Chapter 1 Computer Networks and the Internet Last

Chapter 1: Computer Networks and the Internet Last Update: Sep 29, 2011 Introduction 1

Outline q Internet as an example Computer Network q What is Internet Component view v Services view q What is a protocol, what is layering? v Protocols v Protocol layers and their service models q Network structure - edge: applications and some services q Network structure - core: v v Routers Circuit switching and packet switching q Delay, loss and throughput in packet switched networks q Access networks and physical media q Internet infrastructure v Introduction 2

Internet as an example Computer Network Introduction 3

What’s the Internet PC q millions of connected computing devices: hosts = end systems wireless laptop v running network cellular handheld apps q communication links v fiber, copper, access points radio, satellite wired links v transmission rate = bandwidth q routers: forward router packets (chunks of data) Mobile network server Global ISP Home network Regional ISP Institutional network Introduction 4

“Cool” internet appliances Web-enabled toaster + weather forecaster IP picture frame http: //www. ceiva. com/ World’s smallest web server http: //www-ccs. umass. edu/~shri/i. Pic. html Internet phones Introduction 5

What’s the Internet q protocols control sending, Mobile network receiving of msgs v e. g. , TCP, IP, HTTP, Skype, Ethernet q Internet: “network of networks” v v loosely hierarchical public Internet versus private intranet Global ISP Home network Regional ISP Institutional network q Internet standards v RFC: Request for comments v IETF: Internet Engineering Task Force Introduction 6

What’s the Internet: a service view q communication infrastructure enables distributed applications: v Web, Vo. IP, email, games, e-commerce, file sharing q communication services provided to apps: v reliable data delivery from source to destination v “best effort” (unreliable) data delivery Introduction 7

What’s a protocol? human protocols: q “what’s the time? ” q “I have a question” q introductions … specific msgs sent … specific actions taken when msgs received, or other events network protocols: q machines rather than humans q all communication activity in Internet governed by protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt Introduction 8

What’s a protocol? a human protocol and a computer network protocol: Hi TCP connection request Hi TCP connection response Got the time? Get http: //www. awl. com/kurose-ross 2: 00 time Q: Other human protocols? Introduction 9

Protocol “Layers” Networks are complex! q many “pieces”: v hosts v routers v links of various media v applications v protocols v hardware, software Question: Is there any hope of organizing structure of network? Or at least our discussion of networks? Introduction 10

Protocol Suite q The complexity of the communication task is reduced by using multiple protocol layers: • Each protocol is implemented independently • Each protocol is responsible for a specific subtask • Protocols are grouped in a hierarchy q A structured set of protocols is called a communications architecture or protocol suite Introduction 11

Internet protocol stack q application: supporting network applications v FTP, SMTP, HTTP q transport: process-process data transfer v TCP, UDP q network: routing of datagrams from source to destination v IP, routing protocols q link: data transfer between application transport network link physical neighboring network elements v PPP, Ethernet q physical: bits “on the wire” Introduction 12

Assignment of Protocols to Layers Introduction 13

Layers in the Example Introduction 14

Layers and Services q Service provided by TCP to HTTP: v reliable transmission of data over a logical connection q Service provided by IP to TCP: v unreliable transmission of IP datagrams across an IP network q Service provided by Ethernet to IP: v transmission of a frame across an Ethernet segment q Other services: v v DNS: translation between domain names and IP addresses ARP: Translation between IP addresses and MAC addresses Introduction 15

Encapsulation and Demultiplexing q As data is moving down the protocol stack, each protocol is adding layer-specific control information Introduction 16

To Summarize: Why layering? Dealing with complex systems: q explicit structure allows identification, relationship of complex system’s pieces v layered reference model for discussion q modularization eases maintenance, updating of system v change of implementation of layer’s service transparent to rest of system v e. g. , change in gate procedure doesn’t affect rest of system q layering considered harmful? Introduction 17

Network Structure: a closer look Introduction 18

A closer look at network structure q network edge: applications and hosts q access networks, physical media: wired, wireless communication links q network core: v interconnected routers v network of networks Introduction 19

