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Versions: 00 01 02 03 04 05 06 RFC 1970

INTERNET-DRAFT                                        Thomas Narten, IBM
September 15, 1995                       Erik Nordmark, Sun Microsystems
                                                 W A Simpson, Daydreamer



               Neighbor Discovery for IP Version 6 (IPv6)

                  <draft-ietf-ipngwg-discovery-02.txt>


Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as ``work in progress.''

   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
   Directories on ds.internic.net (US East Coast), nic.nordu.net
   (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific
   Rim).

   Distribution of this memo is unlimited.

   This Internet Draft expires March 15, 1996.



Abstract

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









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Contents

   Status of this Memo..........................................    1

   1.  INTRODUCTION.............................................    4

   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.  CONCEPTUAL MODEL OF A HOST...............................   16
      4.1.  Conceptual Data Structures..........................   16
      4.2.  Conceptual Sending Algorithm........................   18
      4.3.  Garbage Collection and Timeout Requirements.........   19

   5.  ROUTER AND PREFIX DISCOVERY..............................   20
      5.1.  Message Formats.....................................   21
         5.1.1.  Router Solicitation Message Format.............   21
         5.1.2.  Router Advertisement Message Format............   22
      5.2.  Router Specification................................   24
         5.2.1.  Router Configuration Variables.................   24
         5.2.2.  Validation of Router Solicitation Messages.....   27
         5.2.3.  Router Behavior................................   28
         5.2.4.  Router Advertisement Consistency...............   32
         5.2.5.  Link-local Address Change......................   33
      5.3.  Host Specification..................................   33
         5.3.1.  Host Configuration Variables...................   33
         5.3.2.  Host Variables.................................   34
         5.3.3.  Validation of Router Advertisement Messages....   34
         5.3.4.  Host Behavior..................................   35

   6.  ADDRESS RESOLUTION AND NEIGHBOR UNREACHABILITY DETECTION.   39
      6.1.  Message Formats.....................................   39
         6.1.1.  Neighbor Solicitation Message Format...........   39
         6.1.2.  Neighbor Advertisement Message Format..........   42
      6.2.  Address Resolution..................................   44
         6.2.1.  Node Specification.............................   44
         6.2.2.  Sending Neighbor Solicitations.................   44
         6.2.3.  Validation of Neighbor Solicitations...........   45
         6.2.4.  Receipt of Neighbor Solicitations..............   46
         6.2.5.  Sending Solicited Neighbor Advertisements......   46
         6.2.6.  Validation of Neighbor Advertisements..........   47



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         6.2.7.  Receipt of Neighbor Advertisments..............   47
         6.2.8.  Sending Unsolicited Neighbor Advertisements....   48
         6.2.9.  Anycast Neighbor Advertisements................   49
         6.2.10.  Proxy Neighbor Advertisements.................   50
      6.3.  Neighbor Unreachability Detection...................   50
         6.3.1.  Reachability Confirmation......................   51
         6.3.2.  Node Behavior..................................   52

   7.  REDIRECT FUNCTION........................................   55
      7.1.  Redirect Message Format.............................   55
      7.2.  Router Specification................................   57
      7.3.  Host Specification..................................   58
         7.3.1.  Validation of Redirect Messages................   58
         7.3.2.  Host Behavior..................................   59

   8.  OPTIONS..................................................   59
      8.1.  Source/Target Link-layer Address....................   61
      8.2.  Prefix Information..................................   62
      8.3.  Redirected Header...................................   64
      8.4.  MTU.................................................   64

   9.  MULTIHOMED HOSTS.........................................   65

   10.  PROTOCOL CONSTANTS......................................   67

   11.  FUTURE EXTENSIONS.......................................   68

   12.  OPEN ISSUES.............................................   68

   13.  SECURITY CONSIDERATIONS.................................   68

   REFERENCES...................................................   71

   AUTHORS' ADDRESSES...........................................   72

   CHANGES SINCE PREVIOUS DOCUMENT..............................   73















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1.  INTRODUCTION

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

This document is a revision of <draft-ietf-ipngwg-discovery-01.txt>
which was itself based on the protocol specified in the two documents:
     <draft-simpson-ipv6-discov-formats-02.txt>, and
     <draft-simpson-ipv6-discov-process-02.txt>

The authors would like to acknowledge the contributions the IPNGWG
working group an, in particular, (in alphabetical order) Ran Atkinson,
Jim Bound, Scott Bradner, Stephen Deering, Robert Hinden, Allison
Mankin, Dan McDonald, Charles Perkins, and Sue 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,



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                 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; and internet (or higher) layer "tunnels",
                 such as tunnels over IPv4 or IPv6 itself.

   interface   - a node's attachment to a link.

   neighbors   - nodes attached to the same link.

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

   anycast address
               - an identifier for a set of interfaces (typically
                 belonging to different nodes).  A packet sent to an
                 anycast address is delivered to one of the interfaces
                 identified by that address (the "nearest" one,
                 according to the routing protocol's measure of
                 distance).  See [ADDR-ARCH].

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

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

                   - it is covered by one of the link's prefixes, or
                   - a neighboring router specifies the address as the
                     target of a Redirect message, or
                   - a Neighbor Advertisement message is received for
                     the (target) address, or
                   - a 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.

   reachability



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               - 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 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
                 potentially acts 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 sender is notified through an
                 appropriate mechanism (e.g., return value from a
                 procedure call).  Note, however, that an implementation
                 may find it convenient in some cases to return errors
                 to the sender by taking the offending packet,
                 generating an ICMP error message, and then delivering
                 it (locally) through the generic error handling
                 routines.








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2.2.  Link Types

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

   multicast      - a link that supports some mechanism at the link
                    layer for sending packets to all (i.e. broadcast) or
                    a subset of all neighbors.  Multicast/broadcast can
                    be provided by a variety of link layer mechanisms
                    such as the physical link layer itself (for example,
                    Ethernet), replicated unicast packets sent by the
                    link layer software, or multicast servers (such as
                    in ATM).  Note that all point-to-point links are
                    multicast links.

   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 any form of multicast or
                    broadcast (e.g., X.25).

   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.

   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



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                    properties.



2.3.  Addresses

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

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

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

   solicited-node multicast address
               - a link-local scope multicast address that is computed
                 as a function of the solicited target's address.  The
                 solicited-node multicast address is formed by taking
                 the low-order 32 bits of the target IP address and
                 appending those bits to the 96-bit prefix
                 FF02:0:0:0:0:1 to produce a multicast address within
                 the range FF02::1:0:0 to FF02::1:FFFF:FFFF.  For
                 example, the solicited node multicast address
                 corresponding to the IP address 4037::01:800:200E:8C6C
                 is FF02::1:200E:8C6C.  IP addresses that differ only in
                 the high-order bits, e.g. due to multiple high-order
                 prefixes associated with different providers, will map
                 to the same solicited-node address thereby reducing the
                 number of multicast addresses a node must join.

   link-local address
               - a unicast address having link-only scope that can be
                 used to reach neighbors.  All interfaces MUST have a
                 link-local address.  Routers MUST NOT forward packets
                 with a link-local source address.  See [ADDR-ARCH].

   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.




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2.4.  Requirements

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

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

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

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

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

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



3.  PROTOCOL OVERVIEW

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

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

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



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               reachable through a router.)

    Parameter Discovery: How a node learns such link parameters as the
               link MTU or such Internet parameters as the maximum 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.

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




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

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

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


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

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

Router Advertisements (and per-prefix flags) allow routers to inform
hosts how to perform Address Autoconfiguration.  For example, routers
can specify whether hosts should use 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 maximum hop that hosts should use in outgoing packets and,
optionally, link parameters such as the link MTU.  This facilitates
centralized administration of critical parameters that can be set on
routers and automatically propagated to all attached hosts.

