[Basic computer network 1] Understand the computer network architecture and the general model of the computer network (below)

Preface

       In the previous article, we mainly introduced the overview of computer networks. In the following article, we will introduce the content of computer network architecture and reference models. This blog is closely related to the previous blog.

The main content of        this blog is: computer network architecture and reference model , mainly the concepts of computer network hierarchical structure , protocols , interfaces , services , ISO/OSI reference model and TCP/IP model .

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[Exam syllabus content]

Computer Network Architecture and Reference Model

1. Computer network layered structure; concept of computer network protocols, interfaces, services
2. ISO/OSI reference model and TCP/IP model

【pay attention】 

       This chapter mainly introduces the basic concepts of computer network architecture , and readers can memorize them appropriately based on their understanding. Focus on mastering the hierarchical structure of the network (including 5-layer and 7-layer structures) , especially the functions of each layer of the ISO/OSI reference model and related concepts such as protocols, interfaces and services . Understand various performance indicators related to the network . Especially the calculation of delay , bandwidth , rate and throughput .

 1. Computer network hierarchical structure

1.1 Architecture

       First, let’s get to know the architecture: the layers in a computer network , the protocols at each layer , and the collection of interfaces between layers . The hierarchy is represented by a vertical layering model . Except for real communication on the physical media , all other peer entities are virtual communications . Virtual communication at the peer layer must follow the protocol of that layer .

What is real communication and what is virtual communication?

•        To give a vivid example, I want to send a pen to someone. When someone gets the pen, this is virtual communication, because it is not me holding the pen to give it to someone, but it is done by express delivery.
•        I give the pen to the courier, and the courier gives it to the sorter, and then sends it to someone's place via high-speed rail or airplane. This is real communication.

1.2 The meaning and basic principles of layering (but the topic is not covered in the current position)

1.2.1 The meaning of layering

       Communication between entities in two systems is a very complex process . In order to reduce the complexity of the protocol design and debugging process , and to facilitate the research , implementation and maintenance of the network , and promote standardization work , the architecture of the computer network is usually Modeling in a layered manner.

1.2.2 Principles of hierarchical structure division

1. The functions of each layer should be clear and independent of each other . When the specific implementation method of a certain layer is updated, as long as the interfaces of the upper and lower layers remain unchanged, it will not affect the neighbors, and the most appropriate technology can be used to implement it;
2. Inter-layer interfaces must be clear , and the amount of information on cross-domain interfaces should be as small as possible;
3. The number of layers should be moderate . If there are too few layers, the protocol at each layer will be too complex; if there are too many layers, the architecture will be too complex, making it difficult to describe and implement the functions of each layer;
4. The interface between each layer is natural and clear , easy to understand, and there is as little communication as possible;
5. Maintain the independence of the lower layer from the upper layer , and the upper layeruses the services provided by the lower layer in one direction ;
6. The entire layered structure should facilitate standardization efforts .

1.3 Characteristics of the architecture

1. Use function as the basis for dividing levels;
2. When the nth layer entity implements its own defined functions, it can only use the services provided by the n-1th layer ;
3. When the nth layer provides services to the n+1th layer, this service not only includes the functions of the nth layer itself, but also includes the functions provided by the lower layer services ;
4. There are interfaces only between adjacent layers , and the specific implementation details of the provided services are completely shielded from the upper layer .

1.4 Conceptual understanding

1. Network protocols are organized in a hierarchical structure ;
2. The collection of network hierarchical structure models and protocols at each layer is called network architecture;
3. Network architecture precisely defines the functions that a computer network should implement ;
4. Architecture is the abstraction , while implementation refers to some hardware and software that can run.

2. Concepts of computer network protocols, interfaces, and services

2.1 Agreement

The concept of agreement:

• Network protocols are rules, conventions and standards developed for network data exchange . (Just like when a courier company sends a courier, it has to put the courier tracking number on the courier. The agreement is actually a language.)

