1 an overview of networks outline contemporary networks networks: single-link network to an...

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

Contemporary networks1. Ethernet switched networks2. 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 networks12. Public-Switched Telephone Network (PSTN)

2

Netw

ork

s at

the

edges

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

Increasingly complex networks

• Unshared single-link network• Shared single-link network• Multiple-link network (with

switches)• Internetwork (with gateways)

5

Single-link network with one sender and one receiver

Host Host

Simplest type of network:Unshared single-link

network

e.g., private road

Hosts are data sources and sinks

6

Shared single-link network

Single-link network with multiple senders and multiple receivers

e.g., shared road

Symbol for wireless link

Host

Host

Host

http://en.wikipedia.org/wiki/File:Wireless-icon.png

http://en.wikipedia.org/wiki/File:Wireless-icon.png

7Courtesy: 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

Host Host

HostHost

Switch Switch

One network – same type of switches

Multiple-link network

e.g., roadways network (an intersection is comparable to a switch)

Switch

9

Analogous to a switch on roadways network

Courtesy: http://en.wikipedia.org/wiki/Image:Cloverleaf.jpgRoad intersection with traffic lights

10

Train station analogous to a switch

Courtesy: Washington DC Metro web site

11

Host

Host

Switch Switch

Network 1

Host

Host

Switch Switch

Network 2Gateway

An internetwork

Internetwork:Multiple networks

e.g. roadways network

e.g. airport

e.g. airlines network

Network 3

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

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

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

Outline

• Outline• Contemporary networks• Networks• For unshared single-link network

• TasksLayers

• Mechanisms for each task• Protocols and Protocol reference models• Standards

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Layer

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

grouping of tasks

17

Layers in an unshared single-link network

Tasks Layer

Application-specific tasks, e.g. email, web

Application layer

Error control and flow control

Data-link layer(DLL)

Send/receive data bits Physical layer (PHY)

18

Why the name "layers?"

• Because each layer uses the services of the layer below it

Host

Data link layer

Physicallayer

Applicationlayer

Host

Data link layer

Physicallayer

Applicationlayer

1

23

4

5

Draw in response arrows 6 through 10

19

Increasingly complex networks

• Unshared single-link networkShared single-link network

What additional tasks are required?How are these grouped into additional

layers?

• Multiple-link network (with switches)• Internetwork (with gateways)

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

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Shared single-link network

Single-link network with multiple senders and multiple receivers

Let’s call thisScenario 1for creatinga shared single link

Wireless link:what is sent by onecan be heard by all

Host

Host

Host

22

Other scenarios for shared single-link networks

Host HostHost

Shared single wired link

App. 1 App. 2 App. 1 App. 2

Multiple applications

Host 1 Host 2Scenario 2

Scenario 3

Multipoint repeater

HubMultiple links, correct?So, why is it labeled "single" link?

Because only one host can sendat a time

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

“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

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Hub: multipoint repeatingsend 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 DHost A Host C

packet10

0000

1

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 1 step 2

step 2

step 2B

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• 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 A Host C

packet

Cost of no buffers

1000

001

B1100001

C

Differences between hub and switch

27

Buffers to hold packets

Forwards packets to

Hub No all links except input link

Switch Yes specific link based on forwarding table

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 A Host C

packet

Step 2: Switch looks up forwarding table

step 1 step 3

I II III IVSWITCH

Destination Output port

B II

Forwarding table

Step 3: Switch sends the packet on only port II unlike the hub which transmits it on all ports

1000

001

B

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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

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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?

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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

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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

Layers in an shared single-link network

Tasks Layer


Application layer

Error control, flow control, and multiplexing/MAC

Data-link layer (with MAC sublayer)

Send/receive data bits Physical layer(PHY)

Compare this slide with slide 15

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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 ........

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Increasing complexityof networks

• Unshared single-link network• Shared single-link networkMultiple-link network (with

switches)• Additional tasks?• Additional layers?

• Internetwork (with gateways)

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Host Host

HostHost

Switch Switch

One network – same type of switches

Multiple-link network

e.g., roadways network (with road intersection comparable to a switch)

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

Additional layers in an multiple-link network

Tasks Layer

Switching Network layer

End-to-end error control and flow control

Transport layer

39

Where are these layers implemented?

Host Host

HostHost

Switch Switch

Data link layer

Physicallayer

Networklayer

Transportlayer

Applicationlayer

Data link layer

Physicallayer

Networklayer

Transportlayer

Applicationlayer

Data link layer

Networklayer

Data link layer

Physicallayer

Data link layer

Physicallayer

Networklayer

Data link layer

Physical layer

Physical layer

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

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Interesting to note

• That the network-layer tasks implemented at the switch are different from the network-layer tasks implemented at end hosts

42

Layers in a multiple-link network

Tasks LayerApplication-specific tasks, e.g. email, web Application layer


Transport layer


Error control, flow control, multiplexing/MAC

Data-link layer(with MAC sublayer)

Send/receive data bits Physical layer

43

Increasing complexityof networks

• Unshared single-link network• Shared single-link network• Multiple-link network (with

switches)Internetwork (with gateways)

44

Host

Host

Switch Switch

Network 1

Host

Host

Switch Switch

Network 2Gateway

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

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)

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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

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Host

Host

Switch Switch

Network 1

Host

Host

Switch Switch

Network 2Gateway

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-linkleft 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

Network management tasks

• FCAPS:– Fault management– Configuration management– Accounting– Performance monitoring– Security

48

49

Outline


• from a simple single-link network to an internetwork

• TasksLayers

Two ends of a layer• Mechanisms for each task• Protocols and Protocol reference models• Standards

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

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

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Half duplex

Transmitter (Tx) Transmitter (Tx)

Receiver (Rx) Receiver (Rx)



