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Versions: (draft-durand-softwire-dual-stack-lite) 00 01 02 03 04 05 06 07 08 09 10 11 RFC 6333

Internet Engineering Task Force                                A. Durand
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                                R. Droms
Expires: September 4, 2011                                         Cisco
                                                             J. Woodyatt
                                                                   Apple
                                                                  Y. Lee
                                                                 Comcast
                                                           March 3, 2011


    Dual-Stack Lite Broadband Deployments Following IPv4 Exhaustion
                 draft-ietf-softwire-dual-stack-lite-07

Abstract

   This document revisits the dual-stack model and introduces the dual-
   stack lite technology aimed at better aligning the costs and benefits
   of deploying IPv6 in service provider networks.  Dual-stack lite
   enables a broadband service provider to share IPv4 addresses among
   customers by combining two well-known technologies: IP in IP (IPv4-
   in-IPv6) and Network Address Translation (NAT).

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 September 4, 2011.

Copyright Notice

   Copyright (c) 2011 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 . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Requirements language  . . . . . . . . . . . . . . . . . . . .  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Deployment scenarios . . . . . . . . . . . . . . . . . . . . .  5
     4.1.  Access model . . . . . . . . . . . . . . . . . . . . . . .  5
     4.2.  CPE  . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.3.  Directly connected device  . . . . . . . . . . . . . . . .  7
   5.  B4 element . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     5.1.  Definition . . . . . . . . . . . . . . . . . . . . . . . .  7
     5.2.  Encapsulation  . . . . . . . . . . . . . . . . . . . . . .  7
     5.3.  Fragmentation and Reassembly . . . . . . . . . . . . . . .  7
     5.4.  AFTR discovery . . . . . . . . . . . . . . . . . . . . . .  8
     5.5.  DNS  . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     5.6.  Interface initialization . . . . . . . . . . . . . . . . .  8
     5.7.  Well-known IPv4 address  . . . . . . . . . . . . . . . . .  9
   6.  AFTR element . . . . . . . . . . . . . . . . . . . . . . . . .  9
     6.1.  Definition . . . . . . . . . . . . . . . . . . . . . . . .  9
     6.2.  Encapsulation  . . . . . . . . . . . . . . . . . . . . . .  9
     6.3.  Fragmentation and Reassembly . . . . . . . . . . . . . . .  9
     6.4.  DNS  . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     6.5.  Well-known IPv4 address  . . . . . . . . . . . . . . . . . 10
     6.6.  Extended binding table . . . . . . . . . . . . . . . . . . 10
   7.  Network Considerations . . . . . . . . . . . . . . . . . . . . 10
     7.1.  Tunneling  . . . . . . . . . . . . . . . . . . . . . . . . 10
     7.2.  VPN  . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     7.3.  Multicast considerations . . . . . . . . . . . . . . . . . 11
   8.  NAT considerations . . . . . . . . . . . . . . . . . . . . . . 11
     8.1.  NAT pool . . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.2.  NAT conformance  . . . . . . . . . . . . . . . . . . . . . 11
     8.3.  Application Level Gateways (ALG) . . . . . . . . . . . . . 11
     8.4.  Sharing global IPv4 addresses  . . . . . . . . . . . . . . 11
     8.5.  Port forwarding / keep alive . . . . . . . . . . . . . . . 12
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     12.1. Normative references . . . . . . . . . . . . . . . . . . . 14
     12.2. Informative references . . . . . . . . . . . . . . . . . . 14



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   Appendix A.  Deployment considerations . . . . . . . . . . . . . . 16
     A.1.  AFTR service distribution and horizontal scaling . . . . . 16
     A.2.  Horizontal scaling . . . . . . . . . . . . . . . . . . . . 16
     A.3.  High availability  . . . . . . . . . . . . . . . . . . . . 16
     A.4.  Logging  . . . . . . . . . . . . . . . . . . . . . . . . . 16
   Appendix B.  Examples  . . . . . . . . . . . . . . . . . . . . . . 17
     B.1.  Gateway based architecture . . . . . . . . . . . . . . . . 17
       B.1.1.  Example message flow . . . . . . . . . . . . . . . . . 19
       B.1.2.  Translation details  . . . . . . . . . . . . . . . . . 23
     B.2.  Host based architecture  . . . . . . . . . . . . . . . . . 24
       B.2.1.  Example message flow . . . . . . . . . . . . . . . . . 27
       B.2.2.  Translation details  . . . . . . . . . . . . . . . . . 31
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31






































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1.  Introduction

   The common thinking for more than 10 years has been that the
   transition to IPv6 will be based solely on the dual stack model and
   that most things would be converted this way before we ran out of
   IPv4.  However, this has not happened.  The IANA free pool of IPv4
   addresses has now depleted, well before sufficient IPv6 deployment
   had taken place.  As a result, many IPv4 services have to continue to
   be provided even under severely limited address space.

   This document specifies the dual-stack lite technology which is aimed
   at better aligning the costs and benefits in service provider
   networks.  Dual-stack lite will enable both continued support for
   IPv4 services and incentives for the deployment of IPv6.  It also de-
   couples IPv6 deployment in the service provider network from the rest
   of the Internet, making incremental deployment easier.

   Dual-stack lite enables a broadband service provider to share IPv4
   addresses among customers by combining two well-known technologies:
   IP in IP (IPv4-in-IPv6) and NAT.

   This document makes a distinction between a dual-stack capable and a
   dual-stack provisioned device.  The former is a device that has code
   that implements both IPv4 and IPv6, from the network layer to the
   applications.  The latter is a similar device that has been
   provisioned with both an IPv4 and an IPv6 address on its
   interface(s).  This document will also further refine this notion by
   distinguishing between interfaces provisioned directly by the service
   provider from those provisioned by the customer.

   Pure IPv6-only devices (i.e. devices that do not include an IPv4
   stack) are outside of the scope of this document.

   This document will first present some deployment scenario and then
   define the behavior of the two elements of the dual-stack lite
   technology: the B4 and the AFTR.  It will then go into networking and
   NAT-ing considerations.


2.  Requirements language

   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].







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3.  Terminology

   The technology described in this document is known as dual-stack
   lite.  The abbreviation DS-Lite will be used along this text.

   This document also introduces two new terms: the DS-Lite Basic
   Bridging BroadBand element (B4) and the DS-Lite Address Family
   Transition Router element (AFTR).

