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QUIC                                                               T. Li
Internet-Draft                                                  K. Zheng
Intended status: Experimental                                  R. Jadhav
Expires: October 22, 2020                                        J. Kang
                                                     Huawei Technologies
                                                          April 20, 2020


                   Optimizing ACK mechanism for QUIC
                draft-li-quic-optimizing-ack-in-wlan-00

Abstract

   This document analyzes the problems caused by contentions and
   collisions on wireless medium between data packets and ACKs in WLAN
   and it proposes an optimized ACK mechanism that can minimize the
   intensity of ACK Frame in QUIC, improving the performance of
   transport layer connection.

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
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   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 October 22, 2020.

Copyright Notice

   Copyright (c) 2020 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
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   include Simplified BSD License text as described in Section 4.e of



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Requirements Language . . . . . . . . . . . . . . . . . . . .   2
   2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   2
   3.  ACK Mechanism in Current QUIC . . . . . . . . . . . . . . . .   2
   4.  Optimized ACK Mechanism for QUIC  . . . . . . . . . . . . . .   3
     4.1.  Reducing ACK intensity  . . . . . . . . . . . . . . . . .   3
     4.2.  OWD-based RTTmin estimation . . . . . . . . . . . . . . .   4
     4.3.  Sender-Side Operation . . . . . . . . . . . . . . . . . .   6
     4.4.  Receiver-side Operation . . . . . . . . . . . . . . . . .   7
     4.5.  Generating ACK  . . . . . . . . . . . . . . . . . . . . .   7
     4.6.  Modification to QUIC Protocol . . . . . . . . . . . . . .   7
       4.6.1.  Transport Parameter: ack-intensity-support  . . . . .   7
       4.6.2.  ACK-INTENSITY Frame . . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Problem Statement

   High-throughput transport over wireless local area network (WLAN)
   becomes a demanding requirement with the emergence of 4K wireless
   projection, VR/AR-based interactive gaming, and more.  However, the
   shared nature of the wireless medium induces contention between data
   transport and backward signaling, such as acknowledgement.  ACKs
   share the same medium route with data packets, causing similar medium
   access overhead despite the much smaller size of the ACKs.
   Contentions and collisions, as well as the wasted wireless resources
   by ACKs, lead to significant throughput decline on the data path.

3.  ACK Mechanism in Current QUIC

   [QUIC-TRANSPORT] specifies a simple delayed ACK mechanism that a
   receiver can send an ACK for every other packet, and for every packet
   when reordering is observed, or when the max_ack_delay timer expires.
   However, this ACK mechanism may not match the number of ACKs to the



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   transport's required intensity under different network conditions.
   For example, when the data throughput of a WLAN transport is
   extremely high, QUIC will generate a large number of ACKs.  In this
   case, minimizing the ACK intensity of QUIC is not only a win for data
   throughput improvement but also a win for energy and CPU efficiency.

4.  Optimized ACK Mechanism for QUIC

4.1.  Reducing ACK intensity

   ACK intensity can be quantified by the unit of Hz, i.e., number of
   ACKs per second.  Byte-counting ACK and periodic ACK are two
   fundamental ways to reduce ACK intensity on the transport layer.

   1.  Byte-counting ACK: ACK intensity is controlled by sending an ACK
   for every L (L >= 2) incoming full-sized packets, in which the packet
   size equals to the Max Packet Size (set in the max_packet_size
   parameter in QUIC).  The intensity of byte-counting ACK (f_b) is
   proportional to data throughput (bw):

   f_b = bw/L*max_packet_size (1)

   In general, f_b can be reduced by setting a large value of L.
   However, for a given L, f_b increases with bw.  This means when data
   throughput is extremely high, the ACK intensity still might be
   comparatively large.  In other words, the intensity of byte-counting
   ACK changes proportionately with bandwidth.

   2.  Periodic ACK: Byte-counting ACK's unbounded intensity can be
   attributed to the coupling between ACK sending and packet arrivals.
   Periodic ACK can decouple ACK intensity from packet arrivals,
   achieving a bounded ACK intensity when bw is high.  The intensity of
   periodic ACK (f_pack) is:

   f_pack = 1/alpha (2)

   Where alpha is the time interval between two ACKs and is a function
   of RTT.  However, when bw is extremely low, the ACK intensity is
   always as high as that in the case of a high throughput.  In other
   words, the intensity of periodic ACK is unadaptable to bandwidth
   change, which wastes resources.

