QUIC Acknowledgement Frequency

Document Type Active Internet-Draft (quic WG)
Authors Jana Iyengar  , Ian Swett 
Last updated 2021-07-12
Stream Internet Engineering Task Force (IETF)
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QUIC                                                          J. Iyengar
Internet-Draft                                                    Fastly
Intended status: Standards Track                                I. Swett
Expires: 13 January 2022                                          Google
                                                            12 July 2021

                     QUIC Acknowledgement Frequency


   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 (https://github.com/quicwg/

   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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 13 January 2022.

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Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   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
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

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.  Multiple ACK_FREQUENCY Frames . . . . . . . . . . . . . . . .   6
   6.  Sending Acknowledgments . . . . . . . . . . . . . . . . . . .   7
     6.1.  Response to Reordering  . . . . . . . . . . . . . . . . .   7
     6.2.  Expediting Congestion Signals . . . . . . . . . . . . . .   8
     6.3.  Batch Processing of Packets . . . . . . . . . . . . . . .   8
   7.  Computation of Probe Timeout Period . . . . . . . . . . . . .   8
   8.  Implementation Considerations . . . . . . . . . . . . . . . .   9
     8.1.  Loss Detection  . . . . . . . . . . . . . . . . . . . . .   9
     8.2.  New Connections . . . . . . . . . . . . . . . . . . . . .   9
     8.3.  Window-based Congestion Controllers . . . . . . . . . . .   9
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  10
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  10
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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",
   "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.  The delaying of 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 acknowledgement packets can improve
   connection and endpoint performance in the following ways:

   *  Sending UDP packets can be noticeably CPU intensive on some
      platforms.  Reducing the number of packets that only contain
      acknowledgements can therefore reduce the amount of CPU consumed
      at a data receiver.  Experience shows that this cost reduction can
      be significant 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.

   *  Severely asymmetric link technologies, such as DOCSIS, LTE, and
      satellite links, connection throughput in the data direction
      becomes constrained when the reverse bandwidth is filled by
      acknowledgment packets.  When traversing such links, reducing the
      number of acknowledgments allows connection throughput to scale
      much further.

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   As discussed in Section 8 however, there are 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] currently specifies a simple delayed acknowledgement
   mechanism that a receiver can use: send an acknowledgement for every
   other packet, and for every packet when reordering is observed.  This
   simple mechanism does not allow a sender to signal its 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

   min_ack_delay (0xff02de1a):  A variable-length integer representing
      the minimum amount of time in microseconds by which the endpoint
      can delay an acknowledgement.  Values of 2^24 or greater are
      invalid, and receipt of these values MUST be treated as a
      connection error of type TRANSPORT_PARAMETER_ERROR.

   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.

   This Transport Parameter is encoded as per Section 18 of

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   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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |                            0xaf (i)                         ...
   |                      Sequence Number (i)                    ...
   |                  ACK-eliciting threshold (i)                ...
   |                    Update Max Ack Delay (i)                 ...
   | Ignore Order (8)|

   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, see Section 5.

   ACK-eliciting threshold:  A variable-length integer representing the
      maximum number of ack-eliciting packets after which the receiver
      sends an acknowledgement.  A value of 1 will result in an
      acknowledgement being sent for every ack-eliciting packet
      received.  A value of 0 is invalid.  Receipt of an invalid value
      MUST be treated as a connection error of type

   Update Max Ack Delay:  A variable-length integer representing an
      update to the peer's "max_ack_delay" transport parameter
      (Section 18.2 of [QUIC-TRANSPORT]).  The value of this field is in
      microseconds.  Any value smaller than the "min_ack_delay"
      advertised by this endpoint is invalid.  Receipt of an invalid
      value MUST be treated as a connection error of type

   Ignore Order:  An 8-bit field representing a boolean truth value.

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      This field MUST have the value 0x00 (representing "false") or 0x01
      (representing "true").  This field can be set to "true" by an
      endpoint that does not wish to receive an immediate
      acknowledgement when the peer observes reordering (Section 6.1).
      Receipt of any other value MUST be treated as a connection error

   ACK_FREQUENCY frames are ack-eliciting.  However, their loss does not
   require retransmission if an ACK_FREQUENCY frame with a larger
   Sequence Number value has been sent.

   An endpoint MAY send ACK_FREQUENCY frames multiple times during a
   connection and with different values.

   An endpoint will have committed a "max_ack_delay" value to the peer,
   which specifies the maximum amount of time by which the endpoint will
   delay sending acknowledgments.  When the endpoint receives an
   ACK_FREQUENCY frame, it MUST update this maximum time to the value
   proposed by the peer in the Update Max Ack Delay field.

