In the IP package, there is DF flag: Don't Fragment.
I have doubt about it, is it mean let all the L3 device do not to fragment the IP packet (if the L3 device found it is bigger than MTU, device will drop it)?
Or, is it mean this IP packet did not been fragmented at the front path?
The IPv4 DF flag means that an intermediate host (router) cannot fragment the packet if necessary, and it would then need to drop the packet and can send an ICMP message stating that.
RFC 791, Internet Protocol says:
If the Don't Fragment flag (DF) bit is set, then internet fragmentation of this datagram is NOT permitted, although it may be discarded. This can be used to prohibit fragmentation in cases where the receiving host does not have sufficient resources to reassemble internet fragments.
IPv6 does not have the concept of intermediate host fragmentation, so the flag does not exist in the IPv6 header.
The first one, it instructs routers on the path to the destination to not fragment the packet.
So if the packet is to be sent through an interface with a MTU too small, the packet will be dropped, and an ICMP message will (normally) be sent back to the sender.
