diff --git a/config/Makefile.in b/config/Makefile.in
index 19b136c2..d7d29ef5 100644
--- a/config/Makefile.in
+++ b/config/Makefile.in
@@ -432,10 +432,11 @@ FASTTRACK_SRCS= \
   $(SRC_FASTTRACK)/fasttrackClients.mlt \
   $(SRC_FASTTRACK)/fasttrackHandler.ml \
   $(SRC_FASTTRACK)/fasttrackServers.ml \
-  $(SRC_FASTTRACK)/fasttrackPandora.ml \
   $(SRC_FASTTRACK)/fasttrackInteractive.mlt \
   $(SRC_FASTTRACK)/fasttrackMain.mlt
 
+  #$(SRC_FASTTRACK)/fasttrackPandora.ml \
+
 $(BITSTRING)/bitstring_persistent.cmo: $(BITSTRING)/bitstring_persistent.ml $(BITSTRING)/bitstring_persistent.cmi build/bitstring.cma
 	$(OCAMLC) -I $(BITSTRING) -I +camlp4 camlp4lib.cma -c $<
 
diff --git a/src/daemon/common/commonComplexOptions.ml b/src/daemon/common/commonComplexOptions.ml
index b5088d52..0986c3ce 100644
--- a/src/daemon/common/commonComplexOptions.ml
+++ b/src/daemon/common/commonComplexOptions.ml
@@ -1156,9 +1156,9 @@ let backup_tar archive files =
               t_devminor = 0;
               t_prefix = "";
               t_gnu = None;} in
-            let s = String.create size in
+            let s = Bytes.create size in
             Pervasives.really_input ic s 0 size;
-            header, s) in
+            header, Bytes.unsafe_to_string s) in
         Tar.output otar header s
       with
       | e ->
diff --git a/src/daemon/common/commonDownloads.ml b/src/daemon/common/commonDownloads.ml
index be9aa3a6..1c266ad3 100644
--- a/src/daemon/common/commonDownloads.ml
+++ b/src/daemon/common/commonDownloads.ml
@@ -87,7 +87,7 @@ module Make(M: sig
           in
           let final_pos = Unix32.seek64 (file_fd file) d.download_pos
               Unix.SEEK_SET in *)
-        Unix32.write (file_fd file) d.download_pos b.buf b.pos b.len;
+        Unix32.write_bytes (file_fd file) d.download_pos b.buf b.pos b.len;
 (*        end; *)
 (*      lprintf "DIFF %d/%d\n" nread b.len; *)
         d.download_pos <- d.download_pos ++ (Int64.of_int b.len);
diff --git a/src/daemon/common/commonFile.ml b/src/daemon/common/commonFile.ml
index 769a1749..46778767 100644
--- a/src/daemon/common/commonFile.ml
+++ b/src/daemon/common/commonFile.ml
@@ -975,7 +975,7 @@ let recover_bytes file =
     if pos = max then
       iter_file_out file_pos segments
     else
-    if s.[pos] = '\000' then
+    if Bytes.get s pos = '\000' then
       iter_string_out file_pos (pos+1) max segments
     else
     let begin_pos = file_pos -- (Int64.of_int (max - pos)) in
@@ -995,7 +995,7 @@ let recover_bytes file =
     if pos = max then
       iter_file_in file_pos begin_pos segments
     else
-    if s.[pos] = '\000' then
+    if Bytes.get s pos = '\000' then
       let end_pos = file_pos -- (Int64.of_int (max - pos)) in
 (*      lprintf "  0 byte at %Ld\n" end_pos; *)
       iter_string_out file_pos (pos+1) max 
diff --git a/src/daemon/common/commonMultimedia.ml b/src/daemon/common/commonMultimedia.ml
index e68bc310..37aa2ae9 100644
--- a/src/daemon/common/commonMultimedia.ml
+++ b/src/daemon/common/commonMultimedia.ml
@@ -41,10 +41,7 @@ let input_int ic =
   let i1 = input_int16 ic in
   i0 lor (i1 lsl 16)
 
-let input_string4 ic =
-  let s = String.create 4 in
-  really_input ic s 0 4;
-  s
+let input_string4 ic = really_input_string ic 4
 
 let print_string4 v s =
   lprintf "%s :" v;
@@ -230,7 +227,7 @@ let rec page_seek ic s pos =
     else begin
       really_input ic s 0 4;
       seek_in ic (pos_in ic - 3);
-      if s = "OggS"
+      if Bytes.unsafe_to_string s = "OggS"
         then seek_in ic (pos_in ic + 3)
         else page_seek ic s pos
   end
@@ -278,12 +275,10 @@ let rec next_ogg_stream ic ogg_infos str stream_number =
   lprintf "Stream Serial Number: %0.f\n" stream_number;
 *)
   seek_in ic (pos+24);
-  let content_type = String.create 1 in
-  really_input ic content_type 0 1;
+  let content_type = really_input_string ic 1 in
   let content_type = int_of_char content_type.[0] in
   seek_in ic (pos+25);
-  let stream_type = String.create 8 in
-  really_input ic stream_type 0 8;
+  let stream_type = really_input_string ic 8 in
   let stream_type = normalize_stream_type stream_type content_type in
   incr stream_number;
   let pos = pos_in ic in
@@ -299,8 +294,7 @@ let rec next_ogg_stream ic ogg_infos str stream_number =
     | OGG_THEORA_STREAM -> get_ogg_theora_info ic ogg_infos str stream_number
 
 and get_ogg_video_info  ic ogg_infos str sizeof_packet stream_number =
-  let s = String.create sizeof_packet in
-  really_input ic s 0 sizeof_packet;
+  let s = really_input_string ic sizeof_packet in
   let codec = String.lowercase (String.sub s 0 4) in
   let time_unit = read64 (String.sub s 8 8) in
   let video_width =
@@ -326,8 +320,7 @@ and get_ogg_video_info  ic ogg_infos str sizeof_packet stream_number =
   next_ogg_stream ic ogg_infos str stream_number
 
 and get_ogg_audio_info  ic ogg_infos str sizeof_packet stream_number =
-  let s = String.create sizeof_packet in
-  really_input ic s 0 sizeof_packet;
+  let s = really_input_string ic sizeof_packet in
   let codec = get_audio_codec (String.sub s 0 4) in
   let sample_per_unit = read64 (String.sub s 16 8) in
   let channels =
@@ -359,8 +352,7 @@ and get_ogg_audio_info  ic ogg_infos str sizeof_packet stream_number =
 
 and get_ogg_vorbis_info  ic ogg_infos str stream_number =
   seek_in ic (pos_in ic - 2); (* ogm sets 8 octets in the common header as vorbis uses 6 octects for 'vorbis' *)
-  let s = String.create 22 in
-  really_input ic s 0 22;
+  let s = really_input_string ic 22 in
   let version = read32 (String.sub s 0 4) in
   let audio_channels = int_of_char s.[4] in
   let sample_rate = read32 (String.sub s 5 4) in
@@ -389,8 +381,7 @@ and get_ogg_vorbis_info  ic ogg_infos str stream_number =
 
 and get_ogg_theora_info  ic ogg_infos str stream_number =
   seek_in ic (pos_in ic - 2); (* ogm sets 8 octets in the common header as theora uses 6 octects for 'theora' *)
-  let s = String.create 34 in
-  really_input ic s 0 34;
+  let s = really_input_string ic 34 in
   let vmaj = int_of_char s.[0] in
   let vmin = int_of_char s.[1] in
   let vrev = int_of_char s.[2] in
@@ -451,7 +442,7 @@ and get_ogg_index_info ic ogg_infos str stream_number =
 
 let search_info_ogg ic =
     let stream_number = ref 0 in
-    let str = String.create 4 in
+    let str = Bytes.create 4 in
     let ogg_infos = ref [] in
     (* make sure the current reading position is at the file beginning *)
     seek_in ic 0;
diff --git a/src/daemon/common/commonSwarming.ml b/src/daemon/common/commonSwarming.ml
index 4a417c2a..a81a51bf 100644
--- a/src/daemon/common/commonSwarming.ml
+++ b/src/daemon/common/commonSwarming.ml
@@ -2792,7 +2792,7 @@ let range_range r = (r.range_begin, r.range_end)
 let received up file_begin str string_begin string_len =
   assert (string_begin >= 0);
   assert (string_len >= 0);
-  assert (string_begin + string_len <= Bytes.length str);
+  assert (string_begin + string_len <= String.length str);
 
   let t = up.up_t in
   let s = t.t_s in
@@ -2909,7 +2909,7 @@ let received up file_begin str string_begin string_len =
                             (Printexc2.to_string e) 
                             r.range_begin file_end
                             (file_best_name t.t_file));
-                        file_write_bytes tprim.t_file
+                        file_write tprim.t_file
                           r.range_begin
                           str string_pos string_length;
                         range_received (Some t) r r.range_begin file_end;
diff --git a/src/daemon/common/commonUploads.ml b/src/daemon/common/commonUploads.ml
index d63fc1fc..a1b5f538 100644
--- a/src/daemon/common/commonUploads.ml
+++ b/src/daemon/common/commonUploads.ml
@@ -290,7 +290,7 @@ let shared_tree = new_shared_dir ""
 
 let md4_of_list md4s =
   let len = List.length md4s in
-  let s = String.create (len * 16) in
+  let s = Bytes.create (len * 16) in
   let rec iter list i =
     match list with
       [] -> ()
@@ -300,7 +300,7 @@ let md4_of_list md4s =
         iter tail (i+16)
   in
   iter md4s 0;
-  Md4.string s
+  Md4.string @@ Bytes.unsafe_to_string s
 
 let rec tiger_of_array array pos block =
   if block = 1 then
@@ -312,11 +312,11 @@ let rec tiger_of_array array pos block =
   else
   let d1 = tiger_of_array array pos (block/2) in
   let d2 = tiger_of_array array (pos+block/2) (block/2) in
-  let s = String.create (1 + Tiger.length * 2) in
+  let s = Bytes.create (1 + Tiger.length * 2) in
   s.[0] <- '\001';
   String.blit (TigerTree.direct_to_string d1) 0 s 1 Tiger.length;
   String.blit (TigerTree.direct_to_string d2) 0 s (1+Tiger.length) Tiger.length;
-  let t = Tiger.string s in
+  let t = Tiger.string @@ Bytes.unsafe_to_string s in
   let t = TigerTree.direct_of_string (Tiger.direct_to_string t) in
   t
 
@@ -345,11 +345,11 @@ let rec tiger_pos2 nblocks =
   pos, list
 
 let tiger_node d1 d2 =
-  let s = String.create (1 + Tiger.length * 2) in
+  let s = Bytes.create (1 + Tiger.length * 2) in
   s.[0] <- '\001';
   String.blit (TigerTree.direct_to_string d1) 0 s 1 Tiger.length;
   String.blit (TigerTree.direct_to_string d2) 0 s (1+Tiger.length) Tiger.length;
-  let t = Tiger.string s in
+  let t = Tiger.string @@ Bytes.unsafe_to_string s in
   let t = TigerTree.direct_of_string (Tiger.direct_to_string t) in
   t
 
@@ -386,12 +386,12 @@ let rec fill_tiger_tree s list =
 
 let flatten_tiger_array array =
   let len = Array.length array in
-  let s = String.create ( len * TigerTree.length) in
+  let s = Bytes.create ( len * TigerTree.length) in
   for i = 0 to len - 1 do
     String.blit (TigerTree.direct_to_string array.(i)) 0
       s (i * TigerTree.length) TigerTree.length
   done;
-  s
+  Bytes.unsafe_to_string s
 
 let unflatten_tiger_array s =
   let len = String.length s / TigerTree.length in
@@ -504,9 +504,9 @@ computation ??? *)
             let file_size = Unix32.getsize64 fd in
             let len64 = min 307200L file_size in
             let len = Int64.to_int len64 in
-            let s = String.create len in
+            let s = Bytes.create len in
             Unix32.read fd zero s 0 len;
-            Md5Ext.string s
+            Md5Ext.string @@ Bytes.unsafe_to_string s
           with e ->
               current_job := None;
               raise e
diff --git a/src/daemon/common/commonWeb.ml b/src/daemon/common/commonWeb.ml
index d109d115..4cae772c 100755
--- a/src/daemon/common/commonWeb.ml
+++ b/src/daemon/common/commonWeb.ml
@@ -265,7 +265,7 @@ let _ =
               while true do
                 let nread = Unix.read pipe_out buffer 0 buffersize in
                 if nread = 0 then raise End_of_file;
-                Buffer.add_substring output buffer 0 nread
+                Buffer.add_subbytes output buffer 0 nread
               done
              with 
              | End_of_file -> ()
diff --git a/src/daemon/common/giftDecoding.ml b/src/daemon/common/giftDecoding.ml
index 8e9fe8af..c5434795 100755
--- a/src/daemon/common/giftDecoding.ml
+++ b/src/daemon/common/giftDecoding.ml
@@ -11,11 +11,11 @@ let gui_cut_messages f sock nread =
   try
     let rec iter pos len =
       if len = 0 then raise Not_found;
-      if b.buf.[pos] = ';' && (
+      if Bytes.get b.buf pos = ';' && (
           pos = b.pos || 
-          (pos > b.pos && b.buf.[pos-1] <> '\\')) then begin
+          (pos > b.pos && Bytes.get b.buf (pos-1) <> '\\')) then begin
           let len = pos - b.pos+1 in
-          let s = String.sub b.buf b.pos len in
+          let s = Bytes.sub_string b.buf b.pos len in
           buf_used b len;
           f s;
           iter b.pos b.len
diff --git a/src/daemon/common/guiDecoding.ml b/src/daemon/common/guiDecoding.ml
index 4fc5af22..60e2b431 100644
--- a/src/daemon/common/guiDecoding.ml
+++ b/src/daemon/common/guiDecoding.ml
@@ -42,10 +42,10 @@ let gui_cut_messages f sock nread =
   let b = buf sock in
   try
     while b.len >= 4 do
-      let msg_len = get_int b.buf b.pos in
+      let msg_len = get_int (Bytes.unsafe_to_string b.buf) b.pos in
       if b.len >= 4 + msg_len then
         begin
-          let s = String.sub b.buf (b.pos+4) msg_len in
+          let s = Bytes.sub_string b.buf (b.pos+4) msg_len in
           buf_used b (msg_len + 4);
           let opcode = get_int16 s 0 in
           (f opcode s : unit)
diff --git a/src/daemon/common/guiEncoding.ml b/src/daemon/common/guiEncoding.ml
index eddfbd39..b3f7788e 100644
--- a/src/daemon/common/guiEncoding.ml
+++ b/src/daemon/common/guiEncoding.ml
@@ -44,10 +44,10 @@ let gui_send writer sock t =
     Buffer.reset buf;
     buf_int buf 0;
     writer buf t;
-    let s = Buffer.contents buf in
-    let len = String.length s - 4 in
+    let s = Buffer.to_bytes buf in
+    let len = Bytes.length s - 4 in
     str_int s 0 len;
-    write_string sock s;
+    write_bytes sock s
   with UnsupportedGuiMessage -> ()
       
 (***************
diff --git a/src/daemon/driver/driverCommands.ml b/src/daemon/driver/driverCommands.ml
index 731789bc..7cfbfc3e 100644
--- a/src/daemon/driver/driverCommands.ml
+++ b/src/daemon/driver/driverCommands.ml
@@ -1389,12 +1389,12 @@ let _ =
                  (* can't close pipe_out in the already forked+executed process... *)
                  let output = Buffer.create 1024 in
                  let buffersize = 1024 in
-                 let buffer = String.create buffersize in
+                 let buffer = Bytes.create buffersize in
                  (try
                     while true do
                       let nread = Unix.read pipe_out buffer 0 buffersize in
                       if nread = 0 then raise End_of_file;
-                      Buffer.add_substring output buffer 0 nread
+                      Buffer.add_subbytes output buffer 0 nread
                     done
                   with 
                   | End_of_file -> ()
@@ -3893,7 +3893,7 @@ let _ =
            let num = int_of_string arg in
            let file = file_find num in
            let swarmer = CommonSwarming.file_swarmer file in
-           let prio = CommonSwarming.get_swarmer_block_priorities swarmer in
+           let prio = Bytes.unsafe_to_string @@ CommonSwarming.get_swarmer_block_priorities swarmer in
            let downloaded = CommonSwarming.get_swarmer_block_verified swarmer in
            pr "\\<code\\>";
            pr "priorities: ";
diff --git a/src/daemon/driver/driverControlers.ml b/src/daemon/driver/driverControlers.ml
index 59f71f86..1499ae1f 100644
--- a/src/daemon/driver/driverControlers.ml
+++ b/src/daemon/driver/driverControlers.ml
@@ -464,7 +464,7 @@ let user_reader o telnet sock nread =
   let b = TcpBufferedSocket.buf sock in
   let rec iter () =
     if b.len > 0 then
-      let c = b.buf.[b.pos] in
+      let c = Bytes.get b.buf b.pos in
       buf_used b 1;
 (*      lprintf "char %d\n" (int_of_char c); *)
       if c = '\255' && not telnet.telnet_iac then begin
@@ -766,9 +766,7 @@ let read_theme_page page =
   let theme_page = get_theme_page page in
   Unix2.tryopen_read theme_page (fun file ->
     let size = (Unix.stat theme_page).Unix.st_size in
-    let s = String.make size ' ' in
-    really_input file s 0 size;
-    s)
+    really_input_string file size)
 
 let http_add_gen_header r =
   add_reply_header r "Server" ("MLdonkey/"^Autoconf.current_version);
@@ -929,8 +927,8 @@ let send_preview r file fd size filename exten =
 
   add_reply_header r "Content-Disposition"
   (Printf.sprintf "inline;filename=\"%s\"" (Filename.basename filename)); 
-  let s = String.create 200000 in
-  set_max_output_buffer r.sock (String.length s);
+  let s = Bytes.create 200000 in
+  set_max_output_buffer r.sock (Bytes.length s);
   set_rtimeout r.sock 10000.;
   let rec stream_file file pos sock =
     let max = (max_refill sock) - 1 in
diff --git a/src/networks/bittorrent/bTClients.ml b/src/networks/bittorrent/bTClients.ml
index 5ecfbdc5..3ec28cc3 100644
--- a/src/networks/bittorrent/bTClients.ml
+++ b/src/networks/bittorrent/bTClients.ml
@@ -114,6 +114,7 @@ let string_of_event = function
   Better create single global udp socket and use it for all 
   tracker requests and distinguish trackers by txn? FIXME?
   *)
+
 let talk_to_udp_tracker host port args file t need_sources =
   let interact ip =
     let socket = create (Ip.to_inet_addr !!client_bind_addr) 0 (fun sock event ->
@@ -137,10 +138,10 @@ let talk_to_udp_tracker host port args file t need_sources =
     BasicSocket.set_rtimeout (sock socket) 60.;
     let txn = Random.int32 Int32.max_int in
 (*     lprintf_nl "udpt txn %ld for %s" txn host; *)
-    write socket false (connect_request txn) ip port;
+    write socket false (Bytes.unsafe_of_string @@ connect_request txn) ip port;
     set_reader begin fun () ->
       let p = read socket in
-      let conn = connect_response p.udp_content txn in
+      let conn = connect_response (Bytes.unsafe_to_string p.udp_content) txn in
 (*       lprintf_nl "udpt connection_id %Ld for %s" conn host; *)
       let txn = Random.int32 Int32.max_int in
 (*       lprintf_nl "udpt txn' %ld for host %s" txn host; *)
@@ -159,7 +160,7 @@ let talk_to_udp_tracker host port args file t need_sources =
         ~numwant:(if need_sources then try Int32.of_string (List.assoc "numwant" args) with _ -> -1l else 0l)
         (int_of_string (List.assoc "port" args))
       in
-      write socket false req ip port;
+      write socket false (Bytes.unsafe_of_string req) ip port;
       set_reader (fun () ->
         let p = read socket in
 
