diff --git a/Imports.hs b/Imports.hs
index ddbf0217..7ec1ced1 100644
--- a/Imports.hs
+++ b/Imports.hs
@@ -8,10 +8,11 @@ module Imports (
   , module Data.List
   , module Data.Foldable
   , module Data.Int
+  , module Data.Maybe
   , module Data.Monoid
   , module Data.Ord
+  , module Data.String
   , module Data.Word
-  , module Data.Maybe
   , module Numeric
   , GCBuffer
   , withForeignPtr
@@ -29,6 +30,7 @@ import Data.Int
 import Data.List
 import Data.Maybe
 import Data.Monoid
+import Data.String
 import Data.Ord
 import Data.Word
 import Foreign.ForeignPtr
diff --git a/Network/HTTP2/Arch/HPACK.hs b/Network/HTTP2/Arch/HPACK.hs
index 89d725cf..023cf31a 100644
--- a/Network/HTTP2/Arch/HPACK.hs
+++ b/Network/HTTP2/Arch/HPACK.hs
@@ -69,15 +69,16 @@ hpackDecodeHeader hdrblk sid ctx = do
     if isClient ctx || checkRequestHeader vt then
         return tbl
       else
-        E.throwIO $ StreamErrorIsSent ProtocolError sid
+        E.throwIO $ StreamErrorIsSent ProtocolError sid "illegal header"
 
 hpackDecodeTrailer :: HeaderBlockFragment -> StreamId -> Context -> IO HeaderTable
 hpackDecodeTrailer hdrblk sid Context{..} = decodeTokenHeader decodeDynamicTable hdrblk `E.catch` handl
   where
     handl IllegalHeaderName =
-        E.throwIO $ StreamErrorIsSent ProtocolError sid
-    handl _ =
-        E.throwIO $ StreamErrorIsSent CompressionError sid
+        E.throwIO $ StreamErrorIsSent ProtocolError sid "illegal trailer"
+    handl e = do
+        let msg = fromString $ show e
+        E.throwIO $ StreamErrorIsSent CompressionError sid msg
 
 {-# INLINE checkRequestHeader #-}
 checkRequestHeader :: ValueTable -> Bool
diff --git a/Network/HTTP2/Arch/Receiver.hs b/Network/HTTP2/Arch/Receiver.hs
index cd994263..d867776a 100644
--- a/Network/HTTP2/Arch/Receiver.hs
+++ b/Network/HTTP2/Arch/Receiver.hs
@@ -73,10 +73,10 @@ frameReceiver ctx@Context{..} conf@Config{..} = loop 0 `E.catch` sendGoaway
           let frame = goawayFrame sid err $ Short.fromShort msg
           enqueueControl controlQ $ CFrames Nothing [frame]
           enqueueControl controlQ $ CFinish e
-      | Just e@(StreamErrorIsSent err sid) <- E.fromException se = do
+      | Just e@(StreamErrorIsSent err sid msg) <- E.fromException se = do
           let frame = resetFrame err sid
           enqueueControl controlQ $ CFrames Nothing [frame]
-          let frame' = goawayFrame sid err "closing a connection after sending a stream error"
+          let frame' = goawayFrame sid err $ Short.fromShort msg
           enqueueControl controlQ $ CFrames Nothing [frame']
           enqueueControl controlQ $ CFinish e
       | Just e@(StreamErrorIsReceived err sid) <- E.fromException se = do
@@ -174,7 +174,7 @@ controlOrStream ctx@Context{..} conf@Config{..} ftyp header@FrameHeader{streamId
 processState :: StreamState -> Context -> Stream -> StreamId -> IO Bool
 processState (Open (NoBody tbl@(_,reqvt))) ctx@Context{..} strm@Stream{streamInput} streamId = do
     let mcl = fst <$> (getHeaderValue tokenContentLength reqvt >>= C8.readInt)
-    when (just mcl (/= (0 :: Int))) $ E.throwIO $ StreamErrorIsSent ProtocolError streamId
+    when (just mcl (/= (0 :: Int))) $ E.throwIO $ StreamErrorIsSent ProtocolError streamId "no body but content-length is not zero"
     halfClosedRemote ctx strm
     tlr <- newIORef Nothing
     let inpObj = InpObj tbl (Just 0) (return "") tlr
@@ -184,14 +184,14 @@ processState (Open (NoBody tbl@(_,reqvt))) ctx@Context{..} strm@Stream{streamInp
       else
         putMVar streamInput inpObj
     return False
-processState (Open (HasBody tbl@(_,reqvt))) ctx@Context{..} strm@Stream{streamInput} streamId = do
+processState (Open (HasBody tbl@(_,reqvt))) ctx@Context{..} strm@Stream{streamInput} _streamId = do
     let mcl = fst <$> (getHeaderValue tokenContentLength reqvt >>= C8.readInt)
     bodyLength <- newIORef 0
     tlr <- newIORef Nothing
     q <- newTQueueIO
     setStreamState ctx strm $ Open (Body q mcl bodyLength tlr)
     incref <- newIORef 0
-    bodySource <- mkSource q $ informWindowUpdate controlQ streamId incref
+    bodySource <- mkSource q $ informWindowUpdate ctx strm incref
     let inpObj = InpObj tbl mcl (readSource bodySource) tlr
     if isServer ctx then do
         let si = toServerInfo roleInfo
@@ -245,8 +245,8 @@ getStream' ctx@Context{..} ftyp streamId Nothing
                 mMaxConc <- maxConcurrentStreams <$> readIORef mySettings
                 case mMaxConc of
                   Nothing      -> return ()
-                  Just maxConc ->  when (cnt >= maxConc) $
-                    E.throwIO $ StreamErrorIsSent RefusedStream streamId
+                  Just maxConc -> when (cnt >= maxConc) $
+                    E.throwIO $ StreamErrorIsSent RefusedStream streamId "exceeds max concurrent"
             Just <$> openStream ctx streamId ftyp
   | otherwise = undefined -- never reach
 
