Networks & The Internet

Week 7 – Lecture 2

Yesterday we discussed how:

• Data is carried over various media in analogue or digital signals

• A physical link within the network has a given bandwidth and can be divided into a number of discreet channels by multiplexing

• The messages from a number of users can be broken into packets, addressed and transmitted along the same channel – known as packet switching

What do we want from a communications network?

• We want to – transfer messages from a process in one

computer– to a process in another computer

• reliably

• quickly

• and in an understandable form.

We also know that:

• Messages have to pass through a number of routers and switches from one computer to another, and thus the network has to route messages correctly

• The various links may be of different media at different bandwidths

• The signals carrying data are subject to interference and degradation, and thus messages may be corrupted or lost

• The byte coding structure in one computer may be different from the other

We need a series of protocols to address these issues

• “A protocol defines the format and order of messages exchanged between two communicating entities, and the actions taken on receipt or transmission of a message.”

• Those “communicating entities” may be as limited as the two devices on either end of a physical link.

Network Architectures

• Data communication developed in the 60’s

• By early 70s suppliers were developing own architectures – IBM/SNA DEC/DECnet

• But these did not help inter-organisation data communications

• Two approaches:

– OSI model

– Internet’s TCP/IP

OSI – Open Systems Interconnect

• Developed by the International Standards Organisation in 1974

• It is a reference model• Describes a network and a framework for

developing network protocols• Incorporates work done by a number of

organisations, particularly DEC• Its real benefit is that is defines a layered

architecture and thus the model is still used

Application

Presentation

Session

Transport

Network

Data Link

Physical

To provide application services – FTP, X400, X500

To translate, encryptAnd compress data

Establish and manageConnection.

Provide end-to-enderror free deliveryof messages End-to-end routing of

packets through thenetwork.

Reliable transfer ofa frame of data between two devices

Transmits bits acrossa physical medium

Application ApplicationProcess Message

Transport TransportSegmentHost

Network NetworkHost & Router

Datagram Datagram

Link/Physical Link/Physical

Host, router& switch

Frame Frame

LinkLink LinkLink

LinkNetwork

Switch

Router

Key points about a layered approach

• The source process only wants to know that the message it sent gets to the destination process – it does not need to know how

• The Network “stack” is implemented in the operating system of both hosts and the various switches & routers

• As such the software in each operating system will be different code – – but implementing the same protocol

Task management(scheduler)

Memory management

Device drivers

Command language(Shell)

MiddlewareDBMS

Application processes

TCP/IP

EthernetATM etc

Filemanager

Why is OSI not the prevailing standard?

• A case of De facto prevailing over De jure

• While OSI was defined, manufacturers were slow to implement it

• Europeans were keen, but the US had TCP/IP

• US government proclaimed GOSIP (Government OSI Profile) in 1992

• But TCP/IP took off with the Internet in the early 90s

• It worked, was common, and most organisations just accepted it

TCP/IP, Internet & The WWW

• TCP/IP are two protocols at the Transport and Network levels

• The Internet is a “Network or Networks” that use TCP/IP as key layers in its protocol stack

• The World Wide Web is an application that runs on the Internet

TCP/IP

• TCP (Transmission Control Protocol• IP (Internet Protocol)• Developed at the same time as OSI, but as a

product not an international standard• Developed for the ARPANET – Dept of Defence,

defence contractors, Universities and the Military• To enable communication across analogue lines,

packet radios and Ethernet networks• To be a Network of networks

The Principles set out for TCP/IP

• Autonomy – a network should be able to work on its own without change

• Best effort service – Lost messages would be retransmitted

• Stateless servers – Routers should not need to maintain the state of a connection

• Decentralised control – No global control over the Internet

Comparing The Internet & OSI

Addressing within the Internet

• We have talked about applications passing messages through the network

• We know it is more accurate to say that Processes on each machine pass messages

• Thus for two processes to communicate we need:• Source & destination host addresses specified by the IP address

• A processes on the hosts specified by a port number. Common processes have specific post numbers

– HTTP is port 80– SMTP is port 25

Domain Name Service - DNS

• We prefer to use host names such as “usyd.edu.au”

• But the Internet uses a hierarchical IP address• In IPv4 this is a 32 bit address shown as a four byte

address eg 193.92.216.9, where each byte has a range 0-255 i.e. 9=00001001

• In IPv6 this will become a 128 bit address, providing around 3 devices per square metre of the globe

• Network Address Translation (NAT) where many numbers internal to an organisation are not externally visible has reduced the move to 128 bit addresses

Defining a Network

A Network within the Internet

• A group of hosts isolated on one side of a router

• The path between two routers

UDP• At the Transport layer it does very little. Passes

the message straight to the IP packet service• It is connectionless

• No overhead in handshaking• No connection state in end systems

• It has a smaller packet overhead• No congestion control• Trades off risk of data loss against higher

throughput• Used in multimedia – telephony, video• Some apps. provide own error control

TCP – Reliable transmission service

• To provide a reliable data transmission service it needs:

• An error detection mechanism. This is based on a checksum calculation

• A receiver feedback mechanism. The receiver sends an ACK(nowledge) when an error free packet is received, and sends a NAK when the packet has errors.

