Hi Chris, I used a different approach than you did to solve the issue of working NACKs into the stop & wait protocol, and I'm wondering if it's equally valid (it certainly works, though might not scale in context of later content?). Instead of testing if you're an outstanding (unacknowledged) sender by checking if 'nextdatatosend != ackexpected' I'm testing if 'lasttimer != NULLTIMER', and updating 'lasttimer = NULLTIMER' when I stop it after receiving a valid acknowledgement. I wanted to get your thoughts on testing against the timers for this, it seems like an equivalent solution, though to see if it's a bad approach for any reason (like upcoming protocol features / considerations). Let me know your thoughts, Henry