The U.S.-Canada border is the longest undefended national border in the world, and the two countries have always prided themselves on the friendliest of relationships and the ease of crossing between them. But since 9/11, security measures have resulted in an ever-growing mountain of paperwork for any artist or musician looking to cross the border for a performance. The hassles are now so extensive that many venues near the border will no longer book acts from the other side, for fear that the performers won’t be able to get official permission in time for the show.