Network Working Group B. Carpenter Internet-Draft Univ. of Auckland Intended status: Experimental October 28, 2007 Expires: April 30, 2008 Shimmed IPv4/IPv6 Address Network Translation Interface (SHANTI) draft-carpenter-shanti-00 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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. This Internet-Draft will expire on April 30, 2008. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract There is a pragmatic need for a packet-level translation mechanism between IPv4 and IPv6, for scenarios where no other mode of IPv4 to IPv6 interworking is possible. The mechanism defined here uses a shim in both the translator and the IPv6 host to mitigate the problems introduced by stateless translation. Carpenter Expires April 30, 2008 [Page 1] Internet-Draft Shimmed IPv4/IPv6 Translation October 2007 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Requirements notation . . . . . . . . . . . . . . . . . . 4 2. Summary of scenario . . . . . . . . . . . . . . . . . . . . . 4 3. General walkthroughs . . . . . . . . . . . . . . . . . . . . . 6 4. Placement of the shim . . . . . . . . . . . . . . . . . . . . 8 5. DNS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 6. ICMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 7. Unresolved issues . . . . . . . . . . . . . . . . . . . . . . 9 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 11.1. Normative References . . . . . . . . . . . . . . . . . . . 10 11.2. Informative References . . . . . . . . . . . . . . . . . . 10 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10 Intellectual Property and Copyright Statements . . . . . . . . . . 12 Carpenter Expires April 30, 2008 [Page 2] Internet-Draft Shimmed IPv4/IPv6 Translation October 2007 1. Introduction There has long been a defined mechanism for stateless translation betweeen IPv4 and IPv6 packet formats [RFC2765]. Its intended use is any scenario whether dual stack coexistence between IPv4 and IPv6, possibly accompanied by dual stack application level proxies, is insufficient. In the most stringent case, this will occur when communication is needed between unmodified ("legacy") IPv4 hosts and IPv6-only hosts that have no IPv4 code, and no dual stack proxy is available for the application protocol of interest. The previously proposed solution for this scenario, NAT-PT [RFC2766] has known issues and has been deprecated [RFC4966]. The present proposal does not resolve all of those issues; a later section will identify the issues believed to remain open. This proposal aims to resolve those issues that can be handled if the IPv6 protocol stack communicating with a translator can exchange information with the translator that is specific to the translation process. The objectives are to ensure that a. from the IPv4 host's point of view, nothing is worse than in the case of an IPv4-to-IPv4 translation b. from the IPv6 host's point of view, no special code is generally required in the transport layer or above. However, information about the translation is available in the IPv6 host's network stack, if needed. This is the crucial difference from NAT-PT. To achieve these goals, a shim is inserted in the protocol stack at both the IPv6 host and at the translator. Its objective is to allow the IPv6 stack at the host to be aware of the presence of the translator, of the addresses involved in the translation, and of any other information known by the translator that may be of value to the IPv6 host. A shim model is chosen, as in SHIM6 [I-D.ietf-shim6-proto], so that upper layer protocols (ULPs) have no need to be aware of anything unusual. The mechanism is known as SHimmed Address Network Translation Interface (SHANTI). As in SHIM6, ULPs are presented with an upper layer identifier (ULID) in the form of an IPv6 address which is independent of any manipulation of addresses in the shim or translator. The reader is assumed to have a general understanding of SHIM6. Although this early draft does not assume that the SHIM6 mechanisms defined in [I-D.ietf-shim6-proto] would be used unchanged, they form a proof of concept for the type of communication required between two network-layer shims. It should be noted that this mechanism adds complexity to an IPv6- only host. This has to be balanced against the complexity of a dual- Carpenter Expires April 30, 2008 [Page 3] Internet-Draft Shimmed IPv4/IPv6 Translation October 2007 stack host. In this model, no residual IPv4 code is needed in the IPv6 host. The shim has to handle the rewriting of addresses and port numbers, but nothing else. It should also be noted that this mechanism strenuously avoids any impact whatever on IPv6 addressing and routing "on the wire". DISCLAIMER: This draft is incomplete. It is posted to seek comments on plausibility; much more work is needed to make it implementable. 