ESE250: Digital Audio Basics. Week 13: April 2nd, 2013 Networking

Transcription

1 1 ESE250: Digital Audio Basics Week 13: April 2nd, 2013 Networking

2 2 Teaser 2006 Debate Former Senator Stevens Jon Stewart Reporting 2010 Ruling U.S. Court of Appeals for the District of Columbia Decision favors Comcast FCC has no authority to impose net neutrality obligations on broadband providers

3 Networked Systems Today We expect our computers to be networked Google, wikipedia, , IM, Can work stand alone Airplane mode But, are crippled when not connected Phone isn t a phone unless its networked!

4 Course Map 2, Numbers correspond to course weeks

5 Outline Communicating between machines Technology and Costs Layering Transport Network Routing and what can go wrong and what to do about it Physical physical layer independence What can go wrong and what to do about it Running simulation will punctuate points (no interlude)

6 6 Communicating between Machines

7 Minimal Setup Have two computers think raw processors for the moment Want them to communicate Send an mp3 file.

8 Physical Connection Place an IO peripheral in each computer String wire between computers E.g. one wire from A B, another B A

9 Signaling Communicate with Voltage pulses A pulls line low B senses low line Encode data as series of pulses or voltages on the line

10 Communication Start program on B to receive data (file) Start program on A to send data (file) B waits for valid characters A sends data B receives A sends out-of-band signal to indicate end of data

11 Multiple Tasks E.g. download song and X session Could have a separate interface/wire for each application Process allocates hardware when needs to communicate Holds exclusive access during transfer

12 Connect to Multiple Machines Add interface/wire for every machine want to talk to Talk to machine through its dedicated wire

13 Scalability Do we like where this is going? How many machines are there? July Million AL/2000ten.html September Million nternet/timeline/ How many in 2015? 2020?

14 How many connections? What machines do you connect to regularly? server Eniac Web servers? Yahoo, CNN, google, YouTube, facebook itunes How many friends do you IM? How many wires would you need if each of these required its own?

15 1 5 Technology and Costs

16 Wires How fast can I send data over a wire? Consider a Category-5 ethernet cable Bandwidth (bits/s) 100Mbits/s 100Base-T (100Mb ethernet) 1Gbit/s 1000Base-T (Gigabit ethernet) Latency/transit time (distance/time) 0.64 c [c=speed of light = m/s] m/ns or roughly 5ns/m [image:

17 Comparison: computer parts Transit time to back of room? Hard or Solid-State Disk Bandwidth Gbits/s

18 Comparison: computer parts Transit time to back of room? 5m 25ns 3GHz processor 75 machine instructions Hard or Solid-State Disk Bandwidth Gbits/s

19 Comparison: Audio Real-Time, single-channel Audio 16b = 704Kbits/s How many can share 1Gbit/s link? Real-Time single-channel MP3 64Kbits/s How many can share 1Gbit/s link?

20 Comparison: Audio Real-Time, single-channel Audio 16b = 704Kbits/s How many can share 1Gbit/s link? 1Gbits/s / 0.7Mbits/s = 1430 Real-Time single-channel MP3 64Kbits/s How many can share 1Gbit/s link?

21 Comparison: Audio Real-Time, single-channel Audio 16b = 704Kbits/s How many can share 1Gbit/s link? 1Gbits/s / 0.7Mbits/s = 1430 Real-Time single-channel MP3 64Kbits/s How many can share 1Gbit/s link? 1Gbits/s / 64Kbits/s = 16,000

22 Comparison: Video HDTV (uncompressed) 1920x1080 ~= 2M pixel 3 16b/pixel 100Mb/frame 30 frames/second 3Gbits/second How many can share 1Gbit/s link? HDTV compressed Around 36Mbits/s How many can share 1Gbit/s link?

