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

What Is Congestion? Data network is a network of queues If the arrival rate is larger than the transmission rate queues grow to infinity If the arrival rate is smaller than the transmission rate queues grow as the arrival rate approaches to the tranmission rate Generally 80% utilization is critical growth in queue = more delay queue may overflow since it is of finite capacity Congestion control aims to keep number of packets below level at which performance falls off dramatically

Queues at a Node

Effects of Congestion Packets arriving are stored at input buffers Routing decision made Packet moves to appropriate output buffer Packets queued for output transmitted as fast as possible If packets arrive to fast to be routed or arrive faster than can be trnamistted, buffers will fill Precautions discard packets flow control over neighbors

Effects of Congestion Flow control can propagate congestion through network

Ideal Network Performance Performance under the assumptions of infinite buffers (queues) and zero overhead for congestion control

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

Practical Performance Point A: Packet discard starts at heavily congested nodes re-routing and congestion control messages cause overhead Point B: PANIC Retransmission of discarded packets causes heavier traffic More buffers overflow delay increase even transmitted packets are retransmitted due to timeout

Mechanisms for Congestion Control

Backpressure If node becomes congested it can slow down or halt flow of packets from its neighbors That may cause longer queues at neighbors and they do the same Propagates back to source Used in connection oriented networks that allow hop by hop congestion control (e.g. X.25) Not used in ATM and frame relay

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 messages Sent for every discarded packet, or when congestion is anticipated

Implicit Congestion Signaling Transmission delay may increase with congestion Packet may be discarded Source can detect these as implicit indications of congestion Used in TCP and frame relay

Explicit Congestion Signaling Network alerts end systems of increasing congestion End systems take steps to reduce offered load Backward notifies the source about congestion Forward notifies the destination about congestion Could be added to data packets Mechanisms binary notification credit based rate based

Traffic Management Fairness Quality of service Reservations “discard the last received” is not fair multiple queues for multiple connections Quality of service different priorities for different connection types (voice, video, email, etc.) Reservations congestion avoidance mechanism traffic contract between user and network network makes necessary reservations to keep its promise user tries not to overuse the reserved capacity