SDNRG Working Group                                         M. Boucadair
Internet-Draft                                              C. Jacquenet
Intended status: Informational                            France Telecom
Expires: October 12, 2013                                 April 10, 2013

     Software-Defined Networking: A Service Provider's Perspective


   Software-Defined Networking (SDN) has been one of the major buzz
   words of the networking industry for the past couple of years.  And
   yet, no clear definition of what SDN actually covers has been broadly
   admitted so far.  This document aims at contributing to the
   clarification of the SDN landscape.

   It is not meant to endlessly discuss what SDN truly means, but rather
   to suggest a functional taxonomy of the techniques that can be used
   under a SDN umbrella and to elaborate on the various pending issues
   the combined activation of such techniques inevitably raises.  As
   such, a definition of SDN is only mentioned for the sake of

Status of This Memo

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   This Internet-Draft will expire on October 12, 2013.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Context . . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.2.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  What is In and What is Out? . . . . . . . . . . . . . . . . .   4
     2.1.  Remember the Past . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Be Pragmatic  . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Measure Experience Against Expectations . . . . . . . . .   5
     2.4.  Design Carefully  . . . . . . . . . . . . . . . . . . . .   6
     2.5.  There is Life Beyond OpenFlow . . . . . . . . . . . . . .   6
     2.6.  Non Goals . . . . . . . . . . . . . . . . . . . . . . . .   7
   3.  A Definition of Software-Defined Networking . . . . . . . . .   7
     3.1.  A Tautology . . . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  On Flexibility  . . . . . . . . . . . . . . . . . . . . .   8
     3.3.  A Tentative Definition  . . . . . . . . . . . . . . . . .   8
     3.4.  Functional Meta-Domains . . . . . . . . . . . . . . . . .   9
   4.  Disscussion . . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  Full Automation: a Viable Objective?  . . . . . . . . . .   9
     4.2.  The Intelligence resides in the PDP . . . . . . . . . . .  10
     4.3.  Simplicity and Adaptability vs. Complexity  . . . . . . .  11
     4.4.  Performance & Scalability . . . . . . . . . . . . . . . .  11
     4.5.  Risk Assessement  . . . . . . . . . . . . . . . . . . . .  12
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

1.1.  Context

   The Internet has become the federative network that supports a wide
   range of service offerings.  The delivery of network services such as
   IP VPNs assumes the combined activation of various capabilities that
   include (but are not necessarily limited to) forwarding and routing
   capabilities (e.g., customer-specific addressing scheme management,
   dynamic path computation to reach a set of destination prefixes,
   dynamic establishment of tunnels, etc.), quality of service

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   capabilities (e.g., traffic classification and marking, traffic
   conditioning and scheduling), security capabilities (e.g., filters to
   protect customer premises from network-originated attacks, to avoid
   malformed route announcements, etc.)  and management capabilities
   (e.g., detection and processing of faults).

   As these services not only grow in variety but also in complexity,
   their design, delivery and operation have become a complex alchemy
   that often requires various levels of expertise.  This situation is
   further aggravated by the wide variety of (network) protocols and
   tools, as well as recent Any Time Any-Where Any Device
   (ATAWAD)-driven convergence trends that are meant to make sure an
   end-user can access the whole range of services he/she has subscribed
   to, whatever the access and device technologies, wherever the end-
   user is connected to the network, and whether this end-user is in
   motion or not.

   Yet, most of these services have been deployed for the past decade,
   based solely on often static service production procedures that are
   more and more exposed to the risk of malformed configuration
   commands.  In addition, most of these services do not assume any
   specific negotiation between the customer and the service provider or
   between service providers besides the typical financial terms.

   At best, five-year master plans are referred to as the network
   planning policy that will be enforced by the service provider, given
   the foreseen business development perspectives, manually-computed
   traffic forecasts and the market coverage (fixed/mobile, residential/
   corporate).  This so-called network planning policy may very well
   affect the way resources are allocated in a network, but clearly
   fails to be adequately responsive to highly dynamic customer
   requirements in an "always-on" fashion.  These needs are more
   critical for corporate customers.

   In addition, various tools are used for different, sometimes service-
   centric, management purposes but their usage is not necessarily
   coordinated for the sake of event aggregation, correlation and
   processing.  At the cost of extra complexity and possible customer's
   Quality of Experience degradation.

