Internet Engineering Task Force G. Fairhurst
Internet-Draft A. Custura
Intended status: Informational T. Jones
Expires: March 18, 2021 University of Aberdeen
September 14, 2020
Changing the Default QUIC ACK Policy
draft-fairhurst-quic-ack-scaling-03
Abstract
Acknowledgement packets (ACKs) are used by transport protocols to
confirm the delivery of packets, and their reception is used in a
variety of other ways (to measure path round trip time, to gauge path
congestion, etc). However, the transmission of ACKs also consumes
resources at the receiver, forwarding resource in the network and
processing resources at the sender.
On network paths with significant path asymmetry, transmission of
ACKs can limit the available throughput or can reduce the efficient
use of network capacity. This occurs when the return capacity is
significantly more constrained than the forward capacity, and/or the
cost of transmission per packet is a significant component of the
total transmission cost. In these cases, reducing the ratio of ACK
packets to data packets can improve link utilisation and reduce link
transmission costs. It can also reduce processing overhead at the
sender and receiver.
This document proposes a change to the default acknowledgement policy
of the QUIC transport protocol to improve performance over paths with
appreciable asymmetry.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. ACK Ratio Impact on Asymmetric Paths . . . . . . . . . . 3
2.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Adapting the ACK Ratio in Current Transport
Specifications . . . . . . . . . . . . . . . . . . . . . 4
2.4. Adapting the TCP ACK Ratio in the Network . . . . . . . . 4
2.5. QUIC ACKs . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Updating the Default ACK Ratio for QUIC . . . . . . . . . . . 6
3.1. Considerations Updating the Default QUIC ACK Policy . . . 6
3.2. Mitigating the Impact of ACKs during Slow Start . . . . . 7
3.3. ACKs after Slow Start . . . . . . . . . . . . . . . . . . 8
3.4. ACKs after re-ordering is Detected . . . . . . . . . . . 8
3.5. Further Tuning of the ACK Policy . . . . . . . . . . . . 8
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Normative References . . . . . . . . . . . . . . . . . . 9
6.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Summary of Recommended ACK Policy for TCP . . . . . 11
Appendix B. Understanding ACKs in Slow Start . . . . . . . . . . 12
Appendix C. Experiments Exploring an ACK Ratio of 1:10 . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
Acknowledgement packets (ACKs) are used by transport protocols to
confirm the delivery of packets, and their reception is used in a
variety of other ways (to measure path round trip time, to gauge path
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congestion, etc) [Cust20]. However, the transmission of ACKs also
consumes resources at the receiver, forwarding resource in the
network and processing resources at the sender.
The current design of QUIC [I-D.ietf-quic-transport] currently
proposes a default acknowledgement (ACK) ratio of 1:2 (at least one
ACK for every 2 ack-eliciting packets) inspired by current
recommendations for TCP, see Appendix A.
This document motivates an increase in the ratio of ACK packets to
data packets from 1:2 to 1:10 for QUIC flows.
2. Motivation
When the characteristics of the forward and return paths are not
symmetric [RFC3449], the transmission of ACKs can adversely impact
either transport performance and/or the cost of sending data across a
link.
2.1. ACK Ratio Impact on Asymmetric Paths
TCP Performance Implications of Network Path Asymmetry [RFC3449]
describes a series of problems and mitigations when transports use an
asymmetric path. Performance problems arise in several access
networks, including bandwidth-asymmetric networks (such as broadband
satellite access, DOCSIS cable networks, cellular mobile, WiFi, etc)
[Cust20].
Where the ACK rate is limited by the capacity of the return path,
this constrains the maximum throughput for the forward path. ACK
traffic also competes for capacity and/or transmission opportunities
with other traffic that shares a constrained return path. This
motivates a need to reduce the volume of ACK traffic (increase the
number of segments/packets that are acknowledged by each ACK).
Capacity is not the only asymmetric path constraint. Sending ACKs
can consume significant transmission resources and the cost of
transmitting ACKs can become a significant part of the cost of
transmission when using a network segment. In many wireless
technologies, there is appreciable overhead for the transmission of
each packet burst (data and ACK). There can also be associated costs
(e.g., in radio resource management and transmission scheduling) that
are often different for the forward and return paths because they use
different technologies or configurations.
