Oh, you should look at the early BBN TCP for V6 Unix - they would have faced the same issue, with their TCP process. They did have the capac() call (which kind of alleviates the need for non-blocking I/O), but that may have only been available for ports/pipes; I'm not sure if the ARPANET device supported it. (With the NCP as well, that did some amount of demultiplexing in the kernel, and probably had buffering there, so, if so, in theory capac() could have been done there. Of course, with the ARPANET link being only 100Kbit/sec maximum - although only to a host on the same IMP - the overhead of copying buffered data made kernel buffering more 'affordable'.) Noel