WDIFF

draft-ietf-v6ops-mech-v2-00.txt --> draft-ietf-v6ops-mech-v2-02.txt

INTERNET-DRAFT E. Nordmark
February 24, 2003
January 30, 2004 Sun Microsystems, Inc.
Obsoletes: 2893 R. E. Gilligan
Intransa, Inc.

Basic Transition Mechanisms for IPv6 Hosts and Routers
<draft-ietf-v6ops-mech-v2-00.txt>
<draft-ietf-v6ops-mech-v2-02.txt>

Status of this Memo

This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

This draft expires on August 24, 2003. July 30, 2004.

Abstract

This document specifies IPv4 compatibility mechanisms that can be
implemented by IPv6 hosts and routers. These Two mechanisms include are specified,
"dual stack" and configured tunneling. Dual stack implies providing
complete implementations of both versions of the Internet Protocol
(IPv4 and IPv6), IPv6) and configured tunneling provides a means to carry
IPv6 packets over unmodified 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 2893.

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INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 January 2004

Contents

Status of this Memo.......................................... 1

1. Introduction............................................. 3
1.1. Terminology......................................... 3
1.2. Structure of this Document.......................... 5

2. Dual IP Layer Operation.................................. 5
2.1. Address Configuration............................... 6 5
2.2. DNS................................................. 6
2.3. Advertising Addresses in the DNS.................... 7 5

3. Common Configured Tunneling Mechanisms.............................. 8 Mechanisms.......................... 7
3.1. Encapsulation....................................... 10 8
3.2. Tunnel MTU and Fragmentation........................ 11 9
3.2.1. Static Tunnel MTU.............................. 10
3.2.2. Dynamic Tunnel MTU............................. 10
3.3. Hop Limit........................................... 13 12
3.4. Handling IPv4 ICMP errors........................... 13 ICMPv4 errors.............................. 12
3.5. IPv4 Header Construction............................ 14
3.6. Decapsulation....................................... 16 15
3.7. Link-Local Addresses................................ 18
3.8. Neighbor Discovery over Tunnels..................... 18
3.9. Ingress Filtering................................... 19

4. Configured Tunneling..................................... 20
4.1. Ingress Filtering................................... 20 Threat Related to Source Address Spoofing................ 19

5. Acknowledgments.......................................... 21

6. Security Considerations.................................. 21 20

6. Acknowledgments.......................................... 22

7. Authors' Addresses....................................... 21

8. References............................................... 22
8.1.
7.1. Normative References................................ 22
8.2.
7.2. Non-normative References............................ 22

8. Authors' Addresses....................................... 24

9. Changes from RFC 2893.................................... 24

<draft-ietf-v6ops-mech-v2-00.txt> 25
9.1. Changes from draft-ietf-v6ops-mech-v2-00............ 27
9.2. Changes from draft-ietf-v6ops-mech-v2-01............ 28

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INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 January 2004

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
two 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.

The mechanisms specified here include: are:

- 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 A technique for
establishing point-to-point tunnels
made by encapsulating IPv6
packets within IPv4 headers to carry them over IPv4 routing
infrastructures.

The mechanisms defined here are intended to be the core 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 basic set of transition mechanisms, but
these are not the only tools available. Additional transition and
compatibility mechanisms are specified in other documents.

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

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of IPv4 hosts and routers existing before the transition

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INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002
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. in this memo.

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 no longer used by
this specification, thus this definition is preserved in
the specification merely to clarify their non-use.

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 encapsulator. All tunnels can are assumed to be either
unidirectional or bidirectional. Bidirectional

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INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002

configured tunnels behave
bidirectional, behaving as virtual point-to-point links.

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

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 [RFC2119].

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.

- Section 3 discusses the common mechanisms used in some IPv6-
over-IPv4 tunneling techniques, including configured tunneling.

- Section 4 discusses configured tunneling.

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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. Here we use
a rather loose notion of "stack". A stack being enabled has IP
addresses assigned etc, but whether or not any particular application
is available on the stacks is explicitly not defined. 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.

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- 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 IPv6-over-IPv4 configured tunneling techniques, technique, which are is described in
sections 3 and 4, section 3,
may or may not be used in addition to the dual IP layer operation. An IPv6/IPv4 node MAY support configured tunneling.

2.1. Address Configuration

Because they the nodes support both protocols, IPv6/IPv4 nodes may be
configured with both IPv4 and IPv6 addresses. IPv6/IPv4 nodes use
IPv4 mechanisms (e.g., DHCP) to acquire their IPv4 addresses, and
IPv6 protocol mechanisms (e.g., stateless address autoconfiguration
and/or DHCPv6) to acquire their IPv6 addresses.

2.2. DNS

The Domain Naming System (DNS) is used in both IPv4 and IPv6 to map
between hostnames and IP addresses. A new resource record type named

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"AAAA" has been defined for IPv6 addresses [RFC1886]. [RFC3596]. Since
IPv6/IPv4 nodes must be able to interoperate directly with both IPv4
and IPv6 nodes, they must provide resolver libraries capable of
dealing with IPv4 "A" records as well as IPv6 "AAAA" records. Note
that the lookup of A versus AAAA records is independent of whether
the DNS packets are carried in IPv4 or IPv6 packets. packets, and that there
is no assumption that the DNS server know the IPv4/IPv6 capabilities
of the requesting node.

The issues and operational guidelines for using IPv6 with DNS are
described at more length in other documents [DNSOPV6].

DNS resolver libraries on IPv6/IPv4 nodes MUST be capable of handling
both AAAA and A records. However, when a query locates an AAAA
record holding an IPv6 address, and an A record holding an IPv4
address, the resolver library MAY filter or order the results
returned to the application in order to influence the version of IP
packets used to communicate with that node. In terms of filtering,
the resolver library has three alternatives:

- Return only the IPv6 address(es) to the application.

- Return only the IPv4 address(es) to the application.

- Return both types of addresses to the application.

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If it returns only the IPv6 address(es), the application will
communicate with the node using IPv6. If it returns only the IPv4
address(es), the application will communicate with the node using
IPv4. If it returns both types of addresses, the application will
have the choice which address to 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 addresses
in the order they are returned by the resolver, this can affect the
IP version "preference" of applications.

A resolver library performing filtering or ordering of addresses
might also want to take into account external factors such as,
whether IPv6 interfaces have been configured on the node.

The decision to filter or order DNS results is implementation
specific. IPv6/IPv4 nodes MAY provide policy configuration to
control filtering or ordering of addresses returned by the resolver, resolver
-- i.e., which addresses to filter or which order to sort -- or leave
the decision entirely up to the application.

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An implementation MUST allow the application to control whether or
not such filtering takes place.

More details on this subject are specified in [RFC3484].

2.3. Advertising Addresses in the DNS

There are some constraint placed on the use relative preferences of the DNS during
transition. The constraints allow nodes to prefer either IPv6 or IPv4 and IPv6 addresses when both types of addresses are returned by the DNS.
Most of these are obvious but
are stated here for completeness.

The recommendation is that AAAA records for a node should not be
added to specified in the DNS until all of these are true:

1) The default address is assigned to selection document [RFC3484].

3. Configured Tunneling Mechanisms

In most deployment scenarios, the interface on IPv6 routing infrastructure will be
built up over time. While the node.

2) The address IPv6 infrastructure is configured on being deployed,
the interface.