@@ -300,10 +300,9 @@ control FrameGoAway header bs _ _ = do
       else
         E.throwIO $ ConnectionErrorIsReceived err sid $ Short.toShort msg
 
-control FrameWindowUpdate header@FrameHeader{streamId} bs ctx _ = do
+control FrameWindowUpdate header bs ctx _ = do
     WindowUpdateFrame n <- guardIt $ decodeWindowUpdateFrame header bs
-    w <- increaseConnectionWindowSize ctx n
-    when (isWindowOverflow w) $ E.throwIO $ ConnectionErrorIsSent FlowControlError streamId "control window should be less than 2^31"
+    increaseConnectionWindowSize ctx n
 
 control _ _ _ _ _ =
     -- must not reach here
@@ -321,7 +320,7 @@ guardIt x = case x of
 {-# INLINE checkPriority #-}
 checkPriority :: Priority -> StreamId -> IO ()
 checkPriority p me
-  | dep == me = E.throwIO $ StreamErrorIsSent ProtocolError me
+  | dep == me = E.throwIO $ StreamErrorIsSent ProtocolError me "priority depends on itself"
   | otherwise = return ()
   where
     dep = streamDependency p
@@ -367,11 +366,10 @@ stream FrameHeaders header@FrameHeader{flags,streamId} bs ctx (Open (Body q _ _
 
 -- ignore data-frame except for flow-control when we're done locally
 stream FrameData
-       FrameHeader{flags,payloadLength}
+       FrameHeader{flags}
        _bs
-       ctx s@(HalfClosedLocal _)
+       _ctx s@(HalfClosedLocal _)
        _ = do
-    informConnectionWindowUpdate ctx payloadLength
     let endOfStream = testEndStream flags
     if endOfStream then do
         return HalfClosedRemote
@@ -383,7 +381,6 @@ stream FrameData
        bs
        ctx@Context{emptyFrameRate} s@(Open (Body q mcl bodyLength _))
        _ = do
-    informConnectionWindowUpdate ctx payloadLength
     DataFrame body <- guardIt $ decodeDataFrame header bs
     len0 <- readIORef bodyLength
     let len = len0 + payloadLength
@@ -400,7 +397,7 @@ stream FrameData
     if endOfStream then do
         case mcl of
             Nothing -> return ()
-            Just cl -> when (cl /= len) $ E.throwIO $ StreamErrorIsSent ProtocolError streamId
+            Just cl -> when (cl /= len) $ E.throwIO $ StreamErrorIsSent ProtocolError streamId "actual body length is not the same as content-length"
         -- no trailers
         atomically $ writeTQueue q ""
         return HalfClosedRemote
@@ -434,10 +431,9 @@ stream FrameContinuation FrameHeader{flags,streamId} frag ctx s@(Open (Continued
           else
             return $ Open $ Continued rfrags' siz' n' endOfStream
 
