Internet-Draft K. Yamamoto IIJ Research Laboratory Expires in six months M. Sumikawa Category: Informational Hitachi, Ltd. March, 2000 Overview of Transition Techniques for IPv6-only to Talk to IPv4-only Communication Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This memo discusses translators to enable direct communication between IPv4 hosts and IPv6 hosts. Three translation mechanisms are described. From the address mapping point of view, the translators are categorized into four types and each feasibility is considered. This memo is based on a paper appeared in Proceedings of INET98[INET]. 1. Introduction In the early stage of the migration from IPv4[IPv4] to IPv6[IPv6], it is expected that IPv6 sites will be connected to the IPv4 Internet. On the other hand, in the late stage of the migration, IPv4 sites will be connected to the IPv6 Internet. IPv4 hosts need to be connected to the Internet even after the IPv4 address space is exhausted. So, it is necessary to develop translators to enable direct communication between IPv4 hosts and IPv6 hosts. Yamamoto [Page 1] Internet-Draft IPv4/IPv6 translators March 2000 This memo assumes the following for the practical migration scenario from IPv4 to IPv6: (1) We cannot modify both IPv4 hosts and IPv6 hosts in typical environments. (2) A small space of IPv4 address is also assigned to an IPv6 site according to the current severe address assignment policy. (3) An IPv4 site can also obtain a large space of IPv6 address. In this memo, the word "translator" is used as an intermediate component between an IPv4 host and an IPv6 host to enable direct communication between them, without requiring any modifications to them according to the assumption (1) above. This memo is organized as follows: Three translation techniques are described in Section 2. Address mapping between IPv4 and IPv6 is discussed in Section 3. Both SIIT[SIIT] and SOCKS[SOCKS] are a kind of translator between IPv4 and IPv6. This memo, however, does not cover such technologies because they require specific modifications to IPv4 and/or IPv6 hosts. BIS[BIS] is a technology to make an IPv4 host be dual-stack. So, BIS is outside the scope of this memo. 2. Translation Techniques of IPv4 and IPv6 For translation between IPv4 and IPv6, three technologies are available: header conversion, transport relay, and application level gateway(ALG). 2.1 Header Conversion Header conversion refers to converting IPv6 packet headers to IPv4 packet headers, or vice versa, and adjusting (or re-calculating) checksums if necessary. This is IP level translation. (Note that NAT[NAT] is an IPv4-to-IPv4 header converter.) The procedure to translate IPv4 packets to IPv6 packets, or vice versa, is defined as a part of SIIT. NATPT[NATPT] (excluding its ALG portion) is an example this kind of translator, which is based on SIIT. (Note that generic concerns about header translation were originally raised in [IPNGWP]. ) Header conversion could be fast enough, but it has disadvantages in common with NAT. A good example is difficulty in the translation of network layer addresses embedded in application layer protocols, which are typically found in FTP and FTP Extensions[EFTP]. Yamamoto [Page 2] Internet-Draft IPv4/IPv6 translators March 2000 Also, header conversion has problems which are not found in NAT: a large IPv4 packet is fragmented to IPv6 packets because the header length of IPv6 is typically 20 bytes larger than that of IPv4. Also not all the semantics of ICMP[ICMP] and that of ICMPv6[ICMPv6] are inter-changeable. However, the latter problem is believed minor in practical cases. 2.2 Transport Relay Transport relay refers to relaying a {TCP, UDP}/IPv4 session and a {TCP, UDP}/IPv6 session in the middle. This is transport level translation. For example, a typical TCP relay server works as follows: when a TCP request reaches a relay server, the network layer tosses it up to the TCP layer even if the destination is not the server's address. The server accepts this TCP packet and establishes a TCP connection with the source host. Then the server also makes one more TCP connection to the real destination. When two connections are established, the server reads data from one of the two connections and writes the data to the other. Transport relay does not have problems like fragmentation or ICMP conversion, since each session is closed in IPv4 and IPv6, respectively, but it does have problems like the translation of network layer addresses embedded in application layer protocols. 