Congestion Control

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Congestion Control

• Tanenbaum 5.3, 6.5
• Peterson Davie Chapter 6

2
Outline

• Example
• What, why, how?
• Why is it a difficult problem?

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Example 1 A Simple Case
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Congestion Control

• Network Layer
• Congestion control point to point
• Transport Layer
• Congestion control end-to-end

Maximum capacity of bottleneck link
Packets Delivered
Objective
Congestion
Congestion Collapse
Packets Sent
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Congestion ControlStrategies

• Open loop
• Prevent congestion by design
• E.g., admission control
• Closed loop
• Monitor some characteristics of system
• Inform key elements that can take action
• Adjust system operation whenever needed

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Impact of Policies on Congestion Control

• Link Layer policies
• Retransmission
• Out-of-order caching
• Acknowledgement
• Flow control

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Impact of Policies on Congestion Control (2)

• Network Layer policies
• Virtual circuit/datagram
• Packet scheduling
• Queue management
• Routing algorithm
• Packet lifetime management

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Impact of Policies on Congestion Control (3)

• Transport Layer policies
• Retransmission
• Out-of-order caching
• Acknowledgement
• Flow control
• Timeout determination

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Congestion Control in Packet Switched Networks

• Monitor some system state variable u
• u may be link utilization, loss rate
• Instantaneous unow
• Average value u computed as
• unew a.ulast (1- a) unow
• 0 lt a lt 1
• Use of thresholds

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Techniques at Link Layer

• DECBit (Warning bit)
• Source Quench packets (choke packets)
• Hop-by-hop choke packets
• Load shedding
• RED