In the ISO/OSI reference model, what are the main functions of the network layer?

Main functions: 1. Packet and packet switching, encapsulate the data messages received from the transport layer into packets and then transmit them down to the data link layer; 2. Routing selection, through the algorithm for packet communication The subnet selects the most appropriate path; 3. Network connection multiplexing, creating logical links for the transmission of packets between nodes in the communication subnet, and multiplexing multiple network connections on one data link; 4. Error detection and Recovery; 5. Service selection; 6. Network management, managing data communication in the network; 7. Flow control; 8. Congestion control; 9. Network interconnection; 10. Fragmentation and reorganization.

The operating environment of this tutorial: Windows 7 system, Dell G3 computer.

The network layer is the third layer in the OSI reference model, between the transport layer and the data link layer. It provides the transmission of data frames between two adjacent endpoints in the data link layer. Functionally, it further manages data communication in the network and manages to transmit data from the source to the destination through several intermediate nodes, thereby providing the most basic end-to-end data transmission service to the transport layer.

The network layer mainly provides services for the transport layer. In order to provide services to the transport layer, the network layer must use the services provided by the data link layer.

The main role of the data link layer is to solve the communication between two directly adjacent nodes, but it is not responsible for solving the communication problem when the data passes through multiple transfer nodes in the communication subnet. Therefore, , in order to realize transparent transmission of data between two end systems, so that the data from the source end can transparently reach the destination end through multiple transfer nodes in the communication subnet through the optimal path, so that the transport layer does not need to care about the topology configuration of the network As well as the communication media and switching technology used, the network layer must have the following functions:

• Packet and packet switching: Encapsulate the data messages received from the transport layer into packets (Packet, Also called "packets") are then passed down to the data link layer.

• Routing: Routing algorithms are used to select the most appropriate path for packets through the communication subnet.

• Network connection multiplexing: Create logical links for the transmission of packets between nodes in the communication subnet, and multiplex multiple network connections on one data link (multiple time division is used) reuse technology).

• Error detection and recovery: Generally, the header checksum in the packet is used for error checking, and the acknowledgment and retransmission mechanism is used for error recovery.

• Service selection: The network layer can provide datagram and virtual circuit services for the transport layer, but the network layer of the Internet only provides datagram services for the transport layer.

• Network management: Manage the data communication process in the network, try to transmit the data from the source to the destination through several intermediate nodes, and provide the most basic end-to-end data for the transport layer Delivery Service.

• Traffic control: Traffic control is achieved through traffic shaping technology to prevent performance degradation of the communication subnet caused by excessive traffic.

• Congestion control: When the data traffic of the network exceeds the rated capacity, network congestion will occur, causing the network's throughput capacity to drop sharply. Appropriate control measures are therefore required for diversion.

• Network interconnection: Connect one network to another network to achieve cross-network communication between users.

• Fragmentation and reassembly: If the packet to be sent exceeds the allowed length of the protocol data unit, the network layer of the source node will fragment the packet and the fragments will reach the destination. After the host, the network layer of the destination node reassembles the original packet.

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