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rfc:rfc8568

Internet Research Task Force (IRTF) CJ. Bernardos Request for Comments: 8568 UC3M Category: Informational A. Rahman ISSN: 2070-1721 InterDigital

                                                            JC. Zuniga
                                                                SIGFOX
                                                         LM. Contreras
                                                                   TID
                                                             P. Aranda
                                                                  UC3M
                                                              P. Lynch
                                                 Keysight Technologies
                                                            April 2019
             Network Virtualization Research Challenges

Abstract

 This document describes open research challenges for network
 virtualization.  Network virtualization is following a similar path
 as previously taken by cloud computing.  Specifically, cloud
 computing popularized migration of computing functions (e.g.,
 applications) and storage from local, dedicated, physical resources
 to remote virtual functions accessible through the Internet.  In a
 similar manner, network virtualization is encouraging migration of
 networking functions from dedicated physical hardware nodes to a
 virtualized pool of resources.  However, network virtualization can
 be considered to be a more complex problem than cloud computing as it
 not only involves virtualization of computing and storage functions
 but also involves abstraction of the network itself.  This document
 describes current research and engineering challenges in network
 virtualization including the guarantee of quality of service,
 performance improvement, support for multiple domains, network
 slicing, service composition, device virtualization, privacy and
 security, separation of control concerns, network function placement,
 and testing.  In addition, some proposals are made for new activities
 in the IETF and IRTF that could address some of these challenges.
 This document is a product of the Network Function Virtualization
 Research Group (NFVRG).

Bernardos, et al. Informational [Page 1] RFC 8568 Network Virtualization Research Challenges April 2019

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Research Task Force
 (IRTF).  The IRTF publishes the results of Internet-related research
 and development activities.  These results might not be suitable for
 deployment.  This RFC represents the consensus of the Network
 Function Virtualization Research Group of the Internet Research Task
 Force (IRTF).  Documents approved for publication by the IRSG 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/rfc8568.

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.

Bernardos, et al. Informational [Page 2] RFC 8568 Network Virtualization Research Challenges April 2019

Table of Contents

 1.  Introduction and Scope  . . . . . . . . . . . . . . . . . . .   4
 2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
 3.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.1.  Network Function Virtualization . . . . . . . . . . . . .   6
   3.2.  Software-Defined Networking . . . . . . . . . . . . . . .   9
   3.3.  ITU-T Functional Architecture of SDN  . . . . . . . . . .  13
   3.4.  Multi-Access Edge Computing . . . . . . . . . . . . . . .  15
   3.5.  IEEE 802.1CF (OmniRAN)  . . . . . . . . . . . . . . . . .  15
   3.6.  Distributed Management Task Force (DMTF)  . . . . . . . .  15
   3.7.  Open-Source Initiatives . . . . . . . . . . . . . . . . .  16
 4.  Network Virtualization Challenges . . . . . . . . . . . . . .  18
   4.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  18
   4.2.  Guaranteeing Quality of Service . . . . . . . . . . . . .  18
     4.2.1.  Virtualization Technologies . . . . . . . . . . . . .  18
     4.2.2.  Metrics for NFV Characterization  . . . . . . . . . .  19
     4.2.3.  Predictive Analysis . . . . . . . . . . . . . . . . .  20
     4.2.4.  Portability . . . . . . . . . . . . . . . . . . . . .  20
   4.3.  Performance Improvement . . . . . . . . . . . . . . . . .  21
     4.3.1.  Energy Efficiency . . . . . . . . . . . . . . . . . .  21
     4.3.2.  Improved Link Usage . . . . . . . . . . . . . . . . .  21
   4.4.  Multiple Domains  . . . . . . . . . . . . . . . . . . . .  22
   4.5.  5G and Network Slicing  . . . . . . . . . . . . . . . . .  22
     4.5.1.  Virtual Network Operators . . . . . . . . . . . . . .  23
     4.5.2.  Extending Virtual Networks and Systems to the
             Internet of Things  . . . . . . . . . . . . . . . . .  24
   4.6.  Service Composition . . . . . . . . . . . . . . . . . . .  25
   4.7.  Device Virtualization for End Users . . . . . . . . . . .  27
   4.8.  Security and Privacy  . . . . . . . . . . . . . . . . . .  27
   4.9.  Separation of Control Concerns  . . . . . . . . . . . . .  29
   4.10. Network Function Placement  . . . . . . . . . . . . . . .  29
   4.11. Testing . . . . . . . . . . . . . . . . . . . . . . . . .  30
     4.11.1.  Changes in Methodology . . . . . . . . . . . . . . .  30
     4.11.2.  New Functionality  . . . . . . . . . . . . . . . . .  31
     4.11.3.  Opportunities  . . . . . . . . . . . . . . . . . . .  32
 5.  Technology Gaps and Potential IETF Efforts  . . . . . . . . .  33
 6.  NFVRG Focus Areas . . . . . . . . . . . . . . . . . . . . . .  34
 7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  35
 8.  Security Considerations . . . . . . . . . . . . . . . . . . .  35
 9.  Informative References  . . . . . . . . . . . . . . . . . . .  35
 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  41
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  41

Bernardos, et al. Informational [Page 3] RFC 8568 Network Virtualization Research Challenges April 2019

1. Introduction and Scope

 The telecommunications sector is experiencing a major revolution that
 will shape the way networks and services are designed and deployed
 for the next few decades.  In order to cope with continuously
 increasing demand and cost, network operators are taking lessons from
 the IT paradigm of cloud computing.  This new approach of
 virtualizing network functions will enable multi-fold advantages by
 moving communication services from bespoke hardware in the operator's
 core network to Commercial Off-The-Shelf (COTS) equipment distributed
 across data centers.
 Some of the network virtualization mechanisms that are being
 considered include the following: sharing of network infrastructure
 to reduce costs, virtualization of core and edge servers/services
 running in data centers as a way of supporting their load-aware
 elastic dimensioning, and dynamic energy policies to reduce the
 electricity consumption.
 This document presents research and engineering challenges in network
 virtualization that need to be addressed in order to achieve these
 goals, spanning from pure research and engineering/standards space.
 The objective of this memo is to document the technical challenges
 and corresponding current approaches and to expose requirements that
 should be addressed by future research and standards work.
 This document represents the consensus of the Network Function
 Virtualization Research Group (NFVRG).  It has been reviewed by the
 RG members active in the specific areas of work covered by the
 document.

2. Terminology

 The following terms used in this document are defined by the ETSI
 Network Function Virtualization (NFV) Industrial Study Group (ISG)
 [etsi_gs_nfv_003], the Open Networking Foundation (ONF) [onf_tr_521],
 and the IETF [RFC7426] [RFC7665]:
 Application Plane:  The collection of applications and services that
    program network behavior.
 Control Plane (CP):  The collection of functions responsible for
    controlling one or more network devices.  The CP instructs network
    devices with respect to how to process and forward packets.  The
    control plane interacts primarily with the forwarding plane and,
    to a lesser extent, with the operational plane.

Bernardos, et al. Informational [Page 4] RFC 8568 Network Virtualization Research Challenges April 2019

 Forwarding Plane (FP):  The collection of resources across all
    network devices responsible for forwarding traffic.
 Management Plane (MP):  The collection of functions responsible for
    monitoring, configuring, and maintaining one or more network
    devices or parts of network devices.  The management plane is
    mostly related to the operational plane (it is related less to the
    forwarding plane).
 NFV Infrastructure (NFVI):  Totality of all hardware and software
    components that build up the environment in which VNFs are
    deployed.
 NFV Management and Orchestration (NFV-MANO):  Functions collectively
    provided by NFVO, VNFM, and VIM.
 NFV Orchestrator (NFVO):  Functional block that manages the Network
    Service (NS) life cycle and coordinates the management of NS life
    cycle, VNF life cycle (supported by the VNFM) and NFVI resources
    (supported by the VIM) to ensure an optimized allocation of the
    necessary resources and connectivity.
 Operational Plane (OP):  The collection of resources responsible for
    managing the overall operation of individual network devices.
 Physical Network Function (PNF):  Physical implementation of a
    network function in a monolithic realization.
 Service Function Chain (SFC):  For a given service, the abstracted
    view of the required service functions and the order in which they
    are to be applied.  This is somehow equivalent to the Network
    Function Forwarding Graph (NF-FG) at ETSI.
 Service Function Path (SFP):  The selection of specific service
    function instances on specific network nodes to form a service
    graph through which an SFC is instantiated.
 Virtualized Infrastructure Manager (VIM):  Functional block that is
    responsible for controlling and managing the NFVI compute,
    storage, and network resources, usually within one infrastructure
    operator's domain.
 Virtualized Network Function (VNF):  Implementation of a Network
    Function that can be deployed on a Network Function Virtualization
    Infrastructure (NFVI).
 Virtualized Network Function Manager (VNFM):  Functional block that
    is responsible for the life-cycle management of VNF.

Bernardos, et al. Informational [Page 5] RFC 8568 Network Virtualization Research Challenges April 2019

3. Background

 This section briefly describes some basic background technologies as
 well as other Standards Developing Organizations (SDOs) and open-
 source initiatives working on network virtualization or related
 topics.

3.1. Network Function Virtualization

 The ETSI ISG Network Function Virtualization (NFV) is a working group
 that, since 2012, has aimed to evolve quasi-standard IT
 virtualization technology to consolidate many network equipment types
 into industry standard high-volume servers, switches, and storage.
 It enables implementing network functions in software that can run on
 a range of industry-standard server hardware and can be moved to, or
 loaded in, various locations in the network as required, without the
 need to install new equipment.  The ETSI NFV is one of the
 predominant NFV reference framework and architectural footprints
 [nfv_sota_research_challenges].  The ETSI NFV framework architecture
 is composed of three domains (Figure 1):
 o  Virtualized Network Function, running over the NFVI.
 o  NFVI, including the diversity of physical resources and how these
    can be virtualized.  NFVI supports the execution of the VNFs.
 o  NFV Management and Orchestration, which covers the orchestration
    and life-cycle management of physical and/or software resources
    that support the infrastructure virtualization, and the life-cycle
    management of VNFs.  NFV Management and Orchestration focuses on
    all virtualization-specific management tasks necessary in the NFV
    framework.

