rfc1933.txt

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                The datalink address is the datalink address of the end
                node.

          2.2)  If the destination is located off-link, then:

                2.2.1)  If there is an IPv4 router on an attached link,
                        then send an IPv6 packet encapsulated in IPv4.
                        The IPv6 destination address is the address of
                        the end node.  The IPv4 destination address is
                        the low-order 32-bits of the end node's address.
                        The datalink address is the datalink address of
                        the IPv4 router.

                2.2.2)  Else, if there is an IPv6 router on an attached
                        link, then send an IPv6 format packet.  The IPv6
                        destination address is the IPv6 address of the
                        end node.  The datalink address is the datalink
                        address of the IPv6 router.

                2.2.3)  Else, the destination is treated as
                        "unreachable" because it is located off-link and
                        there are no on-link routers.




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   3)   If the address of the end node is an IPv6-only address, then:

          3.1)  If the destination is located on an attached link, then
                send an IPv6 format packet.  The IPv6 destination
                address is the IPv6 address of the end node.  The
                datalink address is the datalink address of the end
                node.

          3.2)  If the destination is located off-link, then:

                3.2.1)  If there is an IPv6 router on an attached link,
                        then send an IPv6 format packet.  The IPv6
                        destination address is the IPv6 address of the
                        end node.  The datalink address is the datalink
                        address of the IPv6 router.

                3.2.2)  Else, if the destination is reachable via a
                        configured tunnel, and there is an IPv4 router
                        on an attached link, then send an IPv6
                        packet encapsulated in IPv4.  The IPv6
                        destination address is the address of the end
                        node.  The IPv4 destination address is the
                        configured IPv4 address of the tunnel endpoint.
                        The datalink address is the datalink address of
                        the IPv4 router.

                3.2.3)  Else, the destination is treated as
                        "unreachable" because it is located off-link and
                        there are no on-link IPv6 routers.

A summary of these sending rules are given in the table below:




















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End         | End     | IPv4    | IPv6    | Packet |      |      |
Node        | Node    | Router  | Router  | Format | IPv6 | IPv4 | DLink
Address     | On      | On      | On      | To     | Dest | Dest | Dest
Type        | Link?   | Link?   | Link?   | Send   | Addr | Addr | Addr
------------+---------+---------+---------+--------+------+------+------
IPv4        | Yes     |  N/A    |  N/A    | IPv4   |  N/A |  E4  | EL
------------+---------+---------+---------+--------+------+------+------
IPv4        | No      |  Yes    |  N/A    | IPv4   |  N/A |  E4  | RL
------------+---------+---------+---------+--------+------+------+------
IPv4        | No      |  No     |  N/A    | UNRCH  |  N/A |  N/A | N/A
------------+---------+---------+---------+--------+------+------+------
IPv4-compat | Yes     |  N/A    |  N/A    | IPv6   |  E6  |  N/A | EL
------------+---------+---------+---------+--------+------+------+------
IPv4-compat | No      |  Yes    |  N/A    | IPv6/4 |  E6  |  E4  | RL
------------+---------+---------+---------+--------+------+------+------
IPv4-compat | No      |  No     |  Yes    | IPv6   |  E6  |  N/A | RL
------------+---------+---------+---------+--------+------+------+------
IPv4-compat | No      |  No     |  No     | UNRCH  |  N/A |  N/A | N/A
------------+---------+---------+---------+--------+------+------+------
IPv6-only   | Yes     |  N/A    |  N/A    | IPv6   |  E6  |  N/A | EL
------------+---------+---------+---------+--------+------+------+------
IPv6-only   | No      |  N/A    |  Yes    | IPv6   |  E6  |  N/A | RL
------------+---------+---------+---------+--------+------+------+------
IPv6-only   | No      |  Yes    |  No     | IPv6/4 |  E6  |  T4  | RL
------------+---------+---------+---------+--------+------+------+------
IPv6-only   | No      |  No     |  No     | UNRCH  |  N/A |  N/A | N/A
------------+---------+---------+---------+--------+------+------+------

        Key to Abbreviations
        --------------------
        N/A:    Not applicable or does not matter.
        E6:     IPv6 address of end node.
        E4:     IPv4 address of end node (low-order 32-bits of
                IPv4-compatible address).
        EL:     Datalink address of end node.
        T4:     IPv4 address of the tunnel endpoint.
        R6:     IPv6 address of router.
        R4:     IPv4 address of router.
        RL:     Datalink address of router.
        IPv4:   IPv4 packet format.
        IPv6:   IPv6 packet format.
        IPv6/4: IPv6 encapsulated in IPv4 packet format.
        UNRCH:  Destination is unreachable.  Don't send a packet.








