Exo implements a display server on top of the Aura Shell. It uses the Wayland protocol to communicate with clients. For a general introduction to Wayland see https://wayland-book.com/.

Current clients of Exo include:

• ARC++ (Android apps on Chrome OS)
• Crostini (Linux apps on Chrome OS)
• Lacros
• PluginVM

In addition to the core Wayland protocol, Exo supports a number of protocol extensions. Some are third-party; see //third_party/wayland-protocols/README.chromium. Others are Chromium-specific.

A few noteworthy extensions (this list is not at all exhaustive):

• zaura_shell
  • A Chromium-specific protocol used by all Exo clients. See //components/exo/wayland/protocol/aura-shell.xml and //components/exo/wayland/zaura_shell.h
• zcr_remote_shell
  • A Chromium-specific protocol used exclusively by ARC++. See //components/exo/wayland/zcr_remote_shell.h and //components/exo/client_controlled_shell_surface.h
• zwp_fullscreen_shell
  • A third-party protocol, used in Chromium only by Chromecast. See //components/exo/wayland/zwp_fullscreen_shell.h