Mobile IP Home Agent Registration and Registration Messages
(Page 2 of 4)
There are two different procedures defined for registration, depending on the type of care-of address used by the mobile node and other specifics we will get into shortly. The first is the direct registration method, which has just two steps:
In some cases, however, a slightly more complex process is required, where the foreign agent conveys messages between the home agent and the mobile node. In this situation, the process has four steps:
The first, simpler method is normally used when a mobile node is using a co-located care-of address. In that situation, the node can easily communicate directly with the home agent, and the mobile node is also set up to directly receive information and datagrams from the home agent. When there is no foreign agent, this is obviously the method that must be used. It is also obviously the method used when a mobile node is de-registering with its home agent after it arrives back on the home network.
The second method is required when a mobile node is using a foreign care-of address. Recall that in this situation, the mobile node doesn't have its own unique IP address at all; it is using a shared address given it by the foreign agent, which precludes direct communication between the node and the home agent. Also, if a mobile node receives an Agent Advertisement with the R flag set, it also should go through the foreign agent, even if it has a co-located care-of address.
Note that the foreign agent really is just a middleman; the exchange is still really between the home agent and the mobile node. However, the foreign agent can deny registration if the request violates whatever rules are in place for using the foreign network. It is for this reason that some foreign agents may require that they be the conduit for registrations even if the mobile node has a co-located care-of address. Of course, if the foreign agent can't contact the home agent the registration will not be able to proceed.
The description above is really a highly simplified explanation of the basics of registration. The Mobile IP standard specifies many more details on exactly how agents and nodes perform registration, including particulars on when requests and replies are sent, how to handle various special conditions such as invalid requests, rules for how home agents maintain a table of mobility bindings, and much more. The standard covers the definition of extensions to the regular registration messages to support authentication, which is required for secure communications (see the topic on security issues for more details). It also includes the ability to have a mobile node maintain more than one concurrent binding, when needed.
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