3) The interface is on a link which is connected existing IPv4 routing infrastructure can remain functional, and
can be used to the carry IPv6
infrastructure.

If traffic. Tunneling provides a way to
utilize an existing IPv4 routing infrastructure to carry IPv6 node is isolated from an
traffic.

IPv6/IPv4 hosts and routers can tunnel IPv6 perspective (e.g., it is not
connected to datagrams over regions of
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 IPv4
infrastructure can tunnel IPv6 packets between themselves. In
this case, the tunnel spans one segment of the end-to-end path
that the 6bone IPv6 packet takes.

- Host-to-Router. IPv6/IPv4 hosts can tunnel IPv6 packets to take a concrete example) constraint #3
would mean an
intermediary IPv6/IPv4 router that it should not have is reachable via an address in the DNS. IPv4
infrastructure. This works great when other dual stack nodes try to contact type of tunnel spans the
isolated dual stack node. There is no first segment of
the packet's end-to-end path.

- Host-to-Host. IPv6/IPv4 hosts that are interconnected by an
IPv4 infrastructure can tunnel IPv6 address in packets between themselves.
In this case, the DNS thus tunnel spans the peer doesn't even try communicating using entire end-to-end path that
the packet takes.

- Router-to-Host. IPv6/IPv4 routers can tunnel IPv6 packets to
their final destination IPv6/IPv4 host. This tunnel spans only
the last segment of the end-to-end path.

Configured tunneling can be used in all of the above cases, but goes directly

<draft-ietf-v6ops-mech-v2-00.txt> is
most likely to be used router-to-router due to the need to explicitly
configure the tunneling endpoints.

The underlying mechanisms for tunneling are:

- The entry node of the tunnel (the encapsulator) creates an

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to January 2004

encapsulating IPv4 (we are assuming both nodes have A records in the DNS.)

However, this does not work well when header and transmits the isolated encapsulated packet.

- The exit node is trying to
establish communication. Even though it does not have an IPv6
address in of the DNS it will find AAAA records in the DNS for tunnel (the decapsulator) receives the peer.
Since
encapsulated packet, reassembles the isolated node has IPv6 addresses assigned to at least one
interface it will try to communicate using IPv6. If it has no IPv6
route to packet if needed, removes
the 6bone (e.g., because IPv4 header, and processes the local router was upgraded to
advertise IPv6 addresses using Neighbor Discovery but that router
doesn't have any received IPv6 routes) this communication will fail.
Typically this means a few minutes of delay as TCP times out. packet.

- The
TCP specification [RFC1122] says that ICMP unreachable messages could
be due encapsulator may need to routing transients thus they should not immediately
terminate the TCP connection. This means that maintain soft state information for
each tunnel recording such parameters as the normal TCP timeout MTU of a few minutes apply. Once TCP times out the application will
hopefully try tunnel
in order to process IPv6 packets forwarded into the IPv4 addresses based on tunnel.

In configured tunneling, the A records tunnel endpoint address is determined
from configuration information in the DNS,
but this will be painfully slow.

A possible implication of encapsulator. For each tunnel,
the recommendations above is that, if one
enables encapsulator must store the tunnel endpoint address. When an
IPv6 on a node on packet is transmitted over a link without IPv6 infrastructure, and
choose to add AAAA records to tunnel, the DNS tunnel endpoint address
configured for that node, then external
IPv6 nodes that might see these AAAA records will possibly try to
reach that node using IPv6 and suffer delays or communication failure
due to unreachability. (A delay tunnel is incurred if used as the application
correctly falls back to using IPv4 if it can not establish
communication using IPv6 addresses. If this fallback is not done destination address for the
application would fail
encapsulating IPv4 header.

The determination of which packets to communicate in this case.) Thus it tunnel is
suggested that either the recommendations be followed, or care be
taken to only do so with nodes that will not be impacted usually made by external
accessing delays and/or communication failure.

In
routing information on the future, when encapsulator. This is usually done via a node discontinues its use of IPv4, analogous
constraints apply with respect to the node's A records in the DNS;
the removal of the A records should be tied to when the node can no
longer be reached using IPv4.

3. Common Tunneling Mechanisms

In most deployment scenarios, the IPv6
routing infrastructure will be
built up over time. While table, which directs packets based on their destination
address using the prefix mask and match technique.

3.1. Encapsulation

The encapsulation of an IPv6 infrastructure datagram in IPv4 is being deployed,
the existing shown below:

Encapsulating IPv6 in IPv4

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IPv6/IPv4 hosts and routers can tunnel IPv6 datagrams over regions of
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 January 2004

In addition to adding an IPv4
infrastructure can tunnel IPv6 packets between themselves. In
this case, the tunnel spans one segment of the end-to-end path
that header, the IPv6 packet takes.

- Host-to-Router. IPv6/IPv4 hosts can tunnel IPv6 packets encapsulator also has to
handle some more complex issues:

- Determine when to fragment and when to report an
intermediary IPv6/IPv4 router that is reachable via an IPv4
infrastructure. This type of tunnel spans the first segment of ICMPv6 "packet
too big" error back to the packet's end-to-end path. source.

- Host-to-Host. IPv6/IPv4 hosts that are interconnected by an
IPv4 infrastructure can tunnel IPv6 packets between themselves.
In this case, How to reflect ICMPv4 errors from routers along the tunnel spans the entire end-to-end path that
back to the packet takes.

- Router-to-Host. IPv6/IPv4 routers can tunnel source as ICMPv6 errors.

Those issues are discussed in the following sections.

3.2. Tunnel MTU and Fragmentation

Naively the encapsulator could view encapsulation as IPv6 packets using IPv4
as a link layer with a very large MTU (65535-20 bytes to
their final destination IPv6/IPv4 host. This tunnel spans only be exact; 20
bytes "extra" are needed for the last segment of encapsulating IPv4 header). The
encapsulator would only need to report ICMPv6 "packet too big" errors
back to the end-to-end path.

Tunneling techniques are usually classified according source for packets that exceed this MTU. However, such a
scheme would be inefficient or non-interoperable for three reasons
and therefore MUST NOT be used:

1) It would result in more fragmentation than needed. IPv4 layer
fragmentation should be avoided due to the
mechanism performance problems
caused by which the encapsulating node determines the address of
the node at loss unit being smaller than the end of retransmission
unit [KM97].

2) Any IPv4 fragmentation occurring inside the tunnel. In tunnel, i.e. between
the first two tunneling
methods listed above -- router-to-router encapsulator and host-to-router -- the
IPv6 packet is being tunneled decapsulator, would have to be
reassembled at the tunnel endpoint. For tunnels that terminate
at a router. The endpoint of router, this type
of tunnel is an intermediary router which must decapsulate the IPv6
packet would require additional memory and forward it on to its final destination. When tunneling other
resources to
a router, the endpoint of the tunnel is different from the
destination of the packet being tunneled. In some cases, the
addresses in reassemble the IPv4 fragments into a complete IPv6
packet being tunneled can not provide the IPv4
address of the tunnel endpoint. In those cases, the tunnel endpoint
address must before that packet could be determined from configuration information on the node
performing the encapsulation. We use the term "configured tunneling"
to describe the type forwarded onward.

3) The encapsulator has no way of tunneling where knowing that the endpoint decapsulator is explicitly
configured.

In the last two tunneling methods -- host-to-host
able to defragment such IPv4 packets (see Section 3.7 for
details), and router-to-host
-- has no way of knowing that the IPv6 packet decapsulator is tunneled all the way
able to its final destination.
In this case, handle such a large IPv6 Maximum Receive Unit (MRU).

