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Chapter 1: roadmap 1.1 what is the Internet? 1.2 network edge end systems, access networks, links 1.3 network core packet switching, circuit switching, network structure 1.4 delay, loss, throughput in networks 1.5 protocol layers, service models 1.6 end-to-end throughput analysis Introduction 1-1 What’s the Internet: a service view Communication infrastructure • enables distributed applications • Web, VoIP, email, games, ecommerce, social nets, … mobile network global ISP home network regional ISP Communication services • provides programming interface to apps • hooks that allow sending and receiving app programs to “connect” to Internet • provides service options, analogous to postal service institutional network Introduction 1-2 What’s the Internet: “nuts and bolts” view hosts or end systems: PC mobile network • billions of connected computing devices • running network apps server wireless laptop global ISP smartphone communication links: wireless links wired links home network regional ISP • fiber, copper, radio, satellite • transmission rate: bandwidth routers & switches: router forward packets (chunks of data) institutional network Introduction 1-3 What’s a protocol? protocols define format, order of messages sent and received among network entities, TCP connection request TCP connection response <file> and actions taken on message transmission, receipt Get http://www.awl.com/kurose-ross Introduction 1-4 What’s the Internet: “nuts and bolts” view Internet: “network of networks” mobile network • Interconnected ISPs global ISP Internet standards • IETF: Internet Engineering Task Force RFC: Request for comments home network regional ISP • IEEE for links/hardware Ethernet, 802.11, protocols • control sending, receiving of messages • e.g., TCP, IP, HTTP, Skype, 802.11 institutional network Introduction 1-5 Chapter 1: roadmap 1.1 what is the Internet? 1.2 network edge end systems, access networks, links 1.3 network core packet switching, circuit switching, network structure 1.4 delay, loss, throughput in networks 1.5 protocol layers, service models 1.6 end-to-end throughput analysis Introduction 1-6 A closer look at network structure: network edge: mobile network • hosts: clients and servers • servers often in data centers access networks, physical media: global ISP home network regional ISP wired, wireless communication links network core: • interconnected routers • network of networks institutional network Introduction 1-7 Access networks Q: How to connect end systems to edge router? residential access nets enterprise/institutional access networks school, company, mobile/wireless access networks Introduction 1-8 Residential access networks wireless devices to/from headend or central office often combined in single box cable or DSL modem wireless access point (54 Mbps) router, firewall, NAT wired Ethernet (1 Gbps) Introduction 1-9 Enterprise access networks (Ethernet) institutional link to ISP (Internet) institutional router Ethernet switch institutional mail, web servers typically used in companies, universities, etc. 10 Mbps, 100Mbps, 1Gbps, 10Gbps transmission rates today, end systems typically connect into Ethernet switch Introduction 1-10 Mobile/wireless access networks shared wireless access network connects end system to router • via base station aka “access point” wide-area wireless access wireless LANs: within building (100 ft.) 802.11b/g/n (WiFi): 11, 54, 450 Mbps transmission rate provided by telco (cellular) operator, 10’s km between 1 and 10 Mbps 3G, 4G: LTE to Internet to Internet Introduction 1-11 Host: sends packets of data host sending function: takes application message breaks into smaller chunks, known as packets, of length L bits transmits packet into access network at transmission rate R • link transmission rate, aka link capacity, aka link bandwidth packet transmission delay = two packets, L bits each 2 1 R: link transmission rate host time needed to transmit L-bit packet into link = L (bits) R (bits/sec) Introduction 1-12 Physical media bit: propagates between transmitter/receiver pairs physical link: what lies between transmitter & receiver guided media: • signals propagate in solid media: copper (twisted pair), fiber, coax unguided media: • signals propagate freely, e.g., wireless Twisted pair cable Coaxial cable fiber optic cable Introduction 1-13 Unguided media signal carried in electromagnetic spectrum no physical “wire” bidirectional propagation environment effects: • reflection • obstruction by objects • interference radio link types: terrestrial microwave • e.g. up to 45 Mbps channels LAN (e.g., WiFi) • 54 Mbps wide-area (e.g., cellular) • 4G cellular: ~ 10 Mbps satellite • Kbps to 45Mbps channel (or multiple smaller channels) • 270 msec end-end delay Introduction 1-14 Chapter 1: roadmap 1.1 what is the Internet? 