Network Working Group P. Conrad
Internet-Draft Temple University
Expires: May 2, 2003 P. Lei
Cisco Systems, Inc.
November 1, 2002
Services Provided By Reliable Server Pooling
draft-conrad-rserpool-service-03.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
RSerPool [1] is a framework to provide highly available services
between clients and servers. This is achieved by grouping servers
into pools, each with an identifier and pooling policy. Three
classes of entities are defined: Pool Users (clients), Pool Elements
(servers), and Name Servers.
This memo defines the services provided by this framework to upper
layer protocols and applications for Pool Users and Pool Elements.
It describes the service primitives that the framework provides and
describes example scenarios.
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It also describes the requirements for mapping (or adaption or
"shim") layers for a variety of transport protocols (SCTP, TCP, or
others) such that upper layer protocols and applications may use a
common framework/API to utilize the services provided.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used In This Document . . . . . . . . . . . . . 4
3. Example Application Scenarios . . . . . . . . . . . . . . . 4
3.1 Example Scenario for Failover Without RSerPool . . . . . . . 4
3.2 Example Scenario Using RSerPool Name Services Only . . . . . 5
3.3 Example Scenario Using Full RSerPool Services . . . . . . . 7
4. Service Primitives . . . . . . . . . . . . . . . . . . . . . 8
4.1 Initialization . . . . . . . . . . . . . . . . . . . . . . . 8
4.2 PE Registration Services . . . . . . . . . . . . . . . . . . 8
4.3 Failover Callback Function . . . . . . . . . . . . . . . . . 9
4.4 PE Selection Services . . . . . . . . . . . . . . . . . . . 10
4.5 Upper Layer/Application Level Acknowledgements . . . . . . . 11
4.6 RSerPool Managed Data Channel . . . . . . . . . . . . . . . 11
5. Transport Mappings . . . . . . . . . . . . . . . . . . . . . 12
5.1 Defined Transport Mappings . . . . . . . . . . . . . . . . . 12
5.2 Transport Mappings Requirements . . . . . . . . . . . . . . 13
5.2.1 Mappings: Mandatory Requirements . . . . . . . . . . . . . . 13
5.2.2 Mappings: Optional Requirements . . . . . . . . . . . . . . 13
5.2.3 Mappings: Other Requirements . . . . . . . . . . . . . . . . 14
6. Security Considerations . . . . . . . . . . . . . . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 14
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
References . . . . . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 15
Full Copyright Statement . . . . . . . . . . . . . . . . . . 16
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1. Introduction
The Reliable Server Pooling architecture is defined in [1]. The
architecture provides highly available services by defining three
classes of entities: pool users (clients), pool elements (servers),
and name servers. Pool elements are grouped into server pools and
can be used by pool users via its pool name (or "handle") and can be
selected by following the pool's pool element selection policy.
This memo describes how an upper layer protocol or application for a
pool user or pool element uses this architecture and associated
protocols to achieve these goals described in that document.
Specifically, it describes how the ASAP protocol [5] and transport
protocols (SCTP, TCP, etc.) can be utilized to realize highly
available services between pool users and pool elements.
There are tradeoffs between the amount of application modification
required, the features and restrictions that the underlying transport
is required to support, and the richness of the feature set provided
by RSerPool. In order to provide support for both existing/legacy
(non-RSerPool) and new applications, several service primitives are
defined in which an upper layer protocol can interact with the
RSerPool framework. Depending on the number of services utilized,
the upper layer protocol achieve a range of reliability from simple
pool element selection to a fully automatic failover capability.
Utilizing a limited set of RSerPool services provides the capability
for legacy upper layer software to use a few RSerPool services with
relatively minor modifications, and allows a broad range of
underlying transport protocols to be supported. To achieve a richer
and more complete failover model, however, a majority of the RSerPool
services should be used, which places certain requirements and
restrictions on the transport layers that can be supported.