Hence, the destination address encapsulator MUST NOT treat the tunnel as an interface
with an MTU of both 64 kilobytes, but instead either use the IPv6 packet and fixed static
MTU or OPTIONAL dynamic MTU determination based on the
encapsulating IPv4 header identify the same node. However, path MTU
to the
tunneling mechanism specified in this document does not handle these
cases any differently; tunnel endpoint.

If both the IPv4 addresses is still determined using
configuration information using configured tunneling.

The underlying mechanisms for tunneling are:

<draft-ietf-v6ops-mech-v2-00.txt> are implemented, the decision which to use
SHOULD be configurable on a per-tunnel endpoint basis.

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- The entry January 2004

3.2.1. Static Tunnel MTU

A node of the using static tunnel (the encapsulating node) creates an
encapsulating IPv4 header and transmits the encapsulated packet.

- The exit node of MTU treats the tunnel (the decapsulating node) receives
the encapsulated packet, reassembles the packet if needed,
removes the IPv4 header, updates interface as having
a fixed interface MTU. By default, the IPv6 header, MTU MUST be between 1280 and processes
1480 bytes (inclusive), but it SHOULD be 1280 bytes. If the received IPv6 packet.

- The encapsulating node MAY need default
is not 1280 bytes, the implementation MUST have a configuration knob
which can be used to maintain soft state
information for each tunnel recording such parameters as change the MTU
of the tunnel in order value.

A node must be able to process accept a fragmented IPv6 packets forwarded into
the tunnel. In cases where packet that, after
reassembly, is as large as 1500 octets [RFC2460]. This memo also
includes requirements (see Section 3.6) for the number amount of tunnels IPv4
reassembly and IPv6 MRU that MUST be supported by all the
decapsulators. These ensure correct interoperability with any one
host or router is using is large, fixed
MTUs between 1280 and 1480 bytes.

A larger fixed MTU than supported by these requirements, must not be
configured unless it is helpful to observe has been administratively ensured that
this state information the
decapsulator can be cached and discarded when not in
use. reassemble or receive packets of that size.

The remainder selection of this section discusses a good tunnel MTU depends on many factors; at least:

- Whether the common mechanisms. A
subsequent section discusses how IPv4 protocol-41 packets will be transported over
media which may have a lower path MTU (e.g., IPv4 Virtual
Private Networks); then picking too high a value might lead to
IPv4 fragmentation.

- Whether the tunnel endpoint address is
determined for configured tunneling.

3.1. Encapsulation

The encapsulation of an used to transport IPv6 datagram in IPv4 is shown below:

+-------------+
| IPv4 |
| Header |
+-------------+ +-------------+
| IPv6 | | IPv6 |
| Header | | Header |
+-------------+ +-------------+
| Transport | | Transport |
| Layer | ===> | Layer |
| Header | | Header |
+-------------+ +-------------+
| | | |
~ Data ~ ~ Data ~
| | | |
+-------------+ +-------------+

Encapsulating IPv6 in IPv4

In addition to adding an IPv4 header, the encapsulating tunneled packets
(e.g., a mobile node also has
to handle some more complex issues:

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- Determine when to fragment with an IPv4-in-IPv6 configured tunnel, and when to report
an ICMP "packet IPv6-in-IPv6 tunnel interface); then picking too big" error back low a value
might lead to the source.

- How IPv6 fragmentation.

If layered encapsulation is believed to reflect IPv4 ICMP errors from routers along the tunnel
path back be present, it may be prudent
to the source as IPv6 ICMP errors.

Those issues are discussed in the following sections.

3.2. Tunnel consider supporting dynamic MTU and Fragmentation

The encapsulating node could view encapsulation determination instead as IPv6 it is
able to minimize fragmentation and optimize packet sizes.

When using IPv4 as
a link layer with a very large the static tunnel MTU (65535-20 bytes to the Don't Fragment bit MUST NOT be exact; 20
bytes "extra" are needed for
set in the encapsulating IPv4 header). The
encapsulating node would need only to report IPv6 ICMP header. As a result the encapsulator
should not receive any ICMPv4 "packet too big" errors back to messages as a result
of the source for packets that exceed this MTU. it has encapsulated.

3.2.2. Dynamic Tunnel MTU

The dynamic MTU determination is OPTIONAL. However, such a scheme would be inefficient for two reasons and if it is
therefor NOT RECOMMENDED:

1) It would result in more fragmentation than needed. IPv4 layer
fragmentation
implemented, it SHOULD be avoided due to the performance problems
caused by the loss unit being smaller than the retransmission
unit [KM97].

2) Any IPv4 fragmentation occurring inside the tunnel, i.e. between
the encapsulating node and the decapsulating node, would have to
be reassembled at the tunnel endpoint. For tunnels that
terminate at a router, behavior described in this would require additional memory to
reassemble the IPv4 fragments into a complete IPv6 packet before
that packet could be forwarded onward.

Hence, the encapsulating node MUST NOT treat the tunnel as an
interface with an MTU of 64 kilobytes, but instead use the smaller
MTU specified below. document.

The fragmentation inside the tunnel can be reduced to a minimum by

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INTERNET DRAFT Basic IPv6 Transition Mechanisms January 2004

having the encapsulating node encapsulator track the IPv4 Path MTU across the tunnel,
using the IPv4 Path MTU Discovery Protocol [RFC1191] and recording
the resulting path MTU. The IPv6 layer in the
encapsulating node encapsulator can then
view a tunnel as a link layer with an MTU equal to the IPv4 path MTU,
minus the size of the encapsulating IPv4 header.

Note that this does not eliminate IPv4 fragmentation in the case when
the IPv4 path MTU would result in an IPv6 MTU less than 1280 bytes.
(Any link layer used by IPv6 has to have an MTU of at least 1280
bytes [RFC2460].) In this case the IPv6 layer has to "see" a link

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INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002
layer with an MTU of 1280 bytes and the encapsulating node encapsulator has to use IPv4
fragmentation in order to forward the 1280 byte IPv6 packets.

This dynamic MTU determination is OPTIONAL. However, if it is
implemented it

The encapsulator SHOULD have the behavior described in this document.
If it is not implemented instead the node MUST instead limit the size
of employ the IPv6 packets it tunnels following algorithm to 1280 bytes i.e., treat the tunnel
interface as having a fixed interface MTU of 1280 bytes. An
implementation MAY have a configuration knob which can be used determine
when to set
a forward an IPv6 packet that is larger value of the tunnel MTU than 1280 bytes, but if so the
default MUST be 1280 bytes. A larger fixed MTU should not be
configured unless it has been administratively ensured that the
decapsulating node can reassemble packets of that size.

