1-61) The IP packet received from the IPv4 network interface is sent to the IP protocol stack, which thenchecks the protocol number in the IP header.2) If the protocol number is IPv4, the IP packet is sent to the tunnel module for decapsulation.3) The decapsulated IP packet is sent back to the IP protocol stack for processing.IPv4/IPv6 over IPv6 TunnelIntroduction to IPv4/IPv6 over IPv6 tunneling protocolThe IPv4/IPv6 over IPv6 tunneling protocol (RFC 2473) is developed for IPv4 or IPv6 data packetencapsulation so that encapsulated packets can be transmitted over an IPv6 network. Theencapsulated packets are IPv6 tunnel packets.Figure 1-4 Principle of IPv4/IPv6 over IPv6 tunnelThe original data in Figure 1-4 refers to an IPv4 or IPv6 packet.Encapsulation and decapsulationThe encapsulation process is as follows:1) After receiving the original packet, the interface of Router A connecting private network A submits itto the corresponding data module for processing. The data module then determines how to routethe packet.2) If the packet needs to be routed to Host B connected to Router B, the packet is sent to Router A’stunnel interface that is connected to Router B.3) After receiving the packet, the tunnel interface adds an IPv6 header to it and submits it to the IPv6module for processing.4) The IPv6 module re-determines a route according to the destination address in the IPv6 header.Contrary to the encapsulation process, the decapsulation process is as follows:5) The packet received from the IPv6 network interface is sent to the IPv6 module for processing.6) If the passenger protocol is IPv4 or IPv6, the packet is sent to the tunnel processing module fordecapsulation.7) The decapsulated packet is sent to the corresponding protocol module for the secondary routingprocess.GRE tunnelGeneric Routing Encapsulation (GRE) is a protocol designed for encapsulating and carrying thepackets of one network layer protocol (for example, IP or IPX) over another network layer protocol (forexample, IP). GRE is a tunneling technology and serves as a Layer 3 tunneling protocol.