Note that regardless of the number of service primitives actually
utilized by any given upper layer protocol, this document assumes
that the upper layer protocol/application is operating on a platform
that has a full running, implmentation of ASAP.
The following figure illustrates the protocol stacks when using the
RSerPool framework (Pool Element perspective shown). Note that the
mapping layer MAY be a "NULL" layer, if no control channel is
utilized and/or the data channel is not utilized (e.g. application
specific data).
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+--------------------------------------+
| Application/Upper Layer Protocol |
+--------------------------------------+
| RSerPool API |
|(control channel) | (data channel) |
+------------------+--------+----------+
| ASAP layer | |
+------------------+--------+ |
| mapping/adaption layer |
+------------------+-------------------+
| transport protocol |
+------------------+-------------------+
The purpose of this document is to describe:
1. the precise services provided by RSerPool to the upper layer,
2. the tradeoffs in choosing which services to utilize,
3. how applications must be designed for each of these services,
4. how applications written over various transports (SCTP, TCP, and
others) can be mapped into these services.
2. Conventions Used In This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [2].
3. Example Application Scenarios
To illustrate the differences among an application without RSerPool,
an application using limited RserPool services, and an application
using a full suite of RSerPool services, this section provides an
informal description of how failover may be handled in each of these
cases.
3.1 Example Scenario for Failover Without RSerPool
Consider a typical client/server application that does not use a
reliable server pooling framework of any kind. Typically, the server
is specified by a DNS name. At some point, the application
translates this name to an IP address (via DNS), and subsequently
makes initial contact with the server to begin a session, via SCTP,
TCP, UDP, or some other protocol. If the client loses contact or
fails to make contact with the server (either due to server failure,
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or a failure in the network) the client must either abandon the
session, or try to contact another server.
In this scenario, the client must first determine that a failure took
place. There are several ways that a client application may
determine that a server failed, including the following:
1. The client may have sent a request to the server, and may time
out waiting for a response, or may receive a message such as "no
route to host", "port not available", or "connection refused".
2. The client may have sent a request to the server, or may have
tried to initiate a connection or association and may have
received a connection/association failure error.
3. The client may already have established a connection to server,
but at some point receives an indication from the transport layer
that the connection failed.
Suppose that the client application has a feature by which the user
can enter the hostname of a secondary server to contact in the event
of failure. Once the application determines that a failure took
place on the primary server, the application can then attempt to
resolve the hostname of the secondary server, and contact the
secondary server to establish a session there. This process can be
iterated to a tertiary server, and so forth.
A limitation of this model is that there is no provision (other than
static client configuration, plus the capabilities of DNS) to
determine which server to contact initially (other than DNS) or which
server to contact next in the event of server failure. (See [3] for
a discussion of the limitations of using DNS for this purpose.)
3.2 Example Scenario Using RSerPool Name Services Only
Now consider the same client/server application mentioned in Section
3.1. First we describe what the application programmer must do to
modify the code to use RSerPool name services. We then describe the
benefits that these modifications provide.
For pool user ("client") applications, there are typically only three
modifications are required along with adding ASAP:
1. Instead of specifying the hostnames of primary, secondary,
tertiary servers, etc., the application user specifies a pool
handle (or pool name).
2. Instead of using a DNS based service (e.g. the Unix library
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function gethostbyname()) to translate from a hostname to an IP
address, the application will invoke an RSerPool service
primitive "GetPrimaryServer" that takes as input a pool handle,
and returns the IP address of the primary server. The
application then uses that IP address just as it would have used
the IP address returned by the DNS in the previous scenario.
3. Without the use of additional RSerPool services, failure
detection is application specific just as in the previous
scenario. However, when failure is detected on the primary
server, instead of invoking DNS translation again on the hostname
of a secondary server, the application invokes the service
primitive "GetNextServer", which has a dual meaning. First it
indicates to the RSerPool layer the failure of the server
returned by a previous "GetPrimaryServer" or "GetNextServer"
call. Second, it provides the IP address of the next server that
should be contacted, according to the best information available
to the RSerPool layer at the present time (e.g. set of available
pool elements, pool element policy in effect for the pool, etc.).
