rfc2893.txt

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Network Working Group                                        R. Gilligan
Request for Comments: 2893                                FreeGate Corp.
Obsoletes: 1933                                              E. Nordmark
Category: Standards Track                         Sun Microsystems, Inc.
                                                             August 2000


            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.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

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.  This document
   obsoletes RFC 1933.



















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Table of Contents

   1.  Introduction.............................................    2
      1.1.  Terminology.........................................    3
      1.2.  Structure of this Document..........................    5
   2.  Dual IP Layer Operation..................................    6
      2.1.  Address Configuration...............................    7
      2.2.  DNS.................................................    7
      2.3.  Advertising Addresses in the DNS....................    8
   3.  Common Tunneling Mechanisms..............................    9
      3.1.  Encapsulation.......................................   11
      3.2.  Tunnel MTU and Fragmentation........................   11
      3.3.  Hop Limit...........................................   13
      3.4.  Handling IPv4 ICMP errors...........................   13
      3.5.  IPv4 Header Construction............................   15
      3.6.  Decapsulation.......................................   16
      3.7.  Link-Local Addresses................................   17
      3.8.  Neighbor Discovery over Tunnels.....................   18
   4.  Configured Tunneling.....................................   18
      4.1.  Default Configured Tunnel...........................   19
      4.2.  Default Configured Tunnel using IPv4 "Anycast Address" 19
      4.3.  Ingress Filtering...................................   20
   5.  Automatic Tunneling......................................   20
      5.1.  IPv4-Compatible Address Format......................   20
      5.2.  IPv4-Compatible Address Configuration...............   21
      5.3.  Automatic Tunneling Operation.......................   22
      5.4.  Use With Default Configured Tunnels.................   22
      5.5.  Source Address Selection............................   23
      5.6.  Ingress Filtering...................................   23
   6.  Acknowledgments..........................................   24
   7.  Security Considerations..................................   24
   8.  Authors' Addresses.......................................   24
   9.  References...............................................   25
   10.  Changes from RFC 1933...................................   26
   11.  Full Copyright Statement................................   29

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



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   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:

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

   -  Configured tunneling of IPv6 over IPv4:  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
   initial 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 documents being written to specify them.

1.1.  Terminology

   The following terms are used in this document:

   Types of Nodes

      IPv4-only node:

         A host or router that implements only IPv4.  An IPv4-only node
         does not understand IPv6.  The installed base of IPv4 hosts and
         routers existing before the transition begins are IPv4-only
         nodes.





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      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 infrastructures.

      Configured tunneling:

         IPv6-over-IPv4 tunneling where the IPv4 tunnel endpoint address
         is determined by configuration information on the encapsulating
         node.  The tunnels can be either unidirectional or
         bidirectional.  Bidirectional configured tunnels behave as
         virtual point-to-point links.




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      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.

      IPv4 multicast tunneling:

         IPv6-over-IPv4 tunneling where the IPv4 tunnel endpoint address
         is determined using Neighbor Discovery [7].  Unlike configured
         tunneling this does not require any address configuration and
         unlike automatic tunneling it does not require the use of
         IPv4-compatible addresses.  However, the mechanism assumes that
         the IPv4 infrastructure supports IPv4 multicast.  Specified in
         [3] and not further discussed in this document.

   Other transition mechanisms, including other tunneling mechanisms,
   are outside the scope of this document.

   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.

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [16].

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.





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   -  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
   called "IPv6/IPv4 nodes."  IPv6/IPv4 nodes have the ability to send
   and receive both IPv4 and IPv6 packets.  They can directly
   interoperate with IPv4 nodes using IPv4 packets, and also directly
   interoperate with IPv6 nodes using IPv6 packets.

   Even though a node may be equipped to support both protocols, one or
   the other stack may be disabled for operational reasons.  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
   provide 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: