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DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
98 Switch Management and Operating Concepts
The header also contains a sequence number that is used to
ensure the packets are received in the correct order. The
packets are not numbered, but rather the octets the packets
contain are. If there are 100 octets of data in each packet, the
first packet is numbered 0, the second 100, the third 200, etc.
To insure that the data in a packet is received uncorrupted,
TCP adds the binary value of all the octets in the packet and
writes the sum in the checksum field. The receiving TCP
recalculates the checksum and if the numbers are different, the
packet is dropped.
Figure 5-10. 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.