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Chapter 2 OSI reference model and TCP/IP Model
Tags: computer technology, computer network, information service, scale, communication technology, original works, can be reprinted, please be sure to mark the original source of the article, author information and this statement in the form of a hyperlink. Otherwise, legal liability will be held. http://3157957.blog.51cto.com/3147957/1718737OSI The advantages of the layer-7 Reference Model Open standardization excuse for multi-vendor compatibility easy to understand, learn, and update protocol standards to implement modular engineering, reducing the complexity of development implementation and facilitating troubleshooting 2. the relationship between the layers of the OSI reference model and the Data encapsulation each layer of the OSI reference model defines the implemented functions to complete certain communication tasks, and only exchange data with the adjacent upper and lower layers. The physical layer involves the original bit streams transmitted over communication channels. It defines the mechanical, electrical, functional, and procedural features required for data transmission, including voltage, cable, data transmission rate, interface definition, etc. The main task of the data link layer is to provide control over the physical layer, detect and correct possible errors, and control traffic. The data link layer is related to physical addresses, network topology, cable planning, error detection, and traffic control. The Network Layer determines the optimal route of the transmission package. The key issue is to determine how to select a route from the source to the destination. The network layer uses the route selection protocol to calculate the route. The basic function of the transport layer is to accept data from the Session Layer and divide the data into smaller units when necessary, and transmit the data to the network layer. This ensures that the information of each segment arriving at the other party is correct, the transmission layer establishes and maintains virtual circuits, and performs error detection and traffic control. The Session Layer allows users on different machines to establish, manage, and terminate session relationships between applications. When coordinating communications between different applications, the Session Layer is required, each application in this layer knows the status of other applications. The Session Layer also provides duplex negotiation and session synchronization. The presentation layer focuses on the syntax and semantics of the transmitted information. It processes computer-related data formats from the application layer into computer-independent formats, to ensure that the Peer device can accurately understand the data of the sender. At the same time, the presentation layer also has complex data encryption. The application layer is the layer closest to the user of the OSI reference model and is responsible for providing network services for applications. Network services include file transmission, text management, and email message processing. 3. Physical Layer: describes and defines the interconnection between physical devices through physical media. Defining mechanical properties such as voltage, interface, cable standard, and transmission distance: Describes the shape and size, number of leads, and arrangement of connectors used for interfaces, for example, we have seen strict rules on the sizes of various specifications of the power plug. Electrical Characteristics: describes the range of voltage and current on which line of the interface cable appears. Feature: describes the meaning of the voltage of an electric flat on a single line. Procedure features: describes the sequence of events that may occur for different functions. Common Physical layer standard for physical layer media twisted pair wires, coaxial cables, optical fiber cables, radio signal lights, lan physical layer standard: 10Base-T, 100Base-TX/FX, 1000Base-T, 1000Base-SX/LX common equipment: repeater, Hub Wan common physical layer standard: RS-232, V.24, V.35 common equipment: Modem 4. the data link layer transmits data on a specific media or link. The main functions of the data link layer are frame encoding and frame recognition: bit encoding is made into frames, and frames are recognized from a series of bit streams, which are unbound and transmitted to a network layer. Data Link establishment, maintenance, and release: When devices in the network need to communicate, the communication parties sometimes have to establish a data link first, and ensure security when establishing the link, data links should be maintained during transmission, and data links should be released after communication ends. Transmission resource control: In some shared media, multiple terminal devices may need to send data at the same time. In this case, the data link layer protocol must decide on resource allocation. Traffic control: In order to ensure normal data transmission and receiving, prevent sending data too quickly, cause the buffer space of the recipient to overflow, and the network is congested, the data transmission rate of the sender must be controlled in a timely manner. Error Control: errors may occur during bit transmission, but the physical layer cannot identify errors. Therefore, the data link layer protocol must implement Error Detection in frames. Addressing: the data link layer protocol should be able to identify all nodes on the media and find the target node to send data to the correct destination. Identify upper-layer data: the data link layer uses transparent transmission to transmit network layer packets. It presents the network layer as a line without errors. To support multiple network layer protocols on the same link, the sender must identify the network layer protocol to which the load (that is, the packet) belongs in the frame control information, in this way, the receiver can submit the load to the correct upper-layer protocol for processing. The IEEE data link layer standard is the most popular LAN standard today. These standards are collectively referred to as IEEE802 standard 802.1, which describes the basic problems to be solved by LAN, for example, 802.1d describes the Decision-Making protocol 802.2 team responsible for the development of the LLC sub-layer standard 802.3 team responsible for the development of the MAC Sub-layer standard, typical Technologies such as CSMA/CD802.4 team is responsible for the formulation of the Token Bus standard 802.5 team is responsible for the formulation of the specification ring network standard Wide Area Network common link layer standards include HDLC, PPP, X.25, Frame Relay HDLC is ISO-developed one data link layer protocol for bit synchronization, it specifies the Data encapsulation method for synchronous serial links using frame characters and checksum. PPP is described by RFC1661. Supports synchronous and asynchronous connections and multiple network layer protocols. Frame Relay is an exchange-type data link protocol. 