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Module 14: Network Structures. Module 14: Network Structures. Motivation Types of Distributed Operating Systems Network Structure Network Topology Communication Structure Communication Protocols Robustness Design Issues An Example: Networking. Chapter Objectives.
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Module 14: Network Structures • Motivation • Types of Distributed Operating Systems • Network Structure • Network Topology • Communication Structure • Communication Protocols • Robustness • Design Issues • An Example: Networking
Chapter Objectives • To provide a high-level overview of distributed systems and the networks that interconnect them • To discuss the general structure of distributed operating systems
Motivation • Resource sharing • sharing and printing files at remote sites • processing information in a distributed database • using remote specialized hardware devices • Computation speedup – load sharing • Reliability – detect and recover from site failure, function transfer, reintegrate failed site • Communication – message passing
Motivation • 컴퓨터 네트워크와 OS의 관련성 • Types of Network • Remote access network • dumb terminal connection • Shared resource network • intelligent node (예, PC) • file transfer와 IPC • Network OS vs. Distributed OS
Network-Operating Systems • Users are aware of multiplicity of machines. Access to resources of various machines is done explicitly by: • Remote logging into the appropriate remote machine. • Transferring data from remote machines to local machines, via the File Transfer Protocol (FTP) mechanism.
Distributed-Operating Systems • Users not aware of multiplicity of machines. Access to remote resources similar to access to local resources. • Data Migration – transfer data by transferring entire file, or transferring only those portions of the file necessary for the immediate task. • Computation Migration – transfer the computation, rather than the data, across the system.
Distributed-Operating Systems (Cont.) • Process Migration – execute an entire process, or parts of it, at different sites. • Load balancing – distribute processes across network to even the workload. • Computation speedup – subprocesses can run concurrently on different sites. • Hardware preference – process execution may require specialized processor. • Software preference – required software may be available at only a particular site. • Data access – run process remotely, rather than transfer all data locally.
Topology • Sites in the system can be physically connected in a variety of ways; they are compared with respect to the following criteria: • Basic cost. How expensive is it to link the various sites in the system? • Communication cost. How long does it take to send a message from site A to site B? • Reliability. If a link or a site in the system fails, can the remaining sites still communicate with each other? • The various topologies are depicted as graphs whose nodes correspond to sites. An edge from node A to node B corresponds to a direct connection between the two sites.
Network Types • Local-Area Network (LAN) – designed to cover small geographical area. • Multiaccess bus, ring, or star network. • Speed 10 megabits/second, or higher. • Broadcast is fast and cheap. • Nodes: • usually workstations and/or personal computers • a few (usually one or two) mainframes.
Network Types (Cont.) • Depiction of typical LAN:
Network Types (Cont.) • Wide-Area Network (WAN) – links geographically separated sites. • Point-to-point connections over long-haul lines (often leased from a phone company). • Speed 100 kilobits/second. • Broadcast usually requires multiple messages. • Nodes: • usually a high percentage of mainframes
Communication • Naming and name resolution: How do two processes locate each other to communicate? • Routing strategies. How are messages sent through the network? • Connection strategies. How do two processes send a sequence of messages? • Contention. The network is a shared resource, so how do we resolve conflicting demands for its use? The design of a communication network must address four basic issues:
Naming and Name Resolution • Name systems in the network • Address messages with the process-id. • Identify processes on remote systems by <host-name, identifier> pair. • Domain name service (DNS) – specifies the naming structure of the hosts, as well as name to address resolution (Internet).
Routing Strategies • Fixed routing. A path from A to B is specified in advance; path changes only if a hardware failure disables it. • Since the shortest path is usually chosen, communication costs are minimized. • Fixed routing cannot adapt to load changes. • Ensures that messages will be delivered in the order in which they were sent. • Virtual circuit. A path from A to B is fixed for the duration of one session. Different sessions involving messages from A to B may have different paths. • Partial remedy to adapting to load changes. • Ensures that messages will be delivered in the order in which they were sent.
Routing Strategies (Cont.) • Dynamic routing. The path used to send a message form site A to site B is chosen only when a message is sent. • Usually a site sends a message to another site on the link least used at that particular time. • Adapts to load changes by avoiding routing messages on heavily used path. • Messages may arrive out of order. This problem can be remedied by appending a sequence number to each message.
