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SDNRG                                                      E. Haleplidis
Internet-Draft                                                S. Denazis
Intended status: Informational                      University of Patras
Expires: June 8, 2014                                     K. Pentikousis
                                                                    EICT
                                                           J. Hadi Salim
                                                       Mojatatu Networks
                                                                D. Meyer
                                                                 Brocade
                                                          O. Koufopavlou
                                                    University of Patras
                                                        December 5, 2013


                SDN Layers and Architecture Terminology
              draft-haleplidis-sdnrg-layer-terminology-03

Abstract

   Software-Defined Networking (SDN) can in general be defined as a new
   approach for network programmability.  Network programmability refers
   to the capacity to initialize, control, change, and manage network
   behavior dynamically via open interfaces as opposed to relying on
   closed-box solutions and propietary-defined interfaces.  SDN
   emphasizes the role of software in running networks through the
   introduction of an abstraction for the data forwarding plane and, by
   doing so, separates it from the control plane.  This separation
   allows faster innovation cycles at both planes as experience has
   already shown.  However, there is increasing confusion as to what
   exactly SDN is, what is the layer structure in an SDN architecture
   and how do layers interface with each other.  This document aims to
   answer these questions and provide a concise reference document for
   SDNRG, in particular, and the SDN community, in general, based on
   relevant peer-reviewed literature and documents in the RFC series.

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/.







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   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 June 8, 2014.

Copyright Notice

   Copyright (c) 2013 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
   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
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  SDN Layers and Architecture . . . . . . . . . . . . . . . . .   5
     2.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  Network Devices . . . . . . . . . . . . . . . . . . . . .   8
     2.3.  Control Plane . . . . . . . . . . . . . . . . . . . . . .   9
     2.4.  Management Plane  . . . . . . . . . . . . . . . . . . . .  10
     2.5.  Service Abstraction Layer . . . . . . . . . . . . . . . .  11
     2.6.  Application Plane . . . . . . . . . . . . . . . . . . . .  11
   3.  SDN Model View  . . . . . . . . . . . . . . . . . . . . . . .  12
     3.1.  ForCES  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     3.2.  NETCONF . . . . . . . . . . . . . . . . . . . . . . . . .  13
     3.3.  OpenFlow  . . . . . . . . . . . . . . . . . . . . . . . .  14
     3.4.  I2RS  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
     3.5.  BFD . . . . . . . . . . . . . . . . . . . . . . . . . . .  15
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   7.  Informative References  . . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction





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   Software-Defined Networking (SDN) is a relevant new term for the
   programmable networks paradigm [PNSurvey99][OF08].  In short, SDN
   refers to the ability to use software to program individual network
   devices dynamically and therefore control the behavior of the network
   as a whole [NV09].  A key element in SDN is the introduction of an
   abstraction between the (traditional) Forwarding and the Control
   planes in order to separate them and provide applications with the
   means necessary to programmatically control the network.  The goal is
   to leverage on this separation, and the associated programmability,
   in order to reduce complexity and enable faster innovation at both
   planes [A4D05].

   Current and earlier research in SDN often focuses on varying aspects
   of programmability, and we are frequently confronted with conflicting
   points of view regarding what exactly SDN is.  For instance, we find
   that for various reasons (e.g. work focusing on one domain and
   therefore not necessarily applicable as-is to other domains), certain
   well-accepted definitions do not correlate well with each other.  For
   example, both OpenFlow [OpenFlow] and NETCONF [RFC6241] have been
   characterized as SDN interfaces, but they refer to control and
   management respectively.

   This motivates us to consolidate the definitions of SDN in the
   literature and correlate them with earlier work in IETF and the
   research community.  Of particular interest, for example, is to
   determine which layers comprise the SDN architecture and which
   interfaces and their corresponding attributes are best suitable to be
   used between them.  As such, the aim of this document is not to
   standardize any particular layer or interface but rather to provide a
   concise reference document which reflects current approaches
   regarding the SDN layers architecture.  We expect that this document
   would be useful to upcoming work in SDNRG as well as future
   discussions within the SDN community as a whole.

