in Redirect messages.
    Redirected Header
                   As much as possible of the IP packet that triggered
                   the sending of the Redirect without making the
                   redirect packet exceed 1280 octets.

Narten, et. al. Standards Track [Page 27] RFC 2461 Neighbor Discovery for IPv6 December 1998

4.6. Option Formats

 Neighbor Discovery messages include zero or more options, some of
 which may appear multiple times in the same message.  All options are
 of the form:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |    Length     |              ...              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                              ...                              ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
    Type           8-bit identifier of the type of option.  The
                   options defined in this document are:
                         Option Name                             Type
                      Source Link-Layer Address                    1
                      Target Link-Layer Address                    2
                      Prefix Information                           3
                      Redirected Header                            4
                      MTU                                          5
    Length         8-bit unsigned integer.  The length of the option
                   (including the type and length fields) in units of
                   8 octets.  The value 0 is invalid.  Nodes MUST
                   silently discard an ND packet that contains an
                   option with length zero.

4.6.1. Source/Target Link-layer Address

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |    Link-Layer Address ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
    Type
                   1 for Source Link-layer Address
                   2 for Target Link-layer Address

Narten, et. al. Standards Track [Page 28] RFC 2461 Neighbor Discovery for IPv6 December 1998

    Length         The length of the option (including the type and
                   length fields) in units of 8 octets.  For example,
                   the length for IEEE 802 addresses is 1 [IPv6-
                   ETHER].
    Link-Layer Address
                   The variable length link-layer address.
                   The content and format of this field (including
                   byte and bit ordering) is expected to be specified
                   in specific documents that describe how IPv6
                   operates over different link layers.  For instance,
                   [IPv6-ETHER].
 Description
                   The Source Link-Layer Address option contains the
                   link-layer address of the sender of the packet.  It
                   is used in the Neighbor Solicitation, Router
                   Solicitation, and Router Advertisement packets.
                   The Target Link-Layer Address option contains the
                   link-layer address of the target.  It is used in
                   Neighbor Advertisement and Redirect packets.
                   These options MUST be silently ignored for other
                   Neighbor Discovery messages.

4.6.2. Prefix Information

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type      |    Length     | Prefix Length |L|A| Reserved1 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Valid Lifetime                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Preferred Lifetime                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           Reserved2                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    |                                                               |
    +                            Prefix                             +
    |                                                               |
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Narten, et. al. Standards Track [Page 29] RFC 2461 Neighbor Discovery for IPv6 December 1998

 Fields:
    Type           3
    Length         4
    Prefix Length  8-bit unsigned integer.  The number of leading bits
                   in the Prefix that are valid.  The value ranges
                   from 0 to 128.
    L              1-bit on-link flag.  When set, indicates that this
                   prefix can be used for on-link determination.  When
                   not set the advertisement makes no statement about
                   on-link or off-link properties of the prefix.  For
                   instance, the prefix might be used for address
                   configuration with some of the addresses belonging
                   to the prefix being on-link and others being off-
                   link.
    A              1-bit autonomous address-configuration flag.  When
                   set indicates that this prefix can be used for
                   autonomous address configuration as specified in
                   [ADDRCONF].
    Reserved1      6-bit unused field.  It MUST be initialized to zero
                   by the sender and MUST be ignored by the receiver.
    Valid Lifetime
                   32-bit unsigned integer.  The length of time in
                   seconds (relative to the time the packet is sent)
                   that the prefix is valid for the purpose of on-link
                   determination.  A value of all one bits
                   (0xffffffff) represents infinity.  The Valid
                   Lifetime is also used by [ADDRCONF].
    Preferred Lifetime
                   32-bit unsigned integer.  The length of time in
                   seconds (relative to the time the packet is sent)
                   that addresses generated from the prefix via
                   stateless address autoconfiguration remain
                   preferred [ADDRCONF].  A value of all one bits
                   (0xffffffff) represents infinity.  See [ADDRCONF].
    Reserved2      This field is unused.  It MUST be initialized to
                   zero by the sender and MUST be ignored by the
                   receiver.

Narten, et. al. Standards Track [Page 30] RFC 2461 Neighbor Discovery for IPv6 December 1998

    Prefix         An IP address or a prefix of an IP address.  The
                   Prefix Length field contains the number of valid
                   leading bits in the prefix.  The bits in the prefix
                   after the prefix length are reserved and MUST be
                   initialized to zero by the sender and ignored by
                   the receiver.  A router SHOULD NOT send a prefix
                   option for the link-local prefix and a host SHOULD
                   ignore such a prefix option.
 Description
                   The Prefix Information option provide hosts with
                   on-link prefixes and prefixes for Address
                   Autoconfiguration.
                   The Prefix Information option appears in Router
                   Advertisement packets and MUST be silently ignored
                   for other messages.

4.6.3. Redirected Header

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type      |    Length     |            Reserved           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           Reserved                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    ~                       IP header + data                        ~
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
    Type           4
    Length         The length of the option in units of 8 octets.
    Reserved       These fields are unused.  They MUST be initialized
                   to zero by the sender and MUST be ignored by the
                   receiver.
    IP header + data
                   The original packet truncated to ensure that the
                   size of the redirect message does not exceed 1280
                   octets.

Narten, et. al. Standards Track [Page 31] RFC 2461 Neighbor Discovery for IPv6 December 1998

 Description
                   The Redirected Header option is used in Redirect
                   messages and contains all or part of the packet
                   that is being redirected.
                   This option MUST be silently ignored for other
                   Neighbor Discovery messages.

4.6.4. MTU

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type      |    Length     |           Reserved            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                              MTU                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
    Type           5
    Length         1
    Reserved       This field is unused.  It MUST be initialized to
                   zero by the sender and MUST be ignored by the
                   receiver.
    MTU            32-bit unsigned integer.  The recommended MTU for
                   the link.
 Description
                   The MTU option is used in  Router Advertisement
                   messages to insure that all nodes on a link use the
                   same MTU value in those cases where the link MTU is
                   not well known.
                   This option MUST be silently ignored for other
                   Neighbor Discovery messages.
                   In configurations in which heterogeneous
                   technologies are bridged together, the maximum
                   supported MTU may differ from one segment to
                   another.  If the bridges do not generate ICMP
                   Packet Too Big messages, communicating nodes will
                   be unable to use Path MTU to dynamically determine
                   the appropriate MTU on a per-neighbor basis.  In
                   such cases, routers use the MTU option to specify

Narten, et. al. Standards Track [Page 32] RFC 2461 Neighbor Discovery for IPv6 December 1998

                   the maximum MTU value that is supported by all
                   segments.

5. CONCEPTUAL MODEL OF A HOST

 This section describes a conceptual model of one possible data
 structure organization that hosts (and to some extent routers) will
 maintain in interacting with neighboring nodes.  The described
 organization is provided to facilitate the explanation of how the
 Neighbor Discovery protocol should behave.  This document does not
 mandate that implementations adhere to this model as long as their
 external behavior is consistent with that described in this document.
 This model is only concerned with the aspects of host behavior
 directly related to Neighbor Discovery.  In particular, it does not
 concern itself with such issues as source address selection or the
 selecting of an outgoing interface on a multihomed host.

5.1. Conceptual Data Structures

 Hosts will need to maintain the following pieces of information for
 each interface:
    Neighbor Cache
                 - A set of entries about individual neighbors to
                   which traffic has been sent recently.  Entries are
                   keyed on the neighbor's on-link unicast IP address
                   and contain such information as its link-layer
                   address, a flag indicating whether the neighbor is
                   a router or a host (called IsRouter in this
                   document), a pointer to any queued packets waiting
                   for address resolution to complete, etc.
                   A Neighbor Cache entry also contains information
                   used by the Neighbor Unreachability Detection
                   algorithm, including the reachability state, the
                   number of unanswered probes, and the time the next
                   Neighbor Unreachability Detection event is
                   scheduled to take place.
    Destination Cache
                 - A set of entries about destinations to which
                   traffic has been sent recently.  The Destination
                   Cache includes both on-link and off-link
                   destinations and provides a level of indirection
                   into the Neighbor Cache; the Destination Cache maps
                   a destination IP address to the IP address of the
                   next-hop neighbor.  This cache is updated with

Narten, et. al. Standards Track [Page 33] RFC 2461 Neighbor Discovery for IPv6 December 1998

                   information learned from Redirect messages.
                   Implementations may find it convenient to store
                   additional information not directly related to
                   Neighbor Discovery in Destination Cache entries,
                   such as the Path MTU (PMTU) and round trip timers
                   maintained by transport protocols.
    Prefix List  - A list of the prefixes that define a set of
                   addresses that are on-link.  Prefix List entries
                   are created from information received in Router
                   Advertisements.  Each entry has an associated
                   invalidation timer value (extracted from the
                   advertisement) used to expire prefixes when they
                   become invalid.  A special "infinity" timer value
                   specifies that a prefix remains valid forever,
                   unless a new (finite) value is received in a
                   subsequent advertisement.
                   The link-local prefix is considered to be on the
                   prefix list with an infinite invalidation timer
                   regardless of whether routers are advertising a
                   prefix for it.  Received Router Advertisements
                   SHOULD NOT modify the invalidation timer for the
                   link-local prefix.
    Default Router List
                 - A list of routers to which packets may be sent.
                   Router list entries point to entries in the
                   Neighbor Cache; the algorithm for selecting a
                   default router favors routers known to be reachable
                   over those whose reachability is suspect.  Each
                   entry also has an associated invalidation timer
                   value (extracted from Router Advertisements) used
                   to delete entries that are no longer advertised.
 Note that the above conceptual data structures can be implemented
 using a variety of techniques.  One possible implementation is to use
 a single longest-match routing table for all of the above data
 structures.  Regardless of the specific implementation, it is
 critical that the Neighbor Cache entry for a router is shared by all
 Destination Cache entries using that router in order to prevent
 redundant Neighbor Unreachability Detection probes.
 Note also that other protocols (e.g., IPv6 Mobility) might add
 additional conceptual data structures.  An implementation is at
 liberty to implement such data structures in any way it pleases.  For
 example, an implementation could merge all conceptual data structures
 into a single routing table.

Narten, et. al. Standards Track [Page 34] RFC 2461 Neighbor Discovery for IPv6 December 1998

 The Neighbor Cache contains information maintained by the Neighbor
 Unreachability Detection algorithm.  A key piece of information is a
 neighbor's reachability state, which is one of five possible values.
 The following definitions are informal; precise definitions can be
 found in Section 7.3.2.
    INCOMPLETE  Address resolution is in progress and the link-layer
                address of the neighbor has not yet been determined.
    REACHABLE   Roughly speaking, the neighbor is known to have been
                reachable recently (within tens of seconds ago).
    STALE       The neighbor is no longer known to be reachable but
                until traffic is sent to the neighbor, no attempt
                should be made to verify its reachability.
    DELAY       The neighbor is no longer known to be reachable, and
                traffic has recently been sent to the neighbor.
                Rather than probe the neighbor immediately, however,
                delay sending probes for a short while in order to
                give upper layer protocols a chance to provide
                reachability confirmation.
    PROBE       The neighbor is no longer known to be reachable, and
                unicast Neighbor Solicitation probes are being sent to
                verify reachability.

5.2. Conceptual Sending Algorithm

 When sending a packet to a destination, a node uses a combination of
 the Destination Cache, the Prefix List, and the Default Router List
 to determine the IP address of the appropriate next hop, an operation
 known as "next-hop determination".  Once the IP address of the next
 hop is known, the Neighbor Cache is consulted for link-layer
 information about that neighbor.
 Next-hop determination for a given unicast destination operates as
 follows.  The sender performs a longest prefix match against the
 Prefix List to determine whether the packet's destination is on- or
 off-link.  If the destination is on-link, the next-hop address is the
 same as the packet's destination address.  Otherwise, the sender
 selects a router from the Default Router List (following the rules
 described in Section 6.3.6).  If the Default Router List is empty,
 the sender assumes that the destination is on-link.
 For efficiency reasons, next-hop determination is not performed on
 every packet that is sent.  Instead, the results of next-hop
 determination computations are saved in the Destination Cache (which

Narten, et. al. Standards Track [Page 35] RFC 2461 Neighbor Discovery for IPv6 December 1998

 also contains updates learned from Redirect messages).  When the
 sending node has a packet to send, it first examines the Destination
 Cache.  If no entry exists for the destination, next-hop
 determination is invoked to create a Destination Cache entry.
 Once the IP address of the next-hop node is known, the sender
 examines the Neighbor Cache for link-layer information about that
 neighbor.  If no entry exists, the sender creates one, sets its state
 to INCOMPLETE, initiates Address Resolution, and then queues the data
 packet pending completion of address resolution.  For multicast-
 capable interfaces Address Resolution consists of sending a Neighbor
 Solicitation message and waiting for a Neighbor Advertisement.  When
 a Neighbor Advertisement response is received, the link-layer
 addresses is entered in the Neighbor Cache entry and the queued
 packet is transmitted.  The address resolution mechanism is described
 in detail in Section 7.2.
 For multicast packets the next-hop is always the (multicast)
 destination address and is considered to be on-link.  The procedure
 for determining the link-layer address corresponding to a given IP
 multicast address can be found in a separate document that covers
 operating IP over a particular link type (e.g., [IPv6-ETHER]).
 Each time a Neighbor Cache entry is accessed while transmitting a
 unicast packet, the sender checks Neighbor Unreachability Detection
 related information according to the Neighbor Unreachability
 Detection algorithm (Section 7.3).  This unreachability check might
 result in the sender transmitting a unicast Neighbor Solicitation to
 verify that the neighbor is still reachable.
 Next-hop determination is done the first time traffic is sent to a
 destination.  As long as subsequent communication to that destination
 proceeds successfully, the Destination Cache entry continues to be
 used.  If at some point communication ceases to proceed, as
 determined by the Neighbor Unreachability Detection algorithm, next-
 hop determination may need to be performed again.  For example,
 traffic through a failed router should be switched to a working
 router.  Likewise, it may be possible to reroute traffic destined for
 a mobile node to a "mobility agent".
 Note that when a node redoes next-hop determination there is no need
 to discard the complete Destination Cache entry.  In fact, it is
 generally beneficial to retain such cached information as the PMTU
 and round trip timer values that may also be kept in the Destination
 Cache entry.

