The problem here isn’t really middleboxes - it’s that people who build middleboxes tend to break protocols. The flags field is specifically designed so that you

The problem here isn’t really middleboxes - it’s that people who build middleboxes tend to break protocols. The flags field is specifically designed so that you can add stuff to it - if the middleboxes were to be built properly, they would fall back to “don’t touch anything” rather than “break the connection” when they don’t understand what’s going on. Middlebox vendors brought this upon themselves.