MOBILE NETWORK LAYER

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MOBILE NETWORK LAYER

• Mobile IP

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Mobile IP (I)

• Mobile IP adds mobility support to the Internet
  network layer protocol IP.
• The Internet started at a time when no-one had a
  concept of mobile computers.
• The Internet of today lacks mechanisms for the
  support of users traveling through the world.
• IP is the common base for thousands of
  applications and runs over dozens of different
  networks this is the reason for supporting
  mobility at the IP layer.
• Motivation for Mobile IP
• Routing
• based on IP destination address, network prefix
  determines physical subnet
• Change of physical subnet implies change of IP
  address to have a topological correct address
  (standard IP) or needs special entries in the
  routing tables

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Mobile IP (II)

• Create specific routes to end-systems mobile
  nodes?
• change of all routing table entries to forward
  packets to the right destination
• does not scale with the number of mobile hosts
  and frequent changes in the location
• Changing the IP address?
• adjust the host IP address depending on the
  current location
• almost impossible to find a mobile host, DNS has
  not been built for frequent updates
• TCP connection break

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Mobile IP (III)

• Requirements to Mobile IP
• Transparency
• mobile end-systems keep their IP address
• continuation of communication after interruption
  of link possible
• point of connection to the fixed network can be
  changed
• Compatibility
• support of the same layer 2 protocols as IP does
• no changes to current end-systems and routers
  required
• Mobile end-systems can communicate with fixed
  systems

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Mobile IP (IV)

• Security
• authentication of all registration messages
• Efficiency and scalability
• only little additional messages to the mobile
  system required (connection typically via a low
  bandwidth radio link)
• world-wide support of a large number of mobile
  systems in the whole Internet

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Real-life Solution

• Take up the analogy of you moving from one
  apartment to another. What do you do?
• Leave a forwarding address with your old
  post-office
• The old post-office forwards mail to your new
  post-office, which then delivers it to you

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Mobile IP - Definition

• Mobile IP (MIP) is a modification to IP that
  allows nodes to continue to receive datagrams no
  matter where they happen to be attached to the
  Internet

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Mobile IP (V)

• Terminology
• Mobile Node (MN)
• system (node) that can change the point of
  connection to the network without changing its IP
  address
• Home Agent (HA)
• system in the home network of the MN, typically a
  router
• registers the location of the MN, tunnels IP
  datagrams to the COA
• Foreign Agent (FA)
• system in the current foreign network of the MN,
  typically a router
• forwards the tunneled datagrams to the MN,
  typically also the default router of the MN

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Mobile IP (VI)

• Care-of Address (COA)
• address of the current tunnel end-point for the
  MN (at FA or MN)
• actual location of the MN from an IP point of
  view
• can be chosen, e.g., via DHCP
• Correspondent Node (CN)
• communication partner

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Mobile IP in detail

• Combination of 3 separable mechanisms
• Discovering the care-of address
• Registering the care-of address
• Tunneling to the care-of address

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Mobile IP in detail
MIPv4
MIPv6
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Discovering the care-of address

• Discovery process built on top of an existing
  standard protocol router advertisements
• Router advertisements extended to carry available
  care-of addresses called agent advertisements
• Foreign agents (and home agents) send agent
  advertisements periodically
• A mobile host can choose not to wait for an
  advertisement, and issue a solicitation message

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Agent advertisements

• Foreign agents send advertisements to advertise
  available care-of addresses
• Home agents send advertisements to make
  themselves known
• Mobile hosts can issue agent solicitations to
  actively seek information
• If mobile host has not heard from a foreign agent
  its current care-of address belongs to, it seeks
  for another care-of address

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Registering the Care-of Address

• Once mobile host receives care-of address, it
  registers it with the home agent
• A registration request is first sent to the home
  agent (through the foreign agent)
• Home agent then approves the request and sends a
  registration reply back to the mobile host
• Security?

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Registration Illustration
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Home agent discovery

• If the mobile host is unable to communicate with
  the home agent, a home agent discovery message is
  used
• The message is sent as a broadcast to the home
  agents in the home network

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Tunneling to the Care-of address

• When home agent receives packets addressed to
  mobile host, it forwards packets to the care-of
  address
• How does it forward it? - encapsulation
• The default encapsulation mechanism that must be
  supported by all mobility agents using mobile IP
  is IP-within-IP
• Using IP-within-IP, home agent inserts a new IP
  header in front of the IP header of any datagram

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Tunneling (contd.)

• Destination address set to the care-of address
• Source address set to the home agents address
• After stripping out the first header, IP
  processes the packet again

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Tunneling Illustration
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Mobile IP (VII)

• Example network

Internet
(current physical network for the MN)
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Mobile IP (VIII)

• Data transfer to the mobile system

Internet
1. Sender sends to the IP address of MN, HA
intercepts packet
2. HA tunnels packet to COA, here FA, by
encapsulation
3. FA forwards the packet to the MN
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Mobile IP (IX)

• Data transfer from the mobile system

Internet
1. Sender sends to the IP address of the
receiver as usual, FA works as default router
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Mobile IP (XIII)

• Optimization of packet forwarding
• Triangular routing
• sender sends all packets via HA to MN
• higher latency and network load
• Solutions optimization
• HA informs a sender about the location of MN
• sender learns the current location of MN
• direct tunneling to this location
• big security problems!

