INTERNET-DRAFT                                                 Jim Bound
NGTRANS Tools Working Group                                       Compaq
Obsoletes draft-toutain-dstm-00.txt                      Laurent Toutain
Expires April 2000                                          Hossam Afifi
                                                           ENST Bretagne

                 Dual Stack Transition Mechanism (DSTM)


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Distribution of this memo is unlimited.


   The initial deployment of IPv6 will require a tightly coupled use of
   IPv4 addresses to support the interoperation of IPv6 and IPv4, within
   an IPv6 Network.  Nodes will be able to be deployed with IPv6
   addresses, but will still need to communicate with IPv4 nodes that do
   not have a dual IP layer supporting both IPv4 and IPv6.  The Dual
   Stack Transition Method (DSTM) provides a set of mechanisms to assign
   temporary Global IPv4 Addresses to IPv6 nodes, use of dynamic tunnels
   within an IPv6 Network to carry IPv4 traffic, and a defined set of
   processes and architecture for the supporting infrastructure required
   for this transition mechanism.

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Table of Contents:

1. Introduction.................................................3
2. Terminology..................................................4
2.1 IPv6 AIIH Terminology.......................................4
2.2 Specification Language......................................4
3. DSTM Overview................................................5
4. Scenarios....................................................7
4.1 IPv6 node to an IPv4 node...................................8
4.2 IPv4 node to an IPv6 node...................................9
4.3 IPv4 compiled application between to IPv6 nodes............10
5. AIIH Server Design Model....................................10
5.1 AIIH DHCPv6/DNS Server.....................................10
5.1.1. Requesting an IPv4 Global Address.......................11
5.1.2 AIIH DHCPv6 Client IPv4 Global Address Requests..........12
5.1.3 AIIH DNS Query and DHCPv6 Processing.....................12
5.1.4. Cleaning up the AIIH IPv4 Assigned Address..............13
5.2 Links with other DNS.......................................13
6. DTI.........................................................14
6.1. DTI Architecture..........................................14
6.1 Assignment of the IPv4 address to the DTI..................14
6.2 Encapsulation of IPv4 packets..............................15
6.2.1 IPv6 source address......................................15
6.2.2 IPv6 destination address.................................15 Dynamic TEP............................................15 Static TEP.............................................16
7. AIIH DHCPv6 Requirements....................................17
7.1 DHCPv6 IPv4 Global Address Extension.......................17
7.2 AIIH Server Processing of an IPv4 Global Address Extension.17
7.3 AIIH Client Processing of an IPv4 Global Address Extension.18
8. Security Considerations.....................................19
9. Year 2000 Considerations....................................19
Appendix A:  DSTM Discussion and Issues........................19

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1. Introduction

The initial deployment of IPv6 will require a tightly coupled use of
IPv4 addresses to support the interoperation of IPv6 and IPv4, within an
IPv6 Network.  Nodes will be able to be deployed with IPv6 addresses,
but will still need to communicate with IPv4 nodes that do not have a
dual IP layer supporting both IPv4 and IPv6.  The Dual Stack Transition
Method (DSTM) provides a set of mechanisms to assign temporary Global
IPv4 Addresses to IPv6 nodes, use of dynamic tunnels within an IPv6
Network to carry IPv4 traffic, and a defined set of processes and
architecture for the supporting infrastructure required for this
transition mechanism.  In the dual stack approach defined in RFC 1933,
every node needs both an IPv4 and an IPv6 address to exchange
information with the IPv4 and the IPv6 world. Use of the dual stack
approach can be acceptable during a short period for testing IPv6
applications and initial network deployment, but does not scale since it
does not solve the lack of IPv4 addresses, once IPv6 begins production

The DSTM assigns, when needed an IPv4 address to an IPv6 host. This will
allow either IPv6 hosts to communicate with IPv4-only hosts, or for
IPv4-only applications to run without modification on an IPv6 host. This
allocation mechanism is coupled with the ability to perform dynamic
tunneling of an IPv4 packet inside an IPv6 packet, to suppress the
exposure of IPv4 native packets within some areas of an IPv6 network.
This will simplify the network management of IPv6 deployment, since
routers need only IPv6 routing tables to move IPv4 packets across an
IPv6 network.

DSTM is targeted to help the interoperation of IPv6 newly deployed
networks with existing IPv4 networks. The main theme of DSTM is to avoid
situations where the introduction of IPv6 in a network, is delayed
because IPv6 will have to interoperate with IPv4 networks and
applications for some time.

DSTM is composed of a DHCPv6 server coupled with a DNS server (called
AIIH server, for Assignment of IPv4 Global Addresses to IPv6 Hosts).
This server will allocate temporary IPv4 addresses to IPv6 hosts using
DHCPv6. This server will also be used to maintain the mapping between
the allocated IPv4 address and the permanent IPv6 address of the host.
Every IPv6 host will have an IPv4 interface called DTI (Dynamic
Tunneling Interface) designed to encapsulate IPv4 packets into IPv6
packets and resolve the address space mechanics, between IPv4 and IPv6.

