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Summary
The establishment of computer network architecture standards enables two computers to understand each other accurately and respond elegantly like two close friends. This paper firstly outlines the motivation of the computer network architecture, and introduces the design concept combined with the postal system in daily life, and gives the relevant basic concepts and standards. Further, we highlight the layering principle of computer network architecture and its most important component - protocol, so that readers can have a new and systematic understanding of computer network architecture.
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Why do you need a computer network architecture?
As we all know, computer network is a very complex system. For example, when two computers connected on the network need to communicate, due to the complexity and heterogeneity of the computer network, many complicated factors need to be considered, such as:
(1). There must be a data transmission link between the two computers
(2) Tell the network how to identify the computer receiving the data;
(3) The computer that initiates the communication must ensure that the data to be transmitted can be sent and received correctly on this path; (
4). Errors and accidents, such as data transmission errors, failure of a node switch in the network, etc., should have reliable and complete measures to ensure that the other computer can finally receive the data correctly.
The establishment of computer network architecture standards is to solve these problems so that two computers (network devices) can accurately understand each other's meaning and respond elegantly like two close friends. That is to say, in order to complete this kind of network communication, it is necessary to ensure that the two computer systems that communicate with each other reach a high degree of tacit understanding. In fact, in the field of network communication, the communication between two computers (network devices) is not as natural as the communication between people. This highly tacit communication (communication) between computers needs to be very complex and complete. network architecture as support. Then, what method can be used to organize the structure of the network reasonably, so as to ensure that it has a clear structure, simplified design and implementation, easy updating and maintenance, strong independence and adaptability, so that there is such a "high degree of" between network devices. tacit agreement"?
The answer is divide-and-conquer, and further, layered thinking.
Advantages of layered thinking
(1) Low coupling (strong independence)
The upper layer only needs to use the services implemented by the lower layer through the interface provided by the lower layer for the upper layer, and does not need to care about the specific implementation of the lower layer. In other words, the lower layer is a black box with certain functions to the upper layer.
(2). Strong adaptability
As long as the services and interfaces provided by each layer for the upper layer remain unchanged, the implementation details of each layer can be changed arbitrarily.
(3). Easy to implement and maintain.
The complex system is decomposed into several sub-units with small scope and simple functions, so that the system structure is clear, and the implementation, debugging and maintenance become simple and easy. That is to say, for designers/developers, this method enables designers/developers to concentrate on designing and developing the functional modules they care about; for debug/maintainers, this method is also convenient for debug/maintainers Process the functional modules they are responsible for.
Network Hierarchical Reference Model
OSI Network Hierarchical Reference Model
Network protocol designers should not design a single, gigantic protocol that specifies complete details for all forms of communication. Instead, divide the communication problem into smaller problems. Then design a separate protocol for each small problem. Doing so makes the design, analysis, timing and testing of each protocol relatively easy. A major principle of protocol partitioning is to ensure that the target system is effective and efficient. To improve efficiency. Each protocol should only pay attention to the part of the communication that is not handled by other protocols; in order to implement the main protocol more efficiently, the protocols should be able to share specific data structures; at the same time, the combination of these protocols should be able to handle all possible hardware errors and other exceptions.
In order to ensure the synergy of the work of these protocols, the protocols should be designed and developed into a complete, collaborative protocol series (ie, protocol family), rather than developing each protocol in isolation.
in the early days of web history. The International Organization for Standardization (ISO) and the Consultative Committee on International Telegraph and Telephone (CCITT) jointly published a seven-layer reference model for the interconnection of open systems. The network process in a computer operating system involves from application requests (at the top of the protocol stack) to network media (bottom), and the OSI reference model divides the functionality into seven discrete levels. Figure 2.1 shows the OSI layered model.
The seven layers of the OSI model perform the following operations:
The first layer: the physical layer
is responsible for finally encoding the information into current pulses or other signals for online transmission. It consists of the actual interface between the computer and the network medium that defines electrical signals, symbols, wire states and clocking requirements, data encoding and connectors used to transmit data. For example, the most frequently used RS-232 specification, 10BASE-T's Manchester encoding, and RJ-45 belong to the first layer. All layers higher than the physical layer talk to it through predefined interfaces. Like the Attachment Unit Interface (AUI) for Ethernet, a DB-15 connector can be used to connect Layer 1 and Layer 2.
Layer 2: Data Link Layer
Provides reliable transmission of data over physical network links. Different data link layers define different network and protocol characteristics, including physical addressing, network topology, error checking, frame sequence, and flow control. Physical addressing (correspondingly, network addressing) defines how devices are addressed at the data link layer; network topology defines how devices are physically connected. Such as bus topology and ring topology; error checking alerts the upper-layer protocol where transmission errors occur; the data frame sequence is organized again and transmits frames other than the sequence; flow control may delay the transmission of data. so that the receiving device does not crash due to receiving more traffic than it can handle at one point. The data link layer actually consists of two independent parts, the media access control (Media Access Control, MAC) and the logical link control layer (Logical Link Control, LLC). The MAC description describes how to schedule, generate and receive data for stations in a shared medium environment. The MAC ensures reliable transmission of information across the link. Synchronize transmitted data. Identify errors and control the flow of data.
