SCONE S. Mishra
Internet-Draft Verizon
Intended status: Informational Z. Sarker
Expires: 23 April 2026 Nokia
A. Tomar
Meta
K. Abbas
Verizon
20 October 2025
Applicability & Manageability consideration for SCONE
draft-mishra-scone-applicability-manageablity-03
Abstract
This document describes the applicability and manageability
considerations for providing throughput guidance to application
endpoints in telecommunications service provider networks supporting
the Standard Communication with Network Elements (SCONE) protocol.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terms and Definitions . . . . . . . . . . . . . . . . . . . . 4
3. Generic Applicability and Manageability considerations . . . 7
3.1. Flow session awareness . . . . . . . . . . . . . . . . . 7
3.2. Per-Flow Signaling . . . . . . . . . . . . . . . . . . . 7
3.3. QoS awareness . . . . . . . . . . . . . . . . . . . . . . 7
3.4. SCONE Hint to the Network . . . . . . . . . . . . . . . . 8
3.5. Retransmission of Advised Bit-Rate . . . . . . . . . . . 8
3.6. Frequency of Updates . . . . . . . . . . . . . . . . . . 8
3.7. Dynamic Updates . . . . . . . . . . . . . . . . . . . . . 8
3.8. Monitoring and Logging . . . . . . . . . . . . . . . . . 9
3.9. Conformance Monitoring . . . . . . . . . . . . . . . . . 9
3.10. Standards Compliance . . . . . . . . . . . . . . . . . . 9
3.11. Interworking with Other Congestion Management
Mechanisms . . . . . . . . . . . . . . . . . . . . . . . 9
4. SCONE Usage in a 5G Network . . . . . . . . . . . . . . . . . 9
4.1. 5G specific considerations . . . . . . . . . . . . . . . 11
4.1.1. 3GPP Defined PDU Session Establishment Procedures . . 11
4.1.2. PDU Session Awareness . . . . . . . . . . . . . . . . 12
4.1.3. Per-Flow Signaling . . . . . . . . . . . . . . . . . 12
4.1.4. QoS Considerations . . . . . . . . . . . . . . . . . 12
4.1.5. Dynamic Updates . . . . . . . . . . . . . . . . . . . 13
4.1.6. Operations Monitoring and Logging . . . . . . . . . . 13
5. SCONE Usage in a 4G/LTE Network . . . . . . . . . . . . . . . 13
5.1. Applicability of SCONE in a 4G/LTE Network . . . . . . . 14
5.2. 4G specific considerations . . . . . . . . . . . . . . . 14
6. SCONE usage in a Wireline Network . . . . . . . . . . . . . . 14
6.1. Wireline specific considerations . . . . . . . . . . . . 15
7. SCONE usage in a Wifi Networks . . . . . . . . . . . . . . . 15
8. Security Considerations . . . . . . . . . . . . . . . . . . . 15
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
10.1. Normative . . . . . . . . . . . . . . . . . . . . . . . 15
10.2. Informative References . . . . . . . . . . . . . . . . . 15
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
11.1. Normative References . . . . . . . . . . . . . . . . . . 16
11.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. Appendix A. Additional Background details on role of
UPF in 5G Mobile Packet Core . . . . . . . . . . . . . . 16
A.1. 5G Mobile Network Architecture . . . . . . . . . . . . . 17
A.2. N3 Interface . . . . . . . . . . . . . . . . . . . . . . 17
A.3. N4 Interface . . . . . . . . . . . . . . . . . . . . . . 18
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A.4. N6 Interface . . . . . . . . . . . . . . . . . . . . . . 18
A.5. N9 Interface . . . . . . . . . . . . . . . . . . . . . . 18
A.6. User Plane Interface Between UPF and UE . . . . . . . . . 18
Appendix B. Appendix B. Non-ASCII Characters . . . . . . . . . 19
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
The SCONE protocol [I-D.ietf-scone-protocol] provides a signaling
mechanism that enables on-path SCONE-capable network elements to
communicate the maximum allowable bit rate to application endpoints,
which is particularly relevant for adaptive bit-rate applications.
This document addresses the applicability and manageability
considerations for deploying the SCONE protocol within
telecommunications service provider networks.
SCONE operates based on a UDP 4-tuple. Network elements capable of
rate limiting at this granularity can send notifications of the
maximum allowable bit rate in each direction of the observed traffic.