Bernardos, et al. Informational [Page 6] RFC 8568 Network Virtualization Research Challenges April 2019

 +-------------------------------------------+  +---------------+
 |   Virtualized Network Functions (VNFs)    |  |               |
 |  -------   -------   -------   -------    |  |               |
 |  |     |   |     |   |     |   |     |    |  |               |
 |  | VNF |   | VNF |   | VNF |   | VNF |    |  |               |
 |  |     |   |     |   |     |   |     |    |  |               |
 |  -------   -------   -------   -------    |  |               |
 +-------------------------------------------+  |               |
                                                |               |
 +-------------------------------------------+  |               |
 |         NFV Infrastructure (NFVI)         |  |      NFV      |
 | -----------    -----------    ----------- |  |  Management   |
 | | Virtual |    | Virtual |    | Virtual | |  |      and      |
 | | Compute |    | Storage |    | Network | |  | Orchestration |
 | -----------    -----------    ----------- |  |               |
 | +---------------------------------------+ |  |               |
 | |         Virtualization Layer          | |  |               |
 | +---------------------------------------+ |  |               |
 | +---------------------------------------+ |  |               |
 | | -----------  -----------  ----------- | |  |               |
 | | | Compute |  | Storage |  | Network | | |  |               |
 | | -----------  -----------  ----------- | |  |               |
 | |          Hardware resources           | |  |               |
 | +---------------------------------------+ |  |               |
 +-------------------------------------------+  +---------------+
                     Figure 1: ETSI NFV Framework
 The NFV architectural framework identifies functional blocks and the
 main reference points between such blocks.  Some of these are already
 present in current deployments, whilst others might be necessary
 additions in order to support the virtualization process and
 consequent operation.  The functional blocks are (Figure 2):
 o  Virtualized Network Function (VNF)
 o  Element Management (EM)
 o  NFV Infrastructure, including: Hardware and virtualized resources
    as well as the Virtualization Layer.
 o  Virtualized Infrastructure Manager(s) (VIM)
 o  NFV Orchestrator
 o  VNF Manager(s)
 o  Service, VNF and Infrastructure Description

Bernardos, et al. Informational [Page 7] RFC 8568 Network Virtualization Research Challenges April 2019

 o  Operational Support Systems and Business Support Systems (OSS and
    BSS)
                                                +--------------------+
 +-------------------------------------------+  | ----------------   |
 |                 OSS/BSS                   |  | | NFV          |   |
 +-------------------------------------------+  | | Orchestrator +-- |
                                                | ---+------------ | |
 +-------------------------------------------+  |    |             | |
 |  ---------     ---------     ---------    |  |    |             | |
 |  | EM 1  |     | EM 2  |     | EM 3  |    |  |    |             | |
 |  ----+----     ----+----     ----+----    |  | ---+----------   | |
 |      |             |             |        |--|-|    VNF     |   | |
 |  ----+----     ----+----     ----+----    |  | | manager(s) |   | |
 |  | VNF 1 |     | VNF 2 |     | VNF 3 |    |  | ---+----------   | |
 |  ----+----     ----+----     ----+----    |  |    |             | |
 +------|-------------|-------------|--------+  |    |             | |
        |             |             |           |    |             | |
 +------+-------------+-------------+--------+  |    |             | |
 |         NFV Infrastructure (NFVI)         |  |    |             | |
 | -----------    -----------    ----------- |  |    |             | |
 | | Virtual |    | Virtual |    | Virtual | |  |    |             | |
 | | Compute |    | Storage |    | Network | |  |    |             | |
 | -----------    -----------    ----------- |  | ---+------       | |
 | +---------------------------------------+ |  | |        |       | |
 | |         Virtualization Layer          | |--|-| VIM(s) +-------- |
 | +---------------------------------------+ |  | |        |         |
 | +---------------------------------------+ |  | ----------         |
 | | -----------  -----------  ----------- | |  |                    |
 | | | Compute |  | Storage |  | Network | | |  |                    |
 | | | hardware|  | hardware|  | hardware| | |  |                    |
 | | -----------  -----------  ----------- | |  |                    |
 | |          Hardware resources           | |  |  NFV Management    |
 | +---------------------------------------+ |  | and Orchestration  |
 +-------------------------------------------+  +--------------------+
               Figure 2: ETSI NFV Reference Architecture

Bernardos, et al. Informational [Page 8] RFC 8568 Network Virtualization Research Challenges April 2019

3.2. Software-Defined Networking

 The Software-Defined Networking (SDN) paradigm pushes the
 intelligence currently residing in the network elements to a central
 controller implementing the network functionality through software.
 In contrast to traditional approaches, in which the network's control
 plane is distributed throughout all network devices, with SDN, the
 control plane is logically centralized.  In this way, the deployment
 of new characteristics in the network no longer requires complex and
 costly changes in equipment or firmware updates, but only a change in
 the software running in the controller.  The main advantage of this
 approach is the flexibility it provides operators to manage their
 network, i.e., an operator can easily change its policies on how
 traffic is distributed throughout the network.
 One of the most well-known protocols for the SDN control plane
 between the central controller and the networking elements is the
 OpenFlow Protocol (OFP), which is maintained and extended by the Open
 Network Foundation (ONF) <https://www.opennetworking.org/>.
 Originally, this protocol was developed specifically for IEEE 802.1
 switches conforming to the ONF OpenFlow Switch specification
 [OpenFlow].  As the benefits of the SDN paradigm have reached a wider
 audience, its application has been extended to more complex scenarios
 such as wireless and mobile networks.  Within this area of work, the
 ONF is actively developing new OFP extensions addressing three key
 scenarios: (i) wireless backhaul, (ii) cellular Evolved Packet Core
 (EPC), and (iii) unified access and management across enterprise
 wireless and fixed networks.

Bernardos, et al. Informational [Page 9] RFC 8568 Network Virtualization Research Challenges April 2019

 +----------+
 | -------  |
 | |Oper.|  |            O
 | |Mgmt.|  |<........> -+- Network Operator
 | |Iface|  |            ^
 | -------  |      +----------------------------------------+
 |          |      | +------------------------------------+ |
 |          |      | | ---------  ---------     --------- | |
 |--------- |      | | | App 1 |  | App 2 | ... | App n | | |
 ||Plugins| |<....>| | ---------  ---------     --------- | |
 |--------- |      | | Plugins                            | |
 |          |      | +------------------------------------+ |
 |          |      | Application Plane                      |
 |          |      +----------------------------------------+
 |          |                         A
 |          |                         |
 |          |                         V
 |          |      +----------------------------------------+
 |          |      | +------------------------------------+ |
 |--------- |      | |     ------------  ------------     | |
 || Netw. | |      | |     | Module 1 |  | Module 2 |     | |
 ||Engine | |<....>| |     ------------  ------------     | |
 |--------- |      | | Network Engine                     | |
 |          |      | +------------------------------------+ |
 |          |      | Control Plane                          |
 |          |      +----------------------------------------+
 |          |                         A
 |          |                         |
 |          |                         V
 |          |      +----------------------------------------+
 |          |      |  +--------------+   +--------------+   |
 |          |      |  | ------------ |   | ------------ |   |
 |----------|      |  | | OpenFlow | |   | | OpenFlow | |   |
 ||OpenFlow||<....>|  | ------------ |   | ------------ |   |
 |----------|      |  | NE           |   | NE           |   |
 |          |      |  +--------------+   +--------------+   |
 |          |      | Data Plane                             |
 |Management|      +----------------------------------------+
 +----------+
               Figure 3: High-Level SDN ONF Architecture
 Figure 3 shows the blocks and the functional interfaces of the ONF
 architecture, which comprises three planes: data, controller, and
 application.  The data plane comprehends several Network Entities
 (NEs), which expose their capabilities toward the control plane via a
 Southbound API.  The control plane includes several cooperating
 modules devoted to the creation and maintenance of an abstracted

Bernardos, et al. Informational [Page 10] RFC 8568 Network Virtualization Research Challenges April 2019

 resource model of the underlying network.  Such a model is exposed to
 the applications via a Northbound API where the application plane
 comprises several applications/services, each of which has exclusive
 control of a set of exposed resources.
 The management plane spans its functionality across all planes
 performing the initial configuration of the network elements in the
 data plane, the assignment of the SDN controller and the resources
 under its responsibility.  In the control plane, the management needs
 to configure the policies defining the scope of the control given to
 the SDN applications, to monitor the performance of the system and to
 configure the parameters required by the SDN controller modules.  In
 the application plane, the management plane configures the parameters
 of the applications and the service-level agreements.  In addition to
 these interactions, the management plane exposes several functions to
 network operators that can easily and quickly configure and tune the
 network at each layer.
 In RFC 7426 [RFC7426], the IRTF Software-Defined Networking Research
 Group (SDNRG) documented a layer model of an SDN architecture.  This
 was due to the following controversial discussion topics (among
 others).  What exactly is SDN?  What is the layer structure of the
 SDN architecture?  How do layers interface with each other?
 Figure 4 reproduces the figure included in RFC 7426 [RFC7426] to
 summarize the SDN architecture abstractions in the form of a
 detailed, high-level schematic.  In a particular implementation,
 planes can be collocated with other planes or can be physically
 separated.
 In SDN, a controller manipulates controlled entities via an
 interface.  Interfaces, when local, are mostly API invocations
 through some library or system call.  However, such interfaces may be
 extended via some protocol definition, which may use local
 interprocess communication (IPC) or a protocol that could also act
 remotely; the protocol may be defined as an open standard or in a
 proprietary manner.
 SDN expands multiple planes: forwarding, operational, control,
 management, and application.  All planes mentioned above are
 connected via interfaces.  Additionally, RFC 7426 [RFC7426] considers
 four abstraction layers: the Device and resource Abstraction Layer
 (DAL), the Control Abstraction Layer (CAL), the Management
 Abstraction Layer (MAL), and the Network Services Abstraction Layer
 (NSAL).

