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rfc:rfc7596

Internet Engineering Task Force (IETF) Y. Cui Request for Comments: 7596 Tsinghua University Category: Standards Track Q. Sun ISSN: 2070-1721 China Telecom

                                                          M. Boucadair
                                                        France Telecom
                                                               T. Tsou
                                                   Huawei Technologies
                                                                Y. Lee
                                                               Comcast
                                                             I. Farrer
                                                   Deutsche Telekom AG
                                                             July 2015
Lightweight 4over6: An Extension to the Dual-Stack Lite Architecture

Abstract

 Dual-Stack Lite (DS-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 is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7596.

Cui, et al. Standards Track [Page 1] RFC 7596 Lightweight 4over6 July 2015

Copyright Notice

 Copyright (c) 2015 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 ....................................................3
 2. Conventions .....................................................4
 3. Terminology .....................................................5
 4. Lightweight 4over6 Architecture .................................6
 5. Lightweight B4 Behavior .........................................7
    5.1. Lightweight B4 Provisioning with DHCPv6 ....................7
    5.2. Lightweight B4 Data-Plane Behavior ........................10
         5.2.1. Fragmentation Behavior .............................11
 6. Lightweight AFTR Behavior ......................................12
    6.1. Binding Table Maintenance .................................12
    6.2. lwAFTR Data-Plane Behavior ................................13
 7. Additional IPv4 Address and Port-Set Provisioning Mechanisms ...14
 8. ICMP Processing ................................................14
    8.1. ICMPv4 Processing by the lwAFTR ...........................15
    8.2. ICMPv4 Processing by the lwB4 .............................15
 9. Security Considerations ........................................15
 10. References ....................................................16
    10.1. Normative References .....................................16
    10.2. Informative References ...................................17
 Acknowledgements ..................................................19
 Contributors ......................................................19
 Authors' Addresses ................................................21

Cui, et al. Standards Track [Page 2] RFC 7596 Lightweight 4over6 July 2015

1. Introduction

 Dual-Stack Lite (DS-Lite) [RFC6333] defines a model for providing
 IPv4 access over an IPv6 network using two well-known technologies:
 IP in IP [RFC2473] and Network Address Translation (NAT).  The
 DS-Lite architecture defines two major functional elements as
 follows:
 Basic Bridging BroadBand (B4) element:  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 (AFTR) element:  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 a 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.  Maintaining per-flow state can also place a large NAPT
 logging overhead on the service provider in countries where logging
 is a legal requirement.
 This document describes a mechanism called "Lightweight 4over6"
 (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 realized:
 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.
 Operators' IPv6 and IPv4 addressing architectures remain independent
 of each other.  Therefore, flexible IPv4/IPv6 addressing schemes can
 be deployed.

Cui, et al. Standards Track [Page 3] RFC 7596 Lightweight 4over6 July 2015

 Lightweight 4over6 is a solution designed specifically for complete
 independence between IPv6 subnet prefixes and IPv4 addresses 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.  "Mapping of Address and
 Port with Encapsulation (MAP-E)" [RFC7597] 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.
 One of the features of Lightweight 4over6 is to keep per-subscriber
 state in the service provider's network.  This technique is
 categorized as a "binding approach" [Unified-v4-in-v6] that 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
 Address plus Port (A+P) called "Binding Table Mode" (see Section 4.4
 of [RFC6346]).
 This document focuses on architectural considerations, particularly
 on the expected behavior of the involved functional elements and
 their interfaces.  Deployment-specific issues such as redundancy and
 provisioning policy are out of scope for this document.

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].

Cui, et al. Standards Track [Page 4] RFC 7596 Lightweight 4over6 July 2015

3. Terminology

 This document defines the following terms:
 Lightweight 4over6 (lw4o6):   An IPv4-over-IPv6 hub-and-spoke
                               mechanism that extends DS-Lite by
                               moving the IPv4 translation (NAPT44)
                               function from the AFTR to the B4.
 Lightweight B4 (lwB4):        A B4 element [RFC6333] that supports
                               Lightweight 4over6 extensions.  An lwB4
                               is a function implemented on a
                               dual-stack-capable node -- either a
                               directly connected device or a CPE --
                               that supports port-restricted IPv4
                               address allocation, implements NAPT44
                               functionality, and creates a tunnel to
                               an lwAFTR.
 Lightweight AFTR (lwAFTR):    An AFTR element [RFC6333] that supports
                               the Lightweight 4over6 extension.  An
                               lwAFTR is an IPv4-in-IPv6 tunnel
                               endpoint that 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, the Stream Control
                               Transmission Protocol (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" and "AFTR"
 should be understood to refer specifically to a DS-Lite
 implementation.  The terms "lwB4" and "lwAFTR" refer to a Lightweight
 4over6 implementation.

