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ITC242 – Introduction to Data Communications

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Last Week

Topic 1• Networking is critical to the new corporate

structures emerging in today’s businesses• Strong trends are driving the development of

communications technology• Business information requirements are

constantly changing - voice, data, image, video• An introduction at a simple level to the different

types of networks

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Last Week

Topic 2• distinguished between digital and analogue

information sources;• understand the nature of four major forms of

business information: audio, data, image and video; and

• described response and throughput as two key performance measures of communications.

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Topic 3 – Distributed Data Processing

Activities

• Read all of Chapter 3 of the text

• Attempt all review questions from chapter 3 of the text

• Attempt to answer problems 3.5,3.6, 3.7, 3.8, 3.12, 3.13

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Topic 3 – Distributed Data Processing

Learning Objectives• Describe the differences between centralised

and distributed data processing• Describe different types of distributed data

processing for applications and distributed databases

• Describe the implications for data comms of distributed data processing

• Understand the motivation towards client server architectures

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Centralised Data Processing

Centralised Data Processing:

Data processing support is provided by one cluster of computers, generally large computers, located in a cental data

processing facility.

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Centralised Data Processing

• Centralised computers: one or more computers located in a central facility.

• Centralised processing: applications are run on the central data processing facility.

• Centralised data: Most data is stored, accessible and controlled at a central facility.

• Centralised control: Control of data, applications, processes and infrastructure maintained centrally.

• Centralised support: Technical support and development staff located in a central facility.

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Centralised Data Processing

• What are the advantages?– Economies of scale (equipment and

personnel)– Lack of duplication– Ease in enforcing standards, security

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Distributed Data Processing

• Computers are dispersed throughout organization

• Allows greater flexibility in meeting individual needs

• More redundancy

• More autonomy

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Definition

• “A distributed system is a collection of independent computers that appears to its users as a single coherent system.”

• Important Points:

1. The machines are autonomous.

2. Users think they are dealing with a single system.

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Distributed Systems

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Distributed System Examples

• The biggest example is the WWW.• The model presented to users to

somewhat uniform – go to this URL and get a web page.

• Most users are (or should be) unable to tell what kind of hardware or software is being used to deliver the web page.

• For the most part, the client should not make a difference either.

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Goals

• There are four main goals for distributed systems to make them worth building.

1. Connect users and resources

2. Make the system transparent

3. Make the system open

4. Make the system scalable

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Connecting Users and Resources

• We want to make it easier for users to access remote resources and to share resources with other users.

• Examples: printers, files, Web pages, etc• A distributed system should also make it

easier for users to exchange information.• Easier resource and data exchange could

cause security problems – a distributed system should deal with this problem.

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Transparency

• Transparency is the ability of the distributed system to hide that fact that it is actually a distributed system.

• The more the distributed system looks like a single computer, the more transparent it is.

• There are several different types of transparency that we should consider

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System Openness

• Openness describes how well a system offers services according to some standard rules for the distributed system.

• An open system will have a set of well-defined standards that describe the syntax and semantics for system services.

• If the system is not open, it will be difficult to expand.

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System Scalability

• A distributed system should be scalable.

• Scalability can be measured in three different ways:– size (add more users and resources)– geographically (add users and resources that

are far away)– administration (easily able to manage if

scaled)

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Hardware Concepts

• Multiprocessors are machines that share memory.

• Multicomputers are machines that do not share memory

• Multicomputer distributed systems can be either homogeneous or heterogeneous– homogeneous: all computers are the same type– heterogeneous: the computers can be different

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Hardware

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Middleware

• Middleware sits between the distributed application and the network OS services.

• Middleware is responsible for hiding the underlying details of each computer while providing a unified view of services to the user.

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The Middleware View

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Client-Server Model

• The basic client-server model has two parts:– the server, a process that is installed on a

particular machine and provides a service (A process is running program)

– the client, a process that requests services from the server by sending a request in a predefined format and waiting for a response.

• Processes interact by sending each other messages.

