1. Introduction1-1 Topic 1 Introduction Part A The majority of the slides in this course are adapted from the accompanying slides to the books by Larry Peterson and Bruce Davie and by Jim Kurose and Keith Ross. Additional slides and/or figures from other sources and from Vasos Vassiliou are also included in this presentation.

2. Introduction1-2 Outline Our goal: r get “feel” and terminology r more depth, detail later in course r approach: m use Internet as example Network Requirements? r Connectivity r Services r Resource sharing r Performance

3. Introduction1-3 What is the Objective of Networking ? r Communication between applications on different computers r Must understand the different requirements that exist r Who defines the requirements?

4. Introduction1-4 Requirement Definition r Requirements and Constraints depend on your perspective: m Network users want the network to provide services that their applications need; e.g., guarantee that each message will be delivered in order, without errors, and within a pre-defined delay m Network designers want a cost-effective design; e.g., network resources are efficiently utilized and fairly allocated to users m Network providers want a system that is easy to administer and manage; e.g., faults can be easily found, system can be hotswapped, and easy to track usage of users

5. Introduction1-5 Four Steps to Networking r Communicating across a link r Connecting together multiple links (internetworking) r Finding and routing data to nodes on internetwork r Matching application requirements

6. Introduction1-6 A First Step r Creating a link between nodes r Link: path followed by bits m Wired or wireless m Broadcast or point-to-point (or both) m copper wires (coax cable, twisted pair), optical fiber, r Node: any device connected to a link m Computers or servers m Routers or switches m Mobile terminal

7. Introduction1-7 Types of Links Point-to-PointMultiple Access …

8. Introduction1-8 Connectivity Requirement r “A network is two or more nodes connected by a direct link, or two or more networks connected by one or more nodes” r Hosts connected directly or indirectly m Need global addressability m Need routing ability m Unicast/Broadcast/Multicast r Network Edge vs. Network Core; does it make a difference?

9. Introduction1-9 Two or more connected nodesTwo or more connected networks Types of Networks

10. Introduction1-10 Internet structure: network of networks r roughly hierarchical r at center: “tier-1” ISPs (e.g., UUNet, BBN/Genuity, Sprint, AT&T), national/international coverage m treat each other as equals Tier 1 ISP Tier-1 providers interconnect (peer) privately NAP Tier-1 providers also interconnect at public network access points (NAPs)

11. Introduction1-11 Tier-1 ISP: e.g., Sprint Sprint US backbone network

12. Introduction1-12 Tier-1 ISP: e.g., BBN/GTE

13. Introduction1-13 Tier-1 ISP: e.g., UUnet

14. Introduction1-14 Internet structure: network of networks r a packet passes through many networks! Tier 1 ISP NAP Tier-2 ISP local ISP local ISP local ISP local ISP local ISP Tier 3 ISP local ISP local ISP local ISP

15. Introduction1-15 Internet[work] Second Step: Internet[work] r A collection of interconnected networks r Host: network endpoints (computer, PDA, light switch, …) r Router: node that connects networks

16. Introduction1-16 Challenge r Many differences between networks m Address formats m Performance – bandwidth/latency m Packet size m Loss rate/pattern/handling m Routing r How to translate between various network technologies

17. Introduction1-17 Third Step: How To Find Nodes? internet Computer 1Computer 2

18. Introduction1-18 Naming r Humans use readable host names m E.g. www.gatech.edu, www.ucy.ac.cy m Globally unique (can correspond to multiple hosts) r Naming system translates to physical address m E.g. DNS translates name to IP Address (e.g. 128.2.11.43) m Address reflects location in network

19. Introduction1-19 Domain Name System What’s the IP address for www.cmu.edu? It is 128.2.11.43 DNS server address manually configured into OS Local DNS ServerComputer 1

20. Introduction1-20 Packet Routing/Delivery r Each network technology has different local delivery methods r Address resolution provides delivery information within network m E.g., ARP maps IP addresses to Ethernet addresses m Local, works only on a particular network r Routing protocol provides path through an internetwork

21. Introduction1-21 Network:Address Resolution Protocol Ethernet Broadcast: who knows the Ethernet address for 128.2.11.43? Ethernet Broadcast: Yes, it is 08-00-2c-19-dc-45

22. Introduction1-22 Internetwork: Datagram Routing R R R R R H H H H R R H R Routers send packet to next closest point H: Hosts R: Routers

23. Introduction1-23 Routing r Forwarding tables at each router populated by routing protocols. r Original Internet: manually updated r Routing protocols update tables based on “cost” m Exchange tables with neighbors or everyone m Use neighbor leading to shortest path

24. Introduction1-24 Fourth Step: Application Demands r Reliability m Corruption m Lost packets r Flow and congestion control r Fragmentation r In-order delivery r Etc…

25. Introduction1-25 What if the Data gets Corrupted? Internet GET windex.htmlGET index.html Solution: Add a checksum Problem: Data Corruption 0,996,7,8214,571,2,36 X

26. Introduction1-26 What if Network is Overloaded? Problem: Network Overload r Short bursts: buffer r What if buffer overflows? m Packets dropped m Sender adjusts rate until load = resources r Called “congestion control” Solution: Buffering and Congestion Control

27. Introduction1-27 What if the Data gets Lost? Internet GET index.html Problem: Lost Data Internet GET index.html Solution: Timeout and Retransmit GET index.html

28. Introduction1-28 Problem: Packet size Solution: Fragment data across packets What if the Data Doesn’t Fit? r On Ethernet, max IP packet is 1.5kbytes r Typical web page is 10kbytes GETindex.html GET index.html

29. Introduction1-29 Solution: Add Sequence Numbers Problem: Out of Order What if the Data is Out of Order? GETx.htindeml GET x.htindeml GET index.html ml4inde2x.ht3GET1

30. Introduction1-30 Network Functionality Summary r Link r Multiplexing r Routing r Addressing/naming (locating peers) r Reliability r Flow control r Fragmentation r Etc….