Cui, et al. Standards Track [Page 5] RFC 7596 Lightweight 4over6 July 2015

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:
 +--------+   +---------+  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|
 +--------+   +---------+                +------+   +-------------+
                         lw4o6 NAPT model:
         subscriber state in the lwAFTR, NAPT state in the 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 IPv4/IPv6
    encapsulation/decapsulation.
 o  The lwAFTR, which performs the IPv4/IPv6 encapsulation/
    decapsulation.
 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 through a provisioning mechanism such as
 DHCP, the Port Control Protocol (PCP) [RFC6887], or the Broadband
 Forum's TR-69 specification [TR069].
 The lwAFTR needs to know the binding between the IPv6 address of
 each subscriber as well as the IPv4 address and port set allocated to
 each 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.

Cui, et al. Standards Track [Page 6] RFC 7596 Lightweight 4over6 July 2015

 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.
                           +------------+
                   /-------|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 [RFC7598].  This
 means that the lifetime of the softwire and the derived configuration
 information (e.g., IPv4 shared address, IPv4 address) are 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.

Cui, et al. Standards Track [Page 7] RFC 7596 Lightweight 4over6 July 2015

 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 Customer
 Edge (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 be used for sourcing the lw4o6 tunnel.  The full
 /128 address is then constructed in the same manner as [RFC7597].
  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, it is 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.

Cui, et al. Standards Track [Page 8] RFC 7596 Lightweight 4over6 July 2015

 An lwB4 MUST support dynamic port-restricted IPv4 address
 provisioning.  The port-set algorithm for provisioning this is
 described in Section 5.1 of [RFC7597].  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
 provisioning [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) not be
 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 lwB4
 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 Sections 5.5 and 6.4 of [RFC6333]
 apply to Lightweight 4over6; lw4o6 implementations MUST comply with
 all requirements stated there.

Cui, et al. Standards Track [Page 9] RFC 7596 Lightweight 4over6 July 2015

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
                                   decapsulation of tunneled traffic
 5.3 Fragmentation and Reassembly  Covering MTU and fragmentation
                                   considerations (referencing
                                   [RFC2473])
 7.1 Tunneling                     Covering tunneling and Traffic
                                   Class mapping between IPv4 and IPv6
                                   (referencing [RFC2473]).  Also see
                                   [RFC2983]
 The lwB4 element performs IPv4 address translation (NAPT44) as well
 as encapsulation and decapsulation.  It runs standard NAPT44
 [RFC3022] using the allocated port-restricted address as its external
 IPv4 address and range of source ports.
 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.

Cui, et al. Standards Track [Page 10] RFC 7596 Lightweight 4over6 July 2015

 When the lwB4 receives an IPv4-in-IPv6 packet from the lwAFTR, it
 decapsulates 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 3, Code 13 -- Destination Unreachable,
 Communication Administratively Prohibited) MAY be sent back to the
 lwAFTR.  The ICMP policy SHOULD be configurable.
 The lwB4 is responsible for performing Application Layer Gateway
 (ALG) functions (e.g., SIP, FTP) and other NAPT traversal mechanisms
 (e.g., Universal Plug and Play (UPnP) IGD (Internet Gateway Device),
 the NAT Port Mapping Protocol (NAT-PMP), manual binding
 configuration, PCP) for the internal hosts, if necessary.  This
 requirement is typical for NAPT44 gateways available today.
 It is possible that an lwB4 is co-located in a host.  In this case,
 the functions of NAPT44 and encapsulation/decapsulation are
 implemented inside the host.