1.2 network edge end systems, access networks, links 1.3 network core packet switching, circuit switching, network structure 1.4 delay, loss, throughput in networks 1.5 protocol layers, service models 1.6 end-to-end throughput analysis Introduction 1-15 The network core The fundamental question: how is data transferred through net? circuit-switching: dedicated circuit per call: telephone nets packet-switching: data sent thru net in discrete “chunks” Introduction 1-16 Network core: packet-switching 100 Mb/s Ethernet A B C 1.5 Mb/s Packet-switching: Source breaks application-layer message into packets Forward from one router to the next until reaching destination Introduction 1-17 Network core: packet-switching 100 Mb/s Ethernet A B C 1.5 Mb/s Packet-switching: no dedication/reservation: all streams share resources no setup is required resources used as needed each packet uses full link bandwidth aggregate resource demand can exceed capacity no guarantee Introduction 1-18 Packet Switching: statistical multiplexing A B R = 100 Mb/s statistical multiplexing R = 1.5 Mb/s C D E queue of packets waiting for output link Sequence of A & B packets does not have fixed pattern, shared on demand statistical multiplexing Introduction 1-19 Packet-switching: store-and-forward L bits per packet source 3 2 1 R bps takes L/R sec to transmit (push out) L-bit packet into link at R bps store and forward: entire packet must arrive at router before it can be transmitted on next link end-end delay = 2L/R (assuming zero propagation delay) R bps destination one-hop numerical example: L = 7.5 Mbits R = 1.5 Mbps one-hop transmission delay = 5 sec more on delay shortly … Introduction 1-20 Packet Switching: queueing delay, loss A C R = 100 Mb/s R = 1.5 Mb/s B queue of packets waiting for output link D E queuing and loss: if arrival rate (in bits) to link exceeds transmission rate of link for a period of time, then there will be consequences: • delay: packets will queue, wait to be transmitted on link • loss: packets can be dropped if memory (buffer) fills up Introduction 1-21 Two key network-core functions routing: determines sourcedestination route taken by packets routing algorithms forwarding: move packets from router’s input to appropriate router output routing algorithm local forwarding table header value output link 0100 0101 0111 1001 3 2 2 1 1 3 2 destination address in arriving packet’s header Introduction 1-22 Alternative core: circuit switching reserved resources: • resources divided into ‘pieces’ • end-end resources are reserved for each “call” • in diagram, each link has four circuits • call gets 2nd circuit in top link and 1st circuit in right link. no sharing: • dedicated resources: guarantees: • circuit-like performance Introduction 1-23 Alternative core: circuit switching wasted if not used: • circuit segment idle if not used by call (no sharing) overhead: • call setup is required • admission control same path for all chunks commonly used in traditional telephone networks Introduction 1-24 Circuit switching: FDM versus TDM Multiplexing: Combining multiple signals such that individual signals can be retrieved at the destination. Signals occupy the same channel, so the channel must be shared Two ways of dividing bandwidth into “pieces” frequency division time division Introduction 1-25 Circuit switching: FDM versus TDM Example: Freq. Division Multiplexing (FDM) 4 users frequency time Time Division Multiplexing (TDM) frequency time Introduction 1-26 Circuit switching: FDM versus TDM Frequency Division: Divides the channel into multiple, but smaller frequency ranges. (+) Supports more user at one time (+) Low latency, no time turn, preferred for real-time applications, limited (-) But limited in terms of frequency availability Time Division: Allocates certain time periods to each channel in an alternating manner. (+) Much more flexible, can support many users but at different times (-) Slower, have to wait the turn What is the best option? Introduction 1-27 Packet switching versus circuit switching A Circuit switching B B: has no packets to send A 2 Mb/s • 2 circuits (use TDM) • A reserves 1 circuit • B reserves 1 circuit Utilization = 50% only = 1 Mb/s Packet switching B 2 Mb/s • statistical multiplex. • B uses full link since A is not using it Utilization = 100% = 2 Mb/s Introduction 1-28 Numerical example How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network? • The link’s transmission rate = 0.64 Mbps • Each link uses TDM with 10 slots/sec • 0.5 sec to establish end-to-end circuit Solution: Bandwidth of circuit (in kbps)= .64x1000/10 = 64 kbps Time to send: 640 kbits/64 kbps + 0.5s = 10.5s Introduction 1-29 Packet switching versus circuit switching Packet switching Circuit switching resources sharing no sharing congestion - may lead to needs congestion control admission control overhead - less overhead no connection setup - more overhead need to reserve resources first guarantee - best-effort model no guarantees - provides guarantees good for multimedia Introduction 1-30 Packet switching versus circuit switching packet switching allows more users to use network! 