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Versions: (draft-huang-behave-bih) 00 01 02 03 04 05 06 07 08 09 RFC 6535

Behave WG                                                       B. Huang
Internet-Draft                                                   H. Deng
Obsoletes: 3338, 2767                                       China Mobile
(if approved)                                              T. Savolainen
Intended status: Standards Track                                   Nokia
Expires: April 14, 2011                                 October 11, 2010


            Dual Stack Hosts Using "Bump-in-the-Host" (BIH)
                     draft-ietf-behave-v4v6-bih-00

Abstract

   This document describes the "Bump-In-the-Host" (BIH), a host based
   protocol translation mechanism that allows a subset of applications
   supporting only one IP address family to communicate with peers that
   are reachable or supporting only the other address family.

   This specification addresses scenarios where a host is provided dual
   stack or IPv6 only network connectivity.  In the dual stack network
   case, single address family applications in the host sometime will
   communicate directly with other hosts using the different address
   family.  In the case of IPv6 only network or IPv6 only destination,
   IPv4 originated communications have to be translated into IPv6.  The
   BIH makes the IPv4 applications think they talk to IPv4 peers and
   hence hides the IPv6 from those applications.

   Acknowledgement of previous work

   This document is an update to and directly derivative from Kazuaki
   TSHUCHIYA, Hidemitsu HIGUCHI, and Yoshifumi ATARASHI [RFC2767] and
   from Seungyun Lee, Myung-Ki Shin, Yong-Jin Kim, Alain Durand, and
   Erik Nordmark's [RFC3338], which similarly provides a dual stack host
   means to communicate with other IPv6 host using existing IPv4
   appliations.  This document combines and updates both [RFC2767] and
   [RFC3338].

   The changes in this document reflect five components

      1.  Supporting IPv6 only network connections

      2.  IPv4 address pool use private address instead of the
      unassigned IPv4 addresses (0.0.0.1 - 0.0.0.255)

      3.  Extending ENR and address mapper to operate differently

      4.  Adding an alternative way to implement the ENR




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      5.  Going for standards track instead of experimental/
      informational


Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   This Internet-Draft will expire on April 14, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
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   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.




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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Components of the Bump-in-the-Host . . . . . . . . . . . . . .  6
     2.1.  Function Mapper  . . . . . . . . . . . . . . . . . . . . .  7
     2.2.  Translator . . . . . . . . . . . . . . . . . . . . . . . .  8
     2.3.  Extension Name Resolver  . . . . . . . . . . . . . . . . .  8
       2.3.1.  Reverse DNS lookup . . . . . . . . . . . . . . . . . .  9
     2.4.  Address Mapper . . . . . . . . . . . . . . . . . . . . . .  9
   3.  Behavior and network Examples  . . . . . . . . . . . . . . . . 12
   4.  Considerations . . . . . . . . . . . . . . . . . . . . . . . . 16
     4.1.  Socket API Conversion  . . . . . . . . . . . . . . . . . . 16
     4.2.  ICMP Message Handling  . . . . . . . . . . . . . . . . . . 16
     4.3.  IPv4 Address Pool and Mapping Table  . . . . . . . . . . . 16
     4.4.  Internally Assigned IPv4 Addresses . . . . . . . . . . . . 16
   5.  Considerations due ALG requirements  . . . . . . . . . . . . . 18
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 20
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 21
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 21
   Appendix A.  Implementation option for the ENR . . . . . . . . . . 22
   Appendix B.  API list intercepted by BIH . . . . . . . . . . . . . 23
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25



























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1.  Introduction

   While IPv6 support is being widely introduced throughout the
   Internet, classes of applications are going to remain IPv4-only.
   This document describes a Bump-in-the-Host (BIH), successor and
   combination of Bump-in-the-Stack (BIS) [RFC2767] and Bump-in-the-API
   (BIA) [RFC3338] technologies, which enables accommodation of
   significant set of the legacy IPv4-only applications in the IPv6-
   world.

