Service Models Explained
draft-ietf-opsawg-service-model-explained-05

OPS Area Working Group                                             Q. Wu
Internet-Draft                                                    W. Liu
Intended status: Informational                       Huawei Technologies
Expires: December 31, 2017                                     A. Farrel
                                                        Juniper Networks
                                                           June 29, 2017


                        Service Models Explained
              draft-ietf-opsawg-service-model-explained-01

Abstract

   The IETF has produced a considerable number of data modules in the
   YANG modelling language.  The majority of these modules are used to
   construct data models to model devices or monolithic functions and
   they allow access for configuration and to read operational status.

   A small number of YANG modules have been defined to model services
   (for example, the Layer Three Virtual Private Network Service Model
   produced by the L3SM working group and documented in RFC 8049).

   This document briefly sets out the scope of and purpose of an IETF
   service model, and it also shows where a service model might fit into
   a Software Defined Networking architecture.  Note that service models
   do not make any assumption of how a service is actually engineered
   and delivered for a customer; details of how network protocols and
   devices are engineered to deliver a service are captured in other
   models that are not exposed through the Customer-Provider Interface.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on December 31, 2017.





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Copyright Notice

   Copyright (c) 2017 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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terms and Concepts  . . . . . . . . . . . . . . . . . . . . .   3
   3.  Using Service Models  . . . . . . . . . . . . . . . . . . . .   6
   4.  Service Models in an SDN Context  . . . . . . . . . . . . . .   8
   5.  Possible Causes of Confusion  . . . . . . . . . . . . . . . .  10
   6.  Comparison With Other Work  . . . . . . . . . . . . . . . . .  11
     6.1.  Comparison With Network Service Models  . . . . . . . . .  12
     6.2.  Service Delivery and Network Element Model Work . . . . .  13
     6.3.  Customer Service Model Work . . . . . . . . . . . . . . .  14
     6.4.  The MEF Architecture  . . . . . . . . . . . . . . . . . .  15
   7.  Further Concepts  . . . . . . . . . . . . . . . . . . . . . .  16
     7.1.  Technology Agnostic . . . . . . . . . . . . . . . . . . .  16
     7.2.  Relationship to Policy  . . . . . . . . . . . . . . . . .  16
     7.3.  Operator-Specific Features  . . . . . . . . . . . . . . .  17
     7.4.  Supporting Multiple Services  . . . . . . . . . . . . . .  17
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   9.  Manageability Considerations  . . . . . . . . . . . . . . . .  18
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     12.2.  Informative References . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   In recent years the number of data modules written in the YANG
   modelling language [RFC6020] for configuration and monitoring has
   blossomed.  Many of these are used for device-level configuration
   (for example, [RFC7223]) or for control of monolithic functions or
   protocol instances (for example, [RFC7407]).



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   Within the context of Software Defined Networking (SDN) [RFC7426]
   YANG data models may be used on Southbound Interfaces (SBIs) between
   a controller and network devices, and between network orchestrators
   and controllers.  There may also be a hierarchy of such components
   with super-controllers, domain controllers, and device controllers
   all exchanging information and instructions using YANG models.

   Recently there has been interest in using YANG to define and document
   data models that describe services in a portable way that is
   independent of which network operator uses the model.  For example,
   the Layer Three Virtual Private Network Service Model (L3SM)
   [RFC8049].  Such models may be used in manual and even paper-driven
   service request processes with a gradual transition to IT-based
   mechanisms.  Ultimately they could be used in online, software-driven
   dynamic systems.

   This document explains the scope and purpose of service models within
   the IETF and describes how a service model can be used by a network
   operator.  Equally, this document clarifies what a service model is
   not, and dispels some common misconceptions.

   The document also shows where a service model might fit into an SDN
   architecture, but it is important to note that a service model does
   not require or preclude the use of SDN.  Note that service models do
   not make any assumption of how a service is actually engineered and
   delivered to a customer; details of how network protocols and devices
   are engineered to deliver a service are captured in other models that
   are not exposed through the Customer- Provider Interface.