Nodes accomplish Address Resolution by multicasting a Neighbor
Solicitation that asks the target node to return its link-layer address.
Neighbor Solicitation messages are multicast to the solicited-node



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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 IP address and link-layer address in the Neighbor
Solicitation.

Neighbor Solicitation messages can also be used to determine if more
than one node has been configured to use a particular 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 explicit 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 (but unreliably)
          update cached link-layer addresses that have become invalid.
          Note that the sending of unsolicited advertisements is a
          performance enhancement only (e.g., unreliable).  The Neighbor
          Unreachability Detection algorithm ensures that all nodes will
          reliably discover the new address, though the delay may be
          somewhat longer.

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



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          Router Advertisement packets, thereby forcing neighbors to use
          Neighbor Solicitation messages to learn the link-layer
          addresses.  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 "Secondary" 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 Secondary 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:

     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



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     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.

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

     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].



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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 is
available:

   point-to-point - Neighbor Discovery handles such links just like
                    multicast links.  (Multicast can be trivially
                    provided on point to point links, and interfaces can
                    be assigned link-local addresses.)

   multicast      - All aspects of Neighbor Discovery are available.

   non-broadcast multiple access (NBMA)
                  - The only Neighbor Discovery mechanisms available on
                    these links are Redirect handling and Neighbor
                    Unreachability Detection.

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

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

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

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

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

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


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

   variable MTU   - Neighbor Discovery allows routers to specify a MTU
                    for the link, which all nodes then use.  All nodes
                    on a link must use the same MTU (or Maximum Receive



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                    Unit) in order for multicast to work properly.  When
                    multicasting, a sender has no way of knowing which
                    nodes will receive the packet, and cannot determine
                    a minimum packet size all receivers can process.

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

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

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



4.  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 behavior is
consistent with the protocol specification.

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.


4.1.  Conceptual Data Structures

Hosts will need to maintain the following pieces of information about an
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 IP address and contain such
                  information as its link-layer address, a flag
                  indicating whether the neighbor is a router or a host
                  (called "is_router" in this document), a pointer to
                  any queued packets waiting for Address Resolution to



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                  complete, etc.

                  A Neighbor Cache entry also contains information used
                  by the Neighbor Unreachability Detection algorithm.
                  This includes 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 for each destination 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.
                  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 delete prefixes that routers
                  stop advertising.  A special "infinity" timer value
                  specifies that a prefix remains valid forever, unless
                  a new (finite) value is received in a subsequent
                  advertisement.

   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.
However, in all cases it is important to not duplicate the conceptual



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Neighbor Cache entry for a router in order to prevent redundant Neighbor
Unreachability Detection probes.

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 three possible values:

   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).

   PROBE       The neighbor may be reachable, but the last explicit
               reachability confirmation was received long enough ago
               that verification is now actively sought.



4.2.  Conceptual Sending Algorithm

When sending a packet, 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-level information about that
neighbor.

Next-hop determination operates as follows for unicast packets.  The
sender examines 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.  If
the destination is off-link, the sender selects a router from the
Default Router List (following the rules described in Section 5.3.4).
If the Default Router List is empty, the sender assumes that the
destination is on-link.

For multicast packets the next-hop is always the (multicast) destination
address.

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.  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



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the Neighbor Cache for link-level information about that neighbor.  If
no entry exists for a multicast destination, an entry is created using
the link specific mapping to a multicast link-layer address (see e.g.
[IPv6-ETHER]).  If no entry exists for a unicast destination, the sender
creates a new one, sets its state to INCOMPLETE, sends a Neighbor
Solicitation message, and then queues the data packet pending completion
of Address Resolution.  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 6.2.

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 6.3), unless the upper-layer has indicated that such
checks are not needed.  For instance, the Neighbor Discovery protocol
itself when sending packets should pass an indication to IP that the
packet should not trigger Neighbor Unreachability Detection.  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.


4.3.  Garbage Collection and Timeout Requirements

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

=46rom 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.




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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 Default Router List or 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.


5.  ROUTER AND PREFIX DISCOVERY

This section describes message formats, router behavior 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 configuration parameters related to address
autoconfiguration.

Prefix Discovery provides a mechanism through which hosts learn of
ranges of IP addresses that reside on-link and thus can be reached
directly without going through a router.  Routers advertise a set of
prefixes that cover those IP addresses that are on-link.  Prefix
discovery is logically separate from Router Discovery.  In practice,
prefix information is included in options piggybacked on Router
Advertisement messages to reduce network traffic.

Address Autoconfiguration information is also logically separate from
Router Discovery.  To reduce network traffic, however, autoconfiguration
information is piggybacked on Router Discovery messages.  In fact, the
same prefixes can be advertised for on-link determination and address
autoconfiguration by specifying the appropriate flags in the Prefix
Information options.  This document does not define how
autoconfiguration information is processed.  See [ADDRCONF] for details.






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5.1.  Message Formats


5.1.1.  Router Solicitation Message Format

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     Code      |          Checksum             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Reserved                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Options ...
     +-+-+-+-+-+-+-+-+-+-+-+-

IP Fields:

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

   Destination Address
                  The all-routers link-local multicast address.

   Hop Count      1

   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.

   Routing Header MUST NOT be sent.

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 ignored by the receiver.

Options:

   Source link-layer address
                  The link-layer address for the sender.  This option
                  SHOULD be included on link layers that have addresses
                  so that routers responding to the request can unicast



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                  a response without the need to first perform address
                  resolution.


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



5.1.2.  Router Advertisement Message Format

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     Code      |          Checksum             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Max 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
                  Either the Source Address of an invoking Router
                  Solicitation or the all-nodes link-local multicast
                  address.

   Hop Count      1

   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.

   Routing Header MUST NOT be sent.

ICMP Fields:

   Type           134




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   Code           0

   Checksum       The ICMP checksum.  See [ICMPv6].

   Max Hop Limit  8-bit unsigned integer.  The maximum hop limit that
                  the router suggests that hosts use when sending IP
                  packets.  A value of zero means unspecified.

   M              1-bit "Managed address configuration" flag.  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 configuration" flag.  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 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 does not apply to information
                  contained in any options in the message.  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
                  receiving some reachability confirmation.  Used by the
                  Neighbor Unreachability Detection algorithm (see
                  Section 6.3).  A value of zero means unspecified (by
                  the 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 6.2
                  and 6.3).  A value of zero means unspecified (by the
                  router).




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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.  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 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 chose the correct
                  outgoing interface when sending traffic to its
                  neighbors.


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



5.2.  Router Specification


5.2.1.  Router Configuration Variables

A router MUST allow for the following variables to be configured by
system management; default values are specified so as to make it
unnecessary to configure any of these variables in many cases.

For each multicast interface:

     AdvertiseDefault
                    A flag indicating whether or not the router should
                    advertise itself as a default router on the
                    interface.

                    Default: TRUE




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     ManagedFlag    The true/false value to be placed in the "Managed
                    address configuration" field in the Router
                    Advertisement.  See [ADDRCONF].

                    Default: FALSE

     OtherFlag      The true/false value to be placed in the "Other
                    configuration" field in the Router Advertisement.
                    See [ADDRCONF].

                    Default: FALSE

     LinkMTU        The value to be placed in MTU options sent by the
                    router.  If the value is set to zero 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: REACHABLE_TIME milliseconds

     ReachableTime  The time a neighbor is considered reachable after
                    receiving a reachability confirmation.

                    Default: If AdvReachableTime is non-zero (specified)
                    a uniformly-distributed random value between
                    MIN_RANDOM_FACTOR and MAX_RANDOM_FACTOR times
                    AdvReachableTime milliseconds.  Otherwise, A
                    uniformly-distributed random value between
                    MIN_RANDOM_FACTOR and MAX_RANDOM_FACTOR times
                    REACHABLE_TIME milliseconds.

