Wireless TCP IntroductionThe problems of standard TCP in wireless networksBasic ideas to overcome the problem in wireless networkBasics topologyI-TCP: Indirect TCPI-TCP indirect TCP (Cont.)PowerPoint PresentationI-TCP Result in LANI-TCP Result in WANSnoop ProtocolThree kinds of data packets from FHFlowchart for snoop_dataProcess three kinds of ACKsFlowchart for snoop_ackData from MH to FHRouting in snoop protocolSlide 17Handoff process in snoop protocolHandoff process Cont.Handoff processTopology for experimentSlide 22Slide 23Delayed Duplicate ACKDelayed Duplicate ACK (cont.)Reduce interferenceDelay interval TSimulation ResultSlide 29Slide 30Multiple Acknowledgements ProtocolMultiple Acknowledgements protocol (Cont.)Slide 33TCP on sender and receiverSnoop protocol on BSSnoop protocol on BS (Cont.)Implementation of ACKp and ACKcFreeze-TCPHandle frequent handoffHandle frequent handoff (cont.)Avoid long freeze timeIllustration of throughputFreeze vs Regular TCP In LANFreeze vs Regular TCP In WANComments and discussionComments and discussion Cont.Slide 47Comparison between different protocolsTCP propagation delay introductionEffect of propagation delaySolutions overview for propagation delaySolutions for propagation delayTCP level solutions for propagation delayApplication level solutions for propagation delayNetwork level solutions for propagation delayNetwork level solutions for propagation delay (contd)Network level solutions for propagation delay (contd.)TCP bandwidth asymmetry introductionAsymmetry network topologyEffect of bandwidth asymmetrySolution for asymmetry effect in one-way transferAck Congestion Control (ACC) - decrease frequency of acksAck Congestion Control (ACC) (contd.)Ack Filtering (AF) - decrease the number of acksThe drawback of receiver solutionTCP Sender Adaptation (SA)TCP Sender Adaptation (SA) (contd.)Ack Reconstruction (AR)Ack Reconstruction (AR) (contd.)Asymmetry effect in two-way transferSolution for asymmetry effect in two-way transferAsymmetry effect summaryFigure: one-way transfer comparisonFigure: one-way throughputFigure: congestion windowFigure: two-way transfer comparisonReferencesReference (contd)Slide 791Wireless TCP IntroductionThe problems of Standard TCP over wireless link and basic solutionSeveral implementations of wireless TCPI-TCP (Indirect TCP)Snoop ProtocolDelayed duplicated ACKMultiple AcknowledgementsFreeze TCPComments and discussion2The problems of standard TCP in wireless networksStandard TCP assume that the packet losses are due to the congestion.(99%)This not valid considering the characteristic of wireless link sporadic high bit-error rate, high latencies  temporary disconnection due to handoff Misinterpretation packet loss for congestion3Basic ideas to overcome the problem in wireless networkHide any non-congestion-related losses from TCPSplit TCP connectionLocal retransmissionsIP LayerLink layerLet TCP aware the non-congestion-related losses and not apply congestion control4Basics topology Figure 1 Topology5I-TCP: Indirect TCPBreaks the TCP connection between FH and MH into two connections at MSRConnection between FH and MSR is regular TCPConnection between MSR and MH is any transport layer protocol tuned for wireless links.6I-TCP indirect TCP (Cont.)BS cache packets from FH and send back ACK for MH.if MH switches to another cell the center point of the connection moves to the new MSR, no need of reconnection.FH is completely unaware of the indirection and is not affected even when the MH switches cells.7Figure 2 I-TCP setup and handoff8I-TCP Result in LANTABLE 1 I-TCP Throughput Performance over Local Area9I-TCP Result in WANTable 2 I-TCP Throughput Performance over Wide Area10Snoop ProtocolBasic ideaModify the IP layer in the BS, and let BS cache the TCP packets sent from FH before route it to the MH.If packet lost on wireless link, IP layer on the BS will retransmit the packet .BS suppress DUPACKs sent from MH to FH.BS use shorter local timer for local timeout.11Three kinds of data packets from FHA new packet in the normal TCP sequence. (common case)An out-of-sequence packet that has been cached earlier. (sender retransmission)An out-of-sequence packet that has not been cached earlier. (congestion loss)12Flowchart for snoop_dataNew Packets?Packet arrives1. Forward Packet2. Reset local rexmit counterIn sequence?1. Mark as cong. loss2. Forward packet1. Cache packet2. Forward to mobile hostFigure 3. Flowchart for snoop_data()Sender rexmitionCongestion lossCommon caseNoNoYesYesAcknowledgedSend back lasted ACKYesNo13Process three kinds of ACKsA new ACK from MHA spurious ACK, sequence number less than the lasted acknowledged oneA duplicate ACK (DUPACK)14Flowchart for snoop_ackAck arrivesNew ACK?Dup ACK?First one?1. Free buffers2. Update RTT3. Propagate ACK to senderRetransmit lostpackets with high priorityDiscardDiscardYesNoYesYesNoNoSpurious AckLater dup Acks, for lost packetNext packet lostCommon caseFigure 4. Flowchart for snoop_ack15Data from MH to FHCan not differentiate between BER loss or Congestion loss. Using SACK (selective acknowledge)BS keep track packet losses in a transmit window.BS send NACK to MH with SACK option indicate the boundary of the lost packets.16Routing in snoop protocolSimilar to Mobile IP.Using multicast and intelligent buffering in nearby base stations.One prime BS forward packetsSeveral nearby BS cache the last several packets17Figure: Intelligent cache18Handoff process in snoop protocolInitiated by mobile hostMH send control messages to the various BS, request them either begin or end forwarding and buffering of packetsThe activated forwarding BS synchronizing its buffer using information in the control message19Handoff process Cont.since snoop module does not change any of the end-to-end semantics of TCP, it is resistant the random packet losses when handoff.20Handoff processBS_OLD MH BS_NEWpacket1beaconStronger beaconBuffer requestForward requestpacket2packet3packet4packet5Handoff latency21Topology for experimentTransmitterBS1 BS2Home AgentMHAT&T Wavelan2Mbits/sEthernet22Throughput: Snoop vs. Regular TCP23Sequence number: Snoop vs. Regular TCP24Delayed Duplicate ACKMotivationsTCP may be encryptedACK may go different paths from that of data.Basic ideasThe idea is to delay the third and subsequent duplicate