path: root/doc/rfc/rfc7426.txt

Internet Research Task Force (IRTF)                   E. Haleplidis, Ed.
Request for Comments: 7426                          University of Patras
Category: Informational                              K. Pentikousis, Ed.
ISSN: 2070-1721                                                     EICT
                                                              S. Denazis
                                                    University of Patras
                                                           J. Hadi Salim
                                                       Mojatatu Networks
                                                                D. Meyer
                                                                 Brocade
                                                          O. Koufopavlou
                                                    University of Patras
                                                            January 2015


 Software-Defined Networking (SDN): Layers and Architecture Terminology

Abstract

   Software-Defined Networking (SDN) refers to a new approach for
   network programmability, that is, the capacity to initialize,
   control, change, and manage network behavior dynamically via open
   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 the layer structure is in an SDN
   architecture, and how layers interface with each other.  This
   document, a product of the IRTF Software-Defined Networking Research
   Group (SDNRG), addresses these questions and provides a concise
   reference for the SDN research community based on relevant peer-
   reviewed literature, the RFC series, and relevant documents by other
   standards organizations.

















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Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Research Task Force
   (IRTF).  The IRTF publishes the results of Internet-related research
   and development activities.  These results might not be suitable for
   deployment.  This RFC represents the consensus of the Software-
   Defined Networking Research Group of the Internet Research Task Force
   (IRTF).  Documents approved for publication by the IRSG are not a
   candidate for any level of Internet Standard; see Section 2 of RFC
   5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7426.

Copyright Notice

   Copyright (c) 2015 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.






















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Table of Contents

   1. Introduction ....................................................4
   2. Terminology .....................................................5
   3. SDN Layers and Architecture .....................................7
      3.1. Overview ...................................................9
      3.2. Network Devices ...........................................12
      3.3. Control Plane .............................................13
      3.4. Management Plane ..........................................14
      3.5. Discussion of Control and Management Planes ...............16
           3.5.1. Timescale ..........................................16
           3.5.2. Persistence ........................................16
           3.5.3. Locality ...........................................16
           3.5.4. CAP Theorem Insights ...............................17
      3.6. Network Services Abstraction Layer ........................18
      3.7. Application Plane .........................................19
   4. SDN Model View .................................................19
      4.1. ForCES ....................................................19
      4.2. NETCONF/YANG ..............................................20
      4.3. OpenFlow ..................................................21
      4.4. Interface to the Routing System ...........................21
      4.5. SNMP ......................................................22
      4.6. PCEP ......................................................23
      4.7. BFD .......................................................23
   5. Summary ........................................................24
   6. Security Considerations ........................................24
   7. Informative References .........................................25
   Acknowledgements ..................................................33
   Contributors ......................................................34
   Authors' Addresses ................................................34





















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1.  Introduction

   "Software-Defined Networking (SDN)" is a term of the programmable
   networks paradigm [PNSurvey99] [OF08].  In short, SDN refers to the
   ability of software applications to program individual network
   devices dynamically and therefore control the behavior of the network
   as a whole [NV09].  Boucadair and Jacquenet [RFC7149] point out that
   SDN is a set of techniques used to facilitate the design, delivery,
   and operation of network services in a deterministic, dynamic, and
   scalable manner.

   A key element in SDN is the introduction of an abstraction between
   the (traditional) forwarding and control planes in order to separate
   them and provide applications with the means necessary to
   programmatically control the network.  The goal is to leverage this
   separation, and the associated programmability, in order to reduce
   complexity and enable faster innovation at both planes [A4D05].

   The historical evolution of the research and development area of
   programmable networks is reviewed in detail in [SDNHistory]
   [SDNSurvey], starting with efforts dating back to the 1980s.  As
   documented in [SDNHistory], many of the ideas, concepts, and concerns
   are applicable to the latest research and development in SDN (and SDN
   standardization) and have been under extensive investigation and
   discussion in the research community for quite some time.  For
   example, Rooney, et al. [Tempest] discuss how to allow third-party
   access to the network without jeopardizing network integrity or how
   to accommodate legacy networking solutions in their (then new)
   programmable environment.  Further, the concept of separating the
   control and forwarding planes, which is prominent in SDN, has been
   extensively discussed even prior to 1998 [Tempest] [P1520] in SS7
   networks [ITUSS7], Ipsilon Flow Switching [RFC1953] [RFC2297], and
   ATM [ITUATM].

   SDN research 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 the Network Configuration Protocol (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 at the IETF and the
   research community.  Of particular interest is, for example, to
   determine which layers comprise the SDN architecture and which



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   interfaces and their corresponding attributes are best suited 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 that reflects current approaches regarding the SDN
   layer 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 addresses the work item in the SDNRG charter titled
   "Survey of SDN approaches and Taxonomies", fostering better
   understanding of prominent SDN technologies in a technology-impartial
   and business-agnostic manner but does not constitute a new IETF
   standard.  It is meant as a common base for further discussion.  As
   such, we do not make any value statements nor discuss the
   applicability of any of the frameworks examined in this document for
   any particular purpose.  Instead, we document their characteristics
   and attributes and classify them, thus providing a taxonomy.  This
   document does not intend to provide an exhaustive list of SDN
   research issues; interested readers should consider reviewing
   [SLTSDN] and [SDNACS].  In particular, Jarraya, et al. [SLTSDN]
   provide an overview of SDN-related research topics, e.g., control
   partitioning, which is related to the Consistency, Availability and
   Partitioning (CAP) theorem discussed in Section 3.5.4.

   This document has been extensively reviewed, discussed, and commented
   by the vast majority of SDNRG members, a community that certainly
   exceeds 100 individuals.  It is the consensus of SDNRG that this
   document should be published in the IRTF stream of the RFC series
   [RFC5743].

   The remainder of this document is organized as follows.  Section 2
   explains the terminology used in this document.  Section 3 introduces
   a high-level overview of current SDN architecture abstractions.
   Finally, Section 4 discusses how the SDN layer architecture relates
   to prominent SDN-enabling technologies.

2.  Terminology

   This document uses the following terms:

   o  Software-Defined Networking (SDN) - A programmable networks
      approach that supports the separation of control and forwarding
      planes via standardized interfaces.

   o  Resource - A physical or virtual component available within a
      system.  Resources can be very simple or fine-grained (e.g., a
      port or a queue) or complex, comprised of multiple resources
      (e.g., a network device).



