Introduction to Computer Networks PDF

Computer Networks and Applications COMP 3331/COMP 9331 Key Topics Internet as a network of networks The protocol stack and layering principle Edge vs. Core Loss, delay and throughput Packet switching vs. Circuit switching...

Computer Networks and Applications COMP 3331/COMP 9331 Key Topics Internet as a network of networks The protocol stack and layering principle Edge vs. Core Loss, delay and throughput Packet switching vs. Circuit switching Week 1 Introduction to Computer Networks Reading Guide: Chapter 1, Sections 1.1 - 1.4 1 Acknowledgment v Majority of lecture slides are from the author’s lecture slide set § Enhancements + additional material 2 Introduction Our goal: Overview/roadmap: v What is the Internet? v Get “feel,” “big picture,” v What is a protocol? introduction to terminology v Network edge: hosts, access network, § more depth, detail later in physical media course v Network core: packet/circuit switching, internet structure v Approach: v Performance: loss, delay, throughput § use Internet as example v Protocol layers, service models v Security (self study, not on exam) v History (self study, not on exam) Hobbe’s Internet Timeline - http://www.zakon.org/robert/internet/timeline/ 3 Quiz: What is the Internet? A. One single homogenous network B. An interconnection of different computer networks C. An infrastructure that provides services to networked applications D. Something else Answer: B and C as explained on the next few slides 4 The Internet: a “nuts and bolts” view Billions of connected mobile network computing devices: national or global ISP § hosts = end systems § running network apps at Internet’s “edge” Packet switches: forward local or packets (chunks of data) Internet regional § routers, switches ISP home network content Communication links provider network datacenter § fiber, copper, radio, satellite network § transmission rate: bandwidth Networks enterprise § collection of devices, routers, network links: managed by an organization 5  “Fun” Internet appliances Web-enabled toaster + weather forecaster Security Camera car Picture frame Fitbit pacemaker Tweet-a-watt: monitor energy use Networked TV Set top Boxes Amazon Echo sensorized, bed AR devices Internet mattress refrigerator Smart Lightbulbs 6 7 The Internet: a “nuts and bolts” view mobile network 4G v Internet: “network of networks” national or global ISP § Interconnected ISPs § protocols are everywhere Skype IP Streaming video control sending, receiving of messages local or regional e.g., HTTP (Web), streaming video, ISP Skype, TCP, IP, WiFi, 4G, Ethernet home network content provider § Internet standards HTTP network datacenter network Ethernet RFC: Request for Comments IETF: Internet Engineering Task TCP Force enterprise network WiFi 8 The Internet: a “service” view v Infrastructure that provides mobile network services to applications: national or global ISP § Web, streaming video, multimedia teleconferencing, email, games, e- Streaming commerce, social media, inter- Skype video connected appliances, … local or regional § provides programming interface to ISP distributed applications: home network content provider “hooks” allowing sending/receiving HTTP network datacenter network apps to “connect” to, use Internet transport service provides service options, analogous enterprise to postal service network 9  while'(...)'{' while'(...)'{' '''message'='receive'('...');' '''message'='...;' } '''send'('message,'...');' } Bob Alice Computer)Networks,)Fall)2015 8 10  while'(...)'{' while'(...)'{' '''message'='receive'('...');' '''message'='...;' } '''send'('message,'...');' } ApplicaGon- Bob Programming- Alice Interface Computer)Networks,)Fall)2015 9 11  facebook- server world-of-warcraE- server instant-messaging instant-messaging world-of-warcraE- firefox-accessing- Computer)Networks,)Fall)2015 client facebook 7 12 What’s a protocol? Human protocols: Network protocols: § “what’s the time?” § computers (devices) rather than humans § “I have a question” § all communication activity in Internet § introductions governed by protocols … specific messages sent Protocols define the format, order of … specific actions taken when message received, messages sent and received among or other events network entities, and actions taken on msg transmission, receipt 13 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://gaia.cs.umass.edu/kurose_ross 2:00 time Q: other human protocols? 