-stream FrameWindowUpdate header@FrameHeader{streamId} bs _ s strm = do
+stream FrameWindowUpdate header bs _ s strm = do
     WindowUpdateFrame n <- guardIt $ decodeWindowUpdateFrame header bs
-    w <- increaseStreamWindowSize strm n
-    when (isWindowOverflow w) $ E.throwIO $ StreamErrorIsSent FlowControlError streamId
+    increaseStreamWindowSize strm n
     return s
 
 stream FrameRSTStream header@FrameHeader{streamId} bs ctx _ strm = do
@@ -458,8 +454,8 @@ stream FrameContinuation FrameHeader{streamId} _ _ _ _ = E.throwIO $ ConnectionE
 stream _ FrameHeader{streamId} _ _ (Open Continued{}) _ = E.throwIO $ ConnectionErrorIsSent ProtocolError streamId "an illegal frame follows header/continuation frames"
 -- Ignore frames to streams we have just reset, per section 5.1.
 stream _ _ _ _ st@(Closed (ResetByMe _)) _ = return st
-stream FrameData FrameHeader{streamId} _ _ _ _ = E.throwIO $ StreamErrorIsSent StreamClosed streamId
-stream _ FrameHeader{streamId} _ _ _ _ = E.throwIO $ StreamErrorIsSent ProtocolError streamId
+stream FrameData FrameHeader{streamId} _ _ _ _ = E.throwIO $ StreamErrorIsSent StreamClosed streamId $ fromString ("illegal data frame for " ++ show streamId)
+stream _ FrameHeader{streamId} _ _ _ _ = E.throwIO $ StreamErrorIsSent ProtocolError streamId $ fromString ("illegal frame for " ++ show streamId)
 
 ----------------------------------------------------------------
 
diff --git a/Network/HTTP2/Arch/Types.hs b/Network/HTTP2/Arch/Types.hs
index 121dcfe3..727495d6 100644
--- a/Network/HTTP2/Arch/Types.hs
+++ b/Network/HTTP2/Arch/Types.hs
@@ -212,7 +212,7 @@ data HTTP2Error =
   | ConnectionErrorIsReceived ErrorCode StreamId ReasonPhrase
   | ConnectionErrorIsSent     ErrorCode StreamId ReasonPhrase
   | StreamErrorIsReceived     ErrorCode StreamId
-  | StreamErrorIsSent         ErrorCode StreamId
+  | StreamErrorIsSent         ErrorCode StreamId ReasonPhrase
   | BadThingHappen E.SomeException
   deriving (Show, Typeable)
 
diff --git a/Network/HTTP2/Arch/Window.hs b/Network/HTTP2/Arch/Window.hs
index 6dabf088..04689d0e 100644
--- a/Network/HTTP2/Arch/Window.hs
+++ b/Network/HTTP2/Arch/Window.hs
@@ -4,6 +4,7 @@
 module Network.HTTP2.Arch.Window where
 
 import Data.IORef
+import qualified UnliftIO.Exception as E
 import UnliftIO.STM
 
 import Imports
@@ -15,48 +16,6 @@ import Network.HTTP2.Arch.Stream
 import Network.HTTP2.Arch.Types
 import Network.HTTP2.Frame
 
