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https://networklessons.com/cisco/ccie-routing-switching-written/tcp-header


FLAGSthere are 9 bits for flags, we also call them control bits. We use them to establish connections,
send data and terminate connections:

URG

urgent pointer. When this bit is set, the data should be treated as priority over other data

ACK

used for the acknowledgment.
PSH

this is the push function. This tells an application that the data should be transmitted
immediately and that we don’t want to wait to fill the entire TCP segment.

RST

this resets the connection, when you receive this you have to terminate the connection right away. T
his is only used when there are unrecoverable errors and it’s not a normal way to finish the TCP connection.

SYN

we use this for the initial three way handshake and it’s used to set the initial sequence number.

FINthis finish bit is used to end the TCP connection. TCP is full duplex so both parties will have to
use the FIN bit to end the connection. This is the normal method how we end an connection.