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Versions: (draft-rajtar-dhc-v4configuration) 00 01 02 03 04 05 06

DHC WG                                                         B. Rajtar
Internet-Draft                                          Hrvatski Telekom
Intended status: Informational                                 I. Farrer
Expires: July 19, 2014                               Deutsche Telekom AG
                                                        January 15, 2014


        Provisioning IPv4 Configuration Over IPv6 Only Networks
                   draft-ietf-dhc-v4configuration-04

Abstract

   As IPv6 becomes more widely adopted, some service providers are
   choosing to deploy IPv6 only networks without dual-stack
   functionality for IPv4.  However, as access to IPv4 based services
   will continue to be a requirement for the foreseeable future, IPv4
   over IPv6 mechanisms, such as softwire tunnels are being developed.

   In order to provision end-user's hosts with the IPv4 configuration
   necessary for such mechanisms, a number of different approaches have
   been proposed.  This memo discusses each of the proposals, identifies
   the benefits and drawbacks and recommends a single approach as the
   basis for future deployment and development.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

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 July 19, 2014.





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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Overview of IPv4 Parameter Configuration Approaches . . .   4
     1.2.  DHCPv4o6 Based Provisioning - Functional Overview . . . .   5
     1.3.  DHCPv6 Based Provisioning - Functional Overview . . . . .   6
     1.4.  DHCPv6 + Stateless DHCPv4oSW Based Provisioning -
           Functional Overview . . . . . . . . . . . . . . . . . . .   6
     1.5.  DHCPv4oDHCPv6 Based Provisioning - Functional Overview  .   7
   2.  Requirements for the Solution Evaluation  . . . . . . . . . .   8
   3.  Comparison of the Five Approaches . . . . . . . . . . . . . .   9
     3.1.  DHCPv4o6 Based Provisioning . . . . . . . . . . . . . . .   9
       3.1.1.  Pros  . . . . . . . . . . . . . . . . . . . . . . . .   9
       3.1.2.  Cons  . . . . . . . . . . . . . . . . . . . . . . . .  10
     3.2.  DHCPv6 Based Provisioning . . . . . . . . . . . . . . . .  10
       3.2.1.  Pros  . . . . . . . . . . . . . . . . . . . . . . . .  10
       3.2.2.  Cons  . . . . . . . . . . . . . . . . . . . . . . . .  10
     3.3.  DHCPv6 + Stateless DHCPv4oSW Based Provisioning . . . . .  11
       3.3.1.  Pros  . . . . . . . . . . . . . . . . . . . . . . . .  11
       3.3.2.  Cons  . . . . . . . . . . . . . . . . . . . . . . . .  11
     3.4.  DHCPv4oSW Based Provisioning  . . . . . . . . . . . . . .  12
       3.4.1.  Pros  . . . . . . . . . . . . . . . . . . . . . . . .  12
       3.4.2.  Cons  . . . . . . . . . . . . . . . . . . . . . . . .  12
     3.5.  DHCPv4oDHCPv6 Based Provisioning  . . . . . . . . . . . .  13
       3.5.1.  Pros  . . . . . . . . . . . . . . . . . . . . . . . .  13
       3.5.2.  Cons  . . . . . . . . . . . . . . . . . . . . . . . .  13
   4.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .  13
   5.  Transporting Unmodified DHCPv4      Messages over an IPv6
       Link Layer  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     5.1.  Combined Hub and DHCPv4 Relay Required Functionality  . .  14
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
     7.1.  DHCPv4oIPv6 . . . . . . . . . . . . . . . . . . . . . . .  15



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     7.2.  DHCPv6  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     7.3.  DHCPv6+DHCPv4oSW  . . . . . . . . . . . . . . . . . . . .  15
     7.4.  DHCPv4oDHCPv6 . . . . . . . . . . . . . . . . . . . . . .  16
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   A service provider with an IPv6-only network must also be able to
   provide customers with access to the IPv4 Internet and other
   IPv4-only services.  IPv4 over IPv6 tunneling / translation
   mechanisms are an obvious example of this, such as the ones described
   in:

   o  [I-D.ietf-softwire-lw4over6]

   o  [I-D.ietf-softwire-map]

   o  [I-D.ietf-softwire-map-t]

