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OPS Area Working Group                                             Q. Wu
Internet-Draft                                                    W. Liu
Intended status: Informational                       Huawei Technologies
Expires: January 19, 2017                                      A. Farrel
                                                        Juniper Networks
                                                           July 18, 2016


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

Abstract

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

   A small number of YANG models are used to model services (for
   example, the Layer Three Virtual Private Network Service Model
   produced by the L3SM working group).

   This document briefly sets out the scope of and purpose of an IETF
   service model, and it shows where a service model might fit into a
   Software Defined Networking architecture or deployment.

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
   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 January 19, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   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
   to this document.  Code Components extracted from this document must
   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  . . . . . . . . . . . . . . . . . . . .   4
   4.  Service Models in an SDN Context  . . . . . . . . . . . . . .   5
   5.  Possible Causes of Confusion  . . . . . . . . . . . . . . . .   8
   6.  Further Concepts  . . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Technology Agnostic . . . . . . . . . . . . . . . . . . .   9
     6.2.  Relationship to Policy  . . . . . . . . . . . . . . . . .  10
     6.3.  Operator-Specific Features  . . . . . . . . . . . . . . .  10
     6.4.  Supporting Multiple Services  . . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   8.  Manageability Considerations  . . . . . . . . . . . . . . . .  11
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     11.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

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

   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.

   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.  These models
   may be used in manual and even paper-driven service request processes



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   moving 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 dispells some common misconceptions.

2.  Terms and Concepts

   The following terms are used in this document:

   Network Operator:  This term is used interchangeably to refer to the
      company that owns a network that provides Internet connectivity
      and services, or the individual who performs operations and
      management on that network.

   Customer:  Someone who purchases connectivity and other services from
      a network operator.  In the context of this document, a customer
      is usually the 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 is the customer of an optical transport
      network.

   Service:  A network operator delivers one or more services to a
      customer.  A 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.  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.

   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 all of the parameters of the service in a
      portable, operator-independent way.  It can be used by a human or
      by software to configure or request a service and may equally be
      consumed by a human or by a software component.

   It needs to be repeatedly clarified that a 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 service model is not
   a data model that describes how a network operator realizes and
   delivers the service described by the model.  This issue is discussed
   further in later sections.

3.  Using Service Models

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

   The language in which a 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 service
   model between customer and network operator are deployment- and
   implementation-specific.  The IETF recommends the use of the NETCONF
   Configuration Protocol [RFC4741] with data encoded in XML or JSON for
   interactions "on the wire" between software components.  However, co-
   located software components might use an API, while systems with more
   direct huan interactions might use web pages or even paper forms.





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


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

   How a network operator processes a service request described be a
   service model will depend on the commercial and operational tools,
   processes, and policies used by the 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 network to deliver the requested services.  That means
   that the network operator (or software run by the network operator)
   takes the information in the service model and determines how to
   deliver the service by enabling and configuring network protocols and
   devices.

4.  Service Models in an SDN Context

   In an SDN system, the control and configuration of network resources
   and protocols is performed by software systems that determine how
   best to utilize the network.  Figure 2 shows common architectural
   view of an SDN system where network elements are programmed by a
   component called a Controller, and where Controllers are instructed
   by an Orchestrator that has a wider view of the whole of, or part of,
   a network.



















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



                    Figure 2: A Common SDN Architecture

   But a service request is (or should be) network-agnostic.  That is,
   there should be an independence between the behavior and unctions
   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 to use depending on what
   technologies are available and which service features have been
   requested.

   This mapping can be achieved by splitting the orchestration function
   between a "Service Orchestrator" and a "Network Orchestrator" as
   shown in Figure 3.  In a system that is fully implemented in
   software, this could lead to agile service delivery or service
   automation.













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                              ------------------   Service  ----------
                             |                  |  Model   |          |
                             |     Service      |<-------->| Customer |
                             |   Orchestrator   |          |          |
                             |                  |           ----------
                              ------------------
                                 .          .               -----------
                                .            .             |           |
                               .              .      ......|Application|
                              .                .     :     |           |
                             .                  .    :      -----------
                            .                    .   :
                   ------------------    ------------------
                  |                  |  |                  |
                  |     Network      |  |     Network      |
                  |   Orchestrator   |  |   Orchestrator   |
                  |                  |  |                  |
                  .------------------    ------------------.
                 .           :                   :          .
                .            :                   :           .
        ------------     ------------     ------------    ------------
       |            |   |            |   |            |  |            |
       | Controller |   | Controller |   | Controller |  | Controller |
       |            |   |            |   |            |  |            |
        ------------     ------------     ------------    ------------
           :              .       .               :               :
           :             .         .              :               :
           :            .           .             :               :
       ---------     ---------   ---------     ---------      ---------
      | Network |   | Network | | Network |   | Network |    | Network |
      | Element |   | Element | | Element |   | Element |    | Element |
       ---------     ---------   ---------     ---------      ---------



         Figure 3: An SDN Architecture with a Service Orchestrator

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

   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.






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

   This can lead to some confusion around the definition of a "service
   interface".  Some previous literature considers the interface that is
   northbound of the Network Orchestrator to be a "service interface"
   used by an application (as shown in Figure 3), 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 where the service is described by a
   service model and the interaction is between customer and 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 also offer value-added services to their customers.

   o  Network operation is completely out of scope in the discussion of
      service models.  That means that the 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
      followed by the tunnel.  Of 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 te service model.

   o  The network operator may use further data 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 in the service model



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      alongside protocol parameters and device configuratin information.
      It is important that the Service Orchestrator should be able to
      map from a service model to these data 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 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 sevices present in service models.  Requests for
      specific bandwidth, for example, might be present in a service
      model, and agreement to deliver a service is a commitment to the
      description of the service in the 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.

6.  Further Concepts

   This section introduces a few further, more advanced concepts

6.1.  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
   definition 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, or protocols for exchanging routing
      information) need to be selected and configured as part of the
      service description.





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6.2.  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, but which 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.

   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.

6.3.  Operator-Specific Features

   When work in Layer Three Virtual Private Network Service Model (L3SM)
   ws 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.  Thus, when a basic description of the 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 vary from one
   environment to another.

6.4.  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.



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7.  Security Considerations

   TBD

8.  Manageability Considerations

   TBD

9.  IANA Considerations

   This document makes no requests for IANA action

10.  Acknowledgements

   Thanks to Daniel King for review comments.

11.  References

11.1.  Normative References

   [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>.

11.2.  Informative References

   [RFC4741]  Enns, R., Ed., "NETCONF Configuration Protocol", RFC 4741,
              DOI 10.17487/RFC4741, December 2006,
              <http://www.rfc-editor.org/info/rfc4741>.

   [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>.

   [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>.






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   [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>.

Authors' Addresses

   Qin Wu
   Huawei Technologies

   Email: bill.wu@huawei.com


   Will Liu
   Huawei Technologies

   Email: liushucheng@huawei.com


   Adrian Farrel
   Juniper Networks

   Email: adrian@olddog.co.uk
























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