Chapter 18Mobile IPJose AlcidDavid ChapmanAaron Trank

COMP429 Spring 2006

OverviewWhat is Mobile IP?Mobility, Routing, and AddressingMobile IP CharacteristicsMobile IP OperationMobile Addressing DetailsForeign Agent DiscoveryAgent Registration

OverviewRegistration Message FormatCommunication With Foreign AgentDatagram Transmission And ReceptionThe Two-Crossing ProblemCommunication With Computers On the Home Network

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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 AddressingMobile ComputingRefers to a system that allows computers to move from one location to another

The IP addressing scheme makes mobility difficultThe hosts address must changeRouters must propagate a host-specific route across the entire Internet

Neither alternative works well

Mobility, Routing and AddressingChanging 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 CharacteristicsIETF devised a technology to permit IP mobility

Officially named IP Mobility Support

Popularly called:

Mobile IP CharacteristicsGeneral Characteristics include:TransparencyMobility 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 IPv4A host using mobile IP can interoperate with stationary hosts that run conventional IPv4 software

Mobile IP CharacteristicsScalabilityThe solution permits mobility across the Internet

SecurityMobile 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 MobilityMobile IP focuses on the problem of long-duration moves (e.g., a user who takes a portable computer on a business trip)

Mobile IP OperationWhat 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 simultaneouslyA permanent and fixed PRIMARY ADDRESSAnd a SECONDARY ADDRESS that is temporary

Mobile IP

Mobile IP OperationMobile IP is designed for macroscopic mobility rather than continuous, high-speed movement

WHY?

Mobile IP OperationThe 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!

Theres More!

Mobile IP versus Standard IPIP assumes end hosts are in fixed physical locationsWhat 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 networksThis 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 dont want to know that they are moving between networksWhy cant mobile users change IP when running an application?

Mobile IP versus Standard IPMobile IP was developed as a means for transparently dealing with problems of mobile usersEnables hosts to stay connected to the Internet regardless of their locationEnables hosts to be tracked without needing to change their IP addressRequires no changes to software of non-mobile hosts/routersRequires addition of some infrastructureHas no geographical limitationsRequires no modifications to IP addresses or IP address formatSupports securityCould be even more important than physically connected routing

Mobile IP EntitiesMobile Node (MN)The entity that may change its point of attachment from network to network in the InternetDetects it has moved and registers with best FAAssigned a permanent IP called its home address to which other hosts send packets regardless of MNs locationSince this IP doesnt change it can be used by long-lived applications as MNs location changesHome Agent (HA)This is router with additional functionalityLocated on home network of MNDoes mobility binding of MNs IP with its COAForwards packets to appropriate network when MN is awayDoes this through encapsulation (IP in IP Tunneling)

Mobile IP EntitiesForeign Agent (FA)Another router with enhanced functionalityIf MN is away from HA the it uses an FA to send/receive data to/from HAAdvertises itself periodicallyForwards MNs registration requestDecapsulates messages for delivery to MNCare-of-address (COA)Address which identifies MNs current locationSent by FA to HA when MN attachesUsually the IP address of the FACorrespondent Node (CN)End host to which MN is corresponding (eg. a web server)

Mobile IP versus DHCP

Two types of care-of addresses Co-LocatedMobile computer handles all forwarding and tunneling itselfMobile obtains a local address on foreign network (e.g. via DHCP)Handles details of contacting the home agent to registerAdvantage: portabilityDisadvantage: Must have special softwareForeign AgentRequires active participant on foreign networkMobile discovers agent when arrives on foreign networkObtains care-of address from foreign agent Advantage: Mobile computer does not need extra softwareDisadvantage: 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 DiscoveryUses ICMP router discovery mechanismRouters periodically send ICMP router advertisement messagesHosts may send an ICMP router solicitation to prompt for the advertisementA 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 extensionThe 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 extensionFieldsTypeType field ICMP = 16 (information reply)LengthSize of the extension msg in octets excluding Type & Length fieldsSequenceSequence number for the message, allows recipient to determine when a msg is lostCodeDefines a specific feature of the agent LifetimeSpecifies a max amount of time in secs that the agent is willing to accept registration requestsCare-of-addrSecond address for the Mobile host0 8 16 24 31

Mobility agent advertisement extensionCode Bits0 Agent supports reversed tunnelingUnused (must be zero)Agent uses Generic Route EncapsulationAgent uses minimal encapsulationAgent functions as foreign agentAgent functions as home agentThe agent is busy and is not accepting registrationsRegistration with an agent is required even when using a co-located care-of-address

0 8 16 24 31

Communicating with a Foreign AgentForeign Agent may use its address as the secondary address for the Mobile HostHow 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 NetworkWhen a host is at a foreign site the home agent can intercept datagrams that arrive from external sources and forward them without problemsA special case arises when the Mobile is at a foreign site and hosts from the home network attempt to forward datagrams to the Mobile unitThose datagrams will be sent via direct delivery and not be intercepted by the home agentTherefore the home agent must arrange to intercept the ARP requests on behalf of the Mobile host which acts as a proxy

Agent RegistrationBefore it can receive datagrams at the foreign location a Mobile host must register with an agentRegistration is done via UDPThe procedure allows a host to:Register with a foreign agentRegister with the home agent to arrange forwardingRenew a registration that is due to expireDeregister with the home agent

Registration Message Format0 8 16 24 31FieldsType 1 = registration request, 3 = registration replyFlags/CodeBits 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.LifetimeSpecifies number of seconds the registration is validHome addrMobiles static IP home addressHome agentHome agents IP address

Registration Message Format0 8 16 24 31Fields contCare-of addrMobiles temporary foreign addressIdentification64 bit number generated by the Mobile. Used to match requests with incoming replies. Prevents Mobile from accepting old messages.Extensionsvariable-length field. Each request is required to contain a mobile-home authentication extension that allows the home agent to verify the mobiles identity

Two Crossing ProblemPoor performance within a foreign networkSpatial locality of referenceVisiting mobile will tend to communicate with hosts local to the foreign networkCrossing internet is more expensive than local deliveryAKA 2X problem

Mobile IP: Encapsulation OptionsIP-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 1The outer header is a full IP header.

Mobile IP: Encapsulation OptionsNew IP HeaderOld IP Header

Mobile IP: Encapsulation OptionsMinimal 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 OptionsModified IP HeaderMinimal forwardingHeader

Mobile IP TunnelingAcross Internet

Security in Mobile IPAuthentication can be performed by all partiesOnly authentication between MN and HA is requiredMD5 is the defaultReplay protectionTimestamps are mandatoryHA and FA do not have to share any security information.

References www.cs.wisc.edu/~pb/640/www.cs.okstate.edu/~sarangawww.wi-fiplanet.com/tutorials/article.php/2205821 www.cisco.com/univercd/cc/td/doc/product/software/ios120/120newft/120t/120t1/mobileip.htm