   In today's home networks, each residential user is allocated a single
   global IPv4 address which is used for NAT44.  Decentralizing NAT44
   allows for much better scaling and, when combined with stateless
   network functions, can simplify redundancy and logging.  This results
   in the need to provision a number of configuration parameters to the
   CPE, such as the external public IPv4 address and a restricted port-
   range to use for NAT.  Other parameters may also be necessary,
   depending on the underlying transport technology that is in use.  In
   IPv4 only networks, DHCPv4 has often been used to provide IPv4
   configuration, but in an IPv6 only network, DHCPv4 messages cannot be
   transported natively without either IPv6 encapsulation or
   translation.

   DHCPv4 messages can be transported, unmodified, over a broadcast
   capable link-layer, depending on the underlying IPv4 in IPv6
   technology, network topology and DHCPv4 client capabilities.  A
   functional description of how unmodified DHCPv4 can be used is
   provided in Section 5.  This approach is RECOMMENDED for service
   providers whose network and clients can support this DHCPv4
   architecture.

   For the most simple IPv4 provisioning case, where the client only
   needs to receive a static IPv4 address assignment (with no dynamic
   address leasing or additional IPv4 configuration), DHCPv6 based




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   approaches (e.g. [I-D.ietf-softwire-map-dhcp]) may provide a suitable
   solution.

   This document is concerned with more complex IPv4 configuration
   scenarios, to bring IPv4 configuration over IPv6-only networks in
   line with the functionality offered by DHCPv4 in IPv4 native
   networks.  DHCPv4 options may also need to be conveyed to clients for
   configuring IPv4 based services, e.g., SIP server addresses.

   Although IPv4-in-IPv6 softwire tunnel and translation clients are
   currently the only use-case for DHCP based configuration of IPv4
   parameters in IPv6 only networks, a suitable approach must not be
   limited to only supporting softwires configuration or be reliant on
   specific underlying IPv4 over IPv6 architectures or mechanisms.

   This document describes and compares four different methods which
   have been proposed as solutions to this problem.

1.1.  Overview of IPv4 Parameter Configuration Approaches

   The following approaches for transporting IPv4 configuration
   parameters over IPv6 only networks have been suggested:

   1.  Adapt DHCPv4 format messages to be transported over IPv6 as
       described in [I-D.ietf-dhc-dhcpv4-over-ipv6].  For brevity, this
       is referred to as DHCPv4o6.

   2.  Extend DHCPv6 to support IPv4 address leasing and other DHCPv4
       options.

   3.  Use DHCPv6 as above for external IPv4 address and source port
       configuration.  Use DHCPv4 over IPv4 messages within an IPv6
       softwire for configuring additional parameters.  This is referred
       to as DHCPv6 + Stateless DHCPv4oSW.

   4.  Use DHCPv4 format messages, transporting them within a new DHCPv6
       message type as described in [I-D.ietf-dhc-dhcpv4-over-dhcpv6].
       This is referred to as DHCPv4oDHCPv6.

   At the time of writing, working examples of the first two methods
   have been developed and successfully tested in several different
   operators networks.  The remaining two methods are still theoretical.

   The following sections provide describe each of the approaches in
   more detail.






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1.2.  DHCPv4o6 Based Provisioning - Functional Overview

   In order to receive IPv4 configuration parameters, IPv4-only clients
   initiate and exchange DHCPv4 messages with the DHCPv4 server.  To
   adapt this for an IPv6-only network, an existing DHCPv4 client
   implements a 'Host Client Relay' (HCRA) function, which takes DHCPv4
   messages and puts them into UDPv6 and IPv6.

   As the mechanism involves unicast based communications, the IPv6
   address of the server must be provisioned to the client.  A DHCPv6
   option for provisioning client's with this address is described in
   [I-D.mrugalski-softwire-dhcpv4-over-v6-option].

