What protocols function is simply to inform the transmitter of what piece of data is expected next?

The protocol that primarily serves to inform the transmitter of the expected next piece of data is known as the acknowledgment (ACK) protocol, often used in communication systems like TCP (Transmission Control Protocol). In this context, the receiver sends back an acknowledgment message indicating which data packet has been successfully received, thereby guiding the transmitter on what to send next. This mechanism helps ensure reliable data transfer and flow control in network communications.