



Internet Engineering Task Force (IETF)                    A. Farrel, Ed.
Request for Comments: 9543                            Old Dog Consulting
Category: Informational                                    J. Drake, Ed.
ISSN: 2070-1721                                               Individual
                                                                R. Rokui
                                                                   Ciena
                                                                S. Homma
                                                                     NTT
                                                            K. Makhijani
                                                               Futurewei
                                                            L. Contreras
                                                              Telefonica
                                                             J. Tantsura
                                                                  Nvidia
                                                              March 2024


A Framework for Network Slices in Networks Built from IETF Technologies

Abstract

   This document describes network slicing in the context of networks
   built from IETF technologies.  It defines the term "IETF Network
   Slice" to describe this type of network slice and establishes the
   general principles of network slicing in the IETF context.

   The document discusses the general framework for requesting and
   operating IETF Network Slices, the characteristics of an IETF Network
   Slice, the necessary system components and interfaces, and the
   mapping of abstract requests to more specific technologies.  The
   document also discusses related considerations with monitoring and
   security.

   This document also provides definitions of related terms to enable
   consistent usage in other IETF documents that describe or use aspects
   of IETF Network Slices.

Status of This Memo

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

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are 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/rfc9543.

Copyright Notice

   Copyright (c) 2024 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.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Background
   3.  Terms and Abbreviations
     3.1.  Abbreviations
     3.2.  Core Terminology
   4.  IETF Network Slice
     4.1.  Definition and Scope of IETF Network Slice
     4.2.  IETF Network Slice Service
       4.2.1.  Connectivity Constructs
       4.2.2.  Mapping Traffic Flows to Network Realizations
       4.2.3.  Ancillary CEs
   5.  IETF Network Slice System Characteristics
     5.1.  Objectives for IETF Network Slices
       5.1.1.  Service Level Objectives
       5.1.2.  Service Level Expectations
     5.2.  IETF Network Slice Service Demarcation Points
     5.3.  IETF Network Slice Composition
   6.  Framework
     6.1.  IETF Network Slice Stakeholders
     6.2.  Expressing Connectivity Intents
     6.3.  IETF Network Slice Controller (NSC)
       6.3.1.  IETF Network Slice Controller Interfaces
       6.3.2.  Management Architecture
   7.  Realizing IETF Network Slices
     7.1.  An Architecture to Realize IETF Network Slices
     7.2.  Procedures to Realize IETF Network Slices
     7.3.  Applicability of ACTN to IETF Network Slices
     7.4.  Applicability of Enhanced VPNs to IETF Network Slices
     7.5.  Network Slicing and Aggregation in IP/MPLS Networks
     7.6.  Network Slicing and Service Function Chaining (SFC)
   8.  Isolation in IETF Network Slices
     8.1.  Isolation as a Service Requirement
     8.2.  Isolation in IETF Network Slice Realization
   9.  Management Considerations
   10. Security Considerations
   11. Privacy Considerations
   12. IANA Considerations
   13. Informative References
   Appendix A.  Examples
     A.1.  Multi-Point to Point Service
     A.2.  Service Function Chaining and Ancillary CEs
     A.3.  Hub and Spoke
     A.4.  Layer 3 VPN
     A.5.  Hierarchical Composition of Network Slices
     A.6.  Horizontal Composition of Network Slices
   Acknowledgments
   Contributors
   Authors' Addresses

1.  Introduction

   A number of use cases would benefit from a network service that
   supplements connectivity, such as that offered by a VPN service, with
   an assurance of meeting a set of specific network performance
   objectives.  This connectivity and resource commitment is referred to
   as a "network slice" and is expressed in terms of connectivity
   constructs (see Section 4) and service objectives (see Section 5).
   Since the term "network slice" is rather generic and has wider or
   different interpretations within other standards bodies, the
   qualifying term "IETF" is used in this document to limit the scope of
   the network slices described to network technologies defined and
   standardized by the IETF.  This document defines the concept of "IETF
   Network Slices" that provide connectivity coupled with a set of
   specific commitments of network resources between a number of
   endpoints (known as Service Demarcation Points (SDPs); see Sections
   3.2 and 5.2) over a shared underlay network that utilizes IETF
   technology.  The term "IETF Network Slice Service" is also introduced
   to describe the service requested by and provided to the service
   provider's customer.

   It is intended that the terms "IETF Network Slice" and "IETF Network
   Slice Service" be used only in this document.  Other documents that
   need to indicate the type of network slice or network slice service
   described in this document can use the terms "RFC 9543 Network Slice"
   and "RFC 9543 Network Slice Service".

   This document also provides a general framework for requesting and
   operating IETF Network Slices.  The framework is intended as a
   structure for discussing interfaces and technologies.

   Services that might benefit from IETF Network Slices include but are
   not limited to:

   *  5G services (e.g., enhanced Mobile Broadband (eMBB), Ultra-
      Reliable and Low Latency Communications (URLLC), and massive
      Machine Type Communications (mMTC) -- see [TS23.501])

   *  Network wholesale services

   *  Network infrastructure sharing among operators

   *  Network Function Virtualization (NFV) [NFVArch] connectivity and
      Data Center Interconnect

   Further analysis of the needs of IETF Network Slice Service customers
   is provided in [USE-CASES].

   IETF Network Slices are created and managed within the scope of one
   or more network technologies (e.g., IP, MPLS, and optical) that use
   an IETF-specified data plane and/or management/control plane.  They
   are intended to enable a diverse set of applications with different
   requirements to coexist over a shared underlay network.  A request
   for an IETF Network Slice Service is agnostic to the technology in
   the underlay network so as to allow customers to describe their
   network connectivity objectives in a common format, independent of
   the underlay technologies used.

   Many preexisting approaches to service delivery and traffic
   engineering already use mechanisms that can be considered as network
   slicing.  For example, Virtual Private Networks (VPNs) have served
   the industry well as a means of providing different groups of users
   with logically isolated access to a common network.  The common or
   base network that is used to support the VPNs is often referred to as
   an "underlay network", and the VPN is often called an "overlay
   network".  An overlay network may, in turn, serve as an underlay
   network to support another overlay network.

   Note that it is conceivable that extensions to IETF technologies are
   needed in order to fully support all the capabilities that can be
   implemented with network slices.  Evaluation of existing
   technologies, proposed extensions to existing protocols and
   interfaces, and creation of new protocols or interfaces are outside
   the scope of this document.

2.  Background

   The concept of network slicing has gained traction, driven largely by
   needs surfacing from 5G (see [NGMN-NS-Concept], [TS23.501], and
   [TS28.530]).  In [TS23.501], a Network Slice is defined as a "logical
   network that provides specific network capabilities and network
   characteristics", and a Network Slice Instance is defined as a "set
   of Network Function instances and the required resources (e.g.
   compute, storage and networking resources) which form a deployed
   Network Slice".  According to [TS28.530], an end-to-end (E2E) network
   slice consists of three major types of network segments: Radio Access
   Network (RAN), Transport Network (TN), and Core Network (CN).  An
   IETF Network Slice provides the required connectivity between
   different entities in RAN and CN segments of an end-to-end network
   slice, with a specific performance commitment (for example, serving
   as a TN slice).  For each end-to-end network slice, the topology and
   performance requirement on a customer's use of an IETF Network Slice
   can be very different, which requires the underlay network to have
   the capability of supporting multiple different IETF Network Slices.

   While network slices are commonly discussed in the context of 5G, it
   is important to note that IETF Network Slices are a narrower concept
   with a broader usage profile and focus primarily on particular
   network connectivity aspects.  Other systems, including 5G
   deployments, may use IETF Network Slices as a component to create
   entire systems and concatenated constructs that match their needs,
   including end-to-end connectivity.

   An IETF Network Slice could span multiple technologies and multiple
   administrative domains.  Depending on the IETF Network Slice Service
   customer's requirements, an IETF Network Slice could be isolated from
   other, often concurrent, IETF Network Slices in terms of data,
   control, and management planes.

   The customer expresses requirements for a particular IETF Network
   Slice Service by specifying what is required rather than how the
   requirement is to be fulfilled.  That is, the IETF Network Slice
   Service customer's view of an IETF Network Slice Service is an
   abstract one.

   Thus, there is a need to create logical network structures with
   required characteristics.  The customer of such a logical network can
   require a level of isolation and performance that previously might
   not have been satisfied by overlay VPNs.  Additionally, the IETF
   Network Slice Service customer might ask for some level of control
   to, e.g., customize the service paths in a network slice.

   This document specifies definitions and a framework for the provision
   of an IETF Network Slice Service.  Section 7 briefly indicates some
   candidate technologies for realizing IETF Network Slices.

3.  Terms and Abbreviations

3.1.  Abbreviations

   The following abbreviations are used in this document.

   NSC:   Network Slice Controller

   SDP:   Service Demarcation Point

   SLA:   Service Level Agreement

   SLE:   Service Level Expectation

   SLI:   Service Level Indicator

   SLO:   Service Level Objective

   The meaning of these abbreviations is defined in greater detail in
   the remainder of this document.

3.2.  Core Terminology

   The following terms are presented here to give context.  Other
   terminology is defined in the remainder of this document.

   Customer:  The requester of an IETF Network Slice Service.  Customers
      may request monitoring of SLOs.  A customer may be an entity such
      as an enterprise network or a network operator, an individual
      working at such an entity, a private individual contracting for a
      service, or an application or software component.  A customer may
      be an external party (classically, a paying customer) or a
      division of a network operator that uses the service provided by
      another division of the same operator.  Other terms that have been
      applied to the customer role are "client" and "consumer".

   Provider:  The organization that delivers an IETF Network Slice
      Service.  A provider is the network operator that controls the
      network resources used to construct the network slice (that is,
      the network that is sliced).  The provider's network may be a
      physical network or a virtual network created within the
      operator's network or supplied by another service provider.

   Customer Edge (CE):  The customer device that provides connectivity
      to a service provider.  Examples include routers, Ethernet
      switches, firewalls, 4G/5G RAN or Core nodes, application
      accelerators, server load balancers, HTTP header enrichment
      functions (such as proxy components adding the Forwarded HTTP
      Extension Header [RFC7239]), and Performance Enhancing Proxies
      (PEPs).  In some circumstances, CEs are provided to the customer
      and managed by the provider.

   Provider Edge (PE):  The device within the provider network to which
      a CE is attached.  A CE may be attached to multiple PEs, and
      multiple CEs may be attached to a given PE.

   Attachment Circuit (AC):  A channel connecting a CE and a PE over
      which packets that belong to an IETF Network Slice Service are
      exchanged.  An AC is, by definition, technology specific: that is,
      the AC defines how customer traffic is presented to the provider
      network.  The customer and provider agree (for example, through
      configuration) on which values in which combination of Layer 2
      (L2) and Layer 3 (L3) header and payload fields within a packet
      identify to which {IETF Network Slice Service, connectivity
      construct, and SLOs/SLEs} that packet is assigned.  The customer
      and provider may agree to police or shape traffic, based on the
      specific IETF Network Slice Service including connectivity
      construct and SLOs/SLEs, on the AC in both the ingress (CE to PE)
      direction and egress (PE to CE) direction.  This ensures that the
      traffic is within the capacity profile that is agreed upon in an
      IETF Network Slice Service.  Excess traffic is dropped by default,
      unless specific out-of-profile policies are agreed upon between
      the customer and the provider.  As described in Section 5.2, the
      AC may be part of the IETF Network Slice Service or may be
      external to it.  Because SLOs and SLEs characterize the
      performance of the underlay network between a sending SDP and a
      set of receiving SDPs, the traffic policers and traffic shapers
      apply to a specific connectivity construct on an AC.

