CSCI 5273Computer Networks

Mobile Internet ProtocolThe Basics

Dirk GrunwaldAssoc. Professor

Dept. of Computer ScienceUniversity of Colorado, Boulder

References

2.1: C. Perkins and A. Myles, "Mobile IP," technical report. 2.2: B. Lancki, A. Dixit, V. Gupta, "Mobile-IP: Supporting Transparent Host

Migration on the Internet," Linux Journal, June 1996. 2.3: D. Johnson and D. Maltz. "Protocols for Adaptive Wireless and Mobile

Networking", IEEE Personal Communication, 3(1), February 1996 2.4: C. Perkins and D. Johnson. "Mobility Support in IPv6," Proceedings of the

Second Annual International Conference on Mobile Computing and Networking (MobiCom'96), November 1996.

2.5: M. Baker, X. Zhao, S. Cheshire, J. Stone, Stanford University, "Supporting Mobility in MosquitoNet", USENIX Winter 1996

Mobile IP Basics

The problem Mobility vs. Portability

Proposed Solution Terminology Registration & maintaince Tunnels Security

Basic Goal of Portable Networking

WirelessRouter

The Internet

Node

Node

Important Problems in Portable Networking

Wireless media has different properties than wired medias

Packet loss may not indicatecontention

IEEE 802.11 is the “wireless ethernet”standard

1 & 11 Mb/s

HomeRF (www.homerf.org) is designed for home networking

Bluetooth (www.bluetooth.org) is designed as “cable replacement”

Some Solutions in Portable/Wireless Networking

802.11 implementations use a MAC addresses and “SSIDS” to identify nodes and networks

Hand-off protocol transferscontrol from one basestation to another

This provides“MAC (L2) LayerMobility”

Mobile IP providesnetwork layer mobility

Node Mobility In Mobile Networking

WirelessRouter

The Internet

WiredRouter

Node

Node

Router Mobility (e.g. Airplane Network)

WirelessRouter

The Internet

WirelessRouter

WiredRouter

WiredRouter

What are the problems?

Nodes in the Internet are identified by a specified IP address

Routing is performed using that same IP address

Some alternatives The node must change its IP address whenever it changes its

point of attachment• Requires upper-level protocols to handle address changes

Host specific routes must be propagated through the network• Requires significant routing tables & doesn’t scale well

Use another level of indirection...

Mobile IP Design Goals

A mobile node must be able to communicate with other nodes after changing it’s link-layer attachment, yet without changing its IP address

A mobile node must be able to communicate with other nodes that do not implement mobile IP

Mobile IP must use authentication to offer security against redirectment attacks

The number of administrative messages should be small to save bandwidth & power

Mobile IP must impose no additional constraints on the assignment of IP addresses

Terminology

Mobile node - a host or router that changes its point of attachment from one network or subnetwork to another. A mobile node may change its location without changing its IP address. It may continue to communicate with other Internet nodes at any location using its (constant) IP address

Home Agent - a router on a mobile nodes home network that tunnels datagrams to the mobile node when it is away from home

Foreign Agent - a router on a mobile nodes visited network that provides routing services to the mobile node while registered

Terminology

The mobile node is assigned a “care-of address” on the foreign network. This address is used to deliver the datagrams for the mobile node.

This can either be the foreign agent (e.g. a route) Or, it can be “co-located” with the mobile node

Terminology

Internet

Home Network

A

HomeAgent(HA)

Visited Network

A

ForeignAgent(FA)

Solution In A Nutshell

Internet

Home Network

A

HomeAgent(HA)

Visited Network

A

ForeignAgent(FA)

Source

Solution In A Nutshell

Internet

Home Network

A

HomeAgent(HA)

Visited Network

A

ForeignAgent(FA)

Source

Solution In A Nutshell

Internet

Home Network

A

HomeAgent(HA)

Visited Network

A

ForeignAgent(FA)

Source

Tunnel

Solution In A Nutshell

Internet

Home Network

A

HomeAgent(HA)

Visited Network

A

ForeignAgent(FA)

Source

Tunnel

Solution In A Nutshell

Internet

Home Network

A

HomeAgent(HA)

Visited Network

A

ForeignAgent(FA)

Source

Tunnel

More Abstractly

HA FA

NodeSource

Protocol Overview

Advertisement Mobility Agents (Foreign Agents and Home Agents) should

advertise their services A mobile node can solicit for mobility agents

Registration - when a mobile node is away from home, it must register its care-of address with it’s home agent

Delivering Datagrams Datagrams must be forwarded by the Home Agent to the

Foreign Agent for delivery to the care-of address. The delivery mechanism must handle all packets (including

broadcast and multicast) A tunnel is used for this

Advertisement & Solicitation

The router discovery ICMP protocol was adapted for advertisement and solicitation

Routers broadcast or multicast every few seconds Uses limited (255.255.255.255) bcast or all-systems-on-this-link

multicast (224.0.0.1)

Mobile nodes also send out solicitation messages, which will cause a router to broadcast or multicast their advertisement

Registration

Request forwarding services when visiting a foreign network

This allocates a local (foreign) node address

Inform home agent of their current care-of address This creates a binding of the foreign node address to the home

address

Renew a binding that’s about to expire Bindings have lifetimes

De-register when they return home

Registration and Security

The home node and the mobile node have conducted some form of prior key exchange

This defines a “secret” between the two nodes The authentication mechanism must defend against replay

attacks

The mobile node uses the keys to authenticate the redirection request

This is not the same as encrypting the communication channel

Replay Attacks & Signatures

A replay attack occurs when a 3rd party can capture your packets and then “replay” them, fooling you into thinking they are correctly authenticated.

