DummyNet

Dummynet is a FreeBSD system facility that simulates network delay and loss by manipulating the IP queue inside the kernel. Dummynet makes use
of " ipfw" which supports various types of IP filters for system network interfaces. The general idea is to create a "pipe" object and configure
it with the desired delay and loss parameters. The pipe gets added to a list of filtering "rules" which are used by dummynet to modify the
treatment of IP packets in desired ways.

Dummy net simulates the effect of 

• finite queues

• bandwidth limitations

• communications delays

• bounded size queues

• packet losses

• multipath

Pipes are fixed-bandwidth channels. Queues represent queues of packets, associated with a weight, which share the bandwidth of the pipe they are connected to proportionally to their weight. These rules are numbered from 1 to 65535 and they are processed in the order that they are listed.

A Simple Tutorial on Dummy net

A FreeBsd implementation of dummynet is available from the author at Dummynet home.

References:

Dummynet: A simple approach to the evaluation of network protocols.