Mar'20, 2006

1
Network Layer/Mobile IP

• By
• Mar.20, 2006

Guihua Jia
2
What is TCP/IP

• TCP/IP is a collection of protocols that
  facilitates communications among servers and
  terminals that are hooked to different networks
• TCP Transport Control Protocol
• IP Internet Protocol

3
What is TCP/IP? (cont.)

• IP addresses are divided into two portions
• network identifier (network ID)
• specifies which network a host is on
• host identifier (host ID)
• uniquely specifies hosts within a network

4
Motivation for Mobile IP

• When a host moves from one network to another,
  what can we do under conventional IP?

5
Motivation for Mobile IP (cont.)

• Two Options
• Change IP Address
• The node must change its IP address whenever it
  changes its point of attachment
• Decouple IP Routing From Address
• Instead of routers sending datagrams based on the
  network ID, they route based on the entire
  address

6
Motivation for Mobile IP (cont.)

• Both of them are often unacceptable
• Change IP Address
• makes it impossible for a node to maintain
  transport and higher-layer connections when the
  node changes location.
• Decouple IP Routing From Address
• severe scaling problems, especially relevant
  considering the explosive growth in sales of
  notebook (mobile) computers.

7
Requirements to Mobile IP (RFC 3344, was 3220,
was 2002)

• 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
• no changes to current end-systems and routers
  required
• mobile end-systems can communicate with fixed
  systems

8
Requirements to Mobile IP (cont.)

• Security
• authentication of all registration messages
• Efficiency and scalability
• only little additional messages to the mobile
  system required
• world-wide support of a large number of mobile
  systems in the whole Internet

9
Applicability of Mobile IP

• Mobile IP is intended to enable nodes to move
  from one IP subnet to another
• It is less well suited for more "micro" mobility
  management applications -- for example, handoff
  amongst wireless transceivers.
• As long as node movement does not occur between
  different IP subnets, link-layer mechanisms for
  mobility (i.e., link-layer handoff) may offer
  faster convergence and far less overhead than
  Mobile IP.

10
Three New Entities In Mobile IP

• Mobile Node (MN)
• Home Agent (HA)
• Foreign Agent (FA)

11
Three New Entities In Mobile IP (cont.)

• Mobile Node (MN)
• A host or router that changes its point of
  attachment from one network or subnetwork to
  another
• It may change its location without changing its
  IP address
• It may continue to communicate with other
  Internet nodes at any location using its constant
  IP address, assuming link-layer connectivity to a
  point of attachment is available

12
Three New Entities In Mobile IP (cont.)

• Home Agent (HA)
• A router on a mobile node's home network
• Tunnels datagrams for delivery to the mobile node
  when it is away from home
• Maintains current location information for the
  mobile node

13
Three New Entities In Mobile IP (cont.)

• Foreign Agent (FA)
• A router on a mobile node's visited network
• Provides routing services to the mobile node
  while registered
• Detunnels and delivers datagrams to the mobile
  node that were tunneled by the mobile node's
  home agent.
• For datagrams sent by a mobile node, the foreign
  agent may serve as a default router for
  registered mobile nodes.

14
Example network
mobile end-system
HA
MN
Internet
router
home network
(physical home network for the MN)
foreign network
FA
router
CN
(current physical network for the MN)
router
end-system
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Data Transfer The Mobile Node
to
HA
2
MN
Internet
home network
receiver
3
FA
foreign network
1. Sender sends to the IP address of MN, HA
intercepts packet (proxy ARP) 2. HA tunnels
packet to COA, here FA, by encapsulation 3.
FA forwards the packet to the MN
1
CN
sender
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Data Transfer The Mobile Node
from
HA
MN
1
Internet
home network
sender
FA
foreignnetwork
1. Sender sends to the IP address of the
receiver as usual, FA works as default router
CN
receiver
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Overview
COA
foreign network
router FA
MN
router HA
home network
Internet
CN
router
foreign network
3.
router FA
MN
router HA
home network
2.
4.
Internet
1.
CN
router
18
Care-of Address

• The termination point of a tunnel toward a mobile
  node, for datagrams forwarded to the mobile node
  while it is away from home.
• Two different types
• Foreign agent care-of address An address of a
  foreign agent with which the mobile node is
  registered
• Co-located care-of addressAn externally obtained
  local address which the mobile node has
  associated with one of its own network interfaces

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Example of Foreign Agent Care-of Address

• The foreign agent is the endpoint of the tunnel,
  decapsulates tunneled datagrams and delivers the
  inner datagram to the mobile node.
• This mode is preferred because it does not place
  unnecessary demands on the already limited IPv4
  address space

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Example of Co-located Care-of Address

• The mobile node serves as the endpoint of the
  tunnel and itself performs decapsulation of the
  datagrams tunneled to it
• Allows a mobile node to function without a
  foreign agent
• Place additional burden on the IP address space

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Agent Discovery

• Agent Discovery is the method by which a mobile
  node determines whether it is currently connected
  to its home network or to a foreign network.
• Agent Advertisement
• This is a message transmitted regularly by a
  router acting as a Mobile IP agent. It consists
  of a regular Router Advertisement message that
  has one or more extensions added that contain
  Mobile-IP-specific information for mobile nodes.
• Agent Solicitation
• This message can be sent by a mobile IP device to
  nudge a local agent to send an Agent
  Advertisement.

