





Internet Engineering Task Force (IETF)                      M. Boucadair
Request for Comments: 7149                                  C. Jacquenet
Category: Informational                                   France Telecom
ISSN: 2070-1721                                               March 2014


            Software-Defined Networking: A Perspective from
                 within a Service Provider Environment

Abstract

   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 to clarify the SDN landscape by
   providing a perspective on requirements, issues, and other
   considerations about SDN, as seen from within a service provider
   environment.

   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 an 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
   clarification.

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 Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are 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/rfc7149.










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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
   2. Introducing Software-Defined Networking .........................4
      2.1. A Tautology? ...............................................4
      2.2. On Flexibility .............................................4
      2.3. A Tentative Definition .....................................5
      2.4. Functional Metadomains .....................................6
   3. Reality Check ...................................................6
      3.1. Remember the Past ..........................................7
      3.2. Be Pragmatic ...............................................8
      3.3. Measure Experience against Expectations ....................8
      3.4. Design Carefully ...........................................9
      3.5. On OpenFlow ................................................9
      3.6. Non-goals .................................................10
   4. Discussion .....................................................11
      4.1. Implications of Full Automation ...........................11
      4.2. Bootstrapping an SDN ......................................12
      4.3. Operating an SDN ..........................................14
      4.4. The Intelligence Resides in the PDP .......................15
      4.5. Simplicity and Adaptability vs. Complexity ................16
      4.6. Performance and Scalability ...............................16
      4.7. Risk Assessment ...........................................17
   5. Security Considerations ........................................17
   6. Acknowledgements ...............................................18
   7. Informative References .........................................18










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1.  Introduction

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

   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 convergence trends driven by Any Time, Any
   Where, Any Device (ATAWAD); ATAWADs are meant to make sure that 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 or not this end user is in
   motion.

   Yet, most of these services have been deployed for the past decade,
   primarily based upon often static service production procedures that
   are more and more exposed to the risk of erroneous 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 market coverage (fixed/mobile and residential/
   corporate).  This so-called network planning policy may very well
   affect the way resources are allocated in a network, but it clearly
   fails to be adequately responsive to highly dynamic customer
   requirements in an "always-on" fashion.  The need for improved
   service delivery procedures (including the time it takes to deliver
   the service once the possible negotiation phase is completed) is even
   more critical for corporate customers.








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   In addition, various tools are used for different, sometimes service-
   centric, management purposes, but their usage is not necessarily
   coordinated for event aggregation, correlation, and processing.  This
   lack of coordination may come at the cost of extra complexity and
   possible customer 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
   providers.

   This document aims to clarify the SDN landscape by providing a
   perspective on the functional taxonomy of the techniques that can be
   used in SDN, as seen from within a service provider environment.

2.  Introducing Software-Defined Networking

2.1.  A Tautology?

   The separation of the forwarding and control planes (beyond
   implementation considerations) has almost become a gimmick to promote
   flexibility as a key feature of the SDN approach.  Technically, most
   of the current router implementations have been assuming this
   separation for decades.  Routing processes (such as IGP and BGP route
   computation) have often been software based, while forwarding
   capabilities are usually implemented in hardware.

   As such, at the time of writing, what is considered to be 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 optimized route computation before the Forwarding
   Information Base (FIB) population is certainly another story.

2.2.  On Flexibility

   Promoters of SDN have argued that it provides additional flexibility
   in how the network is operated.  This 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
   requests for 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.






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   For example, the ability to accommodate short duration extra
   bandwidth requirements so that end users can stream a video file to
   their 4G terminal device is an example of the flexibility that
   several mobile operators are currently investigating.

   From this perspective, the ability to predict the network behavior as
   a function of the network services to be delivered is of paramount
   importance for service providers, so that they can assess the impact
   of introducing new services or activating additional network features
   or enforcing a given set of (new) policies from both financial and
   technical standpoints.  This argues in favor of investigating
   advanced network emulation engines, which can be fed with information
   that can be derived from [LS-DISTRIB], for example.

