diff --git a/h2.mjs b/h2.mjs
index cfb63a1..ed66178 100644
--- a/h2.mjs
+++ b/h2.mjs
@@ -869,7 +869,6 @@ const handleConnection =
 
     const hpackDecode = new HPackCtx();
     const hpackEncode = new HPackCtx();
-    let connectionFlowControlWindow = DEFAULT_FLOW_CONTROL_WINDOW_SIZE;
     const peerSettings = new Map();
     const streams = new Map();
 
@@ -968,19 +967,30 @@ const handleConnection =
 
             const request = buildRequest(rawH2Request);
 
-            const writeDataFrame = (body, flags) => {
+            const writeDataFrame = (body, flags, cb) => {
               const payload = Buffer.from(body);
-              streams.get(frame.streamIdentifier).flowControlWindowSize -=
-                payload.length;
+              const h2stream = streams.get(frame.streamIdentifier);
 
-              socket.write(
-                encodeFrame({
-                  type: FRAME_TYPE.DATA,
-                  flags: flags,
-                  payload,
-                  streamIdentifier: frame.streamIdentifier,
-                })
-              );
+              const doWrite = () => {
+                h2stream.flowControlWindowSize -= payload.length;
+
+                socket.write(
+                  encodeFrame({
+                    type: FRAME_TYPE.DATA,
+                    flags: flags,
+                    payload,
+                    streamIdentifier: frame.streamIdentifier,
+                  })
+                );
+                cb();
+              };
+
+              if (h2stream.flowControlWindowSize < payload.length) {
+                h2stream.waitingForFlowControlWindowSize = payload.length;
+                h2stream.waitingForFlowControlWindowCb = doWrite;
+              } else {
+                setImmediate(doWrite);
+              }
             };
 
             const bodyStream = new stream.Writable({
@@ -993,18 +1003,22 @@ const handleConnection =
                   throw new Error("only buffers are supported");
                 }
 
-                while (chunk.length > 0) {
+                const writePart = () => {
                   const chunkchunk = chunk.subarray(0, maxPayloadSize);
-
-                  writeDataFrame(chunkchunk, 0);
-
                   chunk = chunk.subarray(maxPayloadSize);
-                }
 
-                next();
+                  writeDataFrame(chunkchunk, 0, () => {
+                    if (chunk.length > 0) {
+                      writePart();
+                    } else {
+                      next();
+                    }
+                  });
+                };
+                writePart();
               },
               final: (cb) => {
-                writeDataFrame(Buffer.from([]), DATA_FLAG.END_STREAM);
+                writeDataFrame(Buffer.from([]), DATA_FLAG.END_STREAM, () => {});
                 cb(null);
               },
             });
@@ -1106,7 +1120,27 @@ const handleConnection =
         case FRAME_TYPE.WINDOW_UPDATE: {
           // whatever
           const increment = frame.payload.readUint32BE();
-          console.log(increment, frame.streamIdentifier);
+
+          if (frame.streamIdentifier) {
+            const h2stream = streams.get(frame.streamIdentifier);
+            if (h2stream) {
+              h2stream.flowControlWindowSize += increment;
+
+              if (
+                typeof h2stream.waitingForFlowControlWindowSize === "number" &&
+                h2stream.flowControlWindowSize >=
+                  h2stream.waitingForFlowControlWindowSize
+              ) {
+                h2stream.waitingForFlowControlWindowSize = undefined;
+                const cb = h2stream.waitingForFlowControlWindowCb;
+                h2stream.waitingForFlowControlWindowCb = undefined;
+                cb();
+              }
+            } else {
+              socket.destroy();
+            }
+          }
+
           break;
         }
         case FRAME_TYPE.GOAWAY: {
@@ -1254,7 +1288,9 @@ server.on(
             headers: [["content-type", contentType]],
           });
 
-          const fileStream = fs.createReadStream(filepath);
+          const fileStream = fs.createReadStream(filepath, {
+            highWaterMark: 1_048_576,
+          });
 
           fileStream.pipe(res.body);
         });