Internet-Draft Transport Network aware Mobility for 5G September 2021
Chunduri, et al. Expires 26 March 2022 [Page]
DMM Working Group
Intended Status:
U.C. Chunduri, Ed.
J.K. Kaippallimalil, Ed.
S.B. Bhaskaran
J.T. Tantsura
P.M. Muley

Transport Network aware Mobility for 5G


This document specifies a framework and mapping of slices in 5G mobile systems to transport network slices in IP, Layer 2 or Layer 1 transport networks. Slices in 5G systems are characterized by latency bounds, reservation guarantees, jitter, data rates, availability, mobility speed, usage density, criticality and priority. These characteristics mapped to transport network slice include bandwidth, latency and criteria such as isolation, directionality and disjoint routes. Mobile slice criteria are mapped to the appropriate transport slice and capabilities offered in backhaul, midhaul and fronthaul connectivity segments between radio side network functions and user plane function(gateway).

This document describes how mobile network functions map its slice criteria to identifiers in IP and Layer 2 packets that transport network segments use to grant transport layer services during UE mobility scenarios. Applicability of this framework and underlying transport networks, which can enable different slice properties are also discussed. This is based on mapping between mobile and transport underlays (L2, Segment Routing, IPv6, MPLS and IPv4).

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC2119 [RFC2119].

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

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 26 March 2022.

Table of Contents

1. Introduction

The 3GPP architecture for 5GS defined in [TS.23.501-3GPP], [TS.23.502-3GPP] and [TS.23.503-3GPP] for 5GC (5G Core) and the NG-RAN architecture and procedures defined in [TS.38.300-3GPP] and [TS.38.401-3GPP] include procedures for setting up network slices in the 3GPP network. The 5GS (5G System) architecture also defines a comprehensive set of functions for access mobility, session handling and related functions for subscription management, authentication and policy among others. These network functions (NF) are defined using a service-based architecture (SBA) that allows NFs to expose their functions via an API and common service framework.

IP transport is used to interconnect the data forwarding entities UPFs, gNB-CU and gNB-DU in the 5GC and NG-RAN architecture but 3GPP specifications only define the interfaces (N3, N9, F1U etc.) and the 3GPP transport end points [TS.28.541-3GPP]. The architecture allows the placement of Branching Point (BP) and Uplink Classifier (ULCL) UPFs closer to the access network (5G-AN). The gNB-CU and gNB-DU are the centralized unit (CU) and distributed unit (DU) of a 5G radio access network (NG-RAN) with gNB. The 5G-AN can be a radio access network (NG-RAN) or any non-3GPP access network, for example, WLAN. The IP address is anchored by a PDU session anchor UPF (PSA UPF). 3GPP slicing and RAN aspects are further described in Appendix A.1.

5GS allows more than one UPF on the path for a PDU (Protocol Data Unit) session that provides various functionality including session anchoring, uplink classification and branching point for a multihomed IPv6 PDU session. The interface between the BP/ULCL UPF and the PSA UPF is called N9 [TS.23.501-3GPP]. 3GPP has adopted GTP-U for the N9 and N3 interfaces between the various UPF instances and the (R)AN and also, for the F1-U interface between the DU and the CU in the NG-RAN. 3GPP has specified control and user plane aspects in [TS.23.501-3GPP] to provide slice and QoS support. 3GPP has defined three broad slice types to cover enhanced mobile broadband (eMBB) communications, ultra-reliable low latency communications(URLLC) and massive internet of things (mIoT). ATIS [ATIS075] has defined an additional slice type for V2X services. 3GPP slice types and multiple instances of a slice type satisfy various characteristics for 5G network resources The slice details in 3GPP, ATIS or NGMN do not specify how slice characteristics for QoS, hard /soft isolation, protection and other aspects should be satisfied in IP transport networks.

A transport underlay across each 3GPP segment may have multiple technologies or providers on path and the slice in 3GPP domain should have a corresponding mapping in the transport domain. This document proposes to map a slice in the 3GPP domain to a transport domain slice. This document also proposes to carry this provisioned mapping in an IP packet so that the IP transport domain can classify and provide the required service. This is explored further in this document.

1.1. IETF Network Slicing Terminology

[I-D.ietf-teas-ietf-network-slices] draft defines the 'IETF Network slice', its scope and characteristics. It lists use cases where IETF technologies can be used for slicing solutions, for various connectivity segments. Transport slice terminology as used in this document refers to the connectivity segment between various 5G systems (i.e. 5G-AN which includes NG-RAN, ULCL UPF, BP UPF and PSA UPF) and some of these segments are referred to as IETF Network slices.

[I-D.ietf-teas-ietf-network-slices] also defines a generic framework and how abstract requests to set up slices can be mapped to more specific technologies (e.g., VPN and traffic-engineering technologies). This document is aimed to be specific to 3GPP use case where many such connectivity segments are used in E2E slicing solutions. Some of the terminologies defined in these referred drafts and applicability to this document are further described in Section 2.1.1.

1.2. Problem Statement

5GS defines network slicing as one of the core capabilities of 5GC with slice awareness from Radio and 5G Core (5GC) network. The 5G System (5GS) as defined in 3GPP specifications, does not consider the resources and functionalities needed from the transport network for the selection of UPF. This is seen as independent functionality and is currently not part of 5GS.