Bernardos, et al. Informational [Page 11] RFC 8568 Network Virtualization Research Challenges April 2019

                o--------------------------------o
                |                                |
                | +-------------+   +----------+ |
                | | Application |   |  Service | |
                | +-------------+   +----------+ |
                |       Application Plane        |
                o---------------Y----------------o
                                |
  *-----------------------------Y---------------------------------*
  |           Network Services Abstraction Layer (NSAL)           |
  *------Y------------------------------------------------Y-------*
         |                                                |
         |               Service Interface                |
         |                                                |
  o------Y------------------o       o---------------------Y------o
  |      |    Control Plane |       | Management Plane    |      |
  | +----Y----+   +-----+   |       |  +-----+       +----Y----+ |
  | | Service |   | App |   |       |  | App |       | Service | |
  | +----Y----+   +--Y--+   |       |  +--Y--+       +----Y----+ |
  |      |           |      |       |     |               |      |
  | *----Y-----------Y----* |       | *---Y---------------Y----* |
  | | Control Abstraction | |       | | Management Abstraction | |
  | |     Layer (CAL)     | |       | |      Layer (MAL)       | |
  | *----------Y----------* |       | *----------Y-------------* |
  |            |            |       |            |               |
  o------------|------------o       o------------|---------------o
               |                                 |
               | CP                              | MP
               | Southbound                      | Southbound
               | Interface                       | Interface
               |                                 |
  *------------Y---------------------------------Y----------------*
  |         Device and resource Abstraction Layer (DAL)           |
  *------------Y---------------------------------Y----------------*
  |            |                                 |                |
  |    o-------Y----------o   +-----+   o--------Y----------o     |
  |    | Forwarding Plane |   | App |   | Operational Plane |     |
  |    o------------------o   +-----+   o-------------------o     |
  |                       Network Device                          |
  +---------------------------------------------------------------+
                   Figure 4: SDN-Layer Architecture
 While SDN is often directly associated to OpenFlow, this is just one
 (relevant) example of a southbound protocol between the central
 controller and the network entities.  Other relevant examples of
 protocols in the SDN family are NETCONF [RFC6241], RESTCONF
 [RFC8040], and ForCES [RFC5810].

Bernardos, et al. Informational [Page 12] RFC 8568 Network Virtualization Research Challenges April 2019

3.3. ITU-T Functional Architecture of SDN

 The ITU-T (the Telecommunication standardization sector of the
 International Telecommunication Union) has also looked into SDN
 architectures, defining a slightly modified one from what other SDOs
 have done.  In ITU-T recommendation Y.3302 [itu-t-y.3302], the ITU-T
 provides a functional architecture of SDN with descriptions of
 functional components and reference points.  The described functional
 architecture is intended to be used as an enabler for further studies
 on other aspects such as protocols and security as well as being used
 to customize SDN in support of appropriate use cases (e.g., cloud
 computing, mobile networks).  This recommendation is based on ITU-T
 Y.3300 [itu-t-y.3300] and ITU-T Y.3301 [itu-t-y.3301].  While the
 first describes the framework of SDN (including definitions,
 objectives, high-level capabilities, requirements, and the high-level
 architecture of SDN), the second describes more-detailed
 requirements.
 Figure 5 shows the SDN functional architecture defined by the ITU-T.
 It is a layered architecture composed of the SDN application layer
 (SDN-AL), the SDN control layer (SDN-CL), and the SDN resource layer
 (SDN-RL).  It also has multi-layer management functions (MMF), which
 provide the ability to manage the functionalities of SDN layers,
 i.e., SDN-AL, SDN-CL, and SDN-RL.  MMF interacts with these layers
 using Multi-layer Management Functions Application (MMFA), Multi-
 layer Management Functions Control (MMFC), and Multi-layer Management
 Functions Resource (MMFR) reference points.
 The SDN-AL enables a service-aware behavior of the underlying network
 in a programmatic manner.  The SDN-CL provides programmable means to
 control the behavior of SDN-RL resources (such as data transport and
 processing) following requests received from the SDN-AL according to
 MMF policies.  The SDN-RL is where the physical or virtual network
 elements perform transport and/or processing of data packets
 according to SDN-CL decisions.

Bernardos, et al. Informational [Page 13] RFC 8568 Network Virtualization Research Challenges April 2019

        MMFO                      MMFA
 +-----+ . +---------------------+ . +--------------------+
 |     | . |+---+ +---+ +-------+| . |+---------+ +-----+ |
 |     | . ||   | |   | |       || . ||   AL.   | |     | |
 |     | . || E | |   | |  App. || . || Mngmt.  | | SDN | | SDN-AL
 |     | . || x | | M | | Layer || . || Support | | App | |
 |     | . || t.| | u | | Mngmt.|| . || & Orch. | |     | |
 |     | . ||   | | l | +-------+| . |+---------+ +-----+ |
 |     | . || R | | t |          | . +--------------------+
 |     | . || e | | i |          |MMFC ..................... ACI
 |     | . || l | | - |          | . +--------------------+
 |     | . || a | | l | +-------+| . |+------+ +---------+|
 | OSS/| . || t | | a | |       || . ||      | |   App.  ||
 | BSS | . || i | | y | |       || . ||      | | Support ||
 |     | . || o | | e | |       || . ||      | +---------+|
 |     | . || n | | r | |       || . ||  CL  | +---------+|
 |     | . || s | |   | |Control|| . ||Mngmt.| | Control ||
 |     | . || h | | M | | Layer || . || Supp.| |  Layer  || SDN-CL
 |     | . || i | | a | | Mngmt.|| . || and  | |  Serv.  ||
 |     | . || p | | n | |       || . || Orch.| +---------+|
 |     | . ||   | | a | |       || . ||      | +---------+|
 |     | . || M | | g | |       || . ||      | | Resource||
 |     | . || n | | e | |       || . ||      | | Abstrac.||
 |     | . || g | | m | +-------+| . |+------+ +---------+|
 |     | . || m | | e |          | . +--------------------+
 |     | . || t.| | n |          |MMFR ..................... RCI
 |     | . ||   | | t |          | . +--------------------+
 +-----+ . |+---+ |   | +-------+| . |+------++----------+|
           |      | O | |       || . ||      ||RL Control||
           |      | r | |Resour.|| . ||  RL  |+----------+|
      MMF  |      | c | | Layer || . ||Mngmt.|+----++----+| SDN-RL
           |      | h.| | Mngmt.|| . || Supp.||Data||Data||
           |      |   | |       || . ||      ||Tran||Proc||
           |      +---+ +-------+| . |+------++----++----+|
           +---------------------+ . +--------------------+
 Legend:
   ACI:  Application Control Interface
   MMFA: Multi-layer Management Functions Application
   MMFC: Multi-layer Management Functions Control
   MMFO: Multi-layer Management Functions OSS/BSS
   MMFR: Multi-layer Management Functions Resource
   RCI:  Resource Control Interfaces
   RL:   Resource Layer
              Figure 5: ITU-T SDN Functional Architecture

Bernardos, et al. Informational [Page 14] RFC 8568 Network Virtualization Research Challenges April 2019

3.4. Multi-Access Edge Computing

 Multi-access Edge Computing (MEC) -- formerly known as Mobile Edge
 Computing -- capabilities deployed in the edge of the mobile network
 can facilitate the efficient and dynamic provision of services to
 mobile users.  The ETSI ISG MEC working group, operative from end of
 2014, intends to specify an open environment for integrating MEC
 capabilities with service providers' networks, also including
 applications from third parties.  These distributed computing
 capabilities provide IT infrastructure as in a cloud environment for
 the deployment of functions in mobile access networks.  It can be
 seen then as a complement to both NFV and SDN.

3.5. IEEE 802.1CF (OmniRAN)

 The IEEE 802.1CF Recommended Practice [omniran] specifies an access
 network that connects terminals to their access routers utilizing
 technologies based on the family of IEEE 802 Standards (e.g., 802.3
 Ethernet, 802.11 Wi-Fi, etc.).  The specification defines an access
 network reference model, including entities and reference points
 along with behavioral and functional descriptions of communications
 among those entities.
 The goal of this project is to help unify the support of different
 interfaces, enabling shared-network control and use of SDN
 principles, thereby lowering the barriers to new network
 technologies, to new network operators, and to new service providers.

3.6. Distributed Management Task Force (DMTF)

 The DMTF <https://www.dmtf.org/> is an industry standards
 organization working to simplify the manageability of network-
 accessible technologies through open and collaborative efforts by
 some technology companies.  The DMTF is involved in the creation and
 adoption of interoperable management standards, supporting
 implementations that enable the management of diverse traditional and
 emerging technologies including cloud, virtualization, network, and
 infrastructure.
 There are several DMTF initiatives that are relevant to the network
 virtualization area, such as the Open Virtualization Format (OVF) for
 VNF packaging; the Cloud Infrastructure Management Interface (CIMI)
 for cloud infrastructure management; the Network Management (NETMAN),
 for VNF management; and the Virtualization Management (VMAN), for
 virtualization infrastructure management.

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3.7. Open-Source Initiatives

 The open-source community is especially active in the area of network
 virtualization and orchestration.  We next summarize some of the
 active efforts:
 o  OpenStack.  OpenStack is a free and open-source cloud-computing
    software platform.  OpenStack software controls large pools of
    compute, storage, and networking resources throughout a data
    center, managed through a dashboard or via the OpenStack API.
 o  Kubernetes.  Kubernetes is an open-source system for automating
    deployment, scaling and management of containerized applications.
    Kubernetes can schedule and run application containers on clusters
    of physical or virtual machines.  Kubernetes allows (i) Scale on
    the fly, (ii) Limit hardware usage to required resources only,
    (iii) Load-balancing Monitoring, and (iv) Efficient life-cycle
    management.
 o  OpenDayLight.  OpenDayLight (ODL) is a highly available, modular,
    extensible and scalable multiprotocol controller infrastructure
    built for SDN deployments on modern heterogeneous multi-vendor
    networks.  It provides a model-driven service abstraction platform
    that allows users to write apps that easily work across a wide
    variety of hardware and southbound protocols.
 o  ONOS.  The Open Network Operating System (ONOS) project is an
    open-source community hosted by The Linux Foundation.  The goal of
    the project is to create an SDN operating system for
    communications service providers that is designed for scalability,
    high performance, and high availability.
 o  OpenContrail.  OpenContrail is a licensed Apache 2.0 project that
    is built using standards-based protocols and that provides all the
    necessary components for network virtualization: an SDN
    controller, a virtual router, an analytics engine, and published
    northbound APIs.  It has an extensive Representational State
    Transfer (REST) API to configure and gather operational and
    analytics data from the system.
 o  OPNFV.  The Open Platform for NFV (OPNFV) is a carrier-grade,
    integrated, open-source platform to accelerate the introduction of
    new NFV products and services.  By integrating components from
    upstream projects, the OPNFV community aims at conducting
    performance and use case-based testing to ensure the platform's
    suitability for NFV use cases.  The scope of OPNFV's initial
    release is focused on building NFV Infrastructure (NFVI) and
    Virtualized Infrastructure Manager (VIM) by integrating components

