QUIC Acknowledgement Frequency

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Table of Contents

1. Introduction

This document describes a QUIC extension for an endpoint to control its peer's delaying of acknowledgements.¶

2. Motivation

A receiver acknowledges received packets, but it can delay sending these acknowledgements. Delaying acknowledgements can impact connection throughput, loss detection and congestion controller performance at a data sender, and CPU utilization at both a data sender and a data receiver.¶

Reducing the frequency of acknowledgements can improve connection and endpoint performance in the following ways:¶

• Sending UDP packets can be very CPU intensive on some platforms. Reducing the number of packets that only contain acknowledgements reduces the CPU consumed at a data receiver. Experience shows that this reduction can be critical for high bandwidth connections.¶

• Similarly, receiving and processing UDP packets can also be CPU intensive, and reducing acknowledgement frequency reduces this cost at a data sender.¶

• For asymmetric link technologies, such as DOCSIS, LTE, and satellite, connection throughput in the forward path can become constrained when the reverse path is filled by acknowledgment packets [RFC3449]. When traversing such links, reducing the number of acknowledgments can achieve higher connection throughput, lower the impact on other flows or optimise the overall use of transmission resources [Cus22].¶

• The rate of acknowledgment packets can reduce link efficiency, including transmission opportunities or battery life, as well as transmission opportunities available to other flows sharing the same link.¶

As discussed in Section 8 however, there can be undesirable consequences to congestion control and loss recovery if a receiver unilitaerally reduces the acknowledgment frequency. A sender's constraints on the acknowledgement frequency need to be taken into account to maximize congestion controller and loss recovery performance.¶

[QUIC-TRANSPORT] specifies a simple delayed acknowledgement mechanism that a receiver can use: send an acknowledgement for every other packet, and for every packet that is received out of order (Section 13.2.1 of [QUIC-TRANSPORT]). This does not allow a sender to signal its preferences or constraints. This extension provides a mechanism to solve this problem.¶

3. Negotiating Extension Use

Endpoints advertise their support of the extension described in this document by sending the following transport parameter (Section 7.2 of [QUIC-TRANSPORT]):¶

min_ack_delay (0xff04de1a):

A variable-length integer representing the minimum amount of time in microseconds by which the endpoint that is sending this value is able to delay an acknowledgement. This limit could be based on the receiver's clock or timer granularity. 'min_ack_delay' is used by the peer to avoid requesting too small a value in the Request Max Ack Delay field of the ACK_FREQUENCY frame.¶

An endpoint's min_ack_delay MUST NOT be greater than its max_ack_delay. Endpoints that support this extension MUST treat receipt of a min_ack_delay that is greater than the received max_ack_delay as a connection error of type TRANSPORT_PARAMETER_ERROR. Note that while the endpoint's max_ack_delay transport parameter is in milliseconds (Section 18.2 of [QUIC-TRANSPORT]), min_ack_delay is specified in microseconds.¶

The min_ack_delay transport parameter is a unilateral indication of support for receiving ACK_FREQUENCY frames. If an endpoint sends the transport parameter, the peer is allowed to send ACK_FREQUENCY frames independent of whether it also sends the min_ack_delay transport parameter or not.¶

Receiving a min_ack_delay transport parameter indicates that the peer might send ACK_FREQUENCY frames in the future. Until an ACK_FREQUENCY frame is received, receiving this transport parameter does not cause the endpoint to change its acknowledgement behavior.¶

Endpoints MUST NOT remember the value of the min_ack_delay transport parameter they received for use in a subsequent connection. Consequently, ACK_FREQUENCY frames cannot be sent in 0-RTT packets, as per Section 7.4.1 of [QUIC-TRANSPORT].¶

This Transport Parameter is encoded as per Section 18 of [QUIC-TRANSPORT].¶

4. ACK_FREQUENCY Frame

Delaying acknowledgements as much as possible reduces both work done by the endpoints and network load. An endpoint's loss detection and congestion control mechanisms however need to be tolerant of this delay at the peer. An endpoint signals the frequency it wants to receive ACK frames to its peer using an ACK_FREQUENCY frame, shown below:¶

ACK_FREQUENCY Frame {
  Type (i) = 0xaf,
  Sequence Number (i),
  Ack-Eliciting Threshold (i),
  Request Max Ack Delay (i),
  Reordering Threshold (i)
}

