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INFORMATIONAL

Independent Submission                                     LM. Contreras
Request for Comments: 8597                                    Telefonica
Category: Informational                                    CJ. Bernardos
ISSN: 2070-1721                                                     UC3M
                                                                D. Lopez
                                                              Telefonica
                                                            M. Boucadair
                                                                  Orange
                                                              P. Iovanna
                                                                Ericsson
                                                                May 2019


Cooperating Layered Architecture for Software-Defined Networking (CLAS)

Abstract

   Software-Defined Networking (SDN) advocates for the separation of the
   control plane from the data plane in the network nodes and its
   logical centralization on one or a set of control entities.  Most of
   the network and/or service intelligence is moved to these control
   entities.  Typically, such an entity is seen as a compendium of
   interacting control functions in a vertical, tightly integrated
   fashion.  The relocation of the control functions from a number of
   distributed network nodes to a logical central entity conceptually
   places together a number of control capabilities with different
   purposes.  As a consequence, the existing solutions do not provide a
   clear separation between transport control and services that rely
   upon transport capabilities.

   This document describes an approach called Cooperating Layered
   Architecture for Software-Defined Networking (CLAS), wherein the
   control functions associated with transport are differentiated from
   those related to services in such a way that they can be provided and
   maintained independently and can follow their own evolution path.
















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Status of This Memo

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

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not candidates for any level of Internet Standard;
   see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8597.

Copyright Notice

   Copyright (c) 2019 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
   (https://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.  Architecture Overview . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Functional Strata . . . . . . . . . . . . . . . . . . . .   9
       3.1.1.  Transport Stratum . . . . . . . . . . . . . . . . . .   9
       3.1.2.  Service Stratum . . . . . . . . . . . . . . . . . . .  10
       3.1.3.  Recursiveness . . . . . . . . . . . . . . . . . . . .  10
     3.2.  Plane Separation  . . . . . . . . . . . . . . . . . . . .  10
       3.2.1.  Control Plane . . . . . . . . . . . . . . . . . . . .  11
       3.2.2.  Management Plane  . . . . . . . . . . . . . . . . . .  11
       3.2.3.  Resource Plane  . . . . . . . . . . . . . . . . . . .  11
   4.  Required Features . . . . . . . . . . . . . . . . . . . . . .  11
   5.  Communication between SDN Controllers . . . . . . . . . . . .  12
   6.  Deployment Scenarios  . . . . . . . . . . . . . . . . . . . .  12
     6.1.  Full SDN Environments . . . . . . . . . . . . . . . . . .  13
       6.1.1.  Multiple Service Strata Associated with a Single
               Transport Stratum . . . . . . . . . . . . . . . . . .  13
       6.1.2.  Single Service Stratum Associated with Multiple
               Transport Strata  . . . . . . . . . . . . . . . . . .  13
     6.2.  Hybrid Environments . . . . . . . . . . . . . . . . . . .  13
       6.2.1.  SDN Service Stratum Associated with a Legacy
               Transport Stratum . . . . . . . . . . . . . . . . . .  13
       6.2.2.  Legacy Service Stratum Associated with an SDN
               Transport Stratum . . . . . . . . . . . . . . . . . .  13
     6.3.  Multi-domain Scenarios in the Transport Stratum . . . . .  14
   7.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  Network Function Virtualization (NFV) . . . . . . . . . .  14
     7.2.  Abstraction and Control of TE Networks  . . . . . . . . .  15
   8.  Challenges for Implementing Actions between Service and
       Transport Strata  . . . . . . . . . . . . . . . . . . . . . .  15
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     11.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Appendix A.  Relationship with RFC 7426 . . . . . . . . . . . . .  19
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20











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1.  Introduction

   Network softwarization advances are facilitating the introduction of
   programmability in the services and infrastructures of
   telecommunications operators.  This is generally achieved through the
   introduction of Software-Defined Networking (SDN) [RFC7149] [RFC7426]
   capabilities in the network, including controllers and orchestrators.

   However, there are concerns of a different nature that these SDN
   capabilities have to resolve.  On the one hand, actions focused on
   programming the network to handle the connectivity or forwarding of
   digital data between distant nodes are needed.  On the other hand,
   actions devoted to programming the functions or services that process
   (or manipulate) such digital data are also needed.

