rtt matters

RTTmatters

Matsuzaki ‘maz’Yoshinobu<maz@iij.ad.jp>

InternetInitiativeJapanInc.(IIJ/AS2497) 12015/11/10

RoundTripTime(RTT)

RTT

t0

t1

InternetInitiativeJapanInc.(IIJ/AS2497) 22015/11/10

TCP3wayhandshakeandRTT

RTTRTT

InternetInitiativeJapanInc.(IIJ/AS2497) 32015/11/10

TCPandRTT

RTT

windowsize=TCPRXbuffer

RTT

RTT

InternetInitiativeJapanInc.(IIJ/AS2497) 42015/11/10

packetlossandRTT

RTO

RTOiscomputedbasedonRTT-- seerfc6298

retransmissiontimeout

InternetInitiativeJapanInc.(IIJ/AS2497) 52015/11/10

RTT

• thelower,thebetter– fasterTCPhandshake– betterTCPthroughput– fasterTCPrecoveryfromapacketloss

• TherearevariouseffortstoimproveTCPperformanceeveninahighlatencysituation

InternetInitiativeJapanInc.(IIJ/AS2497) 62015/11/10

Wi-Fiisgettingpopular

wirelesswired

Internet

server2

server1

InternetInitiativeJapanInc.(IIJ/AS2497) 72015/11/10

Ifacedatrouble

wirelesswired• RTTwasnotstablefromawirelessclienttotheserver2.L

64bytestime=398.122ms64bytestime=316.330ms64bytestime=337.251ms64bytestime=358.155ms64bytestime=299.480ms64bytestime=387.789ms

server2

server1

InternetInitiativeJapanInc.(IIJ/AS2497) 82015/11/10

let’strytoisolatetheproblem

wireless

64bytestime=169.588ms64bytestime=170.666ms64bytestime=169.793ms64bytestime=168.185ms64bytestime=169.783ms64bytestime=169.957ms

server164bytestime=3.757ms64bytestime=3.789ms64bytestime=3.768ms64bytestime=3.718ms64bytestime=3.644ms64bytestime=3.762ms

1) thelocalwifi networklooksprettystable

2) RTTtoserver1looksalsostable

server2

InternetInitiativeJapanInc.(IIJ/AS2497) 92015/11/10

hmmm...

wirelesswired

server2

server1

InternetInitiativeJapanInc.(IIJ/AS2497) 10

64bytestime=296.040ms64bytestime=296.105ms64bytestime=296.442ms64bytestime=296.186ms64bytestime=296.103ms64bytestime=296.070ms

3) fromthewiredhostinthesamenetwork,RTTtotheserver2looksstable

2015/11/10

so...

wireless

server1

server2

wired

• thisstrangebehaviorhappensonlyforthiscombinationL

InternetInitiativeJapanInc.(IIJ/AS2497) 112015/11/10

RTTdistributiontoserver1

fromwiredhosttoserver1 fromwifi hosttoserver1

wired wifi

InternetInitiativeJapanInc.(IIJ/AS2497) 122015/11/10

RTTdistributiontoserver2

fromwiredhosttoserver2 fromwifi hosttoserver2

wired wifi

InternetInitiativeJapanInc.(IIJ/AS2497) 132015/11/10

thewifi APwasbufferingpackets

wirelesswired

Internet

server2

server1

InternetInitiativeJapanInc.(IIJ/AS2497) 14

• andthiscausedtheunstableRTTL

2015/11/10

Mywifi adapterdoessleep

• toreducebatteryusage• beforesleeping,theclientsendanotificationtothewifi AP,andtheAPkeepspacketsuntiltheclientwakeup

• so,myPCwasaskingthebuffering!

InternetInitiativeJapanInc.(IIJ/AS2497) 152015/11/10

wifi APsendsbeacon

• beaconinterval– timeintervalbetweenbeacontransmissions– usually100msec,butit’sconfigurable

• TIM(TrafficIndicationMap)– totellanysleepingclientsiftheAPhasanybufferedframespresentforit

• wifi adaptercansleepbetweenbeacons,andwakeuptocheckabeacon(TIMcanindicateiftheadapterneedtoreceivedataornot)

InternetInitiativeJapanInc.(IIJ/AS2497) 162015/11/10

thescenario

• Mywifi adapterwenttosleepafter200msecofnotraffic– that’swhytheunstableRTThappensonlywhenIwascommunicatingwithserver2(averageRTTis300msec)

• Basedonthebeaconintervalinformation(whichwas100msecinmycase),itwokeupandreceivedaresponse– that’swhymostRTTdistributioniswithin100msec

InternetInitiativeJapanInc.(IIJ/AS2497) 172015/11/10

sleepingandbuffering

InternetInitiativeJapanInc.(IIJ/AS2497) 18

200msecnotraffic

goestosleepuntilthenextbeacon buffered

2015/11/10

Summary• StrangeRTTbehaviorhappensifyourcommunicationisbetween:– ahostconnectedtoawifi networkand– afar endhost(RTT>200msec)

• Yourwifi adaptergoestosleep– “200msecofnotraffic”seemsacommontrigger

• Thesleepdurationismanageablebysettingbeaconintervalofyourwifi AP– 100msecwouldbereasonable– Youmightbeabletoreducebatteryusagebysettingitas1000msec,butthiscouldintroducemoreRTTpenalty

InternetInitiativeJapanInc.(IIJ/AS2497) 192015/11/10