As per Relevance of the word tunneling, we have this rfc below:
Network Working Group R.
Request for Comments: 2893 FreeGate Corp
Obsoletes: 1933 E.
Category: Standards Track Sun Microsystems, Inc
August 2000
Transition Mechanisms for IPv6 Hosts and
Status of this
This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited
Copyright
Copyright (C) The Internet Society (2000). All Rights Reserved
This document specifies IPv4 compatibility mechanisms that can
implemented by IPv6 hosts and routers. These mechanisms
providing complete implementations of both versions of the
Protocol (IPv4 and IPv6), and tunneling IPv6 packets over IPv
routing infrastructures. They are designed to allow IPv6 nodes
maintain complete compatibility with IPv4, which should
simplify the deployment of IPv6 in the Internet, and facilitate
eventual transition of the entire Internet to IPv6. This
obsoletes RFC 1933.
Gilligan & Nordmark Standards Track [Page 1]
RFC 2893 IPv6 Transition Mechanisms August 2000
Table of
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.
The key to a successful IPv6 transition is compatibility with
large installed base of IPv4 hosts and routers.
compatibility with IPv4 while deploying IPv6 will streamline the
of transitioning the Internet to IPv6. This specification defines
set of mechanisms that IPv6 hosts and routers may implement in
to be compatible with IPv4 hosts and routers
The mechanisms in this document are designed to be employed by IPv
hosts and routers that need to interoperate with IPv4 hosts
utilize IPv4 routing infrastructures. We expect that most nodes
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RFC 2893 IPv6 Transition Mechanisms August 2000
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
with IPv4 is not required. IPv6 nodes that are designed to be
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
providing complete support for both Internet protocols -- IPv4
IPv6 -- in hosts and routers
- Configured tunneling of IPv6 over IPv4: Point-to-point
made by encapsulating IPv6 packets within IPv4 headers to
them over IPv4 routing infrastructures
- IPv4-compatible IPv6 addresses: An IPv6 address format
employs embedded IPv4 addresses
- Automatic tunneling of IPv6 over IPv4: A mechanism for
IPv4-compatible addresses to automatically tunnel IPv6
over IPv4 networks
The mechanisms defined here are intended to be part of a "
toolbox" -- a growing collection of techniques which
and users may employ to ease the transition. The tools may be
as needed. Implementations and sites decide which techniques
appropriate to their specific needs. This document defines
initial core set of transition mechanisms, but these are not
to be the only tools available. Additional transition
compatibility mechanisms are expected to be developed in the future
with new documents being written to specify them
1.1.
The following terms are used in this document
Types of
IPv4-only node
A host or router that implements only IPv4. An IPv4-only
does not understand IPv6. The installed base of IPv4 hosts
routers existing before the transition begins are IPv4-
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
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
IPv4-compatible IPv6 address
An IPv6 address bearing the high-order 96-bit
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
perform automatic tunneling
IPv6-native address
The remainder of the IPv6 address space. An IPv6 address
bears a prefix other than 0:0:0:0:0:0.
Techniques Used in the
IPv6-over-IPv4 tunneling
The technique of encapsulating IPv6 packets within IPv4 so
they can be carried across IPv4 routing infrastructures
Configured tunneling
IPv6-over-IPv4 tunneling where the IPv4 tunnel endpoint
is determined by configuration information on the
node. The tunnels can be either unidirectional
bidirectional. Bidirectional configured tunnels behave
virtual point-to-point links
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Automatic tunneling
IPv6-over-IPv4 tunneling where the IPv4 tunnel endpoint
is determined from the IPv4 address embedded in the IPv4-
compatible destination address of the IPv6 packet
tunneled
IPv4 multicast tunneling
IPv6-over-IPv4 tunneling where the IPv4 tunnel endpoint
is determined using Neighbor Discovery [7]. Unlike
tunneling this does not require any address configuration
unlike automatic tunneling it does not require the use
IPv4-compatible addresses. However, the mechanism assumes
the IPv4 infrastructure supports IPv4 multicast. Specified
[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
IPv6-only operation
An IPv6/IPv4 node with its IPv6 stack enabled and its IPv
stack disabled
IPv4-only operation
An IPv6/IPv4 node with its IPv4 stack enabled and its IPv
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
document, are to be interpreted as described in [16].
1.2. Structure of this
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
IPv6-over-IPv4 tunneling techniques
- Section 4 discusses configured tunneling
- Section 5 discusses automatic tunneling and the IPv4-
IPv6 address format
2. Dual IP Layer
The most straightforward way for IPv6 nodes to remain compatible
IPv4-only nodes is by providing a complete IPv4 implementation. IPv
nodes that provide a complete IPv4 and IPv6 implementations
called "IPv6/IPv4 nodes." IPv6/IPv4 nodes have the ability to
and receive both IPv4 and IPv6 packets. They can
interoperate with IPv4 nodes using IPv4 packets, and also
interoperate with IPv6 nodes using IPv6 packets
Even though a node may be equipped to support both protocols, one
the other stack may be disabled for operational reasons.
