Mobility aware Transport Network Slicing for 5G
draft-ietf-dmm-tn-aware-mobility-08
The information below is for an old version of the document.
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|---|---|---|---|
| Authors | Uma Chunduri , John Kaippallimalil , Sridhar Bhaskaran , Jeff Tantsura , Praveen Muley | ||
| Last updated | 2023-10-18 (Latest revision 2023-07-05) | ||
| Replaces | draft-clt-dmm-tn-aware-mobility | ||
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draft-ietf-dmm-tn-aware-mobility-08
DMM Working Group U. Chunduri, Ed.
Internet-Draft Intel Corporation
Intended status: Informational J. Kaippallimalil, Ed.
Expires: 20 April 2024 Futurewei
S. Bhaskaran
Rakuten Symphony
J. Tantsura
Microsoft
P. Muley
Nokia
18 October 2023
Mobility aware Transport Network Slicing for 5G
draft-ietf-dmm-tn-aware-mobility-08
Abstract
Network slicing in 5G supports logical networks for communication
services of multiple 5G customers to be multiplexed over the same
infrastructure. While 5G slicing covers logical separation of
various aspects of 5G services, user's data plane packets over the
radio access network (RAN) and mobile core network (5GC) use IP
transport in many segments of the end-to-end 5G slice. When end-to-
end slices in a 5G system use IP network resources, they are mapped
to corresponding IP transport network slice(s) which in turn provide
the bandwidth, latency, isolation and other criteria requested by the
5G slice.
This document describes mapping of 5G slices to IP or Layer 2
transport network slices when the IP transport network (slice
provider) is separated from the networks in which the 5G network
functions are deployed, for example, 5G functions that are
distributed across data centers. The slice mapping proposed here is
supported transparently when a 5G user device moves across 5G
attachment points and session anchors.
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.
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Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Mapping 3GPP Slices to IP Network Slices . . . . . . . . . . 6
3.1. Overview of 5G End-to-End Network Slicing . . . . . . . . 6
3.2. Fronthaul and Mid-Haul Transport Network . . . . . . . . 9
3.3. Backhaul Transport Network . . . . . . . . . . . . . . . 10
3.4. Slice Mapping using UDP Source Port . . . . . . . . . . . 11
4. Transport Network Underlays . . . . . . . . . . . . . . . . . 15
4.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 15
4.2. Transport Network Technologies . . . . . . . . . . . . . 17
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
7. Security Considerations . . . . . . . . . . . . . . . . . . . 18
8. Contributing Authors . . . . . . . . . . . . . . . . . . . . 18
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. Normative References . . . . . . . . . . . . . . . . . . 19
9.2. Informative References . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
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1. Introduction
3GPP architecture for 5GS in [TS.23.501-3GPP], [TS.23.502-3GPP] and
[TS.23.503-3GPP] for 5GC (5G Core), and the NG-RAN architecture
defined in [TS.38.300-3GPP] and [TS.38.401-3GPP] describe slicing as
one of the capabilities for the communication services that 5G
systems offer. Slice types defined in 3GPP and offered to its
clients (UEs) include enhanced mobile broadband (eMBB)
communications, ultra-reliable low latency communications(URLLC),
massive internet of things (MIoT), vehicle-to-X (V2X) and high
performance machine type communications (HMTC) and can be extended in
future to include new slice types.
Slices in 3GPP are logical sets of 3GPP resources that may include
multiple segments across 3GPP control and user plane functions in the
core and radio access networks. For example, a 5G slice instance
requested by an end-user may be realized by multiple slice subnets
that span user plane network functions including the UPF (User Plane
Function), gNB-CU (generalized Node-B Centralized Unit) and gNB-DU
(generalized Node-B Distributed Unit) and corresponding 3GPP
interfaces N3, N9, F1-U. However, considering the IP transport
network aspects for realizing 3GPP slicing:
* 3GPP standards do not specify the capabilities needed by
underlying IP transport network or slices.
* 3GPP standards define how interfaces N3, N9, F1-U are (re)selected
but they do not specify the underlying transport network
(re)selection.
