Transmission of IPv6 over IPv4 Domains without Explicit Tunnels
RFC 2529

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Network Working Group                                       B. Carpenter
Request for Comments: 2529                                           IBM
Category: Standards Track                                        C. Jung
                                                                    3Com
                                                              March 1999

    Transmission of IPv6 over IPv4 Domains without Explicit Tunnels

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 (1999).  All Rights Reserved.

Abstract

   This memo specifies the frame format for transmission of IPv6 [IPV6]
   packets and the method of forming IPv6 link-local addresses over IPv4
   domains.  It also specifies the content of the Source/Target Link-
   layer Address option used in the Router Solicitation, Router
   Advertisement, Neighbor Solicitation, and Neighbor Advertisement and
   Redirect messages, when those messages are transmitted on an IPv4
   multicast network.

   The motivation for this method is to allow isolated IPv6 hosts,
   located on a physical link which has no directly connected IPv6
   router, to become fully functional IPv6 hosts by using an IPv4 domain
   that supports IPv4 multicast as their virtual local link. It uses
   IPv4 multicast as a "virtual Ethernet".

Table of Contents

   1. Introduction....................................................2
   2. Maximum Transmission Unit.......................................2
   3. Frame Format....................................................3
   4. Stateless Autoconfiguration and Link-Local Addresses............3
   5. Address Mapping -- Unicast......................................4
   6. Address Mapping -- Multicast....................................4
   7. Scaling and Transition Isues....................................5
   8. IANA Considerations.............................................6
   9. Security Considerations.........................................6

Carpenter & Jung            Standards Track                     [Page 1]
RFC 2529         Transmission of IPv6 Packets over IPv4       March 1999

   Acknowledgements...................................................7
   References.........................................................7
   APPENDIX A: IPv4 Multicast Addresses for Neighbor Discovery........8
   Authors' Addresses.................................................9
   Full Copyright Notice.............................................10

1. Introduction

   This memo specifies the frame format for transmission of IPv6 [IPV6]
   packets and the method of forming IPv6 link-local addresses over IPv4
   multicast "domains".  For the purposes of this document, an IPv4
   domain is a fully interconnected set of IPv4 subnets, within the same
   local multicast scope, on which there are at least two IPv6 nodes
   conforming to this specification.  This IPv4 domain could form part
   of the globally-unique IPv4 address space, or could form part of a
   private IPv4 network [RFC 1918].

   This memo also specifies the content of the Source/Target Link-layer
   Address option used in the Router Solicitation, Router Advertisement,
   Neighbor Solicitation, Neighbor Advertisement and Redirect messages
   described in [DISC], when those messages are transmitted on an IPv4
   multicast domain.

   The motivation for this method is to allow isolated IPv6 hosts,
   located on a physical link which has no directly connected IPv6
   router, to become fully functional IPv6 hosts by using an IPv4
   multicast domain as their virtual local link.  Thus, at least one
   IPv6 router using the same method must be connected to the same IPv4
   domain if IPv6 routing to other links is required.

   IPv6 hosts connected using this method do not require IPv4-compatible
   addresses or configured tunnels.  In this way IPv6 gains considerable
   independence of the underlying links and can step over many hops of
   IPv4 subnets. The mechanism is known formally as "IPv6 over IPv4" or
   "6over4" and colloquially as "virtual Ethernet".

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2. Maximum Transmission Unit

   The default MTU size for IPv6 packets on an IPv4 domain is 1480
   octets.  This size may be varied by a Router Advertisement [DISC]
   containing an MTU option which specifies a different MTU, or by
   manual configuration of each node.

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