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Softwire Working Group                                          Y.C. Cui
Internet-Draft                                       Tsinghua University
Intended status: Standards Track                                Q.S. Sun
Expires: April 23, 2013                                    China Telecom
                                                          M.B. Boucadair
                                                          France Telecom
                                                               T.T. Tsou
                                                     Huawei Technologies
                                                                  Y. Lee
                                                                 Comcast
                                                             I.F. Farrer
                                                     Deutsche Telekom AG
                                                        October 22, 2012

      Lightweight 4over6: An Extension to the DS-Lite Architecture
              draft-cui-softwire-b4-translated-ds-lite-09

Abstract

   DS-Lite [RFC6333] describes an architecture for transporting IPv4
   packets over an IPv6 network.  This document specifies an extension
   to DS-Lite called Lightweight 4over6 which moves the Network Address
   Translation function from the DS-Lite AFTR to the B4, removing the
   requirement for a Carrier Grade NAT function in the AFTR.  This
   reduces the amount of centralized state that must be held to a per-
   subscriber level.  In order to delegate the NAPT function and make
   IPv4 Address sharing possible, port-restricted IPv4 addresses are
   allocated to the B4s.

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 April 23, 2013.

Copyright Notice

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



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

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Lightweight 4over6 Architecture  . . . . . . . . . . . . . . .  5
   5.  Lightweight B4 Behavior  . . . . . . . . . . . . . . . . . . .  6
     5.1.  Lightweight B4 Provisioning  . . . . . . . . . . . . . . .  6
     5.2.  Lightweight B4 Data Plane Behavior . . . . . . . . . . . .  7
   6.  Lightweight AFTR Behavior  . . . . . . . . . . . . . . . . . .  8
     6.1.  Binding Table Maintenance  . . . . . . . . . . . . . . . .  8
     6.2.  lwAFTR Data Plane Behavior . . . . . . . . . . . . . . . .  9
   7.  Provisioning using DHCPv4 over IPv6 Transport  . . . . . . . . 10
     7.1.  lwB4 DHCPv4 Based Provisioning . . . . . . . . . . . . . . 10
     7.2.  lwAFTR DHCPv4 Based Provisioning . . . . . . . . . . . . . 11
   8.  ICMP Processing  . . . . . . . . . . . . . . . . . . . . . . . 11
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   11. Author List  . . . . . . . . . . . . . . . . . . . . . . . . . 12
   12. Acknowledgement  . . . . . . . . . . . . . . . . . . . . . . . 14
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     13.1.  Normative References  . . . . . . . . . . . . . . . . . . 14
     13.2.  Informative References  . . . . . . . . . . . . . . . . . 15
   Appendix A. Alternatives for Port-Restricted Address Allocation  . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17

1.  Introduction

   Dual-Stack Lite (DS-Lite, [RFC6333]) defines a model for providing
   IPv4 access over an IPv6 network using two well-known technologies:
   IP in IP [RFC2473] and Network Address Translation (NAT). The DS-Lite
   architecture defines two major functional elements as follows:

   Basic Bridging BroadBand element: A B4 element is a function
                                     implemented on a dual-stack capable
                                     node, either a directly connected
                                     device or a CPE, that creates a
                                     tunnel to an AFTR.







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   Address Family Transition Router: An AFTR element is the combination
                                     of an IPv4-in-IPv6 tunnel endpoint
                                     and an IPv4-IPv4 NAT implemented on
                                     the same node.

   As the AFTR performs the centralized NAT44 function, it dynamically
   assigns public IPv4 addresses and ports to requesting host's traffic
   (as described in [RFC3022]). To achieve this, the AFTR must
   dynamically maintain per-flow state in the form of active NAPT
   sessions.  For service providers with a large number of B4 clients,
   the size and associated costs for scaling the AFTR can quickly become
   prohibitive.  It can also place a large NAPT logging overhead upon
   the service provider in countries where legal requirements mandate
   this.

