



Softwire Working Group                                            Y. Cui
Internet-Draft                                       Tsinghua University
Intended status: Standards Track                                  Q. Sun
Expires: May 17, 2015                                      China Telecom
                                                            M. Boucadair
                                                          France Telecom
                                                                 T. Tsou
                                                     Huawei Technologies
                                                                  Y. Lee
                                                                 Comcast
                                                               I. Farrer
                                                     Deutsche Telekom AG
                                                       November 13, 2014


      Lightweight 4over6: An Extension to the DS-Lite Architecture
                    draft-ietf-softwire-lw4over6-13

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 May 17, 2015.



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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  . . . . . . . . . . .   9
       5.2.1.  Fragmentation Behaviour . . . . . . . . . . . . . . .  11
   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 . . . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  ICMPv4 Processing by the lwAFTR . . . . . . . . . . . . .  14
     8.2.  ICMPv4 Processing by the lwB4 . . . . . . . . . . . . . .  14
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   11. Author List . . . . . . . . . . . . . . . . . . . . . . . . .  15
   12. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  19
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     13.2.  Informative References . . . . . . . . . . . . . . . . .  20
   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:
   IP in IP [RFC2473] and Network Address Translation (NAT).  The DS-
   Lite architecture defines two major functional elements as follows:



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   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 an
                                     IPv4-in-IPv6 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 is a solution designed specifically for complete
   independence between IPv6 subnet prefix and IPv4 address with or
   without IPv4 address sharing.  This is accomplished by maintaining
   state for each softwire (per-subscriber state) in the central lwAFTR
   and a hub-and-spoke forwarding architecture.  [I-D.ietf-softwire-map]



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   also offers these capabilities or, alternatively, allows for a
   reduction of the amount of centralized state using rules to express
   IPv4/IPv6 address mappings.  This introduces an algorithmic
   relationship between the IPv6 subnet and IPv4 address.  This
   relationship also allows the option of direct, meshed connectivity
   between users.

   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 extends the mechanism defined in [RFC7040] by allowing
   address sharing.  The solution in this document 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



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                                 connected device or a CPE), that
                                 supports port-restricted IPv4 address
                                 allocation, implements NAPT44
                                 functionality and creates a tunnel to
                                 an lwAFTR.

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

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

   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.  The following figure shows the data plane
   with the main functional change between DS-Lite and lw4o6:












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  +--------+   +---------+  IPv4-in-IPv6  +---------+    +-------------+
  |IPv4 LAN|---|    B4   |================|AFTR/NAPT|----|IPv4 Internet|
  +--------+   +---------+                +---------+    +-------------+
                 DS-Lite NAPT model: all state in the AFTR


  +--------+   +---------+  IPv4-in-IPv6  +------+    +-------------+
  |IPv4 LAN|---|lwB4/NAPT|================|lwAFTR|----|IPv4 Internet|
  +--------+   +---------+                +------+    +-------------+
                        LW4over6 NAPT model:
             subscriber state in the lwAFTR, NAPT state in lwB4

   Figure 1 Comparison of DS-Lite and Lightweight 4over6 Data Plane

   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,
   Port Control Protocol (PCP, [RFC6887]) 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 precise
   mechanism whereby this synchronization occurs is out of scope for
   this document.

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



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

   Figure 2 Lightweight 4over6 Provisioning Synchronization

5.  Lightweight B4 Behavior

5.1.  Lightweight B4 Provisioning with 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, 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

   o  IPv6 Binding Prefix

   The lwB4 MUST implement DHCPv6 based configuration using
   OPTION_S46_CONT_LW as described in section 5.3 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, DHCP 4o6 [RFC7341] MUST be used.

   Although it would be possible to extend lw4o6 to have more than one
   active lw4o6 tunnel configured simultaneously, this document is only
   concerned with the use of a single tunnel.

   The IPv6 binding prefix field is provisioned so that the CE can
   identify the correct prefix to use as the tunnel source.  On receipt
   of the necessary configuration parameters listed above, the lwB4
   performs a longest prefix match between the IPv6 binding prefix and
   its currently active IPv6 prefixes.  The result forms the subnet to



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   be used for sourcing the lw4o6 tunnel.  The full /128 address is then
   constructed in the same manner as [I-D.ietf-softwire-map].