Such network elements may also drop or delay packets within the
corresponding UDP 4-tuple flows. This implies that on-path SCONE-
capable network elements (referred to as SCONE Network Elements in
the rest of this document) are assumed to have the following
capabilities: detect and maintain UDP 4-tuple flows, be aware of or
configurable with rate-limiting policies, and identify flows that
carry SCONE packets in order to insert throughput advice into those
packets.
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In this document, on-path SCONE Network Elements are generally
considered within the _access_ portion of the telecommunications
provider’s network. However, multiple SCONE Network Elements may
exist along a path between the communicating peers. Depending on
their configuration and roles they are likely to generate different
throughput advices for the SCONE enabled application traffic flows,
specially when differnet _access_ technologies are in use. SCONE
protocol For example, a wireless access network element may operate
differently from one in a fixed broadband network. Wi-Fi networks
provide another example, where enforcement is often per user or per
Service Set Identifier (SSID), but visibility into individual UDP
4-tuples may be limited. Among access networks, mobile networks
offer the most fine-grained visibility into traffic flows and can act
on individual flows. In mobile networks, the User Plane Function
(UPF) in 5G and the Packet Data Network Gateway (P-GW) in 4G can
generate throughput advice to guide adaptive applications on a per-
flow basis. In contrast, wireline broadband networks typically apply
rate limiting at a centralized Broadband Network Gateway (BNG) or at
aggregation points serving multiple Customer Premises Equipment (CPE)
devices.
Accordingly, applicability and manageability considerations must
encompass a wide range of access-network scenarios, each of which
handles per-flow rate limiting differently. This document first
presents generic considerations for the SCONE protocol and then
provides network-specific guidance where throughput advisory
signaling can enhance both resource utilization and user experience.
2. Terms and Definitions
This document uses terms and definitions described in
[I-D.ietf-scone-protocol], some more terms and definitions are
described below in this section.
* 4G - Fourth Generation mobile network technology, also known as
Long-Term Evolution (LTE), defined by the 3rd Generation
Partnership Project (3GPP).
* 5G - Fifth Generation Mobile Networks The fifth generation of
cellular mobile network technology defined by 3GPP.
* APN - Access Point Name The Access Point Name (APN) determines the
specific Packet Data Network Gateway (PDN-GW in 4G/LTE) that the
mobile device should use to access a service. The gateway acts as
the access point to external networks such as the public internet
or a private network. Different APNs can be used to provide
different services, access privileges, or Quality of Service to a
user's device.
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* Adaptive Bit-Rate (ABR) Video Video streaming technology that
adjusts video quality dynamically based on network conditions.
* BNG (Broadband Network Gateway) A network element that serves as
the access point for subscribers in wireline broadband networks.
It establishes and manages subscriber sessions, aggregates traffic
from multiple subscriber access nodes, and routes this traffic to
the service provider's core network. BNG functions include
subscriber authentication, IP address assignment, policy
enforcement, and quality of service management. It typically
supports subscriber session protocols such as DHCP, PPPoE, or
IPoE, and interacts with AAA and DHCP servers to enable secure and
managed access to broadband services.
* Client App The user-facing application running on an operating
system, which receives network throughput advice.
* Content Provider Entity or service that delivers media and data
content accessed by end-users.
* CPE - Customer Premise Equipment CPE refers to networking hardware
located at the customer's site and used to connect to a service
provider’s network. Typical CPE includes routers, modems, or
gateways that provide access and management for residential or
enterprise services.
* DHCP - Dynamic Host Configuration Protocol A network management
protocol used to dynamically assign IP addresses and other
configuration parameters to devices on a network, enabling
automatic and centralized network configuration.
* DNN - Data Network Name A Data Network Name (DNN) identifies the
external data network that a User Equipment (UE) connects to
within a 5G system. The DNN specifies the target data network
(for example, the Internet or an enterprise network) and is used
by the 5G Core to establish and manage the corresponding PDU
session. It is functionally equivalent to the Access Point Name
(APN) used in 4G/LTE systems.
* EPC - The Evolved Packet Core Is the all-IP core architecture for
4G/LTE, responsible for managing user sessions, mobility, and the
integration of data and voice traffic over packet-switched
networks.
