Application viewpoint: Practical application of the network layer in basic router Functions

It is very useful to know some basic router functions. Here we mainly analyze the practical application of the network layer in the basic router functions. Traditionally, a router operates on Layer 3 of the OSI Layer 7 Protocol. Its main task is to receive data packets from a network interface and decide to forward the data packets to the next destination address based on the destination address.

Therefore, the router first needs to find its destination address in the forwarding route table. If the destination address is found, add the next MAC address before the frame of the packet, at the same Time, the TTL Time To Live domain of the IP data packet header is also reduced, and the checksum is re-calculated. When a data packet is sent to the output port, it needs to wait in order to be transmitted to the output link.

When working, a router can find the route table of a device based on a certain route communication protocol. If there is more than one path to a specific node, the pre-determined routing principle is to select the optimal or most economical transmission path. Because various network segments and their interconnection conditions may change due to environment changes, the routing information is generally updated on a regular basis according to the provisions of the used routing information protocol.

In the network, the basic functions of each vro dynamically update the route table maintained by the vro according to certain rules to maintain the validity of the route information. To facilitate the transmission of packets between networks, the router always first splits large data into appropriate data packets according to the predefined rules, and then sends these data packets through the same or different paths. After these packets arrive at the destination in order, the decomposed packets are packaged into original packets in a certain order. The layered addressing function of a vro is one of the important functions of a vro. The basic functions of this vro can help networks with many node stations store addressing information, at the same time, packets sent to remote network segments can be intercepted between networks to play a forwarding role. The most reasonable route is selected, and the guiding communication is also the basic function of the router; A multi-protocol router can also connect to a network segment using different communication protocols to become a communication platform between different communication protocol network segments. Generally, the main task of a router is to store and forward data packets. The specific process is as follows:

Step 1: when a data packet arrives at the vro, The vro calls the corresponding link layer function module based on the type of the network physical interface to explain the link layer protocol header that processes the data packet. This step is simple, mainly to verify data integrity, such as CRC verification and frame length check.

Step 2: After the data frame integrity is verified at the link layer, the router starts to process the IP layer of the data frame. This process is the core of the basic router functions. Based on the destination IP address of the IP packet header in the data frame, the router searches for the IP address of the next hop in the routing table. At the same time, the TTLTime To Live domain of the IP packet header starts To be reduced, and the Checksum is re-calculated ).

Step 3: Send the IP packet to the corresponding output link layer according to the next hop IP address found in the route table, encapsulate the corresponding link layer header, and finally send the packet through the physical interface of the output network.

Simply put, the main task of a vro is to find an optimal transmission path for each data packet passing through the vro and transmit the data packet to the destination site effectively. It can be seen that selecting the optimal path policy or the best routing algorithm is the key to the router. To do this, the route Table (Routing Table) is stored in the vro for route selection. The above process describes the main and key work processes of the vro, but does not describe other additional performance, such as Access Control, network address translation, and queuing priority.