The encapsulating node SHOULD employ the following algorithm to
determine when to forward an IPv6 packet that is larger than the
tunnel's path tunnel's path
MTU using IPv4 fragmentation, and when to return an
IPv6 ICMP ICMPv6 "packet
too big" message per [RFC1981]:

if (IPv4 path MTU - 20) is less than or equal to 1280
if packet is larger than 1280 bytes
Send IPv6 ICMP ICMPv6 "packet too big" with MTU = 1280.
Drop packet.
else
Encapsulate but do not set the Don't Fragment
flag in the IPv4 header. The resulting IPv4
packet might be fragmented by the IPv4 layer on
the encapsulating node encapsulator or by some router along
the IPv4 path.
endif
else
if packet is larger than (IPv4 path MTU - 20)
Send IPv6 ICMP ICMPv6 "packet too big" with
MTU = (IPv4 path MTU - 20).
Drop packet.
else
Encapsulate and set the Don't Fragment flag
in the IPv4 header.
endif
endif

Encapsulating nodes

Encapsulators that have a large number of tunnels might not be
able to store the IPv4 Path may choose between
dynamic versus static tunnel MTU for all tunnels. Such nodes can, at on a per-tunnel endpoint basis. In
cases where the expense number of additional fragmentation in the network, avoid tunnels that any one node is using
the IPv4 Path MTU algorithm across the tunnel is
large, it is helpful to observe that this state information can be
cached and instead use the MTU
of the link layer (under IPv4) discarded when not in the above algorithm instead of the
IPv4 path MTU. In use.

Note that case the IPv6 MTU for the using dynamic tunnel MUST be

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INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002

limited to 1280 unless it has explicitly been configured to be
larger.

In this case January 2004

should the Don't Fragment bit MUST NOT ICMPv4 "packet too big" messages be set in dropped by firewalls
or not generated by the
encapsulating IPv4 header. routers. [RFC1435, RFC2923]

3.3. Hop Limit

IPv6-over-IPv4 tunnels are modeled as "single-hop". That is, "single-hop" from the IPv6 hop limit is decremented by 1 when an IPv6 packet traverses the
tunnel. The single-hop model serves to hide the existence of a
tunnel.
perspective. The tunnel is opaque to users of the network, and is not
detectable by network diagnostic tools such as traceroute.

The single-hop model is implemented by having the encapsulating encapsulators and
decapsulating nodes
decapsulators process the IPv6 hop limit field as they would if 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 hop
limit.)

The TTL of the encapsulating IPv4 header is selected in an
implementation dependent manner. The current suggested value is
published in the "Assigned Numbers" RFC [RFC3232][ASSIGNED]. The
implementations MAY also consider using the value 255, as it could be
used as a hint in the decapsulation checks in the future [GTSM].
Implementations MAY provide a mechanism to allow the administrator to
configure the IPv4 TTL such as the one specified in the IP Tunnel MIB [RFC2667].

3.4. Handling IPv4 ICMP ICMPv4 errors

In response to encapsulated packets it has sent into the tunnel, the
encapsulating node
encapsulator might receive IPv4 ICMP ICMPv4 error messages from IPv4 routers
inside the tunnel. These packets are addressed to the
encapsulating node encapsulator
because it is the IPv4 source of the encapsulated packet.

ICMPv4 error handling is only applicable to dynamic MTU
determination, even though the functions could be used with static
MTU tunnels as well.

The ICMP ICMPv4 "packet too big" error messages are handled according to
IPv4 Path MTU Discovery [RFC1191] and the resulting path MTU is
recorded in the IPv4 layer. The recorded path MTU is used by IPv6 to
determine if an IPv6 ICMP ICMPv6 "packet too big" error has to be generated as
described in section 3.2. 3.2.2.

The handling of other types of ICMP ICMPv4 error messages depends on how
much information is included in the "packet in error" field, which

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holds the encapsulated packet that caused the error.

Many older IPv4 routers return only 8 bytes of data beyond the IPv4
header of the packet in error, which is not enough to include the
address fields of the IPv6 header. More modern IPv4 routers are
likely to return enough data beyond the IPv4 header to include the
entire IPv6 header and possibly even the data beyond that.

If the offending packet includes enough data, the encapsulating node encapsulator MAY
extract the encapsulated IPv6 packet and use it to generate an
IPv6 ICMP ICMPv6
message directed back to the originating IPv6 node, as shown below:

IPv4 ICMP

ICMPv4 Error Message Returned to Encapsulating Node

When receiving ICMPv4 errors as above and the errors are not "packet
too big" it would be useful to log the error as an error related to
the tunnel. Also, if sufficient headers are included in the error,
then the originating node MAY send an ICMPv6 error of type
"unreachable" with code "address unreachable" to the IPv6 source.
(The "address unreachable" code is appropriate since, from the
perspective of IPv6, the tunnel is a link and that code is used for
link-specific errors [RFC2463]).

Note that when IPv4 path MTU is exceeded, and ICMPv4 errors of only 8

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INTERNET DRAFT Basic IPv6 Transition Mechanisms January 2004

bytes of payload are generated, or ICMPv4 errors do not cause the
generation of ICMPv6 errors in case there is enough payload, there
will be at least two packet drops instead of at least one (the case
of a single layer of MTU discovery). Consider a case where an IPv6
host is connected to an IPv4/IPv6 router, which is connected to a
network where an ICMPv4 error about too big packet size is generated.
First the router needs to learn the tunnel (IPv4) MTU which causes at
least one packet loss, and then the host needs to learn the (IPv6)
MTU from the router which causes at least one packet loss. Still, in
all cases there can be more than one packet loss if there are
multiple large packets in flight at the same time.

3.5. IPv4 Header Construction

When encapsulating an IPv6 packet in an IPv4 datagram, the IPv4
header fields are set as follows:

Version:

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INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002

IP Header Length in 32-bit words:

5 (There are no IPv4 options in the encapsulating
header.)

Type of Service:

0 unless otherwise specified. (See [RFC2983] and
[RFC3168] section 9.1 for issues relating to the ToS Type-
of-Service byte and tunneling.)

Total Length:

Payload length from IPv6 header plus length of IPv6 and
IPv4 headers (i.e., IPv6 payload length plus a constant
60 bytes).

Identification:

Generated uniquely as for any IPv4 packet transmitted by
the system.

Flags:

Set the Don't Fragment (DF) flag as specified in section

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3.2. Set the More Fragments (MF) bit as necessary if
fragmenting.

Fragment offset:

Set as necessary if fragmenting.

Time to Live:

Set in an implementation-specific manner.

Protocol:

41 manner, as described
in section 3.3.

Protocol:

41 (Assigned payload type number for IPv6) IPv6).

Header Checksum:

Calculate the checksum of the IPv4 header. [RFC791]

Source Address:

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INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002

IPv4 address of outgoing interface of the encapsulating
node. The source address MAY alternatively be encapsulator
or an administratively specified to be a specific IPv4 address
assigned to the encapsulating node. as described
below.

Destination Address:

IPv4 address of the tunnel endpoint.

Any IPv6 options

When encapsulating the packets, the nodes must ensure that they will
use the source address that the tunnel peer has configured, so that
the source addresses are preserved acceptable to the decapsulator. This may be
a problem with multi-addressed, and in particular, multi-interface
nodes, especially when the packet (after routing is changed from a stable
condition, as the IPv6 header). source address selection may be adversely affected.
Therefore, it SHOULD be possible to administratively specify the
source address of a tunnel.

3.6. Decapsulation

When an IPv6/IPv4 host or a router receives an IPv4 datagram that is
addressed to one of its own IPv4 address, addresses, and the value of the
protocol field is 41, it reassembles if the packet if it is potentially part of a tunnel and
needs to be verified to belong to one of the configured tunnel
interfaces (by checking source/destination addresses), reassembled

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(if fragmented at the IPv4 level, then it removes level), have the IPv4 header removed and
submits
the resulting IPv6 datagram be submitted to its the IPv6 layer code. code on
the node.