However, the lack of underlying Transport Network (TN) awareness may lead to selection of sub-optimal UPF(s) and/or 5G-AN during various procedures in 5GS (e.g., session establishment and various mobility scenarios). Meeting the specific slice characteristics on the F1-U, N3 and N9 interfaces depends on the IP transport underlay providing these resources and capabilities. This could also lead to the inability in meeting SLAs for real-time, mission-critical or latency sensitive services.

The 5GS provides slices to its clients (UEs). The UE's PDU session spans the access network (radio access network including the F1-U) and N3 and N9 transport segments which have an IP transport underlay. The 5G operator needs to obtain slice capability from the IP transport provider. Several UE sessions that match a slice may be mapped to an IP transport segment. Thus, there needs to be a mapping between the slice capability offered to the UE (S-NSSAI) and what is provided by the IP transport.

1.3. Solution Approach

This document specifies an approach to fulfil the needs of 5GS to transport user plane traffic from 5G-AN (including NG-RAN) to UPF in an optimized fashion. This is done by keeping establishment and mobility procedures aware of the underlying transport network along with slicing requirements.

Section 2 describes in detail on how TN aware mobility can be built irrespective of underlying TN technology used. How other IETF TE technologies applicable for this draft is specified in Section 3.2.

1.4. Acronyms

5QI -
5G QoS Indicator
5G-AN -
5G Access Network
Access and Mobility Management Function (5G)
BP -
Branch Point (5G)
Cell Site Router
CP -
Control Plane (5G)
CU -
Centralized Unit (5G, gNB)
DN -
Data Network (5G)
DU -
Distributed Unit (5G, gNB)
eMBB -
enhanced Mobile Broadband (5G)
Fast ReRoute
gNB -
5G NodeB
Guaranteed Bit Rate (5G)
GPRS Tunneling Protocol - User plane (3GPP)
Interior Gateway Protocols (e.g. IS-IS, OSPFv2, OSPFv3)
Loop Free Alternatives (IP FRR)
mIOT -
Massive IOT (5G)
Multi Protocol Label Switching
Next Generation Radio Access Network (i.e., gNB, NG-eNB - RAN functions which connect to 5GC)
Network Slice Selection Management Function
QoS Flow ID (5G)
Preferred Path Routing
Protocol Data Unit (5G)
PW -
Pseudo Wire
Radio Access Network (i.e 3GPP radio access network used synonymously with NG-RAN in this document)
Radio Access Network
Reflective QoS Indicator (5G)
Service Based Interface (5G)
Segment Identifier
Session Management Function (5G)
Session and Service Continuity (5G)
Slice and Service Types (5G)
SR -
Segment Routing
TE -
Traffic Engineering
Uplink Classifier (5G)
UP -
User Plane(5G)
User Plane Function (5G)
Ultra reliable and low latency communications (5G)

2. Transport and Slice aware Mobility in 5G Networks

3GPP architecture [TS.23.501-3GPP], [TS.23.502-3GPP] describe slicing in 5GS and is provided here for information. The application of 5GS slices in transport network for backhaul, mid-haul and front haul are not explicitly covered in 3GPP and is the topic here. To support specific characteristics in backhaul (N3, N9), mid-haul (F1) and front haul, it is necessary to map and provision corresponding resources in the transport domain. This section describes how to provision the mapping information in the transport network and apply it so that user plane packets can be provided the transport resources (QoS, isolation, protection, etc.) expected by the 5GS slices.

The figure shows the entities on path for 3GPP Network Functions (gNB-DU, gNB-CU, UPF) to obtain slice aware classification from an IP/L2 transport network.

                5G Control and Management Planes
    | +--------------------------------------------------------------------+ |
    | |                    (TNF)    5G Management Plane  (TNF)             | |
    | +----+-----------------+-------------+---------------+-----------+---+ |
    |      |                 |             |               |           |     |
    | +----+-----+           |   F1-C +----+-----+         |   N2 +----+---+ |
    | |          |----------(---------|gNB-CU(CP)|--------(-------| 5GC CP | |
    | |          |           |        +----+-----+         |      +----+---+ |
    +-|          |-----------|-------------|---------------|-----------|-----+
      |          |           |             |               |           |
      |          |           |             |               |           |
      |          |           | ACTN        |               | ACTN      |
      |          |       +---+---+         |           +---+---+       |
      |          |       |       |         |           |       |       |
      | gNB-DU   |       | SDN-C |         E1          | SDN-C |       |
      |          |       |       |         |           |       |       |
      |          |       +---+---+         |           +---+---+       |
      |          |           |             |               |           |
      |          |           |             |               |           |
      |          |        __ +__           |            ___+__         |
      |          |     __/      \__     +--+---+     __/      \__    +-+-+
      |          |    /     IP     \    | gNB  |    /     IP     \   |   |
 UE--*|          |-(PE) Mid-haul (PE)---+CU(UP)+--(PE) Backhaul(PE)--+UPF|--DN
      +----------+    \__        __/    +------+    \__        __/   +---+
                         \______/                      \______/