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   o  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.  A device can also be considered as a
      container for resources and can be a resource in itself.

   o  Interface - A point of interaction between two entities.  When the
      entities are placed at different locations, the interface is
      usually implemented through a network protocol.  If the entities
      are collocated in the same physical location, the interface can be
      implemented using a software application programming interface
      (API), inter-process communication (IPC), or a network protocol.

   o  Application (App) - An application in the context of SDN is a
      piece of software that utilizes underlying services to perform a
      function.  Application operation can be parameterized, for
      example, by passing certain arguments at call time, but it is
      meant to be a standalone piece of software; an App does not offer
      any interfaces to other applications or services.

   o  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.

   o  Forwarding Plane (FP) - The collection of resources across all
      network devices responsible for forwarding traffic.

   o  Operational Plane (OP) - The collection of resources responsible
      for managing the overall operation of individual network devices.

   o  Control Plane (CP) - The collection of functions responsible for
      controlling one or more network devices.  CP instructs network
      devices with respect to how to process and forward packets.  The
      control plane interacts primarily with the forwarding plane and,
      to a lesser extent, with the operational plane.

   o  Management Plane (MP) - The collection of functions responsible
      for monitoring, configuring, and maintaining one or more network
      devices or parts of network devices.  The management plane is
      mostly related to the operational plane (it is related less to the
      forwarding plane).

   o  Application Plane - The collection of applications and services
      that program network behavior.




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   o  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-plane
      resources.

   o  Control Abstraction Layer (CAL) - The control plane's abstraction
      layer.  CAL provides access to the Control-Plane Southbound
      Interface.

   o  Management Abstraction Layer (MAL) - The management plane's
      abstraction layer.  MAL provides access to the Management-Plane
      Southbound Interface.

   o  Network Services Abstraction Layer (NSAL) - Provides service
      abstractions that can be used by applications and services.

3.  SDN Layers and Architecture

   Figure 1 summarizes the SDN architecture abstractions in the form of
   a detailed, high-level schematic.  Note that in a particular
   implementation, 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
   invocations through some library or system call.  However, such
   interfaces may be extended via some protocol definition, which may
   use local inter-process communication (IPC) or a protocol that could
   also act remotely; the protocol may be defined as an open standard or
   in a proprietary manner.

   Day [PiNA] explores the use of IPC as the mainstay for the definition
   of recursive network architectures with varying degrees of scope and
   range of operation.  The Recursive InterNetwork Architecture [RINA]
   outlines a recursive network architecture based on IPC that
   capitalizes on repeating patterns and structures.  This document does
   not propose a new architecture -- we simply document previous work
   through a taxonomy.  Although recursion is out of the scope of this
   work, Figure 1 illustrates a hierarchical model in which layers can
   be stacked on top of each other and employed recursively as needed.








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                   o--------------------------------o
                   |                                |
                   | +-------------+   +----------+ |
                   | | Application |   |  Service | |
                   | +-------------+   +----------+ |
                   |       Application Plane        |
                   o---------------Y----------------o
                                   |
     *-----------------------------Y---------------------------------*
     |           Network Services Abstraction Layer (NSAL)           |
     *------Y------------------------------------------------Y-------*
            |                                                |
            |               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









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3.1.  Overview

   This document follows a network-device-centric approach: control
   mostly refers to the device packet-handling capability, while
   management typically refers to aspects of the overall device
   operation.  We view a network device as a complex resource that
   contains and is part of multiple resources similar to [DIOPR].
   Resources can be simple, single components of a network device, for
   example, a port or a queue of the device, and can also be aggregated
   into complex resources, for example, a network card or a complete
   network device.

   The reader should keep in mind that we make no distinction between
   "physical" and "virtual" resources or "hardware" and "software"
   realizations in this document, as we do not delve into implementation
   or performance aspects.  In other words, a resource can be
   implemented fully in hardware, fully in software, or any hybrid
   combination in between.  Further, we do not distinguish whether a
   resource is implemented as an overlay or as a part/component of some
   other device.  In general, network device software can run on so-
   called "bare metal" or on a virtualized substrate.  Finally, this
   document does not discuss how resources are allocated, orchestrated,
   and released.  Indeed, orchestration is out of the scope of this
   document.

   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:

   o  Forwarding Plane - Responsible for handling packets in the data
      path based on the instructions received from the control plane.
      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.  The forwarding plane is also
      widely referred to as the "data plane" or the "data path".

   o  Operational Plane - Responsible for managing the operational state
      of the network device, e.g., whether the device is active or
      inactive, the number of ports available, the status of each port,
      and so on.  The operational plane is usually the termination point
      for management-plane services and applications.  The operational
      plane relates to network device resources such as ports, memory,
      and so on.  We note that some participants of the IRTF SDNRG have
      a different opinion in regards to the definition of the
      operational plane.  That is, one can argue that the operational
      plane does not constitute a "plane" per se, but it is, in



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      practice, an amalgamation of functions on the forwarding plane.
      For others, however, a "plane" allows one to distinguish between
      different areas of operations; therefore, the operational plane is
      included as a "plane" in Figure 1.  We have adopted this latter
      view in this document.

   o  Control Plane - Responsible for making decisions on how packets
      should be forwarded by one or more network devices and pushing
      such decisions down to the network devices for execution.  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 instance, the current state of a particular port or
      its capabilities.  The control plane's main job is to fine-tune
      the forwarding tables that reside in the forwarding plane, based
      on the network topology or external service requests.

   o  Management Plane - Responsible for monitoring, configuring, and
      maintaining network devices, e.g., making decisions regarding the
      state of a network device.  The management plane usually focuses
      mostly on the operational plane of 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 through 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 taken
      sparingly.

   o  Application Plane - The plane where applications and services that
      define network behavior reside.  Applications that directly (or
      primarily) support the operation of the forwarding plane (such as
      routing processes within the control plane) are not considered
      part of the application plane.  Note that applications may be
      implemented in a modular and distributed fashion and, therefore,
      can often span multiple planes in Figure 1.

   [RFC7276] has defined the data, control, and management planes in
   terms of Operations, Administration, and Maintenance (OAM).  This
   document attempts to broaden the terms defined in [RFC7276] in order
   to reflect all aspects of an SDN architecture.

   All planes mentioned above are connected via interfaces (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 collocated on the




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   same device, then the interface could be implemented via an open/
   proprietary protocol, an open/proprietary software inter-process
   communication API, or operating system kernel system calls.

   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.  For instance, applications or services can span
   both the control and management planes and thus be able to use both
   the Control-Plane Southbound Interface (CPSI) and Management-Plane
   Southbound Interface (MPSI), although this is only implicitly
   illustrated in Figure 1.  An example of such a case would be an
   application that uses both [OpenFlow] and [OF-CONFIG].

   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 explicitly in Figure 1, services, applications, and
   entire planes can be placed in a recursive manner, thus providing
   overlay semantics to the model.  For example, application-plane
   services can be provided to other applications or services through
   NSAL.  Additional examples include virtual resources that are
   realized on top of a physical resources and hierarchical control-
   plane controllers [KANDOO].

   Note that the focus in this document is, of course, on the north/
   south communication between entities in different planes.  But this,
   clearly, does not exclude entity communication within any one plane.

