1. Final Exam Review Will release at 10:00am Dec. 4th,
Due on Webcourse at 11:59pm the next day

2. Final Exam Review Knowledge questions
True or false statement (explain why)
Protocol
Calculation
Cover the contents after midterm coverage

3. Knowledge Question Examples
Three classes of switch fabric, speed relationship
Where can queue occur in router?
TCP header size? IP header size? UDP header size?
How many bits in IP of IPv6? Address space size? Why it is very slow to be deployed? (enough IP space, hard upgrading and compatible)
What is DHCP? NAT? Their pros and cons?
Routing: what are Link state, distance vector?
Internet two-level routing? (inter-AS, intra-AS)
RIP, OSPF, BGP? Used where?
OSPF uses link state, BGP/RIP uses distance vector
RIP, OSPF -> intra-AS, BGP -> inter-AS
Which is better? pure ALOHA, slotted ALOHA, CSMA/CD?
What are their assumptions? (collision detection, time syn)
CSMA/CD? CSMA/CA? Why wireless use CSMA/CA instead of CSMA/CD?
Ethernet Broadcast MAC addr.? What the broadcast address for? What is ARP?
Why Ethernet is much better than aloha in efficiency?
Carrier sense, collision detection, exp. backoff

4. Knowledge Question Examples
Hub vs. Switch?
802.11a, b, g: speed? Working frequency?
802.15? (personal area network, example: bluetooth)
Wireless no collision detection?
listen while sending, fading, hidden terminal
Network security three elements:
Confidentiality, authentication, integrity
What is public/symmetric key cryptography? Pro vs. con?
According to the textbook notation, how to represent a node A’s digital signature? Digital certificate? Message digest?
What is a “Session key”?
Usage of firewall? (block outside active traffic to inside)
IP spoofing? SYN flood DoS attack?

5. Protocol Problem Examples
NAT address translation procedure
Digital signature procedure
HTTPS connection procedure
CA, public key
Secure (assume known public key)
Confidentiality
Integrity

6. Calculation Examples link state, distance vector parity checking
CRC calculation
wireless MAC protocol
Caesar cipher decrypt, Vigenere cipher, one-time pad decrypt (given the pad)
Slotted ALOHA probability calculation (chapter5-part1.ppt)

7. Slotted ALOHA probability calculation example
There are two nodes and each of them has one packet to send at the same time, what is the probability that both packets can be successfully sent within the first 2 time slots? Suppose the transmission probability is p.

8. Three types of switching fabrics
Property? Speed order?

9. Routing Algorithm classification
Global or decentralized information?
Global:
all routers have complete topology, link cost info
“link state” algorithms
Decentralized:
router knows physically-connected neighbors, link costs to neighbors
iterative process of computation, exchange of info with neighbors
“distance vector” algorithms

10. NAT: Network Address Translation
NAT translation table
WAN side addr LAN side addr
1: host
sends datagram to
, 80
2: NAT router
changes datagram
source addr from
, 3345 to
, 5001,
updates table
, , 3345
…… ……
S: , 3345
D: , 80 1
S: , 80
D: , 3345 4
S: , 5001
D: , 80 2
S: , 80
D: , 5001 3
4: NAT router
changes datagram
dest addr from
, 5001 to , 3345
3: Reply arrives
dest. address:
, 5001

11. Intra-AS and Inter-AS routing
between
A and B a b C A B d c
A.a
A.c
C.b
B.a
Host h2
Host h1
Intra-AS routing
within AS B
Intra-AS routing
within AS A
RIP: Routing Information Protocol
OSPF: Open Shortest Path First
BGP: Border Gateway Protocol (Inter-AS)

12. ARP protocol: Same LAN (network)
A wants to send datagram to B, and B’s MAC address not in A’s ARP table.
A broadcasts ARP query packet, containing B's IP address
Dest MAC address =
FF-FF-FF-FF-FF-FF
all machines on LAN receive ARP query
B receives ARP packet, replies to A with its (B's) MAC address
frame sent to A’s MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state: information that times out (goes away) unless refreshed
ARP is “plug-and-play”:
nodes create their ARP tables without intervention from net administrator

13. 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
Virus really from your friends?
The website really belongs to the bank?
Message Integrity: sender, receiver want to ensure message not altered (in transit, or afterwards) without detection
Digital signature