   This document aims to address the potential work item in the SDNRG
   charter named "Survey of SDN approaches and Taxonomies", fostering
   better understanding of prominent SDN technologies in an technology-
   impartial and business-agnostic manner.  As such we do not make any
   value statements nor discuss the applicability of any of the
   frameworks examined for any particular purpose.  Instead, we document
   their characteristics and attributes and classify them thus providing
   a taxonomy.

   This document does not constitute a new IETF standard nor a new
   specification, and aims to receive rough consensus within SDNRG to be
   published in the IRTF Stream as per [RFC5743].





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   The remainder of this document is organized as follows.  Section 1.1
   explains the terminology used in this document.  Figure 1 introduces
   a high-level overview of current SDN architecture abstractions.
   Finally, Section 3 discusses how the SDN Layer Architecture relates
   with prominent SDN-enabling technologies

1.1.  Terminology

   This document uses the following terms:

      Software-Defined Networking (SDN) - A programmable networks
      approach that supports the separation of Control and Forwarding
      Planes via standardized interfaces.

      Network Device - A device that performs one or more network
      operations related to packet manipulation and forwarding.  This
      reference model makes no distinction whether a network device is
      physical or virtual.

      Interface - A point of interaction between two entities.  In case
      the entities are not in the same physical location, the interface
      is usually implemented as a network protocol.  In case the
      entities are collocated in the same physical location the
      interface can be a protocol or an open/proprietary software inter-
      process communication API.

      Application (App) - A piece of software that utilizes underlying
      services to perform a function.  Application operation can be
      parametrized, typically by passing certain arguments at call time,
      but it is meant to be a standalone piece of software and it does
      not offer any interfaces to other applications or services.

      Service - A piece of software that performs one or more functions
      and provides one or more APIs to applications or other services of
      the same or different layers to make use of said functions and
      returns one or more results.  Services can be combined with other
      services, or called in a certain serialized manner, to create a
      new service.

      Forwarding Plane (FP) - The network device part responsible for
      forwarding traffic.

      Operational Plane (OP) - The network device part responsible for
      managing the overall device operation.

      Control Plane (CP) - Part of the network functionality that is
      assigned to control one or more network devices.  CP instructs
      network devices with respect to how to treat and forward packets.



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      The control plane interacts primarily with the forwarding plane
      and less with the operational plane.

      Management Plane (MP) - Part of the network functionality
      responsible for monitoring, configuring and maintaining one or
      more network devices.  The management plane is mostly related with
      the operational plane and less with the forwarding plane.

      Device and resource Abstraction Layer (DAL) - The device's
      resource abstraction layer based on one or more models.  If it is
      a physical device it may be referred to as the Hardware
      Abstraction Layer (HAL).  DAL provides a uniform point of
      reference for the device's forwarding and operational resources.

      Control Abstraction Layer (CAL) - The control plane's abstraction
      layer.  CAL provides access to the control plane southbound
      interface.

      Management Abstraction Layer (MAL) - The management plane's
      abstraction layer.  MAL provides access to the management plane
      southbound interface.

2.  SDN Layers and Architecture

   Figure 1 provides a detailed high-level overview of the current SDN
   architecture abstractions.  Note that planes can be collocated with
   other planes or can be physically separated, as we discuss below.

   SDN is based on the concept of separation between a controlled entity
   and a controller entity.  The controller manipulates the controlled
   entity via an Interface.  Interfaces, when local, are mostly API
   calls through some library or system call.  However, such interfaces
   may be extended via some protocol definition (which may be local IPC
   or protocol that could also act remotely; the protocol may be defined
   as an open standard or in proprietary manner).

                 o--------------------------------o
                 |                                |
                 | +-------------+   +----------+ |
                 | | Application |   |  Service | |
                 | +-------------+   +----------+ |
                 |       Application Plane        |
                 o---------------Y----------------o
                                 |
   *-----------------------------Y---------------------------------*
   |               Service Abstraction Layer (SAL)                 |
   *------Y------------------------------------------------Y-------*
          |                                                |



