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Week 12 Notes

by: Taylor Kahl

Week 12 Notes CSC 2310

Taylor Kahl
GPA 4.21

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About this Document

This covers all of the lecture and textbook material from chapter 4 - Networks and the Internet
Princliples of Computer Programming
Kebina Manandhar
Class Notes
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This 11 page Class Notes was uploaded by Taylor Kahl on Sunday April 10, 2016. The Class Notes belongs to CSC 2310 at Georgia State University taught by Kebina Manandhar in Winter 2016. Since its upload, it has received 15 views. For similar materials see Princliples of Computer Programming in ComputerScienence at Georgia State University.

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Date Created: 04/10/16
Week 12 Notes: Chapter 4 – Networking & the Internet  Networks – linked computer systems o allow connected computers to exchange data & share resources  Network classifications by scope: o PAN (personal area network) – short-range, less than a few meters  Example: btw a wireless mouse & a PC o LAN (local area network) – a connection of computers in a single building or complex  Example: computers on a university campus o MAN (metropolitan area network) – intermediate size, like a local community o WAN (wide area network) – over large distances, like in different cities or countries  Network classifications by ownership: o open – network’s internal operation is based on designs in the public domain  freely circulated o closed or proprietary – network’s internal operation is based on designs owned & controlled by an individual or corporation  pay/sign contracts to use o The Internet is an open system  Internet communication is governed by the TCP/IP protocol suite, which is free to use  Network classification by topology (the pattern in which machines are connected): o bus – all machines connected to a common communication line, the bus  machines communicate directly w/ each other via the bus  Example: Ethernet o star – one machine is central point that all others connect to  machines communicate indirectly via the central machine  Example: wireless network with machine coordinated by the central machine (also called the access point or AP)  Protocols – rules for activity within a network o by adopting protocol standards, companies can make products that are compatible with other companies’ to work in the same network o CSMA/CD (Carrier Sense, Multiple Access with Collision Detection) protocols  used in bus networks for transmitting messages  each message is sent to every machine on the bus  but each machine only keeps messages addressed to itself  if a machine has to transmit a message, it waits until the bus is silent and begins transmitting while continuing to check for new transmissions  if another machine begins transmitting, they both pause and retry after a random time interval  these protocols are not compatible with wireless star networks where machines communicate through a central AP bc machines can’t detect when they are simultaneously trying to transmit messages  hidden terminal problem – when machines on a wireless star network can’t detect that their transmissions are colliding because the machines’ signals are blocked from each other by objects or distance o Therefore, wireless networks use protocols to avoid collision, rather than detect it o CSMA/CA (Carrier Sense, Multiple Access with Collision Avoidance) protocols  used in WiFi  won’t entirely eliminate the hidden terminal problem  if messages collide, they must be retransmitted  gives priority to machines that were already waiting to transmit  when a machine detects that the communication channel is silent, it waits a brief period – if channel is still silent, it begins transmitting  if the channel is busy, the machine waits for a random time until trying again  after the wait period, the machine can claim a silent channel  if new machines try to transmit, they’ll be blocked  Solving the hidden terminal problem:  some WiFi networks require machines to send a request to the AP to transmit  if the AP is busy with another transmission, it will ignore the request  otherwise, the AP acknowledges the request  Combining networks: o most devices connect networks that are the same type (or “compatible”) o repeater – passes all signals btw 2 connected buses o bridge – also passes signals btw 2 buses  but checks the destination of messages and only sends messages across the connection if the final destination is on the other side  more efficient than a repeater o switch – a bridge w/ multiple connections; can connect more than 2 buses  like a bridge, also checks the messages’ final destinations and only forwards to other buses if that’s where they’re addressed o these connections form a single large network  the system operates using the same protocols as the original smaller networks  Internet – a network of networks with incompatible types o the original networks remain independent ***This term is different from “the Internet,” aka the World Wide Web o routers – special computers for forwarding messages  connect the networks of an internet o Every machine in the internet has 2 addresses:  its original local address within the network  its internet address  Inter-process communication – how different processes being executed on computers within a network, or within the same machine, communicate & coordinate w/ each other o client-server model – processes are considered clients or servers  clients make requests of other processes; server satisfies the requests  one server for many clients  the clients initiate communication  server must execute continuously so it can serve a client at any time  Example: a printer is a server that executes requests from computers (clients) all over the network o peer-to-peer model (P2P) – processes provide and receive services to/from each other  processes only have to execute temporarily  Example: instant messaging; interactive gaming; file sharing  Distributed systems – software systems with different parts on different computers o cluster computing – many independent computers work together to provide computation ability that compares to a much larger machine o grid computing – multiple computers working together, but more loosely coupled  computers can volunteer power to the grid when they’re not otherwise being used o cloud computing – huge pools of shared computers can be allocated for client programs as needed  Amazon’s