1. Jose Alcid David Chapman Aaron Trank
Chapter 18 Mobile IP
Jose Alcid
David Chapman
Aaron Trank
COMP429 Spring 2006

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

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

4. QUIZ
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5. 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

6. 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 host’s address must change
Routers must propagate a host-specific route across the entire Internet
Neither alternative works well

7. 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

8. Mobile IP Characteristics
IETF devised a technology to permit IP mobility
Officially named IP Mobility Support
Popularly called:
MOBILE IP

9. 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

10. 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)

11. 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

12. Mobile IP

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

14. 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)

15. Mobile IP
But Wait!
There’s More!

16. 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

17. Mobile IP versus Standard IP

18. Mobile IP versus Standard IP
What if a user wants to roam between networks?
Mobile users don’t want to know that they are moving between networks
Why can’t mobile users change IP when running an application?

19. 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

20. Mobile IP Entities Mobile Node (MN) Home Agent (HA)
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 MN’s location
Since this IP doesn’t change it can be used by long-lived applications as MN’s location changes
Home Agent (HA)
This is router with additional functionality
Located on home network of MN
Does mobility binding of MN’s IP with its COA
Forwards packets to appropriate network when MN is away
Does this through encapsulation (IP in IP Tunneling)

21. Mobile IP Entities Foreign Agent (FA) Care-of-address (COA)
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
Forward’s MN’s registration request
Decapsulates messages for delivery to MN
Care-of-address (COA)
Address which identifies MN’s 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)

22. Mobile IP versus DHCP

23. 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.

25. 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 ( )
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

26. Mobility agent advertisement extension
Type (16)
Length
Sequence Num
Lifetime
Code
Reserved
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

27. Mobility agent advertisement extension
Type (16)
Length
Sequence Num
Lifetime
Code
Reserved
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

28. 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

29. 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

30. 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

31. Registration Message Format
Type (1 or 3)
Flags/Code
Lifetime
Home Address
Home Agent
Care-Of-Address (request only)
Identification (64bits)
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 Mobile’s static IP home address
Home agent Home agents IP address

32. Registration Message Format
Type (1 or 3)
Flags/Code
Lifetime
Home Address
Home Agent
Care-Of-Address (request only)
Identification (64bits)
Extensions…
Fields cont…
Care-of addr Mobile’s 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 mobile’s identity

34. 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

36. 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.

37. Mobile IP: Encapsulation Options
Version = 4
IHL
Type of service
Total Length
Identification
Flags
Fragment Offset
Time To Live
Protocol = 4
Header Checksum
Source Address (home agent address)
Destination Address (care-of-address)
Protocol
IP Payload
New IP Header
Old IP Header

38. 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.

39. Mobile IP: Encapsulation Options
Version = 4
IHL
Type of Service
Total Length
Identification
Flags
Fragment Offset
Time To Live
Protocol = 55
Header checksum
Source Address (home agent address)
Destination Address (care-of-address)
Protocol S
Retrieved
Destination address (home address)
Source Address(original sender may not be present)
IP Payload (e.g. TCP segment)
Modified IP Header
Minimal forwarding
Header

40. Mobile IP Tunneling
Across Internet

41. 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.

42. References www.cs.wisc.edu/~pb/640/ www.cs.okstate.edu/~saranga