Softwire Provisioning Using DHCPv4 over DHCPv6
RFC 8539

Document Type RFC - Proposed Standard (March 2019; No errata)
Updates RFC 7598
Last updated 2019-03-15
Replaces draft-fsc-softwire-dhcp4o6-saddr-opt
Stream IETF
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Stream WG state Submitted to IESG for Publication
Document shepherd Bernie Volz
Shepherd write-up Show (last changed 2018-06-21)
IESG IESG state RFC 8539 (Proposed Standard)
Consensus Boilerplate Yes
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Responsible AD Suresh Krishnan
Send notices to Bernie Volz <volz@cisco.com>
IANA IANA review state IANA OK - Actions Needed
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Internet Engineering Task Force (IETF)                         I. Farrer
Request for Comments: 8539                           Deutsche Telekom AG
Updates: 7598                                                     Q. Sun
Category: Standards Track                                         Y. Cui
ISSN: 2070-1721                                                   L. Sun
                                                     Tsinghua University
                                                              March 2019

             Softwire Provisioning Using DHCPv4 over DHCPv6

Abstract

   DHCPv4 over DHCPv6 (RFC 7341) is a mechanism for dynamically
   configuring IPv4 for use as an over-the-top service in an IPv6-only
   network.  Softwires are an example of such a service.  For DHCPv4
   over DHCPv6 (DHCP 4o6) to function with some IPv4-over-IPv6 softwire
   mechanisms and deployment scenarios (e.g., RFC 7596 or RFC 7597), the
   operator needs to know the IPv6 address that the client will use as
   the source of an IPv4-in-IPv6 softwire tunnel.  This address, in
   conjunction with the client's IPv4 address, and (in some deployments)
   the Port Set ID are used to create a binding table entry in the
   operator's softwire tunnel concentrator.  This memo defines a DHCPv6
   option to convey IPv6 parameters for establishing the softwire tunnel
   and a DHCPv4 option (to be used only with DHCP 4o6) to communicate
   the source tunnel IPv6 address between the DHCP 4o6 client and
   server.  It is designed to work in conjunction with the IPv4 address
   allocation process.

   "DHCPv6 Options for Configuration of Softwire Address and Port-Mapped
   Clients" (RFC 7598) describes a deterministic DHCPv6-based mechanism
   for provisioning softwires.  This document updates RFC 7598, allowing
   OPTION_S46_BR (90) to be enumerated in the DHCPv6 client's Option
   Request Option (ORO) request and to appear directly within subsequent
   messages sent by the DHCPv6 server.

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RFC 8539           Softwire Provisioning with DHCP 4o6        March 2019

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 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8539.

Copyright Notice

   Copyright (c) 2019 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
   (https://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.

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RFC 8539           Softwire Provisioning with DHCP 4o6        March 2019

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   4.  Solution Overview . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Updating RFC 7598 to Permit the Reuse of
           OPTION_S46_BR (90)  . . . . . . . . . . . . . . . . . . .   5
   5.  DHCP 4o6 IPv6/IPv4 Binding Message Flow . . . . . . . . . . .   6
   6.  DHCP Options  . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  DHCPv6 Softwire Source Binding Prefix Hint Option . . . .   7
     6.2.  DHCP 4o6 Softwire Source Address Option . . . . . . . . .   8
   7.  Client Behavior . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Client Initialization . . . . . . . . . . . . . . . . . .   9
     7.2.  Renewing or Rebinding the IPv4 Address Lease and
           Softwire Source Address . . . . . . . . . . . . . . . . .  10
       7.2.1.  Changing the Bound IPv6 Softwire Source Address . . .  10
     7.3.  Releasing the IPv4 Address Lease and Softwire
           Source Address  . . . . . . . . . . . . . . . . . . . . .  11
     7.4.  OPTION_S46_BIND_IPV6_PREFIX Validation Behavior . . . . .  11
     7.5.  Client and Server Softwire Source Address Mismatch  . . .  11
     7.6.  Use with Dynamic, Shared IPv4 Addresses . . . . . . . . .  12
   8.  Server Behavior . . . . . . . . . . . . . . . . . . . . . . .  12
     8.1.  Changing the Bound IPv6 Source Address  . . . . . . . . .  12
     8.2.  Handling Conflicts between Clients' Bound IPv6 Source
           Addresses . . . . . . . . . . . . . . . . . . . . . . . .  12
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
     9.1.  Client Privacy Considerations . . . . . . . . . . . . . .  14
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     11.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   Deterministic IPv4-over-IPv6 transition technologies require that
   elements be preconfigured with binding rules for routing traffic to
   clients.  This places a constraint on the choice of address used as
   the client's softwire source address: it must use a predetermined
   prefix, which is usually configured on the home gateway device.
   [RFC7598] describes a DHCPv6-based mechanism for provisioning such
   deterministic softwires.

