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Versions: (draft-taylor-pcn-cl-edge-behaviour) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 RFC 6661

Internet Engineering Task Force                                A. Charny
Internet-Draft                                             Cisco Systems
Intended status: Informational                                  F. Huang
Expires: April 29, 2010                              Huawei Technologies
                                                          G. Karagiannis
                                                               U. Twente
                                                                M. Menth
                                                 University of Wuerzburg
                                                          T. Taylor, Ed.
                                                     Huawei Technologies
                                                        October 26, 2009


    PCN Boundary Node Behaviour for the Controlled Load (CL) Mode of
                               Operation
                  draft-ietf-pcn-cl-edge-behaviour-01

Status of this Memo

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   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   Precongestion notification (PCN) is a means for protecting quality of
   service for inelastic traffic admitted to a Diffserv domain.  The
   overall PCN architecture is described in RFC 5559.  This memo is one
   of a series describing possible boundary node behaviours for a PCN
   domain.  The behaviour described here is that for three-state
   measurement-based load control, known informally as Controlled Load
   (CL).


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Assumed Core Network Behaviour for CL  . . . . . . . . . . . .  4
   3.  Node Behaviours  . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Behaviour of the PCN-Egress-Node . . . . . . . . . . . . .  6
       3.2.1.  Flow Admission . . . . . . . . . . . . . . . . . . . .  6
       3.2.2.  Flow Termination . . . . . . . . . . . . . . . . . . .  7
       3.2.3.  Reporting the PCN Data . . . . . . . . . . . . . . . .  8
     3.3.  Behaviour at the Decision Point  . . . . . . . . . . . . .  8
       3.3.1.  Flow Admission . . . . . . . . . . . . . . . . . . . .  8
       3.3.2.  Flow Termination . . . . . . . . . . . . . . . . . . .  8
     3.4.  Behaviour of the Ingress Node  . . . . . . . . . . . . . .  9
   4.  Identifying Ingress-Egress-Aggregates and Their Edge Points  .  9
   5.  Specification of Diffserv Per-Domain Behaviour . . . . . . . . 10
     5.1.  Applicability  . . . . . . . . . . . . . . . . . . . . . . 10
     5.2.  Technical Specification  . . . . . . . . . . . . . . . . . 10
     5.3.  Attributes . . . . . . . . . . . . . . . . . . . . . . . . 10
     5.4.  Parameters . . . . . . . . . . . . . . . . . . . . . . . . 10
     5.5.  Assumptions  . . . . . . . . . . . . . . . . . . . . . . . 10
     5.6.  Example Uses . . . . . . . . . . . . . . . . . . . . . . . 11
     5.7.  Environmental Concerns . . . . . . . . . . . . . . . . . . 11
     5.8.  Security Considerations  . . . . . . . . . . . . . . . . . 11
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13





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1.  Introduction

   The objective of Pre-Congestion Notification (PCN) is to protect the
   quality of service (QoS) of inelastic flows within a Diffserv domain,
   in a simple, scalable, and robust fashion.  Two mechanisms are used:
   admission control, to decide whether to admit or block a new flow
   request, and (in abnormal circumstances) flow termination to decide
   whether to terminate some of the existing flows.  To achieve this,
   the overall rate of PCN-traffic is metered on every link in the
   domain, and PCN-packets are appropriately marked when certain
   configured rates are exceeded.  These configured rates are below the
   rate of the link thus providing notification to boundary nodes about
   overloads before any congestion occurs (hence the "pre" part of pre-
   congestion notification).  The level of marking allows boundary nodes
   to make decisions about whether to admit or terminate.  For more
   details see [RFC5559].

   Boundary node behaviours specify a detailed set of algorithms and
   edge node behaviours used to implement the PCN mechanisms.  Since the
   algorithms depend on specific metering and marking behaviour at the
   interior nodes, it is also necessary to specify the assumptions made
   about interior node behaviour.  Finally, because PCN uses DSCP values
   to carry its markings, a specification of boundary node behaviour
   must include the per domain behaviour (PDB) template specified in
   [RFC3086], filled out with the appropriate content.  The present
   document accomplishes these tasks for the controlled load (CL) mode
   of operation.

   Some aspects of this specification are necessary for
   interoperability, while others are simply suggestions.  This document
   attempts to make the distinction as it proceeds.

1.1.  Terminology

   RFC 2119 requirements language does not seem appropriate for an
   Informational document.  This document uses three levels of
   requirement:

   o  "must" applies to requirements that affect the integrity of
      operation of the complete system;

   o  "recommended" applies to procedures that appear to give superior
      results at time of writing, but may be replaced by other
      procedures directed to the same objective without affecting the
      integrity of operation of the complete system;

   o  "suggested" applies to procedures that are not seen as superior at
      time of writing, but appear to be valid approaches for meeting a



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      particular objective.

