1. I NTRODUCTION TO C OMPUTER N ETWORKS By Dr. Nawaporn Wisitpongphan Presentation Slides Courtesy of: Prof Nick McKeown, Stanford University

2. O UTLINE OSI Model and Layering. How does data travel through a network? Packet Switching and Circuit Switching Some terminologies in Performance Analysis Data rate, “Bandwidth” and “throughput” Propagation delay Packet, header, address Bandwidth-delay product, RTT Queuing Model 2

3. L AYERING : T HE OSI M ODEL Session Network Link Physical Application Presentation Transport Network Link Network Transport Session Presentation Application Network Link Physical Peer-layer communication layer-to-layer communication Router 1 2 3 4 5 6 7 1 2 3 4 5 6 7 3

4. L AYERING : FTP E XAMPLE Network Link Transport Application Presentation Session Transport Network Link Physical The 7-layer OSI Model The 4-layer Internet model Application FTP ASCII/Binary IP TCP Ethernet 4

5. L AYERING : D ATA E NCAPSULATING 5

6. O UTLINE OSI Model and Layering. How does data travel through a network? Packet Switching and Circuit Switching Some terminologies in Performance Analysis Data rate, “Bandwidth” and “throughput” Propagation delay Packet, header, address Bandwidth-delay product, RTT Queuing Model 6

7. E XAMPLE : FTP OVER THE I NTERNET U SING TCP/IP AND E THERNET App OS R2 R3 R4 R1 R5 Ethernet “A” KMUTNB “B” CMU Ethernet App OS 1 234234 6 7 20 19 18 17 5 9 10 8 12 13 11 15 16 14 7

8. SENDING HOST : S TEP 1-2 1. Application-Programming Interface (API) Application at “A” requests TCP connection to “B” 2. Transmission Control Protocol (TCP) Creates TCP “Connection setup” packet Hand the message over to IP TCP Data TCP Header TCP Packet Type = Connection Setup Empty 8

9. SENDING HOST : S TEP 3 3. Internet Protocol (IP) Creates IP packet with correct addresses. IP requests packet to the Ethernet driver. IP Data TCP Packet Encapsulation IP Header IP Packet Destination Address: IP “B” Source Address: IP “A” Protocol = TCP TCP Data TCP Header 9

10. SENDING HOST : S TEP 4 4. Link Layer Protocol (Ethernet Driver/ MAC) Creates MAC frame with Frame Check Sequence (FCS). Wait for Access to the line. MAC requests PHY to transmit the frame bit by bit Ethernet Data IP Packet Ethernet FCS Ethernet Header Ethernet Packet Destination Address: MAC “R1” Source Address: MAC “A” Protocol = IP IP Data IP Header Encapsulation 10

11. R OUTER R1: S TEP 5 5. Link Layer Protocol (Ethernet Driver/ MAC) Accept MAC frame, check address and Frame Check Sequence (FCS). Remove Ethernet Header and Pass data to IP Protocol. Ethernet Data IP Packet Ethernet FCS Ethernet Header Ethernet Packet Destination Address: MAC “R1” Source Address: MAC “A” Protocol = IP IP Data IP Header Decapsulation 11

12. R OUTER R1: S TEP 6 6. Internet Protocol (IP) Use IP destination address to decide where to send packet next (“next-hop routing”). Request Link Layer Protocol to transmit a packet. IP Data IP Header IP Packet Destination Address: IP “B” Source Address: IP “A” Protocol = TCP 12

13. R OUTER R1: S TEP 7 7. Link Layer Protocol (Ethernet Driver/ MAC) Re-Creates MAC frame with Frame Check Sequence (FCS). Wait for Access to the line. MAC requests PHY to send the frame. Ethernet Data IP Packet Ethernet FCS Ethernet Header Ethernet Packet Destination Address: MAC “R2” Source Address: MAC “R1” Protocol = IP IP Data IP Header Encapsulation 13

14. R OUTER R5: S TEP 16 16. Link Layer Protocol Creates MAC frame with Frame Check Sequence (FCS). Wait for Access to the line. MAC requests PHY to send the frame to B Ethernet Data IP Packet Ethernet FCS Ethernet Header Ethernet Packet Destination Address: MAC “B” Source Address: MAC “R5” Protocol = IP IP Data IP Header Encapsulation 14

15. RECEIVING HOST B: S TEP 17 17. Link Layer Protocol (Ethernet Driver/MAC) Accept MAC frame, check address and Frame Check Sequence (FCS). Remove Ethernet Header, Pass data to IP Protocol. Ethernet Data IP Packet Ethernet FCS Ethernet Header Ethernet Packet Destination Address: MAC “B” Source Address: MAC “R5” Protocol = IP IP Data IP Header Decapsulation 15

