MOBILITY

Beyond Third Generation Cellular

Feb 5 1998

Overview

• Mobile IP

• IMHP

• Mobility in GSM

Mobility & IP: The Problem

• Hierarchical IP addressing based on physical location

• The same address is used for identification as well as location

• For identification, an unchanging address is required

• For mobility, a dynamic address is required

Solution

• A level of indirection– Use two addresses– Home address (identification)– Foreign address (location)

• How to obtain a foreign (care-of) address?

• How to manage the binding between the two addresses?

• How to forward packets?

Why handle mobility at the Network Layer?

• Higher layers should not be concerned with mobility

• Mobility management independent of physical layer– can move from one physical network to another

• The problem of mobility is transformed to one of routing

Goals

• Mobility

• Communicate with nodes that run old IP

• Messages about the location of a mobile should be authenticated

• No constraints on assignment of IP addresses

Previous Approaches

• Sony MHP, Columbia MHP, IBM’s MHP

• Use IP options

• Use a mobility router backbone or home gateways

• Use of multicast in the backbone

• Propagation of bindings for route optimization

Mobile IP: Architecture

Home Network

Foreign Network

Specifications

• Agent Discovery

• Registration

• Tunneling

Agent Discovery

• Agents advertise their presence

• Mobile can send solicitation messages

• Mobile-IP modifies the ICMP router discovery procedure

Registration

• Mobile registers its care-of address with its HA

• This could be strongly authenticated– shared secret between mobile & HA

• Registration request & reply messages

• UDP port 434 is used

Tunneling (forwarding)

• HA should intercept messages coming for the mobile (may use proxy ARP)

• HA tunnels the IP packet to the care-of address

Care-of Address

• Could be FA’s address– the FA de-tunnels the packet and sends it to the

mobile

• Mobile could have its own foreign address (obtained through DHCP)– the mobile de-tunnels the packets itself

New Concerns

• Inefficiency: triangle routing

• Security concerns: Any node on the internet can do “remote redirection”

FA

HA

MobileHost talkingto mobile

Internet Mobile Host Protocol

• Aims to provide– routing efficiency– authentication

• Route optimization + Security is difficult

• Security == Current Internet security

• Ideal solution would require key distribution

Route Optimization

• Cache Agents (CA):– cache bindings– cache entries are authenticated– entries are timed out

• A node that wishes to optimize its communication should function as CA

Binding Management

• Lazy notification

• Mobile host always notifies its HA when it moves (registration)

• Node N (CA/HA/LA) receives a packet to be tunneled to the mobile ==> N sends binding notification to source node S

• Binding notifications are re-sent with back-off

Authentication

• Mobile to HA:– strong authentication based on shared secret– secret exchanged while mobile is at home

• Authenticating a binding at CA– send request to mobile/HA with random

number– get reply and check random number

Authentication (Continued…)

• Visitor-List entries at FA– need not be authenticated (since binding is

authenticated)

• Visitor-List entry deletion– mobile exchanges a secret with the LA when

entry is created– this shared secret is used to authenticate visitor-

entry deletion

Other features

• Binding advertisement may be suppressed optionally

• Intermediate CA’s may provide partial optimization– snoop to detect location update messages

Mobility in GSM

• Designed for mobility: integrated approach– Identification is not tied with location

information– Cryptographic keys for authentication

• Other differences– scale– connection oriented nature

Architecture

HLR

VLR

EIR

Location UpdateMobile Base-Station

channel request

location update request

immediate assignment

authentication request

authentication response

location updating accept

TMSI allocation complete

channel release

When a mobile• moves from one cell to another• or when it powers upit initiates a location update procedure

Hand-overMobile Base-Station

conversation

conversation

measurement report

hand-over command

hand-over access

physical information

hand-over complete

conversation

Hand-over may involve:• Only one BSC• Only one MSC• More than one MSC

When more than one MSC is involved, the old MSC is still in control of call-management

Authentication

• Authentication request, response, reject messages

• Ki: Secret Authentication key in SIM

• Identification request, response messages:– IMSI, IMEI, TMSI

• Ki is used to compute Kc - to encrypt data & control messages