Implement TCP Connection Teardown (FIN/ACK Sequence)

Four-Way Graceful Close

TCP is full-duplex: each direction closes independently. A normal teardown takes four segments. For example, the active closer on port 8080 sends FIN, the passive side ACKs, then sends its own FIN, and the active side sends the final ACK.

State Transitions

Active closer: ESTABLISHED -> FIN_WAIT_1 -> FIN_WAIT_2 -> TIME_WAIT -> CLOSED

Passive closer: ESTABLISHED -> CLOSE_WAIT -> LAST_ACK -> CLOSED

The FIN flag consumes one sequence number, just like SYN. After the final ACK, the active closer sits in TIME_WAIT for 2 x MSL (typically 60-120 seconds) to catch retransmitted FINs and reject stale segments from old connections.

Abortive Close with RST

RST aborts immediately with no graceful shutdown. Common triggers:

• SYN arrives for a closed port (e.g. port 9999 with no listener)
• Segment arrives for an already-closed connection
• Application crash

Why This Exercise

This task requires you to simulate the FIN/ACK sequence with state transitions. TIME_WAIT accumulation from this close pattern is a well-known production incident for high-throughput servers like nginx, which is why SO_REUSEADDR exists. Operators tune net.ipv4.tcp_tw_reuse and related sysctls because thousands of connections to port 443 can leave the server holding sockets in TIME_WAIT for 60-120 seconds each.