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Mobile IP (XIV)

• Change of FA
• Packets on-the-fly during the change can be lost
• new FA informs old FA to avoid packet loss, old
  FA forwards remaining packets to new FA
• this information also enables the old FA to
  release resources for the MN

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Mobile IP (XV)

• Change of the foreign agent with the optimized
  mobile IP

MN changeslocation
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Mobile IP (XVI)

• Reverse tunneling

HA
2
MN
Internet
home network
sender
1
FA
foreignnetwork
1. MN sends to FA 2. FA tunnels packets to HA
by encapsulation 3. HA forwards the packet to
the receiver (standard case)
3
CN
receiver
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Mobile IP (XVII)

• Mobile IP with reverse tunneling
• Router accept often only topological correct
  addresses (firewall!)
• a packet from the MN encapsulated by the FA is
  now topological correct
• furthermore multicast and TTL problems solved
  (TTL in the home network correct, but MN is to
  far away from the receiver)
• Reverse tunneling does not solve
• problems with firewalls, the reverse tunnel can
  be abused to circumvent security mechanisms
  (tunnel hijacking)
• optimization of data paths, i.e. packets will be
  forwarded through the tunnel via the HA to a
  sender (double triangular routing)
• The standard is backwards compatible
• the extensions can be implemented easily and
  cooperate with current implementations without
  these extensions
• Agent Advertisements can carry requests for
  reverse tunneling

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Mobile IP in detail
modified from Ericsson Tech. Rep. 11/0362-FCB,
Dec 2000
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Route Optimizations

• Enable direct notification of the corresponding
  host
• Direct tunneling from the corresponding host to
  the mobile host
• Binding cache maintained at corresponding host

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Route optimizations (contd.)

• 4 types of messages
• Binding update
• Binding request
• Binding warning
• Binding acknowledge

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Binding Update

• When a home agent receives a packet to be
  tunneled to a mobile host, it sends a binding
  update message to the corresponding host
• When a home agent receives a binding request
  message, it replies with a binding update message
  
• Also used in the the smooth-handoffs optimization

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Binding Update (Contd.)

• Corresponding host caches binding and uses it for
  tunneling subsequent packets
• Lifetime of binding?
• Corresponding host that perceives a near-expiry
  can choose to ask for a binding confirmation
  using the binding request message
• Home agent can choose to ask for an
  acknowledgement to which a corresponding host has
  to reply with a binding ack message

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Binding warning

• When a foreign agent receives a tunneled message,
  but sees no visitor entry for the mobile host, it
  generates a binding warning message to the
  appropriate home agent
• When a home agent receives a warning, it issues
  an update message to the corresponding host
• What if the foreign agent does not have the home
  agent address (why?) ?

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Binding Update and Warning
Home Agent
Foreign Agent
Corresponding Host
Mobile Host
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Mobile IP and IPv6

• Mobile IP was developed for IPv4, but IPv6
  simplifies the protocols
• security is integrated and not an add-on,
  authentication of registration is included
• COA can be assigned via auto-configuration
  (DHCPv6 is one candidate), every node has address
  autoconfiguration
• no need for a separate FA, all routers perform
  router advertisement which can be used instead of
  the special agent advertisement addresses are
  always co-located
• MN can signal a sender directly the COA, sending
  via HA not needed in this case (automatic path
  optimization)
• soft hand-over, i.e. without packet loss,
  between two subnets is supported
• MN sends the new COA to its old router
• the old router encapsulates all incoming packets
  for the MN and forwards them to the new COA
• authentication is always granted

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Problems with mobile IP

• Security
• authentication with FA problematic, for the FA
  typically belongs to another organization
• no protocol for key management and key
  distribution has been standardized in the
  Internet
• patent and export restrictions
• Firewalls
• typically mobile IP cannot be used together with
  firewalls, special set-ups are needed (such as
  reverse tunneling)
• QoS
• many new reservations in case of RSVP
• tunneling makes it hard to give a flow of packets
  a special treatment needed for the QoS
• Security, firewalls, QoS etc. are topics of
  current research and discussions!