     RetransTimer   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: RETRANS_TIMER milliseconds

     MaximumHopLimit
                    The value to be placed in the Max Hop Limit field in
                    the Router Advertisement messages sent by the



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                    router.  The value zero means unspecified (by this
                    router).

                    Default: The value specified in the most recent
                    "Assigned Numbers" RFC [ASSIGNED].

     MaxRtrAdvInterval
                    The maximum time allowed between sending unsolicited
                    multicast Router Advertisements from the interface,
                    in seconds.  MUST be no less than 1 second 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 0.1 seconds and no
                    greater than .75 * MaxRtrAdvInterval.

                    Default: 0.33 * MaxRtrAdvInterval

     RtrAdvLifetime
                    The value to be placed in the Router Lifetime field
                    of Router Advertisements sent from the interface, in
                    seconds.  MUST be no less than MaxRtrAdvInterval and
                    no greater than 9000 seconds.

                    Note: if AdvertiseDefault is false, the value of
                    RtrAdvLifetime is irrelevant; a Lifetime value of 0
                    MUST be placed in outgoing Router Advertisements
                    messages so that hosts do not use the router as a
                    default router.

                    Default: 3 * MaxRtrAdvInterval

     PrefixList
                    A list of prefixes to be placed in Prefix
                    Information options in Router Advertisement messages
                    sent from the interface.

                    Default: The PrefixList contains all prefixes that
                    the router advertises via routing protocols as being
                    on-link for the interface from which the
                    advertisement is sent.

                    Each prefix is associated with:




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                       InvalidationLifetime
                            The value to be placed in the Invalidation
                            Lifetime in the Prefix Information option,
                            in seconds.  The designated value of all 1's
                            (0xffffffff) represents infinity.

                            Default: infinity.

                       OnLinkFlag
                            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:

                       DeprecationLifetime
                            The value to be placed in the Deprecation
                            Lifetime in the Prefix Information option,
                            in seconds.  The designated value of all 1's
                            (0xffffffff) represents infinity.  See
                            [ADDRCONF].

                            Default: 604800 seconds (7 days)

                       AutonomousFlag
                            The value to be placed in the Autonomous
                            Flag field in the Prefix Information option.
                            See [ADDRCONF].

                            Default: TRUE



Protocol constants are defined in Section 10.


5.2.2.  Validation of Router Solicitation Messages

A router MUST silently discard any received Router Solicitation messages
that do not satisfy all of the following validity checks:

   - IP Source Address is a link-local address.




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   - IP Destination Address is a link-local address or a multicast
     address with link-local scope.

   - IP Routing Header is not present.

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

   - ICMP Checksum is valid.

   - ICMP Code is 0.

   - ICMP length (derived from the IP length) is 8 or more octets.

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


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.

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

Routers MUST also validate Router Advertisements as described in Section
5.3.3.


5.2.3.  Router Behavior

A router MUST join the all-routers multicast address on all multicast
capable interfaces.

The term "advertising interface" refers to any functioning and enabled
interface that has at least one IP address assigned to it.  From each
advertising interface, the router transmits periodic, multicast Router
Advertisements, containing the following values consistent with the
message format above:

   - In the Router Lifetime field: the interface's configured
     RtrAdvLifetime.  If the router's AdvertiseDefault flag is set to
     false, the Router Lifetime field MUST be set to 0.

   - In the M and O flags: the interface's configured ManagedFlag and
     OtherFlag, respectively.  See [ADDRCONF].

   - In the Max Hop Limit field: the interface's configured



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     MaximumHopLimit.

   - In the Reachable Time field: the interface's configured
     AdvReachableTime.

   - In the Retrans Timer field: the interface's configured
     RetransTimer.

   - 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 MTU value that all nodes should be using.

        o Prefix Information options: one Prefix Information option for
          each prefix listed in PrefixList with the option fields set
          from the information in the PrefixList entry as follows:

             - In the "on-link" flag: the entry's OnLinkFlag.

             - In the Invalidation Lifetime field: the entry's
               InvalidationLifetime.

             - In the "Autonomous address configuration" flag: the
               entry's AutonomousFlag.

             - In the Deprecation Lifetime field: the entry's
               DeprecationLifetime.




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, that interface's
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 from
the interface, and a new random value to be chosen.  (It is recommended
that routers include some unique value, such as one of their IP or
link-layer addresses, in the seed used to initialize their pseudo-random
number generators.  Although the randomization range is configured in
units of seconds, the actual randomly-chosen values should not be in
units of whole seconds, but rather in units of the highest available



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timer resolution.)

For the first few advertisements sent from an interface (up to
MAX_INITIAL_RTR_ADVERTISEMENTS), 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 this 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.

In addition to the periodic, unsolicited advertisements, a router sends
advertisements in response to valid solicitations received on any of its
advertising interfaces.  A router MAY choose to unicast the response
directly to the soliciting host's address, or multicast it to the all-
nodes address; in the latter case, the interface's interval timer is
reset to a new random value, as with unsolicited advertisements.  A
unicast response MAY be delayed, and a multicast response MUST be
delayed, for a small random interval not greater than
MAX_RTR_RESPONSE_DELAY, in order to prevent synchronization with other
responding routers, and to allow multiple, closely-spaced solicitations
to be answered with a single multicast advertisement.  A router that
chooses to delay responses behaves as follows:

 - Upon receipt of a Router Solicitation, start a timer taken from a
   random value within the range 0-MAX_RTR_RESPONSE_DELAY.

 - When the timer expires, send out the Router Advertisement.  If no
   other Router Solicitation was received while waiting for the timer to
   expire, unicast the advertisement.  Otherwise, multicast the response
   and reset the interface timer to a new random value, as is done when
   multicasting an unsolicited response.


Note that a router is permitted to send multicast Router Advertisements
more frequently than indicated by the MinRtrAdvInterval configuration
variable if the additional advertisements are responses to explicit
solicitations.  In all cases, however, unsolicited multicast
advertisements MUST NOT be sent more frequently than indicated by
MinRtrAdvInterval.

When a router receives a Router Solicitation it records that the source
of the packet is a neighbor.  If the solicitation contains a Source
Link-Layer Address option, and the router has a Neighbor Cache entry for
the neighbor, the link-layer address SHOULD be updated in the Neighbor
Cache and the entry's "is_router" flag SHOULD be set to false.  If a
Neighbor Cache entry is created for the source its reachability state
MUST be set to PROBE as specified in Section 6.3.2.




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It should be noted that an interface may become an advertising interface
at times other than system startup, as a result of recovery from an
interface failure or through actions of system management such as:

   - enabling the interface, if it had been administratively disabled,
     and its AdvertiseDefault flag is TRUE, or

   - enabling IP forwarding capability (i.e., changing the system from
     being a host to being a router), when the interface's
     AdvertiseDefault flag is TRUE, or

   - changing the AdvertiseDefault flag from FALSE to TRUE.


In such cases the router MUST commence transmission of periodic
advertisements on the new advertising interface, limiting the first few
advertisements to intervals no greater than
MAX_INITIAL_RTR_ADVERT_INTERVAL.  In the case of a host becoming a
router, the system MUST also join the all-routers IP multicast group on
all interfaces on which the router supports IP multicast (whether or not
they are advertising interfaces).

An interface may also cease to be an advertising interface, through
actions of system management such as:

   - administratively disabling the interface, or

   - shutting down the system, or disabling the IP forwarding capability
     (i.e., changing the system from being a router to being a host), or

   - setting the AdvertiseDefault flag of the interface to FALSE.


In such cases the router SHOULD transmit a final multicast Router
Advertisement on the interface with a Router Lifetime field of zero.  In
the case of a router becoming a host, the system MUST 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.

