Appendix 1 Appendix q Networking basics o Protocol

Appendix 1

Appendix q Networking basics o Protocol stack o Layers, etc. q Math basics o o Modular arithmetic Permutations Probability Linear algebra Appendix 2

Networking Basics Appendix 3

Network q Includes o o o q Computers Servers Routers Wireless devices Etc. Purpose is to transmit data Appendix 4

Network Edge Network edge includes q Hosts q o o o Computers Laptops Servers Cell phones Etc. , etc. Appendix 5

Network Core q Network core consists of o Interconnected mesh of routers q Purpose is to move data from host to host Appendix 6

Packet Switched Network q Usual telephone network is circuit switched o For each call, a dedicated circuit is established o Dedicated bandwidth q Modern data networks are packet switched o o o Data is chopped up into discrete packets Packets are transmitted independently No real circuit is established More efficient bandwidth usage But more complex than circuit switched Appendix 7

Network Protocols Study of networking focused on protocols q Networking protocols precisely specify the communication rules q Details are given in RFCs q o RFC is essentially an Internet standard Stateless protocols don’t remember q Stateful protocols do remember q Many security problems related to state q Do. S easier against stateful protocols q Appendix 8

Protocol Stack q q Application layer protocols o HTTP, FTP, SMTP, etc. Transport layer protocols transport o IP, routing protocols network Network layer protocols q Link layer protocols q application o TCP, UDP q o Ethernet, PPP Physical layer Appendix user space OS link NIC card physical 9

Layering in Action data application router transport host q q data application network link physical host At source, data goes down the protocol stack Each router processes packet up to network layer o That’s where routing info lives Router then passes packet down the protocol stack Destination processes up to application layer o That’s where the data lives Appendix 10

Encapsulation q q X = application data at the source As X goes down protocol stack, each layer adds header information: o o q q Application layer: (H, X) Transport layer: (H, X)) Network layer: (H, (H, X))) Link layer: (H, (H, X)))) Header has info required by layer Note that app header is on the inside data X application transport network link physical packet (H, (H, X)))) Appendix 11

Application Layer q Applications o Web browsing, email, P 2 P, etc. o Running on hosts o Hosts want network to be transparent q Application layer protocols o HTTP, SMTP, IMAP, Gnutella, etc. q Protocol is one part of an application o For example, HTTP only a part of web browsing Appendix 12

Client-Server Model q Client “speaks first” q Server tries to respond to request q Hosts are clients and/or servers q Example: Web browsing o You are the client (request web page) o Web server is the server Appendix 13

Peer-to-Peer Model Hosts act as clients and servers q For example, when sharing music q o You are client when requesting a file o You are a server when someone downloads a file from you In P 2 P model, more difficult for client to find a server q Many different P 2 P models q Appendix 14

HTTP Example HTTP request HTTP response HTTP --- Hyper. Text Transfer Protocol q Client (you) request a web page q Server responds to your request q Appendix 15

initial session cookie HTTP r HTTP cookie later session q q Web Cookies equest respon se, coo kie o c , t s e equ HTTP r nse o p s e r HTTP Cookie database HTTP is stateless --- cookies used to add state Initially, cookie sent from server to browser Browser manages cookie, sends it to server Server looks in cookie database to “remember” you Appendix 16

Web Cookies q Web cookies can be used for o Shopping carts o Recommendations, etc. o A weak form of authentication q Privacy concerns o Web site can learn a lot about you o Multiple web sites could learn even more Appendix 17

SMTP used to send email from sender to recipient’s mail server q Then use POP 3, IMAP or HTTP (Web mail) to get messages from server q As with many application protocols, SMTP commands are human readable q Sender SMTP Appendix SMTP Recipient POP 3 18

