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rfc:rfc4213

Network Working Group E. Nordmark Request for Comments: 4213 Sun Microsystems, Inc. Obsoletes: 2893 R. Gilligan Category: Standards Track Intransa, Inc.

                                                          October 2005
       Basic Transition Mechanisms for IPv6 Hosts and Routers

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2005).

Abstract

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

Nordmark & Gilligan Standards Track [Page 1] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

Table of Contents

 1. Introduction ....................................................2
    1.1. Terminology ................................................3
 2. Dual IP Layer Operation .........................................4
    2.1. Address Configuration ......................................5
    2.2. DNS ........................................................5
 3. Configured Tunneling Mechanisms .................................6
    3.1. Encapsulation ..............................................7
    3.2. Tunnel MTU and Fragmentation ...............................8
         3.2.1. Static Tunnel MTU ...................................9
         3.2.2. Dynamic Tunnel MTU ..................................9
    3.3. Hop Limit .................................................11
    3.4. Handling ICMPv4 Errors ....................................11
    3.5. IPv4 Header Construction ..................................13
    3.6. Decapsulation .............................................14
    3.7. Link-Local Addresses ......................................17
    3.8. Neighbor Discovery over Tunnels ...........................18
 4. Threat Related to Source Address Spoofing ......................18
 5. Security Considerations ........................................19
 6. Acknowledgements ...............................................21
 7. References .....................................................21
    7.1. Normative References ......................................21
    7.2. Informative References ....................................21
 8. Changes from RFC 2893 ..........................................23

1. Introduction

 The key to a successful IPv6 transition is compatibility with the
 large installed base of IPv4 hosts and routers.  Maintaining
 compatibility with IPv4 while deploying IPv6 will streamline the task
 of transitioning the Internet to IPv6.  This specification defines
 two mechanisms that IPv6 hosts and routers may implement in order to
 be compatible with IPv4 hosts and routers.
 The mechanisms in this document are designed to be employed by IPv6
 hosts and routers that need to interoperate with IPv4 hosts and
 utilize IPv4 routing infrastructures.  We expect that most nodes in
 the Internet will need such compatibility for a long time to come,
 and perhaps even indefinitely.
 The mechanisms specified here are:
  1. Dual IP layer (also known as dual stack): A technique for

providing complete support for both Internet protocols – IPv4 and

    IPv6 -- in hosts and routers.

Nordmark & Gilligan Standards Track [Page 2] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

  1. Configured tunneling of IPv6 over IPv4: A technique for

establishing point-to-point tunnels by encapsulating IPv6 packets

    within IPv4 headers to carry them over IPv4 routing
    infrastructures.
 The mechanisms defined here are intended to be the core of a
 "transition toolbox" -- a growing collection of techniques that
 implementations and users may employ to ease the transition.  The
 tools may be used as needed.  Implementations and sites decide which
 techniques are appropriate to their specific needs.
 This document defines the basic set of transition mechanisms, but
 these are not the only tools available.  Additional transition and
 compatibility mechanisms are specified in other documents.

1.1. Terminology

 The following terms are used in this document:
 Types of Nodes
    IPv4-only node:
       A host or router that implements only IPv4.  An IPv4-only node
       does not understand IPv6.  The installed base of IPv4 hosts and
       routers existing before the transition begins are IPv4-only
       nodes.
    IPv6/IPv4 node:
       A host or router that implements both IPv4 and IPv6.
    IPv6-only node:
       A host or router that implements IPv6 and does not implement
       IPv4.  The operation of IPv6-only nodes is not addressed in
       this memo.
    IPv6 node:
       Any host or router that implements IPv6.  IPv6/IPv4 and IPv6-
       only nodes are both IPv6 nodes.
    IPv4 node:
       Any host or router that implements IPv4.  IPv6/IPv4 and IPv4-
       only nodes are both IPv4 nodes.

Nordmark & Gilligan Standards Track [Page 3] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

 Techniques Used in the Transition
    IPv6-over-IPv4 tunneling:
       The technique of encapsulating IPv6 packets within IPv4 so that
       they can be carried across IPv4 routing infrastructures.
    Configured tunneling:
       IPv6-over-IPv4 tunneling where the IPv4 tunnel endpoint
       address(es) are determined by configuration information on
       tunnel endpoints.  All tunnels are assumed to be bidirectional.
       The tunnel provides a (virtual) point-to-point link to the IPv6
       layer, using the configured IPv4 addresses as the lower-layer
       endpoint addresses.
 Other transition mechanisms, including other tunneling mechanisms,
 are outside the scope of this document.
 The key words MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
 SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
 document, are to be interpreted as described in [RFC2119].