The specification will begin by defining the terminology (section 2),
then section 3 provides a technical overview of the DSTM methodology as
a transition mechanism.  Then in section 4 we discuss three scenarios
depicting the use of DSTM mechanisms in different configuration
settings. Section 5 describes the relation between DHCPv6 and DNS
Servers, which constitutes the AIIH Server.  Section 6 discusses the DTI
architecture and mechanisms. Section 7 discusses the DHCPv6 extension

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2. Terminology

2.1 IPv6 AIIH Terminology

   DSTM Domain             The network areas on an Intranet where an
                           AIIH Server has access to IPv6 nodes
                           in DSTM for that network.

   DSTM Border Router      A borderd router within a DSTM domain and
                           an IPv4-ONLY domain (Internet or Intranet).

   IPv6 Protocol Terms:    See [3]

   IPv6 Transition Terms:  See [15]

   DHCPv6 Terms:           See [4,5]

   DTI:                    Dynamic Tunneling Interface. An interface
                           encapsulating IPv4 packets into IPv6 packets.

   AIIH Server:            A Server that supports DNS [2] and DHCPv6 [4,5]
                           and communications between DNS and DHCPv6, which
                           is implementation defined.

   IPv4 Global Address:    An IPv4 address that is globally routable on
                           the Internet.

   Tunnel End Point (TEP)  Destination of the IPv6 packet containing an
                           IPv4 packet.

2.2 Specification Language

   In this document, several words are used to signify the requirements
   of the specification, in accordance with RFC 2119 [9]. These words
   are often capitalized.

      MUST          This word, or the adjective "required", means that
                    the definition is an absolute requirement of the

      MUST NOT      This phrase means that the definition is an absolute
                    prohibition of the 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 must be understood and carefully
                    weighed before choosing a different course.
                    Unexpected results may result otherwise.

      MAY           This word, or the adjective "optional", means that
                    this item is one of an allowed set of alternatives.

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                    An implementation which does not include this option
                    MUST be prepared to interoperate with another
                    implementation which does include the option.

      silently discard
                    The implementation discards the packet without
                    further processing, and without indicating an error
                    to the sender. The implementation SHOULD provide
                    the capability of logging the error, including the
                    contents of the discarded packet, and SHOULD record
                    the event in a statistics counter.

3. DSTM Overview

DSTM as discussed in the introduction are a set of mechanisms which use
existing protocols to support the operations within DSTM.  DSTM does not
specify a protocol, except for defining some new DHCPv6 Extensions for

The reason for DSTM is to provide IPv6 nodes a means to acquire an IPv4
address, for communications with IPv4-only nodes or IPv4 applications.

The core assumption within this mechanism is that the user does not want
to use translation as a mechanism when an IPv6 node, which has also an
IPv4 stack, to communicate with an IPv4-only node or an IPv4
application.  It is the authors viewpoint that the user in this case,
has deployed IPv6 to support end-2-end computing, without translation.

The DSTM model is as follows:

  - The DSTM domain is within an Intranet not on the Internet.

  - IPv6 nodes do not maintain IPv4 addresses except on a temporary basis,
    to communicate with IPv4-only and IPv4 Applications.

  - Standard DNS is used to cause access to a DNS server that will
    know the request is an IPv4 address for an IPv6 node above.

  - Standard DHCPv6 is used to support the extensions to provide
    and accept from DHCPv6 clients Global IPv4 Addresses.

  - The network for the IPv6 nodes would like to keep IPv4 routing
    tables to a minimum and use IPv6 routing whenever possible,
    as an initial transition mechanism for IPv6.

  - Once IPv6 nodes have IPv4 addresses Dynamic Tunneling is used
    to move the IPv4 packet within IPv6 to an IPv6 TEP, where the
    packet will be forwarded using IPv4.  DHCPv6 is used to provide
    TEPs to IPv6 nodes supporting DTI.

  - Implementation defined software must exist within DSTM to support
    the following processes:

    o  Software on a DNS implementation to inform a DHCPv6 server
       that a request is being made for an IPv4 address for an
       IPv6 node.  This specifications initial assumption is that

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       the DNS and DHCPv6 are co-located on the same node. This
       eliminates the need for a network protocol for DHCPv6
       and DNS to communicate over a wireless or wired medium.

    o  Software on a DHCPv6 implementation to support speaking
       with a DNS implementation for the above purposes. In addition
       software within DHCPv6 to maintain configuration information
       about tunnel endpoints for encapsulating IPv4 packets between
       IPv6 nodes that can forward IPv4 packets to an IPv4 routing

    o  The DHCPv6 and DNS processes and implementation defined parts
       above are collectively named the AIIH Server in this model
       and the specification.

    o  Software within an IPv6 node to support the dynamic tunneling
       mechanisms in this specification to encapsulate IPv4 packets
       within IPv6 to send IPv4 packets on an IPv6 node.  In addition
       a daemon must exist to access an AIIH server for addresses
       and TEPs.

    o  Software in DSTM domain routers to recall or be able to cache
       the association of IPv6 and IPv4 addresses of nodes during
       decapsulation and encapsulation.

