MN- Mobile IP

7/30/2019 MN- Mobile IP

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Mobile IP

Lec#6



Lecture Objectives

● Present the basic principles of addressing in IPnetworks

● Describe the problem of mobility with IP and

consider alternative solutions

● Describe the operation of Mobile IP



Mobile IP

● Mobile IP (or IP mobility) is an Internet EngineeringTask Force (IETF) standard communications protocol

that is designed to allow mobile device users to

move from one network to another while maintaining

a permanent IP address● Mobile IP for IPv4 is described in IETF RFC 3344, and

updates are added in IETF RFC 4721

● Mobile IPv6, the IP mobility implementation for the

next generation of the Internet Protocol, IPv6, isdescribed in RFC 3775



Mobile IP

● Mobile IP protocol allows location-independentrouting of IP datagrams on the Internet

● Each mobile node is identified by its home address

disregarding its current location in the Internet

● While away from its home network, a mobile node isassociated with a care-of address which identifies its

current location and its home address is associated

with the local endpoint of a tunnel to its home agent

● Mobile IP specifies how a mobile node registers withits home agent and how the home agent routes

datagrams to the mobile node through the tunnel



Applications

● Mobile IP is most often found in wired and wirelessenvironments where users need to carry their mobile

devices across multiple LAN subnets

● Examples of use are in roaming between overlapping

wireless systems, e.g., IP over DVB, WLAN, WiMAX and

BWA

● Currently, Mobile IP is not required within cellular

systems such as 3G, to provide transparency when

Internet users migrate between cellular towers, since

these systems provide their own data link layer handover and roaming mechanisms

● However, it is often used in 3G systems to allow seamless

IP mobility between different packet data serving node

(PDSN) domains



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 thehost identifier is only used within the destination’s

subnet

● IPv6 uses 128-bit addresses

Network Identifier Host Identifier

0 31



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 B 1 0 netid hostid

Class C 1 1 0 hostidnetid

Class D 1 1 1 0 multicast address

Class E 1 1 1 1 reserved for future use

Class A 0 netid0 1 2 3 4 8 16 24 31

hostid



CIDR

● Classless Inter-Domain Routing (CIDR) wasintroduced to remedy problems with the rigid classesof IP addresses■ Defined in RFCs 1517-1520

● Generalize the notion of subnet addressing

● Allows a flexible definition of the boundary betweenthe network identifier and the host identifier

● Example■ IP address: 10.1.9.52

■ Subnet mask: 255.255.252.0 (22-bit network identifier)■ Network: 10.1.8.0/22

■ Packets with address in the range 10.1.8.0-10.1.11.255 will berouted to network 10.1.8.0/22 based on the first 22 bits



A Problem With IP Addressing

● An IP address serves two different functions… ■ The name for an interface (host) and

■ The locat ion (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 oneor 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

● Router uses routing table to direct packets to theappropriate interface

a

b

c

3.0.0.2 3.0.0.3 3.0.0.4

4.0.0.5 4.0.0.6

Router Dest = 3.0.0.4

Target Interface

2.0.0.0/24 a

3.0.0.0/24 b

4.0.0.0/24 c



Traditional Routing for a Mobile Host

● Host moving to another network is unreachable

a

b

c

3.0.0.2 3.0.0.3 3.0.0.4

4.0.0.5 4.0.0.6

Router Dest = 3.0.0.4

3.0.0.4

X

Mobi le

Host

Target Interface

2.0.0.0/24 a

3.0.0.0/24 b

4.0.0.0/24 c



Definitions

● Home link – Link assigned the same network prefixas the prefix of the host’s IP address

■ For example, link 3.0.0.0/24

● Foreign l ink – Any link where the network prefix

differs from the prefix of the host’s IP address ■ For example, link 4.0.0.0/24

● Mobil i ty – The ability of a host to change its

attachment from one link to another while

maintaining communications and not changing its IPaddress (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)

● 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 mobi l i ty , but it

does enable nomadic i ty



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

● 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

● Really need two addresses …■ One address for locat ing (routing to) the host

■ Another address for ident i fy ing (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 netwo rk (foreign link) – Network associated with thevisited or foreign link

■ Care-of address – IP address on the foreign link used to

locate the host



Mobile IP Overview: Data Flow

HomeAgent

Foreign

Agent

Correspondent

Node (Host)