2. Dual IP Layer Operation

 The most straightforward way for IPv6 nodes to remain compatible with
 IPv4-only nodes is by providing a complete IPv4 implementation.  IPv6
 nodes that provide complete IPv4 and IPv6 implementations are called
 "IPv6/IPv4 nodes".  IPv6/IPv4 nodes have the ability to send and
 receive both IPv4 and IPv6 packets.  They can directly interoperate
 with IPv4 nodes using IPv4 packets, and also directly interoperate
 with IPv6 nodes using IPv6 packets.
 Even though a node may be equipped to support both protocols, one or
 the other stack may be disabled for operational reasons.  Here we use
 a rather loose notion of "stack".  A stack being enabled has IP
 addresses assigned, 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:
  1. With their IPv4 stack enabled and their IPv6 stack disabled.
  1. With their IPv6 stack enabled and their IPv4 stack disabled.
  1. With both stacks enabled.
 IPv6/IPv4 nodes with their IPv6 stack disabled will operate like
 IPv4-only nodes.  Similarly, IPv6/IPv4 nodes with their IPv4 stacks

Nordmark & Gilligan Standards Track [Page 4] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

 disabled will operate like IPv6-only nodes.  IPv6/IPv4 nodes MAY
 provide a configuration switch to disable either their IPv4 or IPv6
 stack.
 The configured tunneling technique, which is described in Section 3,
 may or may not be used in addition to the dual IP layer operation.

2.1. Address Configuration

 Because the nodes support both protocols, IPv6/IPv4 nodes may be
 configured with both IPv4 and IPv6 addresses.  IPv6/IPv4 nodes use
 IPv4 mechanisms (e.g., DHCP) to acquire their IPv4 addresses, and
 IPv6 protocol mechanisms (e.g., stateless address autoconfiguration
 [RFC2462] and/or DHCPv6) to acquire their IPv6 addresses.

2.2. DNS

 The Domain Naming System (DNS) is used in both IPv4 and IPv6 to map
 between hostnames and IP addresses.  A new resource record type named
 "AAAA" has been defined for IPv6 addresses [RFC3596].  Since
 IPv6/IPv4 nodes must be able to interoperate directly with both IPv4
 and IPv6 nodes, they must provide resolver libraries capable of
 dealing with IPv4 "A" records as well as IPv6 "AAAA" records.  Note
 that the lookup of A versus AAAA records is independent of whether
 the DNS packets are carried in IPv4 or IPv6 packets and that there is
 no assumption that the DNS servers know the IPv4/IPv6 capabilities of
 the requesting node.
 The issues and operational guidelines for using IPv6 with DNS are
 described at more length in other documents, e.g., [DNSOPV6].
 DNS resolver libraries on IPv6/IPv4 nodes MUST be capable of handling
 both AAAA and A records.  However, when a query locates an AAAA
 record holding an IPv6 address, and an A record holding an IPv4
 address, the resolver library MAY order the results returned to the
 application in order to influence the version of IP packets used to
 communicate with that specific node -- IPv6 first, or IPv4 first.
 The applications SHOULD be able to specify whether they want IPv4,
 IPv6, or both records [RFC3493].  That defines which address families
 the resolver looks up.  If there is not an application choice, or if
 the application has requested both, the resolver library MUST NOT
 filter out any records.
 Since most applications try the addresses in the order they are
 returned by the resolver, this can affect the IP version "preference"
 of applications.

Nordmark & Gilligan Standards Track [Page 5] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

 The actual ordering mechanisms are out of scope of this memo.
 Address selection is described at more length in [RFC3484].

3. Configured Tunneling Mechanisms

 In most deployment scenarios, the IPv6 routing infrastructure will be
 built up over time.  While the IPv6 infrastructure is being deployed,
 the existing IPv4 routing infrastructure can remain functional and
 can be used to carry IPv6 traffic.  Tunneling provides a way to
 utilize an existing IPv4 routing infrastructure to carry IPv6
 traffic.
 IPv6/IPv4 hosts and routers can tunnel IPv6 datagrams over regions of
 IPv4 routing topology by encapsulating them within IPv4 packets.
 Tunneling can be used in a variety of ways:
  1. Router-to-Router. IPv6/IPv4 routers interconnected by an IPv4

infrastructure can tunnel IPv6 packets between themselves. In

    this case, the tunnel spans one segment of the end-to-end path
    that the IPv6 packet takes.
  1. Host-to-Router. IPv6/IPv4 hosts can tunnel IPv6 packets to an

intermediary IPv6/IPv4 router that is reachable via an IPv4

    infrastructure.  This type of tunnel spans the first segment of
    the packet's end-to-end path.
  1. Host-to-Host. IPv6/IPv4 hosts that are interconnected by an IPv4

infrastructure can tunnel IPv6 packets between themselves. In

    this case, the tunnel spans the entire end-to-end path that the
    packet takes.
  1. Router-to-Host. IPv6/IPv4 routers can tunnel IPv6 packets to

their final destination IPv6/IPv4 host. This tunnel spans only

    the last segment of the end-to-end path.
 Configured tunneling can be used in all of the above cases, but it is
 most likely to be used router-to-router due to the need to explicitly
 configure the tunneling endpoints.
 The underlying mechanisms for tunneling are:
  1. The entry node of the tunnel (the encapsulator) creates an

encapsulating IPv4 header and transmits the encapsulated packet.

  1. The exit node of the tunnel (the decapsulator) receives the

encapsulated packet, reassembles the packet if needed, removes the

    IPv4 header, and processes the received IPv6 packet.

