sockets · advanced

Sockets: TCP Handshake, Teardown & TIME_WAIT

TCP establishes a connection with a three-way handshake: the client sends SYN with an initial sequence number x, the server replies SYN+ACK (its own sequence y, acknowledging x+1), and the client sends ACK (acknowledging y+1), after which both sides have synchronized sequence numbers in each direction and the connection is ESTABLISHED — three packets being the minimum needed to synchronize both ways (connect() returns on the client, accept() on the server). Because TCP is full-duplex, teardown is independent per direction and takes four segments: the side calling close() sends FIN, the peer ACKs it (closing that half while still able to send), then the peer sends its own FIN which is ACKed; shutdown(fd, SHUT_WR) can send a FIN while continuing to read, a legal half-close. The side that closes actively (sends the last ACK) then enters TIME_WAIT for 2*MSL (roughly 30-120 seconds) so that delayed or duplicate packets expire and the peer's final FIN can be re-acknowledged. This is why immediately restarting a server often fails to bind with EADDRINUSE ('Address already in use') — its port is still in TIME_WAIT — and the fix is to set SO_REUSEADDR before bind(); a large number of TIME_WAIT sockets generally indicates that this side actively closes many short-lived connections.

The code

// 3-way handshake (connection setup)
client --- SYN(seq=x) ------------> server     // I want to talk
client <-- SYN(seq=y) ACK(x+1) ---- server     // ok, and you?
client --- ACK(y+1) -------------> server      // established

// 4-way teardown (each side closes its half independently)
FIN -> ; <- ACK ; <- FIN ; ACK ->              // then TIME_WAIT

What this lesson walks through

1. 01The 3-way handshake — CLOSED → ESTABLISHED
2. 024-way teardown — each half closes independently
3. 03TIME_WAIT — the active closer waits 2·MSL
4. 04Gotcha — port exhaustion & SO_REUSEADDR

Connection setup takes three packets. The client sends SYN (seq x); the server replies SYN+ACK; the client ACKs. Three messages and both sides agree on sequence numbers — now ESTABLISHED.