Network Working Group                                       T. Dreibholz
Internet-Draft                                           J. Pulinthanath
Expires: September 25, 2006                 University of Duisburg-Essen
                                                            Mar 24, 2006

    Applicability of Reliable Server Pooling for SCTP-Based Endpoint

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   Copyright (C) The Internet Society (2006).


   This document describes a novel mobility concept based on a
   combination of SCTP with Dynamic Address Reconfiguration extension
   and Reliable Server Pooling (RSerPool).

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1.  Introduction

   An increasing amount of Internet devices is getting mobile.
   Therefore, there is a growing demand for software solutions allowing
   for a seamless handover of communication sessions between multiple
   networks, e.g. to allow for a laptop or PDA to use a fast Ethernet
   connection when available, hand over to a WLAN when moving and hand
   over again to UMTS when the WLAN becomes unreachable - without
   interrupting the running communication sessions.

   Mobility handling is a deficiency of the common IP-based networks.
   Most of the available solutions are based on the network layer.  The
   disadvantage of such solutions is that fundamental changes in the
   network infrastructure are needed.  Therefore, we propose a new
   solution based on the upper layers to overcome these disadvantages.
   In this document, we present our mobility solution based on the SCTP
   protocol with Dynamic Address Reconfiguration extension and Reliable
   Server Pooling (RSerPool).

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2.  Existing Mobility Solutions

2.1.  Mobile IP and Mobile IPv6

   In the concept of Mobile IP [10] every node must register to a Home-
   Agent (HA) in its own home network.  Then, the nodes are reachable
   under their home addresses managed by the HA.  When a node leaves its
   home network, it must also register at a Foreign Agent (FA) in the
   new network.  After that, a tunnel is established between the HA and
   the FA.  Any traffic to the mobile node is then tunneled by its HA to
   the FA and forwarded by the FA to the node itself.  Clearly, the
   detour of all traffic via HA and FA is inefficient and results in an
   increased transmission delay.

   Mobile IPv6 [11] is an extension of Mobile IP.  In Mobile IPv6, the
   FA is not needed.  The packets will be tunneled from the HA to the
   Gateway Router in the foreign network, which forwards the packets to
   the endpoint.  The inefficiency due to the detour of traffic as
   described for Mobile IP remains.

2.2.  SCTP with Dynamic Address Reconfiguration

   Using the SCTP protocol (see [6], [9]) together with its Dynamic
   Address Reconfiguration extension (Add-IP, see [12]), it is possible
   for a mobile endpoint to inform its peer on address changes.  That
   is, when a moving mobile client gets in the vicinity of an additional
   radio station, it sends an "ASCONF Add Address Request" to tell its
   peer that it is now reachable under an additional network-layer
   address.  After that, the peer endpoint can use this additional
   address for a new SCTP path.  When the first radio station becomes
   unreachable, the node can send an "ASCONF Delete Address Request" to
   the peer endpoint.  After that, the peer removes the corresponding
   SCTP path to the unusable network-layer address.

   The following two cases for handovers are possible:

   o  Make-before-Break: An additional SCTP path can be used before the
      original path becomes unusable.  This case is trivial, since there
      is a continuous connectivity.

   o  Break-before-Make: The original SCTP path becomes unusable before
      a new SCTP path can be used.  For the case that only one endpoint
      performs a handover procedure at the same time, the mobile
      endpoint can always use Add-IP to communicate its new address to
      its peer endpoint.  However, when both endpoints perform a
      handover simultaneously, no endpoint is able to tell its
      corresponding peer the new address.

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3.  Solutions for Simultaneous Handovers

3.1.  SCTP with Add-IP and Mobile-IP

   Using SCTP with Add-IP and Mobile IP/Mobile IPv6, the ASCONF messages
   will be sent to the home address of the peer node.  That is, even
   when both nodes are mobile, each endpoint is able to reach its peer
   endpoint using the corresponding home address.  However, this
   solution still requires the full Mobile IP/Mobile IPv6

3.2.  SCTP with Add-IP and RSerPool

   Using RSerPool (see [7], [13], [14], [15], [17], [16]), at least one
   node registers as a Pool Element (PE) at an ENRP server under a Pool
   Handle (PH) known to both endpoints.  Upon handover, it is simply
   necessary for the PE endpoint to re-register, i.e. to update its
   registration with its new address.  The other endpoint can - in the
   role of a Pool User (PU) - ask an ENRP server for its peer node's new
   addresses.  After the new address is known, it is able to create a
   new SCTP path and continue the communication.

