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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.
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Lab Activity
Lab Exercise: Multiple Active Host Sessions
This lab will show port usage on a
single host attached to a router.
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Interactive Media Activity
Interactivity: TCP Sliding Windows
After completing this activity, the student will be able to understand window
size.
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Web Links