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          |               Service Interface                |
          |                                                |
   o------Y------------------o       o---------------------Y------o
   |      |    Control Plane |       | Management Plane    |      |
   | +----Y----+   +-----+   |       |  +-----+       +----Y----+ |
   | | Service |   | App |   |       |  | App |       | Service | |
   | +----Y----+   +--Y--+   |       |  +--Y--+       +----Y----+ |
   |      |           |      |       |     |               |      |
   | *----Y-----------Y----* |       | *---Y---------------Y----* |
   | | Control Abstraction | |       | | Management Abstraction | |
   | |     Layer (CAL)     | |       | |      Layer (MAL)       | |
   | *----------Y----------* |       | *----------Y-------------* |
   |            |            |       |            |               |
   o------------|------------o       o------------|---------------o
                |                                 |
                | CP                              | MP
                | Southbound                      | Southbound
                | Interface                       | Interface
                |                                 |
   *------------Y---------------------------------Y----------------*
   |         Device and resource Abstraction Layer (DAL)           |
   *------------Y---------------------------------Y----------------*
   |            |                                 |                |
   |    o-------Y----------o   +-----+   o--------Y----------o     |
   |    | Forwarding Plane |   | App |   | Operational Plane |     |
   |    o------------------o   +-----+   o-------------------o     |
   |                       Network Device                          |
   +---------------------------------------------------------------+

                     Figure 1: SDN Layer Architecture

2.1.  Overview

   This document follows a network device centric approach: Control
   refers to the device's packet handling while Management refers to the
   device's operation.  The reader should keep in mind throughout this
   document that we make no distinction between "physical" and "virtual"
   network devices, as we do not delve into implementation or
   performance aspects.  In other words, a network device can be
   implemented fully in hardware, fully in software, or any hybrid
   combination in between.  Similarly, network device software can run
   on "bare metal" or on a virtualized substrate.  Finally, we do not
   distinguish on whether a device is implemented as an overlay or as a
   part/component of some other device.

   SDN spans multiple planes as illustrated in Figure 1.  Starting from
   the bottom part of the figure and moving towards the upper part, we
   identify the following planes:



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   o  Forwarding Plane - Responsible for handling packets in the
      datapath.  Actions of the forwarding plane include, but are not
      limited to, forwarding, dropping and changing packets.  The
      forwarding plane is usually the termination point for control
      plane services and applications.  The forwarding plane can contain
      forwarding resources such as classifiers.

   o  Operational Plane - Responsible for managing the operational state
      of the Network Device, e.g. active/inactive, number of ports, port
      status, etc.  The Operational Plane is usually the termination
      point for management plane services and applications.  The
      operational plane relates to (operational aspects of) Network
      Device resources such as ports, memory, and so on.

   o  Control Plane - Responsible for taking decisions on how packets
      should be forwarded by one or more Network Devices and pushing
      such decisions down to the Network Devices to be executed.  The
      control plane usually focuses mostly on the forwarding plane and
      less on the operational plane of the device.  The control plane
      may be interested in operational plane information which could
      include, for example, the current state of a particular port or
      its capabilities.  The control plane job is to fine tune the
      forwarding plane.

   o  Management Plane - Responsible for monitoring, configuring and
      maintaining network devices, e.g. taking decisions regarding the
      state of a Network Device.  The management plane usually focuses
      mostly on the operational plane on the device and less on the
      forwarding plane.  The management plane may be used to configure
      the forwarding plane, but it does so infrequently and in a more
      wholesale approach than the control plane.  For instance, the
      management plane may set up all or part of the forwarding rules at
      once, although such action would be expected to be used sparingly.

   o  Application Plane - The plane where applications and services
      reside.  Note that applications may be implemented in a modular
      fashion and therefore can often span multiple planes in Figure 1.

