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Versions: 00                                                            
IPng Transition                                           Dan Harrington
Internet Draft                                   Digital Equipment Corp.
                                                           Quaizar Vohra
                                             University of New Hampshire
                                                           November 1996

         Limiting the role of IPv4-compatible Addresses in IPv6



   This draft presents a proposal to limit IPv4-compatible IPv6
   addresses to tunnelling interfaces in the transition from IPv4 to
   IPv6. The reasons and context for restricting the usage in this
   manner will be presented.

Status of This Memo

   This document is a submission to the NGtrans Working Group of the
   Internet Engineering Task Force (IETF).  Comments should be
   submitted to the ngtrans@sunroof.end.sun.com mailing list.  The
   authors invite discussion and feedback on this topic.

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time.  It is inappropriate to use Internet-Drafts as
   reference material or to cite them other than as ``work in

   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the Internet-Drafts
   Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
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   ftp.isi.edu (US West Coast).

   Distribution of this document is unlimited.

Table Of Contents

       1. Introduction                                                 3
       2. Architectural and Philosophical Issues                       3
       3. Isolated Hosts                                               4
          3.1. Class 1 Isolated Nodes                                  4
          3.2. Class 2 Isolated Nodes                                  4
       4. Other Issues                                                 5
          4.1. Host Issues                                             5

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          4.2. Router Issues                                           5
       5. Acknowledgements                                             6
       6. References                                                   6
       7. Author's Addresses                                           6

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1. Introduction

   IPv4-compatible addresses are designed to ease the transition of
   IPv4 to IPv6, by utilizing the readily available IPv4 address space
   and protocols to provide IPv6 connectivity.  They currently serve
   two roles, both related to tunnelling:

         - To allow isolated IPv6 nodes to come up on the Internet
           and communicate with other IPv6 nodes via automatic tunneling,
           which requires a minimal amount of configuration.

         - Identifying an IPv6 router's next-hop interface address over
           a manually configured tunnel.

   These tasks both require implementations to treat an IPv4 tunnel as
   a pseudo-NBMA link, where ::/96 is treated as an on-link IPv6 prefix
   for the tunnel interface.  In this model, all IPv4-compatible
   addresses are on-link to the tunnel interface and the IPv4 Internet
   forms one large link layer, in which address resolution is a trivial
   function. Manually configured tunnels are used with static routes to
   IPv6 prefixes, where the next-hop is an IPv4-compatible address on
   the link. While this link type does not use the standard link-local
   prefix of FE80:: or Neighbor Discovery protocols, it does have its
   own characteristics and rules [V6TUNNELS].  Conceptually, then, it
   can be seen that IPv6 packets using IPv4-compatible addresses could
   be treated as using a special type of link-local address, and the
   Hop Limit could be set to a value of 1 with no dire consequences.

   The current Transition Mechanisms specification [RFC1933], however,
   also include a provision to allow an IPv4-compatible address to be
   assigned to an interface for native IPv6 communications, with all
   the requirements of Neighbor Discovery.  It is this usage which we
   wish to prohibit, for the sake of reduced complexity and increased

2. Architectural and Philosophical Issues

   Although IPv4 and IPv6 represent different network protocols, IPv4
   addresses can be represented as IPv6 addresses.  However, they still
   define an IPv4 endpoint, that is, an interface on a link connected
   to an IPv4 network, using IPv4 protocols.  Using them in multiple
   fashions, for both IPv4 and IPv6 packets on a given interface as
   well as for tunnelling, can and will lead to interoperability
   problems, as has been reported on the NGTRANS mailing list [NGLIST].
   This dual usage also leads to unnecessary implementation complexity;
   for example, the source address selection algorithm should not
   permit the use of an IPv4-compatible address (as source or
   destination) with a global IPv6 address (as destination or source).

   As mentioned above, the encapsulation of IPv6 packets in IPv4
   packets essentially uses the IPv4 network as a specialized media
   type. The "Generic Packet Tunneling in IPv6" [V6TUNNELS]
   specification gives the mechanism by which one protocol may be run
   over another.  In keeping with the general IP philosophy of an

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   address being associated with a particular interface [RFC1122], it
   should be held that a tunnel interface is not merely an abstraction,
   but a "real" interface to a specific media type, with its own rules
   and behaviours.

   Finally, restricting the usage of IPv4-compatible addresses will
   simplify the definition, implementation, and usage of this address
   form, and smooth the IPv4 to IPv6 transition.  Simple, clear
   definitions are easy to explain; special cases and asterisks are
   not.  If IPv6 is to be widely accepted and deployed, the training
   and educational aspects of the architecture must not be ignored.

3. Isolated Hosts

   Two interpretations of the term "Isolated Host" have been proposed
   in the course of discussing IPv4-compatible address usage. Both  are
   presented below, and hopefully these definitions can be clarified,
   and consensus reached, through further discussion.

3.1. Class 1 Isolated Nodes

   The first interpretation of an isolated host is a host which does
   not have an on-link IPv6 router, and which thus must encapsulate all
   packets to off-link destinations.  But this node is connected to an
   IPv6-capable Internet Service Provider (ISP) and thus has a provider
   based IPv6 address [RFC1897][V6PROVIDER], which we will refer to as
   PBA. This PBA is assigned to the tunnel interface and is used as
   source address in outgoing packets. The node has a manually
   configured tunnel to an ISP router. This PBA is based upon the ISP's
   prefix and the IPV4 address of the IPv4 interface through which the
   encapsulated packets get forwarded to the ISP.  Note that the
   IPv4-compatible might be usable as the link-local address in a
   routing protocol, but this is yet to be determined.

