draft-ietf-ngtrans-mech-v2-00.txt   draft-ietf-ngtrans-mech-v2-01.txt 
INTERNET-DRAFT E. Nordmark INTERNET-DRAFT E. Nordmark
July 17, 2002 Sun Microsystems, Inc. November 4, 2002 Sun Microsystems, Inc.
Obsoletes: 2893 R. E. Gilligan Obsoletes: 2893 R. E. Gilligan
Intransa, Inc. Intransa, Inc.
Transition Mechanisms for IPv6 Hosts and Routers Basic Transition Mechanisms for IPv6 Hosts and Routers
<draft-ietf-ngtrans-mech-v2-00.txt> <draft-ietf-ngtrans-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 January 17, 2003. This draft expires on May 4, 2003.
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
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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 1.2. Structure of this Document.......................... 5
2. Dual IP Layer Operation.................................. 6 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.............................. 9 3. Common Tunneling Mechanisms.............................. 9
3.1. Encapsulation....................................... 10 3.1. Encapsulation....................................... 10
3.2. Tunnel MTU and Fragmentation........................ 11 3.2. Tunnel MTU and Fragmentation........................ 11
3.3. Hop Limit........................................... 13 3.3. Hop Limit........................................... 13
3.4. Handling IPv4 ICMP errors........................... 13 3.4. Handling IPv4 ICMP errors........................... 14
3.5. IPv4 Header Construction............................ 14 3.5. IPv4 Header Construction............................ 15
3.6. Decapsulation....................................... 16 3.6. Decapsulation....................................... 17
3.7. Link-Local Addresses................................ 17 3.7. Link-Local Addresses................................ 18
3.8. Neighbor Discovery over Tunnels..................... 18 3.8. Neighbor Discovery over Tunnels..................... 19
3.9. Ingress Filtering................................... 19 3.9. Ingress Filtering................................... 19
4. Configured Tunneling..................................... 19 4. Configured Tunneling..................................... 20
4.1. Ingress Filtering................................... 20 4.1. Ingress Filtering................................... 20
5. Acknowledgments.......................................... 20 5. Acknowledgments.......................................... 21
6. Security Considerations.................................. 20 6. Security Considerations.................................. 21
7. Authors' Addresses....................................... 20 7. Authors' Addresses....................................... 22
8. References............................................... 21 8. References............................................... 22
8.1. Normative References................................ 22
8.2. Non-normative References............................ 22
9. Changes from RFC 2893.................................... 22 9. Changes from RFC 2893.................................... 23
10. Changes from RFC 2893................................... 23 10. Open issues............................................. 25
11. TODO.................................................... 25
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.
The mechanisms in this document are designed to be employed by IPv6 The mechanisms in this document are designed to be employed by IPv6
hosts and routers that need to interoperate with IPv4 hosts and hosts and routers that need to interoperate with IPv4 hosts and
utilize IPv4 routing infrastructures. We expect that most nodes in utilize IPv4 routing infrastructures. We expect that most nodes in
the Internet will need such compatibility for a long time to come, the Internet will need such compatibility for a long time to come,
and perhaps even indefinitely. and perhaps even indefinitely.
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.
The mechanisms specified here include: The mechanisms specified here include:
- Dual IP layer (also known as Dual Stack): A technique for - Dual IP layer (also known as Dual Stack): A technique for
providing complete support for both Internet protocols -- IPv4 providing complete support for both Internet protocols -- IPv4
and IPv6 -- in hosts and routers. and IPv6 -- in hosts and routers.
- Configured tunneling of IPv6 over IPv4: Point-to-point tunnels - Configured tunneling of IPv6 over IPv4: Point-to-point tunnels
made by encapsulating IPv6 packets within IPv4 headers to carry made by encapsulating IPv6 packets within IPv4 headers to carry
them over IPv4 routing infrastructures. them over IPv4 routing infrastructures.
The mechanisms defined here are intended to be part of a "transition The mechanisms defined here are intended to be the core of a
toolbox" -- a growing collection of techniques which implementations "transition toolbox" -- a growing collection of techniques which
and users may employ to ease the transition. The tools may be used implementations and users may employ to ease the transition. The
as needed. Implementations and sites decide which techniques are tools may be used as needed. Implementations and sites decide which
appropriate to their specific needs. This document defines the techniques are appropriate to their specific needs.
initial core set of transition mechanisms, but these are not expected
to be the only tools available. Additional transition and This document defines the basic set of transition mechanisms, but
compatibility mechanisms are expected to be developed in the future, these are not the only tools available. Additional transition and
with new documents being written to specify them. compatibility mechanisms are specified in other documents.
