Mapping of Address and Port using Translation (MAP-T)

Internet Engineering Task Force (IETF)                             X. Li
Request for Comments: 7599                                        C. Bao
Category: Standards Track                            Tsinghua University
ISSN: 2070-1721                                              W. Dec, Ed.
                                                                O. Troan
                                                           Cisco Systems
                                                           S. Matsushima
                                                        SoftBank Telecom
                                                             T. Murakami
                                                             IP Infusion
                                                               July 2015


         Mapping of Address and Port using Translation (MAP-T)

Abstract

   This document specifies the solution architecture based on "Mapping
   of Address and Port" stateless IPv6-IPv4 Network Address Translation
   (NAT64) for providing shared or non-shared IPv4 address connectivity
   to and across an IPv6 network.

Status of This Memo

   This is an Internet Standards Track document.

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

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7599.

Copyright Notice

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

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

Table of Contents

   1. Introduction ....................................................4
   2. Conventions .....................................................4
   3. Terminology .....................................................5
   4. Architecture ....................................................6
   5. Mapping Rules ...................................................8
      5.1. Destinations outside the MAP Domain ........................8
   6. The IPv6 Interface Identifier ...................................9
   7. MAP-T Configuration ............................................10
      7.1. MAP CE ....................................................10
      7.2. MAP BR ....................................................11
   8. MAP-T Packet Forwarding ........................................11
      8.1. IPv4 to IPv6 at the CE ....................................11
      8.2. IPv6 to IPv4 at the CE ....................................12
      8.3. IPv6 to IPv4 at the BR ....................................12
      8.4. IPv4 to IPv6 at the BR ....................................13
   9. ICMP Handling ..................................................13
   10. Fragmentation and Path MTU Discovery ..........................14
      10.1. Fragmentation in the MAP Domain ..........................14
      10.2. Receiving IPv4 Fragments on the MAP Domain Borders .......14
      10.3. Sending IPv4 Fragments to the Outside ....................14
   11. NAT44 Considerations ..........................................15
   12. Usage Considerations ..........................................15
      12.1. EA-Bit Length 0 ..........................................15
      12.2. Mesh and Hub-and-Spoke Modes .............................15
      12.3. Communication with IPv6 Servers in the MAP-T Domain ......15
      12.4. Compatibility with Other NAT64 Solutions .................16
   13. Security Considerations .......................................16
   14. References ....................................................17
      14.1. Normative References .....................................17
      14.2. Informative References ...................................18
   Appendix A. Examples of MAP-T Translation .........................21
   Appendix B. Port-Mapping Algorithm ................................24
   Acknowledgements ..................................................25
   Contributors ......................................................25
   Authors' Addresses ................................................26

1.  Introduction

   Experiences from initial service provider IPv6 network deployments,
   such as [RFC6219], indicate that successful transition to IPv6 can
   happen while supporting legacy IPv4 users without a full end-to-end
   dual-IP-stack deployment.  However, due to public IPv4 address
   exhaustion, this requires an IPv6 technology that supports IPv4 users
   utilizing shared IPv4 addressing, while also allowing the network
   operator to optimize their operations around IPv6 network practices.
   The use of double NAT64 translation-based solutions is an optimal way
   to address these requirements, especially in combination with
   stateless translation techniques that minimize operational challenges
   outlined in [Solutions-4v6].

   The Mapping of Address and Port using Translation (MAP-T)
   architecture specified in this document is such a double stateless
   NAT64-based solution.  It builds on existing stateless NAT64
   techniques specified in [RFC6145], along with the stateless
   algorithmic address and transport-layer port-mapping scheme defined
   in the Mapping of Address and Port with Encapsulation (MAP-E)
   specification [RFC7597].  The MAP-T solution differs from MAP-E in
   that MAP-T uses IPv4-IPv6 translation, rather than encapsulation, as
   the form of IPv6 domain transport.  The translation mode is
   considered advantageous in scenarios where the encapsulation
   overhead, or IPv6 operational practices (e.g., the use of IPv6-only
   servers, or reliance on IPv6 + protocol headers for traffic
   classification) rule out encapsulation.  These scenarios are
   presented in [MAP-T-Use-Cases].

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-T:                  Mapping of Address and Port using
                           Translation.

   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 IPv6-addressed
                           interface and one or more LAN-side interfaces
                           addressed using private IPv4 addressing.