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RFC 8539           Softwire Provisioning with DHCP 4o6        March 2019

   A dynamic provisioning model, such as using DHCPv4 over DHCPv6 (DHCP
   4o6) [RFC7341], allows much more flexibility in the location of the
   IPv4-over-IPv6 softwire source address.  In this model, the IPv6
   address is dynamically communicated back to the service provider,
   allowing the corresponding softwire configuration to be created in
   the border relay (BR).

   The DHCP 4o6 client and softwire client could be run on end devices
   attached to a network segment using any routable IPv6 prefix
   allocated to an end user, located anywhere within an arbitrary home
   network topology.  Dynamic allocation also helps to optimize IPv4
   resource usage, because only clients that are actively renewing their
   IPv4 lease hold on to the address.

   This document describes a mechanism for dynamically provisioning
   softwires created using DHCP 4o6, including provisioning the client
   with the address of the softwire BR and informing the service
   provider of a client's binding between the dynamically allocated IPv4
   address and Port Set ID and the IPv6 address that the softwire
   initiator will use for accessing IPv4-over-IPv6 services.

   The mechanism operates alongside the DHCP 4o6 message flows to
   communicate the binding information over the IPv6-only network.  The
   DHCP 4o6 server provides a single point in the network that holds the
   current client binding information.  The service provider can then
   use this binding information to provision other functional elements,
   such as the BR(s).

2.  Applicability

   The mechanism described in this document is only suitable for use for
   provisioning softwire clients via DHCP 4o6.  The options described
   here are only applicable within the DHCP 4o6 message-exchange
   process.  Current softwire technologies suitable for extending to
   incorporate DHCP 4o6 with dynamic IPv4 address leasing include
   [RFC7597] and [RFC7596].

3.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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4.  Solution Overview

   In order to provision a softwire, both IPv6 and IPv4 configurations
   need to be passed to the client.  To map this to the DHCP 4o6
   configuration process, the IPv6 configuration is carried in DHCPv6
   options [RFC8415], carried inside the DHCPv6 message DHCPV4-RESPONSE
   (21) sent by the server.  OPTION_S46_BR (90) is used to provision the
   remote IPv6 address for the softwire BR (see Section 4.1).
   OPTION_S46_BIND_IPV6_PREFIX (137) is optionally sent by the DHCP 4o6
   server to indicate to the client a preferred IPv6 prefix for binding
   the received IPv4 configuration and sourcing tunnel traffic.  This
   may be necessary if there are multiple IPv6 prefixes in use in the
   customer network (e.g., Unique Local Addresses (ULAs)) or if the
   specific IPv4-over-IPv6 transition mechanism requires the use of a
   particular prefix for any reason.

   IPv4 configuration is carried in DHCPv4 messages [RFC2131] (inside
   the DHCP 4o6 option OPTION_DHCPV4_MSG (87)) using the mechanism
   described in [RFC7341].

   In order for the client to communicate the softwire source address, a
   new DHCPv4 option OPTION_DHCP4O6_S46_SADDR (109) is defined in this
   document.  This is included in DHCPREQUEST messages sent by the
   client and is stored by the server for the lifetime of the IPv4
   address lease.