@@ -169,7 +170,7 @@ let talk_to_udp_tracker host port args file t need_sources =
         t.tracker_min_interval <- 600;
         if need_sources then t.tracker_last_clients_num <- 0;
 
-        let (interval,clients) = announce_response p.udp_content txn in
+        let (interval,clients) = announce_response (Bytes.unsafe_to_string p.udp_content) txn in
         if !verbose_msg_servers then
           lprintf_nl "udpt got interval %ld clients %d for host %s" interval (List.length clients) host;
         if interval > 0l then
@@ -529,7 +530,7 @@ let is_bit_set s n =
 
 let set_bit s n =
   let i = n lsr 3 in
-  s.[i] <- Char.unsafe_chr (Char.code s.[i] lor bits.(n land 7))
+  s.[i] <- Char.unsafe_chr (Char.code (Bytes.get s i) lor bits.(n land 7))
 
 (* Official client seems to use max_range_request 5 and max_range_len 2^14 *)
 (* How much requests in the 'pipeline' *)
@@ -537,10 +538,10 @@ let max_range_requests = 5
 (* How much bytes we can request in one Piece *)
 
 let reserved () =
-  let s = String.make 8 '\x00' in
+  let s = Bytes.make 8 '\x00' in
   s.[7] <- (match !bt_dht with None -> '\x00' | Some _ -> '\x01');
   s.[5] <- '\x10'; (* TODO bep9, bep10, notify clients about extended*)
-  s
+  Bytes.unsafe_to_string s
 
 (** handshake *)
 let send_init client_uid file_id sock =
@@ -579,20 +580,20 @@ let send_bitfield c =
             lprintf_nl "Sending completed verified bitmap";
           let nchunks = Array.length c.client_file.file_chunks in
           let len = (nchunks+7)/8 in
-          let s = String.make len '\000' in
+          let s = Bytes.make len '\000' in
           for i = 0 to nchunks - 1 do
             set_bit s i
           done;
-          s
+          Bytes.unsafe_to_string s
       | Some swarmer ->
           let bitmap = CommonSwarming.chunks_verified_bitmap swarmer in
           if !verbose_download then 
             lprintf_nl "Sending verified bitmap: [%s]" (VB.to_string bitmap);
           let len = (VB.length bitmap + 7)/8 in
-          let s = String.make len '\000' in
+          let s = Bytes.make len '\000' in
           VB.iteri (fun i c ->
             if c = VB.State_verified then set_bit s i) bitmap;
-          s
+          Bytes.unsafe_to_string s
     ))
 
 let counter = ref 0
@@ -1920,7 +1921,7 @@ let rec iter_upload sock c =
                   end
           in
 (*          lprintf "sending piece\n"; *)
-          send_client c (Piece (num, pos, upload_buffer, 0, len));
+          send_client c (Piece (num, pos, Bytes.unsafe_to_string upload_buffer, 0, len));
           iter_upload sock c
         with exn -> 
           if !verbose then 
diff --git a/src/networks/bittorrent/bTOptions.ml b/src/networks/bittorrent/bTOptions.ml
index 4915e583..d38fa190 100644
--- a/src/networks/bittorrent/bTOptions.ml
+++ b/src/networks/bittorrent/bTOptions.ml
@@ -36,13 +36,13 @@ let client_port = define_option bittorrent_section ["client_port"]
 *)
 let generate_client_uid =
   let client_uid_from_version = "-ML" ^ Autoconf.current_version ^ "-" in
-  let client_uid_random_tail = String.create (20 - (String.length client_uid_from_version)) in
-  let sl_client_uid_random_tail = String.length client_uid_random_tail in
+  let client_uid_random_tail = Bytes.create (20 - (String.length client_uid_from_version)) in
+  let sl_client_uid_random_tail = Bytes.length client_uid_random_tail in
   if sl_client_uid_random_tail > 0  then
     for i = 0 to sl_client_uid_random_tail - 1 do
       client_uid_random_tail.[i] <- char_of_int (Random.int 256)
     done;
-  client_uid_from_version ^ client_uid_random_tail
+  client_uid_from_version ^ Bytes.unsafe_to_string client_uid_random_tail
 
 let client_uid = define_option bittorrent_section ["client_uid"]
   "The UID of this client"
diff --git a/src/networks/bittorrent/bTProtocol.ml b/src/networks/bittorrent/bTProtocol.ml
index 6c91ef72..be610410 100644
--- a/src/networks/bittorrent/bTProtocol.ml
+++ b/src/networks/bittorrent/bTProtocol.ml
@@ -329,9 +329,9 @@ module TcpMessages = struct
         | DHT_Port n -> buf_int8 buf 9; buf_int16 buf n
         | Extended (n,msg) -> buf_int8 buf 20; buf_int8 buf n; Buffer.add_string buf msg
       end;
-      let s = Buffer.contents buf in
-      str_int s 0 (String.length s - 4);
-      s
+      let s = Buffer.to_bytes buf in
+      str_int s 0 (Bytes.length s - 4);
+      Bytes.unsafe_to_string s
   end
 
 (*************************************************************************)
@@ -456,7 +456,7 @@ let bt_handler parse_fun handler c sock =
            then drop the connection) *)
         if b.len >= 20 then
           begin
-            let payload = String.sub b.buf b.pos 20 in
+            let payload = Bytes.sub_string b.buf b.pos 20 in
             let p = parse_fun (-1) payload in
             buf_used b 20;
             c.client_received_peer_id <- true;
@@ -477,13 +477,13 @@ let bt_handler parse_fun handler c sock =
         raise (Wait_for_more "after_peer_id");
       end;
     while b.len >= 4 do
-        let msg_len = get_int b.buf b.pos in
+        let msg_len = get_int (Bytes.unsafe_to_string b.buf) b.pos in
         if msg_len < 0 then
           begin
             let (ip,port) = (TcpBufferedSocket.peer_addr sock) in
             lprintf_nl "BT: Unknown message from %s:%d dropped!! peerid:%b data_len:%i msg_len:%i software: %s"
                 (Ip.to_string ip) port c.client_received_peer_id b.len msg_len (brand_to_string c.client_brand);
-            dump (String.sub b.buf b.pos (min b.len 30));
+            dump (Bytes.sub_string b.buf b.pos (min b.len 30));
             buf_used b b.len;
             close sock Closed_by_user;
           end
@@ -494,7 +494,7 @@ let bt_handler parse_fun handler c sock =
             let (ip,port) = (TcpBufferedSocket.peer_addr sock) in
             lprintf_nl "btprotocol.bt_handler: closed connection from %s:%d because of too much data!! data_len:%i msg_len:%i software: %s"
                 (Ip.to_string ip) port b.len msg_len (brand_to_string c.client_brand);
-            dump (String.sub b.buf b.pos (min b.len 30));
+            dump (Bytes.sub_string b.buf b.pos (min b.len 30));
             buf_used b b.len;
             close sock Closed_by_user
           end
@@ -503,9 +503,9 @@ let bt_handler parse_fun handler c sock =
             buf_used b 4;
             (* lprintf "Message complete: %d\n" msg_len;  *)
             if msg_len > 0 then
-                let opcode = get_int8 b.buf b.pos in
+                let opcode = get_int8 (Bytes.unsafe_to_string b.buf) b.pos in
                 (* FIXME sub *)
-                let payload = String.sub b.buf (b.pos+1) (msg_len-1) in
+                let payload = Bytes.sub_string b.buf (b.pos+1) (msg_len-1) in
                 buf_used b msg_len;
                 (* lprintf "Opcode %d\n" opcode; *)
                 try
@@ -543,7 +543,7 @@ let handlers info gconn =
       match gconn.gconn_handler with
       | BTHeader h ->
           (* dump (String.sub b.buf b.pos (min b.len 100)); *)
-          let slen = get_int8 b.buf b.pos in
+          let slen = get_int8 (Bytes.unsafe_to_string b.buf) b.pos in
           if slen + 29 <= b.len then
             begin
               (* get proto and file_id from handshake,
@@ -552,14 +552,14 @@ let handlers info gconn =
                  *)
 (*              let proto = String.sub b.buf (b.pos+1) slen in *)
               let file_id = Sha1.direct_of_string
-                (String.sub b.buf (b.pos+9+slen) 20) in
-              let proto,pos = get_string8 b.buf b.pos in
-              let rbits = (String.sub b.buf (b.pos+pos) 8) in
+                (Bytes.sub_string b.buf (b.pos+9+slen) 20) in
+              let proto,pos = get_string8 (Bytes.unsafe_to_string b.buf) b.pos in
+              let rbits = (Bytes.sub_string b.buf (b.pos+pos) 8) in
               buf_used b (slen+29);
               h gconn sock (proto, rbits, file_id);
             end
           else
-            if (String.sub b.buf b.pos (min b.len 100)) = "NATCHECK_HANDSHAKE" then
+            if (Bytes.sub_string b.buf b.pos (min b.len 100)) = "NATCHECK_HANDSHAKE" then
                 write_string sock (Printf.sprintf "azureus_rand_%d" !azureus_porttest_random)
           else if (TcpBufferedSocket.closed sock) then
               let (ip,port) = (TcpBufferedSocket.peer_addr sock) in
diff --git a/src/networks/bittorrent/bTTorrent.ml b/src/networks/bittorrent/bTTorrent.ml
index 26ff2096..97c6b167 100644
--- a/src/networks/bittorrent/bTTorrent.ml
+++ b/src/networks/bittorrent/bTTorrent.ml
@@ -328,11 +328,12 @@ let decode_torrent s =
 let encode_torrent torrent =
 
   let npieces = Array.length torrent.torrent_pieces in
-  let pieces = String.create (20 * npieces) in
+  let pieces = Bytes.create (20 * npieces) in
   for i = 0 to npieces - 1 do
     String.blit (Sha1.direct_to_string torrent.torrent_pieces.(i)) 0
       pieces (i*20) 20
   done;
+  let pieces = Bytes.unsafe_to_string pieces in
 
   let encode_file (filename, size) =
     Dictionary [
diff --git a/src/networks/bittorrent/bT_DHT.ml b/src/networks/bittorrent/bT_DHT.ml
index 71569ed2..dab49460 100644
--- a/src/networks/bittorrent/bT_DHT.ml
+++ b/src/networks/bittorrent/bT_DHT.ml
@@ -125,7 +125,7 @@ let send sock stats (ip,port as addr) txnmsg =
   if !debug then lprintf_nl "KRPC to %s : %S" (show_addr addr) s;
   stats_add stats `Sent 1;
   stats_add stats `SentBytes (String.length s);
-  write sock false s ip port
+  write sock false (Bytes.unsafe_of_string s) ip port
 
 type stats_key = [ `Timeout | `Sent | `SentBytes | `Recv | `RecvBytes | `Decoded | `Handled | `NoTxn ]
 type t =
@@ -197,16 +197,16 @@ let create port enabler bw_control answer : t =
       let addr = (Ip.of_inet_addr inet_addr, port) in
       let ret = ref None in
       try
-        stats_add stats `RecvBytes (String.length p.udp_content);
+        stats_add stats `RecvBytes (Bytes.length p.udp_content);
         stats_add stats `Recv 1;
-        let r = decode_exn p.udp_content in
+        let r = decode_exn @@ Bytes.unsafe_to_string p.udp_content in
         stats_add stats `Decoded 1;
         ret := Some r;
         handle addr r;
         stats_add stats `Handled 1;
       with exn ->
         let version = match !ret with Some (_,Some s,_) -> sprintf " client %S" s | _ -> "" in
-        if !verb then lprintf_nl ~exn "handle packet from %s%s : %S" (show_addr addr) version p.udp_content;
+        if !verb then lprintf_nl ~exn "handle packet from %s%s : %S" (show_addr addr) version (Bytes.unsafe_to_string p.udp_content);
         let error txn code str = send socket stats addr (txn,(Error (Int64.of_int code,str))) in
         match exn,!ret with
         | Malformed_packet x, Some (txn, _, _)
@@ -308,20 +308,20 @@ let make_peer (ip,port) =
   assert (port <= 0xffff);
   let (a,b,c,d) = Ip.to_ints ip in
   let e = port lsr 8 and f = port land 0xff in
-  let s = String.create 6 in
+  let s = Bytes.create 6 in
   let set i c = s.[i] <- char_of_int c in
   set 0 a; set 1 b; set 2 c; set 3 d; set 4 e; set 5 f;
-  s
+  Bytes.unsafe_to_string s
 
 let make_nodes nodes =
-  let s = String.create (26 * List.length nodes) in
+  let s = Bytes.create (26 * List.length nodes) in
   let i = ref 0 in
   List.iter (fun (id,addr) ->
-    String.blit (H.direct_to_string id) 0 s (!i*26) 20;
-    String.blit (make_peer addr) 0 s (!i*26+20) 6;
+    Bytes.blit_string (H.direct_to_string id) 0 s (!i*26) 20;
+    Bytes.blit_string (make_peer addr) 0 s (!i*26+20) 6;
     incr i
     ) nodes;
-  s
+  Bytes.unsafe_to_string s
 
 let parse_response_exn q dict =
   let get k = List.assoc k dict in
diff --git a/src/networks/bittorrent/kademlia.ml b/src/networks/bittorrent/kademlia.ml
index c6d901f3..017b5bd6 100644
--- a/src/networks/bittorrent/kademlia.ml
+++ b/src/networks/bittorrent/kademlia.ml
@@ -72,14 +72,14 @@ let cmp id1 id2 =
 let inside node hash = not (cmp hash node.lo = LT || cmp hash node.hi = GT)
 
 let middle =
-  let s = String.make 20 (Char.chr 0xFF) in
+  let s = Bytes.make 20 (Char.chr 0xFF) in
   s.[0] <- Char.chr 0x7F;
-  H.direct_of_string s
+  H.direct_of_string @@ Bytes.unsafe_to_string s
 
 let middle' =
-  let s = String.make 20 (Char.chr 0x00) in
+  let s = Bytes.make 20 (Char.chr 0x00) in
   s.[0] <- Char.chr 0x80;
-  H.direct_of_string s
+  H.direct_of_string @@ Bytes.unsafe_to_string s
 
 let last =
   H.direct_of_string (String.make 20 (Char.chr 0xFF))
@@ -95,16 +95,16 @@ let big_int_of_hash h =
   !n
 
 let hash_of_big_int n =
-  let s = String.create H.length in
+  let s = Bytes.create H.length in
   let n = ref n in
   let div = big_int_of_int 256 in
-  for i = String.length s - 1 downto 0 do
+  for i = Bytes.length s - 1 downto 0 do
     let (d,m) = quomod_big_int !n div in
     s.[i] <- Char.chr (int_of_big_int m);
     n := d
   done;
   assert (eq_big_int zero_big_int !n);
-  H.direct_of_string s
+  H.direct_of_string @@ Bytes.unsafe_to_string s
 
