Mobile IP Agent Discovery, and Agent Advertisement and Solicitation Messages
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Agent Advertisement and Agent Solicitation Messages
Provision already exists for exchanges of data between a router and a node, in the form of ICMP messages that are used for the regular IP router discovery process. Two messages are used for this purpose: Router Advertisement messages that let routers tell local nodes that they exist and describe their capabilities, and Router Solicitation messages that let a node prompt a router to send an advertisement. These are described in their own topic in the ICMP section.
Given the similarity to normal router discovery, it made sense to implement agent discovery as a modification to the existing process rather than set up a whole new system. The messages used in the agent discovery process are:
The use of these messages is described in the Mobile IP standard in detail, and unsurprisingly, is very similar to how regular Router Advertisement and Router Solicitation messages are employed. Agents are normally configured to send out Agent Advertisements on a regular basis, with the rate set to ensure reasonably fast contact with mobile nodes without consuming excessive network bandwidth. They are required to respond to any Agent Solicitation messages they receive by sending an Advertisement. It is possible that some agents may be configured to send Advertisements only upon receipt of a Solicitation.
Mobile nodes are required to accept and process Agent Advertisements. They distinguish these from regular Router Advertisements by looking at the size of the message. They then parse the extension(s) to learn the capabilities of the local agent. They determine whether they are on their home network or a foreign network, and in the case of a foreign agent, how the agent should be used. Mobile nodes are required to use Agent Advertisements to detect when they have moved, using one of two algorithms defined in the standard, and also to detect when they have returned back to their home network after they have been traveling. They are also required to be able to send Agent Solicitation messages if they don't receive an Agent Advertisement after a certain period of time. They are restricted to sending these only infrequently, however, again to keep traffic manageable.
Now let's look at the formats of the two message types.
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