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RFC 7597

Proposed STD
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Mapping of Address and Port with Encapsulation (MAP-E)

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Internet Engineering Task Force (IETF)                     O. Troan, Ed.
Request for Comments: 7597                                        W. Dec
Category: Standards Track                                  Cisco Systems
ISSN: 2070-1721                                                    X. Li
                                                                  C. Bao
                                                     Tsinghua University
                                                           S. Matsushima
                                                        SoftBank Telecom
                                                             T. Murakami
                                                             IP Infusion
                                                          T. Taylor, Ed.
                                                     Huawei Technologies
                                                               July 2015


         Mapping of Address and Port with Encapsulation (MAP-E)

Abstract

   This document describes a mechanism for transporting IPv4 packets
   across an IPv6 network using IP encapsulation.  It also describes a
   generic mechanism for mapping between IPv6 addresses and IPv4
   addresses as well as transport-layer ports.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7597.

Page 2 
Copyright Notice

   Copyright (c) 2015 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.

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Table of Contents

   1. Introduction ....................................................4
   2. Conventions .....................................................5
   3. Terminology .....................................................5
   4. Architecture ....................................................7
   5. Mapping Algorithm ...............................................8
      5.1. Port-Mapping Algorithm ....................................10
      5.2. Basic Mapping Rule (BMR) ..................................11
      5.3. Forwarding Mapping Rule (FMR) .............................14
      5.4. Destinations outside the MAP Domain .......................14
   6. The IPv6 Interface Identifier ..................................15
   7. MAP Configuration ..............................................15
      7.1. MAP CE ....................................................15
      7.2. MAP BR ....................................................16
   8. Forwarding Considerations ......................................17
      8.1. Receiving Rules ...........................................17
      8.2. ICMP ......................................................18
      8.3. Fragmentation and Path MTU Discovery ......................18
           8.3.1. Fragmentation in the MAP Domain ....................18
           8.3.2. Receiving IPv4 Fragments on the MAP Domain
                  Borders ............................................19
           8.3.3. Sending IPv4 Fragments to the Outside ..............19
   9. NAT44 Considerations ...........................................19
   10. Security Considerations .......................................20
   11. References ....................................................21
      11.1. Normative References .....................................21
      11.2. Informative References ...................................21
   Appendix A. Examples ..............................................25
   Appendix B. A More Detailed Description of the Derivation of the
               Port-Mapping Algorithm ................................29
     B.1. Bit Representation of the Algorithm ........................31
     B.2. GMA Examples ...............................................32
   Acknowledgements ..................................................32
   Contributors ......................................................33
   Authors' Addresses ................................................34

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1.  Introduction

   Mapping of IPv4 addresses in IPv6 addresses has been described in
   numerous mechanisms dating back to the mid-1990s [RFC1933] [RFC4213].
   The "automatic tunneling" mechanism as first described in [RFC1933]
   assigned a globally unique IPv6 address to a host by combining the
   host's IPv4 address with a well-known IPv6 prefix.  Given an IPv6
   packet with a destination address with an embedded IPv4 address, a
   node could automatically tunnel this packet by extracting the IPv4
   tunnel endpoint address from the IPv6 destination address.

   There are numerous variations of this idea, as described in 6over4
   [RFC2529], 6to4 [RFC3056], the Intra-Site Automatic Tunnel Addressing
   Protocol (ISATAP) [RFC5214], and IPv6 Rapid Deployment on IPv4
   Infrastructures (6rd) [RFC5969].

   The commonalities of all of these IPv6-over-IPv4 mechanisms are as
   follows:

   o  Automatic provisioning of an IPv6 address for a host or an IPv6
      prefix for a site.

   o  Algorithmic or implicit address resolution of tunnel endpoint
      addresses.  Given an IPv6 destination address, an IPv4 tunnel
      endpoint address can be calculated.

   o  Embedding of an IPv4 address or part thereof into an IPv6 address.

