A URL/resource name whose content describes the type of the serialized protocol buffer message.

For URLs which use the scheme `http`, `https`, or no scheme, the following restrictions and interpretations apply:

* If no scheme is provided, `https` is assumed. * The last segment of the URL's path must represent the fully

qualified name of the type (as in `path/google.protobuf.Duration`).
The name should be in a canonical form (e.g., leading "." is
not accepted).

* An HTTP GET on the URL must yield a [google.protobuf.Type][]

value in binary format, or produce an error.

* Applications are allowed to cache lookup results based on the

URL, or have them precompiled into a binary to avoid any
lookup. Therefore, binary compatibility needs to be preserved
on changes to types. (Use versioned type names to manage
breaking changes.)

Schemes other than `http`, `https` (or the empty scheme) might be used with implementation specific semantics.