 let big_int_2 = big_int_of_int 2
 (* hash <-> number *)
diff --git a/src/networks/direct_connect/dcClients.ml b/src/networks/direct_connect/dcClients.ml
index ccd70c52..4d350e6a 100644
--- a/src/networks/direct_connect/dcClients.ml
+++ b/src/networks/direct_connect/dcClients.ml
@@ -1321,16 +1321,16 @@ let client_downloaded c sock nread = (* TODO check tth while loading, abort if e
         let downloaded =
           if c.client_preread_bytes_left > 0 then begin                (* if precheck not yet done *)
             let check_bytes = min nread c.client_preread_bytes_left in (* which is smaller... *) 
-            let check_buffer = String.create check_bytes in
+            let check_buffer = Bytes.create check_bytes in
             Unix32.read (file_fd file) (c.client_pos -- (Int64.of_int c.client_preread_bytes_left))
               check_buffer 0 check_bytes;
-            let str2 = String.sub b.buf b.pos check_bytes in
-            if (String.compare check_buffer str2) = 0 then begin      (* if downloaded is ok *) 
+            let str2 = Bytes.sub b.buf b.pos check_bytes in
+            if (Bytes.compare check_buffer str2) = 0 then begin      (* if downloaded is ok *) 
               c.client_preread_bytes_left <- c.client_preread_bytes_left - check_bytes;
               if c.client_preread_bytes_left = 0 then begin            (* if checked all preread bytes *)
                 let downloaded = b.len - check_bytes in
                 if downloaded > 0 then begin                           (* check if buffer has bytes to write to file *) 
-                  Unix32.write (file_fd file) c.client_pos b.buf (b.pos+check_bytes) downloaded
+                  Unix32.write_bytes (file_fd file) c.client_pos b.buf (b.pos+check_bytes) downloaded
                 end;
                 Int64.of_int downloaded           
               end else Int64.zero
@@ -1342,7 +1342,7 @@ let client_downloaded c sock nread = (* TODO check tth while loading, abort if e
               Int64.zero
             end
           end else begin (* precheck done, normal flow *)
-        Unix32.write (file_fd file) c.client_pos  b.buf b.pos b.len;
+        Unix32.write_bytes (file_fd file) c.client_pos b.buf b.pos b.len;
             Int64.of_int b.len
           end
           in
@@ -1370,7 +1370,7 @@ let client_downloaded c sock nread = (* TODO check tth while loading, abort if e
     | DcDownloadList filelist_fd ->                   (* downloading file list *)
         let b = TcpBufferedSocket.buf sock in        
         let len = Int64.of_int b.len in
-        Unix32.write filelist_fd c.client_pos b.buf b.pos b.len;
+        Unix32.write_bytes filelist_fd c.client_pos b.buf b.pos b.len;
         c.client_pos <- c.client_pos ++ len;
         (match c.client_user with
         | Some u -> u.user_downloaded <- u.user_downloaded ++ len
@@ -1456,7 +1456,7 @@ let udp_send ip port m =
     Buffer.reset buf;
     dc_write buf m; 
     Buffer.add_char buf '|';
-    let s = Buffer.contents buf in
+    let s = Buffer.to_bytes buf in
     (match !dc_udp_sock with
     | Some sock -> 
         (*if !verbose_udp || !verbose_msg_clients then lprintf_nl "UDP Send: (%s)" s;*)
@@ -1473,9 +1473,9 @@ let udp_handler sock event =
       UdpSocket.read_packets sock (fun p ->
         (try
           let pbuf = p.UdpSocket.udp_content in
-          let len = String.length pbuf in
+          let len = Bytes.length pbuf in
           if len > 0 then
-            udp_parse pbuf sock
+            udp_parse (Bytes.unsafe_to_string pbuf) sock
         with e -> () ) 
       ) 
           | _ -> ()
diff --git a/src/networks/direct_connect/dcProtocol.ml b/src/networks/direct_connect/dcProtocol.ml
index 9e408b45..cc0ab678 100644
--- a/src/networks/direct_connect/dcProtocol.ml
+++ b/src/networks/direct_connect/dcProtocol.ml
@@ -1122,10 +1122,10 @@ let dc_handler_server f sock nread =
   (try
     let rec iter nread =
       if nread > 0 then begin
-        let pos = String.index_from b.buf b.pos '|' in
+        let pos = Bytes.index_from b.buf b.pos '|' in
         if pos < (b.pos + b.len) then begin
           let len = pos - b.pos in
-          let s = String.sub b.buf b.pos len in
+          let s = Bytes.sub_string b.buf b.pos len in
           buf_used b (len+1);
           begin 
             try f (dc_parse true s) sock
@@ -1150,10 +1150,10 @@ let dc_handler_client c fm nm dm sock nread = (* fm = (read_first_message false)
         | Some c when c.client_receiving <> Int64.zero -> (* if we are downloading from client ...*)
             dm c sock nread 
         | _ ->                                            (* or message is a new connection ... *)
-            let pos = String.index_from b.buf b.pos '|' in
+            let pos = Bytes.index_from b.buf b.pos '|' in
             if pos < (b.pos + b.len) then begin
               let len = pos - b.pos in
-              let s = String.sub b.buf b.pos len in
+              let s = Bytes.sub_string b.buf b.pos len in
               let msg = dc_parse false s in
               buf_used b (len+1);
               begin try
diff --git a/src/networks/direct_connect/dcShared.ml b/src/networks/direct_connect/dcShared.ml
index 716577e2..4506245d 100644
--- a/src/networks/direct_connect/dcShared.ml
+++ b/src/networks/direct_connect/dcShared.ml
@@ -98,9 +98,9 @@ let file_to_che3_to_string filename =
     let rec read pos =
       let rlen = int64_min_int (flen -- pos) slen in
       let npos = Int64.add pos (Int64.of_int rlen) in
-      let str = String.create slen in
+      let str = Bytes.create slen in
       Unix32.read file_fd pos str 0 rlen;
-      Buffer.add_string buf str;
+      Buffer.add_bytes buf str;
       if npos < flen then read npos
     in
     read Int64.zero;
@@ -146,12 +146,12 @@ let file_to_bz2_to_buffer filename =
       getchar ()
     in getchar ();*)
     let rec decompress () =
-      let str = String.create 4096 in 
-      let n = Bzip2.input ic str 0 (String.length str) in
+      let str = Bytes.create 4096 in 
+      let n = Bzip2.input ic str 0 (Bytes.length str) in
       if n = 0 then ()
       else begin 
         (*let ss = (String.sub str 0 n) in*)
-        Buffer.add_string buf (String.sub str 0 n);
+        Buffer.add_string buf (Bytes.sub_string str 0 n);
         (*lprintf_nl "(%s)" ss;*)
         decompress ()
       end
@@ -180,7 +180,7 @@ let buffer_to_bz2_to_file buf filename =
         else slen
       in
       let npos = pos + len in
-      let str = Buffer.sub buf pos len in      
+      let str = Bytes.unsafe_of_string @@ Buffer.sub buf pos len in      
       Bzip2.output oc str 0 len;
       if npos < blen then compress npos 
     in compress 0;
diff --git a/src/networks/donkey/donkeyClient.ml b/src/networks/donkey/donkeyClient.ml
index 8d439a9b..ee8ad202 100644
--- a/src/networks/donkey/donkeyClient.ml
+++ b/src/networks/donkey/donkeyClient.ml
@@ -1603,7 +1603,7 @@ is checked for the file.
           if !verbose_download then
             lprintf_nl "Complete compressed block received!";
           
-          let s = String.create comp.comp_len in
+          let s = Bytes.create comp.comp_len in
           let rec iter list =
             match list with
               [] -> 0
@@ -1615,6 +1615,7 @@ is checked for the file.
           in
           let pos = iter comp.comp_blocs in
           assert (pos = comp.comp_len);
+          let s = Bytes.unsafe_to_string s in
             let s = Zlib2.uncompress_string2 s in
             if !verbose_download then
         lprintf_nl "Decompressed: %d/%d" (String.length s) comp.comp_len;
diff --git a/src/networks/donkey/donkeyFiles.ml b/src/networks/donkey/donkeyFiles.ml
index b9b2af84..422e66e6 100644
--- a/src/networks/donkey/donkeyFiles.ml
+++ b/src/networks/donkey/donkeyFiles.ml
@@ -98,7 +98,7 @@ module NewUpload = struct
               impl.impl_shared_uploaded <- 
                 impl.impl_shared_uploaded ++ uploaded);
         
-        write_string sock upload_buffer;
+        write_bytes sock upload_buffer;
         check_end_upload c sock
       with
       | End_of_file -> lprintf_nl "Can not send file %s to %s, file removed?"
diff --git a/src/networks/donkey/donkeyGlobals.ml b/src/networks/donkey/donkeyGlobals.ml
index fee7f679..420cf609 100644
--- a/src/networks/donkey/donkeyGlobals.ml
+++ b/src/networks/donkey/donkeyGlobals.ml
@@ -298,7 +298,7 @@ let md4_of_array md4s =
   Array.iteri (fun i v -> 
     String.blit (Md4.direct_to_string v) 0 s (i*16) 16
   ) md4s;
-  Md4.string s
+  Md4.string @@ Bytes.unsafe_to_string s
 
 (* compute the name used to save the file *)
 
diff --git a/src/networks/donkey/donkeyImport.ml b/src/networks/donkey/donkeyImport.ml
index f337d181..1ade5eba 100644
--- a/src/networks/donkey/donkeyImport.ml
+++ b/src/networks/donkey/donkeyImport.ml
@@ -37,7 +37,7 @@ let dump_file filename =
       let n = input ic s 0 20 in
       lprintf "pos = %d\n" !pos; 
       if n = 0 then raise Exit;
-      dump (String.sub s 0 n);
+      dump (Bytes.sub_string s 0 n);
       pos := !pos + n;
     done
   with End_of_file | Exit -> ())
diff --git a/src/networks/donkey/donkeyInteractive.ml b/src/networks/donkey/donkeyInteractive.ml
index fbe412ac..43a9b4d9 100644
--- a/src/networks/donkey/donkeyInteractive.ml
+++ b/src/networks/donkey/donkeyInteractive.ml
@@ -467,9 +467,8 @@ let import_config dirname =
 
 let broadcast msg =
   let s = msg ^ "\n" in
-  let len = String.length s in
   List.iter (fun sock ->
-      TcpBufferedSocket.write sock s 0 len
+      TcpBufferedSocket.write_string sock s
   ) !user_socks
 
   (*
diff --git a/src/networks/donkey/donkeyMftp.ml b/src/networks/donkey/donkeyMftp.ml
index 0f3d3d64..7ab1d364 100644
--- a/src/networks/donkey/donkeyMftp.ml
+++ b/src/networks/donkey/donkeyMftp.ml
@@ -122,13 +122,7 @@ let read_request ic =
   assert (c = 227);
   let len32 = read_uint64_32 ic in
   let len = Int64.to_int len32 in
-  let s = String.create len in
-  really_input ic s 0 len;
-  (*
-  lprintf "read_request %d [%s]" len (String.escaped s); 
-lprint_newline ();
-  *)
-  s
+  really_input_string ic len
 
 let output_request oc s =
   output_char oc (char_of_int 227);
diff --git a/src/networks/donkey/donkeyOvernetImport.ml b/src/networks/donkey/donkeyOvernetImport.ml
index ff21f58f..76cbb377 100644
--- a/src/networks/donkey/donkeyOvernetImport.ml
+++ b/src/networks/donkey/donkeyOvernetImport.ml
@@ -24,7 +24,7 @@ open LittleEndian
 
 let dump_file filename =
   Unix2.tryopen_read_bin filename (fun ic ->
-  let s = String.create 20 in
+  let s = Bytes.create 20 in
   try
     lprintf "file: %s\n" filename; 
     let pos = ref 0 in
@@ -32,7 +32,7 @@ let dump_file filename =
       let n = input ic s 0 20 in
       lprintf "pos = %d\n" !pos; 
       if n = 0 then raise Exit;
-      dump (String.sub s 0 n);
+      dump (Bytes.sub_string s 0 n);
       pos := !pos + n;
     done
   with End_of_file -> ())
diff --git a/src/networks/donkey/donkeyPandora.ml b/src/networks/donkey/donkeyPandora.ml
index 19934ac2..a48e38df 100644
--- a/src/networks/donkey/donkeyPandora.ml
+++ b/src/networks/donkey/donkeyPandora.ml
@@ -172,7 +172,7 @@ let client_parse c opcode s =
             if comp.comp_len = comp.comp_total then begin
                 lprintf "Compressed bloc received !!!!!!\n";
                 
-                let s = String.create comp.comp_len in
+                let s = Bytes.create comp.comp_len in
                 let rec iter list =
                   match list with
                     [] -> 0
@@ -183,6 +183,7 @@ let client_parse c opcode s =
                       pos + len
                 in
                 let pos = iter comp.comp_blocs in
+                let s = Bytes.unsafe_to_string s in
                 assert (pos = comp.comp_len);
                   let s = Zlib2.uncompress_string2 s in
                   lprintf "Decompressed: %d/%d\n" (String.length s) comp.comp_len;
diff --git a/src/networks/donkey/donkeyProtoCom.ml b/src/networks/donkey/donkeyProtoCom.ml
index bc03edf7..1f68a98b 100644
--- a/src/networks/donkey/donkeyProtoCom.ml
+++ b/src/networks/donkey/donkeyProtoCom.ml
@@ -42,11 +42,11 @@ let client_msg_to_string emule_version msg =
   buf_int8 buf 0;
   buf_int buf 0;
   let magic = DonkeyProtoClient.write emule_version buf msg in
-  let s = Buffer.contents buf in
-  let len = String.length s - 5 in
+  let s = Buffer.to_bytes buf in
+  let len = Bytes.length s - 5 in
   s.[0] <- char_of_int magic;
   str_int s 1 len;
-  s
+  Bytes.unsafe_to_string s
 
 let server_msg_to_string msg =
   Buffer.reset buf;
@@ -60,10 +60,10 @@ let server_msg_to_string msg =
       lprint_newline ();
     end;
 
-  let s = Buffer.contents buf in
-  let len = String.length s - 5 in
+  let s = Buffer.to_bytes buf in
+  let len = Bytes.length s - 5 in
   str_int s 1 len;
-  s
+  Bytes.unsafe_to_string s
 
 let server_send sock m =
 (*
@@ -101,11 +101,11 @@ let client_handler2 c ff f =
             None -> emule_proto ();
           | Some c -> c.client_emule_proto
         in
-        let opcode = get_uint8 b.buf b.pos in
-        let msg_len = get_int b.buf (b.pos+1) in
+        let opcode = get_uint8 (Bytes.unsafe_to_string b.buf) b.pos in
+        let msg_len = get_int (Bytes.unsafe_to_string b.buf) (b.pos+1) in
         if b.len >= 5 + msg_len then
           begin
-            let s = String.sub b.buf (b.pos+5) msg_len in
+            let s = Bytes.sub_string b.buf (b.pos+5) msg_len in
             buf_used b  (msg_len + 5);
             let t = M.parse emule_version opcode s in
 (*          M.print t;
@@ -124,11 +124,11 @@ let cut_messages parse f sock nread =
   let b = TcpBufferedSocket.buf sock in
   try
     while b.len >= 5 do
-      let opcode = get_uint8 b.buf b.pos in
-      let msg_len = get_int b.buf (b.pos+1) in
+      let opcode = get_uint8 (Bytes.unsafe_to_string b.buf) b.pos in
+      let msg_len = get_int (Bytes.unsafe_to_string b.buf) (b.pos+1) in
       if b.len >= 5 + msg_len then
         begin
-          let s = String.sub b.buf (b.pos+5) msg_len in
+          let s = Bytes.sub_string b.buf (b.pos+5) msg_len in
           buf_used b (msg_len + 5);
           let t = parse opcode s in
           f t sock
@@ -148,7 +148,7 @@ let really_udp_send t ip port msg isping =
   try
     Buffer.reset buf;
     DonkeyProtoUdp.write buf msg;
-    let s = Buffer.contents buf in
+    let s = Buffer.to_bytes buf in
     UdpSocket.write t isping s ip port
   with e ->
       lprintf_nl "Exception %s in udp_send" (Printexc2.to_string e)
@@ -163,10 +163,10 @@ let udp_handler f sock event =
       UdpSocket.read_packets sock (fun p ->
           try
             let pbuf = p.UdpSocket.udp_content in
-            let len = String.length pbuf in
+            let len = Bytes.length pbuf in
             if len > 0 then
-              let t = M.parse (int_of_char pbuf.[0])
-                (String.sub pbuf 1 (len-1)) in
+              let t = M.parse (int_of_char (Bytes.get pbuf 0))
+                (Bytes.sub_string pbuf 1 (len-1)) in
 (*              M.print t; *)
               f t p
           with e -> ()
@@ -178,7 +178,7 @@ let udp_basic_handler f sock event =
     UdpSocket.READ_DONE ->
       UdpSocket.read_packets sock (fun p ->
           try
-            let pbuf = p.UdpSocket.udp_content in
+            let pbuf = Bytes.unsafe_to_string p.UdpSocket.udp_content in
             let len = String.length pbuf in
             if len = 0 ||
               int_of_char pbuf.[0] <> DonkeyOpenProtocol.udp_magic then begin
@@ -198,7 +198,7 @@ let udp_basic_handler f sock event =
 
 
 let new_string msg s =
-  let len = String.length s - 5 in
+  let len = Bytes.length s - 5 in
   str_int s 1 len
 
 let empty_string = ""
@@ -315,9 +315,9 @@ let server_send_share compressed sock msg =
           ( make_tagged_server compressed (Some sock) msg )
           0 max_len
       in
-      let s = Buffer.contents buf in
+      let s = Buffer.to_bytes buf in
       str_int s 0 nfiles;
-      let s = String.sub s 0 prev_len in
+      let s = Bytes.sub_string s 0 prev_len in
       if !verbose_share || !verbose then
          lprintf_nl "Sending %d share%s to server %s:%d%s"
            nfiles (Printf2.print_plural_s nfiles) (Ip.to_string (peer_ip sock)) (peer_port sock)
@@ -337,7 +337,7 @@ let server_send_share compressed sock msg =
           buf_int buf 0;
           buf_int8 buf 21; (* ShareReq *)
           Buffer.add_string buf s_c;
-          Buffer.contents buf
+          Buffer.to_bytes buf
         end
       else
         begin
@@ -345,12 +345,12 @@ let server_send_share compressed sock msg =
           buf_int buf 0;
           buf_int8 buf 21; (* ShareReq *)
           Buffer.add_string buf s;
-          Buffer.contents buf
+          Buffer.to_bytes buf
         end
   in
-  let len = String.length s - 5 in
+  let len = Bytes.length s - 5 in
   str_int s 1 len;
-  write_string sock s
+  write_bytes sock s
 
 let client_send_files sock msg =
   let max_len = !!client_buffer_size - 100 -
@@ -363,12 +363,11 @@ let client_send_files sock msg =
   let nfiles, prev_len = DonkeyProtoClient.ViewFilesReply.write_files_max buf (
       make_tagged (Some sock) msg)
     0 max_len in
-  let s = Buffer.contents buf in
-  let s = String.sub s 0 prev_len in
-  let len = String.length s - 5 in
+  let s = Bytes.unsafe_of_string @@ Buffer.sub buf 0 prev_len in
+  let len = Bytes.length s - 5 in
   str_int s 1 len;
   str_int s 6 nfiles;
-  write_string sock s
+  write_bytes sock s
 
 let client_send_dir sock dir files =
   let max_len = !!client_buffer_size - 100 -
@@ -383,13 +382,12 @@ let client_send_dir sock dir files =
   let nfiles, prev_len = DonkeyProtoClient.ViewFilesReply.write_files_max buf (
       make_tagged (Some sock) files)
     0 max_len in
-  let s = Buffer.contents buf in
-  let s = String.sub s 0 prev_len in
-  let len = String.length s - 5 in begin
+  let s = Bytes.unsafe_of_string @@ Buffer.sub buf 0 prev_len in
+  let len = Bytes.length s - 5 in begin
     str_int s 1 len;
     str_int s (pos-4) nfiles;
   end;
-  write_string sock s
+  write_bytes sock s
 
 let udp_server_send s t =
   udp_send (get_udp_sock ()) s.server_ip (s.server_port+4) t
diff --git a/src/networks/donkey/donkeyProtoCom.mli b/src/networks/donkey/donkeyProtoCom.mli
index 7f8312bc..63e6e030 100644
--- a/src/networks/donkey/donkeyProtoCom.mli
+++ b/src/networks/donkey/donkeyProtoCom.mli
@@ -75,7 +75,7 @@ val client_send_files :
 val client_send_dir :
     TcpBufferedSocket.t -> string -> file list -> unit
 
