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Network Working Group                              B. Niven-Jenkins, Ed.
Internet-Draft                                                 G. Watson
Intended status: Informational                  Velocix (Alcatel-Lucent)
Expires: December 12, 2012                                      N. Bitar
                                                                 Verizon
                                                               J. Medved
                                                              S. Previdi
                                                           Cisco Systems
                                                           June 10, 2012


                     Use Cases for ALTO within CDNs
                  draft-jenkins-alto-cdn-use-cases-03

Abstract

   For some time, Content Distribution Networks (CDNs) have been used in
   the delivery of some Internet services (e.g. delivery of websites,
   software updates and video delivery) as they provide numerous
   benefits including reduced delivery cost for cacheable content,
   improved quality of experience for end users and increased robustness
   of delivery.

   In order to derive the optimal benefit from a CDN it is preferable to
   deliver content from the servers (caches) that are "closest" to the
   End User requesting the content, where "closest" may be as simple as
   "geographical or network distance" combined with CDN server load
   within a location, but may also consider other more complex
   combinations of metrics and CDN or Network Service Provider (NSP)
   policies.

   There are a number of different ways in which a CDN may obtain the
   necessary network topology and/or cost information to allow it to
   serve End Users from the most optimal servers/locations, such as
   static configuration, passively listening to routing protocols
   directly, active probing of underlying network(s), or obtaining
   topology and cost by querying an information service such as the ALTO
   map & cost services.

   This document describes the use cases for a CDN to be able to obtain
   network topology and cost information from an ALTO server(s).

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering



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   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on December 12, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.


























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  CDN overview . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  CDN & ALTO Use Cases . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  Exposing NSP End User Reachability to a CDN  . . . . . . .  8
     3.2.  Exposing CDN End User Reachability to CSPs . . . . . . . .  9
     3.3.  CDN deployed within a Broadband network  . . . . . . . . . 10
     3.4.  CDN delivering Over-The-Top of a NSP's network . . . . . . 11
     3.5.  CDN acquiring content from multiple upstream sources
           (Origins)  . . . . . . . . . . . . . . . . . . . . . . . . 11
     3.6.  Additional Use Cases . . . . . . . . . . . . . . . . . . . 12
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   6.  Contributing Authors . . . . . . . . . . . . . . . . . . . . . 13
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   8.  Normative References . . . . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
































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1.  Introduction

   For some time, Content Distribution Networks (CDNs) have been used in
   the delivery of some Internet services (e.g. delivery of websites,
   software updates and video delivery) as they provide numerous
   benefits including reduced delivery cost for cacheable content,
   improved quality of experience for end users and increased robustness
   of delivery.

   A CDN typically consists of a network of servers often attached to
   Network Service Provider (NSP) networks.  The point of attachment is
   often as close to content consumers and peering points as
   economically or operationally feasible in order to decrease traffic
   load on the NSP backbone and to provide better user experience
   measured by reduced latency and higher throughput.

   As the volume of video and multimedia content delivered over the
   Internet is rapidly increasing and expected to continue doing so in
   the future, existing CDN providers are scaling up their
   infrastructure and many NSPs are deploying their own CDNs.  The
   result of such deployments is typically that more CDN servers are
   being deployed within NSP networks and those CDN servers are being
   deployed in locations that are "deeper" (i.e. geographically closer
   to the NSP's End Users) than was previously the case.

   In order to derive the optimal benefit from a CDN it is preferable to
   deliver content from the servers (caches) that are "closest" to the
   End User requesting the content, where "closest" may be as simple as
   "geographical or network distance" combined with CDN server load
   within a location, but may also consider other more complex
   combinations of metrics and CDN or NSP policies.

   When CDN servers are deployed outside of an NSP's network or in a
   small number of central locations within an NSP's network a
   simplified view of the NSP's topology or an approximation of
   proximity is typically sufficient to enable the CDN to serve End
   Users from the optimal server/location.  As CDN servers are deployed
   deeper within NSP networks it becomes necessary for the CDN to have
   more detailed knowledge of the underlying network topology and costs
   between network locations in order to enable the CDN to serve End
   Users from the most optimal servers for the NSP.

   There are a number of different ways in which a CDN may obtain the
   necessary network topology and/or cost information to allow it to
   serve End Users from the most optimal servers/locations, such as
   static configuration, passively listening to routing protocols
   directly, active probing of underlying network(s), or obtaining
   topology and cost by querying an information service such as the ALTO



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   map & cost services.

