Manual: tc-dualpi2

DUALPI2(8) Linux DUALPI2(8)

NAME
DUALPI2 - Dual Queue Proportional Integral Controller AQM - Improved with a square

SYNOPSIS
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 ]

DESCRIPTION
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 heuris‐
tics 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 la‐
tency 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 re‐
sults 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 tar‐
get) 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.

ALGORITHM
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 prob‐
ability 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.

PARAMETERS
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 com‐
patible with, e.g., DCTCP. Note DCTCP should only be used for intra-DC traffic with very low RTTs and AQM de‐
lay 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 ex‐
ceeds 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 DU‐
ALPI2. 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 follow‐
ing relationship: max_rtt = typical_rtt * 6. If any of these parameters is given, it will be used to automat‐
ically 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 cal‐
culated 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 pro‐
vided, 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

EXAMPLES
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

FILTERS
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 probabil‐
ity (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.

Manual: tc-dualpi2

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