Network Working Group A. Barbir
Internet-Draft Nortel Networks
Expires: March 31, 2004 Oct. 2003
OPES entities and end points communication
draft-ietf-opes-end-comm-04
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at http://
www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on March 31, 2004.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This memo documents tracing and non-blocking requirements for Open
Pluggable Edge Services (OPES).
Barbir Expires March 31, 2004 [Page 1]
Internet-Draft OPES entities and end points communication Oct. 2003
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. OPES System . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Requirements for OPES Tracing . . . . . . . . . . . . . . . . 5
3.1 What is traceable in an OPES Flow? . . . . . . . . . . . . . . 5
3.2 Requirements for OPES System . . . . . . . . . . . . . . . . . 6
3.3 Requirements for OPES processors . . . . . . . . . . . . . . . 7
3.4 Requirements for callout servers . . . . . . . . . . . . . . . 7
4. Requirements for OPES System Bypass (Non-blocking feature) . . 8
4.1 What can be bypassed in an OPES Flow? . . . . . . . . . . . . 8
4.2 Bypass requirements for OPES System . . . . . . . . . . . . . 9
4.3 Bypass requirements for OPES processors . . . . . . . . . . . 9
4.4 Bypass requirements for callout servers . . . . . . . . . . . 10
5. Protocol Binding . . . . . . . . . . . . . . . . . . . . . . . 11
6. IANA considerations . . . . . . . . . . . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7.1 Tracing security considerations . . . . . . . . . . . . . . . 13
7.2 Bypass security considerations . . . . . . . . . . . . . . . . 14
Normative References . . . . . . . . . . . . . . . . . . . . . 16
Informative References . . . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 17
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
Intellectual Property and Copyright Statements . . . . . . . . 19
Barbir Expires March 31, 2004 [Page 2]
Internet-Draft OPES entities and end points communication Oct. 2003
1. Introduction
The Open Pluggable Edge Services (OPES) architecture [7] enables
cooperative application services (OPES services) between a data
provider, a data consumer, and zero or more OPES processors. The
application services under consideration analyze and possibly
transform application-level messages exchanged between the data
provider and the data consumer.
This work specifies the requirements for providing tracing
functionality for the OPES architecture [7]. Tracing functionality
enables a data provider or a data consumer application to detect
inappropriate actions that are performed by OPES entities. The work
also develops requirements that can be used to fulfill IAB
Notification and Bypass (Non-Blocking) requirements [2].
The architecture document requires [7] that tracing is supported
in-band. This design goal limits the type of application protocols
that OPES can support. The details of what a trace record can convey
are also dependent on the choice of the application level protocol.
For these reasons, this work documents requirements for application
protocols that need to support OPES traces and non-blocking
mechanism. However, the architecture does not prevent implementers of
developing out-of-band protocols and techniques to address these
limitation.
Barbir Expires March 31, 2004 [Page 3]
Internet-Draft OPES entities and end points communication Oct. 2003
2. OPES System
This sections provides a definition of OPES System. This is needed in
order to define what is traceable in an OPES Flow.
Definition: An OPES System is a set of all OPES entities [7]
authorized by either the data provider or the data consumer
application to process a given application message.
The nature of the authorization agreement determines if authority
delegation is transitive (meaning an authorized entity is authorized
to include other entities).
If specific authority agreements allow for re-delegation, an OPES
system can be formed by induction. In this case, an OPES system
starts with entities directly authorized by a data provider (or a
data consumer) application. The OPES system then includes any OPES
entity authorized by an entity that is already in the OPES system.
The authority delegation is always viewed in the context of a given
application message.
An OPES System is defined on an application message basis. Having an
authority to process a message does not imply being involved in
message processing. Thus, some OPES system members may not
participate in processing of a message. Similarly, some members may
process the same message several times.
The above definition implies that there can be no more than two OPES
systems [Client-side and server-side OPES systems can process the
same message at the same time] processing the same message at a given
time. This is based on the assumption that there is a single data
provider and a single data consumer as far as a given application
message is concerned.
For example, consider a Content Delivery Network (CDN) delivering an
image on behalf of a busy web site. OPES processors and services that
the CDN uses to adapt and deliver the message comprise an OPES
System. In a more complex example, an OPES System would contain CDN
entries as well as 3rd-party OPES entities that CDN engages to
perform adaptations (e.g., to adjust image quality).
