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INTERNET-DRAFT                                             Laurent Toutain
NGTRANS Tools Working Group                                Hossam Afifi
Expired December 1999                                      ENST Bretagne

                                                           Jim Bound
                                                           Compaq



                Dual Stack Transition Mechanism (DSTM)

                  <draft-toutain-ngtrans-dstm-00.txt>


Status of this Memo

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

     This document is a submission by the Next Generation Transition
     Working Group of the Internet Engineering Task Force (IETF).
     Comments should be submitted to the ngtrans@sunroof.eng.sun.com
     mailing list.

     Internet-Drafts are working documents of the Internet Engineering
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     Distribution of this memo is unlimited.

Abstract

   The initial deployment of IPv6 will require a tightly coupled use of
   IPv4 addresses to support the interoperation of IPv6 and IPv4. 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. This specification defines a
   mechanism called "Assignment of IPv4 Global Addresses to IPv6 hosts"
   (AIIH), which will assign an IPv6 host a temporary IPv4 Global
   Address, which can be used to communicate with a host that supports
   IPv4 or IPv4/IPv6. This document includes also the definition of a
   Dynamic Tunneling Interface (DTI) to ease the automatic IPv4 address
   assignment and to remove the IPv4 routing table from routers.
   Another objective is to demonstrate that IPv6 Addresses can
   be deployed now instead of non-Global IPv4 Addresses within an
   Intranet.

Table of Contents:

1. Introduction

   The initial deployment of IPv6 will require a tightly coupled use of
   IPv4 addresses to support the interoperation of IPv6 and IPv4. 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.


   This specification defines a
   mechanism called "Assignment of IPv4 Global Addresses to IPv6 hosts"
   (AIIH), which will assign an IPv6 host a temporary IPv4 Global
   Address, which can be used to communicate with a host that supports
   IPv4 or IPv4/IPv6.  A AIIH  Server combines the functionality of a
   extended DHCPv6 server and a DNS server. An AIIH DHCPv6 server assigns
   dynamically temporary IPv4 addresses to Dual Stack Equipments.
   The AIIH DNS server is used to keep a mapping between the
   name, the IPv4 address and the IPv6 address of a Dual Stack Equipment.

   Another objective of this document is to define the functionality
   of a dynamic tunneling interface (DTI) encapsulating
   IPv4 packets into IPv6 packets. This will ease the assignment of
   dynamic IPv4 address since the
   network topology is hidden. This allows, most of the time, a
   flat addressing plan.
   The second advantage is that IPv4 packets will not
   be directly forwarded anymore. The IPv4 routing table can be
   suppressed.

   This document also proposes some steps to migrate from the
   dual environment described in RFC 1933 to an IPv6 only domain.
   It exhibits some scenarios
   to validate the introduction of AIIH servers and DTI interfaces.

   The methods described in this document may not be used
   for the general case.  The best way is to migrate as quickly as
   possible hosts and applications to IPv6 or to use Application
   Level gateways (ALG).  This document proposes a way to remove a possible
   blocking situation during the migration period, which would
   postpone the introduction of IPv6.

1.1. Scenarios

   To study the behavior of the AIIH Server and the DTI interface, we
   focus on the following scenarios:

   -   The first scenario is the case of an IPv6 application running on a
       IPv6 host initiating a dialog with an IPv4 equipment.

   -   The second scenario is an IPv4 application, running on an IPv6 host
       initiating a dialog with an IPv4-only host.

   -   The third scenario is an IPv4-only application running on an IPv4-only
       host initiating a dialog with a IPv6 host.

1.3. Architecture model

   The design model supports the following network configuration abstraction:

      <------- domain --------------------------><-provider-v4-only---->
      |                                         |                       |
      host X ----------------------- Router Y ------------------- host Z
      (Intranet)                 (Intranet & Internet)        (Intranet)

      Host X represents an IPv4/IPv6 implementation, that has an
      IPv6 address. The IPv6 address is denoted as X6 and, if
      available, the IPv4 address will be denoted as X4.

