diff --git a/async/websocket_async.ml b/async/websocket_async.ml
index 229a7d6..cf19d95 100644
--- a/async/websocket_async.ml
+++ b/async/websocket_async.ml
@@ -210,7 +210,9 @@ let src =
 
 let server ?(name = "websocket.server")
     ?(check_request = fun _ -> Deferred.return true)
-    ?(select_protocol = fun _ -> None) ~reader ~writer ~app_to_ws ~ws_to_app ()
+    ?(select_protocol = fun _ -> None)
+    ?max_frame_length
+    ~reader ~writer ~app_to_ws ~ws_to_app ()
     =
   let handshake r w =
     (Request.read r >>= function
@@ -276,7 +278,9 @@ let server ?(name = "websocket.server")
       handshake reader writer)
   |> Deferred.Or_error.bind ~f:(fun () ->
          set_tcp_nodelay writer;
-         let read_frame = make_read_frame ~mode:Server reader writer in
+         let read_frame =
+           make_read_frame ?max_len:max_frame_length ~mode:Server reader writer
+         in
          let rec loop () = read_frame () >>= Pipe.write ws_to_app >>= loop in
          let transfer_end =
            let buf = Buffer.create 128 in
@@ -303,7 +307,9 @@ let server ?(name = "websocket.server")
          >>= Deferred.Or_error.return)
 
 let upgrade_connection ?(select_protocol = fun _ -> None)
-    ?(ping_interval = Time_ns.Span.of_int_sec 50) ~app_to_ws ~ws_to_app ~f
+    ?(ping_interval = Time_ns.Span.of_int_sec 50)
+    ?max_frame_length
+    ~app_to_ws ~ws_to_app ~f
     request =
   let headers = Cohttp.Request.headers request in
   let key =
@@ -328,7 +334,9 @@ let upgrade_connection ?(select_protocol = fun _ -> None)
       ()
   in
   let handler reader writer =
-    let read_frame = make_read_frame ~mode:Server reader writer in
+    let read_frame =
+      make_read_frame ?max_len:max_frame_length ~mode:Server reader writer
+    in
     let rec loop () =
       try_with read_frame >>= function
       | Error _ -> Deferred.unit
diff --git a/async/websocket_async.mli b/async/websocket_async.mli
index 79a4cca..31ab68f 100644
--- a/async/websocket_async.mli
+++ b/async/websocket_async.mli
@@ -55,13 +55,14 @@ val server :
   ?name:string ->
   ?check_request:(Cohttp.Request.t -> bool Deferred.t) ->
   ?select_protocol:(string -> string option) ->
+  ?max_frame_length:int ->
   reader:Reader.t ->
   writer:Writer.t ->
   app_to_ws:Frame.t Pipe.Reader.t ->
   ws_to_app:Frame.t Pipe.Writer.t ->
   unit ->
   unit Deferred.Or_error.t
-(** [server ?request_cb reader writer app_to_ws
+(** [server ?request_cb ?max_frame_length reader writer app_to_ws
     ws_to_app ()] returns a thread that expects a websocket client
     connected to [reader]/[writer] and, after performing the
     handshake, will resp. read outgoing frames from [app_to_ws] and
@@ -71,17 +72,19 @@ val server :
     reception of the client HTTP request, [request_cb] will be called
     with the request as its argument. If [request_cb] returns true,
     the connection will proceed, otherwise, the result is immediately
-    determined to [Error Exit]. *)
+    determined to [Error Exit]. If [max_frame_length] is specified and
+    the server receives a frame above this size, the connection is closed. *)
 
 val upgrade_connection :
   ?select_protocol:(string -> string option) ->
   ?ping_interval:Core.Time_ns.Span.t ->
+  ?max_frame_length:int ->
   app_to_ws:Frame.t Pipe.Reader.t ->
   ws_to_app:Frame.t Pipe.Writer.t ->
   f:(unit -> unit Deferred.t) ->
   Cohttp.Request.t ->
   Cohttp.Response.t * (Reader.t -> Writer.t -> unit Deferred.t)
-(** [upgrade_connection ?select_protocol ?ping_interval
+(** [upgrade_connection ?select_protocol ?ping_interval ?max_frame_length
     app_to_ws ws_to_app f request] returns a {!Cohttp_async.Server.response_action}.
 