   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 Prefix                     |
   .                        (64-bits)                              .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Zero Padding          |         IPv4 Address          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       IPv4 Addr cont.         |             PSID              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Figure 3 Construction of the lw4o6 /128 Prefix

   Operator Assigned Prefix:  IPv6 prefix allocated to the client.  If
                 the prefix length is less than 64, right padded with
                 zeros to 64-bits.

   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 IPv6 encapsulation source address is
   changed for any reason (such as the DHCPv6 lease expiring), the
   lwB4's dynamic provisioning process MUST be re-initiated.  When the
   lwB4's public IPv4 address or port set ID is changed for any reason,
   the lwB4 MUST flush its NAPT table.

   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, thus allocating a single contiguous
   port set to each lwB4.

   Provisioning of the lwB4 using DHCPv6 as described here allocates a
   single PSID to the client.  In the event that the client is
   concurrently using all of the provisioned L4 ports it may be unable
   to initiate any additional outbound connections.  DHCPv6 based
   provisioning does not provide a mechanism for the client to request
   more L4 port numbers.  Other provisioning mechanisms (e.g.  PCP based



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   provisioning [I-D.ietf-pcp-port-set]) provide this function.  Issues
   relevant to IP address sharing are discussed in more detail in
   [RFC6269].

   Unless an lwB4 is being allocated a full IPv4 address, it is
   RECOMMENDED that PSIDs containing the system ports (0-1023) are not
   allocated to lwB4s.  The reserved ports are more likely to be
   reserved by middleware, and therefore we recommend that they not be
   issued to clients other than as a deliberate assignment.
   Section 5.2.2 of [RFC6269] provides analysis of allocating system
   ports to clients with IPv4 address sharing.

   In the event that the lwB4 receives an ICMPv6 error message (type 1,
   code 5) originating from the lwAFTR, the lwB4 interprets this to mean
   that no matching entry in the lwAFTR's binding table has been found,
   so the IPv4 payload is not being forwarded by the lwAFTR.  The lwB4
   MAY then re-initiate the dynamic port-restricted provisioning
   process.  The lwB4's re-initiation policy SHOULD be configurable.

   On receipt of such an ICMP error message, the lwB4 MUST validate the
   source address to be the same as the lwAFTR address that is
   configured.  In the event that these addresses do not match, the
   lwAFTR MUST discard the ICMP error message.

   In order to prevent forged ICMP messages (using the spoofed lwAFTR
   address as the source) from being sent to lwB4s, the operator can
   implement network ingress filtering as described in [RFC2827].

   The DNS considerations described in Section 5.5 and Section 6.4 of
   [RFC6333] apply to Lightweight 4over6; lw4o6 implementations MUST
   comply with all requirements stated there.

5.2.  Lightweight B4 Data Plane Behavior

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

   5.2 Encapsulation                 Covering encapsulation and de-
                                     capsulation of tunneled traffic

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

   7.1 Tunneling                     Covering tunneling and traffic
                                     class mapping between IPv4 and IPv6




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                                     (referencing [RFC2473] and
                                     [RFC2983])

   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

   Hosts connected to the customer's network behind the lwB4 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.

   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, if
   necessary.  This requirement is typical for NAPT44 gateways available
   today.



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   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.  Fragmentation Behaviour

   For TCP and UDP traffic the NAPT44 implemented in the lwB4 MUST
   conform with the behaviour and best current practices documented in
   [RFC4787], [RFC5508], and [RFC5382].  If the lwB4 supports DCCP, then
   the requirements in [RFC5597] MUST be implemented.

   The NAPT44 in the lwB4 MUST implement ICMP message handling behaviour
   conforming to the best current practice documented in [RFC5508].  If
   the lwB4 receives an ICMP error (for errors detected inside the IPv6
   tunnel), the node relays the ICMP error message to the original
   source (the lwAFTR).  This behaviour SHOULD be implemented conforming
   to the section 8 of [RFC2473].

   If IPv4 hosts behind different lwB4s sharing the same IPv4 address
   send fragments to the same IPv4 destination host outside the
   Lightweight 4over6 domain, those hosts may use the same IPv4
   fragmentation identifier, resulting in incorrect reassembly of the
   fragments at the destination host.  Given that the IPv4 fragmentation
   identifier is a 16-bit field, it could be used similarly to port
   ranges: A lwB4 could rewrite the IPv4 fragmentation identifier to be
   within its allocated port-set, if the resulting fragment identifier
   space is large enough related to the rate fragments are sent.
   However, splitting the identifier space in this fashion would
   increase the probability of reassembly collision for all connections
   through the lwB4.  See also Section 5.3.1 of [RFC6864].