   In later phases of IPv4-to-IPv6 migration, it is expected that
   IPv6-only networks will be common, while there will still be a need
   for residual IPv4 deployment.  This document describes a generic
   mapping of IPv4 to IPv6 and a mechanism for encapsulating IPv4
   over IPv6.

   Just as for the IPv6-over-IPv4 mechanisms referred to above, the
   residual IPv4-over-IPv6 mechanism must be capable of:

   o  Provisioning an IPv4 prefix, an IPv4 address, or a shared IPv4
      address.

   o  Algorithmically mapping between an IPv4 prefix, an IPv4 address,
      or a shared IPv4 address and an IPv6 address.

   The mapping scheme described here supports encapsulation of IPv4
   packets in IPv6 in both mesh and hub-and-spoke topologies, including
   address mappings with full independence between IPv6 and IPv4
   addresses.

Top      ToC       Page 5 
   This document describes the delivery of IPv4 unicast service across
   an IPv6 infrastructure.  IPv4 multicast is not considered in this
   document.

   The Address plus Port (A+P) architecture of sharing an IPv4 address
   by distributing the port space is described in [RFC6346].
   Specifically, Section 4 of [RFC6346] covers stateless mapping.  The
   corresponding stateful solution, Dual-Stack Lite (DS-Lite), is
   described in [RFC6333].  The motivations for this work are described
   in [Solutions-4v6].

   [RFC7598] defines DHCPv6 options for the provisioning of MAP.  Other
   means of provisioning are possible.  Deployment considerations are
   described in [MAP-Deploy].

   MAP relies on IPv6 and is designed to deliver dual-stack service
   while allowing IPv4 to be phased out within the service provider's
   (SP's) network.  The phasing out of IPv4 within the SP network is
   independent of whether the end user disables IPv4 service or not.
   Further, "greenfield" IPv6-only networks may use MAP in order to
   deliver IPv4 to sites via the IPv6 network.

2.  Conventions

   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 RFC 2119 [RFC2119].

3.  Terminology

   MAP domain:             One or more MAP Customer Edge (CE) devices
                           and Border Relays (BRs) connected to the same
                           virtual link.  A service provider may deploy
                           a single MAP domain or may utilize multiple
                           MAP domains.

   MAP Rule:               A set of parameters describing the mapping
                           between an IPv4 prefix, IPv4 address, or
                           shared IPv4 address and an IPv6 prefix or
                           address.  Each domain uses a different
                           mapping rule set.

   MAP node:               A device that implements MAP.

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   MAP Border Relay (BR):  A MAP-enabled router managed by the service
                           provider at the edge of a MAP domain.  A BR
                           has at least an IPv6-enabled interface and an
                           IPv4 interface connected to the native IPv4
                           network.  A MAP BR may also be referred to as
                           simply a "BR" within the context of MAP.

   MAP Customer Edge (CE): A device functioning as a Customer Edge
                           router in a MAP deployment.  A typical MAP CE
                           adopting MAP Rules will serve a residential
                           site with one WAN-side interface and one or
                           more LAN-side interfaces.  A MAP CE may also
                           be referred to as simply a "CE" within the
                           context of MAP.

   Port set:               Each node has a separate part of the
                           transport-layer port space; this is denoted
                           as a port set.

   Port Set ID (PSID):     Algorithmically identifies a set of ports
                           exclusively assigned to a CE.

   Shared IPv4 address:    An IPv4 address that is shared among multiple
                           CEs.  Only ports that belong to the assigned
                           port set can be used for communication.  Also
                           known as a port-restricted IPv4 address.

   End-user IPv6 prefix:   The IPv6 prefix assigned to an End-user CE by
                           means other than MAP itself, e.g.,
                           provisioned using DHCPv6 Prefix Delegation
                           (PD) [RFC3633], assigned via Stateless
                           Address Autoconfiguration (SLAAC) [RFC4862],
                           or configured manually.  It is unique for
                           each CE.

   MAP IPv6 address:       The IPv6 address used to reach the MAP
                           function of a CE from other CEs and from BRs.

