Chapter 18 Mobile IP

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Chapter 18Mobile IP

• Jose Alcid
• David Chapman
• Aaron Trank

COMP429 Spring 2006
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Overview

• What is Mobile IP?
• Mobility, Routing, and Addressing
• Mobile IP Characteristics
• Mobile IP Operation
• Mobile Addressing Details
• Foreign Agent Discovery
• Agent Registration

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Overview

• Registration Message Format
• Communication With Foreign Agent
• Datagram Transmission And Reception
• The Two-Crossing Problem
• Communication With Computers On the Home Network

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QUIZ

• Youll have the chance to win 2 AMC movie
  tickets!!

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What is Mobile IP?

• IETF standard protocol
• Designed to allow mobile users to move from one
  network to another while maintaining their
  permanent IP address.
• Described in IETF RFC 3344

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Mobility, Routing and Addressing

• Mobile Computing
• Refers to a system that allows computers to move
  from one location to another
• The IP addressing scheme makes mobility difficult
• The hosts address must change
• Routers must propagate a host-specific route
  across the entire Internet
• Neither alternative works well

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Mobility, Routing and Addressing

• Changing an address breaks all existing
  transport-layer connections and may require
  restarting some network services
• If the host contacts a server that uses reverse
  DNS lookup to authenticate, an additional change
  to DNS may be required
• A host-specific routing approach cannot scale
  because communicating and storing a route for
  each host requires excessive bandwidth and memory

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Mobile IP Characteristics

• IETF devised a technology to permit IP mobility
• Officially named IP Mobility Support
• Popularly called

MOBILE IP
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Mobile IP Characteristics

• General Characteristics include
• Transparency
• Mobility is transparent to applications and
  transport layer protocols
• A TCP connection can survive a change in location
  provided the connection is not used during
  transition
• Interoperability With IPv4
• A host using mobile IP can interoperate with
  stationary hosts that run conventional IPv4
  software

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Mobile IP Characteristics

• Scalability
• The solution permits mobility across the Internet
• Security
• Mobile IP provides security facilities that can
  be used to ensure all messages are authenticated
  (i.e. to prevent an arbitrary computer from
  impersonating a mobile host)
• Macro Mobility
• Mobile IP focuses on the problem of long-duration
  moves (e.g., a user who takes a portable computer
  on a business trip)

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Mobile IP Operation

• What is the Biggest Challenge?
• Biggest challenge is allowing a host to retain
  its address without requiring routers to learn
  host-specific routes.
• Mobile IP solves the problem by
• Allowing a computer to hold two addresses
  simultaneously
• A permanent and fixed PRIMARY ADDRESS
• And a SECONDARY ADDRESS that is temporary

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Mobile IP

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Mobile IP Operation

• Mobile IP is designed for macroscopic mobility
  rather than continuous, high-speed movement
• WHY?

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Mobile IP Operation

• The reason should be clear
• OVERHEAD
• Because it requires considerable overhead after
  each move, Mobile IP is intended for situations
  in which a host moves infrequently and remains at
  a given location for a relatively long period of
  time (e.g, hours or days)

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Mobile IP

• But Wait!
• Theres More!

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Mobile IP versus Standard IP

• IP assumes end hosts are in fixed physical
  locations
• What happens if we move a host between networks?
• IP addresses enable IP routing algorithms to get
  packets to the correct network
• -DHCP is used to get packets to end hosts in
  networks
• This still assumes a fixed end host

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Mobile IP versus Standard IP

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Mobile IP versus Standard IP

• What if a user wants to roam between networks?
• Mobile users dont want to know that they are
  moving between networks
• Why cant mobile users change IP when running an
  application?

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Mobile IP versus Standard IP

• Mobile IP was developed as a means for
  transparently dealing with problems of mobile
  users
• Enables hosts to stay connected to the Internet
  regardless of their location
• Enables hosts to be tracked without needing to
  change their IP address
• Requires no changes to software of non-mobile
  hosts/routers
• Requires addition of some infrastructure
• Has no geographical limitations
• Requires no modifications to IP addresses or IP
  address format
• Supports security
• Could be even more important than physically
  connected routing

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Mobile IP Entities

• Mobile Node (MN)
• The entity that may change its point of
  attachment from network to network in the
  Internet
• Detects it has moved and registers with best FA
• Assigned a permanent IP called its home address
  to which other hosts send packets regardless of
  MNs location
• Since this IP doesnt change it can be used by
  long-lived applications as MNs location changes
• Home Agent (HA)
• This is router with additional functionality
• Located on home network of MN
• Does mobility binding of MNs IP with its COA
• Forwards packets to appropriate network when MN
  is away
• Does this through encapsulation (IP in IP
  Tunneling)

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Mobile IP Entities

• Foreign Agent (FA)
• Another router with enhanced functionality
• If MN is away from HA the it uses an FA to
  send/receive data to/from HA
• Advertises itself periodically
• Forwards MNs registration request
• Decapsulates messages for delivery to MN
• Care-of-address (COA)
• Address which identifies MNs current location
• Sent by FA to HA when MN attaches
• Usually the IP address of the FA
• Correspondent Node (CN)
• End host to which MN is corresponding (eg. a web
  server)

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Mobile IP versus DHCP

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Two types of care-of addresses

• Co-Located
• Mobile computer handles all forwarding and
  tunneling itself
• Mobile obtains a local address on foreign network
  (e.g. via DHCP)
• Handles details of contacting the home agent to
  register
• Advantage portability
• Disadvantage Must have special software

• Foreign Agent
• Requires active participant on foreign network
• Mobile discovers agent when arrives on foreign
  network
• Obtains care-of address from foreign agent
• Advantage Mobile computer does not need extra
  software
• Disadvantage Limited access for Mobile computers

