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Network Working Group                                              X. Li
Internet-Draft                                                    C. Bao
Intended status: Experimental          CERNET Center/Tsinghua University
Expires: August 14, 2014                                     W. Dec, Ed.
                                                                O. Troan
                                                           Cisco Systems
                                                           S. Matsushima
                                                        SoftBank Telecom
                                                             T. Murakami
                                                             IP Infusion
                                                       February 10, 2014


         Mapping of Address and Port using Translation (MAP-T)
                      draft-ietf-softwire-map-t-05

Abstract

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

Status of This Memo

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

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

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

   This Internet-Draft will expire on August 14, 2014.

Copyright Notice

   Copyright (c) 2014 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



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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Mapping Rules . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Destinations outside the MAP domain . . . . . . . . . . .   7
   6.  The IPv6 Interface Identifier . . . . . . . . . . . . . . . .   7
   7.  MAP-T Configuration . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  MAP CE  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.2.  MAP BR  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   8.  MAP-T Packet Forwarding . . . . . . . . . . . . . . . . . . .   9
     8.1.  IPv4 to IPv6 at the CE  . . . . . . . . . . . . . . . . .   9
     8.2.  IPv6 to IPv4 at the CE  . . . . . . . . . . . . . . . . .  10
     8.3.  IPv6 to IPv4 at the BR  . . . . . . . . . . . . . . . . .  11
     8.4.  IPv4 to IPv6 at the BR  . . . . . . . . . . . . . . . . .  11
   9.  ICMP Handling . . . . . . . . . . . . . . . . . . . . . . . .  11
   10. Fragmentation and Path MTU Discovery  . . . . . . . . . . . .  12
     10.1.  Fragmentation in the MAP domain  . . . . . . . . . . . .  12
     10.2.  Receiving IPv4 Fragments on the MAP domain borders . . .  12
     10.3.  Sending IPv4 fragments to the outside  . . . . . . . . .  13
   11. NAT44 Considerations  . . . . . . . . . . . . . . . . . . . .  13
   12. Usage Considerations  . . . . . . . . . . . . . . . . . . . .  13
     12.1.  EA-bit length 0  . . . . . . . . . . . . . . . . . . . .  13
     12.2.  Mesh and Hub and spoke modes . . . . . . . . . . . . . .  13
     12.3.  Communication with IPv6 servers in the MAP-T domain  . .  14
     12.4.  Compatibility with other NAT64 solutions . . . . . . . .  14
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  14
   15. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  15
   16. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     17.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     17.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Appendix A.  Examples of MAP-T translation  . . . . . . . . . . .  19
   Appendix B.  Port mapping algorithm . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22






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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-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
   optimalise 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 [I-D.ietf-softwire-stateless-4v6-motivation].

   The Mapping of Address and Port - Translation (MAP-T) architecture
   specified in this draft is such a double stateless NAT64 based
   solution.  It builds on existing stateless NAT64 techniques specified
   in [RFC6145], along with the stateless algorithmic address &
   transport layer port mapping scheme defined in MAP-E
   [I-D.ietf-softwire-map].  The MAP-T solution differs from MAP-E in
   the use of 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. use of IPv6 only servers, or reliance on
   IPv6 + protocol headers for traffic classification) rule out
   encapsulation.  These scenarios are presented in
   [I-D.maglione-softwire-map-t-scenarios]

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 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
                           Border Relay router has at least an
                           IPv6-enabled interface and an IPv4 interface



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                           connected to the native IPv4 network.  A MAP
                           BR may also be referred to simply as 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 out of all the MAP
                           Rules communicated to a device, that are
                           intended for determining the devices' traffic
                           forwarding operations.  A set has at least
                           one entry, known as a default map rule.  The
                           Rule set is interchangeably referred to in
                           this document as a Rule table.

   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; denoted as a
                           port-set.

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

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

   End-user IPv6 prefix:   The IPv6 prefix assigned to an End-user CE by
                           other means than MAP itself.  E.g.
                           Provisioned using DHCPv6 PD [RFC3633],
                           assigned via SLAAC [RFC4862], or configured
                           manually.  It is unique for each CE.

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



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   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 BR.  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 IPv6 network as
   follows:

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

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


















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         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)
   |   |   +--------+ |  (w/ v4-embedded-v6 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 is expanded using MAP procedures to form a MAP
   IPv6 address and an IPv4 address.  To allow for IPv4 address sharing,
   the CE may also have be configured with a TCP/UDP port-range that is
   codified by means of a MAP Port Set Identifier (PSID) value.  Each CE
   is responsible for forwarding traffic between a given users' private
   IPv4 address space and the CE's MAP derived IPv4 address + port set,
   as well as adapting traffic between IPv4 and IPv6 using stateless
   NAT64.

