Internet-Draft Implementing 5G Transport Slices June 2024
Szarkowicz, et al. Expires 3 December 2024 [Page]
Workgroup:
TEAS
Internet-Draft:
draft-ietf-teas-5g-ns-ip-mpls-08
Published:
Intended Status:
Informational
Expires:
Authors:
K. G. Szarkowicz, Ed.
Juniper Networks
R. Roberts, Ed.
Juniper Networks
J. Lucek
Juniper Networks
M. Boucadair, Ed.
Orange
L. M. Contreras
Telefonica

A Realization of Network Slices for 5G Networks Using Current IP/MPLS Technologies

Abstract

Slicing is a feature that was introduced by the 3rd Generation Partnership Project (3GPP) in mobile networks. Realization of 5G slicing implies requirements for all mobile domains, including the Radio Access Network (RAN), Core Network (CN), and Transport Network (TN).

This document describes a Network Slice realization model for IP/MPLS networks with a focus on the Transport Network fulfilling 5G slicing connectivity service objectives. The realization model reuses many building blocks currently commonly used in service provider networks.

Discussion Venues

This note is to be removed before publishing as an RFC.

Discussion of this document takes place on the Traffic Engineering Architecture and Signaling Working Group mailing list (teas@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/teas/.

Source for this draft and an issue tracker can be found at https://github.com/boucadair/5g-slice-realization.

Status of This Memo

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

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

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

This Internet-Draft will expire on 3 December 2024.

1. Introduction

This document focuses on network slicing for 5G networks, covering the connectivity between Network Functions (NFs) across multiple domains such as edge clouds, data centers, and the Wide Area Network (WAN). The document describes a Network Slice realization approach that fulfills 5G slicing requirements by using existing IP/MPLS technologies to optimally control connectivity Service Level Agreements (SLAs) offered for 5G slices. To that aim, this document describes the scope of the Transport Network in 5G architectures (Section 3.1), disambiguates 5G Network Slicing versus Transport Network Slicing (Section 3.2), draws the perimeter of the various orchestration domains to realize slices (Section 3.4), and identifies the required coordination between these orchestration domains for adequate setup of Attachment Circuits (ACs) (Section 3.4.2).

This work is compatible with the framework defined in [RFC9543] which describes network slicing in the context of networks built from IETF technologies. Specifically, this document explains how RFC 9543 Network Slices are realized within provider networks and how such slices are stitched to Transport Network resources in a customer site in the context of Transport Network Slices (Figure 1). Concretely, the realization of an RFC 9543 Network Slice (i.e., connectivity with performance commitments) involves the provider network and partially the AC (the PE-side of the AC). This document assumes that the customer site infrastructure is over-provisioned and involves short distances (low latency) where basic QoS/scheduling logic is sufficient to comply with the Service Level Objectives (SLOs).

      |------------------TN Slice------------------|

                        RFC 9543 Network Slice
                        +-----SDP Type 3----+
                        |  +- SDP Type 4-+  |
                        |  |             |  |
                        v  v             v  v
  +------------+          +---------------+         +------------+
  |  Customer  |          |    Provider   |         |  Customer  |
  |   Site 1   |          |    Network    |         |   Site 2   |
  |            |        +-+--+          +-+--+      |            |
  |+---+    +--+-+  AC  |    |          |    | AC +-+-+          |
  ||NF +....+ CE +------+ PE |          | PE +----+NF |          |
  |+---+    +--+-+      |    |          |    |    +-+-+          |
  |            |        +-+--+          +-+--+      |            |
  |            |          |               |         |            |
  +------------+          +---------------+         +------------+
Figure 1: Transport Network Slice & RFC 9543 Network Slice Scopes

The realization approach described in this document is typically triggered by Network Slice Service requests. How a Network Slice Service request is placed for realization, including how it is derived from a 5G Slice Service request, is out of scope. Mapping considerations between 3GPP and IETF Network Slice Service (e.g., mapping of service parameters) are discussed, e.g., in [I-D.ietf-teas-5g-network-slice-application].

The 5G control plane uses the Single Network Slice Selection Assistance Information (S-NSSAI) for slice identification [TS-23.501]. Because S-NSSAIs are not visible to the transport domain, 5G domains can expose the 5G slices to the transport domain by mapping to explicit data plane identifiers (e.g., Layer 2, Layer 3, or Layer 4). The realization of the mapping between customer sites and provider networks is refered to as the "hand-off". Section 4 lists a set of such hand-off methods.

The realization model described in this document uses a set of building blocks commonly used in service provider networks. Concretely, the model uses (1) Layer 2 Virtual Private Network (L2VPN) [RFC4664] and/or Layer 3 Virtual Private Network (L3VPN) [RFC4364] service instances for logical separation, (2) fine-grained resource control at the Provider Edges (PEs), (3) coarse-grained resource control at within the provider network, and (4) capacity management. More details are provided in Sections 3.7, 5, 6, and 7.

This realization model uses a single Network Resource Partition (NRP) (Section 7.1 of [RFC9543]). The applicability to multiple NRPs is out of scope.

Although this document focuses on 5G, the realizations are not fundamentally constrained by the 5G use case. The document is not intended to be a BCP and does not claim to specify mandatory mechanisms to realize network slices. Rather, a key goal of the document is to provide pragmatic implementation approaches by leveraging existing readily-available, widely-deployed techniques. The document is also intended to align the mobile and the IETF perspectives of slicing from a realization perspective.

A brief 5G overview is provided in Appendix B for the reader's convenience. For a definitive description of 3GPP network architectures, the reader should refer to [TS-23.501]. More details can be found in [_5G-Book].

2. Definitions

The document uses the terms defined in [RFC9543]. See Section 3.3 for the contextualization of some of these terms.

An extended list of abbreviations used in this document is provided in Appendix A.

"5G Network Slicing" (or "5G Network Slice") refers to "Network Slicing" (or "Network Slice") as defined in the 3GPP [TS-28.530].

This document makes use of the following terms:

Customer:

An entity that is responsible for managing and orchestrating the end-to-end 5G Mobile Network, notably the Radio Access Network (RAN) and Core Network (CN).

This entity is distinct from the customer of a 5G Network Slice Service.

Customer site:

A customer manages and deploys 5G NFs (e.g., gNodeB (gNB) and 5G Core (5GC)) in customer sites. A customer site can be either a physical or a virtual location.

Examples of customer sites are a customer private locations (Point of Presence (PoP), Data Center (DC)), a Virtual Private Cloud (VPC), or servers hosted within the provider network or colocation service.

Provider:

An entity responsible for interconnecting customer sites.

A provider orchestrates and manages a provider network.

3. 5G Network Slicing Integration in Transport Networks

3.1. Scope of the Transport Network

Appendix B provides an overview of 5G network building blocks: the Radio Access Network (RAN), Core Network (CN), and Transport Network (TN). The Transport Network is defined by the 3GPP as:

"part supporting connectivity within and between CN and RAN parts" (Section 1 of [TS-28.530]).

As discussed in Section 4.4.1 of [TS-28.530], the 3GPP management system does not directly control the Transport Network: it is considered as a non-3GPP managed system.

"The non-3GPP part includes TN parts. The 3GPP management system provides the network slice requirements to the corresponding management systems of those non-3GPP parts, e.g. the TN part supports connectivity within and between CN and AN parts." (Section 4.4.1 of [TS-28.530])

In practice, the TN may not map to a monolithic architecture and management domain. It is frequently segmented, non-uniform, and managed by different entities. For example, Figure 2 depicts an NF instance that is deployed in an edge data center (DC) connected to an NF located in a Public Cloud via a WAN (e.g., MPLS-VPN service). In this example, the TN can be seen as an abstraction representing an end-to-end connectivity based upon three distinct domains: DC, WAN, and Public Cloud. A model for the Transport Network based on orchestration domains is introduced in Section 3.4.

      +----------------------------------+
 +----+      5G RAN or Core Network      +----+
 |    +----------------------------------+    |
 |                                            |
 v                                            v
+--+  +----------------------------------+  +--+
|NF+--+        Transport Network         +--+NF|
+--+  +--+---------------+------------+--+  +--+
         |               |            |
         v               v            v
 +-- Data Center -+  +-MPLS VPN-+   +-Public-+
 |                |  | Backbone |   |  Cloud |
 |.-----. .-----. | +--+      +--+ +--+      |
 |'-----' '-----' | |PE|      |PE| |GW|      |
 |.-. .-. .-. .-. | +--+      +--+ +--+      |
 |'-' '-' '-' '-' |  |          |   |        |
 |                | +--+      +--+  |        |
 |                | |PE|      |PE|  |        |
 |                | +--+      +--+  |        |
 |                |  |          |   |        |
 +----------------+  +----------+   +--------+
Figure 2: An Example of Transport Network Decomposition

3.2. 5G Network Slicing versus Transport Network Slicing

Network slicing has a different meaning in the 3GPP mobile world and transport world. This difference can be seen from the descriptions below that set out the objectives of 5G Network Slicing (Section 3.2.1) and Transport Network Slicing (Section 3.2.2). These descriptions are not intended to be exhaustive.

3.2.1. 5G Network Slicing

5G Network Slicing is defined by the 3GPP [TS-28.530] as an approach:

"where logical networks/partitions are created, with appropriate isolation, resources and optimized topology to serve a purpose or service category (e.g. use case/traffic category, or for MNO internal reasons) or customers (logical system created "on demand")."

These resources are from the TN, RAN, CN domains, and the underlying infrastructure.

Section 3.1 of [TS-28.530] defines 5G Network Slice as:

"a logical network that provides specific network capabilities and network characteristics, supporting various service properties for network slice customers."

3.2.2. Transport Network Slicing

The term "TN slice" refers to a slice in the Transport Network domain of the 5G architecture.

The objective of Transport Network Slicing is to isolate, guarantee, or prioritize Transport Network resources for Slice Services. Examples of such resources are: buffers, link capacity, or even Routing Information Base (RIB) and Forwarding Information Base (FIB).

Transport Network Slicing provides various degrees of sharing of resources between slices. For example, the network capacity can be shared by all slices, usually with a guaranteed minimum per slice, or each individual slice can be allocated dedicated network capacity. Parts of a given network may use the former, while others use the latter. For example, in order to satisfy local engineering guidelines and specific service requirements, shared TN resources could be provided in the backhaul (or midhaul), and dedicated TN resources could be provided in the midhaul (or backhaul). The capacity partitioning strategy is deployment specific.

There are different components to implement TN slices based upon mechanisms such as Virtual Routing and Forwarding instances (VRFs) for logical separation, Quality of Service (QoS), and Traffic Engineering (TE). Whether all or a subset of these components are enabled is a deployment choice.

3.3. Transport Network Reference Design

Figure 3 depicts the reference design used in this document for modelling the Transport Network based on management perimeters (Customer vs. Provider).

      Customer                 Provider                     Customer
   Orchestration            Orchestration                 Orchestration
      Domain                   Domain                       Domain
+----------------+      +---------------------+       +----------------+
|    Customer    |      |  Provider Network   |       |    Customer    |
|      Site 1    |      |                     |       |      Site 2    |
|          +----+|      |+----+         +----+|       |+----+          |
|+--+      |    ||  AC  ||    |         |    ||  AC   || NF |          |
||NF|......| CE +--------+ PE |         | PE +---------+(CE)|          |
|+--+      |    ||      ||    |         |    ||       ||    |          |
|          +----+|      |+----+         +----+|       |+----+          |
|                |      |                     |       |                |
+----------------+      +---------------------+       +----------------+

     <-----------------Transport Network--------------->
Figure 3: Reference Design with Customer Site and Provider Network

The description of the main components shown in Figure 3 is provided in the following subsections.

3.3.1. Customer Site

On top of 5G NFs, a customer may manage additional TN elements (e.g., servers, routers, and switches) within a customer site.

NFs may be hosted on a CE, directly connected to a CE, or be located multiple IP hops from a CE.

The orchestration of the TN within a customer site involves a set of controllers for automation purposes (e.g., Network Functions Virtualization Infrastructure (NFVI), Container Network Interface (CNI), Fabric Managers, or Public Cloud APIs). It is out of scope to document how these controllers are implemented.

3.3.2. Customer Edge (CE)

A CE is a function that provides logical connectivity of a customer site (Section 3.3.1) to the provider network (Section 3.3.3). The logical connectivity is enforced at Layer 2 and/or Layer 3 and is denominated an Attachment Circuit (AC) (Section 3.3.5). Examples of CEs include TN components (e.g., router, switch, and firewalls) and also 5G NFs (i.e., an element of the 5G domain such as Centralized Unit (CU), Distributed Unit (DU), or User Plane Function (UPF)).

A CE is typically managed by the customer, but it can also be co-managed with the provider. A co-managed CE is orchestrated by both the customer and the provider. In this case, the customer and provider usually have control on distinct device configuration perimeters. A co-managed CE has both PE and CE functions and there is no strict AC connection, although one may consider that the AC stitching logic happens internally within the CE itself. The provider manages the AC between the CE and the PE.

This document generalizes the definition of a CE with the introduction of "Distributed CE"; that is, the logical connectivity is realized by configuring multiple devices in the customer domain. The CE function is distributed. An example of distributed CE is the realization of an interconnection using a L3VPN service based on a distributed CE composed of a switch (Layer 2) and a router (Layer 3) (Figure 4). Another example of distributed CE is shown in Figure 5.

+--------------+                    +--------------+
|   Customer   |                    |   Provider   |
|     Site     |                    |    Network   |
|.................                  |              |
||+-----+ +----+ |               +----+            |
|||     | |    ==================     |            |
|||     +------------AC---------+ PE  |            |
||| RTR | | SW ==================     |            |
||+-----+ +----+ |               +----+            |
|'..Distributed..'                  |              |
|       CE     |                    |              |
+--------------+                    +--------------+
Figure 4: Example of Distributed CE

While in most cases CEs connect to PEs using IP (e.g., VLANs subinterface on a Layer 3 interface), a CE may also connect to the provider network using other technologies such as MPLS -potentially over IP tunnels- or Segment Routing over IPv6 (SRv6) [RFC8986]. The CE has thus awareness of provider services configuration (e.g., control plane identifiers such as Route Targets (RTs) and Route Distinguishers (RDs)). However, the CE is still managed by the customer and the AC is based on MPLS or SRv6 data plane technologies. The complete termination of the AC within the provider network may happen on distinct routers: this is another example of distributed PE. Service-aware CEs are used, for example, in the deployments discussed in Sections 4.3.2 and 4.3.3.

3.3.3. Provider Network

A provider uses a provider network to interconnect customer sites. This document assumes that the provider network is based on IP, MPLS, or both.

3.3.4. Provider Edge (PE)

PE is a device managed by a provider that is connected to a CE. The connectivity between a CE and a PE is achieved using one or multiple ACs (Section 3.3.5).

This document generalizes the PE definition with the introduction of "Distributed PE"; that is, the logical connectivity is realized by configuring multiple devices in the provider network (i.e., provider orchestration domain). The PE function is distributed.

An example of a distributed PE is the "Managed CE service". For example, a provider delivers VPN services using CEs and PEs which are both managed by the provider (case (i) in Figure 5). The managed CE can also be a Data Center Gateway as depicted in the example (ii) of Figure 5. A provider-managed CE may attach to CEs of multiple customers. However, this device is part of the provider network.

+--------------+                    +--------------+
|   Customer   |                    |   Provider   |
|     Site     |                    |    Network   |
|              |                .................  |
|          +----+               |+----+   +----+|  |
|          |    ==================Mngd|   |    ||  |
|          | CE +--------AC------+ CE +---+ PE ||  |
|          |    ==================    |   |    ||  |
|          +----+               |+----+   +----+|  |
|              |                '..Distributed..'  |
|              |                    |  PE          |
+--------------+                    +--------------+
                  (i) Distributed PE

+--------------+                    +--------------+
|   Customer   |                    |   Provider   |
|     Site     |                    |    Network   |
|  ..................           .................. |
|  |    IP Fabric   |           |+----+   +----+ | |
|  |.-----. .-----. ============== DC |   |    | | |
|  |'-----' '-----' +-----AC-----+ GW +---+ PE | | |
|  |.-. .-. .-. .-. ==============    |   |    | | |
|  |'-' '-' '-' '-' |           |+----+   +----+ | |
|  '...Distributed..'           '...Distributed..' |
|          CE  |                    |  PE          |
|              |                    |              |
+--Data Center-+                    +--------------+
              (ii) Distributed PE and CE
Figure 5: Examples of Distributed PE

In subsequent sections of this document, the terms CE and PE are used for both single and distributed devices.

3.3.5. Attachment Circuit (AC)

The AC is the logical connection that attaches a CE (Section 3.3.2) to a PE (Section 3.3.4). A CE is connected to a PE via one or multiple ACs.

This document uses the concept of distributed CE and PE (Sections 3.3.2) and (3.3.4) to consolidate a CE/AC/PE definition that is consistent with the orchestration perimeters (Section 3.4). The CEs and PEs delimit respectively the customer and provider orchestration domains, while an AC interconnects these domains.

