INTERNET-DRAFT Erik Nordmark, Sun Microsystems
July 30,November 20, 1997 Site prefixes in Neighbor Discovery <draft-ietf-ngtrans-header-trans-00.txt>Stateless IP/ICMP Translator (SIIT) <draft-ietf-ngtrans-header-trans-01.txt> Status of this Memo This document is an Internet-Draft. 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.'' To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow Directories on ds.internic.net (US East Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). Distribution of this memo is unlimited. This Internet Draft expires January 30,May 20, 1998. Abstract This document specifies a transition mechanism in addition to those already specified in RFC 1933. The new mechanism can be used as part of a solution that allows IPv6 hosts that do not have a permanently assigned IPv4 address to communication with IPv4-only hosts. Acknowledgements Some text has been pulled from an old Internet Draft titled "IPAE: The SIPP Interoperability and Transition Mechanism" authored by R. Gilligan, E. Nordmark, and B. Hinden. George Tsirtsis provides the figures for Section 1. Contents Status of this Memo.......................................... 1 1. INTRODUCTION AND MOTIVATION.............................. 23 1.1. Applicability and Limitations....................... 45 1.2. Impact Outside the Network Layer.................... 56 2. TERMINOLOGY.............................................. 67 2.1. Requirements........................................ 68 3. OVERVIEW................................................. 68 3.1. Assumptions......................................... 78 4. TRANSLATING FROM IPv4 TO IPv6............................ 79 4.1. Translating IPv4 Headers............................ 810 4.2. Translating ICMPv4.................................. 1012 4.3. Translating ICMPv4 Error Messages................... 1214 4.4. Knowing when to Translate........................... 1314 5. TRANSLATING FROM IPv6 TO IPv4............................ 1315 5.1. Translating IPv6 Headers............................ 1416 5.2. Translating ICMPv6.................................. 1618 5.3. Translating ICMPv6 Error Messages................... 1719 5.4. Knowing when to Translate........................... 1820 6. SECURITY CONSIDERATIONS.................................. 1820 7. OPEN ISSUES.............................................. 20 REFERENCES................................................... 1921 AUTHOR'S ADDRESS............................................. 2022 APPENDIX A: CHANGES SINCE PREVIOUS DRAFT..................... 22 1. INTRODUCTION AND MOTIVATION The transition mechanisms specified in [TRANS-MECH] handle the case of dual IPv4/IPv6 hosts interoperating with both dual hosts and IPv4-only hosts which is needed early in the transition to IPv6. The dual hosts are assigned both an IPv4 and one or more IPv6 addresses. As the pool of globally unique IPv4 addresses becomes smaller and smaller as the Internet grows there will be a desire to take advantage of the large IPv6 address and not require that every new Internet node have a permanently assigned IPv4 address. There are several different scenarios where there might be IPv6-only hosts that need to communicate with IPv4-only hosts. [Note: the terminology is somewhat unclear is this area. Is a node with that implements both IPv4 andThese IPv6 but has nohosts might be IPv4-capable, i.e. include an IPv4 addressimplementation but not be assigned to any of its interfaces a dual hostan IPv4 address, or they might not even include an IPv6-only host?]IPv4 implementation. - A completely new network with new devices that all support IPv6. In this case it might be beneficial to not have to configure the routers within the new network to route IPv4 since none of the hosts in the new network are configured with IPv4 addresses. But these new IPv6 devices might occasionally need to communicate with some IPv4 nodes out on the Internet. - An existing network where a large number of IPv6 devices are added. The IPv6 devices might have both an IPv4 and an IPv6 protocol stack but there is not enough global IPv4 address space to give each one of them a permanent IPv4 address. In this case it is more likely that the routers in the network already route IPv4 and are upgraded to dual routers. If there is no IPv4 routing inside the network i.e., the cloud that contains the new devices, some possible solutions are to either use the translators specified in this document at the boundary of the cloud, or to use Application