Physical Mobility 67
network from which the packet actually originated. This problem is tackled in mobile IPv6
by putting the care-of address as the source address, not the home address. There is also a
home address destination option, which allows the use of the care-of address to be transpar-
ent over the IP layer.
Security is automatically provided in IPv6 nodes since they are expected to implement cryp-
tographic and strong authentication and encryption features.
Some additional features supported by IPv6 are as follows:
smooth handoff, which in IPv4, is specified only for the FA, as part of route optimization
renumbering of home networks if the network addresses in the home network have changed
automatic home agent discovery for the mobile node. Here, a home agent, who may want to
stop operations for reasons like overloading, etc., sends switch messages to the mobile node
to get a new home agent. This may also be done if the old home agent has changed or its
address has changed.
5.4 Cellular IP
In mobile IP, packets addressed to an MH are delivered using the regular IP routing mechanism,
to a temporary address assigned to the MH at its actual point of attachment. This approach
results in a simple and scalable scheme that offers global mobility. Mobile IP is not appropriate,
however, for seamless mobility. This is because after each migration, a local address must be
obtained and communicated to a possibly distant location directory or home agent. Support for
seamless mobility is needed in order to provide good service quality to mobile users, particularly
in pico-cellular environments, where the rate of handoff and associated signalling load grows
rapidly.
Thus, mobile IP is not suited to environments where mobility is frequent and restricted
to small cells (micro-mobility). With frequent handoff, micro-mobility protocols have been
proposed to handle local movement of MHs without interaction with the mobile-IP-enabled
Internet. This has the benefit of reducing delay and packet loss during handoff, and eliminating
registration between MHs and distant home agents when MHs remain inside their local coverage
areas.
Eliminating registration reduces the signalling load on the core network in support of mobil-
ity. Similarly, reducing signalling is necessary for the wireless Internet to scale to very large vol-
umes of wireless subscribers. With cellular IP (as in HAWAII and EMA), a visiting mobile device
can use the same care-of address in the whole domain.
As in the case of the cellular phones, wireless IP nodes do not actively communicate most of
the time. They are only switched on, ready for service and constantly reachable by the wireless
Internet. Hence, MHs are normally in an idle state but passively connected to the network infra-
structure. It is sufficient for the wireless Internet to know only the approximate location of its
idle users. Their exact location becomes important only when data need to be forwarded to
them. The network then should be able to efficiently search and find these users in a scalable and
timely manner. In cellular telephony systems, this process is called paging.
As the number of mobile subscribers grows, the need to provide efficient location tracking of
idle users and paging of active communications also grows. In order to achieve scalable location
management, the wireless Internet must handle the location tracking of active and idle MHs inde-
pendently. Support for passive connectivity balances a number of important design considerations.

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