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Mobile IP Agent Discovery, and Agent Advertisement and Solicitation Messages
(Page 3 of 5)
Agent Solicitation Message Format
Lets 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
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
Agent Advertisement Message Format
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 (188.8.131.52)
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.
Note: It is possible that a device may wish to advertise its ability to handle Mobile IP messages, but not act as a regular router. In this case it changes the normal Code field in the header of the Router Advertisement message from 0 to 16.
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.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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