draft-ietf-v6ops-mech-v2-01.txt   draft-ietf-v6ops-mech-v2-02.txt 
INTERNET-DRAFT E. Nordmark INTERNET-DRAFT E. Nordmark
October 27, 2003 Sun Microsystems, Inc. January 30, 2004 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-v2-01.txt> <draft-ietf-v6ops-mech-v2-02.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 April 27, 2004. This draft expires on July 30, 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. Two mechanisms are specified,
providing complete implementations of both versions of the Internet "dual stack" and configured tunneling. Dual stack implies providing
Protocol (IPv4 and IPv6), and tunneling IPv6 packets over IPv4 complete implementations of both versions of the Internet Protocol
routing infrastructures. They are designed to allow IPv6 nodes to (IPv4 and IPv6) and configured tunneling provides a means to carry
maintain complete compatibility with IPv4, which should greatly IPv6 packets over unmodified IPv4 routing infrastructures.
simplify the deployment of IPv6 in the Internet, and facilitate the
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
2. Dual IP Layer Operation.................................. 5 2. Dual IP Layer Operation.................................. 5
2.1. Address Configuration............................... 6 2.1. Address Configuration............................... 5
2.2. DNS................................................. 6 2.2. DNS................................................. 5
2.3. Advertising Addresses in the DNS.................... 7
3. Configured Tunneling Mechanisms.......................... 8 3. Configured Tunneling Mechanisms.......................... 7
3.1. Encapsulation....................................... 10 3.1. Encapsulation....................................... 8
3.2. Tunnel MTU and Fragmentation........................ 10 3.2. Tunnel MTU and Fragmentation........................ 9
3.2.1. Static Tunnel MTU.............................. 11 3.2.1. Static Tunnel MTU.............................. 10
3.2.2. Dynamic Tunnel MTU............................. 11 3.2.2. Dynamic Tunnel MTU............................. 10
3.3. Hop Limit........................................... 13 3.3. Hop Limit........................................... 12
3.4. Handling IPv4 ICMP errors........................... 13 3.4. Handling ICMPv4 errors.............................. 12
3.5. IPv4 Header Construction............................ 14 3.5. IPv4 Header Construction............................ 14
3.6. Decapsulation....................................... 16 3.6. Decapsulation....................................... 15
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
4. Threat Related to Source Address Spoofing................ 19 4. Threat Related to Source Address Spoofing................ 19
5. Security Considerations.................................. 20 5. Security Considerations.................................. 20
6. Acknowledgments.......................................... 21 6. Acknowledgments.......................................... 22
7. References............................................... 21
7.1. Normative References................................ 21
7.2. Non-normative References............................ 21
8. Authors' Addresses....................................... 23 7. References............................................... 22
7.1. Normative References................................ 22
7.2. Non-normative References............................ 22
9. Changes from RFC 2893.................................... 23 8. Authors' Addresses....................................... 24
9.1. Changes from draft-ietf-v6ops-mech-v2-00............ 25
10. Open Issues............................................. 26 9. Changes from RFC 2893.................................... 25
9.1. Changes from draft-ietf-v6ops-mech-v2-00............ 27
9.2. Changes from draft-ietf-v6ops-mech-v2-01............ 28
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
set of mechanisms that IPv6 hosts and routers may implement in order two mechanisms that IPv6 hosts and routers may implement in order to
to be compatible with IPv4 hosts and routers. 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.
The mechanisms specified here include: The mechanisms specified here are:
- 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: A technique for
made by encapsulating IPv6 packets within IPv4 headers to carry establishing point-to-point tunnels by encapsulating IPv6
them over IPv4 routing infrastructures. packets within IPv4 headers to carry them over IPv4 routing
infrastructures.
The mechanisms defined here are intended to be the core of a The mechanisms defined here are intended to be the core of a
"transition toolbox" -- a growing collection of techniques which "transition toolbox" -- a growing collection of techniques which
implementations and users may employ to ease the transition. The implementations and users may employ to ease the transition. The
tools may be used as needed. Implementations and sites decide which tools may be used as needed. Implementations and sites decide which
techniques are appropriate to their specific needs. techniques are appropriate to their specific needs.
This document defines the basic set of transition mechanisms, but This document defines the basic set of transition mechanisms, but
these are not the only tools available. Additional transition and these are not the only tools available. Additional transition and
compatibility mechanisms are specified in other documents. compatibility mechanisms are specified in other documents.
skipping to change at page 4, line 14 skipping to change at page 4, line 15
begins are IPv4-only nodes. begins are IPv4-only nodes.
IPv6/IPv4 node: IPv6/IPv4 node:
A host or router that implements both IPv4 and IPv6. A host or router that implements both IPv4 and IPv6.
IPv6-only node: IPv6-only node:
A host or router that implements IPv6, and does not A host or router that implements IPv6, and does not
implement IPv4. The operation of IPv6-only nodes is not implement IPv4. The operation of IPv6-only nodes is not
addressed here. addressed in this memo.
IPv6 node: IPv6 node:
Any host or router that implements IPv6. IPv6/IPv4 and Any host or router that implements IPv6. IPv6/IPv4 and
IPv6-only nodes are both IPv6 nodes. IPv6-only nodes are both IPv6 nodes.
IPv4 node: IPv4 node:
Any host or router that implements IPv4. IPv6/IPv4 and Any host or router that implements IPv4. IPv6/IPv4 and
IPv4-only nodes are both IPv4 nodes. IPv4-only nodes are both IPv4 nodes.
Types of IPv6 Addresses
IPv4-compatible IPv6 address:
An IPv6 address bearing the high-order 96-bit prefix
0:0:0:0:0:0, and an IPv4 address in the low-order 32-
bits. IPv4-compatible addresses are no longer used by
this specification, thus this definition is preserved in
the specification merely to clarify their non-use.
Techniques Used in the Transition Techniques Used in the Transition
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 encapsulator. The tunnels can be either the encapsulator. All tunnels are assumed to be
unidirectional or bidirectional. Bidirectional bidirectional, behaving as virtual point-to-point links.
configured tunnels behave as virtual point-to-point
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].
