



Network Working Group                                      O. Troan, Ed.
Internet-Draft                                                    W. Dec
Intended status: Standards Track                           Cisco Systems
Expires: September 10, 2015                                        X. Li
                                                                  C. Bao
                                       CERNET Center/Tsinghua University
                                                           S. Matsushima
                                                        SoftBank Telecom
                                                             T. Murakami
                                                             IP Infusion
                                                          T. Taylor, Ed.
                                                     Huawei Technologies
                                                          March 09, 2015


          Mapping of Address and Port with Encapsulation (MAP)
                       draft-ietf-softwire-map-13

Abstract

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

Status of This Memo

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

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

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

   This Internet-Draft will expire on September 10, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Mapping Algorithm . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Port mapping algorithm  . . . . . . . . . . . . . . . . .   8
     5.2.  Basic mapping rule (BMR)  . . . . . . . . . . . . . . . .  10
     5.3.  Forwarding mapping rule (FMR) . . . . . . . . . . . . . .  12
     5.4.  Destinations outside the MAP domain . . . . . . . . . . .  13
   6.  The IPv6 Interface Identifier . . . . . . . . . . . . . . . .  13
   7.  MAP Configuration . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  MAP CE  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.2.  MAP BR  . . . . . . . . . . . . . . . . . . . . . . . . .  15
   8.  Forwarding Considerations . . . . . . . . . . . . . . . . . .  15
     8.1.  Receiving Rules . . . . . . . . . . . . . . . . . . . . .  15
     8.2.  ICMP  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
     8.3.  Fragmentation and Path MTU Discovery  . . . . . . . . . .  17
       8.3.1.  Fragmentation in the MAP domain . . . . . . . . . . .  17
       8.3.2.  Receiving IPv4 Fragments on the MAP domain borders  .  17
       8.3.3.  Sending IPv4 fragments to the outside . . . . . . . .  18
   9.  NAT44 Considerations  . . . . . . . . . . . . . . . . . . . .  18
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  18
   12. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  19
   13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  20
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     14.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  23
   Appendix B.  A More Detailed Description of the Derivation of the
                Port Mapping Algorithm . . . . . . . . . . . . . . .  27
     B.1.  Bit Representation of the Algorithm . . . . . . . . . . .  29
     B.2.  GMA examples  . . . . . . . . . . . . . . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30





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1.  Introduction

   Mapping of IPv4 addresses in IPv6 addresses has been described in
   numerous mechanisms dating back to 1995 [RFC1933].  The Automatic
   tunneling mechanism 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 end-point address
   from the IPv6 destination address.

   There are numerous variations of this idea, described in 6over4
   [RFC2529], 6to4 [RFC3056], ISATAP [RFC5214], and 6rd [RFC5969].

   The commonalities of all these IPv6 over IPv4 mechanisms are:

   o  Automatically provisions an IPv6 address for a host or an IPv6
      prefix for a site

   o  Algorithmic or implicit address resolution of tunnel end point
      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 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 map between either 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.

   This document describes delivery of IPv4 unicast service across an
   IPv6 infrastructure.  IPv4 multicast is not considered further in
   this document.



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   The A+P (Address and Port) 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 DS-lite is described in [RFC6333].  The motivation
   for this work is described in
   [I-D.ietf-softwire-stateless-4v6-motivation].

   A companion document defines a DHCPv6 option for provisioning of MAP
   [I-D.ietf-softwire-map-dhcp].  Other means of provisioning are
   possible.  Deployment considerations are described in
   [I-D.ietf-softwire-map-deployment].

   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) 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 CEs and 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.

   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.




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   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:               The 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 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
                           other means than MAP itself.  E.g.,
                           Provisioned using DHCPv6 PD [RFC3633],
                           assigned via 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.

   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.





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   The MAP mechanism uses existing standard building blocks.  The
   existing 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.  Secondly, the NAPT is not
   required to restrict the ports used on outgoing packets.

