As internet is now evolving itself into IPv6 protocol addressing at IP layer, Mobile IP is also evolving itself accordingly. IPv6 is the future of internet. As IPv6 has almost an infinite addressing space, it will increase internet users enormously. Obviously, mobile users will also increase accordingly. So, Mobile IPv6 must also be enough capable to bear such large amount of mobile users.

Although, the underlying mechanism is same in IPv6 but all protocols must adapt themselves according to new enhancements and mechanisms that IPv6 provide.

Now what changes will be made to Mobile IPv4 to make it Mobile IPv6.

Mobile IPv6 is integral part of IPv6
Mobile IPv4 was a protocol which used IPv4 at IP Layer. Its messages and signaling was dependent on UDP protocol at transport layer. On the contrary, Mobile IPv6 is itself nothing. It is a part of IPv6 protocol. So, a node which has the capability to work with IPv6 networks has also got the capability of being mobile because all the signaling required is the provided by IPv6 instead of separate UDP packets.

Absence of Foreign Agent
Foreign Agent plays a very important role in IPv4 Mobile IP communications. Its duty is to provide Care of Address to the mobile node. It receives all the traffic of Correspondent Node through Home Agent, decapsulates it, and send it to Mobile Node.

This Foreign Agent is completely removed in Mobile IPv6. So the first change which will occur after transitioning to Mobile IPv6 is that Mobile Node must know that there is Foreign Agent available to provide it a Care of Address. So, a Mobile IPv6 complaint node can neither discover that its network is changed nor it can ask for its Care of Address with simple Agent Discovery Message.

IPv6 Neighbor Discovery Protocol instead of Gratuitous ARP messages
In IPv6 movement detection automatically happens with the help of IPv6 Neighbor Discovery protocol. It allows a Mobile Node to learn if its current network is changed. It also allows Mobile Node to discover current network settings. After detecting that the network has been changed, Mobile Node gets an IP Address through IPv6 auto-configuration. After getting IP address Mobile Node registers directly with its Home Agent. So, no foreign agent comes into action.

In IPv4, the Home Agent sends gratuitous ARP messages on the home subnet. This is to ensure that packets destined for the Mobile Node are received by the Home Agent, which tunnels them to the mobile node.

On the contrary, this entire problem is solved by IPv6 Neighbor Discovery Protocol. ARP is a Ethernet protocol and IPv6 is IPv6 own protocol.

Provision for every Mobile Node to have its own Care of Address
In Mobile IPv6 foreign agent has been removed because of multiple reasons. The main reason is that, the IPv4 does not have a large address space. This makes it unpractical for Mobile Nodes to have multiple addresses. That is why; they use Foreign Agent’s address as a Care of Address. This problem is removed in IPv6 as it provides infinite addressing space.
Decapsulation facility at Mobile Node.

Another change due to unavailability of Foreign Agent in Mobile IPv6 scenario is that, the mobile node must have the capability to decapsulate the packet from Home Agent. In Mobile IPv4 it was decapsulated by Foreign Agent. Only a single packet use to come on Foreign Network. But now complete encapsulated packet will traverse to Mobile Node. It is the duty of Mobile Node to remove the tunnel header.

Hierarchical Mobile IPv6 (HMIPv6)
When a Mobile Node moves from its Home Network to a Foreign Network, control messages are passed between Mobile Node and Home Agent to update routing of its data. Until the update occurs, there is a time period, during which many packets are sent to old location of Mobile Node. The number of such packets directly depends on the latency of update.

More than this, if the route optimization of IPv6 is being used then Mobile Node will have to tell every Corresponding Node about its new location. This increases the update time and hence increases the number of dropped packets.

To avoid the mentioned problem Mobile IPv6 introduces another concept known as Hierarchical Mobile IPv6. In this scenario a Mobility Anchor Point (MAP) router is provided which hides the movement of mobile node within a network’s own subnets from Corresponding Node. This reduces number of signaling messages between Mobile Node and its Corresponding Node.

This means a Mobile Node must know how to utilize Hierarchical facility of current network. When a Mobile Node enters a network it first learns weather current network supports HMIPv6. If it finds that HMIPv6 is present then it will use MAP router’s address as its Care of Address.

Route Optimization as an integral component
The routing path between Mobile Node and Corresponding Node can be optimized so that they can directly talk to each other. Same facility is available in Mobile IPv4 but there are many differences.

Firstly, in Mobile IPv6 this facility is available in IPv6 protocol itself. IPv4 itself does not support such optimizations.