   All planes mentioned above are connected via Interfaces (as indicated
   with "Y" in Figure 1.  An Interface may take multiple roles depending
   on whether the connected planes reside on the same (physical or
   virtual) device.  If the respective planes are designed so that they
   do not have to reside in the same device, then the Interface can only
   take the form of a protocol.  If the planes are co-located on the
   same device, then the Interface could either be an open/proprietary
   protocol, an open/proprietary software inter-process communication
   API, or operating system kernel system calls.




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   Applications, i.e. software programs that perform specific
   computations that consume services without providing access to other
   applications, can be implemented natively inside a plane or can span
   multiple planes.

   Services, i.e. software programs that provide APIs to other
   applications or services, can also be natively implemented in
   specific planes.  Services that span multiple planes belong to the
   application plane as well.

   While not shown in Figure 1, services, applications or even planes,
   can be placed in a recursive manner thus providing overlay semantics
   to the model.

   Additionally, this document considers four abstraction layers:

      The Device and resource Abstraction Layer (DAL) abstracts the
      device's forwarding and operational plane resources to the control
      and management plane, respectively.  Variations of DAL may
      abstract both planes or either of the two.

      The Control Abstraction Layer (CAL) abstracts the CP southbound
      interface and the DAL from the applications and services of the
      Control Plane.

      The Management Abstraction Layer (MAL) abstracts the MP southbound
      interface and the DAL from the applications and services of the
      Management Plane.

      The Service Abstraction Layer (SAL) provides service abstractions
      for use by applications and other services.

2.2.  Network Devices

   A Network Device is an entity that receives packets on its ports and
   performs one or more network functions on them.  For example, the
   network device could forward a received packet, drop it, alter the
   packet header (or payload) and forward the packet, and so on.  NDs
   can be implemented in hardware or software and can be either a
   physical or virtual network element.  As mentioned above, this
   document makes no distinction between these.  Each network device has
   both a Forwarding Plane and an Operational Plane.

   The Forwarding Plane, commonly referred to as the "data path", is
   responsible for handling and forwarding packets.  The Forwarding
   Plane provides switching, routing transformation and filtering
   functions.  Resources of the forwarding plane include but are not
   limited to filters, meters, markers and classifiers.



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   The Operational Plane is responsible for operational state of the ND,
   for example, with respect to status of network ports and interfaces.
   Resources of the operational plane include but are not limited to
   memory, CPU, ports, interfaces and queues.

   The Forwarding and the Operational Planes can be exposed via a Device
   and resource Abstraction Layer (DAL), which may be comprised of one
   or more abstraction models.  Examples of Forwarding Plane abstraction
   models are the ForCES model [RFC5812] and the OpenFlow switch model
   [OpenFlow].  Examples of the Operational Plane abstraction model
   include the ForCES model [RFC5812], the YANG model [RFC6020] and SNMP
   MIBs [RFC3418].

   Examples of Network Devices include switches and routers.  Additional
   examples include network elements that may operate at a layer above
   IP, such as firewalls, load balancers and video transcoders.

   Note that applications can also reside in a network device.  Examples
   of such applications include event monitoring, and handling
   (offloading) topology discovery or ARP [RFC0826] in the device itself
   instead of forwarding such traffic to the control plane.

2.3.  Control Plane

   The Control plane is usually distributed and is responsible mainly
   for the configuration of the Forwarding Plane using a Control Plane
   Southbound Interface (CPSI) with DAL as a point of reference.  The
   Control Plane is responsible for instructing the Forwarding Plane
   about how to handle network packets.

   Control Plane functionalities usually include:

   o  Topology discovery and maintenance

   o  Packet route selection and instantiation

   o  Path failover mechanisms

   The CPSI is usually defined with the following characteristics:

   o  As a time-critical interface which requires low latency and
      sometimes high bandwidth in order to perform many operations in
      short order.

   o  Oriented towards wire efficiency and device representation instead
      of human readability





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   Examples include fast and high frequency of flow or table updates,
   high throughput and robustness for packet handling and events.

   CPSI can be implemented using a protocol, an API or even interprocess
   communication.  If the Control Plane and the Network Device are not
   collocated, then this interface is certainly a protocol.  Examples of
   CPSIs are ForCES [RFC5810] and the Openflow protocol [OpenFlow].