                   |------ F1-U -------|        |------ N3 OR N9 ------|

          * Radio and Fronthaul
Figure 1: Backhaul and Mid-haul Transport Network for 5G

2.1. Backhaul and Mid-Haul Transport Network

Figure 1 depicts IP Xhaul network with SDN-C and PE (Provider Edge) routers providing IP transport service to 5GS user plane entities 5G-AN (e.g. gNB) and UPF. 5GS architecture with high level management, control and user plane entities and its interaction with the IP transport plane is shown here. The slice capability required in IP transport networks are estimated and provisioned by the functionality as specified in Section 2.3 (TNF) with support from various other control plane functions such as the Network Data Analytics Function (NWDAF), Network Function Repository Function (NRF) and Policy Control Function (PCF). The TNF is only a logical function that may be realized in a 3GPP management function such as Network Slice Selection Management Function (NSSMF) defined in [TS.28.533-3GPP]. The TNF requests the SDN-C to provision the IP XHaul network using ACTN [RFC8453].

The 5G management plane in Figure 1 interacts with the 5G control plane - the 5GC (5G Core), gNB-CU (5G NodeB Centralized Unit) and gNB-DU (5G Node B Distributed Unit). Non-access stratum (NAS) signaling from the UE for session management, mobility is handled by the 5GC. When a UE initiates session establishment, it indicates the desired slice type in the S-NSSAI (Specific Network Slice Selection Assistance Information) field. The AMF uses the S-NSSAI, other subscription information and configuration in the NSSF to select the appropriate SMF and the SMF in turn selects UPFs (User Plane Functions) that are able to provide the specified slice resources and capabilities.

The AMF, SMF, NSSF, PCF, NRF, NWDAF and other control functions in 5GC are described in [TS.23.501-3GPP]. Some of the slice capabilities along the user plane path between the (R)AN and UPFs (F1-U, N3, N9 segments) such as a low latency path, jitter, protection and priority needs these to be provided by the IP transport network.

The 5G user plane from UE to DN (Data Network) includes a mid-haul segment (F1-U between gNB DU(UP), gNB CU(UP)) and backhaul (N3 between gNB - UPF; N9 between UPFs). If the RAN uses lower layer split architecture as specified by O-RAN alliance, then the user plane path from UE to DN also includes the fronthaul interface. The fronthaul interface carries the radio frames in the form of In-phase (I) and Quadrature (Q) samples using eCPRI encapsulation over Ethernet or UDP over IP.

The N3, N9 and F1-U user planes use GTP-U [TS.29.281-3GPP] to transport UE PDUs (IPv4, IPv6, IPv4v6, Ethernet or Unstructured). For the front-haul described further in Section 2.1.2, an Ethernet transport with VLANs can be expected to be the case in many deployments.

Figure 1 also depicts the PE router, where transport paths are initiated/terminated can be deployed separately with UPF or both functionalities can be in the same node. The TNF provisions this in the SDN-C of the IP XHaul network using ACTN [RFC8453]. When a GTP-U encapsulated user packet from the (R)AN (gNB) or UPF with the slice information traverses the F1-U/N3/N9 segment, the PE router of the IP transport underlay can inspect the slice information and provide the provisioned capabilities. This is elaborated further in Section 2.4.

2.1.1. IETF Network Slicing Applicability

Some of the functional elements depicted in the Figure 1 can be mapped to the terminology set forth in the [I-D.ietf-teas-ietf-network-slices]. From 3GPP perspective, UE and UPF are the network slice endpoints and routers, gNB-DU, gNB-CU, switches, PE nodes are the slice realization endpoints. The TNF represented in the Figure 1 can be seen as IETF Network Slice Controller (NSC) functionality and SDN-C maps to Network Controller (NC). NSC-NBI interface is the interface from 3GPP Management plane (e.g., NSSMF) and NSC-SBI interface is the interface between TNF and SDN-C. Various possibilities for implementation of these interfaces and the relation to ACTN are also further described in the [I-D.ietf-teas-ietf-network-slices].

2.1.2. Fronthaul Transport Network

The O-RAN Alliance has specified the fronthaul interface between the O-RU and the O-DU in [ORAN-WG4.CUS-O-RAN]. The radio layer information, in the form of In-phase (I) and Quadrature (Q) samples are transported using Enhanced Common Public Radio Interface (eCPRI) framing over Ethernet or UDP. On the Ethernet based fronthaul interface, the slice information can be carried in the Ethernet header through the VLAN tags. The Ethernet switches in the fronthaul transport network can inspect the slice information (VLAN tag) in the Ethernet header and provide the provisioned capabilities. The mapping of I and Q samples of different radio resources (radio resource blocks or carriers etc.,.) to different slices and to their respective VLAN tags on the fronthaul interface is controlled by the O-RAN fronthaul C-Plane and M-Plane interfaces. On a UDP based fronthaul interface, the slice information can be carried in the IP or UDP header. The PE routers of the fronthaul transport network can inspect the slice information in the IP or UDP header and provide the provisioned capabilities. The fronthaul transport network is latency and jitter sensitive. The provisioned slice capabilities in the fronthaul transport network MUST take care of the latency and jitter budgets of the specific slice for the fronthaul interface. The provisioning of the fronthaul transport network is handled by the SDN-C pertaining to the fronthaul transport.

