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.

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.

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].

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.

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.

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

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, RFC959, October 1985.

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

[RFC2608] Guttman, E., Perkins, C., Veizades, J. and M. Day, "Service
Location Protocol, Version 2", RFC2608, 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", RFC2719,
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",
RFC2960, November 2000.

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

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

7. Full Copyright Statement

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

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