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
IPv4-only nodes. Similarly, IPv6/IPv4 nodes with their IPv4
disabled will operate like IPv6-only nodes. IPv6/IPv4 nodes
provide a configuration switch to disable either their IPv4 or IPv
stack
The dual IP layer technique may or may not be used in
with the IPv6-over-IPv4 tunneling techniques, which are described
sections 3, 4 and 5. An IPv6/IPv4 node that supports tunneling
support only configured tunneling, or both configured and
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
tunneling
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2.1. Address
Because they support both protocols, IPv6/IPv4 nodes may
configured with both IPv4 and IPv6 addresses. IPv6/IPv4 nodes
IPv4 mechanisms (e.g. DHCP) to acquire their IPv4 addresses, and IPv
protocol mechanisms (e.g. stateless address autoconfiguration)
acquire their IPv6-native addresses. Section 5.2 describes
mechanism by which IPv6/IPv4 nodes that support automatic
MAY use IPv4 protocol mechanisms to acquire their IPv4-
IPv6 address
2.2.
The Domain Naming System (DNS) is used in both IPv4 and IPv6 to
between hostnames and IP addresses. A new resource record type
"A6" has been defined for IPv6 addresses [6] with support for
earlier record named "AAAA". Since IPv6/IPv4 nodes must be able
interoperate directly with both IPv4 and IPv6 nodes, they
provide resolver libraries capable of dealing with IPv4 "A"
as well as IPv6 "A6" and "AAAA" records
DNS resolver libraries on IPv6/IPv4 nodes MUST be capable of
both A6/AAAA and A records. However, when a query locates an A6/
record holding an IPv6 address, and an A record holding an IPv
address, the resolver library MAY filter or order the
returned to the application in order to influence the version of
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
with the node using IPv6. If it returns only the IPv4 address,
application will communicate with the node using IPv4. If it
both addresses, the application will have the choice which address
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
in the order they are returned by the resolver, this can affect
IP version "preference" of applications
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The decision to filter or order DNS results is
specific. IPv6/IPv4 nodes MAY provide policy configuration
control filtering or ordering of addresses returned by the resolver
or leave the decision entirely up to the application
An implementation MUST allow the application to control whether
not such filtering takes place
2.3. Advertising Addresses in the
There are some constraint placed on the use of the DNS
transition. Most of these are obvious but are stated here
completeness
The recommendation is that A6/AAAA records for a node should not
added to the DNS until all of these are true
1) The address is assigned to the interface on the node
2) The address is configured on the interface
3) The interface is on a link which is connected to the IPv
infrastructure
If an IPv6 node is isolated from an IPv6 perspective (e.g. it is
connected to the 6bone to take a concrete example) constraint #3
would mean that it should not have an address in the DNS
This works great when other dual stack nodes tries to contact
isolated dual stack node. There is no IPv6 address in the DNS
the peer doesn't even try communicating using IPv6 but goes
to IPv4 (we are assuming both nodes have A records in the DNS.)
However, this does not work well when the isolated node is trying
establish communication. Even though it does not have an IPv
address in the DNS it will find A6/AAAA records in the DNS for
peer. Since the isolated node has IPv6 addresses assigned to
least one interface it will try to communicate using IPv6. If it
no IPv6 route to the 6bone (e.g. because the local router
upgraded to advertise IPv6 addresses using Neighbor Discovery
that router doesn't have any IPv6 routes) this communication
fail. Typically this means a few minutes of delay as TCP times out
The TCP specification says that ICMP unreachable messages could
due to routing transients thus they should not immediately
the TCP connection. This means that the normal TCP timeout of a
minutes apply. Once TCP times out the application will hopefully
the IPv4 addresses based on the A records in the DNS, but this
be painfully slow
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A possible implication of the recommendations above is that, if
enables IPv6 on a node on a link without IPv6 infrastructure,
choose to add A6/AAAA records to the DNS for that node, then
IPv6 nodes that might see these A6/AAAA records will possibly try
reach that node using IPv6 and suffer delays or communication
due to unreachability. (A delay is incurred if the
correctly falls back to using IPv4 if it can not
communication using IPv6 addresses. If this fallback is not done
application would fail to communicate in this case.) Thus it
suggested that either the recommendations be followed, or care
taken to only do so with nodes that will not be impacted by
accessing delays and/or communication failure
In the future when a site or node removes the support for IPv4
above recommendations apply to when the A records for the node(s
should be removed from the DNS
3. Common Tunneling
In most deployment scenarios, the IPv6 routing infrastructure will
built up over time. While the IPv6 infrastructure is being deployed
the existing IPv4 routing infrastructure can remain functional,
can be used to carry IPv6 traffic. Tunneling provides a way
utilize an existing IPv4 routing infrastructure to carry IPv
traffic
IPv6/IPv4 hosts and routers can tunnel IPv6 datagrams over regions
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 IPv
infrastructure can tunnel IPv6 packets between themselves.
this case, the tunnel spans one segment of the end-to-end
that the IPv6 packet takes
- Host-to-Router. IPv6/IPv4 hosts can tunnel IPv6 packets to
intermediary IPv6/IPv4 router that is reachable via an IPv
infrastructure. This type of tunnel spans the first segment
the packet's end-to-end path
- Host-to-Host. IPv6/IPv4 hosts that are interconnected by an IPv
infrastructure can tunnel IPv6 packets between themselves.