   Given the rather broad scope that the term "flexibility" suggests:

   o  Current SDN-labeled solutions are claimed to be flexible, although
      the notion is hardly defined.  The exact characterization of what
      flexibility actually means is yet to be provided.  Further work
      needs, therefore, to be conducted so that flexibility can be
      precisely defined in light of various criteria such as network
      evolution capabilities as a function of the complexity introduced
      by the integration of SDN techniques and seamless capabilities
      (i.e., the ability to progressively introduce SDN-enabled devices
      without disrupting network and service operation, etc.).

   o  The exposure of programmable interfaces is not a goal per se;
      rather, it is a means to facilitate configuration procedures for
      improved flexibility.

2.3.  A Tentative Definition

   We define Software-Defined Networking as the set of techniques used
   to facilitate the design, delivery, and operation of network services
   in a deterministic, dynamic, and scalable manner.  The said
   determinism refers to the ability to completely master the various
   components of the service delivery chain, so that the service that
   has been delivered complies with what has been negotiated and
   contractually defined with the customer.

   As such, determinism implies that the ability to control how network
   services are structured, designed, and delivered and where traffic
   should be forwarded in the network is for optimized resource usage.
   Although not explicitly restated in the following sections of the
   document, determinism lies beneath any action that may be taken by a
   service provider once service parameter negotiation is completed,
   from configuration tasks to service delivery, fulfillment, and
   assurance (see Section 2.4 below).



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

   Because networking is software driven by nature, the above definition
   does not emphasize the claimed "software-defined" properties of SDN-
   labeled solutions.

2.4.  Functional Metadomains

   SDN techniques can be classified into the following functional
   metadomains:

   o  Techniques for the dynamic discovery of network topology, devices,
      and capabilities, along with relevant information and data models
      that are meant to precisely document such topology, devices, and
      their capabilities.

   o  Techniques for exposing network services and their characteristics
      and for dynamically negotiating the set of service parameters that
      will be used to measure the level of quality associated with the
      delivery of a given service or a combination thereof.  An example
      of this can be seen in [CPP].

   o  Techniques used by service-requirement-derived dynamic resource
      allocation and policy enforcement schemes, so that networks can be
      programmed accordingly.  Decisions made to dynamically allocate
      resources and enforce policies are typically the result of the
      correlation of various inputs, such as the status of available
      resources in the network at any given time, the number of customer
      service subscription requests that need to be processed over a
      given period of time, the traffic forecasts, the possible need to
      trigger additional resource provisioning cycles according to a
      typical multi-year master plan, etc.

   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.

3.  Reality Check

   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 to
   denial-of-service attacks.




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

3.1.  Remember the Past

   SDN techniques are not the next big thing per se but rather a kind of
   rebranding of proposals that have been investigated for several
   years, like active or programmable networks [AN] [PN].  As a matter
   of fact, some of the claimed "new" SDN features have been already
   implemented (e.g., Network Management System (NMS) and Path
   Computation Element (PCE) [RFC4655]) and supported by vendors for
   quite some time.

   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 Forwarding and Control Element
   Separation (ForCES) [RFC5810] [RFC5812].

   Also, the policy-based management framework [RFC2753] introduced in
   the early 2000's 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).

   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.

   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 techniques as a whole are an instantiation of the policy-based
   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), triggers (e.g., dynamic notification of a link failure),
   etc.




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3.2.  Be Pragmatic

   SDN approaches should be holistic, i.e., global and network wide.  It
   is not a matter of configuring devices one by one to enforce a
   specific forwarding policy.  Instead, SDN techniques are about
   configuring and operating a whole range of devices at the scale of
   the network for automated service delivery [AUTOMATION], from service
   negotiation (e.g., [CPNP]) and creation (e.g., [SLA-EXCHANGE]) to
   assurance and fulfillment.

   Because the complexity of activating SDN capabilities is largely
   hidden from the end user and is software handled, a clear
   understanding of the 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 as (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 technique activation and operation remain limited
   to what is supported by embedded software and hardware.  One cannot
   expect SDN techniques to support unlimited customizable features.

3.3.  Measure Experience against Expectations

   Because several software modules may be controlled by external
   entities (typically, a PDP), there is a need for a means to make sure
   that what has been delivered complies with what has been negotiated.
   Such means belong to the set of SDN techniques.