These provides an incentive to reduce the rate of ACK traffic
generated by a transport protocol.
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2.2. Terminology
The key words "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.
2.3. Adapting the ACK Ratio in Current Transport Specifications
We define the ACK Ratio as the ratio between the number of packets
that are received by a transport endpoint (on the forward path) and
the number of ACK packets that are returned (on the return path). A
simple protocol may send an ACK for each data packet, resulting in an
ACK Ratio of 1:1. One that acknowledges alternate data packets has a
ratio of 1:2. Most IETF-defined protocols specify a default ACK
Ratio. Note on the use of ACKs in TCP and the resulting ACK Ratio
are provided in Appendix A.
Various methods have been proposed to modify the ACK Ratio used by
transport protocols. Two examples follow, based on a receiver
understanding whether the return path has become congested:
Various proposals and analyses methods to allow TCP to dynamically
adjust the AR (e.g., [Lan08]). ACK-CC [RFC5690] proposed a method to
control the rate of ACKs to avoid the return path from becoming
congested, but this did not achieve wide-scale deployment.
The Datagram Congestion Control Protocol (DCCP) [RFC4340] has methods
to control the ACK ratio at the receiver. DCCP specifies a TCP-
friendly congestion control [RFC4341], which includes the ability to
use signalling that allows this sender to adjust the receiver ACK
ratio within certain parameters. This also was not widely used.
These methods are suggested to help when there is congestion on the
return path. However, end-to-end transport methods do not have a way
to detect and account for the cost of ACK transmission on a link
along the return path, and are difficult to tune to adapt to delays
introduced by links (e.g., the variations produced by radio resource
management). Transport adaptation therefore can only provide a
partial mitigation to the impacts when sending ACKs over an
asymmetric paths.
2.4. Adapting the TCP ACK Ratio in the Network
An alternative approach has been deployed for TCP that uses a
middlebox in the network to "thin" the rate of ACKs. This method is
used with paths that exhibit significant ACK asymmetry to improve
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performance of TCP. They can modify the ACK Ratio experienced by a
network link from the default TCP ACK Ratio of 1:2 [RFC3449].
Performance Enhancing Proxies (PEPs) implement these functions when
they detect/know an upstream link is (or is likely to be) filled with
TCP ACKs, or on cross-layer information provided by the physical
layer.
Removing redundant TCP ACKs (also known as "ACK Thinning") leads to
stretch ACKs (where a single ACK acknowledges more than two TCP
segments). Stretch ACKs have been observed on Internet paths [All98]
and are are now common [Din15] for various reasons.
The introduction of TCP Appropriate Byte Counting (ABC) [RFC3465]
mostly mitigates the impact of stretch ACKs, and also recommends
burst mitigation techniques at a TCP sender.
2.5. QUIC ACKs
QUIC, like other transports, generates ACK information and sends this
in QUIC packets on the return path.
ACK Thinning methods can only be used when ACKs can be observed by a
network device on the path. In contrast, QUIC uses an encrypted
feedback packet to communicate an ACK. This use of encryption
intentionally prevents such in-network optimisations.
A typical QUIC ACK is around 25% larger than a corresponding TCP ACK,
and can still be larger when there is loss/reordering. This means
additional processing overhead and link usage for all Internet paths,
with a significant impact on asymmetric links, where this can also
limit throughput:
o The smallest IPv4 TCP ACK is 20 bytes, resulting in a 40 byte IPv4
TCP ACK packet, although this could be up to 40 bytes larger when
TCP options are present.
o The TCP ACK size often increases following a reordering event due
to the presence of SACK option blocks. The maximum size for IPv4
is max 60 bytes.
o The smallest IPv4 QUIC ACK packet is 51 bytes. The ACK frame is
only 4 bytes, but is sent using a QUIC packet. For IPv4 this is:
20 (IP) + 8 (UDP) + 3 (1+1+1 QUIC header ... Assuming a 1 B CID) +
4 (minimum ACK frame size) + 16 (crypto overhead). Other types of
frames may also be sent in the same packet, increasing this size.
