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03 Congestion control algorithms

3 min read

  • 1. Introduction
  • 2. General Principles of Congestion
    • [[#2. General Principles of Congestion#What Causes Congestion?|What Causes Congestion?]]
    • [[#2. General Principles of Congestion#Effects of Congestion|Effects of Congestion]]
  • 3. Congestion Prevention Policies
    • [[#3. Congestion Prevention Policies#A. Open-Loop Congestion Control (Prevention Before It Happens)|A. Open-Loop Congestion Control (Prevention Before It Happens)]]
      • [[#A. Open-Loop Congestion Control (Prevention Before It Happens)#Methods:|Methods:]]
    • [[#3. Congestion Prevention Policies#B. Closed-Loop Congestion Control (After Congestion Occurs)|B. Closed-Loop Congestion Control (After Congestion Occurs)]]
      • [[#B. Closed-Loop Congestion Control (After Congestion Occurs)#Methods:|Methods:]]
  • 4. Load Shedding
    • [[#4. Load Shedding#Definition:|Definition:]]
    • [[#4. Load Shedding#How It Works:|How It Works:]]
    • [[#4. Load Shedding#Example:|Example:]]
    • [[#4. Load Shedding#Advantages:|Advantages:]]
    • [[#4. Load Shedding#Disadvantages:|Disadvantages:]]
  • 5. Jitter Control
    • [[#5. Jitter Control#Definition:|Definition:]]
    • [[#5. Jitter Control#Jitter Control Techniques:|Jitter Control Techniques:]]
    • [[#5. Jitter Control#Example:|Example:]]
  • 6. Summary

Congestion Control in Networks

1. Introduction

Network congestion occurs when too many packets try to pass through a network, leading to packet loss, delays, and reduced performance. Congestion control mechanisms help prevent, detect, and resolve congestion to ensure efficient data transmission.

2. General Principles of Congestion

What Causes Congestion?

  1. High Traffic Load – More packets than the network can handle.
  2. Insufficient Bandwidth – Data transmission exceeds network capacity.
  3. Slow Processing – Routers or switches take too long to process packets.
  4. Packet Collision – Data packets collide and must be resent.
  5. Queuing Delays – Too many packets waiting in buffers.

Effects of Congestion

  • Increased latency (delay).
  • Packet loss (data never reaches its destination).
  • Low throughput (reduced network performance).

3. Congestion Prevention Policies

A. Open-Loop Congestion Control (Prevention Before It Happens)

  • Controls traffic flow before congestion occurs.
  • Does not react to congestion directly but prevents it.

Methods:

  1. Traffic Shaping – Limits the rate of data transmission.
    • Example: Leaky Bucket and Token Bucket algorithms.
  2. Admission Control – Restricts new connections if the network is busy.
  3. Packet Prioritization – Important packets get higher priority.
    • Example: QoS (Quality of Service) in VoIP.
  4. Load Balancing – Distributes traffic across multiple routes.
    • Example: CDN (Content Delivery Network) for web services.

B. Closed-Loop Congestion Control (After Congestion Occurs)

  • Detects congestion and adjusts the traffic flow dynamically.

Methods:

  1. Backpressure – Temporarily stops data flow when congestion is detected.
  2. Choke Packets – Router sends a warning (choke packet) to slow down traffic.
  3. Explicit Congestion Notification (ECN) – Marks packets to indicate congestion.
  4. Window Adjustment – TCP reduces the window size to control traffic.

4. Load Shedding

Definition:

Load shedding is a method of reducing congestion by discarding low-priority packets when the network is overloaded.

How It Works:

  • When the buffer is full, routers drop non-essential packets first.
  • Critical data (e.g., emergency messages, VoIP calls) is protected.

Example:

  • Streaming services like Netflix drop some video frames if congestion occurs, preventing buffering delays.

Advantages:

✔ Keeps the network responsive during congestion.
✔ Prevents total network failure.

Disadvantages:

✘ Some data is lost permanently.
✘ May affect low-priority services (e.g., file downloads).

5. Jitter Control

Definition:

Jitter is the variation in packet arrival time. High jitter causes choppy audio in VoIP and buffering in video streaming.

Jitter Control Techniques:

  1. Packet Buffers – Store packets temporarily to ensure smooth delivery.
  2. Traffic Shaping – Uses algorithms like Token Bucket to regulate packet flow.
  3. Priority Scheduling – Assigns higher priority to real-time packets (e.g., VoIP).
  4. Network Optimization – Uses faster routing paths to reduce delays.

Example:

  • Zoom and Microsoft Teams use jitter buffers to smooth video calls.

6. Summary

  • Congestion happens when network traffic exceeds capacity.
  • Congestion prevention methods include traffic shaping, prioritization, and admission control.
  • Load shedding drops low-priority packets to keep the network stable.
  • Jitter control ensures smooth transmission for real-time applications.

Would you like a detailed example of TCP congestion control (AIMD, Slow Start, etc.)?

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