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Versions: 00 01

Internet Engineering Task Force                                 J. Arkko
Internet-Draft                                                  Ericsson
Intended status: Informational                               J. Tantsura
Expires: September 6, 2018                                 Nuagenetworks
                                                              J. Halpern
                                                                B. Varga
                                                                Ericsson
                                                           March 5, 2018


          Considerations on Network Virtualization and Slicing
                   draft-arkko-arch-virtualization-01

Abstract

   This document makes some observations on the effects of
   virtualization on Internet architecture, as well as provides some
   guidelines for further work at the IETF relating to virtualization.

   This document also provides a summary of IETF technologies that
   relate to network virtualization.  An understanding of what current
   technologies there exist and what they can or cannot do is the first
   step in developing plans for possible extensions.

Status of This Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 6, 2018.

Copyright Notice

   Copyright (c) 2018 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



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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  General Observations  . . . . . . . . . . . . . . . . . . . .   4
   4.  Virtualization in 5G Networks . . . . . . . . . . . . . . . .   6
   5.  Overview of IETF Virtualization Technologies  . . . . . . . .   6
     5.1.  Selection of Virtual Instances  . . . . . . . . . . . . .   7
     5.2.  Traffic Separation in VPNs  . . . . . . . . . . . . . . .   7
     5.3.  Traffic Engineering and QoS . . . . . . . . . . . . . . .   9
     5.4.  Service Chaining  . . . . . . . . . . . . . . . . . . . .  10
     5.5.  Management Frameworks and Data Models . . . . . . . . . .  10
   6.  Architectural Observations  . . . . . . . . . . . . . . . . .  12
   7.  Further Work  . . . . . . . . . . . . . . . . . . . . . . . .  14
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   9.  Informative References  . . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   Network virtualization is network management pertaining to treating
   different traffic categories in separate virtual networks, with
   independent lifecycle management and resource, technology, and
   topology choices.

   This document makes some observations on the effects of
   virtualization on Internet architecture, as well as provides some
   guidelines for further work at the IETF relating to virtualization.

   This document also provides a summary of IETF technologies that
   relate to network virtualization.  An understanding of what current
   technologies there exist and what they can or cannot do is the first
   step in developing plans for possible extensions.

   In particular, many IETF discussions earlier in the summer of 2017
   started from a top-down view of new virtualization technologies, but
   were often unable to explain the necessary delta to the wealth of
   existing IETF technology in this space.  This document takes a
   different, bottom-up approach to the topic and attempts to document
   existing technology, and then identify areas of needed development.



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   In particular, whether one calls a particular piece of technology
   "virtualization", "slicing", "separation", or "network selection"
   does not matter at the level of a system.  Any modern system will use
   several underlying technology components that may use different terms
   but provide some separation or management.  So, for instance, in a
   given system you may use VLAN tags in an ethernet segment, MPLS or
   VPNs across the domain, NAIs to select the right AAA instance, and
   run all this top of virtualized operating system and software-based
   switches.  As new needs are being recognised in the developing
   virtualization technology, what should drive the work is the need for
   specific capabilities rather than the need to distinghuish a
   particular term from another term.

2.  Definitions

   Network function virtualization is defined in Wikipedia as follows:

      "Network function virtualization or NFV is a network architecture
      concept that uses the technologies of IT virtualization to
      virtualize entire classes of network node functions into building
      blocks that may connect, or chain together, to create
      communication services.

      NFV relies upon, but differs from, traditional server-
      virtualization techniques, such as those used in enterprise IT.  A
      virtualized network function, or VNF, may consist of one or more
      virtual machines running different software and processes, on top
      of standard high-volume servers, switches and storage devices, or
      even cloud computing infrastructure, instead of having custom
      hardware appliances for each network function."

   We should not confuse NFV and network virtualization, the former, as
   the name suggests is about functions virtualization, and not the
   network.

   The idea of network virtualization is almost as old as the networking
   technology itself.  Network virtualization is hierarchical and
   multilayer in its nature, from layer 1 up to services on top.  When
   talking about virtualization we usually define overlay to underlay
   relationship between different layers, bottom up.  A VPN (Virtual
   Private Network) [RFC4026] is the most common form of network
   virtualization.  The general benefits and desirability of VPNs have
   been described many times and in many places ([RFC4110] and
   [RFC4664]).