The network edge q end systems (hosts): v v v run application programs e. g. Web, email at “edge of network” peer-peer q client/server model v v client host requests, receives service from always-on server client/server e. g. Web browser/server; email client/server q peer-peer model: v v minimal (or no) use of dedicated servers e. g. Skype, Bit. Torrent Introduction 20

Network edge: reliable data transfer service Goal: data transfer between end systems q handshaking: setup (prepare for) data transfer ahead of time v v Hello, hello back human protocol set up “state” in two communicating hosts q TCP - Transmission Control Protocol v Internet’s reliable data transfer service TCP service [RFC 793] q reliable, in-order byte- stream data transfer v loss: acknowledgements and retransmissions q flow control: v sender won’t overwhelm receiver q congestion control: v senders “slow down sending rate” when network congested Introduction 21

Network edge: best effort (unreliable) data transfer service Goal: data transfer between end systems v same as before! q UDP - User Datagram Protocol [RFC 768]: v connectionless v unreliable data transfer v no flow control v no congestion control App’s using TCP: q HTTP (Web), FTP (file transfer), Telnet (remote login), SMTP (email) App’s using UDP: q streaming media, teleconferencing, DNS, Internet telephony Introduction 22

The Network Core q mesh of interconnected routers q the fundamental question: how is data transferred through net? v circuit switching: dedicated circuit per call: telephone net v packet-switching: data sent thru net in discrete “chunks” Introduction 23

Network Core: Circuit Switching End-end resources reserved for “call” q link bandwidth, switch capacity q dedicated resources: no sharing q circuit-like (guaranteed) performance q call setup required Introduction 24

Network Core: Circuit Switching network resources (e. g. , bandwidth) divided into “pieces” q pieces allocated to calls q dividing link bandwidth into “pieces” v frequency division v time division q resource piece idle if not used by owning call (no sharing) Introduction 25

Circuit Switching: FDM and TDM Example: FDM 4 users frequency time TDM frequency time Introduction 26

Numerical example q How long does it take to send a file of 640, 000 bits from host A to host B over a circuit-switched network? All links are 1. 536 Mbps v Each link uses TDM with 24 slots/sec v 500 msec to establish end-to-end circuit v Total Delay = 500 msec+ 640 Kbits/(1. 536 Mbps/24) = 10. 5 sec Introduction 27

Network Core: Packet Switching each end-end data stream divided into packets q user A, B packets share network resources q each packet uses full link bandwidth q resources used as needed Bandwidth division into “pieces” Dedicated allocation Resource reservation resource contention: q aggregate resource demand can exceed amount available q congestion: packets queue, wait for link use q store and forward: packets move one hop at a time v Node receives complete packet before forwarding Introduction 28

Packet Switching: Statistical Multiplexing 100 Mb/s Ethernet A B statistical multiplexing C 1. 5 Mb/s queue of packets waiting for output link D E Sequence of A & B packets does not have fixed pattern, bandwidth shared on demand statistical multiplexing. TDM: each host gets same slot in revolving TDM frame. Introduction 29

Packet switching versus circuit switching Packet switching allows more users to use network! q 1 Mb/s link q each user: v 100 kb/s when “active” v active 10% of time q circuit-switching: v 10 users q packet switching: v with 35 users, probability > 10 active at same time is less than. 0004 N users 1 Mbps link Introduction 30

Packet switching versus circuit switching Is packet switching always a winner? q great for bursty data resource sharing v simpler, no call setup q excessive congestion: packet delay and loss v protocols needed for reliable data transfer, congestion control q Q: How to provide circuit-like behavior? v bandwidth guarantees needed for audio/video apps v still an unsolved problem (chapter 7) v Introduction 31

Packet-switching: store-and-forward L R q takes L/R seconds to R transmit (push out) packet of L bits on to link at R bps q store and forward: entire packet must arrive at router before it can be transmitted on next link q delay = 3 L/R (assuming zero propagation delay) R Example: q L = 7. 5 Mbits q R = 1. 5 Mbps q transmission delay = 15 sec Introduction 32

How do delay and loss occur? packets queue in router buffers q packet arrival rate to link exceeds output link capacity q packets queue, wait for turn packet being transmitted (delay) A B packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers Introduction 33