   In addition to the terms defined in [RFC5559], this document uses the
   following terms:

   decision point
      The node that makes the decision about which flows to admit and to
      terminate.  In a given network deployment, this may be the ingress
      node or a centralized control node.  Of course, regardless of the
      location of the decision point, the ingress node is the point
      where the decisions are enforced.

   PCN-admission-state
      The state ("admit" or "block") derived by the PCN-egress-node for
      a given ingress-egress-aggregate based on PCN packet marking
      statistics.  The decision point decides to admit or block new
      flows offered to the aggregate based on the current value of the
      PCN-admission-state.  For further details see Section 3.2.1 and
      Section 3.3.1.

   Congestion level estimate (CLE)
      A value derived from the measurement of PCN packets received at a
      PCN-egress-node for a given ingress-egress-aggregate, representing
      the ratio of marked to total PCN traffic (measured in octets) over
      a short period.  This specification suggests that the CLE be
      calculated as an exponentially weighted moving average of the
      ratios observed in successive fixed-length measurement intervals,
      but the exact algorithms used are not critical to
      interoperability.  For further details see Section 3.2.1.

   Admission decision threshold
      A fractional value to which the CLE is compared to determine the
      PCN-admission-state.  If the CLE is below the admission decision
      threshold the PCN-admission-state is set to "admit".  If the CLE
      is above the admission decision threshold the PCN-admission-state
      is set to "block".  For further details see Section 3.2.1.


2.  Assumed Core Network Behaviour for CL

   This section describes the assumed behaviour for nodes of the PCN-
   domain when acting in their role as PCN-interior-nodes.  The CL mode
   of operation assumes that:

   o  encoding of PCN status within individual packets is based on
      [ID.PCN-baseline], extended to provide a third PCN encoding state.
      Possible extensions for this purpose are documented in
      [ID.PCN3state] or alternatively [ID.PCN3in1];



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   o  the domain satisfies the conditions specified in the applicable
      encoding extension document;

   o  each link has been configured with a PCN-threshold-rate having a
      value equal to the PCN-admissible-rate for the link;

   o  each link has been configured with a PCN-excess-rate having a
      value equal to the PCN-supportable-rate for the link;

   o  PCN-interior-nodes perform threshold-marking and excess-traffic-
      marking of packets according to the rules specified in
      [ID.PCN-marking], and any additional rules specified in the
      applicable encoding extension document;

   According to [ID.PCN-baseline], the encoding extension documents
   should specify the allowable transitions between marking states.
   However, to be absolutely clear, these allowable transitions are
   specified here.  At any interior node, the only permitted transitions
   are these:

   o  a PCN packet which is not-marked (NM) MAY be threshold-marked
      (ThM) or excess-traffic-marked (ETM);

   o  a PCN packet which is threshold-marked (ThM) MAY be excess-
      traffic-marked (ETM).

   An interior node MUST NOT re-mark a packet from PCN to non-PCN, or
   vice versa.


3.  Node Behaviours

3.1.  Overview

   The Controlled Load (CL) mode of operation supports flow admission
   based on the ratio of threshold-marked to total PCN-traffic observed
   by the PCN-egress-node (the congestion level estimate, see
   Section 1.1) for each ingress-egress-aggregate.  The PCN-egress-node
   reports the latest value of the PCN-admission-state to the decision
   point at regular intervals.  The decision point decides to admit or
   block new PCN flows offered to a given ingress-egress-aggregate based
   on the PCN-admission-state.

   Flow termination is triggered when the PCN-egress-node observes
   excess-traffic-marked packets within a given ingress-egress-
   aggregate.  When this happens, the PCN-egress-node reports not only
   the PCN-admission-state, but also an estimate of the current edge-to-
   edge supportable rate of PCN traffic.  The decision point interprets



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   the presence of this rate in the report as an indication that flow
   termination is required.  The amount of traffic to be terminated is
   calculated as the difference between the rate of admitted PCN traffic
   measured at the PCN-ingress-node and the estimated supportable rate.
   The decision point selects previously-admitted flows within the
   affected ingress-egress-aggregate for termination until no more
   excess-marked packets are observed at the PCN-egress-node.

   When Equal Cost Multipath (ECMP) routing is operating in the network,
   it is possible that some flows within a given ingress-egress-
   aggregate pass through the bottleneck that is resulting in excess-
   traffic-marking, while others do not.  To ensure that the right set
   of flows is terminated, the PCN-egress-node supplies a list of
   excess-traffic-marked flows along with its estimate of the edge-to-
   edge supportable PCN traffic rate.  The decision point should look
   first to this list when deciding which flows to terminate.