   The usage of RSerPool to provide support for mobile endpoints
   provides the following advantages:

   o  Simplicity: No Mobile IP/Mobile IPv6 infrastructure is needed.  In
      particular, it is not necessary that the providers of used
      networks (e.g. public WLAN access points, UMTS providers, etc.)
      provide any support for the mobility solution.

   o  Efficiency: No tunneling of traffic is necessary.

   o  Deployability: All major SCTP implementations already support the
      Dynamic Address Reconfiguration extension.  It is only necessary
      to provide support for RSerPool, e.g. in the form of a userspace
      library, which is much easier to deploy than kernel extensions.

   o  Flexibility: RSerPool provides a complete session layer.  That is,
      providing applications on top of RSerPool makes the support for
      high availability simple.

   A more detailed description of our approach for endpoint mobility, as
   well as a performance analysis using a prototype implementation, can
   be found in our paper [1].

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4.  Normative References

   [1]   Dreibholz, T., Jungmaier, A., and M. Tuexen, "A new Scheme for
         IP-based Internet Mobility", Proceedings of the 28th IEEE Local
         Computer Networks Conference, November 2003.

   [2]   Dreibholz, T., "An efficient approach for state sharing in
         server pools", Proceedings of the 27th IEEE Local Computer
         Networks Conference, October 2002.

   [3]   Dreibholz, T. and E. Rathgeb, "An Application Demonstration of
         the Reliable Server Pooling Framework", Proceedings of the 24th
         IEEE Infocom, March 2005.

   [4]   Dreibholz, T. and E. Rathgeb, "Implementing the Reliable Server
         Pooling Framework", Proceedings of the 8th IEEE International
         Conference on Telecommunications, June 2005.

   [5]   Ong, L., Rytina, I., Garcia, M., Schwarzbauer, H., Coene, L.,
         Lin, H., Juhasz, I., Holdrege, M., and C. Sharp, "Framework
         Architecture for Signaling Transport", RFC 2719, October 1999.

   [6]   Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
         H., Taylor, T., Rytina, I., Kalla, M., Zhang, L., and V.
         Paxson, "Stream Control Transmission Protocol", RFC 2960,
         October 2000.

   [7]   Tuexen, M., Xie, Q., Stewart, R., Shore, M., Ong, L., Loughney,
         J., and M. Stillman, "Requirements for Reliable Server
         Pooling", RFC 3237, January 2002.

   [8]   Coene, L., "Stream Control Transmission Protocol Applicability
         Statement", RFC 3257, April 2002.

   [9]   Stone, J., Stewart, R., and D. Otis, "Stream Control
         Transmission Protocol (SCTP) Checksum Change", RFC 3309,
         September 2002.

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

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

   [12]  Stewart, R., "Stream Control Transmission Protocol (SCTP)
         Dynamic Address  Reconfiguration",
         draft-ietf-tsvwg-addip-sctp-14 (work in progress), March 2006.

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   [13]  Tuexen, M., "Architecture for Reliable Server Pooling",
         draft-ietf-rserpool-arch-10 (work in progress), July 2005.

   [14]  Stewart, R., "Aggregate Server Access Protocol (ASAP)",
         draft-ietf-rserpool-asap-13 (work in progress), February 2006.

   [15]  Stewart, R., "Endpoint Handlespace Redundancy Protocol (ENRP)",
         draft-ietf-rserpool-enrp-13 (work in progress), February 2006.

   [16]  Stewart, R., "Aggregate Server Access Protocol (ASAP) and
         Endpoint Handlespace Redundancy  Protocol (ENRP) Parameters",
         draft-ietf-rserpool-common-param-10 (work in progress),
         February 2006.

   [17]  Tuexen, M. and T. Dreibholz, "Reliable Server Pooling
         Policies", draft-ietf-rserpool-policies-02 (work in progress),
         February 2006.

   [18]  Conrad, P. and P. Lei, "Services Provided By Reliable Server
         Pooling", draft-ietf-rserpool-service-02 (work in progress),
         October 2005.

   [19]  Dreibholz, T., "Thomas Dreibholz's RSerPool Page",

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Authors' Addresses

   Thomas Dreibholz
   University of Duisburg-Essen, Institute for Experimental Mathematics
   Ellernstrasse 29
   45326 Essen, Nordrhein-Westfalen

   Phone: +49-201-1837637
   Fax:   +49-201-1837673

   Jobin Pulinthanath
   University of Duisburg-Essen, Institute for Experimental Mathematics
   Ellernstrasse 29
   45326 Essen, Nordrhein-Westfalen

   Phone: +49-201-1837637
   Fax:   +49-201-1837673

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