   The Control Abstraction Layer (CAL) provides access to control
   applications and services to various CPSIs.  The Control Plane may
   support more than one CPSIs.

   Control applications can use CAL to control a network device without
   providing any service to upper layers.  Examples include applications
   that perform control functions, such as OSPF, BGP, etc.

   Control Plane Services examples include a virtual private LAN
   service, service tunnels, topology services, etc.

2.4.  Management Plane

   The Management Plane is usually centralized and aims to ensure that
   the network, which consists of network devices, is running optimally
   by communicating with the network devices's Operational Plane using a
   Management Plane Southbound Interface (MPSI) with DAL as a point of
   reference.

   Management plane functionalities are typically initiated, based on an
   overall network view, and traditionally have been human-centric.
   However, lately algorithms are replacing most human intervention.
   Management plane functionalities [FCAPS] [RFC3535] usually include:

   o  Fault and Monitoring management

   o  Configuration management

   Normally MSPI, in contrast to the CPSI, is not a time-critical
   interface and does not share the CPSI requirements.

   MSPI is [RFC3535] typically closer to human interaction than the
   control plane and therefore the MSPI usually has the following
   characteristics:

   o  It is oriented more towards usability, with optimal wire
      performance being a secondary concern.

   o  Messages tend to be less frequent than in the CPSI




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   As an example of usability versus performance, we refer to the
   consensus of the 2002 IAB Workshop [RFC3535], as mentioned in
   [RFC6632], where textual configuration files should be able to
   contain international characters.  Human-readable strings should
   utilize UTF-8, and protocol elements should be in case-insensitive
   ASCII which require more processing capabilities to parse.

   The MPSI can range from a protocol, to an API or even interprocess
   communication.  If the Management Plane is not embedded in the
   network device, the MSPI is certainly a protocol.  Examples of MPSIs
   are ForCES [RFC5810], NETCONF [RFC6241], OVSDB
   [I-D.pfaff-ovsdb-proto] and SNMP [RFC3411].

   The Management Abstraction Layer (MAL) provides access to management
   applications and services to various MPSIs.  The Management Plane may
   support more than one MPSI.

   Management Applications can use MAL to manage the network device
   without providing any service to upper layers.  Examples of
   management applications include network monitoring and fault
   detection and recovery applications.

   Management Plane Services provide access to other services or
   applications above the Management Plane.

2.5.  Service Abstraction Layer

   The Service Abstraction Layer (SAL) provides access from services of
   the control, management and application planes to services and
   applications of the application plane.  We note that the term (as
   well as the acronym) is overloaded, as it is often used in several
   contexts ranging from system design to service-oriented
   architectures.  We emphasize that this term relates to Figure 1 and
   we map it accordingly in Section 3 to prominent SDN approaches.

   Service Interfaces can take many forms pertaining to their specific
   requirements.  Examples of service interfaces include but are not
   limited to, RESTful APIs, open or proprietary protocols such as
   NETCONF, inter-process communications, CORBA interfaces, etc.

2.6.  Application Plane

   Applications and services that use services from the control and/or
   management plane form the Application Plane.

   Additionally, services residing in the Application Plane may provide
   services to other services and applications that reside in the
   application plane via the service interface.



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   Examples of applications include network topology discovery, network
   provisioning, path reservation, etc.

3.  SDN Model View

   We advocate that the SDN southbound interface should encompass both
   CSPI and MSPI.

   The SDN northbound interface is implemented in the Service
   Abstraction Layer of Figure 1.

   The above model can be used to describe in a concise manner all
   prominent SDN-enabling technologies, as we explain in the following
   subsections.

3.1.  ForCES

   The Forwarding and Control Element Separation (ForCES [RFC5810]) is
   an IETF framework consisting of a model and two protocols.  ForCES
   separates the Forwarding from the Control Plane via an open
   interface, namely the ForCES protocol which operates on entities of
   the forwarding plane that have been modeled using the ForCES model.