   Multi-service, multi-protocol, multi-technology convergent and
   dynamically-adaptive networking environments of the near future have
   therefore become one of the major challenges faced by service

1.2.  Scope

   This document is a contribution to clarify the SDN landscape:

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   o  Section 2 clarifies items which are considered as viable goals and
      exclude others from the scope.
   o  Section 3 provides a tentative definition from a service provider
   o  Section 3 discusses several issues and identifies some

2.  What is In and What is Out?

   The networking ecosystem has become awfully complex and highly
   demanding in terms of robustness, performance, scalability,
   flexibility, agility, etc.  This means in particular that service
   providers and network operators must deal with such complexity and
   operate networking infrastructures that can evolve easily, remain
   scalable, guarantee robustness and availability, and are resilient
   against denial-of-service attacks.

   The introduction of new SDN-based networking features should
   obviously take into account this context, especially from a cost
   impact assessment perspective.

2.1.  Remember the Past

   SDN techniques cannot be seen as a brand new solution but rather as
   some kind of rebranding of proposals that have been investigated for
   several years, like Active or Programmable Networks.  As a matter of
   fact, some of the claimed "new" SDN features have been already
   implemented (e.g., NMS (Network Management System), PCE (Path
   Computation Element, [RFC4655])), and supported by vendors for quite
   some time (references can be added if needed).

   Some of these features have also been standardized (e.g., DNS-based
   routing [RFC1383] that can be seen as an illustration of separated
   control and forwarding planes or ForCES ([RFC5810][RFC5812])).

2.2.  Be Pragmatic

   SDN approaches should be holistic.  This means that it must be
   global, network-wise.  It is not a matter of configuring devices one
   by one to enforce a specific forwarding policy.  It is about
   configuring and operating a whole range of devices at the scale of
   the network for the sake of automated service delivery
   ([I-D.boucadair-network-automation-requirements]), from service
   negotiation and creation (e.g., [I-D.ietf-idr-sla-exchange]) to
   assurance and fulfillment.

   Because the complexity of activating SDN capabilities is hidden (to
   the user) and pushed to software, a clear understanding of the

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   overall ecosystem is needed to figure out how to manage this
   complexity and to what extent this hidden complexity does not have
   side effects on network operation.

   As an example, SDN designs that assume a central decision-making
   entity must avoid single points of failure.  They must not affect
   packet forwarding performances either (e.g., transit delays must not
   be impacted).

   SDN techniques are not necessary to develop new network services per
   se.  The basic service remains (IP) connectivity that solicits
   resources located in the network.

   SDN techniques can thus be seen as another means to interact with
   network service modules and invoke both connectivity and storage
   resources accordingly in order to meet service-specific requirements.

   By definition, SDN techniques remain limited to what is supported by
   embedded software and hardware.  One cannot expect SDN techniques to
   support unlimited customizable features.

   Policy-based management framework[RFC2753] was designed to
   orchestrate available resources, by means of a typical Policy
   Decision Point (PDP) which masters advanced offline traffic
   engineering capabilities.  As such, this framework has the ability to
   interact with in-band software modules embedded in controlled devices
   (or not).

   SDN techniques as a whole are an instantiation of the policy-based
   network management framework.  Within this context, SDN techniques
   can be used to activate capabilities on demand, to dynamically invoke
   network and storage resources and to operate dynamically-adaptive
   networks according to events (e.g., alteration of the network
   topology) and triggers (e.g., dynamic notification of a link
   failure), etc.

2.3.  Measure Experience Against Expectations

   Because several software modules may be controlled by external
   entities, means to ensure the experienced outcome complies with the
   expected outcome belong to the set of SDN techniques.

   These techniques, as an instantiation of Policy-Based Management,
   should interact with Service Structuring engines and the network to
   continuously assess whether the experienced network behavior is
   compliant with the objectives set by the Service Structuring engine,
   and which may have been dynamically negotiated with the customer
   (e.g., captured in a CPP

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   [I-D.boucadair-connectivity-provisioning-profile]).  This requirement
   applies to several regions of a network:

   1.  At the interface between two adjacent IP network providers.
   2.  At the access interface between a service provider and an IP
       network provider.
   3.  At the interface between a customer and the IP network provider.

   Ideally, a fully automated service delivery procedure from
   negotiation and ordering, through order processing, to delivery,
   assurance and fulfillment, should be supported.  This approach
   assumes widely adopted standard data and information models, let
   alone interfaces.