   Dual-stack is defined in [RFC4213].

   NAT related terminology is defined in [RFC4787].

   CPE stands for Customer Premise Equipment.  This is the layer 3
   device in the customer premise that is connected to the service
   provider network.  That device is often a home gateway.  However,
   sometimes computers are directly attached to the service provider
   network.  In such cases, such computers can be viewed as CPEs as
   well.


4.  Deployment scenarios

4.1.  Access model

   Instead of relying on a cascade of NATs, the dual-stack lite model is
   built on IPv4-in-IPv6 tunnels to cross the network to reach a
   carrier-grade IPv4-IPv4 NAT (the AFTR) where customers will share
   IPv4 addresses.  There are numbers of benefits to this approach:

   o  This technology decouples the deployment of IPv6 in the service
      provider network (up to the customer premise equipment or CPE)
      from the deployment of IPv6 in the global Internet and in customer
      applications & devices.

   o  The management of the service provider access networks is
      simplified by leveraging the large IPv6 address space.
      Overlapping private IPv4 address spaces are not required to
      support very large customer bases.

   o  As tunnels can terminate anywhere in the service provider network,
      this architecture leads itself to horizontal scaling and provides
      great flexibility to adapt to changing traffic load.

   o  Tunnels provide a direct connection between B4 and the AFTR.  This
      can be leveraged to enable customers and their applications to
      control how the NAT function of the AFTR is performed.




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   A key characteristic of this approach is that communications between
   end-nodes stay within their address family.  IPv6 sources only
   communicate with IPv6 destinations, IPv4 sources only communicate
   with IPv4 destinations.  There is no protocol family translation
   involved in this approach.  This simplifies greatly the task of
   applications that may carry literal IP addresses in their payload.

4.2.  CPE

   This section describes home Local Area networks characterized by the
   presence of a home gateway, or CPE, provisioned only with IPv6 by the
   service provider.

   A DS-Lite CPE is an IPv6 aware CPE with a B4 Interface implemented in
   the WAN interface.

   A DS-Lite CPE SHOULD NOT operate a NAT function between an internal
   interface and a B4 interface, as the NAT function will be performed
   by the AFTR in the service provider's network.  That will avoid
   accidentally operating in a double NAT environment.

   However, it SHOULD operate its own DHCP(v4) server handing out
   [RFC1918] address space (e.g. 192.168.0.0/16) to hosts in the home.
   It SHOULD advertise itself as the default IPv4 router to those home
   hosts.  It SHOULD also advertise itself as a DNS server in the DHCP
   Option 6 (DNS Server).  Additionally, it SHOULD operate a DNS proxy
   to accept DNS IPv4 requests from home hosts and send them using IPv6
   to the service provider DNS servers, as described in Section 5.5.

   Note: if an IPv4 home host decides to use another IPv4 DNS server,
   the DS-Lite CPE will forward those DNS requests via the B4 interface,
   the same way it forwards any regular IPv4 packets.  However, each DNS
   request will create a binding in the AFTR.  A large number of DNS
   requests may have direct impact to the AFTR's NAT table utilization.

   IPv6 capable devices directly reach the IPv6 Internet.  Packets
   simply follow IPv6 routing, they do not go through the tunnel, and
   are not subject to any translation.  It is expected that most IPv6
   capable devices will also be IPv4 capable and will simply be
   configured with an IPv4 RFC1918 style address within the home network
   and access the IPv4 Internet the same way as the legacy IPv4-only
   devices within the home.

   Pure IPv6-only devices (i.e. devices that do not include an IPv4
   stack) are outside of the scope of this document.






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4.3.  Directly connected device

   In broadband home networks, some devices are directly connected to
   the broadband service provider.  They are connected straight to a
   modem, without a home gateway.  Those devices are, in fact, acting as
   CPEs.

   Under this scenario, the customer device is a dual-stack capable host
   that is only provisioned by the service provider with IPv6 only.  The
   device itself acts as a B4 element and the IPv4 service is provided
   by an IPv4-in-IPv6 tunnel, just as in the home gateway/CPE case.
   That device can run any combinations of IPv4 and/or IPv6
   applications.

   A directly connected DS-Lite device SHOULD send its DNS requests over
   IPv6 to the IPv6 DNS server it has been configured to use.

   Similarly to the previous sections, IPv6 packets follow IPv6 routing,
   they do not go through the tunnel, and are not subject to any
   translation.

   The support of IPv4-only devices and IPv6-only devices in this
   scenario is out of scope for this document.


5.  B4 element

5.1.  Definition

   The 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.

5.2.  Encapsulation

   The tunnel is a multi-point to point IPv4-in-IPv6 tunnel ending on a
   service provider AFTR.

   See section 7.1 for additional tunneling considerations.

   Note: at this point, DS-Lite only defines IPv4-in-IPv6 tunnels,
   however other types of encapsulation could be defined in the future.

5.3.  Fragmentation and Reassembly

   Using an encapsulation (IPv4-in-IPv6 or anything else) to carry IPv4
   traffic over IPv6 will reduce the effective MTU of the datagram.
   Unfortunately, path MTU discovery [RFC1191] is not a reliable method



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   to deal with this problem.

   A solution to deal with this problem is for the service provider to
   increase the MTU size of all the links between the B4 element and the
   AFTR elements by at least 40 bytes to accommodate both the IPv6
   encapsulation header and the IPv4 datagram without fragmenting the
   IPv6 packet.

   However, as not all service providers will be able to increase their
   link MTU, the B4 element MUST perform fragmentation and reassembly if
   the outgoing link MTU cannot accommodate for the extra IPv6 header.
   Fragmentation MUST happen after the encapsulation on the IPv6 packet.
   Reassembly MUST happen before the decapsulation of the IPv6 header.
   Detailed procedure has been specified in [RFC2473] Section 7.2.

5.4.  AFTR discovery

   In order to configure the IPv4-in-IPv6 tunnel, the B4 element needs
   the IPv6 address of the AFTR element.  This IPv6 address can be
   configured using a variety of methods, ranging from an out-of-band
   mechanism, manual configuration or a variety of DHCPv6 options.

   In order to guarantee interoperability, a B4 element SHOULD implement
   the DHCPv6 option defined in
   [I-D.ietf-softwire-ds-lite-tunnel-option].

