Mobile Communications Chapter 9: Mobile Transport Layer

1 Prof. Jochen Schiller, Communications Chapter 9: Mobile Transport LayerqMotivationqTCP-mechanismsqClassica l approachesqIndirect TCPqSnooping TCPqMobile TCPqPEPsin generalqAdditional optimizationsqFast retransmit/recoveryqTransmission freezingqSelective retransmissionqTransaction oriented TCPqTCP for wirelessProf. Jochen Schiller, HTTP (used by web services) typically uses TCPqReliable Transport between client and server requiredTCPqSteam oriented, not transaction orientedqNetwork friendly: time-out congestion slow down transmissionWell known TCP wrongly assumes congestion in wireless and Mobile networks whenqPacket lossesdue to transmission errorsqPacket loss due to change of networkResultqSevere performancedegradationClientServerConnec tionsetupDatatransmissionConnectionrelea seTCP SYNTCP SYN/ACKTCP ACKHTTP requestHTTP responseGPRS: 500ms!>15 sno dataProf. Jochen Schiller, ITransport protocols typically designed forqFixed end-systemsqFixed, wired networksResearch activitiesqHow to improve TCP performance in wireless networksqMaintain congestion control behaviorqEfficient retransmissionsTCP congestion control in fixed networksqTimeouts/Packet loss typically due to (temporary) overloadqRouters discard packets when buffers are full qTCP recognizes congestion only indirectly via missing ACKs, retransmissions unwise, since they increase congestionqslow-start algorithm as reactionProf.

2 Jochen Schiller, IITCP slow-start algorithmqsender calculates a congestion window for a receiverqstart with a congestion window size equal to one segment (packet)qExponentially increase congestion window till congestion threshold, then linear increaseqTimeout/missing acknowledgement causes reduction of congestion threshold to half of the current congestion window qcongestion window starts again with one segmentTCP fast retransmit/fast recoveryqTCP sends an ACK only after receiving a packetqIf sender receives duplicate ACKs, this is due to gap in received packets at the receiverqReceiver got all packets up to the gap and is actually receivingpacketsqConclusion: packet loss notdue to congestion, retransmit, continue with current congestion window (do not use slow-start)Prof. Jochen Schiller, of Wireless/mobility on TCP-mechanismsTCP assumes congestion if packets are droppedqtypically wrong in wireless networks, here we often have packet loss due to transmission errorsqfurthermore, mobilitycan cause packet loss, if a Mobile node roams from one access point ( foreign agent in Mobile IP) toanother while packets in transit to the old access point and forwarding is not possibleThe performance of an unchanged TCP degrades severelyqTCP cannot be changed fundamentally due to large installed base in the fixed network, TCP for mobility has to remain compatibleqthe basic TCP mechanisms keep the whole Internet togetherProf.

3 Jochen Schiller, approach: Indirect TCP IIndirect TCP or I-TCP segments the connectionqno changes to the TCP protocol for hosts connected to the wired Internet, millions of computers use (variants of) this protocolqoptimized TCP protocol for Mobile hostsqsplitting of the TCP connection at, , the foreign agent into2 TCP connections, no real end-to-end connection any longerqhosts in the fixed part of the net do not notice the characteristics of the wireless partmobile hostaccess point (foreign agent) wired Internet wireless TCPstandard TCPProf. Jochen Schiller, socketand statemigrationmobile hostaccess point2 Internetaccess point1socketmigrationand statetransferProf. Jochen Schiller, TCP IIAdvantagesqNo changes in the fixed network necessary, no changes for the hosts (TCP protocol) necessary, all current optimizations to TCP stillworkqWireless link transmission errors isolated from those in fixed networkqsimple to control, Mobile TCP is used only for one hop between, , a foreign agent and Mobile hostqtherefore, a very fast retransmission of packets is possible, the short delay on the Mobile hop is knownDisadvantagesqloss of end-to-end semantics, an acknowledgement to a sender does now not any longer mean that a receiver really got a packet, foreign agents might crashqhigher latency possible due to buffering of data within the foreign agent and forwarding to a new foreign agentProf.

4 Jochen Schiller, approach: Snooping TCP I Transparent extension of TCP within the foreign agentqbuffering of packets sent to the Mobile hostqlost packets on the wireless link (both directions!) will be retransmitted immediately by the Mobile host or foreign agent, respectively (so called local retransmission)qthe foreign agent therefore snoops the packet flow and recognizes acknowledgements in both directions, it also filters ACKsqchanges of TCP only within the foreign agent wired Internetbuffering of dataend-to-end TCP connectionlocal retransmissioncorrespondenthostforeignag entmobilehostsnooping of ACKsProf. Jochen Schiller, TCP IIData transfer to the Mobile hostqFA buffers data until it receives ACK of the MH, FA detects packet loss via duplicated ACKsor time-outqfast retransmission possible, transparent for the fixed networkData transfer from the Mobile hostqFA detects packet loss on the wireless link via sequence numbers, FA answers directly with a NACK to the MHqMH can now retransmit data with only a very short delayIntegration with MAC layerqMAC Layer often has similar mechanisms to those of TCPqthus, the MAC Layer can already detect duplicated packets due toretransmissions and discard them Problemsqsnooping TCP does not isolate the wireless link as good as I-TCPqsnooping might be tough if packets are encryptedProf.

