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 SourceIP, the entry point of the tunnel and the Destination point, 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 is greater than the Path MTU, the packet is fragmented in the encapsulator, as the outer header should be included. The decapsulator will reassemble the packet.