3.2.  Behaviour of the PCN-Egress-Node

   The PCN-egress-node must meter received PCN traffic in order to
   derive periodically the following rates for each ingress-egress-
   aggregate passing through it:

   o  NM-rate: octets per second of PCN traffic in PCN-unmarked packets;

   o  ThM-rate: octets per second of PCN traffic in PCN-threshold-marked
      packets;

   o  ETM-rate: octets per second of PCN traffic in PCN-excess-marked
      packets.

   This specification recommends that the interval between calculation
   of these quantities be in the range of 100 to 500ms to provide a
   reasonable tradeoff between signalling demands on the network and the
   time taken to react to impending congestion.

   This specification suggests that PCN-traffic be metered continuously,
   that the counts of the number of octets of PCN traffic needed to
   calculate the above rates be accumulated continuously throughout the
   interval, and that the intervals themselves be of equal length, to
   minimize the statistical variance introduced by the measurement
   process itself.

3.2.1.  Flow Admission

   Each time the egress node has calculated the rates listed above, the
   egress node must calculate a ratio R of marked to total traffic.  If
   all of the rates are zero for the interval, the ratio R must be set



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   to zero.  Otherwise, the egress node must calculate the ratio as:

      R = (ThM-rate + ETM-rate) / (NM-rate + ThM-rate + ETM-rate).

   The egress node must then use this ratio to update a congestion level
   estimate (CLE, see Section 1.1).

   Exponential smoothing is suggested for this purpose, so that

      updated CLE = w*R + (1-w)*previous CLE.

   The value of w depends on the length of the measurement interval: for
   the equivalent system memory, a shorter interval calls for a smaller
   smoothing constant.  Simulation results
   ([I-D.babiarz-pcn-explicit-marking],
   [I-D.zhang-pcn-performance-evaluation]) show that the effectiveness
   of PCN is not sensitive to the specific value of w used.

   The egress node now compares the updated CLE against a decision
   threshold.  If the CLE is less than the threshold, the PCN-admission-
   state for the ingress-egress-aggregate is determined to be "admit",
   otherwise it is determined to be "block".

      Simulation results ([I-D.zhang-pcn-performance-evaluation] and
      [Menth08f]) show that the process is also not sensitive to the
      value of the decision threshold.

3.2.2.  Flow Termination

   If the PCN-egress-node observes any excess-traffic-marked packets for
   a given ingress-egress-aggregate, it must immediately set aside the
   measurements it has accumulated and begin a new measurement interval.
   In addition, it must send a report to the decision point indicating
   that the PCN-admission-state is "block"

      If the measurement data being set aside represent more than half
      of the normal calculation interval, it is suggested that the PCN-
      egress-node perform the end-of-interval calculations described in
      the previous section and report the PCN-admission-state value thus
      obtained before starting the new interval.

      Restarting the measurement interval ensures that the estimate of
      edge-to-edge supportable rate described below is not biased by
      lower PCN traffic rates prevailing before the onset of excess-
      traffic-marking.

   For subsequent intervals, as long as the calculated rate of excess-
   marked traffic is greater than zero, the PCN-egress-node must



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   calculate and report to the decision point an estimate of the edge-
   to-edge supportable traffic rate for PCN traffic as well as the
   latest PCN-admission-state.  It is recommended that this estimate be
   calculated as the sum:

      NM-rate + ThM-rate.

   In networks with multipath routing (e.g., ECMP in IP networks), the
   PCN-egress-node records flow identifiers of the individual flows for
   which excess-marked packets have been observed.  These will be used
   by the decision point when it selects flows for termination.

3.2.3.  Reporting the PCN Data

   The PCN-egress-node must report the latest value of the PCN-
   admission-state to the decision point each time it calculates it.  If
   flow termination is required (because PCN-excess-marked packets have
   been observed), the egress node must also report the estimate of the
   supportable edge-to-edge rate of PCN traffic calculated in the
   previous section.

   If so configured, the PCN-egress-node must also report the set of
   flow identifiers of flows for which excess-marked packets were
   observed during the calculation interval.

3.3.  Behaviour at the Decision Point

3.3.1.  Flow Admission

   When the decision point (e.g., the PCN-ingress-node) receives a
   report indicating that the PCN-admission-state for a given ingress-
   egress-aggregate is "admit", it admits new flows to that aggregate.
   When the decision point receives a report indicating that the PCN-
   admission-state for a given ingress-egress-aggregate is "block", it
   ceases to admit new flows to that aggregate.  These actions may be
   modified by policy in specific cases.