   The only immutable infrastructure is the "physical" medium, that
   could be dedicated or "sliced" to provide services(VPNs) in a multi-
   tenant environment.



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   The term slicing has been used to describe a virtualization concept
   in planned 5G networks.  The 3GPP architecture specification
   [TS-3GPP.23.501] defines network slices as having potentially
   different "supported features and network functions optimisations",
   and spanning functions from core network to radio access networks.

   [I-D.king-teas-applicability-actn-slicing] defined slicing as "an
   approach to network operations that builds on the concept of network
   abstraction to provide programmability, flexibility, and modularity.
   It may use techniques such as Software Defined Networking (SDN) and
   Network Function Virtualization (NFV) to create multiple logical
   (virtual) networks, each tailored for a set of services that are
   sharing the same set of requirements, on top of a common network.

   And, [I-D.geng-coms-problem-statement] defines slicing as a
   management mechanism that an service provider can use to allocate
   dedicated network resources from shared network infrastructures to a
   tenant.

3.  General Observations

   Software vs. Protocols

      Many of the necessary tools for using virtualization are software,
      e.g., tools that enable running processes or entire machines in a
      virtual environment decoupled from physical machines and isolated
      from each other, virtual switches that connect systems together,
      management tools to set up virtual environments, and so on.  From
      a communications perspective these tools operate largely in the
      same fashion as their real-world counterparts do, except that
      there may not be wires or other physical communication channels,
      and that connections can be made in the desired fashion.

      In general, there is no reason for protocols to change just
      because a function or a connection exists on a virtual platform.
      However, sometimes there are useful underlying technologies that
      facilitiate connection to virtualized systems, or optimised or
      additional tools that are needed in the the virtualized
      environment.

      For instance, many underlying technologies enable virtualization
      at hardware or physical networking level.  For instance, Ethernet
      networks have Virtual LAN (VLAN) tags and mobile networks have a
      choice of Access Point Names (APNs).  These techniques allow users
      and traffic to be put on specific networks, which in turn may
      comprise of virtual components.





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      Other examples of protocols providing helpful techniques include
      virtual private networking mechanisms or management mechanisms and
      data models that can assist in setting up and administering
      virtualized systems.

      There may also be situations where scaling demands changes in
      protocols.  An ability to replicate many instances may push the
      limits of protocol mechanisms that were designed primarily or
      originally for physical networks.

   Selection vs. Creation and Orchestration

      Two primary tasks in virtualization should be differentiated:
      selection of a particular virtual instance, and the tasks related
      to how that virtual instance was created and continues to be
      managed.

      Selection involves choosing a particular virtual instance, or an
      entrypoint to a virtual network.  In its simplest form, a customer
      could be hardwired by configuration to a particular virtual
      instance.  In more complex cases, the connecting devices may have
      some settings that affect the choice.  In the general case, both
      the connecting devices and the network they are connecting to it
      have a say in the choice.

      The selection choice may even be dynamic in some cases.  For
      instance, traffic pattern analysis may affect the selection.

      Typically, however, connecting devices do not have a say in what
      the virtual instance does.  This is directed by the network
      operator and its customers.  An instance is specified, created,
      and needs to be continously managed and orchestrated.  The
      creation can be manual and occur rarely, or be more dynamic, e.g.,
      an instance can actually be instantiated automatically, and only
      when the first connecting device connects to it.

   Protocols vs. Representations of Virtual Networks

      Some of virtualization technology benefits from protocol support
      either in the data or control plane.  But there are also
      management constructs, such as data models representing virtual
      services or networks and data models useful in the construction of
      such services.

      There are also conceptual definitions that may be needed when
      constructing either protocols or data models or when discussing
      service agreements between providers and consumers.




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4.  Virtualization in 5G Networks

   Goals for the support of virtualization in 5G relate to both the use
   of virtualized network functions to build the 5G network, and to
   enabling the separation of different user or traffic classes into
   separate network constructs called slices.

   Slices enable a separation of concerns, allow the creation of
   dedicated services for special traffic types, allow faster evolution
   of the network mechanisms by easing gradual migration to new
   functionality, and enable faster time to market for new new
   functionality.

