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Network Working Group                                        R. GilliganRequest for Comments: 1933                                   E. NordmarkCategory: Standards Track                         Sun Microsystems, Inc.                                                              April 1996Transition 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.Abstract   This document specifies IPv4 compatibility mechanisms that can be   implemented by IPv6 hosts and routers.  These mechanisms include   providing complete implementations of both versions of the Internet   Protocol (IPv4 and IPv6), and tunneling IPv6 packets over IPv4   routing infrastructures.  They are designed to allow IPv6 nodes to   maintain complete compatibility with IPv4, which should greatly   simplify the deployment of IPv6 in the Internet, and facilitate the   eventual transition of the entire Internet to IPv6.1. 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 a   set of 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.   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.   The mechanisms specified here include:Gilligan & Nordmark         Standards Track                     [Page 1]

RFC 1933               IPv6 Transition Mechanisms             April 1996   -    Dual IP layer.  Providing complete support for both IPv4 and        IPv6 in hosts and routers.   -    IPv6 over IPv4 tunneling.  Encapsulating IPv6 packets within        IPv4 headers to carry them over IPv4 routing infrastructures.        Two types of tunneling are employed: configured and automatic.   Additional transition and compatibility mechanisms may be developed   in the future.  These will be specified in other documents.1.2. 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 here.        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.Gilligan & Nordmark         Standards Track                     [Page 2]

RFC 1933               IPv6 Transition Mechanisms             April 1996   Types of IPv6 Addresses        IPv4-compatible IPv6 address:                An IPv6 address, assigned to an IPv6/IPv4 node, which                bears the high-order 96-bit prefix 0:0:0:0:0:0, and an                IPv4 address in the low-order 32-bits.  IPv4-compatible                addresses are used by the automatic tunneling mechanism.        IPv6-only address:                The remainder of the IPv6 address space.  An IPv6                address that bears a prefix other than 0:0:0:0:0:0.   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.        IPv6-in-IPv4 encapsulation:                IPv6-over-IPv4 tunneling.        Configured tunneling:                IPv6-over-IPv4 tunneling where the IPv4 tunnel endpoint                address is determined by configuration information on                the encapsulating node.        Automatic tunneling:                IPv6-over-IPv4 tunneling where the IPv4 tunnel endpoint                address is determined from the IPv4 address embedded in                the IPv4-compatible destination address of the IPv6                packet.1.3. Structure of this Document   The remainder of this document is organized into three sections:   -Section 2 discusses the IPv4-compatible address format.   -Section 3 discusses the operation of nodes with a dual IP        layer, IPv6/IPv4 nodes.Gilligan & Nordmark         Standards Track                     [Page 3]

RFC 1933               IPv6 Transition Mechanisms             April 1996   -Section 4 discusses IPv6-over-IPv4 tunneling.2. Addressing   The automatic tunneling mechanism uses a special type of IPv6   address, termed an "IPv4-compatible" address.  An IPv4-compatible   address is identified by an all-zeros 96-bit prefix, and holds an   IPv4 address in the low-order 32-bits.  IPv4-compatible addresses are   structured as follows:        |              96-bits                 |   32-bits    |        +--------------------------------------+--------------+        |            0:0:0:0:0:0               | IPv4 Address |        +--------------------------------------+--------------+                 IPv4-Compatible IPv6 Address Format   IPv4-compatible addresses are assigned to IPv6/IPv4 nodes that   support automatic tunneling.  Nodes that are configured with IPv4-   compatible addresses may use the complete address as their IPv6   address, and use the embedded IPv4 address as their IPv4 address.   The remainder of the IPv6 address space (that is, all addresses with   96-bit prefixes other than 0:0:0:0:0:0) are termed "IPv6-only   Addresses."3. Dual IP Layer   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 a complete IPv4 implementation in addition to   their IPv6 implementation 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.   The dual IP layer technique may or may not be used in conjunction   with the IPv6-over-IPv4 tunneling techniques, which are described insection 4.  An IPv6/IPv4 node that supports tunneling may support   only configured tunneling, or both configured and automatic   tunneling.  Thus three configurations are possible:   -    IPv6/IPv4 node that does not perform tunneling.   -    IPv6/IPv4 node that performs configured tunneling only.   -    IPv6/IPv4 node that performs configured tunneling andGilligan & Nordmark         Standards Track                     [Page 4]