23 Comparison: Video HDTV (uncompressed) 1920x1080 ~= 2M pixel 3 16b/pixel 100Mb/frame 30 frames/second 3Gbits/second How many can share 1Gbit/s link? HDTV compressed Around 36Mbits/s How many can share 1Gbit/s link? 1Gbits/s / 36Mbits/s ~ 30

24 Costs Cat 5e per foot ~ $0.20/foot Say $0.60/m Raw wire Ignoring handling to run Ignoring rent/lease/buy land to run Philly San Francisco: ~4,000km $2.4M for wire run

25 Implications? Today s wire bandwidth exceeds the throughput needs of any real-time single-stream data Can afford to share the wire Wires not cheap Cannot afford not to share the wire

26 2 6 Sharing (Virtualizing) Connections

27 Sharing Link Idea: Tag data with target this is for process 34 this is for process 45 Have transport layer deal with Mixing data from separate streams Separating data out into individual streams Delivering to individual processes

28 Packet Divide data up into discrete blocks Begin to form a packet Header: says where to go Data payload: the data to send Header: Added, consumed by network handling in routing Data payload: Only thing seen by the application processes

29 Packets

30 Transport Layer Call this the Transport Layer responsible for delivering data to the individual application process on the computer

31 3 1 Possible Role Assignment SA1 SA2 T1 T2 SA3 SA4 N2 T3 N1 R1 W1/R2 R3 N2 N4 T4

32 3 2 Simulation 1 Send 4 verses or digits from each SA1 SA2 T1 from song-server-app, π-server-app to song-listener-app, π-consumer W1 T2 CA1 CA2

33 3 3 Virtualize Physical Wires

34 Start Simple Add more computers to same pair of wires Like the memory bus How do I know who the message is for?

35 Extended Packet Extend our packet header: Destination computer Process on destination computer Sending computer Process on sending computer Penn ESE250 Spring DeHon 35

36 Network Layer responsible for end-to-end (source to destination) packet delivery

37 Virtualization Effect Each pair of processes on different computers Has the view of a point-to-point connection

38

39 Simulation 2 Send 4 verses or digits from each from song-server serving 2 songs And digit-server serving 2 fundamental constants To two clients

40 4 0 Simulation 2 SA1 SA2 SA3 SA4 T1 T3 N1 N3 W1 CA1 N2 N4 T2 T4 CA2 CA3 CA4

41 4 1 Simulation 2: Dataflows setup SA1 SA2 SA3 SA4 T1 T3 N1 N3 W1 CA1 N2 N4 T2 T4 CA2 CA3 CA4

42 4 2 Extending the Virtual Link

43 Indirect Connections A and B are connected B and C are connected How get message from A to C?

44 Indirect Connection Run process on B to forward messages from A to C

45 Routing B runs a general program If packet destined for B, takes it Otherwise, sends on to (toward) destination Extension of the network handling process that is sorting data to processes

46 Routing (UART: Universal Asynchronous Receiver/Transmitter)

47 Reachability If everyone plays along Can communicate with any computer reachable transitively from my computer Don t need direct connections

48 Routing Route Tables To make efficient Each computer should route close to destination and not route in circles E.g. compute all-pairs shortest paths Store result, each machines knows where to send packet next How much storage? Cleverness to compress/summarize What happens when links break, new computers added? Cleverness to incrementally recompute

49 Network Layer responsible for end-to-end (source to destination) packet delivery including routing through intermediate hosts

50 Simulation 3 Send 4 verses or digits from each from song-server serving 2 songs And digit-server serving 2 fundamental constants To two clients

51 5 1 SA1 Sim 3 SA2 T1 SA3 SA4 T3 N1 N3 R1 R2 CA1 N2 N4 T2 T4 CA2 CA3 CA4

52 Simulation 4 Send 4 verses or digits from each from song-server serving 2 songs And digit-server serving 2 fundamental constants To two clients Roles: 4 server apps Network Interface for each of 2 servers 3 routers Network Interface for each of 2 clients 4 client apps

53 5 3 SA1 Sim 4 SA2 T1 SA3 SA4 T3 N1 N3 R1 R2 CA1 R3 N2 N4 T2 T4 CA2 CA3 CA4

54 Where are we now? Can communicate From one process on a computer To any other process on any other computer If the two are transitively connected By a set of participating computers which route data Abstraction Processes just see streams of data between the endpoints

