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Network Working Group                                        E. NordmarkRequest for Comments: 4213                        Sun Microsystems, Inc.Obsoletes:2893                                              R. GilliganCategory: Standards Track                                 Intransa, Inc.                                                            October 2005Basic Transition Mechanisms for IPv6 Hosts and RoutersStatus of This Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2005).Abstract   This document specifies IPv4 compatibility mechanisms that can be   implemented by IPv6 hosts and routers.  Two mechanisms are specified,   dual stack and configured tunneling.  Dual stack implies providing   complete implementations of both versions of the Internet Protocol   (IPv4 and IPv6), and configured tunneling provides a means to carry   IPv6 packets over unmodified IPv4 routing infrastructures.   This document obsoletesRFC 2893.Nordmark & Gilligan         Standards Track                     [Page 1]

RFC 4213            Basic IPv6 Transition Mechanisms        October 2005Table of Contents1. Introduction ....................................................21.1. Terminology ................................................32. Dual IP Layer Operation .........................................42.1. Address Configuration ......................................52.2. DNS ........................................................53. Configured Tunneling Mechanisms .................................63.1. Encapsulation ..............................................73.2. Tunnel MTU and Fragmentation ...............................83.2.1. Static Tunnel MTU ...................................93.2.2. Dynamic Tunnel MTU ..................................93.3. Hop Limit .................................................113.4. Handling ICMPv4 Errors ....................................113.5. IPv4 Header Construction ..................................133.6. Decapsulation .............................................143.7. Link-Local Addresses ......................................173.8. Neighbor Discovery over Tunnels ...........................184. Threat Related to Source Address Spoofing ......................185. Security Considerations ........................................196. Acknowledgements ...............................................217. References .....................................................217.1. Normative References ......................................217.2. Informative References ....................................218. Changes fromRFC 2893 ..........................................231.  Introduction   The key to a successful IPv6 transition is compatibility with the   large installed base of IPv4 hosts and routers.  Maintaining   compatibility with IPv4 while deploying IPv6 will streamline the task   of transitioning the Internet to IPv6.  This specification defines   two mechanisms that IPv6 hosts and routers may implement in order to   be compatible with IPv4 hosts and routers.   The mechanisms in this document are designed to be employed by IPv6   hosts and routers that need to interoperate with IPv4 hosts and   utilize IPv4 routing infrastructures.  We expect that most nodes in   the Internet will need such compatibility for a long time to come,   and perhaps even indefinitely.   The mechanisms specified here are:   -  Dual IP layer (also known as dual stack):  A technique for      providing complete support for both Internet protocols -- IPv4 and      IPv6 -- in hosts and routers.Nordmark & Gilligan         Standards Track                     [Page 2]

RFC 4213            Basic IPv6 Transition Mechanisms        October 2005   -  Configured tunneling of IPv6 over IPv4:  A technique for      establishing point-to-point tunnels by encapsulating IPv6 packets      within IPv4 headers to carry them over IPv4 routing      infrastructures.   The mechanisms defined here are intended to be the core of a   "transition toolbox" -- a growing collection of techniques that   implementations and users may employ to ease the transition.  The   tools may be used as needed.  Implementations and sites decide which   techniques are appropriate to their specific needs.   This document defines the basic set of transition mechanisms, but   these are not the only tools available.  Additional transition and   compatibility mechanisms are specified in other documents.1.1.  Terminology   The following terms are used in this document:   Types of Nodes      IPv4-only node:         A host or router that implements only IPv4.  An IPv4-only node         does not understand IPv6.  The installed base of IPv4 hosts and         routers existing before the transition begins are IPv4-only         nodes.      IPv6/IPv4 node:         A host or router that implements both IPv4 and IPv6.      IPv6-only node:         A host or router that implements IPv6 and does not implement         IPv4.  The operation of IPv6-only nodes is not addressed in         this memo.      IPv6 node:         Any host or router that implements IPv6.  IPv6/IPv4 and IPv6-         only nodes are both IPv6 nodes.      IPv4 node:         Any host or router that implements IPv4.  IPv6/IPv4 and IPv4-         only nodes are both IPv4 nodes.Nordmark & Gilligan         Standards Track                     [Page 3]

RFC 4213            Basic IPv6 Transition Mechanisms        October 2005   Techniques Used in the Transition      IPv6-over-IPv4 tunneling:         The technique of encapsulating IPv6 packets within IPv4 so that         they can be carried across IPv4 routing infrastructures.      Configured tunneling:         IPv6-over-IPv4 tunneling where the IPv4 tunnel endpoint         address(es) are determined by configuration information on         tunnel endpoints.  All tunnels are assumed to be bidirectional.         The tunnel provides a (virtual) point-to-point link to the IPv6         layer, using the configured IPv4 addresses as the lower-layer         endpoint addresses.   Other transition mechanisms, including other tunneling mechanisms,   are outside the scope of this document.   The key words 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 [RFC2119].2.  Dual IP Layer Operation   The most straightforward way for IPv6 nodes to remain compatible with   IPv4-only nodes is by providing a complete IPv4 implementation.  IPv6   nodes that provide complete IPv4 and IPv6 implementations are called   "IPv6/IPv4 nodes".  IPv6/IPv4 nodes have the ability to send and   receive both IPv4 and IPv6 packets.  They can directly interoperate   with IPv4 nodes using IPv4 packets, and also directly interoperate   with IPv6 nodes using IPv6 packets.   Even though a node may be equipped to support both protocols, one or   the other stack may be disabled for operational reasons.  Here we use   a rather loose notion of "stack".  A stack being enabled has IP   addresses assigned, but whether or not any particular application is   available on the stacks is explicitly not defined.  Thus, IPv6/IPv4   nodes may be operated in one of three modes:   -  With their IPv4 stack enabled and their IPv6 stack disabled.   -  With their IPv6 stack enabled and their IPv4 stack disabled.   -  With both stacks enabled.   IPv6/IPv4 nodes with their IPv6 stack disabled will operate like   IPv4-only nodes.  Similarly, IPv6/IPv4 nodes with their IPv4 stacksNordmark & Gilligan         Standards Track                     [Page 4]

