Chapter 8 Network Security - IRIFhf/verif/ens/an08-09/internet/cours5-12.pdf · 8: Network Security...

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8: Network Security 8-481

Chapter 8 Network Security

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Thanks and enjoy! JFK/KWR

All material copyright 1996-2004

J.F Kurose and K.W. Ross, All Rights Reserved

Computer Networking: A Top Down Approach Featuring the Internet, 3rd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2004.


Chapter 8: Network Security

Chapter goals:

understand principles of network security: cryptography and its many uses beyond “confidentiality”

authentication

message integrity

key distribution

security in practice: firewalls

security in application, transport, network, link layers

2


Chapter 8 roadmap

8.1 What is network security?

8.2 Principles of cryptography

8.3 Authentication

8.4 Integrity

8.5 Key Distribution and certification

8.6 Access control: firewalls

8.7 Attacks and counter measures

8.8 Security in many layers


What is network security?

Confidentiality: only sender, intended receiver should “understand” message contents

sender encrypts message

receiver decrypts message

Authentication: sender, receiver want to confirm identity of each other

Message Integrity: sender, receiver want to ensure message not altered (in transit, or afterwards) without detection

Access and Availability: services must be accessible and available to users

3


Friends and enemies: Alice, Bob, Trudy well-known in network security world

Bob, Alice (lovers!) want to communicate “securely”

Trudy (intruder) may intercept, delete, add messages

secure sender

secure receiver

channel data, control messages

data data

Alice Bob

Trudy


Who might Bob, Alice be?

… well, real-life Bobs and Alices! Web browser/server for electronic transactions (e.g., on-line purchases) on-line banking client/server DNS servers routers exchanging routing table updates other examples?

4


There are bad guys (and girls) out there!

Q: What can a “bad guy” do? A: a lot!

eavesdrop: intercept messages actively insert messages into connection impersonation: can fake (spoof) source address in packet (or any field in packet) hijacking: “take over” ongoing connection by removing sender or receiver, inserting himself in place denial of service: prevent service from being used by others (e.g., by overloading resources)

more on this later ……


Chapter 8 roadmap



8.3 Authentication

8.4 Integrity





5


The language of cryptography

symmetric key crypto: sender, receiver keys identical

public-key crypto: encryption key public, decryption key secret (private)

plaintext plaintext ciphertext

K A

encryption algorithm

decryption algorithm

Alice’s encryption key

Bob’s decryption key

K B


Symmetric key cryptography

substitution cipher: substituting one thing for another monoalphabetic cipher: substitute one letter for another

plaintext: abcdefghijklmnopqrstuvwxyz

ciphertext: mnbvcxzasdfghjklpoiuytrewq

Plaintext: bob. i love you. alice

ciphertext: nkn. s gktc wky. mgsbc

E.g.:

Q: How hard to break this simple cipher?: brute force (how hard?) other?

6


Symmetric key cryptography

symmetric key crypto: Bob and Alice share know same (symmetric) key: K e.g., key is knowing substitution pattern in mono alphabetic substitution cipher Q: how do Bob and Alice agree on key value?

plaintext ciphertext

K A-B

encryption algorithm


A-B

K A-B

plaintext message, m

K (m) A-B

K (m) A-B

m = K ( ) A-B


Symmetric key crypto: DES

DES: Data Encryption Standard

US encryption standard [NIST 1993]

56-bit symmetric key, 64-bit plaintext input

How secure is DES?

DES Challenge: 56-bit-key-encrypted phrase (“Strong cryptography makes the world a safer place”) decrypted (brute force) in 4 months

no known “backdoor” decryption approach

making DES more secure:

use three keys sequentially (3-DES) on each datum

use cipher-block chaining

7


Symmetric key crypto: DES

initial permutation

16 identical “rounds” of function application, each using different 48 bits of key

final permutation

DES operation


AES: Advanced Encryption Standard

new (Nov. 2001) symmetric-key NIST standard, replacing DES

processes data in 128 bit blocks

128, 192, or 256 bit keys

brute force decryption (try each key) taking 1 sec on DES, takes 149 trillion years for AES

8


Public Key Cryptography

symmetric key crypto

requires sender, receiver know shared secret key

Q: how to agree on key in first place (particularly if never “met”)?

public key cryptography

radically different approach [Diffie-Hellman76, RSA78]

sender, receiver do not share secret key

public encryption key known to all

private decryption key known only to receiver


Public key cryptography

plaintext message, m

ciphertext encryption algorithm


Bob’s public key

plaintext message K (m)

B

+

K B

+

Bob’s private key

K B

-

m = K (K (m)) B

+ B

-

9


Public key encryption algorithms

need K ( ) and K ( ) such that B B

. .

given public key K , it should be impossible to compute private key K B

B

Requirements:

1

2

RSA: Rivest, Shamir, Adelson algorithm

+ -

K (K (m)) = m B B

- +

+

-


RSA: Choosing keys

1. Choose two large prime numbers p, q. (e.g., 1024 bits each)

2. Compute n = pq, z = (p-1)(q-1)

3. Choose e (with e<n) that has no common factors with z. (e, z are “relatively prime”).

4. Choose d such that ed-1 is exactly divisible by z. (in other words: ed mod z = 1 ).

5. Public key is (n,e). Private key is (n,d).

K B + K

B -

10


RSA: Encryption, decryption

0. Given (n,e) and (n,d) as computed above

1. To encrypt bit pattern, m, compute

c = m mod n e (i.e., remainder when m is divided by n) e

2. To decrypt received bit pattern, c, compute

m = c mod n d (i.e., remainder when c is divided by n) d

m = (m mod n) e mod n d Magic happens!