   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      |  /".         ,-'                 `-.       ,-'
   | +-----+--------+ | /   `----+--'                       ------'



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   | 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 sub-nets to
   exist within the domain and optimize forwarding between them.

   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 CE's
       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 Rules
       table for the Rule IPv4 prefix.  Given a destination IPv4 address
       and port within the MAP domain, a MAP node can use the matching



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       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-bits 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 Rules table for all
   the Forwarding Mapping Rules, and a default route is installed to the
   MAP BR (see Section 5.4).

   Forwarding Mapping Rules are used to allow direct communication
   between MAP CEs, 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 CIDR [RFC4632].  For example the 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 0xFF00/8 (PSID = 0xFF).  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).





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   When the PSID is embedded in the End-user IPv6 prefix, then to
   minimize dependencies between the End-user IPv6 prefix and the
   assigned port-set, it is desirable to minimize the restrictions of
   possible PSID values.  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 bitfield (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   |   M   |
    the CE port-set  |    > 0    |           |       |
                     +-----------+-----------+-------+
                     |  a bits   |  k bits   |m bits |

            Figure 2: Structure of a port-restricted port field

   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' excludes 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.



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   M: 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, 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-bits length (in bits)

   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 (which is part of the End-user IPv6 prefix) that
   is common among all CEs using the same Basic Mapping Rule within the
   MAP domain.  The EA bits encode the CE specific IPv4 address and port
   information.  The EA bits, which are unique for a given Rule IPv6
   prefix, can contain a full or part of an IPv4 address and, in the
   shared IPv4 address case, a Port-Set Identifier (PSID).  An EA-bit
   length of 0 signifies that all relevant MAP IPv4 addressing
   information is passed directly in the BMR, 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 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).




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   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 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, and can be between 0 and 48.  A
   length of 48 means that the complete IPv4 address and port is
   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-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.  This case is shown in Figure 4.

                               IPv4 prefix:


                   |   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.

                          Complete IPv4 address:






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                   |   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 EA bits is: q = o - p.

                           Shared IPv4 address:


       |   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 also the PSID is provisioned using e.g., the DHCP
   option.

   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
   enable direct CE to CE connectivity.

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


   |        32 bits           |         |    16 bits        |
   +--------------------------+         +-------------------+
   | IPv4 destination address |         |  IPv4 dest port   |
   +--------------------------+         +-------------------+



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                  :           :           ___/       :
                  |  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.

   The BR forwards traffic received from the outside to CE's 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  |
   +--------+----+-----------+--------+




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                                 Figure 8

   In the case of an IPv4 prefix, the IPv4 address field is right-padded
   with 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.

   If the End-user IPv6 prefix length is larger than 64, the most
   significant parts of the interface identifier is 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
   manners, including provisioning methods such as the Broadband Forum's
   "TR-69" Residential Gateway management interface, an XML-based object
   retrieved after IPv6 connectivity is established, or manual
   configuration by an administrator.  IPv6 DHCP options for MAP
   configuration is defined in [I-D.ietf-softwire-map-dhcp].  Other
   configuration and management methods may use the format described by
   this option 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.







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   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 a given CE operates within 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.

   The MAP DHCP option is specified in [I-D.ietf-softwire-map-dhcp].

7.2.  MAP BR

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

   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, neither in IPv6 nor IPv4
   BGP.  However, if anycast is used for the MAP IPv6 relays, the
   anycast addresses must be advertised in the service provider's IGP.

8.  Forwarding Considerations

   Figure 1 depicts the overall MAP architecture with IPv4 users (N and
   M) networks connected to a routed IPv6 network.

   MAP uses Encapsulation mode as specified in [RFC2473].

   For a shared IPv4 address, a MAP CE forwarding IPv4 packets from the
   LAN performs NAT44 functions first and creates appropriate NAT44
   bindings.  The resulting IPv4 packets MUST contain the source IPv4
   address and source transport identifiers specified by the MAP
   provisioning parameters.  The IPv4 packet is forwarded using the CE's
   MAP forwarding function.  The IPv6 source and destination addresses
   MUST then be derived as per Section 5 of this draft.

