rfc1933.txt

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Network Working Group                                        R. Gilligan
Request for Comments: 1933                                   E. Nordmark
Category: Standards Track                         Sun Microsystems, Inc.
                                                              April 1996


            Transition Mechanisms for IPv6 Hosts and Routers

Status 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:




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   -    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.









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   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.




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   -    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 in
   section 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 and



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        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 address



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   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], section
   3.2.1.3, 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