14 Quiz: Internet of Things How many Internet-connected devices do you have in your home (include your computers, phones, tablets)? A. Less than 10 B. Between 10 to 20 C. Between 20 to 50 D. Between 50 to 100 E. More than 100 15 Introduction: roadmap v What is the Internet? v What is a protocol? v Network edge: hosts, access network, physical media v Network core: packet/circuit switching, internet structure v Performance: loss, delay, throughput v Security v Protocol layers, service models v History 16 A closer look at Internet structure mobile network Network edge: national or global ISP vhosts: clients and servers vservers often in data centers local or regional ISP home network content provider network datacenter network enterprise network 17 A closer look at Internet structure mobile network Network edge: national or global ISP vhosts: clients and servers vservers often in data centers Access networks, physical media: local or regional ISP vwired, wireless communication home network links content provider network datacenter network enterprise network 18 A closer look at Internet structure mobile network Network edge: national or global ISP vhosts: clients and servers vservers often in data centers Access networks, physical media: local or regional ISP vwired, wireless communication home network links content provider network datacenter network Network core: § interconnected routers enterprise § network of networks network 19 Access networks and physical media Q: How to connect end systems to mobile network edge router? national or global ISP v residential access nets v institutional access networks (school, company) v mobile access networks (WiFi, 4G/5G) local or regional ISP What to look for: home network content provider § transmission rate (bits per second) of access network datacenter network network? § shared or dedicated access among users? enterprise network 20 Access networks: digital subscriber line (DSL) central office telephone network DSL splitter modem DSLAM voice, data transmitted ISP at different frequencies over DSL access dedicated line to central office multiplexer § use existing telephone line to central office DSLAM data over DSL phone line goes to Internet voice over DSL phone line goes to telephone net 21 Access net: digital subscriber line (DSL) DSL splitter modem Hig h-p er for as lt ADSL over POTS da s filt ass fi ta er -p ic e voice, data transmitted for Low vo at different frequencies over dedicated line to central office Different data rates for upload and download (ADSL) § 24-52 Mbps dedicated downstream transmission rate § 3.5-16 Mbps dedicated upstream transmission rate 22 Access net: digital subscriber line (DSL) 23 Access networks: cable-based access cable headend … cable splitter modem C O V V V V V V N I I I I I I D D T D D D D D D A A R E E E E E E T T O O O O O O O A A L 1 2 3 4 5 6 7 8 9 Channels frequency division multiplexing (FDM): different channels transmitted in different frequency bands 24 Access networks: cable-based access cable headend … cable splitter cable modem modem CMTS termination system data, TV transmitted at different frequencies over shared cable ISP distribution network § HFC: hybrid fiber coax asymmetric: up to 40 Mbps – 1.2 Gbs downstream transmission rate, 30-100 Mbps upstream transmission rate § network of cable, fiber attaches homes to ISP router homes share access network to cable headend Unlike DSL, which has dedicated access to central office 25 Fiber to the home/premise/curb v Fully optical fiber path all the way to the home (or premise or curb) § e.g., NBN, Google, Verizon FIOS § ~30 Mbps to 1Gbps 26 Access networks: home networks wireless devices to/from headend or central office often combined in single box cable or DSL modem WiFi wireless access router, firewall, NAT point (54, 450 Mbps) wired Ethernet (1 Gbps) -27 Wireless access networks Shared wireless access network connects end system to router § via base station aka “access point” Wireless local area networks Wide-area cellular access networks (WLANs) § provided by mobile, cellular network § typically within or around building operator (10’s km) (~100 ft) § 10’s Mbps § 802.11b/g/n (WiFi): 11, 54, 450 § 4G cellular networks (5G coming) Mbps transmission rate to Internet to Internet 28 Access networks: enterprise networks