8.1.  Receiving Rules




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   A MAP CE receiving an IPv6 packet to its MAP IPv6 address sends this
   packet to the CE's MAP function where it is decapsulated.  The
   resulting IPv4 packet is then forwarded to the CE's NAT44 function
   where it is handled according to the NAT's translation table.

   A MAP BR receiving IPv6 packets selects a best matching MAP domain
   rule (Rule IPv6 prefix) based on a longest address match of the
   packet's IPv6 source address, as well as a match of the packet
   destination address against the configured BR IPv6 address(es).  The
   selected MAP rule allows the BR to determine the EA-bits from the
   source IPv6 address.

   To prevent spoofing of IPv4 addresses, any MAP node (CE and BR) MUST
   perform the following validation upon reception of a packet.  First,
   the embedded IPv4 address or prefix, as well as PSID (if any), are
   extracted from the source IPv6 address using the matching MAP rule.
   These represent the range of what is acceptable as source IPv4
   address and port.  Secondly, the node extracts the source IPv4
   address and port from the IPv4 packet encapsulated inside the IPv6
   packet.  If they are found to be outside the acceptable range, the
   packet MUST be silently discard and a counter incremented to indicate
   that a potential spoofing attack may be underway.  The source
   validation checks just described are not done for packets whose
   source IPv6 address is that of the BR (BR IPv6 address).

   By default, the CE router MUST drop packets received on the MAP
   virtual interface (i.e., after decapsulation of IPv6) for IPv4
   destinations not for its own IPv4 shared address, full IPv4 address
   or IPv4 prefix.

8.2.  ICMP

   ICMP message should be supported in MAP domain.  Hence, the NAT44 in
   MAP CE MUST implement the behavior for ICMP message conforming to the
   best current practice documented in [RFC5508].

   If a MAP CE receives an ICMP message having ICMP identifier field in
   ICMP header, NAT44 in the MAP CE MUST rewrite this field to a
   specific value assigned from the port set.  BR and other CEs must
   handle this field similar to the port number in the TCP/UDP header
   upon receiving the ICMP message with ICMP identifier field.

   If a MAP node receives an ICMP error message without the ICMP
   identifier field for errors that is detected inside a IPv6 tunnel, a
   node should relay the ICMP error message to the original source.
   This behavior SHOULD be implemented conforming to the section 8 of
   [RFC2473].




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8.3.  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.

8.3.1.  Fragmentation in the MAP domain

   Encapsulating an IPv4 packet to carry it across the MAP domain will
   increase its size (typically by 40 bytes).  It is strongly
   recommended that the MTU in the MAP domain be well managed and that
   the IPv6 MTU on the CE WAN side interface be set so that no
   fragmentation occurs within the boundary of the MAP domain.

   Fragmentation on MAP domain entry is described in section 7.2 of
   [RFC2473].

   The use of an anycast source address could lead to an ICMP error
   message generated on the path being sent to a different BR.
   Therefore, using dynamic tunnel MTU Section 6.7 of [RFC2473] is
   subject to IPv6 Path MTU black-holes.  A MAP BR using an anycast
   source address SHOULD NOT by default use Path MTU discovery across
   the MAP domain.

   Multiple BRs using the same anycast source address could send
   fragmented packets to the same CE at the same time.  If the
   fragmented packets from different BRs happen to use the same fragment
   ID, incorrect reassembly might occur.  See [RFC4459] for an analysis
   of the problem.  Section 3.4 suggests solving the problem by
   fragmenting the inner packet.

8.3.2.  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.
   Implementers of MAP should be aware that there are a number of well-
   known attacks against IP fragmentation; see [RFC1858] and [RFC3128].



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   Implementers should also be aware of additional issues with
   reassembling packets at high rates, as described in [RFC4963].

