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Transport QUIC Internet-Draft QUIC defines a RESET_STREAM frame to abort sending on a stream. When a sender resets a stream, it also stops retransmitting STREAM frames for this stream in the event of packet loss. On the receiving side, there is no guarantee that any data sent on that stream is delivered. This document defines a new QUIC frame, the RESET_STREAM_AT frame, that allows resetting a stream, while guaranteeing delivery of stream data up to a certain byte offset. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the QUIC Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction QUIC version 1 () allows streams to be reset. When a stream is reset, the sender does not retransmit stream data for the respective stream. On the receiving side, the QUIC stack is free to surface the stream reset to the application immediately, without providing any stream data it has received for that stream. Some applications running on top of QUIC use bytes at the beginning of the stream to communicate critical information related to that stream. For example, WebTransport () uses a variable-length encoded integer to associate a stream with a particular WebTransport session. Since QUIC does not provide guaranteed delivery of stream data for reset streams, it is possible that a receiver is unable to read critical information. In the example above, a reset stream can cause the receiver to fail to associate incoming streams with their respective subcomponent of the application. Therefore, it is desirable to allow a receiver to rely on the delivery of critical information to applications, even if the QUIC stream is reset before this data is read by the application. Another use case is relaying data from an external data source. When a relay is sending data being read from an external source and encounters an error, it might want to use a stream reset to signal that error, while at the same time guaranteeing that all data received from the source is delivered to the peer. This document extends QUIC with a variant of stream resets that reliably delivers the beginning of a stream up to a sender-specified offset, communicated using the RESET_STREAM_AT frame. It can be considered a form of range-based partial reliability. For the receiving application, RESET_STREAM_AT remains an abrupt termination: after providing the stream data guaranteed by the Reliable Size, the QUIC implementation signals a stream reset rather than a clean end of the stream; see .

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. The frame format diagram uses the notation described in .

Transport Parameter Support for receiving RESET_STREAM_AT frames is advertised by sending the reset_stream_at (0x1d) transport parameter () with an empty value. An implementation that understands this transport parameter MUST treat the receipt of a non-empty value as a connection error of type TRANSPORT_PARAMETER_ERROR. An endpoint MUST NOT send a RESET_STREAM_AT frame unless the peer advertised support using the reset_stream_at transport parameter. When using 0-RTT, both endpoints MUST remember whether this extension was advertised by the server (). This allows use of this extension in 0-RTT packets. When the server accepts 0-RTT data, the server MUST NOT disable this extension on the resumed connection.

RESET_STREAM_AT Frame Conceptually, the RESET_STREAM_AT frame is a RESET_STREAM frame with an added Reliable Size field.

RESET_STREAM_AT Frame Format

The RESET_STREAM_AT frame contains the following fields:

Stream ID:

A variable-length integer encoding of the stream ID of the stream being terminated.

Application Protocol Error Code:

A variable-length integer containing the application protocol error code () that indicates why the stream is being closed.

Final Size:

A variable-length integer indicating the final size of the stream by the sender, in units of bytes; see .

Reliable Size:

A variable-length integer indicating the minimum amount of data that needs to be delivered to the application even though the stream is reset.

If the Reliable Size is larger than the Final Size, the receiver MUST close the connection with a connection error of type FRAME_ENCODING_ERROR. As with RESET_STREAM (), Final Size is subject to stream and connection-level flow control. An endpoint MUST NOT send a RESET_STREAM_AT frame that exceeds the largest maximum stream data value advertised by the receiver for the stream, or that violates the receiver's maximum data limit. Consequently, the sender might need to defer sending RESET_STREAM_AT until enough stream or connection-level flow control credit is available. If an endpoint receives a RESET_STREAM_AT frame that violates either flow control limit, it MUST close the connection with an error of type FLOW_CONTROL_ERROR. RESET_STREAM_AT frames are ack-eliciting, and MUST only be sent in the application data packet number space. When lost, they MUST be retransmitted, unless the stream state has transitioned to "Data Recvd" or "Reset Recvd" due to transmission and acknowledgement of other frames (see ).

Resetting Streams A sender that wants to reset a stream while retaining reliable delivery of certain data sends a RESET_STREAM_AT frame with the Reliable Size field specifying the amount of data to be delivered. When using a RESET_STREAM_AT frame, the initiator MUST guarantee reliable delivery of stream data of at least Reliable Size bytes. If STREAM frames containing data up to that byte offset are lost, the initiator MUST retransmit this data, as described in . Data sent beyond that byte offset SHOULD NOT be retransmitted. As described in , an implementation might suppress or withhold a stream reset signal from the application. The same applies to a stream reset signal carried in a RESET_STREAM_AT frame. Similarly, the Reliable Size of the RESET_STREAM_AT frame does not prevent a QUIC stack from delivering data beyond the specified offset to the receiving application. Note that a Reliable Size value of zero is valid. For purposes of data delivery, a RESET_STREAM_AT frame with this value is logically equivalent to a RESET_STREAM frame (). When resetting a stream without the intent to deliver any data to the receiver, the sender MAY use either RESET_STREAM or RESET_STREAM_AT with a Reliable Size of zero. As stated in , the final size for a stream cannot change once it is known.

Sending RESET_STREAM_AT after FIN Similar to how it is possible to send a RESET_STREAM frame after a STREAM frame carrying the FIN bit, it is possible to send a RESET_STREAM_AT frame after a STREAM frame carrying the FIN bit. Due to packet reordering, it is possible for a receiver to receive the RESET_STREAM_AT frame before receiving the STREAM frame carrying the FIN bit.