   In 5G, slice selection happens as a combination of settings in the
   User Equipment (UE) and the network.  Settings in the UE include, for
   instance, the Access Point Name (APN), Dedicated Core Network
   Indicator (DCN-ID) [TS-3GPP.23.401], and, with 5G, a slice indicator
   (Network Slice Selection Assistance Information or NSSAI)
   [TS-3GPP.23.501].  This information is combined with the information
   configured in the network for a given subscriber and the policies of
   the networks involved.  Ultimately, a slice is selected.

   A 5G access network carries a user's connection attempt to the 5G
   core network and the Access Management Function (AMF) network
   function.  This function collects information provided by the UE and
   the subscriber database from home network, and consults the Network
   Slice Selection Function (NSSF) to make a decision of the slice
   selected for the user.  When the selection has been made, this may
   also mean that the connection is moved to a different AMF; enabling
   separate networks to have entirely different network-level service.

   The creation and orchestration of slices does not happen at this
   signalling plane, but rather the slices are separately specified,
   created, and managed, typically with the help of an orchestrator
   function.

   The exact mechanisms for doing this continue to evolve, but in any
   case involve multiple layers of technology, ranging from underlying
   virtualization software to network component configuration mechanisms
   and models (often in YANG) to higher abstraction level descriptions
   (often in TOSCA), to orchestrator software.

5.  Overview of IETF Virtualization Technologies

   General networking protocols are largely agnostic to virtualization.
   TCP/IP does not care whether it runs on a physical wire or on a
   computer-created connection between virtual devices.




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   As a result, virtualization generally does not affect TCP/IP itself
   or applications running on top.  There are some exceptions, though,
   such as when the need to virtualize has caused previously held
   assumptions to break, and the Internet community has had to provide
   new solutions.  For instance, early versions of the HTTP protocol
   assumed a single host served a single website.  The advent of virtual
   hosting and pressure to not use large numbers of IPv4 addresses lead
   to HTTP 1.1 adopting virtual hosting, where the identified web host
   is indicated inside the HTTP protocol rather than inferred from the
   reception of a request at particular IP address [VirtualHosting]
   [RFC2616].

   But where virtualization affects the Internet architecture and
   implementations is at lower layers, the physical and MAC layers, the
   systems that deal with the delivery of IP packets to the right
   destination, management frameworks controlling these systems, and
   data models designed to help the creation, monitoring, or management
   of virtualized services.

   What follows is an overview of existing technologies and technologies
   currently under development that support virtualization in its
   various forms.

5.1.  Selection of Virtual Instances

   Some L2 technology allows the identification of traffic belonging to
   a particular virtual network or connection.  For instance, Ethernet
   VLAN tags.

   There are some IETF technologies that also allow similar
   identification of connections setup with the help of IETF protocols.
   For instance, Network Access Identifiers may identify a particular
   customer or virtual service within AAA, EAP or IKEv2 VPN connections.

5.2.  Traffic Separation in VPNs

   Technologies that assist separation and engineering of networks
   include both end-point and provider-based VPNs.  End-point VPN
   tehchnologies include, for instance, IPsec-based VPNs [RFC4301].

   For providing virtualized services, however, provider-based solutions
   are often the most relevant ones.  L1VPN facilitates virtualization
   of the underlying L0 "physical" medium.  L2[IEEE802.1Q] facilitates
   virtualization of the underlying Ethernet network Tunneling over IP
   (MPLS, GRE, VxLAN, IPinIP, L2TP, etc) facilitates virtualization of
   the underlying IP network - MPLS LSP's - either traffic engineered or
   not belong here L2VPN facilitates virtualization of a L2 network
   L3VPN facilitates virtualization of a L3 network.



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   The IETF has defined a multiplicity of technologies that can be used
   for provider-based VPNs.  The technologies choices available can be
   described along two axes, control mechanisms and dataplane
   encapsulation mechanisms.  The two are not compeltely orthogonal.

   In the data plane, for provider based VPNs, the first important
   observation is that the most obvious encapsulation is NOT used.
   While IPSec could be used for provider-based VPNs, it does not appear
   to be used in practice, and is not the focus for any of the available
   control mechanisms.  Often, when end2end encryption is required it is
   used as an overlay over MPLS based L3VPN

   The common encapsulation for provider-based VPNs is to use MPLS.
   This is particularly common for VPNs within one operator, and is
   sometimes supported across operators.

   Keyed GRE can be used, particularly for cross-operator cases.
   However, it seems to be rare in practice.

