Comment at the top of ns-3.38/examples/tcp/tcp-pacing.cc

// The following network topology is used in this example, and is taken from
// Figure 2 of https://homes.cs.washington.edu/~tom/pubs/pacing.pdf
//
//    n0                          n4
//    |                           |
//    |(4x Mbps, 5ms)             |(4x Mbps, 5ms)
//    |                           |
//    |                           |
//    |      (x Mbps, 40ms)       |
//    n2 ------------------------ n3
//    |                           |
//    |                           |
//    |(4x Mbps, 5ms)             |(4x Mbps, 5ms)
//    |                           |
//    n1                          n5
//
//

// This example illustrates how TCP pacing can be enabled on a socket.
// Two long-running TCP flows are instantiated at nodes n0 and n1 to
// send data over a bottleneck link (n2->n3) to sink nodes n4 and n5.
// At the end of the simulation, the IP-level flow monitor tool will
// print out summary statistics of the flows.  The flow monitor detects
// four flows, but that is because the flow records are unidirectional;
// the latter two flows reported are actually ack streams.
//
// At the end of this simulation, data files are also generated
// that track changes in Congestion Window, Slow Start threshold and
// TCP pacing rate for the first flow (n0). Additionally, a data file
// that contains information about packet transmission and reception times
// (collected through TxTrace and RxTrace respectively) is also produced.
// This transmission and reception (ack) trace is the most direct way to
// observe the effects of pacing.  All the above information is traced
// just for the single node n0.
//
// A small amount of randomness is introduced to the program to control
// the start time of the flows.
//
// This example has pacing enabled by default, which means that TCP
// does not send packets back-to-back, but instead paces them out over
// an RTT. The size of initial congestion window is set to 10, and pacing
// of the initial window is enabled. The available command-line options and
// their default values can be observed in the usual way by running the
// program to print the help info; i.e.: ./ns3 run 'tcp-pacing --PrintHelp'
//
// When pacing is disabled, TCP sends eligible packets back-to-back. The
// differences in behaviour when pacing is disabled can be observed from the
// packet transmission data file. For instance, one can observe that
// packets in the initial window are sent one after the other simultaneously,
// without any inter-packet gaps. Another instance is when n0 receives a
// packet in the form of an acknowledgement, and sends out data packets without
// pacing them.
//
// Although this example serves as a useful demonstration of how pacing could
// be enabled/disabled in ns-3 TCP congestion controls, we could not observe
// significant improvements in throughput for the above topology when pacing
// was enabled. In future, one could try and incorporate models such as
// TCP Prague and ACK-filtering, which may show a stronger performance
// impact for TCP pacing.