Introduction1-1 Introduction To Computer Networks מבוא לרשתות תקשורת פרופ ' אמיר הרצברג Computer Networking: A Top Down Approach Featuring the Internet, 2 nd edition. Jim Kurose, Keith Ross Addison-Wesley, July Based on foils by Kurose & Ross ©, see: My site: Course site:

Introduction1-2 Introduction To Computer Networks תקשורת ומחשוב פרטים  Lecturer: Prof. Amir Herzberg m Research in applied cryptography, secure communication and commerce, networking, dist. algs.,… m Reception: Monday 2:00-3:45pm, room 324 m Physical mail: bin 23 m Will normally answer within a week  Assistants: Mor Brosh, Igal Ioffe, Fridel Fainshtain  Use the book and foils – read at home before lectures!!

Introduction1-3 Introduction To Computer Networks תקשורת ומחשוב ציונים  80% : exam  10% : theoretical assignments (few) m Will not consider the lowest-graded m But submit all – assignments are (rarely) lost  10%: programming assignment m Great for work interviews, experience  Mandatory to pass both assignments and exam m Copying will lead to immediate disciplinary action (to all parties) – please do not.  Class attendance optional  But no noise… m You can pass notes, send – but no noise! m Offenders expelled from lecture – no warning Additional penalties possible (points, etc.)

Introduction1-4 Books  Main book: Computer Networking: A Top Down Approach Featuring the Internet, 2nd edition. Jim Kurose, Keith Ross Addison-Wesley, July m In library and should (soon?) be in shop  Also use: Tanenbaum: Computer Networks m Now in 4 th edition m Some copies in library

Introduction1-5 Online Content  Lectures, exercises and other stuff will be posted at Hi-Learn system:  Also: submit (most) exercises via Hi-Learn  To login: m Use your `regular` BIU user-ID m Your password consists of 6 characters: XYnnnn X is the first letter of your first name, e.g. a for Avi Y is the first letter of your surname, e.g. c for Cohen nnnn is 4 digits code you received in the printed timetable you (should have) received, used to access the BIU information kiosks, e.g Example password is ac1234  Problems/not registered:

Introduction1-6 LectureK&R Chapter Topic 1,21Introduction 3,42Application layer protocols 5-73Transport layer, reliability, congestion 7,84Network (internet) layer 9,105Data-Link layer 11,127, but use notes Network Security 13 Overview Schedule

Introduction1-7 Chapter 1: Introduction (this and next lecture) Our goal:  get context, overview, “feel” of networking  more depth, detail later in course  approach: m descriptive m use Internet as example Overview:  what’s the Internet  what’s a protocol?  network edge  network core  access net, physical media  Internet/ISP structure  performance: loss, delay  protocol layers, service models  history

Introduction1-8 What’s the Internet: “nuts and bolts” view  millions of connected computing devices: hosts, end-systems m PCs workstations, servers m PDAs phones, toasters running network apps  communication links m fiber, copper, radio, satellite m transmission rate = bandwidth  router/switch: forward packets (chunks of data) local ISP company network regional ISP router/switch workstation server mobile

Introduction1-9 What’s the Internet: “nuts and bolts” view  Internet: specific standard technologies: m TCP, UDP, IP – and others… m Allowing interoperability m RFC: Request for comments m IETF: Internet Engineering Task Force  An internet: “network of networks” m loosely hierarchical m public Internet versus private intranet local ISP company network regional ISP router/switch workstation server mobile

Introduction1-10 Computer Communication Network  Connection btw computers  Each pair can communicate (using `physical` network addresses)  Different implementations  Most ensure reliable communication

Introduction1-11 Network Types Each pair can communicate (using network addresses)

Introduction1-12 An internet: Connection of Networks (Using TCP/IP; `Internet` is the `big, public` internet.) Each pair can communicate (using Internet Protocol addresses (IP addresses))

Introduction1-13 What’s the Internet: a service view  communication infrastructure enables distributed applications: m Web, , games, e- commerce, database., voting, file (MP3) sharing  Communication transport services provided to apps: m connectionless m connection-oriented  Use protocols to control send, receive messages m e.g., TCP, IP, HTTP, FTP, POP3, SMTP

Introduction1-14 What’s a protocol? human protocols:  “what’s the time?”  “I have a question”  introductions … specific msgs sent … specific actions taken when msgs received, or other events network protocols:  machines rather than humans  all communication activity in Internet governed by protocols protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt

Introduction1-15 What’s a protocol? a human protocol and a computer network protocol: Q: Other human protocols? Hi Got the time? 2:00 TCP connection req TCP connection response Get time Q: What is this protocol?

