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Versions: (draft-cui-softwire-b4-translated-ds-lite) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 7596

Softwire Working Group                                            Y. Cui
Internet-Draft                                       Tsinghua University
Intended status: Standards Track                                  Q. Sun
Expires: August 15, 2014                                   China Telecom
                                                            M. Boucadair
                                                          France Telecom
                                                                 T. Tsou
                                                     Huawei Technologies
                                                                  Y. Lee
                                                                 Comcast
                                                               I. Farrer
                                                     Deutsche Telekom AG
                                                       February 11, 2014


      Lightweight 4over6: An Extension to the DS-Lite Architecture
                    draft-ietf-softwire-lw4over6-06

Abstract

   Dual-Stack Lite (RFC 6333) 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 and Port Translation (NAPT) function from the
   centralized DS-Lite tunnel concentrator to the tunnel client located
   in the Customer Premises Equipment (CPE).  This removes the
   requirement for a Carrier Grade NAT function in the tunnel
   concentrator and 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 CPEs.

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 15, 2014.



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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
   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 . . . . . . . . . . . . . . . . . . .   7
     5.1.  Lightweight B4 Provisioning with DHCPv6 . . . . . . . . .   7
     5.2.  Lightweight B4 Data Plane Behavior  . . . . . . . . . . .   8
       5.2.1.  Changes to RFC2473 and RFC6333 Fragmentation
               Behaviour . . . . . . . . . . . . . . . . . . . . . .  10
   6.  Lightweight AFTR Behavior . . . . . . . . . . . . . . . . . .  11
     6.1.  Binding Table Maintenance . . . . . . . . . . . . . . . .  11
     6.2.  lwAFTR Data Plane Behavior  . . . . . . . . . . . . . . .  12
   7.  Additional IPv4 address and Port Set Provisioning
       Mechanisms  . . . . . . . . . . . . . . . . . . . . . . . . .  13
   8.  ICMP Processing . . . . . . . . . . . . . . . . . . . . . . .  13
     8.1.  ICMPv4 Processing by the lwAFTR . . . . . . . . . . . . .  14
     8.2.  ICMPv4 Processing by the lwB4 . . . . . . . . . . . . . .  14
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   11. Author List . . . . . . . . . . . . . . . . . . . . . . . . .  15
   12. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  18
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     13.2.  Informative References . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   Dual-Stack Lite (DS-Lite, [RFC6333]) defines a model for providing
   IPv4 access over an IPv6 network using two well-known technologies:




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   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.

   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.  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



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   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.

   Lightweight 4over6 features keeping per-subscriber state in the
   service provider's network.  It is categorized as Binding approach in
   [I-D.ietf-softwire-unified-cpe] which defines a unified IPv4-in-IPv6
   Softwire CPE.

   This document is an extended case, which covers address sharing for
   [RFC7040].  It is also a variant of A+P called Binding Table Mode
   (see Section 4.4 of [RFC6346]).

   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):   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



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                                 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.)

   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 the 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



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   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.

   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 document.

   The solution specified in this document allows the assignment of
   either a full or a shared IPv4 address requesting CPEs.  [RFC7040]
   provides a mechanism for assigning a full IPv4 address only.

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

   Figure 2 Lightweight 4over6 Provisioning Synchronization






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5.  Lightweight B4 Behavior

5.1.  Lightweight B4 Provisioning with DHCPv6

   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

   o  IPv4 External (Public) Address for NAPT44

   o  Restricted port-set to use for NAPT44

   For DHCPv6 based configuration of these parameters, the lwB4 SHOULD
   implement OPTION_SW46_LW as described in section 6.3 of
   [I-D.ietf-softwire-map-dhcp].  This means that the lifetime of the
   softwire and the derived configuration information (e.g. IPv4 shared
   address, IPv4 address) is bound to the lifetime of the DHCPv6 lease.
   If stateful IPv4 configuration or additional IPv4 configuration
   information is required, DHCPv4 [RFC2131] must be used.

   Some other mechanisms which may be adapted for the provisioning of
   IPv4 addresses and port-sets are described in section 7 below.

   An IPv6 address from an assigned prefix is also required for the lwB4
   to use as the encapsulation source address for the softwire.  In
   order to enable end-to-end provisioning, the IPv6 address is
   constructed by taking a /64 prefix assigned to the WAN interface and
   suffixing 64-bits for the interface identifier.  As there may be
   multiple WAN prefixes, of which only one can be used for lw4o6, the
   CPE is provisioned with the logic to select the correct one.  The /
   128 prefix is then constructed as follows:













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   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Operator assigned (64-bits)                   |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Zero Padding          |         IPv4 Address          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       IPv4 Addr cont.         |             PSID              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 3 Construction of the lw4o6 /128 Prefix

   Padding:      Padding (all zeros)

   IPv4 Address: Public IPv4 address allocated to the client

   PSID:         Port Set ID allocated to the client, left padded with
                 zeros to 16-bits.  If no PSID is provisioned, all
                 zeros.