c


RSA example: Bob chooses p=5, q=7. Then n=35, z=24.

e=5 (so e, z relatively prime). d=29 (so ed-1 exactly divisible by z.

letter m m e c = m mod n e

l 12 1524832 17

c m = c mod n d

17 481968572106750915091411825223071697 12

c d

letter

l

encrypt:

decrypt:

11


RSA: Why is that m = (m mod n) e mod n d

(m mod n) e mod n = m mod n d ed

Useful number theory result: If p,q prime and n = pq, then:

x mod n = x mod n y y mod (p-1)(q-1)

= m mod n ed mod (p-1)(q-1)

= m mod n 1

= m

(using number theory result above)

(since we chose ed to be divisible by (p-1)(q-1) with remainder 1 )


RSA: another important property

The following property will be very useful later:

K (K (m)) = m B B

- + K (K (m))

B B + -

=

use public key first, followed by private key

use private key first, followed by public key

Result is the same!

12


Chapter 8 roadmap



8.3 Authentication

8.4 Integrity






Authentication

Goal: Bob wants Alice to “prove” her identity to him

Protocol ap1.0: Alice says “I am Alice”

Failure scenario?? “I am Alice”

13


Authentication

Goal: Bob wants Alice to “prove” her identity to him

Protocol ap1.0: Alice says “I am Alice”

in a network, Bob can not “see” Alice, so Trudy simply declares herself to be Alice

“I am Alice”


Authentication: another try

Protocol ap2.0: Alice says “I am Alice” in an IP packet containing her source IP address

Failure scenario??

“I am Alice” Alice’s IP address

14



Protocol ap2.0: Alice says “I am Alice” in an IP packet containing her source IP address

Trudy can create a packet “spoofing” Alice’s address “I am Alice”

Alice’s IP address



Protocol ap3.0: Alice says “I am Alice” and sends her secret password to “prove” it.

Failure scenario??

“I’m Alice” Alice’s IP addr

Alice’s password

OK Alice’s IP addr

15



Protocol ap3.0: Alice says “I am Alice” and sends her secret password to “prove” it.

playback attack: Trudy records Alice’s packet and later plays it back to Bob


Alice’s password



Alice’s password


Authentication: yet another try

Protocol ap3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it.

Failure scenario??


encrypted password


16



Protocol ap3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it.

record and playback still works!


encrypted password



encrypted password


Authentication: yet another try

Goal: avoid playback attack

Failures, drawbacks?

Nonce: number (R) used only once –in-a-lifetime

ap4.0: to prove Alice “live”, Bob sends Alice nonce, R. Alice must return R, encrypted with shared secret key

“I am Alice”

R

K (R) A-B

Alice is live, and only Alice knows key to encrypt nonce, so it must be Alice!

17


Authentication: ap5.0

ap4.0 requires shared symmetric key

can we authenticate using public key techniques?

ap5.0: use nonce, public key cryptography

“I am Alice”

R Bob computes

K (R) A -

“send me your public key”

K A

+

(K (R)) = R A

- K A

+

and knows only Alice could have the private key, that encrypted R such that

(K (R)) = R A

- K A

+


ap5.0: security hole Man (woman) in the middle attack: Trudy poses as

Alice (to Bob) and as Bob (to Alice)

I am Alice I am Alice

R

T K (R)

-

Send me your public key

T K

+ A

K (R) -

Send me your public key

A K

+

T K (m) +

T m = K (K (m))

+

T

- Trudy gets

sends m to Alice encrypted with Alice’s public key

A K (m) +

A m = K (K (m))

+

A

-

R

18


ap5.0: security hole Man (woman) in the middle attack: Trudy poses as

Alice (to Bob) and as Bob (to Alice)

Difficult to detect: Bob receives everything that Alice sends, and vice

versa. (e.g., so Bob, Alice can meet one week later and recall conversation)

problem is that Trudy receives all messages as well!


Chapter 8 roadmap



8.3 Authentication

8.4 Message integrity





19


Digital Signatures

Cryptographic technique analogous to hand-written signatures. sender (Bob) digitally signs document, establishing he is document owner/creator.

verifiable, nonforgeable: recipient (Alice) can prove to someone that Bob, and no one else (including Alice), must have signed document


Digital Signatures

Simple digital signature for message m: Bob signs m by encrypting with his private key KB, creating “signed” message, KB(m) - -

Bob’s message, m

Public key encryption algorithm

Bob’s private key

K B -

K B - (m)

20


Digital Signatures (more)

Suppose Alice receives msg m, digital signature KB(m)

Alice verifies m signed by Bob by applying Bob’s public key KB to KB(m) then checks KB(KB(m) ) = m.

If KB(KB(m) ) = m, whoever signed m must have used Bob’s private key.

Alice thus verifies that: Bob signed m. No one else signed m. Bob signed m and not m’.

Non-repudiation: Alice can take m, and signature KB(m) to court and prove that Bob signed m.