Elastic Compute Cloud – lets clients rent virtual computers by the hour  Google Drive – allows users to collaborate & build web services  cloud computing lets people use programs even when they’re nowhere near the computer hardware  raises concerns about privacy and security when you don’t know who owns the computers you’re using  The Internet – a world-wide internet; combination of PANs, LANs, MANs, WANs  Internet Architecture: o Internet Service Providers (ISPs) maintain the organization of networks  classified in hierarchal relationship by their role in overall Internet structure  tier-1: high-speed, high-capacity international WANs  “backbone” of the Internet  operated by large communications businesses  tier-2: more regional, lower capabilities  also usually operated by communications businesses  tier-1 & tier-2 ISPs are networks of routers that collectively provide the Internet’s communication infrastructure  tier-3 or access ISP – provide users access to the Internet  end system – devices users use to access ISPs (laptops, cell phones, etc)  hot spots provide access within an AP’s range  telephone lines and cable systems provide direct access  fiber optics provide high-speed digital access  Internet Addressing: o IP addresses – unique address given to each computer in the system  originally 32-bit patterns; currently converting to 128-bit addresses  written in dotted decimal notation  each byte (8 bits) is expressed as an integer and separated by periods  Example: 17.12.25 represents the bit pattern 00010001 00001100 00011001 o The Internet has alternative addressing which is easier for people to remember – gives every machine a mnemonic name o this system is based on the concept of a domain – a “region” of the Internet operated by a single entity  each domain is assigned a unique mnemonic domain name  Example:  top-level domains (TLDs) – the suffix in a domain name  edu for educational domains  com for commercial  gov for government o When transmitting messages, machines must translate the mnemonic address into its IP address  name servers – directories that translate the addresses  domain name system (DNS) – an Internet-wide directory made up of name servers  DNS lookup – the process of using DNS to perform a translation  ICANN (Internet Corporation for Assigned Names and Numbers) – a nonprofit organization which coordinates the Internet’s operation o gives blocks of consecutively numbered IP addresses to ISPs  ISPs then assign those addresses within their region o oversees domain registration and assigns domain names to registered domains  Early Internet applications: o At first, most applications each followed their own network protocols  Newsreader applications contacted servers using the NNTP (Network News Transfer Protocol)  Applications that copied and listed files across the network used the FTP (File Transfer Protocol)  Applications for accessing other computers over large distances used the Telenet or SSH (Secure Shell) protocol o As the Internet becomes more sophisticated, more applications are handled by webpages with the powerful HTTP (Hyper Text Transfer Protocol)  Email (electronic mail) – domain servers collect incoming mail & transmit outgoing mail o transmission is run via the SMTP (Simple Mail Transfer Protocol)  First a mail handling process from one domain contacts the mail server process of the other domain, using DNS to find the mail server name & its IP address  when the recipient server process answers, it identifies itself to the caller o Example SMTP exchange: 220 SMTP Sendmail Gallifrey-1.0; Fri, 23 Aug 2413 14:34:10 (the remote mail server answers the caller by announcing its name, the protocol it speaks, and optional info like date/time) HELO (the sending mail server introduces itself) 250 Hello, pleased to meet you MAIL From: (sending server announces that it has a new message, and the sender) 250 2.1.0 Sender ok RCPT To: (sending server identifies the recipient) 250 2.1.5 Recipient ok DATA (the sending server is ready to send the message) 354 Enter mail, end with "." on a line by itself (the remote server provides instructions for how to send) Subject: Extermination. EXTERMINATE! Regards, Dalek . (ends message) 250 2.0.0 r7NJYAEl028071 Message accepted for delivery QUIT 221 2.0.0 closing connection o protocols for accessing email that has arrived and accumulated at a user’s mail server:  POP3 (Post Office Protocol version 3) – lets the user transfer messages to his local computer, using its mass storage  IMAP (internet Mail Access Protocol) – lets the user store and manipulate messages on the same machine as the mail server  Later Internet Applications: o VoIP (Voice over Internet Protocol) – transmit audio data over the Internet o Internet multimedia streaming – transporting audio and video across the Internet in real-time o Different techniques for streaming:  N-unicast – 1 server sends messages to each client who requests them  burdens the server, and the server’s Internet neighbors  Multicast – transfers distribution to Internet routers, which forward copies of the message to their destinations  on-demand streaming – lets users use media at any time o large-scale streaming uses CDNs (content delivery networks) with groups of servers distributed around the Internet to stream copies of content to nearby users  The World Wide Web – combination of internet technology w/ the concept of hypertext – linked-documents o a format for embedding hyperlinks to other documents  Software packages that allow users to access hypertext on the Internet are either browsers or webservers o browsers – reside on user’s computer. Obtain materials requested by the user & present them in an organized manner o webserver – resides on computer containing the hypertext documents to be accessed. Provides access to documents requested by browsers o hypertext documents are transferred btw browsers & webservers using HTTP o each hypertext document is given a unique URL (Uniform Resource Locator) address o Example URL:  http – protocol required to access the document  – mnemonic name of host holding the document  authors/Shakespeare – directory path, the location of the document  Julias_Ceaser.html – name of the document  HTML (Hypertext Markup Language) o a traditional hypertext document is encoded as a text file o but unlike text files, it also contains tags – symbols that describe:  how the document should appear on a display screen  Example: <h1> starts a level 1 heading  <p> starts a new paragraph  what multimedia should accompany the document  Example: <img src … > inserts an image  which items within it are linked to other documents  Example: <a href … > links to other content o HTML is the system that uses tags o browsers read the tags to learn how to present the text on screen  Example of a page encoded using HTML: <html> (this begins the page) <head> (this begins the header) <title>demonstration page</title> (naming the title – the title won’t be displayed on the webpage) </head> (ends the header) <body> (begins the body) <img src=””> (inserts the JPEG image from this webserver) <h1>My Web Page</h1> (“My Web Page” is a level-one heading – should be displayed prominently) <p>Click (begins a paragraph of text, starting with “Click”) <a href=””> (a link to this webpage) here (“here” will be displayed as a hyperlink) </a> (<a> and </a> are anchor tags – the text inside them becomes hyperlinked) for another page.