   It must be noted, however, that in Figure 1, we present an abstract
   view of the various planes, which is devoid of implementation
   details.  Many implementations in the past have opted for placing the
   management plane on top of the control plane.  This can be
   interpreted as having the control plane acting as a service to the
   management plane.  Further, in many networks, especially in Internet
   routers and Ethernet switches, the control plane has been usually
   implemented as tightly coupled with the network device.  When taken
   as a whole, the control plane has been distributed network-wide.  On
   the other hand, the management plane has been traditionally
   centralized and has been responsible for managing the control plane
   and the devices.  However, with the adoption of SDN principles, this
   distinction is no longer so clear-cut.







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   Additionally, this document considers four abstraction layers:

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

   o  The Control Abstraction Layer (CAL) abstracts the Control-Plane
      Southbound Interface and the DAL from the applications and
      services of the control plane.

   o  The Management Abstraction Layer (MAL) abstracts the Management-
      Plane Southbound Interface and the DAL from the applications and
      services of the management plane.

   o  The Network Services Abstraction Layer (NSAL) provides service
      abstractions for use by applications and other services.

   At the time of this writing, SDN-related activities have begun in
   other SDOs.  For example, at the ITU, work on architectural [ITUSG13]
   and signaling requirements and protocols [ITUSG11] has commenced, but
   the respective study groups have yet to publish their documents, with
   the exception of [ITUY3300].  The views presented in [ITUY3300] as
   well as in [ONFArch] are well aligned with this document.

3.2.  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), forward the packet, and so on.  A network
   device is an aggregation of multiple resources such as ports, CPU,
   memory, and queues.  Resources are either simple or can be aggregated
   to form complex resources that can be viewed as one resource.  The
   network device is in itself a complex resource.  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) or below IP (such
   as Layer 2 switches and optical or microwave network elements).

   Network devices can be implemented in hardware or software and can be
   either physical or virtual.  As has already been mentioned before,
   this document makes no such distinction.  Each network device has a
   presence in a forwarding plane and an operational plane.






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

   The operational plane is responsible for the operational state of the
   network device, for instance, with respect to status of network ports
   and interfaces.  Operational-plane resources include, but are not
   limited to, memory, CPU, ports, interfaces, and queues.

   The forwarding and the operational planes are exposed via the Device
   and resource Abstraction Layer (DAL), which may be expressed by one
   or more abstraction models.  Examples of forwarding-plane abstraction
   models are Forwarding and Control Element Separation (ForCES)
   [RFC5812], OpenFlow [OpenFlow], YANG model [RFC6020], and SNMP MIBs
   [RFC3418].  Examples of the operational-plane abstraction model
   include the ForCES model [RFC5812], the YANG model [RFC6020], and
   SNMP MIBs [RFC3418].

   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.

3.3.  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.  CP is
   responsible for instructing FP about how to handle network packets.

   Communication between control-plane entities, colloquially referred
   to as the "east-west" interface, is usually implemented through
   gateway protocols such as BGP [RFC4271] or other protocols such as
   the Path Computation Element (PCE) Communication Protocol (PCEP)
   [RFC5440].  These corresponding protocol messages are usually
   exchanged in-band and subsequently redirected by the forwarding plane
   to the control plane for further processing.  Examples in this
   category include [RCP], [SoftRouter], and [RouteFlow].

   Control-plane functionalities usually include:

   o  Topology discovery and maintenance

   o  Packet route selection and instantiation

   o  Path failover mechanisms



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   The CPSI is usually defined with the following characteristics:

   o  time-critical interface that 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

   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 inter-
   process 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 CPSI.

   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, IS-IS, and BGP.

   Control-plane service examples include a virtual private LAN service,
   service tunnels, topology services, etc.

3.4.  Management Plane

   The management plane is usually centralized and aims to ensure that
   the network as a whole is running optimally by communicating with the
   network devices' 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] typically include:

   o  Fault and monitoring management

   o  Configuration management

   In addition, management-plane functionalities may also include
   entities such as orchestrators, Virtual Network Function Managers
   (VNF Managers) and Virtualised Infrastructure Managers, as described
   in [NFVArch].  Such entities can use management interfaces to



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   operational-plane resources to request and provision resources for
   virtual functions as well as instruct the instantiation of virtual
   forwarding functions on top of physical forwarding functions.  The
   possibility of a common abstraction model for both SDN and Network
   Function Virtualization (NFV) is explored in [SDNNFV].  Note,
   however, that these are only examples of applications and services in
   the management plane and not formal definitions of entities in this
   document.  As has been noted above, orchestration and therefore the
   definition of any associated entities is out of the scope of this
   document.

   The MPSI, in contrast to the CPSI, is usually not a time-critical
   interface and does not share the CPSI requirements.

   MPSI is typically closer to human interaction than CPSI (cf.
   [RFC3535]); therefore, MPSI 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.

   As an example of usability versus performance, we refer to the
   consensus of the 2002 IAB Workshop [RFC3535]: the key requirement for
   a network management technology is ease of use, not performance.  As
   per [RFC6632], 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 requires more processing capabilities to parse.

   MPSI can range from a protocol, to an API or even inter-process
   communication.  If the management plane is not embedded in the
   network device, the MPSI is certainly a protocol.  Examples of MPSIs
   are ForCES [RFC5810], NETCONF [RFC6241], IP Flow Information Export
   (IPFIX) [RFC7011], Syslog [RFC5424], Open vSwitch Database (OVSDB)
   [RFC7047], 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, fault detection,
   and recovery applications.





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   Management-plane services provide access to other services or
   applications above the management plane.

3.5.  Discussion of Control and Management Planes

   The definition of a clear distinction between "control" and
   "management" in the context of SDN received significant community
   attention during the preparation of this document.  We observed that
   the role of the management plane has been earlier largely ignored or
   specified as out-of-scope for the SDN ecosystem.  In the remainder of
   this subsection, we summarize the characteristics that differentiate
   the two planes in order to have a clear understanding of the
   mechanics, capabilities, and needs of each respective interface.

3.5.1.  Timescale

   A point has been raised regarding the reference timescales for the
   control and management planes regarding how fast the respective plane
   is required to react to, or how fast it needs to manipulate, the
   forwarding or operational plane of the device.  In general, the
   control plane needs to send updates "often", which translates roughly
   to a range of milliseconds; that requires high-bandwidth and low-
   latency links.  In contrast, the management plane reacts generally at
   longer time frames, i.e., minutes, hours, or even days; thus, wire
   efficiency is not always a critical concern.  A good example of this
   is the case of changing the configuration state of the device.

3.5.2.  Persistence

   Another distinction between the control and management planes relates
   to state persistence.  A state is considered ephemeral if it has a
   very limited lifespan and is not deemed necessary to be stored on
   non-volatile memory.  A good example is determining routing, which is
   usually associated with the control plane.  On the other hand, a
   persistent state has an extended lifespan that may range from hours
   to days and months, is meant to be used beyond the lifetime of the
   process that created it, and is thus used across device reboots.
   Persistent state is usually associated with the management plane.

3.5.3.  Locality

   As mentioned earlier, traditionally, the control plane has been
   executed locally on the network device and is distributed in nature
   whilst the management plane is usually executed in a centralized
   manner, remotely from the device.  However, with the advent of SDN
   centralizing, or "logically centralizing", the controller tends to
   muddle the distinction of the control and management plane based on
   locality.