Layer Gateways (ALG) on dual nodes at the cloud's boundary. The ALG solution is less flexible in that it is application protocol specific and it is also less robust since a the ALG box is likely to be a single point of failure for a connection using that box. If there IPv4 routing is supported inside the cloud and the implementations support both IPv6 and IPv4 it might suffice to have a mechanism for allocating temporary IPv4 and use IPv4 end to end when communicating with IPv4-only nodes. However, it would seem that such a solution would require the pool of temporary IPv4 addresses to be partitioned across all the subnets in the cloud which would either require a larger pool of IPv4 addresses or result in cases where communication would fail due to no available IPv4 address for the node's subnet. This document specifies a mechanism by which IPv6-only nodes can interoperate with IPv4-only nodes by having the IPv6-only nodes somehow acquire a temporary IPv4 address. That IPv4 address will be used as an IPv4-compatible IPv6 address and the packets will travel through a stateless IP headerIP/ICMP translator that will translate the packet headers between IPv4 and IPv6 and translate the addresses in those headers between IPv4 addresses on one side and IPv4-compatible or IPv4-mapped IPv6 addresses on the other side. This specification does not cover how an IPv6 node can acquire a temporary IPv4 address and how such a temporary address be registered in the DNS. The DHCP protocol, perhaps with some extensions, could probably be used to acquire temporary addresses with short leases but that is outside the scope of this document. The mechanism for routing this temporary IPv4 address (or the IPv4-compatible IPv6 address) in the site is currently not specified in this document. The figures below show how the Stateless IP/ICMP Translator (SIIT) can be used initially for small networks (e.g., a single subnet) and later for a site which has IPv6-only hosts in a dual IPv4/IPv6 network. This use assumes a mechanism for the IPv6 nodes to acquire an temporary address from the pool of IPv4 addresses. Note that SIIT is not likely to be useful later during transition when most of the Internet is IPv6 and there are only small islands of IPv4 nodes, since such use would either require the IPv6 nodes to acquire temporary IPv4 addresses from a "distant" SIIT box operated by a different administration, or require that the IPv6 routing contain routes for IPv6-mapped addresses. (The latter is known to be a very bad idea.) ___________ / \ [IPv6 Host]---[SIIT]---------< IPv4 network>--[IPv4 Host] | \___________/ (pool of v4 addresses) Figure 1. Using SIIT for a single IPv6-only subnet. ___________ ___________ / \ / \ [IPv6 Host]--< Dual network>--[SIIT]--< IPv4 network>--[IPv4 Host] \___________/ | \___________/ (pool of v4 addresses) Figure 2. Using SIIT for an IPv6-only or dual cloud (e.g. a site) which contains some IPv6-only hosts as well as IPv4 hosts. 1.1. Applicability and Limitations The IPv6 protocol [IPv6] has been designed so that the transport pseudo-header checksums are not affected by such a translation thus the translator does not need to modify TCP and UDP headers. However, ICMPv6 include a pseudo-header checksum but it is not present in ICMPv4 thus the checksum in ICMP messages need to be modified by the translator. In addition, ICMP error messages contain an IP header as part of the payload thus the translator need to rewrite those parts of the packets to make the receiver be able to understand the included IP header. However, all of the translators operations, including path MTU discovery, are stateless in the sense that the translator operates independently of each packet and does not retain any state from one packet to another. This allows redundant translator boxes without any coordination and a given TCP connection can have the two directions of packets go through different translator boxes. The translating function as specified in this document does not translate any IPv4 options and it does not translate IPv6 routing headers, hop-by-hop extension headers, or destination options headers. It could be possible to define a translation between source routing in IPv4 and IPv6. However such a translation would not be semantically correct since the IPv4 source routing option performs a "record route" function as the nodes listed in the source route are traversed and the IPv6 routing header does not include the record route aspect. Also, the usefulness of source routing