e02e86faa7
This move prepares RED to become a queue disc. Building of RED model, tests and examples is disabled until the conversion is done.
118 lines
3.9 KiB
ReStructuredText
118 lines
3.9 KiB
ReStructuredText
Queues
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------
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.. heading hierarchy:
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------------- Chapter
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************* Section (#.#)
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============= Subsection (#.#.#)
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############# Paragraph (no number)
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This section documents the queue object, typically associated with
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NetDevice models, that is maintained as part of the ``network`` module:
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* DropTail
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Model Description
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*****************
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The source code for the new module lives in the directory ``src/network/utils``.
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ns-3 provides a couple of classic queue models and the ability to
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trace certain queue operations such as enqueuing, dequeuing, and dropping.
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These may be added to certain NetDevice objects that take a Ptr<Queue>
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pointer.
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Note that not all device models use these queue models.
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In particular, WiFi, WiMax, and LTE use specialized device queues.
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The queue models described here are more often used with simpler ns-3
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device models such as PointToPoint and Csma.
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Design
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======
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An abstract base class, class Queue, is typically used and subclassed
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for specific scheduling and drop policies. A class QueueItem is introduced
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to model the items stored in a queue. The base class QueueItem only contains
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a pointer to a packet. Subclasses may be defined to store additional information.
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Common operations provided by the base class Queue include:
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* ``bool Enqueue (Ptr<QueueItem> item)``: Enqueue a packet
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* ``Ptr<QueueItem> Dequeue (void)``: Dequeue a packet
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* ``uint32_t GetNPackets (void)``: Get the queue depth, in packets
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* ``uint32_t GetNBytes (void)``: Get the queue depth, in packets
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as well as tracking some statistics on queue operations.
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There are three trace sources that may be hooked:
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* ``Enqueue``
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* ``Dequeue``
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* ``Drop``
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DropTail
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########
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This is a basic first-in-first-out (FIFO) queue that performs a tail drop
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when the queue is full.
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Usage
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*****
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Helpers
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=======
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A typical usage pattern is to create a device helper and to configure
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the queue type and attributes from the helper, such as this example:
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.. sourcecode:: cpp
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PointToPointHelper p2p;
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p2p.SetQueue ("ns3::DropTailQueue");
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p2p.SetDeviceAttribute ("DataRate", StringValue ("10Mbps"));
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p2p.SetChannelAttribute ("Delay", StringValue ("2ms"));
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NetDeviceContainer devn0n2 = p2p.Install (n0n2);
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p2p.SetQueue ("ns3::DropTailQueue");
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p2p.SetDeviceAttribute ("DataRate", StringValue ("10Mbps"));
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p2p.SetChannelAttribute ("Delay", StringValue ("3ms"));
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NetDeviceContainer devn1n2 = p2p.Install (n1n2);
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p2p.SetQueue ("ns3::DropTailQueue",
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"LinkBandwidth", StringValue (linkDataRate),
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"LinkDelay", StringValue (linkDelay));
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p2p.SetDeviceAttribute ("DataRate", StringValue (linkDataRate));
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p2p.SetChannelAttribute ("Delay", StringValue (linkDelay));
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NetDeviceContainer devn2n3 = p2p.Install (n2n3);
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Output
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======
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The ns-3 ascii trace helpers used by many of the NetDevices will hook
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the Enqueue, Dequeue, and Drop traces of these queues and print out
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trace statements, such as the following from ``examples/udp/udp-echo.cc``:
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.. sourcecode:: text
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+ 2 /NodeList/0/DeviceList/1/$ns3::CsmaNetDevice/TxQueue/Enqueue ns3::EthernetHeader
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( length/type=0x806, source=00:00:00:00:00:01, destination=ff:ff:ff:ff:ff:ff)
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ns3::ArpHeader (request source mac: 00-06-00:00:00:00:00:01 source ipv4: 10.1.1.1
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dest ipv4: 10.1.1.2) Payload (size=18) ns3::EthernetTrailer (fcs=0)
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- 2 /NodeList/0/DeviceList/1/$ns3::CsmaNetDevice/TxQueue/Dequeue ns3::EthernetHeader
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( length/type=0x806, source=00:00:00:00:00:01, destination=ff:ff:ff:ff:ff:ff)
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ns3::ArpHeader (request source mac: 00-06-00:00:00:00:00:01 source ipv4: 10.1.1.1
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dest ipv4: 10.1.1.2) Payload (size=18) ns3::EthernetTrailer (fcs=0)
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which shows an enqueue "+" and dequeue "-" event at time 2 seconds.
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Users are, of course, free to define and hook their own trace sinks to
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these trace sources.
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Examples
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========
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The drop-tail queue is used in several examples, such as
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``examples/udp/udp-echo.cc``.
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