* 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 3GPP interfaces N3, N9 and F1U may use
multiple technologies or network providers on path and the slice in
3GPP domain is mapped in each corresponding transport domain. 5G
system slices for distributed infrastructure that make up the 5G
system, and 5G slices offered to its end users (UE) are mapped to
transport domain slices to provide the corresponding level of
bandwidth, isolation and other capabilities. The gNB-CU maintains
the upper layer functionality of the radio access network while the
gNB-DU runs the lower layers of the radio access network stack (e.g.,
RLC, MAC and PHY based on the split), typically called the BBU (Base
Band Unit). Thus, a 3GPP F1-U slice subnet instance would typically
be used to carry all user traffic between a gNB-DU and gNB-CU. The
N3 and N9 transport segments between the gNB and UPF(s) on the other
hand handle user traffic based on the subscribed/offered level of
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service. Thus, an end-user's traffic for eMBB service and assigned
3GPP slice may be mapped to a different IP transport slice across N3
and N9 segments than traffic for URLLC service. Mapping and binding
between 3GPP slice and a new IP transport slice happens transparently
as the end-user moves across attachment points in the radio network
and session anchors in 5GC. Unlike mapping of a fronthaul 3GPP slice
to an IP transport slice, the IP transport slice that F1-U/N3/N9
slice is mapped to is aware of the slice characteristics of the UE
session during initial setup (user initiates 3GPP connectivity
session) and following mobility. For example, a UE served by the
3GPP system for high throughput, low latency service and related 3GPP
slice should be mapped to an IP transport slice that offers the
corresponding characteristics even after handover.
Different network scenarios and mechanisms to map 3GPP and IETF
network slices are found in
[I-D.ietf-teas-5g-network-slice-application].
[I-D.ietf-teas-5g-network-slice-application] also describes the
relationship between slice handling in the 3GPP management plane
([TS.28.541-3GPP]) and IETF network slice management. This document
focuses on a specific set of options outlined in
[I-D.ietf-teas-5g-network-slice-application]. The use of UDP source
port in GTP-U outer header and L2 VLAN to map between a 5G slice and
corresponding IETF network slice segments is described in detail
here. The main considerations in the mapping methods proposed here
include simplicity (i.e., use of L2 VLAN across a Layer-2 network)
and efficiency of inspecting the slice mapping parameters on a per
packet basis (i.e., source UDP port across routed IP networks) when
the IP transport network (slice provider) is separated from the
networks in which the 5G network functions are deployed (for example,
5G functions distributed across data centers). Section 3 describes
these methods in detail. How other IETF TE technologies applicable
for this draft is described in Section 4.2.
[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. IETF Network slice in
[I-D.ietf-teas-ietf-network-slices] and IP transport slice in this
document are the same in the context of descriptions here. When
applied to 5G, they both refer to slices for the connectivity
segments between various 5G functions (i.e. 5G-AN which includes NG-
RAN, ULCL UPF, BP UPF and PSA UPF).
2. Terminology
5G-AN – 5G Access Network
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AC – Attachment Circuit
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)
GTP-U – GPRS Tunneling Protocol - User plane (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
NG-RAN – Next Generation Radio Access Network (i.e., gNB, NG-eNB -
RAN functions which connect to 5GC)
NSC – Network Slice Controller
NSSAI – Network Slice Selection Assistance Information
NSSF – Network Slice Selection Function
NSSMF – Network Slice Selection Management Function
PCP – Priority Code Point (Ethernet)
PPR – Preferred Path Routing
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PDU – Protocol Data Unit (5G)
PW – Pseudo Wire
RQI – Reflective QoS Indicator (5G)
SBI – Service Based Interface (5G)
SDP – Service Demarcation Point
SID – Segment Identifier
SMF – Session Management Function (5G)
S-NSSAI – Single Network Slice Selection Assistance Information
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)
3. Mapping 3GPP Slices to IP Network Slices
3.1. Overview of 5G End-to-End Network Slicing
3GPP architecture in [TS.23.501-3GPP], [TS.23.502-3GPP] specify
slicing in 5GS and an overview is provided here. 5GS comprises of
control plane network functions and user plane network functions.
Communication services offered to 3GPP clients (UE) are associated to
one or more slices represented by NSSAI (Network Slice Selection
Assistance Information) both on the control plane and the user plane.