   The rest of this document describes the use cases for a CDN to be
   able to obtain network topology and cost information from an ALTO
   server(s).

1.1.  Terminology

   The following terms are taken from [I-D.ietf-cdni-problem-statement]
   and repeated here for completeness.

   Content Distribution Network (CDN) / Content Delivery Network (CDN):
   Network infrastructure in which the network elements cooperate at
   layers 4 through layer 7 for more effective delivery of Content to
   User Agents.  Typically a CDN consists of a Request Routing system, a
   Distribution System (that includes a set of Surrogates), a Logging
   System and a CDN control system.

   Content Service Provider (CSP): Provides a Content Service to End
   Users (which the End Users access via a User Agent).  A CSP may own
   the Content made available as part of the Content Service, or may
   license content rights from another party.

   End User (EU): The 'real' user of the system, typically a human but
   maybe some combination of hardware and/or software emulating a human
   (e.g. for automated quality monitoring etc.)

   Network Service Provider (NSP): Provides network-based connectivity/
   services to Users.

   Surrogate: A device/function that interacts with other elements of
   the CDN for the control and distribution of Content within the CDN
   and interacts with User Agents for the delivery of the Content.

   User Agent (UA): Software (or a combination of hardware and software)
   through which the End User interacts with the Content Service.  The
   User Agent will communicate with the CSP's Service for the selection
   of content and one or more CDNs for the delivery of the Content.
   Such communication is not restricted to HTTP and may be via a variety
   of protocols.  Examples of User Agents (non-exhaustive) are:
   Browsers, Set Top Boxes (STB), Dedicated content applications (e.g.
   media players), etc.


2.  CDN overview

   This section provides a high level and simplified overview of the
   operation of a CDN to help put the ALTO & CDN use cases into context.



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   A typical CDN consists of a number of functional components, however
   in the context of ALTO three functional components are of interest:
   The Request Routing function, the Surrogate (i.e. caching) function
   and the Origin function.

   The Request Routing function within a CDN is responsible for
   receiving content requests from User Agents, obtaining and
   maintaining necessary information about a set of candidate
   Surrogates, and for selecting and redirecting the User Agent to the
   appropriate Surrogate.

   The Surrogate function interacts with other elements of the CDN for
   the control and distribution of Content within the CDN and interacts
   with User Agents for the delivery of the Content.

   The figure below shows a high level call flow showing the interaction
   between a User Agent, Request Router and Surrogate for the delivery
   of content in a single CDN.

User Agent                    Request Router                   Surrogate
     |                               |                             |
     |      (1) Initial Request      |                             |
     +------------------------------>|                             |
     |                               +--+                          |
     |                               |  | (2) Surrogate Selection  |
     |                               |<-+                          |
     |    (3) Redirection Response   |                             |
     |<------------------------------+                             |
     |                               |                             |
     |      (4) Content Request      |                             |
     +------------------------------------------------------------>|
     |                               |                             |
     |                               |         (5) Content         |
     |<------------------------------------------------------------+
     |                               |                             |


   1.  The User Agent makes an initial request to the CDN.  Depending on
       the type of content being delivered and the configuration of the
       CDN this request may be an application (e.g.  HTTP, RTMP, etc.)
       level request directly from the User Agent or may be a DNS
       request via the User Agent's assigned DNS proxy.
   2.  The Request Router selects an appropriate Surrogate (or set of
       Surrogates) based on the User Agent's (or its proxy's) IP
       address, the Request Router's knowledge of the network topology
       and reachability cost between CDN caches and end users, and any
       additional CDN policies.




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   3.  The Request Router responds to the UA's initial request with an
       appropriate response containing a redirection to the selected
       cache, for example by returning an appropriate DNS A/AAAA record,
       a HTTP 302 redirect, etc.
   4.  The User Agent uses the information provided in the Redirection
       Response to connect directly to the Surrogate and request the
       desired content.
   5.  If CDN policy allows the User Agent to receive the requested
       content, the Surrogate delivers the content to the User Agent.
       A.  [Not Shown] If the Surrogate does not have a copy of the
           requested content then it obtains it from the appropriate
           Origin Server.
          Note: A Surrogate may not communicate with the Origin directly
          and instead obtain the requested content from other surrogates
          or caching layers in the CDN hierarchy.  The details of how
          content requests filter through the CDN hierarchy to the
          Origin are internal to a specific CDN and are out of scope of
          this document.


