Network Working Group                                              X. Li
Internet-Draft                                                    C. Bao
Intended status: Experimental                     CERNET Center/Tsinghua
Expires: April 15, August 22, 2013                                      University
                                                                  W. Dec
                                                                O. Troan
                                                           Cisco Systems
                                                           S. Matsushima
                                                        SoftBank Telecom
                                                             T. Murakami
                                                             IP Infusion
                                                        October 12, 2012
                                                       February 18, 2013

         Mapping of Address and Port using Translation (MAP-T)
                      draft-ietf-softwire-map-t-00
                      draft-ietf-softwire-map-t-01

Abstract

   This document specifies the "Mapping of Address and Port" double
   stateless NAT64 translation based solution (MAP-T) for providing
   shared or uniquely addressed IPv4 host device connectivity to and across
   an IPv6
   domain, domain.

Status of this Memo

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   This Internet-Draft will expire on April 15, August 22, 2013.

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   Copyright (c) 2012 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3  4
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  3  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3  4
   4.  Architecture . . . . . . . . . . . . . . . . . . . . . . . . .  5  6
   5.  Mapping Rules  . . . . . . . . . . . . . . . . . . . . . . . .  6  8
     5.1.  Basic  Port mapping rule (BMR) algorithm . . . . . . . . . . . . . . . . .  8 . 10
     5.2.  Forwarding  Basic mapping rule (FMR) (BMR) . . . . . . . . . . . . . . . . . 11
     5.3.  Default  Forwarding mapping rule (DMR) . . (FMR)  . . . . . . . . . . . . . . 12 14
     5.4.  MAP IPv6 Interface Identifier  . . . . . . . . . . . . . . 13
     5.5.  Port  Default mapping algorithm . . . . . . . . . . . rule (DMR) . . . . . . . 14
       5.5.1.  Bit Representation of the Algorithm . . . . . . . . . 14
       5.5.2.  GMA examples . . . . . . .
     5.5.  The IPv6 Interface Identifier  . . . . . . . . . . . . . . 15
       5.5.3.  GMA Excluded Ports . . . . . . . . . . . . . . . . . . 16
   6.  Configuration and Packet Forwarding  . . . . . . . . . . . . . . . . . . . . . . 16 15
     6.1.  IPv4 to IPv6 at the CE . . . . . . . . . . . . . . . . . . 16 15
     6.2.  IPv6 to IPv4 at the CE . . . . . . . . . . . . . . . . . . 17 16
     6.3.  IPv6 to IPv4 at the BR . . . . . . . . . . . . . . . . . . 17 16
     6.4.  IPv4 to IPv6 at the BR . . . . . . . . . . . . . . . . . . 17
   7.  ICMP Handling  . . . . . . . . . . . . . . . . . . . . . . . . 18 17
   8.  Fragmentation and Path MTU Discovery . . . . . . . . . . . . . 19 18
     8.1.  Fragmentation in the MAP domain  . . . . . . . . . . . . . 19 18
     8.2.  Receiving IPv4 Fragments on the MAP domain borders . . . . 19 18
     8.3.  Sending IPv4 fragments to the outside  . . . . . . . . . . 19
   9.  Usage Considerations . . . . . . . . . . . . . . . . . . . . . 20 19
     9.1.  Address Independence . . . . . . . . . . . . . . . . . . . 19
     9.2.  Mesh vs Hub and spoke with per subscriber rules mode . . . . . . . . . 20
     9.2. . . . . . . . 19
     9.3.  Communication with IPv6 servers in the MAP-T domain  . . . 20
     9.3. 19
     9.4.  Backwards compatibility  . . . . . . . . . . . . . . . . . 20
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 21
   13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23 22
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 23
     14.2. Informative References . . . . . . . . . . . . . . . . . . 23
   Appendix A.  Example  Examples of MAP-T translation . . . . . . . . . . . . 25
   Authors' Addresses
   Appendix B.  Port mapping algorithm  . . . . . . . . . . . . . . . 29
     B.1.  Bit Representation of the Algorithm  . . . . . . . . . 28

1.  Introduction

   Experiences from initial IPv6 deployments indicate that transitioning
   a network providers' domain fully to IPv6 requires not only the
   continued support of legacy IPv4 users connected to the boundary of
   that domain, allowing IPv4 address sharing, but also the need for
   carrying out IPv6-only operational practices in that domain [, also . . 30
     B.2.  GMA examples . . . . . . . . . . . . . . . . . . . . . . . 31
     B.3.  GMA Excluded Ports . . . . . . . . . . . . . . . . . . . . 31

   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 32

1.  Introduction

   Experiences from initial IPv6 deployments indicate that transitioning
   a network providers' domain fully to IPv6 requires not only the
   continued support of legacy IPv4 users connected to the boundary of
   that domain, allowing IPv4 address sharing, but also the need for
   carrying out IPv6-only operational practices in that domain [, also
   for traffic from IPv4 users.  The use of an double NAT64 translation
   based solutions is an optimal way to address these requirements,
   particularly in combination with stateless translation techniques
   that seek to minimize challenges outlined in
   [I-D.ietf-softwire-stateless-4v6-motivation].

   The Mapping of Address and Port - Translation (MAP-T) solution
   defined in this document is such a solution, that builds on existing
   stateless NAT64 techniques specified in [RFC6145], along with a
   stateless algorithmic address & transport layer port mapping scheme
   to allow the sharing of IPv4 addresses across an IPv6 network.  The
   MAP-T solution is closely related to MAP-E [I-D.ietf-softwire-map],
   with both utilizing the same algorithmic address and port mapping method, but
   differing in their choice of translation IPv6 domain transport, i.e.  Translation
   [RFC6145] and encapsulation [RFC2473]based [RFC2473].  The translation mode is
   required for environments where the IP encapsulation overhead or IPv6
   traffic interaction & processing (eg use of IPv6
   transports. only servers)
   requirements, or both, make the use of the encapsulation solution not
   attractive.

   A companion draft defines the DHCPv6 options for provisioning of MAP
   [I-D.mdt-softwire-map-dhcp-option], applicable to both MAP-T and
   MAP-E.

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 CEs and BRs connected to the
                           same IPv6 network.  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 MAP domain uses a different
                           mapping rule set.

   MAP node:               A device that implements MAP.

   MAP Border Relay (BR):  A MAP enabled router managed by the service
                           provider at the edge of a MAP domain.  A
                           Border Relay router 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 simply as 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 simply as a "CE" within the
                           context of MAP.

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

   Port-set ID (PSID):     Algorithmically identifies a set of ports
                           exclusively assigned to the 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
                           other means than MAP itself.  E.g.
                           Provisioned using DHCPv6 PD [RFC3633] 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.

   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.

   MRT:                    MAP Rule table.  Address and Port aware data
                           structure, supporting longest match lookups.
                           The MRT is used by the MAP forwarding
                           function.

4.  Architecture

   Figure 1 depicts the overall MAP-T architecture with IPv4 users (N N and M) networks
   M connected by means of MAP MAP-T CEs to an IPv6 network that is equipped
   with one or more MAP MAP-T BR.

         User N
       Private IPv4
      |  Network
      |
   O--+---------------O
   |  | MAP-T CE      |
   | +-----+--------+ |
   | NAPT44|  MAP-T | `-.
   | +-----+      | |  -._   ,-------.                     .------.
   |       +--------+ |   ,-'         `-.                ,-'       `-.
   O------------------O  /              \   O---------O /   Public   \
                         /   IPv6 only   \  |  MAP-T  |/     IPv4     \
                        (    Network      --+  Border +-   Network     )
                         \ (MAP-T Domain)/  |  Relay  |\              /
   O------------------O  \              /   O---------O \             /
   |    MAP-T CE      |   ;".         ,-'                `-.       ,-'
   | +-----+--------+ | ,"   `----+--'                      ------'
   | NAPT44|  MAP-T | | ,"        |
   | +-----+        | |        IPv6 Server(s)
   |   |   +--------+ |         (w/ v4 mapped
   O---.--------------O          address)
       |
         User M
       Private IPv4
         Network

                        Figure 1: Network Topology

   The MAP-T CE is responsible for connecting translating a users' private IPv4,
   along with any native IPv6 network IPv4
   space to a shared IPv4 address that is then mapped into the IPv6-only MAP-T domain. IPv6
   domain using stateless NAT64.

   The MAP-T BR is responsible for connecting external IPv4 networks to
   all devices in one or more MAP-T domains, using stateless NAT64 as
   extended by the MAP-T rules in this document.