      Router Y represents an IPv4/IPv6 implementation that has both
      an IPv4 Global addresse and an IPv6 Address. The IPv4 address
      is denoted as Y4 and the IPv6 address is denoted as Y6. Router Y
      implements two routing tables, one for IPv4 and one for IPv6.
      Router Y belongs to the same domain as host X.

      Host Z represents an IPv4 or IPv4/IPv6 implementation that has
      an IPv4 Global Address, and MAY have an IPv6 Address. The IPv4
      address is denoted as Z4 and if an IPv6 address exists it is
      denoted as Z6.

1.2. Migration steps

   RFC 1933 describes the Dual Stack approach and defines a way to
   introduce compatibility between IPv4 and IPv6 applications. If the
   operating system and the applications have been "v6fied", dialogs
   between IPv6 hosts will use the IPv6 protocol. Otherwise dialogs with
   at least one IPv4 host or application will use IPv4 protocol.
   IPv6 applications can use both stacks with IPv4-mapped addresses.

   Nevertheless, this requires a dual configuration either for the hosts or
   for the intermediary equipments. This does not solve the problem for
   the lack of IPv4 addresses since each equipment still needs a IPv4
   address.

   This is the first step of the transition. It is more or less the
   state of IPv6 platforms now deployed in the 6bone.

   The second step is to remove the static configuration of IPv4 addresses
   when possible.  When it will be necessary,
   an AIIH server will assign a temporary IPv4 address to a host
   that needs to communicate with an IPv4-only equipment or with a
   IPv4-only application. The rest of the time, the IPv6 stack will
   be used.

   The configuration during this step will be difficult since an
   addressing plan will still be necessary for the IPv4 protocol and
   routers will have to manage the IPv6 and the IPv4 routing plan.

   The third step is to remove the IPv4 routing functions inside routers
   and keep only the IPv6 routing plan. The IPv4 packets produced by IPv4
   applications or hosts will be encapsulated inside IPv6 packets. DTI
   interfaces will establish the mapping between the IPv4 address and the
   IPv6 address of the destination by using the AIIH server of the
   destination (if available). The IPv4 source address will be,
   as in step 2, assigned temporary by the AIIH server.

   Note that DTI interface can be deployed without any dynamic
   address allocation, without a AIIH Server. In this case manual
   configuration is  needed to assign
   address to the DTI interface and to configure the DNS. So it is more
   logic in a migration process to start with dynamic IPv4
   address allocation and then use DTI to remove IPv4 routing.

   In the fourth step, the mechanisms described in step 3 are the same,
   but they are managed by the
   IPv6-only provider which carries IPv4 packets using tunnels. This
   allows a company to get a unique provider, which manages the inter-
   connectivity with the IPv4 world. Some security measures must be taken
   to avoid attacks like deny of service by requesting the entire IPv4
   address pool of the provider. These measures are not in the scope of
   this document.

1.3. Document architecture

   The specification will begin by defining the terminology (section 2),
   then discuss the AIIH design model (section 3), then the DTI
   architecture model is described with its interaction with the
   AIIH Server (section 4). Section 5 completes the mechanism by
   defining the DHCPv6 Extension needed to assign a temporary IPv4 address
   to an IPv6 node. The specification then discusses Security (section
   5) and Year 2000 considerations (section 6). Appendix A will enumerates
   Open Issues that need to be discussed in the ngtrans Tools Working
   Group, and maintain the state of Open Issues as STILL OPEN, RESOLVED,
   or PARTIALLY RESOLVED during the draft updates to AIIH. Appendix B
   will keep a historical account of changes to the draft and rationale
   for those changes as they occur, and maintain consistence with the
   Open Issues in Appendix A.

2. Terminology

2.1 IPv6 AIIH Terminology

   AIIH Domain             An area where AIIH Server can access to IPv6
                           equipments.

   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.

   DTI encapsulation box:  A intermediary equipment doing the IPv6 tunneling
                           when the end-system is unable to do it.

   DTI resolver:           An application that finds the IPv6 destination
                           address using the IPv4 address of the packet
                           being encapsulated. As ARP or Neighbor discovery
                           the DTI resolver is only called for the first
                           packet.

   DTI daemon              synomyn to DTI resolver

   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.