3 Mb/s link each user: • 1 Mb/s when “active” • active 1/3 of time circuit-switching: N users 3 Mbps link • 3 users packet switching: • With N=4 users, what are the chances that a user won’t get 1 Mb/s? I.e., what is the prob. that more than 3 (strictly) users are active? • With N=5 users, what are the chances that a user won’t get 1 Mb/s? ……. Introduction 1-31 Internet structure: network of networks Question: given millions of access ISPs, how to connect them together? access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net Introduction 1-32 Internet structure: network of networks Option: connect each access ISP to every other access ISP? access net access net access net access net access net access net access net connecting each access ISP to each other directly doesn’t scale: O(N2) connections. access net access net access net access net access net access net access net access net access net Introduction 1-33 Internet structure: network of networks Option: connect each access ISP to one global transit ISP? Customer and provider ISPs have economic agreement. access net access net access net access net access net access net access net global ISP access net access net access net access net access net access net access net access net access net Introduction 1-34 Internet structure: network of networks But if one global ISP is viable business, there will be competitors …. access net access net access net access net access net access net access net ISP A ISP B access net access net access net ISP C access net access net access net access net access net access net Introduction 1-35 Internet structure: network of networks But if one global ISP is viable business, there will be competitors …. which must be interconnected Internet exchange point access access net net access net access net access net IXP access net ISP A access net ISP B IXP access net access net access net ISP C access net peering link access net access net access net access net access net Introduction 1-36 Internet structure: network of networks … and regional networks may arise to connect access nets to ISPs access net access net access net access net access net IXP access net ISP A access net ISP B IXP access net access net access net ISP C access net access net regional net access net access net access net access net Introduction 1-37 Internet structure: network of networks … and content provider networks (e.g., Google, Microsoft, Akamai) may run their own network, to bring services, content close to end users access net access net access net access net access net IXP access net ISP A Content provider network IXP ISP B access net access net access net access net ISP C access net access net regional net access net access net access net access net Introduction 1-38 Internet structure: network of networks Tier 1 ISP Tier 1 ISP IXP IXP Regional ISP access ISP access ISP Google access ISP access ISP IXP Regional ISP access ISP access ISP access ISP access ISP at center: small # of well-connected large networks • “tier-1” commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT), national & international coverage • content provider network (e.g., Google): private network that connects it data centers to Internet, often bypassing tier-1, regional ISPs Introduction 1-39 Tier-1 ISP: e.g., Sprint Introduction 1-40 Chapter 1: roadmap 1.1 what is the Internet? 