   Besides the CE and BR, the MAP-T domain can contain any regular IPv6-
   only hosts/servers (i.e.
   Not equipped with MAP-T capabilities) IPv6-only hosts or servers that
   have an IPv4 mapped IPv6 address (IPv4-
   translatable (IPv4-translatable address per
   [RFC6052]) using a the prefix assigned to the MAP-T domain.  Communication with such devices is naturally possible
   in the domain, e.g.  An
   internal web server, or cache.  The MAP-T architecture support
   communication initiated towards such devices from both inside or
   outside the MAP-T domain including from any IPv4-only hosts.  In this mode, any IPv6-only
   servers SHOULD have both A and AAAA DNS server records.  An
   optional (not shown) DNS64 [RFC6147] becomes component would be required only when if
   the said IPv6 servers in the MAP-T domain devices are expected to themselves to initiate
   communication to internal/
   external IPv4-only entities. entities outside of the domain.

   Functionally the MAP-T CE and BR use existing standard functional extend well established building blocks, or extensions to these
   blocks as follows:

   o  A standard regular (NAT44) NAPT [RFC2663] function on a MAP CE is extended
      with support for restricting the allowable TCP/UDP ports for a
      given IPv4 address.  The enforcement of NAPT function, as well as the IPv4 address and port restriction is conditional upon the MAP CE's configuration as
      applied range used are
      determined by the network operator. MAP provisioning process and identical to MAP-E
      [I-D.ietf-softwire-map].

   o  A standard stateless NAT64 function [RFC6145]is [RFC6145] is extended for
      translating IPv4 traffic to IPv6, based on a lookup allow
      stateless mapping of source/
      destination IPv4 address + TCP/UDP and transport layer port or ICMP id information
      that is then mapped ranges to the source/destination IPv6 address.
      address space.  This algorithmic mapping is specified in section
      5.

   The operation of the above functions is modelled by means of Mapping
   Rules covered in Section 5.  Section 6 describes how these functional blocks are combined with the functions
   are used in packet forwarding operations.

5.  Mapping Rules

   EDITORIAL NOTE: This section is effectively identical of Section 4 to enable IPv4-IPv6 communication between
   the CE and BR and IPv6-only servers in the domain.

5. Mapping
   Rules

   A section[I-D.ietf-softwire-map], and will be re-factored &
   reconciled as the MAP-E draft finalizes.

   Any MAP node is needs to be provisioned with one or more mapping rules.

   Mapping rules are used differently depending on their function. rules,
   that form a mapping rule table.

   Every MAP node must MUST be provisioned with a Basic mapping rule.  This
   is used by Mapping Rule (BMR).
   All node's sharing a BMR are said to be part of the same MAP domain.
   On a CE this rule in combination with its natively assigned IPv6
   prefix, allows the CE node to configure determine its IPv4 address, IPv4 prefix or
   shared IPv4 address. address and port
   range.  This same basic rule BMR, when can also be used for
   forwarding, where to enable direct
   communication (a.k.a. mesh mode), that bypasses the BR, between CEs
   in the same MAP-T domain.  In practical terms this equates to the CE
   having an IPv4 destination address and optionally a
   destination port is mapped into an IPv6 address or prefix.
   Additional mapping rules are specified to allow route entry for e.g.  Multiple
   different the IPv4 subnets prefix assigned to exist that
   domain, and forwarding corresponding traffic using the parameters
   defined in the rule.  Additional mapping rules, termed Forward
   Mapping Rules (FMRs), are used to allow for multiple different IPv4
   subnets to exist within the domain and optimize forwarding between
   them.

   Traffic  These are equivalent to more specific IPv4 routes.

   Destination outside of the a MAP domain (i.e.  When the are reached (represented) by a
   Default Mapping Rule, that directs traffic to a MAP BR. i.e.  Traffic
   for destination IPv4
   address does addresses that do not match (using using a longest matching prefix)
   lookup to any Rule IPv4 prefix in the Rules database) will be forward using database, is forwarded to the Default
   mapping rule.  The Default mapping rule maps outside destinations
   MAP BR.  In MAP-T terms the CE uses stateless NAT64 to map such
   traffic to the BR's IPv6 address or prefix.

   In summary,  While there are can be only one Default
   Mapping Rule within a MAP domain, however there can be multiple BR's
   operating on that rule.

   In specific terms the three types of mapping rules: rules are defined as:

   1.  Basic Mapping Rule (BMR) - used for configuring the CE's IPv4
       address and/or port set assignment as well as deriving the MAP
       IPv6 address that the CE is to use.  There  For a given IPv6 prefix
       there can only be only one Basic
       Mapping BMR.  By default a BMR MUST NOT be used to
       create an IPv4 route entry for the Rule per End-user IPv6 prefix.  The IPv4 prefix.The BMR is
       composed of the following parameters:

       *  Rule IPv6 prefix (including prefix length)

       *  Rule IPv4 prefix (including prefix length)

       *  Rule EA-bits length (in bits)

       *  Optional Rule Port Parameters (optional)

   2.  Forwarding Mapping Rule - used for forwarding in within the MAP
       domain.
       The Basic Mapping Rule is also a Forwarding Mapping Rule.  Each Forwarding Mapping Rule will result in an entry in
       the MRT mapping rules table for the Rule IPv4 prefix + any port
       range.  The FMR consists of the following parameters: parameters (an
       attentive reader will note that a BMR can be set as an FMR,
       thereby enabling mesh-mode communication):

       *  Rule IPv6 prefix (including prefix length)

       *  Rule IPv4 prefix (including prefix length)

       *  Rule EA-bits length (in bits)

       *  Optional Rule Port Parameters (optional)

   3.  Default Mapping Rule - used for reaching destinations outside the
       MAP domain.  An  A DMR will result in an IPv4 0.0.0.0/0 entry is installed in the MRT for this
       rule.
       rules table.

       *  IPv6 prefix of the BR

       *  Rule BR IPv4 address (Optional - can be used for testing a
          BR's reachability)

   4.  Optional Rule Parameters - used to represent additional
       configuration settings.  Currently defined parameters are:

       *  Offset: Specifies the numeric value for the MAP algorithm's
          excluded port range/offset bits (A-bits).  Unless explicitly
          defined this value MUST default to 4.

   A MAP node finds its Basic Mapping Rule by doing a longest match
   between the End-user its assigned IPv6 prefix (e.g. via DHCPv6 PD) and the Rule
   IPv6 prefix in the Mapping Rule database.  The rule is then used for IPv4 prefix,
   address or shared address assignment.

   A MAP assigned IPv6 address prefix,
   is formed from the BMR Rule IPv6 prefix.  This
   address MUST prefix that the operator assigns to the CE using regular means
   like DHCP-PD.  It should be recognized by noted that this prefix is simply the
   prefix that any device, including non MAP node, typically a CE, CEs get assigned, i.e.  MAP
   does not require an additional prefix to be assigned.

   The selected BMR rule is then used for determining the IPv4 address
   and port range assignment as well as forming the CE's MAP IPv6
   address within the assigned IPv6 prefix.  This IPv6 address MUST be
   used
   to terminate for sending and receiving all MAP traffic received by the node. CE.

   Port-aware IPv4 entries in the MRT rules table are installed for all the
   Forwarding Mapping Rules and an IPv4 default route for the Default
   Mapping Rule.

   Forwarding rules are used to allow direct communication between MAP
   CEs, known as mesh mode.  In hub and spoke mode, there are no
   forwarding rules, all traffic MUST be forwarded directly to the BR
   using the Default Mapping Rule.

   The following subsections specify the MAP algorithm and Rule
   processing.

5.1.  Basic  Port mapping rule (BMR) algorithm

   The BMR port mapping algorithm is used in combination with the CE's IPv6 prefix domains whose rules allow IPv4
   address sharing.

   The simplest way to derive represent a port range is using a notation
   similar to CIDR [RFC4632].  For example the MAP IPv4 address, port-set first 256 ports are
   represented as port prefix 0.0/8.  The last 256 ports as 255.0/8.  In
   hexadecimal, 0x0000/8 (PSID = 0) and also 0xFF00/8 (PSID = 0xFF).

   To minimise dependencies between the MAP End-user IPv6 address.  The
   structure of prefix and the
   resulting port set, a MAP CE's IPv6 address is shown below, along with PSID of 0, would, in the naive representation
   assign the system ports [I-D.ietf-tsvwg-iana-ports] to the
   Interface-identifer user.
   Instead using an infix representation, and requiring that the first
   bit field (A) is defined greater than 0, the well known ports are excluded.