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3.5.4.  CAP Theorem Insights

   The CAP theorem views a distributed computing system as composed of
   multiple computational resources (i.e., CPU, memory, storage) that
   are connected via a communications network and together perform a
   task.  The theorem, or conjecture by some, identifies three
   characteristics of distributed systems that are universally
   desirable:

   o  Consistency, meaning that the system responds identically to a
      query no matter which node receives the request (or does not
      respond at all).

   o  Availability, i.e., that the system always responds to a request
      (although the response may not be consistent or correct).

   o  Partition tolerance, namely that the system continues to function
      even when specific nodes or the communications network fail.

   In 2000, Eric Brewer [CAPBR] conjectured that a distributed system
   can satisfy any two of these guarantees at the same time but not all
   three.  This conjecture was later proven by Gilbert and Lynch [CAPGL]
   and is now usually referred to as the CAP theorem [CAPFN].

   Forwarding a packet through a network correctly is a computational
   problem.  One of the major abstractions that SDN posits is that all
   network elements are computational resources that perform the simple
   computational task of inspecting fields in an incoming packet and
   deciding how to forward it.  Since the task of forwarding a packet
   from network ingress to network egress is obviously carried out by a
   large number of forwarding elements, the network of forwarding
   devices is a distributed computational system.  Hence, the CAP
   theorem applies to forwarding of packets.

   In the context of the CAP theorem, if one considers partition
   tolerance of paramount importance, traditional control-plane
   operations are usually local and fast (available), while management-
   plane operations are usually centralized (consistent) and may be
   slow.

   The CAP theorem also provides insights into SDN architectures.  For
   example, a centralized SDN controller acts as a consistent global
   database and specific SDN mechanisms ensure that a packet entering
   the network is handled consistently by all SDN switches.  The issue
   of tolerance to loss of connectivity to the controller is not
   addressed by the basic SDN model.  When an SDN switch cannot reach
   its controller, the flow will be unavailable until the connection is
   restored.  The use of multiple non-collocated SDN controllers has



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   been proposed (e.g., by configuring the SDN switch with a list of
   controllers); this may improve partition tolerance but at the cost of
   loss of absolute consistency.  Panda, et al. [CAPFN] provide a first
   exploration of how the CAP theorem applies to SDN.

3.6.  Network Services Abstraction Layer

   The Network Services Abstraction Layer (NSAL) provides access from
   services of the control, management, and application planes to other
   services and applications.  We note that the term "SAL" is
   overloaded, as it is often used in several contexts ranging from
   system design to service-oriented architectures; therefore, we
   explicitly add "Network" to the title of this layer to emphasize that
   this term relates to Figure 1, and we map it accordingly in Section 4
   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 protocols such as NETCONF, inter-
   process communication, CORBA [CORBA] interfaces, and so on.  The two
   leading approaches for service interfaces are RESTful interfaces and
   Remote Procedure Call (RPC) interfaces.  Both follow a client-server
   architecture and use XML or JSON to pass messages, but each has some
   slightly different characteristics.

   RESTful interfaces, designed according to the representational state
   transfer design paradigm [REST], have the following characteristics:

   o  Resource identification - Individual resources are identified
      using a resource identifier, for example, a URI.

   o  Manipulation of resources through representations - Resources are
      represented in a format like JSON, XML, or HTML.

   o  Self-descriptive messages - Each message has enough information to
      describe how the message is to be processed.

   o  Hypermedia as the engine of application state - A client needs no
      prior knowledge of how to interact with a server, as the API is
      not fixed but dynamically provided by the server.

   Remote procedure calls (RPCs) [RFC5531], e.g., XML-RPC and the like,
   have the following characteristics:

   o  Individual procedures are identified using an identifier.

   o  A client needs to know the procedure name and the associated
      parameters.



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3.7.  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.

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

4.  SDN Model View

   We advocate that the SDN southbound interface should encompass both
   CPSI and MPSI.

   SDN controllers such as [NOX] and [Beacon] are a collection of
   control-plane applications and services that implement a CPSI ([NOX]
   and [Beacon] both use OpenFlow) and provide a northbound interface
   for applications.  The SDN northbound interface for controllers is
   implemented in the Network Services Abstraction Layer (NSAL) of
   Figure 1.

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

4.1.  ForCES

   The IETF Forwarding and Control Element Separation (ForCES) framework
   [RFC3746] consists of one model and two protocols.  ForCES separates
   the forwarding plane from the control plane via an open interface,
   namely the ForCES protocol [RFC5810], which operates on entities of
   the forwarding plane that have been modeled using the ForCES model
   [RFC5812].

   The ForCES model [RFC5812] 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.

   ForCES models the forwarding plane using Logical Functional Blocks
   (LFBs), which, when connected in a graph, compose the Forwarding
   Element (FE).  LFBs are described in XML, based on an XML schema.





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   LFB definitions include base and custom-defined datatypes; metadata
   definitions; input and output ports; operational parameters or
   components; and capabilities and event definitions.

   The ForCES model can be used to define LFBs from fine- to coarse-
   grained as needed, irrespective of whether they are physical or
   virtual.

   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(ete).  The
   ForCES protocol has been designed for high throughput and fast
   updates.

   With respect to Figure 1, the ForCES model [RFC5812] is suitable for
   the DAL, both for the operational and the forwarding plane, using
   LFBs.  The ForCES protocol [RFC5810] has been designed and is
   suitable for the CPSI, although it could also be utilized for the
   MPSI.

4.2.  NETCONF/YANG

   The Network Configuration Protocol (NETCONF) [RFC6241] is an IETF
   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 XML-based data encoding for the
   configuration data as well as the protocol messages.  Recent studies,
   such as [ESNet] and [PENet], have shown that NETCONF performs better
   than SNMP [RFC3411].

   Additionally, the YANG data modeling language [RFC6020] 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 the NETCONF protocol, 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
   datatypes and allows custom-defined datatypes as well.






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

   With respect to Figure 1, the YANG model [RFC6020] is suitable for
   specifying DAL for the forwarding and operational planes.  NETCONF
   [RFC6241] is suitable for the MPSI.  NETCONF is a management protocol
   [RFC6632], which was not (originally) designed for fast CP updates,
   and it might not be suitable for addressing the requirements of CPSI.

4.3.  OpenFlow

   OpenFlow is a framework originally developed at Stanford University
   and currently under active standards development [OpenFlow] through
   the Open Networking Foundation (ONF).  Initially, the goal was to
   provide a way for researchers to run experimental protocols in a
   production network [OF08].  OpenFlow has undergone many revisions,
   and additional revisions are likely.  The following description
   reflects version 1.4 [OpenFlow].  In short, OpenFlow defines a
   protocol through which a logically centralized controller can control
   an OpenFlow switch.  Each OpenFlow-compliant switch maintains one or
   more flow tables, which are used to perform packet lookups.  Distinct
   actions are to be taken regarding packet lookup and forwarding.  A
   group table and an OpenFlow channel to external controllers are also
   part of the switch specification.