2.3 Application Level Gateway (ALG) An ALG for a transaction service is used to hide site information and improve service performance with a cache mechanism. An ALG can be a translator between IPv4 and IPv6 if it supports both protocols. This is application level translation. Since each service is closed in IPv4 and IPv6, respectively, there are no disadvantages found in header conversion, but ALGs for each service must be capable of running over both IPv4 and IPv6 3. Address Mapping Address mapping refers to the allocation of an IPv6 destination address for a given IPv4 destination address, and vice versa. It also includes the allocation of an IPv6 source address for a given IPv4 source address, and vice verca. If translation is performed at the Internet protocol level or transport level, address mapping is an essential issue. If an FQDN(Fully Qualified Domain Name) is used to specify a target host, address mapping is not necessary. So, the ALG is free of this problem. Yamamoto [Page 3] Internet-Draft IPv4/IPv6 translators March 2000 In the case that address mapping is dynamic, it must be implemented in interaction with DNS. If it is static and proliferation of mapped addresses is limited to a small region(i.e. Translator A, described later), it can be implemented by extending resolver libraries on local hosts. However, this violates the assumption (1). So, it is recommended that DNS is used for address mapping even in the static case. Examples: An example of static mapping: suppose that an application tries resolving AAAA/A6[A6] records against a host name. A DNS server receives this query but it can resolve only A record. In this case, the server converts them to AAAA/A6 records embedding them into the pre-configured prefix. Then it returns these records to the application. ([NATPT] also discusses this mechanism.) An example of dynamic mapping: if a DNS server receives a request to return A records for a host name, but only an AAAA/A6 record is resolved, the server picks up an IPv4 address from its address pool then returns it as A record. There are two criteria for addresses to be assigned: (1) the assigned addresses must be reachable between a connection initiating host and a translator, and (2) if addresses are assigned dynamically by DNS, it must be ensured that the DNS cache doesn't cause problems for further communications. If transport relay is used for translation, address mapping is necessary only for destination addresses since source address mapping is closed in the relay server. In other words, the protocol association of the first transport session is mapped to a local port number on the relay server. For header conversion, source address mapping is not essential, either. A protocol association can be represented by a local port of the conversion router or by an address out of the pool or by both. Yamamoto [Page 4] Internet-Draft IPv4/IPv6 translators March 2000 3.1. Translator Categories This memo categorizes IPv4/IPv6 translators from the address mapping point of view. The first picture illustrates the Internet in the early stage of the migration. The second one does that in the late stage. In the early stage +----------------------+ +-------------+ Translator A | | | |--------------->| | | IPv6 site | | The IPv4 Internet | | |<---------------| | +-------------+ Translator B | | +----------------------+ In the late stage +----------------------+ +-------------+ Translator C | | | |--------------->| | | IPv4 site | | The IPv6 Internet | | |<---------------| | +-------------+ Translator D | | +----------------------+ For simplicity, IPv4/IPv6 translators are categorized into four types. Note that practical translation stories could be combination of these four types. Translator A: It is used in the early stage of transition to establish a connection from an IPv6 host in an IPv6 site to an IPv4 host in the IPv4 Internet. Translator B: It is used in the early stage of transition to establish a connection from an IPv4 host in the IPv4 Internet to an IPv6 host in an IPv6 site. Translator C: It is used in the late stage of transition to establish a connection from an IPv4 host in an IPv4 site to an IPv6 host in the IPv6 Internet. Translator D: It is used in the late stage of transition to establish a connection from an IPv6 host in the IPv6 Internet to an IPv4 host in an IPv4 site. Yamamoto [Page 5] Internet-Draft IPv4/IPv6 translators March 2000 3.2. Observations on Address Mapping for Each Translator Here are observations on address mapping for each translator: Translator