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U. Glaeser

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according to its cache so that the new packet will include information of TIP and can be routed to the<br />

mobile host’s current location. If the packet does not reach any intermediate router that supports VIP<br />

and has cache information about the mobile host, the packet will be routed to mobile host’s home<br />

network. The gateway in mobile host’s home network can modify this packet according to this gateway’s<br />

cache and route the packet to the mobile host’s current network. This gateway always has the mobile<br />

host’s current location, because each time when the mobile host moves to a new network, the mobile<br />

host notifies its home network about the mobile host’s current location. The optimized path in VIP is<br />

mainly based on the number of intermediate VIP routers. If many intermediate routers support VIP, the<br />

optimized routing path should be obtained. The option VIP uses to carry information of VIP is an option<br />

of IP, and not all of the routers will support this option. Some computers even discard all of the options<br />

IP packets carry. VIP also needs many extra IP addresses for foreign network to assign to the mobile hosts.<br />

LSRIP<br />

Loose source routing IP uses one of the IP options to cause the IP packets to be routed through a series<br />

of intermediate routers to the destination. This IP option is loose source and record route (LSRR). For<br />

example, if host A wants to send a packet to host C with LSRR option, the packet can reach host B through<br />

general IP routing. Then host B replaces the destination IP address in IP header with the first IP address<br />

C in LSRR option and routes the packet to a new destination C. Also host B puts the pointer to the second<br />

IP address in LSRR option. Host C can perform the same procedure: replace destination IP address with<br />

next IP address in LSRR option, increase pointer to next IP address in LSRR option, and reroute the<br />

packet to a new destination. Until the pointer points to the last IP address in LSRR option, the packet is<br />

sent to its destination address. LSRIP uses LSRR option to carry the information of a mobile host.<br />

When a mobile host moves to a foreign network, the information about the mobile host’s current<br />

location is sent to the mobile host’s home network. During the transmission, each intermediate router<br />

that supports LSRIP protocol can receive this information and update this router’s cache.<br />

If a host wants to send a packet to a mobile host, which is not at its home area, the packet will be first<br />

sent to the mobile host’s home network. During the path to the home network, if an intermediate router<br />

has the cache of the mobile host, this router can put the LSRIP option into the packet and cause it to be<br />

routed to the current network of the mobile host. The gateway of current network reads information<br />

from LSRIP option and can determine that the mobile host is the destination of this packet’s destination.<br />

Thus, the gateway can route the packet to mobile host that is connected to current network.<br />

If the packet does not reach any intermediate router that supports LSRIP and has cache information<br />

about the mobile host, the packet will be routed to mobile host’s home network. The gateway in mobile<br />

host’s home network can add the LSRIP option to this packet according to the gateway’s cache and route<br />

the packet to the mobile host’s current network. This gateway always has the mobile host’s current location<br />

because each time when the mobile host moves to a new network, the mobile host notifies its home<br />

network about the mobile host’s current location.<br />

LSRIP needs more intermediate routers to achieve optimized routing path. Also, the option used by<br />

LSRIP is not compatible with current routers. This can be tested by sending a packet using traceroute, a<br />

tool to check the path one packet has passed. Traceroute uses LSRR option to record the path of the packet<br />

and the transmission time to each intermediate host. If any intermediate host does not support LSRR<br />

option, traceroute bypasses this host. After sending a packet, it can be found from the messages sent back<br />

that some sites are not displayed correctly, which means that these sites do not support LSRR option.<br />

IETF-MIP<br />

IETF-MIP is the most usable protocol for mobile IP in the Internet. The basic idea is to use two agents<br />

to handle the job related to the mobile host. When the mobile host moves to the other networks, it will<br />

notify foreign network’s agent, foreign agent, and its home agent about its current location. Then when<br />

the packet to mobile host is sent to home agent using general IP, the home agent will modify the header<br />

of IP packet: change the destination address to foreign agent’s address and add some fields to the packet<br />

including the mobile host’s permanent address. When the foreign agent receives this packet, it will know<br />

© 2002 by CRC Press LLC

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