Bernardos, et al. Informational [Page 16] RFC 8568 Network Virtualization Research Challenges April 2019

    from upstream projects such as OpenDayLight, OpenStack, Ceph
    Storage, Kernel-based Virtual Machine (KVM), Open vSwitch, and
    Linux.  These components, along with APIs to other NFV elements,
    form the basic infrastructure required for Virtualized Network
    Functions (VNFs) and Management and Orchestration (MANO)
    components.  OPNFV's goal is to (i) increase performance and power
    efficiency, (ii) improve reliability, availability, and
    serviceability, and (iii) deliver comprehensive platform
    instrumentation.
 o  OSM.  Open Source Mano (OSM) is an ETSI-hosted project to develop
    an Open Source NFV Management and Orchestration (MANO) software
    stack aligned with ETSI NFV.  OSM is based on components from
    previous projects, such Telefonica's OpenMANO or Canonical's Juju,
    among others.
 o  OpenBaton.  OpenBaton is a Network Function Virtualization
    Orchestrator (NFVO) that is ETSI NFV compliant.  OpenBaton was
    part of the OpenSDNCore project started with the objective of
    providing a compliant implementation of the ETSI NFV
    specification.
 o  ONAP.  Open Network Automation Platform (ONAP) is an open-source
    software platform that delivers capabilities for the design,
    creation, orchestration, monitoring, and life-cycle management of
    (i) Virtual Network Functions (VNFs), (ii) The carrier-scale
    Software-Defined Networks (SDNs) that contain them, and (iii)
    higher-level services that combine the above.  ONAP (derived from
    the AT&T's ECOMP) provides for automatic, policy-driven
    interaction of these functions and services in a dynamic, real-
    time cloud environment.
 o  SONA.  The Simplified Overlay Network Architecture (SONA) is an
    extension to ONOS to have an almost full SDN network control in
    OpenStack for virtual tenant network provisioning.  Basically,
    SONA is an SDN-based network virtualization solution for cloud DC.
 Among the main areas that are being developed by the aforementioned
 open-source activities that relate to network virtualization
 research, we can highlight policy-based resource management,
 analytics for visibility and orchestration, and service verification
 with regard to security and resiliency.

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4. Network Virtualization Challenges

4.1. Overview

 Network virtualization is changing the way the telecommunications
 sector will deploy, extend, and operate their networks.  These new
 technologies aim at reducing the overall costs by moving
 communication services from specific hardware in the operators' cores
 to server farms scattered in data centers (i.e., compute and storage
 virtualization).  In addition, the networks interconnecting the
 functions that compose a network service are fundamentally affected
 in the way they route, process, and control traffic (i.e., network
 virtualization).

4.2. Guaranteeing Quality of Service

 Achieving a given QoS in an NFV environment with virtualized and
 distributed computing, storage, and networking functions is more
 challenging than providing the equivalent in discrete non-virtualized
 components.  For example, ensuring a guaranteed and stable forwarding
 data rate has proven not to be straightforward when the forwarding
 function is virtualized and runs on top of COTS server hardware
 [openmano_dataplane] [NFV-COTS] [etsi_nfv_whitepaper_3].  Again, the
 comparison point is against a router or forwarder built on optimized
 hardware.  We next identify some of the challenges that this poses.

4.2.1. Virtualization Technologies

 The issue of guaranteeing a network QoS is less of an issue for
 "traditional" cloud computing because the workloads that are treated
 there are servers or clients in the networking sense and hardly ever
 process packets.  Cloud computing provides hosting for applications
 on shared servers in a highly separated way.  Its main advantage is
 that the infrastructure costs are shared among tenants and that the
 cloud infrastructure provides levels of reliability that can not be
 achieved on individual premises in a cost-efficient way
 [intel_10_differences_nfv_cloud].  NFV has very strict requirements
 posed in terms of performance, stability, and consistency.  Although
 there are some tools and mechanisms to improve this, such as Enhanced
 Performance Awareness (EPA), Single Root I/O Virtualization (SR-IOV),
 Non-Uniform Memory Access (NUMA), Data Plane Development Kit (DPDK),
 etc., these are still unsolved challenges.  One open research issue
 is finding out technologies that are different from Virtual Machines
 (VMs) and more suitable for dealing with network functionalities.
 Lately, a number of lightweight virtualization technologies including
 containers, unikernels (specialized VMs) and minimalistic
 distributions of general-purpose OSes have appeared as virtualization

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 approaches that can be used when constructing an NFV platform.
 [LIGHT-NFV] describes the challenges in building such a platform and
 discusses to what extent these technologies, as well as traditional
 VMs, are able to address them.

4.2.2. Metrics for NFV Characterization

 Another relevant aspect is the need for tools for diagnostics and
 measurements suited for NFV.  There is a pressing need to define
 metrics and associated protocols to measure the performance of NFV.
 Specifically, since NFV is based on the concept of taking centralized
 functions and evolving them to highly distributed software (SW)
 functions, there is a commensurate need to fully understand and
 measure the baseline performance of such systems.
 The IP Performance Metrics (IPPM) WG defines metrics that can be used
 to measure the quality and performance of Internet services and
 applications running over transport-layer protocols (e.g., TCP and
 UDP) over IP.  It also develops and maintains protocols for the
 measurement of these metrics.  While the IPPM WG is a long-running WG
 that started in 1997, at the time of writing, it does not have a
 charter item or active Internet-Drafts related to the topic of
 network virtualization.  In addition to using IPPM to evaluate QoS,
 there is a need for specific metrics for assessing the performance of
 network-virtualization techniques.
 The Benchmarking Methodology Working Group (BMWG) is also performing
 work related to NFV metrics.  For example, [RFC8172] investigates
 additional methodological considerations necessary when benchmarking
 VNFs that are instantiated and hosted in general-purpose hardware,
 using bare-metal hypervisors or other isolation environments (such as
 Linux containers).  An essential consideration is benchmarking
 physical and VNFs in the same way when possible, thereby allowing
 direct comparison.
 There is a clear motivation for the work on performance metrics for
 NFV [etsi_gs_nfv_per_001], as stated in [RFC8172] (and replicated
 here):
    I'm designing and building my NFV Infrastructure platform.  The
    first steps were easy because I had a small number of categories
    of VNFs to support and the VNF vendor gave HW recommendations that
    I followed.  Now I need to deploy more VNFs from new vendors, and
    there are different hardware recommendations.  How well will the
    new VNFs perform on my existing hardware?  Which among several new
    VNFs in a given category are most efficient in terms of capacity
    they deliver?  And, when I operate multiple categories of VNFs
    (and PNFs) *concurrently* on a hardware platform such that they

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    share resources, what are the new performance limits, and what are
    the software design choices I can make to optimize my chosen
    hardware platform?  Conversely, what hardware platform upgrades
    should I pursue to increase the capacity of these concurrently
    operating VNFs?
 Lately, there are also some efforts looking into VNF benchmarking.
 The selection of an NFV Infrastructure Point of Presence to host a
 VNF or allocation of resources (e.g., virtual CPUs, memory) needs to
 be done over virtualized (abstracted and simplified) resource views
 [vnf_benchmarking] [VNF-VBAAS].

4.2.3. Predictive Analysis

 On top of diagnostic tools that enable an assessment of the QoS,
 predictive analyses are required to react before anomalies occur.
 Due to the SW characteristics of VNFs, a reliable diagnosis framework
 could potentially enable the prevention of issues by a proper
 diagnosis and then a reaction in terms of acting on the potentially
 impacted service (e.g., migration to a different compute node,
 scaling in/out, up/down, etc.).

4.2.4. Portability

 Portability in NFV refers to the ability to run a given VNF on
 multiple NFVIs, that is, guaranteeing that the VNF would be able to
 perform its functions with a high and predictable performance given
 that a set of requirements on the NFVI resources is met.  Therefore,
 portability is a key feature that, if fully enabled, would contribute
 to making the NFV environment achieve a better reliability than a
 traditional system.  Implementing functionality in SW over
 "commodity" infrastructure should make it much easier to port/move
 functions from one place to another.  However, this is not yet as
 ideal as it sounds, and there are aspects that are not fully tackled.
 The existence of different hypervisors, specific hardware
 dependencies (e.g., EPA related), or state-synchronization aspects
 are just some examples of troublemakers for portability purposes.
 The ETSI NFV ISG is doing work in relation to portability.
 [etsi_gs_nfv_per_001] provides a list of minimal features that the VM
 Descriptor and Compute Host Descriptor should contain for the
 appropriate deployment of VM images over an NFVI (i.e., a "telco data
 center"), in order to guarantee high and predictable performance of
 data-plane workloads while assuring their portability.  In addition,
 [etsi_gs_nfv_per_001] provides a set of recommendations on the
 minimum requirements that hardware (HW) and hypervisor should have
 for a "telco data center" suitable for different workloads (data
 plane, control plane, etc.) present in VNFs.  The purpose of

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 [etsi_gs_nfv_per_001] is to provide the list of VM requirements that
 should be included in the VM Descriptor template, and the list of HW
 capabilities that should be included in the Compute Host Descriptor
 (CHD) to assure predictable high performance.  ETSI NFV assumes that
 the MANO functions will make the mix & match.  Therefore, there are
 still several research challenges to be addressed here.

4.3. Performance Improvement

4.3.1. Energy Efficiency

 Virtualization is typically seen as a direct enabler of energy
 savings.  Some of the enablers for this that are often mentioned
 [nfv_sota_research_challenges] are (i) the multiplexing gains
 achieved by centralizing functions in data centers reduce the overall
 energy consumed and (ii) the flexibility brought by network
 programmability enables to switch off infrastructure as needed in a
 much easier way.  However, there is still a lot of room for
 improvement in terms of virtualization techniques to reduce the power
 consumption, such as enhanced-hypervisor technologies.
 Some additional examples of research topics that could enable energy
 savings are [nfv_sota_research_challenges]:
 o  Energy-aware scaling (e.g., reductions in CPU speeds and partially
    turning off some hardware components to meet a given energy
    consumption target.
 o  Energy-aware function placement.
 o  Scheduling and chaining algorithms, for example, adapting the
    network topology and operating parameters to minimize the
    operation cost (e.g., tracking energy costs to identify the
    cheapest prices).
 Note that it is also important to analyze the trade-off between
 energy efficiency and network performance.

4.3.2. Improved Link Usage

 The use of NFV and SDN technologies can help improve link usage.  SDN
 has already shown that it can greatly increase average link
 utilization (e.g., Google example [google_sdn_wan]).  NFV adds more
 complexity (e.g., due to service-function chaining / VNF forwarding
 graphs), which needs to be considered.  Aspects like the ones
 described in [NFVRG-TOPO] (on NFV data center topology design) have
 to be looked at carefully as well.

