The concept of a worldwide of networks of information was introduced long before the technology used to build the internet. The first workable prototype came in the late 1960s with the creation of ARPANET(The Advanced Research Projects Agency Network). The famous TCP/IP, or Transmission Control Protocol and Internet Protocol, was developed by Robert Kahn and Vinton Cerf in the 1970s. In the 1980s, research by Tim Berners-Lee gave birth to the World Wide Web, linking hypertext documents into an information system, making them accessible from any node on the network (History of Internet). The implementation and the evolution of the internet has improved ever since. Today, for most users, the internet feels like smoke and mirrors since requiring everyone to understand the technical implementation will be way too harsh. However, as software developers, they are much more likely to deal with networks sometime in life. This article is meant to unveil technical details of networks mainly from a programmer’s perspective so the focus will be put on the software side.
The OS View of Networks
For the operating system, the network is perceived as an extra device. The Network Interface Controller(NIC), which is a hardware device used to connect the computer to a computer network, is added to the bus.The data can be transferred to/from memory to NIC through two methods: DMA or memory-mapped I/O. DMA refers to direct memory access. The name suggests that the hardware is able to read or write memory without the involvement of CPU. On the other hand, memory-mapped I/O means the CPU can control the hardware to read or write specific memory addresses, which means the CPU is doing the job of writing/reading to/from memory. DMA is usually used for high-bandwidth operations such as disk I/O while memory-mapped I/O is used in low-bandwidth operations like change control bits.
Layers of Network
The OSI (Open Source Interconnection) 7 Layer Model divides network communication into seven layers. Here I will discuss each layer and its corresponding function. Starting from the lowest one:
Layer 1: This layer is the physical layer which is concerned with the transmission of unconstructed raw bit stream over the physical medium. Thus the protocol data unit(PDU) is bit for this layer.
Layer 2: This is the data link layer. Its function is in charge of reliably transfer the data frames between two nodes connected by a physical layer. The PDU of this layer is frame.
Layer 3: This is the network layer. This layer is in charge of structuring and managing a multi-node network. Examples includes addressing, routing, and load control. The PDU for this layer is packet.
Layer 4: This is the transport layer. It is used to deliver messages error-free, in sequence, and without duplications or losses. The PDU of this layer is segment/datagram(segment for TCP, datagram for UDP).
Layer 5: This is the session layer. It allows session establishment between process running on different stations. Layer 5 is often OS/Library. From here, the PDU will generalized to data.
Layer 6: This is the presentation layer. As the name suggests, it formats the data and present it to the application layer. It can be viewed as the translator for the network. It is usually OS/Library.
Layer 7: The final layer is called the application layer. It serves as the window for the users and application processes to access the network.
Note the Department of Defense Four-Layer Model has only four catagories: the Network Access Layer(layer 1-2), the Internet Layer(layer 3), the Host-to-Host Layer(layer 4), and the Process Layer(layer 5-7).
More on Layer 2 Network
There are three types of layer 2 networks: System Area Network(SAN), Local Area Network(LAN), and Wide Area Network(WAN).