1. 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. 2 Protocol Stack 의 예

3. 3 Protocol “ Layers ” Networks are complex! many “ pieces ” : hosts routers links of various media applications protocols hardware, software

4. 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

5. 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

6. 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 Departing airport arriving airport intermediate air traffic sites airplane routing

7. 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. 8 OSI model  OSI 모델은 분산 응용 처리를 위하여 개방화 시스템 (open 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. 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. 10 Nested Protocol Header

11. 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. 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. 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. 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/IP 는 인터넷과 연결된 컴퓨터들의 공용어 [ 그림 3.3]

15. 15 인터네트 유래 미국방성에서 지원하는 알파네트 (Arpanet) 라 불리는 네트워크에서 유래 1970 년대 초에 TCP/IP 프로토콜 개발 1980 년대 슈퍼 컴퓨터들간의 네트워크인 NSFNET 와 연결되면서 오늘날 말하는 인터네트 (Internet) 라는 네트워크의 외향을 갖춤

16. 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 IP 주소  예 : htlim@kowon.dongseo.ac.kr  호스트 이름 형식 : 호스트 이름. 소속 단체. 단체 성격. 소속 국가 ---> 도메인 네임구조 준수

17. 17 도메인 네임 (Domain Name) 의 예

18. 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- Point Protocol) 사용

19. 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. 20 Layering: logical communication application transport network link physical application transport network link physical application transport network link physical application transport network link physical network link physical Each layer: distributed “ entities ” implement layer functions at each node entities perform actions, exchange messages with peers

21. 21 Layering: logical communication application transport network link physical application transport network link physical application transport network link physical application transport network link physical network link physical data E.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 ack

22. 22 Layering: physical communication application transport network link physical application transport network link physical application transport network link physical application transport network link physical network link physical data

23. 23 Protocol layering and data Each layer takes data from above adds header information to create new data unit passes new data unit to layer below application transport network link physical application transport network link physical source destination M M M M H t H t H n H t H n H l M M M M H t H t H n H t H n H l message segment datagram frame

24. 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 regional ISP local ISP local ISP

25. 25 National Backbone Provider e.g. BBN/GTE US backbone network

26. 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. 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. 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. 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. 30 Access networks and physical media Q: 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. 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. 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. 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. 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 base station mobile hosts router