LRP requires that the network interface be able to identify the destination socket of an incoming network packet, so that the packet can be placed on the correct NI channel. Ideally, this function should be performed by the NI itself. Incidentally, many commercial high-speed network adaptors contain an embedded CPU, and the necessary demultiplexing function can be performed by this CPU. We call this approach LRP with NI demux. In the case of network adaptors that lack the necessary support (e.g., inexpensive Fast Ethernet adaptors), the demultiplexing function can be performed in the network driver's interrupt handler. We call this approach soft demux. Here, some amount of host interrupt processing is necessary to demultiplex incoming packets. Fortunately, with current technology, this overhead appears to be small enough to still maintain good stability under overload. The advantage of this approach is that it will work with any network adaptor, i.e., it is hardware independent. We will quantitatively evaluate both demultiplexing approaches in Section 4.
Our demultiplexing function is self-contained, and has minimal requirements on its execution environment (non-blocking, no dynamic memory allocation, no timers). As such, it can be readily integrated in a network interface's firmware, or the device's host interrupt handler. The function can efficiently demultiplex all packets in the TCP/IP protocol family, including IP fragments. In rare cases, an IP fragment does not contain enough information to allow demultiplexing to the correct endpoint. This happens when the fragment containing the transport header of a fragmented IP packet does not arrive first. In this case, the offending packet is placed on a special NI channel reserved for this purpose. The IP reassembly function checks this channel queue when it misses fragments during reassembly.
Throughout this paper, whenever reference is made to actions performed by the network interface, we mean that the action is performed either by the NI processor (in the case of NI demux), or the host interrupt handler (in the case of soft demux).