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domain, it attaches to a new leaf in the hierarchy.<br />
Establishment of the correct path then<br />
essentially consists of propagating a new route<br />
establishment message up the tree towards the<br />
least common ancestor (LCA) of the current and<br />
old leaf nodes. Thus, the latency involved in the<br />
intra-domain update process is defined by the<br />
latency of communication between the leaf and<br />
the LCA (In the worst case it is as high as the<br />
latency between the leaf node and the ingress<br />
point). Cellular IP and Hawaii differ in the way<br />
the route update and refreshes are provided to<br />
the nodes. We further describe these protocols in<br />
the following sections. Additionally they differ<br />
in the way they interwork with Mobile IP. We<br />
note that these schemes were designed for IPv4<br />
but can be adapted for IPv6. Here we only describe<br />
the version for IPv4.<br />
3.2.2 Cellular IP<br />
Cellular IP [4] provides local mobility and fast<br />
handoff support, relying on Mobile IP for<br />
macro-mobility. The Cellular IP network consists<br />
of a set of Cellular IP routers, several base<br />
stations which are also Cellular IP enabled, and<br />
a Cellular IP gateway (see Figure 3-1). Every<br />
base station is IP addressable and the Gateway<br />
acts as a Foreign Agent. The main idea behind<br />
Cellular IP (CIP) is that the IP address (Home<br />
Address) of the Terminal equipment is used as<br />
the user identity within the Cellular IP domain<br />
(one domain per gateway), and uplink user traffic<br />
packets (from the MS to the Gateway) update<br />
the path to the user. The following provides a<br />
summary of the main procedures specific to Cellular<br />
IP.<br />
Mobility States: The MH is either in idle state<br />
or in active state. When an idle MH receives an<br />
IP packet (paged from the network) it moves<br />
from idle mode to active mode and sends a route<br />
update packet. When a subsequent packet is<br />
received, the active state timer is reset. When<br />
this timer times out, the MS goes back to idle.<br />
In active state, any Packet Data Unit (PDU) sent<br />
uplink will update the route from the MS to the<br />
gateway. A PDU can be a traffic PDU or a control<br />
PDU (route update message) if the route<br />
update timer has timed out. Whenever a traffic-<br />
PDU is sent this timer is reset. Thus in active<br />
state at least one IP packet is sent during a time<br />
equal to the value of the route update timer. In<br />
idle state paging updates are sent instead of route<br />
updates. The paging update is meant to update<br />
the paging cache of the nodes on the path. Route<br />
updates or paging updates uses ICMP.<br />
Cellular IP routing: When a node receives an<br />
uplink packet, it reads the source address and<br />
updates the corresponding cache and then routes<br />
the packet towards the gateway. When a downlink<br />
packet is received, a node checks the cache.<br />
If the routing cache has not been updated the<br />
node is paged. Otherwise it sends to the next<br />
node that is indicated in the binding table.<br />
Handovers: Inter-gateway handoff is like a<br />
new registration at a new gateway. The location<br />
information at the old gateway expires and is not<br />
explicitly removed. There are two options for<br />
intra domain handoffs, hard and semi-soft.<br />
• In hard handoff, when the MH decides to<br />
handoff it sends a hard handoff route update<br />
message (for security reasons, an uplink<br />
packet is not sufficient) to the new base station.<br />
This route update heads towards the gateway.<br />
Each Cellular IP node along the way creates<br />
a new entry in its routing cache for the<br />
mobile node (or updates the existing entry).<br />
When the message reaches the LCA, the old<br />
entry is replaced by the new entry, pointing<br />
towards the new base station. At this point,<br />
the handoff is complete, and the route cache<br />
entries along the path from the LCA to BS 1<br />
are left to expire when they time-out. Meanwhile<br />
packets are sent to the old base station<br />
and are thus lost.<br />
• Semi-soft handoff addresses the problem of<br />
packet loss by requiring the mobile to switch<br />
back to the old base station for a short period,<br />
the semi-soft period, while waiting for the<br />
semi-soft route update to reach the crossover<br />
node. When the LCA is reached, a new entry<br />
(second entry) for the mobile node is created<br />
in the route cache. Hence, during the semi-soft<br />
period, packets are forwarded to both base stations.<br />
When the semi-soft period is over, the<br />
mobile node switches back to the new base<br />
station and sends a hard handoff route update<br />
which eventually removes the entry towards<br />
the old base station in the LCA.<br />
Security<br />
Cellular IP ensures the integrity and confidentiality<br />
of mobility related control messages.<br />
To ensure that, only authenticated packets can<br />
establish or change cache mappings in a Cellular<br />
IP network and all control packets, i.e. paging<br />
and routing update messages, must be authenticated.<br />
Data packets can only refresh existing<br />
mappings. This of course reduces the signalling<br />
efficiency of the original scheme in which the<br />
main benefit was to have the data packets acting<br />
as control packets to update the route.<br />
3.2.3 Hawaii<br />
In Hawaii [5] the addressing is different from<br />
CIP. The mobile host is assigned a co-located<br />
care-of address from its foreign domain and<br />
packets are tunnelled to the care-of address by a<br />
Home Agent in its home domain. When moving<br />
within the foreign domain, the mobile host keeps<br />
Telektronikk 1.2001
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