   The ForCES model is based on the fact that a network element is
   composed of numerous logically separate entities that cooperate to
   provide a given functionality -such as routing or IP switching- and
   yet appear as a normal integrated network element to external
   entities and secondly with a protocol to transport information.

   ForCES models the Forwarding Plane using Logical Functional Blocks
   (LFBs) which are connected in a graph, composing the Forwarding
   Element (FE).  LFBs are described in an XML language, based on an XML
   schema.

   LFB definitions include:

   o  Base and custom-defined datatypes

   o  Metadata definitions

   o  Input and Output ports

   o  Operational parameters, or components

   o  Capabilities

   o  Event definitions




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   The ForCES model can be used to define LFBs from fine- to coarse-
   grained as needed.

   The ForCES protocol is agnostic to the model and can be used to
   monitor, configure and control any ForCES-modeled element.  The
   protocol has very simple commands: Set, Get and Del. ForCES is a
   protocol designed for high throughput and fast updates.

   ForCES [RFC5810] can be mapped to the framework illustrated in Figure
   1 as follows:

   o  The ForCES model can be used to describe DAL, both for the
      Operational and the Forwarding Plane, using LFBs.

   o  The ForCES protocol can then be both the CPSI and the MPSI.
      ForCES is inherently specified for the CPSI and satisfies its
      requirements, however it can also be utilized for the MPSI.

   o  CAL and MAL must be able to utilize the ForCES protocol.

3.2.  NETCONF

   The Network Configuration Protocol (NETCONF [RFC6241]), is an IETF-
   standardized network management protocol [RFC6632].  NETCONF provides
   mechanisms to install, manipulate, and delete the configuration of
   network devices.

   NETCONF protocol operations are realized as remote procedure calls
   (RPCs).  The NETCONF protocol uses an Extensible Markup Language
   (XML) based data encoding for the configuration data as well as the
   protocol messages.

   Additionally, the YANG data modeling language has been developed for
   specifying NETCONF data models and protocol operations.  YANG is a
   data modeling language used to model configuration and state data
   manipulated by NETCONF, NETCONF remote procedure calls, and NETCONF
   notifications.

   YANG models the hierarchical organization of data as a tree, in which
   each node has either a value or a set of child nodes.  Additionally,
   YANG structures data models into modules and submodules allowing
   reusability and augmentation.  YANG models can describe constraints
   to be enforced on the data.  Additionally YANG has a set of base
   datatype and allows custom defined datatypes as well.







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   YANG allows the definition of NETCONF RPCs allowing the protocol to
   have an extensible number of commands.  For RPC definition, the
   operations names, input parameters, and output parameters are defined
   using YANG data definition statements.

   NETCONF can be mapped to the framework illustrated in Figure 1 as
   follows:

   o  The YANG model [RFC6020] is suitable for specifying DAL for the
      operational plane and NETCONF [RFC6241] for the MPSI.

   o  Technically, the YANG model [RFC6020] can be used to specify DAL
      for the Forwarding plane as well, but NETCONF [RFC6241] being a
      management protocol not designed for fast updates is likely not
      suitable for the requirements of CPSI.

3.3.  OpenFlow

   OpenFlow is a framework developed by Standford, currently run by the
   Open Networking Foundation, initially to provide a way for
   researchers to run experimental protocols in the network.  OpenFlow
   provides a protocol with which a controller may manage a static model
   of an OpenFlow switch.

   An OpenFlow Switch consists of one or more flow tables which perform
   packet lookups and forwarding, a group table and an OpenFlow channel
   to an external controller.  The switch communicates with the
   controller which manages the switch via the OpenFlow protocol.

   OpenFlow can be mapped to the framework illustrated in Figure 1 as
   follows:

   o  The Openflow switch specifications [OpenFlow] covers DAL for the
      Forwarding Plane and provides the specification for CPSI.

   o  The OF-CONFIG protocol [OF-CONFIG] based on the YANG model
      [RFC6020], provides DAL for the Operational Plane and specifies
      NETCONF [RFC6241] as the MPSI.  OF-CONFIG overlaps with the
      OpenFlow DAL, but with NETCONF [RFC6241] as the transport protocol
      it shares the limitations described in the previous section.

   o  CAL must be able to utilize the OpenFlow protocol.

   o  MAL must be able to utilize the NETCONF protocol.