For pool element ("server") applications, two additions in are
required along with adding ASAP:
1. The server should invoke the REGISTER service primitive upon
startup to add itself into the server pool using an appropriate
pool handle. This also includes the address(es) protocol or
mapping id, port (if required by the mapping), and pooling
policy(s).
2. The server should invoke the DEREGISTER service primitive to
remove itself from the server pool when shutting down.
When using these RSerPool services, RSerPool provides benefits that
are limited (as compared to utilizing all services, described in
Section 3.3), but nevertheless quite useful as compared to not using
RSerPool at all (as in Section 3.1). First, the client user need
only supply a single string, i.e. the pool handle, rather than a
list of servers. Second, the decision as to which server is to be
used can be determined dynamically by the server selection mechanism
(i.e. a "pool policy" performed by ASAP; see [1]). Finally, when
failures occur, these are reported to the pool via signaling present
in ASAP [5]) and ENRP [4], other clients will eventually know (once
this failure is confirmed by other elements of the RSerPool
architecture) that this server has failed.
Utilizing this subset of services is useful for applications built
over connectionless protocols such as UDP that cannot easily be
adapted to the transport layer requirements required for full
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failover services (see section Section 5) or for an expedient way to
provide some of the benefits of RSerPool to legacy applications
(regardless of the transport protocol used). However, to take full
advantage of the RSerPool framework, utilization of the full suite of
services as described in the next section is recommended.
3.3 Example Scenario Using Full RSerPool Services
Finally, consider the same client/server application as in Section
3.1, but this time, modified to all RSerPool provided services. As
in the Section 3.1, we first describe the modifications needed, then
we describe the benefits provided.
When the full suite of RSerPool services are used, all communication
between the pool user and the pool element is mediated by the
RSerPool framework, including not only session establishment and
teardown, but also the sending and receiving of data. Accordingly,
it is necessary to modify the application to use the service
primitives (i.e. the API) provided by RSerPool, rather than the
transport layer primitives provided by TCP, SCTP, or whatever
transport protocol is being used.
As in the previous case, sessions (rather than connections or
associations) are established, and the destination endpoint is
specified as a pool handle rather than as a list of IP addresses with
a port number. However, failover from one pool element to another is
fully automatic, and can be transparent to the application:
The RSerPool framework control channel provides maintainance
functions to keep pool element lists, policies, etc. current.
Since the application data (e.g. data channel) is managed by the
RSerPool framework, any unsent and unacknowledged transport data
can be automatically re-sent to the newly selected pool element
upon failover. This is enhanced by providing the application an
"upper layer acknowledegment" service.
The application can provide a callback function (described in
Section 4.3) that is invoked in the case of a failover. This
callback function can execute any application specific failover
code, such as generating a special message (or sequence of
messages) that helps the new pool element construct any state
needed to continue an in-process session.
Retrofitting an existing application to this mode of RSerPool
requires more effort on the part of the application programmer than
retrofitting an application to use just the pool selection services;
all use of the transport layer's primitives (e.g. the calls to the
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sockets API) must be modified to use the RSerPool primitives (e.g.
the RSerPool API). This can be mitigated by making the API for
RSerPool as close to existing transport APIs as possible. However,
failure detection and failover is automated in this case.
Furthermore, since the primitives provided by RSerPool are similar to
those of existing transport protocols (and, it is hoped, the APIs
will be also) for developers of new applications, writing to the
RSerPool failover mode primitives is not significantly different in
terms of programmer effort or learning curve than writing the same
applications over existing transport layer primitives.