   The IPv6 Transport Server (TSV) provides an IPv6 interface to the
   client.  This interface may be directly on the server and/or via an
   intermediary 'Transport Relay Agent' (TRA) device acting as the
   gateway between the IPv4 and IPv6 domains.

   For the dynamic allocation of IPv4 addresses, the DHCPv4 server
   function needs to be extended to add DHCPv4o6 TSV capabilities, such
   as the storing the IPv6 address of DHCPv4o6 clients and implementing
   the CRA6ADDR option.

   This approach currently uses functional elements for ingress and
   egress of the IPv6-only transport domain - the HCRA on the host and
   the TRA or TSV on the server.  As a result, this approach has
   sometimes been referred to as a tunneling approach.  However, relay
   agent encapsulation is not a tunnel, since it carries only DHCP
   traffic; it would be more accurate to describe it as an encapsulation
   based transport.

   [I-D.ietf-dhc-dhcpv4-over-ipv6] also defines an On-Link Client Relay
   agent (LCRA), which is a Client Relay Agent located on the same link
   as an unmodified DHCPv4 client.  It is worth noting that there is no
   technical reason for using relay encapsulation for DHCPv4o6; this
   approach was taken because the authors of the draft originally
   imagined that it might be used to provide configuration information
   for an unmodified DHCPv4 client.  However, this turns out not to be a
   viable approach: in order for this to work, there would have to be
   IPv4 routing on the local link to which the client is connected.  In
   that case, there's no need for DHCPv4o6.

   Given that this is the case, there is no technical reason why
   DHCPv4o6 can't simply use the IPv6 transport directly, without any
   relay encapsulation.  This would greatly simplify the specification
   and the implementation, and would still address the requirements
   stated in this document.




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   [I-D.ietf-dhc-dhcpv4-over-ipv6] describes this solution in detail.

   The protocol stack is as follows:

   DHCPv4/UDPv6/IPv6

1.3.  DHCPv6 Based Provisioning - Functional Overview

   In this approach, DHCPv6 [RFC3315] would be extended with new DHCPv6
   options for configuring all IPv4 based services and functions (i.e.
   IPv4 address assignment and any necessary DHCPv4 options).  DHCPv4
   options needed by IPv4 clients connected to the IPv6 network are
   updated as new DHCPv6 native options carrying IPv4 configuration
   parameters.  IPv4 address leasing would also need to be managed by
   the DHCPv6 server.

   At the time of writing, it is not known which or how many such
   options would need to be ported from DHCPv4 to DHCPv6.

   The protocol stack is as follows:

   DHCPv6/UDPv6/IPv6

1.4.  DHCPv6 + Stateless DHCPv4oSW Based Provisioning - Functional
      Overview

   In this approach, the configuration of IPv4 address and source ports
   (if required) is carried out using DHCPv6, e.g. using
   [I-D.ietf-softwire-map-dhcp].  Any additional IPv4 configuration
   parameters that are required are then provisioned using DHCPv4
   messages transported within IPv6 in the configured softwire in the
   same manner as any other IPv4 based traffic.  Broadcast based DHCPv4
   DHCPDISCOVER messages (necessary for IPv4 address assignment) can not
   be transported as they are not compatible with the existing, unicast
   based softwire architecture.

   On receipt by the tunnel concentrator (e.g. MAP Border Router or a
   Lightweight 4over6 lwAFTR), the DHCPv4 message is extracted from the
   IPv6 packet and forwarded to the DHCPv4 server in the same way as any
   other IPv4 forwarding plane packet is handled.

   As the client is already configured with its external IPv4 address
   and source ports (using DHCPv6 or a well-known IPv4 address for DS-
   Lite clients), the messages exchanged between the DHCPv4 client and
   server would be strictly DHCPINFORM/DHCPACK messages.  These would be
   used for the configuration of any additional IPv4 parameters.





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   For this approach to function, a mechanism for the DHCPv4 client to
   learn the IPv4 address of the DHCPv4 server is also required.  This
   could be via a well-known IPv4 address for the DHCPv4 server, a
   DHCPv4 relay function within the tunnel concentrator or other
   methods.