The QUIC ACK packet size also becomes larger for very long
connections due to the varint encoding.
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o The packet size for an IPv4 QUIC ACK increases after a reordering
event due to the inclusion of ACK range information. The maximum
size of this information is limited to the size of a QUIC packet.
Implementation may further limit the size (often observed as 100's
of bytes depending on the pattern of loss/reordering).
Compared to TCP, the performance of QUIC is therefore disadvantaged
when QUIC uses an ACK Ratio of 1:2.
3. Updating the Default ACK Ratio for QUIC
Any ACK policy that changes the ACK ratio from 1:2 needs to consider
three issues:
o A reduced frequency of feedback can increase the time to detect
loss, impacting the loss recovery algorithm, and potentially
leading to cases of spurious retransmission. QUIC mitigates this
by using timer-based retransmission (using the PTO).
o A reduced frequency of feedback can increase the time to detect
and react to congestion, impacting the congestion control
algorithm. The speed of loss detection can be impacted by delayed
acknowledgements. Timer-based methods (e.g., using the QUIC Probe
Timeout (PTO), see section 5.1 of [I-D.ietf-quic-recovery]) can
reduce the reliance on ACKs to detect loss.
o A reduced ACK rate can lead to bursts of acknowledged packets, and
introduces a need for burst mitigation at the sender.
3.1. Considerations Updating the Default QUIC ACK Policy
A QUIC receiver can generate one ACK frame for every received ack-
eliciting packet, but normally aggregates the ACK information into a
cumulative ACK. The QUIC transport protocol currently recommends a
default ACK Ratio of 1:2 [I-D.ietf-quic-transport]. Additionally,
QUIC relies on pacing, rather than ACK-Clocking as a burst
mitigation. Hence reception of larger cumulative ACKs does not
normally have a significant impact on the sender's traffic.
Since the introduction of the specification to allow a larger TCP
Initial Window (IW) [RFC6928], there has been deployment experience
using TCP with an IW of 10 segments at startup. QUIC continues this
practice, which in part motivates the need for a QUIC transport
protocol to operate when a burst of acknowledgements for up to ten
packets is received. QUIC transport recommends the use of pacing to
mitigate packet bursts generated by a sender (see section 6.8 of
[I-D.ietf-quic-recovery])
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This document proposes changing the QUIC behaviour to send an ACK for
at least every 10 received packets. Further background to the
proposed method is detailed in the Annexe (Appendix C).
The QUIC transport protocol also currently specifies a maximum ACK
Delay, which is communicated by the sender at connection setup to
indicate the maximum time an endpoint will delay sending
acknowledgements. A default of 25 milliseconds is recommended
[I-D.ietf-quic-transport]. The ACK Delay timer ensures ACKs are not
unduly delayed (e.g., when data packets are spaced in time, or at the
end of a packet burst). The effect of a large delay could be
significant when a stretch ACK acknowledges more QUIC packets.
When the ACK Ratio is increased, an appropriate choice of the maximum
ACK Delay becomes more important - because it could otherwise
withhold ACK information for a time long enough to impact protocol
performance or operation. The proposed therefore method also
triggers ACK feedback at least four times per RTT (this additional
rule is less important when the RTT is greater than 100ms and the ACK
Delay forms a small part of the total RTT).
3.2. Mitigating the Impact of ACKs during Slow Start
A sender during slow start is often cwnd-limited, so any additional
delay in returning an ACK has the effect of increasing the duration
of the slow-start phase (see Appendix B). The requested change is:
The receiver can provide a higher rate of acknowledge for the
first 100 ACK-eliciting packets, where it acknowledges at least
every second received ACK-eliciting packet. A suitable method
might send an ACK frame for every two received ack-eliciting
packets for the first 100 received packets if max_ack_delay time
has passed since the oldest unacknowledged data was received.
