Problem Statement: Dual Stack Mobility
RFC 4977

Network Working Group                                        G. Tsirtsis
Request for Comments: 4977                                      Qualcomm
Category: Informational                                       H. Soliman
                                                    Elevate Technologies
                                                             August 2007

                 Problem Statement: Dual Stack Mobility

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Abstract

   This document discusses the issues associated with mobility
   management for dual stack mobile nodes.  Currently, two mobility
   management protocols are defined for IPv4 and IPv6.  Deploying both
   in a dual stack mobile node introduces a number of problems.
   Deployment and operational issues motivate the use of a single
   mobility management protocol.  This document discusses such
   motivations.  The document also discusses requirements for the Mobile
   IPv4 (MIPv4) and Mobile IPv6 (MIPv6) protocol so that they can
   support mobility management for a dual stack node.

Table of Contents

   1.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 2
   2.  Introduction and Motivation . . . . . . . . . . . . . . . . . . 2
   3.  Problem Description . . . . . . . . . . . . . . . . . . . . . . 3
     3.1.  The Impossibility of Maintaining IP Connectivity  . . . . . 4
     3.2.  Implementation Burdens  . . . . . . . . . . . . . . . . . . 4
     3.3.  Operational Burdens . . . . . . . . . . . . . . . . . . . . 4
     3.4.  Mobility Management Inefficiencies  . . . . . . . . . . . . 4
     3.5.  IPv4 to IPv6 Transition Mechanisms  . . . . . . . . . . . . 5
   4.  Conclusions and Recommendations . . . . . . . . . . . . . . . . 5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     6.2.  Informative References  . . . . . . . . . . . . . . . . . . 6

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RFC 4977         Problem Statement: Dual Stack Mobility      August 2007

1.  Terminology

   This document uses the following terms as defined in Stateless IP/
   ICMP Translation (SIIT) [RFC2765]: IPv4-capable node, IPv4-enabled
   node, IPv6-capable node, IPv6-enabled node.

   The following terms are introduced in this document:

   - MIPv4-capable node:

      A node that supports MIPv4 [RFC3344] in its implementation.  This
      allows the mobile node to configure a home address (statically or
      dynamically) and use such address in its Mobile IPv4 signaling.  A
      MIPv4-capable node may also be IPv6-capable or IPv6-enabled and
      must be IPv4-capable.

   - MIPv6-capable node:

      A node that supports MIPv6 [RFC3775] by configuring a home address
      and using such address in its Mobile IPv6 signaling.  A MIPv6-
      enabled node may also be IPv4-capable or IPv4-enabled and must be
      IPv6-capable.

2.  Introduction and Motivation

   A MIPv4-capable node can use Mobile IPv4 [RFC3344] to maintain
   connectivity while moving between IPv4 subnets.  Similarly, a MIPv6-
   capable node can use Mobile IPv6 [RFC3775] to maintain connectivity
   while moving between IPv6 subnets.

   One of the ways of migrating to IPv6 is to deploy nodes that are both
   IPv4 and IPv6 capable.  Such nodes will be able to get both IPv4 and
   IPv6 addresses and thus can communicate with the current IPv4
   Internet as well as any IPv6 nodes and networks as they become
   available.

   A node that is both IPv4 and IPv6 capable can use Mobile IPv4 for its
   IPv4 stack and Mobile IPv6 for its IPv6 stack so that it can move
   between IPv4 and IPv6 subnets.  While this is possible, it does not
   ensure connectivity since that also depends on the IP version support
   of the network accessed.  Supporting Mobile IPv4 and Mobile IPv6 is
   also more inefficient since it requires:

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RFC 4977         Problem Statement: Dual Stack Mobility      August 2007

   -  Mobile nodes to be both MIPv4 and MIPv6 capable.

   -  Mobile nodes to send two sets of signaling messages on every
      handoff.

   -  Network Administrators to run and maintain two sets of mobility
      management systems on the same network, with each of these systems
      requiring its own set of optimizations.

   This document discusses the potential inefficiencies, IP connectivity
   problems, and operational issues that are evident when running both
   mobility management protocols simultaneously.  It also proposes a
   work area to be taken up by the IETF on the subject and discusses
   requirements for appropriate solutions.

