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Network Working Group                                      LM. Contreras
Internet-Draft                                                Telefonica
Intended status: Informational                             CJ. Bernardos
Expires: May 25, 2019                                               UC3M
                                                                D. Lopez
                                                              Telefonica
                                                            M. Boucadair
                                                                  Orange
                                                              P. Iovanna
                                                                Ericsson
                                                       November 21, 2018


Cooperating Layered Architecture for Software Defined Networking (CLAS)
                     draft-contreras-layered-sdn-03

Abstract

   Software Defined Networking adheres to 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 sevice intelligence is moved to these control
   entities.  Typically, such entity is seen as a compendium of
   interacting control functions in a vertical, tight 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 relies
   upon transport capabilities.

   This document describes an approach named Cooperating Layered
   Architecture for Software Defined Networking.  The idea behind that
   is to differentiate the control functions associated to transport
   from those related to services, in such a way that they can be
   provided and maintained independently, and can follow their own
   evolution path.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.




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   Internet-Drafts are draft documents valid for a maximum of six months
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   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 . . . . . . . . . . . . . . . . . .  12
       6.1.1.  Multiple Service Strata Associated to a Single
               Transport Stratum . . . . . . . . . . . . . . . . . .  13
       6.1.2.  Single Service Stratum associated to multiple
               Transport Strata  . . . . . . . . . . . . . . . . . .  13
     6.2.  Hybrid Environments . . . . . . . . . . . . . . . . . . .  13
       6.2.1.  SDN Service Stratum associated to a Legacy Transport
               Stratum . . . . . . . . . . . . . . . . . . . . . . .  13
       6.2.2.  Legacy Service Stratum Associated to an SDN Transport
               Stratum . . . . . . . . . . . . . . . . . . . . . . .  13



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     6.3.  Multi-domain Scenarios in Transport Stratum . . . . . . .  13
   7.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  Network Function Virtualization (NFV) . . . . . . . . . .  14
     7.2.  Abstraction and Control of Transport Networks . . . . . .  14
   8.  Challenges for Implementing Actions Between Service and
       Transport Strata  . . . . . . . . . . . . . . . . . . . . . .  15
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     12.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Appendix A.  Relationship with RFC7426  . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

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

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

   SDN adheres to the separation of the control plane from the data
   plane in the network nodes by introducing abstraction among both
   planes, allowing to centralize the control logic on a functional
   entity which is commonly referred as SDN Controller; 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.  Additional capabilities (e.g.,
   performance monitoring, fault management, etc.) could be expected to
   be supported through such kind of programmatic interface [RFC7149].

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

   The approach of considering an omnipotent control entity governing
   the overall aspects of a network, especially both the transport



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   network and the services to be supported on top of it, presents a
   number of issues:

   o  From a provider perspective, where usually different departments
      are responsible of 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.

   o  Difficult interoperability and interchangeability of functional
      components.

   o  Blurred business boundaries among providers, especially in
      situations where a provider provides just 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 segment 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 also
   defined in Section 2 of this document.

   This document describes an approach named Cooperating Layered
   Architecture for SDN (CLAS).  The idea behind that is to
   differentiate the control functions associated to transport 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 it is required a close cooperation
   between service and transport layers (or strata in [Y.2011]) and
   associated components to provide an efficient usage of the resources.








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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 assumption on the functional
      perimeter of a service that can be built above a transport
      infrastructure.  As such, a service can be an offering that is
      offered to customers or be invoked for the delivery of another
      (added-value) service.

   o  Layer: refers to the set of elements comprised for enabling either
      transport or service capabilities as defined before.  In [Y.2011],
      this is referred to as stratum, and both are used interchangeably.

   o  Domain: is a set of elements which share a common property or
      characteristic.  In this document this applies to administrative
      domain (i.e., elements pertaining to the same organization),
      technological domain (elements implementing the same kind of
      technology, as for example 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.

      The SDN intelligence relies on inputs form 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 in this document, is out of scope.

   Additionally, the following acronyms are used in this document:

      CLAS: Cooperating Layered Architecture for SDN

      FCAPS: Fault, Configuration, Accounting, Performance and Security




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      SDN: Software Defined Networking

      SLA: Service Level Agreement

3.  Architecture Overview

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

   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, and similarly, different
   technologies can accommodate the connectivity requirements of a
   certain service.  A close coordination among them is required for a
   consistent service delivery (inter-layer cooperation).