NOTE: The original design of TCP used each ACK as a trigger to
increase the congestion window, motivating an initial ACK Ratio of
1:1 (as in DAASS Appendix A).
NOTE: A receiver typically has no understanding of the sender's
congestion control state, so the number 100 reflects a trade-off,
corresponding to an appreciable opening of the sender's congestion
window.
NOTE: A congestion controller typically re-enters slow start after
congestion is detected, if the congestion window is collapsed to a
small value, this could motivate again using this method. However,
the receiver is typically unaware of this event, and we do not
propose this is considered.
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3.3. ACKs after Slow Start
We recommend an ACK frame should be generated for at least every
tenth ack-eliciting packet. The requested change is:
The receiver SHOULD send an ACK frame if a period more than
MIN(max_ack_delay,min_rtt/4) has passed since receiving the oldest
unacknowledged data or when the received has accumulated at most
10 unacknowledged ack-eliciting packets.
NOTE: The maximum of receiving not more than 10 ack-eliciting packets
is derived from the recommended TCP Initial Window [RFC6928].
NOTE: This utilises the receiver's estimated min_rtt, when this is
known.
3.4. ACKs after re-ordering is Detected
Prompt and reliable communication of ACK ranges after loss is
important for efficient loss recovery. This suggests that additional
ACKs may be needed after a reordering event to protect the sender
from potential ACK loss in the return direction. However, such ACKs
also consume return path capacity and the number of additional
packets needs to be considered.
Following detected loss or congestion, a receiver sends ACKs
according to section 13.2.1 of QUIC transport
[I-D.ietf-quic-transport].
NOTE: A future recommendation could recommend reducing the ACK
frequency after loss/re-ordering.
3.5. Further Tuning of the ACK Policy
In situations where the default method is not sufficient, the ACK
Ratio might be further tuned by server, as described in
[I-D.iyengar-quic-delayed-ack]. This could permit the ACK method to
be adapted to match the behaviour of a new congestion control
algorithm. Reducing the rate of ACKs can also lower the
computational effort required to process ACKs at the sender and
receiver, important for some high speed connections. For instance,
this could reduce the workload for high speed network interfaces by
reducing the rate of cache ejection for Generic Receiver Offload
(GRO).
The change to the default motivated in this document is not related
to such further tuning of the ACK policy.
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4. IANA Considerations
This memo includes no request to IANA.
5. Security Considerations
The security considerations for the QUIC transport protocol are
described in [I-D.ietf-quic-transport].
6. References
6.1. Normative References
[I-D.ietf-quic-recovery]
Iyengar, J. and I. Swett, "QUIC Loss Detection and
Congestion Control", draft-ietf-quic-recovery-30 (work in
progress), September 2020.
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-30 (work
in progress), September 2020.
[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>.
[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>.
6.2. Informative References
[All98] Allman, M., "On the Generation and use of TCP
Acknowledgments; ACM SIGCOMM CCR, 28 (5) p4-21.", October
1998.
[Cust20] Custura, A., Jones, T., and G. Fairhurst, "Rethinking ACKs
at the Transport Layer; FIT Workshop, IEEE IFIP
Networking", June 2020.
[Din15] Ding, H., "TCP Stretch Acknowledgements and Timestamps,
Findings and Implications for Passive RTT Measurement; ACM
SIGCOMM CCR, 45 (3) p20-27.", July 2015.
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[I-D.iyengar-quic-delayed-ack]
Iyengar, J. and I. Swett, "Sender Control of
Acknowledgement Delays in QUIC", draft-iyengar-quic-
delayed-ack-00 (work in progress), January 2020.
[Lan08] Landstroem, S., "TCP/IP Technology for Modern Network
Environments; PhD Thesis", 2008.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.
[RFC2525] Paxson, V., Allman, M., Dawson, S., Fenner, W., Griner,
J., Heavens, I., Lahey, K., Semke, J., and B. Volz, "Known
TCP Implementation Problems", RFC 2525,
DOI 10.17487/RFC2525, March 1999,
<https://www.rfc-editor.org/info/rfc2525>.
[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/info/rfc3449>.
