A Discrete-Event Network Simulator

FqCoDel queue disc

This chapter describes the FqCoDel ([Hoe16]) queue disc implementation in ns-3.

The FlowQueue-CoDel (FQ-CoDel) algorithm is a combined packet scheduler and Active Queue Management (AQM) algorithm developed as part of the bufferbloat-fighting community effort ([Buf16]). FqCoDel classifies incoming packets into different queues (by default, 1024 queues are created), which are served according to a modified Deficit Round Robin (DRR) queue scheduler. Each queue is managed by the CoDel AQM algorithm. FqCoDel distinguishes between “new” queues (which don’t build up a standing queue) and “old” queues, that have queued enough data to be around for more than one iteration of the round-robin scheduler.

Model Description

The source code for the FqCoDel queue disc is located in the directory src/traffic-control/model and consists of 2 files fq-codel-queue-disc.h and fq-codel-queue-disc.cc defining a FqCoDelQueueDisc class and a helper FqCoDelFlow class. The code was ported to ns-3 based on Linux kernel code implemented by Eric Dumazet.

  • class FqCoDelQueueDisc: This class implements the main FqCoDel algorithm:
    • FqCoDelQueueDisc::DoEnqueue (): This routine uses the configured packet filters to classify the given packet into an appropriate queue. If the filters are unable to classify the packet, the packet is dropped. Otherwise, it is handed over to the CoDel algorithm for timestamping. Then, if the queue is not currently active (i.e., if it is not in either the list of new or the list of old queues), it is added to the end of the list of new queues, and its deficit is initiated to the configured quantum. Otherwise, the queue is left in its current queue list. Finally, the total number of enqueued packets is compared with the configured limit, and if it is above this value (which can happen since a packet was just enqueued), packets are dropped from the head of the queue with the largest current byte count until the number of dropped packets reaches the configured drop batch size or the backlog of the queue has been halved. Note that this in most cases means that the packet that was just enqueued is not among the packets that get dropped, which may even be from a different queue.
    • FqCoDelQueueDisc::DoDequeue (): The first task performed by this routine is selecting a queue from which to dequeue a packet. To this end, the scheduler first looks at the list of new queues; for the queue at the head of that list, if that queue has a negative deficit (i.e., it has already dequeued at least a quantum of bytes), it is given an additional amount of deficit, the queue is put onto the end of the list of old queues, and the routine selects the next queue and starts again. Otherwise, that queue is selected for dequeue. If the list of new queues is empty, the scheduler proceeds down the list of old queues in the same fashion (checking the deficit, and either selecting the queue for dequeuing, or increasing deficit and putting the queue back at the end of the list). After having selected a queue from which to dequeue a packet, the CoDel algorithm is invoked on that queue. As a result of this, one or more packets may be discarded from the head of the selected queue, before the packet that should be dequeued is returned (or nothing is returned if the queue is or becomes empty while being handled by the CoDel algorithm). Finally, if the CoDel algorithm does not return a packet, then the queue must be empty, and the scheduler does one of two things: if the queue selected for dequeue came from the list of new queues, it is moved to the end of the list of old queues. If instead it came from the list of old queues, that queue is removed from the list, to be added back (as a new queue) the next time a packet for that queue arrives. Then (since no packet was available for dequeue), the whole dequeue process is restarted from the beginning. If, instead, the scheduler did get a packet back from the CoDel algorithm, it subtracts the size of the packet from the byte deficit for the selected queue and returns the packet as the result of the dequeue operation.
    • FqCoDelQueueDisc::FqCoDelDrop (): This routine is invoked by FqCoDelQueueDisc::DoEnqueue() to drop packets from the head of the queue with the largest current byte count. This routine keeps dropping packets until the number of dropped packets reaches the configured drop batch size or the backlog of the queue has been halved.
  • class FqCoDelFlow: This class implements a flow queue, by keeping its current status (whether it is in the list of new queues, in the list of old queues or inactive) and its current deficit.

In Linux, by default, packet classification is done by hashing (using a Jenkins hash function) on the 5-tuple of IP protocol, and source and destination IP addresses and port numbers (if they exist), and taking the hash value modulo the number of queues. The hash is salted by modulo addition of a random value selected at initialisation time, to prevent possible DoS attacks if the hash is predictable ahead of time. Alternatively, any other packet filter can be configured. In ns-3, at least one packet filter must be added to an FqCoDel queue disc. The Linux default classifier is provided via the FqCoDelIpv{4,6}PacketFilter classes. Finally, neither internal queues nor classes can be configured for an FqCoDel queue disc.


  1. Hoeiland-Joergensen, P. McKenney, D. Taht, J. Gettys and E. Dumazet, The FlowQueue-CoDel Packet Scheduler and Active Queue Management Algorithm, IETF draft. Available online at https://tools.ietf.org/html/draft-ietf-aqm-fq-codel
[Buf16]Bufferbloat.net. Available online at http://www.bufferbloat.net/.


The key attributes that the FqCoDelQueue class holds include the following:

  • Interval: The interval parameter to be used on the CoDel queues. The default value is 100 ms.
  • Target: The target parameter to be used on the CoDel queues. The default value is 5 ms.
  • MaxSize: The limit on the maximum number of packets stored by FqCoDel.
  • Flows: The number of flow queues managed by FqCoDel.
  • DropBatchSize: The maximum number of packets dropped from the fat flow.

Note that the quantum, i.e., the number of bytes each queue gets to dequeue on each round of the scheduling algorithm, is set by default to the MTU size of the device (at initialisation time). The FqCoDelQueueDisc::SetQuantum () method can be used (at any time) to configure a different value.


A typical usage pattern is to create a traffic control helper and to configure type and attributes of queue disc and filters from the helper. For example, FqCodel can be configured as follows:

TrafficControlHelper tch;
uint16_t handle = tch.SetRootQueueDisc ("ns3::FqCoDelQueueDisc", "DropBatchSize", UintegerValue (1));
tch.AddPacketFilter (handle, "ns3::FqCoDelIpv4PacketFilter", "Perturbation", UintegerValue (256));
tch.AddPacketFilter (handle, "ns3::FqCoDelIpv6PacketFilter");
QueueDiscContainer qdiscs = tch.Install (devices);


The FqCoDel model is tested using FqCoDelQueueDiscTestSuite class defined in src/test/ns3tc/codel-queue-test-suite.cc. The suite includes 5 test cases:

  • Test 1: The first test checks that packets that cannot be classified by any available filter are dropped.
  • Test 2: The second test checks that IPv4 packets having distinct destination addresses are enqueued into different flow queues. Also, it checks that packets are dropped from the fat flow in case the queue disc capacity is exceeded.
  • Test 3: The third test checks the dequeue operation and the deficit round robin-based scheduler.
  • Test 4: The fourth test checks that TCP packets with distinct port numbers are enqueued into different flow queues.
  • Test 5: The fifth test checks that UDP packets with distinct port numbers are enqueued into different flow queues.

The test suite can be run using the following commands:

$ ./waf configure --enable-examples --enable-tests
$ ./waf build
$ ./test.py -s fq-codel-queue-disc


$ NS_LOG="FqCoDelQueueDisc" ./waf --run "test-runner --suite=fq-codel-queue-disc"

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