31. Introduction1-31 The network edge: r end systems (hosts): m run application programs m e.g. Web, email m at “edge of network” r client/server model m client host requests, receives service from always-on server m e.g. Web browser/server; email client/server r peer-peer model: m minimal (or no) use of dedicated servers m e.g. Gnutella, KaZaA

32. Introduction1-32 “Cool” internet appliances World’s smallest web server http://www-ccs.cs.umass.edu/~shri/iPic.html IP picture frame http://www.ceiva.com/ Web-enabled toaster+weather forecaster

33. Introduction1-33 What’s the Internet: a service view r communication infrastructure enables distributed applications: m Web, email, games, e- commerce, file sharing r communication services provided to apps: m Connectionless unreliable m connection-oriented reliable

34. Introduction1-34 Application Support Requirement r How should we go about building the network that will realize our wish-list of requirements? m Many requirements, including general, efficient, fair, reliable, high performance connectivity among a large number of computers m Technology and application demands constantly changing m Hardware is heterogeneous

35. Introduction1-35 Why layering? Dealing with complex systems: r Modular approach to network functionality r explicit structure allows identification, relationship of complex system’s pieces m layered reference model for discussion r modularization eases maintenance, updating of system m change of implementation of layer’s service transparent to rest of system m e.g., change in gate procedure doesn’t affect rest of system

36. Introduction1-36 Layering Characteristics r Each layer relies on services from layer below and exports services to layer above r Interface defines interaction r Hides implementation - layers can change without disturbing other layers (black box)

37. Introduction1-37 Protocols r Module in layered structure r Set of rules governing communication between network elements (applications, hosts, routers) r Protocols define: m Interface to higher layers (API) m Interface to peer Format and order of messages sent and received among network entities Actions taken on receipt or transmission of a message

38. Introduction1-38 Interfaces

39. Introduction1-39 ISO Architecture

40. Introduction1-40 Summary of layers

41. Introduction1-41 Μοντέλο OSI r Φυσικό στρώμα (Physical Layer) m Μετάδοση ακατέργαστων bits (0 ή 1) από τον αποστολέα στον δέκτη. r Στρώμα Ζεύξης Δεδομένων (Data Link Layer) m Τεμαχίζει τα δεδομένα σε πλαίσια δεδομένων (frames) m Επιβεβαιώνει ότι η επικοινωνία του Φυσικού στρώματος είναι αξιόπιστη (Πλαίσια επαλήθευσης - acknowledgement frames) m Ανίχνευση και επιδιόρθωση λαθών (Error detection and correction). m Έλεγχος ροής (flow control).

42. Introduction1-42 Μοντέλο OSI r Στρώμα Δικτύου (Network Layer) m Δρομολόγηση πακέτων m Έλεγχος συμφόρησης m Έκδοση λογαριασμών (billing) r Στρώμα Μεταφοράς (Transport Layer) m Τεμαχίζει τα μηνύματα σε μικρότερες μονάδες m Επιβεβαιώνει ότι όλες οι μονάδες φτάνουν στο άλλο άκρο και επανασυναρμολογεί το μήνυμα. m Πολυπλεξία συνδέσεων/συρμών (steams) m Υπηρεσίες μεταφοράς πακέτων από άκρο σε άκρο (end- to-end). (π.χ., αξιόπιστη μεταφορά δεδομένων στον δέκτη). m Έλεγχος συμφόρησης (congestion) και ροής πακέτων

43. Introduction1-43 Μοντέλο OSI r Στρώμα Συνόδου (Session Layer) m Αποκατάσταση συνόδων μεταξύ διαφόρων μηχανών (sessions) m Διαχείριση σκυτάλης (token management) m Συγχρονισμός (synchronization) r Στρώμα Παρουσίασης (Presentation Layer) m Κωδικοποίηση δεδομένων r Στρώμα Εφαρμογή (Application Layer) m Συμβατότητα μεταξύ εφαρμογών

44. Introduction1-44 TCP/IP Protocol stack

45. Introduction1-45 TCP/IP and OSI model

46. Introduction1-46 Relation- ship of layers and addresses in TCP/IP

47. Introduction1-47 Is Layering Harmful? r Sometimes.. m Layer N may duplicate lower level functionality (e.g., error recovery) m Layers may need same info (timestamp, MTU) m Strict adherence to layering may hurt performance

48. Introduction1-48 Design Considerations r How to determine split of functionality m Across protocol layers m Across network nodes r Assigned Reading m [SRC84] End-to-end Arguments in System Design m [Cla88] Design Philosophy of the DARPA Internet Protocols