The protocol was a simple stop-and-wait protocol with explicit acknowledgment of packets. Packets had a start marker, a sequence number, a 16-bit CRC, the data, and a stop marker. If the start or stop marker occurred within the data, it had to be escaped. All good packets received in the proper order were acknowledged. Any packet out of sequence cause a NAK for the missing packets that came before it. Any packet with a bad CRC also got a NAK. If a NAK was received, or an ACK did not occur within a reasonable amount of time, the packet was retransmitted.
They later moved to a sliding window version of the same protocol to cut down on the number of ACKs that were sent. This didn't change the packet format, but it did cut back on retransmissions.
They later moved to a sliding window version of the same protocol to cut down on the number of ACKs that were sent. This didn't change the packet format, but it did cut back on retransmissions.