* EPS Bearer - Evolved Packet System Bearer In 4G LTE networks, an
EPS bearer is a virtual transmission path with specific Quality of
Service (QoS) parameters that carries user data between the User
Equipment (UE) and the Packet Data Network Gateway (P-GW). The
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EPS bearer ensures end-to-end delivery of IP packets with
particular handling characteristics, such as priority, latency,
and guaranteed bit rate. There are two main types: the Default
EPS Bearer which provides always-on best-effort connectivity, and
Dedicated EPS Bearers configured for services with specialized QoS
requirements, such as voice or video.
* EPS Gateway In 4G LTE networks, the EPS Gateway primarily refers
to the combination of the Serving Gateway (S-GW) and the Packet
Data Network Gateway (P-GW). The Serving Gateway routes and
forwards user data packets between the E-UTRAN access network and
the Packet Data Network, acting as a mobility anchor during
handovers. The Packet Data Network Gateway provides connectivity
from the user equipment (UE) to external packet data networks,
performing functions such as policy enforcement, charging, and
lawful interception. Together, these gateways form the core user-
plane interface of the Evolved Packet System (EPS).
* gNB - Next Generation Node B 5G radio access network node
connecting user equipment to the 5G core network.
* IPoE IP over Ethernet A protocol that delivers IP packets directly
over Ethernet without requiring a login or session establishment,
commonly used in broadband networks in conjunction with DHCP for
IP address assignment.
* LTE - Long-Term Evolution 4G wireless broadband technology and
related network architecture.
* P-GW - Public Data Network Gateway Is the network function within
the Evolved Packet Core (EPC) that provides connectivity between
the user equipment and external packet data networks, such as the
Internet.
* PDU - Protocol Data Unit In 3GPP terminology, a PDU is a unit of
information at a given protocol layer, such as an IP packet at the
network layer. Specifically in 5G, a PDU Session represents a
logical connection that carries one or more PDUs between the User
Equipment (UE) and a Data Network (DN) through the User Plane
Function (UPF). PDU Sessions support multiple types of PDUs,
including IPv4, IPv6, Ethernet frames, and unstructured data, and
are associated with one or more QoS Flows that define handling and
quality requirements. The PDU framework is essential for managing
application data transport and quality of service within the 3GPP
system architecture.
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* PPP - Point-to-Point Protocol A data link layer communication
protocol used to establish a direct connection between two nodes,
commonly used for dial-up and broadband internet connections to
provide authentication, encryption, and compression.
* SCONE - Standard Communication with Network Elements Protocol
allowing throughput or rate advice signaling from the network to
application endpoints.
* SMF - Session Management Function 5G network function that manages
sessions and enforces policies.
* UE - User Equipment The mobile device or endpoint used by the
subscriber to access the network.
* UPF - User Plane Function 5G core network element responsible for
user-plane traffic routing and applying policy decisions.
* Wireline Network Broadband network based on fixed infrastructure
(e.g., DSL, cable, fiber).
3. Generic Applicability and Manageability considerations
3.1. Flow session awareness
SCONE signaling operates only over established sessions. SCONE
Network Elements ought to be able to unambiguously associate
throughput advice with application flows. Each session is bound to
an IP address and port, ensuring SCONE packets are routed precisely
without affecting unrelated traffic.
3.2. Per-Flow Signaling
Throughput advice is applied on a per–4-tuple basis. SCONE Network
Elements ought to maintain flow-specific context to ensure signaling
correctness. This enables applications to receive targeted
throughput advice while preventing unintended impact on unrelated
flows.
3.3. QoS awareness
Networks can enforce Quality of Service (QoS) using various
techniques. In some cases, operators may wish to apply separate QoS
policies to SCONE-enabled flows. The SCONE Network Element that
inserts SCONE advice does not need to interpret or enforce QoS
policies directly; it only provides the advice. Operators should be
able to identify SCONE-enabled flows and apply differentiated QoS
treatment when desired.
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3.4. SCONE Hint to the Network
SCONE-aware applications ought to provide hints to the SCONE Network
Elements, enabling it to generate appropriate throughput advice for a
given 4-tuple. Such hints prevent unnecessary default rate-limiting,
allow the network to signal the maximum allowable bit rate, and
reduce CPU overhead by eliminating additional classification steps.
3.5. Retransmission of Advised Bit-Rate
Packet loss or non-delivery of SCONE advice reduces its
effectiveness. Both SCONE Network Elements and applications should
support retransmission or periodic re-sending of SCONE packets to
ensure reliable delivery. Conformance depends on the behavior of
both network and endpoint.