   These typical policy-based techniques should interact with both
   Service Structuring engines (that are meant to expose the service
   characteristics and possibly negotiate those characteristics) and the
   network to continuously assess whether the experienced network
   behavior is compliant with the objectives set by the Service
   Structuring engine and those that may have been dynamically
   negotiated with the customer (e.g., as captured in a CPP [CPP]
   [CPNP]).  This requirement applies to several regions of a network,
   including:



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   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, ordering, and order processing to delivery, assurance,
   and fulfillment, should be supported at the cost of implications that
   are discussed in Section 4.1.  This approach also assumes widely
   adopted standard data and information models in addition to
   interfaces.

3.4.  Design Carefully

   Exposing open and programmable interfaces has a cost from both
   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 and forwarding) without
   requiring any in-between adaptation layers (generic objects to
   vendor-specific command line interfaces (CLIs), for instance).
   Nevertheless, the use of vendor-specific access means to some engines
   that it could be beneficial from a performance standpoint, at the
   cost of increasing the complexity of configuration tasks.

   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-wide deployment is, therefore, very unlikely.
   Instead, multiple instantiations of SDN techniques will be
   progressively deployed and adapted to various network and service
   segments.

3.5.  On OpenFlow

   Empowering networking with in-band controllable modules may rely upon
   the OpenFlow protocol but also use other protocols to exchange
   information between a control plane and a data plane.




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   Indeed, there are many other candidate protocols that can be used for
   the same or even a broader purpose (e.g., resource reservation
   purposes).  The forwarding of the configuration information can, for
   example, rely upon protocols like the Path Computation Element (PCE)
   Communication Protocol (PCEP) [RFC5440], the Network Configuration
   Protocol (NETCONF) [RFC6241], COPS Usage for Policy Provisioning
   (COPS-PR) [RFC3084], Routing Policy Specification Language (RPSL)
   [RFC2622], etc.

   There is, therefore, 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.

3.6.  Non-goals

   There are inevitable trade-offs to be found between operating the
   current networking ecosystem and introducing some SDN techniques,
   possibly at the cost of introducing new technologies.  Operators do
   not have to choose between the two as both environments will have to
   coexist.

   In particular, the following considerations cannot justify the
   deployment of SDN techniques:

   o  Fully flexible software implementations because the claimed
      flexibility remains limited by the 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 are likely to raise some
      scalability issues.  Distributed protocols and their ability to
      react to some events (e.g., link failure) in a timely manner
      remains a cornerstone of 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).











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4.  Discussion

4.1.  Implications of Full Automation

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

   o  Making sure automation is well implemented so as to facilitate
      testing (including validation checks) and troubleshooting.

      *  This suggests the need for simulation tools that accurately
         assess the impact of introducing a high level of automation in
         the overall service delivery procedure to avoid a typical "mad
         robot" syndrome, whose consequences can be serious from control
         and QoS standpoints, among others.

      *  This also suggests careful management of human expertise, so
         that network operators can use robust, flexible means to
         automate repetitive or error-prone tasks and then build on
         automation or stringing together multiple actions to create
         increasingly complex tasks that require less human interaction
         (guidance and input) to complete.

   o  Simplifying and fostering service delivery, assurance, and
      fulfillment, as well as network failure detection, diagnosis, and
      root cause analysis for cost optimization.

      *  Such cost optimization relates to improved service delivery
         times as well as optimized human expertise (see above) and
         global, technology-agnostic service structuring and delivery
         procedures.  In particular, the ability to inject new functions
         in existing devices should not assume a replacement of the said
         devices but rather allow smart investment capitalization.

      *  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 means, and
         operational procedures.

      *  The main intelligence resides in the PDP, which suggests that
         an important part of the SDN-related development effort should
         focus on a detailed specification of the PDP function,
         including algorithms and behavioral state machineries that are
         based upon a complete set of standardized data and information
         models.





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      *  These information models and data need to be carefully
         structured for efficiency and flexibility.  This probably
         suggests that a set of simplified pseudo-blocks can be
         assembled as per the nature of the service to be delivered.

   o  The need for abstraction layers -- clear interfaces between
      business actors and between layers, let alone cross-layer
      considerations, etc.  Such abstraction layers are invoked within
      the context of service structuring and packaging and are meant to
      facilitate the emergence of the following:

      *  IP connectivity service exposure to customers, peers,
         applications, content/service providers, etc.  (an example of
         this can be seen in [CPP]).

