1. 1 An overview of networks Outline Contemporary networks Networks: single-link network to an internetwork For each type of network: tasks & layers Mechanisms for each task Protocols and Protocol reference models Standards Malathi Veeraraghavan Univ. of Virginia Updated: Aug. 29, 2013

2. Contemporary networks 1.Ethernet switched networks 2.Wireless LANs: IEEE 802.11 3.Cellular networks: 3G and 4G (WiMAX and LTE) 4.Residential access networks, such as Passive Optical Networks (PONs) and cable (DOCSIS) 5.Data center networks, such as InfiniBand and FibreChannel 6.Wireless sensor networks: Zigbee, IEEE 802.15.4 7.Vehicular networks: IEEE 1609.3 over 802.11p 8.Satellite networks and disruption tolerant networks 9.Supervisory Control and Data Acquisition (SCADA) networks for electric grid, water/sewage, etc. 10.Backbone circuit-switched networks (SONET, WDM) 11.Dynamic circuit networks 12.Public-Switched Telephone Network (PSTN) 2 Networks at the edges

3. Where/what is the Internet? In this listing of contemporary networks –No mention of the Internet Internet is “THE” global internetwork –Connects enterprise, home, provider networks How? Using gateways called IP routers –IP routers are gateways that interconnect networks that are owned and operated by different enterprises, homes, and providers 3

4. 4 Increasingly complex networks Unshared single-link network Shared single-link network Multiple-link network (with switches) Internetwork (with gateways)

5. 5 Single-link network with one sender and one receiver Host Simplest type of network: Unshared single-link network e.g., private road Hosts are data sources and sinks

6. 6 Shared single-link network Single-link network with multiple senders and multiple receivers e.g., shared road Symbol for wireless link Host http://en.wikipedia.org/wiki/File:Wireless-icon.png

7. 7 Courtesy: http://mars.gmu.edu/dspace/bitstream/1920/2497/1/pca_608_23_16n.jpg Analogy of a shared single link: a shared road with multiple sources of traffic (cars) - multiple driveways connected to single shared road

8. 8 Host Switch One network – same type of switches Multiple-link network e.g., roadways network (an intersection is comparable to a switch) Switch

9. 9 Analogous to a switch on roadways network Courtesy: http://en.wikipedia.org/wiki/Image:Cloverleaf.jpg Road intersection with traffic lights

10. 10 Train station analogous to a switch Courtesy: Washington DC Metro web site

11. 11 Host Switch Network 1 Host Switch Network 2 Gateway An internetwork Internetwork: Multiple networks e.g. roadways network e.g. airport e.g. airlines network Network 3

12. 12 Tasks What tasks are required for successful communication in each of these types of networks? –where should the hardware or software carrying out these tasks be implemented? in hosts? in switches? in gateways?

13. 13 Tasks in an unshared single-link network What tasks are required for successful communication across one link, with one sender and one receiver? Where are these tasks executed? –Only possibility: Hosts

14. 14 Tasks in an unshared single-link network Three sets of tasks: –Tasks specific to the application programs generating data and receiving data Example? –Transmit and receive data bits across a link –Error control and flow control Error control: Detect and recover from errors –bit errors caused by interference and electrical noise Flow control: Handle mismatches in speed –between receiving application program and sending application program

15. 15 Outline Contemporary networks Networks For unshared single-link network Tasks  Layers Mechanisms for each task Protocols and Protocol reference models Standards

16. 16 Layer What is a layer? –A grouping of a set of tasks –Hardware/software that implements this grouping of tasks

17. 17 Layers in an unshared single-link network TasksLayer Application-specific tasks, e.g. email, web Application layer Error control and flow control Data-link layer (DLL) Send/receive data bitsPhysical layer (PHY)

18. 18 Why the name "layers?" Because each layer uses the services of the layer below it Host Data link layer Physical layer Application layer Host Data link layer Physical layer Application layer 1 2 3 4 5 Draw in response arrows 6 through 10

19. 19 Increasingly complex networks Unshared single-link network  Shared single-link network  What additional tasks are required?  How are these grouped into additional layers? Multiple-link network (with switches) Internetwork (with gateways)

