Introduction1-1 Chapter 1 Computer Networks and the Internet Computer Networking: A Top Down Approach Featuring the Internet, 2 nd edition. Jim Kurose, Keith Ross Addison-Wesley, July A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:  If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!)  If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. All material copyright J.F. Kurose and K.W. Ross, All Rights Reserved Small modifications copyright 2003, B.D. Davison

Introduction1-2 Chapter 1: Introduction 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

Introduction1-3 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

Introduction1-4 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  routers: forward packets (chunks of data) local ISP company network regional ISP router workstation server mobile

Introduction1-5 “Cool” internet appliances World’s smallest web server IP picture frame Web-enabled toaster+weather forecaster

Introduction1-6 What’s the Internet: “nuts and bolts” view  protocols control sending, receiving of msgs m e.g., TCP, IP, HTTP, FTP, PPP  Internet: “network of networks” m loosely hierarchical m public Internet versus private intranet  Internet standards m RFC: Request for comments m IETF: Internet Engineering Task Force local ISP company network regional ISP router workstation server mobile

Introduction1-7 What’s the Internet: a service view  communication infrastructure enables distributed applications: m Web, , games, e- commerce, database., voting, file (MP3) sharing  communication services provided to apps: m connectionless m connection-oriented  cyberspace [Gibson]: “a consensual hallucination experienced daily by billions of operators, in every nation,...."

Introduction1-8 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

Introduction1-9 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

Introduction1-10 A closer look at network structure:  network edge: applications and hosts  network core: m routers m network of networks  access networks, physical media: communication links

Introduction1-11 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

Introduction1-12 The network edge:  end systems (hosts): m run application programs m e.g. Web, m at “edge of network”  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 minimal (or no) use of dedicated servers m e.g. Gnutella, KaZaA

Introduction1-13 Network edge: connection-oriented service Goal: 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

Introduction1-14 Network edge: connectionless 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 m no flow control m no congestion control App’s using TCP:  HTTP (Web), FTP (file transfer), Telnet (remote login), SMTP ( ) App’s using UDP:  streaming media, teleconferencing, DNS, Internet telephony

Introduction1-15 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

Introduction1-16 The Network Core  mesh of interconnected routers  the fundamental question: 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”

Introduction1-17 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

Introduction1-18 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” m frequency division m time division

Introduction1-19 Circuit Switching: FDMA and TDMA FDMA frequency time TDMA frequency time 4 users Example:

Introduction1-20 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

Introduction1-21 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

Introduction1-22 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

Introduction1-23 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 provide circuit-like behavior? m bandwidth guarantees needed for audio/video apps m still an unsolved problem (chapter 6) Is packet switching a “slam dunk winner?”

Introduction1-24 Packet-switching: store-and-forward  Takes L/R seconds to transmit (push out) packet of L bits on to link or R bps  Entire packet must arrive at router before it can be transmitted on next link: store and forward  delay = 3L/R Example:  L = 7.5 Mbits  R = 1.5 Mbps  delay = 15 sec R R R L

Introduction1-25 Packet Switching: Message Segmenting 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  Delay reduced from 15 sec to sec

Introduction1-26 Packet-switched networks: forwarding  Goal: move packets through routers from source to destination m we’ll study several path selection (i.e. routing) algorithms (chapter 4)  datagram network: m destination address in packet determines next hop m routes may change during session m analogy: driving, asking directions  virtual circuit network: m each packet carries tag (virtual circuit ID), tag determines next hop m fixed path determined at call setup time, remains fixed throughout call m routers maintain per-call state

Introduction1-27 Network Taxonomy Telecommunication networks Circuit-switched networks FDM TDM Packet-switched networks Networks with VCs Datagram Networks Datagram network is not either connection-oriented or connectionless. Internet provides both connection-oriented (TCP) and connectionless services (UDP) to apps.