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Client-Server Architecture I

©Pearson Education 2001

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Client-Server Architecture II• Clients send requests to servers (i.e., invocation)• Servers send responses to clients (i.e., result)• Servers may be clients of other servers

– A web server is often a client of a file server– An Internet service is a client of a DNS server –

a server that translates DNS names to IP addresses

• Potential problem: a single server is a scalability bottleneck and a single point of failure

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Peer-to-Peer Architecture I

©Pearson Education 2001

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Peer-to-Peer Architecture II

• All processes play similar roles – i.e., they interact as peers

• No central component – potentially better scalability and resiliency to failures

• Use the power of modern desktops to implement a large-scale distributed system

• Examples: Napster, Kazaa, Skype, Bittorrent

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Services by multiple servers

• Multiple servers provide services to clients

• Servers may partition the service objects or replicate them

• WWW: partitioned objects• Sun NIS: replica of a

password file maintained at each server

©Pearson Education 2001

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Proxy Servers and Caches

• A cache is a store of recently used data objects that is closer than the main store

• A newly accessed object is added to the cache• When that object is accessed again, it is fetched from the cache,

if there is an up-to-date copy in the cache• Proxy servers intercept communication with the real server to

provide faster service (e.g., deliver cached data), better security (e.g., a proxy configured as a firewall)

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Multitiered Architectures

• The simplest client-server architecture has only two machines involved – the Client and the Server.

• With a multitiered architecture, it is possible to have more than just two machines involved.

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Multitiered Possibilities

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Multitiered Example (b)

• The client connects to a web server to retrieve some data.

• The web server will then connect to a database in order to get the information needed.

• The database server gives the information to the web server.

• The web server sends the results back to the client.

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Distribution

• The multitiered examples that we have seen are referred to as vertical distribution because the work is split up across “vertical” machines.

• We can distribute work across multiple servers as well, and this is known as horizontal distribution.

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Horizontal Distribution

teh

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More Horizontal Distribution

• In the previous example, the horizontal distribution was applied to the servers.

• It is possible to have horizontal distribution across the “client” side for those cases where the client and the server are the same program.

• This can be seen in “peer-to-peer” distributed systems.

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Topic 4 – The Internet

Learning Objectives

• Discuss the history of the Internet and explain it’s explosive growth

• Describe the overall Internet architecture including key components

• Explain the components and operation of the Domain Name System

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Topic 4 – The Internet

Activities

• Real all of Chapter 4 of text

• Attempt all review questions of chapter 4

• Attempt problems: 4.1, 4.6, 4.7, 4.10, 4.11, 4.12.

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Internet History

• Evolved from ARPANet (Defense Department’s Advanced Research Projects Agency Network)

• ARPANet was developed in 1969, the first packet-switching network

• Initially, included only four nodes: UCLA, UCSB, Utah, and SRI

• In the 1980s, packet-switched networking extended to non-ARPA organization; eventually replaced ARPANet

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Early Applications & Protocols

• Telnet/FTP (1972/73)

• Distributed Email (1972)

• TCP/IP (1982-83)

• DNS (1984)

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Switching Methods

• Circuit Switching: Requires a dedicated communication path for duration of transmission; wastes bandwidth, but minimizes delays

• Packet Switching: Specialized message switching, with very little delay

• Message Switching: Entire path is not dedicated, but long delays result from intermediate storage and repetition of message

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Circuit SwitchingA B

Source Destination

It’s the method used by the telephone network. A call has three phases:

1. Establish circuit from end-to-end (“dialing”),2. Communicate,3. Close circuit (“tear down”).

Originally, a circuit was an end-to-end physical wire. Nowadays, a circuit is like a virtual private wire: each

call has its own private, guaranteed data rate from end-to-end.

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Circuit Switching Telephone Network

Source“Caller”

Central Office“C.O.”

Destination“Callee”

Central Office“C.O.”

TrunkExchange

Each phone call is allocated 64kb/s. So, a 2.5Gb/s trunk line can carry about 39,000

calls.

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Packet Switching

A

R1

R2

R4

R3

B

Source Destination

It’s the method used by the Internet. Each packet is individually routed packet-by-packet,

using the router’s local routing table. The routers maintain no per-flow state. Different packets may take different paths. Several packets may arrive for the same output link at

the same time, therefore a packet switch has buffers.