   Bump-In-the-Host is not a general purpose translation solution.  The
   class of IPv4-only applications the described host-based NAT46
   protocol translation solution provides Internet connectivity over
   IPv6-only network access includes those applications that use DNS for
   IP address resolution and that do not embed IP address literals in
   protocol payloads.  This includes essentially all DNS using legacy
   client-server model applications, which are agnostic on IP address
   family used by the destination, but not other classes of
   applications.  The transition towards IPv6-only Internet is made
   easier by decreasing number of key applications that must be updated
   to IPv6.

   BIH technique includes two major implementation options: inserts a
   protocol translator between the IPv4 and the IPv6 stacks of a host or
   between the socket API module and the TCP/IP module.  Essentially,
   IPv4 is translated into IPv6 at the socket API level or at the IP
   level.

   When the BIH is implemented at the socket API layer, and IPv4
   applications communicate with IPv6 peers, the API translator
   intercepts the socket API functions from IPv4 applications and
   invokes the IPv6 socket API functions to communicate with the IPv6
   hosts, and vice versa.

   When the BIH is implemented at the networking layer, the IPv4 packets
   are intercepted and converted to IPv6 using the IP conversion
   mechanism defined in SIIT [I-D.ietf-behave-v6v4-xlate].  The
   translation has the same benefits and drawbacks as SIIT.

   In order to support communication between IPv4 applications and the
   target IPv6 hosts, pooled IPv4 addresses will be assigned through the
   extension name resolver.

   The BIH can be used whenever an IPv4-only application needs to
   communicate with an IPv6 peer, independently of the address families
   supported by the access network.  Hence the access network can be
   IPv6-only or dual-stack capable.




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   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119] .

   This document uses terms defined in [RFC2460] , [RFC2893] , [RFC2767]
   and [RFC3338].













































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2.  Components of the Bump-in-the-Host

   Figure 1 shows the architecture of the host in which BIH is
   implemented as socket API layer translator, i.e. as the original
   "Bump-in-the-API".


                  +----------------------------------------------+
                  | +------------------------------------------+ |
                  | |                                          | |
                  | |            IPv4 applications             | |
                  | |                                          | |
                  | +------------------------------------------+ |
                  | +------------------------------------------+ |
                  | |           Socket API (IPv4, IPv6)        | |
                  | +------------------------------------------+ |
                  | +-[ API translator]------------------------+ |
                  | | +-----------+ +---------+ +------------+ | |
                  | | | Ext. Name | | Address | | Function   | | |
                  | | | Resolver  | | Mapper  | | Mapper     | | |
                  | | +-----------+ +---------+ +------------+ | |
                  | +------------------------------------------+ |
                  | +--------------------+ +-------------------+ |
                  | |                    | |                   | |
                  | |    TCP(UDP)/IPv4   | |   TCP(UDP)/IPv6   | |
                  | |                    | |                   | |
                  | +--------------------+ +-------------------+ |
                  +----------------------------------------------+

     Figure 1: Architecture of the dual stack host using BIH at socket
                                   layer

   Figure 2 shows the architecture of the host in which BIH is
   implemented as network layer translator, i.e. as the original "Bump-
   in-the-Stack".
















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        +-------------------------------------------------------------+
        |  +-------------------------------------------------------+  |
        |  | IPv4 applications                                     |  |
        |  +-------------------------------------------------------+  |
        |  +-------------------------------------------------------+  |
        |  | TCP/IPv4                                              |  |
        |  |   +---------------------------------------------------+  |
        |  |   |  +-----------+    +---------+  +---------------+     |
        |  |   |  | extension |    | address |  |  translator   |     |
        |  |   |  | name      |    | mapper  |  +---------------+     |
        |  |   |  | resolver  |    |         |  +---------------+     |
        |  |   |  |           |    |         |  |     IPv6      |     |
        |  +---+  +-----------+    +---------+  +---------------+     |
        |  +-------------------------------------------------------+  |
        |  |                  Network card drivers                 |  |
        |  +-------------------------------------------------------+  |
        +-------------------------------------------------------------+
        +-------------------------------------------------------------+
        |                        Network cards                        |
        +-------------------------------------------------------------+

    Figure 2: Architecture of the dual-stack host using BIH at network
                                   layer

   Dual stack hosts defined in RFC2893 [RFC2893] need applications,
   TCP/IP modules and addresses for both IPv4 and IPv6.  The proposed
   hosts in this document have an API or network layer translator to
   communicate with other IPv6 hosts using existing IPv4 applications.
   The BIH translator consists of an extension name resolver, an address
   mapper, and depending on implementation either a function mapper or a
   protocol translator.

