D-Link DES-3250TG User Manual page 63

DES-3250TG Layer 2 Fast Ethernet Switch User's Guide
Figure 5- 8. TCP Packet Header
When packets have been successfully received, TCP sends an
acknowledgement. This is simply a packet that has the
acknowledgement number field filled in.
An acknowledgement number of 1000 indicates that all of the
data up to octet 1000 has been received. If the transmitting
TCP does not receive an acknowledgement in a reasonable
amount of time, the data is resent.
The window field controls the amount of data being sent at any
one time. It would require too much time and overhead to
acknowledge each packet received. Each end of the TCP
connection declares how much data it is able to receive at any
one time by writing this number of octets in the window field.
The transmitting TCP decrements the number in the window
field and when it reaches zero, the transmitting TCP stops
sending data. When the receiving TCP can accept more data, it
increases the number in the window field. In practice, a single
packet can acknowledge the receipt of data and give permission
for more data to be sent.
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