Tcpreplay is aimed at testing the performance of a Network Intrusion Detection
System by replaying real background network traffic in which to hide attacks.

Tcpreplay allows you to control the speed at which the traffic is replayed,
and can replay arbitrary libpcap traces.

Unlike programmatically-generated artificial traffic which doesn't exercise
the application/protocol inspection that a NIDS performs, and doesn't
reproduce the real-world anomalies that appear on production networks
(asymmetric routes, traffic bursts/lulls, fragmentation, retransmissions,
etc.), tcpreplay allows for exact replication of real traffic seen on real
networks. 

Tcpprep is tcpreplay with patches can send traffic out two network cards,
however it requires the calculations be done in real-time.  These calculations
can be expensive and can significantly reduce the throughput of tcpreplay.

tcpprep is a libpcap pre-processor for tcpreplay which enables this 
feature in tcpreplay, and significantly increases throughput through 
the use of a cache file.

WWW: http://synfin.net/aturner/tcpreplay/