draft-ietf-v6ops-mech-v2-00.txt   draft-ietf-v6ops-mech-v2-01.txt 
INTERNET-DRAFT E. Nordmark INTERNET-DRAFT E. Nordmark
February 24, 2003 Sun Microsystems, Inc. October 27, 2003 Sun Microsystems, Inc.
Obsoletes: 2893 R. E. Gilligan Obsoletes: 2893 R. E. Gilligan
Intransa, Inc. Intransa, Inc.
Basic Transition Mechanisms for IPv6 Hosts and Routers Basic Transition Mechanisms for IPv6 Hosts and Routers
<draft-ietf-v6ops-mech-v2-00.txt> <draft-ietf-v6ops-mech-v2-01.txt>
Status of this Memo Status of this Memo
This document is an Internet-Draft and is subject to all provisions This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026. of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
skipping to change at page 1, line 32 skipping to change at page 1, line 32
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
This draft expires on August 24, 2003. This draft expires on April 27, 2004.
Abstract Abstract
This document specifies IPv4 compatibility mechanisms that can be This document specifies IPv4 compatibility mechanisms that can be
implemented by IPv6 hosts and routers. These mechanisms include implemented by IPv6 hosts and routers. These mechanisms include
providing complete implementations of both versions of the Internet providing complete implementations of both versions of the Internet
Protocol (IPv4 and IPv6), and tunneling IPv6 packets over IPv4 Protocol (IPv4 and IPv6), and tunneling IPv6 packets over IPv4
routing infrastructures. They are designed to allow IPv6 nodes to routing infrastructures. They are designed to allow IPv6 nodes to
maintain complete compatibility with IPv4, which should greatly maintain complete compatibility with IPv4, which should greatly
simplify the deployment of IPv6 in the Internet, and facilitate the simplify the deployment of IPv6 in the Internet, and facilitate the
eventual transition of the entire Internet to IPv6. eventual transition of the entire Internet to IPv6.
This document obsoletes RFC 2893. This document obsoletes RFC 2893.
Contents Contents
Status of this Memo.......................................... 1 Status of this Memo.......................................... 1
1. Introduction............................................. 3 1. Introduction............................................. 3
1.1. Terminology......................................... 3 1.1. Terminology......................................... 3
1.2. Structure of this Document.......................... 5
2. Dual IP Layer Operation.................................. 5 2. Dual IP Layer Operation.................................. 5
2.1. Address Configuration............................... 6 2.1. Address Configuration............................... 6
2.2. DNS................................................. 6 2.2. DNS................................................. 6
2.3. Advertising Addresses in the DNS.................... 7 2.3. Advertising Addresses in the DNS.................... 7
3. Common Tunneling Mechanisms.............................. 8 3. Configured Tunneling Mechanisms.......................... 8
3.1. Encapsulation....................................... 10 3.1. Encapsulation....................................... 10
3.2. Tunnel MTU and Fragmentation........................ 11 3.2. Tunnel MTU and Fragmentation........................ 10
3.2.1. Static Tunnel MTU.............................. 11
3.2.2. Dynamic Tunnel MTU............................. 11
3.3. Hop Limit........................................... 13 3.3. Hop Limit........................................... 13
3.4. Handling IPv4 ICMP errors........................... 13 3.4. Handling IPv4 ICMP errors........................... 13
3.5. IPv4 Header Construction............................ 14 3.5. IPv4 Header Construction............................ 14
3.6. Decapsulation....................................... 16 3.6. Decapsulation....................................... 16
3.7. Link-Local Addresses................................ 18 3.7. Link-Local Addresses................................ 18
3.8. Neighbor Discovery over Tunnels..................... 18 3.8. Neighbor Discovery over Tunnels..................... 18
3.9. Ingress Filtering................................... 19
4. Configured Tunneling..................................... 20 4. Threat Related to Source Address Spoofing................ 19
4.1. Ingress Filtering................................... 20
5. Acknowledgments.......................................... 21 5. Security Considerations.................................. 20
6. Security Considerations.................................. 21 6. Acknowledgments.......................................... 21
7. Authors' Addresses....................................... 21 7. References............................................... 21
7.1. Normative References................................ 21
7.2. Non-normative References............................ 21
8. References............................................... 22 8. Authors' Addresses....................................... 23
8.1. Normative References................................ 22
8.2. Non-normative References............................ 22
9. Changes from RFC 2893.................................... 24 9. Changes from RFC 2893.................................... 23
9.1. Changes from draft-ietf-v6ops-mech-v2-00............ 25
10. Open Issues............................................. 26
1. Introduction 1. Introduction
The key to a successful IPv6 transition is compatibility with the The key to a successful IPv6 transition is compatibility with the
large installed base of IPv4 hosts and routers. Maintaining large installed base of IPv4 hosts and routers. Maintaining
compatibility with IPv4 while deploying IPv6 will streamline the task compatibility with IPv4 while deploying IPv6 will streamline the task
of transitioning the Internet to IPv6. This specification defines a of transitioning the Internet to IPv6. This specification defines a
set of mechanisms that IPv6 hosts and routers may implement in order set of mechanisms that IPv6 hosts and routers may implement in order
to be compatible with IPv4 hosts and routers. to be compatible with IPv4 hosts and routers.
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IPv6-over-IPv4 tunneling: IPv6-over-IPv4 tunneling:
The technique of encapsulating IPv6 packets within IPv4 The technique of encapsulating IPv6 packets within IPv4
so that they can be carried across IPv4 routing so that they can be carried across IPv4 routing
infrastructures. infrastructures.
Configured tunneling: Configured tunneling:
IPv6-over-IPv4 tunneling where the IPv4 tunnel endpoint IPv6-over-IPv4 tunneling where the IPv4 tunnel endpoint
address is determined by configuration information on address is determined by configuration information on
the encapsulating node. The tunnels can be either the encapsulator. The tunnels can be either
unidirectional or bidirectional. Bidirectional unidirectional or bidirectional. Bidirectional
configured tunnels behave as virtual point-to-point configured tunnels behave as virtual point-to-point
links. links.
Other transition mechanisms, including other tunneling mechanisms, Other transition mechanisms, including other tunneling mechanisms,
are outside the scope of this document. are outside the scope of this document.
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [RFC2119]. 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.
2. Dual IP Layer Operation 2. Dual IP Layer Operation
The most straightforward way for IPv6 nodes to remain compatible with The most straightforward way for IPv6 nodes to remain compatible with
IPv4-only nodes is by providing a complete IPv4 implementation. IPv6 IPv4-only nodes is by providing a complete IPv4 implementation. IPv6
nodes that provide a complete IPv4 and IPv6 implementations are nodes that provide a complete IPv4 and IPv6 implementations are
called "IPv6/IPv4 nodes." IPv6/IPv4 nodes have the ability to send called "IPv6/IPv4 nodes." IPv6/IPv4 nodes have the ability to send
and receive both IPv4 and IPv6 packets. They can directly and receive both IPv4 and IPv6 packets. They can directly
interoperate with IPv4 nodes using IPv4 packets, and also directly interoperate with IPv4 nodes using IPv4 packets, and also directly
interoperate with IPv6 nodes using IPv6 packets. interoperate with IPv6 nodes using IPv6 packets.
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- With their IPv6 stack enabled and their IPv4 stack disabled. - With their IPv6 stack enabled and their IPv4 stack disabled.
