RFC3142 - An IPv6-to-IPv4 Transport Relay Translator

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.

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.

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.)

destination host

==+=======+== outer network

TCP relay system

==+=======+== inner network

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

does not own the destination IP address (X). The TCP relay system

pretends to having IP address X, and establishes TCP connection with

the initiating host as X. The TCP relay system then makes a another

TCP connection from Y to X, and relays traffic from A to X, and the

other way around.

Thus, two TCP connections are established in the picture: from A to B

(as X), and from Y to X, like below:

TCP/IPv4: the initiating host (A) --> the TCP relay system (as X)

address on IPv4 header: A -> X

TCP/IPv4: the TCP relay system (Y) --> the destination host (X)

address on IPv4 header: Y -> X

The TCP relay system needs to capture some of TCP packets that is not

destined to its address. The way to do it is implementation

dependent and outside the scope of this memo.

2.2. UDP relay

If you can recognize UDP inbound and outbound traffic pair in some

way, UDP relay can be implemented in similar manner as TCP relay. An

implementation can recognize UDP traffic pair like NAT systems does,

by recording address/port pairs onto an table and managing table

entries with timeouts.

3. IPv6-to-IPv4 transport relay translator

We propose a transport relay translator for IPv6-to-IPv4 protocol

translation, TRT. In the following description, TRT for TCP is

described. TRT for UDP can be implemented in similar manner.

For address mapping, we reserve an IPv6 prefix referred to by

C6::/64. C6::/64 should be a part of IPv6 unicast address space

assigned to the site. Routing information must be configured so that

packets to C6::/64 are routed toward the TRT system. The following

diagram shows the network configuration. The subnet marked as "dummy

prefix" does not actually exist. Also, now we assume that the

initiating host to be IPv6-only, and the destination host to be

IPv4-only.

destination host

==+=======+== outer network

TRT system --- dummy prefix (C6::/64)

==+=======+== inner network

initiating host

When the initiating host (whose IPv6 address is A6) wishes to make a

connection to the destination host (whose IPv4 address is X4), it

needs to make an TCP/IPv6 connection toward C6::X4. For example, if

C6::/64 equals to fec0:0:0:1::/64, and X4 equals to 10.1.1.1, the

destination address to be used is fec0:0:0:1::10.1.1.1. The packet

is routed toward the TRT system, and is captured by it. The TRT

system accepts the TCP/IPv6 connection between A6 and C6::X4, and

communicate with the initiating host, using TCP/IPv6. Then, the TRT

system investigates the lowermost 32bit of the destination address

(IPv6 address C6::X4) to get the real IPv4 destination (IPv4 address

X4). It makes an TCP/IPv4 connection from Y4 to X4, and forward

traffic across the two TCP connections.

There are two TCP connections. One is TCP/IPv6 and another is

TCP/IPv4, in the picture: from A6 to B6 (as C6::X4), and Y4 to X4,

like below:

TCP/IPv6: the initiating host (A6) --> the TRT system (as C6::X4)

address on IPv6 header: A6 -> C6::X4

TCP/IPv4: the TRT system (Y4) --> the destination host (X4)

address on IPv4 header: Y4 -> X4

4. Address mapping

As seen in the previous section, an initiating host must use a

special form of IPv6 address to connect to an IPv4 destination host.

The special form can be resolved from a hostname by static address

mapping table on the initiating host (like /etc/hosts in UNIX),

special DNS server implementation, or modified DNS resolver

implementation on initiating host.

5. Notes to implementers

TRT for UDP must take care of path MTU issues on the UDP/IPv6 side.

The good thing is that, as we do not relay IP layer packets between

IPv4 and IPv6, we can decide IPv6 path MTU independently from IPv4

traffic. A simple solution would be to always fragment packets from

the TRT system to UDP/IPv6 side to IPv6 minimum MTU (1280 octets), to

eliminate the need for IPv6 path MTU discovery.

