The reachability component runs network health checks to determine the service level of networks known to the host. It exposes an API fuchsia.net.reachability, that components can use to subscribe to changes in network service levels. The intricacies of the API behavior are located in the API's [FIDL definition].
The rest of the README will outline the architecture for the reachability component and how it is used to calculate reachability state for different network conditions.
Reachability is expected to run while the device is running, so panicking or returning out of the loop is avoided when error handling is possible.
Its state calculations are dependent on real-time data from its sources. These sources include:
fuchsia.net.interfaces: provides interface eventsfuchsia.net.neighbor: provides neighbor cache updatesfuchsia.net.routes.ipv4: provides IPv4 route change eventsfuchsia.net.routes.ipv6: provides IPv6 route change eventsThe data retrieved from these sources are used to make more informed decisions surrounding the device's level of reachability.
Reachability conducts health checks to determine the service level of network services and reports its findings via the API.
To accomplish this, the component creates a state machine for each interface to perform a network check at a constant cadence. The state machine runs each interface through a network check, which is a suite of health checks for typical network services. The network check state machine is re-entrant and asynchronous to allow Reachability to schedule network service checks and run them alongside the intake of new data from the watchers. After a network check has completed, another one can be started for the associated interface.
There are several services tested in reachability:
These signals are exported via the fuchsia.net.reachability API.
Gateway and internet state are determined in sequence. For example, if a system does not have gateway reachability, internet availability will not be evaluated.
Gateway reachability is successful if one of the two checks pass:
fuchsia.net.neighbor contain a healthy router>= 1 attempted ICMP echo requests to a gateway address responded to the hostInternet is available if >= 1 attempted ICMP echo requests to external IPv4 and IPv6 addresses respond to the host.
DNS is checked separately from internet and gateway signals. To determine whether DNS is active, reachability performs a call using [fuchsia.net.name/Lookup] to determine whether IP addresses can be discovered by using a hostname.
State Machine Diagram 