The NSSAI is realized trhough the 5G management plane using network
slice subnet (NSS), for example, a network slice subnet that contains
network function instances of the core network control plane
functions (e.g., SMF, NRF), user plane network functions (e.g., UPF),
radio network slice common functions (e.g., gNB-DU, gNB-CU-CP) and
and radio network (e.g,. gNB-CU-UP). Within the 3GPP domain, the
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control plane slicing is end-to-end while the user plane slicing ends
at the UPF. User plane slicing outside of the UPF towards IP
services is outside the scope of 3GPP and is addressed in
[I-D.mcd-rtgwg-extension-tn-aware-mobility]. 3GPP documents mention
transport network in the context of network slice subnets, but do not
provide any details. 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 provision corresponding resources in the transport
domain and carry a slice identifier that is understood by both the
customer (3GPP network) and the provider (transport network). This
section provides an overiew and subsequent sections describe 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.
5G Control and Management Planes
+--------------------------------------------------------------------+
| +----------------------------------------------------------------+ |
| | 5G Management Plane (NSSMF) | |
| +---+--------------+-------------+---------------+-----------+---+ |
| | | | | | |
| +---+--+ | F1-C +----+-----+ | N2 +----+---+ |
| | |----------(---------|gNB-CU(CP)|--------(-------| 5GC CP | |
| | | | +----+-----+ | +----+---+ |
+-| |-----------|-------------|---------------|-----------|-----+
| | | | | |
| | | e.g. | | e.g. |
| | | ACTN | | ACTN |
| | +---+---+ | +---+---+ |
| | | | | | | |
|gNB-DU| | NSC | E1 | NSC | |
| | | | | | | |
| | +---+---+ | +---+---+ |
| | | | | |
| | | | | |
| | __ +__ | ___+__ |
| | __/ \__ +--+---+ __/ \__ +-+-+
| | / IP/L2 \ | gNB | / IP \ | |
UE--+ +--(PE) Mid-haul (PE)--+CU(UP)+--(PE) Backhaul(PE)--+UPF+--DN
+------+ \__ __/ +------+ \__ __/ +---+
\______/ \______/
|------ F1-U -------| |----- N3 or N9 ------|
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Figure 1: Backhaul and Mid-haul Transport Network for 5G
Figure 1 depicts a 5G System (5GS) expanded to show the IP transport
and PE (Provider Edge) routers providing IP transport service to 5GS
user plane entities 5G-AN (e.g. gNB) and UPF. The Provider Edge (PE)
represents the Service Demarcation Point (SDP) to the transport
network that provides the slice capabilities. The IETF Network Slice
Controller (NSC) interfaces with the 3GPP network (customer network)
that requests for transport network slices (IETF network slice). The
5G management plane in turn requests the Network Slice Controller
(NSC) to setup resources and connectivity in the transport network to
realize the particular network slice. 5G network slice orchestration
is defined in [TS.28.533-3GPP] and is represented in Figure 1 as
Network Slice Selection Management Function (NSSMF)) which is a part
of 5G Management system responsible for managing network slice
subnets. The 3GPP network (customer network) requests for IP
transport network slice (slice provider network) based on estimated
demand. The 5G management plane slice orchestration functionality in
3GPP requests for transport slices via the NSC and may use ACTN
[RFC8453]. The Network Data Analytics Function (NWDAF), Network
Slice Selection Management Function (NSSMF) and other 3GPP functions
in the control and management planes provide data and functionality
to estimate slice capabilities required from the transport network.
The slices provisioned in the IP transport network correspond to 3GPP
slices represented by NSSAI (set of 3GPP slices) available at 3GPP
access/data networks and locations. During 3GPP procedures for
session initialization, the network grants an S-NSSAI (single one out
of the NSSAI) based on the user's session request. The S-NSSAI for
the UE's session is signaled to the user plane nodes (gNB, UPF)
during the session setup and used to associate to the corresponding
IP transport slice. An overview of the sequence of operations from
when a user (UE) requests during session setup to how it relates to
the fronthaul, mid-haul and backhaul transport network slices is
provided below.
Prior to 3GPP user (UE) signaling to setup a session, the UE attaches
to the radio access network and has the parameters for operation
configured. During this sequence of operation, the signaling is
between the UE and the gNB. When the gNB functionality is split
between a central unit (CU) and a distributed unit (DU), a mid-haul
transport segment provides the connectivity as shown in Figure 1.