this case, the tunnel spans the entire end-to-end path that
packet takes
- Router-to-Host. IPv6/IPv4 routers can tunnel IPv6 packets
their final destination IPv6/IPv4 host. This tunnel spans
the last segment of the end-to-end path
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Tunneling techniques are usually classified according to
mechanism by which the encapsulating node determines the address
the node at the end of the tunnel. In the first two
methods listed above -- router-to-router and host-to-router --
IPv6 packet is being tunneled to a router. The endpoint of this
of tunnel is an intermediary router which must decapsulate the IPv
packet and forward it on to its final destination. When tunneling
a router, the endpoint of the tunnel is different from
destination of the packet being tunneled. So the addresses in
IPv6 packet being tunneled can not provide the IPv4 address of
tunnel endpoint. Instead, the tunnel endpoint address must
determined from configuration information on the node performing
tunneling. We use the term "configured tunneling" to describe
type of tunneling where the endpoint is explicitly configured
In the last two tunneling methods -- host-to-host and router-to-
-- the IPv6 packet is tunneled all the way to its final destination
In this case, the destination address of both the IPv6 packet and
encapsulating IPv4 header identify the same node! This fact can
exploited by encoding information in the IPv6 destination
that will allow the encapsulating node to determine tunnel
IPv4 address automatically. Automatic tunneling employs
technique, using an special IPv6 address format with an embedded IPv
address to allow tunneling nodes to automatically derive the
endpoint IPv4 address. This eliminates the need to
configure the tunnel endpoint address, greatly
configuration
The two tunneling techniques -- automatic and configured --
primarily in how they determine the tunnel endpoint address. Most
the underlying mechanisms are the same
- The entry node of the tunnel (the encapsulating node) creates
encapsulating IPv4 header and transmits the encapsulated packet
- The exit node of the tunnel (the decapsulating node) receives
encapsulated packet, reassembles the packet if needed, removes
IPv4 header, updates the IPv6 header, and processes the
IPv6 packet
- The encapsulating node MAY need to maintain soft state
for each tunnel recording such parameters as the MTU of the
in order to process IPv6 packets forwarded into the tunnel.
the number of tunnels that any one host or router may be using
grow to be quite large, this state information can be cached
discarded when not in use
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RFC 2893 IPv6 Transition Mechanisms August 2000
The remainder of this section discusses the common mechanisms
apply to both types of tunneling. Subsequent sections discuss
the tunnel endpoint address is determined for automatic
configured tunneling
3.1.
The encapsulation of an IPv6 datagram in IPv4 is shown below
+-------------+
| IPv4 |
| Header |
+-------------+ +-------------+
| IPv6 | | IPv6 |
| Header | | Header |
+-------------+ +-------------+
| Transport | | Transport |
| Layer | ===> | Layer |
| Header | | Header |
+-------------+ +-------------+
| | | |
~ Data ~ ~ Data ~
| | | |
+-------------+ +-------------+
Encapsulating IPv6 in IPv
In addition to adding an IPv4 header, the encapsulating node also
to handle some more complex issues
- Determine when to fragment and when to report an ICMP "packet
big" error back to the source
- How to reflect IPv4 ICMP errors from routers along the tunnel
back to the source as IPv6 ICMP errors
Those issues are discussed in the following sections
3.2. Tunnel MTU and
The encapsulating node could view encapsulation as IPv6 using IPv4
a link layer with a very large MTU (65535-20 bytes to be exact; 20
bytes "extra" are needed for the encapsulating IPv4 header).
encapsulating node would need only to report IPv6 ICMP "packet
big" errors back to the source for packets that exceed this MTU
However, such a scheme would be inefficient for two reasons
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RFC 2893 IPv6 Transition Mechanisms August 2000
1) It would result in more fragmentation than needed. IPv4
fragmentation SHOULD be avoided due to the performance
caused by the loss unit being smaller than the retransmission
[11].
2) Any IPv4 fragmentation occurring inside the tunnel would have
be reassembled at the tunnel endpoint. For tunnels that
at a router, this would require additional memory to
the IPv4 fragments into a complete IPv6 packet before that
could be forwarded onward
The fragmentation inside the tunnel can be reduced to a minimum
having the encapsulating node track the IPv4 Path MTU across
tunnel, using the IPv4 Path MTU Discovery Protocol [8] and
the resulting path MTU. The IPv6 layer in the encapsulating node
then view a tunnel as a link layer with an MTU equal to the IPv4
MTU, minus the size of the encapsulating IPv4 header
Note that this does not completely eliminate IPv4 fragmentation
the case when the IPv4 path MTU would result in an IPv6 MTU less
1280 bytes. (Any link layer used by IPv6 has to have an MTU of
least 1280 bytes [4].) In this case the IPv6 layer has to "see"
link layer with an MTU of 1280 bytes and the encapsulating node
to use IPv4 fragmentation in order to forward the 1280 byte IPv
packets
The encapsulating node can employ the following algorithm
determine when to forward an IPv6 packet that is larger than
tunnel's path MTU using IPv4 fragmentation, and when to return
IPv6 ICMP "packet too big" message
if (IPv4 path MTU - 20) is less than or equal to 1280
if packet is larger than 1280
Send IPv6 ICMP "packet too big" with MTU = 1280.