Narten, et. al. Standards Track [Page 36] RFC 2461 Neighbor Discovery for IPv6 December 1998

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

5.3. Garbage Collection and Timeout Requirements

 The conceptual data structures described above use different
 mechanisms for discarding potentially stale or unused information.
 From the perspective of correctness there is no need to periodically
 purge Destination and Neighbor Cache entries.  Although stale
 information can potentially remain in the cache indefinitely, the
 Neighbor Unreachability Detection algorithm ensures that stale
 information is purged quickly if it is actually being used.
 To limit the storage needed for the Destination and Neighbor Caches,
 a node may need to garbage-collect old entries.  However, care must
 be taken to insure that sufficient space is always present to hold
 the working set of active entries.  A small cache may result in an
 excessive number of Neighbor Discovery messages if entries are
 discarded and rebuilt in quick succession.  Any LRU-based policy that
 only reclaims entries that have not been used in some time (e.g., ten
 minutes or more) should be adequate for garbage-collecting unused
 entries.
 A node should retain entries in the Default Router List and the
 Prefix List until their lifetimes expire.  However, a node may
 garbage collect entries prematurely if it is low on memory.  If not
 all routers are kept on the Default Router list, a node should retain
 at least two entries in the Default Router List (and preferably more)
 in order to maintain robust connectivity for off-link destinations.
 When removing an entry from the Prefix List there is no need to purge
 any entries from the Destination or Neighbor Caches.  Neighbor
 Unreachability Detection will efficiently purge any entries in these
 caches that have become invalid.  When removing an entry from the
 Default Router List, however, any entries in the Destination Cache
 that go through that router must perform next-hop determination again
 to select a new default router.

6. ROUTER AND PREFIX DISCOVERY

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

Narten, et. al. Standards Track [Page 37] RFC 2461 Neighbor Discovery for IPv6 December 1998

 configuration parameters related to address autoconfiguration.
 Prefix Discovery is the process through which hosts learn the ranges
 of IP addresses that reside on-link and can be reached directly
 without going through a router.  Routers send Router Advertisements
 that indicate whether the sender is willing to be a default router.
 Router Advertisements also contain Prefix Information options that
 list the set of prefixes that identify on-link IP addresses.
 Stateless Address Autoconfiguration must also obtain subnet prefixes
 as part of configuring addresses.  Although the prefixes used for
 address autoconfiguration are logically distinct from those used for
 on-link determination, autoconfiguration information is piggybacked
 on Router Discovery messages to reduce network traffic.  Indeed, the
 same prefixes can be advertised for on-link determination and address
 autoconfiguration by specifying the appropriate flags in the Prefix
 Information options.  See [ADDRCONF] for details on how
 autoconfiguration information is processed.

6.1. Message Validation

6.1.1. Validation of Router Solicitation Messages

 Hosts MUST silently discard any received Router Solicitation
 Messages.
 A router MUST silently discard any received Router Solicitation
 messages that do not satisfy all of the following validity checks:
  1. The IP Hop Limit field has a value of 255, i.e., the packet

could not possibly have been forwarded by a router.

  1. If the message includes an IP Authentication Header, the message

authenticates correctly.

  1. ICMP Checksum is valid.
  1. ICMP Code is 0.
  1. ICMP length (derived from the IP length) is 8 or more octets.
  1. All included options have a length that is greater than zero.
  1. If the IP source address is the unspecified address, there is no

source link-layer address option in the message.

Narten, et. al. Standards Track [Page 38] RFC 2461 Neighbor Discovery for IPv6 December 1998

 The contents of the Reserved field, and of any unrecognized options,
 MUST be ignored.  Future, backward-compatible changes to the protocol
 may specify the contents of the Reserved field or add new options;
 backward-incompatible changes may use different Code values.
 The contents of any defined options that are not specified to be used
 with Router Solicitation messages MUST be ignored and the packet
 processed as normal.  The only defined option that may appear is the
 Source Link-Layer Address option.
 A solicitation that passes the validity checks is called a "valid
 solicitation".

6.1.2. Validation of Router Advertisement Messages

 A node MUST silently discard any received Router Advertisement
 messages that do not satisfy all of the following validity checks:
  1. IP Source Address is a link-local address. Routers must use

their link-local address as the source for Router Advertisement

      and Redirect messages so that hosts can uniquely identify
      routers.
  1. The IP Hop Limit field has a value of 255, i.e., the packet

could not possibly have been forwarded by a router.

  1. If the message includes an IP Authentication Header, the message

authenticates correctly.

  1. ICMP Checksum is valid.
  1. ICMP Code is 0.
  1. ICMP length (derived from the IP length) is 16 or more octets.
  1. All included options have a length that is greater than zero.
 The contents of the Reserved field, and of any unrecognized options,
 MUST be ignored.  Future, backward-compatible changes to the protocol
 may specify the contents of the Reserved field or add new options;
 backward-incompatible changes may use different Code values.
 The contents of any defined options that are not specified to be used
 with Router Advertisement messages MUST be ignored and the packet
 processed as normal.  The only defined options that may appear are
 the Source Link-Layer Address, Prefix Information and MTU options.

Narten, et. al. Standards Track [Page 39] RFC 2461 Neighbor Discovery for IPv6 December 1998

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

6.2. Router Specification

6.2.1. Router Configuration Variables

 A router MUST allow for the following conceptual variables to be
 configured by system management.  The specific variable names are
 used for demonstration purposes only, and an implementation is not
 required to have them, so long as its external behavior is consistent
 with that described in this document.  Default values are specified
 to simplify configuration in common cases.
 The default values for some of the variables listed below may be
 overridden by specific documents that describe how IPv6 operates over
 different link layers.  This rule simplifies the configuration of
 Neighbor Discovery over link types with widely differing performance
 characteristics.
 For each multicast interface:
    AdvSendAdvertisements
                   A flag indicating whether or not the router sends
                   periodic Router Advertisements and responds to
                   Router Solicitations.
                   Default: FALSE
                   Note that AdvSendAdvertisements MUST be FALSE by
                   default so that a node will not accidentally start
                   acting as a router unless it is explicitly
                   configured by system management to send Router
                   Advertisements.
    MaxRtrAdvInterval
                   The maximum time allowed between sending
                   unsolicited multicast Router Advertisements from
                   the interface, in seconds.  MUST be no less than 4
                   seconds and no greater than 1800 seconds.
                   Default: 600 seconds
    MinRtrAdvInterval
                   The minimum time allowed between sending
                   unsolicited multicast Router Advertisements from
                   the interface, in seconds.  MUST be no less than 3
                   seconds and no greater than .75 *

Narten, et. al. Standards Track [Page 40] RFC 2461 Neighbor Discovery for IPv6 December 1998

                   MaxRtrAdvInterval.
                   Default: 0.33 * MaxRtrAdvInterval
    AdvManagedFlag
                   The TRUE/FALSE value to be placed in the "Managed
                   address configuration" flag field in the Router
                   Advertisement.  See [ADDRCONF].
                   Default: FALSE
    AdvOtherConfigFlag
                   The TRUE/FALSE value to be placed in the "Other
                   stateful configuration" flag field in the Router
                   Advertisement.  See [ADDRCONF].
                   Default: FALSE
    AdvLinkMTU     The value to be placed in MTU options sent by the
                   router.  A value of zero indicates that no MTU
                   options are sent.
                   Default: 0
    AdvReachableTime
                   The value to be placed in the Reachable Time field
                   in the Router Advertisement messages sent by the
                   router.  The value zero means unspecified (by this
                   router).  MUST be no greater than 3,600,000
                   milliseconds (1 hour).
                   Default: 0
    AdvRetransTimer The value to be placed in the Retrans Timer field
                   in the Router Advertisement messages sent by the
                   router.  The value zero means unspecified (by this
                   router).
                   Default: 0
    AdvCurHopLimit
                   The default value to be placed in the Cur Hop Limit
                   field in the Router Advertisement messages sent by
                   the router.  The value should be set to that
                   current diameter of the Internet.  The value zero
                   means unspecified (by this router).

Narten, et. al. Standards Track [Page 41] RFC 2461 Neighbor Discovery for IPv6 December 1998

                   Default:  The value specified in the "Assigned
                   Numbers" RFC [ASSIGNED] that was in effect at the
                   time of implementation.
    AdvDefaultLifetime
                   The value to be placed in the Router Lifetime field
                   of Router Advertisements sent from the interface,
                   in seconds.  MUST be either zero or between
                   MaxRtrAdvInterval and 9000 seconds.  A value of
                   zero indicates that the router is not to be used as
                   a default router.
                   Default: 3 * MaxRtrAdvInterval
    AdvPrefixList
                   A list of prefixes to be placed in Prefix
                   Information options in Router Advertisement
                   messages sent from the interface.
                   Default: all prefixes that the router advertises
                   via routing protocols as being on-link for the
                   interface from which the advertisement is sent.
                   The link-local prefix SHOULD NOT be included in the
                   list of advertised prefixes.
                   Each prefix has an associated:
                      AdvValidLifetime
                           The value to be placed in the Valid
                           Lifetime in the Prefix Information
                           option, in seconds.  The designated value
                           of all 1's (0xffffffff) represents
                           infinity.  Implementations MUST allow
                           AdvValidLifetime to be specified in two
                           ways:
  1. a time that decrements in real time,

that is, one that will result in a

                               Lifetime of zero at the specified
                               time in the future, or
  1. a fixed time that stays the same in

consecutive advertisements.

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

Narten, et. al. Standards Track [Page 42] RFC 2461 Neighbor Discovery for IPv6 December 1998

                      AdvOnLinkFlag
                           The value to be placed in the on-link
                           flag ("L-bit") field in the Prefix
                           Information option.
                           Default: TRUE
                 Automatic address configuration [ADDRCONF]
                 defines additional information associated with
                 each the prefixes:
                      AdvPreferredLifetime
                           The value to be placed in the Preferred
                           Lifetime in the Prefix Information
                           option, in seconds.  The designated value
                           of all 1's (0xffffffff) represents
                           infinity.  See [ADDRCONF] for details on
                           how this value is used.  Implementations
                           MUST allow AdvPreferredLifetime to be
                           specified in two ways:
  1. a time that decrements in real time,

that is, one that will result in a

                               Lifetime of zero at a specified time
                               in the future, or
  1. a fixed time that stays the same in

consecutive advertisements.

                           Default: 604800 seconds (7 days), fixed
                           (i.e., stays the same in consecutive
                           advertisements).
                      AdvAutonomousFlag
                           The value to be placed in the Autonomous
                           Flag field in the Prefix Information
                           option.  See [ADDRCONF].
                           Default: TRUE
 The above variables contain information that is placed in outgoing
 Router Advertisement messages.  Hosts use the received information to
 initialize a set of analogous variables that control their external
 behavior (see Section 6.3.2).  Some of these host variables (e.g.,
 CurHopLimit, RetransTimer, and ReachableTime) apply to all nodes
 including routers.  In practice, these variables may not actually be
 present on routers, since their contents can be derived from the
 variables described above.  However, external router behavior MUST be

Narten, et. al. Standards Track [Page 43] RFC 2461 Neighbor Discovery for IPv6 December 1998

 the same as host behavior with respect to these variables.  In
 particular, this includes the occasional randomization of the
 ReachableTime value as described in Section 6.3.2.
 Protocol constants are defined in Section 10.

6.2.2. Becoming An Advertising Interface

 The term "advertising interface" refers to any functioning and
 enabled multicast interface that has at least one unicast IP address
 assigned to it and whose corresponding AdvSendAdvertisements flag is
 TRUE.  A router MUST NOT send Router Advertisements out any interface
 that is not an advertising interface.
 An interface may become an advertising interface at times other than
 system startup.  For example:
  1. changing the AdvSendAdvertisements flag on an enabled interface

from FALSE to TRUE, or

  1. administratively enabling the interface, if it had been

administratively disabled, and its AdvSendAdvertisements flag is

      TRUE, or
  1. enabling IP forwarding capability (i.e., changing the system

from being a host to being a router), when the interface's

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

6.2.3. Router Advertisement Message Content

 A router sends periodic as well as solicited Router Advertisements
 out its advertising interfaces.  Outgoing Router Advertisements are
 filled with the following values consistent with the message format
 given in Section 4.2:
  1. In the Router Lifetime field: the interface's configured

AdvDefaultLifetime.

  1. In the M and O flags: the interface's configured AdvManagedFlag

and AdvOtherConfigFlag, respectively. See [ADDRCONF].

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

CurHopLimit.

Narten, et. al. Standards Track [Page 44] RFC 2461 Neighbor Discovery for IPv6 December 1998

  1. In the Reachable Time field: the interface's configured

AdvReachableTime.

