Abstract: A system simulator simulates operations of a plurality of interconnected devices in a simulation of a computing system. The system simulator implements a communication runtime in the simulation to receive a packet generated by a simulation of a first one of the plurality of devices to be sent to a simulation of a second one of the plurality of devices in the simulation. The communication runtime buffers the packet in its internal buffer and receives a query from the simulation of the second device based on buffer capacity in the simulation of the second device has capacity. The packet is sent from the communication runtime buffer to the simulation of the second device based on the query to simulate transmission of the packet from the first device to the second device on a link.