Modifications to Neighbor Discovery Messages and Options
Mobile IPv6 defines the following changes to Neighbor Discovery (ND) messages and options:
- Modified Router Advertisement message
- Modified Prefix Information option
- New Advertisement Interval option
- New Home Agent Information option
Modifications to the Router Advertisement Message
Mobile IPv6 defines an additional flag in the Router Advertisement message to help facilitate home agent discovery by the home agents on a home link. The new flag, known as the Home Agent (H) flag, indicates whether the advertising router is capable of being a home agent. Each of the home agents on the home link sets this flag when it sends its router advertisements, and each home agent receives each router advertisement. Therefore, each home agent can compile the list of possible home agents.
Figure below shows the structure of the modified Router Advertisement message.
Additionally, Mobile IPv6 allows a router advertisement to be sent more frequently than every three seconds, as specified in RFC 4861. By sending router advertisements more frequently, mobile nodes can use a newly received router advertisement to detect movement to a foreign link more quickly. Recommended values for the pseudo-periodic router advertisement process for routers that might provide connectivity for mobile IPv6 nodes are a minimum of 0.03 seconds and a maximum of 0.07 seconds.
Modified Prefix Information Option
To indicate the global address of the advertising router, Mobile IPv6 defines an additional flag and a redefined use of the Prefix field in the Prefix Information option.
Figure below shows the structure of the modified Prefix Information option.
According to RFC 4861, router advertisements are sent from the link-local address. However, the global address for a home agent must be indicated in the router advertisement it sends so that each home agent can compile a list of home agents. Mobile IPv6 defines the Router Address (R) flag in the Prefix Information option. When set to 1, the R flag indicates to the receiver that the Prefix field contains the global address of the advertising router. In the originally defined Prefix field, the high-order bits corresponding to the value of the Prefix Length field are set to the appropriate values for the advertised prefix and the bits beyond the indicated prefix length are set to 0. With this new definition, the Prefix Length field is advertised in the same way, only the Prefix field contains the entire 128-bit global address of the advertising router.
Advertisement Interval Option
The Advertisement Interval option is sent in Router Advertisement messages to specify how often the router sends unsolicited multicast router advertisements. A mobile node that receives a router advertisement with the Advertisement Interval option can use the advertisement interval to detect whether it has moved to another link.
Figure below shows the structure of the Advertisement Interval option.
The Advertisement Interval option contains a 32-bit field that indicates the maximum number of milliseconds between consecutive unsolicited multicast Router Advertisement messages sent by the router using the pseudo-periodic advertising scheme.
Home Agent Information Option
The Home Agent Information option is sent in Router Advertisement messages sent by a home agent to specify the home agent's configuration. Figure below shows the structure of the Home Agent Information option.
Included in the Home Agent Information option are the home agent preference (a number indicating a preference level for the advertising router to be a home agent) and the home agent lifetime (how long the home agent is acting as a home agent).
The home agents on a home link use the home agent preference values to order the list of home agents that are sent to the mobile node during home agent address discovery.
