Dealing with Mobility -- Mobile IP

References

J. Kurose and K. Ross, Computer Networking: A Top-Down Approach Featuring the Internet, 2nd edition

•C. Perkins and A. Myles, " Mobile IP. " technical report. •Alex C. Snoeren and Hari Balakrishnan, " An End-to-End Approach to Host Mobility." Proc. 6th ACM MOBICOM, August 2000.

Network protocol stack application: supporting network

applications FTP, SMTP, STTP

transport: host-host data transfer TCP, UDP

network: routing of datagrams from source to destination IP, routing protocols

link: data transfer between neighboring network elements PPP, Ethernet

physical: bits “on the wire”

application

transport

network

link

physical

What is mobility?

spectrum of mobility, from the network perspective:

no mobility high mobility

mobile user, usingsame access point

mobile user, passing through multiple access point while maintaining ongoing connections (like cell phone)

mobile user, connecting/ disconnecting from network using DHCP.

Accommodating Mobility

A user might want to turn off an office laptop, bring the laptop home, power up and work from home. The user is primarily interested in e-mail, web browsing.

Not an issue. DHCP provides this functionality. DHCP only allows for a limited form of mobility

since it can’t run networked applications while moving between points of attachment.

In fact, DHCP requires the rebooting of the mobile device.

Accommodating Mobility

If you want to maintain an uninterrupted TCP connection to a remote application while zipping along the autobahn, it would be convenient to maintain the same IP address. Remember that an Internet application

needs to know the IP address and port number of the remote entity with which it is communicating with.

Mobility should be invisible from the application’s viewpoint.

Mobility: Vocabularyhome network: permanent “home” of mobile(e.g., 128.119.40/24)

Permanent address(PA): address in home network, can always be used to reach mobilee.g., 128.119.40.186

home agent(ha): entity that will perform mobility functions on behalf of mobile, when mobile is remote

wide area network

Correspondent

Mobility: more vocabulary

Care-of-address(CoA): address in visited network.(e.g., 79,129.13.2)

wide area network

visited network: network in which mobile currently resides (e.g., 79.129.13/24)

Permanent address: remains constant (e.g., 128.119.40.186)

foreign agent(FA): entity in visited network that performs mobility functions on behalf of mobile.

Correspondent node (CN): wants to communicate with mobile

I wonder where Alice moved to?

Consider friend frequently changing addresses, how do you find her?

Mobility at Which Layer

Where can you manage mobility? Application Transport Network Data-link

Mobile-IP: an extension to current IP architecture To manage mobility at the IP layer To hide mobility from the upper layers

Alternatively, we can also look at the transport layer.

Mobility approaches

Let routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange. Routing tables indicate where each mobile

located No changes to end-systems Scalability is a problem The routers potentially would have to maintain

forwarding table entries for millions of mobile nodes.

Mobility approaches

Let end-systems handle it: indirect routing: communication from

correspondent to mobile goes through home agent, then forwarded to remote

direct routing: correspondent gets foreign address of mobile, sends directly to mobile node

Mobility: registration

End result: Foreign agent knows about mobile Home agent knows location of mobile

wide area network

home network

visited network

1

mobile contacts foreign agent on entering visited network

2

foreign agent contacts home agent home: “this mobile is resident in my network”

Mobility via Indirect Routing

wide area network

homenetwork

visitednetwork

3

2

41

correspondent addresses packets using home address of mobile

home agent intercepts packets, forwards to foreign agent

foreign agent receives packets, forwards to mobile

mobile replies directly to correspondent

Indirect Routing: comments Mobile uses two addresses:

permanent address: used by correspondent (hence mobile location is transparent to correspondent)

care-of-address: used by home agent to forward datagrams to mobile

Routing is based on tunneling

Triangle routing: correspondent-home-network-mobile inefficient when correspondent, mobile are in same network

Forwarding datagrams to remote mobile

Permanent address: 128.119.40.186

Care-of address: 79.129.13.2

dest: 128.119.40.186

packet sent by correspondent

dest: 79.129.13.2 dest: 128.119.40.186

packet sent by home agent to foreign agent: a packet within a packet

dest: 128.119.40.186

foreign-agent-to-mobile packet

Indirect Routing: moving between networks

Suppose mobile user moves to another network Registers with new foreign agent New foreign agent registers with home agent Home agent update care-of-address for

mobile Packets continue to be forwarded to mobile

(but with new care-of-address) Mobility, changing foreign networks

transparent: on going connections can be maintained!