Nordmark & Gilligan Standards Track [Page 6] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

  1. The encapsulator may need to maintain soft-state information for

each tunnel recording such parameters as the MTU of the tunnel in

    order to process IPv6 packets forwarded into the tunnel.
 In configured tunneling, the tunnel endpoint addresses are determined
 in the encapsulator from configuration information stored for each
 tunnel.  When an IPv6 packet is transmitted over a tunnel, the
 destination and source addresses for the encapsulating IPv4 header
 are set as described in Section 3.5.
 The determination of which packets to tunnel is usually made by
 routing information on the encapsulator.  This is usually done via a
 routing table, which directs packets based on their destination
 address using the prefix mask and match technique.
 The decapsulator matches the received protocol-41 packets to the
 tunnels it has configured, and allows only the packets in which IPv4
 source addresses match the tunnels configured on the decapsulator.
 Therefore, the operator must ensure that the tunnel's IPv4 address
 configuration is the same both at the encapsulator and the
 decapsulator.

3.1. Encapsulation

 The encapsulation of an IPv6 datagram in IPv4 is shown below:
                                           +-------------+
                                           |    IPv4     |
                                           |   Header    |
           +-------------+                 +-------------+
           |    IPv6     |                 |    IPv6     |
           |   Header    |                 |   Header    |
           +-------------+                 +-------------+
           |  Transport  |                 |  Transport  |
           |   Layer     |      ===>       |   Layer     |
           |   Header    |                 |   Header    |
           +-------------+                 +-------------+
           |             |                 |             |
           ~    Data     ~                 ~    Data     ~
           |             |                 |             |
           +-------------+                 +-------------+
                    Encapsulating IPv6 in IPv4

Nordmark & Gilligan Standards Track [Page 7] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

 In addition to adding an IPv4 header, the encapsulator also has to
 handle some more complex issues:
  1. Determine when to fragment and when to report an ICMPv6 "packet

too big" error back to the source.

  1. How to reflect ICMPv4 errors from routers along the tunnel path

back to the source as ICMPv6 errors.

 Those issues are discussed in the following sections.

3.2. Tunnel MTU and Fragmentation

 Naively, the encapsulator could view encapsulation as IPv6 using IPv4
 as a link layer with a very large MTU (65535-20 bytes at most; 20
 bytes "extra" are needed for the encapsulating IPv4 header).  The
 encapsulator would only need to report ICMPv6 "packet too big" errors
 back to the source for packets that exceed this MTU.  However, such a
 scheme would be inefficient or non-interoperable for three reasons
 and therefore MUST NOT be used:
 1) It would result in more fragmentation than needed.  IPv4 layer
    fragmentation should be avoided due to the performance problems
    caused by the loss unit being smaller than the retransmission unit
    [KM97].
 2) Any IPv4 fragmentation occurring inside the tunnel, i.e., between
    the encapsulator and the decapsulator, would have to be
    reassembled at the tunnel endpoint.  For tunnels that terminate at
    a router, this would require additional memory and other resources
    to reassemble the IPv4 fragments into a complete IPv6 packet
    before that packet could be forwarded.
 3) The encapsulator has no way of knowing that the decapsulator is
    able to defragment such IPv4 packets (see Section 3.6 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
 with an MTU of 64 kilobytes, but instead either use the fixed static
 MTU or OPTIONAL dynamic MTU determination based on the IPv4 path MTU
 to the tunnel endpoint.
 If both the mechanisms are implemented, the decision of which to use
 SHOULD be configurable on a per-tunnel endpoint basis.

Nordmark & Gilligan Standards Track [Page 8] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

3.2.1. Static Tunnel MTU

 A node using static tunnel MTU treats the tunnel interface as having
 a fixed-interface MTU.  By default, the MTU MUST be between 1280 and
 1480 bytes (inclusive), but it SHOULD be 1280 bytes.  If the default
 is not 1280 bytes, the implementation MUST have a configuration knob
 that can be used to change the MTU value.
 A node must be able to accept a fragmented IPv6 packet that, after
 reassembly, is as large as 1500 octets [RFC2460].  This memo also
 includes requirements (see Section 3.6) for the amount of IPv4
 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:
  1. Whether the IPv4 protocol-41 packets will be transported over

media that may have a lower path MTU (e.g., IPv4 Virtual Private

    Networks); then picking too high a value might lead to IPv4
    fragmentation.
  1. Whether the tunnel is used to transport IPv6 tunneled packets

(e.g., a mobile node with an IPv6-in-IPv4 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
 set in the encapsulating IPv4 header.  As a result, the encapsulator
 should not receive any ICMPv4 "packet too big" messages as a result
 of the packets it has encapsulated.

