Basic OSI Layer-7 model knowledge and common applications at each layer (1)

Basic OSI Layer-7 model knowledge and common applications at each layer (1)

OSI Open Source Initiative (OSI) is a non-profit organization designed to promote the development of Open Source software. The full name of the OSI Reference Model (OSI/RM) is the Open System Interconnection Reference Model (OSI/RM ), it is a network system interconnection model proposed by ISO. It is the foundation of network technology and the basis for analyzing and judging various network technologies. It opens the secrets of the network, so that it can be justified and evidence-based.

I. Key Points of OSI reference model knowledge

Figure 1: OSI model Basic Knowledge Overview

The model divides network communication into seven layers. Layers 1 to 4 are considered as low layers, which are closely related to data movement. Layer 5 to Layer 7 are high-level and contain application-level data. Each layer is responsible for a specific task, and then transmits data to the next layer. It can be divided into the physical layer, data link layer, network layer, transmission layer, Session Layer, presentation layer, and application layer.

Layer-3 Application Layer-provides services directly to applications, which can be changed, but must include electronic message transmission.

Layer 3 Presentation Layer-format data to provide common interfaces for applications. This can include the encryption service

Layer 2 Session Layer-establish an end connection between two nodes. This service includes setting up connections in full or half duplex mode, although the duplex mode can be processed in Layer 4.

Layer 2 Transport Layer-conventional data delivery-connection oriented or connectionless. Includes full or half duplex, flow control, and error recovery services

Layer 3 network layer-this layer establishes a connection between two nodes through addressing, including routing and relay data through the interconnection network.

Layer 3 data link layer-in this layer, data is divided into frames and traffic control is processed. This layer specifies the topology and provides hardware addressing.

Layer 3 physical layer-transmission of original bit streams

When electronic signal transmission and hardware interface data are sent, it is transferred from the seventh layer to the first layer, while the receiver is the opposite.

The typical devices for each layer are as follows:

Application Layer ................... COMPUTER: applications, such as FTP, SMTP, and HTTP

Presentation Layer ................... COMPUTER: encoding method, image codec, URL field Transfer Encoding

Session Layer ................... COMPUTER: SESSION creation, SESSION authentication, resumable Data Transfer

Transport Layer ................... COMPUTER: process and Port

Network Layer ..................... Networks: routers, firewalls, and multi-layer Switches

Data link layer ........... Network: Nic, bridge, switch

Physical Layer ..................... Network: repeater, HUB, network cable, HUB

Ii. OSI basic knowledge

Proposal of OSI/RM Reference Model

The world's first network architecture was proposed by IBM (SNA in 74 years). In the future, other companies have successively proposed their network architecture, such as Digital's DNA, the TCP/IP address of the U. S. Anti-DDoS Department coexist in a variety of network architectures. The result is that if an IBM structure is adopted, only IBM products can be used and only networks with the same structure can be interconnected.

In order to promote the development of computer networks, ISO established a committee in 1977. Based on the existing network, it proposed a network architecture not based on specific models, operating systems or companies, known as the open system interconnection Model (OSI Reference, open system interconnection)

OSI design objective

The OSI model is designed to be an open network model that can be implemented by all vendors to overcome the difficulties and inefficiency caused by the use of many private network models. OSI was completed with the participation of a highly respected international standards group, which is ISO (International Standardization Organization ). OSI stands for Open System Interconnection, which is the reference model for Open System Interconnection. Prior to the emergence of OSI, there were numerous architectures in the computer Network, with IBM's SNA (System Network Architecture) and DEC's DNA (Digital Network Architecture) the digital network architecture is the most famous. In order to solve the Interconnection problem between networks of different architectures, ISO (Be careful not to mix with OSI) developed an Open System Interconnection Reference Model (Open System Interconnection Reference Model) in 1981, OSI/RM ). This model divides Network communication into seven layers, which are Physical Layer (Physical Layer), Data Link Layer (Data Link Layer), and Network Layer (Network Layer ), transport Layer, Session Layer, Presentation Layer, and Application Layer ). The layer 1 to Layer 3 belongs to the lower layer of the OSI reference model and is responsible for creating network communication links. The Layer 4 to Layer 7 is the high-level layer of the OSI reference model, which is responsible for end-to-end data communication. Each layer provides certain functions, each layer provides services for its upper layer, and all layers support each other, while network communication can be top-down (on the sending end) or two-way implementation from the bottom up (at the receiving end. Of course, not every message needs to pass through all Layer 7 of OSI, and some even only need a layer corresponding to both parties. The transfer between physical interfaces and the connection between the repeater and the repeater only needs to be performed in the physical layer. The connection between the router and the router only needs to pass through the Layer 3 below the network layer. In general, the communication between the two parties is at the peer level and cannot be at the asymmetric level.

The method used in OSI Standard Setup is to divide the entire large and complex problem into several small problems that are easy to handle. This is a hierarchical architecture approach. Three levels of abstraction are used in OSI, including architecture, service definition, and Protocol Specification Description.

OSI hierarchy principle

Each node in the network has the same level

Different nodes have the same level of functionality

Communication between adjacent layers at the same node through interfaces

Each layer can use the services provided by the lower layer and provide services to the upper layer.

Communication between peer and peer layers through protocols between different nodes

OSI/RM layered structure

Information Flow During Inter-peer layer object Communication

Essence of peer-to-peer communication:

Virtual Communication between peer-layer entities; the lower layer provides services to the upper layer; the actual communication is completed at the bottom layer, and the sender data is gradually transferred to the lower layer, and the receiver data is gradually transferred to the upper layer from the lowest layer.

Protocol Data Unit PDU

In the SI reference model, information units exchanged between peer-to-peer protocols are collectively referred to as Protocol Data units (PDUS and Protocol Data units ).

The PDU at the transport layer and the following layers also have specific names:

Transport Layer-Data Segment (Segment)

Network Layer-group (Packet)

Data link layer-data Frame)

Physical Layer-Bit)