Cobalt queue disc

This chapter describes the COBALT (CoDel BLUE Alternate) ([Cake16]) queue disc implementation in ns-3.

COBALT queue disc is an integral component of CAKE smart queue management system. It is a combination of the CoDel ([Kath17]) and BLUE ([BLUE02]) Active Queue Management algorithms.

Model Description

The source code for the COBALT model is located in the directory src/traffic-control/model and consists of 2 files: cobalt-queue-disc.h and defining a CobaltQueueDisc class and a helper CobaltTimestampTag class. The code was ported to ns-3 by Vignesh Kanan, Harsh Lara, Shefali Gupta, Jendaipou Palmei and Mohit P. Tahiliani based on the Linux kernel code.

Stefano Avallone and Pasquale Imputato helped in verifying the correctness of COBALT model in ns-3 by comparing the results obtained from it to those obtained from the Linux model of COBALT. A detailed comparison of ns-3 model of COBALT with Linux model of COBALT is provided in ([Cobalt19]).

  • class CobaltQueueDisc: This class implements the main Cobalt algorithm:
  • CobaltQueueDisc::DoEnqueue (): This routine tags a packet with the current time before pushing it into the queue. The timestamp tag is used by CobaltQueue::DoDequeue() to compute the packet’s sojourn time. If the queue is full upon the packet arrival, this routine will drop the packet and record the number of drops due to queue overflow, which is stored in m_stats.qLimDrop.
  • CobaltQueueDisc::ShouldDrop (): This routine is CobaltQueueDisc::DoDequeue()’s helper routine that determines whether a packet should be dropped or not based on its sojourn time. If L4S mode is enabled then if the packet is ECT1 is checked and if delay is greater than CE threshold then the packet is marked and returns false. If the sojourn time goes above m_target and remains above continuously for at least m_interval, the routine returns true indicating that it is OK to drop the packet. Otherwise, it returns ``false. If L4S mode is turned off and CE threshold marking is enabled, then if the delay is greater than CE threshold, packet is marked. This routine decides if a packet should be dropped based on the dropping state of CoDel and drop probability of BLUE. The idea is to have both algorithms running in parallel and their effectiveness is decided by their respective parameters (Pdrop of BLUE and dropping state of CoDel). If either of them decide to drop the packet, the packet is dropped.
  • CobaltQueueDisc::DoDequeue (): This routine performs the actual packet drop based on ``CobaltQueueDisc::ShouldDrop ()’s return value and schedules the next drop. Cobalt will decrease BLUE’s drop probability if the queue is empty. This will ensure that the queue does not underflow. Otherwise Cobalt will take the next packet from the queue and calculate its drop state by running CoDel and BLUE in parallel till there are none left to drop.


[Cake16]Linux implementation of Cobalt as a part of the cake framework. Available online at
[Kath17]Controlled Delay Active Queue Management (draft-ietf-aqm-fq-codel-07) Available online at
[BLUE02]Feng, W. C., Shin, K. G., Kandlur, D. D., & Saha, D. (2002). The BLUE Active Queue Management Algorithms. IEEE/ACM Transactions on Networking (ToN), 10(4), 513-528.
[Cobalt19]Jendaipou Palmei, Shefali Gupta, Pasquale Imputato, Jonathan Morton, Mohit P. Tahiliani, Stefano Avallone and Dave Taht (2019). Design and Evaluation of COBALT Queue Discipline. IEEE International Symposium on Local and Metropolitan Area Networks (LANMAN), July 2019.


The key attributes that the CobaltQueue Disc class holds include the following:

  • MaxSize: The maximum number of packets/bytes accepted by this queue disc.
  • Interval: The sliding-minimum window. The default value is 100 ms.
  • Target: The Cobalt algorithm target queue delay. The default value is 5 ms.
  • Pdrop: Value of drop probability.
  • Increment: Increment value of drop probability. Default value is 1./256 .
  • Decrement: Decrement value of drop probability. Default value is 1./4096 .
  • CeThreshold: The CoDel CE threshold for marking packets.
  • UseL4s: True to use L4S (only ECT1 packets are marked at CE threshold).
  • Count: Cobalt count.
  • DropState: Dropping state of Cobalt. Default value is false.
  • Sojourn: Per packet time spent in the queue.
  • DropNext: Time until next packet drop.


An example program named is located in src/traffic-control/examples. Use the following command to run the program.

$ ./waf --run cobalt-vs-codel


The COBALT model is tested using CobaltQueueDiscTestSuite class defined in src/traffic-control/test/ The suite includes 2 test cases:

  • Test 1: Simple enqueue/dequeue with no drops.
  • Test 2: Change of BLUE’s drop probability upon queue full (Activation of Blue).
  • Test 3: This test verfies ECN marking.
  • Test 4: CE threshold marking test.

The test suite can be run using the following commands:

$ ./waf configure --enable-examples --enable-tests
$ ./waf build
$ ./ -s cobalt-queue-disc


$ NS_LOG="CobaltQueueDisc" ./waf --run "test-runner --suite=cobalt-queue-disc"