5.2.1. Fragmentation Behavior

 For TCP and UDP traffic, the NAPT44 implemented in the lwB4 MUST
 conform to the behavior and best current practices documented in
 [RFC4787], [RFC5508], and [RFC5382].  If the lwB4 supports the
 Datagram Congestion Control Protocol (DCCP), then the requirements in
 [RFC5597] MUST be implemented.
 The NAPT44 in the lwB4 MUST implement ICMP message handling behavior
 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 behavior SHOULD be implemented conforming
 to 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: An 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 at which fragments are

Cui, et al. Standards Track [Page 11] RFC 7596 Lightweight 4over6 July 2015

 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 the
 restricted port set.  Unlike the DS-Lite extended binding table,
 which is a 5-tuple NAPT table and is defined in Section 6.6 of
 [RFC6333], each entry in the Lightweight 4over6 binding table
 contains the following 3-tuples:
 o  IPv6 address for a single lwB4
 o  Public IPv4 address
 o  Restricted port set
 The entry has two functions: the IPv6 encapsulation of inbound
 IPv4 packets destined to the lwB4 and the validation of outbound
 IPv4-in-IPv6 packets received from the lwB4 for decapsulation.
 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.

Cui, et al. Standards Track [Page 12] RFC 7596 Lightweight 4over6 July 2015

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
                                   decapsulation of tunneled traffic
 6.3 Fragmentation and Reassembly  Fragmentation and reassembly
                                   considerations (referencing
                                   [RFC2473])
 7.1 Tunneling                     Covering tunneling and Traffic
                                   Class mapping between IPv4 and IPv6
                                   (referencing [RFC2473]).  Also see
                                   [RFC2983]
 When the lwAFTR receives an IPv4-in-IPv6 packet from an lwB4, it
 decapsulates 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), the lwAFTR MUST discard or implement a policy (such as
 redirection) on the packet.  An ICMPv6 Type 1, Code 5 (Destination
 Unreachable, 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 look up 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.

Cui, et al. Standards Track [Page 13] RFC 7596 Lightweight 4over6 July 2015

 The lwAFTR MUST support hairpinning of traffic between two lwB4s, by
 performing decapsulation 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 provider's network.  This
    enables leasing of IPv4 addresses and makes DHCPv4 options
    available to the DHCPv4-over-DHCPv6 client.  An lwB4 MAY implement
    [RFC7341] and [Dyn-Shared-v4Alloc] to retrieve a shared IPv4
    address with a set of ports.
 o  PCP [RFC6887]: an lwB4 MAY use [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 use 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., IPv4 address/PSID pools, as well as 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 of address
 sharing, Lightweight 4over6 requires specific ICMP message handling
 not required by DS-Lite.

Cui, et al. Standards Track [Page 14] RFC 7596 Lightweight 4over6 July 2015

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 field 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 look up 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 the 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.

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 that 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 sets (which may be
 generated in a pseudorandom way [RFC6431]), the randomness of the

Cui, et al. Standards Track [Page 15] RFC 7596 Lightweight 4over6 July 2015

 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 that 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 other security considerations specific to these
 provisioning protocols.

10. References

10.1. Normative References

 [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
            and E. Lear, "Address Allocation for Private Internets",
            BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,
            <http://www.rfc-editor.org/info/rfc1918>.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
            IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473,
            December 1998, <http://www.rfc-editor.org/info/rfc2473>.
 [RFC4787]  Audet, F., Ed., and C. Jennings, "Network Address
            Translation (NAT) Behavioral Requirements for Unicast
            UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787,
            January 2007, <http://www.rfc-editor.org/info/rfc4787>.
 [RFC5382]  Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.
            Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
            RFC 5382, DOI 10.17487/RFC5382, October 2008,
            <http://www.rfc-editor.org/info/rfc5382>.

Cui, et al. Standards Track [Page 16] RFC 7596 Lightweight 4over6 July 2015

 [RFC5508]  Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
            Behavioral Requirements for ICMP", BCP 148, RFC 5508,
            DOI 10.17487/RFC5508, April 2009,
            <http://www.rfc-editor.org/info/rfc5508>.
 [RFC5597]  Denis-Courmont, R., "Network Address Translation (NAT)
            Behavioral Requirements for the Datagram Congestion
            Control Protocol", BCP 150, RFC 5597,
            DOI 10.17487/RFC5597, September 2009,
            <http://www.rfc-editor.org/info/rfc5597>.
 [RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee,
            "Dual-Stack Lite Broadband Deployments Following IPv4
            Exhaustion", RFC 6333, DOI 10.17487/RFC6333, August 2011,
            <http://www.rfc-editor.org/info/rfc6333>.
 [RFC7598]  Mrugalski, T., Troan, O., Farrer, I., Perreault, S., Dec,
            W., Bao, C., Yeh, L., and X. Deng, "DHCPv6 Options for
            Configuration of Softwire Address and Port-Mapped
            Clients", RFC 7598, DOI 10.17487/RFC7598, July 2015,
            <http://www.rfc-editor.org/info/rfc7598>.

