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Network Working Group                                             Y. Cui
Internet-Draft                                                     P. Wu
Intended status: Standards Track                                   J. Wu
Expires: January 12, 2012                            Tsinghua University
                                                           July 11, 2011

                   DHCPv4 Behavior over IP-IP tunnel


   This document analyzes the scenario in which DHCPv4 interaction is
   performed over IP-IP tunnel, and proposes methods to keep DHCP
   working under such situation.  The main issue is encapsulation of
   DHCP packets on server side, and there are both in-protocol and out-
   of-protocol solutions for this issue.  The in-protocol solution is to
   have DHCP carrying the encapsulation address information, and the
   out-of-protocol solution is to have the DHCP server keeping track of
   the address mapping by inspecting DHCP packets.

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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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 12, 2012.

Copyright Notice

   Copyright (c) 2011 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

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Teminology . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Problem Analysis . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  In-protocol and Out-of-protocol Solutions  . . . . . . . . . .  7
     4.1.  Address mapping with session id  . . . . . . . . . . . . .  7
     4.2.  Leveraging Relay Agent Option  . . . . . . . . . . . . . .  8
   5.  Acknowledgement  . . . . . . . . . . . . . . . . . . . . . . .  9
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11

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1.  Introduction

   The DHC protocol[RFC2131] wasn't designed with tunnel environment
   considerations.  However, due to the development of tunnel-based
   mechanisms, the demand to apply DHCP in tunnel environment arises,
   especially in the context of IPv6 transition.  A typical application
   scenario is IP-IP Hub and spoke tunnel[RFC4925].  In this type of
   scenario, IP-IP tunnel is used to provide non-native IP connectivity
   to hosts, across a heterogenous network.  If the non-native IP
   addresses of the clients are provided by the concentrator side, this
   address provisioning needs to cross the heterogeneous network, too.

   One transition mechanism that requires DHCP over tunnel is documented
   in [I-D.cui-softwire-host-4over6].  In this mechanism, users in IPv6
   network get IPv4 access by IPv4-in-IPv6 tunnel with 4over6
   concentrator.  Every user employs a public IPv4 address to get full
   bidirectional IPv4 communication.  This IPv4 address is allocated by
   the ISP over the IPv6 network.  The document suggests to achieve this
   by tunneling DHCPv4 between the 4over6 initiator(DHCPv4 client) and
   4over6 concentrator(DHCPv4 server).

   Two main flavours of solutions may be considered:

   o  Use DHCPv6 to provision IPv4-related connectivity, since IPv4
      address can be embedded into IPv6 address field.  To achieve this
      mode, dedicated options are needed to convey IPv4-related
      information, such as IPv4 address of DNS server, NTP server, etc.

   o  Use DHCPv4 and sustain it in the tunnel environment.  Unlike the
      previous approach where only DHCPv6 is used for both IPv4 and IPv6
      connectivity, this approach consists in maintaining the separation
      between IPv4 and IPv6 connectivity information.  It allows to
      maintain the IPv4 service without requiring major modification of
      IPv6-related provisioning resources, and perserves DHCP as an
      IPv4-related information carrier.  This document focuses on this

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2.  Teminology

   This document makes use of the following terms:

   o  DHCPv4 refers to IPv4 DHCP [RFC2131].

   o  DHCPv4 client (or client) denotes a node that initiates requests
      to obtain configuration parameters from one or more DHCP servers

   o  DHCPv4 server (or server) refers to a node that responds to
      requests from DHCP clients [RFC2131].

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3.  Problem Analysis

   The scenario of DHCPv4 over IP-IP tunnel is shown in Figure 1.
   DHCPv4 client and DHCPv4 server(could be a relay) are separated by an
   IPv6 or IPv4 network, with no DHCP relay in the middle.  DHCP
   DISCOVER and DHCP REQUEST packets cannot reach the other end since
   they are broadcast packets; DHCP OFFER and DHCP ACK/NAK packets
   cannot reach the other end either, when they are broadcast packets or
   unicast packets forwarded by MAC address.  Therefore a tunnel between
   the client and server is required to build a virtual link.  Besides,
   when the middle network is IPv6-only, all DHCPv4 packets can not go
   through the network.

        |     IPv6/IPv4 Network   |
     +------+                     |
     |DHCPv4|                     |
     |client|                 +-------+   +-----------+
     +------+=================|DHCPv4 |   | IPv4      |
        |       IP-IP tunnel  |server |---| domain    |
     +------+=================|       |   |           |
     |DHCPv4|                 +-------+   +-----------+
     |client|                     |
     +------+                     |
        |                         |

   Figure 1 Scenario of DHCPv4 over tunnel

   For the above reasons, we need to build the whole DHCP procedure on
   an IP-IP tunnel.  The client(tunnel initiator) and server(tunnel
   concentrator) will encapsulate the E-IP(External-IP, IPv4) DHCP
   packets into I-IP(Internal-IP, could be IPv4 or IPv6) before sending
   them to remote end; the remote end(server or client) will decapsulate
   the packets to get the original E-IP DHCP packet before handing them
   to the DHCP process.  The encapsulation on the client is natural: the
   client will use the server's I-IP address as encapsulation
   destination address, which is usually known beforehand.  The problem
   is the encapsulation on the server.  The server serves more than one
   clients, and it must send every DHCP packet to the right client, each
   with different I-IP address.