5.5.  DNS

   A B4 element is only configured from the service provider with IPv6.
   As such, it can only learn the address of a DNS recursive server
   through DHCPv6 (or other similar method over IPv6).  As DHCPv6 only
   defines an option to get the IPv6 address of such a DNS recursive
   server, the B4 element cannot easily discover the IPv4 address of
   such a recursive DNS server, and as such will have to perform all DNS
   resolution over IPv6.

   The B4 element can pass this IPv6 address to downstream IPv6 nodes,
   but not to downstream IPv4 nodes.  As such, the B4 element SHOULD
   implement a DNS proxy, following the recommendations of [RFC5625].

5.6.  Interface initialization

   Initialization of the interface including a B4 element is out-of-
   scope in this specification.







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5.7.  Well-known IPv4 address

   Any locally unique IPv4 address could be configured on the IPv4-in-
   IPv6 tunnel to represent the B4 element.  Configuring such an address
   is often necessary when the B4 element is sourcing IPv4 datagrams
   directly over the tunnel.  In order to avoid conflicts with any other
   address, IANA has defined a well-known range, 192.0.0.0/29.

   192.0.0.0 is the reserved subnet address. 192.0.0.1 is reserved for
   the AFTR element.  The B4 element MAY use any other addresses within
   the 192.0.0.0/29 range.

   Note: a range of addresses has been reserved for this purpose.  The
   intent is to accommodate nodes implementing multiple B4 elements.


6.  AFTR element

6.1.  Definition

   An AFTR element is the combination of an IPv4-in-IPv6 tunnel end-
   point and an IPv4-IPv4 NAT implemented on the same node.

6.2.  Encapsulation

   The tunnel is a point-to-multipoint IPv4-in-IPv6 tunnel ending at the
   B4 elements.

   See section 7.1 for additional tunneling considerations.

   Note: at this point, DS-Lite only defines IPv4-in-IPv6 tunnels,
   however other types of encapsulation could be defined in the future.

6.3.  Fragmentation and Reassembly

   As noted previously, fragmentation and reassembly need to be taken
   care of by the tunnel end-points.  As such, the AFTR MUST perform
   fragmentation and reassembly if the underlying link MTU cannot
   accommodate the extra IPv6 header of the tunnel.  Fragmentation MUST
   happen after the encapsulation on the IPv6 packet.  Reassembly MUST
   happen before the decapsulation of the IPv6 header.  Detailed
   procedure has been specified in [RFC2473] Section 7.2.

   Fragmentation at the Tunnel Entry-Point is a light-weight operation.
   In contrast, reassembly at the Tunnel Exit-Point can be expensive.
   When the Tunnel Exit-Point receives the first fragmented packet, it
   must wait for the second fragmented packet to arrive in order to
   reassemble the two fragmented IPv6 packets for decapsulation.  This



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   requires the Tunnel Exit-Point to buffer and keep track of fragmented
   packets.  Consider that the AFTR is the Tunnel Exit-Point for many
   tunnels.  If many clients simultaneously source large number of
   fragmented packets to the AFTR, this will require the AFTR to buffer
   and consume enormous resources to keep track of the flows.  This
   reassembly process will significantly impact the AFTR performance.
   However, this impact only happens when many clients simultaneously
   source large IPv4 packets.  Since we believe that majority of the
   clients will receive large IPv4 packets (such as watching video
   streams) instead of sourcing large IPv4 packets (such as sourcing
   video streams), so reassembly is only a fraction of the overall
   AFTR's workload.

   Methods to avoid fragmentation, such as rewriting the TCP MSS option
   or using technologies such as Subnetwork Encapsulation and Adaptation
   Layer defined in [RFC5320] are out of scope for this document.

6.4.  DNS

   As noted previously, DS-Lite node implementing a B4 elements will
   perform DNS resolution over IPv6.  As such, very few, if any, DNS
   packets will flow through the AFTR element.

6.5.  Well-known IPv4 address

   The AFTR MAY use the well-known IPv4 address 192.0.0.1 reserved by
   IANA to configure the IPv4-in-IPv6 tunnel.  That address can then be
   used to report ICMP problems and will appear in traceroute outputs.

6.6.  Extended binding table

   The NAT binding table of the AFTR element is extended to include the
   source IPv6 address of the incoming packets.  This IPv6 address is
   used to disambiguate between the overlapping IPv4 address space of
   the service provider customers.

   By doing a reverse look-up in the extended IPv4 NAT binding table,
   the AFTR knows how to reconstruct the IPv6 encapsulation when the
   packets comes back from the Internet.  That way, there is no need to
   keep a static configuration for each tunnel.


7.  Network Considerations

7.1.  Tunneling

   Tunneling MUST be done in accordance to [RFC2473] and [RFC4213].
   Traffic classes ([RFC2474]) from the IPv4 headers SHOULD be carried



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   over to the IPv6 headers and vice versa.

7.2.  VPN

   Dual-stack lite implementations SHOULD NOT interfere with the
   functioning of IPv4 or IPv6 VPNs.

7.3.  Multicast considerations

   Multicast is out-of-scope in this document.


8.  NAT considerations

8.1.  NAT pool

   The AFTR MAY be provisioned with different NAT pools.  The address
   range in the pools may be disjoint but must not be overlapped.
   Operators may implement policies in the AFTR to assign clients in
   different pools.  For example, a AFTR can have two interfaces.  Each
   interface will have a disjoint pool NAT assigned to it.  In another
   case, a policy can apply to the AFTR that a set of B4s will use NAT
   pool 1 and a different set of B4s will use NAT pool 2.

8.2.  NAT conformance

   A dual-stack lite AFTR SHOULD implement behavior conforming to the
   best current practice, currently documented in [RFC4787] and
   [RFC5382].  Other discusions about carrier-grade NATs can be found in
   [I-D.nishitani-cgn].

8.3.  Application Level Gateways (ALG)

   AFTR performs NAT-44 and inherits the limitations of NAT.  Some
   protocols required ALGs in the NAT device to traverse through the
   NAT.  For example: SIP and ICMP require ALG to work properly.  ALGs
   consume resources and there are many different types of ALGs.  The
   AFTR is a shared network device that supports a large number of B4
   elements.  It is impossible for the AFTR to implement every current
   and future ALGs.  This specification only requires that the AFTR MUST
   support [RFC5508].  Implementers can decide to implement other ALGs
   in their implementations.

