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Chapter: Computer Networks

Application layer

1. Domain name system (DNS) 2. E- mail 2.1 Peer-peer model 3. World wide web 3.1 Hypertext data 3.2 Clustering and classification 3.3 Hyperlink analysis 4. SNMP 5. FTP 6. Web services 7. Multimedia applications 8. Overlay network





There are 3 components:

Name Space:

Specifications for a structured name space and data associated with the names



Client programs that extract information from Name Servers.


Name Servers:

Server programs which hold information about the structure and the names.


A Resolver maps a name to an address and vice versa.


Iterative Resolution


Recursive Resolution


2. E- MAIL


·         What is an Email – an electronic message transmitted over a network from one user to another.


·         Can be as simple as a few lines of text, or include attachments such as pictures or documents.

·         Email made up 75% of network traffic soon after the introduction of the internet.


·         The Header


     Who sent the email.


     To whom the mail is sent.


     When the email was sent.


     The email subject.


     The size of the email.


·         The Body


     Contains the message.


     May also contain an attachment.


·         Attachments Different Architectural Models exist for constructing computer systems.


·         Some models include:


·         Peer-Peer


·         Pipe and Filter


·         Implicit Invocation


·         Client-Server


If not embedded within the body, attachments are sent along with the email. How Email Works


2.1 Peer-Peer Model

     Forms in which clients appear:


·         Application based - these are installed onto user’s machines and include Microsoft Outlook and the freely available Outlook Express and Eudora.

·         Web based - these appear in a web browser’s window and include Hotmail, Yahoo and Outlook web client.


     Clients vary greatly in functionality, but all provide a basic level of functionality that assists the user.

     Basic functions include:


·         Ability to create new emails.


·         Display and store received emails.


·         Hold address lists of contacts, a calendar, journal and other extra functions that help organize the user’s working day.


·         The client is also configured with the account information and names or IP addresses of the email servers with which it will be communicating.


     An email server is typically a combination of processes running on a server with a large storage capacity – a list of users and rules, and the capability to receive, send and store emails and attachments.


     These servers are designed to operate without constant user intervention.


     Should process emails for months as sending, receiving and maintenance tasks are carried out at scheduled times. The client only has to connect to the email server when it sends and checks/receives new email.


     Sometimes it may be permanently connected to the server to allow access to shared address books or calendar information – this is typical of a LAN-based email server.


     Most email servers conduct email services by running two separate processes on the same machine.


     One process is the POP3 (Post Office protocol 3) server, which holds emails in a queue and delivers emails to the client when they are requested.


     The other is the SMTP (simple mail transfer protocol) server that receives outgoing emails from clients and sends and receives email from other SMTP servers.


     These two processes are linked by an internal mail delivery mechanism that moves mail between the POP3 and SMTP servers.


     When the client calls the email server to send or check for mail it connects to the server on certain TCP/IP ports:

·         SMTP on port 25

·         POP3 on port 110.




·         Hypertext documents




·         Web


            billions of documents

            authored by millions of diverse people

            edited by no one in particular

            distributed over millions of computers, connected by variety of media

·         Citation,



·         Ramayana, Mahabharata, Talmud


            branching, non-linear discourse, nested commentary,

·         Dictionary, encyclopedia


            self-contained networks of textual nodes

            joined by referential links

            Memex [Vannevar Bush]

            stands for “memory extension”

            photoelectrical-mechanical storage and computing device

            Aim: to create and help follow hyperlinks across documents


            Coined by Ted Nelson


            Xanadu hypertext: system with


            robust two-way hyperlinks, version management, controversy management, annotation and copyright management.


            Initiated at CERN (the European Organization for Nuclear Research)

            By Tim Berners-Lee


            Berners-Lee (1990)

            Erwise and Viola(1992), Midas (1993)

            Mosaic (1993)

            a hypertext GUI for the X-window system

            HTML: markup language for rendering hypertext


·        HTTP: hypertext transport protocol for sending HTML and other data over the Internet


·        CERN HTTPD: server of hypertext documents


            Hypertext data

Semi-structured or unstructured


·        No schema

Large number of attributes


Purpose of crawling and indexing


·        quick fetching of large number of Web pages into a local repository

·        indexing based on keywords


·        Ordering responses to maximize user’s chances of the first few responses satisfying his information need.


Earliest search engine: Lycos (Jan 1994)


Followed by….


Alta Vista (1995), HotBot and Inktomi, Excite

Yahoo! directory


·        to locate useful Web sites

Efforts for organizing knowledge into ontologies


·        Centralized: (Yahoo!)

·        Decentralized: About.COM and the Open Directory


3.2 Clustering and classification



·        discover groups in the set of documents such that documents within a group are more similar than documents across groups.