-val new_string :  DonkeyProtoClient.t -> string -> unit
+val new_string :  DonkeyProtoClient.t -> bytes -> unit
 
 val tag_file : file -> CommonTypes.tag list
 
diff --git a/src/networks/donkey/donkeyProtoKademlia.ml b/src/networks/donkey/donkeyProtoKademlia.ml
index a8da9b8c..b0cb041c 100644
--- a/src/networks/donkey/donkeyProtoKademlia.ml
+++ b/src/networks/donkey/donkeyProtoKademlia.ml
@@ -84,7 +84,7 @@ module P = struct
       ss.[14] <- s.[pos+13];
       ss.[15] <- s.[pos+12];
 
-      Md4.direct_of_string ss
+      Md4.direct_of_string @@ Bytes.unsafe_to_string ss
 
     let buf_md4 buf s =
       let s = Md4.direct_to_string s in
@@ -112,7 +112,7 @@ module P = struct
       ss.[14] <- s.[pos+13];
       ss.[15] <- s.[pos+12];
 
-      Buffer.add_string buf ss
+      Buffer.add_bytes buf ss
 
 
 (* Strange: why was the IP format changed for Kademlia ? *)
@@ -458,7 +458,7 @@ module P = struct
           end;
 *)
 
-        UdpSocket.write sock ping s ip port
+        UdpSocket.write sock ping (Bytes.unsafe_of_string s) ip port
       with
       | MessageNotImplemented -> ()
       | e -> lprintf_nl "Exception %s in udp_send" (Printexc2.to_string e)
@@ -476,7 +476,7 @@ module P = struct
                       Ip.of_inet_addr inet, port
                   | _ -> assert false
                 in
-                let t = parse_message ip port pbuf in
+                let t = parse_message ip port (Bytes.unsafe_to_string pbuf) in
                 let is_not_banned ip =
                   match !Ip.banned (ip, None) with
                     None -> true
@@ -491,7 +491,7 @@ module P = struct
                 begin
                   lprintf_nl "Error %s in udp_handler, dump of packet:"
                     (Printexc2.to_string e);
-                  dump p.UdpSocket.udp_content;
+                  dump (Bytes.unsafe_to_string p.UdpSocket.udp_content);
                   lprint_newline ()
                 end
           );
diff --git a/src/networks/donkey/donkeyProtoOvernet.ml b/src/networks/donkey/donkeyProtoOvernet.ml
index 6157c45d..3ff18328 100644
--- a/src/networks/donkey/donkeyProtoOvernet.ml
+++ b/src/networks/donkey/donkeyProtoOvernet.ml
@@ -387,7 +387,7 @@ module Proto = struct
         UdpSocket.READ_DONE ->
           UdpSocket.read_packets sock (fun p ->
               try
-                let pbuf = p.UdpSocket.udp_content in
+                let pbuf = Bytes.unsafe_to_string p.UdpSocket.udp_content in
                 let len = String.length pbuf in
                 if len < 2 ||
                   int_of_char pbuf.[0] <> 227 then
@@ -420,7 +420,7 @@ module Proto = struct
                 if !verbose_unknown_messages then begin
                   lprintf_nl "Error %s in udp_handler, dump of packet:"
                     (Printexc2.to_string e);
-                  dump p.UdpSocket.udp_content;
+                  dump (Bytes.unsafe_to_string p.UdpSocket.udp_content);
                   lprint_newline ()
                 end
           );
@@ -447,7 +447,7 @@ module Proto = struct
             lprintf_nl "UDP to %s:%d op 0x%02X len %d type %s"
               (Ip.to_string ip) port (get_uint8 s 1) (String.length s) (message_to_string msg);
           end;
-        UdpSocket.write sock ping s ip port
+        UdpSocket.write sock ping (Bytes.unsafe_of_string s) ip port
       with e ->
           lprintf_nl "Exception %s in udp_send" (Printexc2.to_string e)
 
diff --git a/src/networks/fasttrack/fasttrackNetwork.ml b/src/networks/fasttrack/fasttrackNetwork.ml
index b792e58c..405dacfa 100644
--- a/src/networks/fasttrack/fasttrackNetwork.ml
+++ b/src/networks/fasttrack/fasttrackNetwork.ml
@@ -78,7 +78,7 @@ type file_uid = Md5Ext.t
 type file_uri = string
 
 external create_cipher : unit -> cipher = "ml_create_cipher"
-external apply_cipher : cipher -> string -> int -> int -> unit
+external apply_cipher : cipher -> bytes -> int -> int -> unit
   = "ml_apply_cipher"
 external init_cipher : cipher -> unit = "ml_init_cipher"
 external set_cipher : cipher -> int32 -> int -> unit = "ml_set_cipher"
@@ -87,9 +87,9 @@ external get_cipher_from_packet : string -> int -> cipher -> unit
 external xor_ciphers : cipher -> cipher -> unit = "ml_xor_ciphers"
 external xor_ciphers2 : cipher -> cipher -> unit = "ml_xor_ciphers2"
 
-external cipher_packet_set : cipher -> string -> int -> unit
+external cipher_packet_set : cipher -> bytes -> int -> unit
   = "ml_cipher_packet_set"
-external cipher_packet_set_xored : cipher -> string -> int -> cipher -> unit
+external cipher_packet_set_xored : cipher -> bytes -> int -> cipher -> unit
   = "ml_cipher_packet_set_xored"
 external cipher_free : cipher -> unit = "ml_cipher_free"
 external cipher_enc_type : cipher -> int = "ml_cipher_enc_type"
diff --git a/src/networks/fasttrack/fasttrackProto.ml b/src/networks/fasttrack/fasttrackProto.ml
index 71057f5b..5d88d006 100644
--- a/src/networks/fasttrack/fasttrackProto.ml
+++ b/src/networks/fasttrack/fasttrackProto.ml
@@ -65,8 +65,9 @@ let ip_to_string ip = Ip.to_string ip
 let crypt_and_send sock out_cipher str =
   if !verbose_msg_raw || monitored sock then
     lprintf "crypt_and_send: to send [%s]\n" (String.escaped str);
-  let str = String.copy str in
-  apply_cipher out_cipher str 0 (String.length str);
+  let str = Bytes.of_string str in
+  apply_cipher out_cipher str 0 (Bytes.length str);
+  let str = Bytes.unsafe_to_string str in
   if !verbose_msg_raw || monitored sock then
     lprintf "crypt_and_send: [%s] sent\n" (String.escaped str);
   write_string sock str
@@ -1800,7 +1801,7 @@ module UdpMessages = struct
 
       try
         let s = write msg in
-        UdpSocket.write t ping s ip port
+        UdpSocket.write t ping (Bytes.unsafe_of_string s) ip port
       with e ->
           lprintf "FT: Exception %s in udp_send\n" (Printexc2.to_string e)
 
@@ -1888,13 +1889,13 @@ let check_primitives () =
       let cipher = create_cipher () in
       set_cipher cipher 123456789l 0x29;
       init_cipher cipher;
-      let s = String.make 12 '0' in
+      let s = Bytes.make 12 '0' in
       cipher_packet_set cipher s 0;
-      assert (s = "\007\091\205\021\110\233\135\1870000"); 
+      assert (Bytes.to_string s = "\007\091\205\021\110\233\135\1870000"); 
       (* lprintf "cipher_packet_set s = \"%s\"\n" (String.escaped s); *)
-      let s = "123456789abcdefghijklm\233\234\235" in
-      apply_cipher cipher s 0 (String.length s);
-      assert (s = "\016\210\245\241\144Ug\028Z\229\1928\176\167\192\008\139\019\018Z\1937\226\250i"); 
+      let s = Bytes.of_string "123456789abcdefghijklm\233\234\235" in
+      apply_cipher cipher s 0 (Bytes.length s);
+      assert (Bytes.to_string s = "\016\210\245\241\144Ug\028Z\229\1928\176\167\192\008\139\019\018Z\1937\226\250i"); 
       (* lprintf "apply_cipher s = \"%s\"\n" (String.escaped s); *)
       cipher_free cipher;
     with _ ->
diff --git a/src/networks/fasttrack/fasttrackProtocol.ml b/src/networks/fasttrack/fasttrackProtocol.ml
index fe64101f..a0ceb94d 100644
--- a/src/networks/fasttrack/fasttrackProtocol.ml
+++ b/src/networks/fasttrack/fasttrackProtocol.ml
@@ -87,13 +87,13 @@ let rec iter len n =
     s.[len] <- char_of_int (0x80 lor (Int64.to_int (Int64.logand n int64_7f)));
     s
   else
-  let s = String.create (len+1) in
+  let s = Bytes.create (len+1) in
   s.[len] <- char_of_int (Int64.to_int n);
   s
 
 let buf_dynint b data =
   let data = Int64.logand bits32 data in
-  Buffer.add_string b (iter 0 data)
+  Buffer.add_bytes b (iter 0 data)
 
 let buf_dynint b data =
   let data = Int64.logand bits32 data in
@@ -116,7 +116,7 @@ let buf_dynint b data =
           (Int64.logand !data int64_7f)));
     data := Int64.shift_right_logical !data  7;
   done;
-  Buffer.add_string b (String.sub buf 0 len)
+  Buffer.add_string b (Bytes.sub_string buf 0 len)
 
 let dynint v =
   let b = Buffer.create 10 in
diff --git a/src/networks/fasttrack/fasttrackServers.ml b/src/networks/fasttrack/fasttrackServers.ml
index 556f11c0..04208ecd 100644
--- a/src/networks/fasttrack/fasttrackServers.ml
+++ b/src/networks/fasttrack/fasttrackServers.ml
@@ -92,12 +92,12 @@ let server_parse_after s gconn sock =
        let rec iter () =
           let len = b.len in
           if len > 0 then
-            let size = TcpMessages.packet_size ciphers b.buf b.pos b.len in
+            let size = TcpMessages.packet_size ciphers (Bytes.unsafe_to_string b.buf) b.pos b.len in
             match size with
               None -> ()
             | Some size ->
                 if len >= size then
-                  let msg = String.sub b.buf b.pos size in
+                  let msg = Bytes.sub_string b.buf b.pos size in
                   buf_used b size;
                   let addr, t = TcpMessages.parse ciphers msg in
                   FasttrackHandler.server_msg_handler sock s addr t;
@@ -129,13 +129,13 @@ let server_parse_netname s gconn sock =
   let start_pos = b.pos in
   let end_pos = start_pos + len in
   let buf = b.buf in
-  let net = String.sub buf start_pos len in
+  let net = Bytes.sub_string buf start_pos len in
   if !verbose_msg_raw then
     lprintf "net:[%s]\n" (String.escaped net);
   let rec iter pos =
     if pos < end_pos then
-      if buf.[pos] = '\000' then begin
-          let netname = String.sub buf start_pos (pos-start_pos) in
+      if Bytes.get buf pos = '\000' then begin
+          let netname = Bytes.sub_string buf start_pos (pos-start_pos) in
           if !verbose_msg_raw then
             lprintf "netname: [%s]\n" (String.escaped netname);
           buf_used b (pos-start_pos+1);
@@ -159,7 +159,7 @@ let server_parse_cipher s gconn sock =
     | Some ciphers ->
         if !verbose_msg_raw then
           lprintf "Cipher received from server\n";
-        get_cipher_from_packet b.buf b.pos ciphers.in_cipher;
+        get_cipher_from_packet (Bytes.unsafe_to_string b.buf) b.pos ciphers.in_cipher;
         init_cipher ciphers.in_cipher;
 
         xor_ciphers ciphers.out_cipher ciphers.in_cipher;
@@ -241,7 +241,7 @@ let connect_server h =
               };
               set_cipher out_cipher (client_cipher_seed ()) 0x29;
 
-              let s = String.create 12 in
+              let s = Bytes.create 12 in
 
               (match !connection_header_hook with
                   None ->
@@ -253,6 +253,8 @@ let connect_server h =
 
               cipher_packet_set out_cipher s 4;
 
+              let s = Bytes.unsafe_to_string s in
+
               if !verbose_msg_raw then begin
                   lprintf "SENDING %s\n" (String.escaped s);
                   AnyEndian.dump s;
diff --git a/src/networks/fileTP/fileTPFTP.ml b/src/networks/fileTP/fileTPFTP.ml
index f5ba897b..08763f2b 100644
--- a/src/networks/fileTP/fileTPFTP.ml
+++ b/src/networks/fileTP/fileTPFTP.ml
@@ -79,7 +79,7 @@ end_pos !counter_pos b.len to_read;
 
             let old_downloaded = CommonSwarming.downloaded swarmer in
 
-            CommonSwarming.received up !counter_pos b.buf b.pos to_read_int;
+            CommonSwarming.received up !counter_pos (Bytes.unsafe_to_string b.buf) b.pos to_read_int;
             let new_downloaded = CommonSwarming.downloaded swarmer in
 
             c.client_total_downloaded <- c.client_total_downloaded ++ (new_downloaded -- old_downloaded);
@@ -183,15 +183,15 @@ let ftp_send_range_request c (x,y) sock d =
   TcpBufferedSocket.set_reader sock (fun sock nread ->
       let b = TcpBufferedSocket.buf sock in
       if !verbose then 
-        AnyEndian.dump_hex (String.sub b.buf b.pos b.len);
+        AnyEndian.dump_hex (Bytes.sub_string b.buf b.pos b.len);
       let rec iter i =
         if i < b.len then
-          if b.buf.[b.pos + i] = '\n' then begin
-              let slen = if i > 0 && b.buf.[b.pos + i - 1] = '\r' 
+          if Bytes.get b.buf (b.pos + i) = '\n' then begin
+              let slen = if i > 0 && Bytes.get b.buf (b.pos + i - 1) = '\r' 
                 then i - 1
                 else i 
               in
-              let line = String.sub b.buf b.pos slen in
+              let line = Bytes.sub_string b.buf b.pos slen in
               if !verbose then lprintf_nl "SRR LINE [%s]" line;
               buf_used b (i+1);
               if slen > 3 then begin
@@ -382,16 +382,16 @@ let ftp_check_size file url start_download_file =
       TcpBufferedSocket.set_reader sock (fun sock nread ->
           let b = TcpBufferedSocket.buf sock in
           if !verbose then
-             AnyEndian.dump_hex (String.sub b.buf b.pos b.len);
+             AnyEndian.dump_hex (Bytes.sub_string b.buf b.pos b.len);
           let rec iter i =
             if i < b.len then
-              if b.buf.[b.pos + i] = '\n' then begin
-                  let slen = if i > 0 && b.buf.[b.pos + i - 1] = '\r' 
+              if Bytes.get b.buf (b.pos + i) = '\n' then begin
+                  let slen = if i > 0 && Bytes.get b.buf (b.pos + i - 1) = '\r' 
                     then i - 1
                     else i 
                   in
 