Drop packet
Encapsulate but do not set the Don't
flag in the IPv4 header. The resulting IPv
packet might be fragmented by the IPv4 layer
the encapsulating node or by some router
the IPv4 path
if packet is larger than (IPv4 path MTU - 20)
Send IPv6 ICMP "packet too big"
MTU = (IPv4 path MTU - 20).
Drop packet
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RFC 2893 IPv6 Transition Mechanisms August 2000
Encapsulate and set the Don't Fragment
in the IPv4 header
Encapsulating nodes that have a large number of tunnels might not
able to store the IPv4 Path MTU for all tunnels. Such nodes can,
the expense of additional fragmentation in the network, avoid
the IPv4 Path MTU algorithm across the tunnel and instead use the
of the link layer (under IPv4) in the above algorithm instead of
IPv4 path MTU
In this case the Don't Fragment bit MUST NOT be set in
encapsulating IPv4 header
3.3. Hop
IPv6-over-IPv4 tunnels are modeled as "single-hop". That is,
IPv6 hop limit is decremented by 1 when an IPv6 packet traverses
tunnel. The single-hop model serves to hide the existence of
tunnel. The tunnel is opaque to users of the network, and is
detectable by network diagnostic tools such as traceroute
The single-hop model is implemented by having the encapsulating
decapsulating nodes process the IPv6 hop limit field as they would
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 decrement the
limit.)
The TTL of the encapsulating IPv4 header is selected in
implementation dependent manner. The current suggested value
published in the "Assigned Numbers RFC. Implementations MAY
a mechanism to allow the administrator to configure the IPv4 TTL
as the one specified in the IP Tunnel MIB [17].
3.4. Handling IPv4 ICMP
In response to encapsulated packets it has sent into the tunnel,
encapsulating node might receive IPv4 ICMP error messages from IPv
routers inside the tunnel. These packets are addressed to
encapsulating node because it is the IPv4 source of the
packet
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The ICMP "packet too big" error messages are handled according
IPv4 Path MTU Discovery [8] and the resulting path MTU is recorded
the IPv4 layer. The recorded path MTU is used by IPv6 to
if an IPv6 ICMP "packet too big" error has to be generated
described in section 3.2.
The handling of other types of ICMP error messages depends on
much information is included in the "packet in error" field,
holds the encapsulated packet that caused the error
Many older IPv4 routers return only 8 bytes of data beyond the IPv
header of the packet in error, which is not enough to include
address fields of the IPv6 header. More modern IPv4 routers
likely to return enough data beyond the IPv4 header to include
entire IPv6 header and possibly even the data beyond that
If the offending packet includes enough data, the encapsulating
MAY extract the encapsulated IPv6 packet and use it to generate
IPv6 ICMP message directed back to the originating IPv6 node,
shown below
+--------------+
| IPv4 Header |
| dst = encaps |
| node |
+--------------+
| ICMP |
| Header |
- - +--------------+
| IPv4 Header |
| src = encaps |
IPv4 | node |
+--------------+ - -
Packet | IPv6 |
| Header | Original IPv
in +--------------+ Packet -
| Transport | Can be used
Error | Header | generate
+--------------+ IPv6
| | error
~ Data ~ back to the source
| |
- - +--------------+ - -
IPv4 ICMP Error Message Returned to Encapsulating
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RFC 2893 IPv6 Transition Mechanisms August 2000
3.5. IPv4 Header
When encapsulating an IPv6 packet in an IPv4 datagram, the IPv
header fields are set as follows
Version
4
IP Header Length in 32-bit words
5 (There are no IPv4 options in the encapsulating header.)
Type of Service
0. [Note that work underway in the IETF is redefining the
of Service byte and as a result future RFCs might define
different behavior for the ToS byte when tunneling.]
Total Length
Payload length from IPv6 header plus length of IPv6 and IPv
headers (i.e. a constant 60 bytes).
Identification
Generated uniquely as for any IPv4 packet transmitted by
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)
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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
Any IPv6 options are preserved in the packet (after the IPv6 header).
3.6.
When an IPv6/IPv4 host or a router receives an IPv4 datagram that
addressed to one of its own IPv4 address, and the value of
protocol field is 41, it reassembles if the packet if it
fragmented at the IPv4 level, then it removes the IPv4 header
submits the IPv6 datagram to its IPv6 layer code
The decapsulating node MUST be capable of reassembling an IPv4
that is 1300 bytes (1280 bytes plus IPv4 header).
