A Use Case for SCONE Implementation

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Table of Contents

1. Introduction

This document proposes utilizing the User Plane Function (UPF) in 5G networks and packet data network gateway in 4G networks (PDN-GW or P-GW and also referred as a PGW) to transport SCONE signal between the client-application endpoint on a User Equipment (UE) and the network element (UPF/PDN-GW) in the mobile networks. Specifically, this use case focuses on using UPF and PDN-GW to exchange bi-directional communications with client-application end-point on the UE. The mechanism described focuses on mobile networks including 4G and 5G but the mechanism is generic and applicable to other network architectures.¶

2. Conventions and Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3. Overview of User Plane Network Element in Mobile Packet Core

This section describes 5G mobile packet core to explain the role of user-plane network element in mobile packet core and reasons why the 5G User Plane Function (UPF) and 4G P-GW as network elements can be considered candidates for signaling the "throughput advice" to client-application-endpoint. However, the applicability extends to network architectures beyond 4G/5G networks.¶

The user plane network element in the 5G packet core is the UPF, as shown in Figure 1. In the 4G packet core, the P-GW (as shown in Figure 2) performs the same role as the UPF does in the 5G mobile packet core.¶

The UPF is a fundamental component of the 3GPP's 5G packet core network architecture. UPF is the data path between the end-user and the Internet, has access to subscriber policy via standard 3GPP 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:¶

• Allocation of User Equipment (UE) IP Address/prefix¶

• Packet routing, forwarding, and provides interconnection to the Data Network (DN)¶

• Quality of Service: does enforcement of QoS policies and handles traffic filtering¶

• Traffic usage reporting¶

• Packet inspection¶

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 below:¶

               +-----+  Nudm/Nudr  +---------+
               | PCF +-------------+ UDM/UDR |
               +--+--+             +----+----+
                   |                    |
              Npcf |      +-----+       |Nudm
                   +------+ SMF +-------+
                          +--+--+      ___  __
                             | N4     /   )(  \
   +----+   +--------+    +--+--+    (         )    +------------------+
   | UE |---| gNodeB |----| UPF |----(    DN    )---| Content Provider |
   +----+   +--------+ N3 +- -+-+ N6  (        _)   +------------------+
                              | N9     (__(___)
                            +-+---+
                            | UPF |
                            +-----+

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.¶

4. 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). IP flows (aka service data flows or SDFs) may belong to one or more services. All the service data flows with the same QoS maps onto one PDU session. Below is an example of data flow to/from a UE to the UPF.¶

1. Uplink Data Flow¶

   • Apps that are hosted on UE that generate application packets for communication (e.g. web brownsing, video streaming).¶

   • These packets are transmitted to the gNB over the air interface.¶

   • 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.¶

   • The UPF encapsulates incoming data using GTP-U and sends it back over the N3 interface to the gNB.¶

   • 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, subscriber policy enforcement, and QoS handling. For instance, shallow packet inspection, deep packet inspection, traffic optimization, and inline services (NAT, firewall, DNS, and so on) and external PDU session for interconnecting Data Network.¶

                    +-----+
                    | HSS |
                    +-----+
                       |
                    +-----+          +------+
                    | MME |          | PCRF |
                   /+-----+\         +------+
                  /         \            |
                 /           \           |         ___  __
                /             \          |        /   )(  \
   +----+   +-----+        +------+  +------+    (         )    +----------+
   | UE |---| eNB |--------| S-GW |--| P-GW |----(    DN    )---| Content  |
   +----+   +-----+   S1u  +------+  +------+ SGi (        _)   | Provider |
                                                   (__(___)     +----------+

5. Implementing SCONE In the Mobile Network

As described in sections above, UPF is the 3GPP on-path "network element" that has access to subscriber policy via standard 3GPP N4 interface and provides the data pipe connectivity between UE and DN over the air interface and the N3 interface. UPF is a network element that is capable of SCONE signaling over the data path. SCONE signaling shall use IP tubples of the flow to send the "throughput advice signal". SCONE signal will be sent over the same PDU session (N3 interface + air interface) that is carrying the IP flow.¶

Below is a high-level view of SCONE signal path in a 5G network. Please see [Mishra-2025] for a more complete version of this diagram.¶

                          +---------+
                          |   PCF   |
                          +---------+
                               | Subscriber
                               V Policy Rules
                          +---------+
                          |   SMF   |
                          +----+----+
                               | Subscriber
                               v Policy Rules
+--------+               + +---------+-+
| Client |/--------------\ |  SCONE  | |       __
|   App  |\--------------/ | Endpoint| |    __(  )__
+--------+     SCONE     | +---------+ |   (        )   +----------+
|   OS   |  (advised bit |             +--( Internet )--+ Content  |
+--------+   rate and    |     UPF     |   (         )  | Provider |
|  Modem |   other IEs)  |             |    (__)(___)   +----------+
+----+---+               +------+------+
     |                          |
     |         +-----+          |
     +---------+ gNB +----------+
               +-----+

Similarly, the SCONE signal for 4G network is shown below. Please see [Mishra-2025] for a more complete version of this diagram.¶

                          +---------+
                          |  PCRF   |
                          +----+----+
                               | Subscriber
                               v Policy Rules
+--------+               + +---------+-+
| Client |/--------------\ |  SCONE  | |       __
|   App  |\--------------/ | Endpoint| |    __(  )__
+--------+     SCONE     | +---------+ |   (        )   +----------+
|   OS   |  (advised bit |             +--( Internet )--+ Content  |
+--------+   rate and    |     P-GW    |   (         )  | Provider |
|  Modem |   other IEs)  |             |    (__)(___)   +----------+
+----+---+               +------+------+
     |                          |
     |         +-----+       +--+---+
     +---------+ eNB +-------+ S-GW |
               +-----+       +------+

6. SCONE Signal Requirements

• SCONE signal MUST be a client-application endpoint initiated to assist the network element (UPF/5G or PGW/4G) with the implicit flow detection.¶

• UPF/P-GW SHOULD send "throughput advice" and other metadata using on-path SCONE signaling to the client-application-endpoint based on subscriber data-plans.¶

• Client-application endpoint SHOULD send acknowledgement receipt of throughput advisory signal from the network element using the SCONE signal.¶

• SCONE signaling MUST NOT require changes to how a CSP determintes its video policy for a given flow. (No dependency between a CSP's video policy and the SCONE protocol).¶

• Dynamic update - "throughput advice" MAY change during the ongoing flow and UPF/PGW SHOULD be able to send "throughput advice" to client-application-endpoint as soon as possible.¶

• Applications SHOULD self-adapt the video flow max bit-rate to "throughput advice" value.¶

• SCONE signal MUST be extensible to networks beyond 4G/5G network.¶

7. Security Considerations

Security considerations are included separately in the SCONE protocol documents. Specific to the use case description in this document, there are no additional security considerations.¶

8. IANA Considerations

This document has no IANA actions.¶

9. References

Acknowledgments

This document represents collaboration, comments, and inputs from others, including:¶

• Wesley Eddy¶

Authors' Addresses