   Rule IPv6 prefix:       An IPv6 prefix assigned by a service provider
                           for a mapping rule.

   Rule IPv4 prefix:       An IPv4 prefix assigned by a service provider
                           for a mapping rule.

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   Embedded Address (EA) bits:
                           The IPv4 EA-bits in the IPv6 address identify
                           an IPv4 prefix/address (or part thereof) or a
                           shared IPv4 address (or part thereof) and a
                           Port Set Identifier.

4.  Architecture

   In accordance with the requirements stated above, the MAP mechanism
   can operate with shared IPv4 addresses, full IPv4 addresses, or IPv4
   prefixes.  Operation with shared IPv4 addresses is described here,
   and the differences for full IPv4 addresses and prefixes are
   described below.

   The MAP mechanism uses existing standard building blocks.  The
   existing Network Address and Port Translator (NAPT) [RFC2663] on the
   CE is used with additional support for restricting transport-protocol
   ports, ICMP identifiers, and fragment identifiers to the configured
   port set.  For packets outbound from the private IPv4 network, the CE
   NAPT MUST translate transport identifiers (e.g., TCP and UDP port
   numbers) so that they fall within the CE's assigned port range.

   The NAPT MUST in turn be connected to a MAP-aware forwarding function
   that does encapsulation/decapsulation of IPv4 packets in IPv6.  MAP
   supports the encapsulation mode specified in [RFC2473].  In addition,
   MAP specifies an algorithm to do "address resolution" from an IPv4
   address and port to an IPv6 address.  This algorithmic mapping is
   specified in Section 5.

   The MAP architecture described here restricts the use of the shared
   IPv4 address to only be used as the global address (outside) of the
   NAPT running on the CE.  A shared IPv4 address MUST NOT be used to
   identify an interface.  While it is theoretically possible to make
   host stacks and applications port-aware, it would be a drastic change
   to the IP model [RFC6250].

   For full IPv4 addresses and IPv4 prefixes, the architecture just
   described applies, with two differences: first, a full IPv4 address
   or IPv4 prefix can be used as it is today, e.g., for identifying an
   interface or as a DHCP pool, respectively.  Second, the NAPT is not
   required to restrict the ports used on outgoing packets.

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   This architecture is illustrated in Figure 1.

         User N
       Private IPv4
      |  Network
      |
   O--+---------------O
   |  |  MAP CE       |
   | +-----+--------+ |
   | NAPT44|  MAP   | |
   | +-----+        | |\     ,-------.                      .------.
   |       +--------+ | \ ,-'         `-.                 ,-'       `-.
   O------------------O  /              \   O---------O  /   Public   \
                        /    IPv6-only  \  |  MAP    | /     IPv4      \
                       (    Network      --+  Border +-     Network    )
                        \  (MAP Domain) /  |  Relay  | \               /
   O------------------O  \              /   O---------O  \            /
   |    MAP   CE      |  /".         ,-'                 `-.       ,-'
   | +-----+--------+ | /   `----+--'                       ------'
   | NAPT44|  MAP   | |/
   | +-----+        | |
   |   |   +--------+ |
   O---+--------------O
       |
        User M
      Private IPv4
        Network

                        Figure 1: Network Topology

   The MAP BR connects one or more MAP domains to external IPv4
   networks.

5.  Mapping Algorithm

   A MAP node is provisioned with one or more mapping rules.

   Mapping rules are used differently, depending on their function.
   Every MAP node must be provisioned with a Basic Mapping Rule.  This
   is used by the node to configure its IPv4 address, IPv4 prefix, or
   shared IPv4 address.  This same basic rule can also be used for
   forwarding, where an IPv4 destination address and, optionally, a
   destination port are mapped into an IPv6 address.  Additional mapping
   rules are specified to allow for multiple different IPv4 subnets to
   exist within the domain and optimize forwarding between them.