   This document describes a mechanism called Lightweight 4 over 6
   (lw4o6), which provides a solution for these problems.  By relocating
   the NAPT functionality from the centralized AFTR to the distributed
   B4s, a number of benefits can be realised:

   o  NAPT44 functionality is already widely supported and used in
      today's CPE devices.  Lw4o6 uses this to provide private<->public
      NAPT44, meaning that the service provider does not need a
      centralized NAT44 function.

   o  The amount of state that must be maintained centrally in the AFTR
      can be reduced from per-flow to per-subscriber.  This reduces the
      amount of resources (memory and processing power) necessary in the
      AFTR.

   o  The reduction of maintained state results in a greatly reduced
      logging overhead on the service provider.

   Operator's IPv6 and IPv4 addressing architectures remain independent
   of each other as in DS-Lite.  Therefore, flexible IPv4/IPv6
   addressing schemes can be deployed.

   Lightweight 4over6 provides a solution for a hub-and-spoke softwire
   architecture only.  It does not offer direct, meshed IPv4
   connectivity between subscribers without packets traversing the AFTR.
   If this type of meshed interconnectivity is required, [I-D.ietf-
   softwire-map] provides a suitable solution.

   The tunneling mechanism remains the same for DS-Lite and Lightweight
   4over6. This document describes the changes to DS-Lite that are
   necessary to implement Lightweight 4over6. These changes mainly
   concern the configuration parameters and provisioning method
   necessary for the functional elements.

   This document is an extended case, which covers address sharing for
   [I-D.ietf-softwire-public-4over6].  It is also a variant of A+P
   called Binding Table Mode (see Section 4.4 of [RFC6346]).



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   This document focuses on architectural considerations and
   particularly on the expected behavior of the involved functional
   elements and their interfaces.  Deployment-specific issues are
   discussed in a companion document.  As such, discussions about
   redundancy and provisioning policy are out of scope.

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 [RFC2119].

3.  Terminology

   The document defines the following terms:

   Lightweight 4over6 (lw4o6):   Lightweight 4over6 is an IPv4-over-IPv6
                                 hub and spoke mechanism, which extends
                                 DS-Lite by moving the IPv4 translation
                                 (NAPT44) function from the AFTR to the
                                 B4.

   Lightweight B4 (lwB4):        A B4 element (Basic Bridging BroadBand
                                 element [RFC6333]), which supports
                                 Lightweight 4over6 extensions.  An lwB4
                                 is a function implemented on a dual-
                                 stack capable node, (either a directly
                                 connected device or a CPE), that
                                 supports port-restricted IPv4 address
                                 allocation, implements NAPT44
                                 functionality and creates a tunnel to
                                 an lwAFTR

   Lightweight AFTR (lwAFTR):    An AFTR element (Address Family
                                 Transition Router element [RFC6333]),
                                 which supports Lightweight 4over6
                                 extension.  An lwAFTR is an IPv4-in-
                                 IPv6 tunnel endpoint which maintains
                                 per-subscriber address binding only and
                                 does not perform a NAPT44 function.

   Restricted Port-Set:          A non-overlapping range of allowed
                                 external ports allocated to the lwB4 to
                                 use for NAPT44.  Source ports of IPv4
                                 packets sent by the B4 must belong to
                                 the assigned port-set.  The port set is
                                 used for all port aware IP protocols
                                 (TCP, UDP, SCTP etc.)






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   Port-restricted IPv4 Address: A public IPv4 address with a restricted
                                 port-set.  In Lightweight 4over6,
                                 multiple B4s may share the same IPv4
                                 address, however, their port-sets must
                                 be non-overlapping.

   Throughout the remainder of this document, the terms B4/AFTR should
   be understood to refer specifically to a DS-Lite implementation.  The
   terms lwB4/lwAFTR refer to a Lightweight 4over6 implementation.