   SDN advocates for the separation of the control plane from the data
   plane in the network nodes by introducing abstraction among both
   planes, allowing the control logic on a functional entity, which is
   commonly referred as SDN Controller, to be centralized; one or
   multiple controllers may be deployed.  A programmatic interface is
   then defined between a forwarding entity (at the network node) and a
   control entity.  Through that interface, a control entity instructs
   the nodes involved in the forwarding plane and modifies their
   traffic-forwarding behavior accordingly.  Support for additional
   capabilities (e.g., performance monitoring, fault management, etc.)
   could be expected through this kind of programmatic interface
   [RFC7149].

   Most of the intelligence is moved to this kind of functional entity.
   Typically, such an entity is seen as a compendium of interacting
   control functions in a vertical, tightly integrated fashion.

   The approach of considering an omnipotent control entity governing
   the overall aspects of a network, especially both the transport
   network and the services to be supported on top of it, presents a
   number of issues:

   o  From a provider perspective, where different departments usually
      are responsible for handling service and connectivity (i.e.,
      transport capabilities for the service on top), the mentioned
      approach offers unclear responsibilities for complete service
      provision and delivery.

   o  Complex reuse of functions for the provision of services.

   o  Closed, monolithic control architectures.





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   o  Difficult interoperability and interchangeability of functional
      components.

   o  Blurred business boundaries among providers, especially in
      situations where one provider provides only connectivity while
      another provider offers a more sophisticated service on top of
      that connectivity.

   o  Complex service/network diagnosis and troubleshooting,
      particularly to determine which layer is responsible for a
      failure.

   The relocation of the control functions from a number of distributed
   network nodes to another entity conceptually places together a number
   of control capabilities with different purposes.  As a consequence,
   the existing SDN solutions do not provide a clear separation between
   services and transport control.  Here, the separation between service
   and transport follows the distinction provided by [Y.2011] and as
   defined in Section 2 of this document.

   This document describes an approach called Cooperating Layered
   Architecture for SDN (CLAS), wherein the control functions associated
   with transport are differentiated from those related to services in
   such a way that they can be provided and maintained independently and
   can follow their own evolution path.

   Despite such differentiation, close cooperation between the service
   and transport layers (or strata in [Y.2011]) and the associated
   components are necessary to provide efficient usage of the resources.

2.  Terminology

   This document makes use of the following terms:

   o  Transport: denotes the transfer capabilities offered by a
      networking infrastructure.  The transfer capabilities can rely
      upon pure IP techniques or other means, such as MPLS or optics.

   o  Service: denotes a logical construct that makes use of transport
      capabilities.

      This document does not make any assumptions about the functional
      perimeter of a service that can be built above a transport
      infrastructure.  As such, a service can be offered to customers or
      invoked for the delivery of another (added-value) service.






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   o  Layer: refers to the set of elements that enable either transport
      or service capabilities, as defined previously.  In [Y.2011], this
      is referred to as a "stratum", and the two terms are used
      interchangeably.

   o  Domain: is a set of elements that share a common property or
      characteristic.  In this document, it applies to the
      administrative domain (i.e., elements pertaining to the same
      organization), technological domain (elements implementing the
      same kind of technology, such as optical nodes), etc.

   o  SDN Intelligence: refers to the decision-making process that is
      hosted by a node or a set of nodes.  These nodes are called SDN
      controllers.

      The intelligence can be centralized or distributed.  Both schemes
      are within the scope of this document.

      An SDN Intelligence relies on inputs from various functional
      blocks, such as: network topology discovery, service topology
      discovery, resource allocation, business guidelines, customer
      profiles, service profiles, etc.

      The exact decomposition of an SDN Intelligence, apart from the
      layering discussed here, is out of the scope of this document.

