Failover
In simple terms, when A is unable to serve customers, the system can automatically switch to B, allowing it to take over and continue providing services to customers without them noticing the change. Here, A and B can exist in various fields, but failover generally refers to the failover of databases, application services, hardware devices, and other computer-related fields.

Failback
In a cluster network system (a network with two or more interconnected servers), when a server needs to be maintained, network resources and services need to be temporarily redirected to a backup system. The process of restoring network resources and servers to be provided by the original host is called automatic recovery.

MTU
Maximum Transmission Unit, the maximum limit of data frame length. If the length of a data packet exceeds this value, it needs to be fragmented at the IP layer. Any data block exceeding the MTU will be divided into smaller transmission units before transmission. MTU has two measurement levels: network layer and link layer. For example, the standard Internet MTU at the network layer is 1500 bytes, while at the link layer it is 1518 bytes. Unless otherwise specified, MTU usually refers to the network layer.

A common reason to increase the MTU of a network interface is to increase the throughput of high-speed Internet. The standard Internet MTU of 1500 bytes is for backward compatibility with 10M and 100M networks, but it is far from sufficient for current 1G and 10G networks. New network devices can handle larger MTUs, but they need to be explicitly set. These larger MTU frames are called "jumbo frames," typically 9000 bytes. MTU cannot be mixed with any current Internet protocol, but different MTU segments can be connected using a router.

source: TCP/IP model