Section 11.1. Overview | IPv6 Essentials

11.1. Overview

Mobile IPv6 is a protocol that allows a mobile node to move from one network to another without loosing its connections. It is specified in RFC 3775.

Most Internet traffic uses TCP connections. A TCP connection is defined by the combination of IP address and port number of both endpoints of the communication. If one of these four numbers changes, the communication is disrupted and has to be reestablished. If a mobile node connects to a different access point to the network, it needs a new IP address. Mobile IP addresses the challenge of moving a node to a different connection point without changing its IP address by assigning the mobile node two different IP addresses. One is the home address, which is static and does not change, and it is therefore used to identify the TCP connection. The second IP address is called the care-of address. It changes depending on the network to which the node is currently attached. So this works within homogeneous networks (if the node moves from an Ethernet segment to another Ethernet segment) but also in heterogeneous networks (if the node moves from an Ethernet segment to a wireless LAN).

In a wireless network, we are familiar with the handover, the event where a device moves from one access point to another. This is a handover on the Link layer. Mobile IPv6 solves the handover issue on the Network layer and maintains connections to applications and services if a device changes its temporary IP address.

11.1.1. Mobile IPv6 Terms

Here comes the definition of some terms that are used throughout the chapter.

Home address: A global unicast address assigned to a mobile node. It is used as the permanent address for this node and is within the mobile node's home link. Regular routing mechanisms deliver packets to the home address of the mobile node.
Home subnet prefix: The IP subnet prefix corresponding to a mobile node's home address.
Home link: The link on which a mobile node's home subnet prefix is defined.
Mobile node: A node that can change its point of attachment from one link to another while still being reachable via its home address.
Correspondent node: A peer node with which a mobile node is communicating. The correspondent node may be either mobile or stationary.
Foreign subnet prefix: Any IP subnet prefix other than the mobile node's home subnet prefix.
Foreign link: Any link other than the mobile node's home link.
Care-of address: A global unicast address for the mobile node while it is in a foreign network (away from home). The subnet prefix of the care-of address is the foreign subnet prefix. A mobile node may have multiple care-of addresses. The one being registered with its home agent is the primary care-of address.
Home agent: A router on a mobile node's home link with which the mobile node has registered its current care-of address. While the mobile node is away from home, the home agent intercepts packets on the home link destined to the mobile node's home address, encapsulates them (IPv6 encapsulation), and tunnels them to the mobile node's registered care-of address.
Binding: The association of the home address of a mobile node with a care-of address for that mobile node, along with the remaining lifetime of that association.
Registration: The process during which a mobile node sends a Binding Update to its home agent or a correspondent node, causing a binding for the mobile node to be registered. The registration with the correspondent node is called Correspondent Registration.
Binding authorization: A registration with a correspondent node needs to be authorized to allow the recipient to ensure that the sender has the right to specify a new binding.
Return routability procedure: A procedure that authorizes registrations by the use of a cryptographic token exchange.
Keygen token: A number supplied by a correspondent node in the return routability procedure to enable the mobile node to compute the necessary binding management key for authorizing a Binding Update.
Nonce: Random numbers used internally by the correspondent node in the creation of keygen tokens related to the return routability procedure. The nonces are not specific to a mobile node and are kept secret within the correspondent node.
Nonce index: Used to indicate which nonces have been used when creating keygen token values without revealing the nonces themselves.

11.1.2. How Mobile IPv6 Works

Figure 11-1 shows the components of Mobile IPv6 and how they interact.

Figure 11-1. Overview Mobile IPv6

The home address is the IPv6 address within the home link prefix of a mobile node (MN). As long as the mobile node is at home, it receives packets through regular IP routing mechanisms and behaves like any other regular IP host. When the mobile node is away from home on a foreign link, it has an additional care-of address. It receives the care-of address through regular IPv6 mechanisms such as Stateless autoconfiguration or DHCPv6 when connecting to the new link.

The association of a home address and a care-of address is called a binding. While away from home, the mobile node registers its care-of address with a router on its home link, its home agent (HA). To register its care-of address, the mobile node sends a binding update message to the home agent. The home agent responds with a binding acknowledgement. Every node communicating with a mobile node is called a correspondent node (CN). Mobile nodes can also send registrations to the correspondent node directly (called a correspondent registration). A correspondent node can also be a mobile node.

There are two ways to communicate for a correspondent node and a mobile node:

Bidirectional Tunneling: Packets from the correspondent node are sent to the home agent, which encapsulates them in IPv6 and sends them to the care-of address of the mobile node. Packets from the mobile node are sent through the reverse tunnel to the home agent that forwards them to the correspondent node through regular routing mechanisms. This mode does not require any Mobile IPv6 support on the correspondent node and works without correspondent registration.
Route Optimization: With Route Optimization, the communication between mobile node and correspondent node can be direct without going through the home agent. This is one of the main advantages of Mobile IPv6 over Mobile IPv4, where route optimization is not possible. Route optimization requires that the mobile node registers its care-of address with the correspondent node (correspondent registration) and that this binding is authorized through the return routability procedure. The correspondent node uses a special routing header (type 2) when it sends packets to the mobile node directly. The mobile node uses the Home Address option (defined for Mobile IPv6) when sending packets to the correspondent node. The whole process is described in more detail later in the chapter.

The advantage of route optimization is that the shortest available path can be used between correspondent node and mobile node. The packets do not have to go through the home agent. This not only ensures shorter communication paths but also reduces the load on the home agent and the home link. This becomes very important when we talk about high numbers of mobile nodes constantly moving around, for instance in a VoIP (Voice over IP) scenario.

Mobile IPv6 also supports the option to have multiple home agents, and the mobile node can learn about reconfiguration of its home link or a change of IP address of its home agent through Dynamic Home Agent Address Discovery. If the prefix of its home link changes, the mobile node uses the Mobile Prefix Discovery mechanism to learn about the new prefix.

The following sections describe the protocol and new messages, options, and flags in more detail. After this, I dive into the communication flows that have just been described in an overview. Some people prefer to learn this way; other people prefer to learn about the processes and flows first and then about the technical details. Please read the sections in the order that best fits your preference.