TCP Timers

1. TCP Timers Chia-tai Tsai tai@cis.nctu.edu.tw

2. Introduction • The 7 Timers for each Connection • Connection-Establishment Timer • Establish a new connection. • If a response isn’t received within 75 seconds, the connection establishment is aborted. • Retransmission Timer • Based on Round-Trip Time (RTT). • TCP sends data. • When this timer expires, TCP retransmit the data.

3. Introduction cont. • Delay ACK Timer • Need not be acknowledged immediately. • TCP waits up to 200 ms. • Piggybacking • Persist Timer • Based on RTT • Triggered when a null window is advertised • If timer expires, sender issues a probe (between 5 and 60 s, never gives up)

4. Introduction cont. • Keepalive Timer • socket option SO_KEEPALIVE. • Tests if the other end is still up and running • Expires after 2 hours • FIN_WAIT_2 Timer • FIN_WAIT_1 -> FIN_WAIT_2 • Avoid a connection in the FIN_WAIT_2 state forever.

5. Introduction cont. • 2MSL Timer (TIME_WAIT Timer) • MSL (Maximum Segment Life) 30s to 4 min • Triggered when connection (client) enters TIME_WAIT state • During 2MSL, the socket pair cannot be reused. (packets arriving during this state are discarded) • Timer Function • Fast timer • Triggered every 200 ms • Delay ACK Timer • Slow timer • Triggered every 500 ms

6. Implements of the 7 TCP timers

7. tcp_canceltimers Function • tcp_timer.c • Triggered when enters TIME_WAIT state. • All 4 timers counters are set to 0.

8. tcp_fasttime Function • tcp_timer.c • Delay ACK Timer • If TF_DELACK flag is set, it is cleared and TF_ACKNOW flag is set instead.

9. tcp_slowtime Function • tcp_timer.c • Handle the other 6 Timers • Connection establishment、retransmission、persist、keepalive、FIN_WAIT_2、2MSL • Check each timer counter in all TCPCB • Check if TCPCB has been deleted

10. tcp_slowtime Function cont. • Count idle time • Keepalive • FIN_WAIT_2 • Increment RTT counter • Increment RFC 1323 timestamp value • tcp_now

11. tcp_timers Function • tcp_timer.c • Called by TCP’s PRU_SLOWTIMO request

12. tcp_timers Function cont. • Case TCPT_2MSL(FIN_WAIT_2 and 2MSL Timers) • 2MSL Timers

13. tcp_timers Function cont. • FIN_WAIT_2 timer

14. tcp_timers Function cont. • Case TCPT_PERSIST (Persist Timer) • Force window probe segment • Time line of persist time

15. tcp_timers Function cont. • Case TCPT_KEEP (Connection Establishment and Keepalive Timers) • Connection establishment timer and retransmission timer after SYN is sent

16. tcp_timers Function cont. • Keepalive timer • Expired after 2 hours of idle time • Drop connection who no response • Send a keepalive probe • Reset keepalive timer

17. Retransmission Timer Calculations

18. Retransmission Timer Calculations

19. tcp_setpersist Function • tcp_output.c • Check retransmission timer not enabled • Calculate RTO

20. tcp_setpersist Function cont. • Exponential backoff array • {1, 2, 4, 8, 16, 32, 64, 64, 64, 64, 64, 64, 64}

21. tcp_xmit_timer Function • tcp_input.c • rtt is the RTT measurement to be applied • Timestamp option (tcp_now) • RTT counter (t_rtt)

22. tcp_xmit_timer Function cont. • Update smoothed estimators and RTO

23. Retransmission Timeout(tcp_timers Function cont.) • tcp_timer.c

24. Retransmission Timeout cont. • Source introduction • Increment shift count (t_rxtshift) • Calculate new RTO • Ask IP to find a new route • Clear estimators • Force retransmission of oldest unacknowledged data