



Internet Engineering Task Force                               R. Despres
Internet-Draft                                                 RD-IPtech
Intended status: Experimental                              S. Jiang, Ed.
Expires: June 11, 2015                      Huawei Technologies Co., Ltd
                                                                R. Penno
                                                     Cisco Systems, Inc.
                                                                  Y. Lee
                                                                 Comcast
                                                                 G. Chen
                                                            China Mobile
                                                                 M. Chen
                                                        Freebit Co, Ltd.
                                                        December 8, 2014


     IPv4 Residual Deployment via IPv6 - a Stateless Solution (4rd)
                       draft-ietf-softwire-4rd-10

Abstract

   For service providers to progressively deploy IPv6-only network
   domains while still offering IPv4 service to customers, this document
   specifies a stateless solution.  Its distinctive property is that
   TCP/UDP IPv4 packets are valid TCP/UDP IPv6 packets during domain
   traversal, and that IPv4 fragmentation rules are fully preserved end-
   to-end.  Each customer can be assigned one public IPv4 address, or
   several, or a shared address with a restricted port set.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on June 11, 2015.







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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  The 4rd Model . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Protocol Specifications . . . . . . . . . . . . . . . . . . .   8
     4.1.  NAT44 on CE . . . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Mapping rules and other Domain parameters . . . . . . . .   8
     4.3.  Reversible Packet Translations at Domain entries and
           exits . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.4.  Address Mapping from CE IPv6 Prefixes to 4rd IPv4
           prefixes  . . . . . . . . . . . . . . . . . . . . . . . .  14
     4.5.  Address Mapping from 4rd IPv4 addresses to 4rd IPv6
           Addresses . . . . . . . . . . . . . . . . . . . . . . . .  16
     4.6.  Fragmentation Processing  . . . . . . . . . . . . . . . .  21
       4.6.1.  Fragmentation at Domain Entry . . . . . . . . . . . .  21
       4.6.2.  Ports of Fragments addressed to Shared-Address CEs  .  21
       4.6.3.  Packet Identifications from Shared-Address CEs  . . .  22
     4.7.  TOS and Traffic-Class Processing  . . . . . . . . . . . .  23
     4.8.  Tunnel-Generated ICMPv6 Error Messages  . . . . . . . . .  23
     4.9.  Provisioning 4rd Parameters to CEs  . . . . . . . . . . .  24
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  27
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
   7.  Relationship with Previous Works  . . . . . . . . . . . . . .  28
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  29
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  30
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  31
   Appendix A.  Textual representation of Mapping rules  . . . . . .  32
   Appendix B.  Configuring multiple Mapping Rules . . . . . . . . .  33
   Appendix C.  ADDING SHARED IPv4 ADDRESSES TO AN IPv6 NETWORK  . .  35
     C.1.  With CEs within CPEs  . . . . . . . . . . . . . . . . . .  35
     C.2.  With some CEs behind Third-party Router CPEs  . . . . . .  36



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   Appendix D.  REPLACING DUAL-STACK ROUTING BY IPv6-ONLY ROUTING  .  37
   Appendix E.  ADDING IPv6 AND 4rd SERVICE TO A NET-10 NETWORK  . .  38
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  39

1.  Introduction

   For service providers to progressively deploy IPv6-only network
   domains while still offering IPv4 service to customers, the need for
   a stateless solution, i.e. one where no per-customer state is needed
   in IPv4-IPv6 gateway nodes of the provider, is expressed in
   [I-D.ietf-softwire-stateless-4v6-motivation].  This document
   specifies such a solution, named "4rd" for IPv4 Residual Deployment.

   Using the solution, IPv4 packets are transparently tunneled across
   IPv6 networks (reverse of 6rd [RFC5969] in which IPv6 packets are
   statelessly tunneled across IPv4 networks).

   While IPv6 headers are too long to be mapped into IPv4 headers (why
   6rd requires encapsulation of full IPv6 packets in IPv4 packets),
   IPv4 headers can be reversibly translated into IPv6 headers in such a
   way that, during IPv6 domain traversal, UDP packets having checksums
   and TCP packets are valid IPv6 packets.  IPv6-only middle boxes that
   perform deep-packet- inspection can operate on them, in particular
   for port inspection and web caches.

   In order to deal with the IPv4-address shortage, customers can be
   assigned shared public IPv4 addresses, with statically assigned
   restricted port sets.  As such, it is a particular application of the
   A+P approach of [RFC6346].

   Deploying 4rd in the networks that have enough public IPv4 address,
   customer sites can also be assigned full public IPv4 addresses. 4rd
   also supports the scenarios that a set of public IPv4 addresses are
   assigned to customer sites.

   The design of 4rd builds on a number of previous proposals made for
   IPv4-via-IPv6 transition technologies listed in Section 8.

   In some use cases, IPv4-only applications of 4rd-capable customer
   nodes can also work with stateful NAT64s of [RFC6146], provided these
   are upgraded to support 4rd tunnels in addition their IP/ICMP
   translation of [RFC6145].  The advantage is then a more complete IPv4
   transparency than with double translation.

   How the 4rd model fits in the Internet architecture is summarized in
   Section 3.  The protocol specification is detailed in Section 4.
   Section 5 and Section 6 respectively deal with security and IANA
   considerations.  Previous proposals that influenced this



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   specification are listed in Section 8.  A few typical 4rd use cases
   are presented in Appendices.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   ISP: Internet-Service Provider.  In this document, the service it
        offers can be DSL, fiber-optics, cable, or mobile.  The ISP can
        also be a private-network operator.

   4rd (IPv4 Residual Deployment):  An extension of the IPv4 service
        where public IPv4 addresses can be statically shared among
        several customer sites, each one being assigned an exclusive
        port set.  This service is supported across IPv6-routing
        domains.

   4rd domain (or Domain):  An ISP-operated IPv6 network across which
        4rd is supported according to the present specification.

   Tunnel packet:  An IPv6 packet that transparently conveys an IPv4
        packet across a 4rd domain.  Its header has enough information
        to reconstitute the IPv4 header at Domain exit.  Its payload is
        the original IPv4 payload.

   CE (Customer Edge):  A customer-side tunnel endpoint.  It can be in a
        node that is a host, a router, or both.

   BR (Border Relay):  An ISP-side tunnel-endpoint.  Because its
        operation is stateless (neither per CE nor per session state) it
        can be replicated in as many nodes as needed for scalability.

   4rd IPv6 address:  IPv6 address used as destination of a Tunnel
        packet sent to a CE or a BR.

   NAT64+:  An ISP NAT64 of [RFC6146] that is upgraded to support 4rd
        tunneling when IPv6 addresses it deals with are 4rd IPv6
        addresses.

   4rd IPv4 address:  A public IPv4 address or, in case of a shared
        public IPv4 address, a public transport address (public IPv4
        address plus port number).

   PSID (Port-Set Identifier):  A flexible-length field that
        algorithmically identifies a port set.




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   4rd IPv4 prefix:  A flexible-length prefix that may be a a public
        IPv4 prefix, a public IPv4 address, or a public IPv4 address
        followed by a PSID.

   Mapping rule:  A set of parameters that are used by BRs and CEs to
        derive 4rd IPv6 addresses from 4rd IPv4 addresses.  Mapping
        rules are also used by each CE to derive a 4rd IPv4 prefix from
        an IPv6 prefix that it has been delegated.

   EA bits (Embedded Address bits):  Bits that are the same in a 4rd
        IPv4 address and in the 4rd IPv6 address derived from it.

   BR mapping rule:  The mapping rule applicable to off-domain IPv4
        addresses (addresses reachable via BRs).  It can also apply to
        some or all of CE-assigned IPv4 addresses.

   CE mapping rule:  A mapping rule that is applicable only to CE-
        assigned IPv4 addresses (shared or not).

   NAT64+ mapping rule:  Mapping rule applicable to IPv4 addresses
        reachable via a NAT64+.

   CNP (Checksum Neutrality preserver):  A field of 4rd IPv6 addresses
        that ensures that TCP-like checksums do not change when IPv4
        addresses are replaced by 4rd IPv6 addresses.

   4rd Tag:  A 16-bit tag whose value permits, in 4rd CEs, BRs, and
        NAT64+s, to distinguish 4rd IPv6 addresses from other IPv6
        addresses.

3.  The 4rd Model




















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                                    4rd DOMAIN
                       +-----------------------------+
                       |        IPv6 routing         |
                       |  Enforced ingress filtering | +----------
                  ...  |                             | |
                       |                          +------+
        Customer site  |                          |BR(s) |  IPv4
        +------------+ |      BR IPv6 prefix  --> |and/or| Internet
        | dual-stack | |                          |N4T64+|
        |         +--+ |                          +------+
        |         |CE+-+ <-- a CE IPv6 prefix        | |
        |         +--+ |                             | +----------
        |            | |                             |
        +------------+ |     <--IPv4 tunnels-->      +------------
          => Derived   |  (Mesh or hub-and-spoke     |
        4rd IPv4 prefix|         topologies)         |    IPv6
                       |                             |  Internet
                  ...  |                             |
                       |                             +------------
                       +-----------------------------+
                      <== one or several Mapping rules
                  (e.g. announced to CEs in stateless DHCPv6 )

                                 Figure 1

   How the 4rd model fits in the Internet architecture is represented in
   Figure 1.