  1. In the Retrans Timer field: the interface's configured

AdvRetransTimer.

  1. In the options:
         o Source Link-Layer Address option: link-layer address of the
           sending interface.  This option MAY be omitted to
           facilitate in-bound load balancing over replicated
           interfaces.
         o MTU option: the interface's configured AdvLinkMTU value if
           the value is non-zero.  If AdvLinkMTU is zero the MTU
           option is not sent.
         o Prefix Information options: one Prefix Information option
           for each prefix listed in AdvPrefixList with the option
           fields set from the information in the AdvPrefixList entry
           as follows:
  1. In the "on-link" flag: the entry's AdvOnLinkFlag.
  1. In the Valid Lifetime field: the entry's

AdvValidLifetime.

  1. In the "Autonomous address configuration" flag: the

entry's AdvAutonomousFlag.

  1. In the Preferred Lifetime field: the entry's

AdvPreferredLifetime.

 A router might want to send Router Advertisements without advertising
 itself as a default router.  For instance, a router might advertise
 prefixes for address autoconfiguration while not wishing to forward
 packets.  Such a router sets the Router Lifetime field in outgoing
 advertisements to zero.
 A router MAY choose not to include some or all options when sending
 unsolicited Router Advertisements.  For example, if prefix lifetimes
 are much longer than AdvDefaultLifetime, including them every few
 advertisements may be sufficient.  However, when responding to a
 Router Solicitation or while sending the first few initial
 unsolicited advertisements, a router SHOULD include all options so
 that all information (e.g., prefixes) is propagated quickly during
 system initialization.

Narten, et. al. Standards Track [Page 45] RFC 2461 Neighbor Discovery for IPv6 December 1998

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

6.2.4. Sending Unsolicited Router Advertisements

 A host MUST NOT send Router Advertisement messages at any time.
 Unsolicited Router Advertisements are not strictly periodic: the
 interval between subsequent transmissions is randomized to reduce the
 probability of synchronization with the advertisements from other
 routers on the same link [SYNC].  Each advertising interface has its
 own timer.  Whenever a multicast advertisement is sent from an
 interface, the timer is reset to a uniformly-distributed random value
 between the interface's configured MinRtrAdvInterval and
 MaxRtrAdvInterval; expiration of the timer causes the next
 advertisement to be sent and a new random value to be chosen.
 For the first few advertisements (up to
 MAX_INITIAL_RTR_ADVERTISEMENTS) sent from an interface when it
 becomes an advertising interface, if the randomly chosen interval is
 greater than MAX_INITIAL_RTR_ADVERT_INTERVAL, the timer SHOULD be set
 to MAX_INITIAL_RTR_ADVERT_INTERVAL instead.  Using a smaller interval
 for the initial advertisements increases the likelihood of a router
 being discovered quickly when it first becomes available, in the
 presence of possible packet loss.
 The information contained in Router Advertisements may change through
 actions of system management.  For instance, the lifetime of
 advertised prefixes may change, new prefixes could be added, a router
 could cease to be a router (i.e., switch from being a router to being
 a host), etc.  In such cases, the router MAY transmit up to
 MAX_INITIAL_RTR_ADVERTISEMENTS unsolicited advertisements, using the
 same rules as when an interface becomes an advertising interface.

6.2.5. Ceasing To Be An Advertising Interface

 An interface may cease to be an advertising interface, through
 actions of system management such as:
  1. changing the AdvSendAdvertisements flag of an enabled interface

from TRUE to FALSE, or

  1. administratively disabling the interface, or
  1. shutting down the system.

Narten, et. al. Standards Track [Page 46] RFC 2461 Neighbor Discovery for IPv6 December 1998

 In such cases the router SHOULD transmit one or more (but not more
 than MAX_FINAL_RTR_ADVERTISEMENTS) final multicast Router
 Advertisements on the interface with a Router Lifetime field of zero.
 In the case of a router becoming a host, the system SHOULD also
 depart from the all-routers IP multicast group on all interfaces on
 which the router supports IP multicast (whether or not they had been
 advertising interfaces).  In addition, the host MUST insure that
 subsequent Neighbor Advertisement messages sent from the interface
 have the Router flag set to zero.
 Note that system management may disable a router's IP forwarding
 capability (i.e., changing the system from being a router to being a
 host), a step that does not necessarily imply that the router's
 interfaces stop being advertising interfaces.  In such cases,
 subsequent Router Advertisements MUST set the Router Lifetime field
 to zero.

6.2.6. Processing Router Solicitations

 A host MUST silently discard any received Router Solicitation
 messages.
 In addition to sending periodic, unsolicited advertisements, a router
 sends advertisements in response to valid solicitations received on
 an advertising interface.  A router MAY choose to unicast the
 response directly to the soliciting host's address (if the
 solicitation's source address is not the unspecified address), but
 the usual case is to multicast the response to the all-nodes group.
 In the latter case, the interface's interval timer is reset to a new
 random value, as if an unsolicited advertisement had just been sent
 (see Section 6.2.4).
 In all cases, Router Advertisements sent in response to a Router
 Solicitation MUST be delayed by a random time between 0 and
 MAX_RA_DELAY_TIME seconds. (If a single advertisement is sent in
 response to multiple solicitations, the delay is relative to the
 first solicitation.)  In addition, consecutive Router Advertisements
 sent to the all-nodes multicast address MUST be rate limited to no
 more than one advertisement every MIN_DELAY_BETWEEN_RAS seconds.
 A router might process Router Solicitations as follows:
  1. Upon receipt of a Router Solicitation, compute a random delay

within the range 0 through MAX_RA_DELAY_TIME. If the computed

    value corresponds to a time later than the time the next multicast
    Router Advertisement is scheduled to be sent, ignore the random
    delay and send the advertisement at the already-scheduled time.

Narten, et. al. Standards Track [Page 47] RFC 2461 Neighbor Discovery for IPv6 December 1998

  1. If the router sent a multicast Router Advertisement (solicited or

unsolicited) within the last MIN_DELAY_BETWEEN_RAS seconds,

    schedule the advertisement to be sent at a time corresponding to
    MIN_DELAY_BETWEEN_RAS plus the random value after the previous
    advertisement was sent.  This ensures that the multicast Router
    Advertisements are rate limited.
  1. Otherwise, schedule the sending of a Router Advertisement at the

time given by the random value.

 Note that a router is permitted to send multicast Router
 Advertisements more frequently than indicated by the
 MinRtrAdvInterval configuration variable so long as the more frequent
 advertisements are responses to Router Solicitations.  In all cases,
 however, unsolicited multicast advertisements MUST NOT be sent more
 frequently than indicated by MinRtrAdvInterval.
 Router Solicitations in which the Source Address is the unspecified
 address MUST NOT update the router's Neighbor Cache; solicitations
 with a proper source address update the Neighbor Cache as follows. If
 the router already has a Neighbor Cache entry for the solicitation's
 sender, the solicitation contains a Source Link-Layer Address option,
 and the received link-layer address differs from that already in the
 cache, the link-layer address SHOULD be updated in the appropriate
 Neighbor Cache entry, and its reachability state MUST also be set to
 STALE.  If there is no existing Neighbor Cache entry for the
 solicitation's sender, the router creates one, installs the link-
 layer address and sets its reachability state to STALE as specified
 in Section 7.3.3.  Whether or not a Source Link-Layer Address option
 is provided, if a Neighbor Cache entry for the solicitation's sender
 exists (or is created) the entry's IsRouter flag MUST be set to
 FALSE.

6.2.7. Router Advertisement Consistency

 Routers SHOULD inspect valid Router Advertisements sent by other
 routers and verify that the routers are advertising consistent
 information on a link.  Detected inconsistencies indicate that one or
 more routers might be misconfigured and SHOULD be logged to system or
 network management.  The minimum set of information to check
 includes:
  1. Cur Hop Limit values (except for the unspecified value of zero).
  1. Values of the M or O flags.
  1. Reachable Time values (except for the unspecified value of zero).

Narten, et. al. Standards Track [Page 48] RFC 2461 Neighbor Discovery for IPv6 December 1998

  1. Retrans Timer values (except for the unspecified value of zero).
  1. Values in the MTU options.
  1. Preferred and Valid Lifetimes for the same prefix. If

AdvPreferredLifetime and/or AdvValidLifetime decrement in real

    time as specified in section 6.2.7 then the comparison of the
    lifetimes can not compare the content of the fields in the Router
    Advertisement but must instead compare the time at which the
    prefix will become deprecated and invalidated, respectively.  Due
    to link propagation delays and potentially poorly synchronized
    clocks between the routers such comparison SHOULD allow some time
    skew.
 Note that it is not an error for different routers to advertise
 different sets of prefixes.  Also, some routers might leave some
 fields as unspecified, i.e., with the value zero, while other routers
 specify values.  The logging of errors SHOULD be restricted to
 conflicting information that causes hosts to switch from one value to
 another with each received advertisement.
 Any other action on reception of Router Advertisement messages by a
 router is beyond the scope of this document.

6.2.8. Link-local Address Change

 The link-local address on a router SHOULD change rarely, if ever.
 Nodes receiving Neighbor Discovery messages use the source address to
 identify the sender.  If multiple packets from the same router
 contain different source addresses, nodes will assume they come from
 different routers, leading to undesirable behavior.  For example, a
 node will ignore Redirect messages that are believed to have been
 sent by a router other than the current first-hop router.  Thus the
 source address used in Router Advertisements sent by a particular
 router must be identical to the target address in a Redirect message
 when redirecting to that router.
 Using the link-local address to uniquely identify routers on the link
 has the benefit that the address a router is known by should not
 change when a site renumbers.
 If a router changes the link-local address for one of its interfaces,
 it SHOULD inform hosts of this change.  The router SHOULD multicast a
 few Router Advertisements from the old link-local address with the
 Router Lifetime field set to zero and also multicast a few Router
 Advertisements from the new link-local address.  The overall effect
 should be the same as if one interface ceases being an advertising
 interface, and a different one starts being an advertising interface.

Narten, et. al. Standards Track [Page 49] RFC 2461 Neighbor Discovery for IPv6 December 1998

6.3. Host Specification

6.3.1. Host Configuration Variables

 None.

6.3.2. Host Variables

 A host maintains certain Neighbor Discovery related variables in
 addition to the data structures defined in Section 5.1.  The specific
 variable names are used for demonstration purposes only, and an
 implementation is not required to have them, so long as its external
 behavior is consistent with that described in this document.
 These variables have default values that are overridden by
 information received in Router Advertisement messages.  The default
 values are used when there is no router on the link or when all
 received Router Advertisements have left a particular value
 unspecified.
 The default values in this specification may be overridden by
 specific documents that describe how IP operates over different link
 layers.  This rule allows Neighbor Discovery to operate over links
 with widely varying performance characteristics.
 For each interface:
      LinkMTU        The MTU of the link.
                     Default: The valued defined in the specific
                     document that describes how IPv6 operates over
                     the particular link layer (e.g., [IPv6-ETHER]).
      CurHopLimit    The default hop limit to be used when sending
                     (unicast) IP packets.
                     Default: The value specified in the "Assigned
                     Numbers" RFC [ASSIGNED] that was in effect at the
                     time of implementation.
      BaseReachableTime
                     A base value used for computing the random
                     ReachableTime value.
                     Default: REACHABLE_TIME milliseconds.
      ReachableTime  The time a neighbor is considered reachable after
                     receiving a reachability confirmation.

Narten, et. al. Standards Track [Page 50] RFC 2461 Neighbor Discovery for IPv6 December 1998

                     This value should be a uniformly-distributed
                     random value between MIN_RANDOM_FACTOR and
                     MAX_RANDOM_FACTOR times BaseReachableTime
                     milliseconds.  A new random value should be
                     calculated when BaseReachableTime changes (due to
                     Router Advertisements) or at least every few
                     hours even if no Router Advertisements are
                     received.
      RetransTimer   The time between retransmissions of Neighbor
                     Solicitation messages to a neighbor when
                     resolving the address or when probing the
                     reachability of a neighbor.
                     Default: RETRANS_TIMER milliseconds

6.3.3. Interface Initialization

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

6.3.4. Processing Received Router Advertisements

 When multiple routers are present, the information advertised
 collectively by all routers may be a superset of the information
 contained in a single Router Advertisement.  Moreover, information
 may also be obtained through other dynamic means, such as stateful
 autoconfiguration.  Hosts accept the union of all received
 information; the receipt of a Router Advertisement MUST NOT
 invalidate all information received in a previous advertisement or
 from another source.  However, when received information for a
 specific parameter (e.g., Link MTU) or option (e.g., Lifetime on a
 specific Prefix) differs from information received earlier, and the
 parameter/option can only have one value, the most recently-received
 information is considered authoritative.
 Some Router Advertisement fields (e.g., Cur Hop Limit, Reachable Time
 and Retrans Timer) may contain a value denoting unspecified.  In such
 cases, the parameter should be ignored and the host should continue
 using whatever value it is already using.  In particular, a host MUST
 NOT interpret the unspecified value as meaning change back to the
 default value that was in use before the first Router Advertisement
 was received.  This rule prevents hosts from continually changing an
 internal variable when one router advertises a specific value, but
 other routers advertise the unspecified value.
 On receipt of a valid Router Advertisement, a host extracts the
 source address of the packet and does the following:

Narten, et. al. Standards Track [Page 51] RFC 2461 Neighbor Discovery for IPv6 December 1998

