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IETF RFC 2529
Transmission of IPv6 over IPv4 Domains without Explicit Tunnels
Last modified on Thursday, March 4th, 1999
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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 © 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 [RFC 2119].
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.
Carpenter & Jung Standards Track PAGE 2
RFC 2529 Transmission of IPv6 Packets over IPv4 March 1999
Note that if by chance the IPv6 MTU size proves to be too large for
some intermediate IPv4 subnet, IPv4 fragmentation will ensue. While
undesirable, this is not disastrous. However, the IPv4 "do not
fragment" bit MUST NOT be set in the encapsulating IPv4 header.
3. Frame Format
IPv6 packets are transmitted in IPv4 packets [RFC 791] with an IPv4
protocol type of 41, the same as has been assigned in [RFC 1933] for
IPv6 packets that are tunneled inside of IPv4 frames. The IPv4
header contains the Destination and Source IPv4 addresses. The IPv4
packet body contains the IPv6 header followed immediately by the
payload.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| IHL |Type of Service| Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |Flags| Fragment Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time to Live | Protocol 41 | Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 header and payload ... /
+-------+-------+-------+-------+-------+------+------+
If there are IPv4 options, then padding SHOULD be added to the IPv4
header such that the IPv6 header starts on a boundary that is a 32-
bit offset from the end of the datalink header.
The Time to Live field SHOULD be set to a low value, to prevent such
packets accidentally leaking from the IPv4 domain. This MUST be a
configurable parameter, with a recommended default of 8.
4. Stateless Autoconfiguration and Link-Local Addresses
The Interface Identifier [AARCH] of an IPv4 interface is the 32-bit
IPv4 address of that interface, with the octets 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 Identifer is not
globally unique. When the host has more than one IPv4 address in use
Carpenter & Jung Standards Track PAGE 3
RFC 2529 Transmission of IPv6 Packets over IPv4 March 1999
on the physical interface concerned, an administrative choice of one
of these IPv4 addresses is made.
An IPv6 address prefix used for stateless autoconfiguration [CONF] of
an IPv4 interface MUST have a length of 64 bits except for a special
case mentioned in Section 7.
The IPv6 Link-local address [AARCH] 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 |
+-------+-------+-------+-------+-------+-------+------+------+
5. Address Mapping -- Unicast
The procedure for mapping IPv6 addresses into IPv4 virtual link-layer
addresses is described in [DISC]. The Source/Target Link-layer
Address option has the following form when the link layer is IPv4.
Since the length field is in units of 8 bytes, the value below is 1.
+-------+-------+-------+-------+-------+-------+-------+-------+
| Type |Length | must be zero | IPv4 Address |
+-------+-------+-------+-------+-------+-------+-------+-------+
Type:
1 for Source Link-layer address.
2 for Target Link-layer address.
Length:
1 (in units of 8 octets).
IPv4 Address:
The 32 bit IPv4 address, in network byte order. This is the address
the interface currently responds to, and may be different from the
Interface Identifier for stateless autoconfiguration.
6. Address Mapping -- Multicast
IPv4 multicast MUST be available. An IPv6 packet with a multicast
destination address DST MUST be transmitted to the IPv4 multicast
address of Organization-Local Scope using the mapping below. These
IPv4 multicast addresses SHOULD be taken from the block
Carpenter & Jung Standards Track PAGE 4
RFC 2529 Transmission of IPv6 Packets over IPv4 March 1999
239.192.0.0/16, a sub-block of the Organization-Local Scope address
block, or, if all of those are not available, from the expansion
blocks defined in [ADMIN]. Note that when they are formed using the
expansion blocks, they use only a /16 sized block.
+-------+-------+-------+-------+
| 239 | OLS | DST14 | DST15 |
+-------+-------+-------+-------+
DST14, DST15 last two bytes of IPv6 multicast address.
OLS from the configured Organization-Local
Scope address block. SHOULD be 192,
see [ADMIN] for details.
No new IANA registration procedures are required for the above. See
appendix A. for a list of all the multicast groups that must be
joined to support Neighbor Discovery.
7. Scaling and Transition Issues
The multicast mechanism described in Section 6 above appears to have
essentially the same scaling properties as native IPv6 over most
media, except for the slight reduction in MTU size which will
slightly reduce bulk throughput. On an ATM network, where IPv4
multicast relies on relatively complex mechanisms, it is to be
expected that IPv6 over IPv4 over ATM will perform less well than
native IPv6 over ATM.
The "IPv6 over IPv4" mechanism is intended to take its place in the
range of options available for transition from IPv4 to IPv6. In
particular it allows a site to run both IPv4 and IPv6 in coexistence,
without having to configure IPv6 hosts either with IPv4-compatible
addresses or with tunnels. Interfaces of the IPv6 router and hosts
will of course need to be enabled in "6over4" mode.
A site may choose to start its IPv6 transition by configuring one
IPv6 router to support "6over4" on an interface connected to the
site's IPv4 domain, and another IPv6 format on an interface connected
to the IPv6 Internet. Any enabled "6over4" hosts in the IPv4 domain
will then be able to communicate both with the router and with the
IPv6 Internet, without manual configuration of a tunnel and without
the need for an IPv4-compatible IPv6 address, either stateless or
stateful address configuration providing the IPv6 address to the IPv6
host.
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RFC 2529 Transmission of IPv6 Packets over IPv4 March 1999
During transition, routers may need to advertise at least two IPv6
prefixes, one for the native LAN (e.g. Ethernet) and one for
"6over4". As with any IPv6 prefix assigned to an IPv6 subnet, the
latter MUST be unique within its scope, whether site-local or global
addressing is used.
