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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Mobility Support (Mobile IP)

Mobile IP Addressing: Home and "Care-Of" Addresses 1 2 3 4 5 Mobile IP Home Agent Registration and Registration Messages

Agent Solicitation Message Format

Let’s start with the Agent Solicitation message because it is much simpler. In fact, there is no new message format defined for this at all; it is identical to the format of a Router Solicitation message.

The reason no new message type is required here is that a solicitation is an extremely simple message: “hey, if there are any routers out there, please tell me who you are and what you can do”. No extra Mobile IP information needs to be sent. When a regular IP router receives a Router Solicitation it will send a Router Advertisement, but a Mobile IP router automatically sends the longer Agent Advertisement instead when prompted by any solicitation, whether it comes from a Mobile IP node or a regular IP device.

The Agent Advertisement begins with the normal fields of an ICMP Router Advertisement message. The destination of the message is either the “all devices” multicast address (224.0.0.1) if multicast is supported on the local network, or the broadcast address (255.255.255.255) otherwise. The Router Address fields are filled in with the address(es) of the agent.

Following the regular fields, one or more extensions are added to the message format. There are three extensions defined: the Mobility Agent Advertisement Extension, Prefix-Lengths Extension, and One-Byte Padding Extension.

Mobile IP Addressing: Home and "Care-Of" Addresses 1 2 3 4 5 Mobile IP Home Agent Registration and Registration Messages