3.2.2. Dynamic Tunnel MTU

 The dynamic MTU determination is OPTIONAL.  However, if it is
 implemented, it SHOULD have the behavior described in this document.
 The fragmentation inside the tunnel can be reduced to a minimum by
 having the encapsulator track the IPv4 path MTU across the tunnel,
 using the IPv4 Path MTU Discovery Protocol [RFC1191] and recording

Nordmark & Gilligan Standards Track [Page 9] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

 the resulting path MTU.  The IPv6 layer in the encapsulator can then
 view a tunnel as a link layer with an MTU equal to the IPv4 path MTU,
 minus the size of the encapsulating IPv4 header.
 Note that this does not eliminate IPv4 fragmentation in the case when
 the IPv4 path MTU would result in an IPv6 MTU less than 1280 bytes.
 (Any link layer used by IPv6 has to have an MTU of at least 1280
 bytes [RFC2460].)  In this case, the IPv6 layer has to "see" a link
 layer with an MTU of 1280 bytes and the encapsulator has to use IPv4
 fragmentation in order to forward the 1280 byte IPv6 packets.
 The encapsulator SHOULD employ the following algorithm to determine
 when to forward an IPv6 packet that is larger than the tunnel's path
 MTU using IPv4 fragmentation, and when to return an ICMPv6 "packet
 too big" message per [RFC1981]:
       if (IPv4 path MTU - 20) is less than 1280
               if packet is larger than 1280 bytes
                       Send ICMPv6 "packet too big" with MTU = 1280.
                       Drop packet.
               else
                       Encapsulate but do not set the Don't Fragment
                       flag in the IPv4 header.  The resulting IPv4
                       packet might be fragmented by the IPv4 layer
                       on the encapsulator or by some router along
                       the IPv4 path.
               endif
       else
               if packet is larger than (IPv4 path MTU - 20)
                       Send ICMPv6 "packet too big" with
                       MTU = (IPv4 path MTU - 20).
                       Drop packet.
               else
                       Encapsulate and set the Don't Fragment flag
                       in the IPv4 header.
               endif
       endif
 Encapsulators that have a large number of tunnels may choose between
 dynamic versus static tunnel MTUs 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 path MTU
 blackholes should the ICMPv4 "packet too big" messages be dropped by
 firewalls or not generated by the routers [RFC1435, RFC2923].

Nordmark & Gilligan Standards Track [Page 10] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

3.3. Hop Limit

 IPv6-over-IPv4 tunnels are modeled as "single-hop" from the IPv6
 perspective.  The tunnel is opaque to users of the network, and it is
 not detectable by network diagnostic tools such as traceroute.
 The single-hop model is implemented by having the encapsulators and
 decapsulators process the IPv6 hop limit field as they would if they
 were forwarding a packet on to any other datalink.  That is, they
 decrement the hop limit by 1 when forwarding an IPv6 packet.  (The
 originating node and final destination do not decrement the hop
 limit.)
 The TTL of the encapsulating IPv4 header is selected in an
 implementation-dependent manner.  The current suggested value is
 published in the "Assigned Numbers" RFC [RFC3232][ASSIGNED].
 Implementations MAY provide a mechanism to allow the administrator to
 configure the IPv4 TTL as the IP Tunnel MIB [RFC4087].

3.4. Handling ICMPv4 Errors

 In response to encapsulated packets it has sent into the tunnel, the
 encapsulator might receive ICMPv4 error messages from IPv4 routers
 inside the tunnel.  These packets are addressed to the encapsulator
 because it is the IPv4 source of the encapsulated packet.
 ICMPv4 error handling is only applicable to dynamic MTU
 determination, even though the functions could be used with static
 MTU tunnels as well.
 The ICMPv4 "packet too big" error messages are handled according to
 IPv4 Path MTU Discovery [RFC1191] and the resulting path MTU is
 recorded in the IPv4 layer.  The recorded path MTU is used by IPv6 to
 determine if an ICMPv6 "packet too big" error has to be generated as
 described in Section 3.2.2.
 The handling of other types of ICMPv4 error messages depends on how
 much information is available from the encapsulated packet that
 caused the error.
 Many older IPv4 routers return only 8 bytes of data beyond the IPv4
 header of the packet in error, which is not enough to include the
 address fields of the IPv6 header.  More modern IPv4 routers are
 likely to return enough data beyond the IPv4 header to include the
 entire IPv6 header and possibly even the data beyond that.  See
 [RFC1812].

Nordmark & Gilligan Standards Track [Page 11] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

 If sufficient data bytes from the offending packet are available, the
 encapsulator MAY extract the encapsulated IPv6 packet and use it to
 generate an ICMPv6 message directed back to the originating IPv6
 node, as shown below:
                       +--------------+
                       | IPv4 Header  |
                       | dst = encaps |
                       |       node   |
                       +--------------+
                       |    ICMPv4    |
                       |    Header    |
                - -    +--------------+
                       | IPv4 Header  |
                       | src = encaps |
               IPv4    |       node   |
                       +--------------+   - -
               Packet  |    IPv6      |
                       |    Header    |   Original IPv6
                in     +--------------+   Packet -
                       |  Transport   |   Can be used to
               Error   |    Header    |   generate an
                       +--------------+   ICMPv6
                       |              |   error message
                       ~     Data     ~   back to the source.
                       |              |
                - -    +--------------+   - -
           ICMPv4 Error Message Returned to Encapsulating Node
 When receiving ICMPv4 errors as above and the errors are not "packet
 too big", it would be useful to log the error as an error related to
 the tunnel.  Also, if sufficient headers are available, then the
 originating node MAY send an ICMPv6 error of type "unreachable" with
 code "address unreachable" to the IPv6 source.  (The "address
 unreachable" code is appropriate since, from the perspective of IPv6,
 the tunnel is a link and that code is used for link-specific errors
 [RFC2463]).
 Note that when the IPv4 path MTU is exceeded, and sufficient bytes of
 payload associated with the ICMPv4 errors are not available, 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 that causes at least one packet

Nordmark & Gilligan Standards Track [Page 12] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

 loss, and then the host needs to learn the (IPv6) MTU from the router
 that causes at least one packet loss.  Still, in all cases there can
 be more than one packet loss if there are multiple large packets in
 flight at the same time.