   With respect to Figure 1, the OpenFlow switch specifications
   [OpenFlow] define a DAL for the forwarding plane as well as for CPSI.
   The OF-CONFIG protocol [OF-CONFIG], based on the YANG model
   [RFC6020], provides a DAL for the forwarding and operational planes
   of an OpenFlow switch 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.

4.4.  Interface to the Routing System

   Interface to the Routing System (I2RS) provides a standard interface
   to the routing system for real-time or event-driven interaction
   through a collection of protocol-based control or management
   interfaces.  Essentially, one of the main goals of I2RS, is to make
   the Routing Information Base (RIB) programmable, thus enabling new
   kinds of network provisioning and operation.

   I2RS did not initially intend to create new interfaces but rather
   leverage or extend existing ones and define informational models for
   the routing system.  For example, the latest I2RS problem statement



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   [I2RSProb] discusses previously defined IETF protocols such as ForCES
   [RFC5810], NETCONF [RFC6241], and SNMP [RFC3417].  Regarding the
   definition of informational and data models, the I2RS working group
   has opted to use the YANG [RFC6020] modeling language.

   Currently the I2RS working group is developing an Information Model
   [I2RSInfo] in regards to the Network Services Abstraction Layer for
   the I2RS agent.

   With respect to Figure 1, the I2RS architecture [I2RSArch]
   encompasses the control and application planes and uses any CPSI and
   DAL that is available, whether that may be ForCES [RFC5810], OpenFlow
   [OpenFlow], or another interface.  In addition, the I2RS agent is a
   control-plane service.  All services or applications on top of that
   belong to either the Control, Management, or Application plane.  In
   the I2RS documents, management access to the agent may be provided by
   management protocols like SNMP and NETCONF.  The I2RS protocol may
   also be mapped to the service interface as it will even provide
   access to services and applications other than control-plane services
   and applications.

4.5.  SNMP

   The Simple Network Management Protocol (SNMP) is an IETF-standardized
   management protocol and is currently at its third revision (SNMPv3)
   [RFC3417] [RFC3412] [RFC3414].  It consists of a set of standards for
   network management, including an application-layer protocol, a
   database schema, and a set of data objects.  SNMP exposes management
   data (managed objects) in the form of variables on the managed
   systems, which describe the system configuration.  These variables
   can then be queried and set by managing applications.

   SNMP uses an extensible design for describing data, defined by
   Management Information Bases (MIBs).  MIBs describe the structure of
   the management data of a device subsystem.  MIBs use a hierarchical
   namespace containing object identifiers (OIDs).  Each OID identifies
   a variable that can be read or set via SNMP.  MIBs use the notation
   defined by Structure of Management Information Version 2 [RFC2578].

   An early example of SNMP in the context of SDN is discussed in
   [Peregrine].

   With respect to Figure 1, SNMP MIBs can be used to describe DAL for
   the forwarding and operational planes.  Similar to YANG, SNMP MIBs
   are able to describe DAL for the forwarding plane.  SNMP, similar to
   NETCONF, is suited for the MPSI.





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4.6.  PCEP

   The Path Computation Element (PCE) [RFC4655] architecture defines an
   entity capable of computing paths for a single service or a set of
   services.  A PCE might be a network node, network management station,
   or dedicated computational platform that is resource-aware and has
   the ability to consider multiple constraints for a variety of path
   computation problems and switching technologies.  The PCE
   Communication Protocol (PCEP) [RFC5440] is used between a Path
   Computation Client (PCC) and a PCE, or between multiple PCEs.

   The PCE architecture represents a vision of networks that separates
   path computation for services, the signaling of end-to-end
   connections, and actual packet forwarding.  The definition of online
   and offline path computation is dependent on the reachability of the
   PCE from network and Network Management System (NMS) nodes and the
   type of optimization request that may significantly impact the
   optimization response time from the PCE to the PCC.

   The PCEP messaging mechanism facilitates the specification of
   computation endpoints (source and destination node addresses),
   objective functions (requested algorithm and optimization criteria),
   and the associated constraints such as traffic parameters (e.g.,
   requested bandwidth), the switching capability, and encoding type.

   With respect to Figure 1, PCE is a control-plane service that
   provides services for control-plane applications.  PCEP may be used
   as an east-west interface between PCEs that may act as domain control
   entities (services and applications).  The PCE working group is
   specifying extensions [PCEActive] that allow an active PCE to
   control, using PCEP, MPLS or GMPLS Label Switched Paths (LSPs), thus
   making it applicable for the CPSI for MPLS and GMPLS switches.

4.7.  BFD

   Bidirectional Forwarding Detection (BFD) [RFC5880] is an IETF-
   standardized network protocol designed for detecting path failures
   between two forwarding elements, including physical interfaces,
   subinterfaces, data link(s), and, to the extent possible, the
   forwarding engines themselves, with potentially very low latency.
   BFD can provide low-overhead failure detection on any kind of path
   between systems, including direct physical links, virtual circuits,
   tunnels, MPLS LSPs, multihop routed paths, and unidirectional links
   where there exists a return path as well.  It is often implemented in
   some component of the forwarding engine of a system, in cases where
   the forwarding and control engines are separated.





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   With respect to Figure 1, a BFD agent can be implemented as a
   control-plane service or application that would use the CPSI towards
   the forwarding plane to send/receive BFD packets.  However, a BFD
   agent is usually implemented as an application on the device and uses
   the forwarding plane to send/receive BFD packets and update the
   operational-plane resources accordingly.  Services and applications
   of the control and management planes that monitor or have subscribed
   to changes of resources can learn about these changes through their
   respective interfaces and take any actions as necessary.

5.  Summary

   This document has been developed after a thorough and detailed
   analysis of related peer-reviewed literature, the RFC series, and
   documents produced by other relevant standards organizations.  It has
   been reviewed publicly by the wider SDN community, and we hope that
   it can serve as a handy tool for network researchers, engineers, and
   practitioners in the years to come.

   We conclude this document with a brief summary of the terminology of
   the SDN layer architecture.  In general, we consider a network
   element as a composition of resources.  Each network element has a
   forwarding plane (FP) that is responsible for handling packets in the
   data path and an operational plane (OP) that is responsible for
   managing the operational state of the device.  Resources in the
   network element are abstracted by the Device and resource Abstraction
   Layer (DAL) to be controlled and managed by services or applications
   that belong to the control or management plane.  The control plane
   (CP) is responsible for making decisions on how packets should be
   forwarded.  The management plane (MP) is responsible for monitoring,
   configuring, and maintaining network devices.  Service interfaces are
   abstracted by the Network Services Abstraction Layer (NSAL), where
   other network applications or services may use them.  The taxonomy
   introduced in this document defines distinct SDN planes, abstraction
   layers, and interfaces; it aims to clarify SDN terminology and
   establish commonly accepted reference definitions across the SDN
   community, irrespective of specific implementation choices.