when going through a header translator might be limited since all the routers would need to have an IPv4 address (or an IPv4-compatible IPv6 address) since the IPv4-only node will send a source option containing only IPv4 addresses. At first sight it might appear that the IPsec functionality [IPv6-SA, IPv6-ESP, IPv6-AH] can not be carried across the translator. However, since the translator does not modify any headers above the logical IP layer (IP headers, IPv6 fragment headers, and ICMP messages) packets encrypted using ESP in Transport-mode can be carried through the translator. [Note that this assumes that the key management can operate between the IPv6-only and the IPv4-only node.] The use of AH headers is more complex since the AH computation covers most of the fields in the IP header. Should it be possible for the IPv6 node to predict the value of all the IPv4 header fields on the other side of the translator then the IPv6 node could calculate the authentication data using an IPv4 header instead of the IPv6 header even though it is sending and receiving IPv6 packets. [Currently this is not possible since the IP fragment identification field is not carried end-to-end through the translator in all cases.]cases. This could be resolved by changing AH to not include the fragment identification field in the AH computation for either IPv4 or IPv6.] For ESP Tunnel-mode the IPv6 node would have to be able to parse and generate "inner" IPv4 headers since the inner IP will be encrypted together with the transport protocol. IPv4 multicast addresses can not be mapped to IPv6 multicast addresses. For instance, ::ffff:22.214.171.124 is an IPv4 mapped IPv6 address with a class D address, however it is not an IPv6 multicast address. While the IP/ICMP translation aspect of this draft works for multicast packets this address mapping limitation makes it hard to the techniques in this draft for multicast traffic. 1.2. Impact Outside the Network Layer The potential existence of headerstateless IP/ICMP translators is already taken care of from a protocol perspective in [IPv6]. However, an IPv6 node that wants to be able to use translators need some additional logic in the network layer. The network layer in an IPv6-only node when presented with either an IPv4 destination address or an IPv4-mapped IPv6 destination address by the application is likely to drop the packet and return some error message to the application. In order to take advantage of translators such a node should instead send an IPv6 packet where the destination address is the IPv4-mapped address and the source address is the nodes temporarily assigned IPv4-compatible address. If the node does not have a temporarily assigned IPv4-compatible address it should acquire one using mechanisms that are not discussed in this document. Note that the above also applies to a dual implementation node which is not configured with any IPv4 address. There are no extra changes needed to applications to operate through a translator. The applications that have been modified to work on a dual node already have the mechanisms to determine whether they are communicating with an IPv4 or an IPv6 peer. Thus if the applications need to modify their behavior depending on the type of the peer, such as ftp determining which flavor of PORT command to use, they already need to do that when running on dual nodes and the presense of translators does not add anything. For example, when using the socket API [RFC 2133] the applications know that the peer is IPv6 if they get an AF_INET6 address from the name service and the address is not an IPv4-mapped address (i.e., IN6_IS_ADDR_V4MAPPED returns false). If this is not the case, i.e., the address is AF_INET or an IPv4-mapped IPv6 address, the peer is IPv4. One way of viewing the translator, which might help clarify why applications do not need to know that a translator is used, is to look at the information that is passed from the transport layer to the network layer. If the transport passes down an IPv4 address (whether or not is in the IPv4-mapped encoding) this means that at some point there will be IPv4 packets generated. In a dual node the generation of the IPv4 packets takes place in the sending node. In an IPv6-only node conceptually the only difference is that the IPv4 packet is generated by the translator - all the information that the transport layer passedtransport layer passed to the network layer will be conveyed to the translator in some form. That form just "happens" to be in the form of an IPv6 header. 