


Behave WG                                                       B. Huang
Internet-Draft                                                   H. Deng
Obsoletes: 3338, 2767                                       China Mobile
(if approved)                                              T. Savolainen
Intended status: Standards Track                                   Nokia
Expires: July 19, 2012                                  January 16, 2012


            Dual Stack Hosts Using "Bump-in-the-Host" (BIH)
                     draft-ietf-behave-v4v6-bih-09

Abstract

   Bump-In-the-Host (BIH) is a host-based IPv4 to IPv6 protocol
   translation mechanism that allows a class of IPv4-only applications
   that work through NATs to communicate with IPv6-only peers.  The host
   on which applications are running may be connected to IPv6-only or
   dual-stack access networks.  BIH hides IPv6 and makes the IPv4-only
   applications think they are talking with IPv4 peers by local
   synthesis of IPv4 addresses.  This document obsoletes RFC 2767 and
   RFC 3338.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on July 19, 2012.

Copyright Notice

   Copyright (c) 2012 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   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
   material may not have granted the IETF Trust the right to allow
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   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   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
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
     1.2.  Acknowledgement of previous work . . . . . . . . . . . . .  5
   2.  Components of the Bump-in-the-Host . . . . . . . . . . . . . .  6
     2.1.  Function Mapper  . . . . . . . . . . . . . . . . . . . . .  8
     2.2.  Protocol translator  . . . . . . . . . . . . . . . . . . .  8
     2.3.  Extension Name Resolver  . . . . . . . . . . . . . . . . .  8
       2.3.1.  Special exclusion sets for A and AAAA records  . . . .  9
       2.3.2.  DNSSEC support . . . . . . . . . . . . . . . . . . . . 10
       2.3.3.  Reverse DNS lookup . . . . . . . . . . . . . . . . . . 10
       2.3.4.  DNS caches and synthetic IPv4 addresses  . . . . . . . 10
     2.4.  Address Mapper . . . . . . . . . . . . . . . . . . . . . . 11
   3.  Behavior and Network Examples  . . . . . . . . . . . . . . . . 12
   4.  Considerations . . . . . . . . . . . . . . . . . . . . . . . . 16
     4.1.  Socket API Conversion  . . . . . . . . . . . . . . . . . . 16
     4.2.  Socket bindings  . . . . . . . . . . . . . . . . . . . . . 16
     4.3.  ICMP Message Handling  . . . . . . . . . . . . . . . . . . 16
     4.4.  IPv4 Address Pool and Mapping Table  . . . . . . . . . . . 16
     4.5.  Multi-interface  . . . . . . . . . . . . . . . . . . . . . 17
     4.6.  Multicast  . . . . . . . . . . . . . . . . . . . . . . . . 18
   5.  Application-Level Gateway requirements considerations  . . . . 19
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
     7.1.  Implications on End-to-End Security  . . . . . . . . . . . 21
     7.2.  Filtering  . . . . . . . . . . . . . . . . . . . . . . . . 21
     7.3.  Attacks on BIH . . . . . . . . . . . . . . . . . . . . . . 21
     7.4.  DNS considerations . . . . . . . . . . . . . . . . . . . . 22
   8.  Changes since RFC2767 and RFC3338  . . . . . . . . . . . . . . 23
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 24
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 25
     10.2. Informative References . . . . . . . . . . . . . . . . . . 25
   Appendix A.  API list intercepted by BIH . . . . . . . . . . . . . 27
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29















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1.  Introduction

   This document describes Bump-in-the-Host (BIH), a successor and
   combination of the Bump-in-the-Stack (BIS)[RFC2767] and Bump-in-the-
   API (BIA) [RFC3338] technologies, which enable IPv4-only legacy
   applications to communicate with IPv6-only servers by synthesizing
   IPv4 addresses from AAAA records.  Section 8 describes the reasons
   for making RFC2767 and RFC3338 obsolete.

   The supported class of applications includes those that use DNS for
   IP address resolution and that do not embed IP address literals in
   application-protocol payloads.  This includes legacy client-server
   applications using the DNS that are agnostic to the IP address family
   used by the destination and that are able to do NAT traversal.  The
   synthetic IPv4 addresses shown to applications are taken from the
   RFC1918 private address pool in order to ensure that possible NAT
   traversal techniques will be initiated.