   The usage of MPLS for provider-based VPNs generally follows a pattern
   of using two (or more) MPLS labels, top (transport) label to
   represent the remote end point/egress provider-edge device, and
   bottom (service) label to signal the different VPNs on the remote end
   point.  Using TE might result in a deeper label stack.

   L2 VPNs could be signaled thru LDP[RFC4762] or MP-BGP[RFC4761], L3
   VPN is signaled thru MP-BGP[RFC4364]

   The LDP usage to control VPN establishment falls within the PALS
   working group, and is used to establish pseudo-wires to carry
   Ethernet (or lower layer) traffic.  The Ethernet cases tend to be
   called VPLS (Virtual Private LAN Service) for multi-point
   connectivity and VPWS (Virtual Private Wire Service) for point-to-
   point connectivity.  These mechanism do augment the data plane
   capabilites with control words that support additional features.  In
   operation, LDP is used to signal the communicating end-points that
   are interested in communicating with each other in support of
   specific VPNs.  Information about the MAC addresses used behind the
   provider edges is exchanged using classic Ethernet flooding
   technology.  It has been proposed to use BGP to bootstrap the exchang
   eof information as to who the communicating endpoints are.

   BGP can be used to establish Layer 2 or Layer 3 VPNs.  Originally,
   the BGP based MPLS VPN technology was developed to support layer 3
   VPNs. the BGP exchanges uses several different features in MP-BGP
   (specifically route distinguishers and route targets) to control the
   distribution of information about VPN end-points.  The BGP
   information carries the VPN IP address prefixes, and the MPLS labels



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   to be used to represent the VPN.  This technolgoy combination is
   generally known as L3VPN.

   This usage of BGP for VPNs has been extended to support Layer 2 VPNs.
   This is known as EVPN.  The BGP exchanges are used to carry the MAC
   address reachability behind each provider edge router, providing an
   Ethernet multipoint service without a need to flood unkown-
   destination Ethernet packets.

   In theory, the BGP mechanisms can also be used to support other
   tunnels such as keyed GRE.  That is not widely practiced.

   There are also hybrid variations, such as adding an ARP / ND proxy
   service so that an L3VPN can be used with an L2 Access, when the only
   desired service is IP.

5.3.  Traffic Engineering and QoS

   Traffic Engineering (TE) is the term used to refer to techniques that
   enable operators to control how specific traffic flows are treated
   within their networks.

   The TEAS working group works on enhancements to traffic-engineering
   capabilities for MPLS and GMPLS networks:

      TE is applied to packet networks via MPLS TE tunnels and LSPs.
      The MPLS-TE control plane was generalized to additionally support
      non-packet technologies via GMPLS.  RSVP-TE is the signaling
      protocol used for both MPLS-TE and GMPLS.

      The TEAS WG is responsible for:

      *  Traffic-engineering architectures for generic applicability
         across packet and non-packet networks.

      *  Definition of protocol-independent metrics and parameters.

      *  Functional specification of extensions for routing (OSPF,
         ISIS), for path computation (PCE), and RSVP-TE to provide
         general enablers of traffic-engineering systems.

      *  Definition of control plane mechanisms and extensions to allow
         the setup and maintenance of TE paths and TE tunnels that span
         multiple domains and/or switching technologies.

   A good example of work that is currently considered in the TEAS WG is
   the set of models that detail earlier IETF-developed topology models
   with both traffic engineering information and connection to what



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   services are running on top of the network
   [I-D.bryskin-teas-use-cases-sf-aware-topo-model]
   [I-D.bryskin-teas-sf-aware-topo-model].  These models enable
   reasoning about the state of the network with respect to those
   services, and to set up services with optimal network connectivity.

   Traffic engineering is a common requirement for many routing systems,
   and also discussed, e.g., in the context of LISP.

5.4.  Service Chaining

   The SFC working group has defined the concept of Service Chaining:

      Today, common deployment models have service functions inserted on
      the data-forwarding path between communicating peers.  Going
      forward, however, there is a need to move to a different model,
      where service functions, whether physical or virtualized, are not
      required to reside on the direct data path and traffic is instead
      steered through required service functions, wherever they are
      deployed.

      For a given service, the abstracted view of the required service
      functions and the order in which they are to be applied is called
      a Service Function Chain (SFC).  An SFC is instantiated through
      selection of specific service function instances on specific
      network nodes to form a service graph: this is 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.).

