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Web and Internet Transport SCONEPRO Adaptive Bit-Rate Video This document describes API considerations to provide applications with SCONE-PRO metadata, notifying them of network-supplied information about acceptable network flow rates. Since this information is signalled from the network within the stack below the application, it needs to be made accessible to applications in order for them to pick proper video rates, or to otherwise confine the application behavior within network-defined limits.

Introduction The general problem statement for SCONE-PRO is described in the video optimization requirements document , including the shaping or throttling that Communication Service Providers (CSPs) perform. While this problem currently has especially large impact on a few large content providers, solutions for SCONE-PRO are generally applicable to any applications that use QUIC and are subject to throttling within CSP networks. The purpose of this document is only to demonstrate that this is achievable without prescribing a specific solution. It is envisioned that one or more specific API solutions will be defined either through IETF or W3C. At least in its current form, this document is not intended to be published as an RFC.

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.

Application Stack Designs There are a variety of different application stack designs that are relevant. The assumption for SCONE-PRO in general is that QUIC is used. Applications could either use QUIC directly through a linked software library, or applications could be running within a browser, using QUIC indirectly via browser APIs. The SCONE-PRO network rate-limiting information could be discovered by an end-system in several different ways; potential network signaling approaches are described in other documents (e.g. , , or others). General classes of information signaling include:

1. Via the QUIC stack, with the information inserted by an on-path SCONE-PRO network element.
2. Via other IP or transport methods below QUIC (e.g. IP extension headers, UDP options, etc.) that might be inserted on the path.
3. Via the OS, with the information coming in network advertisements separate from the transport connection (e.g. via Router Advertisements or DHCP).

These methods are not necessarily mutually exclusive. For instance, a DHCP option could indicate that certain classes of applications are throttled at a particular rate, while a SCONE-PRO on-path mechanism could also provide more explicit per-connection indications of when throttling is active as well.

SCONE-PRO Solution Type	Information Visibility	API Definitions
QUIC-based	QUIC library or web browser	Specific to each QUIC-library, or browser APIs.
IP or UDP-based	OS and possibly QUIC library	Socket API extension may be needed, e.g. to expose via socket options or other means. For mobile apps using native stacks, OS extensions may be needed.
Other approaches that are not on-path or per-connection	OS	Per-OS and/or socket API extensions

For solution types that are not QUIC-based, the QUIC implementation could use socket or OS API extensions in order to get the data and convey it up to the applications via its own API. However, QUIC library APIs are not standardized; they differ between common QUIC libraries, and so this document only suggests in a general sense of how the QUIC stack should convey this information to applications.

Potential QUIC API Inclusion While there are no standard QUIC APIs, and there are multiple different styles in use, many QUIC implementations include objects in the API that represent the QUIC connections directly, and allow setting callbacks for connection-related events, or allow direct querying of connection state. SCONE-PRO metadata could either be supported as a type of callback event, triggered when the metadata is received, or it could be included within other connection state in a polled or interrogated data structure.

Potential Browser API Extensions For browser applications, there are multiple different browser APIs that might be used; notably including:

• WebTransport using QUIC
• Any Javascript HTTP requests that directly or indirectly use HTTP/3.

In either of these cases, the corresponding W3C API definitions are the proper place for API extensions.

Potential WebTrans API Extension WebTransport (WEBTRANS) is anticipated to often be used by SCONE-PRO's targeted types of applications in the future, such as browser-based adaptive streaming. The IETF WEBTRANS working group is liasing with W3C as the IETF defines the protocol specification, whereas the W3C defines the API to use it. This case is similar to the IETF RTCWEB and W3C WebRTC WG coordination in the past. The same model of collaboration between IETF and W3C should work for SCONE-PRO metadata, and the information provided could be discussed with the WEBTRANS WG in the IETF and notified to the W3C later, either through common participation and/or formal liason statement. The existing WebTrans API definition from W3C includes a "getStats()" method, that is defned in order to asynchronously gather and report statistics for a WebTransport underlying connection. It returns a WebTransportConnectionStats object that is defined as a dictionary, including a number of items such as:

• bytesSent
• packetsSent
• bytesLost
• packetsLost
• bytesReceived
• packetsReceived
• smoothedRtt
• rttVariation
• minRtt
• WebTransportDatagramStats datagrams
• estimatedSendRate

The estimatedSendRate is an unsigned long long, in bits per second, calculated by the congestion control algorithm in the user agent. This would be in the "upstream" direction to a CSP, though, rather than the "downstream" from a CSP, so is not useful to a client application in receiving indication of a downstream throttling rate from the network. Since other measurements are already included and the WebTransportConnectionStats is a dictionary, it seems natural to extend it to include additional optional fields, such as an allowed media rate, or other types of fields providing the application information that the underlying host or stack have discovered about the presence of throttling or explicit signaling of allowed media rate on a path. Such extensions might be including at a "MAY" level of conformance statement (rather than "SHALL" as used by all of the currently-defined information elements), as the allowed media rate will not be universally present or even useful for all WebTransport applications. Alternatively, it could be set to a "null" value similar to how the estimatedSendRate is sent when it is unknown by the user agent.