The information advertised in Router Advertisements may change through
actions of system management.  For instance, the lifetime of advertised
prefixes may change, the advertised MTU may change, etc.  In such cases,
the router MAY transmit a few (no more than
MAX_INITIAL_RTR_ADVERTISEMENTS) Router Advertisements separated by an
interval of MAX_INITIAL_RTR_ADVERT_INTERVAL.



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A router might want to send Router Advertisements without advertising
itself as being a default router.  For instance, a router might
advertise prefixes for address autoconfiguration while not wishing to
forward packets.  Such a router MUST set the Router Lifetime field to
zero in its advertisements.

A router MAY choose not to include all Prefix Information options in
every Router Advertisement.  For example, if prefix lifetimes are much
longer than RtrAdvLifetime, including them every few advertisements may
be sufficient.  However, when responding to a Router Solicitation the
router SHOULD transmit all prefixes to allow hosts to quickly discover
the prefixes during system initialization.


5.2.4.  Router Advertisement Consistency

Routers SHOULD inspect valid Router Advertisements sent by other routers
on the link and verify that the routers are advertising consistent
information.  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 that should be checked
includes:

 - Max Hop Limit values (except for the unspecified value of zero).

 - Values of the M or O flags.

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

 - Retrans Timer values (except for the unspecified value of zero).

 - Values in the MTU options.

 - Invalidation Lifetimes for the same prefix.

 - Deprecation Lifetimes for the same prefix.


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.  The logging of errors SHOULD
be restricted to conflicting information that causes hosts to
continually switch from one value to another.

In addition, routers can optionally examine the source address of Router
Advertisements to determine which of a neighboring router's addresses is
its link-local address.




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Any other action on reception of Router Advertisement messages by a
router is beyond the scope of this document.


5.2.5.  Link-local Address Change

The link-local address on a router SHOULD change infrequently.  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 nodes,
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 must be identical to the target address in a
Redirect message when redirecting to the router.

Using the link-local address to uniquely identify routers on the link
has the benefit that the link-local address does 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 with Router Lifetime field set to zero for the old
link-local address and also multicast a few Router Advertisements for
the new link-local address.  The exact procedures SHOULD be the same as
when an interface ceases being an advertising interface, and when an
interface becomes an advertising interface, respectively.

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.  This may require that routing protocols exchange link-local
addresses.  Alternatively, routers could listen to Router Advertisements
messages to determine link-local addresses of neighboring routers.
However, doing so only works if all routers are sending out Router
Advertisements.


5.3.  Host Specification


5.3.1.  Host Configuration Variables

None.







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5.3.2.  Host Variables

A host maintains certain Neighbor Discovery related variables in
addition to the data structures defined in Section 4.1.  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.

For each interface:

     LinkMTU        The MTU of the link.

                    Default: The valued defined in the specific document
                    that describe how IPv6 operates over the particular
                    link layer (e.g., [IPv6-ETHER]).

     MaximumHopLimit
                    The maximum Hop Count to be used when sending IP
                    packets.

                    Default: The value specified in the most recent
                    "Assigned Numbers" RFC [ASSIGNED].

     ReachableTime  The time a neighbor is considered reachable after
                    receiving a reachability confirmation.

                    Default: A uniformly-distributed random value
                    between MIN_RANDOM_FACTOR and MAX_RANDOM_FACTOR
                    times REACHABLE_TIME milliseconds.


     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



5.3.3.  Validation of Router Advertisement Messages

A node MUST silently discard any received Router Advertisement messages
that do not satisfy all of the following validity checks:

   - IP Source Address is a link-local address.




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   - IP Destination Address is a link-local address or a multicast
     address with link-local scope.

   - IP Routing Header is not present.

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

   - ICMP Checksum is valid.

   - ICMP Code is 0.

   - ICMP length (derived from the IP length) is 16 or more octets.

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


The contents of the Reserved field, and of any unrecognized options,
MUST be ignored.  Future, backward-compatible changes to the protocol
may specify the contents of the Reserved field or add new options;
backward-incompatible changes may use different Code values.

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

A host MUST silently discard any received Router Solicitation messages.


5.3.4.  Host Behavior

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

A host MUST NOT send a Router Advertisement message at any time.

To process a valid Router Advertisement, a host extracts the source
address of the packet and does the following:

   - 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 timer value from the
     advertisement's Router Lifetime field.

   - If the address is already present in the host's Default Router List
     as a result of a previously-received advertisement, reset its timer
     to the Router Lifetime value in the newly-received advertisement.

   - If the address is already present in the host's Default Router List



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     and the received Router Lifetime value is zero, time-out the entry
     immediately and remove it from the Default Router list.


If the received Max Hop Limit value is non-zero the host SHOULD set its
MaximumHopLimit variable to the received value.  Hosts use the last Max
Hop Limit value they have received; routers should be configured to
advertise identical values to avoid hosts switching between different
values.

The host SHOULD set its ReachableTime variable based on the Reachable
Time field, if the received value is non-zero.  The value is computed as
a uniformly-distributed random value between MIN_RANDOM_FACTOR and
MAX_RANDOM_FACTOR times the value received in the Reachable Time field.
Reception of another Router Advertisement causes a new random value to
be chosen.  This avoids any synchronization of Neighbor Unreachability
Detection messages.

The RetransTimer SHOULD be set to the Retrans Timer field, if the
received value is non-zero.

Hosts use the last Reachable Time and Retrans Timer values they have
received; routers should be configured to advertise identical values to
avoid having hosts switch between different values as they receive
advertisements from different routers.

After extracting information from the fixed part of the Router
Advertisement message, the advertisement MUST be scanned for valid
options.  If the advertisement contains a source link-layer address
option the link-layer address MUST be recorded in the Neighbor Cache
entry for the router (creating an entry if necessary) and the
"is_router" flag in the Neighbor Cache entry MUST be set to true.  The
"is_route" 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 PROBE as specified in
Section 6.3.2.

Received MTU options are handled as specified in Section 8.4.

For each Prefix Information option that has the "on-link" (L) flag set,
the host does the following:

   - If the prefix is not already present in the Prefix List, create a
     new entry for the prefix and initialize its invalidation timer to
     the Invalidation Lifetime value in the Prefix Information option.

   - If the prefix is already present in the host's Prefix List as the



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     result of a previously-received advertisement, reset its
     invalidation timer to the Invalidation Lifetime value in the Prefix
     Information option.  If the new Lifetime value is zero, time-out
     the prefix immediately.

   - If the received Invalidation Lifetime value is zero, and the prefix
     is not present in the host's Prefix List, silently ignore the
     option.


   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.


Whenever the invalidation timer expires for a Prefix List entry, that
entry is discarded.  No existing Destination Cache entries are affected,
however.

Whenever a timer expires for an entry in the Default Router List, that
entry is discarded.  Any entries in the Destination Cache going through
that router will continue to be used.  Neighbor Unreachability Detection
will purge them if appropriate.

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).

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 6.3.
The algorithm for selecting a default router is invoked only when a
Destination Cache entry is incomplete 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, with subsequent traffic for that destination using the same
router as indicated in the Destination Cache.  The policy for selecting
routers from the Default Router List is as follows:

  1) Routers reachable or probably reachable (e.g., in the REACHABLE or



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     PROBE state) MUST be preferred over routers whose reachability is
     unknown or suspect.  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 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 the destination is
     on-link as specified in Section 4.2.


A host is permitted (but not required) to transmit up to
MAX_RTR_SOLICITATIONS Router Solicitation messages from any of its
multicast interfaces after any of the following events:

   - The interface is initialized at system startup time.

   - The interface is reinitialized after a temporary interface failure
     or after being temporarily disabled by system management.