2.1.  Function Mapper

   Function mapper translates an IPv4 socket API function into an IPv6
   socket API function, and vice versa.

   When detecting IPv4 socket API function calls from IPv4 applications,
   function mapper intercepts the function calls and invokes new IPv6
   socket API functions which correspond to the IPv4 socket API
   functions.  Those IPv6 API functions are used to communicate with the
   target IPv6 peers.  When detecting IPv6 socket API function calls
   triggered by the data received from the IPv6 peers, function mapper
   works symmetrically in relation to the previous case.







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2.2.  Translator

   Translator translates IPv4 into IPv6 and vice versa using the IP
   conversion mechanism defined in SIIT [I-D.ietf-behave-v6v4-xlate].

   When receiving IPv4 packets from IPv4 applications, translator
   converts IPv4 packet headers into IPv6 packet headers, then, if
   required, fragments the IPv6 packets (because header length of IPv6
   is typically 20 bytes larger than that of IPv4), and sends them to
   IPv6 networks.  When receiving IPv6 packets from the IPv6 networks,
   translator works symmetrically to the previous case, except that
   there is no need to fragment the packets.

2.3.  Extension Name Resolver

   Extension Name Resolver returns a proper answer in response to the
   IPv4 or IPv6 application's name resolution request.

   In the case of socket API implementation option, when an IPv4
   application in an IPv6 only network tries to do forward lookup to
   resolve names via the resolver library (e.g. gethostbyname()), BIH
   intercept the function call and instead calls the IPv6 equivalent
   functions (e.g. getnameinfo()) that will resolve both A and AAAA
   records.

   If only AAAA record is available for the name queried, ENR requests
   the address mapper to assign an IPv4 address corresponding to the
   IPv6 address, creates an A record for the assigned IPv4 address, and
   returns the A record to the IPv4 application.

   If both A and AAAA record are available in the IPv6 only network, ENR
   does not require address mapper to assign IPv4 address, but instead
   asks address mapper to store relationship between the A and AAAA
   records, and then directly passes the received A record to the IPv4
   application.

        Application | Network  | ENR behaviour
        query       | response |
        ------------+----------+---------------------
           A        |   A      |  <return A record>
           A        |   AAAA   |  <synthesize A record>
           A        |  A/AAAA  |  <return A record>

                   Figure 3: ENR behaviour illustration

   NOTE: An implementation option is to have ENR support in host's
   (stub) DNS resolver itself as described in [DNS64], in which case
   record synthesis is not needed and advanced functions such as DNSSEC



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   are possible.  If the ENR is implemented at the network layer, same
   limitations arise as when DNS record synthesis is done on the
   network.  A host also has an option to implement recursive DNS server
   function.

2.3.1.  Reverse DNS lookup

   When an application initiates a reverse DNS query for a PTR record,
   to find a name for an IP address, the ENR MUST check whether the
   queried IP address can be found in the cache of the Address Mapper
   and is a local IP address.  If an entry is found and queried address
   is locally generated, the ENR must initiate corresponding reverse DNS
   query for the real IP address.

   For example, when application initiates reverse DNS query for a
   synthesized locally valid IPv4 address, the ENR needs to intercept
   that query.  The ENR shall perform reverse query procedure for the
   destination's IPv6 address and return the name received as response
   to IPv6 reverse query to application that initiated the IPv4 query.

2.4.  Address Mapper

   Address mapper ("the mapper" later on), maintains an IPv4 address
   pool in the case of dual stack network and IPv6 only network.  The
   pool consists of private IPv4 addresses as per [RFC1918].  Also,
   mapper maintains a table consisting of pairs of these locally
   selected IPv4 addresses and destinationss' IPv6 addresses.