   Additionally, the following acronyms are used in this document:

      CLAS: Cooperating Layered Architecture for SDN

      FCAPS: Fault, Configuration, Accounting, Performance, and Security

      SDN: Software-Defined Networking

      SLA: Service Level Agreement

3.  Architecture Overview

   Current operator networks support multiple services (e.g., Voice over
   IP (VoIP), IPTV, mobile VoIP, critical mission applications, etc.) on
   a variety of transport technologies.  The provision and delivery of a
   service independent of the underlying transport capabilities require
   a separation of the service-related functionalities and an
   abstraction of the transport network to hide the specifics of the
   underlying transfer techniques while offering a common set of
   capabilities.





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   Such separation can provide configuration flexibility and
   adaptability from the point of view of either the services or the
   transport network.  Multiple services can be provided on top of a
   common transport infrastructure; similarly, different technologies
   can accommodate the connectivity requirements of a certain service.
   Close coordination among these elements is required for consistent
   service delivery (inter-layer cooperation).

   This document focuses particularly on the means to:

   o  expose transport capabilities to services.

   o  capture transport requirements of services.

   o  notify service intelligence of underlying transport events, for
      example, to adjust a service decision-making process with
      underlying transport events.

   o  instruct the underlying transport capabilities to accommodate new
      requirements, etc.

   An example is guaranteeing some Quality-of-Service (QoS) levels.
   Different QoS-based offerings could be present at both the service
   and transport layers.  Vertical mechanisms for linking both service
   and transport QoS mechanisms should be in place to provide quality
   guarantees to the end user.

   CLAS architecture assumes that the logically centralized control
   functions are separated into two functional layers.  One of the
   functional layers comprises the service-related functions, whereas
   the other one contains the transport-related functions.  The
   cooperation between the two layers is expected to be implemented
   through standard interfaces.

   Figure 1 shows the CLAS architecture.  It is based on functional
   separation in the Next Generation Network (NGN) architecture defined
   by the ITU-T in [Y.2011], where two strata of functionality are
   defined.  These strata are the Service Stratum, comprising the
   service-related functions, and the Transport Stratum, covering the
   transport-related functions.  The functions of each of these layers
   are further grouped into the control, management, and user (or data)
   planes.

   CLAS adopts the same structured model described in [Y.2011] but
   applies it to the objectives of programmability through SDN
   [RFC7149].  In this respect, CLAS advocates for addressing services
   and transport in a separated manner because of their differentiated
   concerns.



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                                       Applications
                                            /\
                                            ||
                                            ||
      +-------------------------------------||-------------+
      | Service Stratum                     ||             |
      |                                     \/             |
      |                       ...........................  |
      |                       . SDN Intelligence        .  |
      |                       .                         .  |
      |  +--------------+     .        +--------------+ .  |
      |  | Resource Pl. |     .        |   Mgmt. Pl.  | .  |
      |  |              |<===>.  +--------------+     | .  |
      |  |              |     .  |  Control Pl. |     | .  |
      |  +--------------+     .  |              |-----+ .  |
      |                       .  |              |       .  |
      |                       .  +--------------+       .  |
      |                       ...........................  |
      |                                     /\             |
      |                                     ||             |
      +-------------------------------------||-------------+
                                            ||    Standard
                                         -- || --    API
                                            ||
      +-------------------------------------||-------------+
      | Transport Stratum                   ||             |
      |                                     \/             |
      |                       ...........................  |
      |                       . SDN Intelligence        .  |
      |                       .                         .  |
      |  +--------------+     .        +--------------+ .  |
      |  | Resource Pl. |     .        |   Mgmt. Pl.  | .  |
      |  |              |<===>.  +--------------+     | .  |
      |  |              |     .  |  Control Pl. |     | .  |
      |  +--------------+     .  |              |-----+ .  |
      |                       .  |              |       .  |
      |                       .  +--------------+       .  |
      |                       ...........................  |
      |                                                    |
      |                                                    |
      +----------------------------------------------------+

            Figure 1: Cooperating Layered Architecture for SDN








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   In the CLAS architecture, both the control and management functions
   are considered to be performed by one or a set of SDN controllers
   (due to, for example, scalability, reliability), providing the SDN
   Intelligence in such a way that separated SDN controllers are present
   in the Service and Transport Strata.  Management functions are
   considered to be part of the SDN Intelligence to allow for effective
   operation in a service provider ecosystem [RFC7149], although some
   initial propositions did not consider such management as part of the
   SDN environment [ONFArch].

