Part one of this article described how both Mobile IPv4 and IPv6 are similar in the way connections are initiated. This article will describe how IPv6 provides increased efficiency and scalability.
In part one I described how, when using Mobile IPv4, packets traveling from the mobile node to the correspondent node must pass through the home agent. In IPv6, the mobile node can send packets directly to the correspondent node. It sends a message called a binding update to the correspondent node to inform it of the mobile node's current care-of-address. From that point on, traffic between the mobile node and correspondent node travels directly with no need to pass through the home agent. When the mobile node moves to another foreign network, it sends another binding update to the correspondent node. It also sends binding updates to the home agent to enable any new correspondent nodes to initiate communication.
Scalability and Reliability
Direct communication between mobile nodes and correspondent nodes makes Mobile IPv6 a scalable, reliable protocol for supporting very large numbers of mobile nodes. With Mobile IPv4, all packets must first go to the home agent and then to the mobile node. It is possible to modify IPv4 nodes to respond to binding updates and communicate directly with a mobile node, but only modified nodes would be able to take advantage of the optimization. Mobile operation is an integral part of IPv6, so every IPv6 node can communicate with mobile nodes.
An IPv4 service provider supporting hundreds of thousands of VoIP cell phones would need to maintain a very large number of home nodes and very high network bandwidth to support thousands of simultaneous connections. Any disruption of the home network would impact potentially thousands of connections.
Anycast, Multicast and Auto-Configuration
The anycast, multicast and auto-configuration facilities built into IPv6 provide additional efficiencies for mobile operation. A mobile node acquires a home agent by sending a binding update to the home agent anycast address on the home network. One home agent receives the request and responds with a prioritized list of home agents on that network. The mobile node selects the highest priority home agent. The order of the list changes as the loads on the home agents change, providing a mechanism for load sharing. The selected home agent multicasts a Neighbor Discover packet onto the home network to direct incoming packets addressed to the mobile node's home address to the home agent.
A mobile node uses the auto-configuration facilities in IPv6 to acquire a care-of-address without the need for DHCP on the foreign network; so no need to configure and maintain DHCP on the foreign network.
An IPv6 router periodically multicasts an advertisement containing a list of the network prefixes it supports. A mobile node detects that it has moved into the range of a different foreign network when it receives a router advertisement with a different set of prefixes.
Security is essential for mobile communications. IPSec is an integral part of IPv6. In addition to it protecting the content of messages, IPSec is used to protect binding updates to prevent an attacker from intercepting them or sending false updates that would cause packets to be misdirected.
When IPv4 was developed in the 1970's, no one could have predicted mobile network nodes. IPv6 development began in response to IPv4's limited address space, but the developers resolved to use the opportunity to design a protocol that would meet the needs of the future. The primary goal was expansion of address space, but the developers also understood the need for efficient support for mobile nodes. IPv6 has been carefully designed to provide that support.
More information about IPv6 Mobility can be found in RFC 3775 .
David B. Jacobs has more than twenty years of networking industry experience. He has managed leading-edge software development projects and consulted to Fortune 500 companies as well as software start-ups.