   When the extension name resolver, translator, or the function mapper
   requests mapper to assign an IPv4 address corresponding to an IPv6
   address, mapper selects and returns an IPv4 address out of the pool,
   and registers a new entry into the table dynamically.  The
   registration occurs in the following 3 cases:

   (1) When the extension name resolver gets only an 'AAAA' record for
   the target host name in the dual stack or IPv6 only network and there
   is not a mapping entry for the IPv6 address.

   (2) When the extension name resolver gets both an 'A' record and an
   'AAAA' record for the target host name in the IPv6 only network and
   there is not a mapping entry for the IPv6 address.  But ENR does not
   need an IPv4 address out of the pool, just to register both IPv4 and
   IPv6 addresses from 'A' and 'AAAA' records into a new entry into the
   table.

   (3) When the function mapper gets a socket API function call from the
   data received and there is not a mapping entry for the IPv6 source
   address.



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   When the resolver, translator, or the function mapper requests mapper
   to assign an IPv4 address corresponding to an IPv6 address, mapper,
   if required, selects and returns an IPv4 address out of the pool, and
   registers a new entry into the table dynamically.  The following
   table describes how mappings are created into the table in each
   possible scenario:

          Mapping table | Access    | Peer   | Created
          entry for     |link type  | support| address mapping
       -------------------+-------------+-------------------------------
       (1) real IPv4    |IPv4 or DS | v4     | < no mapping needed >
       (2) real IPv6    |IPv6 or DS | v6     | < no mapping needed >
       (3) real IPv4    |IPv6       | v4 & v6| real IPv4 -> real IPv6
       (4) real IPv6    |IPv4       | v4 & v6| < out of scope >
       (5) local IPv4   |IPv6 or DS | v6     | local IPv4 -> real IPv6
       (6) local IPv6   |IPv4 or DS | v4     | < out of scope >
       (7) real IPv4    |IPv6       | v4     | < out of scope >
       (8) real IPv6    |IPv4       | v6     | < out of scope >

           Figure 4: Address Mapper's mapping table illustration

   Below are examples for all eight scenarios:

   (1) When the resolver gets an 'A' reply for application's 'A' query
   on access network supporting IPv4, there is no need to create mapping
   (or just stub mapping real IPv4 -> real IPv4).

   (2) When the resolver gets an 'AAAA' reply for application's 'AAAA'
   query on access network supporting IPv6, there is no need to create
   mapping (or just stub mapping real IPv6 -> real IPv6).

   (3) When the resolver gets both 'A' and 'AAAA' replies for
   application's 'A' query on IPv6-only access, there shall be mapping
   for real IPv4 to real IPv6.

   (4) The scenario where the resolver gets both 'A' and 'AAAA' replies
   for application's 'AAAA' query on IPv4-only access is out of scope.

   (5) When the resolver gets only an 'AAAA' record for the target host
   name for application's 'A' request on IPv6 only or DS access network,
   a local IPv4 address will be given to application and mapping for
   local IPv4 address to real IPv6 address is created.

   (6) The scenario where the resolver gets only an 'A' record for the
   target host name for application's 'AAAA' request on IPv4 only or DS
   access network is out of scope.

   (7) The scenario where the resolver gets only an 'A' record for the



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   target host name for application's 'A' request on IPv6 only access
   network is out of scope.

   (8) The scenario where the resolver gets only an 'AAAA' record for
   the target host name for application's 'AAAA' request on IPv4 only
   access network is out of scope.

   NOTE: There is only one exception.  When initializing the table,
   mapper registers a pair of its own IPv4 address and IPv6 address into
   the table statically.









































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3.  Behavior and network Examples

   Figure 5 illustrates the very basic network scenario.  An IPv4-only
   application is running on a host attached to IPv6-only Internet and
   is talking to IPv6 enabled server.  A communication is made possible
   by bump in the host.

