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Mobile IP. Lec#9. Lecture Objectives. Present the basic principles of addressing in IP networks Describe the problem of mobility with IP and consider alternative solutions Describe the operation of Mobile IP. Agenda. IP addressing Node mobility and IP Mobile IP Foreign agent discovery

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Mobile ip

Mobile IP


Lecture objectives
Lecture Objectives

  • Present the basic principles of addressing in IP networks

  • Describe the problem of mobility with IP and consider alternative solutions

  • Describe the operation of Mobile IP


  • IP addressing

  • Node mobility and IP

  • Mobile IP

    • Foreign agent discovery

    • Home agent registration

    • Packet delivery through tunneling

    • Route optimization

Ip addressing
IP Addressing

  • IPv4 addresses…

    • Uniquely identify an interface

    • 32 bits long

    • Consist of a network identifier and a host identifier

  • Routing outside of the destination host’s subnet is usually based on the network identifier, while the host identifier is only used within the destination’s subnet

  • IPv6 uses 128-bit addresses



Network Identifier

Host Identifier

Five classes of ip addresses
Five Classes of IP Addresses

  • IP addressing was originally based on five classes of addresses

    • A router can interpret the network and host fields by examining the first few bits of the IP address










Class A




Class B





Class C






Class D





multicast address

Class E





reserved for future use

Mobile ip

  • Classless Inter-Domain Routing (CIDR) was introduced to remedy problems with the rigid classes of IP addresses

    • Defined in RFCs 1517-1520

  • Generalize the notion of subnet addressing

  • Allows a flexible definition of the boundary between the network identifier and the host identifier

  • Example

    • IP address:

    • Subnet mask: (22-bit network identifier)

    • Network:

    • Packets with address in the range will be routed to network based on the first 22 bits

A problem with ip addressing
A Problem With IP Addressing

  • An IP address serves two different functions…

    • The name for an interface (host) and

    • The location (subnet) of the interface (host) in the network

  • IP address is the only “name” carried in an IP datagram

    • DNS can be used to map one or more symbolic names to one or more IP addresses, but a symbolic name is not carried in the datagram and has no meaning once the DNS lookup is completed

  • Network identifier in the IP address is used by routers to deliver to the destination subnet

    • The IP address is associated with the location or subnet of the destination host

  • IP address used by the applications to identify the endpoints

Ip routing
IP Routing

  • Router uses routing table to direct packets to the appropriate interface

TargetInterface a b c




Dest =


Traditional routing for a mobile host
Traditional Routing for a Mobile Host

  • Host moving to another network is unreachable

TargetInterface a b c





Dest =




  • Home link – Link assigned the same network prefix as the prefix of the host’s IP address

    • For example, link

  • Foreign link– Any link where the network prefix differs from the prefix of the host’s IP address

    • For example, link

  • Mobility– The ability of a host to change its attachment from one link to another while maintaining communications and not changing its IP address (transparently to corresponding host)

    • Host can change from home link to foreign link (or foreign link to another foreign link) without a change in IP address and without a disruption in communication

Solutions for mobile hosts 1
Solutions for Mobile Hosts (1)

  • Host-specific routing

    • Add routes for the mobile host to routing tables at routers

    • Solution is not scalable since updates and unique entries would be needed for every mobile host

  • Change IP address

    • Mobile host can change its address to the foreign link’s network prefix

    • Need to register new IP address with DNS, resulting in added load on the DNS server and network

    • Communications, e.g., TCP connections, would be disrupted

    • Changing host IP address does not enable mobility, but it does enable nomadicity

Solutions for mobile hosts 2
Solutions for Mobile Hosts (2)

  • Source routing

    • Use loose source routing to specify a path to the foreign link (router interface) and then to the mobile node’s interface

    • Source host must determine address of foreign link, which is not a standard function for a host

  • Use link level (Layer 2) mobility

    • Some Layer 2 protocols support mobility (e.g., between access points in IEEE 802.11 infrastructure networks)

    • Requires that the mobile host not leave the local IP subnet

  • Mobile IP

    • Extension to IP routing to support mobile nodes in a scaleable and secure manner

    • Mobile IP allows a host to move to a foreign network, but still maintain its home IP address

Abstract functions needed to support mobility
Abstract Functions Needed to Support Mobility

  • Readdressing at the home network

  • Associating (in the location directory) the home address and the care-of-address of the mobile node and maintaining up-to-date values for the association

  • Delivering the datagram to the care-of-address

  • Inverting the readdressing operation once the datagram arrives at the care-of-address

Mobile ip addressing
Mobile IP Addressing

  • Really need two addresses …

    • One address for locating (routing to) the host

    • Another address for identifying (naming) a communications end-point

    • Standard IP uses one address for both functions

  • Addresses in Mobile IP

    • Home address– Known IP address for the host

    • Home network (home link) – Destination network associated with the home address