- With both stacks enabled. - With both stacks enabled.
IPv6/IPv4 nodes with their IPv6 stack disabled will operate like IPv6/IPv4 nodes with their IPv6 stack disabled will operate like
IPv4-only nodes. Similarly, IPv6/IPv4 nodes with their IPv4 stacks IPv4-only nodes. Similarly, IPv6/IPv4 nodes with their IPv4 stacks
disabled will operate like IPv6-only nodes. IPv6/IPv4 nodes MAY disabled will operate like IPv6-only nodes. IPv6/IPv4 nodes MAY
provide a configuration switch to disable either their IPv4 or IPv6 provide a configuration switch to disable either their IPv4 or IPv6
stack. stack.
The IPv6-over-IPv4 tunneling techniques, which are described in The configured tunneling technique, which is described in section 3,
sections 3 and 4, may or may not be used in addition to the dual IP may or may not be used in addition to the dual IP layer operation.
layer operation. An IPv6/IPv4 node MAY support configured tunneling. An IPv6/IPv4 node MAY support configured tunneling.
2.1. Address Configuration 2.1. Address Configuration
Because they support both protocols, IPv6/IPv4 nodes may be Because they support both protocols, IPv6/IPv4 nodes may be
configured with both IPv4 and IPv6 addresses. IPv6/IPv4 nodes use configured with both IPv4 and IPv6 addresses. IPv6/IPv4 nodes use
IPv4 mechanisms (e.g., DHCP) to acquire their IPv4 addresses, and IPv4 mechanisms (e.g., DHCP) to acquire their IPv4 addresses, and
IPv6 protocol mechanisms (e.g., stateless address autoconfiguration IPv6 protocol mechanisms (e.g., stateless address autoconfiguration
and/or DHCPv6) to acquire their IPv6 addresses. and/or DHCPv6) to acquire their IPv6 addresses.
2.2. DNS 2.2. DNS
The Domain Naming System (DNS) is used in both IPv4 and IPv6 to map 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 between hostnames and IP addresses. A new resource record type named
"AAAA" has been defined for IPv6 addresses [RFC1886]. Since "AAAA" has been defined for IPv6 addresses [RFC3596]. Since
IPv6/IPv4 nodes must be able to interoperate directly with both IPv4 IPv6/IPv4 nodes must be able to interoperate directly with both IPv4
and IPv6 nodes, they must provide resolver libraries capable of and IPv6 nodes, they must provide resolver libraries capable of
dealing with IPv4 "A" records as well as IPv6 "AAAA" records. Note dealing with IPv4 "A" records as well as IPv6 "AAAA" records. Note
that the lookup of A versus AAAA records is independent of whether that the lookup of A versus AAAA records is independent of whether
the DNS packets are carried in IPv4 or IPv6 packets. the DNS packets are carried in IPv4 or IPv6 packets, and that there
is no assumption that the DNS server know the IPv4/IPv6 capabilities
of the requesting node.
DNS resolver libraries on IPv6/IPv4 nodes MUST be capable of handling DNS resolver libraries on IPv6/IPv4 nodes MUST be capable of handling
both AAAA and A records. However, when a query locates an AAAA both AAAA and A records. However, when a query locates an AAAA
record holding an IPv6 address, and an A record holding an IPv4 record holding an IPv6 address, and an A record holding an IPv4
address, the resolver library MAY filter or order the results address, the resolver library MAY filter or order the results
returned to the application in order to influence the version of IP returned to the application in order to influence the version of IP
packets used to communicate with that node. In terms of filtering, packets used to communicate with that node. In terms of filtering,
the resolver library has three alternatives: the resolver library has three alternatives:
- Return only the IPv6 address(es) to the application. - Return only the IPv6 address(es) to the application.
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address(es), the application will communicate with the node using address(es), the application will communicate with the node using
IPv4. If it returns both types of addresses, the application will IPv4. If it returns both types of addresses, the application will
have the choice which address to use, and thus which IP protocol to have the choice which address to use, and thus which IP protocol to
employ. employ.
If it returns both, the resolver MAY elect to order the addresses -- If it returns both, the resolver MAY elect to order the addresses --
IPv6 first, or IPv4 first. Since most applications try 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 in the order they are returned by the resolver, this can affect the
IP version "preference" of applications. 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 The decision to filter or order DNS results is implementation
specific. IPv6/IPv4 nodes MAY provide policy configuration to specific. IPv6/IPv4 nodes MAY provide policy configuration to
control filtering or ordering of addresses returned by the resolver, control filtering or ordering of addresses returned by the resolver,
or leave the decision entirely up to the application. or leave the decision entirely up to the application.
An implementation MUST allow the application to control whether or An implementation MUST allow the application to control whether or
not such filtering takes place. not such filtering takes place.
More details on this subject are specified in [RFC3484]. More details on the relative preferences of IPv4 and IPv6 addresses
are specified in the default address selection document [RFC3484].
2.3. Advertising Addresses in the DNS 2.3. Advertising Addresses in the DNS
There are some constraint placed on the use of the DNS during There are some constraint placed on the use of the DNS during
transition. The constraints allow nodes to prefer either IPv6 or transition. The constraints allow nodes to prefer either IPv6 or
IPv4 addresses when both types of addresses are returned by the DNS. IPv4 addresses when both types of addresses are returned by the DNS.
Most of these are obvious but are stated here for completeness. Most of these are obvious but are stated here for completeness,
especially given that there is some existing practise in IPv4 to
advertise unreachable IPv4 addresses in the DNS.
The recommendation is that AAAA records for a node should not be The recommendation is that AAAA records for a node should not be
added to the DNS until all of these are true: added to the DNS until all of these are true:
1) The address is assigned to the interface on the node. 1) The address is assigned to the interface on the node.
2) The address is configured on the interface. 2) The address is configured on the interface.
3) The interface is on a link which is connected to the IPv6 3) The interface is on a link which is connected to the IPv6
infrastructure. infrastructure.
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application would fail to communicate in this case.) Thus it is application would fail to communicate in this case.) Thus it is
suggested that either the recommendations be followed, or care be suggested that either the recommendations be followed, or care be
taken to only do so with nodes that will not be impacted by external taken to only do so with nodes that will not be impacted by external
accessing delays and/or communication failure. accessing delays and/or communication failure.
In the future, when a node discontinues its use of IPv4, analogous In the future, when a node discontinues its use of IPv4, analogous
constraints apply with respect to the node's A records in the DNS; 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 the removal of the A records should be tied to when the node can no
longer be reached using IPv4. longer be reached using IPv4.
3. Common Tunneling Mechanisms 3. Configured Tunneling Mechanisms
In most deployment scenarios, the IPv6 routing infrastructure will be In most deployment scenarios, the IPv6 routing infrastructure will be
built up over time. While the IPv6 infrastructure is being deployed, built up over time. While the IPv6 infrastructure is being deployed,
the existing IPv4 routing infrastructure can remain functional, and the existing IPv4 routing infrastructure can remain functional, and
can be used to carry IPv6 traffic. Tunneling provides a way to can be used to carry IPv6 traffic. Tunneling provides a way to
utilize an existing IPv4 routing infrastructure to carry IPv6 utilize an existing IPv4 routing infrastructure to carry IPv6
traffic. traffic.