Barbir Expires March 31, 2004 [Page 4]
Internet-Draft OPES entities and end points communication Oct. 2003
3. Requirements for OPES Tracing
In an OPES System tracing is defined as the inclusion of necessary
information within a message in an OPES Flow that identify the
collection of transformations or adaptations that have been performed
on it before its delivery to an end point (for example, the data
consumer application). An OPES trace represents a snap shot of the
tracing information that have been added to a given application
message. A trace represents the collections of transformations on an
application message in the order that were performed. A traceable
entity can appear multiple times in a trace (every time it acts on a
message).
In an OPES System tracing is performed on per message basis. Trace
format is dependent on the application protocol that is being adapted
by OPES. A data consumer application can use OPES trace to infer the
actions that have been performed by the OPES System. Actions are the
set of OPES services that were performed by OPES entities in an OPES
System.
In an OPES System, the task of providing tracing information, can
depend on many factors. Some considerations are:
o Providers may be hesitant to reveal information about their
internal network infrastructure.
o Within a service provider network, OPES processors may be
configured to use non-routable, private IP addresses.
o A data consumer applications would prefer to have a single point
of contact regarding the trace information.
In an OPES System some OPES services are message-agnostic and operate
on message content or parts of a message. Such services do not
manipulate message headers. Other services can manipulate message
headers. OPES providers require some freedom in the way they deliver
tracing information to an end point.
3.1 What is traceable in an OPES Flow?
This section focuses on identifying traceable entities in an OPES
Flow.
Tracing information provides a data consumer application (or a data
provider application) with information about OPES entities that
adapted the data. There are two distinct uses of OPES traces. First,
a trace enables an "end (content provider or consumer) to detect the
presence of OPES processors within an OPES System. Such "end" should
Barbir Expires March 31, 2004 [Page 5]
Internet-Draft OPES entities and end points communication Oct. 2003
be able to see a trace entry, but does not need to be able to
interpret it beyond identification of the OPES System.
Second, the OPES System administrator is expected to be able to
interpret the contents of an OPES trace. The trace can be relayed to
the administrator by an end (data consumer or provider) without
interpretation, as opaque data (e.g., a TCP packet or an HTTP message
snapshot). The administrator can use the trace information to
identify the participating OPES entities. The administrator can use
the trace to identify the applied adaptation services along with
other message-specific information.
Since the administrators of various OPES Systems can have various
ways of looking into tracing, they require the choice of freedom in
what to put in trace records and how to format them. Trace records
should be easy to extend beyond basic OPES requirements. Trace
management algorithms should treat trace records as opaque data to
the extent possible.
At the implementation level, for a given trace, an OPES entity
involved in handling the corresponding application message is
traceable or traced if information about it appears in that trace.
OPES entities have different levels of traceability requirements.
Specifically,
o An OPES processor SHOULD add its entry to the trace.
o An OPES service May add its entry to the trace.
o An OPES entity MAY delegate addition of its trace entry to another
OPES entity. For example, an OPES System can have a dedicated OPES
processor for adding System entries; an OPES processor can use a
callout service to manage all OPES trace manipulations (since such
manipulations are OPES adaptations).
In an OPES context, a good tracing approach is similar to a trouble
ticket ready for submission to a known address. The address is
printed on the ticket. The trace in itself is not necessarily a
detailed description of what has happened. It is the responsibility
of the operator to resolve the problems.
3.2 Requirements for OPES System
The following requirements apply for information as related to an
OPES System:
o An OPES system MUST add its identification to the trace.
Barbir Expires March 31, 2004 [Page 6]
Internet-Draft OPES entities and end points communication Oct. 2003
o An OPES System MUST include information about its privacy policy.
o An OPES System MUST identify the party responsible for setting and
enforcing that policy.
o An OPES System MUST include information pointing to a technical
contact.
o An OPES System MUST include information that identifies, to the
technical contact, the OPES processors involved in processing the
message.
In addressing the above requirements and in order to reduce the size
of the trace, an OPES System can provide a URL to the OPES System web
page that has links to privacy and other policies.
3.3 Requirements for OPES processors
Tracing requirements for OPES processors are:
o Each OPES processor MUST support tracing, policy can be used to
turn tracing on and to determine its granularity.
o If tracing is turned on, OPES processor SHOULD add its unique
identification to the trace. Here, uniqueness scope is the OPES
System containing the processor.
o OPES processor SHOULD be able to trace it's own invocation and
service(s) execution since it understands the application
protocol.