8.3.3.  Sending IPv4 fragments to the outside

   If two IPv4 host behind two different MAP CEs 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 could
   rewrite the IPv4 fragmentation identifier to be within its allocated
   port-set, if the resulting fragment identifier space was large enough
   related to the rate fragments was sent.  However, splitting the
   identifier space in this fashion would increase the probability of
   reassembly collision for all connections through the CPE.  See also
   [RFC6864]

9.  NAT44 Considerations

   The NAT44 implemented in the MAP CE SHOULD conform with the behavior
   and best current practice documented in [RFC4787], [RFC5508], and
   [RFC5382].  In MAP address sharing mode (determined by the MAP domain
   /rule configuration parameters) the operation of the NAT44 MUST be
   restricted to the available port numbers derived via the basic
   mapping rule.

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 deployment, 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



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      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
      [RFC4787], Section 5.  Furthermore, the MAP CEs SHOULD use a DNS
      transport proxy [RFC5625] function to handle DNS traffic, and
      source such traffic from IPv6 interfaces not assigned to MAP.

   [RFC6269] outlines general issues with IPv4 address sharing.

12.  Contributors

   This document is the result of the IETF Softwire MAP design team
   effort and numerous previous individual contributions in this area:

      Chongfeng Xie (China Telecom)
      Room 708, No.118, Xizhimennei Street Beijing 100035
      People's Republic of China
      Phone: +86-10-58552116
      Email: xiechf@ctbri.com.cn


      Qiong Sun (China Telecom)
      Room 708, No.118, Xizhimennei Street Beijing 100035
      People's Republic of China
      Phone: +86-10-58552936
      Email: sunqiong@ctbri.com.cn


      Gang Chen (China Mobile)
      53A,Xibianmennei Ave. Beijing 100053
      People's Republic of China
      Email: chengang@chinamobile.com


      Yu Zhai
      CERNET Center/Tsinghua University
      Room 225, Main Building, Tsinghua University



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      Beijing 100084
      People's Republic of China
      Email: jacky.zhai@gmail.com


      Wentao Shang (CERNET Center/Tsinghua University)
      Room 225, Main Building, Tsinghua University Beijing 100084
      People's Republic of China
      Email: wentaoshang@gmail.com


      Guoliang Han (CERNET Center/Tsinghua University)
      Room 225, Main Building, Tsinghua University Beijing 100084
      People's Republic of China
      Email: bupthgl@gmail.com


      Rajiv Asati (Cisco Systems)
      7025-6 Kit Creek Road Research Triangle Park NC 27709 USA
      Email: rajiva@cisco.com


13.  Acknowledgments

   This document is based on the ideas of many, including Masakazu
   Asama, Mohamed Boucadair, Gang Chen, Maoke Chen, Wojciech Dec,
   Xiaohong Deng, Jouni Korhonen, Tomasz Mrugalski, Jacni Qin, Chunfa
   Sun, Qiong Sun, and Leaf Yeh. The authors want in particular to
   recognize Remi Despres, who has tirelessly worked on generalized
   mechanisms for stateless address mapping.

   The authors would like to thank Lichun Bao, Guillaume Gottard, Dan
   Wing, Jan Zorz, Necj Scoberne, Tina Tsou, Kristian Poscic, and
   especially Tom Taylor and Simon Perreault for the thorough review and
   comments of this document.  Useful IETF Last Call comments were
   received from Brian Weis and Lei Yan.

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.

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





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   [RFC5625]  Bellis, R., "DNS Proxy Implementation Guidelines", BCP
              152, RFC 5625, August 2009.

14.2.  Informative References

   [I-D.ietf-softwire-map-deployment]
              Qiong, Q., Chen, M., Chen, G., Tsou, T., and S. Perreault,
              "Mapping of Address and Port (MAP) - Deployment
              Considerations", draft-ietf-softwire-map-deployment-03
              (work in progress), October 2013.

