Computer Networks Set 9 Congestion in Data Networks

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

Queues at a Node

Effects of Congestion ÑPackets arriving are stored at input buffers ÑRouting decision made ÑPacket moves to output buffer ÑPackets queued for output transmitted as fast as possible ÑStatistical time division multiplexing ÑIf packets arrive to fast to be routed, or to be output, buffers will fill ÑCan discard packets ÑCan use flow control ÑCan propagate congestion through network

Interaction of Queues

Ideal Performance

Practical Performance ÑIdeal assumes infinite buffers and no overhead ÑBuffers are finite ÑOverheads occur in exchanging congestion control messages

Effects of Congestion - No Control

Congestion control ÑMany policies affect congestion control: acks, retransmits, routing algorithms, Virtual circuits... ÑTwo major families fo congestion control: Open- loop, closed-loop. ÑOpen-loop: Prevent conditions that lead to congestion, e.g. Leacky bucket, token bucket: Traffic shaping. ÑClosed-loop: Deal with congestion when it happens: Use current information about network to deal with congestion.

Mechanisms for Congestion Control

Backpressure ÑIf node becomes congested it can slow down or halt flow of packets from other nodes ÑMay mean that other nodes have to apply control on incoming packet rates ÑPropagates back to source ÑCan restrict to logical connections generating most traffic ÑUsed in connection oriented that allow hop by hop congestion control (e.g. X.25) ÑNot used in ATM nor frame relay ÑOnly recently developed for IP

Choke Packet ÑWarning is either generated by monitoring outgoing line utilization or queue lengths (weighted). ÑControl packet ÑGenerated at congested node ÑSent to source node Ñe.g. ICMP source quench ÑFrom router or destination ÑSource cuts back until no more source quench message ÑSent for every discarded packet, or anticipated ÑRather crude mechanism

Implicit Congestion Signaling ÑTransmission delay may increase with congestion ÑPacket may be discarded ÑSource can detect these as implicit indications of congestion ÑUseful on connectionless (datagram) networks Ñe.g. IP based Ñ(TCP includes congestion and flow control - see chapter 17) ÑUsed in frame relay LAPF

Explicit Congestion Signaling ÑNetwork alerts end systems of increasing congestion ÑEnd systems take steps to reduce offered load ÑBackwards ÑCongestion avoidance in opposite direction to packet required ÑForwards ÑCongestion avoidance in same direction as packet required

Categories of Explicit Signaling ÑBinary ÑA bit set in a packet indicates congestion ÑCredit based ÑIndicates how many packets source may send ÑCommon for end to end flow control ÑRate based ÑSupply explicit data rate limit Ñe.g. ATM

Traffic Management ÑFairness ÑQuality of service ÑMay want different treatment for different connections ÑReservations Ñe.g. ATM ÑTraffic contract between user and network

Congestion Control in Packet Switched Networks ÑSend control packet to some or all source nodes ÑRequires additional traffic during congestion ÑRely on routing information ÑMay react too quickly ÑEnd to end probe packets ÑAdds to overhead ÑAdd congestion info to packets as they cross nodes ÑEither backwards or forwards

Required Reading ÑStallings chapter 12