Connection Strategies • Circuit switching. A permanent physical link is established for the duration of the communication (i.e., telephone system). • Message switching. A temporary link is established for the duration of one message transfer (i.e., post-office mailing system). • Packet switching. Messages of variable length are divided into fixed-length packets which are sent to the destination. Each packet may take a different path through the network. The packets must be reassembled into messages as they arrive. • Circuit switching requires setup time, but incurs less overhead for shipping each message, and may waste network bandwidth. Message and packet switching require less setup time, but incur more overhead per message.
Contention • CSMA/CD. Carrier sense with multiple access (CSMA); collision detection (CD) • A site determines whether another message is currently being transmitted over that link. If two or more sites begin transmitting at exactly the same time, then they will register a CD and will stop transmitting. • When the system is very busy, many collisions may occur, and thus performance may be degraded. • CSMA/CD is used successfully in the Ethernet system, the most common network system. Several sites may want to transmit information over a link simultaneously. Techniques to avoid repeated collisions include:
Contention (Cont.) • Token passing. A unique message type, known as a token, continuously circulates in the system (usually a ring structure). • A site that wants to transmit information must wait until the token arrives. • When the site completes its round of message passing, it retransmits the token. • A token-passing scheme is used by the IBM and Apollo systems. • Message slots. A number of fixed-length message slots continuously circulate in the system (usually a ring structure). • Since a slot can contain only fixed-sized messages, a single logical message may have to be broken down into a number of smaller packets, each of which is sent in a separate slot. • This scheme has been adopted in the experimental Cambridge Digital Communication Ring
네트워크 표준 • 표준기구 • ISO(Int’l Organization for Standardization) • CCITT(Int’l Telegraph and Telephone Consultative Committee) • ANSI, EIA, IEEE, NIST • ISO 7498 (OSI 참조모델) • ISO 9646 (Conformance Standards)
네트워크 구조 : OSI와 다른 참조 모델 • OSI • 7 layer • peer-to-peer 접근법 • 세계적 표준 • TCP/IP • UNIX, Internet에 사용 • SNA • 층 구조 • master-slave 기법
Communication Protocol • Physical layer – handles the mechanical and electrical details of the physical transmission of a bit stream. • Data-link layer – handles the frames, or fixed-length parts of packets, including any error detection and recovery that occurred in the physical layer. • Network layer – provides connections and routes packets in the communication network, including handling the address of outgoing packets, decoding the address of incoming packets, and maintaining routing information for proper response to changing load levels. The communication network is partitioned into the following multiple layers;
Communication Protocol (Cont.) • Transport layer – responsible for low-level network access and for message transfer between clients, including partitioning messages into packets, maintaining packet order, controlling flow, and generating physical addresses. • Session layer – implements sessions, or process-to-process communications protocols. • Presentation layer – resolves the differences in formats among the various sites in the network, including character conversions, and half duplex/full duplex (echoing). • Application layer – interacts directly with the users’ deals with file transfer, remote-login protocols and electronic mail, as well as schemas for distributed databases.
응용층 (층 7) • 분산 OS 기능에 대한 인터페이스 • open a remote file • call a remote procedure • SASE(Specific Application Service Elements) • 예) MHS(Message Handling Subsystem), FTAM(File Transfer, Access and Mangmt), VTP(Virtual Terminal Protocol), MMS(Manufacturing Message Service), DS(Directory Service),TP(Transaction Processing), NM(Network Management)
응용층 (층 7) • 대표적 SASE : FTAM • ISO 8571 • 단순한 파일 전송과 상이한 분산 데이터베이스 환경에서 복잡한 조작에도 적용 • VFS(Virtual File Store) 이용 • 개념적인 계층적 데이터 구조 • 연산은 원격시스템의 VFS에 대해 수행됨 • Association Control Service Elements • 하부의 presentation 서비스를 이용하여 두 응용간의 연관을 설정
Presentation Layer (층 6) • End 시스템 간에 응용 데이터가 제시되는 구문을 선택 Session Layer (층 5) • 통신처리가 아니라 응용처리에 주안을 두어 데이터의 흐름을 조직화 • presentation entity가 대화를 구성하고 조직하며 데이터 교환을 관리하는 수단을 제공 • dialog control(half duplex, full duplex) • 대화 내에 동기화 지점 설정