4. Service Primitives
Upper layer protocols and applications may "choose" to use these
primitive services as needed. By selecting and using the appropriate
set of service primitives, a range of failover scenarios may be
supported. These service primitives are described in the sub-
sections that follow.
4.1 Initialization
[OPEN TBD: what primitive(s) does a PU indicate what mappings can be
used (are supported), whether automatic rollover, message retrieval
are desired, etc. These will likely be in the form of a
initialization call]
4.2 PE Registration Services
Pool Elements ("server") must use the following services to add or
remove themselves from server pools:
REGISTER, to add the pool element into a server pool using {pool
handle, mapping mode, protocol or mapping id, port, policy info}
where mapping mode is defined in Section 5. A response result
code is returned.
DEREGISTER, to remove the pool element from a server pool using
{pool handle, mapping mode, protocol or mapping id, port, policy
info} where mapping mode is defined in Section 5. A response
result code is returned.
TBD: if REGISTER also returns an opaque instance id, the
application can just use that id for DEREGISTER, instead of
passing in the (same) parameters used in REGISTER.
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4.3 Failover Callback Function
The charter of the RSerPool Working Group specifically states that
transaction failover is out of scope for RSerPool, i.e. "if a server
fails during processing of a transaction this transaction may be
lost. Some services may provide a way to handle the failure, but
this is not guaranteed." Accordingly, the RSerPool framework
provides a "hook" for applications to provide their own application-
specific failover mechanism(s).
Specifically, an application can specify a callback function that is
invoked whenever a failover has taken place. This callback function
is invoked immediately after the new transport layer connection/
association is established with a new server, and gives the
application the opportunity to send one or more messages that may
help the server to resume any transaction or session that was in
progress when the first server failed.
As a simple example of how such a callback is useful, consider a file
transfer service built using RSerPool. Let us assume that some FTP
mirroring software is used to maintain mirrored sites, and that the
actual mirroring is out of scope. However, we would like to use
RSerPool to select a server from among the available mirror sites,
and to failover in the middle of a file transfer if a primary server
fails.
For this example, assume that a simple request/response protocol is
used, where one request message results in one or more response
messages. Each request message contains the filename, and the offset
desired within the file, (default zero.) Each response message
contains some portion of the file, along with the offset, length of
the portion in this message, and the length of the entire file.
A single request results is sufficient to result in a sequence of
response messages from the requested offset to the end of the file.
For simplicity, assume that the response messages are delivered by
the underlying transport strictly in order (although this requirement
could be relaxed if a small amount of extra complexity were
introduced.)
In this protocol, all that is needed for failover is for the
application to keep track of the number of bytes that it has read
from the server, and to provide a callback function that reissues the
request to the new server, replacing the offset with this number.
When there is no failover, only one request message is sent and the
minimum number of response messages are returned; in the event of
failover(s), single new request message is sent for each failover
that occurs.
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While this is a simple example, for more complex application
requirements, the failover callback could be used in a variety of
ways:
The client might send security credentials for authentication by
the server, and/or to provide a "key" by which the server could
locate and setup state by accessing some application-specific (and
out-of-scope) state sharing mechanism used by the servers.
The client might keep track of various synchronization points in
the transaction, and use the failover callback to replay message
from a recent synchronization point.
[Open Issue TBD: Are there others to add to this list?]
4.4 PE Selection Services
When automatic failover is enabled, selection of a new pool element
according to the pool policy in place is automatically performed by
the RSerPool framework in case of a detected failure (e.g. provides
automatic failover). No application intervention is required.
Automatic failover may be enabled by setting the appropriate send
flag when used in conjuction with data channel services (described in
Section 4.6) or explicitly during initialization when data channel
services are not used.
FAILOVER_INDICATION, delivered by callback, indicates that a
failover has occurred and that any required application level
state recovery should be performed. The newly selected pool
element handle is provided.
Business Card services: when automatic failover is used, the
exchange of business cards for rendezvous services is
automatically performed by the RSerPool framework (e.g. no
application intervention is required.
