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NAME | SYNOPSIS | DESCRIPTION | ALGORITHM | PARAMETERS | EXAMPLES | FILTERS | SEE ALSO | SOURCES | AUTHORS | COLOPHON |
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DUALPI2(8) Linux DUALPI2(8)
DUALPI2 - Dual Queue Proportional Integral Controller AQM -
Improved with a square
tc qdisc ... dualpi2
[ limit PACKETS ]
[ memlimit BYTES ]
[ coupling_factor NUMBER ]
[ step_thresh TIME|PACKETS ]
[ min_qlen_step PACKETS ]
[ drop_on_overload | overflow ]
[ drop_enqueue | drop_dequeue ]
[ l4s_ect | any_ect ]
[ classic_protection PERCENTAGE ]
[ max_rtt TIME [ typical_rtt TIME ]]
[ target TIME ]
[ tupdate TIME ]
[ alpha float ]
[ beta float ]
[ split_gso | no_split_gso ]
DUALPI2 AQM is a combination of the DUALQ Coupled-AQM with a PI2
base-AQM. The PI2 AQM (details can be found in the paper cited
below) is in turn both an extension and a simplification of the
PIE AQM. PI2 makes quite some PIE heuristics unnecessary, while
being able to control scalable congestion controls like TCP-
Prague. With PI2, both Reno/Cubic can be used in parallel with
Prague, maintaining window fairness. DUALQ provides latency
separation between low latency Prague flows and Reno/Cubic flows
that need a bigger queue. The main design goals are:
• L4S - Low Loss, Low Latency and Scalable congestion control
support
• DualQ option to separate the L4S traffic in a low latency
queue (L-queue), without harming remaining traffic that is
scheduled in classic queue (C-queue) due to congestion-
coupling
• Configurable overload strategies
• Use of sojourn time to reliably estimate queue delay
• Simple implementation
• Guaranteed stability and fast responsiveness
The detailed PI2 parameters (alpha, beta, and tupdate) of DualPI2
are hard to get right and typically give bad results if just tried
or guessed. These parameters need to be calculated to a coherent
set with a typical objective in mind. DualPI2 has a set of default
parameters that can be used for the general Internet, where the
maximum RTT is around 100ms and the typical RTT is around 15ms. It
is highly recommended to use max_rtt and typical_rtt (or target)
helper parameters if your deployment is deviating from the above
objectives (e.g., in a data center). These helpers are used to
provide the theoretically optimal PI2 parameters (alpha, beta, and
tupdate) for those objectives, and that can be used as a basis for
further finetuning, experimentation, and testing if desired.
DUALPI2 is designed to provide low loss and low latency to L4S
traffic, without harming classic traffic. Every update interval, a
new internal base probability is calculated based on queue delay.
The base probability is updated with a delta based on the
difference between the current queue delay and the target delay,
and the queue growth compared with the queuing delay during the
previous tupdate interval. The integral gain factor alpha is used
to correct slowly enough any persistent standing queue error to
the user specified target delay, while the proportional gain
factor beta is used to quickly compensate for queue changes
(growth or shrink).
The updated base probability is used as input to decide to mark
and drop packets. DUALPI2 scales the calculated probability for
each of the two queues accordingly. For the L-queue, the
probability is multiplied by a coupling_factor , while for the C-
queue, it is squared to compensate the squareroot rate equation of
Reno/Cubic. The ECT identifier ( l4s_ect|any_ect ) is used to
classify traffic into respective queues.
If DUALPI2 AQM has detected overload (when excessive non-
responsive traffic is sent), it can signal congestion solely using
drop , irrespective of the ECN field, or alternatively limit the
drop probability and let the queue grow and eventually overflow
(like tail-drop).
Additional details can be found in the RFC cited below.
limit PACKETS
Limit the number of packets that can be enqueued. Incoming
packets will be dropped once this limit is reached. This
limit is common for the L-queue and C-queue. Defaults to
10000 packets. This is about 125ms delay on a 1Gbps link.
memlimit BYTES
Limit the total amount of memory that can be used. Incoming
packets will be dropped once this memlimit is reached. This
memlimit is common for the L-queue and C-queue. Defaults to
10000 * interface MTU bytes.
coupling_factor NUMBER
Set the coupling rate factor between Classic and L4S.
Defaults to 2
l4s_ect|any_ect
Configures the ECT classifier. Packets whose ECT codepoint
matches this are sent to the L-queue, where they receive a
scalable marking. Defaults to l4s_ect , i.e., the L4S
identifier ECT(1). Setting this to any_ect causes all
packets whose ECN field is not zero to be sent to the L-
queue. This enables it to be backward compatible with,
e.g., DCTCP. Note DCTCP should only be used for intra-DC
traffic with very low RTTs and AQM delay targets bigger
than those RTTs, separated from Internet traffic (also if
Prague compliant CC), as it does not support all Prague
requirements that make sure that a congestion control can
work well with the range of RTTs on the Internet.