For consistency with the AC data models terminology (e.g., [I-D.ietf-opsawg-teas-attachment-circuit] and [I-D.ietf-opsawg-ntw-attachment-circuit]), this document assumes that an AC is configured on a "bearer", which represents the underlying connectivity. For example, the bearer is illustrated with "===" in Figures 4 and 5.

An AC is technology-specific. Examples of ACs are Virtual Local Area Networks (VLANs) (AC) configured on a physical interface (bearer) or an Overlay VXLAN EVI (AC) configured on an IP underlay (bearer).

Deployment cases where the AC is also managed by the provider are not discussed in the document because the setup of such an AC does not require any coordination between the customer and provider orchestration domains.

3.4. Orchestration Overview

3.4.1. 5G End-to-End Slice Orchestration Architecture

This section introduces a global framework for the orchestration of a 5G end-to-end slice (a.k.a. 5G Network Slice) with a zoom on TN parts. This framework helps to delimit the realization scope of RFC 9543 Network Slices and identify interactions that are required for the realization of such slices.

This framework is consistent with the management coordination example shown in Figure 4.7.1 of [TS-28.530].

In reference to Figure 6, a 5G End-to-End Network Slice Orchestrator (5G NSO) is responsible for orchestrating 5G Network Slices end-to-end. The details of the 5G NSO are out of the scope of this document. The realization of the 5G Network Slices spans RAN, CN, and TN. As mentioned in Section 3.1, the RAN and CN are under the responsibility of the 3GPP Management System, while the TN is not. The orchestration of the TN is split into two sub-domains in conformance with the reference design in Section 3.3:

Provider Network Orchestration domain:

As defined in [RFC9543], the provider relies on a Network Slice Controller (NSC) to manage and orchestrate RFC 9543 Network Slices in the provider network. This framework permits to manage connectivity together with SLOs.

Customer Site Orchestration domain:

The Orchestration of TN elements of the customer sites relies upon a variety of controllers (e.g., Fabric Manager, Element Management System, or Virtualized Infrastructure Manager (VIM)).

A TN slice relies upon resources that can involve both the provider and customer TN domains. More details are provided in Section 3.4.2.

A TN slice might be considered as a variant of horizontal composition of Network Slices mentioned in Appendix A.6 of [RFC9543].

                         +-----------+
                         |  5G NSO   |
                         +--+---+----+
                            |   |
                            v   |
              +---------------+ |
              | 3GPP domains  | |
  +-----------+ Orchestration +-|--------------------------+
  |           | (RAN and CN)  | |                          |
  |           +---------------+ |                          |
  |                             |                          |
  |    +------------------------|----------------------+   |
  |    |TN Orchestration        |                      |   |
  |    |        +------------------------------+       |   |
  |    |        |               |              |       |   |
  |    |        v               v              v       |   |
  |    |+---------------++-----------++---------------+|   |
  |    || Customer Site ||RFC9543 NSC|| Customer Site ||   |
  |    || Orchestration ||           || Orchestration ||   |
  |    |+---------------++-----------++---------------+|   |
  |    +---|-------------------|---------------------|-+   |
  |        |                   |                     |     |
  |        |                   |                     |     |
  |        v                   v                     v     |
+-|-----------+         +-----------------+         +------|---+
| |           |         |    Provider     |         |      |   |
| v           |       +----+  Network  +----+      +----+  |   |
|+--+     +----+   AC |    |           |    |  AC  | NF |<-+   |
||NF+.....+ CE +------+ PE |           | PE +------+(CE)|      |
|+--+     +----+      |    |           |    |      +----+      |
|             |       +----+           +----+       |          |
|  Customer   |         |                 |         | Customer |
|    Site     |         |                 |         |   Site   |
+-------------+         +-----------------+         +----------+
                              RFC 9543
                      |-----Network Slice---|

    |--------------------TN Slice-------------------|

Figure 6: 5G End-to-End Slice Orchestration with TN

The various orchestration depicted in Figure 6 encompass the 3GPP's Network Slice Subnet Management Function (NSSMF) mentioned, e.g., in Figure 5 of [I-D.ietf-teas-5g-network-slice-application].

3.4.2. Transport Network Segments and Network Slice Instantiation

This document focuses on deployments where the Service Demarcation Points (SDPs) are located per Types 3 and 4 of Figure 1 of [RFC9543]. The concept of distributed PE (Section 3.3.4) assimilates CE-based SDPs defined in Section 5.2 of [RFC9543] (i.e., Types 1 and 2) as SDP Type 3 or 4 in this document.

In reference to the architecture depicted in Section 3.4.1, the connectivity between NFs can be decomposed into three main segment types that are as follows:

Customer Site:

Either connects NFs located in the same customer site or connects an NF to a CE.

This segment may not be present if the NF is the CE. In this case the AC connects the NF to a PE.

The realization of this segment is driven by the 5G Network Orchestration (e.g., NFs instantiation) and the Customer Site Orchestration for the TN part.

Provider Network:

Represents the connectivity between two PEs. The realization of this segment is controlled by an NSC (Section 6.3 of [RFC9543]).

Attachment Circuit:

The orchestration of this segment relies partially upon an NSC for the configuration of the AC on the PE customer-facing interfaces and the Customer Site Orchestration for the configuration of the AC on the CE.

PEs and CEs that are connected via an AC need to be provisioned with consistent data plane and control plane information (VLAN- IDs, IP addresses/subnets, BGP Autonomous System (AS) Number, etc.). Hence, the realization of this interconnection is technology-specific and requires coordination between the Customer Site Orchestration and an NSC. Automating the provisioning and management of the AC is thus key to automate the overall service provisioning. Aligned with [RFC8969], this document assumes that this coordination is based upon standard YANG data models and APIs.

The provisioning of a Network Slice may rely on new or existing ACs.

Figure 7 is a basic example of a Layer 3 CE-PE link realization with shared network resources (such as VLAN-IDs and IP prefixes) which are passed between Orchestrators via a dedicated interface, e.g., the Network Slice Service Model (NSSM) [I-D.ietf-teas-ietf-network-slice-nbi-yang] or the Attachment Circuit-as-a-Service (ACaaS) [I-D.ietf-opsawg-teas-attachment-circuit].

  +---------------+                   +------------------+
  |               |                   |   RFC9543 NSC    |
  | Customer Site |                   |                  |
  | Orchestration |    IETF APIs/DM   |(Provider Network |
  |               |<----------------->|  Orchestration)  |
  +---------------+                   +------------------+
                |                        |
                |                        |
+---------------|-+                    +-|---------------+
|               v |                    | v               |
| +--+      +--+.1|    192.0.2.0/31    |.0+--+           |
| |NF+......+CE+--------------------------+PE|           |
| +--+      +--+  |      VLAN 100      |  +--+           |
|    Customer     |                    |     Provider    |
|      Site       |                    |     Network     |
+-----------------+                    +-----------------+

               |----------- AC -----------|
Figure 7: Coordination of Transport Network Resources for the AC Provisioning

3.5. Mapping 5G Network Slices to Transport Network Slices

There are multiple options for mapping 5G Network Slices to TN slices:

  • 1 to N: A single 5G Network Slice can be mapped to multiple TN slices (1 to N). For instance, consider the scenario depicted in Figure 8, illustrating the separation of the 5G control plane and user plane in TN slices for a single 5G Enhanced Mobile Broadband (eMBB) network slice. It is important to note that this mapping can serve as an interim step to M to N mapping. Further details about this scheme are described in Section 3.6.

  • M to 1: Multiple 5G Network Slices may rely upon the same TN slice. In such a case, the Service Level Agreement (SLA) differentiation of slices would be entirely controlled at the 5G control plane, for example, with appropriate placement strategies: this use case is represented in Figure 9, where a User Plane Function (UPF) for the Ultra Reliable Low Latency Communication (URLLC) slice is instantiated at the edge cloud close to the gNB Centralized Unit User Plane (CU-UP) for better latency/jitter control, while the 5G control plane and the UPF for eMBB slice are instantiated in the regional cloud.

  • M to N: The 5G to TN slice mapping combines both approaches with a mix of shared and dedicated associations.

    In this scenario, a subset of the TN slices can be intended for sharing by multiple 5G Network Slices (e.g., the control plane TN slice is shared by multiple 5G network Slices).

    In practice, for operational and scaling reasons, typically M to N would be used, with M >> N.

+---------------------------------------------------------------+
|                        5G Slice eMBB                          |
|            +------------------------------------+             |
| +-----+ N3 | +---------------------------------+|  N3 +-----+ |
| |CU-UP+------+ RFC 9543 Network Slice UP_eMBB  +------+ UPF | |
| +-----+    | +---------------------------------+|     +-----+ |
|            |                                    |             |
| +-----+ N2 | +---------------------------------+|  N2 +-----+ |
| |CU-CP+------+    RFC 9543 Network Slice CP    +------+ AMF | |
| +-----+    | +---------------------------------+|     +-----+ |
+------------|------------------------------------|-------------+
             |                                    |
             |           Transport Network        |
             +------------------------------------+
Figure 8: 1 (5G Slice) to N (RFC 9543 Network Slice) Mapping
                  +-------------+
                  |  Edge Cloud |
                  |             |
                  | +---------+ |
                  | |UPF_URLLC| |
                  | +-----+---+ |
                  +-------|-----+
+---------------+ +-------|----------------------+
|   Cell Site   | | +-----+--------------------+ | +--------------+
|               | | |                            | |   Regional   |
| +-----------+ | | |                          | | |     Cloud    |
| |CU-UP_URLLC+-----+                          | | | +-----------+|
| +-----------+ | | |     RFC 9543 Network     +-----+  5GC CP  | |
|               | | |        Slice ALL         | | | +-----------+|
| +-----------+ | | |                          | | |              |
| |CU-UP_eMBB +-----+                          | | | +-----------+
| +-----------+ | | |                          +-----+ UPF_eMBB | |
+---------------+ | |                          | | | +-----------+|
                  | +--------------------------+ | |              |
                  |                              | +--------------+
                  |      Transport Network       |
                  +------------------------------+
Figure 9: N (5G Slice) to 1 (RFC 9543 Network Slice) Mapping

Note that the actual realization of the mapping depends on several factors, such as the actual business cases, the NF vendor capabilities, the NF vendor reference designs, as well as service provider or even legal requirements.

Mapping approaches that preserve the 5G slice identification in the TN (e.g., Section 4.2) may simplify required operations to map back TN slices to 5G slices. However, such considerations are not detailed in this document because these are under the responsibility of the 3GPP orchestration domain.

3.6. First 5G Slice versus Subsequent Slices

An operational 5G Network Slice incorporates both 5G control plane and user plane capabilities. For instance, consider a slice based on split-CU in the RAN, both CU-UP and Centralized Unit Control Plane (CU-CP) need to be deployed along with the associated interfaces E1, F1-c, F1-u, N2, and N3 which are conveyed in the TN. In this regard, the creation of the "first slice" can be subject to a specific logic that does not apply to subsequent slices. Let us consider the example depicted in Figure 10 to illustrate this deploloyment. In this example, the first 5G slice relies on the deployment of NF-CP and NF-UP functions together with two TN slices for control and user planes (INS-CP and INS-UP1). Next, the deployment of a second slice relies solely on the instantiation of a UPF (NF-UP2) together with a dedicated user plane TN slice (INS-UP2). In this example, the control plane of the first 5G slice is also updated to integrate the second slice: the TN slice (INS-CP) and Network Functions (NF-CP) are shared.

At the time of writing (2024), Section 6.1.2 of [NG.113] specifies that the eMBB slice (SST-1 and no Slice Differentiator (SD)) should be supported globally. This 5G slice would be the first slice in any 5G deployment.

+---------------------------------------------------------------+
|                  +------------------------------+             |
|  1    +-----+    | +--------------------------+ |    +-----+  |
|  s S  |NF-CP+------+   CP TN Slice (TNS-CP)   +------+NF-CP|  |
|  t l  +-----+    | +--------------------------+ |    +-----+  |
|    i             |                              |             |
|  5 c  +-----+    | +--------------------------+ |    +-----+  |
|  G e  |NF-UP+------+  UP TN Slice (TNS-UP1)   +------+NF-UP|  |
|       +-----+    | +--------------------------+ |    +-----+  |
+------------------|------------------------------|-------------+
                   |                              |
                   |      Transport Network       |
                   +------------------------------+
                      Deployment of first 5G slice
                                  | |
                                  | |
                                --+ +--
                                 \   /
                                  \ /
+---------------------------------------------------------------+
|                  +------------------------------+             |
|  1    +-----+    | +--------------------------+ |    +-----+  |
|  s S  |NF-CP+------+   CP TN Slice (TNS-CP)   +------+NF-CP|  |
|  t l  +-----+    | +--------------------------+ |    +-----+  |
|    i             |                              |             |
|  5 c  +-----+    | +--------------------------+ |    +-----+  |
|  G e  |NF-UP+------+  UP TN Slice (TNS-UP1)   +------+NF-UP|  |
|       +-----+    | +--------------------------+ |    +-----+  |
+------------------|------------------------------|-------------+
                   |                              |
+------------------|------------------------------|-------------+
|  2               |                              |             |
|  n S  +------+   | +--------------------------+ |   +------+  |
|  d l  |NF-UP2+-----+  UP TN Slice (TNS-UP2)   +-----+NF-UP2|  |
|    i  +------+   | +--------------------------+ |   +------+  |
|  5 c             |                              |             |
|  G e             |                              |             |
+------------------|------------------------------|-------------+
                   |                              |
                   |      Transport Network       |
                   +------------------------------+
    Deployment of additional 5G slice with shared Control Plane
Figure 10: First and Subsequent Slice Deployment

Overall, policies might be provided by an operator (e.g., to Network Slice Controllers) to indicate whether the same or dedicated CP NFs are allowed when processing a new slice creation request. Providing such a policy is meant to better automate the realization of 5G slices and minimize the realization delay that might be induced by extra cycles to seek for operator validation.

3.7. Overview of the Transport Network Realization Model

The realization model described in this document is depicted in Figure 11. The following building blocks are used:

  • L2VPN [RFC4664] and/or L3VPN [RFC4364] service instances for logical separation:

    This realization model of transport for 5G slices assumes Layer 3 delivery for midhaul and backhaul transport connections, and a Layer 2 or Layer 3 delivery for fronthaul connections. Enhanced Common Public Radio Interface (eCPRI) [ECPRI] supports both delivery models. L2VPN/L3VPN service instances might be used as a basic form of logical slice separation. Furthermore, using service instances results in an additional outer header (as packets are encapsulated/decapsulated at the nodes hosting service instances) providing clean discrimination between 5G QoS and TN QoS, as explained in Section 5.

    The use of VPNs for realizing Network Slices is briefly described in Appendix A.4 of [RFC9543].

  • Fine-grained resource control at the PE:

    This is sometimes called 'admission control' or 'traffic conditioning'. The main purpose is the enforcement of the bandwidth contract for the slice right at the edge of the provider network where the traffic is handed-off between the customer site and the provider network.

    The method used here is granular ingress policing (rate limiting) to enforce contracted bandwidths per slice and, potentially, per traffic class within the slice. Traffic above the enforced rate might be immediately dropped, or marked as high drop-probability traffic, which is more likely to be dropped somewhere inside the provider network if congestion occurs. In the egress direction at the PE node, hierarchical schedulers/shapers can be deployed, providing guaranteed rates per slice, as well as guarantees per traffic class within each slice.

    For managed CEs, edge admission control can be distributed between CEs and PEs, where a part of the admission control is implemented on the CE and other part of the admission control is implemented on the PE.

  • Coarse-grained resource control at the transit (non-attachment circuits) links in the provider network, using a single NRP (called "base NRP" in Figure 11), spanning the entire provider network. Transit nodes in the provider network do not maintain any state of individual slices. Instead, only a flat (non-hierarchical) QoS model is used on transit links in the provider network, with up to 8 traffic classes. At the PE, traffic-flows from multiple slice services are mapped to the limited number of traffic classes used on provider network transit links.

  • Capacity planning/management for efficient usage of provider network resources:

    The role of capacity management is to ensure the provider network capacity can be utilized without causing any bottlenecks. The methods used here can range from careful network planning, to ensure a more or less equal traffic distribution (i.e., equal cost load balancing), to advanced TE techniques, with or without bandwidth reservations, to force more consistent load distribution even in non-ECMP friendly network topologies. See also Section 8 of [RFC9522]).

            .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ...
      +-----:----+              Base NRP              +----:-----+
      | PE  :    |                                    |    :  PE |
   ...|.....:....|....................................|....:.....|...
   |  *<---+:    |      RFC 9543 Network Slice 1      |    :+--->*  |
   |  |    |:    |       +-----+        +-----+       |    :|    |  |
   |  *<---+:    |       |  P  |        |  P  |       |    :+--->*  |
    .......+---->o<----->o<--->o<------>o---->o<----->o<----+....|..
   |  *<---+:    |       |     |        |     |       |    :+--->*  |
   |  |    |:    |       +-----+        +-----+       |    :|    |  |
   |  *<---+:    |      RFC 9543 Network Slice 2      |    :+--->*  |
   ....|....:....|....................................|....:.....|...
       |    :    |                                    |    :     |
       +----:----+                                    +----:-----+
            '.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..'
 * SDP, with fine-grained QoS (dedicated resources per Network Slice)
 o Coarse-grained QoS, with resources shared by all Network Slices
Figure 11: Resource Allocation Slicing Model with a Single NRP

P nodes shown in Figure 11 are routers that do no interface with customer devices. See Section 5.3.1 of [RFC4026].

