Communication apparatus includes multiple ports for connection to a packet data network. A memory contains, for each port, a respective first, fixed headroom allocation to hold packets received from the network through the port and to contain a shared headroom buffer, which is available to be shared among a plurality of the ports. Flow-control logic allocates to each of the ports, within the shared headroom buffer, a respective second, variable headroom allocation, which varies responsively to fill levels of the respective first headroom allocation and of the shared headroom buffer, thereby defining, for each of the ports, a respective total headroom allocation comprising the respective first and second headroom allocations. The logic is configured to apply flow-control operations in response to the packets received from the network through each port responsively to a total fill level of the respective total headroom allocation of the port.