RFC 4213            Basic IPv6 Transition Mechanisms        October 2005   disabled will operate like IPv6-only nodes.  IPv6/IPv4 nodes MAY   provide a configuration switch to disable either their IPv4 or IPv6   stack.   The configured tunneling technique, which is described inSection 3,   may or may not be used in addition to the dual IP layer operation.2.1.  Address Configuration   Because the nodes support both protocols, IPv6/IPv4 nodes may be   configured with both IPv4 and IPv6 addresses.  IPv6/IPv4 nodes use   IPv4 mechanisms (e.g., DHCP) to acquire their IPv4 addresses, and   IPv6 protocol mechanisms (e.g., stateless address autoconfiguration   [RFC2462] and/or DHCPv6) to acquire their IPv6 addresses.2.2.  DNS   The Domain Naming System (DNS) is used in both IPv4 and IPv6 to map   between hostnames and IP addresses.  A new resource record type named   "AAAA" has been defined for IPv6 addresses [RFC3596].  Since   IPv6/IPv4 nodes must be able to interoperate directly with both IPv4   and IPv6 nodes, they must provide resolver libraries capable of   dealing with IPv4 "A" records as well as IPv6 "AAAA" records.  Note   that the lookup of A versus AAAA records is independent of whether   the DNS packets are carried in IPv4 or IPv6 packets and that there is   no assumption that the DNS servers know the IPv4/IPv6 capabilities of   the requesting node.   The issues and operational guidelines for using IPv6 with DNS are   described at more length in other documents, e.g., [DNSOPV6].   DNS resolver libraries on IPv6/IPv4 nodes MUST be capable of handling   both AAAA and A records.  However, when a query locates an AAAA   record holding an IPv6 address, and an A record holding an IPv4   address, the resolver library MAY order the results returned to the   application in order to influence the version of IP packets used to   communicate with that specific node -- IPv6 first, or IPv4 first.   The applications SHOULD be able to specify whether they want IPv4,   IPv6, or both records [RFC3493].  That defines which address families   the resolver looks up.  If there is not an application choice, or if   the application has requested both, the resolver library MUST NOT   filter out any records.   Since most applications try the addresses in the order they are   returned by the resolver, this can affect the IP version "preference"   of applications.Nordmark & Gilligan         Standards Track                     [Page 5]

RFC 4213            Basic IPv6 Transition Mechanisms        October 2005   The actual ordering mechanisms are out of scope of this memo.   Address selection is described at more length in [RFC3484].3.  Configured Tunneling Mechanisms   In most deployment scenarios, the IPv6 routing infrastructure will be   built up over time.  While the IPv6 infrastructure is being deployed,   the existing IPv4 routing infrastructure can remain functional and   can be used to carry IPv6 traffic.  Tunneling provides a way to   utilize an existing IPv4 routing infrastructure to carry IPv6   traffic.   IPv6/IPv4 hosts and routers can tunnel IPv6 datagrams over regions of   IPv4 routing topology by encapsulating them within IPv4 packets.   Tunneling can be used in a variety of ways:   -  Router-to-Router.  IPv6/IPv4 routers interconnected by an IPv4      infrastructure can tunnel IPv6 packets between themselves.  In      this case, the tunnel spans one segment of the end-to-end path      that the IPv6 packet takes.   -  Host-to-Router.  IPv6/IPv4 hosts can tunnel IPv6 packets to an      intermediary IPv6/IPv4 router that is reachable via an IPv4      infrastructure.  This type of tunnel spans the first segment of      the packet's end-to-end path.   -  Host-to-Host.  IPv6/IPv4 hosts that are interconnected by an IPv4      infrastructure can tunnel IPv6 packets between themselves.  In      this case, the tunnel spans the entire end-to-end path that the      packet takes.   -  Router-to-Host.  IPv6/IPv4 routers can tunnel IPv6 packets to      their final destination IPv6/IPv4 host.  This tunnel spans only      the last segment of the end-to-end path.   Configured tunneling can be used in all of the above cases, but it is   most likely to be used router-to-router due to the need to explicitly   configure the tunneling endpoints.   The underlying mechanisms for tunneling are:   -  The entry node of the tunnel (the encapsulator) creates an      encapsulating IPv4 header and transmits the encapsulated packet.   -  The exit node of the tunnel (the decapsulator) receives the      encapsulated packet, reassembles the packet if needed, removes the      IPv4 header, and processes the received IPv6 packet.Nordmark & Gilligan         Standards Track                     [Page 6]

RFC 4213            Basic IPv6 Transition Mechanisms        October 2005   -  The encapsulator may need to maintain soft-state information for      each tunnel recording such parameters as the MTU of the tunnel in      order to process IPv6 packets forwarded into the tunnel.   In configured tunneling, the tunnel endpoint addresses are determined   in the encapsulator from configuration information stored for each   tunnel.  When an IPv6 packet is transmitted over a tunnel, the   destination and source addresses for the encapsulating IPv4 header   are set as described inSection 3.5.   The determination of which packets to tunnel is usually made by   routing information on the encapsulator.  This is usually done via a   routing table, which directs packets based on their destination   address using the prefix mask and match technique.   The decapsulator matches the received protocol-41 packets to the   tunnels it has configured, and allows only the packets in which IPv4   source addresses match the tunnels configured on the decapsulator.   Therefore, the operator must ensure that the tunnel's IPv4 address   configuration is the same both at the encapsulator and the   decapsulator.3.1.  Encapsulation   The encapsulation of an IPv6 datagram in IPv4 is shown below:                                             +-------------+                                             |    IPv4     |                                             |   Header    |             +-------------+                 +-------------+             |    IPv6     |                 |    IPv6     |             |   Header    |                 |   Header    |             +-------------+                 +-------------+             |  Transport  |                 |  Transport  |             |   Layer     |      ===>       |   Layer     |             |   Header    |                 |   Header    |             +-------------+                 +-------------+             |             |                 |             |             ~    Data     ~                 ~    Data     ~             |             |                 |             |             +-------------+                 +-------------+                      Encapsulating IPv6 in IPv4Nordmark & Gilligan         Standards Track                     [Page 7]