   A 4rd domain is an IPv6 network that includes one or several 4rd BRs
   or NAT64+s at its border with the public IPv4 Internet, and can
   advertise its IPv4-IPv6 Mapping rule(s) to CEs according to
   Section 4.9.

   BRs of a 4rd Domain are all identical as far as 4rd is concerned.  In
   a 4rd CE, the IPv4 packets will be transformed (detailed in
   Section 4.3) into IPv6 packets that have the same anycast IPv6
   prefix, which is the 80-bit BR prefix, in their destination
   addresses.  They are then routed to any of the BRs.  The 80-bit BR
   IPv6 prefix is an arbitrarily chosen /64 prefix from the IPv6 address
   space of the network operator and appended 0x0300 (16-bit 4rd Tag,
   see R-9 in Section 4.5).

   Using the Mapping rule that applies, each CE derives its 4rd IPv4
   prefix from its delegated IPv6 prefix, or one of them if it has
   several, details in Section 4.4.  If the obtained IPv4 prefix has
   more than 32 bits, the assigned IPv4 address is shared among several
   CEs.  Bits beyond the first 32 specify a set of ports whose use is
   reserved for the CE.



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   IPv4 traffic is automatically tunnelled across the Domain, either in
   mesh topology or in Hub&spoke topology [RFC4925].  By default, IPv4
   traffic between two CEs follows a direct IPv6 route between them
   (mesh topology).  If the ISP configures the Hub&spoke option, each
   IPv4 packet from a CE to another is routed via a BR.

   During Domain traversal, each tunnelled TCP/UDP IPv4 packet looks
   like a valid TCP/UDP IPv6 packet.  Thus, TCP/UDP access-control lists
   that apply to IPv6, and possibly some other functions using deep
   packet inspections, also apply to IPv4.

   For IPv4 anti-spoofing protection, as is in CEs and BRs, to remain
   effective when combined with 4rd tunneling, ingress filtering has to
   be effective in IPv6 (R-12 and Section 5).

   If an ISP wishes to support dynamic IPv4 address sharing, in addition
   or in place of 4rd stateless address sharing, it can do it by means
   of a stateful NAT64.  By upgrading this NAT to add 4rd-tunnels
   support, which makes it a NAT64+, CEs that are assigned no static
   IPv4 space can benefit from complete IPv4 transparency between CE and
   NAT64.  (Without this NAT64 upgrade, IPv4 traffic is translated to
   IPv6 and back to IPv4, which looses the DF=MF=1 combination of IPv4,
   that which is recommended for host fragmentation in Section 8 of
   [RFC4821].)

   IPv4 packets are kept unchanged by Domain traversal except that:

   o  The IPv4 Time to live (TTL), unless it is 1 or 255 at Domain
      entry, decreases during Domain traversal by the number of
      traversed routers.  This is acceptable because it is undetectable
      end to end, and because TTL values that can be used with some
      protocols to test adjacency of communicating routers are preserved
      ([RFC4271], [RFC5082] ).  Effect on the traceroute utility, which
      uses TTL expiry to discover routers of end-to-end paths, is noted
      in Section 4.3.

   o  IPv4 packets whose lengths are =< 68 octets always have their
      "Don't fragment flags" DF=1 at Domain exit even if they had DF=0
      at Domain entry.  This is acceptable because these packets are too
      short to be fragmented [RFC0791] so that their DF bits have no
      meaning.  Besides, both [RFC1191] and [RFC4821] recommend that
      sources always set DF=1.

   o  Unless the Tunnel-traffic-class option applies to a Domain
      (Section 4.2), IPv4 packets may have their TOS fields modified
      after Domain traversal (Section 4.7).





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4.  Protocol Specifications

   This section describes detailed 4rd protocol specifications.  They
   are mainly organized by functions.  As a brief summary, a 4rd CE MUST
   follow R-1, R-2, R-3, R-4, R-6, R-7, R-8, R-9, R-10, R-11, R-12,
   R-13, R-14, R-16, R-17, R-18, R-19, R-20, R-21, R-22, R-23, R-24,
   R-25, R-26 and R-27; while a 4rd BR MUST follow R-2, R-3, R-4, R-5,
   R-6, R-9, R-12, R-13, R-14, R-15, R-19, R-20, R-21, R-22 and R-24.

4.1.  NAT44 on CE

   R-1:  A CE node that is assigned a shared public IPv4 address MUST
         include a NAT44 [RFC3022].  This NAT44 MUST only use external
         ports that are in the CE assigned port set.

   NOTE: This specification only concerns IPv4 communication between
   IPv4-capable endpoints.  For communication between IPv4-only
   endpoints and IPv6 only remote endpoints, the BIH specification of
   [RFC6535] can be used.  It can coexist in a node with the CE
   function, including if the IPv4-only function is a NAT44 [RFC3022].

4.2.  Mapping rules and other Domain parameters

   R-2:  CEs and BRs MUST be configured with the following Domain
         parameters:

         A.  One or several Mapping rules, each one comprising:

             1.  Rule IPv4 prefix

             2.  EA-bits length

             3.  Rule IPv6 prefix

             4.  WKPs authorized (OPTIONAL)

         B.  Domain PMTU

         C.  Hub&spoke topology (Yes or No)

         D.  Tunnel traffic class (OPTIONAL)


   "Rule IPv4 prefix" is used to find, by a longest match, which Mapping
   rule applies to a 4rd IPv4 address (Section 4.5).  A Mapping rule
   whose Rule IPv4 prefix is longer than /0 is a CE mapping rule.  BR
   and NAT64+ mapping rules, which must apply to all off-domain IPv4
   addresses, have /0 as their Rule IPv4 prefixes.



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   "EA-bits length" is the number of bits that are common to 4rd IPv4
   addresses and 4rd IPv6 addresses derived from them.  In a CE mapping
   rule, it is also the number of bits that are common to a CE delegated
   IPv6 prefix and the 4rd IPv4 prefix derived from it.  BR and NAT64+
   mapping rules have EA-bits lengths equal to 32.

   "Rule IPv6 prefix" is the prefix that is substituted to the Rule IPv4
   prefix when a 4rd IPv6 address is derived from a 4rd IPv4 address
   (Section 4.5).  In a BR mapping rule or a NAT64+ mapping rule, it
   MUST be a /80 prefix whose 64~79 bits are the 4rd Tag.

   "WKPs authorized" may be set for mapping rules that assign shared
   IPv4 addresses to CEs.  (These rules are those whose length of the
   Rule IPv4 prefix plus the EA-bits length exceeds 32.)  If set, well-
   known ports may be assigned to some CEs having particular IPv6
   prefixes.  If not set, fairness is privileged: all IPv6 prefixes
   concerned with the Mapping rule have ports sets having identical
   values (no port set includes any of the well known ports).

   "Domain PMTU" is the IPv6 path MTU that the ISP can guarantee for all
   its IPv6 paths between CEs and between BRs and CEs.  It MUST be at
   least 1280 [RFC2460].

   "Hub&spoke topology", if set to Yes, requires CEs to tunnel all IPv4
   packets via BRs.  If set to No, CE-to-CE packets take the same routes
   as native IPv6 packets between the same CEs (mesh topology).

   "Tunnel traffic class", if provided, is the IPv6 traffic class that
   BRs and CEs MUST set in Tunnel packets.  In this case, evolutions of
   the IPv6 traffic class that may occur during Domain traversal are not
   reflected in TOS fields of IPv4 packets at Domain exit (Section 4.7).

4.3.  Reversible Packet Translations at Domain entries and exits

   R-3:  Domain-entry nodes that receive IPv4 packets with IPv4 options
         MUST discard these packets, and return ICMPv4 error messages to
         signal IPv4-option incompatibility (Type = 12, Code = 0,
         Pointer = 20) [RFC0792].  This limitation is acceptable because
         there are a lot firewalls in current IPv4 Internet also filter
         IPv4 packets with IPv4 options.

   R-4:  Domain-entry nodes that receive IPv4 packets without IPv4
         options MUST convert them to Tunnel packets, with or without
         IPv6 fragment headers depending on what is needed to ensure
         IPv4 transparency (Figure 2).  Domain-exit nodes MUST convert
         them back to IPv4 packets.





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         An IPv6 fragmentation header MUST be included at tunnel entry
         (Figure 2) if, and only if, one or several of the following
         conditions hold:

         *  The Tunnel_traffic_class option applies to the Domain.

         *  TTL = 1 OR TTL = 255.

         *  The IPv4 packet is already fragmented, or may be fragmented
            later on, i.e. if MF=1 OR Offset>0 OR (Total length > 68 AND
            DF=0).
         In order to optimize cases where fragmentation headers are
         unnecessary, the NAT44 of a CE that has one SHOULD send packets
         with TTL = 254.

   R-5:  In Domains whose chosen topology is Hub&spoke, BRs that receive
         4rd IPv6 packets whose embedded destination IPv4 addresses
         match a CE mapping rule MUST do the equivalent of reversibly
         translating their headers to IPv4 and then reversibly translate
         them back to IPv6 as though packets would be entering the
         Domain.