  1. If the address is not already present in the host's Default

Router List, and the advertisement's Router Lifetime is non-

      zero, create a new entry in the list, and initialize its
      invalidation timer value from the advertisement's Router
      Lifetime field.
  1. If the address is already present in the host's Default Router

List as a result of a previously-received advertisement, reset

      its invalidation timer to the Router Lifetime value in the
      newly-received advertisement.
  1. If the address is already present in the host's Default Router

List and the received Router Lifetime value is zero, immediately

      time-out the entry as specified in Section 6.3.5.
 To limit the storage needed for the Default Router List, a host MAY
 choose not to store all of the router addresses discovered via
 advertisements.  However, a host MUST retain at least two router
 addresses and SHOULD retain more.  Default router selections are made
 whenever communication to a destination appears to be failing.  Thus,
 the more routers on the list, the more likely an alternative working
 router can be found quickly (e.g., without having to wait for the
 next advertisement to arrive).
 If the received Cur Hop Limit value is non-zero the host SHOULD set
 its CurHopLimit variable to the received value.
 If the received Reachable Time value is non-zero the host SHOULD set
 its BaseReachableTime variable to the received value.  If the new
 value differs from the previous value, the host SHOULD recompute a
 new random ReachableTime value.  ReachableTime is computed as a
 uniformly-distributed random value between MIN_RANDOM_FACTOR and
 MAX_RANDOM_FACTOR times the BaseReachableTime.  Using a random
 component eliminates the possibility Neighbor Unreachability
 Detection messages synchronize with each other.
 In most cases, the advertised Reachable Time value will be the same
 in consecutive Router Advertisements and a host's BaseReachableTime
 rarely changes.  In such cases, an implementation SHOULD insure that
 a new random value gets recomputed at least once every few hours.
 The RetransTimer variable SHOULD be copied from the Retrans Timer
 field, if the received value is non-zero.
 After extracting information from the fixed part of the Router
 Advertisement message, the advertisement is scanned for valid
 options.  If the advertisement contains a Source Link-Layer Address
 option the link-layer address SHOULD be recorded in the Neighbor

Narten, et. al. Standards Track [Page 52] RFC 2461 Neighbor Discovery for IPv6 December 1998

 Cache entry for the router (creating an entry if necessary) and the
 IsRouter flag in the Neighbor Cache entry MUST be set to TRUE.  If no
 Source Link-Layer Address is included, but a corresponding Neighbor
 Cache entry exists, its IsRouter flag MUST be set to TRUE.  The
 IsRouter flag is used by Neighbor Unreachability Detection to
 determine when a router changes to being a host (i.e., no longer
 capable of forwarding packets).  If a Neighbor Cache entry is created
 for the router its reachability state MUST be set to STALE as
 specified in Section 7.3.3.  If a cache entry already exists and is
 updated with a different link-layer address the reachability state
 MUST also be set to STALE.
 If the MTU option is present, hosts SHOULD copy the option's value
 into LinkMTU so long as the value is greater than or equal to the
 minimum link MTU [IPv6] and does not exceed the default LinkMTU value
 specified in the link type specific document (e.g., [IPv6-ETHER]).
 Prefix Information options that have the "on-link" (L) flag set
 indicate a prefix identifying a range of addresses that should be
 considered on-link.  Note, however, that a Prefix Information option
 with the on-link flag set to zero conveys no information concerning
 on-link determination and MUST NOT be interpreted to mean that
 addresses covered by the prefix are off-link.  The only way to cancel
 a previous on-link indication is to advertise that prefix with the
 L-bit set and the Lifetime set to zero.  The default behavior (see
 Section 5.2) when sending a packet to an address for which no
 information is known about the on-link status of the address is to
 forward the packet to a default router; the reception of a Prefix
 Information option with the "on-link " (L) flag set to zero does not
 change this behavior.  The reasons for an address being treated as
 on-link is specified in the definition of "on-link" in Section 2.1.
 Prefixes with the on-link flag set to zero would normally have the
 autonomous flag set and be used by [ADDRCONF].
 For each Prefix Information option with the on-link flag set, a host
 does the following:
  1. If the prefix is the link-local prefix, silently ignore the

Prefix Information option.

  1. If the prefix is not already present in the Prefix List, and the

Prefix Information option's Valid Lifetime field is non-zero,

      create a new entry for the prefix and initialize its
      invalidation timer to the Valid Lifetime value in the Prefix
      Information option.
  1. If the prefix is already present in the host's Prefix List as

the result of a previously-received advertisement, reset its

Narten, et. al. Standards Track [Page 53] RFC 2461 Neighbor Discovery for IPv6 December 1998

      invalidation timer to the Valid Lifetime value in the Prefix
      Information option.  If the new Lifetime value is zero, time-out
      the prefix immediately (see Section 6.3.5).
  1. If the Prefix Information option's Valid Lifetime field is zero,

and the prefix is not present in the host's Prefix List,

      silently ignore the option.
 Stateless address autoconfiguration [ADDRCONF] may in some
 circumstances increase the Valid Lifetime of a prefix or ignore it
 completely in order to prevent a particular denial of service attack.
 However, since the effect of the same denial of service targeted at
 the on-link prefix list is not catastrophic (hosts would send packets
 to a default router and receive a redirect rather than sending
 packets directly to a neighbor) the Neighbor Discovery protocol does
 not impose such a check on the prefix lifetime values.
    Note: Implementations can choose to process the on-link aspects of
    the prefixes separately from the address autoconfiguration aspects
    of the prefixes by, e.g., passing a copy of each valid Router
    Advertisement message to both an "on-link" and an "addrconf"
    function.  Each function can then operate independently on the
    prefixes that have the appropriate flag set.

6.3.5. Timing out Prefixes and Default Routers

 Whenever the invalidation timer expires for a Prefix List entry, that
 entry is discarded.  No existing Destination Cache entries need be
 updated, however.  Should a reachability problem arise with an
 existing Neighbor Cache entry, Neighbor Unreachability Detection will
 perform any needed recovery.
 Whenever the Lifetime of an entry in the Default Router List expires,
 that entry is discarded.  When removing a router from the Default
 Router list, the node MUST update the Destination Cache in such a way
 that all entries using the router perform next-hop determination
 again rather than continue sending traffic to the (deleted) router.

6.3.6. Default Router Selection

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

Narten, et. al. Standards Track [Page 54] RFC 2461 Neighbor Discovery for IPv6 December 1998

 with subsequent traffic for that destination using the same router as
 indicated in the Destination Cache modulo any changes to the
 Destination Cache caused by Redirect messages.
 The policy for selecting routers from the Default Router List is as
 follows:
   1) Routers that are reachable or probably reachable (i.e., in any
      state other than INCOMPLETE) SHOULD be preferred over routers
      whose reachability is unknown or suspect (i.e., in the
      INCOMPLETE state, or for which no Neighbor Cache entry exists).
      An implementation may choose to always return the same router or
      cycle through the router list in a round-robin fashion as long
      as it always returns a reachable or a probably reachable router
      when one is available.
   2) When no routers on the list are known to be reachable or
      probably reachable, routers SHOULD be selected in a round-robin
      fashion, so that subsequent requests for a default router do not
      return the same router until all other routers have been
      selected.
      Cycling through the router list in this case ensures that all
      available routers are actively probed by the Neighbor
      Unreachability Detection algorithm.  A request for a default
      router is made in conjunction with the sending of a packet to a
      router, and the selected router will be probed for reachability
      as a side effect.
   3) If the Default Router List is empty, assume that all
      destinations are on-link as specified in Section 5.2.

6.3.7. Sending Router Solicitations

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

failure or after being temporarily disabled by system

      management.

Narten, et. al. Standards Track [Page 55] RFC 2461 Neighbor Discovery for IPv6 December 1998

  1. The system changes from being a router to being a host, by

having its IP forwarding capability turned off by system

      management.
  1. The host attaches to a link for the first time.
  1. The host re-attaches to a link after being detached for some

time.

 A host sends Router Solicitations to the All-Routers multicast
 address.  The IP source address is set to either one of the
 interface's unicast addresses or the unspecified address.  The Source
 Link-Layer Address option SHOULD be set to the host's link-layer
 address, if the IP source address is not the unspecified address.
 Before a host sends an initial solicitation, it SHOULD delay the
 transmission for a random amount of time between 0 and
 MAX_RTR_SOLICITATION_DELAY.  This serves to alleviate congestion when
 many hosts start up on a link at the same time, such as might happen
 after recovery from a power failure.  If a host has already performed
 a random delay since the interface became (re)enabled (e.g., as part
 of Duplicate Address Detection [ADDRCONF]) there is no need to delay
 again before sending the first Router Solicitation message.
 Once the host sends a Router Solicitation, and receives a valid
 Router Advertisement with a non-zero Router Lifetime, the host MUST
 desist from sending additional solicitations on that interface, until
 the next time one of the above events occurs.  Moreover, a host
 SHOULD send at least one solicitation in the case where an
 advertisement is received prior to having sent a solicitation.
 Unsolicited Router Advertisements may be incomplete (see Section
 6.2.3); solicited advertisements are expected to contain complete
 information.
 If a host sends MAX_RTR_SOLICITATIONS solicitations, and receives no
 Router Advertisements after having waited MAX_RTR_SOLICITATION_DELAY
 seconds after sending the last solicitation, the host concludes that
 there are no routers on the link for the purpose of [ADDRCONF].
 However, the host continues to receive and process Router
 Advertisements messages in the event that routers appear on the link.

7. ADDRESS RESOLUTION AND NEIGHBOR UNREACHABILITY DETECTION

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

Narten, et. al. Standards Track [Page 56] RFC 2461 Neighbor Discovery for IPv6 December 1998

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

7.1. Message Validation

7.1.1. Validation of Neighbor Solicitations

 A node MUST silently discard any received Neighbor Solicitation
 messages that do not satisfy all of the following validity checks:
  1. The IP Hop Limit field has a value of 255, i.e., the packet

could not possibly have been forwarded by a router.

  1. If the message includes an IP Authentication Header, the message

authenticates correctly.

  1. ICMP Checksum is valid.
  1. ICMP Code is 0.
  1. ICMP length (derived from the IP length) is 24 or more octets.
  1. Target Address is not a multicast address.
  1. All included options have a length that is greater than zero.
  1. If the IP source address is the unspecified address, the IP

destination address is a solicited-node multicast address.

  1. If the IP source address is the unspecified address, there is no

source link-layer address option in the message.

 The contents of the Reserved field, and of any unrecognized options,
 MUST be ignored.  Future, backward-compatible changes to the protocol
 may specify the contents of the Reserved field or add new options;
 backward-incompatible changes may use different Code values.
 The contents of any defined options that are not specified to be used
 with Neighbor Solicitation messages MUST be ignored and the packet
 processed as normal.  The only defined option that may appear is the
 Source Link-Layer Address option.

Narten, et. al. Standards Track [Page 57] RFC 2461 Neighbor Discovery for IPv6 December 1998

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

7.1.2. Validation of Neighbor Advertisements

 A node MUST silently discard any received Neighbor Advertisement
 messages that do not satisfy all of the following validity checks:
  1. The IP Hop Limit field has a value of 255, i.e., the packet

could not possibly have been forwarded by a router.

  1. If the message includes an IP Authentication Header, the message

authenticates correctly.

  1. ICMP Checksum is valid.
  1. ICMP Code is 0.
  1. ICMP length (derived from the IP length) is 24 or more octets.
  1. Target Address is not a multicast address.
  1. If the IP Destination Address is a multicast address the

Solicited flag is zero.

  1. All included options have a length that is greater than zero.
 The contents of the Reserved field, and of any unrecognized options,
 MUST be ignored.  Future, backward-compatible changes to the protocol
 may specify the contents of the Reserved field or add new options;
 backward-incompatible changes may use different Code values.
 The contents of any defined options that are not specified to be used
 with Neighbor Advertisement messages MUST be ignored and the packet
 processed as normal.  The only defined option that may appear is the
 Target Link-Layer Address option.
 A Neighbor Advertisements that passes the validity checks is called a
 "valid advertisement".

7.2. Address Resolution

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

Narten, et. al. Standards Track [Page 58] RFC 2461 Neighbor Discovery for IPv6 December 1998

7.2.1. Interface Initialization

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

7.2.2. Sending Neighbor Solicitations

 When a node has a unicast packet to send to a neighbor, but does not
 know the neighbor's link-layer address, it performs address
 resolution.  For multicast-capable interfaces this entails creating a
 Neighbor Cache entry in the INCOMPLETE state and transmitting a
 Neighbor Solicitation message targeted at the neighbor.  The
 solicitation is sent to the solicited-node multicast address
 corresponding to the target address.
 If the source address of the packet prompting the solicitation is the
 same as one of the addresses assigned to the outgoing interface, that
 address SHOULD be placed in the IP Source Address of the outgoing
 solicitation.  Otherwise, any one of the addresses assigned to the
 interface should be used.  Using the prompting packet's source
 address when possible insures that the recipient of the Neighbor
 Solicitation installs in its Neighbor Cache the IP address that is
 highly likely to be used in subsequent return traffic belonging to
 the prompting packet's "connection".
 If the solicitation is being sent to a solicited-node multicast
 address, the sender MUST include its link-layer address (if it has
 one) as a Source Link-Layer Address option.  Otherwise, the sender
 SHOULD include its link-layer address (if it has one) as a Source
 Link-Layer Address option.  Including the source link-layer address
 in a multicast solicitation is required to give the target an address
 to which it can send the Neighbor Advertisement.  On unicast
 solicitations, an implementation MAY omit the Source Link-Layer
 Address option. The assumption here is that if the sender has a
 peer's link-layer address in its cache, there is a high probability
 that the peer will also have an entry in its cache for the sender.
 Consequently, it need not be sent.