   This algorithm allocates ports to a given CE as a series of
   contiguous ranges.

                        0                   1
                        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
                        +-------+-----------+-----------+
          Ports in Section 5.4.      | p bits   A   |    PSID   |   q bits     M     |
                   +----------+         +------------+
                   |IPv4  sufx|         |Port-Set ID
       the CE port set  |
                   +----------+         +------------+
                   \          /    ____/    ________/
                     \       :  __/   _____/
                       \     : /     /  > 0  |           | any value |
                        +-------+-----------+-----------+
                        |a bits |  k bits   |  m bits   |

                              Figure 2: PSID

   A  For a > 0, A MUST be larger than 0.  This ensures that the
      algorithm excludes the system ports.

   a-bits  The number of offset bits.  The default Offset bits (a) are:
      4.  To simplify the port mapping algorithm the defaults are chosen
      so that the PSID field starts on a nibble boundary and the
      excluded port range (0-1023) is extended to 0-4095.

   PSID  The Port Set Identifier.  Different Port-Set Identifiers (PSID)
      MUST have non-overlapping port-sets.

   k-bits  The length in bits of the PSID field.  The sharing ratio is
      k^2.  The number of ports assigned to the user is 2^(16-k) - 2^m
      (excluded ports)

   M  The contiguous ports.

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

   This algorithm allocates ports to a given CE as a series of
   contiguous ranges.

5.2.  Basic mapping rule (BMR)

   The Basic Mapping Rule is mandatory, used by the CE to provision
   itself with an IPv4 prefix, IPv4 address or shared IPv4 address.

    |     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 2: 3: IPv6 address format

   The Rule IPv6 prefix is the part of the End-user IPv6 prefix (i.e.
   the regular IPv6 prefix or address that is assigned to any IPv6
   device) that is
   common among all CEs using the same Basic Mapping Rule within the MAP
   domain.  The
   Embedded Address EA bits (EA bits) are encode the CE specific IPv4 address and port
   information.  The EA bits, which are unique per end user within
   that Rule IPv6 prefix (some readers may want to think of these as
   "MAP customer identifier" bits for a given MAP domain covered by the Rule IPv6 prefix).  When present, the EA bits encode the CE specific
   prefix, can contain a full or part of an IPv4 prefix or address, and address and, in the
   shared IPv4 address case contain case, a Port-Set Identifier (PSID) that ultimately
   determines the allowed port-range for the CE. (PSID).  An EA-bit
   length of 0 signifies that all relevant MAP IPv4 addressing
   information is passed directly in the BMR rule rule, and not derived from
   the End-user IPv6 prefix.

   The MAP IPv6 address is created by concatenating the End-user IPv6
   prefix with the MAP subnet-id (if the End-user IPv6 prefix is shorter
   than 64 bits) and the interface-id as specified in Section 5.5.

   The MAP subnet ID is defined to be the first subnet (all bits set to
   zero) of length s required to make n+o+s=64.  A
   zero).  Unless configured differently, a MAP CE node MUST reserve the
   first IPv6 prefix in a an End-user IPv6 prefix for the purpose of MAP.

   CE's

   The MAP IPv6 address is created by concatenating combining the End-user End-User IPv6 prefix with
   the all zeros MAP subnet-id and the interface-id as
   specified in Section 5.4.

   The CE's IPv4 address is synthesized from the CE's MAP IPv6 address and
   the parameters obtained in the BMR, namely by extracting the IPv4
   suffix from the EA-bits, if any, and combining it with the Rule's
   IPv4 prefix.  Figures below show the synthesis for cases of a Shared
   IPv4 address, and a non-shared, complete, IPv4 address: interface identifier.

                           Shared IPv4 address:

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

                       Figure 3: 4: Shared IPv4 address

                          Complete IPv4 address:

                   |   r bits    |        p bits       |
                   +-------------+---------------------+
                   |  Rule IPv4  | IPv4 Address suffix |
                   +-------------+---------------------+
                   |            32 bits                |

                      Figure 4: 5: Complete IPv4 address
                               IPv4 prefix:

                   |   r bits    |        p bits       |
                   +-------------+---------------------+
                   |  Rule IPv4  | IPv4 Address suffix |
                   +-------------+---------------------+
                   |           < 32 bits               |

                           Figure 5: 6: IPv4 prefix

   The length of 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, are concatenated with the Rule IPv4 prefix (r
   bits).

   The offset of the EA bits field in the IPv6 address is equal to the
   BMR Rule IPv6 prefix length.  The length of the EA bits field (o) is
   given by the BMR Rule EA-bits length. length, and can be between 0 and 48.
   The sum of the Rule IPv6 Prefix length and the Rule EA-bits length
   MUST be less or equal than the End-user IPv6 prefix length.

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

   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.

   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 EA bits is: o - p.

   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 also the PSID is provisioned using e.g.  The DHCP
   option.

   In the following examples, only the suffix (last 8 bits)

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

5.3.  Forwarding mapping rule (FMR)

   The Forwarding Mapping Rule is optional, and used in mesh mode to
   merit direct CE to CE connectivity.

   On adding an FMR rule, an IPv4
   address route is embedded installed in the EA bits (r = 24), while Rules table
   for the Rule IPv4 prefix.

   On forwarding an IPv4 packet, a best matching prefix
   (first 24 bits) look up is given done
   in the BMR Rule Rules table and the correct FMR is chosen.

   |        32 bits           |         |    16 bits        |
   +--------------------------+         +-------------------+
   | IPv4 prefix.

   Example:

     Given:
      End-user IPv6 prefix:  2001:db8:0012:3400::/56
      Basic Mapping Rule:    {2001:db8:0000::/40 (Rule IPv6 prefix),
                             192.0.2.0/24 (Rule IPv4 prefix),
                             16 (Rule EA-bits length)}
      Sharing ratio:         256 (16 - (32 - 24) = 8. 2^8 = 256)
      PSID offset:           4 (default value as per section 5.1.3)

     We get IPv4 address and port-set:
      EA bits offset:       40
      IPv4 suffix bits (p): Length of IPv4 address (32) -
                            IPv4 prefix length (24) = 8
      IPv4 address:         192.0.2.18 (0x12)

      PSID start:           40 + p = 40 + 8 = 48
      PSID length:          o - p = 16 (56 - 40) - 8 = 8
      PSID:                 0x34
      Port-set-1:        4928, 4929, 4930, 4931, 4932, 4933, 4934, 4935,
                         4936, 4937, 4938, 4939, 4940, 4941, 4942, 4943
      Port-set-2:        9024, 9025, 9026, 9027, 9028, 9029, 9030, 9031,
                         9032, 9033, 9034, 9035, 9036, 9037, 9038, 9039
      ...
      Port-set-15:          62272, 62273, 62274, 62275,
                            62276, 62277, 62278, 62279,
                            62280, 62281, 62282, 62283,
                            62284, 62285, 62286, 62287,

5.2.  Forwarding mapping rule (FMR)

   On adding an FMR rule, an IPv4 route is installed in the MRT for the
   Rule IPv4 prefix.

   On forwarding an IPv4 packet, a best matching prefix lookup is done
   in the MRT and the correct FMR is chosen.

   |        32 bits           |         |    16 bits        |
   +--------------------------+         +-------------------+
   | IPv4 destination address |         | destination address |         |  IPv4 dest port   |
   +--------------------------+         +-------------------+
                   :          :           ___/       :
                   | p bits   |          /  q bits   :
                   +----------+         +------------+
                   |IPv4  sufx|         |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  --->|

     Given:
      IPv4 destination address: 192.0.2.18
      IPv4 destination port:    9030
      Forwarding Mapping Rule:  {2001:db8:0000::/40 (Rule IPv6 prefix),
                                 192.0.2.0/24 (Rule IPv4 prefix),
                                 16 (Rule EA-bits length)}
      PSID offset:              4 (default value as per section 5.1.3)

     We get IPv6 address:
      IPv4 suffix bits (p): 32 - 24 = 8 (18 (0x12))
      PSID length:          8
      PSID:                 0x34 (9030 (0x2346))
      EA bits:              0x1234
      MAP IPv6 address:     2001:db8:0012:3400:00c0:0002:1200:3400

                  Figure 6: 7: Deriving of MAP IPv6 address

5.3.

