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Versions: 00 01 02 03 04 05 06 rfc2893                                  
INTERNET-DRAFT                                            R. E. Gilligan
7 August 1998                                             FreeGate Corp.
                                                             E. Nordmark
                                                  Sun Microsystems, Inc.

            Transition Mechanisms for IPv6 Hosts and Routers
                    <draft-ietf-ngtrans-mech-01.txt>


Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."

   To view the entire list of current Internet-Drafts, please check the
   "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), ftp.nordu.net (Europe),
   munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or
   ftp.isi.edu (US West Coast).

   This draft expires on 7 February 1999

Abstract

   This document specifies IPv4 compatibility mechanisms that can be
   implemented by IPv6 hosts and routers.  These mechanisms include pro-
   viding complete implementations of both versions of the Internet Pro-
   tocol (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 tran-
   sition 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 compa-
     tibility 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



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     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 interoperabil-
     ity 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:

   -    Dual IP layer (also known as Dual Stack):  A technique for pro-
        viding complete support for both Internet protocols -- IPv4 and
        IPv6 -- in hosts and routers.

   -    Configured tunneling of IPv6 over IPv4:  Unidirectional point-
        to-point tunnels made by encapsulating IPv6 packets within IPv4
        headers to carry them over IPv4 routing infrastructures.

   -    IPv4-compatible IPv6 addresses:  An IPv6 address format that
        employs embedded IPv4 addresses.

   -    Automatic tunneling of IPv6 over IPv4: A mechanism for using
        IPv4-compatible addresses to automatically tunnel IPv6 packets
        over IPv4 networks.

   The mechanisms defined here are intended to be part of a "transition
   toolbox" -- a growing collection of techniques which 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 ini-
   tial core set of transition mechanisms, but these are not expected to
   be the only tools available.  Additional transition and compatibility
   mechanisms are expected to be developed in the future, with new docu-
   ments being written to specify them.

1.1.  Terminology

     The following terms are used in this document:

   Types of Nodes

        IPv4-only node:




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                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.

   Types of IPv6 Addresses

        IPv4-compatible IPv6 address:

                An IPv6 address bearing 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 IPv6/IPv4
                nodes which perform automatic tunneling,

        IPv6-native 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 infras-
                tructures.

        Configured tunneling:



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                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 being tunneled.

   Modes of operation of IPv6/IPv4 nodes

        IPv6-only operation:

                An IPv6/IPv4 node with its IPv6 stack enabled and its
                IPv4 stack disabled.

        IPv4-only operation:

                An IPv6/IPv4 node with its IPv4 stack enabled and its
                IPv6 stack disabled.

        IPv6/IPv4 operation:

                An IPv6/IPv4 node with both stacks enabled.


1.2.  Structure of this Document

   The remainder of this document is organized as follows:

   -    Section 2 discusses the operation of nodes with a dual IP layer,
        IPv6/IPv4 nodes.

   -    Section 3 discusses the common mechanisms used in both of the
        IPv6-over-IPv4 tunneling techniques.

   -    Section 4 discusses configured tunneling.

   -    Section 5 discusses automatic tunneling and the IPv4-compatible
        IPv6 address format.

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 a complete IPv4 and IPv6 implementations are



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   called "IPv6/IPv4 nodes."  IPv6/IPv4 nodes have the ability to send
   and receive both IPv4 and IPv6 packets.  They can directly intero-
   perate with IPv4 nodes using IPv4 packets, and also directly intero-
   perate 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.  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 stacks
   disabled will operate like IPv6-only nodes.  IPv6/IPv4 nodes may pro-
   vide a configuration switch to disable either their IPv4 or IPv6
   stack.

   The dual IP layer technique may or may not be used in conjunction
   with the IPv6-over-IPv4 tunneling techniques, which are described in
   sections 3, 4 and 5.  An IPv6/IPv4 node that supports tunneling may
   support only configured tunneling, or both configured and automatic
   tunneling.  Thus three modes of tunneling support 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 and automatic
        tunneling.


2.1.  Address Configuration

   Because they support both protocols, IPv6/IPv4 nodes may be config-
   ured with both IPv4 and IPv6 addresses.  IPv6/IPv4 nodes use IPv4
   mechanisms (e.g. DHCP) to acquire their IPv4 addresses, and IPv6 pro-
   tocol mechanisms (e.g. stateless address autoconfiguration) to
   acquire their IPv6-native addresses.  Section 5.2 describes a mechan-
   ism by which IPv6/IPv4 nodes that support automatic tunneling may use
   IPv4 protocol mechanisms to acquire their IPv4-compatible IPv6
   address.