     +----+                                   +-------------+
     | H1 |----------- IPv6 Internet -------- | IPv6 server |
     +----+                                   +-------------+
     v4 only
     application

                       Figure 5: Network Scenario #1

   Figure 6 illustrates a possible network scenario where an IPv4-only
   application is running on a host attached to a dual-stack network,
   but the destination server is running on a private site that is
   numbered with public IPv6 addresses and private IPv4 addresses
   without port forwarding setup on NAT44.  The only means to contact to
   server is to use IPv6.

     +----------------------+  +------------------------------+
     | Dual Stack Internet  |  | IPv4 Private site (Net 10)   |
     |                      |  |                              |
     |                      |  |                 +----------+ |
     |                      |  |                 |          | |
     |  +----+           +---------+             |          | |
     |  | H1 |--------   |  NAT44  |-------------|  Server  | |
     |  +----+           +---------+             |          | |
     | v4 only              |  |                 +----------+ |
     | application          |  |                  Dual Stack  |
     |                      |  |                etc. 10.1.1.1 |
     |                      |  |                 AAAA:2009::1 |
     |                      |  |                              |
     +----------------------+  +------------------------------+

                       Figure 6: Network Scenario #2

   Illustrations of host behavior in both implementation options are
   given here.  Figure 7 illustrates the setup where BIH is implemented
   as a bump in the API, and figure 8 illustrates the setup where BIH is
   implemented as a bump in the stack.

 "dual stack"                                                "host6"
 IPv4    Socket |     [ API Translator ]    | TCP(UDP)/IP          Name
 appli-  API    | ENR      Address  Function| (v6/v4)             Server
 cation         |          Mapper   Mapper  |



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  |        |        |        |        |         |              |       |
 <<Resolve an IPv4 address for "host6".>>       |              |       |
  |        |        |        |        |         |              |       |
  |--------|------->|  Query of 'A' records for host6.         |       |
  |        |        |        |        |         |              |       |
  |        |        |--------|--------|---------|--------------|------>|
  |        |        |  Query of 'A' records and 'AAAA' for host6       |
  |        |        |        |        |         |              |       |
  |        |        |<-------|--------|---------|--------------|-------|
  |        |        |  Reply with the 'AAAA' record.           |       |
  |        |        |        |        |         |              |
  |        |        |<<The 'AAAA' record is resolved.>>        |
  |        |        |        |        |         |              |
  |        |        |+++++++>|  Request one IPv4 address       |
  |        |        |        |  corresponding to the IPv6 address.
  |        |        |        |        |         |              |
  |        |        |        |<<Assign one IPv4 address.>>     |
  |        |        |        |        |         |              |
  |        |        |<+++++++|  Reply with the IPv4 address.   |
  |        |        |        |        |         |              |
  |        |        |<<Create 'A' record for the IPv4 address.>>
  |        |        |        |        |         |              |
  |<-------|--------| Reply with the 'A' record.|              |
  |        |        |        |        |         |              |
  |        |        |        |        |         |              |
 <<Call IPv4 Socket API function >>   |         |              |
  |        |        |        |        |         |              |
  |========|========|========|=======>|An IPv4 Socket API function Call
  |        |        |        |        |         |              |
  |        |        |        |<+++++++|  Request IPv6 addresses|
  |        |        |        |        |  corresponding to the  |
  |        |        |        |        |  IPv4 addresses.       |
  |        |        |        |        |         |              |
  |        |        |        |+++++++>| Reply with the IPv6 addresses.
  |        |        |        |        |         |              |
  |        |        |        |        |<<Translate IPv4 into IPv6.>>
  |        |        |        |        |         |              |
  |  An IPv6 Socket API function call.|=========|=============>|
  |        |        |        |        |         |              |
  |        |        |        |        |<<Reply an IPv6 data    |
  |        |        |        |        |  to dual stack.>>      |
  |        |        |        |        |         |              |
  |  An IPv6 Socket API function call.|<========|==============|
  |        |        |        |        |         |              |
  |        |        |        |        |<<Translate IPv6 into IPv4.>>
  |        |        |        |        |         |              |
  |        |        |        |<+++++++|  Request IPv4 addresses|
  |        |        |        |        |  corresponding to the  |



