C09-Transport_Protocols - Mobile Communications Chapter 9:...

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Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS05 9.1 Mobile Communications Chapter 9: Mobile Transport Layer ± Motivation ± TCP-mechanisms ± Classical approaches ± Indirect TCP ± Snooping TCP ± Mobile TCP ± PEPs in general ± Additional optimizations ± Fast retransmit/recovery ± Transmission freezing ± Selective retransmission ± Transaction oriented TCP ± TCP for 2.5G/3G wireless
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Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS05 9.2 Transport Layer E.g. HTTP (used by web services) typically uses TCP ± Reliable transport between client and server required TCP ± Steam oriented, not transaction oriented ± Network friendly: time-out Î congestion Î slow down transmission Well known – TCP guesses quite often wrong in wireless and mobile networks ± Packet loss due to transmission errors ± Packet loss due to change of network Result ± Severe performance degradation Client Server Connection setup Data transmission TCP SYN TCP SYN/ACK TCP ACK HTTP request Connection release HTTP response GPRS: 500ms! >15 s no data
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Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS05 9.3 Motivation I Transport protocols typically designed for ± Fixed end-systems ± Fixed, wired networks Research activities ± Performance ± Congestion control ± Efficient retransmissions TCP congestion control ± packet loss in fixed networks typically due to (temporary) overload situations ± router have to discard packets as soon as the buffers are full ± TCP recognizes congestion only indirect via missing acknowledgements, retransmissions unwise, they would only contribute to the congestion and make it even worse ± slow-start algorithm as reaction
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Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS05 9.4 Motivation II TCP slow-start algorithm ± sender calculates a congestion window for a receiver ± start with a congestion window size equal to one segment ± exponential increase of the congestion window up to the congestion threshold, then linear increase ± missing acknowledgement causes the reduction of the congestion threshold to one half of the current congestion window ± congestion window starts again with one segment TCP fast retransmit/fast recovery ± TCP sends an acknowledgement only after receiving a packet ± if a sender receives several acknowledgements for the same packet, this is due to a gap in received packets at the receiver ± however, the receiver got all packets up to the gap and is actually receiving packets ± therefore, packet loss is not due to congestion, continue with current congestion window (do not use slow-start)
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Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS05 9.5 Influences of mobility on TCP-mechanisms TCP assumes congestion if packets are dropped ± typically wrong in wireless networks, here we often have packet loss due to transmission errors ± furthermore, mobility itself can cause packet loss, if e.g. a mobile node roams from one access point (e.g. foreign agent in Mobile IP)
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This note was uploaded on 02/28/2008 for the course CSCI 6360 taught by Professor Wu during the Spring '08 term at University of Texas at Dallas, Richardson.

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C09-Transport_Protocols - Mobile Communications Chapter 9:...

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