The first segment is a SYN from the client. It carries the client's initial sequence number and starts the state change toward an open connection.
highlighted = computed this step
Why the client speaks first
The opening SYN declares the client's starting sequence number. It asks the server to acknowledge that starting point before data appears.
seq=1000
The SYN segment
The client sends SYN with sequence number 1000. There is no acknowledgment yet. These starting numbers are illustrative; a real stack picks each initial sequence number randomly, which matters for security.
SYN seq=1000
Why the client state changes
After sending the SYN, the client has left CLOSED and is waiting for the server's reply. The server lane is still listening for a connection request.
ack=∅
Summary
The first segment pins the client's initial sequence number and starts the ordering. Connection setup ordering and exact sequence numbers only; timing, retransmission, and round-trip time are not modeled here.