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Security in Mobile IP

• Security requirements (Security Architecture for
  the Internet Protocol, RFC 1825)
• Integrityany changes to data between sender and
  receiver can be detected by the receiver
• Authenticationsender address is really the
  address of the sender and all data received is
  really data sent by this sender
• Confidentialityonly sender and receiver can read
  the data
• Non-Repudiationsender cannot deny sending of
  data
• Traffic Analysiscreation of traffic and user
  profiles should not be possible
• Replay Protectionreceivers can detect replay of
  messages

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IP Micro-mobility support

• Micro-mobility support
• Efficient local handover inside a foreign
  domainwithout involving a home agent
• Reduces control traffic on backbone
• Especially needed in case of route optimization
• Example approaches
• Cellular IP
• HAWAII
• Hierarchical Mobile IP (HMIP)
• Important criteria Security Efficiency,
  Scalability, Transparency, Manageability

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Cellular IP

• Operation
• CIP Nodes maintain routing entries (soft state)
  for MNs
• Multiple entries possible
• Routing entries updated based on packets sent by
  MN
• CIP Gateway
• Mobile IP tunnel endpoint
• Initial registration processing
• Security provisions
• all CIP Nodes sharenetwork key
• MN key MD5(net key, IP addr)
• MN gets key upon registration

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Cellular IP Security

• Advantages
• Initial registration involves authentication of
  MNsand is processed centrally by CIP Gateway
• All control messages by MNs are authenticated
• Replay-protection (using timestamps)
• Potential problems
• MNs can directly influence routing entries
• Network key known to many entities(increases
  risk of compromise)
• No re-keying mechanisms for network key
• No choice of algorithm (always MD5, prefixsuffix
  mode)
• Proprietary mechanisms (not, e.g., IPSec AH)

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Cellular IP Other issues

• Advantages
• Simple and elegant architecture
• Mostly self-configuring (little management
  needed)
• Integration with firewalls / private address
  support possible
• Potential problems
• Not transparent to MNs (additional control
  messages)
• Public-key encryption of MN keys may be a
  problemfor resource-constrained MNs
• Multiple-path forwarding may cause inefficient
  use of available bandwidth

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HAWAII

• Operation
• MN obtains co-located COAand registers with HA
• Handover MN keeps COA,new BS answers Reg.
  Requestand updates routers
• MN views BS as foreign agent
• Security provisions
• MN-FA authentication mandatory
• Challenge/Response Extensions mandatory

1
2
3
4
BS
3
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HAWAII Security

• Advantages
• Mutual authentication and C/R extensions
  mandatory
• Only infrastructure components can influence
  routing entries
• Potential problems
• Co-located COA raises DHCP security issues(DHCP
  has no strong authentication)
• Decentralized security-critical
  functionality(Mobile IP registration processing
  during handover)in base stations
• Authentication of HAWAII protocol messages
  unspecified(potential attackers stationary
  nodes in foreign network)
• MN authentication requires PKI or AAA
  infrastructure

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HAWAII Other issues

• Advantages
• Mostly transparent to MNs(MN sends/receives
  standard Mobile IP messages)
• Explicit support for dynamically assigned home
  addresses
• Potential problems
• Mixture of co-located COA and FA concepts may not
  besupported by some MN implementations
• No private address support possiblebecause of
  co-located COA

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Hierarchical Mobile IPv6 (HMIPv6)

• Operation
• Network contains mobility anchor point (MAP)
• mapping of regional COA (RCOA) to link COA (LCOA)
• Upon handover, MN informsMAP only
• gets new LCOA, keeps RCOA
• HA is only contacted if MAPchanges
• Security provisions
• no HMIP-specificsecurity provisions
• binding updates should be authenticated

HA
RCOA
MAP
binding update
AR
AR
LCOAold
LCOAnew
MN
MN
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Hierarchical MIP
HMIPv6
HMIPv4
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Hierarchical Mobile IP Security

• Advantages
• Local COAs can be hidden,which provides some
  location privacy
• Direct routing between CNs sharing the same link
  is possible (but might be dangerous)
• Potential problems
• Decentralized security-critical
  functionality(handover processing) in mobility
  anchor points
• MNs can (must!) directly influence routing
  entries via binding updates (authentication
  necessary)

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Hierarchical Mobile IP Other issues

• Advantages
• Handover requires minimum numberof overall
  changes to routing tables
• Integration with firewalls / private address
  support possible
• Potential problems
• Not transparent to MNs
• Handover efficiency in wireless mobile scenarios
• Complex MN operations
• All routing reconfiguration messages sent over
  wireless link

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Smooth Hand-offs

• When a mobile host moves from one foreign agent
  to another
• Packets in flight to the old FA are lost and are
  expected to be recovered through higher layer
  protocols (e.g. TCP)
• How can these packets be saved?

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Smooth Hand-offs

• Make previous FA forward packets to the new FA
• Send binding updates to the old FA through the
  new FA
• Such forwarding will be done for a pre-specified
  amount of time (registration lifetime)
• Update can also help old FA free any reserved
  resources immediately
• Why better?

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Recap

• Host mobility and Internet addresses
• Post-office analogy
• Home agent, foreign agent, care-of address, home
  address
• Registration and Tunneling
• Mobile IP problems
• Mobile IP Optimizations
• Other options