3.4 Requirements for callout servers
In an OPES system, it is the task of an OPES processor to add trace
records to application messages. However, in some cases, callout
servers May add trace information to application messages. This
should be done under the control of the OPES System provider.
Barbir Expires March 31, 2004 [Page 7]
Internet-Draft OPES entities and end points communication Oct. 2003
4. Requirements for OPES System Bypass (Non-blocking feature)
IAB recommendation (3.3) [2] requires that the OPES architecture does
not prevent a data consumer application from retrieving non-OPES
version of content from a data provider application, provided that
the non-OPES content exists. IAB recommendation (3.3) suggests that
the Non-blocking feature (Bypass) be used to bypass faulty OPES
intermediaries (once they have been identified, by some method).
In addressing IAB consideration (3.3), one need to specify what
constitute non-OPES content. In some cases, the definition of
"non-OPES" content is provider-dependent and may include content
adapted by OPES. Examples include content that is adapted for Black
and White hand held devices or logging services. In other cases, the
availability of certain content can be a function of the internal
policy of a given organization that has contracted the services of an
OPES provider. For example, Company A has as contract with an OPES
provider to perform virus checking on all e-mail attachments. An
employee X of Company A can issue a non-blocking request for the
virus scanning service. However, the request could be ignored by the
OPES provider since it contradicts its agreement with Company A.
The above examples illustrates that the availability of non-OPES
content can be a function of content providers (or consumers or both)
policy and deployment scenarios [1]. For this reason, this work does
not attempt to define what is an OPES content as opposed to non-OPES
content. The meaning of OPES versus non-OPES content is assumed to be
determined through various agreements between the OPES provider, data
provider and data consumer application. The agreement will also
determine what OPES services can be bypassed and in what order (if
applicable).
In an OPES System a Bypass request is defined as the act of avoiding
the invocation of a service(s) that is identified by a URI within a
message in an OPES Flow before its delivery to an end point (for
example, the data consumer application).
4.1 What can be bypassed in an OPES Flow?
In this work, the focus is on developing a bypass feature that allow
a user to instruct the OPES System to bypass some or all of its
services. The collection of OPES services that can be bypassed is a
function of the agreement of the OPES provider with either (or both)
the content provider or the content consumer applications. In the
general case, a Bypass request is viewed as a bypass instruction that
contains a URI that identifies an OPES entity or a group of OPES
entities that perform a service (or services) to be bypassed. An
instruction may contain more than one such URI. A special wildcard
Barbir Expires March 31, 2004 [Page 8]
Internet-Draft OPES entities and end points communication Oct. 2003
identifier can be used to represent all possible URIs (i.e., all
possible OPES services).
4.2 Bypass requirements for OPES System
In an OPES System the Bypass feature is an end-to-end operation as
opposed to a hop-by-hop operation. Bypass requests are generally
client centric and go in the opposite direction of tracing requests.
Bypass can be performed out of band or in-band. This work requires
that the Bypass feature be performed in-band as an extension to an
application specific protocol. Non-OPES entities should be able to
safely ignore these extensions. The work does not prevent OPES
Systems from developing their own out of band protocols.
The following requirements apply for Bypass feature as related to an
OPES System:
o An OPES system MUST support a Bypass feature. This means that the
OPES System bypasses an entity whose URI is identified by an OPES
end (usually data consumer application).
o An OPES System MUST treat a Bypass request as an end-to-end
operation. This applies to the whole system.
o An OPES System MUST include information about its bypass policy.
o An OPES System MUST identify the party responsible for setting and
enforcing the bypass policy.
o An OPES System MUST include information that identifies, to a
technical contact, the OPES processors involved in processing the
bypass request.
In addressing the above requirements an OPES System can provide a URL
to the OPES System web page that has links to Bypass and other
policies.
In order to facilitate the debugging (or data consumer user
experience) of the bypass feature in an OPES System, it would be
beneficial if non-bypassed entities include information related to
why they ignored the bypass instruction. It is important to note that
in some cases the tracing facility itself may be broken and the whole
OPES System (or part) may need to be bypassed through the issue of a
bypass instruction.
4.3 Bypass requirements for OPES processors
For a given application protocol, in an OPES System there can be
Barbir Expires March 31, 2004 [Page 9]
Internet-Draft OPES entities and end points communication Oct. 2003
services that operate on application message headers and those that
just operate on content. This mix of service requires that an OPES
processor that is calling the service(s) to handle the bypass
request. In some cases, the first OPES processor that will get the
bypass request may not be the first OPES processor that will know
whether a non-OPES version of the content is available or not.