2.2. Mobile Transport Network Context (MTNC) and Scalability

The MTNC represents a slice of a transport path for a tenant between two 3GPP user plane functions. The Mobile-Transport Network Context Identifier (MTNC-ID) is generated by the TNF to be unique for each instance (for a tenant) and per traffic class (including QoS and slice aspects). Thus, there may be more than one MTNC-ID for the same QoS and instance if there is a need to provide isolation (slice) of the traffic. It should be noted that MTNC are per class/instance and not per user session. The MTNC-IDs are configured by the TNF to be unique within a provisioning domain.

Since the MTNC-IDs are generated per instance / tenant, there is no need for unique MTNC-IDs per flow/session. In addition, since the traffic estimation is performed prior to UE's session establishment, there is no provisioning delay experienced by the UE during its session setup. For an instance/tenant, the MTNC-ID space scales roughly as a square of the number sites between which 3GPP user plane functions have paths. If there are T traffic classes across N sites, the number of MTNC-IDs in a fully meshed network is (N*(N-1)/2) * T. For example, if there are 3 traffic classes between 25 sites, there would be at most 900 MTNC-IDs required. Multiple instances/tenants that need to be fully isolated, will add to the MTNC provisioning. An MTNC-ID space of 16 bits (65K identifiers) can be expected to be sufficient.

2.3. Transport Network Function (TNF)

Figure 1 shows a view of the functions and interfaces for provisioning the MTNC-IDs. The focus is on provisioning between the 3GPP management plane (NSSMF), transport network (SDN-C) and carrying the MTNC-IDs in PDU packets for the transport network to grant the provisioned resources.

In Figure 1, the TNF (logical functionality within the NSSMF) requests the SDN-C in the transport domain to program the TE path using ACTN [RFC8453]. The SDN-C programs the Provider Edge (PE) routers and internal routers according to the underlay transport technology (e.g., MPLS, SR, PPR). The PE router inspects incoming PDU data packets for the UDP SRC port which mirrors the MTNC-ID, classifies and provides the VN service provisioned across the transport network.

The detailed mechanisms by which the NSSMF provides the MTNC-IDs to the control plane and user plane functions are for 3GPP to specify. Two possible options are outlined below for completeness. The NSSMF may provide the MTNC-IDs to the 3GPP control plane by either providing it to the Session Management Function (SMF), and the SMF in turn provisions the user plane functions (UP-NF1, UP-NF2) during PDU session setup. Alternatively, the user plane functions may request the MTNC-IDs directly from the TNF/NSSMF. Figure 1 shows the case where user plane entities request the TNF/NSSMF to translate the Request and get the MTNC-ID. Another alternative is for the TNF to provide a mapping of the 3GPP Network Instance Identifier, described in Section 2.6 and the MTNC-ID to the user plane entities via configuration.

The TNF should be seen as a logical entity that can be part of NSSMF in the 3GPP management plane [TS.28.533-3GPP]. The NSSMF may use network configuration, policies, history, heuristics or some combination of these to derive traffic estimates that the TNF would use. How these estimates are derived are not in the scope of this document. The focus here is only in terms of how the TNF and SDN-C are programmed given that slice and QoS characteristics across a transport path can be represented by an MTNC-ID. The TNF requests the SDN-C in the transport network to provision paths in the transport domain based on the MTNC-ID. The TNF is capable of providing the MTNC-ID provisioned to control and user plane functions in the 3GPP domain. Detailed mechanisms for programming the MTNC-ID should be part of the 3GPP specifications.

2.4. Transport Provisioning

Functionality of transport provisioning for an engineered IP transport that supports 3GPP slicing and QoS requirements in [TS.23.501-3GPP] is described in this section.

During a PDU session setup, the AMF using input from the NSSF selects a network slice and SMF. The SMF with user policy from Policy Control Function (PCF) sets 5QI (QoS parameters) and the UPF on the path of the PDU session. While QoS and slice selection for the PDU session can be applied across the 3GPP control and user plane functions as outlined in Section 2, the IP transport underlay across F1-U, N3 and N9 segments do not have enough information to apply the resource constraints represented by the slicing and QoS classification. Current guidelines for interconnection with transport networks [IR.34-GSMA] provide an application mapping into DSCP. However, these recommendations do not take into consideration other aspects in slicing like isolation, protection and replication.

IP transport networks have their own slice and QoS configuration based on domain policies and the underlying network capability. Transport networks can enter into an agreement for virtual network services (VNS) with client domains using the ACTN [RFC8453] framework. An IP transport network may provide such slice instances to mobile network operators, CDN providers or enterprises for example. The 3GPP mobile network, on the other hand, defines a slice instance for UEs as are the mobile operator's 'clients'. The Network Slice Selection Management Function (NSSMF) [TS 28.533] that interacts with a TN controller like an SDN-C (that is out of scope of 3GPP).

The ACTN VN service can be used across the IP transport networks to provision and map the slice instance and QoS of the 3GPP domain to the IP transport domain. An abstraction that represents QoS and slice instances in the mobile domain and mapped to ACTN VN service in the transport domain is represented here as MTNC-IDs. Details of how the MTNC-IDs are derived are up to functions that can estimate the level of traffic demand.

The 3GPP network/5GS provides slices instances to its clients (UE) that include resources for radio and mobile core segments. The UE's PDU session spans the access network (radio) and F1-U/N3/N9 transport segments which have an IP transport underlay. The 5G operator needs to obtain slice capability from the IP transport provider since these resources are not seen by the 5GS. Several UE sessions that match a slice may be mapped to an IP transport segment. Thus, there needs to be a mapping between the slice capability offered to the UE (NSSAI) and what is provided by the IP transport.

