This read-write string sets the minimum, default, and maximum packet sizes the PPP interface will accept from its peer, given as three numbers separated by hyphens. Administrators tune it to match the receive capability advertised during LCP negotiation, which helps avoid a peer sending oversized frames the local interface would have to drop. For instance, the default "46-1500-4096" allows the router to negotiate a receive MRU anywhere from 46 up to 4096 bytes.