Final Exam Review Knowledge questions True or false

Final Exam Review • Knowledge questions • True or false statement (explain why) • Protocol • Calculation • Cover the contents after midterm coverage – Homework 3, 4, security

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 IPv 6? 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? • 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

Knowledge Question Examples • • Hub vs. Switch? 802. 11 a, 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? • Why use “nonce” in security? (replay attack) What is man -in-the-middle attack? • Usage of firewall? (block outside active traffic to inside) • IP spoofing? SYN flood Do. S attack?

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

Calculation Examples • subnet addressing – Figure out subnet based on host’s IP and subnet mask • • • link state, distance vector parity checking CRC calculation wireless MAC protocol Caesar cipher decrypt, Vigenere cipher, one-time pad decrypt (given the pad)

Three types of switching fabrics Property? Speed order?

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

NAT: Network Address Translation 2: NAT router changes datagram source addr from 10. 0. 0. 1, 3345 to 138. 76. 29. 7, 5001, updates table 2 NAT translation table WAN side addr LAN side addr 1: host 10. 0. 0. 1 sends datagram to 128. 119. 40. 186, 80 138. 76. 29. 7, 5001 10. 0. 0. 1, 3345 …… …… S: 10. 0. 0. 1, 3345 D: 128. 119. 40. 186, 80 S: 138. 76. 29. 7, 5001 D: 128. 119. 40. 186, 80 138. 76. 29. 7 S: 128. 119. 40. 186, 80 D: 138. 76. 29. 7, 5001 3: Reply arrives dest. address: 138. 76. 29. 7, 5001 3 1 10. 0. 0. 4 S: 128. 119. 40. 186, 80 D: 10. 0. 0. 1, 3345 10. 0. 0. 1 10. 0. 0. 2 4 10. 0. 0. 3 4: NAT router changes datagram dest addr from 138. 76. 29. 7, 5001 to 10. 0. 0. 1, 3345

Intra-AS and Inter-AS routing C. b a Host h 1 C b A. a Inter-AS routing between A and B A. c a d c b A Intra-AS routing within AS A B. a a c B Host h 2 b Intra-AS routing within AS B – RIP: Routing Information Protocol – OSPF: Open Shortest Path First – BGP: Border Gateway Protocol (Inter-AS)

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 – 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

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 email 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

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

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) public Internet administered network firewall

ap 5. 0: security hole Man (woman) in the middle attack: Trudy poses as Alice (to Bob) and as Bob (to Alice) Difficult to detect: q Bob receives everything that Alice sends, and vice versa. (e. g. , so Bob, Alice can meet one week later and recall conversation) q problem is that Trudy receives all messages as well!

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 C A SYN SYN SYN B SYN

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

Secure e-mail q Alice wants to send confidential e-mail, m, to Bob. KS m KS K (. ) S + K+ B Alice: q q . K B( ) KS(m ) + Internet + KB(KS ) 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.

Secure e-mail q Alice wants to send confidential e-mail, m, to Bob. KS m KS K (. ) S + K+ B Bob: . K B( ) KS(m ) + + KB(KS ) . K S( ) - Internet + KB(KS ) q uses his private key to decrypt and recover K S q uses KS to decrypt KS(m) to recover m KS - . K B( ) KB m

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

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

How SSL (https) works? Three-way handshake Request server certificate K-CA(K+B) KB+ Server B Client Certificate from CA K+B(KA-B) Symmetric session key KA-B(m) time

Forwarding table Destination Address Range Link Interface 11001000 00010111 00010000 through 11001000 00010111 1111 0 11001000 00010111 00011000 0000 through 11001000 00010111 00011000 1111 1 11001000 00010111 00011001 0000 through 11001000 00010111 00011111 2 otherwise 3

Longest prefix matching Prefix Match 11001000 00010111 00010 11001000 00010111 00011000 11001000 00010111 00011 otherwise Link Interface 0 1 2 3 Examples DA: 11001000 00010111 00010110 10100001 Which interface? DA: 11001000 00010111 00011000 1010 Which interface? DA: 11001000 00010111 10011000 1010 Which interface?

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

Dijkstra’s algorithm: example Step N 0 A 1 AD 2 ADE 3 ADEB 4 ADEBC 5 ADEBCF D(B), p(B) D(C), p(C) D(D), p(D) D(E), p(E) D(F), p(F) 2, A 5, A 1, A infinity, 2, A 4, D 1, A 2, D infinity, 2, A 3, E 1, A 2, D 4, E 5 A 1 2 B 2 D 3 C 3 1 5 F 1 E 2

Dx(y) = min{c(x, y) + Dy(y), c(x, z) + Dz(y)} = min{2+0 , 7+1} = 2 node x table cost to x y z x ∞∞ ∞ y ∞∞ ∞ z 71 0 from x 0 2 7 y 2 0 1 z 7 1 0 cost to x y z x 0 2 7 y 2 0 1 z 3 1 0 x 0 2 3 y 2 0 1 z 3 1 0 cost to x y z from x ∞ ∞ ∞ y 2 0 1 z ∞∞ ∞ node z table cost to x y z x 0 2 3 y 2 0 1 z 7 1 0 cost to x y z from x 0 2 7 y ∞∞ ∞ z ∞∞ ∞ node y table cost to x y z Dx(z) = min{c(x, y) + Dy(z), c(x, z) + Dz(z)} = min{2+1 , 7+0} = 3 x 0 2 3 y 2 0 1 z 3 1 0 time x 2 y 7 1 z

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

Subnet calculation • Suppose an ISP has a chunk of IP addresses of 128. 119. 0. 0/17, it allocates this space to three companies. Two companies get equal size space, the third company gets half of the space with higher IP addresses. Show the IP space allocated to the three companies.