5.5.  Management Frameworks and Data Models

   There have been two working groups at the IETF, focusing on data
   models describing VPNs.  The IETF and the industry in general is
   currently specifying a set of YANG models for network element and
   protocol configuration [RFC6020].

   YANG is a powerful and versatile data modeling language that was
   designed from the requirements of network operators for an easy to
   use and robust mechanism for provisioning devices and services across
   networks.  It was originally designed at the Internet Engineering
   Task Force (IETF) and has been so successful that it has been adopted
   as the standard for modeling design in many other standards bodies
   such as the Metro Ethernet Forum, OpenDaylight, OpenConfig, and
   others.  The number of YANG modules being implemented for interfaces,
   devices, and service is growing rapidly.





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   (It should be noted that there are also other description formats,
   e.g., Topology and Orchestration Specification for Cloud Applications
   (TOSCA) [TOSCA-1.0] [TOSCA-Profile-1.1], common in many higher
   abstract level network service descriptions.  The ONAP open source
   project plans to employ it for abstract mobile network slicing
   models, for instance.)

   A service model is an abstract model, at a higher level than network
   element or protocol configuration.  A service model for VPN service
   describes a VPN in a manner that a customer of the VPN service would
   see it.

   It needs to be clearly understood that such a service model is not a
   configuration model.  That is, it does not provide details for
   configuring network elements or protocols: that work is expected to
   be carried out in other protocol-specific working groups.  Instead,
   service models contain the characteristics of the service as
   discussed between the operators and their customers.  A separate
   process is responsible for mapping this customer service model onto
   the protocols and network elements depending on how the network
   operator chooses to realise the service.

   The L2SM WG specifies a service model for L2-based VPNs:

      The Layer Two Virtual Private Network Service Model (L2SM) working
      group is a short-lived WG.  It is tasked to create a YANG data
      model that describes a L2VPN service (a L2VPN customer service
      model).  The model can be used for communication between customers
      and network operators, and to provide input to automated control
      and configuration applications.

      It is recognized that it would be beneficial to have a common base
      model that addresses multiple popular L2VPN service types.  The
      working group derives a single data model that includes support
      for the following:

      *  point-to-point Virtual Private Wire Services (VPWS),

      *  multipoint Virtual Private LAN services (VPLS) that use LDP-
         signaled Pseudowires,

      *  multipoint Virtual Private LAN services (VPLS) that use a
         Border Gateway Protocol (BGP) control plane as described in
         [RFC4761] and [RFC6624],

      *  Ethernet VPNs specified in [RFC7432].





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      Other L2VPN service types may be included if there is consensus in
      the working group.

   Similarly, the L3SM WG specified a sevice model for L3-based VPNs.

      The Layer Three Virtual Private Network Service Model (L3SM)
      working group is a short-lived WG tasked to create a YANG data
      model that describes a L3VPN service (a L3VPN service model) that
      can be used for communication between customers and network
      operators, and to provide input to automated control and
      configuration applications.

      It needs to be clearly understood that this L3VPN service model is
      not an L3VPN configuration model.  That is, it does not provide
      details for configuring network elements or protocols.  Instead it
      contains the characteristics of the service.

6.  Architectural Observations

   This section makes some observations about architectural trends and
   issues.

   Role of Software

      An obvious trend is that bigger and bigger parts of the
      functionality in a network is driven by software, e.g.,
      orchestration or management tools that figure out how to control
      relatively simple network element functionality.  The software
      components are where the intelligence is, and a smaller fraction
      of the intelligence resides in network elements, nor is the
      intelligence encoded in the behaviour rules of the protocols that
      the network elements use to communicate with each other.

   Centralization of Functions

      An interesting architectural trend is that virtualization and data
      /software driven networking technologies are driving network
      architectures where functionality moves towards central entities
      such as various controllers, path computation servers, and
      orchestration systems.

      A natural consequence of this is the simplification (and perhaps
      commoditization) of network elements, while the "intelligent" or
      higher value functions migrate to the center.

      The benefits are largely in the manageability, control, and speed
      of change.  There are, however, potential pitfalls to be aware of
      as well.  First off, networks need to continue to be operate even



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      under partial connectivity situations and breakage, and it is key
      that designs can handle those situations as well.