                     (A) Without IPv6 fragment header
            IPv4 packet                          Tunnel packet
       +--------------------+ :            : +--------------------+
     20|     IPv4 Header    | :    <==>    : |     IPv6 Header    | 40
       +--------------------+ :            : +--------------------+
       |     IP Payload     |      <==>      |     IP Payload     |
       |                    |     layer 4    |                    |
       +--------------------+    unchanged   +--------------------+

                     (B) With IPv6 fragment header
                                                 Tunnel packet
                                           : +--------------------+
            IPv4 packet                    : |     IPv6 Header    | 40
       +--------------------+ :            : +--------------------+
     20|     IPv4 Header    | :    <==>    : |IPv6 Fragment Header|  8
       +--------------------+ :            : +--------------------+
       |     IP Payload     |      <==>      |     IP Payload     |
       |                    |     layer 4    |                    |
       +--------------------+    unchanged   +--------------------+

                       Reversible Packet Translation

                                 Figure 2

   R-6:  Values to be set in IPv6-header fields at Domain entry are
         detailed in Table 1 (no-fragment-header case) and Table 2



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         (fragment-header case).  Those to be set in IPv4 header fields
         at Domain exit are detailed in Table 3 (no-fragment-header
         case) and Table 4 (fragment-header case).

         To convey IPv4-header informations that have no equivalent in
         IPv6, some ad-hoc fields are placed in IPv6 flow labels and in
         Identification fields of IPv6 fragment headers, as detailed in
         Figure 3.

                    |0      |4                            19|
                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    |   0   |         Addr_Prot_Cksm        |
                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                               IPv6 FLOW LABEL

       0 1 2          |8              |16                           31|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |.|.|.|    0    |    IPv4_TOS   |             IPv4_ID           |
      /-+-\-\-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /     \ TTL_255         IPv6 IDENTIFICATION FIELD
   IPv4_DF  TTL_1            (in Fragment header if needed)

            4rd Identification fields of IPv6 Fragment headers

                                 Figure 3

     +---------------------+----------------------------------------+
     | IPv6 FIELD          | VALUE (fields from IPv4 header)        |
     +---------------------+----------------------------------------+
     | Version             | 6                                      |
     | Traffic class       | TOS                                    |
     | Addr_Prot_Cksm      | Sum of Addresses and Protocol (Note 1) |
     | Payload length      | Total length - 20                      |
     | Next header         | Protocol                               |
     | Hop limit           | Time to live                           |
     | Source address      | See Section 4.5                        |
     | Destination address | See Section 4.5                        |
     +---------------------+----------------------------------------+

   IPv4-to-IPv6 Reversible Header Translation (without Fragment header)

                                  Table 1









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    +-----------------+----------------------------------------------+
    | IPv6 FIELD      | VALUE (fields from IPv4 header)              |
    +-----------------+----------------------------------------------+
    | Version         | 6                                            |
    | Traffic class   | TOS OR Tunnel_traffic_class (Section 4.7)    |
    | Addr_Prot_Cksm  | Sum of Addresses and Protocol (Note 1)       |
    | Payload length  | Total length - 12                            |
    | Next header     | 44 (Fragment header)                         |
    | Hop limit       | IF Time to live = 1 or 255 THEN 254          |
    |                 | ELSE Time to live (Note 2)                   |
    | Source address  | See Section 4.5                              |
    | Dest. address   | See Section 4.5                              |
    | 2nd next header | Protocol                                     |
    | Fragment offset | IPv4 Fragment offset                         |
    | M               | More-fragments flag (MF)                     |
    | IPv4_DF         | Don't-fragment flag (DF)                     |
    | TTL_1           | IF Time to live = 1 THEN 1 ELSE 0 (Note 2)   |
    | TTL_255         | IF Time to live = 255 THEN 1 ELSE 0 (Note 2) |
    | IPv4_TOS        | Type of service (TOS)                        |
    | IPv4_ID         | Identification                               |
    +-----------------+----------------------------------------------+

     IPv4-to-IPv6 Reversible Header Translation (with Fragment header)

                                  Table 2

         +-----------------+------------------------------------+
         | IPv4 FIELD      | VALUE (fields from IPv6 header)    |
         +-----------------+------------------------------------+
         | Version         | 4                                  |
         | Header length   | 5                                  |
         | TOS             | Traffic class                      |
         | Total Length    | Payload length + 20                |
         | Identification  | 0                                  |
         | DF              | 1                                  |
         | MF              | 0                                  |
         | Fragment offset | 0                                  |
         | Time to live    | Hop count                          |
         | Protocol        | Next header                        |
         | Header checksum | Computed as per [RFC0791] (Note 3) |
         | Source address  | Bits 80-111 of source address      |
         | Dest. address   | Bits 80-111 of source address      |
         +-----------------+------------------------------------+

   IPv6-to-IPv4 Reversible Header Translation (without Fragment header)

                                  Table 3




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    +-----------------------+-----------------------------------------+
    | IPv4 FIELD            | VALUE (fields from IPv6 headers)        |
    +-----------------------+-----------------------------------------+
    | Version               | 4                                       |
    | Header length         | 5                                       |
    | TOS                   | Traffic class OR IPv4_TOS (Section 4.7) |
    | Total Length          | Payload length + 12                     |
    | Identification        | IPv4_ID                                 |
    | DF                    | IPv4_DF                                 |
    | MF                    | M                                       |
    | Fragment offset       | Fragment offset                         |
    | Time to live (Note 2) | IF TTL_255 = 1 THEN 255TTL_1 = 1 THEN 1 |
    |                       | ELSEIF TTL_1 = 1 THEN 1 ELSE Hop count  |
    | Protocol              | 2nd Next header                         |
    | Header checksum       | Computed as per [RFC0791] (Note 3)      |
    | Source address        | Bits 80-111 of source address           |
    | Destination address   | Bits 80-111 of destination address      |
    +-----------------------+-----------------------------------------+

     IPv6 to IPv4 Reversible Header Translation (with Fragment header)

                                  Table 4

   NOTE 1: The need to save in the IPv6 header a checksum of both IPv4
   addresses and the IPv4 protocol field results from the following
   facts: (1) Header checksums, present in IPv4 but not in IPv6, protect
   addresses or protocol integrity; (2) In IPv4, ICMP messages and null-
   checksum UDP datagram depend on this protection because, unlike other
   datagrams, they have no other address-and-protocol integrity
   protection.  The sum MUST be performed in ordinary 2's complement
   arithmetic.

   IP-layer Packet length is another field covered by the IPv4 IP-header
   checksum.  It is not included in the saved checksum because: (1)
   doing so would have conflicted with [RFC6437] (flow labels must be
   the same in all packets of each flow); (2) ICMPv4 messages have good
   enough protection with their own checksums; (3) the UDP length field
   provides to null-checksum UDP datagrams the same level of protection
   after Domain traversal as without Domain traversal (consistency
   between IP-layer and UDP-layer lengths can be checked).


   NOTE 2: TTL treatment has been chosen to permit adjacency tests
   between two IPv4 nodes situated at both ends of a 4rd tunnel.  TTL
   values to be preserved for this are TTL=255 and TTL=1.  For other
   values, TTL decrease between to IPv4 nodes is the same as though
   traversed IPv6 routers would be IPv4 routers.




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   Effect of this TTL treatment on IPv4 traceroute is specific: (1) the
   number of routers of the end-to-end path includes traversed IPv6
   routers; (2) IPv6 routers of a Domain are listed after IPv4 routers
   of Domain entry and exit; (3) the IPv4 address shown for an IPv6
   router is the IPv6-only dummy IPv4 address of Section 4.8; (4) the
   response time indicated for an IPv6 router is that of the next
   router.

   NOTE 3: Provided the sum of obtained IPv4 addresses and protocol
   matches Addr_Prot_Cksm.  If not, the packet MUST be silently
   discarded.

4.4.  Address Mapping from CE IPv6 Prefixes to 4rd IPv4 prefixes






































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     +--------------------------------------+
     |             CE IPv6 prefix           |
     +--------------------------+-----------+
     :     Longest match        :           :
     :  with a Rule IPv6 prefix :           :
     :           ||             :  EA-bits  :
     :           \/             :   length  :
     +--------------------------+     |     :
     |    Rule IPv6 prefix      |<----'---->:
     +--------------------------+           :
                   ||           :           :
                   \/           :           :
              +-----------------+-----------+
              |Rule IPv4 prefix |  EA bits  |
              +-----------------+-----------+
              :                             :
              +-----------------------------+
              |     CE 4rd IPv4 prefix      |
              +-----------------------------+
     ________/ \_________                   :
    /                    \                  :
   :                  ____:________________/ \__
   :                 /    :                     \
   :    =< 32       :     :          > 32        :
   +----------------+     +-----------------+----+
   |IPv4 prfx or add|  OR |   IPv4 address  |PSID|
   +----------------+     +-----------------+----+
                          :       32        : || :
                                              \/
                    (by default)          (If WKPs authorized)
                        :    :                     :    :
                    +---+----+---------+           +----+-------------+
      Ports in      |> 0|PSID|any value|    OR     |PSID|  any value  |
   the CE port set  +---+----+---------+           +----+-------------+
                    : 4 :     12       :           :        16        :

           From CE IPv6 prefix to 4rd IPv4 address and Port set

                                 Figure 4

   R-7:  A CE whose delegated IPv6 prefix matches the Rule IPv6 prefix
         of one or several Mapping rules MUST select the CE mapping rule
         for which the match is the longest.  It then derives its 4rd
         IPv4 prefix as shown in Figure 4: (1) the CE replaces the Rule
         IPv6 prefix by the Rule IPv4 prefix.  The result is the CE 4rd
         IPv4 prefix. (2) If this CE 4rd IPv4 prefix has less than 32
         bits, the CE takes it as its assigned IPv4 prefix.  If it has
         exactly 32 bits, the CE takes it as its IPv4 address.  If it



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         has more than 32 bits, the CE MUST takes the first 32 bits as
         its shared public IPv4 address, and bits beyond the first 32 as
         its Port-set identifier (PSID).  Ports of its restricted port
         set are by default those that have any non-zero value in their
         first 4 bits (the PSID offset), followed by the PSID, and
         followed by any values in remaining bits.  If the WKP
         authorized option applies to the Mapping rule, there is no
         4-bit offset before the PSID so that all ports can be assigned.

