IPV6 TO IPV4 隧道

Tunneling

Introduction to Tunneling

The expansion of Internet results in scarce IPv4 addresses. Although the technologies

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 technology must be developed to ensure the smooth transition from IPv4

to IPv6. In addition, the interworking technology 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

technologies 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 technologies: dual stack (RFC2893),

tunneling (RFC2893), and NAT-PT (RFC2766).

Tunneling is an encapsulation technology, 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.

IPv6 over IPv4 Tunnel

I. 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 1.

Caution:

The devices at both ends of an IPv6 over IPv4 tunnel must support IPv4/IPv6 dual

stack.

IPv6 host IPv6 host Figure 1 Principle of IPv6 over IPv4 tunnel

The IPv6 over IPv4 tunnel processes packets in the following way:

1) A host in the IPv6 network sends an IPv6 packet to the device at the source end of

the tunnel.

2) After determining according to the routing table that the packet needs to be

forwarded through the tunnel, the device at the source end of the tunnel

encapsulates the IPv6 packet with an IPv4 header the and forwards it through the

physical interface of the tunnel.

3) The encapsulated packet goes through the tunnel to reach the device at the

destination end of the tunnel. The device at the destination end decapsulates the

packet if the destination address of the encapsulated packet is the device itself.

4) The destination device forwards the packet according to the destination address in

the decapsulated IPv6 packet. If the destination address is the device itself, the

device forwards the IPv6 packet to the upper-layer protocol for processing.