Advanced Topics in Next-Generation WirelessNetworksTransport Protocols in Ad hoc NetworksQian ZhangHKUST
Qian Zhang HKUST Advanced Topics in NextGeneration Wireless Networks Transport Protocols in Ad hoc Networks
Transmission Control Protocol(TCP)Reliable ordered delivery Implements congestion avoidance and controlReliability achieved by means ofretransmissions if necessaryEnd-to-end semantics- Acknowledgements sent to TCP sender confirmdelivery of data received by TCP receiver- Ack for data sent only after data has reachedreceiver
Transmission Control Protocol (TCP) • Reliable ordered delivery • Implements congestion avoidance and control • Reliability achieved by means of retransmissions if necessary • End-to-end semantics − Acknowledgements sent to TCP sender confirm delivery of data received by TCP receiver − Ack for data sent only after data has reached receiver
Overview of TcP concepts Conventional TCP Tahoe, Reno, New-Reno, SACKSending rate is controlled byCongestion window (cwnd):limits the # of packets in flight Slow-start threshold (ssthresh): when CA startLoss detection343duplicateACKs(faster,moreoneeso3230efficient)228Retransmissiontimerexpires(slower,2624lessefficient)2320Overview of congestion control1816mechanismshreshiol4Slow-startphase:cwndstartfrom1Duo10可市and increase exponentiallyCongestionavoidance(CA):increaselinearlyFast retransmit and fast recovery:2Triggerby3duplicateACKsSlow-startCongestionavoidance
Overview of TCP concepts • Conventional TCP • Sending rate is controlled by − Congestion window (cwnd): limits the − Slow-start threshold (ssthresh): when • Loss detection − 3 duplicate ACKs (faster, more efficient) − Retransmission timer expires (slower, less efficient) • Overview of congestion control mechanisms − Slow-start phase: cwnd start from 1 and increase exponentially − Congestion avoidance (CA): increase linearly − Fast retransmit and fast recovery: Trigger by 3 duplicate ACKs Slow start Slow start Congestion avoidance Congestion detected Congestion avoidance Fast retransmit/ fast recovery 1 2 3 4 threshold threshold Time 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 Congestion windows size Overview Slow-start Congestion avoidance − Tahoe, Reno, New-Reno, SACK # of packets in flight CA start
ChallengesThroughput unfairness-UnfairnessatMAClayerTransportlayer should takethis into accountResource constraints-Powerandbandwidthconstraints Separation of congestion control and reliabilitycontrolCompletelydecoupledtransportlayer-Wired network: transport protocol completely separatedfrom underlying layerAdhoc network:interactionwithnetworkand MAClayeris expected foradaptivity
Challenges • Throughput unfairness − Unfairness at MAC layer − Transport layer should take this into account • Resource constraints − Power and bandwidth constraints • Separation of congestion control and reliability control • Completely decoupled transport layer − Wired network: transport protocol completely separated from underlying layer − Ad hoc network: interaction with network and MAC layer is expected for adaptivity
Challenges (Cont.)Misinterpretation of congestion- Traditional mechanism: packet loss, timeout- Ad hoc loss/delay due to. High bit error rate due to varying link condition. Packet collisions due to contention and hiddenterminal. Path breaks due to node mobility. Dynamically changing topology-Frequent path breaks- Partitioning and merging of networks- High delay in reestablishment of path
Challenges (Cont.) • Misinterpretation of congestion − Traditional mechanism: packet loss, timeout − Ad hoc loss/delay due to • High bit error rate due to varying link condition • Packet collisions due to contention and hidden terminal • Path breaks due to node mobility • Dynamically changing topology − Frequent path breaks − Partitioning and merging of networks − High delay in reestablishment of path