Message Digests

Computationally expensive to public-key-encrypt long messages

Goal: fixed-length, easy- to-compute digital “fingerprint”

apply hash function H to m, get fixed size message digest, H(m).

Hash function properties:

many-to-1

produces fixed-size msg digest (fingerprint)

given message digest x, computationally infeasible to find m such that x = H(m)

large message m

H: Hash Function

H(m)

21


Internet checksum: poor crypto hash function

Internet checksum has some properties of hash function:

produces fixed length digest (16-bit sum) of message

is many-to-one

But given message with given hash value, it is easy to find another message with same hash value:

I O U 10 0 . 99 B O B

49 4F 55 3130 30 2E 3939 42 D2 42

message ASCII format

B2 C1 D2 AC

I O U 90 0 . 19 B O B

49 4F 55 3930 30 2E 3139 42 D2 42

message ASCII format

B2 C1 D2 ACdifferent messages but identical checksums!


large message m

H: Hash function H(m)

digital signature (encrypt)

Bob’s private

key K B -

+

Bob sends digitally signed message:

Alice verifies signature and integrity of digitally signed message:

KB(H(m)) -

encrypted msg digest

KB(H(m)) -

encrypted msg digest

large message m

H: Hash function

H(m)

digital signature (decrypt)

H(m)

Bob’s public

key K B +

equal ?

Digital signature = signed message digest

22


Hash Function Algorithms

MD5 hash function widely used (RFC 1321)

computes 128-bit message digest in 4-step process.

arbitrary 128-bit string x, appears difficult to construct msg m whose MD5 hash is equal to x.

SHA-1 is also used.

US standard [NIST, FIPS PUB 180-1]

160-bit message digest


Chapter 8 roadmap



8.3 Authentication

8.4 Integrity

8.5 Key distribution and certification




23


Trusted Intermediaries

Symmetric key problem: How do two entities establish shared secret key over network?

Solution: trusted key distribution center (KDC) acting as intermediary between entities

Public key problem: When Alice obtains Bob’s public key (from web site, e-mail, diskette), how does she know it is Bob’s public key, not Trudy’s?

Solution: trusted certification authority (CA)


Key Distribution Center (KDC)

Alice, Bob need shared symmetric key.

KDC: server shares different secret key with each registered user (many users)

Alice, Bob know own symmetric keys, KA-KDC KB-KDC , for

communicating with KDC.

KB-KDC

KX-KDC

KY-KDC

KZ-KDC

KP-KDC

KB-KDC

KA-KDC

KA-KDC

KP-KDC

KDC

24


Key Distribution Center (KDC)

Alice knows R1

Bob knows to use R1 to communicate with Alice

Alice and Bob communicate: using R1 as session key for shared symmetric encryption

Q: How does KDC allow Bob, Alice to determine shared symmetric secret key to communicate with each other?

KDC generates R1


Certification Authorities

Certification authority (CA): binds public key to particular entity, E.

E (person, router) registers its public key with CA. E provides “proof of identity” to CA.

CA creates certificate binding E to its public key.

certificate containing E’s public key digitally signed by CA – CA says “this is E’s public key”

Bob’s public

key K B +

Bob’s identifying

information

digital signature (encrypt)

CA private

key K CA -

K B +

certificate for Bob’s public key,

signed by CA

25


Certification Authorities

When Alice wants Bob’s public key:

gets Bob’s certificate (Bob or elsewhere).

apply CA’s public key to Bob’s certificate, get Bob’s public key

Bob’s public

key K B +

digital signature (decrypt)

CA public

key K CA +

K B +


A certificate contains:

Serial number (unique to issuer)

info about certificate owner, including algorithm and key value itself (not shown)

info about certificate issuer

valid dates

digital signature by issuer

26


Chapter 8 roadmap



8.3 Authentication

8.4 Integrity






Firewalls

isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others.

firewall

administered network

public Internet

firewall

27


Firewalls: Why

prevent denial of service attacks:

SYN flooding: attacker establishes many bogus TCP connections, no resources left for “real” connections.

prevent illegal modification/access of internal data.

e.g., attacker replaces CIA’s homepage with something else

allow only authorized access to inside network (set of authenticated users/hosts)

two types of firewalls:

application-level

packet-filtering


Packet Filtering

internal network connected to Internet via router firewall router filters packet-by-packet, decision to forward/drop packet based on:

source IP address, destination IP address TCP/UDP source and destination port numbers ICMP message type TCP SYN and ACK bits

Should arriving packet be allowed in? Departing packet let out?

28


Packet Filtering

Example 1: block incoming and outgoing datagrams with IP protocol field = 17 and with either source or dest port = 23.

All incoming and outgoing UDP flows and telnet connections are blocked.

Example 2: Block inbound TCP segments with ACK=0.

Prevents external clients from making TCP connections with internal clients, but allows internal clients to connect to outside.


Application gateways

Filters packets on application data as well as on IP/TCP/UDP fields.

Example: allow select internal users to telnet outside.

host-to-gateway telnet session

gateway-to-remote host telnet session

application gateway

router and filter

1. Require all telnet users to telnet through gateway.

2. For authorized users, gateway sets up telnet connection to dest host. Gateway relays data between 2 connections

3. Router filter blocks all telnet connections not originating from gateway.

29


Limitations of firewalls and gateways

IP spoofing: router can’t know if data “really” comes from claimed source

if multiple app’s. need special treatment, each has own app. gateway.

client software must know how to contact gateway.

e.g., must set IP address of proxy in Web browser

filters often use all or nothing policy for UDP.

tradeoff: degree of communication with outside world, level of security

many highly protected sites still suffer from attacks.