The decapsulating node decapsulator MUST be capable of reassembling an IPv4 packet verify that is the maximum of 1280 bytes and tunnel source address is
correct before further processing packets, to mitigate the largest interface MTU problems
with address spoofing (see section 4). This check also applies to
packets which are delivered to transport protocols on the
decapsulator. The 1280 byte number This is a result of encapsulators done by verifying that use the static MTU in section 3.2, while encapsulators that use source address is
the dynamic scheme in section 3.2 can cause up to IPv4 address of the largest
interface MTU other end of a tunnel configured on the decapsulator to be received. (Note that it is
strictly node.
Packets for which the interface MTU on IPv4 source address does not match MUST be
discarded and an ICMP message SHOULD NOT be generated; however, if
the implementation normally sends an ICMP message when receiving an
unknown protocol packet, such an error message MAY be sent (e.g.,
ICMPv4 Protocol 41 Unreachable).

A side effect of this address verification is that the node will
silently discard packets with a wrong source address, and packets
which were received by the node but not directly addressed to it
(e.g., broadcast addresses).

In addition, the node MAY perform ingress filtering [RFC2827] on the
IPv4 source address, i.e., check that the packet is arriving from the
interface in the direction of the route towards the tunnel end-point,
similar to a Strict Reverse Path Forwarding (RPF) check [BCP38UPD].
If done, it is RECOMMENDED that this check is disabled by default.
The packets caught by this check SHOULD be discarded; an ICMP message
SHOULD NOT be generated by default.

The decapsulator MUST be capable of having, on the tunnel interfaces,
an IPv6 MRU of at least the maximum of of 1500 bytes and the largest
(IPv6) interface MTU on the decapsulator.

The decapsulator MUST be capable of reassembling an IPv4 packet that
is (after the reassembly) the maximum of 1500 bytes and the largest
(IPv4) interface MTU on the decapsulator. The 1500 byte number is a
result of encapsulators that use the static MTU scheme in section
3.2.1, while encapsulators that use the dynamic scheme in section
3.2.2 can cause up to the largest interface MTU on the decapsulator
to be received. (Note that it is strictly the interface MTU on the
last IPv4 router *before* the decapsulator that matters, but for most
links the MTU is the same between all neighbors.)

This reassembly limit allows dynamic tunnel MTU determination by the
encapsulator to take advantage of larger IPv4 path MTUs. An
implementation MAY have a configuration knob which can be used to set
a larger value of the tunnel reassembly buffers than the above
number, but it MUST NOT be set below the above number.

The decapsulation is shown below:

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INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 January 2004

The decapsulation is shown below:

Decapsulating IPv6 from IPv4

When decapsulating the packet, the IPv6 header is not modified. (See
(However, see [RFC2983] and [RFC3168] section 9.1 for issues relating
to the Type of Service byte and tunneling.) If the packet is
subsequently forwarded, its hop limit is decremented by one.

As part of

The decapsulator performs IPv4 reassembly before decapsulating the decapsulation the node SHOULD silently discard a
packet with an invalid IPv4 source address such as a multicast
address, a broadcast address, 0.0.0.0, and 127.0.0.1. In general it
SHOULD apply the rules for martian filtering in [RFC1812] and ingress
filtering [RFC2267] on the IPv4 source address.
IPv6 packet.

The decapsulating node performs encapsulating IPv4 reassembly before decapsulating header is discarded. When reconstructing the
IPv6 packet. All packet the length MUST be determined from the IPv6 options are preserved even if payload
length since the
encapsulating IPv4 packet might be padded (thus have a length
which is fragmented.

The encapsulating larger than the IPv6 packet plus the IPv4 header is discarded. being
removed).

After the decapsulation the node SHOULD MUST silently discard a packet with
an invalid IPv6 source address. This includes IPv6 The list of invalid source addresses
SHOULD include at least:

- all multicast
addresses, the unspecified address, and addresses (FF00::/8)

- the loopback address but also (::1)

- all the IPv4-compatible IPv6 source addresses where the IPv4 part of [RFC3513] (::/96),
excluding the unspecified 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 martian
filtering in [RFC1812] and ingress filtering [RFC2267] on the IPv4
address embedded in IPv4-compatible source addresses.

After the IPv6 packet is decapsulated, it is processed almost Duplicate Address
Detection (::/128)

- all the
same as any received IPv4-mapped IPv6 packet. The difference being that a

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INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002

decapsulated packet MUST NOT be accepted (and delivered locally or
forwarded) unless January 2004

In addition, the node has been explicitly configured to accept
tunneled packets with should perform ingress filtering [RFC2827] on
the given IPv4 IPv6 source address. This
configuration can be implicit in e.g., having a bidirectional
configured address, similar to on any of its interfaces, e.g.:

- if the tunnel which matches is towards the Internet, check that the site's
IPv6 prefixes are not used as the IPv4 source address. This
restriction addresses, or

- if the tunnel is needed to prevent tunneling towards an edge network, check that the source
address belongs to be used as a tool to
circumvent ingress filtering [RFC2267] when ingress filtering is used
in IPv4 and IPv6 on both "sides" of the decapsulator. that edge network.

3.7. Link-Local Addresses

The configured tunnels are IPv6 interfaces (over the IPv4 "link
layer") and thus MUST have link-local addresses. The link-local
addresses are used by by, e.g., routing protocols operating over the
tunnels.

The Interface Identifier [RFC2373] interface identifier [RFC3513] for such an Interface SHOULD interface may be based
on the 32-bit IPv4 address of that an underlying interface, or formed
using some other means, as long as it's unique from the other tunnel
endpoint with a reasonably high probability.

If an IPv4 address is used for forming the bytes in IPv6 link-local address,
the same
order in which they would appear in interface identifier is the header of an IPv4 packet,
padded at the left with zeros to a total of 64 bits. address, prepended by zeros.
Note that the "Universal/Local" bit is zero, indicating that the Interface
Identifier
interface identifier is not globally unique. The link-local address
is formed by appending the interface identifier to the prefix
FE80::/64.

When the host has more than one IPv4 address in use on the physical
interface concerned, an administrative choice of one of these IPv4
addresses is made.

The IPv6 Link-local address [RFC2373] for an IPv4 virtual interface
is formed by appending the Interface Identifier, as defined above, to made when forming the prefix FE80::/64. link-local address.

+-------+-------+-------+-------+-------+-------+------+------+
| FE 80 00 00 00 00 00 00 |
+-------+-------+-------+-------+-------+-------+------+------+
| 00 00 | 00 | 00 | IPv4 Address |
+-------+-------+-------+-------+-------+-------+------+------+

3.8. Neighbor Discovery over Tunnels

For unidirectional configured tunnels most of Neighbor Discovery
[RFC2667] and Stateless Address Autoconfiguration [RFC2462] does not
apply; only the formation of the link-local address applies.

If an implementation provides bidirectional configured tunnels it

Configured tunnel implementations MUST at least accept and respond to
the probe packets used by Neighbor Unreachability Detection (NUD)

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INTERNET DRAFT Basic IPv6 Transition Mechanisms January 2004

[RFC2461]. Such The implementations SHOULD also send NUD probe packets to
detect when the configured

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INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 tunnel fails at which point the
implementation can use an alternate path to reach the destination.
Note that Neighbor Discovery allows that the sending of NUD probes be
omitted for router to router links if the routing protocol tracks
bidirectional reachability.

