Entities and terminology:
The following defines several entities and terms needed to understand mobile IP as defined in RFC 3344.
Mobile Node (MN):
A mobile node is an end-system or router that can change its point of attachment to the internet using mobile IP.
The MN keeps its IP address and can continuously communicate with any other system in the internet as long as link-layer connectivity is given. Examples are laptop, mobile phone, router on an aircraft etc.
Correspondent node (CN):
- At least one partner is needed for communication. In the following the CN represents this partner for the MN.
- The CN can be a fixed or mobile node.
The home network is the subnet the MN belongs to with respect to its IP address. No mobile IP support is needed within the home network.
The foreign network is the current subnet the MN visits and which is not the home network.
Foreign agent (FA):
The FA can provide several services to the MN during its visit to the foreign network.
The FA can have the COA, acting as tunnel endpoint and forwarding packets to the MN.
The FA can be the default router for the MN.
FAs can also provide security services because they belong to the foreign network as opposed to the MN which is only visiting.
FA is implemented on a router for the subnet the MN attaches to.
Care-of address (COA):
The COA defines the current location of the MN from an IP point of view.
All IP packets sent to the MN are delivered to the COA, not directly to the IP address of the MN.
Packet delivery toward the MN is done using a tunnel, i.e., the COA marks the tunnel endpoint, i.e., the address where packets exit the tunnel.
There are two different possibilities for the location of the COA:
a. Foreign agent COA:
The COA could be located at the FA, i.e., the COA is an IP address of the FA.
The FA is the tunnel end-point and forwards packets to the MN. Many MN using the FA can share this COA as common COA.
b. Co-located COA:
The COA is co-located if the MN temporarily acquired an additional IP address which acts as COA.
This address is now topologically correct, and the tunnel endpoint is at the MN. Co-located addresses can be acquired using services such as DHCP. IP packet delivery