[Boost-users] Why is basic_stream_socket::get_io_service not const?

According to the documentation [1], the member function get_io_service has the following synopsis: boost::asio::io_service & get_io_service(); Why is this function not constant? What does it change within the socket? [1] <URL:http://www.boost.org/doc/libs/1_41_0/doc/html/boost_asio/reference/basic_stream_socket/get_io_service.html>