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Agent Discovery (cont.)

• Agent Advertisement Format

0
7
8
15
16
31
24
23
type 16 length 6 4 COAs R registration
required B busy, no more registrations H home
agent F foreign agent M minimal
encapsulation G GRE encapsulation r 0, ignored
(former Van Jacobson compression) T FA supports
reverse tunneling reserved 0, ignored
type
checksum
code
addresses
addr. size
lifetime
router address 1
preference level 1
router address 2
preference level 2
. . .
type 16
sequence number
length
registration lifetime
R
B
H
F
M
G
r
reserved
T
COA 1
COA 2
. . .
23
Registration

• It is the method by which mobile nodes
• request forwarding services when visiting a
  foreign network
• inform their home agent of their current care-of
  address
• renew a registration which is due to expire
• deregister when they return home

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Registration (cont.)

• Two different registration procedures

MN
FA
HA
MN
HA
registration request
registration request
registration request
registration reply
registration reply
t
registration reply
t
25
Registration (cont.)

• Registration Request

0
7
8
15
16
31
24
23
type 1
lifetime
T x
home address
home agent
COA
identification
extensions . . .
S simultaneous bindings B broadcast
datagrams D decapsulation by MN M mininal
encapsulation G GRE encapsulation r 0,
ignored T reverse tunneling requested x 0,
ignored
26
Registration (cont.)

• Registration Reply

0
7
8
15
16
31
type 3
lifetime
code
home address
home agent
identification
extensions . . .
Example codes registration successful 0
registration accepted 1 registration accepted,
but simultaneous mobility bindings
unsupported registration denied by FA 65
administratively prohibited 66 insufficient
resources 67 mobile node failed
authentication 68 home agent failed
authentication registration denied by HA 129
administratively prohibited 131 mobile node
failed authentication 133 registration
Identification mismatch
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Encapsulation

• Encapsulation of one packet into another as
  payload
• IP-in-IP-encapsulation
• minimal encapsulation
• GRE (Generic Record Encapsulation)

original IP header
original data
new data
new IP header
outer header
inner header
original data
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Encapsulation (cont.)

• IP-in-IP-encapsulation (mandatory, RFC 2003)

length
DS (TOS)
ver.
IHL
IP identification
flags
fragment offset
TTL
IP-in-IP
IP checksum
IP address of HA
Care-of address COA
length
DS (TOS)
ver.
IHL
IP identification
flags
fragment offset
TTL
lay. 4 prot.
IP checksum
IP address of CN
IP address of MN
TCP/UDP/ ... payload
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Encapsulation (cont.)

• Minimal encapsulation (optional)
• avoids repetition of identical fields
• only applicable for unfragmented packets, no
  space left for fragment identification

length
DS (TOS)
ver.
IHL
IP identification
flags
fragment offset
TTL
min. encap.
IP checksum
IP address of HA
care-of address COA
S
lay. 4 protoc.
IP checksum
reserved
IP address of MN
original sender IP address (if S1)
TCP/UDP/ ... payload
30
Mobile IPv6

• Mobile IP was developed for IPv4, but IPv6
  simplifies the protocols
• COA can be assigned via auto-configuration, 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
• soft hand-over, i.e. without packet loss,
  between two subnets is supported

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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
• tunneling makes it hard to give a flow of packets
  a special treatment needed for the QoS

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References

• Ivan Stojmenovic, Note for CSI5169, Winter 2006.
• Ivan Stojmenovic, Handbook of Wireless Networks
  and Mobile Computing, 2002.
• C. Perkins (Ed.), IP Mobility Support for IPv4 ,
  IETF RFC3344. Oct 2005.
• D. Johnson, C. Perkins (Ed.), Mobility Support
  for Ipv6, IETF RFC3775. Jun 2004.
• http//www.tcpipguide.com/free/t_InternetProtocolM
  obilitySupportMobileIP.htm

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Three Questions

• What are the three new entities in Mobile IP?
• Answer
• Mobile Node, Home Agent, Foreign Agent.
• What is the advantage of Co-located care-of
  address in Mobile IP?
• Answer
• It allows a mobile node to function without a
  foreign agent.
• When may link-layer mechanisms for mobility
  (i.e., link-layer handoff) be better suited than
  Mobile IP?
• Answer
• As long as node moves within the same IP
  subnet.

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Thank You!