1.2 network edge end systems, access networks, links 1.3 network core packet switching, circuit switching, network structure 1.4 delay, loss, throughput in networks 1.5 protocol layers, service models 1.6 end-to-end throughput analysis Introduction 1-41 Four sources of packet delay transmission A propagation B nodal processing queueing dnodal = dproc + dqueue + dtrans + dprop dproc: nodal processing check bit errors determine output link typically < msec dqueue: queueing delay time waiting at output link for transmission depends on congestion level of router Introduction 1-42 Four sources of packet delay transmission A propagation B nodal processing queueing dnodal = dproc + dqueue + dtrans + dprop dtrans: transmission delay: L: packet length (bits) R: link bandwidth (bps) dtrans and dprop dtrans = L/R very different dprop: propagation delay: d: length of physical link s: propagation speed (~2x108 m/sec) dprop = d/s * Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive/ * Check out the Java applet for an interactive animation on trans vs. prop delay Introduction 1-43 “Real” Internet delays and routes what do “real” Internet delay & loss look like? traceroute program: provides delay measurement from source to router along endend Internet path towards destination. For all i: • sends three packets that will reach router i on path towards destination • router i will return packets to sender • sender times interval between transmission and reply. 3 probes 3 probes 3 probes Introduction 1-44 “Real” Internet delays, routes traceroute: gaia.cs.umass.edu to www.eurecom.fr 3 delay measurements from gaia.cs.umass.edu to cs-gw.cs.umass.edu 1 cs-gw (128.119.240.254) 1 ms 1 ms 2 ms 2 border1-rt-fa5-1-0.gw.umass.edu (128.119.3.145) 1 ms 1 ms 2 ms 3 cht-vbns.gw.umass.edu (128.119.3.130) 6 ms 5 ms 5 ms 4 jn1-at1-0-0-19.wor.vbns.net (204.147.132.129) 16 ms 11 ms 13 ms 5 jn1-so7-0-0-0.wae.vbns.net (204.147.136.136) 21 ms 18 ms 18 ms 6 abilene-vbns.abilene.ucaid.edu (198.32.11.9) 22 ms 18 ms 22 ms 7 nycm-wash.abilene.ucaid.edu (198.32.8.46) 22 ms 22 ms 22 ms trans-oceanic 8 62.40.103.253 (62.40.103.253) 104 ms 109 ms 106 ms link 9 de2-1.de1.de.geant.net (62.40.96.129) 109 ms 102 ms 104 ms 10 de.fr1.fr.geant.net (62.40.96.50) 113 ms 121 ms 114 ms 11 renater-gw.fr1.fr.geant.net (62.40.103.54) 112 ms 114 ms 112 ms 12 nio-n2.cssi.renater.fr (193.51.206.13) 111 ms 114 ms 116 ms 13 nice.cssi.renater.fr (195.220.98.102) 123 ms 125 ms 124 ms 14 r3t2-nice.cssi.renater.fr (195.220.98.110) 126 ms 126 ms 124 ms 15 eurecom-valbonne.r3t2.ft.net (193.48.50.54) 135 ms 128 ms 133 ms 16 194.214.211.25 (194.214.211.25) 126 ms 128 ms 126 ms 17 * * * * means no response (probe lost, router not replying) 18 * * * 19 fantasia.eurecom.fr (193.55.113.142) 132 ms 128 ms 136 ms Introduction 1-45 Exercise 1 Packet length = L bits trans. rate R = 1 Mbps Host A Host B distance = 1 km, speed = 2x108m/s Question: Which bit is being transmitted at the time the first bit arrives at Host B Answer: First bit arrives after 1/R + d/s = 1/106 + 103/(2x108) = 10-6 + 5x10-6 = 6x10-6 = 6 µsec After 6 µsec 6 bits are already transmitted; so 7th bit is being transmitted Introduction 1-46 Exercise 2 Transmission vs. propagation L=100Bytes trans. rate R = ? Host A Host B distance = 2 km, speed = 2x108m/s Question: At what rate (bandwidth) R would the propagation delay equal the transmission delay of packet? Answer: Propagation delay = 2x103 (m)/2x108 (m/s) = 10-5 sec Transmission delay = 100x8 (bits)/R Prop. delay = trans. delay => R=105x100x8 = 80 Mbps Introduction 1-47 Chapter 1: roadmap 1.1 what is the Internet? 1.2 network edge end systems, access networks, links 1.3 network core packet switching, circuit switching, network structure 1.4 delay, loss, throughput in networks 1.5 protocol layers, service models 1.6 end-to-end throughput analysis Introduction 1-50 Protocol “layers” Networks are complex, with many “pieces”: hosts routers links of various media applications protocols hardware, software Question: is there any hope of organizing structure of network? Introduction 1-51 Internet protocol stack application: supporting network applications • FTP, SMTP, HTTP transport: process-process data transfer • TCP, UDP network: routing of datagrams from source to destination • IP, routing protocols link: data transfer between neighboring network elements application transport network link physical • Ethernet, 802.11 (WiFi), PPP physical: bits “on the wire” Introduction 1-52 Encapsulation source message segment M Ht M datagram Hn Ht M frame M Hl Hn Ht application transport network link physical link physical switch M Ht M Hn Ht M Hl Hn Ht M destination Hn Ht M application transport network link physical Hl Hn Ht M network link physical Hn Ht M router Introduction 1-53 ISO/OSI Model application presentation session transport application transport network network link data link physical physical 7-layer ISO/OSI model (OSI: open system interconnections) 5-layer Internet Protocol Stack Introduction 1-54 Chapter 1: roadmap 1.1 what is the Internet? 