Mobility support in IP v4

Internet Computing (CS-413) 2

 The term mobile computing refers to a system that allows computers to move from one location to another  IP addressing scheme which was designed and optimized for a stationary environment, makes mobility difficult, because host’s IP includes a network prefix. Moving the host to a new network means  The host’s address must change (reconfiguration) OR  Routers must propagate a host specific route across the entire network (large routing table) Internet Computing (CS-413) 3

 Also known as mobility support  Include  Transparency: Mobility is transparent to application and transport layer protocols as well as to routers not involved in the change  Interoperability with IP v4: A host running mobile IP can interoperate with a host running IPv4 software. Addresses assigned to mobile hosts do not differ from those assigned to fixed hosts.  Scalability: Solution scales to large internets  Security: Security features include message authentication  Macro mobility Mobile IP focuses on the problem of long-duration moves rather than attempting to handle rapid network transitions Internet Computing (CS-413) 4

 Mobile IP allows a single computer to hold two addresses simultaneously.  A mobile host obtains a primary address (home address) (permanent/fixed address) on its original home network.  After it moves to a foreign network and obtains a secondary address(care of address)( temporary address), the secondary address must be sent to an agent(usually a router) at home.  The agent agrees to intercept datagrams sent to the mobile host’s primary address, and uses an IP-in-IP encapsulation to tunnel each datagram to its secondary address Internet Computing (CS-413) 5

 If a mobile host moves again, it obtains a new secondary address and informs the home agent of its new location.  When the mobile host returns home, it must contact the home agent to deregister (stop intercepting datagrams) Internet Computing (CS-413) 6

 A mobile host’s home address is assigned and administered by the network administrator of the home network. Applications on a mobile computer always use home address.  The care of address is never known to the applications. Only IP software on mobile host and agents on the home or foreign networks use this address.  A care of address is administered like any other address on a foreign network, and a route to the care of address is propagated through conventional routing protocols Internet Computing (CS-413) 7

 There are two types of care of addresses  Co-located care of address Requires a mobile computer to handle all forwarding itself The mobile host has software that uses two addresses simultaneously; applications use home address; lower layer software uses care of address to receive datagrams  Foreign agent care of address A mobile host must discover the identity of a foreign agent (agent on the foreign network, also a router), then contact this agent to obtain a care of address. Internet Computing (CS-413) 8

 Uses the ICMP router discovery mechanism  Agent discovery piggybacks additional information, called mobility agent extension, on router discovery messages to allow a foreign agent to advertise its presence OR a mobile to solicit an advertisement on a multicast address  The datagram length specified in IP header of agent discovery message is greater than the length of ICMP router discovery message. Internet Computing (CS-413) 9

Type (value =16)lengthSequence number Life timecodereserved Care-of-address Internet Computing (CS-413) 10 Format of mobility agent advertisement message This extension is appended to an ICMP router advertisement Fielddescription Type (8b)This field is set to 16 Length (8b)Total length of extension message ; excluding the type and length octets Sequence #(16b) Holds the message #, so that the recipient can determine if a message is lost Life time (16b)The maximum amount of time in seconds that the agent is willing to accept registration requests Care of addressList of addresses available for use as care of addresses code8 bit flag, that specifies feature of the agent

 Format of mobility agent advertisement  THE CODE FIELD Internet Computing (CS-413) 11 Bitmeaning 0Registration required, no co-located care-of address 1Agent is busy and not accepting registrations 2Agent acts as a home agent 3Agent acts as a foreign agent 4Agent uses minimal encapsulation 5Agent uses generic routing encapsulation (GRE) 6Agent supports header compression 7Unused (0)

 The registration procedure allows a host to:  Register with an agent on a foreign network  Register directly with its home agent to request forwarding  Renew a registration that is due to expire  De-register after returning home Internet Computing (CS-413) 12

Internet Computing (CS-413) Type(1 or 3)FlagsLifetime Home address Home agent Care-of- address Identification (64b) extension Fielddescription TypeType of message. For request message, the value is 1 LifetimeThe number of seconds the registration is valid. 0 = request for de-registration, all 1s= infinite lifetime Home addressPermanent address of the mobile host Home agentAddress of home agent Care-of-addressSecondary address of the mobile host IdentificationInserted into a request message and repeated in reply. Used to match request with a reply extensionVariable length extensions used by home agent to authenticate a mobile host

The FLAGS field  defines forwarding information Internet Computing (CS-413) 14 Bitmeaning 0Mobile host requests that home agent retain its prior care-of-address 1Mobile host requests that home agent tunnel any broadcast message 2Mobile host is using co-located care-of-address 3Mobile host requests that home agent use minimal encapsulation 4Mobile host requests that home agent use GRE 5Mobile host requests header compression 6-7Reserved bits

 A foreign agent can assign one of its IP addresses for use as a care-of-address. Q)How can a foreign agent and a mobile host communicate over a network when the mobile host does not have a valid IP address? Answer  A foreign agent uses the mobile IP’s home address for communication  Instead of relying on ARP for address binding, the agent records the host’s hardware address when a request arrives and uses the recorded information to supply the necessary binding. Internet Computing (CS-413) 15

 To communicate with an arbitrary computer, the mobile host creates a datagram, that has the computer’s address in the destination field and mobile’s home address in the source field. This datagram will follow the shortest path from the foreign network to the destination’s network.  The reply will travel to mobile host’s home network; the home agent intercepts the datagram and uses IP-in-IP encapsulation to tunnel the datagram to the care-of-address. Internet Computing (CS-413) 16

 Two possibilities for the reply  If the mobile host has a co-located care-of- address, the encapsulated datagram passes directly to the mobile, which discards the outer datagram and processes the inner datagram  If the mobile host uses a foreign agent (F.A) for communication, care-of-address of outer datagram specifies the foreign agent. When F.A receives the datagram from a home agent, it decapsulates the datagram, consults its table of registered mobile hosts, and transmits the datagram along the local network to the appropriate mobile Internet Computing (CS-413) 17

 Disadvantage of mobile IP : In-efficient routing Internet Computing (CS-413) 18

 Eliminating this problem requires propagating host-specific routes; the problem remains for any destination that does not receive the host specific routes Internet Computing (CS-413) 19

 Before forwarding a datagram to a mobile host, the home agent examines its table of mobile hosts to determine whether the destination host is currently at home or visiting a foreign network. What about the datagrams that originate locally??  The sender will NOT forward the datagram to the router.  It will ARP for the mobile host’s hardware address, encapsulate the datagram and transmit it !! Internet Computing (CS-413) 20

 SOLUTION  The home agent uses proxy ARP. It must listen for ARP requests that specify the mobile agent as target, and must answer the requests by supplying its own hardware address. Proxy ARP is completely transparent to local computers This also solves the problem of multiple connections. Only one out of multiple routers connecting the home network to the internet, acts as home agent Internet Computing (CS-413) 21

 Foreign agent lookup table format/entries  Home agent lookup table format and entries  IP-in-IP encapsulation Internet Computing (CS-413) 22

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