4.  Lightweight 4over6 Architecture

   The Lightweight 4over6 architecture is functionally similar to DS-
   Lite.  lwB4s and an lwAFTR are connected through an IPv6-enabled
   network.  Both approaches use an IPv4-in-IPv6 encapsulation scheme to
   deliver IPv4 connectivity services.  The following figure shows the
   data plane with main functional change between DS-Lite and lw4o6:


   +--------+   +---------+   IPv4-in-IPv6    +------+      +-------------+
   |IPv4 LAN|---|lwB4/NAPT|===================|lwAFTR|------|IPv4 Internet|
   +--------+   +---------+                   +------+      +-------------+
                       ^                          |
                       +--------------------------+
                         NAPT function relocated
                            to lwB4 in lw4o6


   Figure 1 Lightweight 4over6 Data Plane Overview

   There are three main components in the Lightweight 4over6
   architecture:

   o  The lwB4, which performs the NAPT function and encapsulation/de-
      capsulation IPv4/IPv6.

   o  The lwAFTR, which performs the encapsulation/de-capsulation IPv4/
      IPv6.

   o  The provisioning system, which tells the lwB4 which IPv4 address
      and port set to use.

   The lwB4 differs from a regular B4 in that it now performs the NAPT
   functionality.  This means that it needs to be provisioned with the
   public IPv4 address and port set it is allowed to use.  This
   information is provided though a provisioning mechanism such as DHCP,
   PCP or TR-69.







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   The lwAFTR needs to know the binding between the IPv6 address of each
   subscriber and the IPv4 address and port set allocated to that
   subscriber.  This information is used to perform ingress filtering
   upstream and encapsulation downstream.  Note that this is per-
   subscriber state as opposed to per-flow state in the regular AFTR
   case.

   The consequence of this architecture is that the information
   maintained by the provisioning mechanism and the one maintained by
   the lwAFTR MUST be synchronized (See figure 2). The details of this
   synchronization depend on the exact provisioning mechanism and will
   be discussed in a companion draft.

                            +------------+
                    /-------|Provisioning|<-------\
                    |       +------------+        |
                    |                             |
                    V                             V
   +--------+   +---------+    IPv4/IPv6      +------+      +-------------+
   |IPv4 LAN|---|lwB4/NAPT|===================|lwAFTR|------|IPv4 Internet|
   +--------+   +---------+                   +------+      +-------------+

   Figure 2 Lightweight 4over6 Provisioning Synchronization

5.  Lightweight B4 Behavior

5.1.  Lightweight B4 Provisioning

   With DS-Lite, the B4 element only needs to be configured with a
   single DS-Lite specific parameter so that it can set up the softwire
   (the IPv6 address of the AFTR). Its IPv4 address can be taken from
   the well-known range 192.0.0.0/29.

   In lw4o6, due to the distributed nature of the NAPT function, a
   number of lw4o6 specific configuration parameters must be provisioned
   to the lwB4. These are:

   o  IPv6 Address for the lwAFTR (as in DS-Lite)

   o  IPv4 External (Public) Address for NAPT44

   o  Restricted port-set to use for NAPT44

   An IPv6 address from an assigned prefix is also required for the lwB4
   to use as the encapsulation source address for the softwire.
   Normally, this is the lwB4's globally unique WAN interface address
   which can be obtained via an IPv6 address allocation procedure such
   as SLAAC, DHCPv6 or manual configuration.

   In the event that the lwB4's encapsulation source address is changed




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   for any reason (such as the DHCPv6 lease expiring), the lwB4's
   dynamic provisioning process must be re-initiated.

   For learning the IPv6 address of the lwAFTR, the lwB4 SHOULD
   implement the method described in section 5.4 of [RFC6333] and
   implement the DHCPv6 option defined in [RFC6334].  Other methods of
   learning this address are also possible.

   An lwB4 MUST support dynamic port-restricted IPv4 address
   provisioning (unlike a DS-Lite B4). Several different mechanisms can
   be used for provisioning the lwB4 with its port-restricted IPv4
   address such as: DHCPv4, DHCPv6, PCP, PPP and IPCP. Some alternatives
   are mentioned in Appendix A of this document.

   In this document, it is RECOMMENDED that the DHCPv4 provisioning
   method is implemented as it is widely deployed in services providers
   networks and supports all IPv4 and IPv6 addressing models.  The
   DHCPv4 based provisioning model is described in section 7 of this
   document.