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

5.4.  Default mapping rule (DMR)

   The Default Mapping rule is used to represent as IPv6 destinations reach all IPv4 destinations
   outside of the MAP IPv4 MAP-T domain.  For MAP-T, the DMR is specified in
   terms of the BR IPv6 prefix.  The Rule IPv4 prefix in the MRT is: 0.0.0.0/0, i.e. the default that MAP-T CEs will use to form IPv6
   addresses out of IPv4 route.

   There MUST be only one Default Mapping Rule within a MAP domain. destination addresses.

    Default Mapping Rule:
         {2001:db8:0001::/Prefix-length (Rule IPv6 prefix),
          0.0.0.0/0 (Rule IPv4 prefix)}
                       Example: Default Mapping Rule

   The Deafult Rule's IPv6 prefix is combined by a MAP-T CE with the
   IPv4 destination addresses, using RFC6052, to form a full IPv6
   destination address for any IPv4 destination following the IPv4
   default route.  Figure 7 below shows such an address format.

   Note that the BR prefix-length is variable and can be both shorter or
   longer than 64 bits, up to 96 bits.  In the respective cases the IPv4
   address and the BR prefix are shifted and "bit spread" across the
   fixed u-octet boundary as per [RFC6052].  All trailing bits after the
   IPv4 address are set to 0x0.

   <---------- 64 ------------>< 8 ><----- 32 -----><--- 24 --->
   +--------------------------+----+---------------+-----------+
   |      BR IPv6 prefix      | u  | IPv4 address  |     0     |
   +--------------------------+----+---------------+-----------+

                    Figure 7: IPv6-IPv4 Mapped Address

5.4.  MAP

5.5.  The IPv6 Interface Identifier

   The IPv6 Interface identifier format of a MAP node is based on the
   format specified in section 2.2 of [RFC6052], with the added PSID
   field if present, as shown in Figure 8.

   +--+---+---+---+---+---+---+---+---+
   |PL|   8 described below.

   |        128-n-o-s bits            |
   | 16  24 bits|    32  40  48  56 bits     |
   +--+---+---+---+---+---+---+---+---+
   |64| u 16 bits|
   +--------+----------------+--------+
   |   0    |  IPv4 address  |  PSID  | 0 |
   +--+---+---+---+---+---+---+---+---+
   +--------+----------------+--------+

                                 Figure 8: MAP IPv6 Interface Identifier

   The encoding of the full IPv4 address into the interface identifier
   into the IPv6 addresses have been shown to be useful for
   troubleshooting. 8

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

5.5.  Port mapping algorithm

   The Generalized Modulus Algorithm (GMA)

   If the End-user IPv6 prefix length is used in MAP domains whose larger than 64, the most
   significant parts of the interface identifier is overwritten by the
   prefix.

6.  Configuration and Packet Forwarding

   The mapping rules allow IPv4 address sharing.  Each and architectural building blocks are combined at
   the CE in such a domain has an
   IPv4 address and a unique Port-Set Identifier (PSID), that is derived
   by means BR to enable IPv4-IPv6 communication as follows.

   The MAP-T CE and BR are set-up as described in Section 7 of
   [I-D.ietf-softwire-map] with the BMR.  For a given only difference being that they are
   set-up to operate in translation mode rather than encapsulation.

6.1.  IPv4 address, the algorithm allows
   each PSID to be processed to reveal a set of unique non-overlapping
   ports, or alternatively for any given IPv6 at the CE

   A MAP-T CE receiving IPv4 packets SHOULD perform NAT44 function first
   and create appropriate NAT44 stateful bindings.  The resulting IPv4
   packets MUST contain the source IPv4 address and source transport
   port number assigned to derive the PSID it
   corresponds to.  Two extreme cases supported CE by algorithm are: (1) means of the MAP Basic Mapping Rule
   (BMR).

   The IPv4 traffic is subject to a longest IPv4 address + port numbers are not contiguous for each PSID, but uniformly
   distributed across match
   MAP rule selection using the port range (0-65535); (2) MRT, which then determines the port numbers are
   contiguous in a single range for each PSID.

   For a given sharing ratio (R)
   subsequent NAT64 operation.  By default, all traffic is matched to
   default mapping rule (DMR), and subject to the maximum number of contiguous
   ports (M), stateless NAT64
   operation using the GMA DMR parameters for the MAP algorithm is defined as:

   1.  The port (P) of and NAT64.

   An optional mapping rule, known as a given PSID (K) is composed of:

   P = R * M * j + M * K + i

       Where:

       *  PSID: K = 0 forward mapping rule (FMR), can
   be used when forwarding to R - 1

       *  Port range index: j = (4096 / M) / R destinations that correspond to ((65536 / M) / R) - 1,
          if a specific
   IPv4+port range in the MAP-T domain i.e.  Typically the IPv4 address
   and port numbers (0 - 4095) are excluded.

       *  Contiguous Port index: i = 0 to M - 1

   2.  The PSID (K) range of a given port number (P) is determined by:

   K = (floor(P/M)) % R

       Where:

       *  % is the modulus operator

       *  floor(arg) another MAP-T CE, aka mesh-mode.  Traffic that is
   matched to such a function that returns the largest integer not
          greater than arg.

5.5.1.  Bit Representation of rule is subject to the Algorithm

   Given a sharing ratio (R=2^k), stateless NAT64 operation
   using the maximum number of contiguous ports
   (M=2^m), FMR parameters for any PSID (K) the MAP algorithm and available ports (P) can be represented
   as:

   0                          8                         15
   +---------------+----------+------+-------------------+
   |                       Port  (P)                     |
   ----------------+-----------------+-------------------+
   |        j      |   PSID (K)      |        M  (i)     |
   +---------------+----------+------+-------------------+
   |<----a bits--->|<-----k bits---->|<------m bits----->|

                       Figure 9: Bit representation

   Where j stateless NAT64.

   A MAP-T CE MUST support a default mapping rule and i are the same indexes defined in the port SHOULD support one
   or more forward mapping
   algorithm.

   For any port number, rules.

6.2.  IPv6 to IPv4 at the PSID can be obtained by a bit mask
   operation.

   For a > 0, j MUST be larger than 0.  This ensures CE

   A MAP-T CE receiving an IPv6 packet performs its regular IPv6
   operations, whereby only packets that are addressed to the algorithm
   excludes MAP-T BMR
   addresses are forwarded to the system ports ([I-D.ietf-tsvwg-iana-ports]).  For a = 0,
   j MAY CE's stateless NAT64 function.  All
   other IPv6 traffic SHOULD be 0 to allow forwarded as per the CE's IPv6 routing
   rules.  The CE SHOULD check that MAP-T received packets' transport-
   layer destination port number is in the range configured by MAP for
   the provisioning of CE and the system ports.

5.5.2.  GMA examples

   For example, for R = 1024, PSID offset: a = 4 and PSID length: k = 10
   bits

             Port-set-1                Port-set-2
   PSID=0   | 4096, 4097, 4098, 4099, | 8192,  8193,  8194,  8195, | ...
   PSID=1   | 4100, 4101, 4102, 4103, | 8196,  8197,  8198,  8199, | ...
   PSID=2   | 4104, 4105, 4106, 4107, | 8200,  8201,  8202,  8203, | ...
   PSID=3   | 4108, 4109, 4110, 4111, | 8204,  8205,  8206,  8207, | ...
   ...
   PSID=1023| 8188, 8189, 8190, 8191, | 12284, 12285, 12286, 12287,| ...

                    Example 1: with offset = 4 (a = 4)

   For example, for R = 64, a = 0 (PSID offset = 0 CE SHOULD drop any non conforming packet and PSID length = 6
   bits):

             Port-set
   PSID=0   | [   0 - 1023]
   PSID=1   | [1024 - 2047]
   PSID=2   | [2048 - 3071]
   PSID=3   | [3072 - 4095]
   ...
   PSID=63  | [64512 - 65535]
                    Example 2: respond
   with offset = 0 (a = 0)

5.5.3.  GMA Excluded Ports

   By default an ICMPv6 "Address Unreachable" (Type 1, Code 3).

   The CE's stateless NAT64 function MUST derive the GMA ensures that a number IPv4 source and
   destination addresses as per Section 5 of "well known" ports are
   excluded from use by this document and MUST
   replace the algorithm.  This number IPv6 header with an IPv4 header in accordance with
   [RFC6145].  The resulting IPv4 packet is determined by then forwarded to the
   number of offset bits (a), in CE's
   NAPT function, when this is enabled, where the figure above.  This value can destination IPv4
   address and port number MUST be
   optionally provisioned via the "Rule Port Mapping Parameters" in mapped to their original value,
   before being forwarded according to the
   Basic Mapping Rule.  In CE's regular IPv4 rules.
   When the absence of such provisioning, NAPT function is not enabled, the
   defaults are:

   o  Excluded ports : 0-4095

   o  Offset bits (a) : 4

   For (a) offset bits, traffic from the range of excluded ports stateless
   NAT64 function is 0 directly forwarded according to 2 ^ (16-a) -
   1.