When the 3GPP user plane function (5G-AN, UPF) does not terminate the transport underlay protocol (e.g., MPLS), it needs to be carried in the IP protocol header from end-to-end of the mobile transport connection (N3, N9). [I-D.ietf-dmm-5g-uplane-analysis] discusses these scenarios in detail.

2.5. MTNC-ID in the Data Packet

When the 3GPP user plane function (5G-AN, UPF) and transport provider edge is on different nodes, the PE router needs to have the means by which to classify the PDU packet. The mapping information is provisioned between the 5G provider and IP transport network and corresponding information should be carried in each IP packet on the F1-U, N3, N9 interface. To allow the IP transport edge nodes to inspect the transport context information efficiently, it should be carried in an IP header field that is easy to inspect. It may be noted that the F1-U, N3 and N9 interfaces in 5GS are IP interfaces. If the fronthaul, midhaul or backhaul IP path is bounded by an L2 network, one option maybe to use VLANs to carry the MTNC-ID. 3GPP specifications for management plane defines transport end-points configuration in [TS.28.541-3GPP]. However, Layer 2 alternatives such as VLAN will fail in L3/routed networks on the F1-U, N3 or N9 path. GTP-U (F1-U, N3, N9 encapsulation header) field extensions offer a possibility, however these extensions are not always easy for a transport edge router to parse efficiently on a per packet basis. Other IP header fields like DSCP are not suitable as it only conveys the QoS aspects (but not other aspects like isolation, protection, etc.)

While IPv6 extension headers like SRv6 may be an option to carry the MTNC-ID that requires the end-to-end network to be IPv6 as well as the capability to lookup the extension header at line rate. To minimise the protocol changes and make this underlay transport independent (IPv4/IPv6/MPLS/L2), an option is to provision a mapping of MTNC-ID to a UDP port range of the GTP encapsulated user packet. A simple mapping table between the MTNC-ID and the source UDP port number can be configured to ensure that ECMP /load balancing is not affected adversely by encoding the UDP source port with an MTNC-ID mapping. The UDP port information containing MTNC-ID is a simple extension that can be provisioned in 3GPP transport end-points defined in [TS.28.541-3GPP]. This mapping is configured in 3GPP user plane functions (5G-AN, UPF) and Provider Edge (PE) Routers that process MTNC-IDs.

PE routers can thus provision a policy based on the source UDP port number (which reflects the mapped MTNC-ID) to the underlying transport path and then deliver the QoS/slice resource provisioned in the transport network. The source UDP port that is encoded is the outer IP (corresponding to GTP-U header) while the inner IP packet (UE payload) is unaltered. The source UDP port is encoded by the node that creates the GTP-U encapsulation and therefore, this mechanism has no impact on UDP checksum calculations.

3GPP network operators may use IPSec gateways (SEG) to secure packets between two sites - for example over an F1-U, N3 or N9 segment. The MTNC identifier in the GTP-U packet should be in the outer IP source port even after IPSec encryption for PE transport routers to inspect and provide the level of service provisioned. Tunnel mode - which is the case for SEG/IPSec gateways - adds an outer IP header in both AH (Authenticated Header) and ESP (Encapsulated Security Payload) modes. The GTP-U / UDP source port with encoded MTNC identifier should be copied to the IPSec tunnel ESP header. One option is to use 16 bits from the SPI field of the ESP header to encode the MTNC identifier and use the remaining 16 bits in SPI field to identify an SA. Load balancing entropy for ECMP will not be affected as the MTNC encoding mechanism already accounts for this.

If the RAN uses O-RAN Alliance lower layer split architecture, then a fronthaul network is involved. On an Ethernet based fronthaul transport network, VLAN tag may be an option to carry the MTNC-ID. The VLAN ID provides a 12 bit space and is sufficient to support up to 4096 slices on the fronthaul transport network. The mapping of fronthaul traffic to corresponding network slices is based on the radio resource for which the fronthaul carries the I and Q samples. The mapping of fronthaul traffic to the VLAN tag corresponding to the network slice is specified in Section 2.1.2. On the UDP based fronthaul transport network, the UDP source port can be used to carry the MTNC-ID.

2.6. Functionality for E2E Management

With the TNF functionality in 5GS Service Based Interface, the following additional functionalities are required for end-2-end slice management including the transport network:

  • The Specific Network Slice Selection Assistance Information (S-NSSAI) of PDU session SHOULD be mapped to the assigned transport VPN and the TE path information for that slice.
  • For transport slice assignment for various SSTs (eMBB, URLLC, MIoT) corresponding underlay paths need to be created and monitored from each transport endpoint (CSR and PE@UPF).
  • During PDU session creation, apart from radio and 5GC resources, transport network resources needed to be verified matching the characteristics of the PDU session traffic type.
  • The TNF MUST provide an API that takes as input the source and destination 3GPP user plane element address, required bandwidth, latency and jitter characteristics between those user plane elements and returns as output a particular TE path's identifier, that satisfies the requested requirements.
  • Mapping of PDU session parameters to underlay SST paths need to be done. One way to do this is to let the SMF install a Forwarding Action Rule (FAR) in the UPF via N4 with the FAR pointing to a "Network Instance" in the UPF. A "Network Instance" is a logical identifier for an underlying network. The "Network Instance" pointed by the FAR can be mapped to a transport path (through L2/L3 VPN). FARs are associated with Packet Detection Rule (PDR). PDRs are used to classify packets in the uplink (UL) and the downlink (DL) direction. For UL procedures specified in Section 2.4, Section 2.5 can be used for classifying a packet belonging to a particular slice characteristic. For DL, at a PSA UPF, the UE IP address is used to identify the PDU session, and hence the slice a packet belongs to and the IP 5 tuple can be used for identifying the flow and QoS characteristics to be applied on the packet at UPF. If a PE is not co-located at the UPF then mapping to the underlying TE paths at PE happens based on the encapsulated GTP-U packet as specified in Section 2.5.
  • In some SSC modes [I-D.chunduri-dmm-5g-mobility-with-ppr], if segmented path (CSR to PE@staging/ULCL/BP-UPF to PE@anchor-point-UPF) is needed, then corresponding path characteristics MUST be used. This includes a path from CSR to PE@UL-CL/BP UPF [TS.23.501-3GPP] and UL-CL/BP UPF to eventual UPF access to DN.
  • Continuous monitoring of the underlying transport path characteristics should be enabled at the endpoints (technologies for monitoring depends on traffic engineering technique used as described in Section 3.2). If path characteristics are degraded, reassignment of the paths at the endpoints should be performed. For all the affected PDU sessions, degraded transport paths need to be updated dynamically with similar alternate paths.
  • During UE mobility events similar to 4G/LTE i.e., gNB mobility (F1 based, Xn based or N2 based), for target gNB selection, apart from radio resources, transport resources MUST be factored. This enables handling of all PDU sessions from the UE to target gNB and this require co-ordination of gNB, AMF, SMF with the TNF module.

Integrating the TNF as part of the 5GS Service Based Interfaces, provides the flexibility to control the allocation of required characteristics from the TN during a 5GS signaling procedure (e.g. PDU Session Establishment). If TNF is seen as separate and in a management plane, this real time flexibility is lost. Changes to detailed signaling to integrate the above for various 5GS procedures as defined in [TS.23.502-3GPP] is beyond the scope of this document.

3. Transport Network Underlays

Apart from the various flavors of IETF VPN technologies to share the transport network resources and capacity, TE capabilities in the underlay network is an essential component to realize the 5G TN requirements. This section focuses on various transport underlay technologies (not exhaustive) and their applicability to realize Midhaul/Backhaul transport networks. Focus is on the user/data plane i.e., F1-U/N3/N9 interfaces as laid out in the framework Figure 1.

3.1. Applicability

  • For 3 different SSTs, 3 transport TE paths can be signaled from any node in the transport network. For Uplink traffic, the 5G-AN will choose the right underlying TE path of the UPF based on the S-NSSAI the PDU Session belongs to and/or the UDP Source port (corresponds to the MTNC-ID Section 2.4) of the GTP-U encapsulation header. Similarly in the Downlink direction matching Transport TE Path of the 5G-AN is chosen based on the S-NSSAI the PDU Session belongs to. The table below shows a typical mapping:

      |GTP/UDP SRC PORT|   SST      |   Transport Path | Transport Path  |
      |                | in S-NSSAI |   Info           | Characteristics |
      | Range Xx - Xy  |            |                  |                 |
      | X1, X2(discrete|  MIOT      | PW ID/VPN info,  | GBR (Guaranteed |
      | values)        |  (massive  |   TE-PATH-A      |       Bit Rate) |
      |                |   IOT)     |                  |   Bandwidth: Bx |
      |                |            |                  |   Delay:     Dx |
      |                |            |                  |   Jitter:    Jx |
      | Range Yx - Yy  |            |                  |                 |
      | Y1, Y2(discrete|  URLLC     | PW ID/VPN info,  | GBR with Delay  |
      | values)        | (ultra-low |   TE-PATH-B      |     Req.        |
      |                |  latency)  |                  |   Bandwidth: By |
      |                |            |                  |   Delay:     Dy |
      |                |            |                  |   Jitter:    Jy |
      | Range Zx - Zy  |            |                  |                 |
      | Z1, Z2(discrete|  EMBB      | PW ID/VPN info,  |   Non-GBR       |
      | values)        | (broadband)|  TE-PATH-C       |   Bandwidth: Bx |

Figure 2: Mapping of Transport Paths on F1-U/N3/N9
  • It is possible to have a single TE Path for multiple input points through a MP2P TE tree structure separate in UL and DL direction.
  • Same set of TE Paths are created uniformly across all needed 5G-ANs and UPFs to allow various mobility scenarios.
  • Any modification of TE parameters of the path, replacement path and deleted path needed to be updated from TNF to the relevant ingress points. Same information can be pushed to the NSSF, and/or SMF as needed.
  • TE Paths support for native L2, IPv4 and IPv6 data/user planes with optional TE features are desirable in some network segments. As this is an underlay mechanism it can work with any overlay encapsulation approach including GTP-U as defined currently for F1-U/N3/N9 interface.