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Packet SwitchingSimple router model

R1Link 1

Link 2

Link 3

Link 4

Link 1, ingress Link 1, egress

Link 2, ingress Link 2, egress

Link 3, ingress Link 3, egress

Link 4, ingress Link 4, egress

ChooseEgress

ChooseEgress

ChooseEgress

ChooseEgress

“4”

“4”

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Internet Components

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The World Wide Web

• Concept proposed by Tim Berners-Lee in 1989, prototype WWW developed at CERN in 1991

• First graphical browser (Mosaic) developed by Mark Andreessen at NCSA

• Client-server system with browsers as clients, and a variety of media types stored on servers

• Uses HTTP (hypertext transfer protocol) for retrieving files

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Connecting to the Internet

• End users get connectivity from an ISP (internet service provider)– Home users use dial-up, ADSL, cable

modems, satellite– Businesses use dedicated circuits connected

to LANs

• ISPs use “wholesalers” called network service providers and high speed (T-3 or higher) connections

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Commercial Internet Use

• ARPANet and NSF limited use to research and development

• Early commercial use primarily information dissemination

• EDI transactions gradually moved to the Internet

• WWW growth in 1990s has led to increased direct sales

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IP address

• People: many identifiers:– SSN, name, passport #

• A unique IP address for every host and router on the network

• 32-bits long IP addresses-- four octets of eight bits each • Dotted decimal notation

– 11000000 11100100 00010001 00111001 (binary)– 192.228.17.57 (decimal)– Each octet ranges between 0 and 255. The lowest IP address is

0.0.0.0 and the highest is 255.255.255.255

• “name”, e.g., ww.yahoo.com - used by humansQ: map between IP addresses and name ?

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Domain Name System

• 32-bit IP addresses have drawbacks– Routers can’t keep track of every network path– Users can’t remember dotted decimals easily– It is not easy to resolve host name conflicts

• Domain names address these problems by mapping domain names into IP address

• DNS is a group of protocols and services widely used over the Internet and TCP/IP-based networks. It is an application layer protocol and uses a hierarchical, domain-based naming scheme and a distributed database system for implementing this naming scheme.

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Mapping Computer Names to IP addresses

The Domain Naming System (DNS)Names are hierarchical and belong to a domain:

– e.g. elaine17.stanford.edu

– Common domain names: .com, .edu, .gov, .org, .net, .uk (or other country-specific domain).

– Top-level names are assigned by the Internet Corporation for Assigned Names and Numbers (ICANN).

– A unique name is assigned to each organization.

DNS Client-Server Model– DNS maintains a hierarchical, distributed database of names.– Servers are arranged in a hierarchy.– Each domain has a “root” server.

– An application needing an IP address is a DNS client.

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Root DNS Servers

com DNS servers org DNS servers edu DNS servers

poly.eduDNS servers

umass.eduDNS servers

yahoo.comDNS servers

amazon.comDNS servers

pbs.orgDNS servers

Distributed, Hierarchical Database

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DNS Components

• Domain name space– Tree-structured name space to identify all internet

resources

• DNS database– Stored in a distributed database

• Name servers– Server programs that hold information about a

specific portion of the domain name tree

• Resolvers– Programs that extract information from name servers

based on client requests

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DNS Database

• Hierarchical database containing resource records (RRs) (name, IP address, other info about hosts).

• Variable-depth hierarchy for names– essentially unlimited levels– uses . as the level delimiter in names

• Distributed database: – resides in DNS servers throughout the Internet

• Distribution controlled by the database– database divided into thousands of separately managed

zones, – distribution and update of records controlled by database

software.

