The network has smoothed out the burst of data

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Unformatted text preview: TCP (Windows) Hear about congestion early Need to infer congestion Router indication TCPs with Explicit Congestion Notification Hear about congestion early Require router support 45 20 11/12/13 TCP Tahoe/Reno •  Avoid conges7on collapse without changing routers (or even receivers) •  Idea is to fix 7meouts and introduce a conges7on window (cwnd) over the sliding window to limit queues/loss •  TCP Tahoe/Reno implements AIMD by adap7ng cwnd using packet loss as the network feedback signal 51 TCP Tahoe/Reno (2) •  TCP behaviors we will study: –  ACK clocking –  Adap7ve 7meout (mean and variance) –  Slow ­start –  Fast Retransmission –  Fast Recovery •  Together, they implement AIMD 52 21 11/12/13 Introduc7on to Computer Networks TCP Ack Clocking (§6.5.10) Computer Science & Engineering Sliding Window ACK Clock •  Each in ­order ACK advances the sliding window and lets a new segment enter the network –  ACKs “clock” data segments 20 19 18 17 16 15 14 13 12 11 Data Ack 1 2 3 4 5 6 7 8 9 10 57 22 11/12/13 Benefit of ACK Clocking •  Consider what happens when sender injects a burst of segments into the network Queue Fast link Slow (bo`leneck) link Fast link 58 Benefit of ACK Clocking (2) •  Segments are buffered and spread out on slow link Segments “spread out” Fast link Slow (bo`leneck) link Fast link 59 23 11/12/13 Benefit of ACK Clocking (3) •  ACKs maintain the spread back to the original sender Slow link Acks maintain spread 60 Benefit of ACK Clocking (4) •  Sender clocks new segments with the spread –  Now sending at the bo`leneck link without queuing! Segments spread Queue no longer builds Slow link 61 24 11/12/13 Benefit of ACK Clocking (4) •  Helps the network run with low levels of loss and delay! •  The network has smoothed out the burst of data segments •  ACK clock transfers this smooth 7ming back to the sender •  Subsequent data segments are not sent in bursts so do not queue up in the network 62 Introduc7on to Computer Networks TCP Slow Start (§6.5.10) Computer Science & Engineering 25 11/12/13 TCP Startup Problem •  We want to quickly near the right rate, cwndIDEAL, but it varies greatly –  Fixed sliding window doesn’t adapt and is rough on the network (loss!) –  AI with small bursts adapts cwnd gently to the network, but might take a long 7me to become efficient 67 Slow ­Start Solu7on •  Start by doubling cwnd every RTT Window (cwnd) –  Exponen7al growth (1, 2, 4, 8, 16, …) –  Start slow, quickly reac...
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