1. CompSci 001 4.1 The Internet l How valuable is a network? ä Metcalfe’s Law l Domain Name System: translates betweens names and IP addresses l Properties of the Internet ä Heterogeneity ä Redundancy ä Packet-switched ä 1.08 billion online (Computer Industry Almanac 2005) l Warriors of the Net! Warriors of the Net l Who has access? l How important is access?

2. CompSci 001 4.2 Tim Berners-Lee I want you to realize that, if you can imagine a computer doing something, you can program a computer to do that. Unbounded opportunity... limited only by your imagination. And a couple of laws of physics. l TCP/IP, HTTP ä How, Why, What, When?

3. CompSci 001 4.3 Graphs: Structures and Algorithms l How do packets of bits/information get routed on the internet ä Message divided into packets on client (your) machine ä Packets sent out using routing tables toward destination Packets may take different routes to destination What happens if packets lost or arrive out-of-order? ä Routing tables store local information, not global (why?) l What about The Oracle of Bacon, Erdos Numbers, and Word Ladders?The Oracle of BaconErdos Numbers ä All can be modeled using graphs ä What kind of connectivity does each concept model? l Graphs are everywhere in the world of algorithms (world?)

4. CompSci 001 4.4 Vocabulary l Graphs are collections of vertices and edges (vertex also called node) ä Edge connects two vertices Direction can be important, directed edge, directed graph Edge may have associated weight/cost l A vertex sequence v 0, v 1, …, v n-1 is a path where v k and v k+1 are connected by an edge. ä If some vertex is repeated, the path is a cycle ä A graph is connected if there is a path between any pair of vertices NYC Phil Boston Wash DC 204 78 190 268 394 LGALAX ORD DCA $186 $412 $1701 $441

5. CompSci 001 4.5 Network/Graph questions/algorithms l What vertices are reachable from a given vertex? ä Two standard traversals: depth-first, breadth-first ä Find connected components, groups of connected vertices l Shortest path between any two vertices (weighted graphs?)! l Longest path in a graph ä No known efficient algorithm ä Longest shortest path: Diameter of graph l Visit all vertices without repeating? Visit all edges? ä With minimal cost? Hard! l What are the properties of the network? ä Structural: Is it connected? ä Statistical: What is the average number of neighbors?

6. CompSci 001 4.6 Network Nature of Society l Slides from Michael Kearns - Univ. of Pennsylvania

7. CompSci 001 4.7 Emerging science of networks l Examining apparent similarities between many human and technological systems & organizations l Importance of network effects in such systems l How things are connected matters greatly l Structure, asymmetry and heterogeneity l Details of interaction matter greatly l The metaphor of viral spread l Dynamics of economic and strategic interaction l Qualitative and quantitative; can be very subtle l A revolution of ä measurement ä theory ä breadth of vision (M. Kearns)

8. CompSci 001 4.8 Business & Economic Networks l Example: eBay bidding ä vertices: eBay users ä links: represent bidder-seller or buyer-seller ä fraud detection: bidding rings l Example: corporate boards ä vertices: corporations ä links: between companies that share a board member l Example: corporate partnerships ä vertices: corporations ä links: represent formal joint ventures l Example: goods exchange networks ä vertices: buyers and sellers of commodities ä links: represent “permissible” transactions (M. Kearns)

9. CompSci 001 4.9 Enron

10. CompSci 001 4.10 Physical Networks l Example: the Internet ä vertices: Internet routersInternet routers ä links: physical connections ä vertices: Autonomous Systems (e.g. ISPs)Autonomous Systems ä links: represent peering agreements ä latter example is both physical and business network l Compare to more traditional data networkstraditional data networks l Example: the U.S. power gridU.S. power grid ä vertices: control stations on the power grid ä links: high-voltage transmission lines ä August 2003 blackout: classic example of interdependenceinterdependence (M. Kearns)

11. CompSci 001 4.11 US Power Grid

12. CompSci 001 4.12 Content Networks l Example: Document similarity ä Vertices: documents on web ä Edges: Weights defined by similarity ä See TouchGraph GoogleBrowser l Conceptual network: thesaurus ä Vertices: words ä Edges: synonym relationships

13. CompSci 001 4.13 Social networks l Example: Acquaintanceship networks ä vertices: people in the world ä links: have met in person and know last names ä hard to measure l Example: scientific collaboration ä vertices: math and computer science researchers ä links: between coauthors on a published paper ä Erdos numbers : distance to Paul Erdos ä Erdos was definitely a hub or connector; had 507 coauthors l How do we navigate in such networks?

14. CompSci 001 4.14

15. CompSci 001 4.15 Acquaintanceship & more

16. CompSci 001 4.16 Network Models (Barabasi) l Differences between Internet, Kazaa, Chord ä Building, modeling, predicting l Static networks, Dynamic networks ä Modeling and simulation l Random and Scale-free ä Implications? l Structure and Evolution ä Modeling via Touchgraph

17. CompSci 001 4.17 Web-based social networks http://trust.mindswap.org l Myspace73,000,000 l Passion.com23,000,000 l Friendster21,000,000 l Black Planet17,000,000 l Facebook8,000,000 l Who’s using these, what are they doing, how often are they doing it, why are they doing it?

18. CompSci 001 4.18 Golbeck’s Criteria l Accessible over the web via a browser l Users explicitly state relationships ä Not mined or inferred l Relationships visible and browsable by others ä Reasons? l Support for users to make connections ä Simple HTML pages don’t suffice