Transport Layer
TCP Handshake Timeline
Start in CCNA mode for the 3-way handshake (SYN, SYN/ACK, ACK). Switch to Expert mode for PSH data segments, FIN teardown, and full sequence/ACK math.
TCP Handshake Timeline
3-way handshake — SYN, SYN/ACK, and ACK only.
Step 1 of 3
Step 1: SYN
Client sends SYN
- Direction
- Client → Server
- Flags
- SYN
- Src port
- 49152
- Dst port
- 80
- Seq
- 1000
- Ack
- —
- Payload
- 0
The client starts the TCP connection with SYN. SYN consumes one sequence number, so the server should acknowledge 1001.
Next expected
Client ISN → next seq: 1001
Server ISN → next seq: 5000
Server should send ACK 1001
Seq/Ack rule box
- SYN consumes 1 sequence number.
- FIN consumes 1 sequence number.
- ACK by itself does not consume a sequence number.
TCP header explainer
Click a field for a CCNA-focused note.
Sequence Number: The number assigned to the first byte of data in this TCP segment. SYN and FIN each consume one sequence number even when no data is carried.