   So this isolated node has global IPv6 connectivity via the ISP. This
   isolated node has a default IPv6 route (::/0) with the ISP router as
   next-hop, which may be identifed by an IPv4-compatible address.
   Examples of this class of isolated node can be found on the current
   6-bone. [6BONE]

3.2. Class 2 Isolated Nodes

   The second form of isolated nodes are those nodes which are not
   connected to an IPv6-capable ISP, i.e. they don't have a PBA. All
   they have is an IPv4-compatible address and they communicate with
   other IPv6 nodes which have IPv4-compatible addresses using
   end-to-end automatic tunneling.  This requires that the destination
   node also has an IPv4-compatible address, and implies that the
   packet will make a single hop (i.e. the IPv6 packet will not be

   In the evolution of the 6bone, the second class of host is not
   represented.  It remains to be seen how common this type of host
   will be as IPv6 is deployed commercially. For these nodes to

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   communicate with other IPv6 nodes on the Internet, the remote IPv6
   system must have automatic tunneling enabled on every IPv6 node on
   the Internet. At some point in transition, when the IPv4 address
   space is exhausted, new IPv6 nodes will not be able to get
   IPv4-compatible addresses to do automatic tunneling.  These nodes
   will only have PBAs and would not be able to communicate with class
   2 isolated nodes. So while this class of system represents a simple
   configuration, it can be seen that from the beginning these nodes
   may only be able to communicate with a subset of the IPv6 network,
   and the percentage of unreachable hosts will likely increase over
   time. Also, the extensive use of IPv4-compatible addresses for
   communications between IPv6 systems will exercise the IPv4 routing
   infrastructure, without promoting the use of IPv6 hierarchical
   routing, thereby taxing an overburdened service without any gain in
   operational experience in the new technology.

4. Other Issues

   One important issue is whether IPv4-compatible addresses should be
   assigned to all physical interfaces having IPv4 addresses. We
   believe that this is not a good idea as it creates several problems
   without  being a solution for any existing problem.  There are other
   issues to consider as well.

   Another disadvantage is that IPv4-compatible addresses will have to
   be treated specially in name services like DNS and DHCP, with
   duplication of data and potential operational confusion resulting.

4.1. Host Issues

   Hosts may have to deal with multiple mechanisms for obtaining
   addresses, and support dual address lifetime (or lease) constructs.
   While DHCP is commonly used to obtain IPv4 addresses,  DHCPv6 does
   not support the assignment of IPv4-compatible addresses, and thus
   the server will not recognize such addresses as belonging to any
   given client.  [DHCPv6]

   Also, assigning an IPv4-compatible address to the interface on which
   IPv4 is running may not be generally possible.  For example, an IPv4
   host using SLIP could support an IPv6 implementation using
   tunnelling, but not a native interface.  There may be other examples
   of media types which support one protocol but not the other.

4.2. Router Issues

   In addition to the issues presented above, which focus largely on
   the impact to IPv6 hosts, there are various concerns related to dual
   IPv6/IPv4 routers.  In the current RFC 1933 model, dual protocol
   routers at the borders of IPv6 islands may be called upon to perform
   routing of packets using IPv4-compatible source and destination
   addresses.  There are several reasons why this is not a good idea:

    - While encapsulation of IPv6 packets in IPv4 tunnels will be a
      necessary function of dual IPv4/IPv6 routers, it would be best

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      to reduce the need for this function by having the originating
      host use automatic tunnelling.

    - The routers may have greater memory requirements than otherwise.
      See the draft "IPv6 Routing Table Issues" [RJA] for details.

5. Acknowledgements

   The authors wish to thank Pedro Roque, Jim Bound, Ran Atkinson, Bill
   Lenharth and Matt Thomas for their input and consideration, as well
   as the growing community of IPv6 developers.

6. References

   [RFC1933]    R. Gilligan, E. Nordmark, "Transition Mechanisms for
                IPv6 Hosts and Routers", RFC 1933, April 1996.

   [V6TUNNELS]  A. Conta, S. Deering, "Generic Packet Tunneling in IPv6",
                <draft-ietf-ipngwg-ipv6-tunnel-04.txt>, Work in Progress,
                October 1996.

   [RFC1122]    R. Braden, "Requirements for Internet Hosts - Communication
                Layers", RFC 1122, October 1989.

   [NGLIST]     Interoperability problem described on ngtrans mailing
                list, Wednesday March 13, 1996.

   [RFC1897]    R. Hinden, "IPv6 Testing Address Allocation", RFC 1897,
                January 1996.

   [V6PROVIDER] Y. Rekhter et al, "An IPv6 Provider-Based Unicast Address
                Format", <draft-ietf-ipngwg-unicast-addr-fmt-04.txt>,
                Work in Progress, March 1996.

   [6BONE]              http://www-cnr.lbl.gov/6bone

   [DHCPv6]     J. Bound, C. Perkins, "Dynamic Host Configuration Protocol
                for IPv6 (DHCPv6)", <draft-ietf-dhc-dhcpv6-07.txt>,
                Work in Progress, August 1996.

   [RJA]                R. Atkinson, "IPv6 Routing Table Size Issues",
                <draft-ietf-ipngwg-ipv6-routing-00.txt>, Work in
                Progress, October 1996.

7. Author's Addresses

        Dan Harrington
        P.O. Box 81W
        W. Townsend, MA

        Quaizar Vohra
        Interoperability Lab
        7 Leavitt Lane

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        University of New Hampshire
        Durham, NH  03824

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