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  |        |        |        |        |  IPv6 addresses.       |
  |        |        |        |        |         |              |
  |        |        |        |+++++++>| Reply with the IPv4 addresses.
  |        |        |        |        |         |              |
  |<=======|========|========|========|  An IPv4 Socket function call.
  |        |        |        |        |         |              |

                 Figure 7: Example of BIH as API addition


      "dual stack"                                            "host6"
      IPv4    TCP/    ENR     address  translator  IPv6
      appli-  IPv4            mapper
      cation
        |      |       |         |       |           |         |
      <<Resolve an IPv4 address for "host6".>>       |         |
        |      |       |         |       |           |         |
        |------|------>|  Query of 'A' records for "host6".    | Name
        |      |       |         |       |           |         | Server
        |      |       |---------|-------|-----------|---------|--->|
        |      |       |  Query of 'A' records and 'AAAA' for "host6"
        |      |       |         |       |           |         |    |
        |      |       |<--------|-------|-----------|---------|----|
        |      |       |  Reply only with 'AAAA' record.       |
        |      |       |         |       |           |         |
        |      |       |<<Only 'AAAA' record is resolved.>>    |
        |      |       |         |       |           |         |
        |      |       |-------->|  Request one IPv4 address   |
        |      |       |         |  corresponding to the IPv6 address.
        |      |       |         |       |           |         |
        |      |       |         |<<Assign one IPv4 address.>> |
        |      |       |         |       |           |         |
        |      |       |<--------|  Reply with the IPv4 address.
        |      |       |         |       |           |         |
        |      |       |<<Create 'A' record for the IPv4 address.>>
        |      |       |         |       |           |         |
        |<-----|-------|  Reply with the 'A' record. |         |
        |      |       |         |       |           |         |
        |      |       |         |       |           |         |
      <<Send an IPv4 packet to "host6".>>|           |         |
        |      |       |         |       |           |         |
        |======|=======|=========|======>|  An IPv4 packet.    |
        |      |       |         |       |           |         |
        |      |       |         |<------|  Request IPv6 addresses
        |      |       |         |       |  corresponding to the IPv4
        |      |       |         |       |  addresses.         |
        |      |       |         |       |           |         |
        |      |       |         |------>|  Reply with the IPv6|



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        |      |       |         |       |  addresses.         |
        |      |       |         |       |           |         |
        |      |       |         |       |<<Translate IPv4 into IPv6.>>
        |      |       |         |       |           |         |
        |      |       |An IPv6 packet.  |===========|========>|
        |      |       |         |       |           |         |
        |      |       |         |     <<Reply an IPv6 packet to
        |      |       |         |       "dual stack".>>       |
        |      |       |         |       |           |         |
        |      |       |An IPv6 packet.  |<==========|=========|
        |      |       |         |       |           |         |
        |      |       |         |       |<<Translate IPv6 into IPv4.>>
        |      |       |         |       |           |         |
        |<=====|=======|=========|=======|  An IPv4 packet.    |
        |      |       |         |       |           |         |

                 Figure 8: Example of BIH at network layer


































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4.  Considerations

4.1.  Socket API Conversion

   IPv4 socket API functions are translated into semantically as same
   IPv6 socket API functions as possible and vice versa.  See Appendix B
   for the API list intercepted by BIH.  However, IPv4 socket API
   functions are not fully compatible with IPv6 since the IPv6 has new
   advanced features.

4.2.  ICMP Message Handling

   When an application needs ICMP messages values (e.g., Type, Code,
   etc.) sent from a network layer, ICMPv4 message values MAY be
   translated into ICMPv6 message values based on SIIT
   [I-D.ietf-behave-v6v4-xlate], and vice versa.  It can be implemented
   using raw socket.

