The network layer is the third layer in the OSI reference model, between the transport layer and the data link layer. It further manages the network by transmitting data frames between two adjacent endpoints provided by the data link layer. In the data communication, the data is managed to be transmitted 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 main contents are: virtual circuit packet switching and data packet switching, routing algorithm, blocking control method, X.25 protocol, integrated service data network (ISDN), asynchronous transmission mode (ATM) and internet interconnection principle and implementation.
The purpose of the network layer is to achieve transparent data transfer between two end systems, including addressing and routing, connection establishment, hold, and termination. It provides services that make it unnecessary for the transport layer to understand the data transfer and switching technologies in the network. If you want to remember the network layer with as few words as possible, it is "path selection, routing, and logical addressing."
To illustrate the function of the network layer, as shown in Figure 4.1, the switching network topology is formed by connecting several network nodes to each other according to any topology. The network layer is related to the operation control of the communication subnet, which embodies the way in which the resource subnet accesses the communication subnet in the network application environment. The network layer is generally geographically broad and logically complex, and is therefore the most complex and critical layer in the lower three layers of data communication (ie, communication subnets) in the OSI model.
Heterogeneous network interconnection
The so-called network interconnection refers to connecting two or more computer networks with one or more communication processing devices (intermediate devices) through a certain method to form a larger network system.
Physical layer relay system: repeater, integrator.
Data link layer: bridge or switch.
Network layer relay system: router.
Relay system above the network layer: gateway
2. Routing and forwarding
Routing: Refers to complex distributed algorithms. Dynamically changing the selected route based on the topology of the entire network obtained from each neighboring router
Packet forwarding: The router forwards the user's IP datagram from the appropriate port according to the forwarding table.
3. Congestion control
The phenomenon of network performance degradation due to excessive packet formation is called congestion.
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