   Transition Box          An equipment managing the encapsulation of
                           IPv4 packets either when one of the links is
                           IPv4-only or when the destination has only an
                           IPv4 stack.

   Tunnel End Point        Destination of the IPv6 packet containing a
                           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
                    specification.

      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.
                    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. AIIH 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 enterprise on a temporary basis for use
   by hosts in the site. The design model also assumes that the site has an
   IPv4/IPv6 router in the site that is used to send and receive packets
   over the Internet.

   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 IPv6-only stack implementation. For
   embedded system type nodes that support only an IPv6 stack, AIIH
   cannot be a solution.

3.1 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 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
         Server.

     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 a IPv6 host requests through DHCPv6 an IPv4 address to
   configure its IPv4 stack. The second event is when the AIIH DNS Server
   fails to response to a A RR query. The temporary IPv4 address is sent
   by the AIIH DNS Server which keeps the
   mapping with the IPv6 address and the name of the equipment in the
   AIIH domain. The temporary IPv4 address is stored in the AIIH DNS Server
   as a A record.

3.1.1. 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 5. 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.

   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 assigned.

3.1.2 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 5 of this specification.

   In case of an IPv4 addressing plan (i.e. step 2 of the migration
   process), the AIIH Server MUST be configured to allocate IPv4
   address in regard with the network topology.

   The AIIH DHCPv6 Server sends a Dynamic Update to the AIIH DNS server.
   The TTL must be shorter than the duration of the allocation to
   the client.

3.1.3 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 5).

   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 a acknowledgment from the client to be sure
   that the host is up before validate an IPv4 address assignation.
   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 while.

   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.

   The Dynamic Update will be done for the direct queries, this will allows
   other queries for the IPv4 address to get the same answer. If DTI is
   present, another Dynamic Update will be done for the reverse queries.
   The type recorded should be TEP (Tunnel End Point). See discussion
   paragraph 4.1.1.

3.1.4. 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, thru the Dynamic Updates to DNS mechanism, if an A record was
   added to the relative Primary DNS Server.

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

3.2 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 "v4host.abc.com" queries for "v6host1.xyz.com"

     Query reaches Primary DNS Server for "v6host1.xyz.com".

-----------
     xyz.com.  IN SOA primary.xyz.com.  etc etc.
     .
     .
     xyz.com          IN  NS  primary.xyz.com
     aiih.xyz.com     IN  NS  v6trans.aiih.xyz.com
     .
     .
     primary.xyz.com          IN   A    202.13.12.6
     v6trans.aiih.xyz.com     IN   A    202.13.12.8
      .
      .
      .
     v6host1.xyz.com        IN   CNAME   v6host1.aiih.xyz.com
     v6host2.xyz.com        IN   CNAME   v6host2.aiih.xyz.com
     v6host3.xyz.com        IN   CNAME   v6host3.aiih.xyz.com

   DNS query will end up going to the authoritative server
   v6trans.aiih.xyz.com looking for v6host1.aiih.xyz.com. 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.

3.3 Scenarios

   These scenarios take place during the step 2 of the migration process.
   IPv6 equipments have a dual stack, but only the IPv6 stack is
   configured. Routers have both of their stacks configured.

   Notation of the equipment is defined in paragraph 1.2.

   ==>  means a IPv6 packet
   -->  means a IPv4 packet
   ..>  means a DNS query or response. The path taken by this
        packet is unknown
   "Z"  means the DNS name of "Z"

3.3.1 X6 (with a v6 application) to Z4

         AIIH   Y4           Z4
   X6           Y6
    |            |            |
    |            |            |    - X6 asks the DNS for a AAAA for "Z"
    |            |            |    - the DNS answers a error
    |            |            |    - X6 asks for the A RR for "Z"
    |            |            |    - the answer is Z4
    |            |            |    - X6 needs a IPv4 address
    |====>       |            |    - X6 queries the AIIH server for an
    |            |            |      IPv4 address using DHCPv6
    |<====       |            |    - The DHCP server locates the client
    |            |            |      and attributes temporally a v4
    |            |            |      address. (the tunnel end-point is
    |            |            |      not set in the response)
    |            |            |    - The AIIH Server may register IPv4
    |            |            |      address to the DNS through
    |            |            |      a Dynamic Update
    |------------+----------->|    - X4 can send the IPv4 packet to Z4
    |<-----------+----------->|    - and vice versa.

