computer networks notes on chapter 13, Essays (university) of Computer Networks

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William Stallings

Data and Computer

Communications

7 th^ Edition

Chapter 1 3

Congestion in Data Networks

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

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

Interaction of Queues

Practical Performance

• Ideal assumes infinite buffers and no overhead

• Buffers are finite

• Overheads occur in exchanging congestion

control messages

Effects of

Congestion -

No 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

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

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

Categories of Explicit Signaling

• Binary

—A bit set in a packet indicates congestion

• Credit based

—Indicates how many packets source may send —Common for end to end flow control

• Rate based

—Supply explicit data rate limit —e.g. ATM

Congestion Control in Packet

Switched Networks

• Send control packet to some or all source nodes

—Requires additional traffic during congestion

• Rely on routing information

—May react too quickly

• End to end probe packets

—Adds to overhead

• Add congestion info to packets as they cross

nodes

—Either backwards or forwards

Frame Relay

Congestion Control

• Minimize discards
• Maintain agreed QoS
• Minimize probability of one end user monopoly
• Simple to implement — Little overhead on network or user
• Create minimal additional traffic
• Distribute resources fairly
• Limit spread of congestion
• Operate effectively regardless of traffic flow
• Minimum impact on other systems
• Minimize variance in QoS

Traffic Rate Management

• Must discard frames to cope with congestion

—Arbitrarily, no regard for source —No reward for restraint so end systems transmit as fast as possible —Committed information rate (CIR)

• Data in excess of this liable to discard
• Not guaranteed
• Aggregate CIR should not exceed physical data rate

• Committed burst size

• Excess burst size

Operation of CIR