Module 14: Network Structures. 16.2 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Module 14: Network Structures Motivation Types of Distributed

Module 14: NetworkModule 14: Network StructuresStructures

16.2 Silberschatz, Galvin and Gagne ©2005Operating System Concepts

Module 14: Network StructuresModule 14: Network Structures

Motivation

Types of Distributed Operating Systems

Network Structure

Network Topology

Communication Structure

Communication Protocols

Robustness

Design Issues

An Example: Networking


Chapter ObjectivesChapter 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


A Distributed SystemA Distributed System


MotivationMotivation

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


컴퓨터 네트워크와 OS 의 관련성

Types of Network

Remote access network

dumb terminal connection

Shared resource network

intelligent node ( 예 , PC)

file transfer 와 IPC

Network OS vs. Distributed OS

MotivationMotivation


Network-Operating SystemsNetwork-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 SystemsDistributed-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.)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.


TopologyTopology

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 TopologyNetwork Topology


Network TypesNetwork 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.)Network Types (Cont.)

Depiction of typical LAN:


Network Types (Cont.)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


CommunicationCommunication

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 ResolutionNaming 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 StrategiesRouting 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.)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 StrategiesConnection 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.


ContentionContention

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.)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 표준네트워크 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 네트워크 구조 : : OSIOSI 와 다른 참조 모델와 다른 참조 모델 OSI

7 layer

peer-to-peer 접근법 세계적 표준

TCP/IP

UNIX, Internet 에 사용

SNA

층 구조 master-slave 기법


Communication ProtocolCommunication 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.)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.


Communication Via ISO Network Communication Via ISO Network ModelModel


The ISO Protocol LayerThe ISO Protocol Layer


응용층 응용층 (( 층 층 7)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 응용층 (( 층 층 7)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 VFS 에 대해 수행됨

Association Control Service Elements

하부의 presentation 서비스를 이용하여 두 응용간의 연관을 설정


End 시스템 간에 응용 데이터가 제시되는 구문을 선택

Presentation Layer (Presentation Layer ( 층 층 6)6)

통신처리가 아니라 응용처리에 주안을 두어 데이터의 흐름을 조직화 presentation entity 가 대화를 구성하고 조직하며 데이터 교환을

관리하는 수단을 제공 dialog control(half duplex, full duplex) 대화 내에 동기화 지점 설정

Session Layer ( 층 5)


transport 서비스는 우편 서비스와 유사 층 3 과 층 4 는 응용지향 또는 운영체제 지향의 상부 층과 물리적

지향의 하부 층간의 인터페이스를 제공 flow control : 전송속도를 제어하여 수신자의 용량을 넘지 않게함

Transport Transport 층 층 (( 층 층 4)4)

지능형 서브네트워크를 연결하여 end-to-end 트랜스포트 커넥션을 형성하는 기법을 제공

두가지 기법 연결지향 (Connection-oriented, ISO 8878) 무연결형 (Connectionless, ISO 8473)

네트워크 층 ( 층 3)


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 데이터 링크 층 (( 층 층 2)2)


Local Area Networks : 논리적 링크제어 LAN 은 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 protocol 플로우 제어나 에러시 재전송 기능 없음

class II : connection oriented protocol 위의 두 기능 모두 있음



Local Area Networks : Media Access Control

아래의 종류가 있음 contention (IEEE 802.3)

– CSMA/CD

– Ethernet 에 기반 token ring (IEEE 802.5)

– FDDI (Fiber Distributed Data Interface, ANSI X3.139) 는 100MBPS 의 고속 LAN 기술로 토큰 - 링 MAC 층을 사용

token bus (IEEE 802.4)

– 공장 자동화에 사용– 예 , MAP (Manufacturing Automation Protocol)



전송매체와의 ‘인터페이스’를 규정 MAU (Media Attachment Unit) : 표준적인 물리적 인터페이스를

비표준적 전송 매체와 인터페이스 시키는 장치 WAN 의 경우 : 모뎀

EIA RS-232C -- 모뎀 -- 전화선

물리 층 물리 층 (( 층 층 1)1)

baseband or broadband : 동축 케이블 twisted pair optical fiber

전송매체


The ISO Network MessageThe ISO Network Message


The TCP/IP Protocol LayersThe TCP/IP Protocol Layers


TCP/IP (Transmission Control Protocol / Internet Protocol)

DoD 가 개발한 프로토콜 suite

SMTP(Simple Mail Transfer Protocol), FTP 도 포함 ISODE (ISO development environment) : TCP/IP 상에 OSI 의

upper layer 들을 구현한 프로그램 대단히 널리 사용


Possible streams implementation of TCP/IP


The AT/T UNIX Networking Model


AT/T Streams Model


OSI Network ManagementOSI Network Management

Common Management Information Protocol 이 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)



RobustnessRobustness

Failure detection

Reconfiguration


Failure DetectionFailure 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)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.


ReconfigurationReconfiguration

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 IssuesDesign 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 ExampleNetworking 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.


An Ethernet PacketAn Ethernet Packet

Documents

Module 14: Network Structures. 16.2 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Module 14: Network Structures Motivation Types of Distributed