2. TERMINOLOGY This documents uses the terminology defined in [IPv6] and [TRANS- MECH] with these clarifications: IPv4 capable node: A node which has an IPv4 protocol stack. In order for the stack to be usable the node must be assigned one or more IPv4 addresses. IPv4 enabled node: A node which has an IPv4 protocol stack and is assigned one or more IPv4 addresses. Both IPv4-only and IPv6/IPv4 nodes are IPv4 enabled. IPv6 capable node: A node which has an IPv6 protocol stack. In order for the stack to the network layer willbe conveyed tousable the translator in some form. That form just "happens" tonode must be in the form ofassigned one or more IPv6 addresses. IPv6 enabled node: A node which has an IPv6 header. 2. TERMINOLOGY This documents uses the terminology defined in [IPv6]protocol stack and [TRANS- MECH].is assigned one or more IPv6 addresses. Both IPv6-only and IPv6/IPv4 nodes are IPv6 enabled. 2.1. Requirements The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in [KEYWORDS]. 3. OVERVIEW The protocol translators are assumed to fit around some piece of topology that includes some IPv6-only nodes and can also include IPv4 nodes and dual nodes. There has to be a translator on each path in and out of this cloud to ensure that the packets always get translated. This does not require a translator at every physical connection between the cloud and the rest of the Internet since the routing can be used to deliver the packets to the translator. For outbound packets i.e., packets that need to be translated from IPv6 to IPv4, it is sufficient to have a route for the IPv4-mapped address prefix (::ffff:0:0/96) injected in the internal IPv6 routing tables. This route will deliver packets to the translator since all IPv6 packets that need translation will have an IPv4-mapped IPv6 destination address. Inbound IPv4 packets needing translation are likely to have some temporary IPv4 address that is drawn from a pool of such addresses. Thus the internal IPv4 routing tables could have one or more routes for the whole pool that direct the packets to the translator. 3.1. Assumptions The IPv6 nodes using the translator must have an IPv4-compatible IPv6 address while it is communicating with IPv4 nodes. 4. TRANSLATING FROM IPv4 TO IPv6 When an IPv4-to-IPv6 translator receives an IPv4 datagram addressed to a destination that lies outside of the attached IPv4 island, it translates the IPv4 header of that packet into an IPv6 header. It then forwards the packet based on the IPv6 destination address. The original IPv4 header on the packet is removed and replaced by a IPv6 header. Except for ICMP packets the transport layer header and data portion of the packet are left unchanged. +-------------+ +-------------+ | IPv4 | | IPv6 | | Header | | Header | +-------------+ +-------------+ | Transport | | Fragment | | Layer | ===> | Header | | Header | |(not always) | +-------------+ +-------------+ | | | Transport | ~ Data ~ | Layer | | | | Header | +-------------+ +-------------+ | | ~ Data ~ | | +-------------+ IPv4-to-IPv6 HeaderTranslation One of the differences between IPv4 and IPv6 is that in IPv6 path MTU discovery is mandatory but it is optional in IPv4. This implies that IPv6 routers will never fragment a packet - only the sender can do fragmentation. When the IPv4 node performs path MTU discovery (by setting the DF bit in the header) the path MTU discovery can operate end-to-end i.e. across the translator. In this case either IPv4 or IPv6 routers might send back ICMP "packet too big" messages to the sender. When these ICMP errors are sent by the IPv6 routers they will pass through a translator which will translate the ICMP error to a form that the IPv4 sender can understand. In this case an IPv6 fragment header is only included if the IPv4 packet is already fragmented. However, when the IPv4 sender does not perform path MTU discovery the translator has to ensure that the packet does not exceed the path MTU on the IPv6 side. This is done by fragmenting the IPv4 packet so that it fits in 576 byte IPv6 packet since IPv6 guarantees that 576 byte packets never need to be fragment. Also, when the IPv4 sender does not perform path MTU discovery the translator MUST always include an IPv6 fragment header to indicate that the sender allows fragmentation. That is needed should the packet pass through an IPv6-to-IPv4 translator. The above rules ensure that when packets are fragmented either by the sender or by IPv4 routers that the low-order 16 bits of the fragment identification is carried end-end to ensure that packets are correctly reassembled. In addition, the rules use the presence of an IPv6 fragment header to indicate that the sender might not be using path MTU discovery i.e. the packet should not have the DF flag set should it later be translated back to IPv4. Other than the special rules for handling fragments and path MTU discovery the actual translation of the packet header consists of a simple mapping as defined below. Note that ICMP packets require special handling in order to translate the content of ICMP error message and also to add the ICMP pseudo-header checksum. 