   In the event that the lwB4 receives and ICMPv6 error message (type 1,
   code 5) originating from the lwAFTR, the lwB4 SHOULD interpret this
   to mean that no matching entry in the lwAFTR's binding table has been
   found.  The lwB4 MAY then re-initiate the dynamic port-restricted
   provisioning process.  The lwB4's re-initiation policy SHOULD be
   configurable.

   The DNS considerations described in Section 5.5 and Section 6.4 of
   [RFC6333] SHOULD be followed.

5.2.  Lightweight B4 Data Plane Behavior

   Several sections of [RFC6333] provide background information on the
   B4's data plane functionality and MUST be implemented by the lwB4 as
   they are common to both solutions.  The relevant sections are:

   5.2. Encapsulation                Covering encapsulation and de-
                                     capsulation of tunneled traffic

   5.3. Fragmentation and Reassembly Covering MTU and fragmentation
                                     considerations (referencing
                                     [RFC2473])

   7.1. Tunneling                    Covering tunneling and traffic
                                     class mapping between IPv4 and IPv6
                                     (referencing [RFC2473] and
                                     [RFC4213])

   The lwB4 element performs IPv4 address translation (NAPT44) as well
   as encapsulation and de-capsulation.  It runs standard NAPT44
   [RFC3022] using the allocated port-restricted address as its external




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   IPv4 address and port numbers.

   Internally connected hosts source IPv4 packets with an [RFC1918]
   address.  When the lwB4 receives such an IPv4 packet, it performs a
   NAPT44 function on the source address and port by using the public
   IPv4 address and a port number from the allocated port-set.  Then, it
   encapsulates the packet with an IPv6 header.  The destination IPv6
   address is the lwAFTR's IPv6 address and the source IPv6 address is
   the lwB4's IPv6 tunnel endpoint address.  Finally, the lwB4 forwards
   the encapsulated packet to the configured lwAFTR.

   When the lwB4 receives an IPv4-in-IPv6 packet from the lwAFTR, it de-
   capsulates the IPv4 packet from the IPv6 packet.  Then, it performs
   NAPT44 translation on the destination address and port, based on the
   available information in its local NAPT44 table.

   The lwB4 is responsible for performing ALG functions (e.g., SIP,
   FTP), and other NAPT traversal mechanisms (e.g., UPnP, NAPT-PMP,
   manual binding configuration, PCP) for the internal hosts.  This
   requirement is typical for NAPT44 gateways available today.

   It is possible that a lwB4 is co-located in a host.  In this case,
   the functions of NAPT44 and encapsulation/de-capsulation are
   implemented inside the host.

   If the lwB4 is provisioned with a full port-set (e.g.  all ports from
   0 to 65535), then it SHOULD behave as a 4 over 6 Initiator as
   described in [I-D.ietf-softwire-public-4over6].

6.  Lightweight AFTR Behavior

6.1.  Binding Table Maintenance

   The lwAFTR maintains an address binding table containing the binding
   between the lwB4's IPv6 address, the allocated IPv4 address and
   restricted port-set.  Unlike the DS-Lite extended binding table
   defined in section 6.6 of [RFC6333] which is a 5-tuple NAT table,
   each entry in the Lightweight 4over6 binding table contains the
   following 3-tuples:

   o  IPv6 Address for a single lwB4

   o  Public IPv4 Address

   o  Restricted port-set

   The entry has two functions: the IPv6 encapsulation of inbound IPv4
   packets destined to the lwB4 and the validation of outbound IPv4-in-
   IPv6 packets received from the lwB4 for de-capsulation.

   The lwAFTR does not perform NAPT and so does not need session
   entries.



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   The lwAFTR MUST synchronize the binding information with the port-
   restricted address provisioning process.  If the lwAFTR does not
   participate in the port-restricted address provisioning process, the
   binding MUST be synchronized through other methods (e.g.  out-of-band
   static update).

   If the lwAFTR participates in the port-restricted provisioning
   process, then its binding table MUST be created as part of this
   process.

   For all provisioning processes, the lifetime of binding table entries
   MUST be synchronized with the lifetime of address allocations.


