diff --git a/projects/miragesdk/src/sdk/IO.ml b/projects/miragesdk/src/sdk/IO.ml
index 2d9345729..a03189370 100644
--- a/projects/miragesdk/src/sdk/IO.ml
+++ b/projects/miragesdk/src/sdk/IO.ml
@@ -4,18 +4,20 @@ let src = Logs.Src.create "IO" ~doc:"IO helpers"
 module Log = (val Logs.src_log src : Logs.LOG)
 
 let rec really_write fd buf off len =
-  Log.debug (fun l -> l "really_write");
   match len with
   | 0   -> Lwt.return_unit
   | len ->
+    Log.debug (fun l -> l "really_write off=%d len=%d" off len);
     Lwt_unix.write fd buf off len >>= fun n ->
     really_write fd buf (off+n) (len-n)
 
+let write fd buf = really_write fd buf 0 (String.length buf)
+
 let rec really_read fd buf off len =
-  Log.debug (fun l -> l "really_read");
   match len with
   | 0   -> Lwt.return_unit
   | len ->
+    Log.debug (fun l -> l "really_read off=%d len=%d" off len);
     Lwt_unix.read fd buf off len >>= fun n ->
     really_read fd buf (off+n) (len-n)
 
@@ -33,7 +35,7 @@ let read_all fd =
   String.concat "" bufs
 
 let read_n fd len =
-  Log.debug (fun l -> l "read_n");
+  Log.debug (fun l -> l "read_n len=%d" len);
   let buf = Bytes.create len in
   let rec loop acc len =
     Lwt_unix.read fd buf 0 len >>= fun n ->
diff --git a/projects/miragesdk/src/sdk/IO.mli b/projects/miragesdk/src/sdk/IO.mli
index 121ba33c7..b07954d18 100644
--- a/projects/miragesdk/src/sdk/IO.mli
+++ b/projects/miragesdk/src/sdk/IO.mli
@@ -3,6 +3,9 @@
 val really_write: Lwt_unix.file_descr -> string -> int -> int -> unit Lwt.t
 (** [really_write fd buf off len] writes exactly [len] bytes to [fd]. *)
 
+val write: Lwt_unix.file_descr -> string -> unit Lwt.t
+(** [write fd buf] writes all the buffer [buf] in [fd]. *)
+
 val really_read: Lwt_unix.file_descr -> string -> int -> int -> unit Lwt.t
 (** [really_read fd buf off len] reads exactly [len] bytes from [fd]. *)
 
diff --git a/projects/miragesdk/src/sdk/ctl.ml b/projects/miragesdk/src/sdk/ctl.ml
index d561e067b..4bad89563 100644
--- a/projects/miragesdk/src/sdk/ctl.ml
+++ b/projects/miragesdk/src/sdk/ctl.ml
@@ -41,7 +41,7 @@ module Message = struct
   type t = {
     operation: operation;
     path     : string;
-    payload  : string option;
+    payload  : string;
   }
 
   [%%cstruct type message = {
@@ -53,48 +53,65 @@ module Message = struct
 
   (* to avoid warning 32 *)
   let _ = hexdump_message
-  let _ = operation_to_string
   let _ = string_to_operation
 
-  let read_message fd =
-    IO.read_n fd 4 >>= fun buf ->
-    let len =
-      Cstruct.LE.get_uint32 (Cstruct.of_string buf) 0
-      |> Int32.to_int
-    in
-    IO.read_n fd len >>= fun buf ->
-    let buf = Cstruct.of_string buf in
+  let pp ppf t =
+    Fmt.pf ppf "%s:%S:%S" (operation_to_string t.operation) t.path t.payload
+
+  (* FIXME: allocate less ... *)
+
+  let of_cstruct buf =
+    Log.debug (fun l -> l "Message.of_cstruct %S" @@ Cstruct.to_string buf);
     let operation = match int_to_operation (get_message_operation buf) with
       | None   -> failwith "invalid operation"
       | Some o -> o
     in
     let path_len = get_message_path buf in
     let payload_len = get_message_payload buf in
-    IO.read_n fd path_len >>= fun path ->
-    (match payload_len with
-     | 0 -> Lwt.return None
-     | n -> IO.read_n fd n >|= fun x -> Some x)
-    >|= fun payload ->
+    Log.debug (fun l -> l "XXX path=%d len=%d" path_len payload_len);
+    let path =
+      Cstruct.sub buf sizeof_message path_len
+      |> Cstruct.to_string
+    in
+    let payload =
+      Cstruct.sub buf (sizeof_message+path_len) payload_len
+      |> Cstruct.to_string
+    in
     { operation; path; payload }
 