A simplistic overview of DSTM is as follows:

                                              |    IPv4 Internet/Intranet
        DSTM Domain Intranet                  |      IPv4 Applications
                                              |         Domain
                _____________________         |
               |                     |        |
               |    AIIH Server      |        |
               |_____________________|        |
                             ^    ^           |
                             |    |           |
   __________________        |    |          _|_______
  |                  |       |    |         |        |
  | IPv6/IPv4 Node   |       |    --------->|  DSTM  |
  |------------------|       |              | Router |
  |  AIIH Daemon     |<-------              |  IPv6  |
  |------------------|                      |    &   |
  |    DTI/Route     |<-------------------->|  IPv4  |
   -------------------                       ---------

Both the IPv6/IPv4 node and the DSTM Router have occasion to access the
AIIH Server.  For more depth please read the following sections of this

For an IPv6 host to participate in the AIIH mechanism it MUST have a
dual IP layer, supporting both an IPv4 and an IPv6 stack. This
specification makes the assumption that for IPv6 initial deployment host
nodes will not be shipped with an IPv6-only stack implementation. For
embedded system type nodes that support only an IPv6 stack, AIIH cannot

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be a solution.

4. Scenarios

These different scenarios illustrate interoperability problems occurring
during the interoperation between IPv4 and IPv6. IPv6 end nodes have a
dual stack, but only the IPv6 stack is configured through an IPv6 auto-
configuration mechanisms. Intermediary routers have only an IPv6 routing
table configured.

In the example below, the following notation will be used:

   X    will designate an IPv6 host with a dual stack, X6 will be the IPv6
        address of this host and X4 the IPv4 address
   Y    will designate a DSTM border router at the boundary between an
        IPv6 DSTM domain and an IPv4-only domain.
   Z    will designate an IPv4-only host and Z4 its address.
   ==>  means an IPv6 packet
   -->  means an IPv4 packet
   ++>  means a tunneled IPv4 packet is encapsulated in an IPv6 packet
   ..>  means a DNS query or response. The path taken by this
        packet does not matter in the examples
   "a"  means the DNS name of a host

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4.1 IPv6 node to an IPv4 node

This scenario describes the case where an application (either compiled
for the IPv6 or IPv4 API) running on an IPv6 host (X6) wants to
establish a session with an IPv4 application on an IPv4-only host (Z4).

The IPv6 host is configured with the IPv6 address of a tunnel end-point,
where an IPv4 encapsulated packet will be sent.

The IPv4 routing table of node X is configured to send IPv4 packets to
the DTI interface.

         AIIH         TB     Z4
   X6          Y6/Y4
    |            |            |
    |. . . . . . . .>    Z    |    - X6 asks the DNS for an AAAA for "Z"
    |<. . . . . . . .   error |    - the DNS answers with an error
    |. . . . . . . .>    Z    |    - X6 asks for the A RR for "Z"
    |<. . . . . . . .    Z4   |    - the answer is Z4
    |            |            |
    |            |            |    - The application sends its first IPv4
    |            |            |      packet which arrives to the DTI
    |            |            |      (If the application is compiled for
    |            |            |      this can be done through an IPv4-mapped
    |            |            |      address).
    |            |            |
    |            |            |    - X6 needs an IPv4 address (first use)
    |====>       |            |    - X6 queries the AIIH server for an
    |            |            |      IPv4 address using DHCPv6
    |<====       |            |    - The DHCPv6 server locates the client
    |            |            |      and provides temporarily an IPv4
    |            |            |      address.
    |            |            |    - the DHCPv6 Server sends a Dynamic
    |            |            |      to the DNS to register the association
    |            |            |      x4<->x6.
    |            |            |
    |+++++++++++>|            |    - The DTI sends the IPv6 packet to the
    |            |            |      tunnel end-point
    |            |----------->|    - Y sends the packet to the destination
    |            |            |    - Y MAY cache the association between
    |            |            |      the IPv4 and IPv6 address of X.

When Z answers two cases are possible either the packet comes back
through Y and Y has cached the association between the IPv4 and the IPv6
address of X, or the packet arrives through another router within the
IPv6 network.

