Hi Chris, Thanks for the reply, yep that's what I was thinking. :) I haven't gotten to duplicate retransmissions yet (though was thinking about it, maybe we store checksums and check against multiple matches? Maybe that's naive...). Timing makes sense, I can see how we: 1. start timer whenever sending 2. stop timer whenever receiving 3. resend & restart timer when negative acknowledgement / junk received 4. resend & restart timer whenever timer runs out 5. success = acknowledgement received (timer already stopped, tell network layer to proceed)