      *  Solutions that accommodate IP connectivity service requirements
         with network engineering objectives.

      *  Dynamically adaptive decision-making processes, which can
         properly operate according to a set of input data and metrics,
         such as current resource usage and demand, traffic forecasts
         and matrices, etc., all for the sake of highly responsive
         dynamic resource allocation and policy enforcement schemes.

   o  Better accommodation of technologically heterogeneous networking
      environments through the following:

      *  Vendor-independent configuration procedures based upon the
         enforcement of vendor-agnostic generic policies instead of
         vendor-specific languages.

      *  Tools to aid manageability and orchestrate resources.

      *  Avoiding proxies and privileging direct interaction with
         engines (e.g., routing and forwarding).

4.2.  Bootstrapping an SDN

   Means to dynamically discover the functional capabilities of the
   devices that will be steered by a PDP intelligence for automated
   network service delivery need to be provided.  This is because the
   acquisition of the information related to what the network is
   actually capable of will help structure the PDP intelligence so that
   policy provisioning information can be derived accordingly.

   A typical example would consist in documenting a traffic engineering
   policy based upon the dynamic discovery of the various functions
   supported by the network devices, as a function of the services to be



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   delivered, thus yielding the establishment of different routes
   towards the same destination depending on the nature of the traffic,
   the location of the functions that need to be invoked to forward such
   traffic, etc.

   Such dynamic discovery capability can rely upon the exchange of
   specific information by means of an IGP or BGP between network
   devices or between network devices and the PDP in legacy networking
   environments.  The PDP can also send unsolicited commands towards
   network devices to acquire the description of their functional
   capabilities in return and derive network and service topologies
   accordingly.

   Of course, SDN techniques (as introduced in Section 2.4) could be
   deployed in an IGP-/BGP-free networking environment, but the SDN
   bootstrapping procedure in such an environment still assumes the
   support of the following capabilities:

   o  Dynamically discover SDN participating nodes (including the PDP)
      and their respective capabilities in a resilient manner, assuming
      the mutual authentication of the PDP and the participating devices
      Section 5.  The integrity of the information exchanged between the
      PDP and the participating devices during the discovery phase must
      also be preserved;

   o  Dynamically connect the PDP to the participating nodes and avoid
      any forwarding loops;

   o  Dynamically enable network services as a function of the device
      capabilities and (possibly) what has been dynamically negotiated
      between the customer and the service provider;

   o  Dynamically check connectivity between the PDP and the
      participating nodes and between participating nodes for the
      delivery of a given network service (or a set thereof);

   o  Dynamically assess the reachability scope as a function of the
      service to be delivered;

   o  Dynamically detect and diagnose failures, and proceed with
      corrective actions accordingly.

   Likewise, the means to dynamically acquire the descriptive
   information (including the base configuration) of any network device
   that may participate in the delivery of a given service should be
   provided so as to help the PDP structure the services that can be
   delivered as a function of the available resources, their location,
   etc.



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   In IGP-/BGP-free networking environments, a specific bootstrap
   protocol may thus be required to support the aforementioned
   capabilities for proper PDP- and SDN-capable device operation, in
   addition to the possible need for a specific additional network that
   would provide discovery and connectivity features.

   In particular, SDN design and operation in IGP-/BGP-free environments
   should provide performances similar to those of legacy environments
   that run an IGP and BGP.  For example, the underlying network should
   remain operational even if connection with the PDP has been lost.
   Furthermore, operators should assess the cost of introducing a new,
   specific bootstrap protocol compared to the cost of integrating the
   aforementioned capabilities in existing IGP/BGP protocol machineries.

   Since SDN-related features can be grafted into an existing network
   infrastructure, they may not be all enabled at once from a
   bootstrapping perspective; a gradual approach can be adopted instead.

   A typical deployment example would be to use an SDN decision-making
   process as an emulation platform that would help service providers
   and operators make appropriate technical choices before their actual
   deployment in the network.

   Finally, the completion of the discovery procedure does not
   necessarily mean that the network is now fully operational.  The
   operationality of the network usually assumes a robust design based
   upon resilience and high availability features.

