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Variety of TCP1－Tahoe Internet Engineering
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Window Control Adjust transmission rate by changing Window Size
Window Size = １segment Window Size = ４ segment Sequence number Sequence number 1 2 3 1 2 3 4 5 6 7 8 9 10 11 12 Sender Sender Data Packet Data Packet ACK ACK Receiver Receiver 2 3 4 2 3 4 5 6 7 8 9 10 Next is Next is ACK number ACK number
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Window Size Control Rule 1 Rule 2
Sent window size is the minimum of window size which calculated by sender (congestion window size) and buffer size of receiver (maximum window size) Sent window size = min (congestion window size, maximum window size) Rule 2 Congestion window increases until network congestion occurs Since network state is unknown when start sending data, window size is set to a small value and increases rapidly by time If the window size is large enough, it increase little by little If congestion occurs, it decreases immediately
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TCP ― The change of Window size
Receiver’s window size (rwnd) = 8 segments Ideally Case Actuality Adjust transmission rate by changing window size Slow Start phase Congestion Avoidance phase RTT: Round Trip Time RTT Sender Theoretical maximum throughput [Mbps] Data packet ACK rwnd [bytes] RTT [s] Receiver ※ byte = 8 bits
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Slow Start Phase Sending a large amount data from startNetwork will congest rapidly Window Size 1 2 4 8 16 Sender Data Packet ACK Receiver However, window size will increase exponentially How to cope this problem？
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Congestion Avoidance Phase
If congestion window exceeds a threshold (ssthresh: slow start threshold) Window Size 1 2 4 8 9 10 11 Sender Data Packet ACK Receiver In this case, ssthresh = 8 Window size increases linearly
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Transition of Window Size
Congestion Window Calculated by sender Window Size Slow start threshold Exponentially increase Linearly increase Maximum Window Size Send Window Size Time Slow Start Congestion Avoidance Rule 1: Send Window Size = min( Congestion Window Size, Maximum Window Size)
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Transition of Window Size
Congestion Window Calculated by sender Window Size Window Size Slow start threshold Maximum Window Size Send Window Size Time Slow Start Congestion Avoidance Rule 1: Send Window Size = min( Congestion Window Size, Maximum Window Size)
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Network Congestion Case
Receive three duplicate ACKs Time Out Packet was dropped ? Network is congested？ Congestion window size is decrease to 1Slow start phase Ssthresh is changed to a half of send window size Window Sizeｃ = ssthresh Window Sizeｃ ssthresh (old) ssthresh (old) = ssthresh Time Time
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Summarize of TCP Window Control (Tahoe)
Window Size Maximum Window Size Packet loss Packet loss Packet loss ssthresh Time (retransmission) (retransmission)
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Experiment 7 4 FTP/TCP 2 3 1 5 25Mbps 5ms 25Mbps 5ms FTP Download
4 FTP/TCP 25Mbps 5ms 25Mbps 5ms 2 3 Bandwidth: Bw Delay: d 1 5 FTP Download TCP Agent TCPSink Agent Network Router Network
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