Flow Control & Congestion Control Study Notes

Nottingham Trent International College - Flow Control & Congestion Control Notes

Overview

  • Module Requirements:

    • To pass the module: ≥ 40%

    • To excel on the module: > 70%

  • Background Information: Essential for understanding the topics of flow control, congestion control, and network topologies.

Learning Outcomes

  • Gain an understanding of:

    • How congestion occurs and strategies to overcome it.

    • Various approaches to handle congestion.

Flow Control

  • Definition:

    • Flow control mechanisms are needed to prevent the sending host from overwhelming the receiver’s buffer by sending data too quickly.

  • Concepts:

    • Free Space in Buffer: Refers to the available capacity in the receiver’s buffer that may contain data.

    • Coordination:

    • Flow control synchronizes the transmission of packets; i.e., how many packets can be sent before receiving acknowledgments.

    • Flow control consists of procedures enabling the source to know the number of packets it can transmit before awaiting an acknowledgment from the destination.

    • Destination Limitation:

    • The destination must inform the source when it nears its capacity limit, requesting either a reduction in data sending or a temporary halt.

Congestion

  • Definition:

    • Congestion arises when the volume of packets being transmitted nears or exceeds the network's packet processing capacity.

  • Impact of Congestion:

    • (i) Packets experience significant delays reaching their destinations.

    • (ii) In severe cases, packets may never be delivered to their intended destinations.

Influences of Congestion

  • Key Factors:

    • Mismatched network infrastructure relative to demand.

    • Simultaneous packet influx from multiple sources.

Overcoming Congestion

  • Congestion Control Methods:

    • Aims to maintain data levels on the network below thresholds where performance deteriorates.

    • Strategies include:

    • Increasing available resources (e.g., routers, bandwidth).

    • Utilizing multiple routers to accommodate additional data loads.

    • Decreasing network load by removing certain services or reducing the baseline service provided to users.

TCP Congestion Control

  • Mechanism Overview:

    • The TCP protocol utilizes a decentralized model where each sender regulates its data transmission rate based on feedback from the network.

    • Feedback Types:

    • Acknowledgment Received: Indicates no congestion, leading the sender to increase the transmission rate.

    • Missing Acknowledgment: Suggests potential network congestion, prompting the sender to reduce the transmission rate.

  • Difference Between Flow Control and Congestion Control:

    • Flow control typically operates at the destination, managing how much data the source can push to the network.

    • Congestion control primarily functions at the source, recognizing network conditions and modulating outbound data rates accordingly.

Additional Congestion Control Approaches

  • Prediction of Congestion:

    • Preventative measures include forecasting potential congestion and proactively reducing packet transmission before the network hits capacity.

  • Network Structure:

    • Design networks to accommodate anticipated traffic levels, ensuring an adequate number of routers, cables, and sufficient bandwidth.

    • Not every network component needs to be active continuously.

  • Limit Data Transmission:

    • The sender should match the data amount with the network capability, receiving instructions to reduce sending amounts as needed (e.g., by adjusting window size).

Network Topologies

  • Types of Network Topologies:

    • Bus, Star, Token Ring, Ring, Mesh, Tree: These are fundamental layouts determining how devices are interconnected in a network.

  • Transmission Types:

    • Broadcast: Packets sent to all devices within a network, suitable for small geographical scopes.

    • Point-to-Point: Involves direct connections between two devices, typically larger networks.

Point-to-Point Communication

  • Definition:

    • Directly connects two nodes without any intermediary devices.

    • Used for WAN connections, linking computers to switches, or connecting switches and routers.

Shared Communication

  • Broadcast Networks:

    • Single communication channel shared among multiple devices.

    • Data packets broadcast to all devices containing address data to determine if the packet should be read or discarded based on the receiving device's address.

Hybrid Topology

  • Definition:

    • Combines multiple topologies into one larger schema, frequently found in extensive WANs.

  • Advantages:

    • Speed benefits from integrating strengths from different topologies.

  • Disadvantages:

    • Increased installation and configuration complexity.

Logical vs. Physical Topology

  • Physical Topology:

    • Represents the tangible arrangement of cabling and devices in a network.

  • Logical Topology:

    • Describes how data travels between nodes, which may not correspond with the physical layout of the network.

Switching Techniques

  • Definition:

    • Determines how connections are formed and data is transmitted across the network, categorized into three types:

    1. Circuit Switching: Establishes a dedicated path for communication for the entire duration (e.g., telephone call).

    2. Message Switching: Involves storing entire messages at intermediate devices before forwarding (e.g., emails).

    3. Packet Switching: Data is segmented into packets with necessary metadata, allowing varied routing for each packet based on efficiency (e.g., Internet).

Advantages and Disadvantages of Each Switching Approach:

  • Circuit Switching:

    • Advantages: Provides guaranteed data rates; effective for time-critical transmissions.

    • Disadvantages: Inefficient for non-continuous data flows due to dedicated channel.

  • Message Switching:

    • Advantages: Permits asynchronous communication; manages network traffic efficiently.

    • Disadvantages: Higher latency due to need for complete message storage and forwarding.

  • Packet Switching:

    • Advantages: Enables dynamic routing, optimizing bandwidth; allows for packet re-routing in case of path failure.

    • Disadvantages: Packets may arrive out of order; require more processing at destination.

Key Takeaways

  • Understanding Congestion: Knowledge of congestion and methods for overcoming it is critical for network design and management.

  • Topology Insight: Familiarity with topologies and switching methods is essential for optimizing network performance.

  • TCP Mechanisms: Recognizing TCP's approach to flow and congestion control provides insight into network reliability and data transmission efficiency.