5. For tasks at the network layer, select a proper path and forward data packets so that the packets can be correctly transmitted from the sender to the receiver. Address: the network layer assigns an identifier for each node. This is the address of the network layer. Address Allocation also provides the basis from the source to the destination path. Route Selection: A key role of the network layer is to determine how to select a route for data transmission from the source to the destination. After the network layer device computes the route, it forwards data packets according to the route information. Congestion Control: if the network transmits too many data packets at the same time, congestion may occur, resulting in data loss or delay. The network layer is also responsible for controlling network congestion. Heterogeneous network interconnection: communication links and media types are diverse. Each link has special communication requirements. The network layer must be able to work on a variety of links and media types, to provide communication services across multiple CIDR blocks. The network layer is between the transport layer and the data link layer. It is responsible for providing services to the transport layer and translating network addresses into corresponding physical addresses. The network layer protocol can also coordinate the unbalanced processing capabilities of the sending, transmitting, and receiving devices. For example, the network layer can segment and reorganize data, so that the length of the data packet can meet the maximum length of the data frame supported by the data link layer protocol of the link. The routed protocol is a network-layer encapsulation protocol that defines the format and usage of fields in a data packet. It allows data packets to be forwarded from one network device to another. Common routing protocols include the IP protocol in the TCP/IP protocol family and the IPX protocol in the Novell IPX/SPX protocol family. The routing protocol (routing protocol) runs on a vro and transmits information between vrouters. It calculates the route used for forwarding and forms a route table to provide routing selection services for the routing protocol. The routing protocol enables route information to be transmitted between neighboring routers, so that all routers can understand the paths to each destination. In computer communication, connection-oriented services (Connect-oriented Service) and Connectionless services are an important concept. Generally, connection-oriented services provide reliable message sequences. The receiver confirms each packet received so that the sender is sure that the packet sent has reached the destination. The validation process adds additional overhead and latency, but if the message is lost, the sender can resend the message. After a connection is established, each user can send a variable-length packet, which is sent to the remote entity in order. Under normal circumstances, when two packets are sent to the same destination, the first packet is received first, but the first packet may be delayed in transit, resulting in the first packet to be sent. The receiver uses the serial number to determine whether the received packets are in disorder and arrange them in the correct order. Connection-oriented services are more suitable for sending many packets to the same destination within a certain period of time. For short packet data transmission, connection-oriented services seem overhead. In the connectionless service, the communication between two entities does not need to establish a connection first, so the resources at the lower layer do not need to be reserved in advance, these resources are dynamically allocated during data transmission. The connectionless service is based on the postal system. Each packet carries a complete destination address, and each message is independent of other packets and transmitted through the route selected by the system. The connectionless service provides the best-effort service, that is, the network tries its best to forward packets based on the current resources, but does not guarantee the exact service quality. 6. The function of the transport layer is to provide an error-free transport link for the Session Layer, ensuring that the information transmitted between the two devices is correct. The data transmission layer at the upper layer receives data from the Session Layer and transmits the data to the network layer. If the Session Layer is too large, the transport layer splits the data into smaller data units (segments) for transmission. The end-to-end connection transport layer is responsible for creating end-to-end communication connections. Through this layer, applications on the host of both parties communicate directly through the address information of the other party, regardless of the number of intermediate nodes on the network. The transparent transmission transport layer can establish a separate connection for each session layer request. It can also create a separate connection for Multiple Session Layer requests based on the connection usage, this is called Multiplexing ). However, in any case, the transport layer service is transparent and reliable for the Session Layer. An important task of the transmission layer is error checksum and retransmission. In order to avoid sending speeds exceeding the processing capability of the network or receiver, the transport layer is also responsible for implementing flow control to reduce traffic when resources are insufficient, when resources are sufficient, the traffic is increased. 7. The Session Layer, presentation Layer, and application Layer are end-to-end services provided by the transport Layer to provide Session services to the presentation Layer or Session users. The Session Layer establishes a session relationship and ensures that the session process is smooth. It determines whether the communication is interrupted and where to re-send the next communication. The Session Layer also handles error recovery. The Presentation Layer (Presentation Layer) is responsible for "representing" the information at the application Layer into a format that allows the peer device to correctly identify. It focuses on the semantics and syntax of the transmitted information. In the presentation layer, data will be encoded in accordance with a method agreed by consensus, so that computers using the same presentation layer protocol can recognize data. The presentation layer is also responsible for data encryption and compression. The Application Layer is the top Layer of OSI. It deals directly with users and applications and provides interfaces to the software to enable the program to use network services.
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