2. Dual IP Layer Operation 2. Dual IP Layer Operation
skipping to change at page 5, line 45 skipping to change at page 5, line 36
- 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 configured tunneling technique, which is described in section 3, The configured tunneling technique, which is described in section 3,
may or may not be used in addition to the dual IP layer operation. may or may not be used in addition to the dual IP layer operation.
An IPv6/IPv4 node MAY support configured tunneling.
2.1. Address Configuration 2.1. Address Configuration
Because they support both protocols, IPv6/IPv4 nodes may be Because the nodes 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 [RFC3596]. 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, and that there 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 is no assumption that the DNS server know the IPv4/IPv6 capabilities
of the requesting node. of the requesting node.
The issues and operational guidelines for using IPv6 with DNS are
described at more length in other documents [DNSOPV6].
DNS resolver libraries on IPv6/IPv4 nodes MUST be capable of handling 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.
skipping to change at page 7, line 11 skipping to change at page 6, line 48
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 A resolver library performing filtering or ordering of addresses
might also want to take into account external factors such as, might also want to take into account external factors such as,
whether IPv6 interfaces have been configured on the node. 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. -- i.e., which addresses to filter or which order to sort -- 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 the relative preferences of IPv4 and IPv6 addresses More details on the relative preferences of IPv4 and IPv6 addresses
are specified in the default address selection document [RFC3484]. are specified in the default address selection document [RFC3484].
2.3. Advertising Addresses in the DNS
There are some constraint placed on the use of the DNS during
transition. The constraints allow nodes to prefer either IPv6 or
IPv4 addresses when both types of addresses are returned by the DNS.
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
added to the DNS until all of these are true:
1) The address is assigned to the interface on the node.
2) The address is configured on the interface.
3) The interface is on a link which is connected to the IPv6
infrastructure.
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
would mean that it should not have an address in the DNS.
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
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.)
However, this does not work well when the isolated node is trying to
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.
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
route to the 6bone (e.g., because the local router was upgraded to
advertise IPv6 addresses using Neighbor Discovery but that router
doesn't have any IPv6 routes) this communication will fail.
Typically this means a few minutes of delay as TCP times out. The
TCP specification [RFC1122] says that ICMP unreachable messages could
be due to routing transients thus they should not immediately
terminate the TCP connection. This means that the normal TCP timeout
of a few minutes apply. Once TCP times out the application will
hopefully try the IPv4 addresses based on the A records in the DNS,
but this will be painfully slow.
A possible implication of the recommendations above is that, if one
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
IPv6 nodes that might see these AAAA records will possibly try to
reach that node using IPv6 and suffer delays or communication failure
due to unreachability. (A delay is incurred if the application
correctly falls back to using IPv4 if it can not establish
communication using IPv6 addresses. If this fallback is not done the
application would fail to communicate in this case.) Thus it is
suggested that either the recommendations be followed, or care be
taken to only do so with nodes that will not be impacted by external
accessing delays and/or communication failure.
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;
the removal of the A records should be tied to when the node can no
longer be reached using IPv4.
3. Configured 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
skipping to change at page 9, line 33 skipping to change at page 8, line 8
most likely to be used router-to-router due to the need to explicitly most likely to be used router-to-router due to the need to explicitly
configure the tunneling endpoints. configure the tunneling endpoints.
The underlying mechanisms for tunneling are: The underlying mechanisms for tunneling are:
- The entry node of the tunnel (the encapsulator) 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 decapsulator) receives the - The exit node of the tunnel (the decapsulator) receives the
encapsulated packet, reassembles the packet if needed, removes encapsulated packet, reassembles the packet if needed, removes
the IPv4 header, updates the IPv6 header, and processes the the IPv4 header, and processes the received IPv6 packet.
received IPv6 packet.
- The encapsulator MAY need to maintain soft state information for - The encapsulator may need to maintain soft state information for
each tunnel recording such parameters as the MTU of the tunnel each tunnel recording such parameters as the MTU of the tunnel
in order to process IPv6 packets forwarded into the tunnel. In in order to process IPv6 packets forwarded into the tunnel.
cases where the number of tunnels that any one host or router is
using is large, it is helpful to observe that this state
information can be cached and discarded when not in use.
In configured tunneling, the tunnel endpoint address is determined In configured tunneling, the tunnel endpoint address is determined
from configuration information in the encapsulator. For each tunnel, from configuration information in the encapsulator. For each tunnel,
the encapsulator must store the tunnel endpoint address. When an the encapsulator must store the tunnel endpoint address. When an
IPv6 packet is transmitted over a tunnel, the tunnel endpoint address IPv6 packet is transmitted over a tunnel, the tunnel endpoint address
configured for that tunnel is used as the destination address for the configured for that tunnel is used as the destination address for the
encapsulating IPv4 header. encapsulating IPv4 header.
The determination of which packets to tunnel is usually made by The determination of which packets to tunnel is usually made by
routing information on the encapsulator. This is usually done via a routing information on the encapsulator. This is usually done via a
skipping to change at page 10, line 28 skipping to change at page 9, line 4
| Transport | | Transport | | Transport | | Transport |
| 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 encapsulator also has to In addition to adding an IPv4 header, the encapsulator also has 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 ICMPv6 "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 ICMPv4 errors from routers along the tunnel path
path back to the source as IPv6 ICMP errors. back to the source as ICMPv6 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
Naively the encapsulator could view encapsulation as IPv6 using IPv4 Naively the encapsulator could view encapsulation as IPv6 using IPv4
as 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
encapsulator would need only to report IPv6 ICMP "packet too big" encapsulator would only need to report ICMPv6 "packet too big" errors
errors back to the source for packets that exceed this MTU. However, back to the source for packets that exceed this MTU. However, such a
such a scheme would be inefficient for two reasons and therefor MUST scheme would be inefficient or non-interoperable for three reasons
NOT be used: and therefore MUST 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 encapsulator and the decapsulator, would have to be the encapsulator and the decapsulator, would have to be
reassembled at the tunnel endpoint. For tunnels that terminate reassembled at the tunnel endpoint. For tunnels that terminate
at a router, this would require additional memory to reassemble at a router, this would require additional memory and other
the IPv4 fragments into a complete IPv6 packet before that resources to reassemble the IPv4 fragments into a complete IPv6
packet could be forwarded onward. packet before that packet could be forwarded onward.