Further, the gNB central unit can be split into the control plane
(CP) and user plane (UP) logical functions as shown in Figure 1. 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. An important point
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to note is that signaling and data transport over the the fronthaul
transport has no notion of an end-user/UE session, but is rather
defined by low latency and other requirements required for processing
radio signaling and data transport between the network entities that
compose gNB. For the front-haul described further in Section 3.2, an
Ethernet transport with VLANs can be expected to be the case in many
deployments.
Folowing the radio access setup and attach, the 3GPP user (UE)
signals to setup a session. 5G core network (5GC) functionality to
handle access mobility (AMF), UE session management (SMF), policy
(PCF) and various other assisting functionality including 3GPP slice
selection (NSSF) are used to authenticate the UE and setup the data
plane to transport the UE PDU (Protocol Data Units). 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). From an IP transport
network perspective, these GTP-U connections can be viewed as
multiple overlay connection segments between each of the 3GPP data
plane entities (gNB, UPF) on a per UE basis. The GTP-U/overlay
transport capabilities required are signaled between the RAN and 5GC
during UE session setup. Note that unlike the slice requirements for
mid-haul segment (F1-U), the slice requirements for the backhaul (N3,
N9) are setup in the 3GPP network on a per UE basis. Thus, in the
backhaul, an IP transport slice with capabilities corresponding to
the S-NSSAI negotiated between UE 5GC is provided. For example, a UE
that sets up an eMBB session may require high bandwidth but can
tolerate delay whereas a URLLC session requires low latency, low
jitter and low error rates. Slice capabilities along the user plane
path between the (R)AN and UPFs such as a low latency path, jitter,
protection and priority needs to be provided by the IP transport
network. 3GPP core network entities may be deployed across multiple
data centers and in such cases require the IP transport network to
provide the resources and connectivity for each of the slice
segments.
3.2. Fronthaul and Mid-Haul Transport Network
The O-RAN Alliance defined a lower layer split at the physical layer
of the radio access network whereby the gNB-DU is split into two
entities (O-RU and O-DU) and 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 an Ethernet based fronthaul
interface, the network slice instance (NSI) information is carried in
the Ethernet header through the VLAN tags and/or PCP marking. The
Ethernet switches in the fronthaul transport network inspects the
slice information (VLAN tag and/or PCP marking) in the Ethernet
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header and provide the provisioned capabilities. The mapping of I
and Q samples of different radio resources (radio resource blocks or
carriers) to different slices and to their respective VLAN tags and
or PCP marking on the fronthaul interface is controlled by the O-RAN
fronthaul C-Plane and M-Plane interfaces. 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 NC pertaining to the fronthaul transport. 3GPP
functions gNB-CU (user plane) and gNB-DU may also be distributed and
have a mid-haul transport between the two 3GPP network functions. If
an IP based mid-haul interface is used, the network slice instance
(NSI) information can be MPLS, SRv6 based as defined in
[TS.28.541-3GPP]. However if the 3GPP network function (slice
customer) is physically separated from the slice provider network
(e.g., a gNB-CU (user plane) with baseband units deployed in a data
center), the MPLS, SRv6 information may not be practical to carry
across to the separated IP transport network (slice provider). In
this case, the source UDP port of the GTP-U can be used to indicate
the slice in the IP transport network (slice provider).
3.3. Backhaul Transport Network
The backhaul transport over which the protocols for N3 and N9
interfaces run are described in [TS.23.501-3GPP] and
[TS.23.502-3GPP]. The end-user (UE) sessions (IP, Ethernet,
unstructured) are carried over GTP-U transport protocol over the N3
and N9 interfaces. GTP-U between the the 3GPP network functions
(gnB, UPF) serves as an overlay protocol across one or more MPLS,
SRv6 or Ethernet transport networks in between. During UE session
setup, a number of parameters for context management are configured
in the gNB, UPF and that includes allowed network slice (S-NSSAI).
On an Ethernet based backhaul interface, the slice information is
carried in the Ethernet header through the VLAN tags. If an IP based
backhaul interface is used, the network slice instance (NSI)
information can be MPLS, SRv6 based as defined in [TS.28.541-3GPP].
However, if the 3GPP network function (slice customer) is physically
separated from the slice provider network (e.g., a gNB-CU (user
plane) or UPF, or both are deployed in a data center), the MPLS, SRv6
information may not be practical to carry across to the separated IP
transport network (slice provider). In this case, the source UDP
port of the GTP-U can be used to indicate the slice in the IP
transport network (slice provider).
