> These interposing network elements, called middleboxes, often unwittingly disallow changes to TCP headers and behavior, even if the server and the client are both willing.

There is nothing worse than finding out that someone not even at the company anymore decided years before to deploy some crap like this. Drives me absolutely crazy to impose stuff like this where silos in companies means transitioning involves on the order of 4-5 different "components" need to change.