3.3.2 X6 (with a v4 application) to Z4

   Same behavior as 3.3.1, except that X will request directly a A RR
   to the DNS instead of going first through a AAAA query.

3.3.3 Z4 to X6

         AIIH   Y4     DNS   Z4
   X6           Y6
    |            |            |
    |            |      <-----|   - Z asks for ôXö
    |       <-...|            |   - The request reaches to the AIIH
    |            |            |     Server
    |<=====      |            |   - The AIIH Server assigns a v4
    |            |            |     address to X
    |=====>      |            |   - X acknowledges
    |      ..................>|   - The AIIH server answers with the
    |            |            |     newly assigned v4 address
    |            |            |   - The AIIH Server may register the
    |            |            |     IPv4 address through a Dynamic Update
    |<-----------+------------|   - Z4 can send the packet to X4
    |<-----------+----------->|   - and vice versa
    |            |            |

4. DTI

4.1. DTI Architecture

   The DTI interface will be used to send IPv4 packets during the migration
   process. The routing table of the host forwards the information to
   that interface. It is possible to send all the IPv4 packets through this
   interface. Some other prefixes can be used to send directly native IPv4
   packets.

   The DTI interface is placed between the IPv4 API and the IPv6 layer,
   as shown in the following figure.

          +------------||------------+------------||------------+
          |         IPv6 API         |          IPv4 API        |
          |                          |                          |
          |                          +--------------------------+
          |                                                     |
          +-----------------------------------------------------+

   The following example gives the configuration of a routing
   table using DTI. The addresses in this example are private, but the
   use of global IPv4 addresses gives a similar result. With this routing
   table, if the destination address contains the prefix 10.35.3/24
   IPv4 packet are send directly on the link. If the destination prefix is
   10.34.3.0/24, packets are sent to the DTI interface. Otherwise the
   packet is sent to the default router.

Routing tables

Internet:
Destination   Gateway            Flags    Refs     Use   Mtu  Netif Expire
default       10.35.3.3          UGSc       3        0  1500    le0
10.34.3/24    10.34.3.2          UXc        0       10  1460   dti0
10.35.3/24    link#3             UC         0        0  1500    le0   -
10.35.3.3     8:0:2b:1c:af:15    UHLW       4        0  1500    le0    649
127.0.0.1     127.0.0.1          UHl        1      102 16384    lo0

   In this example, the DTI already has an IPv4 address. But this address
   can be dynamically acquired using the AIIH Server as explained in
   chapter 3.

   When a DTI has to encapsulate a IPv4 packet into 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
   or, if the destination host is IPv4-only, a IPv6 address of a transition
   box.

   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 proceed by the
   IPv4 layer. The receiver should memorize the association between IPv4
   destination address and TEP.

   This document propose two ways for resolving tunnel end point. The first
   one is dynamic and use the AIIH DNS Server, the second one is static and
   is returned in the DHCPv6 packet when a temporary IPv4 address is
   allocated to the interface. The dynamic resolution is mandatory. The
   tunnel end point in the DHCPv6 message is optional. This TEP is used
   when dynamic TEP fails (for example, the destination does not have a
   AIIH server).

   Dynamic TEP should be used when IPv4 host or application are
   spread inside a domain. Static TEP should be used when the boundary
   between IPv4 and IPv6 domain is clear (for example an IPv6
   domain, connected to an IPv4-only provider).

4.1.1 Dynamic TEP

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

   In 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 DNS 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.

   The AIIH DNS Server MUST be reachable in the reverse query
   DNS tree for the range of IPv4 addresses managed by this server.

   When the resolver daemon receives a query from the kernel, it sends a
   reverse query to the DNS to get the record for this host. Three kinds
   of records can be proceeded by the daemon:

   - PTR record: the daemon sends another query to the DNS to get the
     AAAA record of this host and returns the value to the kernel.