   Other work on classifying YANG data models has been done in
   [I-D.ietf-netmod-yang-model-classification].  That document provides
   an important reference for this document, and also uses the term
   "service model".  Section 6.1 provides a comparison between these two
   uses of the same terminology.

2.  Terms and Concepts

   Readers should familiarize themselves with the description and
   classification of YANG models provided in
   [I-D.ietf-netmod-yang-model-classification].

   The following terms are used in this document:

   Network Operator:  This term is used to refer to the company that
      owns and operates one or more networks that provide Internet
      connectivity services and/or other services.  The term is also
      used to refer to an individual who performs operations and
      management on those networks.



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   Customer:  This term refers to someone who purchases a service
      (including connectivity) from a network operator.  In the context
      of this document, a customer is usually a company that runs their
      own network or computing platforms and wishes to connect to the
      Internet or between sites.  Such a customer may operate an
      enterprise network or a data center.  Sometimes this term may also
      be used to refer to the individual in such a company who contracts
      to buy services from a network operator.  A customer as described
      here is a separate commercial operation from the network operator,
      but some companies may operate with internal customers so that,
      for example, an IP/MPLS packet network may be the customer of an
      optical transport network.

   Service:  A network operator delivers one or more services to a
      customer.  A service in the context of this document (sometimes
      called a Network Service) is some form of connectivity between
      customer sites and the Internet, or between customer sites across
      the network operator's network and across the Internet.  However,
      a distinction should be drawn between the parameters that describe
      a service as included in a customer service model (q.v.) and a
      Service Level Agreement (SLA) as discussed in Section 5 and
      Section 7.2.

      A service may be limited to simple connectivity (such as IP-based
      Internet access), may be a tunnel (such as a virtual circuit), or
      may be a more complex connectivity model (such as a multi-site
      virtual private network).  Services may be further enhanced by
      additional functions providing security, load-balancing,
      accounting, and so forth.  Additionally, services usually include
      guarantees of quality, throughput, and fault reporting.

      This document makes a distinction between a service as delivered
      to a customer (that is, the service as discussed on the interface
      between a customer and the network operator) and the service as
      realized within the network (as described in
      [I-D.ietf-netmod-yang-model-classification]).  This distinction is
      discussed further in Section 6.

      Readers may also refer to [RFC7297] for an example of how an IP
      connectivity service may be characterized.

   Data Model:  The concepts of information models and data models are
      described in [RFC3444].  That document defines a data model by
      contrasting it with the definition of an information model, so it
      may be helpful to quote some text to give context within this
      document.





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         The main purpose of an information model is to model managed
         objects at a conceptual level, independent of any specific
         implementations or protocols used to transport the data.  The
         degree of specificity (or detail) of the abstractions defined
         in the information model depends on the modeling needs of its
         designers.  In order to make the overall design as clear as
         possible, an information model should hide all protocol and
         implementation details.  Another important characteristic of an
         information model is that it defines relationships between
         managed objects.

         Data models, conversely, are defined at a lower level of
         abstraction and include many details.  They are intended for
         implementors and include protocol-specific constructs.

   Service Model:  A service model is a specific type of data model.  It
      describes a service and the parameters of the service in a
      portable way.  The service model may be divided into two
      categories:

      Customer Service Model:  A customer service model is used to
         describe a service as offered or delivered to a customer by a
         network operator.  It can be used by a human (via a user
         interface such as a GUI, web form, or CLI) or by software to
         configure or request a service, and may equally be consumed by
         a human (such as via an order fulfillment system) or by a
         software component.  Such models are sometimes referred to
         simply as "service models" [RFC8049].  A customer service model
         is expressed as a core set of parameters that are common across
         network operators: additional features that are specific to the
         offerings of individual network operators would be defined in
         extensions or augmentations of the model.  Except where
         specific technology details (such as encapsulations, or
         mechanisms applied on access links) are directly pertinent to
         the customer, customer service models are technology agnostic
         so that the customer does not have influence over or knowledge
         of how the network operator engineers the service.



         An example of where such details are relevant to the customer
         are when they describe the behavior or interactions on the
         interface between the equipment at the customer site (often
         referred to as the Customer Edge or CE equipment) and the
         equipment at the network operator's site (usually referred to
         as the Provider Edge or PE equipment).