20. 20 Sharing analogy How are roads shared? –multiple lanes frequency-based sharing - radio AM/FM stations –back-to-back cars on a single lane time-based sharing

21. 21 Shared single-link network Single-link network with multiple senders and multiple receivers Let’s call this Scenario 1 for creating a shared single link Wireless link: what is sent by one can be heard by all Host

22. 22 Other scenarios for shared single-link networks Host Shared single wired link App. 1App. 2App. 1App. 2 Multiple applications Host 1 Host 2 Scenario 2 Scenario 3 Multipoint repeater Hub Multiple links, correct? So, why is it labeled "single" link? Because only one host can send at a time

23. 23 What is a hub? Multipoint repeater or hub –Simple physical-layer device that forwards all packets received on one link to all other links –So how does a host receiving a packet know whether the packet is meant for it?

24. 24 “Sharing”  Need for addresses On a Shared single-link network –A sending host needs to indicate destination for its packet –How is this done? By adding an address to the packet header. –What is a packet? A set of data bits –What is a header? A few bits tagged on at the front of the packet Example: Ethernet (MAC) address – 6 bytes

25. 25 Hub: multipoint repeating send to all links except input link Packet carries destination address “B” in its header; example packet is carrying the data bits 1000001 (which is ASCII for ‘A’) Hub or multipoint repeater Host B Host D Host AHost C packet 1000001 Step 3: All three hosts compare their address to that in the arriving packet header; only B finds a match and delivers data to higher layer; Rest drop the packet step 1step 2 B

26. 26 If hosts A and D simultaneously send packets, they will collide, even if they are headed to different destinations because all packets are sent on all other links Bits cannot be deciphered: both packets are “lost” No buffers in the hub to hold one while processing the other Hub or multipoint repeater Host B Host D Host AHost C packet Cost of no buffers 1000001 B 1100001 C

27. Differences between hub and switch 27 Buffers to hold packets Forwards packets to HubNoall links except input link SwitchYesspecific link based on forwarding table

28. 28 What a switch does instead (recall the term "switch" was used in multiple-link networks) Packet carries destination address “B” in its header; example packet is carrying the data bits 1000001 (which is ASCII for ‘A’) Host B Host D Host AHost C packet Step 2: Switch looks up forwarding table step 1step 3 IIIIII IV SWITCH DestinationOutput port BII Forwarding table Step 3: Switch sends the packet on only port II unlike the hub which transmits it on all ports 1000001 B

29. 29 Single link vs. multiple links Hub: –Single-link network because at a time only one sender can send data Switch: –Multiple-link network because multiple senders can simultaneously send data even if two packets are destined to the same host, the switch can buffer one packet (hold in memory) while sending the first

30. 30 Coming back to our tasks/layers thread Recap: Having understood the three scenarios for shared single-link networks: –Wireless link –Two applications between hosts –Single wired link with a hub What are the additional tasks and layers needed in these shared single-link networks?

31. 31 What additional tasks are needed in a shared single-link network? Tasks related to sharing, e.g., decide which sender gets to send next –a.k.a (also known as) multiplexing schemes Medium Access Control (MAC) schemes

32. 32 What additional layers are needed in shared single-link networks? MAC tasks are implemented in a –sub-layer within the data-link layer In other words, data-link layer in a shared single-link network consists of –error control –flow control –multiplexing/MAC

33. 33 Layers in an shared single-link network TasksLayer Application-specific tasks, e.g. email, web Application layer Error control, flow control, and multiplexing/MAC Data-link layer (with MAC sublayer) Send/receive data bitsPhysical layer (PHY) Compare this slide with slide 15

34. 34 Where are these layers implemented? All three layers at the hosts –For all three scenarios In scenario 3: –Hubs implement only the physical layer which consists of the tasks of........