IPv6/IPv4 hosts and routers can tunnel IPv6 datagrams over regions of IPv6/IPv4 hosts and routers can tunnel IPv6 datagrams over regions of
IPv4 routing topology by encapsulating them within IPv4 packets. IPv4 routing topology by encapsulating them within IPv4 packets.
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- Host-to-Host. IPv6/IPv4 hosts that are interconnected by an - Host-to-Host. IPv6/IPv4 hosts that are interconnected by an
IPv4 infrastructure can tunnel IPv6 packets between themselves. IPv4 infrastructure can tunnel IPv6 packets between themselves.
In this case, the tunnel spans the entire end-to-end path that In this case, the tunnel spans the entire end-to-end path that
the packet takes. the packet takes.
- Router-to-Host. IPv6/IPv4 routers can tunnel IPv6 packets to - Router-to-Host. IPv6/IPv4 routers can tunnel IPv6 packets to
their final destination IPv6/IPv4 host. This tunnel spans only their final destination IPv6/IPv4 host. This tunnel spans only
the last segment of the end-to-end path. the last segment of the end-to-end path.
Tunneling techniques are usually classified according to the Configured tunneling can be used in all of the above cases, but is
mechanism by which the encapsulating node determines the address of most likely to be used router-to-router due to the need to explicitly
the node at the end of the tunnel. In the first two tunneling configure the tunneling endpoints.
methods listed above -- router-to-router and host-to-router -- the
IPv6 packet is being tunneled to a router. The endpoint of this type
of tunnel is an intermediary router which must decapsulate the IPv6
packet and forward it on to its final destination. When tunneling to
a router, the endpoint of the tunnel is different from the
destination of the packet being tunneled. In some cases, the
addresses in the IPv6 packet being tunneled can not provide the IPv4
address of the tunnel endpoint. In those cases, the tunnel endpoint
address must be determined from configuration information on the node
performing the encapsulation. We use the term "configured tunneling"
to describe the type of tunneling where the endpoint is explicitly
configured.
In the last two tunneling methods -- host-to-host and router-to-host
-- the IPv6 packet is tunneled all the way to its final destination.
In this case, the destination address of both the IPv6 packet and the
encapsulating IPv4 header identify the same node. However, the
tunneling mechanism specified in this document does not handle these
cases any differently; the IPv4 addresses is still determined using
configuration information using configured tunneling.
The underlying mechanisms for tunneling are: The underlying mechanisms for tunneling are:
- The entry node of the tunnel (the encapsulating node) creates an - The entry node of the tunnel (the encapsulator) creates an
encapsulating IPv4 header and transmits the encapsulated packet. encapsulating IPv4 header and transmits the encapsulated packet.
- The exit node of the tunnel (the decapsulating node) receives - The exit node of the tunnel (the decapsulator) receives the
the encapsulated packet, reassembles the packet if needed, encapsulated packet, reassembles the packet if needed, removes
removes the IPv4 header, updates the IPv6 header, and processes the IPv4 header, updates the IPv6 header, and processes the
the received IPv6 packet. received IPv6 packet.
- The encapsulating node MAY need to maintain soft state - The encapsulator MAY need to maintain soft state information for
information for each tunnel recording such parameters as the MTU each tunnel recording such parameters as the MTU of the tunnel
of the tunnel in order to process IPv6 packets forwarded into in order to process IPv6 packets forwarded into the tunnel. In
the tunnel. In cases where the number of tunnels that any one cases where the number of tunnels that any one host or router is
host or router is using is large, it is helpful to observe that using is large, it is helpful to observe that this state
this state information can be cached and discarded when not in information can be cached and discarded when not in use.
use.
The remainder of this section discusses the common mechanisms. A In configured tunneling, the tunnel endpoint address is determined
subsequent section discusses how the tunnel endpoint address is from configuration information in the encapsulator. For each tunnel,
determined for configured tunneling. the encapsulator must store the tunnel endpoint address. When an
IPv6 packet is transmitted over a tunnel, the tunnel endpoint address
configured for that tunnel is used as the destination address for the
encapsulating IPv4 header.
The determination of which packets to tunnel is usually made by
routing information on the encapsulator. This is usually done via a
routing table, which directs packets based on their destination
address using the prefix mask and match technique.
3.1. Encapsulation 3.1. Encapsulation
The encapsulation of an IPv6 datagram in IPv4 is shown below: The encapsulation of an IPv6 datagram in IPv4 is shown below:
+-------------+ +-------------+
| IPv4 | | IPv4 |
| Header | | Header |
+-------------+ +-------------+ +-------------+ +-------------+
| IPv6 | | IPv6 | | IPv6 | | IPv6 |
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| Layer | ===> | Layer | | Layer | ===> | Layer |
| Header | | Header | | Header | | Header |
+-------------+ +-------------+ +-------------+ +-------------+
| | | | | | | |
~ Data ~ ~ Data ~ ~ Data ~ ~ Data ~
| | | | | | | |
+-------------+ +-------------+ +-------------+ +-------------+
Encapsulating IPv6 in IPv4 Encapsulating IPv6 in IPv4
In addition to adding an IPv4 header, the encapsulating node also has In addition to adding an IPv4 header, the encapsulator also has to
to handle some more complex issues: handle some more complex issues:
- Determine when to fragment and when to report an ICMP "packet - Determine when to fragment and when to report an ICMP "packet
too big" error back to the source. too big" error back to the source.
- How to reflect IPv4 ICMP errors from routers along the tunnel - How to reflect IPv4 ICMP errors from routers along the tunnel
path back to the source as IPv6 ICMP errors. path back to the source as IPv6 ICMP errors.
Those issues are discussed in the following sections. Those issues are discussed in the following sections.
3.2. Tunnel MTU and Fragmentation 3.2. Tunnel MTU and Fragmentation
The encapsulating node could view encapsulation as IPv6 using IPv4 as Naively the encapsulator could view encapsulation as IPv6 using IPv4
a link layer with a very large MTU (65535-20 bytes to be exact; 20 as a link layer with a very large MTU (65535-20 bytes to be exact; 20
bytes "extra" are needed for the encapsulating IPv4 header). The bytes "extra" are needed for the encapsulating IPv4 header). The
encapsulating node would need only to report IPv6 ICMP "packet too encapsulator would need only to report IPv6 ICMP "packet too big"
big" errors back to the source for packets that exceed this MTU. errors back to the source for packets that exceed this MTU. However,
However, such a scheme would be inefficient for two reasons and is such a scheme would be inefficient for two reasons and therefor MUST
therefor NOT RECOMMENDED: NOT be used:
1) It would result in more fragmentation than needed. IPv4 layer 1) It would result in more fragmentation than needed. IPv4 layer
fragmentation SHOULD be avoided due to the performance problems fragmentation SHOULD be avoided due to the performance problems
caused by the loss unit being smaller than the retransmission caused by the loss unit being smaller than the retransmission
unit [KM97]. unit [KM97].
2) Any IPv4 fragmentation occurring inside the tunnel, i.e. between 2) Any IPv4 fragmentation occurring inside the tunnel, i.e. between
the encapsulating node and the decapsulating node, would have to the encapsulator and the decapsulator, would have to be
be reassembled at the tunnel endpoint. For tunnels that reassembled at the tunnel endpoint. For tunnels that terminate
terminate at a router, this would require additional memory to at a router, this would require additional memory to reassemble
reassemble the IPv4 fragments into a complete IPv6 packet before the IPv4 fragments into a complete IPv6 packet before that
that packet could be forwarded onward. packet could be forwarded onward.