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      Service Delivery Model:  A service delivery model is used by a
         network operator to define and manage how a service is
         engineered in the network.  It can be used by a human operator
         (such as via a management station) or by a software tool to
         instruct network components.  Such models are sometimes
         referred to as "network service models"
         [I-D.ietf-netmod-yang-model-classification] and are consumed by
         "external systems" such as Operations Support System (OSS).  A
         service delivery model is expressed as a core set of parameters
         that are common across a network type and technology:
         additional features that are specific to the configuration of
         individual vendor equipment or proprietary protocols would be
         defined in extensions or augmentations of the model.  Service
         delivery models include technology-specific modules.

   The distinction between a customer service model and a service
   delivery model needs to be repeatedly clarified.  A customer service
   model is not a data model used to directly configure network devices,
   protocols, or functions: it is not something that is sent to network
   devices (i.e., routers or switches) for processing.  Equally, a
   customer service model is not a data model that describes how a
   network operator realizes and delivers the service described by the
   model.  This distinction is discussed further in later sections.

3.  Using Service Models

   As already indicated, customer service models are used on the
   interface between customers and network operators.  This is shown
   simply in Figure 1

   The language in which a customer service model is described is a
   choice for whoever specifies the model.  The IETF uses the YANG data
   modeling language defined in [RFC6020]

   The encoding and communication protocol used to exchange a customer
   service model between customer and network operator are deployment-
   and implementation-specific.  The IETF has standardized the NETCONF
   protocol [RFC6241] and the RESTCONF protocol [RFC8040] for
   interactions "on the wire" between software components with data
   encoded in XML or JSON.  However, co-located software components
   might use an API, while systems with more direct human interactions
   might use web pages or even paper forms.









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            --------------       Customer        ----------------------
           |              |    Service Model    |                      |
           |   Customer   | <-----------------> |   Network Operator   |
           |              |                     |                      |
            --------------                       ----------------------


    Figure 1: The Customer Service Models used on the Interface between
                      Customers and Network Operators

   How a network operator processes a customer's service request
   described with a customer service model depends on the commercial and
   operational tools, processes, and policies used by the network
   operator.  These may vary considerably from one network operator to
   another.

   However, the intent is that the network operator maps the service
   request into configuration and operational parameters that control
   one or more networks to deliver the requested services.  That means
   that the network operator (or software run by the network operator)
   takes the information in the customer service model and determines
   how to deliver the service by enabling and configuring network
   protocols and devices.  They may achieve this by constructing service
   delivery models and passing them to network orchestrators or
   controllers.  The use of standard customer service models eases
   service delivery by means of automation.

   The practicality of customer service models has been repeatedly
   debated.  It has been suggested that network operators have such
   radically different business modes and such diverse commercial
   offerings that a common customer service model is impractical.
   However, the L3SM [RFC8049] results from the consensus of multiple
   individuals working at network operators and offers a common core of
   service options that can be augmented according to the needs of
   individual network operators.

   It has also been suggested that there should be a single, base
   customer service module, and that details of individual services
   should be offered as extensions or augmentations of this.  It is
   quite possible that a number of service parameters (such as the
   identity and postal address of a customer) will be common and it
   would be a mistake to define them multiple times, once in each
   customer service model.  However, the distinction between a 'module'
   and a 'model' should be considered at this point: modules are how the
   data for models is logically broken out and documented especially for
   re-use in multiple models.





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4.  Service Models in an SDN Context

   In an SDN system, the management of network resources and protocols
   is performed by software systems that determine how best to utilize
   the network.  Figure 2 shows a sample architectural view of an SDN
   system where network elements are programmed by a component called an
   "SDN controller" (or "controller" for short), and where controllers
   are instructed by an orchestrator that has a wider view of the whole
   of, or part of, a network.  The internal organization of an SDN
   control plane is deployment-specific.