35. 35 Increasing complexity of networks Unshared single-link network Shared single-link network  Multiple-link network (with switches) Additional tasks? Additional layers? Internetwork (with gateways)

36. 36 Host Switch One network – same type of switches Multiple-link network e.g., roadways network (with road intersection comparable to a switch)

37. 37 What additional tasks are needed in multiple-link networks? Switching: forward data units (groups of bits) from one link to another End-to-end error control and flow control –Why? Switches can drop data due to buffer overflows

38. 38 Additional layers in an multiple-link network TasksLayer SwitchingNetwork layer End-to-end error control and flow control Transport layer

39. 39 Where are these layers implemented? Host Switch Data link layer Physical layer Network layer Transport layer Application layer Data link layer Physical layer Network layer Transport layer Application layer Data link layer Network layer Data link layer Physical layer Data link layer Physical layer Network layer Data link layer Physical layer Physical layer

40. 40 Why is "network layer" shown at the end hosts? Answer: –Network-layer (NL) implementation at a host supports the network-layer switching task implemented at the switch –at the sending host: NL adds destination address to packet headers this allows the switch to forward packets to different output ports (links) based on destination addresses analogy: who writes the address on an envelope sent through USPS? –at the receiving host: NL checks if destination address in packet header corresponds to host's address receive the packet it there is a match otherwise, drop the packet

41. 41 Interesting to note That the network-layer tasks implemented at the switch are different from the network-layer tasks implemented at end hosts

42. 42 Layers in a multiple-link network TasksLayer Application-specific tasks, e.g. email, webApplication layer End-to-end error control and flow controlTransport layer SwitchingNetwork layer Error control, flow control, multiplexing/MAC Data-link layer (with MAC sublayer) Send/receive data bitsPhysical layer

43. 43 Increasing complexity of networks Unshared single-link network Shared single-link network Multiple-link network (with switches)  Internetwork (with gateways)

44. 44 Host Switch Network 1 Host Switch Network 2 Gateway An internetwork Internetwork: Multiple networks e.g. roadways network e.g. airport e.g. airlines network Gateways are the "switches" of the internetwork forwarding data units between networks

45. 45 Layers in an internetwork Complex in a general context Example: the Internet –TCP/IP model –Gateway: IP router Another example: VoIP –Public Switched Telephone Network (PSTN) connected to the Internet –Media Gateway (with controller)

46. 46 Putting together the four “types” of networks introduced in lesson Four types of (increasingly complex) networks described in this lesson was just for purposes of understanding all the various tasks involved in communications In reality, most hosts are connected to the Internet (which is an internetwork) These "four types" are just types of communication instances rather than networks

47. 47 Host Switch Network 1 Host Switch Network 2 Gateway An internetwork Communication instances Hub Host Comm. instance 3: multiple-link Comm. instance 2: shared single-link Comm. instance 4: inter-network Comm. instance 1: unshared single-link left out of the figure because it is usually only in labs that two hosts are directly connected to each other by an unshared link Host

48. Network management tasks FCAPS: –Fault management –Configuration management –Accounting –Performance monitoring –Security 48

49. 49 Outline Contemporary networks Networks: from a simple single-link network to an internetwork Tasks  Layers  Two ends of a layer Mechanisms for each task Protocols and Protocol reference models Standards

50. 50 Are the implementations of a layer at the two communicating ends the same? Sometimes yes. Sometimes no. Physical layer example Application layer example Network layer example: already shown to be different at hosts and switches

51. 51 Physical layer example Bidirectional links (half-duplex or duplex) –both ends have transmit and receive capabilities –Same layer-1 implementation at both ends Unidirectional link (simplex link) –one end has transmit capability –other end has receive capability

52. 52 Half duplex Transmitter (Tx) Receiver (Rx) Transmitter (Tx) Receiver (Rx) One way at a time

53. 53 Full duplex Transmitter (Tx) Receiver (Rx) Example optical fiber Recall the two connectors (blue and red) at each end Simultaneous

54. 54 Application layer Example in which the required tasks differ at the two ends: –Web browsing The web server program waits for clients to connect to it and responds to requests by serving out files The web client program receives inputs from human users and sends corresponding requests to the web server

55. 55 At the application layer Two modes of interaction: –Client-server the application-layer subroutine at the server is different from that at the client –Peer-to-peer the application-layer subroutines are the same at both end hosts

