Course Overview Spring 2012 ECE/CS 372 Introduction to Computer Networks Lecture School of Electrical Engineering and Computer Science Oregon State University Chapter 1, slide: Lecture/Office/Lab Hours  Course website  http://web.engr.oregonstate.edu/~qassim/index_files/ComputerNetwo rks.htm  Please write down this URL—all course material and information will be provided thru this site  Lectures  Everyday 1-1:50pm  Instructor  Yousef Qassim (qassim@eecs.oregonstate.edu)  Office hours: TR 2:30-3:20pm @KEC lounge Chapter 1, slide: Lecture/Office/Lab Hours  Lab Assistant  Location: Dearborn 205  Lab hours: to help you with your labs 205 DEAR Information can be found in course’s website  Lab  Location: Dearborn 205  Access code: will be written on board Chapter 1, slide: Prerequisite/Textbook Prerequisite:  CS or ECE 271 or an equivalent course  Basic Linux familiarity Textbook  Textbook is Required Computer Networking: A Top-Down Approach Featuring the Internet, 6th Edition, Games F Kurose, Keith W Ross Chapter 1, slide: Grading Policy  Assignments: 15%   Each student must hand in one copy assignments: approx every two weeks  Labs: 15%   Each student must hand in one copy labs: approx every two weeks  One midterm exam: 30%  Final exam: 40% Chapter 1, slide: Topics To Be Covered  Architecture of the Internet, and network protocols  Delay analysis  Packet-switching and circuit-switching  Congestion and flow control: TCP  Routing algorithms: IP and datagram  Data link layers and Ethernet: ARP, CSMA/CD  Medium access control and local area networks Chapter 1, slide: Lectures & assignments Objective  Deep understanding of basic and fundamental networking concepts, architectures, and philosophies  IMPORTANT: this course is NOT about setting up your router at home, or writing a twitter program!! Approach: how to well in this course  Easy: attend ALL lectures and ALL assignments  Do your assignments individually  Do NOT miss any Bonus Quiz (i.e., not miss class)  Some hw problems will be solved in class: this gives you the opportunity to clarify things further Chapter 1, slide: Labs Objective  Understand how Internet protocols work  Force network protocols to perform certain actions  Observe and analyze protocols’ behavior Approach  Software tool: Wireshark   already installed in Lab DEAR 205 To run, type: sudo wireshark then enter your eecs psswd  Allows you to sniff and analyze traffic sent/received from/by your end system: real measurement of Internet traffic Chapter 1, slide: Chapter 1: Introduction Our goal:  learn basic network terminologies  more depth, detail later in course  approach:  use Internet as example Acknowledgement: slides drawn heavily from Kurose & Ross Chapter 1, slide: Chapter 1: roadmap What is the Internet? Network edge Network core Internet structure and ISPs Protocol layers, service models Delay & loss in packet-switched networks Chapter 1, slide: 10 Exercise Transmission vs propagation L=100Bytes Host A Question: trans rate R = ? Host B distance = km, speed = 2x108m/s At what rate (bandwidth) R would the propagation delay equal the transmission delay? Answer: Propagation delay = 2x103 (m)/2x108 (m/s) = 10-5 sec Transmission delay = 100x8 (bits)/R Prop delay = trans delay => R=105x100x8 = 80 Mbps Chapter 1, slide: 56 Exercise Voice over IP L=48 Bytes a=64Kbps Host A trans rate R = 1Mbps Host B delay_prop = 2msec Host A    converts analog to digital at a=64Kbps groups bits into L=48Byte packets sends packet to Host B as soon it gathers a packet Host B  As soon as it receives the whole pckt, it converts it to analog Question:  How much time elapses from the 1st bit of 1st packet is created until the last bit of the 1st packet arrives at Host B? Chapter 1, slide: 57 Exercise Voice over IP L=48 Bytes a=64Kbps Host A trans rate R = 1Mbps delay_prop = 2msec Host B Answer: Time to gather 1st pkt: 