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4.4. Multiple Domains

 Market fragmentation has resulted in a multitude of network operators
 each focused on different countries and regions.  This makes it
 difficult to create infrastructure services spanning multiple
 countries, such as virtual connectivity or compute resources, as no
 single operator has a footprint everywhere.  Cross-domain
 orchestration of services over multiple administrations or over
 multi-domain single administrations will allow end-to-end network and
 service elements to mix in multi-vendor, heterogeneous technology,
 and resource environments [multi-domain_5GEx].
 For the specific use case of 'Network as a Service', it becomes even
 more important to ensure that Cross Domain Orchestration also takes
 care of hierarchy of networks and their association, with respect to
 provisioning tunnels and overlays.
 Multi-domain orchestration is currently an active research topic,
 which is being tackled, among others, by ETSI NFV ISG and the 5GEx
 project <https://www.5gex.eu/> [MULTI-NMRG] [multi-domain_5GEx].
 Another side of the multi-domain problem is the integration/
 harmonization of different management domains.  A key example comes
 from Multi-access Edge Computing, which, according to ETSI, comes
 with its own MANO system and would require integration if
 interconnected to a generic NFV system.

4.5. 5G and Network Slicing

 From the beginning of all 5G discussions in the research and industry
 fora, it has been agreed that 5G will have to address many more use
 cases than the preceding wireless generations, which first focused on
 voice services and then on voice and high-speed packet data services.
 In this case, 5G should be able to handle not only the same (or
 enhanced) voice and packet data services, but also emerging services
 like tactile Internet and the Internet of Things (IoT).  These use
 cases take the requirements to opposite extremes, as some of them
 require ultra-low latency and higher-speed, whereas some others
 require ultra-low power consumption and high-delay tolerance.
 Because of these very extreme 5G use cases, it is envisioned that
 selective combinations of radio access networks and core network
 components will have to be combined into a given network slice to
 address the specific requirements of each use case.
 For example, within the major IoT category, which is perhaps the most
 disrupting one, some autonomous IoT devices will have very low
 throughput, will have much longer sleep cycles (and therefore high

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 latency), and a battery life time exceeding by a factor of thousands
 that of smartphones or some other devices that will have almost
 continuous control and data communications.  Hence, it is envisioned
 that a customized network slice will have to be stitched together
 from virtual resources or sub-slices to meet these requirements.
 The actual definition of a "network slice" from an IP infrastructure
 viewpoint is currently undergoing intense debate; see [COMS-PS],
 [NETSLICES], [SLICE-3GPP], and [ngmn_5G_whitepaper].  Network slicing
 is a key for introducing new actors in existing markets at a low cost
 -- by letting new players rent "blocks" of capacity, if the new
 business model enables performance that meets the application needs
 (e.g., broadcasting updates to many sensors with satellite
 broadcasting capabilities).  However, more work needs to be done to
 define the basic architectural approach of how network slices will be
 defined and formed.  For example, is it mostly a matter of defining
 the appropriate network models (e.g., YANG) to stitch the network
 slice from existing components?  Or do end-to-end timing,
 synchronization, and other low-level requirements mean that more
 fundamental research has to be done?

4.5.1. Virtual Network Operators

 The widespread use/discussion/practice of system and network
 virtualization technologies has led to new business opportunities,
 enlarging the offer of IT resources with virtual network and
 computing resources, among others.  As a consequence, the network
 ecosystem now differentiates between the owner of physical resources,
 the Infrastructure Provider (InP), and the intermediary that conforms
 and delivers network services to the final customers, the Virtual
 Network Operator (VNO).
 VNOs aim to exploit the virtualized infrastructures to deliver new-
 and-improved services to their customers.  However, current network
 virtualization techniques offer poor support for VNOs to control
 their resources.  It has been considered that the InP is responsible
 for the reliability of the virtual resources, but there are several
 situations in which a VNO requires a finer control on its resources.
 For instance, dynamic events, such as the identification of new
 requirements or the detection of incidents within the virtual system,
 might urge a VNO to quickly reform its virtual infrastructure and
 resource allocation.  However, the interfaces offered by current
 virtualization platforms do not offer the necessary functions for
 VNOs to perform the elastic adaptations they need to conduct in
 dynamic environments.

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 Beyond their heterogeneity, which can be resolved by software
 adapters, current virtualization platforms do not have common methods
 and functions, so it is difficult for the virtual network controllers
 used by the VNOs to actually manage and control virtual resources
 instantiated on different platforms, not even considering different
 InPs.  Therefore, it is necessary to reach a common definition of the
 functions that should be offered by underlying platforms to give such
 overlay controllers the possibility to allocate and deallocate
 resources dynamically and get monitoring data about them.
 Such common methods should be offered by all underlying controllers,
 regardless of being network-oriented (e.g., ODL, ONOS, and Ryu) or
 computing-oriented (e.g., OpenStack, OpenNebula, and Eucalyptus).
 Furthermore, it is important for those platforms to offer some "PUSH"
 function to report resource state, avoiding the need for the VNO's
 controller to "POLL" for such data.  A starting point to get proper
 notifications within current REST APIs could be to consider the
 protocol proposed by the WEBPUSH WG [RFC8030].
 Finally, in order to establish a proper order and allow the
 coexistence and collaboration of different systems, a common ontology
 regarding network and system virtualization should be defined and
 agreed upon, so different and heterogeneous systems can understand
 each other without requiring reliance on specific adaptation
 mechanisms that might break with any update on any side of the
 relation.

4.5.2. Extending Virtual Networks and Systems to the Internet of Things

 The Internet of Things (IoT) refers to the vision of connecting a
 multitude of automated devices (e.g., lights, environmental sensors,
 traffic lights, parking meters, health and security systems, etc.) to
 the Internet for purposes of reporting and remote command and control
 of the device.  This vision is being realized by a multi-pronged
 approach of standardization in various forums and complementary open-
 source activities.  For example, in the IETF, support of IoT web
 services has been defined by an HTTP-like protocol adapted for IoT
 called "CoAP" [RFC7252]; and, lately, a group has been studying the
 need to develop a new network layer to support IP applications over
 Low-Power Wide Area Networks (LPWAN).
 Elsewhere, for 5G cellular evolution, there is much discussion on the
 need for supporting virtual network slices for the expected massive
 numbers of IoT devices.  A separate virtual network slice is
 considered necessary for different 5G IoT use cases because devices
 will have very different characteristics than typical cellular

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 devices like smartphones [ngmn_5G_whitepaper], and the number of IoT
 devices is expected to be at least one or two orders of magnitude
 higher than other 5G devices (see Section 4.5).
 The specific nature of the IoT ecosystem, particularly reflected in
 the Machine-to-Machine (M2M) communications, leads to the creation of
 new and highly distributed systems which demand location-based
 network and computing services.  A specific example can be
 represented by a set of "things" that suddenly require the setup of a
 firewall to allow external entities to access their data while
 outsourcing some computation requirements to more powerful systems
 relying on cloud-based services.  This representative use case
 exposes important requirements for both NFV and the underlying cloud
 infrastructures.
 In order to provide the aforementioned location-based functions
 integrated with highly distributed systems, the so-called fog
 infrastructures should be able to instantiate VNFs, placing them in
 the required place, e.g., close to their consumers.  This requirement
 implies that the interfaces offered by virtualization platforms must
 support the specification of location-based resources, which is a key
 function in those scenarios.  Moreover, those platforms must also be
 able to interpret and understand the references used by IoT systems
 to their location (e.g., "My-AP" or "5BLDG+2F") and also the
 specification of identifiers linked to other resources, such as the
 case of requiring the infrastructure to establish a link between a
 specific Access Point (AP) and a specific virtual computing node.  In
 summary, the research gap is exact localization of VNFs at far
 network edge infrastructure, which is highly distributed and dynamic.

4.6. Service Composition

 Current network services deployed by operators often involve the
 composition of several individual functions (such as packet
 filtering, deep-packet inspection, load-balancing).  These services
 are typically implemented by the ordered combination of a number of
 service functions that are deployed at different points within a
 network, not necessarily on the direct data path.  This requires
 traffic to be steered through the required service functions,
 wherever they are deployed [RFC7498].
 For a given service, the abstracted view of the required service
 functions and the order in which they are to be applied is called
 "Service Function Chaining" (SFC) [sfc_challenges], which is called
 "Network Function Forwarding Graph" (NF-FG) in ETSI.  SFC is
 instantiated through the selection of specific service function
 instances on specific network nodes to form a service graph: this is

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 called a "Service Function Path" (SFP).  The service functions may be
 applied at any layer within the network protocol stack (network
 layer, transport layer, application layer, etc.).
 Service composition is a powerful means that can provide significant
 benefits when applied in a softwarized network environment.  However,
 there are many research challenges in this area; for example, the
 ones related to composition mechanisms and algorithms to enable load-
 balancing and improve reliability.  The service composition should
 also act as an enabler to gather information across all hierarchies
 (underlays and overlays) of network deployments that may span across
 multiple operators for faster serviceability, thus facilitating
 accomplishing aforementioned goals of "load-balancing and improving
 reliability".
 As described in [dynamic_chaining], different algorithms can be used
 to enable dynamic service composition that optimizes a QoS-based
 utility function (e.g., minimizing the latency per-application
 traffic flows) for a given composition plan.  Such algorithms can
 consider the computation capabilities and load status of resources
 executing the VNF instances, either deduced through estimations from
 historical usage data or collected through real-time monitoring
 (i.e., context-aware selection).  For this reason, selections should
 include references to dynamic information on the status of the
 service instance and its constituent elements, i.e., monitoring
 information related to individual VNF instances and links connecting
 them as well as derived monitoring information at the chain level
 (e.g., end-to-end delay).  At runtime, if one or more VNF instances
 are no longer available or QoS degrades below a given threshold, the
 service selection task can be rerun to perform service substitution.
 There are different research directions that relate to the previous
 point.  For example, the use of Integer Linear Programming (ILP)
 techniques can be explored to optimize the management of diverse
 traffic flows.  Deep-machine learning can also be applied to optimize
 service chains using information parameters, such as some of the ones
 mentioned above.  Newer scheduling paradigms, like co-flows, can also
 be used.
 The SFC working group is working on an architecture for SFC [RFC7665]
 that includes the necessary protocols or protocol extensions to
 convey the SFC and SFP information to nodes that are involved in the
 implementation of service functions and SFCs as well as mechanisms
 for steering traffic through service functions.
 In terms of actual work items, the SFC WG has not yet considered
 working on the management and configuration of SFC components related
 to the support of SFC.  This part is of special interest for

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 operators and would be required in order to actually put SFC
 mechanisms into operation.  Similarly, redundancy and reliability
 mechanisms for SFC are currently not dealt with by any WG in the
 IETF.  While this was the main goal of the VNFpool BoF efforts, it
 still remains unaddressed.

