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!

2 >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.

3 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.

4 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.

5 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.

6 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.

7 Foreign agents might crashqhigher latency possible due to buffering of data within the foreign agent and forwarding to a new foreign agentProf. 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.

8 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.

9 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.

10 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.