   - AAAA record: the value is returned to the kernel

   - TEP record: this record must be introduced for the DTI interface to
     avoid confusion between the destination and the tunnel end point (see
     paragraph 4.2.1). It contains the address of the tunnel end point. Its
     value is returned to the kernel. We recommend the use of this record.
     Only the AIIH server will have to manage such records. They are,
     most of the time, created by the AIIH DHCP Dynamic Update when a
     temporary address is allocated to an IPv6 host.

   The IPv6 address is stored in a cache for a duration indicated in the
   TTL field of the DNS answer. The following example shows a entry for
   destination 10.34.3.1

/homes/toutain>netstat2 -rnf inet
Routing tables

Internet:
Destination   Gateway            Flags  Refs   Use   Mtu  Netif Expire
default       10.35.3.3          UGSc     3     21  1500    le0
10.34.3/24    10.34.3.2          UXc      0    109  1460   dti0
10.34.3.1     3ffe:305:1002:4:a00:2bff:fe1b:8942 UHLS  0  0 1460 dti0 27
10.35.3/24    link#3             UC       0      0  1500    le0   -
10.35.3.2     8:0:2b:1c:11:1f    UHLWl    0     29  1500    lo0
10.35.3.3     8:0:2b:1c:af:15    UHLW     4      0  1500    le0    304
10.35.3.255   ff:ff:ff:ff:ff:ff  UHLWb    0      2  1500    le0
127.0.0.1     127.0.0.1          UHl      1    298 16384    lo0

4.1.2 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. This
   will be the case when the DTI daemon asks for a TEP RR on a non
   AIIH DNS Server.

   Static TEP is used to tunnel packets to a transition box linked to
   a IPv4 network. In some
   domains where the delimitation between the IPv6 and the IPv4 is strict
   it is sub-optimal to wait for the failure of the DNS query
   before using the
   static TEP. DHCPv6 configuration message should contain a flag to
   force the use of static TEP.

4.1.3 IPv4-only hosts

  It is not possible to modify IPv4-only hosts or the applications running
  on such hosts. These hosts are configured to send IPv4 packet on the
  network to a transition box that will encapsulate IPv4 packet into IPv6
  packets. For an IPv4-only host, this equipment is viewed has a default
  router.

  This means that an addressing plan is required for these hosts. At least
  two IPv4 addresses are needed. This will depend on the number of IPv4
  addresses available. One extreme possibility is to keep the addressing
  plan that existed before DTI, but this could lead to a waste of IPv4
  addresses. The other possibility is, if the capability of the IPv4-only
  allows it, to assign a prefix length of 30 to that link.

  The IPv4 address is configured manually in the reverse DNS tree
  in association
  with a TEP record that gives the IPv6 address of the tunnel end point.

  Depending on the DF bit of the IPv4 packet, the translation box will do
  the fragmentation (i.e. use the IPv6 fragmentation extension) or will
  send a ICMP message to the IPv4-only host.

4.2 Examples

   The notation +++> means a IPv4 packet encapsulated in a IPv6 packet.

4.2.1 X6 (with v6 application) to Z4 with TEP dynamic resolution

   Z4 is in the same domain as X6. The DNS for "Z" is configured in the
   reverse query DNS database as follow:

   128.3.4.6  PTR   Z.aiih...  ;the database is statically configured for Z
              TEP   3ffe:..... ;address of the Tunnel End Point

  The DNS has been configured with the address of Z

   Z  A  128.3.4.6



         AIIH         TB     Z4
   X6          Y6/Y4
    |            |            |
    |            |            |    - X6 asks the DNS for a AAAA for "Z"
    |            |            |    - the DNS answers a error
    |            |            |    - X6 asks for the A RR for "Z"
    |            |            |    - the answer is Z4
    |            |            |    - routing protocol has been previously
    |            |            |      configured in X to route v4 through
    |            |            |      dti (compatible with IPv4-mapped
    |            |            |      addresses).
    |            |            |    - X6 needs a IPv4 address (first use)
    |====>       |            |    - X6 queries the AIIH server for an
    |            |            |      IPv4 address using DHCPv6
    |<====       |            |    - The DHCP server locates the client
    |            |            |      and attributes temporally a v4
    |            |            |      address. (the tunnel end-point is
    |            |            |      not set in the response).
    |            |            |    - the DHCP Server sends a Dynamic Update
    |            |            |      to the DNS to memorize the association
    |            |            |      x4<->x6 (x4 TEP X6).
    |            |            |    - dti has to find the IPv6 address of
    |            |            |      the tunnel end-point for Z4
    |.....................>   |    - dti daemon asks the dns for the IPv6
    |            |            |      TEP for Z4 (transition box)
    |<.....................   |    - AIIH DNS answers the TEP
    |+++++++++++++++++>       |    - The dti sends the v6 packet to the
    |            |            |      tunnel end-point
    |            |     ------>|    - The TB sends the packet to
    |            |            |      the destination

   If the tunnel end point for Z4 had been recorded in the DNS with a
   AAAA record, then the source would have been confused and would have
   sent the packet directly in IPv6 to the transition box.

4.2.2 X6 (with v4 application) to Z4 with TEP dynamic resolution

   The dialog is the same as shown in paragraph 4.2.1 when an IPv4
   application wants to talk with a IPv4 application on Z4.

   To maintain compatibility between two v4 application, a v4 application
   running on a IPv6 host may wish to send IPv4 packets to another
   application running also on an IPv6 host, called Z6. Y6 is not used in
   this model. It was kept to show that X and Z can belong to two separate
   AIIH domains.

         AIIH         AIIH
   X6           Y6            Z6
    |            |            |
    |............|.....>      |   - X asks for the v4 address of ôZö.
    |            |      =====>|   - AIIH Server assigns a v4 address to Z
    |            |            |   - AIIH registers this address to
    |            |            |     its DNS server
    |<...........|.....       |   - Z4 is returned to X
    |            |            |   - The v4 address of Z is used by the
    |            |            |     application, which sends v4 packet
    |            |            |     to the kernel
    |            |            |   - routing table has been previously
    |            |            |     configured in X to route
    |            |            |     v4 through dti
    |=====>      |            |   - dti receives its first packets, asks
    |<=====      |            |     the AIIH server to assign
    |            |            |     the v4 address to the DTI interface
    |            |            |   - AIIH registers this address
    |            |            |     to the dns server
    |            |            |   - dti has to find the IPv6 address
    |            |            |     of the tunnel end-point for Z4
    |..................>      |   - dti daemon asks the dns for the
    |<..................      |     TEP RR for Z4
    |++++++++++++++++++++++++>|   - dti tunnels the packet to Z6

4.2.3 Z4 to X6 with TEP dynamic resolution

   This example covers any scenario where a IPv4-only host wants to reach
   an IPv6 host. This could be any application, but in this example, we
   will focus on a DNS query for a IPv4-only host to the DNS server of the
   domain.

   The IPv4-only host is configured with an IPv4 address and a default
   router. The DNS is also configured with the IPv4 address of the DNS
   server. Therefore, the DNS server must have a statically assigned
   IPv4 address. This configuration could be stored in the AIIH Server
   or directly on the host running the name server. We will suppose in
   this example that the configuration is stored in the AIIH Server.

   DNSv4  AIIH   Y4          Z4
   DNSv6         Y6
    |            |            |
    |            |            |   - Z4 wants to know the IPv4 address
    |            |            |     of some equipment in the Internet
    |            |            |   - Z4 has been configured with Y4 as
    |            |            |     default router and DNSv4 as resolver
    |            |<-----------|   - Z4 sends a query to the default route
    |            |            |   - Y receive the packet, Y routing table
    |            |            |     lead packets (except for the link where
    |            |            |     Z4 is connected) to the DTI interface.
    |      <=====|            |   - DTI has to find the TEP. It sends a
    |      =====>|            |     query to the AIIH server for the TEP
    |            |            |     for DNSv4.
    |<=====      |            |   - AIIH assigns the IPv4 address to DNSv6
    |<++++++++++ |            |   - The query is tunneled to DNSv6
    |            |            |

4.2.3 X6 to Z4 with static TEP resolution

   This example covers the case where X6 wants to reach a host outside the
   AIIH domain. Y is the last router for the IPv6 domain and is connected
   to the Internet v4. In this example, Y belongs to the domain.

   This scenario is used when a web browser in the IPv6 domain contact a
   IPv4 HTTP server.

         AIIH         DNS    Z4
   X6          Y6/Y4
    |            |            |
    |=====>      |            |   - X6 after the first DNS query to get
    |            |            |     Z4 address, sends a request to the
    |            |            |     AIIH server to obtain a temporary
    |            |            |     IPv4 address.
    |<=====      |            |   - AIIH returns the IPv4 address and
    |            |            |     the tunnel end-point
    |.....................>   |   - dti daemon asks the dns for the IPv6
    |            |            |     TEP for Z4 (transition box)
    |            |            |
    |            |            |
    |            |            |   - no answer, the DTI use the static TEP
    |+++++++++++>|            |   - Packet is tunneled to the static TEP
    |            |----------->|   - and sent with IPv4 to Z4

   When Z4 replies, the packet will not necessary reach the router Y.
   Routing in the internet is not symmetrical and can change. The AIIH
   Server does not participate to the routing protocol, so the given TEP
   can be sub-optimal. The IPv4 packet sent by Z4 will reach a router
   YÆ (by definition YÆ is at the boundary between a IPv4-only domain
   and an IPv6 domain). YÆ can find out the TEP to reach X6 by using
   the dynamic TEP resolution.

   To avoid the time-out when the dynamic TEP resolution fails, the DTI
   can be configured to send directly packets to the static TEP.

5. 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 5.1) 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.

5.1 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
   protocol.

<< NOTE >>
   flags are missing in this specification
<<END of NOTE>>

5.2 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
   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.

5.3 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 5.2.

   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
       implementation.

   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].



6. 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.

7. Year 2000 Considerations

   There are no Year 2000 issues in this specification.

Appendix A - Open Issues

   -  Need to add Examples for the new A6 Record types and how
      AAAA records can be used initially and references.

      OPEN 1/99

   -  Should use new Basic API terms for APIs.

      OPEN 1/99

   -  Need to add references for IPsec.

      OPEN 1/99

   -  Need to change references for DNS SEC esp solutions
      for Dynamic Updates to DNS.

      OPEN 1/99

   -  Need to look at issues for TCP TIME_WAIT state and other
      issues of addresses timing out.

      OPEN 1/99

   -  Need to add words to the design objective of preserving the
      end-to-end model for IPv6.

      OPEN 1/99

   -  The draft does not speak of PTR records for the IPv6 node
      A record once its created.  But its only useful during the
      lifetime of the assigned IPv4 address.

      STILL OPEN 3/98 Draft.  Closed - New A6 Records

   -  RFC 1183 RT Record is Experimental and there is some concern
      its obsolete.  Though some implementations still support some
      code for the RT record.  Also the Route Through semantics
      specified may need to strongly state the query is passed thru
      to the AIIH server.  This needs to be discussed.

      RESOLVED 3/98 Draft RT record deprecated.

   -  The Primary Server must look for the IPv6 node A record first
      before finding the RT record.  This needs to be verified
      as an implementation issue.

      RESOLVED 3/98 Draft - Use CNAME Records.

   -  When the AIIH Server responds to the query it may not be
      authoritative.  This needs to be verified and checked.
      RESOLVED 3/98 Draft - Use CNAME Records and AIIH Server will
      be authoritative for the AIIH ZONE.

   -  Use of TTL for DNS Caches can cause problems for existing IPv4
      applications that cache IPv4 addresses.

      PARTIALLY RESOLVED - 3/98 Draft do not update DNS unless
      application will be permanent as opposed to transient.
      But TTL's that are updated still need some thought for
      legacy applications.  This also points to possibly adding
      new fields to the hostent structure which will at least
      help new IPv6 applications and legacy IPv4 applications
      to change to act in a well behaved manner.

   -  Specification needs a design example to get packets from
      the IPv6 node to an egress IPv4 router.

      PARTIALLY RESOLVED - 3/98 Draft added Design Section discussing
      tunneling mechanisms to be used and added Tunnel End Point address
      supplied by the AIIH DHCPv6 Server.  Still needs more discussion.

   -  NNAT name does not state what the specification does.

      RESOLVED - 3/98 Draft changed name to AIIH.

