Mobile IP

Polytechnic University

Anthony Scalera

Heine Nzumafo

Duminda Wickramasinghe

Edited by: Malathi Veeraraghavan 12/05/01

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Overview• 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

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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.

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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

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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 a 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

- 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 co-located address. Address obtained via DHCP

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Registration - details

• Authentication – registration request contains a mobility security association

• 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

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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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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. Foreign agent may also be used.

– 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

HA

Inter-net Host

F A

Encapsulation

Packet from Internet Host routed indirectly through HA

Packets to Internet Host routed OK

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• Forward handoff

– New foreign agent sends the registration to the home agent.

• Mixture of overlay and integrated approach– IP routing uses the “integrated” approach– Registration messages are new (to support mobility).

Hence “overlay”– New foreign agent analogous to new BS in forward

handoff scenario.– New FA informs the “system” of the move.

Deregistration sent to 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• Hybrid scheme as with handoff management• Operations

– Move operation performed by mobile sending registrations (overlay)

– Find handled by tunneling from HA to LA (integrated)

• 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

• 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)