In some E2E scenarios, security is desired granularly in the underlying transport network. In such cases, there would be a need to have separate sub-ranges under each SST to provide the TE path in preserving the security characteristics. The UDP Source Port range captured in Figure 2 would be sub-divided to maintain the TE path for the current SSTs with the security. The current solution doesn't provide any mandate on the UE traffic in selecting the type of security.

3.2. Transport Network Technologies

While there are many Software Defined Networking (SDN) approaches available, this section is not intended to list all the possibilities in this space but merely captures the technologies for various requirements discussed in this document.

RSVP-TE [RFC3209] provides a lean transport overhead for the TE path for MPLS user plane. However, it is perceived as less dynamic in some cases and has some provisioning overhead across all the nodes in N3 and N9 interface nodes. Also, it has another drawback with excessive state refresh overhead across adjacent nodes and this can be mitigated with [RFC8370].

SR-TE [RFC8402] does not explicitly signal bandwidth reservation or mechanism to guarantee latency on the nodes/links on SR path. But SR allows path steering for any flow at the ingress and particular path for a flow can be chosen. Some of the issues and suitability for mobile use plane are documented at Section 5.3 of [I-D.bogineni-dmm-optimized-mobile-user-plane]. However, [I-D.ietf-dmm-srv6-mobile-uplane] presents various options for optimized mobile user plane with SRv6 with or without GTP-U overhead along with traffic engineering capabilities. SR-MPLS allows reduction of the control protocols to one IGP (without needing for LDP and RSVP-TE).

Preferred Path Routing (PPR) is an integrated routing and TE technology and the applicability for this framework is described in [I-D.chunduri-dmm-5g-mobility-with-ppr]. PPR does not remove GTP-U, unlike some other proposals laid out in [I-D.bogineni-dmm-optimized-mobile-user-plane]. Instead, PPR works with the existing cellular user plane (GTP-U) for F1-U/N3 and N9. In this scenario, PPR will only help provide TE benefits needed for 5G slices from a transport domain perspective. It does so for any underlying user/data plane used in the transport network (L2/IPv4/IPv6/MPLS).

As specified with the integrated transport network function (TNF), a particular RSVP-TE path for MPLS or SR path for MPLS and IPv6 with SRH user plane or PPR with PPR-ID [I-D.chunduri-dmm-5g-mobility-with-ppr], can be supplied to SMF for mapping a particular PDU session to the transport path.

4. Acknowledgements

Thanks to Young Lee for discussions on this document including ACTN applicability for the proposed TNF. Thanks to Sri Gundavelli, Kausik Majumdar and 3GPP delegates who provided detailed feedback on this document.

5. IANA Considerations

This document has no requests for any IANA code point allocations.

6. Security Considerations

This document does not introduce any new security issues.

7. Contributing Authors

The following people contributed substantially to the content of this document and should be considered co-authors.

Richard Li
2330 Central Expressway
Santa Clara
CA 95050
Luis M. Contreras
Sur-3 building, 3rd floor
Madrid 28050
Xavier De Foy
InterDigital Communications, LLC
1000 Sherbrooke West


8. References

8.1. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.

8.2. Informative References

Alliance for Telecommunications Industry Solutions (ATIS), "IOT Categorization: Exploring the Need for Standardizing Additional Network Slices ATIS-I-0000075", .
Bogineni, K., Akhavain, A., Herbert, T., Farinacci, D., Rodriguez-Natal, A., Carofiglio, G., Auge, J., Muscariello, L., Camarillo, P., and S. Homma, "Optimized Mobile User Plane Solutions for 5G", Work in Progress, Internet-Draft, draft-bogineni-dmm-optimized-mobile-user-plane-01, , <>.
Homma, S., Miyasaka, T., Matsushima, S., and D. Voyer, "User Plane Protocol and Architectural Analysis on 3GPP 5G System", Work in Progress, Internet-Draft, draft-ietf-dmm-5g-uplane-analysis-04, , <>.
Matsushima, S., Filsfils, C., Kohno, M., Garvia, P. C., Voyer, D., and C. E. Perkins, "Segment Routing IPv6 for Mobile User Plane", Work in Progress, Internet-Draft, draft-ietf-dmm-srv6-mobile-uplane-16, , <>.
Farrel, A., Gray, E., Drake, J., Rokui, R., Homma, S., Makhijani, K., Contreras, L. M., and J. Tantsura, "Framework for IETF Network Slices", Work in Progress, Internet-Draft, draft-ietf-teas-ietf-network-slices-04, , <>.
GSM Association (GSMA), "Guidelines for IPX Provider Networks (Previously Inter-Service Provider IP Backbone Guidelines, Version 14.0", .
O-RAN Alliance (O-RAN), "O-RAN Fronthaul Working Group; Control, User and Synchronization Plane Specification; v2.0.0", .
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, , <>.
Beeram, V., Ed., Minei, I., Shakir, R., Pacella, D., and T. Saad, "Techniques to Improve the Scalability of RSVP-TE Deployments", RFC 8370, DOI 10.17487/RFC8370, , <>.
Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., Decraene, B., Litkowski, S., and R. Shakir, "Segment Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, , <>.
Ceccarelli, D., Ed. and Y. Lee, Ed., "Framework for Abstraction and Control of TE Networks (ACTN)", RFC 8453, DOI 10.17487/RFC8453, , <>.
3rd Generation Partnership Project (3GPP), "System Architecture for 5G System; Stage 2, 3GPP TS 23.501 v2.0.1", .
3rd Generation Partnership Project (3GPP), "Procedures for 5G System; Stage 2, 3GPP TS 23.502, v2.0.0", .
3rd Generation Partnership Project (3GPP), "Policy and Charging Control System for 5G Framework; Stage 2, 3GPP TS 23.503 v1.0.0", .
3rd Generation Partnership Project (3GPP), "Management and Orchestration Architecture Framework (Release 15)", .
3rd Generation Partnership Project (3GPP), "Management and orchestration; 5G Network Resource Model (NRM); Stage 2 and stage 3 (Release 17)", .
3rd Generation Partnership Project (3GPP), "GPRS Tunneling Protocol User Plane (GTPv1-U), 3GPP TS 29.281 v15.1.0", .
3rd Generation Partnership Project (3GPP), "NR; NR and NG-RAN Overall Description; Stage 2; v15.7.0", .
3rd Generation Partnership Project (3GPP), "NG-RAN; Architecture description; v15.7.0", .