   Furthermore, the generic user- or data-plane functions included in
   the NGN architecture are referred to here as resource-plane
   functions.  The resource plane in each stratum is controlled by the
   corresponding SDN Intelligence through a standard interface.

   The SDN controllers cooperate in the provision and delivery of
   services.  There is a hierarchy in which the Service SDN Intelligence
   makes requests of the Transport SDN Intelligence for the provision of
   transport capabilities.

   The Service SDN Intelligence acts as a client of the Transport SDN
   Intelligence.

   Furthermore, the Transport SDN Intelligence interacts with the
   Service SDN Intelligence to inform it about events in the transport
   network that can motivate actions in the service layer.

   Despite not being shown in Figure 1, the resource planes of each
   stratum could be connected.  This will depend on the kind of service
   provided.  Furthermore, the Service Stratum could offer an interface
   to applications to expose network service capabilities to those
   applications or customers.

3.1.  Functional Strata

   As aforementioned, there is a functional split that separates
   transport-related functions from service-related functions.  Both
   strata cooperate for consistent service delivery.

   Consistency is determined and characterized by the service layer.

3.1.1.  Transport Stratum

   The Transport Stratum comprises the functions focused on the transfer
   of data between the communication endpoints (e.g., between end-user
   devices, between two service gateways, etc.).  The data-forwarding
   nodes are controlled and managed by the Transport SDN component.




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   The control plane in the SDN Intelligence is in charge of instructing
   the forwarding devices to build the end-to-end data path for each
   communication or to make sure the forwarding service is appropriately
   set up.  Forwarding may not be rely solely on the preconfigured
   entries; means can be enabled so that involved nodes can dynamically
   build routing and forwarding paths (this would require that the nodes
   retain some of the control and management capabilities for enabling
   this).  Finally, the management plane performs management functions
   (i.e., FCAPS) on those devices, like fault or performance management,
   as part of the Transport Stratum capabilities.

3.1.2.  Service Stratum

   The Service Stratum contains the functions related to the provision
   of services and the capabilities offered to external applications.
   The resource plane consists of the resources involved in the service
   delivery, such as computing resources, registries, databases, etc.

   The control plane is in charge of controlling and configuring those
   resources as well as interacting with the control plane of the
   Transport Stratum in client mode to request transport capabilities
   for a given service.  In the same way, the management plane
   implements management actions on the service-related resources and
   interacts with the management plane in the Transport Stratum to
   ensure management cooperation between layers.

3.1.3.  Recursiveness

   Recursive layering can happen in some usage scenarios in which the
   Transport Stratum is itself structured in the Service and Transport
   Strata.  This could be the case in the provision of a transport
   service complemented with advanced capabilities in addition to the
   pure data transport (e.g., maintenance of a given SLA [RFC7297]).

   Recursiveness has also been discussed in [ONFArch] as a way of
   reaching scalability and modularity, where each higher level can
   provide greater abstraction capabilities.  Additionally,
   recursiveness can allow some multi-domain scenarios where single or
   multiple administrative domains are involved, such as those described
   in Section 6.3.

3.2.  Plane Separation

   The CLAS architecture leverages plane separation.  As mentioned in
   Sections 3.1.1 and 3.1.2, three different planes are considered for
   each stratum.  The communication among these three planes (with the
   corresponding plane in other strata) is based on open, standard
   interfaces.



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3.2.1.  Control Plane

   The control plane logically centralizes the control functions of each
   stratum and directly controls the corresponding resources.  [RFC7426]
   introduces the role of the control plane in an SDN architecture.
   This plane is part of an SDN Intelligence and can interact with other
   control planes in the same or different strata to perform control
   functions.

3.2.2.  Management Plane

   The management plane logically centralizes the management functions
   for each stratum, including the management of the control and
   resource planes.  [RFC7426] describes the functions of the management
   plane in an SDN environment.  This plane is also part of the SDN
   Intelligence and can interact with the corresponding management
   planes residing in SDN controllers of the same or different strata.