This document does not describe in detail how to manage an L2VPN or L3VPN, as this is already well-documented. For example, the reader may refer to [RFC4176] and [RFC6136] for such details.

4. Hand-off Between Domains

The 5G control plane relies upon 32-bit S-NSSAIs for slice identification. The S-NSSAI is not visible to the transport domain. So instead, 5G network functions can expose the 5G slices to the transport domain by mapping to explicit Layer 2 or Layer 3 identifiers, such as VLAN-IDs, IP addresses, or Differentiated Services Code Point (DSCP) values. These section lists few hand-off methods for slice mapping between customer sites and provider networks.

More details about the mapping between 3GPP and RFC 9543 Network Slices is provided in [I-D.ietf-teas-5g-network-slice-application].

4.1. VLAN Hand-off

In this option, the RFC 9543 Network Slice, fulfilling connectivity requirements between NFs that belong to a 5G slice, is represented at an SDP by a VLAN ID (or double VLAN IDs, commonly known as QinQ), as depicted in Figure 12.

VLANs representing slices           VLANs representing slices

           |     +------------------+     |             |
           |     |                  |     |             |
+------+   v   +-+---+ Provider +---+-+   v   +-----+   v   +------+
|      +-------+*    |          |    *+-------+     +.......+      |
| NF   +-------+* PE |          | PE *+-------+L2/L3+.......+   NF |
|      +-------+*    |          |    *+-------+     +.......+      |
+------+   AC  +-+---+  Network +---+-+   AC  +-----+       +------+
                 |                  |
                 +------------------+

 + Logical interface represented by a VLAN on a physical interface
 * SDP
Figure 12: Example of 5G Slice with VLAN Hand-off Providing End-to-End Connectivity

Each VLAN represents a distinct logical interface on the ACs; hence it provides the possibility to place these logical interfaces in distinct Layer 2 or Layer 3 service instances and implement separation between slices via service instances. Since the 5G interfaces are IP-based interfaces (with an exception of the F2 fronthaul-interface, where eCPRI with Ethernet encapsulation is used), this VLAN is typically not transported across the provider network. Typically, it has only local significance at a particular SDP. For simplification, a deployment may rely on the same VLAN identifier for all ACs. However, that may not be always possible. As such, SDPs for a same slice at different locations may use different VLAN values. Therefore, a VLAN to RFC 9543 Network Slice mapping table is maintained for each AC, and the VLAN allocation is coordinated between customer orchestration and provider orchestration.

While VLAN hand-off is simple for NFs, it adds complexity at the provider network because of the requirement of maintaining mapping tables for each SDP and performing a configuration task for new VLANs and IP subnet for every slice on every AC.

4.2. IP Hand-off

In this option, an explicit mapping between source/destination IP addresses and slice's specific S-NSSAI is used. The mapping can have either local (e.g., pertaining to single NF attachment) or global TN significance. The mapping can be realized in multiple ways, including (but not limited to):

  • S-NSSAI to a dedicated IP address for each NF

  • S-NSSAI to a pool of IP addresses for global TN deployment

  • S-NSSAI to a subset of bits of an IP address

  • S-NSSAI to a DSCP value

  • Use a deterministic algorithm to map S-NSAAI to an IP subnet, prefix, or pools. For example, adaptations to the algorithm defined in [RFC7422] may be considered.

Mapping S-NSSAIs to IP addresses makes IP addresses an identifier for slice-related policy enfocement in the Transport Network (e.g., Differentiated Services, traffic steering, bandwidth allocation, security policies, or monitoring).

One example of the IP hand-off realization is the arrangement, where the slices in the TN domain are instantiated using IP tunnels (e.g., IPsec or GTP-U tunnels) established between NFs, as depicted in Figure 13. The transport for a single 5G slice might be constructed with multiple such tunnels, since a typical 5G slice contains many NFs - especially DUs and CUs. If a shared NF (i.e., an NF that serves multiple slices, for example, a shared DU) is deployed, multiple tunnels from shared NF are established, each tunnel representing a single slice.

                                        Tunnels representing slices
                 +------------------+                   |
                 |                  |                   |
+------+       +--+--+ Provider +---+-+       +-----+   v   +------+
|    o============*================*==========================o    |
| NF   +-------+ PE  |          | PE  +-------+L2/L3+.......+   NF |
|    o============*================*==========================o    |
+------+  AC   +-+---+  Network +---+-+  AC   +-----+       +------+
                 |                  |
                 +------------------+

o Tunnel (IPsec, GTP-U, ...) termination point
* SDP
Figure 13: Example of 5G Slice with IP Hand-off Providing End-to-End Connectivity

As opposed to the VLAN hand-off case (Section 4.1), there is no logical interface representing a slice on the PE, hence all slices are handled within a single service instance. The IP and VLAN hand-offs are not mutually exclusive, but instead could be used concurrently. Since the TN doesn't recognize S-NSSAIs, a mapping table similar to the VLAN Hand-off solution is needed (Section 4.1).

The mapping table can be simplified if, for example, IPv6 addressing is used to address NFs. An IPv6 address is a 128-bit long field, while the S-NSSAI is a 32-bit field: Slice/Service Type (SST): 8 bits, Slice Differentiator (SD): 24 bits. 32 bits, out of 128 bits of the IPv6 address, may be used to encode the S-NSSAI, which makes an IP to Slice mapping table unnecessary.

The S-NSSAI/IPv6 mapping is a local IPv6 address allocation method to NFs not disclosed to on-path nodes. IP forwarding is not altered by this method and is still achieved following BCP 198 [RFC7608]. Concretely, intermediary TN nodes are not required to associate any additional semantic with IPv6 address.

However, operators using such mapping methods should be aware of the implications of any change of S-NSSAI on the IPv6 addressing plans. For example, modifications of the S-NSSAIs in-use will require updating the IP addresses used by NFs involved in the associated slices.

4.2.1. An Example of Local IPv6 Addressing Plan for Network Functions

Different IPv6 address allocation schemes following the above approach may be used, with one example allocation shown in Figure 14.

             NF-specific          Reserved
        (not slice specific)     for S-NSSAI
   <----------------------------><--------->
   +----+----+----+----+----+----+----+----+
   |xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:ttdd:dddd|
   +----+----+----+----+----+----+----+----+
   <------------------128 bits------------->

    tt     - SST (8 bits)
    dddddd - SD (24 bits)
Figure 14: An Example of S-NSSAI Embedded into an IPv6 Address

In reference to Figure 14, the most significant 96 bits of the IPv6 address are unique to the NF, but do not carry any slice-specific information. The S-NSSAI information is embedded in the least significant 32 bits. The 96-bit part of the address may be structured by the provider, for example, on the geographical location or the DC identification. Refer to Section 2.1. of [RFC9099] for a discussion on the benefits of structuring an address plan around both services and geographic locations for more structured security policies in a network.

Figure 15 uses the example from Figure 14 to demonstrate a slicing deployment, where the entire S-NSSAI is embedded into IPv6 addresses used by NFs. Let us consider that "NF-A" has a set of tunnel termination points with unique per-slice IP addresses allocated from 2001:db8:a:0::/96, while "NF-B" uses a set of tunnel termination points with per-slice IP addresses allocated from 2001:db8:b:0::/96. This example shows two slices: "customer A eMBB" (SST-01, SD-00001) and "customer B Massive Internet of Things (MIoT)" (SST-03, SD-00003). For "customer A eMBB" slice, the tunnel IP addresses are auto-derived as the IP addresses {2001:db8:a::100:1, 2001:db8:b::100:1}, where {:0100:0001} is used as the last two octets. "customer B MIoT" slice (SST-3, SD-00003) tunnel uses the IP addresses {2001:db8:a::300:3, 2001:db8:b::300:3} and simply adds {:0300:0003} as the last two octets. Leading zeros are not represented in the resulting IPv6 addresses as per [RFC5952].

 2001:db8:a::/96 (NF-A)                      2001:db8:b::/96 (NF-B)

 2001:db8:a::100:1/128                2001:db8:b::100:1/128
     |                                                        |
     |            + - - - - - - - - +   eMBB (SST=1)          |
     |            |                 |      |                  |
+----v-+       +--+--+ Provider +---+-+    v  +-----+       +-v----+
|    o============*================*==========================o    |
| NF   +-------+ PE  |          | PE  +-------+L2/L3+.......+   NF |
|    o============*================*==========================o    |
+----^-+       +--+--+  Network +---+-+    ^  +-----+       +-^----+
     |            |                 |      |                  |
     |            + - - - - - - - - + MIoT (SST=3)            |
     |                                                        |
 2001:db8:a::300:3/128               2001:db8:b::300:3/128

 o Tunnel (IPsec, GTP-U, etc) termination point
 * SDP
Figure 15: Deployment Example with S-NSSAI Embedded into IPv6 Addresses

4.3. MPLS Label Hand-off

In this option, the service instances representing different slices are created directly on the NF, or within the customer site hosting the NF, and attached to the provider network. Therefore, the packet is encapsulated outside the provider network with MPLS encapsulation or MPLS-in-UDP encapsulation [RFC7510], depending on the capability of the customer site, with the service label depicting the slice.

There are three major methods (based upon Section 10 of [RFC4364]) for interconnecting MPLS services over multiple service domains:

Option A (Section 4.3.1):

VRF-to-VRF connections.

Option B (Section 4.3.2):

redistribution of labeled VPN routes with next-hop change at domain boundaries.

Option C (Section 4.3.3):

redistribution of labeled VPN routes without next-hop change and redistribution of labeled transport routes with next-hop change at domain boundaries.

Figure 16 illustrates the use of service-aware CE (Section 3.3.2) for the deployment discussed in Sections 4.3.2 and 4.3.3.

+--------------+                      +--------------+
|   Customer   |                      |   Provider   |
|     Site     |                      |    Network   |
|              |                      |              |
|              |                      |              |
|              |  <------MP-BGP-----> |              |
|           +--+-+                  +-+--+           |
|           |    |   MPLS-based AC  |    |           |
|           | CE +------------------+ PE |           |
|        +--+----+--+               |    |           |
|        | VRF foo  |               +-+--+           |
+--------+----------+                 +--------------+
Figure 16: Example of MPLS-based Attachment Circuit

4.3.1. Option A

This option is not based on MPLS label hand-off, but VLAN hand-off, described in Section 4.1.

4.3.2. Option B

In this option, L3VPN service instances are instantiated outside the provider network. These L3VPN service instances are instantiated in the customer site which could be, for example, either on the compute that hosts mobile NFs (Figure 17, left hand side) or within the DC/cloud infrastructure itself (e.g., on the top of the rack or leaf switch within cloud IP fabric (Figure 17, right hand side)). On the AC connected to a PE, packets are already MPLS encapsulated (or MPLS-in-UDP/MPLS-in-IP encapsulated, if cloud or compute infrastructure don't support MPLS encapsulation). Therefore, the PE uses neither a VLAN nor an IP address for slice identification at the SDP, but instead uses the MPLS label.

     <------        <------        <------
     BGP VPN        BGP VPN        BGP VPN
       COM=1, L=A"    COM=1, L=A'    COM=1, L=A
       COM=2, L=B"    COM=2, L=B'    COM=2, L=B
       COM=3, L=C"    COM=3, L=C'    COM=3, L=C
     <-------------><------------><------------->
               nhs  nhs      nhs  nhs
                                                        VLANs
service instances                service instances  representing
representing slices              representing slices    slices
      |                                       |         |
+---+ |           +--------------+           +|---------|----------+
|   | |           |     Provider |           ||         |          |
|+--+-v-+       +-+---+       +--+--+      +-+v----+    v  +------+|
||    # |       |*    |       |    *|      |  #<><>x.......x      ||
|| NF # +-------+* PE |       | PE *+------+  #<><>x.......x   NF ||
||    # |   AC  |*    |       |    *|   AC |  #<><>x.......x      ||
|+---+--+       +-+---+       +---+-+      +-+-----+       +------+|
| CS1|            |      Network  |          | L2/L3    CS2        |
+----+            +---------------+          +---------------------+

  x Logical interface represented by a VLAN on a physical interface
  # Service instances (with unique MPLS labels)
  * SDP
Figure 17: Example of MPLS Hand-off with Option B

MPLS labels are allocated dynamically in Option B deployments, where at the domain boundaries service prefixes are reflected with next-hop self, and a new label is dynamically allocated, as visible in Figure 17 (e.g., labels A, A', and A" for the first depicted slice). Therefore, for any slice-specific per-hop behavior at the provider network edge, the PE needs to determine which label represents which slice. In the BGP control plane, when exchanging service prefixes over an AC, each slice might be represented by a unique BGP community, so tracking label assignment to the slice might be possible. For example, in Figure 17, for the slice identified with COM-1, the PE advertises a dynamically allocated label A". Since, based on the community, the label to slice association is known, the PE can use this dynamically allocated label A" to identify incoming packets as belonging to "slice 1" and execute appropriate edge per-hop behavior.

It is worth noting that slice identification in the BGP control plane might be with per-prefix granularity. In the extreme case, each prefix can have different community representing a different slice. Depending on the business requirements, each slice could be represented by a different service instance as outlined in Figure 17. In that case, the route target extended community (Section 4 of [RFC4360]) might be used as slice differentiator. In other deployments, all prefixes (representing different slices) might be handled by a single 'mobile' service instance, and some other BGP attribute (e.g., a standard community [RFC1997]) might be used for slice differentiation. There could be also a deployment option that groups multiple slices together into a single service instance, resulting in a handful of service instances. In any case, fine-grained per-hop behavior at the edge of provider network is possible.

4.3.3. Option C

Option B relies upon exchanging service prefixes between customer sites and the provider network. This may lead to scaling challenges in large scale 5G deployments as the PE node needs to carry all service prefixes. To alleviate this scaling challenge, in Option C, service prefixes are exchanged between customer sites only. In doing so, the provider network is offloaded from carrying, propagating, and programing appropriate forwarding entries for service prefixes.

Option C relies upon exchanging service prefixes via multi-hop BGP sessions between customer sites, without changing the NEXT_HOP BGP attribute. Additionally, IPv4/IPv6 labeled unicast (SAFI-4) host routes, used as NEXT_HOP for service prefixes, are exchanged via direct single-hop BGP sessions between adjacent nodes in a customer site and a provider network, as depicted in Figure 18. As a result, a node in a customer site performs hierarchical next-hop resolution.

     <-------------------------------------------
             BGP VPN
               COM=1, L=A, NEXT_HOP=CS2
               COM=2, L=B, NEXT_HOP=CS2
               COM=3, L=C, NEXT_HOP=CS2
     <------------------------------------------>

      <------        <------        <------
      BGP LU         BGP LU         BGP LU
        CS2, L=X"      CS2, L=X'      CS2, L=X
     <-------------><------------><------------->
                nhs  nhs      nhs  nhs
                                                        VLANs
service instances                service instances  representing
representing slices              representing slices    slices
      |                                       |         |
+---+ |           +--------------+           +|---------|----------+
|   | |           |     Provider |           ||         |          |
|+--+-v-+       +-+---+       +--+--+      +-+v----+    v  +------+|
||    # |       |*    |       |    *|      |  #<><>x.......x      ||
|| NF # +-------+* PE |       | PE *+------+  #<><>x.......x   NF ||
||    # |   AC  |*    |       |    *|   AC |  #<><>x.......x      ||
|+---+--+       +-+---+       +---+-+      +-+-----+       +------+|
| CS1|            |      Network  |          | L2/L3    CS2        |
+----+            +---------------+          +---------------------+

   x Logical interface represented by a VLAN on s physical interface
   # Service instances (with unique MPLS label)
   * SDP
Figure 18: MPLS Hand-off with Option C

This architecture requires an end-to-end Label Switched Path (LSP) leading from a packet's ingress node inside one customer site to its egress inside another customer site, through a provider network. Hence, at the domain (customer site, provider network) boundaries NEXT_HOP attribute for IPv4/IPv6 labeled unicast needs to be modified to "next-hop self" (nhs), which results in new IPv4/IPv6 labeled unicast label allocation. Appropriate label swap forwarding entries for IPv4/IPv6 labeled unicast labels are programmed in the data plane. On the AC there is no additional 'labeled transport' protocol (i.e., no LDP, RSVP, SR, ...).

Packets are transmitted over the AC with the IPv4/IPv6 labeled unicast as the top label, with service label deeper in the label stack. In Option C, the service label is not used for forwarding lookup on the PE. This significantly lowers the scaling pressure on PEs, as PEs need to program forwarding entries only for IPv4/IPv6 labeled unicast host routes, used as NEXT_HOP for service prefixes. Also, since one IPv4/IPv6 labeled unicast host route represent one customer site, regardless of the number of slices in the customer site, the number of forwarding entries on a PE is considerably reduced.