    • Foreign network (foreign link) – Network associated with the visited or foreign link

    • Care-of address– IP address on the foreign link used to locate the host

Mobile ip overview data flow
Mobile IP Overview: Data Flow



CorrespondentNode (Host)




Mobile ip elements
Mobile IP Elements

  • Mobile Host (MH)– Host that changes its attachment point from one network or subnetwork to another

  • Home Agent (HA)

    • Specialized router on mobile node’s home network that tunnels datagrams for delivery to the mobile host

    • Maintains current location information for the mobile node

    • Implements the readdressing function

  • Foreign Agent (FA)

    • Specialized router on foreign network that provides routing services to the mobile host while registered

    • May serve as default router for registered mobile hosts

    • Implements the reverse readdressing function when the datagram is delivered to the care-of-address

  • Correspondent Node (CN)– Communicates with mobile host

Mobile ip operation 1
Mobile IP Operation (1)

  • Mobile (foreign and home) agents advertise their availability using agent-advertisement messages

    • Mobile host may optionally solicit an agent-advertisement message

  • Mobile host receives agent-advertisement message and decides if it is on a foreign or home network

  • If the mobile node is returning to its home network, it “deregisters” with its home agent

  • If the mobile host is on a foreign network, it obtains a care-of address on the foreign network

    • Foreign agent care-of address

    • Colocated care-of address

Mobile ip operation 2
Mobile IP Operation (2)

  • Mobile host registers new care-of address with home agent, possibly via a foreign agent

    • Registration request

    • Registration reply

  • Home agent intercepts datagrams sent to the mobile node’s home address and tunnels datagrams to the registered care-of address

  • Tunneled datagram received

    • At foreign agent and delivered to mobile node, or

    • Directly at the mobile node (colocated)

  • Mobile host can usually send datagrams directly back to the correspondent node

Mobile ip operation 3
Mobile IP Operation (3)

  • Datagrams sent by the mobile node are delivered directly to the destination

    • No need to pass through the home agent

Mobile ip procedures
Mobile IP Procedures

  • Fall into following areas

    • Agent Advertisement/discovery

      • HA and FA may advertise their availability on each link for which they provide service (advertisement)

      • A newly arrived mobile node can send a solicitation on the link to learn if any prospective agents present (discovery)

    • Registration

      • When the MN is away from home, it registers its care-of-address with it HA

        • Either directly with its HA

        • Through a FA, which forwards the registration to the HA

    • Tunneling

      • HA tunnels the datagram to the care-of-address to deliver to MN

Mobile ip procedures1
Mobile IP Procedures

  • Two ways to acquire care-of-address

    • Provides by the FA through its agent advertisement messages

      • Care-of-address is an IP address of the FA

      • Many MNs share the same care-of-address

    • A co-located care-of-address acquired by the MN through some external means, and associates with one of its network interfaces

      • DHCP

      • Preconfigured in MN

Mobile ip procedures2
Mobile IP Procedures

  • Mobile agents (HA or FA) advertise their presence via agent advertisement messages

    • A MN can also solicit an agent advertisement message from any local agent

  • MN receives an agent advertisement message and determines whether it is on its home network or a foreign network

    • At home networks MN operates without mobility services

    • Deregisters it self from HA when returning home from some FA

  • When MN detects an FA, it obtains a care-of-address (either of FA or co-located)

  • MN, away from home registers its new care-of-address with HA through registration requests/reply messages possible through FA

  • HA tunnels the datagrams to MN

  • Datagrams sent by MN may be delivered to their destination using standard IP routing mechanisms, without necessarily passing through the HA

Mobile ip details 1
Mobile IP Details (1)

  • Agent advertisement/discovery

    • ICMP router discovery

    • Mobility agent discovery operation

    • Agent advertisement and solicitation messages

  • Registration

    • Registration operation

    • Authentication

    • Registration request and reply messages

    • Security

    • Example

  • Datagram delivery (Tunneling)

    • Encapsulation principles and schemes

    • ARP issues




Agent advertisement discovery
Agent Advertisement/Discovery


  • Process by which a mobile host …

    • Determines if it is connected to its home network or to a foreign network

    • Determines when it has moved from one network to another

    • Learns the care-of address provided by a foreign agent

  • Based on extensions to Internet Control Message Protocol (ICMP) router discovery mechanism

    • Agent advertisement is formed by including a mobility agent advertisement extension in an ICMP router advertisement message

    • Agent solicitation message is similar to an ICMP Router solicitation

  • Both messages may not necessary for link layers that already provide this functionality

    • Different for each kind of link layer

  • Mobile IP assumes link level connectivity is established by some other means, e.g., association in IEEE 802.11b

  • No authentication is required for agent advertisement and agent solicitation messages