6.  Packet Forwarding

   The mapping rules and architectual building blocks are combined at the CE and BR to enable IPv4-IPv6 communication as follows.

6.1. CE's IPv4 to
   rules.

6.3.  IPv6 to IPv4 at the CE BR

   A MAP-T CE BR receiving IPv4 packets SHOULD perform NAT44 function first
   and create appropriate NAT44 stateful bindings.  The resulting IPv4 IPv6 packets MUST contain the source IPv4 address and source transport
   port number assigned to the CE by means of the select a best matching MAP Basic Mapping Rule
   (BMR).

   The IPv4 traffic is subject to
   rule based on a longest IPv4 address + port match
   MAP rule selection using the MRT, which then determines the
   subsequent NAT64 operation.  By default, all traffic is matched to
   default mapping rule (DMR), and subject to the stateless NAT64
   operation using of the DMR parameters for packets' source address
   against the BR's configured MAP algorithm and NAT64.

   An optional mapping rule, known BMR prefix(es), as well as a forward mapping match of
   the packet destination address against the configured FMR prefix(es).
   The selected MAP rule (FMR), can
   be used when forwarding to destinations that correspond allows the BR to a specific
   IPv4+port determine the CE's range in from
   the MAP-T domain i.e.  Typically port-set-id contained in the IPv4 address
   and port range of another MAP-T CE, aka mesh-mode.  Traffic that is
   matched to such a rule is subject to the stateless NAT64 operation
   using the FMR parameters for the MAP algorithm and stateless NAT64.

   A MAP-T CE source IPv6 address.  The BR MUST support
   perform a default mapping rule and SHOULD support one
   or more forward mapping rules.

6.2.  IPv6 to IPv4 at validation of the CE

   A MAP-T CE receiving an IPv6 packet performs its regular IPv6
   operations, whereby only packets that are addressed to consistency of the MAP-T BMR
   addresses are forwarded to source against the CE's stateless NAT64 function.  All
   other IPv6 traffic SHOULD be forwarded as per
   allowed values from the CE's IPv6 routing
   rules.  The CE SHOULD check that MAP-T received packets' transport-
   layer destination identified port-range port.  If the packets
   source port number is in found to be outside the range configured by MAP allowed for
   the CE this
   CE-index and the CE SHOULD BMR, the BR MUST drop any non conforming the packet and respond with an
   ICMPv6 "Address Unreachable" "Destination Unreachable, Source address failed ingress/egress
   policy" (Type 1, Code 3). 5).

   The CE's stateless NAT64 function BR MUST derive the IPv4 source and destination IPv4 addresses as per
   Section 5 of this document and MUST
   replace translate the IPv6 header with an to IPv4 header in accordance with headers
   following [RFC6145].  The resulting IPv4 packet is packets are then forwarded to the CE's
   NAPT function, when this is enabled, where the destination IPv4
   address and port number MUST be mapped to their original value,
   before being forwarded according passed to the CE's
   regular IPv4 rules.
   When the NAPT function is not enabled, the traffic from the stateless
   NAT64 function is directly forwarded according to forwarding by the CE's BR.

6.4.  IPv4
   rules.

6.3.  IPv6 to IPv4 IPv6 at the BR

   A MAP-T BR receiving IPv6 packets MUST select a best matching MAP
   rule based on a longest address match of the packets' source address
   against the BR's configured MAP BMR prefix(es), as well as a match of
   the packet destination address against the configured FMR prefix(es).
   The selected MAP rule allows the BR to determine the CE's range from
   the port-set-id contained in the source IPv6 address.  The BR MUST
   perform a validation of the consistency of the source against the
   allowed values from the identified port-range port.  If the packets
   source port number is found to be outside the range allowed for this
   CE-index and the BMR, the BR MUST drop the packet and respond with an
   ICMPv6 "Destination Unreachable, Source address failed ingress/egress
   policy" (Type 1, Code 5).

   The BR MUST derive the source and destination IPv4 addresses as per
   Section 5 of this document and translate the IPv6 to IPv4 headers
   following [RFC6145].  The resulting IPv4 packets are then passed to
   regular IPv4 forwarding by the BR.

6.4.  IPv4 to IPv6 at the BR

   A MAP-T BR receiving IPv4 IPv4 packets uses a longest match IPv4 + port
   lookup to select the target MAP-T domain and rule.  The BR MUST then
   derive the IPv6 source and destination addresses from the IPv4 source
   and destination address and port as per Section 5 of this document.
   Following this, the BR MUST translate the IPv4 to IPv6 headers
   following [RFC6145].  The resulting IPv6 packets are then passed to
   regular IPv6 forwarding.

   Note that the operation of a BR when forwarding to MAP-T domains that
   do not utilize IPv4 address sharing, is the same as stateless IPv4/
   IPv6 translation.

7.  ICMP Handling

   ICMP messages need to be supported in MAP-T domain and also across
   it, taking into consideration also the NAPT component and best
   current practice documented in [RFC5508] along with some additional
   specific considerations.

   MAP-T CEs and BRs MUST follow ICMP/ICMPv6 translation as per
   [RFC6145], with the following extension to cover the address sharing/
   port-range feature.

   Unlike TCP and UDP, which each provide two port fields to represent
   both source and destination, the ICMP/ICMPv6 [RFC0792], [RFC4443]
   Query message header has only one ID field which needs to be used to
   identify a sending IPv4 host.

   When receiving IPv4 ICMP messages, the MAP-T CE SHOULD rewrite the ID
   field to a port value derived from the Port-set-id.  A BR MUST
   translate the resulting ICMPv6 packets back to ICMP preserving the ID
   field on its way to an IPv4 destination.

   In the return path, when MAP-T BR receives an ICMP packet containing
   an ID field which is bound for a shared address in the MAP-T domain,
   the MAP-T BR SHOULD use the ID value as a substitute for the
   destination port in determining the IPv6 destination address.  In all
   other cases, the MAP-T BR MUST derive the destination IPv6 address by
   simply mapping the destination IPv4 address without additional port
   info.

   If a MAP BR receives an ICMP error message on its IPv4 interface, the
   MAP BR should translate the ICMP message to an appropriate ICMPv6
   message, as per [RFC6145] and forward it to the intended MAP CE with
   the following considerations.  If IPv4 address is not shared, the MAP
   BR generates a CE IPv6 address from the IPv4 destination address in
   the ICMP error message and encapsulates the ICMP message in IPv6.  If
   the IPv4 address is shared, the MAP BR derives an original IPv4
   packet from the ICMP payload and generates a CE IPv6 address from the
   source address and the source port in the original IPv4 packet.

8.  Fragmentation and Path MTU Discovery

   Due to the different sizes of the IPv4 and IPv6 header, handling the
   maximum packet size is relevant for the operation of any system
   connecting the two address families.  There are three mechanisms to
   handle this issue: Path MTU discovery (PMTUD), fragmentation, and
   transport-layer negotiation such as the TCP Maximum Segment Size
   (MSS) option [RFC0897].  MAP uses all three mechanisms to deal with
   different cases.

8.1.  Fragmentation in the MAP domain

   Translating an IPv4 packet to carry it across the MAP domain will
   increase its size by 20 bytes respectively.  It is strongly
   recommended that the MTU in the MAP domain is well managed and that
   the IPv6 MTU on the CE WAN side interface is set so that no
   fragmentation occurs within the boundary of the MAP domain.

   Fragmentation in MAP-T domain is to be handled as described in
   section 4 and 5 of [RFC6145].

8.2.  Receiving IPv4 Fragments on the MAP domain borders

   Forwarding of an IPv4 packet received from the outside of the MAP
   domain requires the IPv4 destination address and the transport
   protocol destination port.  The transport protocol information is
   only available in the first fragment received.  As described in
   section 5.3.3 of [RFC6346] a MAP node receiving an IPv4 fragmented
   packet from outside has to reassemble the packet before sending the
   packet onto the MAP link.  If the first packet received contains the
   transport protocol information, it is possible to optimize this
   behavior by using a cache and forwarding the fragments unchanged.  A
   description of this algorithm is outside the scope of this document.

