rfc2461.txt

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                 processed properly by the receiving IP layer.  For
                 neighboring routers, reachability means that packets
                 sent by a node's IP layer are delivered to the
                 router's IP layer, and the router is indeed
                 forwarding packets (i.e., it is configured as a
                 router, not a host).  For hosts, reachability means
                 that packets sent by a node's IP layer are delivered
                 to the neighbor host's IP layer.

   packet      - an IP header plus payload.

   link MTU    - the maximum transmission unit, i.e., maximum packet
                 size in octets, that can be conveyed in one piece
                 over a link.

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

   proxy       - a router that responds to Neighbor Discovery query
                 messages on behalf of another node.  A router acting
                 on behalf of a mobile node that has moved off-link
                 could potentially act as a proxy for the mobile
                 node.

   ICMP destination unreachable indication
               - an error indication returned to the original sender
                 of a packet that cannot be delivered for the reasons
                 outlined in [ICMPv6].  If the error occurs on a node
                 other than the node originating the packet, an ICMP
                 error message is generated.  If the error occurs on
                 the originating node, an implementation is not
                 required to actually create and send an ICMP error
                 packet to the source, as long as the upper-layer
                 sender is notified through an appropriate mechanism
                 (e.g., return value from a procedure call).  Note,
                 however, that an implementation may find it
                 convenient in some cases to return errors to the
                 sender by taking the offending packet, generating an
                 ICMP error message, and then delivering it (locally)
                 through the generic error handling routines.





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   random delay
               - when sending out messages, it is sometimes necessary to
                 delay a transmission for a random amount of time in
                 order to prevent multiple nodes from transmitting at
                 exactly the same time, or to prevent long-range
                 periodic transmissions from synchronizing with each
                 other [SYNC].  When a random component is required, a
                 node calculates the actual delay in such a way that the
                 computed delay forms a uniformly-distributed random
                 value that falls between the specified minimum and
                 maximum delay times.  The implementor must take care to
                 insure that the granularity of the calculated random
                 component and the resolution of the timer used are both
                 high enough to insure that the probability of multiple
                 nodes delaying the same amount of time is small.

   random delay seed
               - If a pseudo-random number generator is used in
                 calculating a random delay component, the generator
                 should be initialized with a unique seed prior to being
                 used.  Note that it is not sufficient to use the
                 interface token alone as the seed, since interface
                 tokens will not always be unique.  To reduce the
                 probability that duplicate interface tokens cause the
                 same seed to be used, the seed should be calculated
                 from a variety of input sources (e.g., machine
                 components) that are likely to be different even on
                 identical "boxes".  For example, the seed could be
                 formed by combining the CPU's serial number with an
                 interface token.

2.2.  Link Types

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

   multicast      - a link that supports a native mechanism at the
                    link layer for sending packets to all (i.e.,
                    broadcast) or a subset of all neighbors.

   point-to-point - a link that connects exactly two interfaces.  A
                    point-to-point link is assumed to have multicast
                    capability and have a link-local address.

   non-broadcast multi-access (NBMA)
                  - a link to which more than two interfaces can attach,
                    but that does not support a native form of multicast
                    or broadcast (e.g., X.25, ATM, frame relay, etc.).



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                    Note that all link types (including NBMA) are
                    expected to provide multicast service for IP (e.g.,
                    using multicast servers), but it is an issue for
                    further study whether ND should use such facilities
                    or an alternate mechanism that provides the
                    equivalent ND services.

   shared media   - a link that allows direct communication among a
                    number of nodes, but attached nodes are configured
                    in such a way that they do not have complete prefix
                    information for all on-link destinations.  That is,
                    at the IP level, nodes on the same link may not know
                    that they are neighbors; by default, they
                    communicate through a router.  Examples are large
                    (switched) public data networks such as SMDS and B-
                    ISDN.  Also known as "large clouds".  See [SH-
                    MEDIA].

   variable MTU   - a link that does not have a well-defined MTU (e.g.,
                    IEEE 802.5 token rings).  Many links (e.g.,
                    Ethernet) have a standard MTU defined by the link-
                    layer protocol or by the specific document
                    describing how to run IP over the link layer.

   asymmetric reachability
                  - a link where non-reflexive and/or non-transitive
                    reachability is part of normal operation.  (Non-
                    reflexive reachability means packets from A reach B
                    but packets from B don't reach A.  Non-transitive
                    reachability means packets from A reach B, and
                    packets from B reach C, but packets from A don't
                    reach C.)  Many radio links exhibit these
                    properties.

2.3.  Addresses

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

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

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





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   solicited-node multicast address
               - a link-local scope multicast address that is computed
                 as a function of the solicited target's address.  The
                 function is described in [ADDR-ARCH]. The function is
                 chosen so that IP addresses which differ only in the
                 high-order bits, e.g., due to multiple high-order
                 prefixes associated with different providers, will map
                 to the same solicited-node address thereby reducing the
                 number of multicast addresses a node must join.

   link-local address
               - a unicast address having link-only scope that can be
                 used to reach neighbors.  All interfaces on routers
                 MUST have a link-local address.  Also, [ADDRCONF]
                 requires that interfaces on hosts have a link-local
                 address.

   unspecified address
               - a reserved address value that indicates the lack of an
                 address (e.g., the address is unknown).  It is never
                 used as a destination address, but may be used as a
                 source address if the sender does not (yet) know its
                 own address (e.g., while verifying an address is unused
                 during address autoconfiguration [ADDRCONF]).  The
                 unspecified address has a value of 0:0:0:0:0:0:0:0.

2.4.  Requirements

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [KEYWORDS].

   This document also makes use of internal conceptual variables to
   describe protocol behavior and external variables that an
   implementation must allow system administrators to change.  The
   specific variable names, how their values change, and how their
   settings influence protocol behavior are provided to demonstrate
   protocol behavior.  An implementation is not required to have them in
   the exact form described here, so long as its external behavior is
   consistent with that described in this document.

3.  PROTOCOL OVERVIEW

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





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

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

     Parameter Discovery: How a node learns such link parameters as the
                link MTU or such Internet parameters as the hop limit
                value to place in outgoing packets.

     Address Autoconfiguration: How nodes automatically configure an
                address for an interface.

     Address resolution: How nodes determine the link-layer address of
                an on-link destination (e.g., a neighbor) given only the
                destination's IP address.

     Next-hop determination: The algorithm for mapping an IP destination
                address into the IP address of the neighbor to which
                traffic for the destination should be sent.  The next-
                hop can be a router or the destination itself.

     Neighbor Unreachability Detection: How nodes determine that a
                neighbor is no longer reachable.  For neighbors used as
                routers, alternate default routers can be tried.  For
                both routers and hosts, address resolution can be
                performed again.

     Duplicate Address Detection: How a node determines that an address
                it wishes to use is not already in use by another node.

     Redirect:  How a router informs a host of a better first-hop node
                to reach a particular destination.

   Neighbor Discovery defines five different ICMP packet types: A pair
   of Router Solicitation and Router Advertisement messages, a pair of
   Neighbor Solicitation and Neighbor Advertisements messages, and a
   Redirect message.  The messages serve the following purpose:

     Router Solicitation: When an interface becomes enabled, hosts may
                send out Router Solicitations that request routers to
                generate Router Advertisements immediately rather than
                at their next scheduled time.





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

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

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