Bit-Twist is a simple yet powerful libpcap-based Ethernet packet generator.
It is designed to complement tcpdump, which by itself has done a great job
at capturing network traffic.
With Bit-Twist, you can now regenerate your captured traffic onto a live
network!
Packets are generated from tcpdump trace file (.pcap file).
Bit-Twist also comes with a comprehensive trace file editor to allow you to
change the contents of a trace file.

Generally, packet generator is useful in simulating networking traffic or
scenario, testing firewall, IDS, and IPS, and troubleshooting various network
problems.