1.1. Terminology 1.1. Terminology
The following terms are used in this document: The following terms are used in this document:
Types of Nodes Types of Nodes
IPv4-only node: IPv4-only node:
A host or router that implements only IPv4. An IPv4- A host or router that implements only IPv4. An IPv4-
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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 encapsulating node. 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.
Modes of operation of IPv6/IPv4 nodes
IPv6-only operation:
An IPv6/IPv4 node with its IPv6 stack enabled and its
IPv4 stack disabled.
IPv4-only operation:
An IPv6/IPv4 node with its IPv4 stack enabled and its
IPv6 stack disabled.
IPv6/IPv4 operation:
An IPv6/IPv4 node with both stacks enabled.
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 [16]. document, are to be interpreted as described in [16].
1.2. Structure of this Document 1.2. Structure of this Document
The remainder of this document is organized as follows: The remainder of this document is organized as follows:
- Section 2 discusses the operation of nodes with a dual IP layer, - Section 2 discusses the operation of nodes with a dual IP layer,
IPv6/IPv4 nodes. IPv6/IPv4 nodes.
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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.
Even though a node may be equipped to support both protocols, one or Even though a node may be equipped to support both protocols, one or
the other stack may be disabled for operational reasons. Thus the other stack may be disabled for operational reasons. Here we use
IPv6/IPv4 nodes may be operated in one of three modes: 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 IPv4 stack enabled and their IPv6 stack disabled.
- 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 dual IP layer technique may or may not be used in conjunction The IPv6-over-IPv4 tunneling techniques, which are described in
with the IPv6-over-IPv4 tunneling technique, which are described in sections 3 and 4, may or may not be used in addition to the dual IP
sections 3 and 4. An IPv6/IPv4 node MAY support configured layer operation. An IPv6/IPv4 node MAY support configured tunneling.
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-native addresses. and/or DHCPv6) to acquire their IPv6-native 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 [6]. Since IPv6/IPv4 "AAAA" has been defined for IPv6 addresses [6]. Since IPv6/IPv4
nodes must be able to interoperate directly with both IPv4 and IPv6 nodes must be able to interoperate directly with both IPv4 and IPv6
nodes, they must provide resolver libraries capable of dealing with nodes, they must provide resolver libraries capable of dealing with
IPv4 "A" records as well as IPv6 "AAAA" records. 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.
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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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 [19]. More details on this subject are specified in [19].
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. Most of these are obvious but are stated here for transition. The constraints allow nodes to prefer either IPv6 or
completeness. IPv4 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 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.
If an IPv6 node is isolated from an IPv6 perspective (e.g., it is not If an IPv6 node is isolated from an IPv6 perspective (e.g., it is not
connected to the 6bone to take a concrete example) constraint #3 connected to the 6bone to take a concrete example) constraint #3
would mean that it should not have an address in the DNS. would mean that it should not have an address in the DNS.
This works great when other dual stack nodes tries to contact the This works great when other dual stack nodes try to contact the
isolated dual stack node. There is no IPv6 address in the DNS thus isolated dual stack node. There is no IPv6 address in the DNS thus
the peer doesn't even try communicating using IPv6 but goes directly the peer doesn't even try communicating using IPv6 but goes directly
to IPv4 (we are assuming both nodes have A records in the DNS.) to IPv4 (we are assuming both nodes have A records in the DNS.)
However, this does not work well when the isolated node is trying to However, this does not work well when the isolated node is trying to
establish communication. Even though it does not have an IPv6 establish communication. Even though it does not have an IPv6
address in the DNS it will find AAAA records in the DNS for the peer. address in the DNS it will find AAAA records in the DNS for the peer.
Since the isolated node has IPv6 addresses assigned to at least one Since the isolated node has IPv6 addresses assigned to at least one
interface it will try to communicate using IPv6. If it has no IPv6 interface it will try to communicate using IPv6. If it has no IPv6
route to the 6bone (e.g., because the local router was upgraded to route to the 6bone (e.g., because the local router was upgraded to
advertise IPv6 addresses using Neighbor Discovery but that router advertise IPv6 addresses using Neighbor Discovery but that router
doesn't have any IPv6 routes) this communication will fail. doesn't have any IPv6 routes) this communication will fail.