   IETF recommends using dual-stack or tunneling based solutions for
   IPv6 transition and specifically recommends against deployments
   utilizing double protocol translation.  Use of BIH together with a
   NAT64 is NOT RECOMMENDED [RFC6180].

   BIH includes two major implementation alternatives: a protocol
   translator between the IPv4 and the IPv6 stacks of a host, or an API
   translator between the IPv4 socket API module and the TCP/IP module.
   Essentially, IPv4 is translated into IPv6 at the socket API layer or
   at the IP layer, former of which is the recommended implementation
   alternative.

   When BIH is implemented at the socket API layer, the translator
   intercepts IPv4 socket API function calls and invokes corresponding
   IPv6 socket API function calls to communicate with IPv6 hosts.

   When BIH is implemented at the network layer the IPv4 packets are
   intercepted and converted to IPv6 using the IP conversion mechanism
   defined in Stateless IP/ICMP Translation Algorithm (SIIT) [RFC6145].
   The protocol translation has the same benefits and drawbacks as SIIT.

   The location of the BIH refers to the location of the protocol
   translation function.  The location of the IPv4 address and DNS A
   record synthesis function is orthogonal to the location of the
   protocol translation, and may or may not happen at the same location.

   BIH can be used whenever an IPv4-only application needs to
   communicate with an IPv6-only server, 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", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119] .

   This document uses terms defined in [RFC2460] and [RFC4213].

1.1.  Terminology

   DNS synthesis

      DNS, A record, synthesis is a process where A type of DNS record
      is created by Extension Name Resolver to contain synthetic IPv4
      address.

   Real IPv4 address

      An IPv4 address of a remote node a host has learned, for example,
      from DNS response to an A query.

   Real IPv6 address

      An IPv6 address of a remote node a host has learned, for example,
      from DNS response to an AAAA query.

   Synthetic IPv4 address

      An IPv4 address that has meaning only inside a host and that is
      used to provide IPv4 representation of remote node's real IPv6
      address.

1.2.  Acknowledgement of previous work

   This document is a direct derivative from Kazuaki TSHUCHIYA,
   Hidemitsu HIGUCHI, and Yoshifumi ATARASHI's Bump-in-the-Stack
   [RFC2767] and from Seungyun Lee, Myung-Ki Shin, Yong-Jin Kim, Alain
   Durand, and Erik Nordmark's Bump-in-the-API [RFC3338], which
   similarly provides IPv4-only applications on dual-stack hosts the
   means to operate over IPv6.  Section 8 covers the changes since those
   documents.











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

   Figure 1 shows the architecture of a host in which BIH is implemented
   as a socket API layer translator, i.e., as a "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 a dual stack host using protocol
                        translation at socket layer

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


















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      +------------------------------------------------------------+
      |  +------------------------------------------+              |
      |  |    IPv4 applications                     |              |
      |  |    Host's main DNS resolver              |              |
      |  +------------------------------------------+              |
      |  +------------------------------------------+              |
      |  |    TCP/UDP                               |              |
      |  +------------------------------------------+              |
      |  +------------------------------------------+ +---------+  |
      |  |    IPv4                                  | |         |  |
      |  +------------------------------------------+ | Address |  |
      |  +------------------+ +---------------------+ | Mapper  |  |
      |  |    Protocol      | |   Extension Name    | |         |  |
      |  |    Translator    | |   Resolver          | |         |  |
      |  +------------------+ +---------------------+ |         |  |
      |  +------------------------------------------+ |         |  |
      |  |    IPv4 / IPv6                           | |         |  |
      |  +------------------------------------------+ +---------+  |
      +------------------------------------------------------------+

        Figure 2: Architecture of a dual-stack host using protocol
                     translation at the network layer

   Dual stack hosts defined in RFC 4213 [RFC4213] 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
   allow legacy IPv4 applications to communicate with IPv6-only peers.
   The BIH architecture consists of an Extension Name Resolver, an
   Address Mapper, and depending on implementation either a Function
   Mapper or a Protocol Translator.  It is worth noting that the
   Extension Name Resolver's placement is orthogonal to the placement of
   protocol translation.  For example, the Extension Name Resolver may
   reside in the socket API while protocol translation takes place at
   the network layer.