Enterprise link to ISP (Internet) institutional router Ethernet institutional mail, switch web servers § companies, universities, etc. § mix of wired, wireless link technologies, connecting a mix of switches and routers (we’ll cover differences shortly) § Ethernet: wired access at 100Mbps, 1Gbps, 10Gbps § WiFi: wireless access points at 11, 54, 450 Mbps 29  Sample results Can you explain the differences? Uniwide Wired Network @ CSE NBN (Hybrid Fibre Coaxial or HFC) + WiFi @ my home 4G Network 30 Quiz: Your access network Your residential ISP provides connectivity using the following technology: A. DSL B. Cable C. Fiber to the home/premise/curb D. Mobile (3G/4G/5G) E. Satellite F. Something Else 31  SELF STUDY Links: physical media NOT ON EXAM § bit: propagates between Twisted pair (TP) transmitter/receiver pairs § two insulated copper wires § physical link: what lies Category 5: 100 Mbps, 1 Gbps Ethernet between transmitter & Category 6: 10Gbps Ethernet receiver § guided media: signals propagate in solid media: copper, fiber, coax § unguided media: signals propagate freely, e.g., radio 32  SELF STUDY Links: physical media NOT ON EXAM Coaxial cable: Fiber optic cable: § two concentric copper conductors § glass fiber carrying light pulses, each pulse a bit § bidirectional § high-speed operation: § broadband: high-speed point-to-point multiple frequency channels on cable transmission (10’s-100’s Gbps) 100’s Mbps per channel § low error rate: repeaters spaced far apart immune to electromagnetic noise 33 Links: physical media SELF STUDY NOT ON EXAM Wireless radio Radio link types: § signal carried in § terrestrial microwave electromagnetic spectrum up to 45 Mbps channels § no physical “wire” § Wireless LAN (WiFi) § broadcast and “half-duplex” Up to 100’s Mbps (sender to receiver) § wide-area (e.g., cellular) § propagation environment 4G cellular: ~ 10’s Mbps effects: reflection § satellite up to 45 Mbps per channel obstruction by objects 270 msec end-end delay interference geosynchronous versus low- earth-orbit 34 Introduction: roadmap v What is the Internet? v What is a protocol? v Network edge: hosts, access network, physical media v Network core: packet/circuit switching, internet structure v Performance: loss, delay, throughput v Security v Protocol layers, service models v History 35  The network core v mesh of interconnected routers mobile network national or global ISP v packet-switching: hosts break application- layer messages into packets § forward packets from one router to the next, across links on path from source to destination local or regional § each packet transmitted at full link capacity ISP § Is used in the Internet home network content provider network datacenter network v circuit-switching: an alternative used in legacy telephone networks which was considered during the design of the enterprise Internet network 36 Alternative to packet switching: circuit switching end-end resources allocated to, reserved for “call” between source and destination v in diagram, each link has four circuits. § call gets 2nd circuit in top link and 1st circuit in right link. v dedicated resources: no sharing § circuit-like (guaranteed) performance v circuit segment idle if not used by call (no sharing) v commonly used in traditional telephone networks 37 Circuit switching: FDM and TDM Frequency Division Multiplexing (FDM) 4 users v optical, electromagnetic frequencies frequency divided into (narrow) frequency bands v each call allocated its own band, can transmit at max rate of that narrow band time Time Division Multiplexing (TDM) frequency § time divided into slots § each call allocated periodic slot(s), can transmit at maximum rate of (wider) frequency band, but only during its time time slot(s) 38 Timing in Circuit Switching Circuit Establish ment Transfer Information time Circuit Tear- down 39 Why circuit switching is not feasible? Ø Inefficient Computer communications tends to be very bursty. For example, viewing a sequence of web pages Dedicated circuit cannot be used or shared in periods of silence Cannot adapt to network dynamics Ø Fixed data rate Computers communicate at very diverse rates. For example, viewing a video vs using telnet or web browsing Fixed data rate is not useful Ø Connection state maintenance Requires per communication state to be maintained that is a considerable overhead Not scalable 39 Packet