1. 1 Overview COS 461: Computer Networks Spring 2006 (MW 1:30-2:50 in Friend 109) Jennifer Rexford Teaching Assistant: Mike Wawrzoniak http://www.cs.princeton.edu/courses/archive/spring06/cos461/

2. 2 Goals of Today’s Class Course overview (45 minutes) –Review of the material in the course –Preparation for Wednesday’s exam Course evaluations (15 minutes) –Scan-tron form –Written responses Assignment #3 (20 minutes) –Overview of assignment from Mike W –Opportunity to ask questions

3. 3 Important Dates Second midterm exam: Wednesday May 3 –Exam during class time (1:30-2:50pm) –Room 104 in the CS building, just as last time –Open notes, open book, and open slides –Covering material from lecture #11 onward Assignment #3: Tuesday May 16 at 9pm –Assignment #3 is due on Dean’s Date at 9pm –Office hours and mailing list during reading period Research projects: Tuesday May 16 at 9pm –Write-up of research projects due on Dean’s Date –Send via e-mail to jrex@cs.princeton.edu

4. 4 Goals of This Course Skill: network programming –Socket programming –Designing and implementing protocols Knowledge: how the Internet works –IP protocol suite –Internet architecture –Applications (Web, e-mail, P2P, VoIP, …) Insight: key concepts in networking –Protocols –Layering –Resource allocation –Naming

5. 5 IP Suite: End Hosts vs. Routers HTTP TCP IP Ethernet interface HTTP TCP IP Ethernet interface IP Ethernet interface Ethernet interface SONET interface SONET interface host router HTTP message TCP segment IP packet

6. 6 Shuttling Data at Different Layers Different devices switch different things –Physical layer: electrical signals (repeaters and hubs) –Link layer: frames (bridges and switches) –Network layer: packets (routers) Application gateway Transport gateway Router Bridge, switch Repeater, hub Frame header Packet header TCP header User data

7. 7 Physical Layer: Repeaters and Hubs Analog electronic devices –Continuously monitors electrical signals on each LAN –Transmits an amplified copy hub

8. 8 Link Layer: Bridges and Switches Connects two or more LANs at the link layer –Extracts destination address from the frame –Looks up the destination in a table –Forwards the frame to the appropriate LAN segment host Bridge switch A B C D

9. 9 Self Learning: Building the Table When a frame arrives –Inspect the source MAC address –Associate the address with the incoming interface –Store the mapping in the switch table –Use a time-to-live field to eventually forget the mapping When frame arrives with an unfamiliar destination –Forward out all interfaces –… except where frame arrived –Hopefully won’t happen often A B C D

10. 10 Network Layer: Routers Switching Fabric Processor Line card

11. 11 Hubs, Switches, and Routers Repeaters and hubs –Analog devices –Relay electrical signals Bridges and switches –Forwards frames based on the MAC address –Self-learning to construct the switch table –Constructing a spanning tree to broadcast frames Routers –Forwards packets based on the IP address –Routing protocols to construct the forwarding tables –Require more configuration than switches do

12. 12 Two-Tiered Routing Architecture Goal: distributed management of resources –Internetworking of multiple networks –Networks under separate administrative control Solution: two-tiered routing architecture –Intradomain: inside a region of control  Okay for routers to share topology information  Routers configured to achieve a common goal –Interdomain: between regions of control  Not okay to share complete information  Networks may have different/conflicting goals Led to the use of different protocols…

13. 13 Autonomous Systems (ASes) 1 2 3 4 5 6 7 Client Web server Path: 6, 5, 4, 3, 2, 1

14. 14 Internet Routing Architecture Divided into Autonomous Systems –Distinct regions of administrative control –Routers/links managed by a single “institution” –Service provider, company, university, … Hierarchy of Autonomous Systems –Large, tier-1 provider with a nationwide backbone –Medium-sized regional provider with smaller backbone –Small network run by a single company or university Interaction between Autonomous Systems –Internal topology is not shared between ASes –… but, neighboring ASes interact to coordinate routing

15. 15 Interdomain Routing (Between ASes) Support local routing policies –Advertise the AS-level paths for each prefix –Allow each AS to decide which path to use –… and whether to announce path to neighbors Common business relationships –Customer-provider  Customer can reach all destinations through provider  Provider ensures rest of Internet can reach customer –Peer-peer  Peers transit traffic between respective customers  … but not to other peers and providers

16. 16 Two Kinds of Routing Protocols Topology information is flooded within the routing domain Best end-to-end paths are computed locally at each router. Best end-to-end paths determine next-hops. Based on minimizing some notion of distance Works only if policy is shared and uniform Examples: OSPF, IS-IS Each router knows little about network topology Only best next-hops are chosen by each router for each destination. Best end-to-end paths result from composition of all next-hop choices Does not require any notion of distance Does not require uniform policies at all routers Examples: RIP, BGP Link StateVectoring

17. 17 Overlay Routing Overlay on the network –Hosts serve as nodes and make forwarding decisions –Tunnels serve as links that carry the packets Two-hop (application-level) Berkeley-to-Princeton route application-layer router Princeton Yale Berkeley

18. 18 Peer-to-Peer Protocols End hosts communicating directly with each other –File sharing (music, movies) –Voice over IP (telephone calls) Challenge –Determining who has the data you want –Handling churn as users come and go Three approaches –Central server: client sends query to the server –Flooding: client floods query throughout the network –Hybrid: client sends query to super-node, and super- nodes flood amongst themselves

19. 19 E-Mail End hosts sending e-mail messages –Asynchronous communication Determining how to relay the message to recipient –Mail agents and mail servers –Special DNS query to identify the mail server –Forwarding of messages from one server to the next –Protocols for recipients to retrieve the e-mail user agent mail server user agent mail server user agent user agent

20. 20 Web Simpler client-server paradigm –Clients (e.g., browsers) send requests –Servers send responses –Optional proxies in between Ingredients of the Web –Uniform Resource Locator (URL) –HyperText Markup Language (HTML) –HyperText Transfer Protocol (HTTP) Stateless protocol –Each request-response pair treated independently –Improves the scalability of the server –Separate mechanisms (e.g., cookies) for state

21. 21 Resource Meta-Data Meta-data –Information relating to a resource –… but not part of the resource itself Example meta-data –Size of a resource –Last modification time –Type of the content –Transfer encoding format Concept borrowed from e-mail protocols –Multipurpose Internet Mail Extensions (MIME) –Data format classification (e.g., Content-Type: text/html) –Enables clients to automatically launch a viewer

22. 22 Internet of Today Internet has evolved substantially –From a small research curiousity –To a world-wide communications infrastructure Yet, some early design decisions remain –Packet switching –The “narrow waist” of IP –Best-effort service model –Limited state inside the network –Protocols based on trust And these are meeting real challenges –Demands for quality of service guarantees –Serious security threats –Challenges of network management

23. 23 Internet of the Future A need for a change? –Circuit switching? –Guaranteed service? –Strict notions of identity? –Central authorities? Open question –Is it possible to have an inter-network that supports such rapid evolution of new services –… while providing performance guarantees & security? New initiatives –Clean-slate rethinking of the Internet design –See www.geni.netwww.geni.net