Hence, the encapsulating node MUST NOT treat the tunnel as an Hence, the encapsulator MUST NOT treat the tunnel as an interface
interface with an MTU of 64 kilobytes, but instead use the smaller with an MTU of 64 kilobytes, but instead either use the fixed static
MTU specified below. MTU below, or use OPTIONAL dynamic MTU determination based on the
IPv4 path MTU to the tunnel endpoint.
3.2.1. Static Tunnel MTU
A node using a static tunnel MTU MUST limit the size of the IPv6
packets it tunnels 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 to set a 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 decapsulator can reassemble
packets of that size. Care should be taken when manually configuring
large tunnel MTUs to only do so when the MTU of the IPv4 path to the
tunnel endpoint is large to avoid causing excessive fragmentation.
When using the static tunnel MTU the Don't Fragment bit MUST NOT be
set in the encapsulating IPv4 header. As a result the encapsulator
should not receive any ICMPv4 "packet too big" message as a result of
the packets it has encapsulated.
3.2.2. Dynamic Tunnel MTU
The dynamic MTU determination is OPTIONAL. However, if it is
implemented, it SHOULD have the behavior described in this document.
The fragmentation inside the tunnel can be reduced to a minimum by The fragmentation inside the tunnel can be reduced to a minimum by
having the encapsulating node track the IPv4 Path MTU across the having the encapsulator track the IPv4 Path MTU across the tunnel,
tunnel, using the IPv4 Path MTU Discovery Protocol [RFC1191] and using the IPv4 Path MTU Discovery Protocol [RFC1191] and recording
recording the resulting path MTU. The IPv6 layer in the the resulting path MTU. The IPv6 layer in the encapsulator can then
encapsulating node can then view a tunnel as a link layer with an MTU view a tunnel as a link layer with an MTU equal to the IPv4 path MTU,
equal to the IPv4 path MTU, minus the size of the encapsulating IPv4 minus the size of the encapsulating IPv4 header.
header.
Note that this does not eliminate IPv4 fragmentation in the case when 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. 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 (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 bytes [RFC2460].) In this case the IPv6 layer has to "see" a link
layer with an MTU of 1280 bytes and the encapsulating node has to use layer with an MTU of 1280 bytes and the encapsulator has to use IPv4
IPv4 fragmentation in order to forward the 1280 byte IPv6 packets. fragmentation in order to forward the 1280 byte IPv6 packets.
This dynamic MTU determination is OPTIONAL. However, if it is
implemented it SHOULD have the behavior described in this document.
If it is not implemented instead the node MUST instead limit the size
of the IPv6 packets it tunnels 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 to set
a 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 The encapsulator SHOULD employ the following algorithm to determine
determine when to forward an IPv6 packet that is larger than the 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 MTU using IPv4 fragmentation, and when to return an IPv6 ICMP "packet
IPv6 ICMP "packet too big" message per [RFC1981]: too big" message per [RFC1981]:
if (IPv4 path MTU - 20) is less than or equal to 1280 if (IPv4 path MTU - 20) is less than 1280
if packet is larger than 1280 bytes if packet is larger than 1280 bytes
Send IPv6 ICMP "packet too big" with MTU = 1280. Send IPv6 ICMP "packet too big" with MTU = 1280.
Drop packet. Drop packet.
else else
Encapsulate but do not set the Don't Fragment Encapsulate but do not set the Don't Fragment
flag in the IPv4 header. The resulting IPv4 flag in the IPv4 header. The resulting IPv4
packet might be fragmented by the IPv4 layer on packet might be fragmented by the IPv4 layer on
the encapsulating node or by some router along the encapsulator or by some router along
the IPv4 path. the IPv4 path.
endif endif
else else
if packet is larger than (IPv4 path MTU - 20) if packet is larger than (IPv4 path MTU - 20)
Send IPv6 ICMP "packet too big" with Send IPv6 ICMP "packet too big" with
MTU = (IPv4 path MTU - 20). MTU = (IPv4 path MTU - 20).
Drop packet. Drop packet.
else else
Encapsulate and set the Don't Fragment flag Encapsulate and set the Don't Fragment flag
in the IPv4 header. in the IPv4 header.
endif endif
endif endif
Encapsulating nodes that have a large number of tunnels might not be Encapsulators that have a large number of tunnels can choose between
able to store the IPv4 Path MTU for all tunnels. Such nodes can, at dynamic versus static tunnel MTU on a per-tunnel endpoint basis.
the expense of additional fragmentation in the network, avoid using
the IPv4 Path MTU algorithm across the tunnel and instead use the MTU
of the link layer (under IPv4) in the above algorithm instead of the
IPv4 path MTU. In that case the IPv6 MTU for the tunnel MUST be
limited to 1280 unless it has explicitly been configured to be
larger.
In this case the Don't Fragment bit MUST NOT be set in the Note that using dynamic tunnel MTU is subject to IPv4 PMTU blackholes
encapsulating IPv4 header. should the ICMPv4 "packet too big" messages be dropped by firewalls
or not generated by the routers. [RFC1435, RFC2923]
3.3. Hop Limit 3.3. Hop Limit
IPv6-over-IPv4 tunnels are modeled as "single-hop". That is, the IPv6-over-IPv4 tunnels are modeled as "single-hop". That is, the
IPv6 hop limit is decremented by 1 when an IPv6 packet traverses 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. The single-hop model serves to hide the existence of a
tunnel. The tunnel is opaque to users of the network, and is not tunnel. The tunnel is opaque to users of the network, and is not
detectable by network diagnostic tools such as traceroute. detectable by network diagnostic tools such as traceroute.
The single-hop model is implemented by having the encapsulating and The single-hop model is implemented by having the encapsulators and
decapsulating nodes process the IPv6 hop limit field as they would if decapsulators process the IPv6 hop limit field as they would if they
they were forwarding a packet on to any other datalink. That is, were forwarding a packet on to any other datalink. That is, they
they decrement the hop limit by 1 when forwarding an IPv6 packet. decrement the hop limit by 1 when forwarding an IPv6 packet. (The
(The originating node and final destination do not decrement the hop originating node and final destination do not decrement the hop
limit.) limit.)
The TTL of the encapsulating IPv4 header is selected in an The TTL of the encapsulating IPv4 header is selected in an
implementation dependent manner. The current suggested value is implementation dependent manner. The current suggested value is
published in the "Assigned Numbers" RFC [RFC3232][ASSIGNED]. published in the "Assigned Numbers" RFC [RFC3232][ASSIGNED].
Implementations MAY provide a mechanism to allow the administrator to Implementations MAY provide a mechanism to allow the administrator to
configure the IPv4 TTL such as the one specified in the IP Tunnel MIB configure the IPv4 TTL such as the one specified in the IP Tunnel MIB
[RFC2667]. [RFC2667].
