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Versions: 00 01

Network Working Group                                             Y. Cui
Internet-Draft                                                     P. Wu
Intended status: Standards Track                                   J. Wu
Expires: December 4, 2011                            Tsinghua University
                                                            June 2, 2011


                   DHCPv4 Behavior over IP-IP tunnel
                 draft-cui-softwire-dhcp-over-tunnel-00

Abstract

   This document analyzes the situation where DHCPv4 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're both in-protocol solution 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 packet.

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 December 4, 2011.

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.  Problem analysis . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  In-protocol and out-of-protocol solution . . . . . . . . . . .  6
     3.1.  Address mapping with session id  . . . . . . . . . . . . .  6
     3.2.  Leveraging relay agent option  . . . . . . . . . . . . . .  7
   4.  Acknowledgement  . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     5.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     5.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10

































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1.  Introduction

   The DHCP protocol wasn't designed with tunnel environment
   consideration.  However, due to the development of tunnel-related
   mechanisms, the demand to apply DHCP in tunnel environment arises,
   especially in IPv6 transition scope.  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
   clients, across the heterogenous transit.  If the non-native IP
   addresses of the clients are provided by the concentrator side, this
   address provisioning needs to cross the heterogeneous transit, too.

   One transition mechanism that requires DHCP over tunnel is Public
   4over6[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 draft suggests to achieve this by
   tunneling DHCPv4 between the 4over6 initiator(DHCPv4 client) and
   4over6 concentrator(DHCPv4 server).































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2.  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.  The DHCP
   DISCOVER and DHCP OFFER packets cannot reach the other end since they
   are broadcast packets, so a tunnel between the client and server is
   required to build a virtual link.  Besides, when the middle network
   is IPv6, all DHCPv4 packets can not go through the network since they
   are IPv4 packets.


        +-------------------------+
        |     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 the 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'll
   use the packet's E-IP destination address to look up the I-IP
   encapsulation destination address.  However, this solution doesn't



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   apply to DHCP packets, because the address mapping can only be
   established after the DHCP address allocation, and also because the
   destination address of DHCPOFFER packet are broadcast address.  So we
   need some extra effort to make the encapsulation of DHCP packet work,
   i.e., make the concentrator encapsulate each DHCP packet with the
   I-IP address of the right initiator and hence send them to the right
   initiator.












































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3.  In-protocol and out-of-protocol solution

   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
   discussion.

3.1.  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 differnet clients,
   such as MAC address, transaction-id, etc.  The mapping need 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 do 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

3.2.  Leveraging relay agent option

   Unlike the first solution ,The second solution is an in-protocol
   solution.  We can see that what's actually needed in 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)[RFC4925] 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's necessary.

























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4.  Acknowledgement

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















































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5.  References

5.1.  Normative References

   [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.

5.2.  Informative References

   [I-D.cui-softwire-host-4over6]
              Cui, Y., Wu, J., Wu, P., Metz, C., Vautrin, O., and Y.
              Lee, "Public IPv4 over Access IPv6 Network",
              draft-cui-softwire-host-4over6-04 (work in progress),
              March 2011.


































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Authors' Addresses

   Yong Cui
   Tsinghua University
   Department of Computer Science, Tsinghua University
   Beijing  100084
   P.R.China

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


   Peng Wu
   Tsinghua University
   Department of Computer Science, Tsinghua University
   Beijing  100084
   P.R.China

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


   Jianping Wu
   Tsinghua University
   Department of Computer Science, Tsinghua University
   Beijing  100084
   P.R.China

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





















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