





Network Working Group                                          M. Tuexen
Request for Comments: 3237                                    Siemens AG
Category: Informational                                           Q. Xie
                                                                Motorola
                                                              R. Stewart
                                                                M. Shore
                                                                   Cisco
                                                                  L. Ong
                                                                   Ciena
                                                             J. Loughney
                                                             M. Stillman
                                                                   Nokia
                                                            January 2002


                Requirements for Reliable Server Pooling

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.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This document defines a basic set of requirements for reliable server
   pooling.

   The goal of Reliable Server Pooling (RSerPool) is to develop an
   architecture and protocols for the management and operation of server
   pools supporting highly reliable applications, and for client access
   mechanisms to a server pool.

1.  Introduction

1.1.  Overview

   The Internet is always on.  Many users expect services to be always
   available; many businesses depend upon connectivity 24 hours a day, 7
   days a week, 365 days a year.  In order to fulfill this level of
   performance, many proprietary solutions and operating system
   dependent solutions have been developed to provide highly reliable
   and highly available servers.




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   This document defines requirements for an architecture and protocols
   enabling pooling of servers to support high reliability and
   availability for applications.

   The range of applications that can benefit from reliable server
   pooling includes both mobile and real-time applications.  Reliable
   server pooling mechanisms will be designed to support functionality
   for flexible pooling such as registration and deregistration, and
   load balancing of traffic across the server pool.  Mechanisms will
   need to balance the needs of scalability, overhead traffic and
   response time to changes in pool status, as discussed below.

1.2.  Terminology

   This document uses the following terms:

      Operation scope:
         The part of the network visible to pool users by a specific
         instance of the reliable server pooling protocols.

      Pool (or server pool):
         A collection of servers providing the same application
         functionality.

      Pool handle (or pool name):
         A logical pointer to a pool.  Each server pool will be
         identifiable in the operation scope of the system by a unique
         pool handle or "name".

      Pool element:
         A server entity having registered to a pool.

      Pool user:
         A server pool user.

      Pool element handle (or endpoint handle):
         A logical pointer to a particular pool element in a pool,
         consisting of the name of the pool and one or more destination
         transport addresses for the pool element.

      Name space:
         A cohesive structure of pool names and relations that may be
         queried by an internal or external agent.

      Name server:
         Entity which is responsible for managing and maintaining the
         name space within the RSerPool operation scope.




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      RSerPool:
         The architecture and protocols for reliable server pooling.

1.3.  Abbreviations

      PE:   Pool element

      PU:   Pool user

      SCTP: Stream Control Transmission Protocol

      TCP:  Transmission Control Protocol

2.  Requirements

2.1.  Robustness

   The solution must allow itself to be implemented and deployed in such
   a way that there is no single point of failure in the system.

2.2.  Failover Support

   The RSerPool architecture must be able to detect failure of pool
   elements and name servers supporting the pool, and support failover
   to available alternate resources.

2.3.  Communication Model

   The general architecture should support flexibility of the
   communication model between pool users and pool elements, especially
   allowing for a peer-to-peer relationship to support some
   applications.

2.4.  Processing Power

   It should be possible to use the protocol stack in small devices,
   like handheld wireless devices.  The solution must scale to devices
   with a differing range of processing power.

2.5.  Transport Protocol

   The protocols used for the pool handling should not cause network
   congestion.  This means that it should not generate heavy traffic,
   even in case of failures, and has to use flow control and congestion
   avoidance algorithms which are interoperable with currently deployed
   techniques, especially the flow control of TCP [RFC793] and SCTP
   [RFC2960] and must be compliant with [RFC2914].




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   The architecture should not rely on multicast capabilities of the
   underlying layer.  Nevertheless, it can make use of it if multicast
   capabilities are available.

   Network failures have to be handled and concealed from the
   application layer as much as possible by the transport protocol.
   This means that the underlying transport protocol must provide a
   strong network failure handling capability on top of an acknowledged
   error-free non-duplicated data delivery service.  The failure of a
   network element must be handled by the transport protocol in such a
   way that the timing requirements are still fulfilled.

2.6.  Support of RSerPool Unaware Clients

   The architecture should allow for ease of interaction between pools
   and non-RSerPool-aware clients.  However, it is assumed that only
   RSerPool-aware participants will receive maximum timing and
   notification benefits the architecture offers.

2.7.  Registering and Deregistering

   Another important requirement is that servers should be able to
   register to (become PEs) and deregister from a server pool
   transparently without an interruption in service.  This means that
   after a PE has deregistered, it will continue to serve PUs which
   started their connection before the deregistration of the PE.  New
   connections will be directed towards an alternative PE.

   Servers should be able to register in multiple server pools which may
   belong to different namespaces.

2.8.  Naming

   Server pools are identified by pool handles.  These pool handles are
   only valid inside the operation scope.  Interoperability between
   different namespaces has to be provided by other mechanisms.

2.9.  Name Resolution

   The name resolution should not result in a pool element which is not
   operational.  This might be important for fulfilling the timing
   requirements described below.

2.10.  Server Selection

   The RSerPool mechanisms must be able to support different server
   selection mechanisms.  These are called server pool policies.




