16.Distributed System Structure

Module 16: Distributed SystemModule 16: Distributed System
StructuresStructures

16.2 Silberschatz, Galvin and GagneOperating System Concepts
Module 16: Network StructuresModule 16: 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

MotivationMotivation
Distributed system is collection of loosely coupled processors
interconnected by a communications network
Processors variously called nodes, computers, machines, hosts
Site is location of the processor
Reasons for distributed systems
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

A Distributed SystemA Distributed System

Types of Distributed OperatingTypes of Distributed Operating
SystemsSystems
Network Operating Systems
Distributed Operating Systems

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 (telnet,
ssh)
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 SystemsDistributed-Operating Systems
(Cont.)(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

Network StructureNetwork Structure
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

Depiction of typical LANDepiction 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

Communication Processors in a Wide-AreaCommunication Processors in a Wide-Area
NetworkNetwork

Network TopologyNetwork 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
The following six items depict various network topologies

Network TopologyNetwork Topology

Communication StructureCommunication Structure
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 some IBM and HP/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

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 NetworkCommunication Via ISO Network
ModelModel

The ISO Protocol LayerThe ISO Protocol Layer

The ISO Network MessageThe ISO Network Message

The TCP/IP Protocol LayersThe TCP/IP Protocol Layers

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

Example: NetworkingExample: 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) 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

End of Chapter 16End of Chapter 16

16.Distributed System Structure

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16.Distributed System Structure