udp_write() gets a packet from a user process, adds a udp header and an ip header, and then passes the packet to the next lower layer (i.e., the ip code). udp_write() is called (indirectly) by sr_rwio(), which is called when the network service receives a write request from the file system (FS). Most of the work involved in moving the data from the udp layer to the ip layer is done by restart_write_fd().