6to4

6to4 (sometimes written " 6 to 4") is a system allowing packages IPv6 to be transmitted on a network IPv4. 6to4 is useful when two hosts wish to exchange information in IPv6 but that a portion of the network which separates them support only IPv4.

6to4 can be used by a computer alone or a IPv6 lan. Used by a computer alone, this last must imperatively have a IPv4 connection and a IPv4 address. It is then responsible for the encapsulation of the outgoing IPv6 packages and the decapsulation of the entering IPv6 packages. When 6to4 is used by a lan, the whole of the network does not require that of only one address IPv4 and a computer will be used as footbridge. Inside the network, the hosts recover their IPv6 beams and tables of routings by using their traditional protocols of identification, just as if they were on a IPv6 network.

6to4 does not allow the interoperation between the hosts only IPv4 and the hosts only IPv6.

Operation of 6to4

6to4 carries out 3 tasks:

Assigns a block of addresses IPv6 with any host or network which has an address IPv4
Encapsule the IPv6 packages inside IPv4 packages in order to allow their transition on a network IPv4
Effectue the transit from the traffic between 6to4 and the networks IPv6 " natifs" (acts like a router)

Assignment of blocks of addresses

For any total address 32-bit IPv4 which is assigned with a host or a network, a prefix 48-bit 6to4 IPv6 can be built or used by this host or network by prefixing 2002 (hex) at its IPv4 address. Thus for the total IPv4 address 207.142.131.202 , the prefix 6to4 correspondent would be 2002: CF8E: 83CA:: /48 (the IPv4 addresses are generally noted into decimal while the IPv6 addresses are it in Hexadécimal). Since the IPv6 addresses are on 128 bits and that 6to4 provides a prefix of 48 bits, 6to4 authorizes more 2⁸⁰ IPv6 hosts on a network to communicate with other IPv6 hosts, even if only external connection uses IPv4 and that there is not that only one IPv4 address.

Any IPv6 address which begins with 2002:: /16 is thus recognized as an address 6to4, in opposition to a native address IPv6 which will not use this prefix.

Encapsulation and transmission

6to4 embeds a IPv6 package in the data field (payload) of a IPv4 package with protocol of the type 41. To send a IPv6 package on a IPv4 network towards a TO address 6to4, a IPv4 heading with type of protocol 41 is prefixed with the IPv6 package.

Address IPv4 to for the heading of package prefixed is derived from the IPv6 address of destination which is inside the IPv6 package, by extracting the 32 bits which immediately follow the heading of TO address IPv6 (2002:: /16). The address IPv4 source in the heading of IPv4 package prefixed is the IPv4 address of the host or router who sends the package on the IPv4 network.

Resulting the IPv4 package is directed towards its IPv4 destination like any IPv4 package.

Routing between 6to4 and network IPv6 natives

To allow the hosts and the networks using of the addresses 6to4 to exchange data with the hosts nativement using IPv6 addresses, routers relay (" relay routers") were set up. A router relay connects himself to a IPv4 network and a IPv6 network. The packages 6to4 arriving on the IPv4 interface will have their IPv6 data directed on the IPv6 network, while the packages IPv6 newcomers on the IPv6 interface with a TO address starting with 2002:: /16 will be encapsulated and retransmis on the IPv4 network.

To make it possible a router 6to4 to communicate with Internet native IPv6, it must imperatively have its access point per defect pointing towards the IPv4 address of a router relay 6to4. In order to avoid with the users the need to establish this manually, the address anycast 192.88.99.1 (who, coated in 6to4 with the mask with sub-network to 0 becomes 2002: c058: 6301::) was allocated for the sending of packages to a router relay. For reasons of routing, the entirety of 192.88.99.0 /24 was allocated for roads leading towards routers relays 6to4 who use IP anycast. The suppliers of access wishing to provide this service to their customers or pars on the IP anycast 6to4 can inform of the prefix Anycast like any prefix IP.

The packages forwarding of the Internet IPv6 to systems 6to4 must be sent to a router relay 6to4 by methods of routing IPv6 normals. The specification establishes that the routers relay should inform only of the roads for the 2002:: /16 and not for the subdivisions so as to prevent that the IPv4 roads do not come to pollute the tables of the IPv6 routers. From what, they can be transmitted through the Internet IPv4 towards their destination.

References

B. Carpenter & K. Moore. Connection off IPv6 Domains via IPv4 Clouds . RFC 3056, February 2001.
R. Gilligan & E. Nordmark. Transition Mechanisms for IPv6 Hosts and Routers . RFC 2893, August 2000.
Christian Huitema. Year Anycast Prefix for 6to4 Relay Routers . RFC 3068, June 2001.
P. Savola & C. Patel. Security Considerations for 6to4 . RFC 3964, December 2004.

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