3.  CDN & ALTO Use Cases

   The primary use case for ALTO in a CDN context is to improve the
   selection of a CDN Surrogate or Origin.  The CDN makes use of an ALTO
   server to choose a better CDN Surrogate or Origin than would
   otherwise be the case.  In its simplest form an ALTO server would
   provide an NSP with the capability to offer a service to a CDN which
   provides network map and cost information that the CDN can use to
   enhance its surrogate and/or Origin selection.

   Although it is possible to obtain raw network map and cost
   information in other ways, for example passively listening to the
   NSP's routing protocols, the use of an ALTO service to expose that
   information may provide additional control to the NSP over how their
   network map/cost is exposed.  Additionally it may enable the NSP to
   maintain a functional separation between their routing plane and
   network map computation functions.  This may be attractive for a
   number of reasons, for example:

   o  The ALTO service could provide a filtered view of the network
      and/or cost map that relates to CDN locations and their proximity
      to end users, for example to allow the NSP to control the level of
      topology detail they are willing to share with the CDN.
   o  The ALTO service could apply additional policies to the network
      map and cost information to provide a CDN-specific view of the
      network map/cost, for example to allow the NSP to encourage the
      CDN to use network links that would not ordinarily be preferred by
      a Shortest Path First routing calculation.



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   o  The routing plane may be operated and controlled by a different
      operational entity (even within a single NSP) to the CDN and the
      ALTO service could provide a layer of separation because:
      *  The CDN is not able to passively listen to routing protocols.
      *  The NSP is not willing to allow the CDN to passively listen to
         routing protocols, e.g. because the NSP is concerned the CDN
         may inadvertently interfere with the routing plane or because
         the routing plane and the CDN are operated by different
         operational entities/groups (including different entities
         within the same NSP).
   The use cases in this document are not necessarily specific as to the
   relationship between the commercial/operational entity that "owns"
   the ALTO service and the commercial/operational entity that "owns"
   the CDN service as it is assumed that such relationships will be
   deployment specific.  Although the ownership of each service may
   affect the level of topology detail that the ALTO service will be
   permitted to expose, it is assumed that the general requirements a
   CDN places on the ALTO service should not change provided that the
   ALTO server is able to expose sufficient topology for the CDN to make
   appropriate surrogate and/or Origin selection decisions.

   In general, the ALTO service is expected to be operated by an entity
   or entities that wish to optimize or otherwise influence request
   routing decisions.  Some, non-exhaustive, examples of such entities
   are:

   o  The entity that operates the CDN's underlying network (e.g. the
      "CDN deployed within a Broadband network" described in
      Section 3.3).
   o  An NSP that wishes to optimize over-the-top content delivery from
      a CDN that is deployed outside of its network (e.g. the "CDN
      delivering Over-The-Top of a NSP" described in Section 3.4).
   o  An NSP (that may or may not operate a CDN) or a CDN that wishes to
      advertise which End Users are reachable via its network/CDN (e.g.
      the exposing "End User reachability" use cases described in
      Section 3.1 and Section 3.2).

   The following sections outline some specific, non-exhaustive, example
   use cases, which are subsets of the primary use case outlined above
   but applied to specific usage examples to demonstrate how a CDN could
   make use of ALTO services.

3.1.  Exposing NSP End User Reachability to a CDN

   In order for a Request Router to be able to make surrogate selection
   decisions, the Request Router needs to have information on which End
   User IP subnets are reachable via which networks or network
   locations.  The granularity of location information required depends



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   on the specific deployment of the CDN relative to the End Users.  For
   example, an Over-The-Top CDN whose surrogates are deployed only
   within the Internet "backbone" may only require knowledge of which
   End User IP subnets are reachable via which NSPs' networks, whereas a
   CDN deployed within a particular NSP's network requires a finer
   granularity of knowledge, i.e. which End User IP subnets are
   reachable via which regions within that NSP's network.

   Such reachability information is often available via dynamic routing
   protocols, however it is likely that in a number of deployment
   scenarios that peering of the routing plane of the network with a CDN
   would be deemed unacceptable (e.g. where the CDN is operated by an
   entity other than the NSP(s) operating the underlying network).

   Provided that some common mapping between ALTO PIDs and network
   locations (or entire networks) is known to both the NSP and the CDN,
   the network map services offered by ALTO could be used to expose
   which End User IP subnets are reachable via a particular network or
   network locations in order to export End User reachability to a
   Request Router to enable the NSP to expose End User reachability
   while also giving the NSP the ability to control the granularity of
   any End User reachability to network location mapping while also
   avoiding routing plane peering between the NSP and the CDN.