3.2.3.  Resource Plane

   The resource plane comprises the resources for either the transport
   or the service functions.  In some cases, the service resources can
   be connected to the transport ones (e.g., being the terminating
   points of a transport function); in other cases, it can be decoupled
   from the transport resources (e.g., one database keeping a register
   for the end user).  Both the forwarding and operational planes
   proposed in [RFC7426] would be part of the resource plane in this
   architecture.

4.  Required Features

   Since the CLAS architecture implies the interaction of different
   layers with different purposes and responsibilities, a number of
   features are required to be supported:

   o  Abstraction: the mapping of physical resources into the
      corresponding abstracted resources.

   o  Service-Parameter Translation: the translation of service
      parameters (e.g., in the form of SLAs) to transport parameters (or
      capabilities) according to different policies.

   o  Monitoring: mechanisms (e.g., event notifications) available in
      order to dynamically update the (abstracted) resources' status
      while taking into account, for example, the traffic load.






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   o  Resource Computation: functions able to decide which resources
      will be used for a given service request.  As an example,
      functions like PCE could be used to compute/select/decide a
      certain path.

   o  Orchestration: the ability to combine diverse resources (e.g., IT
      and network resources) in an optimal way.

   o  Accounting: record of resource usage.

   o  Security: secure communication among components, preventing, for
      example, DoS attacks.

5.  Communication between SDN Controllers

   The SDN controllers residing respectively in the Service and
   Transport Strata need to establish tight coordination.  Mechanisms
   for transferring relevant information for each stratum should be
   defined.

   From the service perspective, the Service SDN Intelligence needs to
   easily access transport resources through well-defined APIs to
   retrieve the capabilities offered by the Transport Stratum.  There
   could be different ways of obtaining such transport-aware
   information, i.e., by discovering or publishing mechanisms.  In the
   former case, the Service SDN Intelligence could be able to handle
   complete information about the transport capabilities (including
   resources) offered by the Transport Stratum.  In the latter case, the
   Transport Stratum reveals the available capabilities, for example,
   through a catalog, reducing the amount of detail of the underlying
   network.

   On the other hand, the Transport Stratum must properly capture the
   Service requirements.  These can include SLA requirements with
   specific metrics (such as delay), the level of protection to be
   provided, maximum/minimum capacity, applicable resource constraints,
   etc.

   The communication between controllers must also be secure, e.g., by
   preventing denial of service or any other kind of threat (similarly,
   communications with the network nodes must be secure).

6.  Deployment Scenarios

   Different situations can be found depending on the characteristics of
   the networks involved in a given deployment.





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6.1.  Full SDN Environments

   This case considers that the networks involved in the provision and
   delivery of a given service have SDN capabilities.

6.1.1.  Multiple Service Strata Associated with a Single Transport
        Stratum

   A single Transport Stratum can provide transfer functions to more
   than one Service Stratum.  The Transport Stratum offers a standard
   interface(s) to each of the Service Strata.  The Service Strata are
   the clients of the Transport Stratum.  Some of the capabilities
   offered by the Transport Stratum can be isolation of the transport
   resources (slicing), independent routing, etc.

6.1.2.  Single Service Stratum Associated with Multiple Transport Strata

   A single Service Stratum can make use of different Transport Strata
   for the provision of a certain service.  The Service Stratum invokes
   standard interfaces to each of the Transport Strata, and orchestrates
   the provided transfer capabilities for building the end-to-end
   transport needs.

6.2.  Hybrid Environments

   This case considers scenarios where one of the strata is totally or
   partly legacy.

6.2.1.  SDN Service Stratum Associated with a Legacy Transport Stratum

   An SDN service Stratum can interact with a legacy Transport Stratum
   through an interworking function that is able to adapt SDN-based
   control and management service-related commands to legacy transport-
   related protocols, as expected by the legacy Transport Stratum.

   The SDN Intelligence in the Service Stratum is not aware of the
   legacy nature of the underlying Transport Stratum.

6.2.2.  Legacy Service Stratum Associated with an SDN Transport Stratum

   A legacy Service Stratum can work with an SDN-enabled Transport
   Stratum through the mediation of an interworking function capable of
   interpreting commands from the legacy service functions and
   translating them into SDN protocols for operation with the SDN-
   enabled Transport Stratum.