For the first case, Y simply encapsulates the IPv4 packet inside an IPv6
packet to X6. If the IPv4 packet size is greater than the MTU inside the
IPv6 DSTM domain, the packet is fragmented, if the IPv4 DF bit is set to
0. Otherwise an ICMP message is sent to Z4.

The second case corresponds to the scenario described in the next
scenario, when an IPv4 packet arrives at the DSTM border router within
a DSTM domain.

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4.2 IPv4 node to an IPv6 node

This example covers any scenario where an IPv4-only host wants to
establish a session with an IPv6 host, which does not have an IPv4

No modification can be made to the IPv4 host or to the application,
especially the IPv4-application cannot be recompiled.

   DNSv4  AIIH   Y4          Z4
   DNSv6         Y6
    |      <. . . . . . . . . |   - ask for the IPv4 address of X
    |            |            |   - this request arrives to the AIIH Server
    |            |            |
    |            |            |   - if node X does not have already a
    |            |            |     temporary IPv4 address assigned then the
    |            |            |     AIIH allocates an IPv4 address and
    |<=====      |            |     registers it in the DNS.
    |      . . . . . . . . . >|   - AIIH returns the IPv4 address to node Z4
    |            |            |
    |            |<-----------|   - Z4 sends an IPv4 packet which arrives at
    |      <=====|            |   - Y4 asks the AIIH server for the IPv6
    |            |            |     corresponding to X4.
    |      =====>|            |   - AIIH server responds
    |<++++++++++ |            |   - The packet is tunneled to node X6
    |            |            |

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4.3 IPv4 compiled application between to IPv6 nodes

To maintain compatibility between two IPv4 applications, an IPv4
application running on an IPv6 host may wish to send IPv4 packets to
another application running also on an IPv6 host, called Z6. To allow
end-to-end communication without the use of a static Tunnel End Point,
nodes can use the same mechanism as the DSTM border router in the
previous example.  This means that a DTI interface can ask the AIIH
server to perform address resolution. If the resolution fails, the DTI
interface can still use the static TEP.

   X6                        Z6
    |                         |
    |............>            |   - X asks for the IPv4 address of Z.
    |            ============>|   - AIIH Server assigns an IPv4 address to Z
    |                         |   - AIIH registers this address to
    |                         |     its DNS server
    |<...........             |   - Z4 is returned to X
    |                         |   - The IPv4 address of Z is used by the
    |                         |     application, which sends an IPv4 packet
    |                         |     to the IPv6 IP implementation
    |                         |   - the routing table has been previously
    |                         |     configured in X to route
    |                         |     IPv4 through DTI
    |===========>             |   - DTI receives its first packets, asks
    |<==========              |     the AIIH server to assign
    |                         |     the IPv4 address to the DTI interface
    |                         |   - AIIH registers this address
    |                         |     to the DNS
    |                         |   - DTI has to find the IPv6 address
    |                         |     of the tunnel end-point for Z4
    |===========>             |   - DTI daemon asks the AIIH Server for the
    |<===========             |     temporary address of Z4
    |++++++++++++++++++++++++>|   - DTI tunnels the packet to Z6

5. AIIH Server Design Model

The design model provides two mechanisms to assign an IPv6 host an IPv4
address. The first mechanism is for the host to request an IPv4 address
that is globally routable, and the second is for an AIIH Server to
assign an IPv6 host a globally routable IPv4 address using the DHCPv6
Reconfigure Message. The assumption in this specification is that a site
has a certain number of IPv4 Global Addresses, which can be assigned
within the network on a temporary basis for use by hosts in the site.

5.1 AIIH DHCPv6/DNS Server

The AIIH Server supports a co-located DHCPv6 and DNS Server and other
implementation defined software functions. The AIIH server configuration
files and database is not defined in this specification. There can be

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one or many AIIH Servers on an Intranet and how they maintain
consistency and Tunnel End Point configurations for IPv6 links is
implementation defined.

The AIIH Server is an implementation where DNS, DHCPv6, and
communications between those two applications exists. These applications
MAY be co-located on the same host, but that is not a requirement of
this specification. How DNS and DHCPv6 communicate is implementation
defined . The AIIH Server SHOULD support the following operations:

     1.  Act as the Authoritative DNS Name Server for a set of IPv6
         hosts that can be queried for IPv4 Global Addresses.

     2.  Communications between the AIIH DNS server and the AIIH DHCPv6

     3.  An AIIH DHCPv6 Server that can maintain a pool of IPv4 Global
         Addresses in an implementation defined manner.

     4.  An AIIH DHCPv6 Server that can maintain Tunnel End Points for
         IPv6 Links in an implementation defined manner.

     5.  An AIIH DHCPv6 Server to process DNS AIIH IPv6 host DNS queries,
         and Reconfiguring IPv6 hosts to assign IPv4 Global Addresses to
         their interfaces.