3.2.  Exposing CDN End User Reachability to CSPs

   This use case is similar to the use case described in Section 3.1
   however in this case it is the CDN that wishes to expose which End
   User IP subnets the CDN is capable of delivering services to.

   In some deployments a particular CDN may not have reachability to (or
   may not wish to offer services to) every End User IP subnet reachable
   via the global Internet, for example because the CDN is only deployed
   within certain networks or geographic regions and the CDN is either
   unable (due to lack of reachability) or unwilling (due to cost or
   policy) to serve all End Users reachable via the global Internet.

   The reachability offered by a particular CDN may not include all the
   End User IP subnets that a particular CSP requires in order to serve
   all of that CSP's customers and therefore if the CSP wishes to make
   use of the services offered by a CDN that can only serve a subset of
   their customers the CSP must have knowledge of which End User IP
   subnets a particulart CDN is able to serve, so that they can select
   an appropriate CDN to use to deliver their service to particular
   subsets of their customers.

   In such cases, the network map services offered by ALTO could be used
   to expose to a CSP which End User IP subnets are reachable via a



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   particular CDN.  In the case where the CDN is operated by an NSP
   using ALTO in this way could also enable the NSP to separate the
   exposure of End User subnets reachable via their CDN from those
   reachable via their underlying network.

3.3.  CDN deployed within a Broadband network

   In this use case an NSP is providing Broadband services to its
   customers and has deployed a CDN within its Broadband network to
   alleviate the cost and/or improve the User Experience of content
   services for its Broadband customers.

   The topology of Broadband access/backhaul networks is often much more
   constrained than metro/core networks.  If CDN surrogates are deployed
   within the access/backhaul network, for a given set of End Users, the
   NSP is likely to want to utilise the surrogates deployed in the same
   access/backhaul region as those End Users in preference to surrogates
   deployed within the metro/core or within other access/backhaul
   regions.

   It is common for Broadband subscribers to obtain their IP addresses
   dynamically and in many deployments the IP subnets allocated to a
   particular access/backhaul region can change relatively frequently.
   For example new IP subnets are added as the subscriber base grows, IP
   subnets are moved from one Broadband product in the NSP's portfolio
   to another as customers migrate in order to optimise the NSP's IP
   address utilisation, or they are simply moved as part of IP address
   management, etc.

   Additionally, in certain cases, CDN surrogates deployed in a
   particular network region may become overloaded, leading to the CDN
   selecting alternative surrogates in a different region of the network
   for content delivery.  If this occurs, an NSP may wish to influence
   such a decision, for example because the NSP would prefer a surrogate
   to be selected that is deployed in the the next best (cost-wise to
   the NSP) location.

   In order to meet the NSP's objective of utilising their CDN to
   constrain access/backhaul costs and/or improve User Experience it is
   important that the CDN is able to select the most appropriate
   surrogate for a given set of End User IP subnets.  Although the
   network topology is often reasonably static, in networks where the IP
   subnets allocated to a Broadband region are changing relatively
   frequently, static configuration of End User IP Subnets to CDN
   surrogates is possible but some NSPs may consider the operational
   burden of having to update such static configuration too high and
   would prefer the CDN to be able to dynamically obtain network map and
   cost information.



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   The NSP could make use of an ALTO service to expose a cost mapping/
   ranking between End User IP subnets (within that NSP's network) and
   CDN surrogate IP subnets/locations to meet its requirements while
   avoiding static configuration or direct integration of the CDN into
   its IP routing plane and to avoid the CDN being required to implement
   network layer routing computations.

3.4.  CDN delivering Over-The-Top of a NSP's network

   In this use case a CDN is deployed within one or more NSPs' networks
   but is delivering content "Over-The-Top" into another NSP's network
   (which we will call NSP Z) where the CDN is not deployed.

   The CDN is unlikely to have direct visibility of NSP Z's network
   topology and may have a choice of entry points into NSP Z's network
   from which it could serve content to NSP Z's End Users.  For example
   because NSP Z has direct peering links with the CDN in a number of
   locations or NSP Z has transit and/or peering relationships with
   several other NSPs where the CDN is deployed.  NSP Z may wish to
   influence the locations from which the CDN serves content based on
   some factor(s) that it does not wish to expose directly or that might
   change over time.  For example the available transit/peering capacity
   in different locations, the cost of connectivity to different
   locations, etc.

   For example, a CSP is using NSP A's CDN and another NSP (NSP Z) has
   peering with NSP A in Los Angeles and New York.  NSP Z would like to
   influence which peering location NSP A's CDN delivers content out of
   for NSP Z's end users by using their knowledge of the peering
   capacity they have deployed in LA & NY and the capacity they have
   between those peering locations and groups of end users without
   directly exposing their internal topology to NSP A.