Chapter 8 roadmap



8.3 Authentication

8.4 Integrity





30


Internet security threats Mapping:

before attacking: “case the joint” – find out what services are implemented on network

Use ping to determine what hosts have addresses on network

Port-scanning: try to establish TCP connection to each port in sequence (see what happens)

nmap (http://www.insecure.org/nmap/) mapper: “network exploration and security auditing”

Countermeasures?


Internet security threats Mapping: countermeasures

record traffic entering network

look for suspicious activity (IP addresses, pots being scanned sequentially)

31


Internet security threats Packet sniffing:

broadcast media

promiscuous NIC reads all packets passing by

can read all unencrypted data (e.g. passwords)

e.g.: C sniffs B’s packets

A

B

C

src:B dest:A payload

Countermeasures?


Internet security threats Packet sniffing: countermeasures

all hosts in organization run software that checks periodically if host interface in promiscuous mode. one host per segment of broadcast media (switched Ethernet at hub)

A

B

C


32


Internet security threats IP Spoofing:

can generate “raw” IP packets directly from application, putting any value into IP source address field

receiver can’t tell if source is spoofed

e.g.: C pretends to be B

A

B

C


Countermeasures?


Internet security threats IP Spoofing: ingress filtering

routers should not forward outgoing packets with invalid source addresses (e.g., datagram source address not in router’s network)

great, but ingress filtering can not be mandated for all networks

A

B

C


33


Internet security threats Denial of service (DOS):

flood of maliciously generated packets “swamp” receiver

Distributed DOS (DDOS): multiple coordinated sources swamp receiver

e.g., C and remote host SYN-attack A

A

B

C

SYN

SYN SYN SYN

SYN

SYN

SYN

Countermeasures?


Internet security threats Denial of service (DOS): countermeasures

filter out flooded packets (e.g., SYN) before reaching host: throw out good with bad traceback to source of floods (most likely an innocent, compromised machine)

A

B

C

SYN

SYN SYN SYN

SYN

SYN

SYN

34


Chapter 8 roadmap

8.1 What is network security? 8.2 Principles of cryptography 8.3 Authentication 8.4 Integrity 8.5 Key Distribution and certification 8.6 Access control: firewalls 8.7 Attacks and counter measures

8.8 Security in many layers 8.8.1. Secure email 8.8.2. Secure sockets 8.8.3. IPsec 8.8.4. Security in 802.11


Secure e-mail

Alice: generates random symmetric private key, KS. encrypts message with KS (for efficiency) also encrypts KS with Bob’s public key. sends both KS(m) and KB(KS) to Bob.

Alice wants to send confidential e-mail, m, to Bob.

KS( ) .

KB( ) . +

+ -

KS(m )

KB(KS ) +

m

KS

KS

KB +

Internet

KS( ) .

KB( ) . -

KB -

KS

m KS(m )

KB(KS ) +

35


Secure e-mail

Bob: uses his private key to decrypt and recover KS uses KS to decrypt KS(m) to recover m

Alice wants to send confidential e-mail, m, to Bob.

KS( ) .

KB( ) . +

+ -

KS(m )

KB(KS ) +

m

KS

KS

KB +

Internet

KS( ) .

KB( ) . -

KB -

KS

m KS(m )

KB(KS ) +


Secure e-mail (continued)

• Alice wants to provide sender authentication message integrity.

• Alice digitally signs message. • sends both message (in the clear) and digital signature.

H( ) . KA( ) . -

+ -

H(m ) KA(H(m)) -

m

KA -

Internet

m

KA( ) . +

KA +

KA(H(m)) -

m H( ) . H(m )

compare

36


Secure e-mail (continued)

• Alice wants to provide secrecy, sender authentication, message integrity.

Alice uses three keys: her private key, Bob’s public key, newly created symmetric key

H( ) . KA( ) . -

+

KA(H(m)) -

m

KA -

m

KS( ) .

KB( ) . +

+

KB(KS ) +

KS

KB +

Internet

KS


Pretty good privacy (PGP)

Internet e-mail encryption scheme, de-facto standard.

uses symmetric key cryptography, public key cryptography, hash function, and digital signature as described.

provides secrecy, sender authentication, integrity.

inventor, Phil Zimmerman, was target of 3-year federal investigation.

---BEGIN PGP SIGNED MESSAGE---

Hash: SHA1

Bob:My husband is out of town

tonight.Passionately yours,

Alice

---BEGIN PGP SIGNATURE---

Version: PGP 5.0

Charset: noconv

yhHJRHhGJGhgg/12EpJ

+lo8gE4vB3mqJhFEvZP9t6n7G6m5Gw

2

---END PGP SIGNATURE---

A PGP signed message:

37


Secure sockets layer (SSL)

transport layer security to any TCP-based app using SSL services.

used between Web browsers, servers for e-commerce (shttp).

security services: server authentication

data encryption

client authentication (optional)

server authentication: SSL-enabled browser includes public keys for trusted CAs. Browser requests server certificate, issued by trusted CA. Browser uses CA’s public key to extract server’s public key from certificate.

check your browser’s security menu to see its trusted CAs.