   Combining these two ways, the minimum ACK intensity in a QUIC
   connection can be set as f_quic = min{f_b,f_pack}. Through Equations
   (1) and (2), we have

   f_quic = min{bw/(L*max_packet_size), 1/alpha} (3)




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   We set alpha = RTTmin/beta, which means sending beta ACKs per RTTmin.
   RTTmin is the minimum RTT observed for a given network path.  As a
   consequence, the minimum ACK intensity in a QUIC connection can be
   given as follow:

   f_quic = min{bw/(L*max_packet_size), beta/RTTmin} (4)

   where beta indicates the number of ACKs per RTT, and L indicates the
   number of full-sized data packets counted before sending an ACK.  To
   minimize the ACK intensity, a smaller beta or a larger L is expected.
   Sara Landstrom et al. has given a lower bound of beta in [Sara],
   i.e., beta >= 2.  An upper bound of L can also be derived according
   to the loss rate on the data path (plr_data) and the ack path
   (plr_ack), i.e., L <= feedback_info/(plr_data*plr_ack), where
   feedback_info denotes the amount of information carried by an ACK

   Qualitatively, periodic ACK is applied when bandwidth-delay product
   (bdp) is large (i.e., bdp >= beta*L* max_packet_size), and byte-
   counting ACK is applied when bdp is small (i.e., bdp < beta*L*
   max_packet_size).

   In terms of a transport with a large bdp, beta = 2 should be
   sufficient to ensure utilization, but the large bottleneck buffer
   (i.e., one bdp) makes it necessary to acknowledge data more often.
   In general, the minimum send window (SWNDmin) can be roughly
   estimated as follow:

   SWNDmin = beta*bdp/(beta-1) (5)

   Ideally, the bottleneck buffer requirement is decided by the minimum
   send window, i.e., SWNDmin - bdp.  Since doubling the ACK frequency
   reduces the bottleneck buffer requirement substantially from 1 bdp to
   0.33 bdp, beta = 4 is RECOMMENDED to provide redundancy [Sara], being
   more robust in practice.

4.2.  OWD-based RTTmin estimation

   In this document, the RTTmin is the minimum RTT samples observed at
   the sender for a given network path during a period of time, and
   OWDmin is the minimum OWD samples observed on the same network path
   during a period of time.

   When multiple packets carrying departure timestamps are transported
   between endpoints via the same path, an RTT of this path can be
   sampled at the sender upon receiving an ACK frame.  However, when
   sending fewer ACK frames, more data packets might be received during
   the ACK interval, generating only one RTT sample among multiple
   packets is likely to result in biases.  For example, a larger minimum



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   RTT estimate.  In general, the higher the throughput, the larger the
   biases.  One alternative way to reduce biases can be that, each ACK
   frame carries multiple timestamps (as well as ACK delays in
   [QUIC-RECOVERY] for the sender to generate more RTT samples.
   However, (1) the overhead is high, which is unacceptable especially
   under high-bandwidth transport.  Also, (2) the number of data packets
   might be far more than the maximum number of timestamps that an ACK
   frame is capable to carry.

