TCP Handshake

How two machines open, use, and close a reliable connection. Watch the SYN / SYN-ACK / ACKthree-way handshake, optional data transfer, and the four-way teardown - with each endpoint's state machine and real sequence numbers updating in step.

ClientCLOSEDNo connection.

ServerLISTENWaiting for an incoming connection (passive open).

Client

Server

SYNseq=1000 · len=1

SYN ACKseq=5000 · ack=1001 · len=1

ACKseq=1001 · ack=5001

PSH ACKseq=1001 · ack=5001 · len=24

ACKseq=5001 · ack=1025

FIN ACKseq=1025 · ack=5001 · len=1

ACKseq=5001 · ack=1026

FIN ACKseq=5001 · ack=1026 · len=1

ACKseq=1026 · ack=5002

0 / 9 segments

Speed - NormalData transfer phase

SYNClient → ServerHandshake

seq=1000len=1

Client picks a random ISN (1000) and sends SYN: "let's talk - my byte stream starts here." It moves to SYN_SENT.

HandshakeDataTeardownSYN & FIN each consume one sequence number - that's why acks land on seq+1.