3.4. Handling IPv4 ICMP errors 3.4. Handling IPv4 ICMP errors
In response to encapsulated packets it has sent into the tunnel, the In response to encapsulated packets it has sent into the tunnel, the
encapsulating node might receive IPv4 ICMP error messages from IPv4 encapsulator might receive IPv4 ICMP error messages from IPv4 routers
routers inside the tunnel. These packets are addressed to the inside the tunnel. These packets are addressed to the encapsulator
encapsulating node because it is the IPv4 source of the encapsulated because it is the IPv4 source of the encapsulated packet.
packet.
The ICMP "packet too big" error messages are handled according to The ICMP "packet too big" error messages are handled according to
IPv4 Path MTU Discovery [RFC1191] and the resulting path MTU is 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 recorded in the IPv4 layer. The recorded path MTU is used by IPv6 to
determine if an IPv6 ICMP "packet too big" error has to be generated determine if an IPv6 ICMP "packet too big" error has to be generated
as described in section 3.2. as described in section 3.2.2 if dynamic tunnel MTU is used.
The handling of other types of ICMP error messages depends on how The handling of other types of ICMP error messages depends on how
much information is included in the "packet in error" field, which much information is included in the "packet in error" field, which
holds the encapsulated packet that caused the error. holds the encapsulated packet that caused the error.
Many older IPv4 routers return only 8 bytes of data beyond the IPv4 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 header of the packet in error, which is not enough to include the
address fields of the IPv6 header. More modern IPv4 routers are address fields of the IPv6 header. More modern IPv4 routers are
likely to return enough data beyond the IPv4 header to include the likely to return enough data beyond the IPv4 header to include the
entire IPv6 header and possibly even the data beyond that. entire IPv6 header and possibly even the data beyond that.
If the offending packet includes enough data, the encapsulating node If the offending packet includes enough data, the encapsulator MAY
MAY extract the encapsulated IPv6 packet and use it to generate an extract the encapsulated IPv6 packet and use it to generate an IPv6
IPv6 ICMP message directed back to the originating IPv6 node, as ICMP message directed back to the originating IPv6 node, as shown
shown below: below:
+--------------+ +--------------+
| IPv4 Header | | IPv4 Header |
| dst = encaps | | dst = encaps |
| node | | node |
+--------------+ +--------------+
| ICMP | | ICMP |
| Header | | Header |
- - +--------------+ - - +--------------+
| IPv4 Header | | IPv4 Header |
skipping to change at page 14, line 42 skipping to change at page 14, line 33
| Transport | Can be used to | Transport | Can be used to
Error | Header | generate an Error | Header | generate an
+--------------+ IPv6 ICMP +--------------+ IPv6 ICMP
| | error message | | error message
~ Data ~ back to the source. ~ Data ~ back to the source.
| | | |
- - +--------------+ - - - - +--------------+ - -
IPv4 ICMP Error Message Returned to Encapsulating Node IPv4 ICMP 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]).
3.5. IPv4 Header Construction 3.5. IPv4 Header Construction
When encapsulating an IPv6 packet in an IPv4 datagram, the IPv4 When encapsulating an IPv6 packet in an IPv4 datagram, the IPv4
header fields are set as follows: header fields are set as follows:
Version: Version:
4 4
IP Header Length in 32-bit words: IP Header Length in 32-bit words:
skipping to change at page 16, line 5 skipping to change at page 16, line 5
Protocol: Protocol:
41 (Assigned payload type number for IPv6) 41 (Assigned payload type number for IPv6)
Header Checksum: Header Checksum:
Calculate the checksum of the IPv4 header. Calculate the checksum of the IPv4 header.
Source Address: Source Address:
IPv4 address of outgoing interface of the encapsulating IPv4 address of outgoing interface of the encapsulator.
node. The source address MAY alternatively be The source address MAY alternatively be administratively
administratively specified to be a specific IPv4 address specified to be a specific IPv4 address assigned to the
assigned to the encapsulating node. encapsulator. This is often necessary on encapsulators
with multiple IPv4 addresses to ensure that the IPv4
source address is acceptable to the decapsulator.
Destination Address: Destination Address:
IPv4 address of tunnel endpoint. IPv4 address of tunnel endpoint.
Any IPv6 options are preserved in the packet (after the IPv6 header). Any IPv6 options are preserved in the packet (after the IPv6 header).
3.6. Decapsulation 3.6. Decapsulation
When an IPv6/IPv4 host or a router receives an IPv4 datagram that is When an IPv6/IPv4 host or a router receives an IPv4 datagram that is
addressed to one of its own IPv4 address, and the value of the addressed to one of its own IPv4 address, and the value of the
protocol field is 41, it reassembles if the packet if it is protocol field is 41, the packet is potentially part of a tunnel and
fragmented at the IPv4 level, then it removes the IPv4 header and needs to be verified against the list of acceptable source addresses
submits the IPv6 datagram to its IPv6 layer code. for tunneled packets, reassembled (if fragmented at the IPv4 level),
have the IPv4 header removed and the resulting IPv6 datagram be
submitted to the IPv6 layer code on the node.
The decapsulating node MUST be capable of reassembling an IPv4 packet The decapsulator MUST verify that the tunnel source address is
that is the maximum of 1280 bytes and the largest interface MTU on acceptable before further processing packets to avoid creating a hole
the decapsulator. The 1280 byte number is a result of encapsulators in ingress filtering (see section 4). This check also applies to
that use the static MTU in section 3.2, while encapsulators that use packets which are delivered to transport protocols on the
the dynamic scheme in section 3.2 can cause up to the largest decapsulator. For bidirectional configured tunnels this is done by
verifying that the source address is the IPv4 address of the other
end of a tunnel configured on the node. For unidirectional
configured tunnels the decapsulator MUST be configured with a list of
IPv4 source address prefixes that are acceptable. Such a list MUST
default to having zero entries i.e., the node has to be explicitly
configured to accept encapsulated packets received over
unidirectional configured tunnels. Packets for which the IPv4 source
address does not match SHOULD be silently dropped.
A side effect of this source address verification is that the node
will silently discard packets with an invalid IPv4 source address
such as a multicast address, a broadcast address (255.255.255.255 and
the broadcast addresses configured on the node), 0.0.0.0/8, and
127.0.0.1/8. In general, it SHOULD apply the rules for martian
filtering in [RFC1812] and ingress filtering [RFC2827] on the IPv4
source address. Packets caught by these checks SHOULD be silently
dropped.
The decapsulator MUST be capable of reassembling an IPv4 packet that
is the maximum of 1280 bytes and the largest interface MTU on the
decapsulator. The 1280 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 interface MTU on the decapsulator to be received. (Note that it is
strictly the interface MTU on the last IPv4 router *before* the strictly the interface MTU on the last IPv4 router *before* the
decapsulator that matters, but for most links the MTU is the same decapsulator that matters, but for most links the MTU is the same
between all neighbors.) between all neighbors.)
This reassembly limit allows dynamic tunnel MTU determination by the This reassembly limit allows dynamic tunnel MTU determination by the
encapsulator to take advantage of larger IPv4 path MTUs. An encapsulator to take advantage of larger IPv4 path MTUs. An
implementation MAY have a configuration knob which can be used to set implementation MAY have a configuration knob which can be used to set
a larger value of the tunnel reassembly buffers than the above a larger value of the tunnel reassembly buffers than the above
number, but it MUST NOT be set below the above number. number, but it MUST NOT be set below the above number.
skipping to change at page 17, line 23 skipping to change at page 17, line 41
| Layer | ===> | Layer | | Layer | ===> | Layer |
| Header | | Header | | Header | | Header |
+-------------+ +-------------+ +-------------+ +-------------+
| | | | | | | |
~ Data ~ ~ Data ~ ~ Data ~ ~ Data ~
| | | | | | | |
+-------------+ +-------------+ +-------------+ +-------------+
Decapsulating IPv6 from IPv4 Decapsulating IPv6 from IPv4
When decapsulating the packet, the IPv6 header is not modified. (See When decapsulating the packet, the IPv6 header is not modified.
