Read setup, acknowledgements and closing separately

TCP means Transmission Control Protocol. It provides an ordered,
reliable byte stream; application success still needs application
evidence. SYN synchronizes starting sequence numbers. ACK marks an
acknowledgement, the next sequence position expected from the peer.

Fresh paper calculation, separate from the diagram and later checks:
Client SYN seq=1200; server SYN-ACK seq=6400, ack=1201.
Client ACK acknowledges 6401. Each SYN consumes one sequence position
without carrying application bytes. Each side acknowledges the peer.

A FIN stops its sender from sending more data. The other direction can
remain open: a half-close. One FIN alone does not finish both directions.
UDP is a different transport. UDP itself has no setup handshake or
promises of delivery, ordering or duplicate protection. Applications
can add behavior above UDP, but those are not UDP guarantees.

Procedure cards for the TCP setup shape. Not a packet capture.

Worked order, peers alfa then beta
1 SYN      alfa -> beta
2 SYN-ACK  beta -> alfa
3 ACK      alfa -> beta
These three messages set up the connection. Application data is a later fact.
A reply from the application is not guaranteed by the third message.

Activity cards, peers shelf and desk, labels shuffled
M1  ACK
M2  SYN
M3  SYN-ACK
Predict the setup order before reading the check. Check: M2, then M3, then M1.

Close card, separate from setup
FIN means the sender has no more data. It belongs to closing, not to the third setup step.

UDP card
One datagram from desk to shelf.
No SYN, no SYN-ACK, and no ACK.
UDP does not guarantee delivery or duplicate protection for that datagram.