Following the common frame format described in Section 12.4 of [QUIC-TRANSPORT], ACK_FREQUENCY frames have a type of 0xaf, and contain the following fields:¶

Sequence Number:

A variable-length integer representing the sequence number assigned to the ACK_FREQUENCY frame by the sender to allow receivers to ignore obsolete frames.¶

Ack-Eliciting Threshold:

A variable-length integer representing the maximum number of ack-eliciting packets the recipient of this frame receives before sending an acknowledgment. A receiving endpoint SHOULD send at least one ACK frame when more than this number of ack-eliciting packets have been received. A value of 0 results in a receiver immediately acknowledging every ack-eliciting packet. By default, an endpoint sends an ACK frame for every other ack-eliciting packet, as specified in Section 13.2.2 of [QUIC-TRANSPORT], which corresponds to a value of 1.¶

Request Max Ack Delay:

A variable-length integer representing the value to which the endpoint requests the peer update its max_ack_delay (Section 18.2 of [QUIC-TRANSPORT]). The value of this field is in microseconds, unlike the 'max_ack_delay' transport parameter, which is in milliseconds. Sending a value smaller than the min_ack_delay advertised by the peer is invalid. Receipt of an invalid value MUST be treated as a connection error of type TRANSPORT_PARAMETER_ERROR. On receiving a valid value in this field, the endpoint MUST update its max_ack_delay to the value provided by the peer.¶

Reordering Threshold:

A variable-length integer that indicates the maximum packet reordering before eliciting an immediate ACK, as specified in {#out-of-order}. If no ACK_FREQUENCY frames have been received, the endpoint immediately acknowledges any subsequent packets that are received out-of-order, as specified in Section 13.2 of [QUIC-TRANSPORT], corresponding to a default value of 1. A value of 0 indicates out-of-order packets do not elicit an immediate ACK.¶

ACK_FREQUENCY frames are ack-eliciting. When an ACK_FREQUENCY frame is lost, the sender is encouraged to send another ACK_FREQUENCY frame, unless an ACK_FREQUENCY frame with a larger Sequence Number value has already been sent. However, it is not forbidden to retransmit the lost frame (see Section 13.3 of [QUIC-TRANSPORT]), because the receiver will ignore duplicate or out-of-order ACK_FREQUENCY frames based on the Sequence Number.¶

An endpoint can send multiple ACK_FREQUENCY frames with different values within a connection. A sending endpoint MUST send monotonically increasing values in the Sequence Number field, since this field allows ACK_FREQUENCY frames to be processed out of order. A receiving endpoint MUST ignore a received ACK_FREQUENCY frame if the Sequence Number value in the frame is smaller than the largest currently processed value.¶

5. IMMEDIATE_ACK Frame

A sender can use an ACK_FREQUENCY frame to reduce the number of acknowledgements sent by a receiver, but doing so increases the likelihood that time-sensitive feedback is delayed as well. For example, as described in Section 8.3, delaying acknowledgements can increase the time it takes for a sender to detect packet loss. Sending an IMMEDIATE_ACK frame can help mitigate this problem.¶

An IMMEDIATE_ACK frame can be useful in other situations as well. For example, if a sender wants an immediate RTT measurement or if a sender wants to establish receiver liveness as quickly as possible. PING frames (Section 19.2 of [QUIC-TRANSPORT]) are ack-eliciting, but if a PING frame is sent without an IMMEDIATE_ACK frame, the receiver might not immediately send an ACK based on its local ACK strategy.¶

By definition IMMEDIATE_ACK frames are ack-eliciting. An endpoint SHOULD send a packet containing an ACK frame immediately upon receiving an IMMEDIATE_ACK frame. An endpoint MAY delay sending an ACK frame despite receiving an IMMEDIATE_ACK frame. For example, an endpoint might do this if a large number of received packets contain an IMMEDIATE_ACK or if the endpoint is under heavy load.¶

IMMEDIATE_ACK Frame {
  Type (i) = 0xac,
}

6. Sending Acknowledgments

Prior to receiving an ACK_FREQUENCY frame, endpoints send acknowledgements as specified in Section 13.2.1 of [QUIC-TRANSPORT].¶

