rfc1970.txt

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

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

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

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

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

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



Narten, Nordmark & Simpson  Standards Track                    [Page 11]

RFC 1970       Neighbor Discovery for IP Version 6 (IPv6)    August 1996


   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,
   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,



Narten, Nordmark & Simpson  Standards Track                    [Page 12]

RFC 1970       Neighbor Discovery for IP Version 6 (IPv6)    August 1996


   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.

     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.



Narten, Nordmark & Simpson  Standards Track                    [Page 13]

RFC 1970       Neighbor Discovery for IP Version 6 (IPv6)    August 1996


          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
     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



Narten, Nordmark & Simpson  Standards Track                    [Page 14]

RFC 1970       Neighbor Discovery for IP Version 6 (IPv6)    August 1996


     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 (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].







Narten, Nordmark & Simpson  Standards Track                    [Page 15]

RFC 1970       Neighbor Discovery for IP Version 6 (IPv6)    August 1996


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 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.