This read-write string sets the minimum, default, and maximum packet sizes the PPP interface will send, given as three numbers separated by hyphens. An admin adjusts it to control the largest frame the link transmits, which matters when negotiating MTU with a peer that has a smaller limit or when troubleshooting fragmentation. For example, the value could be set to "64-1500-4096" to keep the default send MTU at 1500 bytes while allowing negotiation between 64 and 4096.