Computing Performance Metrics

NetSolver computes the time evolution of the following instantaneous metrics of a network:

Average buffer occupancy in packets at time t for each link direction ( AvgSize(t) )

Utilization at time t for each link direction ( Util(t) )

Loss probability for each link direction ( LossProb(t) )

Achieved throughput for a traffic class ( Throughput(t) ) To do this, NetSolver models the TCP/IP network by a time-dependent queueing network and numerically solves the queueing differential equations using an approximation technique called the Z-iteration. A TCP source is modeled by a time-dependent stochastic source that puts traffic into the network at a current rate determined by its profile and the current network state. A UDP source is modeled by a Poisson source.

A TCP profile is a function that expresses the instantaneous throughput of a TCP source versus the instantaneous encountered roundtrip time and the instantaneous loss rate. Different TCP implementations have different profiles, which translates to significant differences in performance. In Net Solver 1.0, the user may choose between the TCP Reno implementations in NetBSD 1.2 and in the ns simulator.

To Run the Evaluator

Use the Compute Metrics command from the Computation menu. The program will ask about the parameters of computation and than start the evaluation step. This may take some time, so the status bar indicates the current step of the computation. Once the computation is finished, the user may create plots of the computed metrics, including comparison against the results of ns simulation.

Note:

The TCP implementations in ns have their own unique profiles. NetSolver 1.0 includes the profile of TCP Reno in NSVERSION, and the user should choose this when comparing against ns simulations because small changes in profile cause significant changes in performance.

NetSolver 1.0 does not capture TCP "phase" effects because it distributes the loss probability at a router queue equally among the connections that traverse that queue.

Computation Parameters to be supplied by the user

Evaluation time - The time interval for which the user wants to compute the metrics of the network .

Evaluation step - The evaluation time interval is divided into small steps. This parameter specifies the size (in seconds) of a step. The smaller the value, the more accurate the results but the longer the computation time. Conversely, for larger steps the metrics display some fluctuations but the computation is proportionally faster. Usually a value of 0.05 is optimal.

Dump interval - How often the evaluator dumps the computed metrics to a dump file. Smaller values will decrease the speed and increase the size of the dump file.

Dump file - The name of the file where to dump the metrics. This usually has the extension .dmp.

TCP profile - This determines how TCP connections are modeled. NetSolver 1.0 offers two choices: ns profile and NetBSD 1.2 profile. If you plan to compare against ns we advise you to use the ns profile, because other profiles yield different results.

	The TCP implementations in ns have their own unique profiles. NetSolver 1.0 includes the profile of TCP Reno in NSVERSION, and the user should choose this when comparing against ns simulations because small changes in profile cause significant changes in performance.
	NetSolver 1.0 does not capture TCP "phase" effects because it distributes the loss probability at a router queue equally among the connections that traverse that queue.

	Evaluation time - The time interval for which the user wants to compute the metrics of the network .
	Evaluation step - The evaluation time interval is divided into small steps. This parameter specifies the size (in seconds) of a step. The smaller the value, the more accurate the results but the longer the computation time. Conversely, for larger steps the metrics display some fluctuations but the computation is proportionally faster. Usually a value of 0.05 is optimal.
	Dump interval - How often the evaluator dumps the computed metrics to a dump file. Smaller values will decrease the speed and increase the size of the dump file.
	Dump file - The name of the file where to dump the metrics. This usually has the extension .dmp.
	TCP profile - This determines how TCP connections are modeled. NetSolver 1.0 offers two choices: ns profile and NetBSD 1.2 profile. If you plan to compare against ns we advise you to use the ns profile, because other profiles yield different results.