1.3 Seeding and independent replications

ns-3 simulations can be configured to produce deterministic or random results. If the ns-3 simulation is configured to use a fixed, deterministic seed with the same run number, it should give the same output each time it is run.

By default, ns-3 simulations use a fixed seed and run number. These values are stored in two ns3::GlobalValue instances: g_rngSeed and g_rngRun.

A typical use case is to run a simulation as a sequence of independent trials, so as to compute statistics on a large number of independent runs. The user can either change the global seed and rerun the simulation, or can advance the substream state of the RNG, which is referred to as incrementing the run number.

A class ns3::SeedManager () provides an API to control the seeding and run number behavior. This seeding and substream state setting must be called before any random variables are created; e.g.

  SeedManager::SetSeed (3);  // Changes seed from default of 1 to 3
  SeedManager::SetRun (7);  // Changes run number from default of 1 to 7
  // Now, create random variables
  UniformVariable x(0,10);
  ExponentialVariable y(2902);
  ...

Which is better, setting a new seed or advancing the substream state? There is no guarantee that the streams produced by two random seeds will not overlap. The only way to guarantee that two streams do not overlap is to use the substream capability provided by the RNG implementation. Therefore, use the substream capability to produce multiple independent runs of the same simulation. In other words, the more statistically rigorous way to configure multiple independent replications is to use a fixed seed and to advance the run number. This implementation allows for a maximum of 2.3x10^15 independent replications using the substreams.

For ease of use, it is not necessary to control the seed and run number from within the program; the user can set the NS_GLOBAL_VALUE environment variable as follows:

  NS_GLOBAL_VALUE="RngRun=3" ./waf --run program-name

Another way to control this is by passing a command-line argument; since this is an ns3 GlobalValue instance, it is equivalently done such as follows:

  ./waf --command-template="%s --RngRun=3" --run program-name

or, if you are running programs directly outside of waf:

  ./build/optimized/scratch/program-name --RngRun=3

The above command-line variants make it easy to run lots of different runs from a shell script by just passing a different RngRun index.

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