3.  Problem Description

   Mobile IP (v4 and v6) uses a signaling protocol (Registration
   requests in MIPv4 [RFC3344] and Binding updates in MIPv6 [RFC3775])
   to set up tunnels between two end points.  At the moment, Mobile IP
   signaling is tightly coupled to the address family (i.e., IPv4 or
   IPv6) used, in the connections it attempts to manipulate.  There are
   no fundamental technical reasons for such coupling.  If Mobile IP
   were viewed as a tunnel-setup protocol, it should be able to set up
   IP in IP tunnels, independently of the IP version used in the outer
   and inner headers.  Other protocols -- for example, SIP [RFC3261] --
   are able to use either an IPv4- or IPv6-based signaling plane to
   manipulate IPv4 and IPv6 connections.

   A node that is both MIPv4 and MIPv6 capable, will require the
   following to roam within the Internet:

   -  The network operator needs to ensure that the home agent supports
      both protocols or that it has two separate Home Agents supporting
      the two protocols, each requiring its own management.

   -  Double the amount of configuration in the mobile node and the home
      agent (e.g., security associations).

   -  IP-layer local network optimizations for handovers will also need
      to be duplicated.

   We argue that all of the above will make the deployment of Mobile
   IPv6, as well as any dual stack solution in a mobile environment,
   harder.  We will discuss some of the issues with the current approach
   separately in the following sections.

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RFC 4977         Problem Statement: Dual Stack Mobility      August 2007

3.1.  The Impossibility of Maintaining IP Connectivity

   Even if a mobile node is both MIPv4 and MIPv6 capable, connectivity
   across different networks would not, in fact, be guaranteed since
   that also depends on the IPv4/IPv6 capabilities of the networks the
   mobile is visiting; i.e., a node attempting to connect via a IPv4-
   only network would not be able to maintain connectivity of its IPv6
   applications and vice versa.  This is potentially the most serious
   problem discussed in this document.

3.2.  Implementation Burdens

   As mentioned above, a node that is IPv4 and IPv6 capable must also be
   MIPv4 and MIPv6 capable to roam within the Internet.  The ability to
   employ both IP versions from one mobility protocol makes it possible
   to implement just that one protocol, assuming the protocol choice is
   known.  However, in situations where the mobile node must be capable
   of working in any network, it may still need two protocols.

3.3.  Operational Burdens

   As mentioned earlier, deploying both protocols will require managing
   both protocols in the mobile node and the home agent.  This adds
   significant operational issues for the network operator.  It would
   certainly require the network operator to have deep knowledge in both
   protocols, which is something an operator may not be able to justify
   due to the lack of substantial gains.

   In addition, deploying both protocols will require duplication of
   security credentials on mobile nodes and home agents.  This includes
   IPsec security associations, keying material, and new authentication
   protocols for Mobile IPv6, in addition to the security credentials
   and associations required by Mobile IPv4.  Depending on the security
   mechanisms used and with some further work, it might be possible to
   rely on one set of common credentials.  Assuming nothing else
   changes, however, such duplication is again significant with no gain
   to the operator or the mobile node.

3.4.  Mobility Management Inefficiencies

   Suppose that a mobile node is moving within a dual stack access
   network.  Every time the mobile node moves, it needs to send two
   mobile IP messages to its home agent to allow its IPv4 and IPv6
   connections to survive.  There is no reason for such duplication.  If
   local mobility optimizations were deployed (e.g., Hierarchical Mobile
   IPv6 (HMIPv6) [RFC4140], Fast handovers for Mobile IPv4 [RFC4068]),
   the mobile node will need to update the local agents running each
   protocol.  Ironically, one local agent might be running both HMIPv6

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RFC 4977         Problem Statement: Dual Stack Mobility      August 2007

   and local MIPv4 home agent.  Clearly, it is not desirable to have to
   send two messages and complete two sets of transactions for the same
   fundamental optimization.

   Hence, such parallel operation of Mobile IPv4 and Mobile IPv6 will
   complicate mobility management within the Internet and increase the
   amount of bandwidth needed at the critical handover time for no
   apparent gain.