Though the TRT system only relays {TCP,UDP} traffic, it needs to

check ICMPv6 packets destined to C6::X4 as well, so that it can

recognize path MTU discovery messages and other notifications between

A6 and C6::X4.

When forwarding TCP traffic, a TRT system needs to handle urgent data

[Postel, 1981] carefully.

To relay NAT-unfriendly protocols [Hain, 2000] a TRT system may need

to modify data content, just like any translators which modifies the

IP addresses.

Scalability issues must carefully be considered when you deploy TRT

systems to a large IPv6 site. Scalability parameters would be (1)

number of connections the operating system kernel can accept, (2)

number of connections a userland process can forward (equals to

number of filehandles per process), and (3) number of transport

relaying processes on a TRT system. Design decision must be made to

use proper number of userland processes to support proper number of

connections.

To make TRT for TCP more scalable in a large site, it is possible to

have multiple TRT systems in a site. This can be done by taking the

following steps: (1) configure multiple TRT systems, (2) configure

different dummy prefix to them, (3) and let the initiating host pick

a dummy prefix randomly for load-balancing. (3) can be implemented

as follows; If you install special DNS server to the site, you may

(3a) configure DNS servers differently to return different dummy

prefixes and tell initiating hosts of different DNS servers. Or you

can (3b) let DNS server pick a dummy prefix randomly for load-

balancing. The load-balancing is possible because you will not be

changing destination address (hence the TRT system), once a TCP

connection is established.

For address mapping, the authors recommend use of a special DNS

server for large-scale installation, and static mapping for small-

scale installation. It is not always possible to have special

resolver on the initiating host, and assuming it would cause

deployment problems.

6. Applicability statement

Combined with a special DNS server implementation (which translates

IPv4 addresses into IPv6), TRT systems support IPv6-to-IPv4

translation very well. It requires no change to existing IPv6

clients, nor IPv4 servers, so the TRT system can be installed very

easily to existing IPv6-capable networks.

IPv4-to-IPv6 translation is much harder to support with any of the

translator techniques [Yamamoto, 1998]. While it is possible to use

TRT system for IPv4-to-IPv6 translation, it requires nontrivial

mapping between DNS names to temporary IPv4 addresses, as presented

in NAT-PT RFC[Tsirtsis, 2000].

As presented in the earlier sections, TRT systems use transport layer

(TCP/UDP) relay technique to translate IPv6 traffic to IPv4 traffic.

It gives two major benefits: (1) the implementation of the TRT system

can be done very simple, (2) with the TRT system path MTU discovery

issue is easier to deal with, as we can decide IPv6 path MTU

independently from IPv4 path MTU. Even with the simplicity, the TRT

system can cover most of the daily applications (HTTP, SMTP, SSH, and

many other protocols). For NAT-unfriendly protocols, a TRT system

may need to modify data content, just like any translators/NATs. As

the TRT system reside in transport layer, it is not possible for the

TRT system to translate protocols that are not known to the TRT

system.

Normally users do not want to translate DNS query/reply traffic using

the TRT system. Instead, it makes more sense to run standard DNS

server, or special DNS server that helps TRT system, somewhere in the

site IPv6 network. There are two reasons to it:

o Transport issue - It is a lot easier to provide recursive DNS

server, accessible via IPv6, than to translate DNS queries/replies

across the TRT system. If someone tries to ask TRT to translate

DNS packets, the person would put C6::X (where C6 is TRT reserved

prefix and X is an IPv4 address of a DNS server) into

/etc/resolv.conf. The configuration is rather complicated than we

normally want.

o Payload issue - In some installation it makes more sense to

transmit queries/replies unmodified, across the TRT system. In

some installation it makes more sense to translate IPv4 DNS

queries (like queries for AAAA record) into queries for A record,

and vice versa, to invite traffic into the TRT system. It depends

on the installation/configuration at the user's site.