         NOTE: The choice of the default PSID position in Port fields
         has been guided by the following objectives: (1) for fairness,
         avoid having any of the well-known ports 0-1023 in the port set
         specified by any PSID value; (2) for compatibility RTP/RTCP
         [RFC4961], include in each port set pairs of consecutive ports;
         (3) in order to facilitate operation and training, have the
         PSID at a fixed position in port fields; (4) in order to
         facilitate documentation in hexadecimal notation, and to
         facilitate maintenance, have this position nibble aligned.
         Ports that are excluded from assignment to CEs are 0-4095
         instead of just 0-1023 in a trade-off to favor nibble alignment
         of PSIDs and overall simplicity.

   R-8:  A CE whose delegated IPv6 prefix has its longest match with the
         Rule IPv6 prefix of the BR mapping rule MUST take as IPv4
         address the 32 bit that, in the delegated IPv6 prefix, follow
         this Rule IPv6 prefix.  If this is the case while the Hub&spoke
         option applies to the Domain, or if the Rule IPv6 prefix is not
         a /80, there is a configuration error in the Domain.  An
         implementation-dependent administrative action MAY be taken.

         A CE whose delegated IPv6 prefix matches the Rule IPv6 prefix
         of neither any CE Mapping rule nor the BR mapping rule, and is
         in a Domain that has a NAT64+ mapping rule, MUST be noted as
         having the unspecified IPv4 address.


4.5.  Address Mapping from 4rd IPv4 addresses to 4rd IPv6 Addresses














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   :            32              :  :       16      : \
   +----------------------------+  +---------------+ |
   |         IPv4 address       |  |Port_or_ICMP_ID| |  Shared-address
   +----------------------------+  +---+------+----+ |       case
   :      Longest match         :  : 4 : PSID :      |   (PSID length
   :  with a Rule IPv4 prefix   :      :length:      |  of the rule > 0)
   :       ||                   :      :      :      |    with WKPs
   :       \/                   :      :      :      |  not authorized
   +----------------+-----------+      +------+      | (PSID offset = 4)
   |Rule IPv4 prefix|IPv4 suffix|      | PSID |      |
   +----------------+-----------+      +------+      |
   :       ||        \_______    \____ |      |      |
   :       \/                \        \|      /      |
   +--------------------------+--------+-----+      /
   |    Rule IPv6 prefix      |    EA bits   |
   +--------------------------+--------------+
   :                                         :
   +-----------------------------------------+
   |                 IPv6 prefix             |
   +-----------------------------------------+
   :\_______________________________        / \
   :             ___________________\______/   \_______________
   :            /                    \                         \
   :           / (CE mapping rule)    \   (BR mapping rule)     \
   :   =<64   :                        :          112            :
   +----------+---+---+------+---+     +--------------+---+------+---+
   |CE v6 prfx| 0 |tag|v4 add|CNP|     |BR IPv6 prefix|tag|v4 add|CNP|
   +----------+-|-+---+------+---+     +--------------+---+------+---+
   :   =<64   : | :16 :  32  :16 :     :      64      :16 :  32  :16 :
                |
          Padding to /64

                 From 4rd IPv4 address to 4rd IPv6 address

                                 Figure 5

   R-9:  BRs, and CEs that are assigned public IPv4 addresses, shared or
         not, MUST derive 4rd IPv6 addresses from 4rd IPv4 addresses by
         the steps below or their functional equivalent (Figure 5
         details the shared public IPv4 address case):

         Note: the rules for forming 4rd specific Interface Identifiers
         is obey the latest specification of [RFC7136].  "Specifications
         of forms of 64-bit IID MUST specify how all 64 bits are set".
         And "the whole IID value MUST be viewed as an opaque bit string
         by third parties, except possibly in the local context."





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         (1)  If Hub&spoke topology does not apply to the Domain, or if
              it applies but the IPv6 address to be derived is a source
              address from a CE or a destination address from a BR, find
              the CE mapping rule whose Rule IPv4 prefix has the longest
              match with the IPv4 address.

              If no Mapping rule is thus obtained, take the BR mapping
              rule.

              If the obtained Mapping rule assigns IPv4 prefixes to CEs,
              i.e. if length of the Rule IPv4 prefix plus EA-bits length
              is 32 - k, with k >= 0, delete the last k bits of the IPv4
              address.

              Otherwise, i.e. if length of the Rule IPv4 prefix plus EA-
              bits length is 32 + k, with k > 0, take k as PSID length,
              and append to the IPv4 address the PSID copied from bits p
              to p+3 of the Port_or_ICMP_ID field where: (1) p, the PSID
              offset, is 4 by default, and 0 if the WKPs authorized
              option applies to the rule; (2) The Port_or_ICMP_ID field
              is in bits of the IP payload that depend on whether the
              address is source or destination, on whether the packet is
              ICMP or not, and, if it is ICMP, whether it is an error
              message or an echo message.  This field is:

              a.  If the packet Protocol is not ICMP, the port field
                  associated with the address (bits 0-15 for a source
                  address, and bits 16-31 for a destination address).

              b.  If the packet is an ICMPv4 echo or echo-reply message,
                  the ICMPv4 Identification field (bits 32-47 ).

              c.  If the packet is an ICMPv4 error message, the port
                  field associated with the address in the returned
                  packet header (bits 240-255 for a source address, bits
                  224-239 for a destination address).

              NOTE 1: Using Identification fields of ICMP messages as
              port fields permits to exchange Echo requests and Echo
              replies between shared-address CEs and IPv4 hosts having
              exclusive IPv4 addresses.  Echo exchanges between two
              shared-address CEs remain impossible, but this is a
              limitation inherent to address sharing (one reason among
              many to use IPv6).

              NOTE 2: When the PSID is taken in the port field of the
              IPv4 payload, it is, to avoid dependency on any particular
              layer-4 protocol having port fields, without checking that



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              the protocol is indeed one that has a port field . A
              packet may consequently go, in case of source mistake,
              from a BR to a shared-address CE with a protocol that is
              not supported by this CE.  In this case, the CE NAT44
              returns an ICMPv4 "protocol unreachable" error message.
              The IPv4 source is thus appropriately informed of its
              mistake.

         (2)  Replace in the result the Rule IPv4 prefix by the Rule
              IPv6 prefix.

         (3)  If the result is shorter than a /64, append to the result
              a null padding up to 64 bits, followed by the 4rd tag
              (0x0300), and followed by the IPv4 address.

              NOTE: The 4rd tag is a 4rd-specific mark.  Its function is
              to ensure that 4rd IPv6 addresses are recognizable by CEs
              without any interference with the choice of subnet
              prefixes in CE sites.  (These choices may have been done
              before 4rd is enabled.)

              For this, the 4rd tag has its "u" and "g" bits of
              [RFC4291] both set to 1, so that they maximumly differ
              from these existing IPv6 address schemas.  So far, u=g=1
              has not been used in any IPv6 addressing architecture.

              With the 4rd tage, IPv6 packets can be routed to the 4rd
              function within a CE node based on a /80 prefix that no
              native-IPv6 address can contain.

         (4)  Add to the result a Checksum-neutrality preserver (CNP).
              Its value, in one's complement arithmetic, is the opposite
              of the sum of 16-bit fields of the IPv6 address other than
              the IPv4 address and the CNP themselves (i.e. 5
              consecutive fields in address-bits 0-79).

              NOTE: CNP guarantees that Tunnel packets are valid IPv6
              packets for all layer-4 protocols that use the same
              checksum algorithm as TCP.  This guarantee does not depend
              on where checksum fields of these protocols are placed in
              IP payloads.  (Today, such protocols are UDP [RFC0768],
              TCP [RFC0793], UDP-Lite [RFC3828], and DCCP [RFC5595].
              Should new ones be specified, BRs will support them
              without needing an update.)

   R-10: 4rd-capable CE SHOULD, and 4rd-enbaled CE MUST always prohibit
         all addresses that use its advertised prefix and have IID




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         starting with 0x0300 (4rd Tag), by using Duplicate Address
         Detection [RFC4862].