1.1. Requirements notation The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 2. Summary of scenario Consider an IPv6-only host X and and IPv4-only host Y. Let A(x) be an IPv6 address for X, and let a(y) be an IPv4 address for Y. Let the port in use at X be P(x) and at Y be P(y). We will observe later that it is irrelevant whether a(y) is translated by an IPv4 NAT, and whether P(y) is translated by an IPv4 NAPT. Additionally, consider a translator T between X and Y. On the IPv6 side it has address A(t) and on the IPv4 side it has address a(t). If port translation is in effect, P(x) will become P(tx) on the IPv4 side. We will observe later that the A(t) address can be chosen from an address pool. We cannot assume that a(t) can be chosen from a pool, which is why port translation will be needed. Thus A() is always an IPv6 address and a() is always an IPv4 address. A diagram of the solution follows: Carpenter Expires April 30, 2008 [Page 4] Internet-Draft Shimmed IPv4/IPv6 Translation October 2007 X T Y ___________ A(x) A(t) _______________ a(t) a(y) _______ | | | V6|P(x) P(y)| V6| | | V4|P(tx) P(y)| V4| | | | S | | | | S | S | | | | | | U | H | S | | S | H | I | S | | S | U | | L | I | T |------------| T | I | I | T |-----------| T | L | | P | M | A | | A | M | T | A | | A | P | | | | C | | C | | | C | | C | | | | | K | | K | | | K | | K | | |___|___|___| |___|___|___|___| |___|___| The address set used by the shim for X is conceptually {a(t),A(x)}, and for Y it is conceptually {a(y),A(t)}. In other words the ULP at X sees its own ULID as a(t) and Y's ULID as a(y), both filled out with /96 prefixes. On the wire, the IPv6 packets between X and T use A(x) and A(t) as the actual address pair. The IPv4 packets between T and Y use a(t) and a(y). P(y) can be used everywhere, but we must assume that P(x) will be used on the IPv6 side and P(tx) on the IPv4 side. If there's a NAT with routable address a(n) on the IPv4 path, it won't know anything is special, and a(y) will be replaced by a(n). X, Y and T won't know the NAT is there. X and T will not know if Y has a private [RFC1918] address or if additional port translation takes place. T should have a pool of A(t) addresses, and should probably have a complete /64 to itself for maximum flexibility. When the ULP in X sends a first packet to ::FF:0:0:a(y)/128, we need to start a SHIM6-like process. The shim in X is configured to catch such packets, and carry out a message exchange with the shim in T to discover the relevant a(t), A(t) and P(tx) values. It can then rewrite the packet header and recompute a checksum as needed, and send the packet on to A(t). T needs to select a specific A(t) and P(tx) for each new flow, and do SHIM6-like things to tell X what the addresses are. This should create enough state in the shims to know what to do with outbound and return packets. Since T has a full /64 to work with, it can create a new A(t) for each new X or even for each new flow if that turns out to be needed. Note that unlike SHIM6, SHANTI must perform the shim exchange before sending the first packet of a traffic flow. This is because the source ULID to be used must be expanded from a(t) and is not initially known by the source host. Also, if P(tx) is unequal to Carpenter Expires April 30, 2008 [Page 5] Internet-Draft Shimmed IPv4/IPv6 Translation October 2007 P(x), this must be learned by the shim in X. A consequence of this is that the shim in X must buffer packets until it has completed its shim exchange with T. For this solution to scale, it is important that the SHANTI translator has adequate capacity for the number of IPv6 hosts it serves, and adequate network connectivity to them. OPEN QUESTION: Would it be preferable to configure X with knowledge of a(t)? Even so, when port translation is needed, the value of P(tx) can only be discovered dynamically. When a data packet reaches T from X, there will already be shim state established. The shim will pass the packet on to SIIT for translation and IPv4 transmission. Once the shim state is established, the ULPs in both X and Y will work as normal. Since T uses a specific A(t) for each X, and the shim at X is aware of that A(t), all port numbers are available in each direction on the IPv6 side. Port mapping, if required, will only affect the IPv4 side of T. Also, the shim in X is aware that the ULP in Y believes it is using the address pair {a(t), a(y)} and the ports {P(tx), P(y)}. Thus, address and port dependent fix-ups can be performed by the shim in X. In addition to using ::FF:0:0:a(y)/128 