3) The encapsulator has no way of knowing that the decapsulator is
able to defragment such IPv4 packets (see Section 3.7 for
details), and has no way of knowing that the decapsulator is
able to handle such a large IPv6 Maximum Receive Unit (MRU).
Hence, the encapsulator MUST NOT treat the tunnel as an interface Hence, the encapsulator MUST NOT treat the tunnel as an interface
with an MTU of 64 kilobytes, but instead either use the fixed static with an MTU of 64 kilobytes, but instead either use the fixed static
MTU below, or use OPTIONAL dynamic MTU determination based on the MTU or OPTIONAL dynamic MTU determination based on the IPv4 path MTU
IPv4 path MTU to the tunnel endpoint. to the tunnel endpoint.
If both the mechanisms are implemented, the decision which to use
SHOULD be configurable on a per-tunnel endpoint basis.
3.2.1. Static Tunnel MTU 3.2.1. Static Tunnel MTU
A node using a static tunnel MTU MUST limit the size of the IPv6 A node using static tunnel MTU treats the tunnel interface as having
packets it tunnels to 1280 bytes i.e., treat the tunnel interface as a fixed interface MTU. By default, the MTU MUST be between 1280 and
having a fixed interface MTU of 1280 bytes. An implementation MAY 1480 bytes (inclusive), but it SHOULD be 1280 bytes. If the default
have a configuration knob which can be used to set a larger value of is not 1280 bytes, the implementation MUST have a configuration knob
the tunnel MTU than 1280 bytes, but if so the default MUST be 1280 which can be used to change the MTU value.
bytes. A larger fixed MTU should not be configured unless it has
been administratively ensured that the decapsulator can reassemble A node must be able to accept a fragmented IPv6 packet that, after
packets of that size. Care should be taken when manually configuring reassembly, is as large as 1500 octets [RFC2460]. This memo also
large tunnel MTUs to only do so when the MTU of the IPv4 path to the includes requirements (see Section 3.6) for the amount of IPv4
tunnel endpoint is large to avoid causing excessive fragmentation. reassembly and IPv6 MRU that MUST be supported by all the
decapsulators. These ensure correct interoperability with any fixed
MTUs between 1280 and 1480 bytes.
A larger fixed MTU than supported by these requirements, must not be
configured unless it has been administratively ensured that the
decapsulator can reassemble or receive packets of that size.
The selection of a good tunnel MTU depends on many factors; at least:
- Whether the IPv4 protocol-41 packets will be transported over
media which may have a lower path MTU (e.g., IPv4 Virtual
Private Networks); then picking too high a value might lead to
IPv4 fragmentation.
- Whether the tunnel is used to transport IPv6 tunneled packets
(e.g., a mobile node with an IPv4-in-IPv6 configured tunnel, and
an IPv6-in-IPv6 tunnel interface); then picking too low a value
might lead to IPv6 fragmentation.
If layered encapsulation is believed to be present, it may be prudent
to consider supporting dynamic MTU determination instead as it is
able to minimize fragmentation and optimize packet sizes.
When using the static tunnel MTU the Don't Fragment bit MUST NOT be 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 set in the encapsulating IPv4 header. As a result the encapsulator
should not receive any ICMPv4 "packet too big" message as a result of should not receive any ICMPv4 "packet too big" messages as a result
the packets it has encapsulated. of the packets it has encapsulated.
3.2.2. Dynamic Tunnel MTU 3.2.2. Dynamic Tunnel MTU
The dynamic MTU determination is OPTIONAL. However, if it is The dynamic MTU determination is OPTIONAL. However, if it is
implemented, it SHOULD have the behavior described in this document. 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 encapsulator track the IPv4 Path MTU across the tunnel, having the encapsulator track the IPv4 Path MTU across the tunnel,
using the IPv4 Path MTU Discovery Protocol [RFC1191] and recording using the IPv4 Path MTU Discovery Protocol [RFC1191] and recording
the resulting path MTU. The IPv6 layer in the encapsulator can then the resulting path MTU. The IPv6 layer in the encapsulator can then
skipping to change at page 12, line 21 skipping to change at page 11, line 19
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 encapsulator has to use IPv4 layer with an MTU of 1280 bytes and the encapsulator has to use IPv4
fragmentation in order to forward the 1280 byte IPv6 packets. fragmentation in order to forward the 1280 byte IPv6 packets.
The encapsulator SHOULD employ the following algorithm to determine The encapsulator SHOULD employ the following algorithm to determine
when to forward an IPv6 packet that is larger than the tunnel's path when to forward an IPv6 packet that is larger than the tunnel's path
MTU using IPv4 fragmentation, and when to return an IPv6 ICMP "packet MTU using IPv4 fragmentation, and when to return an ICMPv6 "packet
too big" message per [RFC1981]: too big" message per [RFC1981]:
if (IPv4 path MTU - 20) is less than 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 ICMPv6 "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 encapsulator 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 ICMPv6 "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
Encapsulators that have a large number of tunnels can choose between Encapsulators that have a large number of tunnels may choose between
dynamic versus static tunnel MTU on a per-tunnel endpoint basis. dynamic versus static tunnel MTU on a per-tunnel endpoint basis. In
cases where the number of tunnels that any one node is using is
large, it is helpful to observe that this state information can be
cached and discarded when not in use.
Note that using dynamic tunnel MTU is subject to IPv4 PMTU blackholes Note that using dynamic tunnel MTU is subject to IPv4 PMTU blackholes
should the ICMPv4 "packet too big" messages be dropped by firewalls should the ICMPv4 "packet too big" messages be dropped by firewalls
or not generated by the routers. [RFC1435, RFC2923] 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" from the IPv6
IPv6 hop limit is decremented by 1 when an IPv6 packet traverses the perspective. The tunnel is opaque to users of the network, and is not
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
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 encapsulators and The single-hop model is implemented by having the encapsulators and
decapsulators process the IPv6 hop limit field as they would if they decapsulators 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. (The decrement the hop limit by 1 when forwarding an IPv6 packet. (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]. The
implementations MAY also consider using the value 255, as it could be
used as a hint in the decapsulation checks in the future [GTSM].
Implementations MAY provide a mechanism to allow the administrator to 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 as the IP Tunnel MIB [RFC2667].
[RFC2667].