   In the event that the lwB4's encapsulation source address is changed
   for any reason (such as the DHCPv6 lease expiring), the lwB4's
   dynamic provisioning process must be re-initiated.

   An lwB4 MUST support dynamic port-restricted IPv4 address
   provisioning.  The port set algorithm for provisioning this is
   described in Section 5.1 of [I-D.ietf-softwire-map].  For lw4o6, the
   number of a-bits SHOULD be 0.

   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



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   5.3 Fragmentation and Reassembly  Covering MTU and fragmentation
                                     considerations (referencing
                                     [RFC2473]), with the exception
                                     noted below.

   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
   IPv4 address and port numbers.

   The working flow of the lwB4 is illustrated in figure 4.

                        +-------------+
                        |     lwB4    |
      +--------+  IPv4  |------+------| IPv4-in-IPv6  +----------+
      |IPv4 LAN|------->|      |Encap.|-------------->|Configured|
      |        |<-------| NAPT |  or  |<--------------|  lwAFTR  |
      +--------+        |      |Decap.|               +----------+
                        +------+------+

   Figure 4 Working Flow of the lwB4

   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.

   If the IPv6 source address does not match the configured lwAFTR
   address, then the packet MUST be discarded.  If the decapsulated IPv4
   packet does not match the lwB4's configuration (i.e. invalid
   destination IPv4 address or port) then the packet MUST be dropped.
   An ICMPv4 error message (type 13 - Communication Administratively
   Prohibited) message MAY be sent back to the lwAFTR.  The ICMP policy
   SHOULD be configurable.



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   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.

5.2.1.  Changes to RFC2473 and RFC6333 Fragmentation Behaviour

5.2.1.1.  Processing of Incoming IPv4 Fragments Encapsulated in IPv6

   The following process for handling fragmented packets is taken from
   Section 3.4 of [RFC6146], adapted for use with fragmentation of
   encapsulated IPv4 tunnel traffic and NAT44.

   On receiving an encapsulated packet containing an IPv4 fragment, then
   more processing may be needed than for non-fragmented payloads.  This
   specification leaves open the exact details of how the lwB4 handles
   incoming encapsulated IPv6 packets containing IPv4 fragments, and
   simply requires that the external behavior of the lwB4 be compliant
   with the following conditions:

   The lwB4 MUST handle encapsulated IPv4 fragments.  In particular, the
   lwB4 MUST handle fragments arriving out of order, conditional on the
   following:

   o  The lwB4 MUST limit the amount of resources devoted to the storage
      of fragmented packets in order to protect from DoS attacks.

   o  As long as the lwB4 has available resources, the lwB4 MUST allow
      the fragments to arrive over a time interval.  The time interval
      SHOULD be configurable and the default value MUST be of at least
      FRAGMENT_MIN.

   o  The lwB4 MAY require that the UDP, TCP, or ICMP header be
      completely contained within the fragment that contains fragment
      offset equal to zero.

   For incoming packets carrying TCP or UDP IPv4 fragments with a non-
   zero checksum, after de-capsulation, the lwB4 MAY elect to queue the
   fragments as they arrive and perform NAPT44 on all fragments at the
   same time.  In this case, the incoming 5-tuple is determined by
   extracting the appropriate fields from the received packet.
   Alternatively, a lwB4 MAY translate the de-capsulated fragments as
   they arrive, by storing information that allows it to compute the
   5-tuple for fragments other than the first.  In the latter case,



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   subsequent fragments may arrive before the first, and the rules (in
   the bulleted list above) about how the lwB4 handles (out-of-order)
   fragments apply.

   For incoming de-capsulated IPv4 packets carrying UDP packets with a
   zero checksum, if the lwB4 has enough resources, the lwB4 MUST
   reassemble the packets and MUST calculate the checksum.  If the lwB4
   does not have enough resources, then it MUST silently discard the
   packets.  The handling of fragmented and un-fragmented UDP packets
   with a zero checksum as specified above deviates from that specified
   in [RFC6145].

   Implementers of an lwB4 should be aware that there are a number of
   well-known attacks against IP fragmentation; see [RFC1858] and
   [RFC3128].  Implementers should also be aware of additional issues
   with reassembling packets at high rates, described in [RFC4963].

