Some embodiments provide a method for a managed forwarding element that operates on a host machine to process packets for at least one logical network. The method receives a packet that includes a particular piece of data to maintain with the packet. The particular piece of data is not stored in a payload of the packet and is not protocol-specific data. The method stores the particular piece of data in a register while processing the packet. The method identifies a next destination of the packet that operates on the host machine. The method generates an object to represent the packet for the identified destination. The particular piece of data is stored in a field of the generated object.