The decapsulation is shown below
+-------------+
| IPv4 |
| Header |
+-------------+ +-------------+
| IPv6 | | IPv6 |
| Header | | Header |
+-------------+ +-------------+
| Transport | | Transport |
| Layer | ===> | Layer |
| Header | | Header |
+-------------+ +-------------+
| | | |
~ Data ~ ~ Data ~
| | | |
+-------------+ +-------------+
Decapsulating IPv6 from IPv
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When decapsulating the packet, the IPv6 header is not modified
[Note that work underway in the IETF is redefining the Type
Service byte and as a result future RFCs might define a
behavior for the ToS byte when decapsulating a tunneled packet.]
the packet is subsequently forwarded, its hop limit is decremented
one
As part of the decapsulation the node SHOULD silently discard
packet with an invalid IPv4 source address such as a
address, a broadcast address, 0.0.0.0, and 127.0.0.1. In general
SHOULD apply the rules for martian filtering in [18] and
filtering [13] on the IPv4 source address
The encapsulating IPv4 header is discarded
After the decapsulation the node SHOULD silently discard a
with an invalid IPv6 source address. This includes IPv6
addresses, the unspecified address, and the loopback address but
IPv4-compatible IPv6 source addresses where the IPv4 part of
address is an (IPv4) multicast address, broadcast address, 0.0.0.0,
or 127.0.0.1. In general it SHOULD apply the rules for
filtering in [18] and ingress filtering [13] on the IPv4-
source address
The decapsulating node performs IPv4 reassembly before
the IPv6 packet. All IPv6 options are preserved even if
encapsulating IPv4 packet is fragmented
After the IPv6 packet is decapsulated, it is processed almost
same as any received IPv6 packet. The only difference being that
decapsulated packet MUST NOT be forwarded unless the node has
explicitly configured to forward such packets for the given IPv
source address. This configuration can be implicit in e.g., having
configured tunnel which matches the IPv4 source address.
restriction is needed to prevent tunneling to be used as a tool
circumvent ingress filtering [13].
3.7. Link-Local
Both the configured and automatic tunnels are IPv6 interfaces (
the IPv4 "link layer") thus MUST have link-local addresses.
link-local addresses are used by routing protocols operating over
tunnels
The Interface Identifier [14] for such an Interface SHOULD be
32-bit IPv4 address of that interface, with the bytes in the
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
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RFC 2893 IPv6 Transition Mechanisms August 2000
"Universal/Local" bit is zero, indicating that the
Identifier is not globally unique. When the host has more than
IPv4 address in use on the physical interface concerned,
administrative choice of one of these IPv4 addresses is made
The IPv6 Link-local address [14] for an IPv4 virtual interface
formed by appending the Interface Identifier, as defined above,
the prefix FE80::/64.
+-------+-------+-------+-------+-------+-------+------+------+
| FE 80 00 00 00 00 00 00 |
+-------+-------+-------+-------+-------+-------+------+------+
| 00 00 | 00 | 00 | IPv4 Address |
+-------+-------+-------+-------+-------+-------+------+------+
3.8. Neighbor Discovery over
Automatic tunnels and unidirectional configured tunnels
considered to be unidirectional. Thus the only aspects of
Discovery [7] and Stateless Address Autoconfiguration [5] that
to these tunnels is the formation of the link-local address
If an implementation provides bidirectional configured tunnels
MUST at least accept and respond to the probe packets used
Neighbor Unreachability Detection [7]. Such implementations
also send NUD probe packets to detect when the configured
fails at which point the implementation can use an alternate path
reach the destination. Note that Neighbor Discovery allows that
sending of NUD probes be omitted for router to router links if
routing protocol tracks bidirectional reachability
For the purposes of Neighbor Discovery the automatic and
tunnels specified in this document as assumed to NOT have a link
layer address, even though the link-layer (IPv4) does have address
This means that a sender of Neighbor Discovery
- SHOULD NOT include Source Link Layer Address options or
Link Layer Address options on the tunnel link
- MUST silently ignore any received SLLA or TLLA options on
tunnel link
4. Configured
In configured tunneling, the tunnel endpoint address is
from configuration information in the encapsulating node. For
tunnel, the encapsulating node must store the tunnel
address. When an IPv6 packet is transmitted over a tunnel,
Gilligan & Nordmark Standards Track [Page 18]
RFC 2893 IPv6 Transition Mechanisms August 2000
tunnel endpoint address configured for that tunnel is used as
destination address for the encapsulating IPv4 header
The determination of which packets to tunnel is usually made
routing information on the encapsulating node. This is usually
via a routing table, which directs packets based on their
address using the prefix mask and match technique
4.1. Default Configured
IPv6/IPv4 hosts that are connected to datalinks with no IPv6
MAY use a configured tunnel to reach an IPv6 router. This
allows the host to communicate with the rest of the IPv6
(i.e. nodes with IPv6-native addresses). If the IPv4 address of
IPv6/IPv4 router bordering the IPv6 backbone is known, this can
used as the tunnel endpoint address. This tunnel can be
into the routing table as an IPv6 "default route". That is, all IPv
destination addresses will match the route and could
traverse the tunnel. Since the "mask length" of such a default
is zero, it will be used only if there are no other routes with
longer mask that match the destination. The default
tunnel can be used in conjunction with automatic tunneling,
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
IPv4 address of one IPv6/IPv4 router at the border of the IPv
backbone. Alternatively, the tunnel endpoint could be an IPv
"anycast address". With this approach, multiple IPv6/IPv4 routers
the border advertise IPv4 reachability to the same IPv4 address.