[RFC3465] Allman, M., "TCP Congestion Control with Appropriate Byte
Counting (ABC)", RFC 3465, DOI 10.17487/RFC3465, February
2003, <https://www.rfc-editor.org/info/rfc3465>.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
DOI 10.17487/RFC4340, March 2006,
<https://www.rfc-editor.org/info/rfc4340>.
[RFC4341] Floyd, S. and E. Kohler, "Profile for Datagram Congestion
Control Protocol (DCCP) Congestion Control ID 2: TCP-like
Congestion Control", RFC 4341, DOI 10.17487/RFC4341, March
2006, <https://www.rfc-editor.org/info/rfc4341>.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/info/rfc5681>.
[RFC5690] Floyd, S., Arcia, A., Ros, D., and J. Iyengar, "Adding
Acknowledgement Congestion Control to TCP", RFC 5690,
DOI 10.17487/RFC5690, February 2010,
<https://www.rfc-editor.org/info/rfc5690>.
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[RFC6928] Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
"Increasing TCP's Initial Window", RFC 6928,
DOI 10.17487/RFC6928, April 2013,
<https://www.rfc-editor.org/info/rfc6928>.
Appendix A. Summary of Recommended ACK Policy for TCP
A TCP sender regards reception of a TCP ACK as a positive indication
that data has been received across the path. The congestion control
algorithm uses this to increase the size of the congestion window,
cwnd RFC 5681 [RFC5681].
To reduce the ACK Rate, a receiver can delay sending an ACK for a
period of time called the ACK Delay. This can increase network
efficiency.
RFC 5681 [RFC5681] clarifies the TCP ACK frequency described in
RFC 1122 [RFC1122]. This recommends that a receiver generates an ACK
corresponding to every second maximum segment size, MSS, of received
data, Section 4.2 of RFC 5681 [RFC5681], specifies the ACK policy for
a TCP receiver: "an ACK SHOULD be generated for at least every second
full-sized segment, and MUST be generated within 500 ms of the
arrival of the first unacknowledged packet". However, RFC 3449
[RFC3449] also notes the need for, and deployment of, methods to
further reduce the number of TCP ACKs in networks with asymmetric
paths.
When a receiver decides to delay ACKs, this can reduce the rate of
growth of the cwnd.
Some congestion controllers can benefit from frequent feedback during
an initial slow start period, where the sender is probing for
available path capacity. When a TCP sender uses each ACK to increase
the cwnd, this directly impacts the cwnd growth. TCP implementations
often use heuristics such as DAAS (Delayed ACK after Slow Start) to
mitigate this. This allows the receiver to estimate when the sender
could be in the slow start phase of cwnd growth, and for a period of
time sends an ACK for each received segment/packet (i.e., an ACK
Ratio of 1:1). In contrast, a TCP congestion controller can increase
its congestion window based on the number of bytes acknowledged, not
the number of ACK packets. (QUIC chooses this approach).
A delayed ACK can also increase the time to complete slow start, by
introducing an additional delay when returning ACKs. This effect is
different to a direct change to the cwnd, but never-the-less can
impact the performance, especially over paths where the ACK Delay
period is comparable to the RTT.
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RFC 3465 [RFC3465] further discusses the issue of bursts that may be
caused by the interaction between ACK processing and congestion
control. This motivates a need to deal with bursts within TCP. TCP
senders can mitigate bursts by using Appropriate Byte Counting (ABC),
which increases the congestion window in proportion to the amount of
data sent into the network, rather than upon the arrival of each ACK.
(This also defends against ACK Splitting, where multiple ACKs are
received for parts of the same segment/packet).
ACKs can be lost on the return path (either through packet loss, or
by intentional thinning of the ACK stream). If a sender does not
receive an ACK for every second segment, a stretch ACK has occurred,
similar to using a larger ACK Ratio. RFC2525 [RFC2525] describes the
significance of stretch ACK violations:
"This behavior will cause TCP senders to generate burstier
traffic, which can degrade performance in congested environments.