Typically this means a few minutes of delay as TCP times out. The Typically this means a few minutes of delay as TCP times out. The
TCP specification says that ICMP unreachable messages could be due to TCP specification [10] says that ICMP unreachable messages could be
routing transients thus they should not immediately terminate the TCP due to routing transients thus they should not immediately terminate
connection. This means that the normal TCP timeout of a few minutes the TCP connection. This means that the normal TCP timeout of a few
apply. Once TCP times out the application will hopefully try the minutes apply. Once TCP times out the application will hopefully try
IPv4 addresses based on the A records in the DNS, but this will be the IPv4 addresses based on the A records in the DNS, but this will
painfully slow. be painfully slow.
A possible implication of the recommendations above is that, if one A possible implication of the recommendations above is that, if one
enables IPv6 on a node on a link without IPv6 infrastructure, and enables IPv6 on a node on a link without IPv6 infrastructure, and
choose to add AAAA records to the DNS for that node, then external choose to add AAAA records to the DNS for that node, then external
IPv6 nodes that might see these AAAA records will possibly try to IPv6 nodes that might see these AAAA records will possibly try to
reach that node using IPv6 and suffer delays or communication failure reach that node using IPv6 and suffer delays or communication failure
due to unreachability. (A delay is incurred if the application due to unreachability. (A delay is incurred if the application
correctly falls back to using IPv4 if it can not establish correctly falls back to using IPv4 if it can not establish
communication using IPv6 addresses. If this fallback is not done the communication using IPv6 addresses. If this fallback is not done the
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 site or node removes the support for IPv4 the In the future, when a node discontinues its use of IPv4, analogous
above recommendations apply to when the A records for the node(s) constraints apply with respect to the node's A records in the DNS;
should be removed from 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 3. Common 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.
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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 Tunneling techniques are usually classified according to the
mechanism by which the encapsulating node determines the address of mechanism by which the encapsulating node determines the address of
the node at the end of the tunnel. In the first two tunneling the node at the end of the tunnel. In the first two tunneling
methods listed above -- router-to-router and host-to-router -- the methods listed above -- router-to-router and host-to-router -- the
IPv6 packet is being tunneled to a router. The endpoint of this type IPv6 packet is being tunneled to a router. The endpoint of this type
of tunnel is an intermediary router which must decapsulate the IPv6 of tunnel is an intermediary router which must decapsulate the IPv6
packet and forward it on to its final destination. When tunneling to packet and forward it on to its final destination. When tunneling to
a router, the endpoint of the tunnel is different from the a router, the endpoint of the tunnel is different from the
destination of the packet being tunneled. So the addresses in the destination of the packet being tunneled. In some cases, the
IPv6 packet being tunneled can not provide the IPv4 address of the addresses in the IPv6 packet being tunneled can not provide the IPv4
tunnel endpoint. Instead, the tunnel endpoint address must be address of the tunnel endpoint. In those cases, the tunnel endpoint
determined from configuration information on the node performing the address must be determined from configuration information on the node
tunneling. We use the term "configured tunneling" to describe the performing the encapsulation. We use the term "configured tunneling"
type of tunneling where the endpoint is explicitly configured. 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 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. -- 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 In this case, the destination address of both the IPv6 packet and the
encapsulating IPv4 header identify the same node. However, the encapsulating IPv4 header identify the same node. However, the
tunneling mechanism specified in this document does not handle these tunneling mechanism specified in this document does not handle these
cases any differently; the IPv4 addresses is still determined using cases any differently; the IPv4 addresses is still determined using
configuration information using configured tunneling. configuration information using configured tunneling.
The underlying mechanisms for tunneling are: The underlying mechanisms for tunneling are:
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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 decapsulating node) receives
the encapsulated packet, reassembles the packet if needed, the encapsulated packet, reassembles the packet if needed,
removes the IPv4 header, updates the IPv6 header, and processes removes the IPv4 header, updates the IPv6 header, and processes
the received IPv6 packet. the received IPv6 packet.
- The encapsulating node MAY need to maintain soft state - The encapsulating node MAY need to maintain soft state
information for each tunnel recording such parameters as the MTU information for each tunnel recording such parameters as the MTU
of the tunnel in order to process IPv6 packets forwarded into of the tunnel in order to process IPv6 packets forwarded into
the tunnel. Since the number of tunnels that any one host or the tunnel. In cases where the number of tunnels that any one
router may be using may grow to be quite large, this state host or router is using is large, it is helpful to observe that
information can be cached and discarded when not in use. this state information can be cached and discarded when not in
use.
