Digisol DG-GS4826S, Management Manual

Page: 1160 / 1344

C
HAPTER
47 | IP Interface Commands
IPv6 to IPv4 Tunnels
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IPv6/IPv4 hosts and routers can tunnel IPv6 datagrams over regions of
IPv4 routing topology by encapsulating them within IPv4 packets.
Tunneling can be used in a variety of ways, including the following:
n
Router-to-Router: IPv6/IPv4 routers interconnected by an IPv4
infrastructure can tunnel IPv6 packets between themselves. In this
case, the tunnel spans one segment of the end-to-end path that the
IPv6 packet takes.
n
Host-to-Router: IPv6/IPv4 hosts can tunnel IPv6 packets to an
intermediate IPv6/IPv4 router that is reachable via an IPv4
infrastructure. This type of tunnel spans the first segment of the
packet's end-to-end path.
n
Host-to-Host: IPv6/IPv4 hosts that are interconnected by an IPv4
infrastructure can tunnel IPv6 packets between themselves. In this
case, the tunnel spans the entire end-to-end path that the packet
takes; and a host can be either a 6to4 node or native IPv6 host.
n
Router-to-Host: IPv6/IPv4 routers can tunnel IPv6 packets to their
final destination IPv6/IPv4 host. This tunnel spans only the last
segment of the end-to-end path.
Tunneling techniques are classified according to the mechanism by
which the encapsulating node determines the address of the node at
the end of the tunnel. In the first two tunneling methods listed above –
router-to-router and host-to-router – the IPv6 packet is being tunneled
to a router. The end point of this type of tunnel is an intermediate
router which must decapsulate the IPv6 packet and forward it on to its
final destination. When tunneling to a router, the end point of the
tunnel is different from the destination of the packet being tunneled. So
the addresses in the IPv6 packet being tunneled can not provide the
IPv4 address of the tunnel end point. Instead, the tunnel end-point
address must be determined from information configured on the
encapsulating node. In other words, “configured tunneling” must be
used to explicitly identify the end point.
In the last two tunneling methods – host-to-host and router-to-host –
the IPv6 packet is tunneled all the way to its final destination. In this
case, the destination address of both the IPv6 packet and the
encapsulating IPv4 header identify the same node. This fact can be
exploited by encoding information in the IPv6 destination address that
will allow the encapsulating node to determine the tunnel end point
IPv4 address automatically. “6to4 automatic tunneling” employs this
technique, using an special IPv6 address format with an embedded
IPv4 address to allow tunneling nodes to automatically derive the
tunnel end-point IPv4 address. This eliminates the need to explicitly
configure the tunnel end-point address.
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The two tunneling techniques – configured and automatic – differ
primarily in how they determine the tunnel end-point address. Most of
the underlying mechanisms are the same:
n
The entry node of the tunnel (the encapsulating node) creates an
encapsulating IPv4 header and transmits the encapsulated packet.