Spoofed email with SMTP User types the red lines: > telnet eniac. cs. sjsu. edu 25 220 eniac. sjsu. edu HELO ca. gov 250 Hello ca. gov, pleased to meet you MAIL FROM: <arnold@ca. gov> 250 arnold@ca. gov. . . Sender ok RCPT TO: <stamp@cs. sjsu. edu> 250 stamp@cs. sjsu. edu. . . Recipient ok DATA 354 Enter mail, end with ". " on a line by itself It is my pleasure to inform you that you are terminated. 250 Message accepted for delivery QUIT 221 eniac. sjsu. edu closing connection Appendix 19

Application Layer q DNS --- Domain Name Service o Convert human-friendly names such as www. google. com into 32 -bit IP address o A distributed hierarchical database q Only 13 “root” DNS servers worldwide o A single point of failure for Internet o Attacks on root servers have succeeded o Attacks have not lasted long enough (yet…) Appendix 20

Transport Layer The network layer offers unreliable, “best effort” delivery of packets q Any improved service must be provided by the hosts q Transport layer has two protocols q o TCP better service, more overhead o UDP minimal service, minimal overhead q TCP and UDP run on hosts, not routers Appendix 21

TCP q TCP assures that packets o Arrive at destination o Are processed in order o Are not sent too fast for receiver (flow control) q TCP also provides o Network-wide congestion control q TCP is “connection-oriented” o TCP contacts server before sending data o Orderly setup and take down of “connection” o But no true connection, only a logical connection Appendix 22

TCP Three Way Handshake SYN request SYN-ACK (and data) SYN: synchronization requested q SYN-ACK: acknowledge SYN request q ACK: acknowledge msg 2 and send data q Then TCP “connection” established q o Connection terminated by FIN or RST packet Appendix 23

Denial of Service Attack The TCP 3 -way handshake makes denial of service (Do. S) attacks possible q Whenever SYN packet is received, server must remember “half-open” connection q o Remembering consumes resources o Too many half-open connections and server resources will be exhausted o Then server can’t respond to new connections Appendix 24

UDP q UDP is minimalist, “no frills” service o No assurance that packets arrive o No assurance packets are in order, etc. q Why does UDP exist? o More efficient (smaller header) o No flow control to slow down sender o No congestion control to slow down sender q Packets sent too fast, they will be dropped o Either at intermediate router or at destination o But in some apps this is OK (audio/video) Appendix 25

Network Layer q Core of network/Internet q Purpose of network layer q Network layer protocol is IP o Interconnected mesh of routers o Route packets through this mesh o Follows a “best effort” approach IP runs in every host and every router q Routers also run routing protocols q o Used to determine the path to send packets o Routing protocols: RIP, OSPF, BGP, etc. Appendix 26

IP Addresses IP address is 32 bits q Every host has an IP address q Not enough IP addresses! q o Lots of tricks to extend address space q IP addresses given in dotted decimal notation o For example: 195. 72. 180. 27 o Each number is between 0 and 255 q Host’s IP address can change Appendix 27

Socket Each host has a 32 bit IP address q But many processes on one host q o You can browse web, send email at same time How to distinguish processes on a host? q Each process has a 16 bit port number q o Port numbers < 1024 are “well-known” ports (HTTP port 80, POP 3 port 110, etc. ) o Port numbers above 1024 are dynamic (as needed) q IP address and port number define a socket o Socket uniquely identifies a process Appendix 28

IP Header q IP header used by routers q Time to live (TTL) limits number of “hops” q Fragmentation information in header (next slide) o Note source and destination IP addresses o So packets can’t circulate forever Appendix 29

IP Fragmentation fragmented re-assembled Each link limits maximum size of packets q If packet is too big, router fragments it q Re-assembly occurs at destination q Appendix 30

IP Fragmentation One packet becomes multiple packets q Packets reassembled at destination q o Prevents multiple fragment/re-assemble q Fragmentation is a security issue! o o Fragments may obscure real purpose of packet Fragments can overlap when re-assembled Must re-assemble packet to fully understand it Lots of work for firewalls, for example Appendix 31