[RFC2983] and [RFC3168] for issues relating to the Type of Service (However, see [RFC2983] and [RFC3168] for issues relating to the Type
byte and tunneling.) If the packet is subsequently forwarded, its of Service byte and tunneling.) If the packet is subsequently
hop limit is decremented by one. forwarded, its hop limit is decremented by one.
As part of 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.
The decapsulating node performs IPv4 reassembly before decapsulating The decapsulator performs IPv4 reassembly before decapsulating the
the IPv6 packet. All IPv6 options are preserved even if the IPv6 packet. All IPv6 options are preserved even if the
encapsulating IPv4 packet is fragmented. encapsulating IPv4 packet is fragmented.
The encapsulating IPv4 header is discarded. The encapsulating IPv4 header is discarded. The length of the IPv6
packet MUST be determined from the IPv6 payload length since the IPv4
packet might be padded (thus have a length which is larger than the
IPv6 packet plus the added IPv4 header).
After the decapsulation the node SHOULD silently discard a packet After the decapsulation the node SHOULD silently discard a packet
with an invalid IPv6 source address. This includes IPv6 multicast with an invalid IPv6 source address. This includes IPv6 multicast
addresses, the unspecified address, and the loopback address but also addresses, the IPv6 unspecified address, and the loopback address but
IPv4-compatible IPv6 source addresses where the IPv4 part of the also IPv4-compatible IPv6 source addresses where the IPv4 part of the
address is an (IPv4) multicast address, broadcast address, 0.0.0.0, address is an IPv4 multicast address, broadcast address
or 127.0.0.1. In general it SHOULD apply the rules for martian (255.255.255.255 and the broadcast addresses configured on the node),
filtering in [RFC1812] and ingress filtering [RFC2267] on the IPv4 0.0.0.0/8, or 127.0.0.1/8. In general it SHOULD apply the rules for
address embedded in IPv4-compatible source addresses. martian filtering in [RFC1812] and ingress filtering [RFC2827] on the
IPv4 address embedded in IPv4-compatible source addresses.
After the IPv6 packet is decapsulated, it is processed almost the
same as any received IPv6 packet. The difference being that a
decapsulated packet MUST NOT be accepted (and delivered locally or
forwarded) unless the node has been explicitly configured to accept
tunneled packets with the given IPv4 source address. This
configuration can be implicit in e.g., having a bidirectional
configured tunnel which matches the IPv4 source address. This
restriction is needed to prevent tunneling 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.
3.7. Link-Local Addresses 3.7. Link-Local Addresses
The configured tunnels are IPv6 interfaces (over the IPv4 "link The configured tunnels are IPv6 interfaces (over the IPv4 "link
layer") thus MUST have link-local addresses. The link-local layer") thus MUST have link-local addresses. The link-local
addresses are used by routing protocols operating over the tunnels. addresses are used by routing protocols operating over the tunnels.
The Interface Identifier [RFC2373] for such an Interface SHOULD be The Interface Identifier [RFC2373] for such an Interface SHOULD be
the 32-bit IPv4 address of that interface, with the bytes in the same the 32-bit IPv4 address of that interface, with the bytes in the same
order in which they would appear in the header of an IPv4 packet, order in which they would appear in the header of an IPv4 packet,
padded at the left with zeros to a total of 64 bits. Note that the padded at the left with zeros to a total of 64 bits. Note that the
"Universal/Local" bit is zero, indicating that the Interface "Universal/Local" bit is zero, indicating that the Interface
Identifier is not globally unique. When the host has more than one Identifier is not globally unique. When the host has more than one
IPv4 address in use on the physical interface concerned, an IPv4 address in use on the physical interface concerned, an
administrative choice of one of these IPv4 addresses is made. administrative choice of one of these IPv4 addresses is made when
forming the link-local address.
The IPv6 Link-local address [RFC2373] for an IPv4 virtual interface The IPv6 Link-local address [RFC2373] for an IPv4 virtual interface
is formed by appending the Interface Identifier, as defined above, to is formed by appending the Interface Identifier, as defined above, to
the prefix FE80::/64. the prefix FE80::/64.
+-------+-------+-------+-------+-------+-------+------+------+ +-------+-------+-------+-------+-------+-------+------+------+
| FE 80 00 00 00 00 00 00 | | FE 80 00 00 00 00 00 00 |
+-------+-------+-------+-------+-------+-------+------+------+ +-------+-------+-------+-------+-------+-------+------+------+
| 00 00 | 00 | 00 | IPv4 Address | | 00 00 | 00 | 00 | IPv4 Address |
+-------+-------+-------+-------+-------+-------+------+------+ +-------+-------+-------+-------+-------+-------+------+------+
3.8. Neighbor Discovery over Tunnels 3.8. Neighbor Discovery over Tunnels
For unidirectional configured tunnels most of Neighbor Discovery For unidirectional configured tunnels most of Neighbor Discovery
[RFC2667] and Stateless Address Autoconfiguration [RFC2462] does not [RFC2461] and Stateless Address Autoconfiguration [RFC2462] does not
apply; only the formation of the link-local address applies. apply; only the formation of the link-local address applies.
If an implementation provides bidirectional configured tunnels it If an implementation provides bidirectional configured tunnels it
MUST at least accept and respond to the probe packets used by MUST at least accept and respond to the probe packets used by
Neighbor Unreachability Detection [RFC2461]. Such implementations Neighbor Unreachability Detection (NUD) [RFC2461]. Such
SHOULD also send NUD probe packets to detect when the configured implementations SHOULD also send NUD probe packets to detect when the
tunnel fails at which point the implementation can use an alternate configured tunnel fails at which point the implementation can use an
path to reach the destination. Note that Neighbor Discovery allows alternate path to reach the destination. Note that Neighbor
that the sending of NUD probes be omitted for router to router links Discovery allows that the sending of NUD probes be omitted for router
if the routing protocol tracks bidirectional reachability. to router links if the routing protocol tracks bidirectional
reachability.
For the purposes of Neighbor Discovery the configured tunnels For the purposes of Neighbor Discovery the configured tunnels
specified in this document are assumed to NOT have a link-layer specified in this document are assumed to NOT have a link-layer
address, even though the link-layer (IPv4) does have address. This address, even though the link-layer (IPv4) does have address. This
means that a sender of Neighbor Discovery packets means that:
- SHOULD NOT include Source Link Layer Address options or Target - the sender of Neighbor Discovery packets SHOULD NOT include
Link Layer Address options on the tunnel link. Source Link Layer Address options or Target Link Layer Address
options on the tunnel link.
- MUST silently ignore any received neighbor discovery source link - the receiver MUST, while otherwise processing the neighbor
layer address or target link layer address options received over discovery packet, silently ignore the content of any Source Link
the tunnel link. Layer Address options or Target Link Layer Address options
received on the tunnel link.
