Mobility With IP, implicit assumption that there is no mobility. Addresses -- network part, host part -- so routers determine how to get to correct network. If nodes move network may change How do we cope with this ? Should there be a change in IP addresses with mobility ? If so how ? Should we use DHCP to assign new addresses ? May be adequate in some cases. Within the same network no problems -- no need to change IP addresses -- link layer delivery.

Mobile IP Problem occurs when user switches between networks. Applications may keep running and so the remote end needs to know how to deliver packets to the mobile host. Mobile IP –Need for transparency for the user –No need to change software of majority of routers on the Internet. –Background compatible

The Home Agent However, it doesn’t come for free! Some routers need new functionalities. Home Agent: Permanent IP address somewhere that the mobile calls home. –A router located on the home network of the mobile. –When a node needs to reach the mobile, sends the messages to the home address.

The Foreign Agent The Foreign Agent: Router located in new network to which the mobile attaches itself when it is away from the home network. Mobile registers with foreign agent and provides the address of its home agent. Foreign agent contacts the home agent and provides a “care-of-address” --> IP address of the foreign agent.

Note.... The home and foreign agents have to announce their presence -- they are specialized routers. The attaching mobile may solicit an advertisement by sending a request.

Proxy ARP When the mobile is away, the home agent has to pick up packets meant for the mobile. With Proxy ARP, the home agent (HA) inserts IP address of mobile node instead of its own! –It provides its own hardware address though ! To invalidate old ARP entries in possible caches, as soon as mobile is known to have registered with a FA (foreign agent), HA issues an ARP. – Note that this is not in response to an ARP query and hence is called “the gratuitous ARP”.

Tunnelling Once home agent gets the IP datagram, it tunnels the packet to the mobile. To recollect, by tunneling, it encapsulates the IP packet within another IP packet destined for the foreign agent. The FA strips the IP wrapper, recognizes that the packet was meant for a registered mobile nad uses its own ARP entry to send the frame to the hardware address of the mobile.

Other details Mobile has to dynamically acquire an IP address in the foreign network. Packets in the other direction are simple, use the source’s IP address (a fixed location). If the source was a mobile, similar procedures could be used.

Route optimization Previous approach sub-optimal. Fixed --> home --> foreign - -> mobile. This is called the triangle routing problem. HA will let the sending node know the care-of-address of mobile node. Sending node creates tunnel to mobile. HA S FA

Implementation and Other issues HA sends a “binding update” to the source in addition to forwarding initial packet. Source creates an entry in a “binding cache” which includes mappings of mobile node addresses to care-of addresses. Entries could become stale -- mobile chooses a new FA -- old FA would issue a binding warning. Issue : Can lead to security problems. Mobile IP not widely deployed -- still being researched -- Mobile networking in general an upcoming research area.