   An NSP could make use of an ALTO service to expose a cost mapping/
   ranking between End User IP subnets (within that NSP's network) and
   entry points into that NSP's network in order to try to influence the
   locations from which the CDN serves content into that NSP's network.

3.5.  CDN acquiring content from multiple upstream sources (Origins)

   Before a surrogate within a CDN is able to deliver content to an End
   User it must first have a copy of the content that the End User is
   requesting.  Content may be obtained by surrogates in advance of it
   being requested (pre-positioned) by End Users or it may be obtained
   by surrogates dynamically in response to End User requests for the
   content (on-demand).

   The ultimate source of the content (i.e. where the 'master' copy is



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   permanently stored) is typically referred to as the content's Origin
   (or Origin Server), however CDNs often employ an internal hierarchy
   of caching layers so that surrogates do not necessarily obtain
   content directly from the Origin.  Such a hierarchy provides a number
   of benefits, for example reducing the number of requests for content
   received by the Origin (and therefore reducing the scaling
   requirements on the Origin), more efficient use of the underlying
   network as fewer copies of the content is required to traverse the
   same network links, etc.

   For a particular CSP's content service multiple, possibly
   independently addressable, Origins may be used for resiliency and the
   Origin(s) may be deployed in a distributed manner across multiple
   geographic locations.

   For the rest of this use case "upstream source" is used to mean
   either the Origin itself as well as other sources of the content, for
   example another caching layer within the CDN that has (or will obtain
   on demand) a copy of the content but is not the actual Origin.

   Therefore, for a particular item of content, a surrogate may have a
   choice of upstream sources (both internal to the CDN and external
   Origins) from which it could obtain the content.

   When presented with a choice of upstream sources, a surrogate may
   utilise some combination of policy and heuristics to decide which
   upstream sources (and in which order) it should attempt to use to
   obtain the content.  A CDN may wish to utilise network topology &
   cost information as one of the inputs into such a content source
   selection process, for example to weight upstream sources that are
   topologically close to the surrogate that requires the content.

   Additionally, where the CDN is deployed within one or more NSP
   networks, an NSP may want to try to influence the choice of upstream
   sources, for example the NSP may prefer the CDN to use content
   sources that are deployed within that NSP's network or within
   networks with which it has direct peering agreements with over other
   content sources.

   An NSP (or a CSP) could provide an ALTO service which a CDN could use
   to obtain network topology and/or cost/ranking information to use as
   an input into surrogates' selection decisions for content sources.

3.6.  Additional Use Cases

   The following additional use case may be relevant to ALTO and will be
   described in more detail in a future version of this document:




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   o  Inter-provider CDN / CDN Interconnect.


4.  IANA Considerations

   This document makes no specific request of IANA.

   Note to RFC Editor: this section may be removed on publication as an
   RFC.


5.  Security Considerations

   TBD.


6.  Contributing Authors

   Reinaldo Penno
   Juniper Networks
   Email: rpenno@juniper.net

   Richard Alimi
   Google
   Email: ralimi@google.com

   Richard Yang
   Yale University
   Email: ryr@yale.edu


7.  Acknowledgements

   The authors would like to thank Vijay Gurbani and Volker Hilt for
   their review comments and contributions to the text.


8.  Normative References

   [I-D.ietf-cdni-problem-statement]
              Niven-Jenkins, B., Faucheur, F., and N. Bitar, "Content
              Distribution Network Interconnection (CDNI) Problem
              Statement", draft-ietf-cdni-problem-statement-06 (work in
              progress), May 2012.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.




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Authors' Addresses

   Ben Niven-Jenkins (editor)
   Velocix (Alcatel-Lucent)
   3 Ely Road
   Milton, Cambridge  CB24 6DD
   UK

   Email: ben@velocix.com


   Grant Watson
   Velocix (Alcatel-Lucent)
   3 Ely Road
   Milton, Cambridge  CB24 6DD
   UK

   Email: gwatson@velocix.com


   Nabil Bitar
   Verizon
   40 Sylvan Road
   Waltham, MA  02145
   USA

   Email: nabil.bitar@verizon.com


   Jan Medved
   Cisco Systems


   Email: jmedved@cisco.com


   Stefano Previdi
   Cisco Systems


   Email: sprevidi@cisco.com










Niven-Jenkins, et al.   Expires December 12, 2012              [Page 14]


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