Mobility via Direct Routing

wide area network

homenetwork

visitednetwork

4

2

41correspondent requests, receives foreign address of mobile

correspondent forwards to foreign agent

foreign agent receives packets, forwards to mobile

mobile replies directly to correspondent

3

Mobility via Direct Routing: comments

Overcome triangle routing problem non-transparent to correspondent:

correspondent must get care-of-address from home agent What happens if mobile changes networks? What about security? This approach is not

considered secure enough by the IETF.

Mobile IP

RFC 3220 Has many features we’ve seen:

home agents, foreign agents, foreign-agent registration, care-of-addresses, encapsulation (packet-within-a-packet)

Three components to standard: agent discovery registration with home agent indirect routing of datagrams

Mobile IP: Agent Discovery Agent advertisement: foreign/home agents

advertise service by broadcasting ICMP messages

RBHFMGV bits reserved

type = 16

type = 9 code = 0 = 9

checksum = 9

router address

standard ICMP fields

mobility agent advertisement

extension

length sequence #

registration lifetime

0 or more care-of-addresses

0 8 16 24

R bit: registration required

H,F bits: home and/or foreign agent

Functions of Agent Advertisement

Allow for the detection of mobility agents Let the mobile node know whether the agent is

a host or foreign agent List one or more available care-of addresses Inform the MN about special features provided

by FA Example: Alternative encapsulation techniques (e.g.,

IP packet within IP packet, minimal encapsulation) MN compares the network portion of the

agent’s IP address with the network portion of its home address. If the network portion do not match, then the MN is on a foreign network.

Mobile IP: Registration example

visited network: 79.129.13/ 24 home agent

HA: 128.119.40.7 f oreign agent

COA: 79.129.13.2 COA: 79.129.13.2

….

I CMP agent adv. Mobile agent MA: 128.119.40.186

registration req.

COA: 79.129.13.2 HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 9999 identification:714 ….

registration req.

COA: 79.129.13.2 HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 9999 identification: 714 encapsulation format ….

registration reply

HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 4999 Identification: 714 encapsulation format ….

registration reply

HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 4999 Identification: 714 ….

time

Mobile IP: Registration

The registration process involves 4 steps: The MN requests the forwarding service by

sending a registration request to the foreign agent that the mobile node wants to use.

The FA relays this request to the mobile node’s home agent.

The HA either accepts or denies the request and sends a registration reply to the FA.

The FA relays this reply to the MN.

Mobile IP: Registration

Registration fields include: Lifetime: The number of seconds before the registration

is considered expired. A value of 0 is a request for deregistration.

Home address: The home IP address of the mobile node.

Home agent: The IP address of the mobile node’s home agent.

Care of Address: The home agent should forward IP datagrams that it receives with MN’s home address to this destination address.

Identification: Generated by MN; used for matching registration requests to registration replies (for security). Should be unique for each registration request.

Mobile IP: Registration

The registration reply message includes the following fields: Home address: The home IP address of the

mobile node. Home agent: The IP address of the MN’s

home agent. Identification: Used for matching

registration requests to registration replies.

Mobile IP: Securing Registration

Mobile IP is designed to resist two types of attacks: A node may pretend to be a FA and send a

registration request to a home agent so as to divert traffic intended for a MN to itself.

A node may replay old registration messages, effectively cutting the MN from the network.

Mobile IP: Securing Registration

Each registration request and reply contains an authentication extension with the following fields: Type: Used to designate the type of authentication

extension (mobile-home, mobile-foreign, foreign-home). Length: 4 + the number of bytes in the authenticator Security parameter index (SPI): An index that identifies

a security context between a pair of nodes. The security context is configured so that the two nodes share a secret key and parameters (e.g. algorithm for computing the Authenticator field) relevant to this association.

Authenticator: A variable length string calculated by computing a MD5 message over the shared secret key, the fixed length portion, and all extensions without the Authenticator field

Resisting Denial-of-Service Attack

A Bad Guy generates a bogus Registration Request specifying his own IP address as the COA address for a mobile node. All packets sent by correspondent nodes would be tunneled by the node’s HA to the Bad Guy.

The HA checks the authenticity of the received message by comparing the value of the Authenticator value it computes with the Authenticator value received.