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   Traffic outside of the domain (i.e., when the destination IPv4
   address does not match (using longest matching prefix) any Rule IPv4
   prefix in the Rules database) is forwarded to the BR.

   There are two types of mapping rules:

   1.  Basic Mapping Rule (BMR) - mandatory.  A CE can be provisioned
       with multiple End-user IPv6 prefixes.  There can only be one
       Basic Mapping Rule per End-user IPv6 prefix.  However, all CEs
       having End-user IPv6 prefixes within (aggregated by) the same
       Rule IPv6 prefix may share the same Basic Mapping Rule.  In
       combination with the End-user IPv6 prefix, the Basic Mapping Rule
       is used to derive the IPv4 prefix, address, or shared address and
       the PSID assigned to the CE.

   2.  Forwarding Mapping Rule (FMR) - optional; used for forwarding.
       The Basic Mapping Rule may also be a Forwarding Mapping Rule.
       Each Forwarding Mapping Rule will result in an entry in the rule
       table for the Rule IPv4 prefix.  Given a destination IPv4 address
       and port within the MAP domain, a MAP node can use the matching
       FMR to derive the End-user IPv6 address of the interface through
       which that IPv4 destination address and port combination can be
       reached.  In hub-and-spoke mode, there are no FMRs.

   Both mapping rules share the same parameters:

   o  Rule IPv6 prefix (including prefix length)

   o  Rule IPv4 prefix (including prefix length)

   o  Rule EA-bit length (in bits)

   A MAP node finds its BMR by doing a longest match between the
   End-user IPv6 prefix and the Rule IPv6 prefix in the Mapping Rules
   table.  The rule is then used for IPv4 prefix, address, or shared
   address assignment.

   A MAP IPv6 address is formed from the BMR Rule IPv6 prefix.  This
   address MUST be assigned to an interface of the MAP node and is used
   to terminate all MAP traffic being sent or received to the node.

   Port-restricted IPv4 routes are installed in the rule table for all
   the Forwarding Mapping Rules, and a default route is installed to the
   MAP BR (see Section 5.4).

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   Forwarding Mapping Rules are used to allow direct communication
   between MAP CEs; this is known as "Mesh mode".  In hub-and-spoke
   mode, there are no Forwarding Mapping Rules; all traffic MUST be
   forwarded directly to the BR.

   While an FMR is optional in the sense that a MAP CE MAY be configured
   with zero or more FMRs -- depending on the deployment -- all MAP CEs
   MUST implement support for both rule types.

5.1.  Port-Mapping Algorithm

   The port-mapping algorithm is used in domains whose rules allow IPv4
   address sharing.

   The simplest way to represent a port range is using a notation
   similar to Classless Inter-Domain Routing (CIDR) [RFC4632].  For
   example, the first 256 ports are represented as port prefix 0.0/8 and
   the last 256 ports as 255.0/8.  In hexadecimal, these would be
   0x0000/8 (PSID = 0) and 0xFF00/8 (PSID = 0xFF), respectively.  Using
   this technique but wishing to avoid allocating the system ports
   [RFC6335] to the user, one would have to exclude the use of one or
   more PSIDs (e.g., PSIDs 0 to 3 in the example just given).

   When the PSID is embedded in the End-user IPv6 prefix, it is
   desirable to minimize the restrictions of possible PSID values in
   order to minimize dependencies between the End-user IPv6 prefix and
   the assigned port set.  This is achieved by using an infix
   representation of the port value.  Using such a representation, the
   well-known ports are excluded by restrictions on the value of the
   high-order bit field (A) rather than the PSID.

   The infix algorithm allocates ports to a given CE as a series of
   contiguous ranges spaced at regular intervals throughout the complete
   range of possible port-set values.

                              0                   1
                              0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                             +-----------+-----------+-------+
               Ports in      |     A     |    PSID   |   j   |
            the CE port set  |    > 0    |           |       |
                             +-----------+-----------+-------+
                             |  a bits   |  k bits   |m bits |

            Figure 2: Structure of a Port-Restricted Port Field

Top      ToC       Page 11 
   a bits:  The number of offset bits -- 6 by default, as this excludes
            the system ports (0-1023).  To guarantee non-overlapping
            port sets, the offset 'a' MUST be the same for every MAP CE
            sharing the same address.