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

   MAP domain:             One or more MAP CEs and BRs connected by
                           means of an IPv6 network and sharing a common
                           set of MAP Rules.  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 rule set:           A rule set is composed of all the MAP Rules
                           communicated to a device that are intended to
                           determine the device's IP+port mapping and
                           forwarding operations.  The MAP rule set is
                           interchangeably referred to in this document
                           as a MAP rule table or as simply a "rule
                           table".  Two specific types of rules -- the
                           Basic Mapping Rule (BMR) and the Forwarding
                           Mapping Rule (FMR) -- are defined in
                           Section 5 of [RFC7597].  The Default Mapping
                           Rule (DMR) is defined in this document.

   MAP rule table:         See MAP rule set.

   MAP node:               A device that implements MAP.

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

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

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

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

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

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

   Rule IPv4 prefix:       An IPv4 prefix assigned by a service provider
                           for a MAP 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

   Figure 1 depicts the overall MAP-T architecture, which sees any
   number of privately addressed IPv4 users (N and M) connected by means
   of MAP-T CEs to an IPv6 network that is equipped with one or more
   MAP-T BRs.  CEs and BRs that share MAP configuration parameters,
   referred to as "MAP Rules", form a MAP-T domain.

   Functionally, the MAP-T CE and BR utilize and extend some
   well-established technology building blocks to allow the IPv4 users
   to correspond with nodes on the public IPv4 network or on the IPv6
   network as follows:

   o  A (NAT44) Network Address and Port Translation (NAPT) [RFC2663]
      function on a MAP CE is extended with support for restricting the
      allowable TCP/UDP ports for a given IPv4 address.  The IPv4
      address and port range used are determined by the MAP provisioning
      process and identical to MAP-E [RFC7597].

   o  A stateless NAT64 function [RFC6145] is extended to allow
      stateless mapping of IPv4 and transport-layer port ranges to the
      IPv6 address space.

         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     )
                         \               /  |  Relay  |\              /
   O------------------O  \              /   O---------O \             /
   |    MAP-T CE      |   ;".         ,-'                `-.       ,-'
   | +-----+--------+ | ,"   `----+--'                      ------'
   | NAPT44|  MAP-T | |,          |
   | +-----+        | +        IPv6 node(s)
   |   |   +--------+ |  (with IPv4-embedded IPv6 address)
   O---+--------------O
       |
         User M
       Private IPv4
         Network

                       Figure 1: MAP-T Architecture

   Each MAP-T CE is assigned with a regular IPv6 prefix from the
   operator's IPv6 network.  This, in conjunction with MAP domain
   configuration settings and the use of the MAP procedures, allows the
   computation of a MAP IPv6 address and a corresponding IPv4 address.
   To allow for IPv4 address sharing, the CE may also have to be
   configured with a TCP/UDP port range that is identified by means of a
   MAP Port Set Identifier (PSID) value.  Each CE is responsible for
   forwarding traffic between a given user's private IPv4 address space
   and the MAP domain's IPv6 address space.  The IPv4-IPv6 adaptation
   uses stateless NAT64, in conjunction with the MAP algorithm for
   address computation.

   The MAP-T BR connects one or more MAP-T domains to external IPv4
   networks using stateless NAT64 as extended by the MAP-T behavior
   described in this document.

   In contrast to MAP-E, NAT64 technology is used in the architecture
   for two purposes.  First, it is intended to diminish encapsulation
   overhead and allow IPv4 and IPv6 traffic to be treated as similarly
   as possible.  Second, it is intended to allow IPv4-only nodes to
   correspond directly with IPv6 nodes in the MAP-T domain that have
   IPv4-embedded IPv6 addresses as per [RFC6052].

   The MAP-T architecture is based on the following key properties:

   1.  Algorithmic IPv4-IPv6 address mapping codified as MAP Rules, as
       described in Section 5

   2.  A MAP IPv6 address identifier, as described in Section 6

   3.  MAP-T IPv4-IPv6 forwarding behavior, as described in Section 8

5.  Mapping Rules

   The MAP-T algorithmic mapping rules are identical to those in
   Section 5 of the MAP-E specification [RFC7597], with the following
   exception: the forwarding of traffic to and from IPv4 destinations
   outside a MAP-T domain is to be performed as described in this
   document, instead of Section 5.4 of the MAP-E specification.

