ArrayIndexOutOfBoundsException when Getting Integer from Byte Buffer

Question

I am trying to send a 28KB text file, over UDP with 8 byte prepending header containing sequence number and packet size for each packet sent. When attempting to send the file with packet size of 1000 bytes, after receiving 8KB of the file I get the ArrayIndexOutOfBoundsException when running the line Getting int from the byte buffer that wraps around my byte array:

packetSize = buff.getInt();

The client code is as follows:

         FileOutputStream fos =  null;
         fos = new FileOutputStream(sDstPath);
         FileChannel fc = fos.getChannel();

         while(dataBytesRcvd <= fileSize)
         {
             if((fileSize - dataBytesRcvd) < (packetSize - 8))  //added to adjust last packet's size
                 packetSize = fileSize - dataBytesRcvd + 8;
             DatagramPacket packet = new DatagramPacket(bytes, packetSize, dp.getSocketAddress());
             ds.receive(packet);
             ByteBuffer buff = ByteBuffer.wrap(bytes);
             seqNum = buff.getInt();
             seq[seqNum]=1;
             packetSize = buff.getInt();
             dataBytesRcvd += (packetSize-8);
             fc.write(buff);
             buff.clear();
         }

It should be noted that increasing the byte array size to 4 more bytes than the packet size, didn't help either. What could be wrong with it?