as the ULID for y, the IPv6 stack can use ::FF:0:0:a(t)/128 as the ULID for X, although the IPv6 packets will be sent between A(x) and A(t). This plus the shim's knowledge of P(tx) means that TCP and UDP checksums do not need to be fixed up by T. This has scaling advantages compared to NAT-PT. Additionally, with this knowledge being available in the host rather than being hidden in the translator as in NAT-PT, it is in principle possible for any address and port dependencies in the ULP to be fixed up in the host itself, precluding the need for Application Level Gateways (ALGs). Although this would introduce a layer violation, it is in principle a more robust design than associating ALGs with a "stateless" translator. OPEN QUESTION: In SIIT, an "IPv4-translated" address format is introduced to represent a synthetic IPv4 address for the IPv6 host, with the ::FF:0:0:0/96 prefix. This format, which is not in the IPv6 address architecture [RFC4291], could be used as the ULID for X. But since the shim has explicit knowledge of the addresses in use, is there any reason to use this format in preference to the simpler ::FF:0:0/96 IPv4-mapped format? The latter is assumed here. 3. General walkthroughs Consider first an IPv6 client attempting to contact an IPv4 server Carpenter Expires April 30, 2008 [Page 6] Internet-Draft Shimmed IPv4/IPv6 Translation October 2007 via this mechanism. The main steps that must occur are: 1. ULP in X obtains Y's IPv4-mapped address ::FF:0:0:a(y)/128. See DNS discussion below. 2. ULP sends unsolicited packet to that address. 3. The shim in X recognises the packet as needing attention. 4. The shim creates local state for a(y), P(x), and buffers the packet. Also, it creates a packet to send to T. This is a packet containing nothing but a shim header indicating that a first packet is ready from A(x):P(x) to a(y):P(y). 5. The shim at T receives this shim header and checks for existing state for {A(x):P(x),a(y):P(y)}. 6. If no such state exists, assign an A(t) value from the pool, and create state. Includes the ports. If P(x) is already in use by T, assign a P(tx). Otherwise, P(tx)=P(x). 7. The shim in T creates a packet to return to X. This is a packet containing nothing but a shim header indicating the assigned A(t), a(t) and P(tx). 8. The shim in X records this additional state, in particular recording ::FF:0:0:a(t)/128 as the source ULID and P(tx) as the translated port. 9. The shim in X now applies the following process to buffered and future packets sent from A(x), port P(x) to ::FF:0:0:a(y), port P(y). 1. Compute checksums as for addresses DA=::FF:0:0:a(y), SA=:: FF:0:0:a(t) and ports DP=P(y), SP=P(tx). 2. Rewrite destination address as A(t). 3. Send packet to A(t). At this point its source address is still A(x). 10. The shim in T rewrites the addresses as DA=::FF:0:0:a(y), SA=:: FF:0:0:a(t), and the source port as P(tx), and hands the packet off to SIIT. 11. SIIT translates the packet and sends it on (destination = a(y), source = a(t)). 12. When an IPv4 return packet comes into SIIT, SIIT translates the packet to IPv6 and hands it to the shim in T. 13. The shim performs port demultiplexing on the destination port (which will be P(tx)) to identify the A(x) involved. 14. The shim sends the packet on to A(x). 15. The shim at X receives the packet, rewrites the addresses to restore the original ULIDs and P(x), and sends the packet on up the stack. Now consider an IPv4 client attempting to contact an IPv6 server via T. The main steps that must occur are: 1. T must be pre-configured to admit traffic for P(x) and forward it to A(x). This is a normal port-forwarding issue, to be solved as for NATs or perhaps as proposed in [I-D.woodyatt-ald]. It cannot be performed without pre-existing state. Assuming T has only one Carpenter Expires April 30, 2008 [Page 7] Internet-Draft Shimmed IPv4/IPv6 Translation October 2007 a(t), a given P(x) can only have one IPv6 listener. 2. ULP in Y obtains an IPv4 address for T (believing it to be the actual server X). 3. Y sends an unsolicited packet from a(y) to a(t), port P(x). 4. T performs port demultiplexing and determines that the packet is destined for A(x). It is therefore passed to SIIT in T, translated to IPv6 format, and passed on to the shim in T. 5. The shim inserts a shim header that will tell X the translation in effect, translates the addresses, and sends the packet from A(t) to A(x). 