4.1. Translating IPv4 Headers If the DF flag is not set and the IPv4 packet will result in an IPv6 packet larger than 576 bytes the IPv4 packet MUST be fragmented prior to translating it. Since IPv4 packets with DF not set will always result in a fragment header being added to the packet the IPv4 packets must be fragmented so that their length, excluding the IPv4 header, is at most 528 bytes (576 minus 40 for the IPv6 header and 8 for the Fragment header). The resulting fragments are then translated independently using the logic described below. If the DF bit is set and the packet is not a fragment (i.e., the MF flag is not set and the Fragment Offset is zero) then there is no need to add a fragment header to the packet. The IPv6 header fields are set as follows: Version: 6 Flow ID: 0 (all zero bits) Payload Length: Total length value from IPv4 header, minus the size of the IPv4 header and IPv4 options, if present. Payload Type: Protocol field copied from IPv4 header Hop Limit: TTL value copied from IPv4 header, decremented by one. Source Address: The low-order 32 bits is the IPv4 source address. The high-order 96 bits is the IPv4-mapped prefix (::ffff:0:0/96) Destination Address: The low-order 32 bits is the IPv4 destination address. The high-order 96 bits is the IPv4- compatible prefix (0::0/96) If IPv4 options are present in the IPv4 packet, they are ignored i.e., there is no attempt to translate them. [Discussion: Should the packet be dropped and an ICMP error be generated instead?] If there is need to add a fragment header (the DF bit is not set or the packet is a fragment) the header fields are set as above with the following exceptions: IPv6 fields: Payload Length: Total length value from IPv4 header, plus 8 for the fragment header, minus the size of the IPv4 header and IPv4 options, if present. Payload Type: Fragment Header (44). Fragment header fields: Next Header: Protocol field copied from IPv4 header. Fragment Offset: Fragment Offset copied from the IPv4 header. M flag: More Fragments bit copied from the IPv4 header. Identification: The low-order 16 bits copied from the Identification field in the IPv4 header. The high-order 16 bits set to zero. 4.2. Translating ICMPv4 All ICMP messages that are to be translated require that the ICMP checksum field be updated as part of the translation since ICMPv6ICMPv6, unlike ICMPv4, has a pseudo-header checksum just like UDP and TCP. In addition all ICMP packets needs to have the Type value translated and for ICMP error messages the included IP header also needs translation. The actions needed to translate various ICMPv4 messages are: ICMPv4 query messages: Echo and Echo Reply (Type 8 and Type 0) Adjust the type to 128 and 129, respectively, and adjust the ICMP checksum both take the type change into account and to include the ICMPv6 pseudo-header. Information Request/Reply (Type 15 and Type 16) Obsoleted in ICMPv4. Silently drop. Timestamp and Timestamp Reply (Type 13 and Type 14) Obsoleted in ICMPv6. Silently drop. Address Mask Request/Reply (Type 17 and Type 18) Obsoleted in ICMPv6. Silently drop. ICMP Router Advertisement (Type 9) Single hop message. Silently drop. ICMP Router Solicitation (Type 10) Single hop message. Silently drop. Unknown ICMPv4 types Silently drop. IGMP messages: While there are ICMPv6 counterparts for the IGMP messages all the "normal" IGMP messages are single-hop messages and should be silently dropped by the translator. Other IGMP messages might be used by multicast routing protocols and, since it would be a configuration error to try to have router adjacencies across IPv4/IPv6 translators those packets should also be silently dropped. ICMPv4 error messages: Destination Unreachable (Type 3) For all that are not explicitly listed below set the Type to 1. Translate the code field as follows: Code 0, 1: Set Code to 0 (no route to destination). Code 