   From a transport perspective, the key difference between this method
   and DHCPv4o6 (described above) is the protocol stack.  Here the
   DHCPv4 message is first put into UDPv4 and IPv4 and then into the
   IPv6 softwire, instead of placing the DHCPv4 message directly into
   UDPv6 and IPv6.

   Currently, this approach is only theoretical and does not have a
   corresponding Internet Draft providing more detail.

   The protocol stack used for obtaining an IPv4 address and source
   ports (if required) is as follows:

   DHCPv6/UDPv6/IPv6

   The protocol stack used for obtaining additional IPv4 configuration
   is as follows:

   DHCPv4/UDPv4/IPv4/IPv6

1.5.  DHCPv4oDHCPv6 Based Provisioning - Functional Overview

   [I-D.ietf-dhc-dhcpv4-over-dhcpv6] describes transporting DHCPv4
   messages within two new DHCPv6 messages types: BOOTREQUESTV6 and
   BOOTREPLYV6.  These new messages types must be implemented in both
   the DHCPv4oDHCPv6 client and server.

   In this approach, the configuration of stateless IPv4 addresses and
   source ports (if required) is carried out using DHCPv6 as described
   in section 1.3 above.  Dynamic IPv4 addressing, and/or any additional
   IPv4 configuration, is provided using DHCPv4 messages carried
   (without IPv4/UDPv4 headers) within a new OPTION_BOOTP_MSG DHCPv6
   option.

   OPTION_BOOTP_MSG enables the client and server to send BOOTP/DHCPv4
   messages verbatim across the IPv6 network.  When a DHCPv4oDHCPv6
   server receives a DHCPv6 request containing OPTION_BOOT_MSG within a
   BOOTREQUESTV6 message, it passes it to the DHCPv4 server engine.
   Likewise, the DHCPv4 server place its DHCPv4 response in the payload
   of OPTION_BOOTP_MSG and puts this into a BOOTPRPLYV6 message.

   DHCPv4 messages can be carried within DHCPv6 multicast messages,
   using the All_DHCP_Relay_Agents_and_Servers multicast address.  These



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   can be relayed in exactly the same way as any other DHCPv6
   multicasted messages.

   Optionally, DHCPv6 relays could be updated so that they forward the
   BOOTREQUESTV6 message to a different destination address, allowing
   for the separation of DHCPv4 and DHCPv6 provisioning infrastructure.

   If the DHCPv4oDHCPv6 client is provision with a unicast IPv6
   address(es) for the server(s), then an entirely unicast message flow
   between the client and server is also possible without the need for
   relaying.

   The protocol stack used for obtaining dynamic v4 addressing or
   additional IPv4 configuration is as follows:

   DHCPv4/DHCPv6/UDPv6/IPv6

2.  Requirements for the Solution Evaluation

   The following requirements have been defined to evaluate the
   different approaches:

   1.  Minimize the amount of work necessary to implement the solution
       through re-use of existing standards and implementations as much
       as possible.

   2.  Provide a method of supporting all DHCPv4 options so that they
       can be utilized without the need for further standardization.

   3.  Allow for the dynamic leasing of IPv4 addresses to clients.  This
       allows for more efficient use of limited IPv4 resources.

   4.  Enable the separation of IPv4 and IPv6 host configuration
       infrastructure, i.e. independent DHCPv4 and DHCPv6 server
       functions to restrict provisioning domains to the relevant
       protocol and allow the removal of IPv4 infrastructure in the
       future.

   5.  Avoid leaving legacy IPv4 options in DHCPv6.

   6.  Provide a flexible architecture to give operators the option of
       only deploying the functional elements necessary for their
       specific requirements.

   7.  Not restricted to specific IPv4 over IPv6 transport mechanisms or
       architectures.





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3.  Comparison of the Five Approaches

   The table below provides a comparative evaluation showing how the
   different approaches meet the solution requirements described above.

