rfc1970.txt

来自「RFC 的详细文档！」· 文本 代码 · 共 1,467 行 · 第 1/5 页

   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.

   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



Narten, Nordmark & Simpson  Standards Track                     [Page 6]

RFC 1970       Neighbor Discovery for IP Version 6 (IPv6)    August 1996


                 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.).
                    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



Narten, Nordmark & Simpson  Standards Track                     [Page 7]

RFC 1970       Neighbor Discovery for IP Version 6 (IPv6)    August 1996


                    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

   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



Narten, Nordmark & Simpson  Standards Track                     [Page 8]

RFC 1970       Neighbor Discovery for IP Version 6 (IPv6)    August 1996


                 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 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

   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



Narten, Nordmark & Simpson  Standards Track                     [Page 9]

RFC 1970       Neighbor Discovery for IP Version 6 (IPv6)    August 1996


        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.

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

3.  PROTOCOL OVERVIEW

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

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

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

    Parameter Discovery: How a node learns 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.




Narten, Nordmark & Simpson  Standards Track                    [Page 10]

RFC 1970       Neighbor Discovery for IP Version 6 (IPv6)    August 1996


    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.