RFC 1933               IPv6 Transition Mechanisms             April 1996        automatic tunneling.3.1. Address Configuration   Because they support both protocols, IPv6/IPv4 nodes may be   configured with both IPv4 and IPv6 addresses.  Although the two   addresses may be related to each other, this is not required.   IPv6/IPv4 nodes may be configured with IPv6 and IPv4 addresses that   are unrelated to each other.   Nodes that perform automatic tunneling are configured with IPv4-   compatible IPv6 addresses.  These may be viewed as single addresses   that can serve both as IPv6 and IPv4 addresses.  The entire 128-bit   IPv4-compatible IPv6 address is used as the node's IPv6 address,   while the IPv4 address embedded in low-order 32-bits serves as the   node's IPv4 address.   IPv6/IPv4 nodes may use the stateless IPv6 address configuration   mechanism [5] or DHCP for IPv6 [3] to acquire their IPv6 address.   These mechanisms may provide either IPv4-compatible or IPv6-only IPv6   addresses.   IPv6/IPv4 nodes may use IPv4 mechanisms to acquire their IPv4   addresses.   IPv6/IPv4 nodes that perform automatic tunneling may also acquire   their IPv4-compatible IPv6 addresses from another source: IPv4   address configuration protocols.  A node may use any IPv4 address   configuration mechanism to acquire its IPv4 address, then "map" that   address into an IPv4-compatible IPv6 address by pre-pending it with   the 96-bit prefix 0:0:0:0:0:0.  This mode of configuration allows   IPv6/IPv4 nodes to "leverage" the installed base of IPv4 address   configuration servers.  It can be particularly useful in environments   where IPv6 routers and address configuration servers have not yet   been deployed.   The specific algorithm for acquiring an IPv4-compatible address using   IPv4-based address configuration protocols is as follows:   1)   The IPv6/IPv4 node uses standard IPv4 mechanisms or protocols        to acquire its own IPv4 address.  These include:           -    The Dynamic Host Configuration Protocol (DHCP) [2]           -    The Bootstrap Protocol (BOOTP) [1]           -    The Reverse Address Resolution Protocol (RARP) [9]           -    Manual configuration           -    Any other mechanism which accurately yields the node's                own IPv4 addressGilligan & Nordmark         Standards Track                     [Page 5]

RFC 1933               IPv6 Transition Mechanisms             April 1996   2)   The node uses this address as its IPv4 address.   3)   The node prepends the 96-bit prefix 0:0:0:0:0:0 to the 32-bit        IPv4 address that it acquired in step (1).  The result is an        IPv4-compatible IPv6 address with the node's own IPv4-address        embedded in the low-order 32-bits.  The node uses this address        as its own IPv6 address.3.1.1. IPv4 Loopback Address   Many IPv4 implementations treat the address 127.0.0.1 as a "loopback   address" -- an address to reach services located on the local   machine.  Per the host requirements specification [10],section3.2.1.3, IPv4 packets addressed from or to the loopback address are   not to be sent onto the network; they must remain entirely within the   node.  IPv6/IPv4 implementations may treat the IPv4-compatible IPv6   address ::127.0.0.1 as an IPv6 loopback address.  Packets with this   address should also remain entirely within the node, and not be   transmitted onto the network.3.2.  DNS   The Domain Naming System (DNS) is used in both IPv4 and IPv6 to map   hostnames into addresses.  A new resource record type named "AAAA"   has been defined for IPv6 addresses [6].  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.3.2.1.  Handling Records for IPv4-Compatible Addresses   When an IPv4-compatible IPv6 addresses is assigned to an IPv6/IPv4   host that supports automatic tunneling, both A and AAAA records are   listed in the DNS.  The AAAA record holds the full IPv4-compatible   IPv6 address, while the A record holds the low-order 32-bits of that   address.  The AAAA record is needed so that queries by IPv6 hosts can   be satisfied.  The A record is needed so that queries by IPv4-only   hosts, whose resolver libraries only support the A record type, will   locate the host.   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 IPv4-compatible IPv6 address, and an A record   holding the corresponding IPv4 address, the resolver library need not   necessarily return both addresses.  It has three options:Gilligan & Nordmark         Standards Track                     [Page 6]

