Chapter 16 IPv6 Tunnel Configuration
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Encapsulation: If the packet egress interface is a tunnel interface when an IPv6 host
or router sends an IPv6 flow, the host or router first determines the tunnel type. If
the tunnel is a 6in4 tunnel, the host or router implements IPv4 header encapsulation.
During the encapsulation, the source and destination addresses of the IPv4 header
are manually configured by the user. The encapsulated packet is then sent according
to the IPv4 packet sending process.
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Decapsulation: The process is just contrary to the encapsulation process. If the
protocol number in the IPv4 header of the received IPv4 packet is 41, the host or router
proceeds to the 6in4 decapsulation process and searches for the matched tunnel
number according to the source and destination addresses of the packet. If the tunnel
number is found, the host or router removes the IPv4 header added during tunnel
encapsulation and delivers the remaining IPv6 packet to the IPv6 packet receiving
process for further handling.
6to4 Tunnel
shows the operating principles of a 6to4 tunnel.
Figure 16-4 Principles of a 6to4 Tunnel
A 6to4 tunnel is a point-to-multipoint auto tunnel used to connect multiple isolated IPv6 sites
through an IPv4 network to an IPv6 network. It makes possible the automatic acquisition
of the IPv4 address at the termination point of the tunnel by embedding an IPv4 address
in the destination address of an IPv6 packet.
The 6to4 tunnel assumes a special 6to4 address in the format of 2002:abcd:efgh:Subnet
ID::InterfaceID/64. Of the address, 2002 is a fixed IPv6 address prefix, abcd:efgh is a
globally unique 32-bit IPv4 source address of the 6to4 tunnel in hexadecimal (e.g. 1.1.1.1
can be expressed as 0101:0101), and the rest uniquely identifies the position of a host in
a 6to4 network. As the termination point of the tunnel can be automatically determined by
this embedded IPv4 address, tunnel establishment is very convenient.
Because the 16-bit subnet ID in the 64-bit address prefix of the 6to4 address can be
user-defined whereas the first 48 bits of the prefix are a fixed number or determined by
the IPv4 address of the device at the start or termination point of a tunnel, it is possible to
forward IPv6 packets over the tunnel. The 6to4 tunnel makes possible the interconnection
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