   +--------+----------+--------+----------------------+---------------+
   |  Req.  | DHCPv4o6 | DHCPv6 |  DHCPv6 + Stateless  | DHCPv4oDHCPv6 |
   |  No.   |          |        |      DHCPv4oSW       |               |
   +--------+----------+--------+----------------------+---------------+
   |   1    |    No    |  Yes   |          No          |      Yes      |
   |   2    |   Yes    |   No   |         Yes          |      Yes      |
   |   3    |   Yes    |   No   |          No          |      Yes      |
   |   4    |   Yes    |   No   |         Yes          |      Yes      |
   |   5    |   Yes    |   No   |         Yes          |      Yes      |
   |   6    |    No    |   No   |         Yes          |      Yes      |
   |   7    |   Yes    |  Yes   |          No          |      Yes      |
   +--------+----------+--------+----------------------+---------------+

                       Table 1: Approach Comparison

   The following sections of the document provide more detail on the
   pros and cons of each of the approaches.

3.1.  DHCPv4o6 Based Provisioning

3.1.1.  Pros

   1.  Implementation makes all existing DHCPv4 options available with
       no further ongoing development work necessary.

   2.  IPv4 and IPv6 based provisioning can be separated from each other
       if required, allowing flexibility in network design.

   3.  Easy to implement through minor adaptation of existing DHCPv4
       client, relay and server code.

   4.  No additional functional elements are necessary except the
       DHCPv4o6 client relay agent and server.  If a TSV is used, then a
       TRA is not required.

   5.  Suitable for dynamic IPv4 address leases where the IPv4 address
       lifetime is not linked to the lifetime of a DHCPv6 lease.

   6.  Implementations already exist, proving that the approach works.







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3.1.2.  Cons

   1.  More new functional elements required within the architecture
       (CRA, DHCPv4o6 server and optionally TRA) than are necessary in
       DHCPv6 based provisioning.

   2.  A new DHCPv6 option is necessary in order to provision the IPv6
       address of the DHCPv4 server to the end device.

   3.  The DHCPv4 client host needs to be updated to implement the IPv6
       encapsulation and decapsulation function (i.e. An HCRA).
       Otherwise a separate On-Link CRA (LCRA) functional element must
       be deployed.

   4.  A DHCPv4 server must be deployed and maintained.

   5.  The DHCPv4 server needs to be updated to implement new DHCPv4o6
       functionality.

3.2.  DHCPv6 Based Provisioning

3.2.1.  Pros

   1.  No additional functional elements are required except the DHCPv6
       client and server.

   2.  A single protocol is used to deliver configuration information
       for IPv4 and IPv6.

   3.  Single provisioning point for all configuration parameters.

   4.  Implementations already exist, proving that the approach works.

3.2.2.  Cons

   1.  Any required DHCPv4 options must be ported to DHCPv6, which will
       require re-development work for each option.

   2.  Means that DHCPv4 'legacy' options (which will be of decreasing
       relevance in the future) will remain in DHCPv6 for the lifetime
       of the protocol.

   3.  Each time that a DHCPv4 option is ported to DHCPv6, all clients
       and servers and possibly relays would need to be updated to
       implement the new option.

   4.  Architecture does not allow for the separation of IPv4 and IPv6
       domains.



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   5.  Does not provide a mechanism for dynamic IPv4 address leasing,
       where the lifetime of IPv4 addresses is not linked to the
       lifetime of a DHCPv6 lease.  A DHCPv4 lease lifetime management
       mechanism would need to be added to DHCPv6 to implement this.

3.3.  DHCPv6 + Stateless DHCPv4oSW Based Provisioning

3.3.1.  Pros

   1.  Once implemented, all existing DHCPv4 options will be available
       with no ongoing development work necessary.

   2.  Uses existing DHCPv4 and DHCPv6 architectures in order to provide
       IPv4 configuration in an IPv6 only environment.

   3.  DHCPv4 and DHCPv6 based provisioning can be separated from each
       other if required, allowing flexibility in network design.