   We can see that regular data packet encapsulation on the concentrator
   faces a similar problem.  The solution is to have the concentrator
   maintaining the mapping between each initiator's E-IP address and
   I-IP address.  When the concentrator performs encapsulation, it will

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   use the packet's E-IP destination address to look up the I-IP
   encapsulation destination address.  However, this solution doesn't
   apply to DHCP packets, because the address mapping can only be
   established after the DHCP address allocation, and also because the
   destination address of DHCP packets can be broadcast address.  So we
   need some extra effort to make the encapsulation of DHCP packets
   work, i.e., make the concentrator encapsulate each DHCP packet with
   the I-IP address of the right initiator and hence send it to the
   right initiator.

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4.  In-protocol and Out-of-protocol Solutions

   So far we've come to two solutions for this problem, one is an in-
   protocol solution and the other is an out-of-protocol solution.  In
   this version of draft, we describe both of them for further

4.1.  Address mapping with session id

   This is an out-of-protocol solution.  The basic idea is that the
   concentrator(server) inspects the incoming DHCP packets, keeps track
   of the mapping between the DHCP session id and the I-IP address of
   the packet.  When sending out a DHCP packet, the concentrator will
   use the session id in the packet to look up corresponding I-IP
   address for encapsulation.  Here the session id could be any field in
   the DHCP packet that can be used to distinguish different clients,
   such as MAC address, transaction-id, etc.  The mapping needs to last
   for only the lifetime of two-time handshake.

   Figure 2 provides an example using MAC as session id.  When receiving
   a DHCPDISCOVER message, the concentrator stores the mapping between
   the MAC address and I-IP address in encapsulation header.  Then the
   concentrator decapsulates the packet and hands the packet to upper
   layer.  When the upper layer passes down the corresponding DHCPOFFER
   packet, the concentrator will look up the I-IP address in the mapping
   table, using the MAC address in the DHCPOFFER packet.  This I-IP
   address will be used as encapsulation destination address.  Then the
   mapping can expire.  Similar procedure happens when the concentrator
   receives a DHCPREQUEST and sends out a DHCPACK.

   This method is transparent to the DHCP process.  There's no protocol
   extension required.  However, the concentrator need to inspect every
   encapsulated packet to filter out DHCP packets.

    DHCP EVENT     initi-          concen-       BEHAVIOR
                   ator            trator
   allocating a new |---DHCPDISCOVER-->|  store I-IP-MAC mapping
   network address  |<-----DHCPOFFER---|  look up I-IP using MAC
                    |                  |  mapping expires
                    |---DHCPREQUEST--->|  store I-IP-MAC mapping
                    |<-----DHCPACK-----|  look up I-IP using MAC
                    |        :         |  mapping expires
                    |        :         |
   address renewal  |---DHCPREQUEST--->|  store IPv6-MAC mapping
                    |<-----DHCPACK-----|  look up I-IP using MAC
                    |        :         |  mapping expires
                    |        :         |

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   Figure 2 4over6 concentrator: DHCP behavior

4.2.  Leveraging Relay Agent Option

   Unlike the first solution, the second solution is an in-protocol
   solution.  We can see that what is actually needed to solve this
   problem is an I-IP encapsulation address for every DHCP packet.  We
   can have the DHCP client to include this information in every DHCP
   packet it sends out.  This document suggests to use the Agent Circuit
   ID Sub-option in DHCP Relay Agent Information Option (Option 82)
   [RFC3046] to carry the I-IP address information.

   Having the client doing this, the operations on the concentrator can
   be significantly simplified.  The receiving and decapsulating
   procedure of the DHCP packet can be identical to regular data packet.
   The DHCP server process will not modify Option 82 in a DHCP packet,
   and this option will be included in the DHCP reply packet.  When the
   upper layer passes down the DHCP reply packet, the concentrator will
   look into the packet and find the encapsulation address in Option 82.
   Then the encapsulation can be done easily.

   This method doesn't need per-packet inspecting when decapsulating
   packet, and doesn't need address mapping maintenance, either.
   However, it's a " misuse" of Option 82 in some level, since there's
   actually no DHCP relay involved.  Another possibility is that we can
   define a new DHCP option for this specific usage if it is necessary.

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5.  Acknowledgement

   The authors would like to thank Alain Durand, Yiu L. Lee, Ted Lemmon
   and Mohamed Boucadair for their valuable comments on this draft.

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6.  References

6.1.  Normative References

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

   [RFC3046]  Patrick, M., "DHCP Relay Agent Information Option",
              RFC 3046, January 2001.

   [RFC4925]  Li, X., Dawkins, S., Ward, D., and A. Durand, "Softwire
              Problem Statement", RFC 4925, July 2007.

6.2.  Informative References

              Boucadair, M., Levis, P., Grimault, J., Savolainen, T.,
              and G. Bajko, "Dynamic Host Configuration Protocol
              (DHCPv6) Options for Shared IP Addresses Solutions",
              draft-boucadair-dhcpv6-shared-address-option-01 (work in
              progress), December 2009.

              Cui, Y., Wu, J., Wu, P., Metz, C., Vautrin, O., and Y.
              Lee, "Public IPv4 over Access IPv6 Network",
              draft-cui-softwire-host-4over6-06 (work in progress),
              July 2011.

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Authors' Addresses

   Yong Cui
   Tsinghua University
   Department of Computer Science, Tsinghua University
   Beijing  100084

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

   Peng Wu
   Tsinghua University
   Department of Computer Science, Tsinghua University
   Beijing  100084

   Phone: +86-10-6278-5822
   Email: weapon@csnet1.cs.tsinghua.edu.cn

   Jianping Wu
   Tsinghua University
   Department of Computer Science, Tsinghua University
   Beijing  100084

   Phone: +86-10-6278-5983
   Email: jianping@cernet.edu.cn

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