Appendix B - Draft Changes and Rationale History

Prior to January 1999:

  -  Changed the name of the draft from NNAT to AIIH. This also
      was done to prevent any perceived antagonism towards the NAT
      IETF work, which is not an objective of this work.

   -  Changed the Introduction to be more descriptive of the task
      at hand.

   -  Added IPv4 Global Address definition to terminology section.

   -  Added tunnel routability discussion to Design Model and a
      diagram abstraction to be used by the specification as
      a point of reference.

   -  Added to the architecture the ability for an IPv6 node to
      request an IPv4 Global Address from an AIIH DHCPv6 Server.
      This will permit AIIH to not only be useful for incoming
      IPv4 host communications with IPv6 hosts but also for outgoing
      IPv4 communications to the Internet from IPv6 hosts and for
      Intranet enterprise communications between an IPv6 host and
      IPv4 host.

   -  Hinted that AIIH could be used in future work to define
      the capability for two IPv6 hosts separated by an IPv4 cloud to
      to communicate thru tunnels, like thru a production 6bone
      network on the Internet.

   -  Added new section to define how an IPv6 host can request
      an IPv4 Global Address.

   -  Defined new mechanism for DNS query processing when an IPv6
      host is looked for from an IPv4 host, thru the use of CNAME
      and NS records.  This also permits IPv4 host Intranet queries
      too now.

   -  New text clarifying that within the Intranet processing AIIH
      must only be used with IPv4 Global Addresses and Private
      IPv4 addresses should be retrieved from DHCPv4, via the IPv6
      hosts IPv4 stack.

   -  Added new text defining the AIIH Server and the interaction
      with DHCPv6 and DNS applications.  Also further refined the
      requirements of the AIIH Server model.

   -  Expanded the section on the new DHCPv6 Section defining the
      required Server and Client behavior.  Added support to permit
      AIIH to be used for Intranet and Internet communications from
      within the site.

   -  Changed the DHCPv6 Extension for IPv4 Global Addresses to
      make it an extension which defines the next extension to
      be a request for AIIH processing relative to DHCPv6.

   -  Added a Tunnel End Point address to the new extension so
      IPv6 hosts can configure tunnels to communicate with the
      egress router to transmit or reply with IPv4 on the Internet
      or within the Intranet.

   -  Defined the AIIH side affect requirements for IPv6 hosts using
      this mechanism with DHCPv6.

   -  Updated and added to the Acknowledgment and References Section.

   -  Updated the Open Issues from December 1997 draft and noted
      the status of each issue as STILL OPEN, RESOLVED, or PARTIALLY
      RESOLVED.

   -  Updated the Changes from this draft.

January 1999:

   -  Updated References.

   -  Fixed Edit Issues

   -  Added new Open Issues.

   -  Removed all terms of NNAT except for History.

Acknowledgments

   The author would like to thank Erik Nordmark for spending time
   reviewing with him this idea and suggesting the use of the DHCPv6
   Reconfigure Message, Richard Johnson for suggesting the use of the
   DNS CNAME Record, and Robert Watson who suggested that the AIIH
   DHCPv6 and DNS Server be co-located. George Tsirtsis who suggested
   using AIIH to assign IPv4 Global Addresses to IPv6 hosts in general.
   Richard Draves and Jack McCann who have provided many helpful
   technical suggestions, and the NGTRANS working group for taking the
   time to work on AIIH.

References

   [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-dhcpv6-13.txt March 1998 (work
        in progress).

   [5]  C. Perkins.  Extensions for the Dynamic host Configuration
        Protocol for IPv6.  draft-ietf-dhc-dhcpv6ext-10.txt March
        1998. (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:
        0-201-63357-4).

   [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
    Email: bound@zk3.dec.com

    Laurent Toutain
    ENST Bretagne
    BP 78
    35 512 Cesson S‰vign‰ Cedex
    Phone : +33 2 99 12 70 26
    Email : Laurent.Toutain@enst-bretagne.fr

    Hossam Afifi
    ENST Bretagne
    BP 78
    35 512 Cesson S‰vign‰ Cedex
    Phone : +33 2 99 12 70 36
    Email : Hossam.Afifi@enst-bretagne.fr


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