5.1.  Destinations outside the MAP Domain

   IPv4 traffic sent by MAP nodes that are all within one MAP domain is
   translated to IPv6, with the sender's MAP IPv6 address, derived via
   the Basic Mapping Rule (BMR), as the IPv6 source address and the
   recipient's MAP IPv6 address, derived via the Forwarding Mapping Rule
   (FMR), as the IPv6 destination address.

   IPv4-addressed destinations outside of the MAP domain are represented
   by means of IPv4-embedded IPv6 addresses as per [RFC6052], using the
   BR's IPv6 prefix.  For a CE sending traffic to any such destination,
   the source address of the IPv6 packet will be that of the CE's MAP
   IPv6 address, and the destination IPv6 address will be the
   destination IPv4-embedded IPv6 address.  This address mapping is said
   to be following the MAP-T Default Mapping Rule (DMR) and is defined
   in terms of the IPv6 prefix advertised by one or more BRs, which
   provide external connectivity.  A typical MAP-T CE will install an
   IPv4 default route using this rule.  A BR will use this rule when
   translating all outside IPv4 source addresses to the IPv6 MAP domain.

   The DMR IPv6 prefix length SHOULD be 64 bits long by default and in
   any case MUST NOT exceed 96 bits.  The mapping of the IPv4
   destination behind the IPv6 prefix will by default follow the /64
   rule as per [RFC6052].  Any trailing bits after the IPv4 address are
   set to 0x0.

6.  The IPv6 Interface Identifier

   The interface identifier format of a MAP-T node is the same as the
   format described in Section 6 of [RFC7597].  The format diagram is
   provided here for convenience:

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

                    Figure 2: IPv6 Interface Identifier

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

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

7.  MAP-T Configuration

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

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

   o  Use of hub-and-spoke mode or Mesh mode (if all traffic should be
      sent to the BR, or if direct CE-to-CE correspondence should be
      supported)

   o  Use of IPv4-IPv6 translation (MAP-T)

   o  The BR's IPv6 prefix used in the DMR

7.1.  MAP CE

   For a given MAP domain, the MAP configuration parameters are the same
   across all CEs within that domain.  These values may be conveyed and
   configured on the CEs using a variety of methods, including DHCPv6,
   the Broadband Forum's "TR-69" Residential Gateway management
   interface [TR069], the Network Configuration Protocol (NETCONF), or
   manual configuration.  This document does not prescribe any of these
   methods but recommends that a MAP CE SHOULD implement DHCPv6 options
   as per [RFC7598].  Other configuration and management methods may use
   the data model described by this option for consistency and
   convenience of implementation on CEs that support multiple
   configuration methods.

   Besides the MAP configuration parameters, a CE requires an IPv6
   prefix to be assigned to the CE.  This End-user IPv6 prefix is
   configured as part of obtaining IPv6 Internet access and is acquired
   using standard IPv6 means applicable in the network where the CE is
   located.

   The MAP provisioning parameters, and hence the IPv4 service itself,
   are tied to the End-user IPv6 prefix; 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 DHCPv6.  Each domain within which a given CE operates
   would require its own set of MAP configuration elements and would
   generate its own IPv4 address.  Each MAP domain requires a distinct
   End-user IPv6 prefix.

7.2.  MAP BR

   The MAP BR MUST be configured with the same MAP elements as the MAP
   CEs operating within the same domain.

   For increased reliability and load balancing, the BR IPv6 prefix MAY
   be shared across a given MAP domain.  As MAP is stateless, any BR may
   be used for forwarding to/from the domain at any time.

   Since MAP uses provider address space, no specific IPv6 or IPv4
   routes need to be advertised externally outside the service
   provider's network for MAP to operate.  However, the BR prefix needs
   to be advertised in the service provider's IGP.

8.  MAP-T Packet Forwarding

   The end-to-end packet flow in MAP-T involves an IPv4 or IPv6 packet
   being forwarded by a CE or BR in one of two directions for each such
   case.  This section presents a conceptual view of the operations
   involved in such forwarding.

8.1.  IPv4 to IPv6 at the CE

   A MAP-T CE receiving IPv4 packets SHOULD perform NAPT44 processing
   and create any necessary NAPT44 bindings.  The source address and
   source port range of packets resulting from the NAPT44 processing
   MUST correspond to the source IPv4 address and source transport port
   range assigned to the CE by means of the MAP Basic Mapping Rule
   (BMR).

