1. Introduction and Overview

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최 양 희서울대학교 컴퓨터공학부

1. Introduction and Overview

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Internet Timeline

• 3000 BC : Abacus• 1642 : Pascal : numerical wheel calculator • 1822 : Babbage : difference engine• 1945 : Eckert : ENIAC• 1969 : first ARPANET connection

– UCLA, SRI, UCSB, Utah• 1972 : CYCLADES (France)• 1974 : V. Cerf : TCP

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Internet Timeline• 1977 : e-mail over Telenet• 1980 : IBM : Bitnet• 1981 : IBM : PC• 1982 : Time : Computer

• 1984 : Domain Name System• 1988 : Morris : Worm, and CERT• 1989 : number of hosts exceeds 100,000• 1991 : Al Gore : High Performance Computing Act• 1991 : Tim Berners-Lee : WWW• 1992 : Internet Society• 1993 : Andreessen : Mosaic• 1994 : Yahoo !

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인터넷 등장 배경

• 미 국방성의 요구 : 많은 통신 시설이 파괴되더라도 계속 살아 남는 통신 네트워크를 고안할 것 . 다양한 통신기술을 활용할 수 있을 것 .

• 과학기술계의 결론 – 패킷 (packet) 방식의 통신네트워크 – 연결을 만들지 않음

• 전체의 70% 이상이 파괴되어도 계속 생존하여 통신기능 유지함

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패킷이란 ?• Packet Circuit 대응개념• 보내야 할 정보를 취급하기 쉬운 작은 단위 ( 패킷이라 부름 ) 로 쪼갬

– 예 : 소설을 엽서로 옮겨 적어 보낸다 .– 예 : 많은 화물은 여러 대의 트럭에 나누어 실어 보낸다

• 통신망은 패킷을 일일이 따로 구분하여 처리– 엽서마다 보내는 이 , 받는 이의 주소를 반복하여 적고 우체국은 엽서마다

이를 확인한다• 받는 쪽에서 정보를 모아서 원래대로 재생• 왜 패킷인가 ?

– 전송효율 / 네트워크 구축비용 절감 : 화물회사마다 전용차선이 있다면 낭비

– 다양한 트래픽을 쉽게 지원 : 전보 , 편지 , 음성 , 영상 , 컴퓨터 데이타

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패킷 통신의 예

A A

BB

B BA A A• •

•

• 회선 공유

AA

B B

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Internet Configuration

subnet

subnet

subnet

subnet

subnet

Host

Host

Packet

Packet

Router

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Internet structure: network of networks

• roughly hierarchical• national/international

backbone providers (NBPs)– e.g. BBN/GTE, Sprint, AT&T,

IBM, UUNet– interconnect (peer) with each

other privately, or at public Network Access Point (NAPs)

• regional ISPs– connect into NBPs

• local ISP, company– connect into regional ISPs

NBP A

NBP BNAP NAP

regional ISP

regional ISP

localISP

localISP

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Protocols

• Protocol: rules for communication– Message formats, timing– Describes how a computer responds when a

message arrives– Specifies how a computer handles errors or

other abnormal conditions• All network services are described by

protocols

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What’s a protocol?a human protocol and a computer network protocol:

Hi

HiGot thetime?2:00

TCP connection req.TCP connectionreply.Get http://gaia.cs.umass.edu/index.htm

<file>time

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Key Elements of a Protocol

• Syntax– Data formats– Signal levels

• Semantics– Control information– Error handling

• Timing– Speed matching– Sequencing

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A Three Layer Model

• Network Access Layer• Transport Layer• Application Layer

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Simplified File Transfer Architecture

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Network Access Layer

• Exchange of data between the computer and the network

• Sending computer provides address of destination

• May invoke levels of service• Dependent on type of network used (LAN, packet

switched etc.)