4.1.  Updating RFC 7598 to Permit the Reuse of OPTION_S46_BR (90)

   Section 4.2 of [RFC7598] defines option OPTION_S46_BR (90) for
   communicating remote softwire BR IPv6 address(es) to a client, but it
   mandates that the option can only be used when encapsulated within
   one of the softwire container options: OPTION_S46_CONT_MAPE (94) or
   OPTION_S46_CONT_LW (96).  From Section 3 of [RFC7598]:

      Softwire46 DHCPv6 clients that receive provisioning options that
      are not encapsulated in container options MUST silently ignore
      these options.

   This document updates [RFC7598], removing this restriction for
   OPTION_S46_BR (90), allowing it to be enumerated in the client's ORO
   request and appear directly within subsequent messages sent by the
   DHCPv6 server.

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RFC 8539           Softwire Provisioning with DHCP 4o6        March 2019

5.  DHCP 4o6 IPv6/IPv4 Binding Message Flow

   Figure 1 shows the relevant extensions to the successful DHCP 4o6
   IPv4 allocation client/server message flow for the softwire source
   address function.  The full process, including error handling, is
   described in Section 7.

   In each step, the DHCPv6 portion of the message and any relevant
   option is shown above the arrow.  The DHCP 4o6 content of the message
   and its relevant options are below the arrow.  All the DHCPv4
   messages are encapsulated in DHCPV4-QUERY (20) or DHCPV4-RESPONSE
   (21) messages.  Where relevant, the necessary options and their
   contents are shown.

        DHCP 4o6                                              DHCP 4o6
         Client                                                Server
           |                                                      |
           |       DHCPv6 - DHCPV4-QUERY message containing       |
           |           OPTION_ORO (6) listing (90, 137)           |
    Step 1 |----------------------------------------------------->|
           |            DHCPv4 - DHCPDISCOVER message             |
           |                                                      |
           |                                                      |
           |     DHCPv6 - DHCPV4-RESPONSE message containing      |
           | OPTION_S46_BR(90), OPTION_S46_BIND_IPV6_PREFIX(137)  |
           |     (bind-ipv6-prefix with service provider's        |
           |                  preferred prefix)                   |
    Step 2 |<-----------------------------------------------------|
           |              DHCPv4 - DHCPOFFER message              |
           |         containing an available IPv4 address         |
           |                                                      |
           |             DHCPv6 - DHCPV4-QUERY message            |
    Step 3 |----------------------------------------------------->|
           |     DHCPv4 - DHCPREQUEST message containing the      |
           | requested IPv4 address and OPTION_DHCP4O6_S46_SADDR  |
           |   (softwire-ipv6-src-address with client's bound     |
           |            IPv6 softwire source address)             |
           |                                                      |
           |                                                      |
           |           DHCPv6 - DHCPV4-RESPONSE message           |
    Step 4 |<-----------------------------------------------------|
           |          DHCPv4 - DHCPACK message containing         |
           | the leased IPv4 address and OPTION_DHCP4O6_S46_SADDR |
           |    (softwire-ipv6-src-address with client's bound    |
           |              IPv6 softwire source address)           |
           |                                                      |

                 Figure 1: IPv6/IPv4 Binding Message Flow

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RFC 8539           Softwire Provisioning with DHCP 4o6        March 2019

   Step 1  The client constructs a DHCPv6 "DHCPV4-QUERY (20)" message.
           This message contains two options: DHCPv6 OPTION_ORO (6) and
           OPTION_DHCPV4_MSG (87).  OPTION_ORO lists "90"
           (OPTION_S46_BR) and "137" (OPTION_S46_BIND_IPV6_PREFIX).
           OPTION_DHCPV4_MSG contains a DHCPv4 DHCPDISCOVER message.

   Step 2  The server responds with a DHCPv6 "DHCPV4-RESPONSE (21)"
           message.  This message contains an OPTION_S46_BR (90)
           containing the IPv6 address of the BR for the client's
           softwire configuration.  The message may also optionally
           contain OPTION_S46_BIND_IPV6_PREFIX (137).  OPTION_DHCPV4_MSG
           contains a DHCPv4 DHCPOFFER message.  The DHCPv4 message
           contains an available IPv4 address.

