made a weird deck a long time ago about this kinda stuff

doc/deck_example.ml

@@ -0,0 +1,17 @@
+let rec copy_blocks buffer r w =
+  match%lwt Lwt_io.read_into r buffer 0 (Bytes.length buffer) with
+  | 0 -> Lwt.return_unit
+  | bytes_read ->
+    let%lwt () = Lwt_io.write_from_exactly w buffer 0 bytes_read in
+    copy_blocks buffer r w
+
+let run () =
+  ((let%lwt server =
+      Lwt_io.establish_server (Lwt_unix.ADDR_INET (Unix.inet_addr_any, 8765))
+        (fun (r, w) ->
+           let buffer = Bytes.create (16 * 1024) in
+           copy_blocks buffer r w)
+    in
+    Lwt.return server) : Lwt_io.server Lwt.t) |> ignore
+
+let () =  Lwt_main.run   run ();