lec11b - Performance of rdt3.0 ! ! rdt3.0 works, but...

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Transport Layer 3-42 Performance of rdt3.0 ! rdt3.0 works, but performance stinks ! ex: 1 Gbps link, 15 ms prop. delay, 8000 bit packet: " U sender : utilization – fraction of time sender busy sending U sender = . 008 30.008 = 0.00027 L / R RTT + L / R = " if RTT=30 msec, 1KB pkt every 30 msec -> 33kB/sec thruput over 1 Gbps link " network protocol limits use of physical resources! ds microsecon 8 bps 10 bits 8000 9 = = = R L d trans
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Transport Layer 3-43 rdt3.0: stop-and-wait operation first packet bit transmitted, t = 0 sender receiver RTT last packet bit transmitted, t = L / R first packet bit arrives last packet bit arrives, send ACK ACK arrives, send next packet, t = RTT + L / R U sender = . 008 30.008 = 0.00027 L / R RTT + L / R =
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Transport Layer 3-45 Pipelining: increased utilization first packet bit transmitted, t = 0 sender receiver RTT last bit transmitted, t = L / R first packet bit arrives last packet bit arrives, send ACK ACK arrives, send next packet, t = RTT + L / R last bit of 2 nd packet arrives, send ACK last bit of 3 rd packet arrives, send ACK U sender = . 024 30.008 = 0.0008 3 * L / R RTT + L / R = Increase utilization by a factor of 3!
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Transport Layer 3-51 Selective Repeat ! receiver individually acknowledges all correctly received pkts " buffers pkts, as needed, for eventual in-order delivery to upper layer ! sender only resends pkts for which ACK not received " sender timer for each unACKed pkt ! sender window " N consecutive seq # ` s " again limits seq #s of sent, unACK ` ed pkts
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Transport Layer 3-52 Selective repeat: sender, receiver windows
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Transport Layer 3-53 Selective repeat data from above : ! if next available seq # in window, send pkt timeout(n): ! resend pkt n, restart timer ACK(n) in [sendbase,sendbase+N]: ! mark pkt n as received ! if n smallest unACKed pkt, advance window base to next unACKed seq # sender pkt n in [rcvbase, rcvbase+N-1] ! send ACK(n) ! out-of-order: buffer ! in-order: deliver (also deliver buffered, in-order pkts), advance window to next not-yet-received pkt pkt n in [rcvbase-N,rcvbase-1] ! ACK(n) otherwise: ! ignore receiver
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Transport Layer 3-54 Selective repeat in action
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lec11b - Performance of rdt3.0 ! ! rdt3.0 works, but...

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