10.2. Informative References

 [B4-Trans-DSLite]
            Cui, Y., Sun, Q., Boucadair, M., Tsou, T., Lee, Y., and
            I. Farrer, "Lightweight 4over6: An Extension to the
            DS-Lite Architecture", Work in Progress,
            draft-cui-softwire-b4-translated-ds-lite-11,
            February 2013.
 [DSLite-LW-Ext]
            Deng, X., Boucadair, M., and C. Zhou, "NAT offload
            extension to Dual-Stack lite", Work in Progress,
            draft-zhou-softwire-b4-nat-04, October 2011.
 [Dyn-Shared-v4Alloc]
            Cui, Y., Sun, Q., Farrer, I., Lee, Y., Sun, Q., and
            M. Boucadair, "Dynamic Allocation of Shared IPv4
            Addresses", Work in Progress,
            draft-ietf-dhc-dynamic-shared-v4allocation-09, May 2015.
 [PCP-PORT_SET]
            Sun, Q., Boucadair, M., Sivakumar, S., Zhou, C., Tsou, T.,
            and S. Perreault, "Port Control Protocol (PCP) Extension
            for Port Set Allocation", Work in Progress,
            draft-ietf-pcp-port-set-09, May 2015.

Cui, et al. Standards Track [Page 17] RFC 7596 Lightweight 4over6 July 2015

 [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
            Defeating Denial of Service Attacks which employ IP Source
            Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
            May 2000, <http://www.rfc-editor.org/info/rfc2827>.
 [RFC2983]  Black, D., "Differentiated Services and Tunnels",
            RFC 2983, DOI 10.17487/RFC2983, October 2000,
            <http://www.rfc-editor.org/info/rfc2983>.
 [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
            Address Translator (Traditional NAT)", RFC 3022,
            DOI 10.17487/RFC3022, January 2001,
            <http://www.rfc-editor.org/info/rfc3022>.
 [RFC6269]  Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
            P. Roberts, "Issues with IP Address Sharing", RFC 6269,
            DOI 10.17487/RFC6269, June 2011,
            <http://www.rfc-editor.org/info/rfc6269>.
 [RFC6346]  Bush, R., Ed., "The Address plus Port (A+P) Approach to
            the IPv4 Address Shortage", RFC 6346,
            DOI 10.17487/RFC6346, August 2011,
            <http://www.rfc-editor.org/info/rfc6346>.
 [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,
            DOI 10.17487/RFC6431, November 2011,
            <http://www.rfc-editor.org/info/rfc6431>.
 [RFC6864]  Touch, J., "Updated Specification of the IPv4 ID Field",
            RFC 6864, DOI 10.17487/RFC6864, February 2013,
            <http://www.rfc-editor.org/info/rfc6864>.
 [RFC6887]  Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
            P. Selkirk, "Port Control Protocol (PCP)", RFC 6887,
            DOI 10.17487/RFC6887, April 2013,
            <http://www.rfc-editor.org/info/rfc6887>.
 [RFC7040]  Cui, Y., Wu, J., Wu, P., Vautrin, O., and Y. Lee, "Public
            IPv4-over-IPv6 Access Network", RFC 7040,
            DOI 10.17487/RFC7040, November 2013,
            <http://www.rfc-editor.org/info/rfc7040>.
 [RFC7341]  Sun, Q., Cui, Y., Siodelski, M., Krishnan, S., and I.
            Farrer, "DHCPv4-over-DHCPv6 (DHCP 4o6) Transport",
            RFC 7341, DOI 10.17487/RFC7341, August 2014,
            <http://www.rfc-editor.org/info/rfc7341>.

