Implement TCP Connection Teardown (FIN/ACK Sequence)

Topic Overview

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.

Given a close-segment trace, run the active closer's TCP teardown state machine (RFC 9293). The FIN consumes one sequence number, TIME_WAIT lasts 2*MSL, and RST aborts without any graceful exchange.

Input (stdin): one line: msl initial_seq N (msl in seconds). Then N segment events: FIN, ACK, FIN+ACK, RST. The first FIN (while ESTABLISHED) is ours and consumes one sequence number; later FIN segments are the peer's.

Output (stdout):

• one line per event: EVENT <E> -> <state> when it causes a transition; EVENT FIN seq=<n> -> <state> for FIN events (each consumed FIN advances the sequence number); EVENT <E> IGNORED state=<state> for events that do not apply in the current state; EVENT <E> ERROR state=CLOSED for events arriving after the connection closed
• TIME_WAIT <2*msl> once, if the trace ends with the machine sitting in TIME_WAIT
• final_state=<state>

Transitions: ESTABLISHED + FIN -> FIN_WAIT_1; FIN_WAIT_1 + ACK -> FIN_WAIT_2; FIN_WAIT_1 + FIN -> CLOSING (simultaneous close); FIN_WAIT_1 + FIN+ACK -> TIME_WAIT; FIN_WAIT_2 + FIN -> TIME_WAIT; CLOSING + ACK -> TIME_WAIT; RST -> CLOSED from any state (abort, no TIME_WAIT).