A method for detecting a forwarding problem within an autonomous system, the autonomous system having a plurality of nodes including a source node, an intermediate node, and a destination node, the method comprising: initiating a message from the source node, the message being arranged to be sent to a message destination that is an external address that is not an address located within the autonomous system; identifying a path, using a Border Gateway Protocol, BGP, Next Hop table, the path being arranged to pass from the source node to the external address via the destination node; determining, from an Internal Gateway Protocol, IGP, Routing table, a route for a first path segment, which is a portion of the path between the source node and the destination node comprising a number of nodes including the intermediate node within the autonomous system; storing an indication in the message, the indication being arranged to indicate the number of nodes through which the path segment passes between the source node and the destination node; forwarding the message from the source node along the path, wherein forwarding the message from the source node along the path includes receiving the message on each node in the path segment, each of the nodes in the path segment being arranged to alter the indication to indicate a number of nodes through which the path segment passes between the node and the destination node; receiving the message on the destination node; obtaining at the destination node the indication in order to determine whether to remove the message from the path; removing the message from the path at the destination node; and initiating a response from the destination node, the response being arranged to be sent along the first path segment from the destination node to the source node, wherein the response is arranged to indicate that the intermediate node does not have a forwarding problem. initiating a message from the source node, the message being arranged to be sent to a message destination that is an external address that is not an address located within the autonomous system; identifying a path, using a Border Gateway Protocol, BGP, Next Hop table, the path being arranged to pass from the source node to the external address via the destination node; determining, from an Internal Gateway Protocol, IGP, Routing table, a route for a first path segment, which is a portion of the path between the source node and the destination node comprising a number of nodes including the intermediate node within the autonomous system; storing an indication in the message, the indication being arranged to indicate the number of nodes through which the path segment passes between the source node and the destination node; forwarding the message from the source node along the path, wherein forwarding the message from the source node along the path includes receiving the message on each node in the path segment, each of the nodes in the path segment being arranged to alter the indication to indicate a number of nodes through which the path segment passes between the node and the destination node; receiving the message on the destination node; obtaining at the destination node the indication in order to determine whether to remove the message from the path; removing the message from the path at the destination node; and initiating a response from the destination node, the response being arranged to be sent along the first path segment from the destination node to the source node, wherein the response is arranged to indicate that the intermediate node does not have a forwarding problem.