   R-11: A CE that is assigned the unspecified IPv4 address (see
         Section 4.4) MUST use, for packets tunneled between itself and
         the Domain NAT64+, addresses as detailed in Figure 6: (a) for
         its IPv6 source, (b) as IPv6 destinations that depend on IPv4
         destinations.  A NAT64+, being NAT64 conforming [RFC6146], MUST
         accept IPv6 packets whose destination conforms to Figure 6 (b)
         (4rd tag instead of "u" and 0x00 octets).  In its Binding
         Information Base, it MUST remember whether a mapping was
         created with a "u" or 4rd-tag destination.  In the IPv4 to IPv6
         direction, it MUST use 4rd tunneling, with source address
         conforming to Figure 6 (b), when using a mapping that was
         created with a 4rd-tag destination.

        +---------------------+---------+-------+-------------+------+
    (a) |   CE IPv6 prefix    |    0    |4rd tag|      0      |  CNP |
        +---------------------+---------+-------+-------------+------+
        :      =< 64          :  >= 0   :    16 :     32      :  16  :
            4rd IPv6 address of a CE having no public IPv4 address

        <----------- Rule IPv6 prefix --------->:
        +-------------------------------+-------+-------------+------+
    (b) |      NAT64+ IPv6 prefix       |4rd tag|IPv4 address |  CNP |
        +-------------------------------+-------+-------------+------+
        :               64              :   16  :      32     :  16  :
               4rd IPv6 address of a host reachable via a NAT64+

                                 Figure 6

   R-12: For anti-spoofing protection, CEs and BRs MUST check that the
         source address of each received Tunnel packet is that which,
         according to Section 4.5, is derived from the source 4rd IPv4
         address.  For this, the IPv4 address used to obtain the source
         4rd IPv4 address is that embedded in the IPv6 source address
         (in its bits 80-111).  (This verification is needed because
         IPv6 ingress filtering [RFC3704] applies only to IPv6 prefixes,
         without guarantee that Tunnel packets are built as specified in
         Section 4.5.)

   R-13: For additional protection against packet corruption at a link
         layer that might be undetected at this layer during Domain
         traversal, CEs and BRs SHOULD verify that source and
         destination IPv6 addresses have not been modified.  This can be
         done by checking that they remain checksum neutral (see the
         Note on CNP above).




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4.6.  Fragmentation Processing

4.6.1.  Fragmentation at Domain Entry

   R-14: If an IPv4 packet enters a CE or BR with a size such that the
         derived Tunnel packet would be longer than the Domain PMTU, the
         packet has to be either discarded or fragmented.  The Domain-
         entry node MUST discard it if the packet has DF=1, with an ICMP
         error message returned to the source.  It MUST fragment it
         otherwise, with the payload of each fragment not exceeding PMTU
         - 48.  The first fragment has its offset equal to the received
         offset.  Following fragments have offsets increased by lengths
         of previous-fragments payloads.  Functionally, fragmentation is
         supposed to be done in IPv4 before applying to each fragment
         the reversible header translation of Section 4.3.

4.6.2.  Ports of Fragments addressed to Shared-Address CEs

   Because ports are available only in first fragments of IPv4
   fragmented packets, a BR needs a mechanism to send to the right
   shared-address CEs all fragments of fragmented packets.

   For this, a BR MAY systematically reassemble fragmented IPv4 packets
   before tunneling them.  But this consumes large memory space, opens
   denial-of-service-attack opportunities, and can significantly
   increase forwarding delays.  This is the reason for the following
   requirement:

   R-15: BRs SHOULD support an algorithm whereby received IPv4 packets
         can be forwarded on the fly.  The following is an example of
         such algorithm:


         (1)  At BR initialization, if at least one CE mapping rule
              concerns shared public IPv4 addresses (length of Rule IPv4
              prefix + EA-bits length > 32), the BR initializes an empty
              "IPv4-packet table" whose entries have the following
              items:

                 - IPv4 source

                 - IPv4 destination

                 - IPv4 identification

                 - Destination port





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         (2)  When the BR receives an IPv4 packet whose matching Mapping
              rule is one of shared public IPv4 addresses (length of
              Rule IPv4 prefix + EA-bits length > 32), the BR searches
              the table for an entry whose IPv4 source, IPv4
              destination, and IPv4 Identification, are those of the
              received packet.  The BR then performs actions detailed in
              Table 5 depending on which conditions hold.

       +---------------------------+---+---+---+---+---+---+---+---+
       | - CONDITIONS -            |   |   |   |   |   |   |   |   |
       | First Fragment (offset=0) | Y | Y | Y | Y | N | N | N | N |
       | Last fragment (MF=0)      | Y | Y | N | N | Y | Y | N | N |
       | An entry has been found   | Y | N | Y | N | Y | N | Y | N |
       | ------------------------- |   |   |   |   |   |   |   |   |
       | - RESULTING ACTIONS -     |   |   |   |   |   |   |   |   |
       | Create a new entry        | - | - | - | X | - | - | - | - |
       | Use port of the entry     | - | - | - | - | X | - | X | - |
       | Update port of the entry  | - | - | X | - | - | - | - | - |
       | Delete the entry          | X | - | - | - | X | - | - | - |
       | Forward the packet        | X | X | X | X | X | - | X | - |
       +---------------------------+---+---+---+---+---+---+---+---+

                                  Table 5



      (3)  The BR performs garbage collection for table entries that
           remain unchanged for longer than some limit.  This limit,
           normally longer that the maximum time normally needed to
           reassemble a packet is not critical.  It should however not
           be longer than 15 seconds [RFC0791].

   R-16: For the above algorithm to be effective, CEs that are assigned
         shared public IPv4 addresses MUST NOT interleave fragments of
         several fragmented packets.

   R-17: CEs that are assigned IPv4 prefixes, and are in nodes that
         route public IPv4 addresses rather than only using NAT44s, MUST
         have the same behavior as described just above for BRs.

4.6.3.  Packet Identifications from Shared-Address CEs

   When packets go from CEs that share the same IPv4 address to a common
   destination, a precaution is needed to guarantee that packet
   Identifications set by sources are different.  Otherwise, packet
   reassembly at destination could otherwise be confused because it is
   based only on source IPv4 address and Identification.  Probability of




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   such confusions may in theory be very low but, in order to avoid
   creating new attack opportunities, a safe solution is needed.

   R-18: A CE that is assigned a shared public IPv4 address MUST only
         use packet Identifications that have the CE PSID in their bits
         0 to PSID length - 1.

   R-19: A BR or a CE that receives a packet from a shared-address CE
         MUST check that bits 0 to PSID length - 1 of their packet
         Identifications are equal to the PSID found in source 4rd IPv4
         address.

4.7.  TOS and Traffic-Class Processing

   IPv4 TOS and IPv6 Traffic class have the same semantic, that of the
   differentiated-services field, or DS field, specified in [RFC2474]
   and [RFC6040].  Their first 6 bits contain a differentiated services
   codepoint (DSCP), and their two last bits can convey explicit
   congestion notifications (ECNs), which both may evolve during Domain
   traversal.  [RFC2983] discusses how the DSCP can be handled by tunnel
   end points.  The Tunnel traffic class option permits to ignore DS-
   field evolutions occurring during Domain traversal, if the desired
   behavior is that of generic tunnels conforming to [RFC2473].

   R-20: Unless the Tunnel traffic class option is configured for the
         Domain, BRs and CEs MUST copy the IPv4 TOS into the IPv6
         Traffic class at Domain entry, and copy back the IPv6 Traffic
         class into the IPv4 TOS at Domain exit.

   R-21: If the Tunnel traffic class option is configured for a Domain,
         BRs and CEs MUST at Domain entry take the configured Tunnel
         traffic class as IPv6 Traffic class, and copy the received IPv4
         TOS into the IPv4_TOS of the fragment header (Figure 3).  At
         Domain exit, they MUST copy back the IPv4_TOS of the fragment
         header into the IPv4 TOS.

4.8.  Tunnel-Generated ICMPv6 Error Messages

   If a Tunnel packet is discarded on its way across a 4rd domain
   because of an unreachable destination, an ICMPv6 error message is
   returned to the IPv6 source.  For the IPv4 source of the discarded
   packet to be informed of packet loss, the ICMPv6 message has to be
   converted into an ICMPv4 message.

   R-22: If a CE or BR receives an ICMPv6 error message [RFC4443], it
         MUST synthesize an ICMPv4 error packet [RFC0792].  This packet
         MUST contain the first 8 octets of the discarded-packet IP




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         payload.  The reserved IPv4 dummy address (TBD, (see Section 6)
         MUST be used as its source address .

         Like in [RFC6145], ICMPv6 Type = 1 and Code = 0 (Destination
         unreachable, No route to destination") MUST be translated into
         ICMPv4 Type = 3 and Code = 0 (Destination unreachable, Net
         unreachable), and ICMPv6 Type = 3 and Code = 0 (Time exceeded,
         Hop limit exceeded in transit) MUST be translated into ICMPv4
         Type = 11 and Code = 0 (Destination unreachable, Net
         unreachable).

4.9.  Provisioning 4rd Parameters to CEs

   Domain parameters listed in Section 4.2 are subject to the following
   constraints:

   R-23: Each Domain MUST have a BR mapping rule and/or a NAT64+ mapping
         rule.  (The BR mapping rule is only used by CEs that are
         assigned public IPv4 addresses, shared or not.  The NAT64+
         mapping rule is only used by CEs that are assigned the
         unspecified IPv4 address (Section 4.4), and therefore need an
         ISP NAT64 to reach IPv4 destinations.