3.5. IPv4 Header Construction

 When encapsulating an IPv6 packet in an IPv4 datagram, the IPv4
 header fields are set as follows:
    Version:
       4
    IP Header Length in 32-bit words:
       5 (There are no IPv4 options in the encapsulating header.)
    Type of Service:
       0 unless otherwise specified. (See [RFC2983] and [RFC3168]
       Section 9.1 for issues relating to the Type-of-Service byte and
       tunneling.)
    Total Length:
       Payload length from IPv6 header plus length of IPv6 and IPv4
       headers (i.e., IPv6 payload length plus a constant 60 bytes).
    Identification:
       Generated uniquely as for any IPv4 packet transmitted by the
       system.
    Flags:
       Set the Don't Fragment (DF) flag as specified in Section 3.2.
       Set the More Fragments (MF) bit as necessary if fragmenting.
    Fragment Offset:
       Set as necessary if fragmenting.
    Time to Live:
       Set in an implementation-specific manner, as described in
       Section 3.3.

Nordmark & Gilligan Standards Track [Page 13] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

    Protocol:
       41 (Assigned payload type number for IPv6).
    Header Checksum:
       Calculate the checksum of the IPv4 header [RFC791].
    Source Address:
       An IPv4 address of the encapsulator: either configured by the
       administrator or an address of the outgoing interface.
    Destination Address:
       IPv4 address of the tunnel endpoint.
 When encapsulating the packets, the node must ensure that it will use
 the correct source address so that the packets are acceptable to the
 decapsulator as described in Section 3.6.  Configuring the source
 address is appropriate particularly in cases in which automatic
 selection of source address may produce different results in a
 certain period of time.  This is often the case with multiple
 addresses, and multiple interfaces, or when routes may change
 frequently.  Therefore, it SHOULD be possible to administratively
 specify the source address of a tunnel.

3.6. Decapsulation

 When an IPv6/IPv4 host or a router receives an IPv4 datagram that is
 addressed to one of its own IPv4 addresses or a joined multicast
 group address, and the value of the protocol field is 41, the packet
 is potentially a tunnel packet and needs to be verified to belong to
 one of the configured tunnel interfaces (by checking
 source/destination addresses), reassembled (if fragmented at the IPv4
 level), and have the IPv4 header removed and the resulting IPv6
 datagram be submitted to the IPv6 layer code on the node.
 The decapsulator MUST verify that the tunnel source address is
 correct before further processing packets, to mitigate the problems
 with address spoofing (see Section 4).  This check also applies to
 packets that are delivered to transport protocols on the
 decapsulator.  This is done by verifying that the source address is
 the IPv4 address of the encapsulator, as configured on the
 decapsulator.  Packets for which the IPv4 source address does not
 match MUST be discarded and an ICMP message SHOULD NOT be generated;

Nordmark & Gilligan Standards Track [Page 14] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

 however, if the implementation normally sends an ICMP message when
 receiving an unknown protocol packet, such an error message MAY be
 sent (e.g., ICMPv4 Protocol 41 Unreachable).
 A side effect of this address verification is that the node will
 silently discard packets with a wrong source address and packets that
 were received by the node but not directly addressed to it (e.g.,
 broadcast addresses).
 Independent of any other forms of IPv4 ingress filtering the
 administrator of the node may have configured, the implementation MAY
 perform ingress filtering, i.e., check that the packet is arriving
 from the interface in the direction of the route toward the tunnel
 end-point, similar to a Strict Reverse Path Forwarding (RPF) check
 [RFC3704].  As this may cause problems on tunnels that are routed
 through multiple links, it is RECOMMENDED that this check, if done,
 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 1500 bytes and the largest
 (IPv6) interface MTU on the decapsulator.
 The decapsulator MUST be capable of reassembling an IPv4 packet that
 is (after the reassembly) the maximum of 1500 bytes and the largest
 (IPv4) interface MTU on the decapsulator.  The 1500-byte number is a
 result of encapsulators that use the static MTU scheme in Section
 3.2.1, while encapsulators that use the dynamic scheme in Section
 3.2.2 can cause up to the largest interface MTU on the decapsulator
 to be received. (Note that it is strictly the interface MTU on the
 last IPv4 router *before* the decapsulator that matters, but for most
 links the MTU is the same between all neighbors.)
 This reassembly limit allows dynamic tunnel MTU determination by the
 encapsulator to take advantage of larger IPv4 path MTUs.  An
 implementation MAY have a configuration knob that can be used to set
 a larger value of the tunnel reassembly buffers than the above
 number, but it MUST NOT be set below the above number.

Nordmark & Gilligan Standards Track [Page 15] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

 The decapsulation is shown below:
          +-------------+
          |    IPv4     |
          |   Header    |
          +-------------+                 +-------------+
          |    IPv6     |                 |    IPv6     |
          |   Header    |                 |   Header    |
          +-------------+                 +-------------+
          |  Transport  |                 |  Transport  |
          |   Layer     |      ===>       |   Layer     |
          |   Header    |                 |   Header    |
          +-------------+                 +-------------+
          |             |                 |             |
          ~    Data     ~                 ~    Data     ~
          |             |                 |             |
          +-------------+                 +-------------+
                  Decapsulating IPv6 from IPv4
 The decapsulator performs IPv4 reassembly before decapsulating the
 IPv6 packet.
 When decapsulating the packet, the IPv6 header is not modified.
 (However, see [RFC2983] and [RFC3168] section 9.1 for issues relating
 to the Type of Service byte and tunneling.)  If the packet is
 subsequently forwarded, its hop limit is decremented by one.
 The encapsulating IPv4 header is discarded, and the resulting packet
 is checked for validity when submitted to the IPv6 layer.  When
 reconstructing the IPv6 packet, the length MUST be determined from
 the IPv6 payload length since the IPv4 packet might be padded (thus
 have a length that is larger than the IPv6 packet plus the IPv4
 header being removed).
 After the decapsulation, the node MUST silently discard a packet with
 an invalid IPv6 source address.  The list of invalid source addresses
 SHOULD include at least:
  1. all multicast addresses (FF00::/8)
  1. the loopback address (::1)
  1. all the IPv4-compatible IPv6 addresses [RFC3513] (::/96),

excluding the unspecified address for Duplicate Address Detection

    (::/128)
  1. all the IPv4-mapped IPv6 addresses (::ffff:0:0/96)

Nordmark & Gilligan Standards Track [Page 16] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

 In addition, the node should be configured to perform ingress
 filtering [RFC2827][RFC3704] on the IPv6 source address, similar to
 on any of its interfaces, e.g.:
 1) if the tunnel is toward the Internet, the node should be
    configured to check that the site's IPv6 prefixes are not used as
    the source addresses, or
 2) if the tunnel is toward an edge network, the node should be
    configured to check that the source address belongs to that edge
    network.
 The prefix lists in the former typically need to be manually
 configured; the latter could be verified automatically, e.g., by
 using a strict unicast RPF check, as long as an interface can be
 designated to be toward an edge.
 It is RECOMMENDED that the implementations provide a single knob to
 make it easier to for the administrators to enable strict ingress
 filtering toward edge networks.

3.7. Link-Local Addresses

 The configured tunnels are IPv6 interfaces (over the IPv4 "link
 layer") and thus MUST have link-local addresses.  The link-local
 addresses are used by, e.g., routing protocols operating over the
 tunnels.
 The interface identifier [RFC3513] for such an interface may be based
 on the 32-bit IPv4 address of an underlying interface, or formed
 using some other means, as long as it is unique from the other tunnel
 endpoint with a reasonably high probability.
 Note that it may be desirable to form the link-local address in a
 fashion that minimizes the probability and the effect of having to
 renumber the link-local address in the event of a topology or
 hardware change.
 If an IPv4 address is used for forming the IPv6 link-local address,
 the interface identifier is the IPv4 address, prepended by zeros.
 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, a choice of one of these IPv4 addresses is made

Nordmark & Gilligan Standards Track [Page 17] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

 by the administrator or the implementation when forming the link-
 local address.
    +-------+-------+-------+-------+-------+-------+------+------+
    |  FE      80      00      00      00      00      00     00  |
    +-------+-------+-------+-------+-------+-------+------+------+
    |  00      00      00      00   |        IPv4 Address         |
    +-------+-------+-------+-------+-------+-------+------+------+

3.8. Neighbor Discovery over Tunnels

 Configured tunnel implementations MUST at least accept and respond to
 the probe packets used by Neighbor Unreachability Detection (NUD)
 [RFC2461].  The implementations SHOULD also send NUD probe packets to
 detect when the 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
 specified in this document are assumed to NOT have a link-layer
 address, even though the link-layer (IPv4) does have an address.
 This means that:
  1. the sender of Neighbor Discovery packets SHOULD NOT include Source

Link Layer Address options or Target Link Layer Address options on

    the tunnel link.
  1. the receiver MUST, while otherwise processing the Neighbor

Discovery packet, silently ignore the content of any Source Link

    Layer Address options or Target Link Layer Address options
    received on the tunnel link.
 Not using link-layer address options is consistent with how Neighbor
 Discovery is used on other point-to-point links.

4. Threat Related to Source Address Spoofing

 The specification above contains rules that apply tunnel source
 address verification in particular and ingress filtering
 [RFC2827][RFC3704] in general to packets before they are
 decapsulated.  When IP-in-IP tunneling (independent of IP versions)
 is used, it is important that this not be used to bypass any ingress
 filtering in use for non-tunneled packets.  Thus, the rules in this
 document are derived based on should ingress filtering be used for
 IPv4 and IPv6, the use of tunneling should not provide an easy way to
 circumvent the filtering.