6.2.  lwAFTR Data Plane Behavior

   Several sections of [RFC6333] provide background information on the
   AFTR's data plane functionality and MUST be implemented by the lwAFTR
   as they are common to both solutions.  The relevant sections are:

   6.2. Encapsulation                Covering encapsulation and de-
                                     capsulation of tunneled traffic

   6.3. Fragmentation and Reassembly Fragmentation and re-assembly
                                     considerations (referencing
                                     [RFC2473])

   7.1. Tunneling                    Covering tunneling and traffic
                                     class mapping between IPv4 and IPv6
                                     (referencing [RFC2473] and
                                     [RFC4213])

   When the lwAFTR receives an IPv4-in-IPv6 packet from an lwB4, it de-
   capsulates the IPv6 header and verifies the source addresses and port
   in the binding table.  If both the source IPv4 and IPv6 addresses
   match a single entry in the binding table and the source port in the
   allowed port-set for that entry, the lwAFTR forwards the packet to
   the IPv4 destination.

   If no match is found (e.g., no matching IPv4 address entry, port out
   of range, etc.), the lwAFTR MUST discard the packet.  An ICMPv6 type
   1, code 5 (source address failed ingress/egress policy) error message
   MAY be sent back to the requesting lwB4. The ICMP policy SHOULD be
   configurable.

   When the lwAFTR receives an inbound IPv4 packet, it uses the IPv4
   destination address and port to lookup the destination lwB4's IPv6
   address in its binding table.  If a match is found, the lwAFTR
   encapsulates the IPv4 packet.  The source is the lwAFTR's IPv6





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   address and the destination is the lwB4's IPv6 address from the
   matched entry.  Then, the lwAFTR forwards the packet to the lwB4
   natively over the IPv6 network.

   If no match is found, the lwAFTR MUST discard the packet.  An ICMPv4
   type 3, code 1 (Destination unreachable, host unreachable) error
   message MAY be sent back.  The ICMP policy SHOULD be configurable.

   The lwAFTR MUST support hairpinning of traffic between two lwB4s, by
   performing de-capsulation and re-encapsulation of packets.  The
   hairpinning policy MUST be configurable.

   If the binding table entry has a full port-set (e.g.  all ports from
   0 to 65535) allocated for an lwB4 client, then the lwAFTR SHOULD
   behave as a 4 over 6 concentrator as described in [I-D.ietf-softwire-
   public-4over6].

7.  Provisioning using DHCPv4 over IPv6 Transport

   The DHCPv4 based provisioning model uses DHCPv4 format messages
   within an IPv6 packet as described in [I-D.ietf-dhc-dhcpv4-over-
   ipv6].  This is used for configuring the lwB4's public IPv4 address
   and port-set that will be used for the softwire and NAPT44 function.

7.1.  lwB4 DHCPv4 Based Provisioning

   The lwB4's steps for this configuration model are as follows:

   1.  The lwB4 learns IPv6 Address of DHCPv4 over IPv6 Server

   2.  The lwB4 sends a DHCPv4 over IPv6 request (Discover) message

   3.  The DHCPv4 over IPv6 response contains the public IPv4 address
       and restricted port-set to configure NAPT44 and the softwire

   The lwB4 must implement the Client Relay Agent function described in
   [I-D.ietf-dhc-dhcpv4-over-ipv6].  This function is responsible for
   converting the DHCPv4 message's IPv4 transport to an IPv6 transport.

   To learn the IPv6 unicast address of the DHCPv4 over IPv6 server or
   relay, the lwB4 SHOULD implement the DHCPv6 option defined in [I-D
   .mrugalski-softwire-dhcpv4-over-v6-option].

   If the DHCPv4 over IPv6 client has multiple IPv6 addresses assigned,
   the mechanisms defined in [RFC3484] MUST be applied for selecting the
   correct address as the source of the DHCPv4 over IPv6 request.  A
   DHCPv4 over IPv6 client embedded within the lwB4 MUST use the same
   IPv6 address as the data plane encapsulation source address for all
   DHCPv4 over IPv6 requests.

   To implement this provisioning model, the lwB4 MUST support public
   IPv4 address and restricted port-set allocation over DHCPv4 according
   to the mechanism described in [I-D.sun-dhc-port-set-option] (or
   evolution thereof).

