R3721-F3210-F3171-HP High-End Firewalls VPN Configuration Guide-6PW101

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When an ISATAP tunnel is used, the destination address of an IPv6 packet and the IPv6 address
of a tunnel interface both adopt special ISATAP addresses. The ISATAP address format is
prefix(64bit):0:5EFE:abcd:efgh. The 64-bit prefix is the prefix of a valid IPv6 unicast address, but
abcd:efgh is a 32-bit source IPv4 address in hexadecimal, which might not be globally unique.
Through the embedded IPv4 address, an ISATAP tunnel can be automatically created to transfer
IPv6 packets.
The ISATAP tunnel is mainly used for communication between IPv6 routers or between a host and
an IPv6 router over an IPv4 network.
Figure 66 Principle of ISATAP tunneling
IPv4 over IPv4 tunneling
Introduction
IPv4 over IPv4 tunneling (specified in RFC 1853) is developed for IP data packet encapsulation so that
data can be transferred from one IPv4 network to another IPv4 network.
Encapsulation and de-encapsulation
Figure 67 Principle of IPv4 over IPv4 tunneling
Packets traveling through a tunnel undergo encapsulation and de-encapsulation processes, as shown
in Figure 67.
• E
ncapsulation
The encapsulation follows these steps:
a. Device A receives an IP packet from an IPv4 host and submits it to the IP protocol stack.
b. The IP protocol stack determines how to forward the packet according to the destination
address in the IP header. If the packet is destined for the IPv4 host connected to Device B,
Device A delivers the packet to the tunnel interface.
c. The tunnel interface encapsulates the packet into an IPv4 packet and submitted to the IP
protocol stack for processing. The IP protocol stack determines the outgoing interface of the
tunnel according to the IP header, and sends the packet out.