The way TCP establishes a connection is through the use of different TCP control flags used in a very specific order.

Source:

TCP 3-Way Handshake Process - GeeksforGeeks

TCP control flags

• URG (short for urgent): A value of one here indicates that the segment is considered urgent and that the urgent pointer field has more data about this
  • This feature of TCP has never really had widespread adoption and isn’t normally seen
• ACK (short for acknowledged)
  • A value of one in this field means that the acknowledgment number should be examined
• PSH (short for push)
  • The transmitting device wants the receiving device to push currently-buffered data to the applications on the receiving end as soon as possible
  • It’s normally more meaningful to buffer data and send them in chunks as it’s reduces duplicate data / more reliant?
  • but immediate responses are sometimes needed with small amount of data where the push flag comes in
• RST (short for reset)
  • One of the sides in a TCP connection hasn’t been able to properly recover form a series of missing or malformed segments
  • Can’t put together the segments, start sending from scratch again
• SYN (short for synchronize)
  • It’s used when first establishing a TCP connection and makes sure the receiving end knows to examine the sequence number field
• FIN (short for finish)
  • When this flag is set to one, it means the transmitting computer doesn’t have any more data to send and the connection can be closed

Handshake

A way for two devices to ensure that they’re speaking the same protocol and will be able to understand each other

After establishing a connection Computer A is free to send whatever data it wants to Computer B and vice versa.

Each segment sent in either direction should be responded to by a TCP segment with the ACK field sent. This way the other side always knows what has been received.

In order to close this connection we use the four-way handshake.