RFC 4213            Basic IPv6 Transition Mechanisms        October 2005   In addition to adding an IPv4 header, the encapsulator also has to   handle some more complex issues:   -  Determine when to fragment and when to report an ICMPv6 "packet      too big" error back to the source.   -  How to reflect ICMPv4 errors from routers along the tunnel path      back to the source as ICMPv6 errors.   Those issues are discussed in the following sections.3.2.  Tunnel MTU and Fragmentation   Naively, the encapsulator could view encapsulation as IPv6 using IPv4   as a link layer with a very large MTU (65535-20 bytes at most; 20   bytes "extra" are needed for the encapsulating IPv4 header).  The   encapsulator would only need to report ICMPv6 "packet too big" errors   back to the source for packets that exceed this MTU.  However, such a   scheme would be inefficient or non-interoperable for three reasons   and therefore MUST NOT be used:   1) It would result in more fragmentation than needed.  IPv4 layer      fragmentation should be avoided due to the performance problems      caused by the loss unit being smaller than the retransmission unit      [KM97].   2) Any IPv4 fragmentation occurring inside the tunnel, i.e., between      the encapsulator and the decapsulator, would have to be      reassembled at the tunnel endpoint.  For tunnels that terminate at      a router, this would require additional memory and other resources      to reassemble the IPv4 fragments into a complete IPv6 packet      before that packet could be forwarded.   3) The encapsulator has no way of knowing that the decapsulator is      able to defragment such IPv4 packets (seeSection 3.6 for      details), and has no way of knowing that the decapsulator is able      to handle such a large IPv6 Maximum Receive Unit (MRU).   Hence, the encapsulator MUST NOT treat the tunnel as an interface   with an MTU of 64 kilobytes, but instead either use the fixed static   MTU or OPTIONAL dynamic MTU determination based on the IPv4 path MTU   to the tunnel endpoint.   If both the mechanisms are implemented, the decision of which to use   SHOULD be configurable on a per-tunnel endpoint basis.Nordmark & Gilligan         Standards Track                     [Page 8]

RFC 4213            Basic IPv6 Transition Mechanisms        October 20053.2.1.  Static Tunnel MTU   A node using static tunnel MTU treats the tunnel interface as having   a fixed-interface MTU.  By default, the MTU MUST be between 1280 and   1480 bytes (inclusive), but it SHOULD be 1280 bytes.  If the default   is not 1280 bytes, the implementation MUST have a configuration knob   that can be used to change the MTU value.   A node must be able to accept a fragmented IPv6 packet that, after   reassembly, is as large as 1500 octets [RFC2460].  This memo also   includes requirements (seeSection 3.6) for the amount of IPv4   reassembly and IPv6 MRU that MUST be supported by all the   decapsulators.  These ensure correct interoperability with any fixed   MTUs between 1280 and 1480 bytes.   A larger fixed MTU than supported by these requirements must not be   configured unless it has been administratively ensured that the   decapsulator can reassemble or receive packets of that size.   The selection of a good tunnel MTU depends on many factors, at least:   -  Whether the IPv4 protocol-41 packets will be transported over      media that may have a lower path MTU (e.g., IPv4 Virtual Private      Networks); then picking too high a value might lead to IPv4      fragmentation.   -  Whether the tunnel is used to transport IPv6 tunneled packets      (e.g., a mobile node with an IPv6-in-IPv4 configured tunnel, and      an IPv6-in-IPv6 tunnel interface); then picking too low a value      might lead to IPv6 fragmentation.   If layered encapsulation is believed to be present, it may be prudent   to consider supporting dynamic MTU determination instead as it is   able to minimize fragmentation and optimize packet sizes.   When using the static tunnel MTU, the Don't Fragment bit MUST NOT be   set in the encapsulating IPv4 header.  As a result, the encapsulator   should not receive any ICMPv4 "packet too big" messages as a result   of the packets it has encapsulated.3.2.2.  Dynamic Tunnel MTU   The dynamic MTU determination is OPTIONAL.  However, if it is   implemented, it SHOULD have the behavior described in this document.   The fragmentation inside the tunnel can be reduced to a minimum by   having the encapsulator track the IPv4 path MTU across the tunnel,   using the IPv4 Path MTU Discovery Protocol [RFC1191] and recordingNordmark & Gilligan         Standards Track                     [Page 9]