3.4. Handling IPv4 ICMP errors 3.4. Handling ICMPv4 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
encapsulator might receive IPv4 ICMP error messages from IPv4 routers encapsulator might receive ICMPv4 error messages from IPv4 routers
inside the tunnel. These packets are addressed to the encapsulator inside the tunnel. These packets are addressed to the encapsulator
because it is the IPv4 source of the encapsulated packet. because it is the IPv4 source of the encapsulated packet.
The ICMP "packet too big" error messages are handled according to ICMPv4 error handling is only applicable to dynamic MTU
determination, even though the functions could be used with static
MTU tunnels as well.
The ICMPv4 "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 ICMPv6 "packet too big" error has to be generated as
as described in section 3.2.2 if dynamic tunnel MTU is used. described in section 3.2.2.
The handling of other types of ICMP error messages depends on how The handling of other types of ICMPv4 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 encapsulator MAY If the offending packet includes enough data, the encapsulator MAY
extract the encapsulated IPv6 packet and use it to generate an IPv6 extract the encapsulated IPv6 packet and use it to generate an ICMPv6
ICMP message directed back to the originating IPv6 node, as shown message directed back to the originating IPv6 node, as shown below:
below:
+--------------+ +--------------+
| IPv4 Header | | IPv4 Header |
| dst = encaps | | dst = encaps |
| node | | node |
+--------------+ +--------------+
| ICMP | | ICMPv4 |
| Header | | Header |
- - +--------------+ - - +--------------+
| IPv4 Header | | IPv4 Header |
| src = encaps | | src = encaps |
IPv4 | node | IPv4 | node |
+--------------+ - - +--------------+ - -
Packet | IPv6 | Packet | IPv6 |
| Header | Original IPv6 | Header | Original IPv6
in +--------------+ Packet - in +--------------+ Packet -
| Transport | Can be used to | Transport | Can be used to
Error | Header | generate an Error | Header | generate an
+--------------+ IPv6 ICMP +--------------+ ICMPv6
| | error message | | error message
~ Data ~ back to the source. ~ Data ~ back to the source.
| | | |
- - +--------------+ - - - - +--------------+ - -
IPv4 ICMP Error Message Returned to Encapsulating Node ICMPv4 Error Message Returned to Encapsulating Node
When receiving ICMPv4 errors as above and the errors are not "packet 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 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, the tunnel. Also, if sufficient headers are included in the error,
then the originating node MAY send an ICMPv6 error of type then the originating node MAY send an ICMPv6 error of type
"unreachable" with code "address unreachable" to the IPv6 source. "unreachable" with code "address unreachable" to the IPv6 source.
(The "address unreachable" code is appropriate since, from the (The "address unreachable" code is appropriate since, from the
perspective of IPv6, the tunnel is a link and that code is used for perspective of IPv6, the tunnel is a link and that code is used for
link-specific errors [RFC2463]). link-specific errors [RFC2463]).
Note that when IPv4 path MTU is exceeded, and ICMPv4 errors of only 8
bytes of payload are generated, or ICMPv4 errors do not cause the
generation of ICMPv6 errors in case there is enough payload, there
will be at least two packet drops instead of at least one (the case
of a single layer of MTU discovery). Consider a case where an IPv6
host is connected to an IPv4/IPv6 router, which is connected to a
network where an ICMPv4 error about too big packet size is generated.
First the router needs to learn the tunnel (IPv4) MTU which causes at
least one packet loss, and then the host needs to learn the (IPv6)
MTU from the router which causes at least one packet loss. Still, in
all cases there can be more than one packet loss if there are
multiple large packets in flight at the same time.
3.5. IPv4 Header Construction 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:
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 unless otherwise specified. (See [RFC2983] and 0 unless otherwise specified. (See [RFC2983] and
[RFC3168] for issues relating to the ToS byte and [RFC3168] section 9.1 for issues relating to the Type-
tunneling.) of-Service byte and 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., IPv6 payload length plus a constant IPv4 headers (i.e., IPv6 payload length plus a constant
60 bytes). 60 bytes).
Identification: Identification:
Generated uniquely as for any IPv4 packet transmitted by Generated uniquely as for any IPv4 packet transmitted by
skipping to change at page 15, line 41 skipping to change at page 15, line 13
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
fragmenting. fragmenting.
Fragment offset: Fragment offset:
Set as necessary if fragmenting. Set as necessary if fragmenting.
Time to Live: Time to Live:
Set in implementation-specific manner. Set in an implementation-specific manner, as described
in section 3.3.
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. [RFC791]
Source Address: Source Address:
IPv4 address of outgoing interface of the encapsulator. IPv4 address of outgoing interface of the encapsulator
The source address MAY alternatively be administratively or an administratively specified address as described
specified to be a specific IPv4 address assigned to the below.
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 the tunnel endpoint.
Any IPv6 options are preserved in the packet (after the IPv6 header). When encapsulating the packets, the nodes must ensure that they will
use the source address that the tunnel peer has configured, so that
the source addresses are acceptable to the decapsulator. This may be
a problem with multi-addressed, and in particular, multi-interface
nodes, especially when the routing is changed from a stable
condition, as the source address selection may be adversely affected.
Therefore, it SHOULD be possible to administratively specify the
source address of a tunnel.
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 addresses, and the value of the
protocol field is 41, the packet is potentially part of a tunnel and protocol field is 41, the packet is potentially part of a tunnel and
needs to be verified against the list of acceptable source addresses needs to be verified to belong to one of the configured tunnel
for tunneled packets, reassembled (if fragmented at the IPv4 level), interfaces (by checking source/destination addresses), reassembled
have the IPv4 header removed and the resulting IPv6 datagram be (if fragmented at the IPv4 level), have the IPv4 header removed and
submitted to the IPv6 layer code on the node. the resulting IPv6 datagram be submitted to the IPv6 layer code on
the node.