55 Layers Application (abstraction) Application Transport Transport Network Network Link Link Physical Physical

56 Protocols Mostly sketched key buildings blocks and functionality of IP IP = Internet Protocol Delivery to process (rather than hosts): UDP UDP = Unreliable Datagram Protocol

57 Simulation 5 Send 4 verses or digits from each from song-server serving 2 songs And digit-server serving 2 fundamental constants To two clients

58 5 8 SA1 Sim 5 SA2 T1 SA3 SA4 T3 N1 N3 R1 R2 CA1 R3 R3 delayed N2 N4 T2 T4 CA2 CA3 CA4

59 What can go wrong? Packets arrive out of order Solution?

60 What can go wrong? Packets arrive out of order Solution? Add a sequence number

61 Simulation 6 Send 4 verses or digits from each from song-server serving 2 songs And digit-server serving 2 fundamental constants To two clients

62 6 2 SA1 Sim 6 SA2 T1 SA3 SA4 T3 N1 N3 R1 R2 CA1 R3 R3 delayed N2 N4 T2 T4 CA2 CA3 CA4

63 6 3 Abstracting Physical Layer Application, transport, network Don t really care how the bits are moved from machine-to-machine What are other ways we send bits? Application Application Transport Transport Network Network Link Link Physical Physical

64 6 4 Abstracting Physical Layer Application, transport, network Don t really care how the bits are moved from machine-to-machine What are other ways we send bits? Optically RF/wireless Pneumatic tubes, passing paper notes, SMS Text messages Application Application Transport Transport Network Network Link Link Physical Physical

65 Simulation 7 Send 4 verses or digits from each from song-server serving 2 songs And digit-server serving 2 fundamental constants To two clients Roles: 4 server apps Network Interface for each of 2 servers 3 routers, connect to both servers and endpoints One link is via text messaging Network Interface for each of 2 clients 4 client apps

66 6 6 SA1 Sim 7 SA2 T1 SA3 SA4 T3 N1 N3 R1 wireless R2 CA1 R3 N2 N4 T2 T4 CA2 CA3 CA4

67 What else can go wrong?

68 What else can go wrong? Bits get corrupted Intermediate machines holding messages can crash Messages can get misrouted

69

70 Data Corruption How do we deal with data corruption? Use redundancy Two strategies: Use enough redundancy to correct Use just enough redundancy to detect it Have the sender resend

71 Data Corruption Relatively uncommon Most packets are fine We have efficient (low overhead) ways to detect Compute a hash of the message data Highly unlikely one (few) message bit errors will result in same hash checksum (e.g., parity) reserve first bit of each byte for error detection set first bit so that entire string has even (or odd) # of 0 s

72 Revised Packet Header Data payload Checksum

73 Lost Packet How can we deal with lost packets?

74 Lost Packet Strategy Sender sends packet But keeps a copy Receiver gets packet Checks checksum OK, uses packet and sends ACK got your last packet intact Not ok, discard packet Sender Receives ACK, can discard packet and send next No ACK (after timeout), resend packet

75 Retransmission Discipline Don t depend on receiver to request retransmission Why? Header may be corrupted Not delivered to receiver Only know receiver got it when it says it got it

76 Corrupted ACK What if the ack is lost? Sender resends Receiver receives a second copy Oops, don t want that to be interpreted as new data i.e. send: rm *; cd..\n Receive: rm *; cd..\n rm *; cd..\n

77 Avoid Duplication How can we avoid duplication?

78 Accommodating Duplication Use packet sequence number Keep track of last packet received If receive packet again, Discard the packet

79 Sequence Numbers

80 TCP TCP = Transmission Control Protocol Provides Reliable delivery Deals with Retransmission Duplication Out of sequence / resequence / reconstruction

81 Transport Layer Call this the Transport Layer responsible for reliably delivering data to the individual application process on the computer

82 Layers Application (abstraction) Application Transport Transport Network Network Link Link Physical Physical

83 Learn More Courses ESE404 Intro Networks and Protocols ESE575 Intro to Wireless Systems

84 Big Ideas Sharing Network interface, wires Virtualization datastream abstracts physical point-to-point link Layering Divide-and-conquer functionality Implementation hiding/technology independence Reliable communication link from unreliable elements