One of the problems in Mobile IP version 4 is the asymmetric routing, also called triangle routing. The triangle routing is due to the fact that all packets going to the mobile first have to pass through the HA, while in the reverse direction packets are routed through regular IP routing. Route optimization [3, 13, 14] is the set of extensions to the basic Mobile IP protocol to allow more efficient routing procedures, so that the IP packets can be routed from a correspondent node to a mobile node without first passing through the HA. The home agent sends a binding update message with the COA of the mobile node to all correspondent nodes that require them. This binding is then stored by the correspondent node and is used to tunnel its own IP packets directly to the care-of address indicated in that binding. Thus, passing through the HA is avoided. In the initiation phase, however, the IP packets sent by the correspondent node still use the triangle routing until the moment that the binding update message sent by the HA is received by the correspondent node.
Other extensions are provided to allow IP packets that are sent by a correspondent node with an out-of-date stored binding, or in transit, to be forwarded
directly to the new COA of the MN. The authentication mechanisms used in route optimization are the same as those used in the basic version of Mobile IP. This authentication generally relies on a mobility security association established in advance between the
sender and receiver of such messages. The route optimization protocol operates in four steps:
1. A binding warning control message may be sent to the HA indicating
that the correspondent node is unaware of the new COA of the mobile
node.
2. A binding request message is sent by a correspondent node to the HA
when it determines that its binding should be refreshed.
3. An authenticated binding update message is sent by the HA to those
correspondent nodes that require them, containing the current COA
of the mobile node.
4. When smooth handoffs occur, the mobile node transmits a binding
update and has to be sure that the update has been received. Thereby,
it can request a binding acknowledgment from the correspondent
node.
The procedure of handoff in Mobile IPv4 When a mobile node attempts at undertaking a handoff from one foreign domain to another, it sends a deregistration message to the previous foreign agent (e.g., FA1). The mobile node can send a deregistration message to FA1 or
just make a handoff and let its connection with FA1 time out. After the mobile enters a new foreign network, it waits for an agent advertisement from a FA. As soon as the mobile node receives the advertisement, it sends registration request to the home agent using the address of the new foreign agent (FA2) as care-of address. The HA processes the request and sends back a registration reply.