Chapter 6 slides, Computer Networking, 3rd edition

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SE 410Mobility and Mobile IP
Dr. R.K.Rao Department of Electrical and
Computer Engineering University of Western
Ontario
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What is mobility?

• spectrum of mobility, from the network
  perspective

mobile wireless user, using same access point
mobile user, passing through multiple access
point while maintaining ongoing connections (like
cell phone)
mobile user, connecting/ disconnecting from
network using DHCP.
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Motivation for Mobile IP

• Mobile IP was developed to enable computers to
  maintain Internet connectivity, while moving from
  one Internet attachment point to another
• Mobile IP can also work with wired connections
  like in a computer that is unplugged from
  physical attachment point and plugged into
  another
• However, Mobile IP is more suited to wireless
  connections

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Mobile IP

• The term MOBILE refers to users point of
  attachment that changes dynamically
• And, all connections are automatically
  maintained despite changes
• Consider a business traveler with a portable
  computer, who arrives at a destination and uses
  the portable computer to dial into an ISP.
• In such a case, each time the user moves to a new
  location a new connection is initiated using the
  software at the point of attachment by obtaining
  temporary IP address

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Typical Network Operation

• Routers make use of IP address in the IP datagram
  to perform routing.
• The network portion of the IP address is used by
  routers to move datagram from source network to
  destination network to which target computer is
  attached.
• The final router on the path, which is attached
  to the same network as the target computer, uses
  the host portion of the IP address to deliver the
  IP datagram

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Typical Network Operation

• This IP address is known to the next higher layer
  in the protocol architecture
• Most applications over the Internet are supported
  by TCP connections
• When TCP connection is set up, the TCP entity on
  each side of the connection knows the IP address
  of the correspondent host.
• When a TCP segment is handed down to the IP layer
  for delivery, TCP provides the IP address

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Operation of Mobile IP

• With a mobile host, the IP address may change.
• Mobile IP basically deals with the problem of
  dynamic IP addresses
• Uses the concept known as Care-of-address
• Similar to when we move from one place to
  another, we leave our forwarding address at the
  Post Office

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How do you contact a mobile friend
I wonder where Alice moved to?
Consider friend frequently changing addresses,
how do you find her?

• search all phone books?
• call her parents?
• expect her to let you know where he/she is?

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Mobile IP Operation

• A mobile node is assigned to a particular
  network, known as HOME NETWORK
• Its IP address on home network, known as HOME
  ADDRESS, is static.
• When the mobile node moves its attachment to
  another network, that is considered a FOREIGN
  NETWORK for that host
• Mobile node then makes its presence known by
  registering with the Network Node (a Router) in
  the foreign network

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Mobile IP Operation

• The mobile node then communicates with the HOME
  AGENT, giving it the Care-of-Address of the
  mobile node.
• The care-of-address of the mobile node identifies
  the foreign agents location.
• Routers are responsible for implementing the
  roles of home and foreign agents on the network

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Mobility Vocabulary
home network permanent home of mobile (e.g.,
128.119.40.0/24)
home agent entity that will perform mobility
functions on behalf of mobile, when mobile is
remote
wide area network
Permanent address address in home network, can
always be used to reach mobile e.g.,
128.119.40.186
correspondent
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Mobility more vocabulary
visited network network in which mobile
currently resides (e.g., 79.129.13.0/24)
Permanent address remains constant (e.g.,
128.119.40.186)
Care-of-address address in visited
network. (e.g., 79.129.13.2)
wide area network
Foreign agent entity in visited network that
performs mobility functions on behalf of mobile.
correspondent wants to communicate with mobile
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Mobility approaches

• Let routing handle it routers advertise
  permanent address of mobile-nodes-in-residence
  via usual routing table exchange.
• routing tables indicate where each mobile located
• no changes to end-systems
• Let end-systems handle it
• indirect routing communication from
  correspondent to mobile goes through home agent,
  then forwarded to remote
• direct routing correspondent gets foreign
  address of mobile, sends directly to mobile

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Mobility approaches

• Let routing handle it routers advertise
  permanent address of mobile-nodes-in-residence
  via usual routing table exchange.
• routing tables indicate where each mobile located
• no changes to end-systems
• let end-systems handle it
• indirect routing communication from
  correspondent to mobile goes through home agent,
  then forwarded to remote
• direct routing correspondent gets foreign
  address of mobile, sends directly to mobile

not scalable to millions of mobiles
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Steps for correspondent to communicate with the
mobile
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Steps for correspondent to communicate with the
mobile

• Server X transmits an IP datagram destined for
  mobile node A, with As home address in the IP
  header. The IP datagram is routed to As home
  network.
• At the home network, IP datagram is intercepted
  by the Home Agent who then encapsulates the
  entire datagram inside a new IP datagram and
  sends to the Foreign Agent
• This process of encapsulating an IP datagram into
  another IP datagram is known as TUNNELING

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Steps for correspondent to communicate with the
mobile