   The choice between the socket API and the network layer architectures
   varies case by case.  While the socket API architecture alternative
   is the recommended one, it may not always be possible to choose.
   This may be the case, for example, when the used operating system
   does not allow modifications to be done for API implementations, but
   does allow addition of virtual network interfaces and related
   software modules.  On the other hand, sometimes it may not be
   possible to introduce protocol translators inside the operating
   system, but it may be easy to modify implementations behind the API
   provided for applications.  The choice of architecture also depends
   on who is creating implementation of BIH.  For example, an
   application framework provider, an operating system provider, and a
   device vendor may all choose different approaches due their different



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   positions.

2.1.  Function Mapper

   The function mapper translates an IPv4 socket API function into an
   IPv6 socket API function.

   When detecting IPv4 socket API function calls from IPv4 applications,
   the function mapper MUST intercept the function calls and invoke IPv6
   socket API functions that correspond to the IPv4 socket API
   functions.

   The function mapper MUST NOT perform function mapping when the
   application is initiating communications to the address range used by
   local synthesis and the address mapping table does not have an entry
   mathching the address.

   See Appendix A for an informational list of functions that would be
   appropriate to intercept by the function mapper.

2.2.  Protocol translator

   The protocol translator translates IPv4 into IPv6 and vice versa
   using the IP conversion mechanism defined in SIIT [RFC6145].  To
   avoid unnecessary fragmentation, the host's IPv4 module SHOULD be
   configured with a small enough MTU (MTU of the IPv6 enabled link - 20
   bytes).

   Protocol translation cannot be performed for IPv4 packets sent to the
   IPv4 address range used by local synthesis and for which a mapping
   table entry does not exist.  The implementation SHOULD attempt to
   route such packets via IPv4 interfaces instead.

2.3.  Extension Name Resolver

   The Extension Name Resolver (ENR) returns an answer in response to
   the IPv4 application's name resolution request.

   In the case of the socket API layer implementation alternative, when
   an IPv4 application tries to do a forward lookup to resolve names via
   the resolver library (e.g., gethostbyname()), BIH intercepts the
   function call and instead calls the IPv6 equivalent functions (e.g.,
   getaddrinfo()) that will resolve both A and AAAA records.  This
   implementation alternative is name resolution protocol agnostic, and
   hence supports techniques such as "hosts-file", NetBIOS, mDNS, and
   anything else the underlying operating system uses.

   In the case of the network layer implementation alternative, the ENR



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   intercepts the A query and creates an additional AAAA query with
   similar content.  The ENR will then collect replies to both A and
   AAAA queries and, depending on results, either return an A reply
   unmodified or synthesize a new A reply.  If no reply for A query is
   received after ENR implementation specific timeout, after reception
   of positive AAAA response, the ENR MAY choose to proceed as if there
   were only AAAA record available for the destination.

   The network layer implementation alternative will only be able to
   catch applications' name resolution requests that result in actual
   DNS queries, hence is more limited when compared to the socket API
   layer implementation alternative.  Hence the socket API layer
   alternative is RECOMMENDED.

   In either implementation alternative, if DNS A record reply contains
   non-excluded real IPv4 addresses the ENR MUST NOT synthesize IPv4
   addresses.

   The ENR asks the address mapper to assign a synthetic IPv4 address
   corresponding to each received IPv6 address if the A record query
   resulted in negative response, all received real IPv4 addresses were
   excluded, or the A query timed out.  The timeout value is
   implementation specific and may be short in order to provide good
   user experience.

   In the case of the API layer implementation alternative, the ENR will
   simply make the API (e.g. gethostbyname) return the synthetic IPv4
   address.  In the case of the network-layer implementation
   alternative, the ENR synthesizes an A record for the assigned
   synthetic IPv4 address, and delivers it up the stack.  If the
   response contains a CNAME or a DNAME record, then the CNAME or DNAME
   chain is followed until the first terminating A or AAAA record is
   reached.

   Application    | Network               | ENR behavior
     query        | response              |
   ---------------+-----------------------+----------------------------
 IPv4 address(es) | IPv4 address(es)      | return real IPv4 address(es)
 IPv4 address(es) | IPv6 address(es)      | synthesize IPv4 address(es)
 IPv4 address(es) | IPv4/IPv6 address(es) | return real IPv4 address(es)

                    Figure 3: ENR behavior illustration

2.3.1.  Special exclusion sets for A and AAAA records

   An ENR implementation SHOULD by default exclude certain real IPv4 and
   IPv6 addresses seen on received A and AAAA records.  The addresses to
   be excluded by default MAY include addresses such as those that



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   should not appear in the DNS or on the wire (see [RFC6147] section
   5.1.4 and [RFC5735]).  Additional addresses MAY be excluded based on
   possibly configurable local policies.

