rfc1970.txt

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                     - 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.




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RFC 1970       Neighbor Discovery for IP Version 6 (IPv6)    August 1996


   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
                    Unit) in order for multicast to work properly.
                    Otherwise when multicasting a sender, which can not
                    know which nodes will receive the packet, could not
                    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.  MESSAGE FORMATS

4.1.  Router Solicitation Message Format

   Hosts send Router Solicitations in order to prompt routers to
   generate Router Advertisements quickly.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     Code      |          Checksum             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Reserved                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Options ...
     +-+-+-+-+-+-+-+-+-+-+-+-

IP Fields:

   Source Address
                  An IP address assigned to the sending interface, or
                  the unspecified address if no address is assigned to
                  the sending interface.

   Destination Address
                  Typically the all-routers multicast address.



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RFC 1970       Neighbor Discovery for IP Version 6 (IPv6)    August 1996


   Hop Limit      255

   Priority       15

   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.

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 MUST be ignored by the receiver.

Valid Options:

   Source link-layer address
                  The link-layer address of the sender, if known.

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

4.2.  Router Advertisement Message Format

   Routers send out Router Advertisement message periodically, or in
   response to a Router Solicitation.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     Code      |          Checksum             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Cur Hop Limit |M|O|  Reserved |       Router Lifetime         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Reachable Time                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Retrans Timer                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Options ...
     +-+-+-+-+-+-+-+-+-+-+-+-




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RFC 1970       Neighbor Discovery for IP Version 6 (IPv6)    August 1996


IP Fields:

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

   Destination Address
                  Typically the Source Address of an invoking Router
                  Solicitation or the all-nodes multicast address.

   Hop Limit      255

   Priority       15

   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.

ICMP Fields:

   Type           134

   Code           0

   Checksum       The ICMP checksum.  See [ICMPv6].

   Cur Hop Limit  8-bit unsigned integer.  The default value that should
                  be placed in the Hop Count field of the IP header for
                  outgoing IP packets.  A value of zero means
                  unspecified (by this router).

   M              1-bit "Managed address configuration" flag.  When set,
                  hosts 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 stateful configuration" flag.  When set,
                  hosts 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 MUST be ignored by the receiver.





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RFC 1970       Neighbor Discovery for IP Version 6 (IPv6)    August 1996


   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 applies only to the router's
                  usefulness as a default router; it does not apply to
                  information contained in other message fields or
                  options.  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
                  having received a reachability confirmation.  Used by
                  the Neighbor Unreachability Detection algorithm (see
                  Section 7.3).  A value of zero means unspecified (by
                  this 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 7.2
                  and 7.3).  A value of zero means unspecified (by this
                  router).

Possible 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 (as
                  specified in the document that describes how to run IP
                  over the particular link type).  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 (except
                  the link-local prefix) 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



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RFC 1970       Neighbor Discovery for IP Version 6 (IPv6)    August 1996


                  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 silently ignore any options they do not recognize and
   continue processing the message.

4.3.  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
   reachability of a neighbor.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     Code      |          Checksum             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Reserved                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                       Target Address                          +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Options ...
     +-+-+-+-+-+-+-+-+-+-+-+-

IP Fields:

   Source Address
                  Either an address assigned to the interface from which
                  this message is sent or (if Duplicate Address
                  Detection is in progress [ADDRCONF]) the unspecified
                  address.

   Destination Address
                  Either the solicited-node multicast address
                  corresponding to the target address, or the target
                  address.

   Hop Limit      255