Build a Deficit Round Robin Queue with tc

Attach tc's Deficit Round Robin (DRR) scheduler to an interface and it has no idea which packet belongs where. tc-drr(8) only describes the scheduler, not a classifier policy. This guide builds a classful DRR queue with two child classes ready for traffic, plus the checks that show whether packets are actually being classified. The examples use the installed tc from iproute2 6.1.0-1ubuntu6.4, and the filter step is kept deliberately separate.

Allow about twenty minutes, plus time to choose a real traffic classification rule.

• iproute2 and an interface name.
• Root privileges for queue changes.
• A maintenance window if the interface carries live traffic. DRR is a packet scheduling change: attaching it to a production device can alter latency and packet loss immediately.

1. Check the installed implementation

First confirm the package and command. These are read-only checks and do not need elevated privileges:

$ tc -V
tc utility, iproute2-6.1.0, libbpf 1.3.0
$ dpkg-query -W -f='\${Package} \${Version}\n' iproute2
iproute2 6.1.0-1ubuntu6.4

The manpage's syntax is tc qdisc ... add drr [ quantum bytes ]. Its important default is the interface MTU: unless you provide quantum, each class starts with one MTU's worth of deficit. The minimum explicit quantum is 1 byte. A quantum is not a rate limit and does not reserve bandwidth by itself.

2. Inspect the existing root queue

Look before changing anything:

$ tc qdisc show dev IFACE

You may see a device-specific root qdisc, or a line such as qdisc noqueue 0: root. Record the output. The examples below install a new root qdisc, so any existing root configuration needs a deliberate replacement plan.

There is no harmless dry run for a complete qdisc replacement. If this is a live interface, save the current output and arrange a rollback before continuing.

3. Attach DRR as the root qdisc

This privileged command uses the interface MTU as the quantum:

# tc qdisc add dev IFACE handle 1: root drr

An explicit quantum is also valid. This example makes the scheduling unit visible, but the value must suit the traffic and interface:

# tc qdisc add dev IFACE handle 1: root drr quantum 1500

Use one of those commands, not both. If the command succeeds it normally prints nothing. Verify the qdisc:

# tc qdisc show dev IFACE
qdisc drr 1: root

The displayed line can include additional counters or options.

4. Add child classes

DRR has no built-in queues and no default class. Add at least two classes beneath handle 1::

# tc class add dev IFACE parent 1: classid 1:1 drr
# tc class add dev IFACE parent 1: classid 1:2 drr

These classes are scheduler entries. They do not yet tell tc which packets belong to either entry. A child qdisc can be attached below a class when you need a particular queueing policy, but the installed manual's central warning still applies: DRR is a pure scheduler and does not itself delay packets.

Check the result before writing a filter:

# tc class show dev IFACE

5. Add a filter policy for the classes

Classification is the part that depends on your network policy. The manual shows a filter assigning packets to a class, but its example deliberately uses protocol and classifier placeholders. Choose a real classifier and verify its own tc filter documentation before applying it.

After installing a policy, inspect exactly what is active:

# tc filter show dev IFACE parent 1:

Then generate only the test traffic you intended and inspect statistics:

# tc -s class show dev IFACE
# tc -s qdisc show dev IFACE

Look for packets and bytes increasing on the expected class. A zero counter can mean that the traffic does not match the filter, the filter is attached at the wrong parent, or no traffic has passed yet.

6. Understand the round-robin behaviour

DRR keeps an active list containing classes whose child qdiscs are non-empty. Each class starts with a deficit counter equal to the quantum. If the packet at the head of a class fits within that counter, it can be dequeued. If it does not fit, DRR adds another quantum and moves to the next active class.

This is why a larger quantum can let a class send more bytes per turn, while a small quantum may require several visits before a larger packet fits. It is not a promise that every class receives an identical byte rate. Packet sizes, offered load, child queue limits and the presence of active classes all affect the result.

The manual specifically warns against attaching a non-work-conserving qdisc such as TBF directly as a DRR child. If the child cannot dequeue, other active-list behaviour can be affected. If you need shaping around a scheduler, place DRR inside a suitable hierarchy such as HTB or HFSC and test the resulting topology.

7. Remove the test configuration safely

# tc qdisc del dev IFACE root

Restore the previous root configuration from the output you recorded in step 2, using its original command or configuration manager. Do not guess a replacement. Verify the final state:

# tc qdisc show dev IFACE
# tc class show dev IFACE
# tc filter show dev IFACE

If the interface is managed by NetworkManager, systemd-networkd or another service, make the persistent configuration change there as well. A manual tc command may disappear at the next link restart.