   This document focuses particularly on:

   o  Means to expose transport capabilities to services.

   o  Means to capture service requirements of services.

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

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

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

   CLAS architecture assumes that the logically centralized control
   functions are separated in 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.




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

   CLAS adopts the same structured model described in [Y.2011] but
   applying it to the objectives of programmability through SDN
   [RFC7149].  To 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. |     .        |  Mngmt. Pl.  | .  |
      |  |              |<===>.  +--------------+     | .  |
      |  |              |     .  |  Control Pl. |     | .  |
      |  +--------------+     .  |              |-----+ .  |
      |                       .  |              |       .  |
      |                       .  +--------------+       .  |
      |                       ...........................  |
      |                                     /\             |
      |                                     ||             |
      +-------------------------------------||-------------+
                                            ||    Standard
                                         -- || --    API
                                            ||
      +-------------------------------------||-------------+
      | Transport Stratum                   ||             |
      |                                     \/             |
      |                       ...........................  |
      |                       . SDN Intelligence        .  |
      |                       .                         .  |
      |  +--------------+     .        +--------------+ .  |
      |  | Resource Pl. |     .        |  Mngmt. Pl.  | .  |
      |  |              |<===>.  +--------------+     | .  |
      |  |              |     .  |  Control Pl. |     | .  |
      |  +--------------+     .  |              |-----+ .  |
      |                       .  |              |       .  |
      |                       .  +--------------+       .  |
      |                       ...........................  |
      |                                                    |
      |                                                    |
      +----------------------------------------------------+


            Figure 1: Cooperating Layered Architecture for SDN

   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, e.g., scalability, reliability) providing the SDN



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   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 the
   effective operation in a service provider ecosystem [RFC7149] despite
   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 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 for the provision and delivery of
   services.  There is a hierarchy in which the Service SDN Intelligence
   requests transport capabilities to the Transport SDN Intelligence.

   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 it is not 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
   towards applications to expose network service capabilities to those
   applications or customers.

3.1.  Functional Strata

   As described before, the functional split separates transport-related
   functions from service-related functions.  Both strata cooperate for
   a 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 end points (e.g., between end-user
   devices, between two service gateways, etc.).  The data forwarding
   nodes are controlled and managed by the Transport SDN component.

   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 forwarding service is appropriately
   setup.  Forwarding may not be rely on the sole pre-configured



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   entries; dynamic means can be enabled so that involved nodes can
   build dynamically 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 for requesting 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 for
   a cooperating management between layers.

3.1.3.  Recursiveness

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

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

3.2.  Plane Separation

   The CLAS architecture leverages on plane separation.  As mentioned
   before, three different planes are considered for each stratum.  The
   communication among these three planes (and 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 a SDN architecture.  This
   plane is part of an SDN Intelligence, and can interact with other
   control planes in the same or different strata for accomplishing
   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 a 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) whereas in other cases it can be decoupled
   from the transport resources (e.g., one database keeping some
   register for the end user).  Both 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: 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
      taking in to account, e.g., the traffic load.

   o  Resource computation: functions able to decide which resources
      will be used for a given service request.  As an example,



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      functions like PCE could be used to compute/select/decide a
      certain path.

   o  Orchestration: 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, e.g.,
      DoS attacks.

5.  Communication Between SDN Controllers

   The SDN controllers residing respectively in the Service and the
   Transport Stratum need to establish a tight coordination.  Mechanisms
   for transfer 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 of handling
   complete information about the transport capabilities (including
   resources) offered by the Transport Stratum.  In the latter case, the
   Transport Stratum exposes available capabilities, e.g., through a
   catalog, reducing the amount of detail of the underlying network.

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

   The communication between controllers must be also secure, e.g., by
   preventing denial of service or any other kind of threats (similarly,
   the 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.

6.1.  Full SDN Environments

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





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6.1.1.  Multiple Service Strata Associated to a Single Transport Stratum

   A single Transport Stratum can provide transfer functions to more
   than one Service strata.  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 to multiple Transport Strata

   A single Service stratum can make use of different Transport Strata
   for the provision of a certain service.  The Service stratum
   interfaces each of the Transport Strata with standard protocols, 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 legacy
   totally or in part.

6.2.1.  SDN Service Stratum associated to a Legacy Transport Stratum

   An SDN service stratum can interact with a legacy Transport Stratum
   through some interworking function 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 to an SDN Transport Stratum

   A legacy Service stratum can work with an SDN-enabled Transport
   Stratum through the mediation of and interworking function capable to
   interpret commands from the legacy service functions and translate
   them into SDN protocols for operating with the SDN-enabled Transport
   Stratum.

