University of Aberdeen

School of Engineering Fraser Noble Building Aberdeen AB24 3UE UK ana@erg.abdn.ac.uk

University of Aberdeen

School of Engineering Fraser Noble Building Aberdeen AB24 3UE UK gorry@erg.abdn.ac.uk

Transport Internet Engineering Task Force Transport, Congestion Control, QUIC This document specifies a logging format for a cautious method for Careful Resume when using the IETF quic transport protocol. It defines the logging format for qlog.

This document defines a way to provide logging for Careful Resume when used with QUIC . defines a Congestion Control (CC) mechanism called Careful Resume, which is expected to reduce the time to complete a transfer when the transfer sends significantly more data than allowed by the Initial congestion Window (IW), and where the BDP of the path is also significantly more than the IW. It introduces an alternative mechanism to select initial CC parameters that seeks to more rapidly and safely grow the sending rate controlled by the congestion window (CWND). CC algorithms that are rate-based can make similar adjustments to their target sending rate. When used with the QUIC transport, Careful Resume provides transport services that resemble those that could be implemented in TCP, using methods such as TCP Control Block (TCB) caching.

This subsection provides a brief summary of key terms and the requirements language.

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 document uses language drawn from a range of IETF RFCs. The following terms are defined: Careful Resume (CR): The method specified in . All other terms in this section, except CWND, are defined in the same document. CWND: The congestion window, or equivalent CC variable limiting the maximum sending rate; current_rtt: A sample measurement of the current RTT; PipeSize: A measure of the validated available capacity based on the acknowledged data; saved_cwnd: The preserved capacity derived from observation of a previous connection; saved_rtt: The preserved minimum RTT; Unvalidated Packet: A packet sent when the CWND has been increased beyond the size normally permitted by the CC algorithm; if such a packet is acknowledged, it contributes to the PipeSize, if congestion is detected, it triggers entry to the Safe Retreat Phase.

Careful Resume defines a series of phases that a congestion controller moves through as a connection uses the mechanism. The key phases are illustrated in Figure 1.

Connect -> Reconnaissance --------------------> Normal (Observing) | ^ v | Unvalidated --------------------------+ | | | | +--> Validating --------------+ | | | | | | +---------------+--> Safe Retreat ---+ ]]>

Examples of these transitions between phases are provided in the annexe to .

There are currently no qlog events associated with this phase.

Careful Resume phases are abbreviated as follows:

• Unval = Unvalidated Phase
• Val = Validating Phase
• Recon = Reconnaissance Phase
• SR = Safe Retreat Phase
• Norm = Normal Phase

This section provides definitions that enable a Careful Resume implementation to generate qlog events when using QUIC. It introduces an event to report a phase transition at the sender, and its associated description. The event's state parameters (which include mandatory CR-specific metrics and optionally CWND or ssthresh) are intended to reflect the state after the phase transition. The event and data structure definitions in this section are expressed in the Concise Data Definition Language (CDDL) and its extensions described in . The current convention is to use long names for variables. For example, "CWND" is expanded as "congestion_window" and "saved_cwnd" is expanded as "saved_congestion_window".

Importance: Extra When the CC algorithm changes the Careful Resume Phase described in of this specification. Definition:

The authors would like to thank Robin Marx and Lucas Pardue for their fruitful comments in developing this specification.

Note by RM: This event type is to be added to the "qlog event schema URIs" registered at IANA, but the qlog event schema URIs registry has not yet been actually created at IANA because the qlog registry has not yet been created.

This document does not exhibit specific security considerations.

WG -00 Addresses comments from Mike Heard and Joerg Deutschmann. WG -01 Adds a new trigger, path_changed, to align with the main draft, fixes formatting NiTs. WG -02 Use uint64 for consistecy with qlog-quic-events, expand congestion_window_limited trigger name, suggested by Nicolai Fischer.