of these routers accept packets to this address as their own,
will decapsulate IPv6 packets tunneled to this address. When
IPv6/IPv4 node sends an encapsulated packet to this address, it
be delivered to only one of the border routers, but the sending
will not know which one. The IPv4 routing system will
carry the traffic to the closest router
Using a default tunnel to an IPv4 "anycast address" provides a
degree of robustness since multiple border router can be provided
and, using the normal fallback mechanisms of IPv4 routing,
will automatically switch to another router when one goes down
However, care must be taking when using such a default tunnel
prevent different IPv4 fragments from arriving at different
for reassembly. This can be prevented by either
fragmentation of the encapsulated packets (by ensuring an IPv4 MTU
at least 1300 bytes) or by preventing frequent changes to IPv
routing
Gilligan & Nordmark Standards Track [Page 19]
RFC 2893 IPv6 Transition Mechanisms August 2000
4.3. Ingress
The decapsulating node MUST verify that the tunnel source address
acceptable before forwarding decapsulated packets to
circumventing ingress filtering [13]. Note that packets which
delivered to transport protocols on the decapsulating node SHOULD
be subject to these checks. For bidirectional configured
this is done by verifying that the source address is the IPv4
of the other end of the tunnel. For unidirectional
tunnels the decapsulating node MUST be configured with a list
source IPv4 address prefixes that are acceptable. Such a list
default to not having any entries i.e. the node has to be
configured to forward decapsulated packets received
unidirectional configured tunnels
5. Automatic
In automatic tunneling, the tunnel endpoint address is determined
the IPv4-compatible destination address of the IPv6 packet
tunneled. Automatic tunneling allows IPv6/IPv4 nodes to
over IPv4 routing infrastructures without pre-configuring tunnels
5.1. IPv4-Compatible Address
IPv6/IPv4 nodes that perform automatic tunneling are assigned IPv4-
compatible address. An IPv4-compatible address is identified by
all-zeros 96-bit prefix, and holds an IPv4 address in the low-
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
IPv4-compatible addresses are assigned exclusively to nodes
support automatic tunneling. A node SHOULD be configured with
IPv4-compatible address only if it is prepared to accept IPv6
destined to that address encapsulated in IPv4 packets destined to
embedded IPv4 address
An IPv4-compatible address is globally unique as long as the IPv
address is not from the private IPv4 address space [15].
implementation SHOULD behave as if its IPv4-compatible address(es
are assigned to the node's automatic tunneling interface, even if
implementation does not implement automatic tunneling using a
of interfaces. Thus the IPv4-compatible address SHOULD NOT be
as being attached to e.g. an Ethernet interface i.e.
Gilligan & Nordmark Standards Track [Page 20]
RFC 2893 IPv6 Transition Mechanisms August 2000
should not use the Neighbor Discovery mechanisms like NUD [7] at
Ethernet. Any such interactions should be done using
encapsulated packets i.e. over the automatic tunneling (conceptual
interface
5.2. IPv4-Compatible Address
An IPv6/IPv4 node with an IPv4-compatible address uses that
as one of its IPv6 addresses, while the IPv4 address embedded in
low-order 32-bits serves as the IPv4 address for one of
interfaces
An IPv6/IPv4 node MAY acquire its IPv4-compatible IPv6 addresses
IPv4 address configuration protocols. It MAY use any IPv4
configuration mechanism to acquire its IPv4 address, then "map"
address into an IPv4-compatible IPv6 address by pre-pending it
the 96-bit prefix 0:0:0:0:0:0. This mode of configuration
IPv6/IPv4 nodes to "leverage" the installed base of IPv4
configuration servers
The specific algorithm for acquiring an IPv4-compatible address
IPv4-based address configuration protocols is as follows
1) The IPv6/IPv4 node uses standard IPv4 mechanisms or protocols
acquire the IPv4 address for one of its interfaces.
include
- The Dynamic Host Configuration Protocol (DHCP) [2]
- The Bootstrap Protocol (BOOTP) [1]
- The Reverse Address Resolution Protocol (RARP) [9]
- Manual
- Any other mechanism which accurately yields the node's own IPv
2) The node uses this address as the IPv4 address for this interface
3) The node prepends the 96-bit prefix 0:0:0:0:0:0 to the 32-bit IPv
address that it acquired in step (1). The result is an IPv4-
compatible IPv6 address with one of the node's IPv4-
embedded in the low-order 32-bits. The node uses this address
one of its IPv6 addresses
Gilligan & Nordmark Standards Track [Page 21]
RFC 2893 IPv6 Transition Mechanisms August 2000
5.3. Automatic Tunneling
In automatic tunneling, the tunnel endpoint address is
from the packet being tunneled. If the destination IPv6 address
IPv4-compatible, then the packet can be sent via automatic tunneling
If the destination is IPv6-native, the packet can not be sent
automatic tunneling
A routing table entry can be used to direct automatic tunneling.
implementation can have a special static routing table entry for
prefix 0:0:0:0:0:0/96. (That is, a route to the all-zeros
with a 96-bit mask.) Packets that match this prefix are sent to
pseudo-interface driver which performs automatic tunneling.