     6.  Support DHCPv6 Client's requesting IPv4 Global Addresses.

     7.  Dynamically Updating DNS with an IPv4 Global Address for
         an IPv6 host that supports IPv4/IPv6.

An AIIH Server MUST support a dual IPv4/IPv6 network layer and
implementation of IPv4/IPv6.

The IPv4 address allocation can be triggered by two events. The first
one is when an IPv6 host requests through DHCPv6 an IPv4 address to
configure its IPv4 stack. The second event happens when a DNS A query is
made for a node that only has an IPv6 address (see section 5.2).  fails
to respond to a DNS A RR query.

5.1.1. Requesting an IPv4 Global Address

An IPv4/IPv6 host can request an IPv4 Global Address by using the IPv4
Global Address Extension defined in section 7. The IPv4/IPv6 host MUST
support DHCPv6 [4] and the DHCPv6 Extensions [5]. The Requests/Response
Model of DHCPv6 will process this new extension as any other extension.
There is no need to define a new message type for DHCPv6 for this
processing or add to the DHCPv6 protocol.

Once the host has obtained an IPv4 Global Address it MUST NOT update DNS
to reflect an A type or PTR type record for this address.  The reason is
that the intent is to provide a host with this temporary address to use
for communications with an IPv4 node. Once the reason for obtaining an
IPv4 Global Address has been satisfied the host MUST Release this IPv4
Global Address from the AIIH DHCPv6 Server implementation.

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On the other hand, if the address lifetime is about to expire, the AIIH
client may send another request to the AIIH Server to keep this address

5.1.2 AIIH DHCPv6 Client IPv4 Global Address Requests

An AIIH DHCPv6 Server will receive DHCPv6 Requests for IPv4 Global
Addresses from IPv6 hosts. The AIIH DHCPv6 Server will determine if an
address is available and assign the address to the DHCPv6 Client as
specified in section 7 of this specification.

5.1.3 AIIH DNS Query and DHCPv6 Processing

Once the AIIH DNS finds the IPv6 host being queried the AIIH DNS
requests from its corresponding AIIH DHCPv6 Server to assign an IPv4
Global Address to the IPv6 host being queried.

The AIIH DHCPv6 Server will look within its pool of IPv4 Global
Addresses for an address and if a Tunnel End Point address is required
for the IPv6 host to reach the router to route packets onto the
Internet. If an address is available the DHCPv6 Server will send a
DHCPv6 Reconfigure Message to the IPv6 node to temporarily assign the
node an IPv4 Global Address (see section 7).

Once the AIIH DHCPv6 server is certain that the IPv6 host has assigned
the address to an interface, the AIIH DHCPv6 Server responds back to the
corresponding AIIH DNS Server with the IPv4 Global Address assigned to
the IPv6 host being queried, or that an address could not be assigned to
this IPv6 host.

It is important to wait for an acknowledgment from the client to be sure
that the host is up before validating an IPv4 address has been assigned.
Nevertheless this could introduce a delay incompatible with the timer
used during a DNS query. The dialog could be modified. Just after the
DNSv6 temporary IPv4 address assignment, the AIIH DNS returns this
address but with a small TTL. The real TTL will be used if the
acknowledgment is received, otherwise the IPv4 address is deprecated for
a some period of time.

The AIIH DNS Server will now respond to the IPv4 DNS Query as the
Authoritative DNS Name Server with an address or host not found.

The AIIH DHCPv6 Server MAY send a dynamic update to DNS [6] to add an A
type record to the Primary DNS Server, where the query came from to the
AIIH DNS Server. The Time-To-Live (TTL) field in the update MUST NOT be
set to be greater than the valid lifetime for the IPv4-Compatible
address in the DHCPv6 Extension provided to the DHCPv6 Client. It is
highly recommended to not update the DNS with an A record for the IPv6
host, unless that IPv6 host provides a permanent IPv4 Application
service needed by IPv4 hosts.

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5.1.4. Cleaning up the AIIH IPv4 Assigned Address

Once the IPv4 address expires, the DHCPv6 Server will permit the IPv4
address to be reused. But before the address can be reused the DHCPv6
Server MUST delete the IPv4 address from the Primary DNS Server, through
the Dynamic Updates to DNS mechanism, if an A record was added to the
relative Primary DNS Server.

If an AIIH client wants to keep the temporary IPv4 address after its
expiration time, it MUST send a DHCPv6 request before the address

5.2 Links with other DNS

When the Primary DNS Server for the IPv6 node receives the IPv4 hosts
query, it will do a DNS search for that IPv6 host and find that there is
an Authoritative DNS Server for that specific DNS A record, which
represents an IPv6 host. That DNS Server will be one part of the AIIH
Server software. After the AIIH DHCPv6 Server assigns the IPv6 node a
temporary IPv4 Global Address, the AIIH DNS Server will respond to the
original IPv4 DNS query authoritatively with an IPv4 Global Address for
the IPv6 host or return host Not Found.

