Agent Advertisement and Agent Discovery are key processes in Mobile IP that enable mobile nodes to detect their network location and communicate with Home and Foreign Agents.
Agent Advertisement
Agent Advertisement is an extension of ICMP Router Advertisements that helps mobile nodes determine their network location. Home Agents (HA) and Foreign Agents (FA) periodically broadcast these messages into their physical subnets.
Key characteristics:
- Uses Extended ICMP Router Advertisements (type 9)
- Contains information about available Care-of Addresses (COAs)
- Includes several flag bits indicating agent capabilities:
- R: Registration required
- B: Busy, no more registrations
- H: Home agent
- F: Foreign agent
- M: Minimal encapsulation support
- G: GRE encapsulation support
- T: Foreign agent supports reverse tunneling
The advertisement message structure includes:
- Type and code fields
- Number of addresses being advertised
- Lifetime (validity period)
- Preference levels for each address
- Router addresses
- Available Care-of Addresses
Agent Discovery
Agent Discovery is the process through which a Mobile Node (MN) listens to Agent Advertisement messages to:
- Detect whether it’s in its home network or a foreign network
- Obtain a Care-of Address (COA) from Foreign Agent advertisements
- Determine the capabilities of available agents
- Initiate the registration process with appropriate agents
This discovery mechanism is essential for network integration in mobile environments as it allows mobile nodes to maintain connectivity while moving between different networks without changing their IP addresses.
After discovery and obtaining a COA, the mobile node can register with its Home Agent (with a limited lifetime), enabling proper packet routing regardless of the node’s physical location.