Mobility under IPv6

A new version of the Internet Protocol, IPv6, is being developed with 128-bit addresses. IPv6 remedies perceived flaws in the existing version of IP (that is, IPv4). Mobile computers are likely to account for a substantial fraction of the population of the Internet during the lifetime of IPv6. The development of IPv6 presents a rare opportunity, in that there is no existing installed base of IPv6 hosts or routers with which compatibility must be maintained. All IPv6 nodes may be assumed to perform the few operations needed to support Internet-wide mobility. The most important function needed to support mobility is the reliable and timely notification of a mobile node's current location to those other nodes that need it. The home agent needs this location information in order to redirect packets from the home network to the mobile node. Correspondent nodes need this information in order to send their own packets directly to the mobile node.

The IPv6 Standards Process

It is important to recognize that IPv6 is rapidly evolving. The current base specification for IPv6 is an ``Internet Draft." Internet Drafts are draft documents that may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet Drafts as reference material or to cite them other than as ``work in progress." Therefore, this entire section should be read with the understanding that it provides only a snapshot as of June 1996.

Overview of Mobility Support in IPv6

From the model of operation enabling mobile networking for IPv4, the authors of the Mobile IPv6 draft [PERK96] borrow the concepts of home network, home address, home agent, care-of address, and binding. Mobile computers are assigned (at least) two IPv6 addresses whenever they are roaming away from their home network. One (the home address) is permanent; the other (the IPv6 link-local address) is used temporarily. In addition, the mobile node will typically autoconfigure figure a globally-routable address at each new point of attachment. Every IPv6 router supports encapsulation, so every router is capable of serving as a home agent on the network(s) to which it is attached. Using IPv4 terminology, the basic model of operation in IPv6 assumes that mobile node can always be reached by sending packets to its home (permanent) address. Whenever the mobile node is not present on its home network, packets arriving for it there will be intercepted by the home agent, and tunneled to a care-of address. Care-of addresses can be constructed by the mobile node using the methods of automatic address configuration. If the mobile node receives router advertisements, it must use automatic address configuration to construct a globally unique, routable address. This routable address can be used by the mobile node as its care-of address. After determining its care-of address, a mobile node must send a binding update containing that care-of address to the home agent (and any other correspondent nodes that may have out-of-date bindings in their binding cache). By default, correspondent nodes send packets to mobile nodes by using routing headers instead of encapsulation. As detailed in the next section, correspondent nodes are usually expected to deliver packets directly to the mobile node's care-of address, so that the home agent is rarely involved with packet transmission to the mobile node. It is essential for scalability and minimizing network load that correspondent nodes be able to learn the care-of address for a mobile node, and to be able to cache this information for use in sending future packets to the mobile node's care-of address. By caching the care-of address of a mobile node, optimal routing of packets can be achieved between the correspondent node and the mobile node. Routing packets directly to the mobile node's care-of address also eliminates congestion at the home agent and thus contributes significantly to the overall health of the Internet. Moreover, many communication events between mobile nodes and correspondent nodes can be carried out with no assistance from the home agent. Thus, the impact of failure at the home agent can be drastically reduced; this is important because many administrative domains will have a single home agent to serve a particular home network, and thus a single point of failure for communications to nodes using that home agent. Communications between the home agent and a mobile node may depend on a number of intervening networks. Thus, there are many more ways that packets can fail to reach a mobile node when the home agent is required as an intermediate node. This would be particularly relevant on, say, trans-oceanic links between home agent and mobile node. Caching the binding of a mobile node at the correspondent node enables communication with the mobile nodes even if the home agent fails or is difficult to contact over the Internet. In the typical case, when a mobile node has configured its care-of address at one of its own interfaces, transferring data to the mobile node means no more work for routers on the link at its current point of attachment, than transferring data to any other node on that link. This improves performance further.