   The IPv4 packet is subject to a longest IPv4 destination address +
   port match MAP Rule selection, which then determines the parameters
   for the subsequent NAT64 operation.  By default, all traffic is
   matched to the DMR and is subject to the stateless NAT64 operation
   using the DMR parameters for NAT64 (Section 5.1).  Packets that are
   matched to (optional) Forwarding Mapping Rules (FMRs) are subject to
   the stateless NAT64 operation using the FMR parameters (Section 5)
   for the MAP algorithm.  In all cases, the CE's MAP IPv6 address
   (Section 6) is used as a source address.

   A MAP-T CE MUST support a Default Mapping Rule and SHOULD support one
   or more Forwarding Mapping Rules.

8.2.  IPv6 to IPv4 at the CE

   A MAP-T CE receiving an IPv6 packet performs its regular IPv6
   operations (filtering, pre-routing, etc.).  Only packets that are
   addressed to the CE's MAP-T IPv6 addresses, and with source addresses
   matching the IPv6 MAP Rule prefixes of a DMR or FMR, are processed by
   the MAP-T CE, with the DMR or FMR being selected based on a longest
   match.  The CE MUST check that each MAP-T received packet's
   transport-layer destination port number is in the range allowed for
   by the CE's MAP BMR configuration.  The CE MUST silently drop any
   nonconforming packet and increment an appropriate counter.  When
   receiving a packet whose source IP address longest matches an FMR
   prefix, the CE MUST perform a check of consistency of the source
   address against the allowed values as per the derived allocated
   source port range.  If the source port number of a packet is found to
   be outside the allocated range, the CE MUST drop the packet and
   SHOULD respond with an ICMPv6 "Destination Unreachable, source
   address failed ingress/egress policy" (Type 1, Code 5).

   For each MAP-T processed packet, the CE's NAT64 function MUST compute
   an IPv4 source and destination address.  The IPv4 destination address
   is computed by extracting relevant information from the IPv6
   destination and the information stored in the BMR as per Section 5.
   The IPv4 source address is formed by classifying a packet's source as
   longest matching a DMR or FMR rule prefix, and then using the
   respective rule parameters for the NAT64 operation.

   The resulting IPv4 packet is then forwarded to the CE's NAPT44
   function, where the destination IPv4 address and port number MUST be
   mapped to their original value before being forwarded according to
   the CE's regular IPv4 rules.  When the NAPT44 function is not
   enabled, by virtue of MAP configuration, the traffic from the
   stateless NAT64 function is directly forwarded according to the CE's
   IPv4 rules.

8.3.  IPv6 to IPv4 at the BR

   A MAP-T BR receiving an IPv6 packet MUST select a matching MAP Rule
   based on a longest address match of the packet's source address
   against the MAP Rules present on the BR.  In combination with the
   Port Set ID derived from the packet's source IPv6 address, the
   selected MAP Rule allows the BR to verify that the CE is using its
   allowed address and port range.  Thus, the BR MUST perform a
   validation of the consistency of the source against the allowed
   values from the identified port range.  If the packet's source port
   number is found to be outside the range allowed, the BR MUST drop the
   packet and increment a counter to indicate the event.  The BR SHOULD
   also respond with an ICMPv6 "Destination Unreachable, source address
   failed ingress/egress policy" (Type 1, Code 5).

   When constructing the IPv4 packet, the BR MUST derive the source and
   destination IPv4 addresses as per Section 5 of this document and
   translate the IPv6-to-IPv4 headers as per [RFC6145].  The resulting
   IPv4 packet is then passed to regular IPv4 forwarding.

8.4.  IPv4 to IPv6 at the BR

   A MAP-T BR receiving IPv4 packets uses a longest match IPv4 +
   transport-layer port lookup to identify the target MAP-T domain and
   select the FMR and DMR rules.  The MAP-T BR MUST then compute and
   apply the IPv6 destination addresses from the IPv4 destination
   address and port as per the selected FMR.  The MAP-T BR MUST also
   compute and apply the IPv6 source addresses from the IPv4 source
   address as per Section 5.1 (i.e., using the IPv4 source and the BR's
   IPv6 prefix, it forms an IPv6-embedded IPv4 address).  The generic
   IPv4-to-IPv6 header translation procedures outlined in [RFC6145]
   apply throughout.  The resulting IPv6 packets are then passed to
   regular IPv6 forwarding.