5.2.1.2.  Processing of Outbound IPv4 Packets Requiring Fragmentation
          for Encapsulation

   When an lwB4 receives an IPv4 packet from a connected host that
   exceeds the IPv6 MTU size after encapsulation, the lwB4 SHOULD
   fragment the IPv4 packet before encapsulation.  This lwB4 behavior
   avoids IPv6 fragmentation, so that the lwAFTR is not required to re-
   assemble fragmented IPv6 packets.  If the the Don't Fragment (DF) bit
   is set in the IPv4 packet header (e.g. for PMTUD discovery), then the
   IPv4 packet is dropped by the lwB4 and an ICMP Fragmentation Needed
   (Type 3, Code 4) with the correct tunnel MTU is sent.

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 NAPT 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






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   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.

   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 is 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 or implement a policy (such



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   as redirection) on 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
   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.

7.  Additional IPv4 address and Port Set Provisioning Mechanisms

   In addition to the DHCPv6 based mechanism described in section 5.1,
   several other IPv4 provisioning protocols have been suggested.  These
   protocols MAY be implemented.  These alternatives include:

   o  DHCPv4 over DHCPv6: [I-D.ietf-dhc-dhcpv4-over-dhcpv6] describes
      implementing DHCPv4 messages over an IPv6 only service providers
      network.  This enables leasing of IPv4 addresses and makes DHCPv4
      options available to the DHCPv4 over DHCPv6 client.

   o  PCP[RFC6887]: an lwB4 MAY use [I-D.ietf-pcp-port-set] to retrieve
      a restricted IPv4 address and a set of ports.

   In a Lightweight 4over6 domain, the binding information MUST be
   aligned between the lwB4s, the lwAFTRs and the provisioning server.

8.  ICMP Processing

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

   ICMPv4 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.





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8.1.  ICMPv4 Processing by the lwAFTR

   For inbound ICMP messages 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:

   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.

   Additionally, the lwAFTR MAY implement:

   o  Discarding of all inbound ICMP messages.

   The ICMP policy SHOULD be configurable.

8.2.  ICMPv4 Processing by the lwB4

   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.

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



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   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

      Department of Computer Science, Tsinghua University

      Beijing 100084

      P.R.China

      Phone: +86-10-62785822

      Email: pengwu.thu@gmail.com





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      Qi Sun

      Tsinghua University

      Beijing 100084

      P.R.China

      Phone: +86-10-62785822

      Email: sunqi@csnet1.cs.tsinghua.edu.cn



      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




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      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 and Suresh
   Krishnan 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.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.




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   [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.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol", RFC
              2131, March 1997.

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

   [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.

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

13.2.  Informative References

   [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", draft-cui-softwire-b4-translated-ds-lite-11
              (work in progress), February 2013.

   [I-D.ietf-dhc-dhcpv4-over-dhcpv6]
              Sun, Q., Cui, Y., Siodelski, M., Krishnan, S., and I.
              Farrer, "DHCPv4 over DHCPv6 Transport", draft-ietf-dhc-
              dhcpv4-over-dhcpv6-04 (work in progress), January 2014.

   [I-D.ietf-pcp-port-set]
              Qiong, Q., Boucadair, M., Sivakumar, S., Zhou, C., Tsou,
              T., and S. Perreault, "Port Control Protocol (PCP)
              Extension for Port Set Allocation", draft-ietf-pcp-port-
              set-04 (work in progress), November 2013.









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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-unified-cpe]
              Boucadair, M., Farrer, I., Perreault, S., and S.
              Sivakumar, "Unified IPv4-in-IPv6 Softwire CPE", draft-
              ietf-softwire-unified-cpe-01 (work in progress), May 2013.

   [I-D.penno-softwire-sdnat]
              Penno, R., Durand, A., Hoffmann, L., and A. Clauberg,
              "Stateless DS-Lite", draft-penno-softwire-sdnat-02 (work
              in progress), March 2012.

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

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

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

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

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

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



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

   [RFC6887]  Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
              Selkirk, "Port Control Protocol (PCP)", RFC 6887, April
              2013.

   [RFC7040]  Cui, Y., Wu, J., Wu, P., Vautrin, O., and Y. Lee, "Public
              IPv4-over-IPv6 Access Network", RFC 7040, November 2013.

Authors' Addresses

   Yong Cui
   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






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   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


   Ian Farrer
   Deutsche Telekom AG
   CTO-ATI, Landgrabenweg 151
   Bonn, NRW  53227
   Germany

   Email: ian.farrer@telekom.de

























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