The decapsulator MUST verify that the tunnel source address is The decapsulator MUST verify that the tunnel source address is
acceptable before further processing packets to avoid creating a hole correct before further processing packets, to mitigate the problems
in ingress filtering (see section 4). This check also applies to with address spoofing (see section 4). This check also applies to
packets which are delivered to transport protocols on the packets which are delivered to transport protocols on the
decapsulator. For bidirectional configured tunnels this is done by decapsulator. This is done by verifying that the source address is
verifying that the source address is the IPv4 address of the other the IPv4 address of the other end of a tunnel configured on the node.
end of a tunnel configured on the node. For unidirectional Packets for which the IPv4 source address does not match MUST be
configured tunnels the decapsulator MUST be configured with a list of discarded and an ICMP message SHOULD NOT be generated; however, if
IPv4 source address prefixes that are acceptable. Such a list MUST the implementation normally sends an ICMP message when receiving an
default to having zero entries i.e., the node has to be explicitly unknown protocol packet, such an error message MAY be sent (e.g.,
configured to accept encapsulated packets received over ICMPv4 Protocol 41 Unreachable).
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 A side effect of this address verification is that the node will
will silently discard packets with an invalid IPv4 source address silently discard packets with a wrong source address, and packets
such as a multicast address, a broadcast address (255.255.255.255 and which were received by the node but not directly addressed to it
the broadcast addresses configured on the node), 0.0.0.0/8, and (e.g., broadcast addresses).
127.0.0.1/8. In general, it SHOULD apply the rules for martian
filtering in [RFC1812] and ingress filtering [RFC2827] on the IPv4 In addition, the node MAY perform ingress filtering [RFC2827] on the
source address. Packets caught by these checks SHOULD be silently IPv4 source address, i.e., check that the packet is arriving from the
dropped. interface in the direction of the route towards the tunnel end-point,
similar to a Strict Reverse Path Forwarding (RPF) check [BCP38UPD].
If done, it is RECOMMENDED that this check is disabled by default.
The packets caught by this check SHOULD be discarded; an ICMP message
SHOULD NOT be generated by default.
The decapsulator MUST be capable of having, on the tunnel interfaces,
an IPv6 MRU of at least the maximum of of 1500 bytes and the largest
(IPv6) interface MTU on the decapsulator.
The decapsulator MUST be capable of reassembling an IPv4 packet that The decapsulator MUST be capable of reassembling an IPv4 packet that
is the maximum of 1280 bytes and the largest interface MTU on the is (after the reassembly) the maximum of 1500 bytes and the largest
decapsulator. The 1280 byte number is a result of encapsulators that (IPv4) interface MTU on the decapsulator. The 1500 byte number is a
use the static MTU scheme in section 3.2.1, while encapsulators that result of encapsulators that use the static MTU scheme in section
use the dynamic scheme in section 3.2.2 can cause up to the largest 3.2.1, while encapsulators that use the dynamic scheme in section
interface MTU on the decapsulator to be received. (Note that it is 3.2.2 can cause up to the largest interface MTU on the decapsulator
strictly the interface MTU on the last IPv4 router *before* the to be received. (Note that it is strictly the interface MTU on the
decapsulator that matters, but for most links the MTU is the same last IPv4 router *before* the decapsulator that matters, but for most
between all neighbors.) links the MTU is the same 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.
The decapsulation is shown below: The decapsulation is shown below:
+-------------+ +-------------+
skipping to change at page 17, line 42 skipping to change at page 17, line 26
| 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.
(However, see [RFC2983] and [RFC3168] for issues relating to the Type (However, see [RFC2983] and [RFC3168] section 9.1 for issues relating
of Service byte and tunneling.) If the packet is subsequently to the Type of Service byte and tunneling.) If the packet is
forwarded, its hop limit is decremented by one. subsequently forwarded, its hop limit is decremented by one.
The decapsulator performs IPv4 reassembly before decapsulating the The decapsulator performs IPv4 reassembly before decapsulating the
IPv6 packet. All IPv6 options are preserved even if the IPv6 packet.
encapsulating IPv4 packet is fragmented.
The encapsulating IPv4 header is discarded. The length of the IPv6 The encapsulating IPv4 header is discarded. When reconstructing the
packet MUST be determined from the IPv6 payload length since the IPv4 IPv6 packet the length MUST be determined from the IPv6 payload
packet might be padded (thus have a length which is larger than the length since the IPv4 packet might be padded (thus have a length
IPv6 packet plus the added IPv4 header). which is larger than the IPv6 packet plus the IPv4 header being
removed).
After the decapsulation the node SHOULD silently discard a packet After the decapsulation the node MUST silently discard a packet with
with an invalid IPv6 source address. This includes IPv6 multicast an invalid IPv6 source address. The list of invalid source addresses
addresses, the IPv6 unspecified address, and the loopback address but SHOULD include at least:
also IPv4-compatible IPv6 source addresses where the IPv4 part of the
address is an IPv4 multicast address, broadcast address - all multicast addresses (FF00::/8)
(255.255.255.255 and the broadcast addresses configured on the node),
0.0.0.0/8, or 127.0.0.1/8. In general it SHOULD apply the rules for - the loopback address (::1)
martian filtering in [RFC1812] and ingress filtering [RFC2827] on the
IPv4 address embedded in IPv4-compatible source addresses. - all the IPv4-compatible IPv6 addresses [RFC3513] (::/96),
excluding the unspecified address for Duplicate Address
Detection (::/128)
- all the IPv4-mapped IPv6 addresses (::ffff:0:0/96)
In addition, the node should perform ingress filtering [RFC2827] on
the IPv6 source address, similar to on any of its interfaces, e.g.:
- if the tunnel is towards the Internet, check that the site's
IPv6 prefixes are not used as the source addresses, or
- if the tunnel is towards an edge network, check that the source
address belongs to that edge network.
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") and thus MUST have link-local addresses. The link-local
addresses are used by routing protocols operating over the tunnels. addresses are used by, e.g., routing protocols operating over the
tunnels.
The Interface Identifier [RFC2373] for such an Interface SHOULD be The interface identifier [RFC3513] for such an interface may be based
the 32-bit IPv4 address of that interface, with the bytes in the same on the 32-bit IPv4 address of an underlying interface, or formed
order in which they would appear in the header of an IPv4 packet, using some other means, as long as it's unique from the other tunnel
padded at the left with zeros to a total of 64 bits. Note that the endpoint with a reasonably high probability.
"Universal/Local" bit is zero, indicating that the Interface
Identifier is not globally unique. When the host has more than one
IPv4 address in use on the physical interface concerned, an
administrative choice of one of these IPv4 addresses is made when
forming the link-local address.
