TCP Timers

Chia-tai Tsaitai@cis.nctu.edu.tw

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.

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)

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.

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

Implements of the 7 TCP timers

tcp_canceltimers Function

tcp_timer.c Triggered when enters TIME_WAIT

state. All 4 timers counters are set to 0.

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.

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

tcp_slowtime Function cont. Count idle time

Keepalive FIN_WAIT_2

Increment RTT counter Increment RFC 1323 timestamp value

tcp_now

tcp_timers Function

tcp_timer.c Called by TCP’s PRU_SLOWTIMO

request

tcp_timers Function cont.

Case TCPT_2MSL(FIN_WAIT_2 and 2MSL Timers) 2MSL Timers

tcp_timers Function cont. FIN_WAIT_2 timer

tcp_timers Function cont.

Case TCPT_PERSIST (Persist Timer) Force window probe segment Time line of persist time

tcp_timers Function cont.

Case TCPT_KEEP (Connection Establishment and Keepalive Timers) Connection establishment timer and retr

ansmission timer after SYN is sent

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

Retransmission Timer Calculations

var4

varvarvar

rttsrttRTO

rttdeltarttrtt

deltagsrttsrtt

srttntickdelta

Retransmission Timer Calculations

member

Units Initial value

#sec

Description

t_srtt ticks x 8

0 Smoothed RTT estimator : srtt x 8

t_rttvar ticks x 4

24 3 Smoothed mean deviation estimator : rttvar x 4

t_rxtcur ticks 12 6 Current retransmission timeout: RTO

t_rttmin ticks 2 1 Min. value for retransmissions timeout

t_rxtshift

null 0 Index into tcp_backoff[] array (exponential backoff)

tcp_setpersist Function

tcp_output.c Check retransmission timer not

enabled Calculate RTO

2

var_4_

4

var_2

8

_

var2

rtttsrttt

rtttsrttt

rttsrttRTO

tcp_setpersist Function cont. Exponential backoff array

{1, 2, 4, 8, 16, 32, 64, 64, 64, 64, 64, 64, 64}

tcp_xmit_timer Function

tcp_input.c rtt is the RTT measurement to be appli

ed Timestamp option (tcp_now) RTT counter (t_rtt)

tcp_xmit_timer Function cont. Update smoothed estimators and RT

O

var_8

_

var4

4

var_var_var_

varvarvar

__

88

rtttsrttt

RTO

rttsrttRTO

rtttdeltartttrttt

rttdeltahrttrtt

deltasrtttsrttt

deltasrttsrtt

deltagsrttsrtt

Retransmission Timeout(tcp_timers Function cont.)

tcp_timer.c

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 unacknow

ledged data