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Versions: 00 01 02 03 04 05 06 07 RFC 4213

INTERNET-DRAFT                                               E. Nordmark
June 15, 2004                                     Sun Microsystems, Inc.
Obsoletes: 2893                                           R. E. Gilligan
                                                          Intransa, Inc.


         Basic Transition Mechanisms for IPv6 Hosts and Routers
                   <draft-ietf-v6ops-mech-v2-03.txt>



Status of this Memo


   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance with
   RFC 3668.


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


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


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


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


   This draft expires on December 15, 2004.


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.








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   Contents


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


      1.  Introduction.............................................    3
         1.1.  Terminology.........................................    3


      2.  Dual IP Layer Operation..................................    5
         2.1.  Address Configuration...............................    5
         2.2.  DNS.................................................    5


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


      4.  Threat Related to Source Address Spoofing................   20


      5.  Security Considerations..................................   21


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


      7.  References...............................................   23
         7.1.  Normative References................................   23
         7.2.  Informative References..............................   23


      8.  Authors' Addresses.......................................   25


      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............   29
         9.3.  Changes from draft-ietf-v6ops-mech-v2-02............   30













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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:


   -    Dual IP layer (also known as Dual Stack):  A technique for
        providing complete support for both Internet protocols -- IPv4
        and IPv6 -- in hosts and routers.


   -    Configured tunneling of IPv6 over IPv4:  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 which
   implementations and users may employ to ease the transition.  The
   tools may be used as needed.  Implementations and sites decide which
   techniques are appropriate to their specific needs.


   This document defines the basic set of transition mechanisms, but
   these are not the only tools available.  Additional transition and
   compatibility mechanisms are specified in other documents.




1.1.  Terminology


   The following terms are used in this document:


   Types of Nodes


        IPv4-only node:


                A host or router that implements only IPv4.  An IPv4-
                only node does not understand IPv6.  The installed base




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                of IPv4 hosts and routers existing before the transition
                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.


   Techniques Used in the Transition


        IPv6-over-IPv4 tunneling:


                The technique of encapsulating IPv6 packets within IPv4
                so that they can be carried across IPv4 routing
                infrastructures.


        Configured tunneling:


                IPv6-over-IPv4 tunneling where the IPv4 tunnel endpoint
                address is determined by configuration information on
                the encapsulator.  All tunnels are assumed to be
                bidirectional, behaving as virtual point-to-point links.


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


   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [RFC2119].








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2.  Dual IP Layer Operation


   The most straightforward way for IPv6 nodes to remain compatible with
   IPv4-only nodes is by providing a complete IPv4 implementation.  IPv6
   nodes that provide complete IPv4 and IPv6 implementations are called
   "IPv6/IPv4 nodes."  IPv6/IPv4 nodes have the ability to send and
   receive both IPv4 and IPv6 packets.  They can directly interoperate
   with IPv4 nodes using IPv4 packets, and also directly interoperate
   with IPv6 nodes using IPv6 packets.


   Even though a node may be equipped to support both protocols, one or
   the other stack may be disabled for operational reasons.  Here we use
   a rather loose notion of "stack".  A stack being enabled has IP
   addresses assigned etc, but whether or not any particular application
   is available on the stacks is explicitly not defined.  Thus IPv6/IPv4
   nodes may be operated in one of three modes:


   -    With their IPv4 stack enabled and their IPv6 stack disabled.


   -    With their IPv6 stack enabled and their IPv4 stack disabled.


   -    With both stacks enabled.


   IPv6/IPv4 nodes with their IPv6 stack disabled will operate like
   IPv4-only nodes.  Similarly, IPv6/IPv4 nodes with their IPv4 stacks
   disabled will operate like IPv6-only nodes.  IPv6/IPv4 nodes MAY
   provide a configuration switch to disable either their IPv4 or IPv6
   stack.


   The 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
   and/or DHCPv6) to acquire their IPv6 addresses.




2.2.  DNS


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




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   "AAAA" has been defined for IPv6 addresses [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 [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 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 isn't 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.


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


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


   This is a bare minimum DNS processing implementation for dual-stack
   hosts.  The more extensive algorithm is specified in [RFC3484].












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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:


   -    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.


   -    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.


   -    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.


   -    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 is
   most likely to be used router-to-router due to the need to explicitly
   configure the tunneling endpoints.


   The underlying mechanisms for tunneling are:


   -    The entry node of the tunnel (the encapsulator) creates an
        encapsulating IPv4 header and transmits the encapsulated packet.


