Application Layer

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Application Layer

• Week 4 Lecture 1

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Internet Architecture
Application layer HTTP, SMTP etc
W3C
Transport layer TCP, UDP
IETF
Network layer - IP
Data link Physical layers the territory of
the LANs/Telcos/ISPs
ITU IEEE
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Variables in Internet service

• Bandwidth
• Latency delay sensitivity
• Error free loss tolerance

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Application requirements
Application Loss Tolerant Delay Sensitive Band- Width
Web No No Medium
E-Mail No No Low
Video streaming Some Some High
Telephone calls Some Yes Low
Video conferencing Some Yes Medium/high
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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

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QoS the key to one network

• When packet switched networks can offer the QoS
  of switched circuits, then all major users will
  stop having two networks
• Service providers are aware of this
• The network must be able to differentiate between
  delay sensitive and delay insensitive applications

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QoS approaches

• The ability to request and receive resource
  reservation (Buffers Bandwidth)
• Prioritisation where network traffic is
  classified and priority given according to
  bandwidth management policy
• These features could be applied to
• An individual data stream
• In aggregate, to flows of a particular type

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IETF is working on

• Developing QoS protocols
• Integrated Services IntServ - Resource
  Reservation Protocol (RSVP)
• Guaranteed Quality of Service
• Controlled load network service
• Differentiated Services (DiffServ) Coarse grained
  and simple way to prioritise services
• Defining classes of service similar to ATM
  traffic classes

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World Wide WEB
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Application Layer The Web HTTP The
Application layer HTML The document mark-up
language Browser Netscape Navigator, Internet
Explorer Web server Apache, Microsoft
Internet Information server Netscape server
Transport and Network layers TCP/IP
Data link and Physical layers Ethernet, Frame
Relay, ATM, PPP
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The Web is an application

• To many people The Internet and The Web are
  synonymous
• 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

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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 Neilson
• 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 still
  acknowledged
• Andreeson and a colleague then started Netscape
  in 1994

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Some definitions

• HTML HyperText Mark-up Language describes how the
  document is structured with tags or meta-data
  imbedded in the document. The Browser then uses
  that meta-data to format the document
• HTTP is the application level protocol or
  service, for establishing connections and
  transmitting messages, between the Browser client
  and the Web server

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HTTP message contains

• Request or Response statement
• Header containing information about the client or
  server
• Data

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Data formats

• The header contains a field specifying the MIME
  type (Multipurpose Internet Mail Extension). This
  was a mail standard that has been adopted by the
  Web.
• Browsers can interpret some MIME types natively
  HTML, ASCII, GIF, JPEG etc
• For others a plug-in or helper application is
  required
• The browser checks the registry to find the
  application registered for that type

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HTTP Version

• HTTP Version 1.0 had been used until 1999
• The problem was that if a document contained 10
  embedded graphics then a separate connection and
  exchange was required for each.
• Version 1.1 introduced in 1999 allowed the
  connection to remain open until all were sent
• This version has been implemented in current
  versions of IE and Netscape browsers.
• There are still problems in 1.1 but little active
  work is underway

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TCP Connections

• HTTP/1.0 uses non-persistent connections but
  objects can be requested in parallel
• Each connection has a cost handshaking,
  allocation of buffers and variable, slow start
• HTTP/1.1 uses persistent connections with
  pipelining as the default
• Persistent connections are dropped on a time out

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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

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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

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Cookies some concerns

• Look in the cookies directory under windows and
  you will find hundreds of them
• They are associated with a particular version of
  a browser so will be of no use when the browser
  is upgraded
• Some security privacy concerns
• You do not know what is being stored
• Can be used to track your behaviour over a number
  of sites and then be associated with your identity

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Application Programming Interfaces

• Programming can be invoked at both the server and
  client side.
• Servers can invoke programs and pass the results
  back in an HTML page
• They can be simple scripts or complex programs.

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Server APIs

• Server side APIs include
• CGI Common Gateway Interface) often in UNIX
• Microsofts Internet Server API (ISAPI)
• Netscapes Netscape Server API (NSAPI)
• Server side languages include
• Java
• Perl
• Active Server Pages
• Microsofts Visual Basic Scripting Language
• Oracles Request Broker

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Client side languages

• In addition to browsers using plug-ins or helper
  applications, Web browsers can also execute
  interpreted programs usually called scripts
• These scripts are theoretically platform
  independent but the browser must have a virtual
  machine with which to run the language. Together
  they provide Dynamic HTML
• Examples of scripting languages are
• JavaScript
• Microsofts Jscript
• Java Applets
• Active X objects

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Techniques for improving Web performance

• Caching
• Load balancing
• Content Distribution Networks

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Caching

• Initially implemented near the client in a proxy
  server operated by the organisation all
  requests are first directed at the proxy server.
  If it cannot supply, then the request is passed
  on to the target server.
• Works on the basis that similar users frequently
  access the same pages between 20-70 of
  requests can be satisfied this way, reducing
  bandwidth on the WAN

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Caching Cont.

• Dynamically created pages cannot be cached
• The risk of out-of-date information is reduced by
  time stamping the page with an expiry time when
  it must be refreshed

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Caching Services

• Caching now provided by service providers that
  maintain an array of cache servers
• Akamii has 2000 servers in 40 countries. The site
  owners decide which pages to be cached
• NLANR is another with a hierachy of backbone and
  regional caches
• One cache can obtain an object from another cache
  using ICP (Internet Caching Protocol)
• Large ISPs serving low bandwidth clients provide
  this service
• Caches are being developed to handle streaming
  video and audio eventually supplying on demand
  music, TV and movies over the Internet

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Load Balancing

• This enables groups of servers to service
  incoming requests
• The request is sent to the server with the lowest
  load
• Cookies can be used to identify high priority
  clients and route their request accordingly
• We saw earlier how DNS can be used to provide
  simple load balancing

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Content Distribution Networks

• This takes load balancing one stage further by
  distributing the servers geographically closer to
  the users.
• This
• Reduces network hops
• Increases overall resilience
• Increases scalability