6.  Security Considerations

   This document does not propose a new network architecture or protocol
   and therefore does not have any impact on the security of the
   Internet.  That said, security is paramount in networking; thus, it
   should be given full consideration when designing a network
   architecture or operational deployment.  Security in SDN is discussed
   in the literature, for example, in [SDNSecurity], [SDNSecServ], and





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   [SDNSecOF].  Security considerations regarding specific interfaces
   (such as, for example, ForCES, I2RS, SNMP, or NETCONF) are addressed
   in their respective documents as well as in [RFC7149].

7.  Informative References

   [A4D05]       Greenberg, A., Hjalmtysson, G., Maltz, D., Myers, A.,
                 Rexford, J., Xie, G., Yan, H., Zhan, J., and H. Zhang,
                 "A Clean Slate 4D Approach to Network Control and
                 Management", ACM SIGCOMM Computer Communication Review,
                 Volume 35, Issue 5, pp. 41-54, 2005.

   [ALIEN]       Parniewicz, D., Corin, R., Ogrodowczyk, L., Fard, M.,
                 Matias, J., Gerola, M., Fuentes, V., Toseef, U.,
                 Zaalouk, A., Belter, B., Jacob, E., and K. Pentikousis,
                 "Design and Implementation of an OpenFlow Hardware
                 Abstraction Layer", In Proceedings of the ACM SIGCOMM
                 Workshop on Distributed Cloud Computing (DCC), Chicago,
                 Illinois, USA, pp. 71-76, doi 10.1145/2627566.2627577,
                 August 2014.

   [Beacon]      Erickson, D., "The Beacon OpenFlow Controller", In
                 Proceedings of the second ACM SIGCOMM workshop on Hot
                 Topics in Software Defined Networking, pp. 13-18, 2013.

   [CAPBR]       Brewer, E., "Towards Robust Distributed Systems", In
                 Proceedings of the Symposium on Principles of
                 Distributed Computing (PODC), 2000.

   [CAPFN]       Panda, A., Scott, C., Ghodsi, A., Koponen, T., and S.
                 Shenker, "CAP for Networks", In Proceedings of the
                 second ACM SIGCOMM workshop on Hot Topics in Software
                 Defined Networking, pp. 91-96, 2013.

   [CAPGL]       Gilbert, S. and N. Lynch, "Brewer's Conjecture and the
                 Feasibility of Consistent, Available,
                 Partition-Tolerant Web Services", ACM SIGACT News,
                 Volume 33, Issue 2, pp. 51-59, 2002.

   [CORBA]       Object Management Group, "CORBA Version 3.3", November
                 2012, <http://www.omg.org/spec/CORBA/3.3/>.

   [DIOPR]       Denazis, S., Miki, K., Vicente, J., and A. Campbell,
                 "Designing Interfaces for Open Programmable Routers",
                 In "Active Networks", Springer Berlin Heidelberg,
                 pp. 13-24, 1999.





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   [ESNet]       Yu, J. and I. Al Ajarmeh, "An Empirical Study of the
                 NETCONF Protocol", Sixth International Conference on
                 Networking and Services, pp. 253-258, 2010.

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

   [I2RSArch]    Atlas, A., Halpern, J., Hares, S., Ward, D., and T.
                 Nadeau, "An Architecture for the Interface to the
                 Routing System", Work in Progress,
                 draft-ietf-i2rs-architecture-07, December 2014.

   [I2RSInfo]    Bahadur, N., Folkes, R., Kini, S., and J. Medved,
                 "Routing Information Base Info Model", Work in
                 Progress, draft-ietf-i2rs-rib-info-model-04, December
                 2014.

   [I2RSProb]    Atlas, A., Nadeau, T., and D. Ward, "Interface to the
                 Routing System Problem Statement", Work in Progress,
                 draft-ietf-i2rs-problem-statement-05, January 2015.

   [ITUATM]      ITU, "B-ISDN ATM Layer Specification", ITU
                 Recommendation I.361, 1990,
                 <http://www.itu.int/rec/T-REC-I.361-199902-I/en>.

   [ITUSG11]     ITU, "ITU-T Study Group 11: Protocols and test
                 specifications", <http://www.itu.int/en/ITU-T/
                 studygroups/2013-2016/11/Pages/default.aspx>.

   [ITUSG13]     ITU, "ITU-T Study Group 13: Future networks including
                 cloud computing, mobile and next-generation networks",
                 <http://www.itu.int/en/ITU-T/studygroups/
                 2013-2016/13/Pages/default.aspx>.

   [ITUSS7]      ITU, "Introduction to CCITT Signalling System No. 7",
                 ITU Recommendation Q.700, 1993,
                 <http://www.itu.int/rec/T-REC-Q.700-199303-I/e>.

   [ITUY3300]    ITU, "Framework of software-defined networking", ITU
                 Recommendation Y.3300, June 2014,
                 <http://www.itu.int/rec/T-REC-Y.3300-201406-I/en>.








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   [KANDOO]      Yeganeh, S. and Y. Ganjali, "Kandoo: A Framework for
                 Efficient and Scalable Offloading of Control
                 Applications", In Proceedings of the first ACM SIGCOMM
                 workshop on Hot Topics in Software Defined Networks,
                 pp. 19-24, 2012.

   [NFVArch]     ETSI, "Network Functions Virtualisation (NFV):
                 Architectural Framework", ETSI GS NFV 002, October
                 2013, <http://www.etsi.org/deliver/etsi_gs/
                 nfv/001_099/002/01.01.01_60/gs_nfv002v010101p.pdf>.

   [NOX]         Gude, N., Koponen, T., Pettit, J., Pfaff, B., Casado,
                 M., McKeown, N., and S. Shenker, "NOX: Towards an
                 Operating System for Networks", ACM SIGCOMM Computer
                 Communication Review, Volume 38, Issue 3, pp. 105-110,
                 July 2008.

   [NV09]        Chowdhury, N. and R. Boutaba, "Network Virtualization:
                 State of the Art and Research Challenges",
                 Communications Magazine, IEEE, Volume 47, Issue 7,
                 pp. 20-26, 2009.

   [OF-CONFIG]   Open Networking Foundation, "OpenFlow Management and
                 Configuration Protocol (OF-Config 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, N., Anderson, T., Balakrishnan, H., Parulkar,
                 G., Peterson, L., Rexford, J., Shenker, S., and J.
                 Turner, "OpenFlow: Enabling Innovation in Campus
                 Networks", ACM SIGCOMM Computer Communication Review,
                 Volume 38, Issue 2, pp. 69-74, 2008.

   [ONFArch]     Open Networking Foundation, "SDN Architecture, Version
                 1", June 2014,
                 <https://www.opennetworking.org/images/stories/
                 downloads/sdn-resources/technical-reports/
                 TR_SDN_ARCH_1.0_06062014.pdf>.