IPv 6 Current version of IP is IPv 4 q IPv 6 is a new-and-improved version q IPv 6 provides q o Longer addresses: 128 bits o Real security (IPSec) But difficult to migrate from v 4 to v 6 q So IPv 6 has not taken hold yet q Appendix 32

Link Layer Link layer sends packet from one node to next q Each link can be different q o o Wired Wireless Ethernet Point-to-point… Appendix 33

Link Layer q Implemented in adapter known as network interface card, or NIC o Ethernet card o Wireless 802. 11 card, etc. q NIC is (mostly) out of host’s control o Implements both link and physical layers Appendix 34

Ethernet is a multiple access protocol q Many hosts access a shared media q o On a local area network, or LAN q In ethernet, two packets can collide o o q Then data is corrupted Packets must be resent How to be efficient in distributed environment? Many possibilities, ethernet is most popular We won’t discuss details here Appendix 35

Link Layer Addressing IP addresses live at network layer q Link layer also requires addresses q o MAC address (LAN address, physical address) q MAC address o 48 bits, globally unique o Used to forward packets over one link q Analogy o IP address is like home address o MAC address is like social security number Appendix 36

ARP Address resolution protocol, ARP q Used at link layer to find MAC address of given IP address q Each host has ARP table q o o Generated automatically Entries expire after some time (20 min) ARP used to find ARP table entries ARP table also known as ARP cache Appendix 37

ARP is stateless q ARP sends request and receives ARP reply q Replies used to fill ARP cache q IP: 111. 002 IP: 111. 001 LAN MAC: AA-AA-AA-AA 111. 002 BB-BB-BB -BB ARP cache Appendix MAC: BB-BB-BB-BB 111. 001 AA-AA-AA -AA ARP cache 38

ARP Cache Poisoning ARP is stateless q Accepts any reply, even if no request sent! q 111. 003 CC-CC-CC-CC ARP “reply” 111. 001 CC-CC-CC-CC 111. 002 CC-CC-CC-CC 111. 001 AA-AA-AA-AA 111. 002 CC-CC-CC-CC BB-BB-BB-BB ARP cache LAN 111. 002 BB-BB-BB-BB 111. 001 AA-AA-AA-AA CC-CC-CC-CC ARP cache Host CC-CC-CC-CC is “man-in-themiddle” Appendix q 39

Math Basics Appendix 40

Modular Arithmetic Appendix 41

Modular Arithmetic q q For integers x and n, x mod n is the remainder of x n Examples o o o 7 mod 6 = 1 33 mod 5 = 3 33 mod 6 = 3 51 mod 17 = 0 17 mod 6 = 5 0 1 5 arithmetic mod 6 2 4 3 Appendix 42

Modular Arithmetic q q Notation and facts o o 7 mod 6 = 1 7 = 13 = 1 mod 6 ((a mod n) + (b mod n)) mod n = (a + b) mod n ((a mod n)(b mod n)) mod n = ab mod n o o o 3 + 5 = 2 mod 6 2 + 4 = 0 mod 6 3 + 3 = 0 mod 6 (7 + 12) mod 6 = 19 mod 6 = 1 mod 6 (7 + 12) mod 6 = (1 + 0) mod 6 = 1 mod 6 Addition Examples Appendix 43

Modular Multiplication q Multiplication o o o Examples 3 4 = 0 (mod 6) 2 4 = 2 (mod 6) 5 5 = 1 (mod 6) (7 4) mod 6 = 28 mod 6 = 4 mod 6 (7 4) mod 6 = (1 4) mod 6 = 4 mod 6 Appendix 44

Modular Inverses q Additive inverse of x mod n, denoted -x, is the number that must be added to x to get 0 mod n o -2 mod 6 = 4, since 2 + 4 = 0 mod 6 q Multiplicative inverse of x mod n, denoted x -1, is the number that must be multiplied by x to get 1 mod n o 3 -1 mod 7 = 5, since 3 5 = 1 mod 7 Appendix 45

