Congestion Control

1. Congestion Control Tanenbaum 5.3, 6.5

2. What? Why? • Congestion occurs when • there is no reservation • Demand exceeds Resources • If congestion persists, resources must be increased • If congestion happens in short bursts, buffers can avoid packet losses to some extent. Congestion Control (A Loss Based Technique: TCP)

3. Congestion Control, How? • Network Layer • Congestion control point to point • Transport Layer • Congestion control end-to-end Maximum capacity of bottleneck link Packets Delivered Objective Congestion Congestion Collapse Packets Sent Congestion Control (A Loss Based Technique: TCP)

4. Network As Black Box Network/ Network Element Load State variable u What can U be? Round trip time (e2e) Throughput (e2e) Packet loss (e2e or p2p) Link utilization (p2p) Queue size (p2p) Congestion Control (A Loss Based Technique: TCP)

5. Congestion Control in Packet Switched Networks • Monitor some system state variable u • u may be link utilization (p2p), loss rate (p2p or e2e) • Instantaneous unow • Average value u computed as: • unew = a.ulast + (1- a) unow • 0 <= a <= 1 • Use of thresholds Congestion Control (A Loss Based Technique: TCP)

6. Congestion Control:Strategies • Open loop • Prevent congestion by design • E.g., resource reservation, admission control • Closed loop • Monitor some characteristics of system • Inform key elements that can take action • Adjust system operation whenever needed Congestion Control (A Loss Based Technique: TCP)

7. End-to-end Congestion Control

8. Network As Black Box Network Packet Loss, Round trip time, or Throughput Window Size Or Rate Congestion Control (A Loss Based Technique: TCP)

9. Packet Loss BasedCongestion Control TCP Congestion Control (Read Van Jacobson, “Congestion Avoidance and Control,” Sigcomm’88)

10. System (Connection) In Equilibrium Time Bandwidth Slow long-haul link High Speed LANs Congestion Control (A Loss Based Technique: TCP)

11. Keeping A System in Equilibrium • Suppose that a system is in equilibrium, how to keep it there? • If there are no losses, easy: just send a packet whenever an ack is received. • If losses occur, we need to distinguish between delayed packets (link capacity shortage) and lost packets need an accurate retransmission timer. Congestion Control (A Loss Based Technique: TCP)

12. How to Get A System To Equilibrium? • At the beginning, no packet is sent yet. • Propose a strategy. Let us discuss some strategies: Strategy 1: Blast a guessed number of packets. Strategy 2: Start with sending one packet (WS=1) and wait • when ack is received, send two packets (WS=2) • For each ack, send one packet, plus one packet when all window is acked (WS=3) Congestion Control (A Loss Based Technique: TCP)

13. How to Get A System To Equilibrium? • Strategy 2 may be inefficient • Consider a download of a 21 KB file using a 1 KB segment size over a 45 Mbps link with 25ms one way latency. • Compute the efficiency of such a transfer Congestion Control (A Loss Based Technique: TCP)

14. How to Get A System To Equilibrium? (Strategy 2) Congestion Control (A Loss Based Technique: TCP)

15. A Faster Strategy: Slow Start • For each acked packet, increase window by 1 Congestion Control (A Loss Based Technique: TCP)

16. Slow-Start to Get A System To Equilibrium? (Strategy 2) Congestion Control (A Loss Based Technique: TCP)

17. A System in Equilibrium MayUNDERUTILIZE the Link Capacity • If losses do not happen, this is an indication that link capacity may not be fully utilized • What to do? • Slightly INCREASE the window size after a complete window of data made it safely through the network (this happens every round trip time.)  Congestion avoidance phase Congestion Control (A Loss Based Technique: TCP)

18. TCP Congestion Avoidance and Control • Start with congestion window=1 (CWND) • Use slow start until CWND reaches the threshold sstresh (sstresh is half the congestion window vale reached when last loss occured) • From CWND=sstresh, use congestion avoidance Congestion Control (A Loss Based Technique: TCP)

