def handshake_headers(self):
        """
        List of headers appropriate for the upgrade
        handshake.
        """
        headers = [
            ('Host', self.host),
            ('Connection', 'Upgrade'),
            ('Upgrade', 'WebSocket'),
            ('Sec-WebSocket-Key', self.key.decode('utf-8')),
            # Origin is proxyed from the downstream server, don't set it twice
            # ('Origin', self.url),
            ('Sec-WebSocket-Version', str(max(WS_VERSION)))
        ]

        if self.protocols:
            headers.append(('Sec-WebSocket-Protocol', ','.join(self.protocols)))

        if self.extra_headers:
            headers.extend(self.extra_headers)

        logger.info("Handshake headers: %s", headers)
        return headers
List of headers appropriate for the upgrade
        handshake.