Provisioning IPv4 Configuration Over IPv6 Only Networks
draft-rajtar-dhc-v4configuration-01

DHC WG                                                         B. Rajtar
Internet-Draft                                          Hrvatski Telekom
Intended status: Informational                                 I. Farrer
Expires: August 19, 2013                             Deutsche Telekom AG
                                                       February 15, 2013

        Provisioning IPv4 Configuration Over IPv6 Only Networks
                  draft-rajtar-dhc-v4configuration-01

Abstract

   As IPv6 becomes more widely deployed, some service providers are
   taking the approach of deploying IPv6 only networks, without dual-
   stack functionality for IPv4.  However, access to IPv4 based services
   is still an ongoing requirement and approaches such as IPv4-in-IPv6
   softwire tunnels are being developed to meet this need.

   In order to provision end-user's hosts with the necessary IPv4
   configuration, a number of different mechanisms have been proposed.
   This memo discusses the benefits and drawbacks of each and recommend
   a single approach to use for the basis for future work.

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
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   This Internet-Draft will expire on August 19, 2013.

Copyright Notice

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   Copyright (c) 2013 IETF Trust and the persons identified as the
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Approaches for Configuring IPv4 Parameters  . . . . . . . . . . 3
     2.1.  DHCPv4o6 Based Provisioning - Functional Overview . . . . . 4
     2.2.  DHCPv6 Based Provisioning - Functional Overview . . . . . . 4
     2.3.  DHCPv4oSW Based Provisioning - Functional Overview  . . . . 5
   3.  Comparison of the Three Approaches  . . . . . . . . . . . . . . 5
     3.1.  DHCPv4o6 Based Provisioning . . . . . . . . . . . . . . . . 5
       3.1.1.  Pros  . . . . . . . . . . . . . . . . . . . . . . . . . 5
       3.1.2.  Cons  . . . . . . . . . . . . . . . . . . . . . . . . . 6
     3.2.  DHCPv6 Based Provisioning . . . . . . . . . . . . . . . . . 6
       3.2.1.  Pros  . . . . . . . . . . . . . . . . . . . . . . . . . 6
       3.2.2.  Cons  . . . . . . . . . . . . . . . . . . . . . . . . . 6
     3.3.  DHCPv4oSW Based Provisioning  . . . . . . . . . . . . . . . 7
       3.3.1.  Pros  . . . . . . . . . . . . . . . . . . . . . . . . . 7
       3.3.2.  Cons  . . . . . . . . . . . . . . . . . . . . . . . . . 7
   4.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 9

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1.  Introduction

   A service provider with an IPv6-only core network must also be able
   to provide customers with access to the Internet and other services
   over IPv4.  Softwire based IPv4-in-IPv6 tunneling mechanisms are an
   obvious example of this, such as the ones described in:
   [I-D.cui-softwire-b4-translated-ds-lite], [I-D.ietf-softwire-map] and
   [I-D.bfmk-softwire-unified-cpe].

   A general trend here is to distribute NAT functionality and IPv4
   address sharing from the centralized tunnel concentrator to the CPE
   in order to achieve better scalability.  This results in a number of
   configuration parameters needing to be provisioned to the CPE such as
   the external public IPv4 address and a restricted port-range to use
   for NAT.

   In order to configure customer's devices for softwire function, a
   dynamic provisioning mechanism is necessary.  In IPv4 only networks,
   DHCPv4 has often been used to provide configuration, but in an IPv6
   only network, DHCPv4 messages cannot be transported.

   This document compares three different approaches which have been
   proposed for resolving this problem.

2.  Approaches for Configuring IPv4 Parameters

   In order to resolve the problem described above, the following
   approaches for transporting IPv4 configuration parameters 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 with new options for IPv4 configuration, such as
       [I-D.mdt-softwire-map-dhcp-option] describes.

   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.  For brevity,
       this is referred to as DHCPv4oSW.

   At the time of writing, working examples of the first two approaches
   have been developed and successfully tested in several different
   operators networks.  The third approach is still only theoretical.

   Each of these approaches are described in more detail underneath.

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2.1.  DHCPv4o6 Based Provisioning - Functional Overview

   In order to receive IPv4 configuration parameters, IPv4-only clients
   initiate and exchange DHCPv4 messages with the DHCPv4 server.  In
   order adapt this to an IPv6-only network, an existing DHCPv4 client
   implements a 'Client Relay' (CRA) 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 new
   DHCPv6 option has been defined for this purpose.

   The DHCPv4o6 server must either provide an IPv6 interface to the
   client, or an intermediary 'Transport Relay Agent' device can act as
   the gateway between the IPv4 and IPv6 domains.