Narten, et. al. Standards Track [Page 59] RFC 2461 Neighbor Discovery for IPv6 December 1998

 While waiting for address resolution to complete, the sender MUST,
 for each neighbor, retain a small queue of packets waiting for
 address resolution to complete.  The queue MUST hold at least one
 packet, and MAY contain more.  However, the number of queued packets
 per neighbor SHOULD be limited to some small value.  When a queue
 overflows, the new arrival SHOULD replace the oldest entry.  Once
 address resolution completes, the node transmits any queued packets.
 While awaiting a response, the sender SHOULD retransmit Neighbor
 Solicitation messages approximately every RetransTimer milliseconds,
 even in the absence of additional traffic to the neighbor.
 Retransmissions MUST be rate-limited to at most one solicitation per
 neighbor every RetransTimer milliseconds.
 If no Neighbor Advertisement is received after MAX_MULTICAST_SOLICIT
 solicitations, address resolution has failed.  The sender MUST return
 ICMP destination unreachable indications with code 3 (Address
 Unreachable) for each packet queued awaiting address resolution.

7.2.3. Receipt of Neighbor Solicitations

 A valid Neighbor Solicitation that does not meet any the following
 requirements MUST be silently discarded:
  1. The Target Address is a "valid" unicast or anycast address

assigned to the receiving interface [ADDRCONF],

  1. The Target Address is a unicast address for which the node is

offering proxy service, or

  1. The Target Address is a "tentative" address on which Duplicate

Address Detection is being performed [ADDRCONF].

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

Narten, et. al. Standards Track [Page 60] RFC 2461 Neighbor Discovery for IPv6 December 1998

 If a Neighbor Cache entry is created the IsRouter flag SHOULD be set
 to FALSE.  This will be the case even if the Neighbor Solicitation is
 sent by a router since the Neighbor Solicitation messages do not
 contain an indication of whether or not the sender is a router.  In
 the event that the sender is a router, subsequent Neighbor
 Advertisement or Router Advertisement messages will set the correct
 IsRouter value.  If a Neighbor Cache entry already exists its
 IsRouter flag MUST NOT be modified.
 If the Source Address is the unspecified address the node MUST NOT
 create or update the Neighbor Cache entry.
 After any updates to the Neighbor Cache, the node sends a Neighbor
 Advertisement response as described in the next section.

7.2.4. Sending Solicited Neighbor Advertisements

 A node sends a Neighbor Advertisement in response to a valid Neighbor
 Solicitation targeting one of the node's assigned addresses.  The
 Target Address of the advertisement is copied from the Target Address
 of the solicitation.  If the solicitation's IP Destination Address is
 not a multicast address, the Target Link-Layer Address option MAY be
 omitted; the neighboring node's cached value must already be current
 in order for the solicitation to have been received.  If the
 solicitation's IP Destination Address is a multicast address, the
 Target Link-Layer option MUST be included in the advertisement.
 Furthermore, if the node is a router, it MUST set the Router flag to
 one; otherwise it MUST set the flag to zero.
 If the Target Address is either an anycast address or a unicast
 address for which the node is providing proxy service, or the Target
 Link-Layer Address option is not included, the Override flag SHOULD
 be set to zero.  Otherwise, the Override flag SHOULD be set to one.
 Proper setting of the Override flag ensures that nodes give
 preference to non-proxy advertisements, even when received after
 proxy advertisements, and also ensures that the first advertisement
 for an anycast address "wins".
 If the source of the solicitation is the unspecified address, the
 node MUST set the Solicited flag to zero and multicast the
 advertisement to the all-nodes address.  Otherwise, the node MUST set
 the Solicited flag to one and unicast the advertisement to the Source
 Address of the solicitation.
 If the Target Address is an anycast address the sender SHOULD delay
 sending a response for a random time between 0 and
 MAX_ANYCAST_DELAY_TIME seconds.

Narten, et. al. Standards Track [Page 61] RFC 2461 Neighbor Discovery for IPv6 December 1998

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

7.2.5. Receipt of Neighbor Advertisements

 When a valid Neighbor Advertisement is received (either solicited or
 unsolicited), the Neighbor Cache is searched for the target's entry.
 If no entry exists, the advertisement SHOULD be silently discarded.
 There is no need to create an entry if none exists, since the
 recipient has apparently not initiated any communication with the
 target.
 Once the appropriate Neighbor Cache entry has been located, the
 specific actions taken depend on the state of the Neighbor Cache
 entry, the flags in the advertisement and the actual link-layer
 address supplied.
 If the target's Neighbor Cache entry is in the INCOMPLETE state when
 the advertisement is received, one of two things happens.  If the
 link layer has addresses and no Target Link-Layer address option is
 included, the receiving node SHOULD silently discard the received
 advertisement.  Otherwise, the receiving node performs the following
 steps:
  1. It records the link-layer address in the Neighbor Cache entry.
  1. If the advertisement's Solicited flag is set, the state of the

entry is set to REACHABLE, otherwise it is set to STALE.

  1. It sets the IsRouter flag in the cache entry based on the Router

flag in the received advertisement.

  1. It sends any packets queued for the neighbor awaiting address

resolution.

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

Narten, et. al. Standards Track [Page 62] RFC 2461 Neighbor Discovery for IPv6 December 1998

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

MUST be inserted in the cache (if one is supplied and is different

    than the already recorded address).
  1. If the Solicited flag is set, the state of the entry MUST be set

to REACHABLE. If the Solicited flag is zero and the link-layer

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

Router flag in the received advertisement. In those cases where

    the IsRouter flag changes from TRUE to FALSE as a result of this
    update, the node MUST remove that router from the Default Router
    List and update the Destination Cache entries for all destinations
    using that neighbor as a router as specified in Section 7.3.3.
    This is needed to detect when a node that is used as a router
    stops forwarding packets due to being configured as a host.
 The above rules ensure that the cache is updated either when the
 Neighbor Advertisement takes precedence (i.e., the Override flag is
 set) or when the Neighbor Advertisement refers to the same link-layer
 address that is currently recorded in the cache.  If none of the
 above apply, the advertisement prompts future Neighbor Unreachability
 Detection (if it is not already in progress) by changing the state in
 the cache entry.

Narten, et. al. Standards Track [Page 63] RFC 2461 Neighbor Discovery for IPv6 December 1998

7.2.6. Sending Unsolicited Neighbor Advertisements

 In some cases a node may be able to determine that its link-layer
 address has changed (e.g., hot-swap of an interface card) and may
 wish to inform its neighbors of the new link-layer address quickly.
 In such cases a node MAY send up to MAX_NEIGHBOR_ADVERTISEMENT
 unsolicited Neighbor Advertisement messages to the all-nodes
 multicast address.  These advertisements MUST be separated by at
 least RetransTimer seconds.
 The Target Address field in the unsolicited advertisement is set to
 an IP address of the interface, and the Target Link-Layer Address
 option is filled with the new link-layer address.  The Solicited flag
 MUST be set to zero, in order to avoid confusing the Neighbor
 Unreachability Detection algorithm.  If the node is a router, it MUST
 set the Router flag to one; otherwise it MUST set it to zero.  The
 Override flag MAY be set to either zero or one.  In either case,
 neighboring nodes will immediately change the state of their Neighbor
 Cache entries for the Target Address to STALE, prompting them to
 verify the path for reachability.  If the Override flag is set to
 one, neighboring nodes will install the new link-layer address in
 their caches.  Otherwise, they will ignore the new link-layer
 address, choosing instead to probe the cached address.
 A node that has multiple IP addresses assigned to an interface MAY
 multicast a separate Neighbor Advertisement for each address.  In
 such a case the node SHOULD introduce a small delay between the
 sending of each advertisement to reduce the probability of the
 advertisements being lost due to congestion.
 A proxy MAY multicast Neighbor Advertisements when its link-layer
 address changes or when it is configured (by system management or
 other mechanisms) to proxy for an address.  If there are multiple
 nodes that are providing proxy services for the same set of addresses
 the proxies SHOULD provide a mechanism that prevents multiple proxies
 from multicasting advertisements for any one address, in order to
 reduce the risk of excessive multicast traffic.
 Also, a node belonging to an anycast address MAY multicast
 unsolicited Neighbor Advertisements for the anycast address when the
 node's link-layer address changes.
 Note that because unsolicited Neighbor Advertisements do not reliably
 update caches in all nodes (the advertisements might not be received
 by all nodes), they should only be viewed as a performance
 optimization to quickly update the caches in most neighbors.  The
 Neighbor Unreachability Detection algorithm ensures that all nodes
 obtain a reachable link-layer address, though the delay may be

Narten, et. al. Standards Track [Page 64] RFC 2461 Neighbor Discovery for IPv6 December 1998

 slightly longer.

7.2.7. Anycast Neighbor Advertisements

 From the perspective of Neighbor Discovery, anycast addresses are
 treated just like unicast addresses in most cases.  Because an
 anycast address is syntactically the same as a unicast address, nodes
 performing address resolution or Neighbor Unreachability Detection on
 an anycast address treat it as if it were a unicast address.  No
 special processing takes place.
 Nodes that have an anycast address assigned to an interface treat
 them exactly the same as if they were unicast addresses with two
 exceptions.  First, Neighbor Advertisements sent in response to a
 Neighbor Solicitation SHOULD be delayed by a random time between 0
 and MAX_ANYCAST_DELAY_TIME to reduce the probability of network
 congestion.  Second, the Override flag in Neighbor Advertisements
 SHOULD be set to 0, so that when multiple advertisements are
 received, the first received advertisement is used rather than the
 most recently received advertisement.
 As with unicast addresses, Neighbor Unreachability Detection ensures
 that a node quickly detects when the current binding for an anycast
 address becomes invalid.

7.2.8. Proxy Neighbor Advertisements

 Under limited circumstances, a router MAY proxy for one or more other
 nodes, that is, through Neighbor Advertisements indicate that it is
 willing to accept packets not explicitly addressed to itself.  For
 example, a router might accept packets on behalf of a mobile node
 that has moved off-link.  The mechanisms used by proxy are identical
 to the mechanisms used with anycast addresses.
 A proxy MUST join the solicited-node multicast address(es) that
 correspond to the IP address(es) assigned to the node for which it is
 proxying.
 All solicited proxy Neighbor Advertisement messages MUST have the
 Override flag set to zero.  This ensures that if the node itself is
 present on the link its Neighbor Advertisement (with the Override
 flag set to one) will take precedence of any advertisement received
 from a proxy.  A proxy MAY send unsolicited advertisements with the
 Override flag set to one as specified in Section 7.2.6, but doing so
 may cause the proxy advertisement to override a valid entry created
 by the node itself.

Narten, et. al. Standards Track [Page 65] RFC 2461 Neighbor Discovery for IPv6 December 1998

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

7.3. Neighbor Unreachability Detection

 Communication to or through a neighbor may fail for numerous reasons
 at any time, including hardware failure, hot-swap of an interface
 card, etc.  If the destination has failed, no recovery is possible
 and communication fails.  On the other hand, if it is the path that
 has failed, recovery may be possible.  Thus, a node actively tracks
 the reachability "state" for the neighbors to which it is sending
 packets.
 Neighbor Unreachability Detection is used for all paths between hosts
 and neighboring nodes, including host-to-host, host-to-router, and
 router-to-host communication.  Neighbor Unreachability Detection may
 also be used between routers, but is not required if an equivalent
 mechanism is available, for example, as part of the routing
 protocols.
 When a path to a neighbor appears to be failing, the specific
 recovery procedure depends on how the neighbor is being used.  If the
 neighbor is the ultimate destination, for example, address resolution
 should be performed again.  If the neighbor is a router, however,
 attempting to switch to another router would be appropriate.  The
 specific recovery that takes place is covered under next-hop
 determination; Neighbor Unreachability Detection signals the need for
 next-hop determination by deleting a Neighbor Cache entry.
 Neighbor Unreachability Detection is performed only for neighbors to
 which unicast packets are sent; it is not used when sending to
 multicast addresses.