   R-24: Each CE and each BR MUST support up to 32 Mapping rules.

         This number of is to ensure that independently acquired CEs an
         BR nodes can always interwork.

         ISPs that need Mapping rules for more IPv4 prefixes than this
         number SHOULD split their networks into multiple Domains.
         Communication between these domains can be done in IPv4, or by
         some implementation-dependent but equivalent other means.

   R-25: For mesh topologies, where CE-CE paths don't go via BRs, all
         mapping rules of the Domain MUST be sent to all CEs.  For hub-
         and-spoke topologies, where all CE-CE paths go via BRs, each CE
         MAY be sent only the BR mapping rule of the Domain plus, if
         different, the CE mapping rule that applies to its CE IPv6
         prefix.

   R-26: In a Domain where the chosen topology is Hub&spoke, all CEs
         MUST have IPv6 prefixes that match a CE mapping rule.
         (Otherwise, packets sent to CEs whose IPv6 prefixes would match
         only the BR mapping rule would, with longest-match selected
         routes, be routed directly to these CEs.  This would be
         contrary to the Hub&spoke requirement).




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   R-27: CEs MUST be able to acquire parameters of 4rd domains
         (Section 4.2) in DHCPv6 (ref.  [RFC2131]).  Formats of DHCPv6
         options to be used are detailed in Figure 7, Figure 8, and
         Figure 9 with field values specified after each Figure.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | option-code = OPTION_4RD      |         option-length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 encapsulated 4rd rule options                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           DHCPv6 option for 4rd

                                 Figure 7

   o  option-code: TBD1, OPTION_4RD (see Section 6)

   o  option-length: the length of encapsulated options in octets

   o  encapsulated 4rd rule options: the 4RD DHCPv6 option contains at
      least one encapsulated 4RD_MAP_RULE option and maximum one
      encapsulated 4RD_NON_MAP_RULE option.  Since DHCP servers normally
      send whatever options the operator configures, operators should be
      advised to configure these options appropriately.  DHCP servers
      MAY check to see that the configuration follows these rules and
      notify the operator in an implementation-dependent manner if the
      settings for these options aren't valid.  The length of
      encapsulated options is in octets.





















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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     option = 4RD_MAP_RULE     |         option-length         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  prefix4-len  |  prefix6-len  |    ea-len     |W|   Reserved  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    rule-ipv4-prefix                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                        rule-ipv6-prefix                       |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Encapsulated option for Mapping-rule parameters

                                 Figure 8

   o  option-code: TBD2, encapsulated 4RD_MAP_RULE option (see
      Section 6)

   o  option-length: 20

   o  prefix4-len: number of bits of the Rule IPv4 prefix

   o  prefix6-len: number of bits of the Rule IPv6 prefix

   o  ea-len: EA-bits length

   o  W: WKP authorized, = 1 if set

   o  rule-ipv4-prefix: the Rule IPv4 prefix, left aligned

   o  rule-ipv6-prefix: Rule IPv6 prefix, left aligned

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   option = 4RD_NON_MAP_RULE   |         option-length         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |H|      0    |T| traffic-class |         domain-pmtu           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Encapsulated option for non-mapping-rule parameters of 4rd-domains

                                 Figure 9



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   o  option-code: TBD3, encapsulated 4RD_NON_MAP_RULE option (see
      Section 6)

   o  option-length: 4

   o  H: Hub&spoke topology (= 1 if Yes)

   o  T: Traffic-class flag (= 1 if a Tunnel traffic class is provided)

   o  traffic-class: Tunnel-traffic class

   o  domain-pmtu: Domain PMTU (at least 1280)


   Other means than DHCPv6 that may prove useful to provide 4rd
   parameters to CEs are off-scope for this document.  The same or
   similar parameter formats would however be recommended to facilitate
   training and operation.

5.  Security Considerations

   Spoofing attacks

      With IPv6 ingress filtering effective in the Domain [RFC3704], as
      required in Section 3 (Figure 1 in particular), and with
      consistency checks between 4rd IPv4 and IPv6 addresses of
      Section 4.5, no spoofing opportunity in IPv4 is introduced by 4rd:
      being able to use as source IPv6 address only one that has been
      allocated to him, a customer can only provide as source 4rd IPv4
      address that which derives this IPv6 address according to
      Section 4.5, i.e. one that his ISP has allocated to him.

   Routing-loop attacks

      Routing-loop attacks that may exist in some automatic-tunneling
      scenarios are documented in [RFC6324].  No opportunity for
      routing-loop attacks has been identified with 4rd.


   Fragmentation-related attacks

      As discussed in Section 4.6, each BR of a Domain that assigns
      shared public IPv4 should maintain a dynamic table for fragmented
      packets that go to these shared-address CEs.

      This opens a BNR vulnerability to a denial of service attack from
      hosts that would send very large numbers of first fragments and
      would never send last fragments having the same packet



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      identifications.  This vulnerability is inherent to IPv4 address
      sharing, be it static or dynamic.  Compared to what it is with
      algorithms that reassemble IPv4 packets in BRs, it is however
      significantly mitigated by the algorithm of Section 4.6.2 which
      uses much less memory space.

6.  IANA Considerations

   IANA is requested to allocate the following:

   o  One DHCPv6 option codes TBD1 for OPTION_4RD of Section 4.9
      respectively (to be added to section 24.3 of [RFC3315].
      Encapsulated options of OPTION_4RD, 4RD_MAP_RULE (TBD2) and
      4RD_NON_MAP_RULE (TBD3) should also be recorded into the DHCPv6
      option code space.

         Value   |    Description   |  Reference
      -----------+------------------+---------------
          TBD1   |    OPTION_4RD    | this document
          TBD2   |   4RD_MAP_RULE   | this document
          TBD3   | 4RD_NON_MAP_RULE | this document

   o  A reserved IPv4 address to be used as the "IPv4 dummy address" of
      Section 4.8.  Its proposed value is 192.0.0.8/32 (Section 4.8).

7.  Relationship with Previous Works

   The present specification has been influenced by many previous IETF
   drafts, in particular those accessible at http://tools.ietf.org/html/
   draft-xxxx where xxxx are the following (in order of their first
   versions):

   o  bagnulo-behave-nat64 (2008-06-10)

   o  xli-behave-ivi (2008-07-06)

   o  despres-sam-scenarios (2008-09-28)

   o  boucadair-port-range (2008-10-23)

   o  ymbk-aplusp (2008-10-27)

   o  xli-behave-divi (2009-10-19)

   o  thaler-port-restricted-ip-issues (2010-02-28)

   o  cui-softwire-host-4over6 (2010-05-05)




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   o  xli-behave-divi-pd (2011-07-02)

   o  dec-stateless-4v6 (2011-03-05)

   o  matsushima-v6ops-transition-experience (2011-03-07)

   o  despres-intarea-4rd (2011-03-07)

   o  deng-aplusp-experiment-results (2011-03-08)

   o  murakami-softwire-4rd (2011-07-04)

   o  operators-softwire-stateless-4v6-motivation (2011-05-05)

   o  murakami-softwire-4v6-translation (2011-07-04)

   o  despres-softwire-4rd-addmapping (2011-08-19)

   o  boucadair-softwire-stateless-requirements (2011-09-08)

   o  chen-softwire-4v6-add-format (2011-10-2)

   o  mawatari-softwire-464xlat (2011-10-16)

   o  mdt-softwire-map-dhcp-option (2011-10-24)

   o  mdt-softwire-mapping-address-and-port (2011-11-25)

   o  mdt-softwire-map-translation (2012-01-10)

   o  mdt-softwire-map-encapsulation (2012-01-27)

8.  Acknowledgements

   This specification has benefited over several years from independent
   proposals, questions, comments, constructive suggestions, and useful
   criticisms, coming from numerous IETF contributors.

   Authors would like to express recognition to all these contributors,
   and more especially to the following, in alphabetical order of first
   names: Brian Carpenter, Behcet Sarikaya, Bing Liu, Cameron Byrne,
   Congxiao Bao, Dan Wing, Derek Atkins, Erik Kline, Francis Dupont,
   Gabor Bajko, Gang Chen, Hui Deng, Jan Zorz, Jacni Quin (who was an
   active co-author of some earlier versions of this specification),
   James Huang, Jari Arkko, Kathleen Moriarty, Laurent Toutain, Leaf
   Yeh, Lorenzo Colitti, Mark Townsley, Marcello Bagnulo, Mohamed
   Boucadair, Nejc Skoberne, Olaf Maennel, Ole Troan, Olivier Vautrin,
   Peng Wu, Qiong Sun, Rajiv Asati, Ralph Droms, Randy Bush, Satoru



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   Matsushima, Simon Perreault, Stuart Cheshire, Teemu Savolainen,
   Tetsuya Murakami, Tomasz Mrugalski, Tina Tsou, Tomasz Mrugalski, Ted
   Lemon, Suresh Krishnan, Washam Fan, Wojciech Dec, Xiaohong Deng, Xing
   Li, and Yu Fu.