Modular Arithmetic Q: What is -3 mod 6? q A: 3 q Q: What is -1 mod 6? q A: 5 q Q: What is 5 -1 mod 6? q A: 5 q Q: What is 2 -1 mod 6? q A: No number works! q Multiplicative inverse might not exist q Appendix 46

Relatively Prime qx and y are relatively prime if they have no common factor other than 1 q x-1 mod y exists only when x and y are relatively prime q x-1 mod y is easy to find (when it exists) using the Euclidean Algorithm Appendix 47

Totient Function (n) is the number of numbers (positive integers) less than n, relatively prime to n q Examples q o o o Appendix (4) = 2 since 4 is relatively prime to 3 and 1 (5) = 4 since 5 is relatively prime to 1, 2, 3 and 4 (12) = 4 (p) = p-1 if p is prime (pq) = (p-1)(q-1) if p and q prime 48

Permutations Appendix 49

Permutations q Let S be a set q A permutation of S is an ordered list of the elements of S o Each element of S appears exactly once q Suppose S={0, 1, 2, …, n-1} o Then the number of perms is… o n(n-1)(n-2) (2)(1) = n! Appendix 50

Permutations q Let S = {0, 1, 2, 3} q Then there are 24 perms of S q For example, o (3, 1, 2, 0) is a perm of S o (0, 2, 3, 1) is a perm of S, etc. q Perms Appendix are important in cryptography 51

Probability Basics Appendix 52

Probability q We only require some elementary facts q Suppose that S={0, 1, 2, …, N-1} is the set of all possible outcomes q If each outcome is equally likely, then the probability of event E S is o P(E) = # elements of E / # elements of S Appendix 53

Probability q For example, suppose we flip 2 coins q Then S = {hh, ht, th, tt} o Suppose X = “at least one tail” = {ht, th, tt} o Then P(X) = 3/4 q Often, it’s easier to compute o P(X) = 1 - P(complement of X) Appendix 54

Probability q Again, suppose we flip 2 coins q Let S = {hh, ht, th, tt} o Suppose X = “at least one tail” = {ht, th, tt} o Complement of X is “no tails” = {tt} q Then o P(X) = 1 - P(comp. of X) = 1 - 1/4 = 3/4 q We’ll Appendix make use of this trick often! 55

Linear Algebra Basics Appendix 56

Linear Algebra q Let R be the set of real numbers q Then v Rn is a vector of n elements q For example o v = [v 1, v 2, v 3, v 4] = [2, -1, 3. 2, 7] R 4 q The dot product of u, v Rn is o u v = [u 1 v 1, u 2 v 2, …, unvn] Appendix 57

Linear Algebra q. A matrix is an n x m array q For example, the matrix A is 2 x 3 q The element in row i column j is aij q We can multiply a matrix by a number Appendix 58

Linear Algebra q We can add matrices of the same size q We can also multiply matrices, but this is not so obvious q We do not simply multiply the elements Appendix 59

Linear Algebra q Suppose A is m x n and B is s x t q Then C=AB is only defined if n=s, in which case C is m x t q Why? q The element cij is the dot product of row i of A with column j of B Appendix 60

Linear Algebra q Suppose q Then q And Appendix AB is undefined 61

Linear Algebra q. A matrix is square if it has an equal number of rows and columns q For square matrices, the identity matrix I is the multiplicative identity o AI = IA = A q The Appendix 3 identity matrix is 62

Linear Algebra Block matrices are matrices of matrices q For example q We can do arithmetic with block matrices q Block matrix multiplication works if individual matrix dimensions match q Appendix 63

Linear Algebra Block matrices multiplication example q For matrices q q We have q Where X = U+CT and Y = AU+BT Appendix 64

Linear Algebra q Two vectors u, v Rn are linearly independent if au + bv = 0 implies a=b=0 q For example, q Are Appendix linearly independent 65

Linear Algebra q Linear independence can be extended to more than 2 vectors q If vectors are linearly independent, then none of them can be written as a linear combination of the others o None of the independent vectors is a sum of multiples of the other vectors Appendix 66