   Service Demarcation Point (SDP):  The point at which an IETF Network
      Slice Service is delivered by a service provider to a customer.
      Depending on the service delivery model (see Section 5.2), this
      may be a CE or a PE and could be a device, a software component,
      or an abstract virtual function supported within the provider's
      network.  Each SDP must have a unique identifier (e.g., an IP
      address or Media Access Control (MAC) address) within a given IETF
      Network Slice Service and may use the same identifier in multiple
      IETF Network Slice Services.

      An SDP may be abstracted as a Service Attachment Point (SAP)
      [RFC9408] for the purpose of generalizing the concept across
      multiple service types and representing it in management and
      configuration systems.

   Connectivity Construct:  A set of SDPs together with a communication
      type that defines how traffic flows between the SDPs.  An IETF
      Network Slice Service is specified in terms of a set of SDPs, the
      associated connectivity constructs, and the service objectives
      that the customer wishes to see fulfilled.  Connectivity
      constructs may be grouped for administrative purposes.

4.  IETF Network Slice

   IETF Network Slices are created to meet specific requirements,
   typically expressed as bandwidth, latency, latency variation, and
   other desired or required characteristics.  Creation of an IETF
   Network Slice is initiated by a management system or other
   application used to specify network-related conditions for particular
   traffic flows in response to an actual or logical IETF Network Slice
   Service request.

   Once created, these slices can be monitored, modified, deleted, and
   otherwise managed.

   Applications and components will be able to use these IETF Network
   Slices to move packets between the specified endpoints of the service
   in accordance with specified characteristics.

   A clear distinction should be made between the "IETF Network Slice
   Service" and the IETF Network Slice:

   IETF Network Slice Service:  The function delivered to the customer
      (see Section 4.2).  It is agnostic to the technologies and
      mechanisms used by the service provider.

   IETF Network Slice:  The realization of the service in the provider's
      network achieved by partitioning network resources and by applying
      certain tools and techniques within the network (see Sections 4.1
      and 7).

4.1.  Definition and Scope of IETF Network Slice

   The term "Slice" refers to a set of characteristics and behaviors
   that differentiate one type of user traffic from another within a
   network.  An IETF Network Slice is a logical partition of a network
   that uses IETF technology.  An IETF Network Slice assumes that an
   underlay network is capable of changing the configurations of the
   network devices on demand, through in-band signaling, or via
   controllers.

   An IETF Network Slice enables connectivity between a set of SDPs with
   specific Service Level Objectives (SLOs) and Service Level
   Expectations (SLEs) (see Section 5) over a common underlay network.
   The SLOs and SLEs characterize the performance of the underlay
   network between a sending SDP and a set of receiving SDPs.  Thus, an
   IETF Network Slice delivers a service to a customer by meeting
   connectivity resource requirements and associated network
   capabilities such as bandwidth, latency, jitter, and network
   functions with other resource behaviors such as compute and storage
   availability.

   IETF Network Slices may be combined hierarchically so that a network
   slice may itself be sliced.  They may also be combined sequentially
   so that various different networks can each be sliced and the network
   slices placed into a sequence to provide an end-to-end service.  This
   form of sequential combination is utilized in some services such as
   in 3GPP's 5G network [TS23.501].

   It is intended that the term "IETF Network Slice" be used only in
   this document.  Other documents that need to indicate the type of
   network slice described in this document can use the term "RFC 9543
   Network Slice".

4.2.  IETF Network Slice Service

   A service provider delivers an IETF Network Slice Service for a
   customer by realizing an IETF Network Slice in the underlay network.
   The IETF Network Slice Service is agnostic to the technology of the
   underlay network, and its realization may be selected based upon
   multiple considerations, including its service requirements and the
   capabilities of the underlay network.  This allows an IETF Network
   Slice Service customer to describe their network connectivity and
   relevant objectives in a common format, independent of the underlay
   technologies used.

   The IETF Network Slice Service is specified in terms of a set of
   SDPs, a set of one or more connectivity constructs between subsets of
   these SDPs, and a set of SLOs and SLEs (see Section 5) for each SDP
   sending to each connectivity construct.  A communication type (Point-
   to-Point (P2P), Point-to-Multipoint (P2MP), or Any-to-Any (A2A)) is
   specified for each connectivity construct.  That is, in a given IETF
   Network Slice Service:

   *  There may be one or more connectivity constructs of the same or
      different type.

   *  Each connectivity construct may be between a different subset of
      SDPs.

   *  Each sending SDP has its own set of SLOs and SLEs for a given
      connectivity construct, and the SLOs and SLEs in each set may be
      different.

   Note that different connectivity constructs can be specified in the
   service request, but the service provider may decide how many
   connectivity constructs per IETF Network Slice Service it wishes to
   support such that an IETF Network Slice Service may be limited to one
   connectivity construct or may support many.

   An IETF Network Slice Service customer may provide IETF Network Slice
   Services to other customers in a mode sometimes referred to as
   "carrier's carrier" (see Section 9 of [RFC4364]).  In this case, the
   relationship between IETF Network Slice Service providers may be
   internal to a commercial organization or may be external through
   service provision contracts.  As noted in Section 5.3, network slices
   may be composed hierarchically or serially.

   Section 5.2 provides a description of SDPs as endpoints in the
   context of IETF network slicing.  For a given IETF Network Slice
   Service, the customer and provider agree, on a per-SDP basis, which
   end of the attachment circuit provides the SDP (i.e., whether the
   attachment circuit is inside or outside the IETF Network Slice
   Service).  This determines whether the attachment circuit is subject
   to the set of SLOs and SLEs at the specific SDP.

   It is intended that the term "IETF Network Slice Service" be used
   only in this document.  Other documents that need to indicate the
   type of network slice service described in this document can use the
   term "RFC 9543 Network Slice Service".

4.2.1.  Connectivity Constructs

   The approach of specifying a Network Slice Service as a set of SDPs
   with connectivity constructs results in the following possible
   connectivity constructs:

   *  For a P2P connectivity construct, there is one sending SDP and one
      receiving SDP.  This construct is like a private wire or a tunnel.
      All traffic injected at the sending SDP is intended to be received
      by the receiving SDP.  The SLOs and SLEs apply at the sender (and
      implicitly, at the receiver).

   *  For a P2MP connectivity construct, there is only one sending SDP
      and more than one receiving SDP.  This is like a P2MP tunnel or
      multi-access VLAN segment.  All traffic from the sending SDP is
      intended to be received by all the receiving SDPs.  There is one
      set of SLOs and SLEs that applies at the sending SDP (and
      implicitly, at all receiving SDPs).

   *  With an A2A connectivity construct, any sending SDP may send to
      any one receiving SDP or any set of receiving SDPs in the
      construct.  There is an implicit level of routing in this
      connectivity construct that is not present in the other
      connectivity constructs because the provider's network must
      determine to which receiving SDPs to deliver each packet.  This
      construct may be used to support P2P traffic between any pair of
      SDPs or to support multicast or broadcast traffic from one SDP to
      a set of other SDPs.  In the latter case, whether the service is
      delivered using multicast within the provider's network or using
      "ingress replication" or some other means is out of scope of the
      specification of the service.  A service provider may choose to
      support A2A constructs but to limit the traffic to unicast.

      The SLOs/SLEs in an A2A connectivity construct apply to individual
      sending SDPs regardless of the receiving SDPs, and there is no
      linkage between sender and receiver in the specification of the
      connectivity construct.  A sending SDP may be "disappointed" if
      the receiver is over-subscribed.  If a customer wants to be more
      specific about different behaviors from one SDP to another SDP,
      they should use P2P connectivity constructs.

   A given sending SDP may be part of multiple connectivity constructs
   within a single IETF Network Slice Service, and the SDP may have
   different SLOs and SLEs for each connectivity construct to which it
   is sending.  Note that a given sending SDP's SLOs and SLEs for a
   given connectivity construct apply between it and each of the
   receiving SDPs for that connectivity construct.

   An IETF Network Slice Service provider may freely make a deployment
   choice as to whether to offer a 1:1 relationship between an IETF
   Network Slice Service and connectivity construct or to support
   multiple connectivity constructs in a single IETF Network Slice
   Service.  In the former case, the provider might need to deliver
   multiple IETF Network Slice Services to achieve the function of the
   second case.

4.2.2.  Mapping Traffic Flows to Network Realizations

   A customer traffic flow may be unicast or multicast, and various
   network realizations are possible:

   *  Unicast traffic may be mapped to a P2P connectivity construct for
      direct delivery or to an A2A connectivity construct for the
      service provider to perform routing to the destination SDP.  It
      would be unusual to use a P2MP connectivity construct to deliver
      unicast traffic because all receiving SDPs would get a copy, but
      this can still be done if the receivers are capable of dropping
      the unwanted traffic.

   *  A bidirectional unicast service can be constructed by specifying
      two P2P connectivity constructs.  An additional SLE may specify
      fate-sharing in this case.

   *  Multicast traffic may be mapped to a set of P2P connectivity
      constructs, a single P2MP connectivity construct, or a mixture of
      P2P and P2MP connectivity constructs.  Multicast may also be
      supported by an A2A connectivity construct.  The choice clearly
      influences how and where traffic is replicated in the network.
      With a P2MP or A2A connectivity construct, it is the operator's
      choice whether to realize the construct with ingress replication,
      multicast in the core, P2MP tunnels, or hub-and-spoke.  This
      choice should not change how the customer perceives the service.

   *  The concept of a Multipoint-to-Point (MP2P) service can be
      realized with multiple P2P connectivity constructs.  Note that, in
      this case, the egress may simultaneously receive traffic from all
      ingresses.  The SLOs at the sending SDPs must be set with this in
      mind because the provider's network is not capable of coordinating
      the policing of traffic across multiple distinct source SDPs.  It
      is assumed that the customer, requesting SLOs for the various P2P
      connectivity constructs, is aware of the capabilities of the
      receiving SDP.  If the receiver receives more traffic than it can
      handle, it may drop some and introduce queuing delays.

   *  The concept of a Multipoint-to-Multipoint (MP2MP) service can best
      be realized using a set of P2MP connectivity constructs but could
      be delivered over an A2A connectivity construct if each sender is
      using multicast.  As with MP2P, the customer is assumed to be
      familiar with the capabilities of all receivers.  A customer may
      wish to achieve an MP2MP service using a hub-and-spoke
      architecture where they control the hub; that is, the hub may be
      an SDP or an ancillary CE (see Section 4.2.3), and the service may
      be achieved by using a set of P2P connectivity constructs to the
      hub and a single P2MP connectivity construct from the hub.

   From the above, it can be seen that the SLOs of the senders define
   the SLOs for the receivers on any connectivity construct.  In
   particular, the network may be expected to handle the traffic volume
   from a sender to all destinations.  This extends to all connectivity
   constructs in an IETF Network Slice Service.

   Note that the realization of an IETF Network Slice Service does not
   need to map the connectivity constructs one-to-one onto underlying
   network constructs (such as tunnels).  The service provided to the
   customer is distinct from how the provider decides to deliver that
   service.

   If a CE has multiple attachment circuits to PEs within a given IETF
   Network Slice Service and they are operating in single-active mode,
   then all traffic between the CE and its attached PEs transits a
   single attachment circuit; if they are operating in all-active mode,
   then traffic between the CE and its attached PEs is distributed
   across all of the active attachment circuits.

4.2.3.  Ancillary CEs

   It may be the case that the set of SDPs that delimits an IETF Network
   Slice Service needs to be supplemented with additional senders or
   receivers within the network that are not customer sites.  An
   additional sender could be, for example, an IPTV or DNS server either
   within the provider's network or attached to it, while an extra
   receiver could be, for example, a node reachable via the Internet.
   This is modeled in the Network Slicing architecture as a set of
   ancillary CEs that supplement the other SDPs in one or more
   connectivity constructs or that are linked by their own connectivity
   constructs.  Note that an ancillary CE can either have a resolvable
   address (e.g., an IP address or MAC address), or it may be a
   placeholder (e.g., a named IPTV or DNS service or server) that is
   resolved within the provider's network when the IETF Network Slice
   Service is instantiated.

