QUIC Acknowledgement Frequency
draft-ietf-quic-ack-frequency-03

QUIC                                                          J. Iyengar
Internet-Draft                                                    Fastly
Intended status: Standards Track                                I. Swett
Expires: 11 September 2023                                        Google
                                                            M. Kühlewind
                                                                Ericsson
                                                           10 March 2023

                     QUIC Acknowledgement Frequency
                    draft-ietf-quic-ack-frequency-03

Abstract

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

Note to Readers

   Discussion of this draft takes place on the QUIC working group
   mailing list (quic@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/search/?email_list=quic.  Source
   code and issues list for this draft can be found at
   https://github.com/quicwg/ack-frequency.

   Working Group information can be found at https://github.com/quicwg.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 11 September 2023.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terms and Definitions . . . . . . . . . . . . . . . . . .   3
   2.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Negotiating Extension Use . . . . . . . . . . . . . . . . . .   4
   4.  ACK_FREQUENCY Frame . . . . . . . . . . . . . . . . . . . . .   5
   5.  IMMEDIATE_ACK Frame . . . . . . . . . . . . . . . . . . . . .   6
   6.  Sending Acknowledgments . . . . . . . . . . . . . . . . . . .   7
     6.1.  Response to Out-of-Order Packets  . . . . . . . . . . . .   7
       6.1.1.  Examples  . . . . . . . . . . . . . . . . . . . . . .   8
     6.2.  Expediting Congestion Signals . . . . . . . . . . . . . .   9
     6.3.  Batch Processing of Packets . . . . . . . . . . . . . . .   9
   7.  Computation of Probe Timeout Period . . . . . . . . . . . . .   9
   8.  Determining Acknowledgement Frequency . . . . . . . . . . . .  10
     8.1.  Congestion Control  . . . . . . . . . . . . . . . . . . .  10
     8.2.  Burst Mitigation  . . . . . . . . . . . . . . . . . . . .  11
     8.3.  Loss Detection and Timers . . . . . . . . . . . . . . . .  11
     8.4.  Connection Migration  . . . . . . . . . . . . . . . . . .  12
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     11.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  14
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

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

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1.1.  Terms and Definitions

   The keywords "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 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   In the rest of this document, "sender" refers to a QUIC data sender
   (and acknowledgement receiver).  Similarly, "receiver" refers to a
   QUIC data receiver (and acknowledgement sender).

   An "acknowledgement packet" refers to a QUIC packet that contains
   only an ACK frame.

   This document uses terms, definitions, and notational conventions
   described in Section 1.2 and Section 1.3 of [QUIC-TRANSPORT].

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].

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   *  The rate of acknowledgment packets can impact 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
   uniltaerally 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 (0xff03de1a):  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.

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   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.

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   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 PROTOCOL_VIOLATION.  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.  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 ACKs.

   ACK_FREQUENCY frames are ack-eliciting.  When an ACK_FREQUENCY frame
   is lost, it 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]), as 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
   processed thus far.

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
   chances 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.  The
   IMMEDIATE_ACK frame helps mitigate this problem.

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   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 6.1, Section 6.2, and Section 6.3 describe exceptions to this
   strategy.

6.1.  Response to Out-of-Order Packets

   As specified in Section 13.2.1 of [QUIC-TRANSPORT], endpoints are
   expected to send an acknowledgement immediately on receiving a
   reordered ack-eliciting packet.  This extension modifies that
   behavior when an ACK_FREQUENCY frame with a Reordering Threshold
   value other than 1 has been received.

   Largest Unacked  The largest packet number among all received ack-
      eliciting packets.

   Largest Acked  The Largest Acknowledged value sent in an ACK frame.

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   Unreported Missing  Packets with packet numbers between the Largest
      Unacked and Largest Acked that have not yet been received.

   An endpoint that receives an ACK_FREQUENCY frame with a non-zero
   Reordering Threshold value SHOULD send an immediate ACK when the gap
   between the smallest Unreported Missing packet and the Largest
   Unacked is greater than or equal to the Reordering Threshold value.
   Sending this additional ACK will reset the max_ack_delay timer and
   Ack-Eliciting Threshold counter as any ACK would do.

   In order to ensure timely loss detection, it is optimal to send a
   Reordering Threshold value of 1 less than the packet threshold used
   by the data sender for loss detection.  If the threshold is smaller,
   an ACK_FRAME is sent before the packet can be declared lost based on
   the packet threshold.  If the value is larger, it causes unnecessary
   delays.  (Section 18.2 of [QUIC-RECOVERY]) recommends a default
   packet threshold for loss detection of 3, equivalent to a Reordering
   Threshold of 2.

