Chapter 18 Mobile IP

Jose Alcid David Chapman Aaron Trank

COMP429 Spring 2006

Overview

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

Overview

• Registration Message Format • Communication With Foreign Agent • Datagram Transmission And

Reception

• The Two-Crossing Problem • Communication With Computers On

the Home Network

QUIZ

• You’ll have the chance to win 2 AMC

movie tickets!!

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

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

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

Mobile IP Characteristics

• IETF devised a technology to permit

IP mobility

• Officially named

IP Mobility Support

• Popularly called:

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

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)

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

Mobile IP

Mobile IP Operation

• Mobile IP is designed for macroscopic

mobility rather than continuous, high speed movement WHY?

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)

Mobile IP

But Wait!

There’s More!

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

Mobile IP versus Standard IP

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?

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

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

location applications as MN’s location changes

• Home Agent (HA) – Located on home network of MN

is away home address

• Since this IP doesn’t change it can be used by long-lived – This is router with additional functionality – Does mobility binding of MN’s IP with its COA • Does this through encapsulation (IP in IP Tunneling)

to which other hosts send packets regardless of MN’s

– Forwards packets to appropriate network when MN

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 – 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)

Mobile IP versus DHCP

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.

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

Mobility agent advertisement extension

0 8 16 24 31 Type (16) Length Sequence Num Lifetime Code Care-Of Address Reserved Type Length Sequence Code Lifetime Care-of-addr

Fields

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

0 1 2 3 4 5 6 7

Mobility agent advertisement extension

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

Code Bits

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

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

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

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

Registration Message Format

0 8 16 24 31 Type (1 or 3) Flags/Code Lifetime Home Address Home Agent Care-Of-Address (request only) Identification (64bits) Extensions… Type Flags/Code Lifetime Home addr Home agent

Fields

1 = registration request, 3 = registration reply 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.

Specifies number of seconds the registration is valid Mobile’s static IP home address Home agents IP address

Registration Message Format

0 8 16 24 31 Type (1 or 3) Flags/Code Lifetime Home Address Home Agent Care-Of-Address (request only) Identification (64bits) Extensions… Care-of addr Identification Extensions

Fields cont…

Mobile’s temporary foreign address 64 bit number generated by the Mobile. Used to match requests with incoming replies. Prevents Mobile from accepting old messages.

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

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

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.

Mobile IP: Encapsulation Options

Version = 4 IHL Type of service Time To Live Identification Protocol = 4 Version = 4 IHL Type of service Flags Source Address (home agent address) Destination Address (care-of-address) Total Length Fragment Offset Header Checksum Total Length Identification Flags Fragment Offset Time To Live Protocol Header Checksum Source Address (home agent address) Destination Address (care-of-address) IP Payload

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.

Mobile IP: Encapsulation Options

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

Mobile IP Tunneling

Across Internet

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.

References

• • •

www.cs.wisc.edu/~pb/640/ www.cs.okstate.edu/~saranga www.wi fiplanet.com/tutorials/article.php/2205821

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www.cisco.com/univercd/cc/td/doc/product /software/ios120/120newft/120t/120t1/mobil eip.htm