Introduction to the OSI Layer-7 Model

OSI Layer-7 Model

The definition and functions of the layer-7 model for Open System Interconnection defined by ISO International Standards Organization. It is a stepping stone for network technology creators and a basis for analyzing and judging various network technologies. From now on, the network is no longer mysterious, and it is also well-founded and well-founded.

The layer-7 model is mainly used to solve compatibility problems encountered during the interconnection of heterogeneous networks. Its biggest advantage is that the three concepts of services, interfaces, and Protocols are clearly separated, and different functional modules of the network share different responsibilities.

An important milestone in network development is the definition of OSI Layer-7 network model by ISO (International Standards Organization. It not only serves as the basis for judging and analyzing various previous and subsequent network technologies, but also serves as a reference model for network protocol design and unification.

The main purpose of establishing a layer-7 model is to solve compatibility problems encountered during the interconnection of heterogeneous networks. Its biggest advantage is that it separates the three concepts of services, interfaces, and Protocols: the service shows what functions a layer provides for the previous layer, the interface describes how to use lower-layer services on the previous layer, and how the protocol involves implementing services on the current layer. In this way, each layer is highly independent, there is no limit on the protocols used by each entity in the interconnect network. You only need to provide the same service up without changing the interfaces of the adjacent layers. The layer-7 division of the network aims to share different responsibilities among different functional modules of the network, which brings the following benefits:

Reduces the complexity of the problem. Once a network fault occurs, you can quickly locate the fault level to facilitate searching and error correction;

Standard interfaces are defined at each layer to enable interoperability between different network devices with the same peer layer. Each layer is relatively independent. A high-level protocol can be run on multiple low-layer protocols; it can effectively stimulate network technology innovation, because each update can be carried out in a small scope, without having to perform major surgery on the entire network; it is convenient for research and teaching.

Network layering reflects the structured ideas in many engineering designs and is a reasonable division.

Layer-7 network functions

The layer-7 network includes the physical layer, data link layer, network layer, transmission layer, Session Layer, presentation layer, and application layer. The physical layer, data link layer, and network layer are usually called the media layer, which is the object of network engineers. The transport layer, Session Layer, presentation layer, and application layer are called the host layer, it is the content that users face and care about.

So what are the specific definitions and responsibilities of the layer-7 network? It is the protocol stack diagram of the OSI Layer-7 model. They are:

Level 1-physical layer: the physical layer defines the electrical or mechanical properties of the physical link between communication networks, and the operations for activating, maintaining, and disabling this link. Physical Layer feature parameters include voltage, data transmission rate, maximum transmission distance, and physical connection media.

Layer 2-data link layer: the actual physical link is unreliable and there will always be errors. The function of the data link layer is to divide the data into frames, transmission in data frame units) converts a faulty physical link into a data link that is not faulty for the upper layer. Its characteristic parameters include: physical address, network topology, error warning mechanism, sorting of transmitted data frames, and traffic control. The physical address is relative to the network layer address. It represents the node identification technology of the data link layer. The "Topology" is a term often encountered in the network, indicates how each device is connected, for example, bus-all devices are connected to a bus, and star-all devices are connected through a central node; an error warning is an error reported during data transmission to the upstream protocol. data frame sorting can re-arrange the transmitted data. traffic control is used to adjust the data transmission rate so that the acceptor is not overloaded.

OSL Reference Model (simple figure 1)

Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer

Layer 3-network layer: the network layer divides data into groups of a certain length and identifies the logical addresses of the source and target nodes in the group header. These addresses are like blocks and house numbers, it is the identifier of each node. The core function of the network layer is to obtain the path from the source to the destination based on these addresses. When multiple paths exist, it is also responsible for routing selection.

Layer 4-Transport Layer: provides reliable data transmission that is transparent to the upper layer (independent from the specific network. If the network layer is concerned with point-to-point transfer, it can be said that the transport layer is concerned with the final effect of "end-to-end" (from the source end to the target end. Its functions include flow control, multi-channel technology, virtual circuit management, error correction, and recovery. Among them, multi-channel technology enables the data of multiple different applications to be transmitted together through a single physical link; virtual circuits are the logical channel for data transmission, which is established, maintained, and terminated at the transmission layer; the error correction function can detect errors and take measures (such as retransmission) to solve the problem.

Layer 5-Session Layer: establishes, manages, and terminates communication application service requests and responses among network entities.

Layer 6-presentation layer: defines a series of code and code conversion functions to ensure that the source data can be recognized at the destination, such as the ASCII code of the text data that everyone is familiar, indicates the GIF of an image or the mpeg of an animation.

Layer 7-Application Layer: the application layer is the highest level for users. It enables direct conversations between networks and users through software applications, such as finding the target communication party, identifying available resources, and synchronizing operations.

The layer-7 bottom layer (physical layer, data link layer, and network layer) is usually called the media layer. They are not what users see, but silently play a supporting role in the network. They are the objects of network engineers; the upper layer (transport layer, Session Layer, presentation layer, and application layer) is called the host layer, which is the content that users are facing and concerned about. These programs often combine the functions of each layer, form a whole in front of the user. The web applications WWW, FTP, and telnet that everyone is familiar with are integrated with these multi-layer functions.

In actual data transmission, the sender sends the data to the application layer, and adds the control information of the layer to the presentation layer. The presentation layer is processed as a method, and the data is marked with its own ID to the Session Layer. Likewise, each layer adds the control information of the current layer to the received data and sends it to the next layer. When it finally reaches the physical layer, the data is transmitted to the receiver through the actual physical media. The acceptor performs the opposite operation with the sender. from the bottom up, the IDs are removed layer by layer and restored to the original data. It can be seen that the data communication parties must adopt the same protocol in the Peer layer and define the same data identification format, so as to ensure the correct transmission of data.
OSI and practical application model

The layer-7 model is a theoretical model, and its practical application is ever-changing. It is entirely possible to have variations. For most applications, we just roughly match the protocol family (protocol stack) with the layer-7 model to see if the specific Protocol actually used belongs to a child layer in layer-7, it also includes the upper and lower layers.

Do the protocols actually used in the network strictly follow these seven layers? This is not the case. The layer-7 model is a theoretical model, and its actual application is ever-changing. It is entirely possible to have variations. Some applications have been around for a long time and cannot be overturned after the launch of the layer-7 model. Therefore, for most applications, we just roughly match the protocol family (protocol stack) with the layer-7 model to see if the specific Protocol actually used belongs to a child layer in layer-7, it also includes the upper and lower layers. We have previously introduced the TCP/IP protocol. its correspondence with the layer-7 model is as follows:

Relationship between OSL and TCP/IP model (figure 2)

Application Layer *

Presentation layer application layer

Session Layer *

Transport Layer

Network Layer

Data link layer network interface layer

Physical Layer *

As shown in figure 2, most TCP/IP application protocols combine the functions of the OSI application layer, presentation layer, and Session Layer to form the application layer. Typical protocols include: HTTP, FTP, telnet, etc. The TCP/UDP protocol corresponds to the OSI transport layer and provides upper-layer data transmission assurance. The IP protocol corresponds to the OSI network layer, which defines the well-known IP address format, as the basis for finding the path in the inter-network. The bottom layer of TCP/IP is implemented by the network interface layer, which is equivalent to the OSI physical layer and data link layer, in fact, TCP/IP does not strictly define this layer, but applies the existing underlying network for transmission. This is why it is widely used.