   If the most recent ACK_FREQUENCY frame received from the peer has a
   Reordering Threshold value of 0, the endpoint SHOULD NOT send an
   immediate acknowledgement in response to packets received out of
   order, and instead rely on the peer's Ack-Eliciting Threshold and
   max_ack_delay thresholds for sending acknowledgements.

6.1.1.  Examples

   When the reordering threshold is 1, any time a packet is received and
   there is a missing packet, an immediate ACK is sent.

   If the reordering theshold is 3 and ACKs are only sent due to
   reordering:

   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

   If the reordering threshold is 5 and ACKs are only sent due to
   reordering:

   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

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6.2.  Expediting Congestion Signals

   An endpoint SHOULD send an immediate acknowledgement when a packet
   marked with the ECN Congestion Experienced (CE) [RFC3168] codepoint
   in the IP header is received and the previously received packet was
   not marked CE.

   Doing this maintains the peer's response time to congestion events,
   while also reducing the ACK rate compared to Section 13.2.1 of
   [QUIC-TRANSPORT] during extreme congestion or when peers are using
   DCTCP [RFC8257] or other congestion controllers (e.g.
   [I-D.ietf-tsvwg-aqm-dualq-coupled]) that mark more frequently than
   classic ECN [RFC3168].

6.3.  Batch Processing of Packets

   For performance reasons, an endpoint can receive incoming packets
   from the underlying platform in a batch of multiple packets.  This
   batch can contain enough packets to cause multiple acknowledgements
   to be sent.

   To avoid sending multiple acknowledgements in rapid succession, an
   endpoint can process all packets in a batch before determining
   whether to send an ACK frame in response, as stated in Section 13.2.2
   of [QUIC-TRANSPORT].

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].  The endpoint MUST
   however wait until the ACK_FREQUENCY frame that carries this new
   value is acknowledged by the peer.

   Until the 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 increases 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

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   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'.

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.

8.1.  Congestion Control

   A sender needs to be responsive to notifications of congestion, such
   as a packet loss or an ECN CE marking.  To enable a sender to respond
   to potential network congestion in a timely fashion, usually at least
   one acknowledgement per round trip is needed if there are
   unacknowledged ack-eliciting packets in flight.  A sender can
   accomplish this by setting the Ack-Eliciting Threshold to a value no
   larger than the current congestion window or the Request Max Ack
   Delay value to no more than the estimated round trip.  Note that the
   congestion window particularly but also the RTT are dynamic and
   therefore might require frequent updates if the selected value are
   close to these limits.  Alternatively, a sender can accomplish this
   by sending an IMMEDIATE_ACK frame once per round trip, though if the
   packet containing an IMMEDIATE_ACK is lost, detection of that loss
   will be delayed by the reordering threshold or requested max ack
   delay.

   Note that it is possible that the RTT is smaller than the receiver's
   timer granularity, as communicated via the 'min_ack_delay' transport
   parameter, preventing the receiver from sending an acknowledgment
   every RTT in time.  In these cases, Reordering Threshold values other
   than 1 can be harmful, because it delays fast loss detection.

   A congestion controller that is congestion window limited relies upon
   receiving acknowledgements to send additional data into the network.
   An increase in acknowledgement delay increases the delay in sending
   data, which can reduce the achieved throughput.  Congestion window
   growth can also depend upon receiving acknowledgements.  This can be
   particularly significant in slow start (Section 7.3.1 of
   [QUIC-RECOVERY]), when delaying acknowledgements can delay the
   increase in congestion window and can create larger packet bursts.

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   If the sender is application-limited, acknowledgements can be delayed
   unnecessarily when entering idle periods.  Therefore, if no further
   data is buffered to be sent, a sender can send an IMMEDIATE_ACK frame
   with the last data packet before an idle period to avoid waiting for
   the ack delay.

8.2.  Burst Mitigation

   Receiving an acknowledgement can allow a sender to release new
   packets into the network.  If a sender is designed to rely on the
   timing of peer acknowledgments ("ACK clock"), delaying
   acknowledgments can cause undesirable bursts of data into the
   network.  In keeping with Section 7.7 of [QUIC-RECOVERY], a sender
   can either employ pacing or limit bursts to the initial congestion
   window.

8.3.  Loss Detection and Timers

   Acknowledgements are fundamental to reliability in QUIC.
   Consequently, delaying or reducing the frequency of acknowledgments
   can cause loss detection at the sender to be delayed.

   A QUIC sender detects loss using packet thresholds on receiving an
   acknowledgement (Section 6.1.1 of [QUIC-RECOVERY]); delaying the
   acknowledgement therefore delays this method of detecting losses.