8.4.  Sharing global IPv4 addresses

   AFTR shares a single IP to multiple users.  This helps to increase
   the IPv4 address utilization.  However, it also brings some issues
   such as logging and lawful intercept.  More considerations on sharing



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   the port space of IPv4 addresses can be found in
   [I-D.ietf-intarea-shared-addressing-issues].

8.5.  Port forwarding / keep alive

   Work on a control plane to the carrier-grade NAT is done in the PCP
   working group at IETF.  The PCP protocol enables applications to
   directly negotiate with the NAT to open ports and negotiate liefetime
   values to avoid keep-alive traffic.  More on PCP can be found in
   [I-D.ietf-pcp-base].


9.  Acknowledgements

   The authors would like to acknowledge the role of Mark Townsley for
   his input on the overall architecture of this technology by pointing
   this work in the direction of [I-D.droms-softwires-snat].  Note that
   this document results from a merging of [I-D.durand-dual-stack-lite]
   and [I-D.droms-softwires-snat].Also to be acknowledged are the many
   discussions with a number of people including Shin Miyakawa,
   Katsuyasu Toyama, Akihide Hiura, Takashi Uematsu, Tetsutaro Hara,
   Yasunori Matsubayashi, Ichiro Mizukoshi.  The author would also like
   to thank David Ward, Jari Arkko, Thomas Narten and Geoff Huston for
   their constructive feedback.  Special thanks go to Dave Thaler and
   Dan Wing for their reviews and comments.


10.  IANA Considerations

   This draft request IANA to allocate a well know IPv4 192.0.0.0/29
   network prefix.  That range is used to number the dual-stack lite
   interfaces.  Reserving a /29 allows for 6 possible interfaces on a
   multi-home node.  The IPv4 address 192.0.0.1 is reserved as the IPv4
   address of the default router for such dual-stack lite hosts.


11.  Security Considerations

   Security issues associated with NAT have long been documented.  See
   [RFC2663] and [RFC2993].

   However, moving the NAT functionality from the CPE to the core of the
   service provider network and sharing IPv4 addresses among customers
   create additional requirements when logging data for abuse usage.
   With any architecture where an IPv4 address does not uniquely
   represent an end host, IPv4 addresses and a timestamps are no longer
   sufficient to identify a particular broadband customer.  The AFTR
   should have the capability to log the tunnel-id, protocol, ports/IP



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   addresses, and the creation time of the NAT binding to uniquely
   identify the user sessions.  Exact details of what is logged are
   implementation specific and out of scope for this document.

   The AFTR performs translation functions for interior IPv4 hosts using
   RFC 1918 addresses or the IANA reserved address range (TBA by IANA).
   In some circumstances, ISP may provision policies in the AFTR and
   instructs the AFTR to bypass translation functions based on <IPv4
   Address, port number, protocol>.  When the AFTR receives a packet
   with matching information of the policy from the interior host, the
   AFTR can simply forward without translation.  The addresses, ports
   and protocols information must be provisioned on the AFTR before
   receiving the packet.  The provisioning mechanism is out-of-scope of
   this specification.

   When decapsulating packets, the AFTR MUST only forward packets
   sourced by RFC 1918 addresses, IANA reserved address range, or any
   other out-of-band pre-authorized addresses.  The AFTR MUST drop all
   others packets.  This prevents rogue devices from launching denial of
   service attacks using unauthorized public IPv4 addresses in the IPv4
   source header field or unauthorized transport port range in the IPv4
   transport header field.  For example, rogue devices could bombard a
   public web server by launching a TCP SYN ACK attack [RFC4987].  The
   victim will receive TCP SYN from random IPv4 source addresses at a
   rapid rate and deny TCP services to legitimate users.

   With IPv4 addresses shared by multiple users, ports become a critical
   resource.  As such, some mechanisms need to be put in place by an
   AFTR to limit port usage, either by rate-limiting new connections or
   putting a hard limit on the maximum number of port usable by a single
   user.  If this number is high enough, it should not interfere with
   normal usage and still provide reasonable protection of the shared
   pool.  More considerations on sharing IPv4 addresses can be found in
   [I-D.ietf-intarea-shared-addressing-issues].  Other considerations
   and recommendations on logging can be found in
   [I-D.ietf-intarea-server-logging-recommendations].

   AFTRs should support ways to limit service only to registered
   customers.  One simple option is to implement IPv6 ingress filter on
   the AFTR's tunnel interface to accept only the IPv6 address range
   defined in the filter.


12.  References







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12.1.  Normative references

   [I-D.ietf-softwire-ds-lite-tunnel-option]
              Hankins, D. and T. Mrugalski, "Dynamic Host Configuration
              Protocol for IPv6 (DHCPv6) Option for Dual- Stack Lite",
              draft-ietf-softwire-ds-lite-tunnel-option-09 (work in
              progress), March 2011.

   [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.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              December 1998.

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

   [RFC5625]  Bellis, R., "DNS Proxy Implementation Guidelines",
              BCP 152, RFC 5625, August 2009.

12.2.  Informative references

   [I-D.droms-softwires-snat]
              Droms, R. and B. Haberman, "Softwires Network Address
              Translation (SNAT)", draft-droms-softwires-snat-01 (work
              in progress), July 2008.

   [I-D.durand-dual-stack-lite]
              Durand, A., "Dual-stack lite broadband deployments post
              IPv4 exhaustion", draft-durand-dual-stack-lite-00 (work in
              progress), July 2008.

   [I-D.ietf-intarea-server-logging-recommendations]
              Durand, A., Gashinsky, I., Lee, D., and S. Sheppard,
              "Logging recommendations for Internet facing servers",
              draft-ietf-intarea-server-logging-recommendations-03 (work
              in progress), February 2011.

   [I-D.ietf-intarea-shared-addressing-issues]
              Ford, M., Boucadair, M., Durand, A., Levis, P., and P.
              Roberts, "Issues with IP Address Sharing",
              draft-ietf-intarea-shared-addressing-issues-04 (work in
              progress), February 2011.



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   [I-D.ietf-pcp-base]
              Wing, D., Cheshire, S., Boucadair, M., Penno, R., and F.
              Dupont, "Port Control Protocol (PCP)",
              draft-ietf-pcp-base-06 (work in progress), February 2011.

   [I-D.nishitani-cgn]
              Yamagata, I., Miyakawa, S., Nakagawa, A., and H. Ashida,
              "Common requirements for IP address sharing schemes",
              draft-nishitani-cgn-05 (work in progress), July 2010.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              November 1990.