RFC 4213            Basic IPv6 Transition Mechanisms        October 2005   the resulting path MTU.  The IPv6 layer in the encapsulator can then   view a tunnel as a link layer with an MTU equal to the IPv4 path MTU,   minus the size of the encapsulating IPv4 header.   Note that this does not eliminate IPv4 fragmentation in the case when   the IPv4 path MTU would result in an IPv6 MTU less than 1280 bytes.   (Any link layer used by IPv6 has to have an MTU of at least 1280   bytes [RFC2460].)  In this case, the IPv6 layer has to "see" a link   layer with an MTU of 1280 bytes and the encapsulator has to use IPv4   fragmentation in order to forward the 1280 byte IPv6 packets.   The encapsulator SHOULD employ the following algorithm to determine   when to forward an IPv6 packet that is larger than the tunnel's path   MTU using IPv4 fragmentation, and when to return an ICMPv6 "packet   too big" message per [RFC1981]:         if (IPv4 path MTU - 20) is less than 1280                 if packet is larger than 1280 bytes                         Send ICMPv6 "packet too big" with MTU = 1280.                         Drop packet.                 else                         Encapsulate but do not set the Don't Fragment                         flag in the IPv4 header.  The resulting IPv4                         packet might be fragmented by the IPv4 layer                         on the encapsulator or by some router along                         the IPv4 path.                 endif         else                 if packet is larger than (IPv4 path MTU - 20)                         Send ICMPv6 "packet too big" with                         MTU = (IPv4 path MTU - 20).                         Drop packet.                 else                         Encapsulate and set the Don't Fragment flag                         in the IPv4 header.                 endif         endif   Encapsulators that have a large number of tunnels may choose between   dynamic versus static tunnel MTUs on a per-tunnel endpoint basis.  In   cases where the number of tunnels that any one node is using is   large, it is helpful to observe that this state information can be   cached and discarded when not in use.   Note that using dynamic tunnel MTU is subject to IPv4 path MTU   blackholes should the ICMPv4 "packet too big" messages be dropped by   firewalls or not generated by the routers [RFC1435,RFC2923].Nordmark & Gilligan         Standards Track                    [Page 10]

RFC 4213            Basic IPv6 Transition Mechanisms        October 20053.3.  Hop Limit   IPv6-over-IPv4 tunnels are modeled as "single-hop" from the IPv6   perspective.  The tunnel is opaque to users of the network, and it is   not detectable by network diagnostic tools such as traceroute.   The single-hop model is implemented by having the encapsulators and   decapsulators process the IPv6 hop limit field as they would if they   were forwarding a packet on to any other datalink.  That is, they   decrement the hop limit by 1 when forwarding an IPv6 packet.  (The   originating node and final destination do not decrement the hop   limit.)   The TTL of the encapsulating IPv4 header is selected in an   implementation-dependent manner.  The current suggested value is   published in the "Assigned Numbers" RFC [RFC3232][ASSIGNED].   Implementations MAY provide a mechanism to allow the administrator to   configure the IPv4 TTL as the IP Tunnel MIB [RFC4087].3.4.  Handling ICMPv4 Errors   In response to encapsulated packets it has sent into the tunnel, the   encapsulator might receive ICMPv4 error messages from IPv4 routers   inside the tunnel.  These packets are addressed to the encapsulator   because it is the IPv4 source of the encapsulated packet.   ICMPv4 error handling is only applicable to dynamic MTU   determination, even though the functions could be used with static   MTU tunnels as well.   The ICMPv4 "packet too big" error messages are handled according to   IPv4 Path MTU Discovery [RFC1191] and the resulting path MTU is   recorded in the IPv4 layer.  The recorded path MTU is used by IPv6 to   determine if an ICMPv6 "packet too big" error has to be generated as   described inSection 3.2.2.   The handling of other types of ICMPv4 error messages depends on how   much information is available from the encapsulated packet that   caused the error.   Many older IPv4 routers return only 8 bytes of data beyond the IPv4   header of the packet in error, which is not enough to include the   address fields of the IPv6 header.  More modern IPv4 routers are   likely to return enough data beyond the IPv4 header to include the   entire IPv6 header and possibly even the data beyond that.  See   [RFC1812].Nordmark & Gilligan         Standards Track                    [Page 11]

RFC 4213            Basic IPv6 Transition Mechanisms        October 2005   If sufficient data bytes from the offending packet are available, the   encapsulator MAY extract the encapsulated IPv6 packet and use it to   generate an ICMPv6 message directed back to the originating IPv6   node, as shown below:                         +--------------+                         | IPv4 Header  |                         | dst = encaps |                         |       node   |                         +--------------+                         |    ICMPv4    |                         |    Header    |                  - -    +--------------+                         | IPv4 Header  |                         | src = encaps |                 IPv4    |       node   |                         +--------------+   - -                 Packet  |    IPv6      |                         |    Header    |   Original IPv6                  in     +--------------+   Packet -                         |  Transport   |   Can be used to                 Error   |    Header    |   generate an                         +--------------+   ICMPv6                         |              |   error message                         ~     Data     ~   back to the source.                         |              |                  - -    +--------------+   - -             ICMPv4 Error Message Returned to Encapsulating Node   When receiving ICMPv4 errors as above and the errors are not "packet   too big", it would be useful to log the error as an error related to   the tunnel.  Also, if sufficient headers are available, then the   originating node MAY send an ICMPv6 error of type "unreachable" with   code "address unreachable" to the IPv6 source.  (The "address   unreachable" code is appropriate since, from the perspective of IPv6,   the tunnel is a link and that code is used for link-specific errors   [RFC2463]).   Note that when the IPv4 path MTU is exceeded, and sufficient bytes of   payload associated with the ICMPv4 errors are not available, or   ICMPv4 errors do not cause the generation of ICMPv6 errors in case   there is enough payload, there will be at least two packet drops   instead of at least one (the case of a single layer of MTU   discovery).  Consider a case where an IPv6 host is connected to an   IPv4/IPv6 router, which is connected to a network where an ICMPv4   error about too big packet size is generated.  First, the router   needs to learn the tunnel (IPv4) MTU that causes at least one packetNordmark & Gilligan         Standards Track                    [Page 12]

RFC 4213            Basic IPv6 Transition Mechanisms        October 2005   loss, and then the host needs to learn the (IPv6) MTU from the router   that causes at least one packet loss.  Still, in all cases there can   be more than one packet loss if there are multiple large packets in   flight at the same time.3.5.  IPv4 Header Construction   When encapsulating an IPv6 packet in an IPv4 datagram, the IPv4   header fields are set as follows:      Version:         4      IP Header Length in 32-bit words:         5 (There are no IPv4 options in the encapsulating header.)      Type of Service:         0 unless otherwise specified. (See [RFC2983] and[RFC3168]         Section 9.1 for issues relating to the Type-of-Service byte and         tunneling.)      Total Length:         Payload length from IPv6 header plus length of IPv6 and IPv4         headers (i.e., IPv6 payload length plus a constant 60 bytes).      Identification:         Generated uniquely as for any IPv4 packet transmitted by the         system.      Flags:         Set the Don't Fragment (DF) flag as specified inSection 3.2.         Set the More Fragments (MF) bit as necessary if fragmenting.      Fragment Offset:         Set as necessary if fragmenting.      Time to Live:         Set in an implementation-specific manner, as described inSection 3.3.Nordmark & Gilligan         Standards Track                    [Page 13]