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3.4. Slice Mapping using UDP Source Port
Communication services offered by 3GPP and the concepts used to
provision and manage it are described in [TS.28.530-3GPP]. A brief
overview is given here with the intent to describe how it is related
to an IP transport slice and the mapping between it and the 3GPP
slice. Communication services (e.g., an eMBB service) may be
realized in a 3GPP network using one or more slices identified by
NSSAI (Network Slice Selection Assistance Information) in the 3GPP
control plane. In the 3GPP management plane, the network slice
identified by NSSAI is realized in as a Network Slice Subnet (NSS).
For example, a slice S-NSSAI is available to a user at different
locations (and even PLMNs) and maybe realized in an NSS at that a
location. The NSS consists of sets of functions from 5GC and RAN
that belong to the NSS. Network interfaces of functions in an NSS
may be associated to one or more slice subnets. These relationships
are illustrated in Figure 2. From the viewpoint of IP transport
slicing and mapping to 3GPP slices, an IP transport network slice is
associated to 3GPP core or RAN network functions in a 3GPP Network
Slice Subnet (NSS). Thus, it can represent a slice of a transport
path for a tenant between two 3GPP user plane functions.
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+-------------------+ +-------------------+ +-------------------+
| Comm. Service A | | Comm. Service B | | Comm. Service C |
+-----+-------------+ +--+-----+----------+ +--------+----------+
| ______________/ | \
| / | \
+-----+---+---+ +-------+-----+ +------+------+
|NSSAI = 000A | |NSSAI = 000B | |NSSAI = 000C |
+-------^-----+ +------^------+ +------^------+
/ / |
______/______ _____/_______ ______|_______
\ Net.Slice \ \ Net.Slice \ | Net.Slice |
\ Subnet-A \ \ Subnet-B \ | Subnet-C |
\ (NSS-A) \ \ (NSS-B) \ | (NSS-C) |
\ \ \ \ | |
\ +--------+\ \ +--------+\ | +--------+ |
\ |NSSI=CN1| \ \ |NSSI=CN1| \ | |NSSI=CN3| |
\+-----\--+ \ \+---\----+ \ | +---|----+ |
\ \ \ \ \ \ | | |
\ +===\====+\ \ +==\=====+ \ | +===|====+ |
\ |NS = IP1| \ \|NS = IP2| \ | |NS = IP3| |
\+====\===+ \ +====\===+ \ | +===|====+ |
\ \ \ \ \ \ | | |
\ +--\-----+\ +--------\-----------+ | |
\ |NSSI=AN1| \ \ \ +--\-----+ \ | |
\+--------+ \ \ \|NSSI=AN2+-----------+ |
\____________\ \ +--------+ \ |
+----\------------\--------------+
+------------+
Figure 2: Slice Configuration
Figure 2 shows the slice hierarchy described in [TS.28.530-3GPP] with
3 communication services enhanced to show the IP transport slice
instances (IP1, IP2, IP3). As an example, when a UE registers with
5GC with NSSAI 000A, OOOB and others, the AMF may select NSSAI 000B
in list of NSSAI allowed for the UE. One of the factors in selecting
the NSSAI is whether the UE may move to another location during the
lifetime of the session. In this case, the NSSAI should be such that
it has a mapping to IP transport network slice during initial attach,
and following handover. For example, a UE that attaches to 5GC with
S-NSSAI = 000B and served by user plane instances CN1 and AN2 uses IP
transport network slice NS = IP2 to provide the resources in the IP
network that correspond to the UE session. Following handover with
S-NSSAI = 000B, the UE may be served by user plane instances CN1' and
AN2' over an IP slice IP2' in the new location.
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When the 3GPP user plane function (5G-AN, UPF) and IP transport
provider edge are on different nodes or separated across a network,
the PE router needs to have the means by which to classify the IP
packet from 3GPP entity based on some header information. In
[I-D.ietf-teas-ietf-network-slices] terminology, this is a scenario
where there is an Attachment Circuit (AC) between the 3GPP entity
(customer edge) and the SDP (Service Demarcation Point) in the IP
transport network (provider edge). The Attachment Circuit(AC) may
for example be between a UPF in a data center to a (provider edge)
router that serves as the service demarcation point for the transport
network slice. The identification 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. For 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. This is illustrated in
Figure 3.