      And it is important that network users and peers continue to be
      able to operate and connect in the distributed, voluntary manner
      that we have today.  Today's virtualization technology is
      primarily used to manage single administrative domains and to
      offer specific service to others.  One could imagine centralised
      models being taken too far as well, limiting the ability of other
      network owners to manage their own networks.

   Tailored vs. general-purpose networking

      The interest in building tailored solutions, tailored Quality-of-
      Service offerings vs. building general-purpose "low touch"
      networks seems to fluctuate over time.

      It is important to find the right balance here.  From an economics
      perspective, it may not be feasible to provide specialised service
      -- at least if it requires human effort -- for large fraction of
      use cases.  Even if those are very useful in critical
      applications.

   Need for descriptions

      As networks deal more and more with virtual services, there arises
      a need to have generally understood, portable descriptions of
      these service.  Hence the creation of YANG data models
      representing abstract VPN services, for instance.

   We can also identify some potential architectural principles, such
   as:

   Data model layering

      Given the heterogenuity of networking technologies and the
      differing users that data models are being designed for, it seems
      difficult to provide a single-level model.  It seems preferable to
      construct a layered set of models, for instance abstract, user-
      facing models that specify services that can then be mapped to
      concrete configuration model for networks.  And these can in turn
      be mapped to individual network element configuration models.

      Getting this layered design right is crucial for our ability to
      evolve a useful set of data models.

   Ability to evolve modelling tools and mapping  systems




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      The networks and their models are complex, and mapping from high
      abstraction level specifications to concrete network
      configurations is a hard problem.

      It is important that each of the components can evolve on its own.
      It should be possible to plug in a new language that represents
      network models better.  Or replace a software component that
      performs mapping between layers to one that works better.

      While this should normally be possible, there's room to avoid too
      tight binding between the different aspects of a system.  For
      instance, abstraction layers within software can shield the
      software from being too closely tied with a particular
      representation language.

      Similarly, it would be an advantage to develop algorithms and
      mapping approaches separately from the software that actually does
      that, so that another piece of software could easily follow the
      same guidelines and provide an alternate implementation.  Perhaps
      there's an opportunity for specification work to focus more on
      processing rules than protocol behaviours, for instance.

   General over specific

      In the quick pace of important developments, it is tempting to
      focus on specific concepts and service offerings such as 5G
      slicing.

      But a preferrable approach seems to provide general-purpose tools
      that can be used by 5G and other networks, and whose longetivity
      exceeds that of a version of a specific offering.  The quick
      development pace is likely driving the evolution of concepts in
      any case, and building IETF tools that provide the ability to deal
      with different technologies is most useful.

7.  Further Work

   There may be needs for further work in this area at the IETF.  Before
   discussing the specific needs, it may be useful to classify the types
   of useful work that might come to question.  And perhaps also outline
   some types of work that is not appropriate for the IETF.

   The IETF works primarily on protocols, but in many cases also with
   data models that help manage systems, as well as operational guidance
   documents.  But the IETF does not work on software, such as
   abstractions that only need to exist inside computers or ones that do
   not have an effect on protocols either on real or simulated "wires".




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   The IETF also does not generally work on system-level design.  IETF
   is best at designing components, not putting those components
   together to achieve a particular purpose or build a specific
   application.

   As a result, IETF's work on new systems employing virtualization
   techniques (such as 5G slicing concept) is more at the component
   improvement level than at the level of the concept.  There needs to
   be a mapping between a vision of a system and how it utilizes various
   software, hardware, and protocol tools to achieve the particular
   virtualization capabilities it needs to.  Developing a new concept
   does not necessarily mean that entirely new solutions are needed
   throughtout the stack.  Indeed, systems and concepts are usually
   built on top of solid, well defined components such as the ones
   produced by the IETF.

   That mapping work is necessarily something that those who want to
   achieve some new functionality need to do; it is difficult for others
   to take a position on what the new functionality is.  But at the same
   time, IETF working groups and participants typically have a
   perspective on how their technology should develop and be extended.
   Those two viewpoints must meet.

   The kinds of potential new work in this space falls generally in the
   following classes:

   Virtualization selectors

      Sometimes protocols need mechanisms that make it possible to use
      them as multiple instances.  E.g., VLAN tags were added to
      Ethernet frames, NAIs were added to PPP and EAP, and so on.  These
      cases are rare today, because most protocols and mechanisms have
      some kind of selector that can be used to run multiple instances
      or connect to multiple different networks.