   [I-D.ietf-softwire-map-dhcp]
              Mrugalski, T., Troan, O., Dec, W., Bao, C.,
              leaf.yeh.sdo@gmail.com, l., and X. Deng, "DHCPv6 Options
              for configuration of Softwire Address and Port Mapped
              Clients", draft-ietf-softwire-map-dhcp-06 (work in
              progress), November 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-05 (work in progress),
              November 2012.

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

   [RFC1858]  Ziemba, G., Reed, D., and P. Traina, "Security
              Considerations for IP Fragment Filtering", RFC 1858,
              October 1995.

   [RFC1933]  Gilligan, R. and E. Nordmark, "Transition Mechanisms for
              IPv6 Hosts and Routers", RFC 1933, April 1996.

   [RFC2529]  Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
              Domains without Explicit Tunnels", RFC 2529, March 1999.

   [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.

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



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   [RFC3128]  Miller, I., "Protection Against a Variant of the Tiny
              Fragment Attack (RFC 1858)", RFC 3128, June 2001.

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

   [RFC4459]  Savola, P., "MTU and Fragmentation Issues with In-the-
              Network Tunneling", RFC 4459, April 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.

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 2007.

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

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

   [RFC4963]  Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
              Errors at High Data Rates", RFC 4963, July 2007.

   [RFC5214]  Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
              Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
              March 2008.

   [RFC5382]  Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
              RFC 5382, October 2008.

   [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.

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





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

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

   [RFC6250]  Thaler, D., "Evolution of the IP Model", RFC 6250, 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.

   [RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
              Stack Lite Broadband Deployments Following IPv4
              Exhaustion", RFC 6333, August 2011.

   [RFC6335]  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", BCP 165, RFC
              6335, August 2011.

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

   [RFC6864]  Touch, J., "Updated Specification of the IPv4 ID Field",
              RFC 6864, February 2013.

Appendix A.  Examples

   Example 1 - Basic Mapping Rule













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

      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)}
      PSID length:          (16 - (32 - 24) = 8. (Sharing ratio of 256)
      PSID offset:          6 (default)

      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

      Available ports (63 ranges) : 1232-1235, 2256-2259, ...... ,
                                    63696-63699, 64720-64723

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

      IPv6 address of MAP CE:  2001:db8:0012:3400:0000:c000:0212:0034



   Example 2 - BR:



















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   Another example can be made of a MAP 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:     1232

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

   IPv6 address of MAP BR:              2001:db8:ffff::1

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

   IPv4 suffix bits (p):  32 - 24 = 8 (18 (0x12))
   PSID length:           8
   PSID:                  0x34 (1232)

   The resulting IPv6 packet will have the following key fields:

   IPv6 source address:       2001:db8:ffff::1
   IPv6 destination address:  2001:db8:0012:3400:0000:c000:0212:0034



   Example 3 - Forwarding Mapping Rule:


   An IPv4 host behind the MAP CE (addressed as per the previous
   examples) corresponding with IPv4 host 1.2.3.4 will have its
   packets encapsulated by IPv6 using the IPv6 address of the BR
   configured on the MAP CE as follows:

   IPv6 address of BR:         2001:db8:ffff::1
   IPv4 source address:        192.0.2.18
   IPv4 destination address:   1.2.3.4
   IPv4 source port:           1232
   IPv4 destination port:      80
   MAP CE IPv6 source address: 2001:db8:0012:3400:0000:c000:0212:0034
   IPv6 destination address:   2001:db8:ffff::1





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   Example 4 - Rule with no embedded address bits and no address sharing

      End-User IPv6 prefix: 2001:db8:0012:3400::/56
      Basic Mapping Rule:   {2001:db8:0012:3400::/56 (Rule IPv6 prefix),
                             192.0.2.18/32 (Rule IPv4 prefix),
                             0 (Rule EA-bits length)}
      PSID length:          0 (Sharing ratio is 1)
      PSID offset:          n/a

