An interface adapter for a packet network includes a first plurality of execution engines, coupled to a host interface so as to read from a memory work items corresponding to messages to be sent over the network, and to generate gather entries defining packets to be transmitted over the network responsive to the work items. A scheduling processor assigns the work items to the execution engines for generation of the gather entries. Switching circuitry couples the execution engines to a plurality of gather engines, which generate the packets responsive to the gather entries.