RFC 3142 (rfc3142) - Page 2 of 11
An IPv6-to-IPv4 Transport Relay Translator
Alternative Format: Original Text Document
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