SSL (continued)

Encrypted SSL session:

Browser generates symmetric session key, encrypts it with server’s public key, sends encrypted key to server.

Using private key, server decrypts session key.

Browser, server know session key

All data sent into TCP socket (by client or server) encrypted with session key.

SSL: basis of IETF Transport Layer Security (TLS).

SSL can be used for non-Web applications, e.g., IMAP.

Client authentication can be done with client certificates.

38


IPsec: Network Layer Security

Network-layer secrecy:

sending host encrypts the data in IP datagram

TCP and UDP segments; ICMP and SNMP messages.

Network-layer authentication

destination host can authenticate source IP address

Two principle protocols:

authentication header (AH) protocol

encapsulation security payload (ESP) protocol

For both AH and ESP, source, destination handshake:

create network-layer logical channel called a security association (SA)

Each SA unidirectional.

Uniquely determined by:

security protocol (AH or ESP)

source IP address

32-bit connection ID


Authentication Header (AH) Protocol

provides source authentication, data integrity, no confidentiality

AH header inserted between IP header, data field.

protocol field: 51

intermediate routers process datagrams as usual

AH header includes:

connection identifier

authentication data: source- signed message digest calculated over original IP datagram.

next header field: specifies type of data (e.g., TCP, UDP, ICMP)

IP header data (e.g., TCP, UDP segment) AH header

39


ESP Protocol

provides secrecy, host authentication, data integrity.

data, ESP trailer encrypted.

next header field is in ESP trailer.

ESP authentication field is similar to AH authentication field.

Protocol = 50.

IP header TCP/UDP segment ESP header

ESP trailer

ESP authent.

encrypted authenticated


IEEE 802.11 security

War-driving: drive around Bay area, see what 802.11 networks available?

More than 9000 accessible from public roadways

85% use no encryption/authentication

packet-sniffing and various attacks easy!

Securing 802.11

encryption, authentication

first attempt at 802.11 security: Wired Equivalent Privacy (WEP): a failure

current attempt: 802.11i

40


Wired Equivalent Privacy (WEP):

authentication as in protocol ap4.0

host requests authentication from access point

access point sends 128 bit nonce

host encrypts nonce using shared symmetric key

access point decrypts nonce, authenticates host

no key distribution mechanism

authentication: knowing the shared key is enough


WEP data encryption

Host/AP share 40 bit symmetric key (semi-permanent)

Host appends 24-bit initialization vector (IV) to create 64-bit key

64 bit key used to generate stream of keys, kiIV

kiIV used to encrypt ith byte, di, in frame:

ci = di XOR kiIV

IV and encrypted bytes, ci sent in frame

41


802.11 WEP encryption

Sender-side WEP encryption


Breaking 802.11 WEP encryption

Security hole: 24-bit IV, one IV per frame, -> IV’s eventually reused

IV transmitted in plaintext -> IV reuse detected

Attack:

Trudy causes Alice to encrypt known plaintext d1 d2 d3 d4 …

Trudy sees: ci = di XOR kiIV

Trudy knows ci di, so can compute kiIV

Trudy knows encrypting key sequence k1IV k2

IV k3IV …

Next time IV is used, Trudy can decrypt!

42


802.11i: improved security

numerous (stronger) forms of encryption possible

provides key distribution

uses authentication server separate from access point


AP: access point AS:

Authentication

server

wired

network

STA:

client station

1 Discovery of

security capabilities

3

STA and AS mutually authenticate, together

generate Master Key (MK). AP servers as “pass through” 2

3 STA derives

Pairwise Master

Key (PMK)

AS derives

same PMK,

sends to AP

4 STA, AP use PMK to derive

Temporal Key (TK) used for message

encryption, integrity

802.11i: four phases of operation

43


wired

network

EAP TLS

EAP

EAP over LAN (EAPoL)

IEEE 802.11

RADIUS

UDP/IP

EAP: extensible authentication protocol

EAP: end-end client (mobile) to authentication server protocol

EAP sent over separate “links” mobile-to-AP (EAP over LAN)

AP to authentication server (RADIUS over UDP)


Network Security (summary)

Basic techniques…... cryptography (symmetric and public)

authentication

message integrity

key distribution

…. used in many different security scenarios secure email

secure transport (SSL)

IP sec

802.11

44

6: Wireless and Mobile Networks 6-567

Chapter 6

Wireless and Mobile

Networks

Computer

Networking: A Top

Down Approach

Featuring the

Internet,

3rd edition.

Jim Kurose, Keith

Ross

Addison-Wesley,

July 2004.