7. Security Considerations

Malicious party may try to use TRT systems akin to an SMTP open relay

[Lindberg, 1999] for traffic to IPv4 destinations, which is similar

to circumventing ingress filtering [Ferguson, 1998] , or to achieve

some other improper use. TRT systems should implement some sorts of

access control to prevent such improper usage.

A careless TRT implementation may be subject to buffer overflow

attack, but this kind of issue is implementation dependent and

outside the scope of this memo.

Due to the nature of TCP/UDP relaying service, it is not recommended

to use TRT for protocols that use authentication based on source IP

address (i.e., rsh/rlogin).

A transport relay system intercepts TCP connection between two nodes.

This may not be a legitimate behavior for an IP node. The document

does not try to claim it to be legitimate.

IPsec cannot be used across a relay.

Use of DNS proxies that modify the RRs will make it impossible for

the resolver to verify DNSsec signatures.

References

[Nordmark, 2000.] Nordmark, E., "Stateless IP/ICMP Translator

(SIIT)", RFC2765, February 2000.

[Postel, 1981.] Postel, J., "Transmission Control Protocol", STD 7,

RFC793 September 1981.

[Hain, 2000.] Hain, T., "Architectural Implications of NAT", RFC

2993, November 2000.

[Yamamoto, 1998] K. Yamamoto, A. Kato, M Sumikawa, and J. Murai,

"Deployment and EXPeriences of WIDE 6bone", in

Proceedings of INET98, 1998.

[Tsirtsis, 2000.] Tsirtsis, G. and P. Srisuresh, "Network Address

Translation - Protocol Translation (NAT-PT)", RFC

2766, February 2000.

[Lindberg, 1999.] Lindberg, G., "Anti-Spam Recommendations for SMTP

MTAs", RFC2505, February 1999.

[Ferguson, 1998.] Ferguson, P. and D. Senie, "Network Ingress

Filtering: Defeating Denial of Service Attacks

which employ IP Source Address Spoofing", RFC2267,

January 1998.

Appendix A. Operational experiences

WIDE KAME IPv6 stack implements TRT for TCP, called "FAITH". The

implementation came from WIDE Hydrangea IPv6 stack, which is one of

ancestors of the KAME IPv6 stack.

The FAITH code has been available and operational for more than 5

years. The implementation has been used at WIDE research group

offsite meeting, and IETF ipngwg 1999 Tokyo interim meeting. At the

latter occasion, we configured IPv6-only terminal network cluster

just like we do in IETF meetings, and used a TRT system to support

more than 100 IPv6 hosts on the meeting network to connect to outside

IPv4 hosts. From statistics we gathered SSH, FTP, HTTP, and POP3 are

the most popular protocol we have relayed. The implementation was

also used in the terminal cluster IPv6 network at IETF48, IETF49 and

IETF50.

The source code is available as free software, bundled in the KAME

IPv6 stack kit.

Special DNS server implementations are available as "newbie" DNS

server implementation by Yusuke DOI, and "totd" DNS proxy server from

University of Tromso (Norway).

Acknowledgements

The authors would like to thank people who were involved in

implementing KAME FAITH translator code, including Kei-ichi SHIMA and

Munechika SUMIKAWA.

Authors' Addresses

Jun-ichiro itojun HAGINO

Research Laboratory, Internet Initiative Japan Inc.

Takebashi Yasuda Bldg.,

3-13 Kanda Nishiki-cho,

Chiyoda-ku, Tokyo 101-0054, JAPAN

Phone: +81-3-5259-6350

Fax: +81-3-5259-6351

EMail: itojun@iijlab.net

Kazu YAMAMOTO

Research Laboratory, Internet Initiative Japan Inc.

Takebashi Yasuda Bldg.,

3-13 Kanda Nishiki-cho,

Chiyoda-ku, Tokyo 101-0054, JAPAN

Phone: +81-3-5259-6350

Fax: +81-3-5259-6351

EMail: kazu@iijlab.net

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