   For Example:

     IPv4 node "" queries for ""

     Query reaches Primary DNS Server for "".  IN SOA  etc etc.
     .          IN  NS     IN  NS
     .          IN   A     IN   A
      .        IN   CNAME        IN   CNAME        IN   CNAME

   DNS query will end up going to the authoritative server looking for This permits
   the AIIH Server to now process a request for an IPv4 Global Address
   for an IPv6 host that had no IPv6 DNS AAAA Record [18].

   If DTI is present, the reverse DNS must be linked to the pool of
   addresses managed by the AIIH Server.

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6. DTI

6.1. DTI Architecture

The DTI interface will be used to send IPv4 packets during the
interoperation of IPv4 and IPv6. The routing table of the host forwards
the information to that interface. It is possible to send all the IPv4
packets through this interface by using only the default prefix.

The DTI interface is placed between the IPv4 API and the IPv6 layer, as
shown in the following figure, the architectural model assumes a BSD
UNIX type platform.

                                                             ( AIIH daemon )

          |         IPv6 API         |          IPv4 API        | PF_  |
          |                          |                          | ROUTE|
          |                          +--------------------------+      |

          |                                                     |      |

On every IPv6 node an AIIH daemon is running to manage the allocation of
the IPv4 addresses need and perform the address resolution when needed.

The following example gives the configuration of an IPv4 routing table
with DTI. All IPv4 packets except those to the 192.44.77/24 prefix are
sent through the dti0 interface. They will be encapsulated into IPv6
packets. Packet to the 192.44.77/24 prefix will be sent natively on the

Routing tables

Destination    Gateway          Flags    Refs     Use   Mtu  Netif Expire
default        link#1           UGSc       3        0  1460   dti0      UC         0        0  1500    le0   -    8:0:2b:1c:af:15  UHLW       4        0  1500    le0    649        UHl        1      102 16384    lo0

6.1 Assignment of the IPv4 address to the DTI

When the DTI interface is activated, no IPv4 address is given to that
interface. If the interface is active, but has no IPv4 address, when it
has to send the first IPv4 packet, the interface sends a request to the
daemon. The daemon will send a DHCPv6 request to the AIIH server to get
the temporary IPv4 address.

An IPv6 node can know it needs an IPv4 address if the DNS resolver on

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the node knows that the destination address will be an IPv4 address.
Once the resolver knows this then a query to the interface index of the
node will inform the IPv6 if it has an IPv4 interface configured.  This
is just one example of how an implementation can determine if the AIIH
daemon must be called.

6.2 Encapsulation of IPv4 packets

The protocol value for IPv4 encapsulation is 4 (as for IPv4 tunneling
over IPv4). When a tunneled packet arrives to the IPv6 destination, the
IPv6 header is removed and the packet is processed by the IPv4 layer.
The receiver SHOULD cache the association between the IPv4 and IPv6
source address.

6.2.1 IPv6 source address

The IPv6 source address of an encapsulated packet will be the IPv6
address of the interface on which the IPv6 packet will be sent.

6.2.2 IPv6 destination address

When a DTI has to encapsulate an IPv4 packet into an IPv6 packet. The
DTI as to find the IPv6 address for the destination, called in this
document a Tunnel End Point (TEP). The tunnel end point can be directly
the host destination or, if the destination host is IPv4-only, the IPv6
address of an IPv4/IPv6 router.

This document propose two ways for resolving the tunnel end point. The
first one is dynamic and uses the AIIH Server, the second one is static
and is returned in the DHCPv6 packet when a temporary IPv4 address is
allocated to the interface or by static configuration of the node.

The IPv6 hosts MAY have a static TEP. DSTM border routers SHOULD use
dynamic TEB, but it is possible in the case of a single homed network,
and for exiting traffic only, to avoid dynamic address resolution and
cache only the association of the IPv4 and IPv6 source address of
incoming packets.

For a host in the case of the failure of the dynamic TEP, static TEP
SHOULD be used.

For DSTM border routers, failure of the dynamic TEP SHOULD generate an
ICMPv4 host unreachable message. Dynamic TEP

Dynamic TEP determination is similar to MAC address resolution when
sending a IP packet over an Ethernet link. The only difference is that
no broadcast facilities can be used to find a TEP.

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In the Unix operating systems, this resolution should not be done in the
kernel. Some operating systems offer the possibility to do external
resolution. A query is sent to a daemon in the user space. This daemon
does a DHCPv6 query to find the TEP. In the rest of this document we
will consider this architectural model, but this is not a limitation for
implementing DTI.

When the resolver daemon receives a query from the kernel, it sends a
DHCPv6 query to the AIIH Server to get the IPv4 address for this host.