Switching v Data is sent as chunks of formatted bits (Packets) v Packets consist of a “header” and “payload” We will cover these later 1. Internet Address 2. Age (TTL) 3. Checksum to protect header Data Header header 01000111100010101001110100011001 payload 41 Packet Switching v Data is sent as chunks of formatted bits (Packets) v Packets consist of a “header” and “payload” § payload is the data being carried § header holds instructions to the network for how to handle packet (think of the header as an API) 42 Packet Switching v Data is sent as chunks of formatted bits (Packets) v Packets consist of a “header” and “payload” v Switches “forward” packets based on their headers 43  Peek ahead: Two key network-core functions routing algorithm Routing: Forwarding: local local forwarding forwarding table table § global action: v local action: move header value output link determine source- arriving packets 0100 0101 3 2 destination paths from router’s 0111 1001 2 1 taken by packets input link to § routing algorithms appropriate 1 router output link 3 2 11 01 destination address in arriving packet’s header 44 Timing in Packet Switching h paylo d ad r time What about the time to process the packet at the switch? We’ll assume it’s relatively negligible (mostly true) 45 Timing in Packet Switching h paylo d ad r Could the switch start transmitting time as soon as it has processed the header? 46 Timing in Packet Switching h paylo d ad r Yes! This would be called a “cut through” switch Could the switch start transmit as time soon as it has processed the header? 47 Timing in Packet Switching h paylo d ad r We will always assume a switch processes/forwards time a packet after it has received it entirely. This is called “store and forward” switching 48 Packet Switching v Data is sent as chunks of formatted bits (Packets) v Packets consist of a “header” and “payload” v Switches “forward” packets based on their headers 49 Packet Switching v Data is sent as chunks of formatted bits (Packets) v Packets consist of a “header” and “payload” v Switches “forward” packets based on their headers v Each packet travels independently § no notion of packets belonging to a “circuit” 50 Packet Switching v Data is sent as chunks of formatted bits (Packets) v Packets consist of a “header” and “payload” v Switches “forward” packets based on their headers v Each packet travels independently v No link resources are reserved. Instead, packet switching leverages statistical multiplexing 51 Three Flows with Bursty Traffic Data Rate 1 Time Data Rate 2 Capacity Time Data Rate 3 Time 52 When Each Flow Gets 1/3rd of Capacity Data Rate 1 like circuit switching Time Data Rate 2 Time Data Rate 3 Overloaded Time 53 When Flows Share Total Capacity packet switching Time No Overloading Statistical multiplexing relies on the assumption that not all flows Time burst at the same time Very similar to insurance, and has same failure case Time 54 Three Flows with Bursty Traffic Data Rate 1 Time Data Rate 2 Capacity Time Data Rate 3 Time 55 Three Flows with Bursty Traffic Data Rate 1 Time Data Rate 2 Capacity Time Data Rate 3 Time 56 Three Flows with Bursty Traffic Data Rate 1+2+3 >> Capacity Time Capacity Time What do we do under overload? 57 Statistical multiplexing: pipe view pkt tx time BW à time à 58 Statistical multiplexing: pipe view 59 Statistical multiplexing: pipe view No Overload 60 Statistical multiplexing: pipe view Queue overload into Buffer Transient Overload Not such a rare event 61 Statistical multiplexing: pipe view Queue overload into Buffer Transient Overload Not such a rare event 62 Statistical multiplexing: pipe view Queue overload into Buffer Transient Overload Not such a rare event 63 Statistical multiplexing: pipe view Queue overload into Buffer Transient Overload Not such a rare event 64 Statistical multiplexing: pipe view Queue overload into Buffer Transient Overload Not such a rare event 65 Statistical multiplexing: pipe view Queue overload into Buffer Transient Overload Buffer absorbs Nottransient a rarebursts event! 66 Statistical multiplexing: pipe view Queue overload into Buffer What about persistent overload? Will eventually drop packets 67  Packet switching versus circuit switching packet switching allows more users to use network! example: § 1 Mb/s link § each user: N ….. 