Positive acknowledgments

Positive acknowledgments

10.1.6 Acknowledgment 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. This field is where tracking of transmitted and received bytes are indicated.

One problem with the IP protocol is that there is no verification method to determine if data segments reach their destination. So data segments may be constantly forwarded with no knowledge as to whether or not they were actually received. TCP uses positive acknowledgment and retransmission (PAR) to control data flow and confirm data delivery.

Many protocols use PAR to provide reliability. With PAR, the source sends a packet, starts a timer, and waits for an acknowledgment before it sends the next packet in the session. If the timer expires before the source receives an acknowledgment, the source retransmits the packet and resets the timer. The acknowledgment is provided by the value of Acknowledgment Number and the ACK flag set in the TCP header. TCP uses expectational acknowledgment in which the Acknowledgment Number value refers to the next octet that is expected as part of the TCP session.

Windowing is a flow control mechanism that requires the source device to receive an acknowledgment from the destination after a specific amount of data bytes has been transmitted. With a window size of three, the source device can send three octets to the destination. It must then wait for an acknowledgment of these bytes. If the destination receives the three octets, it sends an acknowledgment to the source device, which can then transmit three more octets. If the destination does not receive the three octets, it does not send an acknowledgment. This may be caused by overflowing buffers or packets lost in transit. Since the source does not receive an acknowledgment, it knows that the octets should be retransmitted and that the window size should be reduced. This window size reduction provides the receiving host less bytes to process from its buffers before more data arrives. This effectively slows the communication between hosts to provide more reliability between the hosts.

The Lab Activity will teach students how to enable and monitor multiple host sessions. The Interactive Media Activity will help students become more familiar with windows.

The next page will explain how UDP works.