3.6. Frequency of Updates
The rate at which SCONE updates are issued depends on flow
characteristics and available computational resources. Excessively
frequent updates may increase CPU load, while infrequent updates may
reduce advisory effectiveness. Network providers can define
adjustable update intervals based on application requirements,
network capacity, and operational constraints. The SCONE protocol
specifies a minimum interval of 67 seconds between updates
[I-D.ietf-scone-protocol].
3.7. Dynamic Updates
Dynamic rate limits can be enforced by the network during active
application sessions due to:
* Changes in access network type (requiring updated throughput
advice)
* Subscriber policy updates (e.g., exceeding usage thresholds)
* Adjustments to maximum allowable throughput
* Periodic refreshes of throughput advice (e.g., timers for maximum
update periodicity)
In such cases, the SCONE Network Elements need to be able to initiate
SCONE packets to provide updated advice, or applications should
generate SCONE packets frequently enough to trigger network
responses.
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3.8. Monitoring and Logging
SCONE signaling can be integrated into existing operational and
management frameworks to enable monitoring, troubleshooting, and
fault isolation. Metrics of interest include:
* Rate of SCONE advisory messages issued per session
* Correlation between SCONE advisories and user-plane throughput
changes
* Error conditions where SCONE signaling fails to reach the intended
endpoints
3.9. Conformance Monitoring
Networks providing SCONE throughput advice ought to implement
mechanisms to measure compliance, either per application flow or in
aggregate. This allows operators to validate advisory effectiveness
and adjust policies. Due flow awareness, such mechanism are
typically implemented in a SCONE Network Element but may also be
implemented elsewhere in the network.
3.10. Standards Compliance
SCONE signaling is expected to traverse the existing data path. For
example, in 3GPP-compliant networks, SCONE packets are carried within
Protocol Data Unit (PDU) sessions established between the User
Equipment (UE) and Internet endpoints.
3.11. Interworking with Other Congestion Management Mechanisms
SCONE operates independently of transport-layer mechanisms such as
Explicit Congestion Notification (ECN) or Low Latency, Low Loss, and
Scalable throughput (L4S). Operators would benefit from harmonizing
multiple congestion signaling methods by policy or scope deployments
to avoid conflicting feedback.
4. SCONE Usage in a 5G Network
5G systems consist of a 5G Radio Access Network (RAN) and a 5G Packet
Core. The 5G Packet Core is built on a cloud-native Service-Based
Architecture (SBA) and introduces the concept of Network Functions
(NFs), which provides flexibility for deploying SCONE in the network.
Appendix A describes the various network components of a 5G network.
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In 5G, the User Plane Function (UPF) is the on-path network element
with access to subscriber policy and user-plane connectivity between
the User Equipment (UE, or client application endpoint) and the
Internet. The UPF is capable of generating SCONE throughput advice
on a per-application-flow basis, enabling endpoints to adjust sending
rates proactively. SCONE signaling occurs over the existing data
path.
For a 5G network, the UPF serves as the natural anchor point for
SCONE signaling. However, due to the flexibility of 5G’s SBA, any
network component capable of meeting the applicability and
manageability considerations may act as a SCONE Network Element.
The following diagram illustrates how throughput advice can be
conveyed within a 5G network, highlighting the role of user-plane
SCONE Network Elements.
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+---------+
| PCF |
+---------+
|
v Policy Rules
+---------+
| SMF |
+----+----+
| Policy Rules
v
+--------+ +------------------------+
| Client |<===============>| |
| App | SCONE | |
+--------+ Advice | UPF |
| OS | | |
+--------+ | |
| Modem | | |
+----+---+ +------------------------+
| | |
| +-----+ | |
+---+ gNB +-------------------+ |
+-----+ |
v
+--------------+
| Internet |
+--------------+
|
|
v
+-----------------+
| Content Provider|
+-----------------+
Figure 1: SCONE Integration within the 5G SA Network
4.1. 5G specific considerations
This section describes how the SCONE protocol can be deployed and
managed within 3GPP [_5G-Arch] networks, including support for SCONE
packets over established PDU sessions.
4.1.1. 3GPP Defined PDU Session Establishment Procedures
The following high-level functions, defined in 3GPP specifications,
are relevant to SCONE manageability as SCONE packets traverse
established PDU sessions:
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1. PDU Session (5G)
A logical connection between the UE and UPF (5G), allowing the UE
to exchange IP packets with external networks such as the
Internet or a private network.