2: Translate to an ICMPv6 Parameter Problem (Type 4, Code 1) and make the Pointer point to the IPv6 Payload Type field. Code 3: Set Code to 4 (port unreachable). Code 4: Translate to an ICMPv6 Packet Too Big message (Type 2) with code 0. The MTU field needs to be adjusted for the difference between the IPv4 and IPv6 header sizes. [TBD: WhatNote that if the IPv4 router did not set the MTU field i.e. the router does not implement [PMTUv4]. Should[PMTUv4], then the translator must use the plateau values as specified?]specified in [PMTUv4] to determine a likely path MTU and include that path MTU in the ICMPv6 packet. (Use the greatest plateau value that is less than the returned Total Length field.) Code 5: Set Code to 2 (not a neighbor). Code 6,7: Set Code to 0 (no route to destination). Code 8: Set Code to 0 (no route to destination). Code 9, 10: Set Code to 1 (communication with destination administratively prohibited) Code 11, 12: Set Code to 0 (no route to destination). Redirect (Type 5) Single hop message. Silently drop. Source Quench (Type 4) Obsoleted in ICMPv6. Silently drop. Time Exceeded (Type 11) Set the Type field to 3. The Code field is unchanged. Parameter Problem (Type 12) Set the Type field to 4. The Pointer needs to be updated to point to the corresponding field in the translated include IP header. 4.3. Translating ICMPv4 Error Messages There are some differences between the IPv4 and the IPv6 ICMP error message formats as detailed above. In addition, the ICMP error messages contain the IP header for the packet in error which needs to be translated just like a normal IP header. This translated is likely to change the length of the datagram thus the Payload Length field in the outer IPv6 header might need to be updated. +-------------+ +-------------+ | IPv4 | | IPv6 | | Header | | Header | +-------------+ +-------------+ | ICMPv4 | | ICMPv6 | | Header | | Header | +-------------+ +-------------+ | IPv4 | ===> | IPv6 | | Header | | Header | +-------------+ +-------------+ | Partial | | Partial | | Transport | | Transport | | Layer | | Layer | | Header | | Header | +-------------+ +-------------+ IPv4-to-IPv6 ICMP Error HeaderTranslation The translation of the inner IP header can be done by recursively invoking the function that translated the outer IP headers. 4.4. Knowing when to Translate The translator is assumed to know the pool(s) of IPv4 address that are used to represent the internal IPv6-only nodes. Thus if the destination address falls in these configured sets of prefixes the packet needs to be translated to IPv6. 5. TRANSLATING FROM IPv6 TO IPv4 When an IPv6-to-IPv4 translator receives an IPv6 datagram addressed to an IPv4-mapped IPv6 address, it translates the IPv6 header of that packet into an IPv7IPv6 header. It then forwards the packet based on the IPv4 destination address. The original IPv6 header on the packet is removed and replaced by a IPv4 header. Except for ICMP packets the transport layer header and data portion of the packet are left unchanged. +-------------+ +-------------+ | IPv6 | | IPv4 | | Header | | Header | +-------------+ +-------------+ | Fragment | | Transport | | Header | ===> | Layer | |(if present) | | Header | +-------------+ +-------------+ | Transport | | | | Layer | ~ Data ~ | Header | | | +-------------+ +-------------+ | | ~ Data ~ | | +-------------+ IPv6-to-IPv4 HeaderTranslation There are some differences between IPv6 and IPv4 in the area of fragmentation and the minimum link MTU that effect the translation. An IPv6 link has to have an MTU of 576 bytes or greater. The corresponding limit for IPv4 is 68 bytes. Thus, unless there were special measures, it would not be possible to do end-to-end path MTU discovery when the path includes an IPv6-to-IPv4 translator since the IPv6 node might receive ICMP "packet too big" messages originated by an IPv4 router that report an MTU less than 576. However, [IPv6] requires IPv6 nodes to handle such ICMP "packet too big" message by reducing the path MTU to 576 and include an IPv6 fragment header with each packet. This allows end-to-end path MTU discovery across the translator as long as the path MTU is 576 bytes or greater and when the path MTU drops below that limit IPv6 sender will originate 576 byte packets that will be fragmented by IPv4 routers along the path after being translated to IPv4. The only drawback with this scheme is that it is not possible to use PMTU to do optimal UDP fragmentation at sender. The presence of an IPv6 Fragment header is interpreted that is it OK to fragment the packet on the IPv4 side thus if the Fragment header is present because UDP wants to send e.g. 8k packets even though the path MTU is smaller the path MTU discovery will not be end-to-end but only up to and including the translator. Other than the special rules for handling fragments and path MTU discovery the actual translation of the packet header consists of a simple mapping as defined below. Note that ICMP packets require special handling in order to translate the content of ICMP error message and also to add the ICMP pseudo-header checksum. 