Not using a link layer address options is consistent with how Not using a link layer address options is consistent with how
neighbor discovery is used on other point-to-point links. neighbor 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 ingress filtering The specification above contains rules that apply ingress filtering
to packets before they are decapsulated. The purpose of ingress to packets before they are decapsulated. The purpose of ingress
filtering in general is specified in [RFC2267]. When IP-in-IP filtering in general is specified in [RFC2827]. When IP-in-IP
tunneling (independent of IP versions) is used it is important that tunneling (independent of IP versions) is used it is important that
this not be a tool to bypass any ingress filtering in use for non- this not be a tool to bypass any ingress filtering in use for non-
tunneled packets. Thus the rules are derived based on the assumption tunneled packets. Thus the rules in this document are derived based
that should ingress filtering be used for IPv4 and IPv6, the use of on should ingress filtering be used for IPv4 and IPv6, the use of
tunneling should not provide an easy way to circumvent the filtering. tunneling should not provide an easy way to circumvent the filtering.
In this case, without specific ingress filtering checks in the In this case, without specific ingress filtering checks in the
decapsulating node, it would be possible for an attacker to inject a decapsulator, it would be possible for an attacker to inject a packet
packet with: with:
- Outer IPv4 source: real IPv4 address of attacker - Outer IPv4 source: real IPv4 address of attacker
- Outer IPv4 destination: IPv4 address of decapsulating node - Outer IPv4 destination: IPv4 address of decapsulator
- Inner IPv6 source: Alice which is either the decapsulator or a
- Inner IPv6 source: Alice which is either the decapsulating node node close to it.
or a node close to it.
- Inner IPv6 destination: Bob - Inner IPv6 destination: Bob
Even if all IPv4 routers between the attacker and the decapsulating Even if all IPv4 routers between the attacker and the decapsulator
node implement IPv4 ingress filtering, and all IPv6 routers between implement IPv4 ingress filtering, and all IPv6 routers between the
the decapsulating node and Bob implement IPv6 ingress filtering, the decapsulator and Bob implement IPv6 ingress filtering, the above
above spoofed packets will not be filtered out unless the spoofed packets will not be filtered out. As a result Bob will
decapsulator performs some checks. receive a packet that looks like it was sent from Alice even though
the sender was some unrelated node.
The solution to this is to have the decapsulating node perform
ingress filtering checks as part of the decapsulation as specified in
section 4.1.
4. Configured Tunneling
In configured tunneling, the tunnel endpoint address is determined
from configuration information in the encapsulating node. For each
tunnel, the encapsulating node must store the tunnel endpoint
address. When an IPv6 packet is transmitted over a tunnel, the
tunnel endpoint address configured for that tunnel is used as the
destination address for the encapsulating IPv4 header.
The determination of which packets to tunnel is usually made by
routing information on 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
to packets which are delivered to transport protocols on the
decapsulating node. For bidirectional configured tunnels this is
done by verifying that the source address is the IPv4 address of the
other end of the tunnel. For unidirectional configured tunnels the
decapsulating node MUST be configured with a list of source IPv4
address prefixes that are acceptable. Such a list MUST default to
not having any entries i.e., the node has to be explicitly configured
to forward decapsulated packets received over unidirectional
configured tunnels.
5. Acknowledgments
We would like to thank the members of the IPv6 working group, the The solution to this is to have the decapsulator only accept
Next Generation Transition (ngtrans) working group, and the v6ops encapsulated packets having explicitly configured source addresses
working group for their many contributions and extensive review of (e.g., in the case of bidirectional tunnels, the other end of the
this document. Special thanks are due to Jim Bound, Ross Callon, Bob tunnel) as specified in section 3.6.
Hinden, John Moy, and Pekka Savola for many helpful suggestions.
6. Security Considerations 5. Security Considerations
Tunneling is not known to introduce any security holes except for the Tunneling is not known to introduce any security holes except for the
possibility to circumvent ingress filtering [RFC2267]. This possibility to circumvent ingress filtering [RFC2827]. This
specification prevent tunneling from introducing additional specification prevent tunneling from introducing additional
weaknesses when IPv4 and/or IPv6 ingress filtering is in used by weaknesses when IPv4 and/or IPv6 ingress filtering is in used by
requiring that decapsulating nodes only accept packets if they have requiring that decapsulators only accept packets if they have been
been configured to accept encapsulated packets from the IPv4 source configured to accept encapsulated packets from the IPv4 source
address in the received packet. Such a check is easy to perform for address in the received packet. Such a check is easy to perform for
bidirectional tunnels, but for uni-directional tunnels it requires a bidirectional tunnels, but for uni-directional tunnels it requires a
separate configuration of the IPv4 source addresses that are separate configuration of the IPv4 source addresses that are
acceptable. acceptable.
An implementation of tunneling needs to be aware that while a tunnel An implementation of tunneling needs to be aware that while a tunnel
is a link (as defined in [RFC2460]), the threat model for a tunnel is a link (as defined in [RFC2460]), the threat model for a tunnel
might be rather different than for other links, since the tunnel might be rather different than for other links, since the tunnel
potentially includes all of the Internet. The recommendations to potentially includes all of the Internet. The recommendations to
verify that the IPv4 addresses in the encapsulated packet matches verify that the IPv4 addresses in the encapsulated packet matches
what has been configured for the tunnel, coupled with use of ingress what has been configured for the tunnel, coupled with use of ingress
filtering in IPv4, ameliorate some of this. In addition, an filtering in IPv4, ameliorate some of this. In addition, an
implementation must treat interfaces to different links as separate implementation must treat interfaces to different links as separate
e.g. to ensure that Neighbor Discovery packets arriving on one link e.g. to ensure that Neighbor Discovery packets arriving on one link
does not effect other links. This is especially important for tunnel does not effect other links. This is especially important for tunnel
links. links.
7. Authors' Addresses When dropping packets due to failing to match the allowed IPv4 source
Erik Nordmark addresses for a tunnel the node SHOULD NOT "acknowledge" the
Sun Microsystems Laboratories existence of a tunnel, otherwise this could be used to probe the
180, avenue de l'Europe acceptable tunnel endpoint addresses. For that reason the
38334 SAINT ISMIER Cedex, France specification says that such packets SHOULD be silently discarded.
Tel : +33 (0)4 76 18 88 03
Fax : +33 (0)4 76 18 88 88
Email : erik.nordmark@sun.com
Robert E. Gilligan 6. Acknowledgments
Intransa, Inc.
2870 Zanker Rd., Suite 100
San Jose, CA 95134
Tel : +1 408 678 8600 We would like to thank the members of the IPv6 working group, the
Fax : +1 408 678 8800 Next Generation Transition (ngtrans) working group, and the v6ops
Email : gilligan@intransa.com, gilligan@leaf.com 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 and
Fred Templin for many helpful suggestions.
8. References 7. References
8.1. Normative References 7.1. Normative References
[RFC1191] Mogul, J., and S. Deering., "Path MTU Discovery", RFC 1191, [RFC1191] Mogul, J., and S. Deering., "Path MTU Discovery", RFC 1191,
November 1990. November 1990.