In addition, generating fewer ACKs increases the amount of time
needed by the slow start algorithm to open the congestion window
to an appropriate point, which diminishes performance in
environments with large bandwidth-delay products. Finally,
generating fewer ACKs may cause needless retransmission timeouts
in lossy environments, as it increases the possibility that an
entire window of ACKs is lost, forcing a retransmission timeout."
There are other scenarios where a change to the TCP ACK policy could
have improved performance. However, the design of TCP, and
ossification of the protocol has made it hard for new mechanisms to
be deployed. QUIC does not suffer from these design constraints.
Appendix B. Understanding ACKs in Slow Start
This section provides diagrams to help explain the way ACKs are using
during slow start. This assumes the sender uses volume-based methods
to increase cwnd, as in QUIC.
Some congestion controllers can benefit from frequent feedback during
an initial slow start period, where the sender is probing for
available path capacity (see Appendix A). Since a QUIC CC is
expected to work in terms of the volume of data acknowledged, rather
than the number of ACKs received, this is not expected to be an
issue.
Consider a burst of 10 packets sent over a forward path. If the path
RTT is shorter than the ACK Delay value, then all packets are
cumulatively acknowledged by a single ACK. This is therefore the ACK
pattern with an ACK Ratio of 1:10, where 10 packets are sent as a
burst: 1 2 ... 10.
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1 2 ... 10
\ \ \ \ /
\ \ \ \ /
\ \ \ \ /
\ \ \ \ /
\ \ \ \ /
\ \ \ \ /
\ \ \ \ /
\ \ \ \ /
\ \ \ \ /
\ \ \ \ /
\ \ \ \ /
\ \ \ X
ACK
Figure B1: Burst; ACK Delay 25ms; RTT 10ms; ACK Ratio 1:10.
The single ACK both cumulatively acknowledges all the data, and
increases the cwnd corresponding to reception of the 10 packets. An
ACK policy that generates more frequent ACKs (e.g., a lower ACK Delay
or ACK Ratio 1:1 or 1:2, would send multiple ACKs when receiving the
10 packets, the duration of the ACK burst is the same as the duration
of the transmission burst: 1 2 ... 10.
1 2 ... 10
\ \ \ \ / ..... /
\ \ \ \ / /
\ \ \ \ / /
\ \ \ \ / /
\ \ \ \ / /
\ \ \ \ / /
\ \ \ \ / /
\ \ \ X /
\ \ \ / \ /
\ \ X \ /
\ \ / \ \ /
\ X \ X
ACK ACK
Figure : Burst; ACK Delay 25ms; RTT 10ms; ACK Ratio 1:2.
The result of using this ACK policy is that it reduces the time to
the first ACK by about the burst size. This is not usually a
significant benefit, because often the RTT is greater than the burst
size. A similar result occurs when ACKs are generated by either the
ACK Ratio or the ACK Delay timer. This also has an advantage that it
generates more than one ACK, preventing progress being a hostage to
fortune on a single ACK. QUIC recommends pacing. If the sender were
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to pace the 10 packets over the RTT, and the receiver ACK Ratio was
1:1, then this is the ACK pattern for the 10 packets paced over the
RTT:
1 .............. 9 10
\ \ \ \ \ \ \ / \/ \/ \/ / / / / /
\ \ \ \ \ \ X /\ /\ /\ / / / / /
\ \ \ \ \ \ / \ / \/ \/ X / / / /
\ \ \ \ \ X X /\ /\ / \ / / / /
\ \ \ \ \/ \ / \ / \/ X X / / /
\ \ \ \ /\ X X /\ / \ / \ / / /
\ \ \ \/ \/ \ / \ / X X X / /
\ \ \ /\ /\ X X / \ / \ / \ / /
\ \ \/ \/ \/ \ / \/ X X \/ /
\ \ /\ /\ /\ X /\ / \ / \ /\ /
\ \/ \/ \/ \/ \/ \/ X \/ X /
\ /\ /\ /\ /\ /\ /\ / \ /\ / \/
ACK ACK ACK ..................... ACK
Figure B2: Pacing; ACK Delay not relevant; RTT 10ms; ACK Ratio 1:1.