56. 56 Client-server mode The server only serves out files while the client only requests files; example: web server access Courtesy: A. Tanenbaum & Prentice Hall

57. 57 Peer-to-peer mode Both ends have the same implementation; example: telephony Courtesy: A. Tanenbaum & Prentice Hall

58. 58 Outline Contemporary networks Networks: from a simple single-link network to an internetwork Tasks and Layers  Mechanisms for each task Protocols and Protocol reference models Standards

59. 59 Application layer: Source coding Block-oriented –Text: ASCII (7bits for each character) and EBCDIC; extended ASCII uses 8 bits per character Compression techniques: "the" "e" occur a lot –Images: Fax of an 8" by 10" page with 400 by 400 pixels per sq. inch results in 38400000bytes if three bytes are used per pixel, one each to represent R, G, and B. –Using 1MB = 2 20 bytes, this is equal to 36.62MB GIF: lossless compression JPEG: lossy compression Stream-oriented –Voice: PCM (Pulse Code Modulation); 8000 samples/sec; with 8 bits/sample, it results in 64kbps signal Compression techniques: –ADPCM Adaptive Differential Pulse Code Modulation - 32 kbps –Residual excited linear predictive coding - 8-16 kbps –Audio (music): needs 32-384kbps –Video: H.261 coding: 176 by 144 or 352 by 258 frames at 10-30 frame/sec Full motion MPEG-2 HDTV - 1920 by 1080 frames at 30 frames/sec (aspect ratio is important 16:9 vs. 4:3)

60. 60 Metric Units

61. 61 Memory vs. transmission rate MemoryExpressed in Megabytes, Gigabytes, Terabytes Transmission ratekilobits/sec, Megabits/sec, Gigabits/sec With main memory or RAM capacity, gigabyte means 1073741824 bytes; To avoid confusion, let’s call this GibiBytes (see next slide) capitals for above 1 and small for below m: milli but M for Mega; kbps: small k is an exception

62. Multiples of bytes SI decimal prefixesIEC binary prefixes Name (Symbol) ValueName (Symbol) Value kilobyte (kB)10 3 kibibyte (KiB)2 10 megabyte (MB)10 6 mebibyte (MiB)2 20 gigabyte (GB)10 9 gibibyte (GiB)2 30 terabyte (TB)10 12 tebibyte (TiB)2 40 petabyte (PB)10 15 pebibyte (PiB)2 50 Exabyte (EB)10 18 exbibyte (EiB)2 60 1-62 SI: International System of units IEC: International Electrotechnical Commission source: http://en.wikipedia.org/wiki/Gibibyte

63. 63 Physical layer Properties of communication channels –Bandwidth –Amplitude response function –Phase shift function –Attenuation –Speed of light in the medium  Shannon's channel capacity

64. 64 Shannon’s channel capacity Shannon's channel capacity, C: The maximum rate of a noisy channel whose bandwidth is H Hz is given by –C=H log 2 (1+S/N) bits/sec S/N is the signal to noise fraction at the receiver –log 2 x = (log 10 x)/log 10 2 If 2 y = x, take log 10 of both sides –then y log 10 2 = log 10 x. Units: The term log 2 (1+S/N) has a unit of bits, which can be seen in a derivation (not included in this course) of Shannon's equation. Therefore the unit of C is bits/sec, since the unit of H is Hz or /sec

65. 65 Physical layer Group of functions needed to move bits across a link: “communications” Channel (line) coding Modulator Demodulator Channel (line) coding Data bits Channel 1011000... Channel (line) coding: is a method for converting a binary information sequence (1s and 0s) into a digital signal in a digital communications systems Modulation: is a method for carrying an information signal on a carrier signal Data bits 1011000...

66. 66 What are the components of physical- layer delay on a single link? Physical-layer delay incurred in moving a data unit (file or packet) of size S bits across an error-free communication link –Propagation delay –Emission (transmission) delay DELAY = TIME Usage of the word: "I was delayed getting to the airport" implies that there was an expectation of the time needed to drive there and the speaker somehow got "delayed," perhaps due to heavy traffic. In networking, we use the term "delay" to represent the time taken to move the file. There is a component called "queuing delay" to represent additional delay incurred by having to wait.

