1

3 장 . OSI 참조모델과 인터넷 OSI 참조모델 (Reference Model)

개념 종전까지 타사의 제품끼리는 네트워크 또는 구조가

서로 호환이 되지 않아 통신이 되지 않은 결과를 초래

1970 년대 후반 국제 표준화 기구 (ISO) 는 네 트워크 설계의 호환성을 증진시키기 위해 개방시스템 상호접속 참조모델 (OSI) 라 불리우는 구조를 제안

[ 그림 3.1]

2

Protocol Stack의 예

3

Protocol “Layers”Networks are complex!

many “pieces”: hosts routers links of various media

applications protocols hardware, software

4

Organization of air travel

a series of steps

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing

5

Organization of air travel: a different view

Layers: each layer implements a service via its own internal-layer actions relying on services provided by layer below

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing

6

Distributed implementation of layer functionality

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing

Dep

art

ing

air

port

arr

ivin

g

air

port

intermediate air traffic sites

airplane routing airplane routing

7

Why layering?Dealing with complex systems: explicit structure allows identification, relationship of complex system’s pieces layered reference model for discussion

modularization eases maintenance, updating of system change of implementation of layer’s service transparent to rest of system

e.g., change in gate procedure doesn’t affect rest of system

layering considered harmful?

8

OSI model OSI 모델은 분산 응용 처리를 위하여 개방화 시스템 (o

pen system) 을 연결하는 데 있어서 기본모델을 제공 7 계층으로 구성

물리 (Physical) 계층 데이타링크 (Datalink) 계층 네트워크 (Network) 계층 트랜스포트 (Transport) 세션 (Session) 계층 표현 (Presentation) 계층 응용 (Application) 계층

9

논리적 구조 [ 그림 3.2] N 계층의 프로토콜 엔티티

상대방의 동일층 사이간의 통신기능 상위층과 하위층의 프로그램 사이의 데이터와 제어정보를 송수신하는 기능

상위계층 (N+1) 은 현재층 (N) 의 서비스 사용자 (service user) 가 되며 현재층은 서비스 제공자 (service provider)

상하위 계층간의 주고 받는 정보를 N 서비스 프리미티브 (primitive)

10

Nested Protocol Header

11

OSI 7 계층 물리 계층 (Physical Layer)

다른 개방 시스템에 전기적 신호를 전달하는 기능 데이터 링크 계층 (Data Link Layer)

순서 제어 오류 제어 흐름 제어 프레임 동기

12

네트워크 계층 (Network Layer) routing, addressing 기능

트랜스포트 계층 (Transport Layer) 종점간 (end-to-end) 전송에만 관계

데이타 교환을 위해 신뢰성 있는 메커니즘을 제공

세션 계층 (Session Layer)

대화 형태 : 전이중 (duplex), 반이중 (half-duplex), 심플렉스 (simplex)

checkpoint 메커니즘을 제공

13

표현 계층 (Presentation Layer)

데이타 표현과 형식의 차이를 해결

데이터를 암호화하거나 압축하는 기능

응용 계층 (Application Layer)

파일 전송 프로토콜 (FTAM : File Transfer Access and Management)

가상 단말 (VT : Virtual Terminal) 프로토콜

14

인터넷 프로토콜 구조 Transmission and Control Protocol/ Internet Protocol TCP/IP 는 다양한 형태의 컴퓨터들이 연결될 때 통신방식을 통일하기 위해 국제적으로 사용하고 있는 통신방식

인터넷과 연결된 모든 컴퓨터는 TCP/IP 통신방식 준수

TCP/IP는 인터넷과 연결된 컴퓨터들의 공용어

[그림 3.3]

15

인터네트 유래 미국방성에서 지원하는 알파네트 (Arpanet)라 불리는 네트워크에서 유래

1970 년대 초에 TCP/IP 프로토콜 개발1980 년대 슈퍼 컴퓨터들간의 네트워크인

NSFNET 와 연결되면서 오늘날 말하는 인터네트 (Internet) 라는 네트워크의 외향을 갖춤

16

인터넷 주소 인터네트에는 모든 컴퓨터 또는 호스트들이 호스트 이름 또는 IP 주소를 소유

예 : kowon.dongseo.ac.kr, 203.241.176.13 전자우편 주소 형식 : 사용자 ID @ 호스트의 I

P 주소 예 : htlim@kowon.dongseo.ac.kr

호스트 이름 형식 : 호스트 이름 . 소속 단체 . 단체 성격 . 소속 국가 ---> 도메인 네임구조 준수

17

도메인 네임 (Domain Name)의 예

com edu gov kr(한국) fr jp

co ac(교육기관) go hm

snu dongguk dongseo(동서대학교) ehwa

syscom kowon www enter conven

예 : kowon.dongseo.ac.kr

인터네트 주소

(미국은 나라이름을 붙이지않는다)

