TCP Handshake

SYN, SYN-ACK, ACK with real sequence numbers — then the data, the FIN teardown, and the TIME_WAIT that protects the next connection.

Open: 1.5 RTT

Close: 4 segments

SYN and FIN each consume 1 seq

ScenarioISN 4200 / 9100

Animation Speed: 1100ms

Step 1 of 21

client

SND.NXT—

RCV.NXT—

server

SND.NXT—

RCV.NXT—

SND.NXT is the next sequence number this side will use; RCV.NXT is the next one it expects — and what it writes in every ack field.

Two machines, no connection yet. Both endpoints start in CLOSED. The client will use initial sequence number (ISN) 4200 and the server 9100 — real stacks pick these pseudo-randomly so a stale segment from an old connection, or an attacker, cannot guess where the byte stream starts.

Both State Machines

client (active)

CLOSED

SYN_SENT

ESTABLISHED

FIN_WAIT_1

FIN_WAIT_2

TIME_WAIT

CLOSED

server (passive)

CLOSED

LISTEN

SYN_RECEIVED

ESTABLISHED

CLOSE_WAIT

LAST_ACK

CLOSED

The side that closes first walks the left-hand path and pays for TIME_WAIT; the side that closes second walks the right-hand one.

Knowledge Check

Question 1 of 3

A client sends SYN with seq=4200. What acknowledgement number does the server put in its SYN-ACK?

DSAverse

TCP Handshake

Both State Machines

Knowledge Check

See also

TCP Handshake

Both State Machines

Knowledge Check

See also