1. William Stallings Data and Computer Communications Chapter 12 Congestion in Data Networks

2. What Is Congestion? zCongestion occurs when the number of packets being transmitted through the network approaches the packet handling capacity of the network zCongestion control aims to keep number of packets below level at which performance falls off dramatically zData network is a network of queues zGenerally 80% utilization is critical zFinite queues mean data may be lost

3. Queues at a Node

4. Effects of Congestion zPackets arriving are stored at input buffers zRouting decision made zPacket moves to output buffer zPackets queued for output transmitted as fast as possible yStatistical time division multiplexing zIf packets arrive to fast to be routed, or to be output, buffers will fill zCan discard packets zCan use flow control yCan propagate congestion through network

5. Interaction of Queues

6. Ideal Performance "Power" = Throughput/Delay Throughput (bits/s) Delay (s) Load/MaximumLoad ->

7. Practical Performance zIdeal assumes infinite buffers and no overhead zBuffers are finite zOverheads occur in exchanging congestion control messages

8. Effects of Congestion - No Control

9. Mechanisms for Congestion Control

10. Backpressure zIf node becomes congested it can slow down or halt flow of packets from other nodes zMay mean that other nodes have to apply control on incoming packet rates zPropagates back to source zCan restrict to logical connections generating most traffic zUsed in connection oriented that allow hop by hop congestion control (e.g. X.25) zNot used in ATM nor frame relay zOnly recently developed for IP

11. Choke Packet zControl packet yGenerated at congested node ySent to source node ye.g. ICMP source quench xFrom router or destination xSource cuts back until no more source quench message xSent for every discarded packet, or anticipated zRather crude mechanism

12. Implicit Congestion Signaling zTransmission delay may increase with congestion zPacket may be discarded zSource can detect these as implicit indications of congestion zUseful on connectionless (datagram) networks ye.g. IP based x(TCP includes congestion and flow control - see chapter 17) zUsed in frame relay LAPF

13. Explicit Congestion Signaling zNetwork alerts end systems of increasing congestion zEnd systems take steps to reduce offered load zBackwards yCongestion avoidance in opposite direction to packet required zForwards yCongestion avoidance in same direction as packet required

14. Categories of Explicit Signaling zBinary yA bit set in a packet indicates congestion zCredit based yIndicates how many packets source may send yCommon for end to end flow control zRate based ySupply explicit data rate limit ye.g. ATM

15. Traffic Management zFairness zQuality of service yMay want different treatment for different connections zReservations ye.g. ATM yTraffic contract between user and network

16. Congestion Control in Packet Switched Networks (Explicit) zSend control packet to some or all source nodes yRequires additional traffic during congestion zRely on routing information yMay react too quickly zEnd to end probe packets yAdds to overhead zAdd congestion info to packets as they cross nodes yEither backwards or forwards

17. Cell Delay Variation zFor ATM voice/video, data is a stream of cells zDelay across network must be short zRate of delivery must be constant zThere will always be some variation in transit zDelay cell delivery to application so that constant bit rate can be maintained to application

18. Origins of Cell Delay Variation

19. Traffic Management and Congestion Control Techniques zResource management using virtual paths zConnection admission control zUsage parameter control zSelective cell discard zTraffic shaping

20. Configuration of VCCs and VPCs

21. Connection Admission Control zFirst line of defense zUser specifies traffic characteristics for new connection (VCC or VPC) by selecting a QoS zNetwork accepts connection only if it can meet the demand zTraffic contract yPeak cell rate yCell delay variation ySustainable cell rate yBurst tolerance

22. Usage Parameter Control zMonitor connection to ensure traffic informs to contract zProtection of network resources from overload by one connection zDone on VCC and VPC zPeak cell rate and cell delay variation zSustainable cell rate and burst tolerance zDiscard cells that do not conform to traffic contract zCalled traffic policing

23. Traffic Shaping zSmooth out traffic flow and reduce cell clumping zToken bucket

24. Token Bucket

25. Cell Flow

26. Required Reading zStallings chapter 12