Appendix A. New Control Plane and User Planes

A.1. Slicing Framework and RAN Aspects

The 3GPP architecture defines slicing aspects where the Network Slice Selection Function (NSSF) assists the Access Mobility Manager (AMF) and Session Management Function (SMF) to assist and select the right entities and resources corresponding to the slice requested by the User Equipment (UE). The User Equipment (UE) indicates information regarding the set of slices it wishes to connect, in the Network Slice Selection Assistance Information (NSSAI) field during network registration procedure (Attach) and the specific slice the UE wants to establish an IP session, in the Specific NSSAI (S-NSSAI) field during the session establishment procedure (PDU Session Establishment). The AMF selects the right SMF and the SMF in turn selects the User Plane Functions (UPF) so that the QoS and capabilities requested can be fulfilled.

The architecture for the Radio Access Network (RAN) is defined in [TS.38.300-3GPP] and [TS.38.401-3GPP]. The 5G RAN architecture allows disaggregation of the RAN into a Distributed Unit (DU) and a Centralized Unit (CU). The CU is further split into control plane (CU-CP) and user plane (CU-UP). The interface between CU-UP and the DU for the user plane traffic is called the F1-U and between the CU-CP and DU for the control plane traffic is called the F1-C. The F1-C and the F1-U together are called the mid-haul interfaces. The DU does not have a CP/UP split. Apart from 3GPP, O-RAN Alliance has specified further disaggregation of the RAN at the lower layer (physical layer). The DU is disaggregated into a ORAN DU (O-DU) which runs the upper part of the physical layer, MAC and RLC and the ORAN Radio Unit (O-RU) which runs the lower part of the physical layer. The interface between the O-DU and the O-RU is called the Fronthaul interface and is specified in [ORAN-WG4.CUS-O-RAN].

A.2. Slice aware Mobility: Discrete Approach

In this approach transport network functionality from the 5G-AN to UPF is discrete and 5GS is not aware of the underlying transport network and the resources available. Deployment specific mapping function is used to map the GTP-U encapsulated traffic at the 5G-AN (e.g. gNB) in UL and UPF in DL direction to the appropriate transport slice or transport Traffic Engineered (TE) paths. These TE paths can be established using RSVP-TE [RFC3209] for MPLS underlay, SR [RFC3209] for both MPLS and IPv6 underlay or PPR [I-D.chunduri-dmm-5g-mobility-with-ppr] with MPLS, IPv6 with SRH, native IPv6 and native IPv4 underlays.

As per [TS.23.501-3GPP] and [TS.23.502-3GPP] the SMF controls the user plane traffic forwarding rules in the UPF. The UPFs have a concept of a "Network Instance" which logically abstracts the underlying transport path. When the SMF creates the packet detection rules (PDR) and forwarding action rules (FAR) for a PDU session at the UPF, the SMF identifies the network instance through which the packet matching the PDR has to be forwarded. A network instance can be mapped to a TE path at the UPF. In this approach, TNF as shown in Figure 1 need not be part of the 5G Service Based Interface (SBI). Only management plane functionality is needed to create, monitor, manage and delete (life cycle management) the transport TE paths/transport slices from the 5G-AN to the UPF (on N3/N9 interfaces). The management plane functionality also provides the mapping of such TE paths to a network instance identifier to the SMF. The SMF uses this mapping to install appropriate FARs in the UPF. This approach provide partial integration of the transport network into 5GS with some benefits.

One of the limitations of this approach is the inability of the 5GS procedures to know, if underlying transport resources are available for the traffic type being carried in PDU session before making certain decisions in the 5G CP. One example scenario/decision could be, a target 5G-AN selection during a N2 mobility event, without knowing if the target 5G-AN is having a underlay transport slice resource for the S-NSSAI and 5QI of the PDU session. The Integrated approach specified below can mitigate this.

Authors' Addresses

Uma Chunduri (editor)
2191 Laurelwood Rd
Santa Clara, CA 95054
United States of America
John Kaippallimalil (editor)
Sridhar Bhaskaran
Jeff Tantsura
Praveen Muley
440 North Bernardo Ave
Mountain View, CA 94043
United States of America