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7.2.  lwAFTR DHCPv4 Based Provisioning

   The DHCPv4 over IPv6 based provisioning process can be considered
   out-of-band from the perspective of the lwAFTR in that the lwAFTR
   does not need to be directly involved for the mechanism to function
   correctly.  However, the contents of the lwAFTR's binding table MUST
   be synchronized with the DHCPv4 over IPv6 server.

   This is necessary to ensure that the IPv4 address and port-set that
   is allocated in response to a specific client's DHCP request (e.g.
   the originating IPv6 address of the request) matches the equivalent
   entry in the lwAFTR's binding table.  If this elements are not kept
   synchronized, then the lwAFTR will either discard or mis-route
   packets it receives.

   The lwAFTR MAY implement a local DHCPv4 over IPv6 server or Relay
   Agent as described in [I-D.ietf-dhc-dhcpv4-over-ipv6].  If one of
   these is implemented, the lwB4s MAY send DHCPv4 over IPv6 messages to
   the lwAFTR which can then learn the bindings between IPv6 address and
   IPv4 address with port set directly.

8.  ICMP Processing

   ICMP does not work in an address sharing environment without special
   handling [RFC6269].  Due to the port-set style address sharing,
   Lightweight 4over6 requires specific ICMP message handling not
   required by DS-Lite.

   The following behavior SHOULD be implemented by the lwAFTR to provide
   ICMP error handling and basic remote IPv4 service diagnostics for a
   port restricted CPE: for inbound ICMP messages, the lwAFTR MAY behave
   in two modes:

   Either:

   1.  Check the ICMP Type field.

   2.  If the ICMP type is set to 0 or 8 (echo reply or request), then
       the lwAFTR MUST take the value of the ICMP identifier field as
       the source port, and use this value to lookup the binding table
       for an encapsulation destination.  If a match is found, the
       lwAFTR forwards the ICMP packet to the IPv6 address stored in the
       entry; otherwise it MUST discard the packet.

   3.  If the ICMP type field is set to any other value, then the lwAFTR
       MUST use the method described in REQ-3 of [RFC5508] to locate the
       source port within the transport layer header in ICMP packet's
       data field.  The destination IPv4 address and source port
       extracted from the ICMP packet are then used to make a lookup in
       the binding table.  If a match is found, it MUST forward the ICMP
       reply packet to the IPv6 address stored in the entry; otherwise
       it MUST discard the packet.


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   Or:

   o  Discard all inbound ICMP messages.

   The ICMP policy SHOULD be configurable.

   The lwB4 SHOULD implement the requirements defined in [RFC5508] for
   ICMP forwarding.  For ICMP echo request packets originating from the
   private IPv4 network, the lwB4 SHOULD implement the method described
   in [RFC6346] and use an available port from its port-set as the ICMP
   Identifier.

   For both the lwAFTR and the lwB4, ICMPv6 MUST be handled as described
   in [RFC2473].

9.  Security Considerations

   As the port space for a subscriber shrinks due to address sharing,
   the randomness for the port numbers of the subscriber is decreased
   significantly.  This means it is much easier for an attacker to guess
   the port number used, which could result in attacks ranging from
   throughput reduction to broken connections or data corruption.

   The port-set for a subscriber can be a set of contiguous ports or
   non-contiguous ports.  Contiguous port-sets do not reduce this
   threat.  However, with non-contiguous port-set (which may be
   generated in a pseudo-random way [RFC6431]), the randomness of the
   port number is improved, provided that the attacker is outside the
   Lightweight 4over6 domain and hence does not know the port-set
   generation algorithm.

   More considerations about IP address sharing are discussed in Section
   13 of [RFC6269], which is applicable to this solution.

10.  IANA Considerations

   This document does not include an IANA request.