Nordmark & Gilligan Standards Track [Page 18] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

 In this case, without specific ingress filtering checks in the
 decapsulator, it would be possible for an attacker to inject a packet
 with:
  1. Outer IPv4 source: real IPv4 address of attacker
  1. Outer IPv4 destination: IPv4 address of decapsulator
  1. Inner IPv6 source: Alice, which is either the decapsulator or a

node close to it

  1. Inner IPv6 destination: Bob
 Even if all IPv4 routers between the attacker and the decapsulator
 implement IPv4 ingress filtering, and all IPv6 routers between the
 decapsulator and Bob implement IPv6 ingress filtering, the above
 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
 the sender was some unrelated node.
 The solution to this is to have the decapsulator accept only
 encapsulated packets from the explicitly configured source address
 (i.e., the other end of the 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

 Generic security considerations of using IPv6 are discussed in a
 separate document [V6SEC].
 An implementation of tunneling needs to be aware that although 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 potentially includes all of the Internet.
 Several mechanisms (e.g., Neighbor Discovery) depend on Hop Count
 being 255 and/or the addresses being link local for ensuring that a
 packet originated on-link, in a semi-trusted environment.  Tunnels
 are more vulnerable to a breach of this assumption than physical
 links, as an attacker anywhere in the Internet can send an IPv6-in-
 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.

Nordmark & Gilligan Standards Track [Page 19] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

 Therefore, this memo specifies that the decapsulators make these
 steps (as described in Section 3.6) to mitigate this threat:
  1. IPv4 source address of the packet MUST be the same as configured

for the tunnel end-point;

  1. Independent of any IPv4 ingress filtering the administrator may

have configured, the implementation MAY perform IPv4 ingress

    filtering to check that the IPv4 packets are received from an
    expected interface (but as this may cause some problems, it may be
    disabled by default);
  1. IPv6 packets with several, obviously invalid IPv6 source addresses

received from the tunnel MUST be discarded (see Section 3.6 for

    details); and
  1. IPv6 ingress filtering should be performed (typically requiring

configuration from the operator), to check that the tunneled 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 (ranging from
 difficult to predictable) 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, e.g., 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.  However, setting up
 secure IPsec IPv6-in-IPv4 tunnels is described in another document
 [V64IPSEC].
 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.
 In addition, an implementation MUST treat interfaces to different
 links as separate, e.g., to ensure that Neighbor Discovery packets
 arriving on one link do not affect other links.  This is especially
 important for tunnel links.
 When dropping packets due to failing to match the allowed IPv4 source
 addresses for a tunnel the node should not "acknowledge" the
 existence of a tunnel, otherwise this could be used to probe the
 acceptable tunnel endpoint addresses.  For that reason, the
 specification says that such packets MUST be discarded, and an ICMP

Nordmark & Gilligan Standards Track [Page 20] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

 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, 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. Acknowledgements

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

7. References

7.1. Normative References

 [RFC791]   Postel, J., "Internet Protocol", STD 5, RFC 791, September
            1981.
 [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
            November 1990.
 [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
            for IP version 6", RFC 1981, August 1996.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
            (IPv6) Specification", RFC 2460, December 1998.
 [RFC2463]  Conta, A. and S. Deering, "Internet Control Message
            Protocol (ICMPv6) for the Internet Protocol Version 6
            (IPv6) Specification", RFC 2463, December 1998.

7.2. Informative References

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

Nordmark & Gilligan Standards Track [Page 21] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

 [DNSOPV6]  Durand, A., Ihren, J., and Savola P., "Operational
            Considerations and Issues with IPv6 DNS", Work in
            Progress, October 2004.
 [KM97]     Kent, C., and J. Mogul, "Fragmentation Considered
            Harmful".  In Proc.  SIGCOMM '87 Workshop on Frontiers in
            Computer Communications Technology.  August 1987.
 [V6SEC]    Savola, P., "IPv6 Transition/Co-existence Security
            Considerations", Work in Progress, October 2004.
 [V64IPSEC] Graveman, R., et al., "Using IPsec to Secure IPv6-over-
            IPv4 Tunnels", Work in Progress, December 2004.
 [RFC1435]  Knowles, S., "IESG Advice from Experience with Path MTU
            Discovery", RFC 1435, March 1993.
 [RFC1812]  Baker, F., "Requirements for IP Version 4 Routers", RFC
            1812, June 1995.
 [RFC2401]  Kent, S. and R. Atkinson, "Security Architecture for the
            Internet Protocol", RFC 2401, November 1998.
 [RFC2461]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor
            Discovery for IP Version 6 (IPv6)", RFC 2461, December
            1998.
 [RFC2462]  Thomson, S. and T. Narten, "IPv6 Stateless Address
            Autoconfiguration", RFC 2462, December 1998.
 [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
            Defeating Denial of Service Attacks which employ IP Source
            Address Spoofing", BCP 38, RFC 2827, May 2000.
 [RFC2890]  Dommety, G., "Key and Sequence Number Extensions to GRE",
            RFC 2890, September 2000.
 [RFC2923]  Lahey, K., "TCP Problems with Path MTU Discovery", RFC
            2923, September 2000.
 [RFC2983]  Black, D., "Differentiated Services and Tunnels", RFC
            2983, October 2000.
 [RFC3056]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains
            via IPv4 Clouds", RFC 3056, February 2001.