all IPv4-compatible IPv6 addresses will match this prefix,
packets to those destinations will be auto-tunneled
Once it is delivered to the automatic tunneling module, the IPv
packet is encapsulated within an IPv4 header according to the
described in section 3. The source and destination addresses of
encapsulating IPv4 header are assigned as follows
Destination IPv4 address
Low-order 32-bits of IPv6 destination
Source IPv4 address
IPv4 address of interface the packet is sent
The automatic tunneling module always sends packets in
encapsulated form, even if the destination is on an
datalink
The automatic tunneling module MUST NOT send to IPv4 broadcast
multicast destinations. It MUST drop all IPv6 packets destined
IPv4-compatible destinations when the embedded IPv4 address
broadcast, multicast, the unspecified (0.0.0.0) address, or
loopback address (127.0.0.1). Note that the sender can only tell
an address is a network or subnet broadcast for broadcast
assigned to directly attached links
5.4. Use With Default Configured
Automatic tunneling is often used in conjunction with the
configured tunnel technique. "Isolated" IPv6/IPv4 hosts --
with no on-link IPv6 routers -- are configured to use
tunneling and IPv4-compatible IPv6 addresses, and have at least
default configured tunnel to an IPv6 router. That IPv6 router
Gilligan & Nordmark Standards Track [Page 22]
RFC 2893 IPv6 Transition Mechanisms August 2000
configured to perform automatic tunneling as well. These
hosts send packets to IPv4-compatible destinations via
tunneling and packets for IPv6-native destinations via the
configured tunnel. IPv4-compatible destinations will match the 96-
bit all-zeros prefix route discussed in the previous section,
IPv6-native destinations will match the default route via
configured tunnel. Reply packets from IPv6-native destinations
routed back to the an IPv6/IPv4 router which delivers them to
original host via automatic tunneling. Further examples of
combination of tunneling techniques are discussed in [12].
5.5. Source Address
When an IPv6/IPv4 node originates an IPv6 packet, it must select
source IPv6 address to use. IPv6/IPv4 nodes that are configured
perform automatic tunneling may be configured with global IPv6-
addresses as well as IPv4-compatible addresses. The selection
which source address to use will determine what form the
traffic is sent via. If the IPv4-compatible address is used,
return traffic will have to be delivered via automatic tunneling,
if the IPv6-native address is used, the return traffic will not
automatic-tunneled. In order to make traffic as symmetric
possible, the following source address selection preference
RECOMMENDED
Destination is IPv4-compatible
Use IPv4-compatible source address associated with IPv4
of outgoing
Destination is IPv6-native
Use IPv6-native address of outgoing
If an IPv6/IPv4 node has no global IPv6-native address, but
originating a packet to an IPv6-native destination, it MAY use
IPv4-compatible address as its source address
5.6. Ingress
The decapsulating node MUST verify that the encapsulated packets
acceptable before forwarding decapsulated packets to
circumventing ingress filtering [13]. Note that packets which
delivered to transport protocols on the decapsulating node SHOULD
be subject to these checks. Since automatic tunnels
encapsulate to the destination (i.e. the IPv4 destination will
the destination) any packet received over an automatic tunnel
NOT be forwarded
Gilligan & Nordmark Standards Track [Page 23]
RFC 2893 IPv6 Transition Mechanisms August 2000
6.
We would like to thank the members of the IPng working group and
Next Generation Transition (ngtrans) working group for their
contributions and extensive review of this document. Special
are due to Jim Bound, Ross Callon, and Bob Hinden for many
suggestions and to John Moy for suggesting the IPv4 "anycast address
default tunnel technique
7. Security
Tunneling is not known to introduce any security holes except for
possibility to circumvent ingress filtering [13]. This is
by requiring that decapsulating routers only forward packets if
have been configured to accept encapsulated packets from the IPv
source address in the receive packet. Additionally, in the case
automatic tunneling, nodes are required by not forwarding
decapsulated packets since automatic tunneling ends the tunnel
the destination
8. Authors'
Robert E.
FreeGate
1208 E. Arques
Sunnyvale, CA 94086
Phone: +1-408-617-1004
Fax: +1-408-617-1010
EMail: gilligan@freegate.
Erik
Sun Microsystems, Inc
901 San Antonio Rd
Palo Alto, CA 94303
Phone: +1-650-786-5166
Fax: +1-650-786-5896
EMail: nordmark@eng.sun.
Gilligan & Nordmark Standards Track [Page 24]
RFC 2893 IPv6 Transition Mechanisms August 2000
9.
[1] Croft, W. and J. Gilmore, "Bootstrap Protocol", RFC 951,
September 1985.
[2] Droms, R., "Dynamic Host Configuration Protocol", RFC 1541,
October 1993.
[3] Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv
Domains without Explicit Tunnels", RFC 2529, March 1999.
[4] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[5] Thomson, S. and T. Narten, "IPv6 Stateless
Autoconfiguration," RFC 2462, December 1998.
[6] Crawford, M., Thomson, S., and C. Huitema. "DNS Extensions
Support IPv6 Address Allocation and Renumbering", RFC 2874,
2000.
[7] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery
IP Version 6 (IPv6)", RFC 2461, December 1998.
[8] Mogul, J. and S. Deering, "Path MTU Discovery", RFC 1191,
November 1990.
[9] Finlayson, R., Mann, T., Mogul, J. and M. Theimer, "
Address Resolution Protocol", STD 38, RFC 903, June 1984.
