13-CongestioninDataNetworks

13-CongestioninDataNetworks - William Stallings Data and...

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William Stallings Data and Computer Communications 8 th Edition Chapter 13 Congestion in Data Networks
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Key Points congestion occurs when number of packets  traveling throught a network approaches  capacity the lack of flow control mechanisms built into  ATM and frame relay protocols makes  congestion control difficult ATM networks establish a traffic contract with  each user ATM networks monitor the cell flow from each  incoming source and may discard or label cells  that exceed the agreed traffic flow
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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
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Queues at a Node
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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 too fast to be routed, or to be output,  buffers will fill Can discard packets Can use flow control Can propagate congestion through network
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Interaction of Queues
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Ideal Network Utilization
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Practical Performance Ideal assumes infinite buffers and no overhead Buffers are finite Overheads occur in exchanging congestion  control messages
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Effects of Congestion - No Control
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Mechanisms for Congestion Control
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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
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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
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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   Link Acces Procedure for Frame Mode Bearer Services
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13-CongestioninDataNetworks - William Stallings Data and...

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