Congestion_Control

Congestion Control

Congestion occurs when network traffic exceeds its capacity, leading to packet loss, increased delay, and reduced throughput. Congestion control mechanisms aim to prevent or mitigate network overload.

Causes of Congestion:

• High Traffic Load: Too many packets sent simultaneously.
• Slow Receivers: If a receiver cannot process data fast enough.
• Bottleneck Links: Low-bandwidth links in the network path.
• Buffer Overflow: Routers dropping packets when queues are full.

Congestion Control vs. Flow Control

Feature	Congestion Control	Flow Control
Scope	Network-wide (prevents traffic overload)	End-to-end (prevents receiver overload)
Mechanism	Adjusts sending rate based on network conditions	Adjusts sending rate based on receiver’s buffer
Example	TCP’s AIMD (Additive Increase, Multiplicative Decrease)	TCP’s Sliding Window

Quality of Service (QoS)

QoS refers to prioritizing certain types of traffic to ensure better performance for critical applications (e.g., VoIP, video streaming).

QoS Parameters:

1. Bandwidth: Minimum guaranteed data rate.
2. Delay: Maximum tolerable latency.
3. Jitter: Variation in delay (critical for real-time apps).
4. Packet Loss: Acceptable loss percentage.

QoS Improving Techniques:

1. Traffic Shaping: Controls the rate of data transmission (e.g., Leaky Bucket, Token Bucket).
2. Packet Scheduling: Prioritizes packets (e.g., Priority Queuing, Weighted Fair Queuing).
3. Resource Reservation: Allocates bandwidth in advance (e.g., RSVP - Resource Reservation Protocol).
4. Admission Control: Rejects new flows if the network is congested.

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Leaky Bucket Algorithm

Concept:

• Models traffic flow as water leaking from a bucket at a constant rate.
• Used for smoothing bursty traffic into a steady stream.

Working:

1. Bucket Analogy:

   • Incoming Packets: Represented as water poured into the bucket.
   • Leak Rate (Output Rate): Fixed, regardless of input burst.
   • Bucket Size: If full, excess packets are discarded or marked.

2. Algorithm Steps:

   • Packets arrive at variable rates.
   • The bucket stores them temporarily.
   • Packets exit at a fixed rate (R).

Advantages:

• Smooths Traffic: Converts bursty traffic into a steady flow.
• Prevents Network Overload: Limits peak data rate.

Disadvantages:

• Does Not Adapt to Bursts: Drops excess packets.
• Fixed Rate: May not be optimal for all applications.

Use Cases:

• VoIP Traffic: Ensures smooth voice transmission.
• Network Traffic Policing: Enforces bandwidth limits.

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4. Token Bucket Algorithm

Concept:

• More flexible than Leaky Bucket.
• Tokens are added to a bucket at a fixed rate.
• A packet can be transmitted only if a token is available.

Working:

1. Bucket Analogy:

   • Tokens: Represent permission to send data.
   • Token Generation Rate (R): Tokens added at fixed intervals.
   • Bucket Capacity (C): Maximum tokens stored.

2. Algorithm Steps:

   • Tokens are added periodically (e.g., 1 token every 1/R seconds).
   • When a packet arrives:
     • If tokens available, it is transmitted, and a token is consumed.
     • If no tokens, the packet is buffered or dropped.
   • Burst Handling: Allows short bursts up to bucket capacity.

Advantages:

• Allows Controlled Bursts: Better for variable traffic.
• More Flexible: Adapts better than Leaky Bucket.

Disadvantages:

• Complexity: Requires token management.
• Not Strictly Rate-Limiting: Bursts can still cause congestion.

Use Cases:

• Video Streaming: Handles variable bitrate traffic.
• Traffic Shaping in ISPs: Manages user bandwidth.

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Leaky Bucket vs. Token Bucket

Feature	Leaky Bucket	Token Bucket
Traffic Shaping	Strictly enforces a fixed rate	Allows controlled bursts
Burst Handling	Drops excess packets	Permits bursts if tokens available
Complexity	Simpler	More complex (token management)
Flexibility	Less flexible	More flexible
Use Cases	VoIP, policing	Video streaming, traffic shaping