def _inbound(self, From, to, protocol, udp_source=None):
        """
        Implementation of L{Inbound}.
        """
        # Verify stuff!

        self.verifyCertificateAllowed(to, From)
        return self.service.verifyHook(From, to, protocol
                                       ).addCallback(self._inboundimpl,
                                                     From,
                                                     to,
                                                     protocol,
                                                     udp_source).addErrback(
            lambda f: f.trap(KeyError) and dict(listeners=[]))
Implementation of L{Inbound}.