                            ------------------
                           |                  |
                           |   Orchestrator   |
                           |                  |
                           .------------------.
                          .          :         .
                         .           :          .
              ------------     ------------     ------------
             |            |   |            |   |            |
             | Controller |   | Controller |   | Controller |
             |            |   |            |   |            |
              ------------     ------------     ------------
                 :              .       .               :
                 :             .         .              :
                 :            .           .             :
             ---------     ---------   ---------     ---------
            | Network |   | Network | | Network |   | Network |
            | Element |   | Element | | Element |   | Element |
             ---------     ---------   ---------     ---------


                    Figure 2: A Sample SDN Architecture

   But a customer's service request is (or should be) technology-
   agnostic.  That is, there should be an independence between the
   behavior and functions that a customer requests and the technology
   that the network operator has available to deliver the service.  This
   means that the service request must be mapped to the orchestrator's
   view, and this mapping may include a choice of which networks and
   technologies to use depending on which service features have been
   requested.

   One implementation option to achieve this mapping is to split the
   orchestration function between a "Service Orchestrator" and a
   "Network Orchestrator" as shown in Figure 3.




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                                                 Customer
                            ------------------   Service  ----------
                           |                  |  Model   |          |
                           |     Service      |<-------->| Customer |
                           |   Orchestrator   |    (a)   |          |
                           |                  |           ----------
                            ------------------
                               .          .
                              .            .              -----------
                             . (b)          .      ......|Application|
                            .                .     :     |  BSS/OSS  |
                           .                  .    :      -----------
                          .  Service Delivery  .   :
                          .       Model        .   :
                 ------------------    ------------------
                |                  |  |                  |
                |     Network      |  |     Network      |
                |   Orchestrator   |  |   Orchestrator   |
                |                  |  |                  |
                .------------------    ------------------.
               .         :                       :        .
              .          : Network Configuration :         .
              .          :        Model          :         .
      ------------     ------------     ------------    ------------
     |            |   |            |   |            |  |            |
     | Controller |   | Controller |   | Controller |  | Controller |
     |            |   |            |   |            |  |            |
      ------------     ------------     ------------    ------------
         :              .       .                 :            :
         :             .         .      Device    :            :
         :            .           . Configuration :            :
         :            .           .     Model     :            :
     ---------     ---------   ---------     ---------      ---------
    | Network |   | Network | | Network |   | Network |    | Network |
    | Element |   | Element | | Element |   | Element |    | Element |
     ---------     ---------   ---------     ---------      ---------


     Figure 3: An Example SDN Architecture with a Service Orchestrator

   Figure 3 also shows where different data models might be applied
   within the architecture.

   The split between control components that exposes a "service
   interface" is present in many figures showing extended SDN
   architectures:





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   o  Figure 1 of [RFC7426] shows a separation of the "Application
      Plane", the "Network Services Abstraction Layer (NSAL)", and the
      "Control Plane".  It marks the "Service Interface" as situated
      between the NSAL and the Control Plane.

   o  [RFC7491] describes an interface between an "Application Service
      Coordinator" and an "Application-Based Network Operations
      Controller".

   o  Figure 1 of [I-D.ietf-netmod-yang-model-classification] shows an
      interface from an OSS or a Business Support System (BSS) that is
      expressed in "Network Service YANG Models".

   This can all lead to some confusion around the definition of a
   "service interface" and a "service model".  Some previous literature
   considers the interface northbound of the Network Orchestrator
   (labeled "(b)" in Figure 3) to be a "service interface" used by an
   application, but the service described at this interface is network-
   centric and is aware of many features such as topology, technology,
   and operator policy.  Thus, we make a distinction between this type
   of service interface and the more abstract service interface (labeled
   "(a)" in Figure 3) where the service is described by a service model
   and the interaction is between customer and network operator.
   Further discussion of this point is provided in Section 5.