   [OpenFlow]    Open Networking Foundation, "The OpenFlow Switch
                 Specification, Version 1.4.0", October 2013,
                 <https://www.opennetworking.org/images/stories/
                 downloads/sdn-resources/onf-specifications/openflow/
                 openflow-spec-v1.4.0.pdf>.






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   [P1520]       Biswas, J., Lazar, A., Huard, J., Lim, K., Mahjoub, S.,
                 Pau, L., Suzuki, M., Torstensson, S., Wang, W., and S.
                 Weinstein, "The IEEE P1520 standards initiative for
                 programmable network interfaces", IEEE Communications
                 Magazine, Volume 36, Issue 10, pp. 64-70, 1998.

   [PCEActive]   Crabbe, E., Minei, I., Medved, J., and R. Varga, "PCEP
                 Extensions for Stateful PCE", Work in Progress,
                 draft-ietf-pce-stateful-pce-10, October 2014.

   [PENet]       Hedstrom, B., Watwe, A., and S. Sakthidharan, "Protocol
                 Efficiencies of NETCONF versus SNMP for Configuration
                 Management Functions", Master's thesis, University of
                 Colorado, 2011.

   [PNSurvey99]  Campbell, A., De Meer, H., Kounavis, M., Miki, K.,
                 Vicente, J., and D. Villela, "A Survey of Programmable
                 Networks", ACM SIGCOMM Computer Communication Review,
                 Volume 29, Issue 2, pp. 7-23, September 1992.

   [Peregrine]   Chiueh, D., Tu, C., Wang, Y., Wang, P., Li, K., and Y.
                 Huang, "Peregrine: An All-Layer-2 Container Computer
                 Network", In Proceedings of the 2012 IEEE 5th
                 International Conference on Cloud Computing,
                 pp. 686-693, 2012.

   [PiNA]        Day, J., "Patterns in Network Architecture: A Return to
                 Fundamentals", Prentice Hall, ISBN 0132252422, 2008.

   [RCP]         Caesar, M., Caldwell, D., Feamster, N., Rexford, J.,
                 Shaikh, A., and J. van der Merwe, "Design and
                 Implementation of a Routing Control Platform", In
                 Proceedings of the 2nd conference on Symposium on
                 Networked Systems Design & Implementation Volume 2,
                 pp. 15-28, 2005.

   [REST]        Fielding, Roy, "Chapter 5: Representational State
                 Transfer (REST)", in Disseration "Architectural Styles
                 and the Design of Network-based Software
                 Architectures", 2000.

   [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,
                 <http://www.rfc-editor.org/info/rfc826>.





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   [RFC1953]     Newman, P., Edwards, W., Hinden, R., Hoffman, E., Ching
                 Liaw, F., Lyon, T., and G. Minshall, "Ipsilon Flow
                 Management Protocol Specification for IPv4 Version
                 1.0", RFC 1953, May 1996,
                 <http://www.rfc-editor.org/info/rfc1953>.

   [RFC2297]     Newman, P., Edwards, W., Hinden, R., Hoffman, E., Liaw,
                 F., Lyon, T., and G. Minshall, "Ipsilon's General
                 Switch Management Protocol Specification Version 2.0",
                 RFC 2297, March 1998,
                 <http://www.rfc-editor.org/info/rfc2297>.

   [RFC2578]     McCloghrie, K., Ed., Perkins, D., Ed., and J.
                 Schoenwaelder, Ed., "Structure of Management
                 Information Version 2 (SMIv2)", STD 58, RFC 2578, April
                 1999, <http://www.rfc-editor.org/info/rfc2578>.

   [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,
                 <http://www.rfc-editor.org/info/rfc3411>.

   [RFC3412]     Case, J., Harrington, D., Presuhn, R., and B. Wijnen,
                 "Message Processing and Dispatching for the Simple
                 Network Management Protocol (SNMP)", STD 62, RFC 3412,
                 December 2002,
                 <http://www.rfc-editor.org/info/rfc3412>.

   [RFC3414]     Blumenthal, U. and B. Wijnen, "User-based Security
                 Model (USM) for version 3 of the Simple Network
                 Management Protocol (SNMPv3)", STD 62, RFC 3414,
                 December 2002,
                 <http://www.rfc-editor.org/info/rfc3414>.

   [RFC3417]     Presuhn, R., "Transport Mappings for the Simple Network
                 Management Protocol (SNMP)", STD 62, RFC 3417, December
                 2002, <http://www.rfc-editor.org/info/rfc3417>.

   [RFC3418]     Presuhn, R., "Management Information Base (MIB) for the
                 Simple Network Management Protocol (SNMP)", STD 62, RFC
                 3418, December 2002,
                 <http://www.rfc-editor.org/info/rfc3418>.

   [RFC3535]     Schoenwaelder, J., "Overview of the 2002 IAB Network
                 Management Workshop", RFC 3535, May 2003,
                 <http://www.rfc-editor.org/info/rfc3535>.




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   [RFC3746]     Yang, L., Dantu, R., Anderson, T., and R. Gopal,
                 "Forwarding and Control Element Separation (ForCES)
                 Framework", RFC 3746, April 2004,
                 <http://www.rfc-editor.org/info/rfc3746>.

   [RFC4271]     Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
                 Protocol 4 (BGP-4)", RFC 4271, January 2006,
                 <http://www.rfc-editor.org/info/rfc4271>.

   [RFC4655]     Farrel, A., Vasseur, J., and J. Ash, "A Path
                 Computation Element (PCE)-Based Architecture", RFC
                 4655, August 2006,
                 <http://www.rfc-editor.org/info/rfc4655>.

   [RFC5424]     Gerhards, R., "The Syslog Protocol", RFC 5424, March
                 2009, <http://www.rfc-editor.org/info/rfc5424>.

   [RFC5440]     Vasseur, JP. and JL. Le Roux, "Path Computation Element
                 (PCE) Communication Protocol (PCEP)", RFC 5440, March
                 2009, <http://www.rfc-editor.org/info/rfc5440>.

   [RFC5531]     Thurlow, R., "RPC: Remote Procedure Call Protocol
                 Specification Version 2", RFC 5531, May 2009,
                 <http://www.rfc-editor.org/info/rfc5531>.

   [RFC5743]     Falk, A., "Definition of an Internet Research Task
                 Force (IRTF) Document Stream", RFC 5743, December 2009,
                 <http://www.rfc-editor.org/info/rfc5743>.

   [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,
                 <http://www.rfc-editor.org/info/rfc5810>.

   [RFC5812]     Halpern, J. and J. Hadi Salim, "Forwarding and Control
                 Element Separation (ForCES) Forwarding Element Model",
                 RFC 5812, March 2010,
                 <http://www.rfc-editor.org/info/rfc5812>.

   [RFC5880]     Katz, D. and D. Ward, "Bidirectional Forwarding
                 Detection (BFD)", RFC 5880, June 2010,
                 <http://www.rfc-editor.org/info/rfc5880>.