Static TEP cache contains the IPv6 address of a node inside the network.
The IPv6 address is stored in a cache for a duration indicated in the
DHCPv6 message. Static TEP

Static TEP may be returned by the AIIH Server with the temporary IPv4
address. This TEP is used when the dynamic TEP resolution fails or has
not been activated. This will be the case when the DTI daemon asks for
an address not registered in the AIIH Server (for example an IPv4
address outside of the DSTM cloud).

Static TEP contains the IPv6 address of a DSTM border router. Static TEP
records are stored as long as the temporary IPv4 address is assigned to
the interface in case of DHCP configuration, and as long as the DTI
interface is active in case of manual configuration.

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7. AIIH DHCPv6 Requirements

The AIIH DHCPv6 processes will use the DHCPv6 protocol and extensions to
communicate between the AIIH DHCPv6 Server and the DHCPv6 Client. A new
extension is required for DHCPv6 (section 4.1) to support AIIH. But
there are some additional requirements placed on the AIIH processes that
are not specific to the DHCPv6 protocol, but as transition and
interoperation mechanisms for the IPv6 hosts.

7.1 DHCPv6 IPv4 Global Address Extension

The DHCPv6 IPv4 Global Address Extension informs a DHCPv6 Server or
Client that the IPv6 Address Extension [5] following this extension will
contain an IPv4- Compatible Address [20], or is a Request for an IPv4
Global Address from a Client, or a Reply assigning a Global IPv4 Address
to a Client from a Server. The extension can also provide an IPv4-
Compatible or IPv6 address to be used as the Tunnel End Point to
encapsulate an IPv6 packet within IPv4, or an IPv4 packet within IPv6.

    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             |             Length            |
   |                          Tunnel End Point                     |
   |                           (If Present)                        |
   |                            (16 octets)                        |

   Type:                   TBD
   Length:                 0 or 16
   Tunnel End Point:       IPv6 Address if Present

   An IPv4 Global Address Extension MUST only apply to the extension
   following and not to any additional extensions in the DHCPv6

7.2 AIIH Server Processing of an IPv4 Global Address Extension

When a DHCPv6 Server receives an IPv4 Global Address Extension it MUST
assume that the next extension in a DHCPv6 Request or Release Message;
the Client is either Requesting an IPv4 Global Address or Releasing an
IPv4 Global Address.  If an address is present in either of these
messages it will be in the form of an IPv4-Compatible Address.

When a DHCPv6 Server sends a Client a Reconfigure Message to assign an
IPv4 Global Address to an interface the Server MUST NOT set the "N" bit
in the Reconfigure Message, so the Client performs the necessary
Request/Reply DHCPv6 processing to obtain the address from the Server.
The Server MUST NOT assume that the Client has assigned the address to
an interface until it has sent the corresponding Reply to the Client.

The Server will no a priori the IPv6 routable address, when sending a

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Reconfiguration Message, of a Client within the Intranet, and should use
that address with its own IPv6 address as the transaction binding cache
until the DHCPv6 Client/Server protocol processing has completed.

The Server will look in its implementation defined IPv4 Global Address
configuration to determine if a Tunnel End Point is required for a
specific IPv6 Address Prefix. If that is the case the Server will put
the address for the Tunnel End Point in the IPv4 Global Address
Extension. If the Tunnel End Point address is an IPv4 address the Server
will put that address in the extension as an IPv4-Compatible address.

7.3 AIIH Client Processing of an IPv4 Global Address Extension

When a DHCPv6 Client receives an IPv4 Global Address Extension it MUST
assume that the next extension in a DHCPv6 Reconfigure or Reply Message;
the Server is either assigning an IPv4 Global Address or supplying an
IPv4 Global Address. The address present in either of these messages
will be in the form of an IPv4-Compatible Address.

When the Client supplies an IPv4 Global Address as a Request or Release
it MUST represent that address as an IPv4-Compatible Address.

The Client MUST not assume it can use the IPv4 Global Address until it
has received a corresponding Reply to the Client Request, which is
required for a Reconfigure Message too as specified in section 7.2.

Once the Client is assured it can use the IPv4 Global Address it can
perform the following operations:

   1.  In an implementation defined manner the Client MUST assign the
       address to an interface, supporting the Client's IPv4 stack

   2.  In an implementation defined manner the Client MUST create an entry
       as an IPv4-Compatible Address supporting the processing required
       for an IPv6 address regarding the valid and preferred lifetimes
       as specified in IPv6 Addrconf [19].  Once the IPv4-Compatible
       address valid lifetime expires the IPv4 address MUST be deleted
       from the respective interface and a DHCPv6 Release Message
       MUST be sent to the AIIH DHCPv6 Server to delete the IPv4 Global
       Address from the Servers bindings.

