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Module 16: Distributed System Structures
Distributed System
 is a collection of loosely coupled processors
interconnected by a communications
network
 Processors variously called nodes,
computers, machines, hosts
 Site is location of the processor
 What does this sound like?
 What would a “tightly coupled” system be?
 Also known as symmetric vs. asymmetric
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Types of Distributed Operating Systems
 Network Operating Systems
 Distributed Operating Systems
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Network-Operating Systems
OS’s like Netware and Windows NT have been
referred to as network operating systems
 Is this appropriate?
 Is Linux a network operating system?
 Is there any such thing as a non-networked
operating system?
 Users are aware of multiplicity of machines.
Access to resources of various machines is
done explicitly by:
 Remote logging into the appropriate remote
machine (telnet, ssh)
 Remote Desktop (Microsoft Windows)
 Transferring data from remote machines to
local machines, via the File Transfer
Protocol (FTP) mechanism

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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
Network (LAN)
Distributed
Operating System
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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
Oftensites
implemented as third party software running on
top of operating
system
Hardware
preference
– process execution may require
•MPI
specialized processor
•PVM
Software preference – required software may be available at
•Cluster software
only a particular site



Data access – run process remotely, rather than transfer all
data locally
Network (LAN)
Distributed
Operating
System
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LAN
 Local-Area Network (LAN) –
designed to cover small
geographical area.

Multiaccess bus, ring, or star
network

Speed  10 – 100
megabits/second

Broadcast is fast and cheap

Nodes:

usually workstations
and/or personal
computers

a few (usually one or
two) mainframes
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WAN
 Wide-Area Network (WAN) –
links geographically separated
sites

Speed  1.544 – 45
megbits/second

Broadcast usually requires
multiple messages
Wan
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WAN Usually Comprised of Routers
 Routers route packets
 Decisions on forwarding packets based
upon a routing protocol
 TCP/IP is a routing protocol
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Naming and Name Resolution
 Name systems in the network
 All about translating addresses into user friendly names
 Example

Domain name service (DNS)
ping www.umt.edu
Pinging mumwww.gs.umt.edu [10.10.4.102] with 32 bytes of data:
Reply from 10.10.4.102: bytes=32 time<1ms TTL=126
Reply from 10.10.4.102: bytes=32 time<1ms TTL=126
Reply from 10.10.4.102: bytes=32 time<1ms TTL=126
Reply from 10.10.4.102: bytes=32 time<1ms TTL=126
Ping statistics for 10.10.4.102:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms
Operating System Concepts – 7th Edition, Apr 4, 2005
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Routing Strategies
 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
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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
Fiber Optic
Networks
•FDDI
•ATM
 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
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Example: ethernet
 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
Network (LAN)
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ISO (international standard organization)
 Layered approach
Each layer represents
a layer of software but
also a header in a
data frame

Application

Presentation

Session

Transport

Network layer

Data-link layer

Physical layer
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The TCP/IP Protocol Layers
 Mapping TCP/IP to ISO
 IP is network layer

Not connection oriented
 TCP at transport layer

Connection oriented
 Which do you think a socket is?
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Example: Networking

The transmission of a network packet
between hosts on an Ethernet network

Every host has a unique IP address and
a corresponding Ethernet (MAC: media
access control: physical address)
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
Which layer is this?
Where are the other headers?

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
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End of Chapter 16