RFC 1933               IPv6 Transition Mechanisms             April 1996   -    Return only the IPv6 address to the application.   -    Return only the IPv4 address to the application.   -    Return both addresses to the application.   The selection of which address type to return in this case, or, if   both addresses are returned, in which order they are listed, can   affect what type of IP traffic is generated.  If the IPv6 address is   returned, the node will communicate with that destination using IPv6   packets (in most cases encapsulated in IPv4); If the IPv4 address is   returned, the communication will use IPv4 packets.   The way that DNS resolver implementations handle redundant records   for IPv4-compatible addresses may depend on whether that   implementation supports automatic tunneling, or whether it is   enabled.  For example, an implementation that does not support   automatic tunneling would not return IPv4-compatible IPv6 addresses   to applications because those destinations are generally only   reachable via tunneling.  On the other hand, those implementations in   which automatic tunneling is supported and enabled may elect to   return only the IPv4-compatible IPv6 address and not the IPv4   address.4. IPv6-over-IPv4 Tunneling   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.Gilligan & Nordmark         Standards Track                     [Page 7]

RFC 1933               IPv6 Transition Mechanisms             April 1996   -    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.   Tunneling techniques are usually classified according to the   mechanism by which the encapsulating node determines the address of   the node at the end of the tunnel.  In the first two tunneling   methods listed above -- router-to-router and host-to-router -- the   IPv6 packet is being tunneled to a router.  The endpoint of this type   of tunnel is an intermediary router which must decapsulate the IPv6   packet and forward it on to its final destination.  When tunneling to   a router, the endpoint of the tunnel is different from the   destination of the packet being tunneled.  So the addresses in the   IPv6 packet being tunneled do not provide the IPv4 address of the   tunnel endpoint.  Instead, the tunnel endpoint address must be   determined from configuration information on the node performing the   tunneling.  We use the term "configured tunneling" to describe the   type of tunneling where the endpoint is explicitly configured.   In the last two tunneling methods -- host-to-host and router-to-host   -- the IPv6 packet is tunneled all the way to its final destination.   The tunnel endpoint is the node to which the IPv6 packet is   addressed.  Since the endpoint of the tunnel is the destination of   the IPv6 packet, the tunnel endpoint can be determined from the   destination IPv6 address of that packet: If that address is an IPv4-   compatible address, then the low-order 32-bits hold the IPv4 address   of the destination node, and that can be used as the tunnel endpoint   address.  This technique avoids the need to explicitly configure the   tunnel endpoint address.  Deriving the tunnel endpoint address from   the embedded IPv4 address of the packet's IPv6 address is termed   "automatic tunneling".   The two tunneling techniques -- automatic and configured -- differ   primarily in how they determine the tunnel endpoint address.  Most of   the underlying mechanisms are the same:   -    The entry node of the tunnel (the encapsulating node) creates an        encapsulating IPv4 header and transmits the encapsulated packet.   -    The exit node of the tunnel (the decapsulating node) receives        the encapsulated packet, removes the IPv4 header, updates the        IPv6 header, and processes the received IPv6 packet.Gilligan & Nordmark         Standards Track                     [Page 8]

RFC 1933               IPv6 Transition Mechanisms             April 1996   -    The encapsulating node 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.  Since the number of tunnels that any one host or        router may be using may grow to be quite large, this state        information can be cached and discarded when not in use.   The next section discusses the common mechanisms that apply to both   types of tunneling.  Subsequent sections discuss how the tunnel   endpoint address is determined for automatic and configured   tunneling.4.1. Common Tunneling Mechanisms   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 IPv4   In addition to adding an IPv4 header, the encapsulating node also has   to handle some more complex issues:  -     Determine when to fragment and when to report an ICMP "packet        too big" error back to the source.  -     How to reflect IPv4 ICMP errors from routers along the tunnel        path back to the source as IPv6 ICMP errors.   Those issues are discussed in the following sections.Gilligan & Nordmark         Standards Track                     [Page 9]

