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Mobile IP allows mobile hosts to move between networks while maintaining the same home IP address. It uses two IP addresses for each mobile host - a permanent home address and a care-of address that changes as the host moves. The three phases of Mobile IP are agent discovery, registration, and data transfer. During registration, the mobile host registers its current care-of address with its home agent so that data packets can be tunneled or redirected to the mobile host's current location. However, Mobile IP introduces some inefficiency in routing packets to mobile hosts.

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Chapter 27 Mobile IP
CONTENTS ADDRESSING AGENTS THREE PHASES AGENT DISCOVERY REGISTRATION DATA TRANSFER INEFFICIENCY IN MOBILE IP
ADDRESSING 27.1
The IP addresses are designed to work with stationary hosts because part of the address defines the network to which the host is attached.
Figure 27-1 Host address and care-of address
Mobile IP has two addresses for a mobile host: one home address and one care-of address. The home address is permanent; the care-of address changes as the mobile host moves from one network to another.
AGENTS 27.2
Figure 27-2 Home agent and foreign agent
When the mobile host and the foreign agent are the same, the care-of address is called a co-located care-of address.
THREE PHASES 27.3
Figure 27-3 Remote host and mobile host communication
AGENT DISCOVERY 27.4
Figure 27-4 Agent advertisement
Mobile IP does not use a new packet type for agent advertisement; it uses the router advertisement packet of ICMP, and appends an agent advertisement message.
Mobile IP does not use a new packet type for agent solicitation; it uses the router solicitation packet of ICMP.
REGISTRATION 27.5
Figure 27-5 Registration request and reply
Figure 27-6 Registration request format
Figure 27-7 Registration reply format
A registration request or reply is sent by UDP using the well-known port 434.
DATA TRANSFER 27.6
Figure 27-8 Data transfer
The movement of the mobile host is transparent to the rest of the Internet.
INEFFICIENCY IN MOBILE IP 27.7
Figure 27-9 Double crossing
Figure 27-10 Triangle routing

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