   Step 3  The client sends a DHCPv6 "DHCPV4-QUERY (20)" message
           containing a DHCPv4 DHCPREQUEST message with the requested
           IPv4 address and OPTION_DHCP4O6_S46_SADDR (109) with the IPv6
           address that the client will use as its softwire source
           address.

   Step 4  The server sends a DHCPv6 "DHCPV4-RESPONSE (21)" message.
           OPTION_DHCPV4_MSG contains a DHCPv4 DHCPACK message with the
           allocated IPv4 address.  OPTION_DHCP4O6_S46_SADDR with the
           client's bound softwire source address is included.

6.  DHCP Options

6.1.  DHCPv6 Softwire Source Binding Prefix Hint Option

   The format of the DHCPv6 source binding prefix hint option is as
   follows:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  OPTION_S46_BIND_IPV6_PREFIX  |         option-length         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |bindprefix6-len|                                               |
     +-+-+-+-+-+-+-+-+             bind-ipv6-prefix                  .
     .                            (variable length)                  .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 2: Format of OPTION_S46_BIND_IPV6_PREFIX

   o  option-code: OPTION_S46_BIND_IPV6_PREFIX (137)

   o  option-length: 1 + length of bind-ipv6-prefix, specified in bytes.

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RFC 8539           Softwire Provisioning with DHCP 4o6        March 2019

   o  bindprefix6-len: 8-bit field expressing the bit mask length of the
      IPv6 prefix specified in bind-ipv6-prefix.  Valid values are 0 to
      128.

   o  bind-ipv6-prefix: The IPv6 prefix indicating the preferred prefix
      for the client to bind the received IPv4 configuration to.  The
      length is (bindprefix6-len + 7) / 8.  The field is padded on the
      right with zero bits up to the next octet boundary when
      bind-ipv6-prefix is not evenly divisible by 8.  These padding bits
      are ignored by the receiver (see Section 7.4).

   OPTION_S46_BIND_IPV6_PREFIX is a singleton.  Servers MUST NOT send
   more than one instance of the OPTION_S46_BIND_IPV6_PREFIX option.

6.2.  DHCP 4o6 Softwire Source Address Option

   The format of the DHCPv4 over DHCPv6 softwire source address option
   is as follows:

              0                             1
              0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |      option-code      |     option-length     |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             +           softwire-ipv6-src-address           +
             .                  (128 bits)                   .
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

               Figure 3: Format of OPTION_DHCP4O6_S46_SADDR

   o  option-code: OPTION_DHCP4O6_S46_SADDR (109)

   o  option-length: 16.

   o  softwire-ipv6-src-address: 16 bytes long; the IPv6 address that is
      associated (either being requested for binding or currently bound)
      with the client's IPv4 configuration.

   Note: The function of OPTION_DHCP4O6_S46_SADDR may seem similar to
   the DHCPv4 message's "chaddr" field or the Client Identifier (61)
   option in that it provides a unique lower-layer address that the
   server can use for identifying the client.  However, as both of these
   are required to remain constant throughout the address lease
   lifetime, they cannot be used with the mechanism described in this
   document.  This is because the client may only be able to construct
   the IPv6 address to use as the source address after it has received
   the first DHCPV4-RESPONSE message from the server containing
   OPTION_S46_BIND_IPV6_PREFIX.

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RFC 8539           Softwire Provisioning with DHCP 4o6        March 2019

7.  Client Behavior

   A client requiring dynamic softwire configuration first enables DHCP
   4o6 configuration using the method described in Section 5 of
   [RFC7341].  If OPTION_DHCP4_O_DHCP6_SERVER is received in the
   corresponding REPLY message, the client MAY continue with the
   configuration process described below.