   - The system changes from being a router to being a host, by having
     its IP forwarding capability turned off by system management.

   - The host is re-attached to a link after being detached for some
     time.


The IP destination address of the solicitations is the all-routers
multicast address.  The IP source address MUST be one of the interface's
addresses and MUST be a link-local address.  The Source Link-Layer
Address option is set to the host's link-layer address.

If a host does choose to send a solicitation after one of the above
events, it SHOULD delay that 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.  (It is recommended



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that hosts include some unique value, such as one of their IP or link-
layer addresses, in the seed used to initialize their pseudo-random
number generators.)  Although the randomization range is specified in
units of seconds, the actual randomly-chosen values should not be in
units of whole seconds, but rather in units of the highest available
timer resolution.

If a host has performed a random delay earlier during the system startup
(e.g. as part of Duplicate Address Detection [ADDRCONF]) there is no
need to randomly delay the first Router Solicitation message.

A host MAY also choose to further postpone its solicitations, subsequent
to one of the above events, until the first time it needs to use a
default router.

Upon receiving a valid advertisement with a non-zero Lifetime, the host
MUST desist from sending any solicitations on that interface (even if
none have been sent yet), until the next time one of the above events
occurs.  The small number of retransmissions of a solicitation, which
are permitted if no such advertisement is received, SHOULD be sent at
intervals of RTR_SOLICITATION_INTERVAL seconds, without randomization.



6.  ADDRESS RESOLUTION AND NEIGHBOR UNREACHABILITY DETECTION

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

These messages are also used for Duplicate Address Detection as
specified by [ADDRCONF].  In particular, Duplicate Address Detection
sends Neighbor Solicitation messages using an unspecified source address
targeting its own address.  This will generate a multicast Neighbor
Advertisement from any node(s) that have been configured with the same
address.


6.1.  Message Formats


6.1.1.  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



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reachability of a neighbor.


     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     Code      |          Checksum             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Reserved                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                       Sender Address                          +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                       Target Address                          +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Options ...
     +-+-+-+-+-+-+-+-+-+-+-+-

IP Fields:

   Source Address
                  MUST be either the link-local address assigned to the
                  interface from which this message is sent, or the
                  unspecified address.  Use of the unspecified address
                  directs the target node to multicast the resultant
                  Neighbor Advertisement as required by Duplicate
                  Address Detection in [ADDRCONF].

   Destination Address
                  Either the solicited-node link-local multicast address
                  corresponding to the target address, or the target
                  address.  Packets unicast to the target address are
                  used to verify reachability.

   Hop Count      1

   Authentication Header
                  If a Security Association for the IP Authentication
                  Header exists between the sender and the destination



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                  address, then the sender SHOULD include this header.

   Routing Header MUST NOT be sent.

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 ignored by the receiver.

   Sender Address
                  An IP address assigned to the interface from which the
                  solicitation is sent.  If the source address of the
                  data packet prompting the solicitation is the same of
                  one of the sending interface's addresses, that address
                  SHOULD be used.  Doing so ensures that the receiver of
                  the solicitation places the data packet's source
                  address in its Neighbor Cache, eliminating the need
                  for address resolution in the likely case that reverse
                  traffic for that destination will follow.

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

Options:

   Source link-layer address
                  The link-layer address for the sender.  MUST NOT be
                  included in unicast solicitations, in order to prevent
                  off-link senders from creating or modifying cached
                  link-layer addresses.  For multicast solicitations
                  sent on link layers that have addresses it SHOULD be
                  included.



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






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6.1.2.  Neighbor Advertisement Message Format

A node MUST send a Neighbor Advertisement in response to a Neighbor
Solicitation for a target IP address that matches an assigned address on
the receiving interface.  A node MAY also send an unsolicited Neighbor
Advertisement if wishes to advertise that its link-layer address has
changed.

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     Code      |          Checksum             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |R|S|N|                     Reserved                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                       Target 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 an invoking Neighbor
                  Solicitation or, if the source address in the
                  solicitation is the unspecified address, the all-nodes
                  link-local multicast address.  For an unsolicited
                  advertisement the destination is typically the all-
                  nodes link-local multicast address.

   Hop Count      1

   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.

   Routing Header MUST NOT be sent.

ICMP Fields:



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   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.  It MUST be
                  zero in a multicast advertisement and in an
                  unsolicited unicast advertisement.

   N              Secondary Advertisement flag.  When set, the N-bit
                  indicates that the advertisement should only be used
                  if no other advertisement has been received i.e. the
                  advertisement will not update a cached link-layer
                  address.  It SHOULD be set in solicited advertisements
                  for anycast addresses and in solicited proxy
                  advertisements.  It SHOULD be zero in other solicited
                  advertisements and in unsolicited advertisements.

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

   Target Address
                  The address from 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.

Options:

   Target link-layer address
                  The link-layer address for the target.  MUST be
                  included on link layers that have addresses.


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





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6.2.  Address Resolution

Address Resolution provides the mechanism through which a node
determines the link-layer address of a neighbor.  Address Resolution is
only used for destinations that are determined to be on-link and for
which the sender does not know the corresponding link-layer address.
Address resolution is never used for multicast destinations.


6.2.1.  Node Specification

When a multicast-capable interface is initialized 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 operation of automatic address configuration [ADDRCONF] may, over
time, change the set of addresses assigned to an interface; new
addresses might be added and old addresses might be removed.  In such
case the node MUST join and leave the solicited-node multicast address
corresponding to the new and old addresses, respectively.  Note that
multiple assigned addresses might correspond to 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.


6.2.2.  Sending Neighbor Solicitations

When a node has a unicast packet to send, but does not know the next-
hop's link-layer address, it performs address resolution by creating a
Neighbor Cache entry in the INCOMPLETE state and transmitting a Neighbor
Solicitation message targeted at the neighbor.  The solicitation must be
sent to the solicited-node multicast address of the target address.

The sender SHOULD include its link-layer address (if it has one) in the
multicast solicitation as a Source Link-Layer Address option.  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 ICMP Sender Address of the outgoing
solicitation.  Otherwise, the interface's link-local address 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
traffic belonging to the prompting packet's "connection".

While waiting for address resolution to complete, the sender MUST retain
packets waiting for address resolution to complete in a small queue.



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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, all queued packets
SHOULD be transmitted.

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

If no 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.



6.2.3.  Validation of Neighbor Solicitations

A node MUST silently discard any received Neighbor Solicitation messages
that do not satisfy all of the following validity checks:

   - IP Source Address is a link-local address or the unspecified
     address.

   - if the IP Destination Address is a multicast address, its scope is
     link-local.

   - IP Routing Header is not present.

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

   - ICMP Checksum is valid.

   - ICMP Code is 0.

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

   - Target Address is not a multicast address.

   - if the Source Address is the unspecified address or the Destination
     Address is a unicast address, there is no Source Link-layer Address
     option.

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



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   - the Target Address matches an address assigned to the receiving
     interface.


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.

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


6.2.4.  Receipt of Neighbor Solicitations

If and the Source Link-Layer Address option is present, the recipient
SHOULD update the Neighbor Cache entries for both the IP Source Address
and the ICMP Sender Address of the solicitation.  In those cases where a
corresponding entry does not already exist, the node SHOULD create a new
one and set its reachability state to PROBE as specified in
Section 6.3.2.  In all cases the source link-layer address option in the
received advertisement SHOULD replace any cached link-layer addresses.

A Neighbor Solicitation that is being used for Duplicate Address
Detection, i.e. with an unspecified source address, can not contain a
source link-layer address option thus it has no effect on the Neighbor
Cache.


