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CS551
Distributed Operating Systems
Colorado State University
at Lockheed-Martin
Lecture 1 -- Spring 2001
CS551: Lecture 1
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Topics
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Introduction; Syllabus; G-25 Forms
 Homework;
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Galli: Chapters 1, 2, 3, 4, 5, 7, 8 (maybe), 10
 Some
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Reading Reports; Project
networking topics included in course
What is a distributed system? A network?
What is a protocol?
 ISO
OSI Protocol
 TCP/IP
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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
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Why have distributed systems?
Price / Performance
 Resource sharing
 Faster response time
 Improved reliability
 Modular expandability
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Distributed system organizations
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Microcomputer model
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several multiuser systems
Workstations/PCs model
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each user has own WS/PC to do work
each user shares files and other resources
Processor pool model
 LANs, MANs, WANs, WWW
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Figure 1.1 Computers in a
Networked Environment. (Galli, p. 3)
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Figure 1.2 Connecting LAN
Subnets with a Backbone. (Galli,
p.6)
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Figure 1.3 Common Wired
LAN Topologies. (Galli, p.7)
Figure 1.3 Common Wired LAN Topologies.
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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
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Issues in Distributed O.S.
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Global Knowledge
Naming
Scalability
Compatibility
Process synchronization
Resource management
Security
Structuring
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Issues: Global Knowledge
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Unable to determine up-to-date global state
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Need device-efficient distributed control
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no global memory
no common clock
unpredictable message delays
e.g. how to get a concensus
Need method for ordering events
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Issues: Naming
All objects are named
 Need to map name onto its location
 Need a directory (or directories)
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replicated (to maintain consistency)
versus
partitioned (which partition helps me?)
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Issues: Scalability, Process Synch
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Scalability
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Can system grow without performance
degradation?
Want to avoid centralized components
Process synchronization
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Enforce mutual exclusion to shared resources
Deal with potential for deadlock
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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
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Issues: Resource management
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Data migration: bring data to the location
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Computation migration
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distributed file system
distributed shared memory
e.g. RPC
e.g. send a query for info computed remotely
instead of requesting raw data
Distributed scheduling
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process migration
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Issues: Security
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Authentication
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verify user identification
Authorization
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determine user privileges
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Issues: Structuring
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Monolithic kernel
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Collective kernel
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each node doesn’t need entire kernel
O.S. services are processes
microkernel supports messages between such
processes
Object-oriented
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O.S. services are a collection of objects
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Client-Server vs. Peer-To-Peer
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Client-Server
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Similar to collective kernel distributed O.S.
Servers respond to requests from clients
Peer-to-Peer
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An extension of client/server model
A many-to-many relationship between nodes
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Figure 1.6 Client/Server Model.
(Galli, p.13)
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Figure 1.7 Peer-to-Peer Model.
(Galli, p.14)
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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
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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
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enveloped while passed down through the local
host layers
stripped down to original message while passed
up through remote host layers
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Networks: Layered Models
Application
Layer
O.S. Layer
Interconnect
Layer
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virtual path
virtual path
physical path
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Layer
O.S. Layer
Interconnect
Layer
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What is a protocol?
A set of rules
 A method for
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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
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ISO / OSI Protocol
Probably most popular network protocol
model
 Implementation often takes efficiencyrelated shortcuts
 Includes seven layers, grouped into 3 types
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application
operating system
communication service
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OSI / ISO Layers
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Application
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Application layer -- user programs
Presentation layer -- common data
transformations
Operating system
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Session layer -- process-to-process
communication
Transport layer -- reliable host-to-host
communication
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ISO / OSI Layers, continued
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Communication service
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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-andforward node
May exist partly as hardware
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Figure 1.4 The ISO/OSI
Reference Model. (Galli, p. 9)
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ISO/OSI Layers: Application
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Miscellaneous applications
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FTP (file transfer protocol)
remote login: rlogin
browsers: Netscape, Internet Explorer
email (via SMTP)
RJE (remote job entry)
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ISO/OSI Layers: Presentation
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Common data transformations
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data compression
encryption
big/little Endian
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ISO/OSI Layers: Session
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Process-to-Process Communication
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buffering
Some synchronization
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synchronous data communication
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ISO/OSI Layers: Transport
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Reliable site-to-site communication
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ISO/OSI Layers: Network
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Logical path for communication
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converts frames --> packets --> frames
X.25 connection-oriented
IP connectionless
used for WANs; redundant for LANs
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ISO/OSI Layers: Data Link
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Reliable data transmission
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message goes out in frames
 character
count -- header specifies length
 character stuffing -- special character at end
 bit stuffing -- special bit sequence at end
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on LANs can put out a special synch signal
adds a checksum to trailer to detect errors
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ISO/OSI Layers: Data Link, cont.
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Flow control
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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
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ISO/OSI Layers: Physical
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Raw bit-stream communication
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circuit switching
 reserves
a fixed communication at start
 releases path at end
 best for long, continuous stream
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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
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Figure 1.5 TCP/IP Relationship
to ISO/OSI Reference Model.
(Galli, p. 12)
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