8.3.  Sending IPv4 fragments to the outside

   If two IPv4 host behind two different MAP CE's with the same IPv4
   address sends fragments to an IPv4 destination host outside the
   domain.  Those hosts may use the same IPv4 fragmentation identifier,
   resulting in incorrect reassembly of the fragments at the destination
   host.  Given that the IPv4 fragmentation identifier is a 16 bit
   field, it could be used similarly to port ranges.  A MAP CE SHOULD
   rewrite the IPv4 fragmentation identifier to be within its allocated
   port set.

9.  Usage Considerations

9.1.  Hub and spoke with per subscriber rules

   Existing IPv4 service can be realized with  Address Independence

   The MAP using a solution supports use and configuration of domains in so
   called 1:1 mode (meaning 1 mapping rule set per subscriber.  By embedding no part of the IPv4 address in CE), which allows
   complete independence between the IPv6
   prefix, no dependency between prefix assigned to the two CE and
   the IPv4 address families is created. and/or port-range it uses.  This may be useful is achieved in all
   cases where when the IPv6 address allocation is
   sparse, or for other reasons it EA-bit length is difficult set to create efficient
   mapping rules. 0.

   The operator has to the choice of provisioning a full IPv4 address to constraint imposed is that each such MAP domain be composed of
   just 1 MAP CE which has a predetermined IPv6 prefix, i.e.  The BR
   would be configured with a rule-set per CPE, where the end-user, or FMR would
   uniquely describe the IPv6 prefix of a shared given CE.  Each CE would have
   a distinct BMR, that would fully describe that CE's IPv4 address by also provisioning the address, and
   PSID
   in the DHCPv6 option.  A hybrid if any.

9.2.  Mesh vs Hub and spoke mode

   The hub and spoke mode of this use case communication, whereby all traffic sent by
   a MAP-T CE is to provision forwarded via a BR, and the
   full IPv4 address mesh mode, whereby a CE is
   directly able to forward traffic to another CE in the DHCPv6 option, while embedding the PSID in same MAP-T
   domain, are governed by the IPv6 prefix.  That activation of a Basic Mapping Rule as a
   Forward Mapping Rule.  By default, a MAP CE will result in one mapping rule per interpret its BMR
   only to setup its IPv4
   address, e.g.  With a sharing ratio of 64, one rule per 64 customers.

9.2. parameters and IPv6 MAP address and not as an
   FMR.

9.3.  Communication with IPv6 servers in the MAP-T domain

   MAP-T allows communication between both IPv4-only and any IPv6
   enabled end hosts, with native IPv6-only servers which are using
   IPv4-mapped IPv6 address based on DMR in the MAP-T domain.  In this
   mode, the IPv6-only servers SHOULD have both A and AAAA records in
   DNS [RFC6219].  DNS64 [RFC6147] become required only when IPv6
   servers in the MAP-T domain are expected themselves to initiate
   communication to external IPv4-only hosts.

9.3.

9.4.  Backwards compatibility

   A MAP-T CE, in all configuration modes, is by default compatible with
   regular [RFC6146] stateful NAT64 devices that are configured to use/
   advertise BR prefixes.  This allows the use of MAP-T CEs in
   environments that require statistical multiplexing of IPv4 addresses
   while being able to compromise on the stateful nature.  Furthermore,
   a MAP-T CE configured to operate without address sharing (no PSID) is
   compatible with any stateless NAT64 [RFC6146] devices positioned as
   BRs.

10.  IANA Considerations

   This specification does not require any IANA actions.

11.  Security Considerations

   Spoofing attacks:  With consistency checks between IPv4 and IPv6
      sources that are performed on IPv4/IPv6 packets received by MAP
      nodes, MAP does not introduce any new opportunity for spoofing
      attacks that would not already exist in IPv6.

   Denial-of-service attacks:  In MAP domains where IPv4 addresses are
      shared, the fact that IPv4 datagram reassembly may be necessary
      introduces an opportunity for DOS attacks.  This is inherent to
      address sharing, and is common with other address sharing
      approaches such as DS-Lite and NAT64/DNS64.  The best protection
      against such attacks is to accelerate IPv6 enablement in both
      clients and servers so that, where MAP is supported, it is less
      and less used.

   Routing-loop attacks:  This attack may exist in some automatic
      tunneling scenarios are documented in [RFC6324].  They cannot
      exist with MAP because each BRs checks that the IPv6 source
      address of a received IPv6 packet is a CE address based on
      Forwarding Mapping Rule.

   Attacks facilitated by restricted port set:  From hosts that are not
      subject to ingress filtering of [RFC2827], some attacks are
      possible by an attacker injecting spoofed packets during ongoing
      transport connections ([RFC4953], [RFC5961], [RFC6056].  The
      attacks depend on guessing which ports are currently used by
      target hosts, and using an unrestricted port set is preferable,
      i.e.  Using native IPv6 connections that are not subject to MAP
      port range restrictions.  To minimize this type of attacks when
      using a restricted port set, the MAP CE's NAT44 filtering behavior
      SHOULD be "Address-Dependent Filtering".  Furthermore, the MAP CEs
      SHOULD use a DNS transport proxy function to handle DNS traffic,
      and source such traffic from IPv6 interfaces not assigned to
      MAP-T.  Practicalities of these methods are discussed in Section
      5.9 of [I-D.dec-stateless-4v6].

   [RFC6269] outlines general issues with IPv4 address sharing.

12.  Contributors

   Mohamed Boucadair, Gang Chen, Maoke Chen, Wojciech Dec, Xiaohong
   Deng, Jouni Korhonen, Tomasz Mrugalski, Jacni Qin, Chunfa Sun, Qiong
   Sun, Leaf Yeh.

   The following are the authors who provided a major contribution to
   this document:

      Chongfeng Xie (China Telecom)

      Room 708, No.118, Xizhimennei Street Beijing 100035 CN

      Phone: +86-10-58552116

      Email: xiechf@ctbri.com.cn

      Qiong Sun (China Telecom)

      Room 708, No.118, Xizhimennei Street Beijing 100035 CN

      Phone: +86-10-58552936

      Email: sunqiong@ctbri.com.cn
      Rajiv Asati (Cisco Systems)

      7025-6 Kit Creek Road Research Triangle Park NC 27709 USA

      Email: rajiva@cisco.com

      Gang Chen (China Mobile)

      53A,Xibianmennei Ave. Beijing 100053 P.R.China

      Email: chengang@chinamobile.com

      Wentao Shang (CERNET Center/Tsinghua University)

      Room 225, Main Building, Tsinghua University Beijing 100084 CN

      Email: wentaoshang@gmail.com

      Guoliang Han (CERNET Center/Tsinghua University)

      Room 225, Main Building, Tsinghua University Beijing 100084 CN

      Email: bupthgl@gmail.com

      Yu Zhai CERNET Center/Tsinghua University

      Room 225, Main Building, Tsinghua University Beijing 100084 CN

      Email: jacky.zhai@gmail.com

13.  Acknowledgements

   This document is based on the ideas of many.  In particular Remi
   Despres, who has tirelessly worked on generalized mechanisms for
   stateless address mapping.

   The authors would like to thank Guillaume Gottard, Dan Wing, Jan
   Zorz, Necj Scoberne, Tina Tsou for their thorough review and
   comments.

14.  References

14.1.  Normative References

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

   [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
              October 2010.

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

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

14.2.  Informative References

   [I-D.dec-stateless-4v6]
              Dec, W., Asati, R., and H. Deng, "Stateless 4Via6 Address
              Sharing", draft-dec-stateless-4v6-04 (work in progress),
              October 2011.

   [I-D.ietf-softwire-map]
              Troan, O., Dec, W., Li, X., Bao, C., Matsushima, S., and
              T. Murakami, "Mapping of Address and Port with
              Encapsulation (MAP)", draft-ietf-softwire-map-02 draft-ietf-softwire-map-04 (work in
              progress), September 2012. February 2013.

   [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-04
              draft-ietf-softwire-stateless-4v6-motivation-05 (work in
              progress), August November 2012.

   [I-D.ietf-tsvwg-iana-ports]
              Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry",
              draft-ietf-tsvwg-iana-ports-10 (work in progress),
              February 2011.

   [I-D.mdt-softwire-map-dhcp-option]
              Mrugalski, T., Troan, O., Bao, C., and W. Dec, "DHCPv6
              Options for Mapping of Address and Port",
              draft-mdt-softwire-map-dhcp-option-03 (work in progress),
              July 2012.

