两次跳转后session丢失

public ActionResult index()
        {
            Session["a"] = "aaa";
            System.Web.HttpContext.Current.Session["b"] = "bbb";            
            Response.Redirect("about");
            //return Content("");
            return Redirect("about");
        }

        public ActionResult about()
        {
            var a1 = Session["a"];
            var a2 = System.Web.HttpContext.Current.Session["a"];

            var b1 = Session["b"];
            var b2 = System.Web.HttpContext.Current.Session["b"];
            return Content(a1.ToString() + b2.ToString());
        }