</p> (ends the paragraph text) </body> (ends the body) </html> (ends the page)  Client-side & server-side activities o browsers act like clients, using info in a URL to request a copy of the page from its webserver o client-side activities are performed by a client (like a browser) o server-side activities are performed by a server (like a webserver) o Example: using a browser to access a search engine, a user requests a particular topic (client-side) – the topic is transferred to the search engine, which constructs a webpage of possible documents of interest (server-side) o Systems for controlling client-side activities:  JavaScript programs within the HTML source document  Macromedia Flash allows multimedia client-side presentations o Systems for controlling server-side activities:  CGI (Common Gateway Interface) – standards used by clients to request the execution of programs stored on a server  Servlets – clients cause servlets (program units) to be executed server-side  JSP (JavaServer Pages) and ASP (Active Server Pages) – webpage templates stored at the webserver; the client provides information to make a customized page  PHP (Personal Home Page Hypertext Processor) – open-source system for implementing server-side functions  The layered approach to Internet software  message transmission occurs through a hierarchy of 4 software layers: o application – where the message originates; provides the address of the final destination o transport – accepts messages from application layer, ensures they’re properly formatted  divides long messages into shorter segments called packets, which are transported individually  adds sequence numbers so the packets can be rearranged properly o network – decides how the packets should be routed through the Internet o link – receives & transmits packets to the next machine o link layer at the other end of the connection receives the packet o network layer decides where the packet needs to go next o packet goes back to the link layer to be transmitted to the next machine  the link & router levels exist within routers o at the final destination, the network layer recognizes that the packet has arrived o transport layer reconstructs the original message o message goes to the application layer  The TCP/IP Protocol Suite o The TCP/IP Protocol Suite is used to implement the transmission hierarchy o Protocol for the transport layer:  TCP (Transmission Control Protocol)  UDP (User Datagram Protocol)  the difference btw these protocols is similar to the difference btw 2 different package shipping companies – same basic task, but different characteristics  TCP is more reliable but less efficient  UDP is more efficient by less reliable o Protocol for the network layer:  IP (Internet Protocol) implements forwarding & routing of packets  IPv4 (version 4) – uses 32-bit Internet addresses o but the Internet is outgrowing this addressing system  IPv6 will use a 128-bit addressing system  Security  Forms of attack: o malware – malicious software  virus – infects a computer by inserting itself into programs that are already on it  when host program is executed, so is virus program  worm – program that transfers itself through a network by getting into computers & forwarding copies of itself to others  Trojan horse – willingly imported, disguised as another program  often through email attachments  spyware (or sniffing software) – collects info about the computer it’s on; sends that info back to the attacker  phishing software – asks for personal info o DoS (Denial of Service) – overload computers with messages to disrupt all of the computers in the network o spam – attempt to overwhelm an individual with excessive amounts of email – often contain phishing & Trojan horses  Protection: o firewall – filters traffic passing through a network  could block incoming messages from certain origin addresses or block outgoing o spam filters – firewalls to block unwanted email o proxy server – an intermediate step btw client & server communication  can shield the client from adverse server activity o antivirus software – detects & removes known viruses  has to be updated constantly to adapt to new viruses  Encryption – encodes information so that, even if others gain access to it, it will remain unknown  traditional Internet applications have been updated to secure versions that incorporate encryption: o HTTPS – secure version of HTTP  gives users secure Internet access to accounts  uses the protocol SSL (Secure Sockets Layer) for secure connections btw servers & Web clients o public-key encryption:  a public key, which is used to encrypt messages is given to anyone needing to send messages to a particular destination  the private key, used to decrypt messages, is kept secret at the destination  so that only the party at the destination can decrypt them o certificates are used to establish that the public key you’re given is in fact from a trusted source, not from an imposter o certificate authorities – trusted Internet sites who maintain accurate lists of parties & their public keys  the authorities, acting as servers, provide clients the public-key information in certificates – packages containing a party’s name & their public key o authentication – ensuring that the author of a message is not an impostor  some public-key encryption systems reverse the keys  the text is encrypted w/ a private key  this encryption pattern produces a digital signature which only the sender knows how to produce  the receiver uses the public key to decrypt the signature to verify that the sender is authentic


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