   Note that the operation of a BR, when forwarding to/from MAP-T
   domains that are defined without IPv4 address sharing, is the same as
   that of stateless NAT64 IPv4/IPv6 translation.

9.  ICMP Handling

   MAP-T CEs and BRs MUST follow ICMP/ICMPv6 translation as per
   [RFC6145]; however, additional behavior is also required due to the
   presence of NAPT44.  Unlike TCP and UDP, which provide two transport-
   protocol port fields to represent both source and destination, the
   ICMP/ICMPv6 [RFC792] [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 MUST rewrite the ID field
   to a port value derived from the CE's Port Set ID.

   A MAP-T BR receiving an IPv4 ICMP packet that contains an ID field
   that is bound for a shared address in the MAP-T domain 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 information.

10.  Fragmentation and Path MTU Discovery

   Due to the different sizes of the IPv4 and IPv6 headers, 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 [RFC879].  MAP can use all three mechanisms to deal with
   different cases.

   Note: The NAT64 [RFC6145] mechanism is not lossless.  When
   IPv4-originated communication traverses a double NAT64 function
   (a.k.a. NAT464), any IPv4-originated ICMP-independent Path MTU
   Discovery, as specified in [RFC4821], ceases to be entirely reliable.
   This is because the DF=1/MF=1 combination as defined in [RFC4821]
   results in DF=0/MF=1 after a double NAT64 translation.

10.1.  Fragmentation in the MAP Domain

   Translating an IPv4 packet to carry it across the MAP domain will
   increase its size (typically by 20 bytes).  The MTU in the MAP domain
   should be well managed, and the IPv6 MTU on the CE WAN-side interface
   SHOULD be configured so that no fragmentation occurs within the
   boundary of the MAP domain.

   Fragmentation in MAP-T domains SHOULD be handled as described in
   Sections 4 and 5 of [RFC6145].

10.2.  Receiving IPv4 Fragments on the MAP Domain Borders

   The forwarding of an IPv4 packet received from 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 SHOULD reassemble the packet before sending the
   packet onto the MAP domain.  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 such a caching algorithm is outside the scope of this
   document.

10.3.  Sending IPv4 Fragments to the Outside

   Two IPv4 hosts behind two different MAP CEs with the same IPv4
   address sending fragments to an IPv4 destination host outside the
   domain may happen to 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 can be used similarly to port ranges.  Thus, a MAP CE
   SHOULD rewrite the IPv4 fragmentation identifier to a value
   equivalent to a port of its allocated port set.

11.  NAT44 Considerations

   The NAT44 implemented in the MAP CE SHOULD conform to the behavior
   and best current practices 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.

12.  Usage Considerations

12.1.  EA-Bit Length 0

   The MAP solution supports the use and configuration of domains where
   a BMR expresses an EA-bit length of 0.  This results in independence
   between the IPv6 prefix assigned to the CE and the IPv4 address
   and/or port range used by MAP.  The k-bits of PSID information may in
   this case be derived from the BMR.

   The constraint imposed is that each such MAP domain be composed of
   just one MAP CE that has a predetermined IPv6 end-user prefix.  The
   BR would be configured with an FMR for each such Customer Premises
   Equipment (CPE), where the rule would uniquely associate the IPv4
   address + optional PSID and the IPv6 prefix of that given CE.

12.2.  Mesh and Hub-and-Spoke Modes

   The hub-and-spoke mode of communication, whereby all traffic sent by
   a MAP-T CE is forwarded via a BR, and the Mesh mode, whereby a CE is
   directly able to forward traffic to another CE, are governed by the
   activation of Forwarding Mapping Rules that cover the IPv4-prefix
   destination and port-index range.  By default, a MAP CE configured
   only with a BMR, as per this specification, will use it to configure
   its IPv4 parameters and IPv6 MAP address without enabling Mesh mode.

12.3.  Communication with IPv6 Servers in the MAP-T Domain

   By default, MAP-T allows communication between both IPv4-only and any
   IPv6-enabled devices, as well as with native IPv6-only servers,
   provided that the servers are configured with an IPv4-mapped IPv6
   address.  This address could be part of the IPv6 prefix used by the
   DMR in the MAP-T domain.  Such IPv6 servers (e.g., an HTTP server or
   a web content cache device) are thus able to serve IPv6 users and
   IPv4-only users alike, utilizing IPv6.  Any such IPv6-only servers
   SHOULD have both A and AAAA records in DNS.  DNS64 [RFC6147] will be
   required only when IPv6 servers in the MAP-T domain are themselves
   expected to initiate communication to external IPv4-only hosts.