Router discovery icmp operation
Router Discovery ICMP Operation


  • Mobile IP extend ICMP router discovery as its primary mechanism for agent discovery

  • Host determines the local router IP address to deliver its datagrams to hosts on any other link and can monitor their continued presence

  • Involves two ICMP messages

    • One transmitted by the router

    • Other transmitted by the hosts themselves

  • Router discovery message is multicast/broadcast by routers to hosts on the subnet/network

    • Preceded by an IP header

    • Normally, the all-systems multicast address ( is the IP destination address with IP Time-To-Live (TTL) of 1

    • Can be unicast directly to a host that sent a router solicitation message

Router discovery icmp operation1
Router Discovery ICMP Operation


  • A MN can also be configured with router addresses by using DHCP

    • Not recommended, DHCP is quite a complex protocol

  • Router discovery messages do not constitute a routing protocol

    • Enable hosts to discover the existence of neighboring routers, but not which router is best used to reach the destination

  • If a host chooses a poor first-step router for a particular destination, it should receive an ICMP redirect from that router, which identifies a better one

  • In fact, it is quite often the case that hosts send all such datagram traffic through a single router – the default router

  • Router preference level - how eager a router is to have new hosts using its services

Router discovery icmp message 1


Router Discovery ICMP Message (1)

IP Header



ICMP Message









num addrs

addr entry sz


router address (1)

preference (1)


Router Advertisement

Router discovery icmp message 2


Router Discovery ICMP Message (2)

  • Type: Type of ICMP message (9)

  • Code: Used by some types to indicate a specific condition (0)

  • Checksum: Checksum over full message

  • Num addrs: Number of router addresses advertised in this message

  • Addr entry size: The number of 32-bit words of information for each router address (two words here)

  • Lifetime: Maximum number of seconds that the router addresses may be considered valid

Router discovery icmp message 3


Router Discovery ICMP Message (3)

  • Router address (i), i = 1…num addrs: Sending router’s IP addresses on the interface from which this message is sent

  • Preference level (i), i = 1…num addrs: Preference of this router address relative to other routers on this subnet (higher values are more preferable)

Router solicitation icmp operation


Router Solicitation ICMP Operation

  • Host can send router solicitation message for immediate information

  • Solicitation message can be broadcast or multicast

    • Broadcast to the limited-broadcast address (

    • Multicast to the all-routers multicast address ( with TTL = 1

  • Routers reply with a router advertisement

    • Unicast to the host sending the solicitation

  • Host processes advertisement in standard way

Router solicitation icmp message
Router Solicitation ICMP Message


  • Type: ICMP type (10)

  • Code: code for this type (0)

  • Checksum: checksum over full message

  • Reserved: sent as 0; ignored by receiver













Agent advertisement
Agent Advertisement


  • An ICMP Router Advertisement that has been extended also to carry mobility agent advertisement extension

  • Mobility agent transmits agent advertisements to advertise its services on a link

    • Mobile nodes use these advertisements to determine their current point of attachment to the Internet

  • Extend router advertisement messages may also carry other extensions

    • Mobility agent advertisement extension

    • Prefix-length extension

    • One-byte padding extension

    • Future extensions

Agent advertisement1
Agent Advertisement


  • Within an agent advertisement message, ICMP router advertisements include the following link layer, IP, and ICMP header fields

    • Link layer – destination address

      • Link layer destination address of a unicast agent advertisement is required to be the same as the source link-layer address of the agent solicitation that prompted the advertisement

    • IP

      • TTL - requires to be set to 1 for all agent advertisements

      • Destination address – all systems on this link mulicast address ( or the limited broadcast address (

    • ICMP (router discovery fields)

Advertisement features 2
Advertisement Features (2)


  • ICMP

    • Type = 9 (router advertisement message)

    • Code

      • Code = 0 if mobility agent handles common traffic, i.e. it is a router for general IP traffic

      • Code = 16 if mobility agent does not route common traffic (but it must route datagrams from a registered mobile host)

    • Lifetime is maximum time this advertisement is considered valid

    • Router addresses are usual router addresses that are advertised (preference may be low)

    • Num addrs is the number of advertised router addresses

Mobility agent advertisement extension 1


Mobility Agent Advertisement Extension (1)

  • Type: 16

  • Length: 6 + 4*N, where N is the number of advertised care-of addresses (4 bytes each)

  • Sequence number: Count of advertisement messages since agent was initialized

(follows ICMP router advertisement field








sequence number

registration lifetime



zero or more care-of addresses

Agent advertisement extension 2
Agent Advertisement Extension (2)


  • Registration lifetime: Lifetime in seconds that this agent is willing to accept a registration request (65,535  infinity)

  • Bit fields

    • R: Registration required - Foreign agent requires registration rather than using colocated care-of address (e.g., for accounting or other policies)