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

   In contrast to MAP-E, NAT64 technology is used in the architecture
   for two purposes.  Firstly, it is intended to diminish encapsulation
   overhead and normalize IPv4 traffic to IPv6 as much as possible.
   Secondly, it is intended to allow IPv4-only nodes to correspond




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   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 i)
   algorithmic IPv4-IPv6 address mapping codified as MAP Rules covered
   in Section 5 ii) A MAP IPv6 address identifier, described in
   Section 6 iii) MAP-T IPv4-IPv6 forwarding behavior 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 [I-D.ietf-softwire-map], with
   the exception of Section 5.4 concerning the forwarding of traffic to/
   from IPv4 nodes outside the MAP-T.

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 BMR, as the IPv6 source address and the recipient's MAP IPv6
   address, derived via the FMR, as the IPv6 destination address.

   IPv4 destinations outside of the MAP domain are represented by means
   of IPv4-Embedded IPv6 address s 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 the CE's MAP IPv6 address and the
   destination IPv6 address will be the destination IPv4-embedded-IPv6
   address.  This representation is termed as the MAP-T Default Mapping
   Rule (DMR) and is specified in terms of an IPv6 prefix advertised by
   one or more BRs.  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.

   It is recommended that the DMR IPv6 prefix-length SHOULD be by
   default 64 bits long, 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
   described in section 6 of [I-D.ietf-softwire-map].  For convenience
   this is cited below:






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


                                 Figure 2

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

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

7.  MAP-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 Translation mode (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 that are the
   same across all CEs within that domain.  These values may be
   configured using a variety of methods, including DHCPv6, Broadband
   Forum's "TR-69" Residential Gateway management interface, 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 [I-D.ietf-softwire-map-dhcp].  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.



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   Besides the MAP configuration parameters, a CE requires an the 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 a given CE operates within would
   require its own set of MAP configuration elements and would generate
   its own IPv4 address.  Each MAP domain requires a distinct End-user
   IPv6 prefix.

   The MAP DHCPv6 option is specified in [I-D.ietf-softwire-map-dhcp]

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 routes need to be
   advertised externally for MAP to operate, neither in IPv6 nor IPv4
   BGP.  However, the BR prefix needs to be advertised in the service
   provider's IGP.

8.  MAP-T Packet Forwarding

   The end-end packet flow in MAP-T involves an IPv4 or IPv6 packet
   being forwarded by a CE of 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 NAPT NAT44 function,
   and create any necessary NAPT44 bindings.  The source address and
   port of the packet obtained as a result of the NAPT44 process MUST
   correspond to the source IPv4 address and source transport port




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   number derived to belong 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 default mapping rule (DMR), and subject to the
   stateless NAT64 operation using the DMR parameters for NAT64
   Section 5.1.  Packets that are matched to (optional) Forward 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 Forward 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 MAP-T received packets' destination
   transport-layer destination port number is in the range allowed for
   by the CE's MAP BMR configuration.  The CE MUST silently drop any non
   conforming packets and an appropriate counter incremented.  For
   packets whose source address longest matches an FMR prefix, the CE
   MUST perform a check of consistency of the source address against the
   allowed values from the derived source port-range.  If the packets'
   source port number is found to be outside the range allowed, 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 packet, the CE's NAT64 function MUST derive the IPv4
   source and destination addresses.  The IPv4 destination address is
   derived 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 the 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 NAPT 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



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   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 IPv6 packets MUST select a matching MAP rule
   based on a longest address match of the packets' source address
   against the BR's configured MAP Rules.  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 packets' source port number is found
   to be outside the range allowed, the BR MUST drop the packet and
   increment a counter to indicate that a potential spoofing attack may
   be underway.

   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 packets are 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).  Throughout the
   generic IPv4 to IPv6 header translation procedures following
   [RFC6145] apply.  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 [RFC0792], [RFC4443] Query message header has only one ID
   field which needs to be used to identify a sending IPv4 host.  When



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

10.  Fragmentation and Path MTU Discovery

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

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.  It is strongly recommended
   that the MTU in the MAP domain be well managed and that the IPv6 MTU
   on the CE WAN side interface be set so that no fragmentation occurs
   within the boundary of the MAP domain.

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

10.2.  Receiving IPv4 Fragments on the MAP domain borders

   Forwarding of an IPv4 packet received from the outside of the MAP
   domain requires the IPv4 destination address and the transport
   protocol destination port.  The transport protocol information is
   only available in the first fragment received.  As described in
   section 5.3.3 of [RFC6346] a MAP node receiving an IPv4 fragmented
   packet from outside has to reassemble the packet before sending the
   packet onto the MAP 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.