2. IP Address Allocation
During PDU Session establishment, the UE is allocated an IP
address (IPv4, IPv6, or both) used for communication with
external networks.
3. Bearer Establishment
Data traffic within a PDU session flows over radio bearers, each
with defined QoS characteristics. IP packets are mapped to QoS
flows based on packet filters at the UPF and UE.
4.1.2. PDU Session Awareness
SCONE signaling operates only over established PDU sessions. This
allows SCONE Network Elements in 5G network to unambiguously
associate throughput advice with specific UEs and application flows.
Each session is bound to a DNN and an allocated IP address, ensuring
SCONE packets are handled precisely without affecting unrelated
traffic.
4.1.3. Per-Flow Signaling
Throughput advice is applied on a per–4-tuple basis. SCONE Network
Elements in 5G network need to maintain flow-specific context to
ensure signaling correctness. This enables applications to receive
targeted throughput advice while preventing unintended impact on
unrelated flows.
4.1.4. QoS Considerations
In 5G, QoS is enforced at the granularity of QoS Flows. A single PDU
session can contain multiple QoS Flows. Operators may configure a
distinct QoS Flow for SCONE packets to ensure predictable handling or
allow SCONE packets to traverse the same QoS Flows as other user-
plane traffic when differentiated treatment is not required.
5G network functions, such as the PCF and SMF, can assign appropriate
QoS attributes to SCONE flows so that the advised throughput is not
degraded under high-load conditions. They can also dynamically
update SCONE rate advice in response to network load variations.
The UPF can be configured to enforce a Maximum Bitrate (MBR) of
traffic calculated across over an averaging window (default
2seconds). The enforcement may be applied on different granularity,
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all traffic carried within a PDU session with default QoS, all
traffic mapped to a specific QoS flow within a PDU session, or just
the traffic of a specific application traffic flow mapped to a
specific QoS flow. The restriction can be separated for upstream and
downstream directions. By default, the throughput advice reflects
the MBR value the UPF is configured for a particular SCONE-capable
traffic flow.
4.1.5. Dynamic Updates
When preferred mobile networks can enforce dynamic rate limits during
active sessions, for example on a QoS Flow basis. In such cases, a
SCONE Network element in 5G network would like to sent dynamics
updates to applications..
4.1.6. Operations Monitoring and Logging
When preferred mobile operators can integrate SCONE signaling into
existing operational and management frameworks to enable monitoring,
troubleshooting, and fault isolation. Metrics of interest include:
* Rate of SCONE advisory messages issued per session
* Correlation between SCONE advisories and user-plane throughput
changes
* Error conditions where SCONE signaling fails to reach the UE
Integration with analytics frameworks (e.g., NWDAF in 5G) can also be
used to assess SCONE effectiveness.
5. SCONE Usage in a 4G/LTE Network
In LTE/Evolved Packet Core (EPC) systems as defined by 3GPP
[_4G-Arch], SCONE can be integrated at the PDN Gateway (P-GW) or the
Serving Gateway (S-GW). Unlike 5G, traffic granularity is bearer-
based rather than per-flow.
The following diagram illustrates SCONE integration within the P-GW:
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+---------+
| PCRF |
+----+----+
| Flow
v Policy Rules
+--------+ +--------------+
| Client |<========>| P-GW |
| App | SCONE | |
+--------+ advice +-------+------+
| OS | |
+--------+ |
| Modem | |
+----+---+ |
| |
v v
+--+---+ +---+---+
| eNB |--------------| S-GW |
+--+---+ +---+---+
|
v
+-------------+
| Internet |
+-------------+
|
v
+-----------------+
| Content Provider|
+-----------------+
Figure 2: SCONE Integration within the 4G Network
5.1. Applicability of SCONE in a 4G/LTE Network
TBD
Editor's NOTE: SCONE signaling maps to EPS bearers, enabling secure
and targeted throughput advice between endpoints and EPC gateways.
5.2. 4G specific considerations
TBD
6. SCONE usage in a Wireline Network
TBD
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Editor's Note: SCONE can be deployed in wireline broadband networks
at key access aggregation points such as Broadband Network Gateways
(BNGs) or equivalent subscriber access nodes. These SCONE network
elements originate throughput advice, signaling maximum sustainable
data rates to application endpoints for each subscriber session,
typically identified by DHCP, PPP, or IPoE session contexts.Session
granularity is typically based on subscriber sessions using PPP,
DHCP, or IPoE protocols. We need to consider if aggregation points
have flow level visibility or not, or whether there is a point to
provide throughput advice at the aggregate level.