2.4.  Design Carefully

   Exposing open and programmable interfaces has a cost, from both a
   scalability and performance standpoints.

   Maintaining hard-coded performance optimization techniques is
   encouraged.  So is the use of interfaces that allow the direct
   control of some engines (e.g., routing, forwarding) without requiring
   any in-between adaptation layer (generic objects to vendor-specific
   CLI commands for instance).

   SDN techniques will have to accommodate vendor-specific components
   anyway.  Indeed, these vendor-specific features will not cease to
   exist mainly because of the harsh competition.

   The introduction of new functions or devices that may jeopardize
   network flexibility should be avoided, or at least carefully
   considered in light of possible performance and scalability impacts.
   SDN-enabled devices will have to coexist with legacy systems.

   One single SDN, network-wise deployment is unlikely.

   Instead, multiple instantiations of SDN techniques will be
   progressively deployed and adapted to various network and service

2.5.  There is Life Beyond OpenFlow

   Empowering networking with in-band controllable modules does not
   necessarily mean the use of the OpenFlow protocol, which is only a
   protocol that helps devices populate their forwarding tables
   according to a set of instructions.

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   OpenFlow is clearly not the "next big thing": there are many, many
   other protocols that have been standardized (think Routing Policy
   Specification Language (RPSL, [RFC2622]), for one) - or not - and
   which have been massively deployed.

   The forwarding of the configuration information can currently rely
   upon a variety of protocols that include (but is not necessarily
   limited to) PCEP [RFC5440], NETCONF [RFC6241], COPS-PR [RFC3084],

   There is no 1:1 relationship between OpenFlow and SDN.  Rather,
   OpenFlow is one of the candidate protocols to convey specific
   configuration information towards devices.  As such, OpenFlow is one
   possible component of the global SDN toolkit.

2.6.  Non Goals

   There are inevitable trade-offs between the current networking
   ecosystem and the proposed SDN paradigm.  Operators do not have to
   choose between the two as both models may be needed.

   In particular, the following considerations can be seen as a non-goal
   to justify the deployment of SDN techniques:

   o  Fully flexible software implementations, whereas the claimed
      flexibility will be limited by respective software and hardware
      limitations, anyway.
   o  Fully modular implementations are difficult to achieve (because of
      the implicit complexity) and may introduce extra effort for
      testing, validation and troubleshooting.
   o  Fully centralized control systems that raise some scalability
      issues.  Distributed protocols and their ability to react to some
      events (e.g., link failure) in a timely manner remains a key to
      scalable networks.  This means that SDN designs can rely upon a
      logical representation of centralized features (an abstraction
      layer that would support inter-PDP communications, for example).

3.  A Definition of Software-Defined Networking

3.1.  A Tautology

   The separation of the forwarding and control planes (beyond
   implementation considerations) have almost become a gimmick to
   promote flexibility as a key feature of the SDN approach.
   Technically, most of current router implementations have been
   assuming this separation for years if not decades.  Routing processes
   (such as IGP and BGP route computation) have often been software-
   based, while forwarding capabilities are hardware-encoded.

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   As such, the current state-of-the-art tends to confirm the said
   separation, which rather falls under a tautology.

   But a somewhat centralized, "controller-embedded", control plane for
   the sake of route computation before FIB population is certainly
   another story.

3.2.  On Flexibility

   This "flexibility argument" that has been put forward by SDN
   promoters is undoubtedly one of the key objectives that must be
   achieved by service providers.  This is because the ability to
   dynamically adapt to a wide range of customer's requests for the sake
   of flexible network service delivery is an important competitive
   advantage.  But flexibility is much, much more than separating the
   control and forwarding planes to facilitate forwarding decision-
   making processes.  Note:

   o  The exact characterization of what flexibility actually means is
      still required.
   o  The exposure of programmable interfaces is not a goal per se,
      rather a means to facilitate configuration procedures.

3.3.  A Tentative Definition

   We define Software-Defined Networking as the set of techniques used
   to facilitate the design, the delivery and the operation of network
   services in a deterministic, dynamic, and scalable manner.

   Such a definition assumes the introduction of a high level of
   automation in the overall service delivery and operation procedures.

   Because networking is by essence software-driven, the above
   definition does not emphasize the claimed "Softwire-Defined" property
   of SDN-labeled solutions.

