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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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7.2.1 Tunneling<br />

The home agent examines the destination <strong>IP</strong> address of all datagrams arriving<br />

on the home network. If the address matches any of the mobile nodes currently<br />

registered as being away from home, the home agent tunnels (using <strong>IP</strong> in <strong>IP</strong><br />

encapsulation) the datagram to the care-of address for that mobile node. It is<br />

likely that the home agent will also be a router on the home network. In this case,<br />

it is likely that it will receive datagrams addressed for a mobile node that is not<br />

currently registered as being away from home. In this case, the home agent<br />

assumes that the mobile node is at home, <strong>and</strong> forwards the datagram to the<br />

home network.<br />

When a foreign agent receives a datagram sent to its advertised care-of address,<br />

it compares the inner destination address with its list of registered visitors. If it<br />

finds a match, the foreign agent forwards the decapsulated datagram to the<br />

appropriate mobile node. If there is no match, the datagram is discarded. (The<br />

foreign agent must not forward such a datagram to the original <strong>IP</strong> header;<br />

otherwise, a routing loop occurs.)<br />

If the mobile node is using a co-located care-of address, the end of the tunnel<br />

lies at the mobile node itself. The mobile node is responsible for decapsulating<br />

the datagrams received from the home agent.<br />

7.2.2 Broadcast datagrams<br />

7.2.3 Move detection<br />

If the home agent receives a broadcast datagram, it must not forward it to mobile<br />

nodes unless the mobile node specifically requested forwarding of broadcasts in<br />

its registration request. In this case, it forwards the datagram in one of the<br />

following manners:<br />

► If the mobile node has a co-located care-of address, the home agent simply<br />

encapsulates the datagram <strong>and</strong> tunnels it directly to the care-of address.<br />

► If the mobile node has a foreign agent care-of address, the home agent first<br />

encapsulates the broadcast in a unicast datagram addressed to the home<br />

address of the node. It then encapsulates <strong>and</strong> tunnels this datagram to the<br />

care-of address. In this way, the foreign agent, when it decapsulates the<br />

datagram, knows to which of its registered mobile nodes it needs to forward<br />

the broadcast.<br />

Mobile <strong>IP</strong> is designed not just for mobile users who regularly move from one site<br />

to another <strong>and</strong> attach their mobile computers to different subnets each time, but<br />

also for truly dynamic mobile users (for example, users of a wireless connection<br />

284 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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