The key of the header field. The key must be 1 to 40 characters in length, and can contain letters, digits, hyphens (-) and underscores (_). The key does not support Cookie or Host.
The value must be 1 to 128 characters in length, and can contain lowercase letters, printable characters, asterisks (*), and question marks (?). The value cannot contain spaces or the following special characters: # [ ] { } \ | <&.