18

인터넷 접속방법 전용선을 이용한 인터넷 접속

주로 기관 ( 대학 , 연구소 , 회사등 ) 에서 이용 대부분 자체 네트웍인 LAN(Local Atrea Network) 을 구축

전용회선의 속도 : 보통 56K BPS(Bit/second) 이상 모뎀을 이용한 인터넷 사용

가정이나 소규모의 회사 SLIP(Serial Line Internet Protocol) 과 PPP(Point-to-P

oint Protocol) 사용

19

Internet protocol stack application: supporting network applications ftp, smtp, http

transport: host-host data transfer tcp, udp

network: routing of datagrams 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

20

Layering: logical communication

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

Each layer: distributed “entities” implement layer functions at each node

entities perform actions, exchange messages with peers

21

Layering: logical communication

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

data

dataE.g.: transport

take data from app

add addressing, reliability check info to form “datagram”

send datagram to peer

wait for peer to ack receipt

analogy: post office

data

transport

transport

ack

22

Layering: physical communication

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

data

data

23

Protocol layering and dataEach layer takes data from above adds header information to create new data unit

passes new data unit to layer below

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

source destination

M

M

M

M

Ht

HtHn

HtHnHl

M

M

M

M

Ht

HtHn

HtHnHl

message

segment

datagram

frame

24

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 B

NAP NAP

regional ISP

regional ISP

localISP

localISP

25

National Backbone Providere.g. BBN/GTE US backbone network

26

Internet History

1961: Kleinrock - queueing theory shows effectiveness of packet-switching

1964: Baran - packet-switching in military nets

1967: ARPAnet conceived by Advanced Reearch Projects Agency

1969: first ARPAnet node operational

1972: ARPAnet demonstrated publicly

NCP (Network Control Protocol) first host-host protocol

first e-mail program

ARPAnet has 15 nodes

1961-1972: Early packet-switching principles

27

Internet History

1970: ALOHAnet satellite network in Hawaii

1973: Metcalfe’s PhD thesis proposes Ethernet

1974: Cerf and Kahn - architecture for interconnecting networks

late70’s: proprietary architectures: DECnet, SNA, XNA

late 70’s: switching fixed length packets (ATM precursor)

1979: ARPAnet has 200 nodes

Cerf and Kahn’s internetworking principles: minimalism, autonomy - no internal changes required to interconnect networks

best effort service model

stateless routers decentralized control

define today’s Internet architecture

1972-1980: Internetworking, new and proprietary nets

28

Internet History

1983: deployment of TCP/IP

1982: smtp e-mail protocol defined

1983: DNS defined for name-to-IP-address translation

1985: ftp protocol defined

1988: TCP congestion control

new national networks: Csnet, BITnet, NSFnet, Minitel

100,000 hosts connected to confederation of networks

1980-1990: new protocols, a proliferation of networks

29

Internet History

Early 1990’s: ARPAnet decomissioned

1991: NSF lifts restrictions on commercial use of NSFnet (decommissioned, 1995)

early 1990s: WWW hypertext [Bush 1945, Nelson 1960’s]

HTML, http: Berners-Lee

1994: Mosaic, later Netscape

late 1990’s: commercialization of the WWW

Late 1990’s: est. 50 million computers on Internet

est. 100 million+ users

backbone links runnning at 1 Gbps

1990’s: commercialization, the WWW

30

Access networks and physical mediaQ: How to connection end systems to edge router?

residential access nets

institutional access networks (school, company)

mobile access networks

Keep in mind: bandwidth (bits per second) of access network?

shared or dedicated?

31

Residential access: point to point access

Dialup via modem up to 56Kbps direct access to router (conceptually)

ISDN: intergrated services digital network: 128Kbps all-digital connect to router

ADSL: asymmetric digital subscriber line up to 1 Mbps home-to-router

up to 8 Mbps router-to-home

32

Residential access: cable modems

HFC: hybrid fiber coax asymmetric: up to 10Mbps upstream, 1 Mbps downstream

network of cable and fiber attaches homes to ISP router shared access to router among home

issues: congestion, dimensioning

deployment: available via cable companies, e.g., MediaOne

33

Institutional access: local area networks

company/univ local area network (LAN) connects end system to edge router

Ethernet: shared or dedicated cable connects end system and router

10 Mbs, 100Mbps, Gigabit Ethernet

deployment: institutions, home LANs soon

34

Wireless access networks shared wireless access network connects end system to router

wireless LANs: radio spectrum replaces wire

e.g., Lucent Wavelan 10 Mbps

wider-area wireless access CDPD(Cellular Digital Packet data): wireless access to ISP router via cellular network

basestation

mobilehosts

router