Dual-Stack Hosts Using "Bump-in-the-Host" (BIH)

Internet Engineering Task Force (IETF)                          B. Huang
Request for Comments: 6535                                       H. Deng
Obsoletes: 2767, 3338                                       China Mobile
Category: Standards Track                                  T. Savolainen
ISSN: 2070-1721                                                    Nokia
                                                           February 2012


            Dual-Stack Hosts Using "Bump-in-the-Host" (BIH)

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 is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6535.

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   document authors.  All rights reserved.

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   than English.

Table of Contents

   1. Introduction ....................................................4
      1.1. Terminology ................................................5
      1.2. Acknowledgment 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 ..................................11
   4. Considerations .................................................15
      4.1. Socket API Conversion .....................................15
      4.2. Socket Bindings ...........................................15
      4.3. ICMP Message Handling .....................................15
      4.4. IPv4 Address Pool and Mapping Table .......................15
      4.5. Multi-Interface ...........................................17
      4.6. Multicast .................................................17
   5. Application-Level Gateway Requirements Considerations ..........17
   6. Security Considerations ........................................17
      6.1. Implications on End-to-End Security .......................18
      6.2. Filtering .................................................18
      6.3. Attacks on BIH ............................................18
      6.4. DNS Considerations ........................................19
   7. Changes since RFC 2767 and RFC 3338 ............................19
   8. Acknowledgments ................................................20
   9. References .....................................................21
      9.1. Normative References ......................................21
      9.2. Informative References ....................................21
   Appendix A. API List Intercepted by BIH ...........................23

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 7 describes the reasons
   for making RFC 2767 and RFC 3338 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
   private address pool of [RFC1918] in order to ensure that possible
   NAT traversal techniques will be initiated.

   The IETF recommends using solutions based on dual stack or tunneling
   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, the 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 the 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.

   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

      The process of creating an A record containing a 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 a 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.  Acknowledgment of Previous Work

   This document is a direct derivative of [RFC2767], "Dual Stack Hosts
   using the "Bump-In-the-Stack" Technique (BIS)" by Kazuaki TSHUCHIYA,
   Hidemitsu HIGUCHI, and Yoshifumi ATARASHI and of [RFC3338], "Dual
   Stack Hosts Using "Bump-in-the-API" (BIA)" by Seungyun Lee, Myung-Ki
   Shin, Yong-Jin Kim, Alain Durand, and Erik Nordmark, which similarly
   provides IPv4-only applications on dual-stack hosts the means to
   operate over IPv6.  Section 7 covers the changes since those
   documents.

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 the 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".

      +------------------------------------------------------------+
      |  +------------------------------------------+              |
      |  |    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 [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 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 the 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
   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
   matching 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;
   hence, it 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
   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 the 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 a 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, it 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 a 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 a 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 a 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 should not appear in the DNS or on the wire (see Section
   5.1.4 of [RFC6147] 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, a host's
   resolver can use DNS Security (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 if it 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
   and at the host's stub resolver.

2.4.  Address Mapper

   The address mapper maintains an IPv4 address pool that can be used
   for IPv4 address synthesis.  The pool consists of the IPv4 addresses
   of [RFC1918] 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 ENR, 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 ENR 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 ENR 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.

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 the addresses of [RFC1918], 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 socket API layer, and Figure 7
   illustrates a setup where BIH (including the ENR) is implemented at
   the network layer.

"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.>>
 |        |        |        |        |         |              |
 |        |        |        |<+++++++|  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.
    |   |    |       |         |       |           |         |
    |   |    |       |         |++++++>|  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

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 A 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, the
   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 private IPv4 addresses of [RFC1918].
   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 address space of RFC 1918 was chosen because legacy applications
   generally understand it as a private address space.  A new dedicated
   address space would run the 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 them.

   The addresses of RFC 1918 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
   address space of RFC 1918.  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
   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.

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.

6.  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 RFC 6146 and RFC 6147 and modified
   for BIH.

6.1.  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.

6.2.  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.

6.3.  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 Denial-
   of-Service (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 synthesize 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 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 is 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.

6.4.  DNS Considerations

   BIH operates in combination with the DNS, and it 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.

7.  Changes since RFC 2767 and RFC 3338

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

   The changes in this document mainly reflect the following:

   1. Addresses of RFC 1918 used for synthesis

      RFC 3338 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 addresses defined in RFC 1918 instead, as
      described in Section 4.4.

   2. Support for reverse (PTR) DNS queries

      Neither RFC 2767 nor RFC 3338 included support for reverse (PTR)
      DNS queries.  This document adds the support in Section 2.3.3.

   3. DNSSEC support

      RFC 2767 did not include DNSSEC considerations, which are now
      included in Section 2.3.2
   4. Architectural recommendation

      This document recommends the socket API-layer implementation
      option over network layer translation, i.e., it recommends the
      approach introduced in RFC 2767 over the approach of RFC 3338.

   5. Standards-Track document

      RFC 2767 is classified as an Informational RFC and RFC 3338 as an
      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

      A 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 behavior updates, updated security
      considerations, example updates, and deployment scenario updates.

8.  Acknowledgments

   The authors are grateful for discussion from Gang Chen, Dapeng Liu,
   Bo Zhou, Hong Liu, Tao Sun, Zhen Cao, and 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 acknowledgments go to Dave Thaler for his extensive review
   and support.

   The authors of RFC 2767 acknowledged WIDE Project, Kazuhiko YAMAMOTO,
   Jun MURAI, Munechika SUMIKAWA, Ken WATANABE, and Takahisa MIYAMOTO.
   The authors of RFC 3338 acknowledged implementation contributions by
   Wanjik Lee (wjlee@arang.miryang.ac.kr) and i2soft Corporation
   (www.i2soft.net).

   The authors of "Bump-in-the-Wire IPv4/IPv6 Translator" (a draft
   document submitted to the v6ops WG in October 2006), P. Moster, L.
   Chin, and D. Green, are acknowledged.  Some ideas and clarifications
   from BIW have been adapted to this document.

9.  References

9.1.  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.

9.2.  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.
              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.

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 as follows:

      bind()

      connect()

      sendmsg()

      sendto()

      gethostbyaddr()

      getnameinfo()

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

      accept()

      recvfrom()

      recvmsg()

      getpeername()

      getsockname()

      gethostbyname()

      getaddrinfo()

   The functions that are related to socket options are as follows:

      getsocketopt()

      setsocketopt()

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

   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.

Authors' Addresses

   Bill Huang
   China Mobile
   No.32 Xuanwumen West Street
   Xicheng District
   Beijing  100053
   China

   EMail: bill.huang@chinamobile.com


   Hui Deng
   China Mobile
   No.32 Xuanwumen West Street
   Xicheng District
   Beijing  100053
   China

   EMail: denghui@chinamobile.com


   Teemu Savolainen
   Nokia
   Hermiankatu 12 D
   FI-33720 TAMPERE
   Finland

   EMail: teemu.savolainen@nokia.com