The remainder of this section discusses the common mechanisms. A The remainder of this section discusses the common mechanisms. A
subsequent section discusses how the tunnel endpoint address is subsequent section discusses how the tunnel endpoint address is
determined for configured tunneling. determined for configured tunneling.
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:
+-------------+ +-------------+
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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 encapsulating node would need only to report IPv6 ICMP "packet too
big" errors back to the source for packets that exceed this MTU. big" errors back to the source for packets that exceed this MTU.
However, such a scheme would be inefficient for two reasons: However, such a scheme would be inefficient for two reasons:
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 [11]. unit [11].
2) Any IPv4 fragmentation occurring inside the tunnel would have to 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 be reassembled at the tunnel endpoint. For tunnels that
terminate at a router, this would require additional memory to terminate at a router, this would require additional memory to
reassemble the IPv4 fragments into a complete IPv6 packet before reassemble the IPv4 fragments into a complete IPv6 packet before
that packet could be forwarded onward. 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 use the smaller MTU
specified below.
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 encapsulating node track the IPv4 Path MTU across the
tunnel, using the IPv4 Path MTU Discovery Protocol [8] and recording tunnel, using the IPv4 Path MTU Discovery Protocol [8] and recording
the resulting path MTU. The IPv6 layer in the encapsulating node can the resulting path MTU. The IPv6 layer in the encapsulating node can
then view a tunnel as a link layer with an MTU equal to the IPv4 path 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. MTU, minus the size of the encapsulating IPv4 header.
Note that this does not completely eliminate IPv4 fragmentation in Note that this does not completely eliminate IPv4 fragmentation in
the case when the IPv4 path MTU would result in an IPv6 MTU less than 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 1280 bytes. (Any link layer used by IPv6 has to have an MTU of at
least 1280 bytes [4].) In this case the IPv6 layer has to "see" a least 1280 bytes [4].) In this case the IPv6 layer has to "see" a
link layer with an MTU of 1280 bytes and the encapsulating node has link layer with an MTU of 1280 bytes and the encapsulating node has
to use IPv4 fragmentation in order to forward the 1280 byte IPv6 to use IPv4 fragmentation in order to forward the 1280 byte IPv6
packets. packets.
This dynamic MTU determination is OPTIONAL. However, if it is
implemented it SHOULD have the behavior described in this document
and the tunnel MTU MUST be not exceed 4400 bytes. 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.
The encapsulating node can employ the following algorithm to The encapsulating node can employ the following algorithm to
determine when to forward an IPv6 packet that is larger than the determine when to forward an IPv6 packet that is larger than the
tunnel's path MTU using IPv4 fragmentation, and when to return an tunnel's path MTU using IPv4 fragmentation, and when to return an
IPv6 ICMP "packet too big" message: IPv6 ICMP "packet too big" message:
if (IPv4 path MTU - 20) is less than or equal to 1280 if (IPv4 path MTU - 20) is less than or equal to 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 encapsulating node 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) or packet
is larger than 4400
Send IPv6 ICMP "packet too big" with Send IPv6 ICMP "packet too big" with
MTU = (IPv4 path MTU - 20). MTU = MIN(IPv4 path MTU - 20, 4400).
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 Encapsulating nodes that have a large number of tunnels might not be
able to store the IPv4 Path MTU for all tunnels. Such nodes can, at able to store the IPv4 Path MTU for all tunnels. Such nodes can, at
the expense of additional fragmentation in the network, avoid using the expense of additional fragmentation in the network, avoid using
the IPv4 Path MTU algorithm across the tunnel and instead use the MTU 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 of the link layer (under IPv4) in the above algorithm instead of the
IPv4 path MTU. IPv4 path MTU.
In this case the Don't Fragment bit MUST NOT be set in the In this case the Don't Fragment bit MUST NOT be set in the
encapsulating IPv4 header. encapsulating IPv4 header.
skipping to change at page 13, line 31 skipping to change at page 14, line 10
The single-hop model is implemented by having the encapsulating and The single-hop model is implemented by having the encapsulating and
decapsulating nodes process the IPv6 hop limit field as they would if decapsulating nodes process the IPv6 hop limit field as they would if
they were forwarding a packet on to any other datalink. That is, they were forwarding a packet on to any other datalink. That is,
they decrement the hop limit by 1 when forwarding an IPv6 packet. they decrement the hop limit by 1 when forwarding an IPv6 packet.
