Network Organization Concepts Operating Systems I

Network Organization Concepts

Operating Systems I

Introduction

In almost every corporate and academic computing environment, “stand-alone” computers are linked together in a network to send and receive data among every user of the system. Examples include e-mail, web servers (in-house or Internet), database applications, and more.

Introduction - 2

A common goal of networks is the sharing of resources (both hardware and software) while at the same time controlling access to them.

There are two basic kinds of operating system configurations in a networked environment: a network operating system or distributed operating system.

Network Operating System

Network is built on top of local computer operating system.

Users are conscious of the assortment of computers on the network. Access is gained by logging on to a specific remote machine or by transferring data from the remote machine to their own.

Distributed Operating System

Users need not be aware of every machine on the network.

Remote resources accessed as if local. Total view across multiple computer systems

for controlling and managing resources. Offers several advantages over traditional

operating system environments.

Basic Terminology

A network is a collection of loosely coupled processors (Chapter 6) interconnected by a communications network.

In a distributed system each host’s own resources are local and other hosts’ resources are remote.

Basic Terminology - 2

Processors are referred to as sites, hosts, and nodes. “Site” is usually a particular network

location containing one or more computer systems.

“Host” is a specific computer system at a site whose resources can be used from remote locations.

“Node” is a unique name for a computer.

Basic Terminology - 3

Typically, a host at one site - called the server - has resources that another machine at the same site or a remote site - the client - wants to use.

A server can be a client and a client can also be a server - depending on the situation. (For example, I might log on to a server ...)

Network Topologies

Sites can be physically or logically connected to one another in a variety of ways (topologies).

Most common topologies are ring, star, bus, tree, and hybrid.

There are tradeoffs between speed, fault tolerance, cost, and difficulty of making a large number of connections.

Network Topologies - 2

Three criteria need to be kept in mind when deciding on a network topology: Basic cost: the expense of linking the

various sites in the system. Communications cost: time required to

send a message from one site to another. Reliability: assurance that other sites can

still communicate if one of the links fails.

Star Topology

Also known as “hub” or “centralized topology” - all data passes through a central controller when going from sender to receiver.

Allows easy routing because the central controller knows the paths to all other sites.

Access to network easily controlled.

Star Topology - 2

Sites can be given priority. However:

Central site must be extremely reliable. Must be able to handle all network traffic,

no matter how heavy.

Star Topology

CentralController

Host 1Host 5

Host 4

Host 3

Host 2

Ring Topology

All sites are connected in a closed loop with the first connected to the last.

Packets (containing source and destination address) are passed from one site to the next in one direction.

Destination copies it to a local buffer. Packet continues to circulate until it

arrives back at source, then is deleted.

Ring Topology - 2

Variations include double loops or rings bridged together: more flexibility, but at a cost.

Disadvantage: all members must be functional for the network to perform properly. (Rings can be designed to allow failed nodes to be bypassed.)

Bus Topology

All sites are connected through a single communications line.

Sites are connected to the central cable but do not run through a controller.

Only one site can send messages at a time.

End controllers send traffic back down the line in the opposite direction.

Tree Topology

A tree is a collection of busses. Tree layout begins at “head end” (where

one or more cables start) - each branch may in turn have other branches.

Message circulates through the line, can be received at any other site, and is absorbed at the end points.

Tree Topology - 2

One advantage of tree and bus topologies is that message traffic still flows through the network even if one node fails.

See Figure 9.8 on page 214.

Hybrid Topology

Some combination of the other four topologies.

Objective is to take advantage of the strengths of each topology and combine them to most effectively meet the communication requirements.

See Figures 9.9 and 9.10 on page 215 for some examples.

Network Types

Networks are generally classified by the geographical areas they encompass.

These distinctions are becoming more blurred as advances occur in telecommunications technology.

Basic types are: local area network (LAN), metropolitan area network (MAN), and wide-area network (WAN).

Local Area Network

Defines a network configuration found within a single building, warehouse, campus, etc.

Typically owned by a single organization and allows computers to communicate through a common communication line.

Local Area Network - 2

Communications not necessarily confined within the LAN - a gateway or bridge can provide access to the outside.

