In this series of six articles, the authors of “Building the Mobile Internet“ provide a tutorial on
extending Internet connectivity into mobile networking by using extensions of
protocols such as IPv4 and IPv6 as well as mobile specific protocols such as
DSMIP, IKEv2 and MoBIKE. Part 4: Mobile IPv6 technology overview.
Mobile IPv6 is standardized in RFC 3775. Much like Mobile IPv4 discussed earlier in Part 1, Part 2 and Part 3, Mobile IPv6 provides transparent mobility support for mobile nodes communicating across IPv6 networks.
Mobile IPv6 shares many of the same features and capabilities as Mobile IPv4 while leveraging the advantages that the IPv6 protocol itself provides. The major differences between Mobile IPv4 and Mobile IPv6 include the following:
1) Mobile IPv4 foreign agents provide local mobility agent function for a mobile node that has roamed into a foreign network. Mobile IPv6 does not require a local mobility anchor, so no foreign agent exists in a Mobile IPv6 network.
2) Because no foreign agent exists, route optimization and reverse tunneling options are not required for Mobile IPv6. The Mobile IPv6 route optimization capability allows the Mobile IPv6 protocol to coexist with ingress filtering devices located at border gateways.
3) The IPv6 protocol supports neighbor unreachability. This detection can be used in Mobile IPv6 to assure symmetric routing between the mobile node and its default router in the foreign network.
4) Rather than using IP in IP or other encapsulation techniques, the majority of traffic sent to a mobile node is done so using the IPv6 routing header.
5) Mobile IPv6 does not create the same challenges with link layer interactions. Instead of relying on ARP, Mobile IPv6 relies on IPv6 neighbor discovery.
Mobile IPv6 Operation
Mobile IPv6 operation is similar to that of Mobile IPv4. The mobile node is always reachable through its HoA, regardless of point of attachment. The mobile node registers its CoA with a home agent in the home network whenever it changes point of attachment within foreign networks.
While the binding between the mobile node’s CoA and home address allows any node corresponding with the mobile node to continue communication, traffic is routed in a nonoptimal manner.
Mobile IPv6 resolves this by allowing the correspondent node to participate in the Mobile IPv6 process. The two nodes (mobile and correspondent) communicate through two different methods-bidirectional tunneling and route optimization-as explained in the sections that follow.
(Note: As you will see later in this series, the operation of route optimization requires that the Correspondent Node supports additional Mobile IPv6 functionality.)
Bidirectional Tunneling Mode
Bidirectional tunneling mode does not require the correspondent node to support Mobile IPv6. In this mode, traffic is routed similarly to a reverse tunneling mode in Mobile IPv4. Figure 5-27 below illustrates bidirectional tunneling mode in Mobile IPv6.
Packets from the mobile node toward the correspondent node are tunneled to the home agent and then routed from the home network to the correspondent node.
on image to enlarge.
Figure 5-27. Mobile IPv6 bidirectional tunneling mode
Packets from the correspondent node are routed to the home agent and then tunneled to the mobile node. Bidirectional tunneling mode requires IPv6 neighbor discovery capability in the home agent.
IPV6 neighbor discovery, defined in RFC 2461, is an important function in Mobile IPv6. IPv6 neighbor discovery allows a network node to discover the link layer address of nodes residing on the same network and a network host to find a default router.
The IPv6 neighbor discovery capability allows Mobile IPv6 to function over any link layer technology and disassociates the home agent from taking part in any link layer communications.
When a mobile node moves outside the home network, it registers with the home agent. The home agent uses proxy neighbor discovery to notify link-adjacent nodes that all traffic destined for the mobile node should be sent to the home agent.