(The originating node and final destination do not decrement the hop (The 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. Implementations MAY provide published in the "Assigned Numbers" RFC. Implementations MAY provide
a mechanism to allow the administrator to configure the IPv4 TTL such a mechanism to allow the administrator to configure the IPv4 TTL such
as the one specified in the IP Tunnel MIB [17]. as the one specified in the IP Tunnel MIB [17].
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 encapsulating node might receive IPv4 ICMP error messages from IPv4
routers inside the tunnel. These packets are addressed to the routers inside the tunnel. These packets are addressed to the
encapsulating node because it is the IPv4 source of the encapsulated encapsulating node because it is the IPv4 source of the encapsulated
packet. packet.
skipping to change at page 15, line 16 skipping to change at page 15, line 46
4 4
IP Header Length in 32-bit words: IP Header Length in 32-bit words:
5 (There are no IPv4 options in the encapsulating 5 (There are no IPv4 options in the encapsulating
header.) header.)
Type of Service: Type of Service:
0. [Note that work underway in the IETF is redefining 0 unless otherwise specified. (See [20] and [21] for
the Type of Service byte and as a result future RFCs issues relating to the ToS byte and tunneling.)
might define a different behavior for the ToS byte when
tunneling.]
Total Length: Total Length:
Payload length from IPv6 header plus length of IPv6 and Payload length from IPv6 header plus length of IPv6 and
IPv4 headers (i.e. a constant 60 bytes). IPv4 headers (i.e., IPv6 payload length plus a constant
60 bytes).
Identification: Identification:
Generated uniquely as for any IPv4 packet transmitted by Generated uniquely as for any IPv4 packet transmitted by
the system. the system.
Flags: Flags:
Set the Don't Fragment (DF) flag as specified in section Set the Don't Fragment (DF) flag as specified in section
3.2. Set the More Fragments (MF) bit as necessary if 3.2. Set the More Fragments (MF) bit as necessary if
skipping to change at page 16, line 8 skipping to change at page 16, line 40
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 encapsulating
node. node. The source address MAY alternatively be
administratively specified to be a specific IPv4 address
assigned to the encapsulating node.
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, it reassembles if the packet if it is
fragmented at the IPv4 level, then it removes the IPv4 header and fragmented at the IPv4 level, then it removes the IPv4 header and
submits the IPv6 datagram to its IPv6 layer code. submits the IPv6 datagram to its IPv6 layer code.
The decapsulating node MUST be capable of reassembling an IPv4 packet The decapsulating node MUST be capable of reassembling an IPv4 packet
that is 1300 bytes (1280 bytes plus IPv4 header). that is 4420 bytes (4400 bytes plus IPv4 header). This 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 MRU than 4420 bytes, but the value MUST not be set below 4420.
The decapsulation is shown below: The decapsulation is shown below:
+-------------+ +-------------+
| IPv4 | | IPv4 |
| Header | | Header |
+-------------+ +-------------+ +-------------+ +-------------+
| IPv6 | | IPv6 | | IPv6 | | IPv6 |
| Header | | Header | | Header | | Header |
+-------------+ +-------------+ +-------------+ +-------------+
skipping to change at page 16, line 47 skipping to change at page 17, line 40
| 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. When decapsulating the packet, the IPv6 header is not modified. (See
[Note that work underway in the IETF is redefining the Type of [20] and [21] for issues relating to the Type of Service byte and
Service byte and as a result future RFCs might define a different tunneling.) If the packet is subsequently forwarded, its hop limit
behavior for the ToS byte when decapsulating a tunneled packet.] If is decremented by one.
the packet is subsequently forwarded, its hop limit is decremented by
one.
As part of the decapsulation the node SHOULD silently discard a As part of the decapsulation the node SHOULD silently discard a
packet with an invalid IPv4 source address such as a multicast 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 address, a broadcast address, 0.0.0.0, and 127.0.0.1. In general it
SHOULD apply the rules for martian filtering in [18] and ingress SHOULD apply the rules for martian filtering in [18] and ingress
filtering [13] on the IPv4 source address. filtering [13] on the IPv4 source address.
The decapsulating node performs IPv4 reassembly before decapsulating
the IPv6 packet. All IPv6 options are preserved even if the
encapsulating IPv4 packet is fragmented.
The encapsulating IPv4 header is discarded. The encapsulating IPv4 header is discarded.