The time to receive the first ACK is similar to that without pacing,
and all later ACKs arrive paced. For the same path (RTT shorter than
the ACK delay), but with the ACK Ratio set to 1:10, then the pattern
for 10 packets paced over an RTT is:
1 2 3 4 5 6 7 8 9 10 11 12 13
\ \ \ \ \ \ \ \ \ \ X \ \
\ \ \ \ \ \ \ \ \ \ / \ \ \
\ \ \ \ \ \ \ \ \ \ / \ \ \
\ \ \ \ \ \ \ \ \ \ /
\ \ \ \ \ \ \ \ \ \ /
\ \ \ \ \ \ \ \ \ \ /
\ \ \ \ \ \ \ \ \ \ /
\ \ \ \ \ \ \ \ \ \ /
\ \ \ \ \ \ \ \ \ \ /
\ \ \ \ \ \ \ \ \ \ /
\ \ \ \ \ \ \ \ \ X
ACK
Figure B3: Pacing; ACK Delay 25ms; RTT 25ms; ACK Ratio 1;10.
The effect is similar to a very large ACK Delay. The time to receive
the first ACK is increased by an RTT (because that was the pacing
target). This causes the sender to wait an additional RTT (during
which the sender becomes idle) before it receives an ACK for the
series of 10 packets, and resumes pacing new data (at the new higher
rate, following the ACK). Because this increases the time for
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receiver to send the first ACK, it therefore slows the window growth.
If the path is much longer than the ACK delay, this pathology does
not occur. The patterns of ACKs is primarily the result of the ACK
Delay setting. The first ACK is returned after waiting for the ACK
Delay, and arrives shortly after one RTT:
1 .............. 9 10
\ \ \ \ \ \ \ / \/ \/ \/ / / / / /
\ \ \ \ \ \ X /\ /\ /\ / / / / /
\ \ \ \ \ \ / \ / \/ \/ X / / / /
\ \ \ \ \ X X /\ /\ / \ / / / /
\ \ \ \ \/ \ / \ / \/ X X / / /
\ \ \ \ /\ X X /\ / \ / \ / / /
\ \ \ \/ \/ \ / \ / X X X / /
\ \ \ /\ /\ X X / \ / \ / \ / /
\ \ \/ \/ \/ \ / \/ X X \/ /
\ \ /\ /\ /\ X /\ / \ / \ /\ /
\ \/ \/ \/ \/ \/ \/ X \/ X /
\ /\ /\ /\ /\ /\ /\ / \ /\ / \/
>ACK >ACK >ACK ................. >ACK
Figure B4: Pacing; ACK Delay 25ms; RTT 650ms; ACK Ratio - not
relevant.
The window growth is slowed only by the additional time of the ACK
delay period. In this case the role of the ACK Ratio becomes
apparent only as the rate increases so that multiple ACKs are
received with the ACK Delay interval.
Although QUIC uses cumulative ACKs to inflate the cwnd, and does not
rely upon ACK-clocking to time the transmission of new packets, it is
still influenced by the rate of ACKs in the time it is building the
congestion window. This could be mitigated by choosing an
appropriate ACK Delay, relative to the RTT, but the RTT is often
unknown at the time when ACK Delay is negotiated. Another way to
mitigate this is to ensure sufficient ACKs are sent for the first "n"
packets.
For example, setting the ACK Ratio as 1:2 for the first 100 received
packets. A value of 1:2 is expected to be a reasonable compromise
between reducing delay and conserving return path capacity, since
QUIC uses volume-based accounting to increase cwnd, it does not need
to ACK each received packet as in DAAS.
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Appendix C. Experiments Exploring an ACK Ratio of 1:10
We used an experimental approach to examine a change to QUIC's ACK
Policy <http://erg.abdn.ac.uk/~downloads/ackscaling.pdf>. This
section provides a few of the many results. These experiments were
performed in January 2020 based on available implementations at that
time.