5.1. Translating IPv6 Headers If there is no IPv6 Fragment header the IPv4 header fields are set as follows: Version: 4 Internet Header Length: 5 (no IPv4 options) Type of Service: All zero. [Discussion: Should this value be derived from the IPv6 priority field?] Total Length: Payload length value from IPv6 header, plus the size of the IPv4 header. Identification: All zero. [Discussion: In order to make IPv4 header compression work better for translated packets it might make sense to make this an incrementing counter. There is no need for the counter to be correct since the packets will not be fragmented.] Flags: The More Fragments flag is set to zero. The Don't Fragments flag is set to one. Fragment Offset: All zero. Time to Live: Hop Limit value copied from IPv6 header, decremented by one. Protocol: Payload Type field copied from IPv6 header. Header Checksum: Computed once the IPv4 header has been created. Source Address: If the IPv6 source address is an IPv4-compatible or an IPv4-mapped address then the low-order 32 bits of the IPv6 source address is copied to the IPv4 source address. Otherwise, the source address is set to 127.0.0.1. [Discussion: An alternative could be to drop the packet. However, it would be useful if a traceroute from the IPv4 node showed something for the IPv6 router hops. Thus either using 127.0.0.1 or 0.0.0.0 seem like reasonable alternatives.] Destination Address: IPv6 packets that are translated have a destination address that is either an IPv4-compatible or an IPv4-mapped address. Thus the low-order 32 bits of the IPv6 destination address is copied to the IPv4 source address. If any of an IPv6 hop-by-hop options header, destination options header, or routing header are present in the IPv6 packet, they are ignored i.e., there is no attempt to translate them. However, the Total Length field and the Protocol field would have to be adjusted to "skip" these extension headers. [Discussion: Should the packet be dropped and an ICMP error be generated instead?] If the IPv6 packet contains a Fragment header the header fields are set as above with the following exceptions: Total Length: Payload length value from IPv6 header, minus 8 for the Fragment header, plus the size of the IPv4 header. Identification: Copied from the low-order 16-bits in the Identification field in the Fragment header. Flags: The More Fragments flag is copied from the M flag in the Fragment header. The Don't Fragments flag is set to zero allowing this packet to be fragmented by IPv4 routers. Fragment Offset: Copied from the Fragment Offset field in the Fragment Header. Protocol: Next Header value copied from Fragment header. 5.2. Translating ICMPv6 All ICMP messages that are to be translated require that the ICMP checksum field be updated as part of the translation since ICMPv6ICMPv6, unlike ICMPv4, has a pseudo-header checksum just like UDP and TCP. In addition all ICMP packets needs to have the Type value translated and for ICMP error messages the included IP header also needs translation. The actions needed to translate various ICMPv6 messages are: ICMPv6 informational messages: Echo Request and Echo Reply (Type 128 and 129) Adjust the type to 0 and 8, respectively, and adjust the ICMP checksum both take the type change into account and to exclude the ICMPv6 pseudo-header. Group Membership Query/Report/Reduction (Type 130, 131, 132) Single hop message. Silently drop. Neighbor Discover messages (Type 133 through 137) Single hop message. Silently drop. Unknown informational messages Silently drop. ICMPv6 error messages: Destination Unreachable (Type 1) Set the Type field to 3. Translate the code field as follows: Code 0: Set Code to 1 (host unreachable). Code 1: Set Code to 10 (communication with destination host administratively prohibited). Code 2: Set Code to 5 (source route failed). Code 3: Set Code to 1 (host unreachable). Code 4: Set Code to 3 (port unreachable). Packet Too