RFC 4213            Basic IPv6 Transition Mechanisms        October 2005      Protocol:         41 (Assigned payload type number for IPv6).      Header Checksum:         Calculate the checksum of the IPv4 header [RFC791].      Source Address:         An IPv4 address of the encapsulator: either configured by the         administrator or an address of the outgoing interface.      Destination Address:         IPv4 address of the tunnel endpoint.   When encapsulating the packets, the node must ensure that it will use   the correct source address so that the packets are acceptable to the   decapsulator as described inSection 3.6.  Configuring the source   address is appropriate particularly in cases in which automatic   selection of source address may produce different results in a   certain period of time.  This is often the case with multiple   addresses, and multiple interfaces, or when routes may change   frequently.  Therefore, it SHOULD be possible to administratively   specify the source address of a tunnel.3.6.  Decapsulation   When an IPv6/IPv4 host or a router receives an IPv4 datagram that is   addressed to one of its own IPv4 addresses or a joined multicast   group address, and the value of the protocol field is 41, the packet   is potentially a tunnel packet and needs to be verified to belong to   one of the configured tunnel interfaces (by checking   source/destination addresses), reassembled (if fragmented at the IPv4   level), and have the IPv4 header removed and the resulting IPv6   datagram be submitted to the IPv6 layer code on the node.   The decapsulator MUST verify that the tunnel source address is   correct before further processing packets, to mitigate the problems   with address spoofing (seeSection 4).  This check also applies to   packets that are delivered to transport protocols on the   decapsulator.  This is done by verifying that the source address is   the IPv4 address of the encapsulator, as configured on the   decapsulator.  Packets for which the IPv4 source address does not   match MUST be discarded and an ICMP message SHOULD NOT be generated;Nordmark & Gilligan         Standards Track                    [Page 14]

RFC 4213            Basic IPv6 Transition Mechanisms        October 2005   however, if the implementation normally sends an ICMP message when   receiving an unknown protocol packet, such an error message MAY be   sent (e.g., ICMPv4 Protocol 41 Unreachable).   A side effect of this address verification is that the node will   silently discard packets with a wrong source address and packets that   were received by the node but not directly addressed to it (e.g.,   broadcast addresses).   Independent of any other forms of IPv4 ingress filtering the   administrator of the node may have configured, the implementation MAY   perform ingress filtering, i.e., check that the packet is arriving   from the interface in the direction of the route toward the tunnel   end-point, similar to a Strict Reverse Path Forwarding (RPF) check   [RFC3704].  As this may cause problems on tunnels that are routed   through multiple links, it is RECOMMENDED that this check, if done,   is disabled by default.  The packets caught by this check SHOULD be   discarded; an ICMP message SHOULD NOT be generated by default.   The decapsulator MUST be capable of having, on the tunnel interfaces,   an IPv6 MRU of at least the maximum of 1500 bytes and the largest   (IPv6) interface MTU on the decapsulator.   The decapsulator MUST be capable of reassembling an IPv4 packet that   is (after the reassembly) the maximum of 1500 bytes and the largest   (IPv4) interface MTU on the decapsulator.  The 1500-byte number is a   result of encapsulators that use the static MTU scheme inSection3.2.1, while encapsulators that use the dynamic scheme inSection3.2.2 can cause up to the largest interface MTU on the decapsulator   to be received. (Note that it is strictly the interface MTU on the   last IPv4 router *before* the decapsulator that matters, but for most   links the MTU is the same between all neighbors.)   This reassembly limit allows dynamic tunnel MTU determination by the   encapsulator to take advantage of larger IPv4 path MTUs.  An   implementation MAY have a configuration knob that can be used to set   a larger value of the tunnel reassembly buffers than the above   number, but it MUST NOT be set below the above number.Nordmark & Gilligan         Standards Track                    [Page 15]

RFC 4213            Basic IPv6 Transition Mechanisms        October 2005   The decapsulation is shown below:            +-------------+            |    IPv4     |            |   Header    |            +-------------+                 +-------------+            |    IPv6     |                 |    IPv6     |            |   Header    |                 |   Header    |            +-------------+                 +-------------+            |  Transport  |                 |  Transport  |            |   Layer     |      ===>       |   Layer     |            |   Header    |                 |   Header    |            +-------------+                 +-------------+            |             |                 |             |            ~    Data     ~                 ~    Data     ~            |             |                 |             |            +-------------+                 +-------------+                    Decapsulating IPv6 from IPv4   The decapsulator performs IPv4 reassembly before decapsulating the   IPv6 packet.   When decapsulating the packet, the IPv6 header is not modified.   (However, see [RFC2983] and[RFC3168] section 9.1 for issues relating   to the Type of Service byte and tunneling.)  If the packet is   subsequently forwarded, its hop limit is decremented by one.   The encapsulating IPv4 header is discarded, and the resulting packet   is checked for validity when submitted to the IPv6 layer.  When   reconstructing the IPv6 packet, the length MUST be determined from   the IPv6 payload length since the IPv4 packet might be padded (thus   have a length that is larger than the IPv6 packet plus the IPv4   header being removed).   After the decapsulation, the node MUST silently discard a packet with   an invalid IPv6 source address.  The list of invalid source addresses   SHOULD include at least:   -  all multicast addresses (FF00::/8)   -  the loopback address (::1)   -  all the IPv4-compatible IPv6 addresses [RFC3513] (::/96),      excluding the unspecified address for Duplicate Address Detection      (::/128)   -  all the IPv4-mapped IPv6 addresses (::ffff:0:0/96)Nordmark & Gilligan         Standards Track                    [Page 16]