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 unspecified address, and the loopback address but also
IPv4-compatible IPv6 source addresses where the IPv4 part of the 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, 0.0.0.0,
or 127.0.0.1. In general it SHOULD apply the rules for martian or 127.0.0.1. In general it SHOULD apply the rules for martian
filtering in [18] and ingress filtering [13] on the IPv4-compatible filtering in [18] and ingress filtering [13] on the IPv4-compatible
source address. source address.
The decapsulating node performs IPv4 reassembly before decapsulating
the IPv6 packet. All IPv6 options are preserved even if the
encapsulating IPv4 packet is fragmented.
After the IPv6 packet is decapsulated, it is processed almost the After the IPv6 packet is decapsulated, it is processed almost the
same as any received IPv6 packet. The only difference being that a same as any received IPv6 packet. The only difference being that a
decapsulated packet MUST NOT be forwarded unless the node has been decapsulated packet MUST NOT be forwarded unless the node has been
explicitly configured to forward such packets for the given IPv4 explicitly configured to forward such packets for the given IPv4
source address. This configuration can be implicit in e.g., having a source address. This configuration can be implicit in e.g., having a
configured tunnel which matches the IPv4 source address. This configured tunnel which matches the IPv4 source address. This
restriction is needed to prevent tunneling to be used as a tool to restriction is needed to prevent tunneling to be used as a tool to
circumvent ingress filtering [13]. circumvent ingress filtering [13].
3.7. Link-Local Addresses 3.7. Link-Local Addresses
skipping to change at page 18, line 19 skipping to change at page 19, line 13
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
Unidirectional configured tunnels are considered to be For unidirectional configured tunnels most of Neighbor Discovery [17]
unidirectional! Thus the only aspects of Neighbor Discovery [7] and and Stateless Address Autoconfiguration [5] does not apply; only the
Stateless Address Autoconfiguration [5] that apply to these tunnels formation of the link-local address applies.
is the formation of the link-local address.
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 [7]. Such implementations SHOULD Neighbor Unreachability Detection [7]. Such implementations SHOULD
also send NUD probe packets to detect when the configured tunnel also send NUD probe packets to detect when the configured tunnel
fails at which point the implementation can use an alternate path to fails at which point the implementation can use an alternate path to
reach the destination. Note that Neighbor Discovery allows that the reach the destination. Note that Neighbor Discovery allows that the
sending of NUD probes be omitted for router to router links if the sending of NUD probes be omitted for router to router links if the
routing protocol tracks bidirectional reachability. 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 as 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 a sender of Neighbor Discovery packets
- SHOULD NOT include Source Link Layer Address options or Target - SHOULD NOT include Source Link Layer Address options or Target
Link Layer Address options on the tunnel link. Link Layer Address options on the tunnel link.
- MUST silently ignore any received SLLA or TLLA options on the - MUST silently ignore any received SLLA or TLLA options on the
tunnel link. tunnel link.
Not using alink layer address options is consistent with how neighbor
discovery is used on other point-to-point links.
3.9. Ingress Filtering 3.9. Ingress Filtering
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 [13]. When IP-in-IP tunneling filtering in general is specified in [13]. When IP-in-IP tunneling
(independent of IP versions) is used it is important that this not be (independent of IP versions) is used it is important that this not be
a tool to bypass ingress filtering for non-tunneled packets. For a tool to bypass ingress filtering for non-tunneled packets. For
instance, without specific ingress filtering checks in the instance, without specific ingress filtering checks in the
decapsulating node, it would be possible for an attacker to inject a decapsulating node, it would be possible for an attacker to inject a
packet with: packet with:
skipping to change at page 20, line 16 skipping to change at page 21, line 6
The decapsulating node MUST verify that the tunnel source address is The decapsulating node MUST verify that the tunnel source address is
acceptable before forwarding decapsulated packets to avoid acceptable before forwarding decapsulated packets to avoid
circumventing ingress filtering [13]. Note that packets which are circumventing ingress filtering [13]. Note that packets which are
delivered to transport protocols on the decapsulating node SHOULD NOT delivered to transport protocols on the decapsulating node SHOULD NOT
be subject to these checks. For bidirectional configured tunnels be subject to these checks. For bidirectional configured tunnels
this is done by verifying that the source address is the IPv4 address this is done by verifying that the source address is the IPv4 address
of the other end of the tunnel. For unidirectional configured of the other end of the tunnel. For unidirectional configured
tunnels the decapsulating node MUST be configured with a list of tunnels the decapsulating node MUST be configured with a list of
source IPv4 address prefixes that are acceptable. Such a list MUST 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 default to not having any entries i.e., the node has to be explicitly
configured to forward decapsulated packets received over configured to forward decapsulated packets received over
unidirectional configured tunnels. unidirectional configured tunnels.
