tcp timers chia-tai tsai [email protected]. introduction the 7 timers for each connection...
TRANSCRIPT
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