4.3.  IPv4 Address Pool and Mapping Table

   The address pool consists of the private IPv4 addresses as per
   [RFC1918].  This pool can be implemented at different granularity in
   the node e.g., a single pool per node, or at some finer granularity
   such as per user or per process.  However, if a number of IPv4
   applications communicate with IPv6 hosts, the available address
   spaces may be exhausted.  As a result, it will be impossible for IPv4
   applications to communicate with IPv6 nodes.  It requires smart
   management techniques for address pool.  For example, it is desirable
   for the mapper to free the oldest entry and reuse the IPv4 address or
   IPv6 address for creating a new entry.  In case of a per-node address
   mapping table, it MAY cause a larger risk of running out of address.

   The RFC1918 address space was chosen because generally legacy
   applications understand that as a private address space.  A new
   dedicated address space would run a risk of not being understood by
   applications as private.  The RFC1918 addresses have a risk of
   conflicting with other interfaces.  The conflicts can be mitigated by
   using least commonly used network number of the RFC1918 address space
   (TO BE SELECTED) and, if possible, cease using BIH on IPv6-interface
   after an IPv4-enabled interface is activated.

4.4.  Internally Assigned IPv4 Addresses

   The IPv4 addresses, which are internally assigned to IPv6 target
   hosts out of the pool, are the private IPv4 addresses.  IPv4
   addresses, which are internally assigned to IPv6 target hosts out of
   the spool, never flow out from the host, and so do not negatively
   affect other hosts.



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   The internally assigned IPv4 address, which applications see as the
   source address, MUST be from different subnet than the IPv4 addresses
   used by the address synthesis function.  This approach ensures legacy
   applications realize they are not on the same link with their
   destination node and if needed, will trigger NAT traversal procedures
   such as keep-alive message sending.













































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5.  Considerations due ALG requirements

   No ALG functionality is specified herein as ALG design is generally
   not encouraged for host based translation and as BIH is intended for
   applications not including IP addresses in protocol payloads.














































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6.  Security Considerations

   The security consideration of BIH mostly relies on that of
   [I-D.ietf-behave-v6v4-xlate-stateful].

   In the socket layer implementation approach the differences are due
   to the address translation occurring at the API and not in the
   network layer.  That is, since the mechanism uses the API translator
   at the socket API level, hosts can utilize the security of the
   network layer (e.g., IPSec) when they communicate with IPv6 hosts
   using IPv4 applications via the mechanism.  As well, there is no need
   for DNS ALG as in NAT-PT, so there is no interference with DNSSEC
   either.

   In the network layer implementation approach hosts cannot utilize the
   security above network layer when they communicate with IPv6 hosts
   using IPv4 applications via BIH and encrypt embedded IP addresses, or
   when the protocol data is encrypted using IP addresses as keys.  In
   these cases it is impossible for the mechanism to translate the IPv4
   data into IPv6 and vice versa.  Therefore it is highly desirable to
   upgrade to the applications modified into IPv6 for utilizing the
   security at communication with IPv6 hosts.

   The use of address pooling may open a denial of service attack
   vulnerability.  So BIH should employ the same sort of protection
   techniques as NAT64 [I-D.ietf-behave-v6v4-xlate-stateful] does.

























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7.  Acknowledgments

   The author thanks the discussion from Gang Chen, Dapeng Liu, Bo Zhou,
   Hong Liu, Tao Sun, Zhen Cao, Feng Cao et al. in the development of
   this document.

   The efforts of Suresh Krishnan, Mohamed Boucadair, Yiu L. Lee, James
   Woodyatt, Lorenzo Colitti, Qibo Niu, Pierrick Seite, Dean Cheng,
   Christian Vogt, Jan M. Melen, in reviewing this document are
   gratefully acknowledged.

   Advice from Dan Wing, Dave Thaler and Magnus Westerlund are greatly
   appreciated

   The authors of RFC2767 acknowledged WIDE Project, Kazuhiko YAMAMOTO,
   Jun MURAI, Munechika SUMIKAWA, Ken WATANABE, and Takahisa MIYAMOTO.
   The authors of RFC3338 acknowledged implementation contributions by
   Wanjik Lee (wjlee@arang.miryang.ac.kr) and i2soft Corporation
   (www.i2soft.net).
