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
                                     [RFC2983])

   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 from one
   lwB4 that need to be sent to another lwB4 associated with the same
   AFTR.  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: [RFC7341] 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.  An lwB4 MAY implement
      [RFC7341] and [I-D.ietf-dhc-dynamic-shared-v4allocation] to
      retrieve a shared IPv4 address with a set of ports.

   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
   synchronized across the lwB4s, the lwAFTRs and the provisioning
   server.

   To prevent interworking complexity, it is RECOMMENDED that an
   operator uses a single provisioning mechanism / protocol for their
   implementation.  In the event that more than one provisioning
   mechanism / protocol needs to be used (for example during a migration
   to a new provisioning mechanism), the operator SHOULD ensure that
   each provisioning mechanism has a discrete set of resources (e.g.




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   IPv4 address/PSID pools and lwAFTR tunnel addresses and binding
   tables).

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.

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.

   Otherwise the lwAFTR MUST discard all inbound ICMPv4 messages.

   The ICMP policy SHOULD be configurable.

8.2.  ICMPv4 Processing by the lwB4

   The lwB4 MUST 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.



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9.  Security Considerations

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

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

   The lwAFTR MUST rate limit ICMPv6 error messages (see Section 5.1) to
   defend against DoS attacks generated by an abuse user.

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

   This document describes a number of different protocols which may be
   used for the provisioning of lw4o6.  In each case, the security
   considerations relevant to the provisioning protocol are also
   relevant to the provisioning of lw4o6 using that protocol.  Lw4o6
   does not add any additional provisioning protocol specific security
   considerations.

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



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      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



      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




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      Xiaohong Deng

      France Telecom

      Email: xiaohong.deng@orange.com



      Cathy Zhou

      Huawei Technologies

      Section B, Huawei Industrial Base, Bantian Longgang

      Shenzhen 518129

      P.R.China

      Email: cathyzhou@huawei.com



      Alain Durand

      Juniper Networks

      1194 North Mathilda Avenue

      Sunnyvale, CA 94089-1206

      USA

      Email: adurand@juniper.net



      Reinaldo Penno

      Cisco Systems, Inc.

      170 West Tasman Drive

      San Jose, California 95134

      USA

      Email: repenno@cisco.com




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      Axel Clauberg

      Deutsche Telekom AG

      CTO-ATI

      Landgrabenweg 151

      Bonn, 53227

      Germany

      Email: axel.clauberg@telekom.de



      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








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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., Farrer, I., Perreault, S., 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-10
              (work in progress), November 2014.

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

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

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

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

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

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

   [RFC5597]  Denis-Courmont, R., "Network Address Translation (NAT)
              Behavioral Requirements for the Datagram Congestion
              Control Protocol", BCP 150, RFC 5597, September 2009.






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   [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-dynamic-shared-v4allocation]
              Cui, Y., Qiong, Q., Farrer, I., Lee, Y., Sun, Q., and M.
              Boucadair, "Dynamic Allocation of Shared IPv4 Addresses",
              draft-ietf-dhc-dynamic-shared-v4allocation-02 (work in
              progress), September 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-07 (work in progress), November 2014.

   [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-11
              (work in progress), October 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.

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



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   [RFC2983]  Black, D., "Differentiated Services and Tunnels", RFC
              2983, October 2000.

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

   [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.

   [RFC6864]  Touch, J., "Updated Specification of the IPv4 ID Field",
              RFC 6864, February 2013.

   [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.

   [RFC7341]  Sun, Q., Cui, Y., Siodelski, M., Krishnan, S., and I.
              Farrer, "DHCPv4-over-DHCPv6 (DHCP 4o6) Transport", RFC
              7341, August 2014.

Authors' Addresses

   Yong Cui
   Tsinghua University
   Beijing  100084
   P.R.China

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










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

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


   Mohamed Boucadair
   France Telecom
   Rennes  35000
   France

   Email: mohamed.boucadair@orange.com


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

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


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

   Email: yiu_lee@cable.comcast.com


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

   Email: ian.farrer@telekom.de







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