5 Jochen Schiller, approach: Mobile TCPS pecial handling of lengthy and/or frequent disconnectionsM-TCP splits as I-TCP doesqunmodified TCP fixed network to supervisory host (SH)qoptimized TCP SH to MHSupervisory hostqno caching, no retransmissionqmonitors all packets, if disconnection detectedlset sender window size to 0lsender automatically goes into persistent modeqold or new SH reopen the windowAdvantagesqmaintains semantics, supports disconnection, no buffer forwardingDisadvantagesqloss on wireless link propagated into fixed networkqadapted TCP on wireless linkProf. Jochen Schiller, retransmit/fast recoveryChange of foreign agent often results in packet loss qTCP reacts with slow-start although there is no congestionForced fast retransmitqas soon as the Mobile host has registered with a new foreign agent, the MH sends duplicated acknowledgements on purposeqthis forces the fast retransmit mode at the communication partnersqadditionally, the TCP on the MH is forced to continue sending with the actual window size and not to go into slow-start after registrationAdvantageqsimple changes result in significant higher performance DisadvantageqCooperation required between IP and TCP, no transparent approachProf.

6 Jochen Schiller, freezingMobile hosts can be disconnected for a longer timeqno packet exchange possible, , in a tunnel, disconnection due to overloaded cells or mux. with higher priority trafficqTCP disconnects after time-out completelyTCP freezingqMAC Layer is often able to detect interruption in advanceqMAC can inform TCP Layer of upcoming loss of connectionqTCP stops sending, but does now not assume a congested link qMAC Layer signals again if reconnected Advantageqscheme is independent of data DisadvantageqTCP on Mobile host has to be changed, mechanism depends on MAC layerProf. Jochen Schiller, retransmissionTCP acknowledgements are often cumulativeqACK n acknowledges correct and in-sequence receipt of packets up to nqif single packets are missing quite often a whole packet sequence beginning at the gap has to be retransmitted (go-back-n), thus wasting bandwidthSelective retransmission as one solutionqRFC2018 allows for acknowledgements of single packets, not only acknowledgements of in-sequence packet streams without gapsqsender can now retransmit only the missing packetsAdvantageqmuch higher efficiencyDisadvantageqmore complex software in a receiver, more buffer needed at the receiverProf.

7 Jochen Schiller, oriented TCPTCP phasesqconnection setup, data transmission, connection release qusing 3-way-handshake needs 3 packets for setup and release, respectivelyqthus, even short messages need a minimum of 7 packets!Transaction oriented TCPqRFC1644, T-TCP, describes a TCP version to avoid this overheadqconnection setup, data transfer and connection release can be combinedqthus, only 2 or 3 packets are neededAdvantageqefficiencyDisadvantageqr equires changed TCPqmobility not longer transparent Prof. Jochen Schiller, of different approaches for a Mobile TCPA pproachMechanismAdvantagesDisadvantagesI ndirect TCPsplits TCP connectioninto two connectionsisolation of wirelesslink, simpleloss of TCP semantics,higher latency athandoverSnooping TCP snoops data andacknowledgements, localretransmissiontransparent for end-to-end connection, MACintegration possibleproblematic withencryption, bad isolationof wireless linkM-TCPsplits TCP connection,chokes sender viawindow sizeMaintains end-to-endsemantics, handleslong term and frequentdisconnectionsBad isolation of wirelesslink, processingoverhead due tobandwidth managementFast retransmit/fast recoveryavoids slow-start afterroamingsimple and efficientmixed layers, nottransparentTransmission/time-out freezingfreezes TCP state atdisconnect, resumesafter reconnectionindependent of contentor encryption, works forlonger interruptschanges in TCPrequired.

8 MACdependantSelectiveretransmissionretra nsmit only lost datavery efficientslightly more complexreceiver software, morebuffer neededTransactionoriented TCPcombine connectionsetup/release and datatransmissionEfficient for certainapplicationschanges in TCPrequired, not transparentProf. Jochen Schiller, Improvements IInitial research workqIndirect TCP, Snoop TCP, M-TCP, T/TCP, SACK, Transmission/time-out freezing, ..TCP over wireless networksqFine tuning today s TCPqLearn to live withlData rates: 64kbit/s up, 115-384kbit/s down; asymmetry: 3-6, but also up to 1000 (broadcast systems), periodic allocation/release of channelslHigh latency, high jitter, packet lossqSuggestionslLarge (initial) sending windows, large maximum transfer unit, selective acknowledgement, explicit congestion notification, time stamp, no header compressionqAlready in useli-mode running over FOMAlWAP ( TCP with wireless profile )pRTTMSSBW** max.

9 TCP BandWidth Max. Segment Size Round Trip Time loss probabilityProf. Jochen Schiller, Improvements IIPerformance enhancing proxies (PEP, RFC 3135)qTransport layerlLocal retransmissions and acknowledgementsqAdditionally on the application layerlContent filtering, compression, picture , Internet/WAP gatewayslWeb service gateways?qBig problem: breaks end-to-end semanticslDisables use of IP securitylChoose between PEP and security!More open issuesqRFC 3150 (slow links)lRecommends header compression, no timestampqRFC 3155 (links with errors)lStates that explicit congestion notification cannot be usedqIn contrast to recommendations! Mobile systemPEPComm. partnerwirelessInternet