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6.3.  Multi-domain Scenarios in the Transport Stratum

   The Transport Stratum can be composed of transport resources that are
   part of different administrative, topological, or technological
   domains.  The Service Stratum can interact with a single entity in
   the Transport Stratum in case some abstraction capabilities are
   provided in the transport part to emulate a single stratum.

   Those abstraction capabilities constitute a service itself offered by
   the Transport Stratum to the services making use of this stratum.
   This service is focused on the provision of transport capabilities,
   which is different from the final communication service using such
   capabilities.

   In this particular case, this recursion allows multi-domain scenarios
   at the transport level.

   Multi-domain situations can happen in both single-operator and multi-
   operator scenarios.

   In single-operator scenarios, a multi-domain or end-to-end
   abstraction component can provide a homogeneous abstract view of the
   underlying heterogeneous transport capabilities for all the domains.

   Multi-operator scenarios at the Transport Stratum should support the
   establishment of end-to-end paths in a programmatic manner across the
   involved networks.  For example, this could be accomplished by each
   of the administrative domains exchanging their traffic-engineered
   information [RFC7926].

7.  Use Cases

   This section presents a number of use cases as examples of the
   applicability of the CLAS approach.

7.1.  Network Function Virtualization (NFV)

   NFV environments offer two possible levels of SDN control
   [GSNFV-EVE005].  One level is the need to control the NFV
   Infrastructure (NFVI) to provide end-to-end connectivity among VNFs
   (Virtual Network Functions) or among VNFs and PNFs (Physical Network
   Functions).  A second level is the control and configuration of the
   VNFs themselves (in other words, the configuration of the network
   service implemented by those VNFs), which benefits from the
   programmability brought by SDN.  The two control concerns are
   separate in nature.  However, interaction between the two can be
   expected in order to optimize, scale, or influence one another.




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7.2.  Abstraction and Control of TE Networks

   Abstraction and Control of TE Networks (ACTN) [RFC8453] presents a
   framework that allows the creation of virtual networks to be offered
   to customers.  The concept of "provider" in ACTN is limited to the
   offering of virtual network services.  These services are essentially
   transport services and would correspond to the Transport Stratum in
   CLAS.  On the other hand, the Service Stratum in CLAS can be
   assimilated as a customer in the context of ACTN.

   ACTN defines a hierarchy of controllers to facilitate the creation
   and operation of the virtual networks.  An interface is defined for
   the relationship between the customers requesting these virtual
   network services and the controller in charge of orchestrating and
   serving such a request.  Such an interface is equivalent to the one
   defined in Figure 1 (Section 3) between the Service and Transport
   Strata.

8.  Challenges for Implementing Actions between Service and Transport
    Strata

   The distinction of service and transport concerns raises a number of
   challenges in the communication between the two strata.  The
   following list reflects some of the identified challenges:

   o  Standard mechanisms for interaction between layers: Nowadays,
      there are a number of proposals that could accommodate requests
      from the Service Stratum to the Transport Stratum.

      Some of the proposals could be solutions like the Connectivity
      Provisioning Negotiation Protocol [CPNP] or the Intermediate-
      Controller Plane Interface (I-CPI) [ONFArch].

      Other potential candidates could be the Transport API [TAPI] or
      the Transport Northbound Interface [TRANS-NORTH].  Each of these
      options has a different scope.

   o  Multi-provider awareness: In multi-domain scenarios involving more
      than one provider at the transport level, the Service Stratum may
      or may not be aware of such multiplicity of domains.

      If the Service Stratum is unaware of the multi-domain situation,
      then the Transport Stratum acting as the entry point of the
      Service Stratum request should be responsible for managing the
      multi-domain issue.