For the purposes of Neighbor Discovery the configured tunnels
specified in this document are assumed to NOT have a link-layer
address, even though the link-layer (IPv4) does have an address.
This means that a that:

- the sender of Neighbor Discovery packets

- SHOULD NOT include
Source Link Layer Address options or Target Link Layer Address
options on the tunnel link.

- MUST the receiver MUST, while otherwise processing the Neighbor
Discovery packet, silently ignore the content of any received neighbor discovery source link
layer address Source Link
Layer Address options or target link layer address Target Link Layer Address options
received over on the tunnel link.

Not using a link layer address options is consistent with how
neighbor discovery
Deighbor Discovery is used on other point-to-point links.

3.9. Ingress Filtering

4. Threat Related to Source Address Spoofing

The specification above contains rules that apply tunnel source
address verification in particular and ingress filtering
[RFC2827][BCP38UPD] in general to packets before they are
decapsulated. The purpose of ingress
filtering in general is specified in [RFC2267]. When IP-in-IP tunneling (independent of IP versions)
is used it is important that this can not be a tool used to bypass any
ingress filtering in use for non-
tunneled non-tunneled packets. Thus the rules in
this document are derived based on the assumption
that should ingress filtering be used
for IPv4 and IPv6, the use of tunneling should not provide an easy
way to circumvent the filtering.

In this case, without specific ingress filtering checks in the
decapsulating node,
decapsulator, it would be possible for an attacker to inject a packet
with:

- Outer IPv4 source: real IPv4 address of attacker

- Outer IPv4 destination: IPv4 address of decapsulating node decapsulator

- Inner IPv6 source: Alice which is either the decapsulating node decapsulator or a node close

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node close to it.

- Inner IPv6 destination: Bob

Even if all IPv4 routers between the attacker and the decapsulating
node decapsulator
implement IPv4 ingress filtering, and all IPv6 routers between the decapsulating node
decapsulator and Bob implement IPv6 ingress filtering, the

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INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 above
spoofed packets will not be filtered out unless out. As a result Bob will
receive a packet that looks like it was sent from Alice even though
the
decapsulator performs sender was some checks. unrelated node.

The solution to this is to have the decapsulating node perform
ingress filtering checks as part decapsulator only accept
encapsulated packets from the explicitly configured source address
(i.e., the other end of the decapsulation tunnel) as specified in section 4.1.

4. Configured Tunneling

In configured tunneling, the tunnel endpoint address is determined
from configuration information 3.6.
While this does not provide complete protection in the encapsulating node. For each
tunnel, the encapsulating node must store case ingress
filtering has not been deployed, it does provide a significant
increase in security. The issue and the remainder threats are
discussed at more length in Security Considerations.

5. Security Considerations

An implementation of tunneling needs to be aware that while a tunnel endpoint
address. When an IPv6 packet
is transmitted over a tunnel, link (as defined in [RFC2460]), the threat model for a tunnel endpoint address configured
might be rather different than for that other links, since the tunnel is used as
potentially includes all of the
destination address Internet.

Several mechanisms (e.g., Neighbor Discovery) depend on Hop Count
being 255 and/or the addresses being link-local for ensuring that a
packet originated on-link, in a semi-trusted environment. Tunnels
are more vulnerable to a breach of this assumption than physical
links, as an attacker anywhere in the encapsulating Internet can send an IPv6-in-
IPv4 header.

The determination packet to the tunnel decapsulator, causing injection of which packets an
encapsulted IPv6 packet to the configured tunnel is usually made by
routing information on interface unless the encapsulating node. This is usually done
via a routing table, which directs packets based on their destination
address using the prefix mask and match technique.

4.1. Ingress Filtering

The decapsulating node MUST verify that the tunnel source address is
acceptable before accepting decapsulated packets to avoid
circumventing ingress filtering [RFC2267]. This check also applies
decapsulation checks are able to discard packets which are delivered injected in such a
manner.

Therefore, this memo specifies strict checks to transport protocols on the
decapsulating node. For bidirectional configured tunnels mitigate this is
done by verifying that the source address is the threat:

- IPv4 source address of the
other end of the tunnel. For unidirectional configured tunnels the
decapsulating node packet MUST be the same as configured with a list of source IPv4
address prefixes that are acceptable. Such a list MUST default to
not having any entries i.e.,
for the node has to tunnel end-point,

- IPv4 ingress filtering MAY be explicitly configured implemented to forward decapsulated check that the IPv4
packets are received over unidirectional
configured tunnels.

<draft-ietf-v6ops-mech-v2-00.txt> from an expected interface,

- IPv6 packets with several, obviously invalid IPv6 source

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INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002

5. Acknowledgments

We would like January 2004

addresses MUST be discarded (see Section 3.6 for details), and

- IPv6 ingress filtering should be performed, to thank the members of check that the
IPv6 working group, the
Next Generation Transition (ngtrans) working group, and packets are received from an expected interface.

Especially the v6ops
working group for their many contributions and extensive review of first verification is vital: to avoid this document. Special thanks are due check, the
attacker must be able to Jim Bound, Ross Callon, Bob
Hinden, John Moy, know the source of the tunnel (difficult)
and Pekka Savola for many helpful suggestions.

6. Security Considerations

Tunneling is not known be able to introduce any security holes except for spoof it (easier).

If the
possibility remainder threats of tunnel source verification are considered
to circumvent ingress filtering [RFC2267]. This
specification prevent be significant, a tunneling from introducing additional
weaknesses when IPv4 and/or IPv6 ingress filtering is in scheme with authentication should be
used by
requiring that decapsulating nodes only accept packets if they have
been configured to accept encapsulated packets from the IPv4 source
address in the received packet. Such a check is easy to perform for
bidirectional tunnels, but instead, for uni-directional tunnels it requires example IPsec [RFC2401] (preferable) or Generic
Routing Encapsulation with a
separate configuration of pre-configured secret key [RFC2890]. As
the IPv4 source addresses that configured tunnels are
acceptable.

An implementation of tunneling needs to be aware that while a tunnel set up more or less manually, setting up
the keying material is probably not a link (as defined in [RFC2460]), problem.

If the threat model for a tunnel
might be rather different than for other links, since tunneling is done inside an administrative domain, proper
ingress filtering at the tunnel
potentially includes all edge of the Internet. The recommendations to
verify that domain can also eliminate the IPv4 addresses in
threat from outside of the encapsulated packet matches
what has been configured for domain. Therefore shorter tunnels are
preferable to longer ones, possibly spanning the tunnel, coupled with use of ingress
filtering in IPv4, ameliorate some of this. In addition, whole Internet.

Additionally, an implementation must treat interfaces to different
links as separate e.g. to ensure that Neighbor Discovery packets
arriving on one link does not effect other links. This is especially
important for tunnel links.

7. Authors' Addresses

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INTERNET DRAFT Basic

When dropping packets due to failing to match the allowed IPv4 source
addresses for a tunnel the node should not "acknowledge" the
existence of a tunnel, otherwise this could be used to probe the
acceptable tunnel endpoint addresses. For that reason, the
specification says that such packets MUST be discarded, and an ICMP
error message SHOULD NOT be generated, unless the implementation
normally sends ICMP destination unreachable messages for unknown
protocols; in such a case, the same code MAY be sent. As should be
obvious, the not returning the same ICMP code if an error is returned
for other protocols may hint that the IPv6 Transition Mechanisms November 2002

Erik Nordmark
Sun Microsystems Laboratories
180, avenue de l'Europe
38334 SAINT ISMIER Cedex, France
Tel : +33 (0)4 76 18 88 03
Fax : +33 (0)4 76 18 88 88
Email : erik.nordmark@sun.com

Robert E. Gilligan
Intransa, Inc.
2870 Zanker Rd., Suite 100
San Jose, CA 95134

Tel : +1 408 678 8600
Fax : +1 408 678 8800
Email : gilligan@intransa.com, gilligan@leaf.com

8. stack (or the protocol 41
tunneling processing) has been enabled -- the behaviour should be
consistent on how the implementation otherwise behaves to be
transparent to probing.