Bypass requirements for OPES processors are:
o There MUST be at least one OPES processor in an OPES System that
knows how to interpret and process a bypass request. This
requirement applies to all bypass instructions, including those
that identify known-to-recipient entities.
o OPES processors that do not know how to process a bypass request
MUST forward the request to the next application hop provided that
the next hop speaks application protocol with OPES bypass support.
o The recipient of a bypass instruction with a URI that does not
identify any known-to-recipient OPES entity MUST treat that URI as
a wildcard identifier (meaning bypass all applicable services).
o OPES processor SHOULD be able to bypass it's own invocation and
service(s) execution since it understands the application
protocol.
4.4 Bypass requirements for callout servers
In an OPES system, it is the task of an OPES processor to process
bypass requests. However, in some cases, callout servers May be
involved in processing Bypass requests. This should be done under the
control of the OPES System provider.
Barbir Expires March 31, 2004 [Page 10]
Internet-Draft OPES entities and end points communication Oct. 2003
5. Protocol Binding
The task of encoding tracing and bypass features is application
protocol specific. Separate documents will address HTTP and other
protocols.
Barbir Expires March 31, 2004 [Page 11]
Internet-Draft OPES entities and end points communication Oct. 2003
6. IANA considerations
This specification contains no IANA considerations. Application
bindings MAY contain application-specific IANA considerations.
Barbir Expires March 31, 2004 [Page 12]
Internet-Draft OPES entities and end points communication Oct. 2003
7. Security Considerations
The security considerations for OPES are documented in [6]. This
document is a requirement document for tracing and Bypass feature.
The requirements that are stated in this document can be used to
extend an application level protocol to support these features. As
such, the work has security precautions.
7.1 Tracing security considerations
The tracing facility for OPES architecture is implemented as a
protocol extension. Inadequate implementations of the tracing
facility may defeat safeguards built into the OPES architecture. The
tracing facility by itself can become a target of malicious attacks
or used to lunch attacks on an OPES System.
Threats caused by or against the tracing facility can be viewed as
threats at the application level in an OPES Flow. In this case, the
threats can affect the data consumer and the data provider
application.
Since tracing information is a protocol extension, these traces can
be injected in the data flow by non-OPES entities. In this case,
there are risks that non-OPES entities can be compromised in a
fashion that threat the overall integrity and effectiveness of an
OPES System. For example, a non-OPES proxy can add fake tracing
information into a trace. This can be done in the form of wrong, or
unwanted, or non existent services. A non-OPES entity can inject
large size traces that may cause buffer overflow in a data consumer
application. The same threats can arise from compromised OPES
entities. An attacker can control an OPES entity and inject wrong, or
very large trace information that can overwhelm an end or the next
OPES entity in an OPES flow. Similar threats can result from bad
implementations of the tracing facility in trusted OPES entities.
Compromised tracing information can be used to launch attacks on an
OPES System that give the impression that unwanted content
transformation was performed on the data. This can be achieved by
inserting wrong entity (such OPES processor) identifiers. A
compromised trace can affect the overall message integrity structure.
This can affect entities that use message header information to
perform services such as accounting, load balancing, or
reference-based services.
Compromised trace information can be used to launch DoS attacks that
can overwhelm a data consumer application or an OPES entity in an
OPES Flow. Inserting wrong tracing information can complicates the
debugging tasks performed by system administrator during trouble
Barbir Expires March 31, 2004 [Page 13]
Internet-Draft OPES entities and end points communication Oct. 2003
shooting of OPES System behavior.
Specific protocol binding documents ought to take these security
threats into consideration. It is recommended that protocol bindings
provide safe features into their specifications. Such features may
include a place holder in the message header that indicates the size
of the trace. Other holders can include the option of signing the
trace information as a proof of authenticity.
As a precaution, OPES entities ought to be capable of verifying that
the inserted traces are performed by legal OPES entities. This can be
done as part of the authorization and authentication face. Policy can
be used to indicate what trace information can be expected from a
peer entity. Other application level related security concerns can be
found in [6].
7.2 Bypass security considerations
The bypass facility for OPES architecture is implemented as a
protocol extension. Inadequate implementations of the bypass facility
may defeat safeguards built into the OPES architecture. The bypass
facility by itself can become a target of malicious attacks or used
to lunch attacks on an OPES System.