4.3.  Operating an SDN

   From an Operations and Management (OAM) standpoint [RFC6291], running
   an SDN-capable network raises several issues such as those listed
   below:

   o  How do SDN service and network management blocks interact?  For
      example, how the results of the dynamic negotiation of service
      parameters with a customer or a set thereof over a given period of
      time will affect the PDP decision-making process (resource
      allocation, path computation, etc.).

   o  What should be the appropriate OAM tools for SDN network operation
      (e.g., to check PDP or PEP reachability)?

   o  How can performance (expressed in terms of service delivery time,
      for example) be optimized when the activation of software modules
      is controlled by an external entity (typically a PDP)?





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   o  To what extent does an SDN implementation ease network
      manageability, including service and network diagnosis?

   o  Should the "control and data plane separation" principle be
      applied to the whole network or a portion thereof, as a function
      of the nature of the services to be delivered or by taking into
      account the technology that is currently deployed?

   o  What is the impact on the service provider's testing procedures
      and methodologies (that are used during validation and pre-
      deployment phases)?  Particularly, (1) how test cases will be
      defined and executed when the activation of customized modules is
      supported, (2) what the methodology is to assess the behavior of
      SDN-controlled devices, (3) how test regression will be conducted,
      (4) etc.

   o  How do SDN techniques impact service fulfillment and assurance?
      How the resulting behavior of SDN devices (completion of
      configuration tasks, for example) should be assessed against what
      has been dynamically negotiated with a customer.  How to measure
      the efficiency of dynamically enforced policies as a function of
      the service that has been delivered.  How to measure that what has
      been delivered is compliant with what has been negotiated.  What
      the impact is of SDN techniques on troubleshooting practice.

   o  Is there any risk to operate frozen architectures because of
      potential interoperability issues between a controlled device and
      an SDN controller?

   o  How does the introduction of SDN techniques affect the lifetime of
      legacy systems?  Is there any risk of (rapidly) obsoleting
      existing technologies because of their hardware or software
      limitations?

   The answers to the above questions are very likely to be service
   provider specific, depending on their technological and business
   environments.

4.4.  The Intelligence Resides in the PDP

   The proposed SDN definition in Section 2.3 assumes an intelligence
   that may reside in the control or the management planes (or both).
   This intelligence is typically represented by a Policy Decision Point
   (PDP) [RFC2753], which is one of the key functional components of the
   policy-based management framework.






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   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.

   The design and the operation of such PDP-based intelligence in a
   scalable manner remains a part of the major areas that need to be
   investigated.

   To avoid centralized design schemes, inter-PDP communication is
   likely to be required, and corresponding issues and solutions should
   be considered.  Several PDP instances may thus be activated in a
   given domain.  Because each of these PDP instances may be responsible
   for making decisions about the enforcement of a specific policy
   (e.g., one PDP for QoS policy enforcement purposes, another one for
   security policy enforcement purposes, etc.), an inter-PDP
   communication scheme is required for global PDP coordination and
   correlation.

   Inter-domain PDP exchanges may also be needed for specific usages.
   Examples of such exchanges are as follows: (1) during the network
   attachment phase of a node to a visited network, the PDP operated by
   the visited network can contact the home PDP to retrieve the policies
   to be enforced for that node, and (2) various PDPs can collaborate in
   order to compute inter-domain paths that satisfy a set of traffic
   performance guarantees.

4.5.  Simplicity and Adaptability vs. Complexity

   The functional metadomains introduced in Section 2.4 assume the
   introduction of a high level of automation, from service negotiation
   to delivery and operation.  Automation is the key to simplicity, but
   it 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
   automation.

4.6.  Performance and 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
   dynamics.




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   For example, networks deployed in Data Centers (DCs) and that rely
   upon OpenFlow switches are unlikely to raise important FIB
   scalability issues.  Conversely, DC interconnect designs that aim to
   dynamically manage Virtual Machine (VM) mobility, possibly based upon
   the dynamic enforcement of specific QoS policies, may raise
   scalability issues.

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

4.7.  Risk Assessment

   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.  From a
      technical standpoint, the ability to dynamically provision
      resources as a function of the services to be delivered may be
      incompatible with legacy routing systems because of their hardware
      limitations, for example.  Likewise, from an economical
      standpoint, the use of SDN solutions for the sake of flexibility
      and automation may dramatically impact Capital Expenditure (CAPEX)
      and Operational Expenditure (OPEX) budgets.

