US Senator Tom Udall talks to Katty Kay about calling for a more rapid withdrawal of all American combat troops from Afghanistan.He is one of three senators who wrote an op-ed piece in the New York Times titled 'Let’s Not Linger in Afghanistan.'"Nothing focuses the mind and the energies of our troops and the leaders by saying 'you've got about 12-18 months to hand this off'," Senator Udall tells the BBC.