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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 [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.

   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 filter or order the results
   returned to the application in order to influence the version of IP
   packets used to communicate with that node.  In terms of filtering,
   the resolver library has three alternatives:

   -    Return only the IPv6 address to the application.

   -    Return only the IPv4 address to the application.

   -    Return both addresses to the application.

   If it returns only the IPv6 address, the application will communicate
   with the node using IPv6.  If it returns only the IPv4 address, the
   application will communicate with the node using IPv4.  If it returns
   both addresses, the application will have the choice which address to
   use, and thus which IP protocol to employ.

   If it returns both, the resolver may elect to order the addresses --
   IPv6 first, or IPv4 first.  Since most applications try the addresses
   in the order they are returned by the resolver, this can affect the
   IP version "preference" of applications.

   The decision to filter or order DNS results is implementation
   specific.  IPv6/IPv4 nodes may provide policy configuration to con-
   trol filtering or ordering of addresses returned by the resolver, or
   leave the decision entirely up to the application.


3.  Common 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 func-
     tional, and can be used to carry IPv6 traffic.  Tunneling provides
     a way to utilize an existing IPv4 routing infrastructure to carry
     IPv6 traffic.



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     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.

   Tunneling techniques are usually classified according to the mechan-
   ism 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 destina-
   tion of the packet being tunneled.  So the addresses in the IPv6
   packet being tunneled can 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 tun-
   neling 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.
   In this case, the destination address of both the IPv6 packet and the
   encapsulating IPv4 header identify the same node!  This fact can be
   exploited by encoding information in the IPv6 destination address
   that will allow the encapsulating node to determine tunnel endpoint
   IPv4 address automatically.  Automatic tunneling employs this tech-
   nique, using an special IPv6 address format with an embedded IPv4
   address to allow tunneling nodes to automatically derive the tunnel
   endpoint IPv4 address.  This eliminates the need to explicitly



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   configure the tunnel endpoint address, greatly simplifying configura-
   tion.

   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.

   -    The encapsulating node may need to maintain soft state informa-
        tion for each tunnel recording such parameters as the MTU of the
        tunnel in order to process IPv6 packets forwarded into the tun-
        nel.  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 remainder of this 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 config-
   ured tunneling.


3.1.  Encapsulation

        The encapsulation of an IPv6 datagram in IPv4 is shown below:





















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                                                     +-------------+
                                                     |    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.


3.2.  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



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        be reassembled at the tunnel endpoint.  For tunnels that ter-
        minate 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 tun-
   nel, 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
   1280 bytes. (Any link layer used by IPv6 has to have an MTU of at
   least 1280 bytes [4].) In this case the IPv6 layer has to "see" a
   link layer with an MTU of 1280 bytes and the encapsulating node has
   to use IPv4 fragmentation in order to forward the 1280 byte IPv6
   packets.

   The encapsulating node can employ the following algorithm to deter-
   mine 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 1280
                   if packet is larger than 1280 bytes
                           Send IPv6 ICMP "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 encapsulating node or by some router along
                           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





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   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 encapsu-
   lating IPv4 header.


3.3.  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 decre-
     ment the hop limit.)

     The TTL of the encapsulating IPv4 header is selected in an imple-
     mentation dependent manner.  The current suggested value is pub-
     lished in the "Assigned Numbers RFC.  Implementations may provide a
     mechanism to allow the administrator to configure the IPv4 TTL.

3.4.  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 the
     encapsulating node because it is the IPv4 source of the encapsu-
     lated 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 deter-
     mine if an IPv6 ICMP "packet too big" error has to be generated as
     described in section 3.2.

     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.



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     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 gen-
     erate 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 Node



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:



<draft-ietf-ngtrans-mech-01.txt>                               [Page 12]


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                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 in section
                3.2.  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)

        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.




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   Any IPv6 options are preserved in the packet (after the IPv6 header).


3.6.  Decapsulation

     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 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.

   The decapsulating node performs IPv4 reassembly before decapsulating
   the IPv6 packet.  All IPv6 options are preserved even if the encapsu-
   lating IPv4 packet is fragmented.

   After the IPv6 packet is decapsulated, it is processed almost the
   same as any received IPv6 packet.  The only difference being that a
   decapsulated packet must not be forwarded unless the node has been
   explicitly configured to forward such packets for the given IPv4
   source address.  This configuration can be implicit in e.g., having a
   configured tunnel which matches the IPv4 source address.  This res-
   triction is needed to prevent tunneling to be used as a tool to



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   circumvent ingress filtering [13].