6.2.5.  Sending Solicited Neighbor Advertisements

A Neighbor Advertisement is sent in response to a valid Neighbor
Solicitation.  The Target Address of the advertisement is copied from
the Target Address of the Solicitation.  The Target Link-Layer Address
option SHOULD be included, using as its value the interface's link-layer
address.  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, the Secondary
Advertisement flag SHOULD be set to one.  Otherwise, it SHOULD be set
to 0.  Proper setting of the Secondary Advertisement flag insures that
nodes give preference to "primary" advertisements, even when received
after "secondary" advertisements.

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



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to one and unicast the advertisement to the link-local Source Address of
the solicitation.


6.2.6.  Validation of Neighbor Advertisements

A node MUST silently discard any received Neighbor Advertisement
messages that do not satisfy all of the following validity checks:

   - IP Source Address is a link-local address.

   - IP Destination Address is a link-local address or a multicast
     address with link-local scope.

   - IP Routing Header is not present.

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

   - ICMP Checksum is valid.

   - ICMP Code is 0.

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

   - Target Address is not a multicast address.

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

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


The contents of the Reserved field, and of any unrecognized options,
MUST be ignored.  Future, backward-compatible changes to the protocol
may specify the contents of the Reserved field or add new options;
backward-incompatible changes may use different Code values.

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


6.2.7.  Receipt of Neighbor Advertisments

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 in this case, since the recipient has



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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.  In
particular, if no link-layer address is cached for the target (e.g., it
is in the INCOMPLETE state), the first received advertisement would be
used.  On the other hand, if we already have a cached link-layer
address, we can safely be more selective about what information is used
in received advertisements.

If the target's Neighbor Cache entry is in the INCOMPLETE state, the
advertisement is the first response to a solicitation.  The receiving
node MUST record the link-layer address in the Neighbor Cache entry and
send any packets queued for the neighbor awaiting address resolution.
If the Solicited flag is set, the reachability state for the neighbor
MUST be set to REACHABLE; otherwise it MUST be set to PROBE. (A more
detailed explanation of reachability state is described in Section
6.3.2).  The Secondary Advertisement flag is ignored if the entry is in
the INCOMPLETE state.

If the target's Neighbor Cache entry is in the REACHABLE or PROBE state,
the Secondary Advertisement flag is examined.  If set, the entry's state
should be set to PROBE, and the packet SHOULD be silently discarded; no
other changes are made to the Neighbor Cache entry.

If the Secondary Advertisement flag is not set, the link-layer address
in the Target Link-Layer Address option should be copied into the
Neighbor Cache entry.  Furthermore, if the Solicited flag is set, the
entry's state should be set to REACHABLE.  Otherwise, the entry's state
should be set to PROBE.

Finally, the receiving node MUST examine the Router flag in the received
advertisement and update the "is_router" flag in the Neighbor Cache
entry to reflect whether the node is a host or router.  In those cases
where the neighbor was previously used as a router, but the
advertisement's Router flag is now set to zero, 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 6.3.2.


6.2.8.  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.



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These advertisements MUST be separated by at least
MIN_NEIGHBOR_ADVERT_INTERVAL 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 Secondary
Advertisement flag MAY be either set or cleared.  In either case,
neighboring nodes will immediately change the state of their Neighbor
Cache entries for the Target Address to PROBE, prompting them to verify
the path for reachability.  If the Secondary Advertisement is set,
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 instead.

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 reliably
obtain the new link-layer address, though the delay may be slightly
longer.


6.2.9.  Anycast Neighbor Advertisements

A node belonging to an anycast address MUST join the solicited-node
multicast address that corresponds to the anycast address.



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When a node responds to a Neighbor Solicitation for an anycast address,
it MUST respond with an Neighbor Advertisement that has the Secondary
Advertisement flag set to one.  In addition, the sender should delay
sending a response for a random time between 0 and
MAX_ANYCAST_DELAY_TIME seconds.

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


6.2.10.  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 potentially accept packets on behalf of a mobile
node that has moved off-link.  The mechanisms used by proxy are
identical to the mechanisms needed for 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
Secondary Advertisement flag set to one.  This ensures that if the node
itself is present on the link its Neighbor Advertisement (with the
Secondary flag set to zero) will take precedence of any advertisement
received from a proxy.  A proxy MAY send unsolicited advertisements with
the Secondary Advertisement flag set to zero as specified in Section
6.2.8, but doing so may cause the proxy advertisement to override a
valid entry created by the node itself.

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.


6.3.  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



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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.
The conceptual model allows an upper-layer to indicate to IP that
Neighbor Unreachability Detection is not needed for a packet being sent.
This is used by Neighbor Discovery to skip these checks when sending
Neighbor Discovery messages.

When a path to a neighbor appears to be failing, the specific recovery
procedure depends on how the neighbor is being used.  For example, the
specific recovery procedure used when the neighbor is used as a router
differs from that used when the neighbor is the destination.

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


6.3.1.  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 an explicit Neighbor Solicitation probe.

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.  For example, receipt of a (new) acknowledgement
indicates that previously sent data reached the peer.  Likewise, the
arrival of a new (non-duplicate) packet indicates that 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 that is a response to
a Neighbor Solicitation probe serves as reachability confirmation, since



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advertisements with the Solicited flag set to one are sent only in
response to a 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 such unsolicited messages only confirm the
one-way path from the neighbor to the recipient node.  In contrast,
Neighbor Unreachability Detection requires that the forward path from
the sender to the neighbor be working.  Note that an advertisement sent
in response to an explicit solicitation confirms that a path is working
in both directions; the solicitation reached the neighbor, prompting it
to generate an advertisement, and the advertisement reached the querying
node.  However, from the perspective of Neighbor Unreachability
Detection, only the reachability of the forward path is of interest.


6.3.2.  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.

A Neighbor Cache entry can be in one of three 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 REACHABLE, no
               special action takes place as packets are sent.

   PROBE More than ReachableTime milliseconds have elapsed since the
               last positive confirmation was received that the forward
               path was functioning properly.  Upon entering the PROBE
               state, no Neighbor Solicitation is sent.  However, a
               timer is set to expire DELAY_FIRST_PROBE_TIME seconds
               later, and a Neighbor Solicitation probe is sent if the
               entry is still in a PROBE state when the timer expires.
               Delaying the sending of the initial Neighbor Solicitation
               gives the upper layers additional time to provide
               reachability confirmation information.  After the initial
               delay, Neighbor Solicitations are retransmitted every
               RetransTimer milliseconds until a reachability
               confirmation is received.



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When an entry is created as a result of needing to perform address
resolution, a Neighbor Solicitation is sent to the solicited-node
multicast address of the target, a timer is started to expire
RETRANS_TIMER milliseconds later and the entry's state is set to
INCOMPLETE.

As specified in Section 6.2.2, when in the INCOMPLETE state, Neighbor
Solicitation messages are retransmitted every RETRANS_TIMER milliseconds
until a response is received.  If no response is received within
RETRANS_TIMER milliseconds after sending MAX_MULTICAST_SOLICIT probes to
the solicited-node multicast address, address resolution fails.  Upon
failure, ICMP destination unreachable indications with code 3 (Address
unreachable) are returned for any queued packets and the entry is
deleted.  Note that deleting the entry implies that all destinations
using that neighbor must perform next-hop resolution again before
sending a subsequent packet.  Thus, if the neighbor is a router, an
alternate router may be selected.  Alternatively, a destination
previously thought to be on-link, may now only be reachable through a
router.

Unreachability detection changes a neighbor's state from REACHABLE to
PROBE only on-demand, as a side effect of sending a data packet to that
neighbor.  If no traffic is sent to a neighbor, no probes are sent
either.  Note that an entry may technically no longer be in a REACHABLE
state, but the condition need not be checked or acted upon until a
packet is sent to the neighbor.