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Thanks and enjoy! JFK/KWR

All material copyright 1996-2004

J.F Kurose and K.W. Ross, All Rights Reserved


Chapter 6: Wireless and Mobile Networks

Background: # wireless (mobile) phone subscribers now exceeds # wired phone subscribers!

computer nets: laptops, palmtops, PDAs, Internet-enabled phone promise anytime untethered Internet access

two important (but different) challenges communication over wireless link

handling mobile user who changes point of attachment to network

45


Chapter 6 outline

6.1 Introduction

Wireless 6.2 Wireless links, characteristics

CDMA

6.3 IEEE 802.11 wireless LANs (“wi-fi”) 6.4 Cellular Internet Access

architecture standards (e.g., GSM)

Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Handling mobility in cellular networks 6.8 Mobility and higher-layer protocols

6.9 Summary


Elements of a wireless network

network

infrastructure

wireless hosts

laptop, PDA, IP phone

run applications

may be stationary (non-mobile) or mobile

wireless does not always mean mobility

46



network

infrastructure

base station

typically connected to wired network

relay - responsible for sending packets between wired network and wireless host(s) in its “area”

e.g., cell towers 802.11 access points



network

infrastructure

wireless link

typically used to connect mobile(s) to base station

also used as backbone link

multiple access protocol coordinates link access

various data rates, transmission distance

47


Characteristics of selected wireless link standards

384 Kbps

56 Kbps

54 Mbps

5-11 Mbps

1 Mbps 802.15

802.11b

802.11{a,g}

IS-95 CDMA, GSM

UMTS/WCDMA, CDMA2000

.11 p-to-p link

2G

3G

Indoor

10 – 30m

Outdoor

50 – 200m

Mid range

outdoor

200m – 4Km

Long range

outdoor

5Km – 20Km



network

infrastructure

infrastructure mode

base station connects mobiles into wired network

handoff: mobile changes base station providing connection into wired network

48



Ad hoc mode

no base stations

nodes can only transmit to other nodes within link coverage

nodes organize themselves into a network: route among themselves


Wireless Link Characteristics

Differences from wired link ….

decreased signal strength: radio signal attenuates as it propagates through matter (path loss) interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well multipath propagation: radio signal reflects off objects ground, arriving ad destination at slightly different times

…. make communication across (even a point to point) wireless link much more “difficult”

49


Wireless network characteristics Multiple wireless senders and receivers create

additional problems (beyond multiple access):

A B

C

Hidden terminal problem B, A hear each other

B, C hear each other

A, C can not hear each other

means A, C unaware of their interference at B

A B C

A’s signal

strength

space

C’s signal

strength

Signal fading: B, A hear each other

B, C hear each other

A, C can not hear each other interferring at B


Code Division Multiple Access (CDMA)

used in several wireless broadcast channels (cellular, satellite, etc) standards unique “code” assigned to each user; i.e., code set partitioning all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode data encoded signal = (original data) X (chipping sequence) decoding: inner-product of encoded signal and chipping sequence allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)

50


CDMA Encode/Decode

slot 1 slot 0

d1 = -1

1 1 1 1

1 - 1 - 1 - 1 -

Zi,m= di.cm d0 = 1

1 1 1 1

1 - 1 - 1 - 1 -

1 1 1 1

1 - 1 - 1 - 1 -

1 1 1 1

1 - 1 - 1 - 1 -

slot 0

channel

output

slot 1

channel

output

channel output Zi,m

sender

code

data

bits

slot 1 slot 0

d1 = -1

d0 = 1

1 1 1 1

1 - 1 - 1 - 1 -

1 1 1 1

1 - 1 - 1 - 1 -

1 1 1 1

1 - 1 - 1 - 1 -

1 1 1 1

1 - 1 - 1 - 1 -

slot 0

channel

output

slot 1

channel

output receiver

code

received

input

Di = Zi,m.cm m=1

M

M


CDMA: two-sender interference

51


Chapter 6 outline

6.1 Introduction


CDMA




6.9 Summary


IEEE 802.11 Wireless LAN

802.11b 2.4-5 GHz unlicensed radio spectrum

up to 11 Mbps

direct sequence spread spectrum (DSSS) in physical layer

• all hosts use same chipping code

widely deployed, using base stations

802.11a 5-6 GHz range

up to 54 Mbps

802.11g 2.4-5 GHz range

up to 54 Mbps

All use CSMA/CA for multiple access

All have base-station and ad-hoc network versions

52


802.11 LAN architecture

wireless host communicates

with base station

base station = access point

(AP)

Basic Service Set (BSS) (aka

“cell”) in infrastructure mode

contains:

wireless hosts

access point (AP): base

station

ad hoc mode: hosts only

BSS

1

BSS 2

Internet

hub, switch

or router AP

AP


802.11: Channels, association

802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies; 3 non-overlapping

AP admin chooses frequency for AP

interference possible: channel can be same as that chosen by neighboring AP!

host: must associate with an AP scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address

selects AP to associate with; initiates association protocol

may perform authentication

will typically run DHCP to get IP address in AP’s subnet

53


IEEE 802.11: multiple access

Like Ethernet, uses CSMA: random access

carrier sense: don’t collide with ongoing transmission

Unlike Ethernet: no collision detection – transmit all frames to completion

acknowledgment – because without collision detection, you don’t know if your transmission collided or not

Why no collision detection? difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

can’t sense all collisions in any case: hidden terminal, fading

Goal: avoid collisions: CSMA/C(ollision)A(voidance)


IEEE 802.11 MAC Protocol: CSMA/CA

802.11 sender

1 if sense channel idle for DIFS then

- transmit entire frame (no CD)

2 if sense channel busy then

- start random backoff time

- timer counts down while channel idle

- transmit when timer expires

- if no ACK, increase random backoff interval, repeat 2

802.11 receiver if frame received OK

- return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

54


RTS/CTS idea: allow sender to “reserve” channel rather than random

access of data frames: avoid collisions of long data frames optional; not typically used sender first transmits small request-to-send (RTS) packets to AP using CSMA

RTSs may still collide with each other (but they’re short) AP broadcasts clear-to-send CTS in response to RTS CTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely

using small reservation packets!