3.3.2.  Cons

   1.  More new functional elements required than are necessary in
       DHCPv6 based provisioning.

   2.  IPv4 over IPv6 softwire approaches that distribute NAT to the CPE
       and allow for IP address sharing (MAP-E & LW4o6) forbid the use
       of reserved TCP/UDP ports (e.g. 0-1024).  Every DHCPv4 client
       sharing the same address needs to have a UDP listener running on
       UDP port 68.  To resolve this would require significant rework to
       either the softwire mechanisms and/or the DHCPv4 client
       implementation.

   3.  From the current specification, DHCPINFORM is not suitable for
       use over a softwire.  Additional work, such as the development of
       'shims' would be necessary.

   4.  The current DHCPINFORM specification has a number of unclear
       points, such as those described in
       [I-D.ietf-dhc-dhcpinform-clarify].  Substantial work would be
       required to resolve this.

   5.  Links the deployment of IPv4 configuration over IPv6 to a
       softwire implementation (e.g. requiring a softwire concentrator
       to act as a DHCPv4 relay).  Whilst softwires are the only
       application for this functionality at the moment, this may not be
       the case in the future, meaning another solution may be required.






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   6.  A new mechanism must be defined in order to provide the DHCPv4
       client with the IPv4 address of the DHCPv4 server so that unicast
       DHCPINFORM messages can be sent.

   7.  As only the DHCPINFORM/DHCPACK DHCPv4 message types are
       supported, dynamic IPv4 address leasing (using DHCPDISCOVER
       messages) cannot be used.

   8.  Restricted to underlying hub-and-spoke IPv4 over IPv6
       architectures.  The 'hub' is necessary for the location of the
       DHCPv4 relay, as all traffic must pass through it.  An underlying
       mesh architecture does not have such a location to deploy the
       relay function.

   9.  The approach is unproven as no existing implementations exist.

3.4.  DHCPv4oSW Based Provisioning

3.4.1.  Pros

   1.  Once implemented, all existing DHCPv4 options will be available
       with no ongoing development work necessary.

   2.  Uses existing DHCPv4 architecture in order to provide IPv4
       configuration in an IPv6 only environment.

   3.  DHCPv4 and DHCPv6 based provisioning can be separated from each
       other if required, allowing flexibility in network design.

3.4.2.  Cons

   1.  Requires the DHCPv4 client, DHCPv4 server and softwire
       concentrator (or other relaying device) to be modified.

   2.  May require the DHCPv4 client and server to be updated to use
       dynamic ports taken from the restricted port set allocated to the
       client instead of the well-known DHCPv4 ports.

   3.  The DHCPv4 client must be modified to identify the properties of
       the interface it is configuring and request parameters
       accordingly (e.g. restricted port-sets cannot be used on Ethernet
       transport interfaces but are allowed for a softwire transport)

   4.  Restricted to underlying hub-and-spoke IPv4 over IPv6
       architectures.  The 'hub' is necessary for the location of the
       DHCPv4 relay, as all traffic, including DHCPDISCOVER messages
       will pass through it.  An underlying mesh architecture does not
       have such a location to deploy the relay.



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3.5.  DHCPv4oDHCPv6 Based Provisioning

3.5.1.  Pros

   1.  Once implemented, all existing DHCPv4 options will be available
       with no ongoing development work necessary.

   2.  Uses existing DHCPv4 and DHCPv6 architectures in order to provide
       IPv4 configuration in an IPv6 only environment.

   3.  DHCPv4 and DHCPv6 based provisioning can be separated from each
       other if required, allowing flexibility in network design.

   4.  Suitable for the provisioning of dynamic IPv4 configuration as
       the existing DHCPv4 leasing mechanism can be used.

3.5.2.  Cons

   1.  More new functional elements within the architecture than are
       necessary in DHCPv6 based provisioning.

   2.  DHCPv6 clients needs to be updated to implement the new DHCPv6
       message types (BOOTPREQUESTv6 and BOOTPREPLYv6).