4.7. Device Virtualization for End Users

 So far, most of the network softwarization efforts have focused on
 virtualizing functions of network elements.  While virtualization of
 network elements started with the core, mobile-network architectures
 are now heavily switching to also virtualize Radio Access Network
 (RAN) functions.  The next natural step is to get virtualization down
 at the level of the end-user device (e.g., virtualizing a smartphone)
 [virtualization_mobile_device].  The cloning of a device in the cloud
 (central or local) bears attractive benefits to both the device and
 network operations alike (e.g., power saving at the device by
 offloading computational-heaving functions to the cloud, optimized
 networking -- both device-to-device and device-to-infrastructure) for
 service delivery through tighter integration of the device (via its
 clone in the networking infrastructure).  This is, for example, being
 explored by the European H2020 ICIRRUS project
 <https://www.icirrus-5gnet.eu>.

4.8. Security and Privacy

 Similar to any other situations where resources are shared, security
 and privacy are two important aspects that need to be taken into
 account.
 In the case of security, there are situations where multiple service
 providers will need to coexist in a virtual or hybrid physical/
 virtual environment.  This requires attestation procedures amongst
 different virtual/physical functions and resources as well as ongoing
 external monitoring.  Similarly, different network slices operating
 on the same infrastructure can present security problems, for
 instance, if one slice running critical applications (e.g., support
 for a safety system) is affected by another slice running a less
 critical application.  In general, the minimum common denominator for
 security measures on a shared system should be equal to or higher
 than the one required by the most-critical application.  Multiple and
 continuous threat model analysis as well as a DevOps model are
 required to maintain a certain level of security in an NFV system.
 Simplistically, DevOps is a process that combines multiple functions
 into single cohesive teams in order to quickly produce quality
 software.  Typically, it relies on also applying the Agile
 development process, which focuses on (among many things) dividing
 large features into multiple, smaller deliveries.  One part of this

Bernardos, et al. Informational [Page 27] RFC 8568 Network Virtualization Research Challenges April 2019

 is to immediately test the new smaller features in order to get
 immediate feedback on errors so that if present, they can be
 immediately fixed and redeployed.
 On the other hand, privacy refers to concerns about the control of
 personal data and the decision of what to reveal to whom.  In this
 case, the storage, transmission, collection, and potential
 correlation of information in the NFV system, for purposes not
 originally intended or not known by the user, should be avoided.
 This is particularly challenging, as future intentions and threats
 cannot be easily predicted and still can be applied on data collected
 in the past.  Therefore, well-known techniques, such as data
 minimization using privacy features as default and allowing users to
 opt in/out, should be used to prevent potential privacy issues.
 Compared to traditional networks, NFV will result in networks that
 are much more dynamic (in function distribution and topology) and
 elastic (in size and boundaries).  Thus, NFV will require network
 operators to evolve their operational and administrative security
 solutions to work in this new environment.  For example, in NFV, the
 network orchestrator will become a key node to provide security
 policy orchestration across the different physical and virtual
 components of the virtualized network.  For highly confidential data,
 for example, the network orchestrator should take into account if
 certain physical HW of the network is considered to be more secure
 (e.g., because it is located in secure premises) than other HW.
 Traditional telecom networks typically run under a single
 administrative domain controlled by (exactly) one operator.  With
 NFV, it is expected that in many cases, the telecom operator will now
 become a tenant (running the VNFs), and the infrastructure (NFVI) may
 be run by a different operator and/or cloud service provider (see
 also Section 4.4).  Thus, there will be multiple administrative
 domains involved, making security policy coordination more complex.
 For example, who will be in charge of provisioning and maintaining
 security credentials such as public and private keys?  Also, should
 private keys be allowed to be replicated across the NFV for
 redundancy reasons?  Alternatively, it can be investigated how to
 develop a mechanism that avoids such a security policy coordination,
 thus making the system more robust.
 On a positive note, NFV may better defend against denial-of-service
 (DoS) attacks because of the distributed nature of the network (i.e.,
 no single point of failure) and the ability to steer (undesirable)
 traffic quickly [etsi_gs_nfv_sec_001].  Also, NFVs that have physical
 HW that is distributed across multiple data centers will also provide

Bernardos, et al. Informational [Page 28] RFC 8568 Network Virtualization Research Challenges April 2019

 better fault isolation environments.  Particularly, this holds true
 if each data center is protected separately via firewalls,
 Demilitarized Zones (DMZs), and other network-protection techniques.
 SDN can also be used to help improve security by facilitating the
 operation of existing protocols, such as Authentication,
 Authorization and Accounting (AAA).  The management of AAA
 infrastructures, namely the management of AAA routing and the
 establishment of security associations between AAA entities, can be
 performed using SDN, as analyzed in [SDN-AAA].

4.9. Separation of Control Concerns

 NFV environments offer two possible levels of SDN control.  One level
 is the need for controlling the NFVI to provide connectivity end-to-
 end among VNFs or among VNFs and Physical Network Functions (PNFs).
 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 advantage 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.
 Clear mechanisms for such interactions are needed in order to avoid
 malfunctioning or interference concerns.  These ideas are considered
 in [etsi_gs_nfv_eve005] and [LAYERED-SDN].

4.10. Network Function Placement

 Network function placement is a problem in any kind of network
 telecommunications infrastructure.  Moreover, the increased degree of
 freedom added by network virtualization makes this problem even more
 important, and also harder to tackle.  Deciding where to place VNFs
 is a resource-allocation problem that needs to (or may) take into
 consideration quite a few aspects: resiliency, (anti-)affinity,
 security, privacy, energy efficiency, etc.
 When several functions are chained (typical scenario), placement
 algorithms become more complex and important (as described in
 Section 4.6).  While there has been research on the topic
 ([nfv_piecing], [dynamic_placement], and [vnf-p]), this still remains
 an open challenge that requires more attention.  The use of multi-
 domains adds another component of complexity to this problem that has
 to be considered.

Bernardos, et al. Informational [Page 29] RFC 8568 Network Virtualization Research Challenges April 2019

4.11. Testing

 The impacts of network virtualization on testing can be divided into
 three groups:
 1.  Changes in methodology
 2.  New functionality
 3.  Opportunities

4.11.1. Changes in Methodology

 The largest impact of NFV is the ability to isolate the System Under
 Test (SUT).  When testing PNFs, isolating the SUT means that all the
 other devices that the SUT communicates with are replaced with
 simulations (or controlled executions) in order to place the SUT
 under test by itself.  The SUT may be comprised of one or more
 devices.  The simulations use the appropriate traffic type and
 protocols in order to execute test cases.
 As shown in Figure 2, NFV provides a common architecture for all
 functions to use.  A VNF is executed using resources offered by the
 NFVI, which have been allocated using the MANO function.  It is not
 possible to test a VNF by itself, without the entire supporting
 environment present.  This fundamentally changes how to consider the
 SUT.  In the case of a VNF (or multiple VNFs), the SUT is part of a
 larger architecture that is necessary in order to run the SUTs.
 Therefore, isolation of the SUT becomes controlling the environment
 in a disciplined manner.  The components of the environment necessary
 to run the SUTs that are not part of the SUT itself become the test
 environment.  In the case of VNFs that are part of the SUT, the NFVI
 and MANO become the test environment.  The configurations and
 policies that guide the test environment should remain constant
 during the execution of the tests, and also from test to test.
 Configurations such as CPU pinning, NUMA configuration, the SW
 versions and configurations of the hypervisor, vSwitch and NICs
 should remain constant.  The only variables in the testing should be
 those controlling the SUT itself.  If any configuration in the test
 environment is changed from test to test, the results become very
 difficult, if not impossible, to compare since the test environment
 behavior may change the results as a consequence of the configuration
 change.
 Testing the NFVI itself also presents new considerations.  With a
 PNF, the dedicated hardware supporting it is optimized for the
 particular workload of the function.  Routing hardware is specially

Bernardos, et al. Informational [Page 30] RFC 8568 Network Virtualization Research Challenges April 2019

 built to support packet forwarding functions, while the hardware to
 support a purely control-plane application (say, a DNS server, or a
 Diameter function) will not have this specialized capability.  In
 NFV, the NFVI is required to support all types of potentially
 different workload types.
 Therefore, testing the NFVI requires careful consideration about what
 types of metrics are sought.  This, in turn, depends on the workload
 type the expected VNF will be.  Examples of different workload types
 are data forwarding, control plane, encryption, and authentication.
 All these types of expected workloads will determine the types of
 metrics that should be sought.  For example, if the workload is
 control plane, then a metric such as jitter is not useful, but
 dropped packets are critical.  In a multi-tenant environment, the
 NFVI could support various types of workloads.  In this case, testing
 with a variety of traffic types while measuring the corresponding
 metrics simultaneously becomes necessary.
 Test beds for any type of testing for an NFV-based system will be
 largely similar to previously used test architectures.  The methods
 are impacted by virtualization, as described above, but the design of
 test beds are similar as in the past.  There are two main new
 considerations:
 o  Since networking is based on software, which has lead to greater
    automation in deployment, the test system should also be
    deployable with the rest of the system in order to fully automate
    the system.  This is especially relevant in a DevOps environment
    supported by a Continuous Integration and Continuous Deployment
    (CI/CD) tool chain (see Section 4.11.3 below).
 o  In any performance test bed, the test system should not share the
    same resources as the SUT.  While multi-tenancy is a reality in
    virtualization, having the test system share resources with the
    SUT will impact the measured results in a performance test bed.
    The test system should be deployed on a separate platform in order
    not to impact the resources available to the SUT.

4.11.2. New Functionality

 NFV presents a collection of new functionality in order to support
 the goal of software networking.  Each component on the architecture
 shown in Figure 2 has an associated set of functionality that allows
 VNFs to run the following: onboarding, life-cycle management for VNFs
 and Network Services (NS), resource allocation, hypervisor functions,
 etc.

