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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 using a new ACK_RECEIVE_TIMESTAMPS frame type. 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 using a new ACK_RECEIVE_TIMESTAMPS frame type.

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.

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.

Extension Negotiation The use of the ACK_RECEIVE_TIMESTAMPS frame is negotiated using the following two transport parameters ():

max_receive_timestamps_per_ack (TBD):

A variable-length integer indicating that the sending endpoint would like to receive ACK_RECEIVE_TIMESTAMPS frames from the peer containing no more than the given maximum number of receive timestamps. Upon receiving this parameter with a non-zero value, the peer SHOULD send ACK_RECEIVE_TIMESTAMPS frames instead of ACK frames if new receive timestamp information is available. The peer MAY still send regular ACK frames (e.g. if no timing information is available), in which case the endpoint MUST still support processing regular ACK frames as defined by . Each ACK_RECEIVE_TIMESTAMPS frame sent MUST NOT contain more than the specified maximum number of receive timestamps, but MAY contain fewer or none.

receive_timestamps_exponent (TBD):

A variable-length integer indicating the exponent to be used when encoding and decoding timestamp delta fields in ACK_RECEIVE_TIMESTAMPS 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 send ACK_RECEIVE_TIMESTAMPS frames must 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. TODO: Discuss (here or below) why receive timestamps are reported relative to the basis, rather than in absolute time to avoid clock synchronization between endpoints.

ACK_RECEIVE_TIMESTAMPS Frame Receivers send ACK_RECEIVE_TIMESTAMPS (type=TBD) frames in place of--and in the same manner as--regular ACK frames as described in . However, ACK_RECEIVE_TIMESTAMPS frames contain additional fields to report packet receive timestamps. ACK_RECEIVE_TIMESTAMPS frames are formatted as shown in .

ACK_RECEIVE_TIMESTAMPS 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 . ACK_RECEIVE_TIMESTAMPS frames contain the following additional fields:

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 Gap 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). Note that reporting receive timestamps for packets received out of order is not supported. Specifically: for any packet number P for which a receive timestamp T is reported, all reports for packet numbers less than P must have timestamps less than or equal to T; and all reports for packet numbers greater than P must have timestamps greater than or equal to T. Timestamp Ranges are structured as shown in .

Timestamp Range Format

The fields that form each Timestamp Range are:

Gap:

A variable-length integer indicating the largest packet number in the Timestamp Range as follows: For the first Timestamp Range: Gap is the difference between (a) the Largest Acknowledged packet number in the frame and (b) the largest packet in the current (first) Timestamp Range. For subsequent Timestamp Ranges: Gap is the difference between (a) the packet number two lower than the smallest packet number in the previous Timestamp Range and (b) the largest packet in the current Timestamp Range.

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_RECEIVE_TIMESTAMPS frame (see ):

Discussion

Best-Effort Behavior Receive timestamps are sent on a best-effort basis and endpoints MUST gracefully handle scenarios where receive timestamp information for sent packets is not received. Examples of such scenarios are:

• The packet containing the ACK_RECEIVE_TIMESTAMP 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. 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.