3.7.  Link-Local Addresses

     Both the configured and automatic tunnels are IPv6 interfaces (over
     the IPv4 "link layer") thus must have link-local addresses.  The
     link-local addresses are used by routing protocols operating over
     the tunnels.

     The Interface Identifier [14] of an IPv4 interface is the 32-bit
     IPv4 address of that interface, with the bytes in the same order in
     which they would appear in the header of an IPv4 packet, padded at
     the left with zeros to a total of 64 bits.  Note that the
     "Universal/Local" bit is zero, indicating that the Interface Iden-
     tifier is not globally unique.  When the host has more than one
     IPv4 address in use on the physical interface concerned, an admin-
     istrative choice of one of these IPv4 addresses is made.

     The IPv6 Link-local address [14] for an IPv4 virtual interface is
     formed by appending the Interface Identifier, as defined above, to
     the prefix FE80::/64.

     +-------+-------+-------+-------+-------+-------+------+------+
     |  FE      80      00      00      00      00      00     00  |
     +-------+-------+-------+-------+-------+-------+------+------+
     |  00      00   |  00   |  00   |   IPv4 Address              |
     +-------+-------+-------+-------+-------+-------+------+------+


3.8.  Neighbor Discovery over Tunnels

     Since both configured and automatic tunnels are considered to be
     unidirectional the only aspects of Neighbor Discovery [7] and
     Stateless Address Autoconfiguration [5] that apply to these tunnels
     is the formation of the link-local address.

     If an implementation provides bidirectional point-to-point tunnels
     by encapsulating IPv6 inside IPv4 packets it should at least accept
     and respond to the probe packets used by Neighbor Unreachability
     Detection [7].  Such implementations may send NUD probe packets.


4.  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



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     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 des-
     tination address using the prefix mask and match technique.


4.1.  Default Configured Tunnel

     IPv6/IPv4 hosts that are connected datalinks with no IPv6 routers
     may use a configured tunnel to reach an IPv6 router.  This tunnel
     allows the host to communicate with the rest of the IPv6 Internet
     (i.e. nodes with IPv6-native addresses).  If the IPv4 address of an
     IPv6/IPv4 router boardering the IPv6 backbone is known, this can be
     used as the tunnel endpoint address.  This tunnel can be configured
     into the routing table as an IPv6 "default route".  That is, all
     IPv6 destination addresses will match the route and could poten-
     tially traverse the tunnel.  Since the "mask length" of such a
     default route is zero, it will be used only if there are no other
     routes with a longer mask that match the destination.  The default
     configured tunnel can be used in conjunction with automatic tunnel-
     ing, as described in section 5.4.


4.2.  Default Configured Tunnel using IPv4 "Anycast Address"

     The tunnel endpoint address of such a default tunnel could be the
     IPv4 address of one IPv6/IPv4 router at the boarder of the IPv6
     backbone.  Alternatively, the tunnel endpoint could be an IPv4
     "anycast address".  With this approach, multiple IPv6/IPv4 routers
     at the boarder 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 boarder
     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 boarder router can be provided,
     and, using the normal fallback mechanisms of IPv4 routing, traffic
     will automatically switch to another router when one goes down.





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5.  Automatic Tunneling

     In automatic tunneling, the tunnel endpoint address is determined
     by the IPv4-compatible destination address of the IPv6 packet being
     tunneled.  Automatic tunneling allows IPv6/IPv4 nodes to communi-
     cate over IPv4 routing infrastructures without pre-configuring tun-
     nels.


5.1.  IPv4-Compatible Address Format

   IPv6/IPv4 nodes that perform automatic tunneling are assigned 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 exclusively to nodes that sup-
   port automatic tunneling.  A node should be configured with an IPv4-
   compatible address only if it is prepared to accept IPv6 packets des-
   tined to that address encapsulated in IPv4 packets destined to the
   embedded IPv4 address.

   An IPv4-compatible address is globally unique as long as the IPv4
   address is not from the private IPv4 space [15].  An implementation
   should behave as if its IPv4-compatible address(es) are assigned to
   the nodes automatic tunneling interfaces, even if the implementation
   does not implement automatic tunneling using a concept of interfaces.


5.2.  IPv4-Compatible Address Configuration

   An IPv6/IPv4 node with an IPv4-compatible address uses that address
   as one of its IPv6 addresses, while the IPv4 address embedded in the
   low-order 32-bits serves as the IPv4 address for one of its inter-
   faces.

   An IPv6/IPv4 node may acquire its IPv4-compatible IPv6 addresses via
   IPv4 address configuration protocols.  It 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



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   IPv6/IPv4 nodes to "leverage" the installed base of IPv4 address con-
   figuration servers.

