cs551 distributed operating systems

CS551Distributed Operating Systems

Colorado State University

at Lockheed-Martin

Lecture 1 -- Spring 2001

24 January 2001 CS-551, Lecture 1 2

CS551: Lecture 1

Topics– Introduction; Syllabus; G-25 Forms

Homework; Reading Reports; Project

– Galli: Chapters 1, 2, 3, 4, 5, 7, 8 (maybe), 10 Some networking topics included in course

– What is a distributed system? A network?– What is a protocol?

ISO OSI Protocol TCP/IP


What is a distributed system?

A collection of independent computers A communication facility to pass messages No shared memory No shared clock Each computer has its own operating

system


Why have distributed systems?

Price / Performance Resource sharing Faster response time Improved reliability Modular expandability


Distributed system organizations

Microcomputer model– several multiuser systems

Workstations/PCs model– each user has own WS/PC to do work– each user shares files and other resources

Processor pool model LANs, MANs, WANs, WWW


Figure 1.1 Computers in a Networked Environment. (Galli, p. 3)


Figure 1.2 Connecting LAN Subnets with a Backbone. (Galli, p.6)


Figure 1.3 Common Wired LAN Topologies. (Galli, p.7)

Figure 1.3 Common Wired LAN Topologies.


Distributed Operating Systems

Appears to users as a single system on a single machine

A virtual uniprocessor Users do not know where files are located Users don’t know where jobs are executed


Issues in Distributed O.S.

– Global Knowledge– Naming– Scalability– Compatibility– Process synchronization– Resource management– Security– Structuring


Issues: Global Knowledge

Unable to determine up-to-date global state– no global memory– no common clock– unpredictable message delays

Need device-efficient distributed control– e.g. how to get a concensus

Need method for ordering events


Issues: Naming

All objects are named Need to map name onto its location Need a directory (or directories)

– replicated (to maintain consistency)– versus– partitioned (which partition helps me?)


Issues: Scalability, Process Synch Scalability

– Can system grow without performance degradation?

– Want to avoid centralized components Process synchronization

– Enforce mutual exclusion to shared resources– Deal with potential for deadlock


Issues: Compatibility

Possible at different levels Binary level: all processing elements run

same binary code Execution level: same source code can be

compiled and run on all nodes Protocol level: all processing elements

support same protocols


Issues: Resource management

Data migration: bring data to the location– distributed file system– distributed shared memory

Computation migration– e.g. RPC– e.g. send a query for info computed remotely instead

of requesting raw data Distributed scheduling

– process migration


Issues: Security

Authentication– verify user identification

Authorization– determine user privileges


Issues: Structuring

Monolithic kernel– each node doesn’t need entire kernel

Collective kernel– O.S. services are processes– microkernel supports messages between such

processes Object-oriented

– O.S. services are a collection of objects


Client-Server vs. Peer-To-Peer

Client-Server– Similar to collective kernel distributed O.S.– Servers respond to requests from clients

Peer-to-Peer– An extension of client/server model– A many-to-many relationship between nodes


Figure 1.6 Client/Server Model. (Galli, p.13)


Figure 1.7 Peer-to-Peer Model.(Galli, p.14)


What is a network?

A form of a distributed system Connected nodes may be homogeneous or

heterogeneous Nodes may be some distance apart A network may consist of other networks LANs, MANs, WANs The Internet is a WAN: the WWW


Layered Network Models

Used to describe network functions Used to reduce network complexity Each layer logically communicates with the

corresponding layer on the remote host Messages

– enveloped while passed down through the local host layers

– stripped down to original message while passed up through remote host layers


Networks: Layered Models

ApplicationLayer

O.S. Layer

ApplicationLayer

InterconnectLayer

O.S. Layer

InterconnectLayer

virtual path

virtual path

physical path


What is a protocol?

A set of rules A method for

– establishing a connection between two sites– sending a communication over the connection– acknowledging receipt of message– terminating the connection

Example: a telephone call Examples: ISO/OSI; TCP/IP; UDP; SMTP


ISO / OSI Protocol

Probably most popular network protocol model

Implementation often takes efficiency-related shortcuts

Includes seven layers, grouped into 3 types– application– operating system– communication service


OSI / ISO Layers

Application– Application layer -- user programs– Presentation layer -- common data transformations

Operating system– Session layer -- process-to-process communication– Transport layer -- reliable host-to-host

communication


ISO / OSI Layers, continued

Communication service– Network layer -- packets, routing– Data Link layer -- reliability, flow control– Physical layer -- hardware to move a bit stream

between nodes– Needed by any network node, even a store-and-

forward node– May exist partly as hardware


Figure 1.4 The ISO/OSI Reference Model. (Galli, p. 9)


ISO/OSI Layers: Application

Miscellaneous applications– FTP (file transfer protocol)– remote login: rlogin– browsers: Netscape, Internet Explorer– email (via SMTP)– RJE (remote job entry)


ISO/OSI Layers: Presentation

Common data transformations– data compression– encryption– big/little Endian


ISO/OSI Layers: Session

Process-to-Process Communication– buffering

Some synchronization– synchronous data communication


ISO/OSI Layers: Transport

Reliable site-to-site communication


ISO/OSI Layers: Network

Logical path for communication– converts frames --> packets --> frames– X.25 connection-oriented– IP connectionless– used for WANs; redundant for LANs


ISO/OSI Layers: Data Link

Reliable data transmission– message goes out in frames

character count -- header specifies length character stuffing -- special character at end bit stuffing -- special bit sequence at end

– on LANs can put out a special synch signal– adds a checksum to trailer to detect errors


ISO/OSI Layers: Data Link, cont.

Flow control– synchronizes message passing activity– stop-and-wait -- sender waits for receiver’s

permission (inefficient for large transmissions)– sliding window -- allows several outstanding

unacknowledged frames (needs sequence #s)– HDLC (high level data link control) -- balanced,

permits two-way simultaneous message passing, acknowledgments in frame headers, errors results in resend requests


ISO/OSI Layers: Physical

Raw bit-stream communication– circuit switching

reserves a fixed communication at start releases path at end best for long, continuous stream

– packet switching demands access when ready to send packet of info packet may contain 10 - 1000 bytes may need several packets best for bursty, short communication


Figure 1.5 TCP/IP Relationship to ISO/OSI Reference Model. (Galli, p. 12)

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cs551 distributed operating systems

Documents

lecture 1cs551

single system

global knowledgeunable

common wired lan topologies

workeach user shares

processing elements

lan subnets

nodesprotocol level