E.g., sending an encrypted password over a network leaves you open to a replay attack. Note that attack didn’t decrypt

Two methods are used Timestamps: the sender includes a timestamp, and receiver

must find that timestamp is close to their local time. Nonces: Each message from A -> B includes a new random

number. When B replies to A, it must include that same random number. Likewise, each B->A message includes a new random number generated by B and echoed by A.

MD5 & Secure Hashes

A secure hash function is a one-way encoding of a document to a particular hash value.

Knowing the hash value provides no information about the document, but you can repeatedly generate the hash value from the document

Probability of collision should be small

MD5 (128 bit hash value) SHA-1 RIPEMD-160

doc1

doc2

hash1

hash2

MD5

Public Key Cryptography

Public key cryptography is way of “signing” a encoding (M) using a private or secret key (sk) yielding a modified document (M’) that can be decoded using a public key (pk).

The public key can decrypt messages encrypted using the secret key

The secret key can decrypt messages encrypted using the public key

gen

crypt decryptM

M’

skpk

M

Digital Signatures

A digital signature is an electronic way of “signing” a document such that

Both you and the sender agree that only the sender could send the doucment

Both you and the sender agree the contents haven’t been modified

Notice that the message doesn’t need to be encrypted

Message MD5 encryptmd5Message

smd5

Message

smd5decrypt

MD5md5

md5

Compare

Authenticating Registration In Mobile IP Uses Private-Key Cryptography

HAA

Key Exchange

HA

A

Datagramkey

MD5

key

Datagrammd5ipDatagrammd5

Datagramkey key

Compare

Diffie-Hellman Public Key Exchange

Public key cryptosystem that allows two parties to establish a shared secret key, such that the secret key cannot be determined by other parties overhearing the public message exchange.

Two public numbers known by both parties, but not kept secret: a prime (p) and a generator (g)

Each side chooses a private random number (x) Computes c = gx, and then computes & sends “c mod p” Each party then computes the shared same secret key using its

own private random number, x and it’s own p. The secret is “cy mod p” where “y” is own private random # Since gx(y)=gy(x), both know a specific value and knowing “c mod

p” doesn’t let you determine gx(y)

Diffie-Hellman

Once you have established the shared key, you can use digital signature mechanisms to authenticate future communication between the parties

Entire process is anonymous

Subject to a “man in the middle” attack

mobile FA

Authentication in Mobile IP

You need to use signatures for timestamp based methods because the encoded value used to defeat replay attacks (time) is easily predictable

Nonce based systems are based on pseudo-random number sequences. As long as the sequence is not predictable (I.e. it’s heavily influenced by the private key), then you may not need to authenticate it

This is the level of authentication provided by DSS / Frequency hopping

Delivering datagrams

Once a mobile agent has registered a care-of address, datagrams must be delivered to that address.

Many options to get messages there... Have source redirect messages Use forwarding with loose source routing Use forward tunnels

And other options to getmessages back..

Have node directly contactsource with spoofed header

Use reverse tunnels

HA FA

NodeSource

Tunneling Basics

HA FA

NodeSource

Tunneling

IP-in-IP encapsulation Minimal encapsulation GRE -- Generic Routing Encapsulation PPTP -- Point to Point Tunnel Protocol [RFC2637] L2TP -- Layer 2 Tunneling Protocol [RFC2661]

IP in IP

IP Header OPTS InnerIP Header Datagram

IP Header DatagramTunnelEndpoints

IP-in-IP Encapsulation

IP in IP

The outer IP header source & destination address identify the tunnel endpoints (I.e., HA & FA).

Outer protocol is ‘4’ (IP protocol) The inner IP header source address and destination

address identify the original sender & recipient Not changed by the encapsulator, except to change TTL

Other headers for authentication might be added to outer header.