5.  Possible Causes of Confusion

   In discussing service models, there are several possible causes of
   confusion:

   o  The services we are discussing are services provided by network
      operators to customers.  This is a completely different thing to
      "Foo as a Service" (for example, Infrastructure as a Service
      (IaaS)) where a service provider offers a service at some location
      that is reached across a network.  The confusion arises not only
      because of the use of the word "service", but also because network
      operators may offer value-added services as well as network
      connection services to their customers.

   o  Network operation is completely out of scope in the discussion of
      services between a network operator and a customer.  That means
      that the customer service model does not reveal to the customer
      anything about how the network operator delivers the service.  The
      model does not expose details of technology or network resources
      used to provide the service.  For example, in the simple case of
      point-to-point virtual link connectivity provided by a network
      tunnel (such as an MPLS pseudowire) the network operator does not
      expose the path through the network that the tunnel follows.  Of



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      course, this does not preclude the network operator from taking
      guidance from the customer (such as to avoid routing traffic
      through a particular country) or from disclosing specific details
      (such as might be revealed by a route trace), but these are not
      standard features of the service as described in the customer
      service model.

   o  The network operator may use further data models (service delivery
      models) that help to describe how the service is realized in the
      network.  These models might be used on the interface between the
      Service Orchestrator and the Network Orchestrator as shown in
      Figure 3 and might include many of the pieces of information from
      the customer service model alongside protocol parameters and
      device configuration information.
      [I-D.ietf-netmod-yang-model-classification] also terms these data
      models as "service models" or "Network Service YANG Models" and a
      comparison is provided in Section 6.1.  It is important that the
      Service Orchestrator should be able to map from a customer service
      model to these service delivery models, but they are not the same
      things.

   o  Commercial terms are generally not a good subject for
      standardization.  It is possible that some network operators will
      enhance standard customer service models to include commercial
      information, but the way this is done is likely to vary widely
      between network operators.

   o  Service Level Agreements (SLAs) have a high degree of overlap with
      the definition of services present in customer service models.
      Requests for specific bandwidth, for example, might be present in
      a customer service model, and agreement to deliver a service is a
      commitment to the description of the service in the customer
      service model.  However, SLAs typically include a number of fine-
      grained details about how services are allowed to vary, by how
      much, and how often.  SLAs are also linked to commercial terms
      with penalties and so forth, and so are also not good topics for
      standardization.

      If a network operator chooses to express an SLA using a data
      model, that model might be referenced as an extension or an
      augmentation of the customer service model.

6.  Comparison With Other Work

   Other work has classified YANG models, produced parallel
   architectures, and developed a range of YANG models.  This section
   briefly examines that other work and shows how it fits with the
   description of service models introduced in this document.



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6.1.  Comparison With Network Service Models

   As previously noted, [I-D.ietf-netmod-yang-model-classification]
   provides a classification of YANG data models.  It introduces the
   term "Network Service YANG Module" to identify the type of model used
   to "describe the configuration, state data, operations and
   notifications of abstract representations of services implemented on
   one or multiple network elements."  These are service delivery models
   as described in this document, that is, they are the models used on
   the interface between the Service Orchestrator or OSS/BSS and the
   Network Orchestrator as shown in Figure 3.

   Figure 1 of [I-D.ietf-netmod-yang-model-classification] can be
   modified to make this more clear and to add an additional example of
   a Network Service YANG model as shown in Figure 4.




































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          +---------------+
          |               |
          |   Customers   |
          |               |
          +---------------+

      - - - - - - - - - - - - - -
     Customer Service YANG Modules

      +--------------------------+     +--------------------------+
      |                          |     |  Operations and Business |
      |   Service Orchestrator   |     |      Support Systems     |
      |                          |     |        (OSS/BSS)         |
      +--------------------------+     +--------------------------+

     - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     Network Service YANG Modules

    +------------+   +-------------+   +-------------+   +-------------+
    |            |   |             |   |             |   |             |
    |  - L2VPN   |   |   - L2VPN   |   |    EVPN     |   |    L3VPN    |
    |  - VPWS    |   |   - VPLS    |   |             |   |             |
    |            |   |             |   |             |   |             |
    +------------+   +-------------+   +-------------+   +-------------+

     - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     Network Element YANG Modules

     +------------+  +------------+  +-------------+  +------------+
     |            |  |            |  |             |  |            |
     |    MPLS    |  |    BGP     |  | IPv4 / IPv6 |  |  Ethernet  |
     |            |  |            |  |             |  |            |
     +------------+  +------------+  +-------------+  +------------+

       L2VPN: Layer 2 Virtual Private Network
       L3VPN: Layer 3 Virtual Private Network
       VPWS: Virtual Private Wire Service
       VPLS: Virtual Private LAN Service


            Figure 4: YANG Module Layers Showing Service Models

6.2.  Service Delivery and Network Element Model Work

   A number of IETF working groups are developing YANG models related to
   services.  These models focus on how the network operator configures
   the network through protocols and devices to deliver a service.  Some
   of these models are classed as service delivery models while others



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   have details that are related to specific element configuration and
   so are classed as network element models.