   3.  The DHCPv6 server needs to be updated to implement new
       DHCPv4oDHCPv6 message types and functionality.

   4.  The approach is currently unproven as no existing implementations
       exist.

4.  Conclusion

   Whilst all of the approaches described here will require some
   development work to realize, it is clear from the above analysis that
   the most sustainable approach capitalizes on existing DHCPv4
   implementations and include them as new DHCPv6 message types.  The
   main rationale for this is that it enables all of DHCPv4's existing
   options to be migrated for use over IPv6 in a single step.

   Porting of all necessary DHCPv4 options to DHCPv6 would require
   ongoing development work, re-implementing existing DHCPv4
   functionality in DHCPv6.  This will result in having legacy DHCPv4
   options in DHCPv6, which will no longer be useful once IPv4 is
   completely abandoned.

   Therefore, the DHCPv6 approach is not suitable for delivering IPv4
   configuration parameters in an efficient, ongoing manner.




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   The dynamic leasing of IPv4 addresses is fundamental to the efficient
   use of remaining IPv4 resources.  This will become increasingly
   important in the future, so a mechanism which supports this is
   necessary.  DHCPv6 + Stateless DHCPv4oSW does not provide this
   function and so is not recommended.

   The DHCPv4o6 approach requires a DHCPv4 server (with DHCPv4o6
   functionality) for all deployment scenarios, even when DHCPv4
   specific functionality (e.g. sending DHCPv4 options) is not required
   by the operator.

   Therefore, this memo recommends DHCPv4oDHCPv6
   [I-D.ietf-dhc-dhcpv4-over-dhcpv6] as the best underlying approach for
   provisioning IPv4 parameters over an IPv6 only network.

5.  Transporting Unmodified DHCPv4 Messages over an IPv6 Link Layer

   DHCPv4 can be transported across a broadcast capable link layer, such
   as a softwire.  Functionally, a DHCPv4 client operates on the link
   layer interface (e.g. the softwire tunnel interface).  As the link
   layer must support broadcasts, DHCPDISCOVER and other broadcast
   DHCPv4 messages can be transported.  The DHCPv4 message flow is then
   the same as described in section 3.1 of [RFC2131].

   For an unmodified DHCPv4 client to function over an IPv6 native
   network, the underlying IPv4 over IPv6 architecture must be based on
   a point-to-point link between the client and a central point (i.e. a
   hub or tunnel concentrator) which all client DHCPv4 broadcast
   messages will pass through.  This hub must function as either the
   DHCPv4 server or a DHCPv4 relay.  The relay forwards broadcast DHCPv4
   DHCPDISCOVER/DHCPREQUEST messages to a separate DHCPv4 server.

5.1.  Combined Hub and DHCPv4 Relay Required Functionality

   When the DHCPv4 relay function is co-located with the IPv4 in IPv6
   hub function, there are some implementation considerations and
   requirements that must be fulfilled.  The following list describes
   these.

   1.  Depending on the underlying IPv4 over IPv6 mechanism that the hub
       is based upon, it may be necessary to modify the encapsulation/
       decapsulation or IPv6/IPv4 translation packet validation policy
       so that IPv4 payload packets sourced from the unspecified address
       (0.0.0.0) are not dropped for broadcast DHCPv4 payload packets.

   2.  The DHCPv4 relay must use the DHCPv4 Relay Information Option
       (option 82) Relay-ID sub-option (2) to convey the client's source




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       IPv6 address.  This is used by the relay to route DHCPv4 response
       packets sent by the DHCPv4 server to the correct client.

   3.  For some IPv4 in IPv6 transition technologies, a client may be
       configured with an IPv4 address which is shared by other clients.
       In these cases, clients using a single IPv4 address are
       differentiated using the combination of the IPv4 address and a
       range of restricted layer 4 source ports unique to each client
       (used for NAPT).  The DHCPv4 client L4 port (68) must not be
       provisioned to any client for NAPT use.

   4.  The DHCPv4 relay must implement the Server Identifier Override
       Suboption described in [RFC5107] to direct all DHCPv4 messages
       through the DHCPv4 relay.  As option 82 is being used to identify
       the destination IPv6 address for messages from the DHCPv4 server
       to the client, the L4 destination port is not required for the
       return path lookup process and is left unchanged as port 68.

