Netstack is a userspace TCP/IP network stack and interfaces with zircon network drivers. Netstack serves as a back-end for fdio socket API.
+-----------+ +-----------+ | FIDL app | | POSIX app | +--+--------+ +-----+-----+ | | | | | | | | | | | | | +----------------------v---------+ | | BSD socket API | | | (//zircon/system/ulib/fdio) | | +---------+----------------------+ | | +--v-------------v----------------------+ | netstack | | (//src/connectivity/network/netstack) | +----------------+----------------------+ | +----------------v-----------------+ | Ethernet driver | | (//zircon/system/udev/ethernet) | +----------------------------------+