        A:  Selects the range of the port number.  For 'a' > 0, A MUST
            be larger than 0.  This ensures that the algorithm excludes
            the system ports.  For the default value of 'a' (6), the
            system ports are excluded by requiring that A be greater
            than 0.  Smaller values of 'a' exclude a larger initial
            range, e.g., 'a' = 4 will exclude ports 0-4095.  The
            interval between initial port numbers of successive
            contiguous ranges assigned to the same user is 2^(16 - a).

   k bits:  The length in bits of the PSID field.  To guarantee
            non-overlapping port sets, the length 'k' MUST be the same
            for every MAP CE sharing the same address.  The sharing
            ratio is 2^k.  The number of ports assigned to the user is
            2^(16 - k) - 2^m (excluded ports).

     PSID:  The Port Set Identifier (PSID).  Different PSID values
            guarantee non-overlapping port sets, thanks to the
            restrictions on 'a' and 'k' stated above, because the PSID
            always occupies the same bit positions in the port number.

   m bits:  The number of contiguous ports is given by 2^m.

        j:  Selects the specific port within a particular range
            specified by the concatenation of A and the PSID.

5.2.  Basic Mapping Rule (BMR)

   The Basic Mapping Rule is mandatory and is used by the CE to
   provision itself with an IPv4 prefix, IPv4 address, or shared IPv4
   address.  Recall from Section 5 that the BMR consists of the
   following parameters:

   o  Rule IPv6 prefix (including prefix length)

   o  Rule IPv4 prefix (including prefix length)

   o  Rule EA-bit length (in bits)

Top      ToC       Page 12 
   Figure 3 shows the structure of the complete MAP IPv6 address as
   specified in this document.

   |     n bits         |  o bits   | s bits  |   128-n-o-s bits      |
   +--------------------+-----------+---------+-----------------------+
   |  Rule IPv6 prefix  |  EA bits  |subnet ID|     interface ID      |
   +--------------------+-----------+---------+-----------------------+
   |<---  End-user IPv6 prefix  --->|

                     Figure 3: MAP IPv6 Address Format

   The Rule IPv6 prefix is common among all CEs using the same Basic
   Mapping Rule within the MAP domain.  The EA bit field encodes the
   CE-specific IPv4 address and port information.  The EA bit field,
   which is unique for a given Rule IPv6 prefix, can contain a full or
   partial IPv4 address and, in the shared IPv4 address case, a PSID.
   An EA bit field length of 0 signifies that all relevant MAP IPv4
   addressing information is passed directly in the BMR and is not
   derived from the EA bit field in the End-user IPv6 prefix.

   The MAP IPv6 address is created by concatenating the End-user IPv6
   prefix with the MAP subnet identifier (if the End-user IPv6 prefix is
   shorter than 64 bits) and the interface identifier as specified in
   Section 6.

   The MAP subnet identifier is defined to be the first subnet (s bits
   set to zero).

   Define:

      r = length of the IPv4 prefix given by the BMR;

      o = length of the EA bit field as given by the BMR;

      p = length of the IPv4 suffix contained in the EA bit field.

   The length r MAY be zero, in which case the complete IPv4 address or
   prefix is encoded in the EA bits.  If only a part of the IPv4
   address / prefix is encoded in the EA bits, the Rule IPv4 prefix is
   provisioned to the CE by other means (e.g., a DHCPv6 option).  To
   create a complete IPv4 address (or prefix), the IPv4 address suffix
   (p) from the EA bits is concatenated with the Rule IPv4 prefix
   (r bits).

