This is a purely informative rendering of an RFC that includes verified errata. This rendering may not be used as a reference.

The following 'Verified' errata have been incorporated in this document: EID 5225, EID 5324, EID 7160
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

   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.
EID 5225 (Verified) is as follows:

Section: 6

Original Text:

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.

Corrected Text:

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.
Notes:
This text has been copied from RFC7597 (MAP-E). While it is correct in the context of MAP-E, for MAP-T the complete IPv4 source address must be embedded in the interface-identifier for correct translation in the case of an IPv4 prefix. Right padding the prefix with zeroes would lead to the translated packet having all zeroes in its source address.
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 either 20 or 28 bytes with fragmentation). The MTU in the MAP domain
EID 7160 (Verified) is as follows:

Section: 10.1

Original Text:

Translating an IPv4 packet to carry it across the MAP domain will increase its size (typically by 20 bytes).

Corrected Text:

Translating an IPv4 packet to carry it across the MAP domain will increase its size (typically either 20 or 28 bytes with fragmentation).
Notes:
In the context of MTU, it is important to account for packet fragmentation. If an IPv4 packet is fragmented, the size will increase by 28 bytes during translation. The IPv6 header is 20 bytes larger than the IPv4 header, and in addition the 8 byte IPv6 Fragmentation Header must be added.

----
Verifier note
==========
with help of Yong Cui: fragmentation indeed needs to be taken into account.
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
EID 5324 (Verified) is as follows:

Section: Appendix A

Original Text:

Example 5:

PSID:                    0x20 (provisioned with DHCPv6)

Corrected Text:

Example 5:

PSID:                    0x34 (provisioned with DHCPv6)
Notes:
In IPv4, IPv6 and port ranges presented in the example the PSID matches to 0x34 and not 0x20:

PSID: 0x34
Available ports (63 ranges): 1232-1235, 2256-2259, ...... ,
63696-63699, 64720-64723
IPv6 address of MAP CE: 2001:db8:0012:3400:0000:c000:0212:0034
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