   A sample set of these models is listed here:

   o  [I-D.dhjain-bess-bgp-l3vpn-yang] defines a YANG model that can be
      used to configure and manage BGP Layer 3 VPNs.

   o  [I-D.ietf-bess-l2vpn-yang] documents a YANG model that it is
      expected will be used by the management tools run by the network
      operators in order to manage and monitor the network resources
      that they use to deliver L2VPN services.

   o  [I-D.ietf-bess-evpn-yang] defines YANG models for delivering an
      Ethernet VPN service.

6.3.  Customer Service Model Work

   Several initiatives within the IETF are developing customer service
   models.  The most advanced presents the Layer Three Virtual Private
   Network (L3VPN) service as described by a network operator to a
   customer.  This L3VPN service model (L3SM) is documented in [RFC8049]
   where its usage is described as in Figure 5 which is reproduced from
   that document.  As can be seen, the L3SM is a customer service model
   as described in this document.


               L3VPN-SVC |
                 MODEL   |
                         |
                      +------------------+         +-----+
                      |   Orchestration  | < --- > | OSS |
                      +------------------+         +-----+
                         |            |
                 +----------------+   |
                 | Config manager |   |
                 +----------------+   |
                         |            |
                         | Netconf/CLI ...
                         |            |
           +------------------------------------------------+
                                Network


                  Figure 5: The L3SM Service Architecture

   A Layer Two VPN service model (L2SM) is defined in
   [I-D.ietf-l2sm-l2vpn-service-model].  That model's usage is described



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   as in Figure 6 which is a reproduction of Figure 5 from that
   document.  As can be seen, the L2SM is a customer service model as
   described in this document.


             ----------------------------
            | Customer Service Requester |
             ----------------------------
                 |
         L2VPN   |
         Service |
         Model   |
                 |
               -----------------------
              | Service Orchestration |
               -----------------------
                 |
                 |     Service             +-------------+
                 |     Delivery    +------>| Application |
                 |     Model       |       |   BSS/OSS   |
                 |                 V       +-------------+
               -----------------------
              | Network Orchestration |
               -----------------------
                 |            |
         +----------------+   |
         | Config manager |   |
         +----------------+   |  Device
                 |            |  Models
                 |            |
      --------------------------------------------
                        Network


                  Figure 6: The L2SM Service Architecture

6.4.  The MEF Architecture

   The MEF Forum has developed an architecture for network management
   and operation.  It is documented as the Lifecycle Service
   Orchestration (LSO) Reference Architecture and illustrated in
   Figure 2 of [MEF-55].

   The work of the MEF Forum embraces all aspects of Lifecycle Service
   Orchestration including billing, SLAs, order management, and life-
   cycle management.  The IETF's work on service models is typically
   smaller offering a simple, self-contained service YANG module.  Thus,
   it may be impractical to fit IETF service models into the MEF Forum



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   LSO architecture.  This does not invalidate either approach, but only
   observes that they are different.

7.  Further Concepts

   This section introduces a few further, more advanced concepts

7.1.  Technology Agnostic

   Service models should generally be technology agnostic.  That is to
   say, the customer should not care how the service is provided so long
   as the service is delivered.

   However, some technologies reach the customer site and make a
   difference to the type of service delivered.  Such features do need
   to be described in the service model.

   Two examples are:

   o  The data passed between customer equipment and network operator
      equipment will be encapsulated in a specific way, and that data
      plane type forms part of the service.

   o  Protocols that are run between customer equipment and network
      operator equipment (for example, Operations, Administration, and
      Maintenance protocols, protocols for discovery, or protocols for
      exchanging routing information) need to be selected and configured
      as part of the service description.