   The DHCPv4o6 server needs to be extended to support the new
   functionality, such as storing the IPv6 address of DHCPv4o6 clients.

   This approach currently uses functional elements for ingress and
   egress of the IPv6-only transport domain--the CRA 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.

   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.

   This solution is described in detail in
   [I-D.ietf-dhc-dhcpv4-over-ipv6].

2.2.  DHCPv6 Based Provisioning - Functional Overview

   In this approach, DHCPv6 would be extended with new DHCPv6 options
   for configuring IPv4 based functions.

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   An example of this approach is described in
   [I-D.mdt-softwire-map-dhcp-option], where a DHCPv6 message is used to
   convey parameters necessary for IPv4 in IPv6 softwire configuration.

2.3.  DHCPv4oSW Based Provisioning - Functional Overview

   In this approach, the configuration of IPv4 address and source ports
   (if required) is carried out as described in section 2.2 above.
   Additional IPv4 configuration parameters are then provisioned using a
   DHCPv4 messages transported within IPv6 in the softwire in the same
   manner as any other IPv4 based traffic.

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

   As the client is already configured with its external IPv4 address
   and source ports, the messages exchanged between the DHCPv4 client
   and server would be strictly DHCPINFORM/DHCPACK messages, for the
   configuration of additional IPv4 parameters.

   For this approach to function, a mechanism for the DHCPv4 client to
   learn the IPv4 address of the DHCPv4 server is needed.  This could be
   done by defining a well-known IPv4 address for the DHCPv4 server,
   implementing a DHCPv4 relay function within the tunnel concentrator
   or other configuration methods.

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

3.  Comparison of the Three Approaches

   The following section of the document provides the pros and cons of
   the approaches.

3.1.  DHCPv4o6 Based Provisioning

3.1.1.  Pros

   1.  Once implemented, all existing DHCPv4 options will be be
       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.

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   3.  Easy to implement through minor adaptation of existing DHCPv4
       client/server code.

   4.  Simple, in that no additional functional elements are necessary
       except the DHCPv4o6 client and server.  The Transport Relay Agent
       is completely optional.

3.1.2.  Cons

   1.  More complex, in that there are more new functional elements
       within the architecture 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.  DHCPv4 clients needs to be updated to implement the IPv6
       encapsulation and decapsulation function.

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

3.2.  DHCPv6 Based Provisioning

3.2.1.  Pros

   1.  Simpler, in that 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.  A single provisioning point for all configuration parameters.

3.2.2.  Cons

   1.  Any required DHCPv4 options must be ported to DHCPv6, which will
       require a large amount of re-development work.  All functional
       elements in the DHCPv6 implementation (clients, servers, relays)
       would need to be updated for each change.

   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 would need to be updated to implement the new option.

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   4.  Does not provide an architecture for keeping IPv4 and IPv6
       domains separated.

3.3.  DHCPv4oSW Based Provisioning

3.3.1.  Pros

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

   2.  Uses the 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 complex, in that there are more new functional elements
       within the architecture than are necessary in DHCPv6 based
       provisioning.

   2.  IPv4 over IPv6 softwire approaches which 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
       always be the case.

   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.

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4.  Conclusion

      Discussion: This chapter will be updated to reflect the consensus
      of the DHC Working Group.

5.  IANA Considerations

   This document makes no request of IANA.

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

6.  Security Considerations

7.  Acknowledgements

8.  References

8.1.  Normative References

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

8.2.  Informative References

   [I-D.bfmk-softwire-unified-cpe]
              Boucadair, M. and I. Farrer, "Unified IPv4-in-IPv6
              Softwire CPE", draft-bfmk-softwire-unified-cpe-02 (work in
              progress), January 2013.

   [I-D.cui-softwire-b4-translated-ds-lite]
              Cui, Y., Sun, Q., Boucadair, M., Tsou, T., Lee, Y., and I.
              Farrer, "Lightweight 4over6: An Extension to the DS-Lite
              Architecture", draft-cui-softwire-b4-translated-ds-lite-09
              (work in progress), October 2012.

   [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-ipv6]
              Cui, Y., Wu, P., Wu, J., and T. Lemon, "DHCPv4 over IPv6

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              Transport", draft-ietf-dhc-dhcpv4-over-ipv6-05 (work in
              progress), September 2012.

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

   [I-D.mdt-softwire-map-dhcp-option]
              Mrugalski, T., Troan, O., Bao, C., and W. Dec, "DHCPv6
              Options for Mapping of Address and Port",
              draft-mdt-softwire-map-dhcp-option-03 (work in progress),
              July 2012.

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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