Variety of TCP1－Tahoe Internet Engineering

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

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

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

Slow Start Phase Sending a large amount data from startNetwork 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？

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

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)

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)

Network Congestion Case Receive three duplicate ACKs Time Out Packet was dropped ? Network is congested？ Congestion window size is decrease to 1Slow start phase Ssthresh is changed to a half of send window size Window Sizeｃ = ssthresh Window Sizeｃ ssthresh (old) ssthresh (old) = ssthresh Time Time

Summarize of TCP Window Control (Tahoe) Window Size Maximum Window Size Packet loss Packet loss Packet loss ssthresh Time (retransmission) (retransmission)

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