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upstream GTP-U packet
=====================================>
customer edge attachment slice provider customer edge
circuit ______
+-------------+ ______ __/ \__ +-------------+
| gNB-CU | __/ \__ / IP \ | UPF |
|N3 IP i/f = +--/ Data Center\---(PE) Backhaul (PE)--+ N3 IP i/f = |
| gNB-AN2-if | \__ Network__/ \__ __/ | UPF-CN1-if |
+-------\-----+ \______/ \___\__/ +-------------+
\ \
\ \
\ \
\ +----------\---------------+
+----------------\---------------+ | Slice Mapping: |
|+-------------------------+ | |Match: |
||3GPP CP Config: | | | src-IP-addr = gNB-AN2-if |
||NSSAI = {000B, 000C, ..} | | | src-port = 5678 |
||NSSI = AN2 | | |Action: |
|+-------------------------+ | | select NS = IP2 |
| | +--------------------------+
|+------------------------------+|
||Slice Mapping to UPF-CN1-if: ||
||EP_Transport S-NSSAI=000B ||
||logicInterfaceType = UDPSrcPrt||
||logicInterfaceId = 5678 ||
||ipAddress = UPF-CN1-if ||
|+------------------------------+|
+--------------------------------+
Figure 3: Slice Mapping using UDP source port
Figure 3 shows the configuration and mapping applied to network
instances in a 3GPP network slice subnet and corresponding IP
transport network instances for sending an upstream GTP packet from
gNB-CU (user plane) to UPF. The gNB-CU (user plane) function is in a
data center and separated from the IP transport slice provider by an
attachment circuit (AC - i.e., the data center network). In this
example, a GTP-U packet at gNB-CU (user plane) belonging a UE session
with S-NSSAI = 000B (and thus associated to 3GPP and IP transport
network instances in the figure for providing slice resources).
Since the GTP-U packet belongs to a session with S-NSSAI = 000B, the
gNB-CU (UP) maps it to UDP port 5678 in the GTP-U header (outer
encapsulation source port). The GTP-U packet is forwarded by the
data center network to the PE router at IP backhaul network. The PE
matches on GTP-U source IP address and port to select the provisioned
slice (NS = IP2). The UPF customer edge may be attached to the PE as
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shown in the figure or alternatively via a data center network. A
similar set of mappings exist for downlink GTP-U, but they do not
necessarily use the same resources.
PE routers can thus provision a policy based on the source UDP port
number 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
IP network slice 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 slice 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 IP network slice 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 slice encoding mechanism
already accounts for this.
4. 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 between 3GPP network functions.
Focus is on the user/data plane i.e., F1-U/N3/N9 interfaces as laid
out in the framework Figure 1.
4.1. Applicability
* For different slice types, there maybe correspondingly different
transport paths. For example, with 3 different SSTs, there are
potentially 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 of the GTP-U
encapsulation header. Similarly in the downlink direction
matching Transport TE Path of the 5G-AN is chosen based on the
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S-NSSAI to which the PDU Session belongs. 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 4: 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 NSSMF 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.
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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 4 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.
4.2. 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-rtgwg-preferred-path-routing]. 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).
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As specified with the integrated transport NSSMF, 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-rtgwg-preferred-path-routing], can be
supplied to SMF for mapping a particular PDU session to the transport
path.
5. Acknowledgements
Thanks to Young Lee for discussions on this document including ACTN
applicability and relation to NSSMF in the early discussions. Thanks
to Sri Gundavelli, Kausik Majumdar, Hannu Flinck, Joel Halpern,
Satoru Matsushima and Tianji Jiang who provided detailed feedback on
this document.
6. IANA Considerations
This document has no requests for any IANA code point allocations.
7. Security Considerations
This document does not introduce any new security issues.
8. Contributing Authors
The following people contributed substantially to the content of this
document and should be considered co-authors.
Richard Li
Futurewei
2330 Central Expressway
Santa Clara
CA 95050
USA
Email: richard.li@futurewei.com
Luis M. Contreras
Telefonica
Sur-3 building, 3rd floor
Madrid 28050
Spain
Email: luismiguel.contrerasmurillo@telefonica.com
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Xavier De Foy
InterDigital Communications, LLC
1000 Sherbrooke West
Montreal
Canada
Email: Xavier.Defoy@InterDigital.com
Reza Rokui
Ciena
Email: rrokui@ciena.com
9. References
9.1. 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>.