Content of Metadata Existing collections of metadata could be appended to the WebTransportConnectionStats, such as (for one specific example), those defined by the CTA-5004 specification and CTA-5006 specification , which standardize JSON objects conveyed by streaming media clients and servers respectively. These CTA specifications are also expected to be usable by intermediaries. Another example that could be built upon is the proposed flow metadata identifying the DownlinkBitrate . Several of the use cases defined by CTA-5006 are relevant, including:

• Provide an estimate of the throughput available between an edge server and a player, such that the player can use that information to enrich its decision about which bitrate to select, especially the initial bitrate decision made at the start of playback.
• Provide a means for a CDN to instruct all connected players to limit their upper bitrate, in response to an ISP request to reduce congestion on the last-mile network.
• Allow a CDN to signal to a player a suggested playback bitrate in order to optimize collective QoE.

The "CDN" notion could be replaced with an ISP's on-path throttling device. As a specific example of metadata for SCONE-PRO, CTA-5004 allows clients to indicate multiple types of rate metadata that are related to adaptive coded rate selection in the face of throttling. Which of these options are used depends upon the client, but could be one of:

CTA Data	Units	CTA Description
Measured Throughput	Integer kilobits per second (kbps)	The throughput between client and server, as measured by the client and MUST be rounded to the nearest 100 kbps. This value, however derived, SHOULD be the value that the client is using to make its next Adaptive Bitrate switching decision. If the client is connected to multiple servers concurrently, it must take care to report only the throughput measured against the receiving server. If the client has multiple concurrent connections to the server, then the intent is that this value communicates the aggregate throughput the client sees across all those connections.
Requested maximum throughput	Integer kbps	The requested maximum throughput that the client considers sufficient for delivery of the asset. Values MUST be rounded to the nearest 100 kbps. ... Note: This can benefit clients by preventing buffer saturation through over-delivery ...

The CTA-5006 also allows the server to convey a maximum suggested bitrate.

CTA Data	Units	CTA Description
Max suggested bitrate	Integer kbps	The maximum bitrate value that the player SHOULD play in its Adaptive Bit Rate (ABR) ladder. If the player is playing a bitrate higher than this value, it SHOULD immediately switch to a bitrate lower than or equal to this value.

There are two high-level approaches that might be viable in reusing CTA data types:

1. Append entire CTA-defined objects as optional components of the WebTransportConnectionStats, e.g. as a "cta5005" object that would be able to fully convey all of the CTA-defined metadata.
2. Append only selected fields from the CTA specs, reusing the definitions, but only including specific items of interest for SCONE-PRO's problem statement, and leaving other CTA-defined metadata to be conveyed through the existing CTA-defined methods.

The API can be agnostic to how the data is conveyed between network or on-path SCONE-PRO devices and either clients or servers, but the same API can convey the information to the applications, regardless of how the network signaling works. The semantics of the maxium suggested bitrate from the CTA-5006 match the needed semantics most closely, though since it is for an ABR ladder, it should be described carefully to apply the proper bitrate definition that the throttler is using for "on the wire" packets (including header overhead, etc.) versus the pure media payload stream bitrate.

Potential MoQ API Extension While there is no standard MoQ API, an MoQ session is currently scoped either to a QUIC connection or a WebTransport session, so it should not be difficult to expose information learned by either transport stack to MoQ applications.

Security Considerations General SCONE-PRO security considerations are discussed in the other documents covering the problem statement and specific network-to-host signaling methods. This document only exposes the SCONE-PRO metadata supplied by the network to applications. There are no additional security considerations raised by this.

IANA Considerations This document has no IANA actions.

References Normative References This document defines the core of the QUIC transport protocol. QUIC provides applications with flow-controlled streams for structured communication, low-latency connection establishment, and network path migration. QUIC includes security measures that ensure confidentiality, integrity, and availability in a range of deployment circumstances. Accompanying documents describe the integration of TLS for key negotiation, loss detection, and an exemplary congestion control algorithm. 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 Orange Cloud Software Group Holdings, Inc. Nokia Cloud Software Group Holdings, Inc. Verizon Inc Traffic exchanged over a network attachment may be subject to rate- limit policies. These policies may be intentional policies (e.g., enforced as part of the activation of the network attachment and typically agreed upon service subscription) or be reactive policies (e.g., enforced temporarily to manage an overload or during a DDoS attack mitigation). Networks already support mechanisms to advertize a set of network properties to hosts using Neighbor Discovery options. Examples of such properties are link MTU (RFC 4861) and PREFIX64 (RFC 8781). This document complements these tools and specifies a Neighbor Discovery option to be used in Router Advertisements (RAs) to communicate these policies to hosts. For address family parity, a new DHCP option is also defined. Cloud Software Group Holdings, Inc. Cloud Software Group Holdings, Inc. Orange Nokia This document defines per-flow and per-packet metadata for both network-to-host and host-to-network signaling in Concise Data Definition Language (CDDL) which expresses both CBOR and JSON. The common metadata definition allows interworking between signaling protocols with high fidelity. The metadata is also self- describing to improve interpretation by network elements and endpoints while reducing the need for version negotiation. Ericsson Ericsson This document specifies a new Capsule (RFC9297) that can be used with CONNECT-UDP (RFC9298), CONNECT-IP (RFC9484), or other future CONNECT extensions to signal the available bitrate for media traffic that is proxied through an HTTP server. This information can be used by a media application to regulate its media bitrate in accordance with a network policy, as an alternative to in-network traffic shaping. Meta Platforms, Inc. Meta Platforms, Inc. Meta Platforms, Inc. Meta Platforms, Inc. These are the requirements for the "Video Optimization" use-case for the SADCDN topic, 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 and inputs from others, including:

• Anoop Tomar
• Matt Joras
• Brian Tan