RFC 1933               IPv6 Transition Mechanisms             April 19964.1.1.  Tunnel MTU and Fragmentation   The encapsulating node could view encapsulation as IPv6 using IPv4 as   a link layer with a very large MTU (65535-20 bytes to be exact; 20   bytes "extra" are needed for the encapsulating IPv4 header).  The   encapsulating node would need only to report IPv6 ICMP "packet too   big" errors back to the source for packets that exceed this MTU.   However, such a scheme would be inefficient for two reasons:  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 [11].  2)    Any IPv4 fragmentation occurring inside the tunnel would have to        be reassembled at the tunnel endpoint.  For tunnels that        terminate at a router, this would require additional memory to        reassemble the IPv4 fragments into a complete IPv6 packet before        that packet could be forwarded onward.   The fragmentation inside the tunnel can be reduced to a minimum by   having the encapsulating node track the IPv4 Path MTU across the   tunnel, using the IPv4 Path MTU Discovery Protocol [8] and recording   the resulting path MTU.  The IPv6 layer in the encapsulating node 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 completely eliminate IPv4 fragmentation in   the case when the IPv4 path MTU would result in an IPv6 MTU less than   576 bytes. (Any link layer used by IPv6 has to have an MTU of at   least 576 bytes [4].) In this case the IPv6 layer has to "see" a link   layer with an MTU of 576 bytes and the encapsulating node has to use   IPv4 fragmentation in order to forward the 576 byte IPv6 packets.   The encapsulating node can 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   IPv6 ICMP "packet too big" message:        if (IPv4 path MTU - 20) is less than or equal to 576                if packet is larger than 576 bytes                        Send IPv6 ICMP "packet too big" with MTU = 576.                        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 encapsulating node or by some router alongGilligan & Nordmark         Standards Track                    [Page 10]

RFC 1933               IPv6 Transition Mechanisms             April 1996                        the IPv4 path.                endif        else                if packet is larger than (IPv4 path MTU - 20)                        Send IPv6 ICMP "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   Encapsulating nodes that have a large number of tunnels might not be   able to store the IPv4 Path MTU for all tunnels. Such nodes can, at   the expense of additional fragmentation in the network, avoid using   the IPv4 Path MTU algorithm across the tunnel and instead use the MTU   of the link layer (under IPv4) in the above algorithm instead of the   IPv4 path MTU.   In this case the Don't Fragment bit must not be set in the   encapsulating IPv4 header.4.1.2.  Hop Limit   IPv6-over-IPv4 tunnels are modeled as "single-hop".  That is, the   IPv6 hop limit is decremented by 1 when an IPv6 packet traverses the   tunnel.  The single-hop model serves to hide the existence of a   tunnel.  The tunnel is opaque to users of the network, and is not   detectable by network diagnostic tools such as traceroute.   The single-hop model is implemented by having the encapsulating and   decapsulating nodes 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.  Implementations may provide   a mechanism to allow the administrator to configure the IPv4 TTL.4.1.3. Handling IPv4 ICMP errors   In response to encapsulated packets it has sent into the tunnel, the   encapsulating node may receive IPv4 ICMP error messages from IPv4   routers inside the tunnel.  These packets are addressed to theGilligan & Nordmark         Standards Track                    [Page 11]

RFC 1933               IPv6 Transition Mechanisms             April 1996   encapsulating node because it is the IPv4 source of the encapsulated   packet.   The ICMP "packet too big" error messages are handled according to   IPv4 Path MTU Discovery [8] and the resulting path MTU is recorded in   the IPv4 layer.  The recorded path MTU is used by IPv6 to determine   if an IPv6 ICMP "packet too big" error has to be generated as   described insection 4.1.1.   The handling of other types of ICMP error messages depends on how   much information is included in the "packet in error" field, which   holds 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 may   return enough data beyond the IPv4 header to include the entire IPv6   header and possibly even the data beyond that.   If the offending packet includes enough data, the encapsulating node   may extract the encapsulated IPv6 packet and use it to generating an   IPv6 ICMP message directed back to the originating IPv6 node, as   shown below:                +--------------+                | IPv4 Header  |                | dst = encaps |                |       node   |                +--------------+                |     ICMP     |                |    Header    |         - -    +--------------+                | IPv4 Header  |                | src = encaps |        IPv4    |       node   |                +--------------+   - -        Packet  |    IPv6      |                |    Header    |   Original IPv6         in     +--------------+   Packet -                |  Transport   |   Can be used to        Error   |    Header    |   generate an                +--------------+   IPv6 ICMP                |              |   error message                ~     Data     ~   back to the source.                |              |         - -    +--------------+   - -        IPv4 ICMP Error Message Returned to Encapsulating NodeGilligan & Nordmark         Standards Track                    [Page 12]

