... In addition, there are a number of configurations involving pulse-per-second (PPS) signals, modem backup services and other special cases, so that a set of mitigation rules becomes necessary to select a single peer from among the survivors. ... The scheme is based on the concept of prefer peer , which is specified by including the prefer keyword with the associated server or peer command in the configuration file. ... The PPS signal can discipline the kernel in both frequency and time. ...