Bernardos, et al. Informational [Page 31] RFC 8568 Network Virtualization Research Challenges April 2019

 One of the new capabilities enabled by NFV is VNF Forwarding Graphs
 (VNFFG).  This refers to the graph that represents a network service
 by chaining together VNFs into a forwarding path.  In practice, the
 forwarding path can be implemented in a variety of ways using
 different networking capabilities: vSwitch, SDN, and SDN with a
 northbound application.  Additionally, the VNFFG might use tunneling
 protocols like Virtual eXtensible Local Area Network (VXLAN).  The
 dynamic allocation and implementation of these networking paths will
 have different performance characteristics depending on the methods
 used.  The path implementation mechanism becomes a variable in the
 network testing of the NSs.  The methodology used to test the various
 mechanisms should largely remain the same; as usual, the test
 environment should remain constant for each of the tests, focusing on
 varying the path establishment method.
 "Scaling" refers to the change in allocation of resources to a VNF or
 NS.  It happens dynamically at run-time, based on defined policies
 and triggers.  The triggers can be network, compute, or storage
 based.  Scaling can allocate more resources in times of need, or
 reduce the amount of resources allocated when the demand is reduced.
 The SUT in this case becomes much larger than the VNF itself: MANO
 controls how scaling is done based on policies, and then allocates
 the resources accordingly in the NFVI.  Essentially, the testing of
 scaling includes the entire NFV architecture components into the SUT.

4.11.3. Opportunities

 Softwarization of networking functionality leads to softwarization of
 the test as well.  As PNFs are being transformed into VNFs, so are
 the test tools.  This leads to the fact that test tools are also
 being controlled and executed in the same environment as the VNFs.
 This presents an opportunity to include VNF-based test tools along
 with the deployment of the VNFs supporting the services of the
 service provider into the host data centers.  Therefore, tests can be
 automatically executed upon deployment in the target environment, for
 each deployment, and each service.  With PNFs, this was very
 difficult to achieve.
 This new concept helps to enable modern concepts like DevOps and
 Continuous Integration and Continuous Deployment in the NFV
 environment.  The CI/CD pipeline supports this concept.  It consists
 of a series of tools, among which immediate testing is an integral
 part, to deliver software from source to deployment.  The ability to
 deploy the test tools themselves into the production environment
 stretches the CI/CD pipeline all the way to production deployment,
 allowing a range of tests to be executed.  The tests can be simple,

Bernardos, et al. Informational [Page 32] RFC 8568 Network Virtualization Research Challenges April 2019

 with a goal of verifying the correct deployment and networking
 establishment, but can also be more complex, like testing VNF
 functionality.

5. Technology Gaps and Potential IETF Efforts

 Table 1 correlates the open network virtualization research areas
 identified in this document to potential IETF and IRTF groups that
 could address some aspects of them.  An example of a specific gap
 that the group could potentially address is identified as a
 parenthetical beside the group name.
 +-------------------------+-----------------------------------------+
 | Open Research Area      | Potential IETF/IRTF Group               |
 +-------------------------+-----------------------------------------+
 | 1) Guaranteeing QoS     | IPPM WG (Measurements of NFVI)          |
 |                         |                                         |
 | 2) Performance          | SFC WG, NFVRG (energy-driven            |
 | improvement             | orchestration)                          |
 |                         |                                         |
 | 3) Multiple Domains     | NFVRG (multi-domain orchestration)      |
 |                         |                                         |
 | 4) Network Slicing      | NVO3 WG, NETSLICES bar BoF (multi-      |
 |                         | tenancy support)                        |
 |                         |                                         |
 | 5) Service Composition  | SFC WG (SFC Mgmt and Config)            |
 |                         |                                         |
 | 6) End-user device      | N/A                                     |
 | virtualization          |                                         |
 |                         |                                         |
 | 7) Security             | N/A                                     |
 |                         |                                         |
 | 8) Separation of        | NFVRG (separation between transport     |
 | control concerns        | control and services)                   |
 |                         |                                         |
 | 9) Testing              | NFVRG (testing of scaling)              |
 |                         |                                         |
 | 10) Function placement  | NFVRG, SFC WG (VNF placement algorithms |
 |                         | and protocols)                          |
 +-------------------------+-----------------------------------------+
   Table 1: Mapping of Open Research Areas to Potential IETF Groups

Bernardos, et al. Informational [Page 33] RFC 8568 Network Virtualization Research Challenges April 2019

6. NFVRG Focus Areas

 Table 2 correlates the currently identified NFVRG topics of interest
 / focus areas to the open network virtualization research areas
 enumerated in this document.  This can help the NFVRG in identifying
 and prioritizing research topics.  The current list of NFVRG focus
 points is the following:
 o  Re-architecting functions, including aspects such as new
    architectural and design patterns (e.g., containerization,
    statelessness, serverless, control/data plane separation), SDN
    integration, and proposals on programmability.
 o  New management frameworks, considering aspects related to new OAM
    mechanisms (e.g., configuration control, hybrid descriptors) and
    lightweight MANO proposals.
 o  Techniques to guarantee low latency, resource isolation, and other
    data-plane features, including hardware acceleration, functional
    offloading to data-plane elements (including NICs), and related
    approaches.
 o  Measurement and benchmarking, addressing both internal
    measurements and external applications.
   +-------------------------------------+-------------------------+
   | NFVRG Focus Point                   | Open Research Area      |
   +-------------------------------------+-------------------------+
   | 1) Re-architecting functions        | - Performance improvem. |
   |                                     | - Network Slicing       |
   |                                     | - Guaranteeing QoS      |
   |                                     | - Security              |
   |                                     | - End-user device virt. |
   |                                     | - Separation of control |
   |                                     |                         |
   | 2) New management frameworks        | - Multiple Domains      |
   |                                     | - Service Composition   |
   |                                     | - End-user device virt. |
   |                                     |                         |
   | 3) Low latency, resource isolation, | - Performance improvem. |
   | etc.                                | - Separation of control |
   |                                     |                         |
   | 4) Measurement and benchmarking     | - Guaranteeing QoS      |
   |                                     | - Testing               |
   +-------------------------------------+-------------------------+
     Table 2: Mapping of NFVRG Focus Points to Open Research Areas

Bernardos, et al. Informational [Page 34] RFC 8568 Network Virtualization Research Challenges April 2019

7. IANA Considerations

 This document has no IANA actions.

8. Security Considerations

 This is an Informational RFC that details research challenges; it
 does not introduce any security threat.  Research challenges and gaps
 related to security and privacy have been included in Section 4.8.

9. Informative References

 [COMS-PS]  Geng, L., Slawomir, S., Qiang, L., Matsushima, S., Galis,
            A., and L. Contreras, "Problem Statement of Common
            Operation and Management of Network Slicing", Work in
            Progress, draft-geng-coms-problem-statement-04, March
            2018.
 [dynamic_chaining]
            Martini, B. and F. Paganelli, "A Service-Oriented Approach
            for Dynamic Chaining of Virtual Network Functions over
            Multi-Provider Software-Defined Networks", Future
            Internet Vol. 8, No. 2, DOI 10.3390/fi8020024, June 2016.
 [dynamic_placement]
            Clayman, S., Maini, E., Galis, A., Manzalini, A., and
            N. Mazzocca, "The dynamic placement of virtual network
            functions", 2014 IEEE Network Operations and Management
            Symposium (NOMS) pp. 1-9, DOI 10.1109/NOMS.2014.6838412,
            May 2014.
 [etsi_gs_nfv_003]
            ETSI NFV ISG, "Network Functions Virtualisation (NFV);
            Terminology for Main Concepts in NFV", ETSI GS NFV 003
            V1.2.1 NFV 003, December 2014,
            <http://www.etsi.org/deliver/etsi_gs/
            NFV/001_099/003/01.02.01_60/gs_NFV003v010201p.pdf>.
 [etsi_gs_nfv_eve005]
            ETSI NFV ISG, "Network Functions Virtualisation (NFV);
            Ecosystem; Report on SDN Usage in NFV Architectural
            Framework", ETSI GS NFV-EVE 005 V1.1.1 NFV-EVE 005,
            December 2015,
            <http://www.etsi.org/deliver/etsi_gs/NFV-EVE/001_099/
            005/01.01.01_60/gs_NFV-EVE005v010101p.pdf>.

Bernardos, et al. Informational [Page 35] RFC 8568 Network Virtualization Research Challenges April 2019

 [etsi_gs_nfv_per_001]
            ETSI NFV ISG, "Network Functions Virtualisation (NFV); NFV
            Performance & Portability Best Practises", ETSI GS NFV-PER
            001 V1.1.2 NFV-PER 001, December 2014,
            <https://www.etsi.org/deliver/etsi_gs/nfv-per/
            001_099/001/01.01.02_60/gs_nfv-per001v010102p.pdf>.
 [etsi_gs_nfv_sec_001]
            ETSI NFV ISG, "Network Functions Virtualisation (NFV); NFV
            Security; Problem Statement", ETSI GS NFV-SEC 001 V1.1.1
            NFV-SEC 001, October 2014, <http://www.etsi.org/deliver/
            etsi_gs/NFV-SEC/001_099/001/01.01.01_60/
            gs_NFV-SEC001v010101p.pdf>.
 [etsi_nfv_whitepaper_3]
            ETSI, "Network Functions Virtualisation (NFV) - White
            Paper #3: Network Operator Perspectives on Industry
            Progress", Issue 1, SDN & OpenFlow World
            Congress Dusseldorf, Germany, October 2014,
            <http://portal.etsi.org/NFV/NFV_White_Paper3.pdf>.
 [google_sdn_wan]
            Jain, S., et al., "B4: experience with a globally-deployed
            Software Defined WAN", SIGCOMM '13: Proceedings of the ACM
            SIGCOMM 2013 conference on SIGCOMM, pp. 3-14, Hong
            Kong China, DOI 10.1145/2486001.2486019, August 2013.
 [intel_10_differences_nfv_cloud]
            Torre, P., "Discover the Top 10 Differences Between NFV
            and Cloud Environments", November 2015,
            <https://software.intel.com/en-us/videos/discover-the-top-
            10-differences-between-nfv-and-cloud-environments>.
 [itu-t-y.3300]
            ITU-T, "Y.3300: Framework of software-defined networking",
            ITU-T Recommendation Y.3300, June 2014,
            <http://www.itu.int/rec/T-REC-Y.3300-201406-I/en>.
 [itu-t-y.3301]
            ITU-T, "Y.3301: Functional requirements of software-
            defined networking", ITU-T Recommendation Y.3301,
            September 2016,
            <http://www.itu.int/rec/T-REC-Y.3301-201609-I/en>.

