The diagram shown below demonstrates the fixed packet structure that is use by TCP/ This fixed packet structure is used to provide flow control, maintain the reliability of the communication and ensure that any data that is missing is resent buy sequencing and acknowledgement.
The Flag field is a 1 byte field. Common flags in this field include
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These flags are used to manage the TCP sessions. SYN and ACK are used in the three way Handshakes . RST and FIN flags are used to denote the finish of a connection. FIN is used during the normal four step setup, but the RST is used to signal the end of a session that is not a normal one.
The checksum is used to ensure the integrity of the packet is intact
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To allow two hosts to communicate and exchange data reliably, TCP performs a three step handshake before any of the data is sent. This is when TCP guarantees that the data will arrive in tact by using a sequence and acknowledgement numbers. After all data is sent, TCP then performs a four step shutdown that concludes the session.
Three Step Startup
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PC |
—————- SYN —————-> |
Server |
| <———— SYN ACK————– | ||
| —————- ACK —————-> |
Four Step Shutdown
|
PC |
—————FIN ACK ————-> |
Server |
| <—————ACK—————— | ||
| <———— FIN ACK————— | ||
| —————- ACK —————-> |
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