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8.  References

8.1.  Normative References

   [I-D.ietf-behave-v6v4-xlate]
              Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", draft-ietf-behave-v6v4-xlate-23 (work in
              progress), September 2010.

   [I-D.ietf-behave-v6v4-xlate-stateful]
              Bagnulo, M., Matthews, P., and I. Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers",
              draft-ietf-behave-v6v4-xlate-stateful-12 (work in
              progress), July 2010.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC2767]  Tsuchiya, K., HIGUCHI, H., and Y. Atarashi, "Dual Stack
              Hosts using the "Bump-In-the-Stack" Technique (BIS)",
              RFC 2767, February 2000.

   [RFC2893]  Gilligan, R. and E. Nordmark, "Transition Mechanisms for
              IPv6 Hosts and Routers", RFC 2893, August 2000.

   [RFC3338]  Lee, S., Shin, M-K., Kim, Y-J., Nordmark, E., and A.
              Durand, "Dual Stack Hosts Using "Bump-in-the-API" (BIA)",
              RFC 3338, October 2002.

8.2.  Informative References

   [RFC2553]  Gilligan, R., Thomson, S., Bound, J., and W. Stevens,
              "Basic Socket Interface Extensions for IPv6", RFC 2553,
              March 1999.









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Appendix A.  Implementation option for the ENR

   It is not necessary to implement the ENR at the kernel level, but it
   can be implemented instead at the user space by setting the host's
   default DNS server to point to 127.0.0.1.  DNS queries would then
   always be sent to the ENR, which furthermore ensures both A and AAAA
   queries are sent to the actual DNS server and A queries are always
   answered and required mappings created.











































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Appendix B.  API list intercepted by BIH

   The following functions are the API list which SHOULD be intercepted
   by BIH module when implemented at socket layer.

   The functions that the application uses to pass addresses into the
   system are:

      bind()

      connect()

      sendmsg()

      sendto()

   The functions that return an address from the system to an
   application are:

      accept()

      recvfrom()

      recvmsg()

      getpeername()

      getsockname()

   The functions that are related to socket options are:

      getsocketopt()

      setsocketopt()

   The functions that are used for conversion of IP addresses embedded
   in application layer protocol (e.g., FTP, DNS, etc.) are:

      recv()

      send()

      read()

      write()

   As well, raw sockets for IPv4 and IPv6 MAY be intercepted.




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   Most of the socket functions require a pointer to the socket address
   structure as an argument.  Each IPv4 argument is mapped into
   corresponding an IPv6 argument, and vice versa.

   According to [RFC2553], the following new IPv6 basic APIs and
   structures are required.

         IPv4                     new IPv6
         ------------------------------------------------
         AF_INET                  AF_INET6
         sockaddr_in              sockaddr_in6
         gethostbyname()          getaddrinfo()
         gethostbyaddr()          getnameinfo()
         inet_ntoa()/inet_addr()  inet_pton()/inet_ntop()
         INADDR_ANY               in6addr_any

                                 Figure 9

   BIH MAY intercept inet_ntoa() and inet_addr() and use the address
   mapper for those.  Doing that enables BIH to support literal IP
   addresses.

   The gethostbyname() call return a list of addresses.  When the name
   resolver function invokes getaddrinfo() and getaddrinfo() returns
   multiple IP addresses, whether IPv4 or IPv6, they SHOULD all be
   represented in the addresses returned by gethostbyname().  Thus if
   getaddrinfo() returns multiple IPv6 addresses, this implies that
   multiple address mappings will be created; one for each IPv6 address.























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Authors' Addresses

   Bill Huang
   China Mobile
   53A,Xibianmennei Ave.,
   Xuanwu District,
   Beijing  100053
   China

   Email: bill.huang@chinamobile.com


   Hui Deng
   China Mobile
   53A,Xibianmennei Ave.,
   Xuanwu District,
   Beijing  100053
   China

   Email: denghui02@gmail.com


   Teemu Savolainen
   Nokia
   Hermiankatu 12 D
   FI-33720 TAMPERE
   Finland

   Email: teemu.savolainen@nokia.com






















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