11

Variability in TCP Round-trip Variability in TCP Round-trip TimesTimes

Extent, Causes, and ImpactExtent, Causes, and Impact

FacultyFaculty::

Jasleen KaurJasleen KaurDon SmithDon SmithDiane Pozefsky (IBM)Diane Pozefsky (IBM)Vidyadhar Kulkarni (OR)Vidyadhar Kulkarni (OR)Michele WeigleMichele WeigleKevin JeffayKevin Jeffay

http://www.cs.unc.edu/~jasleen/research/RTT

StudentsStudents::

Jay AikatJay AikatSushant RewaskarSushant RewaskarVivek SawantVivek SawantLi Cheng (OR)Li Cheng (OR)

TheThe UNIVERSITY UNIVERSITY ofof NORTH CAROLINA NORTH CAROLINA atat CHAPEL HILL CHAPEL HILL

22

TCP Round-trip Times (RTTs)TCP Round-trip Times (RTTs)

Popular belief: Popular belief: RTTs do not vary significantly within TCP RTTs do not vary significantly within TCP

connectionsconnections

Mean RTT can be used to approximate per-segment RTTsMean RTT can be used to approximate per-segment RTTs

TCP analytical models TCP analytical models [Altman00, Kumar98, Lakshman97, Mathis97, Padhye98, [Altman00, Kumar98, Lakshman97, Mathis97, Padhye98,

Zhang02…]Zhang02…]

RTT of initial segments is “typical” for a TCP connectionRTT of initial segments is “typical” for a TCP connection[Jiang02][Jiang02]

Do per-segment RTTs vary significantly within a TCP connection?Do per-segment RTTs vary significantly within a TCP connection?Do per-segment RTTs vary significantly within a TCP connection?Do per-segment RTTs vary significantly within a TCP connection?

33

UNC Campus UNC Campus

Regional health-

care center

Regional health-

care center

Departments

Research labs

Departments

Research labs

Residence halls

Residence halls

Wireless users

Wireless users

VPNs

Modem

VPNs

Modem

ibiblio web servers

ibiblio web servers

Data SourceData Source

Rest of the world

Monitor

ISP router

Rich data source!Rich data source!Rich data source!Rich data source!

44

Trace StatisticsTrace Statistics

ConnectioConnectionsns

Remote Remote hostshosts RTT samplesRTT samples BytesBytes PacketsPackets

All All connectionsconnections

22.7 22.7 millionmillion 962 K962 K 252 million252 million 628 GB628 GB 511 M511 M

Connections Connections with at least with at least 10 samples10 samples

1.1 million1.1 million 258 K258 K 236 million236 million 581 GB581 GB 464 M464 M

Large data set!Large data set!Large data set!Large data set!

55

Variability Within ConnectionsVariability Within Connections

Median RTT:Median RTT: 30% of connections see a median RTT more than twice the min RTT30% of connections see a median RTT more than twice the min RTT

90% RTT:90% RTT: 22% of connections see a 90% RTT more than 5 times the min RTT22% of connections see a 90% RTT more than 5 times the min RTT 90% RTT increases with min RTT90% RTT increases with min RTT

66

Causes of VariabilityCauses of Variability

Correlation with hop-countCorrelation with hop-count

Correlation with locationCorrelation with location Prefix-aggregationPrefix-aggregation

Behavior with timeBehavior with time TimescalesTimescales

CongestionCongestion Path changesPath changes

77

Per-Segment RTTs: Mean or Per-Segment RTTs: Mean or Distributions?Distributions?

Is mean RTT a good approximation for per-segment RTTs?Is mean RTT a good approximation for per-segment RTTs? TCP analytical modelsTCP analytical models TCP evaluation (simulations)TCP evaluation (simulations)

88

Impact of variabilityImpact of variability

Models for per-connection RTTsModels for per-connection RTTs Accurate simulation environmentsAccurate simulation environments Accurate analysisAccurate analysis

Impact of RTT variability on past workImpact of RTT variability on past work TCP analytical modelsTCP analytical models Delay-based congestion controlDelay-based congestion control TCP evaluation (simulations)TCP evaluation (simulations) TCP-based analysisTCP-based analysis

99

The SYNThe SYN//(SYN+ACK) RTT(SYN+ACK) RTT

RTT yielded by the SYN and SYN+ACK pairRTT yielded by the SYN and SYN+ACK pair

Differs by more than 10% from min RTT for 14% of connectionsDiffers by more than 10% from min RTT for 14% of connections

Differs by more than 10% from median RTT for 50% of Differs by more than 10% from median RTT for 50% of connectionsconnections