   Thus, an ancillary CE may be a node within the provider network
   (i.e., not a node at the edge of the customer's network).  An example
   is a node that provides a service function.  Another example is a
   node that acts as a hub.  There will be times when the customer
   wishes to explicitly select one of these.  Alternatively, an
   ancillary CE may be a service function at an unknown point in the
   provider's network.  In this case, the function may be a placeholder
   that has its addresses resolved as part of the realization of the
   slice service.

   Appendices A.2 and A.3 give simple worked examples of the use of
   ancillary CEs that may aid understanding the concept.

5.  IETF Network Slice System Characteristics

   The following subsections describe the characteristics of IETF
   Network Slices in addition to the list of SDPs, the connectivity
   constructs, and the technology of the ACs.

5.1.  Objectives for IETF Network Slices

   An IETF Network Slice Service is defined in terms of quantifiable
   characteristics known as Service Level Objectives (SLOs) and
   unquantifiable characteristics known as Service Level Expectations
   (SLEs).  SLOs are expressed in terms Service Level Indicators (SLIs)
   and together with the SLEs form the contractual agreement between
   service customer and service provider known as a Service Level
   Agreement (SLA).

   The terms are defined as follows:

   Service Level Indicator (SLI):  A quantifiable measure of an aspect
      of the performance of a network.  For example, it may be a measure
      of throughput in bits per second, or it may be a measure of
      latency in milliseconds.

   Service Level Objective (SLO):  A target value or range for the
      measurements returned by observation of an SLI.  For example, an
      SLO may be expressed as "SLI <= target" or "lower bound <= SLI <=
      upper bound".  A customer can determine whether the provider is
      meeting the SLOs by performing measurements on the traffic.

   Service Level Expectation (SLE):  An expression of an unmeasurable
      service-related request that a customer of an IETF Network Slice
      Service makes of the provider.  An SLE is distinct from an SLO
      because the customer may have little or no way of determining
      whether the SLE is being met, but they still contract with the
      provider for a service that meets the expectation.

   Service Level Agreement (SLA):  An explicit or implicit contract
      between the customer of an IETF Network Slice Service and the
      provider of the slice.  The SLA is expressed in terms of a set of
      SLOs and SLEs that are to be applied for a given connectivity
      construct between a sending SDP and the set of receiving SDPs.
      The SLA may describe the extent to which divergence from
      individual SLOs and SLEs can be tolerated, and commercial terms as
      well as any consequences for violating these SLOs and SLEs.

5.1.1.  Service Level Objectives

   SLOs define a set of measurable network attributes and
   characteristics that describe an IETF Network Slice Service.  SLOs do
   not describe how an IETF Network Slice Service is implemented or
   realized in the underlying network layers.  Instead, they are defined
   in terms of dimensions of operation (time, capacity, etc.),
   availability, and other attributes.

   An IETF Network Slice Service may include multiple connectivity
   constructs that associate sets of endpoints (SDPs).  SLOs apply to a
   given connectivity construct and apply to a specific direction of
   traffic flow.  That is, they apply to a specific sending SDP and the
   set of receiving SDPs.

5.1.1.1.  Some Common SLOs

   SLOs can be described as "Directly Measurable Objectives"; they are
   always measurable.  See Section 5.1.2 for the description of Service
   Level Expectations, which are unmeasurable service-related requests
   sometimes known as "Indirectly Measurable Objectives".

   Objectives such as guaranteed minimum bandwidth, guaranteed maximum
   latency, maximum permissible delay variation, maximum permissible
   packet loss ratio, and availability are "Directly Measurable
   Objectives".  Future specifications (such as IETF Network Slice
   Service YANG models) may precisely define these SLOs, and other SLOs
   may be introduced as described in Section 5.1.1.2.

   The definition of these objectives are as follows:

   Guaranteed Minimum Bandwidth:  Minimum guaranteed bandwidth between
      two endpoints at any time.  The bandwidth is measured in data rate
      units of bits per second and is measured unidirectionally.

   Guaranteed Maximum Latency:  Upper bound of network latency when
      transmitting between two endpoints.  The latency is measured in
      terms of network characteristics (excluding application-level
      latency).  [RFC7679] discusses one-way metrics.

   Maximum Permissible Delay Variation:  Packet Delay Variation (PDV) as
      defined by [RFC3393] is the difference in the one-way delay
      between sequential packets in a flow.  This SLO sets a maximum
      value PDV for packets between two endpoints.

   Maximum Permissible Packet Loss Ratio:  The ratio of packets dropped
      to packets transmitted between two endpoints over a period of
      time.  See [RFC7680].

   Availability:  The ratio of uptime to the sum of uptime and downtime,
      where uptime is the time the connectivity construct is available
      in accordance with all of the SLOs associated with it.
      Availability will often be expressed along with the time period
      over which the availability is measured and the maximum allowed
      single period of downtime.

5.1.1.2.  Other Service Level Objectives

   Additional SLOs may be defined to provide additional description of
   the IETF Network Slice Service that a customer requests.  These would
   be specified in further documents.

   If the IETF Network Slice Service is traffic-aware, other traffic-
   specific characteristics may be valuable including MTU, traffic type
   (e.g., IPv4, IPv6, Ethernet, or unstructured), or a higher-level
   behavior to process traffic according to user application (which may
   be realized using network functions).

5.1.2.  Service Level Expectations

   SLEs define a set of network attributes and characteristics that
   describe an IETF Network Slice Service but are not directly
   measurable by the customer (e.g., diversity, isolation, and
   geographical restrictions).  Even though the delivery of an SLE
   cannot usually be determined by the customer, the SLEs form an
   important part of the contract between customer and provider.

   Quite often, an SLE will imply some details of how an IETF Network
   Slice Service is realized by the provider, although most aspects of
   the implementation in the underlying network layers remain a free
   choice for the provider.  For example, activating unicast or
   multicast capabilities to deliver an IETF Network Slice Service could
   be explicitly requested by a customer or could be left as an
   engineering decision for the service provider based on capabilities
   of the network and operational choices.

   SLEs may be seen as aspirational on the part of the customer, and
   they are expressed as behaviors that the provider is expected to
   apply to the network resources used to deliver the IETF Network Slice
   Service.  Of course, over time, it is possible that mechanisms will
   be developed that enable a customer to verify the provision of an
   SLE, at which point it effectively becomes an SLO.

   An IETF Network Slice Service may include multiple connectivity
   constructs that associate sets of endpoints (SDPs).  SLEs apply to a
   given connectivity construct and apply to specific directions of
   traffic flow.  That is, they apply to a specific sending SDP and the
   set of receiving SDPs.  However, being more general in nature than
   SLOs, SLEs may commonly be applied to all connectivity constructs in
   an IETF Network Slice Service.

5.1.2.1.  Some Common SLEs

   SLEs can be described as "Indirectly Measurable Objectives"; they are
   not generally directly measurable by the customer.

   Security, geographic restrictions, maximum occupancy level, and
   isolation are example SLEs as follows.

   Security:  A customer may request that the provider applies
      encryption or other security techniques to traffic flowing between
      SDPs of a connectivity construct within an IETF Network Slice
      Service.  For example, the customer could request that only
      network links that have Media Access Control Security (MACsec)
      [MACsec] enabled are used to realize the connectivity construct.

      This SLE may include a request for encryption (e.g., [RFC4303])
      between the two SDPs explicitly to meet the architectural
      recommendations in [TS33.210] or for compliance with the HIPAA
      Security Rule [HIPAA] or the PCI Data Security Standard [PCI].

      Whether or not the provider has met this SLE is generally not
      directly observable by the customer and cannot be measured as a
      quantifiable metric.

      Please see further discussion on security in Section 10.

   Geographic Restrictions:  A customer may request that certain
      geographic limits are applied to how the provider routes traffic
      for the IETF Network Slice Service.  For example, the customer may
      have a preference that its traffic does not pass through a
      particular country for political or security reasons.

      Whether or not the provider has met this SLE is generally not
      directly observable by the customer and cannot be measured as a
      quantifiable metric.

   Maximal Occupancy Level:  The maximal occupancy level specifies the
      number of flows to be admitted and optionally a maximum number of
      countable resource units (e.g., IP or MAC addresses) an IETF
      Network Slice Service can consume.  Because an IETF Network Slice
      Service may include multiple connectivity constructs, this SLE
      should state whether it applies to all connectivity constructs, a
      specified subset of them, or an individual connectivity construct.

      Again, a customer may not be able to fully determine whether this
      SLE is being met by the provider.

   Isolation:  As described in Section 8, a customer may request that
      its traffic within its IETF Network Slice Service is isolated from
      the effects of other network services supported by the same
      provider.  That is, if another service exceeds capacity or has a
      burst of traffic, the customer's IETF Network Slice Service should
      remain unaffected, and there should be no noticeable change to the
      quality of traffic delivered.

      In general, a customer cannot tell whether a service provider is
      meeting this SLE.  They cannot tell whether the variation of an
      SLI is because of changes in the underlay network or because of
      interference from other services carried by the network.  If the
      service varies within the allowed bounds of the SLOs, there may be
      no noticeable indication that this SLE has been violated.

   Diversity:  A customer may request that different connectivity
      constructs use different underlay network resources.  This might
      be done to enhance the availability of the connectivity constructs
      within an IETF Network Slice Service.

      While availability is a measurable objective (see
      Section 5.1.1.1), this SLE requests a finer grade of control and
      is not directly measurable (although the customer might become
      suspicious if two connectivity constructs fail at the same time).

5.2.  IETF Network Slice Service Demarcation Points

   As noted in Section 4.1, an IETF Network Slice provides connectivity
   between sets of SDPs with specific SLOs and SLEs.  Section 4.2 goes
   on to describe how the IETF Network Slice Service is composed of a
   set of one or more connectivity constructs that describe connectivity
   between the Service Demarcation Points (SDPs) across the underlay
   network.

   The characteristics of IETF Network Slice SDPs are as follows.

   *  An SDP is the point of attachment to an IETF Network Slice
      Service.  As such, SDPs serve as the IETF Network Slice ingress/
      egress points.

   *  An SDP is identified by a unique identifier in the context of an
      IETF Network Slice Service customer.

   *  The provider associates each SDP with a set of provider-scope
      identifiers such as IP addresses, encapsulation-specific
      identifiers (e.g., VLAN tag and MPLS Label), interface/port
      numbers, node ID, etc.

   *  SDPs are mapped to endpoints of services/tunnels/paths within the
      IETF Network Slice during its initialization and realization.

      -  A combination of the SDP identifier and SDP provider-network-
         scope identifiers define an SDP in the context of the Network
         Slice Controller (NSC) (see Section 6.3).

      -  The NSC will use the SDP provider-network-scope identifiers as
         part of the process of realizing the IETF Network Slice.

   Note that an ancillary CE (see Section 4.2.3) is the endpoint of a
   connectivity construct and so is an SDP in this discussion.

   For a given IETF Network Slice Service, the customer and provider
   agree where the SDP is located.  This determines what resources at
   the edge of the network form part of the IETF Network Slice and are
   subject to the set of SLOs and SLEs for a specific SDP.

