1. Spring 2000John Kristoff1 Congestion Control Computer Networks

2. Spring 2000John Kristoff2 Where are we?

3. Spring 2000John Kristoff3 Recall zData Link Layer yLink level specific transmission zNetwork Layer yEnd-to-End host addressing and routing zTransport Layer yEnd-to-End application multiplexing and message flow-control zThe expert: Sally Floyd http://www.aciri.org/floyd/

4. Spring 2000John Kristoff4 Note Flow control is a subset of congestion control. The former attempts to properly match the rate of the sender with that of the network and receiver. The later deals with the sustained overload of intermediate network elements such as internetwork routers.

5. Spring 2000John Kristoff5 Congestion Collapse zAs the network load increases, packet drops and thus packet retransmissions increase zFragments dropped are especially annoying, the remaining fragments get sent, but cannot be used zAs retransmissions increase, less actual work gets done

6. Spring 2000John Kristoff6 Some Congestion Fixes zWhen congestion increases, slow down! yAdditive Increase, Multiplicative Decrease is used in TCP zSetup reservations or service classes yPackets failing to adhere to their class or reservation are simply discarded or put onto a low priority queue/link zDiscover end-to-end MTU if fragments are getting dropped

7. Spring 2000John Kristoff7 Fairness zEqual share bandwidth to end stations zFair share based on application zFair share based on timeliness of data zFair share based on value of data zFair share based on price paid z...and so on

8. Spring 2000John Kristoff8 Active Congestion Control Mechanisms zEligible discard zQueue management zNetwork Signaling and Notification zEnd station avoidance zClass of service signaling zQuality of service reservations

9. Spring 2000John Kristoff9 Eligible Discard zFrames, cells or packets are marked according to a drop priority zSource or edge intermediate device may mark based on some policy ywatermark/threshold reached ydata type ysource ydestination ycost zUsually implemented at data link or network layer

10. Spring 2000John Kristoff10 Eligible Discard Illustrated

11. Spring 2000John Kristoff11 Queue Management zFirst in, first dropped (FIFO) zTail drop (LIFO) yLeaky bucket yToken bucket zRandom early detection (RED) zWeighted Fair Queueing zUsually implemented in intermediate devices such as routers and switches

12. Spring 2000John Kristoff12 First In, First Out Illustrated zQueue pointers need to be updated zSender learns of drop sooner

13. Spring 2000John Kristoff13 Last In, First Out Illustrated zSimple - no queue pointers to update zSource cannot react as quick

14. Spring 2000John Kristoff14 Leaky Bucket Illustrated zFrom Tanenbaum Figure 5-24, graphic will print to a Postscript printer

15. Spring 2000John Kristoff15 Token Bucket Illustrated zFrom Tanenbaum Figure 5-26, graphic will print to a Postscript printer

16. Spring 2000John Kristoff16 RED Illustrated zProbability marking applied to each packet based on queue length, packet being dropped

17. Spring 2000John Kristoff17 Weighted Fair Queueing

18. Spring 2000John Kristoff18 Network Signaling and Notification zAlso called choke packets zIn Frame Relay yForward Explicit Congestion Notification (FECN) yBackward Explicit Congestion Notification (BECN) yBit in frame set zExperimental Internet mechanism yExplicit Congestion Notification (ECN) yBits set in packets to hosts

19. Spring 2000John Kristoff19 End Station Avoidance zAlso called end-to-end control zTCP ySlow start yCongestion avoidance yFast Retransmit yFast Recovery

20. Spring 2000John Kristoff20 Class of Service Signaling zPackets marked to a particular traffic class zIEEE 802.1p zDifferentiated Services (DiffServ) zRe-defines IP Type of Service (ToS) bit fields zAsynchronous Transfer Mode

21. Spring 2000John Kristoff21 Quality of Service Reservations zResource ReSerVation Protocol yReserve resources in routers yRequires stateful path zAsynchronous Transfer Protocol (ATM)