Tcpreplay1 was designed to replay traffic previously captured in the pcap format back onto the wire for testing NIDS and other passive devices. Over time, it was enhanced to be able to test in-line network devices. However, a re-occurring feature request for tcpreplay is to connect to a server in order to test applications and host TCP/IP stacks. It was determined early on, that adding this feature to tcpreplay was far too complex, so I decided to create a new tool specifically designed for this.
Flowreplay is designed to replay traffic at Layer 4 or 7 depending on the protocol rather then at Layer 2 like tcpreplay does. This allows flowreplay to connect to one or more servers using a pcap savefile as the basis of the connections. Hence, flowreplay allows the testing of applications running on real servers rather then passive devices.