67. 67 Propagation delay Propagation delay = L/v, where L is the length of the link and v is the speed of light in the physical medium (v) of the link 1 bit propagation delay bit travels at the speed of light in the medium hence the dependence on length of the link

68. 68 Emission (transmission) delay Emission (transmission) delay = S/r, where S is the size of the data unit being transmitted in bits, and r is the transmission rate in bits/sec 1 data unit of S bits: a file or packet emission (transmission) delay: time to emit (transmit) the data unit on to the link Let’s say the link transmitter can emit out 10 million bits/sec; this is r, the transmission rate of the link. Hence the size of the data unit, S, and the transmitter rate, r, determine the emission delay

69. 69 Physical-layer delay to move a data unit of size S bits Physical-layer delay = emission delay + propagation delay Why do we not need to multiply propagation delay by the number of bits?

70. 70 Packetization (AL) delay (only for streamed data) Packetization delay is the time taken to create data for the payload of a packet –Packetization delay = S/r codec –r codec : codec rate; S = packet payload size –Packet consists of Header and Payload –Payload is the user-generated data Example: ADPCM voice codec fills packets with a 32-byte payload size –what is the packetization delay to fill one packet payload? codec: coder/decoder

71. 71 Examples of physical media Types of media: –Twisted pair –Coaxial cable –Optical fiber –Wireless Radio frequencies (RF) Infra-red (IR)

72. 72 Electromagnetic spectrum http://www.lbl.gov/MicroWorlds/ALSTool/EMSpec/EMSpec2.html

73. 73 Layers in an unshared single-link network TasksLayer Application-specific tasks, e.g. email, web Application layer Error control and flow control Data-link layer (DLL) Send/receive data bitsPhysical layer (PHY)

74. 74 Data link layer: Error control Error detection –Parity check –Cyclic Redundancy Code (CRC) or polynomial codes Error correction –Automatic Repeat reQuest (ARQ) –Forward Error Correction (FEC)

75. 75 Error correction: Different ARQ schemes Stop-and-Wait Sliding window –Go-Back N –Selective repeat

76. 76 ARQ error/loss detection and recovery Send a frame Hold frame in a retransmission buffer at the sender so that if there is a loss/error, the frame can be resent Wait for Acknowledgment (ACK) from receiver If a received frame had errors, the receiver detects the presence of errors using CRC, and then sends a notification –sender resends errored frame But what happens if the frame itself was lost or the receiver's notification of an error is lost? Solution: –Start a timer at the sender upon sending a frame –If timer times out before ACK arrives, retransmit frame

77. 77 Flow control problem Rates of the transmitter and receiver at the physical layer are matched. The flow control problem arises because the layer above the DLL at the receiver does not deplete the buffer at the same rate at which data is being passed to the DLL at the sender (R snd  R rcv ) Data-link layer (DLL) Physical layer HostSwitch or host Data units Receive buffer H T TH H T R snd R rcv transmission rate: r

78. 78 Different rates R snd : Rate at which the higher layer passes data to the DLL at the sender R rcv : Rate at which the higher layer removes data from the DLL buffer at the receiver r: physical-layer transmission rate

79. 79 Techniques for flow control Flow control mechanisms prevent buffer overflow by regulating the rate at which source is allowed to send information –Stop-and-wait flow control –ON-OFF flow control –Sliding window flow control –Rate based flow control (skip for this class)

80. 80 Layers in an shared single-link network TasksLayer Application-specific tasks, e.g. email, web Application layer Error control, flow control, and multiplexing/MAC Data-link layer (with MAC sublayer) Send/receive data bitsPhysical layer (PHY)

81. MAC MAC: Medium Access Control or Media Access Control –Set of functions to support the sharing of a single link by multiple endpoints MAC vs. Multiplexing –The term "MAC" is used to describe sharing techniques on multi-access links –The term "multiplexing" is used to describe sharing techniques on point-to-point links 81