On receiving an ACK_FREQUENCY frame and updating its max_ack_delay and Ack-Eliciting Threshold values (Section 4), the endpoint sends an acknowledgement when one of the following conditions are met:¶

• Since the last acknowledgement was sent, the number of received ack-eliciting packets is greater than the Ack-Eliciting Threshold.¶

• Since the last acknowledgement was sent, max_ack_delay amount of time has passed.¶

Section 6.2, Section 6.4, and Section 6.5 describe exceptions to this strategy.¶

  Receive 1
  Receive 3 -> 2 Missing
  Receive 4 -> 2 Missing
  Receive 5 -> Send ACK because of 2
  Receive 8 -> 6,7 Missing
  Receive 9 -> Send ACK because of 6, 7 Missing
  Receive 10 -> Send ACK because of 7

  Receive 1
  Receive 3 -> 2 Missing
  Receive 5 -> 2 Missing, 4 Missing
  Receive 6 -> 2 Missing, 4 Missing
  Receive 7 -> Send ACK because of 2, 4 Missing
  Receive 8 -> 4 Missing
  Receive 9 -> Send ACK because of 4

7. Computation of Probe Timeout Period

On sending an update to the peer's max_ack_delay, an endpoint can use this new value in later computations of its Probe Timeout (PTO) period; see Section 5.2.1 of [QUIC-RECOVERY].¶

Until the packet carrying this frame is acknowledged, the endpoint MUST use the greater of the current max_ack_delay and the value that is in flight when computing the PTO period. Doing so avoids spurious PTOs that can be caused by an update that decreases the peer's max_ack_delay.¶

While it is expected that endpoints will have only one ACK_FREQUENCY frame in flight at any given time, this extension does not prohibit having more than one in flight. When using max_ack_delay for PTO computations, endpoints MUST use the maximum of the current value and all those in flight.¶

When the number of in-flight ack-eliciting packets is larger than the ACK-Eliciting Threshold, an endpoint can expect that the peer will not need to wait for its max_ack_delay period before sending an acknowledgement. In such cases, the endpoint MAY therefore exclude the peer's 'max_ack_delay' from its PTO calculation. When Ignore Order is enabled and loss causes the peer to not receive enough packets to trigger an immediate acknowledgement, the receiver will wait 'max_ack_delay', increasing the chances of a premature PTO. Therefore, if Ignore Order is enabled, the PTO MUST be larger than the peer's 'max_ack_delay'.¶

When sending PTO packets, one can include an IMMEDIATE_ACK frame to elicit an immediate acknowledgement. This avoids waiting the ack delay for acknowledgements of PTO packets, reducing tail latency and allowing the sender to exclude the peer's 'max_ack_delay' from subsequent PTO calculations.¶

8. Determining Acknowledgement Frequency

This section provides some guidance on a sender's choice of acknowledgment frequency and discusses some additional considerations. Implementers can select an appropriate strategy to meet the needs of their applications and congestion controllers.¶

9. Security Considerations

An improperly configured or malicious data sender could cause a data receiver to acknowledge more frequently than its available resources permit. However, there are two limits that make such an attack largely inconsequential. First, the acknowledgement rate is bounded by the rate at which data is received. Second, ACK_FREQUENCY and IMMEDIATE_ACK frames can only request an increase in the acknowledgment rate, but cannot force it.¶

In general, with this extension, a sender cannot force a receiver to acknowledge more frequently than the receiver considers safe based on its resource constraints.¶

10. IANA Considerations

This document defines a new transport parameter to advertise support of the extension described in this document and two new frame types to registered by IANQ in the respective "QUIC Protocol" registries under https://www.iana.org/assignments/quic/quic.xhtml.¶

The following entry in Table 1 should be added to the "QUIC Transport Parameters" registry under the "QUIC Protocol" heading.¶

The following frame types should be added to the "QUIC Frame Types" registry under the "QUIC Protocol" heading.¶

Appendix A. Change Log

• RFC Editor's Note: Please remove this section prior to publication of a final version of this document.¶

Acknowledgments

The following people directly contributed key ideas that shaped this draft: Bob Briscoe, Kazuho Oku, Marten Seemann.¶

Authors' Addresses