   -    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.


   -    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.




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   In configured tunneling, the tunnel endpoint address is determined
   from configuration information in the encapsulator.  For each tunnel,
   the encapsulator must store the tunnel endpoint address.  When an
   IPv6 packet is transmitted over a tunnel, the tunnel endpoint address
   configured for that tunnel is used as the destination address for the
   encapsulating IPv4 header.


   The determination of which packets to tunnel is usually made by
   routing information on the encapsulator.  This is usually done via a
   routing table, which directs packets based on their destination
   address using the prefix mask and match technique.




3.1.  Encapsulation


   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



   In addition to adding an IPv4 header, the encapsulator also has to
   handle some more complex issues:


   -    Determine when to fragment and when to report an ICMPv6 "packet
        too big" error back to the source.


   -    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.




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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 to be exact; 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 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
   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 which to use
   SHOULD be configurable on a per-tunnel endpoint basis.




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
   which 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




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   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
   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
   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




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   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 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
   should the ICMPv4 "packet too big" messages be dropped by firewalls
   or not generated by the routers. [RFC1435, RFC2923]





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 is not
   detectable by network diagnostic tools such as traceroute.




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   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].  The
   implementations MAY also consider using the value 255.
   Implementations MAY provide a mechanism to allow the administrator to
   configure the IPv4 TTL as the IP Tunnel MIB [RFC2667].




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 included in the "packet in error" field, which
   holds the encapsulated packet that caused the error.


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


   If the offending packet includes enough data, the 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:






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                   +--------------+
                   | 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 included in the error,
   then the originating node MAY send an ICMPv6 error of type
   "unreachable" with code "address unreachable" to the IPv6 source.
   (The "address unreachable" code is appropriate since, from the
   perspective of IPv6, the tunnel is a link and that code is used for
   link-specific errors [RFC2463]).


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




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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.


        Protocol:





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                41 (Assigned payload type number for IPv6).


        Header Checksum:


                Calculate the checksum of the IPv4 header. [RFC791]


        Source Address:


                IPv4 address of outgoing interface of the encapsulator
                or an administratively specified address as described
                below.


        Destination Address:


                IPv4 address of the tunnel endpoint.


   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


   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 part of a tunnel 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), 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 which are delivered to transport protocols on the
   decapsulator.  This is done by verifying that the source address is
   the IPv4 address of the other end of a tunnel configured on the node.
   Packets for which the IPv4 source address does not match MUST be
   discarded and an ICMP message SHOULD NOT be generated; however, if
   the implementation normally sends an ICMP message when receiving an
   unknown protocol packet, such an error message MAY be sent (e.g.,




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   ICMPv4 Protocol 41 Unreachable).


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


   Independent of any other forms of IPv4 ingress filtering the
   administrator of the node may have configured to perform, the
   implementation MAY perform ingress filtering, i.e., check that the
   packet is arriving from the interface in the direction of the route
   towards the tunnel end-point, similar to a Strict Reverse Path
   Forwarding (RPF) check [RFC3704].  As this may cause problems on
   tunnels which 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 of 1500 bytes and the largest
   (IPv6) interface MTU on the decapsulator.


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


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


   The decapsulation is shown below:













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


                       Decapsulating IPv6 from IPv4



   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 decapsulator performs IPv4 reassembly before decapsulating the
   IPv6 packet.


   The encapsulating IPv4 header is discarded.  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
   which 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:


    -   all multicast addresses (FF00::/8)


    -   the loopback address (::1)


    -   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)





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   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 towards 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 towards 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 towards 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 towards 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's 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




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   interface concerned, a choice of one of these IPv4 addresses is made
   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:


    -   the sender of Neighbor Discovery packets SHOULD NOT include
        Source Link Layer Address options or Target Link Layer Address
        options on the tunnel link.


    -   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 a link layer address options is consistent with how
   Neighbor Discovery is used on other point-to-point links.













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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 can 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.


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


    -   Outer IPv4 source: real IPv4 address of attacker


    -   Outer IPv4 destination: IPv4 address of decapsulator


    -   Inner IPv6 source: Alice which is either the decapsulator or a
        node close to it.


    -   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 only accept
   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.













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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 while 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.