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

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

   [RFC2993]  Hain, T., "Architectural Implications of NAT", RFC 2993,
              November 2000.

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

   [RFC4987]  Eddy, W., "TCP SYN Flooding Attacks and Common
              Mitigations", RFC 4987, August 2007.

   [RFC5320]  Templin, F., "The Subnetwork Encapsulation and Adaptation
              Layer (SEAL)", RFC 5320, February 2010.

   [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.

   [RFC5571]  Storer, B., Pignataro, C., Dos Santos, M., Stevant, B.,
              Toutain, L., and J. Tremblay, "Softwire Hub and Spoke
              Deployment Framework with Layer Two Tunneling Protocol
              Version 2 (L2TPv2)", RFC 5571, June 2009.





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Appendix A.  Deployment considerations

A.1.  AFTR service distribution and horizontal scaling

   One of the key benefits of the dual-stack lite technology lies in the
   fact it is tunnel based.  That is, tunnel end-points may be anywhere
   in the service provider network.

   Using the DHCPv6 tunnel end-point option, service providers can
   create groups of users sharing the same AFTR.  Those groups can be
   merged or divided at will.  This leads to an horizontally scaled
   solution, where more capacity is added simply by adding more boxes.
   As those groups of users can evolve over time, it is best to make
   sure that AFTRs do not require per-user configuration in order to
   provide service.

A.2.  Horizontal scaling

   A service provider can start using just a few AFTR centrally located.
   Later, when more capacity is needed, more boxes can be added and
   pushed to the edges of the access network.  In case of a spike of
   traffic, for example during the Olympic games or an important
   political event, capacity can be quickly added in any location of the
   network (tunnels can terminate anywhere) simply by splitting user
   groups.  Extra capacity can be later removed when the traffic returns
   to normal by resetting the DHCPv6 tunnel end-point settings.

A.3.  High availability

   An important element in the design of the dual-stack lite technology
   is the simplicity of implementation on the customer side.  A simple
   IP4-in-IPv6 tunnel and a default route over it is all is needed to
   get IPv4 connectivity.  Dealing with high availability is the
   responsibility of the service provider, not the customer devices
   implementing dual-stack lite.  As such, a single IPv6 address of the
   tunnel end-point is provided in the DHCPv6 option defined in
   [I-D.ietf-softwire-ds-lite-tunnel-option].  The service provider can
   use techniques such as anycast or various types of clusters to ensure
   availability of the IPv4 service.  The exact synchronization (or lack
   thereof) between redundant AFTRs is out of scope for this document.

A.4.  Logging

   DS-Lite AFTR implementation should offer the possibility to log NAT
   binding creations or other ways to keep track of the ports/IP
   addresses used by customers.  This is both to support
   troubleshooting, which is very important to service providers trying
   to figure out why something may not be working, as well as to meet



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   region-specific requirements for responding to legally-binding
   requests for information from law enforcement authorities.


Appendix B.  Examples

B.1.  Gateway based architecture

   This architecture is targeted at residential broadband deployments
   but can be adapted easily to other types of deployment where the
   installed base of IPv4-only devices is important.

   Consider a scenario where a Dual-Stack lite CPE is provisioned only
   with IPv6 in the WAN port, no IPv4.  The CPE acts as an IPv4 DCHP
   server for the LAN network (wireline and wireless) handing out
   RFC1918 addresses.  In addition, the CPE may support IPv6 Auto-
   Configuration and/or DHCPv6 server for the LAN network.  When an
   IPv4-only device connects to the CPE, that CPE will hand it out a
   RFC1918 address.  When a dual-stack capable device connects to the
   CPE, that CPE will hand out a RFC1918 address and a global IPv6
   address to the device.  Besides, the CPE will create an IPv4-in-IPv6
   softwire tunnel [RFC5571]to an AFTR that resides in the service
   provider network.

   When the device accesses IPv6 service, it will send the IPv6 datagram
   to the CPE natively.  The CPE will route the traffic upstream to the
   default gateway.

   When the device accesses IPv4 service, it will source the IPv4
   datagram with the RFC1918 address and send the IPv4 datagram to the
   CPE.  The CPE will encapsulate the IPv4 datagram inside the IPv4-in-
   IPv6 softwire tunnel and forward the IPv6 datagram to the AFTR.  This
   contrasts what the CPE normally does today, which is, NAT the RFC1918
   address to the public IPv4 address and route the datagram upstream.
   When the AFTR receives the IPv6 datagram, it will decapsulate the
   IPv6 header and perform an IPv4-to-IPv4 NAT on the source address.

   As illustrated in Figure 1, this dual-stack lite deployment model
   consists of three components: the dual-stack lite home router with a
   B4 element, the AFTR and a softwire between the B4 element acting as
   softwire initiator (SI) [RFC5571] in the dual-stack lite home router
   and the softwire concentrator (SC) [RFC5571] in the AFTR.  The AFTR
   performs IPv4-IPv4 NAT translations to multiplex multiple subscribers
   through a pool of global IPv4 address.  Overlapping address spaces
   used by subscribers are disambiguated through the identification of
   tunnel endpoints.





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                   +-----------+
                   |    Host   |
                   +-----+-----+
                         |10.0.0.1
                         |
                         |
                         |10.0.0.2
               +---------|---------+
               |         |         |
               |    Home router    |
               |+--------+--------+|
               ||       B4        ||
               |+--------+--------+|
               +--------|||--------+
                        |||2001:db8:0:1::1
                        |||
                        |||<-IPv4-in-IPv6 softwire
                        |||
                 -------|||-------
               /        |||        \
              |   ISP core network  |
               \        |||        /
                 -------|||-------
                        |||
                        |||2001:db8:0:2::1
               +--------|||--------+
               |        AFTR       |
               |+--------+--------+|
               ||   Concentrator  ||
               |+--------+--------+|
               |       |NAT|       |
               |       +-+-+       |
               +---------|---------+
                         |192.0.2.1
                         |
                 --------|--------
               /         |         \
              |       Internet      |
               \         |         /
                 --------|--------
                         |
                         |198.51.100.1
                   +-----+-----+
                   | IPv4 Host |
                   +-----------+

                   Figure 1: gateway-based architecture




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   Notes:

   o  The dual-stack lite home router is not required to be on the same
      link as the host

   o  The dual-stack lite home router could be replaced by a dual-stack
      lite router in the service provider network

   The resulting solution accepts an IPv4 datagram that is translated
   into an IPv4-in-IPv6 softwire datagram for transmission across the
   softwire.  At the corresponding endpoint, the IPv4 datagram is
   decapsulated, and the translated IPv4 address is inserted based on a
   translation from the softwire.