-----------------------------------------------------------------
--- Receiving window update
-
-increaseStreamWindowSize :: Stream -> Int -> IO WindowSize
-increaseStreamWindowSize Stream{streamWindow} n = atomically $ do
-    w0 <- readTVar streamWindow
-    let w1 = w0 + n
-    writeTVar streamWindow w1
-    return w1
-
-increaseConnectionWindowSize :: Context -> Int -> IO WindowSize
-increaseConnectionWindowSize Context{txConnectionWindow} n = atomically $ do
-    w0 <- readTVar txConnectionWindow
-    let w1 = w0 + n
-    writeTVar txConnectionWindow w1
-    return w1
-
-----------------------------------------------------------------
--- Sending window update
-
-decreaseWindowSize :: Context -> Stream -> WindowSize -> IO ()
-decreaseWindowSize Context{txConnectionWindow} Stream{streamWindow} siz = do
-    atomically $ modifyTVar' txConnectionWindow (subtract siz)
-    atomically $ modifyTVar' streamWindow (subtract siz)
-
-informWindowUpdate :: TQueue Control -> StreamId -> IORef Int -> Int -> IO ()
-informWindowUpdate _        _   _      0   = return ()
-informWindowUpdate controlQ sid incref len = do
-    -- incref is occupied by the receiver thread
-    w0 <- readIORef incref
-    let w1 = w0 + len
-    if w1 >= defaultWindowSize then do -- fixme
-        let frame = windowUpdateFrame sid w1
-        enqueueControl controlQ $ CFrames Nothing [frame]
-        writeIORef incref 0
-      else
-        writeIORef incref w1
-
-informConnectionWindowUpdate :: Context -> Int -> IO ()
-informConnectionWindowUpdate Context{..} =
-    informWindowUpdate controlQ 0 rxConnectionInc
-
 getStreamWindowSize :: Stream -> IO WindowSize
 getStreamWindowSize Stream{streamWindow} = readTVarIO streamWindow
 
@@ -73,6 +32,54 @@ waitConnectionWindowSize Context{txConnectionWindow} = do
     w <- readTVar txConnectionWindow
     checkSTM (w > 0)
 
+----------------------------------------------------------------
+-- Receiving window update
+
+increaseWindowSize :: StreamId -> TVar WindowSize -> WindowSize -> IO ()
+increaseWindowSize sid tvar n = do
+    w <- atomically $ do
+        w0 <- readTVar tvar
+        let w1 = w0 + n
+        writeTVar tvar w1
+        return w1
+    when (isWindowOverflow w) $ do
+        let msg = fromString ("window update for stream " ++ show sid ++ " is overflow")
+            err = if isControl sid then ConnectionErrorIsSent
+                                   else StreamErrorIsSent
+        E.throwIO $ err FlowControlError sid msg
+
+increaseStreamWindowSize :: Stream -> WindowSize -> IO ()
+increaseStreamWindowSize Stream{streamNumber,streamWindow} n =
+    increaseWindowSize streamNumber streamWindow n
+
+increaseConnectionWindowSize :: Context -> Int -> IO ()
+increaseConnectionWindowSize Context{txConnectionWindow} n =
+    increaseWindowSize 0 txConnectionWindow n
+
+decreaseWindowSize :: Context -> Stream -> WindowSize -> IO ()
+decreaseWindowSize Context{txConnectionWindow} Stream{streamWindow} siz = do
+    atomically $ modifyTVar' txConnectionWindow (subtract siz)
+    atomically $ modifyTVar' streamWindow (subtract siz)
+
+----------------------------------------------------------------
+-- Sending window update
+
+informWindowUpdate :: Context -> Stream -> IORef Int -> Int -> IO ()
+informWindowUpdate _        _   _       0   = return ()
+informWindowUpdate Context{controlQ,rxConnectionInc} Stream{streamNumber} streamInc len = do
+    join $ atomicModifyIORef rxConnectionInc $ modify 0
+    join $ atomicModifyIORef streamInc       $ modify streamNumber
+  where
+    modify sid w0
+      | w1 < thresh = (w1, return ())
+      | otherwise   = let frame = windowUpdateFrame sid w1
+                          cframe = CFrames Nothing [frame]
+                          action = enqueueControl controlQ cframe
+                      in (0, action)
+      where
+        thresh = defaultWindowSize -- fixme
+        w1 = w0 + len
+
 ----------------------------------------------------------------
 
 -- max: 2,147,483,647 (2^31-1) is too large.