The layering type of a route.
Plain routes are established by connecting or tunnelling.
Layered routes are established by layering a protocol such as TLS/SSL
over an existing connection.
Protocols can only be layered over a tunnel to the target, or
or over a direct connection without proxies.
Layering a protocol
over a direct connection makes little sense, since the connection
could be established with the new protocol in the first place.
But we don't want to exclude that use case.
Returns the enum constant of this type with the specified name.
The string must match exactly an identifier used to declare an
enum constant in this type. (Extraneous whitespace characters are
name - the name of the enum constant to be returned.