-  let write_message fd msg =
+  let to_cstruct msg =
+    Log.debug (fun l -> l "Message.to_cstruct %a" pp msg);
     let operation = operation_to_int msg.operation in
     let path = String.length msg.path in
-    let payload = match msg.payload with
-      | None   -> 0
-      | Some x -> String.length x
-    in
+    let payload = String.length msg.payload in
     let len = sizeof_message + path + payload in
     let buf = Cstruct.create len in
     set_message_operation buf operation;
     set_message_path buf path;
-    set_message_payload buf path;
+    set_message_payload buf payload;
     Cstruct.blit_from_bytes msg.path 0 buf sizeof_message path;
-    let () = match msg.payload with
-      | None   -> ()
-      | Some x -> Cstruct.blit_from_bytes x 0 buf (sizeof_message+path) payload
+    Cstruct.blit_from_bytes msg.payload 0 buf (sizeof_message+path) payload;
+    buf
+
+  let read fd =
+    IO.read_n fd 4 >>= fun buf ->
+    Log.debug (fun l -> l "Message.read len=%S" buf);
+    let len =
+      Cstruct.LE.get_uint32 (Cstruct.of_string buf) 0
+      |> Int32.to_int
     in
-    IO.really_write fd (Cstruct.to_string buf) 0 len
+    IO.read_n fd len >|= fun buf ->
+    of_cstruct (Cstruct.of_string buf)
+
+  let write fd msg =
+    let buf = to_cstruct msg |> Cstruct.to_string in
+    let len =
+      let len = Cstruct.create 4 in
+      Cstruct.LE.set_uint32 len 0 (Int32.of_int @@ String.length buf);
+      Cstruct.to_string len
+    in
+    IO.write fd len >>= fun () ->
+    IO.write fd buf
 
 end
 
@@ -118,9 +135,7 @@ module Dispatch = struct
         match msg.operation with
         | Write ->
           let info = infof "Updating %a" KV.Key.pp key in
-          (match msg.payload with
-           | None   -> Fmt.kstrf Lwt.fail_with "dispatch: missing payload"
-           | Some v -> KV.set db ~info key v)
+          KV.set db ~info key msg.payload
         | _ -> failwith "TODO"
       )
 
@@ -128,7 +143,7 @@ module Dispatch = struct
     let msgs = Queue.create () in
     let cond = Lwt_condition.create () in
     let rec listen () =
-      read_message fd >>= fun msg ->
+      Message.read fd >>= fun msg ->
       Queue.add msg msgs;
       Lwt_condition.signal cond ();
       listen ()
diff --git a/projects/miragesdk/src/sdk/ctl.mli b/projects/miragesdk/src/sdk/ctl.mli
index 5af101be8..452e65cca 100644
--- a/projects/miragesdk/src/sdk/ctl.mli
+++ b/projects/miragesdk/src/sdk/ctl.mli
@@ -15,14 +15,24 @@ module Message: sig
   type t = {
     operation: operation;
     path     : string;
-    payload  : string option;
+    payload  : string;
   }
 
-  val write_message: Lwt_unix.file_descr -> t -> unit Lwt.t
-  (** [write_message fd t] writes a control message. *)
+  val pp: t Fmt.t
+  (** [pp] is the pretty-printer for messages. *)
 
-  val read_message: Lwt_unix.file_descr -> t Lwt.t
-  (** [read_message fd] reads a control message. *)
+  val of_cstruct: Cstruct.t -> t
+  (** [of_cstruct buf] is the message [t] such that the serialization
+      of [t] is [buf]. *)
+
+  val to_cstruct: t -> Cstruct.t
+  (** [to_cstruct t] is the serialization of [t]. *)
+
+  val write: Lwt_unix.file_descr -> t -> unit Lwt.t
+  (** [write fd t] writes a control message. *)
+
+  val read: Lwt_unix.file_descr -> t Lwt.t
+  (** [read fd] reads a control message. *)
 
 end
 
diff --git a/projects/miragesdk/src/sdk/init.ml b/projects/miragesdk/src/sdk/init.ml
index b9e2cd7be..c44e7ae84 100644
--- a/projects/miragesdk/src/sdk/init.ml
+++ b/projects/miragesdk/src/sdk/init.ml
@@ -97,6 +97,8 @@ module Pipe = struct
 
   type t = Fd.t * Fd.t
 
+  let name (x, _) = x.Fd.name
+
   let priv = fst
   let calf = snd
 
diff --git a/projects/miragesdk/src/sdk/init.mli b/projects/miragesdk/src/sdk/init.mli
index fc50863cd..1e2828a26 100644
--- a/projects/miragesdk/src/sdk/init.mli
+++ b/projects/miragesdk/src/sdk/init.mli
@@ -63,6 +63,9 @@ module Pipe: sig
   (** The type for pipes. Could be either uni-directional (normal
       pipes) or a bi-directional (socket pairs). *)
 
+  val name: t -> string
+  (** [name t] is [t]'s name. *)
+
   val priv: t -> Fd.t
   (** [priv p] is the private side of the pipe [p]. *)
 
diff --git a/projects/miragesdk/src/test/test.ml b/projects/miragesdk/src/test/test.ml
index 0aa18877a..925ed7c04 100644
--- a/projects/miragesdk/src/test/test.ml
+++ b/projects/miragesdk/src/test/test.ml
@@ -2,35 +2,121 @@ open Astring
 open Lwt.Infix
 open Sdk
 
-let random_string n = Bytes.create n
+let random_string n =
+  Bytes.init n (fun _ -> char_of_int (Random.int 255))
+
+(* workaround https://github.com/mirage/alcotest/issues/88 *)
+exception Check_error of string
+
+let check_raises msg exn f =
+  Lwt.catch (fun () ->
+      f () >>= fun () ->
+      Lwt.fail (Check_error msg)
+    ) (function
+      | Check_error e    -> Alcotest.fail e
+      | e ->
+        if exn e then Lwt.return_unit
+        else Fmt.kstrf Alcotest.fail "%s raised %a" msg Fmt.exn e)
+
+let is_unix_error = function
+  | Unix.Unix_error _ -> true
+  | _ -> false
+
+let escape = String.Ascii.escape
+
+let write fd strs =
+  Lwt_list.iter_s (fun str ->
+      IO.really_write fd str 0 (String.length str)
+    ) strs
 
 let test_pipe pipe () =
   let calf = Init.Fd.fd @@ Init.Pipe.(calf pipe) in
   let priv = Init.Fd.fd @@ Init.Pipe.(priv pipe) in
-  let test str =
-    (* check the the pipe is unidirectional *)
-    IO.really_write calf str 0 (String.length str) >>= fun () ->
+  let name = Init.Pipe.name pipe in
+  let test strs =
+    let escape_strs = String.concat ~sep:"" @@ List.map escape strs in
+    (* pipes are unidirectional *)
+    (* calf -> priv works *)
+    write calf strs >>= fun () ->
     IO.read_all priv >>= fun buf ->
-    Alcotest.(check string) "stdout"
-      (String.Ascii.escape str) (String.Ascii.escape buf);
-    Lwt.catch (fun () ->
-        IO.really_write priv str 0 (String.length str) >|= fun () ->
-        Alcotest.fail "priv side is writable!"
-      ) (fun _ -> Lwt.return_unit)
-    >>= fun () ->
-    Lwt.catch (fun () ->
-        IO.read_all calf >|= fun _ ->
-        Alcotest.fail "calf sid is readable!"
-      ) (fun _ -> Lwt.return_unit)
-    >>= fun () ->
+    let msg = Fmt.strf "%s: calf -> priv" name in
+    Alcotest.(check string) msg escape_strs (escape buf);
+    (* priv -> calf don't *)
+    check_raises (Fmt.strf "%s: priv side is writable!" name) is_unix_error
+      (fun () -> write priv strs) >>= fun () ->
+    check_raises (Fmt.strf "%s: calf sid is readable!" name) is_unix_error
+      (fun () -> IO.read_all calf >|= ignore) >>= fun () ->
     Lwt.return_unit
   in
-  test (random_string 1) >>= fun () ->
-  test (random_string 100) >>= fun () ->
-  test (random_string 10241) >>= fun () ->
+  test [random_string 1] >>= fun () ->
+  test [random_string 1; random_string 1; random_string 10] >>= fun () ->
+  test [random_string 100] >>= fun () ->
+  test [random_string 10241] >>= fun () ->
 
   Lwt.return_unit
 
+let test_socketpair pipe () =
+  let calf = Init.Fd.fd @@ Init.Pipe.(calf pipe) in
+  let priv = Init.Fd.fd @@ Init.Pipe.(priv pipe) in
+  let name = Init.Pipe.name pipe in
+  let test strs =
+    let escape_strs = String.concat ~sep:"" @@ List.map escape strs in
+    (* socket pairs are bi-directional *)
+    (* calf -> priv works *)
+    write calf strs >>= fun () ->
+    IO.read_all priv >>= fun buf ->
+    Alcotest.(check string) (name ^ " calf -> priv") escape_strs (escape buf);
+    (* priv -> cal works *)
+    write priv strs >>= fun () ->
+    IO.read_all calf >>= fun buf ->
+    Alcotest.(check string) (name ^ " priv -> calf") escape_strs (escape buf);
+    Lwt.return_unit
+  in
+  test [random_string 1] >>= fun () ->
+  test [random_string 1; random_string 1; random_string 10] >>= fun () ->
+  test [random_string 100] >>= fun () ->
+  (* note: if size(writes) > 8192 then the next writes will block (as
+     we are using SOCK_STREAM *)
+  let n = 8182 / 4 in
+  test [
+    random_string n;
+    random_string n;
+    random_string n;
+    random_string n;
+  ] >>= fun () ->
+
+  Lwt.return_unit
+
+let message = Alcotest.testable Ctl.Message.pp (=)
+
+let test_message_serialization () =
+  let test m =
+    let buf = Ctl.Message.to_cstruct m in
+    let m' = Ctl.Message.of_cstruct buf in
+    Alcotest.(check message) "to_cstruct/of_cstruct" m m'
+  in
+  List.iter test [
+    { operation = Read  ; path = "/foo/bar"; payload = ""    };
+    { operation = Write ; path = ""        ; payload = "foo" };
+    { operation = Delete; path = ""        ; payload = ""    };
+    { operation = Delete; path = "foo"     ; payload = "foo" };
+  ]
+
+let test_message_send () =
+  let calf = Init.Fd.fd @@ Init.Pipe.(calf ctl) in
+  let priv = Init.Fd.fd @@ Init.Pipe.(priv ctl) in
+  let test m =
+    Ctl.Message.write calf m >>= fun () ->
+    Ctl.Message.read priv >|= fun m' ->
+    Alcotest.(check message) "write/read" m m'
+  in
+  Lwt_list.iter_s test [
+    { operation = Read  ; path = "/foo/bar"; payload = ""    };
+    { operation = Write ; path = ""        ; payload = "foo" };
+    { operation = Delete; path = ""        ; payload = ""    };
+    { operation = Delete; path = "foo"     ; payload = "foo" };
+  ]
+
 let run f () =
   try Lwt_main.run (f ())
   with e ->
@@ -40,9 +126,13 @@ let run f () =
 let test_stderr () = ()
 
 let test = [
-  "stdout" , `Quick, run (test_pipe Init.Pipe.stdout);
-  "stdout" , `Quick, run (test_pipe Init.Pipe.stderr);
-  ]
+  "stdout is a pipe"    , `Quick, run (test_pipe Init.Pipe.stdout);
+  "stdout is a pipe"    , `Quick, run (test_pipe Init.Pipe.stderr);
+  "net is a socket pair", `Quick, run (test_socketpair Init.Pipe.net);
+  "ctl is a socket pair", `Quick, run (test_socketpair Init.Pipe.ctl);
+  "seralize messages"   , `Quick, test_message_serialization;
+  "send messages"       , `Quick, run test_message_send;
+]
 
 let reporter ?(prefix="") () =
   let pad n x =