An IPv6-to-IPv4 Transport Relay Translator
RFC 3142
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
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