11.  Author List

   The following are extended authors who contributed to the effort:

      Jianping Wu
   Tsinghua University
   Department of Computer Science, Tsinghua University
   Beijing 100084
   P.R.China

      Phone: +86-10-62785983
   Email: jianping@cernet.edu.cn

      Peng Wu
   Tsinghua University

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   Department of Computer Science, Tsinghua University
   Beijing 100084
   P.R.China

      Phone: +86-10-62785822
   Email: pengwu.thu@gmail.com

      Chongfeng Xie
   China Telecom
   Room 708, No.118, Xizhimennei Street
   Beijing 100035
   P.R.China

      Phone: +86-10-58552116
   Email: xiechf@ctbri.com.cn

      Xiaohong Deng
   France Telecom

      Email: xiaohong.deng@orange.com

      Cathy Zhou
   Huawei Technologies
   Section B, Huawei Industrial Base, Bantian Longgang
   Shenzhen 518129
   P.R.China

      Email: cathyzhou@huawei.com

      Alain Durand
   Juniper Networks
   1194 North Mathilda Avenue
   Sunnyvale, CA 94089-1206
   USA

      Email: adurand@juniper.net

      Reinaldo Penno
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, California 95134
   USA

      Email: repenno@cisco.com

      Alex Clauberg
   Deutsche Telekom AG
   GTN-FM4
   Landgrabenweg 151
   Bonn, CA 53227
   Germany

      Email: axel.clauberg@telekom.de


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      Lionel Hoffmann
   Bouygues Telecom
   TECHNOPOLE
   13/15 Avenue du Marechal Juin
   Meudon 92360
   France

      Email: lhoffman@bouyguestelecom.fr

      Maoke Chen
   FreeBit Co., Ltd.
   13F E-space Tower, Maruyama-cho 3-6
   Shibuya-ku, Tokyo 150-0044
   Japan

      Email: fibrib@gmail.com

12.  Acknowledgement

   The authors would like to thank Ole Troan, Ralph Droms for their
   comments and feedback.

   This document is a merge of three documents: [I-D.cui-softwire-b4
   -translated-ds-lite], [I-D.zhou-softwire-b4-nat] and [I-D.penno-
   softwire-sdnat].

13.  References

13.1.  Normative References

   [I-D.sun-dhc-port-set-option]
              Sun, Q., Lee, Y., Sun, Q., Bajko, G. and M. Boucadair,
              "Dynamic Host Configuration Protocol (DHCP) Option for
              Port Set Assignment", Internet-Draft draft-sun-dhc-port-
              set-option-00, October 2012.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G. and
              E. Lear, "Address Allocation for Private Internets", BCP
              5, RFC 1918, February 1996.

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

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

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022, January
              2001.

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484, February 2003.


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   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213, October 2005.

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

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

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

   [RFC6334]  Hankins, D. and T. Mrugalski, "Dynamic Host Configuration
              Protocol for IPv6 (DHCPv6) Option for Dual-Stack Lite",
              RFC 6334, August 2011.

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

   [RFC6431]  Boucadair, M., Levis, P., Bajko, G., Savolainen, T. and T.
              Tsou, "Huawei Port Range Configuration Options for PPP IP
              Control Protocol (IPCP)", RFC 6431, November 2011.

13.2.  Informative References

   [I-D.boucadair-dhcpv6-shared-address-option]
              Boucadair, M., Levis, P., Grimault, J., Savolainen, T. and
              G. Bajko, "Dynamic Host Configuration Protocol (DHCPv6)
              Options for Shared IP Addresses Solutions", Internet-Draft
              draft-boucadair-dhcpv6-shared-address-option-01, December
              2009.

   [I-D.cui-softwire-b4-translated-ds-lite]
              Cui, Y., Sun, Q., Boucadair, M., Tsou, T., Lee, Y. and I.
              Farrer, "Lightweight 4over6: An Extension to the DS-Lite
              Architecture", Internet-Draft draft-cui-softwire-b4
              -translated-ds-lite-07, July 2012.

   [I-D.ietf-dhc-dhcpv4-over-ipv6]
              Cui, Y., Wu, P., Wu, J. and T. Lemon, "DHCPv4 over IPv6
              Transport", Internet-Draft draft-ietf-dhc-dhcpv4-over-
              ipv6-03, May 2012.

   [I-D.ietf-pcp-base]
              Wing, D., Cheshire, S., Boucadair, M., Penno, R. and P.
              Selkirk, "Port Control Protocol (PCP)", Internet-Draft
              draft-ietf-pcp-base-26, June 2012.