3.4.  I2RS





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   I2RS, although still work in progress at the IETF, can be mapped to
   the framework illustrated in Figure 1 as follows:

   o  The I2RS architecture [I-D.ietf-i2rs-architecture] is concerned
      with the Control and Application Planes and uses whatever CPSI and
      DAL are available, whether that is ForCES, OpenFlow or another
      Interface.

   o  The I2RS agent is a Control Plane Service.  All services or
      applications on top of that belong to either the Control or the
      Application plane.

   o  Currently the I2RS working group if developing an Information
      Model [I-D.ietf-i2rs-rib-info-model] in regards to the Service
      Abstraction Layer for the I2RS agent.

3.5.  BFD

   Bidirectional Forwarding Detection (BFD) [RFC5880], is an IETF
   network protocol designed for detecting communication failures
   between two forwarding elements which are directly connected.  It is
   intended to be implemented in some component of the forwarding engine
   of a system, in cases where the forwarding and control engines are
   separated.

   BFD provides low-overhead detection of faults even on physical media
   that do not support failure detection of any kind, such as Ethernet,
   virtual circuits, tunnels and MPLS Label Switched Paths.

   BFD could be mapped to the framework illustrated in Figure 1 either
   as:

   1.  A control plane service or application that would use the CPSI
       towards the forwarding plane to send/receive BFD packets.

   2.  Or, better, as it was intended for, i.e. as an application that
       runs on the device itself and uses the forwarding plane to send/
       receive BFD packets and update the operational plane resources
       accordingly.

4.  Acknowledgements

   The authors would like to acknowledge David Meyer, Salvatore Loreto
   and Sudhir Modali for the initial discussion on the SDNRG mailing
   list as well as their draft-specific comments that helped put this
   document in a better shape.





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   Additionally the authors would like to acknowledge Russ White, Linda
   Dunbar, Robert Raszuk, Pedro Martinez-Julia, Lee Young, Yaakov Stein,
   Shivleela Arlimatti, Gurkan Deniz, Scott Brim, Carlos Pignataro,
   Ramki Krishnan, Bless Roland, Tim Copley, Francisco Javier Ros Munoz,
   Sriganesh Kini, Alan Clark, Erik Nordmark for their critical comments
   and discussions at the IETF 88 meeting (and the SDNRG mailing list),
   which we took into consideration while revising this document.

5.  IANA Considerations

   This memo makes no requests to IANA.

6.  Security Considerations

   TBD

7.  Informative References

   [A4D05]    Greenberg, Albert, et al., "A clean slate 4D approach to
              network control and management", ACM SIGCOMM Computer
              Communication Review 35.5 (2005): 41-54 , 2005.

   [FCAPS]    International Telecommunication Union, "X.700: Management
              Framework For Open Systems Interconnection (OSI) For CCITT
              Applications", September 1992,
              <http://www.itu.int/rec/T-REC-X.700-199209-I/en>.

   [I-D.ietf-i2rs-architecture]
              Atlas, A., Halpern, J., Hares, S., Ward, D., and T.
              Nadeau, "An Architecture for the Interface to the Routing
              System", draft-ietf-i2rs-architecture-00 (work in
              progress), August 2013.

   [I-D.ietf-i2rs-rib-info-model]
              Bahadur, N., Folkes, R., Kini, S., and J. Medved, "Routing
              Information Base Info Model", draft-ietf-i2rs-rib-info-
              model-01 (work in progress), October 2013.

   [I-D.pfaff-ovsdb-proto]
              Pfaff, B. and B. Davie, "The Open vSwitch Database
              Management Protocol", draft-pfaff-ovsdb-proto-04 (work in
              progress), October 2013.

