In that case, "ACK" is just conceptual* (and the "concept" is pretty self-evident methinks -- you have pretty much explained it yourself). As Zuk points out, you could use anything as the actual signal. When I do client server stuff, I like to use single byte signals (255 is enough...), usually capital letters. So "A" could mean "acknowledged".
Originally Posted by daghenningsorbo
On the other hand, since an ACK will not have any further content, you could make it more elaborate to prevent confusion:
* the literal "ACK" is from http & the SYN-ACK "handshake" used by web servers.
#define ACK "<!Recieved!>"