Congestion Control

Introduction

Congestion Control is a mechanism used to prevent and reduce network congestion by controlling the amount of data entering the network. Its main objective is to ensure that the traffic load on the network does not exceed the network's capacity, thereby maintaining efficient and reliable communication.

In packet-switched networks such as the Internet, congestion occurs when too many packets are transmitted into the network at the same time, causing routers and switches to become overloaded.

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Definition

Congestion Control is the set of techniques and mechanisms used to regulate the flow of packets in a network so that the network load remains below its capacity, preventing performance degradation and packet loss.

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What is Network Congestion?

A network becomes congested when the number of packets entering the network is greater than the number of packets the network can process or forward.

In simple terms,

Network Load > Network Capacity

When this happens:

• Routers become overloaded.
• Buffers (queues) become full.
• Packets are delayed or dropped.
• Network performance decreases.

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Why Does Congestion Occur?

Congestion occurs because routers and switches have limited buffer (queue) space.

Each router maintains:

• Input Queue – Stores incoming packets before processing.
• Output Queue – Stores packets waiting to be transmitted.

If packets arrive faster than the router can process or forward them, the queues begin to fill up.

Incoming Packets
       │
       ▼
+------------------+
|  Input Queue     |
+------------------+
        │
     Router
   Processing
        │
+------------------+
| Output Queue     |
+------------------+
        │
        ▼
Next Router

When these queues become full, congestion occurs.

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Real-Life Analogy

Consider a busy highway during rush hour.

• Cars represent packets.
• The highway represents the network.
• Toll booths or traffic signals represent routers.

If more cars enter the highway than it can handle, traffic slows down, vehicles form long queues, and traffic jams occur.

Similarly, in a computer network, when too many packets are sent, routers become congested, resulting in delays and packet loss.

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Effects of Congestion

When congestion occurs:

• Packets experience longer delays.
• Buffers overflow, causing packet loss.
• Retransmissions increase, creating even more traffic.
• Network throughput decreases.
• Overall network performance degrades.

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Congestion at the Transport Layer

Although congestion actually occurs inside the network layer (at routers and switches), its effects are experienced at the transport layer.

For example, in TCP:

• Packet loss due to congestion is detected.
• TCP reduces its transmission rate.
• Transmission speed is increased again only when the network becomes less congested.

Thus, TCP implements its own congestion control mechanism to avoid overloading the network.

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Congestion Control vs Flow Control

Feature	Flow Control	Congestion Control
Purpose	Prevents the sender from overwhelming the receiver	Prevents the network from becoming overloaded
Concern	Receiver's processing capacity	Network capacity
Operates Between	Sender and Receiver	Entire Network
Main Problem Solved	Receiver buffer overflow	Router queue overflow

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Key Points to Remember

• Congestion Control prevents excessive traffic in the network.
• Congestion occurs when network load exceeds network capacity.
• Routers and switches become congested because their input and output queues are limited.
• Congestion leads to packet loss, increased delay, and reduced throughput.
• Although congestion occurs in the network layer, transport protocols like TCP implement congestion control mechanisms to minimize its effects.

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Summary

Congestion control is a mechanism that regulates the amount of traffic entering a packet-switched network so that the network operates within its capacity. Congestion occurs when routers receive packets faster than they can process them, causing their input and output buffers to overflow. This results in increased delays, packet loss, and poor network performance. Reliable transport protocols such as TCP detect signs of congestion and adjust their transmission rate to reduce network load and improve overall communication efficiency.