     Just wrap the return value of this function with [`Expert].
diff --git a/core/websocket.ml b/core/websocket.ml
index 86ec131..222e577 100644
--- a/core/websocket.ml
+++ b/core/websocket.ml
@@ -155,6 +155,7 @@ module type S = sig
   type mode = Client of (int -> string) | Server
 
   val make_read_frame :
+    ?max_len:int ->
     ?buf:Buffer.t -> mode:mode -> IO.ic -> IO.oc -> unit -> Frame.t IO.t
 
   val write_frame_to_buf : mode:mode -> Buffer.t -> Frame.t -> unit
@@ -177,6 +178,7 @@ module type S = sig
     type t
 
     val create :
+      ?max_len:int ->
       ?read_buf:Buffer.t ->
       ?write_buf:Buffer.t ->
       Cohttp.Request.t ->
@@ -265,7 +267,7 @@ module Make (IO : Cohttp.S.IO) = struct
     write_frame_to_buf ~mode buf @@ Frame.close code;
     write oc @@ Buffer.contents buf
 
-  let read_frame ic oc buf mode hdr =
+  let read_frame ?max_len ic oc buf mode hdr =
     let hdr_part1 = EndianString.BigEndian.get_int8 hdr 0 in
     let hdr_part2 = EndianString.BigEndian.get_int8 hdr 1 in
     let final = is_bit_set 7 hdr_part1 in
@@ -291,6 +293,10 @@ module Make (IO : Cohttp.S.IO) = struct
     else if extension <> 0 then
       close_with_code mode buf oc 1002 >>= fun () ->
       proto_error "unsupported extension"
+    else if (match max_len with Some max -> payload_len > max | None -> false)
+    then
+      close_with_code mode buf oc 1009 >>= fun () ->
+      proto_error "frame payload too big"
     else if Frame.Opcode.is_ctrl opcode && payload_len > 125 then
       close_with_code mode buf oc 1002 >>= fun () ->
       proto_error "control frame too big"
@@ -315,11 +321,11 @@ module Make (IO : Cohttp.S.IO) = struct
             let frame = Frame.of_bytes ~opcode ~extension ~final payload in
             return frame)
 
-  let make_read_frame ?(buf = Buffer.create 128) ~mode ic oc () =
+  let make_read_frame ?max_len ?(buf = Buffer.create 128) ~mode ic oc () =
     Buffer.clear buf;
     read_exactly ic 2 buf >>= function
     | None -> raise End_of_file
-    | Some hdr -> read_frame ic oc buf mode hdr
+    | Some hdr -> read_frame ?max_len ic oc buf mode hdr
 
   module Request = Cohttp.Request.Make (IO)
   module Response = Cohttp.Response.Make (IO)
@@ -337,9 +343,9 @@ module Make (IO : Cohttp.S.IO) = struct
 
     let source { endp; _ } = endp
 
-    let create ?read_buf ?(write_buf = Buffer.create 128) http_request endp ic
+    let create ?max_len ?read_buf ?(write_buf = Buffer.create 128) http_request endp ic
         oc =
-      let read_frame = make_read_frame ?buf:read_buf ~mode:Server ic oc in
+      let read_frame = make_read_frame ?max_len ?buf:read_buf ~mode:Server ic oc in
       {
         buffer = write_buf;
         endp;
diff --git a/core/websocket.mli b/core/websocket.mli
index 04f1689..359761b 100644
--- a/core/websocket.mli
+++ b/core/websocket.mli
@@ -84,6 +84,7 @@ module type S = sig
   type mode = Client of (int -> string) | Server
 
   val make_read_frame :
+    ?max_len:int ->
     ?buf:Buffer.t -> mode:mode -> IO.ic -> IO.oc -> unit -> Frame.t IO.t
 
   val write_frame_to_buf : mode:mode -> Buffer.t -> Frame.t -> unit
@@ -106,6 +107,7 @@ module type S = sig
     type t
 
     val create :
+      ?max_len:int ->
       ?read_buf:Buffer.t ->
       ?write_buf:Buffer.t ->
       Cohttp.Request.t ->
diff --git a/lwt/websocket_cohttp_lwt.ml b/lwt/websocket_cohttp_lwt.ml
index 434ae3e..c9de49e 100644
--- a/lwt/websocket_cohttp_lwt.ml
+++ b/lwt/websocket_cohttp_lwt.ml
@@ -29,12 +29,12 @@ let send_frames stream oc =
   in
   Lwt_stream.iter_s send_frame stream
 
-let read_frames ic oc handler_fn =
-  let read_frame = Lwt_IO.make_read_frame ~mode:Server ic oc in
+let read_frames ?max_len ic oc handler_fn =
+  let read_frame = Lwt_IO.make_read_frame ?max_len ~mode:Server ic oc in
   let rec inner () = read_frame () >>= Lwt.wrap1 handler_fn >>= inner in
   inner ()
 
-let upgrade_connection request incoming_handler =
+let upgrade_connection ?max_frame_length request incoming_handler =
   let headers = Cohttp.Request.headers request in
   (match Cohttp.Header.get headers "sec-websocket-key" with
   | None ->
@@ -61,7 +61,7 @@ let upgrade_connection request incoming_handler =
       [
         (* input: data from the client is read from the input channel
          * of the tcp connection; pass it to handler function *)
-        read_frames ic oc incoming_handler;
+        read_frames ?max_len:max_frame_length ic oc incoming_handler;
         (* output: data for the client is written to the output
          * channel of the tcp connection *)
         send_frames frames_out_stream oc;
diff --git a/lwt/websocket_cohttp_lwt.mli b/lwt/websocket_cohttp_lwt.mli
index b05cf57..3043f0f 100644
--- a/lwt/websocket_cohttp_lwt.mli
+++ b/lwt/websocket_cohttp_lwt.mli
@@ -19,6 +19,7 @@
 open Websocket
 