3.3.2.  Flow Termination

   When the report from the egress node includes an estimate of the
   edge-to-edge supportable PCN traffic rate for the given ingress-
   egress-aggregate, the decision point must fetch the rate at which
   PCN-traffic has been admitted to the aggregate from the PCN-ingress-
   node.  If the rate of admitted traffic is greater than the estimate
   of the edge-to-edge supportable PCN traffic rate for the given
   ingress-egress-aggregate, the decision point must select flows to
   terminate using its knowledge of the bandwidth required by individual
   flows gained, e.g., from resource signalling, until it determines



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   that the PCN traffic admission rate will no longer be greater than
   the estimated edge-to-edge supportable PCN traffic rate provided by
   the egress node.

      Flow termination may be spread out over multiple rounds to avoid
      over-termination.  If this is done, it is recommended that enough
      time elapse between successive rounds of termination to allow the
      effects of previous rounds to be reflected in the measurements
      upon which the termination decisions are based (See
      [I-D.satoh-pcn-performance-termination] and sections 4.2 and 4.3
      of [Menth08-sub-9].)

   If the egress node has supplied a list of flow identifiers
   (Section 3.2.2), the decision point first looks to terminate flows
   from that list.  Flow selection may be guided by policy in specific
   cases.

3.4.  Behaviour of the Ingress Node

   In a specific deployment, the PCN-ingress-node may be the decision
   point.  If so, it carries out the procedures described in the
   previous section.

   Aside from those procedures, the PCN-ingress-node has the
   responsibility to provide the rate of admitted PCN traffic (octets
   per second) on a specific ingress-egress-aggregate when the decision
   point must determine how much flow to terminate in that aggregate.
   The rate that the PCN-ingress-node supplies may be based on a quick
   sample taken at the time the information is required.  It is
   recommended that such a sample be based on observation of at least 30
   PCN packets to achieve reasonable statistical reliability.


4.  Identifying Ingress-Egress-Aggregates and Their Edge Points

   The operation of PCN depends on the ability of the ingress and egress
   nodes to identify the aggregate to which each flow belongs.  The
   egress node also needs to associate an aggregate with the address of
   the ingress node for receiving reports, if the ingress node is the
   decision point.

   The means by which this is done depends on the packet routing
   technology in use in the network.  In general, classification of
   individual packets at the ingress node (for enforcement and metering
   of admission rates) and at the egress node must use the content of
   the outer packet header.  The process may well require configuration
   of routing information in the ingress and egress nodes.  Some cases
   will be particularly challenging, as when a packet is carried by an



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   MPLS tunnel through the ingress node to some node short of the egress
   node, and then turns into an ordinary IP packet.


5.  Specification of Diffserv Per-Domain Behaviour

   This section provides the specification required by [RFC3086] for a
   per-domain behaviour.

5.1.  Applicability

   This section draws heavily upon points made in the PCN architecture
   document, [RFC5559].

   The PCN CL boundary node behaviour specified in this document is
   applicable to inelastic traffic (particularly video and voice) where
   quality of service for admitted flows is protected primarily by
   admission control at the ingress to the domain.  In exceptional
   circumstances (e.g. due to network failures) already-admitted flows
   may be terminated to protect the quality of service of the remainder.
   The CL boundary node behaviour is less likely to terminate too many
   flows under such circumstances than the SM boundary node behaviour
   ([I-D.SM-edge-behaviour]).

5.2.  Technical Specification

   The technical specification of the PCN CL per domain behaviour is
   provided by the contents of [RFC5559], [ID.PCN-baseline],
   [ID.PCN-marking], the specification of the encoding extension (e.g.
   [ID.PCN3state], [ID.PCN3in1]), and the present document.

5.3.  Attributes

   TBD -- basically low loss, low jitter.  Low delay would be nice but
   has to be quantified

5.4.  Parameters

   TBD.  Don't think RFC 3068 is looking for the list of configurable
   parameters given in the architecture document.

5.5.  Assumptions

   Assumed that a specific portion of link capacity has been reserved
   for PCN traffic.  Assumed that recovery from overloads by flow
   termination should happen within 1-3 seconds.





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5.6.  Example Uses

   The PCN CL behaviour may be used to carry real-time traffic,
   particularly voice and video.

5.7.  Environmental Concerns

   TBD

5.8.  Security Considerations

   Please see the security considerations in Section 6 as well as those
   in [RFC2474] and [RFC2475].


6.  Security Considerations

   [RFC5559] provides a general description of the security
   considerations for PCN.  This memo introduces no new considerations.