3.5.  IPv4 to IPv6 Transition Mechanisms

   The IETF has standardized a number of transition mechanisms to allow
   networks and end nodes to gain IPv6 connectivity while the Internet
   is migrating from IPv4 to IPv6.  However, while some transition
   mechanisms can be combined with Mobile IPv4 or Mobile IPv6, none of
   the known mechanisms have been shown to assist with the issues
   described in this document.

4.  Conclusions and Recommendations

   The points above highlight the tight coupling in both Mobile IPv4 and
   Mobile IPv6 between signaling and the IP addresses used by upper
   layers.  Given that Mobile IPv4 is currently deployed and Mobile IPv6
   is expected to be deployed, there is a need for gradual transition
   from IPv4 mobility management to IPv6.  Running both protocols
   simultaneously is inefficient and has the problems described above.
   The gradual transition can be done when needed or deemed appropriate
   by operators or implementers.  In the meantime, it is important to
   ensure that the problems listed above can be avoided.  Hence, this
   section lists some actions that should be taken by the IETF to
   address the problems listed above, without mandating the use of two
   mobility management protocols simultaneously.

   The Mobile IPv6 Working Group has reached the view that to allow for
   a gradual transition based on current standards and deployment, the
   following work areas would be reasonable:

   -  It should be possible to run one mobility management protocol that
      can manage mobility for both IPv4 and IPv6 addresses used by upper
      layers.  Both Mobile IPv4 and Mobile IPv6 should be able to
      perform such tasks.  It may not be possible to support route
      optimization for Mobile IPv6 in all cases; however, mobility
      management and session continuity can be supported.

   -  It should be possible to create IPv4 extensions to Mobile IPv6 so
      that an IPv4 and IPv6 capable mobile node can register its IPv4
      and IPv6 home addresses to an IPv4- and IPv6-enabled Home Agent
      using MIPv6 signaling only.

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RFC 4977         Problem Statement: Dual Stack Mobility      August 2007

   -  It should be possible to create IPv6 extensions to Mobile IPv4 so
      that an IPv4 and IPv6 capable mobile node can register its IPv4
      and IPv6 home addresses to an IPv4- and IPv6-enabled Home Agent
      using Mobile IPv4 signaling only.

   -  It should also be possible to extend MIPv4 [RFC3344] and MIPv6
      [RFC3775] so that a mobile node can register a single care-of
      address (IPv4 or IPv6) to which IPv4 and/or IPv6 packets can be
      tunneled.

   If the IETF chooses to pursue all these paths, a vendor could choose
   to support one mobility management protocol while avoiding the
   incompatibility and inefficiency problems listed in this document.
   Similarly, operators could decide to continue using one mobility
   management protocol throughout the period of IPv4 and IPv6
   coexistence.  However, a mobile node would be forced to choose one
   approach or the other, or nevertheless to install both and use one or
   the other according to circumstances.

5.  Security Considerations

   This document is a problem statement that does not by itself
   introduce any security issues.

6.  References

6.1.  Normative References

   [RFC2765]  Nordmark, E., "Stateless IP/ICMP Translation Algorithm
              (SIIT)", RFC 2765, February 2000.

   [RFC3344]  Perkins, C., "IP Mobility Support for IPv4", RFC 3344,
              August 2002.

   [RFC3775]  Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
              in IPv6", RFC 3775, June 2004.

6.2.  Informative References

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC4068]  Koodli, R., "Fast Handovers for Mobile IPv6", RFC 4068,
              July 2005.

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RFC 4977         Problem Statement: Dual Stack Mobility      August 2007

   [RFC4140]  Soliman, H., Castelluccia, C., El Malki, K., and L.
              Bellier, "Hierarchical Mobile IPv6 Mobility Management
              (HMIPv6)", RFC 4140, August 2005.

Authors' Addresses

   George Tsirtsis
   Qualcomm

   Phone: +908-443-8174
   EMail: tsirtsis@qualcomm.com

   Hesham Soliman
   Elevate Technologies

   Phone: +614-111-410-445
   EMail: hesham@elevatemobile.com

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RFC 4977         Problem Statement: Dual Stack Mobility      August 2007

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