[10] Braden, R., "Requirements for Internet Hosts -
Layers", STD 3, RFC 1122, October 1989.
[11] Kent, C. and J. Mogul, "Fragmentation Considered Harmful".
Proc. SIGCOMM '87 Workshop on Frontiers in
Communications Technology. August 1987.
[12] Callon, R. and D. Haskin, "Routing Aspects of IPv6 Transition",
RFC 2185, September 1997.
[13] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Denial of Service Attacks which employ IP Source
Spoofing", RFC 2267, January 1998.
[14] Hinden, R. and S. Deering, "IP Version 6
Architecture", RFC 2373, July 1998.
Gilligan & Nordmark Standards Track [Page 25]
RFC 2893 IPv6 Transition Mechanisms August 2000
[15] Rechter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.J.
E. Lear, "Address Allocation for Private Internets", BCP 5,
1918, February 1996.
[16] Bradner, S., "Key words for use in RFCs to Indicate
Levels", BCP 14, RFC 2119, March 1997.
[17] Thaler, D., "IP Tunnel MIB", RFC 2667, August 1999.
[18] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812,
June 1995.
10. Changes from RFC 1933
- Deleted section 3.1.1 (IPv4 loopback address) in order to
it from being mis-construed as requiring routers to filter
address ::127.0.0.1, which would put another test in
forwarding path for IPv6 routers
- Deleted section 4.4 (Default Sending Algorithm). This
allowed nodes to send packets in "raw form" to IPv4-
destinations on the same datalink. Implementation experience
shown that this adds complexity which is not justified by
minimal savings in header overhead
- Added definitions for operating modes for IPv6/IPv4 nodes
- Revised DNS section to clarify resolver filtering and
options
- Re-wrote the discussion of IPv4-compatible addresses to
that they are used exclusively in conjunction with the
tunneling mechanism. Re-organized document to place definition
IPv4-compatible address format with description of
tunneling
- Changed the term "IPv6-only address" to "IPv6-native address"
current usage
- Updated to algorithm for determining tunnel MTU to reflect
change in the IPv6 minimum MTU from 576 to 1280 bytes [4].
- Deleted the definition for the term "IPv6-in-IPv4 encapsulation."
It has not been widely used
- Revised IPv4-compatible address configuration section (5.2)
recognize multiple interfaces
Gilligan & Nordmark Standards Track [Page 26]
RFC 2893 IPv6 Transition Mechanisms August 2000
- Added discussion of source address selection when using IPv4-
compatible addresses
- Added section on the combination of the default
tunneling technique with hosts using automatic tunneling
- Added prohibition against automatic tunneling to IPv4 broadcast
multicast destinations
- Clarified that configured tunnels can be unidirectional
bidirectional
- Added description of bidirectional virtual links as another
of tunnels. Nodes MUST respond to NUD probes on such links
SHOULD send NUD probes
- Added reference to [16] specification as an alternative
tunneling over a multicast capable IPv4 cloud
- Clarified that IPv4-compatible addresses are assigned
to nodes that support automatic tunnels i.e. nodes that
receive such packets
- Added text about formation of link-local addresses and use
Neighbor Discovery on tunnels
- Added restriction that decapsulated packets not be
unless the source address is acceptable to the
router
- Clarified that decapsulating nodes MUST be capable of
an IPv4 packet that is 1300 bytes (1280 bytes plus IPv4 header).
- Clarified that when using a default tunnel to an IPv4 "
address" the network must either have an IPv4 MTU of least 1300
bytes (to avoid fragmentation of minimum size IPv6 packets) or
configured to avoid frequent changes to IPv4 routing to
"anycast address" (to avoid different IPv4 fragments arriving
different tunnel endpoints).
- Using A6/AAAA instead of AAAA to reference IPv6 address records
the DNS
- Specified when to put IPv6 addresses in the DNS
- Added reference to the tunnel mib for TTL specification for
tunnels
Gilligan & Nordmark Standards Track [Page 27]
RFC 2893 IPv6 Transition Mechanisms August 2000
- Added a table of contents
- Added recommendations for use of source and target link
address options for the tunnel links
- Added checks in the decapsulation checking both an IPv4-
IPv6 source address and the outer IPv4 source addresses
multicast, broadcast, all-zeros etc
Gilligan & Nordmark Standards Track [Page 28]
RFC 2893 IPv6 Transition Mechanisms August 2000
11. Full Copyright
Copyright (C) The Internet Society (2000). All Rights Reserved
This document and translations of it may be copied and furnished
others, and derivative works that comment on or otherwise explain
or assist in its implementation may be prepared, copied,
and distributed, in whole or in part, without restriction of
kind, provided that the above copyright notice and this paragraph
included on all such copies and derivative works. However,
document itself may not be modified in any way, such as by
the copyright notice or references to the Internet Society or
Internet organizations, except as needed for the purpose
developing Internet standards in which case the procedures
copyrights defined in the Internet Standards process must
followed, or as required to translate it into languages other
English
The limited permissions granted above are perpetual and will not
revoked by the Internet Society or its successors or assigns
This document and the information contained herein is provided on
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED,
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE
Funding for the RFC Editor function is currently provided by
Internet Society
Gilligan & Nordmark Standards Track [Page 29]
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