3Com Switch 8800 Advanced Software V5 Configuration Guide

33
TUNNELING CONFIGURATION
Introduction to
Tunneling
The expansion of Internet results in scarce IPv4 addresses. Although the
techniques such as temporary IPv4 address allocation and network address
translation (NAT) relieve the problem of IPv4 address shortage to some extent, they
not only increase the overhead in address resolution and processing, but also lead
to high-level application failures. Furthermore, they will still face the problem that
IPv4 addresses will eventually be used up. Internet protocol version 6 (IPv6)
adopting the 128-bit addressing scheme completely solves the above problem.
Since significant improvements have been made in address space, security,
network management, mobility, and QoS, IPv6 becomes one of the core standards
for the next generation Internet protocol. IPv6 is compatible with all protocols
except IPv4 in the TCP/IP suite. Therefore, IPv6 can completely take the place of
IPv4.
Before IPv6 becomes the dominant protocol, the network using the IPv6 protocol
stack is expected to communicate with the Internet using IPv4. Therefore, an
IPv6-IPv4 interworking technique must be developed to ensure the smooth
transition from IPv4 to IPv6. In addition, the interworking technique should
provide efficient, seamless information transfer. The Internet Engineering Task
Force (IETF) set up the next generation transition (NGTRANS) working group to
study problems about IPv4-to-IPv6 transition and efficient, seamless IPv4-IPv6
interworking. Currently, multiple transition techniques and interworking solutions
are available. With their own characteristics, they are used to solve communication
problems in different transition stages under different environments.
Currently, there are three major transition techniques: dual stack (RFC 2893),
tunneling (RFC 2893), and NAT-PT (RFC 2766).
Tunneling is an encapsulation technique, which utilizes one network transport
protocol to encapsulate packets of another network transport protocol and
transfer them over the network. A tunnel is a virtual point-to-point connection. In
practice, the virtual interface that supports only point-to-point connections is
called tunnel interface. One tunnel provides one channel to transfer encapsulated
packets. Packets can be encapsulated and decapsulated at both ends of a tunnel.
Tunneling refers to the whole process from data encapsulation to data transfer to
data decapsulation.
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For related configuration about the dual protocol stack, refer to “Dual Stack
Overview” on page 373.
IPv6 over IPv4 Tunnel Principle
The IPv6 over IPv4 tunneling mechanism encapsulates an IPv4 header in IPv6 data
packets so that IPv6 packets can pass an IPv4 network through a tunnel to realize
interworking between isolated IPv6 networks, as shown in
Figure 112.