   Having a predictable network behavior is important to consider.  This
   argues in favor of investigating advanced network emulation engines
   which would assess the impact of enforcing a new policy.  Network
   emulation function would be a helper for operators.  A network
   emulation engine can be fed with information collected using for
   instance [I-D.ietf-idr-ls-distribution].

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3.4.  Functional Meta-Domains

   SDN techniques can be classified into the following functional meta-

   o  Techniques for the dynamic discovery of network topology, devices
      and capabilities, along with relevant information models that are
      meant to precisely document such topology, devices and
   o  Techniques for exposing network services (and their
      characteristics; e.g.,
      [I-D.boucadair-connectivity-provisioning-profile]) and for dynamic
      negotiation of the set of corresponding parameters that will be
      used to measure the level of quality associated to the delivery of
      a given service or a combination thereof.
   o  Techniques used by service requirements-derived dynamic resource
      allocation and policy enforcement schemes, so that networks can be
      programmed accordingly.
   o  Dynamic feedback mechanisms that are meant to assess how
      efficiently a given policy (or a set thereof) is enforced from a
      service fulfillment and assurance perspective.

4.  Disscussion

4.1.  Full Automation: a Viable Objective?

   The path towards full automation is paved with numerous challenges
   and requirements, including:

   o  Simplify and foster service delivery, assurance and fulfillment,
      as well as network failure detection, diagnosis and root cause

      *  This can be achieved thanks to automation, possibly based upon
         a logically centralized view of the network infrastructure (or
         a portion thereof), yielding the need for highly automated
         topology, device and capabilities discovery as well as
         operational procedures.
      *  The main intelligence resides in the PDP, which suggests that
         an important part of the investigation effort should focus on a
         detailed specification of the PDP function, including
         algorithms and behavioral details, based upon a complete set of
         standardized data and information models.
   o  Need for abstraction layers: clear interfaces between business
      actors, clear interaction between layers, cross-layer
      considerations, etc.

      *  Ability to build and package differentiated (network) services.

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      *  Need for IP connectivity service exposure to customers, peers,
         applications, content/service providers, etc.  (e.g.,
      *  Need for a solution to map IP connectivity service requirements
         with network engineering objectives.
      *  Need for dynamically-adaptive objectives based on current
         resource usage and demand, for the sake of highly responsive
         dynamic resource allocation and policy enforcement schemes.
   o  Better accommodate technologically heterogeneous networking

      *  Need for vendor-independent configuration procedures, based
         upon the enforcement of vendor-agnostic generic policies
         instead of vendor-specific languages.
      *  Need for tools to aid manageability and orchestrate resources.
      *  Avoid proxies and privileged direct interaction with engines
         (e.g., routing, forwarding).

4.2.  The Intelligence resides in the PDP

   The proposed SDN definition in Section 3.3 assumes an intelligence
   that may reside in the control or management planes (or both).  This
   intelligence is typically represented by a Policy Decision Point,
   which is one of the key functional components of Policy-Based
   Management architectures [RFC2753].

   The Policy Decision Point (PDP) is where policy decisions are made.
   PDPs use a directory service for policy repository purposes.  The
   policy repository stores the policy information that can be retrieved
   and updated by the PDP.  The PDP delivers policy rules to the Policy
   Enforcement Point (PEP) in the form of policy-provisioning
   information that includes configuration information.

   The Policy Enforcement Point (PEP) is where policy decisions are
   applied.  PEPs are embedded in (network) devices, which are
   dynamically configured based upon the policy-formatted information
   that has been processed by the PEP.  PEPs request configuration from
   the PDP, store the configuration information in the Policy
   Information Base (PIB), and delegate any policy decision to the PDP.

   SDN networking therefore relies upon PDP functions that are capable
   of processing various input data (traffic forecasts, outcomes of
   negotiation between customers and service providers, resource status
   (as depicted in appropriate information models instantiated in the
   PIB, etc.)  to make appropriate decisions.

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   The design and the operation of such PDP-based intelligence in a
   scalable manner remains of the major areas that needs to be
   investigated within SDN environments.

   To avoid centralized design schemes, inter-PDP communication means
   should be considered.

   Several PDP instances may be activated in a given domain.  Because
   each of these PDP instances may be in charge of a given functional
   perimeter, an inter-PDP communication may be required to ease
   collaboration between these PDPs to provision a network service.

