Function invented_sent_by 

pub fn invented_sent_by() -> String

A Via sent-by for an endpoint that can never be connected back to (RFC 7118 §5.2).

.invalid is reserved by RFC 2606 and guaranteed never to resolve, which is the whole point: nothing anywhere must ever try to open a connection to it. Advertising a real address here would be worse than useless — it would send a proxy off to a port that is not listening while the connection it should have used sits open.