   The offset of the EA bit field in the IPv6 address is equal to the
   BMR Rule IPv6 prefix length.  The length of the EA bit field (o) is
   given by the BMR Rule EA-bit length and can be between 0 and 48.  A
   length of 48 means that the complete IPv4 address and port are

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   embedded in the End-user IPv6 prefix (a single port is assigned).  A
   length of 0 means that no part of the IPv4 address or port is
   embedded in the address.  The sum of the Rule IPv6 Prefix length and
   the Rule EA-bit length MUST be less than or equal to the End-user
   IPv6 prefix length.

   If o + r < 32 (length of the IPv4 address in bits), then an IPv4
   prefix is assigned.  This case is shown in Figure 4.

                   |   r bits    |  o bits =  p bits   |
                   +-------------+---------------------+
                   |  Rule IPv4  | IPv4 address suffix |
                   +-------------+---------------------+
                   |           < 32 bits               |

                           Figure 4: IPv4 Prefix

   If o + r is equal to 32, then a full IPv4 address is to be assigned.
   The address is created by concatenating the Rule IPv4 prefix and the
   EA-bits.  This case is shown in Figure 5.

                   |   r bits    |  o bits = p bits    |
                   +-------------+---------------------+
                   |  Rule IPv4  | IPv4 address suffix |
                   +-------------+---------------------+
                   |            32 bits                |

                      Figure 5: Complete IPv4 Address

   If o + r is > 32, then a shared IPv4 address is to be assigned.  The
   number of IPv4 address suffix bits (p) in the EA bits is given by
   32 - r bits.  The PSID bits are used to create a port set.  The
   length of the PSID bit field within the EA bits is q = o - p.

       |   r bits    |        p bits       |         |   q bits   |
       +-------------+---------------------+         +------------+
       |  Rule IPv4  | IPv4 address suffix |         |Port Set ID |
       +-------------+---------------------+         +------------+
       |            32 bits                |

                       Figure 6: Shared IPv4 Address

   The length of r MAY be 32, with no part of the IPv4 address embedded
   in the EA bits.  This results in a mapping with no dependence between
   the IPv4 address and the IPv6 address.  In addition, the length of o
   MAY be zero (no EA bits embedded in the End-user IPv6 prefix),
   meaning that the PSID is also provisioned using, for example, DHCP.

Top      ToC       Page 14 
   See Appendix A for an example of the Basic Mapping Rule.

5.3.  Forwarding Mapping Rule (FMR)

   The Forwarding Mapping Rule is optional and is used in Mesh mode to
   enable direct CE-to-CE connectivity.

   On adding an FMR rule, an IPv4 route is installed in the rule table
   for the Rule IPv4 prefix (Figures 4, 5, and 6).

   |        32 bits           |         |    16 bits        |
   +--------------------------+         +-------------------+
   | IPv4 destination address |         |  IPv4 dest port   |
   +--------------------------+         +-------------------+
                  :           :           ___/       :
                  |  p bits   |          /  q bits   :
                  +-----------+         +------------+
                  |IPv4 suffix|         |Port Set ID |
                  +-----------+         +------------+
                   \          /    ____/    ________/
                     \       :  __/   _____/
                       \     : /     /
   |     n bits         |  o bits   | s bits  |   128-n-o-s bits      |
   +--------------------+-----------+---------+------------+----------+
   |  Rule IPv6 prefix  |  EA bits  |subnet ID|     interface ID      |
   +--------------------+-----------+---------+-----------------------+
   |<---  End-user IPv6 prefix  --->|

                 Figure 7: Derivation of MAP IPv6 Address

   See Appendix A for an example of the Forwarding Mapping Rule.

5.4.  Destinations outside the MAP Domain

   IPv4 traffic between MAP nodes that are all within one MAP domain is
   encapsulated in IPv6, with the sender's MAP IPv6 address as the IPv6
   source address and the receiving MAP node's MAP IPv6 address as the
   IPv6 destination address.  To reach IPv4 destinations outside of the
   MAP domain, traffic is also encapsulated in IPv6, but the destination
   IPv6 address is set to the configured IPv6 address of the MAP BR.

