flowreplay has the simple goal of reading a pcap file, and taking the client side of the connection(s) and replaying that data using a standard TCP/UDP socket to connect to a server. This is because tcpreplay is unable to establish a valid TCP session with a server since it is unable to syncronize Seq/Ack? numbers or maintain any TCP state.
flowreplay is currently an early work in progress and doesn't really work. Right now, I'm looking for a simple method to do IP defragmentation and TCP stream reassembly. In the past, I looked at libnids, but it has a number of serious issues such as being non-thread safe and having a crappy API. Right now, I've put flowreplay development on indefinate hold while I stabilize the 3.x series.