the dcc standard specifies that the sender sends one packet of data, and waits for the receiver to ACK it, before sending the second packet. that's rather superfluous over a tcp connection. as it happens, there are no irc clients that can tell the difference between a reall fast connection and a sender that sends before receiving ACKs. epic takes advantage of this by sending multiple packets at a time (a "window"), and then re-filling the window every time it receives an ACK. this keeps more packets in flight, and increases throughput, while not being too violating of the standards. the default value is (iirc) 1, which is the proper value to be standards-compliant. you can set it higher, if you feel like you want to. a value of 2 greatly increases the throughput, and 3-5 give a little bit more, and you probably shouldn't expect to see much improvement beyond 5.