I'm enjoying VarAC - the functionality is very good and it works well with reasonably strong signals. But the underlying VARA protocol seems to get in the way as the signals drop. I'm an old-time PSK31 and RTTY user - zero error detection and correction. I could manage a QSO under some pretty terrible conditions - it's surprising how much corruption to the data stream you can accept and still work out what the guy at the other end means. I'm finding that many of my VarAC QSOs fail because the signal drops down into the noise floor and VARA goes into a sequence of repeated NACs and retransmissions. If I look at the monitor window, I can see that most of the text is getting through but it's failing the data integrity checks. I would settle for some garbled text rather than a dropped connection!
Does the client application - VarAC in this case - have any control over the low level protocol? Are you able to do anything to accept the errors and let the QSO continue?
the VARA protocol is ARQ, so if you are not interested in data integrity you'd better use any of the FEC modes. The ARQ takes places in the transport / datalink layers, so it's part of the protocol, you can't just turn it off.... It would be interesting to have the capability to read the packets at these layers in the monitor window, and see what's going on in the communication between the modems. It is somehow possible, as many years ago we were doing just that for work on other ARQ protocols, but I guess it would require more work and maybe only few would be interested.
Perhaps the mode for you is Olivia. She works with very weak signals and has FEC for perfect print down to a very weak signal.