For any slice-specific per-hop behavior at the provider network edge, as described in details in Section 3.7, the PE need to determine which label in the packet represents which slice. This can be achieved, for example, by allocating non-overlapping service label ranges for each slice, and use these ranges for slice identification purposes on PE.

5. QoS Mapping Realization Models

5.1. QoS Layers

The resources are managed via various QoS policies deployed in the network. QoS mapping models to support 5G slicing connectivity implemented over packet switched provider network uses two layers of QoS that are discussed in Section 5.1.

5.1.1. 5G QoS Layer

QoS treatment is indicated in the 5G QoS layer by the 5G QoS Indicator (5QI), as defined in [TS-23.501]. A 5QI is an identifier that is used as a reference to 5G QoS characteristics (e.g., scheduling weights, admission thresholds, queue management thresholds, and link layer protocol configuration) in the RAN domain. Given that 5QI applies to the RAN domain, it is not visible to the provider network. Therefore, if 5QI-aware treatment is desired in the provider network as well, 5G network functions might set DSCP with a value representing 5QI so that differentiated treatment can implemented in the provider network as well. Based on these DSCP values, at SDP of each provider network segment used to construct transport for given 5G slice, very granular QoS enforcement might be implemented.

The exact mapping between 5QI and DSCP is out of scope for this document. Mapping recommendations are documented, e.g., in [I-D.cbs-teas-5qi-to-dscp-mapping].

Each slice service might have flows with multiple 5QIs. 5QIs (or, more precisely, corresponding DSCP values) are visible to the provider network at SDPs (i.e., at the edge of the provider network).

In this document, this layer of QoS is referred to as '5G QoS Class' ('5G QoS' in short) or '5G DSCP'.

5.1.2. TN QoS Layer

Control of the TN resources on provider network transit links, as well as traffic scheduling/prioritization on provider network transit links, is based on a flat (non-hierarchical) QoS model in this Network Slice realization. That is, RFC 9543 Network Slices are assigned dedicated resources (e.g., QoS queues) at the edge of the provider network (at SDPs), while all RFC 9543 Network Slices are sharing resources (sharing QoS queues) on the transit links of the provider network. Typical router hardware can support up to 8 traffic queues per port, therefore the document assumes 8 traffic queues per port support in general.

At this layer, QoS treatment is indicated by a QoS indicator specific to the encapsulation used in the provider network. Such an indicator may be DSCP or MPLS Traffic Class (TC). This layer of QoS is referred to as 'TN QoS Class', or 'TN QoS' for short, in this document.

5.2. QoS Realization Models

While 5QI might be exposed to the provider network via the DSCP value (corresponding to specific 5QI value) set in the IP packet generated by NFs, some 5G deployments might use 5QI in the RAN domain only, without requesting per-5QI differentiated treatment from the provider network. This might be due to an NF limitation (e.g., no capability to set DSCP), or it might simply depend on the overall slicing deployment model. The O-RAN Alliance, for example, defines a phased approach to the slicing, with initial phases utilizing only per-slice, but not per-5QI, differentiated treatment in the TN domain (Annex F of [O-RAN.WG9.XPSAAS]).

Therefore, from a QoS perspective, the 5G slicing connectivity realization defines two high-level realization models for slicing in the TN domain: a 5QI-unaware model and a 5QI- aware model. Both slicing models in the TN domain could be used concurrently within the same 5G slice. For example, the TN segment for 5G midhaul (F1-U interface) might be 5QI-aware, while at the same time the TN segment for 5G backhaul (N3 interface) might follow the 5QI-unaware model.

These models are further elaborated in the following two subsections.

5.2.1. 5QI-unaware Model

In 5QI-unaware mode, the DSCP values in the packets received from NF at SDP are ignored. In the provider network, there is no QoS differentiation at the 5G QoS Class level. The entire RFC 9543 Network Slice is mapped to a single TN QoS Class, and, therefore, to a single QoS queue on the routers in the provider network. With a small number of deployed 5G slices (for example, only two 5G slices: eMBB and MIoT), it is possible to dedicate a separate QoS queue for each slice on transit routers in the provider network. However, with the introduction of private/enterprises slices, as the number of 5G slices (and thus corresponding RFC 9543 Network Slices) increases, a single QoS queue on transit links in the provider network serves multiple slices with similar characteristics. QoS enforcement on transit links is fully coarse-grained (single NRP, sharing resources among all RFC 9543 Network Slices), as displayed in Figure 19.

+------------------------------------------------------------+
+-----------------+         PE                               |
|+ - - - - - - - +|                                          |
||  SDP          ||              +---------------------------+
||  +----------+ ||              |       Transit link        |
||  |     NS 1 +------------+    |+------------------------+ |
||  +----------+ ||         |----->     TN QoS Class 1     | |
|+ - - - - - - - +|         |    |+------------------------+ |
|+ - - - - - - - +|         |    |+------------------------+ |
||  SDP          ||         |    ||     TN QoS Class 2     | |
||  +----------+ ||         |    |+------------------------+ |
|   |     NS 2 +--------+   |    |+------------------------+ |
||  +----------+ ||     |   |    ||     TN QoS Class 3     | |
|+ - - - - - - - +|     |   |    |+------------------------+ |
|+ - - - - - - - +|     |   |    |+------------------------+ |
||  SDP          ||     +--------->     TN QoS Class 4     | |
||  +----------+ ||         |    |+------------------------+ |
||  |     NS 3 +------------+    |+------------------------+ |
||  +----------+ ||     +--------->     TN QoS Class 5     | |
|+ - - - - - - - +|     |        |+------------------------+ |
|+ - - - - - - - +|     |        |+------------------------+ |
||  SDP          ||     |        ||     TN QoS Class 6     | |
||  +----------+ ||     |        |+------------------------+ |
||  |     NS 4 +--------+        |+------------------------+ |
||  +----------+ ||     |        ||     TN QoS Class 7     | |
|+ - - - - - - - +|     |        |+------------------------+ |
|+ - - - - - - - +|     |        |+------------------------+ |
||  SDP          ||     |        ||     TN QoS Class 8     | |
||  +----------+ ||     |        |+------------------------+ |
||  |     NS 5 +--------+        |     Max 8 TN Classes      |
||  +----------+ ||              +---------------------------+
|+ - - - - - - - +|                                          |
+-----------------+                                          |
+------------------------------------------------------------+
Fine-grained QoS enforcement   Coarse-grained QoS enforcement
  (dedicated resources per     (resources shared by multiple
   RFC 9543 Network Slice)       RFC 9543 Network Slices)
Figure 19: Slice to TN QoS Mapping (5QI-unaware Model)

When the IP traffic is handed over at the SDP from the AC to the provider network, the PE encapsulates the traffic into MPLS (if MPLS transport is used in the provider network), or IPv6 - optionally with some additional headers (if SRv6 transport is used in the provider network), and sends out the packets on the provider network transit link.

The original IP header retains the DCSP marking (which is ignored in 5QI-unaware model), while the new header (MPLS or IPv6) carries QoS marking (MPLS Traffic Class bits for MPLS encapsulation, or DSCP for SRv6/IPv6 encapsulation) related to TN Class of Service (CoS). Based on TN CoS marking, per-hop behavior for all RFC 9543 Network Slices is executed on provider network transit links. Provider network transit routers do not evaluate the original IP header for QoS-related decisions. This model is outlined in Figure 20 for MPLS encapsulation, and in Figure 21 for SRv6 encapsulation.

                                 +--------------+
                                 | MPLS Header  |
                                 +-----+-----+  |
                                 |Label|TN TC|  |
+--------------+ - - - - - - - - +-----+-----+--+
|  IP Header   |         |\      |  IP Header   |
|      +-------+         | \     |      +-------+
|      |5G DSCP|---------+  \    |      |5G DSCP|
+------+-------+             \   +------+-------+
|              |              \  |              |
|              |               \ |              |
|              |                 |              |
|   Payload    |               / |   Payload    |
|(GTP-U/IPsec) |              /  |(GTP-U/IPsec) |
|              |             /   |              |
|              |---------+  /    |              |
|              |         | /     |              |
|              |         |/      |              |
+--------------+ - - - - - - - - +--------------+
Figure 20: QoS with MPLS Encapsulation
                                 +--------------+
                                 | IPv6 Header  |
                                 |      +-------+
                                 |      |TN DSCP|
                                 +------+-------+
                                 :   Optional   :
                                 :     IPv6     :
                                 :    Headers   :
+--------------+ - - - - - - - - +-----+-----+--+
|  IP Header   |         |\      |  IP Header   |
|      +-------+         | \     |      +-------+
|      |5G DSCP|---------+  \    |      |5G DSCP|
+------+-------+             \   +------+-------+
|              |              \  |              |
|              |               \ |              |
|              |                 |              |
|   Payload    |               / |   Payload    |
|(GTP-U/IPsec) |              /  |(GTP-U/IPsec) |
|              |             /   |              |
|              |---------+  /    |              |
|              |         | /     |              |
|              |         |/      |              |
+--------------+ - - - - - - - - +--------------+
Figure 21: QoS with IPv6 Encapsulation

From a QoS perspective, both options are similar. However, there is one difference between the two options. The MPLS TC is only 3 bits (8 possible combinations), while DSCP is 6 bits (64 possible combinations). Hence, SRv6 provides more flexibility for TN CoS design, especially in combination with soft policing with in-profile/ out-profile traffic, as discussed in Section 5.2.1.1.

Provider network edge resources are controlled in a granular, fine-grained manner, with dedicated resource allocation for each RFC 9543 Network Slice. The resource control/enforcement happens at each SDP in two directions: inbound and outbound.

5.2.1.1. Inbound Edge Resource Control

The main aspect of inbound provider network edge resource control is per-slice traffic volume enforcement. This kind of enforcement is often called 'admission control' or 'traffic conditioning'. The goal of this inbound enforcement is to ensure that the traffic above the contracted rate is dropped or deprioritized, depending on the business rules, right at the edge of provider network. This, combined with appropriate network capacity planning/management (Section 7) is required to ensure proper isolation between slices in a scalable manner. As a result, traffic of one slice has no influence on the traffic of other slices, even if the slice is misbehaving (e.g., Distributed Denial-of-Service (DDoS) attacks or node/link failures) and generates traffic volumes above the contracted rates.

The slice rates can be characterized with following parameters [I-D.ietf-teas-ietf-network-slice-nbi-yang]:

  • CIR: Committed Information Rate (i.e., guaranteed bandwidth)

  • PIR: Peak Information Rate (i.e., maximum bandwidth)

These parameters define the traffic characteristics of the slice and are part of SLO parameter set provided by the 5G NSO to an NSC. Based on these parameters, the provider network's inbound policy can be implemented using one of following options:

  • 1r2c (single-rate two-color) rate limiter

    This is the most basic rate limiter, described in Section 2.3 of [RFC2475]. It meters at the SDP a traffic stream of given slice and marks its packets as in-profile (below CIR being enforced) or out-of-profile (above CIR being enforced). In-profile packets are accepted and forwarded. Out-of profile packets are either dropped right at the SDP (hard rate limiting), or remarked (with different MPLS TC or DSCP TN markings) to signify 'this packet should be dropped in the first place, if there is a congestion' (soft rate limiting), depending on the business policy of the provider network. In the second case, while packets above CIR are forwarded at the SDP, they are subject to being dropped during any congestion event at any place in the provider network.

  • 2r3c (two-rate three-color) rate limiter

    This was initially defined in [RFC2698], and its improved version in [RFC4115]. In essence, the traffic is assigned to one of the these three categories:

    • Green, for traffic under CIR

    • Yellow, for traffic between CIR and PIR

    • Red, for traffic above PIR

    An inbound 2r3c meter implemented with [RFC4115], compared to [RFC2698], is more 'customer friendly' as it doesn't impose outbound peak-rate shaping requirements on customer edge (CE) devices. 2r3c meters in general give greater flexibility for provider network edge enforcement regarding accepting the traffic (green), de- prioritizing and potentially dropping the traffic on transit during congestion (yellow), or hard dropping the traffic (red).

Inbound provider network edge enforcement model for 5QI-unaware model, where all packets belonging to the slice are treated the same way in the provider network (no 5Q QoS Class differentiation in the provider) is outlined in Figure 22.

            Slice
           policer     +---------+
              |    +---|--+      |
              |    |      |      |
              |    |    S |      |
              |    |    l |      |
              v    |    i |      |
-------------<>----|--> c |      |
                   |    e |  A   |
                   |      |  t   |
                   |    1 |  t   |
                   |      |  a   |
                    ------   c   |
                   |      |  h   |
                   |    S |  m   |
                   |    l |  e   |
                   |    i |  n   |
-------------<>----|--> c |  t   |
                   |    e |      |
                   |      |  C   |
                   |    2 |  i   |
                   |      |  r   |
                    ------   c   |
                   |      |  u   |
                   |    S |  i   |
                   |    l |  t   |
                   |    i |      |
-------------<>----|--> c |      |
                   |    e |      |
                   |      |      |
                   |    3 |      |
                   |      |      |
                   +---|--+      |
                       +---------+
Figure 22: Ingress Slice Admission Control (5QI-unware Model)
5.2.1.2. Outbound Edge Resource Control

While inbound slice admission control at the provider network edge is mandatory in the architecture described in this document, outbound provider network edge resource control might not be required in all use cases. Use cases that specifically call for outbound provider network edge resource control are:

  • Slices use both CIR and PIR parameters, and provider network edge links (ACs) are dimensioned to fulfil the aggregate of slice CIRs. If at any given time, some slices send the traffic above CIR, congestion in outbound direction on the provider network edge link (AC) might happen. Therefore, fine-grained resource control to guarantee at least CIR for each slice is required.

  • Any-to-Any (A2A) connectivity constructs are deployed, again resulting in potential congestion in outbound direction on the provider network edge links, even if only slice CIR parameters are used. This again requires fine-grained resource control per slice in outbound direction at the provider network edge links.

As opposed to inbound provider network edge resource control, typically implemented with rate-limiters/policers, outbound resource control is typically implemented with a weighted/priority queuing, potentially combined with optional shapers (per slice). A detailed analysis of different queuing mechanisms is out of scope for this document, but is provided in [RFC7806].

Figure 23 outlines the outbound provider network edge resource control model for 5QI-unaware slices. Each slice is assigned a single egress queue. The sum of slice CIRs, used as the weight in weighted queueing model, should not exceed the physical capacity of the AC. Slice requests above this limit should be rejected by the NSC, unless an already established slice with lower priority, if such exists, is preempted.

      +---------+        QoS output queues
      |     +---|--+- - - - - - - - - - - - - - - - - - - - - - - - - -
      |     | S    |                            \|/
      |     | l    |                             |
      |     | i    |                             |
      |  A  | c    |                             |  weight-Slice-1-CIR
      |  t  | e  +-|--------------------------+  | shaping-Slice-1-PIR
   ---|--t--|---->                            |  |
      |  a  | 1  +-|--------------------------+ /|\
      |  c   ------ - - - - - - - - - - - - - - - - - - - - - - - - - -
      |  h  | S    |                            \|/
      |  m  | l    |                             |
      |  e  | i    |                             |
      |  n  | c    |                             |  weight-Slice-2-CIR
      |  t  | e  +-|--------------------------+  | shaping-Slice-2-PIR
   ---|-----|---->                            |  |
      |  C  | 2  +-|--------------------------+ /|\
      |  i   ------ - - - - - - - - - - - - - - - - - - - - - - - - - -
      |  r  | S    |                            \|/
      |  c  | l    |                             |
      |  u  | i    |                             |
      |  i  | c    |                             |  weight-Slice-3-CIR
      |  t  | e  +-|--------------------------+  | shaping-Slice-3-PIR
   ---|-----|---->                            |  |
      |     | 3  +-|--------------------------+ /|\
      |     +---|--+- - - - - - - - - - - - - - - - - - - - - - - - - -
      +---------+
Figure 23: Ingress Slice Admission control (5QI-unaware Model)

5.2.2. 5QI-aware Model

In the 5QI-aware model, potentially a large number of 5G QoS Classes, represented via the DSCP set by NFs (the architecture scales to thousands of 5G slices) is mapped (multiplexed) to up to 8 TN QoS Classes used in a provider network transit equipment, as outlined in Figure 24.

