Mobile IP
• Why mobile IP?
• How mobile IP works
– Introduction
– Agent discovery/Advertisement
– Registration
– Routing considerations
– Security
• Mobility management
– Handoff management
– Location management
• DHCP and mobile IP
Original by: Anthony Scalera, H. Nzumafo, D. Wickramasinghe, EL604, Fall 2001;
Modified by Prof. M. Veeraraghavan
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Why Mobile IP?
• Increasing variety of mobile devices offer
IP connectivity, such as PDAs, handhelds,
laptops, and digital cellular phones.
• Overcomes technical obstacles of the IP
protocol, which was designed for fixed end
points.
2
How mobile IP works:
Introduction
• IPv4 assumes that a node’s address uniquely
identifies point of attachment to the Internet
• To overcome this limitation and allow for mobility
there are two possibilities:
– The node could change its IP address whenever it
changes its point of attachment along with DNS
updates (needed to allow other hosts to reach it)
– Host-specific routes could be propagated throughout
the Internet routing fabric
• Both solutions are often unacceptable so a new
scalable mechanism was developed for
accommodating node mobility within the Internet
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Introduction contd.
• Applicability – enables nodes to move from
one IP subnet to another
• Architectural entities
– Mobile node
– Home agent
– Foreign agent
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Protocol overview
•
•
•
•
•
Agent discovery
Registration
Tunneling (encapsulation)
Routing consideration
Security considerations
5
Terminology
•
•
•
•
•
•
Care-of address
Agent advertisement
Correspondent node
Foreign network
Home address
Home network
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Illustration of terms
• Agent advertisement – An advertisement message constructed
by typing a special extension to a router advertisement message
• Care-of address – The termination point of tunnel toward a
mobile node, for datagrams forwarded to the mobile node while
it is away from home
• Correspondent node – A peer with which a mobile is
communicating
• Foreign network – Any network other than the mobile node’s
Home network
• Home address – An IP address that is assigned for an extended
period of time to a mobile node. It remains unchanged
regardless of where the node is attached to the Internet
• Home network – A network having an address prefix matching
that of a mobile node’s home address
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Mechanisms of Mobile IP
• Discovering the care-of address
• Registering the care-of address
• Tunneling to the care-of address
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Discovering the care-of address
• Agent advertisement – Needed for mobile
node to discover mobility agent. It is an
advertisement message constructed by
attaching a special extension to an ICMP
router advertisement.
• If advertisement not received or needed
sooner by mobile a solicitation may be
generated.
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Agent advertisement
• Allows for the detection of mobility agents
• Lists one or more available care-of addresses
• Informs mobile node about special feature
provided by foreign agents, for example,
alternative encapsulation techniques
• Allows mobile nodes to determine the network
number and status of their link to the Internet
• Allows mobile node to determine whether the
agent has the functionality to serve as a HA, a FA
or both
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Agent solicitation
• A mobile node may optionally solicit an
agent advertisement message from any
locally attached mobility agent with an
agent solicitation message.
• Identical to an ICMP router solicitation with
the further restriction that the IP TTL field
be set to 1.
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Mobile IP care-of addresses
• A foreign agent care-of address is acquired by a mobile
from the broadcast agent advertisement received from an
FA. In this mode, the FA is the end of the tunnel and does
decapsulation; many mobiles share one care-of address;
hence this is the preferred mode
• Collocated care-of address is a local IP address acquired
directly by the mobile through some means, such as DHCP
or owned by mobile as a long-term address for its use only
when visiting some foreign network. In this mode, mobile
sends registration directly to HA and it is the end of the
tunnel, perfoming decapsulation
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Registering the care-of address
- Mobile node sends a
registration request with
Care-of address
information (uses UDP)
- HA receives request and
adds the necessary info to
its routing table
- HA approves the request
- HA sends reply to mobile
node
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Two types of registration
• Via foreign agent – registration sent from
mobile to foreign agent. If accepted agent
forwards request to HA. List maintained of
visited nodes
• Directly from mobile to home with a colocated address. Address obtained via
DHCP
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Registration - details
• When HA accepts the request it associates the
home address of the mobile node with the care-of
address
• Association maintained until registration lifetime
expires
• Triplet that contains the home address, care-of
address and registration lifetimes is called a
binding
• A registration request can be considered a binding
update sent by the mobile node
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Tunneling to the care-of-address
• IP datagram
encapsulated to route
packets from home
agent to care-of
address
• Types of
encapsulation:
– IP in IP
– Minimal encapsulation
– Generic routing
encapsulation (GRE)
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Tunneling types
• IP in IP
– An outer IP header is
inserted before the
datagram’s existing IP
header
– Protocol number is set to 4
(protocol number for IP)
– Source address is the IP
address of the encapsulator;
tunnel entry point
– Destination address is the IP
address of the decapsulator
(FA or mobile). This is the
tunnel exit point.
• Minimal encapsulation
– Protocol field is 55.
– Some of the
information of the
tunnel header is
combined with the
information in the
inner minimal
encapsulation header to
reconstitute the
original IP header.
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Example
IP packet destined to 123.238.45.134
Corresponent node
1.
2.
3.
Router III
Agent ad
Registration
Tunneling
Reg.
