Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

Figure 15.7 Mobile IP operation

HOME

AGENT

FOREIGN

AGENT

MOBILE

NODE

CORRESPONDING

HOST

1 – Datagram to

mobile node is

forwarded to home

network via standard

IP routing

2 – Datagram is tunneled by the

home agent to care-of address

3 – Datagram is de-tunneled

and delivered to mobile node

4 – Standard IP routing delivers datagrams from mobile

node to the corresponding host via the foreign agent. The

foreign agent is the default router for the mobile node.

Direct path with

route optimization in

one direction

Triangle Routing

264 Chapter 15

MOBILE IP OPERATION

Mobile IP maintains the IP address of the mobile host, when away from home.

It works in the following way. The mobile host has a permanent IP address

assigned in the home network. A router in the home network (called the home

agent) will route IP packets to and from the host using its IP home address,

while the host is still in the home network. When away from home, the mobile

host will register with a local router (called the foreign agent) and will receive

a temporary care-of address. The foreign agent will do the following for the

mobile host:

1. The care-of address is communicated to the home agent.


2. An IP tunnel is used to forward IP packets (datagrams) between to the
   home agent and the mobile host.
   Mobile IP works with both UDP and TCP transport and keeps the
   applications unaware of mobility.
   The flow of IP packets in both directions, between the mobile host and a
   corresponding host on the Internet, is shown in Figure 15.7. Note that
   communications between the mobile host and the corresponding host always
   go via the home network, although this route may not be optimal.
   If, for example, the two hosts are quite near geographically, but the home
   network is far away, the nonoptimal routing becomes a problem. Route
   optimization for mobile IP is described in [16] and provides extensions to
   Mobile IP for the corresponding host to cache the care-of address of the
   mobile host and to bypass the home network, so that packets use normal IP
   routes to the mobile host. Packets from the mobile host to the corresponding
   host will, however, still take the longer route via the home network.
   The nonoptimal routing, at least in one direction, may introduce an
   undesirable delay for interactive communications. The encapsulation in the IP
   tunnel shown in Figure 15.7 also adds to the overhead for RTP/UDP/IP packets.