Networking
Explainer
How TCP Starts a Reliable Connection
Before sending one byte of data, TCP exchanges three messages. Why?
Quick check · tap an answer to continue
Lesson notes
Key points
- TCP is a connection-oriented transport protocol, so both ends must agree on starting sequence numbers first.
- The client sends SYN, the server replies SYN-ACK, and the client confirms with ACK.
- Each ACK number equals the other side's sequence number plus one, proving the SYN was received.
- Two messages are not enough, because the server would never know its own sequence number arrived.
Definitions
- Sequence number
- A counter labelling each byte sent, letting the receiver order data and detect losses.
Takeaway: SYN, SYN-ACK, ACK: both sides confirm sequence numbers.
Quick check: Client SYN has seq 100. What ack does the server send?
- 100
- 200
- 101
Connection-orientedThree-way handshakeACK numberWhy three
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