   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 the IPv4 address for one of its interfaces.  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 address

   2)   The node uses this address as the IPv4 address for this inter-
        face.

   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 one of the node's IPv4-
        addresses embedded in the low-order 32-bits.  The node uses this
        address as one of its IPv6 address.


5.3.  Automatic Tunneling Operation

   In automatic tunneling, the tunnel endpoint address is determined
   from the packet being tunneled.  If the destination IPv6 address is
   IPv4-compatible, then the packet can be sent via automatic tunneling.
   If the destination is IPv6-native, the packet can not be sent via
   automatic tunneling.

   A routing table entry can be used 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 pack-
   ets to those destinations will be auto-tunneled.

   Once it is delivered to the automatic tunneling module, the IPv6



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   packet is encapsulated within an IPv4 header according to the rules
   described in section 3.  The source and destination addresses of the
   encapsulating IPv4 header are assigned as follows:

        Destination IPv4 address:

                Low-order 32-bits of IPv6 destination address

        Source IPv4 address:

                IPv4 address of interface the packet is sent via

   The automatic tunneling module always sends packets in this encapsu-
   lated form, even if the destination is on an attached datalink.

   The automatic tunneling module must not send to IPv4 broadcast or
   multicast destinations.  It must drop all IPv6 packets destined to
   IPv4-compatible destinations when the embedded IPv4 address is broad-
   cast or multicast.

5.4.  Use With Default Configured Tunnels

   Automatic tunneling is often used in conjunction with the default
   configured tunnel technique.  "Isolated" IPv6/IPv4 hosts -- those
   with no on-link IPv6 routers -- are configured to use automatic tun-
   neling and IPv4-compatible IPv6 addresses, and have at least one
   default configured tunnel to an IPv6 router.  That IPv6 router is
   configured to perform automatic tunneling as well.  These isolated
   hosts send packets to IPv4-compatible destinations via automatic tun-
   neling and packets for IPv6-native destinations via the default con-
   figured tunnel.  IPv4-compatible destinations will match the 96-bit
   all-zeros prefix route discussed in the previous section, while
   IPv6-native destinations will match the default route via the config-
   ured tunnel.  Reply packets from IPv6-native destinations are routed
   back to the an IPv6/IPv4 router which delivers them to the original
   host via automatic tunneling.  Further examples of the combination of
   tunneling techniques are discussed in [12].

5.5.  Source Address Selection

   When an IPv6/IPv4 node originates an IPv6 packet, it must select the
   source IPv6 address to use.  IPv6/IPv4 nodes that are configured to
   perform automatic tunneling may be configured with global IPv6-native
   addresses as well as IPv4-compatible addresses.  The selection of
   which source address to use will determine what form the return
   traffic is sent via.  If the IPv4-compatible address is used, the
   return traffic will have to be delivered via automatic tunneling, but
   if the IPv6-native address is used, the return traffic will not be



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   automatic-tunneled.  In order to make traffic as symmetric as possi-
   ble, the following source address selection preference is recom-
   mended:

        Destination is IPv4-compatible:

                Use IPv4-compatible source address associated with IPv4
                address of outgoing interface

        Destination is IPv6-native:

                Use IPv6-native address of outgoing interface

   If an IPv6/IPv4 node has no global IPv6-native address, but is ori-
   ginating a packet to an IPv6-native destination, it may use its
   IPv4-compatible address as its source address.


6.  Acknowledgments

     We would like to thank the members of the IPng working group and
     the Next Generation Transition (ngtrans) working group for their
     many contributions and extensive review of this document.  Special
     thanks are due to Jim Bound, Ross Callon, and Bob Hinden for many
     helpful suggestions and to John Moy for suggesting the IPv4 "any-
     cast address" default tunnel technique.


7.  Security Considerations

     Tunneling is not known to introduce any security holes except for
     the possibility to circumvent ingress filtering [13].  This is
     prevented by requiring that decapsulating routers only forward
     packets if they have been configured to accept encapsulated packets
     from the IPv4 source address in the receive packet.


8.  Authors' Addresses













<draft-ietf-ngtrans-mech-01.txt>                               [Page 20]


INTERNET DRAFT         IPv6 Transition Mechanisms          7 August 1998



   Robert E. Gilligan
   FreeGate Corp
   1208 E. Arques Ave
   Sunnyvale, CA 94086
   USA

   Phone:  +1-408-617-1004
   Fax:    +1-408-617-1010
   Email:  gilligan@freegate.com

   Erik Nordmark
   Sun Microsystems, Inc.
   901 San Antonio Rd.
   Palo Alto, CA 94303
   USA

   Phone:  +1-650-786-5166
   Fax:    +1-650-786-5896
   Email:  nordmark@eng.sun.com



9.  References

  [1]   Croft, W., and J. Gilmore, "Bootstrap Protocol", RFC 951, Sep-
        tember 1985.