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      On the contrary, if the Service Stratum is aware of the multi-
      domain situation, it should be in charge of orchestrating the
      requests to the different underlying Transport Strata to compose
      the final end-to-end path among service endpoints (i.e., service
      functions).

   o  SLA mapping: Both strata will handle SLAs, but the nature of those
      SLAs could differ.  Therefore, it is required for the entities in
      each stratum to map service SLAs to connectivity SLAs in order to
      ensure proper service delivery.

   o  Association between strata: The association between strata could
      be configured beforehand, or both strata could require the use of
      a discovery mechanism that dynamically establishes the association
      between the strata.

   o  Security: As reflected before, the communication between strata
      must be secure to prevent attacks and threats.  Additionally,
      privacy should be enforced, especially when addressing multi-
      provider scenarios at the transport level.

   o  Accounting: The control and accountancy of resources used and
      consumed by services should be supported in the communication
      among strata.

9.  IANA Considerations

   This document has no IANA actions.

10.  Security Considerations

   The CLAS architecture relies upon the functional entities that are
   introduced in [RFC7149] and [RFC7426].  As such, security
   considerations discussed in Section 5 of [RFC7149], in particular,
   must be taken into account.

   The communication between the service and transport SDN controllers
   must rely on secure means that achieve the following:

   o  Mutual authentication must be enabled before taking any action.

   o  Message integrity protection.

   Each of the controllers must be provided with instructions regarding
   the set of information (and granularity) that can be disclosed to a
   peer controller.  Means to prevent the leaking of privacy data (e.g.,
   from the Service Stratum to the Transport Stratum) must be enabled.
   The exact set of information to be shared is deployment specific.



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   A corrupted controller may induce some disruption on another
   controller.  Protection against such attacks should be enabled.

   Security in the communication between the strata described here
   should apply to the APIs (and/or protocols) to be defined among them.
   Consequently, security concerns will correspond to the specific
   solution.

11.  References

11.1.  Normative References

   [Y.2011]   International Telecommunication Union, "General principles
              and general reference model for Next Generation Networks",
              ITU-T Recommendation Y.2011, October 2004,
              <https://www.itu.int/rec/T-REC-Y.2011-200410-I/en>.

11.2.  Informative References

   [CPNP]     Boucadair, M., Jacquenet, C., Zhang, D., and
              P. Georgatsos, "Connectivity Provisioning Negotiation
              Protocol (CPNP)", Work in Progress, draft-boucadair-
              connectivity-provisioning-protocol-15, December 2017.

   [GSNFV-EVE005]
              ETSI, "Network Functions Virtualisation (NFV); Ecosystem;
              Report on SDN Usage in NFV Architectural Framework", ETSI
              GS NFV-EVE 005, V1.1.1, December 2015,
              <https://www.etsi.org/deliver/etsi_gs/
              NFV-EVE/001_099/005/01.01.01_60/
              gs_nfv-eve005v010101p.pdf>.

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

   [RFC7149]  Boucadair, M. and C. Jacquenet, "Software-Defined
              Networking: A Perspective from within a Service Provider
              Environment", RFC 7149, DOI 10.17487/RFC7149, March 2014,
              <https://www.rfc-editor.org/info/rfc7149>.

   [RFC7297]  Boucadair, M., Jacquenet, C., and N. Wang, "IP
              Connectivity Provisioning Profile (CPP)", RFC 7297,
              DOI 10.17487/RFC7297, July 2014,
              <https://www.rfc-editor.org/info/rfc7297>.





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   [RFC7426]  Haleplidis, E., Ed., Pentikousis, K., Ed., Denazis, S.,
              Hadi Salim, J., Meyer, D., and O. Koufopavlou, "Software-
              Defined Networking (SDN): Layers and Architecture
              Terminology", RFC 7426, DOI 10.17487/RFC7426, January
              2015, <https://www.rfc-editor.org/info/rfc7426>.

   [RFC7926]  Farrel, A., Ed., Drake, J., Bitar, N., Swallow, G.,
              Ceccarelli, D., and X. Zhang, "Problem Statement and
              Architecture for Information Exchange between
              Interconnected Traffic-Engineered Networks", BCP 206,
              RFC 7926, DOI 10.17487/RFC7926, July 2016,
              <https://www.rfc-editor.org/info/rfc7926>.

   [RFC8453]  Ceccarelli, D., Ed. and Y. Lee, Ed., "Framework for
              Abstraction and Control of TE Networks (ACTN)", RFC 8453,
              DOI 10.17487/RFC8453, August 2018,
              <https://www.rfc-editor.org/info/rfc8453>.

