OSI Reference Model OSI Reference Model is a recommendation of ISO, which is developed for the international standardization of the protocols on each layer. The OSI Reference Model full name is the Open Systems Interconnect Reference Model (open System Interconnection Reference model). This reference model is divided into seven layers: physical layer, Data link layer, network layer, transport layer, Session layer, presentation layer and Application layer physical layer (physical layer) are mainly dealing with mechanical, electrical and process interfaces, as well as physical transport media under the physical layer. The task of the data link layer is to strengthen the capabilities of the physical layer so that it appears as an error-free line to the network layer. The network layer determines the routing of packets from the source side to the destination. Routing can be fixed in the Network static routing table, can also be determined at each session, but also according to the current network load situation, the flexibility for each group to decide separately. The Transport layer (Transport layer) receives data from the session layer and transmits it to the network layer, while ensuring that the pieces of information that arrive at the destination are correct, and that the session layer is not affected by hardware changes. Typically, the session layer creates a single transport connection for each request, and a separate network connection is created for it by the transport layer. However, if a transport connection requires a higher throughput, the transport layer can also create multiple network connections for it to divert data across these network connections to improve throughput. On the other hand, if creating or maintaining a separate network connection is not advantageous, the transport layer can also reuse several transport connections to the same network connection to reduce costs. In addition to multiplexing, the transport layer also needs to address the establishment and removal of cross-network connections, and has a flow control mechanism. The session layer allows you to establish a session relationship between users on different machines, which can be used for common data transfers similar to the transport layer, or to telnet to a time-sharing system or transfer files between two machines. The presentation layer (Presentation layer) is used to accomplish certain functions that are often requested, so that people want a common solution rather than being implemented by each individual user. The application layer (application layer) contains a large number of protocols that are commonly needed. Different file systems have different file naming principles and different lines of text representations, etc., the transfer of files between different systems There are various incompatibilities, which will be handled by the application layer. In addition, the application layer has a variety of common and dedicated functions, such as virtual terminals, e-mail, and newsgroups.TCP/IP Reference Model The  TCP/IP Reference model is the network architecture that is used first by ARPANET. This architecture is referred to as the TCP/IP Reference Model (TCP/IP Reference models) after its two major protocols appear. This network protocol is divided into four layers: the network access layer, the Internet layer, the transport layer and the application layer . the network access layer, which is not described in detail in the TCP/IP reference model, but indicates that the host must be connected to the network using some kind of protocol. The Internet tier (Internet layer) is a key part of the entire architecture, and its function is to enable the host to send packets to any network, and to enable the grouping to be transmitted independently to the target. These groupings may pass through different networks, and the order of arrival and the order in which they are sent may vary. High-level if you need to send and receive sequentially, you must handle the sorting of the groupings yourself. Internet layer uses Internet Protocol (Ip,internet Protocol). The network layer of the TCP/IP Reference Model's Internet layer and the OSI Reference Model are functionally similar. The Transport layer (Tramsport layer) enables a session to be made by peer entities on the source and destination machines. At this level, two end-to-end protocols are defined: Transmission Control Protocol (Tcp,transmission-Protocol) and User Datagram Protocol (udp,user Datagram Protocol). TCP is a connection-oriented protocol that provides reliable message transmission and connection services to upper-level applications. For this reason, in addition to basic data transmission, it also has the functions of reliability assurance, flow control, multiplexing, priority and security control. UDP is a non-connected, unreliable transport protocol that is used primarily for applications that do not require the ability to sort and flow control of TCP. The application layer (application layer) contains all the high-level protocols, including: Virtual Terminal Protocol (telnet,telecommunications NETwork), File Transfer Protocol (Ftp,file Transfer Protocol), e-Mail Transfer Protocol (smtp,simple Mail Transfer Protocol), Domain Name Services (dns,domain name Service), online News Transfer Protocol (Nntp,net News Transfer Protocol) and Hypertext Transfer Protocol (Http,hypertext Transfer Protocol). Telnet allows a user on a machine to log on to a remote machine and work; FTP provides a way to effectively move files from one machine to another; SMTP is used to send and receive e-mails; DNS is used to map hostnames to network addresses, NNTP is used for publishing, retrieving, and fetching of news, and HTTP is used to get home pages on www.
Network Reference Model