RFC 1933               IPv6 Transition Mechanisms             April 19964.1.4.  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        Total Length:                Payload length from IPv6 header plus length of IPv6 and                IPv4 headers (i.e. 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 4.1.1. Set the More Fragments (MF) bit as                necessary if fragmenting.        Fragment offset:                Set as necessary if fragmenting.        Time to Live:                Set in implementation-specific manner.        Protocol:                41 (Assigned payload type number for IPv6)Gilligan & Nordmark         Standards Track                    [Page 13]

RFC 1933               IPv6 Transition Mechanisms             April 1996        Header Checksum:                Calculate the checksum of the IPv4 header.        Source Address:                IPv4 address of outgoing interface of the                encapsulating node.        Destination Address:                IPv4 address of tunnel endpoint.   Any IPv6 options are preserved in the packet (after the IPv6 header).4.1.5. Decapsulating IPv6-in-IPv4 Packets   When an IPv6/IPv4 host or a router receives an IPv4 datagram that is   addressed to one of its own IPv4 address, and the value of the   protocol field is 41, it removes the IPv4 header and submits the IPv6   datagram to its IPv6 layer code.   The decapsulation is shown below:        +-------------+        |    IPv4     |        |   Header    |        +-------------+                 +-------------+        |    IPv6     |                 |    IPv6     |        |   Header    |                 |   Header    |        +-------------+                 +-------------+        |  Transport  |                 |  Transport  |        |   Layer     |      ===>       |   Layer     |        |   Header    |                 |   Header    |        +-------------+                 +-------------+        |             |                 |             |        ~    Data     ~                 ~    Data     ~        |             |                 |             |        +-------------+                 +-------------+                    Decapsulating IPv6 from IPv4   When decapsulating the IPv6-in-IPv4 packet, the IPv6 header is not   modified.  If the packet is subsequently forwarded, its hop limit is   decremented by one.   The encapsulating IPv4 header is discarded.Gilligan & Nordmark         Standards Track                    [Page 14]

RFC 1933               IPv6 Transition Mechanisms             April 1996   The decapsulating node performs IPv4 reassembly before decapsulating   the IPv6 packet.  All IPv6 options are preserved even if the   encapsulating IPv4 packet is fragmented.   After the IPv6 packet is decapsulated, it is processed the same as   any received IPv6 packet.4.2. Configured Tunneling   In configured tunneling, the tunnel endpoint address is determined   from configuration information in the encapsulating node.  For each   tunnel, the encapsulating node must store the tunnel endpoint   address.  When an IPv6 packet is transmitted over a tunnel, the   tunnel endpoint address configured for that tunnel is used as the   destination address for the encapsulating IPv4 header.   The determination of which packets to tunnel is usually made by   routing information on the encapsulating node.  This is usually done   via a routing table, which directs packets based on their destination   address using the prefix mask and match technique.4.2.1. Default Configured Tunnel   Nodes that are connected to IPv4 routing infrastructures may use a   configured tunnel to reach an IPv6 "backbone".  If the IPv4 address   of an IPv6/IPv4 router bordering the backbone is known, a tunnel can   be configured to that router.  This tunnel can be configured into the   routing table as a "default route".  That is, all IPv6 destination   addresses will match the route and could potentially traverse the   tunnel.  Since the "mask length" of such default route is zero, it   will be used only if there are no other routes with a longer mask   that match the destination.   The tunnel endpoint address of such a default tunnel could be the   IPv4 address of one IPv6/IPv4 router at the border of the IPv6   backbone.  Alternatively, the tunnel endpoint could be an IPv4   "anycast address".  With this approach, multiple IPv6/IPv4 routers at   the border advertise IPv4 reachability to the same IPv4 address.  All   of these routers accept packets to this address as their own, and   will decapsulate IPv6 packets tunneled to this address.  When an   IPv6/IPv4 node sends an encapsulated packet to this address, it will   be delivered to only one of the border routers, but the sending node   will not know which one.  The IPv4 routing system will generally   carry the traffic to the closest router.   Using a default tunnel to an IPv4 "anycast address" provides a high   degree of robustness since multiple border router can be provided,   and, using the normal fallback mechanisms of IPv4 routing, trafficGilligan & Nordmark         Standards Track                    [Page 15]