Bernardos, et al. Informational [Page 36] RFC 8568 Network Virtualization Research Challenges April 2019

 [itu-t-y.3302]
            ITU-T, "Y.3302: Functional architecture of software-
            defined networking", ITU-T Recommendation Y.3302, January
            2017, <http://www.itu.int/rec/T-REC-Y.3302-201701-I/en>.
 [LAYERED-SDN]
            Contreras, L., Bernardos, C., Lopez, D., Boucadair, M.,
            and P. Iovanna, "Cooperating Layered Architecture for
            Software Defined Networking (CLAS)", Work in Progress,
            draft-contreras-layered-sdn-03, November 2018.
 [LIGHT-NFV]
            Sriram, N., Krishnan, R., Ghanwani, A., Krishnaswamy, D.,
            Willis, P., Chaudhary, A., and F. Huici, "An Analysis of
            Lightweight Virtualization Technologies for NFV", Work in
            Progress, draft-natarajan-nfvrg-containers-for-nfv-03,
            July 2016.
 [multi-domain_5GEx]
            Bernardos, C., Gero, B., Di Girolamo, M., Kern, A.,
            Martini, B., and I. Vaishnavi, "5GEx: Realizing a Europe-
            wide Multi-domain framework for software-defined
            infrastructures", Transactions on Emerging
            Telecommunications Technologies Vol. 27, No. 9,
            pp. 1271-1280, DOI 10.1002/ett.3085, July 2016.
 [MULTI-NMRG]
            Bernardos, C., Contreras, L., Vaishnavi, I., Szabo, R.,
            Li, X., Paolucci, F., Sgambelluri, A., Martini, B.,
            Valcarenghi, L., Landi, G., Andrushko, D., and A. Mourad,
            "Multi-domain Network Virtualization", Work in Progress,
            draft-bernardos-nmrg-multidomain-00, March 2019.
 [NETSLICES]
            Galis, A., Dong, J., Makhijani, K., Bryant, S., Boucadair,
            M., and P. Martinez-Julia, "Network Slicing - Introductory
            Document and Revised Problem Statement", Work in
            Progress, draft-gdmb-netslices-intro-and-ps-02, February
            2017.
 [NFV-COTS] Mo, L. and B. Khasnabish, "NFV Reliability using COTS
            Hardware", Work in Progress, draft-mlk-nfvrg-nfv-
            reliability-using-cots-01, October 2015.

Bernardos, et al. Informational [Page 37] RFC 8568 Network Virtualization Research Challenges April 2019

 [nfv_piecing]
            Luizelli, M., Bays, L., Buriol, L., Barcellos, M., and
            L. Gaspary, "Piecing together the NFV provisioning puzzle:
            Efficient placement and chaining of virtual network
            functions", 2015 IFIP/IEEE International Symposium on
            Integrated Network Management (IM) pp. 98-106,
            DOI 10.1109/INM.2015.7140281, May 2015.
 [nfv_sota_research_challenges]
            Mijumbi, R., Serrat, J., Gorricho, J-L., Bouten, N.,
            De Turck, F., and R. Boutaba, "Network Function
            Virtualization: State-of-the-art and Research Challenges",
            IEEE Communications Surveys & Tutorials Volume: 18,
            Issue: 1, pp. 236-262, DOI 10.1109/COMST.2015.2477041,
            September 2015.
 [NFVRG-TOPO]
            Bagnulo, M. and D. Dolson, "NFVI PoP Network Topology:
            Problem Statement", Work in Progress, draft-bagnulo-nfvrg-
            topology-01, March 2016.
 [ngmn_5G_whitepaper]
            NGMN Alliance, "NGMN 5G White Paper", Version 1.0,
            February 2015,
            <https://www.ngmn.org/fileadmin/ngmn/content/
            images/news/ngmn_news/NGMN_5G_White_Paper_V1_0.pdf>.
 [omniran]  IEEE, "Recommended Practice for Network Reference Model
            and Functional Description of IEEE 802 Access Network",
            P802.1CF IEEE Draft, December 2017.
 [onf_tr_521]
            Open Networking Foundation, "SDN Architecture", ONF
            TR-521 TR-521, Issue 1.1, February 2016,
            <https://www.opennetworking.org/images/stories/downloads/
            sdn-resources/technical-reports/
            TR-521_SDN_Architecture_issue_1.1.pdf>.
 [OpenFlow] Open Networking Foundation, "OpenFlow Switch
            Specification", ONF TS-025, Version 1.5.1 (Protocol
            version 0x06), March 2015.
 [openmano_dataplane]
            Lopez, D., "OpenMANO: The Dataplane Ready Open Source NFV
            MANO Stack", March 2015, <https://www.ietf.org/
            proceedings/92/slides/slides-92-nfvrg-7.pdf>.

Bernardos, et al. Informational [Page 38] RFC 8568 Network Virtualization Research Challenges April 2019

 [RFC5810]  Doria, A., Ed., Hadi Salim, J., Ed., Haas, R., Ed.,
            Khosravi, H., Ed., Wang, W., Ed., Dong, L., Gopal, R., and
            J. Halpern, "Forwarding and Control Element Separation
            (ForCES) Protocol Specification", RFC 5810,
            DOI 10.17487/RFC5810, March 2010,
            <https://www.rfc-editor.org/info/rfc5810>.
 [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
            and A. Bierman, Ed., "Network Configuration Protocol
            (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
            <https://www.rfc-editor.org/info/rfc6241>.
 [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
            Application Protocol (CoAP)", RFC 7252,
            DOI 10.17487/RFC7252, June 2014,
            <https://www.rfc-editor.org/info/rfc7252>.
 [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>.
 [RFC7498]  Quinn, P., Ed. and T. Nadeau, Ed., "Problem Statement for
            Service Function Chaining", RFC 7498,
            DOI 10.17487/RFC7498, April 2015,
            <https://www.rfc-editor.org/info/rfc7498>.
 [RFC7665]  Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
            Chaining (SFC) Architecture", RFC 7665,
            DOI 10.17487/RFC7665, October 2015,
            <https://www.rfc-editor.org/info/rfc7665>.
 [RFC8030]  Thomson, M., Damaggio, E., and B. Raymor, Ed., "Generic
            Event Delivery Using HTTP Push", RFC 8030,
            DOI 10.17487/RFC8030, December 2016,
            <https://www.rfc-editor.org/info/rfc8030>.
 [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
            Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
            <https://www.rfc-editor.org/info/rfc8040>.
 [RFC8172]  Morton, A., "Considerations for Benchmarking Virtual
            Network Functions and Their Infrastructure", RFC 8172,
            DOI 10.17487/RFC8172, July 2017,
            <https://www.rfc-editor.org/info/rfc8172>.

Bernardos, et al. Informational [Page 39] RFC 8568 Network Virtualization Research Challenges April 2019

 [SDN-AAA]  Lopez, R. and G. Lopez-Millan, "Software-Defined
            Networking (SDN)-based AAA Infrastructures Management",
            Work in Progress, draft-marin-sdnrg-sdn-aaa-mng-00,
            November 2015.
 [sfc_challenges]
            Medhat, A., Taleb, T., Elmangoush, A., Carella, G.,
            Covaci, S., and T. Magedanz, "Service Function Chaining in
            Next Generation Networks: State of the Art and Research
            Challenges", IEEE Communications Magazine vol. 55, no. 2,
            pp. 216-223, DOI 10.1109/MCOM.2016.1600219RP, February
            2017.
 [SLICE-3GPP]
            Foy, X. and A. Rahman, "Network Slicing - 3GPP Use Case",
            Work in Prgoress, draft-defoy-netslices-3gpp-network-
            slicing-02, October 2017.
 [virtualization_mobile_device]
            Sproule, W. and A. Fernando, "Virtualization of Mobile
            Device User Experience", US Patent 9.542.062 B2, filed
            October 2013 and issued December 2014, Current
            Assignee: Microsoft Technology Licensing LLC.
 [vnf-p]    Moens, H. and , "VNF-P: A model for efficient placement of
            virtualized network functions", 10th International
            Conference on Network and Service Management (CNSM) and
            Workshop pp. 418-423, DOI 10.1109/CNSM.2014.7014205,
            November 2014.
 [VNF-VBAAS]
            Rosa, R., Rothenberg, C., and R. Szabo, "VNF Benchmark-as-
            a-Service", Work in Progress, draft-rorosz-nfvrg-vbaas-00,
            October 2015.
 [vnf_benchmarking]
            Rosa, R., Rothenberg, C., and R. Szabo, "A VNF Testing
            Framework Design, Implementation and Partial Results",
            NFVRG IETF 97, November 2016,
            <https://www.ietf.org/proceedings/97/slides/
            slides-97-nfvrg-06-vnf-benchmarking-00.pdf>.

Bernardos, et al. Informational [Page 40] RFC 8568 Network Virtualization Research Challenges April 2019

Acknowledgments

 The authors want to thank Dirk von Hugo, Rafa Marin, Diego Lopez,
 Ramki Krishnan, Kostas Pentikousis, Rana Pratap Sircar, Alfred
 Morton, Nicolas Kuhn, Saumya Dikshit, Fabio Giust, Evangelos
 Haleplidis, Angeles Vazquez-Castro, Barbara Martini, Jose Saldana,
 and Gino Carrozzo for their very useful reviews and comments to the
 document.  Special thanks to Pedro Martinez-Julia, who provided text
 for the network slicing section.
 The authors want to also thank Dave Oran and Michael Welzl for their
 very detailed IRSG reviews.
 The work of Carlos J. Bernardos and Luis M. Contreras is partially
 supported by the H2020 5GEx (Grant Agreement no. 671636) and
 5G-TRANSFORMER (Grant Agreement no. 761536) projects.

Authors' Addresses

 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/
 Akbar Rahman
 InterDigital Communications, LLC
 1000 Sherbrooke Street West, 10th floor
 Montreal, Quebec  H3A 3G4
 Canada
 Email: Akbar.Rahman@InterDigital.com
 URI:   http://www.InterDigital.com/
 Juan Carlos Zuniga
 SIGFOX
 425 rue Jean Rostand
 Labege  31670
 France
 Email: j.c.zuniga@ieee.org
 URI:   http://www.sigfox.com/

Bernardos, et al. Informational [Page 41] RFC 8568 Network Virtualization Research Challenges April 2019

 Luis M. Contreras
 Telefonica I+D
 Ronda de la Comunicacion, S/N
 Madrid  28050
 Spain
 Email: luismiguel.contrerasmurillo@telefonica.com
 Pedro Aranda
 Universidad Carlos III de Madrid
 Av. Universidad, 30
 Leganes, Madrid  28911
 Spain
 Email: pedroandres.aranda@uc3m.es
 Pierre Lynch
 Keysight Technologies
 800 Perimeter Park Dr, Suite A
 Morrisville, NC  27560
 United States of America
 Email: pierre.lynch@keysight.com
 URI:   http://www.keysight.com

Bernardos, et al. Informational [Page 42]

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