   Figure 1 shows different potential scopes of an IETF Network Slice
   that are consistent with the different SDP locations.  For the
   purpose of this discussion and without loss of generality, the figure
   shows Customer Edge (CE) and Provider Edge (PE) nodes connected by
   Attachment Circuits (ACs).  Notes after the figure give some
   explanations.

          |<---------------------- (1) ---------------------->|
          |                                                   |
          | |<-------------------- (2) -------------------->| |
          | |                                               | |
          | |        |<----------- (3) ----------->|        | |
          | |        |                             |        | |
          | |        |  |<-------- (4) -------->|  |        | |
          | |        |  |                       |  |        | |
          V V   AC   V  V                       V  V   AC   V V
      +-----+   |    +-----+                 +-----+    |   +-----+
      |     |--------|     |                 |     |--------|     |
      | CE1 |   |    | PE1 |. . . . . . . . .| PE2 |    |   | CE2 |
      |     |--------|     |                 |     |--------|     |
      +-----+   |    +-----+                 +-----+    |   +-----+
         ^              ^                       ^              ^
         |              |                       |              |
         |              |                       |              |
      Customer       Provider                Provider       Customer
      Edge 1         Edge 1                  Edge 2         Edge 2

           Figure 1: Positioning IETF Service Demarcation Points

   Explanatory notes for Figure 1 are as follows:

   1.  If the CE is operated by the IETF Network Slice Service provider,
       then the edge of the IETF Network Slice may be within the CE.  In
       this case, the IETF Network Slicing process may utilize resources
       from within the CE such as buffers and queues on the outgoing
       interfaces.

   2.  The IETF Network Slice may be extended as far as the CE to
       include the AC but not to include any part of the CE.  In this
       case, the CE may be operated by the customer or the provider.
       Slicing the resources on the AC may require the use of traffic
       tagging (such as through Ethernet VLAN tags) or may require
       traffic policing at the AC link ends.

   3.  The SDPs of the IETF Network Slice are the customer-facing ports
       on the PEs.  This case can be managed in a way that is similar to
       a port-based VPN: each port (AC) or virtual port (e.g., VLAN tag)
       identifies the IETF Network Slice and maps to an IETF Network
       Slice SDP.

   4.  Finally, the SDP may be within the PE.  In this mode, the PE
       classifies the traffic coming from the AC according to
       information (such as the source and destination IP addresses,
       payload protocol and port numbers, etc.) in order to place it
       onto an IETF Network Slice.

   The choice of which of these options to apply is entirely up to the
   network operator.  It may limit or enable the provisioning of
   particular managed services, and the operator will want to consider
   how they want to manage CEs and what control they wish to offer the
   customer over AC resources.

   Note that Figure 1 shows a symmetrical positioning of SDPs, but this
   decision can be taken on a per-SDP basis through agreement between
   the customer and provider.

   In practice, it may be necessary to map traffic not only onto an IETF
   Network Slice but also onto a specific connectivity construct if the
   IETF Network Slice supports more than one with a source at the
   specific SDP.  The mechanism used will be one of the mechanisms
   described above, dependent on how the SDP is realized.

   Finally, note (as described in Section 3.2) that an SDP is an
   abstract endpoint of an IETF Network Slice Service and as such may be
   a device, interface, or software component.  An ancillary CE
   (Section 4.2.3) should also be thought of as an SDP.

5.3.  IETF Network Slice Composition

   Operationally, an IETF Network Slice may be composed of two or more
   IETF Network Slices as specified below.  Decomposed network slices
   are independently realized and managed.

   Hierarchical (i.e., recursive) composition:  An IETF Network Slice
      can be further sliced into other network slices.  Recursive
      composition allows an IETF Network Slice at one layer to be used
      by the other layers.  This type of multi-layer vertical IETF
      Network Slice associates resources at different layers.

   Sequential composition:  Different IETF Network Slices can be placed
      into a sequence to provide an end-to-end service.  In sequential
      composition, each IETF Network Slice would potentially support
      different data planes that need to be stitched together.

6.  Framework

   A number of IETF Network Slice Services will typically be provided
   over a shared underlay network infrastructure.  Each IETF Network
   Slice consists of both the overlay connectivity and a specific set of
   dedicated network resources and/or functions allocated in a shared
   underlay network to satisfy the needs of the IETF Network Slice
   Service customer.  In at least some examples of underlay network
   technologies, integration between the overlay and various underlay
   resources is needed to ensure the guaranteed performance requested
   for different IETF Network Slices.

   This section sets out the principal stakeholders in an IETF Network
   Slice and describes how the IETF Network Slice Service customer
   requests connectivity.  It then introduces the IETF Network Slice
   Controller (the functional component responsible for receiving
   requests from customers and converting them into network
   configuration commands) and describes its interfaces.

6.1.  IETF Network Slice Stakeholders

   An IETF Network Slice and its realization involve the following
   stakeholders.

   Orchestrator:  An orchestrator is an entity that composes different
      services, resource, and network requirements.  It interfaces with
      the IETF NSC when composing a complex service such as an end-to-
      end network slice.

   IETF Network Slice Controller (NSC):  The NSC realizes an IETF
      Network Slice in the underlay network and maintains and monitors
      the run-time state of resources and topologies associated with it.
      A well-defined interface is needed to support interworking between
      different NSC implementations and different orchestrator
      implementations.

   Network Controller:  The Network Controller is a form of network
      infrastructure controller that offers network resources to the NSC
      to realize a particular network slice.  This may be an existing
      network controller associated with one or more specific
      technologies that may be adapted to the function of realizing IETF
      Network Slices in a network.

   The IETF Network Slice Service customer and IETF Network Slice
   Service provider (see Section 3.2) are also stakeholders.  Clearly,
   the service provider operates the network that is sliced to provide
   the IETF Network Slice Service to the customer.  The Network
   Controller and NSC are management components used by the service
   provider to operate their networks and deliver IETF Network Slice
   Services.  As indicated in Figures 2 and 3, the Orchestrator may be a
   component in the customer environment that requests and coordinates
   IETF Network Slice Services from one or more service providers.  In
   other circumstances, however, the Orchestrator may be a component
   used by the service provider to request and administer IETF Network
   Slices to deliver them to customers or to construct an infrastructure
   to deliver other services to the customer.

6.2.  Expressing Connectivity Intents

   An IETF Network Slice Service customer communicates with the NSC
   using the IETF Network Slice Service Interface.

   An IETF Network Slice Service customer may be a network operator who,
   in turn, uses the IETF Network Slice to provide a service for another
   IETF Network Slice Service customer.

   Using the IETF Network Slice Service Interface, a customer expresses
   requirements for a particular slice by specifying what is required
   rather than how that is to be achieved.  That is, the customer's view
   of a slice is an abstract one.  Customers normally have limited (or
   no) visibility into the provider network's actual topology and
   resource availability information.

   This should be true even if both the customer and provider are
   associated with a single administrative domain, in order to reduce
   the potential for adverse interactions between IETF Network Slice
   Service customers and other users of the underlay network
   infrastructure.

   The benefits of this model can include the following.

   Security:  The underlay network components are less exposed to attack
      because the underlay network (or network operator) does not need
      to expose network details (topology, capacity, etc.) to the IETF
      Network Slice Service customers.

   Layered Implementation:  The underlay network comprises network
      elements that belong to a different layer network than customer
      applications.  Network information (advertisements, protocols,
      etc.) that a customer cannot interpret or respond to is not
      exposed to the customer.  (Note that a customer should not rely on
      network information not exposed directly to the customer by the
      network operator, such as via the IETF Network Slice Service
      Interface.)

   Scalability:  Customers do not need to know any information
      concerning network topology, capabilities, or state beyond that
      which is exposed via the IETF Network Slice Service Interface.
      This protects the customer site from having to hold and process
      extra information and from receiving frequent updates about the
      status of the network.

   The general issues of abstraction in a Traffic Engineered (TE)
   network are described more fully in [RFC7926].

   This framework document does not assume any particular technology
   layer at which IETF Network Slices operate.  A number of layers
   (including virtual L2, Ethernet, or IP connectivity) could be
   employed.

   Data models and interfaces are needed to set up IETF Network Slices,
   and specific interfaces may have capabilities that allow creation of
   slices within specific technology layers.

   Layered virtual connections are comprehensively discussed in other
   IETF documents.  For instance, GMPLS-based networks are discussed in
   [RFC5212] and [RFC4397], and Abstraction and Control of TE Networks
   (ACTN) is discussed in [RFC8453] and [RFC8454].  The principles and
   mechanisms associated with layered networking are applicable to IETF
   Network Slices.

   There are several IETF-defined mechanisms for expressing the need for
   a desired logical network.  The IETF Network Slice Service Interface
   carries data either in a protocol-defined format or in a formalism
   associated with a modeling language.

   For instance:

   *  The Path Computation Element (PCE) Communication Protocol (PCEP)
      [RFC5440] and GMPLS User-Network Interface (UNI) using RSVP-TE
      [RFC4208] use a TLV-based binary encoding to transmit data.

   *  The Network Configuration Protocol (NETCONF) [RFC6241] and
      RESTCONF Protocol [RFC8040] use XML and JSON encoding.

   *  gRPC and gRPC Network Management Interface (gNMI) [GNMI] use a
      binary encoded programmable interface.  ProtoBufs can be used to
      model gRPC and gNMI data.

   *  For data modeling, YANG [RFC6020] [RFC7950] may be used to model
      configuration and other data for NETCONF, RESTCONF, and gNMI,
      among others.

   While several generic formats and data models for specific purposes
   exist, it is expected that IETF Network Slice management may require
   enhancement or augmentation of existing data models.  Further, it is
   possible that mechanisms will be needed to determine the feasibility
   of service requests before they are actually made.

6.3.  IETF Network Slice Controller (NSC)

   An IETF NSC takes requests for IETF Network Slice Services and
   implements them using a suitable underlay technology.  An IETF NSC is
   the key component for control and management of the IETF Network
   Slice.  It provides the creation/modification/deletion, monitoring,
   and optimization of IETF Network Slices in a multi-domain, multi-
   technology, and multi-vendor environment.

   The main task of an IETF NSC is to map abstract IETF Network Slice
   Service requirements to concrete technologies and establish required
   connectivity, ensuring that resources are allocated to the IETF
   Network Slice as necessary.

   The IETF Network Slice Service Interface is used for communicating
   details of an IETF Network Slice Service (configuration, selected
   policies, operational state, etc.) as well as information about
   status and performance of the IETF Network Slice.  The details for
   this IETF Network Slice Service Interface are not in scope for this
   document, but further considerations of the requirements are
   discussed in [USE-CASES].

   The controller provides the following functions.

   *  Exposes an IETF Network Slice Service Interface for
      creation/modification/deletion of the IETF Network Slices that are
      agnostic to the technology of the underlay network.  This API
      communicates the Service Demarcation Points of the IETF Network
      Slice, SLO parameters (and possibly monitoring thresholds),
      applicable input selection (filtering), and various policies.  If
      SLEs have been agreed between the customer and the network
      operator, and if they are supported for the IETF Network Slice
      Service, the API will also allow SLEs to be selected for the IETF
      Network Slice and will allow any associated parameters to be set.
      The API also provides a way to monitor the slice.

   *  Determines an abstract topology connecting the SDPs of the IETF
      Network Slice that meets criteria specified via the IETF Network
      Slice Service Interface.  The NSC also retains information about
      the mapping of this abstract topology to underlay components of
      the IETF Network Slice as necessary to monitor IETF Network Slice
      status and performance.

   *  Supports "Mapping Functions" for the realization of IETF Network
      Slices.  In other words, it will use the mapping functions that:

      -  Map IETF Network Slice Service Interface requests that are
         agnostic to the technology of the underlay network to
         technology-specific network configuration interfaces.

      -  Map filtering/selection information to entities in the underlay
         network so that those entities are able to identify which
         traffic is associated with which connectivity construct and
         IETF Network Slice.