Introduction1-16 A closer look at network structure:  network edge: applications and hosts  network core: m routers/switches m network of networks

Introduction1-17 Chapter 1: roadmap 1.1 What is the Internet? 1.3 Network core 1.2 Network edge 1.4 Network access and physical media 1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched networks 1.7 Protocol layers, service models 1.8 History

Introduction1-18 The Network Core  mesh of interconnected routers and switches  Hi speed, minimize work  Routing: how is data transferred through net? m circuit switching: dedicated circuit per call: telephone net m packet-switching: data sent thru net in discrete “chunks”

Introduction1-19 Network Core: Circuit Switching End-end resources reserved for “call”  link bandwidth, switch capacity  dedicated resources: no sharing  circuit-like (guaranteed) performance  call setup required  Example: phone network

Introduction1-20 Network Core: Circuit Switching network resources (e.g., bandwidth) divided into “pieces”  pieces allocated to calls  resource piece idle if not used by owning call (no sharing)  dividing link bandwidth into “pieces”  Division for Multiple Access: m Frequency division (FDMA) m Time division (TDMA) m Code division (CDMA) – much later

Introduction1-21 Circuit Switching: TDMA and FDMA FDMA frequency time TDMA frequency time 4 users Example:

Introduction1-22 Network Core: Packet Switching each end-end data stream divided into packets  user A, B packets share network resources  each packet uses full link bandwidth  resources used as needed resource contention:  aggregate resource demand can exceed amount available  congestion: packets queue, wait for link use  store and forward: packets move one hop at a time m transmit over link m wait turn at next link Bandwidth division into “pieces” Dedicated allocation Resource reservation

Introduction1-23 Packet Switching: Statistical Multiplexing Sequence of A & B packets does not have fixed pattern  statistical multiplexing. In TDM each host gets same slot in revolving TDM frame. A B C 10 Mbs Ethernet 1.5 Mbs D E statistical multiplexing queue of packets waiting for output link

Introduction1-24 Packet switching versus circuit switching  1 Mbit link  each user: m 100 kbps when “active” m active 10% of time  circuit-switching: m 10 users  packet switching: m with 35 users, probability > 10 active less than.0004 Packet switching allows more users to use network! N users 1 Mbps link

Introduction1-25 Packet switching versus circuit switching  Great for bursty data m resource sharing m simpler, no call setup  Excessive congestion: packet delay and loss m protocols needed for reliable data transfer, congestion control  Q: How to guarantee bandwidth and Quality Of Service (QOS)? (easy with circuit switching!) m Needed for audio/video apps m Still an unsolved problem (chapter 6) Is packet switching always better?

Introduction1-26 Packet-switching: store-and-forward  Takes L/R seconds to transmit (push out) packet of L bits on to link of R bps  Entire packet must arrive at router before it can be transmitted on next link: store and forward  End to end delay = (#hops)*L/R Example:  L = 7.5 Mbits  R = 1.5 Mbps  One link (hop): L/R=5sec  End to end delay = 3*L/R=15 sec R R R L

Introduction1-27 Packet Switching: Message Segmenting and Pipelining Now break up the message into 5000 packets  Each packet 1,500 bits  1 msec to transmit packet on one link  pipelining: each link works in parallel  5 sec for each link  E2E Delay= *2  Delay reduced from 15 sec to sec

Introduction1-28 Packet-switched networks: routing  Goal: move packets through routers from source to destination (using which path?) m we’ll study several path selection (i.e. routing) algorithms (chapter 4)  datagram network/routing: m destination address in packet determines next hop m routes may change during session m analogy: driving, asking directions  virtual circuit network/routing: m each packet carries tag (virtual circuit ID), tag determines next hop m fixed path determined at call setup time, remains fixed thru call m routers maintain per-call state

Introduction1-29 Chapter 1: roadmap 1.1 What is the Internet? 1.3 Network core 1.2 Network edge 1.4 Network access and physical media 1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched networks 1.7 Protocol layers, service models 1.8 History