When automatic failover is not enabled, failover detection and
selection of an alternate PE must be done by the upper layer/
application. The following primitives are provided:
GET_PRIMARY_SERVER, takes as input a pool handle and returns the
{IP address, transport protocol, transport protocol port} of the
primary server.
GET_NEXT_SERVER has a dual meaning. First, it indicates to the
RSerPool layer the failure of the server returned by a previous
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GET_PRIMARY_SERVER or GET_NEXT_SERVER call. Second, it provides
the {IP address, transport protocol, transport protocol port} of
the next server that should be contacted, according to the best
information available to the RSerPool layer at the present time.
The appropriate pool policy for server selection for the pool
should be used for selecting the next server.
4.5 Upper Layer/Application Level Acknowledgements
The RSerPool framework provides an upper layer/application level ack
service. The upper layer protocol may request that the peer
acknowledge receipt and successful processing of its sent data,
providing an additional degree of confidence over transport level
message retrieval. When used in conjuction with the data channel
services (described in Section 4.6), any unacknowledged data will be
automatically sent to a new pool element in case of failover, if
desired (e.g. automatic failover is enabled). The following service
primitive is used to acknowledge an upper layer acknowledgement
request.
ULP_ACK, responds to a received upper layer acknowledgement
request.
4.6 RSerPool Managed Data Channel
The RSerPool framework provides these services to send and receive
application layer data, which are used in place of the direct call of
transport level system functions (e.g. send/sendto, recv/recvfrom)
and provides additional functionality to those calls.
DATA_SEND, to send data to a pool element by using a pool handle,
specific pool element handle, or by transport address. An upper
layer acknowledgement may be requested with this service.
Appropriate error code(s) are returned. When sending to a pool
handle, the specific pool element handle is returned.
DATA_INDICATION, delivered by callback, to indicate that data has
been received from a pool element and to pass that data to the
application layer protocol. An application layer acknowledgement
request can be indicated along with the data.
The application MAY direct that the RSerPool framework multiplex both
the control and data channels onto the same SCTP association/TCP
connection/ etc., if desired.
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5. Transport Mappings
While SCTP is the preferred transport layer protocol for applications
built for RSerPool failover mode (for reasons explained shortly), it
is also possible to use other transport protocols as well (e.g. TCP)
if an SCTP implementation is not available on the client and/or
server. However, there are certain features present in SCTP that are
required if the RSerPool framework is to function in failover mode.
When a transport protocol other than SCTP is used, these features
must be provided by an "adaption layer" (also called a "shim
protocol") that sits between the base transport protocol (e.g. TCP)
and the RSerPool layer. We refer to these "adaptation layers" or
"shim protocols" as "mappings" as the idea is that the requirements
of the RSerPool framework are "mapped" onto the capabilities of the
underlying protocol (e.g. SCTP or TCP).
5.1 Defined Transport Mappings
In order to support the RSerPool framework over a variety of
transport protocols and configurations, several mappings are defined
to provide RSerPool services over a given transport protocol. Each
mapping translates the requirements of the RSerPool framework onto
the capabilities of the transport protocol desired (e.g. SCTP, TCP,
etc.). Initially, three mappings are defined:
NO_MAPPING (0x00): With this mapping, no RserPool control channel
is provided and the application specific communication between a
pool user and the pool element (e.g. data channel) is out of
scope of RSerPool. However, pool elements can register the
application specific communication "protocol" and "port", and thus
can be provided to pool users.
SCTP (0x01): SCTP transport is used for the RSerPool control
channel. The data channel MAY be multiplexed onto the same SCTP
association, if desired. This mapping is the preferred mapping.
TCP (0x02): TCP transport is used for the RSerPool control
channel. The data channel MAY be multiplexed onto the same TCP
connection, if desired.