      -  Depending on the realization solution, map to entities in the
         underlay network according to how traffic should be treated to
         meet the SLOs and SLEs of the connectivity construct.

   *  Collects telemetry data (e.g., Operations, Administration, and
      Maintenance (OAM) results, statistics, states, etc.) via a network
      configuration interface for all elements in the abstract topology
      used to realize the IETF Network Slice.

   *  Evaluates the current performance against IETF Network Slice SLO
      parameters using telemetry data from the underlying realization of
      an IETF Network Slice (e.g., services, paths, and tunnels).
      Exposes this performance to the IETF Network Slice Service
      customer via the IETF Network Slice Service Interface.  The IETF
      Network Slice Service Interface may also include the capability to
      provide notifications if the IETF Network Slice performance
      reaches threshold values defined by the IETF Network Slice Service
      customer.

6.3.1.  IETF Network Slice Controller Interfaces

   The interworking and interoperability among the different
   stakeholders to provide common means of provisioning, operating, and
   monitoring the IETF Network Slices is enabled by the following
   communication interfaces (see Figure 2).

   IETF Network Slice Service Interface:  An interface between a
      customer's higher-level operation system (e.g., a network slice
      orchestrator or a customer network management system) and an NSC.
      It is agnostic to the technology of the underlay network.  The
      customer can use this interface to communicate the requested
      characteristics and other requirements for the IETF Network Slice
      Service, and an NSC can use the interface to report the
      operational state of an IETF Network Slice Service to the
      customer.  More discussion of the functionalities for the IETF
      Network Slice Service Interface can be found in [USE-CASES].

   Network Configuration Interface:  An interface between an NSC and
      network controllers.  It is technology specific and may be built
      around the many network models already defined within the IETF.

   These interfaces can be considered in the context of the Service
   Model and Network Service Model described in [RFC8309] and, together
   with the Device Configuration Interface used by the Network
   Controllers, provides a consistent view of service delivery and
   realization.

        +------------------------------------------+
        | Customer higher-level operation system   |
        |  (e.g., E2E network slice orchestrator,  |
        |     customer network management system)  |
        +------------------------------------------+
                             A
                             | IETF Network Slice Service Interface
                             V
        +------------------------------------------+
        |    IETF Network Slice Controller (NSC)   |
        +------------------------------------------+
                             A
                             | Network Configuration Interface
                             V
        +------------------------------------------+
        |           Network Controllers            |
        +------------------------------------------+

         Figure 2: Interfaces of the IETF Network Slice Controller

6.3.1.1.  IETF Network Slice Service Interface

   The IETF Network Slice Controller provides an IETF Network Slice
   Service Interface that allows customers to manage IETF Network Slice
   Services.  Customers operate on abstract IETF Network Slice Services,
   with details related to their realization hidden.

   The IETF Network Slice Service Interface is also independent of the
   type of network functions or services that need to be connected,
   i.e., it is independent of any specific storage, software, protocol,
   or platform used to realize physical or virtual network connectivity
   or functions in support of IETF Network Slices.

   The IETF Network Slice Service Interface uses protocol mechanisms and
   information passed over those mechanisms to convey desired attributes
   for IETF Network Slices and their status.  The information is
   expected to be represented as a well-defined data model and should
   include at least SDP and connectivity information, SLO/SLE
   specification, and status information.

6.3.2.  Management Architecture

   The management architecture described in Figure 2 may be further
   decomposed as shown in Figure 3.  This should also be seen in the
   context of the component architecture shown in Figure 4 and
   corresponds to the architecture in [RFC8309].

   Note that the customer higher-level operation system of Figure 2 and
   the Network Slice Orchestrator of Figure 3 may be considered
   equivalent to the Service Management & Orchestration (SMO) of [ORAN].

                  --------------
                 | Network      |
                 | Slice        |
                 | Orchestrator |
                  --------------
                   | IETF Network Slice
                   | Service Request
                   |                       Customer view
               ....|................................
                  -v-------------------    Operator view
                 |Controller           |
                 |  ------------       |
                 | | IETF       |      |
                 | | Network    |      |--> Virtual Network
                 | | Slice      |      |
                 | | Controller |      |
                 | | (NSC)      |      |
                 |  ------------       |
               ..|     | Network       |............
                 |     | Configuration |   Underlay Network
                 |     v               |
                 |  ------------       |
                 | | Network    |      |
                 | | Controller |      |
                 | | (NC)       |      |
                 |  ------------       |
                  ---------------------
                   | Device Configuration
                   v

     Figure 3: Interface of IETF Network Slice Management Architecture

7.  Realizing IETF Network Slices

   Realization of IETF Network Slices is a mapping of the definition of
   the IETF Network Slice to the underlying infrastructure and is
   necessarily technology specific and achieved by an NSC over the
   Network Configuration Interface.  Details of how realizations may be
   achieved is out of scope of this document; however, this section
   provides an overview of the components and processes involved in
   realizing an IETF Network Slice.

7.1.  An Architecture to Realize IETF Network Slices

   The architecture described in this section is deliberately at a high
   level.  It is not intended to be prescriptive: implementations and
   technical solutions may vary freely.  However, this approach provides
   a common framework that other documents may reference in order to
   facilitate a shared understanding of the work.

   Figure 4 shows the architectural components of a network managed to
   provide IETF Network Slices.  The customer's view is of individual
   IETF Network Slice Services with their SDPs and connectivity
   constructs.  Requests for IETF Network Slice Services are delivered
   to an NSC.

   Figure 4 shows, without loss of generality, the CEs, ACs, and PEs
   that exist in the network.  The SDPs are not shown and can be placed
   in any of the ways described in Section 5.2.

                         --      --      --
                        |CE|    |CE|    |CE|
                         --      --      --
                       AC :    AC :    AC :
                       ----------------------       -------
                      ( |PE|....|PE|....|PE| )     ( IETF  )
     IETF Network    (   --:     --     :--   )   ( Network )
     Slice Service   (     :............:     )   (  Slice  )
     Request          (  IETF Network Slice  )     (       )  Customer
       v               ----------------------       -------     View
       v        ............................\........./...............
       v                                     \       /        Provider
       v    >>>>>>>>>>>>>>>  Grouping/Mapping v     v           View
       v   ^             -----------------------------------------
       v   ^            ( |PE|.......|PE|........|PE|.......|PE|  )
      ---------        (   --:        --         :--         --    )
     |         |       (     :...................:                 )
     |   NSC   |        (        Network Resource Partition       )
     |         |         -----------------------------------------
     |         |                             ^
     |         |>>>>>  Resource Partitioning |
      ---------        of Filtered Topology  |
       v   v                                 |
       v   v            -----------------------------      --------
       v   v           (|PE|..-..|PE|... ..|PE|..|PE|)    (        )
       v   v          ( :--  |P|  --   :-:  --   :--  )  (  Filter  )
       v   v          ( :.-   -:.......|P|       :-   )  ( Topology )
       v   v          (  |P|...........:-:.......|P|  )   (        )
       v   v           (  -    Filtered Topology     )     --------
       v   v            -----------------------------       ^
       v    >>>>>>>>>>>>  Topology Filter ^                /
       v        ...........................\............../...........
       v                                    \            /  Underlay
      ----------                             \          /  (Physical)
     |          |                             \        /    Network
     | Network  |    ----------------------------------------------
     |Controller|   ( |PE|.....-.....|PE|......    |PE|.......|PE| )
     |          |  (   --     |P|     --      :-...:--     -..:--   )
      ----------  (    :       -:.............|P|.........|P|        )
          v       (    -......................:-:..-       -         )
           >>>>>>> (  |P|.........................|P|......:        )
       Program the  (  -                           -               )
         Network     ----------------------------------------------

              Figure 4: Architecture of an IETF Network Slice

   The network itself (at the bottom of Figure 4) comprises an underlay
   network.  This could be a physical network but may be a virtual
   network.  The underlay network is provisioned through network
   controllers [RFC8309] that may, themselves, utilize device
   controllers.

   The underlay network may optionally be filtered or customized by the
   network operator to produce a number of network topologies that we
   call "Filtered Topologies".  Customization is just a way of selecting
   specific resources (e.g., nodes and links) from the underlay network
   according to their capabilities and connectivity in the underlay
   network.  Filtering and customization are configuration options or
   operator policies that preselect links and nodes with certain
   performance characteristics to enable easier construction of Network
   Resource Partitions (NRPs; see below) that can reliably support
   specific IETF Network Slice SLAs, for example, preselection of links
   with certain security characteristics, preselection of links with
   specific geographic properties, or mapping to colored topologies.
   The resulting topologies can be used as candidates to host IETF
   Network Slices and provide a useful way for the network operator to
   know in advance that all of the resources they are using to plan an
   IETF Network Slice would be able to meet specific SLOs and SLEs.  The
   creation of a Filtered Topology could be an offline planning activity
   or could be performed dynamically as new demands arise.  The use of
   Filtered Topologies is entirely optional in the architecture, and
   IETF Network Slices could be hosted directly on the underlay network.

   Recall that an IETF Network Slice is a service requested by and/or
   provided for the customer.  The IETF Network Slice Service is
   expressed in terms of one or more connectivity constructs.  An
   implementation or operator is free to limit the number of
   connectivity constructs in an IETF Network Slice to exactly one.
   Each connectivity construct is associated within the IETF Network
   Slice Service request with a set of SLOs and SLEs.  The set of SLOs
   and SLEs does not need to be the same for every connectivity
   construct in the IETF Network Slice, but an implementation or
   operator is free to require that all connectivity constructs in an
   IETF Network Slice have the same set of SLOs and SLEs.

   An NRP is a subset of the buffer/queuing/scheduling resources and
   associated policies on each of a connected set of links in the
   underlay network (for example, as achieved in
   [RESOURCE-AWARE-SEGMENTS]).  The connected set of links could be the
   entire set of links with all of their buffer/queuing/scheduling
   resources and behaviors in the underlay network, and in this case,
   there would be just one NRP supported in the underlay network.  The
   amount and granularity of resources allocated in an NRP is flexible
   and depends on the operator's policy.  Some NRP realizations may
   build NRPs with dedicated topologies, while other realizations may
   use a shared topology for multiple NRPs.  Realizations of an NRP may
   be built on a range of existing or new technologies, and this
   document does not constrain solution technologies.

   One or more connectivity constructs from one or more IETF Network
   Slices are mapped to an NRP.  A single connectivity construct is
   mapped to only one NRP (that is, the relationship is many to one).
   Thus, all traffic flows in a connectivity construct assigned to an
   NRP are assigned to that NRP.  Further, all PEs connected by a
   connectivity construct must be present in the NRP to which that
   connectivity construct is assigned.

   An NRP may be chosen to support a specific connectivity construct
   because of its ability to support a specific set of SLOs and SLEs,
   its ability to support particular connectivity constructs, or any
   administrative or operational reason.  An implementation or operator
   is free to map each connectivity construct to a separate NRP,
   although there may be scaling implications depending on the solution
   implemented.  Thus, the connectivity constructs from one slice may be
   mapped to one or more NRPs.  By implication from the above, an
   implementation or operator is free to map all the connectivity
   constructs in a slice to a single NRP and to not share that NRP with
   connectivity constructs from another slice.

   An NRP may use work-conserving schedulers, non-work-conserving
   schedulers, or both (see Section 2 of [RFC3290]) according to the
   function that it needs to deliver.  The choice of how network
   resources are allocated and managed for an NRP, and whether a work-
   conserving scheduling approach or a non-work-conserving scheduling
   approach is adopted, is technology specific: an implementation or
   operator is free to choose the set of techniques for NRP realization.

