Transport Network aware Mobility for 5G
draft-clt-dmm-tn-aware-mobility-09
DMM Working Group U. Chunduri, Ed.
Internet-Draft R. Li
Intended status: Informational Futurewei
Expires: August 16, 2021 S. Bhaskaran
Altiostar
J. Kaippallimalil, Ed.
Futurewei
J. Tantsura
Apstra, Inc.
L. Contreras
Telefonica
P. Muley
Nokia
February 12, 2021
Transport Network aware Mobility for 5G
draft-clt-dmm-tn-aware-mobility-09
Abstract
This document specifies a framework and mapping from slices in 5G
mobile systems to transport slices in IP, Layer 2 and 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 should be mapped to the transport
network slice characteristics that include bandwidth, latency and
criteria such as isolation, directionality and disjoint routes.
Mobile slice criteria need to be 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].
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Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 16, 2021.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. IETF Network Slicing Terminology . . . . . . . . . . . . 4
1.2. Problem Statement . . . . . . . . . . . . . . . . . . . . 4
1.3. Solution Approach . . . . . . . . . . . . . . . . . . . . 5
1.4. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Transport and Slice aware Mobility in 5G Networks . . . . . . 7
2.1. Backhaul and Mid-Haul Transport Network . . . . . . . . . 8
2.1.1. IETF Network Slicing Applicability . . . . . . . . . 10
2.1.2. Front Haul Transport Network . . . . . . . . . . . . 10
2.2. Mobile Transport Network Context (MTNC) and Scalability . 10
2.3. Transport Network Function (TNF) . . . . . . . . . . . . 11
2.4. Transport Provisioning . . . . . . . . . . . . . . . . . 12
2.5. MTNC-ID in the Data Packet . . . . . . . . . . . . . . . 13
2.6. Functionality for E2E Management . . . . . . . . . . . . 14
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3. Transport Network Underlays . . . . . . . . . . . . . . . . . 16
3.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 16
3.2. Transport Network Technologies . . . . . . . . . . . . . 18
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
6. Security Considerations . . . . . . . . . . . . . . . . . . . 19
7. Contributing Authors . . . . . . . . . . . . . . . . . . . . 19
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.1. Normative References . . . . . . . . . . . . . . . . . . 19
8.2. Informative References . . . . . . . . . . . . . . . . . 19
Appendix A. New Control Plane and User Planes . . . . . . . . . 22
A.1. Slicing Framework and RAN Aspects . . . . . . . . . . . . 22
A.2. Slice aware Mobility: Discrete Approach . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
The 3GPP architecture for 5GS is defined in [TS.23.501-3GPP],
[TS.23.502-3GPP] and [TS.23.503-3GPP]. The architecture 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.
UPFs are the data forwarding entities in the 5GC architecture. The
architecture allows the placement of Branching Point (BP) and Uplink
Classifier (ULCL) UPFs closer to the access network (5G-AN). The 5G-
AN can be a radio access network 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 interface between the various UPF instances and the (R)AN and
also, for the F1-U interface between the DU and the CU in the 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. There may be multiple instances of a slice
type to satisfy some characteristics like isolation. The slice
details in 3GPP, ATIS or NGMN do not specify how slice
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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. The document
proposes to map a slice in the 3GPP domain to a transport domain
slice. The document also proposes to carry this provisioned mapping
in an IP packet so that the IP transport domain can classify and
provide the requisite service.This is explored further in this
document.
1.1. IETF Network Slicing Terminology
[I-D.ietf-teas-ietf-network-slice-definition] 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 and some of these segments are referred to as IETF Network
slices.
[I-D.nsdt-teas-ns-framework] defines a generic framework based on the
[I-D.ietf-teas-ietf-network-slice-definition] 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, does not consider the resources and
functionalities needed from the transport network for the selection
of UPF. This is seen as independent functionality and 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, 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.