5. Acknowledgments 5. Acknowledgments
We would like to thank the members of the IPng working group and the We would like to thank the members of the IPng working group and the
Next Generation Transition (ngtrans) working group for their many Next Generation Transition (ngtrans) working group for their many
contributions and extensive review of this document. Special thanks contributions and extensive review of this document. Special thanks
are due to Jim Bound, Ross Callon, Bob Hinden, and John Moy for many are due to Jim Bound, Ross Callon, Bob Hinden, John Moy, and Pekka
helpful suggestions. Savola for many helpful suggestions.
6. Security Considerations 6. 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 [13]. This is prevented possibility to circumvent ingress filtering [13]. This is prevented
by requiring that decapsulating routers only forward packets if they by requiring that decapsulating routers only forward packets if they
have been configured to accept encapsulated packets from the IPv4 have been configured to accept encapsulated packets from the IPv4
source address in the receive packet. source address in the receive packet.
An implementation of tunneling needs to be aware that while a tunnel
is a link (as defined in [4]), the threat model for a tunnel might be
rather different than for other links, since the tunnel potentially
includes all of the Internet. The recommendations to verify that the
IPv4 addresses in the encapsulated packet matches what has been
configured for the tunnel, coupled with use of ingress filtering in
IPv4, ameliorate some of this. In addition, 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 7. Authors' Addresses
Erik Nordmark Erik Nordmark
Sun Microsystems Laboratories Sun Microsystems Laboratories
29, Chemin du Vieux Chene 180, avenue de l'Europe
38240 Meylan, France 38334 SAINT ISMIER Cedex, France
Tel : +33 (0)4 76 18 88 03
phone: +33 (0)4 76 18 88 03 Fax : +33 (0)4 76 18 88 88
fax: +33 (0)4 76 18 88 88 EMail : erik.nordmark@sun.com
email: erik.nordmark@sun.com
Robert E. Gilligan Robert E. Gilligan
Intransa, Inc. Intransa, Inc.
1393 Geneva Drive 2870 Zanker Rd., Suite 100
Sunnyvale, CA 94089-1121 San Jose, CA 95134
phone: 408.548.5140 Tel : +1 408 678 8600
fax: 408.548.5196 Fax : +1 408 678 8800
email: gilligan@intransa.com, gilligan@leaf.com Email : gilligan@intransa.com, gilligan@leaf.com
8. References 8. References
[1] Croft, W., and J. Gilmore, "Bootstrap Protocol", RFC 951, 8.1. Normative References
September 1985.
[2] Droms, R., "Dynamic Host Configuration Protocol", RFC 1541.
October 1993.
[4] Deering, S., and Hinden, R. "Internet Protocol, Version 6 (IPv6) [4] Deering, S., and Hinden, R. "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998. Specification", RFC 2460, December 1998.
[8] Mogul, J., and Deering, S., "Path MTU Discovery", RFC 1191,
November 1990.
[14] Hinden, R., and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
8.2. Non-normative References
[5] Thomson, S., and Narten, T. "IPv6 Stateless Address [5] Thomson, S., and Narten, T. "IPv6 Stateless Address
Autoconfiguration," RFC 2462, December 1998. Autoconfiguration," RFC 2462, December 1998.
[6] Thomson, S., and Huitema C. "DNS Extensions to support IP [6] Thomson, S., and Huitema C. "DNS Extensions to support IP
version 6", RFC 1886, December 1995. version 6", RFC 1886, December 1995.
[7] Narten, T., Nordmark, E., and Simpson, W. "Neighbor Discovery [7] Narten, T., Nordmark, E., and Simpson, W. "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998. for IP Version 6 (IPv6)", RFC 2461, December 1998.
[8] Mogul, J., and Deering, S., "Path MTU Discovery", RFC 1191,
November 1990.
[9] Finlayson, R., Mann, T., Mogul, J., and M. Theimer, "Reverse
Address Resolution Protocol", RFC 903, June 1984.
[10] Braden, R., "Requirements for Internet Hosts - Communication [10] Braden, R., "Requirements for Internet Hosts - Communication
Layers", STD 3, RFC 1122, October 1989. Layers", STD 3, RFC 1122, October 1989.
