D-Link DES-3326SR Manual page 205

D-Link DES-3326SR Layer 3 Switch
TCP must know how large a packet the network can process. To do this, the TCP protocols at each end of a connection state
how large a packet they can handle and the smaller of the two is selected.
The TCP header contains at least 20 octets. The source and destination TCP port numbers are the most important fields.
These specify the connection between two TCP protocols on two network devices.
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.
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.
IP
TCP sends its packets to IP with the source and destination IP addresses. IP is only concerned with these IP addresses. It is
not concerned with the contents of the packet or the TCP header.
IP finds a route for the packet to get to the other end of the TCP connection. IP adds its own header to the packet to
accomplish this.
The IP header contains the source and destination addresses, the protocol number, and another checksum.
The protocol number tells the receiving IP which protocol to give the packet to. Although most IP traffic uses TCP, other
protocols can be used (such as UDP).
The checksum is used by the receiving IP in the same way as the TCP checksum.
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