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Web and Internet Transport SCONE SCONE This document describes a TCP option that permits network elements to provide TCP endpoints advice on rate limits within the network. The functionality for TCP is analogous to SCONE packets within the QUIC protocol.

SCONE Background and Introduction Standard Communication with Network Elements (SCONE) is an extension to QUIC that provides the capability for network elements to provide QUIC application endpoints with guidance on potential permitted throughput, e.g. in order to make explicit the presence of traffic limiting mechanisms within the network that can be problematic for video streaming and other applications. In QUIC, SCONE is negotiated between endpoints using a transport parameter, and the QUIC endpoints send SCONE packets periodically. The SCONE packets are visible to network elements that modify the throughput guidance within them.

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.

TCP Option for SCONE This document defines a TCP option to provide analogous SCONE functionality for TCP. This could be viewed as similar to the way that TCP MSS clamping works traditionally, with the TCP MSS options included by endpoints being inspected and modified en-route by elements on the network path that can provide more pertinent guidance.

Option Format The TCP option kind value (TBD) indicates the SCONE-TCP option. The length value of 4 is always used, along with a two byte throughput guidance. TODO: Explain the throughput guidance values used, which could be based on the QUIC values. TODO: note assumed interaction with other options, namely TCP-AO.

Use During Handshake Like other TCP options, SCONE-TCP is sent during connection establishment on SYN and SYN-ACK segments, and then only used subsequently if it has been successfully negotiated via use on the handshake. Since it is used on the initial SYN, the SCONE-TCP option can serve as a "client hint" that informs the behavior of traffic limiting mechanisms within the network. Since it is used on the SYN-ACK, the SCONE-TCP option can provide an immediate signal to the endpoint application about the advised bitrate that can help to inform the selection of media content to be requested subsequently within the application.

Use Post-Handshake After TCP connection establishment with successful SCONE-TCP negotiation, the option can be used at any time. It does not need to be sent on every segment, and providing an update may be the sole reason for sending a segment. Since it takes up valuable header space, and will be inspected and operated on by network elements, it is not advisable to set on every segment that is transmitted. Instead, SCONE-TCP options may be included either periodically by an endpoint (e.g. every 10 seconds as a probe before requesting new media chunks) or generated by network elements on-demand when significant conditions change.

Endpoint-Included When endpoints desire to send the SCONE-TCP option, they may either include it within the header of segment carrying data (if there is user data to be sent), or may include SCONE-TCP on any generated non-data-carrying ACK segment, when no gaps exist in the received data. Pure ACKs (without data or SACK information) MUST NOT carry SCONE-TCP when there are gaps in the received data, because the other TCP peer would in-general be challenged to distinguish them from duplicate ACKs that could towards fast retransmission, for instance. Endpoints receiving segments with the SCONE-TCP option MUST NOT treat any pure ACKs that have SCONE-TCP as potential indicators of loss. ACKs that carry SACK information MAY include the SCONE-TCP option. Endpoints receiving these MAY use the SACK information to determine reordering, loss inference, and retransmission behavior.

Network-Initiated An on-path network element aware of the live TCP flow MAY generate a pure ACK matching the last observed sequence and acknowledgement values sent to the same endpoint, that includes the SCONE-TCP option with new or updated throughput advice, if changed from the last value that it has sent. In general, network devices are highly discouraged from generating TCP segments to the endpoints of connections passing through them. This can be viewed as a type of attack, and could be prevented by use of methods like IPsec, TCP-AO, or by other means between the end hosts. However, in the cases of applications that SCONE serves, these network devices are on-path and may drop, delay, or otherwise manipulate the packet stream.

API for TCP Applications TODO: describe informationally a possible socket option to request SCONE use. TODO: describe informationally a possible socket option to get SCONE info.

Security Considerations The security considerations for SCONE-TCP are similar to those for SCONE as present in QUIC, however, some differences arise because TCP security lacks the same cryptographic methods that are present in QUIC.

IANA Considerations TODO: TCP option kind value is TBD.

References Normative References This document specifies the Transmission Control Protocol (TCP). TCP is an important transport-layer protocol in the Internet protocol stack, and it has continuously evolved over decades of use and growth of the Internet. Over this time, a number of changes have been made to TCP as it was specified in RFC 793, though these have only been documented in a piecemeal fashion. This document collects and brings those changes together with the protocol specification from RFC 793. This document obsoletes RFC 793, as well as RFCs 879, 2873, 6093, 6429, 6528, and 6691 that updated parts of RFC 793. It updates RFCs 1011 and 1122, and it should be considered as a replacement for the portions of those documents dealing with TCP requirements. It also updates RFC 5961 by adding a small clarification in reset handling while in the SYN-RECEIVED state. The TCP header control bits from RFC 793 have also been updated based on RFC 3168. 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 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. Meta Platforms, Inc. Meta Platforms, Inc. Meta Platforms, Inc. Meta Platforms, Inc. These are the requirements for the "Video Optimization" use-case for the SCONE 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. Mozilla Private Octopus Inc. Fastly Meta Ericsson This document describes a protocol where on-path network elements can give endpoints their perspective on what the maximum achievable throughput might be for QUIC flows.

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

• Alan Frindell
• Bryan Tan
• Anoop Tomar