  +------------------------------------------------------------+
  +-----------------+        PE                                |
  |+ - - - - - - - +|                                          |
R ||  SDP          ||              +---------------------------+
F ||  +----------+ ||              |       Transit link        |
C ||  |5G DSCP A +---------------+ |+------------------------+ |
9 ||  +----------+ ||            +-->     TN QoS Class 1     | |
5 ||  +----------+ ||            | |+------------------------+ |
4 ||  |5G DSCP B +-----------+   | |+------------------------+ |
3 ||  +----------+ ||        |   | ||     TN QoS Class 2     | |
  ||  +----------+ ||        |   | |+------------------------+ |
N ||  |5G DSCP C +--------+  |   | |+------------------------+ |
S ||  +----------+ ||     |  |   | ||     TN QoS Class 3     | |
  ||  +----------+  |     |  |   | |+------------------------+ |
1 ||  |5G DSCP D +-----+  |  |   | |+------------------------+ |
  ||  +----------+  |  |  |  +------>     TN QoS Class 4     | |
  |+ - - - - - - - +|  |  |  |   | |+------------------------+ |
R |+ - - - - - - - +|  |  |  |   | |+------------------------+ |
F ||  +----------+  |  |  +--------->     TN QoS Class 5     | |
C ||  |5G DSCP A +-----|--|--|---+ |+------------------------+ |
9 ||  +----------+ ||  |  |  |     |+------------------------+ |
5 ||  +----------+ ||  |  |  |     ||     TN QoS Class 6     | |
4 ||  |5G DSCP E +-----|--|--+     |+------------------------+ |
3 ||  +----------+ ||  |  |        |+------------------------+ |
  ||  +----------+ ||  |  |        ||     TN QoS Class 7     | |
N ||  |5G DSCP F +-----|--+        |+------------------------+ |
S ||  +----------+ ||  |           |+------------------------+ |
  ||  +----------+ ||  +------------>     TN QoS Class 8     | |
2 ||  |5G DSCP G +-----+           |+------------------------+ |
  ||  +----------+ ||              |     Max 8 TN Classes      |
  ||  SDP          ||              +---------------------------+
  |+ - - - - - - - +|                                          |
  +-----------------+                                          |
  +------------------------------------------------------------+
  Fine-grained QoS enforcement   Coarse-grained QoS enforcement
    (dedicated resources per     (resources shared by multiple
     RFC 9543 Network Slice)        RFC 9543 Network Slices)
Figure 24: Slice 5Q QoS to TN QoS Mapping (5QI-aware Model)

Given that in deployments with a large number of 5G slices, the number of potential 5G QoS Classes is much higher than the number of TN QoS Classes, multiple 5G QoS Classes with similar characteristics - potentially from different slices - would be grouped with common operator-defined TN logic and mapped to a same TN QoS Class when transported in the provider network. That is, common Per-hop Behavior (PHB) [RFC2474] is executed on transit provider network routers for all packets grouped together. An example of this approach is outlined in Figure 25. A provider may decide to implement Diffserv-Intercon PHBs at the boundaries of its network domain [RFC8100].

Note:

The numbers indicated in Figure 25 (S-NSSAI, 5QI, DSCP, queue, etc.) are provided for illustration purposes only and should not be considered as deployment guidance.

                      +-------------  PE  -----------------+
+------ NF-A ------+  |                                    |
|                  |  | + - - - - +                        |
| 3GPP S-NSSAI 100 |  | |   SDP   |                        |
|.------. .-------.|  | |.-------.|                        |
||5QI=1 +->DSCP=46+------>DSCP=46+---+                     |
|'------' '-------'|  | |'-------'|  |                     |
|.------. .-------.|  | |.-------.|  |                     |
||5QI=65+->DSCP=46+------>DSCP=46+|--+                     |
|'------' '-------'|  | |'-------'|  |                     |
|.------. .-------.|  | |.-------.|  |                     |
||5QI=7 +->DSCP=10+------>DSCP=10------+  .--------------. |
|'------' '-------'|  | |'-------'|  | |  |TN QoS Class 5| |
+------------------+  | +- - - - -+  +-|-->   Queue 5    | |
                      |              | |  '--------------' |
+------ NF-B ------+  |              | |                   |
|                  |  | + - - - - +  | |                   |
| 3GPP S-NSSAI 200 |  | |   SDP   |  | |                   |
|.------. .-------.|  | |.-------.|  | |                   |
||5QI=1 +->DSCP=46+------>DSCP=46+---+ |  .--------------. |
|'------' '-------'|  | |'-------'|  | |  |TN QoS Class 1| |
|.------. .-------.|  | |.-------.|  | +-->   Queue 1    | |
||5QI=65+->DSCP=46+------>DSCP=46+|--+ |  '--------------' |
|'------' '-------'|  | |'-------'|    |                   |
|.------. .-------.|  | |.-------.|    |                   |
||5QI=7 +->DSCP=10+------>DSCP=10+-----+                   |
|'------' '-------'|  | |'-------'|                        |
+------------------+  | +- - - - -+                        |
                      +------------------------------------+
Figure 25: Example of 3GPP QoS Mapped to TN QoS

In current SDO progress of 3GPP (Release 17) and O-RAN, the mapping of 5QI to DSCP is not expected to be in a per-slice fashion, where 5QI to DSCP mapping may vary from 3GPP slice to 3GPP slice, hence the mapping of 5G QoS DSCP values to TN QoS Classes may be rather common.

Like in the 5QI-unaware model, the original IP header retains the DCSP marking corresponding to 5QI (5G QoS Class), while the new header (MPLS or IPv6) carries QoS marking related to TN QoS Class. Based on TN QoS Class marking, per-hop behavior for all aggregated 5G QoS Classes from all RFC 9543 Network Slices is executed on the provider network transit links. Provider network transit routers do not evaluate the original IP header for QoS related decisions. The original DSCP marking retained in the original IP header is used at the PE for fine-grained per slice and per 5G QoS Class inbound/outbound enforcement on the AC.

In the 5QI-aware model, compared to the 5QI-unware model, provider network edge resources are controlled in an even more granular, fine-grained manner, with dedicated resource allocation for each RFC 9543 Network Slice and dedicated resource allocation for number of traffic classes (most commonly up 4 or 8 traffic classes, depending on the Hardware capability of the equipment) within each RFC 9543 Network Slice.

5.2.2.1. Inbound Edge Resource Control

Compared to the 5QI-unware model, admission control (traffic conditioning) in the 5QI-aware model is more granular, as it enforces not only per slice capacity constraints, but may as well enforce the constraints per 5G QoS Class within each slice.

A 5G slice using multiple 5QIs can potentially specify rates in one of the following ways:

  • Rates per traffic class (CIR or CIR+PIR), no rate per slice (sum of rates per class gives the rate per slice).

  • Rate per slice (CIR or CIR+PIR), and rates per prioritized (premium) traffic classes (CIR only). Best effort traffic class uses the bandwidth (within slice CIR/PIR) not consumed by prioritized classes.

In the first option, the slice admission control is executed with traffic class granularity, as outlined in Figure 26. In this model, if a premium class doesn't consume all available class capacity, it cannot be reused by non-premium (i.e., Best Effort) class.

                     Class             +---------+
                    policer         +--|---+     |
                                    |      |     |
5Q-QoS-A: CIR-1A ------<>-----------|--> S |     |
5Q-QoS-B: CIR-1B ------<>-----------|--> l |     |
5Q-QoS-C: CIR-1C ------<>-----------|--> i |     |
                                    |    c |     |
                                    |    e |     |
   BE CIR/PIR-1D ------<>-----------|-->   |  A  |
                                    |    1 |  t  |
                                    |      |  t  |
                                     ------   a  |
                                    |      |  c  |
5Q-QoS-A: CIR-2A ------<>-----------|->  S |  h  |
5Q-QoS-B: CIR-2B ------<>-----------|->  l |  m  |
5Q-QoS-C: CIR-2C ------<>-----------|->  i |  e  |
                                    |    c |  n  |
                                    |    e |  t  |
   BE CIR/PIR-2D ------<>-----------|->    |     |
                                    |    2 |  C  |
                                    |      |  i  |
                                     ------   r  |
                                    |      |  c  |
5Q-QoS-A: CIR-3A ------<>-----------|->  S |  u  |
5Q-QoS-B: CIR-3B ------<>-----------|->  l |  i  |
5Q-QoS-C: CIR-3C ------<>-----------|->  i |  t  |
                                    |    c |     |
                                    |    e |     |
   BE CIR/PIR-3D-------<>-----------|->    |     |
                                    |    3 |     |
                                    |      |     |
                                    +--|---+     |
                                       +---------+
Figure 26: Ingress Slice Admission Control (5QI-aware Model)

The second model combines the advantages of 5QI-unaware model (per slice admission control) with the per traffic class admission control, as outlined in Figure 26. Ingress admission control is at class granularity for premium classes (CIR only). Non-premium class (i.e., Best Effort) has no separate class admission control policy, but it is allowed to use the entire slice capacity, which is available at any given moment. I.e., slice capacity, which is not consumed by premium classes. It is a hierarchical model, as depicted in Figure 27.

                              Slice
                             policer   +---------+
                   Class        .   +--|---+     |
                  policer      ; :  |      |     |
5Q-QoS-A: CIR-1A ----<>--------|-|--|--> S |     |
5Q-QoS-B: CIR-1B ----<>--------|-|--|--> l |     |
5Q-QoS-C: CIR-1C ----<>--------|-|--|--> i |     |
                               | |  |    c |     |
                               | |  |    e |     |
   BE CIR/PIR-1D --------------|-|--|-->   |  A  |
                               | |  |    1 |  t  |
                               : ;  |      |  t  |
                                .    ------   a  |
                               ; :  |      |  c  |
5Q-QoS-A: CIR-2A ----<>--------|-|--|--> S |  h  |
5Q-QoS-B: CIR-2B ----<>--------|-|--|--> l |  m  |
5Q-QoS-C: CIR-2C ----<>--------|-|--|--> i |  e  |
                               | |  |    c |  n  |
                               | |  |    e |  t  |
   BE CIR/PIR-2D --------------|-|--|-->   |     |
                               | |  |    2 |  C  |
                               : ;  |      |  i  |
                                .    ------   r  |
                               ; :  |      |  c  |
5Q-QoS-A: CIR-3A ----<>--------|-|--|--> S |  u  |
5Q-QoS-B: CIR-3B ----<>--------|-|--|--> l |  i  |
5Q-QoS-C: CIR-3C ----<>---- ---|-|--|--> i |  t  |
                               | |  |    c |     |
                               | |  |    e |     |
   BE CIR/PIR-3D --------------|-|--|-->   |     |
                               | |  |    3 |     |
                               : ;  |      |     |
                                '   +--|---+     |
                                       +---------+
Figure 27: Ingress Slice Admission Control (5QI-aware) - Hierarchical
5.2.2.2. Outbound Edge Resource Control

Figure 28 outlines the outbound edge resource control model at the transport network layer for 5QI-aware slices. Each slice is assigned multiple egress queues. The sum of queue weights, which are 5Q QoS queue CIRs within the slice, should not exceed the CIR of the slice itself. And, similarly to the 5QI-aware model, the sum of slice CIRs should not exceed the physical capacity of the AC.

   +---------+        QoS output queues
   |      ------ - - - - - - - - - - - - - - - - - - - - - - - - - -
   |     |   |.-|--------------------------. \|/
---|-----|----> 5Q-QoS-A: w-5Q-QoS-A-CIR   |  |
   |     | S |'-|--------------------------'  |
   |     | l |.-|--------------------------.  |
---|-----|-i--> 5Q-QoS-B: w-5Q-QoS-B-CIR   |  |
   |     | c |'-|--------------------------'  |  weight-Slice-1-CIR
   |     | e |.-|--------------------------.  | shaping-Slice-1-PIR
---|-----|----> 5Q-QoS-C: w-5Q-QoS-C-CIR   |  |
   |     | 1 |'-|--------------------------'  |
   |     |   |.-|--------------------------.  |
---|-----|----> Best Effort (remainder)    |  |
   |     |   |'-|--------------------------' /|\
   |  A   ------ - - - - - - - - - - - - - - - - - - - - - - - - - -
   |  t  |   |.-|--------------------------. \|/
   |  t  |   ||                            |  |
   |  a  |   |'-|--------------------------'  |
   |  c  | S |.-|--------------------------.  |
   |  h  | l ||                            |  |
   |  m  | i |'-|--------------------------'  |  weight-Slice-2-CIR
   |  e  | c |.-|--------------------------.  | shaping-Slice-2-PIR
   |  n  | e ||                            |  |
   |  t  |   |'-|--------------------------'  |
   |     | 2 |.-|--------------------------.  |
   |  C  |   ||                            |  |
   |  i  |   |'-|--------------------------' /|\
   |  r   ------ - - - - - - - - - - - - - - - - - - - - - - - - - -
   |  c  |   |.-|--------------------------. \|/
   |  u  |   ||                            |  |
   |  i  | S |'-|--------------------------'  |
   |  t  | l |.-|--------------------------.  |
   |     | i ||                            |  |
   |     | c |'-|--------------------------'  |  weight-Slice-3-CIR
   |     | e |.-|--------------------------+  | shaping-Slice-3-PIR
   |     |   ||                            |  |
   |     | 3 |'-|--------------------------'  |
   |     |   |.-|--------------------------.  |
   |     |   ||                            |  |
   |     |   |'-|--------------------------' /|\
   |      ------ - - - - - - - - - - - - - - - - - - - - - - - - - -
   +---------+
Figure 28: Egress Slice Admission Control (5QI-aware)

5.3. Transit Resource Control

Transit resource control is much simpler than Edge resource control in the provider network. As outlined in Figure 24, at the provider network edge, 5Q QoS Class marking (represented by DSCP related to 5QI set by mobile network functions in the packets handed off to the TN) is mapped to the TN QoS Class. Based on TN QoS Class, when the packet is encapsulated with outer header (MPLS or IPv6), TN QoS Class marking (MPLS TC or IPv6 DSCP in outer header, as depicted in Figures 20 and 21) is set in the outer header. PHB in provider network transit routers is based exclusively on that TN QoS Class marking, i.e., original 5G QoS Class DSCP is not taken into consideration on transit.

Provider network transit resource control does not use any inbound interface policy, but only outbound interface policy, which is based on priority queue combined with weighted or deficit queuing model, without any shaper. The main purpose of transit resource control is to ensure that during network congestion events, for example caused by network failures and temporary rerouting, premium classes are prioritized, and any drops only occur in traffic that was de-prioritized by ingress admission control Section 5.2.1.1 or in non-premium (best-effort) classes. Capacity planning and management, as described in Section 7, ensures that enough capacity is available to fulfill all approved slice requests.

6. Inter-PE Transfer Plane Mapping Models

An inter-PE transfer plane (or "transfer plane") refers to a specific path forwarding behavior between PEs in order to provide packet delivery that is consistent with the corresponding SLOs. This realization step focuses on controlling the paths that will be used for packet delivery between PEs, independent of the underlying network resource partitioning.

It is worth noting that TN QoS Classes and inter-PE transfer planes are orthogonal. The TN domain can be operated with, e.g., 8 TN QoS Classes (representing 8 hardware queues in the routers), and 2 transfer planes (e.g., latency optimized transfer plane using link latency metrics for path calculation, and transfer plane following Interior Gateway Protocol (IGP) metrics). TN QoS Class determines the per-hop behavior when the packets are transiting through the provider network, while transfer plane determines the paths for packets through provider network based on the operator's requirements. This path can be optimized or constrained.

A network operator can define multiple inter-PE transfer planes within a single NRP. A transfer plane may be realized in multiple ways such as (but not limited to):

  • A mesh of RSVP-TE [RFC3209] or SR-TE [RFC9256] tunnels created with specific optimization criteria and constraints. For example, mesh "A" might represent tunnels optimized for latency, and mesh "B" might represent tunnels optimized for high capacity.

  • A Flex-Algorithm [RFC9350] with a particular metric-type (e.g., latency), or one that only uses links with particular properties (e.g., MACsec link [IEEE802.1AE]), or excludes links that are within a particular geography.

These protocols can be controlled, e.g., by tuning the protocol list under the "underlay-transport" data node defined in the L3VPN Network Model (L3NM) [RFC9182] and the L2VPN Network Model (L2NM) [RFC9291].

Also, inter-PE transfer planes may be realized using separate NRPs. However, such an approach is left out of the scope given the current state of the technology (2024).

Similar to the QoS mapping models discussed in Section 5, for mapping to transfer planes at the ingress PE, both 5QI-unaware and 5QI-aware models are defined. Essentially, entire slices can be mapped to transfer planes without 5G QoS consideration (5QI-unaware model). For example, flows with different 5G QoS Classes, even from same slice, can be mapped to different transfer planes (5QI-aware model).

Figure 29 depicts an example of a simple network with two transfer planes, each using a mesh of TE tunnels with or without Path Computation Element (PCE) [RFC5440], and with or without bandwidth reservations. Section 7 discusses in detail different bandwidth models that can be deployed in the provider network. However, discussion about how to realize or orchestrate inter-PE transfer planes is out of scope for this document.

