(a) serial ports are not reliable, characters can get dropped and bits corrupted so you need a protocol that will handle error detection and retries

(b) this has all been done before, either choose an implementation that already works and move onto things that have not been done, or choose an existing protocol and implement that, hence you will be able to work with other machines already using those protocols.

(c) Have a look at UUCP, XMODEM, Kermit

(d) Have a look at SLIP, PPP