1.2 network edge end systems, access networks, links 1.3 network core packet switching, circuit switching, network structure 1.4 delay, loss, throughput in networks 1.5 protocol layers, service models 1.6 end-to-end throughput analysis Introduction 1-55 Nodal delay: four delay components d nodal d proc d queue d trans d prop dproc = processing delay typically a few microsecs or less dqueue = queuing delay depends on congestion dtrans = transmission delay = L/R, significant for low-speed links dprop = propagation delay a few microsecs to hundreds of msecs Introduction 1-56 How does packet loss occur? queue (aka buffer) preceding link has finite capacity packets may arrive at rate exceeding output link capacity packets are queued in router buffers packet arriving to full queue are dropped (aka lost) lost packet may be retransmitted by previous node, by source end system, or not at all buffer (waiting area) A packet being transmitted B packet arriving to full buffer is lost * Check out the Java applet for an interactive animation on queuing and loss Introduction 1-57 Queueing delay (more insights) Packet arrival rate = a packets/sec queue Packet length = L bits Link bandwidth = R bits/sec Every second: aL bits arrive to queue Every second: R bits leave the router Question: what happens if aL > R ? Answer: queue will fill up, and packets will get dropped!! aL/R is called traffic intensity Introduction 1-58 Queueing delay (more insights) 1 packet arrives every L/R seconds queue Link bandwidth = R bits/sec Packet length L bits Arrival rate: a = 1/(L/R) = R/L (packet/second) Traffic intensity = aL/R = (R/L) (L/R) = 1 Average queueing delay = 0 (queue is initially empty) Introduction 1-59 Queueing delay (more insights) Packet arrival rate = a packets/sec queue Packet length = L bits La/R ~ 0: avg. queueing delay small Link bandwidth = R bits/sec La/R 1 La/R ~ 0 La/R -> 1: delays become large La/R > 1: more “work” than can be serviced, average delay infinite! (this is when a is random!) Introduction 1-61 Store-and-forward (revisited) L R R R Entire packet must arrive at router before it can be transmitted on next link: store and forward Takes L/R seconds to transmit (push out) packet of L bits on to link of R bps delay = 3L/R (assuming zero propagation delay) more on this next… Introduction 1-62 Store-and-forward (more insights) distance = d meters; speed of propagation = s m/sec transmission rate of link = R bits/s L d R L d/2 R d/2 R delay (one packet only) = L/R + d/s delay (one packet only) Example: d/s = 0.5 sec L = 10 Mbits R = 1 Mbps delay = 10.5 sec Example: d/s = 0.5 sec L = 10 Mbits R = 1 Mbps delay = 20.5 sec = L/R + ½d/s + L/R + ½d/s = 2L/R + d/s Introduction 1-63 Store-and-forward (more insights) distance = d meters; speed of propagation = s m/sec transmission rate of link = R1 and R2 bits/s Consider sending two packets A and B back to back L d/2 R1 Case 1: Assume R1 < R2 d/2 R2 Case 2:Assume R1 > R2 Q: is there a queuing delay? how much is this delay? Answer (queue is empty initially): Time for last bit of 2nd pkt to arrive at router: t1= L/R1 + L/R1 + d/(2s) Time for last bit of 1st pkt to leave router: t2= L/R1 + d/(2s) + L/R2 Queueing delay = t2 – t1 = L/R2 – L/R1 if positive, otherwise 0. Hence: when R1 < R2, queueing delay = t2 – t1 = 0 when R1 > R2, queueing delay = t2 – t1 = L/R2 – L/R1 Introduction 1-64 Throughput throughput: rate (bits/time unit) at which bits transferred between sender/receiver • instantaneous: rate at given point in time • average: rate over longer period of time server, withbits server sends file of into F bitspipe (fluid) to send to client linkpipe capacity that can carry Rs bits/sec fluid at rate Rs bits/sec) linkpipe capacity that can carry Rc bits/sec fluid at rate Rc bits/sec) Introduction 1-65 Throughput (more) Rs < Rc What is average end-end throughput? Rs bits/sec Rc bits/sec Rs > Rc What is average end-end throughput? Rs bits/sec Rc bits/sec bottleneck link Bottleneck link on end-end path is what constrains end-end throughput Introduction 1-66 Throughput: Internet scenario per-connection endend throughput: min(Rc,Rs,R/10) in practice: Rc or Rs is often bottleneck Rs Rs Rs R Rc Rc Rc 10 connections (fairly) share backbone bottleneck link R bits/sec * Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive/ Introduction 1-67 Throughput analysis Host A L R R R Host B Suppose: Host A has huge file of size F bits to send to Host B File is split into N packets, each of length L bits (i.e., N=F/L) Ignore propagation delay for now Question 1: how long it takes to send the file? A: (N+2)L/R = (F+2L)/R Question 2: what is the average end-to-end throughput achieved when sending the file? A: NL/[(N+2)L/R]=NR/(N+2) = FR/(F+2L) = R/(1+2L/F) Introduction 1-68 Introduction: summary Covered a “ton” of material! Internet overview Network protocol Network edge, core, access network Packet-switching versus circuit-switching Internet/ISP structure layering and service models performance: delay and throughput analysis Introduction 1-69