  [2]   Droms, R., "Dynamic Host Configuration Protocol", RFC 1541.
        October 1993.

  [3]   Bound, J., "Dynamic Host Configuration Protocol for IPv6
        (DHCPv6)", Work in Progress, February 1997.

  [4]   Deering, S., and R. Hinden, "Internet Protocol, Version 6 (IPv6)
        Specification", RFC 1883, December 1995.

  [5]   Thomson, S., and T. Narten, "IPv6 Stateless Address Autoconfi-
        guration," RFC 1971, August 1996.

  [6]   Thomson, S., and C. Huitema. "DNS Extensions to support IP ver-
        sion 6", RFC 1886, December 1995.

  [7]   Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
        for IP Version 6 (IPv6)", RFC 1970, August 1996.

  [8]   Mogul, J., and S. Deering, "Path MTU Discovery", RFC 1191,
        November 1990.



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INTERNET DRAFT         IPv6 Transition Mechanisms          7 August 1998


  [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 Communica-
        tions Technology.  August 1987.

 [12]   Callon, R. and Haskin, D., "Routing Aspects of IPv6 Transition",
        RFC 2185.  September 1997.

 [13]   Ferguson, P., and Senie, D., "Network Ingress Filtering: Defeat-
        ing Denial of Service Attacks which employ IP Source Address
        Spoofing", RFC 2267, January 1998.

 [14]   Hinden, R., and S. Deering, "IP Version 6 Addressing Architec-
        ture", RFC 2373, July 1998.

 [15]   Rechter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.J., and
        Lear, E.  Address Allocation for Private Internets.  RFC 1918,
        February 1996.

10.  Changes from RFC 1933

   -    Deleted section 3.1.1 (IPv4 loopback address) in order to
        prevent it from being mis-construed as requiring routers to
        filter the address ::127.0.0.1, which would put another test in
        the forwarding path for IPv6 routers.

   -    Deleted section 4.4 (Default Sending Algorithm).  This section
        allowed nodes to send packets in "raw form" to IPv4-compatible
        destinations on the same datalink.  Implementation experience
        has shown that this adds complexity which is not justified by
        the minimal savings in header overhead.

   -    Added definitions for operating modes for IPv6/IPv4 nodes.

   -    Revised DNS section to clarify resolver filtering and ordering
        options.

   -    Re-wrote the discussion of IPv4-compatible addresses to clarify
        that they are used exclusively in conjunction with the automatic
        tunneling mechanism.  Re-organized document to place definition
        of IPv4-compatible address format with description of automatic
        tunneling.




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   -    Changed the term "IPv6-only address" to "IPv6-native address"
        per current usage.

   -    Updated to algorithm for determining tunnel MTU to reflect the
        anticipated change in the IPv6 minimum MTU to 1280 bytes.

   -    Deleted the definition for the term "IPv6-in-IPv4 encapsula-
        tion."  It has not been widely used.

   -    Revised IPv4-compatible address configuration section (5.2) to
        recognize multiple interfaces.

   -    Added discussion of source address selection when using IPv4-
        compatible addresses.

   -    Added section on the combination of the default configured tun-
        neling technique with hosts using automatic tunneling.

   -    Added prohibition against automatic tunneling to IPv4 broadcast
        or multicast destinations.

11.  Changes from draft-ietf-ngtrans-mech-01.txt

   -    Clarified that configured tunnels are unidirectional.

   -    Clarified that IPv4-compatible addresses are assigned
        exclusively to nodes that support automatic tunnels i.e. can
        receive such packets.

   -    Added text about formation of link-local addresses and (non)
        used of Neighbor Discovery.

   -    Added restriction that decapsulated packets not be forwarded
        unless to source address is acceptable to the decapsulating
        router.

12.  Open Issues

   -    Should we disallow the asymmetric use of default configured tun-
        neling in one direction and automatic tunneling in the reverse
        direction?

   -    Should we require that configured tunnels be bidirectional?

   -    Should we require nodes to respond to NUD probes on (bidirec-
        tional) configured tunnels?

   -    Should we require nodes to send NUD probes on (bidirectional)



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        configured tunnels? (Can be omitted for router-router links as
        specified in RFC 1970).

   -    Should we specify a DHCPv4 option for configuring the tunnel
        destination for default configured tunnels?














































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