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INTERNET DRAFT Basic IPv6 Transition Mechanisms January 2004

6. Acknowledgments

We would like to thank the members of the IPv6 working group, the
Next Generation Transition (ngtrans) working group, and the v6ops
working group for their many contributions and extensive review of
this document. Special thanks are due to Jim Bound, Ross Callon, Bob
Hinden, Bill Manning, John Moy, Mohan Parthasarathy, Pekka Savola,
Fred Templin, Chirayu Patel, and Tim Chown for many helpful
suggestions. Pekka Savola helped in editing the final revisions of
the specification.

7. References

8.1.

7.1. Normative References

[RFC791] J. Postel, "Internet Protocol", RFC 791, September 1981.

[RFC1191] Mogul, J., and S. Deering., "Path MTU Discovery", RFC 1191,
November 1990.

[RFC1981] McCann, J., S. Deering, and J. Mogul. "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.

[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.

[RFC2460] Deering, S., and Hinden, R. "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.

8.2.

[RFC2463] A. Conta, S. Deering, "Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6)
Specification", RFC 2463, December 1998.

7.2. Non-normative References

[ASSIGNED] IANA, "Assigned numbers online database",
http://www.iana.org/numbers.html

<draft-ietf-v6ops-mech-v2-00.txt>

[BCP38UPD] Baker, F., and Savola P., "Ingress Filtering for Multihomed
Networks", draft-savola-bcp38-multihoming-update-03.txt,

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INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 January 2004

work-in-progress, December 2003.

[DNSOPV6] Durand, A., Ihren, J., and Savola P., "Operational
Considerations and Issues with IPv6 DNS", draft-ietf-dnsop-
ipv6-dns-issues-04.txt, work-in-progress, January 2004.

[GTSM] Gill, V., Heasley, J., and D. Meyer, "The Generalized TTL
Security Mechanism (GTSM)", draft-gill-gtsh-04.txt, work-
in-progress, October 2003.

[KM97] Kent, C., and J. Mogul, "Fragmentation Considered Harmful".
In Proc. SIGCOMM '87 Workshop on Frontiers in Computer
Communications Technology. August 1987.

[RFC1122] Braden, R., "Requirements for Internet Hosts - Communication
Layers", STD 3, RFC 1122, October 1989.

[RFC1435] S. Knowles, "IESG Advice from Experience with Path MTU
Discovery", RFC 1435, March 1993.

[RFC1812] F. Baker, "Requirements for IP Version 4 Routers", RFC 1812,
June 1995.

[RFC1886] Thomson,

[RFC1812] Kent, S., Atkinson, R., "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.

[RFC2461] Narten, T., Nordmark, E., and Huitema C. "DNS Extensions to support Simpson, W. "Neighbor
Discovery for IP
version 6", Version 6 (IPv6)", RFC 1886, 2461, December 1995.

[RFC2267] Ferguson, P., and Senie, D., "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", RFC 2267, January 1998.

[RFC2373] Hinden, R., and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.

[RFC2461] Narten, T., Nordmark, E., and Simpson, W. "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998.

[RFC2462] Thomson, S., 1998.

[RFC2462] Thomson, S., and Narten, T. "IPv6 Stateless Address
Autoconfiguration," RFC 2462, December 1998.

[RFC2667] D. Thaler, "IP Tunnel MIB", RFC 2667, August 1999.

[RFC2827] Ferguson, P., and Senie, D., "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", RFC 2827, May 2000.

[RFC2890] Dommety, G., "Key and Sequence Number Extensions to GRE",
RFC 2890, September 2000.

[RFC2923] K. Lahey, "TCP Problems with Path MTU Discovery", RFC 2923,
September 2000.

[RFC2983] D. Black, "Differentiated Services and Tunnels", RFC 2983,
October 2000.

[RFC3056] B. Carpenter, and K. Moore, "Connection of IPv6 Domains via

<draft-ietf-v6ops-mech-v2-02.txt> [Page 23]

INTERNET DRAFT Basic IPv6 Transition Mechanisms January 2004

IPv4 Clouds", RFC 3056, February 2001.

[RFC3168] K. Ramakrishnan, S. Floyd, D. Black, "The Addition of
Explicit Congestion Notification (ECN) to IP", RFC 3168,
September 2001.

[RFC3232] Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by an
On-line Database", RFC 3232, January 2002.

[RFC3484] R. Draves, "Default Address Selection for IPv6", Work in
progress, draft-ietf-ipv6-default-addr-select-09.txt, June
2002.

<draft-ietf-v6ops-mech-v2-00.txt> RFC 3484,
February 2003.

[RFC3513] Hinden, R., and S. Deering, "IP Version 6 Addressing
Architecture", RFC 3513, April 2003.

[RFC3596] Thomson, S., C. Huitema, V. Ksinant, and M. Souissi, "DNS
Extensions to support IP version 6", RFC 3596, October 2003.

8. Authors' Addresses

Erik Nordmark
Sun Microsystems Laboratories
180, avenue de l'Europe
38334 SAINT ISMIER Cedex, France
Tel : +33 (0)4 76 18 88 03
Fax : +33 (0)4 76 18 88 88
Email : erik.nordmark@sun.com

Robert E. Gilligan
Intransa, Inc.
2870 Zanker Rd., Suite 100
San Jose, CA 95134

Tel : +1 408 678 8600
Fax : +1 408 678 8800
Email : gilligan@intransa.com, gilligan@leaf.com

<draft-ietf-v6ops-mech-v2-02.txt> [Page 23] 24]

INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 January 2004

9. Changes from RFC 2893

The motivation for the bulk of these changes are to simplify the
document to only contain
document to only contain the mechanisms of wide-spread use.

RFC 2893 contains a mechanism called automatic tunneling. But a much
more general mechanism is specified in RFC 3056 [RFC3056] which gives
each node with a (global) IPv4 address a /48 IPv6 prefix i.e., enough
for a whole site.

The following changes have been performed since RFC 2893:

- Removed references to A6 and retained AAAA.

- Removed automatic tunneling and use of IPv4-compatible
addresses.

- Removed default Configured Tunnel using IPv4 "Anycast Address"

- Removed Source Address Selection section since this is now
covered by another document ([RFC3484]).

- Removed brief mention of 6over4.

- Split into normative and non-normative references and other
reference cleanup.

- Dropped "or equal" in if (IPv4 path MTU - 20) is less than or
equal to 1280

- Dropped this: 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.

- Described Static MTU and Dynamic MTU cases separately; clarified
that the dynamic path MTU mechanism is OPTIONAL but if it is
implemented it should follow the rules in section 3.2.2.

- Specified Static MTU to default to a MTU of 1280 to 1480 bytes,
and that this may be configurable. Discussed the issues with
using Static MTU at more length.

- Specified minimal rules for IPv4 reassembly and IPv6 MRU to
enhance interoperability and to minimize blacholes.