Threats caused by or against the bypass facility can be viewed as
threats at the application level in an OPES Flow. In this case, the
threats can affect the data consumer and the data provider
application.
There are risks for the OPES System by non-OPES entities, whereby,
these entities can insert bypass instructions into the OPES Flow. The
threat can come from compromised non-OPES entities. The threat might
affect the overall integrity and effectiveness of an OPES System. For
example, a non-OPES proxy can add bypass instruction to bypass
legitimate OPES entities. The attack might result in overwhelming the
original content provider servers, since the attack essentially
bypass any load balancing techniques. In addition, such an attack is
also equivalent to a DoS attack, whereby, a legitimate data consumer
application may not be able to access some content from a content
provider or its OPES version.
Since an OPES Flow may include non-OPES entities, it is susceptible
to man-in-the-middle attacks, whereby an intruder may inject bypass
instructions into the data path. These attacks may affect content
availability or disturb load balancing techniques in the network.
The above threats can also arise by compromised OPES entities. An
intruder can compromise an OPES entities and then use
Barbir Expires March 31, 2004 [Page 14]
Internet-Draft OPES entities and end points communication Oct. 2003
man-in-the-middle techniques to disturb content availability to a
data consumer application or overload a content provider server
(essentially, some form of a DoS attack).
Attackers can use the bypass instruction to affect the overall
integrity of the OPES System. The ability to illegally introduce
bypass instructions into a data flow may effect the accounting of the
OPES System. It may also affect the quality of content that is
delivered to the data consumer applications. Similar threats can
arise from bad implementations of the bypass facility.
Specific protocol binding documents ought to take these security
threats into consideration. It is recommended that protocol bindings
provide safe features into their specifications. Such features may
include a place holder in the message header that indicates who
originated the bypass request. Other holders can include the option
of signing the bypass request as a proof of identity. Other
application level related security concerns can be found in [6].
Barbir Expires March 31, 2004 [Page 15]
Internet-Draft OPES entities and end points communication Oct. 2003
Normative References
[1] A. Barbir et al., "OPES Use Cases and Deployment Scenarios",
Internet-Draft http://www.ietf.org/internet-drafts/
draft-ietf-opes-scenarios-01.txt, August 2002.
Barbir Expires March 31, 2004 [Page 16]
Internet-Draft OPES entities and end points communication Oct. 2003
Informative References
[2] Floyd, S. and L. Daigle, "IAB Architectural and Policy
Considerations for Open Pluggable Edge Services", RFC 3238,
January 2002.
[3] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., Masinter, L.,
Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999.
[4] A. Barbir et al., "Policy, Authorization and Enforcement
Requirements of OPES", Internet-Draft http://www.ietf.org/
internet-drafts/draft-ietf-opes-authorization-02.txt, February
2003.
[5] Rousskov, A., "HTTP adaptation with OPES", Internet-Draft TBD,
September 2003.
[6] A. Barbir et al., "Security Threats and Risks for Open Pluggable
Edge Services", Internet-Draft http://www.ietf.org/
internet-drafts/draft-ietf-opes-threats-02.txt, February 2003.
[7] A. Barbir et al., "An Architecture for Open Pluggable Edge
Services (OPES)", Internet-Draft http://www.ietf.org/
internet-drafts/draft-ietf-opes-architecture-04, December 2002.
[8] A. Barbir et al., "OPES Treatment of IAB Considerations",
Internet-Draft http://www.ietf.org/internet-drafts/
draft-ietf-opes-iab-02.txt, February 2004.
Author's Address
Abbie Barbir
Nortel Networks
3500 Carling Avenue
Nepean, Ontario K2H 8E9
Canada
Phone: +1 613 763 5229
EMail: abbieb@nortelnetworks.com
Barbir Expires March 31, 2004 [Page 17]
Internet-Draft OPES entities and end points communication Oct. 2003
Appendix A. Acknowledgements
Several people has contributed to this work. Many thanks to: Alex
Rousskov, Hilarie Orman, Oscar Batuner, Markus Huffman, Martin
Stecher, Marshall Rose and Reinaldo Penno.
Barbir Expires March 31, 2004 [Page 18]
Internet-Draft OPES entities and end points communication Oct. 2003
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
Full Copyright Statement
Copyright (C) The Internet Society (2003). 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 assignees.
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
Barbir Expires March 31, 2004 [Page 19]
Internet-Draft OPES entities and end points communication Oct. 2003
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Barbir Expires March 31, 2004 [Page 20]