A: Destination address mapping: global IPv4 to global IPv6 Static or dynamic: static Address pool: a part of assigned global IPv6 addresses to the IPv6 site DNS cache problem: not encountered Implementation: straightforward Note: IPv4 addresses can be embedded to pre-configured IPv6 prefix. Translator B: Destination address mapping: global IPv6 to global IPv4 Static or dynamic: dynamic Address pool: assigned global IPv4 addresses to the IPv6 site DNS cache problem: potentially proliferated into the IPv4 Internet Implementation: very hard Note: it is recommended to use static address mapping for several IPv6 hosts(servers) in the IPv6 site to provide their services to the IPv4 Internet or to use dual-stack servers without translators. Translator C: Destination address mapping: global IPv6 to private IPv4 Static or dynamic: dynamic Address pool: a part of private IPv4 addresses DNS cache problem: closed to the IPv4 site Implementation: possible Note: mapped addresses should be reserved as long as possible for UDP applications which can't tell the end of communications and for applications which cache DNS entries. Translator D: Destination address mapping: global IPv4 to global IPv6 Static or dynamic: static Address pool: assigned global IPv6 addresses to the site DNS cache problem: not encountered Implementation: straightforward Note: IPv4 addresses can be embedded to pre-configured IPv6 prefix. Security Consideration When one or more IPv4/IPv6 translators are used in the intermediate path of an IPv4 host and an IPv6 host, end-to-end authentication mechanisms based on IPv4 and/or IPv6 address (including IPsec[IPsec]) is not available. This problem is well-known in the case of NAT. Yamamoto [Page 6] Internet-Draft IPv4/IPv6 translators March 2000 References [A6] M. Crawford, C. Huitema and S. Thomson, "DNS Extensions to Support IP Version 6", , 1999 [BIS] K. Tsuchiya, H. Higuchi and Y. Atarashi, "Dual Stack Hosts using the "Bump-In-the-Stack" Technique (BIS)", RFC 2767, 2000. [EFTP] M. Allman, S. Ostermann, and C. Metz, "FTP Extensions for IPv6 and NATs", RFC 2428, 1998. [ICMP] J. Postel, "Internet Control Message Protocol", RFC 792, 1981. [ICMPv6] A. Conta and S. Deering, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", RFC 2463, 1998. [INET] K. Yamamoto, A. Kato, M Sumikawa, and J. Murai, "Deployment and Experiences of WIDE 6bone", in Proceedings of INET98, 1998. [IPNGWP] B. Carpenter, "IPng White Paper on Transition and Other Considerations", RFC 1671, 1994. [IPsec] S. Kent and R. Atkinson, "Security Architecture for the Internet Protocol", RFC 2401, 1998. [IPv4] J. Postel, "Internet Protocol", RFC 791, 1981. [IPv6] S. Deering and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, 1998 [NAT] K. Egevang and P. Francis, "The IP Network Address Translator (NAT)", RFC 1631, 1994. [NATPT] G. Tsirtsis and P. Srishuresh, "Network Address Translation - Protocol Translation (NAT-PT)", RFC 2766, 2000. [SIIT] E. Nordmark, "Stateless IP/ICMP Translator (SIIT)", RFC 2765, 2000. [SOCKS] M. Leech, M. Ganis, Y. Lee, R. Kuris, D. Koblas and L. Jones, "SOCKS Protocol Version 5", RFC 1928, 1996. Acknowledgements The authors would like to thank many people for their contributions to this memo, especially Rundy Bush, Brian Carpenter, Shin-ichi Fujisawa, Jun-ichiro Ito, Akira Jinzaki, Akira Kato, Atsushi Onoe, Kazushi Sugyo, and Shigeya Suzuki (in alphabetical order). Yamamoto [Page 7] Internet-Draft IPv4/IPv6 translators March 2000 Authors' Addresses Kazuhiko 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 Munechika Sumikawa Hitachi, Ltd. 1 Horiyamashita, Hadano city, Kanagawa 259-1392 JAPAN Phone: +81-463-88-1311 FAX: +81-463-88-8062 EMail: sumikawa@ebina.hitachi.co.jp Changes From 02 to 03: - The title changed from "Categorizing Translators between IPv4 and IPv6" to "Overview of Transition Techniques for IPv6-only to Talk to IPv4-only Communication". - The word "translator" is explicitly defined. - Even in the case of static mapping, DNS is recommended for the address mapping. - Both SIIT and SOCKS are now outside of the scope. - The assumption (1) is changed: IPv6 hosts cannot be modified, either. - Updating references - Sumikawa's address is changed. From 01 to 02: - Updating references - Refering RFC 1671 - Adding the case of A6 records - Adding security consideration From 00 to 01: - Updating references - Refering to NATPT - Replacing TCP relay with transport relay to generalize - Clarify that the library extensions can be used only for Translator A. Yamamoto [Page 8]