The first time a Neighbor Cache entry is referenced and more than
ReachableTime milliseconds have passed since receipt of the last
reachability confirmation, its state changes to PROBE.  However, no
Neighbor Solicitation probe is sent.  Probing is deferred for an
additional DELAY_FIRST_PROBE_TIME seconds, an optimization that gives
the upper-layer protocol additional time to provide a 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.

If no reachability confirmation is received within
DELAY_FIRST_PROBE_TIME seconds after entering the PROBE state, a unicast
Neighbor Solicitation message is sent to the neighbor using the cached
link-layer address.  In addition, the sender starts a timer to
retransmit probe messages every RetransTimer milliseconds until the
desired solicitation is received.  Subsequent 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



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SHOULD be deleted.  Subsequent traffic to that neighbor recreates 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.

A Neighbor Cache entry also enters the PROBE state when created as a
result of receiving packets other than solicited Neighbor Advertisements
(e.g., 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 PROBE state provides assurance that path failures are
detected quickly.  As always, when entering the PROBE state, the first
probe is delayed for DELAY_FIRST_PROBE_TIME to give the upper layer some
time to provide a reachability confirmation thereby suppressing the
sending of a probe.

To detect the case where a router switches from being a router to being
a host (e.g., by having its IP forwarding capability turned off by
system management), a node MUST compare the Router flag field in all
received Neighbor Advertisement messages with the "is_router" 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 so that all entries using that neighbor as a router
switch to another router.  Note that a router may not be listed in the
Default Router List, even though a Destination Cache entry is using it
(e.g., the a host was redirected to it).

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.











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7.  REDIRECT FUNCTION

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


7.1.  Redirect Message Format

A Redirect packet is sent from a router to a host to inform the host of
a better first-hop node on the path to a destination.

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     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 Count      1

   Authentication Header



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

   Routing Header MUST NOT be sent.

ICMP Fields:

   Type           5

   Code           0

   Checksum       The ICMP checksum.  See [ICMPv6].

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

   Target Address An IP address of the node to which traffic for the
                  Destination SHOULD be sent.  When the target is a
                  router, the Target Address MUST be the router's link-
                  local address so that hosts can uniquely identify
                  routers.  When the target is the actual endpoint of
                  communication, the target address field MUST contain
                  the same value as the Destination Address field.

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

Options:

   Target link-layer address
                  The link-layer address for the target.  It MUST be
                  included on non-broadcast links, since the host can
                  not use the multicast Neighbor Solicitation to resolve
                  the address.  If known by the router, it SHOULD be
                  included on all link layers that have addresses.

   Redirected Header
                  As much as possible of the IP packet that triggered
                  the sending of the Redirect without making the
                  redirect packet exceed 576 octets.


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




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7.2.  Router Specification

A router SHOULD send a redirect message, subject to rate limiting,
whenever it forwards a packet in which:

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

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

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

   - the packet is not source routed through the router, i.e. the
     destination address (when the packet was received by the router)
     did not match one of the router's addresses.  Other source routed
     packets, not explicitly source routed through the router, can be
     redirected.


The transmitted redirect packet contains, consistent with the above
message format:

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

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

   - In the options:

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

        o Redirected Header: as much of the forwarded packet as can fit
          without the redirect packet exceeding 576 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



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be found in [ICMPv6].

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


7.3.  Host Specification


7.3.1.  Validation of Redirect Messages

A host MUST silently discard any received Redirect messages that do not
satisfy all of the following validity checks:

   - IP Source Address is a link-local address.

   - IP Routing Header is not present.

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

   - ICMP Checksum is valid.

   - ICMP Code is 0.

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

   - the IP source address of the Redirect is the same as the current
     first-hop router for the specified destination.

   - the Target Address of the redirect is not a multicast address.

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

   - 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.

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.
That is, identical values in the Target and Destination Address fields
indicates that the target destination is on-link.

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



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7.3.2.  Host Behavior

A host receiving a valid redirect SHOULD update its routing information
accordingly.  When a redirect is received, the host updates the
Destination Cache entry for the destination to use to the specified
target as the new next-hop.  If no Destination Cache entry exists for
the destination, such an entry is created (placing it in the PROBE
state).

If the redirect contains a Target Link-Layer Address option the host
either creates or updates the Neighbor Cache entry for the target.  The
link-layer address in the Neighbor Cache entry MUST be copied from the
Target Link-Layer Address option into the appropriate Neighbor Cache
entry.  If a Neighbor Cache entry is created for the target its
reachability state MUST be set to PROBE as specified in Section 6.3.2.
In addition, if the Target Address is the same as the Destination
Address, the host MUST treat the destination as on-link and set the
"is_router" field in the corresponding Neighbor Cache entry to false.
Otherwise it MUST set to true.

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.


8.  OPTIONS

Options provide a mechanism for encoding variable length fields, fields
that may appear multiple times in the same packet, or information that
is optional and 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



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     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 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 the prefix Invalidation Lifetime is high 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 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.


When multiple options are present in a Neighbor Discovery packet, they
may 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, for instance Prefix Information options.

The length of all options is a multiple of 8 octets, ensuring
appropriate alignment without any "pad" options.  The fields in the
options, as well as the fields in ND packets, are defined to align them
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.

All options are of the form:

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |    Length     |              ...              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                              ...                              ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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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 in
                  units of 8 octets.  The value 0 is invalid.  Nodes
                  MUST silently discard an ND packet that contains an
                  option with length zero.


The size of an ND packet including the IP header is limited to the link
MTU (which is at least 576 octets).  When adding options to an ND packet
a node MUST NOT exceed the link MTU.

The only ND packets that can potentially exceed the link MTU are Router
Advertisements and Redirects; the former due to a large number of Prefix
Information options and the latter due to the Redirected Header option.

If there are more Prefix Information options than can fit in a single
Router Advertisement packet the router MUST send multiple separate
advertisements that each contain a subset of the set of prefixes.

The amount of data to include in the Redirected Header option MUST be
limited so that the entire redirect packet does not exceed 576 octets.


8.1.  Source/Target Link-layer Address

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |    Length     |    Link-Layer Address ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:

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




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   Length         The length of the option 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 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.



8.2.  Prefix Information

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |    Length     | Prefix Length |L|A| Reserved1 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Invalidation Lifetime                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Deprecation Lifetime                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Reserved2                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                            Prefix                             +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:

   Type           3




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   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.

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

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

   Invalidation Lifetime
                  32-bit unsigned integer.  The lifetime of the prefix
                  in seconds for the purpose of on-link determination.
                  A value of all one bits (0xffffffff) represents
                  infinity.  This lifetime is also used by [ADDRCONF].

   Deprecation Lifetime
                  32 bits reserved for autonomous address configuration.
                  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 ignored by the receiver.

   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.

Description
                  The Prefix Information option is only used in Router
                  Advertisement packets.  It provide hosts with on-link
                  prefixes and prefixes for Address Autoconfiguration.

                  Implementations can choose to process the on-link
                  aspects of the prefixes separately from the address
                  autoconfiguration aspects of the prefixes e.g. by
                  passing a copy of each valid Router Advertisement
                  message to both an "on-link" and an "addrconf"
                  function.  Each function can then operate on the
                  prefixes that have the appropriate flag set.





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8.3.  Redirected Header

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     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 ignored by the receiver.

   IP header + data
                  The original packet truncated to ensure that the size
                  of the redirect message does not exceed 576 octets.

Description
                  The Redirected Header option MUST be included in
                  Redirect packets.



8.4.  MTU

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |    Length     |           Reserved            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              MTU                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:

   Type           5

   Length         1

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




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

Description
                  The MTU option SHOULD be included in Router
                  Advertisement packets when the link has no well-known
                  MTU and it MAY be included on links with a well-known
                  MTU.

