F-TransportAndVoIP - Mobile Networks Module F Transport...

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Module F Transport Layer over Wireless Networks + Voice over IP (VoIP) JP Hubaux, P. Papadimitratos and M. Poturalski Mobile Networks http://mobnet.epfl.ch Some slides addapted from Jochen H. Schiller, Nitin Vaidya, and James Kurose & Keith Ross
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Outline o TCP in Mobile Networks P Real-time traffic in Mobile Networks 2
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Reminder: Transmission Control Protocol o Reliable, in-order data delivery P Flow control P Congestion avoidance and control P End-to-end semantics Host A Host B SYN, Seq_no = x SYN, Seq_no = y, ACK, Ack_no = x+1 Seq_no = x +1, ACK, Ack_no = y+1 3
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TCP basic operation Send buffer Segments Receive buffer Application writes bytes in send buffer ACKs Sender Receiver Application reads bytes from receive buffer Application layer Transport layer Write 45 bytes Write 15 bytes Write 20 bytes Read 40 bytes Read 40 bytes Internet 4
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TCP flow control o Flow control is a speed-matching service P Sender adjusts the transmission rate to the receiver P Receiver advertises the remaining buffer space ( rwnd) to the sender P The sender keeps unacknowledged data below rwnd LastByteSent – LastByteAcked ≤ rwnd 5
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Congestion Light traffic o Arrival Rate << R P Low delay P Can accommodate more Congestion onset o Arrival rate approaches R P Delay increases rapidly P Throughput begins to saturate Saturation o Arrival rate > R P Large delays, packet loss P Useful application throughput drops Throughput (bps) Delay (sec) R R Arrival Rate Arrival Rate 6
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TCP congestion control o Keeps TCP off the congestion collapse cliff P Congestion window mechanism LastByteSent – LastByteAcked ≤ min{cwnd, rwnd} a Slow Start phase P Increase congestion window size ( cwnd ) by one segment for each received ACK P Congestion window increases exponentially ACK Segment RTTs cwnd 7
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TCP congestion control o Congestion Avoidance phase P Congestion threshold ssthresh w When cwnd > ssthresh , increase cwnd slowly P cwnd++ per round-trip-time (RTT) Each time an ACK arrives, cwnd is increased by 1/ cwnd In one RTT, cwnd segments are sent, so total increase in cwnd is cwnd x 1/ cwnd = 1 cwnd grows linearly RTTs cwnd ssthresh 8
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TCP congestion control o Congestion detection: P Timeout or P Receipt of duplicate ACKs (Fast Retransmit) P Assumption: current cwnd corresponds to available bandwidth P TCP Tahoe P ssthresh = ½ cwnd P cwnd = 1 P Go back to Slow Start P Over several cycles expect to converge to ssthresh equal to about ½ the available bandwidth Congestion window 0 Slow Start Congestion Avoidance Time-out ssthresh 9 Time (expressed in RTTs)
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TCP congestion control o Fast Retransmit mechanism P If a segment is dropped, subsequent segments trigger duplicate ACKs P Sender retransmits segment instantly (without waiting for a timeout) when duplicate ACKs are received (typically 3) P Improves performance P Faster reaction to packet loss P Implemented in TCP-Reno (more recent than TCP-Tahoe) SEQ=1 ACK=2 ACK=2 ACK=2 ACK=2 SEQ=2 SEQ=3 SEQ=4 SEQ=5 10
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Wireless and Mobile Networks Mobile Host (1) Access Point Internet Base Station Mobile Host (2) Wire-line Communication Wire-less Communication 11
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This note was uploaded on 09/01/2010 for the course IT itphd taught by Professor Nitin during the Spring '10 term at Vlerick Leuven Gent Management School.

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F-TransportAndVoIP - Mobile Networks Module F Transport...

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