14. Collision Avoidance: RTS-CTS exchangeB AP
DIFS
RTS(A)
RTS(B)
reservation collision
RTS(A)
CTS(A)
CIFS
CIFS
DATA (A)
ACK(A)
defer
time
CIFS
Textbook Page 522 figure

15. Firewall Block outside-initiated traffic to inside of a local network
Usually do not block any traffic initiated from inside to outside
Have at least two NICs (two IPs)
administered
network
public
Internet
firewall

16. 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 C
SYN
SYN
SYN
SYN
SYN B
SYN
SYN

17. Digital signature = signed message digest
Alice verifies signature and integrity of digitally signed message:
Bob sends digitally signed message:
large
message m
H: Hash
function
KB(H(m)) -
encrypted
msg digest
H(m)
digital
signature
(encrypt)
Bob’s
private
key
large
message m K B -
Bob’s
public
key
digital
signature
(decrypt) K B +
KB(H(m)) -
encrypted
msg digest
H: Hash
function +
H(m)
H(m)
equal ?
No confidentiality !

18. Secure
Alice wants to send confidential , m, to Bob. KS
KS( ) .
KS(m ) m +
Internet
KB( ) . + KS
KB(KS ) + KB +
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.

19. Secure
Alice wants to send confidential , m, to Bob.
KS( ) .
KB( ) + -
KS(m )
KB(KS ) m KS KB
Internet
Bob:
uses his private key to decrypt and recover KS
uses KS to decrypt KS(m) to recover m

20. Secure e-mail (continued)
Alice wants to provide message integrity (unchanged, really written by Alice).
H( ) .
KA( ) - +
H(m )
KA(H(m)) m KA
Internet
compare
Alice digitally signs message.
sends both message (in the clear) and digital signature.

21. Secure e-mail (continued)
Alice wants to provide secrecy, sender authentication, message integrity.
H( ) .
KA( ) - +
KA(H(m)) m KA
KS( )
KB( )
KB(KS ) KS KB
Internet
Alice uses three keys: her private key, Bob’s public key, newly created symmetric key

22. Internet Web Security Architecture
Web Server B CA
K+B
K-CA(K+B)
Client A
Cert Request
K-CA(K+B)
K+B(KAB, R)
KAB(R)
KAB(m)
Network Security

23. Forwarding table Destination Address Range Link Interface
through
through
through
otherwise

24. Longest prefix matching
Prefix Match Link Interface
otherwise
Examples
DA:
Which interface?
DA:
Which interface?
DA:
Which interface?

25. CRC Example Want: D.2r XOR R = nG equivalently: D.2r = nG XOR R
if we divide D.2r by G, want remainder R
D.2r G
R = remainder[ ]

26. Dijkstra’s algorithm: example
Step 1 2 3 4 5 N
D(B),p(B)
D(C),p(C)
D(D),p(D)
D(E),p(E)
D(F),p(F) A
2,A ,A ,A infinity,- infinity,- AD
2,A ,D ,A ,D infinity,-
ADE
2,A ,E ,A ,D ,E
ADEB
2,A ,E ,A ,D ,E
ADEBC
2,A ,E ,A ,D ,E
ADEBCF
2,A ,E ,A ,D ,E 5 3 B C 2 5 A 2 1 F 3 1 2 D E 1

27. z y x Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3
Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2
node x table
x y z x y z ∞
from
cost to
cost to
cost to
x y z
x y z x x
from y
from y z z
node y table
cost to
cost to
cost to x z 1 2 7 y
x y z
x y z
x y z x ∞ ∞ x ∞ x y
from y
from
from y z z ∞ ∞ ∞ z
node z table
cost to
cost to
cost to
x y z
x y z
x y z x x x
∞ ∞ ∞
from y
from y
from y ∞ ∞ ∞ z z z 7 1
time

28. Caesar cipher decrypt: Vigenere cipher
“welcome”, key= +2 
Vigenere cipher
“final exam” key=3,4,-1 (blank space does not change)

29. Subnet calculation Remember each subnet is represented by a.b.c.d/x
A network of /16 has 216 IPs (2x), first address is ; last address is
Don’t use “ to ” to represent a subnet!