What is the function of each layer in the computer network layered model?

1. Layering of computer networks

In the computer network system, the layered idea is adopted to divide the necessary functions in the communication protocol into layers. Each layer receives specific services provided by the layer below it and is responsible for providing specific services to the layer above it. The agreement followed by the interaction between the upper and lower layers is called "interface", and the agreement followed by the interaction between the same layers is called "protocol".

• An "interface" is the connection point where two adjacent layers exchange positions, and is the entry point for the upper layer to use the services of the lower layer.
• "Protocol" refers to some agreements made by both parties in communication, such as how to start communication, the format and order of information, how to end communication, etc. Protocols are very important. When we study computer networks, the focus is to understand various protocols, such as HTTP, DNS, TCP, etc. Only when both parties in communication support the same protocol at the same time can communication be carried out. Just like communication between humans, if a person only speaks English, then speaking Chinese to him will be incomprehensible.

Why do computer networks use a layered model? The main reasons are that layering has the following advantages:

• Layering is a common routine in software design. It is also a common decoupling idea that can effectively reduce the situation of "one move affects the whole body" in the program. Using each layer independently is more flexible. Even if some layers in the system change, it will not affect the entire system.
• Layering can also break down complex network problems into many smaller parts with clear and simple boundaries, making it easier to implement the protocols of each layer separately and define the specific responsibilities and obligations of each layer.

Of course, the more layers, the better. Layering may also have some disadvantages. For example, it may be over-modularized, making the processing heavier, and different modules may have to handle similar logic, resulting in redundancy and other problems.

2. Common reference models

In the early days of computer communication, each computer manufacturer produced its own network products to achieve computer communication. However, due to the lack of standardization, the various protocols set by different manufacturers were incompatible, and cross-manufacturer communication was impossible, which was very inconvenient.

In order to solve the above problems, the International Organization for Standardization (ISO) has developed an international standard OSI reference model, which divides the computer network system into 7 layers and standardizes the communication system. This model has clear concepts, but is very complex and difficult to implement, and has not been popularized.

For practical purposes, ARPA proposed the TCP/IP reference model (which actually appeared earlier than the OSI model), dividing the computer network into four layers. This model prioritizes the technology that can truly achieve communication during the standard-setting process (it focuses more on how to achieve it rather than on theory), and once any problems are found, the program, protocol or corresponding documents are modified in a timely manner. The model is more concise and more practical, and eventually becomes the industry standard. However, the concept of the network interface layer at the bottom of TCP/IP is not clear and there is no specific content.

In order to facilitate the understanding of computer network principles, computer network tutorials usually divide computer networks into 5 layers by combining the advantages and disadvantages of the OSI 7-layer model and the TCP/IP 4-layer model. This is both concise and can explain the concepts more clearly (of course, the TCP/IP model is still actually used).

Schematic diagrams of several computer network reference models are as follows:

3.5 Layer Reference Model Introduction

• Application layer The transmission unit of the application layer is the message. Its task is to complete specific network applications, such as transmitting emails, transferring files, requesting and responding to Web documents, and other specific network functions. This is the layer that users come into direct contact with. Common protocols include HTTP, FTP, DNS, and SMTP.
• Transport layer The transmission unit of the transport layer is the message segment, and its task is to provide communication services between different processes. A host may run many processes (for example, a user may be listening to music while chatting, and running different processes, so it is necessary to know which process the data should be sent to), and they are distinguished by port numbers. Common protocols include TCP and UDP.
• The transmission unit of the network layer is the datagram (packet). Its task is to provide communication services between different network hosts and transmit data from the source to the destination. IP addresses are used to identify devices on the network. In addition to addressing, routing selection is also required. When transmitting data from host A to host B, there are many routes to choose from. The network layer uses the corresponding routing algorithm to calculate a suitable path. Common protocols include IP and ICMP.
• Data Link Layer The transmission unit of the data link layer is the frame. Its task is to provide communication services between different nodes (hosts or routers) on the same link that are interconnected at the physical level. For example, communication between two nodes connected to an Ethernet. Use MAC addresses to identify devices on the network. The data link layer also provides functions such as flow control and error control. The network connects many computers together, while the Internet connects many networks together through routers, so the Internet is also called the "network of networks." Usually a single network is simplified into a link, and the data link layer serves different nodes in the same link. There are many types of communication links (such as Ethernet, WIFI, cable access network), which are composed of different types of physical media (such as optical fiber, twisted pair, coaxial cable). Different network hosts may be far apart and span many links. Just like when we go from Beijing Company to Nanjing Hometown, we may pass through different roads such as highways, city roads, and country roads.
• The transmission unit of the physical layer is bit, and its task is to transparently transmit bit streams (0/1 signals). (Transparent transmission means that no matter what bit combination the transmitted data is, it should be able to be transmitted on the link)

When two computers are communicating, data will pass through different layers of the computer network. When the sender transmits data between layers, each layer will add a header information (encapsulation) belonging to that layer. For example, the transport layer will add information such as the source port number and destination port number, the network layer will add information such as the source IP address and destination IP address, and the data link layer will add the source MAC address and destination MAC address. Conversely, when the receiver transmits data between layers, each layer will remove the corresponding header (decapsulation) when passing through.

This is similar to sending express delivery. When merchants send parcels, they will add packaging boxes, packaging bags, shipping information stickers, etc., layer by layer of packaging. When consumers receive the parcels, they need to remove the packaging layer by layer to get the final product.

References:

1. HTTP in Pictures
2. TCP/IP in Pictures
3. Computer Networks (7th Edition)

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