The IPv6 Link-local address [RFC2373] for an IPv4 virtual interface If an IPv4 address is used for forming the IPv6 link-local address,
is formed by appending the Interface Identifier, as defined above, to the interface identifier is the IPv4 address, prepended by zeros.
the prefix FE80::/64. Note that the "Universal/Local" bit is zero, indicating that the
interface identifier is not globally unique. The link-local address
is formed by appending the interface identifier to the prefix
FE80::/64.
When the host has more than one IPv4 address in use on the physical
interface concerned, an administrative choice of one of these IPv4
addresses is made when forming the link-local address.
+-------+-------+-------+-------+-------+-------+------+------+ +-------+-------+-------+-------+-------+-------+------+------+
| 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 Configured tunnel implementations MUST at least accept and respond to
[RFC2461] and Stateless Address Autoconfiguration [RFC2462] does not the probe packets used by Neighbor Unreachability Detection (NUD)
apply; only the formation of the link-local address applies. [RFC2461]. The implementations SHOULD also send NUD probe packets to
detect when the configured tunnel fails at which point the
If an implementation provides bidirectional configured tunnels it implementation can use an alternate path to reach the destination.
MUST at least accept and respond to the probe packets used by Note that Neighbor Discovery allows that the sending of NUD probes be
Neighbor Unreachability Detection (NUD) [RFC2461]. Such omitted for router to router links if the routing protocol tracks
implementations SHOULD also send NUD probe packets to detect when the bidirectional reachability.
configured tunnel fails at which point the implementation can use an
alternate path to reach the destination. Note that Neighbor
Discovery allows that the sending of NUD probes be omitted for router
to router links if the routing protocol tracks bidirectional
reachability.
For the purposes of Neighbor Discovery the configured tunnels 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 an address.
means that: This means that:
- the sender of Neighbor Discovery packets SHOULD NOT include - the sender of Neighbor Discovery packets SHOULD NOT include
Source Link Layer Address options or Target Link Layer Address Source Link Layer Address options or Target Link Layer Address
options on the tunnel link. options on the tunnel link.
- the receiver MUST, while otherwise processing the neighbor - the receiver MUST, while otherwise processing the Neighbor
discovery packet, silently ignore the content of any Source Link Discovery packet, silently ignore the content of any Source Link
Layer Address options or Target Link Layer Address options Layer Address options or Target Link Layer Address options
received on the tunnel link. 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. Deighbor Discovery is used on other point-to-point links.
4. Threat Related to Source Address Spoofing 4. Threat Related to Source Address Spoofing
The specification above contains rules that apply ingress filtering The specification above contains rules that apply tunnel source
to packets before they are decapsulated. The purpose of ingress address verification in particular and ingress filtering
filtering in general is specified in [RFC2827]. When IP-in-IP [RFC2827][BCP38UPD] in general to packets before they are
tunneling (independent of IP versions) is used it is important that decapsulated. When IP-in-IP tunneling (independent of IP versions)
this not be a tool to bypass any ingress filtering in use for non- is used it is important that this can not be used to bypass any
tunneled packets. Thus the rules in this document are derived based ingress filtering in use for non-tunneled packets. Thus the rules in
on should ingress filtering be used for IPv4 and IPv6, the use of this document are derived based on should ingress filtering be used
tunneling should not provide an easy way to circumvent the filtering. for IPv4 and IPv6, the use of tunneling should not provide an easy
way to circumvent the filtering.
In this case, without specific ingress filtering checks in the In this case, without specific ingress filtering checks in the
decapsulator, it would be possible for an attacker to inject a packet decapsulator, it would be possible for an attacker to inject a 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 decapsulator - 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 decapsulator or a
node close to it. node close to it.
- Inner IPv6 destination: Bob - Inner IPv6 destination: Bob
Even if all IPv4 routers between the attacker and the decapsulator Even if all IPv4 routers between the attacker and the decapsulator
implement IPv4 ingress filtering, and all IPv6 routers between the implement IPv4 ingress filtering, and all IPv6 routers between the
decapsulator and Bob implement IPv6 ingress filtering, the above decapsulator and Bob implement IPv6 ingress filtering, the above
spoofed packets will not be filtered out. As a result Bob will spoofed packets will not be filtered out. As a result Bob will
receive a packet that looks like it was sent from Alice even though receive a packet that looks like it was sent from Alice even though
skipping to change at page 20, line 17 skipping to change at page 20, line 16
- Inner IPv6 destination: Bob - Inner IPv6 destination: Bob
Even if all IPv4 routers between the attacker and the decapsulator Even if all IPv4 routers between the attacker and the decapsulator
implement IPv4 ingress filtering, and all IPv6 routers between the implement IPv4 ingress filtering, and all IPv6 routers between the
decapsulator and Bob implement IPv6 ingress filtering, the above decapsulator and Bob implement IPv6 ingress filtering, the above
spoofed packets will not be filtered out. As a result Bob will spoofed packets will not be filtered out. As a result Bob will
receive a packet that looks like it was sent from Alice even though receive a packet that looks like it was sent from Alice even though
the sender was some unrelated node. the sender was some unrelated node.
The solution to this is to have the decapsulator only accept The solution to this is to have the decapsulator only accept
encapsulated packets having explicitly configured source addresses encapsulated packets from the explicitly configured source address
(e.g., in the case of bidirectional tunnels, the other end of the (i.e., the other end of the tunnel) as specified in section 3.6.
tunnel) as specified in section 3.6. While this does not provide complete protection in the case ingress
filtering has not been deployed, it does provide a significant
increase in security. The issue and the remainder threats are
discussed at more length in Security Considerations.