Secondly, in Mobile IPv6, Mobile Node and Correspondent Node decide whether they want to enable an optimal path and Home Agent decides whether Route Optimization is allowed or not.

Thirdly, in Mobile IPv6 uses new routing header and destination options instead of using tunneling which was used in IPv4 for traffic between Mobile Node and Correspondent Node.

For the traffic arriving from Mobile Node and arriving at the Corresponding Node, the source IP address is the Care of Address and the destination IP address is the correspondent Node address. These source and destination addresses are reversed for traffic from the Correspondent Node to the Mobile Node. So, Correspondent Node will send all packets on Care of Address of Mobile Node.

In original internet scenario, this will not work due to ingress firewalls and routing issues. To solve this issue IPv6 give us its own facility in its IP header. It provides an opportunity to place Mobile Node’s original Home Address in the header. The sender stores the Mobile Node's Home Address in the IP header of the packet and sends packet to the recipient. When the packet is received on the Correspondent Node, it replaced the source IP address by this Home Address so that an application working at application layer believes that the peer is the Mobile Node on its Home Network. Just like that, in other direction, when Mobile Node receives a packet, the destination address is replaced by this Home Address.
No more Tunneling overhead
As mentioned in Route Optimization, IPv6 does not tunnel the data while using Route Optimization.

In IPv4, tunneling is used. Tunneling uses a new IP Header in the packet hence the size of packet grows due to additional IP Headers. Due to this, sometimes, fragmentation is required. When a new header is added the frame overall grows and become larger than the present MTU of the network. This puts the overhead of de-fragmentation and reassembly which adds latency. Secondly, larger packet goes on the network which wastes bandwidth.

In IPv6, a new routing header and destination options is used. Instead of appending entirely a new IP Header in present IP datagram, it simply adds Mobile Node Address through routing header. This adds fewer bits into the original packet. Less bandwidth is used and no de-fragmentation and reassembly is required.
Different security mechanisms
As we know that IPv4 itself does not support any security mechanisms. So in Mobile IPv4, third party security protocols like IPSec and Encapsulating Security Protocol are used. But, as mentioned, IPv4 does not support them by itself, so, it is possible that one of the Mobile Node of Correspondent Node or both may not support these protocols. Hence, they cannot be taken as standard security protocols.

In contrast, IPv6 itself supports IPSec. So theoretically, all nodes on IPv6 have IPsec implemented in them.
IPv6 auto-configuration
In Mobile IPv4 the Mobile Node broadcasts the famous RRQ message. This message is termed as a subnet broadcast message. The Home Agent of that subnet responds with RRP message to inform Mobile Node about its IP Address.

This makes Mobile IPv4 configuration a long and tedious task.

On the other hand, IPv6 supports auto-configuration feature in its nodes. When a IPv6 Mobile Node enters in a network it receives the router advertisement, it auto-configures its IP address accordingly. Nothing more is required! This happens because of the capability of IPv6 nodes to learn their subnet prefix automatically. After learning network prefix Mobile Node appends its unique Message Authentication Code address to obtain routable IP address on the subnet. This mechanism is used along with Duplicate Avoidance Technique to avoid duplicate addresses. This method is a lot faster then the IPv4 traditional DHCP methods.
Introduction of new Control Messages
All nodes must be capable of understanding new control messages like:

Home test init (HoTI) message
Care of test init (CoTI) message
Home test (HoT) message
Care of test (CoT) message
Binding refresh request
Binding error message
Home address destination option
Type 2 routing header option
Binding authorization data option

These messages are required for IPv6 Route Optimization between Mobile Node and Correspondent Node.
Anycast Dynamic Home Agent Discovery
IPv6 also supports Mobile IPv4 Dynamic Home Agent Resolution method in which ICMP message is used to discover Home Agent. But the difference is that this message is sent to an anycast address. On the contrary, in IPv4 uses subnet directed broadcast.
AAA Infrastructure
Deployment of IPv4 was always suffered due to its difficult configuration. The need of client node configuration makes it tedious and overwhelming to implement.

On the contrary, IPv6 uses boot strap, auto-configuration mechanism to configure Home Agent address, security association with the Home Agent etc using AAA infrastructure. AAA server is a separate entity. It is also supported in Mobile IPv4 extensions. But it is the part of Mobile IPv6 itself.

In AAA Infrastructure, Mobile Node sends an extended RRQ message to AAA server to obtain its key. AAA server recognizes the key request and generates a key. It offers a key
in the reply to the Foreign Agent, Home Agent and Mobile Node. The Foreign Agent and Home Agent transfer the AAA information to extensions in RRP message.