+---------------+                                    +------+
|  Ingress PE   |   .------------------------------->| PE-A |
|               |   |   .-------------------------->>|      |
|  +---------+  |   |   '---------------------.      +------+
|  |         x------'   .---------------------'
|  |Transfer x--------------------------------.      +------+
|  | Plane A x-------------.                  '----->| PE-B |
|  |         x-------.  |  |  .---.   .---.   .---->>|      |
|  +---------+  |    |  |  |  |   |   |   |   |      +------+
|               |    |  |  |  |   '---'   '---'
|  +---------+  |    |  |  |  |                      +------+
|  |         o-------|--'  '------------------------>| PE-C |
|  |Transfer o-------|--------'               .---->>|      |
|  | Plane B o-------|-----------------.      |      +------+
|  |         o-----. '---------------. |      |
|  +---------+  |  | .-. .-. .-. .-. | '------'      +------+
|               |  | | | | | | | | | '-------------->| PE-D |
+---------------+  '-' '-' '-' '-' '--------------->>|      |
                                                     +------+
 x----->   Tunnels of Inter-PE Transfer Plane A
 o---->>   Tunnels of Inter-PE Transfer Plane B
Figure 29: Example of Inter-PE Transfer Planes Relying on TE Tunnels

For illustration purposes, Figure 29 shows only single tunnels per transfer plane for (ingress PE, egress PE) pair. However, there might be multiple tunnels within a single transfer plane between any pair of PEs.

6.1. 5QI-unaware Model

As discussed in Section 5.2.1, in the 5QI-unware model, the provider network doesn't take into account 5G QoS during execution of per-hop behavior. The entire slice is mapped to single TN QoS Class, therefore the entire slice is subject to the same per-hop behavior. Similarly, in 5QI-unaware inter-PE transfer plane mapping model, the entire slice is mapped to a single transfer plane, as depicted in Figure 30.

   +-----------------------------------------+
   |.. .. .. .. .. ..                        |
   :        AC       :      PE               |
   :+---------------+:                       |
   :|  SDP          |:                       |
   :|  +----------+ |:                       |
   :|  |     NS 1 +----------+               |
   :|  +----------+ |:       |               |
   :+---------------+:       |               |
   :+---------------+:       |   +---------+ |
   :|  SDP          |:       |   |         | |
   :|  +----------+ |:       |   |Transfer | |
   :|  |     NS 2 +------+   +--->  Plane  | |
   :|  +----------+ |:   |   |   |    A    | |
   :+---------------+:   |   |   |         | |
   :+---------------+:   |   |   +---------+ |
   :|  SDP          |:   |   |               |
   :|  +----------+ |:   |   |               |
   :|   |     NS 3 +-----+   |               |
   :|  +----------+ |:   |   |   +---------+ |
   :+---------------+:   |   |   |         | |
   :+---------------+:   |   |   |Transfer | |
   :|  SDP          |:   +------->  Plane  | |
   :|  +----------+ |:   |   |   |    B    | |
   :|  |     NS 4 +------+   |   |         | |
   :|  +----------+ |:       |   +---------+ |
   :+---------------+:       |               |
   :+---------------+:       |               |
   :|  SDP          |:       |               |
   :|  +----------+ |:       |               |
   :|  |     NS 5 +----------+               |
   :|  +----------+ |:                       |
   :+---------------+:                       |
   '.. .. .. .. .. ..                        |
   +-----------------------------------------+
Figure 30: Network Slice to Inter-PE Transfer Plane Mapping (5QI-unaware Model)

6.2. 5QI-aware Model

In 5QI-aware model, the traffic can be mapped to transfer planes at the granularity of 5G QoS Class. Given that the potential number of inter-PE transfer planes is limited, packets from multiple 5G QoS Classes with similar characteristics are mapped to a common transfer plane, as depicted in Figure 31.

     +-------------------------------------------+
     |.. .. .. .. .. ..                          |
     :        AC       :      PE                 |
     :+---------------+:                         |
   R :|  SDP          |:                         |
   F :|  +----------+ |:                         |
   C :|  | 5G QoS A +------+                     |
   9 :|  +----------+ |:   |                     |
   5 :|  +----------+ |:   |                     |
   4 :|  | 5G QoS B +------+                     |
   3 :|  +----------+ |:   |         +---------+ |
     :|  +----------+ |:   |         |         | |
   N :|  | 5G QoS C +-----------+    |Transfer | |
   S :|  +----------+ |:   +--------->  Plane  | |
     :|  +----------+ |:   |    |    |    A    | |
   1 :|  | 5G QoS D +-----------+    |         | |
     :|  +----------+ |:   |    |    +---------+ |
     :+---------------+:   |    |                |
   R :+---------------+:   |    |                |
   F :|  +----------+ |:   |    |                |
   C :|  | 5G QoS A +------+    |    +---------+ |
   9 :|  +----------+ |:   |    |    |         | |
   5 :|  +----------+ |:   |    |    |Transfer | |
   4 :|  | 5G QoS E +------+    +---->  Plane  | |
   3 :|  +----------+ |:        |    |    B    | |
     :|  +----------+ |:        |    |         | |
   N :|  | 5G QoS F +-----------+    +---------+ |
   S :|  +----------+ |:        |                |
     :|  +----------+ |:        |                |
   2 :|  | 5G QoS G +-----------+                |
     :|  +----------+ |:                         |
     :|  SDP          |:                         |
     :+---------------+:                         |
     '.. .. .. .. .. ..                          |
     +-------------------------------------------+
Figure 31: Network Slice to Inter-PE Transfer Plane mapping (5QI-aware Model)

7. Capacity Planning/Management

7.1. Bandwidth Requirements

This section describes the information conveyed by the 5G NSO to the NSC with respect to slice bandwidth requirements.

Figure 32 shows three DCs that contain instances of network functions. Also shown are PEs that have links to the DCs. The PEs belong to the provider network. Other details of the provider network, such as P-routers and transit links are not shown. Also details of the DC infrastructure in customer sites, such as switches and routers, are not shown.

The 5G NSO is aware of the existence of the network functions and their locations. However, it is not aware of the details of the provider network. The NSC has the opposite view - it is aware of the provider network infrastructure and the links between the PEs and the DCs, but is not aware of the individual network functions at customer sites.

+ - - - - DC 1- - - -+   + - - - - - - - - +   + - - - - DC 2- - - -+
| +------+           |  +----+         +----+  |           +------+ |
| | NF1A |           +--*PE1A|         |PE2A*--+           | NF2A | |
| +------+           |  +----+         +----+  |           +------+ |
| +------+           |   |                 |   |           +------+ |
| | NF1B |           |   |                 |   |           | NF2B | |
| +------+           |   |                 |   |           +------+ |
| +------+           |  +----+         +----+  |           +------+ |
| | NF1C |           +--*PE1B|         |PE2B*--+           | NF2C | |
| +------+           |  +----+         +----+  |           +------+ |
+ - - - - - - - - - -+   |    Provider     |   + - - - - - - - - - -+
                         |                 |
                         |     Network     |   + - - - - DC 3- - - -+
                         |             +----+  |           +------+ |
                         |             |PE3A*--+           | NF3A | |
                         |             +----+  |           +------+ |
                         |                 |   |           +------+ |
                         |                 |   |           | NF3B | |
                         |                 |   |           +------+ |
                         |             +----+  |           +------+ |
                         |             |PE3B*--+           | NF3C | |
                         |             +----+  |           +------+ |
                         + - - - - - - - - +   + - - - - - - - - - -+

  * SDP, with fine-grained QoS (dedicated resources per RFC 9543 NS)
Figure 32: An Example of Multi-DC Architecture

Let us consider 5G slice "X" that uses some of the network functions in the three DCs. If this slice has latency requirements, the 5G NSO will have taken those into account when deciding which NF instances in which DC are to be invoked for this slice. As a result of such a placement decision, the three DCs shown are involved in 5G slice "X", rather than other DCs. For its decision-making, the 5G NSO needs information from the NSC about the observed latency between DCs. Preferably, the NSC would present the topology in an abstracted form, consisting of point-to-point abstracted links between pairs of DCs and associated latency and, optionally, delay variation and link loss values. It would be valuable to have a mechanism for the 5G NSO to inform the NSC which DC-pairs are of interest for these metrics - there may be of order thousands of DCs, but the 5G NSO will only be interested in these metrics for a small fraction of all the possible DC-pairs, i.e. those in the same region of the provider network. The mechanism for conveying the information is out of scope for this document.

Table 1 shows the matrix of bandwidth demands for 5G slice "X". Within the slice, multiple NF instances might be sending traffic from DCi to DCj. However, the 5G NSO sums the associated demands into one value. For example, "NF1A" and "NF1B" in "DC1" might be sending traffic to multiple NFs in "DC2", but this is expressed as one value in the traffic matrix: the total bandwidth required for 5G slice "X" from "DC1" to "DC2" (8 units). Each row in the right-most column in the traffic matrix shows the total amount of traffic going from a given DC into the transport network, regardless of the destination DC. Note that this number can be less than the sum of DC-to-DC demands in the same row, on the basis that not all the NFs are likely to be sending at their maximum rate simultaneously. For example, the total traffic from "DC1" for slice "X" is 11 units, which is less than the sum of the DC-to-DC demands in the same row (13 units). Note, as described in Section 5, a slice may have per-QoS class bandwidth requirements, and may have CIR and PIR limits. This is not included in the example, but the same principles apply in such cases.

Table 1: Inter-DC Traffic Demand Matrix (Slice X)
From/To DC 1 DC 2 DC 3 Total from DC
DC 1 n/a 8 5 11.0
DC 2 1 n/a 2 2.5
DC 3 4 7 n/a 10.0

[I-D.ietf-teas-ietf-network-slice-nbi-yang] can be used to convey all of the information in the traffic matrix to an NSC. The NSC applies policers corresponding to the last column in the traffic matrix to the appropriate PE routers, in order to enforce the bandwidth contract. For example, it applies a policer of 11 units to PE1A and PE1B that face DC1, as this is the total bandwidth that DC1 sends into the provider network corresponding to Slice X. Also, the controller may apply shapers in the direction from the TN to the DC, if otherwise there is the possibility of a link in the DC being oversubscribed. Note that a peer NF endpoint of an AC can be identified using 'peer-sap-id' as defined in [RFC9408].

Depending on the bandwidth model used in the provider network (Section 7.2), the other values in the matrix, i.e., the DC-to-DC demands, may not be directly applied to the provider network. Even so, the information may be useful to the NSC for capacity planning and failure simulation purposes. If, on the other hand, the DC-to-DC demand information is not used by the NSC, the IETF YANG Data Model for L3VPN Service Delivery [RFC8299] or the IETF YANG Data Model for L2VPN Service Delivery [RFC8466] could be used instead of [I-D.ietf-teas-ietf-network-slice-nbi-yang], as they support conveying the bandwidth information in the right-most column of the traffic matrix.

The provider network may be implemented in such a way that it has various types of paths, for example low-latency traffic might be mapped onto a different transport path to other traffic (for example a particular Flex-Algorithm, a particular set of TE paths, or a specific queue [RFC9330]), as discussed in Section 5. The 5G NSO can use [I-D.ietf-teas-ietf-network-slice-nbi-yang] to request low-latency transport for a given slice if required. However, [RFC8299] or [RFC8466] do not support requesting a particular transport-type, e.g., low-latency. One option is to augment these models to convey this information. This can be achieved by reusing the 'underlay- transport' construct defined in [RFC9182] and [RFC9291].

7.2. Bandwidth Models

This section describes three bandwidth management schemes that could be employed in the provider network. Many variations are possible, but each example describes the salient points of the corresponding scheme. Schemes 2 and 3 use TE; other variations on TE are possible as described in [RFC9522].

7.2.1. Scheme 1: Shortest Path Forwarding (SPF)

Shortest path forwarding is used according to the IGP metric. Given that some slices are likely to have latency SLOs, the IGP metric on each link can be set to be in proportion to the latency of the link. In this way, all traffic follows the minimum latency path between endpoints.

In Scheme 1, although the operator provides bandwidth guarantees to the slice customers, there is no explicit end-to-end underpinning of the bandwidth SLO, in the form of bandwidth reservations across the provider network. Rather, the expected performance is achieved via capacity planning, based on traffic growth trends and anticipated future demands, in order to ensure that network links are not over- subscribed. This scheme is analogous to that used in many existing business VPN deployments, in that bandwidth guarantees are provided to the customers but are not explicitly underpinned end to end across the provider network.

A variation on the scheme is that Flex-Algorithm [RFC9350] is used. For example, one Flex-Algorithm could use latency-based metrics and another Flex-Algorithm could use the IGP metric. There would be a many-to-one mapping of Network Slices to Flex-Algorithms.

While Scheme 1 is technically feasible, it is vulnerable to unexpected changes in traffic patterns and/or network element failures resulting in congestion. This is because, unlike Schemes 2 and 3 which employ TE, traffic cannot be diverted from the shortest path.

7.2.2. Scheme 2: TE Paths with Fixed Bandwidth Reservations

Scheme 2 uses RSVP-TE [RFC3209] or SR-TE paths [RFC9256] with fixed bandwidth reservations. By "fixed", we mean a value that stays constant over time, unless the 5G NSO communicates a change in slice bandwidth requirements, due to the creation or modification of a slice. Note that the "reservations" would be in the mind of the transport controller - it is not necessary (or indeed possible for SR-TE) to reserve bandwidth at the network layer. The bandwidth requirement acts as a constraint whenever the controller (re)computes a path. There could be a single mesh of paths between endpoints that carry all of the traffic types, or there could be a small handful of meshes, for example one mesh for low-latency traffic that follows the minimum latency path and another mesh for the other traffic that follows the minimum IGP metric path, as described in Section 5. There would be a many-to-one mapping of slices to paths.

The bandwidth requirement from DCi to DCj is the sum of the DCi-DCj demands of the individual slices. For example, if only slices "X" and "Y" are present, then the bandwidth requirement from "DC1" to "DC2" is 12 units (8 units for slice "X" (Table 1) and 4 units for slice "Y" (Table 2)). When the 5G NSO requests a new slice, the NSC, in its mind, increments the bandwidth requirement according to the requirements of the new slice. For example, in Figure 32, suppose a new slice is instantiated that needs 0.8 Gbps from "DC1" to "DC2". The transport controller would increase its notion of the bandwidth requirement from "DC1" to "DC2" from 12 Gbps to 12.8 Gbps to accommodate the additional expected traffic.

Table 2: Inter-DC Traffic Demand Matrix (Slice Y)
From/To DC 1 DC 2 DC 3 Total from DC
DC 1 n/a 4 2.5 6.0
DC 2 0.5 n/a 0.8 1.0
DC 3 2.6 3 n/a 5.1

In the example, each DC has two PEs facing it for reasons of resilience. The NSC needs to determine how to map the "DC1" to "DC2" bandwidth requirement to bandwidth reservations of TE LSPs from "DC1" to "DC2". For example, if the routing configuration is arranged such that in the absence of any network failure, traffic from "DC1" to "DC2" always enters "PE1A" and goes to "PE2A", the controller reserves 12.8 Gbps of bandwidth on the path from "PE1A" to "PE2A". If, on the other hand, the routing configuration is arranged such that in the absence of any network failure, traffic from "DC1" to "DC2" always enters "PE1A" and is load-balanced across "PE2A" and "PE2B", the controller reserves 6.4 Gbps of bandwidth on the path from "PE1A" to "PE2A" and 6.4 Gbps of bandwidth on the path from "PE1A" to "PE2B". It might be tricky for the NSC to be aware of all conditions that change the way traffic lands on the various PEs, and therefore know that it needs to change bandwidth reservations of paths accordingly. For example, there might be an internal failure within "DC1" that causes traffic from "DC1" to land on "PE1B", rather than "PE1A". The NSC may not be aware of the failure and therefore may not know that it now needs to apply bandwidth reservations to paths from "PE1B" to "PE2A" / "PE2B".

7.2.3. Scheme 3: TE Paths without Bandwidth Reservation

Like Scheme 2, Scheme 3 uses RSVP-TE or SR-TE paths. There could be a single mesh of paths between endpoints that carry all of the traffic types, or there could be a small handful of meshes, for example one mesh for low-latency traffic that follows the minimum latency path and another mesh for the other traffic that follows the minimum IGP metric path, as described in Section 5. There would be a many-to-one mapping of slices to paths.

The difference between Scheme 2 and Scheme 3 is that Scheme 3 does not have fixed bandwidth reservations for the paths. Instead, actual measured data-plane traffic volumes are used to influence the placement of TE paths. One way of achieving this is to use distributed RSVP-TE with auto-bandwidth. Alternatively, the NSC can use telemetry-driven automatic congestion avoidance. In this approach, when the actual traffic volume in the data plane on given link exceeds a threshold, the controller, knowing how much actual data plane traffic is currently travelling along each RSVP or SR-TE path, can tune the paths of one or more paths using the link such that they avoid that link. This approach is similar to that described in Section 4.3.1 of [RFC9522].

It would be undesirable to move a path that has latency as its cost function, rather than another type of path, in order to ease the congestion, as the altered path will typically have a higher latency. This can be avoided by designing the algorithms described in the previous paragraph such that they avoid moving minimum-latency paths unless there is no alternative.

8. Network Slicing OAM

The deployment and maintenance of slices within a network imply that a set of OAM functions ([RFC6291]) need to be deployed by the providers, e.g.:

  • Providers should be able to execute OAM tasks on a per Network Slice basis. These tasks can cover the "full" slice within a domain or a portion of that slice (for troubleshooting purposes, for example).