16. RECEIVING HOST : S TEP 18 18. Internet Protocol (IP) Verify IP address. Remove IP header. Pass TCP packet to TCP Protocol. IP Data TCP Packet Decapsulation IP Header IP Packet Destination Address: IP “B” Source Address: IP “A” Protocol = TCP TCP Data TCP Header 16

17. RECEIVING HOST : S TEP 19-20 19. Transmission Control Protocol (TCP) Accepts TCP “Connection setup” packet Establishes connection by sending ACK 20. Application-Programming Interface (API) Application receives request for TCP connection from “A”. TCP Data TCP Header TCP Packet Type = Connection Setup Empty 17

18. O UTLINE OSI Model and Layering. How does data travel through a network? Packet Switching and Circuit Switching Some terminologies in Performance Analysis Data rate, “Bandwidth” and “throughput” Propagation delay Packet, header, address Bandwidth-delay product, RTT Queuing Model 18

19. C IRCUIT S WITCHING AB SourceDestination  It’s the method used by the telephone network.  A call has three phases: 1.Establish circuit from end-to-end (“dialing”), 2.Communicate, 3.Close circuit (“tear down”).  Originally, a circuit was an end-to-end physical wire.  Nowadays, a circuit is like a virtual private wire: each call has its own private, guaranteed data rate from end-to-end. 19

20. P ACKET S WITCHING A R1 R2 R4 R3 B SourceDestination  It’s the method used by the Internet.  Each packet is individually routed packet-by-packet, using the router’s local routing table.  The routers maintain no per-flow state.  Different packets may take different paths.  Several packets may arrive for the same output link at the same time, therefore a packet switch has buffers. 20

21. 21 P ACKET S WITCHING S IMPLE ROUTER MODEL R1 Link 1 Link 2 Link 3 Link 4 Link 1, ingressLink 1, egress Link 2, ingressLink 2, egress Link 3, ingressLink 3, egress Link 4, ingressLink 4, egress Choose Egress Choose Egress Choose Egress Choose Egress “4” 21

22. S TATISTICAL M ULTIPLEXING B ASIC IDEA time rate One flowTwo flows Average rate Many flows  Network traffic is bursty. i.e. the rate changes frequently.  Peaks from independent flows generally occur at different times.  Conclusion: The more flows we have, the smoother the traffic. Average rates of: 1, 2, 10, 100, 1000 flows. rate 22

23. Link rate, R X(t)X(t) Dropped packets B Queue Length X ( t ) Time Packet buffer Packets for one output P ACKET S WITCHING S TATISTICAL M ULTIPLEXING DataHdr DataHdr DataHdr R R R  Because the buffer absorbs temporary bursts, the egress link need not operate at rate N.R.  But the buffer has finite size, B, so losses will occur. 1 2 N 23

24. W HY DOES THE I NTERNET USE PACKET SWITCHING ? 1. Efficient use of expensive links: The links are assumed to be expensive and scarce. Packet switching allows many, bursty flows to share the same link efficiently. “Circuit switching is rarely used for data networks,... because of very inefficient use of the links” - Gallager 2. Resilience to failure of links & routers: ”For high reliability,... [the Internet] was to be a datagram subnet, so if some lines and [routers] were destroyed, messages could be... rerouted” - Tanenbaum 24

25. O UTLINE A Detailed FTP Example Layering Packet Switching and Circuit Switching Some terminologies in Performance Analysis Data rate, “Bandwidth” and “throughput” Propagation delay Packet, header, address Bandwidth-delay product, RTT Queuing Model 25

26. S OME D EFINITIONS Packet length, P, is the length of a packet in bits. Link length, L, is the length of a link in meters. Data rate, R, is the rate at which bits can be sent, in bits/second, or b/s. 1 Propagation delay, PROP, is the time for one bit to travel along a link of length, L. PROP = L/c. Transmission time, TRANSP, is the time to transmit a packet of length P. TRANSP = P/R. Latency is the time from when the first bit begins transmission, until the last bit has been received. On a link: Latency = PROP + TRANSP. 1. Note that a kilobit/second, kb/s, is 1000 bits/second, not 1024 bits/second. 26

27. P ACKET S WITCHING Host A Host B R1 R2 R3 A R1 R2 R4 R3 B TRANSP 1 TRANSP 2 TRANSP 3 TRANSP 4 PROP 1 PROP 2 PROP 3 PROP 4 SourceDestination “Store-and-Forward” at each Router 27

