Chapter 10 Congestion Control in Data Networks and Internets 1 Chapter 10 Congestion Control in Data Networks and Internets

Chapter 10 Congestion Control in Data Networks and Internets 2 Introduction Congestion occurs when number of packets transmitted approaches network capacity Objective of congestion control: –keep number of packets below level at which performance drops off dramatically

Chapter 10 Congestion Control in Data Networks and Internets 3 Queuing Theory Data network is a network of queues –Packet arrive, they are stored in the input buffer of the corresponding port –The node examines each incoming packet, makes a routing decision, moves the packet to the appropriate output buffer –This is statistical time division multiplexing If arrival rate > transmission rate then queue size grows without bound and packet delay goes to infinity

Chapter 10 Congestion Control in Data Networks and Internets 4 Congestion in Data Networks and Internets Congestion: the number of packets being transmitted through a network begins to approach the packet- handling capacity of the network. Input and output queues at switch or router  Any given node has a number of I/O ports attached to it.  There are two buffers at each port, one to accept arriving packets, and one to hold packets that are waiting to depart.  Buffer size: fixed-size or variable-size  If packets arrive too fast for the node to process them  input buffer overflow  If packets arrive too fast than packets can be cleared from the outgoing buffers  output buffer overflow

Chapter 10 Congestion Control in Data Networks and Internets 5 At Saturation Point, 2 Strategies Discard any incoming packet if no buffer available Saturated node exercises flow control over neighbors –May cause congestion to propagate throughout network

Chapter 10 Congestion Control in Data Networks and Internets 6 Ideal Performance I.e., infinite buffers, no overhead for packet transmission or congestion control Throughput increases with offered load until full capacity Packet delay increases with offered load approaching infinity at full capacity Power = throughput / delay Higher throughput results in higher delay

Chapter 10 Congestion Control in Data Networks and Internets 7 Assumption : buffer is infinite size Power = throughput / delay Delay: propagation delay (from SA to DA) + processing delay (at each node) + queuing delay (variable delay)

Chapter 10 Congestion Control in Data Networks and Internets 8 Practical Performance buffers are finite With no congestion control, increased load eventually causes moderate congestion: throughput increases at slower rate than load Further increased load causes packet delays to increase and eventually throughput to drop to zero

Chapter 10 Congestion Control in Data Networks and Internets 9  In practice, buffers are finite  If congestion, the exchange of control signals for the congestion consume network capacity  Point A: throughput increases rate is slower than the rate of load increase (have control signals)  Point B: buffer overflow, packet discard, retransmission packet increase

Chapter 10 Congestion Control in Data Networks and Internets 10 Congestion Control Backpressure –Request from destination to source to reduce rate –Choke packet: ICMP Source Quench Implicit congestion signaling –Source detects congestion from transmission delays (ACK packet incurs long delay) –Packets are discarded (time out occurs) –  reduces flow

Chapter 10 Congestion Control in Data Networks and Internets 11 Explicit congestion signaling Direction –Backward the congestion avoidance procedures should be initiated in the opposite direction of the received packet –Forward the congestion avoidance procedures should be initiated in the same directions the received packet. The information may echo to the source station with congestion packet or with window size (TCP) Categories –Binary a bit is set in a data packet to indicate the congestion. The source receives a binary congestion indication, it reduces its traffic flow –Credit-based like TCP slice window size, the destination allows the sender an explicit credit that the sender only can send the amount of the credit packets –rate-based provide an explicit data-rate limit to the source. The source may transmit data at a rate up to the set limit. To control congestion, any node along the path of the connection can reduce the data rate limit in a control message to the source.

Chapter 10 Congestion Control in Data Networks and Internets 12 Mechanisms for congestion control

Chapter 10 Congestion Control in Data Networks and Internets 13 Traffic Management Design issues related to congestion control – when a node is saturated and must discard packets, the discard policy may consider following issues Fairness –Various flows suffer from congestion equally –Last-in-first-discarded may not be fair Quality of Service –We might wish to treat different traffic flows differently –Voice, video: delay sensitive, loss insensitive –File transfer, mail: delay insensitive, loss sensitive –Interactive computing: delay and loss sensitive Reservations –One way to avoid congestion and to provide assured service to applications is to use a reservation scheme –The network agrees to give a defined QoS so long as the traffic flow is within contract parameters –Policing: excess traffic discarded or handled on best-effort basis