Reassembly is a term from the OSI world and stands for the reassembly of data packet fragments. If fragments of a datagram are sent over the networks as individual independent data packets, then the individual fragments must be reassembled at the receiver. This mechanism is called reassembly.
Reassembly represents the reverse function of fragmentation. Here, several service data units( SDU) are combined into one protocol data unit( PDU). However, such a reassembly must meet certain requirements, for example the individual data packets must have the same source addresses and destination addresses, they must have the same protocol type identifier and the same identification number. To avoid unnecessary delays, there is also a time limit in which the individual data packets must be available for reassembly. Reassembly is terminated when the last fragment is received. Reassembling takes place at the network layer.