The following information applies to ScreenOS 4.0.0 and later.
The set flow path-mtu command is used to address VPN tunnel fragmentation issues (the command does not apply to packets being sent in the clear). The command determines the smallest allowable MTU of any link on the current path between two hosts.
If a device on the other end of the tunnel uses the Don't Fragment (DF) bit in the header of a packet, and the packet is large enough to require fragmentation (for example, a Microsoft NetBIOS over TCP/IP packet), the NetScreen device will discard the packet. Further communication with this session will be discontinued.
When the set flow path-mtu command is enabled and a host on the other side of the NetScreen sends a packet with DF bit enabled, the NetScreen sends an ICMP packet back to the originator with a "Destination unreachable - fragmentation required" message. When the originator receives the ICMP packet, they will reduce the MTU size to prevent fragmentation.