RFC 4213            Basic IPv6 Transition Mechanisms        October 2005   In addition, the node should be configured to perform ingress   filtering [RFC2827][RFC3704] on the IPv6 source address, similar to   on any of its interfaces, e.g.:   1) if the tunnel is toward the Internet, the node should be      configured to check that the site's IPv6 prefixes are not used as      the source addresses, or   2) if the tunnel is toward an edge network, the node should be      configured to check that the source address belongs to that edge      network.   The prefix lists in the former typically need to be manually   configured; the latter could be verified automatically, e.g., by   using a strict unicast RPF check, as long as an interface can be   designated to be toward an edge.   It is RECOMMENDED that the implementations provide a single knob to   make it easier to for the administrators to enable strict ingress   filtering toward edge networks.3.7.  Link-Local Addresses   The configured tunnels are IPv6 interfaces (over the IPv4 "link   layer") and thus MUST have link-local addresses.  The link-local   addresses are used by, e.g., routing protocols operating over the   tunnels.   The interface identifier [RFC3513] for such an interface may be based   on the 32-bit IPv4 address of an underlying interface, or formed   using some other means, as long as it is unique from the other tunnel   endpoint with a reasonably high probability.   Note that it may be desirable to form the link-local address in a   fashion that minimizes the probability and the effect of having to   renumber the link-local address in the event of a topology or   hardware change.   If an IPv4 address is used for forming the IPv6 link-local address,   the interface identifier is the IPv4 address, prepended by zeros.   Note that the "Universal/Local" bit is zero, indicating that the   interface identifier is not globally unique.  The link-local address   is formed by appending the interface identifier to the prefix   FE80::/64.   When the host has more than one IPv4 address in use on the physical   interface concerned, a choice of one of these IPv4 addresses is madeNordmark & Gilligan         Standards Track                    [Page 17]

RFC 4213            Basic IPv6 Transition Mechanisms        October 2005   by the administrator or the implementation when forming the link-   local address.      +-------+-------+-------+-------+-------+-------+------+------+      |  FE      80      00      00      00      00      00     00  |      +-------+-------+-------+-------+-------+-------+------+------+      |  00      00      00      00   |        IPv4 Address         |      +-------+-------+-------+-------+-------+-------+------+------+3.8.  Neighbor Discovery over Tunnels   Configured tunnel implementations MUST at least accept and respond to   the probe packets used by Neighbor Unreachability Detection (NUD)   [RFC2461].  The implementations SHOULD also send NUD probe packets to   detect when the configured tunnel fails at which point the   implementation can use an alternate path to reach the destination.   Note that Neighbor Discovery allows that the sending of NUD probes be   omitted for router-to-router links if the routing protocol tracks   bidirectional reachability.   For the purposes of Neighbor Discovery, the configured tunnels   specified in this document are assumed to NOT have a link-layer   address, even though the link-layer (IPv4) does have an address.   This means that:   -  the sender of Neighbor Discovery packets SHOULD NOT include Source      Link Layer Address options or Target Link Layer Address options on      the tunnel link.   -  the receiver MUST, while otherwise processing the Neighbor      Discovery packet, silently ignore the content of any Source Link      Layer Address options or Target Link Layer Address options      received on the tunnel link.   Not using link-layer address options is consistent with how Neighbor   Discovery is used on other point-to-point links.4.  Threat Related to Source Address Spoofing   The specification above contains rules that apply tunnel source   address verification in particular and ingress filtering   [RFC2827][RFC3704] in general to packets before they are   decapsulated.  When IP-in-IP tunneling (independent of IP versions)   is used, it is important that this not be used to bypass any ingress   filtering in use for non-tunneled packets.  Thus, the rules in this   document are derived based on should ingress filtering be used for   IPv4 and IPv6, the use of tunneling should not provide an easy way to   circumvent the filtering.Nordmark & Gilligan         Standards Track                    [Page 18]

RFC 4213            Basic IPv6 Transition Mechanisms        October 2005   In this case, without specific ingress filtering checks in the   decapsulator, it would be possible for an attacker to inject a packet   with:   -  Outer IPv4 source: real IPv4 address of attacker   -  Outer IPv4 destination: IPv4 address of decapsulator   -  Inner IPv6 source: Alice, which is either the decapsulator or a      node close to it   -  Inner IPv6 destination: Bob   Even if all IPv4 routers between the attacker and the decapsulator   implement IPv4 ingress filtering, and all IPv6 routers between the   decapsulator and Bob implement IPv6 ingress filtering, the above   spoofed packets will not be filtered out.  As a result, Bob will   receive a packet that looks like it was sent from Alice even though   the sender was some unrelated node.   The solution to this is to have the decapsulator accept only   encapsulated packets from the explicitly configured source address   (i.e., the other end of the tunnel) as specified inSection 3.6.   While this does not provide complete protection in the case ingress   filtering has not been deployed, it does provide a significant   increase in security.  The issue and the remainder threats are   discussed at more length in Security Considerations.5.  Security Considerations   Generic security considerations of using IPv6 are discussed in a   separate document [V6SEC].   An implementation of tunneling needs to be aware that although a   tunnel is a link (as defined in [RFC2460]), the threat model for a   tunnel might be rather different than for other links, since the   tunnel potentially includes all of the Internet.   Several mechanisms (e.g., Neighbor Discovery) depend on Hop Count   being 255 and/or the addresses being link local for ensuring that a   packet originated on-link, in a semi-trusted environment.  Tunnels   are more vulnerable to a breach of this assumption than physical   links, as an attacker anywhere in the Internet can send an IPv6-in-   IPv4 packet to the tunnel decapsulator, causing injection of an   encapsulted IPv6 packet to the configured tunnel interface unless the   decapsulation checks are able to discard packets injected in such a   manner.Nordmark & Gilligan         Standards Track                    [Page 19]