5.  Security Considerations

   Security is an important aspect of any SDN design because it
   conditions the robustness and reliability of the interactions between
   network and applications people for efficient access control
   procedures and optimized protection of SDN resources against any kind
   of attack.  In particular, SDN security policies [SDNSEC] should make
   sure that SDN resources are properly safeguarded against actions that
   may jeopardize network or application operations.

   In particular, service providers should define procedures to assess
   the reliability of software modules embedded in SDN nodes.  Such
   procedures should include the means to also assess the behavior of
   software components (under stress conditions), detect any exploitable
   vulnerability, reliably proceed with software upgrades, etc.  These





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   security guards should be activated during initial SDN node
   deployment and activation but also during SDN operation that implies
   software upgrade procedures.

   Although these procedures may not be SDN-specific (e.g., operators
   are familiar with firmware updates with or without service
   disruption), it is worth challenging existing practice in light of
   SDN deployment and operation.

   Likewise, PEP-PDP interactions suggest the need to make sure that (1)
   a PDP is entitled to solicit PEPs, so that they can apply the
   decisions made by the said PDP, (2) a PEP is entitled to solicit a
   PDP for whatever reason (request for additional configuration
   information, notification about the results of a set of configuration
   tasks, etc.), (3) a PEP can accept decisions made by a PDP, and (4)
   communication between PDPs within a domain or between domains is
   properly secured (e.g., make sure a pair of PDPs are entitled to
   communicate with each other, make sure the confidentiality of the
   information exchanged between two PDPs can be preserved, etc.).

6.  Acknowledgements

   Many thanks to R. Barnes, S. Bryant, S. Dawkins, A. Farrel, S.
   Farrell, W. George, J. Halpern, D. King, J. Hadi Salim, and T. Tsou
   for their comments.  Special thanks to P. Georgatos for the fruitful
   discussions on SDN Interconnection (SDNI) in particular.

7.  Informative References

   [AN]       Tennenhouse, D. and D. Wetherall, "Towards an Active
              Network Architecture", Multimedia Computing and Networking
              (MMCN), January 1996.

   [AUTOMATION]
              Boucadair, M. and C. Jacquenet, "Requirements for
              Automated (Configuration) Management", Work in Progress,
              January 2014.

   [CPNP]     Boucadair, M. and C. Jacquenet, "Connectivity Provisioning
              Negotiation Protocol (CPNP)", Work in Progress, October
              2013.

   [CPP]      Boucadair, M., Jacquenet, C., and N. Wang, "IP/MPLS
              Connectivity Provisioning Profile", Work in Progress,
              September 2012.






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   [LS-DISTRIB]
              Gredler, H., Medved, J., Previdi, S., Farrel, A., and S.
              Ray, "North-Bound Distribution of Link-State and TE
              Information using BGP", Work in Progress, November 2013.

   [PN]       Campbell, A., De Meer, H., Kounavis, M., Kazuho, M.,
              Vincente, J., and D. Villela, "A Survey of Programmable
              Networks", ACM SIGCOMM Computer Communication Review,
              April 1999.

   [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
              2000.

   [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., 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
              2009.

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

   [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.





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   [RFC6291]  Andersson, L., van Helvoort, H., Bonica, R., Romascanu,
              D., and S. Mansfield, "Guidelines for the Use of the "OAM"
              Acronym in the IETF", BCP 161, RFC 6291, June 2011.

   [SDNSEC]   Hartman, S. and D. Zhang, "Security Requirements in the
              Software Defined Networking Model", Work in Progress,
              April 2013.

   [SLA-EXCHANGE]
              Shah, S., Patel, K., Bajaj, S., Tomotaki, L., and M.
              Boucadair, "Inter-domain SLA Exchange", Work in Progress,
              November 2013.

Authors' Addresses

   Mohamed Boucadair
   France Telecom
   Rennes  35000
   France

   EMail: mohamed.boucadair@orange.com


   Christian Jacquenet
   France Telecom
   Rennes
   France

   EMail: christian.jacquenet@orange.com






















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