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
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 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
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
Li, et al. Expires October 22, 2020 [Page 1]
Internet-Draft Optimizing ACK in WLAN April 2020
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
Li, et al. Expires October 22, 2020 [Page 2]
Internet-Draft Optimizing ACK in WLAN April 2020
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)
Li, et al. Expires October 22, 2020 [Page 3]
Internet-Draft Optimizing ACK in WLAN April 2020
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
Li, et al. Expires October 22, 2020 [Page 4]
Internet-Draft Optimizing ACK in WLAN April 2020
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
Li, et al. Expires October 22, 2020 [Page 5]
Internet-Draft Optimizing ACK in WLAN April 2020
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.
Li, et al. Expires October 22, 2020 [Page 6]
Internet-Draft Optimizing ACK in WLAN April 2020
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.
Li, et al. Expires October 22, 2020 [Page 7]
Internet-Draft Optimizing ACK in WLAN April 2020
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.
Li, et al. Expires October 22, 2020 [Page 8]
Internet-Draft Optimizing ACK in WLAN April 2020
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
Li, et al. Expires October 22, 2020 [Page 9]
Internet-Draft Optimizing ACK in WLAN April 2020
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
Li, et al. Expires October 22, 2020 [Page 10]