Reliable Server Pooling Working L. Coene
group Siemens
Internet-Draft March 3, 2003
Expires: September 1, 2003
Reliable Server pool applicability Statement
<draft-coene-rserpool-applic-01.txt>
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document describes the applicability of the reliable server pool
architecture and protocols to applications which want to have High
avialebility services. This is accomplished by using redundant
servers and failover between servers of the same pool in case of
server failure. Processing load in a pool may de distributed/shared
between the members of the pool according to a certain policy. Also
some guidance is given on the choice of underlying transport protocol
(and corresponding transport protocol mapping) for transporting
application data and Rserpool specific control data.
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1. INTRODUCTION
Reliable server pooling provides protocols for providing higly
available services. The services are located in pool of redundant
servers and if a server fails, another server will take over. The
only requirement put on these servers belonging to the pool is that
if state is maintained by the server, this state must be transfered
to the other server taking over. The mechanism for transfering this
state information is NOT part of the Reliable server pooling
architecture and/or protocols and must be provided by other
protocols.
The goal is to provide server based redundancy. Transport and network
level redundancy are handle by the transport and network layer
protcols.
The application may choose to distribute its traffic over the servers
of the pool conforming to a certain policy.
The application wishing to make use of Rserpool protocols may use
different transport layers(such as UDP, TCP and SCTP). However some
transport layers may have restrictions build in in the way they might
be operating in the Rserpool architecture and its protocols.
1.1 Scope
The scope of this document is to explore the different ways that
Reliable server pool protocols can be used in order to provide a
higly available service towards applications with different
requirements.
1.2 Terminology
The terms are commonly identified in related work and can be found in
the Aggregate Server Access Protocol and Endpoint Name Resolution
Protocol Common Parameters documentRFC COMM [5].
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2. Reliable serverpool
2.1 Architecture
A overview of the reliable server pool architecture is given in the
Rserpool architecture document RFC ARCH [2].
The Rserpool architecture is made up of clients(Pool Users - PU) and
servers(Pool Elements - PE). Both PU and PE's can be grouped into a
pool in which a PE provides a service(File transfer, storage, bank
transaction) to a PU. The PU's may try to find out via the endpoint
resolution protocol(ENRP) which PE's are active. The PU can set up a
communication channel with a particular PE(chosen out of the server
pool) by using the Aggregate Server Access Protocol (ASAP) or by
using directly any of the transport protcols(UDP/TCP/SCTP/RTP). ASAP
may be running on top of UDP, TCP or SCTP.
The minimum mode of using Rserpool is to use only the ENRP for
Endpoint name resolution. The PU may setup the client - server
communication WITHOUT ASAP, but using present transport
protocols(such as UDP, TCP..)
The normal use of Rserpool is to use ENRP for Enpoint name resolution
and ASAP for client - server communication. ASAP may be using as
underlying transport protocol UDP, TCP or SCTP.
2.2 ASAP/ENRP applicability
2.2.1 Minimal rserpool service
The minimum service provided by Rserpool is the use of ENRP for
Endpoint name resolution. The ENRP procol may be running over TCP or
SCTP.
o Endpoint name resolution
o no automatic failover from one PE to another, has to be done by
the application itself
o bussinesscard or cookie mechanism not possible
o May be used by allready existing applications which do not want to
change the interface between PU and PE.
o Only PU-NS and PE-NS communication will use Rserpool protocols
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2.2.2 Full Rserpool service
The fullservice provided by Rserpool is the use of ENRP for Endpoint
name resolution and the Use of ASAP for PU - PE communication . ENRP
may be running over TCP or SCTP while ASAP may be running over TCP,
SCTP, UDP or RTP.
o Endpoint name resolution
o automatic failover from one PE to another is transparent for the
application itself
o bussinesscard exhange for determining if a PU is a pool or not. It
allows the PE to treat the PU's as pool and use Rserpool protocols
for it
o cookie mechanism can be used for state transfer between PE's
o May be used by allready existing applications which do not want to
change the interface between PU and PE.
o All entities wil use Rspool protocols for communication withs
their respective peers
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3. Application and Control data Transport
3.1 Rserpool use between 2 pools
Bussinesscards will allow to detect if their peer is part of a pool
itself. Both the PU and the PE can be part of their own pools. If the
PU or PE would fails, then the businesscard will have informed the
respective peer to contact a alternative fellow PE/PU belonging to
the pool.