   An RTT estimation system contains a sender and a receiver.  The
   sender can hardly generate per-packet RTT samples, which is the root
   cause of the minimum RTT estimation biases in the case of sending
   fewer ACKs.  When multiple packets carrying departure timestamps are
   transported between endpoints via the same path , an RTT of this path
   can be sampled at the sender upon receiving an ACK frame.  However,
   when sending fewer ACK frames, more data packets might be received
   during the ACK interval, generating only one RTT sample among
   multiple packets is likely to result in biases.  For example, a
   larger minimum RTT estimate.  In general, the higher the throughput,
   the larger the biases.  One alternative way to reduce biases can be
   that, each ACK frame carries multiple timestamps (as well as ACK
   delays in [QUIC-RECOVERY]) for the sender to generate more RTT
   samples.  However, (1) the overhead is high, which is unacceptable
   especially under high-bandwidth transport.  Also, (2) the number of
   data packets might be far more than the maximum number of timestamps
   that an ACK frame is capable to carry.  Since the receiver is capable
   to monitor per-packet state, the one-way delay (OWD) of each packet
   can be easily computed according to the departure timestamps (carried
   in the packet) and the arrival timestamps of each packet.  In this
   case, QUIC SHOULD adopt the OWD-based RTTmin estimation.  The
   rationale is that the variation of OWD reflects the variation of RTT
   over near-symmetric links.  The OWD-based RTTmin estimation requires
   the sender to record the departure timestamp in each ack-eliciting
   packet.  Meanwhile, at the receiver, the per-packet OWD samples
   SHOULD be computed upon packet arrivals and a function of computing
   the minimum OWD SHOULD be newly added.  The receiver then generates
   an ACK frame to the sender, in which the ACK delay and departure
   timestamp for the packet that achieves the minimum OWD is reported.
   The ACK delay is defined as the delay incurred between when the
   packet is received and when the ACK frame is sent.  Based on the
   information reported by the incoming ACK frames and the ACK arrival
   timestamps, the sender can generate RTT samples and then compute
   RTTmin accordingly.

   In this document, RTTmin is used to update the ACK intensity.  In
   general, RTTmin can also be used by other modules.  For example, some
   congestion controllers depends on RTTmin to estimate the congestion




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   window [Neal].  RTTmin is also used by QUIC loss detection to reject
   implausibly small rtt samples [QUIC-RECOVERY].

4.3.  Sender-Side Operation

   According to Formula (4), the run-time ACK intensity in QUIC are
   decided by bw, and RTTmin.  Generally, the RTTmin and bw are
   calculated at the sender.

   Before estimating the RTTmin, the RTT samples should be computed
   based on the ACK frames collected during a period of time.  Assume
   that a packet is sent by the sender at time t_1 and arrives at time
   t_3, and the ACK frame is sent at time t_4.  The ACK delay can be
   computed at the receiver.  For example, the receiver computes the ACK
   delay delta_t = t_4 - t_3, and syncs the ACK delay to the sender via
   an ACK frame.  The ACK delay can also be computed at the sender.  For
   example, the receiver directly syncs an ACK frame carrying t_4 and
   t_3 to the sender, the sender then computes the ACK delay delta_t =
   t_4 - t_3.

   The sender therefore computes an RTT sample according to delta_t,
   t_1, and the arrival time (t_2) of the ACK frame, i.e., RTT_sample =
   t_2 - t_1 - delta_t.  Measuring delta_t at the receiver assures an
   explicit correction for a more accurate RTT estimate.  RTT samples
   SHOULD be smoothed using exponentially weighted moving average (EWMA)
   as specified in [RFC6298].  The sender then computes the RTTmin
   according to these RTT samples during a period of time.

   The bw estimation can be acquired in a similar manner to BBR [Neal].
   Since minimizing the ACK intensity induces excessive ACK delay, the
   value of bw may be the average value over a long period of time.
   However, the biases introduced in ACK intensity computation is
   limited.

   After computing the f_quic, the sender periodically syncs it to the
   receiver to update the intensity of ACK Frame by sending a new ACK-
   INTENSITY frame.

   The sender SHOULD generate an ACK-INTENSITY frame on a regular basis.
   For example, when the change of f_quic exceeds a threshold, the ACK-
   INTENSITY frame should be sent to update the ACK intensity in time.
   The interval of ACK-INTENSITY frame can also be set according to the
   update window of RTTmin and bw.








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4.4.  Receiver-side Operation

   Currently, the QUIC receiver reports ACK delays for only the largest
   acknowledged packet in an ACK frame, hence an RTT sample is generated
   using only the largest acknowledged packet in the received ACK frame.
   For a more accurate RTTmin estimate when sending fewer ACK frames,
   QUIC SHOULD adopt the OWD-based RTTmin estimation.  The OWD-based
   RTTmin estimation requires the QUIC receiver to filter the departure
   timestamp for the packet that achieves the minimum OWD during the
   interval between two ACK frames and report the ACK delay of this
   packet.  Whether redefining the meaning of ACK delay or not, it
   depends on the negotiation between endpoints of the QUIC connection.

