D-Link DGS-3308FG Product Manual - Page 50

TCP Packet Header

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8-port Gigabit Ethernet Switch User's Guide The TCP header is added to the beginning of each packet. This header contains at least 20 octets including the source and destination TCP port numbers. Each packet is given a sequence number that is used to ensure that the packets are received in the correct order. The packets themselves are not numbered, instead, the octets the packet contains is numbered. So if there are 100 octets of data in each packet, the first packet would be numbered 0, the second 100, the third 200, and so on. A checksum is calculated by adding up all the octets in the packet and the result is put in the header. The receiving TCP calculates its own checksum and if the two checksums differ, the packet is dropped. Figure 5-11. TCP Packet Header An acknowledgement is sent to indicate that the packets have been received. This is simply a packet with its acknowledgement number field filled in. The acknowledgement number is the number of octets of data received at the time the acknowledgement packet is sent. If the sender does not receive an acknowledgement within a reasonable amount of time, the data is resent. The window field controls the amount of data in transit at any one time. Each end of a connection indicates how much data it is currently able to receive by putting that number of octets in the window field. As the computer receives data, the number in the window field is decremented and when it reaches zero, the sender must stops transmitting. As the recipient processes data, it increases its window, indicating that it is ready to receive more data. IP TCP sends packets to IP, along with source and destination IP addresses. IP is not concerned with the contents of the packets or with the TCP header. IP routes the packet from the source to the destination. IP adds its own header to the packet to allow intermediate gateways or other network devices to forward the packet. The header contains the source and destination IP addresses, a protocol number, and a checksum. The protocol number allows IP to pass the packet to the appropriate protocol (usually TCP) at the receiving end. The checksum is calculated in same way as the TCP checksum and allows IP to verify that the data was not corrupted in transit. 40

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8-port Gigabit Ethernet Switch User’s Guide
40
The TCP header is added to the beginning of each packet. This header contains at least 20 octets including the source and
destination TCP port numbers.
Each packet is given a sequence number that is used to ensure that the packets are received in the correct order. The
packets themselves are not numbered, instead, the octets the packet contains is numbered.
So if there are 100 octets of
data in each packet, the first packet would be numbered 0, the second 100, the third 200, and so on.
A checksum is calculated by adding up all the octets in the packet and the result is put in the header. The receiving TCP
calculates its own checksum and if the two checksums differ, the packet is dropped.
Figure 5-11.
TCP Packet Header
An acknowledgement is sent to indicate that the packets have been received. This is simply a packet with its
acknowledgement number field filled in. The acknowledgement number is the number of octets of data received at the time
the acknowledgement packet is sent.
If the sender does not receive an acknowledgement within a reasonable amount of time, the data is resent.
The window field controls the amount of data in transit at any one time. Each end of
a connection indicates how much data
it is currently able to receive by putting that number of octets in the window field.
As the computer receives data, the number in the window field is decremented and when it reaches zero, the sender must
stops transmitting.
As the recipient processes data, it increases its window, indicating that it is ready to receive more
data.
IP
TCP sends packets to IP, along with source and destination IP addresses. IP is not concerned with the contents of the
packets or with the TCP header.
IP routes the packet from the source to the destination. IP adds its own header to the packet to allow intermediate
gateways or other network devices to forward the packet.
The header contains the source and destination IP addresses, a protocol number, and a checksum.
The protocol number allows IP to pass the packet to the appropriate protocol (usually TCP) at the receiving end.
The checksum is calculated in same way as the TCP checksum and allows IP to verify that the data was not corrupted in
transit.