| 10.1 |  | TCP Operation |
| 10.1.6 |  | Positive ACK |

| Acknowledgement is a common step in the synchronization process which includes sliding windows and data sequencing. In a TCP segment, the sequence number field is followed by the acknowledgment number field, also referred to as the code field. This field is where acknowledgments, or ACKs, (as well as SYN) are indicated. One problem with the unreliable IP protocol is that there is no verification method for determining that data segments actually reached their destination. So data segments may be constantly forwarded with no knowledge as to whether or not they were actually received. TCP utilizes positive acknowledgment and retransmission to control data flow and confirm data delivery. Positive acknowledgment and retransmission (PAR) is a common technique many protocols use to provide reliability. With PAR, the source sends a packet, starts a timer, and waits for an acknowledgment before sending the next packet. If the timer expires before the source receives an acknowledgment, the source retransmits the packet and starts the timer over again. TCP uses expectational acknowledgments in which the acknowledgment number refers to the next octet that is expected. Windowing is a flow control mechanism requiring that the source device receive an acknowledgment from the destination after transmitting a certain amount of data. With a window size of three, the source device can send three octets to the destination. It must then wait for an acknowledgment. If the destination receives the three octets, it sends an acknowledgment to the source device, which can now transmit three more octets. If, for some reason, the destination does not receive the three octets, possibly due to overflowing buffers, it does not send an acknowledgment. Because the source does not receive an acknowledgment, it knows that the octets should be retransmitted and that the transmission rate should be slowed.
| Lab Activity Lab Exercise: Multiple Active Host Sessions This lab will show port usage on a single host attached to a router.
| Interactive Media Activity Interactivity: TCP Sliding Windows After completing this activity, the student will be able to understand window size.
| Web Links TCP - Positive Acknowledgment and Re-transmission http://www.it.iitb.ac.in/~jaju/ tutorials/net/tcpip/ node22.html