   Traffic engineering

      A big reason for building specific networks for specific purposes
      is to provide an engineered service level on delay and other
      factors to the given customer.  There are a number of different
      tools in the IETF to help manage and engineer networks, but it is
      also an area that continues to develop and will likely see new
      functionality.

   Virtual service data models






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      Data models -- such as those described by L2SM or L3SM working
      groups can represent a "service" offered by a network, a setup
      built for a specific customer or purpose.

   Some specific areas where work is likely needed include:

   o  The ability to manage heterogenous technologies, e.g., across SDN
      and traditionally built networks, or manage both general-purpose
      and very technology-specific parameters such as those associated
      with 5G radio.

   o  The ability to specify "statistical" rather than hard performance
      parameters.  In some networks -- notably with wireless technology
      -- recent advances have made very high peak rates possible, but
      with increased bursty-ness of traffic and with potential
      bottlenecks on the aggregation parts of the networks.  The ability
      to specify statistical performance in data models and in VPN
      configuration would be important, over different timescales and
      probabilities.

   o  Mapping from high abstraction level specifications to concrete
      network configurations.

      There is a lot of work on data models and templates at various
      levels and in different representations.  There are also many
      systems built to manage these models and orchestrate network
      configuration.  But the mapping of the abstract models to concrete
      network configurations remains a hard problem, and it certainly
      will need more work.

      There are even some questions about how to go about this.  Is it
      enough that we specify models, and leave the mapping to "magic" of
      the software?  Are the connections something that different
      vendors compete in producing good products in?  Or are the mapping
      algorithms something that needs to be specified together, and
      their ability to work with different types of network equipment
      verified in some manner?

   o  Cross-domain: A big problem is that we have little tools for
      cross-domain management of virtualized networks and resources.

   Finally, there is a question of where all this work should reside.
   There's an argument that IETF-based virtualization technologies
   deserve proper management tools, including data models.

   And there's another argument that with the extensive use of
   virtualization technology, solutions that can manage many different
   networks should be general, and as such, potential IETF work



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   material.  Yet, the IETF is not and should not be in the space of
   replacing various tools and open source toolkits that have been
   created for managing virtualization.  It seems though that work on
   commonly usable data models at several layers of abstraction would be
   good work at the IETF.

   Nevertheless, the IETF should understand where the broader community
   is and what tools they use for what purpose, and try to help by
   building on those components.  Virtualization and slicing are
   sometimes represented as issues needing a single solution.  In
   reality, they are an interworking of a number of different tools.

8.  Acknowledgements

   The authors would like to thank Gonzalo Camarillo, Gabriel
   Montenegro, Alex Galis, Adrian Farrell, Liang Geng, Yi Zhao, Hannu
   Flinck, Yi Zhao, Barry Leiba, Georg Mayer, Benoit Claise, Daniele
   Ceccarelli, Warren Kumari, Ted Hardie, and many others for
   interesting discussions in this problem space.

9.  Informative References

   [CC2015]   claffy, kc. and D. Clark, "Adding Enhanced Services to the
              Internet: Lessons from History", September 2015 (https://
              www.caida.org/publications/papers/2015/
              adding_enhanced_services_internet/
              adding_enhanced_services_internet.pdf).

   [I-D.bryskin-teas-sf-aware-topo-model]
              Bryskin, I. and X. Liu, "SF Aware TE Topology YANG Model",
              draft-bryskin-teas-sf-aware-topo-model-01 (work in
              progress), March 2018.

   [I-D.bryskin-teas-use-cases-sf-aware-topo-model]
              Bryskin, I., Liu, X., Guichard, J., Lee, Y., Contreras,
              L., and D. Ceccarelli, "Use Cases for SF Aware Topology
              Models", draft-bryskin-teas-use-cases-sf-aware-topo-
              model-02 (work in progress), March 2018.

   [I-D.geng-coms-problem-statement]
              67, 4., Slawomir, S., Qiang, L., Matsushima, S., Galis,
              A., and L. Contreras, "Problem Statement of Supervised
              Heterogeneous Network Slicing", draft-geng-coms-problem-
              statement-00 (work in progress), September 2017.

   [I-D.ietf-sfc-nsh]





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              Quinn, P., Elzur, U., and C. Pignataro, "Network Service
              Header (NSH)", draft-ietf-sfc-nsh-28 (work in progress),
              November 2017.