B.1.1.  Example message flow

   In the example shown in Figure 2, the translation tables in the AFTR
   is configured to forward between IP/TCP (10.0.0.1/10000) and IP/TCP
   (192.0.2.1/5000).  That is, a datagram received by the dual-stack
   lite home router from the host at address 10.0.0.1, using TCP DST
   port 10000 will be translated a datagram with IP SRC address
   192.0.2.1 and TCP SRC port 5000 in the Internet.





























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                   +-----------+
                   |    Host   |
                   +-----+-----+
                      |  |10.0.0.1
      IPv4 datagram 1 |  |
                      |  |
                      v  |10.0.0.2
               +---------|---------+
               |         |         |
               |    home router    |
               |+--------+--------+|
               ||        B4       ||
               |+--------+--------+|
               +--------|||--------+
                      | |||2001:db8:0:1::1
       IPv6 datagram 2| |||
                      | |||<-IPv4-in-IPv6 softwire
                      | |||
                 -----|-|||-------
               /      | |||        \
              |   ISP core network  |
               \      | |||        /
                 -----|-|||-------
                      | |||
                      | |||2001:db8:0:2::1
               +------|-|||--------+
               |      | AFTR       |
               |      v |||        |
               |+--------+--------+|
               ||  Concentrartor  ||
               |+--------+--------+|
               |       |NAT|       |
               |       +-+-+       |
               +---------|---------+
                      |  |192.0.2.1
      IPv4 datagram 3 |  |
                      |  |
                 -----|--|--------
               /      |  |         \
              |       Internet      |
               \      |  |         /
                 -----|--|--------
                      |  |
                      v  |198.51.100.1
                   +-----+-----+
                   | IPv4 Host |
                   +-----------+




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                        Figure 2: Outbound Datagram

           +-----------------+--------------+-----------------+
           |        Datagram | Header field | Contents        |
           +-----------------+--------------+-----------------+
           | IPv4 datagram 1 |     IPv4 Dst | 198.51.100.1    |
           |                 |     IPv4 Src | 10.0.0.1        |
           |                 |      TCP Dst | 80              |
           |                 |      TCP Src | 10000           |
           | --------------- | ------------ | -------------   |
           | IPv6 Datagram 2 |     IPv6 Dst | 2001:db8:0:2::1 |
           |                 |     IPv6 Src | 2001:db8:0:1::1 |
           |                 |     IPv4 Dst | 198.51.100.1    |
           |                 |     IPv4 Src | 10.0.0.1        |
           |                 |      TCP Dst | 80              |
           |                 |      TCP Src | 10000           |
           | --------------- | ------------ | -------------   |
           | IPv4 datagram 3 |     IPv4 Dst | 198.51.100.1    |
           |                 |     IPv4 Src | 192.0.2.1       |
           |                 |      TCP Dst | 80              |
           |                 |      TCP Src | 5000            |
           +-----------------+--------------+-----------------+

                         Datagram header contents

   When datagram 1 is received by the dual-stack lite home router, the
   B4 function encapsulates the datagram in datagram 2 and forwards it
   to the dual-stack lite carrier-grade NAT over the softwire.

   When it receives datagram 2, the tunnel concentrator in the AFTR
   hands the IPv4 datagram to the NAT, which determines from its
   translation table that the datagram received on Softwire_1 with TCP
   SRC port 10000 should be translated to datagram 3 with IP SRC address
   192.0.2.1 and TCP SRC port 5000.

   Figure 3 shows an inbound message received at the AFTR.  When the NAT
   function in the AFTR receives datagram 1, it looks up the IP/TCP DST
   in its translation table.  In the example in Figure 3, the NAT
   translates the TCP DST port to 10000, sets the IP DST address to
   10.0.0.1 and hands the datagram to the SC for transmission over
   Softwire_1.  The B4 in the home router decapsulates IPv4 datagram
   from the inbound softwire datagram, and forwards it to the host.









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                   +-----------+
                   |    Host   |
                   +-----+-----+
                      ^  |10.0.0.1
      IPv4 datagram 3 |  |
                      |  |
                      |  |10.0.0.2
               +---------|---------+
               |       +-+-+       |
               |    home router    |
               |+--------+--------+|
               ||        B4       ||
               |+--------+--------+|
               +--------|||--------+
                      ^ |||2001:db8:0:1::1
      IPv6 datagram 2 | |||
                      | |||<-IPv4-in-IPv6 softwire
                      | |||
                 -----|-|||-------
               /      | |||        \
              |   ISP core network  |
               \      | |||        /
                 -----|-|||-------
                      | |||
                      | |||2001:db8:0:2::1
               +------|-|||--------+
               |       AFTR        |
               |+--------+--------+|
               ||   Concentrator  ||
               |+--------+--------+|
               |       |NAT|       |
               |       +-+-+       |
               +---------|---------+
                      ^  |192.0.2.1
      IPv4 datagram 1 |  |
                      |  |
                 -----|--|--------
               /      |  |         \
              |       Internet      |
               \      |  |         /
                 -----|--|--------
                      |  |
                      |  |198.51.100.1
                   +-----+-----+
                   | IPv4 Host |
                   +-----------+

                        Figure 3: Inbound Datagram



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           +-----------------+--------------+-----------------+
           |        Datagram | Header field | Contents        |
           +-----------------+--------------+-----------------+
           | IPv4 datagram 1 |     IPv4 Dst | 192.0.2.1       |
           |                 |     IPv4 Src | 198.51.100.1    |
           |                 |      TCP Dst | 5000            |
           |                 |      TCP Src | 80              |
           | --------------- | ------------ | -------------   |
           | IPv6 Datagram 2 |     IPv6 Dst | 2001:db8:0:1::1 |
           |                 |     IPv6 Src | 2001:db8:0:2::1 |
           |                 |     IPv4 Dst | 10.0.0.1        |
           |                 |       IP Src | 198.51.100.1    |
           |                 |      TCP Dst | 10000           |
           |                 |      TCP Src | 80              |
           | --------------- | ------------ | -------------   |
           | IPv4 datagram 3 |     IPv4 Dst | 10.0.0.1        |
           |                 |     IPv4 Src | 198.51.100.1    |
           |                 |      TCP Dst | 10000           |
           |                 |      TCP Src | 80              |
           +-----------------+--------------+-----------------+

                         Datagram header contents

B.1.2.  Translation details

   The AFTR has a NAT that translates between softwire/port pairs and
   IPv4-address/port pairs.  The same translation is applied to IPv4
   datagrams received on the device's external interface and from the
   softwire endpoint in the device.

