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Network Working Group                                          J. HaginoRequest for Comments: 3142                                   K. YamamotoCategory: Informational                          IIJ Research Laboratory                                                               June 2001An IPv6-to-IPv4 Transport Relay TranslatorStatus 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 (seeAppendix 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   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 spaceHagino & Yamamoto            Informational                      [Page 3]

RFC 3142        IPv6-to-IPv4 Transport Relay Translator        June 2001   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        |X4      ==+=======+== outer network                |Y4              TRT system --- dummy prefix (C6::/64)                |B6      ==+=======+== inner network        |A6      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 -> X44.  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.Hagino & Yamamoto            Informational                      [Page 4]

RFC 3142        IPv6-to-IPv4 Transport Relay Translator        June 20015.  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.Hagino & Yamamoto            Informational                      [Page 5]

RFC 3142        IPv6-to-IPv4 Transport Relay Translator        June 20016.  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.Hagino & Yamamoto            Informational                      [Page 6]

RFC 3142        IPv6-to-IPv4 Transport Relay Translator        June 20017.  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)",RFC 2765, February 2000.   [Postel, 1981.]   Postel, J., "Transmission Control Protocol", STD 7,RFC 793 September 1981.   [Hain, 2000.]     Hain, T., "Architectural Implications of NAT",RFC2993, 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)",RFC2766, February 2000.Hagino & Yamamoto            Informational                      [Page 7]

RFC 3142        IPv6-to-IPv4 Transport Relay Translator        June 2001   [Lindberg, 1999.] Lindberg, G., "Anti-Spam Recommendations for SMTP                     MTAs",RFC 2505, February 1999.   [Ferguson, 1998.] Ferguson, P. and D. Senie, "Network Ingress                     Filtering: Defeating Denial of Service Attacks                     which employ IP Source Address Spoofing",RFC 2267,                     January 1998.Hagino & Yamamoto            Informational                      [Page 8]

RFC 3142        IPv6-to-IPv4 Transport Relay Translator        June 2001Appendix 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.Hagino & Yamamoto            Informational                      [Page 9]

RFC 3142        IPv6-to-IPv4 Transport Relay Translator        June 2001Authors' 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.netHagino & Yamamoto            Informational                     [Page 10]

RFC 3142        IPv6-to-IPv4 Transport Relay Translator        June 2001Full Copyright Statement   Copyright (C) The Internet Society (2001).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Hagino & Yamamoto            Informational                     [Page 11]