                  Hosts MUST handle this option by setting the LinkMTU
                  variable for the interface to the received value.  If
                  the routers on the link are advertising different MTU
                  values this will result in hosts switching between the
                  different MTUs.  Therefore, routers SHOULD verify the
                  consistency between the MTU they and other routers
                  advertise, logging a network management event when
                  contradictory advertisements are detected.

                  When a host or its interface is initialized the
                  LinkMTU of the interface SHOULD be set to the
                  predefined value for that type of link.  If the host
                  receives no MTU option it MUST continue to use that
                  predefined value.  The MTU option can be used by
                  routers to both increase and decrease the MTU.

                  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 an MTU value supported by all
                  segments.



9.  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



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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:

  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.



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     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.



10.  PROTOCOL CONSTANTS

Router constants:

         MAX_INITIAL_RTR_ADVERT_INTERVAL  16 seconds

         MAX_INITIAL_RTR_ADVERTISEMENTS    3 transmissions

         MAX_RTR_RESPONSE_DELAY            6 seconds

Host constants:

         MAX_RTR_SOLICITATION_DELAY        1 second

         RTR_SOLICITATION_INTERVAL         3 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

         MIN_NEIGHBOR_ADVERT_INTERVAL     16 seconds

         REACHABLE_TIME               30,000 milliseconds

         RETRANS_TIMER                10,000 milliseconds

         DELAY_FIRST_PROBE_TIME            5 seconds

         MIN_RANDOM_FACTOR                 .5

         MAX_RANDOM_FACTOR                 1.5



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Additional protocol constants are defined with the message formats in
Section 5.1, 6.1, and 7.1.

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


11.  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.



12.  OPEN ISSUES

 o Should the routers listed in Router Advertisements include a
   precedence metric?  What are the semantics of such metrics (e.g.,
   "router preferences" vs. "default router preferences").


13.  SECURITY CONSIDERATIONS

Neighbor Discovery is subject 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.

The protocol reduces the exposure to such threats in the absence of



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authentication by designing ND packets that modify neighbor state (e.g.
cached link-layer addresses) in such a way that routers cannot or will
not forward them.  Limiting the scope of ND packets to a particular link
makes the protocol more robust against the accidental sending of ND
messages with a hop count larger than one.  Specifically:

 - the source address of all packets have link-local scope.  Routers
   MUST NOT forward such packets.  See [ADDR-ARCH].

 - with the exception of Redirects, the destination address in all ND
   packets that can modify any state in the recipient node have link-
   local scope; routers will be unable to forward them.

 - packets containing a Routing Header are ignored upon receipt.  If
   Routing Headers were allowed, it would be possible to forward packets
   through routers, even if the packet's ultimate destination has link-
   local scope.


Note that the use of link-local destination address makes the checks for
link-local source address somewhat redundant for ND messages other than
Redirects.  The Redirect message is the only message type sent to a
global unicast address that can modify the state in the receiving node.
Thus proper robustness for Redirect messages requires that routers not
forward packets with link-local source addresses.

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 nodes are
authorized to send Router Advertisements; any node, presumably even in
the presence of authentication, can send Router Advertisement messages
thereby being able to cause denial of service.  Furthermore, any node
can send proxy Neighbor Advertisements as well as unsolicited Neighbor
Advertisements as a potential denial of service attack.

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



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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].





































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REFERENCES


  [ADDRCONF] S. Thomson, "IPv6 Address Autoconfiguration", Internet
          Draft.

  [ADDR-ARCH] S. Deering, R. Hinden, Editors, "IP Version 6 Addressing
          Architecture", Internet Draft.

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

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

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

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

  [ICMPv6] A. Conta, and S. Deering, "ICMP for the Internet Protocol
          Version 6 (IPv6)", Internet Draft.

  [IPv6] S. Deering, R. Hinden, Editors, "Internet Protocol, Version 6
          (IPv6) Specification", Internet Draft.

  [IPv6-ETHER] M. Crawford. "A Method for the Transmission of IPv6
          Packets over Ethernet Networks", Internet Draft.

  [IPv6-SA] R. Atkinson.  "Security Architecture for the Internet
          Protocol".  RFC 1825, August 1995.

  [IPv6-AUTH] R. Atkinson.  "IP Authentication Header",  RFC 1826,
          August 1995.

  [IPv6-ESP] R. Atkinson.  "IP Encapsulating Security Payload (ESP)",
          RFC 1827, August 1995.

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

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

  [ASSIGNED] J. Reynolds, J. Postel, "ASSIGNED NUMBERS", RFC 1700,
          October 1994.




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  [SYNC] S. Floyd, V. Jacobsen, "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

     Erik Nordmark                Thomas Narten
     Sun Microsystems, Inc.       IBM Corporation
     2550 Garcia Ave              P.O. Box 12195
     Mt. View, CA 94041           Research Triangle Park, NC 27709-2195
     USA                          USA

     phone: +1 415 336 2788       phone: +1 919 254 7798
     fax:   +1 415 336 6015       fax:   +1 919 254 4027
     email: nordmark@sun.com      email: narten@vnet.ibm.com

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

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























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CHANGES SINCE PREVIOUS DOCUMENT

     There are several changes since the previous version documented in:
          <draft-ietf-ipngwg-discovery-01.txt>
     based on feedback from the working group:

          o Link-local source address required for Neighbor Solicitation
            and Neighbor Advertisement messages.  This change implied
            adding an ICMP Sender Address field to the Neighbor
            Solicitation message and a Secondary Advertisement flag to
            the Neighbor Advertisement message.  This change improves
            the robustness of the protocol - it is no longer possible
            for off-link nodes to send ND messages to a link.

          o Made the ReachableTime value random to avoid synchronizing
            Neighbor Unreachability Detection messages when there is
            more of less "constant" traffic (i.e. packets are sent with
            spacing that is very short compared to the ReachableTime
            value).  Without such randomization the NUD probes from all
            nodes on the link would be sent with almost the same spacing
            which can result in synchronization.  There is no need for
            any additional randomization elsewhere in the protocol since
            there is no long-term periodic behavior - at most 3 packets
            are transmitted.

          o Added definitions for MUST, SHOULD, and MAY.

          o Made NUD and address resolution use the same retransmission
            timer (which can be specified in the Router Advertisements).
            Increased the default value of this timer from 3 seconds to
            10 seconds.

          o Restricted ReachableTime so that it can not be set to more
            than 1 hour to prevent misconfiguration that would make ND
            not detect e.g. changed link-layer addresses.

          o Added text about the support for links with multiple MTUs
            (e.g. bridged Ethernet and FDDI).  With unmodified bridges
            the routers must send MTU options containing the smaller
            (smallest) MTU.  If the bridges are made aware of IPv6 they
            can participate in path MTU discovery (for unicast and
            multicast) and send ICMP packet too big errors for IPv6
            packets that cross the bridge.

          o Added additional validatity checks for Router Solicitation
            and Router Advertisement messages: the destination address
            must be a link local address or a multicast address with
            link-local scope.



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          o Added validity check for all messages: no routing header is
            allowed.

          o Changed the use of the "designated address" term to using
            "link-local address".

          o Clarified authentication header text.

          o Clarified how multicast packets are handled in the
            conceptual model.

          o Added text that ND messages are themselves not subject to
            NUD probes.  This avoids an observed problem where a NUD
            NS/NA exchange would result in a subsequent NUD NS/NA
            exchange of packets.

          o Clarified that the neighbor cache entries generated by
            unsolicited information (RS, RA, NA, Redirect) do still get
            DELAY_FIRST_PROBE_TIME seconds before a probe is sent (in
            order to benefit from upper-layer advise).

          o Solicited Proxy/anycast advertisements are delayed 0-1
            second to avoid creating a load on the network and/or
            receiver.



























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