A method for communication in a packet data network including at least first and second subnets interconnected by routers. The method includes defining at least first and second classes of link-layer traffic within the subnets, such that the link-layer traffic in the first class is transmitted among nodes in the network without loss of packets, while at least some of the packets in the second class are dropped in case of network congestion. The routers are configured by transmitting control traffic over the network in the packets of the second class. Data traffic is transmitted between the nodes in the first and second subnets via the configured routers in the packets of the first class.