A foreign agent does not need to assign a
unique address. Instead, the agent may assign its
IP address.
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Foreign Agent Discovery

• Uses ICMP router discovery mechanism
• Routers periodically send ICMP router
  advertisement messages
• Hosts may send an ICMP router solicitation to
  prompt for the advertisement
• A Mobile may also multicast to the all agents
  group (224.0.0.11)
• If the router acts as a foreign agent then in its
  reply it will append a mobility agent extension
• The message type is the same as for ICMP router
  advertisements except that if the datagram length
  specified in the IP header is greater than the
  length specified in the ICMP router discovery
  message, then the extension is present

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Mobility agent advertisement extension
0 8
16 24
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Type (16) Length Sequence Num Sequence Num
Lifetime Lifetime Code Reserved
Care-Of Address Care-Of Address Care-Of Address Care-Of Address

• Fields
• Type Type field ICMP 16 (information reply)
• Length Size of the extension msg in octets
  excluding Type
• Length fields
• Sequence Sequence number for the message, allows
  recipient to determine when a msg is lost
• Code Defines a specific feature of the agent
• Lifetime Specifies a max amount of time in secs
  that the agent is willing to accept
  registration requests
• Care-of-addr Second address for the Mobile host

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Mobility agent advertisement extension
0 8
16 24
31
Type (16) Length Sequence Num Sequence Num
Lifetime Lifetime Code Reserved
Care-Of Address Care-Of Address Care-Of Address Care-Of Address

• Code Bits
• 0 Agent supports reversed tunneling
• Unused (must be zero)
• Agent uses Generic Route Encapsulation
• Agent uses minimal encapsulation
• Agent functions as foreign agent
• Agent functions as home agent
• The agent is busy and is not accepting
  registrations
• Registration with an agent is required even when
  using a co-located care-of-address

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Communicating with a Foreign Agent

• Foreign Agent may use its address as the
  secondary address for the Mobile Host
• How can the foreign agent communicate with the
  Mobile host when it does not have a unique
  address?
• The Mobile host supplies its hardware address
  during the registration.
• The foreign agent will use the hardware address
  and home IP address to communicate with the
  Mobile host

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Communicating with Home Network

• When a host is at a foreign site the home agent
  can intercept datagrams that arrive from external
  sources and forward them without problems
• A special case arises when the Mobile is at a
  foreign site and hosts from the home network
  attempt to forward datagrams to the Mobile unit
• Those datagrams will be sent via direct delivery
  and not be intercepted by the home agent
• Therefore the home agent must arrange to
  intercept the ARP requests on behalf of the
  Mobile host which acts as a proxy

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Agent Registration

• Before it can receive datagrams at the foreign
  location a Mobile host must register with an
  agent
• Registration is done via UDP
• The procedure allows a host to
• Register with a foreign agent
• Register with the home agent to arrange
  forwarding
• Renew a registration that is due to expire
• Deregister with the home agent

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Registration Message Format
0 8
16 24
31
Type (1 or 3) Flags/Code Lifetime
Home Address Home Address Home Address
Home Agent Home Agent Home Agent
Care-Of-Address (request only) Care-Of-Address (request only) Care-Of-Address (request only)
Identification (64bits) Identification (64bits) Identification (64bits)
Extensions Extensions Extensions
Fields Type 1 registration request, 3
registration reply Flags/Code Bits used in both
requests and replies. They are used as result
codes in a registration reply message and
specify forwarding details in a registration
request. Lifetime Specifies number of seconds
the registration is valid Home addr Mobiles
static IP home address Home agent Home agents IP
address
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Registration Message Format
0 8
16 24
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Type (1 or 3) Flags/Code Lifetime
Home Address Home Address Home Address
Home Agent Home Agent Home Agent
Care-Of-Address (request only) Care-Of-Address (request only) Care-Of-Address (request only)
Identification (64bits) Identification (64bits) Identification (64bits)
Extensions Extensions Extensions
Fields cont Care-of addr Mobiles temporary
foreign address Identification 64 bit number
generated by the Mobile. Used to match
requests with incoming replies. Prevents
Mobile from accepting old messages. Extensions
variable-length field. Each request is required
to contain a mobile-home authentication
extension that allows the home agent to
verify the mobiles identity
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Two Crossing Problem

• Poor performance within a foreign network
• Spatial locality of reference
• Visiting mobile will tend to communicate with
  hosts local to the foreign network
• Crossing internet is more expensive than local
  delivery
• AKA 2X problem

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Mobile IP Encapsulation Options

• IP-within-IP The entire original IP packet
  becomes the payload in a new IP packet.
• The original, inner IP header is unchanged except
  that the TTL field is decreased by 1
• The outer header is a full IP header.

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Mobile IP Encapsulation Options
New IP Header
Old IP Header
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Mobile IP Encapsulation Options

• Minimal encapsulation A new, condensed header
  is inserted between the original IP header and
  the original IP payload.
• The original IP header is then modified to form a
  new outer IP header.

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Mobile IP Encapsulation Options
Modified IP Header
Minimal forwarding Header
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Mobile IP Tunneling

• Across Internet

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Security in Mobile IP

• Authentication can be performed by all parties
• Only authentication between MN and HA is required
• MD5 is the default
• Replay protection
• Timestamps are mandatory
• HA and FA do not have to share any security
  information.

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References

• www.cs.wisc.edu/pb/640/
• www.cs.okstate.edu/saranga
• www.wi-fiplanet.com/tutorials/article.php/2205821
  
• www.cisco.com/univercd/cc/td/doc/product/software/
  ios120/120newft/120t/120t1/mobileip.htm