   The process of determining the NRP may be made easier if the underlay
   network topology is first filtered into a Filtered Topology in order
   to be aware of the subset of network resources that are suitable for
   specific NRPs.  In this case, each Filtered Topology is treated as an
   underlay network on which NRPs can be constructed.  The stage of
   generating Filtered Topologies is optional within this framework.

   The steps described here can be applied in a variety of orders
   according to implementation and deployment preferences.  Furthermore,
   the steps may be iterative so that the components are continually
   refined and modified as network conditions change and as service
   requests are received or relinquished, and even the underlay network
   could be extended if necessary to meet the customers' demands.

7.2.  Procedures to Realize IETF Network Slices

   There are a number of different technologies that can be used in the
   underlay, including physical connections, MPLS, Time-Sensitive
   Networking (TSN), Flex-E, etc.

   An IETF Network Slice can be realized in a network, using specific
   underlay technology or technologies.  The creation of a new IETF
   Network Slice will be realized with the following steps:

   1.  An NSC exposes the network slicing capabilities that it offers
       for the network it manages so that the customer can determine
       whether to request services and what features are in scope.

   2.  The customer may issue a request to determine whether a specific
       IETF Network Slice Service could be supported by the network.  An
       NSC may respond indicating a simple yes or no and may supplement
       a negative response with information about what it could support
       were the customer to change some requirements.

   3.  The customer requests an IETF Network Slice Service.  An NSC may
       respond that the slice has or has not been created and may
       supplement a negative response with information about what it
       could support were the customer to change some requirements.

   4.  When processing a customer request for an IETF Network Slice
       Service, an NSC maps the request to the network capabilities and
       applies provider policies before creating or supplementing the
       NRP.

   Regardless of how an IETF Network Slice is realized in the network
   (e.g., using tunnels of different types), the definition of the IETF
   Network Slice Service does not change at all.  The only difference is
   how the slice is realized.  The following sections briefly introduce
   how some existing architectural approaches can be applied to realize
   IETF Network Slices.

7.3.  Applicability of ACTN to IETF Network Slices

   Abstraction and Control of TE Networks (ACTN) [RFC8453] is a
   management architecture and toolkit used to create virtual networks
   (VNs) on top of a TE underlay network.  The VNs can be presented to
   customers for them to operate as private networks.

   In many ways, the function of ACTN is similar to IETF network
   slicing.  Customer requests for connectivity-based overlay services
   are mapped to dedicated or shared resources in the underlay network
   in a way that meets customer guarantees for SLOs and for separation
   from other customers' traffic.  [RFC8453] describes the function of
   ACTN as collecting resources to establish a logically dedicated
   virtual network over one or more TE networks.  Thus, in the case of a
   TE-enabled underlay network, the ACTN VN can be used as a basis to
   realize IETF network slicing.

   While the ACTN framework is a generic VN framework that can be used
   for VN services beyond the IETF Network Slice, it is also a suitable
   basis for delivering and realizing IETF Network Slices.

   Further discussion of the applicability of ACTN to IETF Network
   Slices, including a discussion of the relevant YANG models, can be
   found in [ACTN-NS].

7.4.  Applicability of Enhanced VPNs to IETF Network Slices

   An enhanced VPN is designed to support the needs of new applications,
   particularly applications that are associated with 5G services.  The
   approach is based on existing VPN and TE technologies but adds
   characteristics that specific services require over and above those
   previously associated with VPN services.

   An enhanced VPN can be used to provide enhanced connectivity services
   between customer sites and can be used to create the infrastructure
   to underpin an IETF Network Slice Service.

   It is envisaged that enhanced VPNs will be delivered using a
   combination of existing, modified, and new networking technologies.

   [ENHANCED-VPN] describes the framework for enhanced VPN services.

7.5.  Network Slicing and Aggregation in IP/MPLS Networks

   Network slicing provides the ability to partition a physical network
   into multiple logical networks of varying sizes, structures, and
   functions so that each slice can be dedicated to specific services or
   customers.  The support of resource preemption between IETF Network
   Slices is deployment specific.

   Many approaches are currently being worked on to support IETF Network
   Slices in IP and MPLS networks with or without the use of Segment
   Routing.  Most of these approaches utilize a way of marking packets
   so that network nodes can apply specific routing and forwarding
   behaviors to packets that belong to different IETF Network Slices.
   Different mechanisms for marking packets have been proposed
   (including using MPLS labels and Segment Routing segment IDs), and
   those mechanisms are agnostic to the path control technology used
   within the underlay network.

   These approaches are also sensitive to the scaling concerns of
   supporting a large number of IETF Network Slices within a single IP
   or MPLS network and so offer ways to aggregate the connectivity
   constructs of slices (or whole slices) so that the packet markings
   indicate an aggregate or grouping where all of the packets are
   subject to the same routing and forwarding behavior.

   At this stage, it is inappropriate to cite any of these proposed
   solutions that are currently work in progress and not yet adopted as
   IETF work.

7.6.  Network Slicing and Service Function Chaining (SFC)

   A customer may request an IETF Network Slice Service that involves a
   set of service functions (SFs) together with the order in which these
   SFs are invoked.  Also, the customer can specify the service
   objectives to be met by the underlay network (e.g., one-way delay to
   cross a service function path, one-way delay to reach a specific SF).
   These SFs are considered as ancillary CEs and are possibly
   placeholders (i.e., the SFs are identified, but not their locators).

   Service Function Chaining (SFC) [RFC7665] techniques can be used by a
   provider to instantiate such an IETF Network Slice Service.  An NSC
   may proceed as follows.

   *  Expose a set of ancillary CEs that are hosted in the underlay
      network.

   *  Capture the SFC requirements (including traffic performance
      metrics) from the customer.  One or more service chains may be
      associated with the same IETF Network Slice Service as
      connectivity constructs.

   *  Execute an SF placement algorithm to decide where to locate the
      ancillary CEs in order to fulfill the service objectives.

   *  Generate SFC classification rules to identify part of the slice
      traffic that will be bound to an SFC.  These classification rules
      may be the same as or distinct from the identification rules used
      to bind incoming traffic to the associated IETF Network Slice.

      An NSC also generates a set of SFC forwarding policies that govern
      how the traffic will be forwarded along a Service Function Path
      (SFP).

   *  Identify the appropriate Classifiers in the underlay network and
      provision them with the classification rules.  Likewise, an NSC
      communicates the SFC forwarding policies to the appropriate
      Service Function Forwarders (SFFs).

   The provider can enable an SFC data plane mechanism, such as those
   described in [RFC8300], [RFC8596], or [RFC9491].

8.  Isolation in IETF Network Slices

8.1.  Isolation as a Service Requirement

   An IETF Network Slice Service customer may request that the IETF
   Network Slice delivered to them is such that changes to other IETF
   Network Slices or to other services do not have any negative impact
   on the delivery of the IETF Network Slice.  The IETF Network Slice
   Service customer may specify the extent to which their IETF Network
   Slice Service is unaffected by changes in the provider network or by
   the behavior of other IETF Network Slice Service customers.  The
   customer may express this via an SLE it agrees with the provider.
   This concept is termed "isolation".

   In general, a customer cannot tell whether a service provider is
   meeting an isolation SLE.  If the service varies such that an SLO is
   breached, then the customer will become aware of the problem, and if
   the service varies within the allowed bounds of the SLOs, there may
   be no noticeable indication that this SLE has been violated.

8.2.  Isolation in IETF Network Slice Realization

   Isolation may be achieved in the underlay network by various forms of
   resource partitioning, ranging from dedicated allocation of resources
   for a specific IETF Network Slice to sharing of resources with
   safeguards.  For example, traffic separation between different IETF
   Network Slices may be achieved using VPN technologies, such as L3VPN,
   L2VPN, EVPN, etc.  Interference avoidance may be achieved by network
   capacity planning, allocating dedicated network resources, traffic
   policing or shaping, prioritizing in using shared network resources,
   etc.  Finally, service continuity may be ensured by reserving backup
   paths for critical traffic and dedicating specific network resources
   for a selected number of IETF Network Slices.

9.  Management Considerations

   IETF Network Slice realization needs to be instrumented in order to
   track how it is working, and it might be necessary to modify the IETF
   Network Slice as requirements change.  Dynamic reconfiguration might
   be needed.

   The various management interfaces and components are discussed in
   Section 6.

10.  Security Considerations

   This document specifies terminology and has no direct effect on the
   security of implementations or deployments.  In this section, a few
   of the security aspects are identified.

   Conformance to security constraints:  Specific security requests from
      customer-defined IETF Network Slice Services will be mapped to
      their realization in the underlay networks.  Underlay networks
      will require capabilities to conform to customer's requests as
      some aspects of security may be expressed in SLEs.

   IETF NSC authentication:  Underlay networks need to be protected
      against attacks from an adversary NSC as this could destabilize
      overall network operations.  An IETF Network Slice may span
      different networks; therefore, an NSC should have strong
      authentication with each of these networks.  Furthermore, both the
      IETF Network Slice Service Interface and the Network Configuration
      Interface need to be secured with a robust authentication and
      authorization mechanism and associated auditing mechanism.

   Specific isolation criteria:  The nature of conformance to isolation
      requests means that it should not be possible to attack an IETF
      Network Slice Service by varying the traffic on other services or
      slices carried by the same underlay network.  In general,
      isolation is expected to strengthen the IETF Network Slice
      security.

   Data confidentiality and integrity of an IETF Network Slice:  An IETF
      Network Slice might include encryption and other security features
      as part of the service (for example, as SLEs).  However, a
      customer wanting to guarantee that their data is secure from
      inspection or modification as it passes through the network of the
      operator that provides the IETF Network Slice Service will need to
      provision their own security solutions (e.g., with IPsec) or send
      only already otherwise-encrypted traffic through the slice.

   See [NGMN-SEC] on 5G network slice security for discussion relevant
   to this section.

   IETF Network Slices might use underlying virtualized networking.  All
   types of virtual networking require special consideration to be given
   to the separation of traffic between distinct virtual networks, as
   well as some amount of protection from effects of traffic use of
   underlay network (and other) resources from other virtual networks
   sharing those resources.

   For example, if a service requires a specific upper bound on latency,
   then that service could be degraded with added delay caused by the
   processing of packets from another service or application that shares
   the same network resources.  Thus, without careful planning or
   traffic policing, it may be possible to attack an IETF Network Slice
   Service simply by increasing the traffic on another service in the
   network.

   Similarly, in a network with virtual functions, noticeably impeding
   access to a function used by another IETF Network Slice (for
   instance, compute resources) can be just as service-degrading as
   delaying physical transmission of associated packet in the network.
   Again, careful planning and policing of service demands may mitigate
   such attacks.

   Both of these forms of attack may also be mitigated by reducing the
   access to information about how IETF Network Slice Services are
   supported in a network.

11.  Privacy Considerations

   Privacy of IETF Network Slice Service customers must be preserved.
   It should not be possible for one IETF Network Slice Service customer
   to discover the presence of other customers, nor should sites that
   are members of one IETF Network Slice be visible outside the context
   of that IETF Network Slice.

   In this sense, it is of paramount importance that the system uses the
   privacy protection mechanism defined for the specific underlay
   technologies that support the slice, including in particular those
   mechanisms designed to preclude acquiring identifying information
   associated with any IETF Network Slice Service customer.

12.  IANA Considerations

   This document has no IANA actions.

13.  Informative References

   [ACTN-NS]  King, D., Drake, J., Zheng, H., and A. Farrel,
              "Applicability of Abstraction and Control of Traffic
              Engineered Networks (ACTN) to Network Slicing", Work in
              Progress, Internet-Draft, draft-ietf-teas-applicability-
              actn-slicing-05, 11 February 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-teas-
              applicability-actn-slicing-05>.

