Send data: The data to be sent to the Socket by calling the system call. Socket checks the type of connection to call the appropriate Send function (usually inet). The Send function checks the status of the socket, and the type of the protocol, and then passes the data to the transport layer (usually TCP or UDP). The protocol of the transport layer creates a SK_BUFFER type SKB for these data, copies the data into the SKB, and adds the header of the transport layer protocol (such as a port number, checksum, etc.) to the SKB, and then the SKB down layer ( IP layer) passes. The IP layer adds the IP head to SKB (such as IP addresses, options, checksums, etc.). If the length of the package exceeds the MTU, it is to slide this package. The IP layer then passes the SKB to the link layer, and the link layer is responsible for writing all the data in the SKB to the XMIT parallel of the sending device, and finally, the device (NIC) sends the data.