A particular pool element might support any combination of these
mappings in order to support a variety of pool users with different
capabilities (i.e. different mapping support). In this case, pool
elements should register each mapping that it supports with its
pool(s).
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5.2 Transport Mappings Requirements
5.2.1 Mappings: Mandatory Requirements
These features MUST be present in any mapping of the RSerPool
framework mode to TCP (or any other transport protocol):
1. Message orientation, which facilitates application re-
synchronization during failover. Messages must be "framed" in
order to allow for undelivered message retrieval from the
transport protocol.
2. A heartbeat mechanism to monitor the health of an association or
connection.
3. A retrieval mechanism to allow an application to retrieve unsent
or unacknowledged data from the transport layer upon failover.
4. A mechanism to transport and differentiate between control
channel messages (e.g. ASAP messages) and data channel messages.
For example in SCTP, the payload protocol identifier (PPID) may
be used.
5. [Open issue TBD: Are there others to be included here?]
5.2.2 Mappings: Optional Requirements
There are several additional features that are present in SCTP that
are lacking in TCP. While these features are not crucial to
RSerPool, providing them in the mapping layer makes it easier for an
application layer programmer to write to a single API. This single
API can then be mapped over both SCTP and TCP, as well as any other
transport protocol for which a mapping is provided. Since these
features are not essential for RSerPool, they are optional in any
defined mapping. However, appropriate error messages or indications
should be provided when these features are not available. These
features include:
1. Support for multiple streams
2. Support for unordered delivery of messages
3. [Open issue TBD: Are there others to be included here?]
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5.2.3 Mappings: Other Requirements
There are some features of SCTP that a mapping may not be able to
provide, because they would require access to transport layer
internals, or modifications in the transport layer itself. The
services provided by the RSerPool layer to the application should
therefore provide mechanisms for the upper layer to access these
features when present (e.g. in SCTP), but also provide appropriate
error messages or indications that these features are not available
when they cannot be provided. These features include:
1. Application access to the RTT and RTO estimates
2. Application access to the Path MTU value
3. [Open issue TBD: Are there others to be included here?]
6. Security Considerations
[Open Issue TBD: Security issues are not discussed in this memo at
this time, but will be added in a later version of this draft.]
7. IANA Considerations
[Open Issue TBD: Will there be an enumeration of the various
transport layer mappings that must be registered with IANA?]
8. Acknowledgements
The authors wish to thank Maureen Stillman, Qiaobing Xie, Michael
Tuexen, Randall Stewart, and many others for their invaluable
comments.
References
[1] Ong, L., Shore, M., Stillman, M., Xie, Q., Loughney, J., Tuexen,
M. and M. Stewart, "Architecture for Reliable Server Pooling",
draft-ietf-rserpool-arch-03 (work in progress), July 2002.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Loughney, J., "Comparison of Protocols for Reliable Server
Pooling", draft-ietf-rserpool-comp-04 (work in progress), July
2002.
[4] Stillman, M., Xie, Q. and R. Stewart, "Enpoint Name Resolution
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Protocol (ENRP)", draft-ietf-rserpool-enrp-04 (work in
progress), September 2002.
[5] Stillman, M., Xie, Q., Tuexen, M. and R. Stewart, "Aggregate
Server Access Protocol (ASAP)", draft-ietf-rserpool-asap-04
(work in progress), July 2002.
Authors' Addresses
Phillip T. Conrad
Temple University
CIS Department
Room 303, Computer Building (038-24)
1805 N. Broad St.
Philadelphia, PA 19122
US
Phone: +1 215 204 7910
EMail: conrad@acm.org
URI: http://www.cis.temple.edu/~conrad
Peter Lei
Cisco Systems, Inc.
955 Happfield Dr.
Arlington Heights, IL 60004
US
Phone: +1 847 870 7201
EMail: peterlei@cisco.com
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Conrad & Lei Expires May 2, 2003 [Page 16]