   [SDN-ARCH] Contreras, LM., Bernardos, CJ., Lopez, D., Boucadair, M.,
              and P. Iovanna, "Cooperating Layered Architecture for
              SDN", Work in Progress, draft-irtf-sdnrg-layered-sdn-01,
              October 2016.

   [TAPI]     Open Networking Foundation, "Functional Requirements for
              Transport API", June 2016,
              <https://www.opennetworking.org/wp-content/uploads/
              2014/10/TR-527_TAPI_Functional_Requirements.pdf>.

   [TRANS-NORTH]
              Busi, I., King, D., Zheng, H., and Y. Xu, "Transport
              Northbound Interface Applicability Statement", Work in
              Progress, draft-ietf-ccamp-transport-nbi-app-statement-05,
              March 2019.


















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Appendix A.  Relationship with RFC 7426

   [RFC7426] introduces an SDN taxonomy by defining a number of planes,
   abstraction layers, and interfaces or APIs among them as a means of
   clarifying how the different parts constituent of SDN (network
   devices, control and management) relate.  A number of planes are
   defined, including:

   o  Forwarding Plane: focused on delivering packets in the data path
      based on the instructions received from the control plane.

   o  Operational Plane: centered on managing the operational state of
      the network device.

   o  Control Plane: dedicated to instructing the device on how packets
      should be forwarded.

   o  Management Plane: in charge of monitoring and maintaining network
      devices.

   o  Application Plane: enabling the usage for different purposes (as
      determined by each application) of all the devices controlled in
      this manner.

   Apart from these, [RFC7426] proposes a number of abstraction layers
   that permit the integration of the different planes through common
   interfaces.  CLAS focuses on control, management, and resource planes
   as the basic pieces of its architecture.  Essentially, the control
   plane modifies the behavior and actions of the controlled resources.
   The management plane monitors and retrieves the status of those
   resources.  And finally, the resource plane groups all the resources
   related to the concerns of each stratum.

   From this point of view, CLAS planes can be seen as a superset of
   those defined in [RFC7426].  However, in some cases, not all the
   planes considered in [RFC7426] may be totally present in CLAS
   representation (e.g., the forwarding plane in the Service Stratum).

   That being said, the internal structure of CLAS strata could follow
   the taxonomy defined in [RFC7426].  What is different is the
   specialization of the SDN environments through the distinction
   between service and transport.









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Acknowledgements

   This document was previously discussed and adopted in the IRTF SDN RG
   as [SDN-ARCH].  After the closure of the IRTF SDN RG, this document
   was progressed as an Independent Submission to record (some of) that
   group's discussions.

   The authors would like to thank (in alphabetical order) Bartosz
   Belter, Gino Carrozzo, Ramon Casellas, Gert Grammel, Ali Haider,
   Evangelos Haleplidis, Zheng Haomian, Giorgios Karagianis, Gabriel
   Lopez, Maria Rita Palatella, Christian Esteve Rothenberg, and Jacek
   Wytrebowicz for their comments and suggestions.

   Thanks to Adrian Farrel for the review.

Authors' Addresses

   Luis M. Contreras
   Telefonica
   Ronda de la Comunicacion, s/n
   Sur-3 building, 3rd floor
   Madrid  28050
   Spain

   Email: luismiguel.contrerasmurillo@telefonica.com
   URI:   http://lmcontreras.com


   Carlos J. Bernardos
   Universidad Carlos III de Madrid
   Av. Universidad, 30
   Leganes, Madrid  28911
   Spain

   Phone: +34 91624 6236
   Email: cjbc@it.uc3m.es
   URI:   http://www.it.uc3m.es/cjbc/


   Diego R. Lopez
   Telefonica
   Ronda de la Comunicacion, s/n
   Sur-3 building, 3rd floor
   Madrid  28050
   Spain

   Email: diego.r.lopez@telefonica.com




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   Mohamed Boucadair
   Orange
   Rennes  35000
   France

   Email: mohamed.boucadair@orange.com


   Paola Iovanna
   Ericsson
   Pisa
   Italy

   Email: paola.iovanna@ericsson.com





































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