      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:       0
      IPv4 suffix bits (p): Length of IPv4 address (32) -
                            IPv4 prefix length (32) = 0
      IPv4 address:         192.0.2.18 (0xc0000212)
      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 MAP CE:  2001:db8:0012:3400:0000:c000:0212:0000



   Example 5 - Rule with no embedded address bits and address sharing
   (sharing ratio 256)





















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      End-User IPv6 prefix: 2001:db8:0012:3400::/56
      Basic Mapping Rule:   {2001:db8:0012:3400::/56 (Rule IPv6 prefix),
                             192.0.2.18/32 (Rule IPv4 prefix),
                             0 (Rule EA-bits length)}
      PSID length:          8. (From DHCP. Sharing ratio of 256)
      PSID offset:          6  (Default)
      PSID       :          0x34 (From DHCP.)

      A MAP node can via the Basic Mapping Rule determine the IPv4
      address and 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:          192.0.2.18 (0xc0000212)
      PSID offset:           6
      PSID length:           8
      PSID:                  0x34

      Available ports (63 ranges) : 1232-1235, 2256-2259, ...... ,
                                    63696-63699, 64720-64723

      The Basic Mapping Rule 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:db8:0012:3400:0000:c000:0212:0034

      Note that the IPv4 address and PSID is not derived from the IPv6
      prefix assigned to the CE, but provisioned separately using
      e.g., DHCP.



Appendix B.  A More Detailed Description of the Derivation of the Port
             Mapping Algorithm

   This Appendix describes how the port mapping algorithm described in
   Section 5.1 was derived.  The algorithm is used in domains whose
   rules allow IPv4 address sharing.

   The basic requirement for a port mapping algorithm is that the port-
   sets it assigns to different MAP CEs MUST be non-overlapping.  A
   number of other requirements guided the choice of the algorithm:






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   o  In keeping with the general MAP algorithm the port-set MUST be
      derivable from a port-set identifier (PSID) that can be embedded
      in the End-user IPv6 prefix.

   o  The mapping MUST be reversible, such that, given the port number,
      the PSID of the port-set to which it belongs can be quickly
      derived.

   o  The algorithm MUST allow a broad range of address sharing ratios.

   o  It SHOULD be possible to exclude subsets of the complete port
      numbering space from assignment.  Most operators would exclude the
      system ports (0-1023).  A conservative operator might exclude all
      but the transient ports (49152-65535).

   o  The effect of port exclusion on the possible values of the End-
      user IPv6 prefix (i.e., due to restrictions on the PSID value)
      SHOULD be minimized.

   o  For administrative simplicity, the algorithm SHOULD allocate the
      the same or almost the same number of ports to each CE sharing a
      given IPv4 address.

   The two extreme cases that an algorithm satisfying those conditions
   might support are: (1) the port numbers are not contiguous for each
   PSID, but uniformly distributed across the allowed port range; (2)
   the port numbers are contiguous in a single range for each PSID.  The
   port mapping algorithm proposed here is called the Generalized
   Modulus Algorithm (GMA) and supports both these cases.

   For a given IPv4 address sharing ratio (R) and the maximum number of
   contiguous ports (M) in a port-set, the GMA is defined as:

   a.  The port numbers (P) corresponding to a given PSID are generated
       by:

   (1) ... P = (R * M) * i + M * PSID + j


       where i and j are indices and the ranges of i, j, and the PSID
       are discussed in a moment.

   b.  For any given port number P, the PSID is calculated as:

   (2) ... PSID = trunc((P modulo (R * M)) / M)






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       where trunc() is the operation of rounding down to the nearest
       integer.

