The public Internet is a worldwide computer network that connects hundreds of millions of computing devices all over the world.

The end system is connected to the Internet through Internet Service Providers (ISPs). Each ISP is a network composed of multiple packet switches and multiple communication links. The lower isPs are connected through the national and international higher ISPs. Different ISPs provide different types of network access for the end system.

End systems, packet switches, and other Internet components all run a series of protocols that Control the reception and Transmission of information on the Internet, Transmission Control Protocol (TCP) and Internet Protocol (IP). Internet protocols are the two most important protocols on the Internet. The IP protocol defines the format of packets sent and received on routers and end systems. The main Protocols on the Internet are collectively known as TCP/IP.

To provide a structure for the design of network protocols, network designers organize the protocols and the network hardware and software that implement them in a hierarchical manner. Each protocol belongs to a layer, and each layer provides its services by performing certain actions in that layer, or by using services directly below it. A protocol layer can be implemented using software, hardware, or a combination of both, with protocols distributed among the end systems, packet switches, and other components that make up the network. (Understand layers in terms of protocol functionality, not individual endpoints or hardware devices!)

With layered architectures, we can talk about specific parts of a well-defined, large and complex system. This makes the implementation of services provided by the layers easy to change. The ability to change the implementation of services without affecting other parts is an important advantage of layered architecture.

Layer 5 of the Internet

The Internet protocol stack consists of five layers: application layer, transport layer, network layer, link layer and physical layer. We use a top-down approach to description.

The application layer

The application layer of the Internet includes many protocols, such as HTTP, SMTP and FTP. Application layer protocols are distributed on multiple end systems. Applications in one end system exchange information packets with applications in another end system using protocols. We refer to this grouping of information at the application layer as messages.

Transport layer

The transport layer provides services for sending application-layer messages between application endpoints. On the Internet, there are two transport layer protocols, NAMELY TCP and UDP. The transport layer is grouped into segments.

TCP protocol to its application provides a connection-oriented service, the service includes application layer packet to the destination to ensure that the transfer and flow control (i.e. the sender/recipient rate match), TCP will long message is divided into short message, and provide a congestion control mechanism, when the network congestion occurs, the source restrain its transmission rate.

The UDP protocol provides connectionless services to its applications, provides no reliability, no flow control, and no congestion control.

The network layer

The Internet Transport layer protocol in the source host delivers transport-layer packet segments and destination addresses to the network layer, which is responsible for moving network-layer packets called datagrams from one host to another, just as you provide destination addresses to postcode. The network layer of the Internet also includes routing protocols that determine the route by which packets travel from source to destination.

The link layer

The network layer of the Internet sends packets between source and destination through a series of routers. In order to move packets from one node (host or router) to the next node on the path, the network layer must rely on the services of the link layer. At each node, the network layer passes the datagram down to the link layer, which passes the datagram along the path to the next node, where the link layer passes the datagram to the network layer. We call the link layer groups frames.

The physical layer

The task of the link layer is to move the entire frame from one grid element to a neighboring network element, while the task of the physical layer is to move the bits of the frame from one node to the next. Ethernet has many physical layer protocols: for twisted-pair copper, for coax, for optical fiber, and in each case, a different way to move a bit across those links.

Seven layers of the OSI

The OSI reference model has seven layers: application layer, presentation layer, session layer, transport layer, network layer, link layer, and physical layer, two more than the Internet.

The function of the presentation layer is to enable communicating applications to interpret the meaning of interactive data. The services it provides include data compression, data encryption, and data description. The session layer provides demarcation and synchronization capabilities for data exchange, including methods for establishing checkpoints and recovery schemes.

The Internet assumes that these two extra layers can be left to application developers to implement at their own discretion, so OSI’s presentation and session layers in the Internet also belong to the application layer.

Column in the direction of the wind

This column will not go through the full five levels of detail, but will focus on

  • Data exchange between the link layer and the network layer to clarify the transmission mode of data packets
  • Transport layer TCP protocol, including ensure delivery and congestion control, and learn its design ideas
  • Linux network tools are used for daily troubleshooting