[11] Kent, C., and J. Mogul, "Fragmentation Considered Harmful". In [11] Kent, C., and J. Mogul, "Fragmentation Considered Harmful". In
Proc. SIGCOMM '87 Workshop on Frontiers in Computer Proc. SIGCOMM '87 Workshop on Frontiers in Computer
Communications Technology. August 1987. Communications Technology. August 1987.
[12] Callon, R. and Haskin, D., "Routing Aspects of IPv6 Transition",
RFC 2185. September 1997.
[13] Ferguson, P., and Senie, D., "Network Ingress Filtering: [13] Ferguson, P., and Senie, D., "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", RFC 2267, January 1998. Address Spoofing", RFC 2267, January 1998.
[14] Hinden, R., and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[15] Rechter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.J., and
Lear, E. "Address Allocation for Private Internets", RFC 1918,
February 1996.
[16] S. Bradner, "Key words for use in RFCs to Indicate Requirement [16] S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997. Levels", RFC 2119, March 1997.
[17] D. Thaler, "IP Tunnel MIB", RFC 2667, August 1999. [17] D. Thaler, "IP Tunnel MIB", RFC 2667, August 1999.
[18] F. Baker, "Requirements for IP Version 4 Routers", RFC 1812, [18] F. Baker, "Requirements for IP Version 4 Routers", RFC 1812,
June 1995. June 1995.
[19] R. Draves, "Default Address Selection for IPv6", Work in [19] R. Draves, "Default Address Selection for IPv6", Work in
progress, draft-ietf-ipv6-default-addr-select-08.txt, June progress, draft-ietf-ipv6-default-addr-select-09.txt, June
2002. 2002.
[20] D. Black, "Differentiated Services and Tunnels", RFC 2893,
October 2000.
[21] K. Ramakrishnan, S. Floyd, D. Black, "The Addition of Explicit
Congestion Notification (ECN) to IP", RFC 3168, September 2001.
9. Changes from RFC 2893 9. Changes from RFC 2893
The motivation for the bulk of these changes are to simplify the
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 which gives
each node with a (global) IPv4 address a /48 IPv6 prefix i.e.,
enough for a whole site.
- 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 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 ([19]). covered by another document ([19]).
- Removed brief mention of 6over4. - Removed brief mention of 6over4.
10. Changes from RFC 2893 - Split into normative and non-normative references.
- Should 6to4 be mentioned? How complete should we make the list - Drop "or equal" in if (IPv4 path MTU - 20) is less than or equal
of tunneling techniques in section 1.1? 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.
- Increased the minimum MRU from 1300 to 4420.
- 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 that when the dynamic PMTU is not implemented the sender
MUST NOT by default send IPv6 packets larger than 1280 into the
tunnel.
- Stated that implementations MAY have a knob by which the "min
MRU" and the max MTU" can be set to larger values on a tunnel by
tunnel basis, but that the defaults MUST be the above numbers.
- Restated the "currently underway" language about ToS to loosely
point at [20] and [21].
- Stated that IPv4 source MAY also be administratively specified.
(This is especially useful on multi-interface nodes and with
configured tunneling)
10. Open issues
- Should all of section 2.2 (DNS stuff) be removed? It duplicates - Should all of section 2.2 (DNS stuff) be removed? It duplicates
[19]. [19].
- Should we drop the discussion about unidirectional tunneling?
- Is 1280 a reasonable MTU for encapsulators that do not implement
dynamic tunnel MTU discovery?
- Is 4400 a reasonable cap on the MTU for encapsulators that
implement dynamic tunnel MTU discovery?
- Is 4420 is reasonable minimum MRU for decapsulating nodes?
- IPv6 native addresses include IPv4-mapped as currently defined.
Is this an issue that needs to be dealt with? The term "IPv6-
native" is only used once in the document.
11. TODO
- Section 4.1: Ingress filtering as described does not guard
against the attack described in 3.9. Checking must be done on
the v6 address - attacker can spoof the v4 source address,
unless tunnel is secured. Make it more clear that the
assumption is that *if* IPv4 and IPv6 ingress filtering is
deployed, the decapsulating node can't blindly decapsulate since
it would circumvent the ingress filtering.
- Section 3.3 reference to Assigned Numbers needs to be to online
version (with proper pointer to "Assigned Numbers is obsolete"
RFC)
 End of changes. 

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