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Transport Layer

• Reliable data exchange• Independent of network being used• Independent of application

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Application Layer

• Support for different user applications• e.g. e-mail, file transfer

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OSI• Open Systems Interconnection• Developed by the International Organization for Standardization (ISO)• Seven layers• A theoretical system delivered too late!• TCP/IP is the de facto standard

• A layer model• Each layer performs a subset of the required communication

functions• Each layer relies on the next lower layer to perform more primitive

functions• Each layer provides services to the next higher layer• Changes in one layer should not require changes in other layers

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OSI Layers

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Internet protocol stack• application: supporting network

applications– ftp, smtp, http

• transport: host-host data transfer– tcp, udp

• network: routing of packets from source to destination– ip, routing protocols

• link: data transfer between neighboring network elements– ppp, Ethernet

• physical: bits on the wire

application

transport

network

link

physical

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OSI v TCP/IP

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Some Protocols in TCP/IP Suite

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Access Technologies

• Dialup via modem• DSL: digital subscriber line : ADSL, VDSL• Cable Modem• LAN (wired, wireless : IEEE 802), WiBro (mobile WiMax) 802.16e• Wireless Mesh• Bluetooth• Satellite• Cellular (2, 2.5, 3, 4G)• FTTH• Sensor network

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Data Link Layer• two physically connected devices:

– host-router, router-router, host-host• unit of data: frame

applicationtransportnetwork

linkphysical

networklink

physical

MMMM

HtHtHnHtHnHl MHtHnHl

framephys. link

data linkprotocol

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Link Layer Services

• Framing, link access: – encapsulate datagram into frame, adding header, trailer– implement channel access if shared medium, – ‘physical addresses’ used in frame headers to identify

source, destination • different from IP address

• Reliable delivery between two physically connected devices:

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Link Layer Services (more)

• Flow Control: – pacing between sender and receivers

• Error Detection: – errors caused by signal attenuation, noise. – receiver detects presence of errors:

• signals sender for retransmission or drops frame

• Error Correction: – receiver identifies and corrects bit error(s) without

resorting to retransmission

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Stop and Wait

• The source transmits one frame• The destination receives one frame and replies

with an acknowledgement• The source waits for ACK before sending the next

frame• The destination can stop the flow by not sending

ACK

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Sliding Windows Flow Control

• Allow multiple frames to be in transit• Receiver has buffer (size = W frames)• Transmitter can send up to W frames without ACK• Each frame is numbered• ACK includes the number of next frame expected• Sequence number

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Error Control

• Detection and correction of errors• Lost frames• Damaged frames• Automatic repeat request

– Error detection– Positive acknowledgment– Retransmission after timeout– Negative acknowledgement and retransmission

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Automatic Repeat Request (ARQ)

• Stop and wait• Go back N• Selective reject (selective retransmission)

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Stop and Wait -Diagram

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Go Back N • Based on sliding window• If no error, ACK as usual with the number of the

next frame expected• Use window to control the number of outstanding

frames• If error, reply with rejection

– Discard the bad frame and all future frames until the error frame is received correctly

– Transmitter must go back and retransmit the bad frame and all subsequent frames

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Go Back N : Diagram

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Ethernet : CSMA/CD

• Carriers Sense Multiple Access with Collision Detection• IEEE 802.3

– Random Access• Stations access medium randomly

– Contention• Stations contend for time on medium

1. If medium idle, transmit, otherwise, step 22. If busy, listen for idle, then transmit3. If collision detected, jam then cease transmission4. After jam, wait random time then start from step 1

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Hubs• Physical Layer devices: essentially repeaters operating at

bit levels: repeat received bits on one interface to all other interfaces

• Hubs can be arranged in a hierarchy (or multi-tier design), with backbone hub at its top

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IEEE 802.11 Wireless LAN• IEEE 802.11 standard:

– MAC protocol– unlicensed frequency spectrum: 900Mhz, 2.4Ghz

• Basic Service Set (BSS) – wireless hosts– access point (AP)