networks & the internet week 7 – lecture 2. yesterday we discussed how: data is carried over...
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TRANSCRIPT
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