The so-called port also refers to the transport layer service access point TSAP, that is, the transport layer address. Messages from different processes in the application layer are sent down to the transport layer through different ports, multiplexed into a transport connection by the transport layer, and then sent to the network layer. When these messages arrive at the destination host, the transport layer of the destination host enables the separation function and submits the messages to the corresponding application process for processing through different ports.