RFC 1933               IPv6 Transition Mechanisms             April 1996   will automatically switch to another router when one goes down.4.3. Automatic Tunneling   In automatic tunneling, the tunnel endpoint address is determined   from the packet being tunneled.  The destination IPv6 address in the   packet must be an IPv4-compatible address.  If it is, the IPv4   address component of that address -- the low-order 32-bits -- are   extracted and used as the tunnel endpoint address.  IPv6 packets that   are not addressed to an IPv4-compatible address can not be tunneled   using automatic tunneling.   IPv6/IPv4 nodes need to determine which IPv6 packets can be sent via   automatic tunneling.  One technique is to use the IPv6 routing table   to direct automatic tunneling.  An implementation can have a special   static routing table entry for the prefix 0:0:0:0:0:0/96.  (That is,   a route to the all-zeros prefix with a 96-bit mask.)  Packets that   match this prefix are sent to a pseudo-interface driver which   performs automatic tunneling.  Since all IPv4-compatible IPv6   addresses will match this prefix, all packets to those destinations   will be auto-tunneled.4.4. Default Sending Algorithm   This section presents a combined IPv4 and IPv6 sending algorithm that   IPv6/IPv4 nodes can use.  The algorithm can be used to determine when   to send IPv4 packets, when to send IPv6 packets, and when to perform   automatic and configured tunneling.  It illustrates how the   techniques of dual IP layer, configured tunneling, and automatic   tunneling can be used together.  Note that is just an example to show   how the techniques can be combined; IPv6/IPv6 implementations may   provide different algorithms.  This algorithm has the following   properties:   -    Sends IPv4 packets to all IPv4 destinations.   -    Sends IPv6 packets to all IPv6 destinations on the same link.   -    Using automatic tunneling, sends IPv6 packets encapsulated in        IPv4 to IPv6 destinations with IPv4-compatible addresses that        are located off-link.   -    Sends IPv6 packets to IPv6 destinations located off-link when        IPv6 routers are present.   -    Using the default IPv6 tunnel, sends IPv6 packets encapsulated        in IPv4 to IPv6 destinations with IPv6-only addresses when no        IPv6 routers are present.Gilligan & Nordmark         Standards Track                    [Page 16]

RFC 1933               IPv6 Transition Mechanisms             April 1996The algorithm is as follows:  1)    If the address of the end node is an IPv4 address then:          1.1)  If the destination is located on an attached link, then                send an IPv4 packet addressed to the end node.          1.2)  If the destination is located off-link, then;                1.2.1)  If there is an IPv4 router on link, then send an                        IPv4 format packet.  The IPv4 destination                        address is the IPv4 address of the end node.                        The datalink address is the datalink address of                        the IPv4 router.                1.2.2)  Else, the destination is treated as                        "unreachable" because it is located off link and                        there are no on-link routers.  2)    If the address of the end node is an IPv4-compatible IPv6        address (i.e. bears the prefix 0:0:0:0:0:0), then:          2.1)  If the destination is located on an attached link, then                send an IPv6 format packet (not encapsulated).  The IPv6                destination address is the IPv6 address of the end node.                The datalink address is the datalink address of the end                node.          2.2)  If the destination is located off-link, then:                2.2.1)  If there is an IPv4 router on an attached link,                        then send an IPv6 packet encapsulated in IPv4.                        The IPv6 destination address is the address of                        the end node.  The IPv4 destination address is                        the low-order 32-bits of the end node's address.                        The datalink address is the datalink address of                        the IPv4 router.                2.2.2)  Else, if there is an IPv6 router on an attached                        link, then send an IPv6 format packet.  The IPv6                        destination address is the IPv6 address of the                        end node.  The datalink address is the datalink                        address of the IPv6 router.                2.2.3)  Else, the destination is treated as                        "unreachable" because it is located off-link and                        there are no on-link routers.Gilligan & Nordmark         Standards Track                    [Page 17]