7.2.  Relationship to Policy

   Policy appears as a crucial function in many places during network
   orchestration.  A Service Orchestrator will, for example, apply the
   network operator's policies to determine how to provide a service for
   a particular customer (possibly considering commercial terms).
   However, the policies within a service model are limited to those
   over which a customer has direct influence and that are acted on by
   the network operator.

   The policies that express desired behavior of services on occurrence
   of specific events are close to SLA definitions: they should only be
   included in the base service model where they are common to all
   network operators' offerings.  Policies that describe who at a
   customer may request or modify services (that is, authorization) are
   close to commercial terms: they, too, should only be included in the
   base service model where they are common to all network operators'
   offerings.




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   Nevertheless, policy is so important that all service models should
   be designed to be easily extensible to allow policy components to be
   added and associated with services as needed.

7.3.  Operator-Specific Features

   When work in the L3SM working group was started, there was some doubt
   as to whether network operators would be able to agree on a common
   description of the services that they offer to their customers
   because, in a competitive environment, each markets the services in a
   different way with different additional features.  However, the
   working group was able to agree on a core set of features that
   multiple network operators were willing to consider as "common".
   They also understood that should an individual network operator want
   to describe additional features (operator-specific features) they
   could do so by extending or augmenting the L3SM model.

   Thus, when a basic description of a core service is agreed and
   documented in a service model, it is important that that model should
   be easily extended or augmented by each network operator so that the
   standardized model can be used in a common way and only the operator-
   specific features varied from one environment to another.

7.4.  Supporting Multiple Services

   Network operators will, in general, offer many different services to
   their customers.  Each would normally be the subject of a separate
   service model.

   It is an implementation and deployment choice whether all service
   models are processed by a single Service Orchestrator that can
   coordinate between the different services, or whether each service
   model is handled by a specialized Service Orchestrator able to
   provide tuned behavior for a specific service.

   It is expected that, over time, certain elements of the service
   models will be seen to repeat in each model.  An example of such an
   element is the postal address of the customer.

   It is anticipated that, while access to such information from each
   service model is important, the data will be described in its own
   module and may form part of the service model either by inclusion or
   by index.








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8.  Security Considerations

   The interface between customer and service provider is a commercial
   interface and needs to be subject to appropriate confidentiality.
   Additionally, knowledge of what services are provided to a customer
   or delivered by a network operator may supply information that can be
   used in a variety of security attacks.

   Clearly, the ability to modify information exchanges between customer
   and network operator may result in bogus requests, unwarranted
   billing, and false expectations.  Furthermore, in an automated
   system, modifications to service requests or the injection of bogus
   requests may lead to attacks on the network and delivery of customer
   traffic to the wrong place.

   Therefore it is important that the protocol interface used to
   exchange service request information between customer and network
   operator is subject to authorization, authentication, and encryption.
   This document discusses modeling that information, not how it is
   exchanged.

9.  Manageability Considerations

   This whole document discusses issues related to network management.

   It is important to observe that automated service provisioning
   resulting from use of a customer service model may result in rapid
   and significant changes in traffic load within a network and that
   that might have an effect on other services carried in a network.

   It is expected, therefore, that a Service Orchestration component has
   awareness of other service commitments, that the Network
   Orchestration component will not commit network resources to fulfill
   a service unless doing so is appropriate, and that a feedback loop
   will be provided to report on degradation of the network that will
   impact the service.

   The operational state of a service does not form part of a customer
   service model.  However, it is likely that a network operator may
   want to report some state information about various components of the
   service, and that could be achieved through extensions to the core
   service model.

10.  IANA Considerations

   This document makes no requests for IANA action





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11.  Acknowledgements

   Thanks to Daniel King, Xian Zhang, and Michael Scharf for useful
   review and comments.  Med Boucadair gave thoughtful and detailed
   comments on version -04 of this document.  Thanks to Dean Bogdanovic
   and Tianran Zhou for their help coordinating with [I-D.ietf-netmod-
   yang-model-classification].

   Many thanksto Jerry Bonner for spotting a tiny, one-word, but
   critical typo.