12.4.  Compatibility with Other NAT64 Solutions

   The MAP-T CE's NAT64 function is by default compatible for use with
   [RFC6146] stateful NAT64 devices that are placed in the operator's
   network.  In such a case, the MAP-T CE's DMR prefix is configured to
   correspond to the NAT64 device prefix.  This in effect allows the use
   of MAP-T CEs in environments that need to perform statistical
   multiplexing of IPv4 addresses, while utilizing stateful NAT64
   devices, and can take the role of a customer-side translator (CLAT)
   as defined in [RFC6877].

13.  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 in
      address sharing and is common with other address-sharing
      approaches such as Dual-Stack Lite (DS-Lite) and NAT64/DNS64.  The
      best protection against such attacks is to accelerate IPv6 support
      in both clients and servers.

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

   Attacks facilitated by restricted port set:  From hosts that are not
      subject to ingress filtering [RFC2827], an attacker can inject
      spoofed packets during ongoing transport connections [RFC4953]
      [RFC5961] [RFC6056].  The attacks depend on guessing which ports
      are currently used by target hosts.  Using an unrestricted port
      set is preferable, i.e., using native IPv6 connections that are
      not subject to MAP port-range restrictions.  To minimize these
      types of attacks when using a restricted port set, the MAP CE's
      NAT44 filtering behavior SHOULD be "Address-Dependent Filtering"
      as described in Section 5 of [RFC4787].  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 [Stateless-4Via6].

   ICMP Flooding:  Given the necessity to process and translate ICMP and
      ICMPv6 messages by the BR and CE nodes, a foreseeable attack
      vector is that of a flood of such messages leading to a saturation
      of the node's ICMP computing resources.  This attack vector is not
      specific to MAP, and its mitigation lies in a combination of
      policing the rate of ICMP messages, policing the rate at which
      such messages can get processed by the MAP nodes, and of course
      identifying and blocking off the source(s) of such traffic.

   [RFC6269] outlines general issues with IPv4 address sharing.

14.  References

14.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
              DOI 10.17487/RFC6052, October 2010,
              <http://www.rfc-editor.org/info/rfc6052>.

   [RFC6145]  Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", RFC 6145, DOI 10.17487/RFC6145, April 2011,
              <http://www.rfc-editor.org/info/rfc6145>.

   [RFC6346]  Bush, R., Ed., "The Address plus Port (A+P) Approach to
              the IPv4 Address Shortage", RFC 6346,
              DOI 10.17487/RFC6346, August 2011,
              <http://www.rfc-editor.org/info/rfc6346>.

   [RFC7597]  Troan, O., Ed., Dec, W., Li, X., Bao, C., Matsushima, S.,
              Murakami, T., and T. Taylor, Ed., "Mapping of Address and
              Port with Encapsulation (MAP-E)", RFC 7597,
              DOI 10.17487/RFC7597, July 2015,
              <http://www.rfc-editor.org/info/rfc7597>.

14.2.  Informative References

   [MAP-T-Use-Cases]
              Maglione, R., Ed., Dec, W., Leung, I., and E. Mallette,
              "Use cases for MAP-T", Work in Progress,
              draft-maglione-softwire-map-t-scenarios-05, October 2014.

   [RFC792]   Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,
              <http://www.rfc-editor.org/info/rfc792>.

   [RFC879]   Postel, J., "The TCP Maximum Segment Size and Related
              Topics", RFC 879, DOI 10.17487/RFC0879, November 1983,
              <http://www.rfc-editor.org/info/rfc879>.

   [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and Considerations",
              RFC 2663, DOI 10.17487/RFC2663, August 1999,
              <http://www.rfc-editor.org/info/rfc2663>.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
              May 2000, <http://www.rfc-editor.org/info/rfc2827>.

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              DOI 10.17487/RFC3633, December 2003,
              <http://www.rfc-editor.org/info/rfc3633>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", RFC 4443,
              DOI 10.17487/RFC4443, March 2006,
              <http://www.rfc-editor.org/info/rfc4443>.

