Computer Networking

1. The Internet: A Network of Networks Providing Services

The Internet is a computer network that interconnects billions of computing devices throughout the world.

Global Interconnection. The Internet is a vast, interconnected system linking billions of devices, from traditional computers to smartphones, IoT devices, and servers. It's not just about hardware; it's an infrastructure that provides services to distributed applications.

Service-Oriented Architecture. The Internet offers a socket interface, a set of rules that programs must follow to request data delivery to other programs. This interface allows developers to create distributed applications without needing to understand the underlying network complexities.

Protocols and Standards. The Internet relies on protocols like TCP/IP to govern data transmission. Standards, developed by organizations like the IETF, ensure interoperability, allowing diverse systems to communicate effectively.

2. Network Edge: End Systems, Access Networks, and Physical Media

In Internet jargon, all of these devices are called hosts or end systems.

End Systems and Hosts. Devices connected to the Internet, known as end systems or hosts, include computers, smartphones, and IoT devices. These hosts run applications and are categorized as clients (requesting services) or servers (providing services).

Access Networks. End systems connect to the Internet through access networks, such as DSL, cable, FTTH, Ethernet, WiFi, and cellular. These networks use various physical media, including copper wire, coaxial cable, optical fiber, and radio spectrum.

Physical Media. Physical media, both guided (e.g., fiber-optic cables) and unguided (e.g., radio waves), transmit data between devices. The choice of medium affects transmission rates, costs, and suitability for different environments.

3. Network Core: Packet Switching, Circuit Switching, and Interconnections

A packet switch takes a packet arriving on one of its incoming communication links and forwards that packet on one of its outgoing communication links.

Packet Switching. Data is divided into packets, which are forwarded through the network by packet switches (routers and link-layer switches). This approach allows for efficient use of network resources and flexible routing.

Circuit Switching. In contrast, circuit switching establishes a dedicated connection between end systems for the duration of a communication session. While providing guaranteed bandwidth, it can be less efficient for bursty traffic.

Network of Networks. The Internet is a network of interconnected ISPs, ranging from local access providers to national and international tier-1 providers. These ISPs peer with each other at Internet Exchange Points (IXPs) to exchange traffic.

4. Delay, Loss, and Throughput: Understanding Network Performance

A packet starts in a host (the source), passes through a series of routers, and ends its journey in another host (the destination).

Sources of Delay. Packets experience various delays as they traverse the network, including processing delay, queuing delay, transmission delay, and propagation delay. Understanding these delays is crucial for optimizing network performance.

Queuing and Loss. Queuing delay occurs when packets wait in output buffers at routers. If buffers overflow, packet loss occurs, impacting application performance. Traffic intensity (La/R) is a key factor in determining queuing delay and packet loss.

End-to-End Throughput. Throughput, the rate at which data is transferred between end systems, is limited by the bottleneck link along the path. Factors such as link capacity and intervening traffic affect end-to-end throughput.

5. Protocol Layers: Structuring Network Communication

A protocol defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event.

Layered Architecture. Network protocols are organized into layers, each providing specific services to the layer above. The Internet protocol stack consists of five layers: physical, link, network, transport, and application.

Internet Protocol Stack. Each layer has specific responsibilities:

• Application: Network applications and their protocols (e.g., HTTP, SMTP)
• Transport: Transports application-layer messages (TCP, UDP)
• Network: Moves network-layer packets (datagrams) from one host to another (IP)
• Link: Moves frames from one network element to an adjacent network element (Ethernet, WiFi)
• Physical: Moves individual bits within the frame from one node to the next

Encapsulation. As data moves down the protocol stack, each layer adds header information, encapsulating the data from the layer above. This process ensures that each layer has the necessary information to perform its functions.

6. Security Threats: Defending Against Network Attacks

All activity in the Internet that involves two or more communicating remote entities is governed by a protocol.

Malware. Malicious software, including viruses and worms, can infect devices via the Internet, causing damage, stealing data, or enrolling devices in botnets.

Denial-of-Service (DoS) Attacks. DoS attacks aim to render networks or hosts unusable by legitimate users. These attacks include vulnerability attacks, bandwidth flooding, and connection flooding. Distributed DoS (DDoS) attacks, leveraging botnets, are particularly challenging to defend against.

Packet Sniffing. Attackers can use packet sniffers to capture sensitive information transmitted over networks, especially wireless networks. Encryption is a key defense against packet sniffing.

7. A Brief History: From ARPANET to Today's Internet

Today’s Internet is arguably the largest engineered system ever created by mankind.

Packet Switching Origins. The Internet's roots trace back to the early 1960s and the development of packet switching as an alternative to circuit switching. Key figures include Leonard Kleinrock, Paul Baran, and Donald Davies.

ARPANET and TCP/IP. The ARPANET, the first packet-switched network, was a precursor to the Internet. The development of TCP/IP in the 1970s provided a foundation for interconnecting networks.

The Internet Explosion. The 1980s and 1990s saw the proliferation of networks and the commercialization of the Internet. The emergence of the World Wide Web in the 1990s brought the Internet to the masses.

Last updated: February 25, 2025