def ros_call_service_cmd(service, _id=None, args=None):
    """
    create a rosbridge call_service command object

    :param service: name of the service to call
    :param _id: optional identifier to link the matching service_response to this request
    :param args: optional dict containing named arguments for the service

    not passed on:
    (optional) "fragment_size": <int>,
    (optional) "compression": <string>
    """
    command = {
        "op": "call_service",
        "service": service
    }

    if _id:
        command["id"] = _id

    if args:
        command["args"] = args

    return command