Four sources of packet delay q 1. nodal processing: v check bit errors v determine output link q 2. queueing v time waiting at output link for transmission v depends on congestion level of router transmission A propagation B nodal processing queueing Introduction 34

Delay in packet-switched networks 3. Transmission delay: q R=link bandwidth (bps) q L=packet length (bits) q time to send bits into link = L/R transmission A 4. Propagation delay: q d = length of physical link q s = propagation speed in medium (~2 x 108 m/sec) q propagation delay = d/s Note: s and R are very different quantities! propagation B nodal processing queueing Introduction 35

Nodal delay q dproc = processing delay v typically a few microsecs or less q dqueue = queuing delay v depends on congestion q dtrans = transmission delay v = L/R, significant for low-speed links q dprop = propagation delay v a few microsecs to hundreds of msecs Introduction 36

Queueing delay q R=link bandwidth (bps) q L=packet length (bits) q a=average packet arrival rate traffic intensity = La/R q La/R ~ 0: average queueing delay small q La/R -> 1: delays become large q La/R > 1: more “work” arriving than can be serviced, average delay infinite! Introduction 37

Packet loss q queue (aka buffer) preceding link in buffer has finite capacity q packet arriving to full queue dropped (aka lost) q lost packet may be retransmitted by previous node, by source end system, or not at all buffer (waiting area) A B packet being transmitted packet arriving to full buffer is lost Introduction 38

Timing in Circuit Switching Assume D Number of hops = M Per-hop processing delay = P Link propagation delay = D Transmission speed = R bit/s Message size = L bits Total Delay = total propagation + total transmission + total processing = 4 MD + L/R + (M-1)P P L/R Total Delay Introduction 39

Timing in Datagram Packet Switching Assume: Number of hops = M Per-hop processing delay = P Link propagation delay = D Packet transmission delay = T Message size = N packets Nodal Queueing delay = Q Total Delay = total propagation + total transmission + total store&forward + total processing + total queueing = MD + NT + (M-1)P + (M-1) Q T P+Q T Total Delay P+Q D Introduction 40

Throughput q Another performance metric q Throughput (between source S and destination D): amount of data that can be transferred per second. Some Units: Gbps or Mbps or Kbps. v ~ transfer rate q Depends on v v the transmission rates of the links on the path between S-D the intervening traffic q ~= transmission rate of the bottleneck link on the path from S to D (assuming single flow S-D exists) S r 1 R r 2 R r 3 D tput=min{r 1, r 2, r 3} Introduction 41

Access Networks and Physical Media Introduction 42

Access networks and physical media Q: How to connect end systems to edge router? q residential access nets q institutional access networks (school, company) q mobile access networks Keep in mind: q bandwidth (bits per second) of access network? q shared or dedicated? Introduction 43

Residential access: point to point access q Dialup via modem up to 56 Kbps direct access to router (often less) v Can’t surf and phone at same time: can’t be “always on” v q DSL: digital subscriber line deployment: telephone company (typically) v up to 1 Mbps upstream (today typically < 256 kbps) v up to 8 Mbps downstream (today typically < 1 Mbps) v dedicated physical line to telephone central office v Introduction 44

Residential access: cable modems q HFC: hybrid fiber coax asymmetric: up to 30 Mbps downstream, 2 Mbps upstream q network of cable and fiber attaches homes to ISP router v homes share access to router q deployment: available via cable TV companies v Introduction 45

Company access: local area networks q company/univ local area network (LAN) connects end system to edge router q Ethernet: v 10 Mbs, 100 Mbps, 1 Gbps, 10 Gbps Ethernet v modern configuration: end systems connect into Ethernet switch q LANs: chapter 5 Introduction 46

Wireless access networks q shared wireless access network connects end system to router v via base station aka “access point” q wireless LANs: v 802. 11 b/g (Wi. Fi): 11 or 54 Mbps q wider-area wireless access v provided by telco operator v ~1 Mbps over cellular system (EVDO, HSDPA) v next up (? ): Wi. MAX (10’s Mbps) over wide area router base station mobile hosts Introduction 47

