@HEAD
    public void checkValidity() {
        // Do nothing
    }
Tests whether this session resource represented a valid session at the
time it was created. This function always succeeds. It is possible for
an HTTP request aimed at this operation to fail, but that failure occurs
further up the chain when locating the session.
guacamole/src/main/java/org/apache/guacamole/rest/session/SessionResource.java