   [ENHANCED-VPN]
              Dong, J., Bryant, S., Li, Z., Miyasaka, T., and Y. Lee, "A
              Framework for NRP-based Enhanced Virtual Private Network",
              Work in Progress, Internet-Draft, draft-ietf-teas-
              enhanced-vpn-17, 25 December 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-teas-
              enhanced-vpn-17>.

   [GNMI]     Shakir, R., Shaikh, A., Borman, P., Hines, M., Lebsack,
              C., and C. Morrow, "gRPC Network Management Interface
              (gNMI)", Work in Progress, Internet-Draft, draft-
              openconfig-rtgwg-gnmi-spec-01, 5 March 2018,
              <https://datatracker.ietf.org/doc/html/draft-openconfig-
              rtgwg-gnmi-spec-01>.

   [HIPAA]    HHS, "The Security Rule", <https://www.hhs.gov/hipaa/for-
              professionals/security/index.html>.

   [MACsec]   IEEE, "IEEE Standard for Local and metropolitan area
              networks - Media Access Control (MAC) Security", IEEE Std 
              802.1AE-2018, DOI 10.1109/IEEESTD.2018.8585421, December
              2018, <https://ieeexplore.ieee.org/document/8585421>.

   [NFVArch]  ETSI, "Network Functions Virtualisation (NFV);
              Architectural Framework", V1.1.1, ETSI GS NFV 002, October
              2013, <http://www.etsi.org/deliver/etsi_gs/
              nfv/001_099/002/01.01.01_60/gs_nfv002v010101p.pdf>.

   [NGMN-NS-Concept]
              NGMN Alliance, "Description of Network Slicing Concept",
              January 2016, <https://ngmn.org/wp-content/
              uploads/160113_NGMN_Network_Slicing_v1_0.pdf>.

   [NGMN-SEC] NGMN, "5G security recommendations Package #2 - Network
              Slicing", April 2016, <https://www.ngmn.org/wp-
              content/uploads/Publications/2016/160429_NGMN_5G_Security_
              Network_Slicing_v1_0.pdf>.

   [ORAN]     O-RAN, "O-RAN Working Group 1 Slicing Architecture",
              O-RAN.WG1 v06.00, 2022,
              <https://orandownloadsweb.azurewebsites.net/
              specifications>.

   [PCI]      PCI Security Standards Council, "PCI DSS", March 2022,
              <https://www.pcisecuritystandards.org/document_library>.

   [RESOURCE-AWARE-SEGMENTS]
              Dong, J., Miyasaka, T., Zhu, Y., Qin, F., and Z. Li,
              "Introducing Resource Awareness to SR Segments", Work in
              Progress, Internet-Draft, draft-ietf-spring-resource-
              aware-segments-08, 23 October 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-spring-
              resource-aware-segments-08>.

   [RFC3290]  Bernet, Y., Blake, S., Grossman, D., and A. Smith, "An
              Informal Management Model for Diffserv Routers", RFC 3290,
              DOI 10.17487/RFC3290, May 2002,
              <https://www.rfc-editor.org/info/rfc3290>.

   [RFC3393]  Demichelis, C. and P. Chimento, "IP Packet Delay Variation
              Metric for IP Performance Metrics (IPPM)", RFC 3393,
              DOI 10.17487/RFC3393, November 2002,
              <https://www.rfc-editor.org/info/rfc3393>.

   [RFC4208]  Swallow, G., Drake, J., Ishimatsu, H., and Y. Rekhter,
              "Generalized Multiprotocol Label Switching (GMPLS) User-
              Network Interface (UNI): Resource ReserVation Protocol-
              Traffic Engineering (RSVP-TE) Support for the Overlay
              Model", RFC 4208, DOI 10.17487/RFC4208, October 2005,
              <https://www.rfc-editor.org/info/rfc4208>.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,
              <https://www.rfc-editor.org/info/rfc4303>.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <https://www.rfc-editor.org/info/rfc4364>.

   [RFC4397]  Bryskin, I. and A. Farrel, "A Lexicography for the
              Interpretation of Generalized Multiprotocol Label
              Switching (GMPLS) Terminology within the Context of the
              ITU-T's Automatically Switched Optical Network (ASON)
              Architecture", RFC 4397, DOI 10.17487/RFC4397, February
              2006, <https://www.rfc-editor.org/info/rfc4397>.

   [RFC5212]  Shiomoto, K., Papadimitriou, D., Le Roux, JL., Vigoureux,
              M., and D. Brungard, "Requirements for GMPLS-Based Multi-
              Region and Multi-Layer Networks (MRN/MLN)", RFC 5212,
              DOI 10.17487/RFC5212, July 2008,
              <https://www.rfc-editor.org/info/rfc5212>.

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,
              <https://www.rfc-editor.org/info/rfc5440>.

   [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>.

   [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>.

   [RFC7239]  Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
              RFC 7239, DOI 10.17487/RFC7239, June 2014,
              <https://www.rfc-editor.org/info/rfc7239>.

   [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>.

   [RFC7679]  Almes, G., Kalidindi, S., Zekauskas, M., and A. Morton,
              Ed., "A One-Way Delay Metric for IP Performance Metrics
              (IPPM)", STD 81, RFC 7679, DOI 10.17487/RFC7679, January
              2016, <https://www.rfc-editor.org/info/rfc7679>.

   [RFC7680]  Almes, G., Kalidindi, S., Zekauskas, M., and A. Morton,
              Ed., "A One-Way Loss Metric for IP Performance Metrics
              (IPPM)", STD 82, RFC 7680, DOI 10.17487/RFC7680, January
              2016, <https://www.rfc-editor.org/info/rfc7680>.

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

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,
              <https://www.rfc-editor.org/info/rfc7950>.

   [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>.

   [RFC8300]  Quinn, P., Ed., Elzur, U., Ed., and C. Pignataro, Ed.,
              "Network Service Header (NSH)", RFC 8300,
              DOI 10.17487/RFC8300, January 2018,
              <https://www.rfc-editor.org/info/rfc8300>.

   [RFC8309]  Wu, Q., Liu, W., and A. Farrel, "Service Models
              Explained", RFC 8309, DOI 10.17487/RFC8309, January 2018,
              <https://www.rfc-editor.org/info/rfc8309>.

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

   [RFC8454]  Lee, Y., Belotti, S., Dhody, D., Ceccarelli, D., and B.
              Yoon, "Information Model for Abstraction and Control of TE
              Networks (ACTN)", RFC 8454, DOI 10.17487/RFC8454,
              September 2018, <https://www.rfc-editor.org/info/rfc8454>.

   [RFC8596]  Malis, A., Bryant, S., Halpern, J., and W. Henderickx,
              "MPLS Transport Encapsulation for the Service Function
              Chaining (SFC) Network Service Header (NSH)", RFC 8596,
              DOI 10.17487/RFC8596, June 2019,
              <https://www.rfc-editor.org/info/rfc8596>.

   [RFC9408]  Boucadair, M., Ed., Gonzalez de Dios, O., Barguil, S., Wu,
              Q., and V. Lopez, "A YANG Network Data Model for Service
              Attachment Points (SAPs)", RFC 9408, DOI 10.17487/RFC9408,
              June 2023, <https://www.rfc-editor.org/info/rfc9408>.

   [RFC9491]  Guichard, J., Ed. and J. Tantsura, Ed., "Integration of
              the Network Service Header (NSH) and Segment Routing for
              Service Function Chaining (SFC)", RFC 9491,
              DOI 10.17487/RFC9491, November 2023,
              <https://www.rfc-editor.org/info/rfc9491>.

   [TS23.501] 3GPP, "System architecture for the 5G System (5GS)", 3GPP
              TS 23.501, 2019.

   [TS28.530] 3GPP, "Management and orchestration; Concepts, use cases
              and requirements", 3GPP TS 28.530, 2019.

   [TS33.210] 3GPP, "Network Domain Security (NDS); IP network layer
              security", Release 14, December 2016,
              <https://portal.3gpp.org/desktopmodules/Specifications/
              SpecificationDetails.aspx?specificationId=2279>.

   [USE-CASES]
              Contreras, L. M., Homma, S., Ordonez-Lucena, J. A.,
              Tantsura, J., and H. Nishihara, "IETF Network Slice Use
              Cases and Attributes for the Slice Service Interface of
              IETF Network Slice Controllers", Work in Progress,
              Internet-Draft, draft-ietf-teas-ietf-network-slice-use-
              cases-01, 24 October 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-teas-
              ietf-network-slice-use-cases-01>.

Appendix A.  Examples

   This appendix contains realization examples.  This is not intended to
   be a complete set of possible deployments, nor does it provide
   definitive ways to realize these deployments.

   The examples shown here must not be considered to be normative.  The
   descriptions of terms and concepts in the body of the document take
   precedence.

A.1.  Multi-Point to Point Service

   As described in Section 4.2, an MP2P service can be realized with
   multiple P2P connectivity constructs.  Figure 5 shows a simple MP2P
   service where traffic is sent from any of CE1, CE2, and CE3 to the
   receiver, which is CE4.  The service comprises three P2P connectivity
   constructs: CE1-CE4, CE2-CE4, and CE3-CE4.

                                 CE1
                               ___|________
                              /    \       \
                             (      \______ )
                             (             \)
                       CE2---(--------------)---CE4
                             (      _______/)
                             (     /        )
                              \___|________/
                                  |
                                 CE3

            Figure 5: Example MP2P Service with P2P Connections

A.2.  Service Function Chaining and Ancillary CEs

   Section 4.2.3 introduces the concept of ancillary CEs.  Figure 6
   shows a simple example of IETF Network Slices with connectivity
   constructs that are used to deliver traffic from CE1 to CE3, taking
   in a service function along the path.

                        CE1         CE2         CE3
                        xo*         * *         *ox
                    ____xo*_________*_*_________*ox____
                  _/    xo*         * *         *ox    \_
                 /      xo*********** ***********ox      \
                (       xo                       ox       )
                (       xooooooooo(ACE1)oooooooooox       )
                (       x                         x       )
                (       x   ------------------    x       )
                (       x  | Service Function |   x       )
                (       x  |  ....(ACE2)....  |   x       )
                (       x  | :              : |   x       )
                (       xxxx.:....(ACE3)....:.xxxxx       )
                (          | :              : |           )
                (          |  ....(ACE4)....  |           )
                (          |                  |           )
                (           ------------------            )
                (                                         )
                 \_          Operator Network           _/
                   \___________________________________/

                    Figure 6: Example with Ancillary CEs

   A customer may want to utilize a service where traffic is delivered
   from CE1 to CE3, including a service function sited within the
   customer's network at CE2.  To achieve this, the customer may request
   an IETF Network Slice Service comprising two P2P connectivity
   constructs: CE1-CE2 and CE2-CE3 (represented with "*" in Figure 6).

   Alternatively, the service function for the same CE1 to CE3 flow may
   be hosted at a node within the network operator's infrastructure.
   This is an ancillary CE in the IETF Network Slice Service that the
   customer requests.  This service contains two P2P connectivity
   constructs: CE1-ACE1 and ACE1-CE3 (represented with "o" in Figure 6).
   How the customer knows of the existence of the ancillary CE and the
   service functions it offers is a matter for agreement between the
   customer and the network operator.

   Finally, it may be that the customer knows that the network operator
   is able to provide the service function but does not know the
   location of the ancillary CE at which the service function is hosted.
   Indeed, it may be that the service function is hosted at a number of
   ancillary CEs (ACE2, ACE3, and ACE4 in Figure 6); the customer may
   know the identities of the ancillary CEs but be unwilling or unable
   to choose one, or the customer may not know about the ancillary CEs.
   In this case, the IETF Network Slice Service request contains two P2P
   connectivity constructs: CE1-ServiceFunction and ServiceFunction-CE3
   (represented with "x" in Figure 6).  It is left as a choice for the
   network operator as to which ancillary CE to use and how to realize
   the connectivity constructs.

