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TCP Timers

Chia-tai Tsai

tai@cis.nctu.edu.tw

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Introduction The 7 Timers for each Connection

Connection-Establishment Timer

Establish a new connection. If a response isnt 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.

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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)

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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.

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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

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Implements of the 7 TCP timers

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tcp_canceltimers Function tcp_timer.c

Triggered when enters TIME_WAITstate.

All 4 timers counters are set to 0.

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tcp_fasttime Function tcp_timer.c

Delay ACK Timer

If TF_DELACK flag is set, it is clearedand TF_ACKNOW flag is set instead.

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tcp_slowtime Function tcp_timer.c

Handle the other 6 Timers Connection establishmentretransmission

persistkeepaliveFIN_WAIT_22MSL

Check each timer counter in all TCPCB

Check if TCPCB has been deleted

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tcp_slowtime Function cont. Count idle time

Keepalive

FIN_WAIT_2

Increment RTT counter

Increment RFC 1323 timestamp value

tcp_now

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tcp_timers Function tcp_timer.c

Called by TCPs PRU_SLOWTIMOrequest

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tcp_timers Function cont. Case TCPT_2MSL

(FIN_WAIT_2 and 2MSL Timers)

2MSL Timers

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tcp_timers Function cont. FIN_WAIT_2 timer

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tcp_timers Function cont. Case TCPT_PERSIST (Persist Timer)

Force window probe segment

Time line of persist time

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tcp_timers Function cont. Case TCPT_KEEP (Connection

Establishment and Keepalive Timers)

Connection establishment timer andretransmission timer after SYN is sent

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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

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Retransmission Timer Calculations

var4

varvarvar

rttsrttRTO

rttdeltarttrtt

deltagsrttsrtt

srttntickdelta

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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 deviationestimator : rttvar x 4

t_rxtcur ticks 12 6 Current retransmissiontimeout: RTO

t_rttmin ticks 2 1Min. value for

retransmissions timeout

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

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tcp_setpersist Function

tcp_output.c

Check retransmission timer not enabled

Calculate RTO

2

var_4

_

4

var_

28

_

var2

rtttsrttt

rtttsrttt

rttsrttRTO

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tcp_setpersist Function cont.

Exponential backoff array {1, 2, 4, 8, 16, 32, 64, 64, 64, 64, 64, 64, 64}

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tcp_xmit_timer Function

tcp_input.c

rtt is the RTT measurement to be applied

Timestamp option (tcp_now)

RTT counter (t_rtt)

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tcp_xmit_timer Function cont.

Update smoothed estimators and RTO

var_8

_

var4

4

var_var_var_

varvarvar

__

88

rttt

srttt

RTO

rttsrttRTO

rtttdeltartttrttt

rttdeltahrttrtt

deltasrtttsrttt

deltasrttsrtt

deltagsrttsrtt

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Retransmission Timeout(tcp_timers Function cont.)

tcp_timer.c

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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 oldestunacknowledged data