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4.4.1  On/Off Link Determination

   Part of the process of determining what packet format to use includes
   determining whether a destination is located on an attached link or
   not.  IPv4 and IPv6 employ different mechanisms.  IPv4 uses an
   algorithm in which the destination address and the interface address
   are both logically ANDed with the netmask of the interface and then
   compared.  If the resulting two values match, then the destination is
   located on-link.  This algorithm is discussed in more detail in
   Section 3.3.1.1 of the host requirements specification [10].  IPv6
   uses the neighbor discovery algorithm described in "Neighbor
   Discovery for IP Version 6" [7].

   IPv6/IPv4 nodes need to use both methods:

   -    If a destination is an IPv4 address, then the on/off link
        determination is made by comparison with the netmask, as
        described in RFC 1122 section 3.3.1.1.

   -    If a destination is represented by an IPv4-compatible IPv6
        address (prefix 0:0:0:0:0:0), the decision is made using the
        IPv4 netmask comparison algorithm using the low-order 32-bits
        (IPv4 address part) of the destination address.

  -     If the destination is represented by an IPv6-only address
        (prefix other than 0:0:0:0:0:0), the on/off link determination
        is made using the IPv6 neighbor discovery mechanism.

5. Acknowledgements

   We would like to thank the members of the IPng working group and the
   IPng transition working group for their many contributions and
   extensive review of this document.  Special thanks to Jim Bound, Ross
   Callon, and Bob Hinden for many helpful suggestions and to John Moy
   for suggesting the IPv4 "anycast address" default tunnel technique.

6.  Security Considerations

   Security issues are not discussed in this memo.












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7. Authors' Addresses

   Robert E. Gilligan
   Sun Microsystems, Inc.
   2550 Garcia Ave.
   Mailstop UMTV 05-44
   Mountain View, California 94043

   Phone: [phone]
   Fax:   [phone]
   EMail: [e-mail]


   Erik Nordmark
   Sun Microsystems, Inc.
   2550 Garcia Ave.
   Mailstop UMTV 05-44
   Mountain View, California 94043

   Phone: [phone]
   Fax:   [phone]
   EMail: [e-mail]

7. References

   [1] Croft, W., and J. Gilmore, "Bootstrap Protocol", RFC 951,
       September 1985.

   [2] Droms, R., "Dynamic Host Configuration Protocol", RFC 1541.
       October 1993.

   [3] Bound, J., "Dynamic Host Configuration Protocol for IPv6 for IPv6
       (DHCPv6)", Work in Progress, November 1995.

   [4] Deering, S., and R. Hinden, "Internet Protocol, Version 6 (IPv6)
       Specification", RFC 1883, December 1995.

   [5] Thomson, S., and T. Nartan, "IPv6 Stateless Address
       Autoconfiguration, Work in Progress, December 1995.

   [6] Thomson, S., and C. Huitema. "DNS Extensions to support IP
       version 6", RFC 1886, December 1995.

   [7] Nartan, T., Nordmark, E., and W. Simpson, "Neighbor Discovery for
       IP Version 6 (IPv6)", Work in Progress, November 1995.

   [8] Mogul, J., and S. Deering, "Path MTU Discovery", RFC 1191,
       November 1990.



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   [9] Finlayson, R., Mann, T., Mogul, J., and M. Theimer, "Reverse
       Address Resolution Protocol", RFC 903, June 1984.

  [10] Braden, R., "Requirements for Internet Hosts - Communication
       Layers", STD 3, RFC 1122, October 1989.

  [11] Kent, C., and J. Mogul, "Fragmentation Considered Harmful".  In
       Proc.  SIGCOMM '87 Workshop on Frontiers in Computer
       Communications Technology.  August 1987.










































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