·        Subjective disagreements due to

     different similarity measures


     Large feature sets




·        For assisting human efforts in maintaining taxonomies


·        E.g.: IBM's Lotus Notes text processing system & Universal Database text extenders


3.3 Hyperlink analysis

Take advantage of the structure of the Web graph.


·        Indicators of prestige of a page (E.g. citations)

·        HITS & PageRank



·        bibliographic citation graph of academic papers

Topic distillation


·        Adapting to idioms of Web authorship and linking styles

Federations of crawling and search services


·        each specializing in specific topical areas.

Goal-driven Web resource discovery


·        language analysis does not scale to billions of documents

·        counter by throwing more hardware




·        SNMP is a tool (protocol) that allows for remote and local management of items on the network including servers, workstations, routers, switches and other managed devices.


·        Comprised of agents and managers


·        Agent - process running on each managed node collecting information about the device it is running on.


·        Manager - process running on a management workstation that requests information about devices on the network.


Advantages of using SNMP


universally supported



allows distributed management access

lightweight protocol

SNMP is a “client pull” model


The management system (client) “pulls” data from the agent (server). SNMP is a “server push” model


The agent (server) “pushes” out a trap message to a (client) management system


Ports & UDP


SNMP uses User Datagram Protocol (UDP) as the transport mechanism for SNMP messages



The Three Parts of SNMP

SNMP network management is based on three parts:


SNMP Protocol

Defines format of messages exchanged by management systems and agents.


Specifies the Get, GetNext, Set, and Trap operations Structure of Management Information (SMI)


Rules specifying the format used to define objects managed on the network that the SNMP protocol accesses




Management Information Base (MIB)



·        A map of the hierarchical order of all managed objects and how they are accessed


Items in an SNMP Network are called nodes. There are different types of nodes.

·        Managed nodes

·        Management nodes


·        Typically a workstation running some network management & monitoring software Nodes that are not manageable by SNMP


A node may not support SNMP, but may be manageable by SNMP through a proxy agent running on another machine



5. FTP


The File Transfer Protocol (FTP) is a standard network protocol used to transfer computer files from one host to another host over a TCP-based network, such as the Internet. FTP is built on a client-server architecture and uses separate control and data connections between the client and the server. FTP users may authenticate themselves using a clear-text sign-in protocol, normally in the form of a username and password, but can connect anonymously if the server is configured to allow it. For secure transmission that protects the username and password, and encrypts the content, FTP is often secured with SSL/TLS (FTPS). SSH File Transfer Protocol (SFTP) is sometimes also used instead, but is technologically different.


The first FTP client applications were command-line applications developed before operating systems had graphical user interfaces, and are still shipped with most Windows, Unix, and Linux operating systems. Many FTP clients and automation utilities have since been developed for desktops, servers, mobile devices, and hardware, and FTP has been incorporated into productivity applications, such as Web page editors.


This stands for File Transfer Protocol. This is a popular way to transfer files from machine to machine across a network


Ways to connect to an FTP server



Private user-only site. Allows only system users to connect via FTP and access their files.

Anonymous. Allows anyone on the network to connect to it and transfer files without having an account.

            ftp [sitename]

ftp starts the program and connects to the specified site


cd [directory]


cd stands for change directory. This command will change to the spcified directory pwd


Print working directory(tells the user which directory he/she is in) dir [filespec]


List details about the file specification At the command prompt type :

ftp papa.cdrom.com (this will start the ftp and connection to the site)


The system will respond with the message


>connected to sunsite.cnlab-switch.ch.


>220 warchive.cdrom.com.FTP server (Version wu-2.4.2(18)


>Thu nov 26 09:30:12 MET 2001) ready.


>Name (carchive.cdrom.com:usr):

Use ‘dir’ to find the file:




dir l*

to get a listing of all files which start with ‘l’. You should see:

-rw-rw-r– 1 2066 ftp-game 134868 Jun 30 2001 lan.txt

·        Because there is no ‘d’ at the far left, you know that it is a file, not a directory.


·        The 134868 is the file size, it is 134,868 bytes (approx. 134 Kb). It was last modified on the 30th of June 2001.




n Web services provide interoperability between various software applications running on various platforms.


     “vendor, platform, and language agnostic”


Web services leverage open standards and protocols. Protocols and data formats are text based where possible

·         Easy for developers to understand what is going on.


By piggybacking on HTTP, web services can work through many common firewall security measures without requiring changes to their filtering rules.


First the client discovers the service.


·         More in next lecture!


Typically, client then binds to the server.


·         By setting up TCP connection to the discovered address .


·         But binding not always needed.