5.  Multiple ACK_FREQUENCY Frames

   An endpoint can send multiple ACK_FREQUENCY frames, and each one of
   them can have different values in all fields.  An endpoint MUST use a
   sequence number of 0 for the first ACK_FREQUENCY frame it constructs
   and sends, and a strictly increasing value thereafter.

   An endpoint MUST allow reordered ACK_FREQUENCY frames to be received
   and processed, see Section 13.3 of [QUIC-TRANSPORT].

   On the first received ACK_FREQUENCY frame in a connection, an
   endpoint MUST immediately record all values from the frame.  The
   sequence number of the frame is recorded as the largest seen sequence
   number.  The new ACK-eliciting Threshold and Update Max Ack Delay
   values MUST be immediately used for delaying acknowledgements; see
   Section 6.

   On a subsequently received ACK_FREQUENCY frame, the endpoint MUST
   check if this frame is more recent than any previous ones, as

   *  If the frame's sequence number is not greater than the largest one
      seen so far, the endpoint MUST ignore this frame.

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   *  If the frame's sequence number is greater than the largest one
      seen so far, the endpoint MUST immediately replace old recorded
      state with values received in this frame.  The endpoint MUST start
      using the new values immediately for delaying acknowledgements;
      see Section 6.  The endpoint MUST also replace the recorded
      sequence number.

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 recorded
   "max_ack_delay" and "ACK-eliciting threshold" values (Section 5), the
   endpoint MUST send 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 or equal to the recorded
      "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

   An endpoint is expected to bundle acknowledgements when possible.
   Every time an acknowledgement is sent, bundled or otherwise, all
   counters and timers related to delaying of acknowledgments are reset.

6.1.  Response to Reordering

   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 this

   If the endpoint has not yet received an ACK_FREQUENCY frame, or if
   the most recent frame received from the peer has an "Ignore Order"
   value of "false" (0x00), the endpoint MUST immediately acknowledge
   any subsequent packets that are received out of order.

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   If the most recent ACK_FREQUENCY frame received from the peer has an
   "Ignore Order" value of "true" (0x01), the endpoint does not make
   this exception.  That is, the endpoint MUST NOT send an immediate
   acknowledgement in response to packets received out of order, and
   instead continues to use the peer's "ACK-eliciting threshold" and
   "max_ack_delay" thresholds for sending acknowledgements.

6.2.  Expediting Congestion Signals

   As specified in Section 13.2.1 of [QUIC-TRANSPORT], an endpoint
   SHOULD immediately acknowledge packets marked with the ECN Congestion
   Experienced (CE) codepoint in the IP header.  Doing so reduces the
   peer's response time to congestion events.

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 MAY process all packets in a batch before determining
   whether a threshold has been met and an acknowledgement is to be sent
   in response.

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.  Generally, when using
   "max_ack_delay" for PTO computations, endpoints MUST use the maximum
   of the current value and all those in flight.

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8.  Implementation Considerations

   There are tradeoffs inherent in a sender sending an ACK_FREQUENCY
   frame to the receiver.  As such it is recommended that implementers
   experiment with different strategies and find those which best suit
   their applications and congestion controllers.  There are, however,
   noteworthy considerations when devising strategies for sending
   ACK_FREQUENCY frames.

8.1.  Loss Detection

   A sender relies on receipt of acknowledgements to determine the
   amount of data in flight and to detect losses, e.g. when packets
   experience reordering, see [QUIC-RECOVERY].  Consequently, how often
   a receiver sends acknowledgments determines how long it takes for
   losses to be detected at the sender.

8.2.  New Connections

   Many congestion control algorithms have a startup mechanism during
   the beginning phases of a connection.  It is typical that in this
   period the congestion controller will quickly increase the amount of
   data in the network until it is signalled to stop.  While the
   mechanism used to achieve this increase varies, acknowledgments by
   the peer are generally critical during this phase to drive the
   congestion controller's machinery.  A sender can send ACK_FREQUENCY
   frames while its congestion controller is in this state, ensuring
   that the receiver will send acknowledgments at a rate which is
   optimal for the the sender's congestion controller.

8.3.  Window-based Congestion Controllers

   Congestion controllers that are purely window-based and strictly
   adherent to packet conservation, such as the one defined in
   [QUIC-RECOVERY], rely on receipt of acknowledgments to move the
   congestion window forward and send additional data into the network.
   Such controllers will suffer degraded performance if acknowledgments
   are delayed excessively.  Similarly, if these controllers rely on the
   timing of peer acknowledgments (an "ACK clock"), delaying
   acknowledgments will cause undesirable bursts of data into the

9.  Security Considerations


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10.  IANA Considerations


11.  Normative References

              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>.

              Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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/info/rfc8174>.

Appendix A.  Change Log

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


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

Authors' Addresses

   Jana Iyengar

   Email: jri.ietf@gmail.com

   Ian Swett

   Email: ian.swett@google.com

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