RFC 4213            Basic IPv6 Transition Mechanisms        October 2005   Therefore, this memo specifies that the decapsulators make these   steps (as described inSection 3.6) to mitigate this threat:   -  IPv4 source address of the packet MUST be the same as configured      for the tunnel end-point;   -  Independent of any IPv4 ingress filtering the administrator may      have configured, the implementation MAY perform IPv4 ingress      filtering to check that the IPv4 packets are received from an      expected interface (but as this may cause some problems, it may be      disabled by default);   -  IPv6 packets with several, obviously invalid IPv6 source addresses      received from the tunnel MUST be discarded (seeSection 3.6 for      details); and   -  IPv6 ingress filtering should be performed (typically requiring      configuration from the operator), to check that the tunneled IPv6      packets are received from an expected interface.   Especially the first verification is vital: to avoid this check, the   attacker must be able to know the source of the tunnel (ranging from   difficult to predictable) and be able to spoof it (easier).   If the remainder threats of tunnel source verification are considered   to be significant, a tunneling scheme with authentication should be   used instead, e.g., IPsec [RFC2401] (preferable) or Generic Routing   Encapsulation with a pre-configured secret key [RFC2890].  As the   configured tunnels are set up more or less manually, setting up the   keying material is probably not a problem.  However, setting up   secure IPsec IPv6-in-IPv4 tunnels is described in another document   [V64IPSEC].   If the tunneling is done inside an administrative domain, proper   ingress filtering at the edge of the domain can also eliminate the   threat from outside of the domain.  Therefore, shorter tunnels are   preferable to longer ones, possibly spanning the whole Internet.   In addition, an implementation MUST treat interfaces to different   links as separate, e.g., to ensure that Neighbor Discovery packets   arriving on one link do not affect other links.  This is especially   important for tunnel links.   When dropping packets due to failing to match the allowed IPv4 source   addresses for a tunnel the node should not "acknowledge" the   existence of a tunnel, otherwise this could be used to probe the   acceptable tunnel endpoint addresses.  For that reason, the   specification says that such packets MUST be discarded, and an ICMPNordmark & Gilligan         Standards Track                    [Page 20]

RFC 4213            Basic IPv6 Transition Mechanisms        October 2005   error message SHOULD NOT be generated, unless the implementation   normally sends ICMP destination unreachable messages for unknown   protocols; in such a case, the same code MAY be sent.  As should be   obvious, not returning the same ICMP code if an error is returned for   other protocols may hint that the IPv6 stack (or the protocol 41   tunneling processing) has been enabled -- the behaviour should be   consistent on how the implementation otherwise behaves to be   transparent to probing.6.  Acknowledgements   We would like to thank the members of the IPv6 working group, the   Next Generation Transition (ngtrans) working group, and the v6ops   working group for their many contributions and extensive review of   this document.  Special thanks are due to (in alphabetical order) Jim   Bound, Ross Callon, Tim Chown,  Alex Conta, Bob Hinden, Bill Manning,   John Moy, Mohan Parthasarathy, Chirayu Patel, Pekka Savola, and Fred   Templin for many helpful suggestions.  Pekka Savola helped in editing   the final revisions of the specification.7.  References7.1.  Normative References   [RFC791]   Postel, J., "Internet Protocol", STD 5,RFC 791, September              1981.   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery",RFC 1191,              November 1990.   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery              for IP version 6",RFC 1981, August 1996.   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6              (IPv6) Specification",RFC 2460, December 1998.   [RFC2463]  Conta, A. and S. Deering, "Internet Control Message              Protocol (ICMPv6) for the Internet Protocol Version 6              (IPv6) Specification",RFC 2463, December 1998.7.2.  Informative References   [ASSIGNED] IANA, "Assigned numbers online database",http://www.iana.org/numbers.htmlNordmark & Gilligan         Standards Track                    [Page 21]

RFC 4213            Basic IPv6 Transition Mechanisms        October 2005   [DNSOPV6]  Durand, A., Ihren, J., and Savola P., "Operational              Considerations and Issues with IPv6 DNS", Work in              Progress, October 2004.   [KM97]     Kent, C., and J. Mogul, "Fragmentation Considered              Harmful".  In Proc.  SIGCOMM '87 Workshop on Frontiers in              Computer Communications Technology.  August 1987.   [V6SEC]    Savola, P., "IPv6 Transition/Co-existence Security              Considerations", Work in Progress, October 2004.   [V64IPSEC] Graveman, R., et al., "Using IPsec to Secure IPv6-over-              IPv4 Tunnels", Work in Progress, December 2004.   [RFC1435]  Knowles, S., "IESG Advice from Experience with Path MTU              Discovery",RFC 1435, March 1993.   [RFC1812]  Baker, F., "Requirements for IP Version 4 Routers",RFC1812, June 1995.   [RFC2401]  Kent, S. and R. Atkinson, "Security Architecture for the              Internet Protocol",RFC 2401, November 1998.   [RFC2461]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor              Discovery for IP Version 6 (IPv6)",RFC 2461, December              1998.   [RFC2462]  Thomson, S. and T. Narten, "IPv6 Stateless Address              Autoconfiguration",RFC 2462, December 1998.   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:              Defeating Denial of Service Attacks which employ IP Source              Address Spoofing",BCP 38,RFC 2827, May 2000.   [RFC2890]  Dommety, G., "Key and Sequence Number Extensions to GRE",RFC 2890, September 2000.   [RFC2923]  Lahey, K., "TCP Problems with Path MTU Discovery",RFC2923, September 2000.   [RFC2983]  Black, D., "Differentiated Services and Tunnels",RFC2983, October 2000.   [RFC3056]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains              via IPv4 Clouds",RFC 3056, February 2001.Nordmark & Gilligan         Standards Track                    [Page 22]