   Before the dynamic softwire configuration process can commence, the
   client MUST be configured with a suitable IPv6 prefix to be used as
   the local softwire endpoint.  This could be obtained using DHCPv6,
   Router Advertisement (RA) / Prefix Information Option (PIO), or
   another mechanism.

7.1.  Client Initialization

   When constructing the initial DHCP 4o6 DHCPDISCOVER message, the
   client includes a DHCPv6 OPTION_ORO (6) within the options field of
   the DHCP-QUERY message.  OPTION_ORO contains the option codes for
   OPTION_S46_BR (90) and OPTION_S46_BIND_IPV6_PREFIX (137).

   On receipt of the DHCP 4o6 server's reply (a DHCPV4-RESPONSE
   containing a DHCPOFFER message), the client checks the contents of
   the DHCPv4-RESPONSE for the presence of a valid OPTION_S46_BR option.
   If this option is not present, or does not contain at least one valid
   IPv6 address for a BR, then the client MUST discard the message, as
   without the address of the BR the client cannot configure the
   softwire and so has no interface to request IPv4 configuration for.

   The DHCPV4-RESPONSE message may also include
   OPTION_S46_BIND_IPV6_PREFIX, which is used by the operator to
   indicate a preferred prefix that the client should bind IPv4
   configuration to.  If received, the client first checks the option
   according to Section 7.4.  If valid, the client uses this prefix as
   the "IPv6 binding prefix" and follows to the process described in
   Section 5.1 of [RFC7596] in order to select an active IPv6 prefix to
   construct the softwire.  If no match is found, or the client doesn't
   receive OPTION_S46_BIND_IPV6_PREFIX, the client MAY select any valid
   IPv6 prefix (of a suitable scope) to use as the tunnel source.

   Once the client has selected a suitable prefix, it MAY either use an
   existing IPv6 address that is already configured on an interface or
   create a new address specifically for use as the softwire source
   address (e.g., using an Interface Identifier constructed as per
   Section 6 of [RFC7597]).  If a new address is being created, the
   client MUST complete configuration of the new address, performing
   duplicate address detection (if required) before proceeding.

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RFC 8539           Softwire Provisioning with DHCP 4o6        March 2019

   The client then constructs a DHCPV4-QUERY message containing a DHCPv4
   DHCPREQUEST message.  OPTION_DHCP4O6_S46_SADDR is included in the
   options field of the DHCPREQUEST message with the IPv6 address of its
   softwire source address in the softwire-ipv6-src-address field.

   When the client receives a DHCPv4 DHCPACK message from the server, it
   checks the IPv6 address in OPTION_DHCP4O6_S46_SADDR against its
   active softwire source address.  If they match, the allocation
   process has concluded.  If there is a discrepancy, then the process
   described in Section 7.5 is followed.

   If the client receives a DHCPv4 DHCPNAK message from the server, then
   the configuration process has been unsuccessful.  The client then
   restarts the process from Step 1 of Figure 1.

7.2.  Renewing or Rebinding the IPv4 Address Lease and Softwire Source
      Address

   Whenever the client attempts to extend the lease time of the IPv4
   address, OPTION_DHCP4O6_S46_SADDR with the IPv6 address of its
   softwire source address in the softwire-ipv6-src-address field MUST
   be included in the DHCPREQUEST message.

7.2.1.  Changing the Bound IPv6 Softwire Source Address

   Across the lifetime of the leased IPv4 address, it is possible that
   the client's IPv6 address will change, e.g., if there is an IPv6
   renumbering event.

   In this situation, the client MUST inform the server of the new
   address.  This is done by sending a DHCPREQUEST message containing
   OPTION_DHCP4O6_S46_SADDR with the new IPv6 source address.

   When the client receives a DHCPv4 DHCPACK message from the server, it
   checks the IPv6 address in OPTION_DHCP4O6_S46_SADDR against its
   active softwire source address.  If they match, the allocation
   process has concluded.  If there is a discrepancy, then the process
   described in Section 7.5 is followed.