   [RFC4787]  Audet, F., Ed., and C. Jennings, "Network Address
              Translation (NAT) Behavioral Requirements for Unicast
              UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787,
              January 2007, <http://www.rfc-editor.org/info/rfc4787>.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
              <http://www.rfc-editor.org/info/rfc4821>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <http://www.rfc-editor.org/info/rfc4862>.

   [RFC4953]  Touch, J., "Defending TCP Against Spoofing Attacks",
              RFC 4953, DOI 10.17487/RFC4953, July 2007,
              <http://www.rfc-editor.org/info/rfc4953>.

   [RFC5382]  Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
              RFC 5382, DOI 10.17487/RFC5382, October 2008,
              <http://www.rfc-editor.org/info/rfc5382>.

   [RFC5508]  Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
              Behavioral Requirements for ICMP", BCP 148, RFC 5508,
              DOI 10.17487/RFC5508, April 2009,
              <http://www.rfc-editor.org/info/rfc5508>.

   [RFC5961]  Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
              Robustness to Blind In-Window Attacks", RFC 5961,
              DOI 10.17487/RFC5961, August 2010,
              <http://www.rfc-editor.org/info/rfc5961>.

   [RFC6056]  Larsen, M. and F. Gont, "Recommendations for Transport-
              Protocol Port Randomization", BCP 156, RFC 6056,
              DOI 10.17487/RFC6056, January 2011,
              <http://www.rfc-editor.org/info/rfc6056>.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
              April 2011, <http://www.rfc-editor.org/info/rfc6146>.

   [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,
              DOI 10.17487/RFC6147, April 2011,
              <http://www.rfc-editor.org/info/rfc6147>.

   [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,
              DOI 10.17487/RFC6219, May 2011,
              <http://www.rfc-editor.org/info/rfc6219>.

   [RFC6269]  Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
              P. Roberts, "Issues with IP Address Sharing", RFC 6269,
              DOI 10.17487/RFC6269, June 2011,
              <http://www.rfc-editor.org/info/rfc6269>.

   [RFC6324]  Nakibly, G. and F. Templin, "Routing Loop Attack Using
              IPv6 Automatic Tunnels: Problem Statement and Proposed
              Mitigations", RFC 6324, DOI 10.17487/RFC6324, August 2011,
              <http://www.rfc-editor.org/info/rfc6324>.

   [RFC6877]  Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
              Combination of Stateful and Stateless Translation",
              RFC 6877, DOI 10.17487/RFC6877, April 2013,
              <http://www.rfc-editor.org/info/rfc6877>.

   [RFC7598]  Mrugalski, T., Troan, O., Farrer, I., Perreault, S., Dec,
              W., Bao, C., Yeh, L., and X. Deng, "DHCPv6 Options for
              Configuration of Softwire Address and Port-Mapped
              Clients", RFC 7598, DOI 10.17487/RFC7598, July 2015,
              <http://www.rfc-editor.org/info/rfc7598>.

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

   [Stateless-4Via6]
              Dec, W., Asati, R., Bao, C., Deng, H., and M. Boucadair,
              "Stateless 4Via6 Address Sharing", Work in Progress,
              draft-dec-stateless-4v6-04, October 2011.

   [TR069]    Broadband Forum TR-069, "CPE WAN Management Protocol",
              Amendment 5, CWMP Version: 1.4, November 2013,
              <https://www.broadband-forum.org>.

Appendix A.  Examples of MAP-T Translation

   Example 1 - Basic Mapping Rule:

   Given the following MAP domain information and IPv6 end-user prefix
   assigned to a MAP CE:

   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-bit 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 (q):       o - p = (End-user prefix len -
                          Rule IPv6 prefix len) - 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 End-user IPv6 prefix.

   IPv6 address of MAP CE:  2001:db8:0012:3400:0000:c000:0212:0034
   Example 2 - BR:

   Another example is a MAP-T BR configured with the following FMR
   when receiving a packet with the following characteristics:

   IPv4 source address:       10.2.3.4 (0x0a020304)
   TCP source port:           80
   IPv4 destination address:  192.0.2.18 (0xc0000212)
   TCP destination port:      1232

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

   MAP-T BR Prefix (DMR):     2001:db8:ffff::/64

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

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

   The resulting IPv6 packet will have the following header fields:

   IPv6 source address:      2001:db8:ffff:0:000a:0203:0400::
   IPv6 destination address: 2001:db8:0012:3400:0000:c000:0212:0034
   TCP source port:          80
   TCP destination port:     1232