 val upgrade_connection :
+  ?max_frame_length:int ->
   Cohttp.Request.t ->
   (Frame.t -> unit) ->
   (Cohttp_lwt_unix.Server.response_action * (Frame.t option -> unit)) Lwt.t
diff --git a/lwt/websocket_lwt_unix.ml b/lwt/websocket_lwt_unix.ml
index 4080aa7..94943e1 100644
--- a/lwt/websocket_lwt_unix.ml
+++ b/lwt/websocket_lwt_unix.ml
@@ -101,11 +101,12 @@ let write { write_frame; _ } frame = write_frame frame
 let close_transport { oc; _ } = Lwt_io.close oc
 
 let connect ?(extra_headers = Cohttp.Header.init ())
+    ?max_frame_length
     ?(random_string = Websocket.Rng.init ())
     ?(ctx = Lazy.force Conduit_lwt_unix.default_ctx) ?buf client url =
   let nonce = Base64.encode_exn (random_string 16) in
   connect ctx client url nonce extra_headers >|= fun (ic, oc) ->
-  let read_frame = make_read_frame ?buf ~mode:(Client random_string) ic oc in
+  let read_frame = make_read_frame ?max_len:max_frame_length ?buf ~mode:(Client random_string) ic oc in
   let read_frame () =
     Lwt.catch read_frame (fun exn ->
         Lwt.async (fun () -> Lwt_io.close ic);
@@ -135,7 +136,7 @@ let write_failed_response oc =
   let open Response in
   write ~flush:true (fun writer -> write_body writer body) response oc
 
-let server_fun ?read_buf ?write_buf check_request flow ic oc react =
+let server_fun ?max_len ?read_buf ?write_buf check_request flow ic oc react =
   let read = function
     | `Ok r -> Lwt.return r
     | `Eof ->
@@ -181,11 +182,11 @@ let server_fun ?read_buf ?write_buf check_request flow ic oc react =
   in
   Response.write (fun _writer -> Lwt.return_unit) response oc >>= fun () ->
   let client =
-    Connected_client.create ?read_buf ?write_buf request flow ic oc
+    Connected_client.create ?max_len ?read_buf ?write_buf request flow ic oc
   in
   react client
 
-let establish_server ?read_buf ?write_buf ?timeout ?stop
+let establish_server ?max_frame_length ?read_buf ?write_buf ?timeout ?stop
     ?(on_exn = fun exn -> !Lwt.async_exception_hook exn)
     ?(check_request = check_origin_with_host)
     ?(ctx = Lazy.force Conduit_lwt_unix.default_ctx) ~mode react =
@@ -194,7 +195,8 @@ let establish_server ?read_buf ?write_buf ?timeout ?stop
       set_tcp_nodelay flow;
       Lwt.catch
         (fun () ->
-          server_fun ?read_buf ?write_buf check_request
+          server_fun ?max_len:max_frame_length ?read_buf ?write_buf
+            check_request
             (Conduit_lwt_unix.endp_of_flow flow)
             ic oc react)
         (function
@@ -211,9 +213,11 @@ let mk_frame_stream recv =
   in
   Lwt_stream.from f
 
-let establish_standard_server ?read_buf ?write_buf ?timeout ?stop ?on_exn
+let establish_standard_server ?max_frame_length ?read_buf ?write_buf ?timeout
+    ?stop ?on_exn
     ?check_request ?(ctx = Lazy.force Conduit_lwt_unix.default_ctx) ~mode react
     =
   let f client = react (Connected_client.make_standard client) in
-  establish_server ?read_buf ?write_buf ?timeout ?stop ?on_exn ?check_request
+  establish_server ?max_frame_length ?read_buf ?write_buf ?timeout ?stop
+    ?on_exn ?check_request
     ~ctx ~mode f
diff --git a/lwt/websocket_lwt_unix.mli b/lwt/websocket_lwt_unix.mli
index 2ad2736..96d9b08 100644
--- a/lwt/websocket_lwt_unix.mli
+++ b/lwt/websocket_lwt_unix.mli
@@ -39,6 +39,7 @@ val close_transport : conn -> unit Lwt.t
 
 val connect :
   ?extra_headers:Cohttp.Header.t ->
+  ?max_frame_length:int ->
   ?random_string:(int -> string) ->
   ?ctx:Conduit_lwt_unix.ctx ->
   ?buf:Buffer.t ->
@@ -47,6 +48,7 @@ val connect :
   conn Lwt.t
 
 val establish_server :
+  ?max_frame_length:int ->
   ?read_buf:Buffer.t ->
   ?write_buf:Buffer.t ->
   ?timeout:int ->
@@ -71,6 +73,7 @@ val mk_frame_stream :
     is received, the stream will be closed. *)
 
 val establish_standard_server :
+  ?max_frame_length:int ->
   ?read_buf:Buffer.t ->
   ?write_buf:Buffer.t ->
   ?timeout:int ->