   [NV09]     Chowdhury, NM Mosharaf Kabir, and Raouf Boutaba, "Network
              virtualization: state of the art and research challenges",
              Communications Magazine, IEEE 47.7 (2009): 20-26 , 2009.

   [OF-CONFIG]



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              Open Networking Foundation, "OpenFlow Management and
              Configuration Protocol 1.1.1", March 2013, <https://
              www.opennetworking.org/images/stories/downloads/sdn-
              resources/onf-specifications/openflow-config/of-
              config-1-1-1.pdf>.

   [OF08]     McKeown, Nick, et al., "OpenFlow: enabling innovation in
              campus networks", ACM SIGCOMM Computer Communication
              Review 38.2 (2008): 69-74 , 2008.

   [OpenFlow]
              Open Networking Foundation, "The OpenFlow 1.4
              Specification.", October 2013, <https://
              www.opennetworking.org/images/stories/downloads/sdn-
              resources/onf-specifications/openflow/openflow-
              spec-v1.4.0.pdf>.

   [PNSurvey99]
              Campbell, Andrew T., et al, "A survey of programmable
              networks", ACM SIGCOMM Computer Communication Review 29.2
              (1999): 7-23 , September 1992.

   [RFC0826]  Plummer, D., "Ethernet Address Resolution Protocol: Or
              converting network protocol addresses to 48.bit Ethernet
              address for transmission on Ethernet hardware", STD 37,
              RFC 826, November 1982.

   [RFC3411]  Harrington, D., Presuhn, R., and B. Wijnen, "An
              Architecture for Describing Simple Network Management
              Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
              December 2002.

   [RFC3418]  Presuhn, R., "Management Information Base (MIB) for the
              Simple Network Management Protocol (SNMP)", STD 62, RFC
              3418, December 2002.

   [RFC3535]  Schoenwaelder, J., "Overview of the 2002 IAB Network
              Management Workshop", RFC 3535, May 2003.

   [RFC5743]  Falk, A., "Definition of an Internet Research Task Force
              (IRTF) Document Stream", RFC 5743, December 2009.

   [RFC5810]  Doria, A., Hadi Salim, J., Haas, R., Khosravi, H., Wang,
              W., Dong, L., Gopal, R., and J. Halpern, "Forwarding and
              Control Element Separation (ForCES) Protocol
              Specification", RFC 5810, March 2010.





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   [RFC5812]  Halpern, J. and J. Hadi Salim, "Forwarding and Control
              Element Separation (ForCES) Forwarding Element Model", RFC
              5812, March 2010.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, June 2010.

   [RFC6020]  Bjorklund, M., "YANG - A Data Modeling Language for the
              Network Configuration Protocol (NETCONF)", RFC 6020,
              October 2010.

   [RFC6241]  Enns, R., Bjorklund, M., Schoenwaelder, J., and A.
              Bierman, "Network Configuration Protocol (NETCONF)", RFC
              6241, June 2011.

   [RFC6632]  Ersue, M. and B. Claise, "An Overview of the IETF Network
              Management Standards", RFC 6632, June 2012.

Authors' Addresses

   Evangelos Haleplidis
   University of Patras
   Department of Electrical and Computer Engineering
   Patras  26500
   Greece

   Email: ehalep@ece.upatras.gr


   Spyros Denazis
   University of Patras
   Department of Electrical and Computer Engineering
   Patras  26500
   Greece

   Email: sdena@upatras.gr


   Kostas Pentikousis
   EICT GmbH
   Torgauer Strasse 12-15
   10829 Berlin
   Germany

   Email: k.pentikousis@eict.de






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   Jamal Hadi Salim
   Mojatatu Networks
   Suite 400, 303 Moodie Dr.
   Ottawa, Ontario  K2H 9R4
   Canada

   Email: hadi@mojatatu.com


   David Meyer
   Brocade

   Email: dmm@1-4-5.net


   Odysseas Koufopavlou
   University of Patras
   Department of Electrical and Computer Engineering
   Patras  26500
   Greece

   Email: odysseas@ece.upatras.gr





























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