Collision Avoidance: RTS-CTS exchange

AP A B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation

collision

defer

55


frame

control duration

address

1

address

2

address

4

address

3 payload CRC

2 2 6 6 6 2 6 0 - 2312 4 seq

control

802.11 frame: addressing

Address 2: MAC address

of wireless host or AP

transmitting this frame

Address 1: MAC address

of wireless host or AP

to receive this frame

Address 3: MAC

address

of router interface to

which AP is attached

Address 3: used

only in ad hoc

mode


Internet router

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address 3

802.11 frame

R1 MAC addr AP MAC addr dest. address source address

802.3 frame

802.11 frame: addressing

56


frame

control duration

address

1

address

2

address

4

address

3 payload CRC

2 2 6 6 6 2 6 0 - 2312 4 seq

control

Type From

AP Subtype

To

AP

More

frag WEP

More

data

Power

mgt Retry Rsvd

Protocol

version

2 2 4 1 1 1 1 1 1 1 1

802.11 frame: more

duration of reserved

transmission time (RTS/CTS)

frame seq #

(for reliable ARQ)

frame type

(RTS, CTS, ACK, data)


hub or

switch

AP 2

AP 1

H1 BBS 2

BBS 1

802.11: mobility within same subnet

router H1 remains in same IP subnet: IP address can remain same

switch: which AP is associated with H1?

self-learning: switch will see frame from H1 and “remember” which switch port can be used to reach H1

57


Mradius of

coverage

S

SS

P

P

P

P

M

S

Master device

Slave device

Parked device (inactive

P

802.15: personal area network

less than 10 m diameter

replacement for cables (mouse, keyboard, headphones)

ad hoc: no infrastructure

master/slaves: slaves request permission to send (to master)

master grants requests

802.15: evolved from Bluetooth specification

2.4-2.5 GHz radio band

up to 721 kbps


Chapter 6 outline

6.1 Introduction


CDMA




6.9 Summary

58


Mobile

Switching

Center

Public telephone

network, and

Internet

Mobile

Switching

Center

Components of cellular network architecture

connects cells to wide area net

manages call setup (more later!)

handles mobility (more later!)

MSC

covers

geographical region

base station (BS)

analogous to 802.11

AP

mobile users

attach to network

through BS

air-interface:

physical and link

layer protocol

between mobile and

cell

wired network


Cellular networks: the first hop

Two techniques for sharing mobile-to-BS radio spectrum

combined FDMA/TDMA: divide spectrum in frequency channels, divide each channel into time slots

CDMA: code division multiple access

frequency

bands

time slots

59


Cellular standards: brief survey

2G systems: voice channels IS-136 TDMA: combined FDMA/TDMA (north america)

GSM (global system for mobile communications): combined FDMA/TDMA

most widely deployed

IS-95 CDMA: code division multiple access

IS-136 GSM IS-95 GPRS EDGE CDMA-2000

UMTS

TDMA/FDMA

Don’t drown in a bowl

of alphabet soup: use this

oor reference only



2.5 G systems: voice and data channels for those who can’t wait for 3G service: 2G extensions

general packet radio service (GPRS) evolved from GSM

data sent on multiple channels (if available)

enhanced data rates for global evolution (EDGE) also evolved from GSM, using enhanced modulation

Date rates up to 384K

CDMA-2000 (phase 1) data rates up to 144K

evolved from IS-95

60



3G systems: voice/data Universal Mobile Telecommunications Service (UMTS)

GSM next step, but using CDMA

CDMA-2000

….. more (and more interesting) cellular topics due to mobility (stay tuned for details)


Chapter 6 outline

6.1 Introduction


CDMA




6.9 Summary

61


What is mobility?

spectrum of mobility, from the network perspective:

no mobility high mobility

mobile wireless user,

using same access

point

mobile user, passing

through multiple access

point while maintaining

ongoing connections

(like cell phone)

mobile user,

connecting/

disconnecting from

network using DHCP.


Mobility: Vocabulary home network: permanent

“home” of mobile (e.g., 128.119.40/24)

Permanent address:

address in home network,

can always be used to

reach mobile e.g., 128.119.40.186

home agent: entity that will perform

mobility functions on behalf of

mobile, when mobile is remote

wide area

network

correspondent

62


Mobility: more vocabulary

Care-of-address: 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)

home agent: entity in

visited network that

performs mobility

functions on behalf of

mobile.

correspondent: wants to

communicate with

mobile


How do you contact a mobile friend:

search all phone books?

call her parents?

expect her to let you know where he/she is?

I wonder where

Alice moved to? Consider friend frequently changing

addresses, how do you find her?

63


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

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


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

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

not

scalable

to millions of

mobiles

64


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

home

network

visited

network

3

2

4 1

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

65


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

foreign agent functions may be done by mobile itself

triangle routing: correspondent-home-network-mobile

inefficient when

correspondent, mobile

are in same network


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!

66


Mobility via Direct Routing

wide area

network

home

network

visited

network

4

2

4 1 correspondent

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 if mobile changes visited network?