   If the client receives a DHCPv4 DHCPNAK message in response from the
   server, then the change of the bound IPv6 softwire source address has
   been unsuccessful.  In this case, the client MUST stop using the new
   IPv6 source address.  The client then restarts the process from Step
   1 of Figure 1.

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7.3.  Releasing the IPv4 Address Lease and Softwire Source Address

   When the client no longer requires the IPv4 resource, it sends a
   DHCPv4 DHCPRELEASE message to the server.  As the options field is
   unused in this message type, OPTION_DHCP4O6_S46_SADDR is not
   included.

7.4.  OPTION_S46_BIND_IPV6_PREFIX Validation Behavior

   On receipt of the OPTION_S46_BIND_IPV6_PREFIX option, the client
   makes the following validation checks:

   o  The received bindprefix6-len value is not larger than 128.

   o  The number of bytes received in the bind-ipv6-prefix field is
      consistent with the received bindprefix6-len value (calculated as
      described in Section 6.1).

   If either check fails, the receiver discards the invalid option and
   proceeds to attempt configuration as if the option had not been
   received.

   The receiver MUST only use bits from the bind-ipv6-prefix field up to
   the value specified in the bindprefix6-len when performing the
   longest prefix match. bind-ipv6-prefix bits beyond this value MUST be
   ignored.

7.5.  Client and Server Softwire Source Address Mismatch

   If the client receives a DHCPACK message with an
   OPTION_DHCP4O6_S46_SADDR containing an IPv6 address that differs from
   its active softwire source address, the client SHOULD wait for a
   randomized time interval and then resend the DHCPREQUEST message with
   the correct softwire source address.  Section 4.1 of [RFC2131]
   describes the retransmission backoff interval process.

   The default minimum time for the client to attempt retransmission is
   60 seconds.  If, after this time has expired, the client has not
   received a DHCPACK message with the correct bound IPv6 address,
   client MAY send a DHCPRELEASE message and restart the process
   described in Section 7.  The retry interval should be configurable
   and aligned with any server policy defining the minimum time interval
   for client address updates as described in Section 8.1.

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7.6.  Use with Dynamic, Shared IPv4 Addresses

   [RFC7618] describes a mechanism for using DHCPv4 to distribute
   dynamic, shared IPv4 addresses to clients.  The mechanism described
   in this document is compatible with IPv4 address sharing and can be
   enabled by following the process described in Section 6 of [RFC7618].

8.  Server Behavior

   Beyond the normal DHCP 4o6 functionality defined in [RFC7341], the
   server MUST also store the IPv6 softwire source address of the client
   in the leasing address database, alongside the IPv4 address and
   client identifier.

   An OPTION_DHCP4O6_S46_SADDR containing the bound softwire source
   address MUST be sent in every DHCPACK message sent by the server.

   The binding entry between the client's IPv6 softwire source address
   and the leased IPv4 address is valid as long as the IPv4 lease
   remains valid.

8.1.  Changing the Bound IPv6 Source Address

   In the event that the server receives a DHCPREQUEST message for an
   active IPv4 lease containing an OPTION_DHCP4O6_S46_SADDR with an IPv6
   address that differs from the address that is currently stored, the
   server updates the stored softwire source address with the new
   address supplied by the client and sends a DHCPACK message containing
   the updated softwire source address in OPTION_DHCP4O6_S46_SADDR.

   The server MAY implement a policy enforcing a minimum time interval
   between a client updating its softwire source IPv6 address.  If a
   client attempts to update the softwire source IPv6 address before the
   minimum time has expired, the server can either silently drop the
   client's message or send back a DHCPACK message containing the
   existing IPv6 address binding in OPTION_DHCP4O6_S46_SADDR.  If
   implemented, the default minimum client source address update
   interval is 60 seconds.