6.3.  Multi-domain Scenarios in Transport Stratum

   The Transport Stratum can be composed by transport resources being
   part of different administrative, topological or technological
   domains.  The Service Stratum can yet 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.




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

   In this particular case this recursion allows multi-domain scenarios
   at 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 an 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.  This could be accomplished, for example, by
   the exchange of traffic-engineered information of each of the
   administrative domains [RFC7926].

7.  Use Cases

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

7.1.  Network Function Virtualization (NFV)

   NFV environments offer two possible levels of SDN control
   [ETSI_NFV_EVE005].  One level is the need for controlling the NFV
   Infrastructure (NFVI) to provide connectivity end-to- end 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), taking profit of the
   programmability brought by SDN.  Both control concerns are separated
   in nature.  However, interaction between both could be expected in
   order to optimize, scale or influence each other.

7.2.  Abstraction and Control of Transport Networks

   Abstraction and Control of Transport Networks (ACTN) [RFC8453]
   presents a framework to allow 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.



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   ACTN defines a hierarchy of controllers for facilitating the creation
   and operation of the virtual networks.  An interface is defined for
   the relation of the customers requesting these virtual networks
   services with the controller in charge of orchestrating and serving
   such request.  Such interface is equivalent to the one defined in
   Figure 1 (Section 3) between 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 both strata.  The following
   is a work-in-progress list reflecting 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 them could be solutions like the Connectivity Provisioning
      Protocol [I-D.boucadair-connectivity-provisioning-protocol] or the
      Intermediate-Controller Plane Interface (I-CPI) [ONFArch].

      Other potential candidates could be the Transport API [TAPI] or
      the Transport Transport Northbound Interface
      [I-D.ietf-ccamp-transport-nbi-app-statement].  Each of these
      options has a different status of maturity and scope.

   o  Multi-provider awareness: In multi-domain scenarios involving more
      than one provider at transport level, the service stratum could
      have or not awareness of such multiplicity of domains.

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

      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 for
      composing the final end-to-end path among service end-points
      (i.e., service functions).

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




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   o  Association between strata: The association between strata could
      be configured beforehand, or could be dynamic following mechanisms
      of discovery, that could be required to be supported by both
      strata with this purpose.

   o  Security: As reflected before, the communication between strata
      must be secure preventing attacks and threats.  Additionally,
      privacy should be enforced, especially when addressing multi-
      provider scenarios at 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 does not request any action from IANA.

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 which 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 about the
   set of information (and granularity) that can be disclosed to a peer
   controller.  Means to prevent leaking 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.

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

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






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11.  Acknowledgements

   This document was previously discussed and adopted in the IRTF SDN RG
   as [I-D.irtf-sdnrg-layered-sdn].  After the closure of the IRTF SDN
   RG this document is being progressed as Individual Submission to
   record (some of) that group's disucussions.

   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.

12.  References

12.1.  Normative References

   [Y.2011]   "General principles and general reference model for Next
              Generation Networks", ITU-T Recommendation Y.2011 ,
              October 2004.

12.2.  Informative References

   [ETSI_NFV_EVE005]
              "Report on SDN Usage in NFV Architectural Framework",
              December 2015.

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

   [I-D.ietf-ccamp-transport-nbi-app-statement]
              Busi, I., King, D., Zheng, H., and Y. Xu, "Transport
              Northbound Interface Applicability Statement", draft-ietf-
              ccamp-transport-nbi-app-statement-04 (work in progress),
              November 2018.

   [I-D.irtf-sdnrg-layered-sdn]
              Contreras, L., Bernardos, C., Lopez, D., Boucadair, M.,
              and P. Iovanna, "Cooperating Layered Architecture for
              SDN", draft-irtf-sdnrg-layered-sdn-01 (work in progress),
              October 2016.




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   [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>.

   [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>.

   [TAPI]     "Functional Requirements for Transport API", June 2016.

Appendix A.  Relationship with RFC7426

   [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 among them.  A number of
   planes are defined, namely:

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





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

   o  Control Plane: devoted to instruct 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 that, [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 strata.

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

   Being said that, internal structure of CLAS strata could follow the
   taxonomy defined in [RFC7426].  Which is differential is the
   specialization of the SDN environments, through the distinction
   between service and transport.

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








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   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


   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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