67


wide area

network

1

foreign net visited

at session start anchor

foreign

agent 2

4

new foreign

agent

3 5

correspondent

agent correspondent

new

foreign

network

Accommodating mobility with direct routing

anchor foreign agent: FA in first visited network data always routed first to anchor FA when mobile moves: new FA arranges to have data forwarded from old FA (chaining)


Chapter 6 outline

6.1 Introduction


CDMA




6.9 Summary

68


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: indirect routing of datagrams

agent discovery

registration with home agent


Mobile IP: indirect routing

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

69


Mobile IP: agent discovery

agent advertisement: foreign/home agents advertise service by broadcasting ICMP messages (typefield = 9)

R bit: registration

required

H,F bits: home and/or

foreign agent


Mobile IP: registration example

70


Components of cellular network architecture

correspondent

MSC

MSC

MSC MSC

MSC

wired public

telephone

network

different cellular networks,

operated by different providers

recall:


Handling mobility in cellular networks

home network: network of cellular provider you subscribe to (e.g., Sprint PCS, Verizon)

home location register (HLR): database in home network containing permanent cell phone #, profile information (services, preferences, billing), information about current location (could be in another network)

visited network: network in which mobile currently resides

visitor location register (VLR): database with entry for each user currently in network could be home network

71


Public

switched

telephone network

mobile

user

home

Mobile

Switching Center

HLR home

network

visited

network

correspondent

Mobile

Switching

Center

VLR

GSM: indirect routing to mobile

1 call routed

to home network

2

home MSC consults HLR,

gets roaming number of

mobile in visited network

3

home MSC sets up 2nd leg of call

to MSC in visited network

4

MSC in visited network completes

call through base station to mobile


Mobile

Switching

Center

VLR

old BSS new BSS

old

routing

new

routing

GSM: handoff with common MSC

Handoff goal: route call via new base station (without interruption)

reasons for handoff: stronger signal to/from new BSS (continuing connectivity, less battery drain)

load balance: free up channel in current BSS

GSM doesn’t mandate why to perform handoff (policy), only how (mechanism)

handoff initiated by old BSS

72


Mobile

Switching

Center

VLR

old BSS

1

3

2 4

5 6

7 8

GSM: handoff with common MSC

new BSS

1. old BSS informs MSC of impending

handoff, provides list of 1+ new BSSs

2. MSC sets up path (allocates resources)

to new BSS

3. new BSS allocates radio channel for

use by mobile

4. new BSS signals MSC, old BSS: ready

5. old BSS tells mobile: perform handoff to

new BSS

6. mobile, new BSS signal to activate new

channel

7. mobile signals via new BSS to MSC:

handoff complete. MSC reroutes call

8 MSC-old-BSS resources released


home network

Home

MSC

PSTN

correspondent

MSC

anchor MSC

MSC MSC

(a) before handoff

GSM: handoff between MSCs

anchor MSC: first MSC visited during cal

call remains routed through anchor MSC

new MSCs add on to end of MSC chain as mobile moves to new MSC

IS-41 allows optional path minimization step to shorten multi-MSC chain

73


home network

Home

MSC

PSTN

correspondent

MSC

anchor MSC

MSC MSC

(b) after handoff

GSM: handoff between MSCs

anchor MSC: first MSC

visited during cal

call remains routed through

anchor MSC

new MSCs add on to end of

MSC chain as mobile moves

to new MSC

IS-41 allows optional path

minimization step to shorten

multi-MSC chain


Mobility: GSM versus Mobile IP GSM element Comment on GSM element Mobile IP element

Home system Network to which the mobile user’s permanent

phone number belongs

Home network

Gateway Mobile

Switching Center, or

“home MSC”. Home

Location Register

(HLR)

Home MSC: point of contact to obtain routable

address of mobile user. HLR: database in

home system containing permanent phone

number, profile information, current location of

mobile user, subscription information

Home agent

Visited System Network other than home system where

mobile user is currently residing

Visited network

Visited Mobile

services Switching

Center.

Visitor Location

Record (VLR)

Visited MSC: responsible for setting up calls

to/from mobile nodes in cells associated with

MSC. VLR: temporary database entry in

visited system, containing subscription

information for each visiting mobile user

Foreign agent

Mobile Station

Roaming Number

(MSRN), or “roaming

number”

Routable address for telephone call segment

between home MSC and visited MSC, visible

to neither the mobile nor the correspondent.

Care-of-

address

74


Wireless, mobility: impact on higher layer protocols

logically, impact should be minimal …

best effort service model remains unchanged

TCP and UDP can (and do) run over wireless, mobile

… but performance-wise:

packet loss/delay due to bit-errors (discarded packets, delays for link-layer retransmissions), and handoff

TCP interprets loss as congestion, will decrease congestion window un-necessarily

delay impairments for real-time traffic

limited bandwidth of wireless links


Chapter 6 Summary

Wireless wireless links:

capacity, distance channel impairments CDMA

IEEE 802.11 (“wi-fi”) CSMA/CA reflects wireless channel characteristics

cellular access architecture standards (e.g., GSM, CDMA-2000, UMTS)

Mobility principles: addressing, routing to mobile users

home, visited networks direct, indirect routing care-of-addresses

case studies mobile IP mobility in GSM

impact on higher-layer protocols

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