Mobile IP Chapter 19

Introduction Mobile IP is designed to allow portable computers to move from one network to another Associated with wireless technologies –A notebook computer attached to a wireless LAN does not affect IP –A desktop computer that is unplugged and plugged into another network requires reconfiguring IP

Characteristics Transparency Interoperability with IPv4 –With conventional IP hosts and other mobile hosts Scalability –Permits mobility across the Internet Security –Ensures authentication; others cannot impersonate Macro mobility –Focus is on long-duration moves, not like cellular

Mobile IP Operation The biggest challenge for mobile IP –allowing a host to retain its address without requiring routers to learn host-specific routes The solution –Allowing the host to have two addresses simultaneously the permanent address is fixed –this is its home network the secondary address is temporary, valid only when the computer is in a given location –this is a foreign network –the host sends the secondary address to an agent at home

Mobile IP Operation –The agent intercepts datagrams sent to the mobile host’s primary address and uses IP-in-IP encapsulation to tunnel to the secondary address –If the mobile host moves again, it gets a new secondary address and tells the home agent of its new location –When the mobile host returns home, it contacts the home agent to deregister, and the agent will stop intercepting datagrams –Mobile IP is intended for infrequent moves and a host that remains at a given location for some time

Mobile Addressing The temporary address, a care of address is not known or used by applications –Only IP software on the mobile and agents on the home or foreign networks use the temporary address –Two types of care-of addresses Co-located care-of address –requires the mobile host to handle all forwarding itself –works on existing structure, extra software is required –applications use home address, lower layers use care-of Foreign agent care of address –host discovers a foreign agent (router) and gets a care-of address

Foreign Agent Discovery Agent discovery uses ICMP’s router discovery –Routers periodically send ICMP router advertisement messages and allow hosts to send ICMP router solicitation to prompt for an advertisement A mobile that does not knowan agent’s IP address can multicast to the all agents group ( ) on page 324 –Router dicovery messages can be extended to allow foreign agent advertisement or mobile advertisement solicitation Mobility extensions do not use a separate ICMP message type

Foreign Agent Discovery –A mobile host knows that an extension is present when the datagram length in the IP header is greater than the length of the ICMP router discovery message –The extension in Figure 19.1 is appended to the router discovery message Length of the extension message, not including type and length (8 bits) Lifetime is maximum seconds that the agent will accept registration requests (infinity is all ones) Codes are shown in Figure 19.2

Agent Registration A mobile host must register so that it can receive datagrams at a foreign location –It registers with an agent on the foreign network –It registers with its home agent to request forwarding –It renews a registration if it is about to expire –It deregisters when it gets back home If it has a co-located care-of address, the host handles the registration If the care-of address is from a foreign agent, the host sends registration requests to the foreign agent and the foreign agent contacts the home agent on its behalf

Registration Messages All registration messages are sent using UDP Agents listen to well-known port 434 The message format –Begins with a set of fixed-size fields followed by variable-length extensions as in Figure 19.3 Type: request or reply Flags: forwarding details Lifetime: how long is the registration valid? Home address: permanent IP address of mobile host Home agent address Care-of address: temporary IP address of mobile host Identification: a number used to match requests with replies

Communication with a Foreign Agent How do we overcome the problem of a foreign agent assigning a non-unique IP for a care-of address? –When a mobile host sends to a foreign agent, the host uses its home address as the IP source address –When the foreign agent sends to the mobile host, the agent uses the mobile’s home address as the IP destination address –Address binding is done by using information that was captured by the agent at registration time

Datagram Transmission and Reception The mobile host creates a datagram with the address of the host it is sending to as the destination and the home address as the source –The shortest path is taken from the foreign network to the destination –The reply does not take the shortest path; instead it goes to the home address and is sent to the care-of address using IP-in-IP Using a foreign agent: the care-of address on the outer datagram specifies the foreign agent which decapsulates it Using a co-located address: the encapsulated datagram goes directly to the mobile host

The Two-Crossing Problem Major disadvantage of mobile IP: inefficient routing –Look at Figure 19.5 which shows routing problems when M communicates with D –A route optimization possibility Use a host-specific route for the mobile host –The problem remains for any destination that does not receive the host-specific route

Communication with Computers on the Home Network Normally, the home agent is the router to which the mobile host’s home network connects to the rest of the internet –Before forwarding a datagram, the home agent looks at its table of mobile hosts to determine if the host is at home or at a foreign network –For traffic that is generated local to the home agent, the agent listens for ARP requests that specify the mobile as a target, and answer with their own IP address - this is proxy ARP (see page 150)

Summary Mobile IP allows a computer to move from one network to another –A mobile host obtains a temporary care-of address –Applications use the home address, underlying software uses the care-of address to deliver across a foreign network When a mobile host detects that it has moved: –it obtains a co-located care-of address –or discovers a foreign agent which assigns a

Summary –the mobile registers with its home agent and requests the agent to forward datagrams When registration is complete, mobile hosts can communicate with arbitrary computers on the internet –Datagrams sent by the mobile are forwarded to the specified destination –Datagrams are sent back first to the home network and then to the mobile host on the foreign network

For Next Time Read Chapter 20 Quiz over Chapter 20