   In Figure 2, the translator network interface in the AFTR is on the
   Internet, and the softwire interface connects to the dual-stack lite
   home router.  The AFTR translator is configured as follows:

   Network interface:  Translate IPv4 destination address and TCP
      destination port to the softwire identifier and TCP destination
      port

   Softwire interface:  Translate softwire identifier and TCP source
      port to IPv4 source address and TCP source port

   Here is how the translation in Figure 3 works:

   o  Datagram 1 is received on the AFTR translator network interface.
      The translator looks up the IPv4-address/port pair in its
      translator table, rewrites the IPv4 destination address to
      10.0.0.1 and the TCP source port to 10000, and hands the datagram
      to the SE to be forwarded over the softwire.



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   o  The IPv4 datagram is received on the dual-stack lite home router
      B4.  The B4 function extracts the IPv4 datagram and the dual-stack
      lite home router forwards datagram 3 to the host.

        +------------------------------------+--------------------+
        |         Softwire-Id/IPv4/Prot/Port | IPv4/Prot/Port     |
        +------------------------------------+--------------------+
        | 2001:db8:0:1::1/10.0.0.1/TCP/10000 | 192.0.2.1/TCP/5000 |
        +------------------------------------+--------------------+

            Dual-Stack lite carrier-grade NAT translation table

   The Softwire-Id is the IPv6 address assigned to the Dual-Stack lite
   CPE.  Hosts behind the same Dual-Stack lite home router have the same
   Softwire-Id.  The source IPv4 is the RFC1918 addressed assigned by
   the Dual-Stack home router which is unique to each host behind the
   CPE.  The AFTR would receive packets sourced from different IPv4
   addresses in the same softwire tunnel.  The AFTR combines the
   Softwire-Id and IPv4 address/Port [Softwire-Id, IPv4+Port] to
   uniquely identify the host behind the same Dual-Stack lite home
   router.

B.2.  Host based architecture

   This architecture is targeted at new, large scale deployments of
   dual-stack capable devices implementing a dual-stack lite interface.

   Consider a scenario where a Dual-Stack lite host device is directly
   connected to the service provider network.  The host device is dual-
   stack capable but only provisioned an IPv6 global address.  Besides,
   the host device will pre-configure a well-known IPv4 non-routable
   address (see IANA section).  This well-known IPv4 non-routable
   address is similar to the 127.0.0.1 loopback address.  Every host
   device implemented Dual-Stack lite will pre-configure the same
   address.  This address will be used to source the IPv4 datagram when
   the device accesses IPv4 services.  Besides, the host device will
   create an IPv4-in-IPv6 softwire tunnel to an AFTR.  The Carrier Grade
   NAT will reside in the service provider network.

   When the device accesses IPv6 service, the device will send the IPv6
   datagram natively to the default gateway.

   When the device accesses IPv4 service, it will source the IPv4
   datagram with the well-known non-routable IPv4 address.  Then, the
   host device will encapsulate the IPv4 datagram inside the IPv4-in-
   IPv6 softwire tunnel and send the IPv6 datagram to the AFTR.  When
   the AFTR receives the IPv6 datagram, it will decapsulate the IPv6
   header and perform IPv4-to-IPv4 NAT on the source address.



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   This scenario works on both wireline and wireless networks.  A
   typical wireless device will connect directly to the service provider
   without CPE in between.

   As illustrated in Figure 4, this dual-stack lite deployment model
   consists of three components: the dual-stack lite host, the AFTR and
   a softwire between the softwire initiator B4 in the host and the
   softwire concentrator in the AFTR.  The dual-stack lite host is
   assumed to have IPv6 service and can exchange IPv6 traffic with the
   AFTR.

   The AFTR performs IPv4-IPv4 NAT translations to multiplex multiple
   subscribers through a pool of global IPv4 address.  Overlapping IPv4
   address spaces used by the dual-stack lite hosts are disambiguated
   through the identification of tunnel endpoints.

   In this situation, the dual-stack lite host configures the IPv4
   address 192.0.0.2 out of the well-known range 192.0.0.0/29 (defined
   by IANA) on its B4 interface.  It also configure the first non-
   reserved IPv4 address of the reserved range, 192.0.0.1 as the address
   of its default gateway.






























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               +-------------------+
               |                   |
               |  Host 192.0.0.2   |
               |+--------+--------+|
               ||        B4       ||
               |+--------+--------+|
               +--------|||--------+
                        |||2001:db8:0:1::1
                        |||
                        |||<-IPv4-in-IPv6 softwire
                        |||
                 -------|||-------
               /        |||        \
              |   ISP core network  |
               \        |||        /
                 -------|||-------
                        |||
                        |||2001:db8:0:2::1
               +--------|||--------+
               |       AFTR        |
               |+--------+--------+|
               ||  Concentrator   ||
               |+--------+--------+|
               |       |NAT|       |
               |       +-+-+       |
               +---------|---------+
                         |192.0.2.1
                         |
                 --------|--------
               /         |         \
              |       Internet      |
               \         |         /
                 --------|--------
                         |
                         |198.51.100.1
                   +-----+-----+
                   | IPv4 Host |
                   +-----------+

                     Figure 4: host-based architecture

   The resulting solution accepts an IPv4 datagram that is translated
   into an IPv4-in-IPv6 softwire datagram for transmission across the
   softwire.  At the corresponding endpoint, the IPv4 datagram is
   decapsulated, and the translated IPv4 address is inserted based on a
   translation from the softwire.