A bridge is a device (including software) that connects two or more distinct LANs that use the same protocols - e.g., two Ethernet LANs.

Local Area Network - 3

A gateway is a more complex device (and software) used to connect two or more LANs that use different protocols (e.g., PC network running token-ring and mainframe SNA network).

Physical medium used to construct the network varies. Coaxial cable and fiber optic cable are common.

Metropolitan Area Networks

Spans an area larger than a LAN, such as several blocks or an entire city (up to a 100 km radius).

High-speed network typically configured as a logical ring.

Messages may be transmitted in one or both directions.

Wide Area Networks

Connects networks in different parts of the country or even the world.

Uses communications lines of telecommunications companies (like the phone company).

Broad range of communications media, including satellites, microwaves, etc.

The Internet is the best-known WAN.

Software Design Issues

How do sites use addresses to locate other sites?

How are messages routed? How sent? How do processes communicate with

one another? How are conflicting demands for

resources resolved?

Addressing Conventions

Network sites are not necessarily connected directly to each other so there is a need for names, addresses, and routes.

Addressing protocols are closely related to the geographic location of the site and its network topology.

Local name and global name may differ.

Addressing Conventions - 2

Example of a global name: an e-mail address. [email protected]

DNS (Domain Name Service) Protocol is used to resolve Internet addresses (or, more properly, resolve a name to an address).

Addressing Conventions - 3

DNS works backwards: finds the “edu” domain list. locates the address for “uc” on that list. queries the UC DNS server for host “email”

The name email.uc.edu resolves to an address (129.137.33.153).

E-mail transmission contains that address. E-mail server resolves the name John.Doe.

Routing Strategies

Routing allows data to get from one point to another on a network.

Each destination must be uniquely identified.

When destination network is reached, the router makes sure the data gets to the correct machine.

Routing Strategies - 2

Routing protocols need to consider addressing, address resolution, message format, and error reporting.

Most routing protocols use a network and node number to identify each node.

Router stores internally the addresses of networks directly connected.

Routers communicate with one another.


At regular intervals, routers will broadcast their entire routing table.

Addresses allow communication between networks; they do not allow communication between nodes in the same network.

Address resolution - router relates network address to hardware address.


Routers and routing protocols report error conditions (unable to reach destination, etc.) but do not attempt to correct them. (That’s left up to other protocols at other levels of the network architecture.)

Two of the most widely-used routing protocols are routing information protocol (RIP) and open shortest path first.

Routing Information Protocol

In RIP, selection of a path to a destination is based on the number of intermediate nodes (hops) between the source and destination.

Path with smallest number of hops is always chosen.

Shortest path is not always the best path (may be slow, for example).

RIP – Continued

Routing table is updated and reissued every 30 seconds, whether or not it has changed.

This leads to increased network traffic and could negatively affect delivery of messages.

May take time for changes to propagate to the other end of the network.

Open Shortest Path First

In OSPF, selection of a network path is made only after the state of a network has been determined.

Failed intermediate hop eliminated from consideration until it’s back up.

Routing updates are sent only when changes have occurred.

Memory usage is increased (more info).

OSPF - continued

Savings in bandwidth consumption are offset by increased CPU usage required to compute the shortest path.

Router creates a topological database (data structure) which is updated when failures are reported.

Router checks its database to determine whether a path is available.

OSPF - continued

If the path is not available, the router uses Dijkstra’s algorithm to generate a “shortest path tree” around the failed link.

Connection Models

Data entering the network at one point is routed to its destination by being switched from node to node, either by circuit switching or packet switching.

Circuit switching - dedicated communication path established between hosts before transmission begins. If path fails, so does communication.

Connection Models - 2

(Circuit switching, continued): after a slight delay to set up the connection, the network is transparent to users and data is sent at a fixed rate of speed.

Packet switching - Store-and-forward technology in which messages are divided into equal-sized packets which are then sent, and reassembled at their destination.

Packet Switching

Packet switching is an effective technology for long-distance communication because it permits transmission between devices that send or receive at different rates.

However - no guarantee that all packets will follow the same path or that they will arrive in the “correct” order.

Packet Switching - Continued

A “header” containing information about the packet is attached to it before it is sent.

Provides greater line efficiency because a single node-to-node circuit can be shared by several packets and does not sit idle for long periods of time.