Introduction1-30 The network edge:  end systems (hosts): m Provide transport services m To application programs e.g. Web, m Idea: Do most work at edge  client/server model m client host requests, receives service from always-on server m e.g. Web browser/server; client/server  peer-peer model: m Both sides `equal` m E.g. `ad-hoc network` between palmtop personal devices

Introduction1-31 connection-oriented transport service Goal: reliable data transfer between end systems  handshaking: setup (prepare for) data transfer ahead of time m Hello, hello back human protocol m set up “state” in two communicating hosts  TCP - Transmission Control Protocol m Internet’s connection- oriented service TCP service [RFC 793]  reliable, in-order byte- stream data transfer m loss: acknowledgements and retransmissions  flow control: m sender won’t overwhelm receiver  congestion control: m senders “slow down sending rate” when network congested m Prevents increased congestion m Improves performance to all (if all use it and delay, loss are mainly due to congestion)

Introduction1-32 connectionless transport service Goal: data transfer between end systems m same as before!  UDP - User Datagram Protocol [RFC 768]: Internet’s connectionless service m unreliable data transfer Don’t resend (phone,…) m no flow control m no congestion control TCP congestion control harmful if delay, loss NOT due to congestion! m Simple, small App’s using TCP:  HTTP (Web), FTP (file transfer), Telnet (remote login), SMTP ( ) App’s using UDP:  streaming media, teleconferencing, DNS, Internet telephony Other transport services

Introduction1-33 Network Taxonomy Telecommunication networks Circuit-switched networks FDM TDM Packet-switched networks Networks with VCs Datagram Networks Internet is Packet-switched, Datagram network Transport services (on end-systems): connection-oriented (TCP) connectionless (UDP) Recently: others (DCCP, SCP) Packet-switched networks Networks with VCs Datagram Networks TCP Connection Service UDP Connection-less Service

Introduction1-34 Lecture 1: Main concepts  Network, internet, Internet, intranet  What’s a protocol?  Network edge, core  Network transport services m Connectionless (UDP) and connection (TCP) m Client/server vs. Peer to Peer  Routing and switching m Router: connects two different networks in internet m Packet-switching versus circuit-switching (FDM,TDM) m Packet-switching: Datagram vs. virtual circuit (VC) Packet segmentation and pipelining  Use book and course site (in Hi-Learn system)

Introduction1-35 Chapter 1: roadmap 1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access and physical media 1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched networks 1.7 Protocol layers, service models 1.8 History

Introduction1-36 Internet History  1961: Kleinrock - queueing theory shows effectiveness of packet- switching  1964: Baran - packet- switching in military nets  1967: ARPAnet conceived by Advanced Research Projects Agency  1969: first ARPAnet node operational  1972: m ARPAnet demonstrated publicly m NCP (Network Control Protocol) first host- host protocol m first program m ARPAnet has 15 nodes : Early packet-switching principles

Introduction1-37 Internet History  1970: ALOHAnet satellite network in Hawaii  1973: Metcalfe’s PhD thesis proposes Ethernet  1974: Cerf and Kahn - architecture for interconnecting networks  late70’s: proprietary architectures: DECnet, SNA, XNA  late 70’s: switching fixed length packets (ATM precursor)  1979: ARPAnet has 200 nodes Cerf and Kahn’s internetworking principles: m minimalism, autonomy - no internal changes required to interconnect networks m best effort service model m stateless routers m decentralized control define today’s Internet architecture : Internetworking, new and proprietary nets

Introduction1-38 Internet History  1983: deployment of TCP/IP  1982: SMTP protocol defined  1983: DNS defined for name-to-IP- address translation  1985: FTP protocol defined  1988: TCP congestion control  new national networks: Csnet, BITnet, NSFnet, Minitel  100,000 hosts connected to confederation of networks : new protocols, a proliferation of networks

Introduction1-39 Internet History  Early 1990’s: ARPAnet decommissioned  1991: NSF lifts restrictions on commercial use of NSFnet (decommissioned, 1995)  early 1990s: Web m hypertext [Bush 1945, Nelson 1960’s] m HTML, HTTP: Berners-Lee m 1994: Mosaic, later Netscape m late 1990’s: commercialization of the Web Late 1990’s – 2000’s:  more killer apps: instant messaging, peer2peer file sharing (e.g., Naptser)  network security to forefront  est. 50 million host, 100 million+ users  backbone links running at Gbps 1990, 2000’s: commercialization, the Web, new apps