-                  let line = String.sub b.buf b.pos slen in
+                  let line = Bytes.sub_string b.buf b.pos slen in
                   if !verbose then lprintf_nl "CS LINE [%s]" line;
                   buf_used b (i+1);
                   if slen > 3 then begin
diff --git a/src/networks/fileTP/fileTPHTTP.ml b/src/networks/fileTP/fileTPHTTP.ml
index 62f3ac18..c4130f8c 100644
--- a/src/networks/fileTP/fileTPHTTP.ml
+++ b/src/networks/fileTP/fileTPHTTP.ml
@@ -264,7 +264,7 @@ let rec client_parse_header c gconn sock header =
 
             let old_downloaded = CommonSwarming.downloaded swarmer in
 
-            CommonSwarming.received up !counter_pos b.buf b.pos to_read_int;
+            CommonSwarming.received up !counter_pos (Bytes.unsafe_to_string b.buf) b.pos to_read_int;
             let new_downloaded = CommonSwarming.downloaded swarmer in
 
             c.client_total_downloaded <- c.client_total_downloaded ++ (new_downloaded -- old_downloaded);
diff --git a/src/networks/fileTP/fileTPProtocol.ml b/src/networks/fileTP/fileTPProtocol.ml
index 0cc46b8c..8879948c 100644
--- a/src/networks/fileTP/fileTPProtocol.ml
+++ b/src/networks/fileTP/fileTPProtocol.ml
@@ -48,12 +48,12 @@ let handlers info gconn =
           let begin_pos =  b.pos in
           let rec iter i n_read =
             if i < end_pos then
-              if b.buf.[i] = '\r' then
+              if Bytes.get b.buf i = '\r' then
                 iter (i+1) n_read
               else
-              if b.buf.[i] = '\n' then
+              if Bytes.get b.buf i = '\n' then
                 if n_read then begin
-                    let header = String.sub b.buf b.pos (i - b.pos) in
+                    let header = Bytes.sub_string b.buf b.pos (i - b.pos) in
 (*                    if info then begin
                         lprintf "HEADER : ";
                         dump header; lprint_newline ();
diff --git a/src/networks/fileTP/fileTPSSH.ml b/src/networks/fileTP/fileTPSSH.ml
index a08184b5..6b93c992 100644
--- a/src/networks/fileTP/fileTPSSH.ml
+++ b/src/networks/fileTP/fileTPSSH.ml
@@ -182,14 +182,14 @@ let ssh_check_size file url start_download_file =
       close sock s);
   TcpBufferedSocket.set_reader sock (fun sock nread ->
       let b = TcpBufferedSocket.buf sock in
-      lprintf "SSH reader %d [%s]\n" nread (String.escaped (String.sub b.buf b.pos b.len));
+      lprintf "SSH reader %d [%s]\n" nread (String.escaped (Bytes.sub_string b.buf b.pos b.len));
       let rec iter i =
         if i < b.len then
-          if b.buf.[b.pos + i] = '\n' then begin
-              let slen = if i > 0 && b.buf.[b.pos + i - 1] = '\r' then
+          if Bytes.get b.buf (b.pos + i) = '\n' then begin
+              let slen = if i > 0 && Bytes.get b.buf (b.pos + i - 1) = '\r' then
                   i - 1
                 else i in
-              let line = String.sub b.buf b.pos slen in
+              let line = Bytes.sub_string b.buf b.pos slen in
               lprintf "SSH LINE [%s]\n" line;
               buf_used b (i+1);
               if String2.starts_with line "[SIZE " then begin
@@ -245,7 +245,7 @@ let ssh_connect token c f =
 
             if b.len >= elen then begin
                 segment := SegmentX (file_num, pos, len, elen,
-                  String.sub b.buf b.pos elen);
+                  Bytes.sub_string b.buf b.pos elen);
                 buf_used b elen;
                 iter0 0
               end
@@ -255,11 +255,11 @@ let ssh_connect token c f =
 
       and iter0 i =
         if i < b.len then
-          if b.buf.[b.pos + i] = '\n' then begin
-              let slen = if i > 0 && b.buf.[b.pos + i - 1] = '\r' then
+          if Bytes.get b.buf (b.pos + i) = '\n' then begin
+              let slen = if i > 0 && Bytes.get b.buf (b.pos + i - 1) = '\r' then
                   i - 1
                 else i in
-              let line = String.sub b.buf b.pos slen in
+              let line = Bytes.sub_string b.buf b.pos slen in
 (*              lprintf "SSH LINE [%s]\n" line; *)
               buf_used b (i+1);
 
diff --git a/src/networks/gnutella/gnutellaClients.ml b/src/networks/gnutella/gnutellaClients.ml
index 733b1ab7..ab440993 100644
--- a/src/networks/gnutella/gnutellaClients.ml
+++ b/src/networks/gnutella/gnutellaClients.ml
@@ -215,7 +215,7 @@ save it on disk in the next version. *)
             end;
           let buf = Buffer.create 100 in
           let read_ttr counter_pos b to_read_int =
-            Buffer.add_substring buf b.buf b.pos to_read_int
+            Buffer.add_subbytes buf b.buf b.pos to_read_int
           in
           let read_more () =
             if !verbose_msg_clients then
@@ -266,7 +266,7 @@ and read_some d c counter_pos b to_read_int =
   begin
     try
       CommonSwarming.received up
-        counter_pos b.buf b.pos to_read_int;
+        counter_pos (Bytes.unsafe_to_string b.buf) b.pos to_read_int;
     with e -> 
         lprintf "FT: Exception %s in CommonSwarming.received\n"
           (Printexc2.to_string e);
diff --git a/src/networks/gnutella/gnutellaFunctions.ml b/src/networks/gnutella/gnutellaFunctions.ml
index ff83b287..09e6390c 100644
--- a/src/networks/gnutella/gnutellaFunctions.ml
+++ b/src/networks/gnutella/gnutellaFunctions.ml
@@ -87,7 +87,7 @@ let default_handler gconn sock =
       let b = buf sock in
       if !verbose then
         lprintf "HttpReader: Handler not found for [%s]\n"
-          (String.escaped (String.sub b.buf b.pos b.len));
+          (String.escaped (Bytes.sub_string b.buf b.pos b.len));
       close sock (Closed_for_error "not recognized");
       failwith "Reply is not in the correct protocol"
   | Some f -> f gconn sock
@@ -104,12 +104,12 @@ let handlers info gconn =
     let b = TcpBufferedSocket.buf sock in 
     if monitored sock || !verbose_msg_raw then
       lprintf "iter_read %s :%d/%d\n%s\n" (Ip.to_string (peer_ip sock)) nread b.len
-        (String.escaped (String.sub b.buf b.pos b.len));
+        (String.escaped (Bytes.sub_string b.buf b.pos b.len));
     if b.len > 0 then
       match gconn.gconn_handler with
       | HttpReader (n, hs, default) ->
           if b.len >= n then
-            let head = String.sub b.buf b.pos n in
+            let head = Bytes.sub_string b.buf b.pos n in
             (try                 
                 let rec iter hs =
                   match hs with
@@ -133,9 +133,9 @@ let handlers info gconn =
                   
                   | (proto, h) :: tail ->
                       if String2.starts_with head proto then begin
-                          let i = find_header_end b.buf b.pos b.len in
+                          let i = find_header_end (Bytes.unsafe_to_string b.buf) b.pos b.len in
                           if i <> 0 then
-                            let header = String.sub b.buf b.pos (i - b.pos) in
+                            let header = Bytes.sub_string b.buf b.pos (i - b.pos) in
                             let first_line, headers =        
                               match  Http_client.split_header header  with
                                 [] -> "", []
@@ -197,17 +197,17 @@ let handlers info gconn =
       | CipherReader (cipher, h) ->
           if monitored sock || !verbose_msg_raw then
             lprintf "CipherReader %d: [%s]\n" nread
-              (String.escaped (String.sub b.buf b.pos b.len)); 
+              (String.escaped (Bytes.sub_string b.buf b.pos b.len)); 
           if nread > 0 then begin
 (*              AnyEndian.dump_sub b.buf (b.pos + b.len - nread) nread; *)
               apply_cipher cipher b.buf (b.pos + b.len - nread) nread;
               
               log (LogReceive(peer_ip sock, peer_port sock,
-                  String.sub  b.buf (b.pos + b.len - nread) nread));
+                  Bytes.sub_string  b.buf (b.pos + b.len - nread) nread));
               
               if monitored sock || !verbose_msg_raw then
                 lprintf "   deciphered: [%s]\n"
-                  (String.escaped (String.sub b.buf b.pos b.len));
+                  (String.escaped (Bytes.sub_string b.buf b.pos b.len));
             end;
           let len = b.len in
           (try
diff --git a/src/networks/gnutella/gnutellaMain.ml b/src/networks/gnutella/gnutellaMain.ml
index f7e07e83..7d252f7c 100644
--- a/src/networks/gnutella/gnutellaMain.ml
+++ b/src/networks/gnutella/gnutellaMain.ml
@@ -74,7 +74,7 @@ let udp_handler sock event =
             in
 (*            lprintf "Gnutella: calling udp_client_handler %s:%d\n"
               (Ip.to_string ip) port; *)
-            let buf = p.UdpSocket.udp_content in
+            let buf = Bytes.unsafe_to_string p.UdpSocket.udp_content in
             GnutellaHandler.udp_client_handler ip port buf 
           with e ->
               lprintf "Error %s in udp_handler\n"
diff --git a/src/networks/gnutella/gnutellaTypes.ml b/src/networks/gnutella/gnutellaTypes.ml
index c9ee3fbe..8eda3461 100644
--- a/src/networks/gnutella/gnutellaTypes.ml
+++ b/src/networks/gnutella/gnutellaTypes.ml
@@ -100,7 +100,7 @@ and client = {
 and upload_client = {
     uc_sock : TcpBufferedSocket.t;
     uc_partial : bool;
-    uc_reader : (int64 -> string -> int -> int -> unit);
+    uc_reader : (int64 -> bytes -> int -> int -> unit);
     mutable uc_chunk_pos : int64;
     uc_chunk_len : int64;
     uc_chunk_end : int64;
diff --git a/src/utils/cdk/bzip2.ml b/src/utils/cdk/bzip2.ml
index 07ca4192..89b4e1dd 100644
--- a/src/utils/cdk/bzip2.ml
+++ b/src/utils/cdk/bzip2.ml
@@ -7,7 +7,7 @@ let buffer_size = 1024
 
 type in_channel =
   { in_chan: Pervasives.in_channel;
-    in_buffer: string;
+    in_buffer: bytes;
     mutable in_pos: int;
     mutable in_avail: int;
     mutable in_eof: bool;
@@ -16,7 +16,7 @@ type in_channel =
 
 let open_in_chan ic =
   { in_chan = ic;
-    in_buffer = String.create buffer_size;
+    in_buffer = Bytes.create buffer_size;
     in_pos = 0;
     in_avail = 0;
     in_eof = false;
@@ -32,12 +32,12 @@ let open_in filename =
 let read_byte iz =
   if iz.in_avail = 0 then begin
     let n = Pervasives.input iz.in_chan iz.in_buffer 0
-                             (String.length iz.in_buffer) in
+                             (Bytes.length iz.in_buffer) in
     if n = 0 then raise End_of_file;
     iz.in_pos <- 0;
     iz.in_avail <- n
   end;
-  let c = iz.in_buffer.[iz.in_pos] in
+  let c = Bytes.get iz.in_buffer iz.in_pos in
   iz.in_pos <- iz.in_pos + 1;
   iz.in_avail <- iz.in_avail - 1;
   Char.code c
@@ -53,19 +53,19 @@ let read_int32 iz =
                    (Int32.shift_left (Int32.of_int b4) 24)))
 
 let rec input iz buf pos len =
-  if pos < 0 || len < 0 || pos + len > String.length buf then
+  if pos < 0 || len < 0 || pos + len > Bytes.length buf then
     invalid_arg "Bzip2.input";
   if iz.in_eof then 0 else begin
     if iz.in_avail = 0 then begin
       let n = Pervasives.input iz.in_chan iz.in_buffer 0
-                               (String.length iz.in_buffer) in
+                               (Bytes.length iz.in_buffer) in
       if n = 0 then raise(Error("truncated file"));
       iz.in_pos <- 0;
       iz.in_avail <- n
     end;
     let (finished, used_in, used_out) =
       try
-        Bzlib.decompress iz.in_stream iz.in_buffer iz.in_pos iz.in_avail
+        Bzlib.decompress iz.in_stream (Bytes.unsafe_to_string iz.in_buffer) iz.in_pos iz.in_avail
                                    buf pos len
       with Bzlib.Error(_, e) ->
         raise(Error(Bzlib.string_of_error e)) in
@@ -88,10 +88,10 @@ let rec really_input iz buf pos len =
     really_input iz buf (pos + n) (len - n)
   end
 
-let char_buffer = String.create 1
+let char_buffer = Bytes.create 1
 
 let input_char iz =
-  if input iz char_buffer 0 1 = 0 then raise End_of_file else char_buffer.[0]
+  if input iz char_buffer 0 1 = 0 then raise End_of_file else Bytes.get char_buffer 0
 
 let input_byte iz =
   Char.code (input_char iz)
@@ -106,7 +106,7 @@ let close_in iz =
 
 type out_channel =
   { out_chan: Pervasives.out_channel;
-    out_buffer: string;
+    out_buffer: bytes;
     mutable out_pos: int;
     mutable out_avail: int;
     out_stream: Bzlib.stream;
@@ -115,7 +115,7 @@ type out_channel =
 let open_out_chan ?(level = 6) oc =
   if level < 1 || level > 9 then invalid_arg "Bzip2.open_out: bad level";
   { out_chan = oc;
-    out_buffer = String.create buffer_size;
+    out_buffer = Bytes.create buffer_size;
     out_pos = 0;
     out_avail = buffer_size;
     out_stream = Bzlib.compress_init level 0 0;
@@ -125,18 +125,18 @@ let open_out ?(level = 6) filename =
   open_out_chan ~level (Pervasives.open_out_bin filename)
 
 let rec output oz buf pos len =
-  if pos < 0 || len < 0 || pos + len > String.length buf then
+  if pos < 0 || len < 0 || pos + len > Bytes.length buf then
     invalid_arg "Bzlib2.output";
   (* If output buffer is full, flush it *)
   if oz.out_avail = 0 then begin
   (* Printf.printf "Flushing out_avail\n"; *)
     Pervasives.output oz.out_chan oz.out_buffer 0 oz.out_pos;
     oz.out_pos <- 0;
-    oz.out_avail <- String.length oz.out_buffer
+    oz.out_avail <- Bytes.length oz.out_buffer
   end;
   let (_, used_in, used_out) =
     try
-      Bzlib.compress oz.out_stream buf pos len
+      Bzlib.compress oz.out_stream (Bytes.unsafe_to_string buf) pos len
                                  oz.out_buffer oz.out_pos oz.out_avail
                                  Bzlib.BZ_RUN
     with Bzlib.Error(_, e) ->
@@ -147,7 +147,7 @@ let rec output oz buf pos len =
   if used_in < len then output oz buf (pos + used_in) (len - used_in)
 
 let output_char oz c =
-  char_buffer.[0] <- c;
+  Bytes.set char_buffer 0 c;
   output oz char_buffer 0 1
 
 let output_byte oz b =
@@ -159,10 +159,10 @@ let flush oz =
     if oz.out_avail = 0 then begin
       Pervasives.output oz.out_chan oz.out_buffer 0 oz.out_pos;
       oz.out_pos <- 0;
-      oz.out_avail <- String.length oz.out_buffer
+      oz.out_avail <- Bytes.length oz.out_buffer
     end;
     let (finished, _, used_out) =
-      Bzlib.compress oz.out_stream oz.out_buffer 0 0
+      Bzlib.compress oz.out_stream (Bytes.unsafe_to_string oz.out_buffer) 0 0
                                  oz.out_buffer oz.out_pos oz.out_avail
                                  Bzlib.BZ_FINISH in
     oz.out_pos <- oz.out_pos + used_out;
diff --git a/src/utils/cdk/bzip2.mli b/src/utils/cdk/bzip2.mli
index 40ddf405..01969973 100644
--- a/src/utils/cdk/bzip2.mli
+++ b/src/utils/cdk/bzip2.mli
@@ -22,10 +22,10 @@ val input_byte: in_channel -> int
         (* Same as [Bzip2.input_char], but return the 8-bit integer representing
            the character.
            Raise [End_of_file] if no more compressed data is available. *)
-val input: in_channel -> string -> int -> int -> int
+val input: in_channel -> bytes -> int -> int -> int
         (* [input ic buf pos len] uncompresses up to [len] characters
            from the given channel [ic],
-           storing them in string [buf], starting at character number [pos].
+           storing them in buffer [buf], starting at character number [pos].
            It returns the actual number of characters read, between 0 and
            [len] (inclusive).
            A return value of 0 means that the end of file was reached.
@@ -38,10 +38,10 @@ val input: in_channel -> string -> int -> int -> int
            exactly [len] characters.)
            Exception [Invalid_argument "Bzip2.input"] is raised if
            [pos] and [len] do not designate a valid substring of [buf]. *)
-val really_input: in_channel -> string -> int -> int -> unit
+val really_input: in_channel -> bytes -> int -> int -> unit
         (* [really_input ic buf pos len] uncompresses [len] characters
            from the given channel, storing them in
-           string [buf], starting at character number [pos].
+           buffer [buf], starting at character number [pos].
            Raise [End_of_file] if fewer than [len] characters can be read.
            Raise [Invalid_argument "Bzip2.input"] if
            [pos] and [len] do not designate a valid substring of [buf]. *)
@@ -82,7 +82,7 @@ val output_char: out_channel -> char -> unit
 val output_byte: out_channel -> int -> unit
         (* Same as [Bzip2.output_char], but the output character is given
            by its code.  The given integer is taken modulo 256. *)
-val output: out_channel -> string -> int -> int -> unit
+val output: out_channel -> bytes -> int -> int -> unit
         (* [output oc buf pos len] compresses and writes [len] characters
            from string [buf], starting at offset [pos], and writes the
            compressed data to the channel [oc].
diff --git a/src/utils/cdk/bzlib.ml b/src/utils/cdk/bzlib.ml
index ff17e9ec..459e4ac1 100644
--- a/src/utils/cdk/bzlib.ml
+++ b/src/utils/cdk/bzlib.ml
@@ -23,7 +23,7 @@ type action = BZ_RUN | BZ_FLUSH | BZ_FINISH
 
 external compress_init: int -> int -> int -> stream = "camlzip_bzCompressInit"
 external compress:
-  stream -> string -> int -> int -> string -> int -> int -> action
+  stream -> string -> int -> int -> bytes -> int -> int -> action
          -> bool * int * int
   = "camlzip_bzCompress_bytecode" "camlzip_bzCompress"
 external compress_end: stream -> unit = "camlzip_bzCompressEnd"
@@ -31,7 +31,7 @@ external compress_end: stream -> unit = "camlzip_bzCompressEnd"
 
