IPv6
IPv6
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IPv6 Transition Technologies
by Kaushik Das

Here is a general summary of several IPv6 transition technologies:

6 to 4

RFC 3056 details this IPv6 transition technology. This mechanism allows IPv6 sites to communicate with each other over the IPv4 network without explicit tunnel setup. These IPv6 sites communicate with native IPv6 domains via relay routers. Effectively it treats the wide area IPv4 network as a unicast point-to-point link layer.

The main advantage of 6 to 4 (not to be confused with 6 over 4) is that it requires no end-node reconfiguration and minimal router configuration. This mechanism is intended as a start-up transition tool used during the period of co-existence of IPv4 and IPv6. It is not intended as a permanent solution.
  IPv6

Using 6 to 4, IPv6 sites or hosts do not require IPv4-compatible IPv6 addresses or configured tunnels. Therefore, IPv6 gains considerable independence of the underlying wide area network and can step over many hops of IPv4 subnets.

The 6 to 4 mechanism is typically implemented almost entirely in border routers (edge device), without specific host modifications except a suggested address selection default. Only a modest amount of router configuration is required. The 6 to 4 works well when a 6 to 4 router exists at the edge of the site.
 
IPv6

Tunnelling and Tunnel Brokers

In networking, tunnelling protocol implies enabling new networking functions while still preserving the underlying network as is. There may be several reasons why a network needs tunnelling, for example, to carry a payload over an incompatible delivery network, or to provide a secure path through an untrusted network. IPv6 tunneling enables IPv6 hosts and routers to connect with other IPv6 hosts and routers over the existing IPv4 Internet. The main purpose of IPv6 tunneling is to deploy IPv6 as well as maintain compatibility with large existing base of IPv4 hosts and routers. IPv6 tunneling encapsulates IPv6 datagrams within IPv4 packets. The encapsulated packets travel across an IPv4 Internet until they reach their destination host or router. The IPv6-aware host or router decapsulates the IPv6 datagrams, forwarding them as needed.

IPv6   A tunnel broker is essentially a service that provides a network tunnel. Using these tunnels, encapsulated connectivity is provided over existing infrastructure to a new infrastructure. RFC 3053 defines a very common tunnel broker, called IPv6 tunnel broker.

This tunnel Broker has the capability to tunnel IPv6 packages to over IPv4. In order to support tunnel broker, the OS must have IPv4/IPv6 dual stack and should be capable of establishing IPv6 in IPv4 tunnels. An user can see tunnel brokers as virtual IPv6 ISPs, providing IPv6 connectivity to users already connected to the IPv4 Internet.

Teredo

Teredo, another IPv6 transition technology that provides address assignment and host-to-host automatic tunneling for unicast IPv6 traffic when IPv6/IPv4 hosts are located behind one or multiple IPv4 network address translators (NATs). To traverse IPv4 NATs, IPv6 packets are sent as IPv4-based User Datagram Protocol (UDP) messages. IPv6 traffic from Teredo hosts can flow across NATs because it is sent as an IPv4 UDP message. Note that The main benefit of Teredo is that its a NAT traversal technology for IPv6 traffic. If the NAT supports UDP port translation, then the NAT supports Teredo.

Teredo is losing its importance for designed for IPv6 connectivity. If native IPv6, 6to4, or Intrasite Automatic Tunnel Addressing Protocol (ISATAP) connectivity is present, the host does not act as a Teredo client. As more IPv4 edge devices are upgraded to support 6to4 and IPv6 connectivity becomes ubiquitous,

ISATAP

ISATAP refers to Intra-Site Automatic Tunnel Addressing Protocol; another IPv6 transition mechanism for transmitting IPv6 packets over IPv4 network. The word "automatic" signifies that once an ISATAP server/router has been set up only the clients must be configured to connect to it. This solution enables enterprises to deploy a simple and manageable IPv6 within their infrastructure with little time and effort.

Another advantage is that, within a site, usually only one ISATAP router is needed. The host/router functioning as an ISATAP server should be dualstack and have a connection to the IPv6 internet in order for it to become a gateway for all clients in the ISATAP subnet it serves.
 
IPv6

Using ISATAP, an user can connect isolated IPv6/IPv4-dualstack hosts to the IPv6 internet. Whereas this protocol doesnt require that the IPv4 infrastructure supports multicast, it (ISATAP) uses IPv4 as non-broadcast multiple access data link layer and performs neighbor discovery on top of IPv4. As mentioned in RFC 4214 Dual-stack (IPv6/IPv4) nodes use ISATAP to automatically tunnel IPv6 packets in IPv4, i.e., ISATAPviews the IPv4 network as a link layer for IPv6 and view other nodes on the network as potential IPv6 hosts/routers.

Dual stack configuration

Out of several mechanism that are mentioned in RFC 2893, one important mechanism is to allow IPv6 hosts and routers to be implemented in such a way that these hosts/routers are compatible/and co-exist with IPv4 hosts and routers. This mechanism is called dual IP layer (or dual stack); in this technique both Internet protocols IPv4 and IPv6 are supported.