   On the CE, the path to the BR can be represented as a point-to-point
   IPv4-over-IPv6 tunnel [RFC2473] with the source address of the tunnel
   being the CE's MAP IPv6 address and the BR IPv6 address as the remote
   tunnel address.  When MAP is enabled, a typical CE router will
   install a default IPv4 route to the BR.

Top      ToC       Page 15 
   The BR forwards traffic received from the outside to CEs using the
   normal MAP forwarding rules.

6.  The IPv6 Interface Identifier

   The interface identifier format of a MAP node is described below.

                   |          128-n-o-s bits          |
                   | 16 bits|    32 bits     | 16 bits|
                   +--------+----------------+--------+
                   |   0    |  IPv4 address  |  PSID  |
                   +--------+----------------+--------+

                    Figure 8: IPv6 Interface Identifier

   In the case of an IPv4 prefix, the IPv4 address field is right-padded
   with zeros up to 32 bits.  The PSID field is left-padded with zeros
   to create a 16-bit field.  For an IPv4 prefix or a complete IPv4
   address, the PSID field is zero.

   If the End-user IPv6 prefix length is larger than 64, the most
   significant parts of the interface identifier are overwritten by the
   prefix.

7.  MAP Configuration

   For a given MAP domain, the BR and CE MUST be configured with the
   following MAP elements.  The configured values for these elements are
   identical for all CEs and BRs within a given MAP domain.

   o  The Basic Mapping Rule and, optionally, the Forwarding Mapping
      Rules, including the Rule IPv6 prefix, Rule IPv4 prefix, and
      Length of EA bits.

   o  Hub-and-spoke mode or Mesh mode (if all traffic should be sent to
      the BR, or if direct CE-to-CE traffic should be supported).

   In addition, the MAP CE MUST be configured with the IPv6 address(es)
   of the MAP BR (Section 5.4).

7.1.  MAP CE

   The MAP elements are set to values that are the same across all CEs
   within a MAP domain.  The values may be configured in a variety of
   ways, including provisioning methods such as the Broadband Forum's
   "TR-69" Residential Gateway management interface [TR069], an
   XML-based object retrieved after IPv6 connectivity is established, or
   manual configuration by an administrator.  IPv6 DHCP options for MAP

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   configuration are defined in [RFC7598].  Other configuration and
   management methods may use the formats described by these options for
   consistency and convenience of implementation on CEs that support
   multiple configuration methods.

   The only remaining provisioning information the CE requires in order
   to calculate the MAP IPv4 address and enable IPv4 connectivity is the
   IPv6 prefix for the CE.  The End-user IPv6 prefix is configured as
   part of obtaining IPv6 Internet access.

   The MAP provisioning parameters, and hence the IPv4 service itself,
   are tied to the associated End-user IPv6 prefix lifetime; thus, the
   MAP service is also tied to this in terms of authorization,
   accounting, etc.

   A single MAP CE MAY be connected to more than one MAP domain, just as
   any router may have more than one IPv4-enabled service-provider-
   facing interface and more than one set of associated addresses
   assigned by DHCP.  Each domain within which a given CE operates would
   require its own set of MAP configuration elements and would generate
   its own IPv4 address.  Each MAP domain requires a distinct End-user
   IPv6 prefix.

   MAP DHCP options are specified in [RFC7598].

7.2.  MAP BR

   The MAP BR MUST be configured with corresponding mapping rules for
   each MAP domain for which it is acting as a BR.

   For increased reliability and load balancing, the BR IPv6 address MAY
   be an anycast address shared across a given MAP domain.  As MAP is
   stateless, any BR may be used at any time.  If the BR IPv6 address is
   anycast, the relay MUST use this anycast IPv6 address as the source
   address in packets relayed to CEs.

   Since MAP uses provider address space, no specific routes need to be
   advertised externally for MAP to operate in IPv6 or IPv4 BGP.
   However, if anycast is used for the MAP IPv6 relays, the anycast
   addresses must be advertised in the service provider's IGP.


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