The three elements of the protocol: semantics , syntax and synchronization (timing) .

• Semantics: Used to explain the meaning of each part of the bitstream
• Grammar: Grammar is the structure and format of the user data domain control information , as well as the meaning of the order in which the data appears ; (equivalent to the fixed position and order of the name, address, and contact information written on the express delivery note.)
• Synchronization (timing): detailed description of the sequence of events

Let's take a look at an exam question about timing:

The concept of entity:

• Indicates any hardware or software process that can send or receive information .

Protocol functions:

1. A protocol is a collection of rules governing the communication between two peer entities .
2. Under the control of the protocol, communication between two peer entities enables this layer to provide services to the upper layer.
3. To implement this layer protocol , you also need to use the services provided by the lower layer .

2.2 Services

Service concept:

• In order to ensure that upper-layer peers can communicate with each other, the lower layer provides functions to the upper layer . (remember it’s one way )

2.3 Interface (service access point)

The concept of interface:

• An interface is a connection point for exchanging information between adjacent layers within the same node . 

Interface functions:

• There are clearly defined interfaces between adjacent layers of the same node, and the bottom layer provides services to the upper layer through the interface ;
• As long as the interface conditions remain unchanged and the low-level functions remain unchanged, changes in the specific implementation methods and technologies of the low-level functions will not affect the work of the entire system.

The concept of service access point:

• The place where entities from two adjacent layers of the same system interact is called the service access point SAP. Service access point SAP is an abstract concept, it is actually a logical interface .

The concept of service data unit SDU:

• The unit of data exchanged between layers.

The concept of protocol data unit PDU:

• The data unit transmitted between peer layers is called the protocol data unit (PDU) of that layer.
• SDU can be different from PDU. For example, multiple SDUs can be combined into one PDU, or one SDU can be divided into several PDUs.

2.4 The concepts of protocols and services are different

1. The implementation of the protocol ensures that services can be provided to the upper layer.
2. Service users at this layer can only see the service but not the underlying protocols.
3. Protocols are "horizontal" , that is, protocols are the rules that control communication between peer entities .
4. Services are "vertical" , that is, services areprovided from the lower layer to the upper layer through inter-layer interfaces.
5. The upper layer uses service primitives to obtain the services provided by the lower layer.

What are service primitives?

The concept of service primitives:

• When the upper layer uses the services provided by the lower layer, it must exchange some commands with the lower layer . These commands are called service primitives in the OSI reference model.

The OSI reference model divides primitives into 4 categories:

1. ask. Sent by a service user to a service provider to request the completion of a certain job .
2. instruct. Sent by a service provider to a service user to instruct the user to do something .
3. response. Sent by the service user to the service provider in response to an indication .
4. confirmed. Sent by the service provider to the service user as confirmation of the request .

Maybe the above explanation is a little confusing, let’s explain it graphically :

3. ISO/OSI reference model (seven layers)

       OSI's architecture defines a seven-layer model for communication between processes and serves as a framework to coordinate the formulation of standards at each layer; OSI's service definition describes the services provided by each layer , as well as the relationships between layers. abstract interfaces and service primitives for interaction. Let’s take a look at the functions between each layer:

3.1 Main functions of the physical layer

1. Use transmission media to establish, manage and release physical connections between network nodes for communication ;
2. Achieve transparent transmission of bit streams and provide data transmission services for the data link layer;
3. The data transmission unit of the physical layer is bit .

3.2 Main functions of the data link layer

1. Based on the services provided by the physical layer, the data link layer establishes data link connections between communicating entities;
2. Transmit data packets in "frame" units;
3. Error control and flow control methods are used to turn physical lines with errors into error-free data links.

3.3 Main functions of the network layer

1. Select appropriate paths for packets through the communications subnet through routing algorithms (control plane) ;
2. Create logical links for data transmission (forwarding, data plane) between nodes ;
3. Realize functions such as congestion control and network interconnection .

