rfc2461.txt

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

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

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

   Router Advertisements (and per-prefix flags) allow routers to inform
   hosts how to perform Address Autoconfiguration.  For example, routers
   can specify whether hosts should use stateful (DHCPv6) and/or
   autonomous (stateless) address configuration.  The exact semantics
   and usage of the address configuration-related information is
   specified in [ADDRCONF].

   Router Advertisement messages also contain Internet parameters such
   as the hop limit that hosts should use in outgoing packets and,



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   optionally, link parameters such as the link MTU.  This facilitates
   centralized administration of critical parameters that can be set on
   routers and automatically propagated to all attached hosts.

   Nodes accomplish address resolution by multicasting a Neighbor
   Solicitation that asks the target node to return its link-layer
   address.  Neighbor Solicitation messages are multicast to the
   solicited-node multicast address of the target address.  The target
   returns its link-layer address in a unicast Neighbor Advertisement
   message.  A single request-response pair of packets is sufficient for
   both the initiator and the target to resolve each other's link-layer
   addresses; the initiator includes its link-layer address in the
   Neighbor Solicitation.

   Neighbor Solicitation messages can also be used to determine if more
   than one node has been assigned the same unicast address.  The use of
   Neighbor Solicitation messages for Duplicate Address Detection is
   specified in [ADDRCONF].

   Neighbor Unreachability Detection detects the failure of a neighbor
   or the failure of the forward path to the neighbor.  Doing so
   requires positive confirmation that packets sent to a neighbor are
   actually reaching that neighbor and being processed properly by its
   IP layer.  Neighbor Unreachability Detection uses confirmation from
   two sources.  When possible, upper-layer protocols provide a positive
   confirmation that a connection is making "forward progress", that is,
   previously sent data is known to have been delivered correctly (e.g.,
   new acknowledgments were received recently).  When positive
   confirmation is not forthcoming through such "hints", a node sends
   unicast Neighbor Solicitation messages that solicit Neighbor
   Advertisements as reachability confirmation from the next hop.  To
   reduce unnecessary network traffic, probe messages are only sent to
   neighbors to which the node is actively sending packets.

   In addition to addressing the above general problems, Neighbor
   Discovery also handles the following situations:

     Link-layer address change - A node that knows its link-layer
           address has changed can multicast a few (unsolicited)
           Neighbor Advertisement packets to all nodes to quickly update
           cached link-layer addresses that have become invalid.  Note
           that the sending of unsolicited advertisements is a
           performance enhancement only (e.g., unreliable).  The
           Neighbor Unreachability Detection algorithm ensures that all
           nodes will reliably discover the new address, though the
           delay may be somewhat longer.





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

           Load balancing is handled by allowing routers to omit the
           source link-layer address from Router Advertisement packets,
           thereby forcing neighbors to use Neighbor Solicitation
           messages to learn link-layer addresses of routers.  Returned
           Neighbor Advertisement messages can then contain link-layer
           addresses that differ depending on who issued the
           solicitation.

     Anycast addresses - Anycast addresses identify one of a set of
           nodes providing an equivalent service, and multiple nodes on
           the same link may be configured to recognize the same Anycast
           address.  Neighbor Discovery handles anycasts by having nodes
           expect to receive multiple Neighbor Advertisements for the
           same target.  All advertisements for anycast addresses are
           tagged as being non-Override advertisements.  This invokes
           specific rules to determine which of potentially multiple
           advertisements should be used.

     Proxy advertisements - A router willing to accept packets on behalf
           of a target address that is unable to respond to Neighbor
           Solicitations can issue non-Override Neighbor Advertisements.
           There is currently no specified use of proxy, but proxy
           advertising could potentially be used to handle cases like
           mobile nodes that have moved off-link.  However, it is not
           intended as a general mechanism to handle nodes that, e.g.,
           do not implement this protocol.

3.1.  Comparison with IPv4

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

   The Neighbor Discovery protocol provides a multitude of improvements
   over the IPv4 set of protocols:




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      Router Discovery is part of the base protocol set; there is no
      need for hosts to "snoop" the routing protocols.

      Router advertisements carry link-layer addresses; no additional
      packet exchange is needed to resolve the router's link-layer
      address.

      Router advertisements carry prefixes for a link; there is no need
      to have a separate mechanism to configure the "netmask".

      Router advertisements enable Address Autoconfiguration.

      Routers can advertise an MTU for hosts to use on the link,
      ensuring that all nodes use the same MTU value on links lacking a
      well-defined MTU.

      Address resolution multicasts are "spread" over 4 billion (2^32)
      multicast addresses greatly reducing address resolution related
      interrupts on nodes other than the target.  Moreover, non-IPv6
      machines should not be interrupted at all.

      Redirects contain the link-layer address of the new first hop;
      separate address resolution is not needed upon receiving a
      redirect.

      Multiple prefixes can be associated with the same link.  By
      default, hosts learn all on-link prefixes from Router
      Advertisements.  However, routers may be configured to omit some
      or all prefixes from Router Advertisements.  In such cases hosts
      assume that destinations are off-link and send traffic to routers.
      A router can then issue redirects as appropriate.

      Unlike IPv4, the recipient of an IPv6 redirect assumes that the
      new next-hop is on-link.  In IPv4, a host ignores redirects
      specifying a next-hop that is not on-link according to the link's
      network mask.  The IPv6 redirect mechanism is analogous to the
      XRedirect facility specified in [SH-MEDIA].  It is expected to be
      useful on non-broadcast and shared media links in which it is
      undesirable or not possible for nodes to know all prefixes for
      on-link destinations.

      Neighbor Unreachability Detection is part of the base
      significantly improving the robustness of packet delivery in the
      presence of failing routers, partially failing or partitioned
      links and nodes that change their link-layer addresses.  For
      instance, mobile nodes can move off-link without losing any
      connectivity due to stale ARP caches.




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

      Unlike in IPv4 Router Discovery the Router Advertisement messages
      do not contain a preference field.  The preference field is not
      needed to handle routers of different "stability"; the Neighbor
      Unreachability Detection will detect dead routers and switch to a
      working one.

      The use of link-local addresses to uniquely identify routers (for
      Router Advertisement and Redirect messages) makes it possible for
      hosts to maintain the router associations in the event of the site
      renumbering to use new global prefixes.

      Using the Hop Limit equal to 255 trick Neighbor Discovery is
      immune to off-link senders that accidentally or intentionally send
      ND messages.  In IPv4 off-link senders can send both ICMP
      Redirects and Router Advertisement messages.

      Placing address resolution at the ICMP layer makes the protocol
      more media-independent than ARP and makes it possible to use
      standard IP authentication and security mechanisms as appropriate
      [IPv6-AUTH, IPv6-ESP].

3.2.  Supported Link Types

   Neighbor Discovery supports links with different properties.  In the
   presence of certain properties only a subset of the ND protocol
   mechanisms are fully specified in this document:

     point-to-point - Neighbor Discovery handles such links just like
                      multicast links.  (Multicast can be trivially
                      provided on point to point links, and interfaces
                      can be assigned link-local addresses.)  Neighbor
                      Discovery should be implemented as described in
                      this document.

     multicast      - Neighbor Discovery should be implemented as
                      described in this document.

     non-broadcast multiple access (NBMA)
                    - Redirect, Neighbor Unreachability Detection and
                      next-hop determination should be implemented as
                      described in this document.  Address resolution,
                      and the mechanism for delivering Router
                      Solicitations and Advertisements on NBMA links is
                      not specified in this document.  Note that if



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

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

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

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

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

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

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