6.1. Wireline specific considerations
TBD
7. SCONE usage in a Wifi Networks
TBD
Editor's Note: Home, enterprise, and campus networks commonly use Wi-
Fi access. The SCONE client may remain within the Wi-Fi network for
the duration of a session, or it may be subject to handover or
offloading, moving between a cellular network and a Wi-Fi network,
and vice versa. In such scenarios, rate limiting is typically
applied per user, device, or Service Set Identifier (SSID). These
cases should be considered when defining applicability and
manageability guidelines for SCONE deployments.
8. Security Considerations
Security considerations are included separately in the SCONE protocol
documents.
9. IANA Considerations
This document has no IANA actions.
10. References
10.1. Normative
[I-D.ietf-scone-protocol]
10.2. Informative References
[_4G-Arch] [_5G-Arch]
11. References
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11.1. Normative References
[I-D.ietf-scone-protocol]
Thomson, M., Huitema, C., Oku, K., Joras, M., and M.
Ihlar, "Standard Communication with Network Elements
(SCONE) Protocol", Internet-Draft, draft-ietf-scone-
protocol, Work in Progress , July 2025,
.
11.2. Informative References
[_4G-Arch] 3GPP, "System Architecture for the Evolved Packet Core
(EPC)", 1 June 2020,
.
[_5G-Arch] 3GPP, "System Architecture for the 5G System (5GS)", 7
January 2025,
.
Appendix A. Appendix A. Additional Background details on role of UPF
in 5G Mobile Packet Core
This section describes 5G mobile packet core in mobile packet core
and reasons why the 5G User Plane Function (UPF) as SCONE network
elements can be considered candidates for signaling the "throughput
advice" to client-application-endpoint.
The user plane SCONE network element in the 5G packet core, termed as
the UPF, as shown in Figure 1.
+-----+ Nudm/Nudr +---------+
| PCF +-------------+ UDM/UDR |
+--+--+ +----+----+
| |
Npcf | +-----+ |Nudm
+------+ SMF +-------+
+--+--+ ___ __
| N4 ( )( )
+----+ +--------+ +--+--+ ( ) +------------------+
| UE |---| gNodeB |----| UPF |----( Internet )---| Content Provider |
+----+ +--------+ N3 +- ---+ N6 ( ) +------------------+
| N9 (__(___)
+-+---+
| UPF |
+-----+
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Figure 3: 5G Mobile Network Architecture
A.1. 5G Mobile Network Architecture
The UPF is a fundamental component of the 3GPP's 5G packet core
network architecture. UPF is on the data path between the end-user
and the Internet, has access to subscriber policy via standard 3GPP
N4 interface and is responsible for routing and forwarding user data
packets. UPF is the anchor point between the mobile infrastructure
and the Packet Data Network. The UPF is responsible for functions
such as:
* Packet routing, forwarding, and interconnection to the Data
Network (Internet)
* Allocation of User Equipment (UE) IP Address/prefix, in
conjunction with Session Management Function (SMF)
* Quality of Service policy enforcement
* Handling of traffic filtering, steering and application detection
* Traffic usage reporting
Note: This is not an exhaustive list of UPF functions. For details
refer to [_5G-Arch].
To accomplish above mentioned functions, the UPF has four distinct
reference points (interfaces) as defined by the 3GPP and as shown in
the figure 1 above:
1. The N3 interface is between the UPF and the 5G Base station.
2. The N4 interface is a connection between the UPF and the Session
Management Function (SMF).
3. The N6 interface is between the UPF and the public data network
or the Internet.
4. The N9 interface is between instances of UPFs.
A.2. N3 Interface
The N3 interfaces transfers user plane traffic, that is, user data
packets between the gNodeB and the UPF. It uses GPRS Tunneling
Protocol - User Plane or GTP-U. It replaces the S1-U interfaces from
the 4G mobile packet core.
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A.3. N4 Interface
The N4 interface connects the UPF and the 5G Session Management
Function (SMF). Through N4, the SMF informs the UPF about the
subscriber policy and data plans. Additionally, this interface is
used to manage session setup, modification, deletion, and for
configuring QoS and forwarding rules for user data. The QoS rules
contain parameters such as MBR. The N4 interface among others uses
Packet Forwarding Control Protocol (PFCP).