   [I-D.king-teas-applicability-actn-slicing]
              King, D. and Y. Lee, "Applicability of Abstraction and
              Control of Traffic Engineered Networks (ACTN) to Network
              Slicing", draft-king-teas-applicability-actn-slicing-01
              (work in progress), July 2017.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, DOI 10.17487/
              RFC2616, June 1999, <https://www.rfc-editor.org/info/
              rfc2616>.

   [RFC4026]  Andersson, L. and T. Madsen, "Provider Provisioned Virtual
              Private Network (VPN) Terminology", RFC 4026, DOI 10.17487
              /RFC4026, March 2005, <https://www.rfc-editor.org/info/
              rfc4026>.

   [RFC4110]  Callon, R. and M. Suzuki, "A Framework for Layer 3
              Provider-Provisioned Virtual Private Networks (PPVPNs)",
              RFC 4110, DOI 10.17487/RFC4110, July 2005, <https://www
              .rfc-editor.org/info/rfc4110>.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <https://www.rfc-editor.org/info/rfc4301>.

   [RFC4664]  Andersson, L., Ed. and E. Rosen, Ed., "Framework for Layer
              2 Virtual Private Networks (L2VPNs)", RFC 4664, DOI
              10.17487/RFC4664, September 2006, <https://www.rfc-
              editor.org/info/rfc4664>.

   [RFC4761]  Kompella, K., Ed. and Y. Rekhter, Ed., "Virtual Private
              LAN Service (VPLS) Using BGP for Auto-Discovery and
              Signaling", RFC 4761, DOI 10.17487/RFC4761, January 2007,
              <https://www.rfc-editor.org/info/rfc4761>.

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010, <https://www.rfc-
              editor.org/info/rfc6020>.







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   [RFC6624]  Kompella, K., Kothari, B., and R. Cherukuri, "Layer 2
              Virtual Private Networks Using BGP for Auto-Discovery and
              Signaling", RFC 6624, DOI 10.17487/RFC6624, May 2012,
              <https://www.rfc-editor.org/info/rfc6624>.

   [RFC7432]  Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
              Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
              Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
              2015, <https://www.rfc-editor.org/info/rfc7432>.

   [RFC8049]  Litkowski, S., Tomotaki, L., and K. Ogaki, "YANG Data
              Model for L3VPN Service Delivery", RFC 8049, DOI 10.17487/
              RFC8049, February 2017, <https://www.rfc-editor.org/info/
              rfc8049>.

   [TOSCA-1.0]
              OASIS, "Topology and Orchestration Specification for Cloud
              Applications Version 1.0", OASIS OASIS Standard, http://
              docs.oasis-open.org/tosca/TOSCA/v1.0/os/
              TOSCA-v1.0-os.html, November 2013.

   [TOSCA-Profile-1.1]
              OASIS, "TOSCA Simple Profile in YAML Version 1.1", OASIS
              OASIS Standard, http://docs.oasis-open.org/tosca/TOSCA-
              Simple-Profile-YAML/v1.1/TOSCA-Simple-Profile-
              YAML-v1.1.html, January 2018.

   [TS-3GPP.23.401]
              3GPP, "3rd Generation Partnership Project; Technical
              Specification Group Services and System Aspects; General
              Packet Radio Service (GPRS) enhancements for Evolved
              Universal Terrestrial Radio Access Network (E-UTRAN)
              access; (Release 15)", 3GPP Technical Specification
              23.401, December 2017.

   [TS-3GPP.23.501]
              3GPP, "3rd Generation Partnership Project; Technical
              Specification Group Services and System Aspects; 3G
              Security; Security architecture and procedures for 5G
              System; (Release 15)", 3GPP Technical Specification
              23.501, December 2017.

   [VirtualHosting]
              Wikipedia, "Virtual Hosting", Wikipedia article https://
              en.wikipedia.org/wiki/Virtual_hosting, August 2017.

Authors' Addresses




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   Jari Arkko
   Ericsson
   Kauniainen  02700
   Finland

   Email: jari.arkko@piuha.net


   Jeff Tantsura
   Nuagenetworks

   Email: jefftant.ietf@gmail.com


   Joel Halpern
   Ericsson

   Email: joel.halpern@ericsson.com


   Balazs Varga
   Ericsson
   Budapest  1097
   Hungary

   Email: balazs.a.varga@ericsson.com

























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