   Example 3 - FMR:

   An IPv4 host behind a MAP-T CE (configured as per the previous
   examples) corresponding with IPv4 host 10.2.3.4 will have its
   packets converted into IPv6 using the DMR configured on the MAP-T
   CE as follows:

   Default Mapping Rule:         {2001:db8:ffff::/64 (Rule IPv6 prefix),
                                  0.0.0.0/0 (Rule IPv4 prefix)}

   IPv4 source address:          192.0.2.18
   IPv4 destination address:     10.2.3.4
   IPv4 source port:             1232
   IPv4 destination port:        80
   MAP-T CE IPv6 source address: 2001:db8:0012:3400:0000:c000:0212:0034
   IPv6 destination address:     2001:db8:ffff:0:000a:0203:0400::

   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),
                             192.0.2.1/32 (Rule IPv4 prefix),
                             0 (Rule EA-bit length)}
   PSID length:             0 (sharing ratio is 1)
   PSID offset:             n/a

   A MAP node 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 -
                            IPv4 prefix length = 32 - 32 = 0
   IPv4 address:            192.0.2.18 (0xc0000212)
   PSID start:              0
   PSID length:             0
   PSID:                    null

   The BMR information allows a MAP CE to also 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:c000:0201:0000
   Example 5 - Rule with no embedded address bits and address sharing
   (sharing ratio of 256):

   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-bit length)}
   PSID length:             (16 - (32 - 24)) = 8 (sharing ratio of 256;
                            provisioned with DHCPv6)
   PSID offset:             6 (default)
   PSID:                    0x20 (provisioned with DHCPv6)

   A MAP node can, via the BMR, determine the IPv4 address and port set
   as shown below:

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

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

   The BMR information allows a MAP CE to also 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 are not derived from the IPv6
   prefix assigned to the CE but are provisioned separately, using, for
   example, MAP options in DHCPv6.

Appendix B.  Port-Mapping Algorithm

   The driving principles and the mathematical expression of the mapping
   algorithm used by MAP can be found in Appendix B of [RFC7597].

Acknowledgements

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

   The authors would also like to thank Mohamed Boucadair, Guillaume
   Gottard, Dan Wing, Jan Zorz, Nejc Skoberne, Tina Tsou, Gang Chen,
   Maoke Chen, Xiaohong Deng, Jouni Korhonen, Tomek Mrugalski, Jacni
   Qin, Chunfa Sun, Qiong Sun, Leaf Yeh, Andrew Yourtchenko, Roberta
   Maglione, and Hongyu Chen for their review and comments.

Contributors

   The following individuals authored major contributions to this
   document and made the document possible:

   Chongfeng Xie
   China Telecom
   Room 708, No. 118, Xizhimennei Street
   Beijing  100035
   China
   Phone: +86-10-58552116
   Email: xiechf@ctbri.com.cn

   Qiong Sun
   China Telecom
   Room 708, No. 118, Xizhimennei Street
   Beijing  100035
   China
   Phone: +86-10-58552936
   Email: sunqiong@ctbri.com.cn

   Rajiv Asati
   Cisco Systems
   7025-6 Kit Creek Road
   Research Triangle Park, NC  27709
   United States
   Email: rajiva@cisco.com

   Gang Chen
   China Mobile
   29, Jinrong Avenue
   Xicheng District, Beijing  100033
   China
   Email: phdgang@gmail.com, chengang@chinamobile.com
   Wentao Shang
   CERNET Center/Tsinghua University
   Room 225, Main Building, Tsinghua University
   Beijing  100084
   China
   Email: wentaoshang@gmail.com

   Guoliang Han
   CERNET Center/Tsinghua University
   Room 225, Main Building, Tsinghua University
   Beijing  100084
   China
   Email: bupthgl@gmail.com

   Yu Zhai
   CERNET Center/Tsinghua University
   Room 225, Main Building, Tsinghua University
   Beijing  100084
   China
   Email: jacky.zhai@gmail.com

Authors' Addresses

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

   Email: xing@cernet.edu.cn


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

   Email: congxiao@cernet.edu.cn


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

   Email: wdec@cisco.com
   Ole Troan
   Cisco Systems
   Philip Pedersens vei 1
   Lysaker  1366
   Norway

   Email: ot@cisco.com


   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, CA  94085
   United States

   Email: tetsuya@ipinfusion.com