   Inter-domain PDP exchanges may be needed for some specific usages.
   Examples of such exchanges are: (1) During the network attachment
   phase of a node to a visited network, the PDP belonging to the the
   visited network can contact the home PDP to retrieve the policies to
   be enforced for that node.  (2) Various PDPs can collaborate together
   in order to compute inter-domain paths which satisfy a set of traffic
   performance guarantees.

4.3.  Simplicity and Adaptability vs.  Complexity

   The above meta functional domains assume the introduction of a high
   level of automation, from service negotiation to delivery and

   Automation is the key to simplicity, but must not be seen as a magic
   button that would be hit by a network administrator whenever a
   customer request has to be processed or additional resources need to
   be allocated.

   The need for simplicity and adaptability thanks to automated
   procedures generally assumes some complexity that lies beneath

4.4.  Performance & Scalability

   The combination of flexibility with software inevitably raises
   performance and scalability issues as a function of the number and
   the nature of the services to be delivered and their associated

   While the deployment of a network solely composed of OpenFlow
   switches within a data center environment is unlikely to raise FIB
   scalability issues given the current state-of-the-art, data center
   networking that relies upon complex, possibly IP-based, QoS-inferred,
   interconnect design schemes meant to dynamically manage the mobility
   of Virtual Machines between sites is certainly another scale.

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   The claimed flexibility of SDN networking in the latter context will
   have to be carefully investigated by operators.

4.5.  Risk Assessement

   Various risks are to be assessed such as:

   o  Evaluating the risk of depending on a controller technology rather
      than a device technology.
   o  Evaluating the risk of operating frozen architectures because of
      potential interoperability issues between a controller and a
      controlled device.
   o  Assessing whether SDN-labeled solutions are likely to obsolete
      existing technologies because of hardware limitations.
   o  Etc.

5.  IANA Considerations

   This document does not require any action from IANA.

6.  Security Considerations

   This document does not define any protocol nor architecture.

7.  Acknowledgements

   Many thanks to J.  Halpern and T.  Tsou for their feedback.

   Special thanks to P.  Georgatos for the interesting discussion;
   particularly the discussion on SDNi (SDN Interconnection).

8.  Informative References

              Boucadair, M., Jacquenet, C., and N. Wang, "IP/MPLS
              Connectivity Provisioning Profile", draft-boucadair-
              connectivity-provisioning-profile-02 (work in progress),
              September 2012.

              Boucadair, M. and C. Jacquenet, "Requirements for
              Automated (Configuration) Management", draft-boucadair-
              network-automation-requirements-00 (work in progress),
              December 2012.

              Gredler, H., Medved, J., Previdi, S., Farrel, A., and S.
              Ray, "North-Bound Distribution of Link-State and TE

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              Information using BGP", draft-ietf-idr-ls-distribution-02
              (work in progress), February 2013.

              Shah, S., Patel, K., Bajaj, S., Tomotaki, L., and M.
              Boucadair, "Inter-domain SLA Exchange", draft-ietf-idr-
              sla-exchange-00 (work in progress), January 2013.

   [RFC1383]  Huitema, C., "An Experiment in DNS Based IP Routing", RFC
              1383, December 1992.

   [RFC2622]  Alaettinoglu, C., Villamizar, C., Gerich, E., Kessens, D.,
              Meyer, D., Bates, T., Karrenberg, D., and M. Terpstra,
              "Routing Policy Specification Language (RPSL)", RFC 2622,
              June 1999.

   [RFC2753]  Yavatkar, R., Pendarakis, D., and R. Guerin, "A Framework
              for Policy-based Admission Control", RFC 2753, January

   [RFC3084]  Chan, K., Seligson, J., Durham, D., Gai, S., McCloghrie,
              K., Herzog, S., Reichmeyer, F., Yavatkar, R., and A.
              Smith, "COPS Usage for Policy Provisioning (COPS-PR)", RFC
              3084, March 2001.

   [RFC4655]  Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
              Computation Element (PCE)-Based Architecture", RFC 4655,
              August 2006.

   [RFC5440]  Vasseur, JP. and JL. Le Roux, "Path Computation Element
              (PCE) Communication Protocol (PCEP)", RFC 5440, March

   [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.

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

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

Authors' Addresses

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   Mohamed Boucadair
   France Telecom
   Rennes  35000


   Christian Jacquenet
   France Telecom


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