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   Examples of server pool policies are:

      -  Round Robin

      -  Least used

      -  Most used

   The set of supported policies must be extensible in the sense that
   new policies can be added as required.  Non-stochastic and stochastic
   policies can be supported.

   There must be a way for the client to provide operational status
   feedback to the name server about the pool elements.

   The name server protocols must be extensible to allow more refined
   server selection mechanisms to be implemented as they are developed
   in the future.

   For some applications it is important that a client repeatedly
   connects to the same server in a pool if it is possible, i.e., if
   that server is still alive.  This feature should be supported through
   the use of pool element handles.

2.11.  Timing Requirements and Scaling

   Handling of name resolution must be fast to support real-time
   applications.  Moreover, the name space should reflect pool
   membership changes to the client application as rapidly as possible,
   i.e., not waiting until the client application next reconnects.

   The architecture should support control of timing parameters based on
   specific needs, e.g., of an application or implementation.

   In order to support more rapid and accurate response, the
   requirements on scalability of the mechanism are limited to server
   pools consisting of a suitably large but not Internet-wide number of
   elements, as necessary to support bounded delay in handling real-time
   name resolution.

   Also, there is no requirement to support hierarchical organization of
   name servers for scalability.  Instead, it is envisioned that the set
   of name servers supporting a particular pool is organized as a flat
   space of equivalent servers.  Accordingly, the impact of relatively
   frequent updates to ensure accurate reflection of the status of pool
   elements is limited to the set of name servers supporting a specific
   pool.




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2.12.  Scalability

   The RSerPool architecture should not require a limitation on the
   number of server pools or on the number of pool users, although the
   size of an individual pool may be limited by timing requirements as
   defined above.

2.13.  Security Requirements

2.13.1.  General

   -  The scaling characteristics of the security architecture should be
      compatible with those given previously.

   -  The security architecture should support hosts having a wide range
      of processing powers.

2.13.2.  Name Space Services

   -  It must not be possible for an attacker to falsely register as a
      pool element with the name server either by masquerading as
      another pool element or by registering in violation of local
      authorization policy.

   -  It must not be possible for an attacker to deregister a server
      which has successfully registered with the name server.

   -  It must not be possible for an attacker to spoof the response to a
      query to the name server

   -  It must be possible to protect the privacy of queries to the name
      server and responses to those queries from the name server.

   -  Communication among name servers must be afforded the same
      protections as communication between clients and name servers.

2.13.3.  Security State

   The security context of an application is a subset of the overall
   context, and context or state sharing is explicitly out-of-scope for
   RSerPool.  Because RSerPool does introduce new security
   vulnerabilities to existing applications application designers
   employing RSerPool should be aware of problems inherent in failing
   over secured connections.  Security services necessarily retain some
   state and this state may have to be moved or re-established.
   Examples of this state include authentication or retained ciphertext





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   for ciphers operating in cipher block chaining (CBC) or cipher
   feedback (CFB) mode.  These problems must be addressed by the
   application or by future work on RSerPool.

3.  Security Considerations

   Security issues are discussed in section 2.13.

4.  Acknowledgements

   The authors would like to thank Bernard Aboba, Matt Holdrege, Eliot
   Lear, Christopher Ross, Werner Vogels and many others for their
   invaluable comments and suggestions.

5.  References

   [RFC793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
             793, September 1981.

   [RFC959]  Postel, J. and J. Reynolds, "File Transfer Protocol (FTP)",
             STD 9, RFC 959, October 1985.

   [RFC2026] Bradner, S., "The Internet Standards Process -- Revision
             3", BCP 9, RFC 2026, October 1996.

   [RFC2608] Guttman, E., Perkins, C., Veizades, J. and M. Day, "Service
             Location Protocol, Version 2", RFC 2608, June 1999.

   [RFC2719] 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.

   [RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, RFC
             2914, September 2000.

   [RFC2960] 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, November 2000.











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6.  Authors' Addresses

   Michael Tuexen
   Siemens AG
   ICN WN CS SE 51
   D-81359 Munich
   Germany

   Phone:   +49 89 722 47210
   EMail: Michael.Tuexen@icn.siemens.de


   Qiaobing Xie
   Motorola, Inc.
   1501 W. Shure Drive, #2309
   Arlington Heights, Il 60004
   USA

   Phone: +1 847 632 3028
   EMail: qxie1@email.mot.com


   Randall Stewart
   Cisco Systems, Inc.
   24 Burning Bush Trail
   Crystal Lake, Il 60012
   USA

   Phone: +1 815 477 2127
   EMail: rrs@cisco.com


   Melinda Shore
   Cisco Systems, Inc.
   809 Hayts Rd
   Ithaca, NY 14850
   USA

   Phone: +1 607 272 7512
   EMail: mshore@cisco.com











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   Lyndon Ong
   Ciena
   10480 Ridgeview Court
   Cupertino, CA 95014
   USA

   Phone: +1 408 366 3358
   EMail: lyong@ciena.com


   John Loughney
   Nokia Research Center
   PO Box 407
   FIN-00045 Nokia Group
   Finland

   Phone: +358 50 483 6242
   EMail: john.loughney@nokia.com


   Maureen Stillman
   Nokia
   127 W. State Street
   Ithaca, NY 14850
   USA

   Phone: +1 607 273 0724 62
   EMail: maureen.stillman@nokia.com























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7.  Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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