   Upon packet arrivals, the receiver is capable to generate per-packet
   OWD samples according to the difference between packet departure
   timestamp and packet arrival timestamp.  The receiver then computes
   the minimum OWD by comparing the per-packet OWD samples.  The OWD
   estimation does not require clock synchronization here because the
   relative values are adopted.

   Afterwards, based on the ACK delay and the departure timestamp
   corresponding to the packet that achieves the minimum OWD, the sender
   calculates the RTT of this packet as a minimum RTT sample.
   Ultimately, the minimum RTT is computed according to these minimum
   RTT samples.

   The ACK Delay field SHOULD be carried in the ACK Frame.  Other fields
   carried in the ACK frame have the same meaning as defined in
   [QUIC-RECOVERY].

   The receiver adopts the newly updated ACK intensity once it receives
   the ACK-INTENSITY frame from the sender.

4.5.  Generating ACK

   The newly proposed ACK mechanismSHOULD be applied when there is no
   out-of-order delivery.  When reordering happens, the ACK Frame SHOULD
   be generated immediately.

4.6.  Modification to QUIC Protocol

4.6.1.  Transport Parameter: ack-intensity-support

   A new field named ack-intensity-support should be added for
   negotiation between both parties whether starting the dynamic ACK
   intensity function in QUIC connection.  The endpoints sends this
   parameter during handshakes.  Only when both parties agree, ACK
   intensity refreshment can be adopted.



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   ack-intensity-support (0x XX):This parameter has two values (0 or 1)
   specifying whether the sending endpoint is willing to adopt ACK
   intensity refreshment.  When the value is set as 1, it means that the
   sending endpoint want to start ACK intensity refreshment during
   connection.  When the value is set as 0, it means that the sending
   endpoint does not support this function.

4.6.2.  ACK-INTENSITY Frame

   An ACK-INTENSITY frame is shown in Figure 1.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             0x XX                            ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Sequence Number(i)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       ACK Intensity (i)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 1: ACK-INTENSITY Frame

   An ACK-INTENSITY frame contains the following fields:

   Sequence Number: A variable-length integer indicating the sequence
   number assigned to the ACK-INTENSITY frame by the sender.

   ACK Intensity: A variable-length integer indicating the updated
   f_quic calculated by the sender.

   ACK-INTENSITY frames are ack-eliciting.  However, their loss does not
   require retransmission.

   Multiple ACK-INTENSITY frames SHOULD be generated by the sender
   during a connection to notify the receiver the variation of ACK
   intensity requirement under network dynamics.

5.  Security Considerations

   TBD

6.  IANA Considerations

   The value for ack-intensity-support transport parameter and ACK-
   INTENSITY frame should be allocated.





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7.  References

7.1.  Normative References

   [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/info/rfc2119>.

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              DOI 10.17487/RFC6298, June 2011,
              <https://www.rfc-editor.org/info/rfc6298>.

7.2.  Informative References

   [Neal]     Cardwell, N., Cheng, Y., Gunn, C. S., Yeganeh, S. H., and
              V. Jacobson, "BBR: Congestion-based congestion control",
              ACM QUEUE 14(5):20-53, 2016.

   [QUIC-RECOVERY]
              Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
              and Congestion Control", draft-ietf-quic-recovery-27 (work
              in progress), February 2020.

   [QUIC-TRANSPORT]
              Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", draft-ietf-quic-
              transport-27 (work in progress), March 2020.

   [Sara]     Landstrom, S. and L. Larzon, "Reducing the tcp
              acknowledgment frequency", ACM SIGCOMM CCR 37(3):5-16,
              2007.

Authors' Addresses

   Tong Li
   Huawei Technologies
   D2-03,Huawei Industrial Base
   Longgang District
   Shenzhen
   China

   Email: li.tong@huawei.com







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   Kai Zheng
   Huawei Technologies
   Information Road, Haidian District
   Beijing
   China

   Email: kai.zheng@huawei.com


   Rahul Arvind Jadhav
   Huawei Technologies
   D2-03,Huawei Industrial Base
   Longgang District
   Shenzhen
   China

   Email: rahul.jadhav@huawei.com


   Jiao Kang
   Huawei Technologies
   D2-03,Huawei Industrial Base
   Longgang District
   Shenzhen
   China

   Email: kangjiao@huawei.com
























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