One way at a time

53

Full duplex



Example optical fiberRecall the two connectors (blue and red) at each end

Simultaneous

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

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

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

Peer-to-peer mode

• Both ends have the same implementation; example: telephony


58

Outline



• Tasks and LayersMechanisms for each task• Protocols and Protocol reference models• Standards

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 = 220 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

Metric Units

61

Memory vs. transmission rate

Memory Expressed in Megabytes, Gigabytes, Terabytes

Transmission rate

kilobits/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: millibut M for Mega; kbps: small k is an exception

Multiples of bytes

SI decimal prefixes IEC binary prefixes

Name(Symbol)

Value Name(Symbol)

Value

kilobyte (kB) 103 kibibyte (KiB) 210

megabyte (MB) 106 mebibyte (MiB) 220

gigabyte (GB) 109 gibibyte (GiB) 230

terabyte (TB) 1012 tebibyte (TiB) 240

petabyte (PB) 1015 pebibyte (PiB) 250

Exabyte (EB) 1018 exbibyte (EiB) 260

1-62

SI: International System of unitsIEC: International Electrotechnical Commissionsource: http://en.wikipedia.org/wiki/Gibibyte

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

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 log2(1+S/N) bits/sec

• S/N is the signal to noise fraction at the receiver

– log2x = (log10x)/log102• If 2y = x, take log10 of both sides

– then y log102 = log10x.

Units: The term log2(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

Physical layer• Group of functions needed to move bits across a link:

“communications”

Channel (line)coding

Modulator

DemodulatorChannel (line)coding

Data bits

Channel1011000...

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

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" impliesthat there was an expectation of the time needed to drive there andthe speaker somehow got "delayed," perhaps due to heavy traffic. In networking, we use the term "delay" to represent the time takento move the file. There is a component called "queuing delay" torepresent additional delay incurred by having to wait.

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 bitpropagation delay

bit travels at the speedof light in the medium

hence the dependence onlength of the link

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 canemit 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

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

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/rcodec

– rcodec: 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

Examples of physical media

• Types of media: – Twisted pair – Coaxial cable – Optical fiber – Wireless

• Radio frequencies (RF)• Infra-red (IR)

72

Electromagnetic spectrum

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

73

Layers in an unshared single-link network

Tasks Layer


Application layer

Error control and flow control

Data-link layer(DLL)

Send/receive data bits Physical layer (PHY)

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

Error correction:Different ARQ schemes

• Stop-and-Wait• Sliding window

– Go-Back N– Selective repeat

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

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 (Rsnd Rrcv)

Data-link layer (DLL) Data-link layer (DLL)

Physical layerPhysical layer

Host Switch or host

Data units Receivebuffer

H

T

T H

H

T

Rsnd Rrcv

transmission rate: r

78

Different rates

• Rsnd: Rate at which the higher layer passes data to the DLL at the sender

• Rrcv: Rate at which the higher layer removes data from the DLL buffer at the receiver

• r: physical-layer transmission rate

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

Layers in an shared single-link network

Tasks Layer


Application layer

Error control, flow control, and multiplexing/MAC

Data-link layer (with MAC sublayer)

Send/receive data bits Physical layer(PHY)

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

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 Host Host......

Switch

HostSwitch Host

Host

82

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


• For point-to-point links– Scheduling techniques

• For multi-access links– Random access schemes

84

Examples

Circuit-based multiplexing


Multi-access wireless link Cellular (FDMA/TDMA)

IEEE 802.11 (WiFi)

Multi-access wired link Ethernet hub

Point-to-point switch-to-switch link

PDH, SONET, WDM Ethernet switch

Point-to-point endpoint-to-switch link

Plain Old Telephone Service (POTS)(space division multiplexing)

Ethernet link

Multiplexing/MAC schemesTypes of links

Phone links from residences carry only one phone call andhence it is space-division multiplexing; DSL: new technology for multiplexing data with voice

85

86


Tasks Layer

Application-specific tasks, e.g. email, web Application layer

End-to-end error control and flow control Transport layer





Controller

1

2

3

P

Line card

Line card

Line card

Line card

Spa

ce s

witc

h

Line card

Line card

Line card

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

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

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


Tasks LayerApplication-specific tasks, e.g. email, web Application layer


Transport layer





Mechanisms considered under DLL

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



• Tasks and Layers• Mechanisms for each task Protocols and Protocol reference models• Standards

Status check

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

What’s a protocol?

a human protocol a computer network protocol

Hi

Hi

Got thetime?

2:00

TCP connectionresponse

Get http://www.awl.com/kurose-ross

<file>time

TCP connectionrequest

Kurose and Ross 94

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) Data-link layer (layer 2)

Physical layer (layer 1)Physical layer (layer 1)

Host Switch or host

Adds parity Checks parity

96

Layers, protocols, and interfaces

Physical layer

Data-link layer

Network layer

Transport layer

Application layer


sourceapplicatio

ntransportnetwork

linkphysical

HtHn M

segment Ht

datagram

destination

application

transportnetwork

linkphysical

HtHnHl M

HtHn M

Ht M

M

networklink

physical

linkphysical

HtHnHl M

HtHn M

HtHn M

HtHnHl M

router

switch

Encapsulationmessage M

Ht M

Hn

frame

Kurose and Ross 97

98

Service: interface between layers


99

What is a protocol reference model?

• A grouping of layers• Recall that we defined a layer as a

grouping of tasks.

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

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

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

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

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• Tasks and Layers• Mechanisms for each task • Protocols and Protocol reference modelsStandards

Status check

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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

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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

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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.


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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?

1 an overview of networks outline contemporary networks networks: single-link network to an...

Documents

shared single link

unshared singlelink

psatellite networks

networks sonet

cellular networks

vehicular networks

interconnect networks

types of networks