Some outer IP header fields are copied from the inner IP fields (TOS), most are re-computed (checksum, length) based on new datagram

Minimal Encapsulation

Outer IP Header Minimal Header Datagram

IP Header DatagramTunnelEndpoints

Dest IP Address

Minimal Encapsulation

We can save some space by recognizing that much of the inner header can be derived from the outer header

Copy inner header Modify protocol field to be 55, for the minimal encapsulation

protocol Destination address replaced by tunnel exit If encapsulator isn’t originator of message, replace source

address with address of encapsulator Increment total length by the size of the additional header

(either 12 or 8 octets) Recompute checksum

Minimal Encapsulation Header

Header ChecksumProtocol ResvS

Original Destination Address

Original Source Address

Specifies if sourceaddress provided

GRE

Generic Routing Encapsulation (RFC2784) Implemented in e.g. Linux, Cisco routers, etc

Generalized IP format that can route any protocol over IP Multiple source routes specified by source route records

Formats specified for IP, AppleTalk, IPX, etc Also used for Ethernet bridging

Handling Broadcast & Multicast

HA should forward everything (but not ARP packets) Broadcast packets are either sent directly to co-located

nodes or “double encapsulated”

Mobile nodes can join multicast routes on the foreign network

But, this doesn’t handle link-level or administratively scoped multicast

Or, it can set up a “bi-directional tunnel” with HA

IP Dest(care of)

IP Dest(mobile)

Datagram

Lastly, ARP

When a mobile node is on a foreign net, its HA used proxy ARP to get any messages directed to it

When a mobile node leaves home, the HA used gratuitous ARP to update all ARP tables on the subnet

When a mobile node returns home, it uses gratuitous ARP to recapture its messages

When a mobile node is away from home, it can’t transmit any broadcast ARP or ARP reply messages

This means that even “local” traffic destined for the mobile node on the foreign network goes to the HA and then the FA and then the mobile node!

The need for reverse tunnels

HA FA

NodeSource

128.138. 241.10

161.145.65.58

Ingress filtering discardsdatagrams that appear to

originate from outsidethe domain

128.138. 241.10

128.138. 241.10

Reverse Tunnel In Action

HA FA

NodeSource

Alternate Reverse Tunnel

HA FA

NodeSource

Route Optimization

Obviously, all this indirection has a performance penalty Solution: remove that indirection!

Route optimization tackles three areas Supply a binding update to a correspondent node that needs

one (and has a chance of processing it correctly) Provide a way to create the authenticated so that the recipient

of a binding update can believe it Allow the mobile node and foreign agent to create a registration

key for later use in making a smooth transition to a new point of attachement.

Route Optimization

HA FA

NodeSource

1

2

3

5

BindingUpdate

Route Optimization

Foreign Agent Smooth Handoff

When a mobile node moves & registers with a new foreign agent, the base Mobile IP protocol does not notify the previous FA.

New messages tunneled to new care-of address In-flight datagrams are lost & upper-level layers (e.g. TCP)

should handle that

As part of registration, the mobile node can have the new FA contact the previous FA

New FA builds a binding update message with a “forwarding pointer” to its new location

The new FA and the mobile node need a shared secret, the registration key, used to authenticate the notification sent to the previous foreign agent

Registration Keys

Need to have a way to have an anonymous foreign agent to establish a registration key with the mobile node

Use the mobility “security association” they share if it exists or can be established

Use the mobile nodes public key, if it exists Use the FA public key, if it exists, to enable the HA to create

public keys for both entities (transitive trust) Use the security association between the FA and HA to create

keys for both entities Use the Diffie-Hellman key exchange algorithm

Route Optimizations

Binding warning: Used by old foreign agent, to request the home agent to send current binding to a correspondent host.

When a host moves: Old foreign agent may cache a forwarding pointer to the new

foreign agent: packets re-tunneled along the forwarding pointer + binding warning sent to home agent to update the correspondent with the new binding

Old foreign agent may not cache (or purge) the forwarding pointer: packets forwarded to home agent. Home agent tunnels it to current care-of-address + sends binding update to correspondent

MosquitoNet

No foreign agent Visiting mobile host is assigned a temporary IP address

corresponding to the foreign subnet. Packets are tunneled directly to the mobile host (without

having to go through a foreign agent)

MosquitoNet -- Advantages

Mobile hosts can visit networks that do not have home agents

Foreign agent is no more a single point of failure Scalability: Foreign agent not needed on every network

that a mobile may visit. Home agents only needed on networks with mobile clients

Simpler protocol: Only part of foreign agent functionality needed

MosquitoNet -- Disadvantages

Mobile host needs to acquire a temporary IP on foreign subnet

Security: If a temporary IP address is re-assigned to another mobile too soon, the new mobile agent may receive packets intended for the previous mobile.

But shouldn’t security / authentication issues remove this?

Packet loss: Foreign agents can forward packets destined for a mobile host that has moved to another foreign subnet. Without foreign agents, the packets will simply be lost.

Mobile host is more complex, as it must incorporate some of the functionality of a foreign agent.

Other Protocols: CDPD

CDPD: Cellular Digital Packet Data Similarity to Mobile IP:

Triangular routing approach between mobile host and home and foreign agents

Differences: User IP assigned by CDPD service provider Uses prop. Tunneling, not IP-in-IP or GRE Not strictly above the data link layer

Other Protocols: GPRS

GPRS: General Packet Radio Data

GSN: GPRS Support NodeMSC: Mobile Switching CenterBTS: Base Transciever StationBSC: Base Station Controller

Mobile IP vs. CDPD vs. GPRS

CDPD is slowing down (Jan 1999) Mobile IP is big in the US. IETF is behind it US industry just started adapting Mobile IP Motorola’s iDEN network is Mobile IP based.