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Transport QUIC Internet-Draft This document defines an extension to the QUIC transport protocol which supports reporting multiple packet receive timestamps for post-handshake packets. Discussion Venues Discussion of this document takes place on the QUIC Working Group mailing list (quic@ietf.org), which is archived at . Source for this draft and an issue tracker can be found at .

Introduction The QUIC Transport Protocol provides a secure, multiplexed connection for transmitting reliable streams of application data. This document defines an extension to the QUIC transport protocol which supports reporting multiple packet receive timestamps.

Motivation QUIC congestion control () supports sampling round-trip time (RTT) by measuring the time from when a packet was sent to when it is acknowledged. However, more precise delay signals measured via packet receive timestamps have the potential to improve the accuracy of network bandwidth measurements and the effectiveness of congestion control, especially for latency-critical applications such as real-time video conferencing or game streaming. Numerous existing algorithms and techniques leverage receive receive timestamps to improve transport performance. Examples include:

• The WebRTC congestion control algorithm described in uses the difference between packet inter-departure and packet inter-arrival times as the input to its delay-based controller.
• The pathChirp () technique estimates available bandwidth by measuring inter-arrival time of multiple packets.

Notably, these techniques require receive timestamps for more than one packet per round-trip in order to best measure the network. Additionally, receive timestamps can provide valuable network telemetry, even if they are not used by the congestion controller.

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.

ACK Frame Wire Format Endpoints send ACK frames in 1-RTT packets as they otherwise would, with 0 or more receive timestamps following the Ack Ranges and optional ECN Counts. Receive timestamps MUST NOT be sent in Initial or Handshake packets, because the peer would not know to use the extended wire format. ACK frames are never sent in 0-RTT packets, so there is no change to 0-RTT. Once negotiated, the ACK format is identical to RFC9000, but with an additional section for receive timestamps at the end:

ACK Frame Format

The fields Largest Acknowledged, ACK Delay, ACK Range Count, First ACK Range, and ACK Range are the same as for ACK (type=0x02) frames specified in .

Timestamp Range Count:

A variable-length integer specifying the number of Timestamp Range fields in the frame.

Timestamp Ranges:

Ranges of receive timestamps for contiguous packets in descending packet number order; see .

Timestamp Ranges Each Timestamp Range describes a series of contiguous packet receive timestamps in descending sequential packet number (and descending timestamp) order. Timestamp Ranges consist of a Delta Largest Acknowledged indicating the largest packet number in the range, followed by a list of Timestamp Deltas describing the relative receive timestamps for each contiguous packet in the Timestamp Range (descending). Packets within a range are in descending packet number and timestamp order. Ranges are in descending timestamp order but do not have to be in descending packet number order. Timestamp Ranges are structured as shown in .

Timestamp Range Format

The fields that form each Timestamp Range are:

Delta Largest Acknowledged:

A variable-length integer indicating the largest packet number in the Timestamp Range as a delta to subtract from the Largest Acknowledged in the ACK frame. For example, 0 indicates the range starts with the Largest Acknowledged.

Timestamp Delta Count:

A variable-length integer indicating the number of Timestamp Deltas in the current Timestamp Range. The sum of Timestamp Delta Counts for all Timestamp Ranges in the frame MUST NOT exceed max_receive_timestamps_per_ack as specified in .

Timestamp Deltas:

Variable-length integers encoding the receive timestamp for contiguous packets in the Timestamp Range in descending packet number order as follows: For the first Timestamp Delta of the first Timestamp Range in the frame: the value is the difference between (a) the receive timestamp of the largest packet in the Timestamp Range (indicated by Gap) and (b) the session receive_timestamp_basis (see ), decoded as described below. For all other Timestamp Deltas: the value is the difference between (a) the receive timestamp specified by the previous Timestamp Delta and (b) the receive timestamp of the current packet in the Timestamp Range, decoded as described below. All Timestamp Delta values are decoded by mulitplying the value in the field by 2 to the power of the receive_timestamps_exponent transport parameter received by the sender of the ACK frame (see ):

Extension Negotiation

max_receive_timestamps_per_ack (0xff0a002 temporary value for draft use):

A variable-length integer indicating that the maximum number of receive timestamps the sending endpoint would like to receive in an ACK frame. Each ACK frame sent MUST NOT contain more than the peer's maximum number of receive timestamps.

receive_timestamps_exponent (0xff0a003 temporary value for draft use):

A variable-length integer indicating the exponent to be used when encoding and decoding timestamp delta fields in ACK frames sent by the peer (see ). If this value is absent, a default value of 0 is assumed (indicating microsecond precision). Values above 20 are invalid.

Receive Timestamp Basis Endpoints which negotiate the extension need to determine a value, receive_timestamp_basis, relative to which all receive timestamps for the session will be reported (see ). The value of receive_timestamp_basis MUST be less than the smallest receive timestamp reported, and MUST remain constant for the entire duration of the session. The receive_timestamp_basis is a local value that is not communicated to the peer. Receive timestamps are reported relative to the basis, rather than in absolute time to avoid requiring clock synchronization between endpoints and to make the frame more compact.

Discussion

Best-Effort Behavior Receive timestamps are sent on a best-effort basis. Endpoints MUST gracefully handle scenarios where the receiver does not communicate receive timestamps for acknowledged packets. Examples of such scenarios are:

• A packet containing an ACK frame is lost.
• The sender truncates the number of timestamps sent in order to (a) avoid sending more than max_receive_timestamps_per_ack (); or (b) fit the ACK frame into a packet.

Security Considerations TODO Security

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 This document describes loss detection and congestion control mechanisms for QUIC. Google Google Politecnico di Bari Politecnico di Bari Politecnico di Bari This document describes two methods of congestion control when using real-time communications on the World Wide Web (RTCWEB); one delay- based and one loss-based. It is published as an input document to the RMCAT working group on congestion control for media streams. The mailing list of that working group is rmcat@ietf.org.

Acknowledgments TODO acknowledge.