- Restated the "currently underway" language about Type-of-
Service, and loosely point at [RFC2983] and [RFC3168].

<draft-ietf-v6ops-mech-v2-02.txt> [Page 25]

INTERNET DRAFT Basic IPv6 Transition Mechanisms January 2004

- Fixed reference to Assigned Numbers to be to online version
(with proper pointer to "Assigned Numbers is obsolete" RFC).

- Clarified text about ingress filtering e.g. that it applies to
packet delivered to transport protocols on the decapsulator as
well as packets being forwarded by the decapsulator, and how the
decapsulator's checks help when IPv4 and IPv6 ingress filtering
is in place.

- Removed unidirectional tunneling; assume all tunnels are
bidirectional.

- Removed the guidelines for advertising addresses in DNS as
slightly out of scope, referring to another document for the
details.

- Removed the SHOULD requirement that the link-local addresses
should be formed based on IPv4 addresses.

- Added a SHOULD for implementing a knob to be able to set the
source address of the tunnel, and add discussion why this is
useful.

- Added stronger wording for source address checks: both IPv4 and
IPv6 source addresses MUST be checked, and RPF-like ingress
filtering is optional.

- Rewrote security considerations to be more precise about the
threats of tunneling.

- Added a note that using TTL=255 when encapsulating might be
useful for decapsulation security checks later on.

- Added more discussion in Section 3.2 why using an "infinite"
IPv6 MTU leads to likely interoperability problems.

- Added an explicit requirement that if both MTU determination
methods are used, choosing one should be possible on a per-
tunnel basis.

Clarified that ICMPv4 error handling is only applicable to
dynamic MTU determination.

- Made a lot of miscellaneous editorial cleanups.

<draft-ietf-v6ops-mech-v2-02.txt> [Page 26]

INTERNET DRAFT Basic IPv6 Transition Mechanisms January 2004

9.1. Changes from draft-ietf-v6ops-mech-v2-00

[[ RFC-Editor note: remove the change history between the drafts
before publication. ]]

- Clarified in section 2.2 that there is no assumption that the
DNS server knows the IPv4/IPv6 capabilities of the requesting
node.

- Clarified in section 2.2 that a filtering resolver might want to
take into account external factors e.g., whether IPv6 interfaces
have been configured on the node.

- Clarified in section 2.3 that part of the motivation for the
section is that this is the opposite of common DNS practices in
IPv4; advertising unreachable IPv4 addresses in the DNS is
common.

- Removed the now artificial separation in a section on "common
tunneling techniques" and "configured tunneling" to make one
section on "configured tunneling".

- Restructured the section on tunnel MTU to make the relationship
between static tunnel MTU and dynamic tunnel MTU more clear.
This includes fixing the unclear language about "must be 1280
but may be configurable".

- Added warning about manually configuring large tunnel MTUs
causing excessive fragmentation.

- Added warning about IPv4 PMTU blackholes when using dynamic MTU.

- Clarified that when decapsulating the receiver must be liberal
and allow for padding of the encapsulated packet.

- Added example that when reflecting ICMPv4 errors as ICMPv6
errors it would be appropriate to use ICMPv6 unreachable type
with code "address unreachable" since an error from inside the
tunnel is in effect a link specific problem from IPv6's
perspective.

- Consolidated the text on ingress filtering and created a
separate section on the threat related to source address
spoofing through open decapsulators.

- Clarified "martian" filtering as follows: 0.0.0.0 should be
0.0.0.0/8, same for 127. (per RFC1812), and elaborated that the
broadcast address check includes both the 255.255.255.255

<draft-ietf-v6ops-mech-v2-02.txt> [Page 27]

INTERNET DRAFT Basic IPv6 Transition Mechanisms January 2004

address and all the mechanisms broadcast addresses of wide-spread use.

RFC 2893 contains a mechanism called automatic tunneling. But a
much more general mechanism is specified in RFC 3056 [RFC3056] the decapsulator.

- Clarified that packets which gives each node with a (global) fail the checks (such as verifying
the IPv4 source address, martian, and ingress filtering) on the
decapsulator should be silently dropped.

- Clarified that while source link layer address a /48 IPv6
prefix i.e., enough for a whole site. options and
target link layer address options are ignored in received ND
packets, the ND packets themselves are processed as normal.

9.2. Changes from draft-ietf-v6ops-mech-v2-01

- Removed references to A6 and retained AAAA. unidirectional tunnels; assume all the tunnels are
bidirectional.

- Removed automatic tunneling and the definition of IPv4-compatible IPv6 addresses.

- Removed default Configured Tunnel using IPv4 "Anycast Address" redundant text in the Hop Limit processing rules.

- Removed Source Address Selection section since this is now
covered by the guidelines for advertising addresses in DNS as
slightly out of scope, referring to another document ([RFC3484]). for the
details.

- Removed brief mention of 6over4.

- Split into normative and non-normative references and other
reference cleanup.

- Dropped "or equal" in if (IPv4 path MTU - 20) is less than or
equal to 1280

- Clarified that the dynamic path MTU mechanism in section 3.2 is
OPTIONAL but if it is implemented it should follow the rules in
section 3.2.

- Stated SHOULD requirement that when the dynamic PMTU is not implemented link-local addresses
should be formed based on IPv4 addresses.

- Added more discussion on the sender
MUST NOT by default send IPv6 packets larger than 1280 into ICMPv4/6 Path MTU Discovery and the
tunnel.
required number of packet drops.

- Stated that implementations MAY have Added a SHOULD for implementing a knob by which the MTU can to be set able to larger values on a tunnel by tunnel basis, but that set the default
source address of the tunnel, and add discussion why this is
useful.

- Added stronger wording for source address checks: both IPv4 and
IPv6 source addresses MUST be 1280 checked, and that decapsulators need RPF-like ingress
filtering is optional.

- Rewrote security considerations to be
configured to match more precise about the encapsulaltor's MTU.

<draft-ietf-v6ops-mech-v2-00.txt>
threats of tunneling.

- Added a note that using TTL=255 when encapsulating might be
useful for decapsulation security checks later on.

<draft-ietf-v6ops-mech-v2-02.txt> [Page 24] 28]

INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 January 2004

- Restated the "currently underway" language about ToS Added more discussion in Section 3.2 why using an "infinite"
IPv6 MTU leads to loosely
point at [RFC2983] and [RFC3168]. likely interoperability problems.

- Stated Added an explicit requirement that IPv4 source MAY also if both MTU determination
methods are used, choosing one should be administratively specified.
(This is especially useful possible on multi-interface nodes and with
configured tunneling)

- Fixed reference to Assigned Numbers to be to online version
(with proper pointer to "Assigned Numbers is obsolete" RFC)

- a per-
tunnel basis.

Clarified text about ingress filtering e.g. that it applies ICMPv4 error handling is only applicable to
packet delivered
dynamic MTU determination.

- Specified Static MTU to transport protocols on the decapsulating
node as well as packets being forwarded by the decapsulator, default to a MTU of 1280 to 1480 bytes,
and
how that this may be configurable. Discussed the decapsulator's checks help when issues with
using Static MTU at more length.

- Specified minimal rules for IPv4 reassembly and IPv6 ingress
filtering is in place.

<draft-ietf-v6ops-mech-v2-00.txt> MRU to
enhance interoperability and to minimize blacholes.

- Made a lot of miscellaneous editorial cleanups.

<draft-ietf-v6ops-mech-v2-02.txt> [Page 25] 29]

----