Allows “priority” messages.


Two methods of selecting path: datagrams and virtual circuits.

Datagram approach: destination and sequence number added to the unique message identifier.

Each packet is then handled independently - route selected as each packet is put onto the network.

Packet Switching - Datagrams

At the destination, packets are reassembled in the correct order into one continuous message.

Receiving node requests retransmission of missing or damaged packets.

Two distinct advantages: less congestion by using lighter-used paths more reliable: allows for alternate paths


Virtual circuit approach: destination and packet sequence numbers are not added because a complete circuit between sender and receiver is established before transmission begins.

May be several virtual circuits between nodes. Faster than datagram method - only one routing

decision made.


Disadvantage of virtual circuits: If a node fails then all virtual circuits using

that node become unavailable. Congestion is more difficult to resolve

when circuit experiences heavy traffic.

Conflict Resolution

Some method is necessary to control access to the network so that all users have equal and fair access.

Access control techniques: round robin, reservation, contention.

Medium access control protocols: carrier sense multiple access, token passing, distributed queue dual-bus.

Access Control Techniques

Round robin is much like the round robin processor management (chapter 4).

Each node is allowed to use the medium (network). If it has data to send it is given a certain time to complete sending.

If it finishes sooner or at the end of its time, control goes to the next node.

Access Control (Continued)

Round robin is efficient when there are a lot of nodes transmitting over a long period of time.

Otherwise, overhead of passing control from node to node can be high.

In that case, other techniques may be preferable.


Reservation technique access time to medium divided into slots each node can “reserve” a future time slot

for its use. Good for configuration in which several

terminals are connected to a host through a single I/O port.


Contention technique Good for short, intermittent traffic No attempt made to control access to the

medium Works well with light to moderate traffic Breaks down under heavy load Easy to implement

Medium Access Control

Carrier sense multiple access (CSMA) Easy-to-implement, contention-based

protocol. “Carrier sense” - node listens to (tests)

medium before transmitting any data (prevents collision with another node that’s transmitting).

“Multiple Access” - multiple nodes connected to the same medium.

CSMA (Continued)

Two or more nodes could conclude that all is quiet at the same time, and both begin to transmit at the same time.

Result is damaged messages and unstable situation until bad messages are dissipated.

Failure to receive acknowledgement from receiver causes retransmission.

CSMA (Continued)

Original CSMA modified to include collision detection - CSMA/CD.

Ethernet is the most common CSMA/CD protocol.

Reduces collisions - does not eliminate them.

Hosts “jammed” when collision detected, retry after a random interval.

CSMA (continued)

CSMA/CA (collision avoidance) is another modification of CSMA.

Prevents multiple nodes from colliding during transmission.

Used by Apple’s LocalTalk protocol. Sends out a three-byte packet indicating

it wants to transmit. Other nodes wait until this node is done.

Token Passing

Electronic signal (“token”) is generated when network is turned on.

Token passed from node to node - only the node which has the token can transmit.

Data is sent, with the token. Receiver copies the data, adds the acknowledgement, and returns the packet to the sending node.

Token Passing

Requires higher overhead at each node than CSMA/CD, and nodes may have long waits under certain conditions.

Token ring is the most widely-used protocol for ring topology. Token moves in one direction from node to

node. When not carrying a message it’s a “free” token.

Token Passing

Token ring (continued): Nodes waiting to transmit wait for free

token to come by. Free token becomes “busy” and message

sent right after token. Receiving node copies the message, sets

the “copied” bit, and sends the message on around the ring to the sender.

Sender releases a new free token.

Distributed Queue Dual Bus

Intended for use with a dual bus where each bus sends data in only one direction and has been standardized by one of the IEEE committees as part of its MAN standards.

Transmission is a series of fixed-size slots marked “free” and sent downstream.

DQDB (Continued)

Free slots are marked “busy” and written to by nodes waiting to send data.

Nodes read and copy data from the slots, which then move to the end of the bus and dissipate.

Very effective protocol even under heavy loads.

Transport Protocol Standards

Growth of networks in the 1980s and the need to integrate dissimilar network devices from different vendors led to the push for a single universal network architecture.