2.3.2.  DNSSEC support

   When the ENR is implemented at the network layer, the A record
   synthesis can cause similar issues as are described in [RFC6147]
   section 3.  While running BIH, the main resolver of the host SHOULD
   NOT perform validation of A records as synthetic A records created by
   ENR would fail in validation.  While not running BIH, host's resolver
   can use DNSSEC in the same way that any other resolver can.  The ENR
   MAY support DNSSEC, in which case the (stub) resolver on a host can
   be configured to trust validations done by the ENR located at the
   network layer.  In some cases the host's validating stub resolver can
   implement the ENR by itself.

   When the ENR is implemented at the socket API level, there are no
   issues with DNSSEC use, as the ENR itself uses socket APIs for DNS
   resolution.  This approach is RECOMMENDED.

2.3.3.  Reverse DNS lookup

   When an application requests a reverse lookup (PTR query) for an IPv4
   address, the ENR MUST check whether the queried IPv4 address can be
   found in the Address Mapper's mapping table and is a synthetic IPv4
   address.  If an entry is found and the queried IPv4 address is
   synthetic, the ENR MUST initiate a corresponding reverse lookup for
   the real IPv6 address.  In the case where the application requested a
   reverse lookup for an address not part of the synthetic IPv4 address
   pool, e.g., a global address, the request MUST be passed on
   unmodified.

   For example, when an application requests a reverse lookup for a
   synthetic IPv4 address, the ENR needs to intercept that query.  The
   ENR asks the address mapper for the real IPv6 address that
   corresponds to the synthetic IPv4 address.  The ENR shall perform a
   reverse lookup procedure for the destination's IPv6 address and
   return the name received as a response to the application that
   initiated the IPv4 query.

2.3.4.  DNS caches and synthetic IPv4 addresses

   When BIH shuts down or address mapping table entries are cleared for
   any reason, DNS cache entries for synthetic IPv4 addresses MUST be
   flushed.  There may be a DNS cache in the network-layer ENR itself,
   but also at the host's stub resolver.




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2.4.  Address Mapper

   The address mapper maintains an IPv4 address pool that can be used
   for IPv4 address synthesis.  The pool consists of [RFC1918] IPv4
   addresses as per section 4.4.  Also, the address mapper maintains a
   table consisting of pairs of synthetic IPv4 addresses and
   destinations' real IPv6 addresses.

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

   (1) When the extension name resolver gets only IPv6 addresses for the
   target host name and there is no existing mapping entry for the IPv6
   addresses.  One or more synthetic IPv4 addresses will be returned to
   the application and mappings for synthetic IPv4 addresses to real
   IPv6 addresses are created.

   (2) When the extension name resolver gets both real IPv4 and IPv6
   addresses, but the real IPv4 addresses contain only excluded IPv4
   addresses (e.g., 127.0.0.1).  The behavior will follow case (1).

   (3) When the function mapper is triggered by a received IPv6 packet
   and there is no existing mapping entry for the IPv6 source address
   (for example, the client sent a UDP request to an anycast address but
   a response was received from a unicast address).

   Other possible combinations are outside of BIH and BIH is not
   involved in those.



















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3.  Behavior and Network Examples

   Figure 4 illustrates a very basic network scenario.  An IPv4-only
   application is running on a host attached to the IPv6-only Internet
   and is talking to an IPv6-only server.  Communication is made
   possible by Bump-In-the-Host.

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

                       Figure 4: Network Scenario #1

   Figure 5 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 not globally reachable IPv4
   addresses, such as [RFC1918] addresses, without port forwarding set
   up on the NAT44.  The only means to contact the server is to use
   IPv6.

