Abstract: A communication layer is proposed that, besides improving communication performance on clusters of PCs, by reducing the latencies and increasing the bandwidth figures even for short messages, also meets other requirements such as multiprogramming, portability, protection against corrupted programs, reliable message delivery, direct access to the network for all applications, etc. Instead of removing the operating system kernel from the critical path and creating a user-level network interface, our aim was to optimize the operating system support to provide reliable and efficient network software, avoiding the TCP/IP protocol stack. The communication system was tested in a cluster of PCs with Linux OS and interconnected with Fast Ethernet. The performance figures obtained define the best situation that can be attained without modifying the device drivers or using a user-level network interface approach.