Our tests show that the proposed change to the ACK Ratio did not
negatively impact the protocol. It reduced the amount of IP, UDP and
ACK overhead by a factor of approximately 5. The implemented
congestion control, was also not negatively impacted. Unlike TCP,
QUIC sends other types of data frames in addition to ACK frames,
increasing the total overhead on the return path. On asymmetrical
paths an ACK Ratio of 1:10 may still reduce the ACK traffic, helping
to avoid return path capacity limits impacting the ability to use the
forward path capacity.
Figure 1 presents a table with a set of asymmetric scenarios. The
columns present the rate of ACK traffic required (in kbps) to fill
each of the forward paths. The table shows the results for TCP
(without a PEP), both for lossless communication. It considers the
period after loss, when ACKs communicate the loss information. It
also shows the impact of using an ACK Ratio of 1:10 with QUIC.
An ACK Ratio of 1:10 reduces the utilisation of the return path.
Scenarios where the ACK traffic exceeds the return link capacity
(i.e. where this limits the forward path capacity that can be used)
are marked with a star. Note that the QUIC figure does not include
the encryption overhead, which would be dependent on the ciphers
chosen. This would add several additional bytes for every QUIC
packet.
+-------------------+-------------+---------------+-----------------+
| | 10/2 Mbps | 50/10 Mbps | 250/3 Mbps |
+-------------------+-------------+---------------+-----------------+
| TCP no loss | 133 - 346 | 650 -1,730 | 3250 - 8,650* |
| TCP loss | 346 - 560 | 1,730 - 2,800 | 8,650 - 14,000* |
| QUIC 1:2 no loss | 144 - 438 | 720 - 2,190 | 3,600 - 10,950* |
| QUIC 1:2 loss | 290 - * | 1450 - * | 7,250 - * |
| QUIC 1:10 no loss | 28.8 - 87.6 | 144 - 438 | 720 - 2,190 |
+-------------------+-------------+---------------+-----------------+
Figure 1: ACK traffic required to fill the forward path in different
loss and asymmetry scenarios. The QUIC figures do not include
encryption overhead.
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Figure 2 presents a table with the numbers of packets sent by two
QUIC implementations using a 1:2 and a 1:10 ACK Ratio. This shows
between a four and five time reduction in the number of packets sent.
+-------------------------------+----------------+----------------+
| | Chromium | Quicly |
| | Draft 23 | Draft 23 |
+-------------------------------+----------------+----------------+
| Packets sent | 77,419 | 83,238 |
| Packets on return path (1:2) | 39,089 (50.4%) | 41,108 (49.3%) |
| Packets on return path (1:10) | 10,409 (13.4%) | 9650 (11.5%) |
+-------------------------------+----------------+----------------+
Figure 2: Number of packets sent and received during a 100MB QUIC
transfer using different ACK ratios, for two implementations
Figure 3 presents a table with the number of bytes sent by one of the
QUIC implementations using a 1:2 and a 1:10 ACK Ratio. This shows a
reduction in the number of bytes on the return path from 2.7 to 0.7%
of the total bytes sent. In these scenarios, this is sufficient to
take full advantage of the forward path capacity.
+---------------------------------+----------------+
| | Chromium |
| | Draft 23 |
+---------------------------------+----------------+
| Bytes sent | 110M |
| Bytes on return path (1:2 AR) | 3,056KB (2.7%) |
| Bytes on return path (1:10 AR) | 810KB (0.7%) |
+---------------------------------+----------------+
Figure 3: Number of bytes sent and received during a 100MB Chromium
QUIC transfer using different ACK ratios
The number of bytes and packets reduces, as expected, when using an
ACK Ratio of 1:10, without any increase in loss, on a high-latency
path with an asymmetry of 5:1. This offers a clear benefit for paths
that are capacity-constrained, as well as paths which would benefit
from a reduction in the ACK Rate.
Additional data and analysis is provided in [Cust20].
Authors' Addresses
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Godred Fairhurst
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen AB24 3UE
UK
Email: gorry@erg.abdn.ac.uk
Ana Custura
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen AB24 3UE
UK
Email: ana@erg.abdn.ac.uk
Tom Jones
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen AB24 3UE
UK
Email: tom@erg.abdn.ac.uk
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