7.  IANA Considerations

   This memo includes no request to IANA.


8.  Acknowledgements

   The content of this memo bears a family resemblance to
   [ID.briscoe-CL].  The authors of that document were Bob Briscoe,
   Philip Eardley, and Dave Songhurst of BT, Anna Charny and Francois Le
   Faucheur of Cisco, Jozef Babiarz, Kwok Ho Chan, and Stephen Dudley of
   Nortel, Giorgios Karagiannis of U. Twente and Ericsson, and Attila
   Bader and Lars Westberg of Ericsson.

   Ruediger Geib, Philip Eardley, and Bob Briscoe have helped to shape
   the present document with their comments.


9.  References

9.1.  Normative References

   [ID.PCN-baseline]
              Moncaster, T., Briscoe, B., and M. Menth, "Baseline
              Encoding and Transport of Pre-Congestion Information (Work
              in progress)", September 2009.




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   [ID.PCN-marking]
              Eardley, P., "Metering and marking behaviour of PCN-nodes
              (Work in progress)", August 2009.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              December 1998.

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, December 1998.

   [RFC5559]  Eardley, P., "Pre-Congestion Notification (PCN)
              Architecture", RFC 5559, June 2009.

9.2.  Informative References

   [I-D.SM-edge-behaviour]
              Charny, A., Zhang, J., Karagiannis, G., Menth, M., and T.
              Taylor, "PCN Boundary Node Behaviour for the Single
              Marking (SM) Mode of Operation (Work in progress)",
              October 2009.

   [I-D.babiarz-pcn-explicit-marking]
              Liu, X. and J. Babiarz, "Simulations Results for 3sM
              (expired Internet Draft)", July 2007.

   [I-D.satoh-pcn-performance-termination]
              Satoh, D., Ueno, H., and M. Menth, "Performance Evaluation
              of Termination in CL-Algorithm (Work in progress)",
              July 2009.

   [I-D.zhang-pcn-performance-evaluation]
              Zhang, X., "Performance Evaluation of CL-PHB Admission and
              Termination Algorithms (expired Internet Draft)",
              July 2007.

   [ID.PCN3in1]
              Briscoe, B., "PCN 3-State Encoding Extension in a single
              DSCP (Work in progress)", July 2009.

   [ID.PCN3state]
              Moncaster, T., Briscoe, B., and M. Menth, "A PCN encoding
              using 2 DSCPs to provide 3 or more states (Work in
              progress)", April 2009.

   [ID.briscoe-CL]



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              Briscoe, B., "An edge-to-edge Deployment Model for Pre-
              Congestion Notification:  Admission Control over a
              DiffServ Region (expired Internet Draft)", 2006.

   [Menth08-sub-9]
              Menth, M. and F. Lehrieder, "PCN-Based Measured Rate
              Termination", July 2009, <http://
              www3.informatik.uni-wuerzburg.de/~menth/Publications/
              papers/Menth08-Sub-9.pdf>.

   [Menth08f]
              Menth, M. and F. Lehrieder, "Performance Evaluation of
              PCN-Based Admission Control", in Proceedings of the 16th
              International Workshop on Quality of Service (IWQoS)",
              June 2008, <http://www3.informatik.uni-wuerzburg.de/
              ~menth/Publications/papers/Menth08f.pdf>.

   [RFC3086]  Nichols, K. and B. Carpenter, "Definition of
              Differentiated Services Per Domain Behaviors and Rules for
              their Specification", RFC 3086, April 2001.


Authors' Addresses

   Anna Charny
   Cisco Systems
   300 Apollo Drive
   Chelmsford, MA  01824
   USA

   Email: acharny@cisco.com


   Fortune Huang
   Huawei Technologies
   Section F, Huawei Industrial Base,
   Bantian Longgang, Shenzhen  518129
   P.R. China

   Phone: +86 15013838060
   Email: fqhuang@huawei.com










Charny, et al.           Expires April 29, 2010                [Page 13]

Internet-Draft       PCN CL Boundary Node Behaviour         October 2009


   Georgios Karagiannis
   U. Twente


   Phone:
   Email: karagian@cs.utwente.nl


   Michael Menth
   University of Wuerzburg
   Am Hubland
   Wuerzburg  D-97074
   Germany

   Phone: +49-931-888-6644
   Email: menth@informatik.uni-wuerzburg.de


   Tom Taylor (editor)
   Huawei Technologies
   1852 Lorraine Ave
   Ottawa, Ontario  K1H 6Z8
   Canada

   Phone: +1 613 680 2675
   Email: tom111.taylor@bell.net

























Charny, et al.           Expires April 29, 2010                [Page 14]


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