3.4 Main functions of the transport layer

1. Provide users with reliable end-to-end services ;
2. Handle packet errors, packet order, and other critical transmission issues ;
3. The transport layer shields the details of lower-layer data communication from the upper layers and is a key layer in the computer communication architecture.

3.5 Main functions of the session layer

1. Responsible for the transmission link between two nodes to ensure that point-to-point transmission is not interrupted ;
2. Manage data exchange.

 3.6 Main functions of the presentation layer

1. A representation used to handle information exchanged between two communication systems ;
2. Data format exchange;
3. Data encryption and decryption ;
4. Data compression and recovery.

3.7 Main functions of the application layer

1. Provides network services for applications ;
2. The application layer needs to identify and ensure the availability of the communication partner to enable synchronization between applications working together;
3. Establish a control mechanism to correct transmission errors and ensure data integrity.

4. TCP/IP reference model (layer 4)

4.1 Development of TCP/IP reference model

• Currently used version 4 , its network layer protocol is generally recorded as IPv4;
• The network layer IP protocol of version 6 is generally recorded as IPv6 (or IPng, IP next generation);
• IPv6 is called the next generation IP protocol .

4.2 Characteristics of TCP/IP protocol

1. Open protocol standards;
2. Independent of specific computer hardware and operating systems;
3. Independent of specific network hardware, it can run on LAN, WAN, and is more suitable for the Internet;
4. A unified network address allocation scheme enables the entire TCP/IP device to have a unique address on the Internet ;
5. Standardized high-level protocols can provide a variety of reliable user services.

4.3 Network interface layer

1. The lowest layer of the reference model, responsible for sending and receiving IP datagrams over the network;
2. Allows hosts to connect to the network using a variety of existing and popular protocols;
3. It fully reflects the compatibility and adaptability of the TCP/IP protocol , and it also lays the foundation for the success of TCP/IP.

4.4 Internet layer

1. Equivalent to the OSI reference model network layer connectionless network service ;
2. Handle interconnection routing , flow control and congestion issues;
3. The IP protocol is a connectionless network layer protocol that provides "best effort" services .

4.5 Transport layer

1. Establishing an end-to-end connection for a session between peer entities of a source host and a destination host on the Internet;
2. Transmission Control Protocol TCP is a reliable connection-oriented protocol;
3. User Datagram Protocol UDP is an unreliable connectionless protocol.

4.6 Application layer

The application layer protocols mainly include:

• File Transfer Protocol FTP
• Simple Mail Transfer Protocol SMTP
• Domain Name System DNS
• Hypertext Transfer Protocol HTTP
• Dynamic Address Acquisition Protocol DHCP

Five, five-layer structure

6. Comparison between OSI reference model and TCP/IP reference model

Comments on the OSI reference model:

1. The number of layers and content selection are not very good , the session layer is rarely used, the presentation layer is almost empty, and the data link layer and network layer have many sub-layer insertions;
2. Addressing, flow control and error control are repeated in each layer, reducing system efficiency;
3. Data security, encryption, and network management were neglected early in the design of the reference model;
4. The design of the reference model is more dominated by the idea of ​​communication, which is not suitable for working with computers and software;
5. Software programmed strictly according to the hierarchical model is very inefficient .

Evaluation of the TCP/IP reference model:

1. The differences between services, interfaces and protocols are not very clear . A good software project should distinguish functions from implementation methods. The reference model is not suitable for other non-TCP/IP protocol families;
2. The host-network layer of the TCP/IP reference model is not an actual layer by itself;
3. The division of the physical layer and the data link layer is necessary and reasonable , but the TCP/IP reference model does not do this.

Summarize

       This article mainly tells an overview of computer networks, understanding the layered structure of computer networks, the contents of computer network protocols, interfaces, services, and two important models: OSI/ISO model and TCP/IP model. Thank you, sir. With your likes and attention, I will work harder to simplify my notes.