Router I
(FA)
128.238.58
Router II
(HA)
Reg.
Encapsulated DG
Agent Ad
128.238.45
Mobile
Permanent address: 128.238.45.134
Temporary address: 128.238.58.15
Mobile
permanent address:
128.238.45.134
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Mobile IP Agent Advertisement
ICMP router advertisement (see icmp.ppt)
Agent
advertisement
extension
Type
Length
Lifetime
Sequence number
R BH F M G V
Rsvd
Zero or more care-of addresses
Optional prefix length extensions
• Type: indicates agent advertisement
• Length = 6+4N, where N: number of addresses
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Fields
• Sequence number: count of AA messages sent
since agent was initialized
• Lifetime: in seconds that this agent is willing to
accept a registration request
• R: Must register with this foreign agent
• B: Busy; this FA will not accept any more
registrations
• H: this agent offers home agent services
• F: this agent offers foreign agent services
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Fields contd.
• M: this agent can receive tunneled IP datagrams that use
minimal encapsulation
• G: this agent can receive tunneled IP datagrams that use
GRE encapsulation
• V: this agent support Van Jacobson header compression
(RFC 1144) for compressing TCP and IP headers
• Care-of-addresses: at least one should be present if F bit is
set
• Prefix-length extension: shows the number of leading bits
that define the network number of the corresponding router
address in the ICMP router advertisement part of the
message
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Registration and reply
• See textbook or RFC for exact format of
messages and fields
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Routing considerations
• Mobile node considerations in routing
– Packets destined to mobile node routed to care-of
address. Return packets routed directly from mobile
node to correspondent node
– On foreign network, mobile node chooses default router
from among the router addresses in the ICMP
advertisement portion of the agent advertisement
message.
– If using co-located care-of address, choose router
address that matches its address network prefix (for the
care-of address it obtained) from the set of addresses
received in ICMP router advertisements
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Triangular routing
• Triangular routing occurs in Mobile IP
between the HA, FA, mobile and
correspondent nodes.
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Figure of triangular routing
Packet from Internet Host routed
indirectly through HA
Inter
net
Host
HA
Packets to Internet
Host routed OK
Encapsulation
FA
Mobile client
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FA considerations in routing
• FA examines inner destination address of
encapsulated datagram and compares it with
its visitor list.
– If no match exists packet is dropped so as to
prevent routing loops.
– If match exists datagram is routed to mobile
node.
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HA considerations in routing
• HA intercepts all datagrams on the home network
destined to the mobile node while mobile is away
from home.
• HA examines destination address and compares
this with any entries in its mobility bindings.
• HA tunnels the datagram to mobile node’s current
care-of address. In the case of multiple mobility
bindings it tunnels to each one.
• In the case of no mobility bindings the HA must
not intercept the datagrams. The mobile may be
assumed to be in the home network.
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Security considerations
•
A binding update is sent remotely to the home agent to affect the home
agent’s routing table, hence the need for authentication.
• Each mobile node must share a security association and be able to use
Message Digest 5 (MD5 - RFC 1321) with 128-bit keys to create
unforgeable digitally signatures for registration request.
• Each registration request must contain unique data so that two different
registrations will in practical terms never have the same MD5 hash.
• Each registration message contains a special identification field which
changes with every new registration. There are two ways to make the
identification field unique:
– Timestamp
– Pseudorandom number
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Handoff
• Move detection
– Lifetime expires and no new agent advertisement is received
– New agent advertisement does not have a router address whose
network prefix matches node’s current care-of-address prefix
• Simultanenous bindings
– If S bit is set in registration, then priori binding still maintained in
home agent and mutiple copies are sent for received IP datagrams
– New FA informs the “system” of the move. HA may send
deregistration to the old FA.
• Comparison to generic approach
– No buffering; no connection setup
• COS scheme used: Home switch is the COS.
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Location management
• Operations
– Move operation performed by mobile sending
registrations
– Find handled by tunneling from HA to LA
• Primary mobility messages
– Registrations
– Advertisements
• Registration messages transported on UDP using
port 434.
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Use of DHCP + mobile IP
• If a mobile moves from an AP on one subnet to an AP on
another subnet, it can use DHCP to acquire a second IP
address, and then register this as a “collocated care-of
address” with the router that it was using after it acquired
its first IP address (which now becomes its “home”)
– The router (home agent) creates a mobility binding
between the mobile’s newly acquired address and old
address
– Since the communication session (say TCP) was started
with the old address, packets will continue to arrive
with the old address as the destination. The HA will
perform encapsulation and tunnel packets to new
address. The mobile will perform decapsulation and
receive the packets.
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Separate functions
• DHCP server: allocates IP address
dynamically
• Router specified in DHCP exchange option
field: becomes home router for mobile
• Foreign agent: sends agent advertisements,
receives registrations, etc.
• Router specified in agent advertisements:
chosen as default router by mobile
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References
• Chapter 12 of class textbook
– See appendix 12A for ICMP
• Mobile IP design principles and practices
– By Charles E. Perkins
• Mobile networking through Mobile IP
– (http://www,computer.org/internet/v2n1/perkins.htm)
• Mobile IP (RFC 2002)
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