Generally speaking. MAC is only important in a shared medium environment where multiple nodes can connect to the same transmission medium. IEEE MAC rules define addresses to identify multiple devices in the data link layer. The Logical Link Control sublayer manages communications between devices on a single network link, and the IEEE 802.2 standard defines LLC. LLC supports both connectionless and connection-oriented services.
A number of fields are defined in the information frame of the data link layer. These domains enable multiple higher layer protocols to share a physical data link.
The third layer: the network layer
is responsible for establishing the connection between the source and the destination. It generally includes network routing, and may also include flow control, error checking, etc. The transmission data between different network segments of the same MAC standard generally only involves the data link layer, while the transmission data between different MAC standards all involve the network layer. For example, IP routers work at the network layer, so they can realize interconnection between various networks.
Layer 4: Transport Layer
Provide reliable end-to-end network data flow services to the upper layer. The functions of the transport layer generally include flow control, multiplexing, virtual circuit management, and error checking and recovery. Flow control manages the transmission of data between devices. Ensures that the transmitting device does not send more data than the receiving device can handle; multiplexing enables data from multiple applications to be transmitted over a single physical link; virtual circuits are established, maintained, and terminated by the transport layer; error checking is included for detection The various structures established by transmission errors; while error recovery includes actions taken (eg, requesting data retransmission) to resolve whatever errors occur. Transmission Control Protocol (TCP) is a transport layer protocol in the TCP/IP protocol suite that provides reliable transmission of data.
The fifth layer: Session layer
Establishes, manages and terminates the communication session between the presentation layer and the entity. A communication session includes service requests and service responses that occur between different network application layers, and these requests and responses are implemented through the protocols of the session layer. It also includes creating checkpoints that allow you to return to a previous state if communication is interrupted.
Layer 6: Presentation Layer
Provides a variety of functions for application layer data encoding and transformation to ensure that information sent by one system application layer can be recognized by another system application layer.
The encoding and transformation modes of the presentation layer include common data representation format, performance transformation representation format, common data compression mode and common data encryption mode.
Common data representation formats are standard image, sound and video formats. By using these standard formats. Different types of computer systems are able to exchange data with each other. The transformation mode exchanges information between systems by using different text and data representations, such as ASCII (American Standard Code for Information Interchange); the standard data compression mode ensures that the data compressed on the original device can be stored on the target device. The encryption mode ensures that data encrypted on the original device can be decrypted correctly on the target device.
The presentation layer protocol is generally not associated with a special protocol stack. For example, QuickTime is the video and audio standard for Applet computers, and MPEG is the ISO video compression and encoding standard.
Common graphic image formats PCX, GIF, JPEG are different still image compression and encoding standards.
The seventh layer: the application layer
is the OSI layer closest to the end user, which means that the OSI application layer and the user interact directly through the application software.
Note: The application layer is not composed of actual application software executed on the computer, but is composed of an API (Application Program Interface) that provides applications with access to network resources. Such application software programs are beyond the scope of the OSI model. The functions of the application layer generally include identifying communication partners, defining the availability of resources, and synchronizing communication. Due to possible loss of communication partner. The application layer must define the identification and availability of the communication partners for the application subroutines of the data transfer.
When defining resource availability, the application layer must determine whether sufficient network resources are available in order to request communication. In synchronous communication, the communication between all applications requires the cooperative operation of the application layer.
OSI's application layer protocol includes file transfer, access and management protocol (FTAM), as well as file virtual terminal protocol (VIP) and common management system information (CMIP) and so on.
TCP/IP Layered Model
The TCP/IP Layening Model is called the Internet Layering Model and the Internet Reference Model.
The TCP/IP protocol is organized into four conceptual layers. Three of the layers correspond to the corresponding layers in the ISO reference model. The ICP/IP protocol family does not include the physical layer and the data link layer, so it cannot independently complete the functions of the entire computer network system, and must work together with more protocols.
The four protocol layers of the TCP/IP layered model perform the following functions respectively:
Layer 1: Network Interface Layer
Contains protocols for cooperating IP data transmission on existing network media.
In fact, the TCP/IP standard does not define functions corresponding to the ISO data link layer and physical layer. on the contrary. It defines a protocol like the Address Resolution Protocol (ARP), which provides the interface between the data structures of the TCP/IP protocol and the actual physical hardware.
The second layer: the network layer
corresponds to the network layer of the OSI seven-layer reference model. This layer includes IP protocol and RIP protocol (Routing Information Protocol, routing information protocol), which is responsible for data packaging, addressing and routing. At the same time, the Internet Control Message Protocol (ICMP) is also included to provide network diagnostic information.
Layer 3: Transport Layer
Corresponding to the transport layer of the OSI seven-layer reference model, it provides two end-to-end communication services.
Among them, TCP (Transmission Control Protocol) provides reliable data stream transport services, and UDP (Use Datagram Protocol) provides unreliable user datagram services.
The fourth layer: application layer
Corresponding to the application layer and the expression layer of the OSI seven-layer reference model. The application layer protocols of the Internet include Finger, Whois, FTP (File Transfer Protocol), Gopher, HTTP (Hypertext Transfer Protocol), Telent (Remote Terminal Protocol), SMTP (Simple Mail Transfer Protocol), IRC (Internet Relay Session), NNTP (Network News Transfer Protocol), etc., this is also the focus of this book.
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