100 kb/s when “active” users active 10% of time 1 Mbps link v circuit-switching: § 10 users Q: how did we get value 0.0004? v packet switching: § with 35 users, probability > Q: what happens if > 35 users 10 active at same time is less say 70? than.0004 Hint: Bernoulli Trials and Binomial Distribution 68 Binomial Probability Distribution v A fixed number of observations (trials), n § E.g., 5 tosses of a coin v Binary random variable § E.g., head or tail in a coin toss § Often called as success or failure § Probability of success is p and failure is (1-p) v Constant probability for each observation 69 Binomial Distribution: Example v Q: What is the probability of observing exactly 3 heads in a sequence of 5 coin tosses v A: § One way to get exactly 3 heads is: HHHTT § Probability of this sequence occurring = (1/2) x (1/2) x (1/2) x (1-1/2) x (1-1/2) = (1/2)5 § Another way to get exactly 3 heads is: THHHT § Probability of this sequence occurring = (1-1/2) x (1/2) x (1/2) x (1/2) x (1-1/2) = (1/2)5 § How many such unique combinations exist? 70 Binomial Distribution: Example 5 P (3 heads and 2 tails) = 10 x (1/2)5 = 0.3125 71 Binomial Distribution 72 Packet switching versus circuit switching v Let’s revisit the earlier problem v N = 35 users v Prob (# active users > 10)= 1– Prob (# active = 10) – Prob (# active = 9) – Prob (# active = 8)... – Prob (# active = 0) where Prob (# active = 10) = C(35,10) x 0.110 x 0.925 v Prob (# active users > 10) = 0.0004 (approx) 73 Packet switching versus circuit switching Is packet switching a “slam dunk winner”? § great for “bursty” data – sometimes has data to send, but at other times not resource sharing simpler, no call setup § excessive congestion possible: packet delay and loss due to buffer overflow protocols needed for reliable data transfer, congestion control § Q: How to provide circuit-like behavior? bandwidth guarantees traditionally used for audio/video applications Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet switching)? 74 Quiz: Switching-1 In ____________ resources are allocated on demand A. Packet switching B. Circuit switching C. Both D. None 75 Quiz: Switching-2 A message from device A to B consists of packet X and packet Y. In a circuit switched network, packet Y’s path ___________________ packet X’s path A. is the same B. is independent C. is always different from 76 Internet structure: a “network of networks” vHosts connect to Internet via access Internet Service Providers (ISPs) residential, enterprise (company, university, commercial) ISPs vAccess ISPs in turn must be interconnected so that any two hosts can send packets to each other vResulting network of networks is very complex evolution was driven by economics and national policies vLet’s take a stepwise approach to describe current Internet structure 77 Internet structure: a “network of networks” Question: given millions of access ISPs, how to connect them together? access … access net access net … net access access net net access access net net … … access access net net access net access net access net access net … access net access net … access net 78 Internet structure: a “network of networks” Question: given millions of access ISPs, how to connect them together? access … access net access net … net access access net … … net access access net net connecting each access ISP to … … each other directly doesn’t scale: … access O(N2) connections. access … net net access net access net access net access … net … access net access net … access net 79 Internet structure: a “network of networks” Option: connect each access ISP to one global transit ISP? Customer and provider ISPs have economic agreement. access … access net access net … net access access net net access access net net … … global access net ISP access net access net access net access net access net … access net access net … access net 80 Internet structure: a “network of networks” But if one global ISP is viable business, there will be competitors …. access … access net access net … net access access net net access access net net ISP A … … access net ISP B access net access ISP C net access net access net access net … access net access net … access net 81 Internet structure: a “network of networks” But if one global ISP is viable business, there will be competitors …. who will want to be connected Internet exchange point access … access net access net … net access access net net IXP access access net net ISP A … … access