82. Types of links Multi-access links –Typically used to connect multiple hosts to a switch –Cheaper than point-to-point links –Mostly used in wireless networks –Sometimes in wired networks through hub Point-to-point links –Typically used between switches –Increasing typical between hosts and switches in wired networks Host...... Switch Host Switch Host 82

83. Classification of Multiplexing/MAC techniques Multiplexing/MAC techniques Circuit-based multiplexing Position based: space (reuse: cellular) time frequency Each multiplexed data stream occupies a different position Packet-based multiplexing Packet header based: header carries destination address Each multiplexed data stream consists of packets with headers carrying corresponding destination addresses 83

84. Packet-based multiplexing For point-to-point links –Scheduling techniques For multi-access links –Random access schemes 84

85. Examples Circuit-based multiplexing Packet-based multiplexing Multi-access wireless linkCellular (FDMA/TDMA) IEEE 802.11 (WiFi) Multi-access wired linkEthernet hub Point-to-point switch-to- switch link PDH, SONET, WDMEthernet switch Point-to-point endpoint-to- switch link Plain Old Telephone Service (POTS) (space division multiplexing) Ethernet link Multiplexing/MAC schemes Types of links Phone links from residences carry only one phone call and hence it is space-division multiplexing; DSL: new technology for multiplexing data with voice 85

86. 86 Layers in a multiple-link network TasksLayer Application-specific tasks, e.g. email, webApplication layer End-to-end error control and flow controlTransport layer SwitchingNetwork layer Error control, flow control, multiplexing/MAC Data-link layer (with MAC sublayer) Send/receive data bitsPhysical layer

87. Controller 1 2 3 P Line card Space switch Line card 1 2 3 P Input ports Output ports Data path Control path … … … … Generic switch architecture (circuit or packet) Line cards –Packet switch: header based mux/demux –Circuit switch: position based mux/demux Space switch –Crossbar, Clos Controller –Processor: routing protocols and signaling protocols 87

88. Types of switches Line card (multiplexing) Controller (admission control or not) Circuit-switch (CS) (position-based) Packet-switch (PS) (header-based) Connectionless (CL) (no admission control) e.g., datagram: IP routers; Ethernet switches Connection-oriented (CO) (admission control) e.g., telephone network circuit switches, SONET switches Virtual-circuit switches: MPLS Routing: Required in controller for all three types of switches Signaling: Admission control – hence required only for connection- oriented switches (not included in this course) 88

89. A network of connectionless packet switches Control path –Switch controllers exchange routing information and create forwarding tables Data path –Packets carrying user data and destination addresses in headers are switched from input link to appropriate output link based on forwarding table entry for destination address 89

90. 90 Layers in a multiple-link network TasksLayer Application-specific tasks, e.g. email, webApplication layer End-to-end error control and flow controlTransport layer SwitchingNetwork layer Error control, flow control, multiplexing/MAC Data-link layer (with MAC sublayer) Send/receive data bitsPhysical layer Mechanisms considered under DLL

91. Other functions of transport protocols Transport protocols include functions that augment the services offered by underlying network layers –port-multiplexing to carry data from many processes at the end hosts –if network layer is connectionless packet switched transport layer may include congestion control to handle losses in switch buffers 91

92. 92 Outline Contemporary networks Networks: from a simple single-link network to an internetwork Tasks and Layers Mechanisms for each task  Protocols and Protocol reference models Standards Status check

93. What’s a protocol? human protocols:  “what’s the time?”  “I have a question”  introductions … specific msgs sent … specific actions taken when msgs received, or other events network protocols:  machines rather than humans  all communication activity in Internet governed by protocols protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt Kurose and Ross 93

94. What’s a protocol? a human protocola computer network protocol Hi Got the time? 2:00 TCP connection response Get http://www.awl.com/kurose-ross time TCP connection request Kurose and Ross 94