     +----------------------+  +------------------------------+
     | Dual Stack Internet  |  | IPv4 Private site (Net 10)   |
     |                      |  | IPv6 routed site             |
     |                   +---------+             +----------+ |
     |                 +-|  NAT44  |-------------+          | |
     |  +----+         | +---------+             |          | |
     |  | H1 |---------+    |  |                 |  Server  | |
     |  +----+         | +-----------+           |          | |
     | v4 only         +-|IPv6 Router|-----------+          | |
     | application       +-----------+           +----------+ |
     |                      |  |                  Dual Stack  |
     |                      |  |                    10.1.1.1  |
     |                      |  |                 2001:DB8::1  |
     +----------------------+  +------------------------------+

                       Figure 5: Network Scenario #2

   Illustrations of host behavior in both implementation alternatives
   are given here.  Figure 6 illustrates a setup where BIH (including
   the ENR) is implemented at the sockets API layer, and Figure 7
   illustrates a setup where BIH (including the ENR) is implemented at
   the network layer.






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"dual stack"                                                "host6"
IPv4    Socket |     [ API Translator ]    | TCP(UDP)/IP          Name
appli-  API    | ENR      Address  Function| (v6/v4)             Server
cation         |          Mapper   Mapper  |
 |        |        |        |        |         |              |       |
<<Resolve IPv4 addresses for "host6".>>        |              |       |
 |        |        |        |        |         |              |       |
 |------->|------->|  Query IPv4 addresses for host6.         |       |
 |        |        |        |        |         |              |       |
 |        |        |------------------------------------------------->|
 |        |        |  Query 'A' and 'AAAA' records for host6          |
 |        |        |        |        |         |              |       |
 |        |        |<-------------------------------------------------|
 |        |        |  Reply with the 'AAAA' record.           |       |
 |        |        |        |        |         |              |
 |        |        |<<The 'AAAA' record is resolved.>>        |
 |        |        |        |        |         |              |
 |        |        |+++++++>|  Request synthetic IPv4 address |
 |        |        |        |  corresponding to the IPv6 address.
 |        |        |        |        |         |              |
 |        |        |        |<<Assign one synthetic IPv4 address.>>
 |        |        |        |        |         |              |
 |        |        |<+++++++|  Reply with the synthetic IPv4 address.
 |        |        |        |        |         |              |
 |<-------|<-------| Reply with the IPv4 address              |
 |        |        |        |        |         |              |
 |        |        |        |        |         |              |
<<Call IPv4 Socket API function >>   |         |              |
 |        |        |        |        |         |              |
 |=======>|=========================>|An IPv4 Socket API action
 |        |        |        |        |         |              |
 |        |        |        |<+++++++|  Request IPv6 addresses|
 |        |        |        |        |  corresponding to the  |
 |        |        |        |        |  synthetic IPv4 addresses.
 |        |        |        |        |         |              |
 |        |        |        |+++++++>| Reply with the IPv6 addresses.
 |        |        |        |        |         |              |
 |        |        |        |        |<<Translate IPv4 into IPv6.>>
 |        |        |        |        |         |              |
 |  An IPv6 Socket API action        |=======================>|
 |        |        |        |        |         |              |
 |        |        |        |        |<<IPv6 data received    |
 |        |        |        |        |  from network.>>       |
 |        |        |        |        |         |              |
 |  An IPv6 Socket API action        |<=======================|
 |        |        |        |        |         |              |
 |        |        |        |        |<<Translate IPv6 into IPv4.>>
 |        |        |        |        |         |              |



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 |        |        |        |<+++++++|  Request synthetic IPv4 addresses
 |        |        |        |        |  corresponding to the  |
 |        |        |        |        |  IPv6 addresses.       |
 |        |        |        |        |         |              |
 |        |        |        |+++++++>| Reply with the IPv4 addresses.
 |        |        |        |        |         |              |
 |<=======|<=========================|  An IPv4 Socket API action
 |        |        |        |        |         |              |

                 Figure 6: Example of BIH as API addition


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



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    |   |    |       |         |       |           |         |
    |   |    |       |         |++++++>|  Reply with the IPv6|
    |   |    |       |         |       |  addresses.         |
    |   |    |       |         |       |           |         |
    |   |    |       |         |       |<<Translate IPv4 into IPv6.>>
    |   |    |       |         |       |           |         |
    |   |    |       |An IPv6 packet.  |==========>|========>|
    |   |    |       |         |       |           |         |
    |   |    |       |         |   <<Reply with an IPv6 packet.>>
    |   |    |       |         |       |           |         |
    |   |    |       |An IPv6 packet.  |<==========|<========|
    |   |    |       |         |       |           |         |
    |   |    |       |         |       |<<Translate IPv6 into IPv4.>>
    |   |    |       |         |       |           |         |
    |   |    |       |         |<++++++|  Request synthetic IPv4
    |   |    |       |         |       |  addresses corresponding
    |   |    |       |         |       |  to the IPv6 addresses.
    |   |    |       |         |       |           |         |
    |   |    |       |         |++++++>|  Reply with the IPv4 addresses.
    |   |    |       |         |       |           |         |
    |<=======|=========================|  An IPv4 packet.    |
    |   |    |       |         |       |           |         |