• A retransmission service is a packet is damaged or not received

IP

• Provides a connectionless datagram service

• Source to destination

• Relies on routers to route the datagram through the network

• Routers use the IP addresses together with their internal routing tables to direct datagram down the appropriate link

The WAN and who provides it.

Telstra

Optus

AT&T

NTT

IP Links are router to router

Router

Switch

Background

• Telco’s started to bring WAN to large buildings – Fibre

• Telco’s expanded capacity dramatically in the late 90s – 5 times the fibre X 100 time efficiency = 500 times the capacity

• But usage is only doubling each year

• Hence the Telecom shakeout we are seeing

International Carriers

• Each of these have International “Hubs” in the countries in which they operate

• They own or lease a share of the intercontinental cables

• Many of them “own” the last mile and much of the national networks

• They either provide ISP services direct to users (Telsta’s Bigpond or UUNet’s Ozemail), or

• To smaller ISP that service geographical areas

The major carriers share cables

• PacRim East & PacRim West– Telstra has a share– 1 gbps transmission rate

• Europe to Asia link

• Southern Cross Cable between Australasia and the US

• 600 communication satellites

We tend to think the public Internet and the Web are

dominant.But most large organisations

have their own WAN.

More than 30 companies offer Frame Relay or ATM services in Australia

• AAPT has Optic fibre in most capital cities with fibre direct to 250 buildings. They have 50 FR/ATM POPs

• Cable & Wireless Optus have 38 FR/ATM POPs• CITEC a business unit of the Queensland State

Gov.• Telstra has 300 FR/ATM POPs and 2,200

customers

Figures as at September 2000, Telsyte report

An Organisation’s network can be:

• Leased channels

• VPN Virtual Private Network

• VPN on Public network

• Public Network

• Combination of some or all or these

The main WAN protocols are

• PPP – Point-to-Point Protocol• HDLC – High level Data Link Control• FDDI – Fibre Distributed Data Interface• ATM – Asynchronous Transfer Mode• X.25 & Frame Relay• SONET – Synchronous Optical Network• ISDN – Integrated Services Data Network• POS – Packet over SONET

Quality of Service

• One drawback with the Internet is that it is democratic, and all packets are treated as important as any other.

• It provides “best effort” service• IPv4 has no mechanism to provide priority• This is needed for time critical applications such

as telephony, real time conferencing and high performance transaction processing

• QoS aims for a predictable and specifiable bandwidth and latency

QoS the key to one network

• When packet switched networks can offer the QoS of switched circuits, that will be the day when all major users stop having two networks

• Service providers are aware of this• The network must be able to differentiate

between delay sensitive and delay insensitive applications

QoS requires:

• The ability to reserve resources (Buffers & Bandwidth)

• Prioritisation where network traffic is classified and priority is given according to bandwidth management policy

• These features could be applied to:• An individual data stream

• In aggregate, to flows of a particular type

The Web is an application!

• To many people The Internet and The Web are synonymous

• But we know that The Web is an application that sits at the application level of the Internet

• But is is the biggest, and therefore the most important to most people

• But theoretically it could run over different Transport & Network protocols

A little History

• Tim Berners-Lees from CERN in Switzerland is credited with the invention of the Web in 1989

• He and his colleagues developed the first versions of a Browser, Web server, HTML and HTTP

• Using Hyper-Text work done by Ted Nelson

• Marc Andreeson at the National Center for Supercomputing Applications at the University of Illinois developed the first GUI browser, known as Mosaic. If you look at the “about” Internet Explorer you will see that this work is acknowledged

• Andreeson and a colleague then started Netscape in 1994

Statelessness in HTTP

• HTTP is a stateless protocol – designed to be quick

• When a resource has been sent, the server keeps no record of the exchange, so that if a second request is made by the same client, it is as if this was first contact with that client

• This is not satisfactory for – complex transactions, say completing a multi-page

form– Access requiring identification & authentication

Approaches to keeping track of state in HTTP

• Cookies – small strings of text held in the clients memory or on disk and communicated invisibly between the client and the server

• Hidden fields in an HTML form which are not visible but move back and forward with the form and enabling the server to know the state of the transaction

• Writing complex URLs containing various parameters – used by search engines, allowing the user to request the nest block of data

• Programs in both the browser & the Web server