There are two competing models: OSI and TCP/IP.

OSI Reference Model

Created by International Organization for Standardization (ISO). Framework for services provided by a

network. Basis for connecting “open” systems for

distributed applications processing. (“Open” systems conform to the model and can be connected regardless of vendor.)

OSI Reference Model - 2

Once services were identified, some were collected in logical clusters called layers. Allows changes to one layer and its

protocols (for example, due to advances in hardware or software) without affecting the others.

Seven-layer model is software that handles data transmission from one node to another.


Layer 1 - The Physical Layer Mechanical, electrical, functional

specifications for connecting device to network.

Primary concern is transmitting bits over communication lines.

Only layer concerned with hardware. All data is passed down to layer 1.


Layer 2 - The Data Link Layer Software for this layer stored in some sort

of device. Establishes and controls the physical path

of communications before sending data to the physical layer below it.

Physically assembles packets for transmission.

Checks for/resolves transmission errors.


Layer 3 - The Network Layer Provides services such as addressing and

routing. Accepts packets from layer 4, resizes

them, and routes them to the proper destination.

Database of routing tables (dynamically updated) is stored at this level.


Layer 4 - The Transport Layer Maintains reliable data transmission

between users. Software at this level contains facilities to

handle user addressing and to ensure that all data packets have been received.

Also has a mechanism to control data flow to prevent overrun of slower computer by faster one.

TCP is a layer-4 protocol.


Layer 5 - The Session Layer Provides user-oriented connection service

and transfers data over communication lines.

Creates and maintains logical link between end points.

User interface. Data flow control. Very similar functions to transport layer.


Layer 6 - The Presentation Layer Data manipulation functions: compression,

formatting, encryption. Data conversion, syntax conversion, and

protocol conversion are common tasks performed in layer 6.

IBM mainframe CICS teleprocessing monitor is an example (but it also does much more).


Layer 7 - The Application Layer Application programs, terminals, and

computers access the network at level 7. Provides interface to users and is

responsible for formatting user data before passing it to lower layers.

File transfer and e-mail are two common applications at this layer.

TCP/IP Model

Most widely used network protocol today. (If you use the Internet you use TCP/IP.)

It is a file transfer protocol so there is a good chance your data will get to its destination without error.

The TCP/IP model organizes a system with three main components: processes, hosts, and networks.

TCP/IP Model - 2

Model is divided into four layers instead of seven: Network Access Layer - Equivalent to the

physical data link and part of the network layer of the OSI model.

Protocols at this level provide access to a communication network.

Functions performed here include flow control, error control, and security.

TCP/IP Model - 3

Layer 2 - Internet Layer Equivalent to portion of OSI network layer

not included in Layer 1 of this model, specifically the mechanism that performs routing functions.

Usually implemented within gateways and hosts.

IP is an example of the standard for this layer.

TCP/IP Model - 4

Layer 3 - Host-to-Host Layer Equivalent to OSI Model Transport and

Session Layers. Supports mechanisms to transport data

between two processes on different host computers.

Services also include error checking and flow control.

Well-know standard: TCP.

TCP/IP Model - 5

Layer 4 - Process/Application Layer Equivalent to presentation and application

layers of the OSI Model. Includes protocols for computer-to-

computer resource sharing and terminal-to-computer remote access.

Examples: FTP, SMTP, Telnet.

About the Final Exam

Open book / open notes Covers the entire quarter Objective questions:

Multiple choice Matching

Short Answer questions Exercises in text are good review

questions.

Taking Operating Systems II?

Emphasis on Windows platform Consumer systems (XP Home Edition) Business workstations (Windows 2000 /

XP Pro) Server and network environment (Windows

2000) What’s new in Windows .NET server

Lecture and hands-on labs (at UC, I hope)

Taking Operating Systems II?

Overview of Unix Hope to do a Linux lab – we’ll see

Mainframe (OS/390) Enterprise computing Also:

No new text required! Smaller class size!

A Final Word …

Bring a self-addressed, stamped envelope to the exam if you want your papers back.

Papers are kept for one year, records kept indefinitely.

You can also get papers back if you are in the IT 454 class next quarter.

Please do not e-mail or call me about grades.

Documents

Network Organization Concepts Operating Systems I