net IXP ISP B access net access ISP C net access net access net peering link access net … access net access net … access net 82 Internet structure: a “network of networks” … and regional networks may arise to connect access nets to ISPs access … access net access net … net access access net net IXP access access net net ISP A … … access net IXP ISP B access net access ISP C net access net access net regional ISP access net … access net access net … access net 83 Internet structure: a “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 … access net access net net access access net net IXP access access net net ISP A … … Content provider network access net IXP ISP B access net access ISP C net access net access net regional ISP access net … access net access net … access net 84  Internet structure: a “network of networks” Tier 1 ISP Tier 1 ISP Google IXP IXP IXP Regional ISP Regional ISP access access access access access access access access ISP ISP ISP ISP ISP ISP ISP 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 networks (e.g., Google, Facebook): private network that connects its data centers to Internet, often bypassing tier-1, regional ISPs 85 Tier-1 ISP Network map: Sprint (2019) POP: point-of-presence to/from other Sprint PoPS links to peering networks … … … … … links to/from Sprint customer networks 86 AARNET: Australia’s Academic and Research Network https://www.aarnet.edu.au/ https://www.submarinecablemap.com 87 Introduction: roadmap v What is the Internet? v What is a protocol? v Network edge: hosts, access network, physical media v Network core: packet/circuit switching, internet structure v Performance: loss, delay, throughput v Security v Protocol layers, service models v History 88 How do packet loss and delay occur? packets queue in router buffers § packets queue, wait for turn § arrival rate to link (temporarily) exceeds output link capacity: packet loss packet being transmitted (transmission delay) A B packets in buffers (queueing delay) free (available) buffers: arriving packets dropped (loss) if no free buffers 89 Packet delay: four sources transmission A propagation B nodal processing queueing dnodal = dproc + dqueue + dtrans + dprop dproc: nodal processing dqueue: queueing delay § check bit errors § time waiting at output link for § determine output link transmission § typically < msec § depends on congestion level of router 90 Packet delay: four sources transmission A propagation B nodal processing queueing dnodal = dproc + dqueue + dtrans + dprop dtrans: transmission delay: dprop: propagation delay: § L: packet length (bits) § d: length of physical link § R: link transmission rate (bps) § s: propagation speed (~2x108 m/sec) § dtrans = L/R § dprop = d/s dtrans and dprop very different 91 Caravan analogy 100 km 100 km ten-car caravan toll booth toll booth (aka 10-bit packet) (aka router) § cars “propagate” at 100 km/hr § time to “push” entire caravan § toll booth takes 12 sec to service through toll booth onto car (bit transmission time) highway = 12*10 = 120 sec § car ~ bit; caravan ~ packet § time for last car to propagate from 1st to 2nd toll both: § Q: How long until caravan is lined 100km/(100km/hr) = 1 hr up before 2nd toll booth? § A: 62 minutes 92 Caravan analogy 100 km 100 km ten-car caravan toll booth toll booth (aka 10-bit packet) (aka router) § suppose cars now “propagate” at 1000 km/hr § and suppose toll booth now takes one min to service a car § Q: Will cars arrive to 2nd booth before all cars serviced at first booth? A:Yes! after 7 min, first car arrives at second booth; three cars still at first booth Interactive Java Applet – Propagation vs transmission delay https://www2.tkn.tu-berlin.de/teaching/rn/animations/propagation/ 93 Queueing delay (more insight) queue Packet arrival rate = a packets/sec Packet length Link bandwidth = L bits = R bits/sec v Every second: aL bits arrive to queue v Every second: R bits leave the router v Question: what happens if aL > R ? v Answer: queue will fill up, and packets will get dropped!! aL/R is called traffic intensity 94 Queueing delay: One Scenario Link bandwidth queue = R bits/sec 1 packet arrives every L/R seconds 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) 95 Queueing delay: Another Scenario Link bandwidth queue = R bits/sec N packet arrive simultaneously every LN/R seconds Packet length L bits Arrival rate: a = N/(LN/R) = R/L