   Therefore, this memo specifies that the decapsulators make these
   steps to mitigate this threat:


    -   IPv4 source address of the packet MUST be the same as configured
        for the tunnel end-point,


    -   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),


    -   IPv6 packets with several, obviously invalid IPv6 source
        addresses received from the tunnel MUST be discarded (see
        Section 3.6 for details), and


    -   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, for example IPsec [RFC2401] (preferable) or Generic
   Routing Encapsulation with a pre-configured secret key [RFC2890].  As




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   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.


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


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










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7.  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,
            November 1990.


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


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


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


 [RFC2463]  A. Conta, 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


 [DNSOPV6]  Durand, A., Ihren, J., and Savola P., "Operational
            Considerations and Issues with IPv6 DNS", draft-ietf-dnsop-
            ipv6-dns-issues-07.txt, work-in-progress, May 2004.


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


 [V6SEC]    P. Savola, "IPv6 Transition/Co-existence Security
            Considerations", draft-savola-v6ops-security-overview-
            02.txt, work-in-progress, February 2004.


 [V64IPSEC] Graveman, R., et al., "Using IPsec to Secure IPv6-over-IPv4
            Tunnels", draft-tschofenig-v6ops-secure-tunnels-00.txt,
            work-in-progress, June 2004.




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


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


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


 [RFC2401]  Kent, S., Atkinson, R., "Security Architecture for the
            Internet Protocol", RFC 2401, November 1998.


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


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


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


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


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


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


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


 [RFC3056]  B. Carpenter, and K. Moore, "Connection of IPv6 Domains via
            IPv4 Clouds", RFC 3056, February 2001.


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


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


 [RFC3484]  R. Draves, "Default Address Selection for IPv6", RFC 3484,
            February 2003.


 [RFC3493]  Gilligan, R., et al, "Basic Socket Interface Extensions for
            IPv6", RFC 3493, February 2003.




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


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


 [RFC3704]  Baker, F., and Savola P., "Ingress Filtering for Multihomed
            Networks", RFC 3704, BCP 84, March 2004.




8.  Authors' Addresses


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


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


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





9.  Changes from RFC 2893


   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:


   -    Removed references to A6 and retained AAAA.


   -    Removed automatic tunneling and use of IPv4-compatible




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        addresses.


   -    Removed default Configured Tunnel using IPv4 "Anycast Address"


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


   -    Removed brief mention of 6over4.


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


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


   -    Dropped this: However, IPv6 may be used in some environments
        where interoperability with IPv4 is not required.  IPv6 nodes
        that are designed to be used in such environments need not use
        or even implement these mechanisms.


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


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


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


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


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


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


   -    Removed unidirectional tunneling; assume all tunnels are
        bidirectional.


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




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        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.


   -    Removed/clarified DNS record filtering; an API is a SHOULD and
        if it does not exist, MUST NOT filter anything.  Refer more
        strongly to RFC3484 on the DNS record ordering/processing.


   -    Add a note that the destination IPv4 address could also be a
        multicast address.


   -    Make it RECOMMENDED to provide a toggle to perform strict
        ingress filtering on an interface.


   -    Made a lot of miscellaneous editorial cleanups.






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


   [[ RFC-Editor note: remove the change history between the drafts




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   before publication. ]]


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


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


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


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


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


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


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


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


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


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


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


   -    Clarified that packets which fail the checks (such as verifying




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        the IPv4 source address, martian, and ingress filtering) on the
        decapsulator should be silently dropped.


   -    Clarified that while source link layer address options and
        target link layer address options are ignored in received ND
        packets, the ND packets themselves are processed as normal.






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




   -    Removed unidirectional tunnels; assume all the tunnels are
        bidirectional.


   -    Removed the definition of IPv4-compatible IPv6 addresses.


   -    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.




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   -    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.




9.3.  Changes from draft-ietf-v6ops-mech-v2-02




   -    Removed DNS record filtering, and made it a SHOULD NOT;
        summarized the DNS record ordering.


   -    Further clarified the usage of IPv6 RPF checks, as they
        typically require manual configuration.


   -    Add a note on link-local address formation strategy to point out
        that minimizing the probability of renumbering may be desirable.


   -    Clarify that with multiple IPv4 addresses, and the link-local
        address formed based on one of them, the selection can be done
        administratively or by the implementation.


   -    Refer to separate documents on generic IPv6 security
        considerations and IPsec set-up details.


   -    Clarify that the IPv4 end-point address can be predictable in
        some situations.


   -    Add a note that the destination IPv4 address could also be a
        multicast address.


   -    Make it RECOMMENDED to provide a toggle to perform strict
        ingress filtering on an interface.






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Acknowledgment


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