CSC 581 Communication Networks II Chapter 7c: Congestion Control Dr. Cheer-Sun Yang

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

3 Queues at a Node

4 Effects of Congestion Packets arriving are stored at input buffers Routing decision made Packet moves to output buffer Packets queued for output transmitted as fast as possible –Statistical time division multiplexing If packets arrive to fast to be routed, or to be output, buffers will fill Can discard packets Can use flow control –Can propagate congestion through network

5 Interaction of Queues

6 Ideal Performance

7 Practical Performance Ideal assumes infinite buffers and no overhead Buffers are finite Overheads occur in exchanging congestion control messages

8 Effects of Congestion - No Control

9 Mechanisms for Congestion Control

10 Congestion Control Techniques Packet elimination (implicit congestion signaling) Flow control: using backpressure to regulate the flow. Buffer allocation: used with virtual circuit (connection-oriented) networks. Buffer size is considered a credit-based technique. Choke packets

11 Implicit Congestion Signaling Transmission delay may increase with congestion Packet may be discarded Source can detect these as implicit indications of congestion Useful on connectionless (datagram) networks –e.g. IP based (TCP includes congestion and flow control - see chapter 17) Used in frame relay LAPF

12 Flow Control:Backpressure If node becomes congested it can slow down or halt flow of packets from other nodes May mean that other nodes have to apply control on incoming packet rates Propagates back to source Can restrict to logical connections generating most traffic Used in connection oriented that allow hop by hop congestion control (e.g. X.25) Not used in ATM nor frame relay Only recently developed for IP

13 Explicit Congestion Signaling Network alerts end systems of increasing congestion End systems take steps to reduce offered load Backwards –Congestion avoidance in opposite direction to packet required Forwards –Congestion avoidance in same direction as packet required

14 Categories of Explicit Signaling Binary –A bit set in a packet indicates congestion Credit based: such as a buffer size –Indicates how many packets source may send –Common for end to end flow control Rate based –Supply explicit data rate limit –e.g. ATM

15 Choke Packet Control packet –Generated at congested node –Sent to source node –e.g. ICMP source quench From router or destination Source cuts back until no more source quench message Sent for every discarded packet, or anticipated Rather crude mechanism

16 Two kinds of deadlocks Store-and-forward deadlock Reassembly deadlock

17 A C C’s buffers containing Packets destined for A are full B’s buffers containing Packets destined for C are full A’s buffers containing Packets destined for B are full B

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20 Public Data Networks: The X series protocols X.25 X.3 X.28 X.29

21 Packet-Switched Network Modes Virtual circuits Datagram

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26 Example of Packet Switching Protocol: X Interface between host and packet switched network Almost universal on packet switched networks and packet switching in ISDN Defines three layers –Physical –Link –Packet

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28 X.21 – Physical Layer Interface between attached station and link to node Data terminal equipment DTE (user equipment) Data circuit terminating equipment DCE (node) Uses physical layer specification X.21 Reliable transfer across physical link Sequence of frames

29 LAPB – Data Link Layer Link Access Protocol Balanced (LAPB) –Subset of HDLC –see chapter 5

30 X.25 Network Layer A PDU is called a packet. External virtual circuits Logical connections (virtual circuits) between subscribers

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32 X.25 Use of Virtual Circuits

33 Virtual Circuit Service Virtual Call –Dynamically established Permanent virtual circuit –Fixed network assigned virtual circuit

34 Packet Format

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36 X.25 Packet Types Refer to Table 7.8

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38 Virtual Call

Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks Congestion Figure 7.50

Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 40 Offered load Throughput Controlled Uncontrolled Figure 7.51

Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 41 Time Bits per second Peak rate Average rate Figure 7.52

Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 42 Water drains at a constant rate Leaky bucket Water poured irregularly Figure 7.53

Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 43 Arrival of a packet at time t a X’ = X - (t a - LCT) X’ < 0? X’ > L? X = X’ + I LCT = t a conforming packet X’ = 0 Nonconforming packet X = value of the leaky bucket counter X’ = auxiliary variable LCT = last conformance time Yes No Yes No Figure 7.54

Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 44 I L+I Bucket content Time Packet arrival Nonconforming ********* Figure 7.55

Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 45 Time MBS TLI Figure 7.56

Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 46 Tagged or dropped Untagged traffic Incoming traffic Untagged traffic Leaky bucket 1 PCR and CDVT Leaky bucket 2 SCR and MBS Tagged or dropped Figure 7.57

Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 47 Time Kbps Time Kbps 100 Kbps (a) (b) (c) Figure 7.58

Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 48 Incoming traffic Shaped traffic Size N Packet Server Figure 7.59

Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 49 Incoming traffic Shaped traffic Size N Size K Tokens arrive periodically Server Packet Token Figure 7.60

Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 50 b bytes instantly t r bytes per second Figure 7.61

Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 51 A(t) = b+rt R(t) No backlog of packets bRbR b R - r (a) (b) Buffer 1 0 empty t t Figure 7.62

Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 52 Congestion window Round-trip times Slow start Congestion avoidance Congestion occurs Threshold Figure 7.63

Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 53 CBR VBR ABR Total bandwidth Time Figure 7.64

Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 54 SD Data cell Forward RM cell Backward RM cell Switch S = source D = destination Figure 7.65

Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 55 SD Congested switch Data cell EFCI=0 Data cell EFCI=1 RM cell CI=0 RM cell CI=1 Figure 7.66

Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 56 SD Can only support 5 Mbps ER = 5 MbpsER = 10 Mbps ER = 1 Mbps Can only support 1 Mbps ER = 1 Mbps Figure 7.67

57 Suggested Reading Chapter 7.8