   [I-D.ietf-softwire-map]


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              Troan, O., Dec, W., Li, X., Bao, C., Zhai, Y., Matsushima,
              S. and T. Murakami, "Mapping of Address and Port (MAP)",
              Internet-Draft draft-ietf-softwire-map-01, June 2012.

   [I-D.ietf-softwire-public-4over6]
              Cui, Y., Wu, J., Wu, P., Vautrin, O. and Y. Lee, "Public
              IPv4 over IPv6 Access Network", Internet-Draft draft-ietf-
              softwire-public-4over6-02, July 2012.

   [I-D.mrugalski-softwire-dhcpv4-over-v6-option]
              Mrugalski, T. and P. Wu, "Dynamic Host Configuration
              Protocol for IPv6 (DHCPv6) Option for DHCPv4 over IPv6
              Transport", Internet-Draft draft-mrugalski-softwire-
              dhcpv4-over-v6-option-00, April 2012.

   [I-D.penno-softwire-sdnat]
              Penno, R., Durand, A., Hoffmann, L. and A. Clauberg,
              "Stateless DS-Lite", Internet-Draft draft-penno-softwire-
              sdnat-02, March 2012.

   [I-D.tsou-pcp-natcoord]
              Sun, Q., Boucadair, M., Deng, X., Zhou, C. and T. Tsou,
              "Lightweight 4over6 Port-set Allocation: Using PCP To
              Coordinate Between the CGN and Home Gateway", Internet-
              Draft draft-tsou-pcp-natcoord-07, July 2012.

   [I-D.wu-dhc-port-set-option]
              Wu, P., Lee, Y., Sun, Q. and T. Lemon, "Dynamic Host
              Configuration Protocol (DHCP) Options for Port Set
              Assignment", Internet-Draft draft-wu-dhc-port-set-
              option-00, April 2012.

   [I-D.zhou-softwire-b4-nat]
              Zhou, C., Boucadair, M. and X. Deng, "NAT offload
              extension to Dual-Stack lite", Internet-Draft draft-zhou-
              softwire-b4-nat-04, October 2011.

Appendix A.  Alternatives for Port-Restricted Address Allocation

   Besides DHCPv4, other protocols for address and port-set provisioning
   MAY also be implemented.  Some possible alternatives include:

   o  PCP[I-D.ietf-pcp-base]: a lwB4 MAY use [I-D.tsou-pcp-natcoord] to
      retrieve a restricted IPv4 address and a set of ports.

   o  DHCPv6: the DHCPv6 protocol MAY be extended to support port-set
      allocation [I-D.boucadair-dhcpv6-shared-address-option], along
      with IPv6-mapped IPv4 address allocation.

   o  IPCP: IPCP MAY be extended to carry the port-set (e.g.,
      [RFC6431]).

   In a Lightweight 4over6 domain, the same provisioning mechanism MUST
   be enabled in the lwB4s, the AFTRs and the provisioning server.

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Authors' Addresses

   Yong Cui
   Tsinghua University
   Department of Computer Science, Tsinghua University
   Beijing, 100084
   P.R.China

   Phone: +86-10-62603059
   Email: yong@csnet1.cs.tsinghua.edu.cn


   Qiong Sun
   China Telecom
   Room 708, No.118, Xizhimennei Street
   Beijing, 100035
   P.R.China

   Phone: +86-10-58552936
   Email: sunqiong@ctbri.com.cn


   Mohamed Boucadair
   France Telecom
   Rennes, 35000
   France

   Email: mohamed.boucadair@orange.com


   Tina Tsou
   Huawei Technologies
   2330 Central Expressway
   Santa Clara, CA 95050
   USA

   Phone: +1-408-330-4424
   Email: tena@huawei.com


   Yiu L. Lee
   Comcast
   One Comcast Center
   Philadelphia, PA 19103
   USA

   Email: yiu_lee@cable.comcast.com







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   Ian Farrer
   Deutsche Telekom AG
   GTN-FM4,Landgrabenweg 151
   Bonn, NRW 53227
   Germany

   Email: ian.farrer@telekom.de














































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