packet/second Traffic intensity = aL/R = (R/L) (L/R) = 1 Average queueing delay (queue is empty at time 0) ? {0 + L/R + 2L/R + … + (N-1)L/R}/N = L/(RN){1+2+…+(N-1)} =L(N-1)/(2R) Note: traffic intensity is same as previous scenario, but queueing delay is different 96  Queueing delay: typical behaviour queue Packet arrival rate = a packets/sec Packet length Link bandwidth = L bits = R bits/sec Interactive Java Applet: http://computerscience.unicam.it/marcantoni/reti/applet/QueuingAndLossInteractive/1.html q La/R ~ 0: avg. queueing delay small q La/R -> 1: delays become large q La/R > 1: more “work” than can be serviced, average delay infinite! (this is when a is random!) 97  End to End Delay Packet length = L Propagation speed = s d1,r1 d2,r2 d3,r3 Client R1 R2 Server L/r1 Client L/r2 Time (along horizontal axis) R1 d1/s L/r3 R2 d2/s Server d3/s 98 In the picture, r1 = r2 = r3, you may wish to consider what happens when this is not the case  End to End Delay Packet length = L Propagation speed = s d1,r1 d2,r2 d3,r3 Client R1 R2 Server L/r1 Client L/r2 Time (along horizontal axis) R1 d1/s Queueing Delay (packets queued at R2) L/r3 R2 d2/s Server d3/s 99 In the picture, r1 = r2 = r3, you may wish to consider what happens when this is not the case “Real” Internet delays and routes § what do “real” Internet delay & loss look like? § traceroute program: provides delay measurement from source to router along end-end Internet path towards destination. For all i: sends three packets that will reach router i on path towards destination (with time-to-live field value of i) router i will return packets to sender sender measures time interval between transmission and reply 3 probes 3 probes 3 probes 100 Real Internet delays and 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 3 delay measurements 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 to border1-rt-fa5-1-0.gw.umass.edu 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 link 8 62.40.103.253 (62.40.103.253) 104 ms 109 ms 106 ms 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 looks like delays 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 decrease! Why? 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 * * * 18 * * * * means no response (probe lost, router not replying) 19 fantasia.eurecom.fr (193.55.113.142) 132 ms 128 ms 136 ms * Do some traceroutes from exotic countries at www.traceroute.org 101 “Real” delay variations dnodal = dproc + dqueue + dtrans + dprop End-to-end delay = sum of all dnodal along the path 102  Quiz: Propagation Delay Propagation delay depends on the size of the packet A. True B. False 103  Quiz: Oh these delays Consider a packet that has just arrived at a router. What is the correct order of the delays encountered by the packet until it reaches the next-hop router? A. Transmission, processing, propagation, queuing B. Propagation, processing, transmission, queuing C. Processing, queuing, transmission, propagation D. Queuing, processing, propagation, transmission 104 Packet loss v queue (aka buffer) preceding link in buffer has finite capacity v packet arriving to full queue dropped (aka lost) v lost packet may be retransmitted by previous node, source end system, or not at all buffer (waiting area) packet being transmitted A B packet arriving to full buffer is lost 105  Throughput § throughput: rate (bits/time unit) at which bits are being sent from sender to receiver instantaneous: rate at given point in time average: rate over longer period of time link capacity pipe that can carry linkthat pipe capacity can carry Rsfluid bits/sec at rate R c bits/sec fluid at rate serverserver, sends with bits (Rs bits/sec) (Rc bits/sec) (fluid) fileinto of Fpipe bits to send to client 106 Throughput 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 link on end-end path that constrains end-end throughput 107 Throughput: network scenario § per-connection end-end Rs throughput: Rs Rs min(Rc,Rs,R/10) § in practice: Rc or Rs is R often bottleneck Rc Rc Rc 10 connections (fairly) share backbone bottleneck link R bits/sec 108 Introduction: summary covered a “ton” of material! v Internet overview v what’s a protocol? v network edge, core, access network § packet-switching versus circuit-switching § Internet structure v performance: loss, delay, throughput v Next Week § Protocol layers, service models § Application Layer End of Week 1 109