10.0.8.0/24

1 0 .4 . 5 . 0 / 2 4

10.0.8.510.0.8.5

10.4.5.43

10.4.5.43

Mobile

Host10.0.8.5

10.0.8.5

10.92.2.3

10.92.2.3



Mobile IP Elements

● Mob ile Host (MH) – Host that changes its attachment point fromone network or subnetwork to another

● Home Agent (HA)

■ Specialized r outer 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 A gent (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



Components of Mobile IP



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 messageand 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 acare-of address on the foreign network

■ Foreign agent care-of address

■ Colocated care-of address




● Mobile host registers new care-of address with homeagent, possibly via a foreign agent

■ Registration request

■ Registration reply

● Home agent intercepts datagrams sent to the mobilenode’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




● Datagrams sent by the mobile node are delivereddirectly to the destination

■ No need to pass through the home agent



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




● Two ways to acquire care-of-address1. 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

2. 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




1. Mobile agents (HA or FA) advertise their presence via agentadvertisement messages

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

agent

2. 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

3. When MN detects an FA, it obtains a care-of-address (either of FA

or co-located)

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

5. HA tunnels the datagrams to MN

6. 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)

● 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

1

2

3



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 sol ic i tation 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

1



Router Discovery ICMP Operation

● Mobile IP extend ICMP router discovery as its primarymechanism 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 (224.0.0.1) 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

1



Router Discovery ICMP Operation

● A MN can also be configured with router addresses by usingDHCP

■ 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

1

1



Router Discovery ICMP Message (1)

IP Header

ICMP Message

0 8 16 24 31

num addrs

router address (1)

...

addr entry size lifetime

preference (1)

type code checksum

ICMP

Message

ICMPHeader

1

Router Advertisement

1




● 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

1

1




● Router address (i), i = 1…num addrs: Sendingrouter’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 thissubnet (higher values are more preferable)

1

1



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 (255.255.255.255)

■

Multicast to the all-routers multicast address (224.0.0.2) withTTL = 1

● Routers reply with a unicast router advertisement

sent directly to the soliciting host

● Host processes advertisement as if the

advertisement were unsolicited and received at the

broadcast or multicast address

1



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

0 8 16 24 31

reserved

type code checksum

1

ChecksumCodeType



Agent Advertisement

● An ICMP Router Advertisement that has been extendedalso 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

1



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 uni-cast 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 multicast

address (224.0.0.1) or the limited broadcast address

(255.255.255.255)

■ ICMP (router discovery fields)

1



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

1

1



Mobility Agent Advertisement Extension (1)

0 8 16 24 31

registration lifetime

type length sequence number

reservedRBHFMGV

zero or more care-of addresses

● Type: 16

●

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

● Sequence number: Count of advertisement messages

since agent was initialized

(fol lows ICMP rou ter advertis emen t field

1



Agent Advertisement Extension (2)

● Registration lifetime: Lifetime in seconds that thisagent is willing to accept a registration request

(65,535 infinity)

● Bit fields

■

R: Registration required - Foreign agent requiresregistration 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 thislink

■ F: Foreign agent -- agent offers foreign agent services on

this link

1




● 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 useof 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

1




● HA should never claim to be too busy to server MNs on itshome 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 thatany 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

1




● Either the F bit or the H bit is required to be set in themobility 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

1

1



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

0 8 16 24 31

type length Prefix length

1



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

0 8

type = 0

1



Mobile Agent Solicitation Message

● Same as ICMP router solicitation message■ TTL = 1 required

● Used in a slightly different way

■ Frequency

■

Number of attempts

1



Advertising by Mobility Agents

● If link-layer protocol does not provide agentdiscovery, 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 requiresadditional registration (when R bit is set or as a response to

a specific agent solicitation)

1



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

1




● A MA is required to limit the rate at which it sendsbroadcast 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 differentsubnet 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

1




● 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 givenlink, 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 solicitationindefinitely until a suitable FA finally comes within

range

■ With maximum interval of at least one min

1



Registration Requires

● When a MN receives an agent advertisement with R bitset, 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)

1



Returning Home

● A MN can detect that it has returned to its homenetwork 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 shouldconfigure 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 toARP, proxy ARP, and gratuitous ARP

1

Documents

MN- Mobile IP