A.3.  Hub and Spoke

   Hub and spoke is a popular way to realize A2A connectivity in support
   of multiple P2P traffic flows (where the hub performs routing) or
   P2MP flows (where the hub is responsible for replication).  In many
   cases, it is the network operator's choice whether to use hub and
   spoke to realize a mesh of P2P connectivity constructs or P2MP
   connectivity constructs; this is entirely their business as the
   customer is not aware of how the connectivity constructs are
   supported within the network.

   However, it may be the case that the customer wants to control the
   behavior and location of the hub.  In this case, the hub appears as
   an ancillary CE as shown in Figure 7.

   For the P2P mesh case, the customer does not specify a mesh of P2P
   connectivity constructs (such as CE1-CE2, CE1-CE3, CE2-CE3, and the
   equivalent reverse direction connectivity) but connects each CE to
   the hub with P2P connectivity constructs (as CE1-Hub, CE2-Hub,
   CE3-Hub, and the equivalent reverse direction connectivity).  This
   scales better in terms of provisioning compared to a full mesh but
   requires that the hub is capable of routing traffic between
   connectivity constructs.

   For the P2MP case, the customer does not specify a single P2MP
   connectivity construct (in this case, CE3-{CE1+CE2}) but requests
   three P2P connectivity constructs (as CE3-Hub, Hub-CE1, and Hub-CE2).
   It is the hub's responsibility to replicate the traffic from CE3 and
   send it to both CE1 and CE2.

                               ------------
                         CE1  |    Hub     |   CE2
                         ||    ------------    ||
                      ___||_____||__||__||_____||___
                     /   ||     ||  ||  ||     ||   \
                    (     ======    ||   ======      )
                    (               ||               )
                    (               ||               )
                     \______________||______________/
                                    ||
                                    CE3

           Figure 7: Example Hub and Spoke under Customer Control

A.4.  Layer 3 VPN

   Layer 3 VPNs are a common service offered by network operators to
   their customers.  They may be modeled as an A2A service but are often
   realized as a mesh of P2P connections, or if multicast is supported,
   they may be realized as a mesh of P2MP connections.

   Figure 8 shows an IETF Network Slice Service with a single A2A
   connectivity construct between the SDPs CE1, CE2, CE3, and CE4.  It
   is a free choice how the network operator realizes this service.
   They may use a full mesh of P2P connections, a hub-and-spoke
   configuration, or some combination of these approaches.

                            CE1             CE2
                         ____|_______________|____
                        /    :...............:    \
                       (     :.            . :     )
                       (     : ......     .  :     )
                       (     :       .....   :     )
                      (      :   .... .      :      )
                       (     :  .      ....  :     )
                       (     : .           . :     )
                       (     :...............:     )
                        \____:_______________:____/
                             |               |
                            CE3             CE4

                      Figure 8: Example L3VPN Service

A.5.  Hierarchical Composition of Network Slices

   As mentioned in Section 5.3, IETF Network Slices may be arranged
   hierarchically.  There is nothing special or novel about such an
   arrangement, and it models the hierarchical arrangement of services
   of virtual networks in many other environments.

   As shown in Figure 9, an Operator's Controller (NSC) that is
   requested to provide an IETF Network Slice Service for a customer
   may, in turn, request an IETF Network Slice Service from another
   carrier.  The Operator's NSC may manage and control the underlay IETF
   Network Slice by modifying the requested connectivity constructs and
   changing the SLAs.  The customer is entirely unaware of the hierarchy
   of slices, and the underlay carrier is entirely unaware of how its
   slice is being used.

   This stacking of IETF Network Slice constructs is not different to
   the way virtual networks may be arranged.

              --------------
             | Network      |
             | Slice        |
             | Orchestrator |
              --------------
               | IETF Network Slice
               | Service Request
               |                    Customer view
           ....|................................
              -v----------------    Operator view
             |Controller        |
             |  ------------    |
             | | IETF       |   |
             | | Network    |---|---
             | | Slice      |   |   |
             | | Controller |   |   |
             | | (NSC)      |   |   |
             |  ------------    |   |
              ------------------    |
                                    | IETF Network Slice
                                    | Service Request
                                    |
           .........................|.....................
                          ----------v-------    Carrier view
                         |Controller        |
                         |  ------------    |
                         | | IETF       |   |
                         | | Network    |   |
                         | | Slice      |   |
                         | | Controller |   |
                         | | (NSC)      |   |
                         |  ------------    |
                     ....|  | Network       |............
                         |  | Configuration |   Underlay Network
                         |  v               |
                         |  ------------    |
                         | | Network    |   |
                         | | Controller |   |
                         | | (NC)       |   |
                         |  ------------    |
                          ------------------
                           | Device Configuration
                           v

     Figure 9: Example Hierarchical Arrangement of IETF Network Slices

   In this case, the network hierarchy may also be used to provide
   connectivity between points in the higher-layer network, as shown in
   Figure 10.  Here, an IETF Network Slice may be requested of the
   lower-layer network to provide the desired connectivity constructs to
   supplement the connectivity in the higher-layer network where this
   connectivity might be presented as a virtual link.

               CE1                                       CE2
                |                                         |
                |                                         |
               _|_________________________________________|_
              ( :                                         : )
             (  :..............             ..............:  )
              (_______________:_____________:_______________)
                            __|_____________|__
                           (  :             :  )
                          (   :.............:   )
                           (___________________)

        Figure 10: Example Hierarchical Arrangement of IETF Network
                       Slices to Bridge Connectivity

A.6.  Horizontal Composition of Network Slices

   It may be that end-to-end connectivity is achieved using a set of
   cooperating networks as described in Section 5.3.  For example, there
   may be multiple interconnected networks that provide the required
   connectivity as shown in Figure 11.  The networks may utilize
   different technologies and may be under separate administrative
   control.

               CE1                                       CE2
                |                                         |
               SDP1                                      SDP2
                |                                         |
               _|____       ______       ______       ____|_
              (      )     (      )     (      )     (      )
             (        )---(        )---(        )---(        )
              (______)     (______)     (______)     (______)

        Figure 11: Example Customer View of Interconnected Networks
                     Providing End-to-End Connectivity

   In this scenario, the customer (represented by CE1 and CE2) may
   request an IETF Network Slice Service connecting the CEs.  The
   customer considers the SDPs at the edge (shown as SDP1 and SDP2 in
   Figure 11) and might not be aware of how the end-to-end connectivity
   is composed.

   However, because the various networks may be of different
   technologies and under separate administrative control, the networks
   are sliced individually, and coordination is necessary to deliver the
   desired connectivity.  The Network-to-Network Interfaces (NNIs) are
   present as SDPs for the IETF Network Slices in each network, so that
   each network is individually sliced.  In the example in Figure 12,
   this is illustrated as network 1 (N/w1) being sliced between SDP1 and
   SDPX, N/w2 being sliced between SDPY and SDPU, etc.  The coordination
   activity involves binding the SDPs, and hence the connectivity
   constructs, to achieve end-to-end connectivity with the required SLOs
   and SLEs.  In this way, simple and complex end-to-end connectivity
   can be achieved with a variety of connectivity constructs in the IETF
   Network Slices of different networks "stitched" together.

          CE1                                                CE2
           |                                                  |
          SDP1                                               SDP2
           |                                                  |
          _|____          ______          ______          ____|_
         (      ) SDPX   (      ) SDPU   (      ) SDPS   (      )
        (  N/w1  )------(  N/w2  )------(  N/w3  )------(  N/w4  )
         (______)   SDPY (______)   SDPV (______)   SDPT (______)

    Figure 12: Example Delivery of an End-to-End IETF Network Slice with
                          Interconnected Networks

   The controller/coordinator relationship is shown in Figure 13.

         --------------
        | Network      |
        | Slice        |
        | Orchestrator |
         --------------
          | IETF Network Slice
          | Service Request
          |                    Customer view
      ....|................................
         -v----------------    Coordinator view
        |Coordinator       |
        |                  |
         ------------------
          |             |_________________
          |                               |
          |                               |
      ....|.......................    ....|.....................
         -v--------------                -v--------------
        |Controller1     | Operator1    |Controller2     | Operator2
        |  ------------  |              |  ------------  |
        | | IETF       | |              | | IETF       | |
        | | Network    | |              | | Network    | |
        | | Slice      | |              | | Slice      | |
        | | Controller | |              | | Controller | |
        | | (NSC)      | |              | | (NSC)      | |
        |  ------------  |              |  ------------  |
    ....|  | Network     |............  |  | Network     |............
        |  | Config      | Underlay1    |  | Config      | Underlay2
        |  v             |              |  v             |
        |  ------------  |              |  ------------  |
        | | Network    | |              | | Network    | |
        | | Controller | |              | | Controller | |
        | | (NC)       | |              | | (NC)       | |
        |  ------------  |              |  ------------  |
         ----------------                ----------------
          | Device Configuration
          v

     Figure 13: Example Relationship of IETF Network Slice Coordination

Acknowledgments

   The entire TEAS Network Slicing design team and everyone
   participating in related discussions has contributed to this
   document.  Some text fragments in the document have been copied from
   the [ENHANCED-VPN], for which we are grateful.

   Significant contributions to this document were gratefully received
   from the contributing authors listed in the "Contributors" section.
   In addition, we would like to also thank those others who have
   attended one or more of the design team meetings, including the
   following people not listed elsewhere:

   *  Aihua Guo
   *  Bo Wu
   *  Greg Mirsky
   *  Lou Berger
   *  Rakesh Gandhi
   *  Ran Chen
   *  Sergio Belotti
   *  Stewart Bryant
   *  Tomonobu Niwa
   *  Xuesong Geng

   Further useful comments were received from Daniele Ceccarelli, Uma
   Chunduri, Pavan Beeram, Tarek Saad, Kenichi Ogaki, Oscar Gonzalez de
   Dios, Xiaobing Niu, Dan Voyer, Igor Bryskin, Luay Jalil, Joel
   Halpern, John Scudder, John Mullooly, Krzysztof Szarkowicz, Jingrong
   Xie, Jia He, Reese Enghardt, Dirk Von Hugo, Erik Kline, and Éric
   Vyncke.

   This work is partially supported by the European Commission under
   Horizon 2020 grant agreement number 101015857 Secured autonomic
   traffic management for a Tera of SDN flows (Teraflow).

Contributors

   The following people contributed substantially to the content of this
   document and should be considered coauthors.  Eric Gray was the
   original editor of the foundation documents.

   Eric Gray
   Retired


   Jari Arkko
   Ericsson
   Email: jari.arkko@piuha.net


   Mohamed Boucadair
   Orange
   Email: mohamed.boucadair@orange.com


   Dhruv Dhody
   Huawei
   India
   Email: dhruv.ietf@gmail.com


   Jie Dong
   Huawei
   Email: jie.dong@huawei.com


   Xufeng Liu
   Volta Networks
   Email: xufeng.liu.ietf@gmail.com


Authors' Addresses

   Adrian Farrel (editor)
   Old Dog Consulting
   United Kingdom
   Email: adrian@olddog.co.uk


   John Drake (editor)
   Individual
   United States of America
   Email: je_drake@yahoo.com


   Reza Rokui
   Ciena
   Email: rrokui@ciena.com


   Shunsuke Homma
   NTT
   Japan
   Email: shunsuke.homma.ietf@gmail.com


   Kiran Makhijani
   Futurewei
   United States of America
   Email: kiran.ietf@gmail.com


   Luis M. Contreras
   Telefonica
   Spain
   Email: luismiguel.contrerasmurillo@telefonica.com


   Jeff Tantsura
   Nvidia
   Email: jefftant.ietf@gmail.com
