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8: Network Security 8-481
Chapter 8 Network Security
A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides
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note our copyright of this material.
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.
8: Network Security 8-482
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
8: Network Security 8-483
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: Network Security 8-484
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
8: Network Security 8-485
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
8: Network Security 8-486
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
8: Network Security 8-487
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 ……
8: Network Security 8-488
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
5
8: Network Security 8-489
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
8: Network Security 8-490
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
8: Network Security 8-491
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
decryption algorithm
A-B
K A-B
plaintext message, m
K (m) A-B
K (m) A-B
m = K ( ) A-B
8: Network Security 8-492
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
8: Network Security 8-493
Symmetric key crypto: DES
initial permutation
16 identical “rounds” of function application, each using different 48 bits of key
final permutation
DES operation
8: Network Security 8-494
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
8: Network Security 8-495
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
8: Network Security 8-496
Public key cryptography
plaintext message, m
ciphertext encryption algorithm
decryption 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
8: Network Security 8-497
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
- +
+
-
8: Network Security 8-498
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
8: Network Security 8-499
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
8: Network Security 8-500
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
8: Network Security 8-501
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 )
8: Network Security 8-502
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
8: Network Security 8-503
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: Network Security 8-504
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
8: Network Security 8-505
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”
8: Network Security 8-506
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
8: Network Security 8-507
Authentication: another try
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
8: Network Security 8-508
Authentication: another try
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
8: Network Security 8-509
Authentication: another try
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
“I’m Alice” Alice’s IP addr
Alice’s password
OK Alice’s IP addr
“I’m Alice” Alice’s IP addr
Alice’s password
8: Network Security 8-510
Authentication: yet another try
Protocol ap3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it.
Failure scenario??
“I’m Alice” Alice’s IP addr
encrypted password
OK Alice’s IP addr
16
8: Network Security 8-511
Authentication: another try
Protocol ap3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it.
record and playback still works!
“I’m Alice” Alice’s IP addr
encrypted password
OK Alice’s IP addr
“I’m Alice” Alice’s IP addr
encrypted password
8: Network Security 8-512
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
8: Network Security 8-513
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
+
8: Network Security 8-514
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
8: Network Security 8-515
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!
8: Network Security 8-516
Chapter 8 roadmap
8.1 What is network security?
8.2 Principles of cryptography
8.3 Authentication
8.4 Message integrity
8.5 Key Distribution and certification
8.6 Access control: firewalls
8.7 Attacks and counter measures
8.8 Security in many layers
19
8: Network Security 8-517
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
8: Network Security 8-518
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
8: Network Security 8-519
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.
8: Network Security 8-520
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
8: Network Security 8-521
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!
8: Network Security 8-522
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
8: Network Security 8-523
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
8: Network Security 8-524
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
23
8: Network Security 8-525
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)
8: Network Security 8-526
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
8: Network Security 8-527
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
8: Network Security 8-528
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
8: Network Security 8-529
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 +
8: Network Security 8-530
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
8: Network Security 8-531
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: Network Security 8-532
Firewalls
isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others.
firewall
administered network
public Internet
firewall
27
8: Network Security 8-533
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
8: Network Security 8-534
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
8: Network Security 8-535
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.
8: Network Security 8-536
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
8: Network Security 8-537
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.
8: Network Security 8-538
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
30
8: Network Security 8-539
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?
8: Network Security 8-540
Internet security threats Mapping: countermeasures
record traffic entering network
look for suspicious activity (IP addresses, pots being scanned sequentially)
31
8: Network Security 8-541
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?
8: Network Security 8-542
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
src:B dest:A payload
32
8: Network Security 8-543
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
src:B dest:A payload
Countermeasures?
8: Network Security 8-544
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
src:B dest:A payload
33
8: Network Security 8-545
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?
8: Network Security 8-546
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
8: Network Security 8-547
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
8: Network Security 8-548
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
8: Network Security 8-549
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 ) +
8: Network Security 8-550
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
8: Network Security 8-551
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
8: Network Security 8-552
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
8: Network Security 8-553
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.
8: Network Security 8-554
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
8: Network Security 8-555
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
8: Network Security 8-556
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
8: Network Security 8-557
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
8: Network Security 8-558
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
8: Network Security 8-559
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
8: Network Security 8-560
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
8: Network Security 8-561
802.11 WEP encryption
Sender-side WEP encryption
8: Network Security 8-562
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
8: Network Security 8-563
802.11i: improved security
numerous (stronger) forms of encryption possible
provides key distribution
uses authentication server separate from access point
8: Network Security 8-564
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
8: Network Security 8-565
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)
8: Network Security 8-566
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.