Home networks Typical home network components: q DSL or cable modem q router/firewall/NAT q Ethernet q wireless access point to/from cable headend cable modem router/ firewall Ethernet wireless laptops wireless access point Introduction 48

Physical Media q Bit: propagates between transmitter/rcvr pairs q physical link: what lies between transmitter & receiver q guided media: v signals propagate in solid media: copper, fiber, coax Twisted Pair (TP) q two insulated copper wires v v Category 3: traditional phone wires, 10 Mbps Ethernet Category 5: 100 Mbps Ethernet q unguided media: v signals propagate freely, e. g. , radio Introduction 49

Physical Media: coax, fiber Coaxial cable: q two concentric copper conductors q bidirectional q baseband: v v single channel on cable legacy Ethernet q broadband: v multiple channels on cable v HFC Fiber optic cable: q glass fiber carrying light pulses, each pulse a bit q high-speed operation: v high-speed point-to-point transmission (e. g. , 10’s 100’s Gps) q low error rate: repeaters spaced far apart ; immune to electromagnetic noise Introduction 50

Physical media: radio q signal carried in electromagnetic spectrum q no physical “wire” q bidirectional q propagation environment effects: v v v reflection obstruction by objects interference Radio link types: q terrestrial microwave v e. g. up to 45 Mbps channels q LAN (e. g. , Wifi) v 11 Mbps, 54 Mbps q wide-area (e. g. , cellular) v 3 G cellular: ~ 1 Mbps q satellite v Kbps to 45 Mbps channel (or multiple smaller channels) v 270 msec end-end delay v geosynchronous versus low altitude Introduction 51

Internet Infrastructure Introduction 52

Internet Infrastructure Introduction 53

Internet Infrastructure q The infrastructure of the Internet consists of a federation of connected networks that are each independently managed (“autonomous system”) v Note: Each “autonomous system may consist of multiple IP networks q Hierarchy of network service providers (NSPs) v Tier-1: nation or worldwide network (10 s) v Tier-2: regional networks (100 s) v Tier-3: local Internet service provider (1000 s) Introduction 54

Internet Infrastructure q Location where a network (ISP, corporate network, or regional network) gets access to the Internet is called a Point-of. Presence (POP). q Locations (Tier-1 or Tier-2) networks are connected for the purpose of exchanging traffic are called peering points. Public peering: Traffic is swapped in a specific location, called Internet exchange points (IXPs) v Private peering: Two networks establish a direct link to each other. v Introduction 55

Internet structure: network of networks q at center: “tier-1” ISPs (e. g. , Verizon, Sprint, AT&T, Cable and Wireless), national/international coverage v treat each other as equals Tier-1 providers interconnect (peer) privately Tier 1 ISP Introduction 56

Tier-1 ISP: e. g. , Sprint POP: point-of-presence to/from backbone peering … … … to/from customers Introduction 57

Internet structure: network of networks q “Tier-2” ISPs: smaller (often regional) ISPs v Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet q tier-2 ISP is customer of tier-1 provider Tier-2 ISP Tier 1 ISP Tier-2 ISPs also peer privately with each other. Tier-2 ISP Introduction 58

Internet structure: network of networks q “Tier-3” ISPs and local ISPs v last hop (“access”) network (closest to end systems) local ISP Local and tier 3 ISPs are customers of higher tier ISPs connecting them to rest of Internet Tier 3 ISP Tier-2 ISP local ISP Tier-2 ISP Tier 1 ISP Tier-2 ISP local ISP Introduction 59

Internet structure: network of networks q a packet passes through many networks! local ISP Tier 3 ISP Tier-2 ISP local ISP Tier-2 ISP Tier 1 ISP Tier-2 ISP local ISP Introduction 60

Summary q A network has building-blocks and provides some services q A network operates with protocols q Network architecture is layered and protocols are distributed into layers q Applications can be structured using application models like client-server or p 2 p q Various access networks exist q Various transmission media exist q Internet is a packet switched network v Routers forward packets q Internet has a hierarchical structure v Loosely coupled q Some important network terminology and concepts introduced. Introduction 61

References q J. Kurose and K. Ross, Computer Networking, A top-down approach, Firth Edition, Addison Wesley, 2010. Introduction 62