6. The shim at X receives the packet, and translates the addresses to ::FF:0:0:a(t)/128 and ::FF:0:0:a(y)/128. This should checksum OK. 7. The packet is delivered to the ULP, minus the shim header. State will be created and subsequent packets will flow as in the previous case. 4. Placement of the shim In SHIM6 the shim is logically placed below both the transport and IPsec layers, so that their checksums do not need recalculation. In SHANTI, the transport layer checksum does need to be recalculated by the shim, rather in the manner that a NAT behaves. However, this cannot be done for cryptographic checksums for obvious reasons. The shim should perhaps be regarded as logically below transport, but a better implementation would be for each transport layer to invoke the shim in-line prior to executing its checksum calculation. 5. DNS It is required that the IPv6 hosts "behind" a SHANTI translator either have a resolver that maps A records into AAAA records expanded with ::FF:0:0/96, or a DNS server that actually stores such records, or a DNS ALG that performs this transformation on the fly. On the assumption that hosts behind a translator will need to be configured in any case, in order to activate the shim, a mapping resolver seems likely to be the most robust choice, applying the fate-sharing principle. It would also work in a network with a mixture of SHANTI and dual-stack hosts. The former would see A records mapped as AAAA, and the latter would see native A records. This illustrates that SHANTI is an all-or-nothing approach. It doesn't seem plausible to activate SHANTI on a dual stack host since DNS entries are either mapped, or they aren't. But why would it be needed? Carpenter Expires April 30, 2008 [Page 8] Internet-Draft Shimmed IPv4/IPv6 Translation October 2007 "Outside" the translator, SHANTI hosts must be represented by an A record with the address of their translator. Specifically, the host's FQDN will have one or more AAAA records with its IPv6 address(es) and an A record with its translator's address. 6. ICMP TBD 7. Unresolved issues This section will be expanded to identify issues raised in [RFC4966] that are not resolved by the present specification. 8. Security Considerations As for NAT-PT, there is no obvious way to carry network layer IPsec across a SHANTI translator. There seems to be no reason IKE [RFC4306] cannot run in a SHANTI scenario, using its port agility intended for NAT tolerance. But that in itself isn't very useful The use of a shim layer in SHANTI will raise some of the security issues considered for SHIM6 . More analysis of the potential threats is needed to determine whether a cryptographic solution is needed, or if there is a straightforward way to prevent attackers taking over a session by impersonating the shim. It may be possible to find a simple method of arranging a shared secret between X and T, such that an elementary hash can be used to authenticate the shim headers. 9. IANA Considerations This document has not yet been exhaustively checked for possible action by the IANA. 10. Acknowledgements Vital comments on a very primitive version of this proposal were made by Marcelo Bagnulo Braun and Iljitsch van Beijnum. Contributions and comments by TBD are gratefully acknowledged. This document was produced using the xml2rfc tool [RFC2629]. Carpenter Expires April 30, 2008 [Page 9] Internet-Draft Shimmed IPv4/IPv6 Translation October 2007 11. References 11.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2765] Nordmark, E., "Stateless IP/ICMP Translation Algorithm (SIIT)", RFC 2765, February 2000. [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, February 2006. 11.2. Informative References [I-D.ietf-shim6-proto] Bagnulo, M. and E. Nordmark, "Shim6: Level 3 Multihoming Shim Protocol for IPv6", draft-ietf-shim6-proto-08 (work in progress), April 2007. [I-D.woodyatt-ald] Woodyatt, J., "Application Listener Discovery (ALD) for IPv6", draft-woodyatt-ald-01 (work in progress), June 2007. [RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E. Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, February 1996. [RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629, June 1999. [RFC2766] Tsirtsis, G. and P. Srisuresh, "Network Address Translation - Protocol Translation (NAT-PT)", RFC 2766, February 2000. [RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC 4306, December 2005. [RFC4966] Aoun, C. and E. Davies, "Reasons to Move the Network Address Translator - Protocol Translator (NAT-PT) to Historic Status", RFC 4966, July 2007. Carpenter Expires April 30, 2008 [Page 10] Internet-Draft Shimmed IPv4/IPv6 Translation October 2007 Author's Address Brian Carpenter Department of Computer Science University of Auckland PB 92019 Auckland, 1142 New Zealand Email: brian.e.carpenter@gmail.com Carpenter Expires April 30, 2008 [Page 11] Internet-Draft Shimmed IPv4/IPv6 Translation October 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Carpenter Expires April 30, 2008 [Page 12]