IPv4-IPv6 Transition

The challenge fro an IPv6 is for its transition to be complete before IPv4 routing and addressing break.  The 2 transition requirements are the flexibility of deployment and the ability for IPv4 hosts to communicate with IPv6 hosts.  IPv6 hosts should be able to communicate with IPv4-only hosts globally.  

The first objective to the deployment strategy is to have great flexibility and to implement and maintain the interoperability between the two.  Features designed into an IPv6 have to be backwards compatible with IPv4.  

The second objective is to allow IPv6 to be deployed in a highly diffuse and incremental fashion, with few interdependencies. 

The third objective is that the transition should be easy for users, system administrators, network operators to understand and carry out.

Features include: 
-Incremental upgrade and deployment: individual IPv4 hosts and routers may be upgraded to IPv6 one at a time without any other hosts or routers to be upgraded at the same time. 
-Minimal upgrade dependencies: the only prerequisite is to upgrade DNS server to handle IPv6 address records.
-Easy addressing: When existing installed IPv4 hosts or routers are upgraded to IPv6, they may continue to use their existing address, they do not need to be assigned new addresses.
-Low startup costs: there is no or little work to prepare for the upgrade of IPv4 to IPv6. The mechanisms employed by the IPv6 transition mechanisms include:

• An IPv6 addressing structure that embeds IPv4 addresses within IPv6 addresses, and encodes other information used by the transition mechanisms.  IPv6 nodes are assigned IPv6 unicast addresses that carry an IPv4 address in the low order 32-bits. This type of address has the format:

| 80 bits                                   | 16   | 32 bits                 |
+-----------------------------+--------------------------+
|0000..............................0000|0000| IPV4 ADDRESS |
+----------------------------+----+---------------------+

• A model of deployment where all hosts and routers upgraded to IPv6 in the early transition phase are "dual" capable (i.e. implement complete IPv4 and IPv6 protocol stacks).

• The technique of encapsulating IPv6 packets within IPv4 headers to carry them over segments of the end-to-end path where the routers have not yet been upgraded to IPv6.

• The header translation technique to allow the eventual introduction of routing topologies that route only IPv6 traffic, and the deployment of hosts that support only IPv6. Use of this technique is optional, and would be used in the later phase of transition if it is used at all.

The gradual upgrade features of IPv6 transition mechanisms allow the host and router vendors to integrate IPv6 into their product lines at their own pace, and allows the end users and network operators to deploy IPv6 on their own schedules.