CMPE 257 Wireless and Mobile Networking Katia Obraczka Computer Engineering UCSC Baskin Engineering Lecture 11 CMPE 257 Winter 11 1 Student Presentation Logistics We will source from San Jose Send me your presentations ahead of time CMPE 257 Winter 11 2 Today E2E protocols cont d CMPE 257 Winter 11 3 Reliable Point2Point Transport Layer Outline TCP IP basics Impact of transmission errors on TCP performance Approaches to improve TCP performance on wireless networks Classification TCP on infrastructure based networks cont d TCP on MANETs CMPE 257 Winter 11 4 Strict E2E Schemes CMPE 257 Winter 11 5 Receiver Based Scheme Biaz98Asset MH is TCP receiver Receiver uses heuristics to guess cause of packet loss When receiver believes that packet loss is due to errors it sends notification to sender TCP sender on receiving notification retransmits lost packet without reducing congestion window CMPE 257 Winter 11 6 Heuristics Receiver uses inter arrival time between consecutively received packets to guess cause of packet loss On determining a packet loss as being due to errors the receiver may Tag corresponding dupacks with an ELN bit or Send an explicit notification to sender CMPE 257 Winter 11 7 Receiver Based Scheme Packet loss due to congestion 12 FH 11 10 BS MH T FH BS 12 11 Congestion loss CMPE 257 Winter 11 10 MH 8 Receiver Based Scheme Packet loss due to transmission error 12 FH 11 10 BS MH 2T 12 FH BS 11 Error loss CMPE 257 Winter 11 10 MH 9 Sender Based Discrimination Scheme CMPE 257 Winter 11 10 Sender Based Discrimination Biaz98ic3n Biaz99techrep Sender can attempt to determine cause of a packet loss If packet loss determined to be due to errors do not reduce congestion window Sender can only use statistics based on round trip times window sizes and loss pattern Unless network provides more information example explicit loss notification CMPE 257 Winter 11 11 Heuristics for Congestion Avoidance Define condition C as a function of congestion window size and observed RTTs Condition C evaluated for new RTT If C True reduce congestion window CMPE 257 Winter 11 12 Heuristics for Congestion Avoidance Some proposals TCP Vegas Brakmo94 expected throughput ET W i RTTmin actual throughput AT W i RTT i Condition C ET AT beta CMPE 257 Winter 11 13 Sender Based Heuristics Record latest value evaluated for condition C When a packet loss is detected If last evaluation of C is TRUE assume packet loss due to congestion Else assume packet loss due to transmission errors If packet loss determined to be due to errors do not reduce congestion window CMPE 257 Winter 11 14 Sender Based Heuristics Disadvantage Does not work quite well enough Reason Not much correlation between observed short term statistics and onset of congestion CMPE 257 Winter 11 15 Sender Based Heuristics Advantages Only sender needs to be modified Needs further investigation to develop better heuristics Investigate longer term heuristics CMPE 257 Winter 11 16 Reliable Point2Point Transport Layer Outline TCP IP basics Impact of transmission errors on TCP performance Approaches to improve TCP performance on wireless networks Classification TCP on cellular TCP on MANETs CMPE 257 Winter 11 17 TCP in Mobile Ad Hoc Networks CMPE 257 Winter 11 18 Issues Route changes due to mobility Frequent route changes may cause OOO delivery Wireless transmission errors Problem compounded due to multiple hops MAC MAC protocol can impact TCP performance CMPE 257 Winter 11 19 Throughput over Multi Hop Wireless Paths Gerla99 When contention based MAC protocol is used connections over multiple hops are at a disadvantage compared to shorter connections They have to contend for wireless access at each hop Delay or drop probability increases with number of hops CMPE 257 Winter 11 20 Analysis of TCP Performance over MANETs Holland99 Impact of mobility Simulation study Performance metric throughput Baseline ideal expected throughput Upper bound Static network CMPE 257 Winter 11 21 Throughput versus Hops 1600 1400 1200 1000 800 600 400 200 0 TCP Throughtput Kbps 1 2 3 4 5 6 7 8 9 10 Number of hops TCP throughput over 2 Mbps 802 11 MAC fixed linear MANET CMPE 257 Winter 11 22 Expected Throughput exp ected throughput ti Ti ti i 1 i 1 Ti is measured throughput for i hops using static linear chain topology ti time duration of TCP connection containing i hops CMPE 257 Winter 11 23 Throughput versus speed Throughput decreases with speed Expected Average Throughput Over 50 runs Actual Speed m s CMPE 257 Winter 11 24 Throughput versus Speed But not always 30 m s 20 m s Actual throughput Mobility pattern CMPE 257 Winter 11 25 Impact of Mobility TCP Throughput 10 m s Actual throughput 2 m s Ideal throughput Kbps CMPE 257 Winter 11 26 Impact of Mobility 20 m s Actual throughput 30 m s Ideal throughput CMPE 257 Winter 11 27 Why Throughput Degrades mobility causes link breakage resulting in route failure Route is repaired TCP sender starts sending packets again No throughput No throughput despite route repair TCP data and acks en route discarded CMPE 257 Winter 11 28 Why Throughput Degrades mobility causes link breakage resulting in route failure TCP sender times out Backs off timer Route is repaired TCP sender resumes sending No throughput No throughput despite route repair Larger route repair delays especially harmful TCP data and acks en route discarded CMPE 257 Winter 11 29 Why Throughput Improves Low Speed Scenario C B D C D B A C B D A A 1 5 second route failure Route from A to D is broken for 1 5 second When TCP sender times out after 1 second route still broken TCP times out after another 2 seconds and only then resumes CMPE 257 Winter 11 30 Why Throughput Improves Higher Speed Scenario C B D C D B A C B D A A 0 75 second route failure Route from A to D is broken for 0 75 second Before TCP sender times after 1 second route is repaired CMPE 257 Winter 11 31 Why Throughput Improves General Principle TCP timeout interval somewhat independent of speed Network state at higher speed when timeout occurs may be more favorable than at lower speed Network state Link route status Route caches Congestion CMPE 257 Winter 11 32 How to Improve Throughput Network feedback Inform TCP of route failure explicitly Let TCP know when route is repaired Probing Explicit notification Reduce repeated TCP timeouts and backoff CMPE 257 Winter 11 33 ELFN Explicit Link Failure Notification Piggyback notification onto DSR s route failure message to sender TCP responds by disabling congestion