28. 28 P ACKET S WITCHING W HY NOT SEND THE ENTIRE MESSAGE IN ONE PACKET ? Breaking message into packets allows parallel transmission across all links, reducing end to end latency. It also prevents a link from being “hogged” for a long time by one message. Host A Host B R1 R2 R3 M/R Host A Host B R1 R2 R3 M/R 28

29. P ACKET S WITCHING Q UEUEING D ELAY Host A Host B R1 R2 R3 TRANSP 1 TRANSP 2 TRANSP 3 TRANSP 4 PROP 1 PROP 2 PROP 3 PROP 4 Q2Q2 Because the egress link is not necessarily free when a packet arrives, it may be queued in a buffer. If the network is busy, packets might have to wait a long time. How can we determine the queueing delay? 29

30. 30 Q UEUES AND Q UEUEING D ELAY To understand the performance of a packet switched network, we can think of it as a series of queues interconnected by links. For given link rates and lengths, the only variable is the queueing delay. If we can understand the queueing delay, we can understand how the network performs. 30

31. Q UEUES AND Q UEUEING D ELAY Cross traffic causes congestion and variable queueing delay. 31

32. A ROUTER QUEUE  A(t), D(t)D(t) Model of FIFO router queue Q(t)Q(t) 32

33. A SIMPLE DETERMINISTIC MODEL Properties of A(t), D(t):  A(t), D(t) are non-decreasing  A(t) >= D(t)  A(t), D(t)D(t) Model of FIFO router queue Q(t)Q(t) 33

34. A SIMPLE DETERMINISTIC MODEL BYTES OR “ FLUID ” A(t)A(t) D(t)D(t) Cumulative number of departed bits up until time t. time Service process Cumulative number of bits Cumulative number of bits that arrived up until time t.  A(t)A(t) D(t)D(t) Q(t)Q(t) Properties of A(t), D(t) :  A(t), D(t) are non-decreasing  A(t) >= D(t)  34

35. D(t) A(t) time Q(t) d(t) Queue occupancy: Q(t) = A(t) - D(t). Queueing delay, d(t), is the time spent in the queue by a bit that arrived at time t, and if the queue is served first-come-first-served (FCFS or FIFO) S IMPLE DETERMINISTIC MODEL Cumulative number of bits 35

36. D(t) A(t) time Q(t) d(t) E XAMPLE Cumulative number of bits Example: Every second, a train of 100 bits arrive at rate 1000b/s. The maximum departure rate is 500b/s. What is the average queue occupancy? 0.1s0.2s1.0s 100 36

37. P ROPERTIES OF QUEUES Time evolution of queues. Examples Burstiness increases delay Determinism minimizes delay Little’s Result. The M/M/1 queue. 37

38. 38 T IME EVOLUTION OF A QUEUE P ACKETS  A(t), D(t)D(t) Model of FIFO router queue Q(t)Q(t) time Packet Arrivals: Departures: Q(t)Q(t) 38

39. B URSTINESS INCREASES DELAY Example 1: Periodic arrivals 1 packet arrives every 1 second 1 packet can depart every 1 second Depending on when we sample the queue, it will contain 0 or 1 packets. Example 2: Bursty Arrival N packets arrive together every N seconds (same rate) 1 packet departs every second Queue might contain 0,1, …, N packets. Both the average queue occupancy and the variance have increased. In general, burstiness increases queue occupancy (which increases queueing delay). 39

40. D ETERMINISM MINIMIZES DELAY Example 3: Random arrivals Packets arrive randomly; on average, 1 packet arrives per second. Exactly 1 packet can depart every 1 second. Depending on when we sample the queue, it will contain 0, 1, 2, … packets depending on the distribution of the arrivals. In general, determinism minimizes delay. i.e. random arrival processes lead to larger delay than simple periodic arrival processes. 40

41. L ITTLE ’ S R ESULT 41

42. T HE P OISSON PROCESS 42

43. T HE P OISSON PROCESS Why use the Poisson process? Inter-arrival time of Poisson is simply Exponential. The Poisson process is known to model well systems in which a large number of independent events are aggregated together. e.g. Arrival of new phone calls to a telephone switch Decay of nuclear particles “Shot noise” in an electrical circuit Exponential property makes the math easy. In reality Network traffic is very bursty! Packet arrivals are not Poisson. But it models quite well the arrival of new flows. 43

44. A N M/M/1 QUEUE If A(t) is a Poisson process with rate, and the time to serve each packet is exponentially distributed with rate , then:  A(t), D(t)D(t) Model of FIFO router queue 44

45. W HAT ’ S N EXT !!! Fewer Math Equations!!!!... Promise What are the technologies underlying the Physical Layer? Report Topics WiMAX Bluetooth xDSL Ad Hoc Networks Peer-to-Peer Networks RFID Etc. 45