TCP Handshake Animator

Rendered when the site was built, by the same state machine the browser runs. No JavaScript required. With JavaScript on, this is replaced by the interactive ladder, which reveals one segment at a time.

The 11 states

Every RFC 793 connection state, in the order the machine walks them: CLOSED · LISTEN · SYN-SENT · SYN-RECEIVED · ESTABLISHED · FIN-WAIT-1 · FIN-WAIT-2 · CLOSE-WAIT · CLOSING · LAST-ACK · TIME-WAIT. A connection is only ever in one of them at each end, and the traces below name both ends at every step.

• TCP Three-Way Handshake

  3 segments, 4 steps.

  · (initial) · client=CLOSED server=LISTEN
  step 1/3 · C->S SYN seq=100 ack=0 len=0 · client=SYN-SENT server=LISTEN
  step 2/3 · S->C SYN,ACK seq=500 ack=101 len=0 · client=SYN-SENT server=SYN-RECEIVED
  step 3/3 · C->S ACK seq=101 ack=501 len=0 · client=ESTABLISHED server=ESTABLISHED

  TCP Three-Way Handshake: step by step

  Step	Segment	Client	Server	What happened
  1	none	CLOSED	LISTEN	Client is CLOSED; the server is LISTENing for connections.
  2	SYN	SYN-SENT	LISTEN	Client sends a SYN with its initial sequence number (ISN=100) and enters SYN-SENT.
  3	SYN,ACK	SYN-SENT	SYN-RECEIVED	Server acknowledges the SYN (ack=101) and sends its own SYN (ISN=500), entering SYN-RECEIVED.
  4	ACK	ESTABLISHED	ESTABLISHED	Client acknowledges the server's SYN (ack=501); the handshake is complete and both ends are ESTABLISHED.

• Graceful Connection Teardown

  4 segments, 6 steps.

  · (initial) · client=ESTABLISHED server=ESTABLISHED
  step 1/4 · C->S FIN,ACK seq=200 ack=800 len=0 · client=FIN-WAIT-1 server=ESTABLISHED
  step 2/4 · S->C ACK seq=800 ack=201 len=0 · client=FIN-WAIT-2 server=CLOSE-WAIT
  step 3/4 · S->C FIN,ACK seq=800 ack=201 len=0 · client=FIN-WAIT-2 server=LAST-ACK
  step 4/4 · C->S ACK seq=201 ack=801 len=0 · client=TIME-WAIT server=CLOSED
  · (initial) · client=CLOSED server=CLOSED

  Graceful Connection Teardown: step by step

  Step	Segment	Client	Server	What happened
  1	none	ESTABLISHED	ESTABLISHED	The connection is open; both ends are ESTABLISHED.
  2	FIN,ACK	FIN-WAIT-1	ESTABLISHED	Client begins an active close, sending FIN and entering FIN-WAIT-1.
  3	ACK	FIN-WAIT-2	CLOSE-WAIT	Server acknowledges the FIN (ack=201): the client enters FIN-WAIT-2 and the server enters CLOSE-WAIT.
  4	FIN,ACK	FIN-WAIT-2	LAST-ACK	Server finishes sending and closes, emitting its own FIN and entering LAST-ACK.
  5	ACK	TIME-WAIT	CLOSED	Client acknowledges the server's FIN (ack=801): the server reaches CLOSED and the client enters TIME-WAIT.
  6	none	CLOSED	CLOSED	TIME-WAIT lasts twice the maximum segment lifetime (2·MSL), long enough for any late packets to arrive and be discarded before the client reaches CLOSED.

• Simultaneous Close (Crossing FINs)

  4 segments, 6 steps.

  · (initial) · client=ESTABLISHED server=ESTABLISHED
  step 1/4 · C->S FIN,ACK seq=200 ack=800 len=0 · client=FIN-WAIT-1 server=ESTABLISHED
  step 2/4 · S->C FIN,ACK seq=800 ack=200 len=0 · client=FIN-WAIT-1 server=FIN-WAIT-1
  step 3/4 · C->S ACK seq=201 ack=801 len=0 · client=CLOSING server=CLOSING
  step 4/4 · S->C ACK seq=801 ack=201 len=0 · client=TIME-WAIT server=TIME-WAIT
  · (initial) · client=CLOSED server=CLOSED

  Simultaneous Close (Crossing FINs): step by step

  Step	Segment	Client	Server	What happened
  1	none	ESTABLISHED	ESTABLISHED	The connection is open; both ends are ESTABLISHED.
  2	FIN,ACK	FIN-WAIT-1	ESTABLISHED	Client closes, sending FIN and entering FIN-WAIT-1, but this FIN is still in flight.
  3	FIN,ACK	FIN-WAIT-1	FIN-WAIT-1	Server closes at the same moment, before the client's FIN arrives (ack=200 proves it has not seen it). The two FINs cross on the wire and both ends sit in FIN-WAIT-1.
  4	ACK	CLOSING	CLOSING	The crossed FINs land: each end receives its peer's FIN while still awaiting an ACK for its own, so both enter CLOSING, the simultaneous-close state. The client acknowledges the server's FIN (ack=801).
  5	ACK	TIME-WAIT	TIME-WAIT	The server acknowledges the client's FIN (ack=201). As the two ACKs land, both ends move from CLOSING to TIME-WAIT. In a simultaneous close, neither side can skip TIME-WAIT.
  6	none	CLOSED	CLOSED	Both ends closed actively, so both hold TIME-WAIT for 2·MSL while stray segments drain, then reach CLOSED.

• Connection Reset (RST)

  1 segment, 2 steps.

  · (initial) · client=ESTABLISHED server=ESTABLISHED
  step 1/1 · C->S RST seq=200 ack=0 len=0 · client=CLOSED server=CLOSED

  Connection Reset (RST): step by step

  Step	Segment	Client	Server	What happened
  1	none	ESTABLISHED	ESTABLISHED	The connection is open; both ends are ESTABLISHED.
  2	RST	CLOSED	CLOSED	Client sends RST, tearing the connection down immediately: no handshake, both ends drop to CLOSED.