Note: SMF also interacts with Policy Control Function (PCF) for
functions such as QoS and Charging policy rules, Unified Data
Management (UDM) and Unified Data Repository (UDR) for functions such
as subscription data and policy plans.
A.4. N6 Interface
The N6 interface connects the UPF to external Data Networks, similar
to the SGi interface between the P-GW and the external Data Network
for access to services and applications. The interface supports
various transport protocols over IP.
A.5. N9 Interface
This interface interconnects two or more UPFs when used in a data
path. The interface uses GTP-U protocol for user traffic tunneling
including roaming.
Note: In the scenario of 2 or more UPFs in the data path, only one
UPF that has access to subscriber policy would send "throughput
advice" to the client-application-endpoint.
A.6. User Plane Interface Between UPF and UE
This section describes the N3 interface (between the UPF and gNodeB
or gNB) and the air interface between the gNB and UE. For purposes
of nomenclature, a Protocol Data Unit (PDU) session is a logical path
between a UE and UPF to carry packets belonging to one or more IP
flows between UE and DN. A PDU session within a 5G mobile network
consists of an air-interface between UE and gNB and GTP-U tunnel
between gNB and UPF (N3 interface). Application traffic flows with
different QoS requirements get mapped to different QoS treatments
based on packet filters and QoS rules configured on the UPF and UE.
Below is an example of data flow to/from a UE to the UPF.
1. Uplink Data Flow
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* Apps that are hosted on UE that generate application packets
for communication (e.g. web browsing, video streaming).
* These packets are transmitted to the gNB over the air
interface and get mapped to different QoS treatments based on
packet filters and QoS rules provided to the UE
* N3 Encapsulation and Forwarding
1. The gNB then encapsulates this user-plane data using GTP-
U.
2. It then forwards the encapsulated packets over the N3
interface to the UPF in the 5G mobile packet core.
* UPF Routes Data to External Networks.
1. Within the UPF, UPF then removes the GTP-U header,
processes the packet, and routes it over the N6 interface
toward the destination (Internet, enterprise network,
cloud services, etc.).
2. Downlink Data Flow
* UPF receives incoming data in downlink direction at N6
interface (e.g. from the Internet).
* The UPF encapsulates incoming data using GTP-U and forwards it
over the N3 interface to the gNB. It maps traffic flows with
different QoS requirements to different QoS treatments based
on packet filters and QoS rules configured by SMF.
* The gNB forwards the packets to the UE over the air-interface.
UE-side modem stack then transparently passes the application
packets to the app hosted on the UE.
In summary, the UPF is responsible for packet routing and forwarding,
packet inspection and filtering, participating in subscriber and flow
policy enforcement, inline services (NAT, firewall, DNS etc) and QoS
handling.
Appendix B. Appendix B. Non-ASCII Characters
This document uses the following kramdown-rfc character escapes for
common non-ASCII symbols:
* U+00A0 NO-BREAK SPACE → {nbsp}
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* U+00AD SOFT HYPHEN → {shy}
* U+2011 NON-BREAKING HYPHEN → {nbhy}
* U+200B ZERO WIDTH SPACE → {zwsp}
* U+2060 WORD JOINER → {wj}
* U+2013 EN DASH → {ndash}
* U+2014 EM DASH → {mdash}
* U+201C LEFT DOUBLE QUOTATION MARK → {ldquo}
* U+201D RIGHT DOUBLE QUOTATION MARK → {rdquo}
* U+2018 LEFT SINGLE QUOTATION MARK → {lsquo}
* U+2019 RIGHT SINGLE QUOTATION MARK → {rsquo}
* U+20AC EURO SIGN → {euro}
Acknowledgments
The authors would like to acknowledge and thank the SCONE working
grouup and also the following individuals for their valuable
feedback, discussions, and contributions that helped improve this
document:
* Wesley Eddy
* Renjie Tang
* Kevin Smith
* Tina Tsou
* Tianji Jiang
* Lucas Pardue
* Martin Thomson
Authors' Addresses
Sanjay Mishra
Verizon
Email: sanjay.mishra@verizon.com
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Zaheduzzaman Sarker
Nokia
Email: zaheduzzaman.sarker@nokia.com
Anoop Tomar
Meta
Email: anooptomar@meta.com
Khurram Abbas
Verizon
Email: khurram.abbas@verizonwireless.com
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