9.  References

9.1.  Normative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
              1981.

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, September 1981.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol", RFC
              2131, March 1997.

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

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474, December
              1998.

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

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.



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   [RFC4925]  Li, X., Dawkins, S., Ward, D., and A. Durand, "Softwire
              Problem Statement", RFC 4925, July 2007.

   [RFC5082]  Gill, V., Heasley, J., Meyer, D., Savola, P., and C.
              Pignataro, "The Generalized TTL Security Mechanism
              (GTSM)", RFC 5082, October 2007.

   [RFC6040]  Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", RFC 6040, November 2010.

9.2.  Informative References

   [I-D.ietf-softwire-stateless-4v6-motivation]
              Boucadair, M., Matsushima, S., Lee, Y., Bonness, O.,
              Borges, I., and G. Chen, "Motivations for Carrier-side
              Stateless IPv4 over IPv6 Migration Solutions", draft-ietf-
              softwire-stateless-4v6-motivation-05 (work in progress),
              November 2012.

   [I-D.shirasaki-nat444]
              Yamagata, I., Shirasaki, Y., Nakagawa, A., Yamaguchi, J.,
              and H. Ashida, "NAT444", draft-shirasaki-nat444-06 (work
              in progress), July 2012.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              November 1990.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets", BCP
              5, RFC 1918, February 1996.

   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, December 1998.

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022, January
              2001.

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

   [RFC3828]  Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., and
              G. Fairhurst, "The Lightweight User Datagram Protocol
              (UDP-Lite)", RFC 3828, July 2004.




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   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, March 2007.

   [RFC4961]  Wing, D., "Symmetric RTP / RTP Control Protocol (RTCP)",
              BCP 131, RFC 4961, July 2007.

   [RFC5595]  Fairhurst, G., "The Datagram Congestion Control Protocol
              (DCCP) Service Codes", RFC 5595, September 2009.

   [RFC5969]  Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4
              Infrastructures (6rd) -- Protocol Specification", RFC
              5969, August 2010.

   [RFC6145]  Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", RFC 6145, April 2011.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, April 2011.

   [RFC6324]  Nakibly, G. and F. Templin, "Routing Loop Attack Using
              IPv6 Automatic Tunnels: Problem Statement and Proposed
              Mitigations", RFC 6324, August 2011.

   [RFC6346]  Bush, R., "The Address plus Port (A+P) Approach to the
              IPv4 Address Shortage", RFC 6346, August 2011.

   [RFC6437]  Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
              "IPv6 Flow Label Specification", RFC 6437, November 2011.

   [RFC6535]  Huang, B., Deng, H., and T. Savolainen, "Dual-Stack Hosts
              Using "Bump-in-the-Host" (BIH)", RFC 6535, February 2012.

   [RFC6887]  Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
              Selkirk, "Port Control Protocol (PCP)", RFC 6887, April
              2013.

   [RFC7136]  Carpenter, B. and S. Jiang, "Significance of IPv6
              Interface Identifiers", RFC 7136, February 2014.

Appendix A.  Textual representation of Mapping rules

   In the next sections, each Mapping rule will be represented as
   follows, using 0bXXX to represent binary number XXX, and square
   brackets [ ] for what is optional:



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   {Rule IPv4 prefix, EA-bits length, Rule IPv6 prefix
      [, WKPs authorized]}

   EXAMPLES:
    {0.0.0.0/0, 32, 2001:db8:0:1:300::/80}
                               a BR mapping rule
    {198.16.0.0/14, 22, 2001:db8:4000::/34}
                               a CE mapping rule
    {0.0.0.0/0, 32, 2001:db8:0:1::/80}
                               a NAT64+ mapping rule)
    {198.16.0.0/14, 22, 2001:db8:4000::/34, Yes}
                               a CE mapping rule and Hub&spoke Topology

Appendix B.  Configuring multiple Mapping Rules

   As far as mapping rules are concerned, the simplest deployment model
   is that in which the Domain has only one rule (the BR mapping rule).
   To assign an IPv4 address to a CE in this model, an IPv6 /112 is
   assigned to it comprising the BR /64 prefix, the 4rd tag, and the
   IPv4 address.  This model has however the following limitations: (1)
   shared IPv4 addresses are not supported; (2) IPv6 prefixes used for
   4rd are too long to be used also for native IPv6 addresses; (3) if
   the IPv4 address space of the ISP is split with many disjoint IPv4
   prefixes, the IPv6 routing plan must be as complex as an IPv4 routing
   plan based on these prefixes.

   With more mapping rules, CE prefixes used for 4rd can be those used
   for native IPv6.  How to choose CE mapping rules for a particular
   deployment needs not being standardized.

   The following is only a particular pragmatic approach that can be
   used for various deployment scenarios.  It is used in some of the use
   cases that follow.

   (1)  Select a "Common_IPv6_prefix" that will appear at the beginning
        of all 4rd CE IPv6 prefixes.

   (2)  Choose all IPv4 prefixes to be used, and assign one of them to
        each CE mapping rule i.

   (3)  For each CE mapping rule i, do the following:

        A.  choose the length of its Rule IPv6 prefix (possibly the same
            for all CE mapping rules).

        B.  Determine its PSID_length(i).  A CE mapping rule that
            assigns shared addresses with a sharing ratio 2^Ki, has
            PSID_length = Ki.  A CE mapping rule rule that assigns IPv4



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            prefixes of length L < 32, is considered to have a negative
            PSID_length = L - 32.

        C.  Derive EA-bits length (i) = 32 - L(Rule IPv4 prefix(i)) +
            PSID_length(i).

        D.  Derive the length of Rule_code(i), the prefix to be appended
            to the Common prefix to get the Rule IPv6 prefix of rule i:


              L(Rule_code(i)) = L(CE IPv6 prefix(i))
                               - L(Common_IPv6_prefix]
                               - (32 - L(Rule IPv4 prefix(i)))
                               - PSID_length(i)

        E.  Derive Rule_code(i) with the following constraints: (1) its
            length is L(Rule_code(i); it does not overlap with any of
            the previously obtained Rule codes (for instance, 010, and
            01011 do overlap, while 00, 011, and 010 do not); it has the
            lowest possible value as a fractional binary number (for
            instance, 0100 < 10 < 11011 < 111).  Thus, rules whose
            Rule_code lengths are 4, 3 , 5, and 2, give Rule_codes 0000,
            001, 00010, and 01)

        F.  Take Rule IPv6 prefix(i)= the Common_IPv6_prefix followed by
            Rule_code(i).

   :<--------------------- L(CE IPv6 prefix(i)) --------------------->:
   :                                                                  :
   :                       32 - L(Rule IPv4 prefix(i))  PSID_length(i):
   :                                                \             |   :
   :                                      :<---------'--------><--'-->:
   :                                      :              ||           :
   :                                      :              \/           :
   :                            :<------->:<--- EA-bits length(i) --->:
   :                          L(Rule code(i))
   :                            :         :
   +----------------------------+---------+
   |    Common IPv6 prefix      |Rule code|
   |                            |   (i)   |
   +----------------------------+---------+
   :<------ L(Rule IPv6 prefix(i)) ------>:

                                 Figure 10







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Appendix C.  ADDING SHARED IPv4 ADDRESSES TO AN IPv6 NETWORK

C.1.  With CEs within CPEs

   We consider an ISP that offers IPv6-only service to up to 2^20
   customers.  Each customer is delegated a /56, starting with common
   prefix 2001:db8:0::/36.  It wants to add public IPv4 service to
   customers that are 4rd-capable.  It prefers to do it with stateless
   operation in its nodes, but has largely less IPv4 addresses than IPv6
   addresses so that a sharing ratio is necessary.

   The only IPv4 prefixes it can use are 192.8.0.0/15, 192.4.0.0/16,
   192.2.0.0/16, and 192.1.0.0/16 (neither overlapping nor
   aggregetable).  This gives 2^(32-15) + 3*2^(32-16) IPv4 addresses,
   i.e. 2^18 + 2^16).  For the 2^20 customers to have the same sharing
   ratio, the number of IPv4 addresses to be shared has to be a power of
   2.  The ISP can therefore renounce to use one /16, say the last one.
   (Whether it could be motivated to return it to its Internet Registry
   is off-scope for this document.)  The sharing ratio to apply is then
   2^20 / 2^18 = 2^2 = 4, giving a PSID length of 2.