12.  References

12.1.  Normative References

   [I-D.ietf-netmod-yang-model-classification]
              Bogdanovic, D., Claise, B., and C. Moberg, "YANG Module
              Classification", draft-ietf-netmod-yang-model-
              classification-08 (work in progress), June 2017.

   [RFC3444]  Pras, A. and J. Schoenwaelder, "On the Difference between
              Information Models and Data Models", RFC 3444,
              DOI 10.17487/RFC3444, January 2003,
              <http://www.rfc-editor.org/info/rfc3444>.

   [RFC7426]  Haleplidis, E., Ed., Pentikousis, K., Ed., Denazis, S.,
              Hadi Salim, J., Meyer, D., and O. Koufopavlou, "Software-
              Defined Networking (SDN): Layers and Architecture
              Terminology", RFC 7426, DOI 10.17487/RFC7426, January
              2015, <http://www.rfc-editor.org/info/rfc7426>.

   [RFC8049]  Litkowski, S., Tomotaki, L., and K. Ogaki, "YANG Data
              Model for L3VPN Service Delivery", RFC 8049,
              DOI 10.17487/RFC8049, February 2017,
              <http://www.rfc-editor.org/info/rfc8049>.

12.2.  Informative References

   [I-D.dhjain-bess-bgp-l3vpn-yang]
              Jain, D., Patel, K., Brissette, P., Li, Z., Zhuang, S.,
              Liu, X., Haas, J., Esale, S., and B. Wen, "Yang Data Model
              for BGP/MPLS L3 VPNs", draft-dhjain-bess-bgp-l3vpn-yang-02
              (work in progress), August 2016.








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   [I-D.ietf-bess-evpn-yang]
              Brissette, P., Sajassi, A., Shah, H., Li, Z.,
              Tiruveedhula, K., Hussain, I., and J. Rabadan, "Yang Data
              Model for EVPN", draft-ietf-bess-evpn-yang-02 (work in
              progress), March 2017.

   [I-D.ietf-bess-l2vpn-yang]
              Shah, H., Brissette, P., Chen, I., Hussain, I., Wen, B.,
              and K. Tiruveedhula, "YANG Data Model for MPLS-based
              L2VPN", draft-ietf-bess-l2vpn-yang-05 (work in progress),
              March 2017.

   [I-D.ietf-l2sm-l2vpn-service-model]
              Wen, B., Fioccola, G., Xie, C., and L. Jalil, "A YANG Data
              Model for L2VPN Service Delivery", draft-ietf-l2sm-l2vpn-
              service-model-01 (work in progress), May 2017.

   [MEF-55]   MEF Forum, "Service Operations Specification MEF 55 :
              Lifecycle Service Orchestration (LSO) Reference
              Architecture and Framework", March 2016.

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,
              <http://www.rfc-editor.org/info/rfc6020>.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <http://www.rfc-editor.org/info/rfc6241>.

   [RFC7223]  Bjorklund, M., "A YANG Data Model for Interface
              Management", RFC 7223, DOI 10.17487/RFC7223, May 2014,
              <http://www.rfc-editor.org/info/rfc7223>.

   [RFC7297]  Boucadair, M., Jacquenet, C., and N. Wang, "IP
              Connectivity Provisioning Profile (CPP)", RFC 7297,
              DOI 10.17487/RFC7297, July 2014,
              <http://www.rfc-editor.org/info/rfc7297>.

   [RFC7407]  Bjorklund, M. and J. Schoenwaelder, "A YANG Data Model for
              SNMP Configuration", RFC 7407, DOI 10.17487/RFC7407,
              December 2014, <http://www.rfc-editor.org/info/rfc7407>.

   [RFC7491]  King, D. and A. Farrel, "A PCE-Based Architecture for
              Application-Based Network Operations", RFC 7491,
              DOI 10.17487/RFC7491, March 2015,
              <http://www.rfc-editor.org/info/rfc7491>.



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   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
              <http://www.rfc-editor.org/info/rfc8040>.

Authors' Addresses

   Qin Wu
   Huawei Technologies

   Email: bill.wu@huawei.com


   Will Liu
   Huawei Technologies

   Email: liushucheng@huawei.com


   Adrian Farrel
   Juniper Networks

   Email: afarrel@juniper.net





























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