[RFC1981] McCann, J., S. Deering, and J. Mogul. "Path MTU Discovery [RFC1981] McCann, J., S. Deering, and J. Mogul. "Path MTU Discovery
for IP version 6", RFC 1981, August 1996. for IP version 6", RFC 1981, August 1996.
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate [RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997. Requirement Levels", RFC 2119, March 1997.
[RFC2460] Deering, S., and Hinden, R. "Internet Protocol, Version 6 [RFC2460] Deering, S., and Hinden, R. "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998. (IPv6) Specification", RFC 2460, December 1998.
8.2. Non-normative References [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", [ASSIGNED] IANA, "Assigned numbers online database",
http://www.iana.org/numbers.html http://www.iana.org/numbers.html
[KM97] Kent, C., and J. Mogul, "Fragmentation Considered Harmful". [KM97] Kent, C., and J. Mogul, "Fragmentation Considered Harmful".
In Proc. SIGCOMM '87 Workshop on Frontiers in Computer In Proc. SIGCOMM '87 Workshop on Frontiers in Computer
Communications Technology. August 1987. Communications Technology. August 1987.
[RFC1122] Braden, R., "Requirements for Internet Hosts - Communication [RFC1122] Braden, R., "Requirements for Internet Hosts - Communication
Layers", STD 3, RFC 1122, October 1989. 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, [RFC1812] F. Baker, "Requirements for IP Version 4 Routers", RFC 1812,
June 1995. June 1995.
[RFC1886] Thomson, S., and Huitema C. "DNS Extensions to support IP
version 6", RFC 1886, 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 [RFC2461] Narten, T., Nordmark, E., and Simpson, W. "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998. Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998.
[RFC2462] Thomson, S., and Narten, T. "IPv6 Stateless Address [RFC2462] Thomson, S., and Narten, T. "IPv6 Stateless Address
Autoconfiguration," RFC 2462, December 1998. Autoconfiguration," RFC 2462, December 1998.
[RFC2667] D. Thaler, "IP Tunnel MIB", RFC 2667, August 1999. [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.
[RFC2923] K. Lahey, "TCP Problems with Path MTU Discovery", RFC 2923,
September 2000.
[RFC2983] D. Black, "Differentiated Services and Tunnels", RFC 2983, [RFC2983] D. Black, "Differentiated Services and Tunnels", RFC 2983,
October 2000. October 2000.
[RFC3056] B. Carpenter, and K. Moore, "Connection of IPv6 Domains via [RFC3056] B. Carpenter, and K. Moore, "Connection of IPv6 Domains via
IPv4 Clouds", RFC 3056, February 2001. IPv4 Clouds", RFC 3056, February 2001.
[RFC3168] K. Ramakrishnan, S. Floyd, D. Black, "The Addition of [RFC3168] K. Ramakrishnan, S. Floyd, D. Black, "The Addition of
Explicit Congestion Notification (ECN) to IP", RFC 3168, Explicit Congestion Notification (ECN) to IP", RFC 3168,
September 2001. September 2001.
[RFC3232] Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by an [RFC3232] Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by an
On-line Database", RFC 3232, January 2002. On-line Database", RFC 3232, January 2002.
[RFC3484] R. Draves, "Default Address Selection for IPv6", Work in [RFC3484] R. Draves, "Default Address Selection for IPv6", RFC 3484,
progress, draft-ietf-ipv6-default-addr-select-09.txt, June February 2003.
2002.
[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
9. Changes from RFC 2893 9. Changes from RFC 2893
The motivation for the bulk of these changes are to simplify the The motivation for the bulk of these changes are to simplify the
document to only contain the mechanisms of wide-spread use. document to only contain the mechanisms of wide-spread use.
RFC 2893 contains a mechanism called automatic tunneling. But a RFC 2893 contains a mechanism called automatic tunneling. But a much
much more general mechanism is specified in RFC 3056 [RFC3056] more general mechanism is specified in RFC 3056 [RFC3056] which gives
which gives each node with a (global) IPv4 address a /48 IPv6 each node with a (global) IPv4 address a /48 IPv6 prefix i.e., enough
prefix i.e., enough for a whole site. for a whole site.
The following changes have been performed since RFC 2893:
- Removed references to A6 and retained AAAA. - Removed references to A6 and retained AAAA.
- Removed automatic tunneling and IPv4-compatible addresses. - Removed automatic tunneling and use of IPv4-compatible
addresses.
- Removed default Configured Tunnel using IPv4 "Anycast Address" - Removed default Configured Tunnel using IPv4 "Anycast Address"
- Removed Source Address Selection section since this is now - Removed Source Address Selection section since this is now
covered by another document ([RFC3484]). covered by another document ([RFC3484]).
- Removed brief mention of 6over4. - Removed brief mention of 6over4.
- Split into normative and non-normative references and other - Split into normative and non-normative references and other
reference cleanup. reference cleanup.
skipping to change at page 24, line 48 skipping to change at page 24, line 29
OPTIONAL but if it is implemented it should follow the rules in OPTIONAL but if it is implemented it should follow the rules in
section 3.2. section 3.2.
- Stated that when the dynamic PMTU is not implemented the sender - Stated that when the dynamic PMTU is not implemented the sender
MUST NOT by default send IPv6 packets larger than 1280 into the MUST NOT by default send IPv6 packets larger than 1280 into the
tunnel. tunnel.
- Stated that implementations MAY have a knob by which the MTU can - Stated that implementations MAY have a knob by which the MTU can
be set to larger values on a tunnel by tunnel basis, but that be set to larger values on a tunnel by tunnel basis, but that
the default MUST be 1280 and that decapsulators need to be the default MUST be 1280 and that decapsulators need to be
configured to match the encapsulaltor's MTU. configured to match the encapsulator's MTU.
- Restated the "currently underway" language about ToS to loosely - Restated the "currently underway" language about ToS to loosely
point at [RFC2983] and [RFC3168]. point at [RFC2983] and [RFC3168].
- Stated that IPv4 source MAY also be administratively specified. - Stated that IPv4 source MAY also be administratively specified.
(This is especially useful on multi-interface nodes and with (This is especially useful on multi-interface nodes and with
configured tunneling) configured tunneling)
- Fixed reference to Assigned Numbers to be to online version - Fixed reference to Assigned Numbers to be to online version
(with proper pointer to "Assigned Numbers is obsolete" RFC) (with proper pointer to "Assigned Numbers is obsolete" RFC)
- Clarified text about ingress filtering e.g. that it applies to - Clarified text about ingress filtering e.g. that it applies to
packet delivered to transport protocols on the decapsulating packet delivered to transport protocols on the decapsulator as
node as well as packets being forwarded by the decapsulator, and well as packets being forwarded by the decapsulator, and how the
how the decapsulator's checks help when IPv4 and IPv6 ingress decapsulator's checks help when IPv4 and IPv6 ingress filtering
filtering is in place. is in place.
9.1. Changes from draft-ietf-v6ops-mech-v2-00
- 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
address and all the broadcast addresses of the decapsulator.
- Clarified that packets which 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 options and
target link layer address options are ignored in received ND
packets, the ND packets themselves are processed as normal.
10. Open Issues
The document has some specific text about unidirectional configure
tunnels since they are different with respect to Neighbor Discovery
and ingress filtering. Does anybody implement unidirectional
tunnels? Should we remove the specific text and make the explicit
assumption that all configured tunnels are bidirectional?
 End of changes. 

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