Also note that when a router is handling both native LAN and "6over4"
on the same physical interface, during stateless autoconfiguration,
there is a period when IPv6 link-local addresses are used, in both
cases with the prefix FE80::/64. Note that the prefix-length for
these link-local adddress MUST then be 128 so that the two cases can
be distinguished.
As the site installs additional IPv6 routers, "6over4" hosts which
become physically adjacent to IPv6 routers can be changed to run as
native IPv6 hosts, with the the only impact on IPv6 applications
being a slight increase in MTU size. At some stage during transition,
it might be convenient to dual home hosts in both native LAN and
"6over4" mode, but this is not required.
8. IANA Considerations
No assignments by the IANA are required beyond those in [ADMIN].
9. Security Considerations
Implementors should be aware that, in addition to posssible attacks
against IPv6, security attacks against IPv4 must also be considered.
Use of IP security at both IPv4 and IPv6 levels should nevertheless
be avoided, for efficiency reasons. For example, if IPv6 is running
encrypted, encryption of IPv4 would be redundant except if traffic
analysis is felt to be a threat. If IPv6 is running authenticated,
then authentication of IPv4 will add little. Conversely, IPv4
security will not protect IPv6 traffic once it leaves the IPv6-over-
IPv4 domain. Therefore, implementing IPv6 security is required even
if IPv4 security is available.
There is a possible spoofing attack in which spurious 6over4 packets
are injected into a 6over4 domain from outside. Thus, boundary
routers MUST discard multicast IPv4 packets with source or
destination multicast addresses of organisation local scope as
defined in section 6 above, if they arrive on physical interfaces
outside that scope. To defend against spurious unicast 6over4
packets, boundary routers MUST discard incoming IPv4 packets with
protocol type 41 from unknown sources, i.e. IPv6-in-IPv4 tunnels
must only be accepted from trusted sources. Unless IPSEC
Carpenter & Jung Standards Track PAGE 6
RFC 2529 Transmission of IPv6 Packets over IPv4 March 1999
authentication is available, the RECOMMENDED technique for this is to
configure the boundary router only to accept protocol type 41 packets
from source addresses within a trusted range or ranges.
Acknowledgements
The basic idea presented above is not original, and we have had
invaluable comments from Matt Crawford, Steve Deering, Dan
Harrington, Rich Draves, Erik Nordmark, Quang Nguyen, Thomas Narten,
and other members of the IPNG and NGTRANS working groups.
This document is seriously ripped off from RFC 1972 written by Matt
Crawford. Brian Carpenter was at CERN when the work was started.
References
[AARCH] Hinden, R., and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[ADMIN] Meyer, D., "Administratively Scoped IP Multicast", BCP 23,
RFC 2365, July 1998.
[CONF] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[DISC] Narten, T., Nordmark, E. and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, December
1998.
[IPV6] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC 791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[RFC 1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., de Groot, G.
and E. Lear, "Address Allocation for Private Internets",
RFC 1918, February 1996.
[RFC 1933] Gilligan, R. and E. Nordmark, "Transition Mechanisms for
IPv6 Hosts and Routers", RFC 1933, April 1996.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 1972] Crawford, M., "A Method for the Transmission of IPv6
Packets over Ethernet Networks", RFC 1972, August 1996.
Carpenter & Jung Standards Track PAGE 7
RFC 2529 Transmission of IPv6 Packets over IPv4 March 1999
APPENDIX A: IPv4 Multicast Addresses for Neighbor Discovery
The following IPv4 multicast groups are used to support Neighbor
Discovery with this specification. The IPv4 addresses listed in this
section were obtained by looking at the IPv6 multicast addresses that
Neigbour Discovery uses, and deriving the resulting IPv4 "virtual
link-layer" addresses that are generated from them using the
algorithm given in Section 6.
all-nodes multicast address
- the administratively-scoped IPv4 multicast address used to
reach all nodes in the local IPv4 domain supporting this
specification. 239.OLS.0.1
all-routers multicast address
- the administratively-scoped IPv4 multicast address to reach
all routers in the local IPv4 domain supporting this
specification. 239.OLS.0.2
solicited-node multicast address
- an administratively scoped multicast address that is computed
as a function of the solicited target's address by taking the
low-order 24 bits of the IPv4 address used to form the IPv6
address, and prepending the prefix FF02:0:0:0:0:1:FF00::/104
[AARCH]. This is then mapped to the IPv4 multicast address by
the method described in this document. For example, if the
IPv4 address used to form the IPv6 address is W.X.Y.Z, then
the IPv6 solicited node multicast address is
FF02::1:255.X.Y.Z and the corresponding IPv4 multicast
address is 239.OLS.Y.Z
Carpenter & Jung Standards Track PAGE 8
RFC 2529 Transmission of IPv6 Packets over IPv4 March 1999
Authors' Addresses
Brian E. Carpenter
Internet Division
IBM United Kingdom Laboratories
MP 185, Hursley Park
Winchester, Hampshire S021 2JN, UK
EMail: brian@hursley.ibm.com
Cyndi Jung
3Com Corporation
5400 Bayfront Plaza, Mailstop 3219
Santa Clara, California 95052-8145
EMail: cmj@3Com.com
Carpenter & Jung Standards Track PAGE 9
RFC 2529 Transmission of IPv6 Packets over IPv4 March 1999
Full Copyright Statement
Copyright © The Internet Society (1999). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Carpenter & Jung Standards Track PAGE 10
Transmission of IPv6 over IPv4 Domains without Explicit Tunnels
RFC TOTAL SIZE: 21049 bytes
PUBLICATION DATE: Thursday, March 4th, 1999
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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