 external decompress_init: int -> bool -> stream = "camlzip_bzDecompressInit"
 external decompress:
-  stream -> string -> int -> int -> string -> int -> int -> bool * int * int
+  stream -> string -> int -> int -> bytes -> int -> int -> bool * int * int
   = "camlzip_bzDecompress_bytecode" "camlzip_bzDecompress"
 external decompress_end: stream -> unit = "camlzip_bzDecompressEnd"
 
diff --git a/src/utils/cdk/bzlib.mli b/src/utils/cdk/bzlib.mli
index 74f6d09f..12945b71 100644
--- a/src/utils/cdk/bzlib.mli
+++ b/src/utils/cdk/bzlib.mli
@@ -10,7 +10,7 @@ type action = BZ_RUN | BZ_FLUSH | BZ_FINISH
 
 external compress_init: int -> int -> int -> stream = "camlzip_bzCompressInit"
 external compress:
-  stream -> string -> int -> int -> string -> int -> int -> action
+  stream -> string -> int -> int -> bytes -> int -> int -> action
          -> bool * int * int
   = "camlzip_bzCompress_bytecode" "camlzip_bzCompress"
 external compress_end: stream -> unit = "camlzip_bzCompressEnd"
@@ -18,7 +18,7 @@ external compress_end: stream -> unit = "camlzip_bzCompressEnd"
 
 external decompress_init: int -> bool -> stream = "camlzip_bzDecompressInit"
 external decompress:
-  stream -> string -> int -> int -> string -> int -> int -> bool * int * int
+  stream -> string -> int -> int -> bytes -> int -> int -> bool * int * int
   = "camlzip_bzDecompress_bytecode" "camlzip_bzDecompress"
 external decompress_end: stream -> unit = "camlzip_bzDecompressEnd"
 
diff --git a/src/utils/cdk/file.ml b/src/utils/cdk/file.ml
index c25e50d1..b63ce211 100644
--- a/src/utils/cdk/file.ml
+++ b/src/utils/cdk/file.ml
@@ -21,13 +21,13 @@
 let to_string name =
   Unix2.tryopen_read_bin name (fun chan ->
   let buf_size = 1024 in
-  let buf = String.create buf_size in
+  let buf = Bytes.create buf_size in
   let rec iter buf nb_read =
-    let buf_size = String.length buf in
+    let buf_size = Bytes.length buf in
     let to_read = min (buf_size - nb_read) 8192 in
     let tmp = input chan buf nb_read to_read in
     if tmp = 0 then
-      String.sub buf 0 nb_read
+      Bytes.sub_string buf 0 nb_read
     else
     let nb_read = nb_read + tmp in
     let buf =
diff --git a/src/utils/cdk/filename2.ml b/src/utils/cdk/filename2.ml
index 38032f2f..3b3e49f1 100644
--- a/src/utils/cdk/filename2.ml
+++ b/src/utils/cdk/filename2.ml
@@ -116,19 +116,19 @@ let to_string filename =
   List.fold_left (fun file f -> f file) filename !to_strings
 
 let path_of_filename filename =
-  let filename = String.copy filename in
   let len = String.length filename in
+  let filename = Bytes.of_string filename in
   for i = 0 to len - 1 do
-    if filename.[i] = '\\' then filename.[i] <- '/';
+    if Bytes.get filename i = '\\' then Bytes.set filename i '/';
   done;
   let filename = 
-    if len > 2 && filename.[1]  = ':' &&
-      match filename.[0] with 
+    if len > 2 && Bytes.get filename 1 = ':' &&
+      match Bytes.get filename 0 with 
         'a' .. 'z' | 'A' .. 'Z' -> true
       | _ -> false then
-      Printf.sprintf "%s/%s" (String.sub filename 0 2) 
-      (String.sub filename 2 (len-2))
-    else filename
+      Printf.sprintf "%s/%s" (Bytes.sub_string filename 0 2) 
+      (Bytes.sub_string filename 2 (len-2))
+    else Bytes.unsafe_to_string filename
   in
   split_simplify filename '/'
 
@@ -142,11 +142,12 @@ let filesystem_compliant name fstype namemax =
   (* replace all illegal characters with a valid one.
      assumes all filesystems accept '_'s in filenames *)
   let escape_chars p filename =
-    let s = String.copy filename in
+    let s = Bytes.of_string filename in
     for i = 0 to String.length filename - 1 do
-      if p s.[i] then s.[i] <- '_'
+      if p (Bytes.get s i) then Bytes.set s i '_'
     done;
-    s in
+    Bytes.unsafe_to_string s
+  in
 
   (* remove all illegal characters at the beginning of filename *)
   let trim_left p filename =
diff --git a/src/utils/cdk/genlex2.ml b/src/utils/cdk/genlex2.ml
index de90afe4..dc211dc0 100644
--- a/src/utils/cdk/genlex2.ml
+++ b/src/utils/cdk/genlex2.ml
@@ -28,7 +28,7 @@ type token =
 
 (* The string buffering machinery *)
 
-let initial_buffer = String.create 32
+let initial_buffer = Bytes.create 32
 
 let buffer = ref initial_buffer
 let bufpos = ref 0
@@ -36,16 +36,16 @@ let bufpos = ref 0
 let reset_buffer () = buffer := initial_buffer; bufpos := 0
 
 let store c =
-  if !bufpos >= String.length !buffer then
+  if !bufpos >= Bytes.length !buffer then
     begin
-      let newbuffer = String.create (2 * !bufpos) in
-      String.blit !buffer 0 newbuffer 0 !bufpos; buffer := newbuffer
+      let newbuffer = Bytes.create (2 * !bufpos) in
+      Bytes.blit !buffer 0 newbuffer 0 !bufpos; buffer := newbuffer
     end;
-  String.set !buffer !bufpos c;
+  Bytes.set !buffer !bufpos c;
   incr bufpos
 
 let get_string () =
-  let s = String.sub !buffer 0 !bufpos in buffer := initial_buffer; s
+  let s = Bytes.sub_string !buffer 0 !bufpos in buffer := initial_buffer; s
 
 (* The lexer *)
 
diff --git a/src/utils/cdk/gzip.ml b/src/utils/cdk/gzip.ml
index d170263b..239ece75 100644
--- a/src/utils/cdk/gzip.ml
+++ b/src/utils/cdk/gzip.ml
@@ -28,7 +28,7 @@ type in_channel =
     in_stream: Zlib.stream;
     mutable in_size: int32;
     mutable in_crc: int32;
-    char_buffer: string }
+    char_buffer: bytes }
 
 let open_in ic =
   (* Superficial parsing of header *)
@@ -73,7 +73,7 @@ let open_in ic =
     in_stream = Zlib.inflate_init false;
     in_size = Int32.zero;
     in_crc = Int32.zero;
-    char_buffer = String.create 1 }
+    char_buffer = Bytes.create 1 }
 
 let open_in_file filename =
   let ic = Pervasives.open_in_bin filename in
@@ -176,7 +176,7 @@ type 'a out_channel =
     out_stream: Zlib.stream;
     mutable out_size: int32;
     mutable out_crc: int32;
-    char_buffer: string }
+    char_buffer: bytes }
 
 let open_out ?(level = 6) oc =
   if level < 1 || level > 9 then invalid_arg "Gzip_stream.open_output: bad level";
@@ -195,7 +195,7 @@ let open_out ?(level = 6) oc =
     out_stream = Zlib.deflate_init level false;
     out_size = Int32.zero;
     out_crc = Int32.zero;
-    char_buffer = String.create 1 }
+    char_buffer = Bytes.create 1 }
 
 let open_out_file ?level filename =
   let oc = Pervasives.open_out_bin filename in
diff --git a/src/utils/cdk/string2.ml b/src/utils/cdk/string2.ml
index f51028a7..84f20643 100644
--- a/src/utils/cdk/string2.ml
+++ b/src/utils/cdk/string2.ml
@@ -192,12 +192,12 @@ let of_char c = String.make 1 c
   
   
 let resize s newlen =
-  let len = String.length s in
-  if len > newlen then String.sub s 0 newlen 
+  let len = Bytes.length s in
+  if len > newlen then Bytes.sub s 0 newlen 
   else
   let b = Bytes.create newlen in
-  String.blit s 0 b 0 len;
-  Bytes.unsafe_to_string b
+  Bytes.blit s 0 b 0 len;
+  b
   
 let is_space c = c = ' ' || c = '\n' || c = '\r' || c = '\t'
   
diff --git a/src/utils/cdk/string2.mli b/src/utils/cdk/string2.mli
index b363716c..854c5b1c 100644
--- a/src/utils/cdk/string2.mli
+++ b/src/utils/cdk/string2.mli
@@ -79,7 +79,7 @@ val subcontains : string -> string -> bool
 val of_char : char -> string
 (*d [of_char c] returns the string containing one [c]. *)
   
-val resize : string -> int ->  string
+val resize : bytes -> int ->  bytes
 (*d [resize s len] returns a string of length [len] starting with [s]. *)
   
 val init : int -> (int -> char) -> string
diff --git a/src/utils/cdk/tar.mlcpp b/src/utils/cdk/tar.mlcpp
index 2c2f2b6a..193a5486 100644
--- a/src/utils/cdk/tar.mlcpp
+++ b/src/utils/cdk/tar.mlcpp
@@ -205,9 +205,13 @@ let read_oldgnu_header header =
     t_realsize = extract_int32 header 483 12;
   }
 
+let really_input_string t size =
+  let buf = Bytes.create size in
+  t.chan#really_input buf 0 size;
+  Bytes.unsafe_to_string buf
+
 let read_gnu_header t =
-  let buf = String.create blocksize in
-    t.chan#really_input buf 0 blocksize;
+    let buf = really_input_string t blocksize in
     { t_atime = extract_int32 buf 0 12;
       t_ctime = extract_int32 buf 12 12;
       t_offset = extract_int32 buf 24 12;
@@ -215,9 +219,8 @@ let read_gnu_header t =
     }
 
 let read_header t =
-  align_at_header t;
-  let buf = String.create blocksize in
-    t.chan#really_input buf 0 blocksize;
+    align_at_header t;
+    let buf = really_input_string t blocksize in
     if buf = empty_block then raise End_of_file;
     let head1 = { t_name = c_string buf 0;
                  t_mode = extract_num buf 100 8;
@@ -264,14 +267,11 @@ let read_body t =
   let header = get_header t in
     t.last_header <- None;
     if header.t_size = 0 then "" 
-    else let buf = String.create header.t_size in
-      t.chan#really_input buf 0 header.t_size;
+    else
+      let buf = really_input_string t header.t_size in
       let align = blocksize - (header.t_size mod blocksize) in
-        if align <> blocksize then begin
-          let leftover = String.create blocksize in	    
-            t.chan#really_input leftover 0 align
-        end;
-        buf
+      if align <> blocksize then ignore (really_input_string t align : string);
+      buf
 
 let read_entry t =
   let head = read_header t in
@@ -390,7 +390,7 @@ let write_gnu_header t buf =
   
 let output t head body =
   let size = String.length body in
-  let buf = String.copy empty_block in
+  let buf = Bytes.of_string empty_block in
     write_str buf 0 100 head.t_name;
     write_num8 buf 100 head.t_mode;
     write_num8 buf 108 head.t_uid;
@@ -407,23 +407,23 @@ let output t head body =
     write_str buf 345 155 head.t_prefix;
     if head.t_format = OLDGNU_FORMAT then
       write_oldgnu_header head buf;
-    let chksum = compute_chksum buf in
+    let chksum = compute_chksum @@ Bytes.unsafe_to_string buf in
       write_padded_num buf 148 chksum;
       t.ochan#output buf 0 blocksize;
-      if head.t_format = GNU_FORMAT && isdigit buf.[156] then begin
-        let buf2 = String.copy empty_block in
+      if head.t_format = GNU_FORMAT && isdigit @@ Bytes.get buf 156 then begin
+        let buf2 = Bytes.of_string empty_block in
           write_gnu_header head buf2;
           t.ochan#output buf2 0 blocksize
       end;
       if size > 0 then begin
-    t.ochan#output body 0 size;
+          t.ochan#output (Bytes.unsafe_of_string body) 0 size;
           let align = blocksize - (size mod blocksize) in
             if align > 0 && align < blocksize then 
-              t.ochan#output empty_block 0 align
+              t.ochan#output (Bytes.unsafe_of_string empty_block) 0 align
       end
       
 let flush t =
-  t.ochan#output empty_block 0 blocksize;
+  t.ochan#output (Bytes.unsafe_of_string empty_block) 0 blocksize;
   t.ochan#flush ()
 
 let close_out t =
diff --git a/src/utils/cdk/zlib2.ml b/src/utils/cdk/zlib2.ml
index d2d25abf..5e7d1170 100644
--- a/src/utils/cdk/zlib2.ml
+++ b/src/utils/cdk/zlib2.ml
@@ -10,27 +10,27 @@ let zlib_version_num () =
   end
 
 let grow_buffer s =
-  let s' = String.create (2 * String.length s) in
-  String.blit s 0 s' 0 (String.length s);
+  let s' = Bytes.create (2 * Bytes.length s) in
+  Bytes.blit s 0 s' 0 (Bytes.length s);
   s'
 
 let compress_string ?(level = 6) inbuf =
   let zs = deflate_init level true in
   let rec compr inpos outbuf outpos =
     let inavail = String.length inbuf - inpos in
-    let outavail = String.length outbuf - outpos in
+    let outavail = Bytes.length outbuf - outpos in
     if outavail = 0
     then compr inpos (grow_buffer outbuf) outpos
     else begin
       let (finished, used_in, used_out) =
-        deflate zs inbuf inpos inavail outbuf outpos outavail
+        deflate_string zs inbuf inpos inavail outbuf outpos outavail
                    (if inavail = 0 then Z_FINISH else Z_NO_FLUSH) in
       if finished then 
-        String.sub outbuf 0 (outpos + used_out)
+        Bytes.sub_string outbuf 0 (outpos + used_out)
       else
         compr (inpos + used_in) outbuf (outpos + used_out)
     end in
-  let res = compr 0 (String.create (String.length inbuf)) 0 in
+  let res = compr 0 (Bytes.create (String.length inbuf)) 0 in
   deflate_end zs;
   res
 
@@ -42,20 +42,20 @@ let gzip_string ?(level = 6) inbuf =
   let out_crc = ref Int32.zero in
   let rec compr inpos outbuf outpos =
     let inavail = String.length inbuf - inpos in
-    let outavail = String.length outbuf - outpos in
+    let outavail = Bytes.length outbuf - outpos in
     if outavail = 0
     then compr inpos (grow_buffer outbuf) outpos
     else begin 
       let (finished, used_in, used_out) =
-        deflate zs inbuf inpos inavail outbuf outpos outavail
+        deflate_string zs inbuf inpos inavail outbuf outpos outavail
                    (if inavail = 0 then Z_FINISH else Z_NO_FLUSH) in
-         out_crc := update_crc !out_crc inbuf inpos used_in;
+         out_crc := update_crc_string !out_crc inbuf inpos used_in;
       if finished then 
-        String.sub outbuf 0 (outpos + used_out)
+        Bytes.sub_string outbuf 0 (outpos + used_out)
       else
         compr (inpos + used_in) outbuf (outpos + used_out)
     end in
-  let res = compr 0 (String.create (String.length inbuf)) 0 in
+  let res = compr 0 (Bytes.create (String.length inbuf)) 0 in
   deflate_end zs; 
   let buf = Buffer.create (18 + String.length res) in
   let write_int wbuf n =
@@ -85,18 +85,18 @@ let uncompress_string2 inbuf =
   let zs = inflate_init true in
   let rec uncompr inpos outbuf outpos =
     let inavail = String.length inbuf - inpos in
-    let outavail = String.length outbuf - outpos in
+    let outavail = Bytes.length outbuf - outpos in
     if outavail = 0
     then uncompr inpos (grow_buffer outbuf) outpos
     else begin
       let (finished, used_in, used_out) =
-        inflate zs inbuf inpos inavail outbuf outpos outavail Z_SYNC_FLUSH in
+        inflate_string zs inbuf inpos inavail outbuf outpos outavail Z_SYNC_FLUSH in
       if finished then 
-        String.sub outbuf 0 (outpos + used_out)
+        Bytes.sub_string outbuf 0 (outpos + used_out)
       else
         uncompr (inpos + used_in) outbuf (outpos + used_out)
     end in
-  let res = uncompr 0 (String.create (2 * String.length inbuf)) 0 in
+  let res = uncompr 0 (Bytes.create (2 * String.length inbuf)) 0 in
   inflate_end zs;
   res
 
@@ -105,12 +105,12 @@ let uncompress_string s =
   let pos = ref 0 in
   let len = String.length s in
   uncompress ~header: true (fun b ->
-      let n = min (String.length b) (len - !pos) in
+      let n = min (Bytes.length b) (len - !pos) in
       if n < 1 then 0 else begin
       String.blit s !pos b 0 n;
       pos := !pos + n;
       n end
-  ) (fun s len -> Buffer.add_string buf (String.sub s 0 len));
+  ) (fun s len -> Buffer.add_string buf (Bytes.sub_string s 0 len));
   Buffer.contents buf
   
 
diff --git a/src/utils/lib/bitv.ml b/src/utils/lib/bitv.ml
index 707a06d3..2f82fda4 100644
--- a/src/utils/lib/bitv.ml
+++ b/src/utils/lib/bitv.ml
@@ -455,11 +455,11 @@ let all_ones v =
 
 let to_string v = 
   let n = v.length in
-  let s = String.make n '0' in
+  let s = Bytes.make n '0' in
   for i = 0 to n - 1 do
     if unsafe_get v i then s.[i] <- '1'
   done;
-  s
+  Bytes.unsafe_to_string s
 
 let print fmt v = Format.pp_print_string fmt (to_string v)
 
diff --git a/src/utils/lib/magiclib.ml b/src/utils/lib/magiclib.ml
index b8e19b2b..392b712f 100644
--- a/src/utils/lib/magiclib.ml
+++ b/src/utils/lib/magiclib.ml
@@ -69,13 +69,13 @@ let escape_colon s =
   let n = ref 0 in
   for i = 0 to len - 1 do if s.[i] = ':' then incr n done;
   if !n = 0 then s else
-    let s' = String.create (len + !n) in
+    let s' = Bytes.create (len + !n) in
     let j = ref 0 in
     for i = 0 to len - 1 do
       if s.[i] = ':' then (s'.[!j] <- '\\'; incr j);
       s'.[!j] <- s.[i]; incr j
     done;
-    s'
+    Bytes.unsafe_to_string s'
 
 (* Concatenate the filenames with ":".  If ":" is present in a
    filename, escape it. *)
diff --git a/src/utils/lib/md4.ml b/src/utils/lib/md4.ml
index 40bb0282..146af068 100644
--- a/src/utils/lib/md4.ml
+++ b/src/utils/lib/md4.ml
@@ -45,7 +45,7 @@ module Base16 = struct
       else Char.chr (Char.code '0' + x)
         
     let to_string hash_length s =
-      let p = String.create (hash_length * 2) in
+      let p = Bytes.create (hash_length * 2) in
       for i = 0 to hash_length - 1 do
         let c = s.[i] in
         let n = int_of_char c in
@@ -54,7 +54,7 @@ module Base16 = struct
         p.[2 * i] <- hexa_digit i0;
         p.[2 * i+1] <- hexa_digit i1;
       done;
-      p
+      Bytes.unsafe_to_string p
     
     let hexa_digit_case upper x =
       if x >= 10 then Char.chr (Char.code (
@@ -62,7 +62,7 @@ module Base16 = struct
       else Char.chr (Char.code '0' + x)
 
     let to_string_case upper hash_length s =
-      let p = String.create (hash_length * 2) in
+      let p = Bytes.create (hash_length * 2) in
       for i = 0 to hash_length - 1 do
         let c = s.[i] in
         let n = int_of_char c in
@@ -71,7 +71,7 @@ module Base16 = struct
         p.[2 * i] <- hexa_digit_case upper i0;
         p.[2 * i+1] <- hexa_digit_case upper i1;
       done;
-      p
+      Bytes.unsafe_to_string p
     
     let digit_hexa c =
       let i = int_of_char c in
@@ -82,13 +82,13 @@ module Base16 = struct
     
     let of_string hash_length s =
       assert (String.length s = hash_length*2);
-      let p = String.create hash_length in
+      let p = Bytes.create hash_length in
       for i = 0 to hash_length - 1 do
         let c0 = s.[2*i] in
         let c1 = s.[2*i+1] in
         p.[i] <- char_of_int ((16 * digit_hexa c0) + digit_hexa c1);
       done;
-      p
+      Bytes.unsafe_to_string p
     
   end
 
@@ -103,32 +103,32 @@ module Base32 = struct
         'A' .. 'Z' -> int_of_char n - 65
       | 'a' .. 'z' -> int_of_char n - 97
       | _ -> (int_of_char n+26)-50
-    
+
     let of_string hash_length r =
       let len = String.length r in
       assert (len =  (hash_length * 8 + 4)/5);
-      let s = String.make hash_length '\000' in
+      let s = Bytes.make hash_length '\000' in
       for i = 0 to len - 1 do
         let pos = i * 5 in
         let byte = pos / 8 in
         let bit = pos mod 8 in
         let c = int5_of_char r.[i] in
-        if bit < 3 then 
+        if bit < 3 then
           let x = c lsl (3-bit) in
-          s.[byte] <- char_of_int (int_of_char s.[byte] lor x);
+          s.[byte] <- char_of_int (int_of_char (Bytes.get s byte) lor x);
         else
         let x = (c lsr (bit - 3)) land 0xff in
-        s.[byte] <- char_of_int (int_of_char s.[byte] lor x);
+        s.[byte] <- char_of_int (int_of_char (Bytes.get s byte) lor x);
         if byte+1 < hash_length then
           let y = (c lsl (11 - bit)) land 0xff in
-          s.[byte+1] <- char_of_int (int_of_char s.[byte+1] lor y);
+          s.[byte+1] <- char_of_int (int_of_char (Bytes.get s (byte+1)) lor y);
       done;
-      s    
-    
+      Bytes.unsafe_to_string s
+
     let to_string hash_length s =
       assert (String.length s = hash_length);
       let len = (hash_length * 8 + 4)/5 in
-      let r = String.create len in
+      let r = Bytes.create len in
       for i = 0 to len - 1 do
         let pos = i * 5 in
         let byte = pos / 8 in
@@ -145,16 +145,16 @@ module Base32 = struct
         let c = (x lsr (11 - bit)) land 0x1f in
         r.[i] <- char_of_int5 c
       done;
-      r
+      Bytes.unsafe_to_string r
 
     let char_of_int5 upper n =
       char_of_int (if n < 26 then (if upper then 65 else 97)+n else
           50+(n-26))
-    
+
     let to_string_case upper hash_length s =
       assert (String.length s = hash_length);
       let len = (hash_length * 8 + 4)/5 in
-      let r = String.create len in
+      let r = Bytes.create len in
       for i = 0 to len - 1 do
         let pos = i * 5 in
         let byte = pos / 8 in
@@ -171,7 +171,7 @@ module Base32 = struct
         let c = (x lsr (11 - bit)) land 0x1f in
         r.[i] <- char_of_int5 upper c
       done;
-      r
+      Bytes.unsafe_to_string r
       
   end
 
@@ -181,7 +181,7 @@ module Base6427 = struct
     let _ = assert (String.length base64tbl = 64)
     
     let to_string _ hashbin =
-      let hash64 = String.create 30 in
+      let hash64 = Bytes.create 30 in
       let hashbin n = int_of_char hashbin.[n] in
       hash64.[0] <- '=';
       let j = ref 1 in
@@ -198,16 +198,17 @@ module Base6427 = struct
         done
       done;
       hash64.[!j-1] <- '=';
-      String.sub hash64 0 !j
+      Bytes.sub_string hash64 0 !j
     
-    let base64tbl_inv = String.create 126
-    let _ = 
+    let base64tbl_inv =
+      let table = Bytes.create 126 in
       for i = 0 to 63 do
-        base64tbl_inv.[int_of_char base64tbl.[i]] <- char_of_int i
-      done
+        table.[int_of_char base64tbl.[i]] <- char_of_int i
+      done;
+      Bytes.unsafe_to_string table
     
     let of_string _ hash64 =
-      let hashbin = String.make 20 '\000' in
+      let hashbin = Bytes.make 20 '\000' in
       let hash64 n = 
         let c = hash64.[n] in
         int_of_char base64tbl_inv.[int_of_char c]
@@ -233,7 +234,7 @@ module Base6427 = struct
         hashbin.[!j+1] <- char_of_int ((!tmp lsr  8) land 0xff);
         j := !j + 2;
       done;
-      hashbin
+      Bytes.unsafe_to_string hashbin
       
     let to_string_case _ = to_string
   end
@@ -291,12 +292,12 @@ module Make(M: sig
       val hash_name : string
       
 (* [unsafe_string digest string string_len] *)
-      val unsafe_string : string -> string -> int -> unit
+      val unsafe_string : bytes -> string -> int -> unit
           
 (* [unsafe_file digest filename filesize] *)
-        val unsafe_file : string -> string -> int64 -> unit
+      val unsafe_file : bytes -> string -> int64 -> unit
 (* [unsafe_string digest file_fd offset len] *)
-      val digest_subfile : string -> Unix.file_descr -> int64 -> int64 -> unit 
+      val digest_subfile : bytes -> Unix.file_descr -> int64 -> int64 -> unit 
     
       module Base : Base
     end) = struct
@@ -314,13 +315,13 @@ module Make(M: sig
 
     let string s =
       let len = String.length s in
-      let digest = String.create hash_length in
+      let digest = Bytes.create hash_length in
       unsafe_string digest s len;
-      digest
+      Bytes.unsafe_to_string digest
 
     let to_bits s =
       let len = String.length s in
-      let digest = String.create (8*len) in
+      let digest = Bytes.create (8*len) in
       for i = 0 to len-1 do
         let c = int_of_char s.[i] in
         for j = 7 downto 0 do
@@ -329,39 +330,39 @@ module Make(M: sig
             
         done
       done;
-      digest
+      Bytes.unsafe_to_string digest
       
-    external xor_c : t -> t -> t -> unit = "md4_xor" "noalloc"
+    external xor_c : t -> t -> bytes -> unit = "md4_xor" "noalloc"
     
     let xor m1 m2 =
-      let m3 = String.create hash_length in
+      let m3 = Bytes.create hash_length in
       xor_c m1 m2 m3;
-      m3
+      Bytes.unsafe_to_string m3
     
     let file s =
-      let digest = String.create hash_length in
+      let digest = Bytes.create hash_length in
       let file_size = Unix32.getsize s in
       unsafe_file digest s file_size;
-      digest
+      Bytes.unsafe_to_string digest
     
     let digest_subfile fd pos len =
-      let digest = String.create hash_length in
+      let digest = Bytes.create hash_length in
       Unix32.apply_on_chunk fd pos len 
         (fun fd pos ->
           digest_subfile digest fd pos len);
-      digest
+      Bytes.unsafe_to_string digest
     
-    let create () =  String.create hash_length
+    let create () = String.make hash_length '\x00'
     
     let direct_to_string s = s
     let direct_of_string s = s
     
     let random () =
-      let s = create () in
+      let s = Bytes.create hash_length in
       for i = 0 to hash_length - 1 do
         s.[i] <- char_of_int (Random.int 256)
       done;
-      s
+      Bytes.unsafe_to_string s
     
     let of_string = Base.of_string hash_length
     let to_string = Base.to_string hash_length
@@ -397,9 +398,9 @@ module Md4 = Make(struct
       let hash_length = 16
       let hash_name = "Md4"        
       
-      external unsafe_string : string -> string -> int -> unit = "md4_unsafe_string"
-      external unsafe_file : string -> string -> int64 -> unit = "md4_unsafe_file"
-      external digest_subfile : string -> Unix.file_descr -> int64 -> int64 -> unit =
+      external unsafe_string : bytes -> string -> int -> unit = "md4_unsafe_string"
+      external unsafe_file : bytes -> string -> int64 -> unit = "md4_unsafe_file"
+      external digest_subfile : bytes -> Unix.file_descr -> int64 -> int64 -> unit =
         "md4_unsafe64_fd"
   
       module Base = Base16
@@ -409,9 +410,9 @@ module Md5 = Make(struct
       let hash_length = 16
       let hash_name = "Md5"        
       
-      external unsafe_string : string -> string -> int -> unit = "md5_unsafe_string"
-      external unsafe_file : string -> string -> int64 -> unit = "md5_unsafe_file"
-      external digest_subfile : string -> Unix.file_descr -> int64 -> int64 -> unit =
+      external unsafe_string : bytes -> string -> int -> unit = "md5_unsafe_string"
+      external unsafe_file : bytes -> string -> int64 -> unit = "md5_unsafe_file"
+      external digest_subfile : bytes -> Unix.file_descr -> int64 -> int64 -> unit =
         "md5_unsafe64_fd"
     
       module Base = Base16
@@ -421,9 +422,9 @@ module PreSha1 = Make(struct
       let hash_length = 20
       let hash_name = "Sha1"        
       
-      external unsafe_string : string -> string -> int -> unit = "sha1_unsafe_string"
-      external unsafe_file : string -> string -> int64 -> unit = "sha1_unsafe_file"
-      external digest_subfile : string -> Unix.file_descr -> int64 -> int64 -> unit =
+      external unsafe_string : bytes -> string -> int -> unit = "sha1_unsafe_string"
+      external unsafe_file : bytes -> string -> int64 -> unit = "sha1_unsafe_file"
+      external digest_subfile : bytes -> Unix.file_descr -> int64 -> int64 -> unit =
         "sha1_unsafe64_fd"
       
       module Base = Base32
@@ -464,7 +465,7 @@ module Tiger = Make(struct
       let hash_length = 24
       let hash_name = "Tiger"        
       
-      external unsafe_string : string -> string -> int -> unit = 
+      external unsafe_string : bytes -> string -> int -> unit = 
         "tiger_unsafe_string"
         
       let unsafe_file digest filename = 
@@ -481,8 +482,8 @@ module PreTigerTree = Make(struct
       let hash_length = 24
       let hash_name = "TigerTree"        
       
-      external unsafe_string : string -> string -> int -> unit = "tigertree_unsafe_string"
-      external digest_subfile : string -> Unix.file_descr -> int64 -> int64 -> unit =
+      external unsafe_string : bytes -> string -> int -> unit = "tigertree_unsafe_string"
+      external digest_subfile : bytes -> Unix.file_descr -> int64 -> int64 -> unit =
         "tigertree_unsafe64_fd"
       
       let unsafe_file digest filename file_size = 
@@ -530,10 +531,10 @@ module PreMd5Ext = Make(struct
       let hash_length = 20
       let hash_name = "Md5Ext"        
 
-      external unsafe_string : string -> string -> int -> unit =
+      external unsafe_string : bytes -> string -> int -> unit =
         "fst_hash_string_ml"
         
-      external unsafe_file : string -> string -> int64 -> unit = "fst_hash_file_ml"
+      external unsafe_file : bytes -> string -> int64 -> unit = "fst_hash_file_ml"
       let digest_subfile _ _ _ _ = 
         failwith "Md5Ext.digest_subfile not implemented"
     
diff --git a/src/utils/lib/md4.mli b/src/utils/lib/md4.mli
index 64cee459..b15de4d9 100644
--- a/src/utils/lib/md4.mli
+++ b/src/utils/lib/md4.mli
@@ -80,4 +80,4 @@ module Md5Ext : Digest
 module Base16 : Base
 module Base32 : Base
 module Base6427 : Base
-  
\ No newline at end of file
+  
diff --git a/src/utils/lib/misc.ml b/src/utils/lib/misc.ml
index b670e1e4..bfa7293b 100644
--- a/src/utils/lib/misc.ml
+++ b/src/utils/lib/misc.ml
@@ -92,13 +92,13 @@ let zip_create zipfile files =
 let gz_extract filename =
   let file = ref "" in
   try
-    let buffer = String.create 4096 in
+    let buffer = Bytes.create 4096 in
     let file_out = Filename2.temp_file "arch_" ".tmp" in
     file := file_out;
     Unix2.tryopen_read_gzip filename (fun ic ->
       Unix2.tryopen_write_bin file_out (fun oc ->
         let rec decompress () =
-          let n = Gzip.input ic buffer 0 (String.length buffer) in
+          let n = Gzip.input ic buffer 0 (Bytes.length buffer) in
           if n = 0 then ()
           else
             begin
diff --git a/src/utils/lib/misc2.mlcpp b/src/utils/lib/misc2.mlcpp
index 9ba2f447..6f60f623 100644
--- a/src/utils/lib/misc2.mlcpp
+++ b/src/utils/lib/misc2.mlcpp
@@ -26,14 +26,14 @@ let bz2_extract filename =
   else begin
     let file = ref "" in
     try
-      let buffer = String.create 4096 in
+      let buffer = Bytes.create 4096 in
       let file_out = Filename2.temp_file "arch_" ".tmp" in
       file := file_out;
       let ic = Bzip2.open_in filename
       in
       let oc = open_out_bin file_out in
         let rec decompress () =
-          let n = Bzip2.input ic buffer 0 (String.length buffer) in
+          let n = Bzip2.input ic buffer 0 (Bytes.length buffer) in
             if n = 0 then ()
             else
               begin
diff --git a/src/utils/lib/options.ml4 b/src/utils/lib/options.ml4
index 823372bf..c18137ff 100644
--- a/src/utils/lib/options.ml4
+++ b/src/utils/lib/options.ml4
@@ -208,7 +208,7 @@ and parse_option = parser
 | [< 'Int i >] -> IntValue i
 | [< 'Float f >] -> FloatValue  f
 | [< 'Kwd "@"; 'Int i; v = parse_once_value i >] -> OnceValue v
-| [< 'Char c >] -> StringValue (let s = String.create 1 in s.[0] <- c; s)    
+| [< 'Char c >] -> StringValue (String.make 1 c)
 | [< 'Kwd "["; v = parse_list [] >] -> List v
 | [< 'Kwd "("; v = parse_list [] >] -> List v
 
diff --git a/src/utils/lib/unix32.ml b/src/utils/lib/unix32.ml
index 044ba7c9..6ba7e089 100644
--- a/src/utils/lib/unix32.ml
+++ b/src/utils/lib/unix32.ml
@@ -327,13 +327,13 @@ module FDCache = struct
       check_destroyed t2;
       let buffer_len = 128 * 1024 in
       let buffer_len64 = Int64.of_int buffer_len in
-      let buffer = String.make buffer_len '\001' in
+      let buffer = Bytes.make buffer_len '\001' in
       let rec iter remaining pos1 pos2 =
         let len64 = min remaining buffer_len64 in
         let len = Int64.to_int len64 in
         if len > 0 then begin
             read t1 pos1 buffer 0 len;
-            write t2 pos2 buffer 0 len;
+            write t2 pos2 (Bytes.unsafe_to_string buffer) 0 len;
             iter (remaining -- len64) (pos1 ++ len64) (pos2 ++ len64)
           end
       in
@@ -363,7 +363,7 @@ module type File =   sig
     val mtime64 : t -> float
     val exists : t -> bool
     val remove : t -> unit
-    val read : t -> int64 -> string -> int -> int -> unit
+    val read : t -> int64 -> bytes -> int -> int -> unit
     val write : t -> int64 -> string -> int -> int -> unit
     val destroy : t -> unit
     val is_closed : t -> bool
@@ -1216,6 +1216,8 @@ let write file file_pos string string_pos len =
       | Destroyed -> failwith "Unix32.write on destroyed FD"
   else
     lprintf_nl "Unix32.write: error, invalid argument len = 0"
+
+let write_bytes f fpos b bpos len = write f fpos (Bytes.unsafe_to_string b) bpos len
         
 let buffer = Buffer.create 65000
 
@@ -1349,12 +1351,12 @@ let copy_chunk t1 t2 pos1 pos2 len =
   flush_fd t1;
   flush_fd t2;
   let buffer_size = 128 * 1024 in
-  let buffer = String.make buffer_size '\001' in
+  let buffer = Bytes.make buffer_size '\001' in
   let rec iter remaining pos1 pos2 =
     let len = mini remaining buffer_size in
     if len > 0 then begin
       read t1 pos1 buffer 0 len;
-      write t2 pos2 buffer 0 len;
+      write t2 pos2 (Bytes.unsafe_to_string buffer) 0 len;
       let len64 = Int64.of_int len in
       iter (remaining - len) (pos1 ++ len64) (pos2 ++ len64)
     end
diff --git a/src/utils/lib/unix32.mli b/src/utils/lib/unix32.mli
index 026362db..2145f18b 100644
--- a/src/utils/lib/unix32.mli
+++ b/src/utils/lib/unix32.mli
@@ -55,10 +55,11 @@ val flush_fd : t -> unit
 val buffered_write : t -> int64 -> string -> int -> int -> unit
 val buffered_write_copy : t -> int64 -> string -> int -> int -> unit
 val write : t -> int64 -> string -> int -> int -> unit
+val write_bytes : t -> int64 -> bytes -> int -> int -> unit
 val max_buffered : int64 ref
 val remove : t -> unit
   
-val read : t -> int64 -> string -> int -> int -> unit
+val read : t -> int64 -> bytes -> int -> int -> unit
 (*val allocate_chunk :  t -> int64 -> int -> unit*)
   
 val copy_chunk : t -> t -> int64 -> int64 -> int -> unit
diff --git a/src/utils/lib/url.ml b/src/utils/lib/url.ml
index 03ff7e6c..b7bdb6da 100644
--- a/src/utils/lib/url.ml
+++ b/src/utils/lib/url.ml
@@ -36,7 +36,7 @@ type url = {
 let encode s =
   let pos = ref 0 in
   let len = String.length s in
-  let res = String.create (3*len) in
+  let res = Bytes.create (3*len) in
   let hexa_digit x =
     if x >= 10 then Char.chr (Char.code 'A' + x - 10)
     else Char.chr (Char.code '0' + x) in
@@ -50,7 +50,7 @@ let encode s =
         res.[!pos+2] <- hexa_digit (Char.code c mod 16);
         pos := !pos + 3
   done;
-  String.sub res 0 !pos
+  Bytes.sub_string res 0 !pos
 
 (** decodes a sting according RFC 1738
 or x-www-form-urlencoded ('+' with ' ')
@@ -304,4 +304,4 @@ open Options
 let option = 
   define_option_class "URL"
     (fun v -> of_string (value_to_string v))
-  (fun url -> string_to_value (to_string url))
\ No newline at end of file
+  (fun url -> string_to_value (to_string url))
diff --git a/src/utils/mp3tagui/mp3_info.ml b/src/utils/mp3tagui/mp3_info.ml
index 6d64df9a..0cc2ad99 100644
--- a/src/utils/mp3tagui/mp3_info.ml
+++ b/src/utils/mp3tagui/mp3_info.ml
@@ -78,8 +78,7 @@ let get_xing_header ic header =
     then if mode <> 3 then 32 else 17
     else if mode <> 3 then 17 else 9 in
   seek_in ic (pos_in ic + offset);
-  let buf = String.create 4 in
-  really_input ic buf 0 4;
+  let buf = really_input_string ic 4 in
   if buf <> "Xing" then raise Not_found;
   let flags = read_i4 ic in
   (* 3 = FRAMES_FLAG | BYTES_FLAG *)
@@ -90,12 +89,13 @@ let get_xing_header ic header =
 
 let for_channel ic =
   seek_in ic 0;
-  let buf = String.create 4 in
+  let buf = Bytes.create 4 in
   really_input ic buf 0 4;
-  while not (check_head buf) do
-    String.blit buf 1 buf 0 3;
+  while not (check_head @@ Bytes.unsafe_to_string buf) do
+    Bytes.blit buf 1 buf 0 3;
     buf.[3] <- input_char ic
   done;
+  let buf = Bytes.unsafe_to_string buf in
   let header = 
     (Char.code buf.[1] lsl 16) lor
     (Char.code buf.[2] lsl 8) lor
diff --git a/src/utils/mp3tagui/mp3_tag.ml b/src/utils/mp3tagui/mp3_tag.ml
index f5dd6bbc..c19abecd 100644
--- a/src/utils/mp3tagui/mp3_tag.ml
+++ b/src/utils/mp3tagui/mp3_tag.ml
@@ -46,9 +46,7 @@ module Id3v1 =
       let res =
         if len < 128 then false else begin
           seek_in ic (len - 128);
-          let buffer = String.create 3 in
-          really_input ic buffer 0 3;
-          buffer = "TAG"
+          really_input_string ic 3 = "TAG"
         end in
       close_in ic;
       res
@@ -57,10 +55,7 @@ let read_channel ic =
     let len = in_channel_length ic in
     if len < 128 then raise Not_found;
     seek_in ic (len - 128);
-    let readstring len =
-      let buf = String.create len in
-      really_input ic buf 0 len;
-      Mp3_misc.chop_whitespace buf 0 in
+    let readstring len = Mp3_misc.chop_whitespace (really_input_string ic len) 0 in
     if readstring 3 <> "TAG" then raise Not_found;
     let title = readstring 30 in
     let artist = readstring 30 in
@@ -150,7 +145,7 @@ module Id3v2 = struct
     for i = 0 to len - 1 do
       buff.[i] <- Char.chr (input_byte ic)
     done;
-    buff
+    Bytes.unsafe_to_string buff
 
   let input_int4 ic =
     let b4 = input_byte ic in let b3 = input_byte ic in
@@ -188,8 +183,7 @@ module Id3v2 = struct
 
   let read_channel ic =
     try
-      let header = String.create 10 in
-      really_input ic header 0 10;
+      let header = really_input_string ic 10 in
       if not (valid_header header) then raise Not_found;
       let len = length_header header in
       let startpos = pos_in ic in
@@ -280,16 +274,15 @@ module Id3v2 = struct
     let ic = open_in_bin filename in
     try
       begin try
-        let header = String.create 10 in
-        really_input ic header 0 10;
+        let header = really_input_string ic 10 in
         if not (valid_header header) then raise Not_found;
         seek_in ic (pos_in ic + length_header header)
       with Not_found | End_of_file ->
         seek_in ic 0
       end;
-      let buffer = String.create 4096 in
+      let buffer = Bytes.create 4096 in
       let rec copy_file () =
-        let n = input ic buffer 0 (String.length buffer) in
+        let n = input ic buffer 0 (Bytes.length buffer) in
         if n = 0 then () else begin output oc buffer 0 n; copy_file () end in
       copy_file ();
       close_in ic
diff --git a/src/utils/net/base64.ml b/src/utils/net/base64.ml
index d3d30420..6f788345 100644
--- a/src/utils/net/base64.ml
+++ b/src/utils/net/base64.ml
@@ -57,7 +57,7 @@ let encode_with_options b64 equal s pos len linelen crlf =
   in
 (* l_t': length of the result with CRLF or LF characters *)
   
-  let t = String.make l_t' equal in
+  let t = Bytes.make l_t' equal in
   let j = ref 0 in
   let q = ref 0 in
   for k = 0 to len / 3 - 1 do
@@ -75,10 +75,10 @@ let encode_with_options b64 equal s pos len linelen crlf =
         (Char.code (String.unsafe_get s (p+2))) in
 (* Obviously, 'bits' is a 24 bit entity (i.e. bits < 2**24) *)
     assert(!j + 3 < l_t');
-    String.unsafe_set t !j     (Array.unsafe_get b64 ( bits lsr 18));
-    String.unsafe_set t (!j+1) (Array.unsafe_get b64 ((bits lsr 12) land 63));
-    String.unsafe_set t (!j+2) (Array.unsafe_get b64 ((bits lsr  6) land 63));
-    String.unsafe_set t (!j+3) (Array.unsafe_get b64 ( bits         land 63));
+    Bytes.unsafe_set t !j     (Array.unsafe_get b64 ( bits lsr 18));
+    Bytes.unsafe_set t (!j+1) (Array.unsafe_get b64 ((bits lsr 12) land 63));
+    Bytes.unsafe_set t (!j+2) (Array.unsafe_get b64 ((bits lsr  6) land 63));
+    Bytes.unsafe_set t (!j+3) (Array.unsafe_get b64 ( bits         land 63));
     j := !j + 4;
     if linelen > 3 then begin
         q := !q + 4;
@@ -135,7 +135,7 @@ let encode_with_options b64 equal s pos len linelen crlf =
         end;	
     end;
   
-  t ;;
+  Bytes.unsafe_to_string t
 
 
 
@@ -207,7 +207,7 @@ let decode_substring t ~pos ~len ~url_variant:p_url ~accept_spaces:p_spaces =
   in
   
   let l_s = (l_t / 4) * 3 - pad_chars in       (* sic! *)
-  let s = String.create l_s in
+  let s = Bytes.create l_s in
   
   let decode_char c =
     match c with
@@ -248,9 +248,9 @@ let decode_substring t ~pos ~len ~url_variant:p_url ~accept_spaces:p_spaces =
         let x0 = (n0 lsl 2) lor (n1 lsr 4) in
         let x1 = ((n1 lsl 4) land 0xf0) lor (n2 lsr 2) in
         let x2 = ((n2 lsl 6) land 0xc0) lor n3 in
-        String.unsafe_set s q     (Char.chr x0);
-        String.unsafe_set s (q+1) (Char.chr x1);
-        String.unsafe_set s (q+2) (Char.chr x2);
+        Bytes.unsafe_set s q     (Char.chr x0);
+        Bytes.unsafe_set s (q+1) (Char.chr x1);
+        Bytes.unsafe_set s (q+2) (Char.chr x2);
       done;
     end
   else begin
@@ -269,9 +269,9 @@ let decode_substring t ~pos ~len ~url_variant:p_url ~accept_spaces:p_spaces =
         let x0 = (n0 lsl 2) lor (n1 lsr 4) in
         let x1 = ((n1 lsl 4) land 0xf0) lor (n2 lsr 2) in
         let x2 = ((n2 lsl 6) land 0xc0) lor n3 in
-        String.unsafe_set s q     (Char.chr x0);
-        String.unsafe_set s (q+1) (Char.chr x1);
-        String.unsafe_set s (q+2) (Char.chr x2);
+        Bytes.unsafe_set s q     (Char.chr x0);
+        Bytes.unsafe_set s (q+1) (Char.chr x1);
+        Bytes.unsafe_set s (q+2) (Char.chr x2);
       done;
       cursor := pos + l_t - 4;
     end;
@@ -316,7 +316,7 @@ let decode_substring t ~pos ~len ~url_variant:p_url ~accept_spaces:p_spaces =
     
     end;
   
-  s ;;
+  Bytes.unsafe_to_string s ;;
 
 
 
diff --git a/src/utils/net/cobs.ml b/src/utils/net/cobs.ml
index 6f6fd619..6441343c 100644
--- a/src/utils/net/cobs.ml
+++ b/src/utils/net/cobs.ml
@@ -98,16 +98,16 @@ let encodeData pdest psrc srclen =
 let decode psrc =
   let srclen = String.length psrc in 
   let dstlen = calcDecodedLength psrc srclen in
-  let pdest = String.create dstlen in
+  let pdest = Bytes.create dstlen in
   decodeData pdest psrc srclen;
-  pdest
+  Bytes.unsafe_to_string pdest
   
 let encode psrc =
   let srclen = String.length psrc in 
   let dstlen = calcEncodedLength psrc srclen in
-  let pdest = String.create dstlen in
+  let pdest = Bytes.create dstlen in
   encodeData pdest psrc srclen;
-  pdest
+  Bytes.unsafe_to_string pdest
 
 (*
 ggep:
@@ -352,17 +352,17 @@ let write buf list =
                 String.make 1 (char_of_int up)
               else
               if up land 0xffff = up then
-                let s = String.create 2 in
+                let s = Bytes.create 2 in
                 LittleEndian.str_int16 s 0 up;
-                s
+                Bytes.unsafe_to_string s
               else if up land 0xffffff = up then
-                let s = String.create 3 in
+                let s = Bytes.create 3 in
                 LittleEndian.str_int24 s 0 up;
-                s
+                Bytes.unsafe_to_string s
               else
-              let s = String.create 4 in
+              let s = Bytes.create 4 in
               LittleEndian.str_int s 0 up;
-              s
+              Bytes.unsafe_to_string s
             in
             GGEP.GGEP ("DU", s)
     ) list
@@ -384,4 +384,4 @@ let print list =
       | GGEP_DU_uptime up ->
           lprintf "   Uptime %d seconds\n" up
   ) list
-  
\ No newline at end of file
+  
diff --git a/src/utils/net/http_server.ml b/src/utils/net/http_server.ml
index 4cd2c5b6..e8adad69 100644
--- a/src/utils/net/http_server.ml
+++ b/src/utils/net/http_server.ml
@@ -64,7 +64,7 @@ let decode64 s =
     | _ -> failwith "not a base64 string" in
   let len = String.length s in
   let len_res = len * 3 / 4 in
-  let res = String.create len_res in
+  let res = Bytes.create len_res in
   for i=0 to len/4 - 1 do
     let i1 = 4*i and i2 = 3*i in
     let v1 = (val64 s.[i1]) lsl 18 in
@@ -80,7 +80,7 @@ let decode64 s =
     if s.[len-1] = '=' then
       if s.[len-2] = '=' then 2 else 1
     else 0 in
-  String.sub res 0 (len_res - nb_cut)
+  Bytes.sub_string res 0 (len_res - nb_cut)
 
 
 let debug = ref false
@@ -793,17 +793,17 @@ let request_handler config sock nread =
   let rec iter i =
     let end_pos = b.pos + b.len in
     if i < end_pos then
-      if b.buf.[i] = '\n' && i <= end_pos - 2 then
-        let c = b.buf.[i+1] in
+      if Bytes.get b.buf i = '\n' && i <= end_pos - 2 then
+        let c = Bytes.get b.buf (i+1) in
         if c = '\n' then
           let len = i + 2 - b.pos in
-          let header = String.sub b.buf b.pos len in
+          let header = Bytes.sub_string b.buf b.pos len in
           buf_used b len;
           manage config sock header
         else
-        if c = '\r' && i <= end_pos - 3 && b.buf.[i+2] = '\n' then
+        if c = '\r' && i <= end_pos - 3 && Bytes.get b.buf (i+2) = '\n' then
           let len = i + 3 - b.pos in
-          let header = String.sub b.buf b.pos len in
+          let header = Bytes.sub_string b.buf b.pos len in
           buf_used b len;
           manage config sock header
         else
diff --git a/src/utils/net/mailer.ml b/src/utils/net/mailer.ml
index f22bf0c2..7de49172 100644
--- a/src/utils/net/mailer.ml
+++ b/src/utils/net/mailer.ml
@@ -144,11 +144,11 @@ let canon_addr s =
 
 let string_xor s1 s2 =
   assert (String.length s1 = String.length s2);
-  let s = String.create (String.length s1) in
-  for i = 0 to String.length s - 1 do
+  let s = Bytes.create (String.length s1) in
+  for i = 0 to Bytes.length s - 1 do
     s.[i] <- Char.chr (Char.code s1.[i] lxor Char.code s2.[i]);
   done;
-  s
+  Bytes.unsafe_to_string s
 
 (* HMAC-MD5, RFC 2104 *)
 let hmac_md5 =
@@ -157,8 +157,9 @@ let hmac_md5 =
   let md5 s = Md5.direct_to_string (Md5.string s) in
   fun secret challenge ->
     let secret = if String.length secret > 64 then md5 secret else secret in
-    let k = String.make 64 '\x00' in
+    let k = Bytes.make 64 '\x00' in
     String.blit secret 0 k 0 (String.length secret);
+    let k = Bytes.unsafe_to_string k in
     md5 (string_xor k opad ^ md5 (string_xor k ipad ^ challenge))
 
 let sendmail smtp_server smtp_port new_style mail =
diff --git a/src/utils/net/tcpBufferedSocket.ml b/src/utils/net/tcpBufferedSocket.ml
index 6f8b8767..1e042df4 100644
--- a/src/utils/net/tcpBufferedSocket.ml
+++ b/src/utils/net/tcpBufferedSocket.ml
@@ -1618,7 +1618,8 @@ let write t s pos len =
       _to_deflate t;
       buf_add t wbuf s pos len
 
-let write_string t s = write t (Bytes.of_string s) 0 (String.length s)
+let write_bytes t s = write t s 0 (Bytes.length s)
+let write_string t s = write_bytes t (Bytes.unsafe_of_string s)
 
 (*************************************************************************)
 (*                                                                       *)
diff --git a/src/utils/net/tcpBufferedSocket.mli b/src/utils/net/tcpBufferedSocket.mli
index 3a2ffed4..5e45a0c5 100644
--- a/src/utils/net/tcpBufferedSocket.mli
+++ b/src/utils/net/tcpBufferedSocket.mli
@@ -67,6 +67,7 @@ val set_refill : t -> (t -> unit) -> unit
 val set_rtimer : t ->  (t -> unit) -> unit
 val write: t -> bytes -> int -> int -> unit
 val write_string: t -> string -> unit
+val write_bytes: t -> bytes -> unit
 val connect: token -> string -> Unix.inet_addr -> int -> handler -> t
 val close : t -> BasicSocket.close_reason -> unit
 val closed : t -> bool