   3.  If a Tunnel End Point address is provided in the IPv4 Global
       Address Extension, the Client MUST create a configured tunnel
       to the Tunnel End Point address, in an implementation defined
       manner. If the Tunnel End Point address is an IPv4-Compatible
       address then the encapsulation is IPv4 within IPv4, if the
       Tunnel End Point is an IPv6 address then the encapsulation
       is IPv6 in IPv4. These encapsulation mechanisms are defined
       in other IPv6 specifications [13, 15].

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8. Security Considerations

The AIIH mechanism can use all the defined security specifications for
each functional part of the operation. For DNS the DNS Security
Extensions/Update can be used [10, 11], for DHCPv6 the DHCPv6
Authentication Message can be used [5], and for communications between
the IPv6 node, once it has an IPv4 address, and the remote IPv4 node,
IPSEC [8] can be used as AIIH does not break secure end- to-end
communications at any point in the mechanism.

9. Year 2000 Considerations

There are no Year 2000 issues in this specification.

Appendix A:  DSTM Discussion and Issues

1.  DHCPv6 work needs to be proposed as DHCPv6 additional extension.

2.  Need to review timers for DHCPv6 and DNS for performance and

3.  Can we make the DSTM router a Transition Box with DTI using

4.  Some of the refernences in the spec are not needed.

5.  Need to add acknowledgements to spec from AIIH and DTI previous
    work and the input.

6.  Should DSTM also support tunneling IPv6 within IPv4 at the DTI?

7.  Should the DSTM Overview contain more information.


   [1]  Mockapetris, P., "Domain Names - Concepts and Facilities", STD
        13, RFC 1034, USC/Information Sciences Institute, November 1987.

   [2]  Mockapetris, P., "Domain Names - Implementation and Specifica-
        tion", STD 13, RFC 1035, USC/Information Sciences Institute,
        November 1987.

   [3]  S. Deering and R. Hinden.  Internet Protocol, Version 6 (IPv6)
        Architecture", RFC 2460, December 1998.

   [4]  J. Bound and C. Perkins.  Dynamic host Configuration Protocol
        for IPv6.  draft-ietf-dhc-dhcpv6-14.txt March 1999 (work
        in progress).

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   [5]  C. Perkins.  Extensions for the Dynamic host Configuration
        Protocol for IPv6.  draft-ietf-dhc-dhcpv6ext-11.txt March
        1999. (work in progress).

   [6]  P. Vixie, S. Thomson, Y. Rekhter, and J. Bound. Dynamic Updates
        to the Domain Name System (DNS).  RFC 2136, April 1997.

   [7]  William R. Cheswick and Steven Bellovin.  Firewalls and Internet
        Security.  Addison-Wesley, Reading, MA 1994 (ISBN:

   [8]  IPSEC - This needs to include the Arch, Auth, and ESP specs.

   [9]  S. Bradner.  Key words for use in RFCs to indicate Requirement
        Levels.  RFC 2119, March 1997.

   [10] D. Eastlake and C. Kaufman. Domain Name System Security
        Extensions.  RFC 2065, January 1997.

   [11] D. Eastlake. Secure Domain Name System Dynamic Update.
        RFC 2137, April 1997.

   [12] R. Callon and D. Haskins. Routing Aspects Of IPv6 Transition
        RFC 2185, September 1997.

   [13] A. Conta and S. Deering.  Generic Packet Tunneling in IPv6.
        RFC 2473, December 1998.

   [14] E. Nordmark. Stateless IP/ICMP Translator (SIIT)
         draft-ietf-ngtrans-siit-03.txts, November 1998
        (work in progress)

   [15] R. Gilligan and E. Nordmark.  Transition Mechanisms for IPv6
        hosts and Routers. draft-ietf-ngtrans-trans-mech-01.txt,
        August 1998 (work in progress).

   [16] R. Droms.  Dynamic host Configuration Protocol.
        RFC 2131, March 1997.

   [17] Rekhter, Moskowitz, Karrenburg, Groot.  Address Allocation
        for Private Networks. RFC 1918.  February 1996.

   [18] This needs to reflect the new DNS work for IPv6.

   [19] Thomson, Narten.  IPv6 Stateless Address Configuration.
        RFC 2462, December 1998.

   [20] Hinden, Deering.  IP Version 6 Addressing Architecture.
        RFC 2373, July 1998.

Authors' Address

    Jim Bound
    Compaq Computer Corporation
    110 Spitbrook Road, ZKO3-3/U14
    Nashua, NH 03062
    Phone: (603) 884-0400

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    Laurent Toutain
    ENST Bretagne
    BP 78
    35 512 Cesson Sëvignë Cedex
    Phone : +33 2 99 12 70 26
    Email :

    Hossam Afifi
    ENST Bretagne
    BP 78
    35 512 Cesson Sëvignë Cedex
    Phone : +33 2 99 12 70 36
    Email :

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