RFC 1933               IPv6 Transition Mechanisms             April 1996   3)   If the address of the end node is an IPv6-only address, then:          3.1)  If the destination is located on an attached link, then                send an IPv6 format packet.  The IPv6 destination                address is the IPv6 address of the end node.  The                datalink address is the datalink address of the end                node.          3.2)  If the destination is located off-link, then:                3.2.1)  If there is an IPv6 router on an attached link,                        then send an IPv6 format packet.  The IPv6                        destination address is the IPv6 address of the                        end node.  The datalink address is the datalink                        address of the IPv6 router.                3.2.2)  Else, if the destination is reachable via a                        configured tunnel, and there is an IPv4 router                        on an attached link, then send an IPv6                        packet encapsulated in IPv4.  The IPv6                        destination address is the address of the end                        node.  The IPv4 destination address is the                        configured IPv4 address of the tunnel endpoint.                        The datalink address is the datalink address of                        the IPv4 router.                3.2.3)  Else, the destination is treated as                        "unreachable" because it is located off-link and                        there are no on-link IPv6 routers.A summary of these sending rules are given in the table below:Gilligan & Nordmark         Standards Track                    [Page 18]

RFC 1933               IPv6 Transition Mechanisms             April 1996End         | End     | IPv4    | IPv6    | Packet |      |      |Node        | Node    | Router  | Router  | Format | IPv6 | IPv4 | DLinkAddress     | On      | On      | On      | To     | Dest | Dest | DestType        | Link?   | Link?   | Link?   | Send   | Addr | Addr | Addr------------+---------+---------+---------+--------+------+------+------IPv4        | Yes     |  N/A    |  N/A    | IPv4   |  N/A |  E4  | EL------------+---------+---------+---------+--------+------+------+------IPv4        | No      |  Yes    |  N/A    | IPv4   |  N/A |  E4  | RL------------+---------+---------+---------+--------+------+------+------IPv4        | No      |  No     |  N/A    | UNRCH  |  N/A |  N/A | N/A------------+---------+---------+---------+--------+------+------+------IPv4-compat | Yes     |  N/A    |  N/A    | IPv6   |  E6  |  N/A | EL------------+---------+---------+---------+--------+------+------+------IPv4-compat | No      |  Yes    |  N/A    | IPv6/4 |  E6  |  E4  | RL------------+---------+---------+---------+--------+------+------+------IPv4-compat | No      |  No     |  Yes    | IPv6   |  E6  |  N/A | RL------------+---------+---------+---------+--------+------+------+------IPv4-compat | No      |  No     |  No     | UNRCH  |  N/A |  N/A | N/A------------+---------+---------+---------+--------+------+------+------IPv6-only   | Yes     |  N/A    |  N/A    | IPv6   |  E6  |  N/A | EL------------+---------+---------+---------+--------+------+------+------IPv6-only   | No      |  N/A    |  Yes    | IPv6   |  E6  |  N/A | RL------------+---------+---------+---------+--------+------+------+------IPv6-only   | No      |  Yes    |  No     | IPv6/4 |  E6  |  T4  | RL------------+---------+---------+---------+--------+------+------+------IPv6-only   | No      |  No     |  No     | UNRCH  |  N/A |  N/A | N/A------------+---------+---------+---------+--------+------+------+------        Key to Abbreviations        --------------------        N/A:    Not applicable or does not matter.        E6:     IPv6 address of end node.        E4:     IPv4 address of end node (low-order 32-bits of                IPv4-compatible address).        EL:     Datalink address of end node.        T4:     IPv4 address of the tunnel endpoint.        R6:     IPv6 address of router.        R4:     IPv4 address of router.        RL:     Datalink address of router.        IPv4:   IPv4 packet format.        IPv6:   IPv6 packet format.        IPv6/4: IPv6 encapsulated in IPv4 packet format.        UNRCH:  Destination is unreachable.  Don't send a packet.Gilligan & Nordmark         Standards Track                    [Page 19]