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B.2.1.  Example message flow

   In the example shown in Figure 5, the translation tables in the AFTR
   is configured to forward between IP/TCP (a.b.c.d/10000) and IP/TCP
   (192.0.2.1/5000).  That is, a datagram received from the host at
   address 192.0.0.2, using TCP DST port 10000 will be translated a
   datagram with IP SRC address 192.0.2.1 and TCP SRC port 5000 in the
   Internet.











































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               +-------------------+
               |                   |
               |Host 192.0.0.2     |
               |+--------+--------+|
               ||        B4       ||
               |+--------+--------+|
               +--------|||--------+
                      | |||2001:db8:0:1::1
       IPv6 datagram 1| |||
                      | |||<-IPv4-in-IPv6 softwire
                      | |||
                 -----|-|||-------
               /      | |||        \
              |   ISP core network  |
               \      | |||        /
                 -----|-|||-------
                      | |||
                      | |||2001:db8:0:2::1
               +------|-|||--------+
               |      | AFTR       |
               |      v |||        |
               |+--------+--------+|
               ||  Concentrator   ||
               |+--------+--------+|
               |       |NAT|       |
               |       +-+-+       |
               +---------|---------+
                      |  |192.0.2.1
      IPv4 datagram 2 |  |
                 -----|--|--------
               /      |  |         \
              |       Internet      |
               \      |  |         /
                 -----|--|--------
                      |  |
                      v  |198.51.100.1
                   +-----+-----+
                   | IPv4 Host |
                   +-----------+

                        Figure 5: Outbound Datagram










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           +-----------------+--------------+-----------------+
           |        Datagram | Header field | Contents        |
           +-----------------+--------------+-----------------+
           | IPv6 Datagram 1 |     IPv6 Dst | 2001:db8:0:2::1 |
           |                 |     IPv6 Src | 2001:db8:0:1::1 |
           |                 |     IPv4 Dst | 198.51.100.1    |
           |                 |     IPv4 Src | a.b.c.d         |
           |                 |      TCP Dst | 80              |
           |                 |      TCP Src | 10000           |
           | --------------- | ------------ | -------------   |
           | IPv4 datagram 2 |     IPv4 Dst | 198.51.100.1    |
           |                 |     IPv4 Src | 192.0.2.1       |
           |                 |      TCP Dst | 80              |
           |                 |      TCP Src | 5000            |
           +-----------------+--------------+-----------------+

                         Datagram header contents

   When sending an IPv4 packet, the dual-stack lite host encapsulates it
   in datagram 1 and forwards it to the AFTR over the softwire.

   When it receives datagram 1, the concentrator in the AFTR hands the
   IPv4 datagram to the NAT, which determines from its translation table
   that the datagram received on Softwire_1 with TCP SRC port 10000
   should be translated to datagram 3 with IP SRC address 192.0.2.1 and
   TCP SRC port 5000.

   Figure 6 shows an inbound message received at the AFTR.  When the NAT
   function in the AFTR receives datagram 1, it looks up the IP/TCP DST
   in its translation table.  In the example in Figure 3, the NAT
   translates the TCP DST port to 10000, sets the IP DST address to
   a.b.c.d and hands the datagram to the concentrator for transmission
   over Softwire_1.  The B4 in the dual-stack lite hosts decapsulates
   IPv4 datagram from the inbound softwire datagram, and forwards it to
   the host.
















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               +-------------------+
               |                   |
               |Host 192.0.0.2     |
               |+--------+--------+|
               ||        B4       ||
               |+--------+--------+|
               +--------|||--------+
                      ^ |||2001:db8:0:1::1
      IPv6 datagram 2 | |||
                      | |||<-IPv4-in-IPv6 softwire
                      | |||
                 -----|-|||-------
               /      | |||        \
              |   ISP core network  |
               \      | |||        /
                 -----|-|||-------
                      | |||
                      | |||2001:db8:0:2::1
               +------|-|||--------+
               |       AFTR        |
               |      | |||        |
               |+--------+--------+|
               ||  Concentrator   ||
               |+--------+--------+|
               |       |NAT|       |
               |       +-+-+       |
               +---------|---------+
                      ^  |192.0.2.1
      IPv4 datagram 1 |  |
                 -----|--|--------
               /      |  |         \
              |       Internet      |
               \      |  |         /
                 -----|--|--------
                      |  |
                      |  |198.51.100.1
                   +-----+-----+
                   | IPv4 Host |
                   +-----------+

                        Figure 6: Inbound Datagram










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           +-----------------+--------------+-----------------+
           |        Datagram | Header field | Contents        |
           +-----------------+--------------+-----------------+
           | IPv4 datagram 1 |     IPv4 Dst | 192.0.2.1       |
           |                 |     IPv4 Src | 198.51.100.1    |
           |                 |      TCP Dst | 5000            |
           |                 |      TCP Src | 80              |
           | --------------- | ------------ | -------------   |
           | IPv6 Datagram 2 |     IPv6 Dst | 2001:db8:0:1::1 |
           |                 |     IPv6 Src | 2001:db8:0:2::1 |
           |                 |     IPv4 Dst | a.b.c.d         |
           |                 |       IP Src | 198.51.100.1    |
           |                 |      TCP Dst | 10000           |
           |                 |      TCP Src | 80              |
           +-----------------+--------------+-----------------+

                         Datagram header contents

B.2.2.  Translation details

   The translations happening in the AFTR are the same as in the
   previous examples.  The well known IPv4 address 192.0.0.2 out of the
   192.0.0.0/29 (defined by IANA) range used by all the hosts are
   disambiguated by the IPv6 source address of the softwire.

        +-----------------------------------+--------------------+
        |        Softwire-Id/IPv4/Prot/Port | IPv4/Prot/Port     |
        +-----------------------------------+--------------------+
        | 2001:db8:0:1::1/a.b.c.d/TCP/10000 | 192.0.2.1/TCP/5000 |
        +-----------------------------------+--------------------+

            Dual-Stack lite carrier-grade NAT translation table

   The Softwire-Id is the IPv6 address assigned to the Dual-Stack host.
   Each host has an unique Softwire-Id.  The source IPv4 address is one
   of the well-known IPv4 address.  The AFTR could receive packets from
   different hosts sourced from the same IPv4 well-known address from
   different softwire tunnels.  Similar to the gateway architecture, the
   AFTR combines the Softwire-Id and IPv4 address/Port [Softwire-Id,
   IPv4+Port] to uniquely identify the individual host.











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Authors' Addresses

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

   Email: adurand@juniper.net


   Ralph Droms
   Cisco
   1414 Massachusetts Avenue
   Boxborough, MA  01714
   USA

   Email: rdroms@cisco.com


   James Woodyatt
   Apple
   1 Infinite Loop
   Cupertino, CA  95014
   USA

   Email: jhw@apple.com


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

   Email: yiu_lee@cable.comcast.com















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