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10.3.  Sending IPv4 fragments to the outside

   Two IPv4 hosts behind two different MAP CE's 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 with the behavior
   and best current practice documented in [RFC4787], [RFC5508], and
   [RFC5382].  In MAP address sharing mode (determined by the MAP domain
   /rule configuration parameters) the operation of the NAT44 MUST be
   restricted to the available port numbers derived via the basic
   mapping rule.

12.  Usage Considerations

12.1.  EA-bit length 0

   The MAP solution supports 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 1 MAP CE which has a predetermined IPv6 end-user prefix.  The BR
   would be configured with an FMR for each such 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 Forward Mapping Rule 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.






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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 both IPv6 users as
   well as IPv4-only users alike utilizing IPv6.  Any such IPv6-only
   servers SHOULD have both A and AAAA records in DNS.  DNS64 [RFC6147]
   become required only when IPv6 servers in the MAP-T domain are
   expected themselves to initiate communication to external IPv4-only
   hosts.

12.4.  Compatibility with other NAT64 solutions

   The MAP-T CEs 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 CLAT as defined in [RFC6877].

13.  IANA Considerations

   This specification does not require any IANA actions.

14.  Security Considerations

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

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

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



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

   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 nodes' ICMP computing resources.  This attack vector is not
      specific to MAP, and its mitigation lies 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.

15.  Contributors

   The following individuals authored major contribution to this
   document:

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

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

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

   Gang Chen (China Mobile) 53A,Xibianmennei Ave. Beijing 100053
   P.R.China Email: chengang@chinamobile.com

   Wentao Shang (CERNET Center/Tsinghua University) Room 225, Main
   Building, Tsinghua University Beijing 100084 CN Email:
   wentaoshang@gmail.com





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

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

16.  Acknowledgements

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

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

17.  References

17.1.  Normative References

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

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

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

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

17.2.  Informative References

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









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   [I-D.ietf-softwire-map-dhcp]
              Mrugalski, T., Troan, O., Dec, W., Bao, C.,
              leaf.yeh.sdo@gmail.com, l., and X. Deng, "DHCPv6 Options
              for configuration of Softwire Address and Port Mapped
              Clients", draft-ietf-softwire-map-dhcp-06 (work in
              progress), November 2013.

   [I-D.ietf-softwire-map]
              Troan, O., Dec, W., Li, X., Bao, C., Matsushima, S.,
              Murakami, T., and T. Taylor, "Mapping of Address and Port
              with Encapsulation (MAP)", draft-ietf-softwire-map-10
              (work in progress), January 2014.

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

   [I-D.maglione-softwire-map-t-scenarios]
              Maglione, R., Dec, W., Leung, I., and E. Mallette, "Use
              cases for MAP-T", draft-maglione-softwire-
              map-t-scenarios-03 (work in progress), October 2013.

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

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

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

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

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

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.





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

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

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

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

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

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

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

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, April 2011.

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

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

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

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




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   [RFC6877]  Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
              Combination of Stateful and Stateless Translation", RFC
              6877, April 2013.

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

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

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












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   Example 2 - BR:


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

      IPv4 source address:       1.2.3.4 (0x01020304)
      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-bits length)}

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

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

      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:0:0001:0203:0400::
      IPv6 destination address: 2001:db8:0012:3400:0000:c000:0212:0034
      TCP source Port:          80
      TCP destination Port:     1232


















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   Example 3- FMR:


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

   Default Mapping Rule:         {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:     1.2.3.4
   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:0:0001:0203:0400::


   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-bits 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 also to determine (complete)
   its full IPv6 address by combining the IPv6 prefix with the MAP
   interface identifier (that embeds the IPv4 address).

   IPv6 address of MAP CE:  2001:db8:0012:3400:0000:c000:0201:0000







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   Example 5 - Rule with no embedded address bits and address sharing
   (sharing ratio 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-bits 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 also to determine (complete)
   its full IPv6 address by combining the IPv6 prefix with the MAP
   interface identifier (that embeds the IPv4 address and PSID).

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

   Note that the IPv4 address and PSID is not derived from the IPv6
   prefix assigned to the CE, but provisioned separately using 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
   [I-D.ietf-softwire-map]

Authors' Addresses







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

   Email: xing@cernet.edu.cn


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

   Email: congxiao@cernet.edu.cn


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

   Email: wdec@cisco.com


   Ole Troan
   Cisco Systems
   Oslo
   Norway

   Email: ot@cisco.com


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

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









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   Tetsuya Murakami
   IP Infusion
   1188 East Arques Avenue
   Sunnyvale
   USA

   Email: tetsuya@ipinfusion.com












































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