               Figure 7: Example of BIH at the network layer


























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4.  Considerations

4.1.  Socket API Conversion

   IPv4 socket API functions are translated into IPv6 socket API
   functions that are semantically as identical as possible and vice
   versa.  See Appendix B for the API list intercepted by BIH.  However,
   some IPv4 socket API functions are not fully compatible with IPv6
   since IPv4 supports features that are not present in IPv6, such as
   SO_BROADCAST.

4.2.  Socket bindings

   BIH SHOULD select a source address for a socket from the recommended
   source address pool if a socket used for communications has not been
   explicitly bound to any IPv4 address.

   The binding of an explicitly bound socket MUST NOT be changed by the
   BIH.

4.3.  ICMP Message Handling

   ICMPv4 and ICMPv6 messages MUST be translated as defined by SIIT
   [RFC6145].  In the network layer implementation alternative, protocol
   translator MUST translate ICMPv6 packets to ICMPv4 and vice versa,
   and in the socket API implementation alternative, the socket API MUST
   handle conversions in similar fashion.

4.4.  IPv4 Address Pool and Mapping Table

   The address pool consists of the [RFC1918] private IPv4 addresses.
   This pool can be implemented at different granularities in the node,
   e.g., a single pool per node, or at some finer granularity such as
   per-user or per-process.  In the case of a large number of IPv4
   applications communicating with a large number of IPv6 servers, the
   available address space may be exhausted if the granularity is not
   fine enough.  This should be a rare event and chances will decrease
   as IPv6 support increases.  The applications may use IPv4 addresses
   they learn for a much longer period than DNS time-to-live indicates.
   Therefore, the mapping table entries should be kept active for a long
   period of time.  For example, a web browser may initiate one DNS
   query and then create multiple TCP sessions over time to the address
   it learns.  When address mapping table clean-up is required, the BIH
   may utilize techniques used by network address translators, such as
   described in [RFC2663], [RFC5382], and [RFC5508].

   The RFC1918 address space was chosen because generally legacy
   applications understand it as a private address space.  A new



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   dedicated address space would run a risk of not being understood by
   applications as private. 127/8 and 169.254/16 are rejected due to
   possible assumptions applications may make when seeing those.

   The RFC1918 addresses used by the BIH have a risk of conflicting with
   addresses used in the host's possible IPv4 interfaces and
   corresponding local networks.  The conflicts can be mitigated, but
   not fully avoided, by using less commonly used portions of the
   RFC1918 address space.  Addresses from 172.16/12 are thought to be
   less likely to be in conflict than addresses from 10/8 or 192.168/16
   spaces.  A source address can usually be selected in a non-
   conflicting manner, but a small possibility exists for synthesized
   destination addresses being in conflict with real addresses used in
   attached IPv4 networks.

   The RECOMMENDED IPv4 addresses are following:

   Primary source addresses: 172.21.112.0/20.  Source addresses have to
   be allocated because applications use getsockname() calls and in the
   network layer mode an IP address of the IPv4 interface has to be
   shown (e.g., by 'ifconfig').  More than one address is allocated to
   allow implementation flexibility, e.g., for cases where a host has
   multiple IPv6 interfaces.  The source addresses are from different
   subnets than destination addresses to ensure applications would not
   make on-link assumptions and would instead enable NAT traversal
   functions.

   Secondary source addresses: 10.170.224.0/20.  These addresses are
   recommended if a host has a conflict with primary source addresses.

   Primary destination addresses: 10.170.160.0/20.  The address mapper
   will select destination addresses primarily out of this pool.

   Secondary destination addresses: 172.21.80.0/20.  The address mapper
   will select destination addresses out of this pool if the node has a
   dual-stack connection conflicting with primary destination addresses.