Multiple RESET_STREAM_AT / RESET_STREAM frames The initiator MAY send multiple RESET_STREAM_AT frames for the same stream in order to reduce the Reliable Size. It MAY also send a RESET_STREAM frame, which for purposes of data delivery is equivalent to sending a RESET_STREAM_AT frame with a Reliable Size of zero. The Reliable Size can only decrease; when sending multiple RESET_STREAM_AT or RESET_STREAM frames for the same stream, the initiator MUST NOT increase the Reliable Size. The sender MUST retransmit the information from the RESET_STREAM_AT frame carrying the current Reliable Size, as well as stream data up to that size, until both have been acknowledged. When receiving a RESET_STREAM_AT frame with a lower Reliable Size, the receiver only needs to provide data up to the lower Reliable Size to the application. It MUST NOT expect the sender to deliver any data beyond that byte offset. Reordering of packets might lead to a RESET_STREAM_AT frame with a higher Reliable Size being received after a RESET_STREAM_AT frame with a lower Reliable Size. The receiver MUST ignore the increase in Reliable Size and continue to use the smallest Reliable Size received. For the same stream, the initiator MUST NOT change the Application Error Code in subsequent RESET_STREAM_AT or RESET_STREAM frames, or the Final Size in any such frame or a STREAM frame carrying the FIN bit. A receiver that detects a change MUST close the connection with a connection error of type STREAM_STATE_ERROR or FINAL_SIZE_ERROR, respectively. While multiple RESET_STREAM_AT frames can reduce Reliable Size, some applications might need to ensure that a minimum amount of data is always delivered on a stream. Application protocols can establish rules for streams that ensure that Reliable Size is not reduced below a certain threshold if that is necessary to ensure correct operation of the protocol.

Stream States In terms of stream state transitions (), the effect of a RESET_STREAM_AT frame is equivalent to that of the FIN bit. Both the RESET_STREAM_AT frame and the FIN bit on a STREAM frame serve the same role: signaling the amount of data to be delivered. On the sending side, when the first RESET_STREAM_AT frame is sent, the sending part of the stream enters the "Data Sent" state. Once the RESET_STREAM_AT frame carrying the smallest Reliable Size and all stream data up to that byte offset have been acknowledged, the sending part of the stream enters the "Data Recvd" state. The transition from "Data Sent" to "Data Recvd" happens immediately if the application resets a stream and all bytes up to the specified Reliable Size have already been sent and acknowledged. Conversely, if bytes below that offset still need to be sent or acknowledged, the transition might take multiple network roundtrips. Note that entering the "Data Sent" state might itself be deferred, because flow control can prevent the sender from transmitting the RESET_STREAM_AT frame; see . On the receiving side, when a RESET_STREAM_AT frame is received, the receiving part of the stream enters the "Size Known" state. Once all data up to the smallest Reliable Size have been received, it enters the "Data Recvd" state. Similarly to the sending side, the transition from "Size Known" to "Data Recvd" might happen immediately, or might require additional network roundtrips while the sender transmits remaining bytes up to the smallest Reliable Size.

Handling STOP_SENDING An endpoint responds to a STOP_SENDING frame as described in . If it sends a RESET_STREAM_AT frame in response, it SHOULD set the Reliable Size to zero, since the peer has indicated that it does not intend to process further stream data.

Implementation Guidance In terms of transport machinery, the RESET_STREAM_AT frame is more akin to the FIN bit than to the RESET_STREAM frame (see ). By sending a RESET_STREAM_AT frame, the sender commits to delivering all bytes up to the Reliable Size. To the endpoints, the main differences from closing a stream by using the FIN bit are:

• the offset up to which the sender commits to sending might be smaller than Final Size,
• this offset might get reduced by subsequent RESET_STREAM_AT frames, and
• the closure is accompanied by an error code.

Security Considerations As the RESET_STREAM_AT frame is an extension to the stream machinery defined in QUIC version 1, the security considerations of apply accordingly. Specifically, given that RESET_STREAM_AT frames do not cause data exchange to terminate, endpoints need to continue to monitor for resource commitment and exhaustion attacks even after sending or receiving RESET_STREAM_AT. This persists until all data is delivered, similar to the handling of normal stream termination; see .

IANA Considerations

QUIC Transport Parameter This document registers the reset_stream_at transport parameter in the "QUIC Transport Parameters" registry established in . The following fields are registered:

Value:

0x1d

Parameter Name:

reset_stream_at

Status:

Provisional (will become Permanent once this document is approved)

Specification:

This document

Change Controller:

IETF (iesg@ietf.org)

Contact:

QUIC Working Group (quic@ietf.org)

QUIC Frame Types This document registers one new value in the "QUIC Frame Types" registry established in . The following fields are registered:

Value:

0x24

Frame Type Name:

RESET_STREAM_AT

Status:

Provisional (will become Permanent once this document is approved)

Specification:

This document

Change Controller:

IETF (iesg@ietf.org)

Contact:

QUIC Working Group (quic@ietf.org)

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 Meta Apple Inc. Google WebTransport over HTTP/3 is a binding of the WebTransport protocol framework [OVERVIEW] to HTTP/3 [HTTP3]. It provides support for unidirectional streams, bidirectional streams, and datagrams, all multiplexed within the same HTTP/3 connection. WebTransport enables application clients constrained by the Web security model to communicate with a remote application server using a secure multiplexed transport.

Acknowledgments This document had reviews and input from many contributors in the IETF QUIC Working Group, with substantive input from Lucas Pardue and Martin Thomson.