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Mobile IP
Why do we need it?
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Acknowledgement
• Some figures and texts are from:
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Solomon
Perkins
Govindan
Kurose
Peterson & Davie
Huitema
Heijenk
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Motivation
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What is mobility?
• ability to send and receive communications
anytime, anywhere
• both source and destination devices, applications
and people are free of the constraints imposed by
physical location
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Mobility is not wirelessness
• Mobility is a topological capability
• connectivity is maintained regardless of the location or
motion of the mobile entity
cf) IEEE 802.11 (wireless LAN): geographically constrained
• location independence over an area that is physically too
large for any single medium such as Ethernet or RF
channel
cf) CDPD supports only one medium without changing IP
address
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Mobile IP vs CDPD or wireless
LAN
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area
medium
speed
layer
unique IP
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Mobile IP CDPD
wide
wide
any
cellular
depends on ~11kbps
medium
network
link-layer
yes
yes
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Wireless LAN
local
any LAN medium
1-2 Mbps
link-layer
no
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Routing in the Internet
• Packets flow from link (subnetwork) to link via routers
• Packets are routed individually, based on their IP addresses
(not on DNS name)
• Routing is based on the (sub)network prefix of the IP address
» A mobile host must be assigned a new address when it moves
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Connections between Internet
computers
• TCP connections are defined by source and destination IP addresses
and port numbers
• Change of host address would cause the connection to break
» Host address must be preserved regardless of a hosts location
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The Mobile IP problem
A mobile host must be assigned a new address when it moves
«»
Host address must be preserved regardless of a hosts location
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Route advertisement in ICMP
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Internet Control Message Protocol
(ICMP)
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Echo (ping)
Redirect (from router to source host)
Destination unreachable (protocol, port, or host)
TTL exceeded (so datagrams don’t cycle forever)
Checksum failed
Reassembly failed
Cannot fragment
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Route advertisement in ICMP
• Type 9 code 0: Router adv
• Type 9 code 16:
– Mobile agents
– Ignored when mobile IP is not
installed
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0
3
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Code
0
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0
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0
0
0
0
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description
echo reply (ping)
dest. network unreachable
dest host unreachable
dest protocol unreachable
dest port unreachable
dest network unknown
dest host unknown
source quench (congestion
control - not used)
echo request (ping)
route advertisement
router discovery
TTL expired
bad IP header
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The need for mobility
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The need for mobile-IP
• What happens when a node changes link?
• Can’t you solve this problem with host-specific
routes?
• Why not just change the node’s IP address?
• Can’t you just solve this at the link layer?
• What if I only need nomadicity?
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What happens when a node changes
link?
• network-prefix routing: all nodes with interfaces
on a given link have identical network-prefix
portions of their IP addresses
• IP packets destined to a specific address will be
routed toward the router which advertise
reachability to the network-prefix of that address
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Example
• (#Fig. 3-1 in Solomon)
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Host4 moves from 2.0.0 network to 4.0.0 network
Host1 generates an IP packet for Host4(2.0.0.4)
The IP packet is sent to RouterA (default for Host1)
Router A finds an entry for 2.0.0 network, sends the
packet to router B
– Router B transmits the packet via interface b, only to
find the packet is undeliverable
– Router B sends an ICMP host unreachable error to
Host1
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Can’t you solve this problem with
host-specific routes?
• use the longest-matching routing rule
– place host-specific routes in the routing tables
ex)
Router A: 2.0.0.4/32 next hop = 3.0.0.252, interface = “c”
Router B: 2.0.0.4/32 next hop = 3.0.0.252, interface = “c”
Router C: 2.0.0.4/32 next hop = “direct”, interface = “b”
• home link vs. foreign link
– home link: the link which has been assigned the same
network-prefix as the node’s IP address
– foreign link: any link other than the home link
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Can’t you solve this problem with
host-specific routes?
• host-specific route is not a good solution:
– number of mobile nodes increase --> increase the size of routing
table
– all nodes along the path from a node’s home link to its current
home link must be provided with host-specific routes --> increase
dramatically, as the distance grows
(#Fig. 3-2 in Solomon)
– mobile nodes frequently change links --> a lot of deletion and
addition in the routing tables
– if only minimal update is made, it is not robust
– bad guy can lie about good guy’s location so that good guy can not
receive anything
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Why not just change the node’s IP
address?
• Connection for communication on Internet is
determined by:
– source IP address, source port #, dest. IP address, dest.
port #
• Changing IP disables an existing connection and
starts a new one --> no mobility
• Nomadicity can be supported:
– Nomadic node can terminate and restart everytime the
node moves out with new IP
– DHCP, PPP’s IPCP
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Why not just change the node’s IP
address?
• It’s hard to locate nomadic nodes:
– a node wishing to send something to a nomadic does
not know it’s “current” IP
– If DNS entry is updated to include the change of IP
addresses for nomadic nodes, there will be a lot of
traffic for DNS
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Can’t you just solve this at the link
layer?
• CDPD(Cellular Digital Packet Data):
– differs from cellular modem: charged not for the
duration but for the amount of data actually transmitted
(many CDPD modem shares a channel)
– unique IP throughout the CDPD network
– works with CDPD system only: link-layer solution
– ~11kbps
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Can’t you just solve this at the link
layer?
• IEEE 802.11 (wireless LAN):
– geographically constrained
– transceiver bridges wireless medium and wired
networks
– change of location requires change of IP address
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Can’t you just solve this at the link
layer?
• Problems with link-layer solutions:
– need new IP for new medium
– N solutions for N media: A single solution over all
media types is Mobile IP!
– geographically constrained
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