   Reducing acknowledgement frequency reduces the number of RTT samples
   that a sender receives (Section 5 of [QUIC-RECOVERY]), making a
   sender's RTT estimate less responsive to changes in the path's RTT.
   As a result, any mechanisms that rely on an accurate RTT estimate,
   such as time-threshold loss detection (Section 6.1.2 of
   [QUIC-RECOVERY]) or Probe Timeout (Section 6.2 of [QUIC-RECOVERY]),
   will be less responsive to changes in the path's RTT, resulting in
   either delayed or unnecessary packet transmissions.

   To limit these consequences of reduced acknowledgement frequency, a
   sender SHOULD cause a receiver to send an acknowledgement at least
   once per RTT if there are unacknowledged ack-eliciting packets in
   flight.  A sender can accomplish this by sending an IMMEDIATE_ACK
   frame once per round-trip time (RTT), or it can set the Ack-Eliciting
   Threshold and Request Max Ack Delay values to be no more than a
   congestion window and an estimated RTT, respectively.

   A sender might use timers to detect loss of PMTU probe packets
   (Section 14 of [QUIC-TRANSPORT]).  A sender SHOULD bundle an
   IMMEDIATE_ACK frame with any PTMU probes to avoid triggering such
   timers.

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8.4.  Connection Migration

   To avoid additional delays to connection migration confirmation when
   using this extension, a client can bundle an IMMEDIATE_ACK frame with
   the first non-probing frame (Section 9.2 of [QUIC-TRANSPORT]) it
   sends or it can send only an IMMEDIATE_ACK frame, which is a non-
   probing frame.

   An endpoint's congestion controller and RTT estimator are reset upon
   confirmation of migration (Section 9.4 of [QUIC-TRANSPORT]), which
   can impact the number of acknowledgements received after migration.
   An endpoint that has sent an ACK_FREQUENCY frame earlier in the
   connection SHOULD update and send a new ACK_FREQUENCY frame
   immediately upon confirmation of connection migration.

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
   (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.

             +============+=================+===============+
             | Value      | Parameter Name. | Specification |
             +============+=================+===============+
             | 0xff03de1a | min_ack_delay.  | Section 3     |
             +------------+-----------------+---------------+

                   Table 1: Addition to QUIC Transport
                            Parameters Entries

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

                 +=======+===============+===============+
                 | Value | Frame Name    | Specification |
                 +=======+===============+===============+
                 | 0xaf  | ACK_FREQUENCY | Section 4     |
                 +-------+---------------+---------------+
                 | 0xac  | IMMEDIATE_ACK | Section 5     |
                 +-------+---------------+---------------+

                   Table 2: Addition to QUIC Frame Types
                                  Entries

11.  References

11.1.  Normative References

   [QUIC-TRANSPORT]
              Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/rfc/rfc9000>.

   [QUIC-RECOVERY]
              Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
              and Congestion Control", RFC 9002, DOI 10.17487/RFC9002,
              May 2021, <https://www.rfc-editor.org/rfc/rfc9002>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

11.2.  Informative References

   [Cus22]    Custura, A., Secchi, R., and G. Fairhurst, "Reducing the
              acknowledgement frequency in IETF QUIC",
              DOI 10.1002/sat.1466, name IJSCN, October 2022,
              <https://doi.org/10.1002/sat.1466>.

Iyengar, et al.         Expires 11 September 2023              [Page 13]
Internet-Draft       QUIC Acknowledgement Frequency           March 2023

   [RFC3449]  Balakrishnan, H., Padmanabhan, V., Fairhurst, G., and M.
              Sooriyabandara, "TCP Performance Implications of Network
              Path Asymmetry", BCP 69, RFC 3449, DOI 10.17487/RFC3449,
              December 2002, <https://www.rfc-editor.org/rfc/rfc3449>.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <https://www.rfc-editor.org/rfc/rfc3168>.

   [RFC8257]  Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,
              and G. Judd, "Data Center TCP (DCTCP): TCP Congestion
              Control for Data Centers", RFC 8257, DOI 10.17487/RFC8257,
              October 2017, <https://www.rfc-editor.org/rfc/rfc8257>.

   [I-D.ietf-tsvwg-aqm-dualq-coupled]
              De Schepper, K., Briscoe, B., and G. White, "Dual-Queue
              Coupled Active Queue Management (AQM) for Low Latency, Low
              Loss, and Scalable Throughput (L4S)", Work in Progress,
              Internet-Draft, draft-ietf-tsvwg-aqm-dualq-coupled-25, 29
              August 2022, <https://datatracker.ietf.org/doc/html/draft-
              ietf-tsvwg-aqm-dualq-coupled-25>.

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

   Jana Iyengar
   Fastly
   Email: jri.ietf@gmail.com

   Ian Swett
   Google
   Email: ianswett@google.com

   Mirja Kühlewind
   Ericsson
   Email: mirja.kuehlewind@ericsson.com

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