A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides
(including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the
following: If you use these slides (e.g., in a class) in substantially unaltered form,
that you mention their source (after all, we’d like people to use our book!)
If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and
note our copyright of this material.
Thanks and enjoy! JFK/KWR
All material copyright 1996-2004
J.F Kurose and K.W. Ross, All Rights Reserved
6: Wireless and Mobile Networks 6-568
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
6: Wireless and Mobile Networks 6-569
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
6: Wireless and Mobile Networks 6-570
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
6: Wireless and Mobile Networks 6-571
Elements of a wireless network
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
6: Wireless and Mobile Networks 6-572
Elements of a wireless network
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
6: Wireless and Mobile Networks 6-573
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
6: Wireless and Mobile Networks 6-574
Elements of a wireless network
network
infrastructure
infrastructure mode
base station connects mobiles into wired network
handoff: mobile changes base station providing connection into wired network
48
6: Wireless and Mobile Networks 6-575
Elements of a wireless network
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
6: Wireless and Mobile Networks 6-576
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
6: Wireless and Mobile Networks 6-577
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
6: Wireless and Mobile Networks 6-578
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
6: Wireless and Mobile Networks 6-579
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
6: Wireless and Mobile Networks 6-580
CDMA: two-sender interference
51
6: Wireless and Mobile Networks 6-581
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
6: Wireless and Mobile Networks 6-582
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
6: Wireless and Mobile Networks 6-583
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
6: Wireless and Mobile Networks 6-584
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
6: Wireless and Mobile Networks 6-585
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)
6: Wireless and Mobile Networks 6-586
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
6: Wireless and Mobile Networks 6-587
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!
6: Wireless and Mobile Networks 6-588
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
6: Wireless and Mobile Networks 6-589
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
6: Wireless and Mobile Networks 6-590
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
6: Wireless and Mobile Networks 6-591
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)
6: Wireless and Mobile Networks 6-592
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
6: Wireless and Mobile Networks 6-593
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
6: Wireless and Mobile Networks 6-594
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
58
6: Wireless and Mobile Networks 6-595
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
6: Wireless and Mobile Networks 6-596
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
6: Wireless and Mobile Networks 6-597
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
6: Wireless and Mobile Networks 6-598
Cellular standards: brief survey
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
6: Wireless and Mobile Networks 6-599
Cellular standards: brief survey
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)
6: Wireless and Mobile Networks 6-600
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
61
6: Wireless and Mobile Networks 6-601
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.
6: Wireless and Mobile Networks 6-602
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
6: Wireless and Mobile Networks 6-603
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
6: Wireless and Mobile Networks 6-604
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
6: Wireless and Mobile Networks 6-605
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
6: Wireless and Mobile Networks 6-606
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
6: Wireless and Mobile Networks 6-607
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”
6: Wireless and Mobile Networks 6-608
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
6: Wireless and Mobile Networks 6-609
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
6: Wireless and Mobile Networks 6-610
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
6: Wireless and Mobile Networks 6-611
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
6: Wireless and Mobile Networks 6-612
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
6: Wireless and Mobile Networks 6-613
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)
6: Wireless and Mobile Networks 6-614
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
68
6: Wireless and Mobile Networks 6-615
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
6: Wireless and Mobile Networks 6-616
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
6: Wireless and Mobile Networks 6-617
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
6: Wireless and Mobile Networks 6-618
Mobile IP: registration example
70
6: Wireless and Mobile Networks 6-619
Components of cellular network architecture
correspondent
MSC
MSC
MSC MSC
MSC
wired public
telephone
network
different cellular networks,
operated by different providers
recall:
6: Wireless and Mobile Networks 6-620
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
6: Wireless and Mobile Networks 6-621
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
6: Wireless and Mobile Networks 6-622
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
6: Wireless and Mobile Networks 6-623
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
6: Wireless and Mobile Networks 6-624
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
6: Wireless and Mobile Networks 6-625
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
6: Wireless and Mobile Networks 6-626
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
6: Wireless and Mobile Networks 6-627
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
6: Wireless and Mobile Networks 6-628
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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