3.2 state sharing via the cookie
Every time a response is send back, a cookie could be send along the
response. The cookie is "encrypted" and is stored by the PU, no
modification at all it done to the cookie . If a PE fails then the
cookie is send to a alternate PE, the PE check if the cookie is
valid. The contents of the cookie is only provided and validated by
the PE. It can be used for state sharing between the PE.
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4. Transport protocols used by ENRP & ASAP
4.1 ASAP on top of UDP
UDP is a unreliable message transport delivery protocol, so if a
message gets lost due to a changeover of server(or client), then the
message will not be retransmitted after changeover has occured. New
messages will be sent to alternate server/client within the
serverpool.
This service may be of some importance to services where realtime
constraints apply.(Example video servers: a few lost message ain't
that important as long as the big bulk of messages get through). No
conegstion control is done and as such no real measure of the
congestion status on the server(or client) is taken into account,
thus making loadsharing harder. Only the ENRP server responsible for
that particular server pool will have a up to date view of the load
distribution in the pool.
4.2 ASAP on top of TCP
TCP provides full reliable delivery with congestion control of the
message to its peer node. It provides for a single homed, single
stream delivery of a byte stream from or to the server. Change over
will retrieve the unsent messages and send them on another TCP
connection to a different server of the server pool.
4.3 ASAP on top of SCTP
PR-SCTP is the only know protocol which allows the choice of full,
partial or no reliable delivery with congestion control of the
message to its peer node. If the no-reliable delivery option is
selected of SCTP, then ASAP will function as described in ASAP over
UDP and including congestion control.
if multihoming, streams, unsequenced and/or assured delivery are
required for the application, then SCTP should be used for ASAP. See
SCTP aplicability statement RFC 3257 [9].
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5. Issues for Reliable Server pooling
5.1 State transfer accoss the server pool
Rserpool protocols(ENRP and ASAP) do NOT provide any service for
transfering state information of a application from one Processing
Element(PE) to another.
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6. Security considerations
The protocols used in the Reliable server pool architecture only
tries to increase the availability of the servers in the network.
Rserpool protocols does not contain any protocol mechanisms which are
directly related to user message authentication, integrity and
confidentiality functions. For such features, it depends on the IPSEC
protocols or on Transport Layer Security(TLS) protocols for its own
security and on the architecture and/or security features of its user
protocols.
A overview of possible treats to Reliable Server pooll protcols is
detailed in RFC TREAT [8].
Rserpool architecture allows the use of different Transport protocols
for its application and control data exchange. Those transport
protocols may have mechanisms for reducing the risk of blind
denial-of-service attacks and/or masquerade attacks. If such measures
are required by the applications, then it is advised to check the
SCTP applicability statement[RFC3057] for guidance on this issue.
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7. Acknowledgments
The authors wish to thank X, Y and M. Stillman and many others for
their invaluable comments.
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References
[1] Tuexen, M., Stewart, R., Shore, M., Xie, Q., Ong, L., Loughney,
J. and M. Stillman, "Requirements for Reliable Server Pooling",
RFC 3237, January 2002.
[2] Tuexen, M., Stewart, R., Shore, M., Xie, Q., Ong, L., Loughney,
J. and M. Stillman, "Architecture for Reliable Server Pooling",
Draft in progress , October 2002.
[3] Stewart, R., Xie, Q., Stillman, M. and M. Tuexen, "Aggregate
Server Access Protocol (ASAP)", Draft in progress , October
2002.
[4] Xie, Q., Stewart, R. and M. Stillman, "Endpoint Name Resolution
Protocol (ENRP)", Draft in progress , October 2002.
[5] Stewart, R., Xie, Q., Stillman, M. and M. Tuexen, "Aggregate
Server Access Protocol and Endpoint Name Resolution Protocol
Common Parameters", Draft in progress , October 2002.
[6] Conrad, P. and P. Lei, ""Services Provided By Reliable Server
Pooling", Draft in progress , January 2003.
[7] 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.
[8] Stillman, M., Gopal, R., Sengodan, S., Guttman, E. and M.
Holdrege, ""Threats Introduced by Rserpool and Requirements for
Security in response to Threats"", RFC zzzz, Nov 2002.
[9] Coene, L., ""Stream Control Transmission Protocol Applicability
statement"", RFC 3257, April 2002.
Author's Address
Lode Coene
Siemens
Atealaan 32
Herentals 2200
Belgium
Phone: +32-14-252081
EMail: lode.coene@siemens.com
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