6.  IANA Considerations

   This document does not make any request from IANA.

   Note to RFC Editor: this section may be removed on publication as an
   RFC.

7.  Security Considerations

   The following sections provide pointers to the documented security
   considerations associated with each approach.

7.1.  DHCPv4oIPv6

   Security considerations associated with this approach are described
   in Section 8 of [I-D.ietf-dhc-dhcpv4-over-ipv6].

7.2.  DHCPv6

   Security considerations associated with this approach are described
   in Section 23 of [RFC3315].

7.3.  DHCPv6+DHCPv4oSW

   There is currently no document describing this mechanism, so no
   security considerations have been documented.







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7.4.  DHCPv4oDHCPv6

   Security considerations associated with this approach are described
   in Section 10 of [RFC3315].

8.  Acknowledgements

   Thanks to Ted Lemon, Tomek Mrugalski, Ole Troan and Francis Dupont
   for their input and reviews.

9.  References

9.1.  Normative References

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

9.2.  Informative References

   [I-D.ietf-dhc-dhcpinform-clarify]
              Hankins, D., "Dynamic Host Configuration Protocol
              DHCPINFORM Message Clarifications", draft-ietf-dhc-
              dhcpinform-clarify-06 (work in progress), October 2011.

   [I-D.ietf-dhc-dhcpv4-over-dhcpv6]
              Sun, Q., Cui, Y., Siodelski, M., Krishnan, S., and I.
              Farrer, "DHCPv4 over DHCPv6 Transport", draft-ietf-dhc-
              dhcpv4-over-dhcpv6-03 (work in progress), November 2013.

   [I-D.ietf-dhc-dhcpv4-over-ipv6]
              Cui, Y., Wu, P., Wu, J., Lemon, T., and Q. Sun, "DHCPv4
              over IPv6 Transport", draft-ietf-dhc-dhcpv4-over-ipv6-08
              (work in progress), October 2013.

   [I-D.ietf-softwire-lw4over6]
              Cui, Y., Qiong, Q., Boucadair, M., Tsou, T., Lee, Y., and
              I. Farrer, "Lightweight 4over6: An Extension to the DS-
              Lite Architecture", draft-ietf-softwire-lw4over6-03 (work
              in progress), November 2013.

   [I-D.ietf-softwire-map-dhcp]
              Mrugalski, T., Troan, O., Dec, W., Bao, C.,
              leaf.yeh.sdo@gmail.com, l., and X. Deng, "DHCPv6 Options
              for configuration of Softwire Address and Port Mapped
              Clients", draft-ietf-softwire-map-dhcp-06 (work in
              progress), November 2013.





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   [I-D.ietf-softwire-map-t]
              Li, X., Bao, C., Dec, W., Troan, O., Matsushima, S., and
              T. Murakami, "Mapping of Address and Port using
              Translation (MAP-T)", draft-ietf-softwire-map-t-04 (work
              in progress), September 2013.

   [I-D.ietf-softwire-map]
              Troan, O., Dec, W., Li, X., Bao, C., Matsushima, S.,
              Murakami, T., and T. Taylor, "Mapping of Address and Port
              with Encapsulation (MAP)", draft-ietf-softwire-map-09
              (work in progress), December 2013.

   [I-D.mrugalski-softwire-dhcpv4-over-v6-option]
              Mrugalski, T. and P. Wu, "Dynamic Host Configuration
              Protocol for IPv6 (DHCPv6) Option for DHCPv4 over IPv6
              Endpoint", draft-mrugalski-softwire-
              dhcpv4-over-v6-option-01 (work in progress), September
              2012.

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

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC5107]  Johnson, R., Kumarasamy, J., Kinnear, K., and M. Stapp,
              "DHCP Server Identifier Override Suboption", RFC 5107,
              February 2008.

Authors' Addresses

   Branimir Rajtar
   Hrvatski Telekom
   Zagreb
   Croatia

   Email: branimir.rajtar@t.ht.hr


   Ian Farrer
   Deutsche Telekom AG
   Bonn
   Germany

   Email: ian.farrer@telekom.de





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