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Advanced Computer Networks: Lecture 36. This lecture will cover the following: host solutions - TCP congestion control; TCP congestion control strategy; initialization of congestion window; TCP congestion window trace; congestion avoidance;...
CS716 Advanced Computer Networks By Dr. Amir Qayyum 1 Lecture No. 36 Congestion Control • Basics: problem, terminology, approaches, metrics • Solutions – Routerbased: queuing disciplines – Hostbased: TCP congestion control • Congestion avoidance – DECbit – RED gateways • Quality of service Host Solutions: TCP Congestion Control • Host has very little information – Assumes besteffort network – Acts independently of other hosts • Host infers congestion – From synchronization feedback – e.g. dropped packet timeouts, duplicate ACK’s – Loss on wired lines rarely due to transmission error • Host acts – Reduce transmission rate below congestion threshold – Continuously monitor network for signs of congestion TCP Congestion Control • Add notion of congestion window • Effective sliding window is smaller of – Remote advertised window (flow control) – Local congestion window (congestion control) • Changes in congestion window size – Slow increases to absorb new bandwidth – Quick decreases to eliminate congestion TCP Congestion Control Strategy • Selfclocking – Send data only when outstanding data ACK’d – Equivalent to send window limitation mentioned • Growth – Add one Maximum Segment Size (MSS) per congestion window of data ACK’d – It’s really done this way, at least in Linux: see tcp_cong_avoid in tcp_input.c. Actually, every ack for new data is treated as an MSS ACK’d… any problems with that? – Known as additive increase TCP Congestion Control Strategy • Decrease – Cut window in half when timeout occurs – In practice, set window = window/2 – Known as multiplicative decrease • Additive Increase, Multiplicative Decrease (AIMD) TCP Congestion Control • AIMD sawtooth trace TCP Congestion Control • AIMD fairness for two competing flows – Slope of 1 for additive increase – Proportional decrease (towards origin) for multiplicative decrease Initialization of Congestion Window • Congestion window should start small • Avoid congestion due to new connections • Start at 1 MSS, reset to 1 MSS with each timeout (note that timeouts are coarse grained, ~1/2 seconds) • Known as slow start 10 Initialization of Congestion Window • To make up for slow start, ramp up quickly • Maintain threshold window size • Use multiplicative increase – When congestion window is smaller than threshold – Double window for each window ACK’d 11 Initialization of Congestion Window • Threshold value – Initially set to maximum window size – Set to 1/2 of current window on timeout • In practice, increase congestion window by 1 MSS for each ACK of new data (or N bytes for N bytes) 12 TCP Congestion Control • Coarsegrained timeouts lead to idle periods … • Solution: fast retransmission – Send ACK for each segment received – When duplicate ACK’s received • Resend lost segment immediately • Do not wait for timeout • In practice, retransmit on 3rd duplicate – Fast recovery • When fast retransmission occurs, skip slow start • Congestion window becomes 1/2 of the previous • Start additive increase immediately 13 TCP Congestion Window Trace 14 Congestion Avoidance Flight Planning for Your Air Travel • • • • Planning a vacation for your semester break ? Trying for a trip to scenic Northern Areas No way to go (route is too congested) Delay your program till next semester Islamabad Airport CASE Flights are FULL Peshawer Sakardu 16 Congestion Avoidance • Control vs Avoidance – Control: minimize impact of congestion when it occurs – Avoidance: avoid producing congestion • In terms of operating point limits: 17 .. .Lecture? ?No.? ?36 Congestion Control • Basics: problem, terminology, approaches, metrics • Solutions – Routerbased: queuing disciplines