RFC 4213            Basic IPv6 Transition Mechanisms        October 2005   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition              of Explicit Congestion Notification (ECN) to IP",RFC3168, September 2001.   [RFC3232]  Reynolds, J., "Assigned Numbers:RFC 1700 is Replaced by              an On-line Database",RFC 3232, January 2002.   [RFC3484]  Draves, R., "Default Address Selection for Internet              Protocol version 6 (IPv6)",RFC 3484, February 2003.   [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.              Stevens, "Basic Socket Interface Extensions for IPv6",RFC3493, February 2003.   [RFC3513]  Hinden, R. and S. Deering, "Internet Protocol Version 6              (IPv6) Addressing Architecture",RFC 3513, April 2003.   [RFC3596]  Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,              "DNS Extensions to Support IP Version 6",RFC 3596,              October 2003.   [RFC3704]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed              Networks",BCP 84,RFC 3704, March 2004.   [RFC4087]  Thaler, D., "IP Tunnel MIB",RFC 4087, June 2005.8.  Changes fromRFC 2893   The motivation for the bulk of these changes are to simplify the   document to only contain the mechanisms of wide-spread use.RFC 2893 contains a mechanism called automatic tunneling.  But a much   more general mechanism is specified inRFC 3056 [RFC3056] which gives   each node with a (global) IPv4 address a /48 IPv6 prefix i.e., enough   for a whole site.   The following changes have been performed sinceRFC 2893:   -  Removed references to A6 and retained AAAA.   -  Removed automatic tunneling and use of IPv4-compatible addresses.   -  Removed default Configured Tunnel using IPv4 "Anycast Address"   -  Removed Source Address Selection section since this is now covered      by another document ([RFC3484]).   -  Removed brief mention of 6over4.Nordmark & Gilligan         Standards Track                    [Page 23]

RFC 4213            Basic IPv6 Transition Mechanisms        October 2005   -  Split into normative and non-normative references and other      reference cleanup.   -  Dropped "or equal" in if (IPv4 path MTU - 20) is less than or      equal to 1280.   -  Dropped this: However, IPv6 may be used in some environments where      interoperability with IPv4 is not required.  IPv6 nodes that are      designed to be used in such environments need not use or even      implement these mechanisms.   -  Described Static MTU and Dynamic MTU cases separately; clarified      that the dynamic path MTU mechanism is OPTIONAL but if it is      implemented it should follow the rules insection 3.2.2.   -  Specified Static MTU to default to a MTU of 1280 to 1480 bytes,      and that this may be configurable.  Discussed the issues with      using Static MTU at more length.   -  Specified minimal rules for IPv4 reassembly and IPv6 MRU to      enhance interoperability and to minimize blacholes.   -  Restated the "currently underway" language about Type-of-Service,      and loosely point at [RFC2983] and [RFC3168].   -  Fixed reference to Assigned Numbers to be to online version (with      proper pointer to "Assigned Numbers is obsolete" RFC).   -  Clarified text about ingress filtering e.g., that it applies to      packet delivered to transport protocols on the decapsulator as      well as packets being forwarded by the decapsulator, and how the      decapsulator's checks help when IPv4 and IPv6 ingress filtering is      in place.   -  Removed unidirectional tunneling; assume all tunnels are      bidirectional, between endpoint addresses (not nodes).   -  Removed the guidelines for advertising addresses in DNS as      slightly out of scope, referring to another document for the      details.   -  Removed the SHOULD requirement that the link-local addresses      should be formed based on IPv4 addresses.Nordmark & Gilligan         Standards Track                    [Page 24]

RFC 4213            Basic IPv6 Transition Mechanisms        October 2005   -  Added a SHOULD for implementing a knob to be able to set the      source address of the tunnel, and add discussion why this is      useful.   -  Added stronger wording for source address checks: both IPv4 and      IPv6 source addresses MUST be checked, and RPF-like ingress      filtering is optional.   -  Rewrote security considerations to be more precise about the      threats of tunneling.   -  Added a note about considering using TTL=255 when encapsulating.   -  Added more discussion inSection 3.2 why using an "infinite" IPv6      MTU leads to likely interoperability problems.   -  Added an explicit requirement that if both MTU determination      methods are used, choosing one should be possible on a per-tunnel      basis.   -  Clarified that ICMPv4 error handling is only applicable to dynamic      MTU determination.   -  Removed/clarified DNS record filtering; an API is a SHOULD and if      it does not exist, MUST NOT filter anything.  Decree ordering out      of scope, but refer toRFC3484.   -  Add a note that the destination IPv4 address could also be a      multicast address.   -  Make it RECOMMENDED to provide a toggle to perform strict ingress      filtering on an interface.   -  Generalize the text on the data in ICMPv4 messages.   -  Made a lot of miscellaneous editorial cleanups.Nordmark & Gilligan         Standards Track                    [Page 25]

RFC 4213            Basic IPv6 Transition Mechanisms        October 2005Authors' Addresses   Erik Nordmark   Sun Microsystems   17 Network Circle   Menlo Park, CA 94025   USA   Phone: +1 650 786 2921   EMail: erik.nordmark@sun.com   Robert E. Gilligan   Intransa, Inc.   2870 Zanker Rd., Suite 100   San Jose, CA 95134 USA   Phone : +1 408 678 8600   Fax :   +1 408 678 8800   EMail:  bob.gilligan@acm.orgNordmark & Gilligan         Standards Track                    [Page 26]

RFC 4213            Basic IPv6 Transition Mechanisms        October 2005Full Copyright Statement   Copyright (C) The Internet Society (2005).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM 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.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 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 athttp://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.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Nordmark & Gilligan         Standards Track                    [Page 27]