RFC 1933               IPv6 Transition Mechanisms             April 19964.4.1  On/Off Link Determination   Part of the process of determining what packet format to use includes   determining whether a destination is located on an attached link or   not.  IPv4 and IPv6 employ different mechanisms.  IPv4 uses an   algorithm in which the destination address and the interface address   are both logically ANDed with the netmask of the interface and then   compared.  If the resulting two values match, then the destination is   located on-link.  This algorithm is discussed in more detail inSection 3.3.1.1 of the host requirements specification [10].  IPv6   uses the neighbor discovery algorithm described in "Neighbor   Discovery for IP Version 6" [7].   IPv6/IPv4 nodes need to use both methods:   -    If a destination is an IPv4 address, then the on/off link        determination is made by comparison with the netmask, as        described inRFC 1122 section 3.3.1.1.   -    If a destination is represented by an IPv4-compatible IPv6        address (prefix 0:0:0:0:0:0), the decision is made using the        IPv4 netmask comparison algorithm using the low-order 32-bits        (IPv4 address part) of the destination address.  -     If the destination is represented by an IPv6-only address        (prefix other than 0:0:0:0:0:0), the on/off link determination        is made using the IPv6 neighbor discovery mechanism.5. Acknowledgements   We would like to thank the members of the IPng working group and the   IPng transition working group for their many contributions and   extensive review of this document.  Special thanks to Jim Bound, Ross   Callon, and Bob Hinden for many helpful suggestions and to John Moy   for suggesting the IPv4 "anycast address" default tunnel technique.6.  Security Considerations   Security issues are not discussed in this memo.Gilligan & Nordmark         Standards Track                    [Page 20]

RFC 1933               IPv6 Transition Mechanisms             April 19967. Authors' Addresses   Robert E. Gilligan   Sun Microsystems, Inc.   2550 Garcia Ave.   Mailstop UMTV 05-44   Mountain View, California 94043   Phone: 415-336-1012   Fax:   415-336-6015   EMail: Bob.Gilligan@Eng.Sun.COM   Erik Nordmark   Sun Microsystems, Inc.   2550 Garcia Ave.   Mailstop UMTV 05-44   Mountain View, California 94043   Phone: 415-336-2788   Fax:   415-336-6015   EMail: Erik.Nordmark@Eng.Sun.COM7. References   [1] Croft, W., and J. Gilmore, "Bootstrap Protocol",RFC 951,       September 1985.   [2] Droms, R., "Dynamic Host Configuration Protocol",RFC 1541.       October 1993.   [3] Bound, J., "Dynamic Host Configuration Protocol for IPv6 for IPv6       (DHCPv6)", Work in Progress, November 1995.   [4] Deering, S., and R. Hinden, "Internet Protocol, Version 6 (IPv6)       Specification",RFC 1883, December 1995.   [5] Thomson, S., and T. Nartan, "IPv6 Stateless Address       Autoconfiguration, Work in Progress, December 1995.   [6] Thomson, S., and C. Huitema. "DNS Extensions to support IP       version 6",RFC 1886, December 1995.   [7] Nartan, T., Nordmark, E., and W. Simpson, "Neighbor Discovery for       IP Version 6 (IPv6)", Work in Progress, November 1995.   [8] Mogul, J., and S. Deering, "Path MTU Discovery",RFC 1191,       November 1990.Gilligan & Nordmark         Standards Track                    [Page 21]

RFC 1933               IPv6 Transition Mechanisms             April 1996   [9] Finlayson, R., Mann, T., Mogul, J., and M. Theimer, "Reverse       Address Resolution Protocol",RFC 903, June 1984.  [10] Braden, R., "Requirements for Internet Hosts - Communication       Layers", STD 3,RFC 1122, October 1989.  [11] Kent, C., and J. Mogul, "Fragmentation Considered Harmful".  In       Proc.  SIGCOMM '87 Workshop on Frontiers in Computer       Communications Technology.  August 1987.Gilligan & Nordmark         Standards Track                    [Page 22]