19. Example CWND Round trip time Congestion Control (A Loss Based Technique: TCP)

20. Code When a packet is acked if cwnd < ssthresh* { /* Slow-Start: increment cwnd every ack*/ cwnd = cwnd + 1; } else { /* Congestion avoidance: increment cwnd every RTT */ cwnd = cwnd + 1/cwnd } When a packet is lost (timeout) ssthresh = cwnd / 2; cwnd = init_cwnd; * ssthresh may be dynamically adjusted at start-up (see Janey Hoe work) Congestion Control (A Loss Based Technique: TCP)

21. Fast Recovery/ Fast Retransmit • When 3rd dupack received: • Fast Recovery: • ssthresh = cwnd / 2; • cwnd = ssthresh + 3; • Fast Retransmit: • Retransmit missing packet without waiting for timeout Congestion Control (A Loss Based Technique: TCP)

22. Example CWND 16 8 4 2 Round trip time Congestion Control (A Loss Based Technique: TCP)

23. TCP Performance: A Simple Derivation • Mathis et. Al: "The Macroscopic Behavior of the TCP Congestion Avoidance Algorithm” • Assume • a very simple loss model: periodical loss with loss rate p • No timeout • Neglect the start-up behavior Congestion Control (A Loss Based Technique: TCP)

24. loss loss loss loss TCP Performance: A Simple Derivation (2) cwnd W Startup Before Stabiliza. W/2 0 (RTT) W/2 W 3W/2 Congestion Control (A Loss Based Technique: TCP)

25. TCP Performance (3) • Facts: • A loss happens whenever packets are sent (periodical loss model). • A loss happens every RTTs • The total number of packets sent every cycle (time between consecutive 2 losses) is the area under the curve. • The area is Congestion Control (A Loss Based Technique: TCP)

26. TCP Performance (4) • Facts (cont’d): • The number of packets delivered per cycle is • Solving, • Throughput = # packets received per cycle/cycle duration • Throughput = (packets per second) Congestion Control (A Loss Based Technique: TCP)

27. Taking Into Account Timeout • Jitendra Padhye et. Al “Modeling TCP Reno Performance: A Simple Model and Its Empirical Validation” . • Wmax : Max window size (flow control) • RTT: round trip time • b = # of packets received by an ack. • p: probability of packet loss • To= Duration of first timeout Congestion Control (A Loss Based Technique: TCP)

28. Shortcomings/Limits of TCP • TCP is not suited for streams => how to exercise TCP like congestion control on streams (UDP)? • TCP, as is, will not scale well to very high speeds => How to adapt TCP to very high speed networks? Congestion Control (A Loss Based Technique: TCP)

29. Impact of Policies on Congestion Control • Link Layer policies • Retransmission • Out-of-order caching • Acknowledgement • Flow control Congestion Control (A Loss Based Technique: TCP)

30. Impact of Policies on Congestion Control (2) • Network Layer policies • Virtual circuit/datagram • Packet scheduling • Queue management • Routing algorithm • Packet lifetime management Congestion Control (A Loss Based Technique: TCP)

31. Impact of Policies on Congestion Control (3) • Transport Layer policies • Retransmission • Out-of-order caching • Acknowledgement • Flow control • Timeout determination Congestion Control (A Loss Based Technique: TCP)

32. Congestion Control in Packet Switched Networks • Monitor some system state variable u • u may be link utilization, loss rate • Instantaneous unow • Average value u computed as: • unew = a.ulast + (1- a) unow • 0 <= a <= 1 • Use of thresholds Congestion Control (A Loss Based Technique: TCP)

33. Techniques at Link Layer • DECBit (Warning bit) • Source Quench packets (choke packets) • Hop-by-hop choke packets • Load shedding • RED Congestion Control (A Loss Based Technique: TCP)