IP in IP is an IP tunneling protocol that encapsulates one IP packet in another IP packet. To encapsulate an IP packet in another IP packet, an outer header is added with Source IP, the entry point of the tunnel, and Destination IP, the exit point of the tunnel. While doing this, the inner packet is unmodified (except the TTL field, which is decremented). The Don't Fragment and the Type Of Service fields should be copied to the outer packet. If the packet size, including the outer header, is greater than the Path [[Maximum transmission unit|MTU]], the encapsulator fragments the packet. The decapsulator will reassemble the packet.