A method and apparatus tracks packets and reliably transmits data over a computer transmission system with a reduced amount of memory needed in the transmission interface. The invention eliminates the need to keep all the packets of data queued until the acknowledge message for that data has arrived. Instead, it keeps track of how many packets have been sent and which packets correspond to the end of a message, marking them off when acknowledge messages are received. If packets are lost, the invention determines which packet were lost, backs up to the lost packet, re-fetches the data and resends it. Embodiments of the present invention eliminate the need to queue packets, reducing silicon size and allowing the available bandwidth of the output bus to be more fully utilized.