Big (Type 2) Translate to an ICMPv4 Destination Unreachable with code 4. The MTU field needs to be adjusted for the difference between the IPv4 and IPv6 header sizes taking into account whether or not the packet in error includes a Fragment header. Time Exceeded (Type 3) Set the Type to 11. The Code field is unchanged. Parameter Problem (Type 4) If the Code is 1 translate this to an ICMPv4 protocol unreachable (Type 3, Code 2). Otherwise set the Type to 12 and the Code to zero. The Pointer needs to be updated to point to the corresponding field in the translated include IP header. Unknown error messages Silently drop. 5.3. Translating ICMPv6 Error Messages There are some differences between the IPv4 and the IPv6 ICMP error message formats as detailed above. In addition, the ICMP error messages contain the IP header for the packet in error which needs to be translated just like a normal IP header. This translated is likely to change the length of the datagram thus the Payload Length field in the outer IPv6 header might need to be updated. +-------------+ +-------------+ | IPv6 | | IPv4 | | Header | | Header | +-------------+ +-------------+ | ICMPv6 | | ICMPv4 | | Header | | Header | +-------------+ +-------------+ | IPv6 | ===> | IPv4 | | Header | | Header | +-------------+ +-------------+ | Partial | | Partial | | Transport | | Transport | | Layer | | Layer | | Header | | Header | +-------------+ +-------------+ IPv6-to-IPv4 ICMP Error HeaderTranslation The translation of the inner IP header can be done by recursively invoking the function that translated the outer IP headers. 5.4. Knowing when to Translate When the translator receives a IPv6 packet with an IPv4-mapped destination address the packet will be translated to IPv4. 6. SECURITY CONSIDERATIONS TBDThe use of stateless IP/ICMP translators does not introduce any new security issues beyond the security issues that are already present in the IPv4 and IPv6 protocols and in the routing protocols which are used to make the packets reach the translator. 7. OPEN ISSUES - Can/should we make AH work through a translator e.g. by removing the fragment ID field from the AH computation for both IPv4 and IPv6? - For IPv6 to IPv4 should the Identification be something other than zero to be more friendly to header compression? These packets have DF set thus it is possible to choose e.g. a local incrementing counter without adverse effects on reassembly. - IPv6 to IPv4 when source is pure IPv6 address. What should the IPv4 source address be set to in order to make traceroute usable? If such packets are not translated an IPv4 traceroute would show no responses from IPv6 routers. We could use 127.0.0.1 or 0.0.0.0. REFERENCES [KEYWORDS] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [IPv6] S. Deering, R. Hinden, Editors, "Internet Protocol, Version 6 (IPv6) Specification", RFC 1883, January 1996. [IPv4] J. Postel, "Internet Protocol", RFC 791, September 1981. [ADDR-ARCH] S. Deering, R. Hinden, Editors, "IP Version 6 Addressing Architecture", RFC 1884, January 1996. [TRANS-MECH] R. Gilligan, E. Nordmark, "Transition Mechanisms for IPv6 Hosts and Routers", RFC 1933, April 1996. [DISCOVERY] T. Narten, E. Nordmark, and W. Simpson, "Neighbor Discovery for IP Version 6 (IPv6)", RFC 1970, August 1996. [IPv6-SA] R. Atkinson. "Security Architecture for the Internet Protocol". RFC 1825, August 1995. [IPv6-AUTH] R. Atkinson. "IP Authentication Header", RFC 1826, August 1995. [IPv6-ESP] R. Atkinson. "IP Encapsulating Security Payload (ESP)", RFC 1827, August 1995. [ICMPv4] J. Postel, "Internet Control Message Protocol", RFC 792, September 1981. [ICMPv6] A. Conta, S. Deering, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6)", RFC 1885, January 1996. [IGMP] S. Deering, "Host extensions for IP multicasting", RFC 1112, August 1989. [PMTUv4] J. Mogul, S. Deering, "Path MTU Discovery", RFC 1191, November 1990. [PMTUv6] J. McCann, S. Deering, J. Mogul, "Path MTU Discovery for IP version 6", RFC 1981, August 1996. AUTHOR'S ADDRESS Erik Nordmark Sun Microsystems, Inc. 901 San Antonio Road Palo Alto, CA 94303 USA phone: +1 415650 786 5166 fax: +1 415650 786 5896 email: firstname.lastname@example.org APPENDIX A: CHANGES SINCE PREVIOUS DRAFT The following changes have been made since version 00 of the draft. o Added clarification about applicability for multicast. o Clarified the terminology with IPv4 capable vs. enabled etc. o Changed the title to provide an acronym (SIIT). o Added some figures to explain the applicability.