step_thresh TIME | PACKETS
Set the step threshold for the L-queue. This will cause
packets with a sojourn time exceeding the threshold to
always be marked. This value can either be specified using
time units (i.e., us, ms, s), or in packets (p, pkt,
packet(s)). A value without units is assumed to be in time
(us). If defining the step in packets, be sure to disable
GRO on the ingress interfaces. Defaults to 1ms
min_qlen_step PACKETS
Incoming packets enqueued to the L-queue may apply the step
threshold when the queue length of the L-queue exceeds this
value. Default to 0 packets. This means that every enqueued
packets to the L-queue with a sojourn time exceeds the step
threshold will be marked.
drop_on_overload | overflow
Control the overload strategy. drop_on_overload preserves
the delay in the L-queue by dropping in both queues on
overload. overflow sacrifices delay to avoid losses,
eventually resulting in a taildrop behavior once the limit
is reached. Defaults to drop_on_overload
drop_enqueue | drop_dequeue
Decide when packets are PI-based dropped or marked. The
step_thresh based L4S marking is always at dequeue.
Defaults to drop_dequeue
classic_protection PERCENTAGE
Protects the C-queue from unresponsive traffic in the L-
queue. This bounds the maximal scheduling delay in the C-
queue to be (100 - PERCENTAGE) times greater than the one
in the L-queue. Defaults to 10
typical_rtt TIME
max_rtt TIME
Specify the maximum round trip time (RTT) and/or the
typical RTT of the traffic that will be controlled by
DUALPI2. These values are specified using time units (i.e.,
us, ms, s). A value without units is assumed to be in us.
If either max_rtt or typical_rtt is not specified, the
missing value will be computed from the following
relationship: max_rtt = typical_rtt * 6. If any of these
parameters is given, it will be used to automatically
compute suitable values for alpha, beta, target, and
tupdate, according to the relationship from the appendix
A.1 in the IETF RFC cited below, to achieve a stable
control. Consequently, those derived values will override
their eventual user-provided ones. The default range of
operation for the qdisc uses max_rtt = 100ms and
typical_rtt = 15ms , which is suited to controlling
Internet traffic.
target TIME
Set the expected queue delay. Defaults to 15 ms. A value
without units is assumed to be in us.
tupdate TIME
Set the frequency at which the system drop probability is
calculated. Defaults to 16 ms. A value without units is
assumed to be in microseconds. This should be less than
one-third of the maximum RTT supported.
alpha float
beta float
Set alpha and beta, the integral and proportional gain
factors in Hz for the PI controller. These can be
calculated based on control theory. Defaults are 0.16 and
3.2 Hz, which provide stable control for RTT's up to 100ms
with tupdate of 16ms. Be aware, unlike with PIE, these are
the real unscaled gain factors. If not provided, they will
be automatically derived from typical_rtt and max_rtt , if
one of them or both are provided.
split_gso | no_split_gso
Decide how to handle aggregated packets. Either treat the
aggregate as a single packet (thus all share fate with
respect to marks and drops) with no_split_gso , trading
some tail latency for CPU usage, or treat each packet
individually (i.e., split them) with split_gso to enable
fine-grained marking/dropping of packets to control
queueing latencies. Defaults to split_gso
Setting DUALPI2 for the Internet with default parameters:
# sudo tc qdisc add dev eth0 root dualpi2
Setting DUALPI2 for datacenter with legacy DCTCP using ECT(0):
# sudo tc qdisc add dev eth0 root dualpi2 any_ect
This qdisc can be used in conjunction with tc-filters. More
precisely, it will honor filters "stealing packets", as well as
accept other classification schemes.
Packets whose priority/classid are set to
1 will be enqueued in the L-queue, alongside L4S traffic,
and thus subject to the increased marking probability (or
drops if they are marked not-ECT).
Packets whose priority/classid are set to
2 will also be enqueued in the L-queue, but will never be
dropped if they are not-ECT (unless the qdisc is full and
thus resorts to taildrop).
Finally, all the other classid/priority map to the C-queue.
tc(8), tc-pie(8)
• IETF RFC9332 : https://datatracker.ietf.org/doc/html/rfc9332
• CoNEXT '16 Proceedings of the 12th International on Conference
on emerging Networking Experiments and Technologies : "PI2: A
Linearized AQM for both Classic and Scalable TCP"
DUALPI2 was implemented by Koen De Schepper, Olga Albisser, Henrik
Steen, Olivier Tilmans, and Chia-Yu Chang, also the authors of
this man page. Please report bugs and corrections to the Linux
networking development mailing list at <netdev@vger.kernel.org>.
This page is part of the iproute2 (utilities for controlling
TCP/IP networking and traffic) project. Information about the
project can be found at
⟨http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2⟩.
If you have a bug report for this manual page, send it to
netdev@vger.kernel.org, shemminger@osdl.org. This page was
obtained from the project's upstream Git repository
⟨https://git.kernel.org/pub/scm/network/iproute2/iproute2.git⟩ on
2025-08-11. (At that time, the date of the most recent commit
that was found in the repository was 2025-08-08.) If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a
mail to man-pages@man7.org
iproute2 29 Oct 2024 DUALPI2(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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