Queues

This section documents a few queue objects, typically associated with NetDevice models, that are maintained as part of the network module:

• DropTail
• Random Early Detection

Model Description

The source code for the new module lives in the directory src/network/utils.

ns-3 provides a couple of classic queue models and the ability to trace certain queue operations such as enqueuing, dequeuing, and dropping. These may be added to certain NetDevice objects that take a Ptr<Queue> pointer.

Note that not all device models use these queue models. In particular, WiFi, WiMax, and LTE use specialized device queues. The queue models described here are more often used with simpler ns-3 device models such as PointToPoint and Csma.

Design

An abstract base class, class Queue, is typically used and subclassed for specific scheduling and drop policies. Common operations include:

• bool Enqueue (Ptr<Packet> p): Enqueue a packet
• Ptr<Packet> Dequeue (void): Dequeue a packet
• uint32_t GetNPackets (void): Get the queue depth, in packets
• uint32_t GetNBytes (void): Get the queue depth, in packets

as well as tracking some statistics on queue operations.

There are three trace sources that may be hooked:

• Enqueue
• Dequeue
• Drop

DropTail

This is a basic first-in-first-out (FIFO) queue that performs a tail drop when the queue is full.

Random Early Detection

Random Early Detection (RED) is a queue variant that aims to provide early signals to transport protocol congestion control (e.g. TCP) that congestion is imminent, so that they back off their rate gracefully rather than with a bunch of tail-drop losses (possibly incurring TCP timeout). The model in ns-3 is a port of Sally Floyd’s ns-2 RED model.

Scope and Limitations

The RED model just supports default RED. Adaptive RED is not supported.

References

The RED queue aims to be close to the results cited in: S.Floyd, K.Fall http://icir.org/floyd/papers/redsims.ps

Usage

Helpers

A typical usage pattern is to create a device helper and to configure the queue type and attributes from the helper, such as this example from src/network/examples/red-tests.cc:

PointToPointHelper p2p;

p2p.SetQueue ("ns3::DropTailQueue");
p2p.SetDeviceAttribute ("DataRate", StringValue ("10Mbps"));
p2p.SetChannelAttribute ("Delay", StringValue ("2ms"));
NetDeviceContainer devn0n2 = p2p.Install (n0n2);

p2p.SetQueue ("ns3::DropTailQueue");
p2p.SetDeviceAttribute ("DataRate", StringValue ("10Mbps"));
p2p.SetChannelAttribute ("Delay", StringValue ("3ms"));
NetDeviceContainer devn1n2 = p2p.Install (n1n2);

p2p.SetQueue ("ns3::RedQueue", // only backbone link has RED queue
              "LinkBandwidth", StringValue (redLinkDataRate),
              "LinkDelay", StringValue (redLinkDelay));
p2p.SetDeviceAttribute ("DataRate", StringValue (redLinkDataRate));
p2p.SetChannelAttribute ("Delay", StringValue (redLinkDelay));
NetDeviceContainer devn2n3 = p2p.Install (n2n3);

Attributes

The RED queue contains a number of attributes that control the RED policies:

• Mode (bytes or packets)
• MeanPktSize
• IdlePktSize
• Wait (time)
• Gentle mode
• MinTh, MaxTh
• QueueLimit
• Queue weight
• LInterm
• LinkBandwidth
• LinkDelay

Consult the ns-3 documentation for explanation of these attributes.

Output

The ns-3 ascii trace helpers used by many of the NetDevices will hook the Enqueue, Dequeue, and Drop traces of these queues and print out trace statements, such as the following from examples/udp/udp-echo.cc:

+ 2 /NodeList/0/DeviceList/1/$ns3::CsmaNetDevice/TxQueue/Enqueue ns3::EthernetHeader
( length/type=0x806, source=00:00:00:00:00:01, destination=ff:ff:ff:ff:ff:ff)
ns3::ArpHeader (request source mac: 00-06-00:00:00:00:00:01 source ipv4: 10.1.1.1
dest ipv4: 10.1.1.2) Payload (size=18) ns3::EthernetTrailer (fcs=0)
- 2 /NodeList/0/DeviceList/1/$ns3::CsmaNetDevice/TxQueue/Dequeue ns3::EthernetHeader
( length/type=0x806, source=00:00:00:00:00:01, destination=ff:ff:ff:ff:ff:ff)
ns3::ArpHeader (request source mac: 00-06-00:00:00:00:00:01 source ipv4: 10.1.1.1
dest ipv4: 10.1.1.2) Payload (size=18) ns3::EthernetTrailer (fcs=0)

which shows an enqueue “+” and dequeue “-” event at time 2 seconds.

Users are, of course, free to define and hook their own trace sinks to these trace sources.

Examples

The drop-tail queue is used in several examples, such as examples/udp/udp-echo.cc. The RED queue example is found at src/network/examples/red-tests.cc.

Validation

The RED model has been validated and the report is currently stored at: https://github.com/downloads/talau/ns-3-tcp-red/report-red-ns3.pdf