8.2.  Handling Conflicts between Clients' Bound IPv6 Source Addresses

   In order for traffic to be forwarded correctly, each customer edge's
   (CE's) softwire IPv6 source address must be unique.  To ensure this,
   on receipt of every client DHCPREQUEST message containing
   OPTION_DHCP4O6_S46_SADDR, the DHCP 4o6 server MUST check the received
   IPv6 address against all existing CE source addresses stored for

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   active client IPv4 leases.  If there is a match for any active lease
   other than the lease belonging to the client sending the DHCPREQUEST,
   then the client's IPv6 source address MUST NOT be stored or updated.

   Depending on where the client and server are in the address leasing
   lifecycle, the DHCP 4o6 server then takes the following action:

   o  If the DHCP 4o6 does not have a current, active IPv4 address lease
      for the client, then the DHCP address allocation process has not
      been successful.  The server returns a DHCPNAK message to the
      client.

   o  If the DHCP 4o6 does have a current, active IPv4 address lease,
      then the source address update process (see Section 8.1) has not
      been successful.  The DHCP 4o6 server can either silently drop the
      client's message or return a DHCPACK message containing the
      existing IPv6 address binding in OPTION_DHCP4O6_S46_SADDR.

9.  Security Considerations

   Security considerations that are applicable to [RFC7341] are also
   applicable here.

   A rogue client could attempt to use the mechanism described in
   Section 7.2.1 to redirect IPv4 traffic intended for another client to
   itself.  This would be performed by sending a DHCPREQUEST message for
   another client's active IPv4 lease containing the attacker's softwire
   IPv6 address in OPTION_DHCP4O6_S46_SADDR.

   For such an attack to be effective, the attacker would need to know
   both the client identifier and the active IPv4 address lease
   currently in use by another client.  This could be attempted in three
   ways:

   1.  One customer learning the active IPv4 address lease and client
       identifier of another customer via snooping the DHCP4o6 message
       flow between the client and server.  The mechanism described in
       this document is intended for use in a typical ISP network
       topology with a dedicated Layer 2 access network per client,
       meaning that snooping of another client's traffic is not
       possible.  If the access network is a shared medium, then
       provisioning softwire clients using dynamic DHCP4o6 as described
       here is NOT RECOMMENDED.

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   2.  Learning the active IPv4 address lease and client identifier via
       snooping the DHCP4o6 message flow between the client and server
       in the aggregation or core ISP network.  In this case, the
       attacker requires a level of access to the ISP's infrastructure
       that means they can already intercept or interfere with traffic
       flows to the client.

   3.  An attacker attempting to brute-force guess the IPv4 lease
       address and client identifier tuple.  The risk of this can be
       reduced by using a client identifier format that is not easily
       guessable, e.g., by using a random-based client identifier (see
       Section 3.5 of [RFC7844]).

   An attacker could attempt to redirect existing flows to a client
   unable to process the traffic.  This type of attack can be prevented
   by implementing network ingress filtering [BCP38] in conjunction with
   the BR source address validation processes described in [RFC7596]
   Section 5.2 and [RFC7597] Section 8.1.

   A client may attempt to overload the server by sending multiple
   source address update messages (see Section 7.2.1) in a short time
   frame.  This risk can be reduced by implementing a server policy
   enforcing a minimum time interval between client address changes, as
   described in Section 8.1.

9.1.  Client Privacy Considerations

   [RFC7844] describes anonymity profiles for DHCP clients.  These
   considerations and recommendations are also applicable to clients
   implementing the mechanism described in this document.  As DHCP 4o6
   only uses DHCPv6 as a stateless transport for DHCPv4 messages, the
   "Anonymity Profile for DHCPv4" described in Section 3 is most
   relevant here.

   In addition to the considerations given in [RFC7844], the mechanism
   that the client uses for constructing the interface identifier for
   its IPv6 softwire source address (see Section 7.1) could result in
   the device being trackable across different networks and sessions,
   e.g., if the client's softwire Interface Identifier (IID) is
   immutable.

   This can be mitigated by constructing the softwire source IPv6
   address as per Section 6 of [RFC7597].  Here, the address's IID
   contains only the allocated IPv4 address (and port set identifier if
   [RFC7618] is being used).  This means no additional client
   information is exposed to the DHCP 4o6 server; it also means that the
   IID will change as the leased IPv4 address changes (e.g., between
   sessions when Section 3.5 of [RFC7844] is implemented).

