]> Verizon

sanjay.mishra@verizon.com

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Web and Internet Transport SCONE Adaptive Bit-Rate Video This document is based on the SCONE working group charter that says the SCONE Working Group aims to establish a mechanism for network elements capable of rate-limiting a UDP 4-tuple to communicate an upper bound on achievable bitrate termed "throughput advice".

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.

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 when, and only when, they appear in all capitals, as shown here.

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 . 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:

5G Mobile Network Architecture

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.

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.

N4 Interface The N4 interface connects the UPF and the 5G Session Management Function (SMF). Through N4, the UPF gets access to the subscriber policy and data plans. Additionally, this interface is used to manage session setup, modification, deletion, and for configuring forwarding rules for user data. The N4 interface uses Packet Forwarding Control Protocol (PFCP). Through the N4 interface, UPF gets access to the subscriber policy and data plans. This enables UPF to send the throughput advice based on the subscriber data plan to the client application end point. 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.

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 trasnport protocols over IP.

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. Of the interfaces listed above and for the purpose of SCONE, the N3 and N4 interfaces are the most relevant to SCONE signaling and to clarify that use of N4 by the UPF within the mobile packet core network is agnostic to SCONE signal.

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.

Significance of UPF from SCONE Perspective The UPF is a data path mobile packet core network element that routes and forwards application packets between the gNodeB and the DN and it has access to subscriber policy via standard 3GPP N3 interface. This enables UPF to send the throughput advice to client application end point over data-path.

4G Mobile Network Architecture

4G Mobile Network Architecture

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 for a more complete version of this diagram.

SCONE Integration with Video Policy in 5G SA N/W

Similarly, the SCONE signal for 4G network is shown below. Please see for a more complete version of this diagram.

SCONE Integration with Vido Policy in 4G N/W

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.

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.

IANA Considerations This document has no IANA actions.

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References Meta Platforms, Inc. Meta Platforms, Inc. Meta Platforms, Inc. Meta Platforms, Inc. These are the requirements for the "Video Optimization" use-case for SCONE, which broadly speaking seeks to optimize video playback experience in mobile networks by cooperative communication between video content providers and the providers of network services to end users. Discussion Venues This note is to be removed before publishing as an RFC. Source for this draft and an issue tracker can be found at https://github.com/mjoras/sadcdn-video-optimization-requirements.

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

• Wesley Eddy