5. Security Considerations 5. Security Considerations
Tunneling is not known to introduce any security holes except for the
possibility to circumvent ingress filtering [RFC2827]. This
specification prevent tunneling from introducing additional
weaknesses when IPv4 and/or IPv6 ingress filtering is in used by
requiring that decapsulators only accept packets if they have been
configured to accept encapsulated packets from the IPv4 source
address in the received packet. Such a check is easy to perform for
bidirectional tunnels, but for uni-directional tunnels it requires a
separate configuration of the IPv4 source addresses that are
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.
verify that the IPv4 addresses in the encapsulated packet matches
what has been configured for the tunnel, coupled with use of ingress Several mechanisms (e.g., Neighbor Discovery) depend on Hop Count
filtering in IPv4, ameliorate some of this. In addition, an being 255 and/or the addresses being link-local for ensuring that a
implementation must treat interfaces to different links as separate packet originated on-link, in a semi-trusted environment. Tunnels
e.g. to ensure that Neighbor Discovery packets arriving on one link are more vulnerable to a breach of this assumption than physical
does not effect other links. This is especially important for tunnel links, as an attacker anywhere in the Internet can send an IPv6-in-
links. IPv4 packet to the tunnel decapsulator, causing injection of an
encapsulted IPv6 packet to the configured tunnel interface unless the
decapsulation checks are able to discard packets injected in such a
manner.
Therefore, this memo specifies strict checks to mitigate this threat:
- IPv4 source address of the packet MUST be the same as configured
for the tunnel end-point,
- IPv4 ingress filtering MAY be implemented to check that the IPv4
packets are received from an expected interface,
- IPv6 packets with several, obviously invalid IPv6 source
addresses MUST be discarded (see Section 3.6 for details), and
- IPv6 ingress filtering should be performed, to check that the
IPv6 packets are received from an expected interface.
Especially the first verification is vital: to avoid this check, the
attacker must be able to know the source of the tunnel (difficult)
and be able to spoof it (easier).
If the remainder threats of tunnel source verification are considered
to be significant, a tunneling scheme with authentication should be
used instead, for example IPsec [RFC2401] (preferable) or Generic
Routing Encapsulation with a pre-configured secret key [RFC2890]. As
the configured tunnels are set up more or less manually, setting up
the keying material is probably not a problem.
If the tunneling is done inside an administrative domain, proper
ingress filtering at the edge of the domain can also eliminate the
threat from outside of the domain. Therefore shorter tunnels are
preferable to longer ones, possibly spanning the whole Internet.
Additionally, an implementation must treat interfaces to different
links as separate e.g. to ensure that Neighbor Discovery packets
arriving on one link does not effect other links. This is especially
important for tunnel links.
When dropping packets due to failing to match the allowed IPv4 source When dropping packets due to failing to match the allowed IPv4 source
addresses for a tunnel the node SHOULD NOT "acknowledge" the addresses for a tunnel the node should not "acknowledge" the
existence of a tunnel, otherwise this could be used to probe the existence of a tunnel, otherwise this could be used to probe the
acceptable tunnel endpoint addresses. For that reason the acceptable tunnel endpoint addresses. For that reason, the
specification says that such packets SHOULD be silently discarded. specification says that such packets MUST be discarded, and an ICMP
error message SHOULD NOT be generated, unless the implementation
normally sends ICMP destination unreachable messages for unknown
protocols; in such a case, the same code MAY be sent. As should be
obvious, the not returning the same ICMP code if an error is returned
for other protocols may hint that the IPv6 stack (or the protocol 41
tunneling processing) has been enabled -- the behaviour should be
consistent on how the implementation otherwise behaves to be
transparent to probing.
6. Acknowledgments 6. Acknowledgments
We would like to thank the members of the IPv6 working group, the We would like to thank the members of the IPv6 working group, the
Next Generation Transition (ngtrans) working group, and the v6ops Next Generation Transition (ngtrans) working group, and the v6ops
working group for their many contributions and extensive review of working group for their many contributions and extensive review of
this document. Special thanks are due to Jim Bound, Ross Callon, Bob this document. Special thanks are due to Jim Bound, Ross Callon, Bob
Hinden, Bill Manning, John Moy, Mohan Parthasarathy, Pekka Savola and Hinden, Bill Manning, John Moy, Mohan Parthasarathy, Pekka Savola,
Fred Templin for many helpful suggestions. Fred Templin, Chirayu Patel, and Tim Chown for many helpful
suggestions. Pekka Savola helped in editing the final revisions of
the specification.
7. References 7. References
7.1. Normative References 7.1. Normative References
[RFC791] J. Postel, "Internet Protocol", RFC 791, September 1981.
[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.
[RFC2463] A. Conta, S. Deering, "Internet Control Message Protocol [RFC2463] A. Conta, S. Deering, "Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6) (ICMPv6) for the Internet Protocol Version 6 (IPv6)
Specification", RFC 2463, December 1998. Specification", RFC 2463, December 1998.
7.2. Non-normative References 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
[BCP38UPD] Baker, F., and Savola P., "Ingress Filtering for Multihomed
Networks", draft-savola-bcp38-multihoming-update-03.txt,
work-in-progress, December 2003.
[DNSOPV6] Durand, A., Ihren, J., and Savola P., "Operational
Considerations and Issues with IPv6 DNS", draft-ietf-dnsop-
ipv6-dns-issues-04.txt, work-in-progress, January 2004.
[GTSM] Gill, V., Heasley, J., and D. Meyer, "The Generalized TTL
Security Mechanism (GTSM)", draft-gill-gtsh-04.txt, work-
in-progress, October 2003.
[KM97] Kent, C., and J. Mogul, "Fragmentation Considered Harmful". [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 [RFC1435] S. Knowles, "IESG Advice from Experience with Path MTU
Discovery", RFC 1435, March 1993. 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.
[RFC1812] Kent, S., Atkinson, R., "Security Architecture for the
Internet Protocol", RFC 2401, November 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: [RFC2827] 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 2827, May 2000. Address Spoofing", RFC 2827, May 2000.
[RFC2890] Dommety, G., "Key and Sequence Number Extensions to GRE",
RFC 2890, September 2000.
[RFC2923] K. Lahey, "TCP Problems with Path MTU Discovery", RFC 2923, [RFC2923] K. Lahey, "TCP Problems with Path MTU Discovery", RFC 2923,
September 2000. 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
skipping to change at page 24, line 18 skipping to change at page 25, line 40
reference cleanup. reference cleanup.
- Dropped "or equal" in if (IPv4 path MTU - 20) is less than or - Dropped "or equal" in if (IPv4 path MTU - 20) is less than or
equal to 1280 equal to 1280
- Dropped this: However, IPv6 may be used in some environments - Dropped this: However, IPv6 may be used in some environments
where interoperability with IPv4 is not required. IPv6 nodes where interoperability with IPv4 is not required. IPv6 nodes
that are designed to be used in such environments need not use that are designed to be used in such environments need not use
or even implement these mechanisms. or even implement these mechanisms.