Cui, et al. Standards Track [Page 18] RFC 7596 Lightweight 4over6 July 2015

 [RFC7597]  Troan, O., Ed., Dec, W., Li, X., Bao, C., Matsushima, S.,
            Murakami, T., and T. Taylor, Ed., "Mapping of Address and
            Port with Encapsulation (MAP-E)", RFC 7597,
            DOI 10.17487/RFC7597, July 2015,
            <http://www.rfc-editor.org/info/rfc7597>.
 [Stateless-DS-Lite]
            Penno, R., Durand, A., Clauberg, A., and L. Hoffmann,
            "Stateless DS-Lite", Work in Progress,
            draft-penno-softwire-sdnat-02, March 2012.
 [TR069]    Broadband Forum TR-069, "CPE WAN Management Protocol",
            Amendment 5, CWMP Version: 1.4, November 2013,
            <https://www.broadband-forum.org>.
 [Unified-v4-in-v6]
            Boucadair, M., Farrer, I., Perreault, S., Ed., and S.
            Sivakumar, Ed., "Unified IPv4-in-IPv6 Softwire CPE", Work
            in Progress, draft-ietf-softwire-unified-cpe-01, May 2013.

Acknowledgements

 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: [B4-Trans-DSLite],
 [DSLite-LW-Ext], and [Stateless-DS-Lite].

Contributors

 The following individuals contributed to this effort:
 Jianping Wu
 Tsinghua University
 Department of Computer Science, Tsinghua University
 Beijing  100084
 China
 Phone: +86-10-62785983
 Email: jianping@cernet.edu.cn
 Peng Wu
 Tsinghua University
 Department of Computer Science, Tsinghua University
 Beijing  100084
 China
 Phone: +86-10-62785822
 Email: pengwu.thu@gmail.com

Cui, et al. Standards Track [Page 19] RFC 7596 Lightweight 4over6 July 2015

 Qi Sun
 Tsinghua University
 Beijing  100084
 China
 Phone: +86-10-62785822
 Email: sunqi@csnet1.cs.tsinghua.edu.cn
 Chongfeng Xie
 China Telecom
 Room 708, No. 118, Xizhimennei Street
 Beijing  100035
 China
 Phone: +86-10-58552116
 Email: xiechf@ctbri.com.cn
 Xiaohong Deng
 The University of New South Wales
 Sydney  NSW 2052
 Australia
 Email: dxhbupt@gmail.com
 Cathy Zhou
 Huawei Technologies
 Section B, Huawei Industrial Base, Bantian Longgang
 Shenzhen  518129
 China
 Email: cathyzhou@huawei.com
 Alain Durand
 Juniper Networks
 1194 North Mathilda Avenue
 Sunnyvale, CA  94089-1206
 United States
 Email: adurand@juniper.net
 Reinaldo Penno
 Cisco Systems, Inc.
 170 West Tasman Drive
 San Jose, CA  95134
 United States
 Email: repenno@cisco.com

Cui, et al. Standards Track [Page 20] RFC 7596 Lightweight 4over6 July 2015

 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 (a.k.a. Noriyuki Arai)
 BBIX, Inc.
 Tokyo Shiodome Building, Higashi-Shimbashi 1-9-1
 Minato-ku, Tokyo  105-7310
 Japan
 Email: maoke@bbix.net

Authors' Addresses

 Yong Cui
 Tsinghua University
 Beijing  100084
 China
 Phone: +86-10-62603059
 Email: yong@csnet1.cs.tsinghua.edu.cn
 Qiong Sun
 China Telecom
 Room 708, No. 118, Xizhimennei Street
 Beijing  100035
 China
 Phone: +86-10-58552936
 Email: sunqiong@ctbri.com.cn

Cui, et al. Standards Track [Page 21] RFC 7596 Lightweight 4over6 July 2015

 Mohamed Boucadair
 France Telecom
 Rennes  35000
 France
 Email: mohamed.boucadair@orange.com
 Tina Tsou
 Huawei Technologies
 2330 Central Expressway
 Santa Clara, CA  95050
 United States
 Phone: +1-408-330-4424
 Email: tena@huawei.com
 Yiu L. Lee
 Comcast
 One Comcast Center
 Philadelphia, PA  19103
 United States
 Email: yiu_lee@cable.comcast.com
 Ian Farrer
 Deutsche Telekom AG
 CTO-ATI, Landgrabenweg 151
 Bonn, NRW  53227
 Germany
 Email: ian.farrer@telekom.de

Cui, et al. Standards Track [Page 22]

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