48x8 (bits)/64x1000 (b/s) = msec Time to push 1st pkt to link: 48x8 (bits)/1x106 (b/s) = 0.384 msec Time to propagate: msec Total delay = + 0.384 + = 8.384 msec Chapter 1, slide: 58 ECE/CS 372 – introduction to computer networks Lecture Announcements:  Lab is due Tuesday 2nd week  HW1 is due Monday 3rd week  No class on Friday, Friday is a lab hour 1st week 1-2pm, later on 2:30-3:20pm You don’t have to show up and you can come to me with your lab or class related questions Acknowledgement: slides drawn heavily from Kurose & Ross Chapter 1, slide: 59 Nodal delay 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 Chapter 1, slide: 60 Queueing delay (more insight) Packet arrival rate = a packets/sec Packet length = L bits queue 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 Chapter 1, slide: 61 Queueing delay: illustration 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) = Average queueing delay = (queue is initially empty) Chapter 3, slide: 62 Queueing delay: illustration N packet arrive simultaneously every LN/R seconds queue Link bandwidth = R bits/sec Packet length L bits Arrival rate: a = N/(LN/R) = R/L packet/second Traffic intensity = aL/R = (R/L) (L/R) = Average queueing delay (queue is empty @ 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 Chapter 3, slide: 63 Queueing delay: behavior Packet arrival rate = a packets/sec queue Packet length = L bits Link bandwidth = R bits/sec  La/R ~ 0: avg queuing delay small  La/R -> 1: delays become large  La/R > 1: more “work” than can be serviced, average delay infinite! (this is when a is random!) Chapter 1, slide: 64 Packet-switching: store-and-forward 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… Chapter 1, slide: 65 Store-and-forward: illustration  distance = d meters; speed of propagation = s m/sec  transmission rate of link = R bits/s L d R  delay (one packet only) = L/R + d/s Example: d/s = 0.5 sec L = 10 Mbits R = Mbps delay = 10.5 sec L d/2 R d/2 R  delay (one packet only) = L/R + ½d/s + L/R + ½d/s = 2L/R + d/s Example: d/s = 0.5 sec L = 10 Mbits R = Mbps delay = 20.5 sec Chapter 1, slide: 66 Store-and-forward & queuing delay  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 d L R1  Case 1: Assume R1 < R2 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: d1= L/R1 + L/R1 + d/(2s) Time for last bit of 1st pkt to leave router: d2= L/R1 + d/(2s) + L/R2 Queueing delay = d2 – d1 = L/R2 – L/R1 if positive, otherwise Hence: when R1 < R2, queueing delay = d2 – d1 = when R1 > R2, queueing delay = d2 – d1 = L/R2 – L/R1 Chapter 1, slide: 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 throughput achieved when sending the file? A: NL/[(N+2)L/R]=NR/(N+2) = FR/(F+2L) = R/(1+2L/F) Note: throughput = number of total bits sent / total time taken Chapter 1, slide: 68 Throughput analysis Host A L d/3 R d/3 R d/3 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)  Do NOT ignore propagation delay (assume prop speed = s m/s)  Question 1: how long it takes to send the file? A: (N+2)L/R + d/s = (F+2L)/R + d/s  Question 2: what is the average throughput achieved when sending the file? A: NL/[(N+2)L/R +d/s]=FR/[(N+2)L + dR/s] = FR/[F+2L+dR/s] Chapter 1, slide: 69 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 Chapter 1, slide: 70 ... won’t get Mb/s? Chapter 1, slide: 33 Packet switching versus circuit switching Board … Chapter 1, slide: 34 ECE/ CS 372 – introduction to computer networks Lecture Announcements:  HW1 is posted and... Time to send: 640 kbits/64 kbps + 0.5s = 10 .5s Chapter 1, slide: 29 ECE/ CS 372 – introduction to computer networks Lecture Announcements:  Please make sure to check the course’s website in a regular... protocols  workstation control sending/receiving of messages e.g., TCP, IP, HTTP, FTP, PPP company network Chapter 1, slide: 13 What’s a protocol? a human protocol and a computer network protocol: