The low level interface simply transmits and receives trits. It is very simple, a few DFF and XOR gates.
The higher level can be much more complex, or still simple, depending on the performance, requirements and platform.
The first thing to care about is the timeout and activity of the link. A slow timer (1Hz) sends a ACK trit (when no activity is detected) and if another trit is detected/received before sending another one (within 1s) then the link is deemed active. The higher levels of protocol can initiate transmission.
The protocol transmits unbounded strings of 0s and 1s, and the ACK trit is used for framing : the strings are thus cut and distinguished from others, thus making frames or packets. There is no upper bound for the frame size but let's say 4096 bits (2^12) or 512 bytes is reasonable for now.
A packet's minimal size is 1 bit. Give it the meaning you want.
So from a higher perspective, the protocol sends and receives strings of bits of variable lengths. The length in itself is useful to simplify protocols, in-band or out-of-band signaling, etc.
For example, in a hypothetical protocol, data packets would be identified by a length of 256 bits, control packets would have any other length, and the length can filter which type of command is sent, with the data itself being the arguments. Very short packets are possible for IRQ, command acknowledge...