add: fInterfaceMTU = fMRU;
context: PPPInterface::CalculateInterfaceMTU(){#if DEBUG	printf("PPPInterface: CalculateInterfaceMTU()\n");#endif		fInterfaceMTU = fMRU;	fHeaderLength = 2;		// sum all headers (the protocol field is not counted)	PPPProtocol *protocol = FirstProtocol();	for(; protocol; protocol = protocol->NextProtocol()) {		if(protocol->Level() < PPP_PROTOCOL_LEVEL)			fHeaderLength += protocol->Overhead();	}		fInterfaceMTU -= fHeaderLength;		if(Ifnet()) {		Ifnet()->if_mtu = fInterfaceMTU;		Ifnet()->if_hdrlen = fHeaderLength;	}		if(Parent())		Parent()->CalculateInterfaceMTU();}