   [I-D.xli-behave-divi]
              Shang, W., Li, X., Zhai, Y., and C. Bao, "dIVI: Dual-
              Stateless IPv4/IPv6 Translation", draft-xli-behave-divi-04
              (work in progress), October 2011.

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

   [RFC0897]  Postel, J., "Domain name system implementation schedule",
              RFC 897, February 1984.

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

   [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and Considerations",
              RFC 2663, August 1999.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, May 2000.

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              December 2003.

   [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.

   [RFC4632]  Fuller, V. and T. Li, "Classless Inter-domain Routing
              (CIDR): The Internet Address Assignment and Aggregation
              Plan", BCP 122, RFC 4632, August 2006.

   [RFC4953]  Touch, J., "Defending TCP Against Spoofing Attacks",
              RFC 4953, July 2007.

   [RFC5508]  Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
              Behavioral Requirements for ICMP", BCP 148, RFC 5508,
              April 2009.

   [RFC5961]  Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
              Robustness to Blind In-Window Attacks", RFC 5961,
              August 2010.

   [RFC6056]  Larsen, M. and F. Gont, "Recommendations for Transport-
              Protocol Port Randomization", BCP 156, RFC 6056,
              January 2011.

   [RFC6147]  Bagnulo, M., Sullivan, A., Matthews, P., and I. van
              Beijnum, "DNS64: DNS Extensions for Network Address
              Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
              April 2011.

   [RFC6219]  Li, X., Bao, C., Chen, M., Zhang, H., and J. Wu, "The
              China Education and Research Network (CERNET) IVI
              Translation Design and Deployment for the IPv4/IPv6
              Coexistence and Transition", RFC 6219, May 2011.

   [RFC6269]  Ford, M., Boucadair, M., Durand, A., Levis, P., and P.
              Roberts, "Issues with IP Address Sharing", RFC 6269,
              June 2011.

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

Appendix A. Automatic Tunnels: Problem Statement and Proposed
              Mitigations", RFC 6324, August 2011.

Appendix A.  Examples of MAP-T translation
   Example 1 - BMR:

      Given the MAP domain information and an IPv6 address of
      an endpoint:

      IPv6 prefix assigned to the end user:  2001:db8:0012:3400::/56
      Basic Mapping Rule:  {2001:db8:0000::/40 (Rule IPv6 prefix),
         192.0.2.0/24 (Rule IPv4 prefix), 16 (Rule EA-bits length)}
      PSID length: (16 - (32 - 24) = 8. (Sharing ratio of 256)
      PSID offset:  4

      A MAP node (CE or BR) can via the BMR, or equivalent FMR,
      determine the IPv4 address and port-set as shown below:

      EA bits offset:  40
      IPv4 suffix bits (p)  Length of IPv4 address (32) - IPv4 prefix
         length (24) = 8
      IPv4 address  192.0.2.18 (0xc0000212)
      PSID start:  40 + p = 40 + 8 = 48
      PSID length:  o - p = (56 - 40) - 8 = 8
      PSID:  0x34

      Port-set-1:  4928, 4929, 4930, 4931, 4932, 4933, 4934, 4935, 4936,
         4937, 4938, 4939, 4940, 4941, 4942, 4943
      Port-set-2:  9024, 9025, 9026, 9027, 9028, 9029, 9030, 9031, 9032,
         9033, 9034, 9035, 9036, 9037, 9038, 9039
      ...  ...
      Port-set-15  62272, 62273, 62274, 62275, 62276, 62277, 62278,
         62279, 62280, 62281, 62282, 62283, 62284, 62285, 62286, 62287

      The BMR information allows a MAP CE also to determine (complete)
      its IPv6 address within the indicated IPv6 prefix.

      IPv6 address of MAP-T CE:  2001:db8:0012:3400:00c0:0002:1200:3400

   Example 2:

      Another example can be made of a hypothetical MAP-T BR,
      configured with the following FMR when receiving a packet
      with the following characteristics:

      IPv4 source address:  1.2.3.4 (0x01020304)
      IPv4 source port:  80
      IPv4 destination address:  192.0.2.18 (0xc0000212)
      IPv4 destination port:  9030

      Configured Forwarding Mapping Rule:  {2001:db8:0000::/40
         (Rule IPv6 prefix), 192.0.2.0/24 (Rule IPv4 prefix),
         16 (Rule EA-bits length)}

      MAP-T BR Prefix  2001:db8:ffff::/64

      The above information allows the BR to derive as follows
      the mapped destination IPv6 address for the corresponding
      MAP-T CE, and also the mapped source IPv6 address for
      the IPv4 source.

      IPv4 suffix bits (p)  32 - 24 = 8 (18 (0x12))
      PSID length:  8
      PSID:  0x34 (9030 (0x2346))

      The resulting IPv6 packet will have the following key fields:

      IPv6 source address  2001:db8:ffff:0:0001:0203:0400::
      IPv6 destination address:  2001:db8:0012:3400:00c0:0002:1200:3400
      IPv6 source Port:  80
      IPv6 destination Port:  9030

   Example 3- FMR:

      An IPv4 host behind the MAP-T CE (addressed as per the previous
      examples) corresponding with IPv4 host 1.2.3.4 will have its
      packets converted into IPv6 using the DMR configured on the MAP-T
      CE as follows:

      Default Mapping Rule used by MAP-T CE:  {2001:db8:ffff::/64
      (Rule IPv6 prefix), 0.0.0.0/0 (Rule IPv4 prefix), null (BR IPv4
         address)}

      IPv4 source address (post NAT44 if present)  192.0.2.18
      IPv4 destination address:  1.2.3.4
      IPv4 source port (post NAT44 if present):  9030
      IPv4 destination port:  80
      IPv6 source address of MAP-T translation CE:
                           2001:db8:0012:3400:00c0:0002:1200:3400
      IPv6 destination address:  2001:db8:ffff:0:0001:0203:0400::

   Example 1:

      Given 4 - 1:1 Rule with no address sharing

      IPv6 prefix assigned to the end user:  2001:db8:0012:3400::/56
      Basic Mapping Rule:  {2001:db8:0012:3400::/56 (Rule IPv6 prefix),
         192.0.2.1/32 (Rule IPv4 prefix), 0 (Rule EA-bits length)}
      PSID length: 0 (Sharing ratio is 1)
      PSID offset:  n/a

      A MAP domain information node (CE or BR) can via the BMR or equivalent FMR, determine
      the IPv4 address and an port-set as shown below:

      EA bits offset:  0
      IPv4 suffix bits (p)  Length of IPv4 address (32) - IPv4 prefix
         length (32) = 0
      IPv4 address  192.0.2.1 (0xc0000201)
      PSID start:  0
      PSID length: 0
      PSID:  null

      The BMR information allows a MAP CE also to determine (complete)
      its full IPv6 address by combining the IPv6 prefix with the MAP
      interface identifier (that embeds the IPv4 address).

      IPv6 address of
      an endpoint: MAP CE:  2001:db8:0012:3400:00c0:0002:0100:0000

   Example 5 - 1:1 Rule with address sharing (sharing ratio 256)

      IPv6 prefix assigned to the end user:  2001:db8:0012:3400::/56
      Basic Mapping Rule:  {2001:db8:0000::/40  {2001:db8:0012:3400::/56 (Rule IPv6 prefix),
         192.0.2.0/24
         192.0.2.1/32 (Rule IPv4 prefix), 16 0 (Rule EA-bits length)}
      Sharing ratio:  256
      PSID length: (16 - (32 - 24) = 8. 2^8 = (Sharing ratio of 256)
      PSID offset:  4

      A MAP node (CE or BR) can via the BMR or equivalent FMR determine
      the IPv4 address and port-set as shown below:

      EA bits offset:  40  0
      IPv4 suffix bits (p)  Length of IPv4 address (32) - IPv4 prefix
         length (24) = 8
      IPv4 address  192.0.2.18 (0xc0000212) IPv4 address (32) - IPv4 prefix
         length (32) = 0
      IPv4 address  192.0.2.1 (0xc0000201)
      PSID start:  0
      PSID length: 8
      PSID:  0x34

      Port-set-1:  4928, 4929, 4930, 4931, 4932, 4933, 4934, 4935, 4936,
         4937, 4938, 4939, 4940, 4941, 4942, 4943
      Port-set-2:  9024, 9025, 9026, 9027, 9028, 9029, 9030, 9031, 9032,
         9033, 9034, 9035, 9036, 9037, 9038, 9039
      ...  ...
      Port-set-15  62272, 62273, 62274, 62275, 62276, 62277, 62278,
         62279, 62280, 62281, 62282, 62283, 62284, 62285, 62286, 62287

      The BMR information allows a MAP CE also to determine (complete)
      its full IPv6 address by combining the IPv6 prefix with the MAP
      interface identifier (that embeds the IPv4 address and PSID).