   Formula (1) can be interpreted as follows.  First, the available port
   space is divided into blocks of size R * M. Each block is divided
   into R individual ranges of length M. The index i in formula (1)
   selects a block, PSID selects a range within that block, and the
   index j selects a specific port value within the range.  On the basis
   of this interpretation:

   o  i ranges from ceil(N / (R * M)) to trunc(65536/(R * M)) - 1, where
      ceil is the operation of rounding up to the nearest integer and N
      is the number of ports (e.g., 1024) excluded from the lower end of
      the range.  That is, any block containing excluded values is
      discarded at the lower end, and if the final block has fewer than
      R * M values it is discarded.  This ensures that the same number
      of ports is assigned to every PSID.

   o  PSID ranges from 0 to R - 1;

   o  j ranges from 0 to M - 1.

B.1.  Bit Representation of the Algorithm

   If R and M are powers of 2 (R = 2^k, M = 2^m), formula (1) translates
   to a computationally convenient structure for any port number
   represented as a 16-bit binary number.  This structure is shown in
   Figure 9.

   0                          8                         15
   +---------------+----------+------+-------------------+
   |                     P                               |
   ----------------+-----------------+-------------------+
   |        i      |       PSID      |        j          |
   +---------------+----------+------+-------------------+
   |<----a bits--->|<-----k bits---->|<------m bits----->|


               Figure 9: Bit Representation of a Port Number

   As shown in the figure, the index value i of formula (1) is given by
   the first a = 16 - k - m bits of the port number.  The PSID value is
   given by the next k bits, and the index value j is given by the last
   m bits.

   Because the PSID is always in the same position in the port number
   and always the same length, different PSID values are guaranteed to
   generate different sets of port numbers.  In the reverse direction,



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   the generating PSID can be extracted from any port number by a bit
   mask operation.

   Note that when M and R are powers of 2, 65536 divides evenly by R *
   M. Hence the final block is complete and the upper bound on i is
   exactly 65536/(R * M) - 1.  The lower bound on i is still the minimum
   required to ensure that the required set of ports is excluded.  No
   port numbers are wasted through discarding of blocks at the lower end
   if block size R * M is a factor of N, the number of ports to be
   excluded.

   As a final note, the number of blocks into which the range 0-65535 is
   being divided in the above representation is given by 2^a.  Hence the
   case where a = 0 can be interpreted as one where the complete range
   has been divided into a single block, and individual port-sets are
   contained in contiguous ranges in that block.  We cannot throw away
   the whole block in that case, so port exclusion has to be achieved by
   putting a lower bound equal to ceil(N / M) on the allowed set of PSID
   values instead.

B.2.  GMA examples

   For example, for R = 256, PSID = 0, offset: a = 6 and PSID length: k
   = 8 bits


   Available ports (63 ranges) : 1024-1027, 2048-2051, ...... ,
                                 63488-63491, 64512-64515

                    Example 1: with offset = 6 (a = 6)

   For example, for R = 64, PSID = 0, a = 0 (PSID offset = 0 and PSID
   length = 6 bits), no port exclusion:

   Available ports (1 range) : 0-1023


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

Authors' Addresses

   Ole Troan (editor)
   Cisco Systems
   Philip Pedersens vei 1
   Lysaker  1366
   Norway

   Email: ot@cisco.com



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

   Email: wdec@cisco.com


   Xing Li
   CERNET Center/Tsinghua University
   Room 225, Main Building, Tsinghua University
   Beijing 100084
   People's Republic of China

   Email: xing@cernet.edu.cn


   Congxiao Bao
   CERNET Center/Tsinghua University
   Room 225, Main Building, Tsinghua University
   Beijing 100084
   People's Republic of China

   Email: congxiao@cernet.edu.cn


   Satoru Matsushima
   SoftBank Telecom
   1-9-1 Higashi-Shinbashi, Munato-ku
   Tokyo
   Japan

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


   Tetsuya Murakami
   IP Infusion
   1188 East Arques Avenue
   Sunnyvale
   USA

   Email: tetsuya@ipinfusion.com








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   Tom Taylor (editor)
   Huawei Technologies
   Ottawa
   Canada

   Email: tom.taylor.stds@gmail.com













































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