Nordmark & Gilligan Standards Track [Page 22] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

 [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
            of Explicit Congestion Notification (ECN) to IP", RFC
            3168, September 2001.
 [RFC3232]  Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by
            an On-line Database", RFC 3232, January 2002.
 [RFC3484]  Draves, R., "Default Address Selection for Internet
            Protocol version 6 (IPv6)", RFC 3484, February 2003.
 [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
            Stevens, "Basic Socket Interface Extensions for IPv6", RFC
            3493, February 2003.
 [RFC3513]  Hinden, R. and S. Deering, "Internet Protocol Version 6
            (IPv6) Addressing Architecture", RFC 3513, April 2003.
 [RFC3596]  Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
            "DNS Extensions to Support IP Version 6", RFC 3596,
            October 2003.
 [RFC3704]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed
            Networks", BCP 84, RFC 3704, March 2004.
 [RFC4087]  Thaler, D., "IP Tunnel MIB", RFC 4087, June 2005.

8. 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 [RFC3056] which gives
 each node with a (global) IPv4 address a /48 IPv6 prefix i.e., enough
 for a whole site.
 The following changes have been performed since RFC 2893:
  1. Removed references to A6 and retained AAAA.
  1. Removed automatic tunneling and use of IPv4-compatible addresses.
  1. Removed default Configured Tunnel using IPv4 "Anycast Address"
  1. Removed Source Address Selection section since this is now covered

by another document ([RFC3484]).

  1. Removed brief mention of 6over4.

Nordmark & Gilligan Standards Track [Page 23] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

  1. Split into normative and non-normative references and other

reference cleanup.

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

equal to 1280.

  1. 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.
  1. Described Static MTU and Dynamic MTU cases separately; clarified

that the dynamic path MTU mechanism is OPTIONAL but if it is

    implemented it should follow the rules in section 3.2.2.
  1. 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.
  1. Specified minimal rules for IPv4 reassembly and IPv6 MRU to

enhance interoperability and to minimize blacholes.

  1. Restated the "currently underway" language about Type-of-Service,

and loosely point at [RFC2983] and [RFC3168].

  1. Fixed reference to Assigned Numbers to be to online version (with

proper pointer to "Assigned Numbers is obsolete" RFC).

  1. Clarified text about ingress filtering e.g., that it applies to

packet delivered to transport protocols on the decapsulator as

    well as packets being forwarded by the decapsulator, and how the
    decapsulator's checks help when IPv4 and IPv6 ingress filtering is
    in place.
  1. Removed unidirectional tunneling; assume all tunnels are

bidirectional, between endpoint addresses (not nodes).

  1. Removed the guidelines for advertising addresses in DNS as

slightly out of scope, referring to another document for the

    details.
  1. Removed the SHOULD requirement that the link-local addresses

should be formed based on IPv4 addresses.

Nordmark & Gilligan Standards Track [Page 24] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

  1. 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.
  1. Added stronger wording for source address checks: both IPv4 and

IPv6 source addresses MUST be checked, and RPF-like ingress

    filtering is optional.
  1. Rewrote security considerations to be more precise about the

threats of tunneling.

  1. Added a note about considering using TTL=255 when encapsulating.
  1. Added more discussion in Section 3.2 why using an "infinite" IPv6

MTU leads to likely interoperability problems.

  1. Added an explicit requirement that if both MTU determination

methods are used, choosing one should be possible on a per-tunnel

    basis.
  1. Clarified that ICMPv4 error handling is only applicable to dynamic

MTU determination.

  1. Removed/clarified DNS record filtering; an API is a SHOULD and if

it does not exist, MUST NOT filter anything. Decree ordering out

    of scope, but refer to RFC3484.
  1. Add a note that the destination IPv4 address could also be a

multicast address.

  1. Make it RECOMMENDED to provide a toggle to perform strict ingress

filtering on an interface.

  1. Generalize the text on the data in ICMPv4 messages.
  1. Made a lot of miscellaneous editorial cleanups.

Nordmark & Gilligan Standards Track [Page 25] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

Authors' Addresses

 Erik Nordmark
 Sun Microsystems
 17 Network Circle
 Menlo Park, CA 94025
 USA
 Phone: +1 650 786 2921
 EMail: erik.nordmark@sun.com
 Robert E. Gilligan
 Intransa, Inc.
 2870 Zanker Rd., Suite 100
 San Jose, CA 95134 USA
 Phone : +1 408 678 8600
 Fax :   +1 408 678 8800
 EMail:  bob.gilligan@acm.org

Nordmark & Gilligan Standards Track [Page 26] RFC 4213 Basic IPv6 Transition Mechanisms October 2005

Full Copyright Statement

 Copyright (C) The Internet Society (2005).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
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 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
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 Copies of IPR disclosures made to the IETF Secretariat and any
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 The IETF invites any interested party to bring to its attention any
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 rights that may cover technology that may be required to implement
 this standard.  Please address the information to the IETF at ietf-
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Nordmark & Gilligan Standards Track [Page 27]

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