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The 5GS provides slices to its clients (UEs). The UE's PDU session
spans the access network (radio 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 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
AMF - Access and Mobility Management Function (5G)
BP - Branch Point (5G)
CSR - 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)
FRR - Fast ReRoute
gNB - 5G NodeB
GBR - Guaranteed Bit Rate (5G)
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GTP-U - GPRS Tunneling Protocol - Userplane (3GPP)
IGP - Interior Gateway Protocols (e.g. IS-IS, OSPFv2, OSPFv3)
LFA - Loop Free Alternatives (IP FRR)
mIOT - Massive IOT (5G)
MPLS - Multi Protocol Label Switching
NSSMF - Network Slice Selection Management Function
QFI - QoS Flow ID (5G)
PPR - Preferred Path Routing
PDU - Protocol Data Unit (5G)
PW - Pseudo Wire
RAN - Radio Access Network
RQI - Reflective QoS Indicator (5G)
SBI - Service Based Interface (5G)
SID - Segment Identifier
SMF - Session Management Function (5G)
SSC - Session and Service Continuity (5G)
SST - Slice and Service Types (5G)
SR - Segment Routing
TE - Traffic Engineering
ULCL - Uplink Classifier (5G)
UP - User Plane(5G)
UPF - User Plane Function (5G)
URLLC - Ultra reliable and low latency communications (5G)
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2. Transport and Slice aware Mobility in 5G Networks
3GPP architecture [TS.23.501-3GPP], [TS.23.502-3GPP] describe slicing
in 5GS. However, the application of 5GS slices in transport network
for backhaul, mid-haul and front haul are not explicitly covered. 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 a 3GPP Network Functions
(gNB-DU, gNB-CU, UPF) to obtain slice aware classification from an
IP/L2 transport network.
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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].
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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 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
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.
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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-slice-definition]. 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
further described in the [I-D.nsdt-teas-ns-framework].
2.1.2. Front Haul 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 is 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 is 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, QoS configuration for a transport path
between two 3GPP user plane functions. The Mobile-Transport Network
Context Identifier (MTNC-ID) is generated by the TNF to be unique for
each path and per traffic class (including QoS and slice aspects).
Thus, there may be more than one MTNC-ID for the same QoS and path if
there is a need to provide isolation (slice) of the traffic. It
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should be noted that MTNC are per class/path and not per user session
(nor is it per data path entity). The MTNC-IDs are configured by the
TNF to be unique within a provisioning domain.
Since the MTNC-IDs are not generated per user flow or session, there
is no need for unique MTNC-IDs per flow/session. In addition, since
the traffic estimation is not performed at the time of session
establishment, there is no provisioning delay experienced during
session setup. The MTNC-ID space scales as a square of the number
sites between which 3GPP user plane functions require 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 slices for the same QoS class 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.
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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
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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.
Thus, Layer 2 alternatives such as VLAN will fail if there are
multiple L2 networks on the F1-U or N3 or N9 path. GTP (F1-U, N3, N9
encapsulation header) field extensions offer a possibility, however
these extensions are hard 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.)
IPv6 extension headers like SRv6 may be options to carry the MTNC-ID
when such a mechanism is a viable (if a complete transport network is
IPv6 based). To minimise the protocol changes are required 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
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port with an MTNC-ID mapping. 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 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 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:
o 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.
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o 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).
o 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.
o 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.
o 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.
o 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.
o 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.
o 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
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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
o 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:
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+----------------+------------+------------------+-----------------+
|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
o 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.
o Same set of TE Paths are created uniformly across all needed 5G-
ANs and UPFs to allow various mobility scenarios.
o 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.
o 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
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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.
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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.
Xavier De Foy
InterDigital Communications, LLC
1000 Sherbrooke West
Montreal
Canada
Email: Xavier.Defoy@InterDigital.com
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
8.2. Informative References
[ATIS075] Alliance for Telecommunications Industry Solutions (ATIS),
"IOT Categorization: Exploring the Need for Standardizing
Additional Network Slices ATIS-I-0000075", September 2019.
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[I-D.bogineni-dmm-optimized-mobile-user-plane]
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", draft-bogineni-dmm-
optimized-mobile-user-plane-01 (work in progress), June
2018.