7.3.1. Reachability Confirmation

 A neighbor is considered reachable if the node has recently received
 a confirmation that packets sent recently to the neighbor were
 received by its IP layer.  Positive confirmation can be gathered in
 two ways: hints from upper layer protocols that indicate a connection
 is making "forward progress", or receipt of a Neighbor Advertisement
 message that is a response to a Neighbor Solicitation message.
 A connection makes "forward progress" if the packets received from a
 remote peer can only be arriving if recent packets sent to that peer
 are actually reaching it.  In TCP, for example, receipt of a (new)
 acknowledgement indicates that previously sent data reached the peer.
 Likewise, the arrival of new (non-duplicate) data indicates that

Narten, et. al. Standards Track [Page 66] RFC 2461 Neighbor Discovery for IPv6 December 1998

 earlier acknowledgements are being delivered to the remote peer.  If
 packets are reaching the peer, they must also be reaching the
 sender's next-hop neighbor; thus "forward progress" is a confirmation
 that the next-hop neighbor is reachable.  For off-link destinations,
 forward progress implies that the first-hop router is reachable.
 When available, this upper-layer information SHOULD be used.
 In some cases (e.g., UDP-based protocols and routers forwarding
 packets to hosts) such reachability information may not be readily
 available from upper-layer protocols.  When no hints are available
 and a node is sending packets to a neighbor, the node actively probes
 the neighbor using unicast Neighbor Solicitation messages to verify
 that the forward path is still working.
 The receipt of a solicited Neighbor Advertisement serves as
 reachability confirmation, since advertisements with the Solicited
 flag set to one are sent only in response to a Neighbor Solicitation.
 Receipt of other Neighbor Discovery messages such as Router
 Advertisements and Neighbor Advertisement with the Solicited flag set
 to zero MUST NOT be treated as a reachability confirmation.  Receipt
 of unsolicited messages only confirm the one-way path from the sender
 to the recipient node.  In contrast, Neighbor Unreachability
 Detection requires that a node keep track of the reachability of the
 forward path to a neighbor from the its perspective, not the
 neighbor's perspective.  Note that receipt of a solicited
 advertisement indicates that a path is working in both directions.
 The solicitation must have reached the neighbor, prompting it to
 generate an advertisement.  Likewise, receipt of an advertisement
 indicates that the path from the sender to the recipient is working.
 However, the latter fact is known only to the recipient; the
 advertisement's sender has no direct way of knowing that the
 advertisement it sent actually reached a neighbor.  From the
 perspective of Neighbor Unreachability Detection, only the
 reachability of the forward path is of interest.

7.3.2. Neighbor Cache Entry States

 A Neighbor Cache entry can be in one of five states:
    INCOMPLETE  Address resolution is being performed on the entry.
                Specifically, a Neighbor Solicitation has been sent to
                the solicited-node multicast address of the target,
                but the corresponding Neighbor Advertisement has not
                yet been received.
    REACHABLE   Positive confirmation was received within the last
                ReachableTime milliseconds that the forward path to
                the neighbor was functioning properly.  While

Narten, et. al. Standards Track [Page 67] RFC 2461 Neighbor Discovery for IPv6 December 1998

                REACHABLE, no special action takes place as packets
                are sent.
    STALE       More than ReachableTime milliseconds have elapsed
                since the last positive confirmation was received that
                the forward path was functioning properly.  While
                stale, no action takes place until a packet is sent.
                The STALE state is entered upon receiving an
                unsolicited Neighbor Discovery message that updates
                the cached link-layer address.  Receipt of such a
                message does not confirm reachability, and entering
                the STALE state insures reachability is verified
                quickly if the entry is actually being used.  However,
                reachability is not actually verified until the entry
                is actually used.
    DELAY       More than ReachableTime milliseconds have elapsed
                since the last positive confirmation was received that
                the forward path was functioning properly, and a
                packet was sent within the last DELAY_FIRST_PROBE_TIME
                seconds.  If no reachability confirmation is received
                within DELAY_FIRST_PROBE_TIME seconds of entering the
                DELAY state, send a Neighbor Solicitation and change
                the state to PROBE.
                The DELAY state is an optimization that gives upper-
                layer protocols additional time to provide
                reachability confirmation in those cases where
                ReachableTime milliseconds have passed since the last
                confirmation due to lack of recent traffic.  Without
                this optimization the opening of a TCP connection
                after a traffic lull would initiate probes even though
                the subsequent three-way handshake would provide a
                reachability confirmation almost immediately.
    PROBE       A reachability confirmation is actively sought by
                retransmitting Neighbor Solicitations every
                RetransTimer milliseconds until a reachability
                confirmation is received.

7.3.3. Node Behavior

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

Narten, et. al. Standards Track [Page 68] RFC 2461 Neighbor Discovery for IPv6 December 1998

 When a node needs to perform address resolution on a neighboring
 address, it creates an entry in the INCOMPLETE state and initiates
 address resolution as specified in Section 7.2.  If address
 resolution fails, the entry SHOULD be deleted, so that subsequent
 traffic to that neighbor invokes the next-hop determination procedure
 again.  Invoking next-hop determination at this point insures that
 alternate default routers are tried.
 When a reachability confirmation is received (either through upper-
 layer advice or a solicited Neighbor Advertisement) an entry's state
 changes to REACHABLE.  The one exception is that upper-layer advice
 has no effect on entries in the INCOMPLETE state (e.g., for which no
 link-layer address is cached).
 When ReachableTime milliseconds have passed since receipt of the last
 reachability confirmation for a neighbor, the Neighbor Cache entry's
 state changes from REACHABLE to STALE.
    Note: An implementation may actually defer changing the state from
    REACHABLE to STALE until a packet is sent to the neighbor, i.e.,
    there need not be an explicit timeout event associated with the
    expiration of ReachableTime.
 The first time a node sends a packet to a neighbor whose entry is
 STALE, the sender changes the state to DELAY and a sets a timer to
 expire in DELAY_FIRST_PROBE_TIME seconds.  If the entry is still in
 the DELAY state when the timer expires, the entry's state changes to
 PROBE.  If reachability confirmation is received, the entry's state
 changes to REACHABLE.
 Upon entering the PROBE state, a node sends a unicast Neighbor
 Solicitation message to the neighbor using the cached link-layer
 address.  While in the PROBE state, a node retransmits Neighbor
 Solicitation messages every RetransTimer milliseconds until
 reachability confirmation is obtained.  Probes are retransmitted even
 if no additional packets are sent to the neighbor.  If no response is
 received after waiting RetransTimer milliseconds after sending the
 MAX_UNICAST_SOLICIT solicitations, retransmissions cease and the
 entry SHOULD be deleted.  Subsequent traffic to that neighbor will
 recreate the entry and performs address resolution again.
 Note that all Neighbor Solicitations are rate-limited on a per-
 neighbor basis.  A node MUST NOT send Neighbor Solicitations to the
 same neighbor more frequently than once every RetransTimer
 milliseconds.

Narten, et. al. Standards Track [Page 69] RFC 2461 Neighbor Discovery for IPv6 December 1998

 A Neighbor Cache entry enters the STALE state when created as a
 result of receiving packets other than solicited Neighbor
 Advertisements (i.e., Router Solicitations, Router Advertisements,
 Redirects, and Neighbor Solicitations).  These packets contain the
 link-layer address of either the sender or, in the case of Redirect,
 the redirection target.  However, receipt of these link-layer
 addresses does not confirm reachability of the forward-direction path
 to that node.  Placing a newly created Neighbor Cache entry for which
 the link-layer address is known in the STALE state provides assurance
 that path failures are detected quickly.  In addition, should a
 cached link-layer address be modified due to receiving one of the
 above messages the state SHOULD also be set to STALE to provide
 prompt verification that the path to the new link-layer address is
 working.
 To properly detect the case where a router switches from being a
 router to being a host (e.g., if its IP forwarding capability is
 turned off by system management), a node MUST compare the Router flag
 field in all received Neighbor Advertisement messages with the
 IsRouter flag recorded in the Neighbor Cache entry.  When a node
 detects that a neighbor has changed from being a router to being a
 host, the node MUST remove that router from the Default Router List
 and update the Destination Cache as described in Section 6.3.5.  Note
 that a router may not be listed in the Default Router List, even
 though a Destination Cache entry is using it (e.g., a host was
 redirected to it).  In such cases, all Destination Cache entries that
 reference the (former) router must perform next-hop determination
 again before using the entry.
 In some cases, link-specific information may indicate that a path to
 a neighbor has failed (e.g., the resetting of a virtual circuit).  In
 such cases, link-specific information may be used to purge Neighbor
 Cache entries before the Neighbor Unreachability Detection would do
 so.  However, link-specific information MUST NOT be used to confirm
 the reachability of a neighbor; such information does not provide
 end-to-end confirmation between neighboring IP layers.

8. REDIRECT FUNCTION

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

Narten, et. al. Standards Track [Page 70] RFC 2461 Neighbor Discovery for IPv6 December 1998

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

8.1. Validation of Redirect Messages

 A host MUST silently discard any received Redirect message that does
 not satisfy all of the following validity checks:
  1. IP Source Address is a link-local address. Routers must use

their link-local address as the source for Router Advertisement

      and Redirect messages so that hosts can uniquely identify
      routers.
  1. The IP Hop Limit field has a value of 255, i.e., the packet

could not possibly have been forwarded by a router.

  1. If the message includes an IP Authentication Header, the message

authenticates correctly.

  1. ICMP Checksum is valid.
  1. ICMP Code is 0.
  1. ICMP length (derived from the IP length) is 40 or more octets.
  1. The IP source address of the Redirect is the same as the current

first-hop router for the specified ICMP Destination Address.

  1. The ICMP Destination Address field in the redirect message does

not contain a multicast address.

  1. The ICMP Target Address is either a link-local address (when

redirected to a router) or the same as the ICMP Destination

      Address (when redirected to the on-link destination).
  1. All included options have a length that is greater than zero.
 The contents of the Reserved field, and of any unrecognized options
 MUST be ignored.  Future, backward-compatible changes to the protocol
 may specify the contents of the Reserved field or add new options;
 backward-incompatible changes may use different Code values.

Narten, et. al. Standards Track [Page 71] RFC 2461 Neighbor Discovery for IPv6 December 1998

 The contents of any defined options that are not specified to be used
 with Redirect messages MUST be ignored and the packet processed as
 normal.  The only defined options that may appear are the Target
 Link-Layer Address option and the Redirected Header option.
 A host MUST NOT consider a redirect invalid just because the Target
 Address of the redirect is not covered under one of the link's
 prefixes.  Part of the semantics of the Redirect message is that the
 Target Address is on-link.
 A redirect that passes the validity checks is called a "valid
 redirect".

8.2. Router Specification

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

and

  1. the router determines that a better first-hop node resides on

the same link as the sending node for the Destination Address of

      the packet being forwarded, and
  1. the Destination Address of the packet is not a multicast

address, and

 The transmitted redirect packet contains, consistent with the message
 format given in Section 4.5:
  1. In the Target Address field: the address to which subsequent

packets for the destination SHOULD be sent. If the target is a

      router, that router's link-local address MUST be used.  If the
      target is a host the target address field MUST be set to the
      same value as the Destination Address field.
  1. In the Destination Address field: the destination address of the

invoking IP packet.

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

Narten, et. al. Standards Track [Page 72] RFC 2461 Neighbor Discovery for IPv6 December 1998

         o Redirected Header: as much of the forwarded packet as can
           fit without the redirect packet exceeding 1280 octets in
           size.
 A router MUST limit the rate at which Redirect messages are sent, in
 order to limit the bandwidth and processing costs incurred by the
 Redirect messages when the source does not correctly respond to the
 Redirects, or the source chooses to ignore unauthenticated Redirect
 messages.  More details on the rate-limiting of ICMP error messages
 can be found in [ICMPv6].
 A router MUST NOT update its routing tables upon receipt of a
 Redirect.

8.3. Host Specification

 A host receiving a valid redirect SHOULD update its Destination Cache
 accordingly so that subsequent traffic goes to the specified target.
 If no Destination Cache entry exists for the destination, an
 implementation SHOULD create such an entry.
 If the redirect contains a Target Link-Layer Address option the host
 either creates or updates the Neighbor Cache entry for the target.
 In both cases the cached link-layer address is copied from the Target
 Link-Layer Address option.  If a Neighbor Cache entry is created for
 the target its reachability state MUST be set to STALE as specified
 in Section 7.3.3.  If a cache entry already existed and it is updated
 with a different link-layer address, its reachability state MUST also
 be set to STALE.  If the link-layer address is the same as that
 already in the cache, the cache entry's state remains unchanged.
 If the Target and Destination Addresses are the same, the host MUST
 treat the Target as on-link.  If the Target Address is not the same
 as the Destination Address, the host MUST set IsRouter to TRUE for
 the target.  If the Target and Destination Addresses are the same,
 however, one cannot reliably determine whether the Target Address is
 a router.  Consequently, newly created Neighbor Cache entries should
 set the IsRouter flag to FALSE, while existing cache entries should
 leave the flag unchanged.  If the Target is a router, subsequent
 Neighbor Advertisement or Router Advertisement messages will update
 IsRouter accordingly.
 Redirect messages apply to all flows that are being sent to a given
 destination.  That is, upon receipt of a Redirect for a Destination
 Address, all Destination Cache entries to that address should be
 updated to use the specified next-hop, regardless of the contents of
 the Flow Label field that appears in the Redirected Header option.