Resisting Replay Attacks

A Bad Guy could obtain a copy of a valid Registration Request message, store and then “replay” at a later time, thereby registering a bogus COA address for the mobile node.

To prevent that the Identification field is generated in such a way as to allow the home agent to determine what the next value should be. Timestamps Pseudorandom numbers (at least 32 bits)

If the Bad Guy uses the intercepted message, the Home Agent will recognize it as being out of date.

Security Issues

Can’t deal with a Bad Guy sending a tremendous number of packets to a host that brings the host’s CPU to its knees.

The current standard uses a similar approach for FA/HA authentication but this is not required.

Traffic between HA and MN can be eavesdropped on.

Key distribution No data privacy Firewalls

Home Network

Where Can We Put the Home Agent? At the router? As a separate server?

At the router What if there are multiple routers for the

home network? As a separate server

How can it pick up a packet

Foreign Network

Where is FA? (Router or Separated Server?)

How Can FA deliver MN the packet [CNMN] Normally, [CNMN] would go straight to a

router (because MN is foreign) Is There Adequate Support at A Foreign

Network What if there is no FA at the network you

visit? Co-located FA

Problems

Routing inefficiencies Firewalls

Firewalls filter those packets whose source address is not part of the network; MNs fall into this category.

Users perceptions of reliability Users expect failures; why bother?

Alternative to Mobile IP

Why an alternative? Mobile IP was designed under the principle

that fixed Internet hosts and applications were to remain unmodified and only the underlying IP substrate should change.

An alternative is to require no changes to the IP substrate. Instead, we should modify transport protocols and applications and the end hosts.

Not a hindrance; rather should make it easy to deploy

The alternative discussed was developed by Snoerent and Balakrishnan (MIT)

Characteristics

Similar to Mobile IP in that the issues of obtaining an IP address in a foreign domain from locating and seamlessly communicating with mobile hosts are separated.

The use of DHCP can be assumed. No tunneling is required DNS is used to provide a level of

indirection between a host’s current location and an invariant end-point identifier.

DNS Based Solution

In Mobile IP, a host’s home address is the invariant.

The DNS name is the invariant since a DNS name identifies a host and does not assume anything about the network attachment point to which it may currently be attached.

When the mobile node changes its attachment point, it must detect this and change the hostname to address mapping in the DNS.

DNS based solution

Detecting changes in an attachment point is similar to Mobile IP and is done through a daemon process

Changing the hostname to address mapping (Arecord) is done through the secure DNS update protocol

DNS based solution

DNS provides a mechanism by which name resolvers can cache name mappings for some period of time (specified in TTL field of the Arecord). This can be avoided by setting the TTL field of zero.

Not considered a problem by authors since name lookups for an uncached Arecord do not have to start from a root name server.

What to do if binding changes after connection?

TCP Connection Migration

TCP connection identified by <source address, source port, destination address,

dest port> Need an ID that is address independent

• During initial connection establishment a token is determined.

• Now connection identified by – <source address, source port, token>

Moving end can send migrate SYN message to other end

• With connection ID and new address This message not acked

• Next message from stationary end to new address implicitly acks migrate message

Migrate Architecture

DNS Server

Mobile Hostfoo.bar.edu

Location Query(DNS Lookup)

Connection Initiation

Location Update(Dynamic DNS Update)

Connection Migration

xxx.xxx.xxx.xxxyyy.yyy.yyy.yyy

CorrespondentHost

From snoeren’00

TCP ConnectionMigration

1. Initial SYN

2. SYN/ACK

3. ACK (with data)

4. Normal data transfer

5. Migrate SYN

6. Migrate SYN/ACK

7. ACK (with data)(Note typo in proceedings)

From snoeren’00

Race Conditions

Occurs when a mobile host moves between when a corresponding host receives the result of its its DNS query and when it initiates a TCP connection

The failure of the corresponding host to open a connection to the mobile host will result in another DNS lookup.

Both end points migrate at same time Solution assumes one fixed host

Security Issues Third party can change DNS mapping

Secure DNS needed

Third party can move connection Token prevents this

Replay attack Sequence number of request prevents this

Denial of service SYN Flooding possible since token is known on

all hosts on the route of the migrate message. This can be handled using a timeout period for a token.

Deployment Issues

Problem: Both peers cannot move simultaneously

Problem: System requires changes to the transport protocol