4.5.  Multi-interface

   In the case of dual-stack destinations BIH MUST NOT do protocol
   translation from IPv4 to IPv6 when the host has any IPv4 interfaces,
   native or tunneled, available for use.

   It is possible that an IPv4 interface is activated during BIH
   operation, for example if a node moves to a coverage area of an IPv4-
   enabled network.  In such an event, BIH MUST stop initiating protocol
   translation sessions for new connections and BIH MAY disconnect
   active sessions.  The choice of disconnection is left for



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   implementations and it may depend on whether IPv4 address conflict
   occurs between addresses used by BIH and addresses used by the new
   IPv4 interface.

4.6.  Multicast

   Protocol translation for multicast is not supported.












































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5.  Application-Level Gateway requirements considerations

   No Application-Level Gateway (ALG) functionality is specified herein
   as ALG design is generally not encouraged for host-based translation
   and as BIH is intended for applications that do not include IP
   addresses in protocol payloads.













































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6.  IANA Considerations

   There are no actions for IANA.
















































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7.  Security Considerations

   The security considerations of BIH follows closely, but not
   completely, those of NAT64 [RFC6146] and DNS64 [RFC6147].  The
   following sections are copied from RFC6146 and RFC6147 and modified
   for BIH scenario.

7.1.  Implications on End-to-End Security

   Any protocols that protect IP header information are essentially
   incompatible with BIH.  This implies that end-to-end IPsec
   verification will fail when the Authentication Header (AH) is used
   (both transport and tunnel mode) and when ESP is used in transport
   mode.  This is inherent in any network-layer translation mechanism.
   End-to-end IPsec protection can be restored, using UDP encapsulation
   as described in [RFC3948].  The actual extensions to support IPsec
   are out of the scope of this document.

7.2.  Filtering

   BIH creates binding state using packets flowing from the IPv4 side to
   the IPv6 side.  In accordance with the procedures defined in this
   document following the guidelines defined in [RFC4787], a BIH
   implementation MUST offer "Endpoint-Independent Mapping".

   Implementations MAY also provide support for "Address-Dependent
   Mapping" following the guidelines defined in [RFC4787].

   The security properties, however, are determined by which packets the
   BIH allows in and which it does not.  The security properties are
   determined by the filtering behavior and by the possible filtering
   configuration in the filtering portions of the BIH, not by the
   address mapping behavior.

7.3.  Attacks on BIH

   The BIH implementation itself is a potential victim of different
   types of attacks.  In particular, the BIH can be a victim of DoS
   attacks.  The BIH implementation has a limited number of resources
   that can be consumed by attackers creating a DoS attack.  The BIH has
   a limited number of IPv4 addresses that it uses to create the
   bindings.  Even though the BIH performs address translation, it is
   possible for an attacker to consume the synthetic IPv4 address pool
   by triggering a host to issue DNS queries for names that cause ENR to
   synthesise A records.  DoS attacks can also affect other limited
   resources available in the host running BIH such as memory or link
   capacity.  For instance, it is possible for an attacker to launch a
   DoS attack on the memory of the BIH running device by sending



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   fragments that the BIH will store for a given period.  If the number
   of fragments is large enough, the memory of the host could be
   exhausted.  BIH implementations MUST implement proper protection
   against such attacks, for instance, allocating a limited amount of
   memory for fragmented packet storage.

   Another consideration related to BIH resource depletion refers to the
   preservation of binding state.  Attackers may try to keep a binding
   state alive forever by sending periodic packets that refresh the
   state.  In order to allow the BIH to defend against such attacks, the
   BIH implementation MAY choose not to extend the session entry
   lifetime for a specific entry upon the reception of packets for that
   entry through the external interface.  However, such an action would
   not allow one-way communication sessions to stay alive.

7.4.  DNS considerations

   BIH operates in combination with the DNS, and is therefore subject to
   whatever security considerations are appropriate to the DNS mode in
   which the BIH is operating (i.e. recursive or stub-resolver mode).

   BIH has the potential to interfere with the functioning of DNSSEC,
   because BIH modifies DNS answers, and DNSSEC is designed to detect
   such modifications and to treat modified answers as bogus.



























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8.  Changes since RFC2767 and RFC3338

   This document combines and obsoletes both [RFC2767] and [RFC3338].