Narten, et. al. Standards Track [Page 73] RFC 2461 Neighbor Discovery for IPv6 December 1998

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

9. EXTENSIBILITY - OPTION PROCESSING

 Options provide a mechanism for encoding variable length fields,
 fields that may appear multiple times in the same packet, or
 information that may not appear in all packets.  Options can also be
 used to add additional functionality to future versions of ND.
 In order to ensure that future extensions properly coexist with
 current implementations, all nodes MUST silently ignore any options
 they do not recognize in received ND packets and continue processing
 the packet.  All options specified in this document MUST be
 recognized.  A node MUST NOT ignore valid options just because the ND
 message contains unrecognized ones.
 The current set of options is defined in such a way that receivers
 can process multiple options in the same packet independently of each
 other.  In order to maintain these properties future options SHOULD
 follow the simple rule:
      The option MUST NOT depend on the presence or absence of any
      other options.  The semantics of an option should depend only on
      the information in the fixed part of the ND packet and on the
      information contained in the option itself.
 Adhering to the above rule has the following benefits:
   1) Receivers can process options independently of one another.  For
      example, an implementation can choose to process the Prefix
      Information option contained in a Router Advertisement message
      in a user-space process while the link-layer address option in
      the same message is processed by routines in the kernel.
   2) Should the number of options cause a packet to exceed a link's
      MTU, multiple packets can carry subsets of the options without
      any change in semantics.
   3) Senders MAY send a subset of options in different packets.  For
      instance, if a prefix's Valid and Preferred Lifetime are high
      enough, it might not be necessary to include the Prefix
      Information option in every Router Advertisement.  In addition,
      different routers might send different sets of options.  Thus, a
      receiver MUST NOT associate any action with the absence of an

Narten, et. al. Standards Track [Page 74] RFC 2461 Neighbor Discovery for IPv6 December 1998

      option in a particular packet.  This protocol specifies that
      receivers should only act on the expiration of timers and on the
      information that is received in the packets.
 Options in Neighbor Discovery packets can appear in any order;
 receivers MUST be prepared to process them independently of their
 order.  There can also be multiple instances of the same option in a
 message (e.g., Prefix Information options).
 If the number of included options in a Router Advertisement causes
 the advertisement's size to exceed the link MTU, the router can send
 multiple separate advertisements each containing a subset of the
 options.
 The amount of data to include in the Redirected Header option MUST be
 limited so that the entire redirect packet does not exceed 1280
 octets.
 All options are a multiple of 8 octets of length, ensuring
 appropriate alignment without any "pad" options.  The fields in the
 options (as well as the fields in ND packets) are defined to align on
 their natural boundaries (e.g., a 16-bit field is aligned on a 16-bit
 boundary) with the exception of the 128-bit IP addresses/prefixes,
 which are aligned on a 64-bit boundary.  The link-layer address field
 contains an uninterpreted octet string; it is aligned on an 8-bit
 boundary.
 The size of an ND packet including the IP header is limited to the
 link MTU (which is at least 1280 octets).  When adding options to an
 ND packet a node MUST NOT exceed the link MTU.
 Future versions of this protocol may define new option types.
 Receivers MUST silently ignore any options they do not recognize and
 continue processing the message.

10. PROTOCOL CONSTANTS

 Router constants:
          MAX_INITIAL_RTR_ADVERT_INTERVAL  16 seconds
          MAX_INITIAL_RTR_ADVERTISEMENTS    3 transmissions
          MAX_FINAL_RTR_ADVERTISEMENTS      3 transmissions
          MIN_DELAY_BETWEEN_RAS             3 seconds
          MAX_RA_DELAY_TIME                 .5 seconds

Narten, et. al. Standards Track [Page 75] RFC 2461 Neighbor Discovery for IPv6 December 1998

 Host constants:
          MAX_RTR_SOLICITATION_DELAY        1 second
          RTR_SOLICITATION_INTERVAL         4 seconds
          MAX_RTR_SOLICITATIONS             3 transmissions
 Node constants:
          MAX_MULTICAST_SOLICIT             3 transmissions
          MAX_UNICAST_SOLICIT               3 transmissions
          MAX_ANYCAST_DELAY_TIME            1 second
          MAX_NEIGHBOR_ADVERTISEMENT        3 transmissions
          REACHABLE_TIME               30,000 milliseconds
          RETRANS_TIMER                 1,000 milliseconds
          DELAY_FIRST_PROBE_TIME            5 seconds
          MIN_RANDOM_FACTOR                 .5
          MAX_RANDOM_FACTOR                 1.5
 Additional protocol constants are defined with the message formats in
 Section 4.
 All protocol constants are subject to change in future revisions of
 the protocol.
 The constants in this specification may be overridden by specific
 documents that describe how IPv6 operates over different link layers.
 This rule allows Neighbor Discovery to operate over links with widely
 varying performance characteristics.

11. SECURITY CONSIDERATIONS

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

Narten, et. al. Standards Track [Page 76] RFC 2461 Neighbor Discovery for IPv6 December 1998

 The protocol reduces the exposure to such threats in the absence of
 authentication by ignoring ND packets received from off-link senders.
 The Hop Limit field of all received packets is verified to contain
 255, the maximum legal value.  Because routers decrement the Hop
 Limit on all packets they forward, received packets containing a Hop
 Limit of 255 must have originated from a neighbor.
 An example of denial of service attacks is that a node on the link
 that can send packets with an arbitrary IP source address can both
 advertise itself as a default router and also send "forged" Router
 Advertisement messages that immediately time out all other default
 routers as well as all on-link prefixes.  An intruder can achieve
 this by sending out multiple Router Advertisements, one for each
 legitimate router, with the source address set to the address of
 another router, the Router Lifetime field set to zero, and the
 Preferred and Valid lifetimes set to zero for all the prefixes.  Such
 an attack would cause all packets, for both on-link and off-link
 destinations, to go to the rogue router.  That router can then
 selectively examine, modify or drop all packets sent on the link. The
 Neighbor Unreachability Detection will not detect such a black hole
 as long as the rogue router politely answers the NUD probes with a
 Neighbor Advertisement with the R-bit set.
 Many link layers are also subject to different denial of service
 attacks such as continuously occupying the link in CSMA/CD networks
 (e.g., by sending packets closely back-to-back or asserting the
 collision signal on the link), or originating packets with somebody
 else's source MAC address to confuse, e.g., Ethernet switches.
 The trust model for redirects is the same as in IPv4.  A redirect is
 accepted only if received from the same router that is currently
 being used for that destination.  It is natural to trust the routers
 on the link.  If a host has been redirected to another node (i.e.,
 the destination is on-link) there is no way to prevent the target
 from issuing another redirect to some other destination.  However,
 this exposure is no worse than it was; the target host, once
 subverted, could always act as a hidden router to forward traffic
 elsewhere.
 The protocol contains no mechanism to determine which neighbors are
 authorized to send a particular type of message (e.g., Router
 Advertisements); any neighbor, presumably even in the presence of
 authentication, can send Router Advertisement messages thereby being
 able to cause denial of service.  Furthermore, any neighbor can send
 proxy Neighbor Advertisements as well as unsolicited Neighbor
 Advertisements as a potential denial of service attack.

Narten, et. al. Standards Track [Page 77] RFC 2461 Neighbor Discovery for IPv6 December 1998

 Neighbor Discovery protocol packet exchanges can be authenticated
 using the IP Authentication Header [IPv6-AUTH].  A node SHOULD
 include an Authentication Header when sending Neighbor Discovery
 packets if a security association for use with the IP Authentication
 Header exists for the destination address.  The security associations
 may have been created through manual configuration or through the
 operation of some key management protocol.
 Received Authentication Headers in Neighbor Discovery packets MUST be
 verified for correctness and packets with incorrect authentication
 MUST be ignored.
 It SHOULD be possible for the system administrator to configure a
 node to ignore any Neighbor Discovery messages that are not
 authenticated using either the Authentication Header or Encapsulating
 Security Payload.  The configuration technique for this MUST be
 documented.  Such a switch SHOULD default to allowing unauthenticated
 messages.
 Confidentiality issues are addressed by the IP Security Architecture
 and the IP Encapsulating Security Payload documents [IPv6-SA, IPv6-
 ESP].

12. RENUMBERING CONSIDERATIONS

 The Neighbor Discovery protocol together with IPv6 Address
 Autoconfiguration [ADDRCONF] provides mechanisms to aid in
 renumbering - new prefixes and addresses can be introduced and old
 ones can be deprecated and removed.
 The robustness of these mechanisms is based on all the nodes on the
 link receiving the Router Advertisement messages in a timely manner.
 However, a host might be turned off or be unreachable for an extended
 period of time (i.e., a machine is powered down for months after a
 project terminates).  It is possible to preserve robust renumbering
 in such cases but it does place some constraints on how long prefixes
 must be advertised.
 Consider the following example in which a prefix is initially
 advertised with a lifetime of 2 months, but on August 1st it is
 determined that the prefix needs to be deprecated and removed due to
 renumbering by September 1st.  This can be done by reducing the
 advertised lifetime to 1 week starting on August 1st and as the
 cutoff gets closer the lifetimes can be made shorter until by
 September 1st the prefix is advertised with a zero lifetime.  The
 point is that, if one or more nodes were unplugged from the link
 prior to September 1st they might still think that the prefix is
 valid since the last lifetime they received was 2 months.  Thus if a

Narten, et. al. Standards Track [Page 78] RFC 2461 Neighbor Discovery for IPv6 December 1998

 node was unplugged on July 31st it thinks the prefix is valid until
 September 30th.  If that node is plugged back in prior to September
 30th it may continue to use the old prefix.  The only way to force a
 node to stop using a prefix that was previously advertised with a
 long Lifetime is to have that node receive an advertisement for that
 prefix that changes the lifetime downward.  The solution in this
 example is simple: continue advertising the prefix with a lifetime of
 0 from September 1st until October 1st.
 In general, in order to be robust against nodes that might be
 unplugged from the link it is important to track the furthest into
 the future a particular prefix can be viewed valid by any node on the
 link.  The prefix must then be advertised with a 0 Lifetime until
 that point in future. This "furthest into the future" time is simply
 the maximum, over all Router Advertisements, of the time the
 advertisement was sent plus the prefix's Lifetime contained in the
 advertisement.
 The above has an important implication on using infinite lifetimes.
 If a prefix is advertised with an infinite lifetime, and that prefix
 later needs to be renumbered, it is undesirable to continue
 advertising that prefix with a zero lifetime forever.  Thus either
 infinite lifetimes should be avoided or there must be a limit on how
 long time a node can be unplugged from the link before it is plugged
 back in again.  However, it is unclear how the network administrator
 can enforce a limit on how long time hosts such as laptops can be
 unplugged from the link.
 Network administrators should give serious consideration to using
 relatively short lifetimes (i.e., no more than a few weeks).  While
 it might appear that using long lifetimes would help insure
 robustness, in reality a host will be unable to communicate in the
 absence of properly functioning routers.  Such routers will be
 sending Router Advertisements that contain appropriate (and current)
 prefixes.  A host connected to a network that has no functioning
 routers is likely to have more serious problems than just a lack of a
 valid prefix and address.
 The above discussion does not distinguish between the preferred and
 valid lifetimes.  For all practical purposes it is probably
 sufficient to track the valid lifetime since the preferred lifetime
 will not exceed the valid lifetime.

Narten, et. al. Standards Track [Page 79] RFC 2461 Neighbor Discovery for IPv6 December 1998

REFERENCES

 [ADDRCONF]   Thomson, S. and T. Narten, "IPv6 Address
              Autoconfiguration", RFC 2462, December 1998.
 [ADDR-ARCH]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 2373, July 1998.
 [ANYCST]     Partridge, C., Mendez, T. and W. Milliken, "Host
              Anycasting Service", RFC 1546, November 1993.
 [ARP]        Plummer, D., "An Ethernet Address Resolution Protocol",
              STD 37, RFC 826, November 1982.
 [HR-CL]      Braden, R., Editor, "Requirements for Internet Hosts --
              Communication Layers", STD 3, RFC 1122, October 1989.
 [ICMPv4]     Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, September 1981.
 [ICMPv6]     Conta, A. and S. Deering, "Internet Control Message
              Protocol (ICMPv6) for the Internet Protocol Version 6
              (IPv6) Specification", RFC 2463, December 1998.
 [IPv6]       Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.
 [IPv6-ETHER] Crawford, M., "Transmission of IPv6 Packets over
              Ethernet Networks", RFC 2464, December 1998.
 [IPv6-SA]    Kent, S. and R. Atkinson, "Security Architecture for the
              Internet Protocol", RFC 2401, November 1998.
 [IPv6-AUTH]  Kent, S. and R. Atkinson, "IP Authentication Header",
              RFC 2402, November 1998.
 [IPv6-ESP]   Kent, S. and R. Atkinson, "IP Encapsulating Security
              Payload (ESP)", RFC 2406, November 1998.
 [KEYWORDS]   Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RDISC]      Deering, S., "ICMP Router Discovery Messages", RFC 1256,
              September 1991.
 [SH-MEDIA]   Braden, R., Postel, J. and Y. Rekhter, "Internet
              Architecture Extensions for Shared Media", RFC 1620, May
              1994.

