CS558 Sylabus

IP mobility support using DHCP

Recall the purpose of the DHCP protocol:
- Assign an IP host with a unique IP address (that has the correct network ID for the subnet)
- Obtain a default router
The IP address assignment capability of DHCP can be used to provide mobility support for IP.
Similar to the foreign host approach, a host wants to become nomadic (move around across multiple IP networks), then its home network must implement IP mobility support:
- One of the routers in its home network must become a home agent
In the DHCP approach for providing IP mobility, the IP module in the nomadic host must be upgraded to the specifications given in the Internet document RFC 2002 (which will be explained below - you must realise that the mobile host is not running a plain-vanilla IP module !!!).
Home Agent = router at mobile host's home network which is running the special mobile IP software.
As you know, agents (routers) with mobility support will periodically broadcast agent advertising messages (Ethernet broadcast).
Mobile IP using DHCP proceeds as follows:
1. Before it leaves its home network, the nomadic host listens for agent advertising messages on its own network and record the IP address of the agent on its home network.
  This agent is the home agent for the nomadic host
2. When the nomadic host leaves its home network and enters a foreign network, it acquires a new IP address using the DHCP protocol and this new IP address will be one pertaining to the foreign network.
  After acquiring a new IP address, the nomadic host sends a CareOf message to its home agent, containing:
  - IP address of the nomadic host
  - new IP address acquired for the foreign network
3. Upon receiving the CareOf message, the home agent creates an IP tunnel for the nomadic host. The tunnel will have:
```
    Destination  |  Next
   --------------+---------------
    IP address   |  (Virtual Tag)
    of nomadic   |  IP address of
    host         |  nomadic host
		 |  on foreign network
```
  The effect of this is:
  - When the home agent (a router) receives a message destined for the nomadic host, it will tunnel the message directly to the nomadic home on the foreign network !
  The figure below depicts the situation after successful registering:
  - NOTE: Again, the tunnel is unidirectional (only from home agent to nomadic host).
  - You already have seen before that you don't have to tunnel back to allow the nomadic host to send messages

Here is an example of how the mobile IP host receives messages:

The nomadic host will receive an encapsulated IP message
Thus: the IP module in the nomadic host must be updated with IP mobility support !

Again, a nomadic host has absolutely no trouble sending messages to any destination on the Internet...

Advantage of the DHCP approach:

Foreign network do not need to have IP mobility support - all necessary software support for IP mobility are implemented on equipment pertaining to the home network (the home agent (router) and the nomadic host)
Foreign network need only to provide DHCP support which is commonly available (you can get a DHCP server included in even a $40 router for your home today).