An IPv6-to-IPv4 Transport Relay Translator
RFC 3142

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Network Working Group                                          J. Hagino
Request for Comments: 3142                                   K. Yamamoto
Category: Informational                          IIJ Research Laboratory
                                                               June 2001

               An IPv6-to-IPv4 Transport Relay Translator

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   The document describes an IPv6-to-IPv4 transport relay translator
   (TRT).  It enables IPv6-only hosts to exchange {TCP,UDP} traffic with
   IPv4-only hosts.  A TRT system, which locates in the middle,
   translates {TCP,UDP}/IPv6 to {TCP,UDP}/IPv4, or vice versa.

   The memo talks about how to implement a TRT system using existing
   technologies.  It does not define any new protocols.

1.  Problem domain

   When you deploy an IPv6-only network, you still want to gain access
   to IPv4-only network resources outside, such as IPv4-only web
   servers.  To solve this problem, many IPv6-to-IPv4 translation
   technologies are proposed, mainly in the IETF ngtrans working group.
   The memo describes a translator based on the transport relay
   technique to solve the same problem.

   In this memo, we call this kind of translator "TRT" (transport relay
   translator).  A TRT system locates between IPv6-only hosts and IPv4
   hosts and translates {TCP,UDP}/IPv6 to {TCP,UDP}/IPv4, vice versa.

   Advantages of TRT are as follows:

   o  TRT is designed to require no extra modification on IPv6-only
      initiating hosts, nor that on IPv4-only destination hosts.  Some
      other translation mechanisms need extra modifications on IPv6-only
      initiating hosts, limiting possibility of deployment.

Hagino & Yamamoto            Informational                      [Page 1]
RFC 3142        IPv6-to-IPv4 Transport Relay Translator        June 2001

   o  The IPv6-to-IPv4 header converters have to take care of path MTU
      and fragmentation issues.  However, TRT is free from this problem.

   Disadvantages of TRT are as follows:

   o  TRT supports bidirectional traffic only.  The IPv6-to-IPv4 header
      converters may be able to support other cases, such as
      unidirectional multicast datagrams.

   o  TRT needs a stateful TRT system between the communicating peers,
      just like NAT systems.  While it is possible to place multiple TRT
      systems in a site (see Appendix A), a transport layer connection
      goes through particular, a single TRT system.  The TRT system thus
      can be considered a single point of failure, again like NAT
      systems.  Some other mechanisms, such as SIIT [Nordmark, 2000],
      use stateless translator systems which can avoid a single point of
      failure.

   o  Special code is necessary to relay NAT-unfriendly protocols.  Some
      of NAT-unfriendly protocols, including IPsec, cannot be used
      across TRT system.

   This memo assumes that traffic is initiated by an IPv6-only host
   destined to an IPv4-only host.  The memo can be extended to handle
   opposite direction, if an appropriate address mapping mechanism is
   introduced.

2.  IPv4-to-IPv4 transport relay

   To help understanding of the proposal in the next section, here we
   describe the transport relay in general.  The transport relay
   technique itself is not new, as it has been used in many of
   firewall-related products.

2.1.  TCP relay

   TCP relay systems have been used in firewall-related products.  These
   products are designed to achieve the following goals: (1) disallow
   forwarding of IP packets across a system, and (2) allow {TCP,UDP}
   traffic to go through the system indirectly.  For example, consider a
   network constructed like the following diagram.  "TCP relay system"
   in the diagram does not forward IP packet across the inner network to
   the outer network, vice versa.  It only relays TCP traffic on a
   specific port, from the inner network to the outer network, vice
   versa.  (Note:  The diagram has only two subnets, one for inner and
   one for outer.  Actually both sides can be more complex, and there
   can be as many subnets and routers as you wish.)

Hagino & Yamamoto            Informational                      [Page 2]
RFC 3142        IPv6-to-IPv4 Transport Relay Translator        June 2001

      destination host
        |X
      ==+=======+== outer network
                |Y
              TCP relay system
                |B
      ==+=======+== inner network
        |A
      initiating host

   When the initiating host (whose IP address is A) tries to make a TCP
   connection to the destination host (X), TCP packets are routed toward
   the TCP relay system based on routing decision.  The TCP relay system
   receives and accepts the packets, even though the TCP relay system