TCP/IP Tutorial and Technical Overview - IBM Redbooks

2. The receiving node's network interface layer identifies the incoming packets
(or packets if the IPv4 datagram was fragmented) as belonging to IPv4 and
passes them upward to the IPv4 part of the dual IPv6/IPv4 internetwork layer.
3. The IPv4 layer then receives the datagram in the normal way, reassembling
fragments if necessary, notes the protocol number of 41, removes the IPv4
header, and passes the original IPv6 packet “sideways” to the IPv6 part of the
internetwork layer.
4. The IPv6 code then processes the original packet as normal. Because the
destination IPv6 address in the packet is the IPv6 address of the node (an
IPv4-compatible address matching the IPv4 address used in the
encapsulating IPv4 datagram), the packet is at its final destination. IPv6 then
processes any extension headers as normal and then passes the packet's
remaining payload to the next protocol listed in the last IPv6 header.
Figure 9-26 on page 384 shows two IPv6/IPv4 nodes separated by an IPv4
network. Both workstations have IPv4-compatible IPv6 addresses. Workstation A
sends a packet to workstation B, as follows:
1. Workstation A has received router solicitation messages from an
IPv6-capable router (X) on its local link. It forwards the packet to this router.
2. Router X adds an IPv4 header to the packet, using the IPv4 source and
destination addresses derived from the IPv4-compatible addresses. The
packet is then forwarded across the IPv4 network, all the way to workstation
B. This is router-to-host automatic tunneling.
3. The IPv4 datagram is received by the IPv4 stack of workstation B. Because
the Protocol field shows that the next header is 41 (IPv6), the IPv4 header is
stripped from the datagram and the remaining IPv6 packet is then handled by
the IPv6 stack.
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