Next build the SOAP request: (Marshaling)

Fill in what service is needed, and the arguments. Send it to server side.


·         XML is the standard for encoding the data (but is very verbose and results in HUGE overheads)


SOAP router routes the request to the appropriate server(assuming more than one available server)

Can do load balancing here.

Server unpacks the request, (Demarshaling) handles it, computes result.


Result sent back in the reverse direction: from the server to the SOAP router back to the client.


Marshalling Issues

Data exchanged between client and server needs to be in a platform independent format.


·         “Endian”ness differ between machines.


·         Data alignment issue (16/32/64 bits)


·         Multiple floating point representations.


·         Pointers


·         (Have to support legacy systems too)



This is the problem of finding the “right” service


·         In our example, we saw one way to do it – with a URL


·         Web Services community favors what they call a URN: Uniform Resource Name


But the more general approach is to use an intermediary: a discovery service


Discovery and naming


     Many settings, like the big data centers run by large corporations, have rather standard structure. Can we automate discovery?

     How to debug if applications might sometimes bind to the wrong service?


     Delegation and migration are very tricky


     Should a system automatically launch services on demand?


·         Client has opinions


     “I want current map data for Disneyland showing line-lengths for the rides right now”


·         Service has opinions


     Amazon.com would like requests from Ithaca to go to the NJ-3 datacenter, and if possible, to the same server instance within each clustered service





Multimedia is a combination of text, graphic, sound, animation, and video that is delivered interactively to the user by electronic or digitally manipulated means.

     There are a number of fields where multimedia could be of use. Examples are:-


·         Business


·         Education


·         Entertainment


·         Home


Public Places



·         Use and Applications


     Sales / Marketing Presentation


     Trade show production


     Staff Training Application


     Company Kiosk




·         Use and Applications


     Courseware / Simulations


     E-Learning / Distance Learning


     Information Searching




·         Use and Applications


     Games (Leisure / Educational)




     Video on Demand






·         Use and Applications




     Satellite TV


SMS services (chats, voting, reality TV

     Public Places


·         Use and Applications


     Information Kiosk


     Smart Cards, Security


     Briefing Products


     Reference Products


     Database Products


     Education and Training Products




     Entertainment and Games


     Small, straightforward, linear products used to present information quickly and concisely.


     Characteristic of briefing product:


·         Short Development Cycle


·         Limited Number of Presentations


·         Usage of text to present information with limited use of graphic, audio and video.


·         Have few navigational controls. (mouse click and button press to move from one page to another)


     Content and the format are suitable for the audience and fulfill the purpose of the presentation.

Good briefing presentation depends on:


·         The understanding of the presented subject.


·         Seamless integration of content.


·         Consistent layout




·         Corporate Presentation


·         Sales Presentation


·         Educational Lectures


Basic classes of reference product:


·         Generalized Content (dictionary/encyclopedia)


     Broad treatment of content at a limited depth


·         Detailed Content


Focus on specific area and provide extensive information




Multicast improvement over point to point unicast

Embedding of overlay in native network

Improved throughput

–   BGP doesn’t find shortest path

–   Triangle inequality

A logical network built on top of a physical network

–   Overlay links are tunnels through the underlying network

Many logical networks may coexist at once

–   Over the same underlying network

–   And providing its own particular service

Nodes are often end hosts

–   Acting as intermediate nodes that forward traffic

–   Providing a service, such as access to files

Who controls the nodes providing service?

–   The party providing the service (e.g., Akamai)

–   Distributed collection of end users (e.g., peer-to-peer)


Alternative routing strategies

–   No application-level processing at the overlay nodes

–   Packet-delivery service with new routing strategies

Incremental enhancements to IP

–   IPv6

–   Multicast

–   Mobility

–   Security

Revisiting where a function belongs

–   End-system multicast: multicast distribution by end hosts

Customized path selection

–   Resilient Overlay Networks: robust packet delivery


            IP tunnel is a virtual point-to-point link

–   Illusion of a direct link between two separated nodes

–   Encapsulation of the packet inside an IP datagram

–   Node B sends a packet to node E

–   … containing another packet as the payload

            Routing overlays

–   Experimental versions of IP (e.g., 6Bone)

–   Multicast (e.g., MBone and end-system multicast)

–   Robust routing (e.g., Resilient Overlay Networks)

            Types of peer-to-peer networks

–   Directory-based (e.g., original Napster design)

–   Unstructured (e.g., Gnutella, Kazaa, BitTorrent)

–   Structured (e.g., distributed hash tables)

            Challenges in peer-to-peer


– Legal issues, free riding, fast response to queries, peers coming and going over time, reliability, security, …

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