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DNS Server Hierarchy

• Each name server configured for a specific local zone– Includes subdomains and associated RRs– Authoritative source for that portion of hierarchy

• Root servers are at top of hierarchy– Different root servers for different top level domains– Some redundancy within domain spaces to prevent

bottlenecks

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DNS: Root name servers• contacted by local name server that can not resolve name• root name server:

– contacts authoritative name server if name mapping not known

– gets mapping– returns mapping to local name server

13 root name servers worldwide

b USC-ISI Marina del Rey, CAl ICANN Los Angeles, CA

e NASA Mt View, CAf Internet Software C. Palo Alto, CA (and 36 other locations)

i Autonomica, Stockholm (plus 28 other locations)

k RIPE London (also 16 other locations)

m WIDE Tokyo (also Seoul, Paris, SF)

a Verisign, Dulles, VAc Cogent, Herndon, VA (also LA)d U Maryland College Park, MDg US DoD Vienna, VAh ARL Aberdeen, MDj Verisign, ( 21 locations)

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TLD and Authoritative Servers

• Top-level domain (TLD) servers:– responsible for com, org, net, edu, etc, and all

top-level country domains uk, fr, ca, jp.– Network Solutions maintains servers for com TLD– Educause for edu TLD

• Authoritative DNS servers: – organization’s DNS servers, providing

authoritative hostname to IP mappings for organization’s servers (e.g., Web, mail).

– can be maintained by organization or service provider

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Local Name Server

• does not strictly belong to hierarchy

• each ISP (residential ISP, company, university) has one.– also called “default name server”

• when host makes DNS query, query is sent to its local DNS server– acts as proxy, forwards query into hierarchy

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DNS Operation

• User program requests IP address for a domain name• Resolver module in local host or ISP formulates query

for local name server (same domain as the resolver)• Local name server checks local database/cache

– if found returns IP address to the requestor. – If not found, queries other available name servers, starting down

from the root of the DNS tree or as high up the tree as possible.

• When response is received, local name server stores the name/address mapping in local cache

• User program receives IP address or error message.

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DNS Name Resolution

• Query begins with name resolver located in the user host system

• If requested name not in cache, query sent to local DNS server– returns an address immediately, or– returns address after querying other servers

• Two possible types of queries– Recursive– Iterative

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requesting hostcis.poly.edu

gaia.cs.umass.edu

root DNS server

local DNS serverdns.poly.edu

1

23

4

5

6

authoritative DNS serverdns.cs.umass.edu

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TLD DNS server

DNS name resolution example

• Host at cis.poly.edu wants IP address for gaia.cs.umass.edu

iterated query:• contacted server

replies with name of server to contact

• “I don’t know this name, but ask this server”

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requesting hostcis.poly.edu

gaia.cs.umass.edu

root DNS server

local DNS serverdns.poly.edu

1

2

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6

authoritative DNS serverdns.cs.umass.edu

7

8

TLD DNS server

3recursive query:• puts burden of

name resolution on contacted name server

• heavy load?

DNS name resolution example

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DNS: caching and updating records

• once (any) name server learns mapping, it caches mapping– cache entries timeout (disappear) after

some time– TLD servers typically cached in local

name servers• Thus root name servers not often visited

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Top Level Domains

• http://www.icann.org/registries/listing.html• Country Code TLD’s (ccTLD)

http://www.iana.org/cctld/cctld-whois.htm

.aero .biz .cat .com .coop

.edu .gov .info .int .jobs

.mil .mobi .museum .name .net

.org .pro .til .travel

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.AU TLD.au Delegated country name space for Australia. .au Subdomains are:• .com.au     - commercial entities• .edu.au     - educational institutions• .org.au     - miscellaneous organisations• .gov.au    - government and semi-government entities• .net.au     - network infrastructure and providers• .asn.au     - associations• .id.au      - individual people• .conf.au    - conferences/exhibitions short duration Internet

connectivity• .info.au    - major Australian informational and service resources• .csiro.au   - CSIRO• .oz.au      - entities which are visible within the ACSnet domain• .telememo.au - X.400 entities registered within the

"/C=AU/ADMD=Telememo" administrative domain

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ipconfig • ipconfig in

Windows is a command line utility that displays all current TCP/IP network configuration values and refreshes Dynamic Host Configuration Protocol DHCP and Domain Name System DNS settings.

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nslookup • The

nslookup command can be used in Windows and Unix to find the IP addresses of a particular computer, using DNS lookup. The name means "name server lookup".

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Topic 4 Summary

• Discuss the history of the Internet and explain it’s explosive growth

• Describe the overall Internet architecture including key components

• Explain the components and operation of the Domain Name System