9.2. Informative References
[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", Work in Progress,
Internet-Draft, draft-bogineni-dmm-optimized-mobile-user-
plane-01, 29 June 2018,
<https://datatracker.ietf.org/doc/html/draft-bogineni-dmm-
optimized-mobile-user-plane-01>.
[I-D.chunduri-dmm-5g-mobility-with-ppr]
Chunduri, U., Contreras, L. M., Bhaskaran, S., Tantsura,
J., and P. Muley, "Transport aware 5G mobility with PPR",
Work in Progress, Internet-Draft, draft-chunduri-dmm-5g-
mobility-with-ppr-00, 2 November 2020,
<https://datatracker.ietf.org/doc/html/draft-chunduri-dmm-
5g-mobility-with-ppr-00>.
[I-D.chunduri-rtgwg-preferred-path-routing]
Bryant, S., Chunduri, U., and A. Clemm, "Preferred Path
Routing Framework", Work in Progress, Internet-Draft,
draft-chunduri-rtgwg-preferred-path-routing-03, 7 November
2022, <https://datatracker.ietf.org/doc/html/draft-
chunduri-rtgwg-preferred-path-routing-03>.
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[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", Work in Progress, Internet-Draft, draft-ietf-dmm-
5g-uplane-analysis-04, 2 November 2020,
<https://datatracker.ietf.org/doc/html/draft-ietf-dmm-5g-
uplane-analysis-04>.
[I-D.ietf-dmm-srv6-mobile-uplane]
Matsushima, S., Filsfils, C., Kohno, M., Camarillo, P.,
and D. Voyer, "Segment Routing IPv6 for Mobile User
Plane", Work in Progress, Internet-Draft, draft-ietf-dmm-
srv6-mobile-uplane-24, 17 January 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-dmm-
srv6-mobile-uplane-24>.
[I-D.ietf-teas-5g-network-slice-application]
Geng, X., Contreras, L. M., Rokui, R., Dong, J., and I.
Bykov, "IETF Network Slice Application in 3GPP 5G End-to-
End Network Slice", Work in Progress, Internet-Draft,
draft-ietf-teas-5g-network-slice-application-01, 10 July
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
teas-5g-network-slice-application-01>.
[I-D.ietf-teas-ietf-network-slices]
Farrel, A., Drake, J., Rokui, R., Homma, S., Makhijani,
K., Contreras, L. M., and J. Tantsura, "A Framework for
Network Slices in Networks Built from IETF Technologies",
Work in Progress, Internet-Draft, draft-ietf-teas-ietf-
network-slices-25, 14 September 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-teas-
ietf-network-slices-25>.
[I-D.mcd-rtgwg-extension-tn-aware-mobility]
Majumdar, K., Chunduri, U., and L. Dunbar, "Extension of
Transport Aware Mobility in Data Network", Work in
Progress, Internet-Draft, draft-mcd-rtgwg-extension-tn-
aware-mobility-08, 12 September 2023,
<https://datatracker.ietf.org/doc/html/draft-mcd-rtgwg-
extension-tn-aware-mobility-08>.
[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.
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[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>.
[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.28.530-3GPP]
3rd Generation Partnership Project (3GPP), "Aspects;
Management and Orchestration; Concepts, use cases and
requirements (Release 17)", June 2022.
[TS.28.533-3GPP]
3rd Generation Partnership Project (3GPP), "Management and
Orchestration Architecture Framework (Release 15)", June
2018.
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[TS.28.541-3GPP]
3rd Generation Partnership Project (3GPP), "Management and
orchestration; 5G Network Resource Model (NRM); Stage 2
and stage 3 (Release 17)", June 2020.
[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.
Authors' Addresses
Uma Chunduri (editor)
Intel Corporation
2191 Laurelwood Rd
Santa Clara, CA 95054
United States of America
Email: umac.ietf@gmail.com
John Kaippallimalil (editor)
Futurewei
Email: john.kaippallimalil@futurewei.com
Sridhar Bhaskaran
Rakuten Symphony
Email: sridhar.bhaskaran@rakuten.com
Jeff Tantsura
Microsoft
Email: jefftant.ietf@gmail.com
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Praveen Muley
Nokia
440 North Bernardo Ave
Mountain View, CA 94043
United States of America
Email: praveen.muley@nokia.com
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