Transport 층 (층 4) • transport 서비스는 우편 서비스와 유사 • 층3과 층4는 응용지향 또는 운영체제 지향의 상부 층과 물리적 지향의 하부 층간의 인터페이스를 제공 • flow control : 전송속도를 제어하여 수신자의 용량을 넘지 않게함 네트워크 층 (층 3) • 지능형 서브네트워크를 연결하여 end-to-end 트랜스포트 커넥션을 형성하는 기법을 제공 • 두가지 기법 • 연결지향(Connection-oriented, ISO 8878) • 무연결형(Connectionless, ISO 8473)
데이터 링크 층 (층 2) • 직접 연결된 시스템간의 데이터를 전송하고 에러를 탐지 • Wide Area Networks : High-Level Data Link Control • 사용 통신선 • 전화선 • fiber optic cable • 위성통신 • 아래의 연결 기법 사용 • 전용선 • circuit-switched (예, dial up service) • packet switching
데이터 링크 층 (층 2) • Local Area Networks : 논리적 링크제어 • LAN은 소규모 지역 (예, 캠퍼스, 빌딩)의 고속 통신에 이용 • 구별 기준 • MAC(media access control) method : • 공통매체에 대한 접근을 제어하는 원칙 • transmission method • broad band • base band • medium • twisted-pair cable • 동축 케이블 • 광섬유 케이블 • Logical Link Control (IEEE 802.2) • class I : connection-less link protocol플로우 제어나 에러시 재전송 기능 없음 • class II : connection oriented protocol 위의 두 기능 모두 있음
데이터 링크 층 (층 2) • Local Area Networks : Media Access Control • 아래의 종류가 있음 • contention (IEEE 802.3) • CSMA/CD • Ethernet에 기반 • token ring (IEEE 802.5) • FDDI (Fiber Distributed Data Interface, ANSI X3.139)는 100MBPS의 고속 LAN 기술로 토큰-링 MAC 층을 사용 • token bus (IEEE 802.4) • 공장 자동화에 사용 • 예, MAP (Manufacturing Automation Protocol)
물리 층 (층 1) • 전송매체와의 ‘인터페이스’를 규정 • MAU (Media Attachment Unit) : 표준적인 물리적 인터페이스를 비표준적 전송 매체와 인터페이스 시키는 장치 • WAN의 경우 : 모뎀 • EIA RS-232C -- 모뎀 -- 전화선 전송매체 • baseband or broadband : 동축 케이블 • twisted pair • optical fiber
TCP/IP (Transmission Control Protocol / Internet Protocol) • DoD가 개발한 프로토콜 suite • SMTP(Simple Mail Transfer Protocol), FTP도 포함 • ISODE (ISO development environment) : TCP/IP상에 OSI의 upper layer들을 구현한 프로그램 • 대단히 널리 사용
OSI Network Management • Common Management Information Protocol이 네트워크 관리명령을 규정 • Configuration Management : naming, activating and deactivation of entities at each layer • Fault 관리 • 성능 관리 • Security 관리와 계정관리 • SNMP (Simple Network Management Protocol) • SNMP v2 (= SMP)
Robustness • Failure detection • Reconfiguration
Failure Detection • Detecting hardware failure is difficult. • To detect a link failure, a handshaking protocol can be used. • Assume Site A and Site B have established a link. At fixed intervals, each site will exchange an I-am-up message indicating that they are up and running. • If Site A does not receive a message within the fixed interval, it assumes either (a) the other site is not up or (b) the message was lost. • Site A can now send an Are-you-up? message to Site B. • If Site A does not receive a reply, it can repeat the message or try an alternate route to Site B.
Failure Detection (cont) • If Site A does not ultimately receive a reply from Site B, it concludes some type of failure has occurred. • Types of failures:- Site B is down - The direct link between A and B is down- The alternate link from A to B is down - The message has been lost • However, Site A cannot determine exactly why the failure has occurred.
Reconfiguration • When Site A determines a failure has occurred, it must reconfigure the system: 1. If the link from A to B has failed, this must be broadcast to every site in the system. 2. If a site has failed, every other site must also be notified indicating that the services offered by the failed site are no longer available. • When the link or the site becomes available again, this information must again be broadcast to all other sites.
Design Issues • Transparency – the distributed system should appear as a conventional, centralized system to the user. • Fault tolerance – the distributed system should continue to function in the face of failure. • Scalability – as demands increase, the system should easily accept the addition of new resources to accommodate the increased demand. • Clusters – a collection of semi-autonomous machines that acts as a single system.
Networking Example • The transmission of a network packet between hosts on an Ethernet network. • Every host has a unique IP address and a corresponding Ethernet (MAC) address. • Communication requires both addresses. • Domain Name Service (DNS) can be used to acquire IP addresses. • Address Resolution Protocol (ARP) is used to map MAC addresses to IP addresses. • If the hosts are on the same network, ARP can be used. If the hosts are on different networks, the sending host will send the packet to a router which routes the packet to the destination network.