• Upon receiving the IP datagram the Foreign agent
  strips off the outer IP header, encapsulates the
  original IP datagram in the network-level PDU and
  delivers the original datagram to A
• When A sends IP traffic to X, it uses Xs IP
  address (in our case not mobile) and sends to the
  router (foreign agent) on the Foreign network for
  routing to X
• The IP datagram from A to X travels directly
  across the Internet to X, using Xs IP address

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Mobile IP

• Mobile IP is best understood as the cooperation
  of three separable mechanisms
• Discovering the care-of address
• Registering the care-of address
• Tunneling to the care-of address

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Support Required for Mobile IP

• Discovery A mobile node uses discovery procedure
  to identify prospective home and foreign agents
• Registration A mobile node uses an authenticated
  registration procedure to inform its home agent
  of its care-of-address
• Tunneling Tunneling is used to forward IP
  datagram from Home address to Care-of-address

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Discovery Process

• For purpose of discovery, a router or other
  network node that can act as an agent
  periodically issues a router advertisement ICMP
  message with an advertisement extension
• The router advertisement portion includes the IP
  address of the router.
• The advertisement extension includes additional
  information about routers role as an agent

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Discovery Process

• A mobile node listens for these agent
  advertisement messages
• Foreign agents are expected to issue agent
  advertisement messages periodically.
• If a mobile node needs an agent information
  immediately, it can issue an ICMP router
  solicitation message.
• Any agent receiving this message will issue an
  agent advertisement

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Registration

• Once a mobile has recognized that it is on a
  foreign network and has acquired a
  care-of-address, it needs to alert a home agent
  on home network
• The mobile node requests the forwarding service
  by sending a registration request to foreign
  agent that the mobile node wants to use
• Foreign agent relays this to mobiles home agent

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Registration

• The home agent either accepts or denies the
  request and sends a registration reply to the
  foreign agent
• Foreign agent relies this to the mobile
• A key concern with registration procedure is
  security
• 1. A node may pretend to be a foreign agent and
  send registration request to a home agent so as
  to divert traffic intended for mobile to itself
• 2. A malicious agent may replay old registration
  messages, effectively cutting the mobile node
  from the network

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Mobility registration
visited network
home network
wide area network

• End result
• Foreign agent knows about mobile
• Home agent knows location of mobile

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Tunneling

• To forward an IP datagram to care-of-address, the
  home agent puts the entire IP datagram into an
  outer IP datagram. This is a form of
  encapsulation. Three options for encapsulation
  are allowed for Mobile IP
• 1. IP-within-IP encapsulation (RFC2003)
• 2. Minimal Encapsulation (RFC2004)
• 3. Generic Routing Encapsulation (RFC1701)

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Mobility via Indirect Routing
visited network
home network
wide area network
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Indirect Routing comments

• Mobile uses two addresses
• permanent address used by correspondent (hence
  mobile location is transparent to correspondent)
• care-of-address used by home agent to forward
  datagrams to mobile
• foreign agent functions may be done by mobile
  itself
• triangle routing correspondent-home-network-mobil
  e
• inefficient when
• correspondent, mobile
• are in same network

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Indirect Routing moving between networks

• suppose mobile user moves to another network
• registers with new foreign agent
• new foreign agent registers with home agent
• home agent updates care-of-address for mobile
• packets continue to be forwarded to mobile (but
  with new care-of-address)
• mobility, changing foreign networks transparent
  on going connections can be maintained!

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Mobility via Direct Routing
correspondent forwards to foreign agent
visited network
home network
wide area network
correspondent requests, receives foreign address
of mobile
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Mobility via Direct Routing comments

• overcome triangle routing problem
• non-transparent to correspondent correspondent
  must get care-of-address from home agent
• what if mobile changes visited network?

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Accommodating mobility with direct routing

• anchor foreign agent FA in first visited network
• data always routed first to anchor FA
• when mobile moves new FA arranges to have data
  forwarded from old FA (chaining)

foreign net visited at session start
anchor foreign agent
wide area network
new foreign network
correspondent agent
new foreign agent
correspondent
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Mobile IP

• RFC 3220
• has many features weve seen
• home agents, foreign agents, foreign-agent
  registration, care-of-addresses, encapsulation
  (packet-within-a-packet)
• three components to standard
• indirect routing of datagrams
• agent discovery
• registration with home agent

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Operation of Mobile IP

• Mobile node is assigned to a particular network
  home network
• IP address on home network is static home
  address
• Mobile node can move to another network foreign
  network
• Mobile node registers with network node on
  foreign network foreign agent
• Mobile node gives care-of address to agent on
  home network home agent

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Mobile IP indirect routing
Permanent address 128.119.40.186
Care-of address 79.129.13.2
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Wireless, mobility impact on higher layer
protocols

• logically, impact should be minimal
• best effort service model remains unchanged
• TCP and UDP can (and do) run over wireless,
  mobile
• but performance-wise
• packet loss/delay due to bit-errors (discarded
  packets, delays for link-layer retransmissions),
  and handoff
• TCP interprets loss as congestion, will decrease
  congestion window un-necessarily
• delay impairments for real-time traffic
• limited bandwidth of wireless links