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




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

   [RFC6632]     Ersue, M. and B. Claise, "An Overview of the IETF
                 Network Management Standards", RFC 6632, June 2012,
                 <http://www.rfc-editor.org/info/rfc6632>.

   [RFC7011]     Claise, B., Trammell, B., and P. Aitken, "Specification
                 of the IP Flow Information Export (IPFIX) Protocol for
                 the Exchange of Flow Information", STD 77, RFC 7011,
                 September 2013,
                 <http://www.rfc-editor.org/info/rfc7011>.

   [RFC7047]     Pfaff, B. and B. Davie, "The Open vSwitch Database
                 Management Protocol", RFC 7047, December 2013,
                 <http://www.rfc-editor.org/info/rfc7047>.

   [RFC7149]     Boucadair, M. and C. Jacquenet, "Software-Defined
                 Networking: A Perspective from within a Service
                 Provider Environment", RFC 7149, March 2014,
                 <http://www.rfc-editor.org/info/rfc7149>.

   [RFC7276]     Mizrahi, T., Sprecher, N., Bellagamba, E., and Y.
                 Weingarten, "An Overview of Operations, Administration,
                 and Maintenance (OAM) Tools", RFC 7276, June 2014,
                 <http://www.rfc-editor.org/info/rfc7276>.

   [RINA]        Day, J., Matta, I., and K. Mattar, "Networking is IPC:
                 A Guiding Principle to a Better Internet", In
                 Proceedings of the 2008 ACM CoNEXT Conference, Article
                 No. 67, 2008.

   [RouteFlow]   Nascimento, M., Rothenberg, C., Salvador, M., Correa,
                 C., de Lucena, S., and M. Magalhaes, "Virtual Routers
                 as a Service: The RouteFlow Approach Leveraging
                 Software-Defined Networks", In Proceedings of the 6th
                 International Conference on Future Internet
                 Technologies, pp. 34-37, 2011.

   [SDNACS]      Kreutz, D., Ramos, F., Verissimo, P., Rothenberg, C.,
                 Azodolmolky, S., and S. Uhlig, "Software-Defined
                 Networking: A Comprehensive Survey", Networking and
                 Internet Architecture (cs.NI), arXiv:1406.0440, 2014.






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   [SDNHistory]  Feamster, N., Rexford, J., and E. Zegura, "The Road to
                 SDN: An Intellectual History of Programmable Networks",
                 ACM Queue, Volume 11, Issue 12, 2013.

   [SDNNFV]      Haleplidis, E., Hadi Salim, J., Denazis, S., and O.
                 Koufopavlou, "Towards a Network Abstraction Model for
                 SDN", Journal of Network and Systems Management:
                 Special Issue on Management of Software Defined
                 Networks, pp. 1-19, 2014.

   [SDNSecOF]    Kloti, R., Kotronis, V., and P. Smith, "OpenFlow: A
                 Security Analysis", 21st IEEE International Conference
                 on Network Protocols (ICNP) pp. 1-6, October 2013.

   [SDNSecServ]  Scott-Hayward, S., O'Callaghan, G., and S. Sezer, "SDN
                 Security: A Survey", In IEEE SDN for Future Networks
                 and Services (SDN4FNS), pp. 1-7, 2013.

   [SDNSecurity] Kreutz, D., Ramos, F., and P. Verissimo, "Towards
                 Secure and Dependable Software-Defined Networks", In
                 Proceedings of the second ACM SIGCOMM workshop on Hot
                 Topics in Software Defined Networking, pp. 55-60, 2013.

   [SDNSurvey]   Nunes, B., Mendonca, M., Nguyen, X., Obraczka, K., and
                 T.  Turletti, "A Survey of Software-Defined Networking:
                 Past, Present, and Future of Programmable Networks",
                 IEEE Communications Surveys and Tutorials,
                 DOI:10.1109/SURV.2014.012214.00180, 2014.

   [SLTSDN]      Jarraya, Y., Madi, T., and M. Debbabi, "A Survey and a
                 Layered Taxonomy of Software-Defined Networking", IEEE
                 Communications Surveys and Tutorials, Volume 16, Issue
                 4, pp. 1955-1980, 2014.

   [SoftRouter]  Lakshman, T., Nandagopal, T., Ramjee, R., Sabnani, K.,
                 and T. Woo, "The SoftRouter Architecture", In
                 Proceedings of the ACM SIGCOMM Workshop on Hot Topics
                 in Networking, 2004.

   [Tempest]     Rooney, S., van der Merwe, J., Crosby, S., and I.
                 Leslie, "The Tempest: A Framework for Safe, Resource
                 Assured, Programmable Networks", Communications
                 Magazine, IEEE, Volume 36, Issue 10, pp. 42-53, 1998.








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Acknowledgements

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

   Additionally, we would like to thank (in alphabetical order)
   Shivleela Arlimatti, Roland Bless, Scott Brim, Alan Clark, Luis
   Miguel Contreras Murillo, Tim Copley, Linda Dunbar, Ken Gray, Deniz
   Gurkan, Dave Hood, Georgios Karagiannis, Bhumip Khasnabish, Sriganesh
   Kini, Ramki Krishnan, Dirk Kutscher, Diego Lopez, Scott Mansfield,
   Pedro Martinez-Julia, David E. Mcdysan, Erik Nordmark, Carlos
   Pignataro, Robert Raszuk, Bless Roland, Francisco Javier Ros Munoz,
   Dimitri Staessens, Yaakov Stein, Eve Varma, Stuart Venters, Russ
   White, and Lee Young for their critical comments and discussions at
   IETF 88, IETF 89, and IETF 90 and on the SDNRG mailing list, which we
   took into consideration while revising this document.

   We would also like to thank (in alphabetical order) Spencer Dawkins
   and Eliot Lear for their IRSG reviews, which further refined this
   document.

   Finally, we thank Nobo Akiya for his review of the section on BFD,
   Julien Meuric for his review of the section on PCE, and Adrian Farrel
   and Benoit Claise for their IESG reviews of this document.

   Kostas Pentikousis is supported by [ALIEN], a research project
   partially funded by the European Community under the Seventh
   Framework Program (grant agreement no. 317880).  The views expressed
   here are those of the author only.  The European Commission is not
   liable for any use that may be made of the information in this
   document.


















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Contributors

   The authors would like to acknowledge (in alphabetical order) the
   following persons as contributors to this document.  They all
   provided text, pointers, and comments that made this document more
   complete:

   o  Daniel King for providing text related to PCEP.

   o  Scott Mansfield for information regarding current ITU work on SDN.

   o  Yaakov Stein for providing text related to the CAP theorem and
      SDO-related information.

   o  Russ White for text suggestions on the definitions of control,
      management, and application.

Authors' Addresses

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

   EMail: ehalep@ece.upatras.gr


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

   EMail: k.pentikousis@eict.de


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

   EMail: sdena@upatras.gr







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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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