95. 95 Example of a protocol Parity bit added in data-link layer trailer for error detection Protocol: (1) parity bit is added (2) sent at end (3) even or odd? –Agreement on these aspects is required at the data-link layer implementations at the two ends: sending and receiving –these rules constitute the data-link layer protocol Packet (110010100...) Packet (110010100...) Parity Packet (110010100...) Packet (110010100...) Parity Data-link layer (layer 2) Physical layer (layer 1) HostSwitch or host Adds parity Checks parity

96. 96 Layers, protocols, and interfaces Physical layer Data-link layer Network layer Transport layer Application layer Courtesy: A. Tanenbaum & Prentice Hall

97. source application transport network link physical HtHt HnHn M segment HtHt datagram destination application transport network link physical HtHt HnHn HlHl M HtHt HnHn M HtHt M M network link physical link physical HtHt HnHn HlHl M HtHt HnHn M HtHt HnHn M HtHt HnHn HlHl M router switch Encapsulation message M HtHt M HnHn frame Kurose and Ross 97

98. 98 Service: interface between layers Courtesy: A. Tanenbaum & Prentice Hall

99. 99 What is a protocol reference model? A grouping of layers Recall that we defined a layer as a grouping of tasks.

100. 100 Two protocol reference models OSI (Open Systems Interconnection) reference model –has two more layers, presentation and session The TCP/IP reference model –Used in the Internet

101. 101 OSI protocol reference model (lists tasks for layers) Application layer: –Implements some application involving communications Presentation layer: –Allow applications to interpret meaning of data –Examples: e.g., encryption, compression, machine-specific conventions (Endian) Session layer: –Dialog control (track whose turn it is to send) –Token management (avoid same critical operation at the same time) –Synchronization (checkpoint long transmissions and continue after a crash) Transport layer: –End-to-end error control, flow control Network layer: –Switching Data link layer: –Error control, flow control –Additionally for shared links: multiplexing/MAC Physical layer: –Transmitting and receiving data bits over a communication link

102. 102 TCP/IP Protocol Stack (Protocols in each layer) Application layer protocols: –web: Mozilla client; Apache server; http (hypertext transfer protocol) –email: Outlook client; mail server; smtp (simple mail transfer protocol) –file transfers: SecureFX client; SFTP server; ftp (file transfer protocol) –remote login: telnet, SecureCRT Transport layer protocols: –TCP (Transmission Control Protocol) –UDP (User Datagram Protocol) –RTP (Real-time Transfer Protocol) IP (Internet Protocol): –IP (Internet Protocol) –Provides packet forwarding between networks Link-layer/Physical-layer protocols: –all the protocol layers necessary for communication across a specific network

103. 103 Layers in the TCP/IP model Application Layer TCP (Layer 4) IP (Layer 3) Layer 2 + Layer 1 (industry) This usage of the term “Layer 2” for anything below IP layer has led to industry usage of the term “Layer 2 switch” to describe a switch within a subnetwork, e.g., Ethernet switch

104. 104 Outline Contemporary networks Networks: from a simple single-link network to an internetwork Tasks and Layers Mechanisms for each task Protocols and Protocol reference models  Standards Status check

105. 105 Standards IETF: Internet Engineering Task Force http://www.ietf.org ITU-T: International Telecommunications Union – Telecommunications (part of the United Nations) ANSI: American National Standards Institute IEEE: Institute of Electrical and Electronics Engineers

106. 106 Role of standards bodies To define protocol specifications, which includes –message formats –parameter formats Goal: allow protocol implementations from two different vendors to communicate –analogy: two people speaking the same language have to obey the rules of the language To allow for product differentiation –implementation details are not standardized

107. 107 IEEE 802 Standards The 802 working groups. The important ones are marked with *. The ones marked with  are hibernating. The one marked with † was given up. Courtesy: A. Tanenbaum & Prentice Hall

108. 108 Summary Key points of this lesson: –List the various tasks required for different communication instances (across a unshared single link, shared single link, across a multiple-link path via a switch) –How are these tasks grouped into layers? –List two modes used in application-layer implementations –List mechanisms for each task type –What is the purpose of protocols? Why is a protocol needed for each layer? –What are the two main protocol reference models? –Name the main standards bodies. Why are standards bodies important in this field?