The Multi-Generator (MGEN) is open source software by the Naval Research
Laboratory (NRL) PROTocol Engineering Advanced Networking (PROTEAN)
Research Group.  MGEN provides the ability to perform IP network
performance tests and measurements using UDP/IP traffic (TCP is currently
being developed).
The toolset generates real-time traffic patterns so that the network can
be loaded in a variety of ways.  The generated traffic can also be
received and logged for analyses.  Script files are used to drive the
generated loading patterns over the course of time. These script files can
be used to emulate the traffic patterns of unicast and/or multicast UDP/IP
applications.  The receive portion of this tool set can be scripted to
dynamically join and leave IP multicast groups.  MGEN log data can be used
to calculate performance statistics on throughput, packet loss rates,
communication delay, and more.

WWW: http://mgen.pf.itd.nrl.navy.mil/