Narten, et. al. Standards Track [Page 80] RFC 2461 Neighbor Discovery for IPv6 December 1998

 [ASSIGNED]   Reynolds, J. and J. Postel, "ASSIGNED NUMBERS", STD 2,
              RFC 1700, October 1994. See also:
              http://www.iana.org/numbers.html
 [SYNC]       S. Floyd, V. Jacobson, "The Synchronization of Periodic
              Routing Messages", IEEE/ACM Transactions on Networking,
              April 1994.  ftp://ftp.ee.lbl.gov/papers/sync_94.ps.Z

Authors' Addresses

 Thomas Narten
 IBM Corporation
 P.O. Box 12195
 Research Triangle Park, NC 27709-2195
 USA
 Phone: +1 919 254 7798
 EMail: narten@raleigh.ibm.com
 Erik Nordmark
 Sun Microsystems, Inc.
 901 San Antonio Road
 Palo Alto, CA 94303
 USA
 Phone: +1 650 786 5166
 Fax:   +1 650 786 5896
 EMail: nordmark@sun.com
 William Allen Simpson
 Daydreamer
 Computer Systems Consulting Services
 1384 Fontaine
 Madison Heights, Michigan  48071
 USA
 EMail: Bill.Simpson@um.cc.umich.edu
        bsimpson@MorningStar.com

Narten, et. al. Standards Track [Page 81] RFC 2461 Neighbor Discovery for IPv6 December 1998

APPENDIX A: MULTIHOMED HOSTS

 There are a number of complicating issues that arise when Neighbor
 Discovery is used by hosts that have multiple interfaces.  This
 section does not attempt to define the proper operation of multihomed
 hosts with regard to Neighbor Discovery.  Rather, it identifies
 issues that require further study.  Implementors are encouraged to
 experiment with various approaches to making Neighbor Discovery work
 on multihomed hosts and to report their experiences.
 If a multihomed host receives Router Advertisements on all of its
 interfaces, it will (probably) have learned on-link prefixes for the
 addresses residing on each link.  When a packet must be sent through
 a router, however, selecting the "wrong" router can result in a
 suboptimal or non-functioning path.  There are number of issues to
 consider:
   1) In order for a router to send a redirect, it must determine that
      the packet it is forwarding originates from a neighbor.  The
      standard test for this case is to compare the source address of
      the packet to the list of on-link prefixes associated with the
      interface on which the packet was received.  If the originating
      host is multihomed, however, the source address it uses may
      belong to an interface other than the interface from which it
      was sent.  In such cases, a router will not send redirects, and
      suboptimal routing is likely.  In order to be redirected, the
      sending host must always send packets out the interface
      corresponding to the outgoing packet's source address.  Note
      that this issue never arises with non-multihomed hosts; they
      only have one interface.
   2) If the selected first-hop router does not have a route at all
      for the destination, it will be unable to deliver the packet.
      However, the destination may be reachable through a router on
      one of the other interfaces.  Neighbor Discovery does not
      address this scenario; it does not arise in the non-multihomed
      case.
   3) Even if the first-hop router does have a route for a
      destination, there may be a better route via another interface.
      No mechanism exists for the multihomed host to detect this
      situation.
 If a multihomed host fails to receive Router Advertisements on one or
 more of its interfaces, it will not know (in the absence of
 configured information) which destinations are on-link on the
 affected interface(s).  This leads to a number of problems:

Narten, et. al. Standards Track [Page 82] RFC 2461 Neighbor Discovery for IPv6 December 1998

   1) If no Router Advertisement is received on any interfaces, a
      multihomed host will have no way of knowing which interface to
      send packets out on, even for on-link destinations.  Under
      similar conditions in the non-multihomed host case, a node
      treats all destinations as residing on-link, and communication
      proceeds.  In the multihomed case, however, additional
      information is needed to select the proper outgoing interface.
      Alternatively, a node could attempt to perform address
      resolution on all interfaces, a step involving significant
      complexity that is not present in the non-multihomed host case.
   2) If Router Advertisements are received on some, but not all
      interfaces, a multihomed host could choose to only send packets
      out on the interfaces on which it has received Router
      Advertisements.  A key assumption made here, however, is that
      routers on those other interfaces will be able to route packets
      to the ultimate destination, even when those destinations reside
      on the subnet to which the sender connects, but has no on-link
      prefix information.  Should the assumption be FALSE,
      communication would fail.  Even if the assumption holds, packets
      will traverse a sub-optimal path.

Narten, et. al. Standards Track [Page 83] RFC 2461 Neighbor Discovery for IPv6 December 1998

APPENDIX B: FUTURE EXTENSIONS

 Possible extensions for future study are:
  o Using dynamic timers to be able to adapt to links with widely
    varying delay.  Measuring round trip times, however, requires
    acknowledgments and sequence numbers in order to match received
    Neighbor Advertisements with the actual Neighbor Solicitation that
    triggered the advertisement.  Implementors wishing to experiment
    with such a facility could do so in a backwards-compatible way by
    defining a new option carrying the necessary information.  Nodes
    not understanding the option would simply ignore it.
  o Adding capabilities to facilitate the operation over links that
    currently require hosts to register with an address resolution
    server.  This could for instance enable routers to ask hosts to
    send them periodic unsolicited advertisements.  Once again this
    can be added using a new option sent in the Router Advertisements.
  o Adding additional procedures for links where asymmetric and non-
    transitive reachability is part of normal operations.  Such
    procedures might allow hosts and routers to find usable paths on,
    e.g., radio links.

Narten, et. al. Standards Track [Page 84] RFC 2461 Neighbor Discovery for IPv6 December 1998

APPENDIX C: STATE MACHINE FOR THE REACHABILITY STATE

 This appendix contains a summary of the rules specified in Sections
 7.2 and 7.3.  This document does not mandate that implementations
 adhere to this model as long as their external behavior is consistent
 with that described in this document.
 When performing address resolution and Neighbor Unreachability
 Detection the following state transitions apply using the conceptual
 model:

State Event Action New state

- Packet to send. Create entry. INCOMPLETE

                                      Send multicast NS.
                                      Start retransmit timer

INCOMPLETE Retransmit timeout, Retransmit NS INCOMPLETE

              less than N             Start retransmit timer
              retransmissions.

INCOMPLETE Retransmit timeout, Discard entry -

              N or more               Send ICMP error
              retransmissions.

INCOMPLETE NA, Solicited=0, Record link-layer STALE

              Override=any            address.  Send queued
                                      packets.

INCOMPLETE NA, Solicited=1, Record link-layer REACHABLE

              Override=any            address.  Send queued
                                      packets.

!INCOMPLETE NA, Solicited=1, - REACHABLE

              Override=0
              Same link-layer
              address as cached.

REACHABLE NA, Solicited=1, - STALE

              Override=0
              Different link-layer
              address than cached.

STALE or PROBE NA, Solicited=1, - unchanged

              Override=0
              Different link-layer
              address than cached.

Narten, et. al. Standards Track [Page 85] RFC 2461 Neighbor Discovery for IPv6 December 1998

!INCOMPLETE NA, Solicited=1, Record link-layer REACHABLE

              Override=1              address (if
                                      different).

!INCOMPLETE NA, Solicited=0, - unchanged

              Override=0

!INCOMPLETE NA, Solicited=0, - unchanged

              Override=1
              Same link-layer
              address as cached.

!INCOMPLETE NA, Solicited=0, Record link-layer STALE

              Override=1              address.
              Different link-layer
              address than cached.

!INCOMPLETE upper-layer reachability - REACHABLE

              confirmation

REACHABLE timeout, more than - STALE

              N seconds since
              reachability confirm.

STALE Sending packet Start delay timer DELAY

DELAY Delay timeout Send unicast NS probe PROBE

                                      Start retransmit timer

PROBE Retransmit timeout, Retransmit NS PROBE

              less than N
              retransmissions.

PROBE Retransmit timeout, Discard entry -

              N or more
              retransmissions.

Narten, et. al. Standards Track [Page 86] RFC 2461 Neighbor Discovery for IPv6 December 1998

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

State Event Action New state

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

INCOMPLETE NS, RS, RA, Redirect Record link-layer STALE

                                      address.  Send queued
                                      packets.

!INCOMPLETE NS, RS, RA, Redirect Update link-layer STALE

              Different link-layer    address
              address than cached.

!INCOMPLETE NS, RS, RA, Redirect - unchanged

              Same link-layer
              address as cached.

Narten, et. al. Standards Track [Page 87] RFC 2461 Neighbor Discovery for IPv6 December 1998

APPENDIX D: SUMMARY OF ISROUTER RULES

 This appendix presents a summary of the rules for maintaining the
 IsRouter flag as specified in this document.
 The background for these rules is that the ND messages contain,
 either implicitly or explicitly, information that indicates whether
 or not the sender (or Target Address) is a host or a router.  The
 following assumptions are used:
  1. The sender of a Router Solicitation is implicitly assumed to be a

host since there is no need for routers to send such messages.

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

router.

  1. Neighbor Solicitation messages do not contain either an implicit

or explicit indication about the sender. Both hosts and routers

    send such messages.
  1. Neighbor Advertisement messages contain an explicit "IsRouter

flag", the R-bit.

  1. The target of the redirect, when the target differs from the

destination address in the packet being redirected, is implicitly

    assumed to be a router.  This is a natural assumption since that
    node is expected to be able to forward the packets towards the
    destination.
  1. The target of the redirect, when the target is the same as the

destination, does not carry any host vs. router information. All

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

Narten, et. al. Standards Track [Page 88] RFC 2461 Neighbor Discovery for IPv6 December 1998

APPENDIX E: IMPLEMENTATION ISSUES

Appendix E.1: Reachability confirmations

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

data) not previously acknowledged indicates that the forward path

    was working at the time the data was sent.
  1. Completion of the initial three-way handshake is a special case of

the previous rule; although no data is sent during the handshake,

    the SYN flags are counted as data from the sequence number
    perspective.  This applies to both the SYN+ACK for the active open
    the ACK of that packet on the passively opening peer.
  1. Receipt of new data (i.e., data not previously received) indicates

that the forward-path was working at the time an acknowledgement

    was sent that advanced the peer's send window that allowed the new
    data to be sent.
 To minimize the cost of communicating reachability information
 between the TCP and IP layers, an implementation may wish to rate-
 limit the reachability confirmations its sends IP.  One possibility
 is to process reachability only every few packets.  For example, one
 might update reachability information once per round trip time, if an
 implementation only has one round trip timer per connection.  For
 those implementations that cache Destination Cache entries within
 control blocks, it may be possible to update the Neighbor Cache entry
 directly (i.e., without an expensive lookup) once the TCP packet has
 been demultiplexed to its corresponding control block.  For other
 implementation it may be possible to piggyback the reachability
 confirmation on the next packet submitted to IP assuming that the
 implementation guards against the piggybacked confirmation becoming
 stale when no packets are sent to IP for an extended period of time.
 TCP must also guard against thinking "stale" information indicates
 current reachability.  For example, new data received 30 minutes
 after a window has opened up does not constitute a confirmation that
 the path is currently working.  In merely indicates that 30 minutes

Narten, et. al. Standards Track [Page 89] RFC 2461 Neighbor Discovery for IPv6 December 1998

 ago the window update reached the peer i.e. the path was working at
 that point in time.  An implementation must also take into account
 TCP zero-window probes that are sent even if the path is broken and
 the window update did not reach the peer.
 For UDP based applications (RPC, DNS) it is relatively simple to make
 the client send reachability confirmations when the response packet
 is received.  It is more difficult and in some cases impossible for
 the server to generate such confirmations since there is no flow
 control, i.e., the server can not determine whether a received
 request indicates that a previous response reached the client.
 Note that an implementation can not use negative upper-layer advise
 as a replacement for the Neighbor Unreachability Detection algorithm.
 Negative advise (e.g. from TCP when there are excessive
 retransmissions) could serve as a hint that the forward path from the
 sender of the data might not be working.  But it would fail to detect
 when the path from the receiver of the data is not functioning
 causing, none of the acknowledgement packets to reach the sender.

Narten, et. al. Standards Track [Page 90] RFC 2461 Neighbor Discovery for IPv6 December 1998

APPENDIX F: CHANGES SINCE RFC 1970

  o Removed all references to the IPv6 priority field.
  o Replaced definition of solicited node multicast address with a
    reference to the [ADDR-ARCH] specification.  That specification
    says that "the solicited-node multicast address is formed by
    taking the low-order 24 bits of the address (unicast or anycast)
    and appending those bits to the prefix FF02:0:0:0:0:1:FF00::/104".
  o Updated the references section to list (new) RFC numbers.
  o Updated the text in section 7.2.5 and the tables in appendix C to
    have the receipt of an NS message update the state of an existing
    neighbor cache entry only if the link-layer address is different
    than the recorded link-layer address.
  o Added an explicit check in section 7.1.1 so that received NS
    messages from an unsolicited address must be sent the solicited-
    node multicast address; if sent to unicast destination, silently
    discard.
  o Added a requirement in section 6.2.1 that Lifetimes be
    configurable in either of two ways: as a fixed value that doesn't
    change over time, or one that decrements in real time.
  o Added text in section 6.2.7 to relax the consistency checks on
    prefix lifetimes when the lifetimes are configured to decrement in
    real time.  This is needed to avoid false alarms due to link
    propagation delay and lack of synchronized clocks.
  o Added text to section 6.3.4 to point out that [ADDRCONF] might
    ignore short lifetimes but that Neighbor Discovery does not ignore
    short prefix lifetimes.
  o Clarified the rules for RS and NS packets with an unspecified
    source address. Such packets MUST NOT include source link-layer
    address option; verified by receivers.
  o Clarified in section 7.2.3 that addresses for which the node
    proxies are acceptable in NS messages.  Previously the text only
    mentioned unicast and anycast addresses assigned to the interface
    (i.e., wasn't clear that proxy addresses were allowed).
  o Tightened up ambiguities an inconsistencies regarding when to set
    the IsRouter flag in Neighbor Cache entries.  Added an appendix to
    summarize these rules.

Narten, et. al. Standards Track [Page 91] RFC 2461 Neighbor Discovery for IPv6 December 1998

  o Added a section on renumbering considerations to clarify how long
    prefixes have to be advertised when the lifetime(s) are reduced.
  o Added additional text to the rules in section 7 for the NS/NA
    packets used for NUD probes so that the Link-Layer Address options
    can be omitted from these packets in certain cases without causing
    an infinite NS "recursion". Specifically, added text that permits
    the Link-Layer address to be omitted in unicast solicitations
    (i.e., MAY language).
  o Changed the default AdvValidLifetime from infinity to 30 days.
  o Changed the constant "576" to "1280" in places where its context
    was that of the minimum sized IP packet that all links must be
    able to carry.

Narten, et. al. Standards Track [Page 92] RFC 2461 Neighbor Discovery for IPv6 December 1998

Full Copyright Statement

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