[I-D.ietf-dmm-5g-uplane-analysis]
Homma, S., Miyasaka, T., Matsushima, S., and D. Voyer,
"User Plane Protocol and Architectural Analysis on 3GPP 5G
System", draft-ietf-dmm-5g-uplane-analysis-04 (work in
progress), November 2020.
[I-D.ietf-dmm-srv6-mobile-uplane]
Matsushima, S., Filsfils, C., Kohno, M., Camarillo, P.,
Voyer, D., and C. Perkins, "Segment Routing IPv6 for
Mobile User Plane", draft-ietf-dmm-srv6-mobile-uplane-09
(work in progress), July 2020.
[I-D.ietf-teas-ietf-network-slice-definition]
Rokui, R., Homma, S., Makhijani, K., Contreras, L., and J.
Tantsura, "Definition of IETF Network Slices", draft-ietf-
teas-ietf-network-slice-definition-00 (work in progress),
January 2021.
[I-D.nsdt-teas-ns-framework]
Gray, E. and J. Drake, "Framework for Transport Network
Slices", draft-nsdt-teas-ns-framework-04 (work in
progress), July 2020.
[IR.34-GSMA]
GSM Association (GSMA), "Guidelines for IPX Provider
Networks (Previously Inter-Service Provider IP Backbone
Guidelines, Version 14.0", August 2018.
[ORAN-WG4.CUS-O-RAN]
O-RAN Alliance (O-RAN), "O-RAN Fronthaul Working Group;
Control, User and Synchronization Plane Specification;
v2.0.0", August 2019.
[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, December 2001,
<https://www.rfc-editor.org/info/rfc3209>.
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[RFC8370] 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, May 2018,
<https://www.rfc-editor.org/info/rfc8370>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC8453] Ceccarelli, D., Ed. and Y. Lee, Ed., "Framework for
Abstraction and Control of TE Networks (ACTN)", RFC 8453,
DOI 10.17487/RFC8453, August 2018,
<https://www.rfc-editor.org/info/rfc8453>.
[TS.23.501-3GPP]
3rd Generation Partnership Project (3GPP), "System
Architecture for 5G System; Stage 2, 3GPP TS 23.501
v2.0.1", December 2017.
[TS.23.502-3GPP]
3rd Generation Partnership Project (3GPP), "Procedures for
5G System; Stage 2, 3GPP TS 23.502, v2.0.0", December
2017.
[TS.23.503-3GPP]
3rd Generation Partnership Project (3GPP), "Policy and
Charging Control System for 5G Framework; Stage 2, 3GPP TS
23.503 v1.0.0", December 2017.
[TS.28.533-3GPP]
3rd Generation Partnership Project (3GPP), "Management and
Orchestration Architecture Framework (Release 15)", June
2018.
[TS.29.281-3GPP]
3rd Generation Partnership Project (3GPP), "GPRS Tunneling
Protocol User Plane (GTPv1-U), 3GPP TS 29.281 v15.1.0",
December 2018.
[TS.38.300-3GPP]
3rd Generation Partnership Project (3GPP), "NR; NR and NG-
RAN Overall Description; Stage 2; v15.7.0", September
2019.
[TS.38.401-3GPP]
3rd Generation Partnership Project (3GPP), "NG-RAN;
Architecture description; v15.7.0", September 2019.
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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.
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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)
Futurewei
2330 Central Expressway
Santa Clara, CA 95050
USA
Email: umac.ietf@gmail.com
Richard Li
Futurewei
2330 Central Expressway
Santa Clara, CA 95050
USA
Email: richard.li@futurewei.com
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Sridhar Bhaskaran
Altiostar
Email: sridharb@altiostar.com
John Kaippallimalil (editor)
Futurewei
Email: john.kaippallimalil@futurewei.com
Jeff Tantsura
Apstra, Inc.
Email: jefftant.ietf@gmail.com
Luis M. Contreras
Telefonica
Sur-3 building, 3rd floor
Madrid 28050
Spain
Email: luismiguel.contrerasmurillo@telefonica.com
Praveen Muley
Nokia
440 North Bernardo Ave
Mountain View, CA 94043
USA
Email: praveen.muley@nokia.com
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