      IPv6 address of MAP CE:  2001:db8:0012:3400:00c0:0002:1200:3400

      Note that the IPv4 address and PSID is not derived from the IPv6
      prefix assigned to the CE.

Appendix B.  Port mapping algorithm

   The Generalized Modulus Algorithm (GMA) used in MAP domains can also
   be expressed mathematically.  Each CE in such a domain has an IPv4
   address and a unique Port-Set Identifier (PSID), that is derived by
   means of the BMR.  For a given IPv4 address, the algorithm allows
   each PSID to be processed to reveal a set of unique non-overlapping
   ports, or alternatively for any given port to derive the PSID it
   corresponds to.  Two extreme cases supported by algorithm are: (1)
   the port numbers are not contiguous for each PSID, but uniformly
   distributed across the port range (0-65535); (2) the port numbers are
   contiguous in a single range for each PSID.

   For a given sharing ratio (R) and the maximum number of contiguous
   ports (M), the GMA algorithm is defined as:

   1.  The port (P) of a given PSID start:  40 + p (K) is composed of:

   P = 40 R * M * j + 8 M * K + i

       Where:

       *  PSID: K = 48
      PSID length:  o 0 to R - p 1

       *  Port range index: j = 16 (56 (4096 / M) / R to ((65536 / M) / R) - 40) 1,
          if the port numbers (0 - 8 4095) are excluded.

       *  Contiguous Port index: i = 8
      PSID:  0x34

      Port-set-1:  4928, 4929, 4930, 4931, 4932, 4933, 4934, 4935, 4936,
         4937, 4938, 4939, 4940, 4941, 4942, 4943
      Port-set-2:  9024, 9025, 9026, 9027, 9028, 9029, 9030, 9031, 9032,
         9033, 9034, 9035, 9036, 9037, 9038, 9039
      ...  ...
      Port-set-15  62272, 62273, 62274, 62275, 62276, 62277, 62278,
         62279, 62280, 62281, 62282, 62283, 62284, 62285, 62286, 62287 0 to M - 1

   2.  The BMR information allows PSID (K) of a MAP CE also to determine (complete)
      its IPv6 address within given port number (P) is determined by:

   K = (floor(P/M)) % R

       Where:

       *  % is the modulus operator

       *  floor(arg) is a function that returns the largest integer not
          greater than arg.

B.1.  Bit Representation of the Algorithm

   Given a sharing ratio (R=2^k), the maximum number of contiguous ports
   (M=2^m), for any PSID (K) and available ports (P) can be represented
   as:

   0                          8                         15
   +---------------+----------+------+-------------------+
   |                       Port  (P)                     |
   ----------------+-----------------+-------------------+
   |        j      |   PSID (K)      |        M  (i)     |
   +---------------+----------+------+-------------------+
   |<----a bits--->|<-----k bits---->|<------m bits----->|

                       Figure 9: Bit representation

   Where j and i are the indicated IPv6 prefix.

      IPv6 address of MAP-T CE:  2001:db8:0012:3400:00c0:0002:1200:3400

   Example 2:

      Another example same indexes defined in the port mapping
   algorithm.

   For any port number, the PSID can be made of obtained by a hypothetical MAP-T BR,
      configured with the following FMR when receiving bit mask
   operation.

   For a packet
      with > 0, j MUST be larger than 0.  This ensures that the following characteristics:

      IPv4 source address:  1.2.3.4 (0x01020304)
      IPv4 source port:  80
      IPv4 destination address:  192.0.2.18 (0xc0000212)
      IPv4 destination port:  9030

      Configured Forwarding Mapping Rule:  {2001:db8:0000::/40
         (Rule IPv6 prefix), 192.0.2.0/24 (Rule IPv4 prefix),
         16 (Rule EA-bits length)}

      MAP-T BR Prefix  2001:db8:ffff::/64

      The above information allows algorithm
   excludes the BR system ports ([I-D.ietf-tsvwg-iana-ports]).  For a = 0,
   j MAY be 0 to derive as follows allow for the mapped destination IPv6 address provisioning of the system ports.

B.2.  GMA examples

   For example, for R = 1024, PSID offset: a = 4 and PSID length: k = 10
   bits

             Port-set-1                Port-set-2
   PSID=0   | 4096, 4097, 4098, 4099, | 8192,  8193,  8194,  8195, | ...
   PSID=1   | 4100, 4101, 4102, 4103, | 8196,  8197,  8198,  8199, | ...
   PSID=2   | 4104, 4105, 4106, 4107, | 8200,  8201,  8202,  8203, | ...
   PSID=3   | 4108, 4109, 4110, 4111, | 8204,  8205,  8206,  8207, | ...
   ...
   PSID=1023| 8188, 8189, 8190, 8191, | 12284, 12285, 12286, 12287,| ...

                    Example 1: with offset = 4 (a = 4)

   For example, for R = 64, a = 0 (PSID offset = 0 and PSID length = 6
   bits):

             Port-set
   PSID=0   | [   0 - 1023]
   PSID=1   | [1024 - 2047]
   PSID=2   | [2048 - 3071]
   PSID=3   | [3072 - 4095]
   ...
   PSID=63  | [64512 - 65535]

                    Example 2: with offset = 0 (a = 0)

B.3.  GMA Excluded Ports

   By default the corresponding
      MAP-T CE, and also GMA ensures that a number of "well known" ports are
   excluded from use by the mapped source IPv6 address for algorithm.  This number is determined by the IPv4 source.

      IPv4 suffix
   number of offset bits (p)  32 - 24 = 8 (18 (0x12))
      PSID length:  8
      PSID:  0x34 (9030 (0x2346))

      The resulting IPv6 packet will have (a), in the following key fields:

      IPv6 source address  2001:db8:ffff:0:0001:0203:0400::
      IPv6 destination address:  2001:db8:0012:3400:00c0:0002:1200:3400
      IPv6 source Port:  80
      IPv6 destination Port:  9030

   Example 3:

      An IPv4 host behind figure above.  This value can be
   optionally provisioned via the MAP-T CE (addressed as per "Rule Port Mapping Parameters" in the previous
      examples) corresponding with IPv4 host 1.2.3.4 will have its
      packets converted into IPv6 using
   Basic Mapping Rule.  In the DMR configured on absence of such provisioning, the MAP-T
      CE as follows:

      Default Mapping Rule used by MAP-T CE:  {2001:db8:ffff::/64
      (Rule IPv6 prefix), 0.0.0.0/0 (Rule IPv4 prefix), null (BR IPv4
         address)}

      IPv4 source address (post NAT44 if present)  192.0.2.18
      IPv4 destination address:  1.2.3.4
      IPv4 source port (post NAT44 if present):  9030
      IPv4 destination port:  80
      IPv6 source address
   defaults are:

   o  Excluded ports : 0-4095

   o  Offset bits (a) : 4

   For (a) offset bits, the range of MAP-T CE:
                           2001:db8:0012:3400:00c0:0002:1200:3400
      IPv6 destination address:  2001:db8:ffff:0:0001:0203:0400:: excluded ports is 0 to 2 ^ (16-a) -
   1.

Authors' Addresses

   Xing Li
   CERNET Center/Tsinghua University
   Room 225, Main Building, Tsinghua University
   Beijing 100084
   CN

   Email: xing@cernet.edu.cn

   Congxiao Bao
   CERNET Center/Tsinghua University
   Room 225, Main Building, Tsinghua University
   Beijing 100084
   CN

   Email: congxiao@cernet.edu.cn

   Wojciech Dec
   Cisco Systems
   Haarlerbergpark Haarlerbergweg 13-19
   Amsterdam, NOORD-HOLLAND  1101 CH
   Netherlands

   Phone:
   Email: wdec@cisco.com

   Ole Troan
   Cisco Systems
   Oslo
   Norway

   Email: ot@cisco.com
   Satoru Matsushima
   SoftBank Telecom
   1-9-1 Higashi-Shinbashi, Munato-ku
   Tokyo
   Japan

   Email: satoru.matsushima@tm.softbank.co.jp

   Tetsuya Murakami
   IP Infusion
   1188 East Arques Avenue
   Sunnyvale
   USA

   Email: tetsuya@ipinfusion.com