- Clarified that the dynamic path MTU mechanism in section 3.2 is - Described Static MTU and Dynamic MTU cases separately; clarified
OPTIONAL but if it is implemented it should follow the rules in that the dynamic path MTU mechanism is OPTIONAL but if it is
section 3.2. implemented it should follow the rules in section 3.2.2.
- 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 MTU can - Specified Static MTU to default to a MTU of 1280 to 1480 bytes,
be set to larger values on a tunnel by tunnel basis, but that and that this may be configurable. Discussed the issues with
the default MUST be 1280 and that decapsulators need to be using Static MTU at more length.
configured to match the encapsulator's MTU.
- Restated the "currently underway" language about ToS to loosely - Specified minimal rules for IPv4 reassembly and IPv6 MRU to
point at [RFC2983] and [RFC3168]. enhance interoperability and to minimize blacholes.
- Stated that IPv4 source MAY also be administratively specified. - Restated the "currently underway" language about Type-of-
(This is especially useful on multi-interface nodes and with Service, and loosely point at [RFC2983] and [RFC3168].
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 decapsulator as packet delivered to transport protocols on the decapsulator as
well as packets being forwarded by the decapsulator, and how the well as packets being forwarded by the decapsulator, and how the
decapsulator's checks help when IPv4 and IPv6 ingress filtering decapsulator's checks help when IPv4 and IPv6 ingress filtering
is in place. is in place.
- Removed unidirectional tunneling; assume all tunnels are
bidirectional.
- Removed the guidelines for advertising addresses in DNS as
slightly out of scope, referring to another document for the
details.
- Removed the SHOULD requirement that the link-local addresses
should be formed based on IPv4 addresses.
- Added a SHOULD for implementing a knob to be able to set the
source address of the tunnel, and add discussion why this is
useful.
- Added stronger wording for source address checks: both IPv4 and
IPv6 source addresses MUST be checked, and RPF-like ingress
filtering is optional.
- Rewrote security considerations to be more precise about the
threats of tunneling.
- Added a note that using TTL=255 when encapsulating might be
useful for decapsulation security checks later on.
- Added more discussion in Section 3.2 why using an "infinite"
IPv6 MTU leads to likely interoperability problems.
- Added an explicit requirement that if both MTU determination
methods are used, choosing one should be possible on a per-
tunnel basis.
Clarified that ICMPv4 error handling is only applicable to
dynamic MTU determination.
- Made a lot of miscellaneous editorial cleanups.
9.1. Changes from draft-ietf-v6ops-mech-v2-00 9.1. Changes from draft-ietf-v6ops-mech-v2-00
[[ RFC-Editor note: remove the change history between the drafts
before publication. ]]
- Clarified in section 2.2 that there is no assumption that the - Clarified in section 2.2 that there is no assumption that the
DNS server knows the IPv4/IPv6 capabilities of the requesting DNS server knows the IPv4/IPv6 capabilities of the requesting
node. node.
- Clarified in section 2.2 that a filtering resolver might want to - Clarified in section 2.2 that a filtering resolver might want to
take into account external factors e.g., whether IPv6 interfaces take into account external factors e.g., whether IPv6 interfaces
have been configured on the node. have been configured on the node.
- Clarified in section 2.3 that part of the motivation for the - Clarified in section 2.3 that part of the motivation for the
section is that this is the opposite of common DNS practices in section is that this is the opposite of common DNS practices in
skipping to change at page 26, line 13 skipping to change at page 28, line 14
address and all the broadcast addresses of the decapsulator. address and all the broadcast addresses of the decapsulator.
- Clarified that packets which fail the checks (such as verifying - Clarified that packets which fail the checks (such as verifying
the IPv4 source address, martian, and ingress filtering) on the the IPv4 source address, martian, and ingress filtering) on the
decapsulator should be silently dropped. decapsulator should be silently dropped.
- Clarified that while source link layer address options and - Clarified that while source link layer address options and
target link layer address options are ignored in received ND target link layer address options are ignored in received ND
packets, the ND packets themselves are processed as normal. packets, the ND packets themselves are processed as normal.
10. Open Issues 9.2. Changes from draft-ietf-v6ops-mech-v2-01
The document has some specific text about unidirectional configure - Removed unidirectional tunnels; assume all the tunnels are
tunnels since they are different with respect to Neighbor Discovery bidirectional.
and ingress filtering. Does anybody implement unidirectional
tunnels? Should we remove the specific text and make the explicit - Removed the definition of IPv4-compatible IPv6 addresses.
assumption that all configured tunnels are bidirectional?
- Removed redundant text in the Hop Limit processing rules.
- Removed the guidelines for advertising addresses in DNS as
slightly out of scope, referring to another document for the
details.
- Removed the SHOULD requirement that the link-local addresses
should be formed based on IPv4 addresses.
- Added more discussion on the ICMPv4/6 Path MTU Discovery and the
required number of packet drops.
- Added a SHOULD for implementing a knob to be able to set the
source address of the tunnel, and add discussion why this is
useful.
- Added stronger wording for source address checks: both IPv4 and
IPv6 source addresses MUST be checked, and RPF-like ingress
filtering is optional.
- Rewrote security considerations to be more precise about the
threats of tunneling.
- Added a note that using TTL=255 when encapsulating might be
useful for decapsulation security checks later on.
- Added more discussion in Section 3.2 why using an "infinite"
IPv6 MTU leads to likely interoperability problems.
- Added an explicit requirement that if both MTU determination
methods are used, choosing one should be possible on a per-
tunnel basis.
Clarified that ICMPv4 error handling is only applicable to
dynamic MTU determination.
- Specified Static MTU to default to a MTU of 1280 to 1480 bytes,
and that this may be configurable. Discussed the issues with
using Static MTU at more length.
- Specified minimal rules for IPv4 reassembly and IPv6 MRU to
enhance interoperability and to minimize blacholes.
-
- Made a lot of miscellaneous editorial cleanups.
 End of changes. 

This html diff was produced by rfcdiff 1.23, available from http://www.levkowetz.com/ietf/tools/rfcdiff/