    • B: Busy -- foreign agent will not accept registrations from new mobile hosts if set

    • H: Home agent -- agent offers home agent services on this link

    • F: Foreign agent -- agent offers foreign agent services on this link

Agent advertisement extension 3
Agent Advertisement Extension (3)


  • Bit fields (continued)

    • M: Minimal encapsulation -- agent can receive datagrams that contain minimal encapsulation

    • G: Generic routing encapsulation (GRE) -- agent can receive datagrams that use GRE

    • V: Van Jacobson header compression -- agent supports use of header compression

  • Reserved: sent as 0; ignored by receiver

  • Care-of addresses: care-of addresses provided by this agent

    • Must provide at least one if F = 1

    • Number of addresses determined by length field

Agent advertisement extension 4
Agent Advertisement Extension (4)


  • HA should never claim to be too busy to server MNs on its home network

  • There can be multiple HAs

  • However, an advertisement from any of the HA on the same network will suffice to inform the MN that it is indeed attached to its home network

  • A FA may indicate that it is too busy to allow new MNs to register with it, by setting the B bit in the agent advertisements

    • However, it must continue to send agent advertisements so that any MN already registered with it will know that they have not moved out of the range of FA

  • An agent advertisement message is not allowed to have a B bit set if the F bit is not also set

Agent advertisement extension 5
Agent Advertisement Extension (5)


  • Either the F bit or the H bit is required to be set in the mobility agent advertisement extension

  • When a FA wishes to require registration even from those mobile nodes that have acquired colocated care-of-address, it sets R bit to one

Prefix length extension


Prefix-Length Extension

  • May follow the mobility agent advertisement extension

  • Used to indicate the number of bits of network prefix that apply to each router address listed in the ICMP Router Advertisement portion of the agent advertisement portion

  • Type: 19

  • Length: N, where N is the value of the num addrs fields in the ICMP Router Advertisement portion of the agent advertisement








Prefix length

One byte padding extension


One-byte Padding Extension

  • Use to align ICMP to even number of bytes

  • If present this extension should be the last extension in the agent advertisement



type = 0

Mobile agent solicitation message


Mobile Agent Solicitation Message

  • Same as ICMP router solicitation message

    • TTL = 1 required

  • Used in a slightly different way

    • Frequency

    • Number of attempts

Advertising by mobility agents
Advertising by Mobility Agents


  • If link-layer protocol does not provide agent discovery, mobility agent (HA and FA) must…

    • Send agent advertisement messages (at some maximum rate with 1 second maximum recommended rate)

    • Respond to agent solicitation messages

  • If link-layer protocol does provide agent discovery, mobility agent must…

    • Respond to agent solicitation messages

    • Send agent advertisement messages if site policy requires additional registration (when R bit is set or as a response to a specific agent solicitation)

Agent discovery by mobile hosts
Agent Discovery by Mobile Hosts


  • MH sends solicitation only if…

    • There is no agent advertisement message

    • Care-of address not established by link-layer protocol

  • Agent advertisement provides…

    • Care-of address

    • Foreign agent address

  • Mobile host knows it is on its home link when it sees advertisement messages from its home agent

    • Mobile host reconfigures routing for home network operation

    • Issues gratuitous ARP to update any cached ARP entries

    • Deregisters with home agent

Agent discovery by mobile hosts1
Agent Discovery by Mobile Hosts


  • A MA is required to limit the rate at which it sends broadcast or multicast agent advertisements

    • A recommended maximal rate is once per second

  • A FA must accept router solicitation even when the IP source address appears to reside on a different subnet than the mobility agent’s interface on which the solicitation was received

  • A MA may be configured to send agent advertisements only in response to an agent solicitation message

Agent discovery by mobile hosts2
Agent Discovery by Mobile Hosts


  • Solicitations should only be sent in the absence of agent advertisements and when a care-of-address has not been determined through link-layer protocol or other means

  • A node may send three initial solicitations on a given link, at a maximum rate of one per second

    • After this, solicitation rate is required to be reduced (binary exponential backoff mechanism) so as to limit the overhead on the local link

  • In fact, a MN can continue to send out solicitation indefinitely until a suitable FA finally comes within range

    • With maximum interval of at least one min

Registration requires
Registration Requires


  • When a MN receives an agent advertisement with R bit set, the MN should register through the FA, even when the MN might be able to acquire its own colocated care-of-address

    • Intended to enforce visiting policies (such as accounting)

Returning home
Returning Home


  • A MN can detect that it has returned to its home network when it receives as agent advertisement from its own home agent

  • If so, it should deregister with its home agent

  • Before attempting to deregister, the MN should configure its routing table appropriately for its home network

  • In addition, if the home network is using ARP, the MN is required to follow the procedures with regard to ARP, proxy ARP, and gratuitous ARP