    For example, per-slice OAM tasks can consist of (but not limited to):

    • tracing resources that are bound to a given Network Slice,

    • tracing resources that are invoked when forwarding a given flow bound to a given Network Slice,

    • assessing whether flow isolation characteristics are in conformance with the Network Slice Service requirements, or

    • assessing the compliance of the allocated Network Slice resources against flow/ customer service requirements.

    [RFC7276] provides an overview of available OAM tools. These technology-specific tools can be reused in the context of network slicing. Providers that deploy network slicing capabilities should be able to select whatever OAM technology or specific feature that would address their needs.

  • Providers may want to enable differentiated failure detect and repair features for a subset of network slices. For example, a given Network Slice may require fast detect and repair mechanisms, while others may not be engineered with such means. The provider can use techniques such as [RFC5286], [RFC5714], or [RFC8355].

  • Providers may deploy means to dynamically discover the set of Network Slices that are enabled within its network. Such dynamic discovery capability facilitates the detection of any mismatch between the view maintained by the control/management plane and the actual network configuration. When mismatches are detected, corrective actions should be undertaken accordingly. For example, a provider may rely upon the L3NM [RFC9182] or the L2NM [RFC9291] to maintain the full set of L3VPN/L2VPNs that are used to deliver Network Slice Services. The correlation between an LxVPN instance and a Network Slice Service is maintained using "parent-service-id" attribute (Section 7.3 of [RFC9182]).

  • Means to report a set of network performance metrics to assess whether the agreed slice service objectives are honored. These means are used for SLO monitoring and violation detect purposes. For example, [RFC9375] can be used to report links' one-way delay, one-way delay variation, etc. Both conventional active/passive measurement methods [RFC7799] and more recent telemetry methods (e.g., YANG Push [RFC8641]) can be used.

  • Means to report and expose observed performance metrics and other OAM state to customer. For example, [I-D.ietf-teas-ietf-network-slice-nbi-yang] exposes a set of statistics per SDP, connectivity construct, and connection group.

9. Scalability Implications

The mapping between 5G slice to TN slices (see Section 3.5) is a design choice of service operators that may be a function of, e.g., the number of instantiated slices, requested services, or local engineering capabilities and guidelines. However, operators should carefully consider means to ease slice migration strategies. For example, a provider may initially adopt a 1-to-1 mapping if it has to instantiate just a few Network Slices and accommodate the need of only a few customers. That provider may decide to move to a N-to-1 mapping for aggregation/scalability purposes if sustained increased slice demand is observed.

Putting in place adequate automation means to realize Network Slices (including the adjustment of Slice Services to Network Slices mapping) would ease slice migration operations.

The realization model described in the document inherits the scalability properties of the underlying L2VPN and L3VPN technologies (Section 3.7). Readers may refer, for example, to Section 13 of [RFC4365] or Section 1.2.5 of [RFC6624] for a scalability assessment of some of these technologies. Providers may adjust the mapping model to better handle local scalability constraints.

10. IANA Considerations

This document does not make any IANA request.

11. Security Considerations

Section 10 of [RFC9543] discusses generic security considerations that are applicable to network slicing, with a focus on the following considerations:

  • Conformance to security constraints:

    Specific security requests, such as not routing traffic through a particular geographical region can be met by mapping the traffic to an inter-PE transfer plane that avoids that region.

  • IETF NSC authentication:

    This is out of the scope for this document. It should be addressed in documents that describe IETF NSC realization (e.g., [I-D.ietf-teas-ns-controller-models]).

  • Specific isolation criteria:

    Adequate admission control policies, for example policers as described in Section 5.2.1.1, should be configured in the edge of the provider network to control access to specific slice resources. This prevents the possibility of one slice consuming resources at the expense of other slices. Likewise, access to classification and mapping tables have to be controlled to prevent misbehaviors (an unauthorized entity may modify the table to bind traffic to a random slice, redirect the traffic, etc.). Network devices have to check that a required access privilege is provided before granting access to specific data or performing specific actions.

  • Data Confidentiality and Integrity of an IETF Network Slice:

    As described in Section 5.1.2.1 of [RFC9543], the customer might request an SLE that mandates encryption. As described in Section 6, this can be achieved, e.g., by mapping the traffic to an inter-PE transfer plane that uses only MACsec-encrypted links.

Many of the YANG modules cited in this document define schema for data that is designed to be accessed via network management protocols such as NETCONF [RFC6241] or RESTCONF [RFC8040]. The lowest NETCONF layer is the secure transport layer, and the mandatory-to-implement secure transport is Secure Shell (SSH) [RFC6242]. The lowest RESTCONF layer is HTTPS, and the mandatory-to-implement secure transport is TLS [RFC8446].

The NETCONF access control model [RFC8341] provides the means to restrict access for particular NETCONF or RESTCONF users to a preconfigured subset of all available NETCONF or RESTCONF protocol operations and content.

In order to avoid the need for a mapping table to associate source/destination IP addresses and slices' specific S-NSSAIs, Section 4.2 describes an approach where some or all S-NSSAI bits are embedded in an IPv6 address using an algorithm approach. An attacker from within the transport network who has access to the mapping configuration may infer the slices to which belong a packet. It may also alter these bits which may lead to steering the packet via a distinct network slice, and thus lead to service disruption. Note that such an on-path attacker may make more damage (e.g., randomly drop packets).

Security considerations specific to each of the technologies and protocols listed in the document are discussed in the specification documents of each of these protocols.

12. References

12.1. Normative References

[RFC4364]
Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, , <https://www.rfc-editor.org/rfc/rfc4364>.
[RFC6241]
Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed., and A. Bierman, Ed., "Network Configuration Protocol (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, , <https://www.rfc-editor.org/rfc/rfc6241>.
[RFC6242]
Wasserman, M., "Using the NETCONF Protocol over Secure Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, , <https://www.rfc-editor.org/rfc/rfc6242>.
[RFC7608]
Boucadair, M., Petrescu, A., and F. Baker, "IPv6 Prefix Length Recommendation for Forwarding", BCP 198, RFC 7608, DOI 10.17487/RFC7608, , <https://www.rfc-editor.org/rfc/rfc7608>.
[RFC8040]
Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF Protocol", RFC 8040, DOI 10.17487/RFC8040, , <https://www.rfc-editor.org/rfc/rfc8040>.
[RFC8341]
Bierman, A. and M. Bjorklund, "Network Configuration Access Control Model", STD 91, RFC 8341, DOI 10.17487/RFC8341, , <https://www.rfc-editor.org/rfc/rfc8341>.
[RFC8446]
Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, , <https://www.rfc-editor.org/rfc/rfc8446>.
[RFC9543]
Farrel, A., Ed., Drake, J., Ed., Rokui, R., Homma, S., Makhijani, K., Contreras, L., and J. Tantsura, "A Framework for Network Slices in Networks Built from IETF Technologies", RFC 9543, DOI 10.17487/RFC9543, , <https://www.rfc-editor.org/rfc/rfc9543>.

12.2. Informative References

[ECPRI]
Common Public Radio Interface, "Common Public Radio Interface: eCPRI Interface Specification", n.d., <http://www.cpri.info/downloads/eCPRI_v_2.0_2019_05_10c.pdf>.
[I-D.cbs-teas-5qi-to-dscp-mapping]
Contreras, L. M., Bykov, I., and K. G. Szarkowicz, "5QI to DiffServ DSCP Mapping Example for Enforcement of 5G End-to-End Network Slice QoS", Work in Progress, Internet-Draft, draft-cbs-teas-5qi-to-dscp-mapping-00, , <https://datatracker.ietf.org/doc/html/draft-cbs-teas-5qi-to-dscp-mapping-00>.
[I-D.ietf-opsawg-ntw-attachment-circuit]
Boucadair, M., Roberts, R., de Dios, O. G., Barguil, S., and B. Wu, "A Network YANG Data Model for Attachment Circuits", Work in Progress, Internet-Draft, draft-ietf-opsawg-ntw-attachment-circuit-11, , <https://datatracker.ietf.org/doc/html/draft-ietf-opsawg-ntw-attachment-circuit-11>.
[I-D.ietf-opsawg-teas-attachment-circuit]
Boucadair, M., Roberts, R., de Dios, O. G., Barguil, S., and B. Wu, "YANG Data Models for Bearers and 'Attachment Circuits'-as-a-Service (ACaaS)", Work in Progress, Internet-Draft, draft-ietf-opsawg-teas-attachment-circuit-13, , <https://datatracker.ietf.org/doc/html/draft-ietf-opsawg-teas-attachment-circuit-13>.
[I-D.ietf-teas-5g-network-slice-application]
Geng, X., Contreras, L. M., Rokui, R., Dong, J., and I. Bykov, "IETF Network Slice Application in 3GPP 5G End-to-End Network Slice", Work in Progress, Internet-Draft, draft-ietf-teas-5g-network-slice-application-02, , <https://datatracker.ietf.org/doc/html/draft-ietf-teas-5g-network-slice-application-02>.
[I-D.ietf-teas-ietf-network-slice-nbi-yang]
Wu, B., Dhody, D., Rokui, R., Saad, T., and J. Mullooly, "A YANG Data Model for the RFC 9543 Network Slice Service", Work in Progress, Internet-Draft, draft-ietf-teas-ietf-network-slice-nbi-yang-13, , <https://datatracker.ietf.org/doc/html/draft-ietf-teas-ietf-network-slice-nbi-yang-13>.
[I-D.ietf-teas-ns-controller-models]
Contreras, L. M., Rokui, R., Tantsura, J., Wu, B., Liu, X., Dhody, D., and S. Belotti, "IETF Network Slice Controller and its associated data models", Work in Progress, Internet-Draft, draft-ietf-teas-ns-controller-models-01, , <https://datatracker.ietf.org/doc/html/draft-ietf-teas-ns-controller-models-01>.
[IEEE802.1AE]
IEEE, "802.1AE: MAC Security (MACsec)", n.d., <https://1.ieee802.org/security/802-1ae/>.
[NG.113]
GSMA, "NG.113: 5GS Roaming Guidelines Version 4.0", , <https://www.gsma.com/newsroom/wp-content/uploads//NG.113-v4.0.pdf>.
[O-RAN.WG9.XPSAAS]
O-RAN Alliance, "O-RAN.WG9.XPSAAS: O-RAN WG9 Xhaul Packet Switched Architectures and Solutions Version 04.00", , <https://www.o-ran.org/specifications>.
[RFC1997]
Chandra, R., Traina, P., and T. Li, "BGP Communities Attribute", RFC 1997, DOI 10.17487/RFC1997, , <https://www.rfc-editor.org/rfc/rfc1997>.
[RFC2474]
Nichols, K., Blake, S., Baker, F., and D. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, DOI 10.17487/RFC2474, , <https://www.rfc-editor.org/rfc/rfc2474>.
[RFC2475]
Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., and W. Weiss, "An Architecture for Differentiated Services", RFC 2475, DOI 10.17487/RFC2475, , <https://www.rfc-editor.org/rfc/rfc2475>.
[RFC2698]
Heinanen, J. and R. Guerin, "A Two Rate Three Color Marker", RFC 2698, DOI 10.17487/RFC2698, , <https://www.rfc-editor.org/rfc/rfc2698>.
[RFC3209]
Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V., and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP Tunnels", RFC 3209, DOI 10.17487/RFC3209, , <https://www.rfc-editor.org/rfc/rfc3209>.
[RFC4026]
Andersson, L. and T. Madsen, "Provider Provisioned Virtual Private Network (VPN) Terminology", RFC 4026, DOI 10.17487/RFC4026, , <https://www.rfc-editor.org/rfc/rfc4026>.
[RFC4115]
Aboul-Magd, O. and S. Rabie, "A Differentiated Service Two-Rate, Three-Color Marker with Efficient Handling of in-Profile Traffic", RFC 4115, DOI 10.17487/RFC4115, , <https://www.rfc-editor.org/rfc/rfc4115>.
[RFC4176]
El Mghazli, Y., Ed., Nadeau, T., Boucadair, M., Chan, K., and A. Gonguet, "Framework for Layer 3 Virtual Private Networks (L3VPN) Operations and Management", RFC 4176, DOI 10.17487/RFC4176, , <https://www.rfc-editor.org/rfc/rfc4176>.
[RFC4360]
Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended Communities Attribute", RFC 4360, DOI 10.17487/RFC4360, , <https://www.rfc-editor.org/rfc/rfc4360>.
[RFC4365]
Rosen, E., "Applicability Statement for BGP/MPLS IP Virtual Private Networks (VPNs)", RFC 4365, DOI 10.17487/RFC4365, , <https://www.rfc-editor.org/rfc/rfc4365>.
[RFC4664]
Andersson, L., Ed. and E. Rosen, Ed., "Framework for Layer 2 Virtual Private Networks (L2VPNs)", RFC 4664, DOI 10.17487/RFC4664, , <https://www.rfc-editor.org/rfc/rfc4664>.
[RFC5286]
Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for IP Fast Reroute: Loop-Free Alternates", RFC 5286, DOI 10.17487/RFC5286, , <https://www.rfc-editor.org/rfc/rfc5286>.
[RFC5440]
Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation Element (PCE) Communication Protocol (PCEP)", RFC 5440, DOI 10.17487/RFC5440, , <https://www.rfc-editor.org/rfc/rfc5440>.
[RFC5714]
Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC 5714, DOI 10.17487/RFC5714, , <https://www.rfc-editor.org/rfc/rfc5714>.
[RFC5952]
Kawamura, S. and M. Kawashima, "A Recommendation for IPv6 Address Text Representation", RFC 5952, DOI 10.17487/RFC5952, , <https://www.rfc-editor.org/rfc/rfc5952>.
[RFC6136]
Sajassi, A., Ed. and D. Mohan, Ed., "Layer 2 Virtual Private Network (L2VPN) Operations, Administration, and Maintenance (OAM) Requirements and Framework", RFC 6136, DOI 10.17487/RFC6136, , <https://www.rfc-editor.org/rfc/rfc6136>.
[RFC6291]
Andersson, L., van Helvoort, H., Bonica, R., Romascanu, D., and S. Mansfield, "Guidelines for the Use of the "OAM" Acronym in the IETF", BCP 161, RFC 6291, DOI 10.17487/RFC6291, , <https://www.rfc-editor.org/rfc/rfc6291>.
[RFC6459]
Korhonen, J., Ed., Soininen, J., Patil, B., Savolainen, T., Bajko, G., and K. Iisakkila, "IPv6 in 3rd Generation Partnership Project (3GPP) Evolved Packet System (EPS)", RFC 6459, DOI 10.17487/RFC6459, , <https://www.rfc-editor.org/rfc/rfc6459>.
[RFC6624]
Kompella, K., Kothari, B., and R. Cherukuri, "Layer 2 Virtual Private Networks Using BGP for Auto-Discovery and Signaling", RFC 6624, DOI 10.17487/RFC6624, , <https://www.rfc-editor.org/rfc/rfc6624>.
[RFC7276]
Mizrahi, T., Sprecher, N., Bellagamba, E., and Y. Weingarten, "An Overview of Operations, Administration, and Maintenance (OAM) Tools", RFC 7276, DOI 10.17487/RFC7276, , <https://www.rfc-editor.org/rfc/rfc7276>.
[RFC7422]
Donley, C., Grundemann, C., Sarawat, V., Sundaresan, K., and O. Vautrin, "Deterministic Address Mapping to Reduce Logging in Carrier-Grade NAT Deployments", RFC 7422, DOI 10.17487/RFC7422, , <https://www.rfc-editor.org/rfc/rfc7422>.
[RFC7510]
Xu, X., Sheth, N., Yong, L., Callon, R., and D. Black, "Encapsulating MPLS in UDP", RFC 7510, DOI 10.17487/RFC7510, , <https://www.rfc-editor.org/rfc/rfc7510>.
[RFC7799]
Morton, A., "Active and Passive Metrics and Methods (with Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799, , <https://www.rfc-editor.org/rfc/rfc7799>.
[RFC7806]
Baker, F. and R. Pan, "On Queuing, Marking, and Dropping", RFC 7806, DOI 10.17487/RFC7806, , <https://www.rfc-editor.org/rfc/rfc7806>.
[RFC8100]
Geib, R., Ed. and D. Black, "Diffserv-Interconnection Classes and Practice", RFC 8100, DOI 10.17487/RFC8100, , <https://www.rfc-editor.org/rfc/rfc8100>.
[RFC8299]
Wu, Q., Ed., Litkowski, S., Tomotaki, L., and K. Ogaki, "YANG Data Model for L3VPN Service Delivery", RFC 8299, DOI 10.17487/RFC8299, , <https://www.rfc-editor.org/rfc/rfc8299>.
[RFC8355]
Filsfils, C., Ed., Previdi, S., Ed., Decraene, B., and R. Shakir, "Resiliency Use Cases in Source Packet Routing in Networking (SPRING) Networks", RFC 8355, DOI 10.17487/RFC8355, , <https://www.rfc-editor.org/rfc/rfc8355>.
[RFC8466]
Wen, B., Fioccola, G., Ed., Xie, C., and L. Jalil, "A YANG Data Model for Layer 2 Virtual Private Network (L2VPN) Service Delivery", RFC 8466, DOI 10.17487/RFC8466, , <https://www.rfc-editor.org/rfc/rfc8466>.
[RFC8641]
Clemm, A. and E. Voit, "Subscription to YANG Notifications for Datastore Updates", RFC 8641, DOI 10.17487/RFC8641, , <https://www.rfc-editor.org/rfc/rfc8641>.
[RFC8969]
Wu, Q., Ed., Boucadair, M., Ed., Lopez, D., Xie, C., and L. Geng, "A Framework for Automating Service and Network Management with YANG", RFC 8969, DOI 10.17487/RFC8969, , <https://www.rfc-editor.org/rfc/rfc8969>.
[RFC8986]
Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer, D., Matsushima, S., and Z. Li, "Segment Routing over IPv6 (SRv6) Network Programming", RFC 8986, DOI 10.17487/RFC8986, , <https://www.rfc-editor.org/rfc/rfc8986>.
[RFC9099]
Vyncke, É., Chittimaneni, K., Kaeo, M., and E. Rey, "Operational Security Considerations for IPv6 Networks", RFC 9099, DOI 10.17487/RFC9099, , <https://www.rfc-editor.org/rfc/rfc9099>.
[RFC9182]
Barguil, S., Gonzalez de Dios, O., Ed., Boucadair, M., Ed., Munoz, L., and A. Aguado, "A YANG Network Data Model for Layer 3 VPNs", RFC 9182, DOI 10.17487/RFC9182, , <https://www.rfc-editor.org/rfc/rfc9182>.
[RFC9256]
Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov, A., and P. Mattes, "Segment Routing Policy Architecture", RFC 9256, DOI 10.17487/RFC9256, , <https://www.rfc-editor.org/rfc/rfc9256>.
[RFC9291]
Boucadair, M., Ed., Gonzalez de Dios, O., Ed., Barguil, S., and L. Munoz, "A YANG Network Data Model for Layer 2 VPNs", RFC 9291, DOI 10.17487/RFC9291, , <https://www.rfc-editor.org/rfc/rfc9291>.
[RFC9330]
Briscoe, B., Ed., De Schepper, K., Bagnulo, M., and G. White, "Low Latency, Low Loss, and Scalable Throughput (L4S) Internet Service: Architecture", RFC 9330, DOI 10.17487/RFC9330, , <https://www.rfc-editor.org/rfc/rfc9330>.
[RFC9350]
Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K., and A. Gulko, "IGP Flexible Algorithm", RFC 9350, DOI 10.17487/RFC9350, , <https://www.rfc-editor.org/rfc/rfc9350>.
[RFC9375]
Wu, B., Ed., Wu, Q., Ed., Boucadair, M., Ed., Gonzalez de Dios, O., and B. Wen, "A YANG Data Model for Network and VPN Service Performance Monitoring", RFC 9375, DOI 10.17487/RFC9375, , <https://www.rfc-editor.org/rfc/rfc9375>.
[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, , <https://www.rfc-editor.org/rfc/rfc9408>.
[RFC9522]
Farrel, A., Ed., "Overview and Principles of Internet Traffic Engineering", RFC 9522, DOI 10.17487/RFC9522, , <https://www.rfc-editor.org/rfc/rfc9522>.
[TR-GSTR-TN5G]
ITU-T, "Technical Report GSTR-TN5G", , <https://www.itu.int/dms_pub/itu-t/opb/tut/T-TUT-HOME-2018-PDF-E.pdf>.
[TS-23.501]
3GPP, "TS 23.501: System architecture for the 5G System (5GS)", , <https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId-3144>.
[TS-28.530]
3GPP, "TS 23.530: Management and orchestration; Concepts, use cases and requirements)", , <https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId-3273>.
[_5G-Book]
Peterson, L., Sunay, O., and B. Davie, "5G Mobile Networks: A Systems Approach", , <https://5g.systemsapproach.org/>.