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10.  IANA Considerations

   IANA has assigned the OPTION_S46_BIND_IPV6_PREFIX (137) option code
   from the DHCPv6 "Option Codes" registry maintained at
   <http://www.iana.org/assignments/dhcpv6-parameters> as follows:

       Value:             137
       Description:       OPTION_S46_BIND_IPV6_PREFIX
       Client ORO:        Yes
       Singleton Option:  Yes
       Reference:         RFC 8539

   IANA has assigned the OPTION_DHCP4O6_S46_SADDR (109) option code from
   the "BOOTP Vendor Extensions and DHCP Options" registry maintained at
   <http://www.iana.org/assignments/bootp-dhcp-parameters> as follows:

       Tag:          109
       Name:         OPTION_DHCP4O6_S46_SADDR
       Data Length:  16
       Meaning:      DHCPv4 over DHCPv6 Softwire Source Address Option
       Reference:    RFC 8539

   IANA has updated the entry for DHCPv6 OPTION_S46_BR (90) in the
   "Option Codes" registry maintained at
   <https://www.iana.org/assignments/dhcpv6-parameters> as follows:

   Old Entry:

       Value:             90
       Description:       OPTION_S46_BR
       Client ORO:        No
       Singleton Option:  No
       Reference:         [RFC7598]

   New Entry:

       Value:             90
       Description:       OPTION_S46_BR
       Client ORO:        Yes
       Singleton Option:  No
       Reference:         [RFC7598], [RFC8539]

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11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <https://www.rfc-editor.org/info/rfc2131>.

   [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,
              <https://www.rfc-editor.org/info/rfc7341>.

   [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,
              <https://www.rfc-editor.org/info/rfc7598>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8415]  Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
              Richardson, M., Jiang, S., Lemon, T., and T. Winters,
              "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
              RFC 8415, DOI 10.17487/RFC8415, November 2018,
              <https://www.rfc-editor.org/info/rfc8415>.

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11.2.  Informative References

   [BCP38]    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,
              <https://www.rfc-editor.org/info/bcp38>.

   [RFC7596]  Cui, Y., Sun, Q., Boucadair, M., Tsou, T., Lee, Y., and I.
              Farrer, "Lightweight 4over6: An Extension to the Dual-
              Stack Lite Architecture", RFC 7596, DOI 10.17487/RFC7596,
              July 2015, <https://www.rfc-editor.org/info/rfc7596>.

   [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,
              <https://www.rfc-editor.org/info/rfc7597>.

   [RFC7618]  Cui, Y., Sun, Q., Farrer, I., Lee, Y., Sun, Q., and M.
              Boucadair, "Dynamic Allocation of Shared IPv4 Addresses",
              RFC 7618, DOI 10.17487/RFC7618, August 2015,
              <https://www.rfc-editor.org/info/rfc7618>.

   [RFC7844]  Huitema, C., Mrugalski, T., and S. Krishnan, "Anonymity
              Profiles for DHCP Clients", RFC 7844,
              DOI 10.17487/RFC7844, May 2016,
              <https://www.rfc-editor.org/info/rfc7844>.

Acknowledgements

   The authors would like to thank Ted Lemon, Lishan Li, Tatuya Jinmei,
   Jonas Gorski, and Razvan Becheriu for their contributions and
   comments.

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Authors' Addresses

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

   Email: ian.farrer@telekom.de

   Qi Sun
   Tsinghua University
   Beijing  100084
   China

   Phone: +86-10-6278-5822
   Email: sunqi.ietf@gmail.com

   Yong Cui
   Tsinghua University
   Beijing  100084
   China

   Phone: +86-10-6260-3059
   Email: yong@csnet1.cs.tsinghua.edu.cn

   Linhui Sun
   Tsinghua University
   Beijing  100084
   China

   Phone: +86-10-6278-5822
   Email: lh.sunlinh@gmail.com

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