32.5. Prio queue disc

32.5.1. Model Description

PrioQueueDisc implements a strict priority policy, where packets are dequeued from a band only if higher priority bands are all empty. PrioQueueDisc is a classful queue disc and can have an arbitrary number of bands, each of which is handled by a queue disc of any kind. The capacity of PrioQueueDisc is not limited; packets can only be dropped by child queue discs (which may have a limited capacity). If no packet filter is installed or able to classify a packet, then the packet is enqueued into a priority band based on its priority (modulo 16), which is used as an index into an array called priomap. Users can read Use of Send() vs. SendTo() for details on how to set the packet priority. If a packet is classified by an installed packet filter and the returned value i is non-negative and less than the number of priority bands, then the packet is enqueued into the i-th priority band. Otherwise, the packet is enqueued into the priority band specified by the first element of the priomap array.

If no queue disc class is added by the user before the queue disc is initialized, three child queue discs of type FifoQueueDisc are automatically added. It has to be noted that PrioQueueDisc needs at least two child queue discs.

32.5.1.1. Attributes

The PrioQueueDisc class holds the following attribute:

• Priomap: The priority to band mapping. The default value is the same mapping as the (fixed) one used by PfifoFastQueueDisc.

32.5.1.2. Examples

An example of how to configure PrioQueueDisc with custom child queue discs and priomap is provided by queue-discs-benchmark.cc located in examples/traffic-control:

TrafficControlHelper tch;
uint16_t handle = tch.SetRootQueueDisc("ns3::PrioQueueDisc", "Priomap", StringValue("0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1"));
TrafficControlHelper::ClassIdList cid = tch.AddQueueDiscClasses(handle, 2, "ns3::QueueDiscClass");
tch.AddChildQueueDisc(handle, cid[0], "ns3::FifoQueueDisc");
tch.AddChildQueueDisc(handle, cid[1], "ns3::RedQueueDisc");

The code above adds two classes (bands) to a PrioQueueDisc. The highest priority one is a FifoQueueDisc, the other one is a RedQueueDisc. The attribute Priomap is set to an array containing only 0 and 1 (since PrioQueueDisc only has two bands).

32.5.2. Validation

PrioQueueDisc is tested using PrioQueueDiscTestSuite class defined in src/traffic-control/test/prio-queue-disc-test-suite.cc. The test aims to check that: i) packets are enqueued in the correct band based on their priority and the priomap or according to the value returned by the installed packet filter; ii) packets are dequeued in the correct order.

The test suite can be run using the following commands:

$ ./ns3 configure --enable-examples --enable-tests
$ ./ns3 build
$ ./test.py -s prio-queue-disc

or

$ NS_LOG="PrioQueueDisc" ./ns3 run "test-runner --suite=prio-queue-disc"