Watch TCP open a connection. Three segments cross the wire in order, and each side's state machine advances as they land. Numbers are concrete (client ISN x=1000, server ISN y=3000) so you can see ack=x+1 and ack=y+1 derived. Press Replay, or walk it with Prev / Next.
Client
CLOSED
Server
LISTEN
SYNseq=1000
SYN-ACKseq=3000ack=1001
ACKack=3001
CONNECTION ESTABLISHED
open in both directions
client ISN (x)
1000
server ISN (y)
3000
ack = x + 1
1001
ack = y + 1
3001
step 0 / 3
The OS kernel's TCP stack runs this handshake, not the browser's JavaScript. The seq/ack values are illustrative; real ISNs are randomized (RFC 6528). The SYN also carries the TCP options (MSS, window scale, SACK, timestamps) that form the OS-level TCP fingerprint.