Announcements CMPE 257 Wireless and Mobile Networking n n n Spring 2003 Lecture 13 CMPE 257 Spring 2003 n 1 Project status update Solution for hw 1 posted Hw 3 multicast routing questions posted Project 2 demo logistics CMPE 257 Spring 2003 2 Today n n Transport layer cont d Reliable multicast CMPE 257 Spring 2003 TCP in Mobile Ad Hoc Networks 3 CMPE 257 Spring 2003 4 Throughput over Multi Hop Wireless Paths Gerla99 Issues n Route changes due to mobility n n Wireless transmission errors n MAC n n n Frequent route changes may cause OOO delivery Problem compounded due to multiple hops MAC protocol can impact TCP performance CMPE 257 Spring 2003 n n n CMPE 257 Spring 2003 1600 1400 1200 1000 800 600 400 200 0 Baseline ideal expected throughput n n They have to contend for wireless access at each hop Delay or drop probability increases with number of hops 6 Throughput versus Hops Impact of mobility Simulation study Performance metric throughput n n 5 Analysis of TCP Performance over MANETs Holland99 n When contention based MAC protocol is used connections over multiple hops are at a disadvantage compared to shorter connections Upper bound Static network TCP Throughtput Kbps 1 2 3 4 5 6 7 8 9 1 Number of hops TCP throughput over 2 Mbps 802 11 MAC fixed linear MANET CMPE 257 Spring 2003 7 CMPE 257 Spring 2003 8 Throughput versus speed Expected Throughput Throughput decreases with speed exp ected throughput i 1 ti i 1 n n Ti Expected Average Throughput Over 50 runs ti Ti is measured throughput for i hops using static linear chain topology ti time duration of TCP connection containing i hops CMPE 257 Spring 2003 Speed m s 9 CMPE 257 Spring 2003 But not always 30 m s 2 m s 10 m s Actual throughput 20 m s Ideal throughput Kbps Mobility pattern CMPE 257 Spring 2003 10 Impact of Mobility TCP Throughput Throughput versus Speed Actual throughput Actual 11 CMPE 257 Spring 2003 12 Why Throughput Degrades Impact of Mobility 20 m s mobility causes link breakage resulting in route failure Actual throughput 30 m s Route is repaired TCP sender starts sending packets again No throughput No throughput despite route repair Ideal throughput TCP data and acks en route discarded CMPE 257 Spring 2003 13 CMPE 257 Spring 2003 Why Throughput Improves Low Speed Scenario Why Throughput Degrades mobility causes link breakage resulting in route failure TCP sender times out Backs off timer Route is repaired 14 TCP sender resumes sending C B No throughput C D B A No throughput D C B D A A 1 5 second route failure despite route repair Larger route repair delays especially harmful 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 TCP data and acks en route discarded CMPE 257 Spring 2003 15 CMPE 257 Spring 2003 16 Why Throughput Improves Higher Speed Scenario D C B C D B C Why Throughput Improves General Principle n D B n A A A 0 75 second route failure n 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 n Route from A to D is broken for 0 75 second n n When TCP sender times after 1 second route is repaired CMPE 257 Spring 2003 17 How to Improve Throughput n n n n n n n n Probing Explicit notification 18 Explicit Link Failure Notification Piggyback notification onto DSR s route failure message to sender TCP responds by disabling congestion control until route is fixed n Reduce repeated TCP timeouts and backoff CMPE 257 Spring 2003 CMPE 257 Spring 2003 ELFN Network feedback Inform TCP of route failure explicitly Let TCP know when route is repaired n Link route status Route caches Congestion n 19 Disable retransmission timers When ACK is received TCP restores state and resumes normal operation CMPE 257 Spring 2003 20 Performance Improvement Without network feedback Performance Improvement Without network feedback With feedback Actual throughput Actual throughput With feedback Ideal throughput 2 m s speed Ideal throughput 30 m s speed CMPE 257 Spring 2003 21 throughput as a fraction of ideal Performance with Explicit Notification CMPE 257 Spring 2003 Issues Network Feedback n 1 0 8 22 Network knows best why packets are lost Base TCP Network feedback beneficial Need to modify transport network layer to receive send feedback 0 6 With explicit notification 0 4 0 2 0 2 10 20 n 30 mean speed m s CMPE 257 Spring 2003 23 Need mechanisms for information exchange between layers CMPE 257 Spring 2003 24 Impact of Caching n n To Cache or Not to Cache Route caching has been suggested as a mechanism to reduce route discovery overhead e g DSR Each node may cache one or more routes to given destination When route from S to D detected as broken node S may n n Use another cached route from local cache or Obtain a new route using cached route at another node CMPE 257 Spring 2003 n Average speed m s 25 Why Performance Degrades With Caching n Actual throughput as fraction of expected throughput n CMPE 257 Spring 2003 To Cache or Not to Cache When a route is broken route discovery returns cached route from local cache or from nearby node Cached routes may also be broken n n n timeout due to route failure timeout cached timeout second cached route is broken route also broken CMPE 257 Spring 2003 26 n 27 Caching can result in faster route repair But faster does not necessarily mean correct If incorrect repairs occur often enough caching performs poorly Need mechanisms for determining when cached routes are stale CMPE 257 Spring 2003 28 Caching and TCP Performance n n Caching can reduce overhead of route discovery even if cache accuracy is not very high But if cache accuracy is not high enough gains in routing overhead may be offset by loss of TCP performance due to multiple timeouts CMPE 257 Spring 2003 n n n n n n Reset window to small value after route repair TCP needs to be aware of route repair Route Failure and Route Re establishment Notifications Impact low on paths with small delay bw product CMPE 257 Spring 2003 n Impact of routing algorithm on TCP performance n On demand versus proactive n Example new RTO old RTO new route length old route length Not evaluated yet n n CMPE 257 Spring 2003 When route breaks may be too optimistic or may be too conservative Better be conservative than overly optimistic 30 TCP Over Different Routing Protocols Dyer2001 If new route longer RTO may be too small leading to timeouts New RTO