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.