Appendix A. Acronyms and Abbreviations

3GPP: 3rd Generation Partnership Project

5GC: 5G Core

5QI: 5G QoS Indicator

A2A: Any-to-Any

AC: Attachment Circuit

AMF: Access and Mobility Management Function

AUSF: Authentication Server Function

BBU: Baseband Unit

BH: Backhaul

BS: Base Station

CE: Customer Edge

CIR: Committed Information Rate

CN: Core Network

CoS: Class of Service

CP: Control Plane

CU: Centralized Unit

CU-CP: Centralized Unit Control Plane

CU-UP: Centralized Unit User Plane

DC: Data Center

DDoS: Distributed Denial of Services

DN: Data Network

DSCP: Differentiated Services Code Point

DU: Distributed Unit

eCPRI: enhanced Common Public Radio Interface

FH: Fronthaul

FIB: Forwarding Information Base

GPRS: Generic Packet Radio Service

gNB: gNodeB

GTP: GPRS Tunneling Protocol

GTP-U: GPRS Tunneling Protocol User plane

IGP: Interior Gateway Protocol

L2VPN: Layer 2 Virtual Private Network

L3VPN: Layer 3 Virtual Private Network

LSP: Label Switched Path

MH: Midhaul

MIoT: Massive Internet of Things

MPLS: Multiprotocol Label Switching

NF: Network Function

NRF: Network Function Repository

NRP: Network Resource Partition

NSC: Network Slice Controller

PE: Provider Edge

PIR: Peak Information Rate

QoS: Quality of Service

RAN: Radio Access Network

RIB: Routing Information Base

RSVP: Resource Reservation Protocol

RU: Radio Unit

SD: Slice Differentiator

SDP: Service Demarcation Point

SLA: Service Level Agreement

SLO: Service Level Objective

SMF: Session Management Function

S-NSSAI: Single Network Slice Selection Assistance Information

SST: Slice/Service Type

SR: Segment Routing

SRv6: Segment Routing version 6

TC: Traffic Class

TE: Traffic Engineering

TN: Transport Network

UDM: Unified Data Management

UE: User Equipment

UP: User Plane

UPF: User Plane Function

URLLC: Ultra Reliable Low Latency Communication

VLAN: Virtual Local Area Network

VNF: Virtual Network Function

VPN: Virtual Private Network

VRF: Virtual Routing and Forwarding

VXLAN: Virtual Extensible Local Area Network

Appendix B. An Overview of 5G Networking

This section provides a brief introduction to 5G mobile networking with a perspective on the Transport Network. This section does not intend to replace or define 3GPP architecture, instead its objective is to provide an overview for readers that do not have a mobile background. For more comprehensive information, refer to [TS-23.501].

B.1. Key Building Blocks

[TS-23.501] defines the Network Functions (UPF, Access and Mobility Function (AMF), etc.) that compose the 5G System (5GS) Architecture together with related interfaces (e.g., N1 and N2). This architecture has built-in control and user plane separation, and the control plane leverages a Service- Based Architecture (SBA). Figure 33 outlines an example 5GS architecture with a subset of possible NFs and network interfaces.

  +-----+  +-----+  +-----+    +-----+  +-----+  +-----+
  |NSSF |  | NEF |  | NRF |    | PCF |  | UDM |  | AF  |
  +--+--+  +--+--+  +--+--+    +--+--+  +--+--+  +--+--+
Nnssf|    Nnef|    Nnrf|      Npcf|    Nudm|        |Naf
  ---+--------+--+-----+----------+---+----+--------+----
            Nausf|    Namf|       Nsmf|
              +--+--+  +--+--+     +--+------+
              |AUSR |  | AMF |     |   SMF   |
              +-----+  +--+--+     +--+------+
                       /  |           |      \
Control Plane      N1 /   |N2         |N4     \N4
------------------------------------------------------------
User Plane          /     |           |         \
                +---+  +--+--+  N3 +--+--+ N9 +-----+ N6  .---.
                |UE +--+(R)AN+-----+ UPF +----+ UPF +----( DN  )
                +---+  +-----+     +-----+    +-----+     '---'
Figure 33: 5GS Architecture and Service-based Interfaces

Similar to previous versions of 3GPP mobile networks [RFC6459], a 5G mobile network is split into the following four major domains (Figure 34):

  • UE, MS, MN, and Mobile:

    The terms User Equipment (UE), Mobile Station (MS), Mobile Node (MN), and mobile refer to the devices that are hosts with the ability to obtain Internet connectivity via a 3GPP network. An MS is comprised of a Terminal Equipment (TE) and a Mobile Terminal (MT).

  • Radio Access Network (RAN):

    Provides wireless connectivity to UEs. A RAN is made up of the Antenna that transmits and receives signals to UEs and the Base Station that digitizes the signal and converts the Radio Frequency (RF) data stream to IP packets.

  • Core Network (CN):

    Controls the CP of the RAN and provides connectivity to the Data Network (e.g., the Internet or a private VPN). The Core Network hosts dozens of services such as authentication, phone registry, charging, access to Public Switched Telephony Network (PSTN) and handover.

  • Transport Network (TN):

    Provides connectivity between 5G NFs. The TN may provide connectivity from the RAN to the CN as well as within the RAN or within the CN. The traffic generated by NFs is - mostly - based on IP or Ethernet.

+----------------------------------------------+
|             +------------+    +------------+ |
| +----+      |            |    |            | |   .-------.
| | UE +------+    RAN     |    |     CN     +----(    DN   )
| +----+      |            |    |            | |   '-------'
|             +------+-----+    +------+-----+ |
|                    |                 |       |
|              +-----+-----------------+----+  |
|              |     Transport Network      |  |
|              +----------------------------+  |
|                                              |
|                    5G System                 |
+----------------------------------------------+
Figure 34: Building Blocks of 5G Architecture (A High-Level Representation)

B.2. Core Network (CN)

The 5G Core Network (5GC) is made up of a set of NFs which fall into two main categories (Figure 35):

  • 5GC User Plane:

    The UPF is the interconnect point between the mobile infrastructure and the Data Network (DN). It interfaces with the RAN via the N3 interface by encapsulating/ decapsulating the user plane traffic in GTP tunnels (aka GTP-U or Mobile user plane).

  • 5GC Control Plane:

    The 5G control plane is made up of a comprehensive set of NFs. The description of these entities is out of the scope of this document. The following NFs and interfaces are worth mentioning, since their connectivity may rely on the Transport Network:

    • the AMF connects with the RAN control plane over the N2 interface

    • the SMF controls the 5GC UPF via the N4 interface

  +---------+    +-------------------------+
  |   RAN   |    |      5G Core (5GC)      |
  |         |    |                         |
  |         |    |   [AUSF  NRF  UDM ...]  |
  |         |    |         (SBA)           |
  |         |    |                         |
  |         | N2 |   +-----+ N11 +-----+   |
  |    CP -----------+ AMF +-----+ SMF |   |
  |         |    |   +-----+     +--+--+   |
  |         |    |                  |      |  Control Plane
-----------------------------------------------------------
  |         |    |                  | N4   |  User Plane
  |         | N3 |               +--+--+   | N6  .-------.
  |    UP -----------------------+ UPF +------->(   DN    )
  |         |    |               +-----+   |     `-------'
  +---------+    +-------------------------+
Figure 35: 5G Core Network (CN)

B.3. Radio Access Network (RAN)

The RAN connects cellular wireless devices to a mobile Core Network. The RAN is made up of three components, which form the Radio Base Station:

  • The Baseband Unit (BBU) provides the interface between the Core Network and the Radio Network. It connects to the Radio Unit and is responsible for the baseband signal processing to packet.

  • The Radio Unit (RU) is located close to the Antenna and controlled by the BBU. It converts the Baseband signal received from the BBU to a Radio frequency signal.

  • The Antenna converts the electric signal received from the RU to radio waves

The 5G RAN Base Station is called a gNodeB (gNB). It connects to the Core Network via the N3 (User Plane) and N2 (Control Plane) interfaces.

The 5G RAN architecture supports RAN disaggregation in various ways. Notably, the BBU can be split into a DU (Distributed Unit) for digital signal processing and a CU (Centralized Unit) for RAN Layer 3 processing. Furthermore, the CU can be itself split into Control Plane (CU-CP) and User Plane (CU-UP).

Figure 36 depicts a disaggregated RAN with NFs and interfaces.

            +---------------------------------+    +-----------+
            |                                 | N3 |           |
+----+  NR  |                                 +----+  5G Core  |
| UE +------+             gNodeB              |    |           |
+----+      |                                 +----+   (5GC)   |
            |                                 | N2 |           |
            +---------------------------------+    +-----------+
                            | |
                           .+ +.
                           \   /
                            \ /
            +---------------------------------+    +-----------+
            |           +-------------------+ |    |           |
            |           |                   | |    |           |
+----+  NR  | +----+ F2 |+----+ F1-U +-----+| | N3 |  +-----+  |
| UE +--------+ RU +-----+ DU +------+CU-UP+----------+ UPF |  |
+----+      | +----+    |+-+--+      +--+--+| |    |  +-----+  |
            |           |  |            |E1 | |    |           |
            |           |  | F1-C       |   | |    |           |
            |           |  |         +--+--+| | N2 |  +-----+  |
            |           |  +---------+CU-CP+----------+ AMF |  |
            |           |            +-----+| |    |  +-----+  |
            |           |     BBU split     | |    |  5G Core  |
            |           +-------------------+ |    |           |
            |       Disaggregated gNodeB      |    |           |
            +---------------------------------+    +-----------+
Figure 36: RAN Disaggregation

B.4. Transport Network (TN)

The 5G transport architecture defines three main segments for the Transport Network, which are commonly referred to as Fronthaul (FH), Midhaul (MH), and Backhaul (BH) [TR-GSTR-TN5G]:

  • Fronthaul happens before the BBU processing. In 5G, this interface is based on eCPRI with Ethernet or IP encapsulation.

  • Midhaul is optional: this segment is introduced in the BBU split presented in Appendix B.3, where Midhaul network refers to the DU- CU interconnection (i.e., F1 interface). At this level, all traffic is encapsulated in IP (signaling and user plane).

  • Backhaul happens after BBU processing. Therefore, it maps to the interconnection between the RAN and the CN. All traffic is encapsulated in IP.

Figure 37 illustrates the different segments of the Transport Network with the relevant NFs.

+---------------------------------------------------------+
|                    Transport Network                    |
|                                                         |
|    Fronthaul       Midhaul       Backhaul               |
|  +-----------+ +------------+ +-----------+             |
|  |           | |            | |           |             |
+--|-----------|-|------------|-|-----------|-------------+
 +-+--+      +-+-++         +-+-++        +-+---+     .---.
 | RU |      | DU |         | CU |        | UPF :----( DN  )
 +----+      +----+         +----+        +-----+     `---'
Figure 37: 5G Transport Segments

A given part of the transport network can carry several 5G transport segments concurrently, as outlined in Figure 38. This is because different types of 5G NFs might be placed in the same location (e.g., the UPF from one slice might be placed in the same location as the CU-UP from another slice).

+---------+
|+----+   | Colocated
||RU-1|   | RU/DU
|+-+--+   |
|  | FH-1 |
|+-+--+   |
||DU-1|   |  +----+         +-----+         .---.
|+-+--+   |  |CU-1|         |UPF-1+--------( DN  )
+--|------+  +-+-++         +-+---+         `---'
+--|-----------|-|------------|----------------------------+
|  |    MH-1   | |    BH-1    |          Transport Network |
|  +-----------+ +------------+                            |
|  +-----------+ +------------+ +-----------+              |
|  |    FH-2   | |    MH-2    | |    BH-2   |              |
+--|-----------|-|------------|-|-----------|--------------+
 +-+--+      +-+-++         +-+-++        +-+---+     .---.
 |RU-2|      |DU-2|         |CU-2|        |UPF-2+----( DN  )
 +----+      +----+         +----+        +-----+     `---'
Figure 38: Concurrent 5G Transport Segments

Appendix C. Acknowledgments

The authors would like to thank Adrian Farrel, Joel Halpern, Tarek Saad, Greg Mirsky, Rüdiger Geib, Nicklous D. Morris, Daniele Ceccarelli, Bo Wu, Xuesong Geng, and Deborah Brungard for their review of this document and for providing valuable comments.

Special thanks to Jie Dong and Adrian Farrel for the detailed and careful reviews.

Thanks to Alvaro Retana for the rtg-dir review, Yoshifumi Nishida for the tsv-art review, and Timothy Winters for the int-dir review.

Contributors

John Drake
Sunnyvale,
United States of America
Ivan Bykov
Ribbon Communications
Tel Aviv
Israel
Reza Rokui
Ciena
Ottawa
Canada
Luay Jalil
Verizon
Dallas, TX,
United States of America
Beny Dwi Setyawan
XL Axiata
Jakarta
Indonesia
Amit Dhamija
Rakuten
Bangalore
India
Mojdeh Amani
British Telecom
London
United Kingdom

Authors' Addresses

Krzysztof G. Szarkowicz (editor)
Juniper Networks
Wien
Austria
Richard Roberts (editor)
Juniper Networks
Rennes
France
Julian Lucek
Juniper Networks
London
United Kingdom
Mohamed Boucadair (editor)
Orange
France
Luis M. Contreras
Telefonica
Ronda de la Comunicacion, s/n
Madrid
Spain