   The changes in this document mainly reflect the following:

   1. RFC1918 addresses used used for synthesis

      The RFC3338 used unassigned IPv4 addresses (e.g., 0.0.0.1 -
      0.0.0.255) for synthetic IPv4 addresses.  Those addresses should
      not have been used and that may cause problems with applications.
      It is preferable to use RFC1918 defined addresses instead, as
      described in Section 4.4.

   2. Support for reverse (PTR) DNS queries

      Neither RFC2767 or RFC3338 included support for reverse (PTR) DNS
      queries.  This document adds the support at Section 2.3.3.

   3. DNSSEC support

      RFC2767 did not include DNSSEC considerations, which are now
      included in Section 2.3.2

   4. Architectural recommendation

      This document recommends socket API layer implementation option
      over network layer translation, i.e. recommends approach
      introduced in RFC2767 over the approach of RFC3338.

   5. Standards track document

      RFC2767 is classified as Informational RFC and RFC3338 as
      Experimental RFC.  It was discussed and decided in the IETF that
      this technology should be on the standards track.

   6. Set of other extensions and improvements

      Set of lesser extensions, improvements, and clarifications have
      been introduced.  These include but are not limited to: IPv4 and
      IPv6 address exclusion sets at Section 2.3.1, host's DNS cache
      considerations, ENR behaviour updates, updated security
      considerations, example updates, and deployment scenario updates.








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9.  Acknowledgments

   The authors thank 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 Mohamed Boucadair, Dean Cheng, Lorenzo Colitti, Paco
   Cortes, Ralph Droms, Stephen Farrell, Fernando Gont, Marnix Goossens,
   Wassim Haddad, Ala Hamarsheh, Dave Harrington, Ed Jankiewizh, Suresh
   Krishnan, Julien Laganier, Yiu L. Lee, Jan M. Melen, Qibo Niu,
   Pierrick Seite, Christian Vogt, Magnus Westerlund, Dan Wing, and
   James Woodyatt in reviewing this document are gratefully
   acknowledged.

   Special acknowledgements go to Dave Thaler for his extensive review
   and support.

   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).

   The authors of Bump-in-the-Wire (BIW) (draft-ietf-biw-00.txt, October
   2006), P. Moster, L. Chin, and D. Green, are acknowledged.  Some
   ideas and clarifications from BIW have been adapted to this document.

























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10.  References

10.1.  Normative References

   [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.

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213, October 2005.

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 2007.

   [RFC6145]  Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", RFC 6145, April 2011.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, April 2011.

   [RFC6147]  Bagnulo, M., Sullivan, A., Matthews, P., and I. van
              Beijnum, "DNS64: DNS Extensions for Network Address
              Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
              April 2011.

10.2.  Informative References

   [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and Considerations",
              RFC 2663, August 1999.

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

   [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.



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              Stevens, "Basic Socket Interface Extensions for IPv6",
              RFC 3493, February 2003.

   [RFC3948]  Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
              Stenberg, "UDP Encapsulation of IPsec ESP Packets",
              RFC 3948, January 2005.

   [RFC5382]  Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
              RFC 5382, October 2008.

   [RFC5508]  Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
              Behavioral Requirements for ICMP", BCP 148, RFC 5508,
              April 2009.

   [RFC5735]  Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
              BCP 153, RFC 5735, January 2010.

   [RFC6180]  Arkko, J. and F. Baker, "Guidelines for Using IPv6
              Transition Mechanisms during IPv6 Deployment", RFC 6180,
              May 2011.






























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Appendix A.  API list intercepted by BIH

   The following informational list includes some of the API functions
   that would be appropriate to intercept by BIH module when implemented
   at the socket API layer.  Please note that this list is not fully
   exhaustive, as the function names and services that are available on
   different APIs vary significantly.

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

      bind()

      connect()

      sendmsg()

      sendto()

      gethostbyaddr()

      getnameinfo()

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

      accept()

      recvfrom()

      recvmsg()

      getpeername()

      getsockname()

      gethostbyname()

      getaddrinfo()

   The functions that are related to socket options are:

      getsocketopt()

      setsocketopt()

   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 [RFC3493], 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 8

   BIH may intercept inet_ntoa() and inet_addr() and use the address
   mapper for those.  Doing that enables BIH to support literal IP
   addresses.  However, IPv4 address literals can only be used after a
   mapping entry between the IPv4 address and corresponding IPv6 address
   has been created.

   The gethostbyname() and getaddrinfo() calls 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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