Tunnels

The third way to overcome the barriers caused by dissimilar routed protocols is to use tunnels. Tunnels are relatively simple constructs that can be used to pass data through an otherwise incompatible network region. Datagrams that are to traverse this dissimilar network region must be encapsulated in the packet structure of the dissimilar network protocol. This allows the original datagram to be forwarded through the otherwise incompatible section of the network en route to its destination. The original framing and formatting is retained but treated as data by the encapsulating protocol.

Upon reaching the end of the tunnel, the encapsulating protocol "wrapper" is removed and discarded. This restores the datagram to its original format, complete with its original internetwork addressing. Implied in this description of tunneling is that both the source and destination machines use the same routed protocol, but that some portion of the network interconnecting them uses a different protocol. The tunnel would be constructed between the two routers that form the boundaries between the dissimilar regions. As such, they would be capable of using both routed protocols. These routers, in theory, would be the ones in the network that are closest to both the source and destination machines.

This type of mechanism is used as a transitional device between the two versions of IP: IP Version 4 (IPv4) and IP Version 6 (IPv6).

Figure 14-4 illustrates the tunneling of IPv4 packets through an IPv6 network region. Because of the inherent difference in the length of these two protocols' addresses, they are not directly compatible. To overcome this incompatibility, IPv4 packets are wrapped in IPv6 by Router A for transmission through an IPv6 WAN. Router B removes the IPv6 wrapper and presents the restored IPv4 packet to the destination host in the form that it can recognize.

Internetworking with Dissimilar Protocols

Figure 14-4: Tunneling IPv4 packets through an IPv6 region.

Internetworking with Dissimilar Protocols

Figure 14-4: Tunneling IPv4 packets through an IPv6 region.

This same process works in reverse: IPv6 datagrams can be wrapped in IPv4 packets for delivery through an IPv4-only network.

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