   Applying principles of Appendix B with L[Common IPv6 prefix] = 36,
   L[PSID] = 2 for all rules, and L[CE IPv6 prefix(i)] = 56 for all
   rules, Rule codes and Rule IPv6 prefixes are:

   +--------------+--------+-----------+-----------+-------------------+
   | CE Rule IPv4 | EA     | Rule-Code | Code      | CE Rule IPv6      |
   | prefix       | bits   | length    | (binary)  | prefix            |
   |              | length |           |           |                   |
   +--------------+--------+-----------+-----------+-------------------+
   | 192.8.0.0/15 | 19     | 1         | 0         | 2001:db8:0::/37   |
   | 192.4.0.0/16 | 18     | 2         | 10        | 2001:db8:800::/38 |
   | 192.2.0.0/16 | 18     | 2         | 11        | 2001:db8:c00::/38 |
   +--------------+--------+-----------+-----------+-------------------+

   Mapping rules are then the following:

             {192.8.0.0/15, 19, 2001:0db8:0000::/37}
             {192.4.0.0/16, 18, 2001:0db8:0800::/38}
             {192.2.0.0/16, 18, 2001:0db8:0c00::/38}
             {0.0.0.0/0,    32, 2001:0db8:0000:0001:300::/80}


   The CE whose IPv6 prefix is, for example, 2001:db8:0bbb:bb00::/56,
   derives its IPv4 address and its port set as follows (Section 4.4):






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      CE IPv6 prefix     : 2001:0db8:0bbb:bb00::/56
      Rule IPv6 prefix(i): 2001:0db8:0800::/38 (longest match)
      EA-bits length(i)  : 18
      EA bits            : 0b11 1011 1011 1011 1011
      Rule IPv4 prefix(i): 0b1100 0000 0000 0100 (192.4.0.0/16)
      IPv4 address       : 0b1100 0000 0000 0100 1110 1110 1110 1110
                         : 192.4.238.238
      PSID               : 0b11
      Ports              : 0bYYYY 11XX XXXX XXXX
                           with YYYY > 0, and X...X any value

   An IPv4 packet sent to address 192.4.238.238 and port 7777 is
   tunneled to the IPv6 address obtained as follows (Section 4.5):

      IPv4 address       : 192.4.238.238 (0xC004 EEEE)
                         : 0b1100 0000 0000 0100 1110 1110 1110 1110
      Rule IPv4 prefix(i): 192.4.0.0/16  (longest match)
                         : 0b1100 0000 0000 0100
      IPv4 suffix (i)    : 0b1110 1110 1110 1110
      EA-bits length (i) : 18
      PSID length (i)    : 2  (= 16 + 18 - 32)
      Port field         : 0b 0001 1110 0110 0001 (7777)
      PSID               : 0b11
      Rule IPv6 prefix(i): 2001:0db8:0800::/38
      CE IPv6 prefix     : 2001:0db8:0bbb:bb00::/56
      IPv6 address       : 2001:0db8:0bbb:bb00:300:c004:eeee:YYYY
                           with YYYY = the computed CNP

C.2.  With some CEs behind Third-party Router CPEs

   We now consider an ISP that has the same need as in the previous
   section except that, instead of using only its own IPv6
   infrastructure, it uses that of a third-party provider, and that some
   of its customers use CPEs of this provider to use specific services
   it offers.  In these CPEs, a non-zero index is used to route IPv6
   packets to the physical port to which CEs are attached, say 0x2.
   Each such CPE delegates to the CE nodes the customer-site IPv6 prefix
   followed by this index.

   The ISP is supposed to have the same IPv4 prefixes as in the previous
   use case, 192.8.0.0/15, 192.4.0.0/16, and 192.2.0.0/16, and to use
   the same Common IPv6 prefix, 2001:db8:0::/36.


   We also assume that only a minority of customers use third-party
   CPEs, so that it is sufficient to use only one of the two /16s for
   them.




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   Mapping rules, are then (see Appendix C.1):

             {192.8.0.0/15, 19, 2001:0db8:0000::/37}
             {192.4.0.0/16, 18, 2001:0db8:0800::/38}
             {192.2.0.0/16, 18, 2001:0db8:0c00::/38}
             {0.0.0.0/0,    32, 2001:0db8:0000:0001:300::/80}

   CEs that are behind third-party CPEs derive their own IPv4 addresses
   and port sets as in Appendix C.1.

   In a BR, and also in a CE if the topology is mesh, the IPv6 address
   that is derived from IPv4 address 192.4.238.238 and port 7777 is
   obtained as in the previous section, except for the two last steps
   which are modified:

     IPv4 address       : 192.4.238.238 (0xC004 EEEE)
                        : 0b1100 0000 0000 0100 1110 1110 1110 1110
     Rule IPv4 prefix(i): 192.4.0.0/16  (longest match)
                        : 0b1100 0000 0000 0100
     IPv4 suffix (i)    : 0b1110 1110 1110 1110
     EA-bits length (i) : 18
     PSID length (i)    : 2  (= 16 + 18 - 32)
     Port field         : 0b 0001 1110 0110 0001 (7777)
     PSID               : 0b11
     Rule IPv6 prefix(i): 2001:0db8:0800::/38
     CE IPv6 prefix       : 2001:0db8:0bbb:bb00::/60
     IPv6 address         : 2001:0db8:0bbb:bb00:300:192.4.238.238:YYYY
                            with YYYY = the computed CNP


Appendix D.  REPLACING DUAL-STACK ROUTING BY IPv6-ONLY ROUTING

   In this use case, we consider an ISP that offers IPv4 service with
   public addresses individually assigned to its customers.  It also
   offers IPv6 service, having deployed for this dual-stack routing.
   Because it provides its own CPEs to customers, it can upgrade all its
   CPEs to support 4rd.  It wishes to take advantage of this capability
   to replace dual-stack routing by IPv6-only routing without changing
   any IPv4 address or IPv6 prefix.

   For this, the ISP can use the single-rule model described at the
   beginning of Appendix B.  If the prefix routed to BRs is chosen to
   start with 2001:db8:0:1::/64, this rule is:

      {0.0.0.0/0, 32, 2001:db8:0:1:300::/80}

   All what is needed in the network before disabling IPv4 routing is
   the following:



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   o  In all routers, where there is an IPv4 route toward x.x.x.x/n, add
      a parallel route toward 2001:db8:0:1:300:x.x.x.x::/(80+n)

   o  Where IPv4 address x.x.x.x was assigned to a CPE, now delegate
      IPv6 prefix 2001:db8:0:1:300:x.x.x.x::/112.

   NOTE: In parallel with this deployment, or after it, shared IPv4
   addresses can be assigned to IPv6 customers.  It is sufficient that
   IPv4 prefixes used for this be different from those used for
   exclusive-address assignments.  Under this constraint, Mapping rules
   can be set up according to the same principles as those of
   Appendix C.


Appendix E.  ADDING IPv6 AND 4rd SERVICE TO A NET-10 NETWORK

   In this use case, we consider an ISP that has only deployed IPv4,
   possibly because some of its network devices are not yet IPv6
   capable.  Because it did not have enough IPv4 addresses, it has
   assigned private IPv4 addresses of [RFC1918] to customers, say
   10.x.x.x.  It thus supports up to 2^24 customers (a "Net-10" network,
   using the NAT444 model of [I-D.shirasaki-nat444]).

   Now, it wishes to offer IPv6 service without further delay, using for
   this 6rd [RFC5969].  It also wishes to offer incoming IPv4
   connectivity to its customers with a simpler solution than that of
   PCP [RFC6887].

   This appendix describes an example that adds IPv6 (using 6rd) and 4rd
   services to the "Net-10" private IPv4 network.

   The IPv6 prefix to be used for 6rd is supposed to be 2001:db8::/32,
   and the public IPv4 prefix to be used for shared addresses is
   supposed to be 198.16.0.0/16 (0xc610).  The resulting sharing ratio
   is 2^24 / 2^(32-16) = 256, giving a PSID length of 8.

   The ISP installs one or several BRs, at its border to the public IPv4
   Internet.  They support 6rd, and 4rd above it.  The BR prefix /64 is
   supposed to be that which is derived from IPv4 address 10.0.0.1 (i.e.
   2001:db8:0:100:/64).

   In accordance with [RFC5969], 6rd BRs are configured with the
   following parameters IPv4MaskLen = 8, 6rdPrefix = 2001:db8::/32;
   6rdBRIPv4Address = 192.168.0.1 (0xC0A80001).

   4rd Mapping rules are then the following:





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               {198.16.0.0/16, 24, 2001:db8:0:0:300::/80}
               {0.0.0.0/0,      32, 2001:db8:0:100:300:/80,}

   Any customer device that supports 4rd in addition to 6rd can then use
   its assigned shared IPv4 address with 240 assigned ports.

   If its NAT44 supports port forwarding to provide incoming IPv4
   connectivity (statically, or dynamically with UPnP an/or NAT-PMP), it
   can use it with ports of the assigned port set (a possibility that
   does not exist in Net-10 networks without 4rd/6rd).

Authors' Addresses

   Remi Despres
   RD-IPtech
   3 rue du President Wilson
   Levallois
   France

   Email: despres.remi@laposte.net


   Sheng Jiang (editor)
   Huawei Technologies Co., Ltd
   Q14, Huawei Campus, No.156 BeiQing Road
   Hai-Dian District, Beijing  100095
   P.R. China

   Email: jiangsheng@huawei.com


   Reinaldo Penno
   Cisco Systems, Inc.
   170 West Tasman Drivee
   San Jose, California  95134
   USA

   Email: repenno@cisco.com


   Yiu Lee
   Comcast
   One Comcast Center
   Philadelphia, PA  1903
   USA

   Email: Yiu_Lee@Cable.Comcast.com




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   Gang Chen
   China Mobile
   53A, Xibianmennei Ave.
   Xuanwu District, Beijing  100053
   China

   Email: phdgang@gmail.com


   Maoke Chen
   Freebit Co, Ltd.
   13F E-space Tower, Maruyama-cho 3-6
   Shibuya-ku, Tokyo  150-0044
   Japan

   Email: fibrib@gmail.com



































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